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How changing the formulation and structure of
Carbopol and Pemulen polymers affect the
rheology
Matthew WearonUniversity of Greenwich
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ContentsAbstract.....................................................................................................................................3Introduction...............................................................................................................................4Objectives and Hypothesis....................................................................................................8Materials and Methods...........................................................................................................9Method,...................................................................................................................................10Results....................................................................................................................................11Discussion..............................................................................................................................33Conclusions............................................................................................................................45Further work...........................................................................................................................45Bibliography............................................................................................................................46Appendix.................................................................................................................................47
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AbstractThe research presented in this thesis looks at changes to the structure and formulation of three different polymers and the effect this has on their rheology.
Polyacrylic acid, Carbopol 974P and hydrophobically modified polymers Pemulen TR2-NF and Pemulen TR1-NF where investigated.
This involved changing concentration, pH, addition of salt and addition of SDS surfactant.
The results primarily could be explained by chain expansion and production of microstructures from hydrophobic aggregation.
Similar trends between the polymers could be seen but direct comparisons could not be made due to expected differences in molecular weights.
Pemulen TR2-NF the more modified polymer showed both a decrease in viscosity and shear stress in comparison to Pemulen TR1-NF.
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Introduction
Rheology relates to the deformation of matter from the application of force. One way rheology can be measured is via a rheometer that measures shear stress against shear rate. The two are linked by the well-known property viscosity, where a compound may have high viscosity such as honey or low viscosity such as water.
If we think of matter divided into layers and ball being pushed against these layers, the ball would only interact with the middle two layers in low viscosity. However with high viscosity, for example honey. The ball would interact with the centre layers but also many of them around, causing drag among other layers. This is due to the fact the interactions between the layers are stronger in the honey than they are in water.
The friction between moving molecules in a liquid is called viscosity, the stronger the intermolecular forces such as hydrogen bonds in a liquid higher the viscosity.
(Shaw, 1992) Rheological behaviour can be seen as Newtonian viscous fluids that obey Newton’s law of viscosity, equation 1. Examples of Newtonian fluids include water and honey where the viscosity is only temperature dependent. Plotting shear stress against shear rate gives a straight line showing proportionality. Here in equation 1. Viscosity is shown to equal shear stress multiplied by shear rate.
η = τ γ
Eq.1 Newton’s law of viscosity.
Alternatively many fluids are described as non-Newtonian and their viscosity is dependent on shear rate. Many well-known fluids exhibit Non Newtonian viscosity including mayonnaise, toothpaste, blood and paint. These fluids take advantage of the non-Newtonian behaviour and are dependent on forces placed on them.
Non Newtonian fluids deviate from the Newtonian plot of shear stress against shear rate and are most commonly described as either shear thickening or shear thinning. Various models and computations can be used to characterise further the non-Newtonian properties.
Other non-Newtonian fluids are Bingham plastics and anti-thixotropic fluids. Here the fluid may exhibit the need for a particular shear stress before their properties change or time related shear stress retrospectively.
Shear thickening fluids increase their viscosity according to increase in shear rate. A recent development of this has been in armour protection where the fluid coats Kevlar and provides increase protection upon impact.
Shear thinning fluids have the opposite effect and decrease viscosity according to increasing shear rate. This property is used in paints where the fluid is thinned to apply to the wall at sufficient viscosity. Shear thinning is also seen in sauces where the action of shaking can cause decrease in viscosity and flow out of the bottle.
Polymers are a range of macromolecules of repeating units that give rise to many different useful functions including resistance to other chemicals, thermal and electrical insulating, light weight and strong, various characteristics and properties otherwise unseen. One of the most common polymers others are based on is polyacrylic acid where a carboxylic group is the repeating unit. Variations of
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polyacrylic acid can be produced because of the reactivity of the carboxylic group including polyesters and polynitriles. One modified group of polyacrylic acid that may apply to any polymer is that of hydrophobically modified polymers (HMP’s) here hydrophobic groups are added in between the carboxylate chains for functionality. It is the functionalities of these groups that gives HMP’s another name of associative thickeners.
The polymers Carbopol 974P, Pemulen TR-1 and Pemulen TR-2 are shear thinning polymers, this is seen over a wide range of shear. The polymers are of use in paint formulations, and are large structures with many ionisable parts and in the case of Pemulen TR-1 and Pemulen TR-2 have been hydrophobically modified to increase hydrophobic areas along the chain. This causes increased hydrophobic areas that affect the viscosity of polymer.
Carbopol and Pemulen polymers are described in the literature as polymers of acrylic acid, crosslinked with polyalkenyl ethers or divinyl glycol. Carbopol 9747 is a highly crosslinked polymer. Quantifying the polymers can prove difficult as the Carbopol polymers are produced by free radicals and give random polymers. The pKa is described at 6+/-0.5 (Lubrizol, n.d.).
The three polymers are based on polyacrylic acid and Pemulen TR1 and Pemulen TR2 have ionisable carboxylic groups along the backbone. This hydrophilic back bone allows the polymer to be water soluble. And at low pH cause the polymer to sit in a random coil in solution with little interaction along the chain, whilst the polymer is in this random coil formation it is often insoluble in aqueous solution and gives the solution a very low viscosity. The potential for these groups to be ionised means the polymer can be classed as an anionic polymer or polyelectrolyte. When the polymer is in this state small intramolecular hydrophobic associations take precedent allowing the polymer chain to curl up and gives reduced viscosity. With increased pH the carboxylic groups ionise and give increased electrostatic repulsive forces between the carboxylate groups causing the polymer chain to extend and increase viscosity.
The extension of the polyelectrolyte backbone allows the already cross linked polymers to entangle with other polymer chains increasing the viscosity. Overlap and entanglement of these long chains hinders the movement of the polymer and increased viscosity. When the polymer is in its initial dilute phase there are very few intermolecular interactions between polymer chains, the interactions are of the intramolecular hydrophobic parts that cause the random coil to be tighter than it may be in unmodified polymer. As the concentration increases there are more polymer chains and more chance for intermolecular interactions with other polymer chains to increase viscosity.
The interesting part comes that parts of the polymers chains are hydrophobically modified therefore increasing hydrophobic interactions when the polymer extends. The hydrophobic interactions help dictate the movement of the polymer through solution due to steric hindrance and changes in entropy. The dissolution of hydrocarbons in aqueous conditions gives negative entropy change and means the reaction is non spontaneous. A solvent cage is formed around the hydrophobic part where the water molecules adopt a less disordered arrangement than in the rest of the solution. It is favourable when there is more than one hydrophobic region as with the polymers for these hydrophobic regions to group together, this allows the water molecules to move more freely and preferably increases the entropy of the system.
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This grouping together hinders the movement of the polymer in solution and increases viscosity. The intermolecular hydrophobic association of various chains in solution give the polymer its three dimensional structure. The bond is non covalent and differences in entropy and thermal fluctuations causes the polymer to constantly break and be reformed. (Shedge), n.d.)The self-assembly of hydrophobic groups occurs after a certain concentration, the viscosity can be increased much more than in unmodified polymer. These hydrophobic interactions occur at a lower concentration than the actual polymer entanglement.
Hydrophobically modified polymers where initially studied by Strauss in the 1950s, where the idea originated from the fact that surfactants formed micelles. Hydrophobically modified cellulose was first used as a paint thickener in the 1980s and is probably the most common type of hydrophobically modified polymer.
Literature describes the strength of these hydrophobic interactions as dependent on, length of the hydrophobic groups, and the time each microstructure spends together, as the microstructures are continually at equilibrium due to Brownian motion and thermal fluctuations. Also the type of hydrophobic group determines the intra and inter molecular interactions. (Leif Karlson, n.d.)
The time of hydrophobic association has been discussed by Tripathi (AnubhavTripathi, n.d.). Here they estimate energy barrier of hydrophobic regions to detach spontaneously. This is seen to be dependent on the chain length, and takes into consideration both the natural thermal vibration frequency and 10e8-10e10Hz and the Gibbs free energy of the system.
Because of this hydrophobic property it can be seen that the number of hydrophobic regions and entanglements will increase with concentration therefore a comparison of the system at different concentrations shows the production of these networks. The solvent the polymer is immersed in will affect these hydrophobic parts, as with water there is a greater repulsion of water to hydrophobic region but this repulsion decreases in less polar solvents such as alcohols.
It is obvious that in a model of strings as polymer chains that increasing the polymer length would increase the number of entanglements and intermolecular interactions with other strings. The hydrophobic polymers may either have the hydrophobic moieties adjoined along the backbone of the polymer and be considered comb like or have them positioned at the ends of the chains named end capped. (Leif Karlson,n.d.)
The shear thinning properties of the polymers have been previously noted, many HMP have shear thinning properties because of their sensitivity to shear. The polymers entangled state allow for a microstructure network, this increases the viscosity of the solution and hinders the movement of the polymer. When the polymer solution undergoes shear the polymer chains are physically detangled and line up with the associated flow, thus reducing the viscosity. The ability to shear can be seen by the strength of the interactions (Padua, n.d.) with hydrophobic interactions under 10kcal/mol and electrostatic repulsion between 1-20kcal/mol it’s easy to see how the physical process of shear can break these non-covalent bonds, especially when we compare these interactions to the normal strength of a C-C bond at 86kcal/mol.
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It can be possible in this study the effect of the hydrophobic groups. If we take away the ability of the polymer backbone to extend by the addition of salt chain, the rheology we see is particular only to the hydrophobic regions of coiled polymers.
As discussed previously with the idea that increasing amounts of SDS can flood the system and cause the SDS micelles to outnumber the hydrophobic regions we can see the effect this has on rheology.
Previous studies of these interactions include,
Shear thickening HMP’s have been used in for oil recovery to decrease the mobility of water during the water flooding process. The HMP’s are useful to this process because of their aptitude to harsh conditions such as pH changes and addition of salt source.
The hydrophobic interactions have been shown to provide stiffness and steric hindrance to the polymer network giving changes in the rheology. A.K.M Lau, Rheology of HASE (Lau, n.d.) explains that the hydrophobic regions interact with latex particles and surfactants by adsorption or intermolecular associations giving a network structure
The production of these network structure due to the hydrophobic interactions change the rheology of the polymer solutions (English, 97).
Polyelectrolyte back bones expand over three concentration ranges, random coils, semi entanglement and entanglement (Leif Karlson, n.d.)
Selb et al, (Selb, n.d.) have seen differences between block hydrophobically modified polymers and randomly arranged hydrophobes. Due to their interactions the blocked polymers have been seen to give less viscous solutions.
(Colby, n.d.) Adding salt to the polymer solution allows the sodium to make an ionic bond to the carboxylic group, this decreases repulsions and causes to the chain to recoil. The less interactions along the chain r the more it will be in random coil conformation. It’s the electrostatic repulsions that keep the polymer straight and viscosity high.
There are lots of formulations that involve the use of polymers with surfactants, including hydrophobically modified polymers. These include paints, liquid detergents and shampoos. The involvement of surfactants causes micelles to be produced, when surfactant concentration is increased to beyond the critical aggregation concentration. This critical aggregation constant is lower than what is seen for critical micelle concentration in aqueous conditions (website, n.d.).
The hydrophobic microstructures are strengthened by the SDS surfactant initially and cause increase in the lifetime of the micelle structure.
(Colbe, n.d.)Colby et al explain by increasing the concentration increases the HMP’s intermolecular associations until a maximum concentration. After this the solution is saturated and is dominated by surfactant with isolated mixed HMP’s.
(Somasudarah, n.d.) Sivadasan and Somasudarah, show that surfactants hydrophobic tails interact with polymers hydrophobic parts to give mixed surfactant-polymer hydrophobic micellar complexes.
As seen by Lund (Leif Karlson, n.d.), including surfactants in the solution increases the activation energy to remove the hydrophobic group from the micelle which will
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increase the time the hydrophobic group spends in the micelle giving stronger associations.
Lund (Leif Karlson, n.d.) shows that excess surfactant disrupts the hydrophobic domains along the polymer chain and although the selection of hydrophobes that are joined with SDS to make micelles is random, determining the increase in viscosity can go some way to determining the mechanism for thickening through hindering hydrophobic regions.
Objectives and Hypothesis To understand the changes to rheology of polymers Carbopol, Pemulen 1 and 2 by adjusting pH, concentration, ionic strength and surfactant SDS. To do this the shear stress and viscosity data will be manipulated accordingly.
To be able to appreciate the stages of concentration increase in the system and the changes they produce.
To understand the effect that pH has on the polymer backbone. The hypothesis is to see expansion of polyacrylic backbone that will increase viscosity, for this extension to be disrupted by the addition of salt to the system and decreased viscosity.
To confirm that the three polymers are shear thinning as expected.
To determine that the increased hydrophobicity of Pemulen 2 from Pemulen 1 would be emulated in increased viscosity.
To see that the increased viscosity that hydrophobic associations produce.
To determine that addition of SDS surfactant initially increases the strength and increases the lifetime of the hydrophobic domains and this would show as increased viscosity.
To attempt some understanding as to the mechanism of thickening of the Pemulen polymers.
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Materials and MethodsMaterials, Polymers Pemulen TR-1 NF (Pemulen 1), Pemulen TR-2 NF (Pemulen 2) and Carbopol 974P where sourced from Surfachem Group Limited. Images are included in the appendix.
Sodium Chloride and Sodium dodecyl sulfate where used to alter the formulation of the polymer solution. Along with Hydrochloric acid and Sodium Hydroxide to change the pH.
Diagram 1. Shows the crosslinked polyacrylic network of Carbopol 974P. (Lubrizol,n.d.)
Diagram 1.1 shows the added hydrophobic regions along the polyacrylic backbone. (Maryland, n.d.)
Diagram 1.2 SDS molecule (Bristol, n.d.).
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Method, Samples where made up in vials at a total of 15ml to different weight percentages by dissolving the polymers in distilled water to give different wt% concentrations.
Various pH’s where achieved using Hydrochloric acid and Sodium Hydroxide. The pH of the polymer solutions where chosen relative to the pKa value recorded in the literature as previously discussed.
A range of polymer solutions where made up at different pH between 1 and 7. With concentrations between 0.05wt% to 1wt% for each polymer.
Solutions including sodium chloride were also made up with between 0.5wt% to 3wt% polymer and 1mM to 10mM NaCl concentrations.
Sodium dodecyl sulfate (SDS) solutions were also made of 1% polymer and between 0.005wt% to 10wt% SDS concentration.
It was taken that the polymer solution must form a homogenous mixture to enable good readings from the rheometer and a produce a worthwhile sample.
Samples were analysed by a Bohlin Gemeni Rheometer. A copy of the manual can be found in Nelson 244. Using Viscometry setting with controlled staggered shear rate at 25 degrees. Sample data produced as well as settings are shown in appendix. The data produced was further exported to Excel for manipulation.
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ResultsThe results from the rheometer where extracted from the software and copied to Excel. Primarily only Shear rate1/s, Shear Stress Pa and Viscosity Pas where used and these are the results shown.
The literature values for the rheology of these polymers are that they are all shear thinning (Lubrizol, n.d.) (Lubrizol, n.d.). Most other values for the rheology of the polymers have been measured using a Brookfield Viscometer and are not comparable.
Carbopol, compliance with shear thinning profile,
0 200 400 600 800 1000 12000
200
400
600
800
1000
1200
1400Carbopol at pH7 viscosity decrease upon shear
0.2wt%
0.5wt%
1wt%
Shear Rate 1/s
Visco
sity P
as
Figure 1 viscosity decrease of Carbopol 974
The steep decrease in viscosity is difficult to pick out as it occurs at such low shear rates and the decrease in viscosity is so great.
At pH7 with highest concentration of polymer at 1wt%. The viscosity is at 1180Pas under the lowest shear of 0.09956 1/s and decreases to 136.1 Pas at 1.263 1/s shear.
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Carbopol, change in concentration effect on viscosity and shear stress.
It was seen that polymer concentrations 0.2wt%, 0.5% and 1.0wt% made homogenous mixtures that could be measured using the rheometer.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10
200
400
600
800
1000
1200
1400
Shear Viscosity vs wt% concentration
pH7 pH5
wt%
vis
cosi
ty a
t 0.0
9954
s-1
she
ar
Figure 2 Viscosity at 0.09954s-1 shear, with increase in wt% showing increasing viscosity with concentration.
The viscosity increase shown in figure 2 is linear due to the polymer being fully extended. The non-linearity in pH5 is due to intramolecular interactions along the same polymer chain.
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-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
increasing wt% at pH7
0.2wt% 0.5wt% 1.0wt%
log shear rate
log
visc
osity
Figure 2.1 log shear rate s-1 against log viscosity at increasing concentration.
Figure 2.1 shows us similar log viscosity plots for 0.5wt% and 1.0wt% at pH7. This indicates a maximum for homogenous mixtures to give greatest viscosity at around 0.5wt% after this the viscosity will show little increase. The polymer chains are extended as far as they will go and increasing the concentration does not result in increased shear viscosity.
0 200 400 600 800 1000 12000
100
200
300
400
500
600
700
800
900
Shear stress profile of Carbopol at pH7
C1 0.2wt% pH7 C 0.5wt% pH7 C1 1wt% pH7
Shear Rate 1/s
Shea
r Stre
ss P
a
Figure 2.2 increase in shear stress profile as wt% concentration increases at pH7.
A large increase in shear stress is seen between lower 0.2wt% and higher concentrations compared to the increase between polymer weights 0.5wt% and 1wt%
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Carbopol 974, change in pH effect on viscosity and shear stress.
Carbopol 974 made homogenous mixtures at pH3, 5 and 7 with 1wt% polymer and at pH 7 and pH5 with 0.2wt% and 0.5wt% polymer concentrations.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
pH extending polymer chain at 0.2wt%
pH5 pH7
log shear rate
log
visc
osity
Figure 3 shows Carbopol 0.2 wt%, log shear s-1 against log viscosity at different pH.
Figure 3 shows a small increase in viscosity of Carbopol is seen with the increase from pH5 to pH7. This is due to the extension of polymer chains and increased overlap and stiffness created due to this.
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0 200 400 600 800 1000 12000
20
40
60
80
100
120
140
160
180
Shear stress change due to pH change
0.2 wt% pH7
0.2wt% pH5
Shear Rate 1/s
Shea
r Str
ess P
a
Figure 3.1 shows changes in shear stress profiles due to adjusting pH at 0.2wt%
The increase in shear stress curve of figure 3.1 is relative to the increase in viscosity and creation of increased three dimensional structures.
Carbopol, salt additionAddition of salt produced homogenous samples that could be analysed with Carbopol 3wt% 1mM, 3mM and 5mM NaCl.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
viscosity affected by salt
1mM NaCl 5mM NaCl 10mM NaCl
log shear rate 1/s
log v
iscos
ity P
as
Figure 4 shows decrease in viscosity by addition of salt
The addition of salt in figure 4 has taken away the electrostatic repulsions created by increased pH and polymer extension.
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1mM NaCl 5mM NaCl 10mM NaCl0
10
20
30
40
50
60 55.91
33.5731.02
Viscosity at 0.3546 s-1 shear
Salt Concentration
Visc
osity
Figure 4.1 comparison of viscosity at 0.3546s-1 shear rate.
Figure 4.1 shows the increase in salt has caused decrease in repulsions and leaves no interactions and the polymer able to recoil.
0 200 400 600 800 1000 12000
50
100
150
200
250
300
shear stress of C 3wt% with salt 1,5,10mM
C3wt% 1mM NaCl C3wt% 5mM NaCl C3wt% 10mM NaCl
Shear Rate 1/s
Shea
r Stre
ss P
a
Figure 4.3 shear stress vs shear rate curve.
The decrease in stress curve of each increasing concentration of salt causes less polymer structure to be affected by shear and feel stress.
Pemulen 1, compliance with shear thinning
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0 200 400 600 800 1000 12000
50
100
150
200
250
300
Shear thinning profile of Pemulen 1
0.2wt% 0.5wt% 1wt%
Shear Rate 1/s
Visc
osity
Pas
Figure 5. Shear thinning profile of Pemulen 1
Again Figure 1 shows the expected shear thinning curve of the polymers.
Pemulen 1, change in concentration effect on viscosity and shear stress.
Pemulen 1 solutions made homogenous mixtures at 0.2wt%, 0.5wt% and 1wt%, at pH 7 and pH5.
0.2 0.5 10
50
100
150
200
250
300
44.04
168.5
282.7
Viscosity change of Pemulen 1 at pH7 and 0.09957s-1 shear
wt% Pemulen 1
Visc
osity
Pas
Figure 6 increased concentration on viscosity of Pemulen 1 at pH7
Increased viscosity seen through increasing concentrations of Pemulen 1 at pH7 for initial shear.
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-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
log viscosity vs log shear rate s-1 of Pemulen 1 at pH7
P1 0.2wt% pH 7 p1 0.5wt% pH7 P1 1wt% pH 7
log shear rate s-1
log v
iscos
ity
Figure 6.1 log plot of viscosity of Pemulen 1 at increasing concentrations
Figure 6.1 shows increase in viscosity from concentration. It shows increase from 0.2wt% and small increase from 0.5wt% to 1wt%. Theoretical weights between 0.2 to 0.5wt% could possibly fit between the gap and show regular increase. However the small increase from 0.5wt% to 1wt% shows a maximum is being reached.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
Pemulen 1
P1 0.2wt% pH5 P1 0.5 wt% pH5 p1 1wt% pH 5
Shear Rate 1/s
Visc
osity
Pas
Figure 6.2 shows increase between 0.5wt% and 1wt% at pH5
Figure 6.2 shows the polymer chains not fully extended at pH5, compared to figure 6.1 it shows differences in viscosity at 0.5 and 1.0 wt% polymer.
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10
100
200
300
400
500
600
Pemulen 1 viscosity at 0.009956s-1 shear
pH 5 pH7
wt%
Visc
osity
Pas
Figure 6.3 increase in viscosity of Pemulen 1 at 0.09956s-1 shear
Figure 6.3 shows sudden increase in viscosity at pH5 between 0.5wt% and 1wt%. It shows that increased extension of the polymer does not necessarily give increased viscosity as seen in unmodified polymers.
0 200 400 600 800 1000 12000
50
100
150
200
250
300
350
400
450
500
Shear Stress profile of Pemulen 1 at pH7
0.5wt% 0.2wt% 1.0wt%
Shear Rate 1/s
Shea
r Stre
ss P
a
Figure 6.3 showing increase in shear stress curve with increased wt% Pemulen 1
The greater shear in 6.3 is an effect of the increased viscosity seen previously.
Pemulen 1, change in pH effect on viscosity and shear stress.
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-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
log plot of change in pH of Pemulen 1 at 0.2wt%
P1 0.2wt% pH 7 P1 0.2wt% pH5
Figure 7. No difference in viscosity between pH5 and pH7 of Pemulen at 0.2wt%
Figure 7 and figure 7.1 show a lack of change in viscosity at 0.2wt% but an increase at 0.5wt%. This may be due to the small hydrophobic domains.
-1 -0.5 0 0.5 1 1.5 2 2.5
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
log plot of change in pH of Pemulen 1 at 0.5wt%
p1 0.5wt% pH7 P1 0.5 wt% pH5
Figure 7.1. Difference in viscosity between pH5 and pH7 at 0.5wt% Pemulen 1
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0 200 400 600 800 1000 12000
50
100
150
200
250
300
350
Shear Stress vs Shear Rate plot of Pemulen 1 at 0.5wt%
pH5 pH7
Figure 7.2 shows comparison of stress by adjusting the pH
Figure 7.2 shows increased stress due to the change in pH and increase in viscosity.
Pemulen 1, salt additionOnly 2wt% 5mM NaCl gave homogenous mixtures as seen in figure 8. This is because the polyacrylic backbone is the mechanism for polymer expansion and due to the polymer modification there are less carboxylate groups to be ionised, couple this with the addition of salt there is little chance of the polymer to extend. Also the hydrophobic moieties on the chain also want to group together increasing the probability of a coil structure.
This tells us that for the hydrophobic domains to produce homogenous mixtures there needs to be some expansion of the polymer chain.
0 200 400 600 800 1000 12000
10
20
30
40
50
60
2 wt% Pemulen 1 5mM NaCl
Shear Rate 1/s
Shea
r Str
ess P
a
Figure 8. Addition of salt to Pemulen 1
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Pemulen 1 TR1, addition of SDS
Pemulen 1 made gels at 1% Pemulen TR1 and 1%,3%,5% and 7%SDS.
0 1000
20
40
60
80
100
120Shear thinning viscosity of Pemulen 1 with SDS
P1 1wt% 1wt% SDS P1 1wt% 3wt%SDS P1 1wt% 5%SDS P1 1wt% 7wt%SDS
Shear rate 1/s
Viso
sity P
as
Figure 9 shear thinning profile of Pemulen 1 with SDS
Expected shear thinning profile is seen with the Pemulen 1 polymer. The range of shear and viscosity has been lowered to emphasise the curve. The initial viscosity produced is lowered by each wt% addition of salt.
0 200 400 600 800 1000 12000
20
40
60
80
100
120
140
160
P1 1wt% with SDS Shear Stress
P1 1wt% 3%SDS P1 1wt% 5%SDS P1 1wt% 1%SDS P1 1wt% 7%SDS
Shear Rate 1/s
Shea
r St
ress
Pa
Figure 9.1 shear stress graph of Pemulen 1 with increasing SDS concentration
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The addition of SDS by wt% was seen to decrease the viscosity produced in figure 9. This trend is seen also for the stress felt on the polymer solution over the shear range as expected.
There is an overlap of 1wt% and 3wt% is seen at high shear rate. This may be due to the high salt concentration.
0 1 2 3 4 5 6 7 80
20
40
60
80
100
120
Viscosity at 0.09957 s-1 shear at different SDS wt%
SDS concentration wt%
visc
osity
Pas
Figure 9.2 decrease in viscosity from increasing SDS wt%
The polymer solution experiences a drop in viscosity as expected due to flooding of the hydrophobe domains at increased SDS concentrations.
1 3 5 70
5
10
15
20
25
0
0.5
1
1.5
2
2.5
3
3.5
4
Affect on Viscosity by SDS wt% concentration
at 1.263s-1 shear rate at 11.67s-1 shear rate
Figure 9.3 decrease in viscosity from SDS increase at different shear rates
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High viscosity is initially seen due to strengthening of hydrophobic interactions by production of the mixed micelles from SDS.
0.09957 1.263 11.67 57.1 279.50
50
100
150
200
250
300Comparison of various shear viscosity with and without SDS
1mM SDS pH 7 without SDS
Shear Rate 1/s
Visco
sity P
as
Figure 9.4 comparison of Pemulen 1 viscosity at 1wt% with and without SDS.
Comparison of Pemulen 1 with and without SDS shows great difference, this is attributed to decrease in polarity with addition of SDS as the hydrophobic domains are increased and hydrogen bonds in the aqueous solution are diminished.
Pemulen 2 compliance with shear thinning profile.
0 10 20 30 40 50 60 70 80 90 1000
5
10
15
20
25
30
35
40
45
50
Shear thinning profile of Pemulen 2
P2 0.2wt% pH7 P2 0.5wt% pH7 P2 0.5wt% pH5 P2 0.2wt% pH5
Shear Rate 1/s
Visc
osity
Pas
Figure 10 shear thinning profile of Pemulen 2,
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Figure 10 has been zoomed in to express the wide variations in viscosity across the shear
The expected shear thinning profile is seen as with all of the polymers. The larger weight and higher pH polymer solutions gave increased initial viscosity as expected.
Pemulen 2, change in concentration effect on viscosity and shear stress.
Pemulen 2 formed homogenous mixtures that could be analysed by the rheometer at 0.2wt% and 0.5wt%.
0.2 0.50
20
40
60
80
100
120
140
160
18.97
140.7
Comparison of Shear viscosity at 0.09956 s-1
pH5
wt%
Visc
osity
Pas
Figure 11 shear viscosity at increasing concentration for Pemulen 2 at pH5
A large increase in viscosity is seen in figure 11 at pH5 for Pemulen 2 at 0.2wt% increase to 0.5wt%.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
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0.5
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log plot of Pemulen 2 at pH5
P2 0.2wt% pH5 P2 0.5wt% pH5
log shear rate 1/s
log
visc
osity
Pas
Figure 11.1a, comparison of 0.2 wt% and 0.5wt% Pemulen 2 at pH 5
25
-1.5 -1 -0.5 0 0.5 1 1.5 2
-1.5
-1
-0.5
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3.5
log plot of Pemulen 2 at pH7
0.5 wt% 0.2 wt%
Figure 11.1b, comparison of 0.2wt% and 0.5wt% Pemulen 2 at pH 7
The differences in plots of 11.1a and 11.1b are not linear.
Figure 11.1a shows a larger difference at low shear followed by a decrease in this difference at higher shear. Whereas 11.1b shows larger difference between the two weights at higher shear.
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
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-1
-0.5
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1.5
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2.5
Comparison of P1 and P2 shear viscosity at pH7
P1 0.2wt% pH 7 p1 0.5wt% pH7 P2 0.2wt% pH7 P2 0.5wt% pH7
log shear rate 1/s
log v
iscos
ity P
as
Figure 11.2 viscosity change due to concentration increase at pH7
Figure 11.2 at pH7 shows that the viscosity for Pemulen 2 is lower than of Pemulen 1.
26
0 200 400 600 800 1000 12000
50
100
150
200
250
Change in Shear Stress due to concentration increase at pH7
P2 0.2wt% pH7 P2 0.5wt% pH7
Shear Rate 1/s
Shea
r Stre
ss Pa
Figure 11.3 shear stress curve of increasing concentration of Pemulen 2.
The lower weight polymer solutions gave lower viscosity and as an effect decreased shear stress.
0 200 400 600 800 1000 12000
20
40
60
80
100
120
140
160
180
200
Change in Shear Stress due to concentration increase at pH5
P2 0.2wt% pH5 P2 0.5wt% pH5
Figure 11.4 shear stress curve of increasing concentration of Pemulen 2.
27
0 200 400 600 800 1000 12000
20
40
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80
100
120
140
160
180
200
Shear Stress Comparison of P1 and P2
P1 0.2wt% pH5 P1 0.5 wt% pH5 P2 0.2wt% pH5 P2 0.5wt% pH5
Shear Rate 1/s
Shea
r Str
ess
Pa
Figure 11.5a comparison of P1 and P2 shear stress curves
0 200 400 600 800 1000 12000
50
100
150
200
250
300
350
Change in shear stress at pH7
P1 0.2wt% pH 7 p1 0.5wt% pH7 P2 0.2wt% pH7 P2 0.5wt% pH7
Shear rate 1/s
Shea
r stre
ss P
a
Figure 11.5b comparison of P1 and P2 shear stress curves
Figure 11.5a and b show comparison of the two Pemulen polymers shear stress. It shows that at high weights at pH5 there is a similarity in the shear stress between the two polymers. At pH7 there is a similarity between the shear stress at low weight.
Pemulen 2, change in pH effect on viscosity and shear stress.
28
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1.5
-1
-0.5
0
0.5
1
1.5
Pemulen 2 0.2wt% log plot
pH7 pH5
log shear rate 1/s
log V
iscos
ity P
as
Figure 12. Shows small change in viscosity relating to changes in pH at 0.2wt%
At low weight the hydrophobic associations are the method of thickening, upon higher shear these can be disrupted
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
-1
-0.5
0
0.5
1
1.5
2
2.5
Pemulen 2 0.5wt% log plot
pH2 pH5
Figure 12.1 small difference in viscosity between pH 5 and 7 at 0.5wt% Pemulen 2
29
0 200 400 600 800 1000 12000
50
100
150
200
250
Pemulen 2 Shear Stress Curve at 0.5wt%
pH7 pH5
Figure 12.2 shear stress changes due to adjusting pH
0 200 400 600 800 1000 12000
20
40
60
80
100
120
Pemulen 2 Shear Stress Curve at 0.2wt%
pH5 pH7
Figure 12.3a shear stress changes of P2 at 0.2wt% due to adjusting the pH
0 200 400 600 800 1000 12000
10
20
30
40
50
60
70
80
90
100
Pemulen 1 Shear Stress Curve at 0.2wt%
P1 0.2wt% pH5 P1 0.2wt% pH 7
Figure 12.3b shear stress changes of P1 at 0.2wt% due to adjusting the pH
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Pemulen 2, addition of saltPemulen 2 didn’t make any homogenous mixtures with the addition of salt. This is understood that the polymer needs to be able to extend a certain amount to allow the hydrophobic areas to group together, as seen previously.
As there are fewer carboxylic groups on the Pemulen 2 this failure to make homogenous mixes including salt is no surprise as the decreased carboxylic groups will have bonded ionically with the Na+ ions.
Pemulen 2, addition of SDS
0 10 20 30 40 50 60 70 80 90 1000
5
10
15
20
25
30
35
40
45
50
Shear thinning profile of Pemulen 2 with SDS
P2 1wt% 1%SDS P2 1wt% 3%SDS P2 1wt% 5%SDSP2 1wt% 7%SDS P2 1wt% 10%SDS
Figure 13. Shear thinning of Pemulen 2 with varying SDS concentrations
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1 3 5 7 100
10
20
30
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50
60
70
0
1
2
3
4
5
6
7
Viscosity change from SDS increase
0.009957 4.5
Figure 13.1 increased SDS concentration effect on viscosity.
The viscosity decrease at shear rate 0.009957s-1 in figure 13.2 is seen in other parts of the shear including 4.5 s-1.
0 2 4 6 8 10 120
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Viscosity change from SDS increase
279.5 107.8 203.4 22.02
Figure 13.3 variation in viscosity at different shear rates from increased SDS concentration
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0 200 400 600 800 1000 12000
20
40
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100
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140
160
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P2 SDS variance shear stress
1 wt% 1%SDS 1wt% 3%SDS 1wt% 5%SDS 1wt% 7%SDSFigure 13.4 shear stress curve of Pemulen 2 with increasing SDS concentration.
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DiscussionCarbopol, compliance with shear thinning profile.
As from the objectives and from the literature it is expected that Carbopol 974 is shear thinning. It is also expected that increased concentrations will give increased viscosity.
These predictions can be seen in the results, Carbopol 974 polyacrylic acid can be seen to be shear thinning from Figure 1. This shows the dramatic viscosity drop occur at very low shear.
This shows the relatively weak polymeric entanglements created by polymer extension and increase in concentration can be easily disrupted by shear. The polymer chain entanglements are able to elongate and be disentangled along with the momentum of shear and cause the viscosity to drop.
The electrostatic repulsions from the carboxylate groups that cause extension are not necessarily affected by the shear but are allowed to be squished in a more compact way causing polymer entanglements to go from a circular shape to a more elongated shape.
Carbopol, change in concentration effect on viscosity and shear stress.
Expected results are that increased concentration gives more entanglements and increases the viscosity. And that this increase number of polymer chains would deform more under shear and give increased stress.
Figure 1 and Figure 2 show the effect of increasing concentration gives more interactions with other polymer chains. This is shown in the increased initial viscosity by each increase in wt% at 0.09956s-1 shear,
As discussed previously, increasing the concentration of the polymer gives more interactions and leads to entanglements that allow networks of polymer chains to be formed. The more that these entanglements are populated gives rise to stiffness and friction between the polymer chains which is seen as increased viscosity.
Figure 2 shows linear increase in viscosity at pH7 due to the expansion of the polymer chain in solution. The nonlinear increase at pH5 may be attributed to the polymer not being fully expanded.
The polymer at pH7 does not show three increasing concentration ranges as explained in Lund (Leif Karlson, n.d.) This is due to the polymer being extended to optimum. This leaves less room for further entanglements in the aqueous solution.
Figure 2 shows increase in viscosity at pH5 and this gives a better representation of the three stages of concentration increase. It could be assumed that the concentration of entanglement is around 0.5wt%. It is after this concentration that the solution begins to form networks and give increased viscosity. The polymer chains can be seen through these increments in figure D1.
34
Figure D1. (Leif Karlson, n.d.)
The effect of increased concentration on the shear stress of the solution can be seen in figure 2.2, here the characteristic shear thinning curve can be seen at the various concentrations but is more exaggerated at higher concentrations.
Again this can be related to the increased microstructures of the more concentrated polymer solutions. Increased entanglements and increased viscosity is reflected in the way the polymer solution reacts to shear from the rheometer.
As with the shear thinning nature of the polymer more stress is felt on the polymer solution due to the increased number and size of entanglements when they move and align with the shear force. Overall the entanglements are larger and deform to a greater extent with shear as the concentration is increased.
Carbopol, change in pH effect on viscosity and shear stress.
It was understood earlier that increasing the pH allows the polymer to be further extended and this caused more areas of hindrance and friction between the polymer chains increasing the viscosity.
The polyacrylic acid chain is well known to be flexible and its expansion can be seen in figure 3 where at 0.2wt% the pH is increased from 5 to 7. Here increased viscosity can be seen as a result of this expansion, showing that the viscosity of the solution is related to change in pH as well as the concentration.
As seen in figure 3.1 with the increase in viscosity of increased pH. Figure 3.1 shows the effect this has on shear stress, due to the increased viscosity the polymer solution undergoes increased shear at higher pH. From this it can be seen that increasing the pH of the polymer allows for the polymer to take up more of the solution and increase the amount of interactions with other chains. This causes and more stress released from these interacting polymer chains when they are subjected to shear.
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Comparing figure 3.1 of increased pH to figure 2.4 which shows shear stress related to increased concentration. Increasing concentration has had a greater impact on the relative shear stress curve compared to increasing the pH.
This can also be seen in the change of viscosity by the log plot figure 3 which shows small increase in shear viscosity due to pH change and figure 2.1 where there is a large increase in viscosity between lower and higher concentrations. The shear stress felt by higher concentrations is much greater than it is for the increase in pH.
Carbopol, salt addition
As discussed previously, the addition of salt is expected to decrease the shear viscosity achieved due to reducing the electrostatic repulsions of the carboxylate groups along the chain.
The decrease in viscosity can be seen from figure 4 where increasing concentrations of NaCl show decreased viscosity.
Both Figure 4 and 4.1 show decrease in viscosity due to increased salt concentration. As discussed this is due to the Na+ ions binding with the carboxylate anions to remove the electrostatic repulsions and allow the polymer to coil up. As seen before the coiled polymer is then allowed to move around the solution with little interaction with other polymer chains giving reduced viscosity.
Figure 4.2 further highlights the decrease in viscosity from addition of salt by taking the viscosity after 0.3546s-1 shear. The viscosity can be seen by large decrease from 1mM to 3mM NaCl and then begin to level off when the polymer chains have recoiled due to the lack of interactions along the chain.
The interesting part is that at 3wt% there must be entanglements as it was seen previously that when the polymer chain was extended at pH7/pH5 at 0.5wt% there was little difference in the viscosity and it was seen that the chains took up most of the aqueous solution. Here at 3wt% the random polymer coils that are not extended due to lack of repulsions must still entangle with other polymer coils to give enough viscosity to form the homogenous mixtures.
Again the relationship between viscosity, shear rate and shear stress can be seen in figure 4.3. Where the increased salt concentration has hindered polymer extension and increase in viscosity this is seen in decreasing amount of shear. The lower viscosity and decrease in the 3D polymeric structure gives less stress when shear is performed on it.
Comparing figure 4.3 to figure 2.1 the maximum shear stress values achieved when the salt is involved is much lower than when it is not, even when the Carbopol concentration is higher. This shows that ability for the polymer chain to extend is of great importance to the formulation in terms of giving the polymer the increased shear stress.
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Pemulen 1, compliance with shear thinning
It can be seen from figure 5 that the shear thinning profile can be seen as expected of Pemulen 1. Again the decrease in viscosity is dramatic at low shear and the starting viscosity is dependent on the wt% concentration of polymer.
Increased concentration causes the viscosity to be increased. The increased concentration is expected to be more abrupt than it was for the polyacrylic acid Carbopol 974, due to the aggregation of hydrophobic regions before the entanglement of the polymers. The viscosity for Pemulen 1 is lower than Carbopol throughout the increase in concentration range.
Pemulen 1, change in concentration effect on viscosity and shear stress.
Figure 6 shows comparison of viscosity at very low shear rate, this follows the general trend of increased viscosity related to increased concentration.
(Leif Karlson, n.d.) Literature explains that the hydrophilic polymer backbone bends around the hydrophobic region to decrease interactions with the aqueous solution. .
Figure 6.1 shows large difference in the viscosity at 0.2wt% and the higher weights 0.5 and 1 wt% which show very similar viscosity. This shows at pH7 the viscosity of Pemulen 1 reaches somewhat of a maximum at 0.5wt% where only small increase is felt at 1wt%. From this it may be assumed that the polymer has begun to form intermolecular associations at this weight.
Although the polymer structure is undetermined, it may be assumed from the changes in rheology that the polymer is a block polymer. The block polymer encourages intramolecular hydrophobic associations rather that intermolecular as the hydrophobic moieties are grouped close together on the chain and therefore the easiest association is intramolecularly. This would make the hydrophobic domains smaller than they would be if they had associated with the other hydrophobe chains.
The smaller domains are more suited to polymer chains at lower pH. At the lower pH the polymer backbone can adopt a smaller space and not worry so much about the electrostatic repulsions felt close by.
Figure 6.1 shows that Pemulen 1 gives low viscosity at 0.2wt% and does not form intermolecular associations at this weight. This may be due to the size or number or hydrophobes along the chain not being close enough to associate. Figure 6.1 shows that the large extended chain that is produced by electrostatic repulsions doesn’t minimalize the interactions of the low weight polymer. However at higher weights the large polymer chain is of use as it can bend round the larger intermolecular hydrophobic associations.
This model does not necessarily fit at pH5 seen in figure 6.2 where viscosity is much increased for 1wt% polymer. This shows a large jump from shear viscosity at 0.5wt% to 1wt%.
At pH5 the block polymer chains may favour intramolecular associations due to the lack of chain expansion provided. The intramolecular associations give smaller hydrophobic domains and will consequently give lower viscosities.
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The model of effective polymer chain size related to pH is seen in figure 6.3. Here the ability of the chain at pH5 to fit between the hydrophobic domains and aqueous solution is highlighted in figure 6.3. It show almost doubled viscosity at 1wt% from the polymer chain at pH7.
The polymer backbone at the higher pH7 gives much more repulsion along the chain and in small areas, causing it to expand and lose the shape of the hydrophobe and adopting its own structure. This gives the effect of decreased viscosity..
The higher viscosity seen for pH5 in figure 6.3 would agree with the idea that Pemulen 1 makes smaller intramolecular hydrophobic associations and is a block polymer. This has been seen to prefer the pH at 5 as the backbone can adopt the favoured tighter confirmation due to lack of repulsions.
(Leif Karlson, n.d.)Lund continues to explain the variations in hydrophobe dictate the shape of the micelle and consequently its bonding. Favourable is a flower shape where the hydrophilic backbone loops away from the hydrophobes.
The hydrophobic domains may be considered clumps if the polymer is actually block type where the hydrophobic groups are all very close together. When the intramolecular bonded domains meet with other domains they may associate together by crosslinking which will allow the polymer backbone to fit to more favoured confirmations and lower the energy of the system. This is something seen at higher concentrations and is the cause for the increase in viscosity.
The shear stress has been affected by the changes in viscosity seen. The Pemulen 1 polymer gives a lower stress curve and viscosity readings than was seen for the Carbopol polymer. This could be to do with differences in molecular weights but also due to increased hydrophobicity giving decreased polarity and decreased viscosity.
Pemulen 1, change in pH effect on viscosity and shear stress.
Figure 7 shows there is no change to the viscosity at 0.2wt% due to change in pH.
This may be due to the hydrophobic domains having not formed many crosslinks. The hydrophobic associations are made up of small intramolecular domains and the polymer backbone can shape around these domains at both pH5 and pH7.
At 0.5wt% in figure 7.1 more hydrophobic domains have been formed along with intermolecular associations. This increases the size the polymer chain with increased repulsion is favoured.
The effect of changing pH to viscosity of Pemulen is much smaller than effect seen by change in concentration.
The change in shear stress seen in figure 7.2 shows that the increase in pH has an effect on the shear stress of Pemulen 1. The increase in shear stress cannot be caused by the very small changes in viscosity seen by pH change.
This is understood that at higher weights the polymer backbone cannot shield all of the hydrophobic domains and as the concentrations increase there is more hydrophobic interactions with water. This causes a lower viscosity to be seen, but
38
has the effect that the hydrophobic regions are still there and due to their lower energy they are more easily sheared. This lack of shielding can also be affected by increased pH where more repulsions along the chain in small spaces cause the backbone to not sit cleanly around the hydrophobes, it may also not be conformational favourable for the chain to adopt such positions. The polymer backbone is suited to high pH for large hydrophobic domains.
The areas with polymer shielding will feel a different stress than those that are subject to hydrophobic-aqueous interactions.
The water molecules at hydrophobic planes lose hydrogen bonds and feel increased enthalpy. The water network expands as a result of this and gives lower entropy and density which causes the viscosity to be lowered. (LSBU, n.d.)
Pemulen 1, salt addition
Only 2wt% 5mM NaCl gave homogenous mixtures as seen in figure 8.
The addition of salt to the system increases the polarity and promotes hydrophobic association. Unfortunately it also causes the polymer backbone to coil up and this gives many hydrophobic hydrophilic interactions which destroy the viscosity.
Pemulen 1 TR1, addition of SDS
Pemulen 1 made gels at 1% Pemulen TR1 and 1%,3%,5% and 7%SDS.
Figure 9 shows viscosity against shear rate and confirms addition of SDS to Pemulen 1 keeps the system shear thinning as expected. Figure 9 shows that at very low shears small addition of SDS surfactant initially gives increased viscosity reading. It can be seen that upon further addition of SDS surfactant this decreases the starting viscosity under shear. This was something that was expected to be seen due to the addition of SDS causes individual hydrophobic domains to become larger mixed SDS/hydrophobic micelles.
The SDS is also seen to lengthen the lifetime of these hydrophobic associations which are constantly being broken and reformed. If the SDS has given this increased strengthening it is not seen in increased viscosity but by lowering the stress felt. The strengthened SDS hydrophobic domains feel less stress from the shear than without the SDS. It is noted that hydrophobic associations are of low energy and consequently are more easily sheared.
As SDS concentration increases in figure 9.1 there is seen a decrease in the stress curve of Pemulen 1 produced. The lowering of the stress curve is also affected by the decrease in viscosity produced by the inclusion of SDS.
The effect of the SDS micelles can be seen in figure 9.2 showing viscosity against SDS concentration at very low shear rate 0.09957s-1. It is seen that at relatively low concentrations of SDS the shear stress of the solution is high but this decreases by around a half at increased concentrations.
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The same decrease in viscosity can be seen at higher shear rates seen in figure 9.3 where the same shape of graph is seen due to the excess of SDS surfactant micelles and has been seen to cause the decrease in viscosity and shear stress.
Literature explains that when the solution exceeds critical overlap concentration caused by the increase in microstructure networks due to the involvement of SDS the viscosity will drop. This overlap concentration is at a similar value to the critical micelle concentration of the aqueous solution. The viscosity increase that is initially seen is caused by the adsorption of individual SDS molecules onto the hydrophobic domains improving their strength and time of life.
It is interesting to compare the viscosity of 1wt Pemulen with and without SDS. With the addition of SDS the viscosity is lower across all shear rates than it is without at pH7 seen by figure 9.4.
This may be explained by SDS increasing the hydrophobicity of the solution. This decreases the strength of hydrogen bonding and other intermolecular forces and ultimately the polarity of the solution. The decrease in polarity causes the viscosity to decrease. Water in the solution is polar due to its uneven shape.
The shear stress curve of Pemulen 1 can be seen in figure 9.1, this shows correlation with the decrease in viscosity due to increased concentration of SDS.
This also shows the slight jump between higher and lower concentrations of the surfactant seen previously in the viscosity graphs due to excess SDS.
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Pemulen 2 compliance with shear thinning profile
Again it can be seen from figure that the polymer solution in this case Pemulen 2 fits in to the shear thinning model. The decrease in viscosity as has been seen before is dramatic at low shear rates. The initial viscosity decrease follows expected weight and pH decrease from 0.5wt% pH7 to 0.2wt% pH5.
Pemulen 2, change in concentration effect on viscosity and shear stress
Pemulen 2 formed homogenous mixtures that could be analysed by the rheometer at 0.2wt% and 0.5wt%.
An increase in viscosity can be seen due to increased concentration in figure 11 for Pemulen 2 at pH5 after 0.09956s-1 shear rate. Here the viscosity undergoes a substantial increase from 18.9Pas to 140.7Pas with increase of 0.2wt% to 0.5wt%. The large increase may be attributed to increase in the strength and number of hydrophobic domains. The increase allows crosslinks of hydrophobic domains to be produced that here are seen as viscosity.
The two log plots of figure 11.1a and 11.1b show large changes to the viscosity by increasing the concentration. By increasing the pH to 7 there is a marked increase in the viscosity of the polymer solution. This could be related to a larger hydrophobic domain created in Pemulen 2 because of the increased number of hydrophobic domains. The differences in viscosity may be due to differences between the intermolecular and intramolecular hydrophobic associations produced at the two different pH. It is seen that increasing the concentration in Pemulen 2 has increased the viscosity in the polymer solution. At increased pH7 the lower weight 0.2wt% gives nearly the same viscosity as the high 0.5wt% at pH5. This shows in importance for the polymer chain to be extended which reaffirms the large hydrophobic regions produced.
The increased pH also allows the hydrophobic regions of the polymer to interact with hydrophobes on other polymer chains and give intermolecular associations. It may be that at pH5 the polymer is not extended enough and gives more intramolecular associations that give lower viscosity as they cause the ring to tighten. The favoured pH7 allows the hydrophobic associations to expand with the polymer chain and this would increase the viscosity.
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Figure D.2
Figure D.2 shows at pH5 there is less repulsion on the polymer backbone and favours intramolecular associations. At pH7 the polymer is more extended and can undergo intermolecular associations and expand both the network and the polymer causing an increase in viscosity.
Figure 11.2b shows clearly the lower viscosity of Pemulen 2 compared to Pemulen 1. As discussed previously this may be due to effect on polarity by the increased hydrophobia of Pemulen 2.
Increase in the stress curve of Pemulen 2 due to increased viscosity can be seen in both figure 11.3 and figure 11.4 the stress curve is roughly increased by the same amount for both at the higher weight. It was seen that the effect of increased concentration gave increased viscosity and this has resulted with increased stress of the polymer solution. The increased viscosity and stress may be seen from increased intermolecular associations.
It may be assumed that intramolecular and intermolecular associations give different amounts of stress when they are sheared due to different strengths in their networks.
Figures 11.5 show comparison of the two Pemulen polymers shear stress. It shows that at high weights at pH5 there is a similarity in the shear stress between the two polymers. At pH7 there is a similarity between the shear stress at low weight. This shows differences to the viscosity changes seen in figure 11.2 where all the polymers show different viscosities. Pemulen 2 has shown increased shear stress at different pH.
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Pemulen 2, change in pH effect on viscosity and shear stress
When the polymers are compared at the same weight and different pH there is only a small change seen. This implies that similar structural differences do occur for both regardless of pH. The previous section was concerned with different concentrations and this showed much greater differences in viscosity achieved.
Figure 12 and 12.1 show changes in viscosity due to increasing in pH, there is a small increase at high shear at 0.2wt% but the difference is very little change at 0.5wt%.
The lack of change due to pH change has previously been seen with Pemulen 1. It can be seen the mechanism of thickening is the hydrophobic association and by continued ionisation of the polymer chain causes further repulsions that don’t allow the hydrophilic chain to coil around the hydrophobic domains.
It was declared that Pemulen 2 was more hydrophobically modified than Pemulen 1 but the two polymers appear to bond intramolecularly to begin with and this leads to decreased viscosity as it means the polymer adopts a smaller shape than it would if the associations where intermolecular. Also, for the increased hydrophobes there does not seem a way to have got them to sit better in aqueous solution.
The similarity in viscosity at 0.5wt% can also be seen via the stress curve in figure 12.2. The difference between the two is smaller than for many of the other stress curves seen previously.
Looking at figure 12.3 the stress curve for 0.2wt% the difference between the higher and lower pH stress curve is much more pronounced. This tells us that at a molecular level the 0.2% solution gives a different structure than the 0.5%. At low concentrations 0.2wt% there is a difference in shear due to change in pH. At higher pH there are more repulsions forcing the chain to be larger and to fit around the increased hydrophobic domains.
Comparing to figure 12.3b this is not seen in Pemulen 1 at this concentration. It can be understood that the difference in shear show the production of a hydrophobic network structure. The increase in hydrophobic moieties in Pemulen 2 allow more hydrophobic association at lower concentration. This allows the hydrophilic polymer chain to surround the hydrophobic regions as understood. This increases the viscosity and here the stress felt by shear.
Pemulen 2, addition of salt
Pemulen 2 didn’t make any homogenous mixtures with the addition of salt. This is understood that the polymer needs to be able to extend a certain amount to allow the hydrophobic areas to group together, as seen previously.
As there are fewer carboxylic groups on the Pemulen 2 this failure to make homogenous mixes including salt is no surprise as the decreased carboxylic groups will have bonded ionically with the Na+ ions.
Pemulen 2, addition of SDS
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Pemulen 2 was expected to give the expected shear thinning property seen before. This is confirmed in figure 13. The dramatic decrease in viscosity is still seen at low shear rates. It is confirmed to us that the hydrophobic moieties still allow the polymer to be shear thinning.
Pemulen 2 was expected to increase the strength of the hydrophobic domains by the addition of SDS, upon increased concentrations of SDS this was expected to decrease in viscosity due to excess SDS surfactant that would cause the hydrophobic regions to become separated and decrease in effect.
Figure 13.1 shows the shear viscosity at 0.009957s-1 and 4.5s-1 shear with different amounts of SDS increasing from 1 to 10wt%. This graph shows the expected decrease in viscosity at low concentrations between 1wt% and 3wt% for the initial 0.009957s-1 shear.
The inclusion of SDS allows the mixed hydrophobic micelle to stay together longer and give the increased viscosity as seen, this also acts as a way of extending the network.
A slight increase in viscosity can be seen at 10%SDS for 4.5s-1 shear rate but only very small. This may be due to the excess SDS giving increased viscosity because there is so much of it.
Figure 13.3 shows the viscosity at some shear rates of different wt% SDS solutions. It’s interesting that they seem to follow the same up down correlation but at a more decreased amount as the shear rate gets larger. The ups and downs of these peaks may be attributed to the same changes seen by SDS previously.
As discussed in the introduction, Pemulen 2 was expected to make micelles with SDS surfactant. It can be seen that the expected shear stress curve is seen when the SDS is included in the solution. The stress curve gives relatively high values for all wt% of SDS/polymer. As the SDS weight % increases the stress curve decreases as seen with Pemulen 1. However when you compare figure 13.4 of Pemulen 2 with figure 9.1 of Pemulen 1, the shear stress curve for Pemulen 2 is greater.
Without knowledge as to the weight of the polymers we cannot draw any comparisons to the differences of the polymers themselves only individually.
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Explanations of refuted hypothesis,
It was expected that the increased hydrophobic polymer Pemulen 2 was to give increased viscosity due to what was seemed as possibility to produce stronger microstructures between the polymer chains. This wasn’t necessarily the case and the microstructure seems to give other properties, including reduced shear stress in Pemulen 2 compared to Pemulen 1.
It is assumed that increased hydrophobic functions on the polymer decrease the number of carboxylic groups. This increased hydrophobicity gives decreased polarity in the solution due to the decrease in hydrogen bonding between O and H2O. This decrease in polarity causes the viscosity to go down. (Padua, n.d.)Literature describes the energy of hydrogen bonds to be at 2-30 kcal/mol telling us that these interactions can be stronger than the hydrophobic ones.
How may methods been improved?
The rheology could have been carried out on more pH and concentration changes to give a better idea as to concentration of entanglement and overall better results.
As now differences have been seen in the rheology produced by hydrophobically modified polymers, further comparison is achieved by comparing the weights of the polymers. This can be done by many techniques including Ubbelhoede viscometry, breaking the polymer apart in concentrated acid then back titrating but most accurately by a form of chromatography, either HPLC or GC. The HPLC technique can be used to quantify the difference in hydrophobic moieties between Pemulen 1 and Pemulen 2, this could then be used to compare with the rheology seen.
When the polymers had been quantified it would be more understandable to compute a pKa of the backbone, this would allow increased accuracy to use the polymers at more pHs.
A determination of the molecular weight of the polymers need to be made as otherwise the rheology cannot be compared. Increased molecular weight gives larger coils in solution and increased entanglements causing increased viscosity. This could without knowing mask the other experimental factors.
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ConclusionsThe basic changes in polymer confirmation and this effect on rheology of the polymer solution where seen.
It was seen that increased concentration gave the greatest change in viscosity compared to increase in pH.
The polymer backbone was seen to change the rheology of the polymer system.
The polymers where confirmed as shear thinning.
The increased hydrophobicity was seen to give decreased viscosity compared to Pemulen 1.
Surfactant SDS did show to initially increase the viscosity of the polymer solutions by increasing the strength and the life time of the hydrophobic domains.
Understanding the thickening mechanism of Pemulen polymers has proved difficult, the viscosity is lowered by increased hydrophobicity. This has been interpreted as that increased hydrophobicity decreases the polarity of the aqueous solution and decreases the viscosity. The polymer backbone depending on pH does a better or worse job at forming a barrier and lowering the interactions between the two.
Further work Further work would certainly be to analyse the polymers further particularly their weights meaning the rheology seen could be quantified. This may be done by a form of chromatography either HPLC or GC, where the hydrophobic regions could be analysed by their retention times.
The properties seen should be attributed to the number of hydrophobic parts and then real comparisons can be made between hydrophobically modified polymers and straight polyacrylic acid.
Manufacturer of the polymer Lubrizol explain a method (TDS-244) to measure the yield value, which is the initial response to flow under stress and is carried out using a Brookfield viscometer. This leads the way to other methods that may be carried out on the rheometer to look at the time it takes polymer solution to relax back to its original shape.
Articles (Guo, n.d.) Describe using cyclodextrin to displace hydrophobe-hydrophobe interactions and to look at the affects that this has on the rheology.
Other experiments that may seem useful to carry out include, potentiometric titration of the polymer backbone to understand the ionisation of the carboxylic groups.
The differences between the intramolecular and intermolecular associations has proven interesting to the changes in rheology, it would be useful to try and quantifiy these further.
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Bibliographyal, C. e., n.d. HMP and Surfactants. al, E. e., 97. Rheology of HMP. al, S. e., n.d. hydrophobically modified polymers. Anon., n.d. http://www.chbe.umd.edu/fpss/slideshows/myslideshow/images/cell-gels.jpg. [Online].Anon., n.d. http://www.chm.bris.ac.uk/motm/SLS/SLSh.htm. [Online].Anon., n.d. http://www.rug.nl/research/portal/files/3189673/c2.pdf. [Online].Anubhav Tripathi, K. C. T. a. G. M., n.d. Rheology and dynamics of associative polymers in shear and extension. Colby, A. J. K. a. R. H., n.d. Polyelectrolyte Charge Effects on Solution Viscosity of Poly(acrylic acid). Filipe E Antunes, U. o. C., n.d. Mixed Systems of Hydrophobically Modified Polyelectrolytes. Guo, X., n.d. Rheology control by modulating hydrophobic and inclusion associations in modified polyacrylic acid. Lau, A., n.d. Rheology of HASE. Leif Karlson, L. U., n.d. Hydrophobically Modified Polymer Rheology and Molecular Associations. LSBU, n.d. [Online].Lubrizol, n.d. TDS 222, s.l.: s.n.Lubrizol, n.d. TDS 93, s.l.: s.n.Padua, U., n.d. http://www.chimica.unipd.it/fabrizio.mancin/pubblica/Suprachem/II%20lezione%20Mancin.pdf. [Online].Shaw, D. J., 1992. Colloid and Surface Chemistry, Fourth Edition, Page 244. s.l.:s.n.Shedge), A. S., n.d. Hydrophobically modified polymers. Somasudarah, S. a., n.d. SDS hydrophobic interactions.
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Appendix
1.Example data produced from Rheomoeter Software
2.Bohlin Rotational Rheometer
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3.Rheometer Sotware
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