Polymer Clay Dispersion

8/12/2019 Polymer Clay Dispersion

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INTRODUCTION

This review aims at reporting on very recent developments in syntheses, properties and

(future) applications of polymer-layered silicate(clay minerals) nanocomposites. This new

type of materials, based on smectite clays usually rendered(cause to became) hydrophobic

through ionic exchange of the sodium interlayer cation with an oniumcation, may be prepared

via various synthetic routes comprising exfoliation adsorption, in situ intercalative

polymerization and melt intercalation. The whole range of polymer matrices is covered, i.e.

thermoplastics, thermosets and elastomers. Two types of structure may be obtained, namely

intercalated nanocompositeswhere the polymer chains are sandwiched in between silicatelayers and exfoliated nanocompositeswhere the separated, individual silicate layers are more

or less uniformly dispersed in the polymer matrix. This new family of materials exhibits

enhanced properties at very low filler level, usually inferior to 5 wt.%, such as increased

Young's modulusand storage modulus, increase in thermal stabilityand gas barrier properties

and good flame retardancy.

Manufacturers fill polymers with particles in order to improve the stiffness and thetoughness of the materials, to enhance their barrier properties, to enhance their

resistance to fire and ignition or simply to reduce cost.

Nanocomposites are a new class of composites, that are particle-filled polymers for which

atleast one dimension of the dispersed particles is in the nanometer range. One can

distinguish threetypes of nanocomposites, depending on how many dimensions of the

dispersed particles are in thenanometer range. When the three dimensions are in the orderof nanometers, we are dealing withisodimensional nanoparticles, such as spherical silica


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nanoparticles obtained by in situ solgelmethods [1,2] or by polymerization promoted

directly from their surface [3], but also can includesemiconductor nanoclusters [4] and

others [2]. When two dimensions are in the nanometer scale and the third is larger,

forming an elongated structure, we speak about nanotubes(2/3, 1/3) or whiskers as,

forexample, carbon nanotubes [5] or cellulose whiskers [6,7] which are extensively studied

asreinforcing nanofillers yielding materials with exceptional properties. The third type

ofnanocomposites is characterized by only one dimension in the nanometer range. In this

case thefiller is present in the form of sheets of one to a few nanometer thick to hundreds to

thousands nanometers long. This family of composites can be gathered under the name of

polymer-layeredcrystal nanocomposites, and their study will constitute the main object of

this contribution. Thesematerials are almost exclusively obtained by the intercalation of the

polymer (or a monomersubsequently polymerized) inside the galleries of layered host

crystals. There is a wide variety ofboth synthetic and natural crystalline fillers that are able,

under specific conditions, to intercalate apolymer. Table 1 presents a non-exhaustive list of

possible layered host crystals.

Amongst all the potential nanocomposite precursors, those based on clay and layered silicates

have been more widely investigated probably because the starting clay materials are easily

available and because their intercalation chemistry has been studied for a long time [16,17].

Owing to the nanometer-size particles obtained by dispersion, these nanocomposites exhibit

markedly improved mechanical, thermal, optical and physico-chemical properties when

compared with the pure polymer or conventional (microscale) composites. Improvements can

include, for example, increased moduli, strength and heat resistance, decreased gas

permeability and flammability.

The aim of this report is to review the different techniquesused to obtainpolymer-layered

silicatesnanocompositesand the improved properties that those materials can display.

Structure of layered silicates

The layered silicates commonly used in nanocomposites belong to the structural family

knownas the 2:1 phyllosilicates. Their crystal lattice consists of two-dimensional layers

where a central octahedral sheet of alumina or magnesia is fused to two external silica


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over the whole crystal. Proportionally, even if a small part of the charge balancing cations islocated on the externalcrystallite surface, the majority of these exchangeable cations islocated inside the galleries. Whenthe hydrated cations are ion-exchanged with organic cationssuch as more bulky alkyammoniums, itusually results in a larger interlayer spacing.

Fig. 2. Alkyl chain aggregation in layered silicates: (a) lateral monolayer; (b) lateral bilayer; (c) paraffin-typemonolayer and (d) paraffin-type bilayer (reproduced from [21] with permission).

In order to describe the structure of the interlayer in organoclays, one has to know that, as the

negative charge originates in the silicate layer, the cationic head group of the alkylammoniummolecule preferentially resides at the layer surface, leaving the organic tail radiating away

from the surface. In a given temperature range, two parameters then define the equilibrium

layer spacing: thecation exchange capacity of the layered silicate, driving the packing of the

chains, and the chain length of organic tail(s). According to X-ray diffraction (XRD) data, the

organic chainshave beenlong thought to lie either parallel to the silicate layer, forming mono

or bilayers or, depending on thepacking density and the chain length, to radiate away

from the surface, forming mono or evenbimolecular tilted `paraffinic' arrangement [20] as

shown in Fig. 2.


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A more realistic description has been proposed by Vaia et al. [21], based on FTIR

experiments. By monitoring frequency shiftsof the asymmetric CH2 stretching andbending

vibrations, they found that the intercalated chains exist in states with varying degrees of

order. In general, as the interlayer packing density or the chain length decreases (or the

temperature increases), the intercalated chains adopt a more disordered, liquid-like structure

resulting from an increase in the gauche/transconformer ratio. When the available surface

area per molecule is within a certain range, the chainsare not completely disordered but retain

Young's modulus at room temperature of nylon-6 nanocomposites obtained by meltintercalation infunction of the filler weight content measured in this case at room temperature [85].

The preparation of nanocomposites by this technique has the advantage to use the samematrix

for each composite, thus with the same Mw and MWD nylon-6. Fig. 31 shows a constant andlargeincrease in the modulus up to ca. 10 wt.% of nanoclay, above this threshold the Young'smodulusseems to level off. This change exactly corresponds to the passage from totally exfoliatedstructure(below 10 wt.%) to partially exfoliatedpartially intercalated structure (for 10 wt.% and upper)asdetermined by XRD and TEM analyses [85].The same behavior can account for the evolution of Young's modulus in polypropylenenanocomposites obtained by melt intercalation when the amount of maleic anhydride-

modified PP

Fig. 31. Effect of clay content on tensile modulus, measured at room temperature, oforganomodifiedmontmorillonite/nylon-6-basednanocomposite obtained by melt intercalation (reproduced from [85] with permission).


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The montmorillonite clay mineral is a 2:1 aluminosilicatecomprising an octahedral layer of alumina fusedbetween two tetrahedral layers of silica. Themodel weused is a Wyoming-like montmorillonite in which isomorphicsubstitutions in the octahedral and tetrahedral


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layers occur (aluminium and silicon atoms are replacedby magnesium and aluminium, respectively). Thesecreate a net negative charge in the clay sheets that iscompensated by the presence of counterions in the interlayerspace. Naturally occurring cations are sodium

and calcium. They can easily be exchanged with othertypes of cations (lithium, potassium or ammoniumcontainingorganic molecules). In this communication,we report results on lithium, sodium and potassiummontmorillonite clays.

The water swelling behaviour of clays is a well-knownphenomenon and has been widely studied in the pastboth experimentally and theoretically [1923]. Therefore,as a benchmark for the current work, we implementedtheTeppen force field within LAMMPS and

calculated the swelling curve of the sodium-montmorillonitefor various water contents (from 0 to 300 mg/g ofclay). The

program [24] (with an isobaricisothermal ensemble at

300 K) typically lie within this hysteresis loop. It is

noteworthy that the swelling curve calculated with

Discover was performed with the Teppen force field

using the same model size whereas for the Monte Carlo

simulation, the TIP4P force field was used for water. It

is now well-known from experiment [19,20] that the

hydration of the sodium-montmorillonite proceeds according

to a typical step-jump behaviour that arises

when one, two and three layers of water molecules are

formed. This trend is well reproduced by similations

(Fig. 2) including LAMMPS with the Teppen force field.

Fig. 2. Swelling curve of sodium-montmorillonite clay showing the

dependence of the d-spacing on the water content of the clay. Experimentaldata from [19]. Monte Carlo simulations using an NPT ensemble

at 300 K (from [22]). Discover and LAMMPS data: this work.

3.2. Simulation of the Li+, Na+- and K+-montmorillonite Clays

The interaction of the clay cations with their environment


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depends on non-bonding electrostatic and van

der Waals potentials. Since no parameters were available

for the lithium cation within the Teppen force field,

the parameters that we have chosen for the van der

Waals potential are 0.003 kcal mol_1 for e and 3.25 _A

for r. Then, a new series of simulations were performed

to

counterions and the water molecules (Fig. 3c): they

clearly show a first sphere of coordination at about 2.0,2.25 and 2.5 _A for Li, NaandK, respectively, and a

second one at about 4.55.0 _A. Several experimental andtheoretical studies validate our results. In aqueous solution

[25] Lihydrates strongly and is surrounded by

an octahedral sphere of water coordination located at

about 1.9 _A. This was also found by Skipper et al. [26],from neutron diffraction studies on hydrated Li-vermiculite.

These

(HPMC), poly(acrylonitrile) (PAN),poly(dimethyldiallylammonium) (PDDA) and poly(aniline) (PANI) [100].

1. To publish a paper on simulation of foam bed reactor .

2. 4. Water soluble polymers

Polymers which have hydrophilic groups such as hydroxyl, carboxyl and sulfonate groups

tend to solve in water. Polysaccharides are usually water soluble. Various kinds ofpolysaccharide molecules accept a random structure in a dilute aqueous solution at a

temperature higher than the critical temperature (get-sol transition temperature). When the

solution temperature is decreased, random molecular chains ordinarily form helical

structures, and the helical chains assemble into hydro gel or liquid-crystal form, depending on

thermal treatment, the concentration, and the presence of various kinds of ion.

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Polymer Clay Dispersion