Polymerisation : Molecular Weight and Structure

There are two major classes of polymer formation mechanismsAddition polymerization: The polymer grows by sequential addition of monomers to a reactive siteChain growth is linearMaximum molecular weight is obtained early in the reactionCondensation Polymerization/Step-Growth polymerization: Monomers react together to make small oligomers. Small oligomers make bigger ones, and big oligomers react to give polymers.Chain growth is exponentialMaximum molecular weight is obtained late in the reaction

Polymerization MethodsAddition polymerization

Polymerization Methods

Polymerization MethodsFree Radical Polymerization3. Termination++Intentional or unintentional molecules/impurities can also terminate.

Addition Polymerization

Step-Growth Polymerization

Step-Growth PolymerizationBecause high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved.Xn = Degree of polymerizationp = mole fraction monomer conversion

Sheet1

pXn

01

0.11.1111111111

0.21.25

0.31.4285714286

0.41.6666666667

0.52

0.62.5

0.73.3333333333

0.85

0.910

0.92513.3333333333

0.9520

0.97540

0.99100

0.9991000

step-growth

1

1.1111111111

1.25

1.4285714286

1.6666666667

2

2.5

3.3333333333

5

10

13.3333333333

20

40

100

1000

Xn

Mole Fraction Conversion (p)

Degree of Polymerization

Sheet2

Sheet3

Nylon-6,6

Nylon-6,6Since the reactants are in different phases, they can only react at the phase boundary. Once a layer of polymer forms, no more reaction occurs. Removing the polymer allows more reaction to occur.

MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION

What is n?n is degree of polymerisation

Molecular WeightWhen we talk about molecular weight in terms of polymers, we are really talking about the length of the individual chains.

The polymerization process is subject to variation so there is no single chain length, there is actually a wide range of lengths, so when we discuss molecular weight, wereally mean the average molecular weight of the material. This average is found by measuring samples of the material as it is produced.

Molecular WeightThere are two different categories of molecular weight average that are commonly used:

The first is the Number Average Molecular Weight ( )

The second is the Weight Average Molecular Weight ( )

Another important aspect of the molecular weight distribution is the shape of the curve.

Molecular Weights Not only are there different structures (molecular arrangements) but there can also be a distribution of molecular weights (i.e. number of monomers per polymer molecule).Average molecular weight =This is what is called number average molecular weight. (Mn)

Molecular WeightThe figure to the right represents a typical molecular weightdistribution.

The vertical axis representsthe number of chains at thatlength.The horizontal axis represents the different chain lengths.

Notice that the longer chains are to the left on the graph and the shorter chains are to the right.

Molecular WeightThe Number Average Molecular Weight ( ) is the total weight of the polymer moleculesdivided by the total numberof polymer molecules.

Molecular WeightNumber Average Molecular Weight ( ) Example:We have: 10 moles of Polyethylene (PE) that are 500 monomers long5 moles of PE that are 100 monomers long5 moles of PE that are 800 monomers long

What is ?

Molecular WeightNumber Average Molecular Weight ( ) Example:Each monomer is 2 Carbons and 4 Hydrogens 10 moles X 500 monomers = 5,0005 moles X 100 monomers = 5005 moles X 800 monomers = 4,000Total number of moles = 20 = (10 + 5 + 5)Total number of monomers = 9,500

What is ? 9500/20 = 475 monomersThe average chain is 475 monomers long

Molecular WeightLook at the numbers from the previous example:10 moles of PE that are 500 monomers long5 moles of PE that are 100 monomers long5 moles of PE that are 800 monomers long

Calculate the

Molecular Weight

Calculate theFind the weight fractions:10 x 500 / 9500 = 52.6% 5 x 100/ 9500 = 5.3%5 x 800 / 9500 = 42.1%

= (0.562 x 500) + (0.053 x 100) + (0.421 x 800) = 605.3 monomers long

Molecular WeightThe Weight Average Molecular Weight ( ) takes into account that the larger molecules contain a much higher amount of the molecular mass of the polymer.The Weight Average Molecular Weight is almost always higher than the Number Average Molecular Weight ( ).

MOLECULAR WEIGHT

Molecular WeightJust knowing the averages is not enough, the distribution of the molecular weights also has a large effect on how the material will process and its properties.A broader or wide spec distribution may make the material unsuitable for processes like injection molding, but better suited for processes like extrusion, blow-molding, or thermoforming.

Molecular WeightFor injection molding grades of material, a narrower distribution is better.When the distribution is narrow, the polymer chains will melt and flow at around the same temperature.The longer the chains, the higher the viscosity or resistance to flow.

Molecular WeightWhen you have a broad or even a bi-modal distribution, the shorter chains melt more quickly and allow some flow, while the longer chains hold the material together. This gives the polymer mixture melt strength which allows it to be used for some of the other processes mentioned other than injection molding.

Molecular WeightYou can have virtually an infinite number or distributions with the same number average molecular weight.

All of these materials will process differently and have at least slightly different properties.

PropertiesWhen making polymers, the goal is to make a material with the ideal properties.

The longer the molecules (or the higher the molecular weight) the higher the entanglement forces:Longer hair is harder to get untangled than shorter hair

PropertiesIncreasing the molecular weight of the material increases many of the properties of the material by increasing the entanglement of the molecules.A higher molecular weight:Increases the chemical resistance - to a pointIt takes more damage to the main chains of the molecules before it will affect the strength of the materialThe big loophole to this is if you have a chemicalthat is very similar to the chemical makeup of themain chain, it will dissolve it much more easilyLike Dissolves Like

PropertiesA higher molecular weight:Increases how far the material can stretch before rupturing (ductility)The higher degree of entanglement allows the material to be pulled further before the chains break

PropertiesA higher molecular weight:Increases ductility: A candle and Polyethylene (PE) have basically the same molecular structure. The chain length of the candle is just much shorter than that of the PE. If you bend a bar of PE in half it will bend, if you bend a candle in half, it will fracture.

PropertiesProcessors want materials that will flow easily in order to form complex geometries, but that can affect the properties of material used to create the product.

Many times it turns out to be a trade-off between the required properties and processability of the material.

CDs and DVDs are made from the same material as most safety glasses, Polycarbonate.

Safety glasses require a higher molecular weight in order to provide the necessary property of impact resistance.CDs and DVDs require a lower molecular weight material in order to fill out the thin walls. CDs and DVDs can shatter, safety glasses dont.

Properties

Molecular weight Physical and mechanical property implications of molecular weight and MWD Characterization of MWD Relate MWD information to perform characteristics of the finished product.

Property/process parametersIncrease the MWTensile strengthincreaseImpact strengthincreaseBrittlenessdecreaseAbrasion resistanceincreaseMelt viscosityincreaseProcessing temperatureincrease

Molecular weight (cont.)

Measurement of MWPrimary methods to obtain absolute values of molecular weight. Accurate but time consuming technique.Osmetry (Mn)Scattering (Mw)Sedimentation (Mz)Secondary methods requires prior determination of empirical relationships that relate the molecular weight to the viscosity of a polymer solution or to the retention volume of polymer solution being eluted from a gel permeation column. Its fast, simple & accurate.Intrinsic viscosityGPC

Gel Permeation Chromatography TheoryUsed to separate liquid phase componentsForm of size exclusion chromatography where small MW components trapped in gel, higher MW components too large, pass around, come out fasterCan estimate MW of unknowns by running standards of known MW

ELUTION VOLUME (mls)16182022242628Mw = 124600, Mn = 33200, MWD = 3.8Mw = 73800, Mn = 37400, MWD = 2.0Molecular Weight Distribution Comparisonby Gel Permeation Chromatography

Properties: Molecular weightsynthetic polymers possess a molecular weight distribution*polydispersity index = Mw/Mn

Molecular weight and dispersionSynthetic polymers always show a distribution in molecular weights. number average :

weight average:

(ni and wi are number and weight fractions, respectively, of molecules with molar mass Mi)

The polydispersity index is given by Mw /Mn

Molecular weight and dispersion - an example:Here are:10 chains of 100 molecular weight20 chains of 500 molecular weight40 chains of 1000 molecular weight5 chains of 10000 molecular weight

What the Weights MeanMn: This gives you the true average weightLet's say you had the following polymer sample:2 chains: 1,000,000 Dalton2,000,0005 chains: 700,000 Dalton3,500,00010 chains: 400,000 Dalton4,000,0004 chains: 100,000 Dalton 400,0002 chains: 50,000 Dalton 100,00010,000,00010,000,000/23 = 435,000 Dalton

1 Dalton = 1 g/mole

Weight Average Molecular WeightMw: Since most of the polymer mass is in the heavier fractions, this gives the average molecular weight of the most abundant polymer fraction by mass.

Examples Light scattering: larger molecules scatter more light than smaller ones.

Infrared absorption properties: larger molecules have more side groups and light absorption (due to vibrational modes of side groups) varies linearly with number of side groups.Molecular Weights Why do we care about weight average MW?some properties are dependent on MW (larger MW polymer chains can contribute to overall properties more than smaller ones).Distribution of polymer weights

Polydispersity and Degree of PolymerizationPolydispersity:When polydispersity = 1, system is monodisperse.Degree of Polymerization:Number avg degree of polymerizationWeight avg degree of polymerization

Compute the number-average degree of polymerization for polypropylene, given that the number-average molecular weight is 1,000,000 g/mol.

What is mer of PP?

Mer molecular weight of PP is Example 1C3H6mo=3AC+6AH =3(12.01 g/mol)+6(1.008 g/mol) = 42.08 g/molNumber avg degree of polymerization

Problem 1 (a, b, and c)A. Calculate the number and weight average degrees of polymerization and polydispersity for a polymer sample (PP) with the following distribution.Avg # of monomers/chainRelative abundance10 5100 25500 501000 305000 1050,000 5

Problem 2 (b) B. If the polymer is PMMA, calculate number and weight average molecular weights.Mw if monomer is methylmethacrylate (5C, 2O, and 8H)So m0= 5(12)+2(16)+8(1)= 100 g/mol CH3 |-CH2-C- | CO2CH3

Problem 3 (c)C. If we add polymer chains with avg # of monomers = 10 such that their relative abundance changes from 5 to 10, what are the new number and weight average degrees of polymerization and polydispersity?Add 5 more monomers of length 10 .

Regularity and symmetry of side groups affect properties Stereoisomerism: (can add geometric isomerism too)Polymer Molecular Configurations- Conversion from one stereoisomerism to another is not possible by simple rotation about single chain bond; bonds must be severed first, then reformed!PolymerizeCan it crystallize?Melting T?

Regularity and symmetry of side groups affect propertiesPolymer Geometrical Isomerismcis-structure trans-structure

with R= CH3 to form rubberCis-polyisoprene trans-polyisopreneConversion from one isomerism to another is not possible by simple rotation about chain bond because double-bond is too rigid!

See Figure 4.8 for taxonomy of polymer structures

Covalent chain configurations and strength:Direction of increasing strengthAdapted from Fig. 14.7, Callister 6e.Polymer MicrostructureVan der Waals, HMore rigid

Random, Alternating, Blocked, and GraftedCoPolymers Synthetic rubbers are often copolymers.

e.g., automobile tires (SBR)Styrene-Butadiene Rubber random polymer

Example 3Nitrile rubber copolymer, co-poly(acrylonitrile-butadiene), has

Calculate the ratio of (# of acrylonitrile) to (# of butadiene). 3 C = 3 x 12.01 g/mol 3 H = 3 x 1.008 g/mol 1 N = 1 x 14.007 g/mol

m1= 53.06 g/mol 4 C = 4 x 12.01 g/mol 6 H = 6 x 1.008 g/mol

m2= 54.09 g/mol1,4-addition productWe need to use an avg. monomer MW:

Crosslinking in elastomers is called vulcanization, and is achieved by irreversible chemical reaction, usually requiring high temperatures. Vulcanization Sulfur compounds are added to form chains that bond adjacent polymer backbone chains and crosslinks them.Unvulcnaized rubber is soft and tacky an poorly resistant to wear.e.g., cis-isopreneStress-strain curvesSee also sect. in Chpt. 8

Molecular weight, Mw: Mass of a mole of chains. Tensile strength (TS): --often increases with Mw. --Why? Longer chains are entangled (anchored) better. % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases.Adapted from Fig. 14.11, Callister 6e.Molecular Weight and Crystallinity

Polymer Crystallinitypolyethylene Some are amorphous. Some are partially crystalline (semi-crystalline). Why is it difficult to have a 100% crystalline polymer?rs = density of specimen in questionra = density of totally amorphous polymerrc = density of totally crystalline polymer

Volume fraction of crystalline component.Using definition of volume fractions:Substituting in fc into the original definition:

Polymer CrystallinityDegree of crystallinity depends on processing conditions (e.g. cooling rate) and chain configuration.

Cooling rate: during crystallization upon cooling through MP, polymers become highly viscous. Requires sufficient time for random & entangled chains to become ordered in viscous liquid.

Chemical groups and chain configuration: More CrystallineSmaller/simper side groupsLinear

Isotactic or syndiotacticLess CrystallineLarger/complex side groupsHighly branchedCrosslinked, networkRandom

Semi-Crystalline PolymersFringed micelle model: crystalline region embedded in amorphous region. A single chain of polymer may pass through several crystalline regions as well as intervening amorphous regions.Crystalline volume fractions Important

Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resinAdapted from Fig. 15.18, Callister 6e. Thermoplastics vs ThermosetsTm: melting over wide range of T depends upon history of sample consequence of lamellar structure thicker lamellae, higher Tm.Tg: from rubbery to rigid as T lowers

Packing of spherical atoms as in ionic and metallic crystals led to crystalline structures.

How polymers pack depend on many factors: long or short, e.g. long (-CH2-)n. stiff or flexible, e.g. bendy C-C sp3. smooth or lumpy, e.g., HDPE. regular or random single or branched slippery or sticky, e.g. C-H covalent (nonpolar) joined via vdW.

Analogy: Consider dried (uncooked) spaghetti (crystalline) vs. cooked and buttered spaghetti (amorphous). pile of long stiff spaghetti forms a random arrangement. cut into short pieces and they align easily.

Candle wax more crystalline than PE, even though same chemical nature.Packing of Polymers

Would you expect melting of nylon 6,6 to be lower than PE?What Are Expected Properties? ++++++++++++nylon 6,6polyethyleneWhat is the source of intermolecular cohesion in Nylon vs PE?How does the source of linking affect temperature? Hydrogen bondsVan der Waals bondsWith H-bonds vs vdW bonds, nylon is expected to have (and does) higher melting T.

Which polymer more likely to crystallize? Can it be decided? What Are Expected Properties? Linear syndiotactic polyvinyl chlorideLinear isotactic polystyrene Linear and syndiotactic polyvinyl chloride is more likely to crystallize. The phenyl side-group for PS is bulkier than the Cl side-group for PVC. Generally, syndiotactic and isotactic isomers are equally likely to crystallize. For linear polymers, crystallization is more easily accomplished as chain alignment is not prevented. Crystallization is not favored for polymers that are composed of chemically complex mer structures, e.g. polyisoprene.

Which polymer more likely to crystallize? Can it be decided? What Are Expected Properties? Linear and highly crosslink cis-isoprene Not possible to decide which might crystallize. Both not likely to do so. Networked and highly crosslinked structures are near impossible to reorient to favorable alignment.++ H20Networked Phenol-Formaldehyde(Bakelite)

Polymer MicrostructurePolyolefins with side chains have stereocenters on every other carbonWith so many stereocenters, the stereochemistry can be complex. There are three main stereochemical classifications for polymers.

TacticityTacticity stereoregularity of chainisotactic all R groups on same side of chain

syndiotactic R groups alternate sides

atactic R groups random

Why is this important?Tacticity affects the physical propertiesAtactic polymers will generally be amorphous, soft, flexible materialsIsotactic and syndiotactic polymers will be more crystalline, thus harder and less flexiblePolypropylene (PP) is a good exampleAtactic PP is a low melting, gooey materialIsoatactic PP is high melting (176), crystalline, tough material that is industrially usefulSyndiotactic PP has similar properties, but is very clear. It is harder to synthesize

Linear polymers with side branchesIsotacticSame side of the linear polymerGreater effect of dispersion forces therefore high density, rigid and tough and a high softening temp.AtacticIrregular points on both sides of the linear polymerChains of molecules cannot get close together, therefore low density.Soft, waxy little use

Poly(propene)This varying degree of randomness will affect the strength and melting point of the polymer.

The less random, the stronger the polymer and the higher the melting point

This is because in a more ordered polymer they chains can get closer together and hence the van der Waals forces will be greater.

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