MM 152: Polymeric Materials

Poly (many)-mer (unit) Large molecules (macromolecules) made of several repeat units

Ethylene molecule

(monomer)

Addition of two ethylene monomers

Polymerization of ethylene monomers gives polyethylene (PE)

Perspective view of a PE molecule

Representation of a PE molecule

Polytetrafluoroethylene (PTFE also known as Teflon)

Poly(vinylchloride) (PVC) Poly(propylene) (PP)

Common polymers

Some examples of bio-polymers

http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookglossg.html

http://www.goldiesroom.org/Note%20Packets/04%20Biochemistry/02%20Biochemistry--Lesson%202.htm

http://www.ucmp.berkeley.edu/glossary/gloss3/dna.html J van Mameren et al, Proc. Natl. Acad. Sci.,2009

Some examples of synthetic polymers

Six major commodity polymers: PE, PP, PVC, PET, PS, PC (98% of the plastics in use)Long molecules in a polymer product make it stable against degradation

PE degradation by chain scission (random breaking of bonds) at 450C

Large amounts of plastics suspended beneath ocean surface700,000 15,000,000 sq km80% from land based sources3.34 pieces/ 5.1 mg per sq m

Molecular weight upto 5 millionC-C bond energy 340 KJ/mol (3.5 eV/atom)

Poly(lactic) acid: derived from renewable sources such as corn starch and sugarcaneCompanies that make PLA: Natureworks (USA), Toyota (Japan), PURAC (Netherlands)

Natureworks Ingeo biopolymer can be made into both woven and non-woven products

PLA does not perform well at high temperatures

Biodegradable polymers

Polymer molecular weight

What is the weight of a polymer molecule? Monomer weight Number of monomersHow many monomers are present in a polymer molecule?

Most of the synthetic polymerization processes do not yield polymer molecules of the same lengthThe molecular weight of different polymer molecules in the same sample may be differentDefine molecular weight averages for polymers instead of absolute molecular weights

Nu

mb

er

frac

tio

n

We

igh

t fr

acti

on

Molecular weight (103 g/mol)

Number-average molecular weight

iin MxM

Weight-average molecular weight

iiw MwM

Degree of polymerization

m

MDP n

Molecular structure

Linear Branched

NetworkCross-linked

Single polymer molecule in solution

Polymer crystallinity

Polyethylene unit cell

Life inside a spherulite

Polyethylene single crystal

TEM showing the spheruliticstructure of polyethylene

Chain-folded structure for a plate-shaped polymer crystallite

Mechanical propertiesSt

ress

(M

Pa)

Strain

Brittle polymer

Plastic polymer

Highly elastic polymer

Stre

ss (

MP

a)

Strain

Influence of temperature on mechanical behaviour of PMMA ductile-to-brittle transition

Mechanical properties

Mechanical Deformation of semi-crystalline polymers

Stages in elastic deformation

Before deformation Elongation of

amorphous tie chains

Increase in lamellar thickness (reversible)

Stage 1 Stage 2

Mechanical Deformation of semi-crystalline polymers

Stages in plastic deformation

Stage 3

Stage 4

Stage 5

Tilting of lamellar chain folds

Separation of crystalline block

segments Orientation of block segments and tie

chains with the tensile axis

State at the end of elastic deformation

Effect of degree of crystallinity

Pe

rce

nt

of

crys

talli

nit

y

Molecular weight

Hard plastics

Soft plastics

Tough waxes

Brittle waxes

Soft waxes

Grease, liquids

Increase in crsytallinity leads to increase in tensile modulus, e.g., for PE modulus increases almost 10 times as percentage crystallinity increases from 0.3 to 0.6.

Increase in molecular weight generally leads to an increase in tensile strength

Rubber Elasticity

Gutta percha (trans-1, 4-polyisoprene) tree

Gutta percha points for root canal treatment

Natural rubber (cis-1, 4-polyisoprene) latex being collected from rubber tree

Rubber Elasticity

Gutta percha points for root canal treatment

Rubber Elasticity

Stress-strain curves for an alloy steel (crystalline material) and natural rubber (elastomer/ rubbery material)

Molecular Structure of a Rubber

Ordered lattice arrangement in crystalline materials Disordered network of

polymer chains in a rubber

Stretching Polymer Chains

Stretching a single polymer chain from its initial dis-ordered arrangement. Stretching reduces the entropy of the polymer

Similarly, polymer segments between network points in a rubber exist in a disordered, coiled state prior to extension. The degree of disorder decreases when a tensile stress is applied.

Affine Deformation

i

li

l0Extension ratio in the i-direction

Stretching of a rubber network

Affine Deformation

NkT

V1

1

1

2

E 11

1

2