Polymers:1. Introduction2. The variety of Polymeric Materials

Dept. Phys., Tunghai Univ.

C. T. Shih

Introduction

Polymers: a giant molecule, made up of many repeat units covalently joined together

Common characteristics: long, string-like molecules

Living polymers: long-chain objects whose subunits are joined by physical bonds, e.g., actin filaments

Variety of Polymeric Materials

Polymer chemistry Stereochemistry Architecture Homopolymers and Copolymers Physical state

Polymer chemistry

Most polymers contains carbon atoms. Simplest polymer: polyethylene （聚乙烯）

Polymer chemistry (conti.)

The main chain can have different side groups: polymethyl methacrylate（聚甲基丙烯酸甲酯）

Polymer chemistry (conti.)

The main chain can incorporate non-carbon atoms, e.g., Nylon

Polymer chemistry (conti.)

The main chain can involve loops, e.g., amylose （澱粉）

Polymer chemistry (conti.)

The polymer can be multiply connected, forming a ladder structure, e.g., poly(dioctyl fluorene)

Polymer chemistry (conti.)

The main chain can contain no carbon atoms at all, e.g., PDMS (silicone oil)

Stereochemistry

If a polymer has more than one type of chemical group attached to each main chain (carbon) atom, then different arrangement of the groups in 3D are possible: isotactic, syndiotactic, and atactic.

Quenched disorder→ Glasses

Architecture

Linear vs. Branched polymers Linear polymers can be characterized by N, the

number of repeated units: degree of polymerization

N is proportional to the relative molecular mass M

Degree of polymerization is not a constant for a group of polymers – a distribution

PDI: polydispersity index ≡ (weight average)/(number average)

PDI=Mw/Mn

Number average: Weight average: Standard error:

i

ii

NMN

ii

ii

i

ii

MNMN

wMw 2

2

222

nwn

i

ii

ii

ii

MMM

N

MN

MN

MN

Branched Polymer

The polymers may be branched The branched points introduce quenched

disorder, and hinder the formation of crystals

A lot of branch points – a network

Copolymers

The building units are all the same – homopolymers

Different building units – copolymers Phase separation and microphase separat

ion DNA and proteins are copolymers

Physical States

Liquid – melts and solutions, usually very viscous/viscoelastic

Glass – is common because of the difficulty of crystallization

Crystalline – usually incomplete/small due to quenched disorder

Liquid crystal