Lecture 6 Atomic packing in

Polymers and Glasses

Jayant Jain Assistant Professor,

Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016

Mark the positions of octahedral and tetrahedral voids in BCC unit cell.

BCC voids Position Voids /

cell

Voids /

atom

Distorted

Tetrahedral Four on each face: [(4/2) 6 = 12] (0, , ) 12 6

Non-regular

Octahedral

Face centre: (6/2 = 3) (, , 0)

Edge centre: (12/4 = 3) (, 0, 0) 6 3

OV TV

VOIDS BCC

a

a3/2

a a3/2

{0, 0, })

Polymer structure: Atomic packing in polymers

Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

Polymers have a carbon-carbon backbone with varying side-groups

Polymer structure

Varying side groups

Common Polymers

Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

Polymer chains bond to each other through weak hydrogen

bonds

Red lines indicate strong cross-linked carbon-carbon bonds

Polymer Structure

Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon

(a): No regular repeating pattern of

polymer chains results in a glassy or amorphous structure

(b): Regions in which polymer chains

line up and register forms crystalline patches

(c): Occasional cross-linking allowing

the polymer to stretch typical of elastomers

(d): Heavily cross-linked polymers

typical of epoxy

Thermoplastics: Secondary bonds can be easily broken with temperature and this allows the polymer to be moulded and shaped readily. They retain its shape on cooling Thermosets: They have many cross-links, making them stiffer and stronger than thermoplastics.The cross-links cannot be broken by heat. So they cannot be thermally moulded.

The temperature at which the polymer experiences the transition from rubbery to rigid states is termed the glass transition temperature, Tg

Tg

Rubberry

Rigid solid

T

Tm

Glass transition temperature

In metals, ceramics Young's modulus generally decreases with increase in temperature The change is modulus is not that significant in these materials as compared to polymers For polymers, however, a temperature change of 30 C may change the elastic modulus by a factor of 1,000.

Elastic deformation of Polymers

The drastic change is associated with the temperature being high enough to weaken the secondary bonds of the polymer allowing more chain movement and therefore decreasing the polymer stiffness.

An example of polysterene

Effect of temperature on Polymer stiffness

Glass structure

Oxygen atoms at the corner of tetrahedron and Si at the centre of the tetrahedron

Structure of silica (Glass)

Amorphous silica is the bases of most glasses

Structure of crystalline and non crystalline silica

If these tetrahedra are arrayed in a regular and ordered manner, a crystalline structure is formed: quartz: clocks

If these tetrahedra are arrayed in a random manner then amorphous structure is formed: Glass

Soda glass

Addition of soda lowers the softening temperature of glass by breaking some strong bonds