Lecture 6 Atomic packing in Polymers and Glasses Jayant Jain Assistant Professor, Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016
Dec 19, 2015
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