Polymers
Hydrocarbons
Covalent bonds between H and C. 
- unsaturated: there are double/triple bonds
- saturated: all single bonds
- isomers: same composition, different arrangement
Polymer Molecules
Are hydrocarbons, but gigantic. 
- Thus, we have repeat units (also called monomer, kind of)
- homopolymer: all repeating units along chain of same type.
- copolymers: chains composed of two or more different repeat units
- have: size, shape, structure 
- size in mol weight (or DP)
- shape in terms of degree of chain twisting, coiling, bending
- structure (linear, branched, crosslinked) in addition to several isometric configs.
Molecular Weight
Number-Average Molecular Weight

Obtained by dividing the chains into a series of size ranges and then determining the number fraction of chains within each size range:

Weight-Average Molecular Weight

Based on the weight fraction of molecules within the various size ranges:

Degree of Polymerization
A way of expressing average chain size of a polymer:
DP = M(n)/m
where M(n) is number-avg mol weight, m is repeat unit molecular weight
Molecular Structure
Rotational flexibility is dependent on repeat unit structure (for example, chain with double bond is rotationally rigid).  
- Introduction of a bulky or large side group of atoms restricts rotational movement.
Types
Linear, branched, crosslinked, network. 
Copolymers
Can be of several types:
- random copolymer
- alternating (exactly alternating)
- block (sections of each)
- gaft (branches of the other)
Polymer Crystallinity
Packing of molecular chains to produce an ordered atomic array. 
- polymers often times are semicrystalline. 
- metals are almost entirely always crystalline
- ceramics either totally crystalline or non-crystalline
- polymers can range 
Formula

where ps = density of specimen, pa = density of totally amorphous polymer, pc = density of perfectly crystalline polymer 

Stress/Strain Behavior 
Three types
A - brittle polymer (fractures while deforming elastically)
B - plastic polymer (like metallic)
C - elastomers (very elastic, rubberlike)
Properties from graph
- Modulus of elasticity, ductility in % elongation - same as metals
- yield point is maximum point right after elastic region
- TS = stress at fracture
VS temp - becomes softer and more ductile 
Semicrystalline
When necking occurs, chains become oriented, which leads to localized strengthening.
Viscoelasticity
Definition
For intermediate temperatures, polymers exhibit viscoelasticity (rubbery solid that exhibits combined mechanical characteristics of both extremes - like a glass and liquid)

Relation to time

As time increases, relaxation modulus decreases.

Viscoelastic Relaxation Modulus
Stress necessary to maintain certain level of strain (as a function of time) at a constant temperature:
Main Idea
Relation to temperature

As temperature increases, relaxation modulus decreases.   At intermediate temps, we see viscoelasticity. 
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