Download Week 10 Day 2-Plastics - Washington State University

Polymers: what they are and how
they work?
Michael R. Kessler
School of Mechanical and Materials Engineering
Washington State University
What are Polymers
• Examples of polymers and where
they are used.
• Often Synthetic Polymers are
called Plastic.
• The volume of polymers produced
is three times larger than that of
all metals!
• Poly (many) mer (parts)  made
up of molecules of very long
chains.
What is a Polymer?
• Poly (many) mer (parts)  made
up of molecules of very long
chains.
repeat
unit
repeat
unit
H H H H H H
C C C C C C
H H H H H H
H H H H H H
C C C C C C
H Cl H Cl H Cl
Polyethylene (PE)
repeat
unit
H
C
H
Poly(vinyl chloride) (PVC)
H H
C C
CH3 H
H H
C C
CH3 H
H
C
CH3
Polypropylene (PP)
Ancient Polymers
• Originally natural polymers were used
– Wood
– Cotton
– Leather
– Rubber
– Wool
– Silk
• Oldest known uses
– Rubber balls used by Incas
– Noah used pitch (a natural polymer)
for the ark
Polymer Composition
Most polymers are hydrocarbons
– i.e., made up of H and C
• Saturated hydrocarbons
– Each carbon singly bonded to four other atoms
– Example:
» Ethane, C2H6
H
H
C
H
H
C
H
H
Chemistry and Structure of
Polyethylene
Note: polyethylene is a long-chain
hydrocarbon
- paraffin wax for candles is short polyethylene
Bulk or Commodity Polymers
Bulk or Commodity Polymers (cont)
Bulk or Commodity Polymers (cont)
Analogy for the Structure of
Polymers
Properties of Polymers
• Glass transition temperature (snake pit
analogy)
• Strength—measured in maximum stress
(force per unit area)
• Stiffness—measured in stress per unit
strain
• Damping—ability to absorb energy
Properties are often temperature
and rate dependent
(Viscoelastic)
Properties (Demonstration)
Happy Ball
Sad Ball
•Compare the damping properties for two
apparently identical polymer balls by
bouncing them on the floor.
•How do their damping behaviors vary
with temperature?
Happy Ball = Neoprene (polychloroprene)
Sad Ball = Norsorex (polynorbornene)
MOLECULAR WEIGHT
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
Not all chains in a polymer are of the same length
— i.e., there is a distribution of molecular
weights
MOLECULAR WEIGHT
DISTRIBUTION
total wt of polymer
Mn 
total # of molecules
M n  xi Mi
M w  wi Mi
Mi = mean (middle) molecular weight of size range i
xi = number fraction of chains in size range i
wi = weight fraction of chains in size range i
Degree of Polymerization, DP
DP = average number of repeat units per chain
H H H H H H
H H H
H H H
H C C (C C ) C C C C C C C C
H H H H H H
H H H
H
DP = 6
H H H
Mn
DP 
m
where m  average molecular weight of repeat unit
for copolymers this is calculated as follows :
m  fi mi
Chain fraction
mol. wt of repeat unit
i
Types of Polymers
• Thermoplastics
–Amorphous
Crystalline
Region
–Semi-crystalline
• Thermosets
Crosslinks
Molecular Structures for Polymers
secondary
bonding
Linear
Branched
Cross-Linked
Network
Polymers – Molecular Shape
Molecular Shape (or Conformation) – chain bending
and twisting are possible by rotation of carbon atoms
around their chain bonds
– note: not necessary to break chain bonds to alter molecular shape
Chain End-to-End Distance, r
Tacticity
Tacticity – stereoregularity or spatial arrangement of R
units along chain
isotactic – all R groups on
same side of chain
syndiotactic – R groups
alternate sides
H H H H H H H H
H H H R H H H R
C C C C C C C C
C C C C C C C C
H R H R H R H R
H R H H H R H H
Tacticity (cont.)
atactic – R groups
randomly
positioned
H H H H H R H H
C C C C C C C C
H R H R H H H R
Copolymers
two or more monomers
polymerized together
• random – A and B randomly
positioned along chain
• alternating – A and B alternate
in polymer chain
• block – large blocks of A units
alternate with large blocks of
B units
• graft – chains of B units
grafted onto A backbone
A–
B–
rando
m
alternatin
g
block
graft
23
Crystallinity in Polymers
• Ordered atomic
arrangements involving
molecular chains
• Crystal structures in terms
of unit cells
• Example shown
– polyethylene unit cell
24
Polymer Crystallinity
• Crystalline regions
– thin platelets with chain folds at faces
– Chain folded structure
10 nm
25
Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline
• Difficult for all regions of all chains to
become aligned
crystalline
region
• Degree of crystallinity
expressed as %
crystallinity.
-- Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity to
amorphous
increase.
region
Semicrystalline Polymers
• Some semicrystalline
polymers form spherulite
structures
• Alternating chain-folded
crystallites and
amorphous regions
• Spherulite structure for
relatively rapid growth
rates
Spherulite
surface
Photomicrograph – Spherulites in
Polyethylene
Cross-polarized light used
-- a maltese cross appears in each spherulite
Adapted from Fig. 14.14, Callister & Rethwisch 8e.