Pull thin polymer rod in tension

1
Pull thin polymer rod in tension
2
Polymer fibers have aligned crystalline
regions - alignment gives greater strength to
fiber
3
Polymer fibers have aligned crystalline
regions - alignment gives greater strength to
fiber
4

• Breaking strength of polymer fibers (tenacity)
• measure denier (wt. in grams of 9000 meters of
  fiber)
• run tensile test

5
Tenacity increases w/ chain length - fewer
crystal defects
6
Stress/strain characteristicsof polymers
Polymer stiffness, strength and toughness vary
over extraordinary range
7
Stress/strain characteristicsof polymers
Polymer stiffness, strength and toughness vary
over extraordinary range Due to structure -
ranges from purely amorphous states to chain
folded semi-crystalline to highly oriented
(fibers)
8
Stress/strain characteristicsof polymers
Polymer stiffness, strength and toughness vary
over extraordinary range Due to structure -
ranges from purely amorphous states to chain
folded semi-crystalline to highly oriented
(fibers) Polymers plastically deform readily,
esp. if temp raised (often less than 1000C )
9
Stress/strain characteristicsof polymers
Glassy polymer or
semi-crystalline polymer
Stress
below Tg
x
Semi-crystalline polymer
(
)
s
above Tg
x
x
Rubber
Strain (
e
)
10
Yielding in flexible semi-crystalline polymers
Flexible semi-crystalline polymers such as
polyethylene (Tg of amorphous domains is below rm
temp) usually display considerable amount of
yielding if not stretched too quickly
11
Relaxation
Yielding due to relaxation Time dependent
molecular transition or rearrangement, such as
change in conformation of a chain, crystalline
slip, chain sliding, usw.
12
Yielding in rigid polymers
Rigid polymers usually don't have yield
point May yield by crazing
13
Crazing
Microscopic cracks form perpendicular to applied
stress
14
Crazing
Microscopic cracks form perpendicular to applied
stress Tiny fibrils span cracks - hold material
together
15
Crazing
Microscopic cracks form perpendicular to applied
stress Tiny fibrils span cracks - hold material
together Polymer whitens
16
Polymers arent very stiff
17
Stiffness dictated by structure
18
Stiffness depends on crystallinity crosslinking
Tg
19
For fibers, stiffness depends on draw ratio
20
Tensile strength
21
Glass transition temperature (Tg)
22
Glass transition temperature (Tg)
23
Glass transition temperature (Tg)
Chemical structure
24
Glass transition temperature (Tg)
Chain stiffness
25
Glass transition temperature (Tg)
Chain stiffness
26
Glass transition temperature (Tg)
Bulky side groups