1. MASE 542/CHEM 442 BIOMATERIALS
POLYMERS

2. What is a Polymer? Poly mer
many repeat unit
repeat
unit
repeat
unit
repeat
unit C H
Polyethylene (PE) Cl C H
Poly(vinyl chloride) (PVC) H
Polypropylene (PP) C
CH3
Adapted from Fig. 14.2, Callister & Rethwisch 8e.
“ Macromolecules that consist of small repeating units added together in long chains ”

3. Ancient Polymers Originally natural polymers were used Wood – Rubber
Cotton – Wool
Leather – Silk
Oldest known uses
Rubber balls used by Incas
Noah used pitch (a natural polymer) for the ark

4. Polymers Types according to origin Natural Synthetic
Types according to processability
Thermoplastics : Linear & processable
Thermosets : Crosslinked & not processable
Types according to mechanical properties
Plastics
Elastomer
Fibers
Types according to method of synthesis
Addition
Condensation

6. Polymer synthesis Addition polymerization (Chain growth) Free radical
Ionic
Ring opening
Condensation Polymerization (Step growth)

7. Polymerization and Polymer Chemistry
Free radical polymerization
Initiator: example - benzoyl peroxide

8. ROP

9. Addition Polymerization

10. Condensation Polymerization

11. Condensation Copolymers

12. Three things that make Polymers Unique
Summation of Intermolecular Forces
The bigger the molecule, the more molecule there is to exert an intermolecular force.
Even when only weak Van der Waals forces are at play, they can be very strong in binding different polymer chains together.
Chain Entanglement
one huge tangled mess
Can uncoil if you heat it up!
Time Scale of Motion
polymers move more slowly than small molecules do
if you dissolve a polymer in a solvent, the solution will be a lot more viscous than the pure solvent
Chain Entanglement
the chains tend to twist and wrap around each other, so the polymer molecules collectively will form one huge tangled mess.
Now when a polymer is molten, the chains will act like spaghetti tangled up on a plate. If you try to pull out any one strand of spaghetti, it slides right out with no problem. But when polymers are cold and in the solid state, they act more like a ball of string.
Summary of Intermolecular Forces
The bigger the molecule, the more molecule there is to exert an intermolecular force. Even when only weak Van der Waals forces are at play, they can be very strong in binding different polymer chains together. This is another reason why polymers can be very strong as materials. Polyethylene , for example is very nonpolar. It only has Van der Waals forces to play with, but it is so strong it's used to make bullet proof vests.
Time Scale of Motion
polymers move more slowly than small molecules do
if you dissolve a polymer in a solvent, the solution will be a lot more viscous than the pure solvent

13. Physical properties of polymers
Mwt
Shape (entanglement/crystallinity)
Structure of the chain
Chemical composition

14. Chemistry and summation of forces Polyethylene
Adapted from Fig. 14.1, Callister & Rethwisch 8e.
Polymer- can have various lengths depending on number of repeat units
polyethylene is a long-chain hydrocarbon
Hydrophobic/vdW interactions/bullet proof vest!
paraffin wax for candles is short polyethylene

15. MOLECULAR WEIGHT • Molecular weight, M: Mass of a mole of chains.
Low M
high M
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
Not all chains in a polymer are of the same length
— i.e., there is a distribution of molecular weights

16. MOLECULAR WEIGHT DISTRIBUTION
Adapted from Fig. 14.4, Callister & Rethwisch 8e.
Simple for small molecules
All the same size
Number of grams/mole
Polymers – distribution of chain sizes
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

17. Molecular Weight Calculation
Example: average mass of a class
Student
Weight
mass (lb) 1
104 2
116 3
140 4
143 5
180 6
182 7
191 8
220 9
225 10
380
What is the average
weight of the students in
this class:
Based on the number fraction of students in each mass range?
Based on the weight fraction of students in each mass range?

18. Molecular Weight Calculation (cont.)
Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table:
Calculate the number and weight fraction of students in each weight range as follows:
For example: for the lb range
total number
total weight

19. Molecular Weight Calculation (cont.)

20. Degree of Polymerization, DP
DP = average number of repeat units per chain
DP = 6

21. 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
Adapted from Fig. 14.5, Callister & Rethwisch 8e.

22. Chain Entanglement
Adapted from Fig. 14.6, Callister & Rethwisch 8e.

23. fig_15_15 Chain Entanglement
STRETCHED
Rubber: elastic extention!
fig_15_15
COILED
Impacts mechanical properties
Rubber: elastic extention!

24. fig_14_07 STRUCTURE OF THE MOLECULAR CHAINS HDPE PS Reduces: PMMA
nylon
Reduces:
Packing,
Density
LDPE B
Linear
ranched
Cross-Linked
Network
Vulcanized rubber
fig_14_07

25. Physical X-links

26. fig_15_15
fig_15_15

27. Molecular Configuration Tacticity
stereoregularity or spatial arrangement of R units along chain
isotactic – all R groups on same side of chain
syndiotactic – R groups alternate sides

28. atactic – R groups randomly positioned
Tacticity (cont.)
atactic – R groups randomly positioned

29. Tacticity
Tacticity is simply the way pendant groups are arranged along the backbone chain of a polymer.

30. Tacticity and crystallinity
If regular arrangement of atoms (isotactic and syndiotactic)
pack together easily into crystals and fiber
molecules pack best with other molecules of the same shape
Syndiotactic PS
Metallocene catalysis vinyl polymerization
Crystalline
Tm 270 ℃
Atactic polystyrene
Can`t pack!
Amorphous
Hard plastic
Free radical polymerization
Isotactic polypropylene (PP)
Ziegler-Natta Polymerization
Crystalline
Fibers for things like indoor-outdoor carpeting.
Atactic polypropylene
soft and sticky, not very strong, and not that good for anything.

31. Composition (Copolymers)
Adapted from Fig. 14.9, Callister & Rethwisch 8e.
Composition (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 –
random
alternating
block
graft

32. Crystallinity in Polymers
Adapted from Fig , Callister & Rethwisch 8e.
Ordered atomic arrangements involving molecular chains
Crystal structures in terms of unit cells
Example shown
polyethylene unit cell (orthorhombic)

33. 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 increase.
amorphous
region
Adapted from Fig , Callister 6e.
(Fig is from H.W. Hayden, W.G. Moffatt,
and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

34. Polymer Crystallinity
Partially crystalline!
Metals: fully crystalline
Ceramics: fully crystalline or fully noncrystalline
Crystalline regions
thin platelets with chain folds at faces
Chain folded structure
10 nm
Adapted from Fig , Callister & Rethwisch 8e.
10micron long

35. Semicrystalline Polymers
Some semicrystalline polymers form spherulite structures
Alternating chain-folded crystallites and amorphous regions
Spherulite structure for relatively rapid growth rates
Spherulite surface
Adapted from Fig , Callister & Rethwisch 8e.

36. Polymer Single Crystals
Electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene
Single crystals – only for slow and carefully controlled growth rates
Adapted from Fig , Callister & Rethwisch 8e.

37. Crystallinity Increases density Act like physical crosslinks Stronger
More resistant to attack by solvent
Cooling from viscous melt
Slow to allow time for chain allignment
Simple structures favor x-ity (PE, TEFLON)
Complex structures, bulky side groups, atacticity and branching prevents x-izatization
polyisoprene can not crystallize
Network or x-linked: amorphous

38. Thermal Properties
Tg: the temperature above which a polymer becomes soft and pliable, and below which it becomes hard and glassy. Temperature where long range motion of molecules cease.
Tm: crystalline melting temperature.

39. Melting & Glass Transition Temps.
What factors affect Tm and Tg?
Both Tm and Tg increase with increasing chain stiffness
Chain stiffness increased by presence of
Bulky sidegroups
Polar groups or sidegroups
Chain double bonds and aromatic chain groups
Regularity of repeat unit arrangements – affects Tm
Adapted from Fig , Callister & Rethwisch 8e. 39 39

40. STRUCTURE-PROPERY Polymer properties depend on:
Chemical structure (composition)
Chain flexibility/rigidity
Side chain size, flexibility
Intermolecular interaction
regularity
Tacticity
Packing ability/density
Crystallinity

41. Structure-Property “pendant groups”

42. Structure-Property “Backbone groups”
poly(phenylene sulfone)
Stiff/rigid backbone
No Tg!
Stay glassy as high as 500 oC, then decompose.
Poly(ether sulfone)                            
flexible ether groups bring the Tg of this one down to a more manageable 190 oC.

43. Processing of Plastics
Thermoplastic
can be reversibly cooled & reheated, i.e. recycled
heat until soft, shape as desired, then cool
ex: polyethylene, polypropylene, polystyrene.
Thermoset
when heated forms a molecular network (chemical reaction)
degrades (doesn’t melt) when heated
a prepolymer molded into desired shape, then chemical reaction occurs
ex: urethane, epoxy
Can be brittle or flexible & linear, branching, etc. 43 43

44. Types according to processability
Thermoplastics : Linear & processable
Thermosets : Crosslinked & not processable

45. Types according to mechanical properties
Fibers
Plastics
Elastomer

46. Polymer Types – Miscellaneous
Coatings – thin polymer films applied to surfaces – i.e., paints, varnishes
protects from corrosion/degradation
decorative – improves appearance
can provide electrical insulation
Adhesives – bonds two solid materials (adherands)
bonding types:
Secondary – van der Waals forces
Mechanical – penetration into pores/crevices
Films – produced by blown film extrusion
Foams – gas bubbles incorporated into plastic 46 46

47. Mechanical Properties of Polymers – Stress-Strain Behavior
brittle polymer
plastic
elastomer
elastic moduli – less than for metals
Adapted from Fig. 15.1, Callister & Rethwisch 8e.
Plastics are like metals: elastic deformation-yielding than plastic deformation
Elastomer: at low stress level large deformation (reversible)
• Fracture strengths of polymers ~ 10% of those for metals
• Deformation strains for polymers > 1000% – for most metals, deformation strains < 10% 47 47

48. Fibers Fibers - length/diameter >100 Primary use is in textiles.
Fiber characteristics:
high tensile strengths
High abrasion resşstance
high degrees of crystallinity
structures containing polar groups
Formed by spinning
extrude polymer through a spinneret (a die containing many small orifices)
the spun fibers are drawn under tension
leads to highly aligned chains - fibrillar structure 48 48

49. Mechanisms of Deformation—Brittle Crosslinked and Network Polymers
(MPa)
Near Failure
Near Failure
Initial x
Initial
network polymer
brittle failure x
plastic failure
aligned, crosslinked
polymer e
Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. 49 49

50. Predeformation by Drawing
• Drawing…(ex: monofilament fishline)
-- stretches the polymer prior to use
-- aligns chains in the stretching direction
• Results of drawing:
-- increases the elastic modulus (E) in the
stretching direction
-- increases the tensile strength (TS) in the
-- decreases ductility (%EL)
• Annealing after drawing...
-- decreases chain alignment
-- reverses effects of drawing (reduces E and TS, enhances %EL)
• Contrast to effects of cold working in metals!
Adapted from Fig , Callister & Rethwisch 8e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp ) 50 50

51. Plastics
either deform permanently, or just plain break, when you stretch them too hard.
it will stay in the shape you stretched it into once you stop stretching it.
Elastomers bounce back when you let go.
plastics resist deformation better than elastomers
Sometimes additives are added to a plastic to make it softer and more pliable. These additives are called plasticizers

52. Mechanical Properties of Polymers – Stress-Strain Behavior
brittle polymer
plastic
elastomer
elastic moduli – less than for metals
Adapted from Fig. 15.1, Callister & Rethwisch 8e.
Plastics are like metals: elastic deformation-yielding than plastic deformation
Elastomer: at low stress level large deformation (reversible)
• Fracture strengths of polymers ~ 10% of those for metals
• Deformation strains for polymers > 1000% – for most metals, deformation strains < 10% 52 52

53. Mechanisms of Deformation — Semicrystalline (Plastic) Polymers
(MPa)
fibrillar structure
near
failure x
brittle failure
Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister & Rethwisch 8e. (15.12 & are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
onset of
necking
plastic failure x
crystalline
regions align
crystalline
block segments
separate
amorphous
regions
elongate
unload/reload
undeformed
structure e 53 53

54. fig_15_02
fracture
Linear elastic regime
fig_15_02

55. table_15_01

56. Higher tensile strength Elastic polymers may elongate over 1000%
METALS
POLYMERS
Rarely elongate over 100%
Higher tensile strength
48-410GPa
Elastic polymers may elongate over 1000%
Lower tensile strength
7MPa-4GPa

57. Polymer Additives
Improve mechanical properties, processability, durability, etc.
Fillers
Added to improve tensile strength & abrasion resistance, toughness & decrease cost
ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.
Plasticizers
Added to reduce the glass transition
temperature Tg below room temperature
Presence of plasticizer transforms brittle polymer to a ductile one
Commonly added to PVC - otherwise it is brittle
Polymers are almost never used as a pure material
Migration of plasticizers can be a problem 57 57

58. Plasticizers New car smell Usually a small molecule
increases the free volume
Tg decreases
softer and more processable
BUT: may leach out
Bis-(2-ethylhexyl)phthlate
dioctyl phthlate (DOP)
New car smell
A small molecule which will get in between the polymer chains, and space them out from each other.
softer and more pliable
This increases the free volume. Chains can slide past each other more easily. In this way, the Tg of a polymer can be lowered, to make a polymer easier to work with.

59. Polymer Additives (cont.)
Stabilizers
Antioxidants
UV protectants
Lubricants
Added to allow easier processing
polymer “slides” through dies easier
ex: sodium stearate
Colorants
Dyes and pigments
Flame Retardants
Substances containing chlorine, fluorine, and boron 59 59