Lecture 20 - Polymers (B)

Question 1

Natural Polymers

Answer 1

• Chitosan
• Collagen
• Cellulose
• DNA
• Natural Rubber

Question 2

Synthetic Polymers

Answer 2

• PE
• PS
• PLA (Polylactic acid)
• PGA (Polyglycolide)
• PLGA
• PCL (Polycaprolactone

Question 3

Addition Polymerization

Answer 3

• Chain reaction
• Unsaturated monomers react (opens bond within)
• Initiation, propagation, termination

Question 4

Initiation

Answer 4

• Activates monomer
• Free radical, cation, anion
• Opens double bond within monomer
• Creates initiation site (site that wants to bond)

Question 5

Propagation

Answer 5

Rapid chain growth

Question 6

Termination

Answer 6

• Reaction with another radical, solvent molecule, fragment of polymer chain
• Depends on probability, more difficult to control

Question 7

Condensation Polymerization

Answer 7

• Stepwise growth
• Two monomers react to form covalent bond
• Elimination of small molecule (H2O, HCl, CO2)
• Often carboxylic acid and amide

Question 8

Synthesis Method

Answer 8

• Strongly affects polymer obtained
• Ex: free radical polymerization (MW difficult to control, MW distribution broad, branched products)
• Ex: anionic polymerization (lower PI, regular linear chains, termination via addition of cation to neutralize process)

Question 9

Homopolymer

Answer 9

All one mer

Question 10

Copolymer

Answer 10

• Alternating mers, no product of polymerization made
• For Addition: random/alternating (blend of properties based on weight % of mers), or block (distinct properties of each)
• For Condensation: depends on monomer used, properties depend on distribution of polymer

Question 11

Solid State Properties

Answer 11

• Determined from synthesis
• Tacticity
• Crystallinity
• Thermal Properties
• Mechanical Properties

Question 12

Thermal Properties

Answer 12

• Melt/Liquid State

- Measured by viscoelastic response (good for rank order, not great quantification), and DSC

Question 13

Thermal Properties: Thermal Energy

Answer 13

• Random chain motion (Brownian motion)
• Cool-temp reached where all chain motion essentially ceases, glass transition temp (T_g), varies from polymer to polymer

Question 14

Thermal Properties: Polymers Below T_g

Answer 14

Hard, glassy

Question 15

Thermal Properties: Polymers Above T_g

Answer 15

Rubbery

Question 16

Thermal Properties: Polymers With Crystallinity

Answer 16

Changes melting temp (T_m), melting of crystallinity phase

Question 17

Differential Scanning Calorimetry (DSC)

Answer 17

• Can provide T_g —> heat capacity increases as energy is absorbed by chain rearrangement
• Can provide T_m —> peak temperature, endothermic can be used to determine % crystallinity
• Sample of known mass heated or cooled, changes in heat capacity tracked
• Used to detect transitions in material —> melt, T_g, phase transformations, curing, loss of solvents, degradation

Question 18

DSC Type #1

Answer 18

Heat Flow

• How much energy put in to raise temperature
• Double oven (2 samples in separate ovens), maintain reference material/sample material @ same T
• Measure difference in power to do this
• During melting, heat absorbed into material
• More power to sample to obtain equal T between sample/reference

Question 19

DSC Type #2

Answer 19

Heat Flux

• Single oven
• Measure T difference between sample and reference
• Cheaper, less accurate
• Sample absorbs heat, cooler than reference

Question 20

Mechanical Properties: Mechanical Testing

Answer 20

• Highly dependent on test conditions
• Temperature —> as T increases, increase in thermal energy results in chain motion and increase in free volume between chains
• Rate —> fast rate means no time for chains to reorient and slide past one another, higher stiffness, shorter elongation to failure, brittle

Question 21

Mechanical Properties: Semi-Crystalline Polymers

Answer 21

• Contains spherulites (ordered structure, polymer chains tightly packed with amorphous regions in between)

Question 22

Mechanical Properties: Exposure to Tensile Force

Answer 22

• Tie chains extend and lamellae slide past each other
• Lamellae become reoriented —> chain folds aligned along axis of loading
• Blocks of crystalline phase become separated from one another (extra energy)
• Blocks and tie molecules oriented along axis of applied tensile force

Question 23

Classes of Polymers

Answer 23

• Homopolymers
• Copolymers
• ——> Non-resorbable and Bioresorbable

Question 24

Non-Resorbable Homopolymers

Answer 24

• PMMA (polymethylmethacrylate)
• HEMA (2-hydroxyethyl methacrylate)
• HDPE (high density polyethylene)
• PP (polypropylene)
• PTFE (polytetrafluoroethylene)
• PVC (polyvinylchloride)
• PDMS (poly(dimethysiloxane))

Question 25

PMMA (polymethylmethacrylate)

Answer 25

- AKA plexiglass, lucite - Hydrophobic - Clear - Tough - Applications: hard contact lenses (not very flexible/comfortable for eye), bone cements (hold in place)

Question 26

HEMA (2-hydroxyethyl methacrylate)

Answer 26

- Hydrophilic - Clear - Soft - Cross-linked to resist degradation - Applications: soft contact lenses (wettable and easy movement along eye, but susceptible for protein deposition)

Question 27

HDPE (high density polyethylene)

Answer 27

- Hydrophobic - Tough - Inexpensive - Applications: tubing, drains, catheters, UHMWPE acetabular cups (articulating surfaces)

Question 28

PP (polypropylene)

Answer 28

- High rigidity (semi-crystalline form) - Chemical resistance - Good tensile strength - Applications: similar to HDPE, storage containers (tubes)

Question 29

PTFE (polytetrafluoroethylene)

Answer 29

- AKA gortex - Very stable - Difficult to process - Very hydrophobic - Low coefficient of friction (don't want protein adsorption) - Applications: vascular grafts

Question 30

PVC (polyvinylchloride)

Answer 30

- Hard - Brittle - Can be softened with plasticizers (plasticizers can be eluted into body, give space between adjacent chains) - Applications: tubing

Question 31

PDMS (poly(dimethysiloxane))

Answer 31

- Extremely flexible (Si-O-Si) - Lower T-g (remain rubbery to lower T) - Applications: catheters, tubing, insulation, blood vessels, heart valves, cosmetic surgery ---> comfortable

Question 32

Non-resorbable Copolymers

Answer 32

- FEP (tetrafluoroethylene-hexafluoropropylene) | - Polyurethane