MASE 542/CHEM 442 BIOMATERIALS

MASE 542/CHEM 442 BIOMATERIALS
POLYMERS

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 ”

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

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

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

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

Condensation Polymerization

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

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

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

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

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

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

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.

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

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

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

Physical X-links

fig_15_15 fig_15_15

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

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

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

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.

cis/trans Isomerism cis trans cis-isoprene (natural rubber)
H atom and CH3 group on same side of chain trans trans-isoprene (gutta percha) H atom and CH3 group on opposite sides of chain

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

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)

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.)

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

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.

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.

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

Photomicrograph – Spherulites in Polyethylene
Cross-polarized light used -- a maltese cross appears in each spherulite Adapted from Fig , Callister & Rethwisch 8e.

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.

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. 36 36

Thermal Properties Elastomers has a Tg below room temperature
They are soft and rubbery at room temperature. Thermoplastics has a Tg above room temperature They are hard and glassy at room temperature.

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

Structure-Property “pendant groups”

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.

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. 41 41

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

Thermoplastics vs. Thermosets
Callister, Fig. 16.9 T Molecular weight Tg Tm mobile liquid viscous rubber tough plastic partially crystalline solid • Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene • Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig , Callister & Rethwisch 8e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) 43 43

fig_15_20 fig_15_20

Types according to mechanical properties
Fibers Plastics Elastomer

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

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

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

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

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

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

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

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

fig_15_02 fracture Linear elastic regime fig_15_02

table_15_01

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

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

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.

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

Tg values (C) PE (low density) PE (high density) PVC PS PC - 110 - 90
- 90 + 87 +100 +150 Selected values from Table 15.2, Callister & Rethwisch 8e.

Mechanisms of Deformation—Elastomers
(MPa) final: chains are straighter, still cross-linked x brittle failure Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. Inset figures along elastomer curve (green) adapted from Fig , Callister & Rethwisch 8e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) plastic failure x x elastomer initial: amorphous chains are kinked, cross-linked. deformation is reversible (elastic)! e 61 61

FRACTURE OF POLYMERS Strength < metals, ceramics
Thermoset : brittle fracture at crack site Thermoplastic: ductile-to-brittle transition Brittle (below Tg): High Tg, high strain, sharp notch

Influence of T and Strain Rate on Thermoplastics
(MPa) • Decreasing T... -- increases E -- increases TS -- decreases %EL • Increasing strain rate... -- same effects as decreasing T. brittle 20 4 6 8 Plots for 4ºC semicrystalline PMMA (Plexiglas) 20ºC 40ºC ductile to 1.3 60ºC e 0.1 0.2 0.3 Adapted from Fig. 15.3, Callister & Rethwisch 8e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) 63 63

fig_15_07 MECHANICAL PROPERTIES ARE TEMP DEPENDENT Rigid-brittle
Time dependent deformation Amorphous (atactic) polystyrene (viscoelastic behavior) Viscous and elastic Low modulus, easy deformation

fig_15_08 Crystalline isotactic PS Lightly crosslinled PS Amorphous PS

Crazing During Fracture of Thermoplastic Polymers
Craze ( regions of high plastic deformation) formation prior to cracking – during crazing, plastic deformation of spherulites – and formation of microvoids and fibrillar bridges fibrillar bridges microvoids crack aligned chains Note: crack spreads by breaking C-C bonds Ductile plastics have large craze zones that absorb large amounts of energy as it spreads Adapted from Fig. 15.9, Callister & Rethwisch 8e. 66 66

Polyphenylene oxide fig_15_10 fig_15_10

fatigue important in applications where dynamic strain is applied.
Artificial heart must be able to withstand many cycles of pulsating motion. The number of cycles to failure decreases as the applied stress level is increased

Advanced Polymers Ultrahigh Molecular Weight Polyethylene (UHMWPE)
Molecular weight ca. 4 x 106 g/mol Outstanding properties Extremely high impact strength resistance to wear/abrasion low coefficient of friction self-lubricating surface Chemical resitance But low Tm (137C)! Important applications bullet-proof vests golf ball covers hip implants (acetabular cup) UHMWPE Adapted from chapter-opening photograph, Chapter 22, Callister 7e. 69 69

Thermoplastic Elastomers
The idea behind thermoplastic elastomers is the notion of a reversible crosslink. Normal crosslinks: covalent The reversible crosslink: noncovalent secondary interactions between the polymer chains these interaction include hydrogen bonding and ionic bonding. Two approaches have been tried: ionomers block copolymers

Thermoplastic Elastomers
Styrene-butadiene block copolymer hard component domain styrene soft component domain butadiene Fig (a), Callister & Rethwisch 8e. Fig , Callister & Rethwisch 8e. (Fig adapted from the Science and Engineering of Materials, 5th Ed., D.R. Askeland and P.P. Phule, Thomson Learning, 2006.) 71

Ionomers Ionic crosslinks
When heated, the ionic groups will lose their attractions for each other and the chains will move around freely. As the temperature increases, the chains move around faster and faster and the groups cannot stay in their clusters. This allows for a polymer with the properties of an elastomer and the processability of a thermoplastic

Summary • Limitations of polymers: • Thermoplastics (PE, PS, PP, PC):
-- E, sy, Kc, Tapplication are generally small. -- Deformation is often time and temperature dependent. • Thermoplastics (PE, PS, PP, PC): -- Smaller E, sy, Tapplication -- Larger Kc -- Easier to form and recycle • Elastomers (rubber): -- Large reversible strains! • Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication -- Smaller Kc Table 15.3 Callister & Rethwisch 8e: Good overview of applications and trade names of polymers.

fig_15_15 fig_15_15

fig_15_12 fig_15_12

fig_15_13 fig_15_13

table_15_02 table_15_02

table_15_03a table_15_03a

table_15_03b table_15_03b

table_15_03c table_15_03c

table_15_04 table_15_04

MASE 542/CHEM 442 BIOMATERIALS

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