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Polymers Dr. Majid Rad 1 Advanced Materials & Manufacturing Systems Chapter 14 - ENS3260/ENS5261 – and Plastics Molding • Effect of molecular weight on properties • Polymer crystals and polymers chains The main reference for the materials of this lecture: Chapters 14 & 15, “Materials Science & Engineering, An Introduction”, W.D. Callister, Jr. & D.G. Rethwisch, 8th edition, Wiley, 2010. Outline: axonomy of polymers • T • Basic microstructural features • Processing of polymers

There may be thousands, even millions of units 2 A compound consisting of long-chain molecules, Chapter 14 - Most polymers are based on carbon and are each molecule made up of repeating units Polymers Defined connected together • in a single polymer molecule • therefore considered organic chemicals

3 Chapter 14 - Most polymers are hydrocarbons made up of H and C atoms. Adapted from Callister & Rethwisch 8e. What explains different boiling points?

H H H H H 4 H C CC CC Chapter 14 - C H H H Saturated & Unsaturated H H H can C n H 2n C n H 2n-2 Hydrocarbons Double & triple bonds relatively reactive – Each carbon bonded to four other atoms C n H 2n+2 ethylene or ethene - acetylene or ethyne - – – 4-bonds, 3 to C’s form new bonds Double bond – Triple bond – •

Some physical properties depend on isomeric state 5 Two compounds with same chemical formula can Boils at -0.5°C Chapter 14 - Boils at 12.3°C Isomerism Isomerism have quite different structures Ex: C 4 H 10 (butane) Normal butane Iso-butane CH 3 = methyl • – –

that 6 in comparison to the H H Chapter 14 - H CCC H H H C H H macromolecules H H H H CC C H H H H Polymer Molecules Polymer molecules are gigantic H H CCC H H The long molecules are composed of structural entities structural entities of long polymer small molecule from which a polymer number of bonds that a given hydrocarbon molecules -- H H monomer can form. (bifuntional-trifunctional, etc.) H CC H are successively repeated along the chain. Repeat unit – is synthesized. H – H C H H molecules. Monomer – Functionality •

7 time in chainlike fashion to form a linear macromolecule Process by which monomer units are attached one at a Active site (unpaired e) Chapter 14 - Addition Polymerization Ethylene monomer (gas at room T) molecule (solid material at room T) See Section 15.20 for more Initiator or catalyst PE Initiation: Propagation: Termination: – – –

Typical Product 8 Chapter 14 - Bulk or Commodity Polymers

Typical Product 9 Chapter 14 -

Typical Product Chapter 14 -10

Adapted from Fig. 14.9, Callister 7e. Chapter 14 - 11 Homopolymer vs. Copolymers random alternating block graft and B randomly and B alternate A large blocks of A alternate with large blocks of B chains of B grafted on Homopolymer: All the repeating units along a chain are of the same type. Copolymers: Two or more monomers polymerized together. A – random • vary in chain – alternating • in polymer chain – •block – •graft to A backbone

ENS3260 higher molecular weights is more sensitive to Chapter 14 -12 Mass of a mole of chains. MOLECULAR WEIGHT higher M M w Adapted from Callister & Rethwisch 8e. polymer molecules fraction of total # of weight fraction of Number-average molecular weight Weight-average molecular weight • Molecular weight, M i : chains molecules of Xi: Wi: of wt # M i i M i i M i i M i i Lower M total total x   w   x   w   n  n w n w M M M M M

i i M i M w i Number-average molecular weight x  M n  Weight-average molecular weight  M w  Chapter 14 -13 Molecular Weight Calculation Problem: Find average mass of a class 0.065 0.078 0.182 0.312 0.233 0.130 w w i M i n x 0.1 0.1 0.2 0.3 0.2 0.1 M i M Kg 50 60 70 80 90 100 # of students 1 1 2 3 2 1 i N

weight Many polymer properties affected by molecular (or length of chains) Melting or softening temperature increase with increasing molecular weight (length) The same polymer material can have quite different properties if produced with different molecular weight ex: E, TS, YS, %EL Chapter 14 -14 Effect of Molecular Weight

Chapter 14 -15 Degree of Polymerization, n n = 6 n = number of repeat units per chain H H C H Number-average molecular weight molecular weight of H H repeat unit H CCC H H H Can you guess what is DP (or n) for a typical PVC? H H H CCC H n H H M m H H  ) H CCC H n H H or ( H H DP H CC H H

Adapted from Callister & Rethwisch 8e. Adapted from Callister & Rethwisch 8e. Chapter 14 -16 Molecular orientation can be changed Molecular Shape End to End Distance, r note: no bond breaking needed by rotation around the bonds and coils Ex: large Polymers – Conformation – – Schematic representation of a single polymer chain molecule that has numerous random kinks produced by chain band rotations. These kinks and coils are responsible for a number of important characteristics of polymers – elastic extensions in rubbers.

Adapted from Callister & Rethwisch 8e. Chapter 14 -17 Network Epoxies, polyurethanes Highly cross-linked. Distinctive mechanical and thermal properties. Molecular Structures configurations and strength: Cross-Linked increasing strength Rubber elastic materials Chains connected by covalent bonding of additive atoms or molecules. Branched HDPE & LDPE Linear structures may also be branched. Lowering density • Covalent chain bonding secondary Linear PE, PVC, PS, nylon Extensive van der waals and hyd bonding between chains. Higher density.

Adapted from Fig. 14.10, Callister 7e. 18 Chapter 14 - Polymer Crystallinity 10 nm Crystals must contain the structure polymer chains in some Ex: polyethylene unit cell Chain folded Adapted from Fig. 14.12, Callister 7e. way – •

19 Adapted from Callister & Rethwisch 8e. Chapter 14 - Polymer Crystallinity region amorphous Too difficult to get all those chains aligned crystalline region causes crystalline regions Polymers rarely 100% crystalline Crystallinity: % of material that is crystalline. Depend on rate of cooling, molecular chemistry, chain configuration. often increase and E with % crystallinity. Annealing to grow. % crystallinity increases. % -- TS -- -- •

Chapter 14 -20 Adapted from Callister & Polymer Crystallinity X 100 Rethwisch 8e.  a )  a ) - - ( s ( c crystalline polymer amorphous polymer  c  s = density of specimen in question density of perfectly density of totally Problem 14.2 % crystallinity =  s =  c =  a See Ex.

Adapted from Callister & Rethwisch 8e. Chapter 14 -21 Glass Transition – and polymers. Due to reaction in motion of large segments of molecular at which polymer makes the transition from a rubbery state semi-crystalline Occurs in amorphous chains with decreasing T. Causes changes in physical properties stiffness, heat capacity, coefficient of thermal expansion. Glass transition temperature to a rigid state. • • • – T

cool down Chapter 14 -22 Thermoplastic vs. Thermoset can be reversibly cooled & reheated, i.e. recycled  shape as desired polyethylene, polypropylene, polystyrene, polycarbonate, till soft  Thermoplastics up heat little crosslinking Ductile soften w/heating –ex: etc. • – – – –

Chapter 14 -23 Thermoplastic vs. Thermoset rubber, Thermosets when heated forms a network degrades (not melts) when heated mold the prepolymer then allow further reaction large crosslinking (10 to 50% of mers) hard and brittle -- soften w/heating do NOT -- ex: epoxy, urethane, bakelite, vulcanized -- polyester resin, phenolic resin Water-based epoxy polymers • – – – --

styrene Chapter 14 -24 and thermoplastic elastomers Polymer Types: Elastomers styrene-butadiene rubber butadiene rubber – Crosslinked materials Natural rubber Synthetic rubber SBR- Silicone rubber Elastomers • – – • –

of polymer ca. 10% that of metals elastomer Adapted from Fig. 15.1, Callister 7e. Chapter 14 -25 Mechanical Properties i.e. stress-strain behavior of polymers plastic ca. 10% or less) brittle polymer  FS elastic modulus less than metal – deformations > 1000% possible (for metals, maximum strain Strains – •

fibrillar structure near failure Chapter 14 -26 Tensile Response: Brittle & Plastic crystalline regions slide  crystalline regions align plastic failure x unload/reload amorphous regions elongate brittle failure onset of necking (MPa) x semi- crystalline case Near Failure Initial networked aligned, case cross- linked case Adapted from Callister & Rethwisch 8e.

Adapted from Callister & Rethwisch 8e. Chapter 14 -27 Tensile Response: Elastomer Case final: chains are straight, still cross-linked x  (aligned, crosslinked & networked polymer) plastic failure elastomer (semi-crystalline polymers) x Deformation is reversible! brittle failure • Compare to responses of other polymers: (MPa) x brittle response plastic response kinked, cross-linked. initial: amorphous chains are -- --

to 1.3 0.3  Chapter 14 -28 and Strain Rate: Thermoplastics Data for the semicrystalline polymer: PMMA (Acrylic) 60°C 0.2 20°C 40°C 0.1 Adapted from Callister & Rethwisch 8e. 4°C (MPa) 80 60 40 20 0 0 • Decreasing T... increases E increases TS decreases %EL strain rate... same effects as decreasing T. T -- -- -- • Increasing --

Chapter 14 -29 Molding Processing Plastics - Compression and transfer molding – thermoplastic or thermoset Adapted from Callister & • Rethwisch 8e.

Chapter 14 -30 Molding Adapted from Callister & Rethwisch 8e. thermoplastic & some thermosets Processing Plastics - Injection molding – •

Extrusion Processing Plastics – Adapted from Callister & Rethwisch 8e. Chapter 14 -31

Adapted from Callister & Rethwisch 8e. Chapter 14 -32 Blown-Film Extrusion

Chapter 14 -33 Summary i.e. made up of H and C by covalent bonding Copolymers: How repeat units arranged along a chain Structures: linear, branched, cross-linked, network temperature Thermoplastics, thermosets and elastomers Good overview of applications and trade names of polymers. Most polymers are hydrocarbons Classification of polymers Molecular Shape and • Homopolymers vs. Glass transition Polymer Crystallinity Moulind methods for polymers able 15.3 Callister 8e: – – T • • • • • • •