LectChap14c.pdf

Chapter 14c: SPolymer tructures ? Structure & chemical characteristics of polymer molecules? ? Common polymeric materials: how do they differ chemically? ? Crystalline state in polymers: difference from that in metals and ceramics Saturated and Unsaturated bonds Isomerism Addition polymerization Condensation Polymerization Some Characteristics that Affect Properties ? Molecular Weight ? Molecular Shape and Structure ? Composition (mers) C lli i? rysta n ty Composition: Copolymers (composed of different mers) Two different types of mers can be arranged differently Synthetic rubber Random copolymer as in car tires are copolymers Styrene Alternating copolymer + butadiene mers Block copolymer Impact resistant polystyrene styrene& butadiene mers Graft copolymer Molecular Configurations Isomerism Configurations ? arrangement of the R groups HH H H H R C C RH C C H R or C C H H RÆ benzene ringÆ styrene mer ; RÆCH3Æ propylene mer E A C C A E Stereoisomers that are mirror images B D D B mirror ? can?t superimpose without breaking a bond plane Geometric (cis/trans) Isomerism C C HCH3 CH2 CH2 C C CH3 CH2 CH2 H cis trans cis-isoprene (natural rubber) trans-isoprene (gutta percha) H atom and CH3 group on same side of chain H atom and CH3 group on opposite sides of chain inelastic natural latex produced from the sap of the gutta percha Used in resins and turpentine 5 Isomerism (Tacticity) Tacticity ?spatial arrangement of R units along chain isotactic ? R groups on same side syndiotactic ? R groups alternate sides C C H H HH CC HH CC HH CC C C H H C C H H C C H R RH CC H R RH RH RH H R H RH H HH 6 Tacticity (cont.) atactic ? R groups randomly positioned C C H H H R R H H H CC R H H H CC R H H H CC Affects ability to crystallize or remain amorphous; melt temperature, nature of light absorption and t i i 7 ransm ss on Stereoisomerism: summary 1 Isotactic configuration: R are on the same side of the chain. 2 Syndiotactic configuration: R alternate sides of the chain. 3 At ti fi ti d i t ti f R iac c con gura on: ran om or en a ons o groups a n. Photoresist & Polymer Degradation Polymers can be crystallized at least partly ? Crystallinity in Polymers ? Ordered atomic arrangements ? Crystal structures Æ unit cells ? Example shown ? polyethylene unit cell More complex to catagorize A Polymer Crystal ? Crystalline regions (?chain-folded? crystals?) ? thin platelets with chain folds at faces 10 nm 12 Polymer Crystallinity Polymers are partially crystalline with crystalline regions dispersed within Polyethylene amorphous material. Aggregates of lamellar crystallites ~ 10 nm thick, separated by amorphous material 14 Polymer Crystallinity (SpherulitesÆ crystalized from a melt) Polymers rarely 100% crystalline Diffi l f ll h i b li d Lamellar chain-folded crystallite ? cu t or a c a ns to ecome a gne ? Degree of crystallinity crystalline region expressed as % crystallinity. -- physical properties depend on % crystallinity. -- Heating can cause crystalline regions to grow & % crystallinity to increase. amorphous region 15 Semicrystalline Polymers ? Some semicrystalline polymers form spherulite structures ? Alternating chain- folded crystallites and amorphous regions Spherulite surface 16 Photomicrograph ? Spherulites in Polyethylene Cross-polarized light used -- a maltese cross appears in each spherulite Like crystalline grains in metals spherical structure changes as spherulites Interact Polyethylene Nylon PVC Etc. 17 Polymer Single Crystals ? Electron micrograph ? multilayered single crystals (chain-folded layers) of polyethylene ? Single crystals ? only for slow and carefully controlled growth rates 18 14.23 For each of the following pairs of polymers, do the following: (1) state whether or not it is possible to determine whether one polymer is more likely to crystallize than the other; (2) if it is possible, note which is the more likely and then cite reason(s) for your choice; and (3) if it is not possible to decide, then state why. 14.26 The density and associated percent crystallinity for two nylon 6 6 materials are as follows:, ? (g/cm3) crystallinity (%) 1.188 67.3 1 152 43 7. . Compute the densities of totally crystalline and totally amorphous nylon 6,6.(Find ?c, ?a) Determine the density of a specimen having C=55.4% crystallinity. C = % crystallinity C = ?c (?s ? ?a) Eq.14.8 100 ?s (?c ? ?a) C(1C )??? += wawcs C - )/(C ??= Comes from wsc Effect of temperature Thermoplastic & Thermosetting Polymers Thermoplastic polymers (thermoplasts): soften reversibly when heated (harden when cooled) At elevated T, inter-chain bonding is weakened allowing deformation at low stresses. Most thermoplasts are linear polymers and some branched structures Polyethylene terephthalate (#1), polyethylene (#2), polypropylene (#5), polystyrene (#6) Thermosetting polymers (thermosets): harden permanently when heated. C l t li k ( 10 50% f ) f d d i h ti C li kiova en cross n s ~ - o mers orme ur ng ea ng. ross- n ng hinder bending and rotations. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts. Examples: vulcanized rubber, epoxies resins, . 14.14 (a) Is it possible to grind up and reuse phenol-formaldehyde? Why or why not? (b) Is it possible to grind up and reuse polypropylene? Why or why not? Mechanical Properties ¾Mechanical properties ¾Viscoelasticity ¾Deformation and Elastomers ¾Elastomers ¾ The description of stress-strain behavior is similar to that of metals Stress ? Strain Behavior ¾ And yet dis-similar A: Brittle Polymer B: Plastic Polymer C: Elastomer Deformation shown by curve C is totally elastic (rubber-like elasticity). This class of polymers - Elastomers Stress ? Strain Behavior ? Moduli of elasticity for polymers are ~ 10 MPa - 4 GPa ? compare to metals ~ 50 - 400 GPa ? Tensile strengths are ~ 10 100 MPa- ? compare to metals, hundreds of MPa to several GPa ? Elongation can be up to 1000 % in some cases ¾ Mechanical properties of polymers change dramatically with temperature, going from glass like brittle behavior at low temperatures to a rubber like ? << 100% for metals - - behavior at high temperatures. ¾ Polymers are also very sensitive to the rate of deformation (strain rate). ¾Decreasing deformation rate has the same effect as increasing T. A way to think about some of these materials is to think of a big glob of cooked spaghetti . Stretch it a bit it is kind of elastic but if you pull hard the noodles , , , start to slide past one another and the whole glob starts to permanently deform. What happens to a PE plastic bag when you stretch it? What must be happening to the microscopic spaghetti the bag is made of! Heat up PE or PS to moderate temperatures, if the chains do not cross linked they will melt and turn into goopy liquids which are- , , called polymer melts. Stress ? Strain Behavior Temperature increase leads to: 9 A decrease in elastic modulus 9 A reduction in tensile strength 9 An increase in ductility polymethyl methacrylate (PMMA) (plexiglass) Plastic Deformation of Semicrystalline Polymers Macroscopic deformation Æ necking: increases strength of material Deformation aligns chains & increases local strength in necked region (up to 2-5 times) Chains in neck align along elongation direction: strengthening Elongation by extension of neck Viscoelasticity Stress & response (I) Elastic Hold s e R e l e a Applied stress (III) Vi (II) Viscoelastic scous Load is applied at ta and released at tr Viscoelasticity Materials that are both visocus & elastic Depending on strain rate Elasticity is the result of bond stretching along crystallographic planes in a metal or chain alignment in polymers Viscosity is the result of the ?diffusion? of molecules inside a typically amorphous solid under strain Elastomers ( materials with memories) Elastomers: deform to very large strains & then spring back elastically first observed in natural rubber To be elastomeric: several criteria: ¾ Resistance to crystallization (elastomers are amorphous) ¾ Relatively free chain rotations (unstressed elastomers have coiled/twisted structure ? uncoil during deformation) ¾ Certain degree of cross-linking (achieved by vulcanization) that increases resistance to plastic deformation ¾ T t i b th l t iti t t (b l T l t bempera ure s a ove e g ass rans on empera ure e ow g e as omer ecomes brittle) Elastomers & Vulcanization ¾ Cross linking a requirement- . ¾ Achieved by vulcanization - irreversible reaction at high T & usually involving addition of sulfur compounds ¾ Sulfur atoms bond with double-bonded C in chain backbones & form the bridge cross-links. cis-isoprene mer l i dnatural rubber vu can ze Unvulcanized rubber softens at high T and hardens at low T. 1839 Charles Goodyear found vulcanization accidentally: heated sulphur-coated rubber: was stable under heating and cooling. SUMMARY ? Thermoplastics (PE, PS, PP, PC): -- Small E, ?y -- Larger Kc -- Easier to form and recycle ? Thermosets (epoxies polyesters):, -- Larger E, ?y, high Tapplication -- Smaller Kc ? Elastomers (rubber): -- Large reversible strains! Plastics Some ubiquitous `mers' are ethylene and styrene and acrylamide (C3H5NO). Polymerized : polyethylene (the soft clear plastic that plastic bags are made of), polystyrene (the stiffer: white plastic that the covers for cups) polyacrylamide (very tough, clear plastic compact discs). 35 Summary: Size ? Shape -Structure 36 Summary ¾ Alternating copolymer understand language and concepts: ¾ Mer ¾ M l l i h¾ Atactic configuration ¾ Bifunctional mer ¾ Block copolymer o ecu ar we g t ¾ Monomer ¾ Network polymer ¾ Branched polymer ¾ Chain-folded model ¾ Cis (structure) ¾ Polymer ¾ Polymer crystallinity ¾ Random copolymer ¾ Copolymer ¾ Crosslinked polymer ¾ Saturated ¾ Spherulite ¾ St i i¾ Crystallite ¾ Degree of polymerization ¾ Graft copolymer ereo somer sm ¾ Syndiotactic configuration ¾ Trans (structure) ¾ Homopolymer ¾ Isomerism ¾ Isotactic configuration ¾ Trifunctional mer ¾ Unsaturated 37 ¾ Linear polymer ¾ Macromolecule robert johnson Microsoft PowerPoint - Chap14c.pptx