Presentation on theme: "Characteristics, Applications, and Processing of Polymers"— Presentation transcript:
1 Characteristics, Applications, and Processing of Polymers Chapter Outline:Characteristics, Applications, and Processing of PolymersMechanical propertiesStress-Strain BehaviorDeformation of Semicrystalline PolymersCrystallization, Melting, Glass TransitionThermoplastic and Thermosetting PolymersViscoelasticityDeformation and ElastomersFracture of PolymersPolymerizationElastomersOptional reading: 15.6, 15.8, , ,
2 Stress – Strain Behavior (I) A: Brittle Polymer B: Plastic Polymer C: ElastomerStress-strain behavior can bebrittle (A), plastic (B), and highly elastic (C)Curve C is totally elastic (rubber-like elasticity). This class of polymers - elastomers
3 Stress – Strain Behavior (II) Elastic Modulus – defined as for metalsDuctility (%EL) – defined as for metalsYield strength – For plastic polymers (B)maximum on curve just after the elastic region (different from metals)Tensile strength is defined at the fracture point and can be lower than the yield strength (different from metals)
4 Stress – Strain Behavior (III) Moduli of elasticityPolymers: ~ 10 MPa - 4 GPaMetals: ~ GPaTensile strengthsPolymers: ~ MPaMetals: 100’s ’s MPaElongationPolymers: up to 1000 % in some casesMetals: < 100%
5 Mechanical properties of polymers change dramatically with temperature, going from glass-like brittle behavior at low temperatures to a rubber-like behavior at high temperatures.Polymers are also very sensitive to the rate of deformation (strain rate). Decreasing rate of deformation has the same effect as increasing T.
6 Stress – Strain Behavior (IV) Temperature increase leads to:Decrease in elastic modulusReduction in tensile strengthIncrease in ductilitypolymethyl methacrylate (PMMA)
7 Deformation of Semicrystalline Polymers Semi-crystalline polymers: crystalline regions separated by amorphous materialElastic deformation:Elongation (straightening) of chain molecules in direction of applied stress.Elastic modulus: determined by elastic properties of amorphous and crystalline regions and the microstructure.
8 Plastic Deformation of Semicrystalline Polymers Plastic deformation: defined by interaction between crystalline and amorphous regions and is partially reversible.Stages of plastic deformation:elongation of amorphous tie chainstilting of lamellar crystallites towards the tensile axisseparation of crystalline block segmentsstretching of crystallites and amorphous regions along tensile axisInitial structure1234
9 Plastic Deformation of Semicrystalline Polymers Necking. Neck gets stronger since deformation aligns the chains and increases local strength in the neck region (up to 2-5 times) neck expandsChains in neck align along elongation direction: strengtheningElongation by extension of neckDifferent from ductile metals where the deformation is confined in the initial neck region.
10 Factors that Influence Mechanical properties (I) Temperature and strain rateChain entanglement, strong intermolecular bonding (van der Waals, cross-links) increase strengthDrawing, analog of work hardening in metals, corresponds to neck extension. Is used in production of fibers and films. Molecular chains become highly oriented properties of drawn material are anisotropic (perpendicular to the chain alignment direction strength is reduced)Heat treatment: changes crystal size and orderundrawn material: Increasing annealing temperature leads toincrease in elastic modulusincrease in yield/tensile strengthdecrease in ductility.(changes are opposite to metals)drawn material: opposite changes (due to recrystallization and loss of chain orientation)
11 Factors that Influence Mechanical properties (II) Tensile strength increases with molecular weight – effect of entanglementHigher degree of crystallinity – stronger secondary bonding - stronger and more brittle
12 Crystallization, Melting, Glass Transition (I) Crystallization: nuclei form and grow, chains align and order. Rates can be defined from the same type of S-curves we saw in the case of metals. Nucleation becomes slower at higher temperatures.Glass transition: polymers become rigid solids (viscosity increasing) upon cooling yet retain disordered molecular structure like a liquid
13 Crystallization, Melting, Glass Transition (II) Melting of semicrystalline polymers is intermediate between that of crystalline materials (sharp density change at a melting temperature) and pure amorphous material (melting can be only defined from viscosity).The glass transition temperature is between 0.5 and 0.8 of the melting temperature.
14 Crystallization, Melting, Glass Transition Melting temperature increases with rate of heating, thickness of lamellae: depends on material history and crystallization temperature.Melting: breaking of the inter-chain bonds:glass and melting temperatures depend on:chain stiffness (e.g., single vs. double bonds) rigid chains -> higher melting temperaturessize/weight of molecule - increasing molecular weight increases Tmsize, shape of side groups, side branches, cross-linking, defects, etc.Essentially the same molecular characteristics raise and lower both melting and glass transition temperatures.
15 Crystallization, Melting, Glass Transition Dependence of melting and glass transition temperatures and polymer properties on molecular weight
16 Thermoplastic and 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 structuresThermosetting polymers (thermosets): harden permanently when heated.Covalent crosslinks (~ % of mers) formed during heating. Cross-linking hinder bending and rotations. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts.Examples: vulcanized rubber, epoxies, resins.
17 Viscoelasticity (I)Amorphous polymer: glass at low temperatures, rubber at intermediate temperatures, viscous liquid at high T.Low temperatures: elastic deformation at small strains ( = E). Deformation is instantaneous when load is applied. Deformation is reversible.High temperatures: viscous. Deformation is time dependent and not reversible.Intermediate temperatures: viscoelastic behavior. Instantaneous elastic strain followed by viscous time dependent strain.Viscoelastic behavior determined by rate of strain (elastic for rapidly applied stress, viscous for slowly applied stress)Rate dependence of viscoelastic properties in a silicone polymer (Silly Putty). Picture by Geon Corp.
18 Load is applied at ta and released at tr Viscoelasticity (II)ElasticViscoelasticViscousLoad is applied at ta and released at tr
19 Viscoelasticity (III) Viscoelasticity characterized byviscoelastic relaxation modulusSample is strained rapidly to pre-determined strainStress required to maintain this strain over time is measured at constant TStress decreases with time due to molecular relaxation processesRelaxation modulusdefined asEr(t) also a function of temperatureEr(t) = (t)/e0Log time,t
20 Viscoelasticity (IV)Influence of temperature: relaxation modulus plotted at a fixed time for different T:Glassy (elastic) region.Chains are “frozen”Leathery/glass transition regionDeformation is time dependentand not fully recoverableRubbery plateauRubbery flow(increasing chain motion)Viscous flow (motionof chains largely independent)
21 Temperature dependence for polymer structures Viscoelasticity (V)Temperature dependence for polymer structuresA : Largely crystalline isotactic polystyrene. Glass transition region limited – small amount of amorphous materialB: Lightly cross-linked atactic polystyrene - leathery region extends to decomposition temperature: no meltingC: Amorphous polystyrene
22 Viscoelastic CreepMany polymers susceptible to time-dependent deformation under constant loadViscoelsatic Creep: may be significant even at room temperature and under moderately low stresses (below yield strength)Results of creep tests are described by time dependent creep modulus:Ec(t) = o / (t)Amount of creep decreases as crystallinity increases
23 ElastomersCan be deformed to very large strains and then spring back elastically to the original length: first observed in natural rubber.Elastomeric polymer:Elastomers are amorphousUnstressed elastomers have coiled/twisted structure – uncoil during deformationSome cross-linking (vulcanization) increases resistance to plastic deformationT above the glass transition temperaturebelow Tg elastomer becomes brittleIn silicone rubbers, the backbone C atoms are replaced by a chain of alternating silicon and oxygen atoms. These elastomers are also cross-linked and are stable to higher temperatures than C-based elastomers
24 Elastomers: Vulcanization (I) Cross linking: required for elastomeric behavior.Vulcanization: irreversible chemical reaction at high T involving addition of sulphur compounds.Sulfur atoms bond with double-bonded C in chain backbones, form the bridge cross-links.Unvulcanized rubber softens at high T and hardens at low T.In 1839 Charles Goodyear found vulcanization by accidentally heating sulphur-coated rubber. He observed it became firm and was stable under heating and cooling.
25 Elastomers: Vulcanization (II) Cross-linking elastomers are thermosetting polymersElastic modulus, tensile strength, oxidation resistance increased by vulcanization.E proportional to number of cross-links. Too many cross-links reduces extensibility600% elongation!
26 Fracture of PolymersFracture strengths are low compared to metals and ceramicsBrittle fracture occurs in thermosetting polymers. Fracture initiated at stress concentrators (scratches, notches, etc). Covalent bonds are severed during fractureIn thermoplastic polymers, both ductile and brittle fracture are possible. Brittle fracture occurs at lower T, higher strain rates, and at stress concentratorsBrittle to ductile transition often occurs with increasing temperature
27 Fracture of Polymers: Crazing Fracture of glassy thermoplasts often proceeds through crazing. Crazing occurs when localized regions yield, forming an interconnected array of microvoids. Fibrillar bridges of oriented molecular chains form between voids.At high enough tensile loads, these bridges elongate and break, enabling crack propagationCrazing absorbs fracture energy and increases fracture toughness
28 Significantly slower than addition polymerization Polymerization is the synthesis of high polymers from raw materials like oil or coal. It may occur by:Addition (chain-reaction) polymerization, where monomer units are attached one at a time (discussed in Chapter 14). Has three distinct stages: initiation, propagation, and termination.Condensation (step reaction) polymerization, by stepwise intermolecular chemical reactions that produce the mer units.Usually there is small by-product that is then eliminatedSignificantly slower than addition polymerizationOften form trifunctional molecules that can form cross-linked and network polymers
29 Summary Make sure you understand language and concepts: Addition polymerizationCondensation polymerizationElastomerGlass transition temperatureRelaxation modulusThermoplastic polymerThermosetting polymerViscoelasticityVulcanization