Lecture notes on Structure and Properties of Engineering ...

1 Textbook: Plastics: Materials and Processing (Third Edition), by A. Brent Young (Pearson, NJ, 2006). Structure and Properties of Engineering PolymersLecture: mechanical Properties : Macro ViewpointNikolai V. PriezjevMechanical Properties : Macro Viewpoint mechanical Properties of solids (forces, elastic behavior) mechanical Properties of liquids viscous flows (Newtonian non-Newtonian regimes, viscosity measurements) Viscoelastic materials (definitions, time dependence, short-range, long-range interactions) Plastic stress-strain behavior definitions, mechanical models Creep, toughness, impact strength, hardness Reinforcements, fillers, toughness modifiersReading: Chapter 4 of Plastics: Materials and Processingby A.

2 Brent Weight, Crystallinity and Properties Molecular weightMw: Mass of a mole of chains. Tensile strength (TS):--often increases with Longer chains are entangled (anchored) better. % Crystallinity: % of material that is and E often increasewith % causescrystalline regionsto grow. % regionamorphous regionsmaller Mw larger Mw Semicrystalline Polymers ~10 nm spacingOriented chains with long-range orderAmorphous disordered polymer chains in the intercrystalline structures in both melt and solid states Schematic representations of the molecular structures in both melt andsolid states for (a) semicrystalline, (b) amorphous, and (c) liquid crystal polymers.

3 (a)(b)(c)Melting vs. Glass Transition Temperature What factors affect Tmand Tg? Both Tmand Tgincrease with increasing chain stiffness Chain stiffness increased groups or bonds or aromatic chain groups Regularity (tacticity) affects of PolymersRandom arrangement = High EntropyStretched = Low EntropyEntropy is a measure of randomness: The more ordered the chains are, the lower is the entropy. Spontaneous processes always tend to increase the entropy, whichmeans that after stretching, the chains will tend to return to a high-entropy of PolymersElastic DeformationcreepCross-linking stops the sliding of chainsrandomSlow DeformationLow entropy Strain Behavior of Polymers Polymers can be brittle (A), plastic (B), or highly elastic (C).

4 Deformation shown by curve C is totally elastic (rubber-like elasticity, large recoverable strain at low stress levels). This class of polymers description of stress-strain behavior is (somewhat) similar to that of metalsStrains deformations > 1000% possible(for metals, maximum strain~100% or less)brittle polymerplasticelastomer FSof polymer ~10% that of metalselastic modulus less than metalABCS tress Strain Behavior of Polymers Characteristics of stress-strain behavior:Modulus of elasticity defined as for metalsDuctility (%EL) defined as for metalsYield strength -For plastic polymers (B), yield strength is defined by the 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) Moduli of elasticity for polymers are ~ 10 MPa 4 GPa (compare to metals ~ 50 -400 GPa) Tensile strengths are ~ 10 -100 MPa (compare tometals, hundreds of MPa to several GPa) Elongation can be up to 1000 % in some cases (< 100% for metals)YOUNG S MODULI.

5 ,AluminumPlatinumSilver, GoldTantalumZinc, TiSteel, NiMolybdenumGraphiteSi crystalGlass-sodaConcreteSi nitrideAl oxidePCWood( grain)AFRE( fibers)*CFRE*GFRE*Glass fibers onlyCarbon fibers onlyAramid fibers onlyEpoxy alloysTungsten<100> <111>Si carbideDiamondPTFEHDPELDPEPPP olyesterPSPETCFRE( fibers)*GFRE( fibers)*GFRE(|| fibers)*AFRE(|| fibers)*CFRE(|| fibers)*MetalsAlloysGraphiteCeramicsSemi condPolymersComposites/fibersE(GPa)109 PaBased on data in Table B2,Callister data based onreinforced epoxy with 60 vol%of alignedcarbon (CFRE),aramid (AFRE), orglass (GFRE) STRENGTH: COMPARISONRoom T values y(ceramics) >> y(metals) >> y(polymers)Based on data in Table B4,Callister = annealedhr = hot rolledag = agedcd = cold drawncw = cold workedqt = quenched & temperedStress Strain Temperature Dependence Temperature increase leads to: Decrease in elastic modulus Reduction in tensile strength Increase in ductilitypolymethyl methacrylate(PMMA) -PlexiglasThe glass transition temperature (Tg) of PMMA ranges from 85 to 165 C all of the above curves are for temperatures below Tg.

6 mechanical Properties of polymers change dramatically with temperature, going from glass-like brittlebehavior 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 temperature (T).Quiz: Stress Strain Temperature Dependence polymethyl methacrylate(PMMA) -PlexiglasFrom the stress strain data for poly(methyl methacrylate) shown in Figure estimate the modulus of elasticity and tensile strength at room temperature Strain Temperature and Rate Dependence Temperature increase leads to: Decrease in elastic modulus Reduction in tensile strength Increase in ductility mechanical Properties of polymers change dramatically with temperature, going from glass-like brittlebehavior at low temperatures to a rubber-like behavior at high temperatures.

7 Polymers are also very sensitive to the rate of deformation(strain rate). Decreasing rate of deformation has the same effect as increasing temperature (T).Temperature is fixedTime dependence of viscoelastic (polymeric) materials showing the changes in stress-strain with strain rateStrain rate is fixedViscoelasticity 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 behavior.

8 Deformation is time dependent and not reversible. Intermediate temperatures: viscoelastic behavior. Instantaneous elastic strain followed by viscous time dependent behavior is 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).Viscoelasticity Load is applied at taand released at trDefinition of the Relaxation Modulus timeStress, 10 s 10DL fixed DLLoEr(0) = E, Young s ModulusEr( ) = 0 Glass-like elasticityRubber-likeelasticityFluid-lik eviscousfixedrsE )10()10( Viscoelastic ModulusModulus of ElasticityRelaxation Modulus Stress relaxation test:--strain to oand decrease in stress with : Relaxation Modulus Viscoelasticity can be characterized by the viscoelastic relaxation modulus:Log time, t Sample is strained rapidly to pre-determined strain.

9 Stress required to maintain this strain 0over time is measured at constant T. Stress decreases with time due to molecular relaxation processes. Relaxation moduluscan be defined as: Er(t) is also a function of temperature. Er(t) (t) oViscoelasticity: Relaxation ModulusTo show the influence of temperature, the relaxation modulus can be plotted at a fixed time for different T:Log time, tTime-Dependent Deformation Stress relaxation test:--strain to oand decrease in stress with )()( Relaxation modulus: Sample Tg( C) values:PE (low density)PE (high density)PVCPSPC-110-90+ 87+100+150 Selected values from Table , Callister 7e.

10 Timestraintensile test o (t) Data:Large drop in Erfor T> Tg.(amorphouspolystyrene)10310110-110-31 0560100140180rigid solid (small relax)transition regionT( C)TgEr(10s)in MPaviscous liquid (large relax) SuperpositionConstruction of the viscoelastic master curve for PIB (Polyisobutylene) at 25 C reference temperature by shifting stress relaxation curves obtained at different temperatures horizontally along the time axis. The shift factor, aTvaries with temperature as shown in the (t,T) = G(aTt, Tref)Viscoelasticity Temperature dependence of the relaxation modulus for different polymer structuresA : Largely crystallineisotactic polystyrene.