Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Reading assignment: Callister Learning objectives: Recognize distinctive aspects of polymers’ mechanical behavior: Temperature dependence w.r.t. T g Deformation mechanism Stress-strain curve: differences from metals & ceramics Dependence on crystallinity and molecular weight Viscoelasticity; relaxation modulus Lecture 27: Mechanical Properties II: Polymers

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University A - brittle B - ductile (plastic) C - elastomeric Callister Fig Mechanical Properties of Polymers [Callister 16.2] Engineering stress – strain curves for polymers

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Engineering stress – strain curve yield strength (σ y ) tensile strength (TS) Callister Fig Note: different definitions than for metals Note: strengthening can be observed after yielding Mechanical Properties of Polymers [Callister 16.2]

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Effect of temperature in a linear polymer (PMMA) Callister Fig increasing temperature T g = 4 °C T>T g : plasticity  T≤T g : stiffness  ; brittleness  Effect of Temperature: Glassy vs. Plastic [Callister 16.2]

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Polymer crystals: Platelets Bundles Spherulites Callister Fig Callister Fig Semicrystalline Polymers [Callister 15.11]

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig Microscopic Mechanism of Deformation [Callister 16.3]

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Macroscopic effects of deformation Callister Fig Callister Fig slope = tensile modulus Mechanical Characteristics of Polymers [Callister 16.3] necking

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Factors influencing tensile strength As molecular weight , tensile strength  Little effect on tensile modulus As degree of crystallinity  : tensile strength  tensile modulus  strain to failure  Callister Fig. 16.6: Polyethylene Mechanical Characteristics of Polymers [Callister 16.3] tensile strength at infinite molecular weight average molecular weight constant

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Factors influencing mechanical properties Predeformation by drawing Modulus  Ductility  Heat treatment Annealing near T m to reverse effects of drawing Crystallization Modulus  Yield strength  Ductility  Mechanical Characteristics of Polymers [Callister 16.3] Callister Fig. 16.5

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Crystallization of a Polymer [Callister 16.5] Polypropylene (T g = –18 °C; T m = 175 °C) Nucleation of crystals, followed by growth from Callister

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Stress Pulse Viscoelastic Viscous Elastic Viscoelasticity in Polymers [Callister 16.7] from Callister

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Viscoelasticity Initial strain ε 0 for a given initial stress σ 0 Stress decreases from σ 0 as material deforms  Relaxation modulus: stress time Relaxation Modulus [Callister 16.7] E r  with time as stress  with time

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Temperature Dependence of Relaxation Modulus [Callister 16.7] Relaxation modulus decreases … … with time … with T  from Callister

Lecture 27, summer 2007 Mechanical Properties II: Polymers ENGR 145, Chemistry of Materials Case Western Reserve University Thermomechanical Characteristics of Polymers [Callister 16.7] Five types of viscoelastic behavior Glassy Leathery Rubbery Rubbery flow Viscous flow from Callister