LECTURE #30 : 3.11 MECHANICS OF MATERIALS F03 INSTRUCTOR : Professor Christine Ortiz OFFICE : 13-4022 PHONE : 452-3084 WWW : http://web.mit.edu/cortiz/www • PLASTICITY OF POLYMERS II SUMMARY : LAST LECTURE I. Molecular Origins of Plasticity in Metals • edge and screw dislocations, 2-D and 3-D movement of dislocations, slip systems, planes, directions, polycrystalline plastic deformation II. Stress versus Strain Curves for Polymers in Uniaxial Tension • structure of semicrystalline polymers: amorphous+cystalline regions, chain folding, lamellar, spherulites (macroscopically isotropic) highly oriented VIII. polymer fiber • II. amorphous, s glassy polymer smax VII. s (T<Tg) Y III. semicrystalline IV. V. polymer s VI. T>Tg elastomer (T>Tg) I. e Lo e f e Semicrystalline polymers (T>Tg): I. linear elastic deformation II. homogeneous plastic deformation, strain hardening III. NECK forms and grows unstably, i.e. Polyethylene : inhomogeneous plastic deformation used in milk jugs, toys, pens, IV. neck stabilizes ice trays V. COLD DRAWING, neck increases Nylon (polyamide) in length by extracting polymer from unnecked region of sample MACROSCOPIC VI. entire sample is drawn VII. begin stretching of completely drawn MECHANICAL sample, strain hardening BEHAVIOR VIII. fracture Plastic Deformation of Semicrystalline Polymers s, e crystalline lamellae amorphous region s, e Stages and Mechanisms of Plastic Deformation 1) elongation of amorphous chains (uncoiling along stress axis) 2) rotation/tilting of lamellae crystallites toward tensile axis 3) separation of crystallites into block segments (partial melting) 4) further stretching of crystallites and orientation of amorphous regions along tensile axis, void formation, stress whitening 5)2nd strain hardening regim: recrystallization to a fiber like oriented structure, high stiffness Comparison of Amorphous and Semicrystalline Polymers Some Applications of Amorphous Polymers Plastic Deformation of Amorphous, Glassy Polymers or Networks (T < Tg) s e Crazing in Amorphous, Glassy Polymers (T<Tg) s,e s,e Crazing in Amorphous, Glassy Polymers (T<Tg) polycarbonate craze bulk bulk s,e s,e craze-bulk interface main fibril cross-tie fibril CRAZE NANO STRUCTURE fibril oriented polymer entanglement chains Crazing in Amorphous, Glassy Polymers (T<Tg) s,e s,e Shear DZ’s in Amorphous, Glassy Polymers (T<Tg) Macroscopic sample ~ necking s,e s,e Microscopic structure CRAZING OR SHEAR DZ’s? s,e s,e s,e s,e CONSEQUENCES FOR IMPACT RESISTANCE bisphenol-A polycarbonate (crazing) Plastic Deformation and Fracture of Amorphous Components Plastic Deformation and Fracture of Amorphous Components TOUGHENING OF AMORPHOUS POLYMERS TOUGHENING OF AMORPHOUS POLYMERS Typical uniaxial tensile stress-strain behavior of polystyrene (PS), medium-impact PS (MIPS), high- impact PS (HIPS), and poly(acrylonitrile-co-styrene- graft-butadiene) ABS. .