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ME 260: Introduction to Engineering Materials CHAPTER 15 Characterization, applications and processing of Polymers Chapter 16. Characterization, applications 15.1 ME 260: Introduction to Engineering Materials Mechanical Characteristics of Polymers v.sensitive to TEMPERATURE STRAIN RATE ENVIRONMENT (presence of water, O2, solvents) Stress at which fracture occurs Modulus of Elasticity = Tensile modulus = modulus For polymers: 7 MPa- 4GPa Metals: 48-410GPa Chapter 16. Characterization, applications 15.2 ME 260: Introduction to Engineering Materials Three typical types of stress strain curves are found for polymers Polymers can experience elongations up to 1000% Metals typically 100% maximum. Chapter 16. Characterization, applications 15.3 ME 260: Introduction to Engineering Materials Typical values Chapter 16. Characterization, applications 15.4 ME 260: Introduction to Engineering Materials Effect of Temperature Tensile modulus Elongation Strength Chapter 16. Characterization, applications PMMA 15.5 ME 260: Introduction to Engineering Materials FACTORS INFLUENCING MECHANICAL PROPERTIES OF POLYMERS Extensive chain entanglement or significant molecular bonding = increase in modulus and strength Effect of Molecular weight Constant TS = TS – A Mn Tensile strength at infinite molecular weight TS increases with increase in molecular weight, WHY???? Increasing chain entanglement Chapter 16. Characterization, applications 15.6 ME 260: Introduction to Engineering Materials DEGREE OF CRYSTALLINITY Important since affects extent of intermolecular secondary bonding Tensile modulus increases with increase in degree of crystallinity As crystallinity increases polymer more brittle Chapter 16. Characterization, applications 15.7 ME 260: Introduction to Engineering Materials CRYSTALLIZATION Occurs by nucleation and growth of ordered and aligned chain folded layers from random and tangled molecules in the melt. Chapter 16. Characterization, applications 15.8 ME 260: Introduction to Engineering Materials MELTING POINT Tm and GLASS TRANSITION TEMPERATURE Tg Tm - occurs in crystalline polymers Tg - occurs in amorphous and semi-crystalline polymers Chapter 16. Characterization, applications 15.9 ME 260: Introduction to Engineering Materials MELTING POINT From ordered to disordered state. Requires rotation and movement of ordered molecules CHAIN STIFFNESS Controlled by ease of rotation about chemical bonds along he chain Double bond chains and aromatic groups reduces chain flexibility increases Tm Size and type of side group affect chain rotational freedom and flexibility Tm increases with increase in Molecular weight (chain length). Why is there a range of Tm rather than a single temperature??? Chapter 16. Characterization, applications 15.10 ME 260: Introduction to Engineering Materials e.g. Polypropylene (Tm = 175 oC), Polyethylene(Tm = 115 oC) Polyethylene MER H H Polypropylene MER H H - C–C- - C – C- H H H- C - H H H Larger side group Tg affected in similar way to Tm Chapter 16. Characterization, applications 15.11 ME 260: Introduction to Engineering Materials Tg = 0.5-0.8 Tm(in K) Chapter 16. Characterization, applications 15.12 ME 260: Introduction to Engineering Materials Chapter 16. Characterization, applications 15.13 ME 260: Introduction to Engineering Materials THERMOPLASTIC AND THERMOSETTING POLYMERS Classification according to mechanical response at elevated temperatures THEMOPLASTIC Repeatable and reversible processes Secondary bonding decreases with increased Temp. due to increased molecular motion MOST LINEAR & SLIGHTLY BRANCHED POLYMERS THEMOSET Heating = harden polymer cross-linking 10-50% of mers x-linked Chapter 16. Characterization, applications X-lined and network polymers e.g. Epoxy 15.14 ME 260: Introduction to Engineering Materials POLYMER TYPES PLASTICS- ELASTOMERS (RUBBERS)- FIBERS, COATINGS, ADHESIVE, FOAMS, FILMS. If plastic is x-linked and used above Tg = good elastomer PLASTICS Majority of polymers e.g. Polyethylene, Polypropylene, PVC, Polystyrene, fluorocarbons, epoxies, polyesters. Brittle and flexible, linear , branched, thermoplastic, thermosetting etc. Fluorocarbons = low coeff. Of friction, extremely resistant o chemical attack Used as coating in cookware. Optical applications = PMMA, polystyrene Chapter 16. Characterization, applications 15.15