Characterization, applications CHAPTER 15 Characterization, applications and processing of Polymers

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

Three typical types of stress strain curves are found for polymers Polymers can experience elongations up to 1000% Metals typically 100% maximum.

Typical values

Effect of Temperature PMMA Tensile modulus Elongation Strength

Effect of Molecular weight Constant 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

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

CRYSTALLIZATION Occurs by nucleation and growth of ordered and aligned chain folded layers from random and tangled molecules in the melt.

Tm - occurs in crystalline polymers MELTING POINT Tm and GLASS TRANSITION TEMPERATURE Tg Tm - occurs in crystalline polymers Tg - occurs in amorphous and semi-crystalline polymers

From ordered to disordered state. 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???

e.g. Polypropylene (Tm = 175 oC), Polyethylene(Tm = 115 oC) Polyethylene MER Polypropylene MER H H H H - C – C - - C – C - H H H - C - H H H Larger side group Tg affected in similar way to Tm

Tg = 0.5-0.8 Tm(in K)

Repeatable and reversible THERMOPLASTIC AND THERMOSETTING POLYMERS Classification according to mechanical response at elevated temperatures THEMOPLASTIC THEMOSET Repeatable and reversible processes Heating = harden polymer  cross-linking Secondary bonding decreases with increased Temp. due to increased molecular motion 10-50% of mers x-linked MOST LINEAR & SLIGHTLY BRANCHED POLYMERS X-lined and network polymers e.g. Epoxy

If plastic is x-linked and used above Tg = good elastomer 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