Polymers 1. What are polymers 2. Polymerization 3. Structure features of polymers 4. Thermoplastic polymers and thermosetting polymers 5. Additives 6. Polymer crystals 7. Mechanical properties of polymers 8. Processing of polymers 1
Periodic table with the elements associated with commercial polymers in color. 2
Characteristics of polymers (plastics) 1. Organic materials 2. Long-chain molecule composed of many mers bonded together 3. mer is building block of the long-chain (e.g. -C 2 H 4 -in polyethylene) 4. Compound of hydrogen and carbon, and/or O, N, F and Si 5. Extensive formability and ductility 6. Light weight, low cost 7. Low strength compared with metals; lower melting point and higher chemical reactivity compared with ceramics Polymerization 1. The critical feature of a monomer in polymerization: The presence of reactive sites - double bonds 2. A saturated hydrocarbon All bonds are single bonds, 3. Unsaturated monomer double or triple covalent bonds 3
Two distinct ways for the process of polymerization 1. Chain growth (addition polymerization) Rapid chain reaction 2. Step growth (condensation polymerization) Chemical reaction between pairs of reactive monomers Much slower Chain growth 4
An initiator An initiator: hydroxyl free radical in Fig. 13.2 A free radical: a reactive atom or group of atoms containing an unpaired electron A terminator A terminator: another hydroxyl free radical Form a stable molecule with n mer units An initiator - terminator pair Hydrogen-peroxide H 2 O 2 fi 2OH Recombination: the termination step Hydrogen abstraction: obtaining a hydrogen aton with unpaired electron Disproportionation: obtaining a monomerlike double bond Copolymer Block copolymer Regular, along a single carbon-bonded chain Blend copolymer irregular 5
The polymerization of formaldehyde to form polyacetal 6
Step growth (condensition polymerization) Extensive polymerization requires this three molecule reaction to be repeated for each unit increase in molecule length. The time required for this process is substantially greater than that for the chain reaction or chain growth. Bifunctional a linear molecule structure, Softer than the network polymer Polyfunctional has several potential points of contact ; a three dimensional network molecule structure Fig. 13.7 After several reaction steps like that in Fig. 13.6, polyfunctional mers form a three-dimensional network molecule structure. 7
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Structure features of polymers Each bond angle between three adjacent C atoms is near 109.5º and the angle can be rotated freely in space. 10
The length of of the polymeric molecule The degree of polymerization A polymeric molecule ( C 2 H 4 ) n n is termed the degree of polymerization (DP) The root-mean-square length L L = l m l: the length of a single bond in the backbone of the hydrocarbon chain m: the number of bonds The extended length L ext L ext = ml sin(109.5º/2) For typical bifunctional linear polymer, m = 2n Bend, coil, kink Intertwining, entanglement 11
Molecular configurations the side groups R: the large side group As the side groups become larger and more irregular, rigidity and melting point tend to rise, because: The side groups serve as hindrances to molecule sliding; The side groups lead to greater secondary bonding forces Polymers structures Schematic representations of (a) linear, (b) branched, (c) crosslinked, and (d) network (three dimensional) molecule structures. Circles designate individual mer units. 12
Thermoplastic polymers 1. Become soft and deformable upon heating 2. Linear polymers including those that are branched but not cross-linked 3. High-temperature plasticity due to the ability of the molecules to slide past one another (thermally activated or Arrhenius process 4. The ductility of thermoplastic polymers is reduced upon cooling 5. High temperature: for polymers ~100ºC, for metals can be ~1000ºC Engineering polymers (see Table 13.1) Retaining good strength and stiffness up to 150-175ºC The general-use polymers, e.g. textile fiber nylon, polyester (textile fiber) Polyethylene Low-density polyethylene (LDPE) High-density polyethylene (HDPE) Ultra-high molecule-weight polyethylene (UHMWPE) Thermoplastic elastomers, With mechanical behaviour analogous to natural rubbers, Synthetic rubbers, vulcanization 13
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Thermosetting polymers 1. Becoming hard and rigid upon heating, the opposite of thermoplastics 2. This phenomenon is not lost upon cooling 3. With network molecule structure, formed by the step-growth mechanism, the chemical reaction are enhanced by high temperatures and are irreversible 4. Commen thermosetting polymers (see Table 13.2) Thermosets With significant strength and stiffness Being common metal substitutes Not being recyclable Elastomers 15
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Additives A plasticizer To soften a polymer Blending with a low-molecular-weight polymer A filler To strengthen a polymer by restricting chain mobility Inert materials are used, e.g. short-fiber cellulose and asbestos, carbon black Stabilizers To reduce polymer degradation, e.g. To retard the room temperature oxidation by adding complex phenol group Flame retardants To reduce the inherent combustibility Halogens e.g. Cl atoms, by terminating free-radical chain reaction Colorants To provide colour to a polymer Pigments (insoluble), and dyes (soluble and provide transparent colour) Polymer crystals 17
Polymer Crystallinity Polymers rarely 100% crystalline Too difficult to get all those chains aligned crystalline region % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. amorphous region Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.) 18
Chain-folded model Polymer Crystal Forms Spherulites fast growth forms lamellar (layered) structures Spherulite surface Adapted from Fig. 14.13, Callister 7e. 19
Flexural modulus or modulus of elasticity in bending E flex = L 3 m / (4bh 3 ) m: the slope of the tangent to the initial straight-line portion of the loaddeflection curve Describe the combined effects of compressive deformation and tensile deformation (on the opposite side of the specimen) The tensile and compressive moduli differ significantly 20
Dynamic modulus of elasticity Some polymers, especially the elastomers, are used in structures for the purpose of isolation and absorption of shock and vibration. For such applications a dynamic elastic modulus is more useful to characterizethe performance of the polymerunder an oscillating mechanical load. E dyn = CIf 2 C: a constant, dependent upon test geometry I: the moment of interia (kg m 2 ) of the beam and weights used in the dynamic test f: the frequency of vibration (in cycles) for the test Mechanical properties of polymers- Stress- strain behaviours (p207) 21
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