Chapter 15: Characteristics, Applications & Processing of Polymers

Transcription

1 Chapter 15: Characteristics, Applications & Processing of Polymers ISSUES TO ADDRESS... What are the tensile properties of polymers and how are they affected by basic microstructural features? Hardening, anisotropy, and annealing in polymers. How does the elevated temperature mechanical response of polymers compare to ceramics and metals? What are the primary polymer processing methods? Chapter 15-1 Mechanical Properties of Polymers Stress-Strain Behavior brittle polymer plastic elastomer elastic moduli less than for metals Adapted from Fig. 15.1, Callister & Rethwisch 8e. Chapter 15-2

2 Mechanisms of Deformation Brittle Crosslinked and Network Polymers Initial Near Failure (MPa) brittle failure Initial Near Failure plastic failure aligned, crosslinked polymer Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. network polymer Chapter 15-3 Mechanisms of Deformation Semicrystalline (Plastic) Polymers Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister & Rethwisch 8e. (15.12 & are from J.M. Schultz, Polymer Materials Science, Prentice- Hall, Inc., 1974, pp ) (MPa) brittle failure onset of necking unload/reload plastic failure fibrillar structure near failure undeformed structure amorphous regions elongate crystalline regions align crystalline block segments separate Chapter 15-4

3 Mechanisms of Deformation Semicrystalline (Plastic) Polymers Elastic Deformation two adjacent chain-folded lamemllae and the interlamellar amorphous material Stage 1 : chain molecules in amorphous regions elongating in the direction of the applied tensile stress Stage 2 : amorphous chains continue to align and become elongated; bending and stretching of the strong chain covalent bonds within the lamellar crystallites. slight, reversible increase in the lamellar crystallite thickness ( t) Chapter 15-5 Mechanisms of Deformation Semicrystalline (Plastic) Polymers Plastic Deformation Stage 3 : adjacent chains in the lamellae slide past one another tilting of the lamellae so that the chain folds become more aligned Stage 4 : crystalline block segments separate from the lamella, with the segments attached to one another by tie chains. Stage 5 : the blocks and tie chains become oriented in the direction of the tensile ais highly oriented structure by drawing. Chapter 15-6

4 Mechanical Properties of Polymers Stress-Strain Behavior Tensile stress-strain curve for a semicrystalline polymer Chapter 15-7 Predeformation by Drawing Drawing (e: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) Annealing after drawing decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) Adapted from Fig , Callister & Rethwisch 8e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp ) Chapter 15-8

5 Degree of Crystallinity Influence of degree of crystallinity and molecular weight on the physical characteristics of polyethylene Increasing the crystallinity of a polymer enhances its strength; however, the material tends to become more brittle Chapter 15-9 Heat-treating of semicrystalline polymers - Heat-treating(annealing) can lead to an increase in the percent crystallinity, and crystallite size and perfection, as well as modifications of the spherulite structure - Heat treatment of undrawn materials; increasing temp. leads to 1) an increase in tensile modulus 2) an increase in yield strength 3) a reduction in ductility - Heat treatment of drawn polymers (e. fibers), Modulus decreases with increased annealing temp. because of a loss of chain orientation and strain-induced crystallinty Chapter 15-10

6 Mechanisms of Deformation Elastomers (MPa) initial: amorphous chains are kinked, cross-linked. brittle failure plastic failure elastomer deformation is reversible (elastic)! final: chains are straighter, still cross-linked Stress-strain curves adapted from Fig. 15.1, Callister & Rethwisch 8e. Inset figures along elastomer curve (green) adapted from Fig , Callister & Rethwisch 8e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) - Moduli of elasticity are quite small and vary with strain; non-linear S-S - Elastic deformation, upon application of a tensile load, is the partial uncoiling, untwisting, and straightening Chapter Mechanisms of Deformation Elastomers the crosslinks act as anchor points between the chains and prevent chain slippage. Entropy is the part of the driving force for elastic deformation. Requirement for elastomers - Must not easily crystallize; amorphous, having molecular chains that are naturally coiled and kinked in the unstressed state. - Chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force - Restricting the motions of chains past one another by crosslinking delay the onset of plastic deformation Chapter 15-12

7 Mechanisms of Deformation Elastomers Vulcanization - Crosslinking process in elastomers - Achieved by a nonreversible chemical reaction at an elevated temp. - Sulfur compounds are added to the heated elastomer Chapter Mechanisms of Deformation Elastomers Vulcanization - Unvulcanized rubber contains very few crosslinks: soft, tacky and has poor resistance to abrasion - Modulus of elasticity, tensile strength, and resistance to degradation by oidation are all enhanced by vulcanization - To produce a rubber that is capable of large etensions without rupture of the primary chain bonds, there must be relatively few crosslinks, and these must be widely seprated. - Increasing the sulfur content further hardens the rubber and reduces the etensibility. Chapter 15-14

8 Crystallization, Melting, and Glasstransition in polymers - Crystallization : the process by which, upon cooling, an ordered (i.e. crystalline) solid phase is produced from a liquid melt having a highly random molecular structure. - Melting : the reverse process that occurs when a polymer is heated. - Glass-transition : when cooled from a liquid melt, amororhpous or noncrystallizable polymer become rigid solids yet retain the disordered molecular structure; named because glass changes from a rigid sheet to a fleible plastic-like material at its transition point. - For semicrystalline polymers, crystalline regions will eperience melting (and crystallization), whereas noncrystalline area pass through the glass transition. Chapter Melting & Glass Transition Temps. - Crystalline material(c): Discontinuous change in specific volume at T m. - Totally amorphous material(a): continuous but eperiences a slight decrease in slope at T g Specific volume versus temp., upon cooling from the liquid melt - Semicrystalline material(b): both T m and T g are observed. Adapted from Fig , Callister & Rethwisch 8e. Chapter 15-16

9 Melting & Glass Transition Temps. What factors affect T m and T g? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by presence of 1. Bulky side groups 2. Polar groups or side groups 3. Chain double bonds and aromatic chain groups Regularity of repeat unit arrangements affects T m only Chapter Melting & Glass Transition Temps. Chapter 15-18

10 Melting & Glass Transition Temps. Dependence of polymer properties as well as melting and glass transition temp. on molecular weight T mobile liquid viscous liquid Callister, rubber Fig tough plastic T m T g T g = 0.5~0.8T m crystalline solid partially crystalline solid Molecular weight Adapted from Fig , Callister & Rethwisch 8e. (Fig is from F.W. Billmeyer, Jr., Tetbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) Chapter Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften w/ heating -- polyethylene, polypropylene, polycarbonate, polystyrene Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard and brittle -- do NOT soften w/ heating -- vulcanized rubber, epoies, polyester resin, phenolic resin Chapter 15-20

11 Influence of T and Strain Rate on Thermoplastics Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. For intermediate temp., the polymer is rubbery solid that ehibits the combined mechanical characteristics of elastic and viscous behaviors viscoelasticity (MPa) ºC 20ºC 40ºC Plots for semicrystalline PMMA (Pleiglas) 60ºC to 1.3 Adapted from Fig. 15.3, Callister & Rethwisch 8e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) Chapter Viscoelastic Materials An amorphous polymer may behave like a glass at low temp.(t<t g ), a viscous liquid at high temp.(t>t g ), and a rubbery solid at intermediate temp.(t T g ) Viscoelasticity Chapter 15-22