Presentation on theme: "CHAPTER 14/15: POLYMER STUDIES Issues: What are the basic microstructural features of a polymer? How are polymer properties affected by molecular weight?"— Presentation transcript:
CHAPTER 14/15: POLYMER STUDIES Issues: What are the basic microstructural features of a polymer? How are polymer properties affected by molecular weight? How do polymeric materials accommodate the polymer chain? What are the tensile properties of polymers and how are they affected by basic microstructural features? Changing Polymer Properties: 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 14 – Polymers What is a polymer? Poly mer many repeat unit Adapted from Fig. 14.2, Callister 7e. CCCCCC HHHHHH HHHHHH Polyethylene (PE) Cl CCCCCC HHH HHHHHH Polyvinyl chloride (PVC) HH HHHH Polypropylene (PP) CCCCCC CH 3 HH H repeat unit repeat unit repeat unit
Ancient Polymer History Originally many natural polymers were used –Wood– Rubber –Cotton– Wool –Leather– Silk Oldest known uses of “Modern Polymers” –Rubber balls used by Incas –Noah used pitch (a natural polymer) for the ark – as had all ancient mariners!
Polymer Composition Most polymers are hydrocarbons – i.e. made up of H and C (we also recognize Si-H ‘silicones’) Saturated hydrocarbons –Each carbon bonded to four other atoms C n H 2n+2
Unsaturated Hydrocarbons Double & triple bonds relatively reactive – can form new bonds –Double bond – ethylene or ethene - C n H 2n 4-bonds, but only 3 atoms bound to C’s –Triple bond – acetylene or ethyne - C n H 2n-2
Isomerism –two compounds with same chemical formula can have quite different structures Ex: C 8 H 18 n-octane 2-methyl-4-ethyl pentane (isooctane)
Chemistry of Polymers Free radical polymerization Initiator: example - benzoyl peroxide
Chemistry of Polymers Free radical polymerization (addition polymerization) Initiator: example - benzoyl peroxide
Condensation Polymerization Some of the original monomer’s materials are shed (condensed out) during polymerization process Process is conducted in the presence of a catalyst Water, CO 2 are commonly condensed out but other compounds can be emitted including HCN or other acids Water is “Condensed out” during polymerization of Nylon
Bulk or Commodity Polymers
NOTE: See Table 15.3 for commercially important polymers – including trade names
MOLECULAR WEIGHT M w is more sensitive to higher molecular weights Molecular weight, M i : Mass of a mole of chains. Lower M higher M Adapted from Fig. 14.4, Callister 7e.
Molecular Weight Calculation Example: average mass of a class
Degree of Polymerization, n n = number of repeat units per chain n i = 6 mol. wt of repeat unit iChain fraction
Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.7, Callister 7e. Molecular Structures BranchedCross-LinkedNetworkLinear secondary bonding
Polymers – Molecular Shape Conformation – Molecular orientation can be changed by rotation around the bonds –note: no bond breaking needed Adapted from Fig. 14.5, Callister 7e.
Polymers – Molecular Shape Configurations – to change must break bonds Stereoisomerism mirror plane
Tacticity Tacticity – stereoregularity of chain isotactic – all R groups on same side of chain syndiotactic – R groups alternate sides atactic – R groups random
cis/trans Isomerism cis cis-isoprene (natural rubber) bulky groups on same side of chain trans trans-isoprene (gutta percha) bulky groups on opposite sides of chain
Copolymers two or more monomers polymerized together random – A and B randomly vary in chain alternating – A and B alternate in polymer chain block – large blocks of A alternate with large blocks of B graft – chains of B grafted on to A backbone A – B – random block graft Adapted from Fig. 14.9, Callister 7e. alternating
Polymer Crystallinity Ex: polyethylene unit cell Crystals must contain the polymer chains in some way –Chain folded structure Adapted from Fig , Callister 7e. Adapted from Fig , Callister 7e.
amorphous region Polymer Crystallinity % Crystallinity: how much is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. Adapted from Fig , Callister 6e. (Fig 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.) crystalline region Polymers rarely exhibit 100% crystalline Too difficult to get all those chains aligned
Mechanical Properties i.e. stress-strain behavior of polymers brittle polymer plastic elastomer FS of polymer ca. 10% that of metals Strains – deformations > 1000% possible (for metals, maximum strain ca. 100% or less) elastic modulus – less than metal Adapted from Fig. 15.1, Callister 7e.
Tensile Response: Brittle & Plastic brittle failure plastic failure (MPa) x x crystalline regions slide fibrillar structure near failure crystalline regions align onset of necking Initial Near Failure semi- crystalline case aligned, cross- linked case networked case amorphous regions elongate unload/reload Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice- Hall, Inc., 1974, pp )
Predeformation by Drawing Drawing…(ex: 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 alignment -- reverses effects of drawing. Comparable to cold working in metals! Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) Tensile Response: Elastomer Case (MPa) initial: amorphous chains are kinked, cross-linked. x final: chains are straight, still cross-linked elastomer Deformation is reversible! brittle failure plastic failure x x
Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) Thermoplastics vs. Thermosets Callister, Fig T Molecular weight TgTg TmTm mobile liquid viscous liquid rubber tough plastic partially crystalline solid crystalline solid
Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. Adapted from Fig. 15.3, Callister 7e. (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.) T and Strain Rate: Thermoplastics °C 20°C 40°C 60°C to 1.3 (MPa) Data for the semicrystalline polymer: PMMA (Plexiglas)
Melting vs. Glass Transition Temp. What factors affect T m and T g ? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by 1.Bulky sidegroups 2.Polar groups or sidegroups 3.Double bonds or aromatic chain groups Regularity (tacticity) – affects T m only Adapted from Fig , Callister 7e.
Stress relaxation test: -- strain to and hold. -- observe decrease in stress with time. Relaxation modulus: Sample T g ( C) values: PE (low density) PE (high density) PVC PS PC Selected values from Table 15.2, Callister 7e. Time Dependent Deformation time strain tensile test oo (t)(t) Data: Large drop in E r for T > T g. (amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) rigid solid (small relax) transition region T(°C) TgTg E r (10s) in MPa viscous liquid (large relax)
Polymer Fracture fibrillar bridges microvoids crack alligned chains Adapted from Fig. 15.9, Callister 7e. Crazing Griffith cracks in metals – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form
Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers –Added to improve tensile strength & abrasion resistance, toughness & decrease cost –ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Plasticizers – Added to reduce the glass transition temperature T g – commonly added to PVC - otherwise it is brittle
Polymer Additives Stabilizers –Antioxidants –UV protectants Lubricants – Added to allow easier processing – “slides” through dies easier – ex: Na stearate Colorants – Dyes or pigments Flame Retardants – Cl/F & B
Processing of Plastics Thermoplastic – –can be reversibly cooled & reheated, i.e. recycled –heat till soft, shape as desired, then cool –ex: polyethylene, polypropylene, polystyrene, etc. Thermoset – when heated forms a network – degrades (not melts) when heated – mold the prepolymer then allow further reaction – ex: urethane, epoxy
Processing Plastics - Molding Compression and transfer molding –thermoplastic or thermoset Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, )
Processing Plastics - Molding Injection molding –thermoplastic & some thermosets Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, )
Processing Plastics – Extrusion Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
Polymer Types: Fibers Fibers - length/diameter >100 Textiles are main use –Must have high tensile strength –Usually highly crystalline & highly polar Formed by spinning – ex: extrude polymer through a spinnerette Pt plate with 1000’s of holes for nylon ex: rayon – dissolved in solvent then pumped through die head to make fibers – the fibers are drawn – leads to highly aligned chains- fibrillar structure
Polymer Types Coatings – thin film on surface – i.e. paint, varnish –To protect item –Improve appearance –Electrical insulation Adhesives – produce bond between two adherands – Usually bonded by: 1. Secondary bonds 2. Mechanical bonding Films – blown film extrusion Foams – gas bubbles in plastic
Blown-Film Extrusion Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
Advanced Polymers Ultrahigh molecular weight polyethylene (UHMWPE) –Molecular weight ca. 4 x 10 6 g/mol –Excellent properties for variety of applications bullet-proof vest, golf ball covers, hip joints, etc. UHMWPE Adapted from chapter- opening photograph, Chapter 22, Callister 7e.
The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC cup made from polyethylene
ABS, Acrylonitrile-Butadiene-Styrene Made up of the 3 materials: acrylonitrile, butadiene and styrene. The material is located under the group styrene plastic. Styrene plastics are in volume one of the most used plastics. Properties The mechanical properties for ABS are good for impact resistance even in low temperatures. The material is stiff, and the properties are kept over a wide temperature range. The hardness and stiffness for ABS is lower than for PS and PVC. Weather and chemical resistance The weather resistance for ABS is restricted, but can be drastically improved by additives as black pigments. The chemical resistance for ABS is relatively good and it is not affected by water, non organic salts, acids and basic. The material will dissolve in aldehyde, ketone, ester and some chlorinated hydrocarbons. Processing ABS can be processed by standard mechanical tools as used for machining of metals and wood. The cutting speed need to be high and the cutting tools has to be sharp. Cooling is recommended to avoid melting of the material. If the surface finish is of importance for the product, the ABS can be treated with varnish, chromium plated or doubled by a layer of acrylic or polyester. ABS can be glued to it self by use of a glue containing dissolvent. Polyurethane based or epoxy based glue can be used for gluing to other materials. ABS – A Polymerized “Alloy”
A Processing Movie: Calloway Golf:
General drawbacks to polymers: -- E, y, K c, T application are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. Thermoplastics (PE, PS, PP, PC): -- Smaller E, y, T application -- Larger K c -- Easier to form and recycle Elastomers (rubber): -- Large reversible strains! Thermosets (epoxies, polyesters): -- Larger E, y, T application -- Smaller K c And Remember: Table 15.3 Callister 7e Is a Good overview of applications and trade names of polymers. Summary