3Polymer Crystallinity: When the monomers are arranged in a neat orderly manner, the polymer is crystalline.Polymers are just like socks. Sometimes they are arranged in a neat orderly manner.An Amorphous solid is a solid in which the molecules have no order or arrangement.Some people will just throw their socks in the drawer in one big tangled mess. Their sock drawers look like this:
4Crystallinity in Polymers Both amorphous and crystalline structures are possible, although the tendency to crystallize is much less than for metals or non‑glass ceramicsNot all polymers can form crystalsFor those that can, the degree of crystallinity (the proportion of crystallized material in the mass) is always less than 100%Lamella = the typical form of a crystallized region
5Amorphous and Crystalline structures lamellaCrystalline structure:Inclusion Compounds of Hexakis (4-cyanophenyl) benzene: Open Networks Maintained by C-H···N Interactions. In Maly et al, 2006.Amorphous structure:Disordered, amorphous solid structure of silica glass, as modeled in computer simulations. Red spheres are oxygen atoms, grey spheres are silicon atoms.
6Crystalline Polymer Structure Figure 8.9 Crystallized regions in a polymer: (a) long molecules forming crystals randomly mixed in with the amorphous material; and (b) folded chain lamella, the typical form of a crystallized region.
7Crystallinity and Properties As crystallinity is increased in a polymerDensity increasesStiffness, strength, and toughness increasesHeat resistance increasesIf the polymer is transparent in the amorphous state, it becomes opaque when partially crystallizedWhen you make the socks in your drawer in order, they occupy less space! When polymer has crystalline structure rather than amorphous, the whole material will have smaller volume (V = smaller), mass is the same, so M/V which is density (with smaller V) is larger!For the same reason, when density is larger, the material is stiffer and stronger and is tougher against deformations.If some people are standing in a line in a narrow hall, you can hardly get through! Heat transfer is done via conduction (transferring the energy from one molecule to another molecules, no mass is transferred). Thermal energy can not be easily transferred, or in another word, there will be higher resistance against heat transfer.If people are standing so close to each other you can not see what is happening in the front row (opaque!).
8Low Density & High Density Polyethylene Polyethylene typeLow density orLDPEHigh density orHDPEDegree of crystallinity55%92%Specific gravity0.920.96(heavier)Modulus of elasticity140 MPa(20,000 lb/in2)700 MPa(100,000 lb/in2)(stiffer)Melting temperature115C(239F)135C(275F)(Resists against temperature)
9Some Observations About Crystallization Linear polymers consist of long molecules with thousands of repeated mers.Crystallization involves folding back and forth of the long chains upon themselves to achieve a very regular arrangement.The crystallized regions are called crystallitesCrystallites take the form of lamellae randomly mixed in with amorphous material:A polymer that crystallizes is a two‑phase system ‑ crystallites interspersed throughout an amorphous matrix.
10Factors for Crystallization Slower cooling promotes crystal formation and growth.Mechanical deformation, as in the stretching of a heated thermoplastic, tends to align the structure and increase crystallization.Plasticizers (chemicals added to a polymer to soften it) reduce the degree of crystallinity.
12Thermoplastic Polymers (TP) A thermoplastic polymer can be heated from a solid state to a viscous liquid state and then cooled back down to solidHeating and cooling can be repeated many times without degrading the polymerThe reason is that TP polymers consist of linear (and/or branched) macromolecules that do not cross‑link upon heating
13Mechanical Properties of Thermoplastics Low modulus of elasticity (stiffness)E is two or three orders of magnitude lower than metals and ceramicsLow tensile strengthabout 10% of the metalMuch lower hardness than metals or ceramicsGreater ductility on averageTremendous range of values, from 1% elongation for polystyrene to 500% or more for polypropylene (PP)
14Physical Properties of Thermoplastics Lower densities than metals or ceramicsTypical specific gravity for polymers are 1.2Ceramics specific gravity = 2.5Metals specific gravity = 7.0Much higher coefficient of thermal expansion: L2 - L1 = αL1(T2-T1)Roughly five times the value for metals and 10 times the value for ceramicsMuch lower melting temperatures(Lower electrical conductivity )Insulating electrical propertiesLower thermal conductivityHigher specific heats than metals and ceramicsH = C .m. (T2 ‑ T1)
15Commercial Thermoplastic Products and Raw Materials Thermoplastic products includeMolded and extruded itemsFibers and filamentsFilms and sheetsPackaging materialsPaints and varnishesThe starting plastic materials are normally supplied to the fabricator in the form of powders or pellets in bags, drums, or larger loads by truck or rail car
17Question What is cross-linking? The formation of covalent side bonds between linear polymer chains. Usually achieved in thermosets and rubbers by vulcanization.Cross-linkingincreases modulus,reduces elongation andreduces flow under stress.
18Thermosetting Polymers (TS) TS polymers are distinguished by their highly cross‑linked three‑dimensional, covalently‑bonded structure.Chemical reactions associated with cross‑linking are called curing or setting.In effect, the formed part (e.g., pot handle, electrical switch cover, etc.) becomes one large macromolecule.Always amorphous and exhibits no glass transition temperature.Elastomer
19General Properties of Thermosets Rigid - modulus of elasticity is two to three times greater than thermoplasticsBrittle, virtually no ductilityLess soluble in common solvents than thermoplasticsCapable of higher service temperatures than thermoplasticsCannot be remelted ‑ instead they degrade or burn
20TS Products TS vs. TP Polymers: Countertops, plywood adhesives, paints, molded parts, printed circuit boards and other fiber reinforced plastics.TS vs. TP Polymers:TS plastics are not as widely used as the TPOne reason is the added processing costs and complications involved in curingLargest market share of TS = phenolic resins with 6% of the total plastics market, Compare polyethylene with 35% market share.
22ElastomersPolymers capable of large elastic deformation when subjected to relatively low stressesSome can be extended 500% or more and still return to their original shapeElastomers consist of long‑chain molecules that are cross‑linked (like thermosetting polymers)Two categories:Natural rubber - derived from biological plantsSynthetic polymers - produced by polymerization processes similar to those used for thermoplastic and thermosetting polymersThermosetmolecule
23Characteristics of Elastomers They owe their impressive elastic properties to two features:Molecules are tightly kinked when unstretchedDegree of cross‑linking is substantially less than thermosetsWhen stretched, the molecules are forced to uncoil and straightenFigure Model of long elastomer molecules, with low degree of cross‑linking: (a) unstretched, and (b) under tensile stress.
25Stiffness of RubberFigure Increase in stiffness as a function of strain for three grades of rubber: natural rubber, vulcanized rubber, and hard rubber.
26Natural Rubber (NR) Latex NR consists primarily of polyisoprene, a high molecular‑weight polymer of isoprene (C5H8)It is derived from latex, a milky substance produced by various plants, most important of which is the rubber tree that grows in tropical climatesLatex is a water emulsion of polyisoprene (about 1/3 by weight), plus various other ingredientsRubber is extracted from latex by various methods that remove the water1/3 Water +2/3 polyisopreneRubberRemoving waterLatex
27Natural Rubber Products Largest single market for NR is automotive tiresOther products: shoe soles, bushings, seals, and shock absorbing components.In tires, carbon black is an important additive; it reinforces the rubber, serving to increase tensile strength and resistance to tear and abrasion.Other additives: clay, kaolin, silica, talc, and calcium carbonate, as well as chemicals that accelerate and promote vulcanization.
28Vulcanization Curing to cross‑link most elastomers Vulcanization = the term for curing in the context of natural rubber (and certain synthetic rubbers)Typical cross‑linking in rubber is one to ten links per hundred carbon atoms in the linear polymer chain, depending on degree of stiffness desiredConsiderably less than cross‑linking in thermosets
29Vulcanization or curing Design and optimization of the chemical formulation and the manufacturing process for the production of complex-shaped rubber parts used in critical engineering applications. (More info: Center for catalyst design, Purdue Uni)
30Vulcanized Natural Rubber Properties: noted among elastomers for high tensile strength, tear strength, resilience (capacity to recover shape), and resistance to wear and fatigueWeaknesses: degrades when subjected to heat, sunlight, oxygen, ozone, and oilSome of these limitations can be reduced by additivesMarket share of NR 22% of total rubber volume (natural plus synthetic)Total rubber volume 15% of total polymer market
31Hard RubberA hard, rigid and shiny resin. It is actually a very hard rubber first obtained by Charles Goodyear by vulcanizing rubber for prolonged periods.It is often used in bowling balls, smoking pipe mouthpieces, fountain pen nib feeds, and high-quality saxophone and clarinet mouthpieces.
32Synthetic RubbersDevelopment of synthetic rubbers was motivated largely by world wars when NR was difficult to obtain.Today, tonnage of synthetic rubbers is more than three times that of NR.The most important synthetic rubber is styrene‑butadiene rubber (SBR), a copolymer of butadiene (C4H6) and styrene (C8H8).As with most other polymers, the main raw material for synthetic rubbers is petroleum.
33Thermoplastic Elastomers (TPE) A thermoplastic that behaves like an elastomerElastomeric properties not from chemical cross‑links, but from physical connections between soft and hard phases in the materialCannot match conventional elastomers in elevated temperature strength and creep resistanceProducts: footwear; rubber bands; extruded tubing, wire coating; molded automotive parts, but no tires
34Guide to the Processing of Polymers Polymers are nearly always shaped in a heated, highly plastic stateCommon operations are extrusion and moldingMolding of thermosets is more complicated because of cross‑linkingThermoplastics are easier to mold and a greater variety of molding operations are availableRubber processing has a longer history than plastics, and rubber industries are traditionally separated from plastics industry, even though processing is similar
35Test yourself! TP TS Elastomer Choose the correct structure for TP, TS, and Elastomer molecules:TPTSElastomer
36Test yourself! Which one has the highest degree of cross-linking? TP TSElastomerWhich one is heavier (has highest degree of crystallinity)?LDPEHDPEWhich one has lower melting temperature?LDPEHDPE
37Test yourself!What type of rubber is used to produce the following products:Rubber: Thermoplastic ElastomerTire: Synthetic rubber(and Natural rubber)Shoe sole: Natural rubber