Classes of Polymeric Materials Elastomers

Transcription

1 Classes of Polymeric Materials Elastomers 1

2 Elastomers Elastomers are rubber like polymers that are thermoset or thermoplastic butyl rubber: natural rubber thermoset: polyurethane, silicone thermoplastic: thermoplastic urethanes (TPU), thermoplastic elastomers (TPE), thermoplastic olefins (TPO), thermoplastic rubbers (TPR) Elastomers exhibit more elastic properties versus plastics which plastically deform and have a lower elastic limit. Rubbers have the distinction of being stretched 200% and returned to original shape. Elastic limit is 200%

3 Rubbers Rubbers have the distinction of being stretched 200% and returned to original shape. Elastic limit is 200% Natural rubber (isoprene) is produced from gum resin of certain trees and plants that grow in southeast Asia, Ceylon, Liberia, and the Congo. The sap is an emulsion containing 40% water & 60% rubber particles Vulcanization occurs with the addition of sulfur (4%). Sulfur produces cross-links to make the rubber stiffer and harder. The cross-linkages reduce the slippage between chains and results in higher elasticity. Some of the double covalent bonds between molecules are broken, allowing the sulfur atoms to form cross-links. Soft rubber has 4% sulfur and is 10% cross-linked. Hard rubber (ebonite) has 45% sulfur and is highly cross-linked.

4 Rubber Additives and Modifiers Fillers can comprise half of the volume of the rubber Silica and carbon black. Reduce cost of material. Increase tensile strength and modulus. Improve abrasion resistance. Improve tear resistance. Improve resistance to light and weathering. Example, Tires produced from Latex contains 30% carbon black which improves the body and abrasion resistance in tires. Additives Antioxidants, antiozonants, oil extenders to reduce cost and soften rubber, fillers, reinforcement

5 Vulcanizable Elastomeric Compounds Rubbers are compounded into practical elastomers The rubber (elastomer) is the major component and other components are given as weight per hundred weight rubber (phr) Sulfur is added in less than 10 phr Accelerators and activators with the sulfur hexamethylene tetramine (HMTA) zinc oxide as activators Protective agents are used to suppress the effects of oxygen and ozone phenyl betabaphthylamine and alkyl paraphenylene diamine (APPD) Reinforcing filler carbon black silica when light colors are required calcium carbonate, clay, kaoilin Processing aids which reduce stiffness and cost Plasticizers, lubricants, mineral oils, paraffin waxes,

6 Vulcanizable Rubber Typical tire tread Natural rubber smoked sheet (100), sulfur (2.5) sulfenamide (0.5), MBTS (0.1), strearic acid (3), zinc oxide (3), PNBA (2), HAF carbon black (45), and mineral oil (3) Typical shoe sole compound SBR (styrene-butadiene-rubber) (100) and clay (90) Typical electrical cable cover polychloroprene (100), kaolin (120), FEF carbon black (15) and mineral oil (12), vulcanization agent

7 Synthetic Rubber Reactive system elastomers Low molecular weight monomers are reacted in a polymerization step with very little cross-linking. Reaction is triggered by heat, catalyst, and mixing Urethanes processed with Reaction Injection Molding (RIM) Silicones processed with injection molding or extrusion Thermoplastic Elastomers Processing involves melting of polymers, not thermoset reaction Processed by injection molding, extrusion, blow molding, film blowing, or rotational molding. Injection molded soles for footwear Advantages of thermoplastic elastomers Less expensive due to fast cycle times More complex designs are possible Wider range of properties due to copolymerization Disadvantage of thermoplastic elastomers Higher creep

8 Thermoplastic Elastomers Four types of elastomers Olefinics and Styrenics Polyurethanes and Polyesters Olefinics (TPOs are used for bumper covers on cars) Produced by Blending copolymers of ethylene and propylene (EPR) or ter polymer of ethylene-propylene diene (EPDM) with PP in ratios that determine the stiffness of the elastomer A 80/20 EPDM/PP ratio gives a soft elastomer (TPO) Styrenic thermoplastic elastomers (STPE) Long triblock copolymer molecules with an elastomeric central block (butadiene, isoprene, ethylene-butene, etc.) and end blocks (styrene, etc.) which form hard segments Other elastomers have varying amounts of soft and hard blocks

9 Thermoplastic Elastomers Polyurethanes Have a hard block segment and soft block segment Soft block corresponds to polyol involved in polymerization in ether based Hard blocks involve the isocyanates and chain extenders Polyesters are etheresters or copolyester thermoplastic elastomer Soft blocks contain ether groups are amorpous and flexible Hard blocks can consist of polybutylene terephthalate (PBT) Polyertheramide or polyetherblockamide elastomer Hard blocks consits of a crystallizing polyamide Soft Hard Soft Hard Soft Hard

10 Commercial Elastomers Diene C=C double bonds and Related Elastomers Polyisoprene- (C 5 H 8 ) 20,000 Basic structure of natural rubber Can be produced as a synthetic polymer Capable of very slow crystallization Tm = 28 C, Tg = -70 C for cis polyisoprene Tm = 68 C, Tg = -70 C for trans polyisoprene Trans is major component of gutta percha, the first plastic Natural rubber was first crosslinked into highly elastic network by Charles Goodyear (vulcanization with sulfur in 1837) Sulfur crosslinked with the unsaturations C=C Natural rubber in unfilled form is widely used for products with very large elastic deformations or very high resilience, resistance to cold flow (low compression set) and resistance to abrasion, wear, and fatigue. Natural rubber does not have good intrinsic resistance to sunlight, oxygen, ozone, heat aging, oils, or fuels.

11 Commercial Elastomers Polybutadiene Basis for synthetic rubber as a major component in copolymers Styrene-Butadiene Rubber (SBR, NBR) or in Blends with other rubbers (NR, SBR) Can improve low-temperature properties, resilence, and abrasion or wear resistance Tg = -50 C Polychloroprene Polychloroprene or neoprene was the very first synthetic rubber Due to polar nature of molecule from Cl atom it has very good resistance to oils and is flame resistant (Cl gas coats surface) Used for fuel lines, hoses, gaskets, cable covers, protective boots, bridge pads, roofing materials, fabric coatings, and adhesives Tg = -65 C.

12 Commercial Elastomers Butyl rubber- addition polymer of isobutylene. Copolymer with a few isoprene units, Tg =-65 C Contains only a few percent double bonds from isoprene Small extent of saturation are used for vulcanization Good regularity of the polymer chain makes it possible for the elastomer to crystallize on stretching Soft polymer is usually compounded with carbon black to increase modulus Nitrile rubber Copolymer of butadiene and acrylonitrile Solvent resistant rubber due to nitrile C N Irregular chain structure will not crystallize on stretching, like SBR vulcanization is achieved with sulfur like SBR and natural rubber Thiokol- ethylene dichloride polymerized with sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing.

13 Thermoplastic Elastomers Thermoplastic Elastomers result from copolymerization of two or more monomers. One monomer is used to provide the hard, crystalline features, whereas the other monomer produces the soft, amorphous features. Combined these form a thermoplastic material that exhibits properties similar to the hard, vulcanized elastomers. Thermoplastic Urethanes (TPU) The first Thermoplastic Elastomer (TPE) used for seals gaskets, etc. Other TPEs Copolyester for hydraulic hoses, couplings, and cable insulation. Styrene copolymers are less expensive than TPU with lower strength Styrene-butadiene (SBR) for medical products, tubing, packaging, etc. Olefins (TPO) for tubing, seals, gaskets, electrical, and automotive.

14 Thermoplastic Elastomers Styrene-butadiene rubber (SBR) Developed during WWII Germany under the name of BUNA-S. North America as GR-S,Government rubber-styrene. Random copolymer of butadiene (67-85%) and styrene (15-33%) Tg of typical 75/25 blend is 60 C Not capable of crystallizing under strain and thus requires reinforcing filler, carbon black, to get good properties. One of the least expensive rubbers and generally processes easily. Inferior to natural rubber in mechanical properties Superior to natural rubber in wear, heat aging, ozone resistance, and resistance to oils. Applications include tires, footwear, wire, cable insulation, industrial rubber products, adhesives, paints (latex or emulsion) More than half of the world s synthetic rubber is SBR World usage of SBR equals natural rubber

15 Acrylonitrile-butadiene rubber (NBR) Also called Nitrile rubber Developed as an oil resistant rubber due to the polar C:::N polar bond. Resistant to oils, fuels, and solvents. Copolymer of acrylonitrile (20-50%) and butadiene(80-50%) Moderate cost and a general purpose rubber. Excellent properties for heat aging and abrasion resistance Poor properties for ozone and weathering resistance. Has high dielectric losses and limited low temperature flexibility Applications include fuel and oil tubing; hose, gaskets, and seals; conveyer belts, print rolls, and pads. Carboxylated nitrile rubbers (COX-NBR) has carboxyl side groups (COOH)which improve Abrasion and wear resistance; ozone resistance; and low temperature flexibility NBR and PVC for miscible, but distinct polymer blend or polyalloy 30% addition of PVC improves ozone and fire resistance

16 Ethylene-propylene rubber (EPR) EPR and EPDM Form a noncrystallizing copolymer with a low Tg. The % PP and PE units determines properties Tg = -60 C for PE/PP of 67/33 to 50/50 Unsaturated polymer since PP and PE are saturated Resistant to ozone, weathering, and heat aging Does not allow for conventional vulcanization Terpolymer with addition of small amount of third monomer (Diene D) has unsaturations referred to as EPDM 1,4, hexadiene (HD); 5-ethylidene-2-norbornene (ENB); diclopentadiene (DCPB) feature unsaturations in a side (pendant) group Feature excellent ozone and weathering resistance and good heat aging Limitations include poor resistance to oils and fuels, poor adhesion to many substrates and reinforcements Applications include exterior automotive parts (TPO is PP/EPDM), construction parts, weather strips, wire and cable insulation, hose and belt products, coated fabrics.

17 Ethylene Related Elastomers Chlorosulfonated Polyethylene (CSPE) Moderate random chlorination of PE (24-43%) Infrequent chlorosulfonic groups (SO2Cl) Sulfur content is 1-1.5%. CSPE is noted for excellent weathering resistance Good resistance to ozones, heat, chemicals, solvents. Good electrical properties, low gas permeability, good adhesion to substrates Applications include hose products, roll covers, tank linings, wire and cable covers, footwear, and building products Chlorinated Polyethylene (CPE) Moderate random chlorination Suppresses crystallinity (rubber) Can be crosslinked with peroxides Cl range is 36-42% versus 56.8% for PVC Properties include good heat, oil, and ozone resistance Used as plasticizer for PVC

18 Ethylene Related Elastomers Ethylene-vinylacetate Copolymer (EVA) Random copolymer of E and VA Amorphous and thus elastomeric VA range is 40-60% Can be crosslinked through organic peroxides Properties include Good heat, ozone, and weather resistance Ethylene-acrylate copolymer (EAR) Copolymer of Ethylene and methacrylate Contains carboxylic side groups (COOH) Properties include Excellent resistance to ozone and Excellent energy absorbers Better than butyl rubbers

19 FluoroElastomers Polyvinylidene fluoride (PVDF) Tg = -35 C Poly chloro tri fluoro ethylene (PCTFE) Tg = 40 C Poly hexa fluoro propylene (PHFP) Tg = 11 C Poly tetra fluoro ethylene (PTFE) Tg = C Fluoroelastomers are produced by random copolymerization that suppresses the crystallinity and provides a mechanism for cross linking by terpolymerization Monomers include VDF, CTFE, HFP, and TFE

20 FluoroElastomers Fluoroelastomers are expensive but have outstanding properties Exceptional resistance to chemicals, especially oils, solvents High temperature resistance, weathering and ozone resistance. Good barrier properties with low permeability to gases and vapors Applications Mechanical seals, packaging, O-rings, gaskets, diaphrams, expansion joints, connectors, hose liners, roll covers, wire and cable insulation. Previous fluoroelastomners are referred to as Fluorohydrocarbon elastomers since they contain F, H, and C atoms with O sometimes Two other classes of elastomers include fluorinated types Fluorosilicone elastomers remain flexible at low temperatures Fluorinated polyorganophosphazenes have good fuel resistance

21 Silicone Polymers Silicone polymers or polysiloxanes (PDMS) Polymeric chains featuring Tg = -125 C Very stable alternating combination of Silicone and oxygen, and a variety of organic side groups attached to Si Two methyl, CH 3, are very common side group generates polydimethylsiloxane (PDMS) Unmodified PDMS has very flexible chains corresponding to low Tg Modified PDMS has substitution of bulky side groups (5-10%) Phenylmethlsiloxane or diphenylsiloxane suppress crystallization Substituted side groups, e.g., vinyl groups (.5%) featuring double bonds (unsaturations ) enables crosslinking to form vinylmethylsiloxane (VMS) Degree of polymerization, DP, of polysiloxane = 200-1,000 for low consistency chains to 3,000-10,000 for high consistency resins. Mechanism of crosslinking can be from a vinyl unsaturation or reactive end groups (alkoxy, acetoxy)

22 Silicone Polymers Silicone polymers or polysiloxanes (PDMS) Properties Mediocre tear properties High temperature resistance from -90 C to 250 C. Surface properties are characterized by very low surface energy (surface tension) giving good slip, lubricity, and release properties (antistick) and water repellency. Excellent adhesion is obtained for curing compounds for caulk

23 Silicones Unmodified PDMS has very flexible chains with a low Tg. Regular structure allows for crystallization below Tm Addition of small amount of bulky side groups are used to suppress crystallization Trifluoropropyl side groups enhance the resistance to solvent swelling and are called fluorosilicones Linear form (uncrosslinked) polysiloxane corresponds to DP of for low consistency to 3,000-10,000 for high consistency resins Mechanism for crosslinking (vulcanization) can be based upon vinyl unsaturations or reactive end groups (alkoxy) Silicone polymers are mostly elastomers with mediocre tear properties, but with addition of silica can have outstanding properties unaffected by a wide temp range from 90 C to 250 C Surface properties have low surface energy, giving good slip, lubricity, release properties, water repellency, excellent adhesion for caulks Good chemical inertness but sensitive to swelling by hydrocarbons Good resistance to oils and solvents, UV radiation, temperature Electrical properties are excellent and stable for insulation and dielectric

24 Silicones Properties Low index of reflection gives silicone contains useful combination of high transmission and low reflectance Can be biologically inert and with low toxicity are well tolerated by body tissue Polymers are normally crosslinked in the vulcanization stage. Four groups Low consistency-room temperature curing resins (RTV) Low consistency-high temperature curing resins (LIM,LSR) High consistency-high temperature curing resins (HTV, HCE), Rigid resins RTV elastomers involve low molecular weight polysiloxanes and rely on reactive end groups for crosslinking at room temperature. One component, or one part, packages rely on atmospheric moisture for curing and are used for thin parts or coatings Two component systems have a catalyst and require a mixing stage and result in a small exotherm where heat is given off.

25 Silicones Properties LSR elastomers involve low molecular weight polysiloxanes but a different curing system Relatively high temperature (150 C) for a faster cure (10-30s) Mixed system is largely unreactive at room temp (long pot life) Suitable for high speed liquid injection molding of small parts. HTV elastomers contain unsaturations that are suitable for conventional rubber processing. Heat curable elastomers (HCE) are cross linked through high temperature vulcanization (HTV) with the use of peroxides. Rigid silicones are cross linked into tight networks. Non-crosslinked systems are stable only in solutions that are limited to paints, varnishes, coatings, and matrices for laminates Cross-linking takes place when the solvent evaporates. Post curing is recommended to complete reaction, e.g., silicone-epoxy systems for electrical encapsulation

26 Silicones Applications Most applications involve elastomeric form. Flexibility and hardness can be adjusted over a wide range Electrical applications high voltage and high or low temperatures Power cable insulation, high voltage leads and insulator boots, ignition cables, spark plug boots, etc.. Semi-conductors are encapsulated in silicone resins for potting. Mechanical applications requiring low and high-temperature flexibility and chemical inertness O-rings, gaskets, seals for aircraft doors and windows, freezers, ovens, and appliances, diaphragms flapper valves, protective boots and bellows. Casting molds and patterns for polyurethane, polyester, or epoxy Sealants and caulking agents Shock absorbers and vibration damping characteristics Silly-Putty : Non-crosslinked, high molecular weight PDMS-based compound modified with fillers and plasticizers. Biomedical field for biological inertness include prosthetic devices

27 Miscellaneous Other Elastomers Acrylic Rubber (AR) Polyethylacrylate (PEA) copolymerized with a small amount (5%) of 2-chloro-ethyl-vinyl-ether CEVE, which is a cure site. The Tg of PEA is about -27 C and acrylic rubber is not suitable for low temperature applications. Polybutylacrylate (PBR) has a Tg of -45 C. Applications Resistant to high temperatures, lubricating oils, including sulfur-bearing oils. Include seals, gaskets, and hoses. Epichlorohydrin Rubber (ECHR) Polymerization of epichlorohydrin with a repeat unit of PECH. Excellent resistant to oils, fuels and flame resistance. (Cl presence) Copolymer with flexible ethyleneoxide (EO) provides Tg = -40C Applications include seals, gaskets, diaphragms, wire covers

28 Miscellaneous Other Elastomers Polysulfide Rubbers (SR) One of the first synthetic rubbers. Tg =-27 C, PES Thiokol A Consists of adjacent ethylene and sulfide units giving a stiff chain. Flexibility is increased with addition of ethylene oxide for polyethylene-ether-sulfide (PEES), Thiokol B Mechanical properties are not very good, but are used for outstanding resistance to many oils, solvents and weathering. Applications include caulking, mastics, and putty. Propylene rubber (PROR) Does not crystallize in its atactic form and has a low Tg = -72 C. Has excellent dynamic properties

29 Miscellaneous Other Elastomers Polynorborene (PNB) Norborene polymerizes into highly molecular weight PNB. Tg = 35 C but can be plasticized with oils and vulcanized into an elastomer with lower Tg = -65 C. Excellent damping properties that can be adjusted. Polyorgano-phosphazenes (PPZ) Form an example of a new class of polymeric materials involving inorganic chains. Atoms of Nitrogen (azo) and Phosphorous form, the chain and a variety of organic side groups, R1 and R2 can be attached to the phosphorous atom. Side groups include halo (Cl or F), amino (NH2 or NHR), alkoxy (methoxy, ethoxy, etc.) and fluoroalkoxy groups. High molecular weight is flexible with a low Tg Excellent inherent fire resistance, weatherability, and water & oil repellency Applications coatings, fibers, and biomedical materials

30 Characteristics Commercial Elastomers Name Chemical Name Vucanization agent Natural rubber cis polyisoprene sulfur Polyisoprene cis polyisoprene sulfur Polybutadiene Polybutadiene sulfur SBR Polybutadiene-styrene sulfur Nitrile Polybutadiene-acrylonitrile sulfur Butyl Poly isobutylene-isoprene sulfur EPR (EPDM) Poly ethylene propylene- diene Peroxides or sulfur Neoprene Polychloroprene MgO Silicone Polydimethylsiloxane peroxides Thiokol Polyslkylenesulfide ZnO Urethanes Polyester or polyether urethanes Diisocycanates

31 Polymerization Methods 4 Methods to produce polymers Some polymers have been produced by all four methods PE, PP and PVC are can be produced by several of these methods The choice of method depends upon the final polymer form, the intrinsic polymer arrangement (isotactic, atactic, etc), and the yield and throughput of the polymer desired. Bulk Polymerization Solution Polymerization Suspension Polymerization Emulsion Polymerization

32 Manufacturing of Emulsion SBR Free-radical emulsion process Developed in 1930s and still in use Typical process (Figure) Soap stabilized water emulsion of two monomers is converted in a train of 10 continuous reactors (4000 gallons each) Water, butadiene, styrene, soaps, initiators, buffers, and modifier are fed continuously Temp is 5 to 10 C and conversion proceeds until 60% of the reactants have polymerized in the last reactor. Shortstop is added in the emulsion to stop the conversion at 60% Unreacted butadiene is flashed off with steam and recycled Unreacted styrene is stripped off in a distillation column that separates liquid rubber emulsion from the gas styrene. Rubber is recovered from the latex in a series of operations. Introduction of antioxidants, blending with oils, dilution with brine, coagulation, dewatering, drying, and packaging the rubber

33 Polymerization of Elastomers Butadiene-Acrylonitrile (Nitrile) Rubber Produced by emulsion polymerization Nitrile rubbers have nitrile contents from 10 to 40%. Chloroprene rubber Produced by emulsion polymerization Produced as a homopolymer that has a high trans 1,4 chain structure and is susceptible to straininduced crystallization, much like natural rubber. Leads to high tensile strength Does not lead itself to copolymerization

34 Polymerization of Elastomers Butyl Rubber- Only important commercial rubber prepared by cationic polymerization Processes with AlCl 3 at 98 to 90 C Copolymer of isobutene and isoprene with isoprene used in 1.5 % quantities The isoprene is introduced to provide sufficient unsaturations for sulfur vulcanization. MW is in the range of 300,000 to 500,000

35 Processing of Elastomers Rubber Products 50% of all rubber produced goes into automobile tires; 50% goes into mechanical parts such as mountings, gaskets, belts, and hoses, as well as consumer products such as shoes, clothing, furniture, and toys Elastomers and Rubbers Thermoset rubbers Compounding the ingredients in recipe into the raw rubber with a mill, calender, or Banbury (internal) mixer Compression molding of tires Thermoplastic elastomers Compression molding, extrusion, injection molding, casting.

36 Polymer Length Polymer Length Polymer notation represents the repeating group Example, -[A]- n where A is the repeating monomer and n represents the number of repeating units. Molecular Weight Way to measure the average chain length of the polymer Defined as sum of the atomic weights of each of the atoms in the molecule. Example, Water (H 2 O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole Methane CH 4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole Polyethylene -(C 2 H 4 ) = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole

37 Molecular Weight Average Molecular Weight Polymers are made up of many molecular weights or a distribution of chain lengths. The polymer is comprised of a bag of worms of the same repeating unit, ethylene (C 2 H 4 ) with different lengths; some longer than others. Example, Polyethylene -(C 2 H 4 ) has some chains (worms) with 1001 repeating ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units. The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole.

38 Molecular Weight Average Molecular Weight Distribution of values is useful statistical way to characterize polymers. For example, Value could be the heights of students in a room. Distribution is determined by counting the number of students in the class of each height. The distribution can be visualized by plotting the number of students on the x-axis and the various heights on the y-axis. Histogram of Heights of Students Frequency Height, inches Series1

39 Molecular Weight Molecular Weight Distribution Count the number of molecules of each molecular weight The molecular weights are counted in values or groups that have similar lengths, e.g., between 100,000 and 110,000 For example, Group the heights of students between 65 and 70 inches in one group, 70 to 75 inches in another group, 75 and 80 inches in another group. The groups are on the x-axis and the frequency on the y-axis. The counting cells are rectangles with the width the spread of the cells and the height is the frequency or number of molecules Figure 3.1 A curve is drawn representing the overall shape of the plot by connecting the tops of each of the cells at their midpoints. The curve is called the Molecular Weight Distribution (MWD)

40 Molecular Weight Average Molecular Weight Determined by summing the weights of all of the chains and then dividing by the total number of chains. Average molecular weight is an important method of characterizing polymers. 3 ways to represent Average molecular weight Number average molecular weight Weight average molecular weight Z-average molecular weight

41 Gel Permeation Chromatography GPC Used to measure Molecular Weights form of size-exclusion chromatography smallest molecules pass through bead pores, resulting in a relatively long flow path largest molecules flow around beads, resulting in a relatively short flow path chromatogram obtained shows intensity vs. elution volume correct pore sizes and solvent critical

42 Gel Permeation Chromatography

43 M n = N i N M i i = Number Average Molecular Weight, M n N 1 M 1 N + 1 N + 2 N M N N 3 3 M where M i is the molecular weight of that species (on the x-axis) where N i is the number of molecules of a particular molecular species I (on the y-axis). Number Average Molecular Weight gives the same weight to all polymer lengths, long and short. Example, What is the molecular weight of a polymer sample in which the polymers molecules are divided into 5 categories. Group Frequency 50, , , , ,000 1 N im i N1M 1 + N 2M 2 + N 3M M n = = N i N1 + N 2 + N (50K) + 4(100 K) + 5(200K ) + 3(500K ) + 1(700 K) M n = ( ) M n = 260,000

44 Molecular Weight Number Average Molecular Weight. The data yields a nonsymmetrical curve (common) The curve is skewed with a tail towards the high MW The Mn is determined experimentally by analyzing the number of end groups (which permit the determination of the number of chains) The number of repeating units, n, can be found by the ratio of the Mn and the molecualr weight of the repeating unit, M 0, for example for polyethylene, M 0 = 28 g/mole The number of repeating units, n, is often called the degree of polymerization, DP. DP relates the amount of monomer that has been converted to polymer. n = M n M 0

45 Weight Average Molecular Weight, M w M w = N N i i M M 2 i i = N 1 N M 1 M N N 2 2 M M N N 3 3 M M Weight Average Molecular Weight, M w Favors large molecules versus small ones Useful for understanding polymer properties that relate to the weight of the polymer, e.g., penetration through a membrane or light scattering. Example, Same data as before would give a higher value for the Molecular Weight. Or, M w = 420,000 g/mole

46 Z- Average Molecular Weight Emphasizes large molecules even more than Mw Useful for some calculations involving mechanical properties. Method uses a centrifuge to separate the polymer = = M N M N M N M N M N M N N M N M M i i i i z

47 Molecular Weight Distribution Molecular Weight Distribution represents the frequency of the polymer lengths The frequency can be Narrow or Broad. Narrow distribution represents polymers of about the same length. Broad distribution represents polymers with varying lengths MW distribution is controlled by the conditions during polymerization MW distributions can be symmetrical or skewed.

48 Physical and Mechanical Property Implications of MW and MWD Higher MW increases Tensile Strength, impact toughness, creep resistance, and melting temperature. Due to entanglement, which is wrapping of polymer chains around each other. Higher MW implies higher entanglement which yields higher mechanical properties. Entanglement results in similar forces as secondary or hydrogen bonding, which require lower energy to break than crosslinks.

49 Physical and Mechanical Property Implications of MW and MWD Higher MW increases tensile strength Resistance to an applied load pulling in opposite directions Tension forces cause the polymers to align and reduce the number of entanglements. If the polymer has many entanglements, the force would be greater. Broader MW Distribution decreases tensile strength Broad MW distribution represents polymer with many shorter molecules which are not as entangled and slide easily. Higher MW increases impact strength Impact toughness or impact strength are increased with longer polymer chains because the energy is transmitted down chain. Broader MW Distribution decreases impact strength Shorter chains do not transmit as much energy during impact

50 Thermal Property Implications of MW & MWD Higher MW increases Melting Point Melting point is a measure of the amount of energy necessary to have molecules slide freely past one another. If the polymer has many entanglements, the energy required would be greater. Low molecular weights reduce melting point and increase ease of processing. Broader MW Distribution decreases Melting Point Broad MW distribution represents polymer with many shorter molecules which are not as entangled and melt sooner. Broad MW distribution yields an easier processed polymer Mechanical Properties Melting Point * Decomposition MW MW

51 Example of High Molecular Weight Ultra High Molecular Weight Polyethylene (UHWMPE) Modifying the MWD of Polyethylene yields a polymer with Extremely long polymer chains with narrow distribution Excellent strength Excellent toughness and high melting point. Material works well in injection molding (though high melt T) Does not work well in extrusion or blow molding, which require high melt strength. Melt temperature range is narrow and tough to process. Properties improved if lower MW polyethylene Acts as a low-melting lubricant Provides bimodal distributions, Figure 3.5 Provides a hybrid material with hybrid properties