Classes of Polymeric Materials Elastomers

1

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%

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.

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

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,

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

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

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

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 Hard Hard

Soft Soft

Commercial Elastomers •  Diene C=C double bonds and Related Elastomers

–  Polyisoprene- (C5H8)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.

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.

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.

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.

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

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

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.

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

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

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 = - 130°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

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

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, CH3, 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)

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

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

200-1000 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

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.

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

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

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

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

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

Commercial Elastomers •  Characteristics

Name Chemical Name Vucanization agentNatural rubber cis polyisoprene sulfurPolyisoprene cis polyisoprene sulfurPolybutadiene Polybutadiene sulfurSBR Polybutadiene-styrene sulfurNitrile Polybutadiene-acrylonitrile sulfurButyl Poly isobutylene-isoprene sulfurEPR (EPDM) Poly ethylene propylene- diene Peroxides or sulfurNeoprene Polychloroprene MgOSilicone Polydimethylsiloxane peroxidesThiokol Polyslkylenesulfide ZnOUrethanes Polyester or polyether urethanes Diisocycanates

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

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

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 strain-induced crystallization, much like natural rubber.

•  Leads to high tensile strength – Does not lead itself to copolymerization

Polymerization of Elastomers •  Butyl Rubber-

– Only important commercial rubber prepared by cationic polymerization

•  Processes with AlCl3 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

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.

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 (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/mole –  Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole –  Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 =

28,000 g/mole

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 (C2H4) with different lengths; some longer than others.

•  Example, –  Polyethylene -(C2H4)-1000 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 .

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

0510152025

60 70 80

Height, inches

Freq

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y

Series1

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)

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

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

Gel Permeation Chromatography

Number Average Molecular Weight, Mn

•  where Mi is the molecular weight of that species (on the x-axis) •  where Ni 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 –  100,000 4 –  200,000 5 –  500,000 3 –  700,000 1

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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, M0, for example for polyethylene, M0 = 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.

0MMn n=

Weight Average Molecular Weight, Mw

•  Weight Average Molecular Weight, Mw –  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, Mw = 420,000 g/mole

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– Emphasizes large molecules even more than Mw – Useful for some calculations involving

mechanical properties. – Method uses a centrifuge to separate the polymer

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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.

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.

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

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

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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