2013-Mechanical Properties Polymer(1)

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    Mechanical Properties

    Outline:

    Mechanical properties

    Stress-Strain Behavior

    Crystallization , Melting, Glass Transition

    Thermoplastic and Thermosetting Polymers

    Viscoelasticity

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    e orma on an as omers

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    Mechanical Properties

    Mechanical Properties

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    Mechanical Properties

    Mechanical Properties

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    .

    A polyethylene sample necking undertension.

    Mechanical Properties

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    Mechanical Properties

    Toughness

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    Mechanical Properties

    Stress Strain Behavior (III)

    Moduli of elasticity Polymers: ~ 10 MPa - 4 GPa Metals: ~ 50 - 400 GPa

    Tensile strengths Polymers: ~ 10 - 100 MPa Metals: 100s - 1000s MPa

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    onga on Polymers: up to 1000 % in some cases Metals: < 100%

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    Tensile Response: Brittle & Plastic

    brittle failure

    lastic failure

    (MPa)x

    fibrillar

    structure

    nearfailureonset of

    necking

    Near Failure

    xInitial

    aligned, networked

    unload/reload

    regionsslide

    crystallineregions align

    semi-crystalline

    case

    cross-linkedcase

    case

    amorphousregions

    elongateStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted fromFigs. 15.12 & 15.13, Callister 7e . (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science , Prentice-Hall, Inc., 1974, pp. 500-501.)

    Stress-strain curvesadapted from Fig. 15.1,Callister 7e. Insetfigures along elastomer

    Tensile Response: Elastomer Case

    (MPa)brittle failurex

    curve (green) adaptedfrom Fig. 15.15, Callister7e . (Fig. 15.15 is fromZ.D. Jastrzebski, TheNature and Properties ofEngineering Materials ,3rd ed., John Wiley andSons, 1987.)

    x

    final: chainsare straight,

    stillcross-linked

    elastomer

    plastic failurex

    Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)

    initial: amorphous chains arekinked, cross-linked.

    Deformationis reversible!

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    Stress-Strain Test

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    machine

    Tensile Test

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    Important Mechanical Propertiesfrom a Tensile Test

    Young's Modulus: This is the slope of the linearportion of the stress-strain curve, it is usuallyspec c to eac mater a ; a constant, nown va ue.

    Yield Strength: This is the value of stress at theyield point, calculated by plotting young's modulusat a specified percent of offset (usually offset =0.2%).

    Ultimate Tensile Strength: This is the highest

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    value of stress on the stress-strain curve.

    Percent Elongation: This is the change in gaugelength divided by the original gauge length.

    Melting vs. Glass Transition Temp.What factors affect Tm

    and Tg? Both T and T increase with

    increasing chain stiffness

    Chain stiffness increased by1. Bulky sidegroups2. Polar groups or

    sidegroups3. Double bonds or aromatic

    c a n groups

    Regularity (tacticity) affectsTm only

    Adapted from Fig. 15.18,Callister 7e .

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    The Glass Transition

    The Glass Transition

    The glass transition is a property of onl y the amorphous portion of asemi-crystalline sol id 34 . The crystalline portion remains crystalline duringthe glass transition.

    a ow empera ure e amorp ous reg ons o a po ymer are n e g assystate . In this state the molecules are frozen on place. They may be able tovibrate slightly, but do not have any segmental motion in which portions ofthe molecule wiggle around. In the glassy state, the motion of the redmolecule in the schematic diagram at the right would NOT occur. When theamorphous regions of a polymer are in the glassy state, it generally will behard, rigid, and brittle.

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    The Glass Transition

    If the polymer is heated it eventually will reach its glass transitiontemperature . At this temperature portions of the molecules can start towiggle around as is illustrated by the red molecule in the diagram above.

    .and flexibility to a polymer.You may have experienced the glass transition of chewing gum. At bodytemperature the gum is soft and pliable, which is characteristic of anamorphous solid in the rubbery state. If you put a cold drink in your mouthor hold an ice cube on the gum, it becomes hard and rigid. The glasstransition temperature of the gum is somewhere between 0 oC and 37 oC.

    The Glass Transition

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    The Glass Transition

    The Glass Transition

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    Factors Influencing T g

    Factors Influencing T g

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    Factors Influencing T g

    Stronger intermolecular forces lead to a higher T g. PVC has

    Intermolecular Forces

    of the dipole-dipole forces from the C-Cl bond.

    Factors Influencing T gPendant Groups

    1. Bulky pendant groups , such as a benzene ring, cancatch on neighboring chains like a "fish hook" and restrict

    . g.

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    Factors Influencing T g2. Flexible pendant groups , such as aliphaticchains, tend to limit how close chains can pack.

    This increases rotational motion and lowers T .

    Factors Influencing T g

    The presence of cross-links between chains restricts rotational motion andraises Tg.

    Cross-Linking

    Plasticizers

    Plasticizers are low molecular weight compounds added to plastics to increasetheir flexibility and workability. They weaken the intermolecular forces betweenthe polymer chains and decrease T g. Plasticizers often are added to semi-crystalline polymers to lower the value of T g below room temperature. In thiscase the amorphous phase of the polymer will be rubbery at normaltemperatures, reducing the brittleness of the material.Plasticizers are added to the lastic used for automobile u holster . In older automobiles, the plasticizer may be distilled from the upholstery during hotweather so that it becomes brittle over time.Some plasticizers have been identified as major health and environmentalproblems. Before 1977, Polychlorinated biphenyls (PCBs) were used asplasticizers in paints and plastics. Because they are toxic and possibleendocrine disruptors, PCBs no longer are used.

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    Crystallization, Melting, Glass Transition

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    Dependence of melting and glass transitiontemperatures and polymer properties on molecular weight

    Thermoplastics:-- little crosslinking-- ductile

    Thermoplastics vs. Thermosets

    Callister,Fig. 16.9

    T

    Tmmobileviscous

    liquidrubber

    -- soften w/heating-- polyethylene

    polypropylenepolycarbonatepolystyrene

    Thermosets:i i i ill

    Molecular weight

    Tgliquid oug

    plastic

    partiallycrystallinesolidcrystallinesolid

    -- large crosslinking(10 to 50% of mers)

    -- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,

    polyester resin, phenolic resin

    i . . , . i . . i . . ill ,Jr., Textbook of Polymer Science , 3rd ed., John Wiley and Sons, Inc.,1984.)

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    Polymer Additives

    Improve mechanical properties, processability,durability, etc.

    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 Tg 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

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    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 mo e prepo ymer en a ow ur er reac on ex: urethane, epoxy

    Processing Plastics - Molding Compression and transfer mold ing

    thermoplastic or thermoset

    Adapted from Fig. 15.23,Callister 7e . (Fig. 15.23 is fromF.W. Billmeyer, Jr., Textbook ofPolymer Science, 3rd ed.,John Wiley & Sons, 1984. )

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    Processing Plastics - Molding

    Injection molding

    Adapted from Fig. 15.24,Callister 7e . (Fig. 15.24 is fromF.W. Billmeyer, Jr., Textbook ofPolymer Science, 2nd edition,John Wiley & Sons, 1971. )

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    Processing Plastics Extrusion

    Adapted from Fig. 15.25,Callister 7e . (Fig. 15.25 is fromEncyclopdia Britannica, 1997.)

    Blown-Film Extrusion

    Adapted from Fig. 15.26, Callister 7e .(Fig. 15.26 is from EncyclopdiaBritannica, 1997.)