1.1 Stress-Strain Curves 1.2 Definition and Classes

1 Stress-Strain CurvesElastomersPlasticsFibersModulus (psi)15 - 1501,500 - 200,000150,000 - 1,500,000 Percent Elongation100 - 100020 - 100< 10 CrystallinityLowModerateHighPolymer exampleNatural Definition and Classes Plastic (thermoplastic) Any material which undergoes a permanent change of shape (plastic deformation) when strained bdtiit(ild it)beyond a certain point (yield point) Plastics can be identified and characterized by the shape of their Stress-Strain Curves Hard-tough Hard-strongg Soft-tough Hard-brittle Definition and Classes Hard = high modulus (steep slope) Tough= high elongation before break, large area under Stress-Strain Hard and Tough High density polyethylene, HDPE Fairly high crystallinity (Tm = 135 oC, Tg= -90 oC) Polypropylene PP Polypropylene, PP Tm= 175 oC, Tg= -18 oC Slightly harder, higher tensile modulus than HDPE.

2 Why? Poly(ethylene terephthalate), PETy(yp) Tm= 265 oC, Tg= 70 oC Stiffer, higher tensile modulus than HDPE. Why? Typical applications Definition and Classes Hard= high modulus (steep slope) Strong= moderate elongation and high Hard and Strong Poly(vinyl chloride), PVC Tm= 212 oC, Tg= 85 oC Would you expect this polymer to be brittle or pliable at 0 oC? How can you change the flexibility of PVC? Typical Definition and Classes Soft= low modulus (shallow slope) Tough= high Soft and Tough Low-Density Polyethylene, LDPE Tm= 115 oC, highly branched, lower crystallinity Linear Low-Density Polyethylene, LLDPE Moderate degree of branching More crystalline than LDPE?

3 Higher tensile strength (stronger) than LDPE Higher tensile strength (stronger) than LDPE Typical applications The Three PE The Three PE s Stress-Strain Curves Which is which? Definition and Classes Hard= high modulus (steep slope) Brittle= low Hard and Brittle Polystyrene, PS Properties Tm= 240 oC, Tg= 100 oCg Typical applications Requirements for Fibers High tensile strength/modulus (tenacity) Relatively low elongation High melting point (esp. for clothing)ggp(pg) Tm> 200 oC (iron without damage) but < 300 oCto enable spinning from melt Tg< 100 oC so fibers soften when ironed at 150 oC Creases removedPlT(oC)T(oC)PolymerTg (oC)Tm(oC)PET70265 Nylon 6, Spinning of Requirements for Fibers Symmetrical, unbranched polymer High crystallinity promotes linear molecular alignment this is critical Strong intermolecular forces High tenacity, fiber strength, low elongationCHOCNHOCOOCHCN hydrogen bonding:dipole-dipole.

4 - + -+HOHCHOHNCOOCOOCNNCCH cellulosicsproteins, nylonspolyestersacrylics - + + - + Requirements for Fibers Rigid backbone structures Aramids(aromatic CH2CH2CH2CH2 NHCCH2 OCH2CH2CH2 CCH2CH2 NHOn(polyamides) high stiffness heat resistance Compete with wire and glass fibers Form linear,rodlikeNHNHCCH2CH2CH2CH2 OCOnNHNH Form linear, rodlikepolymersCOCnONHNH COCnO Important Fibers NylonNH(CH2)5 COnnylon 6CO(CH2)4 CONH(CH2)6 NHnnylon 6,6 Fiber strength achieved Properties Dyeable, somewhat hydrophilic (absorbs water) Typical Important Fibers PolyesterCOCOOCH2CH2On Fiber strength achieved Properties Choice of diacid (o, m, p) can vary tensile and elongation propertiesproperties Relatively hydrophobic Typical Important Fibers Polypropylene Fiber strength achievedby(CH2CH)nCH3 Fiber strength achieved Properties Low density ( g/cc))

5 Yields lightweight fiber Chemical and abrasion resistance Hydrophobic Typical Properties of Elastomers Requirements for elastomers Completely amorphous, used above their Tg Low intermolecular forces allow for flexibility Large reversible extensions (several hundred percent) High localized chain movements, low overall movement of chains relative to one another Cross-linked chains can help prevent slipslipstressappliedStressremoved Properties of Elastomers Possess high molar mass to allow for chain entanglements or are cross-linked Cross-linked with sulfur Reaction at allylic hydrogensCH3 CCHSxSCH2CH2 CCHCH2nCH2 CCCH2CH2CH2 CCHCH2 Synny = 1 or Important Elastomers Natural Rubber Cis-1.

6 4-polyisopreneCH3 HBiological polymerizationAgricultural cropTg= -70 oCPiit ti PolybutadieneCCCH2)n(CH2 Primary use is auto tiresFree-radical polymerizationTg= -110 oCPrimary use is auto tiresHHyAlso belts, hoses, gasketsHCCHCH2)n(CH2 Why is the Tg40 oC lower than that of natural rubber? Fiber or ElastomerFiber PropertiesCelluloseTenacity (g/denier)Tensile Strength (kg/cm2)Elongation (%)Polymer High-tenacity Cellulose Cellulose 6645604000 600026 Nylon 6, (acrylic) (Spandex) (polyolefin)