Test 3 Notes.docx - Polymers Polymer material whose molecules contain a very large number of atoms linked by covalent bonds which makes polymers

Test 3 Notes.docx - Polymers Polymer material whose...

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Polymers - Polymer: material whose molecules contain a very large number of atoms linked by covalent bonds, which makes polymers macromolecules - Monomer: unit forming the repetitive pattern of a polymer - Synthetic polymers: based on a chain of carbon atoms; manufactured from petroleum hydrocarbons o Alkanes (paraffins): saturated hydrocarbons; single bonds between carbons o Alkenes (olefins): unsaturated hydrocarbons; contain at least one carbon-carbon double bond; chemically reactive o Alkynes: unsaturated hydrocarbons; at least one carbon-carbon triple bond; most chemically reactive; ring shaped o Isomerism: different atomic arrangements for the same composition of hydrocarbon compound Stereoisomerism: atoms linked together in same order but differ in spatial arrangement Geometrical: cis and trans isomers - Polymer molecular structures o Non branching- linear o Branching- when atom attached to main linear chain is removed and replaced by another chain; interferes with movement allowing less deformation LDPE: branching HDPE: non branching o Crosslinking- linked by covalent bonds to form more rigid 3-d structure - Microstructure effects on behavior o Higher degree of crystallization higher strength higher temperature resistance Depends on rate of cooling, chain configuration, molecular chemistry - Polymerization: process of creating long chain hydrocarbons by linking repeated basic unit hydrocarbons o Linear addition: monomers bonded end to end to form exact multiple of original monomer; initiation propagation termination o Linear condensation: reaction of 2+ monomer species; can form long chains o Degree of polymerization: average length to which a chain grows = molecular weight of polymer / molecular weight of mer Multiple: average molecular weight of mer: M = sigma (molecular fraction of monomer * molecular weight of monomer) - Categories: o Thermoplastics: can be softened and reformed at moderately elevated temps; not cross linked; PVC Ductile and brittle stress-strain behavior o Thermosets: cannot be reformed after set (will burn); cross linked Brittle
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o Elastomers: natural or synthetic; coiled in stress free state; a lot of elastic deformation and will theoretically completely recover; rubber bands Low E and non-linear stress/strain behavior o Rigid rod polymers: cannot freely rotate due to multiple bonding along backbone - Crazing: brittle material below glass transition temperature and like a rubbery material near the glass transition temperature o Crazes: localized areas of yielding that form at highly stressed regions; fibrils and small voids form; propagate perpendicular to applied tensile stress; can support loads - High crystallinity: high stress, low strain - Slower load rate = lower strength values - Faster strain rate = more brittle behavior - Temperature o @ T > Tmelt = liquid material o @ T < Tglass transition = glassy, brittle material o
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