Lecture33-Notes - ME 382 Lecture 33 POLYMERS Polymers...

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ME 382 Lecture 33 1 P OLYMERS Polymers consist of long-chain C-C molecules (up to 10 5 atoms / chain) Atoms within a long-chain molecule are covalently bonded Chains consist of a repeated representative unit (“ mer ”) Degree of polymerization = number of repeated units ( molecular weight) Properties of polymers depend on molecular weight & bonding between chains Commercial polymers have a range of molecular weights No unique set of properties Accurate design data for design must be measured or obtained from supplier Unlike metals, no standards or well-defined properties that can be looked up Data-book values are approximate (even for modulus) General properties: (i) ρ 1000 kg/m 3 ; E 1-8 GPa (down to 1 MPa for an elastomer) (ii) Design often stiffness limited (because of low modulus) (iii) Properties sensitive to temperature (around room temperature) and time (iv) Polymers are very sensitive to u/v radiation and O 3 (become stiff & brittle) Three types of polymers determined by nature of bonding between chains (i) Covalent cross links (ii) Secondary bonding (Hydrogen or van der Waals) (iii) Chain entanglements Thermoplastics No covalent cross-links between chains Below glass transition temperature, T g , there is secondary bonding between chains Hydrogen or van der Waals
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ME 382 Lecture 33 2 Above T g secondary bonds melt leaving molecules free to slide over each other
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Lecture33-Notes - ME 382 Lecture 33 POLYMERS Polymers...

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