Polymers - This file is to be used strictly for personal...

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Unformatted text preview: This file is to be used strictly for personal reading and non-commercial teaching purposes. Not for circulation. The file contains images from recent journal papers. Polymers Long chain molecules consisting of a number of small repeating units called mers, Formed by covalent bonding along the backbone Have a partially crystalline structure Have disordered noncrystalline or amorphous region Also contain ordered crystalline regions which contain folded chains Degree of polymerisation (DP) defined as average number of mers per polymer chain Molecular weight of polymer is the product of DP and the MW of the mer units, Higher molecular weight, less chain mobility, higher strength Polymer synthesis Synthesis occurs by condensation polymerisation or addition polymerisation Small molecule such as water is removed during condensation reaction Addition polymerisation: breaking of double bond with an initiator usually a free radical Types of polymers Polymer chains arranged as linear, branched and 3 dimensional crosslinked Branched where there are side chains of certain groups Cross linked where the various chains and branches are linked Copolymers Polymer containing one type of monomer is called homopolymer Copolymers contain two or more types of monomers Random where the order of the monomers does not follow a fixed pattern Alternating where each type alternates with the other regularly Block copolymer where a group of monomer is followed by the other: Can have distinct segments with varying properties Graft where one monomeric type forms a support for the other type Thermal behavior Amorphous rubbery polymers are soft and reversibly extensible Temperature at which all long range segmental motion ceases is the glass transition temperature Tg Polymers below Tg are hard and glassy Above the Tg the polymer is more rubbery Polymers with some crystallinity also show a melting temperature Tm owing to melting of the crystalline phase Polyethylene Homopolymer (-CH2-CH2-)n Made by reacting ethylene gas at high pressure (100-300 MPa) in the presence of peroxide catalyst : Low density polyethylene By using Zigler Natta catalyst, high density polyethylene can be produced at low pressure 10 MPa High density polyethylene does not contain branches as opposed to LDPE HDPE has better packing of chains, increased density and crystallinity Molecular weight of 3X 10 3 for LDPE, 5 X 10 5 for HDPE and 2 X 10 6 for UHMWPE Used for pharmaceutical bottles, catheters, flexible pouches and for acetabular implants UHMWPE in heart valves Tilting disc formed of UHMWPE in Chitra valve More flexible leaflet than the pyrolytic carbon Hip joint replacements UHMWPE used as acetabular cup Avoided with titanium alloy due to wear Low wear with ceramic heads Polymethymethacrylate Acrylic polymer obtained by addition polymerisation of acrylic acid derivatives...
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Polymers - This file is to be used strictly for personal...

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