Types of Polymers, Branching and branching effect Types of Polymers

Types of Polymers, Branchingand branching effectBy hammad MohsinTypes of Polymers• Polymers can be separated into plasticsand rubbers• As engineering materials, it is appropriateto divide them into the following threecategories:1. Thermoplastic polymers2. Thermosetting polymers3. Elastomerswhere (1) and (2) are plastics and (3) arerubbers1

Thermoplastic Polymers -Thermoplastics (TP)• Solid materials at room temperature butviscous liquids when heated totemperatures of only a few hundreddegrees• This characteristic allows them to be easilyand economically shaped into products• They can be subjected to heating andcooling cycles repeatedly withoutsignificant degradationThermosetting Polymers -Thermosets (TS)• Cannot tolerate repeated heating cyclesas thermoplastics can– When initially heated, they soften and flow formolding– But elevated temperatures also produce achemical reaction that hardens the materialinto an infusible solid– If reheated, thermosets degrade and charrather than soften2

ElastomersPolymers that exhibit extreme elasticextensibility when subjected to relativelylow mechanical stress• Also known as rubber• Some elastomers can be stretched by afactor of 10 and yet completely recover totheir original shape• Although their properties are quite differentfrom thermosets, they share a similarmolecular structure that is different fromthe thermoplasticsExamples of Polymers• Thermoplastics:– Polyethylene, polyvinylchloride,polypropylene, polystyrene, and nylon• Thermosets:– Phenolics, epoxies, and certain polyesters• Elastomers:– Natural rubber (vulcanized)– Synthetic rubbers, which exceed thetonnage of natural rubber3

Linear, Branched, and CrosslinkedPolymers• Linear structure – chain-like structure• Branched structure – chain-like but withside branches• Cross-linked structure– Loosely cross-linked,– Tightly cross-linked,Source: R. Esfand, , D.A. Tomalia, , A.E. Beezer, , J.C. Mitchell, M. Hardy, C. Orford,Polymer Preprints, 41 (2), 1324 (2000)4

Linear structure of a thermoplasticpolymerFigure 8.7 - Various structures of polymer molecules:(a) linear, characteristic of thermoplasticsBranched structure that includes sidebranches along the chainFigure 8.7 - Various structures of polymer molecules: (b) branched5

Effect of Branching onProperties• Thermoplastic polymers always possesslinear or branched structures, or a mixtureof the two• Branches increase entanglement amongthe molecules, which makes the polymer:– Stronger in the solid state– More viscous at a given temperature in theplastic or liquid state7