Polystyrene softening behaviors

THERMOPLASTIC. A high polymer that softens when exposed to heat and returns to its original condition when cooled to room temperature. Natural substances that exhibit this behavior are crude rubber and a number of waxes however, the term is usually applied to synthetics such as polyvinyl chloride, nylons, fluorocarbons, linear polyethylene, polyurethane prepolymer, polystyrene, polypropylene, and cellulosic and acrylic resins. See also Plastics. 

All polymers can be divided into two major groups thermoplastics and thermosets) based on their thermal processing behavior. Thermoplastics soften and flow when heated. Upon cooling, thermoplastic polymers harden and assume the shape of the mold. Examples of commercial thermoplastics include polystyrene, polyolefins (e.g. polyethylene and polypropylene), nylon, poly(vinyl) chloride (PVC), and poly (ethylene) terephthalate (PET). Thermoplastics make up 80% of the plastic produced today and these polymers are linear or branched in their structure. 

Both Tg and Tm are important parameters that serve to characterize a given polymer. While Tg sets an upper temperature limit for the use of amorphous thermoplastics like poly(methyl methacrylate) or polystyrene and a lower temperature limit for rubbery behavior of an elastomer-like SBR rubber or 1,4-cw-polybutadiene, Tm or the onset of the melting range determines the upper service temperature for semicrystalline thermoplastics. Between T,n and Tg, these polymers tend to behave as a tough and leathery material. They are generally used at temperatures between Tg and a practical softening temperature that lies above Tg and below Tm-... 

For engineering purposes, the most useful classification of polymers is based on their thermal (thermomechanical) response. Under this scheme, polymers are classified as thermoplastics or thermosets. As the name suggests, thermoplastic polymers soften and flow under the action of heat and pressure. Upon cooling, the polymer hardens and assumes the shape of the mold (container). Thermoplastics, when compounded with appropriate ingredients, can usually withstand several of these heating and cooling cycles without suffering any structural breakdown. This behavior is similar to that of candle wax. Examples of thermoplastic polymers are polyethylene, polystyrene, and nylon. 

Styrene decreases the viscosity of SPS significantly relative to atactic polystyrene and offsets that effect. From a practical standpoint, the dynamic mechanical behavior of SPS reveals that SPS softens appreciably at its glass transition temperature, thus to maintain mechanical strength up to the melting point, the polymer needs to be reinforced. 

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