what is the glass transition temperature?

can anyone tell me what physical characteristics of a polymer changes at the glass temperature? Except for the ones related to the elasticity modulus..

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  • 1 decade ago
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    Polymers generally consist two distinct regions - crystalline and amorphous. The glass transition temperature Tg is a property of the amorphous region only. Below Tg the polymer is hard and glass-like above it is rubbery. The transition is a so called second order as it involves no enthalpy (unlike melting) it is characterised by significant molecular motion in the amorphous regions. At Tg there is generally an increase in volume of the polymer and a change in heat capacity (that can be detected by calorimetry).

  • 1 decade ago

    (m)

    glass transition temperature

    A material’s glass transition temperature, Tg, is the temperature below which molecules have little relative mobility. Tg is usually applicable to wholly or partially amorphous phases such as glasses and plastics. For inorganic or mineral glasses, such as common silicon dioxide (SiO2) glass, it is the mid-point of a temperature range in which they gradually become more viscous and change from being liquid to solid. Thermoplastic (non-crosslinked) polymers are more complex because, in addition to a melting temperature, Tm, above which all their crystalline structure disappears, such plastics have a second, lower Tg below which they become rigid and brittle, and can crack and shatter under stress. Small molecular weight pure substances such as water have just one such condensed-phase temperature, below which they are solid crystals (or amorphous ice if cooled below Tg fast enough) and above which they are liquids.

    Above Tg, the secondary, non-covalent bonds between the polymer chains become weak in comparison to thermal motion, and the polymer becomes rubbery and capable of elastic or plastic deformation without fracture. This behavior is one of the things which make most plastics useful. But such behavior is not exhibited by crosslinked thermosetting plastics which, once cured, are set for life and will shatter rather than deform, never becoming plastic again when heated, nor melting.

  • 1 decade ago

    A material’s glass transition temperature, Tg, is the temperature below which molecules have little relative mobility. Tg is usually applicable to wholly or partially amorphous phases such as glasses and plastics. For inorganic or mineral glasses, such as common silicon dioxide (SiO2) glass, it is the mid-point of a temperature range in which they gradually become more viscous and change from being liquid to solid. Thermoplastic (non-crosslinked) polymers are more complex because, in addition to a melting temperature, Tm, above which all their crystalline structure disappears, such plastics have a second, lower Tg below which they become rigid and brittle, and can crack and shatter under stress. Small molecular weight pure substances such as water have just one such condensed-phase temperature, below which they are solid crystals (or amorphous ice if cooled below Tg fast enough) and above which they are liquids.

  • 4 years ago

    Define Glass Transition Temperature

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