Glass transition temperature

In a composite material, the temperature at which the matrix material changes from a glassy to rubbery state or vice versa is termed the glass transition temperature or . This change in state may result in a two to three orders of magnitude change in modulus. The transition is characteristic of the detailed molecular structure of the matrix polymer. As well as modulus the heat capacity and thermal expansion coefficient also change at the glass transition temperatures. As shown in the schematic diagram, the transition is not sharp but occurs progressively over a temperature range which may be up to some tens of degrees wide. The quoted is usually the mid-point of the range. Different methods of measuring may may give somewhat different values, so the method used should always be quoted.

Polymer matrices will absorb moisture and become plasticised. Plasticisation can reduce the leading to a 'wet' value. The reduction in depends on the initial crosslink density of the matrix material. Measurement of for a composite matrix material requires the specimen to be heated, which in turn, removes the absorbed moisture and hence affects the wet value by increasing it slightly. It is possible to seal the specimens to prevent moisture loss. Also in thick specimens, moisture may be lost on the outside surfaces resulting in a wider glass transition range perhaps with two inflection points representing the inner wet material and the drier outer material.

Polymer matrices in composites are rarely used at temperatures above their because they cannot effectively transfer load in their rubbery state.

The is measured for composites using a number of different pieces of equipment including:

	Differential scanning calorimetry (DSC)
	Thermomechanical analysis (TMA)
	Dynamic mechanical analysis (DMA)