Hard segments glass transition

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

Philips R.A., McKenna J.M., and Cooper S.L., Glass transition and melting behavior of poly(ether-ester) multihlock copol3miers with poly(ethyleneterephthalate) hard segments, J. Polym. Sci. Part B, 32, 791, 1994. 

Ethylene-butylene, glass transition and melting temperature for soft/hard segments, 7 649t Ethylene-butylene-isobutylene,... 

When chain segments can move relatively freely in cured polymers, it is most likely due to low crosslink density or the mobility of the molecular chain structure. The glass transition temperature is a measure of the mobility of the molecular chains in the polymer network as a function of temperature. The glass transition is the reversible change in a polymer from (or to) a rubbery condition to (or from) a hard and relatively glassy state condition (Fig. 3.14). This transition occurs at a temperature called the glass transition temperature or Tg. It is... 

Table I. Soft-Segment, Glass-Transition-Temperature Dependence on Hard-Segment Content for PTMO/MDI/BD(ET) - and PTMA/MDI/BD (ES) -Segmented Copolymers...

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