Low temperature transitions in the three series of polyurethanes (PPG-HMDI, -XDI, -TDI) crosslinked with different glycols (EG, PG, TMG, 1, 3-BD, 1, 4-BD, PD, NPG, HMG) were investigated by measuring dynamic mechanical properties over a temperature range of -150 to 20°C and at 110Hz.
The three relaxations (α, β, and γ-relaxation) were observed for all samples. The α-relaxation was associated with micro-Brownian motions of polymer backbones. By introducing benzene rings into the polymer backbones and by increasing reactant ratio K (NCO mole/OH mole of PPG), the temperature of loss modulus peak, T_α, shifted to higher temperatures. Structure of glycol as curing agents had a little effect on T_α because of its small quantities. The contribution of urethane groups to T_g was slightly larger than that of allophanate groups. However, the contribution of urethane groups to T_α nearly equaled to that of allophanate groups. The β-relaxation (-50-100°C) was associated with the motions of crosslinking sites and/or aggregated urethane segments. The γ-relaxation (-100-150°C) was associated with a complex relaxation including motion of methylene chains, local motion of polyether, and the rotation of methyl side groups.

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The interchain interaction energy is introduced into the existing theories of rubbe elasticity with the most general form which may be represented by the second-order theory of Wang and Guth. The theory is expected to represent the elastic properties of amorphous or little crystalline polymers in rubbery or leathery or hard plastic states, and to include the major theories of rubber elasticity based on the network models. The constitutive equations of the 0th-order to 2nd-order approximations thus derived are similar in shape to those from all of main network theories and are so general as to take the places of them.
It is shown that each expression can represent continuous change in stress-strain relations from those in rubbery state to those in hard plastic state with increase in the interchain cohesive energy.

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