TEMPERATURE-DEPENDENCE OF VISCOELASTICITY OF POLYCAPROLACTONE CRITICAL GELS

A systematic procedure was developed for determining the temperature dependence of critical gel properties. The polymer system was a poly(epsilon-caprolactone) diol (PCL) end-linked with a three-functional isocyanate. Samples varied in prepolymer molecular weight and stoichiometric ratio. Time-temperature superposition was found to apply to the self-similar relaxation modulus G(t) = St(-n). A prerequisite of time-temperature superposition is that the relaxation exponent n is independent of temperature. As a consequence, critical gels have the unique property that their loss tangent is independent of temperature. This was confirmed experimentally. The gel stiffness S(T) = S(T-0)a(T)(n)/b(T) shifts with an Arrhenius type temperature dependence. The temperature shift factors for the PCL critical gels seem to be identical with those for the viscosity of the difunctional prepolymer (eta(0)) and the modulus of the fully cross-linked polymer (G(e)). This suggests that an experimental relationship of S = G(e)(T)[eta(0)(T)/G(e)(T)](n) is valid for our polymers at the experimental temperatures between 90 and 130 degrees C.