Relaxation Transitions and Viscoelasticity of IPNs

The of the most powerful methods in the investigation of heterogeneous polymeric systems is dynamic mechanical spectroscopy (DMS), which enables the estimation of the elastic moduli, mechanical losses, glass transition temperature, relaxation characteristics, etc., and plays a very important role both in the theoretical description of the systems and in their practical application. 

Temperature dependencies of loss modulus G and tan S usually show sharp maxima at the glass transition temperatures that correspond to the cooperative movement of the segments of polymer chains. Low-temperature maxima are ascribed to the movements of short fragments of the main chain or of the side groups. The equilibrium elastic modulus oo could be an important characteristic of the cross-linking density in IPNs and may be presented as consisting of two parts  

When polymer blends and alloys are considered, a problem arises for the application of the principle of temperature-time superposition to the systems consisting of two and more phases. A variant of this superposition has been proposed for such materials [175]. The main feature of the temperature-time reduction in this case is the dependence of the reduction coefficient on the variables—temperature and time. The expression for the reduction coefficient may be obtained via the Taylor expansion of the relaxation function with respect to variable t and T. Phase-separated IPNs relate to thermorheologically complicated materials, hi principle, in these systems two different mechanisms of relaxation exist, each of them being characterized by its own temperature coefficient. Because of this, the application of traditional temperature-frequency superposition to IPNs is restricted. However, even in those cases when this approach is not entirely vahd, it may be used for approximate calculations. Thus, the apphcation of the temperature-time superposition to heterogeneous polymeric materials shows that the method may be very valuable for prediction of the viscoelastic properties, in spite of the necessity of further developing the theory. 

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