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Polymers are the most studied viscoelastic material. The shorter times (high frequencies) corresponding to lower temperatures and long times (low frequencies) to high temperatures. This represents the quantitatively applied time-temperature superposition (TTS). Use of the Williams-Landel-Ferry (WLF) equation and the time-temperature superposition principle enables prediction and modeling of the mechanical and rheological properties of polymeric systems outside the timescale of the experiment. Application and validity of the TTS principle and WLF equation in polymer blends, composites, biopolymers and polymer networks will be discussed in this review.

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A thermoplastic pultrusion process was examined using commercial fiber roving of PET/E glass, to determine the optimum pulling speed and optimal zonal temperatures. Finite element analysis predicted heat transfer through the commingled fibers and air in the pultrusion die. The cross-section of obtained rods was examined, and image analysis was carried out to obtain information about the degree of fiber impregnation, number of voids and uniformity of fiber distribution. Optimizing the temperature field for the pultrusion of poly (ethylene terephthalate) is of significant importance. The pulling speed has the same importance. These two parameters are closely related as evidenced by the analysis of images.