Effect of cross‐link density on the morphology, thermal and mechanical properties of flexible molded polyurea/urethane foams and films

Abstract

AbstractThe effect of cross‐link density on the morphology and properties of two flexible molded foam samples was studied. Film samples based on the same foam formulations were also fabricated to study the feasibility of using them for the characterization of complex foam products. Fourier transform infrared spectroscopy (FTIR) and small angle X‐ray scattering (SAXS) data show that films and foam samples have entirely different hard domain ordering. The results of the study of morphology indicate that an increase in cross‐link density appears to increases phase mixing in film and foam samples. Differential scanning calorimetry (DSC) studies indicate that the soft segment glass transition temperature (Tg) is independent of cross‐link density (at levels studied). But for both film and foam samples, morphology clearly dicates the manner in which moisture interacts with the hard domains. Results of the stress‐strain behavior indicate that an increase in cross‐link density increases the modulus and decreases the elongation at break. Mooney‐Rivilin modeling of the stress‐elongation behavior of film shows that the higher cross‐link density sample gives more nonaffine behavior, possibly due to a heterogeneous distribution of hard domains. Similar modeling of the foams was not possible because of their linear stress response to surprisingly high elongation. The results of the power law modeling of stress relaxation response indicates that with an increase in cross‐link density (covalent and virtual), the power law exponent decreases as expected. At levels of cross‐linking and hard segment content studied, stroke‐controlled equilibrium hysteresis was independent of cross‐link density. Normalized dynamic mechanical spectra (DMS) show that the film samples have higher rubbery plateau modulus. The magnitude of the area under the tan δ curve at Tg indicates greater flexibility of polymer segments in foam sample. Structure‐property relationships of cellular materials can be established by characterizing film samples because a parallel trend exists between each group. © 1994 John Wiley & Sons, Inc.