We are currently investigating the glass transition temperature (Tg) of ABA poly(styrene-ó-isoprene

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The structural evolution with temperature of an anionically synthesized ABA poly(styrene‐b‐isoprene) (SIS) lamellar block copolymer (total molecular weight 45,000; isoprene content 38% by weight) was studied by melt‐rheological measurements, electron microscopy, and x‐ray and light diffraction. Above 225°C, the dynamic viscosity was found to be independent of frequency up to a critical frequency. The variation of the elastic modulus confirmed the occurence of a transition between 215 and 225°C. For the temperature range considered, all results superimposed well on a two‐branch master curve. It was concluded that above 225°C, our SIS behaves like a Newtonian material, whereas for lower temperatures and/or higher frequencies classical non‐Newtonian behavior is found. The melt‐rheological properties were explained by microscopy and diffraction investigations, which allowed us to follow morphological changes as the temperature was raised. It was found that the two‐phase lamellar structure is progressively destroyed, and the transition temperature of 225°C corresponds to the temperature above which complete mixing occurs.

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Dynamic light scattering (photon correlation (PCS) and Brillouin spectroscopy (BS)) and dielectric relaxation (DS) techniques are employed to study local segmental motion in an interpenetrating polymer network (IPN) of poly(methyl methacrylate) (PMMA) with 50 wt % polyurethane (PUR) content and the constituent pure PMMA and PUR networks over the temperature (T) range from 140 to 410 K. For the PMMA network, the dielectric loss e"(w) shows a strong secondary (ß) relaxation, whereas dynamic light scattering arises from both ß and primary (a) relaxation with characteristics very similar to the linear PMMA. The PUR network displays a relatively broad «-relaxation process in the PCS and DS experiments. The IPN exhibits two well-separated primary relaxation processes assigned to local regions of mobility distinguished by different degrees of mixing. ß-Relaxation becomes faster in the IPN, indicating a reduced packing density. At high temperatures the fast «-relaxation process dominates the Brillouin spectra, leading to significant hypersonic attenuation. The hypersonic velocity shows an additional temperature kink characteristic of the upper high glass transition temperature due to the hard PMMA phase. The use of complementary techniques allows the dynamic study of the present IPN over a broad temperature and time range.