(555d) Design of Polymer Blends with Nanoscale Dispersed Phases Using Melt Processing and Solid-State Shear Pulverization

Achievement of nanostructured polymer blends is of interest because such materials may have enhanced properties (optical transparency, toughness, etc.) in comparison to blends with a microscale dispersed phase. Up to the present time, most research in this area has focused on the use of reactive blending or in situ polymerization in the presence of a polymer to achieve such nanoblends. Here we demonstrate two approaches, both starting with polymers and without reaction, that can lead to the development of nanostructured blends.

In the first case, we show that melt mixing can be used to produce nanoblends when hydrogen bonding can be achieved between one of the homopolymers and a block copolymer used as a compatibilizer. In particular, in melt mixing studies of 80/20 wt% polystyrene/polycaprolactone blends compatibilized with the addition of 5 wt% styrene-hydroxystyrene block copolymer, we find that it is possible to produce blends with a number-average dispersed phase domain size of 100 nm that does not change appreciably with long-term, high-temperatuare annealing. This length scale is roughly a factor of 30 smaller than in a blend without block copolymer. Due to nanoscopic confinement of the polycaprolactone phase, we find that the compatibilized blend exhibits fractional or confined crystallization, with the polycaprolactone crystallization onset and peak temperatures being reduced by 30 K and more than 70 K, respectively, relative to the uncompatibilized blend. Thus, addition of a styrene-hydroxystyrene block copolymer can yield a compatiblized nanoblend with rubbery-state polycaprolactone domains at room temperature.

In the second case, we demonstrate that achievement of nanoblends is possible using solid-state shear pulverization (SSSP). SSSP is a continuous process in which polymers are mixed by exposure to high shear and compressive forces in the absence of melt processing. The morphology of the blended SSSP output is obtained by forming a consolidated sample using a cold platen press and then analyzing by field-emission scanning electron microscopy. In the case of polystyrene/poly(methyl methacrylate) blends, the PMMA phase can be etched with acetic acid, revealing the presence of a 3D, nanostructured, irregular morphology. When the blend is made via SSSP with 5 wt% added styrene-methyl methacrylate gradient copolymer, which serves as a blend compatibilizer, the same nanostructure is obtained as in the neat blend. Melt processing of this SSSP output yields a nanostructured blend with spherical dispersed-phase morphology that is stable to long-term, high-temperature annealing.