Reactive compatibilization is commonly used when blending immiscible homopolymers. The compatibilizers formed from the interfacial coupling of two types of reactive chains often have a graft copolymer architecture. Here we consider the case where both reactive chains are multifunctional, leading to a crosslinked copolymer at the interface. Experiments were conducted on a model blend of ϳ30% polydimethylsiloxane drops in a polyisoprene matrix. Compatibilizer was formed by an interfacial reaction between amine-functional polydimethylsiloxane and maleic anhydride-functional polyisoprene. Both species were multifunctional, and therefore capable of interfacial crosslinking. Optical microscopy showed some unusual features including drop clusters, nonspherical drops, and some drops with apparently nonsmooth surfaces. All these features suggest that a crosslinked "skin" covers the interface of the drops. Rheologically, the reactively compatibilized blend showed gel-like behavior in oscillatory experiments, enhanced viscosity and elastic recovery at low stresses, and strong viscosity overshoots in creep experiments, all of which are likely attributable to drop clustering. At the highest stress studied ͑400 Pa͒, the viscosity of the reactively compatibilized blend is comparable to that of a similar blend compatibilized by diblock copolymer. This suggests that, in practical processing operations that occur at even higher stresses, interfacial crosslinking by multifunctional chains will not adversely affect processability.
Reactive compatibilization, in which a compatibilizer is formed by an interfacial coupling between two reactive polymers, is commonly used when blending immiscible homopolymers. We consider reactive compatibilization using two multifunctional reactive polymers, which leads to a crosslinked copolymer at the interface. Experiments were conducted on model blends of polydimethylsiloxane ͑PDMS͒ and polyisoprene ͑PI͒. Compatibilizer was formed by a chemical reaction between amine-functional PDMS and maleic anhydride-functional PI. Droplet-matrix blends with a PI:PDMS ratio of 30:70 or 70:30 and reactive compatibilizer loadings from 0.1% to 3% were examined by optical microscopy and rheometry. Experiments reveal that the effects of interfacial crosslinking are highly asymmetric, with PI-continuous blends showing altogether different behaviors from PDMS-continuous blends. The PI-continuous blends show unusual features including drop clusters and nonspherical drops. In contrast, PDMS-continuous blends displayed a typical droplet-matrix morphology with round drops that do not appear to stick together. The rheological properties are also asymmetric: The PI-continuous blend showed gel-like behavior in oscillatory experiments, high viscosity, and viscosity overshoots during startup of shear flow, whereas PDMS-continuous blends showed liquidlike behavior that is qualitatively similar to that of compatibilizer-free blends. We speculate that the observed structural and rheological asymmetry is attributable to the asymmetry of the compatibilizer architecture on the two sides of the interface.
Most recent developments in polymers from renewable resources have focused on thermoplastics, whereas there has been no comparable development of plastics with elastomeric properties. Here we evaluate the possibility of developing renewable elastomers based on starch. Potato starch plasticized with glycerol (called plasticized starch, or PLS) was melt-blended with small quantities (5 wt % or 15 wt%) of maleated polypropylene (MAPP). The maleic anhydride groups of the polypropylene are expected to react with the hydroxy groups of starch under melt blending conditions. The resulting blends of MAPP and PLS were characterized by mechanical testing, SEM, DMA, and DSC. SEM, solubility and adhesion tests indicate that the blends are two-phase materials, in which the continuous phase PLS is physically crosslinked by polypropylene domains. The materials showed rubbery properties as judged by a low glass transition temperature ($À50 C independent of polypropylene content), and a wide rubbery plateau in DMA experiments that extended from room temperature to as high as 170 C. The tensile properties are also characteristic of elastomers. However, slow aging due to starch crystallization, and extraction of glycerol upon water exposure remain two challenges that must be overcome before the materials can be used as practical elastomers.
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