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Novel, Nanostructured, Immiscible a/b Polymer Blends Made by Conventional Melt Processing: Addition of a/c Gradient Copolymer as An Interfacial Compatibilizer

Robert W. Sandoval, Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, Jungki Kim, Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208, and John M. Torkelson, Chemical and Biological Engineering and Materials Science and Engineering, Northwestern University, 2145 Sheridan Rd, TECH E136, Evanston, IL 60208.

Novel, compatibilized, nanostructured polymer blends have been made using a single melt-mixing step in which a 90/10 wt% A/B blend is mixed with a small amount of an A-C gradient copolymer as compatibilizing agent and in which there is an attractive dipole-dipole interaction between the B and C units. This is the first time that nanostructured blends have been made with such as a large level of the minor blend component without the aid of reactive processing or in situ polymerization of one component in the presence of the other polymer. In particular, when a 90/10 wt% polystyrene (PS)/poly(ethylene oxide) (PEO) mixture is melt blended with 7 wt% styrene (S)-methyl methacrylate (MMA) gradient copolymer, we obtain an average PEO dispersed-phase diameter of 105-110 nm. In contrast, without the addition of the gradient copolymer and using the melt processing conditions, the average PEO dispersed-phase diameter is 520-525 nm. The 90/10 wt% PS/PEO blend with added gradient copolymer is shown to be compatibilized by the virtual absence of coarsening of the dispersed-phase diameter during long-term, high-temperature static annealing (at 443 K for 90 min). However, using the same annealing protocol, the 90/10 wt% PS/PEO blend without added gradient copolymer undergoes significant coarsening, yielding an average PEO dispersed-phase diameter of 735 nm. When compared to random and block copolymer addition, the blends made with gradient copolymer show superior compatibilization efficiency, due to random copolymers unable to prevent coalescence and only very limited amounts of the block copolymer being able to reach the blend interfaces. This is apparent when observing the non-isothermal crystallization behavior. In blends with added gradient copolymer, PEO is confined to nanoscale domains, and the PEO crystallization occurs almost exclusively by homogeneous nucleation (at ~ 258 K) rather than heterogeneous nucleation at temperatures above room temperature. Thus, nanostructured PS/PEO blends that are maintained at temperatures near or above room temperature have a glassy matrix with nanoscale rubbery domains dispersed in the matrix while normal PS/PEO blends have a glassy matrix with semi-crystalline domains dispersed in the matrix. This study illustrates in a simple manner that nanostructured blends can yield major property differences from conventional polymer blends and that gradient copolymers, with a composition gradient along most or all of the copolymer chain length, can be useful in producing nanostructured blends using a very simple, commercially attractive processing method.