Relaxation of Slow Glassy Interphases in Polymeric Systems

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NON-TECHNICAL SUMMARYThe glassy interphases in two model polymeric systems will be studied using two unique techniques: dilatometry to measure the volume and thermal expansion coefficient, and ultrafast calorimetry to measure the heat capacity and other important thermal properties. These complimentary techniques will be used to determine the size, properties, and dynamic behavior of the interphase in several nanostructured materials. For example, semicrystalline polymers will be studied since these materials consist of crystalline and non-crystalline domains. So will the nanoscale-size interphase between the two domains, whose properties are important for understanding the overall material behavior. Through characterization of this interphase the results of the research will provide a fundamental understanding of nanostructured materials for technological applications including but not limited to applications in structural composites and nano/microelectronics. The project includes the training of one graduate student, one postdoctoral researcher, and an undergraduate researcher in research in polymer physics, thermal analysis, behavior of interphases, and nanostructured materials. A strong effort will be made to include minority and female students. The project will include outreach to K-12 students, including organization of several one-week long materials modules for the TTU program "Science – It's a Girl Thing" for junior high school girls. TECHNICAL SUMMARYTwo model polymeric systems with slowly relaxing glassy interphases will be investigated, namely, the rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) and the immobile glassy layer at the particle interface in hairy polystyrene-grafted silica nanoparticles. The properties and relaxation dynamics of the glassy interphases will be studied as a function of morphology in the PET system and as a function of graft density, graft chain length, and silica content in the nanoparticle system. Dilatometry and nanocalorimetry experiments will aim to determine the length scale, glass transition temperature (Tg), and breadth of Tg for the rigid or "immobile" interphases, the temperature dependence of the segmental relaxation times associated with the rigid or "immobile" phases, and in the case of the semicrystalline polymer, the interplay and competition between structural recovery and crystallization as a function of material morphology. A unique temperature perturbation experiment will be implemented to allow structural recovery to be differentiated from crystallization in the semicrystalline system during isothermal aging experiments and to allow changes in the immobilized surface layer thickness to be quantified in the hairy nanoparticle system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.