Coupling Within and Between Nanophases of the Global, Metastable Structure of Polymers

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

The proposed research involves a study of molecularly coupled processes in polymers. In the past there were two ways to describe polymer molecules thermodynamically. One was to treat the whole molecule as a single component, despite the fact that a single polymer molecule may consist of 10,000 chain atoms of which each has a certain amount of independent mobility. The other was to treat each of the chain atoms as an independent component, despite the fact that all chain atoms are linked together. Neither description works for all situations. The new idea is to test the possibility to identify local or temporal decoupling. The local decoupling limits the chain segments that can be treated as an independent component. The temporal decoupling limits successive steps in a process which occur too slowly to be acted upon by a single molecular driving force, a common occurrence in slowly acting polymers. In the last ten years, temperature-modulated calorimetry was developed to a degree to be able to quantitatively distinguish reversible and irreversible processes. The effect of frequency- and amplitude-variation of temperature-changes is studied with this technique and allows to find the time- and position-effect of reversible and irreversible processes such as melting and crystallization, which can then be linked to the decoupling described above. Multi-phase, metastable structures of different component length were found to undergo transitions of different degrees of reversibility. Semicrystalline materials and copolymers of incompatible blocks are characterized as globally metastable systems of micro- and nanophases with strong connections across their boundaries due to polymer molecules that are longer than the phase dimensions and become part of more than one phase with intermediate decoupling. Changes of state involving crystallization, ordering, orienting, melting, disordering, mixing, vitrification, and devitrification are to be studied. The intellectual merit of the proposed research would be an increased understanding of polymers, a part of science which presently involves more than half of all scientists. The broader impact involves a reduction of the need for trial and error in the application to polymeric materials as soon as the connection between structure, properties, and processing is established in areas such as fibers, films, plastics, etc. and including also biological polymers such as proteins and starches. The basis of the quantitative, thermodynamic analyses proposed is the Advanced THermal Analysis System (ATHAS), developed over the last 30 years with NSF support. Extensive teaching efforts reach from scientific publications, discussions, and lectures, to instruction of students and postdoctoral associates, all on an international level. A computer course is available over the internet for study of the new results. Over the last 5 years, the duration of a prior NSF Grant the Data Bank and Computer Course were accessed by more than 25,000 visitors.