Structure-property relationships in the formation of polyphenylsulfone molecular composites and nanocomposites

Paul Joseph Jones

Abstract

In the first portion of this dissertation, semi rigid-rod macromolecules having phenylketone substituted para -phenylene and unsubstituted meta -phenylene recurring units (i.e. SRPs) at two different ratios are blended by rapid coagulation from solution with polyphenylsulfone (PPSU), and the resulting effects on miscibility, morphology and nanomechanical properties are assessed. Initially, the nanomechanical behavior of an SRP having a completely sp 2 hybridized backbone was demonstrated in comparison to conventional high performance engineering thermoplastics as a function of polymer rigidity via nanoprobe instrumentation techniques. Next, various light scattering techniques were employed to obtain key molecular and structural parameters of the SRPs and PPSU in dilute solution, which were related to polymer conformation, theoretical entropic and enthalpic contributions, and predicted blend compatibility. Miscibility was investigated using thermal analysis techniques to monitor the glass transition as a function of blend composition. The bulk and surface morphologies of these blends were analyzed via atomic force microscopy (AFM) to confirm a homogeneous morphology or determine the mechanism of phase separation, and the mechanical properties of these blends were evaluated using nanoindentation. Finally, an understanding of the relationship between the ratio of substituted para and unsubstituted meta recurring units in the SRP copolymer backbone to miscibility, morphology and nanomechanical properties in blends (or molecular composites) with PPSU was developed. A polymer nanocomposite is broadly defined as a polymeric composite material in which one of the phases has dimensions less than 100 nm. These materials are not new since polymer blends often have dimensions much less than 100 nm. However, polymer composites containing nanofillers have experienced a recently renewed interest from the scientific community due to the potential for these materials to exhibit not only superior mechanical properties, but also elevated thermal and dimensional stability and an array of other property improvements at relatively low additions of nanofiller. A special class of nanofillers is polyhedral oligomeric silsesquioxane (POSS ® ) nanostructured chemicals. POSS molecules with their hybrid organic/inorganic structure, well defined threedimensional architecture and mono-disperse particle size have been the subject of a great deal of both academic and scientific interest for their potential to increase the strength and modulus of a polymer matrix without the negative side effects to processing observed with many traditional fillers. In fact, significant enhancements in the rheological and melt flow behavior of amorphous polymers have been observed with only minimal additions of POSS. These enhancements depend upon the interactions of POSS with the amorphous matrix based on the chemical structure of POSS. However, few detailed studies of these relationships have been performed, and the mechanism of this behavior has not been clearly defined. In the second portion of this dissertation improvement in the melt processing and rheological behavior of an amorphous polymer, PPSU, and the resulting thermomechanical properties of the nanocomposite by the addition of different types of POSS at various loading levels is discussed. The relationship of POSS chemical structure to the final properties of the nanocomposite materials was defined in terms of the difference in solubility parameters of POSS and PPSU, the dispersion of POSS within the PPSU matrix and the phase transformations POSS undergoes as a function of temperature. In these studies many new nanoprobe characterization techniques were adapted and utilized for the advanced characterization of polymer films, specifically AFM. Recent advances in these characterization techniques have made possible the direct imaging of molecular events with sub-nanometer resolution. When applied to polymer films they can provide a wealth of knowledge that could not be obtained otherwise. In the Appendix of this dissertation a description of one of these techniques applied to stimuli-responsive polymer systems is included, in which current sensing AFM was used to identify the actuation mechanism in perfluorosulfonated ionic membranes. (Abstract shortened by UMI.)