The research will provide the scientific and technological foundation for the synthesis, characterization, processing, and performance verification of melt-stable, polyacrylonitrile (PAN) copolymers and blends with poly(ethylene oxide) (PEO or PEG). These systems will provide for the first time valuable fibers, films, and molded objects that will have the solvent-free, environmental attractiveness afforded by melt fabrication. The project will largely focus on classical free radical statistical copolymerization of acrylonitrile with ion-containing comonomers and with methacrylate functional poly(ethylene oxide) macromonomer oligomers. They can be efficiently prepared in high molecular weight in DMSO. The rheological properties of the copolymers and blends will be measured as a function of temperature, time, and shear rate to confirm the thermoplastic nature of these systems and to determine compositional limits.
NON-TECHNICAL SUMMARY
This award is based upon the growing need for sustainable, environmentally attractive, advanced macromolecular materials systems for high performance film and fiber applications ranging from energy to packaging materials and wastewater purification systems and polymer matrix carbon composites -- the latter of interest for aerospace, transportation, and windmill power. The first priority will be to investigate systems that are melt-processable and permit very low gas permeabilities and melt-spun acrylic fibers. Such materials would open broad new vistas for textile synthesis and production, which is currently not practiced in the Unites States. A major feature will involve interactions with minority women professors and students at Fisk University and at the University of South Carolina Upstate. The goal will be to develop talented diverse scientists; summer internships will be provided involving faculty and students from HBCUs and 4-year schools to broaden students' perspectives well beyond their own research projects.
The development of commercial polyacrylonitrile (PAN) textile fibers such as OrlonÔ and AcrylonÔ was once well established in the U.S. However currently there is no domestic commercial manufacturing of polyacrylonitrile fibers for textile applications or precursors for carbon fibers and all such manufacturing occurs outside the borders of the United States. Since all PAN fiber fabrication is via environmentally unattractive solution processing, the development of more economical and less environmentally hazardous melt spinning processes could be a powerful driver for domestic commercial development of fibers and films. This NSF-DMR project has explored the synthesis of acrylonitrile/methyl acrylate statistical copolymers utilizing classical emulsion, suspension, and RAFT controlled/living co-polymerization methods to control the molecular weight (MW) and molecular weight distribution (MWD) which are both critical parameters for melt processability. VT has explored a number of variables including comonomer content, MW, MWD, and the application of non-toxic plasticizers. Currently acrylic fibers with relatively high acrylonitrile content are produced using solution spinning techniques which are less economically and environmentally friendly than melt processes. One goal of our program has been to develop a viable option for melt processible polyacrylonitrile copolymers suitable for use in acrylic fibers, as well as in new applications such as barrier films in multilayer packaging. The principal contributors to the program have been Ph.D. candidates Priya Pisipati, Susan Beck, undergraduate Matt Joseph, and Research Scientist Dr. Sue Mecham. Several presentations have been presented at regional, national, and international meetings including the American Chemical Society, SAMPE, and the 6th International Symposium on the Separation and Characterization of Natural and Synthetic Macromolecules in Germany. Two manuscripts have been submitted to Polymer for publication and two additional manuscripts are very close to submission. In summary, two thrusts for developing melt processable materials have been pursued, both of which were based on designing a system where the high temperature endotherm was reduced or eliminated to allow the material to flow at temperatures above Tg but below the degradation temperature of PAN polymers. Firstly, the copolymer backbone composition and MW were controlled to minimize interchain dipole-dipole interaction of the nitrile moieties and provide low melt viscosity (via control of MW and MWD). The second thrust was the use of low molecular weight non-toxic plasticizers including water and glycerin additions which lead to significant depression of the Tm of high molecular weight PAN 93 wt% copolymers. Glycerin appears to further induce crystallization of the acrylonitrile, but water is more efficient at reducing the copolymer Tm and Tg. Blends of the two plasticizers can be utilized possibly with some advantages. Initial FTIR studies demonstrated significant shift of the plasticizer hydroxyl groups suggesting specific interactions with the copolymer, which reduced the processing temperature below the degradation point. Our more recent efforts to explore opportunities for enabling the melt processing of PAN copolymers to make low oxygen permeable films and fibers has included varying the copolymer composition, varying the molecular weight and molecular weight distribution, and incorporation of low molar mass plasticizers into the copolymers to reduce or eliminate the high temperature endotherm, even of high AN content copolymers, that prevents PAN copolymers from flowing before reaching a temperature where degradation occurs. We have synthesized a range of copolymer compositions of AN/MA ranging from 80% AN to 98% AN using the classical free radical heterogeneous polymerization methods of emulsion and suspension and the reversible addition-fragmentation chain-transfer (RAFT) polymerization technique in solution. We controlled MW and MWD using each method from approximately Mw=40K to Mw=2.5M and compared these to the molecular weights of commercial alternatives. The highest molecular weights were obtained using emulsion polymerization and higher molecular weights were obtained with higher conversion in the RAFT polymerization. The lower AN content copolymers show high potential for melt processability and transparent films were melt pressed. Higher AN content copolymers were prepared using water and glycerin as plasticizers and fibers were extruded from the melt. MW was found to be a factor in the potential for melt extrusion of these blends.
