Effect of elastomer functionalized carbon nanofibers on the properties of polyimide nanocomposite and polydimethylsiloxane-carbon nanofiber sheets

Carbon nanofibers (CNF) are ideal candidate for polymer reinforcement to enhance mechanical, electrical and thermal properties. Uniform dispersion and good interfacial interaction between the nanofibers and the polymer matrix is desired to optimize the properties of the resulting composite material. While functionalization of CNF with organic groups is a good way to improve the dispersion within the polymer matrix, this approach has not led to significant improvement in composite properties. In this study a method to engineer the interface between the nanofiber and the polymer matrix by binding a block copolymer on the CNF surface with targeted functionality from each block was investigated.;Specifically, CNF were functionalized with elastomeric block (i.e. aminopropyl terminated polydimethylsiloxane (PDMS-NH2)) and oligomer of polyimide block that mimics structure of matrix polymer. The oligomer block enhances the compatibility of the fiber with the polymer matrix and the elastomeric block is designed to reduce brittleness and improve the toughness of the nanocomposite. The functionalization of the oxidized carbon nanofibers (OCNF) with the amine end capped moieties was achieved using carbodiimide chemistry. The carbodiimide facilitated the formation of amide linkage between the carboxylic acid groups on OCNF and amine group at faster rate and lower temperature than the alternative methods. The functionalized CNF were characterized with X-ray photoelectron spectroscopy, thermal gravimetric analysis (TGA) and UV-vis spectroscopy. Qualitative analysis confirmed that the groups were covalently attached on the CNF surface.;The composite of polyimide with pristine, OCNF, CNF functionalized with PDMS and CNF functionalized with block copolymer were produced using blending approach. The goal was to investigate the effect of surface functionalization of CNF and loading on the flexibility of the composite. The tensile testing results showed increase in modulus with incorporation of OCNF and functionalized fiber. The composite with OCNF became brittle and broke at lower strain value, while the composite with elastomer functionalized CNF retained flexibility similar to base polymer up to 2.5 wt% loading. The elastomer groups appeared to improved the flexibility of the interface and minimize the effect of incorporating rigid CNF on the flexibility of the polymer. The block copolymer functionalized fiber showed improvement in modulus and flexibility of the composite. Scanning electron microscopy (SEM) analysis showed that pristine fiber had poor dispersion and formed agglomerates which results in poor mechanical properties of the composite. The block copolymer functionalized fiber had better dispersion and adhesion with the polymer which translated in to improved properties.;A novel PDMS base technique was developed to fabricate high fiber loading flexible CNF sheets. The sheets produced using elastomer functionalized fibers were more flexible and exhibited better mechanical properties compared to sheet made with pristine CNF and OCNF. The modulus of sheets with 30 wt% loading of elastomer functionalized fibers were 1500% higher than the base PDMS and 250% higher than sheets made with 30% pristine nanofiber. The strength and the toughness of 30 wt% sheet of elastomer functionalized fibers were 650% and 1800% higher compared to sheets with 30% pristine fibers. The electrical conductivity of the sheet with elastomer functionalized fiber were an order of magnitude lower than sheet made with pristine fibers, but achieved percolation at 1.25 vol% compared to 5 vol% for sheets with pristine fibers. The presence of elastomer groups bound to the surface of CNF improved the interfacial interaction and adhesion of the fibers with the polymer, resulting in flexible and strong composite.;Finally hybrid nanofiller were produced using CNF that was functionalized with polyhedral oligomeric silsesquioxane (POSS) nanoparticle and polyamidoamine (PAMAM) dendrimers. The goal was to increase the amount of reactive functional groups on the surface of CNF. The covalent attachment of CNF with POSS and PAMAM was confirmed using several analytical techniques. The reactivity of the CNF functionalized with POSS improved by 4-5 times compared to OCNF. These highly functional groups improved the interaction of the CNF with the polymer which would assist in effective load transfer from the polymer to the fiber. Composite were formed with this functional materials and their properties were measured.