Powell, Felicia M.

The degradation of polymer composites in moist environments is a limiting
factor in the advancement of composite technology. The key to mitigate this
degradation is to maintain the integrity of the fiber/matrix (F/M) interface. In this
study, the F/M interface of carbon/vinyl ester composites has been modified by
treating the carbon fiber with polyhedral oligomeric silsesquioxane (POSS). Two
POSS systems, namely octaisobutyl and trisilanolphenyl, have been
investigated. A set of chemical and mechanical procedures has been developed
to coat carbon fibers with POSS, and fabricate layered composites using vinyl
ester resin. lnterlaminar shear, transverse tension, and low velocity impact tests
on composites have indicated around 10-38% improvement in mechanical
properties with respect to control samples. Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) tests have also shown
significant improvement in glass transition temperature (T9). Hygrothermal tests,
under various environments, have demonstrated that POSS reduces water
absorption by 20-30%.

Enhancement of mechanical, thermal and hygrothermal properties of carbon fiber/vinyl ester (CFVE) composites through nanoparticle reinforcement has been investigated. CFVE composites are becoming more and more attractive for marine applications due to two reasons : high specific strength and modulus of carbon fiber and low vulnerability of vinyl ester resin to sea water. However, the problem with this composite system is that the fiber matrix (F/M) interface is inherently weak. This leads to poor mechanical properties and fast ingress of water at the interface further deteriorating the properties. This investigation attempts to address these deficiencies by inclusion of nanoparticles in CFVE composites. Three routes of nanoparticle reinforcement have been considered : nanoparticle coating of the carbon fiber, dispersion of nanoparticles in the vinyl ester matrix, and nanoparticle modification of both the fiber and the matrix. Flexural, short beam shear and tensile testing was conducted after exposure to dry and wet environments. Differential scanning calorimetry and dynamic mechanical analysis were conducted as well. Mechanical and thermal tests show that single inclusion of nanoparticles on the fiber or in the matrix increases carbon/vinyl ester composite properties by 11-35%. However, when both fiber and matrix were modified with nanoparticles, there was a loss of properties.