Non destructive FTIR-photoacoustic spectroscopy studies on carbon fiber reinforced polyimide composite and water diffusion in epoxy resin

Date of Completion

January 2008

Keywords

Chemistry, Analytical|Chemistry, Polymer|Engineering, Materials Science

Degree

Ph.D.

Abstract

Photo-acoustic (PA) detection is a non-destructive, non-disruptive mode of sample analysis. The principle of PA detection is monitoring the change in thermal properties of the material as a result of optical absorption. The ability to use with any incident radiation source makes it an attractive technique to study molecular excitations, vibrations and defects in any sample. Given the need for non-destructive analysis, the tool can be employed to study plethora of samples ranging from organic to inorganic. In the polymeric domain, there is a significant need for studying samples non-destructively with the architecture intact. For instance, molecular characterization in carbon fiber reinforced polymer, chemical diffusion in polymer resin/membrane and particulate/fillers incorporated thermosets suffer in characterization due to sample make-up. These samples are affected by opacity and thickness, which make them a very difficult set-up to study using conventional spectroscopic tools. We have employed PA mode of detection in tandem with a FTIR source to study the molecular vibrations to get an understanding of the systems considered. ^ The first part of the work involved employing PA spectroscopy to study the curing in carbon fiber reinforced polymer (CFRP). Phenyl-ethynyl terminated oligoamic acid impregnated composite system was studied. The curing of composite and resin was monitored using PAS and compared with Transmission FTIR on resin and dynamic scanning calorimetry (DSC). The composite showed two distinct reactions as a function of thermal treatment. (1) Imidization at low temperatures due to cyclo-dehydration and (2) at high temperatures, crosslinking due to ethynyl addition reaction. Composite exhibited enhanced curing trends compared to neat resin. Our results indicate that the thermal conductivity of the carbon fiber might play a role in heat transfer facilitating the reaction. The activation energy was found to be 23kcal/mol for the crosslinking step. The order of the kinetic rates agreed to a good extent with literature reports implying the mechanistic aspect is retained in composite. ^ The second part of the work focused on the diffusion studies. With the capability of depth profiling using PAS, we were able to demonstrate the sorption trend as a function of depth in epoxies. The samples used were physically aged and the apparent diffusion constant was found to be of the order of 10 -10cm2/s. The lower value of the diffusion constant was attributed to poor relaxation capability due to ageing. The initial sorption trend was modeled on the basis of Fickian diffusion and the trend correlated well with the gravimetry analysis. Time evolution of water sorption in PAS correlated well with gravimetry technique. ^ The third aspect of the work involved curing studies in nanocomposites. With the recent trend being controlling active nano-domains for better applicability and processing of materials. We were able to employ PAS to study the curing trend in carbon nanotube dispersed thermosets. DSC studies showed the existence of two glass transition temperatures, which we believe might be due to relaxation in homopolymers and relaxation of polymeric segment close to the nanotube surface. We were able to study the curing aspect of the filler reinforced thermoset and found that the surface curing displayed a higher crosslinking compared to the bulk. ^

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