When 1:00 PM - 3:00 PM May 08, 2014
Where 2166 HH Dow
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In Situ Light Scattering Analysis of the Curing Behavior and the Mechanical Properties of Thermoset Polymers


Michael Aldridge
Thesis Defense

John Kieffer, advisor

 

Thermoset resins are used in carbon fiber composites because of their superior stiffness and improved thermal resistance over thermoplastics. In the manufacturing of these composites, the resin is infused into a carbon textile, and cured at a proscribed temperature and pressure. The presence of carbon fibers may influence the progress of the cure reaction and the resulting mechanical properties of the resin. Mechanical testing of composite structures reveals that the resulting composite has mechanical properties that diverge from those predicted by assuming bulk mechanical properties of the constituent materials.

 

This dissertation describes the investigation of how chemistry and thermal history influence the mechanical properties of thermoset polymers. The relationship between mechanical properties and molecular structure is probed using a novel combination of, Brillouin and Raman light scattering techniques. Brillouin scattering yields information about the mechanical and transport properties of a material, while Raman scattering is used to detect changes in the molecular structure of the material. These techniques are used concurrently, in-situ during cure, sharing a single optical setup, probing the chemical state and mechanical properties of the same focal volume within the sample.

 

Using this approach, two archetypical thermoset polymers are studied as a function of the initial chemistry and thermal history: Grubb’s catalyzed dicyclopentadiene, and an epoxy resin cured with an amine hardener. Raman spectra are used to develop kinetic models of the polymer cure behavior. However, the mechanical properties do not map linearly onto the degree of cure, but instead require one to account for relaxation processes that occur during and after cure. Finally, concurrent micro-Raman and -Brillouin scattering are used to resolve the spatial variations of the mechanical properties of the fully cured epoxy matrix in between carbon fibers. The elastic modulus decreases linearly with fiber density in regions of the matrix with closely packed fibers. Comparing Raman and Brillouin data reveals that the observed inhomogeneity of elastic properties is not due to residual stresses, but likely results from structural reorganization of the polymer in the interphase region.