Dynamic mechanical analysis of storage modulus development in light-activated polymer matrix composites

Objectives: The goal of this study was to evaluate the potential for using dynamic mechanical analysis of tubular geometry in a three-point flexure fixture for monitoring the storage modulus development of a light-activated polymer matrix composite. Methods: Composite samples were inserted into PTFE tubes and tested in a three-point bend fixture in a dynamic mechanical analyzer (DMA) at 200Hz with 20μm amplitude. Samples were light activated for 60s (385mW/cm² at the composite surface) and storage modulus (E′) was measured continuously for the seven light-activated composites studied (one microfill, four hybrids and two unfilled resins). Cores of composite were removed from the PTFE sheath after 13.5min and evaluated with the same parameters in the DMA. A finite element model of the test configuration was created and used to estimate operating parameters for the DMA. Degree of conversion (DC) was measured using micro-Fourier Transform Infrared (FTIR) spectroscopy for the microfilled composite samples and one hybrid 13.5 and 60min after light activation. Results: The E′ for a generic hybrid and microfilled composite was 13,400±1100 and 5900±200MPa, respectively, when cured within the tube and then removed and tested in the DMA. DC was 54.6% for the hybrid and 60.6% for the microfill. A linear regression of E′ for the sheath and core vs core alone (r₂=0.986) indicated a linear scaling of the sheath and core values for E′ enabling a correction for estimated E′ values of the composite core. Significance: This method estimates the storage modulus growth during light-activated polymerization of highly filled dimethacrylates. Although the approach is phenomenological in that quantitative measurements of E′ are not made directly from the DMA, estimates of early polymerization kinetics appear to be validated by three different approaches.