Direct Measurement of Cure-Induced Stress in Thermosetting Materials by Means of a Dynamic Mechanical Analyzer

Summary: Significant stresses develop during cure in functional and structural applications of polymeric materials ranging from glass fiber composites to advanced functional polymers used in microelectronics, optoelectronics, and biomaterials applications. These stresses arise from a combination of chemical shrinkage and stiffness buildup in a confined geometry. In this paper, a new method for direct measurement of cure-induced stresses during curing of thermosetting materials by using the iso-strain mode of a dynamic mechanical analyzer (DMA) has been developed. A thermal tape was used to facilitate maintaining a constant strain and initiate the iso-strain measurement. Two quartz rods with a small gap were used to contain the material. The top of the quartz rod and one side of the thermal tape were secured by the fixed clamp, while the bottom quartz rod and the other side of the thermal tape were clamped with the moveable force probe. The cure force was thereby directly measured by the probe during the curing process. The cure stress buildup was observed to occur after a certain duration that corresponds to the gel point. Experimental results clearly show that curing at lower temperature could lead to higher cure stress due to the earlier onset of vitrification. An investigation of the stress buildup as a function of degree of cure indicates that a majority of the cure stress was generated in the vitrification regime. The methodology proposed herein provides an accurate experimental approach to investigate the cure-induced stress generated in a thermosetting material in applications ranging from microelectronics and optoelectronics packaging to biomaterials amongst others. Evolution of cure force and heat flow measured by means of DMA and DSC, respectively, at cure temperature 100 °C.