The Effect of Free Radical Inhibitor on the Sensitized Radiation Crosslinking and Thermal Processing Stabilization of Polyurethane Shape Memory Polymers

The effects of free radical inhibitor on the electron beam crosslinking and thermal processing stabilization of novel radiation crosslinkable polyurethane shape memory polymers (SMPs) blended with acrylic radiation sensitizers have been determined. The polyurethane SMPs in this study possess novel processing capabilities. These SMP's have the ability to be melted and processed into complex geometries as thermoplastics and then crosslinked using electron beam irradiation in a final curing step. This novel method is potentially much more cost effective and efficient than current methods when high throughput is desired. To increase the effectiveness of radiation crosslinking, the radiation sensitizer pentaerythritol triacrylate (PETA) was solution blended with thermoplastic polyurethane SMPs synthesized from 2-butene-1,4-diol and trimethylhexamethylene diisocyanate (TMHDI). Since thermoplastic melt processing methods such as injection molding are readily carried out at elevated temperatures, premature crosslinking due to sensitizer thermal instability is a significant processing concern. To overcome sensitizer thermal instability, free radical inhibitor can be added to provide thermal stabilization. However, free radical inhibitor can reduce the effectiveness of radiation crosslinking. In this study, we determined the inhibition effects of the inhibitor 1,4-benzoquinone (BQ) on thermal stabilization and radiation crosslinking of polyurethane SMPs solution blended with PETA. Sol/gel analysis of irradiated samples showed that free radical inhibitor at concentrations of 0-10,000 ppm had little to no inverse effects on gel fraction. Dynamic mechanical analysis (DMA) showed a minor negative correlation between inhibitor composition and rubbery modulus. The study also showed inhibitor BQ was highly effective on the thermal stabilization of acrylic sensitizer PETA. The thermoplastic blends could be heated to 150 °C for up to 5 hours or to 125 °C for up to 24 hours if stabilized with 10,000 ppm BQ without undesired sensitizer reaction occurring. The blended samples could also be heated to 125 °C for up to 5 hours if stabilized with 1000 ppm BQ without reaction. We believe this study provides significant insight into methods for manipulation of the competing mechanisms of radiation crosslinking and thermal stabilization of radiation sensitizers, thereby facilitating further development of radiation crosslinkable thermoplastic SMPs.