The chemistry of xylylenes. XIV. The moldability and thermal stability of poly(p‐xylylene) and related polymers

Abstract

AbstractFreshly prepared α‐poly(p‐xylylene) softens at 220–260°C. and then rehardens as it forms β‐poly(p‐xylylene). Consequently, homogeneous objects can be fabricated at this temperature range by simultaneous application of heat and pressure. This “moldable” poly(p‐xylylene), however, becomes nonmoldable within a week if allowed to age at room temperature. It is speculated that this change in moldability with aging at room temperature may be due to formation of tiny areas of crystalline α‐polymer which serve as seeds for rapid conversion from α‐ to β‐polymer before melt‐flow can occur in the critical temperature range of 220–260°C. of the heating cycle. The shelf life of the moldable polymer is increased considerably, however, by polymerization of the monomer in the presence of a molar equivalent of trialkylphosphines of trialkylphosphites. In vacuum, poly(p‐xylylene) has excellent thermal stability below 400°C., but in air, oxidative degradation is serious even at about 225°C. The moldability of p‐xylylene polymers is improved by copolymerization with a variety of olefinic and inorganic molecules, but the thermal stability, thermal oxidative stability, and sometimes the electrical properties of the resultant product are inferior to those of unmodified poly(p‐xylylene). These polymers appear to be resistant to degradation by γ‐irradiation.