Crystallography of Poly-p-Xylylene

Abstract

Poly-p-xylylene prepared by the vapor-phase pyrolysis of di-p-xylylene affords a linear high-purity polymer whose crystalline transitions were characterized by x-ray and electron diffraction, mechanical creep studies, and differential thermal analysis (DTA). Two of the three endotherms observed by DTA correspond to the known alpha and beta polymorphs of poly-p-xylylene. The third, occurring at 270°C, is a reversible transition which is regarded as a smectic mesomorphic transition of the stable beta modification. Hence: α→ lim 220∘Cβ1⇋ lim 270∘Cβ2→Tm 420∘C (decomposition).Single crystals precipitated from alpha-chloronaphthalene were instrumental in characterizing the alpha and beta unit cells. The alpha modification is best represented by an orthorhombic cell with a=21.3, b=33.6, c=6.58 Å, for which the theoretical density is 1.141 g·cm−3 at 23°C. The beta modification is hexagonal, with a=20.52±0.05 Å and c=6.581±0.020 Å, having a theoretical density at 23°C equal to 1.153±0.005 g·cm−3. The fiber repeat distance consists of one chemical repeat unit in a fully extended configuration for both modifications. The high positive birefringence, particularly for the beta modification, reveals that the planes of benzene rings in neighboring molecules are steeply inclined to one another.