The structure of poly(cyano-p-xylylene)

Films of poly(cyano-p-xylylene) (CN-PPX) synthesized by vapor deposition polymerization are highly crystalline and can be prepared with uni-planar orientation. This orientation is maintained after drawing the film 700% at 350 °C, when the chains become aligned parallel to the direction of draw, leading to a mosaic texture of doubly-oriented crystals. Three-dimensional wide angle X-ray data show that the polymer has a monoclinic unit cell with dimensions a = 5.96 angstroms, b = 13.52 angstroms, c = 6.48 angstroms and β = 135.9°, containing two monomer units from symmetry related chains. The unit cell is almost identical to that of the α-form of the unsubstituted polymer, poly(p-xylylene), except for the increased b dimension (b = 10.64 angstroms in PPX), which is due to the -CN substituent. Molecular modeling and X-ray simulations show that the two polymers have similar crystal structures. The best agreement between the observed and calculated structure amplitudes (R = 0.21) is for a model where there is random substitution of the -CN groups.