Controllable Cross-Linking of Vapor-Deposited Polymer Thin Films and Impact on Material Properties

We report the single-step preparation of controllably cross-linked poly(divinylbenzene) (PDVB) and poly(4-vinylpyridine-co-divinylbenzene) thin films using initiated chemical vapor deposition (iCVD). Fourier transform infrared spectroscopy-based methods for quantifying film composition and degree of cross-linking are elucidated; the validity of these methods is assessed using X-ray photoelectron spectroscopy and nanoindentation. The extent of reaction of divinylbenzene (DVB) pendant vinyl bonds in homo- and copolymer films is unaffected by changes in initiator concentration, suggesting that bond reactivity, rather than radical concentration, is the limiting factor. Analysis of film step coverage (S) over high aspect ratio (AR) features and sticking probability calculations lend insight into the reactivity of both monomers and explain the extreme conformality of PDVB films (S = 0.87 +/- 0.02 at AR = 4.7). In addition, the incorporation and cross linking of DVB moieties in the copolymer are extremely reproducible and can be used to tune the elastic moduli of the films from 3.4 to 5.8 GPa.