(638f) Chemical Vapor Deposition of Dicyanate Ester Polymer Films

Chemical vapor deposition (CVD) has been used to create different classes of polymer thin films, including acrylates, methacrylates, vinyls, silicones, fluoropolymers, and conjugated polymers. These films are used in applications such as hydrophilic surfaces, hydrophobic barriers, enhanced adhesion to substrates, wear and scratch resistance, and conductive coatings. One broad class of polymers that is not commonly found in CVD processes are thermosets. Thermosets have high strength-to-weight ratios, and excellent thermal and mechanical properties. Compared to other thermosets like epoxies, dicyanate esters have better thermal stability, less shrinkage, lower water adsorption, and lower dielectric constant. This work focuses on the chemical vapor deposition of dicyanate esters. But there are two major difficulties in synthesizing dicyanate ester thin films using CVD. First, the heavy monomers are found as solids or viscous resins and therefore are not easily vaporized for a CVD process. Second, fully curing them into highly crosslinked networks are done in bulk polymers using metal-hydroxyl co-catalysts that can be difficult to achieve by CVD. Adaptions need to be made in the CVD process to overcome these challenges.

Methods and results

This work presents a novel approach to synthesizing dicyanate ester thin films using CVD. Two dicyanate ester monomers are employed: a commercial bisphenol E monomer and a 1,4-dicyanobenzene synthesized in-house to be a lower molecular weight and more structurally rigid monomer. The first monomer is expected to yield a 3D random network polymer, while the latter is expected form a rigid, porous, layered network. Monomers are heated to their vaporization point in a crucible seated within a cylindrical CVD reactor at low pressures. A silicon wafer substrate is suspended face down above the crucible and is heated to the cure temperature pre-determined by differential scanning calorimetry (DSC) and temperature-programmed attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. We have successfully deposited dicyanate ester thin films on silicon wafers, and it has been found that, as expected, increasing the final cure temperature and total cure time results in increased conversion of monomer to polymer. This is confirmed by the reduction in monomer cyano peaks in ATR-FTIR and the appearance of triazine ring peaks, which are only present in the polymer. Additionally, XPS high resolution spectra show the presence of bonding environments from both the monomer and polymer, indicating partially cured films in the absence of curing catalysts.

Implications and future work

This work shows that dicyanate ester thins films can be achieved using CVD, adding both to the library of CVD polymers and to the capabilities of thin films and coatings. Dicyanate ester thin films are expected to exhibit similar properties as their bulk counterparts and could be used in applications such as high temperature electronics substrates and scratch resistant coatings. On-going and future work includes the addition of catalysts to promote curing, such as metal-coated or room-temperature ionic liquid (RTIL) coated silicon wafers. The effect of network rigidity will also be studied by comparing polymer properties with the two dicyanate ester monomers.