Initiated chemical vapor deposition (iCVD) of poly( alkyl acrylates): An experimental study

In a two-part investigation, an experimental study and a kinetic model analysis of the initiated chemical vapor deposition (iCVD) of alkyl acrylate polymers are described. In this first part, an experimental study was performed to look at the effect of process parameters on iCVD polymerization. A homologous series of alkyl acrylates, from ethyl up to hexyl acrylate, were iCVD polymerized. The resulting polymers matched well spectroscopically with those from liquid-phase polymerization, demonstrating that stoichiometric polymers with no observable cross-linking can be achieved in a chemical vapor deposition environment. Deposition rate and molecular weight increased by a factor of over 300 and 60, respectively, when monomer saturated vapor pressure, P-sat, was reduced from 42.6 to 0.584 Torr at equal gas pressures, P-M. Over three times increase in deposition rate was observed for ethyl acrylate when substrate temperature was reduced from 29 to 17 degrees C. These trends are attributed to an increase in P-M/ P-sat or, equivalently, monomer surface concentration in Henry's law limit at low P-M/P-sat. Evidence for adsorption-limited iCVD kinetics came from an apparent negative activation energy of - 79.4 kJ/mol obtained experimentally that agreed well with a mathematically derived activation energy of - 81.8 kJ/mol equal to twice the heat of desorption in the negative sense. Adsorption measurements found Henry's law limit to be valid and, when fitted to a BET equation, allowed the heat of desorption to be calculated. On the basis of this experimental study, process guidelines were made to define the appropriate parameter space for future iCVD polymerization, with P-M/P-sat in the range of 0.4-0.7 recommended as an optimal iCVD window.