(638h) Kinetically-Limited Deposition of Polymer Films By Initiated Chemical Vapor Deposition

Relevance and Problem
Initiated chemical vapor deposition (iCVD) has been used to create various classes of thin film polymers, such as acrylates, styrenes, vinyls, ethers, silicones, and fluoropolymers. Thin film polymers from iCVD are used in applications such as hydrophilic surfaces, anti-fouling membranes, anti-microbial coatings, photovoltaics, and batteries. The kinetics of iCVD polymer deposition are typically limited by the monomer adsorption rate, with the deposition rate increasing with decreasing substrate temperature. The necessity for a cooled substrate presents a challenge for scale-up to continuous roll-to-roll iCVD operations due to the difficulty of cooling a moving substrate. Rather than decreasing the substrate temperature to better facilitate monomer adsorption, the reactor pressure can also be increased to facilitate high levels of adsorption at saturated conditions. However, iCVD kinetics at saturated conditions has not been systematically probed previously, raising questions on whether operating at saturated conditions is a viable alternative to substrate cooling.

Methods and Results
This work investigates the kinetics of iCVD at saturated conditions, where the partial pressure of the monomer vapor equals to the monomer saturation pressure at the substrate surface. The monomer N-vinylpyrrolidone (VP) was chosen for this kinetic study due to its intermediate volatility and the applications of its corresponding polymer, poly(vinylpyrrolidone) (PVP), in applications that include biocompatible coatings and battery anodes. Monomer VP and initiator tert-butyl peroxide were fed to an iCVD reactor to deposit PVP films onto silicon substrates at substrate temperatures of 10, 15, 20, and 25 °C with the total reactor pressure adjusted to maintain monomer vapor saturation. Using laser interferometry, the deposition rate was measured in-situ throughout each reaction. The interferometry data revealed that iCVD deposition at saturated conditions operates in two regimes: an initial transient followed by a steady state. In the transient regime, we find a constant deposition rate with substrate temperature at saturation that is consistent with the adsorption-limited kinetics that is typical of iCVD. Unexpectedly, in the steady state regime, we find the deposition rate increasing with substrate temperature at saturation that gives an overall Arrhenius activation energy of 86 kJ/mol. This value is remarkably close to the activation energy of bulk PVP polymerization experiments of 89 kJ/mol from literature and to that predicted by bulk polymerization theory at 91 kJ/mol. These findings suggest that iCVD at saturated conditions operates initially in an adsorption-limited regime where surface polymerization dominates, but subsequently switches to bulk polymerization within the growing film that operates in a kinetically-limited regime.

Implications and Future Work
This work shows that iCVD can operate in a kinetically-limited regime to achieve high deposition rates at higher substrate temperatures, which is in contrast to the current understanding of iCVD kinetics in which high deposition rates can only be achieved with decreasing substrate temperature. With our findings, it is possible for scale-up to roll-to-roll continuous iCVD reactors to achieve high polymer deposition rates at near-room temperatures without necessitating substrate cooling. The high deposition rates also correspond to higher molecular weight polymers, which give rise to desirable film properties such as higher strength. iCVD at saturated conditions has also been shown to form polymer patterns by area selective deposition onto chemical prepatterns, and the findings from the kinetic study at saturated conditions allow for better control over the polymer patterning process.