No abstract

We report a systematic study of the plasma polymerization of ethyl α-bromoisobutyrate (EBIB) to produce thin film coatings capable of serving as ATRP initiation surfaces, for which they must contain α-bromoisobutyryl functional groups. In the deposition of polymeric coatings by plasma polymerization there generally occurs considerable fragmentation of precursor ("monomer") molecules in the plasma; and the retention of larger structural elements is challenging, particularly when they are inherently chemically labile. Empirical principles such as low plasma power and low pressure are usually utilized. However, we show that the α-bromoisobutyryl structural moiety is labile in a plasma gas phase and in low pressure plasma conditions, below the collisional threshold, there is little retention. At higher pressure, in contrast, fragmentation of this structural motif appears to be reduced substantially, and coatings useful for ATRP initiation were obtained. Mass spectrometry analysis of the composition of the plasma phase revealed that the desired structural moiety can be retained through the plasma, if the plasma conditions are steered toward ions of the precursor molecule. Whereas at low pressure the plasma polymer assembles mainly from various neutral (radical) fragments, at higher pressure the deposition occurs from hyperthermal ions, among which the protonated intact molecular ion is the most abundant. At higher pressure, a substantial population of ions has low kinetic energy, leading to "soft landing" and thus less fragmentation. This study demonstrates that relatively complex structural motifs in precursor molecules can be retained in plasma polymerization if the chemical and physical processes occurring in the plasma phase are elucidated and controlled such that desirable larger structural elements play a key role in the film deposition.

While plasma polymerization is used extensively to fabricate functionalized surfaces, the processes leading to plasma polymer growth are not yet completely understood. Thus, reproducing processes in different reactors has remained problematic, which hinders industrial uptake and research progress. Here we examine the crucial role pressure plays in the physical and chemical processes in the plasma phase, in interactions at surfaces in contact with the plasma phase, and how this affects the chemistry of the resulting plasma polymer films using ethanol as the gas precursor. Visual inspection of the plasma reveals a change from intense homogeneous plasma at low pressure to lower intensity bulk plasma at high pressure, but with increased intensity near the walls of the chamber. It is demonstrated that this occurs at the transition from a collision-less to a collisional plasma sheath, which in turn increases ion and energy flux to surfaces at constant RF power. Surface analysis of the resulting plasma polymer films show that increasing the pressure results in increased incorporation of oxygen and lower cross-linking, parameters which are critical to film performance. These results and insights help to explain the considerable differences in plasma polymer properties observed by different research groups using nominally similar processes.

Antimicrobial surface coatings that act through a contact-killing mechanism (not diffusive release) could offer many advantages to the design of medical device coatings that prevent microbial colonization and infections. However, as the authors show here, to prevent arriving at an incorrect conclusion about their mechanism of action, it is essential to employ thorough washing protocols validated by surface analytical data. Antimicrobial surface coatings were fabricated by covalently attaching polyene antifungal drugs to surface coatings. Thorough washing (often considered to be sufficient to remove noncovalently attached molecules) was used after immobilization and produced samples that showed a strong antifungal effect, with a log 6 reduction in Candida albicans colony forming units. However, when an additional washing step using surfactants and warmed solutions was used, more firmly adsorbed compounds were eluted from the surface as evidenced by XPS and ToF-SIMS, resulting in reduction and complete elimination of in vitro antifungal activity. Thus, polyene molecules covalently attached to surfaces appear not to have a contact-killing effect, probably because they fail to reach their membrane target. Without additional stringent washing and surface analysis, the initial favorable antimicrobial testing results could have been misinterpreted as evidencing activity of covalently grafted polyenes, while in reality activity arose from desorbing physisorbed molecules. To avoid unintentional confirmation bias, they suggest that binding and washing protocols be analytically verified by qualitative/quantitative instrumental methods, rather than relying on false assumptions of the rigors of washing/soaking protocols.

Historically, there have been two opposing views regarding deposition mechanisms in plasma polymerisation, radical growth and direct ion deposition, with neither being able to fully explain the chemistry of the resultant coating. Deposition rate and film chemistry are dependent on the chemistry of the plasma phase and thus the activation mechanisms of species in the plasma are critical to understanding the relative contributions of various chemical and physical routes to plasma polymer formation. In this study, we investigate the roles that hydrogen plays in activating and deactivating reactive plasma species. Ethyl trimethylacetate (ETMA) is used as a representative organic precursor, and additional hydrogen is added to the plasma in the form of water and deuterium oxide. Optical emission spectroscopy confirms that atomic hydrogen is abundant in the plasma. Comparison of the plasma phase mass spectra of ETMA/HO and ETMA/DO reveals that (1) proton transfer from hydronium is a common route to charging precursors in plasma, and (2) hydrogen abstraction (activation) and recombination (deactivation) processes are much more dynamic in the plasma than previously thought. Consideration of the roles of hydrogen in plasma chemistry may then provide a more comprehensive view of deposition processes and bridge the divide between the two disparate schools of thought.