Multitechnique investigation into the aqueous behavior of plasma polymers

Abstract

Plasma polymers are often used in applications requiring aqueous immersion; therefore, it is important to understand how this exposure affects the physical and chemical properties of the films. Three different plasma polymer films were deposited at different distances from the electrode, and the film properties were characterized using contact angle, ellipsometry, and x-ray photoelectron spectroscopy. The film behaviors in aqueous solutions were studied via quartz crystal microbalance with dissipation (QCM-D). Exposure to buffer solutions produced significant swelling of the plasma polymerized acrylic acid films, with swelling increasing with distance from the powered electrode, results that could be correlated with changes in film chemistry. Plasma polymerized octadiene and allylamine exhibited little swelling. These films exhibited changes in thickness and contact angle with respect to distance from the electrode, but this had little influence on their behavior in aqueous solution. By combining QCM-D with the more traditional surface chemical analysis techniques, the authors have been able to explore both swelling behavior and the effect that sample position and thus deposition parameters have on film properties and aqueous behavior. This approach gives the authors the basis to define deposition parameters to assist the engineering of thin films for applications such as biosensing and tissue engineering applications where specific chemistries and film behaviors are desired.

Exploiting Reactor Geometry to Manipulate the Properties of Plasma Polymerized Acrylic Acid Films

“…The decreases in film thickness and increases in the COOH/R groups as the distance from the electrode increases are likely to be contributed to by reduced monomer fragmentation further from the electrode. Our previous research has shown decreasing electron flux with increasing distances from the electrode [3], results in decreased electron collisions [34]. A previous study has shown that the optical emission of an ethylene plasma peaks 10–15 mm from the electrode and decreases as the distance from the electrode increases, indicating decreasing plasma density and therefore lower collisions further from the electrode [35].…”

“…Plasma polymerization was undertaken at low pressure with a stainless steel T-piece reactor with a volume of 16.2 L and 170 mm diameter internal aluminium disk electrode, as previously described [3]. Briefly, the electrode was connected to a 13.56 MHz radio frequency (RF) power source generator (Coaxial Power Ltd., Eastbourne, UK) with an impedance-matching network to generate the plasma.…”

“…The properties of plasma polymer films, such as chemistry, thickness and stability are conventionally controlled by varying the input monomer/gas, plasma power, monomer flow rate and deposition time. Deposition at lower powers reduces monomer fragmentation, which results in the deposition of relatively intact monomer molecules on the surface; however, these films tend to be less stable in solution due to their low crosslinking [2] and incorporation of low-molecular-weight fragments within the film [3,4,5]. Increasing the power tends to produce more stable films due to higher crosslinking; however, the number of functional groups are typically reduced due to greater monomer fragmentation [6].…”

“…The properties of plasma polymerized acrylic acid (ppAAc) can also be influenced by other parameters such as co-polymerization [7,8,9,10,11,12], plasma pulsing [13,14,15,16,17,18] and reactor geometry [3,19,20,21]. Acrylic acid is a commonly used monomer for plasma polymerization and produces a negatively charged carboxylic acid terminated surface.…”

“…Previous work by our group has shown that plasma polymerized allylamine and octadiene films deposited in a T-piece reactor increased in film thickness as the distance from the electrode increased, due to decreasing fragmentation, but did not result in changes in the surface chemistry or swelling. ppAAc films, however, exhibited increasing film swelling and carboxylic acid group concentrations but decreasing film thicknesses, as the distance from the electrode was increased [3]. Several studies have deposited ppAAc films at varying sample distances from the monomer inlet.…”