Most of the commonly used polymers are hydrophobic by nature. Hydrophobic surfaces suffer from poor wettability which makes addition of coatings on them challenging. Plasma treatment is one of the most widely utilized surface treatment methods to improve the hydrophilicity of the surfaces. Oxygen and nitrogen plasmas are most commonly used. Plasma treatment is able to increase the wettability of most polymers. However, hydrophobic recovery is often encountered which causes wettability to decrease as a function of time.
What happens in plasma treatment?
In a plasma treatment process, the surface is placed in a chamber under vacuum and bombarded by ionized gas. The ionized gas will induce chemical reactions at the surface of the material being treated, changing its surface energy. Typically, polymer surfaces exposed to oxygen or nitrogen plasmas become more hydrophilic due to formation of high energy surface groups in reactions between the native surface groups of the polymer and the reactive plasma species. In addition to surface chemistry, plasma treatment also often affects the surface topography. This means that in some cases, the surface roughness needs to also be taken into account when wettability of the surface is evaluated.
What causes the hydrophobic recovery?
One common problem with hydrophilization is that it is typically not stable, and either a partial or complete hydrophobic recovery is usually observed. Typically the hydrophobic recovery is a significant drawback for most applications as the bonding ability will decrease when wettability of the surface decreases. The main reasons for the recovery have been identified as being the reorientation of the surface layer, or the migration of the polymer chains from the bulk of the polymer to the surface or from the surface to the bulk.
Contact angle measurements can be used to study the effect of plasma treatment
Contact angle measurements are used to evaluate the efficiency of the plasma treatment as well as the hydrophobic recovery process. Contact angle measurements offer an simple and fast method to check the wettability of the surface. In addition to basic contact angle measurements it is also possible to do combined contact angle and roughness measurements to see the effect of roughness on the contact angle data.
To read more about how contact angle measurements are utilized to determine the plasma treatment time, please download the case study through the link below.
Jokinen, V., Suvanto, P. and Franssila, S., "Oxygen and nitrogen plasma hydrophilization and hydrophobic recovery of polymers", Biomicrofluidics 6 (2012) 016501.
Contact lens should support a continuous tear film for optimum visual clarity. It should also resist the sorption of tear components as deposition can lead to decreased visual clarity and discomfort. Wettability of the surface affects both the tear film as well as biocompatibility of the lens.
In industry, wettability is important for example when manufacturing electronics or when planning how to extract oil from the oil reservoir. In all these applications, knowing the contact angle value gives a strong indication on the performance of the product or a successfulness of the process.