**5. Surface aging**

**Figure 2.** AFM topographic images of HA plasma polymer films deposited with a power of 20 W: (a) as deposited, (b)

Immersed in a solvent, plasma polymers can be subject to numerous processes such as:

However, not many studies focus on the physic-chemical changes that happen to the plasma

Plasma polymer stability behavior depends on the type of the polymer. Muir et al. [72] studied the penetration of water into the films and characterized the swelling of allylamine (Aam) and heptylamine (HA) plasma polymers. When immersed in water, the plasma polymerized Aam film (ppAam) was found to swell by 5% and to contain 3% water whereas the ppHA film did not appear to swell but contained 5% water. The swelling characteristics of other plasma

Moreover, the degree of swelling strongly depends on the plasma process parameters. Zhang et al. [73], demonstrated that ppAam deposited at 20 W only shows a small degree of swelling while ppAam deposited at 5 W shows a large degree of swelling. This is due to the fact that at low powers the plasma polymer contains a large number of oligomers which are not covalently bound to the film; these oligomers can thus be readily extracted in the solvent. In fact, when studying the morphology of ppHA, Vasilev et al. [76] found that pores of several nanometers in diameter were formed after ppHA has been immersed in water for 24 h (see **Figure 2**). And the dimension of the pores was found to depend on the deposition conditions with larger pores obtained at lower powers (see **Figure 3**). This was attributed to oligomer water extraction after low molecular weight fragments were detected in the water. This results in the formation of gaps in the film and leads to ruptures of the polymer chains thus

Förch et al. [77] found that for ppAam, the roughness of the polymer film increased from 0.85 to 1.26 nm after soaking in water which was attributed to the swelling of the film in water;

• delamination from the substrate

polymer films after exposure to aqueous environments.

polymers have also been reported [73–75].

• detachment of oligomers

76 Recent Research in Polymerization

• reaction with the solvent

• swelling

after immersion in water for 24 h.

forming the observed porosity.

It is widely known that the enhancement in surface wettability obtained after plasma activation processes changes with storage time. This phenomenon is referred to as aging or hydrophobic recovery and is due to the tendency of a surface to minimize its surface energy by reverting to its original structure. This leads to a loss of surface polar functional groups that re-orientate to the bulk [18]. Therefore, in the case of plasma activation, in order to avoid the adverse effect of aging, it is advisable to only use freshly prepared samples.

On the other hand, plasma surface grafted polymers and plasma polymerized films show much less modifications after storage in ambient air and are thus considered comparatively stable in time. However, research on plasma polymers show that they are susceptible to oxidation upon storage in air [31]. Since these coatings have shown great potential for many applications including biomedical ones, several studies have been done to better understand this so-called aging process and therefore further evaluate the relevancy of plasma polymers. And since most products are usually stored for a certain period before they are used, the film properties at the time of use are usually considered more important than immediately after treatment.

Major advancements in the understanding of oxidative reaction mechanisms that occur during plasma polymer aging have been made by Gengenbach et al. [80–83].

This was done using XPS, FTIR spectroscopy and contact angle goniometry characterization techniques which allowed significant perception of the eventual surface compositional changes.

Their studies included detailed oxidation investigations of hydrocarbon based plasma polymers [80], fluorocarbon coatings [81], nitrogen coatings [82] and other plasma deposited films [83]. Results showed that the aging process was due to the reaction of ambient oxygen with the residual radicals present in plasma polymers; ESR spectroscopy showed that the free radicals detected in freshly deposited plasma films slowly disappear upon storage in air. Results also showed that, the kinetics, mechanisms and formed oxidative products during aging depend on many factors, such as the structure of the film, the type of functional groups and the mobility of the surface.
