Reactions between Plasmas and Polymer Surface

The characterization of the surface chemistry of the modified polymer is one step in understanding the mechanism for cells adhesion. The next crucial step is to determine the nature and extent of chemical interactions between the overlayer of interest and the modified polymer surface. This step presents a challenge because cmrently there are no techniques available with the sensitivity to characterize chemical interactions for an atomically thin buried interface. Several approaches have been used to analyze buried interfaces. Ion sputter depth profiling (typically done with Ar ions) in conjunction with XPS can be used to evaluate a buried interface for overlayers 10 nm [2]. 

The parameters of treatment were chosen since these led to the most pronounced changes of polymer surface in our previous experiments [70-74]. It was observed elsewhere that plasma treatment of polymer macromolecules results in their cleavage, ablation, alterations of chemical structure and thus affects surface properties e g. solubility [75]. The chemical structure of modified polyethylene (PE) was characterized by FTIR and XPS spectroscopy. Exposition to discharge leads to cleavage of polymeric chains and C-H bonds followed by generation of free radicals which easily oxidize [10,76]. By FTIR spectroscopy the presence of new oxidized structures within whole specimen volume can be detected. IR spectra in the 1710-1745 cm interval [71,77] from PE, exposed to 

At plasma, show the presence of the oxidized structures (carbonyl, carboxyl, ester). 

The chemical structure of plasma-treated PE was examined using XPS, the oxygen concentration profile was determined from RBS measurement and the concentration of free radicals was determined by EPR technique [78], The -CH a -CH2 (of pristine PE), oxidized (-C=0, -COO, -COC-), -NH2, and also -C=C-groups (typical for aromates) were reported present at Ar plasma-treated high-density PE (HOPE) [70], 

Due to plasma treatment the radicals (R, free spin) are generated on polymer chain. Not only C-H but also C-C bonds are likely to brake, the later leading to fragmentation of polymer chain. The radical concentration determined by EPR method is in Table 3. The concentration of free radicals R decreases during the aging to about quarter of the initial after 80 days. The decrease of R is a result of radical recombination [h]. Detailed comment of EPR observation ( aging of radicals) is described too [79]. 

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