Plasma crosslinking

Fig. 12. Schematic of a polymer-coated crosslinked PDMS cap in contact with a polymer-coated flat surface. The PDMS cap is oxidized in 02-plasma, and the polymer layer is coated by solvent casting. On flat surface, the polymer layer is spin coated.

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

In the presence of an inert gas, such as He or Ar, crosslinking can be introduced into the surface layer of material by plasma treatment. Hansen and Schon-horn [60] named this Crosslinking by Activated Species of Inert Gases (CASING). As a result, bond strength is enhanced because crosslinking strengthens the surface layer. 

Fibrin is an elastic filamentous protein elaborated from its precursor, fibrinogen, which is present in plasma at high concentration. Fibrin is formed in response to the actions of thrombin. Thrombin cleaves small peptides from the fibrinogen molecule, forming fibrin monomers that will begin to polymerize and become crosslinked. 

Very thin films may be also obtained through adsorption of a thin layer from solution [11,71,74] or chemical grafting [98] which is achieved by a polymerization reaction at the surface. A polymer film may also be deposited on the surface by plasma polymerization [99]. It is then, however, usually crosslinked and chemically not well-defined. 

FIGURE 19 Daily rabbit blood plasma levels of levonorgestrel from a crosslinked polymer prepared from a 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro[5,5Jundecane)/3-methyl-l,5-pentanediol prepolymer crosslinked with 1,2,6-hexane triol. Polymer rods, 2.4 x 20 mm, containing 30 wt% levonorgestrel and 7.1 mol% Mg(OH)2. Devices implanted subcutaneously in rabbits, (o) 1 device/rabbit, ( ) 3 devices/rabbit. (From Ref. 2.)... 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

Infrared Spectrum. The plasma polymerized organic film shows features distinctive from the conventional polymer. According to ESR measurements (31), the film contains a high concentration of residual free radicals, which showed a relatively long life time. The free radicals were oxidized in air and the oxidization is promoted significantly at elevated temperatures. The film is not soluble in usual solvents and it is more thermally stable than the conventional polymers. These properties are thought to be caused by the highly crosslinked structure of the film (32). 

In contrast to the film prepared in the plasma, a film prepared downsteam in the system shows little evidence for crosslinking and consists of mainly CF type carbon with a lesser amount of CFj, CF and C environments, its overall atomic composition being CiF2,2. The spectrum of this film (which incidentally is not unlike that which would be expected to result from radical or ionic polymerization of the (CFj) species discussed earlier) could be dramatically converted to a structure similar to that recorded for the polymer formed in the plasma by subsequent argon... 

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