Polymer adhesion surface hydrophobicity

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). 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

In addition to PEG, certain polysaccharides, such as hyaluronic acid (HA) and dextran, have been used as low protein-adsorption, low cell adhesion surface coatings. Synthetic polymer surfactants consisting of poly(vinyl amine) with dextran and alkanoyl side chains, which mimicks the glycocalyx—negatively charged sugar layer outside cell membrane, are also shown to reduce protein adsorption to hydrophobic graphite surface. ... 

Untreated, many polymers, in particular polyolefins, are hydrophobic and therefore tend to repel water and water-based substances. The oxidation of the film through flame surface treatment increases the surface energy of the film, making the material less repellent of water, improving film wettability. Increasing wettability is a necessary component of many manufacturing processes that require water-based inks and adhesive materials to be applied to the polymer film surface. 

Therefore, the first and the most important problem is the fibre-matrix adhesion. The role of the matrix in a fibre-reinforced composite is to transfer the load to the stiff fibres through shear stresses at the interface. This process requires a good bond between the polymeric matrix and the fibres. Poor adhesion at the interface means that the full capabilities of the composite cannot be exploited and leaves it vulnerable to environmental attacks that may weaken it, thus reducing its life span. Insufficient adhesion between hydrophobic polymers and hydrophilic fibres result in poor mechanical properties of the natural fibre-reinforced polymer composites. Pre-treatments of the natural fibre can clean the fibre surface. 

Figure 4. Influence of the polymer surface free energy 7sv on the adhesion of hydrophobic 5. epidermidis strains to various polymers. 

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