Modification of bulk properties

The surface modification of polymers for improvement of adhesive bonding, and altering surface properties in general without concomitant modification of bulk properties is an active area of research in both industrial and academic laboratories and has been accomplished by a variety of means ranging from Corona discharge treatment, direct chemical modification and by interaction with plasmas excited in inert gases either capacitively or inductively27. ... 

The concentration of a small molecule reactant inside the polymer coils can be lower than outside when one uses a poor solvent for the polymer. This results in lower local and overall reaction rates. In the extreme, a poor solvent results in reaction occurring only on the surfaces of a polymer. Surface reactions are advantageous for applications requiring modification of surface properties without affecting the bulk physical properties of a polymer, such as modification of surface dyeability, biocompatibility, adhesive and frictional behavior, and coatability [Ward and McCarthy, 1989]. 

Some polymers have a specific set of bulk properties that make them ideal for a certain application, but cannot be used because the surface properties are inappropriate. For example, a material may have excellent elasticity but cannot be used in cardiovascular devices because the polymer surface triggers blood clotting. Alternatively, another polymer may have excellent surface biological compatibility but is too brittle for a cardiovascular application. An answer to this problem, like many others, is to select a polymer for its advantageous bulk properties and then carry out property modification reactions on the polymer surface without affecting the bulk material. 

The principal limitation to the classical theory is seen as the capillarity approximation, the attribution of bulk properties to the critical cluster (see Figure 11.8). Most modifications to the classical theory retain the basic capillarity approximation but introduce correction factors to the model [e.g., see Hale (1986), Dillmann and Meier (1989, 1991), and Delale and Meier (1993)]. 

Semiconducting silicides epitaxially grown on silicon have gained an increased practical interest to be used in novel semiconducting devices due to their high thermal stability, homogeneous interface and smooth surface morphology [1]. Lowdimensional structures are the main object of study and application in nanoelectronics. When the structure size in one direction decreases up to several run, its properties may differ essentially from the bulk properties of a source material. Such modification of the properties looks attractive. 

The action of compression plasma flows (CPF), generated by quasi-stationary plasma accelerators, upon solid surfaces leads to a substantial modification of surface properties of exposed materials [1-3]. It was found that exposure of silicon crystals to CPF causes formation of sub-micron bulk periodic structures on its surface. These structures are of great interest for development of nanoelectronic devices. 

For many industrial applications of plastics that are dependent on adhesive bonding, cold gas plasma surface treatment has rapidly become the preferred industrial process. Plasma surface treatment, which is conducted in a vacuum environment, affords an opportunity to minimize or eliminate the barriers to adhesion through three distinct effects (1) removal of surface contaminants and weakly bound polymer layers, (2) enhancement of wettability through incorporation of functional or polar groups that facilitate spontaneous spreading of the adhesive or matrix resin, and (3) formation of functional groups on the surface that permit covalent bonding between the substrate and the adhesive or matrix resin. Since plasma treatment is a process of surface modification, the bulk properties of the material are retained. The nature of the process also allows precise control of the process parameters and ensures repeatability of the process in industrial applications. Finally, several studies have demonstrated that these surface modifications can be achieved with minimum impact on the environment. 

In surfactant manufacture, propylene oxide (PO) is employed both as a hydrophobe (see Section 2.3 above), and as a modifier for poly(ethylene oxide). Propylene oxide is similar to EO except that it contains an additional methyl group, which adds steric bulk, and is significantly more hydrophobic in nature. If PO is inserted in the middle of an poly(ethylene oxide) chain, different properties (e.g. greater liquidity) are obtained. Since this approach requires three separate alkoxylations, it is only used when modification of specific properties is required. More common is the use of PO to cap the end of the poly(ethylene oxide) chain. This significantly reduces foaming, which can be critical in certain applications (e.g. machine dishwashing). 

Polymerization of adsorbed monomers on solid surfaces results in the formation of ultrathin coating layers. Obtained thin polymer layers are attractive coating materials for many applications due to their wide range of chemical, mechanical, electrical, and optical properties that can be engineered to fit specific needs. Therefore, the GASP method is very suited to the modification of surface properties of bulk materials. 

Solvents and Miscibility of the Liquid Phases. In a biphasic process, one may want to keep the catalyst exclusively in one of the phases, say phase A (Fig. 1). This requires its complete insolubility in the other solvent moreover, no modification of solvent properties of phase B under the actual reaction conditions can be tolerated, leading to dissolution of the catalyst in that phase. In contrast, components of phase B (the solvent, reactants, and products) can have limited solubilities in the catalyst-containing phase, and in fact, in most cases such solubility properties lead to faster reactions due to the better catalyst-substrate contact in the bulk of phase A. For example, the solubility of CH3COOC2H5 in water at 20°C is 6.1 wt% whereas that of water in CH3COOC2H5 is 3.3 wt%. Very similar values were determined for (C2H5)20 in H2O (6.9 wt%) and H2O in (C2H5)20 (3.3 wt%). For other mutual miscibility data with water as one of the phases, see References 49 and 50. 

There is not yet a complete and definitive thermodynamic treatment of colloidal systems which includes all the parameters of importance. In particular the problem of interactions between many particles of different sizes, particle solvent interactions in the presence of an adsorbed layer, etc. Several questions remain unresolved, e.g. the question of whether surface potentials or surface charges are modified as particles approach each other [5]. As the primary effect of surfactants on suspension stability is effected through adsorption and the modification of the properties of the interface, rather than on effects on bulk... 

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