Silane Physisorbed

Reactive surface treatment assumes chemical reaction of the coupling agent with both of the components. The considerable success of silanes in glass reinforced thermosets have led to their application in other fields they are used, or at least experimented with, in all kinds of composites irrespective of the type, chemical composition or other characteristics of the components. Reactive treatment, however, is even more complicated than non-reactive polymerization of the coupling agent, development of chemically bonded and physisorbed layers render the identification of surface chemistry, characterization of the interlayer... 

Removal of reversibly adsorbed (physisorbed) cyanine-dye molecules, D, led to skeletonized, SA silane monolayers having pinholes in the shape of D (Fig. 18) [185]. Such pinholes can, in turn, serve as templates for similarly shaped guest molecules. This approach opens the door for molecular recognition and device construction based on molecular recognition [186-190]. The idea relies upon the construction of a well-packed, SA monolayer from a mixture of OTS molecules and a solvent removable guest species. Removal of the guest molecules leaves pinholes with precise dimensions which only accept molecules with... 

The substrate/silane interphase and the silane/matrix interphase are equally important in considering the mechanism of reinforcement by silane coupling agents in composites. The mineral oxide/silane interphase is more well defined than a metal/silane or a silane/matrix interphase. For example, in the case of a metal substrate, surface oxides may dissolve into the silane layer or form a complex. In the case of the silane/matrix interphase, a diffuse boundary layer may exist due to dispersion of physisorbed silanes in the matrix phase or penetration of the matrix resin into chemisorbed silane layers. Many features of the interaction of a silane coupling agent with a polymer matrix are specific to the system, and thus the chemistry of the silane/matrix interphase must be characterized and defined for each system. 

However, the chemical bonding theory cannot account for the increase in adhesion experienced between non-reactive matrices such as polyolefins and inorganic reinforcements in which chemical bonds will not be formed [4], This observation, among others, leads to an alternative proposal that an interphase composed of various constituents forms surrounding the reinforcement. This third phase in the composite is possibly formed through interdiffusion of physisorbed silane and matrix molecules in the interphase and perhaps via preferential adsorption of both matrix components as well as silane coupling agents on the reinforcement surface [5],... 

Covalent bond formation is not an immediate process. Silane coating layers consist of physisorbed as well as chemisorbed molecules. Physisorbed molecules go into condensation only slowly and chemical stabilization of the coating layer requires a post-reaction curing step. In this step the modified substrate is thermally treated at temperatures generally in the 353 - 473 K range. 

Figure 9.18 Amount of physisorbed silane molecules per g of modified silica as a function of curing time in vacuum, (a) APTS (b) APDMS.

We have shown how hydroxyl groups on the silica surface act as active sites in the modification reaction. The amount of hydroxyls is controlled by the thermal pretreatment of the substrate. APTS molecules are physisorbed to the surface by hydrogen bonding of the amine group to a surface hydroxyl (H). Chemisorption of APTS to the silica surface, in dry conditions, involves the formation of siloxane bonds with release of ethanol (I). Water causes the hydrolysis of the ethoxy groups of the APTS, with formation of silane silanols. These silanols are more reactive than the original alkoxy groups. Siloxane bonds with other silane molecules or with the silica surface are formed with release of water (J). 

The presence of significant quantities of physisorbed water promotes the chemisorption of alkoxysilanes, especially of the ones with two or three alkoxy groups. Engelhardt and Orth90 reported that chemisorption of n-octyltriethoxysilane from a dry toluene solution on a dehydrated silica amounts 0.30 fimol/m2. However, if a toluene solution containing traces of water is used, the amount of chemisorbed silane increases up to 1.25 /tmol/m2. 

Also on theoretical grounds, it can be expected that the chemically bounded, electron donating amine function would have a larger impact on the electronic environment of the Si-H species, than the physisorbed NH4C1 salt. This is probably the reason why Low15 observed no perturbation of the silane peak, when he sublimed NH4C1 on the chlorosilylated silica surface. 

Culler et al. were able to distinguish between chemisorbed and physisorbed APS in DRIFT spectra of APS-coated silicon powder 22). Removal of physisorbed silane by exposure to warm water reveals a hydrolytically stable layer of APS with less than monolayer coverage, covalently bound to the surface. They concluded that intermolecular silane bonds are easily hydrolyzed and that the overall degree of condensation was unimportant to hydrothermal stability. Hydrothermal stability was only enhanced by increasing the number of direct surface bonds. The electro-osmosis study supports this view of APS surface chemistry. 

It presents a comparison of the observed GOD activities with activity of GOD physisorbed on native, underivatized CPG. An increase of enzyme activity over a time of four weeks was observed. The first bar shows the absorption of the sample immobilized without any reagent on native CPG. The second bar results from immobilization using APTES/GA, and the last two bars show the activity of GOD immobilized using aldehyde-functionalized silanes with the same chain length (C7) but differing in the number of substituent efrioxy groups. 

On the first day, all samples show measurable catalytic activity, including the samples where the enzyme was coupled to the native, underivatized CPG. For the latter, however, the activity decreases almost to zcto over the following weeks. This can be explained by desorption of the physisorbed GOD finm unmodified CPG. In contrast, immobilizations using functionalized silanes provided increasing activities arriving at a constant value after two weeks. 

The structure of the physisorbed silane molecules tends to be cyclic oligomers, and their molecular weight strongly depends on the pH of the filler (22). The pH of the filler is conveniently measured by the sliury pH method where a certain amount of filler is boiled in water for a specified time and the pH of the liquid measured. This method provides composite information about both the solubility of the selective surface components as well as the concentration of the siuface acid/alkaline centers. This sluny pH directly relates to the molecular weight of the conden silane rather than the concentration of the surface acid centers alone, since the solubility of the surface species and the concentration of acid centers both contribute to the pH of the silane solution that is on the filler surface. 

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