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Discussion
An explanation of the nature of the APTES surface modification cannot be given that is consistent with the currently held view of the chain-insertion and amidation (amides that have not been spectroscopically observed) of PET. Rather, a new
scheme is proposed (Figure 1), where APTES adsorbs to and then cross-links on PET, forming a multilayer siloxane much as in silane multilayers on silica. The first step in this process (a) is the physisorption of APTES to the polymer surface
through hydrogen-bonding an H-bond acceptor on the surface. Siloxane bond formation through the ethoxy groups with neighbors H-bonded to the surface or with free APTES molecules in solution then occurs as shown in (b). In contrast
with prior models,we assert that covalent bonding between APTES and the polymer substrate does not occur, even with extended reaction time. Furthermore, perfect registry between the donor amine and the acceptor groups is not required. It is likely that some of the APTES molecules are not actually
H-bonded to the surface; however, the lateral bonds ensure that the layer will not come off. The inter-APTES reaction to form siloxane cross-links thus becomes the driving force of the reaction. After (or concurrently with) the lateral cross-linking
occurs, the layer builds up in thickness to form a thin multilayer as evidenced by the lack of angle dependence of the XPS atomic concentrations. Some subtleties concerning the role of water in this proposed mechanism should be addressed. Water does not need to be initially available at the polymer surface to create a
film, as physisorption occurs independent of water content.However, as has been previously established for silane deposition, for the cross-linking reaction to proceed, trace amounts of water are necessary.With respect to the polymer films created, we assume most of the water present in the system is in solution due to an inherent amount of water present in the solvents, and the ability of the solvents to abstract water from the surfaces of the vial and substrate at elevated temperatures. For in-situ cross-linking to occur, either water must diffuse to the already physisorbed surface or some of the APTES molecules react in solution and contribute to film formation en masse. The implication of such small amounts of water being present in the reaction system is that the surfaces as initially
created are poorly hydrolyzed and have a limited degree of cross-linking. This points to the necessity of an additional hydrolyzation step in which the films were immersed in water. Using this model of APTES modification, previous results
can readily be explained, and the reason for all of the polymers except polystyrene and poly(tetrafluoroethylene) to be heavily modified becomes clear. All modified polymers have polar, H-bond accepting groups allowing for the initial adsorption
event of the chain reaction. Lacking groups such as these, polystyrene and poly(tetrafluoroethylene) do not support the reaction. The influence of solvent on the effectiveness of the reaction also supports our proposed model. Protic solvents such as the three with negative results (formamide, ethanol, and 1-octanol) would interfere with H-bonding and prevent APTES modification. Barring this, varying the medium of reaction should have little effect. Exactly this is observed. The effects
of replacing the reagent ethoxy groups are also congruent with our model of reaction. Lateral bond formation is a necessary event. Interfering with this process would have negative consequences on the reaction. With just two possible linking
points, APMDS can at most form linear chains that are not as effective as cross-linked networks at anchoring the layer on thesurface. When two ethoxy groups are replaced, the APDES layer cannot maintain its integrity at all due to its ability to form only dimers. Last, while nPTES/butylamine results do not suggest that previously held models are wrong, the necessity of the amine being attached to the silane also supports the proposed model. In short, the evidence is incongruent with the previous held notions of the reaction yet is consistent with our hydrogenbonding cross-linking model.

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