Cross-link density accessibility

Most MIPs show a heterogeneous distribution of binding sites and can be considered as polyclonal in their nature. In non-covalent imprinting, the amorphous material contains binding sites which are not identical because they may have different cross-linking density or accessibility. Moreover, the monomer (M) and the template molecule (A) may form complexes of different stoichiometry (MnA) in the pre-polymerization mixture [5]... 

Even if this does not reflect the swollen state porosity, it would lead to increased diffusional limitations and a larger specific surface area. The photopolymers probably have a more open pore structure in the swollen state giving the template more rapid access to the sites, which are in this case confined to a smaller surface area. The difference in the conversion of pendant double bonds, and thereby the difference in cross-linking densities between the two types of materials, is probably also a factor that comes into play. An increase in chain flexibility at the sites is likely to cause an increase in the template adsorption-desorption rate coefficients. In this context it is interesting to note that increased rate enhancements were observed upon controlled hydrolysis of the polymer backbone of an imprinted esterase model [73]. 

The density of the cross-links and the fiber-matrix covalent links are dictated by the amotmt of diisocyanate used. It s also important to notice that -OH groups of HPC are more accessible, therefore most of the diisocyanate is consumed in cross-linking of HPC. The maximum permeability obtained for 0.5-1% (w/w HPC) of fibers is probably related to the maximum density of fiber-matrix covalent links allowed in the system, which is likely dependent on the amount of diisocyanate used. The increase of the fiber content leads firstly to the settling of fibers at the bottom surface and then to its distribution over the volume of the composites. Such fiber packing mode may be respmisible for the decrease in the diffusivity of the gases, which in turn causes the decrease in permeability. 

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