Pore structure in the swollen state

Pore size distributions in the swollen state can be obtained by a technique called thermoporosimetry [1,2]. This is based on measurements of the freezing point of solvents inside the pores. This is usually lower in the smaller pores and an empirical relationship has been found  

by scanning calorimetry at lower temperatures, a distribution curve is obtained allowing one to estimate the pore size distribution. Another alternative technique consists of freeze drying of the polymer and thereafter measuring the pore size distribution by the conventional gas sorption technique [2]. 

The swelling is also dependent on the level of cross-linking in the resin, i.e. the higher the cross-linking the lower the swelling. For heterogeneously cross-linked resins, as in the case of permanently porous, mesoporous or macroporous 

Since the weight of the bed volume of the dry particles is known, the apparent density, d pp, can be calculated  

Weight swelling ratio = Volume swelling ratio/c( 

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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]. 

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