Fig. 1. (a) Chemical structures of the non-hydrolysed di-ureasil precursor (d-UPTES) and the CPEs investigated. (b) Schematic representation of the two synthetic routes used to prepare CPE-di-ureasil composites. In the Direct Insertion method, the d-UPTES precursor is mixed with a solution of the CPE prior to initiation of the sol–gel reaction. Hydrolysis and condensation of the silica network proceeds with the CPE in situ, such that it becomes trapped within the di-ureasil matrix. In the solvent Permeation method, a pre-formed di-ureasil sample is immersed in a solution of the CPE. The di-ureasil absorbs the solution, leading to uptake of the CPE. Upon drying the di-ureasil contracts to its initial size.