A combination of vapor sorption and dynamic laser light scattering methods for the determination of the Flory parameter χ and the crosslink density of a powdered polymeric gel

Reliable techniques can be easily used to determine the crosslink density and the Flory interaction parameter of polymeric gels, on condition that monolithic matrices of the crosslinked polymer are available, but they cannot be profitably applied to powdered forms. In the present work we propose an experimental procedure which is particularly suitable to determine the above mentioned parameters in the case of polymeric gel particles. The technique is based on polymer swelling experiments performed in the presence of vapour and liquid external phases, and was applied to four different commercial types of sodium starch glycolate. Such a crosslinked polymer is normally supplied in a powdered form and commonly used in the pharmaceutical field as disintegrant in tablet formulation. For each polymer, the crosslink density ρx and the Flory interaction parameter χ were calculated from the experimental swelling data by solving the relevant equilibrium equations deriving from the Flory theory for gel/liquid and gel/vapor equilibrium conditions. The crosslink density values were then used to estimate the mesh size of the polymeric networks. Such a parameter is of great interest in the pharmaceutical field since it can play an important role in the performance of drug release systems based on polymeric gel matrices or minimatrices.