Structural phenomena in hydrogel-drug systems

Crystallization and structural transition in a cross-linked poly(ethylene o xide) 4000 (PEO) polymer were investigated as functions of water content an d concentration of model drugs (acetaminophen and caffeine) using small ang le X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) technique s. In the hydrated state (300 wt% of water), gel retains an ordering with c haracteristic spacing of 80 Angstrom. With a water concentration of about 2 0%, this structure undergoes transition into lamellae with long spacing of 200 Angstrom and then into semi-crystalline polymer matrix when water conte nt is below 5%. This transition is associated with different type of phase separations, first, between crystalline and amorphous PEO domains in the dr y polymer and, second, between water and hydrophobic cross-linked regions i n the swollen gel. Because of a specific hydrogen bonding, acetaminophen fo rms a molecular complex with the PEG, a drug concentration as small as 1% r esulting in a significant increase of the long spacing and decrease of crys tallinity of the polymer. Caffeine precipitates in the form of crystalline particles and also reduces the crystallinity of the polymer matrix. (C) 199 9 Elsevier Science B.V. All rights reserved.