Influence of external stimuli on the network properties of cationic poly(N-acryloyl-N’-propyl piperazine) hydrogels

Abstract

Stimuli-responsive cationic hydrogels based on N-acryloyl-N′-propyl piperazine (AcrNPP) crosslinked with N,N′-methylenebisacrylamide (Mba) and 1,4-butanedioldiacrylate (Bda) were prepared by UV light-initiated free-radical polymerization in bulk. The effect of external stimuli and type of crosslinker on the equilibrium swelling behavior and dynamic swelling was investigated in detail in buffer solution of various pH and temperatures. The equilibrium swelling capacity of the gels was large in swelling medium at pH 3.0 than at pH 10.0 due to ionization of polymer network under acidic conditions. With increase in temperature from 25 °C to 45 °C, the gels exhibited negative temperature-responsive (thermo-shrinking/thermophobic) behavior with negative activation energy for diffusion of water. The thermodynamic parameters such as Gibbs' free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for the swelling of gels as function of temperature were negative indicating an exothermic swelling process. Water (media) transport mechanism and diffusion process in thin rectangular gels was studied. At pH 3.0, the diffusion process was non-Fickian (anomalous) while at pH 10.0 it was quasi-Fickian. The transport mechanism was partly influenced by the type of crosslinker in the gel. The dynamic swelling data was analyzed using early-time, late-time and Etters diffusion models. From the equilibrium swelling studies the average molecular weight between crosslinks (Mc), the crosslink density (ρc), and the mesh size (ξ) were determined. The Mc was large at pH 3.0 due to ionization of polymer and chain expansion. The experimental Mc was much larger than the theoretical Mc which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 Å in the swollen (ionized) state (pH 3.0), and between 100 and 231 Å in the collapsed (non-ionized) state (pH 10.0). The mesh size increased between three to four times during the pH-dependent swelling transition. The state of water in fully swollen hydrogels which influences many important biomaterial properties was determined by differential scanning calorimetry. The bound water content of gels increased linearly with increase in pH of the swelling medium while the unbound water decreased. These hydrogels have potential to be used as controlled drug delivery systems and sorbents for removal of pollutants from water.