Competitive swelling forces and complexation in pH- and temperature-sensitive interpenetrating network hydrogels [electronic resource]
Keywords: Chemical Engineering

Responsive polymers are of great interest to the scientific community and have been the subject of much research over the years due to their unique behavior. These 'smart' materials undergo a non-linear change in physical behavior in response to a wide variety of external stimuli such as changes in pH, ionic strength, temperature, light intensity, humidity, and electric field. Preparing crosslinked hydrogel networks from stimuli-sensitive polymers is particularly attractive because, in addition to their high water content, elastic nature (similar to natural tissue), and biocompatibility, these materials can mimic biological responsive behavior, making them especially useful for many biomaterial and drug delivery applications. In some instances it may be desirable to design a material that responds to multiple stimuli. This work studies the fundamental swelling properties of dual-responsive interpenetrating network (IPN) hydrogels. The first component is a pH-sensitive, poly(acrylic acid) (PAA) network with high swelling capacities that can be enhanced further by ionization. The second component is a temperature-sensitive, poly(N-isopropyl acrylamide) (PNIPAAm) network that undergoes a transition from the swollen to collapsed state as a result of hydrophobic aggregation above the lower critical solution temperature (LCST), ~32°C. IPN hydrogels tend to have unique physical properties because they are prepared by polymerizing one network in the presence of another, so that they are independently crosslinked, yet inseparable due to physical entanglements. The behavior of the overall material is, thus, more complex, acquiring properties from each polymer, which is particularly advantageous when both components are responsive. For these multi-component systems, however, there may be some conditions under which the swelling behaviors of each network are complementary and other conditions where they may oppose one another. In the case of the latter, physical entanglements can restrict the responsive nature of the overall material and the relative proportion of each component becomes particularly important. To better understand the balance of competitive forces in our hydrogels, we have studied the effects of IPN composition on the swelling properties under various conditions of pH and temperature, and compared their behavior with single network hydrogels of the individual components. Analyzing the swelling behavior of these IPN hydrogels, we have found that hydrogen-bonded complexation occurs between the PNIPAAm and PAA networks, resulting in dehydration of the hydrogel. This effect is most significant when the relative proportion of each component is approximately equal and occurs only at low pH, since the interaction relies on the protonated form of the poly(carboxylic acid). The thermo-responsive behavior of the IPN hydrogel is generally determined by the dominant network. As composition and pH change, the extent of collapse, at temperatures above the LCST, is a function of the molar ratio of NIPAAm to ionized AA residues, indicating that the collapse of the PNIPAAm network is hindered by an increase in osmotic pressure as the PAA network is ionized. We have also shown that the average water content of the IPN hydrogel (over the range of temperatures studied) is a function of the concentration of ionized AA residues, while the magnitude of the heat-induced collapse correlates to the percentage of NIPAAm residues. The results of wide angle x-ray scattering experiments have shown that single network hydrogels of both PAA and PNIPAAm exhibit nanoscale ordering upon dehydration of the network from either evaporation or thermo-sensitive collapse. The observed scattering peak corresponds to a correlation distance of ~1.1 nm, which we believe is a measure of the distance between polymer chains as they become desolvated and pack into bundles. IPN hydrogels exhibit a similar scattering peak; the presence of which is reasonably correlated to their water content based on the bulk swelling studies. Furthermore, when water is removed completely from these samples, we can see that complexation facilities the formation of this local chain ordering.