“Click” PNIPAAm hydrogels – a comprehensive study of structure and properties

A detailed understanding of the structural and chemical features of hydrogels is vital when it comes to modifying and tailoring these materials into potential beneficial tools for different disciplines, such as drug delivery, membrane science, etc. However the characterization of hydrogels using conventional analytical techniques is a major challenge since they are three-dimensional crosslinked molecules of effectively infinite molecular weight. Control of the structure of networks of poly(N-isopropylacrylamide) (PNIPAAm) is especially important considering the reported sensitivity of the thermal properties of linear PNIPAAm to molecular weight and end group structure. Therefore in this work we have studied for the first time a series of telechelic PNIPAAm chains synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization and used these to prepare highly regular networks by end-linking to a tetra-acetylene crosslinker via the copper(I)-catalyzed azide–alkyne cycloaddition reaction. Detailed comments on the reaction conditions required to form hydrogels are provided. The pore structure of the hydrogels was varied by selection of the molecular weight of the PNIPAAm resulting in networks with different mesh sizes. The thermal and transport properties of the hydrogels are especially sensitive to the structure of the hydrogel and have been extensively characterized.