Abstract
AbstractThermoresponsive hydrogels are of great importance as smart materials. They are usually composed of cross‐linked polymers with a lower critical solution temperature (LCST). Although much is known about networks of poly(N‐isopropylacrylamide), all other polymers are somewhat neglected. In this work, the temperature‐dependent swelling behavior of differently cross‐linked thermoresponsive poly(2‐ethyl‐2‐oxazoline) (PEtOx) hydrogels were investigated with regard to varying parameters of the network composition. It was found that the degrees of swelling of the hydrogels converge for a certain polymer/solvent system at a distinct temperature independent of its degree of cross‐linking. Furthermore, this temperature correlates with the LCST of the respective starting PEtOx. Its net chain molecular weight Mc only affects the maximum degree of swelling and thus, the swelling–deswelling rate of the hydrogel. The fundamental structure/property relations found in this study could be useful to predict the behavior of other thermoresponsive hydrogels.
Highlights
Smart or responsive materials react to external stimuli by changing at least one of their properties, such as shape,[1,2] stiffness,[3,4] or optical properties.[5]
The temperature-dependent swelling behavior of differently cross-linked PEtOx hydrogels was investigated with regard to the molecular weight and, respectively, the lower critical solution temperature (LCST) of the applied starting polymer, the net chain molecular weight of the synthesized hydrogel, the kind of solvent, and the cross-linker composition
We found that the degree of swelling S of the hydrogels converges in a break point temperature TBP for a certain polymer/solvent system independent of its degree of cross-linking
Summary
Smart or responsive materials react to external stimuli by changing at least one of their properties, such as shape,[1,2] stiffness,[3,4] or optical properties.[5]. Polymers that undergo a temperature-triggered phase transition in aqueous solution are of great interest, for example, for medical applications in drug delivery,[14,15,16,17,18,19,20] artificial muscles,[21] and desalting/dewatering of proteins[22,23] or for actuators and sensors.[24,25,26,27] One class of such polymers is the lower critical solution temperature (LCST) polymers, which are fully soluble in water at low temperatures and phase-separate upon heating.[28] This phase separation is indicated by clouding appearing at the cloud point temperature (Tcp). Popular thermoresponsive polymers include poly (N-isopropylacrylamide) (PNiPAM),[29,30] poly(vinyl caprolactame)s,[31] poly(vinyl methyl ether)s,[32] and poly (2-alkyl-2-oxazoline)s (POx).[28,33,34,35] POx polymers have
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