Abstract
Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N′-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N′-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van’t Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.
Highlights
Stimuli-responsive polymer hydrogels are great interest for advanced applications as smart materials, because of the ability to change their properties in response to different external stimuli, such as temperature, pH, presence of chemicals, exposure to light, electromagnetic field [1,2,3,4,5]
The transition region from the expanded to collapsed state is slightly shifted to higher temperatures in double network (DN) networks, the steepness of the transition region decreases and magnitude of deswelling of the DN hydrogels is strongly reduced relative to corresponding
These values are practically 2–4 times higher than the composition of reaction mixtures which was established during the synthesis of DN hydrogels (PDEAAm:PAAm monomer molar ratio = 1:2 and PDEAAm:PDMAAm monomer molar ratio = 1:2, see Section 2.1). This discrepancy could be explained as a possible formation of heterogeneous structure in DN hydrogels as we reported for PNIPAm/PAAm hydrogels [35]
Summary
Stimuli-responsive polymer hydrogels are great interest for advanced applications as smart materials, because of the ability to change their properties (volume, transparency, adsorption) in response to different external stimuli, such as temperature, pH, presence of chemicals, exposure to light, electromagnetic field [1,2,3,4,5]. Thermosensivity of polymer hydrogels is associated with a changed balance between various types of interactions, mainly hydrogen bonds and hydrophobic interactions. At temperatures below volume phase transition temperature (VPTT) hydrogels absorb water to reach the swollen state and above VPTT they release water and shrink. Volume phase transition (collapse) in crosslinked hydrogels is assumed to be a macroscopic manifestation of a coil-globule transition, as was shown for poly(N-isopropylacrylamide) (PNIPAm, VPTT ≈307 K) in water by light scattering [12].
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
