Poly(N, N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure

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.

Keywords: NMR spectroscopy; differential scanning calorimetry; double network; poly(N,N′-diethylacrylamide); swelling; thermoresponsive hydrogel.

Figure 1

(a) Schematic model of temperature-dependent deswelling experiment of SN and double network (DN) networks (the figure shows an actual model of DN). The entire system is shown, and dashed-line squares denote the boundary of the polymer network during the collapse. (b) Two-state swollen-collapsed network model with permanently bound water.

Figure 2

Theoretical dependence of swelling ratio r(T) as defined by Equation (16) (also present in the graph). The essential features of the curve are shown, including fitted parameters highlighted by red color.

Figure 3

Schematic model of phase transition of one cooperative unit. Only one cooperative unit of the network sample is shown. Blue lines highlight the enhanced interaction between molecules of the cooperative water unit (containing N_wcu molecules).

Figure 4

Temperature dependence of swelling ratio for hydrogels SN2, DNA2, and DNM2. Experimental points are fitted according to Equation (16).

Figure 5

Differential scanning calorimetry (DSC) curves for SN1 and DNA1 hydrogels obtained during heating.

Figure 6

500.1 MHz ¹H spectra of hydrogels (a) SN1 and (b) DNA1 in D₂O recorded at 25 °C and 39 °C under the same instrumental conditions. Peak assignments are explained in the text.

Figure 7

Temperature dependences of p-fraction as determined from PDEAAm CH₃ signal in (a) SN and (b) DNA hydrogels. Experimental points are fitted according to Equation (2).

Figure 8

Time dependence of methylene blue (MB) concentration for release process for (a) SN2 hydrogel and (b) DNA2 and DNM2 hydrogels at 45 °C. Experimental points are fitted according to Equation (4).