Cross-link Density Dependence of Spatial Inhomogeneities and Dynamic Fluctuations of Poly(N-isopropylacrylamide) Gels

Abstract

Dynamic light scattering studies have been carried out on poly(N-isopropylacrylamide) (NIPA) gels having different cross-link densities. Intensity−intensity time correlation functions obtained at 25 °C were successfully analyzed by taking account of the nonergodic nature of gels. By taking the ensemble average (〈I〉E) of the time average scattered intensity (〈I〉T), the dynamic component of the concentration fluctuations (〈I F〉T) and the spatial inhomogeneities (〈I〉E; the ensemble average) were evaluated as a function of cross-link density. While 〈I F〉T remained more or less constant, 〈I〉E increased with the increasing ratio of cross-linker concentration (C BIS) to the monomer concentration (C NIPA), r ≡ C BIS/C NIPA, where BIS is the abbreviation of the cross-linker, N,N‘-methylenebis(acrylamide). This indicates domination of the static inhomogeneity for large r. The correlation length, ξ, a measure of the range of dynamic fluctuations, was evaluated from the time correlation function. A linear relationship was obtained for ξ-1, i.e., 1/ξ = (1/ξsoln) + (1/ ξgel)r, with ξsoln = 144 Å and ξgel = 8.12 Å. The value of ξgel was found to be in good agreement with the literature value for the segment length of NIPA polymers, 8.12 Å.

Universality and Specificity of Polymer Gels Viewed by Scattering Methods

“…Figure 9 shows speckle patterns obtained from PNIPA hydrogels. 45 The cross-linker (BIS) concentration, C BIS , was varied from 0 to 325 mM, while the NIPA monomer concentration, C NIPA , was fixed to 690 mM. As shown in the figure, the speckle patterns become stronger with C BIS .…”

“…Figure 18 shows the time course of swelling ratio, d=d 0 , for (a) the PNIPA monomer cross-linked gels and (b) the -ray cross-linked gels with different cross-linking concentrations, after a T-jump from 20 to 45 C, where d is the diameter of the gel at observation and d 0 is the capillary diameter at which the gel was formed. 59 As can be seen from the figure, rapid shrinking was attained by simply lowering the cross-linker concentration or the irradiation dose for both types of gels indicated by filled symbols.…”

“…80 The appearance of the plateau region in hIi E is accounted for by suppression of the concentration fluctuations with C. Note that hIi E usually increases with increasing cross-link density. 45 However, in the case of physical gels, such as gelatin, the cross-link density is automatically fixed at a given temperature and concentration. When the ratio of cross-link concentration to the polymer concentration is constant, hIi E was confirmed to be a decreasing function of C.…”

New Thermo-Sensitive Hydrogel Based on Copolymer of 2-hydroxyethyl Acrylate and Ethyl Acrylate

“…In solutions, these polymers undergo phase separation upon heating and their cross-linked analogs (hydrogels) experience sharp contraction or collapse. N-isopropylacrylamide (NIPAAM) is one of the most studied monomers for the preparation of temperature-responsive polymers [5]. The cloud point of homopolymer of NIPAAM is around 32 °C, and copolymerization of NIPAAM with different anionic [6], cationic [7], and non-ionic [8] monomers can significantly change this value.…”

Temperature-responsive poly(N-vinylcaprolactam-co-hydroxyethyl methacrylate) nanogels for controlled release studies of curcumin

“…Typical LCST polymers include poly(N-isopropylacrylamide) (NIPAAm), [11,12] poly(N,N-diethylacrylamide), [13] poly(methylvinylether), [14,15] etc., are the most studied temperature-sensitive polymers. In comparison to their temperature-sensitive counterparts poly(N-vinylcaprolactam) (PNVC) is favored but less investigated.…”