Relationship between Temperature-Induced Changes in Internal Microscopic Structures of Poly(N-isopropylacrylamide) Microgels and Organic Dye Uptake Behavior

Abstract

Temperature-induced changes in the internal structures of poly(N-isopropylacrylamide) (pNIPAm) microgels were evaluated by small-angle X-ray scattering (SAXS), and the results were used to explain organic dye uptake by the microgels. The dye uptake experiments were conducted using two organic dyes: cationic rhodamine 6G (R6G) and anionic erythrosine. In the SAXS investigation, the internal structures of the microgels were characterized in terms of the correlation length, ξ, and the distance, d*, which originated from the local packing of the isopropyl groups of two neighboring chains. With increasing temperature up to the volume phase transition temperature (VPTT) of the microgels, the correlation length, ξ, was increased and the distance, d*, was decreased. At the same time, the amounts of the dyes taken up by the pNIPAm microgels were increased, despite a decrease in the volume of the microgels. The results indicated that the pNIPAm chains were closer to each other due to the hydrophobic association of isopropyl groups, which resulted in the growth of the hydrophobic domains. Thus, the hydrophobic interactions between the dyes and pNIPAm were probably accompanied by the domain formation. With a further increase of temperature above the VPTT, the correlation length, ξ, was decreased and then not defined because the Ornstein-Zernike type contribution disappeared, and the distance, d*, was not largely changed. At the same time, the uptake amounts of the dyes per unit volume of the microgels were also not largely changed, which behaved similar to the distance, d*. It was probably due to the fact that the internal structures of the microgels were not largely changed because the isopropyl groups were in contact with each other. The view was supported by the result of the uptake study of the nonthermoresponsive microgels which did not have the hydrophobic isopropyl groups.

Small-Angle X-ray Scattering Study on Internal Microscopic Structures of Poly(N-isopropylacrylamide-co-tris(2,2′-bipyridyl))ruthenium(II) Complex Microgels

“…A similar trend was seen previously when pNIPAm or poly- Langmuir XXXX, XXX, XXX−XXX (NIPAm-co-3-(methacrylamino) propyltrimethylammonium chloride) microgels were studied; 13,37 however, the present study differs from these microgels. 13,37 Although the NRu microgels in the oxidized state were highly deswollen at 25 or 30°C (Figure 2b), the scattering intensity at q < 1 nm −1 was not able to be explained solely based on Porod's law ( Figure 5), as the contributions from the Guinier and OZ equations were still overlapped. This indicates that the internal structure of the deswollen NRu microgels is still inhomogeneous in length roughly corresponding to q < 1 nm −1 , most likely due to the presence of highly charged Ru(bpy) 3 complexes preventing the partial gel network from collapsing.…”

“…In the previous studies, the best fit curves for the scattering intensity of the pNIPAm-based microgels were obtained by taking five different structural features into account. 13,37 In this study, we analyzed the scattering intensities obtained from the NRu microgel dispersions, where the Ru moieties in the microgels were in either the reduced or oxidized states, according to the method shown in the previous studies. 13,37 Here, high ionic strength (1.5 M) solutions were chosen, as the green color of the oxidized state became orange upon reduction after the SAXS measurements when the ionic strength of the dispersion was below 1.5 M ( Figure S4 Figures 3 and 4, 16°C ≤ T ≤ 35°C in Figure 5).…”

“…13,37 In our previous study, it was demonstrated that hydrophobic interactions occurred between the hydrophobic domains in the pNIPAm microgels and rhodamine 6G or erythrosine (cationic and anionic organic dyes, respectively), with increasing uptake of dye as the hydrophobic domains in the pNIPAm microgels grew. 13 It has also been reported that the correlation length (ξ) showed critical behavior near the volume phase transition temperature (VPTT) of pNIPAm microgels; before which, the occurrence of critical behavior in the microgel system was unclear. 13 Therefore, we expected that the SAXS study on poly(NIPAmco-Ru(bpy) 3 ) microgels would allow us to understand their internal microscopic structures in aqueous media.…”

“…13 It has also been reported that the correlation length (ξ) showed critical behavior near the volume phase transition temperature (VPTT) of pNIPAm microgels; before which, the occurrence of critical behavior in the microgel system was unclear. 13 Therefore, we expected that the SAXS study on poly(NIPAmco-Ru(bpy) 3 ) microgels would allow us to understand their internal microscopic structures in aqueous media.…”

Critical fluctuations and static inhomogeneities in polymer gel volume phase transitions

“…This notice is included in the online and print versions to indicate that both have been corrected on 21 May 2015. functions to capture both solidlike gel inhomogeneity on a lengthscale N and liquid-like thermal concentration fluctuations on a lengthscale n. 18 With this analysis, n was shown to diverge upon approach to the volume phase transition temperature by a power law with an exponent of m 0.6, suggesting this transition to be a critical phenomenon with three-dimensional Ising universality. 19 A similar, extended analytical approach was later applied to comparable SANS and SAXS data recorded on colloidal-scale pNIPAAm microgels by Fernandez-Barbero, 27 Hellweg, 28 Suzuki, 29 and their respective co-authors, partly denoting similar scaling exponents. 27,28 Some years after the pioneering work by Shibayama, Tanaka, and Han, in the mid-1990s, Panyukov and Rabin used replica field theory methods to model potentially inhomogeneous polymer-network structures, leading to a sum expression for the density correlation function and, hence, the structure factor of polymer gels, S(q) 5 G(q) 1 C(q).…”