Energetics and Mechanisms of poly(N-isopropylacrylamide) Phase Transitions in Water–Methanol Solutions

Dynamic Behavior of Poly(N-isopropylmethacrylamide) in Neat Water and in Water/Methanol Mixtures

We investigate the collective dynamics of thermoresponsive polymer poly(N-isopropylmethacrylamide) (PNIPMAM) in aqueous solution and in water/methanol mixtures in the one-phase region. In neat water, the polymer concentration c is varied in a wide range around the overlap concentration c*, that is estimated at 23 g L-1. Using dynamic light scattering (DLS), two decays ("modes") are consistently observed in the intensity autocorrelation functions for c = 2-150 g L-1 with relaxation rates which are proportional to the square of the momentum transfer. Below c*, these are attributed to the diffusion of single chains and to clusters from PNIPMAM that are formed due to hydrophobic interactions. Above c*, they are assigned to the diffusion of the chain segments between overlap points and to long-range concentration fluctuations. From the temperature-dependent behavior of the overall scattering intensities and the dynamic correlation lengths of the fast mode, the critical temperatures and the scaling exponents are determined. The latter are significantly lower than the static values predicted by mean-field theory, which may be related to the presence of the large-scale inhomogeneities. The effect of the cosolvent methanol on the dynamics is investigated for polymer solutions having c = 30 g L-1 and methanol volume fractions in the solvent mixtures of up to 60 vol %. The phase diagram was established by differential scanning calorimetry. The slow mode detected by DLS becomes significantly weaker as methanol is added, i.e., the solutions become more homogeneous. Beyond the minimum of the coexistence line, which is located at 40-50 vol % of methanol, the dynamics is qualitatively different from the one at lower methanol contents. Thus, going from the water-rich to the methanol-rich side of the miscibility gap, the change of interaction of the PNIPMAM chains with the two solvents has a severe effect on the collective dynamics.

"One-pot" preparation and adsorption performance of chitosan-based La3+/Y3+ dual-ion-imprinted thermosensitive hydrogel

Temperature-sensitive materials are increasingly of deep interest to researchers. Ion imprinting technology is widely used in the field of metal recovery. In order to solve the problem of rare earth metal recovery, we designed a temperature-sensitive dual-imprinted hydrogel adsorption product (CDIH) with chitosan as the matrix, N-isopropylacrylamide as a thermally responsive monomer, and La³⁺ and Y³⁺ as the co-templates. The reversible thermal sensitivity and ion-imprinted structure were determined by differential scanning calorimetry, Fourier transform infrared spectrometer, Raman spectra, Thermogravimetric analysis, X-ray photoelectron spectroscopy, Scanning electron microscopy and X-ray energy spectroscopy various characterizations and analyses. The simultaneous adsorption amount of CDIH for La³⁺ and Y³⁺ was 87.04 mg/g and 90.70 mg/g, respectively. The quasi-secondary kinetic model and Freundlich isotherms model well described the adsorption mechanism of CDIH. It's worthy to mention that CDIH could be well regenerated through washing with deionized water at 20 °C, with a desorption rate of 95.29 % for La³⁺ and 96.03 % for Y³⁺. And after 10 cycles of reuse, 70 % of the adsorption amount could be maintained, revealing excellent reusability. Furthermore, CDIH expressed better adsorption selectivity to La³⁺ and Y³⁺ than its non-imprinted counterparts in a solution containing six metal ions.

Thermoresponsive and co-nonsolvency behavior of poly(N-vinyl isobutyramide) and poly(N-isopropyl methacrylamide) as poly(N-isopropyl acrylamide) analogs in aqueous media

Sets of the nonionic polymers poly(N-vinyl isobutyramide) (pNVIBAm) and poly(N-isopropyl methacrylamide) (pNIPMAm) are synthesized by radical polymerization covering the molar mass range from about 20,000 to 150,000 kg mol−1, and their thermoresponsive and solvent-responsive behaviors in aqueous solution are studied. Both polymers feature a lower critical solution temperature (LCST) apparently of the rare so-called type II, as characteristic for their well-studied analogue poly(N-isopropyl acrylamide) (pNIPAm). Moreover, in analogy to pNIPAm, both polymers exhibit co-nonsolvency behavior in mixtures of water with several co-solvents, including short-chain alcohols as well as a range of polar aprotic solvents. While the cloud points of the aqueous solutions are a few degrees higher than those for pNIPAm and increase in the order pNIPAm < pNVIBAm < pNIPMAm, the co-nonsolvency behavior becomes less pronounced in the order pNIPAm > pNVIBAm > pNIPMAm. Exceptionally, pNIPMAm does not show co-nonsolvency in mixtures of water and N,N-dimethylformamide.

Graphical Abstract

Water-Recyclable Chitosan-Based Ion-Imprinted Thermoresponsive Hydrogel for Rare Earth Metal Ions Accumulation

The demand for rare earth metal increases rapidly in the modern high-tech industry and therefore the accumulation of rare earth metal ions from an aqueous environment becomes a significant concern worldwide. In this paper, a water-recyclable chitosan-based La³⁺-imprinted thermoresponsive hydrogel (CLIT) was prepared to accumulate La³⁺ from solution. The CLIT was characterized by DSC, FITR, Raman spectroscopy, XPS, and SEM, which revealed obvious reversible thermosensitivity and imprinted sites of La³⁺ ions. An adsorption capacity of 112.21 mg/g to La³⁺ ions was achieved on CLIT under its optimum adsorption conditions (pH 5, 50 °C, 60 min). The adsorption could be well illustrated by second-order kinetics and Freundlich isotherm models. The La³⁺-adsorbed CLIT could be recycled only by rinsing with 10 °C cold water, with a desorption rate of 96.72%. After ten cycles of adsorption-desorption, CLIT retained good adsorption capability. In the solution containing six ions, the adsorption coefficients k_La3+/Mn+ of CLIT were 2.04–3.51 times that of non-imprinted hydrogel, with k_La3+/Y3+, k_La3+/Gd3+, k_La3+/Al3+, k_La3+/Fe3+ and k_La3+/Cu2+ being 1.67, 2.04, 3.15, 2.72 and 4.84, respectively.

Poly(N-isopropylacrylamide)-Based Hydrogels for Biomedical Applications: A Review of the State-of-the-Art

A prominent research topic in contemporary advanced functional materials science is the production of smart materials based on polymers that may independently adjust their physical and/or chemical characteristics when subjected to external stimuli. Smart hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) demonstrate distinct thermoresponsive features close to a lower critical solution temperature (LCST) that enhance their capability in various biomedical applications such as drug delivery, tissue engineering, and wound dressings. Nevertheless, they have intrinsic shortcomings such as poor mechanical properties, limited loading capacity of actives, and poor biodegradability. Formulation of PNIPAM with diverse functional constituents to develop hydrogel composites is an efficient scheme to overcome these defects, which can significantly help for practicable application. This review reports on the latest developments in functional PNIPAM-based smart hydrogels for various biomedical applications. The first section describes the properties of PNIPAM-based hydrogels, followed by potential applications in diverse fields. Ultimately, this review summarizes the challenges and opportunities in this emerging area of research and development concerning this fascinating polymer-based system deep-rooted in chemistry and material science.

Equilibrium Swelling of Thermo-Responsive Gels in Mixtures of Solvents

Thermo-responsive (TR) gels of the LCST (lower critical solution temperature) type swell in water at temperatures below their volume phase transition temperature Tc and collapse above the critical temperature. When water is partially replaced with an organic liquid, these materials demonstrate three different types of equilibrium solvent uptake diagrams at temperatures below, above, in the close vicinity of Tc. A model is developed for equilibrium swelling of TR gels in binary mixtures of solvents. It takes into account three types of phase transitions in TR gels driven by (i) aggregation of hydrophobic side groups into clusters from which solvent molecules are expelled, (ii) replacement of water with cosolvent molecules in cage-like structures surrounding these groups, and (iii) replacement of water with cosolvent as the main element of hydration shells around backbone chains. The model involves a relatively small number of material constants that are found by matching observations on covalently cross-linked poly(N-isopropylacrylamide) macroscopic gels and microgels. Good agreement is demonstrated between the experimental data and results of numerical analysis. Classification is provided of the phase transition points on equilibrium swelling diagrams.

Cononsolvency of thermoresponsive polymers: where we are now and where we are going

Cononsolvency is an intriguing phenomenon where a polymer collapses in a mixture of good solvents. This cosolvent-induced modulation of the polymer solubility has been observed in solutions of several polymers and biomacromolecules, and finds application in areas such as hydrogel actuators, drug delivery, compound detection and catalysis. In the past decade, there has been a renewed interest in understanding the molecular mechanisms which drive cononsolvency with a predominant emphasis on its connection to the preferential adsorption of the cosolvent. Significant efforts have also been made to understand cononsolvency in complex systems such as micelles, block copolymers and thin films. In this review, we will discuss some of the recent developments from the experimental, simulation and theoretical fronts, and provide an outlook on the problems and challenges which are yet to be addressed.

Photoluminescent polymer micelles with thermo-/pH-/metal responsibility and their features in selective optical sensing of Pd(ii) cations

Photoluminescent polymers can be divided into two types of structures: one is the well-known conventional π-conjugated rigid chain polymers bearing π-conjugated chromophores in their side chains, and the other is the common flexible polymers without π-conjugated chromophores in their main or side chains but with a feature of clustering electron-rich and/or dipole groups in their main and/or side chains. In this work, we found a new photoluminescent polymer comprising theophylline (T) and imidazole (I) residues in a suitable ratio in the side chains on the common polystyrenic block (PVB-T/I). We synthesized a block copolymer (denoted as P2) consisting of hydrophobic PVB-T/I and hydrophilic poly(N-isopropylacrylamide), and we investigated its self-assembly into micelles and their micellar features, such as thermo-responsibility, fluorescence emission, pH, and metal ion-dependent photoluminescence, in detail. Especially, the micelles self-assembled from P2 showed intrinsic blue emission which was emitted from the charge transfer association between T and I residues in the intra-chains. Weakening the association by adjustment of the pH or addition of metal ions could evidently reduce the photoluminescence in the micellar state. Very interestingly, among many metal cations, only Pd2+, which can chelate strongly with theophylline, strongly quenched the photoluminescence from the micelles. Therefore, the polymer micelles functioned as an optical sensor for Pd(ii) ion not only by spectroscopy but also with the naked eye.

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

The mechanism by which arthropods (e.g., spiders and many insects) can produce silk fibres from an aqueous protein (fibroin) solution has remained elusive, despite much scientific investigation. In this work, we used several techniques to explore the role of a hydration shell bound to the fibroin in native silk feedstock (NSF) from Bombyx mori silkworms. Small angle X-ray and dynamic light scattering (SAXS and DLS) revealed a coil size (radius of gyration or hydrodynamic radius) around 12 nm, providing considerable scope for hydration. Aggregation in dilute aqueous solution was observed above 65 °C, matching the gelation temperature of more concentrated solutions and suggesting that the strength of interaction with the solvent (i.e., water) was the dominant factor. Infrared (IR) spectroscopy indicated decreasing hydration as the temperature was raised, with similar changes in hydration following gelation by freezing or heating. It was found that the solubility of fibroin in water or aqueous salt solutions could be described well by a relatively simple thermodynamic model for the stability of the protein hydration shell, which suggests that the affected water is enthalpically favoured but entropically penalised, due to its reduced (vibrational or translational) dynamics. Moreover, while the majority of this investigation used fibroin from B. mori, comparisons with published work on silk proteins from other silkworms and spiders, globular proteins and peptide model systems suggest that our findings may be of much wider significance.