Phase-Separated Nanodroplets Formed below the Cloud Point for the Aqueous Solution of Stereo-Controlled Poly(N-isopropylacrylamide)

Antidehydration and Stable Mechanical Properties during the Phase Transition of the PNIPAM-Based Hydrogel for Body-Temperature-Monitoring Sensors

Poly(N-isopropylacrylamide) (PNIPAM) enhances the reversibility and responsiveness of wearable temperature-sensitive devices. However, an open question is whether and how the hydrogel design can prevent adhesive performance loss caused by phase-transition-induced dehydration and unstable mechanical properties between devices and human skin and reduce interfacial failure. Herein, a gelatin-mesh scaffold-based hydrogel (NAGP-Gel) is constructed to inhibit dehydration and volume change, leading to stable mechanical properties, superior adhesiveness, and thermal sensing sensitivity during the phase transition. NAGP-Gel enhances the polymer chains-water interaction and weakens the degree of aggregation of polymer chains-chains, improving antidehydration properties under 45 °C conditions that are higher than the lower critical solution temperature (LCST; i.e., ∼32 °C). The mesh scaffold greatly restricts the phase-transition-induced polymer chain movement and maintains the mechanical performance. In a 60 °C environment, the maximum water loss and volume retention ratio of NAGP-Gel are only 3.58% and 97.3%, respectively. Additionally, NAGP-Gel serves as a temperature sensor, producing a stable thermal-electrical signal within the LCST range. It also can be assembled into an electronic device enabling the transmission of information and recognition of sign language via Morse code. This work broadens the application of PNIPAM in constructing intelligent hydrogels and opens the door to exploring emerging hydrogels for temperature-monitoring applications.

Unveiling the Factors Influencing Different Nucleation Pathways and Liquid-Liquid Phase Separation

Exploring nucleation pathways has been a research hot spot in the fields of crystal engineering. In this work, vanillin as a model compound was utilized to explore the factors influencing different nucleation pathways with or without liquid-liquid phase separation (LLPS). A thermodynamic phase diagram of vanillin in the mixed solvent system of water and acetone from 10 to 55 °C was determined. It was found that the occurrence of LLPS might be related to different nucleation pathways. Under the guidance of a thermodynamic phase diagram, Raman spectroscopy and molecular simulation were applied to investigate the influencing factors of different nucleation paths. It was found that the degree of solvation is a key factor determining the nucleation path, and strong solvation could lead to LLPS. Additionally, the molecular self-assembly evolution during the crystallization process was further investigated by using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). The findings indicate that larger clusters with a diffuse transition layer may lead to LLPS during the nucleation process.

Physical Gels of Atactic Poly(N-isopropylacrylamide) in Water: Rheological Properties and As-Derived Spinodal Temperature

Aqueous solutions of atactic poly(N-isopropylacrylamide) (a-PNIPAM) undergo complex phase transitions at 20–33 °C. In this temperature range, the a-PNIPAM solution exhibits a phase behavior of lower critical solution temperature at the binodal temperature (Tb) and physical gel formation at the gel temperature (Tgel). On slow heating of the one-phase solution containing linear a-PNIPAM chains, branched chains are gradually developed to proceed with the physical gelation before phase separation considering that Tgel < Tb. Thus, the phase separation temperature determined from the conventional approaches, either by turbidity to derive the Tb or by scattering to derive the spindal temperature (Ts) from the Ornstein–Zernike analysis, is strictly the transition temperature associated with the a-PNIPAM hydrogel (or highly branched chains newly developed at elevated temperatures), rather than the initial a-PNIPAM solution prepared. Herein, the spinodal temperatures of a-PNIPAM hydrogels (Ts,gel) of various concentrations were determined from rheological measurements at a heating rate of 0.2 °C/min. Analyses of the temperature dependence of loss modulus G″ and storage modulus G′ give rise to the Ts,gel, based on the Fredrickson–Larson–Ajji–Choplin mean field theory. In addition, the specific temperature (T1) above which the one-phase solution starts to dramatically form the aggregated structure (e.g., branched chains) was also derived from the onset temperature of G′ increase; this is because as solution temperature approaches the spinodal point, the concentration fluctuations become significant, which is manifested with the elastic response to enhance G′ at T > T1. Depending on the solution concentration, the measured Ts,gel is approximately 5–10 °C higher than the derived T1. On the other hand, Ts,gel is independent of solution concentration to be constant at 32.8 °C. A phase diagram of the a-PNIPAM/H2O mixture is thoroughly constructed together with the previous data of Tgel and Tb.

Electrospinning of Aqueous Solutions of Atactic Poly(N-isopropylacrylamide) with Physical Gelation

The phase diagram of a given polymer solution is used to determine the solution’s electrospinnability. We constructed a phase diagram of an aqueous solution of atactic poly(N-isopropylacrylamide) (a-PNIPAM) based on turbidity measurements and the rheological properties derived from linear viscoelasticity. Several important transition temperatures were obtained and discussed, including the onset temperature for concentration fluctuations T1, gel temperature Tgel, and binodal temperature Tb. On heating from 15 °C, the one-phase a-PNIPAM solution underwent pronounced concentration fluctuations at temperatures above T1. At higher temperatures, the thermal concentration fluctuations subsequently triggered the physical gelation process to develop a macroscopic-scale gel network at Tgel before the phase separation at Tb. Thus, the temperature sequence for the transition is: T1 < Tgel < Tb~31 °C for a given a-PNIPAM aqueous solution. Based on the phase diagram, a low-temperature electrospinning process was designed to successfully obtain uniform a-PNIPAM nanofibers by controlling the solution temperature below T1. In addition, the electrospinning of an a-PNIPAM hydrogel at Tgel < T < Tb was found to be feasible considering that the elastic modulus of the gel was shown to be very low (ca. 10−20 Pa); however, at the jet end, jet whipping was not seen, though the spitting out of the internal structures was observed with high-speed video. In this case, not only dried nanofibers but also some by-products were produced. At T > Tb, electrospinning became problematic for the phase-separated gel because the enhanced gel elasticity dramatically resisted the stretching forces induced by the electric field.