1H NMR study of temperature-induced phase transitions in aqueous solutions of poly(N-isopropylmethacrylamide)/poly(N-vinylcaprolactam) mixtures

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

External Stimuli-Responsive Characteristics of Poly(N,N′-diethylacrylamide) Hydrogels: Effect of Double Network Structure

Swelling experiments and NMR spectroscopy were combined to study effect of various stimuli on the behavior of hydrogels with a single- and double-network (DN) structure composed of poly(N,N′-diethylacrylamide) and polyacrylamide (PAAm). The sensitivity to stimuli in the DN hydrogel was found to be significantly affected by the introduction of the second component and the formation of the double network. The interpenetrating structure in the DN hydrogel causes the units of the component, which is insensitive to the given stimulus in the form of the single network (SN) hydrogel, to be partially formed as globular structures in DN hydrogel. Due to the hydrophilic PAAm groups, temperature- and salt-induced changes in the deswelling of the DN hydrogel are less intensive and gradual compared to those of the SN hydrogel. The swelling ratio of the DN hydrogel shows a significant decrease in the dependence on the acetone content in acetone–water mixtures. A certain portion of the solvent molecules bound in the globular structures was established from the measurements of the ¹H NMR spin–spin relaxation times T₂ for the studied DN hydrogel. The time-dependent deswelling and reswelling kinetics showed a two-step profile, corresponding to the solvent molecules being released and absorbed during two processes with different characteristic times.

Formation of Single Double-Layered Coacervate of Poly(N,N-diethylacrylamide) in Water by a Laser Tweezer

We demonstrate liquid-liquid phase separation involving both coacervation and coil-to-globule phase transition of a thermoresponsive polymer. By focusing a near-infrared laser beam into an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM), a single phase-separated polymer microdroplet can be formed and stably trapped at the focal point. Such droplet formation is induced by a local elevation in temperature (induced by a photothermal effect) and an optical force. The technique allows us to selectively analyze a single polymer droplet trapped at the focal point. In this study, we applied this technique to poly(N,N-diethylacrylamide) (PDEA) in water and generated a double-layered PDEA droplet. Such an inhomogeneous and complex microstructure has not been previously observed both in steady-state heating of a PDEA solution and in the PNIPAM system. Moreover, we used micro-Raman spectroscopy to clarify that PDEA underwent dehydration due to a coil-to-globule phase transition. Despite this, the polymer concentration (C_poly) of the trapped PDEA droplet was very low and was around 30 wt %. C_poly depended on the molecular weight of PDEA and the laser power that regulates the temperature elevation. These results strongly indicate that PDEA undergoes coacervation in addition to a coil-to-globule phase transition. This study will help provide us with a fundamental understanding of the phase separation mechanisms of thermoresponsive polymers.

Temperature Behavior of Aqueous Solutions of Poly(2-oxazoline) Homopolymer and Block Copolymers Investigated by NMR Spectroscopy and Dynamic Light Scattering

1H NMR methods in combination with dynamic light scattering were applied to study temperature behavior of poly(2-isopropyl-2-oxazoline) (PIPOx) homopolymer as well as PIPOx-b-poly(2-methyl-2-oxazoline) (PMeOx) and poly(2-ethyl-2-oxazoline) (PEtOx)-b-PMeOx diblock copolymers in aqueous solutions. 1H NMR spectra showed a different way of phase transition for the main and side chains in PIPOx-based solutions. Additionally, the phase transition is irreversible for PIPOx homopolymer and partially reversible for PIPOx-b-PMeOx copolymer. As revealed by NMR, the phase transition in PEtOx-based copolymers solutions exists despite the absence of solution turbidity. It is very broad, virtually independent of the copolymer composition and reversible with some hysteresis. Two types of water molecules were detected in solutions of the diblock copolymers above the phase transition-"free" with long and "bound" with short spin-spin relaxation times T2. NOESY spectra revealed information about conformational changes observed already in the pre-transition region of PIPOx-b-PMeOx copolymer solution.

Improving the Colloidal Stability of Temperature-Sensitive Poly(N-isopropylacrylamide) Solutions Using Low Molecular Weight Hydrophobic Additives

Poly(N-isopropylacrylamide) (PNIPAM) is an important polymer with stimuli-responsive properties, making it suitable for various uses. Phase behavior of the temperature-sensitive PNIPAM polymer in the presence of four low-molecular weight additives tert-butylamine (t-BuAM), tert-butyl alcohol (t-BuOH), tert-butyl methyl ether (t-BuME), and tert-butyl methyl ketone (t-BuMK) was studied in water (D₂O) using high-resolution nuclear magnetic resonance (NMR) spectroscopy and dynamic light scattering. Phase separation was thermodynamically modeled as a two-state process which resulted in a simple curve which can be used for fitting of NMR data and obtaining all important thermodynamic parameters using simple formulas presented in this paper. The model is based on a modified van't Hoff equation. Phase separation temperatures T _p and thermodynamic parameters (enthalpy and entropy change) connected with the phase separation of PNIPAM were obtained using this method. It was determined that T _p is dependent on additives in the following order: T _p(t-BuAM) > T _p(t-BuOH) > T _p(t-BuME) > T _p(t-BuMK). Also, either increasing the additive concentration or increasing pK _a of the additive leads to depression of T _p. Time-resolved ¹H NMR spin-spin relaxation experiments (T ₂) performed above the phase separation temperature of PNIPAM revealed high colloidal stability of the phase-separated polymer induced by the additives (relative to the neat PNIPAM/D₂O system). Small quantities of selected suitable additives can be used to optimize the properties of PNIPAM preparations including their phase separation temperatures, colloidal stabilities, and morphologies, thus improving the prospects for the application.

Thermoresponsive functional polymers based on 2,6-diaminopyridine motif with tunable UCST behaviour in water/alcohol mixtures

Two thermoresponsive polyacrylamides based on the 2,6-diaminopyridine motif were synthesized and their UCST-type reversible thermoresponsive behaviour was studied in water/alcohol mixtures.

Exploration of Doubly Thermal Phase Transition Process of PDEGA-b-PDMA-b-PVCL in Water

Understanding of phase transition mechanism of thermoresponsive polymers is the basis for the rational design of smart materials with predictable properties. Linear ABC triblock terpolymer poly(di(ethylene glycol)ethyl ether acrylate)-b-poly(N,N-dimethylacrylamide)-b-poly(N-vinylcaprolactam) (PDEGA-b-PDMA-b-PVCL) was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The doubly thermal phase transition of PDEGA-b-PDMA-b-PVCL in aqueous solution was investigated by a combination of nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), turbidimetry, and dynamic light scattering (DLS). The terpolymer self-assembles into micelles with PDEGA being the core-forming block during the first lower critical solution temperature (LCST) transition corresponding to PDEGA, which is followed by a second LCST transition corresponding to PVCL, resulting in the formation of micellar aggregates. The PDMA middle segment plays an important role as an isolation zone to prevent cooperative dehydration of the PDEGA and PVCL segments, and therefore, two independent LCST transitions corresponding to PDEGA and PVCL were observed. Furthermore, FT-IR with perturbation correlation moving window (PCMW) and two-dimensional spectroscopy (2DCOS) was applied to elucidate the two-step phase transition mechanism of this terpolymer. It was observed that the CH, ester carbonyl, and ether groups of PDEGA change prior to the CH and amide carbonyl groups of PVCL, further supporting that the two phase transitions corresponding to PDEGA and PVCL indeed occur without mutual interferences.

Unusual Phase Transition Behavior of Poly(N-isopropylacrylamide)-co-Poly(tetrabutylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in the Poly(N-isopropylacrylamide) Part

In this paper, one LCST-type thermoresponsive poly(ionic liquid) (PIL), poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]), was introduced to poly(N-isopropylacrylamide) (PNIPAM) by two different ways, mixing and copolymerization. Interestingly, they show distinct thermoresponsive phase transition behaviors, evidenced by temperature-variable (1)H nuclear magnetic resonance and Fourier transform infrared in combination with the perturbation correlation moving window (PCMW) technique. The PNIPAM/P[P4,4,4,4][SS] mixture exhibits a sharp and drastic phase transition, similar to that of pure PNIPAM. In the statistical copolymer, PNIPAM-co-P[P4,4,4,4][SS], the thermosensitivity of P[P4,4,4,4][SS] is largely suppressed, resulting in a linear, mild, and incomplete phase transition, which has never been reported before. This abnormal phenomenon is shown to arise from the outstanding hydration ability of P[P4,4,4,4][SS]. Our findings should be conducive to improving our understanding of the interaction between LCST-type polymers with distinct structures and provide a new perspective for preparing thermoresponsive materials with linear phase transition behavior.

Polyplex Micelles with Double-Protective Compartments of Hydrophilic Shell and Thermoswitchable Palisade of Poly(oxazoline)-Based Block Copolymers for Promoted Gene Transfection

Improving the stability of polyplex micelles under physiological conditions is a critical issue for promoting gene transfection efficiencies. To this end, hydrophobic palisade was installed between the inner core of packaged plasmid DNA (pDNA) and the hydrophilic shell of polyplex micelles using a triblock copolymer consisting of hydrophilic poly(2-ethyl-2-oxazoline), thermoswitchable amphiphilic poly(2-n-propyl-2-oxazoline) (PnPrOx) and cationic poly(L-lysine). The two-step preparation procedure, mixing the triblock copolymer with pDNA below the lower critical solution temperature (LCST) of PnPrOx, followed by incubation above the LCST to form a hydrophobic palisade of the collapsed PnPrOx segment, induced the formation of spatially aligned hydrophilic-hydrophobic double-protected polyplex micelles. The prepared polyplex micelles exhibited significant tolerance against attacks from nuclease and polyanions compared to those without hydrophobic palisades, thereby promoting gene transfection. These results corroborated the utility of amphiphilic poly(oxazoline) as a molecular thermal switch to improve the stability of polyplex gene carriers relevant for physiological applications.

Molecular Evolution of Poly(2-isopropyl-2-oxazoline) Aqueous Solution during the Liquid-Liquid Phase Separation and Phase Transition Process

A detailed phase transition process of poly(2-isopropyl-2-oxazoline) (PIPOZ) in aqueous solution was investigated by means of DSC, temperature-variable (1)H NMR, Raman, optical micrographs, and FT-IR spectroscopy measurements. Gradual phase separation accompanied by large dehydration degree and big conformational changes above the lower critical solution temperature (LCST) and facile reversibility were identified. Based on the two-dimensional correlation (2Dcos) and perturbation correlation moving window (PCMW) analyses, the sequence order of chemical group motions in phase transition process was elucidated. Additionally, a newly assigned CH3···O═C intermolecular hydrogen bond at 3008 cm(-1) in the PIPOZ system provides extra information on the interactions between C-H and C═O groups. The formation of cross-linking "bridging" hydrogen bonds C═O···D-O-D···O═C (1631 cm(-1)) is proposed as the key process to induce the liquid-liquid phase separation and polymer-rich phase formation of PIPOZ solution. With slow heating, more and more "bridging" hydrogen bonds were formed and D2O were expelled with an ordered and mostly all-trans conformation adopted in the PIPOZ chains. On the basis of these observations, a physical picture on the molecular evolution of PIPOZ solution during the phase transition process has been derived.

Remarkable distinctions in the heat-induced phase transition processes of two poly(2-isopropyl-2-oxazoline)-based mixed aqueous solutions

Detailed phase transition processes of poly(2-isopropyl-2-oxazoline) (PIPOZ)/poly(N-isopropylacrylamide) (PNIPAM) and PIPOZ/poly(N-vinylcaprolactam) (PVCL) mixtures in aqueous solution were investigated by DSC, temperature-variable (1)H-NMR, optical microscopy and FT-IR spectroscopy measurements accompanied with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) analytical methods. Through the comparison of these two systems, it is revealed that PVCL chains can interact with PIPOZ chains directly through the polymer-water-polymer cross-linking hydrogen bonds (C[double bond, length as m-dash]OD-O-DO[double bond, length as m-dash]C), which induce their transition process as one. However, in the PIPOZ/PNIPAM mixture, the phase transition of the given component (PNIPAM or PIPOZ) is indirectly affected by the presence of the second component, because the strong hydrogen bonds C[double bond, length as m-dash]OD-N in PNIPAM components forbid the direct connection with PIPOZ, which induces two phase transition processes separately with no liquid-liquid phase separation (LLPS). Additionally, the formation of polymer-water-polymer hydrogen bonds (C[double bond, length as m-dash]OD-O-DO[double bond, length as m-dash]C) is highlighted as the key process in macroscopic LLPS.

Comparison of LCST-transitions of homopolymer mixture, diblock and statistical copolymers of NIPAM and VCL in water

The LCST-transitions of linear, well-defined polymers of N-isopropylacrylamide (NIPAM) and N-vinylcaprolactam (VCL), including a homopolymer mixture, diblock and statistical copolymers, in water are explored and compared by applying turbidity and FTIR measurements in combination with two-dimensional correlation spectroscopy (2Dcos). Only one transition is observed in all polymer systems, suggesting a dependent aggregation of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinylcaprolactam) (PVCL) parts in the phase transition processes. With the help of 2Dcos analysis, it is discovered that the hydrophobic interaction among C-H groups is the driving force for simultaneous collapse of the two distinct thermo-responsive segments. Additionally, the delicate differences within the LCST-transitions thereof have been emphasized, where the phase separation temperatures of the homopolymer mixture and the diblock copolymer are close while that of the statistical copolymer is relatively higher. Moreover, both diblock and statistical copolymers exhibit rather sharp phase transitions while the homopolymer mixture demonstrates a moderately continuous one.

Structural investigation of thermo-responsive poly(2-isopropyl-2-oxazoline) hydrogel across the volume phase transition

The deswelling and swelling behaviors of poly(2-isopropyl-2-oxazoline)-based hydrogel synthesized by a one-pot microwave-assisted solvent-free reaction were investigated. A distinct hydrophobic collapse of the hydrogel compared with the corresponding aqueous solution was observed by FT-IR spectroscopy combined with two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window (PCMW) analyses. The volume phase transition (VPT) temperature of 35 °C during heating and the transition temperature range of 41-30 °C during cooling were determined, indicating different dynamic transition mechanisms during heating and cooling. Water expulsion starting from the benzene ring-centered hydrophobic spots to the surroundings was revealed during deswelling. However, during swelling, although the rebuilding of cross-linking hydrogen bond bridges provided a channel-like microstructure to reswell the hydrogel gradually, a slow, unusual recovery of the amide hydrogen bonds to water molecules was observed.