Dissecting the Mechanism of the Heat-Induced Phase Separation and Crystallization of Poly(2-isopropyl-2-oxazoline) in Water through Vibrational Spectroscopy and Molecular Orbital Calculations

Aqueous solutions of amphiphilic polymers often undergo a heat-induced phase separation, which is known as the lower critical solution temperature (LCST) phase transition. In the case of aqueous poly­(2-isopropyl-2-oxazoline) (PIPOZ) solutions, the phase separation is followed, upon prolonged heat treatment, by an irreversible crystallization of the polymer. Optical microscopy observation of a PIPOZ solution (60 g L^–1) in water revealed that liquid–liquid phase separation of the aqueous PIPOZ solution occurs at the cloud point (<i>T</i>_c) and that PIPOZ crystallizes in the polymer-rich liquid phase upon prolonged heating of the mixture at a temperature <i>T</i> > <i>T</i>_c. Vibrational spectroscopy combined with molecular orbital (MO) calculations and spectral measurements with model compounds were employed to monitor water/polymer interactions and changes in polymer conformation during the LCST-type phase separation. The thermally induced spectral variations suggest that the dehydration of the PIPOZ amide functions occurs gradually as the temperature is raised from 20 °C up to <i>T</i>_c. Upon prolonged heating of the phase-separated mixture at constant temperature (<i>T</i>_c + ∼2 °C), the infrared spectrum of the polymer undergoes further changes ascribed to conformational transitions of the polymer backbone. These changes, which are irreversible upon cooling the solution below <i>T</i>_c, lead to the conformation taken by the polymer in the crystalline phase. This situation facilitates crystallization of the polymer by a nucleation/growth mechanism in the polymer-rich phase, a process akin to the crystallization of proteins from solution

Kinetic Study for AB-Type Coupling Reaction of Tetra-Arm Polymers

The reaction rate for the polycondensation in the Tetra-PEG gel system, i.e., A–B type coupling reaction between mutually reactive two four-arm polymers, has been studied by ATR-IR spectroscopy. It was found that (1) the polycondensation kinetics of Tetra-PEG gel can be simply treated as a chemical reaction between mutually reactive end-groups in solution, (2) the reaction undergoes as a simple second-order reaction from beginning to end regardless of gelation threshold, and (3) the gelation mechanism was predicted from the thermodynamic enthalpy and entropy at the transition state estimated by temperature dependence of rate constants. The reson of smooth second-order kinetics is suspected to be that the mean-field approximation can be applied to the reactivity of terminal groups on Tetra-PEGs; i.e., the reactivity of terminal groups on Tetra-PEGs is not affected by the steric hindrance, substitution effet, and gelation threshold

Transient Reciprocating Motion of a Self-Propelled Object Controlled by a Molecular Layer of a <i>N</i>‑Stearoyl‑<i>p</i>‑nitroaniline: Dependence on the Temperature of an Aqueous Phase

The mode-bifurcation of a self-propelled system induced by the property of a <i>N</i>-stearoyl-<i>p</i>-nitroaniline (C₁₈ANA) monolayer developed on an aqueous phase was studied. A camphor disk was placed on a C₁₈ANA monolayer, which indicated a characteristic surface pressure–area (π–<i>A</i>) isotherm. A camphor disk transiently exhibited reciprocating motion at a higher surface density of C₁₈ANA. The amplitude of the reciprocating motion increased with an increase in the temperature of the aqueous phase below 290 K, but reciprocating motion varied to irregular motion over 290 K. The temperature-dependent reciprocating motion is discussed in terms of the π–<i>A</i> curve for C₁₈ANA depending on the temperature. The interaction between C₁₈ANA molecules was measured by Fourier transform IR spectrometry and Brewster-angle microscopy. As an extension of the study, the trajectory of reciprocating motion could be determined by writing with a camphor pen on the C₁₈ANA monolayer

Accelerating the Phase Separation in Aqueous Poly(<i>N</i>‑isopropylacrylamide) Solutions by Slight Modification of the Polymer Stereoregularity: A Single Molecule Fluorescence Study

We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model

Accelerating the Phase Separation in Aqueous Poly(<i>N</i>‑isopropylacrylamide) Solutions by Slight Modification of the Polymer Stereoregularity: A Single Molecule Fluorescence Study

We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model

Accelerating the Phase Separation in Aqueous Poly(<i>N</i>‑isopropylacrylamide) Solutions by Slight Modification of the Polymer Stereoregularity: A Single Molecule Fluorescence Study

We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model

Accelerating the Phase Separation in Aqueous Poly(<i>N</i>‑isopropylacrylamide) Solutions by Slight Modification of the Polymer Stereoregularity: A Single Molecule Fluorescence Study

We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model

SANS and DLS Study of Tacticity Effects on Hydrophobicity and Phase Separation of Poly(<i>N</i>‑isopropylacrylamide)

The tacticity effect on phase separation process of poly­(<i>N</i>-isopropylacrylamide) (PNiPAM) aqueous solutions was investigated by dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements. SANS measurement revealed that hydrophobicity of PNiPAM consisting of meso- and racemo-isomers increased with increasing the meso-content. This result is in accordance with the result of the previous experimental and simulation study on NiPAM dimers (DNiPAM) and trimers (TNiPAM) [Katsumoto, Y.; J. Phys. Chem. B 2010, 114, 13312−13318, and Autieri, E.; J. Phys. Chem. B 2011, 115, 5827–5839]; i.e., meso-diad is more hydrophobic than racemo-diad. In addition, a series of scattering experiments revealed that the ratio of meso-diad does not affect the static structure or the shrinking behavior of a single chain, but strongly affects the aggregation behavior. The PNiPAMs with low meso-content suddenly associate around the phase separation temperature, while that of the high meso-content gradually aggregate with increasing temperature. We propose that phase transition behavior of PNiPAM aqueous solutions can be controlled by changing the stereoregularity of the polymer chain

Accelerating the Phase Separation in Aqueous Poly(<i>N</i>‑isopropylacrylamide) Solutions by Slight Modification of the Polymer Stereoregularity: A Single Molecule Fluorescence Study

We discovered for aqueous thermoresponsive polymer solutions that only a slight change in stereoregularity of the polymer can drastically accelerate phase separation. Single molecule fluorescence tracking (SMT) for an isotactic-slight-rich (meso-diad-rich) polymer sample solution revealed an interpolymer nanonetwork even before phase separation, and also revealed a novel phase in which translational molecular motion was frozen after phase separation. For such systems, fluorescence correlation spectroscopy (FCS) provided quantitative information on molecular diffusion. The results on FCS well agreed with the interpolymer nanonetwork model that was proposed on the basis of SMT measurement. We demonstrate such a novel method to control phase separation dynamics and also the interpolymer nanonetwork model

Effects of Syndiotacticity on the Dynamic and Static Phase Separation Properties of Poly(<i>N</i>‑isopropylacrylamide) in Aqueous Solution

The dynamic and static phase separation behavior in aqueous poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) solutions is highly sensitive to the tacticity of PNIPAM. We investigated the phase separation dynamics of aqueous solutions of PNIPAM with different tacticities (atactic and syndiotactic-rich types) and found that the phase separation dynamics of syndiotactic-rich PNIPAM was much different from that of atactic-type PNIPAM. First, phase separation in syndiotactic-rich PNIPAM was faster. Second, there was a critical point (<i>C</i>_cp) in the concentration dependence of the phase separation rate: the phase separation accelerated dramatically when the solution concentration was higher than 2.0 wt % (= <i>C</i>_cp). Third, syndiotactic-rich PNIPAM required a higher thermal energy for phase separation compared to atactic PNIPAM. Such behavior can be explained on the basis of the high hydrophobicity of syndiotactic-rich PNIPAM in a dehydrated state and a diffusion-controlled aggregation model. The present study shows that precise control of the stereoregularity will open new channels toward the design and development of stimuli-responsive-polymer-based smart materials