The whole process of phase transition and relaxation of poly(N-isopropylacrylamide) aqueous solution (original) (raw)
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Physical Chemistry Chemical Physics, 2012
We observed phase transition and phase relaxation processes of a poly(N-isopropylacrylamide) (PNIPAM) aqueous solution using the heterodyne transient grating (HD-TG) method combined with the laser temperature jump technique. The sample temperature was instantaneously raised by about 1.0 K after irradiation of a pump pulse to crystal violet (CV) molecules for heating, and the phase transition was induced for the sample with an initial temperature just below the lower critical solution temperature (LCST); the following phase relaxation dynamics was observed. Turbidity relaxation was observed in both the turbidity and HD-TG responses, while another relaxation process was observed only in the HD-TG response, namely via the refractive index change. It is suggested that this response is due to formation of globule molecules or their assemblies since they would have nothing to do with turbidity change but would affect the refractive index, which is dependent on the molar volume of a chemical species. Furthermore, the grating spacing dependence of the HD-TG responses suggests that the response was caused by the counter propagating diffusion of the coil molecules as a reactant species and the globule molecules as a product species and the lifetime of the globule molecules ranged from 1.5 to 5 seconds. Thus, we conclude that the turbidity reflects the dynamics of aggregate conditions, not molecular conditions. The coil and globule sizes were estimated from the obtained diffusion coefficient. The sizes of the coil molecules did not change at the initial temperatures below the LCST but increased sharply as it approaches LCST. We propose that the coil-state molecules associate due to hydrophobic interaction when the initial temperature was higher than LCST minus 0.5 K and that the globule-state molecules generated from the coil-state molecules showed a similar trend in temperature. The phase transition was also induced by heating under a microscope, and the relaxation process was followed using the fluorescence peak shift of a fluorescent molecule-labeled PNIPAM. The result also supports the existence of a globule molecule or its assembly remains for several seconds in the phase relaxation.
Coil−Globule Transition of Poly(N-isopropylacrylamide): A Study of Polymer−Surfactant Association
Macromolecules, 1996
The phase behavior of a fractionated high molecular weight sample of poly(N-isopropylacrylamide) in dilute aqueous solution containing a surfactant, sodium dodecyl sulfate (SDS), is studied with temperature and surfactant Concentration as independent variables. Static and dynamic light scattering are used as methods of investigation. Without surfactant, the polymer exhibits a lower critical solution temperature (LCST) of 34 OC, above which it precipitates. At SDS concentrations of only 250 mg/L, aggregation is completely prevented (intermolecular solubilization) and the behavior of isolated polymer molecules can be studied in the whole temperature range: Upon heating, the polymer undergoes a coil-to-globule phase transition with a volume reduction by a factor of more than 300. The transition temperature depends on the surfactant concentration. It is first constant and equal to the LCST, but begins to increase above 300 mg/L surfactant. Therefore when increasing the surfactant concentration at constant temperatures above the LCST, we cross the phase boundary and observe intramolecular solubilization, Le., a surfactant-induced globule-to-coil transition. Below the LCST, the surfactant causes an expansion of the polymer coils also setting on at 300 mg/L.
Macromolecular Rapid Communications, 1995
The coil-globule transition of poly(Nisopropylacry1amide) (systematic name: poly-[ 1-(isopropylaminocarbonyl)ethylene]) has been viscometrically investigated in low-concentration aqueous sodium dodecyl sulfate solution. In this environment, even if the macromolecular coils collapse as the temperature increases above the lower critical solution temperature of the polymer, 3 4 T , the polymer does not precipitate. It seems that the hydrophobically collapsed macromolecular coils do not aggregate, but remain in solution with the aid of surface charges supplied by surfactant ions adsorbed through hydrophobic interactions.
Structure Relaxation of Hydrophobically Aggregated Poly( N -isopropylacrylamide) in Water
Macromolecules, 1996
Introduction. Poly(N-isopropylacrylamide) (PNI-PAM) in water undergoes a coil-to-globule transition at about 32°C with elevating temperature. 1-3 This is mainly due to the dissociation of ordered water molecules surrounding hydrophobic N-isopropyl groups. In the case of PNIPAM gels, which are usually prepared by polymerizing NIPAM monomers in the presence of a cross-linker, a volume transition takes place from a swollen state to a collapsed state at a temperature, T c , which is slightly above the coil-globule transition temperature. The dissociation enthalpy is large enough to be detected with a conventional differential scanning calorimeter (DSC). 4-6 We have conducted a series of studies on polymer concentration dependence 6 and comonomer dependence of PNIPAM gels by DSC. 7 During these experiments, we have found that successive DSC runs for PNIPAM solutions exhibit poor reproducibility in both the endotherm and the transition temperature. This is observed exclusively in PNIPAM solutions but not in PNIPAM gels. However, reproducibility is recovered by aging the solution sample at a temperature lower than the transition temperature. This phenomenon indicates that it takes a certain time for PNIPAM solutions to recover the hydrated structure after a DSC run. In this communication, we focus on the difference in the structure relaxation after thermal treatment (DSC run) and discuss the relaxation phenomenon in conjunction with the cooperativity of gel networks. Experimental Section. NIPAM monomers (Kohjin Chemical Co. Ltd.), purified by recrystallization, were dissolved in deionized water, and a 690 mM NIPAM solution was prepared. Ammonium persulfate (APS) was then added to the solution. The polymerization was initiated with N,N,N′,N′-tetramethylethylenediamine (TEMED) at 20°C for 20 h after the pregel solution was degassed, and a PNIPAM solution was prepared. For the preparation of gel samples, N,N′-methylenebis-(acrylamide) (BIS) (cross-linker) was also added to the NIPAM monomer solution, followed by the same procedure as in the case of the PNIPAM solution. The concentrations of these reagents were 31.4 (BIS), 1.75 (APS), and 8 mM (TEMED). Gels were immersed in deionized water for a week to remove unreacted residue. DSC measurements were carried out with a DSC3100 (Mac Science Co. Ltd.). The polymer concentration was adjusted by moisturizing a dried gel or dried polymers in a sealed DSC sample pan. The sample was allowed to equilibrate for a few days to obtain a homogeneous gel or solution. A piece of thus prepared samples of about 10 mg was crimped in a sealed pan, and DSC thermograms were taken at a heating rate of 3°C/min.
Soft matter, 2018
Small-angle neutron scattering (SANS) and neutron spin-echo (NSE) have been used to investigate the temperature-dependent solution behaviour of highly-branched poly(N-isopropylacrylamide) (HB-PNIPAM). SANS experiments have shown that water is a good solvent for both HB-PNIPAM and a linear PNIPAM control at low temperatures where the small angle scattering is described by a single correlation length model. Increasing the temperature leads to a gradual collapse of HB-PNIPAM until above the lower critical solution temperature (LCST), at which point aggregation occurs, forming disperse spherical particles of up to 60 nm in diameter, independent of the degree of branching. However, SANS from linear PNIPAM above the LCST is described by a model that combines particulate structure and a contribution from solvated chains. NSE was used to study the internal and translational solution dynamics of HB-PNIPAM chains below the LCST. Internal HB-PNIPAM dynamics is described well by the Rouse model...
The Journal of Physical Chemistry B, 2012
Dielectric relaxation spectroscopy has been utilized for studying the molecular dynamics of polymer solutions. 1À6 In the case of polymer solutions composed of polar solvents in a solvent-rich region, relaxation processes due to the reorientation of dipoles of solvents and polymer chains are observed separately at higher and lower frequencies, respectively. 3À5,7,8 Typically, the relaxation process observed at frequencies on the order of 10 GHz is associated with the molecular motion of solvent molecules, while the relaxation process observed at kHz-MHz frequencies is attributed to the relaxation modes of polymer chains. The relaxation process owing to solvent molecules is affected by the addition of polymers. 3,4,9À11 This implies that the dynamical structures of solvent molecules are related to the polymers through interactions at the molecular level. The relaxation process that arises from the polymer chains should also be affected by the solvent molecules. This interdependence of polymer chains and solvent molecules can be analyzed by investigating the dielectric relaxation spectrum as a function of concentration and/or temperature. Dielectric relaxation spectra can be described by, for example, the relaxation time, the relaxation strength, and the shape parameter characterizing the distribution of the relaxation process. Therefore, the relaxation parameters obtained by the variation of polymer concentration or temperature, as well as the solvent species, can provide important information leading to greater understanding of molecular interactions. Recently, the relaxation processes of polymer chains and solvent molecules have been studied systematically for the poly(vinylpyrrolidone) [PVP] system in various polar and nonpolar solvents in broad temperature and frequency ranges. 3À5 It has been revealed that the cooperation between segmental motion and the reorientation of solvent molecules provides intrinsic information about the molecular dynamics of polymer solutions. In this study, we report the experimental results of dielectric relaxation behavior for the systems of poly(N-isopropylacrylamide) (PNiPAM) in protic and aprotic solvents as a function of PNiPAM concentration studied by dielectric relaxation spectroscopy. An aqueous solution of PNiPAM has a Θ-temperature of 30.6°C and undergoes a coilÀglobule transition upon heating. 12À15 The transition of PNiPAM chains in water is also observed upon the addition of a second water-miscible solvent, such as methanol,
Macromolecules, 1996
The intramolecular segmental relaxation behaviors of poly(methyl methacrylate) (PMMA) and poly(methyl acrylate) (PMA) in dilute dichloromethane solutions have been studied using time-resolved fluorescence anisotropy measurements (TRAMS). TRAMS have been made on two different spectrometers, incorporating a picosecond laser source and synchrotron, respectively, as excitation sources. Excellent agreement was achieved between the resultant relaxation data, generating confidence both in the spectroscopic procedures involved and in the various forms of analytical data retrieval applied. The relaxation characteristics of each polymer, over the temperature range 230-310 K, was adequately described by an exponential model for the anisotropy, for both acenaphthylene-and 1-vinylnaphthalenebased labels. The associated correlation times for segmental motion exhibited an Arrhenius dependence in the temperature range studied, giving activation energies of the order of 14 and 11 kJ mol -1 for PMMA and PMA, respectively, in dichloromethane. These values are considerably reduced compared to those which have been reported for either polymer in other solvents. The differences in activation parameters are too great to be explained on the assumption that the solvents function to provide frictional resistance alone to the polymer dynamics. It is tentatively suggested that both PMMA and PMA exhibit specific interactions with dichloromethane and/or other solvents, such as toluene. Alternatively, the naphthyl labels used to interrogate the macromolecular dynamics might experience specific interactions with the dichloromethane which distort the apparent behavior of the polymer.