Multiphysics modelling of volume phase transition of ionic hydrogels responsive to thermal stimulus

This paper presents the analysis of the volume phase transition of ionic thermo-sensitive hydrogels to thermal stimulus through mathematical modelling. The model is termed the multi-effect-coupling thermal-stimulus (MECtherm) model and it considers the effects of multi-phases and multi-physics. Its application to steady-state analysis of the hydrogels in swelling equilibrium is validated against available experimental data for the relation between volume swelling ratio and temperature, in which very good agreement is achieved. The phenomenon of volume phase transition is studied for the thermal-stimulus responsive hydrogel. The numerical studies predict well the influences of initially fixed charge density and initial volume fraction of polymeric network on the swelling equilibrium of the hydrogels.     

Imaging the volume transition in thermosensitive core-shell particles by cryo-transmission electron microscopy

We report a study of colloidal thermosensitive core-shell particles by cryo-transmission electron microscopy (cryo-TEM). The particles consist of a solid core of poly(styrene), onto which a network of cross-linked poly(N-isopropylacrylamide) (PNIPAM) is affixed. In water, the shell of these particles swells when the temperature is low. Raising the temperature above 32 degrees C leads to a marked shrinking of the shell. In this letter, we present the first study of these core-shell particles by cryo-TEM in situ, that is, in aqueous solution. We demonstrate that the core-shell particles are well-defined and exhibit a narrow size distribution. In particular, the PNIPAM shell is compact and has a defined outer surface of a slightly irregular shape. The micrographs show that there are density fluctuations within the network. Cryo-TEM of the system above and below the transition temperature furnishes information about the thermosensitive particles that had not been available through other methods employed in previous investigations.     

Mechanical properties of poly(vinyl methyl ether) hydrogels below and above their volume phase transition

The mechanical properties of radiation cross-linked poly(vinyl methyl ether) hydrogels below and above the volume phase transition (VPT) under isobar conditions were studied. The viscoelastic properties as a function of radiation dose, radiation source and polymer concentration at the state of irradiation were examined. Increased radiation doses led to higher cross-linking densities and higher moduli. Hydrogels irradiated with -rays were much harder than those obtained with electron beam irradiation at the same radiation dose. It was found that the modulus strongly increased by up to 1 order of magnitude at a temperature of the VPT of about 37 °C. In the collapsed state at temperatures well above the VPT a frequency dependence of the E() moduli in the range 0.1–22 Hz was detected, indicating viscoelastic behavior. To study the influence of solvent quality on the modulus of the hydrogels, rheological measurements were performed in water, 2-propanol and cyclohexane. A scaling exponent for the modulus according to de Gennes (G^2.25) was not found. Possible reasons for deviations (G^3.54) on poly(vinyl methyl ether) hydrogels were discussed in the context of deviations from ideal networks.     

Studies on phase transition processes of radiation synthesized hydrogels

PAM hydrogel and P(AM-NaA) (polyacrylamide-sodium acrylate) copolymer hydrogels with different AM/NaA ratios by radiation polymerization of aqueous solutions were synthesized. For further developing sensitive materials, we investigated the phase transition processes of all of the hydrogels in acetone-water solutions (so called A-W).     

Anion effects on the phase transition of N‐isopropylacrylamide hydrogels

NMR spectra were collected for poly(N‐isopropylacrylamide) (PNIPAAm) hydrogel using high‐resolution magic angle spinning (HRMAS) after gel pieces were hydrated in the presence of D₂O, NaF, NaCl, and NaI aqueous solutions. Changes in the peak height intensity of the spectra provide quantitative insight into the phase transition process. The thermodynamic values of the phase transition were calculated using a van't Hoff analysis of the NMR data. Unlike the trend observed for decreases in the (LCST), changes in the enthalpy and entropy did not clearly display a linear dependence with respect to salt concentration. Rather, it was observed that increases in salt concentration did not affect the enthalpy and entropy to the extent as the initial change observed between no salt and 100 mM solutions. Finally, the effect of salts on the hysteresis of the rehydrating process was observed. Hysteresis occurs due to the need for hydrophobic interactions to break down before water is able to infiltrate the polymer matrix. NaF stabilizes hydrophobic interactions while NaI destabilize hydrophobic interactions, causing them to break down at higher temperatures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Theory of laser-stimulated phase transition kinetics in solids

A theory of laser-stimulated solid-state phase transition kinetics is presented. Two cases are treated; the first case corresponds to a situation where the order parameters are directly coupled with the radiation field and in the second case the internal degrees of freedom are excited by the incident radiation and the excitation energy is then transferred to the order parameters.     

Spontaneous volume transition of polyampholyte nanocomposite hydrogels based on pure electrostatic interaction

A circular system is employed in this paper to investigate the swelling behaviors of polyampholyte hydrogels; this circular system can effectively eliminate the disturbance of various factors and keep the surrounding environment constant. It is found that there exists a spontaneous volume transition to the collapsed state of polyampholyte hydrogels, which is attributed to the overshooting effect, and the transition can occur repeatedly under certain conditions. (13)C NMR is employed to investigate the swelling behavior of polyampholyte hydrogels. The swelling kinetics of polyampholyte hydrogels under various circular media and various circular runs are also investigated in this paper. All the results suggest that the spontaneous volume transition to the collapsed state of polyampholyte hydrogels is dominated by pure electrostatic interaction between different charges in polymer chains.     

A diffuse‐interface modeling for liquid solution–solid gel phase transition of physical hydrogel with nonlinear deformation

A diffuse‐interface model is presented in this paper for simulation of the evolution of phase transition between the liquid solution and solid gel states for physical hydrogel with nonlinear deformation. The present domain covers the gel and solution states as well as a diffuse interface between them. They are indicated by the crosslink density in such a way that the solution phase is identified as the state when the crosslink density is small, while the gel as the state if the crosslink density becomes large. In this work, a novel order parameter is thus defined as the crosslink density, which is homogeneous in each distinct phase and smoothly varies over the interface from one phase to another. In this model, the constitutive equations, imposed on the two distinct phases and the interface, are formulated by the second law of thermodynamics, which are in the same form as those derived by a different approach. The present constitutive equations include a novel Ginzburg–Landau type of free energy with a double‐well profile, which accounts for the effect of crosslink density. The present governing equations include the equilibrium of forces, the conservations of mass and energy, and an additional kinetic equation imposed for phase transition, in which nonlinear deformation is considered. The equilibrium state is investigated numerically, where two stable phases are observed in the free energy profile. As case studies, a spherically symmetrical solution‐gel phase transition is simulated numerically for analysis of the phase transition of physical hydrogel.     

Effect of filler networking on the response of thermosensitive composite hydrogels

Several composite hydrogels of poly(N-isopropylacrylamide) (pNIPAAm) with sodium montmorillonite (NaMM) have been synthesized using a fixed polymer/NaMM ratio (4:1 wt./wt.), but various monomer concentrations, in order to obtain hydrogels with different degrees of swelling, and thus different clay contents in the swollen state. For comparison, unfilled pNIPAAm gels have been also prepared at the same concentrations. The equilibrium swelling behaviour of the gels has been studied both in the swollen and in the shrunk state. In the swollen state, the polymer volume fraction increases with the initial monomer concentration C₀. In the shrunk state, the polymer fraction in pNIPAAm hydrogels is dependent on the specimen size and on C₀, whereas in the composite gels a constant polymer content is observed. When subjected to stepwise heating from 25 to 45 °C, unfilled gels undergo only poor deswelling. By contrast, complete deswelling takes place in composite gels. The latter show half-shrinking times varying over two orders of magnitude, depending on the monomer concentration and on the procedure followed to disperse NaMM, which determine the overall dispersion state of the filler, as evidenced by transmission electron microscopy (TEM). In particular, TEM observations show clay networking above a percolation threshold near 2.5 wt.% of NaMM. The effect of the incorporation of clay on the response to thermal stimuli is discussed in terms of the ability of NaMM to hinder the hydrophobic association of pNIPAAm segments and in terms of its dispersion state. It is suggested that, above the percolation threshold, NaMM forms a hydrophilic, physical network, through which water can flow also above the volume transition temperature, where pNIPAAm acquires a hydrophobic character.     

Generalized kinetics of overall phase transition explicit to crystallization

Modeling of process for reaction kinetics is a fashionable subject of publications. This paper tests various phenomenological models starting with the Verhulst logistic functions, Kolmogorov adaptation up to the renowned KJMA and SB variants. The meaning of both the mortality (α) and fertility (1 ? α) terms are mathematically analyzed in detail involving variation of their power exponents, their characteristic points, and shape examination of the rate curves. Kolmogorov derivation of nucleation-growth process is revised.     

Composite hydrogels with temperature sensitive heterogeneities: influence of gel matrix on the volume phase transition of embedded poly-(N-isopropylacrylamide) microgels

The thermo-responsive behaviour of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in covalently cross-linked non-temperature-sensitive polyacrylamide (PAam) hydrogel matrixes with different compositions was investigated by using small angle neutron scattering (SANS). The composition of the composite hydrogel was varied by (a) increasing the cross-linker and acrylamide concentration leading to strong hydrogel matrixes and (b) by increasing the microgel concentration to obtain composite gels with an internal structure. Additionally we synthesized composite hydrogels by using γ-irradiation as initiation for the polymerisation. This leads to the formation of chemical bonds between the PNiPAM microgels and the surrounding polyacrylamide matrix. Thus it is possible to synthesize hydrogels without an additional cross-linker, as well as pure particle networks. Some samples were prepared at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or totally collapsed microgels during the incorporation step. The volume phase transition of microgels is not influenced by a hydrogel matrix with high acrylamide concentration independent of the preparation temperature. However, an increased cross-linker concentration leads to a corset like constraint on microgel swelling. Microgels, which are embedded in the collapsed state (at 50 °C), are not able to swell upon cooling, whereas microgels embedded in the swollen state can collapse upon heating. For samples with an increased microgel concentration, the close microgel packing was disturbed by the formation of the polyacrylamide matrix. The hydrogel matrix squeezes the microgels together and leads to partial aggregation. The experiments demonstrate how composite hydrogels with stimuli-sensitive heterogeneities can be prepared such that the full responsiveness of the embedded microgels is retained while the macroscopic dimensions of the gel are not affected by the volume phase transition of the microgels.     

温敏水凝胶溶胀行为及其分离性能

以甲基丙烯酸钠(SM)为单体与异丙基丙烯酰胺(NPA)共聚,N,N-亚甲基双丙烯酰胺(MBA)为交联剂,得到阴离子型温敏水凝胶;对其热敏性能、溶胀性能及它对甘草酸的分离性能进行了研究,并通过凝胶中水的状态理论对结果进行了分析和解释,还研究了阴离子浓度的变化对上述性能的影响。将其用于甘草酸粗品的分离吸收,探讨它用于分离中草药有效成分的可能性。     

Two-step phase transition via in situ hydrolysis of thermosensitive polymeric micelles with acid-labile core

This communication reports thermoresponsive diblock copolymer micelles composed of a hydrophilic poly(ethylene glycol) (PEG) shell with temperature-responsive functionality and an acid-labile, hydrophobic poly(N-acryloyl-2,2-dimethyl-1,3-oxazolidine) (PADMO) core. Because of the partial in situ hydrolysis of these micelles, an interesting two-step phase transition occurs, exhibiting two separate cloud points. This can be attributed to the dual characters displayed by poly(2-hydroxyethyl acrylamide) (PHEAM), the hydrolysis product of PADMO in an acid medium. The formation of hydrogen bonding between PEG and PHEAM enhances the hydrophobicity of PEG chains and promotes the phase transition that takes place at a lower temperature than before. On the other hand, generation of water-soluble PHEAM increases the hydrophilicity of the whole micelles and requires higher temperature that induces dehydration of these micelles. This finding could help us better understand the relationship between micellar microstructure and thermosensitive behaviors.     

Re-entrant volume phase transition of hydrogel membrane of microcapsule

Re-entrant volume phase transition of hydrogel wall membrane of microcapsules (MC) was first observed using MC suspensions consisting of poly (L-lysine-alt-terephthalic acid) wall and aqueous inner and outer solutions with different pHs. To analyze the dynamics of the re-entrant phase transition, we extended the theory for the swelling and the shrinking dynamics of the microcapsule gel [T. Narita, T. Yamamoto, D. Suzuki, T. Dobashi, Langmuir 19 (2004) 4051]. In the theory, the microcapsule size and the force constant for the driving force which gives rise to the size relaxation were chosen as the thermodynamic variables. The time course of the cross-sectional area of the microcapsules fitted well to the theoretical equations, and the time constants determined as the fitting parameters were discussed in terms of the force constant relaxation and the size relaxation.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

Unperturbed volume transition of thermosensitive poly-(N-isopropylacrylamide) microgel particles embedded in a hydrogel matrix

The thermoresponsive behavior of poly-(N-isopropylacrylamide) (PNiPAM) microgels embedded in a covalently cross-linked polyacrylamide hydrogel matrix was investigated using ultraviolet-visible (UV-vis) spectroscopy, small-angle neutron scattering (SANS), and confocal laser scanning microscopy. The hydrogel synthesis was performed at two different temperatures, below and above the volume phase transition temperature of PNiPAM, resulting in highly swollen or fully collapsed PNiPAM microgel particles during the incorporation step. UV-vis spectroscopy experiments verify that the incorporation of thermosensitive microgels leads to temperature-sensitive optical properties of the composite materials. SANS measurements at different temperatures show that the thermosensitive swelling behavior of the PNiPAM microgels is fully retained in the composite material. Volume and structure criteria of the embedded microgel particles are compared to those of the free microgels in acrylamide solution. To visualize the temperature responsive behavior of larger PNiPAM particles, confocal fluorescence microscopy images of PNiPAM beads, of 40-microm size, were taken at two different temperatures. The micrographs also demonstrate the retained temperature sensitivity of the embedded microgels.     

Temperature-jump investigations of the kinetics of hydrogel nanoparticle volume phase transitions.

The dynamics of the deswelling and swelling processes in thermoresponsive poly-N-isopropylacrylamide (pNIPAm) hydrogel nanoparticles have been studied by using time-resolved transmittance measurements, in combination with a nanosecond laser-induced temperature-jump (T-jump) technique. A decrease in the solution transmittance associated with deswelling of the particles has been observed as the solution temperature traverses the volume phase transition temperature of the particles. Upon inducing the T-jump, the deswelling transition only occurs in a small percentage (<10%) of the particle volume, which was found to be a thin periphery layer of the particles. The particle deswelling occurs on the microsecond time scale, and as shown previously, the collapse time can be tuned via adding small amounts of hydrophobic component to the particle shell. In contrast, the reswelling of the particles was thermodynamically controlled by bath equilibration, and only small differences in particle reswelling kinetics were found due to sluggish heat dissipation (millisecond time scale) from the sample cell.     

Synergistic depression of volume phase transition temperature in copolymer microgels

Microgels consisting of poly-N,N-diethylacrylamide (PDEAAM) and copolymer microgels consisting of N,N-diethylacrylamide-co-N-isopropylacrylamide (PDEAAM-co-PNIPAM) have been synthesized via free radical polymerizations. The volume phase transition of the microgels in aqueous solution was investigated by means of dynamic light scattering. All samples revealed a volume phase transition upon heating and the size change was fully reversible. An unusual dependence of transition temperature on the composition of the copolymer microgels was observed and samples were obtained with a transition temperature that was lower than that of both corresponding homopolymer particles. This synergistic behavior could be caused by strong hydrogen bonding between the mono- and the disubstituted acrylamide repeating units at nearly equimolar composition of the microgel.     

Thermal, phase transition, and thermal kinetics studies of carbamazepine

The thermal, phase transition of carbamazepine dihydrate and the solid-state transformation of carbamazepine from form III to form I were performed by Differential scanning calorimetry (DSC), Thermo gravimetry (TG–DTA), and X-ray powder diffraction.The non-thermal kinetic analysis of carbamazepine dihydrate and form III was carried out by DSC at different heating rates in dynamic nitrogen atmosphere. The model-free model, the Kissinger method, was used to give the Arrhenius parameters. Arrhenius plots from the kinetic model yielded activation energies corresponding to dehydration of dihydrate and melting of anhydrate CBZ form I were 95.28, 966.06 kJ mol^?1, the pre-exponential factors were 8.34E+11 and 1.41E+149, respectively. For the transformation of carbamazepine from form III to form I, activation energies corresponding to the melting of CBZ form III, recrystallization of form I, and melting of form I were 1160.81, 710.89, 1265.89 kJ mol^?1, the pre-exponential factors were 2.29E+144, 4.43E+91, and 1.61E+151, respectively. As a comparison, Ozawa method was used to verify the activation energy values obtained by Kissinger method. The result shows a close activation energy values between two methods.