Kinetics Of Volume Phase Transition Research Articles

Water–Polymer interactions in PVME hydrogels – Raman spectroscopy studies

Raman spectroscopy was used to investigate molecular interactions and structure of water in PVME hydrogels of various crosslinking degrees. Analysis of polymer ν(CH3) and ν(CH2) stretching vibrations has allowed to monitor the changes of hydrophilic and hydrophobic polymer–water interactions in a course of water deswelling and volume phase transition. Formation of water–polymer hydrogen bonds has appeared to influence directly supramolecular structure of the absorbed water. It was found that the polymer network density determines an availability of the hydrophilic sites to water molecules and also influences the kinetics of volume phase transition of the hydrogels. Activation energy of the volume phase transition in the investigated systems was estimated to be ca.88 kJ/mol.

Modeling the nonlinear large deformation kinetics of volume phase transition for the neutral thermosensitive hydrogels

By regarding the neutral thermosensitive hydrogel as a biphasic mixture media, an incompressible deformable porous solid skeleton, and an incompressible interstitial fluid phase saturating the interconnected space, a novel biphasic mixture theory for modeling the nonlinear large deformation kinetics of the volume phase transition was developed from the first law of thermodynamics and the continuum mixture theory. The governing equations consist of the saturated continuity equation, the momentum equations of the fluid, and the momentum equations of the bulk hydrogel. The explicit expressions of the constitutive relationships, which are capable of characterizing the nonlinear large deformation kinetics of the volume phase transition of the N-isopropylacrylamide thermosensitive hydrogels, were derived from the Helmholtz free energy formed by the elastic and mixing contributions. Both the governing equations and the constitutive relationships are in the Lagrangian framework, and special attention was paid to the deformable porous solid skeleton.

Potential application of poly( N-isopropylacrylamide) gel containing polymeric micelles to drug delivery systems

We have investigated rapidly thermo-responsive NIPA gel containing polymer surfactant PMDP (NIPA-PMDP gel) as a potential drug carrier using (+)- l-ascorbic acid as a model drug. In the NIPA-PMDP gel system micelles of polymer surfactant PMDP are trapped by the entanglement of polymer chains inside the gel networks. Therefore, in principle the gel system tightly stores targeted drug in the micelles and rapidly releases controlled amount of the drug by switching on–off of external stimuli such as temperature or infrared laser beam. In our investigation on release profile, the NIPA-PMDP gel system showed completely different releasing behavior from that of the conventional NIPA gel. The NIPA-PMDP gel released rapidly all loaded (+)- l-ascorbic acid above the phase transition temperature (ca. 34 °C), while slowly released the corresponding amount of the drug below the temperature. In contrast, the conventional NIPA gel released more slowly limited amount of the drug above the phase transition temperature while similarly did to the NIPA-PMDP gel below the temperature. The release profile of the NIPA-PMDP gel seems to be governed by only kinetics of volume phase transition of the gel network but not by the hydrophobic domains of the micelles probably because of too hydrophilic nature of (+)- l-ascorbic acid.

Kinetics of Volume Phase Transition in Nematic Gels Coupled with Nematic−Isotropic Phase Transition

We have investigated the dynamics of the phase transition between swollen isotropic and shrunken nematic states for a nematic network swollen in nematic solvent. The kinetics of swelling and shrinking driven by temperature (T)-jumps across the nematic−isotropic (N−I) transition temperature (TNIG) strongly depends on the distance of destination temperature (Td) from TNIG (ΔT). When Td is far above TNIG, the N → I transition completes fast within the T-jump, which yields the swelling process of totally isotropic gel without coupling of N → I transition. If Td is moderately above TNIG, the N → I transition slowly proceeds from surface toward center of gel with accompanying swelling. In this case, the total swelling process is composed of two different stages, i.e., during N → I transition and after N → I transition: The former is a thermally activated process strongly coupled with N → I transition; the latter is the T-independent swelling process of totally isotropic gel. When the T-quench from isotropic ph...

Kinetics of volume phase transition in poly(N-isopropylacrylamide) gels

Kinetics of volume phase transition in poly(N-isopropylacrylamide) (NIPA) gels jumped from a low-temperature swollen phase to a high-temperature shrunken phase was studied as functions of NIPA monomer and crosslinker concentrations. We found for the first time a clear kinematical boundary at which the shrinking relaxation time of gels changes discontinuously by 102–104 times, and that the profile of the boundary correlates with the sol-gel transition line and the contour line of turbidity of gels. A “morphological” boundary which characterizes the emergence of the bubble formation on gel surface was also determined. The theoretical calculation of the phase diagram on the basis of the mean field theory shows qualitatively that the shrinking speed of gels could be connected with the depth of the thermodynamic region of the spinodal instability (K+4μ/3=0) into which they are transferred where K and μ are the bulk and the shear moduli, respectively. A mechanism of discontinuous change of the shrinking speed is discussed in connection with the thermodynamic properties as well as the inhomogeneity of network structure.

Critical kinetics of volume phase transition of gels.

The volume phase transition of submillimeter spherical isopropylacrylamide gels shows critical kinetic behavior near the critical point; the transition takes an infinite time, and the thermal expansion becomes infinitely large. At the critical point the gels also undergo internal critical density fluctuations as revealed by photon-correlation spectroscopy. The observation of critical kinetics along with critical dynamics clarifies the physical principle of the kinetics of gel volume change.