Heat capacities of water swollen hydrophilic polymers above and below 0°C

Abstract

The heat capacities of water swollen poly[2-(2-hydroxyethoxy)ethyl methacrylate] were determined in a DSC-2 calorimeter within the temperature range 220-350K for concentrations from 0 to 1,2g of water per 1 g of the polymer. At temperatures above 0°C the partial specific heat capacity of water in gel is concentration independent and equal to the specific heat capacity of pure liquid water.It seems, therefore, that water does not form stable icelike structures near polymer chains. To analyze phase transformations of water in gel below O"C, general thermodynamic equations were derived and used as a basis for suggesting criteria which allow to decide whether in a given experiment the phase transformation proceeded in an equilibrium way. In measurements below the melting point of ice the conditions for an equilibrium process consist in the preceding heating of the frozen sample to a temperature close to the melting point, followed by cooling to 220K. The assessed composition dependence of the melting point depression is consistent with the dependence of activity on concentration obtained from measurements of water vapour sorption at 35°C. Analysis of data on the heat capacity below 0°C led to the conclusion that at a water content of 0,4g/g and lower, or at temperatures below 250 K the crystallization of water from gel was inhibited by kinetic factors originating probably in the reduced diffusivity of water in gel, due to the reduced, mobility of polymer chains. Hence, non-freezing water need not be identical with "strongly bound" water; in the study of water structure in polymers based on heat capacities, preference should be given to data obtained at usual and elevated temperatures.

Indomethacin release behaviors from pH and thermoresponsive poly(vinyl alcohol) and poly(acrylic acid) IPN hydrogels for site-specific drug delivery

“…This state of the polymer matrix by using DSC. [29][30][31] Scheme 1. Schematic diagrams of swelling/deswelling behaviors of IPN hydrogels dependent on pH change.…”

“…The amount of free water and bound water was calculated from the melting enthalpies. 29,30 The following equation assumes the desired release medium under stirring to rethat the heat of fusion of free water in the hymove the boundary layer. Three-mL aliquots samdrogel was the same as that of the ice.…”

Thermal behavior of water in crosslinked dextran

“…When the hydrogels are examined by means of differential scanning calorimetry (DSC), the states of water are defined on the basis of the thermal transitions observed: crystallization and melting of water denoted as "free" are quite similar to those of bulk water; "intermediate" (or "freezing, loosely bound") water undergoes first order transitions at temperatures lower than bulk water; "unfreezing" ("strongly bound") water exhibits no detectable phase transition over the temperature range normally associated with bulk water. As regards unfreezing water, it has been suggested 5) that crystallization of water from hydrogels may be inhibited by kinetic factors. Consequently, unfreezing water does not necessarily correspond to "strongly bound" water which is immobilized by significant interactions with the polar groups of the polymer chains.…”

Synthesis and characterisation of macroporous poly(methyl methacrylate) with plasma-polymerised hydrophilic coating

“…The BET equation can be obtained from GAB's equation for f = 1 and c =cf when α ≈ β ≈ 1. The GAB equation describes correctly the sorption isotherms in macromolecular systems of synthetic [80,[82][83][84] and biological nature [85]. It has been even recommended by the COST90 project to describe water sorption in food [86].…”

“…The thermal events measured in a DSC scan during the crystallisation and melting of water in a hydrogel include, besides the phase transition heat, the heat of demixing and mixing of the solvent in the polymer [114]. Nevertheless, these heats can be neglected compared with the phase-change enthalpies.…”