Sorption of water in hydrophilic polymers, 2. Thermodynamics of mixing of water with poly(2‐hydroxyethyl methacrylate) and with poly[2‐(2‐hydroxyethoxy)ethyl methacrylate]

Abstract

Sorption isotherms and heats of immersion of water in poly(2-hydroxyethyl methacrylate) (PHEMA) and in poly[2-(2-hydroxyethoxy)ethyl methacrylate] (PHEOEMA) were determined at 25 "C and the Gibbs energy, enthalpy and entropy of mixing were calculated from the data.The results confirm that the physical state of the polymer plays a decisive role in the thermodynamics of sorption. At the temperature of measurement, pure PHEMA is in the glassy state and after sorption of a certain amount of water it passes into the viscoelastic state. Similar to other cases of mixing of a glassy polymer with a low-molecular weight liquid, the entropy and enthalpy changes are negative, with contributions due to the difference between the physical states of both pure components predominating in them. The thermodynamics of liquid mixtures can be used only for PHEOEMA which at 25 "C is in the viscoelastic state. The mixing of water (especially of the first additions) with PHEOEMA is exothermal, being at the same time connected with a negative residual entropy, the effect of which predominates at 25 OC. As a result, the affinity of water to the polymer, expressed through a change in the Gibbs energy, is comparatively weak. This is why in this system no strongly bound water is involved, even if the A H and AS values indicate some sort of increase in ordering during mixing. a) Part 1 : cf. l).

Wide‐angle X‐ray scattering for swollen and glassy poly(2‐hydroxyethyl methacrylate)

“…The structural role of water within a swollen network has also been considered by Pouchly et al 15,16) and by Pedley'?. The latter used quantitative differential scanning calorimetry to show that little of the water, about 2 molecules per repeating unit of 1, was structurally associated (perhaps through hydrogen bonds) with the macromolecular network.…”

Multilayer adsorption by Monte Carlo simulation

“…The best known adsorption equations, i.e. those of Langmuir, Brunauer-Emmett-Teller (BET), Guggenheim-Anderson-de Boer (GAB) have been used to characterize adsorption phenomena in a broad range of fields which include polymer hydrogels and food engineering [1,2,3,4,5]. These equations actually characterize equilibrium states, and relate some measure for the composition of the system with the activity or fugacity of the adsorbed species.…”