Journal Title
Title of Journal: Colloid Polym Sci
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Abbravation: Colloid and Polymer Science
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Publisher
Springer Berlin Heidelberg
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Authors: Marcin Pastorczak Gustavo DominguezEspinosa Lidia Okrasa Marek Pyda Marcin Kozanecki Slawomir Kadlubowski Janusz M Rosiak Jacek Ulanski
Publish Date: 2014/06/18
Volume: 292, Issue: 8, Pages: 1775-1784
Abstract
Water interacting with a polymer reveals a number of properties very different to bulk water These interactions lead to the redistribution of hydrogen bonds in water It results in modification of thermodynamic properties of water and the molecular dynamics of water That kind of water is particularly well observable at temperatures below the freezing point of water when the bulk water crystallizes In this work we determine the amount of water bound to the polymer and of the socalled premelting water in polyvinyl methyl ether hydrogels with the use of Raman spectroscopy dielectric spectroscopy and calorimetry This analysis allows us to compare various physical properties of the bulk and the premelting water We also postulate the molecular mechanism responsible for the premelting of part of water in polyvinyl methyl ether hydrogels We suggest that above −60 °C the first segmental motions of the polymer chain are activated which trigger the process of the premeltingVarious states of water whose physical and chemical properties are modified by interactions with a solute have been discussed and studied since the beginning of twentieth century 1 It is claimed that the states of water interacting with a solute differ from those existing in bulk water in their “structure” rotational and vibrational dynamics and freezing temperature One of the first observations of this distinct water in waterpolymer mixtures was performed with calorimetric method 2 It has been noticed that the enthalpy of melting of ice in waterpolymer mixtures is significantly lower than that which should result from the analytical content of water It was hence concluded that the interactions with a polymer prevent some fraction of water from freezing The presence of liquid water below 0 °C in polymer systems has been confirmed by Xray experiments 3 infrared spectroscopy 4 and more recently by broadband dielectric spectroscopy BDS 5 6 The detection in the last method bases on the presence of dielectric depolarization at around 18 GHz which results from rotational reorientation of nonfreezing water molecules The nonfreezing water is often linked with the water which has rotational or translational motions restricted at room temperature due to its interactions with a solute Although the presence of nonfreezing water in polymeric solution has been confirmed with various experimental techniques the exact relations both qualitative and quantitative between the noncrystallized water the rotationally restricted water and the water hydrogen bonded to solute molecules are still not well understoodIn this work we analyze an evolution of intermolecular interactions between water and selected model polymer at temperatures below the freezing point of water with use of Raman spectroscopy Then we relate the observed evolution of these interactions with changing fraction of noncrystallized water determined by means of dielectric spectroscopy and differential scanning calorimetryAs a model system for these studies we have chosen series of hydrogels with various crosslinking degree prepared by electronbeam irradiation of polyvinyl methyl ether PVME 7 PVME is an amphiphilic amorphous polymer and its aqueous solutions reveal a number of interesting properties particularly the presence of the lower critical solution temperature LCST around 37 °C 8 Our previous work was devoted to molecular relaxations at temperatures between −60 and 50 °C in the PVME hydrogels in particular we have studied the transition revealed around −18 °C whose origin had been so far unclear 9 We have shown therein that this transition is related to the premelting of water induced by the segmental motions of the polymer Such phenomenon should be associated with modification of intermolecular interactions between polymer network and water in this temperature range this hypothesis was one of motivations for the research described in the present workThe selection of PVME hydrogels for these studies gives us an opportunity to investigate an influence of the crosslinking degree on the content of nonfreezing and premelting water Moreover we would like to compare how these two types of water differ in their thermal properties rotational motions and ability to form hydrogen bonds with the polymer in the range of temperatures below the freezing point of water
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