The status and nature of water molecules in hydrogels with a liquid crystalline organization of the polymer network based on a biopolymer were investigated. Liquid crystalline (LC) hydrogels were obtained in situ by the photopolymerization of acrylic acid into the lyotropic liquid crystalline phase of (2-hydroxypropyl)cellulose in a solvent mixture of water and acrylic acid. The photopolymerization of acrylic acid in the lyotropic liquid crystalline phase at room temperature gives a hydrogel in which liquid crystalline order and water are retained. The liquid crystalline hydrogel contains water, which originates from the composition of the lyotropic liquid crystalline phase, and may also contain water after immersion in liquid water. The water molecule characteristics were examined by means of differential scanning calorimetry, dielectric relaxation spectroscopy, and differential scanning calorimetry coupled with thermooptical measurements. The swelling data were obtained by using a weighing method. The investigations reveal the different nature of the two above-mentioned water types. For the water from the composition of the lyotropic liquid crystalline phase, the phase transitions-typical for the bulk water-were not observed, in contract to the water after swelling of the liquid crystalline hydrogel in the liquid water. The results of the measurements suggest that water, which comes from the composition of the lyotropic liquid crystalline phase, forms-together with a polymer network-a microstructure, stabilized by this water. The water, after swelling of the LC hydrogel in the liquid water, is separated in the pores of the hydrogel and exhibits the phase transitions of the bulk water.
Summary: Photopolymerisation of acrylic acid (AA) to poly(acrylic acid) (polyAA) in the lyotropic liquid crystalline (LLC) phase of hydroxypropylcellulose (HPC) in the AA‐water mixture, as well as the influence of mesomorphic organisation of the HPC on the structure of water was studied. Raman and thermo‐optical analysis results reveal that the lyotropic phase of the HPC/AA‐H2O can be used as a template system suitable for the formation of anisotropic hydrogels. The mesomorphic organisation of the HPC/AA‐H2O phase is preserved after photopolymerisation induced by UV radiation. Also the structure of water, strongly disturbed in the LLC‐phase as compared with the structure of liquid water, is preserved after the photopolymerisation. Apart from the water originating from the LLC‐phase of HPC/AA‐H2O, the anisotropic hydrogel may also contain water molecules taken up during immersion of the hydrogel in liquid water. Raman spectroscopy reveals that the water resulting from the swelling has a structure close to that of liquid water. Because the anisotropic hydrogel network is created by hydrogen bonds between HPC, water and polyAA, it is not stable when immersed in water for long time. An increased stability of the LC‐organisation of the hydrogel network can be achieved by crosslinking of the hydrogel with calcium ions, which can form a salt with polyAA, as confirmed by Raman spectroscopy.Comparison of the (a) Raman spectrum of liquid water with (b) that of water in the hydrogel, illustrating the influence of the mesomorphic organization in the hydrogel on the water structure.magnified imageComparison of the (a) Raman spectrum of liquid water with (b) that of water in the hydrogel, illustrating the influence of the mesomorphic organization in the hydrogel on the water structure.
The process of crosslinking of the hydrogel, derived from lyotropic liquid crystalline (LLC) phases of hydroxypropylcellulose/acrylic acid‐water, by calcium ions was studied by means of Raman and dielectric spectroscopy. Formation of salt by poly(acrylic acid) and calcium ions, resulting in hydrogel crosslinking, induces differences in Raman spectra of the hydrogel before and after the crosslinking. The crosslinking results in significant increase in the activation energy of β‐relaxation of poly(acrylic acid). This is a direct consequence of restriction in motions of carboxylic groups of poly(acrylic acid) due to calcium salt formation. Thus, the crosslinking improves polymer network stability in the hydrogel in the swollen state.
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