Structure of Hydrogels Based on Lyotropic Phases of Cellulose Derivative as Studied by Raman Spectroscopy

Abstract

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.

TEMPO-oxidized cellulose nanofibril/polyvalent cations hydrogels: a multifaceted view of network interactions and inner structure

“…In particular, the vibrational profiles indicate a clear decrease of the ice-like component of water for the cellulose gels with 10 mM of Ca 2+ with respect to the pure TOCNF dispersion. This finding suggests that the intermolecular structure of water in the compartmentalized spaces of the TOCNF hydrogel network is different from that found in the dispersion of cellulose, consistently with what has already been observed in other gel systems (Maeda and Kitano 1995;Ratajska-Gadomska and Gadomski 2004;Joachimiak et al 2005;Pastorczak et al 2009). We can argue that, when ions are added and the gel is formed, water molecules hydrating the cellulose fibrils partially lose their tetrahedral organization typical of the bulk.…”

“…Raman spectroscopy and in particular UVRR experiments can be a useful tool for providing details on the H-bond state of water molecules inside hydrogels' phases (Rossi et al 2015b(Rossi et al , a, 2018Bottari et al 2017). In particular, the Raman signal associated with the OH stretching band of water located in the 3000-3800 cm −1 wavenumber range can be used as a sensitive probe of the H-bond organization of water, as demonstrated in previous works (Maeda and Kitano 1995;Ratajska-Gadomska and Gadomski 2004;Joachimiak et al 2005;Pastorczak et al 2009). The use of UV wavelengths for exciting the Raman spectra has the further advantage of strongly reducing the interfering fluorescence background typically affecting the visible Raman spectra of hydrogel systems, thus simplifying the analysis of the vibrational profile.…”

Novel Covalently Cross-Linked Attapulgite/Poly(acrylic acid-co-acrylamide) Hybrid Hydrogels by Inverse Suspension Polymerization: Synthesis Optimization and Evaluation as Adsorbents for Toxic Heavy Metals

“…27 An increase in the number of carboxyl groups results in an increase in the number of hydrogen bonds between the carboxyl groups and water molecules, and the formed hydrogen-bond network structure is more stable. 28 Hydrogen bonds show a certain directional property, which leads to the orientation of the carboxyl groups and water molecules, producing an anisotropic hydrogel network 29 and restricting the motion of the polymer segments. As a result, the distances among chain segments increase, which also decreases the density of the hydrogels.…”

A comparison of different approaches for depth profiling of films and coatings by confocal Raman microscopy

“…It is possible to gather spectroscopic information deep within the sample without the need of physical sectioning, which virtually implies no sample preparation. Many polymers used in applications such as protective coatings, adhesives, packaging or drug delivery can be characterized with this methodology, which can be used not only to access to details of structure, but also to monitor time dependent phenomena such as diffusion, segregation, layering, phase transitions or chemical transformations [1][2][3][4][5][6][7][8].…”