Probing the molecular connectivity of water confined in polymer hydrogels (original) (raw)
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Hydrogen-bond dynamics of water confined in cyclodextrin nanosponges hydrogel
Journal of Inclusion Phenomena and Macrocyclic Chemistry, 2014
Cyclodextrin nanosponges (CDNS) are a very promising class of cross-linked polymers, made up of cyclodextrins. CDNS swollen in aqueous solution give rise to cyclodextrin-based hydrogel in different states-gel or liquid suspension-depending on the hydration level of the system. Here we present a thorough inspection of the vibrational dynamics of these hydrogel by Raman scattering experiments, with the aim of clarifying the role played by the hydrogen-bond dynamics of water molecules confined into the nano-sized pores of nanosponges in determining the rigidity of the hydrogel network and their maximum water-holding capacity. Changes occurring in the spectral shape of the OH stretching band of water were interpreted by accounting the connectivity pattern of water molecules concurring to the gelation process. Spectral deconvolution analysis gives evidence of the existence of a characteristic cross-over hydration level associated to the rearrangement of water molecules in more cooperative, bulk-like networks as a consequence of saturation sites of water confinement of nanosponges. This interpretation is further confirmed by the inspection of the estimated collective intensities. These findings also support the existence of a specific phase diagram of the cyclodextrin nanosponges hydrogel, where the molecular structure of the crosslinking agent used during the synthesis of nanosponge plays a fundamental role in defining the nano-and microscopic properties of the system.
Soft Matter, 2013
The vibrational dynamics of a new class of cyclodextrin-based hydrogel is explored in depth here with the aim to clarify the intimate relationship between the structural and functional properties of these innovative polymeric materials. The thorough quantitative analysis of the FTIR-ATR and Raman spectra of the hydrogel obtained by swelling of cross-linked polymers of cyclodextrins with heavy water is performed in the wavenumber range between 1600 and 1800 cm À1 by using best-fitting and deconvolution procedures. The use of D 2 O instead of H 2 O allowed us to separately examine in the vibrational spectra of the hydrogel the C]O stretching bands assigned to the polymer network and the bending mode of engaged water molecules, giving the possibility to explore the structural changes occurring in the polymer network during the hydration process. The experimental findings were interpreted in the light of a comprehensive model which attempts to understand how physical and covalent cross-links combine to determine the macroscopic properties of the gel, like its water holding capacity and the whole rigidity of the gel network. These results shed light on the complex interplay between physical and chemical interactions which yield the formation and stabilization of the hydrogel network, opening the possibility of a rational design of these innovative soft materials for specific technological applications.
Physical chemistry chemical physics : PCCP, 2015
A detailed experimental and theoretical vibrational analysis of hydrogels of β-cyclodextrin nanosponges (β-CDNS), obtained by polymerization of β-cyclodextrin (β-CD) with the cross-linking agent ethylenediaminetetraacetic acid (EDTA), is reported here. Thorough structural characterization is achieved by exploiting the complementary selection rules of FTIR-ATR and Raman spectroscopies and by supporting the spectral assignments by DFT calculations of the spectral profiles. The combined analysis of the FTIR-ATR spectra of the polymers hydrated with H2O and D2O allowed us to isolate the HOH bending of water molecules not involved in symmetrical, tetrahedral environments. The analysis of the HOH bending mode was carried out as a function of temperature, showing the existence of a supercooled state of the water molecules. The highest level of cooperativity of the hydrogen bond scheme was reached at a value of the β-CD/EDTA molar ratio n = 6. Finally, the connectivity pattern of "unco...
Vibrational spectroscopy investigation of swelling phenomena in cyclodextrin nanosponges
Journal of Raman Spectroscopy, 2013
The swelling behaviour of b-cyclodextrin nanosponges, a new class of polymers obtained by reacting b-cyclodextrin with pyromellitic dianhydride, has been here investigated by the analysis of the vibrational dynamics performed by using Fourier-transform infrared spectroscopy in Attenuated Total Reflectance geometry and Raman scattering. The measurements have been carried out at different hydration levels and in the temperature range 250 K-310 K. The decomposition and assignment of the spectral components of the O-H stretching band have been discussed in terms of water molecules engaged in different hydrogen bonding networks, allowing us to recognize three different 'types' of water, namely 'bonded', 'intermediate'and 'free'. By following this approach, a detailed picture of the connectivity pattern of water molecules inside the polymeric matrix is achieved. The analysis of HOH bending vibrational band gives further information perfectly consistent with the results obtained from the analysis of the O-H stretching spectral region.
Colloid and Polymer Science, 2014
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 so-called pre-melting water in poly(vinyl 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 pre-melting of part of water in poly(vinyl 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 pre-melting.
European Physical Journal E, 2003
We have used a sol-gel technique to obtain optically transparent hydrogels in which water is confined within a 3D silica matrix. In this work we report X-ray scattering and dielectric spectroscopy measurements on samples having different aging times and compare them with previously obtained results with near-infrared (NIR) absorption spectroscopy. X-ray scattering at room temperature enables to characterize the structure and size of the matrix pores and the non-uniform distribution of water inside the hydrogel. Broad band dielectric spectroscopy in the temperature range 130–280 K enables to study water dynamics. In aged hydrogels two relaxations are clearly evident and show characteristic temperature dependence. The faster relaxation has an Arrhenius behavior in the whole temperature range investigated with an activation enthalpy of ∼ 50 kJ/mol; it is attributed to water molecules strongly interacting with the silica matrix. The slower relaxation has a markedly non-Arrhenius behavior which can be fitted with a Vogel-Fulcher-Tamman (VFT) relation with critical temperature of ∼100 K and activation enthalpies of 35 and 95 kJ/mol at 300 and 170 K respectively; it is attributed to water molecules within the pores that do not interact strongly with the matrix and behave collectively. The VFT temperature dependence of the dielectric relaxation time suggests that this water does not crystallize, in agreement with previous results from NIR spectroscopy.
Vibrational spectrum of water confined in and around cyclodextrins
Chemical Physics Letters, 2011
The effects of a-, band nd c-cyclodextrins (ACD, BCD, and GCD) on the low-frequency vibrational spectrum of water present around them and those confined inside their cavities have been investigated from molecular dynamics simulations. Attempts have been made to understand the effects of variation of the number of glucose rings and the ability of these macromolecules to form hydrogen bonds with water on the distribution of the vibrational density of states of water. It is observed that these bands for water in and around the cyclodextrins suffer blue shifts, the extent of the shifts are sensitive to the degree of confinement within the cavities and their hydrogen bonding status.
Water–Polymer interactions in PVME hydrogels – Raman spectroscopy studies
Polymer, 2009
Raman spectroscopy was used to investigate molecular interactions and structure of water in PVME hydrogels of various crosslinking degrees. Analysis of polymer n(CH 3) and n(CH 2) stretching vibrations has allowed to monitor the changes of hydrophilic and hydrophobic polymer-water interactions in a course of water deswelling and volume phase transition. Formation of water-polymer hydrogen bonds has appeared to influence directly supramolecular structure of the absorbed water. It was found that the polymer network density determines an availability of the hydrophilic sites to water molecules and also influences the kinetics of volume phase transition of the hydrogels. Activation energy of the volume phase transition in the investigated systems was estimated to be ca.88 kJ/mol.
Phys. Chem. Chem. Phys., 2015
The molecular dynamics of water and a polymer matrix is here explored in a paradigmatic model of a polysaccharide hydrogel, by the combined use of UV Raman scattering and infrared measurements. The case example of cyclodextrin nanosponges (CDNS)/hydrogel is chosen since the simultaneous presence in the structure of the polymer matrix of both hydrophilic and hydrophobic sites mimics the complexity of polysaccharide hydrogels. In this way, the contributions provided by the balance between the hydrophilicity/ hydrophobicity and the grade of entanglement of the polymer hydrogel to lead to the formation of the gel phase are separately accounted and evaluated. As main results, we found that the hydrophobic CH groups inserted on the aromatic ring of CDNS experience a more pronounced dynamic perturbation with respect to the carbonyl groups due to the collision between the solvent and vibrating atoms of the polymer. The overall results provide a detailed molecular picture of the swelling phenomena occurring when a chemically cross-linked polymer contacts with water or biological fluids and exploits the potentiality of UV Raman spectroscopy to retrieve dynamic information besides their structural counterpart obtained by the classical analysis of the basic features of vibrational spectra.