Structural fluctuations in cross-linked matrices with narrow pore size distribution
Introduction
Polymer hydrogels are two-component systems where water is quantitatively the main one. The interplay with the polymeric network, the other component, is important in many respects: (i) the diffusional properties of water are deeply influenced by the interaction with the matrix [1], [2]; (ii) crystallographic studies on the interface “bulk water-biological macromolecule” of proteins and DNA show the existence of clusters of water networked by hydrogen bonds not resembling the typical ice Ih structure and involving also the macromolecule [3]; (iii) the segregation of water molecules in hydrogels is a state commonly observed in many domains of living systems, as cytoplasm and membranes [4].
The study of the equilibrium and dynamic behaviour of water in crowded polymeric domains in principle can supply valuable information on the functioning of the hydrogels in terms of responsiveness to external stimuli and ability to control the release of guest molecules. Experimentally this is a difficult task that can be accomplished by NMR relaxometry [5], incoherent quasielastic [6] and inelastic neutron scattering [6], [7]. The complementary information supplied on dynamically different protons contained in the hydrogels by these approaches have contributed significantly to assess a picture of the state of water in confining matrices. Spin–lattices and spin–spin NMR relaxometry probe time windows ranging from few milliseconds to few seconds accounting for the proton exchanges between the polymer matrix and water whereas incoherent quasielastic neutron scattering (QENS) has dynamic and spatial windows in the range of hundreds of picoseconds and of few Ångstroms, respectively, thus probing the water diffusion at molecular scale in the matrix. The emerging scenario, common to many hydrogels, is a compartimentalization of water [8], [9] which assumes, depending on the hydration level of the network, the behaviour of a supercooled liquid in the vicinity of the polymer domains and of bulk water in the center of the polymer network meshes.
Usually physical or chemical polymer networks are heterogeneous systems in that the chains are not evenly distributed in the hydrogels [10]. Correspondingly the mesh size distribution is very broad and the observed static and dynamic properties of the system are smeared around averaged values. At the present, we are aiming to establish a correlation between pore size and dynamic properties of engulfed water especially focusing to the use of hydrophilic polymer matrices with different pore size and well characterized in terms of pore size distribution. In this respect, we found suitable for such study cross-linked dextran matrices, commercially available with trade name of Sephadex.
Typically, the dynamic structure factor, S(q,ω), from a QENS experiment [6] is analyzed by means of a model that takes into account the principal dynamic contributions to the quasielastic scattering, i.e., diffusion of segregated or bound water and local motions of the polymeric chains. The collection of the dynamic scattering law, S(q,ω), is a demanding experiment and preliminary explorative work on the hydrogels can be accomplished by a more agile experiment, namely observing the q-dependence of the elastic intensity of scattered neutrons, Iel, at different temperatures. Information concerning the motions of protons in the system at molecular scale are obtained by analysing the departure of the Debye–Waller factor from the Gaussian behaviour in the S(q,0) [11] on the basis of a simplified model that treats the proton displacements in terms of a potential profile with two asymmetric wells [12] representing the sites, with probability p1 and p2, where the jumps take place with an average length d corresponding to the site separation. According to this model, the elastic neutron scattering law is given by:where the exponential term is the Debye–Waller factor describing the Gaussian distribution of the harmonic proton displacement and the second term characterizes the modulation on the elastic incoherent structure factor due to a two minima potential profile with an energy difference of ΔG° and separated by a distance d. It can be useful for the following discussion to point out that with this simplified two state model it can be described also a system characterized by two degenerate states with the same equilibrium occupation probabilities. In Eq. (1) an offset, bkg, was introduced to account for the quasielastic contribution of water, as it will be discussed in Section 3.
In this work, we describe the results obtained by incoherent elastic neutron scattering of G15, G50, G100 Sephadex matrices at fixed hydration and corresponding to a pore size ranging from few to tens of Ångstroms. The fluctuation behaviour of non-exchangeable proton of the matrices can be compared with the NMR relaxometric studies carried out on similar systems.
Section snippets
Experimental
Sephadex G15, G50 and G100 samples from Pharmacia, corresponding in the swollen state to a pore size of 15, 30 and 60 Å, were used for the experiments and combined with double distilled water up to a fixed H2O content of 60% (w/w). The wet samples were equilibrated overnight in capped containers. Pore sizes of different Sephadex samples were evaluated from the characteristics supplied by the manufacturer in terms of the exclusion limits of molecular weight of standards in aqueous solution.
For
Results and discussion
Sephadex matrices are chemically cross-linked dextran hydrophilic networks with very narrow pore size distributions. For this characteristics these materials are used as molecular weight separation phase in size exclusion chromatography and they are available with different pore sizes.
We have investigated three different Sephadex hydrogels at fixed hydration degree differing only in their pore size. The temperature window of our study was restricted from above 273 K up to 320 K, where the
Concluding remarks
In this work, we have studied the incoherent elastic neutron scattering behaviour of hydrated samples presenting narrow pore size distributions. The pore size ranged from 15 to 60 Å. These systems allow to study the effect of the pore size in hydrophilic matrices limiting the structural heterogeneities usually present in hydrated materials and in hydrogels.
The picture emerging from this investigation is the strong interaction with water that behaves differently according to the pore size.
Acknowledgements
We thank Professor A. Deriu for helpful discussions.
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