Structure and dynamics of thin anchored hydrogel layers by fluorescence correlation and photon correlation spectroscopy

Abstract

This thesis reports on the structure and dynamics investigation of hydrogel layers. Photocross-linked thin films of a thermo-responsive polymer anchored onto solid substrates were prepared from copolymers of N-isopropylacrylamide (NIPAAm, 94%), methacrylic acid (MAA, 5%) and 4-methacryloyloxy-benzophenone (MaBP, 1%). FCS measurements were performed on these gel films swollen in two different solvents, ethanol and water, and several different fluorescent probes of various sizes and charges were used. In water, positively charged tracers (e.g. Rh6G, and a positively charged perylene derivative) were strongly attracted into the gel and suffered a hindered diffusion due to attractive electrostatic interactions with the negatively charged polymer. However, in the case of negatively charged tracers (e.g. fluorescein and a negatively charged perylene derivative), the presence of repulsive electrostatic forces prevented their diffusion into the like-charged gels. Different cross-linking densities (corresponding to different irradiation times in the cross-linking process) were examined and the FCS results indicate that the probe molecules are slowed down and the swelling ratio of the gels decreases with increasing cross-linking density. An attempt to extend the FCS diffusion study from rigid small molecules to flexible macromolecules was not successful since the negative charge of the fluorescently labelled macromolecule utilized (labelled PEG) prevented the diffusion inside the gel layer due to repulsive electrostatic interactions. Measurements on the diffusion process of proteins have shown that a positively charged protein (cBSA) was immobilized inside the hydrogel layer, whereas a neutral one (native BSA) diffuses into the hydrogel layer. Increase of the characteristic diffusion time of the protein was observed with increasing cross-linking density. The translational motion of a molecular (Rh6G) and macromolecular (pNIPAAm) tracer in transient pNIPAAm-MAA-MaBP networks (semi-dilute polymer solutions) in ethanol at varying polymer concentrations (up to c ~ 20 vol %) at ambient conditions were found by FCS to exhibit a Brownian diffusion in the absence of specific interactions with the polymer matrix, which is explained by the lack of ion dissociation in ethanol as a solvent. The dynamics of surface anchored and crosslinked pNIPAAm-MAA-MaBP layers with different crosslinking densities swollen in the good solvent ethanol were probed by the newly developed micro-photon correlation spectroscopy in the time range 1 x 10-6 s to 1 s. The cooperative diffusion coefficient describes the fast motion associated with the effective mesh size of the network in analogy to the transient physical network of uncrosslinked pNIPAAm chains. Assuming one dimensional swelling vertical to the surface for an estimate of the polymer volume fraction, the cooperative diffusion was found to increase faster with increasing volume fraction in the swollen gel layers than in the uncrosslinked pNIPAAm semidilute solutions. The present tethered pNIPAAm gel layers also revealed a second, slow diffusive process for the relaxation of the concentration fluctuations which in contrast to the physical network of the linear pNIPAAm chains, is virtually insensitive to the concentration i.e. crosslinking density. Also, unexpected for common unconfined gels, this slow diffusive mode was fully relaxed at times less than 0.1 s at wavevectors larger than 0.008 nm-1. This mode was related to long wavelength dynamic heterogeneities but its nature is still unknown. Finally, the effect of added salt and varying temperature on the dynamics of the surface anchored pNIPAAm crosslinked layers swollen in water was investigated. An initial small increase of the ionic strength (addition of NaCl up to 0.02 mol / L) resulted in an increase of the cooperative diffusion coefficient Dcoop possibly due to the electrostatic screening of the anionic carboxylate groups in the polymer gel. Further increase of the ionic strength resulted in a decrease of Dcoop with increasing NaCl concentration, which may be related to a salting-out effect by decreasing the solvent quality. Very high ionic strengths (>1M) completely erased the dynamic behavior due to the collapse of the gel. Similarly, when the temperature was increased towards the transition temperature (LCST), the collective diffusion coefficient Dcoop exhibited a critical slow down upon the swollen gel approaching the collapse. In conclusion, FCS and PCS have proven to be complementary and powerful optical techniques to investigate the structure and dynamics of thin anchored gel layers.