Modulation of Optical Anisotropy in Chitosan Thin Films: Role of Swelling

Abstract

Thin films of chitosan, a biopolymer, have attracted attention as plausible candidates for several niche applications. Such uses would be better served by a deeper understanding of the physical characteristics of the films and their relationship to polymer confinement. Therefore, we have used spectroscopic ellipsometry to investigate the optical properties of chitosan thin films prepared by spin coating. Thin films of chitosan exhibited uniaxial anisotropy with a higher in-plane refractive index. This suggested that the polymer chains were preferentially oriented in the plane of the thin film. The optical anisotropy, defined as the difference between the in-plane and the outof plane refractive indices, increased upon decreasing the (dry) film thickness and the consequent increase in the one-dimensional confinement of the polymer chains. Upon exposure to solvent vapor, the film thickness increased and was accompanied by an exponential decrease in the optical anisotropy with increasing exposure time. The characteristic time of the exponential decrease was of the order of a few minutes and weakly increased with the dry film thickness. Deswelling experiments suggested that the preparation-induced residual stresses contributed negligibly to the observed optical anisotropy, which, in addition to the initial (dry) film thickness, also depended on the weight fraction of the penetrant in the swollen film. The continued increase in the film thickness, however, indicated that the swelling progressed for several hours before reaching saturation. Assuming the existence of a compact, bound layer of chitosan that could not swell upon exposure to solvent vapor, we defined a time-dependent "effective swelling ratio". The swelling kinetics of chitosan thin films of different initial thicknesses could be described using this single function, which suggested that the swelling process in these films was qualitatively identical.

Probing the tightly bound layer in poly(vinyl alcohol) thin films using swelling measurements

“…We found that the swelling of the thin films could be expressed using an ''effective swelling ratio'', c(t), for all of the investigated films irrespective of their initial (dry) thickness, provided there existed a tightly bound polymer layer of thickness E15 nm at the polymer/substrate interface that could not swell upon exposure to H 2 O vapour. The results for PVA were comparable to our earlier results 18 for a different hydrophilic polymer, chitosan, where the bound layer thickness was found to be E19 nm. XRR measurements were performed on two of the PVA thin films whose initial (dry) thicknesses were 18 nm and 58 nm.…”

“…For all of our samples, adequate fits, as indicated by the mean square error (MSE) values ≤10 (the larger values were obtained for the thicker films) could be obtained. Similarly to earlier investigations, 18,39,41,42,44 we found that adequate fits, as indicated by a negligible increase in the MSE values, could be obtained even by setting C = 0 in eqn (2). Therefore, to reduce the number of fit parameters, C = 0 was used for all of the fits performed in this work.…”

“…Depending on their thickness, spin-coated polymer thin films can exhibit significant optical anisotropy, typically uniaxial ( n x = n y ≠ n z ), as has been observed in chitosan films. 18 Accounting for the anisotropy yields a film thickness of 189.3 ± 0.1 nm, essentially identical to the thickness 189.2 ± 0.1 nm obtained by fitting to the optically isotropic model. Using the optically isotropic model enables fitting the data using fewer fit parameters without deleteriously affecting the quality of the fits and the uncertainty in the obtained thickness values.…”

“…21 Recently, we demonstrated that SE measurements of the time variation of the thin film thickness during the swelling of a polymer thin film upon exposure to solvent vapour can be used to investigate the nature of the aforementioned tightly bound layer. 18 By assuming at the polymer/substrate interface a layer of polymer that is impermeable to the solvent vapour and therefore does not swell, we found that the swelling kinetics of chitosan thin films of different initial thicknesses could be described using a single time-dependent swelling ratio, c(t). For reasons that will become apparent later, we identified the aforementioned layer with the tightly bound layer mentioned above.…”

“…In this context, several investigations have established the existence of a characteristic layer of polymer chains strongly adsorbed and therefore tightly bound to the solid substrate, referred to as the ''bound layer'' in the literature, of thickness between a few to few tens of nm. 7,8,17,18 For instance, Fujii et al 17 have reported the existence of a bound layer at the polymer/substrate interface of a polystyrene thin film spin coated onto a hydrophobic (H-Si) substrate. Using X-ray reflectivity, Vignaud et al 7 observed an increase in the density of polystyrene thin films with a decrease in their thickness and attributed this to the existence of a compact adsorbed layer at the polymer/substrate interface.…”