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Effect of non-solvent on viscoelastic and microstructural properties of cellulose acetate in a ternary system Korehei, Reza

Abstract

In this study, the effects of component composition on rheological and microstructural behaviour of a ternary of cellulose acetate (CA), N,N-dimethylacetamide (DMAc) and non-solvent (1-Propanol, 2-Propanol, 1-Hexanol, 1-Octanol, 1-Decanol, 1,2-Ethanediol, 1,2-Propanediol, 1,3-Propanediol, 1,4-Butanediol, 1,6-Hexanediol, Glycerol) system was examined. In this ternary system, physical gel formation can arise as a result of phase separation, which is characterized by the observation of a gradual to extreme cloudiness in the system. Depending on the non-solvent concentration, structure and polymer concentration, phase separation leads to CA aggregation and the formation of large macromolecular assemblies. Sol-gel transition is observed at a critical non-solvent concentration, which is dependent on the non-solvent structure and CA concentration. Increasing CA and non-solvent concentration resulted in enhanced steady shear viscosity and dynamic viscoelastic properties. Enhanced dynamic viscoelastic property and gelation are due to the intensification of intermolecular hydrogen bonding and hydrophobic interactions. Increasing the available hydrogen-bonding groups within the non-solvent leads to the formation of gels with larger elastic and viscous modulus (G' and G"), and a lower concentration sol-gel transition. Likewise, increasing the hydrophobic component of the non-solvent also enhanced the gel properties and accelerated the sol-gel transition. Although hydrophobic interactions play a role in the gelation process, it appears that gel properties are greatly influenced by competitive hydrogen bonding between system components. Competitive hydrogen bonding interactions between components in the various stages of the phase separation and gel formation was used to explain the weak and strong polymer-network structures observed by rheology. Through the use of Fourier transfer infrared (FTIR) spectroscopy the effect of hydrogen bonding between CA, DMAc and non-solvent were probed. Shifting the hydroxyl (OH) band to a lower wavenumber in the FTIR spectra suggests the intensification of the intermolecular hydrogen bonds in the ternary system. This shift accounts for the phase separation, and development of microstructure in the sample. Increasing the non-solvent content shifts the yield strain of the gels to a lower strain value, suggesting that they are made of floes. These floes consist of aggregated macromolecules with strong-links, and the links between floes are stronger than the links within the floes. The power-law dependence of elastic modulus (G1), together with similar values of fractal dimension for gels observed through confocal microscopy, suggests that the gels are fractal in nature and that they are made through an aggregation mechanism. Scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) revealed differences in the gel microstructure, depending on the constituent composition. Microscopic images showed better uniform packing in the polymer network structure as the CA concentration increases. The LSCM images (fluorescence and reflective mode) confirm the rheological results, and show different texture and aggregated structure for the gel as the structure of non-solvents are varied in the ternary system.

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