Elsevier

European Polymer Journal

Volume 49, Issue 9, September 2013, Pages 2730-2737
European Polymer Journal

Investigation of cellulose acetate viscoelastic properties in different solvents and microstructure

https://doi.org/10.1016/j.eurpolymj.2013.06.007Get rights and content

Highlight

  • Film and membrane properties are influenced by the solvent used in their preparation.

  • A non-solvent addition to cellulose acetate (CA) solutions causes chain agglomeration.

  • A three-dimensional elastic gel behavior is observed in some particular CA solutions.

  • CA agglomerates cause changes in the film microstructure and rigidity.

Abstract

This paper deals with the effect of solvent on the viscoelastic properties and microstructure of cellulose acetate (CA) by means of rheological measurements, dynamic-mechanical analysis and scanning electron microscopy. Acetone, acetic acid, dimethylacetamide (DMAc) and dimethylformamide (DMF) were used as good solvents and water as a non-solvent. The addition of water in the cellulose acetate/solvent mixture led to the development of an intense shear thinning behavior, as it caused the formation of CA agglomerates which were disrupted in a steady shearing condition. Apart from this, a three-dimensional elastic gel behavior in the CA/DMF/water and CA/DMAc/water solutions was observed during dynamical rheological analysis. The formation of CA agglomerates was controlled by the solvent type and resulted in microstructure changes, going from a dense structure to a porous one, and also influenced the CA chain rigidity and consequently the glass transition temperatures of the CA films.

Introduction

Biopolymers have currently received considerable scientific and technical interest, since they can be used as attractive alternatives to replace petroleum-based polymers as is the case of cellulose derivatives [1], [2], [3], [4], [5]. Among these derivatives, cellulose acetate (CA) is one of the most commercially applied. CA is a thermoplastic produced primarily from cellulose, a low cost starting material and the most abundant natural polymer on earth. CA is produced by the esterification of cellulose materials such as cotton, recycled paper, wood cellulose and sugarcane and it has applications in many areas such as support for fibers, packing films, plastic devices, filters, membranes, adhesives, coating for papers, electrical isolation and drug delivery systems [3], [4], [6].

Cellulose derivatives can provide sophisticated network structures through aggregation-induced phase separation for membrane and encapsulation applications. This process can be either thermally or non-solvent induced [3], [6], [7], [8], [9], [10], [11]. The non-solvent induced phase separation process involves three components: a polymer, a solvent and a non-solvent. Due to mass and/or heat transfer during a casting-solvent process, the polymeric systems become thermodynamically unstable and consequently phase separation occurs [12].

CA solubility in a solvent is dependent on a number of factors, including the degree of substitution and molecular weight. However, CA molecules are practically never completely molecularly dispersed in solution, but rather existing as complex molecular associates, which depend on the strength and the amount of intra and intermolecular interactions [7], [13], [14]. Tabe-Mohammadi et al. [15] showed that CA membrane selectivity was affected by the solvent type due to its evaporation rate which caused significant changes in its structure. In addition, Shieh and Chung [16] and Li et al. [17] studied the effect of liquid–liquid demixing on CA membrane structures. They concluded that the solvent polarity and the interaction between the non-solvent and solvent were responsible for different CA membrane structures, such as the presence of macrovoids or the development of a sponge-like structure.

As well known, structures of polymers in bulk are affected by the prior polymer chain shape in solution. Qian et al. [18] demonstrated that CA membrane morphologies were influenced by the presence of a second solvent with lower CA solvation characteristics, where the addition of tetrahydrofuran (THF) to CA/acetone changed the morphology characteristics from dense and homogeneous to an irregular and pore-like one. The authors, based on atomic force microscopy results, observed that the membrane surface presented many nodules, which were considered to be aggregates of polymer chains.

Phase separation can also lead to gel formation [7], [8], [19]. A gel is defined as a three-dimensional network made up of basic elements connected in some arrangement and swollen by a solvent; in other words, a gel structure is defined by large macromolecular associates and clusters [20], [21], [22]. Polymeric gels are classified into two main categories: chemical and physical gels. In chemical gels the connection usually occurs through covalent bonds, whereas in physical gels bonding between chains occurs through van der Waals interactions, especially by hydrogen links [8]. A gel structure has been observed in different CA systems including dimethylacetamide (DMAc)/water mixtures [7], dioxane/water [23] and ethylene glycol/water [24]. Several explanations about gel formation is reported in literature [7], [24], [25], based on van der Waals bonding strength between polymer chains and intramolecular interaction densities, as well as donor–acceptor characteristics of the components.

Rheological properties can generate an insight into the formation of the gel network [7], [9], [26], [27]. Appaw et al. [7] studied the effect of water concentration on the gel behavior and microstructure of a ternary CA/DMAc/water system. The authors concluded that the presence of water was responsible for the formation of a dense gel network and suggested that hydrogen-bonding interactions would be the major driving force for the initiation of the CA sol–gel process. The authors also showed that the increase of water content enhanced the microstructure density and, at higher CA concentration, a fine and more uniform structure was obtained. Kadla and Korehei [9], [28] studied the effect of hydrophilic and hydrophobic interactions on the rheological properties and microstructure of CA/DMAc solutions, using alcohols with different alkyl chain length as non-solvents. They observed that the increase of the alkyl chain length of the non-solvent caused the development of a heterogeneous network structure. In addition, a sol–gel transition was obtained at lower non-solvent concentration and the increase of the available hydrogen bonding groups on the non-solvent led to gels with higher modulus.

The aim of this work was firstly to investigate CA solution behavior in different solvents and solvent/non-solvent mixtures, due to the fact that there is very little literature about the comparison of different solvents or mixtures for CA at the same preparation conditions. The solution behavior was analyzed by means of the viscoelastic properties using rheological measurements. Thus, to complement this study, the resulting bulk structure was evaluated based on dynamic-mechanical analysis and scanning electron microscopy. This study can contribute both towards the understanding of the casting solvent effect on the morphology and consequently the CA film properties and towards the development of membranes and other nanostructured films made from this polymer.

Section snippets

Materials

Cellulose acetate (CA) with a number-average molecular weight of 30,000 g mol−1 and 39.8 wt% acetyl content (degree of substitution of CA = 2.5) was purchased from Aldrich and used as received. Commercially available acetone was obtained from Cromoline, acetic acid (HAc) from Synth, dimethylacetamide (DMAc) from Vetec and dimethylformamide (DMF) from Nuclear. Deionized water was used as the non-solvent. CA was dried at 60 °C, in a vacuum oven, for 24 h prior to use.

Sample preparation

CA powder was dispersed in each

CA solution viscoelastic behavior

Polymer solution behavior is a function of polymer chain conformation which depends on the solvent quality. The differences in the solution viscoelastic properties can be interpreted by chain conformation changes in solvents due to the characteristic solvent interactions with specific functional groups on the polymer chain. The CA chains present acetyl and hydroxyl groups which promote strong intra and intermolecular interactions, mainly by hydrogen links. The primary requisite for CA

Conclusions

Cellulose acetate aggregates were studied in solutions prepared with different solvents as well as in casting films prepared from these solutions. The addition of water to the solvent led to the development of an intense shear thinning behavior, due to the disruption of CA aggregates in the presence of shearing. In the case of CA/DMF/water and CA/DMAc/water solutions, a three-dimensional elastic gel behavior was observed, which is dependent on the following: (i) intramolecular interactions;

Acknowledgements

This research was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The authors would also like to thank Prof. Edvaldo Sabadini and L. Padula for assistance with the rheometer and D.R. Cocco for the dynamic-mechanical analysis.

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