Secondary interactions in blends of lignin and cellulose derivatives: composite morphology and properties

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Virginia Polytechnic Institute and State University


Differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) were used to characterize the morphology of solvent cast hydroxypropyl cellulose (HPC) films. These techniques revealed the existence of three phases in the bulk material: 1) an amorphous phase, 2) a crystalline phase, and 3) a phase of intermediate order which arises as a consequence of liquid crystal mesophase formation during solvent evaporation. Characterization of the effect of crosslinking on the tan peak temperature led to the conclusion that these relaxations (as observed by DMTA) are similar to glass transitions (Tg's) involving large-scale cooperative motions of the main chains.

This three-phase morphology presents a unique system for study with regard to the resultant morphology of binary blends with lignin. This blend system was prepared by solution blending in pyridine and dioxane, as well as melt-mixing followed by extrusion. A partially miscible system was obtained from all preparation methods; however, the injection molded and dioxane-cast materials were generally distinguished from those blends cast from pyridine solution. Their distinction is explained by an enhanced level of superstructure development in these blends as reflected by DMTA analysis and tensile properties. The dramatic improvement in modulus and tensile strength, particularly for the injection-molded samples, leads to the conclusion that lignin serves to reinforce the amorphous matrix of the resulting composite material.

The modification of lignin's polyhydroxy character through ethylation, acetylation, and propoxylation, revealed that specific secondary interactions between the components play a minor role, if any, in determining the state of miscibility in this blend system. However, from the analysis of the interaction parameter, B, it is concluded that the extensive hydrogen bonding within the lignin influences the conformation and chain rigidity of this component which dramatically influences the development of supermolecular morphology, and subsequently the overall morphology of the polymeric blend. This is reflected by a substantial increase in the amorphous volume fraction as detected by both DSC and DMTA.

The characterization of blends prepared from the unmodified organosolv lignin with ethyl cellulose and a cellulose acetate/butyrate ester confirms the minimal role of secondary interactions between components. However, the characteristics of the second phase suggest that the formation of liquid crystal domains in the cellulose derivatives significantly contribute to the overall morphology and properties of this blend system, as was noted for the HPC/lignin blend systems.