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This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

A method for controlling microsphere particle size by regulating the ratio of polymer to solvent concentration and volume fraction, for an emulsion-solvent removal type of microencapsulation system, has been investigated. Viscosity of the external phase was kept constant by using light mineral oil in all experiments. Viscosity of the polymer solution, the internal phase, was modified by changing the ratio of the polymer to solvent concentration. Microspheres were obtained by adding the internal phase to the external phase and stirring the mixture for 30 min. A non-solvent was then added to the system to harden the polymer and recover the microspheres. Polymer concentration was modified by adding extra solvent to the mineral oil, just before the addition of the internal phase and also by adding extra solvent to the polymer phase. Similar, but not identical, results were obtained in both of these systems. A plot of particle size versus polymer to the solvent ratio resulted in sigmoidal curves. The term solvent means the solvent available to the polymer after mixing with mineral oil. A separate curve was obtained for each polymer concentration used in the experiments. When the internal phase volume fraction was incorporated as a variable in the plot of particle size, the three sigmoidal curves merged into a single curve, irrespective of the polymer concentration. The equation developed for controlling the particle size, as a function of polymer and solvent concentration and phase volume ratio, also was tested in systems that contained tartrazine as a model drug. Dissolution experiments were carried out and dissolution rate was correlated to particle size. Microspheres size was controlled by polymer and solvent concentration and phase volume ratio.

The influence of the viscosity of the internal and external phases and the interfacial tension between the two phases, in the emulsion type of microencapsulation system was investigated. The viscosity of mineral oil (external phase) was measured by a capillary viscometer and the viscosity of cellulose acetate trimellitate solution (internal phase) was measured by a Brookfield viscometer. The viscosity of the two phases were measured prior to mixing and at 5 and 60 min after mixing the phases. It was observed that the viscosity of the mineral oil phase prior to mixing had little effect on the average diameter of the microspheres, until a high concentration of light mineral oil was used. A graph of viscosity ratio of the internal phase to the external phase shows that a minimum viscosity ratio may be required before particle size increases. Results are discussed with respect to viscosity effects of mineral oil and polymer solution, as influenced by the solvent uptake by the mineral oil. The interfacial tension between the two phases was measured by pendant drop method. Interfacial tension was measured at 5 and 60 min after the two phases came in contact. The interfacial tension between the mineral oil and polymer solution ranged up to 7 dyne/cm and the particle size was not affected appreciably by the interfacial tension. Particle size and morphological analysis of the microspheres were determined using microscopy and scanning electron microscope.

Phase diagrams were prepared to indicate the region of microcapsule formation for the following system: cellulose acetate trimellitate, light mineral oil, and the solvent mixture (acetone:ethanol), using chloroform as the hardening agent. The effect of sorbitan monoleate, sorbitan monolaurate, and sorbitan trioleate on the region of the phase diagram for the formation of microcapsules was investigated. The results indicate that microcapsules are readily formed when the polymer concentration is in the 0.5-1.5% range and the solvent concentration is in the 5-10% range. Aggregation of microcapsules was minimized by using lower solvent concentration. Low concentrations of sorbitan monooleate in mineral oil (less than or equal to 1%) gave products that had smoother coats and more uniform particle size. Surfactants with low hydrophile:lipophile balance produced larger regions on the phase diagram for microencapsulation compared with a surfactant with higher hydrophile:lipophile balance. A mechanism for microencapsulation is described. Tartrazine microcapsules produced using different concentrations of surfactant were tested for dissolution characteristics in both acidic and neutral conditions. Tartrazine-containing microcapsules prepared by using 3% sorbitan monooleate had the lowest release in acidic conditions. The effect of surfactant and formulation concentration on microcapsule size was studied by analyzing the particle size distribution for both blank and tartrazine-containing microcapsules. The smallest microcapsule size was obtained when the sorbitan monooleate concentration was 3%. It appears that there is an upper limit for the surfactant concentration that could be used to achieve successful microencapsulation.

Phase diagrams were prepared to indicate the region of microcapsule formation for the following system: cellulose acetate phthalate, light mineral oil, acetone: 95% ethanol solvent, and sorbitan monooleate. The microencapsulation of pharmaceuticals having widely different solubility properties was carried out. Various factors affecting microencapsulation, namely trituration, order and time of addition of the pharmaceutical, core size, and the core to coat ratio, were investigated. The evidence for a mechanism of microencapsulation is also presented. The phase diagrams showed that microcapsules readily formed when the cellulose acetate phthalate concentration was in the 0.93-3.85% range and the polymer solvent concentration was in the 7-16% range. Aggregation of microcapsules was minimized at low solvent concentrations. Pharmaceuticals could be microencapsulated regardless of their solubility in the polymer solvent or hardening liquid. The size of the microencapsulated pharmaceutical increased as the core:coat ratio increased to a maximum of 1.5:1. There is an upper size limit of the pharmaceuticals which can be coated.

This work reinforced both a glassy polymer (high stiffness matrix) and a rubbery polymer (low stiffness matrix) with cellulose nanocrystals (CNC) derived from natural sources. CNC addition always increased stiffness while it increased toughness for a rubbery polymer and caused no loss in toughness for a glassy polymer. These results contradict many claims that when stiffness increases, the toughness decreases. We show that these claims depend on how toughness is measured. Our results were based on toughness measured using the essential work of fracture method. In contrast, toughness determined from area under the stress-strain curve shows a significant decrease, but that method may be a poor measure of toughness. Property enhancements usually require a good fiber/matrix interface. We used modeling of stiffness properties to confirm that CNC has a good interface with the studied polymer matrices. [ABSTRACT FROM AUTHOR]