Effect of the Heterogeneous Structure on Mechanical Properties for a Nanocellulose-Reinforced Polymer Composite

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Ultrasonication effects on physicochemical properties of biopolymer-based films: A comprehensive review

Biopolymeric films manufactured from materials such as starch, cellulose, protein, chitosan, gelatin, and polyvinyl alcohol are widely applied due to their complete biodegradability. While biopolymer-based films exhibit good gas barriers and optical properties when used in packaging, poor moisture resistance and mechanical properties limit their further application. Ultrasonication is a promising, effective technology for resolving these shortcomings, with its high efficiency, environmentally friendly nature, and safety. This review briefly introduces basic ultrasonication principles and their main effects on mechanical properties, transparency, color, microstructure, water vapor permeability, and oxygen resistance. We also describe the thermal performance of biopolymeric films. While ultrasonication has many positive effects on the physicochemical properties of biopolymeric films, many factors influence their behavior during film preparation, including power density, amplitude, treatment time, frequency, and the inherent properties of the source materials. This review focuses on biopolymers as film-forming materials and comprehensively discusses the promotional effects of ultrasonication on their physicochemical properties.

Nanocellulose in Emulsions and Heterogeneous Water-Based Polymer Systems: A Review

Nanocelluloses (i.e., bacterial nanocellulose, cellulose nanocrystals, and cellulose nanofibrils) are cellulose-based materials with at least one dimension in the nanoscale. These materials have unique and useful properties and have been shown to assemble at oil-water interfaces and impart new functionality to emulsion and latex systems. Herein, the use of nanocellulose in both emulsions and heterogeneous water-based polymers is reviewed, including dispersion, suspension, and emulsion polymerization. Comprehensive tables describe past work employing nanocellulose as stabilizers or additives and the properties that can be tailored through the use of nanocellulose are highlighted. Even at low loadings, nanocellulose offers an unprecedented level of control as a property modifier for a range of emulsion and polymer applications, influencing, for example, emulsion type, stability, and stimuli-responsive behavior. Nanocellulose can tune polymer particle properties such as size, surface charge, and morphology, or be used to produce capsules and polymer nanocomposites with enhanced mechanical, thermal, and adhesive properties. The role of nanocellulose is discussed, and a perspective for future direction is presented.

Incorporating Functionalized Cellulose to Increase the Toughness of Covalent Adaptable Networks

Covalent adaptable networks (CANs) are cross-linked polymers that have mechanical properties similar to thermosets at operating conditions yet can be reprocessed by cross-link exchange reactions that are activated by a stimulus. Although CAN exchange dynamics have been studied for many polymer compositions, the tensile properties of these demonstration systems are often inferior compared to those of commercial thermosets. In this study, we explore toughening CANs capable of forming covalent bonds with a reactive filler to characterize the trade-off between improved toughness and longer reprocessing times. Polycarbonate (PC) and polyurethane (PU) CANs were toughened by incorporating cellulose modified with cyclic carbonate groups as a reactive filler with loadings from 1.3 to 6.6 wt %. The addition of 6.6 wt % of the cellulose derivative resulted in a 3.2-fold increase in average toughness for the PC CANs, yet it only increased the characteristic relaxation time of stress relaxation (τ*) via disulfide exchange at 180 °C from 63 to 365 s. The cellulose-containing samples also showed >80% recovery in crosslinking density and mechanical properties after reprocessing. The addition of 3.2 wt % of the functionalized cellulose into a polyethylene glycol-based PU CAN led to a 2.3-fold increase in toughness while increasing τ* at 140 °C from 106 to 157 s. These findings demonstrate the promise of functionalized cellulose as an inexpensive, renewable, and sustainable filler that toughens CANs containing hydroxyl groups.

Effect of a heterogeneous network on glass transition dynamics and solvent crack behavior of epoxy resins

In general, it has been widely accepted that the physical properties of an epoxy resin are strongly dependent on how it is prepared. However, a clear understanding of the mechanisms of the relationship at a molecular level has yet to be achieved. We here studied the glass transition dynamics and fracture behavior of four epoxy resins, which were pre-cured at different temperatures and well cured under the same conditions. Fourier-transform infrared spectroscopy revealed that the reaction kinetics for an epoxy-amine mixture were strongly dependent on the pre-curing temperature. The glass transition temperature of epoxy resins with the same cross-linking density was dependent on the pre-curing temperature. Dielectric relaxation spectroscopy and dynamic mechanical analysis revealed that the fragility index of the epoxy resin decreased with increasing pre-curing temperature, indicating that the network structure formed in it became more heterogeneous with increasing pre-curing temperature. Once the epoxy resin was immersed in a good solvent, it was partly swollen and was then macroscopically fractured. The fracture was initiated by the crack generation in an un-swollen region of the resin due to the stress induced upon swelling. The immersion time required to reach the fracture decreased as the extent of the heterogeneity increased. The knowledge here obtained should be useful for understanding and controlling fracture toughness of epoxy resins, leading to the furtherance of their functionalization.

Mesoscopic heterogeneity in a nanocellulose-containing cell storage medium

A nanocellulose (NC)-containing medium is a promising candidate for cell storage that allows cell floating without any stirring. We here found that the NC medium was spatially heterogeneous in terms of its rheological properties. The characteristic length of the heterogeneity was a few tens of micrometers and it decreased upon sonication treatment. The length scale of the heterogeneity affected the cell suspension; the NC medium having a smaller length scale suppressed the cell sedimentation effectively.