A detailed overview on interface design and control in polymer based composite dielectrics for energy storage applications.

Energy harvesting exploits ambient sources of energy such as mechanical loads, vibrations, human motion, waste heat, light or chemical sources and converts them into useful electrical energy. The applications for energy harvesting include low power electronics or wireless sensing at relatively lower power levels (nW to mW) with an aim to reduce a reliance on batteries or electrical power via cables and realise fully autonomous and self-powered systems. This review focuses on flexible energy harvesting system based on polyvinylidene fluoride based polymers, with an emphasis on manipulating and optimising the properties and performance of the polymeric materials and related nanocomposites through structuring the material at multiple scales. Ferroelectric properties are described and the potential of using the polarisation of the materials for vibration and thermal harvesting using piezo- and pyro-electric effects are explained. Approaches to tailor the ferroelectric, piezoelectric and pyroelectric properties of polymer materials are explored in detail; these include the influence of polymer processing conditions, heat treatment, nanoconfinement, blending, forming nanocomposites and electrospinning. Finally, examples of flexible harvesting devices that utilise the optimised ferroelectric polymer or nanocomposite systems are described and potential applications and future directions of research explored.

The concerns about the accumulating plastic waste pollution have stimulated the rapid development of bioplastics, in particular biodegradable bioplastics derived from renewable resources. Driven by a low carbon circular economy, bioplastics production is estimated to reach a 40% share of the plastics market by 2030 (Bioplastics Market Data, 2018). It is expected to substitute petrochemical-based plastics in many applications, from food packaging, pharmaceuticals, electronics, agriculture to textiles. The current biodegradable bioplastics have met challenges in competing with engineering polymers such as PET and Nylon in terms of processing capacity at the industry scale, mechanical robustness, thermal resistance, and stability. Poly(glycolic acid) (PGA) has a similar chemical structure to PLA but without the methyl side group, which allows the polymer chains to pack together tightly and results in a high degree of crystallinity (45–55%), high thermal stability (T_m = 220–230 °C), exceptionally high gas barrier (3 times higher than EVOH), as well as high mechanical strength (115 MPa) and stiffness (7 GPa). Meanwhile, PGA is rapidly biodegradable and 100% compostable, showing a similar biodegradation profile to cellulose. To date, PGA has been mainly used in the form of copolymers, such as poly(lactic-co-glycolic acid) (PLGA). Its unique properties have often been overlooked and are yet to be explored. This is caused by its intrinsic characteristics such as high hydrophilicity, rapid degradation, insolubility in most organic solvents and brittleness that have hindered its practical applications. Here we introduced the synthetic chemistry, processing methods, modification, and applications of PGA, aiming to provide a critical discussion about the technical challenges, development opportunities, and solutions for PGA-based materials. The future direction and perspectives for high-performance PGA are proposed. Given its synthesis diversity and unique properties, PGA shows great potential to substitute engineering petrochemical-based polymers for high temperature and high gas barrier packaging applications.

Polymer/graphene (oxide) nanocomposites exhibit enhanced mechanical properties at low volume fractions of graphene-based nanofillers. An understanding of the reinforcement behaviour was developed through the investigation of interfacial interactions between graphene oxide (GO) nanoplatelets and polymer matrix (PLLA, PCL, PS or HDPE) by combination of microstructure characterization and micromechanical modeling methods. The interfacial interaction determines the degree of dispersion of GO in polymers, interfacial adhesion strength as well as reinforcement efficiency, which can be tailored by the surface chemistry of GO and functionality of polymers. Homogeneous dispersion of GO nanoplatelets with high aspect ratios was found in PLLA and PCL matrices, as well as in lower polar polymer PS due to the preferable interactions between the aromatic rings and the graphene layers, while stacked GO layers with a lower aspect ratio were observed in HDPE matrix even with the presence of an organic compatibilizer. The theoretical elastic moduli of the four kinds of polymer/GO nanocomposites calculated by using the Halpin–Tsai model or a combination of Laminate theory and Mori–Tanaka model underestimated the experimental results. Considering the interfacial interactions, an effective volume fraction was introduced to the above composite models which interpreted the experimental data well. The interphase zone was thus quantified by using micromechanical modeling based on the measured mechanical properties of polymer/GO nanocomposites.