Yew Wei Leong

Stable Dispersions of Hybrid Nanoparticles Induced by Stereocomplexation between Enantiomeric Poly(lactide) Star Polymers

We report the formation and characterization of stable dispersions of hybrid nanoparticles in solution formed via stereocomplexation of enantiomeric poly(lactide) hybrid star polymers. The hybrid starlike polymers, having polyhedral oligomeric silsesquioxane (POSS) nanocages as the core and either poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) as the arms, are synthesized via ring-opening polymerization of lactide using octafunctional POSS as the macroinitiator. In the solid state, differential scanning calorimetry and wide-angle X-ray scattering measurements confirmed the formation of the stereocomplex in the mixture of POSS-star-PLLA and POSS-star-PDLA (50:50, wt %). In a solution of the same mixture in tetrahydrofuran (THF), sterocomplexation leads to formation of hybrid nanaoparticles. Detailed accounts of the nanoparticle formation and influence of aging and concentration have been presented. It was observed that at low concentration the stereocomplexed nanaoparticles remain stable over 45 days and are not sensitive to dilution, suggesting the formation of a stable hybrid nanoparticle dispersion in solution. In contrast, the aggregates of the individual POSS-star-PLLA or POSS-star-PDLA in THF, formed via weak solvophobic interactions, tended to disintegrate into smaller aggregates on dilution. Exploiting the PLLA-PDLA stereocomplexation with an appropriate molecular design can be a versatile route to develop stable organic/inorganic hybrid nanoparticle dispersions.

Tuning self-assembly of hybrid PLA-P(MA-POSS) block copolymers in solution via stereocomplexation

We demonstrate the formation of stable hybrid nanoparticles in solution through self-assembly driven by the stereocomplexation between enantiomeric poly(lactide) (PLA) chains in organic–inorganic hybrid diblock copolymers. The well-defined hybrid diblock copolymers (PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS)) are synthesized via atom transfer radical polymerization of methacrylisobutyl POSS (MA-POSS) using either PLLA or PDLA as a macroinitiator. The structure of the block copolymers is confirmed by 1H NMR and GPC. The mixture of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) in THF solution resulted in the formation of self-assembled nanoparticles as confirmed by the light scattering data. It is further verified that the only driving force for self-assembly in THF solution is the ‘stereocomplexation’ between the PLLA and PDLA blocks as no aggregation could be observed in THF solutions of homochiral polylactides at low concentration (∼1 mg mL−1). A solution mixture of 50:50 weight% of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) for all the samples yields the best stereocomplexation results in terms of particles size and aggregation numbers. For a given composition, the size of the stereocomplexed hybrid nanoparticles, however, decreases with the increasing length of the (MA-POSS) block in the copolymer. As an example, for a 50:50 weight% mixture, the mean hydrodynamic radius Rh and apparent aggregation number Nagg of the particles decreased from 220 nm and 890, respectively, to 72 nm and 290, when the degree of polymerization of P(MA-POSS) increased from ∼2 to 11–12. It is assumed that the bulky POSS nanocages of P(MA-POSS) in the PLA-b-P(MA-POSS) block copolymer sterically hinder the formation of larger nanoparticles by block copolymers with longer P(MA-POSS) blocks. The stereocomplexed nanoparticles remain stable over 30 days and are not sensitive to dilution, suggesting the formation of stable hybrid nanoparticles dispersion. In contrast, the homopolymer mixture of PLLA and PDLA turned cloudy and was no longer stable after 12 days due to the formation of larger macroscopic aggregates. This interesting finding opens up new opportunities to tune the size and stability of the stereocomplexed nanoparticles in solution by manipulating the block length of the inorganic P(MA-POSS) segment.

Sulfonic Acid- and Lithium Sulfonate-Grafted Poly(Vinylidene Fluoride) Electrospun Mats As Ionic Liquid Host for Electrochromic Device and Lithium-Ion Battery

Electrospun polymer nanofibrous mats loaded with ionic liquids (ILs) are promising nonvolatile electrolytes with high ionic conductivity. The large cations of ILs are, however, difficult to diffuse into solid electrodes, making them unappealing for application in some electrochemical devices. To address this issue, a new strategy is used to introduce proton conduction into an IL-based electrolyte. Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) copolymer is functionalized with sulfonic acid through covalent attachment of taurine. The sulfonic acid-grafted P(VDF-HFP) electrospun mats consist of interconnected nanofibers, leading to remarkable improvement in dimensional stability of the mats. IL-based polymer electrolytes are prepared by immersing the modified mats in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM(+)BF4(-)). It is found that the SO3(-) groups can have Lewis acid-base interactions with the cations (BMIM(+)) of IL to promote the dissociation of ILs, and provide additional proton conduction, resulting in significantly improved ionic conductivity. Using this novel electrolyte, polyaniline-based electrochromic devices show higher transmittance contrast and faster switching behavior. Furthermore, the sulfonic acid-grafted P(VDF-HFP) electrospun mats can also be lithiated, giving additional lithium ion conduction for the IL-based electrolyte, with which Li/LiCoO2 batteries display enhanced C-rate performance.

Poly(vinylidene fluoride) nanofibrous mats with covalently attached SiO2 nanoparticles as an ionic liquid host: enhanced ion transport for electrochromic devices and lithium-ion batteries

In this article, it is demonstrated that the electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)) nanofibrous mat functionalized with (3-aminopropyl)triethoxysilane is a versatile platform for the fabrication of hybrid nanofibrous mats by covalently attaching various types of inorganic oxide nanoparticles on the nanofiber surface via a sol–gel process. In particular, SiO2-on-P(VDF–HFP) nanofibrous mats synthesized using this method is an excellent ionic liquid (IL) host for electrolyte applications. The IL-based electrolytes in the form of free-standing mats are obtained by immersing SiO2-on-P(VDF–HFP) mats in two types of liquid electrolytes, namely LiClO4/1-butyl-3-methylimidazolium tetrafluoroborate and bis(trifluoromethane)sulfonimide lithium salt/1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. It is found that the surface attached SiO2 nanoparticles can effectively serve as salt dissociation promoters by interacting with the anions of both ILs and lithium salts through Lewis acid–base interactions. They dramatically enhance the ionic conductivity and lithium transference number of the electrolytes. In addition, better compatibility of the electrolytes with lithium electrodes is also observed in the presence of surface-attached SiO2. Using IL-loaded SiO2-on-P(VDF–HFP) nanofibrous mats as the electrolytes, electrochromic devices display higher transmittance contrast, while Li/LiCoO2 batteries show significantly improved C-rate performance and cycling stability. This class of novel non-volatile electrolytes with high ionic conductivity also has the potential to be used in other electrochemical devices.

Interfacial Characteristics of Film Insert Moldings Consisting of Semicrystalline and Amorphous Polymers

Abstract Film insert moldings were produced from amorphous and crystalline polymers, i. e. PC + ABS and PET respectively. The vastly different characteristics of these polymers provide an interesting platform to study the mechanisms that are responsible for interfacial bond formation, particularly when these materials were molded at different molding conditions. It was observed that intermolecular diffusion between amorphous polymers takes place at virtually any temperature as long as it is kept above the glass transition temperature of the material, though higher temperatures are favorable to accelerate diffusion. As for crystalline polymers, there is an initial need to create free and mobile molecular chains by heating the materials to its melting temperature before any significant diffusion takes place. Certain criteria have to be met, irrespective of the materials used, before good interfacial bonding is achieved. Firstly, the injection molding parameters should be optimized to ensure intimacy between the surfaces of the film and injected resin. Once this is achieved, interdiffusion of molecules should be promoted to ensure extensive film-substrate molecular entanglements. The state of molecular entanglement should then be retained for a sufficient amount of time as the material solidifies in order to obtain good film-substrate adhesion.

Filler treatment effects on the weathering of talc-, CaCO3- and kaolin-filled polypropylene hybrid composites

Talc, calcium carbonate (CaCO3), and kaolin hold considerable promise in the development of polymer composites for good mechanical properties and stability. Comparative studies on the usage of these minerals as single fillers in polypropylene (PP) have shown varying degrees of reinforcement due to their differences in terms of particle geometry, surface energy and affinity towards the matrix polymer. In this study, comparisons were made in terms of mechanical, thermal and weatherability properties between hybrid-filler PP composites (i.e. PP filled with either talc–CaCO3 or talc–kaolin hybrid filler combinations), with particular attention directed towards the effect of surface modification of the fillers. The talc/CaCO3 hybrid composites have shown exceptional performance in terms of flexural and impact properties. The contribution of talc in the talc–kaolin hybrid composite system has been significant in terms of enhancing the overall tensile and flexural properties. The ability of silane and titanate coupling agents in boosting the resistance of the composites to severe damage and degradation due to natural weathering has been shown.

Mechanical Properties of Injection-Moulded Jute/Glass Fibre Hybrid Composites

In order to improve the mechanical performance of natural fibre composites, glass fibres (GF) and natural fibres were simultaneously incorporated into a single matrix system. Such a practice would of course decrease the “degree of green”, which measures the amount (in weight percentage) of natural materials in a composite system. Nevertheless, it was found that hybrid composites containing up to 20 wt.% jute fibres exhibited similar tensile properties to those of glass fibre composites. The distribution of glass and jute fibres was anisotropic, wherein the GF tend to accumulate at the skin regions of the mouldings, thus orientating towards the resin flow direction. Jute fibres, on the other hand, tend to remain at the core regions and were mostly oriented transverse to the flow direction. This anisotropic distribution and orientation are thought to be responsible for the weakening of the composites at high natural fibre loadings.

Internal Structure and Mechanical Properties of Glass Fibre Reinforced PC/ABS Injection Mouldings with Different Fibre Surface Treatments

The microstructure (including the morphology, fibre length, and fibre orientation) of injection moulded glass fibre reinforced polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) blend was investigated. Three grades of commercially available surface treated glass fibres were used; each grade received a treatment that was specially formulated and optimized to enhance the affinity of the glass fibre surface to PC, ABS or both PC and ABS phases. Morphological evaluation confirms that the glass fibres had a preferential affinity to a particular resin depending on the type of surface treatment applied. This preferential affinity could also be detected through the difference in fibre length distribution in the specimens, although their mean fibre lengths and fibre orientations were quite similar. Flexural modulus, flexural strength and impact strength were all affected by differences in the distribution of fibre lengths. It was also discovered that the resin flowed faster than the glass fibres, especially at the core of the mouldings.

Drop Weight Impact Properties of (CO) Injection Molded Short Glass Fiber/Short Carbon Fiber/Polyamide 6 Hybrid Composites

Polyamide 6 (PA 6) composites with short glass fiber (GF)/short carbon fiber (CF) hybrid fiber reinforcement were produced using two types of injection molding techniques, i.e., the conventional and co-injection molding machines. In the latter, sandwich skin-core hybrid composites were produced. The fiber volume fraction for all formulations was fixed at 0.07. The composites were subjected to drop weight impact tests, to determine their impact strength, impact energy, ductility index (DI) and ductility ratio (DR). It was observed that the PA 6 formulation exhibited the highest impact strength and impact energy values whereas carbon fiber/PA 6 (CF/PA 6) exhibited the highest DI and DR values. The carbon fiber skin/glass fiber core/PA 6 (CFS/GFC/PA 6) sandwich skin-core hybrid composites showed the highest impact energy and impact strength values among hybrids. The glass fiber skin/carbon fiber core/PA 6 (GFS/CFC/PA 6) hybrids on the other hand showed the highest DI and DR values among hybrids. Failure mechanisms in the matrix and composites, assessed by scanning electron microscope (SEM), were discussed in relation to the effects of fiber incorporation and its effects on the ductility of the composites, with particular emphasis on the hybrid composites.

Crystallization, mechanical properties and thermal stability of cockleshell-derived CaCO3 filled polypropylene

This study discusses the potential of utilizing waste cockleshell derived-CaCO3 (CS) as filler in polypropylene (PP). Mineral fillers were prepared from cockleshell-derived CaCO3 and used to fill polypropylene. The composites were prepared by melt blending and fabricated by injection and compression molding techniques. The effects of filler on crystal structure, crystallization and thermal degradation characteristics of filled polypropylene composites were elucidated. The cockleshell filler promoted the formation of the β-crystalline phase in PP, which improved the rigidity and toughness of the composites. However, stearic acid treatments on the filler would significantly affect the nucleation process and therefore hindered crystallization. Acceleration in thermal degradation of PP was also noted with increasing filler loading.