Xia Liao

Unique interfacial and confined porous morphology of PLA/PS blends in supercritical carbon dioxide

In this study, a typical immiscible system, a poly(lactic acid) (PLA)/polystyrene (PS) bioblend is used to investigate the effect of interface and phase structure on bubble nucleation and porous morphologies using supercritical carbon dioxide as a physical foaming agent. A unique microcellular skin-core structure with a porous core and surface, and a nonporous skin layer which is embedded in a solid PLA phase are observed. The involved possible mechanism of the interfacial nucleation and confined foaming behavior in multi-phase systems has been discussed. Because of the higher gas concentration and lower activation energy barrier, bubble nucleation preferentially occurs at the interface of the PS and PLA phases. Due to the constrained effect of the crystalline PLA phase, smaller space could be provided for the expansion of the PS phase during the foaming process, hence gas bubbles in the interior are restrained from further growth. The porous structures of the PS phases are similar when the blends have comparable phase morphology. The results indicated that the confinement effect on the foam behavior is not only related to the crystalline PLA phase but also the phase structure of the blend.

Preparation of alumina-coated graphite for thermally conductive and electrically insulating epoxy composites

Herein, highly thermally conductive and insulating epoxy composites were reported. Firstly uniform alumina-coated graphite flakes were successfully prepared by a two-step coating method of chemical precipitation with the aid of a sodium dodecyl sulfonate (SDS) surfactant using an inorganic precursor (aluminum nitrate) as the starting material. Then the alumina-coated graphite particles were incorporated into the epoxy resin. The thermal conductivity value of epoxy/alumina-coated graphite composite shows a significant increase from 0.22 W mK−1 (neat epoxy) to 0.64 W mK−1 by a factor of approximately 3 at the filler loading of 18.4%. Moreover, due to the presence of the alumina nanolayers coating on the graphite surface, epoxy/alumina-coated graphite composites could retain high electrical volume resistivity of >1010 Ω cm up to high filler contents, which was much higher than that of epoxy/graphite composites (<105 Ω cm) at the same filler loadings. And they still could be regarded as insulators.

The effects of viscoelastic properties on the cellular morphology of silicone rubber foams generated by supercritical carbon dioxide

Supercritical fluid foaming technology has been investigated for use in silicone rubber foam production due to its many unique properties. The viscoelastic properties of silicone rubber play a vital role in the supercritical carbon dioxide (scCO2) foaming process. This paper investigated for the first time the effect of silica content, saturation temperature and pressure on the viscoelastic properties of silicone rubber compounds. Further, the cellular morphology of silicone rubber foams generated by scCO2 was investigated with rheological analysis, which will be helpful for uncovering the connection between the cellular structure and the viscoelastic properties. This study could provide an environmentally-friendly and convenient way to better control the cellular morphology of rubber by adjusting experimental conditions.

Unusual hierarchical structures of micro-injection molded isotactic polypropylene in presence of an in situ microfibrillar network and a β-nucleating agent

The microstructural and mechanical properties of isotactic polypropylene (iPP), in situ PET microfibrils, and β-nucleating agent blends obtained from micro-injection molding were investigated via polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, and two-dimensional wide-angle X-ray diffraction. The results indicate that addition of PET microfibrils markedly increases crystallization temperatures, and increases the thickness of the final oriented layer. Introduction of PET microfibrils to β-nucleation agent-nucleated iPP samples leads to formation of oriented β-crystals epiphytic on the surface of PET fibers in the inner region; this feature improves adhesion between the fiber and the matrix and simultaneously improves the strength and toughness of the final PP/0.5/15 microparts (e.g., the tensile strength increased by 12 MPa and the elongation at break increased by 1.2%) compared with those of iPP microparts. Taken together, the results of this study introduce an alternative approach to optimize the properties of MIM parts.

Nanocellular and needle-like structures in poly(L-lactic acid) using spherulite templates and supercritical carbon dioxide

Poly(L-lactic acid) (PLLA) foams with unique nanocellular and needle-like morphology were successfully prepared by combining spherulite templating and supercritical carbon dioxide (CO2) foaming. The corresponding crystalline morphology formed under supercritical CO2 in PLLA was illustrated to investigate the foaming behavior of spherulites in detail. It was found that not only the degree of crystallinity but also the crystalline morphology played a vital role in cell nucleation and growth, thus the foam morphology. Nanocellular structure was primarily generated for the PLLA foamed at 100 °C and 12–24 MPa. Moreover, the morphological transition from approximate circular cells to needle-like cells occurred around 16 MPa at 100 °C because of the constraint of lamellae, and the two different structures coexisted at 100 °C and at pressures ranging from 16 to 24 MPa. The results indicated that the expansion ratio of spherulite was bigger than that of PLLA foam.

Concentric ring-banded spherulites of six-arm star-shaped poly(ε-caprolactone) via subcritical CO2

Birefringent Maltese-cross concentric ring-banded spherulites with radial periodic variation of thicknesses grown from six-arm star-shaped poly(e-caprolactone) (PCL) under the condition of subcritical CO2 were observed in this study. The structure of unique unclassical ring-banded spherulites was found to be different from that of traditional ring-banded spherulites which formed by periodic twisting of lamellar crystals along the spherulite radial direction. Laser scanning confocal microscopy, atomic force microscopy and scanning electronic microscopy images revealed that the ring-banded structure consisted of alternating periodically ridge and valley bands with continuous edge-on lamellae. A two-steps growth mechanism of ring-banded spherulites has been proposed to explain the development of central crystals and periodic ring-bands via CO2.

Introduction of a long-chain branching structure by ultraviolet-induced reactive extrusion to improve cell morphology and processing properties of polylactide foam

In this paper, long-chain branched polylactide (LCB-PLA) prepared by UV-induced reaction extrusion with trimethylolpropane triacrylate (TMPTA) was foamed by supercritical carbon dioxide (scCO2), and the effect of the long-chain branching structure on the cell morphologies of PLA foams was investigated. The LCB-PLA displayed higher complex viscosity, melting point and crystal nucleation potential under scCO2, and these factors could influence the foaming behavior of PLA which was proved by the different cell morphologies of samples foamed after various saturation times. The advantage of LCB-PLA on foaming was remarkable at high temperature and high pressure. LCB-PLA with more than 0.5% TMPTA showed nano-cells while the other samples showed micro-cells at 142 °C under 12 MPa, and the samples displayed elliptic cells with horizontal semimajor axis in linear PLA and circular cells or oval cells with vertical semimajor axis in LCB-PLA with increasing temperature. The improved cell morphology with reduced coalescence, no collapse and uniform cell distribution was also shown in LCB-PLA under higher pressure. All these results were due to the increasing matrix strength and higher crystal nucleation potential of LCB-PLA. The findings indicate that LCB-PLA possesses better foaming behavior at high temperature and high pressure. The wide foaming processing window of LCB-PLA would benefit the high temperature and high pressure foaming of PLA such as bead foaming and continuous extrusion foaming, thus broadening its application.

Creep-resistant behavior of beta-polypropylene with different crystalline morphologies

β-Phase isotactic polypropylene (β-iPP) specimens with different contents of β-phase nucleating agent were employed to investigate the deformation-induced microstructure evolution during creep behavior. Morphological investigations by SEM showed that the crystalline morphologies of β-iPP were controlled by the content of the β-phase nucleating agent, namely, well-developed β-spherulites induced by low content of β-phase nucleating agent, bundle-like morphology with imperfect spherulites induced by medium content of β-phase nucleating agent and needle-like morphology induced by high content of β-phase nucleating agent. It was interesting to observe that all samples with different contents of β-phase nucleating agent showed a similar β/α transformation process. However, well-developed β-spherulites, which have integrated crystalline structure, showed poor creep resistance compared with the crystalline morphology nucleated by higher contents of β-phase nucleating agent. For bundle-like morphology, the crystalline phase was imperfect and obtained larger long spacing, resulting in better creep resistance. With respect to needle-like morphology, the crystalline phase was disordered and displayed largest long spacing, resulting in best creep resistance. The results of this work revealed that the creep resistance would be different with different crystalline morphologies. On the other hand, this work provided the evolution of microstructure during deformation to further explain the molecular mechanism of fatigue failure for creep.

New understanding of the hierarchical distribution of isotactic polypropylene blends formed by microinjection-molded poly(ethylene terephthalate) and β-nucleating agent

Blends of isotactic polypropylene (iPP) and poly(ethylene terephthalate) (PET) were prepared by using a special injection molding process named microinjection molding (MIM). Interestingly, a strong continuous shear flow imposed on the melt of iPP/PET directly promotes the formation of the in situ PET microfibrils under microinjection molding. The hierarchical structures, including the shish-kebab-like structure, β-cylindrite, β-spherulite and α-spherulite are simultaneously formed in the iPP/PET microparts, which are closely related to formation of row-nuclei induced by the strong shear flow that is further amplified by incorporating in situ PET microfibrils. A surprising synergetic effect is observed between PET microfibrils and β-NA, resulting in the coexistence of shish-kebab, shish-kebab-like β-cylindrite, β-cylindrite and oriented β-crystal epiphytic on the surface of PET fibers in PP/0.1/15 microparts for the first time. Mechanical properties (e.g., tensile strength increased by 10.4 MPa) of the specimen are significantly improved compared with that of the iPP microparts because of the abundant hierarchical structures. A schematic model of the formation of hierarchical distribution of β-crystals via PET and β-NA addition is thus proposed.

Effective in situ polyamide 6 microfibrils in isotactic polypropylene under microinjection molding: significant improvement of mechanical performance

Microparts of isotactic polypropylene (iPP)/polyamide 6 (PA6) blends were prepared with a particular injection molding method known as microinjection molding (MIM). Continuous and strong shear action exerted on the melts of iPP/PA6 directly promoted the formation of in situ PA6 microfibril in MIM. Moreover, hierarchical structures, namely, spherulite, cylindrites, and transcrystallization, were observed in the microparts. The synergetic effect of PA6 in situ microfibrils, β-nucleating agent (β-NA), and strong shear action even induced more oriented β-crystals around the surface of PA6 microfibrils in the core layer and markedly increased the β-crystal content. Results showed that adding PA6 and β-NA markedly raised the crystallization temperature of iPP, and the effect of PA6 microfibrils was evidently more pronounced than that of PA6 spherical particles in conventional blends which implies more nucleation sites on the microfibrils. Moreover, strong orientation of iPP molecular chain was also confirmed by 2D-WAXD. It is well worth mentioning that the mechanical property was remarkably improved by these special morphology and crystalline structures.