RESUMO
Poly(lactic acid) (PLA) microneedles have been explored extensively, but the current regular fabrication strategy, such as thermoforming, is inefficient and poorly conformable. In addition, PLA needs to be modified as the application of microneedle arrays made of pure PLA is limited because of their easy tip fracture and poor skin adhesion. For this purpose, in this article, we reported a facile and scalable strategy to fabricate the microneedle arrays of the blend of PLA matrix and poly(p-dioxanone) (PPDO) dispersed phase with complementary mechanical properties through microinjection molding technology. The results showed that the PPDO dispersed phase could be in situ fibrillated under the effect of the strong shear stress field generated in micro-injection molding. These in situ fibrillated PPDO dispersed phases could hence induce the formation of the shish-kebab structures in the PLA matrix. Particularly for PLA/PPDO (90/10) blend, there are the densest and most perfect shish-kebab structures formed. The above microscopic structure evolution could be also advantageous to the enhancement in the mechanical properties of microparts of PLA/PPDO blend (tensile microparts and microneedle arrays), e.g., the elongation at break of the blend is almost double that of pure PLA while still maintaining the high stiffness (Young's modulus of 2.7 GPa) and the high strength (tensile strength of 68.3 MPa) in the tensile test, and relative to pure PLA, there is 100% or more increase in the load and displacement of microneedle in the compression test. This could open up new spaces for expanding the industrial application of the fabricated microneedle arrays.
RESUMO
Microinjection molding is a novel frontier polymer processing strategy different from conventional ones. In this paper, three different cavity-sizes of micro-mold tools were firstly fabricated, and the influences of micro-mold cavity dimension on the phase morphology structure, crystallization and orientation, and mechanical performance of the microinjection molded polylactic acid (PLA)/polycaprolactone (PCL) blend microparts were carefully investigated accordingly. The results show that the reduction of the cavity size would result in much higher shear stress field and cooling temperature gradient, which is advantageous to the fibrillation and orientation of PCL-dispersed phase. Consequently, with decreasing the micro-mold cavity dimension from length 26 mm to 15 mm, the interfacial compatibility is improved, significantly increasing number of PCL fibers with smaller diameter are in situ formed in PLA matrix and their orientation degree also obviously increases, which is verified by SEM and 2D-WAXD measurements. The Differential Scanning Calorimetry (DSC) analysis shows that the decrease in cavity dimension causes the enhancement of PLA crystallization property due to shear-induced crystallization, which is reflected by the decreasing PLA cold crystallization temperature and increasing PLA crystallinity (almost doubling that of conventional macropart). As a result, the dynamic/static mechanical property measurements exhibit that with decreasing the cavity size, the storage modulus, and the loss modulus of PLA/PCL blend micropart increase, and the corresponding tensile strength, elongation at break, and Young's modulus also present an obviously increasing tendency. The related investigations would provide some new spaces and insights for realization of high-performance of PLA/PCL blend micropart.
RESUMO
Formation of a segregated structure in conductive polymer composites is one of the most effective strategies for achieving good electrical conductivity and electromagnetic interference (EMI) shielding performance. Nevertheless, for low-melt-viscosity poly(lactic acid) (PLA), intense molecular motion occurs at the molding temperature, which is detrimental to the fixation of the conductive networks. In this study, a novel molding technique assisted by microwave heating was proposed to construct a segregated structure in a PLA/carbon nanotube (CNT) composite. The coating layer of CNTs acted as the microwave absorber and caused intense localized heating of PLA surfaces upon microwave irradiation. The surface temperature of the PLA granule was precisely regulated by adjusting the coated CNT content, microwave power, and irradiation time. Thus, the coated granules were softened and fused at an optimal sintering zone, which effectively hindered the excessive migration of CNT strips into the interior of PLA phases, and a majority retained the original CNT network in the molded composite. Meanwhile, benefiting from microwave sintering, sufficient chain diffusion and entanglement occurred in the interfacial regions, enhancing the adhesion strength among the neighboring PLA phases. The prepared PLA/CNT composite with only 5.0 wt % CNTs exhibited a high electrical conductivity of 16.3 S/m and an excellent EMI shielding effectiveness (EMI SE) of 36.7 dB at a frequency of 10.0 GHz. The results indicate that microwave-assisted sintering might be a promising alternative for constructing a segregated structure in low-melt-viscosity polymers.
RESUMO
Fabrication of microcellular polymer composite foam based on high-performance plastic is a promising strategy for preparing the lightweight, high-strength and multifunctional materials. Herein, we proposed a facile and green method to prepare microcellular polysulfone/carbon nanotube (PSU/CNTs) composite foams with segregated structure by combining solid-phase milling and supercritical carbon dioxide (scCO2) foaming. The segregated PSU/CNTs foam with as low as 5.0 wt% CNT was provided with a good electrical conductivity of 5.2 S m-1 and an acceptable electromagnetic interference shielding effectiveness (EMI SE) of 23.7 dB, respectively. Moreover, the segregated PSU/CNT foam exhibited an ultralow percolation threshold of 0.06 vol%. An absorption-dominant shielding feature was observed for segregated PSU/CNT foam, which could be attributed to the synergistic effect of the perfect CNT networks and the microcellular structure in PSU domains. In addition, benefitting from the inherent properties of the PSU matrix, foam density dropped to 0.69 g cm-3, and the material still possessed a high specific compression strength of 38.8 MPa cm3 g-1. Therefore, our work provided an insight into the preparation of lightweight, high-strength and multifunctional materials that might have great potential applications in aerospace and military areas.
