Shear and pressure fields unavoidably coexist in practical polymer processing operations, but their combined influence on the crystalline structure of poly(L-lactic acid) (PLLA) has never been studied due to the limit of experiment device. In the current work, we utilized a homemade pressuring and shearing device to study the crystalline morphology and structure of PLLA under the coexistence of shear and pressure. Interestingly, we obtained almost exclusive β-form directly from PLLA melt crystallization at our experimental condition (shear 13.6 s −1 , pressure 100 MPa, and crystallization temperature 160 °C). Undoubtedly, abundant β-form is helpful to tackle the major shortcoming of PLLA performance, i.e., poor toughness. This meaningful result is different from the common viewpoints that PLLA β-form can usually be obtained by hot-drawing or solid coextrusion under a high tensile ratio, suggesting that PLLA βform can be obtained through shear-induced crystallization. In addition, the fraction of β-PLLA strongly depends on supercooling and shear intensity. A higher supercooling (pressure 150 MPa and crystallization temperature 160 °C) could also induce predominant β-form even under a very low shear rate of 1.0 s −1 . While, under a lower supercooling (pressure 50 MPa and crystallization temperature 160 °C), we did not observe any trace of β-form. In the heating experiment to investigate crystal form transformation, we also found that partial β-form transformed into α-form through melting−crystallization, and meanwhile some β-form crystals melted directly without transformation. These results could beyond doubt help to comprehend the relationship between crystallization condition and inner crystal structure and thus afford guidance in practical processing to toughen final PLLA products via controlling crystalline structure.
The conventional approach to improve the thermal conductivity (TC) of polymers by blindly adding inorganic fillers suffers from the limited TC enhancement (<3.0 W/mK) with isotropic TC and poor mechanical performance. Here, highly anisotropic, thermally conductive, and mechanically strong boron nitride (BN)/ultrahigh molecular weight polyethylene composites are prepared via a facile solid-phase extrusion (SPE) technology. The in-plane TC approaches 12.42 W/mK at the BN loading of 50 vol %, which is 242% higher than that of the high-pressure compressed counterpart (3.63 W/mK). The anisotropic index of TC reaches as high as ∼1000%, allowing the heat transfer more readily along in-plane direction than throughplane direction as revealed by infrared imaging results. We attribute the increased TC to the unique nacre-like structure induced by the flow-assisted alignment. BN platelets are highly oriented to form connected thermal conductive pathways for phonon transport along the basal plane. The interfacial thermal resistance is reduced by 2 orders of magnitude as deduced by theoretical calculation. More strikingly, compared to the controlled samples with randomly distributed structure, SPE composites exhibit significantly superior strength and toughness at equivalent filler content. These results demonstrate that the prepared SPE composites have high potential for the engineering application in heat dissipation fields.
Abstract:A knotty issue concerning the poor mechanical properties exists in the porogen leaching approach to porous scaffolds, despite its advantage in tuning pore structure. To address this hurdle, solid state extrusion (SSE) combined with porogen leaching was utilized to engineer porous scaffolds of poly(lactic acid) (PLA). Advances introduced by poly(ethylene glycol) (PEG) caused the PLA ductile to be processed and, on the other hand, enabled the formation of interconnected pores. Thus, a well-interconnected porous architecture with high connectivity exceeding 97% and elevated porosity over 60% was obtained in the as-prepared PLA scaffolds with the composition of NaCl higher than 75.00 wt % and PEG beyond 1.25 wt %. More strikingly, the pore walls of macropores encompassed countless micropores and rough surface topography, in favor of transporting nutrients and metabolites as well as cell attachment. The prominent compressive modulus of the PLA scaffolds was in the range of 85.7-207.4 MPa, matching the normal modulus of human trabecular bone . By means of alkaline modification to improve hydrophilicity, biocompatible porous PLA scaffolds exhibited good cell attachment. These results suggest that the SSE/porogen leaching approach provides an eligible clue for fabricating porous scaffolds with high mechanical performance for use as artificial extracellular matrices.
Surface
nanotopography provides a physical stimulus to direct cell
fate, especially in the case of osteogenic differentiation. However,
fabrication of nanopatterns usually suffers from complex procedures.
Herein, a feasible and versatile method was presented to create unique
nanosheets on a poly(ε-caprolactone) (PCL) substrate via surface
epitaxial crystallization. The thickness, periodic distance, and root-mean-square
nanoroughness of surface nanosheets were tunable by simply altering
the PCL concentration in the growth solution. Epitaxial nanosheets
possessed an identical composition as the substrate, being a prerequisite
to revealing the independent effect of biophysical linkage on the
osteogenic mechanism of the patterned surface. Preosteoblasts’
response to the epitaxial nanosheets was examined in the aspect of
preosteoblast proliferation and osteogenic differentiation. The expression
of alkaline phosphatase, collagen type I, osteopontin, and osteocalcin
as well as mineralization was significantly promoted by the epitaxial
nanosheets. Acceleration of osteogenic differentiation was attributed
to activating the TAZ/RUNX2 signaling pathway. The findings demonstrate
that surface epitaxial crystallization is a feasible approach to design
and construct nanotopography for bone tissue engineering.
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