| Electrospinning for biodegradable polymers have generated great interest in the nanomaterial research field recently. As typical one of such kind of materials, the thermoplastic aliphatic polyester has been extensively used for the electrospinning study, also including polylactide (PLA) and poly(ε-caprolactone)(PCL). Although these two polymers belong to the aliphatic polyester, they show different bulk properties:the glassy PLA with high intensity and rapid rate of degradation is brittle and shows low thermal deformation temperature, while the rubbery PCL with high toughness and deformation temperature shows slow degradation rate and poor strength. But this property complementarity between PLA and PCL just provide a tremendous possibility to obtain new materials with outstanding performance and controllable degradation rate. The work on electrospinning of PLA/PCL blend has been already reported. But those studies mainly focused on the exploration of basic electrospinning technologies and applications on biological tissue engineering field. The hierarchical structure of electrospun PLA/PCL blend fibers, such as orientation structure, phase morphology and microstructure of molecular chains, and the relations between those structures and electrospinning conditions are worthy of further study.Therefore, in this work, biodegradable PLA/PCL blend fibers/membranes were prepared by electrospinning for the study of hierarchical structure-processing-property relations. Firstly the electrospinning of neat PLA was studied in details in terms of basic technological factors. The effects of spinning solution properties and processing parameters on fibers’morphology were deeply explored, aiming at relating macroscopic morphology to electrospinning conditions. Then, the aligned PLA/PCL blend fibers were prepared with different collector devices and the effect of technological conditions on macroscopic alignment of fibers and microscopic orientation of molecular chains was also investigated. For the purpose of application, the PLA/PCL blend fiber membranes were then used as reinforcements to improve the mechanical properties of PCL matrix with layer-by-layer method. The influences of kinds and filler quantity of fibers on the macroscopic performance of composites were studied in details. The preliminary results are as follows.(1) PLA electrospun fibers The beaded structure are easily formed on the surface of elextrospun PLA fibers at lower solution concentrations (<6wt%), while at higher solution concentrations (>12wt%), it is hard to perform spinning continuously. Compared with the fibers from the solution using chloroform (CF) as the single solvent, those from the solutions using mixed solvents show much better appearance. But the surface structures of those fibers are highly dependent on the properties of assitant solvents. Moreover, the mixing ratio of solvents also affects the morphology of fibers: the addition of dimethylformamide (DMF) with strong polarization favors improving fiber morphology and refining of the fiber diameter. At the high DMF content levels, however, the obtained fibers show the conglutination structure. Besides, with increasing voltage, the average diameter of fibers reduces, accompanied by the lowered uniform level of fibers. The average diameter of fibers is also dependent on the flow rate of spinning solution. The best electrospinning conditions for PLA are determined:the solvent is CF/DMF (w/w,80/20); the solution concentration10wt%; the voltage12kV and the (low rate of spinning solution0.2ml/h. In this case, the obtained PLA fibers show best surface structure without any beads or micro-drops and with a narrow diameter distribution.(2) Electrospun membrane from PLA/PCL blendThe morphologies of the electrospun PLA/PCL blend fiber depend strongly on the blending ratio of two polymer components:the presence of higher viscous PLA component favors improvement of the fiber morphology and increase of the fiber diameter. Owning to the thermodynamically immiscibility between PLA and PCL, phase separation can be observed on their electrospun blend fibers. Besides, PCL phase shows an elongated structure along with fiber axis, indicating an orientation of large-scaled structure within the jet of immiscible polymer blend. As for the degradation of blend fiber membranes in phosphoric acid buffer solution, amorphous region degrades prior to crystallization region, and the PCL phase shows lower degradation rate than PLA. Furthermore, the degradation rate of blend fiber membranes in aqueous alkali is far higher than that in hydrochloric acid solution.(3) Electrospun membrane from PLA/PCL blend reinforced PCL compositeIn comparison with that of the neat PLA fibers, the PLA90/PCL10blend fibers show much better phase adhesion with the PCL matrix, this favors improvement of mechanical properties of PCL using electrospun blend fibers as reinforcements. As expected, the addition of PLA90/PCL10fibers improves both Young’s modulus and tensile strength of PCL. As the PLA content achieves up to15wt%, the Young’s modulus and the tensile strength increase by about91.7%and27.3%in contrast to neat PCL. However, PLA80/PCL20and PLA70/PCL30fibers show poorer reinforcement due to the phase separation between PLA and PCL component and the increasing content of PCL. Therefore, used as reinforcements, the best blending ratio of the blend fibers is90/10(PLA/PCL, w/w).(4) Aligned electrospun fibers from PLA/PCL blendThe electrospun fibers obtained from the setup with a collector constituted by two paralleled copper bars, a frame collector or a rotating drum collector plus auxiliary electric field can present aligned structure to a certain extent. Although adjusting processing parameters can increase the alignment of fibers, it is hard to obtain fibers with all high alignment through these three approaches. Using a rotating disk collector, however, can obtain highly aligned fiber bundles. The mean fiber diameter reduces and the degree of order increases with increasing tangential velocity. Furthermore, the results of polarized infrared spectra shows that the carbonyl stretching vibration of aligned fiber has a strong perpendicular dichroism, indicating that carbonyl group on the skeleton is oriented approximately perpendicular to the draw direction of the fiber. Therefore, there are hierarchical orientation behaviors within the PLA/PCL blend fibers after mechanical drawing:the macroscopic fiber alignment and the polymer chain orientation. |
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