We use atomic force microscopy (AFM) to perform a systematic quantitative characterization of the elastic modulus and dielectric constant of poly(L-lactic acid) electrospun nanofibers (PLLA), as well as composites of PLLA fibers with 1.0 wt% embedded multiwall carbon nanotubes (MWCNTs-PLLA). The elastic moduli are measured in the fiber skin region via AFM nanoindentation, and the dielectric constants are determined by measuring the phase shifts obtained via electrostatic force microscopy (EFM). We find that the average value for the elastic modulus for PLLA fibers is (9.8 ± 0.9) GPa, which is a factor of 2 larger than the measured average elastic modulus for MWCNT-PLLA composites (4.1 ± 0.7) GPa. We also use EFM to measure dielectric constants for both types of fibers. These measurements show that the dielectric constants of the MWCNT-PLLA fibers are significantly larger than the corresponding values obtained for PLLA fiber. This result is consistent with the higher polarizability of the MWCNT-PLLA composites. The measurement methods presented are general, and can be applied to determine the mechanical and electrical properties of other polymers and polymer nanocomposites.
Multi-walled carbon nanotubes (MWCNTs) often serve as an effective nucleating agent that facilitates the crystallization of semicrystalline polymers. Here we study the influence of MWCNTs on thermal and structural properties of electrospun fibers of Poly-lactide (PLA), which is well-known as a biodegradable and biocompatible thermoplastic polymer. MWCNTs were co-electrospun with poly(L-lactide) (PLLA, with 100% L-isomer) and poly (D-lactide) (PDLA, containing 4% D-isomer) in weight ratios ranging from 1.0 to 4.0 wt.% MWCNT. Electrospun fibers had average diameters below one micron, and addition of MWCNTs reduced the average fiber diameter and narrowed the diameter distribution. Results of real time synchrotron wide-angle X-ray scattering and calorimetry show that lower content of CNTs contributes to higher speed of crystallization. For the first time, the crystal, mobile amorphous, and rigid amorphous fractions were measured in MWCNT-containing PLA fibers. More rigid amorphous fraction was observed upon increasing the MWCNT concentration to 1.0 wt.% due to the restricted polymer chain mobility induced by MWCNTs. However, at the highest concentration of 4.0 wt.% MWCNTs, the crystal and rigid amorphous fractions were both reduced due to MWCNT aggregation. Fibers were assessed for their efficacy as substrates for growth of human umbilical vein fibroblasts. Cell culture results of fibroblasts plated on oriented PLLA fibers indicated cells grew preferentially in the same direction as the fiber orientation, while metabolic activity was reduced.
Lovastatin (Merck's Mevacor) is a statin drug designed to lower cholesterol, and reduce the risk of heart attack and stroke. We use electrospinning to combine the biomedical properties of lovastatin with the advantages of electrospun fibers to prepare a composite biomaterial for lovastatin delivery. Poly(l‐lactic acid) (PLLA), a biodegradable and biocompatible polymer, was co‐spun with lovastatin. Incorporation of lovastatin at 5 or 10 wt % improved fiber alignment and surface smoothness, and increased fiber diameter. Influence of lovastatin on the phase structure (crystal, mobile amorphous, and rigid amorphous fractions) was investigated using scanning calorimetry and synchrotron X‐ray scattering. Addition of lovastatin resulted in increased crystallinity and reduced mobile amorphous fraction. PLLA fibers were characterized in terms of their drug release kinetics in comparison to PLLA film. High drug entrapment efficiency (ranging from 72% to 82%) and appropriate release profiles were achieved. In vitro drug release studies demonstrated that release occurred in two stages: an initial rapid release over the first day and a slower second stage of release which approached a plateau after 7 days. PLLA fibers have a higher release rate than comparable film. Electrospun biomaterial fibers of PLLA provide a promising new release strategy for delivery of lovastatin. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45287.
Poly(L‐lactic acid) (PLLA)‐aligned fibers with diameters in the nano‐ to micrometer size scale are successfully prepared using the electrospinning technique from two types of solutions, different material parameters and working conditions. The fiber quality is evaluated using scanning electron microscopy (SEM) to judge fiber diameter, diameter uniformity, orientation, and appearance of defects or beads. The smoothest fibers, most uniform in diameter and defect free, were found to be produced from 10% w/v chloroform/dimethylformamide solution using an accelerating voltage from 10–20 kV. Addition of 1.0% multiwalled carbon nanotubes (MWCNT) into the electrospinning solution decreases fiber diameter, improves diameter uniformity, and slightly increases molecular chain alignment. The fibers were cold crystallized at 120°C and compared with their as‐spun counterparts. The influences of the crystalline phase and/or MWCNT addition were examined using fiber shrinkage, temperature‐modulated calorimetry, X‐ray diffraction, and dynamic mechanical analysis. Crystallization increases the glass transition temperature, Tg, slightly, but decreases the overall fiber alignment through shrinkage‐induced buckling of the fibers when heated above Tg. MWCNT addition has little impact on Tg, but significantly increases the orientation of crystallites. MWCNT addition slightly reduces the dynamic modulus, whereas crystallization increases the modulus in both neat‐ and MWCNT‐containing fibers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41779.
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