PLLA/PDLA blend films with only stereocomplex crystallites as a crystalline species together with pure PLLA and PDLA films with only homo‐crystallites as a crystalline species were prepared, and the effects of enantiomeric polymer blending, crystalline species, and crystallinity on the water vapor permeability were investigated. The WVT coefficient P of PLLA/PDLA blend films was 14–23% lower than that of pure PLLA and PDLA films in the crystallinity Xc range of 0–30%. Amorphous PLLA/PDLA blend films have a much lower P than pure PLLA and PDLA films. The dependence of P on Xc for blend films was stronger for Xc = 0–30% than for Xc = 30–100%. This dependence is discussed using the Nielsen model and the concept of “restricted” (or “restrained”) and “free” amorphous regions.
DSC indicated that the nucleation of PLLA is enhanced in the presence of PGA even at a PGA content as low as 0.1 wt.‐%. However, the enhancing behavior of PGA was different to that of other nucleating agents for PLLA. Polarized optical microscopy revealed that the presence of PGA increased the number of PLLA spherulites per unit area. WAXD showed that in the PLLA/PGA films, PLLA and PGA crystallize separately to form their respective crystallites and PGA crystallites were formed at a PGA content at above 3 wt.‐% (at least). FTIR spectroscopy indicated that that there are significant interactions between PLLA and PGA chains in amorphous regions. Such interactions should have enhanced the growth of PLLA crystallites from the surface of PGA crystallites.
A new target collector was designed for taking up aligned nanofibers by electrospinning. The collector consists of a rotor around which several fins were attached for winding electrospun filaments continuously in large amounts. The alignment of the nanofibers wound on the collector was affected by the electrospinning conditions, such as the needle‐to‐collector distance and the applied voltage, but not by the rotation speed of the collector. At a voltage of 0.5 kV · cm?1, about 60% of the fibers were found to be aligned within an angle of ± 5° relative to the rotational direction of the collector. The fiber alignment was improved to 90% by drawing the fiber bundle 2–3 times at 110 °C. The drawing was also effective for crystal orientation of the fibers as revealed by WAXD. The drawn fibers show improved mechanical properties.
The effects of nucleobases, especially uracil, on the nonisothermal and isothermal crystallization, melting behavior, spherulite morphology, and crystalline structure of bio‐based and biodegradable PLLA are studied. The melt‐ and cold‐crystallization rates of PLLA increase with increasing uracil loading. The melting behavior of nonisothermally melt‐ and cold‐crystallized PLLAs depends on the uracil content. The isothermal crystallization kinetics is analyzed based on an Avrami model. The incorporation of uracil changes the t1/2/Tc profile of PLLA due to the more distinct heterogeneous nucleation effects at small supercooling. The crystalline structure of PLLA is not affected by uracil presence. The nucleation density increases and the spherulite size decreases by uracil incorporation.
A new stereo pentablock copolymer consisting of poly(l ‐lactide) (PLLA: A), poly‐d ‐lactide (PDLA: B), and poly(butylene succinate) (PBS: C) is synthesized by two‐step ring‐opening polymerization of d ‐ and l ‐lactides in the presence of bis‐hydroxyl‐terminated PBS prepolymer that has been prepared by the ordinary polycondensation. The pentablock copolymers (PLLA‐PDLA‐PBS‐PDLA‐PLLA) as well as the triblock copolymers (PLLA‐PBS‐PLLA) obtained as the intermediates show different properties depending on the polymer compositions. In the pentablock copolymers, the direct connection of the PLLA and PDLA blocks allows easy formation of the stereocomplex crystals, while the introduction of the semicrystalline PBS block is effective not only for changing the crystallization kinetics but also for imparting an elastomeric property.
TMC/LLA copolymers with several TMC/LLA ratios are synthesized and a composite is obtained by reinforcing with short PLGA fibers. In vitro degradation is studied at 37 °C in pH = 7.4 buffer and compared with a PLLA homopolymer. The degradation of the copolymers appears slower than that of PLLA, showing that TMC units are more resistant to hydrolysis than LLA. Compositional changes indicate a preferential degradation of LLA units as compared to TMC ones. Morphological changes with crystallization of degradation by‐products are observed. The composite degrades much faster than the neat copolymer and PLLA because the faster degradation of PLGA fibers speeds up the degradation of the matrix. The composite appears promising for the fabrication of totally bioresorbable stents.
Biodegradable hyperbranched poly(ester amide) (HBP) was used as a compatibilizer to modify PLA/SiO2 nanocomposites for the first time. The ternary composites displayed dramatically improved mechanical properties including excellent toughness and fairly high stiffness. TEM images revealed that an encapsulation structure was formed by HBP surrounding SiO2 nanoparticles, and their surfaces became flocculent due to the migration process of silica. The linear viscoelastic behavior of the nanocomposites measured by parallel plate rheometer indicated that strong interface adhesion existed between PLA matrix and silica nanofiller after incorporating of HBP. The compatibilization effect of HBP and the enhanced mobility of nanoparticles contributed to the improved mechanical properties.
In an attempt to enhance the biocompatibility and mechanical strength of fibrous polymeric scaffold systems, nanocrystalline hydroxyapatite (HAp) particles were incorporated into the electrospun poly(L ‐lactide) (PLLA) fibers and then mechanically interlocked using a vapor‐phase solvent adsorption method. The solvent‐assisted compression molding substantially increased the tensile strength (from 4.61 to 12.63 MPa) and mechanical modulus (from 50.6 to 627.7 MPa) of the fibrous scaffold, which maintained the interstitial space between the fibers to allow the facile transport of nutrients and waste during cell growth and polymer biodegradation. Macrometer‐sized pores (ca. 100–400 µm) were introduced into the scaffolds in a controlled fashion using the salt leaching/gas forming technique to give desired space for a facile cell implantation and growth. Overall, the developed methodology allows the polymer‐based scaffold systems to be tailored for various applications in light of surface characteristics, mechanical strength, and pore size of engineered scaffolds.