Preparation of poly (l-lactic acid) microspheres by droplet-freezing process

A recently developed process of microsphere preparation, named droplet-freezing process is introduced in this paper. The PLLA microspheres were fabricated by the droplet-freezing process, the diameter and porosity of the microspheres were measured, and the micro-morphologies of the microspheres were characterized by scanning electron microscopy (SEM). The formation process of microspheres was achieved by two steps: first, after droplets dropped off the delivery tube, they became approximately spherical in the air under the effect of the surface tension; second, droplets dropped into liquid condensate and maintained the spherical shape, and were frozen during the free settling process. Experimental results indicated that the microspheres fabricated by the droplet-freezing process have uniform diameters and the diameter can be controlled properly, along with the increase of the PLLA concentration, the size of microspheres increases, but the porosity of the microspheres decreases. The microspheres with high porosity can be obtained with a low concentration of the PLLA solution. SEM analysis revealed that the surfaces and interiors of the microsphere contain plentiful and interconnected micro pores. The microspheres are hopeful to be applied in bone tissue engineering.     

Coating of collagen on a poly(l-lactic acid) sponge surface for tissue engineering

Porous scaffolds play important roles in tissue engineering. Biodegradable synthetic polymers, such as poly(l-lactic acid) (PLLA), frequently are used in the preparation of porous scaffolds. Pretreating the surface of a PLLA porous scaffold is required to increase its wettability for smooth cell seeding due to the hydrophobic property of the scaffold's surface. In this study, a simple coating method was used to modify the surface of the PLLA sponges. The coating method included three steps: filling the PLLA sponge pores with collagen aqueous solution, centrifuging to remove excess collagen, and, finally, freeze-drying. Compared with the uncoated PLLA sponge, the collagen-coated PLLA sponge demonstrated both improved wettability and high water absorption. Cells were smoothly seeded in the collagen-coated PLLA sponges by dropping a cell suspension solution onto the sponges. Cells adhered to the collagen-coated sponge and were distributed homogeneously throughout the collagen-coated PLLA sponge.     

Fabrication of high conductivity dual multi-porous poly (l-lactic acid)/polypyrrole composite micro/nanofiber film

Dual multi-porous PLLA (poly(l-lactic acid))/H₂SO₄-doped PPy (polypyrrole) composite micro/nano fiber films were fabricated by combining electrospinning with in situ polymerization. The morphologies and structures of the resulting samples were analyzed by scanning electron microscopy (SEM). It was found that the composite micro/nano fibers exhibited a core-shell structure and the composite fiber film had a dual multi-pore structure composed of pores both in the fibers and among the fibers. Semiconductor parameter analyzer was used to characterize the electrical properties of the samples. It was interesting to find that all the PLLA/H₂SO₄-doped PPy composite micro/nano fiber films had higher conductivity than H₂SO₄-doped PPy particles when the polymerization time up to 180 min. Effects of the pyrrole synthesis conditions on the pore size and the conductivity of PLLA/PPy composite fiber film were assessed. By optimizing the polymerization conditions, the max conductivity of this composite fiber film was about 179.0 S cm⁻¹ with a pore size of about 250 μm. The possible mechanism of PLLA/H₂SO₄-doped PPy composite micro/nano fiber films had much higher conductivity than H₂SO₄-doped PPy particles was discussed.     

Characterization of vapor-deposited l-leucine nanofilm

X-ray reflective measurements (XRR), atomic force microscopy and single wavelength ellipsometry were used to investigate the optical properties of thin l-leucine films deposited onto silicon substrates. The ellipsometry data (Ψ,Δ) were fitted with a four-layer-model, and the optical refractive index of the l-leucine film measured with ellipsometry was determined to be 1.37. With the conventional effective medium approximation theory and the ellipsometry results, the density of the l-leucine nanofilm was determined to be 70% (0.81 g/cm³) of crystalline l-leucine. This value was in good agreement with the density of 69% (0.80 g/cm³) obtained with XRR measurement. The ellipsometry measurements also enabled us to estimate the surface roughness or absorption layer of the film. This procedure of combined XRR and ellipsometry measurements could be a powerful tool for the determination of the (otherwise hard-to-determine) refractive index in thin organic material films with a rough surface layer.     

Nucleation and crystal growth kinetics of poly(l-lactic acid) with self-assembled DBS nanofibrils

The effect of 1,3:2,4-dibenzylidene-d-sorbitol (DBS) on the crystallization behaviors of poly(l-lactic acid) (PLLA) was examined in this study. A small amount (≤4 wt%) of DBS altered the crystallization rate and regime transition temperature of PLLA. First, the addition of DBS and the formation of self-assembled DBS nanofibrils both increased the nucleation rate of PLLA. Second, the curves of the spherulitic growth rate versus the crystallization temperature of PLLA were discontinuous and did not show the typical bell-shaped behavior for all samples. We found that the change in crystal structures (α′-to-α) affected the regime transition temperatures, which led to the discontinuity. The regime transition (regime II–III) temperatures of PLLA slightly decreased as the DBS amounts were increased. This indicates that the more regular structure (regime II) of PLLA formed at lower temperatures when more DBS was added. In addition, the spherulitic growth rate of PLLA was found to be mainly influenced by the fold surface free energy. When the DBS amounts were increased, the increase in the fold surface free energy decreased the growth rate of PLLA. Nonetheless, the Avrami exponent, n, was not significantly changed because the spherulitic growth geometry and nucleation mechanism of PLLA were basically the same. The Avrami plot also shows that the secondary crystallization began earlier due to the formation of DBS nanofibrils for the samples containing higher DBS amounts.     

Oriented morphology and enhanced mechanical properties of poly(l-lactic acid) from shear controlled orientation in injection molding

This study investigates the effect of operative parameters of shear controlled orientation in injection molding (SCORIM) on poly(l-lactic acid), PLLA, and compared with conventional injection molded (CIM) PLLA. The in situ structure development was carried out with systematic variations of mold temperature and shearing time. The energy at break and maximum stress of all the SCORIM processed PLLA are higher than the CIM processed PLLA without sacrificing the modulus. The overall increments in maximum stress and energy at break were 134% and 641%, respectively. Significant enhancements in ductility for the SCORIM processed PLLA are shown to be a consequence of preferential molecular orientation of large fraction of core. The orientation of core is more pronounced at low mold temperature conditions and was increased with increasing shearing time at both the low and high mold temperatures. The processing–morphology and morphology–mechanical property relationships were then established.     

In vitro hydrolytic degradation of poly(para-dioxanone)/poly(d,l-lactide) blends

A series of biodegradable polymers were prepared by solution coprecipitation of poly(para-dioxanone) (PPDO) and poly(d,l-lactide) (PDLLA) in various blend ratios. Samples were compression molded into bars using a platen vulcanizing press. The in vitro hydrolytic degradation of PPDO/PDLLA blends was studied by examining the changes in weight, water absorption, tensile strength, breaking elongation, thermal properties, and morphology of the blends in phosphate buffered saline (PBS; pH 7.44) at 37 °C for 8 weeks. During the hydrolytic degradation, the weight loss and water absorption increased significantly for all samples, whereas the hydrolysis rate varied with the blend composition. The weight loss of PPDO/PDLLA 80/20, which showed the smallest degradation rate, was lower than that of pure PPDO for almost all of the hydrolytic degradation period. The results showed that the blend composition played an important role in determining the degradation behaviors of blends.     

Influence of fiber surface-treatment on interfacial property of poly(l-lactic acid)/ramie fabric biocomposites under UV-irradiation hydrothermal aging

The present study is devoted to the effect of fiber surface-treatment on the interfacial property of biocomposites based on poly(l-lactic acid) (PLLA) and ramie fabric. Ramie fiber is used as reinforced material because it's lowest water absorption among sisal, jute, kenaf and ramie fiber. Fiber surface-treatment can increase the water absorption of natural fibers. SEM images show that PLLA biocomposites with treated ramie fabric exhibit better interfacial adhesion character. DMA results show that the storage modulus of PLLA biocomposites with treated ramie increase compared to neat PLLA and PLLA biocomposites with untreated ramie. Unexpectedly, fiber surface-treatment can cause an accelerated decline in mechanical properties of PLLA biocomposites after UV-irradiation hydrothermal aging. Finally, GPC results show that there is no obvious decline in the molecular weight of PLLA. The main reason for this decline is the interfacial destructive effect induced by the water absorption of ramie fiber.     

Fabrication and degradation of poly(l-lactic acid) scaffolds with wool keratin

As a natural protein, wool keratin was used to improve the cell affinity of poly(l-lactic acid) (PLLA). Small keratin particles were prepared from keratin solution by the spray-drying process. Keratin particles were blended with PLLA/1,4-dioxane solution and paraffin micro-spheres which were used as progens. After the mixture was molded and dried, the paraffin micro-spheres were removed by cyclohexane. PLLA/keratin scaffolds with controlled pore size and well interconnectivity were fabricated. Keratin releasing rate was detected by Fourier transform infrared (FTIR) after the scaffold was immersed into PBS up to 4 weeks. The surface chemical structure was examined by X-ray photoelectron spectroscope (XPS). The results suggested that the keratin could be held into the scaffold which was expected to improve the interactions between osteoblasts and the polymeric scaffolds.     

Utilization of polymer degradation to modify electrical properties of poly(l-lactide)/poly(methyl methacrylate)/carbon filler composites

This research attempts to utilize polymer degradability in modifying electrical properties of poly(l-lactide) (PLLA)/poly(methyl methacrylate) (PMMA)/carbon fillers composites. Three kinds of carbon particles, i.e. carbon black, vapor-grown carbon fiber, and carbon nanotube, were compounded with PLLA/PMMA blend, followed by hydrolytic degradation of the composites, resulted in degradation of PLLA molecular chain from the surface of samples, with PMMA and carbon particles remained undegraded. By controlling degradation rate, it was possible to prepare samples with low surface resistivity, yet at the same time exhibited high value of volume resistivity. It was also found that final electrical properties of degraded composites depend on the size and the shape of the fillers.     

X-ray diffraction studies on stress transfer of kenaf reinforced poly(l-lactic acid) composite

Mechanical reinforcement of environmentally friendly composite, composed of kenaf fibers as reinforcement and poly-l-lactic acid (PLLA) resin as matrix, was investigated. The stress on the incorporated fibers in the composite under transverse load was monitored in situ and non-destructively using X-ray diffraction. The outer applied stress was found to be well transferred to the incorporated kenaf fibers through the PLLA matrix, which suggests a strong interaction between the fiber and the matrix. In addition, it was also revealed that a silane-coupling treatment to the kenaf fiber was effective for the improvement of interfacial adhesion.     

Synthesis and characterization of biodegradable poly(l-lactide)/layered double hydroxide nanocomposites

This study reports the preparation and physical properties of biodegradable nanocomposites fabricated using poly(l-lactide) (PLLA) and magnesium/aluminum layered double hydroxide (MgAl-LDH). The MgAl-LDH with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method. In order to improve the chemical compatibility between PLLA and LDH, the surface of LDH was organically-modified by polylactide with carboxyl end group (PLA–COOH) using ion-exchange process. Then, the PLLA/LDH nanocomposites were prepared by solution intercalation of PLLA into the galleries of PLA–COOH modified LDH (P-LDH) in tetrahydrofuran solution. Both X-ray diffraction data and Transmission electron microscopy images of PLLA/P-LDH nanocomposites indicate that the P-LDHs are randomly dispersed and exfoliated into the PLLA matrix. Mechanical properties of the fabricated 1.2 wt.% PLLA/P-LDH nanocomposites show significant enhancements in the storage modulus when compared to that of neat PLLA. Adding more P-LDH into PLLA matrix induced a decrease in the storage modulus of PLLA/P-LDH nanocomposites, probably due to the excessive content of PLA–COOH moleculars with low mechanical properties. The thermal stability and degradation activation energies of the PLLA and PLLA/P-LDH nanocomposites can also be discussed.     

Fabrication and characterization of poly-d-l-lactide/nano-hydroxyapatite composite scaffolds with poly (ethylene glycol) coating and dexamethasone releasing

The control of pore size and structure, drug release capacity, and biodegradation of scaffolds is of importance for bone tissue engineering. In this study, a technique combining polymer coagulation, cold compression molding, salt particulate leaching and drug coating method was developed to fabricate poly (ethylene glycol)/dexamethasone coated porous poly-d-l-lactide/nano-hydroxyapatite (PDLLA/nano-HAp) scaffolds. These scaffolds possess homogenous pore networks with high porosity (66-82%) and controllable pore size (200-300 μm). The compressive moduli and strength of the scaffolds after incorporation of nano-HAp were improved by 50% and 20%, respectively. The surface hydrophilicity of the scaffold was significantly improved by poly (ethylene glycol)/dexamethasone coating and nano-HAp addition, leading to a higher initial drug loading amount. The results showed that the drug release behavior of the scaffolds after 35-day immersion in water could be adjusted by varying the porosity level and by incorporation of 20 wt.% of nano-HAp.     

Preparation,crystallization and hydrolytic degradation of biodegradable poly(l-lactide)/polyhedral oligomeric silsesquioxanes nanocomposite

Biodegradable poly(l-lactide) (PLLA)/polyhedral oligomeric silsesquioxanes (POSS) nanocomposite was prepared via solution casting method for the first time in this work. Scanning electron microscopy observation indicates that POSS were homogeneously dispersed in the PLLA matrix. Effect of POSS on the crystal structure, crystallization kinetics, dynamical properties, and hydrolytic degradation of PLLA in the nanocomposite was investigated in detail. It is found that the presence of POSS has enhanced significantly the crystallization rate, improved mechanical properties and accelerated the hydrolytic degradation of PLLA in the nanocomposite with respect to neat PLLA.     

Concentration dependence of solution crystallization for poly(l-lactide) in p-xylene

The macromolecular chain conformation in solution changes with the solvent and temperature, which will affect the formation of crystalline structure in the subsequent crystallization process. The crystallization behavior of poly(l-lactide) samples prepared from solutions with various concentrations was studied by differential scanning calorimetric and X-ray diffraction. It is found that the sample recovered from a dilute solution exhibits higher crystallinity, higher non-isothermal crystallization temperature and faster crystallization rate. The condensation process of polymer chain in dilute solutions has an influence on the crystallization of polymer in the solid state. This results in the acceleration of the melt crystallization rate and the rise of non-isothermal crystallization temperature of PLLA recovered from the dilute solution.     

Structure and formation of self-assembled monolayers of helical poly (γ-benzyl l-glutamate) by surface plasmon resonance and infrared spectroscopy

The self-assembly of a rigid, rod-like, α-helical polypeptide was investigated on gold substrates. A disulfide labeled poly(γ-benzyl-l-glutamate) (PBLG) was prepared by a novel synthetic route wherein the polymerization of the N-carboxyanhydride was initiated with an amine terminated disulfide. Two different SSPBLG materials with molecular weight of 11 and 28 kDa were prepared and characterized using dielectric spectroscopy. The kinetics of self-assembly of the two polypeptides was studied using surface plasmon resonance (SPR) and the organization of the self-assembled monolayers (SAMs) was characterized by polarization-modulation infrared absorption spectroscopy (PM-IRRAS). SPR revealed that self-assembly of the high molecular weight polypeptides on gold substrates proceeds rapidly on a time-scale of minutes and is comparable to the time of assembly of simple n-alkanethiols. The increase in length of the polypeptide led to an increase in the dipole moment from 156 to 363 D. The enhanced electrostatic interactions in the SAMs were manifested as an increase in the average tilt of the helix axis from the normal to the surface and monolayers with smaller thickness.     

The growth of l-Glutamic acid hydrochloride crystals by Sankaranarayanan-Ramasamy (SR) method

An efficient semiorganic nonlinear optical crystal l-Glutamic acid hydrochloride has been grown by using the novel uniaxial crystal growth method of Sankaranarayanan and Ramasamy with a slight modification in the experimental setup. This method allows the crystals to grow in one specified axis with well developed facet. The grown crystal has a cylindrical morphology with good optical quality. The grown crystal has been characterized by powder X-ray diffraction and UV-Vis-NIR analyses. The NLO efficiency of the crystal has been confirmed by using the Kurtz powder technique.     

Heat and compression molded electrospun poly(l-lactide) membranes: Preparation and characterization

The fibrous membranes prepared by electrospinning have great advantages, such as high porosity and high specific surface area. However, low mechanical strength of electrospun membranes has been one of the most difficult technical problems to overcome, resulting in negative impact on the application. In this paper, the heat-assisted compression approach was employed to improve the mechanical performances of electrospun poly(l-lactide)(PLLA) membranes, especially in tensile strength. It is found that the electrospun PLLA membranes crystallize in α form and strong fiber-to-fiber linkages occurred with the aid of heat and compression. The tensile properties including tensile strength and modulus of membranes treated with a press at 6 MPa and a temperature at 60 °C (80 °C and 100 °C) increased by more than 100% compared with those of the as-electrospun membranes.     

Ellipsometric determination of optical properties of carbazole-containing poly(l-glutamate) thin films prepared from different solvents

In this study, the refractive indices (n) and thicknesses of carbazole-containing hole-transport materials such as poly(γ-carbazolylethyl l-glutamate) (PCELG) and poly(N-vinyl carbazole) (PVCz) films were determined by carrying out ellipsometric measurements. The thicknesses of PCELG and PVCz films determined by ellipsometric analysis were in good agreement with those determined by surface profilometry. The dependence of the refractive indices of the PCELG films on film thickness was classified into two types on the basis of the solvent from which the films were prepared: the refractive indices either increased with increasing film thickness, as in the case of PCELG films prepared from 1,2-dichloroethane (DCE) and monochlorobenzene (?-Cl), or were independent of the film thickness, as in the case of films prepared from 1,1,2,2-tetrachloroethane (TCE). A comparison of these results with the structures of the polymers, as determined by ¹H NMR, reveals that the two types of dependences of the refractive indices of the PCELG thin film on the film thickness can be attributed to the two types of aggregation structures and/or orientational characteristics corresponding to the helical conformation of the polymer. In contrast, the refractive indices of PVCz films are governed mainly by the film thickness. Finally, we would like to emphasize that the combination of ellipsometry and other techniques such as NMR and surface profilometry provide information not only on the film thickness and refractive index but also on the aggregation structure in thin films with thicknesses on the order of 50 nm.     

Structural characteristics and thermal properties of plasticized poly(l-lactide)-silica nanocomposites synthesized by sol-gel method

Plasticized poly(l-lactide)-silica nanocomposite materials have been successfully synthesized by sol-gel process. The resultant nanocomposites were characterized by infrared spectra (IR), X-ray diffraction (XRD), thermogravimetry (TG), Tensile testing and scanning electron microscope (SEM). IR measurements show that vibration of C-O-C group is confined by silica network. Also the crystallization of poly(l-lactide) is partly confined by silica network. The presence of even small amount of silica largely improves the tensile strength of the samples. TGA results reveal that the thermal stability of samples is improved with silica loading.