Amorphous PLLA Research Articles

Synthesis, solid-state structure, and surface properties of end-capped poly(L-lactide).

End-capped poly(L-lactide) (PLLA) samples with dodecyl or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester were synthesized by ring-opening polymerization of L-lactide in the presence of zinc dodecanoxide or zinc 2-(2-(2-methoxyethoxy)ethoxy)ethoxide as a catalyst, respectively. On the basis of NMR analysis, it was confirmed that the carboxylic acid chain ends of PLLA molecules were selectively substituted by dodecyl or MEEE ester groups. To evaluate the wettability on the surface of end-capped PLLA films, the advancing contact angle (thetaa) with water was measured. The amorphous PLLA films showed relatively similar thetaa values regardless of the chemical structure of the polymer chain end. In contrast, the thetaa values of semicrystalline films were varied over a wide range, dependent on the chemical structure of the chain end. In addition, the thetaa values of dodecyl ester end-capped PLLA film with low molecular weight increased with an increase in the crystallization temperature. Both the crystallinity and lamellar thickness of dodecyl ester end-capped PLLA films increased with the crystallization temperature. These results suggest that the segregation of the chain ends on the PLLA film surface was strongly affected by the crystallization conditions.

Crystallization Behavior of Amorphous Poly(l-Lactide)

Amorphous poly(l-lactide) (PLLA) was annealed in two different ways: amorphous samples were heated at a given temperature to induce crystallization (one-step annealing); and amorphous samples were first crystallized at a low temperature and subsequently annealed at a higher temperature than the crystallization temperature. Samples thus prepared were measured by DSC. The original amorphous sample exhibited an exothermic peak at about 100°C (exothermic peak I), an exothermic peak just below the melting point (exothermic peak II), and an endothermic peak when it was melted. Exothermic peak I was caused by cold crystallization. When the melting points of PLLA samples, heat-treated in various ways, were plotted as a function of annealing temperature, there was discontinuity at about 120°C. From analyses of wide-angle X-ray diffraction patterns, it was found that when amorphous PLLA was crystallized at a temperature below 120°C, crystallites of the β-form formed, and when annealed at a temperature above 120°C, crystallites of the α-form grew. Thus, exothermic peak I was attributed to cold crystallization of the β-form, and peak II was caused by the phase transition of the β-form to a more stable form.

Environmental degradation of biodegradable polyesters 2. Poly( ε-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(L-lactide) films in natural dynamic seawater

The (bio)degradation of aliphatic polyesters, poly( ε-caprolactone) (PCL), poly[(R)-3-hydroxybutyrate] (R-PHB), and poly(L-lactide) (PLLA) films in natural dynamic seawater was investigated using polarizing optical microscopy, gravimetry, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and tensile testing. The results were compared with those reported for the polyester films biodegraded in the controlled static seawater to reveal the physical effects in the natural dynamic seawater on the degradation of the aliphatic polyesters. The gravimetry and tensile testing showed that the mechanical stresses and strains in the natural dynamic seawater caused mechanical destruction or degradation of the films, resulting in seemingly accelerated (bio)degradation of all the films compared with that in the controlled static seawater. Such mechanical acceleration effects in the natural dynamic seawater were higher for the R-PHB and PLLA films with relatively high T g than for the PCL films with relatively low T g, when their (bio)degradation was monitored by gravimetry. The polarizing microscopy showed that the R-PHB films were (bio)degraded predominantly at the centers of and between the spherulites. The tensile strength and Young's modulus were lower for the initially crystallized PLLA films than for the initially amorphous PLLA films when compared at the same degradation time.

Poly(L-Lactide), 8. High-Temperature Hydrolysis of Poly(L-Lactide) Films with Different Crystallinities and Crystalline Thicknesses in Phosphate-Buffered Solution

Poly(L-lactide) (PLLA) films having different crystallinities (Xc's) and crystalline thicknesses (Lc's) were prepared by annealing at different temperatures (Ta's) from the melt and their high-temperature hydrolysis was investigated at 97°C in phosphate-buffered solution. The changes in remaining weight, molecular weight distribution, and surface morphology of the PLLA films during hydrolysis revealed that their hydrolysis at the high temperature in phosphate-buffered solution proceeds homogeneously along the film cross-section mainly via the bulk erosion mechanism and that the hydrolysis takes place predominantly and randomly at the chains in the amorphous region. The remaining weight was higher for the PLLA films having high initial Xc when compared at the same hydrolysis time above 30 h. However, the difference in the hydrolysis rate between the initially amorphous and crystallized PLLA films at 97°C was smaller than that at 37°C, due to rapid crystallization of the initially amorphous PLLA film by exposure to crystallizable high temperature in phosphate-buffered solution. The hydrolysis constant (k) values of the films at 97°C for the period of 0–8 h, 0.059–0.085 h–1 (1.4–2.0 d–1), were three orders of magnitude higher than those at 37°C for the period of 0–12 months, 2.2–3.4×10–3 d–1. The melting temperature (Tm) and Xc of the PLLA films decreased and increased, respectively, monotonously with hydrolysis time, excluding the initial increase in Tm for the PLLA films prepared at Ta = 100, 120, and 140°C in the first 8, 16, and 16 h, respectively. A specific peak that appeared at a low molecular weight around 1×104 in the GPC spectra was ascribed to the component of one fold in the crystalline region. The relationship between Tm and Lc was found to be Tm (K) = 467·[1–1.61/Lc (nm)] for the PLLA films hydrolyzed at 97°C for 40 h.

Diffusion coefficient and equilibrium solubility of water molecules in biodegradable polymers

The diffusion coefficient and solubility of water molecules were measured in polyglycolide (PGA), poly(L-lactide) (PLLA), poly[(R)-3-hydroxybutyrate] (PHB), poly(ϵ-caprolactone) (PCL), and SkygreenR (SG). The diffusion coefficient and equilibrium solubility decreased in the order SG > PCL > PLLA > PHB > PGA and PGA > SG > PLLA > PHB > PCL, respectively. The diffusion coefficient and solubility of water at low sorption temperature in PHB varied according to the initial crystallinity of the matrix polymer even though crystallization of PHB molecules took place during the sorption experiment. In contrast, the amorphous PLLA and the crystalline PLLA showed an almost identical diffusion coefficient and solubility of water, in spite of the fact that the amorphous PLLA remained practically amorphous during the whole sorption procedure. A strong correlation existed between the water solubility and the surface tension or contact angle of the polymer matrix. The water diffusivity in PGA was almost 2 orders of magnitude lower while water was more soluble in PGA with a lower heat of sorption than that corresponding to the other more hydrophobic polymers, indicating that the transport of water molecules in PGA followed the solution–diffusion model. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1716–1722, 2000

Diffusion coefficient and equilibrium solubility of water molecules in biodegradable polymers

The diffusion coefficient and solubility of water molecules were measured in polyglycolide (PGA), poly(L-lactide) (PLLA), poly[(R)-3-hydroxybutyrate] (PHB), poly(ϵ-caprolactone) (PCL), and SkygreenR (SG). The diffusion coefficient and equilibrium solubility decreased in the order SG > PCL > PLLA > PHB > PGA and PGA > SG > PLLA > PHB > PCL, respectively. The diffusion coefficient and solubility of water at low sorption temperature in PHB varied according to the initial crystallinity of the matrix polymer even though crystallization of PHB molecules took place during the sorption experiment. In contrast, the amorphous PLLA and the crystalline PLLA showed an almost identical diffusion coefficient and solubility of water, in spite of the fact that the amorphous PLLA remained practically amorphous during the whole sorption procedure. A strong correlation existed between the water solubility and the surface tension or contact angle of the polymer matrix. The water diffusivity in PGA was almost 2 orders of magnitude lower while water was more soluble in PGA with a lower heat of sorption than that corresponding to the other more hydrophobic polymers, indicating that the transport of water molecules in PGA followed the solution–diffusion model. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1716–1722, 2000

Uniaxial orientation behavior of poly(L-lactide) films.

Changes of orientation and crystallinity in uniaxially drawn poly (L-lactide) (PLLA) films were studied by differential scanning calorimetry, x-ray diffractometry, optical and viscoelastic methods. After drawing amorphous PLLA films at 90°C in air, we observed that their crystallinity, orientation of crystallites, refractive index, and storage moduli at 30°C increased monotonically with increasing draw ratio (D. R.); however, these physical properties were saturated at D. R. of ca. 2.5. To draw the unoriented film brought about intermolecular crystallization of the PLLA chains effectively at 90°C. Most crystallites thus formed in the drawn films were found to be the a-form and oriented into the drawn direction to a saturated level at D. R.=2.5_??_3.0. Alternatively, when as-crystallized PLLA films were drawn at 150°C to about 10 times of the original length, they showed a unique x-ray fiber diagram where continuous scattering intensities imposed on the diffraction spots were located on three layer lines. Long period of the drawn film annealed at 170°C indicated a larger value than that of the as-drawn film.

Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties

Several samples of poly(lactic acid) with different molecular weights and tacticity have been prepared, and some PLLA injection moulded specimens have been annealed to promote their crystallization. From the characterization data, poly(L-lactide) showed more interesting mechanical properties than poly(D, L-lactide), and its behavior significantly improves with crystallization. In fact, annealed specimens possess higher values of tensional and flexural modulus of elasticity, Izod impact strength, and heat resistance. The plateau region of flexural strength as a function of molecular weights appears around Mv = 35,000 for PDLLA and amorphous PLLA and at higher molecular weight, around Mv = 55,000, for crystalline PLLA. The study of temperature effect shows that at 56°C only crystalline PLLA still exhibits useful mechanical properties. © 1996 John Wiley & Sons, Inc.

Dynamic mechanical and calorimetric analysis of compression‐molded PLLA of different molecular weights: Effect of thermal treatments

AbstractThe effect of thermal treatment on compression‐molded poly(L‐lactic acid) (PLLA) has been investigated by means of viscosimetric molecular weight (\documentclass{article}\pagestyle{empty}\begin{document}$$M_{w_v}$$\end{document}) determination, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). Starting from two initial molecular weight PLLAs, amorphous samples with different \documentclass{article}\pagestyle{empty}\begin{document}$$M_{w_v}$$\end{document} have been obtained due to degradation occurring during the molding. The crystallization capability of the materials after different thermal treatments has been determined as a function of the molecular weight, and their dynamic mechanical properties have been measured. Initially fully amorphous PLLA matrices attained very high degrees of crystallinity (up to 90%) following different annealing processes. Concomitantly, PLLA degrades due to thermal cleavage of the chains. This is an unavoidable effect that must be taken into consideration when defining the material processing and annealing conditions. Crystallization phenomena occurring in the material during the treatment are clearly documented by DMTA.