Crystallization behavior of poly(l-lactic acid)

This article contains a detailed analysis of the crystallization behavior of poly(l-lactic acid) (PLLA). Crystallization rates of PLLA have been measured in a wide temperature range, using both isothermal and non-isothermal methods. The combined usage of multiple thermal treatments allowed to obtain information on crystallization kinetics of PLLA at temperatures almost ranging from glass transition to melting point. Crystallization rate of PLLA is very high at temperatures between 100 and 118 °C, showing a clear deviation from the usual bell-shaped curve. This discontinuity has been ascribed to a sudden acceleration in spherulite growth, and is not associated to morphological changes in the appearance of PLLA spherulites. Experimental data of spherulite growth rates of PLLA have been analyzed with Hoffman-Lauritzen method. Applicability and limitations of this theoretical treatment have been discussed.     

Different enantioselectivity of two types of poly(lactic acid) depolymerases toward poly(l-lactic acid) and poly(d-lactic acid)

Poly(lactic acid) (PLA) depolymerases are categorized into protease-type and lipase-type. Protease-types can hydrolyze poly(l-lactic acid) (PLLA) but not poly(d-lactic acid) (PDLA). Lipase-types, including cutinase-like enzyme (CLE) from Cryptococcus sp. strain S-2 preferentially hydrolyze PDLA. Both enzymes degraded not only PLA emulsion but also PLA film, in which amorphous region is preferentially attacked, but crystalline region can be also attacked. Stereocomplex PLA (sc-PLA) formed by 50:50 blending of PLLA and PDLA included no homo crystals, but a tiny homo crystallization peak appeared and crystallinity increased by 5% when attacked by CLE, although no significant change of molecular weight and crystalline size was found. Enantioselective degradation must occur in amorphous region of PLLA/PDLA film and preferentially hydrolyzed PDLA, resulting in a slightly excess amount of PLLA remained, which must be crystallized.     

Crystalline morphology of poly(l-lactic acid) thin films

Crystalline morphologies of spin-coated poly(l-lactic acid) (PLLA) thin films under different conditions are investigated mainly with atomic force microscopy (AFM) technique. When PLLA concentration in chloroform is varied from 0.01 to 1% gradually, disordered structure, rod-shape and larger spheres aggregates are observed in thin films subsequently. Under different annealing temperature, such as at 78, 102, 122 °C, respectively, we can find most rod-like crystalline aggregates. Interestingly, we observed that nucleation sites locate at the edge of the holes at the original crystalline stage. Then, these holes developed to form chrysanthemum-like and rods subsequently with annealing time meanwhile the size and the shape of crystalline aggregate are changed. In addition, effect of substrate and solvent on morphology is also discussed. On the other hand, the possible mechanism of crystalline morphology evolution is proposed.     

Hydrolysis of poly(l-lactic acid) using microwave irradiation

Poly(l-lactic acid) (PLLA) was hydrolyzed using microwave irradiation, and yields of the resultant lactic acid and reaction time were compared with those obtained by conventional heating. In both cases, the reaction temperature was maintained at 170 °C and the weight ratio of PLLA:H₂O was 3:1. Under conventional heating, the lactic acid yield reached saturation after 800 min at 45%, whereas only 120 min was required to reach the same yield level under microwave irradiation. The optical purity under conventional heating decreased with hydrolysis of the PLLA and dropped to 94%ee when the lactic acid yield reached at 45%. Under microwave irradiation, however, the optical purity decreased only after the equilibrium state of hydrolysis was attained. Therefore, to maintain the optical purity at 98%ee, it was necessary to stop microwave irradiation when the lactic acid yield reached 45%.     

Alcoholysis of Poly(l-lactic acid) under microwave irradiation

Chemical recycling of poly(l-lactic acid) (PLLA) by alcoholysis under microwave irradiation was investigated. Reaction rates in ethanol (140-180 °C) and butanol (130-210 °C) were greater under microwave irradiation than under conventional heating. However, because activation energies were almost the same under both conditions, the reaction mechanisms would be the same, but the reaction rates differed due to the reaction frequency factor.     

Surface properties and enzymatic degradation of end-capped poly(l-lactide)

Surface properties and enzymatic degradation of poly(l-lactide) (PLLA) end-capped with hydrophobic dodecyl and dodecanoyl groups were investigated by means of advancing contact angle (θ_a) measurement, quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The θ_a values of end-capped PLLA films were larger than those of non-end-capped PLLA films, suggesting that the hydrophobic dodecyl and dodecanoyl groups were segregated on the film surface. The weight changes of end-capped PLLA thin films during enzymatic degradation in the presence of proteinase K were monitored by using a QCM technique. The relatively fast weight loss of PLLA film occurred during first few hours of degradation, followed by a decrease in the erosion rate. The erosion rate of PLLA films at the initial stage of degradation was dependent on the chain-end structure of PLLA molecules, and the value decreased with an increase in the amount of hydrophobic functional groups. The surface morphologies of PLLA thin films before and after degradation were characterized by AFM. After the enzymatic degradation, the surface of non-end-capped PLLA films was blemished homogeneously. In contrast, the end-capped PLLA thin films were degraded heterogeneously by the enzyme, and many hollows were formed on the film surface. From these results, it has been concluded that the introduction of hydrophobic functional groups at the chain-ends of PLLA molecules depressed the erosion rate at the initial stage of enzymatic degradation.     

Influence of melting conditions on the thermal behaviour of poly(l-lactic acid)

The influence of melting temperature and time on the thermal behaviour of poly(l-lactic acid) (PLLA) was studied with differential scanning calorimetry (DSC). Different melting conditions were investigated at temperature ranging from 200 to 210 °C, and for time from 2 to 20 min. For lower-molecular-weight PLLA, a single exothermic peak could be observed at cooling rate of 2 °C/min, after melted at different conditions. The obtained peak temperature and degrees of crystallinity dramatically increased with an increase of melting temperature or time. During subsequent heating scans, double melting peaks could be observed, which were significantly affected by prior melting conditions. The degradation of this material in the melt and the melt/re-crystallization mechanism might be responsible for the observations above. Apart from double melting, double cold crystallization peaks were observed during heating traces for this material after fast cooling (20 °C/min) from the melt. Prior melting conditions could significantly influence the cold crystallization behaviour. The competition between the crystallization from the nuclei remained after cooling, and that from spontaneous nucleation might be responsible for the appearance of double peaks. Additionally, the influence of melting conditions on the thermal behaviour of PLLA was dependent on the initial molecular weight.     

Synthesis of poly(l-lactic acid) with improved thermal stability by sulfonic acid-catalyzed melt/solid polycondensation

Melt/solid polycondensation (MP/SSP) is deemed as an alternative synthetic route besides ring-opening polymerization (ROP) in synthesizing poly(l-lactic acid) (PLLA). However, it is found that PLLA synthesized by MP/SSP has much poorer thermal stability than that by ROP due to more residual Sn(II) metallic catalyst in the former, but sulfonic acids does not show any detrimental effect on the thermal stability of PLLA. To synthesizing PLLA with good thermal stability by MP/SSP, a variety of commercially available sulfonic acids were screened as catalysts in MP/SSP of PLLA. Among these nonmetallic catalysts, it was found that 1,3-propanedisulfonic acid (PSA) and 1,5-naphthalene disulfonic acid (NSA) exhibited satisfactory catalytic reactivity and PLLAs with excellent thermal stability, high molecular weight, little coloration and good optical purity were successfully synthesized by MP/SSP. The decomposition temperature was increased by 80–100 °C in comparison to SnCl₂-catalyzed PLLA, and the thermal stability is comparable to commercial PLLA produced by ROP.     

Synthesis of high-molecular-weight poly(l-lactic acid) through the direct condensation polymerization of l-lactic acid in bulk state

A strategy was attempted to produce high-molecular-weight poly(l-lactic acid) (PLLA) through the direct condensation polymerization of l-lactic acid in bulk state. Polymerizations were carried out with titanium(IV) butoxide (TNBT) as a catalyst employing different duration of decompression, esterification and polycondensation. The molecular weights were characterized by using the gel permeation chromatography (GPC). The stereosequences were analyzed from the ¹³C NMR spectra on the basis of the triad fractions.     

Rapid controlled hydrolytic degradation of poly(l-lactic acid) by blending with poly(aspartic acid-co-l-lactide)

Although poly(lactic acid) is known as a biodegradable polymer, its hydrolytic degradation is extremely slow, taking years in water and in the human body. In this study the effects of blending oligomeric poly(aspartic acid-co-lactide) (PALs) on the hydrolytic degradation of poly(l-lactic acid) (PLLA) were studied in detail. It was found that the addition of PAL did not accelerate the hydrolysis of the PLLA in air (25 °C, 60% relative humidity), but significantly accelerated it in a phosphate buffer solution. The degradation rate becomes higher for the blends containing PAL with higher molar ratios of lactide to aspartic acid units, [LA]/[Asp], when PLLA/PAL blends prepared with different PALs are compared at the same PAL concentration. TEM results, in which the distribution of PALs with higher [LA]/[Asp] occurs at a smaller scale in blends, imply that higher miscibility of the PAL with PLLA results in higher contact area between the components, thereby accelerating the degradation efficiently.     

Miscibility and properties of poly(l-lactic acid)/poly(butylene terephthalate) blends

Binary blends of poly(l-lactide) (PLLA) and poly(butylene terephthalate) (PBT) containing PLLA as major component were prepared by melt mixing. The two polymers are immiscible, but display compatibility, probably due to the establishment of interactions between the functional groups of the two polyesters upon melt mixing. Electron microscopy analysis revealed that in the blends containing up to 20% of poly(butylene terephthalate), PBT particles are finely dispersed within the PLLA matrix, with a good adhesion between the phases. The PLLA/PBT 60/40 blend presents a co-continuous multi-level morphology, where PLLA domains, containing dispersed PBT units, are embedded in a PBT matrix. The varied morphology affects the mechanical properties of the material, as the 60/40 blend displays a largely enhanced resistance to elongation, compared to the blends with lower PBT content.     

Swelling and hydrolytic degradation of poly(d,l-lactic acid) in aqueous solutions

Low molecular weight poly(lactic acid) was synthesized by direct polycondensation of lactic acid. The oligomers were characterized by viscometry, light scattering, and gel permeation chromatography (GPC). The swelling behaviour of tablets made of the above polymer immersed in buffer solutions at 37 °C was studied. In the same experiments, the hydrolytic stability of d,l-PLA was assessed by measuring the weight loss after drying the tablets. In order to inhibit any degradation due to bacteria, formaldehyde was added in the solution as biostatic factor. The effect of an incorporated drug on the swelling behaviour of d,l-PLA tablets was also considered. It was found that the incorporation of drug in d,l-PLA tablets increases their swelling index, probably due to the creation of additional porosity in the specimens or other interaction between drug and polymeric matrix.     

Quantitative evaluation of photodegradation and racemization of poly(l-lactic acid) under UV-C irradiation

To obtain details of poly(l-lactic acid) (PLLA) photodegradation behavior, PLLA films were irradiated by UV-C light (λ = 253.7 nm) to directly excite carbonyl groups, resulting in a rapid decrease in the molecular weight accompanying a gradual decrease in the optical purity of monomeric units in the chains. The racemization during the photodegradation was first detected as a result of the chain scission by irradiation. From quantitative analyses of the molecular weight and the monomeric unit composition, it was found that the chain scission ratio and the d-lactate unit ratio increased in parallel during the irradiation, suggesting that approximately one d-lactate unit formed for every chain scission. From a mechanistic consideration, the racemization equilibrium was proposed to occur at both carboxyl and hydroxyl chain ends.     

Comparative study on hydrolytic degradation and monomer recovery of poly(l-lactic acid) in the solid and in the melt

The hydrolytic degradation of poly(l-lactide) (PLLA) and the formation of its monomer in the solid and in the melt were investigated at 120-150 °C (in the solid), at 160 °C (in the solid up to 40 min and in the melt exceeding 40 min), and at 170-190 °C (in the melt). Such state difference caused the difference in the degradation behavior of PLLA and the behavior of lactic acid formation, although the degradation of PLLA proceeds via a bulk erosion mechanism, regardless of its state. The crystalline residues were formed at the degradation temperatures below 140 °C, but not at the degradation temperatures above 160 °C. The lactic acid yield exceeding 95% can be successfully attained for all the temperatures of 120-190 °C. The activation energy for hydrolytic degradation values of PLLA were 69.6 and 49.6 kJ mol⁻¹ for the temperature ranges of 120-160 °C (in the solid) and 170-250 °C (in the melt), respectively, and are compared with the reported values.     

Synthesis and characterization of functional l-lactic acid/citric acid oligomer

Oligomers of l-lactic acid and citric acid (PLCA) were synthesized by reacting lactic acid with citric acid in the presence of stannous chloride. The chemical compositions of these multicarboxylated oligomers were verified by FT-IR and ¹H-NMR spectroscopy. The thermal characteristics of the oligomers, such as glass transition temperature T_g, melting temperature T_m and melting enthalpy, were confirmed by DSC. The crystallinity of the oligomers were determined by DSC and WXRD. Meanwhile, the acid-base surface characteristics of PLCA have been determined by contact angle. The results implicated that these oligomers may be used to entrap the cospecies on PLLA surface in tissue engineering.     

Hydrolytic degradation behavior of poly(l-lactide)/SiO2 composites

In this work, different contents of nano-silica (SiO₂) particles were introduced into poly(l-lactide) (PLLA) to prepare PLLA/SiO₂ composites though a two-step compounding method, i.e. solution compounding (preparing master batch) and subsequent melt compounding (master batch dilution). The dispersion of SiO₂ was characterized using scanning electron microscope (SEM). The hydrophilicity of the material was evaluated by measuring the contact angle of water on the sample surface. The hydrolytic degradation measurements of the nanocomposites were carried out in alkaline solution at two different temperatures, i.e. 37 and 55 °C. Subsequently, microstructure evolution of PLLA matrix during the hydrolytic degradation process was systematically investigated using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The results showed that SiO₂ had good dispersion in the PLLA matrix. Largely enhanced hydrolytic degradation ability was achieved for PLLA/SiO₂ composites. Increasing the content of SiO₂ or enhancing the hydrolytic degradation temperature accelerated the hydrolytic degradation of PLLA matrix. Further results showed that SiO₂ promoted the reorganization of microstructure of PLLLA matrix during the hydrolytic degradation process.     

Miscibility and hydrolytic degradation in alkaline solution of poly(l-lactide) and poly(p-vinyl phenol) blends

Poly(l-lactide) (PLLA) was melt-blended with poly(p-vinyl phenol) (PVPh) using a two-roll mill, and the miscibility between PLLA and PVPh and degradation of the blend films were investigated. It was found that PLLA/PVPh blend has miscibility in the amorphous state because only single T_g was observed in the DSC and DMA measurements. The T_g of the PLLA/PVPh blend could be controlled in the temperature range from 55 °C to 117 °C by changing the PVPh weight fraction. In alkaline solution, degradation rate of PLLA/PVPh blends was faster than that of neat PLLA because PVPh could dissolve in alkaline solution. The surface morphology of degraded PLLA and PLLA/PVPh blend were observed by SEM. The surface morphology of degraded PLLA/PVPh blend was finer than that of PLLA. Young's modulus of PLLA/PVPh blend increased with increasing PVPh content. Yield stress of PLLA/PVPh blends whose PVPh content was less than 30 wt% kept the level of about 55 MPa and that of PLLA/PVPh blend whose PVPh content was 40 wt% is much lower than that of neat PLLA.     

Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s

Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M_n) and d-lactide unit contents (X_d) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X_d, while the k values did not depend on M_n. This means that the effects of X_d on the hydrolytic degradation rate of the films are higher than those of M_n. In contrast, in the period of 32-60 weeks neither X_d nor M_n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks.     

Biodegradable poly(l-lactic acid) nanofiber prepared by a carbon dioxide laser supersonic drawing

Biodegradable poly(l-lactic acid) (PLLA) nanofiber was prepared by a carbon dioxide (CO₂) laser supersonic drawing which was carried out by irradiating the laser on an as-spun fiber in a supersonic jet. The supersonic jet was generated by blowing off air into a vacuum chamber from a fiber supplying orifice. The flow velocity from the orifice can be estimated by applying Graham’s theorem from the pressure difference between the atmospheric pressure and the pressure of the vacuum chamber. The fastest flow velocity estimated was 396 m s⁻¹ when the chamber pressure was 6 kPa. The PLLA nanofiber having an average diameter of 0.132 μm was obtained when the supersonic drawing was carried out by irradiating the laser at 177 W cm⁻² on the as-spun fiber supplied at 0.1 m min⁻¹ in the vacuum chamber at 6 kPa. The obtained nanofiber had a draw ratio of about 323,000 and a degree of crystallinity of 45%, and its diameter uniformity was high. The CO₂ laser supersonic drawing was a new route for preparation of various nanofibers without using any solvent.     

Effects of mechanical load on the degradation of poly(d,l-lactic acid) foam

Degradable behaviors of polymer for implantation in body should be evaluated before clinical application. The effect of continuous mechanical load on the degradation progress of poly(d,l-lactic acid) (PDLLA) foam gasket was investigated in detail by specially designed load-providing devices. While PDLLA degraded in the PBS solution (pH, 7.4) at 37 °C for 3 months, the changes of surface morphology, molecular weight, elastic modulus, tensile strength and mass loss were recorded. The results revealed that the degradation rates of PDLLA under continuous loads were obviously quicker than those without load. Moreover, the influence of tensile plus compressive load was larger than that of tensile load. It was indicated that in vivo degradation of PDLLA would not only be influenced by the local solution, but also by the surrounding load. When regulating the degradation rate of bioabsorbable polymer, one should consider the indispensable effect of load where implanted.