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.     

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%.     

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.     

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.     

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.     

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.     

Characterization and release of triptolide-loaded poly (d,l-lactic acid) nanoparticles

Triptolide (TP), which has immunosuppressive effect, anti-neoplastic activity, anti-fertility function and severe toxicities on digestive, urogenital, blood circulatory system, was used as a model drug in this study. TP-loaded poly (d,l-lactic acid) (PLA) nanoparticles were prepared by the modified spontaneous emulsification solvent diffusion method (modified-SESD method). Dynamic light scattering system (DLS), transmission electron microscope (TEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC), X-ray powder diffractometry and Fourier transform infra-red spectroscopy (FT-IR) were employed to characterize the nanoparticles fabricated for size and size distribution, surface morphology, the physical state of drug in nanoparticles, and the interaction between the drug and polymer. Encapsulation efficiency (EE) and the in vitro release of TP in nanoparticles were measured by the reverse phase high-performance liquid chromatography (RP-HPLC). The produced nanoparticles exhibited a narrow size distribution with a mean size of approximately 150 nm and polydispersity index of 0.088. The morphology of the nanoparticles exhibited a fine spherical shape with smooth surfaces without aggregation or adhesion. TP-entrapped in nanoparticles was found in the form of amorphous or semicrystalline. It was found that a weak interaction existed between the drug and polymer. In all experiments, more than 65% of EE were obtained. The in vitro release profile of TP from nanoparticles exhibited a typical biphasic release phenomenon, namely initial burst release and consequently sustained release. In this case, the particle size played an important role for the drug release. The modified-SESD method was a potential and advantage method to produce an ideal polymer nanoparticles for drug delivery system (DDS).     

Enhanced interlayer interaction in cellulose single nanofibre and poly(l-lactic acid) layered films by plasma-initiated surface grafting of poly(acrylic acid) onto poly(l-lactic acid) films

The surface of a poly(l-lactic acid) (PLLA) film was modified with poly(acrylic acid) (PAA) by plasma-initiated polymerization to increase the interaction between PLLA and cellulose single nanofibres (CSNF). The surface wettability of the PAA grafted PLLA film (PLLA-PAA film) was investigated by contact angle measurements. Modification of the PLLA film with PAA decreased the contact angle from 61° to 50°. The surface morphologies of the PLLA film, PLLA-PAA film and CSNF-coated PLLA-PAA film were studied by atomic force microscopy. The interaction between the CSNF and PLLA layers was strengthened by incorporation of a PAA layer onto the PLLA films and it is higher than 2N as proved by a peeling test. This is probably because the carboxyl groups of PAA form hydrogen bonds with the hydroxyl groups of CSNF.     

Synthesis and characterization of poly(l-lactic acid) reinforced by biomesogenic units

To increase the thermal and mechanical properties of poly(l-lactic acid) (PLA), a nontoxic biomesogen PFBH derived from ferulic acid (FA), 4-hydroxybenzoic acid (HBA) and 1,6-hexanediol (HD) was introduced into the PLA backbones by solution polymerization of PLA, PFBH and chain linker hexamethylene diisocyanate (HDI). The content of PFBH was varied from 0 to 30 mol% so that the effects of the biomesogen content on the thermal and physical properties, morphological textures and enzymatic degradation were examined, respectively. The synthesized materials were characterized by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide angle X-ray diffraction (WAXD), polarizing light microscopy (PLM) and mechanical property measurements. It was found that introducing biomesogenic units could increase the thermal stability and reinforce the elastic properties, while reduced the melting temperature, the degree of crystallinity and the enzymatic degradation rate. The nontoxicity and biocompatibility of degradation would make the products promising candidates for medical applications in the area of tissue engineering.     

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.     

Synthesis, crystallization and hydrolysis of aromatic-aliphatic copolyester: Poly(trimethylene terephthalate)-co-poly(l-lactic acid)

In this study, poly(trimethylene terephthalate)-co-poly(l-lactic acid) (PTT-co-PLLA) copolyesters with different compositions were synthesized by melt polycondensation. The crystal morphologies of PTT-co-PLLA copolyesters were investigated with polarized light microscope (PLM). It was found that PTT-co-PLLA copolyesters exhibited banded spherulites with smaller band spacing at the same degree of super-cooling compared with PTT homopolymer. The PLLA segments in those copolyesters ranged from 0 to 28.4 mol% and did not form crystals during crystallization. Hydrolysis study on PTT homopolymer and PTT-co-PLLA copolyesters was carried out in buffer solutions. PTT-co-PLLA copolyesters represented pronounced hydrolytic degradation, which increased with the content of lactyl units. And it was concluded that degradation of PTT-co-PLLA was mainly attributed to the scission of PLLA segments.     

On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC

The crystallization and melting process of poly(l-lactic acid), PLLA, is investigated by temperature modulated differential scanning calorimetry, TMDSC. The sample is cooled from the melt to different temperatures and the crystallization process is followed by subjecting the material to a modulated quasi-isothermal stage. From the average component of the heat flow and the application of the Lauritzen–Hoffman theory two crystallization regimes are identified with a transition temperature around 118 °C. Besides, the oscillating heat flow allows calculating the crystal growth rate via the model proposed by Toda et al., what gives, in addition, an independent determination of the transition temperature from modulated experiments. Further, the kinetics of melting is studied by modulated heating scans at different frequencies. A strong frequency dependence is found both in the real and imaginary part of the complex heat capacity in the transition region. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al. Finally, step-wise quasi-isothermal TMDSC was used to investigate the reversible surface crystallization and melting both on cooling and heating and a small excess heat capacity is observed.     

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.     

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.     

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.     

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 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.     

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.