Hideto Tsuji

Poly(lactic acid) : synthesis, structures, properties, processing, and applications

This book describes the synthesis, properties, and processing methods of poly(lactic acid) (PLA), an important family of degradable plastics. As the need for environmentally-friendly packaging materials increases, consumers and companies are in search for new materials that are largely produced from renewable resources, and are recyclable. To that end, an overall theme of the book is the biodegradability, recycling, and sustainability benefits of PLA. The chapters, from a base of international expert contributors, describe specific processing methods, spectroscopy techniques for PLA analysis, and and applications in medical items, packaging, and environmental use.

Epitaxial crystallization and crystalline polymorphism of polylactides

Abstract Two crystal phases of poly( l -lactide) and that of the racemate of poly( l -lactide) and poly( d -lactide) can be grown epitaxially on one and the same crystalline substrate, hexamethylbenzene (HMB), which had been shown by Zwiers et al. [Polymer 1983;24:167] to form a eutectic with these polymers. The stable α-crystal modification of the optically active polymer, based on a 103 helix conformation (for PDLA; 107 for PLLA), is obtained for Tc near 155°C. A new crystal modification is produced by epitaxial crystallization at slightly lower Tc (≈140°C). The crystal structure of this new form is established by electron diffraction and packing energy analysis. Two antiparallel helices are packed in an orthorhombic unit-cell of parameters a=9.95 A , b=6.25 A and c=8.8 A . The racemate of poly( l -lactide) and poly( d -lactide) also crystallize epitaxially (at ≈165°C) on HMB, which appears to be a very versatile substrate.

The frustrated structure of poly(L-lactide)

Abstract The crystal structure formed upon stretching or stroking of poly( l -lactide) is determined by electron diffraction and conformational energy analysis. It rests on a frustrated packing of three three-fold helices in a trigonal unit-cell of parameters a=b=1.052 nm, c=0.88 nm, space group P32. The frustrated packing is of the type described as North–South–South (NSS). This structure appears to be formed to accommodate the random up–down orientation of neighbor chains associated with rapid crystallization conditions. This randomness introduces structural disorder (c-axis shifts and azimuthal setting of neighbor helices). The resultant streaking of the diffraction pattern is modeled. Frustrated packings observed in polymeric systems that depart from three-fold symmetry, and in pseudo-racemates of low molecular weight compounds are discussed.

Enhanced thermal stability of poly(lactide)s in the melt by enantiomeric polymer blending

Abstract Poly( l -lactide) (i.e. poly( l -lactic acid) (PLLA)) and poly( d -lactide) (i.e. poly( d -lactic acid) (PDLA)) and their equimolar enantiomeric blend (PLLA/PDLA) films were prepared and the effects of enantiomeric polymer blending on the thermal stability and degradation of the films were investigated isothermally and non-isothermally under nitrogen gas using thermogravimetry (TG). The enantiomeric polymer blending was found to successfully enhance the thermal stability of the PLLA/PDLA film compared with those of the pure PLLA and PDLA films. The activation energies for thermal degradation (Δ E td ) were evaluated at different weight loss values from TG data using the procedure recommended by MacCallum et al. The Δ E td values of the PLLA/PDLA, PLLA, and PDLA films were in the range of 205–297, 77–132, and 155–242xa0kJxa0mol −1 when they were evaluated at weight loss values of 25–90% and the Δ E td value of the PLLA/PDLA film was higher by 82–110xa0kJxa0mol −1 than the averaged Δ E td value of the PLLA and PDLA films. The mechanism for the enhanced thermal stability of the PLLA/PDLA film is discussed.

Autocatalytic hydrolysis of amorphous-made polylactides: effects of l-lactide content, tacticity, and enantiomeric polymer blending

Abstract Poly( dl -lactide), i.e., poly( dl -lactic acid) (PDLLA), poly( l -lactide), i.e. poly( l -lactic acid) (PLLA), and poly( d -lactide), i.e., poly( d -lactic acid) (PDLA) were synthesized to have similar molecular weights. The non-blended PDLLA, PLLA, and PDLA films and PLLA/PDLA(1/1) blend film were prepared to be amorphous state, and the effects of l -lactide unit content, tacticity, and enantiomeric polymer blending on their autocatalytic hydrolysis were investigated in phosphate-buffered solution (pH7.4) at 37xa0°C for up to 24xa0months. The results of gravimetry, gel permeation chromatography (GPC), and tensile testing showed that the autocatalytic hydrolyzabilities of polylactides, i.e. poly(lactic acid)s (PLAs) in the amorphous state decreased in the following order: nonblended PDLLA>nonblended PLLA, nonblended PDLA>PLLA/PDLA(1/1) blend. The high hydrolyzability of the nonblended PDLLA film compared with those of the nonblended PLLA and PDLA films was ascribed to the lower tacticity of PDLLA chains, which decreases their intramolecular interaction and therefore the PDLLA chains are susceptible to the attack from water molecules. In contrast, the retarded hydrolysis of PLLA/PDLA(1/1) blend film compared with those of the nonblended PLLA and PDLA films was attributable to the peculiar strong interaction between PLLA and PDLA chains in the blend film, resulting in the disturbed interaction of PLLA or PDLA chains and water molecules. The X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA films remained amorphous even after the autocatalytic hydrolysis for 16 (PDLLA film) and 24 [nonblended PLLA and PDLA films, PLLA/PDLA(1/1) blend film] months and that the melting peaks observed at around 170 and 220xa0°C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24xa0months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200xa0°C but not during the autocatalytic hydrolysis.

Environmental degradation of biodegradable polyesters 1. Poly(e-caprolactone), poly((R)-3-hydroxybutyrate), and poly(L-lactide) films in controlled static seawater

Abstract Films of biodegradable aliphatic polyesters, poly(e-caprolactone) (PCL), poly[(R)-3-hydroxybutyrate] (R-PHB), and poly(L-lactide) (PLLA) were prepared by solution-casting and annealing from the melt. Their biodegradation in static seawater controlled at 25xa0°C was investigated using polarizing optical microscopy, gravimetry, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and tensile testing. The change in weight loss, tensile strength, and Youngs modulus revealed that the biodegradabilities of the aliphatic polyesters in the controlled seawater decreased in the order: PCL>R-PHB>PLLA. The results of gravimetry, GPC, and DSC showed that the biodegradation of PCL and R-PHB films proceeds via surface erosion mechanisms. The polarizing optical microscopy indicated that the PCL and R-PHB films were biodegraded inhomogeneously on the film surface where the marine microbes attached, resulting in pore formation. The crystalline residues of PCL and R-PHB films could not be traced by GPC even when large weight losses occurred. Polarizing optical microscopy and GPC indicated that the decreased tensile strength and Youngs modulus of the PCL and R-PHB films are attributed to the formation of pores and cracks during biodegradation. The biodegradation of the PLLA films was insignificant even after immersion in the controlled seawater for 10 weeks and the initial crystallinity had no significant effects on the biodegradability of the PLLA films, excluding the tensile properties change. The biodegradation of these aliphatic polyester films in the controlled static seawater could not be traced by GPC and DSC measurements.

Poly(l-lactide): 7. Enzymatic hydrolysis of free and restricted amorphous regions in poly(l-lactide) films with different crystallinities and a fixed crystalline thickness

Poly(l-lactide) (PLLA) films having different initial crystallinities (xc) (0–57%) and a fixed crystalline thickness were prepared by annealing the melt at a fixed temperature for different times. Their enzymatic hydrolysis was investigated in the presence of Proteinase K®. The rate of weight loss decreased rapidly and slowly with an increase in the initial xc for xc below and above 33%, respectively, where the free and the restricted amorphous regions, respectively, are the major amorphous components in the PLLA films. This is ascribed to the higher hydrolysis-resistance of the PLLA chains in the restricted amorphous region than that in the free amorphous region. Gel permeation chromatography (GPC) results revealed that in the restricted amorphous region the folding chains are much more hydrolysis-resistant than the tie chains and the chains with free ends. The increased xc during the enzymatic hydrolysis is due to the preferential hydrolysis and removal of the amorphous regions, but not to the crystallization of the amorphous regions.

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 (Xcs) and crystalline thicknesses (Lcs) were prepared by annealing at different temperatures (Tas) 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.

Blends of aliphatic polyesters. VI. Lipase-catalyzed hydrolysis and visualized phase structure of biodegradable blends from poly(ε-caprolactone) and poly(l-lactide)

Phase-separated biodegradable polymer blends were prepared from poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA), and Rhizopus arrhizus lipase-catalyzed hydrolysis and phase structure of the blend films were investigated. Gravimetry revealed that the lipase-catalyzed hydrolysis of PCL in PCL- and PLLA-rich phases is disturbed by the presence of PLLA. Polarimetry confirmed the occurrence of a predominant hydrolysis of PCL and subsequent removal of the hydrolyzed water-soluble PCL oligomers in the blend films. Gravimetry and gel permeation chromatography of the non-blended PLLA film indicated that R. arrhizus lipase has no catalytic effect on the hydrolysis of PLLA. The phase structure of the blend films could be visualized by selective enzymatic removal of one component and subsequent scanning electron microscopic observation.

Enhanced crystallization of poly(L-lactide-co-ε-caprolactone) during storage at room temperature

Copolymer of L-lactide and e-caprolactone [P(LLA-CL)] (50/50) was synthesized using stannous octoate and was stored at room temperature. The change in physical properties occurring during this storage at room temperature was investigated by differential scanning calorimetry (DSC), X-ray diffractometry, polarizing optical microscopy, tensile and bending tests, and light absorbance measurements. It was concluded that the increase in mechanical properties and light absorbance during storage can be ascribed to gradual selective crystallization of the L-lactide sequence in P(LLA-CL) at room temperature.