Hetero-stereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s

The crystallization behavior of the stereoblock copolymer of substituted and non-substituted poly(lactide)s, i.e., poly(d-2-hydroxybutyrate) and poly(l-lactide) chains having the opposite configurations [P(D-2HB)-b-PLLA] and the reference block copolymer of poly(d-2-hydroxybutyrate) and poly(d-lactide) chains with the identical configurations [P(D-2HB)-b-PDLA] was investigated. At the crystallizable temperature range of 60-160 °C, the crystallized P(D-2HB)-b-PLLA contained solely the hetero-stereocomplex crystallites as a crystalline species, without formation of poly(d-2-hydroxybutyrate) or poly(l-lactide) homo-crystallites, in contrast with their polymer blends. On the other hand, at the crystallizable temperature range of 60-140 °C, the crystallized P(D-2HB)-b-PDLA had only PDLA homo-crystallites as crystalline species, reflecting no co-crystallites formation between poly(d-2-hydroxybutyrate) and poly(d-lactide) chains having the same configurations. The equilibrium melting temperature of hetero-stereocomplex crystallites in P(D-2HB)-b-PLLA was 189.0 °C, which was higher than 171.3 °C of PDLA homo-crystallites in P(D-2HB)-b-PDLA. Although the final crystallinity of P(D-2HB)-b-PLLA was higher than those of P(D-2HB)-b-PDLA, the spherulite growth rate of P(D-2HB)-b-PLLA was lower.The regime analysis indicated unusual nucleation mechanism of P(D-2HB)-b-PLLA.     

Formation of banded and non-banded poly(l-lactic acid) spherulites during crystallization of films of poly(l-lactic acid)/poly(ethylene oxide) blends

In this study, a series of poly(l-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) blends with different PLLA concentrations was prepared. Films of these blends crystallized with and without a coverslip were characterized by the presence and absence of banded structures, respectively. This difference in morphology was observed because the PEO component of the blends was oxidized at a high temperature (125 °C) in air without the protection of a coverslip. X-ray photoelectron spectroscopy (XPS) results showed that the surface of the blends crystallized in nitrogen without a coverslip contained mostly PLLA while the surfaces of the same blends crystallized under a coverslip contained only a moderately higher concentration of PLLA than their bulks. The migration of PLLA to the surface of the blends during crystallization in nitrogen when no coverslip was used was due to its low surface tension. Phase images obtained using atomic force microscopy (AFM) indicated that the banded structures consisted of valleys and ridges, which were in fact flat-on and edge-on lamellae, respectively. A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) examination suggested that PLLA and PEO were located mainly on the surfaces of the ridges and valleys, respectively.     

Small-angle X-ray scattering studies on melting and recrystallization behaviors of poly(oxyethylene) crystallites in poly(d,l-lactide)/poly(oxyethylene) blends

Direct determination of the discrete distribution for crystalline lamellar thickness has been performed for poly(d,l-lactic acid)/poly(oxyethylene) (PDLLA/PEG) blends by conducting small-angle X-ray scattering (SAXS) measurements using synchrotron radiation. The PDLLA used was an random (racemic) copolymer of bio-based poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) with the lactide monomer ratio of l:d = 50:50. It is known that PLA is miscible with PEG in the amorphous state. In the current paper, we report comprehensive results on structural analyses of PDLLA/PEG blends in the course of heating and cooling process using SAXS to elucidate the change in the thickness distribution of the lamellae. As a consequence, it was found that the distribution of the lamellar thickness moves toward the larger value (in other words, lamellar thickening) as temperature approaches the melting point. Typically, the thickness distribution was dispersed in the range of 10–20 nm at room temperature and it changed toward 40 nm in the vicinity of the melting temperature. To the best of our knowledge, this is the first report of direct determination of the discrete distribution for the crystalline lamellar thickness and their in-situ changes in the course of the lamellar thickening process. As a result, the lamellar thickening was found to occur at much lower temperature for the blend samples with 10% and 20% of PDLLA contents as compared to the PEG 100% sample. This phenomenon can be ascribed to the melting point depression owing to the miscibility between PEG and PDLLA. Thereby, thinner lamellae were melted and thicker ones appeared at much lower temperature for the blends than for the PEG 100% sample. As for the average repeating distance (long period) of the lamellar stacks, an abrupt increase similar to the critical divergence was observed (from 25 nm to 50 nm) in the heating process. Not only for the melting behavior but also in the course of recrystallization, change in the lamellar-thickness distribution was uncovered, which shows strong hysteresis depending on what temperature the sample was cooled down from.     

Plasticization of semicrystalline poly(l-lactide) with poly(propylene glycol)

Plasticization of semicrystalline poly(l-lactide) (PLA) with a new plasticizer - poly(propylene glycol) (PPG) is described. PLA was plasticized with PPG with nominal M_w of 425 g/mol (PPG4) and 1000 g/mol (PPG1) and crystallized. The plasticization decreased T_g, which was reflected in a lower yield stress and improved elongation at break. The crystallization in the blends was accompanied by a phase separation facilitated by an increase of plasticizer concentration in the amorphous phase and by annealing of blends at crystallization temperature. The ultimate properties of the blends with high plasticizer contents correlated with the acceleration of spherulite growth rate that reflected accumulation of plasticizer in front of growing spherulites causing weakness of interspherulitic boundaries. In PLA/PPG1 blends the phase separation was the most intense leading to the formation of PPG1 droplets, which facilitated plastic deformation of the blends that enabled to achieve the elongation at break of about 90-100% for 10 and 12.5 wt% PPG1 content in spite of relatively high T_g of PLA rich phase of the respective blends, 46.1-47.6 °C. Poly(ethylene glycol) (PEG), long known as a plasticizer for PLA, with nominal M_w of 600 g/mol, was also used to plasticize PLA for comparison.     

pH-dependent self-assembly of amphiphilic poly(l-glutamic acid)-block-poly(lactic-co-glycolic acid) copolymers

A novel biodegradable AB-type diblock copolymer poly(L-lactic- co-glycolic acid)-block-poly(l-glutamic acid) (PLGA-b-PGA) was synthesized by a macromolecular coupling reaction between carboxyl-terminated PLGA and amino-terminated poly(γ-benzyl-glutamate) (PBLG) and the subsequent elimination of the protecting benzyl group. The structures of PLGA-PGA and its precursors were confirmed by Fourier transform infrared spectroscopy (FT-IR), ¹H nuclear magnetic resonance (¹H NMR) spectroscopy and gel permeation chromatography (GPC). This synthetic strategy simplified a former synthesis process of polypeptide-poly(l-lactic acid)(PLA); by using this new synthetic route the molecular weight and block ratio of PLGA-PGA could be easily controlled by adjusting the chain length of PLGA/PGA. The pH sensitivity and self-assembly behavior of PLGA-PGA copolymer were investigated by environmental scanning electron microscopy (ESEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The results showed that the copolymer exhibited high pH responses, and the morphologies of the copolymer aggregates underwent four stages orderly with the pH increase (pH = 3-9): a disorganized form, micelles, semi-vesicles with thick walls and vesicles. Such a pH-dependent self-assembly process of the copolymer is promising for drug control release and bio-applications.     

Highly accelerated stereocomplex crystallization by blending star-shaped 4-armed stereo diblock poly(lactide)s with poly(d-lactide) and poly(l-lactide) cores

Star-shaped 4-armed stereo diblock poly(lactide)s with the core/shell types of poly(d-lactide) (PDLA)/poly(l-lactide) (PLLA) and PLLA/PDLA (abbreviated as 4-DL and 4-LD, respectively) and the number-average molecular weights of about 1 × 10⁴ g mol⁻¹ were synthesized and the crystallization behavior of neat 4-DL, 4-LD, and their 50/50 blend (abbreviated as 4-DL/4-LD blend) was investigated. Solely stereocomplex (SC) crystallites as crystalline species were formed in the neat 4-DL, 4-LD, and 4-DL/4-LD blend, irrespective of crystallization temperature (100–160 °C). The overall SC crystallization of 4-DL/4-LD blend was highly accelerated compared with that of neat 4-DL and 4-LD, due to the largely elevated spherulite nuclei number per unit mass in the blend. Such high density of nuclei formation in 4-DL/4-LD blend is attributable to the facile intermolecular interaction and subsequent SC nucleation between the PLLA shell of 4-DL and the PDLA shell of 4-LD. The blending method reported in the present study is applicable for various core/shell types of star-shaped stereo diblock stereocomplexationable polymers to accelerate overall SC crystallization and can counterbalance the lowered crystallization rate caused by the star-shaped architecture. Despite the highly accelerated overall SC crystallization of 4-DL/4-LD blend by blending 4-DL and 4-LD, the spherulite growth rate, induction period for spherulite growth, final crystallinity, crystalline species, growth morphology, and crystallization mechanism were not altered by blending 4-DL and 4-LD.     

Effect of the addition of poly(d-lactic acid) on the thermal property of poly(l-lactic acid)

Thermal property and crystallization behavior of PLLA blended with a small amount of PDLA (1-5 wt%) were studied. PDLA molecules added in PLLA formed stereocomplex crystallites in the PLLA matrix. When the blend was cooled to a temperature below T_m of PLLA, stereocomplex crystallites acted as nucleation sites of PLLA and enhanced the crystallization of PLLA significantly (heterogeneous nucleation). Such crystallization enhancement was not observed when the blend with lower PDLA content was cooled from 240 °C at which both PLLA crystal and the stereocomplex disappeared. Low molecular weight PDLA isolated in the matrix of PLLA did not form a stereocomplex crystallite with a large surface area enough to act as a nucleation site. On the other hand, high molecular weight PDLA chains formed a large stereocomplex crystallite. With increasing PDLA content, stereocomplex crystallites were more easily formed and they acted as nucleation sites. PLLA crystal near the stereocomplex crystallites has an incomplete structure and showed a melting peak at a lower temperature than pure PLLA crystal.     

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (M_n) of 3.9 × 10³, 9.3 × 10³, and 1.1 × 10⁴ g mol⁻¹, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (X_c) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle M_n of 9.3 × 10³ g mol⁻¹. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in M_n, but seems to be caused by the upper critical M_n values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.     

Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

A novel biodegradable multiblock poly(ester urethane) containing poly(l-lactic acid) and poly(butylene succinate) blocks

A novel biodegradable multiblock poly(ester urethane) (PEU), consisting of poly(l-lactic acid) (PLLA) and poly(butylene succinate) (PBS) blocks, has been successfully synthesized via chain-extension reaction of dihydroxyl terminated PLLA (PLLA-OH) and PBS prepolymers (PBS-OH) using toluene-2,4-diisocyanate (TDI) as a chain extender. The chemical structures and molecular weights of PEUs, containing different block lengths and weight fractions of PLLA and PBS, were characterized by ¹H NMR and GPC. The effects of the structures on the physical properties of PEUs were systematically studied by means of DSC, TGA, WAXD and tensile testing. The DSC results indicated that PLLA segment was compatible well with PBS segment in amorphous phase and the crystallization of PEU was predominantly caused by PBS segment, which was also confirmed by WAXD. The results of tensile testing showed that the extensibility of PLLA was largely improved by incorporating PBS segment. The PEU can be used as a potential substitute for some petroleum-based thermoplastics.     

Crystallization and morphology of stereocomplexes in nonequimolar mixtures of poly(l-lactic acid) with excess poly(d-lactic acid)

Crystallization of nonequimolar compositions of poly(d-lactic acid) with low-molecular-weight poly(l-lactic acid) (PDLA/LM_w-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LM_w-PLLA blends within the composition window of LM_w-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LM_w-PLLA blends with excess PDLA, e.g. PDLA/LM_w-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites.     

Synthesis of high molecular weight poly(l-lactic acid) and poly(d-lactic acid) with improved thermal stability via melt/solid polycondensation catalyzed by biogenic creatinine

Stereoregular high polymers of poly(l-lactic acid) (PLLA) (M_w 1.2 × 10⁵, isotacticity 96.0%) and poly(d-lactic acid) (PDLA) (M_w 1.0 × 10⁵, isotacticity 98.6%) were successfully synthesized via melt/solid polycondensation (MP/SSP) using a biogenic catalyst creatinine (CR). The follow-up monitor of the polycondensation products with ¹³C NMR technique revealed that the polymerization of MP/SSP proceeded in a stereochemical controlled way throughout the whole process as evidenced by the constant high values of isotacticity (97.8–99.4%) of produced polymers. Thermogravimetric analysis demonstrated that the decomposition temperatures (T_d,init 324.3 °C, T_{d, 5%} 347.0 °C, T_{d, max} 400.2 °C) of PLLA synthesized with catalyst CR are over 100 °C above those of PLLA synthesized with catalyst SnCl₂·2H₂O.     

Controlled crystal nucleation in the melt-crystallization of poly(l-lactide) and poly(l-lactide)/poly(d-lactide) stereocomplex

Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔH_c (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.     

Copoly(γ,dl-glutamate)s containing short and long linear alkyl side chains

Copoly(α-alkyl γ,dl-glutamate)s containing octadecyl and ethyl side groups were prepared from racemic poly(γ,dl-glutamic acid) of bacterial origin, and their structure in the solid state examined by DSC, polarized IR spectroscopy and X-ray diffraction methods. Copolymers with a blocky microstructure were prepared by transesterification of poly(α-ethyl γ,dl-glutamate) with octadecanol for a wide range of compositions. Random copolymers were prepared by sequential alkylation of the polyacid with ethyl bromide and octadecyl bromide in solution. All the multiblock copolymers were found to adopt the layered structure characteristic of comb-like polypeptides with the octadecyl side chains crystallized in extended conformation in a separate phase, and aligned normal to the main chains, which are arranged in helical conformation. Melting of paraffinic crystallites reversibly happened in the 50-55 °C range without significant alteration of the layered arrangement. Upon heating at 150 °C, the structure contracted but no indication on the existence of a second transition, as it is known to occur in comb-like poly(α-alkyl γ-glutamate) homopolymers, was observed by DSC. Random copolymers did show nor thermal transition neither evidence of any organized supramolecular structure.     

Studies on the interactions of CO2 with biodegradable poly(dl-lactic acid) and poly(lactic acid-co-glycolic acid) copolymers using high pressure ATR-IR and high pressure rheology

Amorphous poly(dl-lactic acid) (P_dlLA) and poly(lactic acid-co-glycolic acid) (PLGA) polymers have been used to fabricate porous scaffolds for tissue engineering applications via a supercritical foaming technique. The chemical composition of the polymers and the morphology (pore size, porosity and interconnectivity) of the scaffolds are crucial because they influence cell filtration, migration, nutrient exchange, degradation and drug release rate. To control the morphology of supercritical foamed scaffolds, it is essential to study the interactions of polymers with CO₂ and the consequent solubility of CO₂ in the polymers, as well as the viscosity of the plasticized polymers. In this paper, we are showing for the first time that well known and useful biodegradable polymers can be plasticized easily using high pressure CO₂ and that we can monitor this process easily via a high pressure attenuated total reflection Fourier transform infrared (ATR-IR) and rheology. High pressure ATR-IR has been developed to investigate the interactions of CO₂ with P_dlLA and PLGA polymers with the glycolic acid (GA) content in the copolymers as 15, 25, 35 and 50% respectively. Shifts and intensity changes of IR absorption bands of the polymers in the carbonyl region (∼1750 cm⁻¹) are indicative of the interaction on a qualitative level. A high pressure parallel plate rheometer has also been developed for the shear viscosity measurements of the CO₂-plastisized polymers at a temperature below their glass transition temperatures. The results demonstrate that the viscosities of the CO₂-plasticized polymers at 35 °C and 100 bar were comparable to the values for the polymer melts at 140 °C, demonstrating a significant process advantage through use of scCO₂. The data from the high pressure rheology and high pressure ATR-IR, combined with the sorption and swelling studies reported previously, demonstrate that the interaction and the solubility of CO₂ in PLGA copolymers is related to the glycolic acid content. As the glycolic acid ratio increases the interaction and consequent solubility of CO₂ decreases. The potential applications of this study are very broad, from tissue engineering and drug delivery to much broader applications with other polymers in areas that may range from composites and polymer synthesis through to injection moulding.     

Direct synthesis of poly(l-lactic acid) in supercritical carbon dioxide with dicyclohexyldimethylcarbodiimide and 4-dimethylaminopyridine

The direct synthesis of poly(l-lactic acid) (PLLA) from an l-lactic acid oligomer has been performed in supercritical carbon dioxide (scCO₂) using an esterification promoting agent, dicyclohexyldimethylcarbodiimide (DCC), and 4-dimethylaminopyridine (DMAP) as a catalyst. PLLA within M_n of 13,500 g/mol was synthesised in 90% yield at 3500 psi and 80 °C after 24 h. The molecular weight distribution of the products was narrower than PLLA prepared with melt-solid phase polymerisation under conventional conditions. Both DCC and DMAP showed high solubility in scCO₂ (DCC: 7.6 wt% (1.63×10⁻² mol/mol CO₂) at 80 °C, 3385 psi, DMAP: 4.5 wt% (1.62×10⁻²mol/mol CO₂) at 80 °C, 3386 psi) and supercritical fluid extraction was found to be effective at removing excess DMAP and DCC after the polymerisation was complete. We show that DCC and DMAP are effective esterification promoting reagents with further applications for condensation polymerisations in scCO₂.     

High molecular weight methyl ester of microbial poly(β,l-malic acid): Synthesis and crystallization

High molecular weight poly(α-methyl β,l-malate) (M_n ∼ 25,000, PD ∼ 1.7) was prepared from microbial poly(β,l-malic acid) (M_n ∼ 29,000, PD ∼ 1.3) by methylation with diazomethane in dry acetone without substantial cleavage of the polyester main chain. The thermal properties of this poly(malate) were assessed and its crystal structure was preliminary examined. Two crystal forms were identified by X-ray diffraction, their occurrence being dependent on crystallization conditions. The kinetics of nonisothermal and isothermal crystallizations from the melt were studied and modelled using the Avrami approach. Results were compared to those recently reported by us for low molecular weight poly(α-methyl β,l-malate) (M_n ∼ 3000, PD ∼ 1.3).     

Crystallization behavior of poly(l-lactic acid)

The crystallization behavior of poly(l-lactic acid) was studied in the range of 80-160 °C. The peak crystallization time (τ_p) was defined and obtained from the crystallization isotherm measured with a differential scanning calorimeter (DSC). Isothermal crystallization temperature (T_c) dependence of log(τ_p) discretely changed at 113 °C (= T_b). The linear growth rate of spherulite, G, was measured with a polarizing microscope. The T_c dependence of G and the size of the spherulite also discretely changed at T_b. Crystal structures for samples isothermally crystallized at temperatures which were higher and lower than T_b were orthorhombic (α-form) and trigonal (β-form), respectively. The discrete change of the crystallization behavior was explained by the formation of different crystal.     

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

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 37 °C for up to 24 months. 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 220 °C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24 months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 °C but not during the autocatalytic hydrolysis.