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.     

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.     

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.     

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.     

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.     

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.     

Synthesis and characterization of poly(l-glutamic acid)-block-poly(l-phenylalanine)

Poly(γ-benzyl l-glutamate)-block-poly(l-phenylalanine) was prepared via the ring opening polymerization of γ-benzyl l-glutamate N-carboxyanhydride and l-phenylalanine N-carboxyanhydride using n-butylamine·HCl as an initiator for the living polymerization. Polymerization was confirmed by ¹H-nuclear magnetic resonance spectroscopy and matrix assisted laser desorption ionization time of flight mass spectroscopy. After deprotection, the vesicular nanostructure of poly(l-glutamic acid)-block-poly(l-phenylalanine) particles was examined by transmission electron microscopy and dynamic light scattering. The pH-dependent properties of the nanoparticles were evaluated by means of ζ-potential and transmittance measurements. The results showed that the block copolypeptide could be prepared using simple techniques. Moreover, the easily prepared PGA-PPA block copolypeptide showed pH-dependent properties due to changes in the PGA ionization state as a function of pH; this characteristic could potentially be exploited for drug delivery applications.     

Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.     

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.     

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.     

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.     

Thermal degradation of poly(l-lactide): effect of alkali earth metal oxides for selective l,l-lactide formation

To achieve the feed stock recycling of poly(l-lactide) (PLLA) to l,l-lactide, PLLA composites including alkali earth metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), were prepared and the effect of such metal oxides on the thermal degradation was investigated from the viewpoint of selective l,l-lactide formation. Metal oxides both lowered the degradation temperature range of PLLA and completely suppressed the production of oligomers other than lactides. CaO markedly lowered the degradation temperature, but caused some racemization of lactide, especially in a temperature range lower than 250 °C. Interestingly, with MgO racemization was avoided even in the lower temperature range. It is considered that the effect of MgO on the racemization is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270 °C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred smoothly causing unzipping depolymerization, resulting in selective l,l-lactide production. A degradation mechanism was discussed based on the results of kinetic analysis. A practical approach for the selective production of l,l-lactide from PLLA is proposed by using the PLLA/MgO composite.     

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).     

The effect of surface chemistry and nanoclay loading on the microcellular structure of porous poly(d,l lactic acid) nanocomposites

Three series of polymer nanocomposites, based on poly(d,l lactic acid) (PDLLA) and organically modified montmorillonite, were prepared by the melt and the solution intercalation technique. The first series was prepared by extrusion using different clay loadings. The second series of nanohybrids was obtained using montmorillonite modified with different types of alkylammonium surfactants in terms of carbon-chain lengths (i.e., 4, 8, 12, 16 and 18). In the third series of nanocomposites, the organic cation concentration of the surfactant was varying. Microcellular porous materials were, afterwards, fabricated from these three series of nanocomposites. The porous structures of pure and nanocomposite PDLLA were prepared by isothermal pressure quench using supercritical CO₂ as foaming agent. The morphology of the produced porous materials was investigated by scanning electron microscopy (SEM). Image processing of the samples revealed that the final cellular structure is strongly related to clay loading and, both, the type and the organic cation concentration of the alkylammonium used for the modification of the clay. The results suggest that the size of the pores decreases and the cell density and bulk foam density increase with the increase of clay loading or the surfactant's carbon chain length or the cation concentration in clay. Clay dispersion seems to be enhanced by the supercritical treatment upon foaming.     

Melt intercalation of poly(l-lactide) chains into clay galleries

The melt intercalation of poly(l-lactide) (PLLA) chains into silicate galleries has been investigated via a melting process without any shearing force at elevated temperature. Under the melting process, the incorporation of various types of organo-modified montmorillonites into PLLA matrix lead to the increase in the basal spacing of clay particles in different manner without delamination into individual layers. The changes in layer-stacked structures of the clay particles in the PLLA matrix were examined by use of wide-angle X-ray diffraction and transmission electron microscopy. The effects of clay content in PLLA matrix and clay surfactants on the melt intercalation of PLLA were discussed in terms of chain mobility.     

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.     

Dielectric study on dynamics and conformation of poly(d,l-lactic acid) in dilute and semi-dilute solutions

Fractionated samples of d,l-poly(lactic acid) (PLA) were prepared and the dielectric normal mode relaxation was studied for dilute and semi-dilute solutions of the PLA in a good solvent benzene. Results indicate that in the dilute regime the normal mode relaxation time is proportional to [η]M_w in agreement with the Rouse-Zimm theory, where [η] and M_w denote the intrinsic viscosity and weight average molecular weight, respectively. The dielectric relaxation strength which is proportional to the mean square end-to-end distance 〈r²〉 increases with increasing M_w with the power of 2ν, where ν is the excluded volume parameter determined from [η]. The relaxation time in the semi-dilute regime increases with increasing concentration C due to increases of the entanglement density and the friction coefficient. The relaxation time corrected to the iso-friction state agrees approximately with the dynamic scaling theories. The relaxation strength decreases with increasing concentration indicating that 〈r²〉 decreases on account of the screening of the excluded volume effect. The concentration dependence of 〈r²〉 agrees approximately with the scaling theory proposed by Daoud and Jannink.     

Transcrystalline structures of poly(l-lactide)

Poly(l-lactide) (PLLA) film containing transcrystalline (TC) structures can easily be obtained by placing PLLA films melted between two poly(tetrafluoroethylene) (PTFE) sheets, followed by isothermal crystallization at 122 °C. The fine structures of the PLLA-TC film were studied by various structural techniques such as X-ray diffractometry, optical microscopy and transmission electron microscopy. We also examined the purification effect upon the morphology of PLLA-TC film. The formation of the TC structures revealed that one-dimensional spherulitic growth occurred from the assembling impurities as nucleation agent near the PTFE substrate in the heterogeneous nucleation system. We found that the b-axis of PLLA crystal was parallel to the lamellae growth direction confirmed using X-ray diffraction. The precipitated PLLA film crystallized in a similar process exhibited scanty TC textures, suggesting that the existence of impurity in the PLLA sample was an important factor for the formation of those structures.     

Miscibility and properties of linear poly(l-lactide)/branched poly(l-lactide) copolyester blends

Polymer blends consisting of linear poly(l-lactide) (PLLA) and different proportions of dendritic PLLA-based copolyesters (hb-PLLA) characterized by different degrees of branching (DB) were obtained in melt. The solid-state properties of poly(l-lactide)s and their blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), atomic force microscopy (AFM) and stress-strain measurements. DSC and DMA methods proved miscibility of PLLA/hb-PLLA blends for the studied composition range. AFM indicated that no phase separation occurs in PLLA/hb-PLLA blends and that PLLA and hb-PLLA cocrystallize in one single lamellae type. The mechanical characteristics of PLLA/hb-PLLA blends deteriorated with an increase of the DB and with changing blend composition. Susceptibility of the blends to biodegradation was studied by measuring the weight loss in two different biodegradation media. PLLA/hb-PLLA blends showed more pronounced hydrophilic character and higher susceptibility to biodegradation with an increase in the degree of branching.     

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.