Crystallization Of Polylactide Research Articles

Enhanced Nucleation Rate of Polylactide in Composites Assisted by Surface Acid Oxidized Carbon Nanotubes of Different Aspect Ratios

Biodegradable polylactide (PLA) composites added with acid oxidized multiwalled carbon nanotubes (A-MWCNTs) of two different aspect ratios (length to diameter) were prepared by coagulation. The aspect ratios and surface structures of A-MWCNTs were characterized by TGA, Raman, and SEM measurements. The percolation thresholds for gelation in the PLA composites with A-MWCNTs of large and small aspect ratios are 2.5 and 4.0 wt %, respectively, which were determined by a rheological method, and in turn, the rheological result confirms the aspect ratio differences for the added two types of A-MWCNTs in the composites. Isothermal crystallization kinetics of neat PLA and its composites were further investigated by using polarized optical microscope (POM) and differential scanning calorimetry (DSC) to clarify the effects of A-MWCNTs of different aspect ratios and concentrations. The different aspect ratio A-MWCNTs with the same carboxyl group mass percent show substantial effects on PLA crystallization kinetics. Those with smaller aspect ratios enhance nucleation rate for PLA spherulites much more than those with larger aspect ratios. This phenomenon can be attributed to fewer sidewall carboxyl groups on the surfaces of A-MWCNTs with smaller aspect ratios, which provides more nucleation sites for PLA crystallization than those with larger aspect ratios at the same concentration, resulting in faster PLA nucleation rates for the former one.

Polylactide toughening with epoxy‐functionalized grafted acrylonitrile–butadiene–styrene particles

AbstractGlycidyl methacrylate functionalized acrylonitrile–butadiene–styrene (ABS‐g‐GMA) particles were prepared and used to toughen polylactide (PLA). The characteristic absorption at 1728 cm−1 of the Fourier transform infrared spectra indicated that glycidyl methacrylate (GMA) was grafted onto the polybutadiene phase of acrylonitrile–butadiene–styrene (ABS). Chemical reactions analysis indicated that compatibilization and crosslinking reactions took place simultaneously between the epoxy groups of ABS‐g‐GMA and the end carboxyl or hydroxyl groups of PLA and that the increase of GMA content improved the reaction degree. Scanning electron microscopy results showed that 1 wt % GMA was sufficient to satisfy the compatibilization and that ABS‐g‐GMA particles with 1 wt % GMA dispersed in PLA uniformly. A further increase of GMA content induced the agglomeration of ABS‐g‐GMA particles because of crosslinking reactions. Dynamic mechanical analysis testing showed that the miscibility between PLA and ABS improved with the introduction of GMA onto ABS particles because of compatibilization reactions. The storage modulus decreased for the PLA blends with increasing GMA content. The decrease in the storage modulus was due to the chemical reactions in the PLA/ABS‐g‐GMA blends, which improved the viscosity and decreased the crystallization of PLA. A notched impact strength of 540 J/m was achieved for the PLA/ABS‐g‐GMA blend with 1 wt % GMA, which was 27 times than the impact strength of pure PLA, and a further increase in the GMA content in the ABS‐g‐GMA particles was not beneficial to the toughness improvement. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Effect of an organo-modified montmorillonite on PLA crystallization and gas barrier properties

Amorphous nanocomposite films were prepared from melt-blending of polylactide (PLA) and an organo-modified montmorillonite (O-MMT). The filler amount was varied in the range from 0 to 8 wt.%. The gas permeability decrease measured as a function of the clay content was accurately described by Nielsen law. The influence of O-MMT was studied on PLA non isothermal and isothermal crystallisation. The incorporation of small amounts of OMMTs within PLA favoured the polymer matrix crystallisation by decreasing the cold crystallisation temperature for heating thermal treatments and the crystallisation half-time for isothermal ones. Optimised annealing conditions allowing to obtain high crystallinity degrees combined with high melting crystalline lamellae were proposed. The permeability decrease observed for the annealed nanocomposite with respect to the amorphous reference PLA film was shown to result from the respective contribution of the clays and the crystalline phase.

Preparation and crystallization behavior of polylactide nanocomposites reinforced with POSS-modified montmorillonite

We herein report the preparation and crystallization behavior of polylactide (PLA) nanocomposites reinforced with polyhedral oligomeric silsesquioxane-modified montmorillonite (POSS-MMT), which is prepared by exchanging sodium cations of pristine sodium montmorillonite (Na-MMT) with protonated aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS-NH3+). PLA nanocomposites with 1–10 wt% POSS-MMT contents are manufactured via melt-compounding, and their structures and melt-crystallization behavior are investigated. It is characterized that POSS-MMT nanoparticles in the nanocomposites have an exfoliated structure of MMT silicates with POSS-NH3+ and partial POSS-NH2 crystals. DSC cooling thermograms suggest that the overall melt-crystallization rates of the nanocomposite with only 3 wt% POSS-MMT are remarkably enhanced in comparison with the neat PLA. From the isothermal crystallization analysis based on the Avrami model, the overall melt-crystallization of PLA/POSS-MMT nanocomposites is found to be dominated by the heterogeneous nucleation and three-dimensional spherulite growth. Isothermal melt-crystallization experiments using a polarized optical microscope show that the spherulite nucleation density of PLA/POSS-MMT nanocomposites is much higher than that of the neat PLA, whereas the spherulite growth rates of all the nanocomposites are almost identical with the rate of the neat PLA. It is concluded that the highly enhanced melt-crystallization rates of PLA/POSS-MMT nanocomposites stem from the dominant nucleation effect of POSS-MMT nanoparticles for PLA crystals.

Electrospinning of polylactide and polycaprolactone mixtures for preparation of materials with tunable drug release properties

Electrospun microfibers with a variable ratio between polycaprolactone and polylactide homopolymers were prepared from chloroform/acetone solutions. Thermal properties of both as-processed and melt crystallized samples were studied. Time-resolved WAXD patterns were taken during heating runs in order to evaluate the initial crystallinity and changes occurred during cold crystallization. DSC and WAXD experiments clearly indicated that fiber orientation facilitated the crystallization of polylactide, especially when fibers had a high polycaprolactone content. Triclosan could be effectively loaded by electrospinning and was well mixed in the polycaprolactone and polylactide phases. SAXS patterns allowed inferring that both polymers were also well mixed in the electrospun fibers and that triclosan hindered the lamellar stacking of polycaprolactone. Thermal properties, crystallinities and fiber surface morphologies were also significantly modified by the incorporation of triclosan. The release of drug loaded samples into different mixtures of ethanol and Sorensen medium was evaluated and the different affinity between triclosan and the two studied homopolymers was demonstrated. In this way, it was possible to obtain a series of materials with tuned release behavior and tuned antibacterial effect. The biocompatibility of all triclosan loaded polymer mixtures was evaluated by studying cell adhesion and proliferation.

Rheological properties of crystallizing polylactide: Detection of induction time and modeling the evolving structure and properties

AbstractLinear viscoelastic properties of polymer melts are highly sensitive to any structural changes, including molecular weight changes and the formation and growth of crystallites. Here, we make use of this sensitivity to study the homogeneous crystallization of polylactide. As this polymer is rather quickly susceptible to thermal degradation even at moderate temperatures, it is essentially impossible to study homogeneous crystallization in the absence of degradation. Thus, the evolution of complex viscosity of a polylactide due to thermal degradation in the absence of crystallization was studied. A simple empirical model is used to characterize the variation of complex viscosity due to thermal degradation and to determine the induction time of homogeneous crystallization at wide range of degrees of supercooling. Next, the evolution of complex viscosity due to crystallization was measured at several temperatures. Based on the results, a phenomenological model describing the viscosity evolution during homogeneous crystallization is proposed and validated. Finally, the linear viscoselastic data in the early stages of crystallization are shown to be consistent with gelation due to the formation of a network of tie molecules between spherulites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 812–822, 2010

Poly(ethylene glycol‐co‐propylene glycol) as a macromolecular plasticizing agent for polylactide: Thermomechanical properties and aging

AbstractFinding a suitable plasticizer for polylactide (PLA) is necessary to overcome its brittleness and enlarge its range of applications. In this study, commercial PLA was melt‐blended with a new plasticizer, an ethylene glycol/propylene glycol random copolymer [poly(ethylene glycol‐co‐propylene glycol) (PEPG)] with a typical number‐average molecular weight of 12 kDa and an ethylene glycol content of 78.7 mol %. The thermal properties, crystallization behavior, and mechanical properties of the quenched blends and the properties of the blends after storage for 2 months under the ambient conditions were investigated in detail. The advantage of using PEPG is that it does not crystallize at room temperature and has good compatibility with PLA. The quenched PLA/PEPG blends were homogeneous and amorphous systems. With an increase in the PEPG content (5–20%), the glass‐transition temperature, tensile strength, and modulus of the blends decreased, whereas the elongation at break and crystallizability increased dramatically. The cold crystallization of PLA resulted in phase separation of the PLA/PEPG blends by annealing of the blends at the crystallization temperature. After storage under the ambient conditions for 2 months, the PLA/PEPG blends retained good toughness but still lost some flexibility. The reasons for the aging of the blends were the enthalpy relaxation and cold crystallization of PLA and the induced phase separation. Poly(ethylene glycol) with a typical number‐average molecular weight of 10 kDa was also blended with PLA for comparison. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Polylactide/nano‐ and micro‐scale silica composite films. II. Melting behavior and cold crystallization

AbstractNonisothermal crystallization of polylactide (PLA)/silica composites prepared by (i) directly blending the PLA with nanoscale colloidal silica sol and by (ii) a sol–gel process are studied by differential scanning calorimeter (DSC) at various heating rates. Samples quenched from the molten state exhibited two melting endotherms (Tml and Tmh) due to melt‐recrystallization during the DSC scans. Lower heating rate and the presence of silica particles generate a lower peak intensity ratio of Tml /Tmh. The nonisothermal crystallization kinetics is analyzed by modified Avrami model, Ozawa model, and Liu‐Mo models. The modified Avrami and Liu‐Mo models successfully described the nonisothermal cold crystallization processes, but Ozawa is inapplicable. The nucleation constant (Kg) is calculated by modified Lauritzen‐Hoffman equation and the activation energy by Augis‐Bennett, Kissinger, and Takhor models. These calculated parameters indicate consistently that the nanoscale silica particles seem to form more heterogeneous nucleation to increase crystallization, but microscale one form hindrance to retard crystallization. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Clean and Green Bioplastic Composites: Comparison of Calcium Sulfate and Carbon Nanospheres in Polylactide Composites

AbstractTwo environmentally friendly fillers, carbon nanospheres (CNS) derived from cellulose and calcium sulfate anhydrite (CaSO4), a by‐product of the polylactide (PLA) production process, are compared as nucleating agents in commercial‐grade PLA. CNS and CaSO4 are compounded with PLA using solution blending. Additionally, CaSO4 is melt mixed with PLA. Crystallization kinetics are explored using differential scanning calorimetry (DSC) and polarized optical microscopy. Mechanical properties are examined using dynamic mechanical thermal analysis (DMTA) and morphology is determined using field‐emission scanning electron microscopy (FE‐SEM). It is found that the filler does not increase the PLA crystallization rates significantly at loadings up to 15 wt% (CNS) or 20 wt% (CaSO4). The lack of effect on crystallization kinetics is attributed to the high D‐lactic acid content of commercial‐grade PLA, and also to poor dispersion of the fillers in the PLA matrix. The glassy shear storage modulus of the composites is found to increase by 50% in the highest weight loadings tested. These clean and green bioplastic composites may be able to offset the use of fossil resource‐based materials.

Banded Spherulites in PS−PLLA Chiral Block Copolymers

In this study, thin-film samples of semicrystalline block copolymer (BCP), polystyrene-b-poly(l-lactide) (PS−PLLA), were prepared by solution-casting from dichloromethane solution (1 wt % of PS−PLLA). As observed by polarized light microscopy (PLM), banded spherulites can be found in the PS−PLLA thin-film samples crystallized from melt. By contrast, no banded spherulites can be obtained in crystallized PLLA homopolymer under similar crystallization conditions. However, the formation of banded spherulites for PLLA homopolymer can be achieved by blending compatible poly(ε-caprolactone) (PCL) homopolymer. Also, blends of PS−PLLA and poly(d-lactide) (PDLA) were prepared; no banded spherulites can be obtained in crystallized blends having equimolar PLLA and PDLA (namely, racemic blends). As a result, we suggest that the effect of chirality on polylactide crystallization is necessary for the induction of cooperative lamellar twisting along the radial growth direction to form banded spherulites. Moreover, the chiral effect might be enhanced by the introduction of diluents, such as compatible PCL homopolymer that promotes the formation of banded spherulites; this is referred to the effect of diluents. In contrast to the PLLA/PCL blends, a significant decrease in the band spacing of PS−PLLA banded spherulites (namely, the increase in twisting power) can be found. We speculate that the incompatible PS block chemically jointed with the PLLA block may intensify the effect of diluents so as to increase the imbalanced stress at crystalline lamellar fold surfaces for lamellar twisting. The morphological evolution of the banded spherulites was further investigated by using transmission electron microscopy and field-emission scanning electron microscopy. A hypothetic model was thus proposed to reveal the mechanism for the formation of curved crystalline lamellae in crystallized polylactide BCP.

Crystallization of Polylactide and its Stereocomplex Investigated by Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy Employing Carbonyl Overtones

The band origins and transitions of weak vibrational modes developed in the 3500 cm(-1) region of polylactide (PLA) spectra during crystallization are investigated. The band assignment to the OH stretching mode of terminal hydroxyls is unlikely because the trace amount of chain-ends is negligible considering the long chain of high molecular weight polymer. The band intensity can be enhanced for quantitative study by increasing the sample film thickness. The results show that the transition patterns of these bands mimic those of C=O stretching modes. Therefore, these are assigned to C=O overtones. Two bands associated with crystalline and amorphous characteristics are revealed during cold crystallization. The crystalline C=O bands of PDLA and its stereocomplex counterpart are located at 3510 cm(-1) and 3482 cm(-1), respectively, indicating a weaker C=O bond in the latter crystal structure. Two-dimensional Fourier transform infrared (2D-FT-IR) correlation spectroscopy is then applied to study the correlation between C=O overtones and the crystalline characteristic band located near 900 cm(-1). The transitions of the two vibrational modes observed in crystallization of the stereocomplex are in-phase with each other. This reflects an involvement of short-range hydrogen bonding in the stereocomplex crystal structure. In contrast, crystallization of PDLA shows that the C=O overtone varies prior to that of the C-H character, indicating that dipole-dipole force is a crystal-induced interaction.

Effect of casting solvent on characteristics of hexanoyl chitosan/polylactide blend films

AbstractBlend films of hexanoyl chitosan (H‐chitosan) and polylactide (PLA) were cast from corresponding blend solutions in chloroform, dichloromethane, or tetrahydrofuran. Thermal degradation behavior of the as‐prepared blend films was intermediate to those of the pure components and no significant effect from the type of the casting solvent was observed. All of the blend films exhibited one composition‐dependent glass transition temperature, but the results only suggested partial miscibility of the components in the amorphous phase at “low” contents of H‐chitosan. As revealed by solvent etching technique, the as‐prepared blend films prepared from the blend solutions in chloroform and dichloromethane showed extensive phase separation of the two components, with the minor phase forming into discrete domains throughout the matrix. Both thermal and X‐ray analyses showed that the apparent degree of crystallinity of the PLA component in the blends decreased monotonically with increasing H‐chitosan content and the choice of the casting solvent did not have an effect on the structure of PLA crystals. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Morphology and crystallization kinetics in a mixture of low-molecular weight aliphatic amide and polylactide

Polylactide (PLA)/ N, N-ethylenebis(12-hydroxystearamide) mixture was prepared by using melt extrusion. The detailed crystallization kinetics and morphology of neat PLA and a mixture were studied by using polarized optical microscopy, light scattering, differential scanning calorimetry, and wide-angle X-ray diffraction analyses. The overall crystallization rate and spherulitic texture of PLA were strongly influenced in presence of the organic additive. The overall crystallization rate of matrix PLA increased with addition of WX1. These behaviors indicated that WX1 crystallites, which crystallized at the very early stage of PLA crystallization act as a nucleating agent for PLA crystallization.

Atomic force microscopy of polyethylene and poly-(d-lactide) single crystals

Single crystals of polyethylenes with different molecular weight and of poly(d-lactide) were examined with atomic force microscopy. Morphologies of single crystals and their aggregates formed by overlaying as well as multilayer crystals are shown. Elevated diagonals caused by sectorization were found on crystals of both polymers. The nanoscale images demonstrate a grain structure of the lamellar surfaces with lateral features in the 5–15 nm range and roughness of about 1 nm. Polyethylene and the polylactide crystals exhibit some additional directional surface structure. The results are discussed in terms of surface organization.