Crystallization behavior of poly(lactide)/poly(β‐hydroxybutyrate)/talc composites

The morphology and miscibility of commercial poly(lactide) (PLA)/poly(β‐hydroxybutyrate) (PHB, from 5 to 20 wt %) blends prepared by melt extrusion method, were investigated using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) observations. The results show that for all the studied blend contents, PLA/PHB blends are immiscible. The effects of PHB and talc on the nonisothermal cold crystallization kinetics of PLA were examined using a differential scanning calorimetry (DSC) at different heating rates. PHB acted as a nucleating agent on PLA and the addition of talc to the blend yielded further improvement, since significant increase in the enthalpy peak was observed for samples containing 10 wt % PHB and talc (from 0.5 to 5 phr). The crystallization kinetics were then examined using the Avrami–Jeziorny and Liu–Mo approach. The simultaneous presence of PHB and talc induced a decrease of the crystallization half time. The evolution of activation energies determined with Kissinger's equation suggests that blending with PHB and incorporating talc promote nonisothermal cold crystallization of PLA. The synergistic nucleating effect of PHB and talc was also observed on isothermal crystallization of PLA from the melt. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of molding conditions on crystallization kinetics and mechanical properties of poly(lactic acid)

Although Poly(lactic acid) (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly, to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a post‐molding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate. POLYM. ENG. SCI., 57:306–311, 2017. © 2016 Society of Plastics Engineers     

Polylactide/organoclay nanocomposites: The effect of organoclay types on the structure development and the kinetic of cold crystallization

Polylactide (PLA)/organoclay composites were prepared by melt compounding with 4 phr of two different types of organoclays (cloisite 20A and cloisite 30B). Structure development and nonisothermal kinetic of cold crystallization of PLA/organoclay nanocomposites were examined by using of X‐ray diffraction technique, transmission electron microscopy, melt viscoelastic measurements, and differential scanning calorimetry. XRD results demonstrated that the melt intercalation of PLA chains into the cloisite 30B and cloisite 20A galleries was achieved to the same extent. However, it was shown that, PLA/cloisite 20A sample exhibited a significant viscosity upturn and a pronounced nonterminal low frequency storage modulus whose values were greater than those of PLA/cloisite 30B nanocomposite. A detailed analysis of the linear melt viscoelastic properties for the filled and unfilled samples at low frequencies was conducted by fitting the complex viscosity and storage modulus data with Carreau–Yasuda and Fractional Zener models, respectively. The glass transition, cold crystallization, melting temperature, and degree of crystallinity of virgin PLA and PLA/organoclay nanocomposites were inspected. Subsequently, the cold crystallization kinetics was analyzed by Avrami, Jezioney, and Lauritzen–Hoffman kinetic models. It was shown that, the crystallization rate followed Avrami equation with the exponent n around 2.4. From Lauritzen–Hoffman equation and Kissinger model, the nucleation parameter K_g and activation energy were estimated, respectively. J. VINYL ADDIT. TECHNOL., 25:48–58, 2019. © 2018 Society of Plastics Engineers     

Crystallization behavior of polyphenylenesulfide

The crystallization behavior of poly(phenylene sulfide) (PPS) has been examined by differential scanning calorimetry as a function of melt temperature, residence time in the melt, and the presence of a liquid crystal polymer. The results suggest that the thermal history of the sample plays an important role in determining the crystallization kinetics. Short residence times at low melt temperatures and high melt temperatures alone resulted in a low value for the Avrami exponent n. The former effect was ascribed to incomplete melting of the polymer while the latter effect was attributed to thermal degradation. Blending a liquid crystal polymer, Vectra A950, with PPS had only a minor effect on the crystallization behavior.     

PLA/T-ZnOw等温冷结晶及熔融行为的研究

采用熔融共混法制备了PLA/T-ZnOw复合物。用差示扫描量热仪（DSC）研究了PLA/T-ZnOw的等温冷结晶及其熔融行为,Avrami方程分析表明,Avrami方程能很好的描述PLA/T-ZnOw的等温冷结晶过程,Avrami指数n值在1.78~3.10之间,表明PLA/T-ZnOw是以二维盘状生长和三维球晶生长并存的球状晶体。偏光显微镜观察结果显示,随着T-ZnOw的加入,PLA球晶尺寸变小,数目增多,说明T-ZnOw促进了PLA的成核但未提高其结晶速度。     

Synthetic organofluoromica/poly(lactic acid) nanocomposites: Structure,rheological and thermal properties

Organically modified fluoromica/poly(lactic acid) (PLA) nanocomposites were prepared by melt processing at different compositions in an internal mixer at the same conditions. Gel permeation chromatography (GPC) was used to measure the molar masses of PLA alone, before and after processing. The results indicated that PLA presented some degradation during melt mixing. With the addition of the organically-modified fluoromica, higher reduction in the polymer molar masses was found. The structure of the synthetic mica and of the clay polymer nanocomposites (CPN) was studied by small-angle X-ray scattering (SAXS), which revealed the presence of nanometer-sized clay mineral aggregates. Dynamical rheological measurements were carried out in the linear viscoelastic region for both PLA and CPN molten samples. A predominantly viscous behavior was obtained for the polymer samples, whereas the CPN presented a pronounced elastic behavior. Addition of mica to PLA also had a significant effect on the complex viscosity of PLA, imparting an increasing shear thinning behavior with increasing mica composition. Below T_g, dynamical mechanical analysis results showed a substantial decrease of rigidity of the CPN in relation to PLA alone, attributed to the presence of high content of organic intercalant in the synthetic fluoromica. Lower cold crystallization rate was detected for the CPN in comparison with processed PLA alone.     

Crystallization and thermal behavior of multiwalled carbon nanotube/poly(butylenes terephthalate) composites

Multiwalled carbon nanotube/poly(butylene terephthalate) composites (PCTs) were prepared by melt mixing. The nonisothermal crystallization and thermal behavior of PCTs were respectively investigated by X‐ray diffractometer, polarized optical microscope, differential scanning calorimeter, dynamic mechanical thermal analyzer, and thermogravimetric analyzer. The presence of nanotubes has two disparate effects on the crystallization of PBT: the nucleation effect promotes kinetics, while the impeding effect reduces the chain mobility and retards crystallization. The kinetics was then analyzed using Ozawa, Mo, Kissinger, Lauritzen‐Hoffman, and Ziabicki model, and the results reveal that the nucleation effect is always the dominant role on the crystallization of PBT matrix. Thus the crystallizability increases with increase of nanotube loadings. In addition, the presence of nanotubes nearly has no remarkable contribution to thermal stability because nanotubes also play two disparate roles on the degradation of PBT matrix: the Lewis acid sites to facilitate decomposition and the physical hindrance to retard decomposition. Hence the nanotubes act merely as inert‐like filler to thermal stability. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Degradation induced by nanostructural evolution of polylactide/clay nanocomposites in the isothermal cold crystallization process

BACKGROUND: The aim of the work presented was to study the nanostructural evolution of clay tactoids during the isothermal cold crystallization of polylactide/clay nanocomposites (PLACNs). An interesting degradation behavior of the polylactide (PLA) matrix, however, was observed simultaneously with nanostructural evolution. The possible mechanisms of degradation and structural evolution are discussed. RESULTS: The evolution of the nanostructure and the degradation were studied online or offline using parallel plate rheometry, X‐ray diffraction, transmission electron microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy, gel permeation chromatography and thermogravimetric analysis. The results showed that the intercalation level of the clay tactoids further increased during cold crystallization, leading to simultaneous degradation of the PLA matrix, which was further confirmed by the marked change of small‐ and large‐amplitude oscillatory shear flow behavior of the PLACNs before and after cold crystallization. CONCLUSION: The increase of the intercalation level of the tactoids is due to the diffuse in movement of the PLA chain segments during cold crystallization. In this case, chain‐end scission occurs more easily, especially for those PLA chains around the loose edge and defect site of the tactoids during crystallization, which results in a simultaneous degradation of the PLA matrix. Hence, both the linear and nonlinear dynamic viscoelastic properties show a remarkable change after cold crystallization. Copyright © 2009 Society of Chemical Industry     

Crystallization behavior,morphology, and mechanical properties of poly(lactic acid)/tributyl citrate/treated calcium carbonate composites

In this article, poly(lactic acid) (PLA) composites containing titanate coupling agent treated calcium carbonate (T‐CaCO₃) and tributyl citrate (TBC) were prepared via melt blending. The crystallization, morphology, mechanical properties, and nonisothermal cold crystallization kinetics of PLA composites were studied by means of differential scanning calorimetry (DSC), wide angle X‐ray diffraction (WAXD), polarized light microscopy (PLM), scanning electron microscopy (SEM), and mechanical tests, respectively. The results show that TBC promotes crystallization of PLA. Both T‐CaCO₃ and TBC significantly decrease the spherulitic size, but the crystal structure of PLA is not changed. A synergistic toughening effect is obtained by the combination of T‐CaCO₃ and TBC. The nonisothermal cold crystallization kinetics of PLA composites is well described by Mo's method. The activation energies (ΔE) of nonisothermal cold crystallization were calculated by Kissinger and Takhor methods, respectively. It is found that the addition of T‐CaCO₃ and TBC increases ΔE, but it also increases the cold crystallization rate. POLYM. COMPOS., 35:1570–1582, 2014. © 2013 Society of Plastics Engineers     

Unique isothermal crystallization phenomenon in the ternary blends of biopolymers polylactide and poly[(butylene succinate)-co-adipate] and nano-clay

Melt and cold isothermal crystallization studies were carried out on polylactide (PLA)/poly[butylene succinate)-co-adipate] (PBSA) neat blend and blend-clay composites. The neat blend and blend-clay composites were prepared by melt-blending in a batch mixer. The weight ratio of PLA to PBSA was fixed at 70:30, while the content of the organoclay was varied from 0 to 9 wt%. The spherulitic growth rates and morphologies of PLA and PBSA in the samples were examined through polarized optical microscopy, while the rate of crystallization and the extent of crystallinity were studied through differential scanning calorimetry. The kinetics of melt and cold crystallization of PLA were adequately described by the Avrami model. There was a strong dependence of the rate of crystallization and extent of crystallinity of PLA on the extent of clay loading and flow induced morphology. For composites with 2 and 6 wt% clay loading, uniquely slower crystallization occurred. A qualitative relationship between phase morphology and crystallization, as affected by clay loading, is therefore described.     

Crystallization kinetics,morphology, and hydrolytic degradation of novel bio‐based poly(lactic acid)/crystalline silk nano‐discs nanobiocomposites

In this work, novel biodegradable crystalline silk nano‐discs (CSNs) having a disc‐like morphology have been utilized for fabrication of poly(lactic acid) (PLA) nanocomposites by melt‐extrusion. The main focus is to investigate the effect of CSN on isothermal melt crystallization kinetics, spherulitic growth rates, morphology, and hydrolytic degradation of PLA. Spherulitic morphology and growth rates are examined over a wide range of crystallization temperatures (90–120 °C). With incorporation of CSN, the isothermal crystallization kinetics of PLA/CSN increases, however, the crystallization mechanism remains unaltered. The apparent activation energy and surface energy barrier for crystallization process decreases upon addition of CSNs. At lower isothermal crystallization temperatures (T_c) viz. (90–100 °C), reduced growth rates of PLA spherulites is observed. Both PLA and PLA/CSN exhibit highest crystallization rates at around ～107 °C. The hydrolytic degradation rates calculated from molecular weight reduction shows that PLA/CSN nanocomposites' degradation rates are lower as compared to PLA in acidic, neutral, and alkaline media at pH = 2, 7, and 12, respectively, due to hydrophobic nature of CSN. Scanning electron microscopy study demonstrated the surface erosion mechanism of hydrolytic degradation of PLA and PLA/CSN nanocomposites. This work provides valuable insight for the application and reclamation of PLA/CSN bionanocomposites in moist and wet working environments. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46590.     

Crystallization of poly(lactic acid) enhanced by phthalhydrazide as nucleating agent

The effect of phthalhydrazide compound on the nonisothermal and isothermal crystallization behavior of bio-based and biodegradable poly(lactic acid) (PLA) was investigated by differential scanning calorimetry and polarized optical microscopy. The nonisothermal melt crystallization of PLA started much earlier in the presence of phthalhydrazide even at a phthalhydrazide content as low as 0.1 wt%. The isothermal crystallization kinetics was analyzed by the Avrami model. It was found that the Avrami exponent of the PLA crystallization was not significantly influenced by the addition of phthalhydrazide, indicating that the crystallization mechanism almost did not change in the composites. The crystallization half-time of PLA/phthalhydrazide composites decreased significantly with increase in phthalhydrazide loading. The observation from optical microscopy showed that the presence of phthalhydrazide increased the number of nucleation sites. The above observations indicate that phthalhydrazide is an efficient nucleating agent of PLA.     

Borassus powder-reinforced poly(lactic acid) composites with improved crystallization and mechanical properties

This article reports on the development of biocomposites based on polylactic acid (PLA) and borassus powder. Borassus powder was treated with alkali to remove hemicelluloses and lignin. The treated borassus improved the homogeneous mixing with PLA and increased the crystallinity of PLA. Dispersibility of the borassus was studied by scanning electron microscopy (SEM) and X-ray MicroCT. PLA/borassus composites were prepared by melt mixing of PLA with 5, 10, and 15 wt % treated/untreated borassus. Composites were examined for mechanical properties and crystallization. Composites showed enhanced tensile strength compared to neat PLA. The PLA/treated borassus powder composites displayed higher crystallinity than PLA. The isothermal cold crystallization study showed increase in the crystallization rate of PLA in the presence of treated borassus. The spherulitic growth was studied using polarized optical microscopy. The enhanced performance of the PLA-borassus composites was observed in the presence of borassus. This study demonstrates that the PLA-borassus composites show great promise for bioplastics applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47440.     

Nucleation of polyvinyl alcohol crystallization by single-walled carbon nanotubes

Non-isothermal crystallization experiments were performed on poly(vinyl alcohol) mixed with single-walled carbon nanotubes. Non-isothermal crystallization experiments showed that nanotubes nucleate crystallinity at weight fractions as low as 0.1%. An Ozawa model was applied to the non-isothermal crystallization data, and this approach confirmed the results deduced from the qualitative examination of the data: there was a clear difference in crystallization kinetics for the 0.1 and 1% sample vs. the sample without nanotubes. Nanotubes were also found to promote the thermal degradation of poly(vinyl alcohol).     

Influence of pearlescent pigments on mechanical properties and crystallization behavior of polylactic acid

Polylactic acid (PLA)/pearlescent pigments (PEPs) composites were fabricated by melt blending method and their properties were studied using a rotational rheometry, field emission scanning electron microscopy (FESEM), differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). Because PEP could disperse uniformly in PLA matrix and would form PEP networks, there were improvements made in the flexural strength, tensile strength and impact strength of PLA composites. Interestingly, the viscosity of PLA composites reduced significantly as compared with neat PLA, indicating an improvement in processing properties. The rheological tests confirmed that the storage modulus of PLA composites decreased as PEP content increased at a given frequency. Also, there was a plateau at low frequency range, which was a sign of PEP networks formation. The DMA tests showed that the storage modulus of PLA composite increased with PEP content, suggesting the enhancement of rigidity. According to the XRD results, no change was observed in the crystal structure of PLA in the presence of PEP. The addition of PEP did not change melting temperature, but the glass transition temperature increased a little, and the cold crystallization temperature of PLA decreased largely. The DSC results also showed that nucleation ability of PLA was enhanced by addition of PEP at low cooling rates, but the whole crystallization process of PLA in composite was inhibited at higher cooling rates. This conclusion was also confirmed by the results of crystallization kinetics.     

Nanocomposites of halloysite and polylactide

Naturally occurred halloysite (Hal) nanotubes compounded with polylactide (PLA) via melt mixing formed biodegradable and biocompatible clay polymer nanocomposites (CPN). The hydrogen bonding interactions between Hal and PLA were confirmed by Fourier transform infrared spectroscopy (FTIR). The modulus, strength and toughness of the Hal-PLA nanocomposites were substantially higher than those of neat PLA. Storage modulus and glass transition temperature of the Hal-PLA nanocomposites also increased with Hal loading as observed by dynamic mechanical analysis. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that Hal was uniformly dispersed and oriented in the CPN. X-ray diffraction (XRD) of the CPN showed the absence of Hal reflection at around 20°, indicating interactions of the PLA molecular chains in the interlayer space of Hal. Hal could nucleate PLA, leading to the decreased cold crystallization temperature and increased crystallinity. The vicat softening temperature and the degradation temperature of the CPN increased with Hal loading. Owing to the high performance and biocompatibility of the CPN, the prepared Hal-PLA nanocomposites had potential applications in biodegradable plastic and biomedical areas.     

Isothermal crystallization kinetics and spherulite morphologies of poly(lactic acid)/ethylene acrylate copolymer blends

Isothermal crystallization kinetics and spherulite morphologies of partially immiscible blends of poly(lactic acid) (PLA) and ethylene acrylate copolymer (EAC) were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. The DSC data obtained was analyzed using the Avrami equation. Crystallization kinetics of PLA from the melt was strongly influenced by the blend composition and the crystallization temperature. At a given crystallization temperature, the overall crystallization rate value was greater in the blends than in PLA suggesting that the presence of EAC enhanced crystallization of PLA. Polarized optical micrographs showed that the crystallization of PLA initially took place at the PLA/EAC interface. At high EAC content (>1 wt %), EAC domains acted as hindrance to crystallization reducing the overall crystallization rate of PLA in the blends. Based on the DSC analysis, the crystallization rate was maximum when PLA blend with 1 wt % EAC was isothermally crystallized at 103 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45487.     

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.     

Crystallization kinetics of chain extended poly(lactic acid)/clay nanocomposites

The poly(lactic acid)/clay nanocomposites (PLACNs) were prepared by melt mixing method, then multiepoxide chain extender (CE) was added into PLACNs to induce the branched structure of poly(lactic acid) (PLA) chains. The nonisothermal cold crystallization and isothermal melting crystallization of PLA, PLACNs, and chain extended PLACNs (CEPLACNs) were characterized by DSC and studied by Avrami analysis. The results showed that the inducing of clay and CE affected the crystallization behavior of PLA in different way. Adding CE increased the overall crystallinity of PLA at cooling process, but clay had an opposite effect. Besides that, the addition of CE and clay increased the crystal nuclei number due to the heterogeneous nucleation mechanism. According to the crystallization kinetics study, the inducing of clay almost no effect on the crystal growth rate of PLA, but the branched structure had a pronounced effect for improving crystal growth rate of PLA. POLYM. COMPOS., 36:2123–2134, 2015. © 2014 Society of Plastics Engineer     

Effect of flax fiber content on polylactic acid (PLA) crystallization in PLA/flax fiber composites

Crystallization of polylactic acid (PLA) has a profound effect on its thermal stability and mechanical properties. However, almost no crystallization occurs in actual injection molding process due to rapid cooling program. In this paper, flax fiber was employed as nucleator to enhance the crystallization capability of PLA. Effects of flax fiber content on cold crystallization, melt crystallization, crystallinity, crystal form, morphologies, and size of spherulites of PLA/flax fiber composites were investigated. Dynamic mechanical analysis was innovatively employed to study cold crystallization temperature of PLA/flax fiber composites under dynamic force, and the relationship between cold crystallization temperature (y) and flax fiber content (x) data was fitted by the function y = 34.1 × exp (?x/5.7) + 78.0. The differential scanning calorimetry results showed that the cold crystallization temperature of composites dropped, the melt crystallization temperature of composites increased, and the crystallinity of composites improved with increasing of flax fiber content. Using polarized optical microscopy, it has been found that the spherocrystal size of composites was much smaller than that of neat PLA, and flax fiber induced transcrystallization on the flax fiber surfaces. Wide-angle X-ray diffraction was applied to reveal that flax fiber significantly enhanced the formation of α-form PLA crystals.