Crystallinity Of PLA Research Articles

Bleached extruder chemi‐mechanical pulp fiber‐PLA composites: Comparison of mechanical, thermal, and rheological properties with those of wood flour‐PLA bio‐composites

ABSTRACTAn environmentally friendly bleached extruder chemi‐mechanical pulp fiber or wood flour was melt compounded with poly(lactic acid) (PLA) into a biocomposite and hot compression molded. The mechanical, thermal, and rheological properties were determined. The chemical composition, scanning electron microscopy, and Fourier transform infrared spectroscopy results showed that the hemicellulose in the pulp fiber raw material was almost completely removed after the pulp treatment. The mechanical tests indicated that the pulp fiber increased the tensile and flexural moduli and decreased the tensile, flexural, and impact strengths of the biocomposites. However, pulp fiber strongly reinforced the PLA matrix because the mechanical properties of pulp fiber‐PLA composites (especially the tensile and flexural strengths) were better than those of wood flour‐PLA composites. Differential scanning calorimetry analysis confirmed that both pulp fiber and wood flour accelerated the cold crystallization rate and increased the degree of crystallinity of PLA, and that this effect was greater with 40% pulp fiber. The addition of pulp fiber and wood flour modified the rheological behavior because the composite viscosity increased in the presence of fibers and decreased as the test frequency increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44241.

Thermo‐rheological and interfacial properties of polylactic acid/polyethylene glycol blends toward the melt electrospinning ability

ABSTRACTThe electrospinning ability of PLA/PEG system at the melt state was investigated through the viscoelastic parameters obtained from dynamic shear and extensional rheometers. PLA and PEG were melt‐blended at various composition ratios. Effect of PEG concentration on the PLA thermal behavior was studied by the differential scanning calorimetry (DSC). According to DSC and wide‐angle X‐ray diffraction, the PLA crystallinity increased and the crystalline structure became more completed (α‐crystal form) in the presence of PEG. Viscoelastic parameters such as zero‐shear viscosity and relaxation time as an indication of elasticity were obtained. The results revealed enhanced polymer chain mobility and disentanglement ought to plasticizing effect of PEG. The critical content of PEG about 20–30 wt % at which the solid–liquid phase separation occurred was in good agreement with the viscoelastic properties. Hence, more than 20% PEG the elasticity diminished and the melt strength became zero. The interfacial tension of the PLA and PEG estimated through the rheological and morphological parameters evidenced the good miscibility of PLA/PEG system at the melt electrospinnig temperature. While the high viscose samples (η0 > 1800 Pa/s) PLA and PLA/PEG (95/5) were not spinnable at the spinning temperature of 180 °C, blends containing 10–30% PEG were easily spun. The finest and continuous fiber mats were obtained by electrospinning of PLA/PEG (80/20) blend (df = 4.8 ± 0.8 μm). More than the critical concentration of PEG (Φ > 30%), lacking the elasticity suppressed the melt electro‐spinnability of PLA/PEG. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44120.

The effect of cellulose nanofibers on the crystallinity and nanostructure of poly(lactic acid) composites

Poly(lactic acid) (PLA) is a semi-crystalline polymer with good transparency, mechanical strength, and melt processability. However, the use of PLA in applications like food packaging is limited by its low heat resistance and slow crystallization rate. The addition of cellulose nanofibers can be an efficient solution to enhance PLA crystallinity and stiffness, maintaining the transparency and biodegradability, which are mandatory for packaging applications. The combined effect of annealing and cellulose nanofibers (surface treated—SCN or untreated—CN) on the crystalline structure of PLA was thoroughly studied at both micro- and nano-scales by differential scanning calorimetry (DSC), mechanical tests, X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM), Peak Force QNM (Quantitative Nanomechanical Mapping) mode. The ability of CN to act as nucleating agents was highlighted by DSC and XRD, both methods pointing out similar trends of crystallinity variation. Higher crystallinity was observed after annealing, 59 % instead of 40 % for PLA, 67 % instead of 48 % for PLA-CN and 57 % instead of 39 % for PLA-SCN, respectively. A mixture of α and α′ crystals was detected in all the samples after annealing but the concentration of the more disordered α′ form was higher in PLA-CN than in PLA-SCN. The transformation of the grained structure specific to amorphous PLA into a crystalline lamellar structure (average lamellar thickness of about 22 nm) after annealing was detected by Peak Force QNM. Nanoindentation tests showed an improvement of the surface modulus and hardness for the composites compared to PLA, after annealing. Thus, modulus increased from 5.9 GPa for PLA to 6.2 GPa for PLA-CN and 6.6 GPa for PLA-SCN. The understanding of PLA nanostructure modification by annealing and CN addition may contribute to adjust post-processing treatments, thus making PLA more suitable for packaging and other applications.

Lignin-coated cellulose nanocrystals as promising nucleating agent for poly(lactic acid)

We report the effect of lignin-coated cellulose nanocrystals (L-CNCs) on the crystallization behavior of poly(lactic acid) (PLA). PLA/L-CNC nanocomposites were prepared by melt mixing, and the crystallization behavior of PLA was investigated using differential scanning calorimetry. Isothermal crystallization data were analyzed using Avrami and Lauritzen–Hoffman secondary nucleation theory, while the equilibrium melting temperature was determined using the nonlinear Hoffman–Weeks method. The lignin-coated cellulose nanocrystals acted as a nucleating agent and significantly increased the rate of crystallization and degree of crystallinity of PLA in PLA/L-CNC nanocomposites. The Avrami exponent, n, increased in the presence of L-CNCs, displaying a conversion from lamellar morphology to two-dimensional crystal growth. PLA/L-CNC nanocomposites also gave lower values of the nucleation parameters, $$K_{\text{g}}$$ and σ e, due to a reduction in the activation energy for nucleation.

Isothermal crystallization kinetics of talc-filled poly(lactic acid) and poly(butylene succinate) blends

Blends comprising poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and talc were prepared via a solution casting method. The weight ratios of PLA to PBS (PLA/PBS) blends were 80:20 and 60:40, whereas the talc content was varied from 0 to 5.0 parts per hundred resin (phr). The crystallization halftime (t 1/2) of PLA/PBS (80:20) at 80 °C was higher than those of neat PLA, PBS, and PLA/PBS (60:40); this is due to its high PLA content. The t 1/2 value for neat PLA at 90 °C decreased when the talc content was increased, whereas the value for neat PBS gradually increased. These phenomena can be seen more clearly at 95 and 100 °C. This means that talc not only promotes PLA crystallization but also acts as a crystallization inhibitor for PBS. The isothermal crystallization kinetics and spherulitic morphology of these samples were investigated. The extent of talc loading showed a strong dependence on the isothermal crystallization rate and the extent of PLA crystallinity, whereas the contrary effect was observed for PBS due to the growth of large crystals. This suggests that talc not only promotes the crystallization of PLA but also inhibits the crystallization of PBS. The results are in agreement with the crystallization halftime and polarized optical microscopy analysis.

Recent advances in nucleating agents used for poly (lactic acid)

Poly (lactic acid) (PLA) is a polymer synthesized from lactic acid with good biocompatibility and biodegradability. At present, PLA manufactured on industrial scale is mainly synthesized from L-lactic acid. The obtained products have good transparency but poor heat resistance. Adding nucleating agents could increase the crystallinity of PLA, to improve heat resistance. We reviewed the progress of research on organic and inorganic nucleating agents that can be used for PLA synthesis.

Effect of PCL and Compatibility Contents on the Morphology, Crystallization and Mechanical Properties of PLA/PCL Blends

This study focused on the effect of PCL on crystalline morphology, isothermally crystallization of PLA in PLA/PCL blends using POM on investigation. The blend with Pluronic used as a plasticizer was also studied. The blends were prepared by melt blending in an internal mixer. The contents of PCL incorporated were varied from 0-30% by weight. The content of Pluronic used in this study was at 2.5, 5 and 7.5 phr. The blends were characterized for their morphology, crystallization and mechanical properties. Melt strength of the blends was also investigated. The morphology of the blends confirmed immiscibility of two polymer phases. Scanning electron microscope (SEM) of PLA/PCL blends indicated larger particle size of PCL dispersed in PLA with the increase of PCL contents. Tg of PLA in the blend could not be observed on Differential Scanning Calorimetry (DSC) traces. This was due to the overlapping with Tm of PCL. The presence of Pluronic in PLA/PCL blends resulted in Tms shifting to slightly lower temperatures. The crystallinity of PLA was also decreased. The spherulitic growth of PLA in blends was followed by polarized optical microscope using isothermal condition at 141°C. The spherulite structure was found affected by the presence of Pluronic hence effect on crystallization of PLA. Tensile properties of PLA/PCL blend were suffered with PCL content except for elongation at break which seems to be enhanced. It was found that the presence of pluronic in PLA/PCL blend increased ductility of the blends. At Plurunic of 2.5 phr and PCL content of 10-20% by weight, tensile strength of these blends was increased. This was explained by the fine dispersed PCL particles facilitated orientation of PLA chain after proportional limit and hence increased tensile strength. With higher content both PCL and Pluronic, large PCL particle size was disadvantaged for tensile properties.

Water absorption and hygrothermal aging behavior of short ramie fiber‐reinforced poly(lactic acid) composites

Short ramie/poly(lactic acid) (PLA) composites were prepared by using extrusion and injection moulding method. The water absorption and hygrothermal aging behaviors of ramie/PLA composites at 60°C was analyzed. The water absorption and mechanical properties of the ramie/PLA composites with immersion time were reported and compared to those of neat PLA. Crystallinity and molecular weight of neat PLA and ramie/PLA composites were determined by differential scanning calorimetry and gel permeation chromatography, respectively. The results showed that the water absorption and mechanical properties was affected by the short ramie fiber. Ramie/PLA composites showed higher saturation weight gains and diffusion coefficients than that of neat PLA, which indicated that the addition of short ramie fiber induced higher water absorption caused by the hydrophilic nature of the fibers. Tensile strength and flexural strength severely decreased after aging. Moreover, it was found that PLA was severely degraded in hygrothermal environment, and ramie accelerated the degradation of PLA. The morphologies of fractured surfaces of neat PLA and ramie/PLA composites with different immersion time were observed by scanning electronic microscopy. The micro‐cracks and voids were found which was due to the degradation of PLA and debonding of ramie and PLA. POLYM. COMPOS., 39:1098–1104, 2018. © 2016 Society of Plastics Engineers

Hydrolytic degradation behaviour of sucrose palmitate reinforced poly(lactic acid) nanocomposites

This work discusses the influence of novel biofiller, “sucrose palmitate” (SP) on the hydrolytic degradation behavior of poly(lactic acid) (PLA) nanocomposites. The influence of temperature and pH of the solution on the hydrolytic degradation behavior of PLA and PLA-SP nanocomposites was investigated. The variation in the crystallinity of PLA and PLA composites subjected to the hydrolytic degradation process is verified by XRD and DSC analysis. The morphological changes that occurred during the degradation process are observed by scanning electron microscopy (SEM). Thermo-gravimetric analysis confirms the loss of thermal stability of the neat PLA as well as composites after hydrolytic degradation process. Transparency measurements support the enhancement in opacity of both the PLA and PLA-SP nanocomposites with progress in hydrolytic degradation period.

Performance and Structure Characterization of Poly(lactic acid)/Zinc Oxide Pillared Organic Saponite Nanocomposites by Solution Intercalation

Zinc oxide (ZnO) pillared organic saponite was synthesized with cetyltrimethylammonium bromide (CTAB) via a microwave hydrolysis method. Moreover, poly(lactic acid) (PLA)/ZnO pillared organic saponite nanocomposites were prepared by solution intercalation. The microstructure analysis suggested that ZnO pillared organic saponite appeared at an exfoliated or partially intercalated nanometer layer, which dispersed homogeneously in PLA matrix. The experimental results showed that ZnO pillared organic saponite improved the mechanical properties and thermal stabilities of PLA/ZnO pillared organic saponite nanocomposites. Differential scanning calorimetry (DSC) results demonstrated that the ZnO pillared organic saponite eliminated the cold crystallization process, decreased the crystal size and obviously promoted the crystallinity of PLA, which was in accordance with XRD analysis results. At the same time, the properties of UV absorbance and antibacterial properties of nano ZnO were introduced into PLA.

Poly(lactic acid) plasticized by biodegradable glyceryl lactate

ABSTRACTThe esterification of glycerol with lactic acid was carried out to synthesize glyceryl lactate, which was used as a as a plasticizer with different contents and blended with poly(lactic acid) (PLA) to get a completely biodegradable material with potential applications because of its biodegradability. The structural, morphological, thermal, and mechanical properties of the plasticized PLA were studied to evaluate the plasticizing effects. The results show that the PLA plasticized with glyceryl lactate had a lower glass‐transition temperature and a better ductility. With increasing plasticizer content, the crystallinity of PLA increased. The elongation at break reached 380%, and the impact strength was 1.7 times that of neat PLA. Simultaneously, the migration rate measurements showed that glyceryl lactate presented relatively low migration toward the surface compared with glycerol and citrate esters. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43460.

Solution blow spinning: parameters optimization and effects on the properties of nanofibers from poly(lactic acid)/dimethyl carbonate solutions

Solution blow spinning (SBS) is a process to produce non-woven fiber sheets with high porosity and an extremely large amount of surface area. In this study, a Box–Behnken experimental design (BBD) was used to optimize the processing parameters for the production of nanofibers from polymer solutions consisting of poly(lactic acid) (PLA) dissolved in dimethyl carbonate. In addition, a comparative study between SBS fibers and cast film was performed to verify the influence of the SBS process on the crystallinity and thermal properties of PLA. The PLA concentration in polymer solutions was the most significant parameter affecting fiber diameter. The BBD analysis revealed that small diameter fibers were best obtained by a combination of 8 % w/v PLA concentration, 80 psi air pressure, and a feed rate of 50 µL min−1. The comparative study showed that both the SBS and the film casting processes increased the PLA crystallinity. However, the PLA films had a higher degree of crystallinity compared with the fibers made by the SBS process (39 and 17 %, respectively), which was attributed to the high shear created at the SBS nozzle inducing orientation and chain alignment. During the fiber formation, crystals formed with varied morphology including the α′-crystals, which have a less ordered structure and lower thermal stability compared to the α-crystals. The lower thermal stability of SBS fibers compared to the films can be explained by the lower degree of crystallinity and also by the higher surface area which can accelerate the weight loss process.

Investigation of poly(l-lactic acid)/graphene oxide composites crystallization and nanopore foaming behaviors via supercritical carbon dioxide low temperature foaming

Abstract

Biodegradable poly(lactic acid)/poly(butylene succinate)/wood flour composites: Physical and morphological properties

This work sought to improve the toughness and thermal stability of poly(lactic acid) (PLA) by incorporating poly(butylene succinate) (PBS) and wood flour (WF). The PLA/PBS blends showed a PBS‐dose‐dependent increase in the impact strength, elongation at break, degree of crystallinity, and thermal stability compared to the PLA, but the tensile strength, Young's modulus, and flexural strength were all decreased with increasing PBS content. Based on the optimum impact strength and elongation at break, the 70/30 (w/w) PLA/PBS blend was selected for preparing composites with five loadings of WF (5–30 phr). The impact strength, tensile strength, flexural strength, and thermal stability of the PLA/PBS/WF composites decreased with increasing WF content, and the degree of crystallinity was slightly increased compared to the 70/30 (w/w) PLA/PBS blend. Based on differential scanning calorimetry, the inclusion of PBS and WF into PLA did not significantly change the glass transition and melting temperatures of PLA in the PLA/PBS blends and PLA/PBS/WF composites. From the observed cold crystallization temperature of PLA in the samples, it was evident that the degree of crystallinity of PLA in all the blends and composites was higher than that of PLA. The PLA/PBS blend and PLA/PBS/WF composite degraded faster than PLA during three months in natural soil, which was due to the fast degradation rate of PBS. POLYM. COMPOS., 38:2841–2851, 2017. © 2016 Society of Plastics Engineers

Rheology, mechanical properties and crystallization behavior of glycidyl methacrylate grafted poly(ethylene octene) toughened poly(lactic acid) blends

Poly(lactic acid) (PLA)/poly(ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA denoted as GPOE) blends were prepared via simple melt compounding method at GPOE loadings from 5 to 20wt%. GPOE can significantly affect the physical properties of PLA. Compared to neat PLA, the elongation at break and impact strength of the blends were significantly improved. Scanning electron micrograph analysis revealed large numbers of cavities in the fracture surface of the blends, and the size of the cavities increased along with the increase of GPOE content in the PLA/GPOE blends. Furthermore, the overall crystallization rates were faster in the PLA/GPOE blends than that in neat PLA. However, the crystallization mechanism and crystal structure of these blends remained unchanged despite the presence of GPOE. The addition of GPOE decreased the degree of crystallinity of PLA. The toughened PLA could be of great use and importance for wider practical applications.

Mechanical, Thermal and Morphological Properties of Injection Molded Poly(lactic acid)/Calcium Carbonate Nanocomposites

In this work, mechanical and thermal properties of maleated ethylene-propylene rubber (EPR-g-MAH) toughened poly(lactic acid) (PLA)/nano-precipitated calcium carbonate (NPCC) nanocomposites were investigated. It was found that elongation at break of PLA/NPCC was increased with increasing EPR-g-MAH content up to 15 phr (parts per hundred resin). The investigations on the crystallization behaviours demonstrated that the NPCC acted as nucleating agent for the PLA. However, the degree of crystallinity of PLA was reduced by the addition of EPR-g-MAH which appears to be due to encapsulation of NPCC by the maleated rubber. Dispersion of NPCC was observed using transmission electron microscopy (TEM), and it was revealed that both nano-dispersed and agglomerates NPCC exist in the PLA matrix. Moreover, encapsulation of NPCC was favored in the presence of EPR-g-MAH.

Effect of halloysite on structure and properties of melt-drawn PCL/PLA microfibrillar composites

The study deals with the modification of mechanical properties of poly (!-caprolactone) (PCL)/poly(lactic acid) (PLA) system using the microfibrillar composite (MFC) concept. As the in-situ formation of PLA fibrils by melt drawing was impossible due to flow instability during extrusion, the system was modified by adding halloysite nanotubes (HNT) using different mixing protocols. The resulting favourable effect on the rheological parameters of the components allowed successful melt drawing. Consequently, PLA fibrils formation combined with the reinforcement of components by HNT and increased PLA crystallinity lead to a biocompatible and biodegradable material with good performance suitable for a broad range of applications. The best results, comparable with analogous MFC modified with layered silicate (oMMT), have been achieved at a relatively low content of HNT of 3%, in spite of its lower reinforcing ability in a single nanocomposite. This indicates that modifying MFC by HNT, including fibrils and interface parameters, is more complex in comparison with the undrawn system.

Glass fiber coated with graphene constructed through electrostatic self‐assembly and its application in poly(lactic acid) composite

ABSTRACTA method to construct glass fiber/graphene material via the electrostatic self‐assembly was proposed. The graphene oxide (GO) nanosheets were firstly prepared from graphite according to Hummer's methods. Oppositely charged GO is successfully introduced to the surface of the GF cationized by 3‐aminopropyltriethoxysilane (APTES) treatment in the solution with mild agitation. Sequently, glass fibers coated with graphene (GF/CRG) were obtained after chemical reduction. The graphene content was characterized by TGA and XPS tests and the value of about 0.7 wt % was obtained. Composites of poly(lactic acid) and GF/CRG were prepared through melt blending. Thermogravimetric analysis (TGA) results shows that more than 50% graphene remains on the surface of GF after processing, which indicates a strong binding between GF and graphene. GF/CRG has significant influences on crystallization and mechanical property of PLA: the crystallinity of PLA increases from 27.61 to 51.29%; the tensile strength of the PLA–GF/CRG composite is about 63% larger than the pure PLA at the GF/CRG content of 10 wt %. This new method can apply to making composites with high performance. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43296.

Development of PLA/cellulosic fiber composite foams using injection molding: Crystallization and foaming behaviors

Poly(lactic acid) (PLA) and northern bleached softwood kraft (NBSK) or black spruce medium density fiberboard (MDF) fibers were melt compounded using a co-rotating twin screw extruder and subsequently microcellular injection molded. Poly(ethlylene glycol) (PEG) was used as a lubricant. The microcellular structure, thermal properties, and crystallization behaviors were characterized using scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and wide angle X-ray diffraction. Results show that cellulosic fibers, acting as crystal nucleating agents, increased the crystallization temperature and the crystallinity and decreased the crystallization half time. The dissolved N2, the shear stress, and biaxial stretching during foaming also enhanced the crystallinity of PLA. Compared to PLA/PEG, a finer and more uniform cell structure was achieved in the cellulosic fiber composite foams. The improved foam morphology was attributed to the cell nucleating effects of the fibers and the increased melt strength by the addition of cellulosic fibers and by the gas- and fiber- induced crystallization.

Dielectric Breakdown Properties of Polylactic Acid with Spherulite Growth

SUMMARYPolylactic acid (PLA) is a bioplastic and a biomass material that is made from corn starch, and so on. Having studied the potential of PLA as an electrical insulation material, the authors have demonstrated that PLA has high resistivity, high dielectric breakdown strength, and other outstanding electrical insulation properties, equivalent to those of low‐density polyethylene (LDPE) and other petroleum‐based plastics, within a temperature range from room temperature to approximately 70°C. The degree of crystallinity of PLA was increased by heat treatment, and the dielectric breakdown strength (EB) and mechanical properties of the obtained PLA with different spherulite sizes were studied. PLA heat‐treated at a temperature of 120°C or higher showed rapid increases in spherulite size. At both measurement temperatures of 25°C and 80°C, the EB showed rapid falls when the heat treatment temperature was 120°C or higher. At a measurement temperature of 25°C, the tensile strength at break and breaking elongation showed rapid falls when the heat treatment temperature was 120°C or higher.