Effect of the melting temperature on the crystallization behavior of a poly(l‐lactide)/poly(d‐lactide) equimolar mixture

The effect of the final melting temperature (T_f) on the crystallization of poly(l ‐lactide) (PLLA)/poly(d ‐lactide) (PDLA) was studied via a combination of differential scanning calorimetry, wide‐angle X‐ray scattering, polarized optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. We observed that a residual stereocomplex (SC) crystal induced the formation of SC crystals during cooling from a T_f (230°C) just above the melting peak of the SC crystals. On cooling from a T_f (240°C) just above the endset temperature of SC crystal melting [T_m(S)(E)], the possible order structure and the strong interchain interaction promoted the preferential crystallization of SC crystals; this enhanced the formation of α crystals. During cooling from a T_f (≥250°C) far above T_m(S)(E), the crystallization peaks of α and SC crystals converged. The FTIR results indicated that the residual SC crystals, possible ordered structure, and interchain interactions in the melt might have been the key factors for the different crystallization of PLLA/PDLA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43015.     

Effect of nucleating agents on the crystallization behavior and heat resistance of poly(l‐lactide)

Poly (l ‐lactide) (PLLA) blends with various nucleators were prepared by melt processing. The effect of different nucleators on the crystallization behavior and heat resistance as well as thermomechanical properties of PLLA was studied systematically by differential scanning calorimetry, X‐ray diffraction, heat deflection temperature tester, and dynamic mechanical analysis. It was found that poly(d ‐lactide), talcum powder (Talc), a multiamide compound (TMC‐328, abbreviated as TMC) can significantly improve the crystallization rate and crystallinity of PLLA, thus improving thermal–resistant property. The heat deflection temperature of nucleated PLLA can be as high as 150°C. The storage modulus of nucleated PLLA is higher than that of PLLA at the temperature above T_g of PLLA. Compared with other nucleating agents, TMC was much more efficient at enhancing the crystallization of PLLA and the PLLA containing TMC showed the best heat resistance. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42999.     

Effect of crystallization temperature on crystal modifications and crystallization kinetics of poly(L‐lactide)

The crystalline structure of poly(L ‐lactide) (PLLA) have been found to quite depend on the crystallization temperatures (T_cs), especially in the range of 100?120°C, which is usually used as the crystallization temperature for the industrial process of PLLA. The analysis of wide‐angle X‐ray diffraction and Fourier transformed infrared spectroscopy revealed that 110°C is a critical temperature for PLLA crystallization. At T_c < 110°C and T_c ≥ 110°C, the α′ and α crystals were mainly produced, respectively. Besides, the structural feature of the α′‐form was illustrated, and it was found that the α′‐form has the larger unit cell dimension than that of the α‐form. Moreover, the crystallization kinetics of the α′ and α crystals are different, resulting in the discontinuousness of the curves of spherulite radius growth rate (G) versus T_c and the half time in the melt‐crystallization (t_1/2) versus T_c investigated by Polarized optical microscope and Differential scanning calorimetry, respectively. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Non‐isothermal crystallization kinetics of poly(L‐lactide)

The non‐isothermal crystallization kinetics of poly( L ‐lactide) (PLLA) in comparison with a polylactide stereocopolymer (PLA₉₈) containing 98% L ‐lactyl and 2% D ‐lactyl units were investigated using differential scanning calorimetry to examine the effect of the configurational structure. Avrami, Ozawa and Liu models were applied to describe the crystallization process. The Avrami analysis exhibited two stages in non‐isothermal crystallization, while the Ozawa and Liu models did not successfully describe the crystallization behaviour. The activation energy was calculated with Kissinger's method. The energy barrier was found to be the same for PLLA and PLA₉₈ with a value of 126 kJ mol^?1. Copyright © 2010 Society of Chemical Industry     

Synergistic effect of poly(ethylene glycol) and graphene oxides on the crystallization behavior of poly(l‐lactide)

In this article, we report the combined effects of poly(ethylene glycol) (PEG) and/or graphene oxides (GOs) on the crystallization behavior of poly(l ‐lactide) (PLLA) under different crystallization conditions, such as nonisothermal crystallization, isothermal crystallization, and annealing‐induced cold crystallization. Differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, and polarized optical microscopy were used to study the crystallization kinetics and crystallinity to illustrate the effects of PEG and/or GOs on the crystallization behavior of PLLA. The results show that PEG functioned as a plasticizer and improved the chain mobility of /PLLA during crystallization and the GOs acted as efficient nucleation agents and accelerated the crystallization rate. Finally, both PEG and GOs improved the crystallization ability of PLLA. Importantly, the simultaneous addition of PEG and GOs led to a synergistic effect on the crystallization behavior of PLLA under all conditions. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3498–3508, 2013     

Disclosing the crystallization behavior and morphology of poly(ϵ‐caprolactone) within poly(ϵ‐caprolactone)/poly(l‐lactide) blends

In this work, the effect of poly(l ‐lactide) (PLLA) components on the crystallization behavior and morphology of poly(?‐caprolactone) (PCL) within PCL/PLLA blends was investigated by polarized optical microscopy, DSC, SEM and AFM. Morphological results reveal that PCL forms banded spherulites in PCL/PLLA blends because the interaction between the two polymer components facilitates twisting of the PCL lamellae. Additionally, the average band spacing of PCL spherulites monotonically decreases with increasing PLLA content. With regard to the crystallization behaviors of PCL, the crystallization ability of PCL is depressed with increase of the PLLA content. However, it is interesting to observe that the growth rate of PCL spherulites is almost independent of the PLLA content while the overall isothermal crystallization rate of PCL within PCL/PLLA blends decreases first and then increases at a given crystallization temperature, indicating that the addition of PLLA components shows a weak effect on the growth rate of the PCL but mainly on the generation of nuclei. © 2018 Society of Chemical Industry     

Hydrolytic degradation and crystallization behavior of linear 2‐armed and star‐shaped 4‐armed poly(l‐lactide)s: Effects of branching architecture and crystallinity

“Linear” aliphatic polyesters composed of two poly(l ‐lactide) arms attached to 1,3‐propanediol and “star‐shaped” ones composed of four poly(l ‐lactide) arms attached to pentaerythritol (2‐L and 4‐L polymers, respectively) with number‐average molecular weight (M_n) = 1.4–8.4 × 10⁴g/mol were hydrolytically degraded at 37°C and pH = 7.4. The effects of the branching architecture and crystallinity on the hydrolytic degradation and crystalline morphology change were investigated. The degradation mechanism of initially amorphous and crystallized 2‐L polymers changed from bulk degradation to surface degradation with decreasing initial M_n; in contrast, initially crystallized higher molecular weight 4‐L polymer degraded via bulk degradation, while the degradation mechanism of other 4‐L polymers could not be determined. The hydrolytic‐degradation rates monitored by molecular‐weight decreases decreased significantly with increasing branch architecture and/or higher number of hydroxyl groups per unit mass. The hydrolytic degradation rate determined from the molecular weight decrease was higher for initially crystallized samples than for initially amorphous samples; however, that of 2‐L polymers monitored by weight loss was larger for initially amorphous samples than for initially crystallized samples. Initially amorphous 2‐L polymers with an M_n below 3.5 × 10⁴g/mol crystallized during hydrolytic degradation. In contrast, the branching architecture disturbed crystallization of initially amorphous 4‐L polymers during hydrolytic degradation. All initially crystallized 2‐L and 4‐L polymers had δ‐form crystallites before hydrolytic degradation, which did not change during hydrolytic degradation. During hydrolytic degradation, the glass transition temperatures of initially amorphous and crystallized 2‐L and 4‐L polymers and the cold crystallization temperatures of initially amorphous 2‐L and 4‐L polymers showed similar changes to those reported for 1‐armed poly(l ‐lactide). © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41983.     

Effect of a small amount of poly(3‐hydroxybutyrate) on the crystallization behavior of poly(l‐lactic acid) in their immiscible and miscible blends during physical aging

The miscibility and effect of physical aging on the crystallization behavior of poly(l ‐lactic acid) (PLLA)/poly(3‐hydroxybutyrate) (PHB) blends with a small amount of PHB (≤10 wt%) have been investigated using differential scanning calorimetry and Fourier transform infrared spectroscopy. It is found that the miscibility of PLLA/PHB blends with a very small percentage of PHB can be modulated by varying the molecular weight of the PHB. That is, a PLLA/PHB blend with low‐molecular‐weight PHB is miscible, whereas that with high‐molecular‐weight PHB is immiscible. It is found that physical aging at temperatures far below the glass transition temperature can promote the cold crystallization kinetics of PLLA in PLLA/PHB blends with high‐molecular‐weight PHB rather than in those with low‐molecular‐weight PHB. These findings suggest that the effect of physical aging on the crystallization behavior of the main component in a crystalline/crystalline blend with a small percentage of the second component is strongly dependent on the miscibility of the blend system. Enhanced chain mobility of PLLA in the interface region of PLLA matrix and PHB micro‐domains is proposed to explain the physical aging‐enhanced crystallization rate in immiscible PLLA/PHB blends with high‐molecular‐weight PHB. © 2013 Society of Chemical Industry     

Physical properties and crystallization behavior of poly(lactide)/poly(methyl methacrylate)/silica composites

Poly(lactide)/poly(methyl methacrylate)/silica (PLA/PMMA/SiO₂) composites were fabricated using a twin‐screw extruder. Nanosilica particles were incorporated to improve the toughness of the brittle PLA, and a chain extender reagent (Joncryl ADR 4368S) was used to reduce the hydrolysis of the PLA during fabrication. Highly transparent PLA and PMMA were designated to blend to obtain the miscible and transparent blends. To estimate the performance of the PLA/PMMA/SiO₂ composites, a series of measurements was conducted, including tensile and Izod impact tests, light transmission and haze measurements, thermomechanical analysis, and isothermal crystallization behavior determination. A chain extender increases the ultimate tensile strength of the PLA/PMMA/SiO₂ composites by ～43%, and both a chain extender and nanosilica particles increase Young's modulus and Izod impact strength of the composites. Including 0.5 wt % nanosilica particles increase the elongation at break and Izod impact strength by ～287 and 163%, respectively, compared with those of the neat PLA. On account of the mechanical performances, the optimal blending ratio may be between PLA/PMMA/SiO₂ (90/10) and PLA/PMMA/SiO₂ (80/20). The total light transmittance of the PLA/PMMA/SiO₂ composites reaches as high as 91%, indicating a high miscible PLA/PMMA blend. The haze value of the PLA/PMMA/SiO₂ composites is less than 35%. Incorporating nanosilica particles can increase the crystallization sites and crystallinities of the PLA/PMMA/SiO₂ composites with a simultaneous decrease of the spherulite dimension. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42378.     

Effect of poly(l‐lactide)/poly(d‐lactide) block length ratio on crystallization behavior of star‐shaped asymmetric poly(l(d)‐lactide) stereoblock copolymers

Effect of Poly(l ‐lactide)/Poly(d ‐lactide) (PLLA/PDLA) block length ratio on the crystallization behavior of star‐shaped poly(propylene oxide) block poly(d ‐lactide) block poly (l ‐lactide) (PPO–PDLA–PLLA) stereoblock copolymers with molecular weights (M_n) ranging from 6.2 × 10⁴ to 1.4 × 10⁵ g mol^?1 was investigated. Crystallization behaviors were studied utilizing differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). Only stereocomplex crystallites formed in isothermal crystallization at 140 to 156°C for all samples. On one hand, the overall crystallization rate decreased as PLLA/PDLA block length ratio increased. As PLLA/PDLA block length ratio increased from 7:7 to 28:7, the value of half time of crystallization (t_1/2) delayed form 2.85 to 5.31 min at 140°C. On the other hand, according to the Lauritzen–Hoffman theory, the fold‐surface energy (σ_e) was calculated. σ_e decreased from 77.7 to 73.3 erg/cm² with an increase in PLLA/PDLA block length ratio. Correspondingly increase in nucleation density was observed by the polarized optical microscope. Results indicated that the PLLA/PDLA block length ratio had a significant impact on the crystallization behavior of PPO–PDLA–PLLA copolymers. POLYM. ENG. SCI., 55:2534–2541, 2015. © 2015 Society of Plastics Engineers     

聚乙二醇增塑聚乳酸的非等温结晶动力学研究

采用DSC方法对聚乙二醇(PEG)增塑聚乳酸的非等温结晶动力学进行了研究。结果表明,PEG的加入明显提高了聚乳酸的结晶速度。对所得数据分别用Ozawa方程和莫志深方法进行了处理,发现在给定温度范围里非等温结晶时,PLA/PEG主要是以均相成核的三维生长方式结晶;PLA的结晶速度随着PEG分子质量的增加而升高。     

Effect of the ionic liquid [bmim]PF6 on the nonisothermal crystallization kinetics behavior of poly(ether‐b‐amide)

Blending ionic liquid with crystalline polymer permits the design of new high‐performance composite materials. The final properties of these materials are critically depended on the degree of crystallinity and the nature of crystalline morphology. In this work, nonisothermal crystallization behavior of poly(ether‐b‐amide) (Pebax®1657)/room temperature ionic liquid (1‐butyl‐3‐methylimidazolium hexafluorophosphate, [bmim]PF₆) was investigated by differential scanning calorimetry. The presence of [bmim]PF₆ can retard the nucleation of Pebax®1657 and lead to the crystallization depression of the PA block and the crystalline disappearance of the PEO block. However, the dilution effect of the IL results in a higher growth rate of crystallization of PA block. The influence of [bmim]PF₆ content and cooling rate on crystallization mechanism and spherulitc structures was determined by the Avrami equation modified by Jeziorny and Mo's methods, whereas the Ozawa's approach fails to describe the nonisothermal crystallization behavior of Pebax®1657/[bmim]PF₆ blends. In the modified Avrami analysis, the Avrami exponent of PA blocks, n > 3, for pure Pebax®1657, while 3 > n > 2 for Pebax®1657/[bmim]PF₆ blends testifies the transformation of crystallization growth pattern induced by [bmim]PF₆ from three‐dimensional growth of spherulites to a combination of two‐ and three‐dimensional spherulitic growth. Further, lower activation energy for the nonisothermal crystallization of PA blocks of Pebax®1657 can be observed with the increase of [bmim]PF₆ content. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42137.     

Microstructural characteristics and crystallization behaviors of poly(l‐lactide) scaffolds by thermally induced phase separation

In this study, poly(l ‐lactic acid) (PLLA) was prepared by four typical approach systems, namely, solid–liquid phase‐separation processes from PLLA–dioxane at ?80°C, PLLA–dioxane–water at ?80°C, PLLA–tetrahydrofuran (THF) at ?80°C, and PLLA–THF at 18°C. The microstructural characteristics and crystallization behaviors of PLLA were investigated by scanning electron microscopy, differential scanning calorimetry, X‐ray diffraction, and Fourier transform infrared spectroscopy. In the PLLA–dioxane binary system and PLLA–dioxane–water ternary system, the solvent froze immediately after quenching to a low temperature, and this restricted the PLLA chain arrangement. Thus, the PLLA amorphous phase dominated in the scaffolds, and solid‐walled structures were produced. THF was liquid throughout the entire process, which enabled free PLLA chain arrangement and further crystallization. Single crystals aggregated by crystal nucleation and growth at a critical temperature (T_c) of 18°C; this resulted in its most common and stable polymorph, the α form. However, α′‐form crystals, which were assumed to be limit‐disordered crystals of the α form, were produced at a low T_c (?80°C). Scaffolds with a plateletlike structure were produced at a T_c of 18°C, whereas a nanofibrous network was obtained at ?80°C. PLLA crystallization competed with phase separation; thus, the crystal structure and scaffold morphology depended on the codevelopment of these two processes. Finally, the effects of the scaffold morphologies on the cell behaviors were studied, and the nanofibrous scaffold was found to have better cell adhesion and viability than the other three scaffolds. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39436.     

Nonisothermal crystallization behavior of poly (butylene terephthalate)–poly (ethylene terephthalate‐co‐isophthalate‐co‐sebacate) segmented copolyesters

In this paper, two different analytical methods were applied to investigate nonisothermal crystallization behavior of copolyesters prepared by melting transesterification processing from bulk polyesters involving poly (butylene terephthalate) (PBT) and ternary amorphous random copolyester poly(ethylene terephthalate‐co‐isophthalate‐co‐sebacate) (PETIS). The results show that the half‐time of crystallization of copolyesters depended on the reaction time and decreased with the content of ternary polyesters in the amorphous segment. The modified Avrami model describes the nonisothermal crystallization kinetics very well. The values of the Avrami exponent range from 2.2503 to 3.7632, and the crystallization kinetics constant ranges from 0.0690 to 0.9358, presenting a mechanism of three‐dimensional spherulitic growth with heterogeneous nucleation. Ozawa analysis, however, failed to describe the nonisothermal crystallization behavior of copolyesters, especially at higher cooling rate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1232–1238, 2003     

Effect of plasticizer on the crystallization behavior of poly(lactic acid)

In this article, the spherulitic morphology and growth rate of the neat and plasticized poly(lactic acid) (PLA) with triphenyl phosphate (TPP) were compared and analyzed by polarizing optical microscopy with hot stage at a temperature range of 100?142°C. The spherulitic morphology of the neat PLA underwent a series of changes such as the typical Maltese Cross at less than 132°C, the disappearance of the Maltese Cross at 133°C, the irregular and distorted spherulites at higher than 134 and 142°C, respectively. For plasticized PLA, the spherulitic morphology exhibited the same changes as neat PLA, but these changes were shifted to lower temperature when compared with neat PLA. In the case of the spherulitic growth, neat PLA had the maximum value of 0.28 μm/s at 132°C, and plasticized PLA had higher values than that of neat PLA. Further analysis based on the Lauritzen–Hoffman theory was presented and results showed that the values of nucleation parameter K_g increased with TPP content. The crystallization behavior of PLA was analyzed by differential scanning calorimetry and wide‐angle X‐ray diffraction. The results showed that the degree of crystallinity of plasticized PLA markedly increased when compared with neat PLA sharply with the incorporation of plasticizer. The crystallization kinetics for the neat and plasticized PLA under isothermal crystallization at 114°C was described by the Avrami equation and the Avrami exponent is close to 2, implying that the crystallization mechanism did not change. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Cellulose nanocrystal driven crystallization of poly(d,l‐lactide) and improvement of the thermomechanical properties

The technological exploitation of polylactide in fields requiring wide range of operating conditions is limited by the low crystallization rate of the polymer and therewith the low thermomechanical stability. Here we report the crystallization and consequent improvement of the thermomechanical properties of originally amorphous poly(d,l ‐lactide) (d : l ratio 11 : 89) loaded with cellulose nanocrystals (CNCs). Isothermal treatment of samples with different CNC contents and at various temperatures, showed up to 6 wt % crystalline phase formation, as confirmed by differential scanning calorimetry and X‐ray diffraction measurements. Under a particular set of annealing conditions, CNCs promote the formation of a lamellar structure. This provides the system with higher order and cohesion which in combination with stress‐transfer between CNCs, led to an increase of the storage modulus in the rubbery plateau up to 30 times (from 2.7 MPa up to 79 MPa), a rise of the melting temperature up to 50°C, and an improvement of the Young's modulus up to 40%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41607.     

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     

Accelerated crystallization of poly(L‐lactide) by physical aging

The low‐temperature physical aging of amorphous poly(L ‐lactide) (PLLA) at 25–50°C below glass transition temperature (T_g) was carried out for 90 days. The physical aging significantly increased the T_g and glass transition enthalpy, but did not cause crystallization, regardless of aging temperature. The nonisothermal crystallization of PLLA during heating was accelerated only by physical aging at 50°C. These results indicate that the structure formed by physical aging only at 50°C induced the accelerated crystallization of PLLA during heating, whereas the structure formed by physical aging at 25 and 37°C had a negligible effect on the crystallization of PLLA during heating, except when the physical aging at 37°C was continued for the period as long as 90 days. The mechanism for the accelerated crystallization of PLLA by physical aging is discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Nonisothermal crystallization behavior of a luminescent conjugated polymer,poly(9,9‐dihexylfluorene‐alt‐2,5‐didodecyloxybenzene)

The nonisothermal crystallization kinetics of poly(9,9‐dihexylfluorene‐alt‐2,5‐didodecyloxybenzene) (PF6OC12) from the melt were investigated using differential scanning calorimetry under different cooling rates. Several analysis methods were used to describe the nonisothermal crystallization behavior of PF6OC12. It was found that the modified Avrami method by Jeziorny was only valid for describing the early stage of crystallization but was not able to describe the later stage of PF6OC12 crystallization. Also, the Ozawa method failed to describe the nonisothermal crystallization behavior of PF6OC12. However, the method developed by combining the Avrami and Ozawa equations could successfully describe the nonisothermal crystallization kinetics of PF6OC12. According to the Kissinger method, the activation energy was determined to be 114.9 kJ mol^?1 for the nonisothermal melt crystallization of PF6OC12. Copyright © 2006 Society of Chemical Industry     

The effects on mechanical properties and crystallization of poly (l‐lactic acid) reinforced by cellulosic fibers with different scales

Bacterial cellulose (BC), microcrystalline cellulose (MCC), and bamboo cellulosic fibers (BCFs) were used to reinforce poly(l ‐lactic acid) (PLLA) based bio‐composites. The mechanical properties and crystallization of the composites were studied through mechanical testing, differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and polarizing microscope. The incorporation of all three kinds of cellulose increased the stiffness of the composites compared to pure PLLA. The reinforcing effect of the MCC in the composites is most significant. The Young's modulus and impact toughness of the MCC/PLLA composites were increased by 44.4% and 58.8%, respectively. The tensile strength of the MCC/PLLA composites was increased to 71 MPa from 61 MPa of PLLA. However, the tensile strength of the composites reinforced with BCF or BC was lower than PLLA. The three kinds of cellulosic fibers improved the crystallization of PLLA. The BC with smallest size provided the composites with smallest grain and highest crystallinity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41077.