Non-isothermal crystallization kinetics of poly (lactic acid)/modified carbon black composite

A novel method was employed to modify the surface of carbon black (CB) by an organic small molecule in a Haake Rheomix mixer. Jeziorny equation, the Ozawa model and Mo equation were employed to describe the non-isothermal crystallization process of poly (lactic acid) (PLA), PLA/CB and PLA/modified carbon black (MCB) composites. It is found that the Ozawa model fail to describe the non-isothermal crystallization process for PLA and its composites, while Jeziorny equation and Mo’s theory provide a good fitting. The comparison of crystallization kinetics between PLA/MCB and PLA through Lauritzen–Hoffman model indicates that there appears a transition from regimes II to III in PLA and PLA/MCB. The fold surface free energy σ _e of PLA/MCB composite is higher than that of neat PLA, implying that the existence of nucleating agent is unfavorable for the regular folding of the molecule chain.     

Effect of nucleation and plasticization on the crystallization of poly(lactic acid)

In this paper, different strategies to promote PLA crystallization were investigated with the objective of increasing the crystalline content under typical polymer processing conditions. The effect of heterogeneous nucleation was assessed by adding talc, sodium stearate and calcium lactate as potential nucleating agents. The PLA chain mobility was increased by adding up to 10 wt% acetyl triethyl citrate and polyethylene glycol as plasticizers. The crystallization kinetics were studied using DSC analysis under both isothermal and non-isothermal conditions. The isothermal data showed that talc is highly effective in nucleating the PLA in the 80-120 °C temperature range. In the non-isothermal DSC experiments, the crystallinity developed upon cooling was systematically studied at cooling rates of 10, 20, 40, and 80 °C/min. The non-isothermal data showed that the combination of nucleant and plasticizer is necessary to develop significant crystallinity at high cooling rates. The nucleated and/or plasticized PLA samples were injection molded and the effect of mold temperature on crystallinity was determined. It was possible to mold the PLA formulations using mold temperatures either below 40 °C or greater than 60 °C. At low temperature, the molded parts were nearly amorphous while at high mold temperatures, the PLA formulation with proper nucleation and plasticization was shown to achieve crystallinity levels up to 40%, close to the maximum crystalline content of the material. Tensile mechanical properties and temperature resistance of these amorphous and semi-crystalline materials were examined.     

The synthesis of poly (lactic acid)-fulvic acid graft polymer and its effect on the crystallization and performance of poly (lactic acid)

ABSTRACT

The poly (lactic acid)-fulvic acid graft polymer (PLA-FA) was synthesized with lactic acid and fulvic acid (FA). The optimum parameters were determined by orthogonal experiment. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy exhibited that FA was successfully grafted onto PLA. Then, PLA/PLA-FA composites were prepared with PLA-FA as fillers by melt blending. The structure characterization and performance tests demonstrated that PLA-FA effectively enhanced the comprehensive performance of PLA composites. The rheological analysis demonstrated that PLA-FA had plasticization effect. The non-isothermal crystallization kinetics demonstrated that PLA-FA promoted the crystallization rate of PLA composites, improving toughness of PLA composites.     

Nonisothermal crystallization kinetics and morphology of poly(ethylene terephthalate) modified with poly(lactic acid)

The nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers modified with poly(lactic acid) (PLA) were investigated with differential scanning calorimetry, and a crystal morphology of the samples was observed with scanning electron microscopy. Waste PET (P100) obtained from postconsumer water bottles was modified with a low‐molecular‐weight PLA. The PET/PLA weight ratio was 90/10 (P90) or 50/50 (P50) in the modified samples. The nonisothermal melt‐crystallization kinetics of the modified samples were compared with those of P100. The segmented block copolymer structure (PET‐b‐PLA‐b‐PET) of the modified samples formed by a transesterification reaction between the PLA and PET units in solution and the length of the aliphatic and aromatic blocks were found to have a great effect on the nucleation mechanism and overall crystallization rate. On the basis of the results of the crystallization kinetics determined by several models (Ozawa, Avrami, Jeziorny, and Liu–Mo) and morphological observations, the crystallization rate of the samples decreased in the order of P50 > P90 > P100, depending on the amount of PLA in the copolymer structure. However, the apparent crystallization activation energies of the samples decreased in the order of P90 > P100 > P50. It was concluded that the nucleation rate and mechanism were affected significantly by the incorporation of PLA into the copolymer structure and that these also had an effect on the overall crystallization energy barrier. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

The electrical properties and crystallization of stereocomplex poly(lactic acid) filled with carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) filled poly(l-lactic acid) (PLLA) and PLLA/poly(d-lactic acid) (PDLA) composites were prepared through a directly melt mixing process. A special crystalline structure of stereocomplex was formed by PLLA and PDLA, which was easily found when mixing two polymers with identical chemical composition but different steric structures. The electrical conductivities were greatly improved by the formation of stereocomplex compared to that of PLLA/MWCNT composites at same MWCNT content. The percolation threshold of the PLLA/PDLA/MWCNT composite at a PLLA/PDLA weight ratio of 50/50 was 0.35 wt%, while being 1.43 wt% of PLLA/MWCNT composites. The X-ray diffraction, non-isothermal and isothermal crystallization results showed that the formation of stereocomplex greatly increased the crystallinity of the composites, meanwhile MWCNTs acted as heterogeneous nucleating agent, which significantly accelerated the nucleation and spherulite growth. Therefore, the PLLA/PDLA/MWCNT composites have a very low percolation threshold due to the volume exclusion effect.     

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     

Relationship between the crystallization behavior of poly(ethylene glycol) and stereocomplex crystallization of poly(L‐lactic acid)/poly(D‐lactic acid)

Poly(l ‐lactic acid) (PLLA) is a good biomedical polymer material with wide applications. The addition of poly(ethylene glycol) (PEG) as a plasticizer and the formation of stereocomplex crystals (SCs) have been proved to be effective methods for improving the crystallization of PLLA, which will promote its heat resistance. In this work, the crystallization behavior of PEG and PLLA/poly(d ‐lactic acid) (PDLA) in PLLA/PDLA/PEG and PEG‐b‐PLLA/PEG‐b‐PDLA blends has been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both SCs and homocrystals (HCs) were observed in blends with asymmetric mass ratio of PLLA/PDLA, while exclusively SCs were observed in blends with approximately equal mass ratio of PLLA/PDLA. The crystallization of PEG was only observed for the symmetric blends of PLLA_39k/PDLA_35k/PEG_2k, PLLA_39k/PDLA_35k/PEG_5k, PLLA_69k/PDLA_96k/PEG_5k and PEG‐b‐PLLA_31k/PEG‐b‐PDLA_27k, where the mass ratio of PLLA/PDLA was approximately 1/1. The results demonstrated that the formation of exclusively SCs would facilitate the crystallization of PEG, while the existence of both HCs and SCs could restrict the crystallization of PEG. The crystallization of PEG is related to the crystallinity of PLLA and PDLA, which will be promoted by the formation of SCs. © 2017 Society of Chemical Industry     

不同构型聚乳酸共混体系的立构复合结晶研究进展

左旋聚乳酸(PLLA)和右旋聚乳酸(PDLA)在共混体系中可形成立构复合(sc)结晶,与聚乳酸(PLA)同质结晶材料相比,sc 结晶材料具有良好的耐热性和耐化学稳定性。因此,sc 结晶是改善PLA 综合性能的一种有效手段。但在PLLA/PDLA 共混体系中,存在各自的同质结晶与两者之间sc 结晶的竞争,所以制备高耐热sc 型PLA 材料的关键之一是理解其sc 结晶的形成条件与机理,进而调控和促进其sc 结晶程度。在PLLA/PDLA 共混物中,sc 结晶受聚合物化学结构、结晶与加工条件等诸多因素影响,其影响规律和机理较复杂。根据PLLA/PDLA共混物sc 结晶行为影响因素的不同,从聚合物分子量、立构规整性、共混比例、分子链拓扑结构、结晶方式与条件、加工助剂和其他组分加入6 个方面出发,详细综述了PLLA/PDLA 共混物sc 结晶及其sc 材料制备的研究进展,以期为高耐热生物基PLA 材料的加工制备提供指导。     

Poly(lactic acid) crystallization

Poly(lactic acid) is a biobased and compostable thermoplastic polyester that has rapidly evolved into a competitive commodity material over the last decade. One key bottleneck in extending the use of PLA is the control of its crystallinity. Understanding the crystallization behavior is particularly crucial to control PLA's degradation rate, thermal resistance as well as optical, mechanical and barrier properties. PLA crystallization has also been a particularly rich topic from a fundamental point of view because of the existence of the two enantiomeric forms of lactic acid that can be used to control the crystallization rate but also to form high melting point stereocomplex structures. This article presents an overview of the current understanding on the fundamentals of PLA crystallization in quiescent conditions and on the practical means to enhance its rate. Data from the abundant literature on PLA crystallization were compiled and analyzed to provide comprehensive relationships between crystallization kinetics and the main molecular structure characteristics of PLA. In addition, the most promising efforts in enhancing PLA crystallization kinetics through plasticization or heterogeneous nucleation were reviewed.     

Peculiar crystallization kinetics of biodegradable poly(lactic acid)/poly(propylene carbonate) blends

Binary blends of poly(lactic acid) (PLA) and poly(propylene carbonate) (PPC) were found to display a peculiar crystallization kinetics. The two biodegradable polymers were blended by melt mixing, to obtain binary blends at various compositions. Temperature‐modulated calorimetry and dynamic‐mechanical analysis indicated that the blend components are partially miscible, and display two separate glass transitions, at temperatures intermediate to those of the plain polymers. Electron microscopy analysis disclosed the morphology of PLA/PPC blends, made of PPC‐rich particles finely dispersed within the PLA‐rich matrix. The possible establishment of interactions between the functional groups of the two polymers upon melt mixing has been hypothesized as the reason for partial miscibility and compatibility of the two biodegradable polymers. The PLA/PPC blends display good mechanical properties, with enhanced performance at rupture compared with plain PLA. Most importantly, the addition of PPC affects also the crystallization kinetics of PLA, since the more mobile PPC chains favor diffusion of the stiffer PLA chain segments towards the growing crystals, which fastens the spherulite growth rate of PLA. Such positive influence of an amorphous polymer on crystal growth rate has been demonstrated here for the first time in blends that display phase‐separation in the melt. POLYM. ENG. SCI., 55:2698–2705, 2015. © 2015 Society of Plastics Engineers     

Preparation of poly(lactic acid) and modified starch composites

In this article, the pretreatment before extrusion between the PLA and the modified starch was researched. The research on the composites of PLA and the modified starch focused on improving its compatibility and mechanical properties. The pretreatment samples were characterized by solid ¹³C-NMR and the chemical titration of the carboxyl end (–COOH) groups in PLA. The results illuminated that the pretreatment reaction was successful and the –COOH in PLA had reacted with the –OH in modified starch. The compatibility of the composites was determined by differential scanning calorimetry and scanning electron microscopy. The results showed that pretreatment could improve the compatibility of the composites. The mechanical properties of the composites were also enhanced. This approach is identified as a reasonable method to produce commercial PLA/modified starch composites with economical feasibility.     

The effect of poly(ethylene glycol) mixed with poly(l‐lactic acid) on the crystallization characteristics and properties of their blends

The non‐isothermal and isothermal crystallizations of extruded poly(l ‐lactic acid) (PLLA) blends with 10, 20 and 30 wt% poly(ethylene glycol) (PEG) were investigated with differential scanning calorimetry. The formation of α‐form crystals in the blend films was verified using X‐ray diffraction and an increase in crystallinity indexes using Fourier transformation infrared spectroscopy. Crystallization and melting temperatures and crystallinity of PLLA increased with decreasing cooling rate (CR) and showed higher values for the blends. Although PLLA crystallized during both cooling and heating, after incorporation of PEG and with CR = 2 °C min^?1 its crystallization was completed during cooling. Increasingly distinct with CR, a small peak appeared on the lower temperature flank of the PLLA melting curve in the blends. A three‐dimensional nucleation process with increasing contribution from nuclei growth at higher CR was verified from Avrami analysis, whereas Kissinger's method showed that the diluent effect of 10 and 20 wt% PEG in PLLA decreased the effective energy barrier. During isothermal crystallization, crystallization half‐time increased with temperature (T_ic) for the blends, decreased with PEG content and was lower than that of pure PLLA. In addition, the Avrami rate constants were significantly higher than those of pure PLLA, at the lower T_ic. Different crystal morphologies in the PLLA phase were formed, melting in a broader and slightly higher T_m range than pure PLLA. The crystallization activation energy of PLLA decreased by 56% after the addition of 10 wt% PEG, increasing though with PEG content. Finally, PEG/PLLA blends presented improved flexibility and hydrophilicity. © 2019 Society of Chemical Industry     

Effect of aliphatic diacyl adipic dihydrazides on the crystallization of poly(lactic acid)

A series of aliphatic diacyl adipic dihydrazides (ADHs) with different alkyl moieties were synthesized by the reaction between adipic dihydrazide and acyl chloride. Then these ADHs were solution blended with PLA respectively and were evaluated as nucleating agents. Through the investigation of nonisothermal and isothermal crystallization, it was found that both the crystallization rate and the crystallinity of PLA could be enhanced by adding only 1 wt % of ADHs. Especially for ADH‐Oc (ADH‐Octyl), the crystallization rate of PLA increased about 4 times at 105°C. Optical morphology showed that and the size of PLA spherulites decreased and the nucleation density increased with the existence of ADH‐Oc. Meanwhile, the crystal structure of PLA were not discerniblly affected after the addition of ADHs as found by wide‐angle X‐ray diffraction. Thus, this study suggested these ADHs compounds are simple and potential nucleating agents to enhance crystallization ability of PLA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42028.     

The effect of ionic interaction on the miscibility and crystallization behaviors of poly(ethylene glycol)/poly(L‐lactic acid) blends

The effect of end groups (2NH₂) of poly(ethylene glycol) (PEG) on the miscibility and crystallization behaviors of binary crystalline blends of PEG/poly(L ‐lactic acid) (PLLA) were investigated. The results of conductivity meter and dielectric analyzer (DEA) implied the existence of ions, which could be explained by the amine groups of PEG gaining the protons from the carboxylic acid groups of PLLA. The miscibility of PEG(2NH₂)/PLLA blends was the best because of the ionic interaction as compared with PEG(2OH, 1OH‐1CH₃, and 2CH₃)/PLLA blends. Since the ionic interaction formed only at the chain ends of PEG(2NH₂) and PLLA, unlike hydrogen bonds forming at various sites along the chains in the other PEG/PLLA blend systems, the folding of PLLA blended with PEG(2NH₂) was affected in a different manner. Thus the fold surface free energy played an important role on the crystallization rate of PLLA for the PEG(2NH₂)/PLLA blend system. PLLA had the least fold surface free energy and the fast crystallization rate in the PEG(2NH₂)/PLLA blend system, among all the PEG/PLLA systems studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dyeing poly(lactic acid) fibres in supercritical carbon dioxide

A study has been conducted into the dyeing of poly(lactic acid) fibres in supercritical carbon dioxide. The fibres were completely dyed using disperse dyes at 50 °C as shown by fibre cross-sections, although high colour depths in dark shades still prove challenging. Dye uptake increased significantly at temperatures ≥80 °C. At 95 °C in supercritical carbon dioxide, shrinkage and hardening of raw poly(lactic acid) were observed which could partly be overcome by the supercritical carbon dioxide extraction step. Afterclearing with cold supercritical carbon dioxide (to remove unfixed dye after dyeing) decreased the colour depth and led to non-uniform dyeing results on poly(lactic acid). Wash and rub fastness was good to very good also when poly(lactic acid) was not aftercleared in supercritical carbon dioxide. Fibre damage and elongation at break in supercritical carbon dioxide were similar to water.     

分子筛改性可降解材料聚乳酸的研究

将吸附有聚乳酸齐聚物的分子筛同工业级聚乳酸在双螺杆挤出机中造粒,制备了分子筛改性聚乳酸材料。研究了分子筛对聚乳酸力学性能的影响,添加量在7%~9%左右时可以起到最好的增强效果;复合材料可以在土壤中自然降解,增加分子筛添加量可以加快复合材料的降解。     

Cold crystallization behavior of glassy poly(lactic acid) prepared by rapid compression

The cold crystallization behavior of glassy poly(lactic acid) was studied by comparison among samples obtained through three different treatments, which are naturally cooling (NC), liquid nitrogen quenching (NQ), and rapidly compressing (RC). The crystallization kinetics and structural changes of these glassy samples were analyzed by using in situ Fourier transform infrared (FTIR) and in situ wide angle X‐ray diffraction measurements. The results reveal that, the cold crystallization behavior is very similar between NC and NQ samples, but RC sample exhibits higher crystallization rate and lower crystallization temperature than them. FTIR investigation showed that, during the heating process for all the glassy samples, conformational adjustment in the amorphous phase occurs first, and followed by formation of the disordered crystals and then the disordered crystals further perfecting, while for RC sample the onset temperature of each step is much lower. Furthermore, for RC sample, the crystal grain number is larger and its initial α ′ form crystal induced by the heating is higher ordered than that of the others, and this unique crystallization behavior might be caused by the local ordered domains formed during the RC process. POLYM. ENG. SCI., 55:359–366, 2015. © 2014 Society of Plastics Engineers     

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