Carbon dioxide‐induced crystallization in poly(L‐lactic acid) and its effect on foam morphologies

The effect of carbon dioxide (CO₂) on the physical properties of poly(L ‐lactic acid) (PLLA) and on the formation of crystalline domains was investigated. The presence of CO₂ in the matrix was found to induce crystallization in PLLA, with the crystallinity increasing with increasing CO₂ pressure. The combination of saturation conditions and formation of crystalline domains was studied for its effect on the formation of porous morphologies in PLLA. Moreover, the effect of CO₂ on PLLA properties and formation of porous structures was further exploited by first creating crystalline domains in samples using CO₂ at various pressures at 25 °C and then re‐saturating the same samples with CO₂ at a constant pressure of 2.8 MPa and 0 °C. This paper reports on the solubility of CO₂ at 25 and 0 °C in PLLA, crystallization and subsequent effect on foam morphologies when processed using different saturation cycles. Unique and intriguing morphologies were obtained by specifically controlling the properties of PLLA. Copyright © 2010 Crown in the right of Canada. Published by John Wiley & Sons, Ltd     

Formation of poly‐vinyl‐alcohol structures by supercritical CO2

Poly‐vinyl‐alcohol (PVA) porous structures have been prepared using a supercritical phase inversion process in which supercritical carbon dioxide (SC‐CO₂) acts as the nonsolvent. First, we tested the versatility of the SC‐CO₂ phase inversion process, forming PVA/dimethylsulfoxide (DMSO) solutions with polymer concentrations ranging from 1 to 35% (w/w) and changing the process parameters. We worked at temperatures from 35 to 55°C and pressures from 100 to 200 bar obtaining different membranes morphologies: dense films, membranes with coexisting morphologies, and microparticles. However, we did not produce symmetric or asymmetric porous membranes. To obtain this result, we used casting solutions formed by adding acetone to DMSO with the aim of modifying the affinity between SC‐CO₂ and the liquid solvent. In this series of experiments, we obtained asymmetric membranes with skin layer thicknesses lower than 10 μm. The results obtained in this work have been explained considering that the membranes formation mechanism is related to the kinetics of the process; i.e. the affinity between the solvent (mixture of solvents) and SC‐CO₂. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

楝树杀虫成分苦楝素的超临界CO2萃取

苦楝素是楝科植物中制备植物源农药的主要杀虫成分.针对不同种源楝树的皮、叶及果实,采用溶剂浸提法提取筛选苦楝素含量较高的种源作为实验原料,采用乙醇提取,考察液固质量比、提取时间和提取次数等参数对苦楝素提取率的影响.当液固质量比为25∶ 1、提取时间为4 h、提取次数为3次时,苦楝素的提取率达18%.进一步采用超临界CO2...     

Controlling sandwich‐structure of PET microcellular foams using coupling of CO2 diffusion and induced crystallization

Controlling sandwich‐structure of poly(ethylene terephthalate) (PET) microcellular foams using coupling of CO₂ diffusion and CO₂‐induced crystallization is presented in this article. The intrinsic kinetics of CO₂‐induced crystallization of amorphous PET at 25°C and different CO₂ pressures were detected using in situ high‐pressure Fourier transform infrared spectroscopy and correlated by Avrami equation. Sorption of CO₂ in PET was measured using magnetic suspension balance and the diffusivity determined by Fick's second law. A model coupling CO₂ diffusion in and CO₂‐induced crystallization of PET was proposed to calculate the CO₂ concentration as well as crystallinity distributions in PET sheet at different saturation times. It was revealed that a sandwich crystallization structure could be built in PET sheet, based on which a solid‐state foaming process was used to manipulate the sandwich‐structure of PET microcellular foams with two microcellular or even ultra‐microcellular foamed crystalline layers outside and a microcellular foamed amorphous layer inside. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2512–2523, 2012     

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     

Reaction kinetics and mechanism of catalyzed hydrolysis of waste PET using solid acid catalyst in supercritical CO2

Hydrolysis of waste poly(ethylene terphthalate) (PET) using solid acid catalyst in SCCO₂ is presented in this work for the first time. The mechanism of PET chains scission was proved to be a combination of chain end and random chain scission by Fourier transform ‐ infrared spectroscopy (FT‐IR) and titration analysis. A new reaction kinetics model of PET hydrolysis in SCCO₂ was setup by introducing the Arrhenius equation into an ordinary reaction rate equation, the frequency factor and apparent activation energy were expressed in terms of temperature and CO₂ pressure, respectively. With this reaction kinetics model, the effects of temperature, and pressure were investigated. An interesting mechanism was proposed to describe the reaction process that both water molecules and hydroniums were carried and penetrated into the amorphous regions of the swollen PET by SCCO₂, subsequently hydrolysis reaction preferentially took place in the amorphous regions of both surface and bulk of PET matrix. © 2014 American Institute of Chemical Engineers AIChE J, 61: 200–214, 2015     

Effect of supercritical CO2 on three‐dimensional colloid arrays of PS‐MA–EGDMA

Using emulsifier‐free emulsion polymerization method, monodispersed crosslinked poly(styrene‐co‐methacrylic acid‐co‐ethylene glycol dimethacrylate) colloid microspheres were synthesized. The microspheres were treated in supercritical carbon dioxide (SCCO₂) after they have self‐assembled into ordered three‐dimensional (3D) colloid arrays. The CO₂ absorbed into the polymer microspheres enhances chain segments mobility and reduces the glass transition temperature (T_g) of polymer, which induce the microspheres coalesce at the relatively low temperature. The coalescence degree of microspheres was studied by varying the experimental temperature, pressure, and exposure time in SCCO₂. The results were shown by scanning electron microscopy (SEM). Further, the lattice spacing of the assembled 3D colloid microspheres was calculated from the SEM images. It was illustrated that the coalescence degree enhanced with the increase of CO₂ pressure; however, this tendency became weak when CO₂ pressure reached a certain value. Extending exposure time in SCCO₂ or elevating temperature can also increase coalescence degree, and the effect of temperature is more significant. It is believed that these results will make sense when the polymer microspheres are considered to be used as templates in SCCO₂; meanwhile, it raises a new method about tuning the final morphology of the stabilized colloidal crystals and porous materials via controlling the coalescence degree with the assistance of SCCO₂. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Low‐temperature clean preparation of poly(lactic acid) foams by combining ethyl lactate and supercritical CO2: correlation between processing and foam pore structure

This study reports the low‐temperature and clean fabrication of porous poly(lactic acid) (PLA) through solid‐state foaming using various mixtures of ethyl lactate (EL) and supercritical CO₂ (scCO₂) as the blowing agent. Results showed that adding a small amount of EL (up to 0.2% molar fraction) to scCO₂ enhanced the plasticizing effect of the blowing agent mixture. As a direct consequence, at an operating temperature of 35 °C, PLA foams could be manufactured with homogeneous morphology, density as low as 0.09 ± 0.01 g cm^?3, mean pore sizes up to 519.0 ± 205.0 µm and pore densities in the range 2.0 × 10⁵ to 3.4 × 10⁸ pores cm^?3. Conversely, at a temperature of 40 °C, an increase of plasticizer concentration in the blowing agent mixture up to 0.2% promoted the crystallization of the polymer during sorption stage and, consequently, foaming was slightly reduced. © 2013 Society of Chemical Industry     

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     

Non‐Isothermal Crystallization Behavior of Poly(L‐lactic acid) in the Presence of Various Additives

Summary: The effects of various additives: poly(D ‐lactic acid) (PDLA), talc, fullerene C₆₀, montmorillonite, and various polysaccharides, on the non‐isothermal crystallization behavior of poly(L ‐lactic acid) (PLLA), during both the heating of melt‐quenched films from room temperature, and the cooling of as‐cast films from the melt, was investigated. When the melt‐quenched PLLA films were heated from room temperature, the overall PLLA crystallization was accelerated upon addition of PDLA or the stereocomplex crystallites formed between PDLA and PLLA, the mixtures containing PDLA, and the mixture of talc and montmorillonite. No significant effects on the overall PLLA crystallization were observed for talc, C₆₀, montmorillonite, and the mixtures containing C₆₀. Such rapid completion of the overall PLLA crystallization upon addition of the aforementioned additives can be ascribed to the increased density (number per unit volume or area) of PLLA spherulites. When the as‐cast PLLA films were cooled from the melt, the overall PLLA crystallization completed rapidly, upon addition of PDLA, talc, C₆₀, montmorillonite, and their mixtures. Such rapid overall PLLA crystallization is attributable to the increased density of the PLLA spherulites and the higher nucleation temperature for PLLA crystallization. In contrast, the addition of various polysaccharides has no significant effect, or only a very small effect, on the overall PLLA crystallization during heating from room temperature or during cooling from the melt. This finding means that the polysaccharides can be utilized as low‐cost fillers for PLLA‐based materials, without disturbing the crystallization of the PLLA. The effect of additives in accelerating the overall PLLA crystallization during cooling from the melt, decreased in the following order: PDLA > talc > C₆₀ > montmorillonite > polysaccharides.

Polarization optical photomicrographs of pure PLLA, and the PLLA‐F film, with the fullerene additive, during cooling from the melt (Process IIB). Both of the photomicrographs were taken at 120 °C.     

CO2‐Induced Mechanical Reinforcement of Polyolefin‐Based Nanocellular Foams

The effect of CO₂‐induced crystallization on the mechanical properties, in particular the yield and the ultimate stresses, of polyolefins is studied. PP and SEBS copolymer blends are used as examples and foamed after sorption of CO₂ at temperatures below T_m. CO₂ sorption thickens the crystalline lamellae and consequently increases T_m from 160 to 178 °C for both pure PP and PP/SEBS blend systems. Foams with an average cell size smaller than 250 nm retain the ultimate stress at the level of the polymer before foaming, even without the effect of CO₂‐induced crystallization. Including CO₂‐induced crystallization, the yield and the ultimate stresses of the foam can be improved by 30 and 50% over solid PP and by 22 and 40%, for solid PP/SEBS blends, respectively.

     

Thermally induced crystallization and enzymatic degradation studies of poly (L‐lactic acid) films

Poly(L ‐lactic acid) (PLLA) films with different crystallinities were prepared by solvent casting and subsequently annealed at various temperatures (T_a) (80–110°C). The effects of crystallinity on enzymatic degradation of PLLA films were examined in the presence of proteinase K at 37°C by means of weight loss, DSC, FTIR spectroscopy, and optical microscopy. DSC and the absorbance ratio of 921 and 956 cm^?1 (A₉₂₁/A₉₅₆) were used to evaluate crystallinity changes during thermally induced crystallization and enzymatic hydrolysis. The highest percentage of weight loss was observed for the film with the lowest initial crystallinity and the lowest percentage of weight loss was observed for the film with highest crystallinity. FTIR investigation of degraded films showed a band at 922 cm^?1 and no band at 908 cm^?1 suggested that all degraded samples form α crystals. The rate of degradation was found to depend on the initial crystallinity of PLLA film and shown that enzymatic degradation kinetics followed first‐order kinetics for a given enzyme concentration. DSC crystallinity and IR absorbance ratio, A₉₂₁/A₉₅₆ ratio, showed no significant changes with degradation time for annealed PLLA films whereas as‐cast PLLA film showed an increase in crystallinity with degradation; this revealed that degradation takes place predominantly in the free amorphous region of annealed PLLA films without changing long range and short range order © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nucleating agent for poly(L‐lactic acid)—An optimization of chemical structure of hydrazide compound for advanced nucleation ability

A series of compounds having hydrazide groups was prepared and evaluated as nucleating agent for poly(L ‐lactic acid) by differential scanning calorimetry. Hydrazide compounds derived from benzoic acid, 2‐hydroxybenzoic acid, 3‐tert‐butylbenzoic acid, and 2‐aminobenzoic acid, where two of hydrazide compounds connected by four methylene chain were evaluated in series. Benzoylhydrazide type was found to be more effective on the enhancement of crystallization of poly(L ‐lactic acid). Effects of connecting length of methylene chain numbers between two of benzoylhydrazide on the nucleation ability were also evaluated. Benzoylhydrazide‐type compound having 10 methylenes, that is, decamethylenedicarboxylic dibenzoylhydrazide demonstrated excellent nucleation ability, and the resulted crystallization temperature and enthalpy of PLA with the compound of 1 wt % loading were 131°C and 46 J g^?1. The achieved crystallization temperature and enthalpy were over 10°C and over 10 J g^?1 higher than PLA with conventional nucleating agents, such as talc and ethylenebis (12‐hydroxystearylamide). Thus, the improvement in processability, productivity, and heat resistance of PLA is suggested to be achieved by using decamethylenedicarboxylic dibenzoylhydrazide as a nucleating agent. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 198–203, 2007     

Effect of crystallinity on unimodal and bimodal cell structures,in a continuous production of HDPE microcellular foamed sheets using supercritical CO2 (ScCO2)

Crystallinity is a controlling parameter in the development of microcellular foam final structure. In this research, using a well-controlled continuous microcellular foamed sheet production system, the effect of crystallinity on the final structure of the microcellular foam is studied. To produce microcellular foamed sheets, different levels of the supercritical carbon dioxide (ScCO₂) is dissolved in the polymer at high pressure and the foamed material is stabilized using a four roll apparatus at different roll temperatures. Crystallization occur by delay, however this lag time can be controlled by controlling temperature gradient. The higher decrease in the roll temperature results in higher temperature gradient which increases the crystallization rate during the cell growth, causing bimodal cell structures which indicates secondary nucleation. Moreover, as the amount of gas increases, the cell formation time increases, hence, at even higher temperatures we observed the effect of secondary nucleation, resulting in the occurrence of bimodal cell structures.     

Supercritical CO2‐assisted synthesis of poly(acrylic acid)/nylon1212 blend

PAA/nylon1212 blend was prepared using supercritical CO₂ as substrate‐swelling agent and monomer/initiator carrier. Both supercritical CO₂/nylon1212 binary system and SC CO₂/monomer/nylon1212 ternary system have been studied. Virgin nylon1212 and blends were characterized through differential scanning calorimetry (DSC), infrared spectroscopy (IR), and scanning electron microscopy (SEM). © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2040–2044, 2003     

Synthesis,physical properties,and crystallization of optically active poly(L‐phenyllactic acid) and poly(L‐phenyllactic acid‐co‐L‐lactic acid)

Optically active poly(L ‐phenyllactic acid) (Ph‐PLLA), poly(L ‐lactic acid) (PLLA), and poly(L ‐phenyllactic acid‐co‐L ‐lactic acid) with weight‐average molecular weight exceeding 6 × 10³ g mol^?1 were successfully synthesized by acid catalyzed direct polycondensation of L ‐phenyllactic acid and/or L ‐lactic acid in the presence of 2.5–10 wt % of p‐toluenesulfonic acid. Their physical properties and crystallization behavior were investigated by differential scanning calorimetry, thermogravimetry, and polarimetry. The absolute value of specific optical rotation ([α]) for Ph‐PLLA (?38 deg dm^?1 g^?1 cm³) was much lower than that of [α] for PLLA (?150 deg dm^?1 g^?1 cm³), suggesting that the helical nature was reduced by incorporation of bulky phenyl group. PLLA was crystallizable during solvent evaporation, heating from room temperature, and cooling from the melt. Incorporation of a very low content of bulky phenyllactyl units even at 4 mol % suppressed the crystallization of L ‐lactyl unit sequences during heating and cooling, though the copolymers were crystallizable for L ‐phenylactyl units up to 6 mol % during solvent evaporation. The activation energy of thermal degradation (ΔE_td) for Ph‐PLLA (200 kJ mol^?1) was higher than that for PLLA (158 kJ mol^?1). The ΔE_td for the copolymers increased with an increase in L ‐phenyllactyl unit content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Banded spherulites in poly(L‐lactic acid): Effects of the crystallization temperature and molecular weight

The spherulitic morphology of pure poly(L ‐lactide) (PLLA) was investigated with polarized optical microscopy as a function of the crystallization temperature and molecular weight. After being melted at 210°C for 3 min, samples were cooled quickly to designated temperatures for isothermal crystallization. It was shown for the first time that a clear banding‐to‐nonbanding morphological transition took place at a critical temperature for PLLA with a number‐average molecular weight of 86,000. With the increasing molecular weight of the material, the spherulite growth rates decreased notably, and the band spacing decreased significantly. On the basis of the main‐chain chirality in PLLA and the observation of a nonbanded spherulitic morphology in a certain temperature region, it was suggested that the crystallization temperature might have an effect on the relationship between the sense of lamellar twisting and the main‐chain chiral structure in PLLA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Poly(L‐lactic acid)/silicon dioxide nanocomposite prepared via the in situ melt polycondensation of L‐lactic acid in the presence of acidic silica sol: Isothermal crystallization and melting behaviors

In a previous article, we reported the preparation and characterization of a nanocomposite of poly(L ‐lactic acid) (PLLA) and silica via the in situ melt polymerization of L ‐lactic acid in the presence of acidic silica sol. In this study, the isothermal crystallization and melting behaviors of a PLLA/silicon dioxide (SiO₂) nanocomposite with 5 wt % well‐dispersed SiO₂ nanoparticles (PLLASN5) and pure PLLA were comparatively studied with differential scanning calorimetry and polarized optical microscopy. The SiO₂ nanoparticles acted as nucleation agents in the PLLA matrix and enhanced its nucleation rate and overall crystallization rate, especially at high crystallization temperatures. However, no deleterious effect on the crystal morphology or crystallinity was observed. The crystals that formed at a low temperature were imperfect; therefore, double melting peaks occurred during the second heating scan because of melt recrystallization. With the crystallization temperature increasing, the crystals became increasingly perfect; as a result, the low melting peak increased and shifted to a higher temperature. The existence of SiO₂ nanoparticles had no effect on the equilibrium temperature of the PLLA matrix. Pure PLLA and PLLASN5 have the same equilibrium temperature of 171.5°C. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

超临界CO2流体工艺用于制备多孔分离膜的研究进展

介绍了超临界CO2流体用于微孔分离膜制备机理;分析了压力、聚合物浓度及温度等条件对膜孔结构的影响;论述了CO2对聚合物增塑作用的大小和CO2在聚合物材料中向外扩散的速率是最终影响膜孔结构形态的两个基本因素;综述了超临界CO2流体用于分离膜的制备的独特优势.     

Process control and monitoring of reactive crystallization of L‐glutamic acid

The aim of the present study was to investigate feedback control of a reactive crystallization process. The present study built up a control structure needed to control the driving force of reactive crystallization using the feed rate of added acid. The concentration of the crystallizing compound and pH was used to compute feedback in the closed‐loop control of semi‐batch precipitation. The concentration of L ‐glutamic acid was determined from measured MID‐IR ATR‐FTIR spectra based on a multivariate model. Dynamic change of set value was based on the mass of added sulfuric acid and pH. The studied properties of the product crystals were polymorphism and crystal size. The polymorphic composition was analyzed with a Raman spectrometer and was expressed by mass fraction of the α‐polymorph. The obtained results showed that the developed feedback process control system allows effective control of forming of polymorphs. © 2009 American Institute of Chemical Engineers AIChE J, 2010