聚乳酸成型加工过程中的熔体粘弹特性和结晶行为调控

聚乳酸(PLA)是目前合成生物可降解高分子材料中应用量最大的品种,可望逐渐部分取代聚烯烃而更广泛应用于各个领域。但PLA树脂结构决定的松弛特性导致其加工过程特殊的黏弹特性,使其熔体强度低、成型工艺特性不稳定并进而导致产品尺寸和性能不稳定。此外,PLA极低的结晶速率,使其在挤出和注射成型等较高冷却速率的实际加工条件下呈无定型态,进一步影响了其加工和使用性能。这些问题已成为PLA更大规模商品化应用的瓶颈。本文从通过调控PLA熔体加工过程的黏弹特性而提高其可加工性出发,综述近年来本课题组在PLA成型加工过程中熔体粘弹特性和结晶行为(结晶速率和结晶结构)调控方面的研究进展。     

Crystallinity and dimensional stability of biaxial oriented poly(lactic acid) films

Transparent biaxial oriented poly(lactic acid) (BOPLA) films with improved dimensional stability were successfully prepared by controlling the crystallization of poly(lactic acid) (PLA). The crystalline morphology of PLA films can be manipulated by changing certain processing parameters, such as stretch ratio, heat setting temperatures, and heat setting time. Optical and mechanical properties as well as dimensional stability of the resulting polymer films are governed by their crystallinity and crystalline morphology. Crystallization behavior and kinetics of PLA, therefore, were investigated using wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC) techniques. Mechanical properties and the dimensional stability of the biaxial oriented PLA films were obtained and correlated with their processing conditions. Poly(lactic acid) films prepared by melt extrusion methods have great potential for food packaging, shrink labeling and protective film applications. However, shrinkage at elevated processing temperature should be minimized to avoid puckering of the polymer film. Shrinkage of less than 2% was achieved for a BOPLA film stretched 300% in both directions at 75 °C and then annealed at 160 °C for 30 s. Fabrication, properties, and potential applications of a series of biodegradable films will be described.     

The effect of halloysite nanotubes and N,N′-ethylenebis (stearamide) on morphology and properties of polylactide nanocomposites with crystalline matrix

Poly(lactide)/halloysite nanotubes (PLA/HNT) nanocomposites with crystalline matrix were obtained by cold crystallization and examined. Neat HNT and HNT treated with N,N′- ethylenebis(stearamide) (EBS) were used as nanofillers. Reference materials, PLA and PLA/EBS blend, prepared in the same way, were also considered. The influence of HNT and/or EBS content on the crystallinity and morphology of PLA matrix, as well as on the dynamic mechanical and optical properties of the materials, was determined.The nanocomposites contained well-distributed HNT, with only occasional agglomerates. HNT, EBS-treated HNT and EBS influenced the morphology of the crystalline PLA matrix and the amounts of the disorder α’ (termed also δ) and order α crystallographic forms of PLA. Crystallinity increased stiffness of the materials compared to their counterparts with the amorphous matrix. Owing to the crystallinity and the presence of the nanofillers, the storage modulus at 20 °C and 60 °C increased by up to 30 and 60%, respectively, compared to neat amorphous PLA. Interestingly, at lower nanofiller content the crystalline nanocomposites with EBS were more transparent than neat crystalline PLA.     

The influence of matrix crystallinity,filler grain size and modification on properties of PLA/calcium carbonate composites

Thermal and mechanical properties of polylactide (PLA) composites with different grades of calcium carbonate, 40 nm and 90 nm nanoparticles, and also with submicron particles, unmodified and modified with calcium stearate or stearic acid, obtained by melt mixing, were compared. Films with amorphous and crystalline matrices were prepared and examined.T_g of PLA in the composites remained unaffected whereas its cold crystallization was enhanced by the fillers and predominantly depended on filler content. Filling decreased thermal stability of the materials but their 5% weight loss temperatures well exceeded 250 °C, evidencing stability in the temperature range of PLA processing. The amorphous nanocomposites with modified nanoparticles exhibited improved drawability and toughness without a significant decrease of tensile strength; nearly two-fold increase of the elongation at break and tensile toughness was achieved at 5 wt% content of the modified nanofiller. Lack of surface modification of the filler, larger grain size with an average of 0.9 μm, and matrix crystallinity had a detrimental effect on the drawability. However, the presence of nanofillers and crystallinity improved tensile modulus of the materials by up to 15% compared to neat amorphous PLA.     

Confined crystallization,melting behavior and morphology in PEG-b-PLA diblock copolymers: Amorphous versus crystalline PLA

In diblock copolymers, the constraining effects of different stereochemical structure of high-T_m block on crystallization and melting behaviors of other constituent are supposed to be different. In this work, PEG-b-PDLLA and PEG-b-PLLA were synthesized, and crystallization kinetics, crystalline structure, melting behaviors of PEG blocks and morphology development in these systems were evaluated. Compared to those connected to PLLA, PEG-b-PDLLA exhibited lower crystallization rates, implying that connectivity of amorphous chain exerted more pronounced effect on crystallization rate of PEG than that of steric hindrance of PLLA crystallites. While all PEG-b-PDLLA samples showed a single endothermic peak during heating process, multiple melting peaks were observed in PEG-b-PLLA associated with composition, crystallization temperature and cooling rate of PLLA. A lamellar structure was formed by the crystallization of PEG in all PEG-b-PDLLA, however, when PEG-b-PLLA crystallized at room temperature directly, unexpected results occurred: lamellar for diblock copolymers with 31.5 and 48.0 wt% PLA or cylindrical structure for the diblock copolymers with 56.1 and 63.8 wt% PLA. Depending on composition, PEG-b-PLLA created one or two types of lamellar stacks after sequential crystallization of PLLA and PEG. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 455–465     

Study of the crystallization kinetics in amorphous Fe83P17 alloy

The crystallization kinetics of Fe₈₃P₁₇ amorphous alloy has been studied by Mössbauer spectroscopy and X-ray diffractometry. The samples were annealed isothermally at two different temperatures (315 °C and 325 °C). During isothermal annealing of the samples three phases were observed: crystalline Fe₃P phase, crystalline -Fe phase and the amorphous phase. The value of the Avrami exponent was found to be about 2.0 at each annealing temperature. This suggests that the growth rate of the crystals is controlled by volume diffusion and the nucleation rate decreases during crystallization. The activation energy obtained for the overall crystallization process was 193±43 kJ mol^–1.     

Amorphization of Silicon during Mechanical Treatment of Its Powders: 2. Heats of Recrystallization

Exothermic effects arising during silicon powder heating after their mechanical treatment were measured by synchronous TG–DSC thermal analysis method. Silicon samples possessed semicrystalline structure. As temperature varied from room temperature to 1200°C at a rate of 10 and 50 deg/min, the heat was released in the range 550–800°C. The crystallization of amorphous phase was also observed in the same temperature range using the X-ray diffraction. As the amount of energy consumed during the mechanical treatment of powders increased, the value of the heat effect rose synchronously with the degree of amorphization. In this case, the ratio of the released heat to the content of X-ray amorphous phase was constant and equal to 6 ± 2 kJ/mol.     

Analysis of the nonisothermal crystallization kinetics in three linear aromatic polyester systems

Melt or cold crystallization kinetics has a strong bearing on morphology and the extent of crystallization, which significantly affects the physical properties of polymeric materials. Nonisothermal crystallization kinetics are often analyzed by the classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) model or one of its variants, even though they are based on an isothermal assumption. As a result, during the nonisothermal (e.g. constant heating or cooling rate) crystallization of polymeric material, different sets of model parameters are required to describe crystallization at different rates, thereby increasing the total number of model parameters. In addition, due to the uncorrelated nature of these model parameters with the cooling or heating rate, accurate modeling at any intermediate condition is not possible. In the present work, these two limitations of the conventional approach have been eliminated by exhibiting the existence of a functional relationship between cooling or heating rate and effective activation energy during nonisothermal melt or cold crystallization in three linear aromatic polyesters. Furthermore, it has been shown that when the JMAK model is used in conjunction with this functional relationship, it is possible to precisely predict the experimental nonisothermal melt or cold crystallization kinetics at any linear cooling or heating rate with a single set of model parameters.     

Can Classic Avrami Theory Describe the Isothermal Crystallization Kinetics for Stereocomplex Poly(lactic acid)?

Classic Avrami model and its modifications have found diverse applications in describing the thermal and phase behaviors of inorganic metals and organic polymers.The direct introduction of classic Avrami equation to offer quantitative analyses of crystallization kinetic parameters for enantiomeric poly(lactic acid) (PLA) blends may,however,lead to contradictory conclusions.As revealed by this study,during the characterization of isothermal melt and cold crystallization for stereocomplex PLA containing equal-weight poly(L-lactic acid) and poly(D-lactic acid),the kinetic parameters yielded by Avrami equation are not in line with the classic crystallization hypotheses or the direct morphological observations.The underlying mechanisms,to some extent,lie in the generation of stereocomplex crystals (SCs) during the cooling/heating which affects the subsequent crystallization dynamics.The huge gap between the melting enthalpies of 100％ crystalline SCs (142 J/g) and homo-crystals (HCs,93 J/g) is most likely responsible for the confusing kinetic parameters acquired from the deduction of Avrami equation,which is based on the integration of enthalpies as a function of crystallization time.This prompts for great care that the classic Avrami equation is not applicable to accurately describe the crystallization kinetics of stereocomplex PLA,given the generation of SCs prior to crystallization and the coexistence of HCs and SCs during crystallization.     

A DSC study of the crystallization behaviour of polylactic acid and its nanocomposites

The isothermal crystallization behaviour of polylactic acid (PLA) and a clay nanocomposite of have been examined using differential scanning calorimetry. The data obtained clearly indicates that the presence of the nanocomposite particles in the composite material influences the crystallization kinetics of the PLA when crystallized both from the solid amorphous state as well as from the melt. When crystallized from the melt the presence of the clay nano-particles appears to be influencing the nucleation and crystal growth rate of the PLA such that the crystallization rate is enhanced by a factor of about 15 to 20. This result is of tremendous significance in identifying the processing window for the production of foamed nanocomposites from PLA. In addition the effect of thermal exposure at 200°C on the crystallization behaviour of these materials has been investigated, with the results suggesting that holding these materials at 200°C for periods of time up to 60 min in an inert atmosphere only has a marginal effect.     

Time-resolved SAXS studies of morphological changes in cold crystallized poly(ethylene terephthalate) during annealing and heating

Structural changes occurring during crystallization of quenched amorphous poly(ethylene terephthalate) (PET) and subsequent cooling/heating cycles have been studied by real-time small-angle x-ray scattering (SAXS), using synchrotron radiation. Initial crystallization is found to occur by insertion of new lamellae between the existing ones, while rapid continuous melting/recrystallization happens when the cold-crystallized PET samples are heated above the previous highest annealing temperature. Such melting/recrystalization results in irreversible increases in the lamellar long period, the crystal thickness and the density difference between the crystalline and amorphous regions; in contrast, at temperatures below the prior highest crystallization temperature, the structural changes are dominated by reversible effects such as thermal expansion. However, throughout the entire temperature range up to the melting point around 250 °C, the crystal core thickness remains quite small, less than ca. 50 Å, and the linear crystallinity of lamellar stacks remains nearly constant around 0.3. Such a low crystallinity indicates the presence of thick order-disorder interfacial layers on the lamellar surface, whose thickness increases with temperature.Dedicated to Prof. E. W. Fischer on the occasion of his 65th birthday.     

Thermophysical analysis and modeling of the crystallization and melting behavior of PLA with talc

The crystallization kinetics and the melting behavior of PLA and PLA with talc are investigated by dynamic scanning calorimeter and optical microscopy. The polymorphic aspect of PLA was highlighted by analyzing the melting process throughout heating after isothermal crystallization. The melting process of PLA with 5 mass% talc (PLAT5) shows the same thermal transitions as for PLA alone. The thermodynamic melting temperature of PLA and PLAT5 is determined to be 167.7 °C. The effects of the temperature and the cooling rate on the crystallization kinetics of PLA are analyzed. Finally, a simple and efficient protocol is defined to model the isothermal and the non-isothermal crystallization taking into account the polymorphism of PLA. Good agreement is found between the predictions of the proposed model and the experimental results under isothermal and non-isothermal conditions.     

The Influences of Elastomer toward Crystallization of Poly(lactic acid)

Poly (lactic acid)/elastomer blends were prepared via direct injection molding. In non-isothermal crystallization scan, the crystallinity of PLA increased with a decrease in the heating and cooling rate. The melt crystallization of PLA appeared in the low cooling rate (1, 5 and 7.5°C/min). The presence of elastomer tended also to increase the crystallinity of PLA. However, it started to decrease in 30% of elastomer. It was also showed by the decreasing of cold crystallization activation energy. Elastomer also gave plasticization effect in PLA properties. Thermal treatment through annealing completed after 1 h at 80 °C. In isothermal crystallization scan, the cold crystallization rate increased with increasing crystallization temperature in the blends. The Avrami analysis showed that at low temperatures, the cold crystallization had two regime processes whereas at high temperature only one stage was observed.     

Vitrification and crystallization of low-temperature amorphous condensates of water and methane—water mixture

Low-temperature condensates of water and water-methane mixture are studied in the temperature range of 65–200 K. Amorphous samples are obtained by molecular beam deposition under vacuum conditions on the substrate cooled with liquid nitrogen. The vitrification and crystallization temperatures are determined from the changes in the dielectric properties of the condensates upon heating. The kinetics of crystallization of amorphous water layers is studied by differential thermal analysis. The temperature conditions for the growth of thick methane crystalline hydrate layers during the low-temperature condensation of molecular water-gas mixture beams are found.     

Determination of Crystallization and Melting Behaviour of Poly-lactic Acid and Polypropyleneblends as a Food Packaging Materials by Differential Scanning Calorimeter

The food packaging materials commonly used is made from polymers synthetic base on petroleum derivatives. However, the use of synthetic polymers has negative impacts on the environment, because it is difficult to degrade naturally either by biotic or abiotic process. This is a problem for the environment and therefore it needs to do the assessment of the technology to reduce the degree of difficulty on its degradation or to find a new material that can be degradable naturally. One of the most important properties of food packaging materials is the polymer crystallinity. This refers to the overall level of crystalline component in relationship to its amorphous component. It is directly related to many of key properties exhibited by a semi-crystalline polymer including brittleness, toughness, stiffness or modulus, optical clarity, creep or cold flow, barrier resistance and long-term stability. Thus, in this study, PP blends with the PLA and meltsat 250^oCfor 4 hours, and investigates their crystallization and melting behavior using DSC at cooling rate of 10 and 40^oC/min. The results show that base on their thermograms, with increasing the cooling rate will decreasing the crystallinity or increasing amorphous area underthe peaks.     

Crystallization and Physical Ageing of Poly (2-vinyl pyridine)-b-poly(ethylene oxide) Diblock Copolymers

Summary: A set of melt miscible Poly(2-vinyl pyridine)-b-Poly(ethylene oxide) (P2VP-b-PEO) block copolymers of different compositions were studied. Transmission electron microscopy shows phase separation in the materials during the crystallization process of the PEO block as crystalline lamellae are observed for all compositions evaluated. The isothermal crystallization kinetics of PEO is progressively retarded as the P2VP content in the copolymer increases, since P2VP hinders molecular mobility in the miscible amorphous phase. Polarized light optical microscopy demonstrated that the glassy P2VP block has a negative effect on the secondary nucleation of the PEO. Finally, physical ageing experiments performed in the glassy state of the amorphous mixed phase, at different ageing times, demonstrated that a nucleating effect can be induced in the glassy state as a consequence of the reorganization of the amorphous regions. This nucleating effect significantly alters the cold crystallization rate upon subsequent heating above the glass transition temperature.     

Thermal methods for evaluating polymorphic transitions in nifedipine

The thermal behaviour of nifedipine was studied with the view to understand the various phase transitions between its polymorphs. The focus was on polymorph identification, accompanying morphological changes during crystallization and the nature of the phase transformations. These features were compared to the complexity of the crystallization mechanisms, studied by dynamic differential scanning calorimetry (DSC) heating techniques. DSC, thermogravimetry (TG) established the temperature limits for preparation of amorphous nifedipine from the melt. DSC studies identified that metastable form B, melting point ∼163 °C, was enantiotropically related to a third modification, form C, which existed at lower temperatures. Form C converted endothermically to form B at ∼56 °C on heating and was shown by hot stage microscopy (HSM) to be accompanied by morphological changes. Modulated temperature differential scanning calorimetry (MTDSC) showed discontinuities in the reversing heat flow signal during crystallization of amorphous nifedipine (from ∼92 °C) to form B, which suggested that a number of polymorphs may nucleate from the melt prior to form B formation. Identification of the number of nifedipine polymorphs included the use of combined DSC-powder X-ray diffraction (PXRD) and variable temperature powder X-ray diffraction (VTPXRD). The crystallization kinetics studied by dynamic DSC heating techniques followed by analysis using the Friedman isoconversion method where values of activation energy (E) and frequency factor (A) were estimated as a function of alpha or extent of conversion (α). The variations in E with α, from 0.05 to 0.9, for the amorphous to form B conversion could indicate the formation of intermediate polymorphs prior to form B. The form B to form A conversion showed a constancy in E on kinetic analysis from α 0.05 to 0.9, which suggested that a constant crystallization mechanism operated during formation of the thermodynamically stable form A.     

Influence of a cooling rate on a structure of PA6

The structure of the plate specimens obtained from the molten PA6 that was cooled at rates between 2 and 2000 °C/min have been studied. The cooling rate of 2000 °C/min did not ensure a complete amorphization of the specimens. The amorphous phase created by supercooling is unstable and at room temperature undergoes a noticeable cold crystallization. The access of water to the specimen from the surrounding air accelerates this process.Variations in the cooling rate of the melt reflect in rearrangement of the amorphous phase of PA6. Owing to the effect of interaction with the crystalline phase, the amorphous regions undergo changes in molecular weight and MWD of the chain segments between junctions in topological regions, temperatures of the glass transition and β-relaxation, compaction, etc.     

Thermal degradation of poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and their blends upon melt processing

Poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) are biodegradable aliphatic polyesters, which being semicrystalline and thermoplastic can be processed by conventional methods. Their blends give interesting materials for industrial packaging applications, due to their increased ductility as PBAT content increases. However, like many aliphatic polyesters, the PLA matrix degrades upon melt processing thus affecting the thermo-mechanical features of the blended material. In this work, we studied the effect of processing at high temperature on the molecular weight distribution, morphology, and thermo-mechanical properties of both homopolymers, as well as the PLA/PBAT 75/25 blend. Notably, different processing conditions were adopted in terms of temperature (range 150-200 °C) and other relevant processing parameters (moisture removal and nitrogen atmosphere). Analysis of PLA/PBAT blends indicated that intermolecular chain reactions took place under strong degradative conditions of PLA, yielding PLA/PBAT mixed chains (copolymers). Increasing amounts of copolymers resulted in improved phase dispersion and increased ductility, as SEM and mechanical tests indicated. Conversely, reduced PLA degradation with less copolymer formation, afforded higher modulus materials, owing to poorer dispersion of the soft phase (PBAT) into the PLA matrix.