Crystallization Half-time Research Articles

Promoted formation of stereocomplex in enantiomeric poly(lactic acid)s induced by cellulose nanofibers

Stereocomplex (SC) crystallization between enantiomeric poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) is believed to yield poly(lactic acid) (PLA) with superior physiochemical properties. However, homocrystallization (HC) crystallites are inevitably generated in the PLLA/PDLA blends. Herein, we report a simple approach to fabricate PLLA/PDLA racemic blends with high contents of SC crystallites by introducing cellulose nanofibers (CNFs). The isothermal crystallization results revealed that the half-crystallization time of the PLLA/PDLA blend was significantly decreased by adding CNFs. Additionally, with the incorporation of 3 wt% modified CNFs, the PLLA/PDLA blend was overwhelmingly crystallized into SC crystallites with no HC crystallite formation. Based on Fourier transform infrared spectroscopy findings, it was speculated that the preferred SC crystallization of PLLA/PDLA/CNF was caused by enhanced interchain molecular interactions between CNFs and PLA. This work presents a feasible and efficient method to fabricate PLA with exclusively SC crystallites, which possesses great potential for producing high-performance PLA materials.

Enhancing the crystallization of biodegradable poly(ε-caprolactone) using a polyvinyl alcohol fiber favoring nucleation

In this study, biodegradable poly(ε-caprolactone) (PCL)/polyvinyl alcohol (PVA) fiber composites were prepared using melt processing techniques with different PVA fiber contents. The effect of PVA fiber on the non-isothermal and isothermal melt crystallization kinetics of PCL/PVA fiber composites was studied by using differential scanning calorimetry. Non-isothermal crystallization peak temperatures were enhanced significantly, as exemplified by about 20 K increase for the composite loaded with 5 wt% PVA fiber at a cooling rate of 20 K/min compared to that of neat PCL. The crystallization half-time decreased from 1.67 min for neat PCL to 0.80 min for composite with 5 wt% PVA fiber at a cooling rate of 5 K/min. PVA fiber was also found to dramatically accelerate the isothermal crystallization rate of PCL/PVA fiber composites. The crystallization half-time of composite containing 5 wt% PVA fiber was reduced by more than 10 times compared with neat PCL. Moreover, dynamic mechanical analysis results showed that PVA fiber had a strengthening effect on the PCL.

Combined Effects of Cellulose Nanofiber Nucleation and Maleated Polylactic Acid Compatibilization on the Crystallization Kinetic and Mechanical Properties of Polylactic Acid Nanocomposite.

This work investigated the combined effects of CNF nucleation (3 wt.%) and PLA-g-MA compatibilization at different loadings (1–4 wt.%) on the crystallization kinetics and mechanical properties of polylactic acid (PLA). A crystallization kinetics study was done through isothermal and non-isothermal crystallization kinetics using differential scanning calorimetry (DSC) analysis. It was shown that PLA-g-MA had some effect on nucleation as exhibited by the value of crystallization half time and crystallization rate of the PLA/PLA-g-MA, which were increased by 180% and 172%, respectively, as compared to neat PLA when isothermally melt crystallized at 100 °C. Nevertheless, the presence of PLA-g-MA in PLA/PLA-g-MA/CNF3 nanocomposites did not improve the crystallization rate compared to that of uncompatibilized PLA/CNF3. Tensile strength was reduced with the increased amount of PLA-g-MA. Contrarily, Young’s modulus values showed drastic increment compared to the neat PLA, showing that the addition of the PLA-g-MA contributed to the rigidity of the PLA nanocomposites. Overall, it can be concluded that PLA/CNF nanocomposite has good performance, whereby the addition of PLA-g-MA in PLA/CNF may not be necessary for improving both the crystallization kinetics and tensile strength. The addition of PLA-g-MA may be needed to produce rigid nanocomposites; nevertheless, in this case, the crystallization rate of the material needs to be compromised.

Innovative ionic liquids as functional agent for wood-polymer composites

Chemical modification of lignocellulosic fillers is a hydrophobization process that has been used for years in the production of wood-polymer composites (WPCs). However, finding new, more effective modifiers is still a big challenge and remains the subject of much research. This study involved the chemical modification of wood with the use of newly designed ammonium and imidazolium ionic liquids containing reactive functional groups. The effectiveness of the modification was confirmed using FTIR and XRD techniques. The effect of modification of wood on the supermolecular structure and morphology of wood-polymer composites was investigated by X-ray diffraction, hot stage optical microscopy and differential scanning calorimetry. A significant influence of the modifier structure on the shaping of polymorphic varieties of the polymer matrix was demonstrated. The chemical modification also had significant effect on the nucleating properties of the wood fillers, which was confirmed by the determined crystallization parameters (crystallization half-time, crystallization temperature, crystal conversion). Moreover, the formation of a transcrystalline PP layer was noticed, which showed a large variation depending on the structure of the used ionic liquid. The obtained results correlated very well with the results of mechanical tests. It has been shown that it is possible to precisely design an ionic liquid containing a reactive functional group capable of interacting with hydroxyl groups of cellulose molecules. Moreover, the possibility of functionalizing the lignocellulosic material with innovative ionic liquids without the need to use organic solvents has not been demonstrated so far.Graphic abstract

Utilization of Talc as a Nucleating Agent in Poly(Lactic Acid) and Poly(Butylene Succinate) Blend

In this paper, poly(lactic acid) (PLA) was modified with poly(butylene succinate) (PBS) and talc to obtain PLA formulation with good toughness and high crystallization rate. PBS was added as a toughening agent at 40wt% and talc was added as a nucleating agent from 2 to 10wt%. Experimental results showed that both the tensile modulus and strength of PLA significantly decreased with the presence of PBS. Both values were found to notably increase with talc concentration and reached the maximum value at 8wt%. The tensile elongation at break was found to remarkably increase with PBS blending. However, it was linearly dropped with talc addition. Thermal test results also indicated the faster crystallization rate with the decreased crystallization temperature (Tc) and increased degree of crystallinity (Xc), by more than four times, when talc was added at least 4wt%. The isothermal crystallization half-time (t1/2) was applied to provide the data for injection molding process. The results showed that neat PLA required more than 25 min to obtain its half crystallinity. Minimum t1/2 of 3.45 min was obtained when talc was added to PLA/PBS at 8wt%. Heat distortion temperature (HDT) was also found to increase from 56.8 (neat PLA) to 97.2°C (8wt% talc). Based on the experimental results, the optimum talc concentration was 8wt% which provided the highest crystallization rate and thermal stability. The practical application of this formulation is for the biodegradable injection molding products.

Insights into the nucleation and crystallization analysis of PHB-rubber toughened PLA biocomposites

An insightful understanding of the isothermal cold crystallization, melting behavior, and kinetics of the novel PHB-rubber copolymer (PHB-di-rub)-reinforced polylactide (PLA) were determined. Incorporating 10 wt% PHB-di-rub significantly reduces the crystallization half-times (t1/2) by 32-fold, lowers the kinetic parameter (Kg) and folding surface energy (σe), which show the rapid polymer chains folding and the super-nucleating capability due to simultaneous instigated nucleation and plasticization. Concurrently, the shear-thinning behavior verified the plasticization effect and the convergence of diffraction peak show development of primary α′-crystalline form that contributes to the PLAs' ultra-extensibility. This evocative finding could comprehend a valuable framework for the future development of high-performance PLA/PHB-based copolymer biocomposites with total green physiognomies.

In-situ synthesis and thermal properties of biobased Poly(neopentyl glycol 2,5-furandicarboxylate)/multi-walled carbon nanotubes composites

The in-situ synthesis and thermal properties of biobased poly(neopentyl glycol 2,5-furandicarboxylate) (PNF)/multi-walled carbon nanotubes (MWCNTs) composites were studied in this research. The chemical structure, thermal stability, crystallization behavior, and crystal structure of PNF/MWCNTs composites were extensively investigated and compared with those of PNF. Scanning electron microscopy image revealed that MWCNTs uniformly dispersed in PNF matrix. In the composites, MWCNTs slightly improved the thermal stability of PNF, while the crystallization of PNF was obviously enhanced by MWCNTS under both the nonisothermal and isothermal crystallization conditions as the heterogeneous nucleating agent. Only 0.5 wt% of MWCNTs obviously increased the melt crystallization temperature from 132.9 °C for neat PNF to 141.2 °C for the composite and decreased the crystallization half-time from 3.1 min for neat PNF to 1.9 min for the composite at 165 °C. In addition, MWCNTs did not modify the crystal structure of PNF.

Enhanced formation of stereocomplex crystallites in Poly(l-lactic acid)/Poly(d-lactic acid) blends by silk fibroin nanodisc

Blending of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) is known to form stereocomplex crystallites (SC). In this research, the impact of a biobased filler, silk fibroin nanodisc (SFN), on the crystallizability of the PLLA/PDLA (50/50) blend was examined by differential scanning calorimetry (DSC), polarizing optical microscope (POM), and time-resolved small- and wide-angle X-ray scattering (SWAXS) measurements. The crystallization temperature (Tc), measured by non-isothermal DSC cooling scan, was increased by the addition of 1% SFN in the PLLA/PDLA (50/50) blend. The inclusion of SFN enhanced the formation of SC, as indicated by increasing the melting enthalpy of SC. Isothermal crystallization study at 110 °C by DSC and WAXS suggested that the SFN enhanced the SC crystallization while it suppressed the homocrystal (HC) crystallization, as indicated by crystallization half-time (t0.5) values. It was found that SFN not only enhances the SC crystallizability but also increases the fraction of SC crystals in the PLLA/PDLA blend. The long period of HC and SC lamellar stack was reduced by the addition of SFN, as indicated by the SAXS results. The POM results suggested that SFN significantly enhanced the number of SC nuclei for the isothermal crystallization at 170 °C from the melt.

Analysis of non-isothermal polymer crystallization at constant scan rates based on the Avrami model

When polymer crystallization is initiated by an instantaneous athermal nucleation and controlled by two-dimensional secondary nucleation on the growth face, for crystallization from the melt upon cooling at a constant scan rate β, an analysis based on the Avrami model reveals that a plot of β/(ΔTpeak)2=β/(TM0−Tpeak)2, in which Tpeak represents the peak temperature of crystallization rate and TM0 the equilibrium melting point, is nearly equivalent to that of the inverse of crystallization half-time τ1/2 and peak time τpeak, which are obtained from isothermal crystallization and used as a measure of crystal growth rate. Therefore, a plot of β/(ΔTpeak)2 against Tpeak at different scan rates can be utilized to examine the temperature dependence of the crystal growth rate in the same way as that of τ1/2 and τpeak obtained under isothermal conditions at different temperatures. The applicability was confirmed via numerical calculations and experiments using poly(vinylidene fluoride) and poly(butylene terephthalate). The agreement of the temperature dependence estimated by the plot of β/(ΔTpeak)2 was much better than that of β/(Tpeak)2 used in the well-known Kissinger plot.

Enhanced crystallization and storage stability of mechanical properties of biosynthesized poly (3-hydroxybutyrate-co-3-hydroxyhexanate) induced by self-nucleation

The poor mechanical properties induced by unsatisfactory crystallization ability limit the widespread use of biosynthesized poly (3-hydroxybutyrate-co-3-hydroxyhexanate) (PHBH). In this work, poly (3-hydroxybutyrate) (PHB) with a high melting point was first used as a homogeneous nucleating agent to increase the crystallization rate of PHBH by a self-nucleation method with a wider processing temperature window and crystallization kinetics and storage stability of mechanical properties of the PHBH/PHB mixtures were systematically investigated. By controlling the processing temperature and PHB content, the crystal nucleus density and crystallization rate of PHBH could be greatly increased while secondary crystallization was inhibited. When the processing temperature is 185 °C and PHB content is 20 wt%, the half crystallization time is shortened by 96% and the crystallinity was increased to 37.2%. Meanwhile, the mechanical performance of PHBH and its storage stability are greatly improved. Therefore, this work provides a simple and efficient way to improve the crystallization and mechanical performance of PHBH, which is expected to be applied to industrial production on a large scale.

Isothermal melt crystallization kinetics study of cellulose nanocrystals nucleated biodegradable Poly(ethylene succinate)

The isothermal melt crystallization kinetics of fully biodegradable poly(ethylene succinate) (PES)/cellulose nanocrystals (CNC) composites was studied in this research. Due to the efficient nucleating agent effect, the crystallization rate of PES was remarkably enhanced by low loadings of CNC; consequently, the composites displayed shorter crystallization half-time and greater nucleation density than neat PES. Through the self-nucleation and nonisothermal melt crystallization studies, the nucleation efficiency values of CNC were quantitively evaluated in PES/CNC composites. Despite CNC loading, the nucleation efficiency exceeded 50%. With increasing the CNC loading from 0.25 to 1 wt%, both the crystallization rate of the composites and the nucleation efficiency of CNC at different loadings first increased, reached a maximum, and then decreased, displaying the CNC loading dependence. Such dependence was attributed to the different dispersion states of CNC in the composites.

Lummus Novolen Technology awarded major polypropylene contract

Kinetic model of development of lamellar thickness distribution and average lamellar thickness during melt crystallization is proposed. The model bases on the Avrami–Evans and Hoffman–Lauritzen approaches to the crystallization and crystal growth kinetics and involves a thermal thickening model proposed in this paper which accounts for two processes – lamellar doubling transformation and logarithmic thickening of the doubled crystals. In the thickening model, continuity equation for the lamellar thickness distribution is proposed which includes a term responsible for fast creation of doubled crystals during a doubling time introduced as phenomenological parameter and a term which accounts for slow, logarithmic with time thickening subsequent to the doubling transformation. The model concerns melt crystallization in systems where the undercooling in the crystallization and melting processes refers to the same thermodynamic point – the thermodynamic equilibrium melting temperature.Analytical formulae for time-dependent lamellar thickness distribution vs. crystallization time are obtained by solving the continuity equation for the case of isothermal crystallization, as well as for the development of the average lamellar thickness of the doubled and un-doubled crystals during the crystallization time.The model predictions are illustrated by example computations for isothermal crystallization of PE grades characterized by high differences in the crystallization half-time resulting in high differences in the development of lamellar thickness distribution. Relative contributions of doubled and un-doubled crystals are predicted during the conversion and influence of the doubling time and logarithmic thickening parameters is discussed.

Insights into high molecular weight poly(ethylene 2,5-furandicarboxylate) with satisfactory appearance: Roles of in-situ catalysis of metal zinc

In pursuit of poly(ethylene 2,5-furandicarboxylate) PEF materials with high molecular weight, satisfactory appearance and faster crystallization rate, its preparation from dimethyl furan-2,5-dicarboxylate (DMFD) with ethylene glycol (EG) in the trace presence of metal zinc was performed via transesterification method. Optimization of the main polymerization parameters enabled Zn-catalyzed PEF to gain high molecular weight (Mn, 5.40×104gmol−1) and low content of diethylene glycol furandicarboxylate unit (DEGF, 2.91%). On the basis of experimental phenomena and high performance liquid chromatography (HPLC), the actual catalytic active species of metal zinc in the polymerization was speculated to be a salt of 2, 5-furandicarboxylate derivative (Zn(II)). The in-stiu catalyst can efficaciously inhibit the influence of by-product FDCA on the color of PEF, which displayed quite better appearance (close to white). Differential Scanning Calorimetry (DSC) showed that the in-stiu catalytic system acted as nucleating agent (NA) and the crystallization half-time (t1/2) of PEFs-Zn was only 1/4 of that of PEF-tin. Additionally, series of high molecular weight furan-based polyesters from DMFD and diols with different methylene (3, 5, 6 and 8) were obtained by zinc powder as in-stiu catalyst, which further evidenced it to be efficiency and universality.

Renewable Poly(butene 2, 5-furan dicarboxylate) Nanocomposites Constructed by TiO2 Nanocubes: Synthesis, Crystallization, and Properties

Poly(butene 2,5-furan dicarboxylate) (PBF) is an emerging bio-based polyester with excellent thermal and tensile properties. It is expected to replace its petroleum-based homologous polyester, polybutylene terephthalate (PBT). However, the crystallization rate and strength of PBF need to be improved. In this study, PBF/TiO2 nanocomposites were synthesized by the in-situ polymerization of 2,5-furandicarboxylic acid (FDCA), butanediol, and rutile/anatase TiO2 nanocubes, with a size of 50 nm. The amount of nano-TiO2 added was 1–9‰ of the mass ratio of FDCA. As a result, a series of PBF/TiO2 nanocomposites with high molecular weights was obtained. The crystallization rate of the nanocomposites was significantly improved compared to that of pure PBF. The isothermal and non-isothermal crystallization of the nanocomposites were tested, and the temperature and rates of crystallization were compared. The shortest half-crystallization time was less than 1 min. It is evident from the results that the crystallization rate of the composite increases with an increase in the nano-TiO2 content and reaches a maximum at a nano-TiO2 content of 7‰. In addition, the elongation at break and impact strength of the nanocomposites increased by 118% and 200%, respectively, compared to that of neat PBF. In the range of 200–400 nm, the lowest UV transmittance of nanocomposites is only 5%, which is highly beneficial for the application of PBF in the field of film packaging.

Effect of Cyanuric Acid as an Efficient Nucleating Agent on the Crystallization of Novel Biodegradable Branched Poly(Ethylene Succinate)

Novel biodegradable branched poly(ethylene succinate) (b-PES) composites, i.e., nucleated b-PES samples, were prepared by incorporating low loadings of cyanuric acid (CA) through a solution and casting method to enhance the crystallization rate. As an efficient nucleating agent, CA could remarkably increase the nonisothermal melt crystallization peak temperature, shorten the crystallization half-time, accelerate the overall isothermal melt crystallization, and enhance the nucleation density of b-PES spherulites in the composites. Despite the addition of CA, the crystallization mechanism and crystal structure of b-PES remained unchanged. A possible epitaxial crystallization mechanism may account for the nucleation of b-PES crystals induced by CA.

Non-Isothermal Crystallization Kinetics of Poly(Ethylene Glycol)-Poly(l-Lactide) Diblock Copolymer and Poly(Ethylene Glycol) Homopolymer via Fast-Scan Chip-Calorimeter.

The non-isothermal crystallization kinetics of double-crystallizable poly(ethylene glycol)–poly(l-lactide) diblock copolymer (PEG-PLLA) and poly(ethylene glycol) homopolymer (PEG) were studied using the fast cooling rate provided by a Fast-Scan Chip-Calorimeter (FSC). The experimental data were analyzed by the Ozawa method and the Kissinger equation. Additionally, the total crystallization rate was represented by crystallization half time t1/2. The Ozawa method is a perfect success because secondary crystallization is inhibited by using fast cooling rate. The first crystallized PLLA block provides nucleation sites for the crystallization of PEG block and thus promotes the crystallization of the PEG block, which can be regarded as heterogeneous nucleation to a certain extent, while the method of the PEG block and PLLA block crystallized together corresponds to a one-dimensional growth, which reflects that there is a certain separation between the crystallization regions of the PLLA block and PEG block. Although crystallization of the PLLA block provides heterogeneous nucleation conditions for PEG block to a certain extent, it does not shorten the time of the whole crystallization process because of the complexity of the whole crystallization process including nucleation and growth.

Weak Shear-Induced Slowdown in Crystallization of Less-Entangled Poly(ε-caprolactone)

Self-nucleation (SN) and strong shear methods were employed to obtain less-entangled melts in poly(e-caprolactone) and its blends with poly(styrene-co-acrylonitrile). The effect of shear flow on the isothermal and nonisothermal crystallization behavior of the less-entangled melts was studied by rheology and shear-Raman spectroscopy. Weak shear with a Rouse Weissenberg number WiR < 1 causes a lower crystallization temperature (Tc) and a longer half-crystallization time (t₁/₂) for both SN and strongly sheared melts under nonisothermal and isothermal conditions, respectively. This indicates that the crystallization of the less-entangled melts is decelerated by weak shear, implying that the melt memory effect from the crystallization is weakened. This phenomenon is ascribed to the accelerated chain re-entanglement under a weak shear flow, as verified by the rheological responses under the interrupted shear flow, stress relaxation, and oscillatory time sweep. It is believed that shear flow increases the free energy of polymer chains, which overcomes the free energy barriers for re-entanglement, activates the reptation, and speeds up the chain re-entanglements.

Fast-Scanning Chip-Calorimetry Measurement of Crystallization Kinetics of Poly(Glycolic Acid)

We report fast-scanning chip-calorimetry measurement of isothermal crystallization kinetics of poly(glycolic acid) (PGA) in a broad temperature range. We observed that PGA crystallization could be suppressed by cooling rates beyond −100 K s−1 and, after fast cooling, by heating rates beyond 50 K s−1. In addition, the parabolic curve of crystallization half-time versus crystallization temperature shows that PGA crystallizes the fastest at 130 °C with the minimum crystallization half-time of 4.28 s. We compared our results to those of poly(L-lactic acid) (PLLA) with nearby molecular weights previously reported by Androsch et al. We found that PGA crystallizes generally more quickly than PLLA. In comparison to PLLA, PGA has a much smaller hydrogen side group than the methyl side group in PLLA; therefore, crystal nucleation is favored by the higher molecular mobility of PGA in the low temperature region as well as by the denser molecular packing of PGA in the high temperature region, and the two factors together decide the higher crystallization rates of PGA in the whole temperature range.

Influence of wood thermal modification on the supermolecular structure of polypropylene composites

AbstractThe thermal modification of wood can be an interesting way of improving adhesion in polymer composites, although the mechanical properties of composites with heat treated wood presented so far in many works arouse much controversy. In this work, effects of thermal modification of wood on the supermolecular structure and morphology of wood/polypropylene composites (WPC) were investigated using X‐ray diffraction, hot stage optical microscopy, and differential scanning calorimetry. A significant impact of the conditions of thermal wood modification on the formation of polymorphic varieties of the polymer matrix has been demonstrated. Furthermore, thermal treatment also affected the nucleation activity of wood, which was confirmed by determined parameters such as, crystal conversion, half crystallization time, and crystallization temperature. In addition, differences in the formation of transcrystalline structures responsible for the level of interfacial adhesion were depicted based on hot stage optical microscopy. Mechanical tests have shown that obtaining the appropriate strength properties of composite materials depends on the selection of appropriate conditions for thermal modification, which determine the supermolecular structure and nucleation activity. The obtained results, not reported so far, are extremely important when designing WPC composite materials with strictly defined physicochemical parameters.

Fast-scan chip-calorimetry measurement on crystallization and enthalpy relaxation kinetics of isotactic poly(cyclohexene carbonate)

Isotactic poly(cyclohexene carbonate) (iPCHC) is a newly developed degradable semi-crystalline polymer. In this paper, the crystallization kinetics of iPCHC above the glass transition temperature (Tg) and the enthalpy relaxation kinetics below Tg were studied by using a commercial fast-scan chip-calorimeter—Flash DSC 1. Results showed that half crystallization time of iPCHC as a function of crystallization temperature presented the typical bell shape. And iPCHC showed a maximum crystallization rate at 190 °C, with the half crystallization time 153 s. IPCHC enthalpy relaxation time in 85 °C ~ 100 °C temperature range decreases with isothermal temperature. The above results have great significance to the development of processing and application of novel crystallizable polycarbonate materials.