Isothermal Crystallization Research Articles

Synthesis, Thermal and Mechanical Properties of Fully Biobased Poly (hexamethylene succinate-co-2,5-furandicarboxylate) Copolyesters

Poly (hexamethylene succinate) (PHS) is a biobased and biodegradable polyester. In this research, two fully biobased high-molecular-weight poly (hexamethylene succinate-co-2,5-furandicarboxylate) (PHSF) copolyesters with low hexamethylene furandicarboxylate (HF) unit contents (about 5 and 10 mol%) were successfully synthesized through a two-step transesterification/esterification and polycondensation method. The basic thermal behavior, crystal structure, isothermal crystallization kinetics, melting behavior, thermal stability, and tensile mechanical property of PHSF copolyesters were studied in detail and compared with those of PHS. PHSF showed a decrease in the melt crystallization temperature, melting temperature, and equilibrium melting temperature while showing a slight increase in the glass transition temperature and thermal decomposition temperature. PHSF copolyesters displayed the same crystal structure as PHS. Compared with PHS, PHSF copolyesters showed the improved mechanical property. The presence of about 10 mol% of HF unit increased the tensile strength from 12.9 ± 0.9 MPa for PHS to 39.2 ± 0.8 MPa; meanwhile, the elongation at break also increased from 498.5 ± 4.78% to 1757.6 ± 6.1%.

Catalytic Control of Crystallization in Dynamic Networks.

The effect of catalysts with varying nucleophilic strength on thiol-thioester bond exchange dynamics and concomitant crystallization was studied in a model semicrystalline polymer network. It was found that the characteristic time scale of covalent bond exchange, τ, could be tuned over a ∼101-103 s range simply by changing the nucleophilicity of the catalyst. Using isothermal crystallization measurements via differential scanning calorimetry, thermodynamic and kinetic features of crystallization were considered. A depression in melting temperature was observed with increasing bond exchange rate, suggesting a dependence of crystalline organization on network dynamics. Furthermore, a systematic slowing of crystallization kinetics with faster covalent bond exchange rates was observed. Lauritzen-Hoffman analysis showed a near doubling in the barrier for secondary nucleation for dynamic networks, suggesting that that bond exchange slows crystallization by limiting secondary nucleation and further growth. Finally, longitudinal DSC studies reveal a long-term increase in melting temperature for samples held at ambient temperature with bond exchange activated at room temperature, indicating that while bond exchange slows crystallization on short time scales it facilitates isothermal long-term crystal rearrangement and growth on longer time scales.

Directional polymer crystallisation with a fast-moving sink.

It has previously been shown that non-isothermal directional polymer crystallisation driven by local melting (Zone Annealing), has a close analogy with an equivalent isothermal crystallisation protocol. This surprising analogy is due to the low thermal conductivity of polymers-because they are poor thermal conductors, crystallisation occurs over a relatively narrow spatial domain while the thermal gradient spans a much wider scale. This separation of scales, which occurs in the limit of small sink velocity, allows replacing the crystallinity profile with a step and the temperature at the step acts as an effective isothermal crystallisation temperature. In this paper, we study directional polymer crystallisation under faster moving sinks using both numerical simulations and analytical theory. While, only partial crystallisation occurs, regardless, a steady state exists. At large velocity, the sink quickly moves ahead of a region that is still crystallizing; since polymers are poor thermal conductors, the latent heat dissipation to the sink becomes inefficient, eventually resulting in the temperature increasing back to the melting point thereby resulting in incomplete crystallization. This transition occurs when the two length scales measuring the sink-interface distance and the width of the crystallizing interface become comparable. For steady state and in the limit of large sink velocity, regular perturbation solutions of the differential equations governing heat transport and crystallization in the region between the heat sink and the solid-melt interface are in good agreement with numerical results.

Crystallization and melting of unentangled poly(ε-caprolactone) cycles containing pendants.

The Rouse model provides a basic framework to understand the chain dynamics of polymers, which is confirmed to be more suitable for exploring the linear dynamics of unentangled polymers. The crystalline morphology governed by chain dynamics and crystallization kinetics is expected to differ in linear and cyclic polymers. Cyclic poly(ε-caprolactone)s (c-PCLs) containing two bi-anthracenyl group pendants with molecular weights close to the critical molecular weight (Mc) were synthesized to investigate the chain dynamics based crystallization and melting behavior by DSC, POM, and in situ simultaneous small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) investigations during heating of the isothermally crystallized samples. Double endothermic peaks were observed in the DSC curves with a low heating rate of c-PCLs without entanglement after isothermal crystallization, especially for c-PCLs with Mc. The structure evolution of the crystalline structures observed from the in situ investigations during the heating and double endothermic peaks in DSC heating curves of the c-PCLs indicate the role of pendants in the chain dynamics, which leads to the reorganization of the metastable structures. Banded spherulites of c-PCL without entanglement were observed for the first time, and the uneven growth of spherulites along the radial direction may be caused by the mismatch between chain dynamics and crystallization kinetics.

Effect of temperature conditions of a heated plate on the crystallization of CFRTP

The bed part of a fused-filament-fabrication-based composite 3D printer was simulated, and the use of a hot plate to control the trans-crystallization (TC) thickness in the bed part was evaluated based on the molding conditions under which TC is known to occur. An experiment using plastic, which is commonly used in 3D printers, showed that TC was generated vertically from the surface of carbon fiber. Some resins exhibited an increasing degree of isothermal crystallization as the temperature of the bed was increased, but the effect of the cooling rate on crystallization was significant; TCs with an average thickness of 38.5, 19.3, and 5.2 μm were generated in carbon-filled (CF)/PP, CF/PPS, and CF/PET, respectively. A simulated experiment that investigated the effect of the cooling rate from the melted state showed a clear reduction in TC during rapid cooling, indicating that controlling the cooling rate from the melted state can alter the thickness of the TC at the interface.

Impact of polymer chain packing and crystallization on the emission behavior of curcumin-embedded poly(L-lactide)s.

The development of biodegradable and biocompatible fluorescent materials with tunable emission in the solid state has become increasingly relevant for smart packaging and biomedical applications. Molecular packing and conformations play a critical role in tuning the solid-state photophysical properties of fluorescent materials. In this work, tunable emission of bioactive curcumin was achieved through the manipulation of the crystallization conditions and the polymorphic form of covalently linked poly(L-lactide) in the curcumin-embedded poly(L-lactide) (curcumin-PLLA). In the melt-crystallized curcumin-PLLA, with the increase in the isothermal crystallization temperature, a bathochromic shift in the fluorescence of curcumin-PLLA was observed due to the change in the intramolecular conjugation length of curcumin. The change in the isothermal crystallization temperature of curcumin-PLLA resulted in the rotation of the terminal phenyl rings of curcumin with respect to the central keto-enol group due to the covalently linked helical PLLA chains. In addition, solvent-induced single crystals and a gel of curcumin-PLLA were prepared and the influence of the polymorphic form of PLLA on the emission behavior of curcumin-PLLA was investigated. The results suggest that the polymer chain packing, crystallization conditions, morphology, and polymorphic form could play an influential role in dictating the fluorescence properties of fluorophore-embedded polymers.

Crystallization of Poly(lactic acid) under continuous shear conditions: On-line x-ray analysis in isothermal conditions

In this work, the effect of shear rate on the isothermal crystallization of a poly(lactic acid), PLA, was analyzed. The crystallinity evolution of PLA during continuous shear was assessed by collecting X-ray diffraction data at short time intervals by adopting a synchrotron source. Shear rates in the range from 0.1 s−1 to 1.5 s−1 were analyzed. It was found that increasing the shear rate induced a dramatic decrease of the half crystallization times, more than one order of magnitude in the analyzed range of shear rates. A model for the description of crystallinity evolution was applied to describe the experimental results. It was found that the nucleation rate needed to describe the data are much larger than what measured in a previous work in the same range of shear rates but adopting a short step shear protocol.

A feasible strategy to balance the performance of stereo-complexed polylactide by incorporating poly(butylene adipate-co-terephthalate)

The raw material of polylactide (PLA) is lactic acid obtained by biological fermentation. PLA is the most promising degradable polymer to replace traditional plastics to address the pollution problems caused by their non-degradability. However, the application of PLA is hindered by its low softening temperature, easy hydrolysis, and poor toughness. Herein, the ternary composites with PLLA, PDLA and Poly (butylene adipate-co-terephthalate) (PBAT) were prepared by melt blending to balance its thermal stability, hydrolysis, and toughness. The effects of PBAT content (3 %, 6 %, 9 % and 12 %) and isothermal crystallization temperature on composite properties were fully investigated. The results show that the composite of stereo-complexed PLA (sc-PLA) with 6 % PBAT crystallized at 110 °C exhibits good comprehensive properties. Its vicat softening temperature (VST), mass loss rate under alkaline (pH = 12) and breaking elongation are 166 °C, 21.6 % and 4.40 %, respectively. Compared with the pure PLLA sample crystallized at same condition, the VST, mass loss rate and breaking elongation are 159 °C, 24.7 % and 3.76 % respectively, which increased by nearly 5 %, 13 % and 20 %. This indicates that this strategy is feasible to balance the heat resistance, hydrolysis resistance and toughness of PLA, while it sacrifices the tensile strength a little. This work provides a new way to modify and improve the PLA properties. Nonetheless, it is also necessary to coordinate the compatibility of PLA and PBAT.

Synthesis, Thermal Behavior, and Mechanical Properties of Fully Biobased Poly(Hexamethylene 2,5-Furandicarboxylate-Co-Sebacate) Copolyesters.

In this research, three fully biobased poly(hexamethylene 2,5-furandicarboxylate-co-sebacate) (PHFSe) copolyesters with low contents of hexamethylene sebacate (HSe) unit (10 mol%, 20 mol%, and 30 mol%) were successfully synthesized through a two-step transesterification/esterification and polycondensation method. The chemical structure and actual composition of PHFSe copolyesters were confirmed by hydrogen nuclear magnetic resonance. The thermal behavior and mechanical property of PHFSe copolyesters were investigated and compared with those of the poly(hexamethylene 2,5-furandicarboxylate) (PHF) homopolymer. Both PHFSe copolyesters and PHF showed the high thermal stability. The basic thermal parameters, including glass transition temperature, melting temperature, and equilibrium melting temperature, gradually decreased with increasing the HSe unit content. PHFSe copolyesters crystallized more slowly than PHF under both the nonisothermal and isothermal melt crystallization conditions; however, they crystallized through the same crystallization mechanism and crystal structure. In addition, the mechanical property, especially the elongation at break of PHFSe copolyesters, was obviously improved when the HSe unit content was greater than 10 mol%. In brief, the thermal and mechanical properties of PHF may be easily tuned by changing the HSe unit content to meet various practical end-use requirements.

Molecular Dynamics of Topological Barriers on the Crystallization Behavior of Ring Polyethylene Melts with Trefoil Knots

Topological barriers in ring polyethylene (PE) melts of trefoil knots inhibit crystallization due to self-entanglement, as confirmed by united atom molecular dynamics (UAMD) simulations. In this study, we clarified the decrease in the topological barriers of self-entangled knots with increasing polymer chain length (N) and the localization of the knotted segments in the noncrystallized region. In the UAMD simulations, isothermal crystallization processes were performed at T = 300 K for the trefoil knots using a crystallization time tIC = 200 ns. Here, the trefoil knot gives the simplest nontrivial topology with nonzero crossing number. Crystallization was not observed in trefoil PE knots for N ≤ 120; however, crystallization did take place in the ring PE melt of the trivial knot with trivial topology and zero minimal crossing number. For N = 140, suppression of the growth of a crystalline phase (i.e., the polycrystalline phase) was observed in the trefoil PE melts, whereas such suppression was not detected in the trivial PE melts. In contrast, no significant differences in the crystallization behaviors of the trefoil and trivial PE melts were observed at N = 200. These results indicated that the topological barriers decreased with increasing N. Furthermore, to investigate the relationship between the chain conformation and degree of local crystallization, we developed a new mathematical method for searching the knotted segment of a trefoil knot in a crystallized trefoil PE melt. We found that trefoil knots with large subloops (“large-leaves”) exhibited localization of the knotted segment. In addition, the large leaves of the localized knots dominated in the crystallized region, and the knotted segments of the localized knots were located mainly in the noncrystallized region.

Miscibility, crystallization and properties of bio-based polyamide 56/polyamide 6 blends

The miscibility, crystallization behaviors, and morphology of polyamide 56/polyamide 6 (PA56/PA6) blends were investigated. The blends had only one Tg, which confirmed that PA56 and PA6 are miscible in the blends. The melting point, crystallization temperature, crystallinity of the blends depressed due to the interaction of PA56 and PA6 in the blends. The crystalline structure of both PA56 and PA6 in the blends did not change and crystallized separately in both isothermal and non-isothermal crystallization process. PA56 in the blends crystallized in all blending ratio, while PA6 only crystallized with PA6 content φ6 > 0.2. The crystals of PA56 and PA6 grew in the same crystal skeleton but in different layers. Mechanical properties of the blends indicated that the elongation at break could be improved significantly without scarifying the strength in the case of adding a small amount of PA6 in PA56, which provides a promising approach for fabricating polyamide materials with desired properties.

Magnetic properties evolution and crystallization behaviour of vacuum- and air-long-term-annealed rapidly quenched Fe80.3Co5Cu0.7B14 alloy.

This work aims to investigate the isothermal crystallization behaviour, crystal structure and magnetic properties evolution of long-term (up to 300h) low temperature (210 and 260°C) vacuum- and air-annealed Fe80.3Co5Cu0.7B14alloy. Before the α-Fe(Co) phase crystallization, the primary relaxation process has been identified at a temperature range up to 340°C. The relaxation process performed under 210°C for 300h did not initiate the crystallization process. However, the topological and compositional short-range rearrangements improved magnetic properties remarkably. Annealing 150h at 260°C helps to deliver enough energy to stabilize the glassy state and initiate the crystallization process fully. Structural and magnetic properties evolution of 150h annealing at 260°C corresponds to the evolution presented during isochronal 20min annealing at 310°C. Magnetic properties Bs = 1.75-1.79T, Hc < 20 A/m and P10/50are similar to those for 20min of annealing at 310°C. Comparison of core power losses from up to 400kHz frequency dependences of long-term low temperature annealed alloy with 20min classical annealing at 310°C shown that presented here long-term annealing is energetically insufficient to bring the glassy state system into the same low level of core power losses efficiency.

Role of the Branched PEG-b-PLLA Block Chain in Stereocomplex Crystallization and Crystallization Kinetics for PDLA/MPEG-b-PLLA-g-glucose Blends with Different Architectures.

The isothermal crystallization behavior and corresponding morphology evolution of poly(d-lactic acid) (PDLA) blends with PLLA6.7k or MPEG-b-PLLA6.7k-g-glucose with different architectures and different PLLA-grafted copolymer contents were investigated. The formation of stereocomplexes (SCs) in between the chain branched structure of MPEG-b-PLLA6.7k-g-glucose and PDLA chains acting as the physical crosslinking points slows down the motion of PDLA chains, but the SCs could act as a heterogeneous nucleating agent for the late formation of homocrystals (HCs) in the blend system, accelerating the crystallization kinetics of HCs through enhancing the nucleation density. For PDLA/MPEG-b-PLLA6.7k-g-glucose blends, the mobility of SCs in the blend system and the nucleation density of SCs in the blends exhibit oppositional behavior during the isothermal crystallization at a Tc of 130 °C. The evolution of the crystal growth and structure during the isothermal crystallization process by rheometry has revealed the interesting role of the branched chains of MPEG-b-PLLA6.7k-g-glucose in the mechanism of the crystallization in PDLA blends.

Isothermal Crystallization of Poly(ε–Caprolactone) PCL in Three‐Armed Copolymers

AbstractBranched copolymers are a special class of polymeric materials in which are reflected the combined effects of polymer segments and architectural constraints of the branched architecture. In this work, three‐armed graft copolymers, poly (hydroxyethyl methacrylate‐graft‐poly(caprolactone), [P(HEMA‐g‐PCL)]3, are synthesized by combination of reversible addition‐fragmentation chain‐transfer (RAFT) and ring opening polymerization (ROP) mechanisms in a one‐pot/one‐step protocol. The resulting macromolecules are characterized by 1H Nuclear Magnetic Resonance (NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The preliminary results indicate the success of the methodology and are in agreement with literature reports. In addition, DSC isothermal crystallization are performed, and Avrami's theory is employed in order to obtain kinetics parameters of interest, such as the half‐life for the crystallization process (t1/2), the bulk crystallization constant (k), and the Avrami's exponent (n). Thermal analysis evidences a noticeable reduction in the melting temperature compared with linear PCL homopolymer. However, the presence of branches does not modify the values of the maximum degradation temperatures significantly. Finally, the synthesized copolymers adopt a two‐dimensional crystallization type (disk and cylindrical).

Micro-Raman spectroscopy for a comprehensive understanding of the structural evolution of Basaltic-Andesite and Trachybasalt multiphase systems

The liquid viscosity of multiphase systems is usually retrieved by the use of combined chemical analysis and empirical chemistry-based models. Here we present an alternative approach to study the glassy pools in highly crystallized basaltic-andesite and trachybasalt samples obtained from isothermal crystallization experiment at different shear rates. Polarized micro-Raman spectra were collected at room temperature on the glassy portions. Results of residual glass structures from the high frequency region of Raman spectra show an increasing polymerization degree. The fragility, derived from the evolution of the boson peak, decreases according to the experimental time evolution. Also, the presence of nanolites in residual glasses adds a further degree of complexity that needs to be considered in the frame of melts polymerization and rheological behavior of volcanic systems. On this concern, the MYEGA formalism approach allows to retrieve the residual melt viscosity of the partly crystallized samples, avoiding its direct measurement and reducing the errors induced when general viscosity models, based only on chemical data, are used. We validated our prediction by determining the low-temperature viscosity data converting, through known shift factors, the thermograms obtained by differential scanning calorimetry. This study provides a further route in characterizing multiphase systems, also including the effect of nanosized crystals, and offers the possibility of increasing the spatial resolution through microscopy and ridding viscosity parametrizations of invasive and time-consuming operations.

Description of non-isothermal crystallization kinetics of Fe48Co32P14B6 metallic glass using the isothermal analysis data

The non-isothermal crystallization kinetics of Fe48Co32P14B6 glass was experimentally studied by differential scanning calorimetry (DSC) at heating rates from 5 to 40 K/min. The integral form of the non-isothermal glass crystallization kinetic equation based on the combination of the Kolmogorov's model of mass crystallization kinetics and the Kashchiev's transient nucleation approach involving the classical equations for nucleation rate and growth velocity was derived. The kinetic model describing the interface-controlled mode of crystallization in a wide range of the nucleation rate deviations from its steady-state value involves only the first terms of the Kashchiev's infinite series and of the series obtained by re-summation. Using the thermodynamic and kinetic parameters and their temperature dependencies derived from analysis of the isothermal crystallization of the glass investigated [Vasiliev et al, J. Alloys and Compds. 869 (2021) 159,285] the experimental DSC scans were correctly approximated in the frames of the proposed kinetic model. The values of the transient of nucleation rates at continuous heating were estimated and the effect of non-stationarity on thermal stability of metallic glasses was discussed.

Crystallization Kinetics of a Liquid-Forming 2D Coordination Polymer.

We investigated a mechanism of crystal melting and crystallization behavior of a two-dimensional coordination polymer [Ag2(L1)(CF3SO3)2] (1, L1 = 4,4'-biphenyldicarbonitrile) upon heating-cooling processes. The crystal showed melting at 282 °C, and the following gentle cooling induced the abrupt crystallization at 242 °C confirmed by DSC. A temperature-dependent structural change has been discussed through calorimetric, spectroscopic, and mechanical measurements. They indicated that the coordination-bond networks are partially retained in the melt state, but the melt showed a significantly low viscosity of 9.8 × 10-2 Pa·s at Tm which is six orders lower than that of ZIF-62 at Tm (435 °C). Rheological studies provided an understanding of the fast relaxation dynamics for the recrystallization process, along with that the high Tm provides enough thermal energy to crossover the activation energy barrier for the nucleation. The isothermal crystallization kinetics through calorimetric measurements with applying the Avrami equation identified the nature of the nuclei and its crystal growth mechanism.

Enhanced Separation Performance of Hierarchically Porous Membranes Fabricated via the Combination of Crystallization Template and Foaming.

In this work, poly (vinylidene fluoride) (PVDF) hierarchically porous membranes (HPMs) with isolated large pores and continuous narrow nano-pores have been fabricated from its blend with poly (methyl methacrylate) (PMMA) based on the combination of crystallization template with chemical or supercritical CO2 foaming. On the one hand, the decomposition of azodicarbonamide (ADC, chemical foaming agent) or the release of CO2 can produce isolated large pores. On the other hand, PMMA is expelled during the isothermal crystallization of PVDF in their miscible blend, yielding narrow nano-pores upon etching with a selective solvent. In the case of supercritical CO2, the attained PVDF HPMs fail to improve separation performance because of the compact wall of isolated-large-pore and consequent poor connectivity of hierarchical pores. In the case of ADC, the optimal HPM exhibits much higher flux (up to 20 times) without any loss of selectivity compared with the reference only with nano-pores. The enhanced permeability can be attributed to the shorter diffusion length and lower diffusion barrier from isolated large pores, while the comparable selectivity is determined by narrow nano-pores in THE matrix.

Sustained antioxidant properties of epigallocatechin gallate loaded halloysite for PLA as potentially durable materials

AbstractHalloysite nanotube (HNT) was etched by hydrochloric acid, and epigallocatechin gallate (EGCG) was loaded into the etched HNT (HH) to prepare loaded antioxidant (HH‐E) for polylactic acid (PLA) with improved thermal‐oxidative stability. The loading process was confirmed by TGA and FT‐IR, and the loading capacity was 16.15%. The presence of EGCG on the surface of etched HNT was further confirmed by nitrogen adsorption and TEM. The effect of loaded EGCG on thermal‐oxidative stability of PLA was characterized by the change in OOT values of PLA after the aging process under 140°C. The results showed that HH‐E increased OOT values of PLA from 231.2°C to 293.6°C at first, indicating that HH‐E provided PLA with excellent thermal‐oxidative stability. Furthermore, as can be seen from the trend of OOT changing over aging time, during the 14 days aging process, OOT values of PLA composites with HH‐E decreased more slowly than that of PLA composites with EGCG alone, endowing PLA with sustainable thermal‐oxidative stability. Further kinetic analysis showed that the apparent activation energy of the degradation process (Ea) of PLA composites with HH‐E changed less than that of PLA composites with EGCG alone, which showed that the loading system provided PLA better sustainable stability. Moreover, the isothermal crystallization process of modified PLA was characterized by DSC and POM. The results showed that HH‐E provided heterogeneous crystal nucleus for PLA, reducing the half crystallization time (t0.5) and improvement of crystallization rate for PLA. These results prove that HH‐E exhibited excellent performance in improving the thermal stability and crystallization property of PLA, showing multifunctional characterization.

Influence of Latent Heat of Fusion on the Melt Pool Shape and Size in the Direct Laser Deposition Process

The melt pool calculating method is presented based on the solution of the heat conduction problem in a three-dimensional formulation, taking into account the latent heat of fusion and the change in thermophysical properties with temperature. In this case, the phase transitions of melting and crystallization are accounted for using the source method. Considering the latent heat of fusion in the heat transfer process leads to melt pool elongation, as well as to a slight decrease in its width and depth. Depending on the mode, the melt pool elongation can be up to 22%. The penetration depth is reduced by about 5%. The deposition width does not change practically. The presented model was validated by comparing the experimentally determined melt pool shape and its dimensions with the corresponding theoretically calculated results. Experimental data were obtained as a result of coaxial video recording and the melt pool crystallization. The calculated form of the crystallization isotherm changes from a U-shape to a V-shape with an increase in the power and speed of the process, which coincides with the experimental data.