Isothermal Cold Crystallization Research Articles

Structural evolution of ultrasonic microinjection molded sterecomplexes of poly(lactide) during heating process

Two stereocomplexes (SCs) with different molecular weights in poly(L-lactide) (PLLA-49k and PLLA-163k) were prepared by mixing equal amounts of poly(D-lactide) (PDLA) and PLLA via the solution method. The exclusive formation of SCs was achieved in SC-PLLA-49k, whereas both homocrystallites (HCs) and SCs were observed in SC-PLLA-163k. Two blends were subjected to further processing via the ultrasonic microinjection molding technique at varying mold temperatures, yielding amorphous samples. As the samples were heated, the heating induced structural reorganization occurs with the concurrent SCs and HCs for all samples at ca. 80 °C. The degree of crystallinity for HCs XHC exhibits a positive correlation with the mold temperature, whereas the degree of crystallinity for SCs XSC shows a negative dependence on mold temperature for both systems. This behavior can be tentatively explained as follows. Firstly, the demixing of enantiomeric PLA chains occurs in the melt state as evidenced by twice heating measurements of blends. The degree of chain segregation is expected to increase with the increase in mold temperature due to lower supercooling, resulting in a decrease of PLLA/PDLA clusters (SC nuclei). Secondly, a high mold temperature is beneficial for the formation of HC nuclei as the chain mobility is improved. The reformulation of SC nuclei is difficult because of a longer diffusion distance required for PLA molecules. The coexistence of SC and HC nuclei causes the simultaneous appearance of SCs and HCs during heating of samples. As HCs start to melt, a transition from HCs to SCs occurs accompanied by a continuous increase in XSC. For both systems, a maximum of XSC is reached when SCs begin to melt followed by a mold temperature independent evolution of XSC upon further heating. Such a maximum is larger in the low molar mass system because it is less entangled. The reservation of SC nuclei is demonstrated by a comparison of the structural evolution and isothermal cold crystallization behavior between the two blends and their ultrasonic microinjection molded samples.

Crystallization behaviour of polyamide 4 and its effect on melting point

Polyamide 4 (PA4) has received widespread attention as a bio-based polyamide exhibiting biodegradability. However, the melting and degradation processes of PA4 occur simultaneously, making it difficult to perform thermal processing. To expand the difference between the melting point temperature (Tm) and the thermal decomposition temperature (Ttd), the crystallization and melting processes of PA4 were investigated to design strategies for regulating the crystallization. The isothermal crystallization and cold crystallization properties of PA4 were investigated with flash DSC. A special self-nucleation phenomenon which slowed the rate of recrystallization has been discovered. It confirms the significant differences in the morphology of crystals generated at differences temperatures. By controlling the crystallization process, the end-of-melt temperature (T95) was greatly reduced. Nucleation and crystal growth at different temperatures provided a low melting temperature (T95 = 246 °C), which was 20 °C lower than that for isothermal crystallization.

Influence of low contents of cellulose nanocrystals on the crystallization behavior of biobased Poly(propylene 2,5-furandicarboxylate)

To address the slow crystallization problem of poly(propylene 2,5-furandicarboxylate) (PPF), fully biobased PPF/cellulose nanocrystals (CNC) composites were prepared via a solution and casting method in this work at low CNC contents. The influence of CNC on the crystallization of PPF was studied in detail under various conditions, including nonisothermal and isothermal melt crystallization as well as nonisothermal and isothermal cold crystallization. The results revealed that the crystallization of PPF was enhanced under various crystallization conditions by CNC due to the heterogeneous nucleating agent role. From the isothermal crystallization kinetics study, the presence of CNC did not alter the crystallization mechanism of PPF. In addition, the thermogravimetric analysis results showed that both PPF and PPF/CNC composites displayed an excellent thermal stability. On the basis of the wide angle X-ray diffraction study, CNC did not change the crystal structure of PPF.

Rigid amorphous fraction in isotactic polystyrene induced by cold crystallization: Characterization by differential scanning calorimetry and ellipsometry

AbstractRigid amorphous fraction (RAF) in bulk and thin‐film states of isotactic polystyrene (iPS) was studied as a function of crystalline fraction (CF). The CF increased with isothermal cold crystallization in initially amorphous, melt‐quenched bulk iPS samples. Assessed by differential scanning calorimetry, RAF in bulk iPS increased and then decreased with increasing crystallization time and CF. Specific RAF, that is, RAF/CF, decreased with increasing CF. Using ellipsometry, similar behaviors of RAF and specific RAF as functions of CF were observed in initially amorphous iPS thin films. These results indicate that specific RAF in iPS exhibits a continuous decrease from slightly below 2 at very low CF to ~0.2 at the maximum CF, independent of sample type and characterization method. This trend of decreasing specific RAF is consistent with a progression of crystal perfection during cold crystallization which is associated with lamellae thickening and decoupling between crystalline and amorphous regions. Qualitatively similar behaviors of RAF and specific RAF have been reported in other semi‐crystalline polymers and in our recent studies of syndiotactic polystyrene, indicating that this trend of decreasing specific RAF with increasing CF is general for many semi‐crystalline polymers. Finally, maximum CF is strongly reduced by nanoscale confinement in thin iPS films.

Crystallization kinetics in the chiral compound showing the vitrification of the smectic CA* phase for moderate cooling rates

ABSTRACT The crystallization of the 3F5FPhF6 compound from the liquid crystalline antiferroelectric smectic CA* phase is investigated in various conditions using differential scanning calorimetry and polarizing optical microscopy. The kinetics of isothermal melt and cold crystallization is studied on the basis of the Avrami model. The kinetics of non-isothermal cold crystallization is analysed using the Ozawa model and the activation energy is determined by the Augis-Bennett, Friedman’s differential isoconversional and Matusita methods. The border temperature between the diffusion-controlled crystallization and thermodynamically-controlled crystallization is estimated. The high fragility index of 105 is determined from the α-process in the smectic CA* phase by the broadband dielectric spectroscopy. The metastable, conformationally disordered crystal phase, forming only via cold crystallization, is reported.

Phase Sequence, Kinetics of Crystallization and Molecular Dynamics of the Chiral Liquid Crystalline Compound Forming a Hexatic Smectic Glass

The vitrification of the antiferroelectric hexatic smectic XA* phase and cold crystallization are reported for (S)-4′-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]benzoate. The kinetics of isothermal cold crystallization and melt crystallization are investigated, revealing that both are controlled mainly by diffusion, as indicated by decrease in the characteristic crystallization time with increasing temperature of crystallization, with an activation energy of 114 kJ/mol. A weak relaxation process is detected in a crystal phase, with an activation energy of 38 kJ/mol, implying the conformationally disordered crystal phase. The estimated fragility parameter of the investigated glass former is equal to 94.5, which indicates rather high fragility.

Kinetics of cold crystallization in two liquid crystalline fluorinated homologues exhibiting the vitrified smectic CA* phase

Dielectric relaxation processes in the supercooled antiferroelectric smectic CA* phase and crystallization kinetics of two chiral fluorinated 5HF6 and 6HF6 compounds from the same homologous series are investigated. Fragility parameters are determined from the relaxation time of the α-process, including τHN from the Havriliak-Negami formula and τpeak denoting the position of the absorption peak. The coupling coefficient ξ between the characteristic time of isothermal cold crystallization and relaxation time of the α-process is obtained. Despite similar values of the fragility index, the even 6HF6 homologue undergoes cold crystallization much faster than the odd 5HF6 homologue, with significantly different ξ coefficients. Influence of the relaxation time of the PH process (anti-phase phason) in the smectic CA* phase on the crystallization kinetics is presumed.

Development of rigid amorphous fraction in cold‐crystallized syndiotactic polystyrene films confined near the nanoscale: Novel analysis via ellipsometry

AbstractRigid amorphous fraction (RAF) in syndiotactic polystyrene (sPS) thin films was studied as a function of crystalline fraction (CF). In contrast to bulk sPS, confined films yield close‐to‐amorphous sPS, thus allowing for a complete description of RAF development. To study thin‐film samples, we developed an ellipsometry‐based method to quantify CF and RAF. Maximum crystallinity was thickness dependent, indicating a strong effect of near‐nanoscale confinement on CF. ~360‐nm‐thick spin‐coated films were isothermally cold crystallized to tune CF from 1% to a maximum of 12%. Combining RAF characterized at low CF here by ellipsometry with results from an earlier differential scanning calorimetry (DSC)‐based study on bulk sPS with higher CF, a complete picture was obtained of RAF development as a function of CF in cold‐crystallized sPS. RAF increases from ~4% at 1% CF to ~16% for 10% < CF < 25%; above 25% CF, RAF decreases monotonically with increasing CF, reaching ~7% at a maximum 50% CF. Specific RAF (=RAF/CF) decreases with increasing CF, consistent with increasing crystal perfection with isothermal cold crystallization. This ellipsometry method can be an important tool for evaluating RAF, mobile amorphous fraction (MAF), and CF in semi‐crystalline polymer films and for studying confinement effects on RAF, MAF, and CF.

Composites of a PLA with SBA-15 mesoporous silica: Polymorphism and properties after isothermal cold crystallization

Composites based on an L-rich poly(lactic acid), PLA, and different contents in mesoporous SBA-15 silica have been prepared by extrusion in order to evaluate the influence of incorporation of mesoporous particles on the structural PLA features and on their ultimate mechanical performance. For that, the samples have been isothermally cold crystallized for relatively long times at different temperatures, ranging from 65 to 120 °C. The results from DSC, X-ray diffraction and Raman spectroscopy show that the pure α′ modification is obtained at the lower annealing temperatures, while the α form is in majority above 85 °C. The temperature range at which α′ and α polymorphs were developed is somewhat atypical, being associated with the particular D stereoisomer content of the PLA used. Furthermore, several phase transitions are observed depending on the crystallinity reached and the polymorphs developed during the isothermal crystallization from the glass: an additional cold crystallization, the α′/α transformation and the subsequent melting processes. These transitions have been evaluated thoroughly by DSC measurements and real-time variable temperature experiments with synchrotron radiation. It was also found that rigidity of these materials is influenced by both the presence of SBA-15 particles and by the amount and perfection of crystallites. Thus, the relative reinforcement effect of SBA-15 particles is importantly reduced by differences in long spacing PLA values for the materials crystallized between 85 and 120 °C, mainly in the composites with the lowest SBA-15 content. Those are the temperatures where the α polymorph is the major existing crystalline structure.

Exceptional cold-crystallization kinetics of erythritol-polyelectrolyte enables long-term thermal energy storage

Long-term thermal energy storage balances the seasonal variations in renewable energy supply and demand, but applied storage concepts require improved performance in efficiency, reliability and capacity. In principle, supercooling and cold-crystallization offer a way to store heat for an extensive amount of time. In this approach, crystallization behaviour of the material governs the storage performance, as it directly relates to optimal efficiency, length of the storage period and heat release properties. This work explains the unique cold-crystallization behaviour of erythritol in cross-linked sodium polyacrylate. To this end, isothermal cold-crystallization was measured experimentally and analysed with the Avrami equation. Although the cold-crystallization rate constant follows the Arrhenius equation, it drastically decreases near the glass transition region and diverges from the equation. Thermal history also influences the cold-crystallization behaviour. Increases in cooling end-temperature reduce the subsequent crystallization time and promote metastable polymorph formation. These findings stem from the peculiar energy landscape of erythritol in cross-linked sodium polyacrylate. The landscape is classified as kinetically strong and thermodynamically fragile, which facilitates long-term thermal energy storage. Consistent supercooling and cold-crystallization behaviour of the material enables predicting the time-dependent crystallization rate at different temperatures. This confirms applicability of the two-stage Arrhenius-VFT model for temperature dependence and supports storage design in real-life applications. • Cold-crystallizing PCM for long-term TES with 230–350 MJ/m 3 storage capacity. • Spherical cold-crystallization enables modelling and predicting the kinetics. • Cold-crystallization follows Avrami equation and combined Arrhenius-VFT dependency. • Crystallization rate decreases drastically when approaching glass transition region. • Reducing cooling temperature increases following isothermal crystallization rate.

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.

Isothermal cold crystallization of antiferroelectric liquid crystal 3F5BFBiHex

Isothermal cold crystallization of liquid crystalline substance of (2S)-octano-2-yl4′-(2-fluoro-4-{[5-(1,1,2,2,3,3,3heptafluoropropoxyy)pentylo]oxy}benzoyloxy)-[1,1′-biphenyl]-4-carboxylate (3F5BFBiHex) from antiferroelectric phase has been investigated by complementary method, i.e., differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and polarizing microscopy observations (POM). We employed the Avrami and Avramov approach to determine the dimensionality (n) and crystallization constant rate (k) parameters. The monitored crystal growth occurred to be two-dimensional. Two kinds of crystallization mechanisms were found depending on the temperature range. The kinetic factor dominated for the low-temperature range, while for higher temperatures the importance of the thermodynamic factor increased. The change in crystallization mechanisms was confirmed by the activation energy values, which for the high-temperature range was about 20 kJ/mol (DSC method) and about 30 kJ/mol (BDS method) higher than for the low-temperature range.

Improving the stereocomplexation and toughness of poly(L-lactic acid)/poly(D-lactic acid) blends via melt blending with ethylene/methyl acrylate/glycidyl methacrylate terpolymer

In this study, the ethylene/methyl acrylate/glycidyl methacrylate (EGA) terpolymer was performed to improve the stereocomplexation and toughness of poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA). The effects of EGA presence on the competing formations and crystallization kinetics of homocrystallites (HCs) and stereocomplexes (SCs), phase morphology and mechanical properties of PLLA/PDLA blends were investigated. For non-isothermal melt crystallization, the degree of SC crystallinity of PLLA/PDLA blends increased after PLLA/PDLA blending with EGA. For both isothermal and non-isothermal cold crystallization, the crystallization rate of PLLA/PDLA blends increased as the mass fraction of EGA in the blends grew from 10% to 30%. The incorporation of EGA was observed to facilitate the HC-to-SC melt reorganization of PLLA/PDLA blends; moreover, a significant enhancement in the toughness of the PLLA/PDLA matrix was achieved by incorporating the EGA elastomer. When EGA was added to produce a blend containing 30 wt%, the blend’s impact strength increased from 6.0 (for PLLA/PDLA) to 44.9 kJ/m2. Overall, this study provides effective guidance for achieving the melt-processability of PLLA/PDLA blends characterized by improved stereocomplexation ability and toughness.

Crystallization Behaviours of Poly(vinylidene fluoride) (PVDF) Nanocomposites with MoS₂ Nanosheets with Different Surface Functional Groups.

The crystallization behaviours of amorphous poly(vinylidene fluoride) (PVDF) nanocompositesmodified with two different kinds of molybdenum disulfide (MoS₂) at different filler loadings were investigated in detail in this work. The crystallinity, melting temperature and crystallization temperature of the PVDF/MoS₂ nanocomposites were transformed from α-phase to β-phase with the addition of MoS₂, MoS₂-COOH and MoS₂-NH₂. During isothermal cold crystallization, the overall crystallization rate of PVDF was slowed with increased MoS₂ loading relative to that of neat PVDF. Moreover, the crystallization temperature of the PVDF nanocomposites increased with the addition of MoS₂ despite the cooling rate during nonisothermal cold crystallization. DMA tests showed that the storage modulus of PVDF was decreased with the addition of MoS₂, while those of PVDF/MoS₂-COOH and PVDF/MoS₂-NH₂ were enhanced to different degrees. The decomposition of the PVDF/MoS₂ nanocomposites were also discussed. Relative to neat PVDF, the thermal stability of PVDF was obviously improved with the addition of MoS₂, MoS₂-COOH and MoS₂-NH₂, which could be ascribed to the increased degree of crystallinity.

Mesomorphic and dynamic properties of 3F5BFBiHex antiferroelectric liquid crystal as reflected by polarized optical microscopy, differential scanning calorimetry and broadband dielectric spectroscopy

This paper presents studies of the influence of various thermal treatments on molecular dynamics and phase behavior of 3F5BFBiHex antiferroelectric liquid crystal by a combination of polarized optical microscopy (POM), differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). Three liquid crystalline phases: the paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC*A phases, and vitrification of SmC*A were found upon cooling. The cold crystallization of phase Cr was observed only upon heating at the rate lower than 1.5 K/min. During heating faster than 1.5 K/min 3F5BFBiHex shows the same phase sequence as on cooling. Two relaxation processes in the SmC*A phase obey the Vogel−Fulcher−Tammann (VFT) relation when approaching the glass transition temperature (Tg). An unusual finding that relaxation processes are preserved upon the SmC*A – Cr transition on heating is presented in detail. Finally, the time evolution of relaxation parameters during isothermal cold crystallization process in SmC*A is discussed.

Non-isothermal and isothermal cold crystallization of glass-forming chiral smectic liquid crystal (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-benzoate

The mesomorphic behaviour as well as the kinetics of the isothermal and non-isothermal cold crystallization of the liquid-like smectic SmCA⁎ phase in (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-benzoate (3F7HPhH7) were studied using differential scanning calorimetry, polarized optical microscopy and Fourier transform infrared spectroscopy supported by quantum chemical DFT calculations. The process of cold crystallization under isothermal conditions was treated in terms of Avrami and Avrami-Avramow models, and under non-isothermal conditions it was described by Ozawa, Mo, Kissinger and Augis-Bennett models. Non-isothermal cold crystallization of the 3F7HPhH7 compound was achieved by two different mechanisms: one depending on diffusion rates (for slow heating) and second one based on the nucleation process (for fast heating). The activation energy of non-isothermal cold crystallization was found equal to 130 and 50 kJ/mol for slow and fast heating, respectively. Cold crystallization under isothermal conditions depends primarily on diffusion rates with the activation energy equalled 133 kJ/mol. The obtained values of the Avrami exponent nA and the Ozawa exponent nO suggest that the crystal growth is three-dimensional.

Isothermal cold crystallization kinetics and properties of thermoformed poly(lactic acid) composites: effects of talc, calcium carbonate, cassava starch and silane coupling agents

The effects of three different fillers (i.e., talc, calcium carbonate, and cassava starch) and surface functionalization by 3-aminopropyltriethoxysilane (APTES) and vinyltriethoxysilane (VTES) on morphology, thermal and tensile properties of the poly(lactic acid) (PLA) composites were comparatively examined. Dynamic differential scanning calorimetry (DSC) results revealed that the incorporation of filler can facilitate the cold crystallization of PLA, as confirmed by lowered cold crystallization temperature. By fitting DSC data with Avrami model, the highest isothermal cold crystallization rate constant k and the shortest crystallization half time t1/2 were obtained for the PLA/talc composites under isothermal temperature of 100 °C, implying that talc was the most effective nucleating agent for PLA in this study. The average Avrami index n of neat PLA and its composites lay within the same range of 3.0–3.6, which reflected the three-dimensional spherulitic growth of PLA with the mixture of instantaneous nuclei and sporadic nuclei. In comparison with the composite cast films, the thermoformed films had higher degree of crystallinity as well as higher tensile strength and Young’s modulus owing to the chain orientation upon annealing. Furthermore, the addition of silane-treated talc, especially APTES-treated talc, fastened cold crystallization rate and enhanced tensile properties because of the improved interfacial interaction between talc particles and PLA matrix.

Relaxation dynamics and crystallization study of glass-forming chiral-nematic liquid crystal S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl 9H-fluorene (5P-Am*FLAm*-P5)

.The chiral nematic S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl9H-fluorene (5P-Am*FLAm*-P5) liquid crystal shows a complex phase diagram strongly dependent on thermal treatment as identified by Polarizing Optical Microscopy (POM) and differential scanning calorimeter (DSC). The molecular dynamics in various thermodynamics states was studied by means of broadband dielectric spectroscopy (BDS). The vitrification of a chiral nematic phase (N*) is manifested by a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of structural relaxation time (tau_{alpha}). Three dielectric relaxation processes exhibiting Arrhenius-like thermal activation were found in conformationally disordered (condis) Cr1 and Cr2 structures. The isothermal cold crystallization process of Cr2 occurs in the metastable N* phase; however, in the non-isothermal experiments, the Cr2 phase is formed in the isotropic phase obtained on heating the metastable N* phase. The findings for the isothermal process were compared with those regarding non-isothermal crystallization.Graphical abstract

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Borassus powder‐reinforced poly(lactic acid) composites with improved crystallization and mechanical properties

ABSTRACTThis article reports on the development of biocomposites based on polylactic acid (PLA) and borassus powder. Borassus powder was treated with alkali to remove hemicelluloses and lignin. The treated borassus improved the homogeneous mixing with PLA and increased the crystallinity of PLA. Dispersibility of the borassus was studied by scanning electron microscopy (SEM) and X‐ray MicroCT. PLA/borassus composites were prepared by melt mixing of PLA with 5, 10, and 15 wt % treated/untreated borassus. Composites were examined for mechanical properties and crystallization. Composites showed enhanced tensile strength compared to neat PLA. The PLA/treated borassus powder composites displayed higher crystallinity than PLA. The isothermal cold crystallization study showed increase in the crystallization rate of PLA in the presence of treated borassus. The spherulitic growth was studied using polarized optical microscopy. The enhanced performance of the PLA‐borassus composites was observed in the presence of borassus. This study demonstrates that the PLA‐borassus composites show great promise for bioplastics applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47440.