Cold Crystallization Rate Research Articles

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.

Effect of Organic Modifier and Clay Content on Non-Isothermal Cold Crystallization and Melting Behavior of Polylactide/Organovermiculite Nanocomposites.

In clay/polymer nanocomposites, the crystallization behavior and kinetics of the polymer can be affected by the presence of clay, its content and the degree of miscibility between the clay and the polymer matrix. The effect of two different organomodified vermiculites on the non-isothermal cold crystallization and melting behavior of polylactide (PLA) was studied by differential scanning calorimetry (DSC). In the presence of vermiculites, the cold crystallization of PLA occurred earlier, particularly for the highest content of the most miscible organovermiculite with PLA. The cold crystallinity of PLA decreased at low heating rates, notably at high organoclay loadings, and increased at high heating rates, especially at low vermiculite contents. According to the crystallization half-time, crystallization rate coefficient (CRC), and crystallization rate parameter (CRP) approaches, the cold crystallization rate of PLA increased by incorporating vermiculites, with the effect being most noteworthy for the vermiculite showing better compatibility. The Mo model was successful in describing the non-isothermal cold crystallization kinetics of the PLA/vermiculite composites. The melting behavior was affected by the heating rate and the type and content of clay. The nucleating effect of the most compatible clay resulted in the less perfect crystallites. The activation energy was evaluated using the Kissinger and Takhor methods.

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.

Influence of the combination of nucleating agent and plasticizer on the non-isothermal crystallization kinetics and activation energies of poly(lactic acid)

The influence of the combination of the nucleating agent (talc, N, N′-ethylene bis-stearamide (EBS) and a commercial nucleating agent NT-20) and the plasticizer (poly(ethylene glycol), PEG) on the melting and crystallization behaviors of poly(lactic acid) (PLA) was investigated by means of differential scanning calorimetry (DSC). The Jeziorny’s, Ozawa’s and Mo’s models were used to describe the non-isothermal cold and melt crystallization kinetics of the modified PLA samples. The non-isothermal cold and melt crystallization activation energies were evaluated by Kissinger’s method and Friedman’s method, respectively. The results show that the non-isothermal cold and melt crystallization kinetics of the samples are successfully analyzed by Jeziorny’s and Mo’s models, whereas Ozawa’s model is only suitable for the non-isothermal melt crystallization kinetics of PLA/talc sample. It is shown that the combination of one nucleating agent and PEG results in the synergistic effect on the cold and melting crystallization rates of PLA. Moreover, the combination of dual nucleating agents and PEG improves the cold crystallization rate but hinders the melt crystallization rate. It is indicated that dual nucleating agents act as physical hindrance to the molecular chain movement of PLA which results in the increases in the cold and melt crystallization activation energies.

Electrospun Graphene Nanosheet-Filled Poly(Trimethylene Terephthalate) Composite Fibers: Effects of the Graphene Nanosheet Content on Morphologies, Electrical Conductivity, Crystallization Behavior, and Mechanical Properties.

In this study the effects of increased graphene nanosheet (GNS) concentration on variations in the structure and properties of electrospun GNS-filled poly(trimethylene terephthalate) (PTT/GNS) composite fiber, such as its morphologies, crystallization behavior, mechanical properties, and electrical conductivity, were investigated. The effects of GNS addition on solution rheology and conductivity were also investigated. GNSs were embedded in the fibers and formed protrusions. The PTT cold crystallization rate of PTT/GNS composite fibers increased with the gradual addition of GNSs. A PTT mesomorphic phase was formed during electrospinning, and GNSs could induce the PTT mesomorphic phase significantly during PTT/GNS composite fiber electrospinning. The PTT/GNS composite fiber mats (CFMs) became ductile with the addition of GNSs. The elastic recoveries of the PTT/GNS CFMs with 170 °C annealing were better than those of the as-spun PTT/GNS CFMs. Percolation scaling laws were applied to the magnitude of conductivity to reveal the percolation network of electrospun PTT/GNS CFMs. The electrical conductivity mechanism of the PTT/GNS CFMs differed from that of the PTT/GNS composite films. Results showed that the porous structure of the PTT CFMs influenced the performance of the mats in terms of electrical conductivity.

Preparation and Characteristics of Poly(butylene adipate-co-terephthalate)/Polylactide Blend Films via Synergistic Efficiency of Plasticization and Compatibilization

Polylactide (PLA) films blended with poly(butylene adipate-co-terephthalate) (PBAT) were hot melted using a twin screw extruder with the addition of triethyl citrate (TEC) as a plasticizer and toluene diisocyanate (TDI) as a compatibilizer. The synergistic effects of the two additives on the mechanical, thermal, and morphological properties of the PLA/PBAT blend films were investigated. The influence of TEC content on the plasticized PLA films and the effect of TDI’s presence on the PLA/PBAT blend films were also studied by comparing them with neat PLA. The results showed a pronounced increase in elongation at break of the plasticized PLA films with increasing TEC levels, but a slight reduction in thermal stability. Also, the addition of TEC and TDI to the blend system not only synergistically enhanced the tensile properties and tensile-impact strength of the PLA/PBAT blends, but also affected their crystallinity and cold crystallization rate, a result of the improvement of interfacial interaction between PLA and PBAT, including the enhancement of their chain mobility. The synergy of the plasticization and compatibilization processes led to the improvement of tensile properties, tensile-impact strength, and compatibility of the blends, accelerating cold crystallization without affecting crystallization.

Effect of heat cycling on melting and crystallization of PHB/TiO2 compounds

Compounds of poly(3-hydroxybutyrate) (PHB) and titanium dioxide (TiO2) with filler content between 1% and 10% were prepared in a laboratory internal mixer. The effect of heating and cooling rates on the crystallization and melting of PHB/TiO2 compounds was investigated by differential scanning calorimetry (DSC). Melt and cold crystallization rates rise with increasing cooling/heating rates. A higher cooling rate translates to a lower melt crystallization temperature, while a higher heating rate results in a higher cold crystallization temperature. TiO2 promotes melt crystallization of PHB, behaving as a nucleant agent. The total crystallinity developed after melt and cold crystallization decreases for low levels of TiO2, i.e. 2% per weight, and is almost independent of the heating/cooling rate. The melting temperatures and rates are minimally affected by both the heating rate and filler content. The results suggest that the desired PHB microstructure can be controlled by filler content and adjusted heating/cooling rate.

Thermal behaviors and characteristics of polylactide/poly(butylene succinate) blend films via reactive compatibilization and plasticization

Polylactide (PLA)/poly(butylene succinate) (PBS) blend films modified with a compatibilizer and a plasticizer were hot‐melted through a twin screw extruder and prepared by hydraulic press. Toluene diisocyanate (TDI) and polylactide‐grafted‐maleic anhydride (PLA‐g‐MA) were used as compatibilizers, while triethyl citrate and tricresyl phosphate acted as plasticizers. The effects of the type and content of compatibilizer and plasticizer on the mechanical characteristics, thermal properties, crystallization behavior, and phase morphology of the PLA/PBS blend films were investigated. Reactive compatibilization at increasing levels of TDI improved the compatibility of the PLA and PBS, affecting the toughness of the films. As evidenced by scanning electron microscope, the addition of TDI enhanced the interfacial adhesion of the blends, leading to the appearance of many elongated fibrils at the fracture surface. Furthermore, PLA/PBS blending with both TDI and PLA‐g‐MA led to an acceleration of the cold crystallization rate and an increment of the degree of crystallinity ( ). Toluene diisocyanate could be a more effective compatibilizer than PLA‐g‐MA for PLA/PBS blend films. The synergistic combination of compatibilizer and plasticizer brought a significant improvement in elongation at break and tensile‐impact toughness of the PLA/PBS blends, compared with neat PLA. Their failure mode changed from brittle to ductile due to the improved compatibility and molecular segment mobility of the PLA and PBS phases. Differential scanning calorimeter results revealed that the plasticizers triethyl citrate and tricresyl phosphate changed the thermal behavior of Tcc and Tm, affecting α′ and α crystal formations. However, these plasticizers only slightly improved the thermal stability of the films.

Influence of TiO2 Nanoparticles on the Crystallization Behaviour and Tensile Properties of Biodegradable PLA and PCL Nanocomposites

This paper investigated the influence of TiO2 nanoparticles on the morphologies, as well as crystallization behaviour and kinetics, of neat PLA and PCL, and of these polymers in different PLA/PCL blends. We used transmission electron microscopy to evaluate the morphologies of the systems, while the crystallization behaviour and kinetics were investigated through differential scanning calorimetry (DSC). In addition to standard and modulated (StepScan) DSC analyses, the self-nucleation temperatures of neat PCL and PCL in the different nanocomposites were determined, followed by a self-nucleation and annealing thermal fractionation analysis of PCL crystallization and an Avrami isothermal kinetic analysis of PCL crystallization and PLA cold crystallization. We found that the nanoparticles were well dispersed, but only in the PLA phase of the blends, with only a few on the interface or in the PCL phase. They did nucleate and accelerate, and influence the mechanism of, the PCL crystallization in neat PCL, but had little influence on PCL crystallization in the blends. They strongly influenced the rate of cold crystallization of PLA, but had little influence on this parameter in the blends. The tensile properties were also determined, and changes in these properties could be related to the morphologies of the systems.

Isothermal and nonisothermal cold crystallization kinetics of poly(l-lactide)/functionalized eggshell powder composites

Functionalized eggshell powder (NES) with nucleating surface of calcium phenylphosphonic acid (PPCa) for poly(l-lactide) (PLLA) was compounded with PLLA via melt blending to improve the cold crystallization process of PLLA. The cold crystallization behavior of the PLLA/NES composites was studied by differential scanning calorimetry. The isothermal cold crystallization rates have been enhanced obviously in the PLLA/NES composites than in the neat PLLA, indicative of the excellent nucleating effects of NES on PLLA. For the nonisothermal cold crystallization, the overall crystallization rate of PLLA increased with both increasing NES loadings and heating rate. It was found that the Avrami equation and the combined Ozawa–Avrami model could describe the experiment data successfully.

Melting and crystallization of PHB/ZnO compounds

Compounds of poly(3-hydroxybutyrate) (PHB) and zinc oxide with filler content between 1 and 10% were prepared in a laboratory internal mixer. The effect of heating and cooling rates on the crystallization and melting of PHB/ZnO compounds was investigated by differential scanning calorimetry. Physical aspects of the compounds were examined by means of scanning electron microscopy and optical microscopy. Melt and cold crystallization rates rise with increasing cooling/heating rates. The melt crystallization temperatures and maximum melt crystallization rates are almost unaffected by the filler, independent on the filler concentration. The main effect of ZnO on PHB is to reduce the overall crystallinity obtained after a successive melt and cold crystallization events. For defined filler content, Mo model can be applied for the description of tested and untested heating/cooling rates, provided that a lower accuracy is acceptable.

Enhanced dispersion and interface compatibilization of crystalline nanocellulose in polylactide by surfactant adsorption

In this study, the surface polarity of sulfated crystalline nanocellulose (CNC) was tailored using an ecologically compatible surface modification strategy. Lauric arginate, a novel biologically-derived cationic surfactant, was ionically bonded to the CNC surface sulfate groups forming a monolayer that significantly increased surface hydrophobicity. Both unmodified (P-CNC) and surfactant modified (F-CNC) were incorporated into a non-polar PLA matrix to study their reinforcing effect. The P-CNC, ascribed to its inherent hydrophilic characteristic, had limited nucleating and reinforcing effect on the PLA matrix. Large nanoparticle aggregation and interface debonding were easily discernable in P-CNC/PLA nanocomposite films. The hydrophobic F-CNC, by contrast, had a much better dispersibility and interface compatibility within the PLA matrix. The cold crystallization rate, crystallinity, storage modulus (glassy and rubbery states), glass transition temperature, and tensile strength and modulus of F-CNC/PLA nanocomposite films were remarkably enhanced with appropriate loading level of F-CNC (<5 wt%). These results demonstrate an efficient route to increase the hydrophobicity of CNC for its enhanced nanoreinforcing effect in various non-polar matrices.

Non-isothermal kinetics of cold crystallization in multicomponent PLA/thermoplastic polyurethane/nanofiller system

The effect of platy and tubular nanofillers (NF) in combination with thermoplastic polyurethane (TPU) on cold non-isothermal crystallization (NIC) of poly(lactic acid) (PLA) was studied by differential scanning calorimetry. The data were processed using a recently proposed method of evaluating NIC kinetics. The results indicate that the NF/TPU combination leads to crystallization behaviour of the PLA matrix which is dissimilar from that of the NF-free blend and neat PLA. Addition of organophilized montmorillonite (C30) results in a dual effect on PLA crystallinity—stimulation at lower concentrations and suppression at higher loadings. In the case of halloysite nanotubes (HNT), this effect is practically absent. Crystallinity of injection-moulded samples is significantly increased by cold NIC. This indicates a synergy originating in a complex effect of NF/TPU, e.g. NF-induced changes in morphology and interphase parameters supporting the solid annealing process. TPU partially eliminates negative effect of high NF contents on cold crystallization rate. This effect is more marked in the remelted samples where the rate increases for all NF loadings. This is probably caused by absence of as-prepared crystallites in combination with higher mobility of PLA chains due to presence of TPU. The observed differences between C30 and HNT could be attributed to differences in size, shape, and specific surface. The results indicate that synergistic and antagonistic effects of NF/polymeric modifiers on mechanical and other material parameters are also significantly determined by the effects on crystallization of the matrix.

Isothermal and nonisothermal crystallization kinetics of novel biobased poly(ethylene succinate-co-ethylene sebacate) copolymers from the amorphous state

In this work, the isothermal and nonisothermal crystallization kinetics of three novel biobased poly(ethylene succinate-co-ethylene sebacate) (PESSe) copolymers was systematically investigated with differential scanning calorimetry under different crystallization conditions from the amorphous state. For the isothermal cold crystallization kinetics study, the Avrami equation could well describe the crystallization process of PESSe at various crystallization temperatures. All three PESSe copolymers crystallized through the same crystallization mechanism; moreover, the overall isothermal cold crystallization rate of PESSe decreased with increasing ethylene sebacate (ESe) comonomer content. The nonisothermal cold crystallization kinetics of PESSe was also studied at different heating rates. With increasing ESe content or heating rate, the nonisothermal cold crystallization exotherm of PESSe copolymers shifted to high temperature range. Both the crystallization rate parameter and crystallization rate coefficient of PESSe copolymers decreased with increasing ESe content, indicating that PESSe copolymer with higher ESe content crystallized more slowly than that with lower ESe content. The Ozawa equation was used to analyze the nonisothermal cold crystallization kinetics of PESSe copolymers, which was found to fit the crystallization process very well.

Polylactide/cellulose nanocrystal composites: a comparative study on cold and melt crystallization

Polylactide (PLA) composites with pristine cellulose nanocrystals (CNC) and acetylated one (aCNC) were prepared for the crystallization study. The roles of CNC and aCNC in cold and melt crystallization of PLA were explored. Both CNC and aCNC have good nucleation activity during cold crystallization of PLA, but also highly impede transport of adjacent chain segments to the growing surface, acting as the role of physical barrier in the glassy bulk. Within the experimental temperature range, growth dominates the overall kinetics, rather than nucleation. Therefore, barrier role overwhelms nucleation agent one and as a result, the cold crystallization rates of composites decrease as compared with neat PLA, accompanied by decreased degrees of crystallinity. During melt crystallization, although the presence of CNC and aCNC leads to sharply increased system viscosities, reducing chain mobility, nucleation is the dominant role as the systems crystallize from the melts. Thus, the presence of CNC and aCNC promotes melt crystallization of PLA, and the composites show far higher crystallization rates and degrees of crystallinity than neat PLA. Besides, the surface acetylation of CNC improves its nucleation ability during melt crystallization of PLA, and as a result, the composite with aCNC has denser fold surfaces relative to the one with CNC. But the presence of these two kinds of particles has no evident influence on the lamellar structure of PLA whether in the cold or in melt crystallization. This work can provide useful information on the crystallization control of PLA using CNC.

Bleached extruder chemi‐mechanical pulp fiber‐PLA composites: Comparison of mechanical, thermal, and rheological properties with those of wood flour‐PLA bio‐composites

ABSTRACTAn environmentally friendly bleached extruder chemi‐mechanical pulp fiber or wood flour was melt compounded with poly(lactic acid) (PLA) into a biocomposite and hot compression molded. The mechanical, thermal, and rheological properties were determined. The chemical composition, scanning electron microscopy, and Fourier transform infrared spectroscopy results showed that the hemicellulose in the pulp fiber raw material was almost completely removed after the pulp treatment. The mechanical tests indicated that the pulp fiber increased the tensile and flexural moduli and decreased the tensile, flexural, and impact strengths of the biocomposites. However, pulp fiber strongly reinforced the PLA matrix because the mechanical properties of pulp fiber‐PLA composites (especially the tensile and flexural strengths) were better than those of wood flour‐PLA composites. Differential scanning calorimetry analysis confirmed that both pulp fiber and wood flour accelerated the cold crystallization rate and increased the degree of crystallinity of PLA, and that this effect was greater with 40% pulp fiber. The addition of pulp fiber and wood flour modified the rheological behavior because the composite viscosity increased in the presence of fibers and decreased as the test frequency increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44241.

Non-isothermal cold crystallization kinetics of poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate) (PETG) copolyesters with different compositions

In this work, the non-isothermal crystallization kinetics of poly(ethylene glycol-co-1,4-cyclohexanedimethanol terephthalate) (PETG) random copolyesters with different compositions was first investigated from the glassy state. Regardless of the composition, the value of the Avrami exponent determined from Jeziorny theory was 2.1–2.4, depending on the heating rate, whereas the value of α (the ratio of the Avrami exponent to the Ozawa exponent) obtained from Mo's method was 1.3–1.6, depending on the relative crystallinity. These results indicated that the nucleation and growth mechanism of the PETG copolyesters were apparently identical to those of the poly(ethylene terephthalate) (PET) homopolymer. However, the cold crystallization rate was accelerated by increasing the heating rate, but retarded by incorporating the 1,4-cyclohexanedimethanol (CHDM) unit into the polymer backbone. Based on the crystallization rate coefficient (CRC) and the crystallization rate parameter (CRP) approaches, the cold crystallization rate was in the following order: PET > PETG(30/70) > PETG(85/15) > PETG(50/50) > PETG(70/30).

The effect of Pglass state on the non-isothermal cold and melt crystallization processes of PET matrix

The physical state of phosphate glass (Pglass) has an influence on the non-isothermal crystallization behaviors of PET matrix in the PET/Pglass blends, which has been investigated via heating the glassy state and cooling the melt state of the blends at various scanning rates, respectively, by means of differential scanning calorimetry (DSC) technique. The kinetics models based on the Avrami and Mo equations were used to analyze the non-isothermal crystallization process. Furthermore, the activation energy of non-isothermal crystallization, according to Kissinger theory for heating process and Friedman theory for cooling process, has been evaluated. The results showed that the Pglass accelerated the non-isothermal cold crystallization rate of PET matrix due to its nucleation effect. In contrast, for the non-isothermal melt crystallization, the Pglass hindered the crystallization process due to its large melt viscosity.

Effect of polystyrene on poly(ethylene terephthalate) crystallization

Blends of poly(ethylene terephthalate) (PET) and polystyrene (PS) with PS content ranging from 0% to 60% by weight, were compounded in a laboratory internal mixer, followed by quenching into iced water, resulting in substantially amorphous compounds. Blend morphology was analyzed by scanning electron microscopy (SEM). Standard differential scanning calorimetry (DSC) was used to study the cold crystallization and melting behavior of PET and PET/PS blends. SEM micrographs show a two-phase structure made up with spherical PS particles dispersed in a PET matrix. DSC scans showed that crystallization of PET is affected by the heating rate and by the addition of even small amounts of PS. In particular, the rate of cold crystallization of PET is significantly reduced by the incorporation of 1% of PS, attributed to the anti-nucleating effect of PS on PET. The amount of PET crystallized and the melting characteristics (temperature, rate) are not significantly affected by either PS content or heating rate.

Crystallization behavior and spherulitic morphology of poly(lactic acid) films induced by casting process

A novel way using a polarized optical microscopy and statistical image analysis techniques for direct investigation of the crystallization kinetics and spherulitic morphology of poly (lactic acid) induced by casting process has been proposed. DSC analyses were carried out to explore the effect of drawing in thermal properties of PLA. We found that the rate of cold crystallization was sharply increased with draw ratio. Isothermal calorimetric results reveal that thin films crystallize slowly compared to sheets. Moreover, thin films don't fit the Avrami model demonstrating that more stretching may modify the crystallization kinetics of films. A comparison was made between pure PLA and stretched materials using Hoffman theory and giving a compiled view on the details of crystallization behavior with relation to stretch effect. At draw ratio 167, the growth rate reach its maximum and the crystallization rate go up exponentially. POM experiments showed the existence of two different spherulitic populations in thin films and two nucleation mechanisms was identified. Accordingly, the growth rate was found to be not linear with time. Furthermore, the X-ray diffraction analyses showed the presence of α and α' crystal form in thin films contrarily to sheets with α form. POLYM. ENG. SCI., 55:1881–1888, 2015. © 2014 Society of Plastics Engineers