Exothermic Crystallization Research Articles

Computational Investigation of Heat and Mass Transfer Processes in a Gel-Like Fuel Ignited by a Limited-Capacity Source

We have carried out a computational investigation of the interrelated processes of heat and mass transfer, exothermal (crystallization) and endothermal (melting and evaporation) phase transitions, and chemical reactions resulting from ignition of a gel-like condensed substance. The dependence of the main integral characteristic of the process — the ignition delay time — on the temperature of the local heating source has been established. Limiting ignition conditions have been revealed. The position of the zone of the leading oxidation reaction relative to the interface between the heated particle and the fuel has been determined. A comparison has been made between the ignition conditions of condensed substances in different aggregate states under local heating.

Preparation and properties of PTFE/PAI nanocomposites

Polytetrafluoroethylene/poly(amide‐imide) PTFE/PAI nanocomposites were prepared by precipitation of TORLON® 4000TF (PAI) into a PTFE latex containing nanoparticles with average diameters of 41 nm and spherical shape. Several samples were obtained by varying the relative ratio between PAI and PTFE. SEM and PCS analysis indicates that the precipitation of PAI in the presence of PTFE leads mainly, if not exclusively, to bimodal mixtures of the two homoparticles. The PTFE crystallization process was quite complex and, depending on the sample composition, single or multiple crystallization exotherms were observed. This behavior, discussed within the fractionated crystallization frame, was revealing of the PTFE dispersion degree within the PAI matrix. When the PTFE amount is lower than 20%, a perfect PTFE nanoparticle dispersion is obtained. Partial aggregation is observed for PTFE amounts ranging from 20 to 40% whereas extensive aggregation is found for PTFE amount higher that 40%. The dynamic‐mechanical behavior indicates that the interactions at nanoparticle/polymer interface are rather weak. A greater increase in the relative modulus (Ec/Em) in the rubbery state region with respect to the glassy region is observed and was explained in terms of the particle aggregation state. POLYM. COMPOS., 34:1451–1459, 2013. © 2013 Society of Plastics Engineers

The equilibrium melting temperature and isothermal crystallisation kinetics of cyclic poly(butylene terephthalate) and styrene maleimide (c-PBT/SMI) blends

The equilibrium melting point (\( T_{\text{m}}^{0} \)) and isothermal crystallisation kinetics of cyclic poly(butylene terephthalate) (c-PBT) and styrene maleimide (SMI) blends prepared by solid dispersion and in situ polymerisation of cyclic butylene terephthalate oligomers (CBT) within SMI were investigated. This c-PBT/SMI blend is a miscible semicrystalline–amorphous blend system. The \( T_{\text{m}}^{0} \) of c-PBT/SMI blends was determined using the Hoffman and Weeks method, while Avrami crystallisation kinetic model have been applied to study their isothermal crystallisation kinetics. It was found that \( T_{\text{m}}^{0} \) decreased with increasing SMI content in the blend compositions. All the crystallisation exotherms were obtained from differential scanning calorimetry under isothermal experimental conditions. The average value of Avrami exponent, n, is in the range of 2.4–2.8 for the primary crystallisation process for c-PBT and its blends, which suggest that heterogeneous nucleation of spherulites occurred and growth of spherulites was between two-dimensional and three-dimensional.

Isothermal and nonisothermal crystallization kinetics of poly(ether ether ketone)/gadolinium oxide composites

In this study, poly(ether ether ketone) (PEEK)/gadolinium oxide (Gd2O3) composites were prepared by melt extrusion in combination with a Haake rheometer. Differential scanning calorimetry was used to investigate the crystallization behavior of PEEK and its corresponding composites. The microstructural features of the composites and the interfacial interactions between the filler and PEEK phases were viewed with a scanning electron microscope. Gd2O3 powder was blended with PEEK, and sulfonated poly(ether ether ketone) (SPEEK) was used as the compatibilizer in the PEEK/Gd2O3 composites. Isothermal crystallization exotherms showed that the addition of SPEEK increased the crystallization of PEEK in PEEK/Gd2O3 composites. However, the PEEK crystallization rates decreased with increasing Gd2O3 fractions, which probably resulted from the Gd2O3 that had no heterogeneous nucleation effects on the matrix. The modified Avrami equation and Mo’s equation were used to study the nonisothermal crystallization kinetics. The addition of Gd2O3 did not change the nucleation process of PEEK, but it markedly decreased the crystal growth rate.

Properties of biodegradable poly(butylene carbonate) (PBC) composites with fumed silica nanoparticles

Biodegradable poly(butylene carbonate)/fumed silica (PBC/SiO2) nanocomposites were prepared by melt compounding. The PBC/SiO2 nanocomposites exhibited a good dispersion of aggregates of SiO2 in the PBC matrix, and an improvement in mechanical properties. Nanoparticles affect, also, the thermal properties of PBC and especially the crystallization rate, which in all nanocomposites is faster than that of pure PBC. Due to ongoing crystallization and the crystal perfection during heating process, the melting peak of PBC shifted to higher temperature when heating from amorphous state with decreasing heating rate. With increasing cooling rate, the non-isothermal crystallization exotherms became wider and shifted to lower temperature. At a given cooling rate, the crystallization peak temperature of neat PBC was lower than that of its nanocomposite. Non-isothermal crystallization kinetic procedure, the method of Ozawa, was applied to the first deconvoluted DSC peak only by processing the data related to DSC peak. The average value of Ozawa exponent m of pure PBC is 3.04, while the one of its nanocomposite is about 2.98. Moreover, the thermal stability of the nanocomposites was increased. The Td enhancement of the nanocomposite was remarkable.

Crystallization kinetics of bacterial poly(3‐hydroxylbutyrate) copolyesters with cyanuric acid as a nucleating agent

AbstractEffects of cyanuric acid (CA) on nonisothermal and isothermal crystallization, melting behavior, and spherulitic morphology of bacterial copolyesters of poly(3‐hydroxybutyrate), i.e., poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBH), have been investigated. CA has excellent acceleration effectiveness on the melt crystallization of bacterial PHB, PHBV, and PHBH, better than the nucleating agents reported in the literatures, such as boron nitride, uracil, and orotic acid. PHBV and PHBH do not crystallize upon cooling from the melt at 10°C/min, while they are able to complete crystallization under the same conditions with an addition of 1% CA, with a presence of sharp crystallization exotherm at 75–95°C. Isothermal crystallization kinetics of neat and CA‐containing PHBV and PHBH were analyzed by Avrami model. Crystallization half‐times (t1/2) of PHBV and PHBH decrease dramatically with an addition of CA. The melting behavior of isothermally melt‐crystallized PHBV and PHBH is almost not influenced by CA. Spherulitic numbers of PHBV and PHBH increase and the spherulite sizes reduce with an incorporation of CA. Nucleation densities of PHBV and PHBH increase by 3–4 orders of magnitude with a presence of 1% CA. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Hydrogen sorption enhancement by Nb2O5 and Nb catalysts combined with MgH2

Three Nb materials, synthesized oxide (s-Nb2O5), commercial oxide (c-Nb2O5) and metallic Nb, were applied as catalysts to promote the enhancement of hydrogen sorption in MgH2. A high specific surface area, around 100m2g−1, was obtained for both oxides used as catalysts, s-Nb2O5 and c-Nb2O5 after being heat treated at 350°C. X-ray diffraction (XRD) tests, performed prior to milling, revealed the amorphous structure of both oxides. All the catalysts were mixed for only 20min with MgH2 by mechanical milling forming composites. Differential Scanning Calorimetric curves of the s-Nb2O5 and c-Nb2O5 showed the presence of exothermic crystallization peaks. A total capacity of 5.8wt.% was attained when the hydrogen absorption kinetic tests were conducted at 350°C, with the MgH2+5wt.% s-Nb2O5 sample.

Some studies on nucleation, crystallization, microstructure and mechanical properties of mica glass-ceramics in the system 0.2BaO·0.8K2O·4MgO·Al2O3·6SiO2·2MgF2

The crystallization, microstructure, microhardness and theoretical machinability have been investigated by DTA, XRD, SEM and Microhardness Indenter of resulting glass-ceramics. Two distinct crystallization exotherms in the DTA curve are observed and resolved. The first peak corresponds to the initial formation of potassium fluorophlogopite and the second is due to the formation of barium fluorophlogopite. The activation energy for precipitation of each crystalline phase has been evaluated, and the crystallization mechanism has been studied. DTA analyses were conducted at different heating rates and the activation energy was determined graphically from Kissinger and Ozawa equation. The average activation energy is calculated as 276KJ/mol for the first and 366KJ/mol for the second crystallization peak. The Avrami exponent for first and second crystallization peak temperature determined by Augis and Bennett method is found to be 3 and 3.9, respectively. The results indicate that the growth of mica is a two and three dimensional process, controlled by the crystal-glass interface reaction. The Vicker's hardness decreased steadily at intermediate heat treatment temperature with the formation of barium and potassium fluorophlogopite phase, but the decrease in hardness is more rapid at higher temperature with the development of an interconnected ‘house of cards’ microstructure.

Diblock Poly(ester)-Poly(ester-ether) Copolymers: I. Synthesis, Thermal Properties, and Degradation Kinetics

The synthesis and characterization of polycaprolactone (PCL) and poly(dioxanone-methyl dioxanone) (P(DX-co-MeDX)) block copolymers in a range of compositions of the two segments and with varying methyl dioxanone units is herein reported. The thermal properties of the copolymers were studied by differential scanning calorimetry (DSC) which revealed that copolymers exhibited two melting transitions ranging between 48 and 53 °C for the PCL segment and 71–79 °C for the P(DX-co-MeDX) segment. Copolymers exhibited only one crystallization exotherm which decreased as the MeDX content of the copolymer increased, thereby increasing miscibility of PCL and P(DX-co-MeDX) segments, a result also confirmed by scanning electron micrographs (SEM). Lastly, the kinetics of thermal degradation of PCL-b-P(DX-co-MeDX) copolymers were investigated by thermogravimetric analysis (TGA). Thermal degradation was shown to proceed in three distinct steps with the P(DX-co-MeDX) segment degrading in the first stage followed by the PCL segment in the last two stages most likely via unzipping and random polymerization mechanisms. The activation energies of copolymer degradation were determined and were found to decrease with increasing MeDX content of the copolymer. Overall, increasing MeDX content influenced both thermal properties and degradation kinetics through phase mixing of segments in the copolymers.

Mechanically Induced Amorphization of Drugs: A Study of the Thermal Behavior of Cryomilled Compounds

The purpose of this work was to determine what aspect of the milled compound influences its thermal profile. For this, six different compounds with different properties were chosen and cryomilled for different times to get an amorphous solid. Differential scanning calorimetry (DSC) and X-ray powder diffraction were used to characterize the material and look at the thermal behavior. Melt-quenched samples were also prepared, and the thermal profile upon milling was determined and correlated with the thermal behavior of the cryomilled samples. Growth rates were determined by hot-stage microscopy. Ketoconazole, when cryomilled, showed only one crystallization exotherm in the DSC profile. Ursodiol, and to some extent indomethacin, initially showed a double exotherm which eventually become a single exotherm on further milling. Griseofulvin, carbamazepine, and piroxicam exhibited a double exotherm in the DSC profile upon cryomilling to the amorphous state. Surface crystal growth rates around T (g) were found to be highest for compounds showing the double exotherm in the DSC. Thus, it was seen that compounds which have high surface crystallization tendency will exhibit the double exotherm during heating.

Preparation and Nonisothermal Crystallization Behavior Study of PET/PTT/MMT Nanocomposites

Abstract. Poly(ethylene terephthalate)(PET)/poly(trimethylene terephthalate) (PTT) and PET/PTT /montmorillonite(MMT) nanocomposites were prepared by melt blending through a co-rotating twin screw extruder. Their morphology was characterized by Scanning Electron Microscope(SEM). And the nonisothermal crystallization behavior was studied by Differential Scanning Calorimetry(DSC). The results indicated that MMT dispersed homogeneously in the PTT matrix; the crystallization onset temperature(Tonset) and the crystallization peak temperature(Tp) in crystallization exotherms of PET/ PTT/MMT nanocomposites shifted to higher temperature as MMT content increased, which proved MMT acted as heterogeneous nucleating agent, and the crystallization behavior became imperfect as the cooling rate increased.

X‐ray diffraction and differential scanning calorimetry studies of a BaTiO3/polyvinylidene fluoride composites

AbstractBarium titanate/polyvinylidene fluoride (BaTiO3)x (PVDF)100–x composite samples were prepared and characterized using X‐ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. In this work, the ratio of the constituents of this composite was altered, and the structural and thermal changes were studied. Also, the variation of tetragonality of BaTiO3 (BT) in the composite samples as a function of BT content was studied for the first time. The results show that all the samples are in the α‐phase, and the hindrance to the PVDF crystallization increases with the increase of BaTiO3 (BT) ratio in the composite. Tetragonal distortion of BT nanoparticles in the composite increases with the increase in BT ratio up to 30%, where it gets a saturation value. Also, it seems that stretching the samples enhances the BT tetragonality. Both melting and crystallization behaviors of the composite samples show double‐melting endotherms (reorganization) and crystallization exotherms. The inclusion of BT in the composite samples results in a decrease in the melting temperature of the samples. POLYM. ENG. SCI., 52:1945–1950, 2012. © 2012 Society of Plastics Engineers

Mechanical properties and crystallization behavior of toughened polyamide‐6/carbon nanotube composites

AbstractThis article presents a study on nanocomposite prepared by incorporation of single wall carbon nanotube (SWNT) into the blend of polyamide‐6 (PA‐6) and ethylene–propylene–diene elastomer grafted with maleic anhydride (EPDM‐g‐MA), with varying SWNT content from 1 to 3% with the fixed composition of the matrix (PA‐6: EPDM‐g‐MA being 65 : 35). Tensile properties of the rubber toughened PA‐6 (i.e., PA‐6/EPDM‐g‐MA blend) increased on incorporation of the nanofiller SWNT. Crystallization behavior studied through the analysis of the DSC crystallization exotherm showed distinct changes with varying content of the nanofiller. The SWNT nanofiller showed initially an enhancement of nucleation rate, which was overlapped with slowing down of crystallization growth rate, which results in smaller crystallinity and decreased crystal size. This slowing down of crystalline growth with increasing SWNT content is attributed to the presence of SWNT in the form of network like dispersion as seen by the scanning electron microscopic measurement. The decrease of crystallinity with increasing SWNT content is also shown by X‐ray diffraction (XRD) measurements. The changes in XRD pattern indicate a variation in the relative proportion of α and γ crystalline forms of PA‐6 due to the presence of SWNT nanofiller. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Effect of additives, catalyst residues, and confining substrates on the fractionated crystallization of polypropylene droplets

AbstractFractionated crystallization of Ziegler‐Natta PP (ZN‐PP) droplets produced from ZN‐PP/polystyrene (PS) multilayered films resulted in four crystallization exotherms at 40, 64, 90, and 103°C. The origins of these crystallization peaks were investigated in this article. The nucleation of ZN‐PP droplets at various crystallization exotherms was attributed to (1) various additives in ZN‐PP and PS, (2) catalyst residues present in ZN‐PP, or (3) substrate nucleation by confining matrix. Effect of various additives on the fractionated crystallization an additive‐free grade of ZN‐PP (cPP) was investigated. Crystallization of cPP droplets from 200‐nm layers showed that Irganox was responsible for crystallization of droplets at 103°C. Irgafos and ZnS did not affect the crystallization of cPP droplets. We also investigated three grades of PP, which were synthesized using different catalysts. The PP grades were ZN‐PP, developmental PP (D‐PP) and metallocene PP (M‐PP). D‐PP droplets from 200‐nm layers showed crystallization at 100°C, while ZN‐PP resulted in crystallization at 90 and 103°C. It was suggested that the 90°C peak for ZN‐PP droplets was most likely due to the presence of Ziegler‐Natta catalyst residues. Crystallization of M‐PP droplets produced from 200 nm showed multiple crystallization exotherms (60 to 90°C). The effect of confining substrates on the fractionated crystallization of ZN‐PP droplets revealed heterogeneous nucleation by polycarbonate (PC) substrate. When the confining matrix was changed from polystyrene (PS) to polymethyl methacrylate (PMMA), the crystallization thermograms of ZN‐PP droplets were unaffected. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Preparation and Thermal Characterization of PTFE/PES Nanocomposites

AbstractSummary: PTFE/PES composites were prepared by precipitation of Radel A® into a PTFE latex containing nanoparticles with average diameters of 48 nm and spherical shape. Several samples were prepared by varying the relative ratio between the Radel A® and PTFE content. The combination of SEM and AFM analysis indicates that the precipitation of Radel A in the presence of PTFE leads mainly, if not exclusively, to a bimodal mixture of the two homoparticles. The fractionated crystallization behaviour of these samples is revealing of the PTFE dispersion degree within the Radel A® matrix. When the PTFE amount is lower than 2%, a perfect PTFE nanoparticle dispersion is obtained. When the amount of PTFE is comprised between 5 and 30%, larger PTFE clusters are obtained that, after melting, coalesce and crystallize at higher temperatures depending on the crystallization propensity of their individual heterogeneous nuclei. Finally, in case of samples 40%, only one crystallization exotherm is observed at 310 °C indicating the formation of very large clusters that after melting coalesce into wide domains.

Study of the thermal and mechanical properties of blown films of high‐ and low‐density polyethylene blends

AbstractThe miscibility of the blown film of low‐ and high‐density polyethylene blends and its implications on melting, crystallization, and mechanical behavior throughout the full composition range and the possible role of blending on the miscibility of the binary pair system have been investigated. Using differential scanning calorimetry it was inferred that the films of blends in solid state are miscible in HDPE‐rich compositions, partially miscible at 95% LDPE and completely immiscible at other compositions. The thermal analysis revealed that blending has insignificant effect on lamella thickness of LDPE phase but strong influence on the lamella thickness of HDPE phase. Moreover, analysis of crystallization exotherms was also discussed. XRD measurements also revealed qualitatively similar trend in crystallinity and lamella thickness to DSC results. Measurements on tensile properties in the machine direction of the films showed both positive and negative deviation from linear additivity and improvement of the mechanical properties was found in the film blends containing low amount of LDPE. Especially the strain at break and tensile strength for the film of 5% LDPE blend was higher than that of neat HDPE, which suggests synergistic effects. This supports its larger lamella thickness and better miscibility. Nonetheless, there was negative deviation from the additivity rule in the entire range of composition for the mechanical properties in the transverse direction of the films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Glass-forming ability of soda lime borate liquids

We investigate the composition dependence of glass-forming ability (GFA) of a series of iron-containing soda lime borate liquids by substituting Na2O for B2O3. We have characterized GFA by measuring the glass stability against crystallization using a differential scanning calorimeter (DSC). The results show that the GFA decreases when substituting Na2O for B2O3. Moreover, we find that there is no direct link between the kinetic fragility and GFA for the soda lime borate series studied herein. We have also discovered and clarified a striking thermal history dependence of the glass stability against crystallization. In particular, the two glasses containing 20 and 25 Na2O mol% do not exhibit crystallization exotherms during the second DSC upscan at 10 and 20K/min following prior slow (10 and 20K/min) downscans. This indicates that the glass stability of these compositions can be enhanced by cooling their melts to the glassy state slowly, before any reheating. We explain this phenomenon in terms of the thermal history dependence of boron speciation.

Effect of counterion on the phase behaviour during lyophilization of indomethacin salt forms

This manuscript studies the effect of counterion on the phase behaviour during lyophilization of indomethacin salts. Salt forms of indomethacin namely, sodium, potassium, rubidium and cesium were prepared in situ and analyzed by modulated differential scanning calorimetry (MDSC) for determination of the critical process parameter for lyophilization. At a cooling rate of 17 °C/min, indomethacin salts separated as amorphous form on freeze concentration. T g ′ of indomethacin salts followed the order: sodium (−35.36 °C) > potassium (−38.90 °C) > rubidium (−43.70 °C) > cesium (−47.84 °C) salt. T g ′ was followed by crystallization exotherm during the heating run for all salts, except indomethacin cesium. At slower cooling rates (0.5 and 1 °C/min), indomethacin sodium, potassium and rubidium crystallized in the cooling run, while indomethacin cesium remained amorphous. The differential crystallization behaviour of indomethacin salts was explained in terms of bulkiness of the counterion, molecular interactions and structural relaxation behaviour near T g ′ . The research work has implications in lyophilization processing as well as the stability and performance of final product.

The nucleation and crystallization of MgO–B 2O 3–SiO 2 glass

The nucleation and crystallization of MgO–B 2O 3–SiO 2 (MBS) glass were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The temperature range of the nucleation and the temperature of the maximum nucleation rate for MBS glass were determined from the dependences of the inverse temperature at the DSC peak (1/ T p ) and the maximum intensity of the exothermic DSC crystallization peak (( δT) p ) on the nucleation temperature ( T n ). For MBS glass the nucleation occurred at 600–750 °C, with the maximum nucleation rate at 700 °C, whereas the nucleation and crystal growth processes overlapped at 700 °C < T ≤ 750 °C. The analyses of the non-isothermal data for the bulk MBS glass using the most common models (Ozawa, Kissinger, modified Kissinger, Ozawa–Chen, etc.) revealed that the crystallization of Mg 2B 2O 5 was three-dimensional bulk with a diffusion-controlled crystal growth rate, that n = m = 1.5 and that the activation energy for the crystallization was 410–440 kJ/mol.

Nonisothermal and Isothermal Cold Crystallization Behaviors of Biodegradable Poly(p-dioxanone)

Nonisothermal and isothermal cold crystallization behaviors of poly(p-dioxanone) (PPDO) were investigated by differential scanning calorimetry (DSC). There were two crystallization exotherms—a majo...