Function Of Crystallization Time Research Articles

Monitoring the influence of additives on the crystallization processes of glycine with dynamic nuclear polarization solid-state NMR

Crystallization is fundamental in many domains, and the investigation of the sequence of solid phases produced as a function of crystallization time is thus key to understand and control crystallization processes. Here, we used a solid-state nuclear magnetic resonance strategy to monitor the crystallization process of glycine, which is a model compound in polymorphism, under the influence of crystallizing additives, such as methanol or sodium chloride. More specifically, our strategy is based on a combination of low-temperatures and dynamic nuclear polarization (DNP) to trap and detect transient crystallizing forms, which may be present only in low quantities. Interestingly, our results show that these additives yield valuable DNP signal enhancements even in the absence of glycerol within the crystallizing solution.

Eu3+ heavily doped tellurite glass ceramic as an efficient red phosphor for white LED

To design a phosphor for near ultraviolet (n-UV) based white light emitting diodes (wLED) with low correlated color temperature (CCT) and high CRI, a series of highly Eu3+ activated tellurite samples was prepared. Glass ceramic (GC) containing Eu2Te6O15 microcrystals were obtained as a function of crystallization time from 0 to 4 h at 435 °C. X-ray diffraction (XRD) and cathodoluminescence measurement confirm the existence Eu2Te6O15 microcrystals in translucent glass ceramic, improving the luminescence properties of the samples. Upon 393 nm excitation two intense emission bands centered at 594 nm and 614 nm were observed with an increase of 200% for the crystallized samples at 435 °C for 4 h with respect to as-fabricated glasses. Radiometric properties of heavily (10 mol %) Eu3+ doped samples as a function of crystallization time were investigated and the maximum luminous efficacy (LE) obtained is 12.01 lm/W for a crystallization time of 4 h. A green phosphor (SrBaSiO4:Eu2+) on top of samples was used the construct a white light-emitting diode and the results show that the color coordinate temperature (CCT) can be tuned from 4729 K to 3405 K as crystallization time increase from 0 to 4 h with a diminishing of LE from 34.3 to 19 lm/W. The results show that tellurite glass and GC present enormous potential as phosphor converters in solid state lighting application.

Estimating the Relative Crystallinity of Biodegradable Polylactic Acid and Polyglycolide Polymer Composites by Machine Learning Methodologies.

Biodegradable polymers have recently found significant applications in pharmaceutics processing and drug release/delivery. Composites based on poly (L-lactic acid) (PLLA) have been suggested to enhance the crystallization rate and relative crystallinity of pure PLLA polymers. Despite the large amount of experimental research that has taken place to date, the theoretical aspects of relative crystallinity have not been comprehensively investigated. Therefore, this research uses machine learning methods to estimate the relative crystallinity of biodegradable PLLA/PGA (polyglycolide) composites. Six different artificial intelligent classes were employed to estimate the relative crystallinity of PLLA/PGA polymer composites as a function of crystallization time, temperature, and PGA content. Cumulatively, 1510 machine learning topologies, including 200 multilayer perceptron neural networks, 200 cascade feedforward neural networks (CFFNN), 160 recurrent neural networks, 800 adaptive neuro-fuzzy inference systems, and 150 least-squares support vector regressions, were developed, and their prediction accuracy compared. The modeling results show that a single hidden layer CFFNN with 9 neurons is the most accurate method for estimating 431 experimentally measured datasets. This model predicts an experimental database with an average absolute percentage difference of 8.84%, root mean squared errors of 4.67%, and correlation coefficient (R2) of 0.999008. The modeling results and relevancy studies show that relative crystallinity increases based on the PGA content and crystallization time. Furthermore, the effect of temperature on relative crystallinity is too complex to be easily explained.

Anionic Tuning of Zeolite Crystallization

Zeolites are of great industrial relevance as catalysts, adsorbents, and ion-exchangers and typically synthesized under hydrothermal conditions. Rational regulation of their crystallization process...

Investigation of ZSM-12 nanocrystals evolution derived from aluminosilicate nanobeads for sustainable production of ethyl levulinate from levulinic acid esterification with ethanol

Abstract The evolution of hierarchical ZSM-12 nanosized crystals as a function of crystallization time has been demonstrated using amorphous aluminosilicate (AS) nanobeads as simultaneous silica and alumina sources and cyclic diquaternary ammonium as a structure-directing agent (SDA) in a one-pot hydrothermal synthesis. The physicochemical properties of the derived ZSM-12 products were characterized by means of X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) elemental analysis, N2 physisorption, temperature-programmed desorption of ammonia (NH3-TPD), and pyridine adsorption. It is clearly demonstrated that crystallinity, textural properties, and acidity of the zeolite products are enhanced as a function of crystallization time. The structure of nanocrystalline ZSM-12 is well-developed after 120 h of crystallization and the corresponding zeolite product exhibits the promising catalytic performance for sustainable production of ethyl levulinate (EL) from levulinic acid (LA) esterification with ethanol. This contribution illustrates an alternative way to synthesize hierarchical zeolite nanocrystals with MTW framework via the use of homogeneously dispersed aluminosilicate nanobeads as the starting materials and opens up the perspectives for the sustainable application of zeolites nanosized crystals in a bulky-molecule reaction of bio-based chemical synthesis.

Melting temperature versus crystallinity: new way for identification and analysis of multiple endotherms of poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) materials with different molecular weights were isothermally crystallized from melt by systematically varying the temperature and duration of the treatment performed in the differential scanning calorimeter (DSC). Multiple endotherm peaks were observed on the subsequent heating thermograms that were separated from each other on the basis of their melting temperature versus crystallization temperature and melting temperature versus crystallinity function. By this new approach five sub-peak sets were identified and then comprehensively characterised. Wide-Angle X-Ray Diffraction (WAXD) analyses revealed that the identified sub-peak sets do not differ in crystalline forms. By analysing the crystallinity and the melting temperature of the sub-peak sets as a function of crystallization time, crystallization temperature and intrinsic viscosity, it was concluded that below the crystallization temperature of 460 K the sub-peak sets that were formed during primary or secondary crystallization transform partially or completely to a third sub-peak set during the heating run of the measurement, while above this temperature, the sub-peak set formed during primary crystallization gradually transforms to a more stable structure, with higher melting temperature. These formations and transformations are described with mathematically defined parameters as well.

Investigating the equilibrium melting temperature of linear polyethylene using the non-linear Hoffman-Weeks approach

The melting behavior of three linear polyethylene fractions with number average molecular weights of 11, 29, and 100.5 kg/mol was studied as a function of crystallization time with conventional and ultra-fast calorimetry. The initial melting temperatures of non-thickened lamellae formed under isothermal conditions over a range of crystallization temperatures were analyzed with the non-linear Hoffman-Weeks method to determine the equilibrium melting temperature. Tmeq values of 138.4±0.9°C, 139.7.4±0.9°C, and 140.9±0.8°C were estimated for PE 11K, PE 29K, and PE 100K, respectively, in close agreement with those reported in the literature for the melting of extended-chain crystals or with the Gibbs-Thomson analysis. The Lauritzen-Hoffman theory and the non-linear Hoffman-Weeks treatment were modified to account for the effect of the tilt angle, θ, of the crystallized stems of linear polyethylene on the initial average lamellar thickness. Accuracy of the non-linear Hoffman-Weeks method was examined using initial lamellar thickness, ℓg∗ , data reported for PE 29K in the literature at different crystallization temperatures. The equilibrium melting temperature obtained by the Gibbs-Thomson approach and the C2 value extracted from the ℓg∗vs.1/ΔT plot were similar within the limits of experimental error to those obtained here through the non-linear Hoffman-Weeks method. Using the Huggins equation, the equilibrium melting temperature of an infinitely long linear polyethylene chain is found to be equal to 141.4±0.8°C, the same value proposed by Wunderlich.

Can classic Avrami theory describe the isothermal crystallization kinetics for stereocomplex poly(lactic acid)?

Classic Avrami model and its modifications have found diverse applications in describing the thermal and phase behaviors of inorganic metals and organic polymers. The direct introduction of classic Avrami equation to offer quantitative analyses of crystallization kinetic parameters for enantiomeric poly(lactic acid) (PLA) blends may, however, lead to contradictory conclusions. As revealed by this study, during the characterization of isothermal melt and cold crystallization for stereocomplex PLA containing equal-weight poly(L-lactic acid) and poly(D-lactic acid), the kinetic parameters yielded by Avrami equation are not in line with the classic crystallization hypotheses or the direct morphological observations. The underlying mechanisms, to some extent, lie in the generation of stereocomplex crystals (SCs) during the cooling/heating which affects the subsequent crystallization dynamics. The huge gap between the melting enthalpies of 100% crystalline SCs (142 J/g) and homo-crystals (HCs, 93 J/g) is most likely responsible for the confusing kinetic parameters acquired from the deduction of Avrami equation, which is based on the integration of enthalpies as a function of crystallization time. This prompts for great care that the classic Avrami equation is not applicable to accurately describe the crystallization kinetics of stereocomplex PLA, given the generation of SCs prior to crystallization and the coexistence of HCs and SCs during crystallization.

Insights of the Crystallization Process of Molecular Sieve AlPO4-5 Prepared by Solvent-Free Synthesis.

Crystallization of AlPO4-5 with AFI structure under solvent-free conditions has been investigated. Attention was mainly focused on the characterization of the intermediate phases formed at the early stages during the crystallization. The development in the long-range ordering of the solid phases as a function of crystallization time was monitored by XRD, SEM, IR, UV-Raman, and MAS NMR techniques. Particularly, the UV-Raman spectroscopy was employed to obtain the information on the formation process of the framework. J-HMQC (27)Al/(31)P double-resonance NMR experiments were used to identify the P-O-Al bonded species in the intermediate phases. For the first time the P-O-Al bonded species in the intermediate phases can be correctly described through using this advanced NMR technique. The crystallization under solvent-free conditions appears to follow the pathway: The initial amorphous raw material is converted to an intermediate phase which has four-/six-membered ring species, then gradually transformed into crystalline AlPO4-5. This observation is not consistent with the common idea that the intermediate phase is the semicrystalline intermediates with a three-dimensional structure.

A study of the formation of microporous material SAPO-37

The crystallization of microporous material SAPO-37 was studied under hydrothermal synthesis (HTS) and dry gel conversion (DGC) conditions. The evolution of the intermediate phases as a function of crystallization time was followed by several techniques including PXRD, SEM, AFM, and solid-state NMR. For DGC, SAPO-37 crystallizes from a semi-crystalline layered precursor containing large pores and sodalite cages, whose structure is held by weak nonbonding interactions. Si atoms are only incorporated in the later stage of the crystallization via the formation of aluminosilicate species. Under HTS conditions, the crystallization pathway of SAPO-37 is somewhat similar to that for DGC. The AFM data suggest that the crystal growth mechanism is “birth and spread”, and the final SAPO-37 crystals have rough (111) surfaces terminated by different structural building units. The findings of the present work provide new physical insights into the formation of SAPO-37.

Investigation of the Crystallization Process of SAPO-35 and Si Distribution in the Crystals

The hydrothermal crystallization of SAPO-35 molecular sieve is examined in the present study. Many characterization methods including XRD, SEM, XRF, XPS, EDS, and various solid-state NMR techniques are applied to monitor the formation process of SAPO-35 and the development of the local environments of Si, P, and Al atoms as a function of crystallization time. The results show that an intermediate phase with ordered structure and AlPO compositions first forms. Subsequently, SAPO-35 crystallizes at the expense of the intermediate phase. Si takes part in the formation of SAPO-35 since the initial crystallization. The Si concentration in SAPO-35 shows an increasing trend with time, which leads to the change of both Si and Al coordination environments. More information is obtained from two- dimensional 27 Al MQ MAS and 27 Al → 31 P MQ-MAS HETCOR NMR. Moreover, XPS analysis reveals the enrichment of Si on the crystal surface. On the basis of the characterization results, it is supposed that the distribution of Si in the SAPO-35 crystals is not uniform, showing a gradual increase from the core to the external surface.

Laminated mordenite/ZSM-5 hybrid membranes by one-step synthesis: Preparation, membrane microstructure and pervaporation performance

Laminated mordenite/ZSM-5 hybrid zeolite membranes were fabricated on seeded porous α-alumina tubes with ZSM-5 crystals by one-step hydrothermal treatment for dehydration of industrial solutions. The microstructure was clarified with observations by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometer (XRD) analysis on the membranes synthesized as a function of crystallization time. The synthesized hybrid membranes were composed by two layers. The upper mordenite layer overgrew with 2μm thick above the ZSM-5 lower layer with 3μm thick. The formation sequence was estimated to be that (1) the ZSM-5 layer crystallized in the early stage of 4h; (2) then the mordenite crystals nucleated and grew on the lower ZSM-5 layer; (3) elongated mordenite grains crystallized for 4–8h to form the tightly packed layer.The potential pervaporation (PV) performance of dehydration was investigated for industrial solvents of 2-propanol (IPA) and acetic acid (AAc). In the results of PV at 75°C for a mixture of water (50wt.%)/IPA (50wt.%), the membrane exhibited higher permeation flux up to 6.4kgm−2h−1 and separation factors (α) with 1800. In the results of PV at 80°C for a mixture of water (50wt.%)/AAc (50wt.%), a higher permeation flux with 5.6kgm−2h−1 and a small amount of leakage of AAc with 0.68wt.% into the permeates or the separation factor α=150 were observed. These higher separating features in the laminated mordenite/ZSM-5 hybrid membrane could be caused by tightly assemblage of mordenite crystals that were formed uppermost layer of the membrane.

Investigation of Silicalite-1 Crystallization Using Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy

The crystallization of silicalite-1 in aqueous sols has been investigated using ATR/FTIR spectroscopy as a function of sol composition. The existence of oligomeric siliceous species has been verified for all compositions after the end of tetraethoxysilane (TEOS) hydrolysis. On the other hand, more condensed and highly cross-linked siliceous structures have been observed only at the sols of high SiO2 concentrations. These structures have been considered to be the “primary” nanoparticles reported by other researchers, and their IR spectra revealed that their structures are different from that of the silicalite-1 crystals. The evolution of the nanoparticle structure as a function of crystallization time has been monitored. It has been found that, upon heating of the synthesis sols, the “primary” nanoparticles transform to a new form that has an increased number of SiOSi bonds. These “intermediate” particles do not have the structure of silicalite-1, and they are present for most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in a suspension of these “intermediate” particles.

Characterization of the amorphous phases formed during the synthesis of microporous material AlPO 4-5

We have used several solid-state NMR techniques in conjunction with powder X-ray diffraction (XRD) and Raman spectroscopy to characterize the structure of the intermediate amorphous phases of AlPO 4-5 molecular sieve synthesis. The evolution of the gel phases as a function of crystallization time was followed by 31P and 27Al magic-angle spinning (MAS) NMR to obtain information on the local environments of Al and P atoms and by powder XRD to detect the long-range ordering of the gel samples. We have utilized 27Al/ 31P double-resonance techniques such as heteronuclear correlation spectroscopy (HETCOR) to select the 31P–O– 27Al bonding connectivities and transfer of populations by double-resonance (TRAPDOR) to obtain information regarding the degree of condensation for P sites in the amorphous phases. The 1H → 31P cross-polarization (CP) was used to identify the P sites with hydroxyl groups attached. Raman spectroscopy was also employed to follow the evaluation of the pore system. The 13C CP MAS spectra provide information regarding the chemical environment of template molecule in the gel. The work shows that a combination of the above-mentioned techniques can provide more detailed information on the structure of amorphous phases formed during hydrothermal synthesis.

Isothermal Crystallization Kinetics of Poly(ε‐caprolactone) with Tetramethyl Polycarbonate and Poly(styrene‐co‐acrylonitrile) Blends Using Broadband Dielectric Spectroscopy

AbstractSummary: Phase behavior and isothermal crystallization kinetics of poly(ε‐caprolactone) (PCL) blends with tetramethyl polycarbonate (TMPC) and poly(styrene‐co‐acrylonitrile) with 27.5 wt.‐% acrylonitrile content have been investigated using broadband dielectric spectroscopy and differential scanning calorimeter. An LCST‐type phase diagram has been observed for PCL/SAN blend while all the different blend compositions of PCL/TMPC were optically clear without any phase separation structure even at high temperatures up to 300 °C. The composition dependence of Tgs for both blends has been well described by the Gordon‐Taylor equation. The phase diagram of PCL/SAN was theoretically calculated using the Flory‐Huggins equation considering that the interaction parameter is temperature and composition dependent. The equilibrium melting point of PCL depressed in the blend and the magnitude of the depression was found to be composition dependent. The interaction parameters of PCL with TMPC and SAN could not be calculated from the melting point depression based on Nishi‐Wang approach. The isothermal crystallization kinetics of PCL and in different blends was also investigated as a function of crystallization temperature using broadband dielectric spectroscopy. For pure PCL the rate of crystallization was found to be crystallization temperature (Tc) dependent, i.e., the higher the Tc, the lower the crystallization rate. The crystallization kinetics of PCL/TMPC blend was much slower than that of PCL/SAN at a constant crystallization temperature. This behavior was attributed to the fact that PCL is highly interacted with TMPC than SAN and consequently the stronger the interaction the higher the depression in the crystallization kinetics. It was also attributed to the different values of Tg of TMPC (191 °C) and SAN (100 °C); therefore, the tendency for crystallization decreases upon increasing the Tg of the amorphous component in the blend. The analysis of the isothermal crystallization kinetics was carried out using the theoretical approach of Avrami. The value of Avrami exponent was almost constant in the pure state and in the blends indicating that blending simply retarded the crystallization rate without affecting the crystallization mechanism.Dielectric constant, ε′, of pure PCL, blends of PCL/TMPC = 80/20 and PCL/SAN = 80/20 as a function of crystallization time at 47 °C and 1 kHz.magnified imageDielectric constant, ε′, of pure PCL, blends of PCL/TMPC = 80/20 and PCL/SAN = 80/20 as a function of crystallization time at 47 °C and 1 kHz.

Investigation into the role of MgO in the synthesis of MAPO-11 large single crystals

MgO was introduced into the reaction system of AlPO 4-11 and large MAPO-11 single crystals were obtained by using di- iso-propylamine as the template and HF as the mineralizer. The crystal size of MAPO-11 is about eight times that of AlPO 4-11. Compared with that of AlPO 4-11, the crystallization time of MAPO-11 is shortened greatly while the crystallinity is increased. We followed the crystallization process of AlPO 4-11 and MAPO-11 single crystals and elucidated the role of MgO in the crystallization process. To obtain useful information on understanding the crystallization and growth mechanism of molecular sieve single crystals, we monitored the evolution of the gel phases as a function of crystallization time through XRD, IR, and 27Al Magic Angle Spinning (MAS) NMR. 31P Magic Angle Spinning (MAS) NMR and 1H– 31P CP MAS NMR measurements were conducted to reveal the different sites of phosphorus in the final products. IR spectroscopy also provided insight into the interactions between the organic template and the inorganic gel species.

A Study of the Formation of Molecular Sieve SAPO-44

The formation of a silicoaluminophosphate-based molecular sieve SAPO-44 has been investigated in the present study. Particular attention was directed toward the characterization of the intermediate phases formed at different stages during the synthesis. The changes in the long-range ordering of the gel phases as a function of crystallization time was monitored by powder X-ray diffraction, and the development of the local structure of P, Al, and Si atoms was followed by 31P, 27Al, and 29Si magic-angle spinning NMR. 1H → 31P cross polarization (CP) was employed to differentiate the P sites in different phases coexisting in the sample. Several 27Al/31P and 27Al/29Si double-resonance NMR experiments such as CP and transferred echo double-resonance were also utilized to identify the nature of the intermediate phases, and to select the P−O−Al and Si−O−Al bonding connectivities. FT-Raman spectroscopy was used to gain the information regarding the evolution of the porosity in the framework and the immediate envir...

AlPO4-18 synthesized from colloidal precursors and its use for the preparation of thin films

Nanosized AlPO4-18 is synthesized from clear precursor solutions; a complete transformation from amorphous to crystalline colloidal particles is observed after 60, 20 and 18h at 100, 150, and 170°C, respectively. The evaluation of the long-range ordering of nanocrystals as a function of crystallization time was followed by X-ray diffraction and dynamic light scattering. The time-dependent 27Al NMR measurements in the liquid phase before and after purifications with different crystallization times show that the tetrahedrally coordinated Al is formed at RT prior to hydrothermal treatment indicating the formation of crystalline phase. The mean hydrodynamic radius of the amorphous aluminophosphate entities and the crystalline AlPO4-18 particles does not change during the entire process of crystal growth. Further these AlPO4-18 nanocrystals were employed for the preparation of thin films on silicon wafers via spin coating of stable AlPO4-18/EtOH solutions, and additional crystal growth of the seeded layers at elevated temperature was carried out. The structural evolution of the thin films was studied with grazing incidence diffraction using synchrotron X-ray radiation. The orientation of growing crystals is governed by the preferred orientation of the seeded particles deposited by spin coating approach.

Investigation of the Evolution of Intermediate Phases of AlPO4-18 Molecular Sieve Synthesis

In the present work, we examined the intermediate phases formed during the hydrothermal synthesis of microporous material, AlPO4-18. The evolution of the long-range ordering of the gel samples as a function of crystallization time was followed by powder X-ray diffraction. The development of the local environments of P and Al atoms was monitored by 31P and 27Al magic angle spinning (MAS) NMR. Several representative intermediate phases were further characterized by 27Al multiple quantum MAS and 27Al → 31P cross-polarization experiments. Our results show that the formation of AlPO4-18 undergoes several stages. Mixing phosphorus and aluminum sources together with a template at room temperature yielded a mixture containing an ordered aluminophosphate (AlPO) phase and a phosphate material. These materials were converted to an amorphous AlPO phase under hydrothermal conditions. Continuous heating resulted in the transformation of the amorphous phase into a crystalline microporous material, AlPO4-5. A further inc...

Characterization of the Gel Phases of AlPO4-11 Molecular Sieve Synthesis by Solid-state NMR

We have used several solid-state NMR techniques in conjunction with powder X-ray diffraction (XRD) and FT-Raman spectroscopy to characterize the structures of the intermediate phases of AlPO4-11 molecular sieve synthesis. The evolution of the gel phases as a function of crystallization time was followed by 31P and 27Al magic angle spinning (MAS) NMR to obtain information on the local environments of Al and P atoms and by powder XRD to detect the long range ordering of the gel samples. We have utilized 27Al triple-quantum MAS (3QMAS) to gain additional resolution for monitoring the incorporation of different Al sites into the framework during the crystallization. 27Al/31P double resonance techniques such as cross polarization (CP), transfer of populations by double-resonance, and rotational echo double-resonance have been utilized to select the 31P−O−27Al bonding connectivities within the gel phases. The 1H → 31P CP enables differentiation of the phosphorus sites in different phases coexisting in the same ...