Isothermal Crystallization Kinetics and Melting Behavior of Poly(ethylene terephthalate)/Attapulgite Nanocomposites Studied by Step-scan DSC

The crystallization kinetics of poly(ethylene terephthalate)/attapulgite (AT) nanocomposites and their melting behaviors after isothermal crystallization from the melt were investigated by DSC and analyzed using the Avrami method. The isothermal crystallization kinetics showed that the addition of AT increased both the crystallization rate and the isothermal Avrami exponent of PET. Step-scan differential scanning calorimetry was used to study the influence of AT on the crystallization and subsequent melting behavior in conjunction with conventional DSC. The results revealed that PET and PET/AT nanocomposites experience multiple melting and secondary crystallization processes during heating. The melting behaviors of PET and PET/AT nanocomposites varied in accordance with the crystallization temperature and shifted to higher temperature with the increase of AT content and isothermal crystallization temperature. The main effect of AT nanoparticles on the crystallization of PET was to improve the perfection of PET crystals and weaken its recrystallization behavior.     

Isothermal Crystallization and Subsequent Melting Behavior of Poly(ethylene terephthalate)/Silica Nanocomposites

The crystallization process of poly(ethylene terephthalate)/silica nanocomposites were investigated by differential scanning calorimetry (DSC) and then analyzed using the Avrami method. The results indicated that the crystallization of pure poly(ethylene terephthalate) (PET) was fitted for thermal nucleation and three‐dimensional spherical growth throughout the whole process, whereas the crystallization of PET/silica nanocomposites exhibits two stages. The first stage corresponds to athermal nucleation and three‐dimensional spherical growth, and the second stage corresponds to recrystallization caused by the earlier spherulites impingement. The crystallization rate increases remarkably and the activation energies decrease considerably when silica nanoparticles are added. The subsequent melting behavior of the crystallized samples shows that the melting point (T _m) of nanocomposites is higher than that of pure PET, which might be caused by two factors: (1) The higher melting point might be due to some hindrance to the PET chains caused by the nanoparticles at the beginning of the melting process; (2) it might also be the case that more perfect crystals can be formed due to the higher crystallization temperatures and lower activation energies of PET/silica nanocomposites.     

Synthesis of Polypropylene/Attapulgite Nanocomposites and Their Crystallization Behavior Studied by Step-Scan DSC

Attapulgite (AT) was modified by grafting with butyl acrylate (BA) via polymerizations initiated by Gamma radiation. Polypropylene (PP)/AT nanocomposites were synthesized via melt extrusion in a twin-screw extruder. Fourier transform infrared (FTIR) spectroscopy and thermogravimetry (TG) were used to assess the structure of the hybrid materials and the dispersion of AT was verified by transmission electron microscopy (TEM). The crystallization kinetics of PP/AT nanocomposites were investigated by differential scanning calorimetry (DSC) and analyzed by using the Avrami method. The isothermal crystallization kinetics showed that the addition of AT increased both the crystallization rate and the isothermal Avrami exponent of PP. Step-scan differential scanning calorimetry (SDSC) was used to study the influence of AT on the crystallization and subsequent melting behavior. The results revealed that PP and PP/AT nanocomposites experienced multiple melting and secondary crystallization processes during heating. The melting behaviors of PP and PP/AT nanocomposites varied with the variation of crystallization temperature and AT content.     

Synthesis and Thermal Behavior of Silica‐Graft‐Polypropylene Nanocomposites Studied by Step‐Scan DSC and TGA

Silica graft poly(propylene) (silica‐g‐PP) nanocomposites were successfully prepared by radical grafting copolymerization and ring‐opening reaction. Their thermal properties were studied by step‐scan differential scanning calorimetry (SDSC) and thermogravimetric analysis (TGA). The exothermic peaks in the IsoK baseline (C_p,IsoK, nonreversing signal) of SDSC reveal that PP and silica‐g‐PP nanocomposites undergo melting‐recrystallization‐remelting during heating. The peak temperatures of recrystallization and remelting shift upward with the existence of nanoparticles in the PP matrix. The thermal degradation kinetics of silica‐g‐PP nanocomposites were investigated using nonisothermal TGA and the Flynn‐Wall‐Ozawa method. The results indicate that the thermal stability was significantly improved with increasing silica content, mainly because of the physical‐chemical adsorption of the volatile degradation products on the nanoparticles that delays their volatilization during decomposition, and the covalent interaction between nanoparticles and PP chains, which will also reduce the breakage of PP backbone chains.     

Synthesis and Non-isothermal Crystallization Behavior of PET/Surface-treated TiO 2 Nanocomposites

Poly(ethylene terephthalate) (PET)/TiO ₂ nanocomposites were prepared by melt-blending PET and surface-treated TiO ₂ . The crystallization behavior and the non-isothermal crystallization kinetics of these composites were investigated by differential scanning calorimetry (DSC). Jeziorny and Mo's methods were applied to describe the kinetics of the non-isothermal crystallization process. It was found that the PET matrix with incorporated surface-treated TiO ₂ particles has lower crystallization temperature and melting point than that with incorporated pure nano-TiO ₂ particles. Unlike plain TiO ₂ , surface-treated TiO ₂ particles showed less effect on the degree of crystallization of the PET matrix.     

Nonisothermal Crystallization Behavior of Polypropylene-C60 Nanocomposites

The nonisothermal crystallization behavior of polypropylene (PP) and PP-fullerene (C₆₀) nanocomposites was studied by differential scanning calorimetry (DSC). The kinetic models based on the Jeziorny, Ozawa, and Mo methods were used to analyze the nonisothermal crystallization process. The onset crystallization temperature (T_c), half-time for the crystallization (t_1/2), kinetic parameter (F(T)) by the Mo method and activation energy (ΔE) estimated by the Kissinger method showed that C₆₀ accelerates the crystallization of PP, implying a nucleating role of C₆₀. Furthermore, due to the reduced viscosity of PP by adding 5% C₆₀, the parameters of crystallization kinetics for the PP-5%C₆₀ nanocomposites changed remarkably relative to that of neat PP and when lower contents of C₆₀ were added to PP.     

Melting behavior of the oriented β-phase polypropylene texture crystallized by the temperature slope method

Isotactic polypropylene consisting of uniaxially oriented P-phase lamellae was crystallized in a temperature gradient. The β → α transition was investigated by simultaneous measurements with differential scanning calorimetry (DSC) and X-ray diffraction using synchrotron radiation (SR). To compare the transition mechanism, the β-phase sample was deformed by rolling it along the direction of the crystallization. During rolling, the β-crystal is deformed by interlamellar and interchain slip, which induces c-axis-oriented molecules along the rolling direction. The melting behavior is changed by the rolling deformation. For the as-grown β-crystal, the DSC thermogram has three peaks: the β-melting endotherm at 150°C, an exotherm by recrystallization into the °-form, and the endotherm at 167°C caused by melting of the recrystallized α-form. After the rolling deformation, the β-endotherm is extinguished by the successive exotherm. Simultaneous X-ray measurements reveal that the β → α transition is shifted to a lower temperature and that the recrystallized α-form has a c-axis-orientation caused by the rolling deformation. In the process of the β→ α transition, higher-order lamellar structure is developed earlier than formation of the crystalline structure. In this study, the heating phenomena, such as the β α transition and thickening of the β- and α-lamellae, are consistently explained by a mechanism involving melting and subsequent recrystallization.     

Novel synthesis of highly ordered mesoporous Fe2O3/SiO2 nanocomposites for a room temperature VOC sensor

The controlled synthesis of mesoporous silica and metal oxide nanocomposites with a highly ordered porous structure and large specific surface area for specific applications has been an attractive topic in the field of porous materials. Herein, we introduce a novel method for the fabrication of highly ordered mesoporous structured and large specific surface area Fe₂O₃/SiO₂ nanocomposites, and consider their application in room temperature gas sensors. The mesoporous Fe₂O₃/SiO₂ nanocomposites were synthesised by a two-step method, which combines the hydrothermal growth of Fe₂O₃ nanoparticles and the microemulsion phase of Brij 56 (C₁₆EO₁₀) surfactant as templates in instantly direct-templating synthesis. This synthesis method enables the fabrication of mesoporous Fe₂O₃/SiO₂ nanocomposites without distortion of the ordered porous structure after calcination at high temperature. The synthesised materials were found to be efficient in a room temperature VOC sensor application, with good recovery.     

Crystallization of α and γ Phases in Isotactic Polypropylene with Low Ethylene Content: Isothermal Crystallization and Secondary Crystallization

Crystallization and melting behaviors of isotactic polypropylene (iPP) with low ethylene content during an isothermal process and on heating just after it are investigated by means of in situ x-ray diffraction. At every isothermal temperature investigated here, the formations of α and γ phases are confirmed. The ratio of γ to α increases with an increase in isothermal temperature. On heating just after the isothermal process, the γ starts melting at a lower temperature than the α. This may be due to the small crystallite size of the γ crystallization during the isothermal process. On cooling after the completion of the isothermal crystallization, the secondary crystallization accelerates, and the γ phase crystallizes predominantly. The fractionation is considered to occur in the primary crystallization during the isothermal process. The low-molecular-weight portion of the materials, which remained in amorphous regions of the spherulite during the isothermal process, crystallizes into the γ phase in the secondary crystallization. Furthermore, the fractionation behavior seems to be enhanced at higher temperatures.     

Thermal Behavior of Polypropylene-g-glycidyl Methacrylate Prepared by Melt Grafting

The thermal behaviors of glycidyl methacrylate (GMA)-grafted polypropylene (PP) (PP-g-GMA) with two different grafting degrees, namely, GPP1 and GPP2, were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), dynamic mechanical analysis (DMA), and thermogravimetrical analysis (TGA). DSC results suggested that the GMA grafted PP exhibited higher crystallization temperature T_c, higher melting temperature T_m, and higher crystallinity compared with the neat PP. The isothermal crystallization kinetics was analyzed with the Avrami equation and the total crystallization activation energy was calculated. It was concluded that the crystallization processes of PP and the grafted PP were controlled by nucleation and the values of the crystallization activation energy of PP and the grafted PP were almost identical. POM results suggested that the GMA grafted PP exhibited smaller spherulites size compared with the neat PP. WAXD patterns indicated that the neat PP encouraged the formation of γ phase, compared with the grafted PP, during the crystallization process. DMA results showed that melt grafting did not induce a clear effect on the γ-transition and β-transition of the amorphous phase but resulted in a decrease in mobility of the PP chains in the crystals. TGA curves suggested that the melt grafting slightly improved the thermal stability of PP.     

Kinetics of the apparent isothermal and non-isothermal crystallization of the α-Fe phase within the amorphous Fe81B13Si4C2 alloy

Kinetics of the apparent isothermal and the non-isothermal crystallization of α-Fe phase within the amorphous Fe₈₁B₁₃Si₄C₂ alloy were investigated by an X-ray diffraction (XRD) and by a differential scanning calorimetry (DSC). It was established that the apparent isothermal crystallization of α-Fe phase within amorphous Fe₈₁B₁₃Si₄C₂ alloy could be described by the Johnson-Mehl-Avrami (JMA) kinetic model (with parameter n_iso=4.0). The apparent isothermal crystallization process includes a constant rate of nucleation and three-dimensional growth of nuclei. The results of X-ray diffraction (XRD) data of the isothermally crystallized samples confirmed the above established kinetic model. From the kinetic analysis of the non-isothermal crystallization of the α-Fe phase within this amorphous alloy, it was concluded that the autocatalytic two-parameter Šesták-Berggren (SB) reaction model (with kinetic exponents M=0.72 and N=1.02) describes well the studied process under the given conditions. The non-isothermal crystallization process involves the constant nucleation rate of stable nuclei with additional secondary two-dimensional (surface) nucleation and overlapping of the growing nuclei on account of the non-isothermal activation.     

CO2-induced Crystallization of Isotactic Polypropylene by Annealing Near Its Melting Temperature

CO₂-induced crystallization of isotactic polypropylene (iPP) by annealing had been studied using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). The iPP before annealed was in α-form and amorphous states. At lower temperatures by CO₂ isothermal treatments, iPP chains crystallized from the amorphous phase and only one crystal form, i.e., α-form, was observed. At higher temperatures by CO₂ isothermal treatments, both crystallization from the amorphous phase and thickening of existing crystal lamellae were observed. Moreover, light γ-form crystal appeared in the treated iPP. The crystalline lamellar thickness of iPP annealed at different CO₂ pressures had been determined. Using the Gibbs–Thomson plot method, the equilibrium melting temperature was found to be 187.6°C.     

Polypropylene Nanocomposites Based on Synthetic Organic-Soluble Ag Nanocrystals with Prominent β-nucleating Effect: Quiescent Crystallization and Melting Behavior

Differential scanning calorimetry, x-ray diffraction, and polarized optical microscopy were used to investigate the quiescent crystallization and melting behavior of isotactic polypropylene (iPP) nanocomposites based on synthetic organic-soluble Ag nanocrystals (NCs). The effects of Ag loading and crystallization temperature on the crystallization behavior and crystalline structure were studied. The results showed that the synthetic Ag NCs as a novel effective β-crystal nucleating agent for iPP could promote the overall crystallinity, decrease the size of spherulites, and induce the formation of large amounts of β-crystals in the nanocomposites under quiescent crystallization. The relative content of β-crystals significantly increased with increasing Ag loading, and slightly increased with decreasing crystallization temperature. The quiescent crystallization kinetics was analyzed using the Avrami model. The results showed that the iPP nanocomposites with added Ag NCs had higher crystallization rate constant (k) and lower crystallization half-times (t_1/2) as well as the Avrami exponent (n) than pure iPP, indicating that the presence of Ag NCs acted as heterogeneous nucleating sites and promoted the crystallization rate of iPP.     

Synthesis of Co(II) Complex with Hexamethylenetetramine Using Jet Milling and Its Influence on the Thermal Performance of Poly(l-lactic acid)

Cobalt(II)-hexamethylenetetramine (Co(II)-HMTA) complex was prepared using jet milling. Elemental analysis and thermogravimetric analysis confirmed that the structure of the Co(II)-HMTA complex was Co(HMTA)₂Cl₂·6H₂O (LG). The influence of LG on the thermal performance of poly(l-lactic acid) (PLLA) was investigated. Isothermal crystallization behavior and X-ray diffraction analysis (XRD) results of PLLA/LG showed that LG could improve the crystallization performance of PLLA; 1% LG caused the half time of overall crystallization (t_1/2) of PLLA to decrease from 96.5 min to a minimum value 3.8 min at 100°C. However, the isothermal crystallization kinetics of PLLA/LG described using the Avrami equation and XRD analysis indicated that the isothermal crystallization temperature and the LG concentration significantly affected the isothermal crystallization process of PLLA. In particular, 0.3% LG caused the intensity of the X-ray crystal diffraction peaks of PLLA to decrease with an increase of isothermal crystallization time after increasing for the first 5 min. The thermal decomposition analysis of PLLA/LG showed that the onset decomposition temperature of PLLA with a small amount of LG was higher than that of the neat PLLA and PLLA with a high concentration LG.     

Equilibrium melting temperature as a result of induction time measurements: Isotactic polypropylene

A method for the determination of equilibrium melting temperature from induction time measurements is suggested. Theory of the induction time, t _i (most probable period from the beginning of isothermal crystallization to the instant when a stable crystal nucleus starts growing) involves parameters that influence the nucleation-crystallization process, such as specific interfacial free-energy parameter, specific surface energies of a growing nucleus, enthalpy of crystal melting, diffusion activation energy, undercooling and the equilibrium melting temperature, T_m°. An extrapolation method exploiting the aspect of the induction time that it increases to infinity, that is, 1/t _i decreases to zero at the equilibrium melting temperature, cannot be used to calculate the equilibrium melting temperature. High- or low-temperature approximations of the basic equation yield some simplifications that make it possible to find its parameters via the best fit of the equation with experimental data. This procedure can yield also the value of the equilibrium melting temperature if the measured data are sufficiently precise. Applying that procedure to crystallization data of isotactic polypropylene, we obtained the values of the equilibrium melting temperatures 199.5°C (high-temperature approximation) and 212.7°C (low-temperature approximation). A more detailed discussion of the procedure suggests that from both these reasonable values, the higher one is more justified. This result agrees well with higher T_m° data reported in the literature.     

Effect of TiO2 addition on the crystallization of quenched glasses in the SiO2–Al2O3–ZrO2 system

The glass formation in the SiO₂-rich region of the ternary oxide system Al₂O₃–ZrO₂–SiO₂ with MgO, CaO, and TiO₂ as melting aids was analyzed. The crystallization of glasses with different content of TiO₂ and phase evolution with the temperature was studied by X-ray diffraction, infrared, laser Raman spectroscopy and transmission electron microscopy. The use of TiO₂ favored formation and crystallization of the glasses due to the decrease of the viscosity of melts and acting as a nucleating agent. The crystalline phase of t-ZrO₂ was developed at temperatures as low as 880°C whereas in as prepared specimens without TiO₂ its presence was not detected. For the specimens with TiO₂, t-ZrO₂ and mullite were the principal phases at 1000°C. TiO₂ addition did not change the crystallization sequence but decreased the formation temperature of the crystalline phases. Most of Ti⁴⁺ ions entered into t-ZrO₂ and only a small portion in mullite, but the surplus was detected in ZrTiO₄.     

Study of oxygen vacancy ordering in mullite at high temperatures

High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al₂O₃:SiO₂ ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature T_c > 3000°C.     

Transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si1−xCx nanocomposites mixed with Si nanoparticles and C atoms

Bright-field transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and fast-Fourier transformed electron-diffraction patterns showed that n-butyl terminated Si nanoparticles were aggregated. The formation of Si_1−xC_x nanocomposites was mixed with Si nanoparticles and C atoms embedded in a SiO₂ layer due to the diffusion of C atoms from n-butyl termination shells into aggregated Si nanoparticles. Atomic force microscopy (AFM) images showed that the Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms existed in almost all regions of the SiO₂ layer. The formation mechanism of Si nanoparticles and the transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms are described on the basis of the TEM, HRTEM, and AFM results. These results can help to improve the understanding of the formation mechanism of Si nanoparticles.     

CRYSTALLIZATION IN PARTIALLY MOLTEN ORIENTED BLENDS OF POLYCONDENSATES AS REVEALED BY X-RAY STUDIES*

Oriented fibers or films of binary polymer blends from polycondensates were investigated by two-dimensional (2D) wide-angle X-ray scattering (WAXS) during the finishing process of microfibrillar reinforced composite (MFC) preparation, that is, heating to a temperature between the melting temperatures of the two components, isothermal annealing, and subsequent cooling. It is shown that the crystallization behavior in such MFC from polycondensates depends not only on the blend composition, but also on thermal treatment conditions. Poly(ethylene terephthalate)/polyamide 12 (PET/PA12), poly(butylene terephthalate)/poly(ether ester) (PBT/PEE), and PET/PA6 (polyamide 6) composites were prepared in various compositions from the components. Materials were investigated using rotating anode and synchrotron X-ray source facilities. The effect of the annealing time on the expected isotropization of the lower melting component was studied in the PET/PA6 blend. It was found that PA6 isotropization took place after 2 h; shorter (up to 30 min) and longer (up to 8 h) melt annealing results in oriented crystallization due to different reasons. In PET/PA12 composites, the effect of PA12 transcrystallization with reorientation was confirmed for various blend compositions. The relative strength of the effect decreases with progressing bulk crystallization. Earlier presumed coexistence of isotropic and highly oriented crystallites of the same kind with drawn PBT/PEE blend was confirmed by WAXS from a synchrotron source.

     

Crystallization of carboxylic acids as studied by NMR Spectrometry

The temperature dependences of the narrow ¹H NMR signals, in the temperature range below phase transition point from the plastic to brittle crystal for pivalic acid (PA) confined in the controlled pore size glasses with mean diameters of 7.9 and 23.9 nm have been investigated in order to determine the depression of the crystallization temperature and the pore size distribution parameters. The obtained parameters and the temperature dependences of the narrow ¹H NMR signals in the solid phases of PA and other carboxylic acids (hexadecanoic (C₁₆), heptadecanoic (C₁₇), octadecanoic (C₁₈), nonadecanoic (C₁₉)) have been used for studying the pore size distributions appeared during the natural crystallization. It was found that the pore size distribution for the even-numbered acids (C₁₆ and C₁₈) is spread over much wider range of the sizes comparing with those for odd-numbered acids (C₁₇ and C₁₉). The maximal value of the pore radius R?=?40.4 nm has been found at the temperature just below the melting point of C₁₈, which is about eight times bigger than that found for C₁₇.