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.     

Studies on the Nonisothermal Crystallization Behavior of Polypropylene/Multiwalled Carbon Nanotubes Nanocomposites

Polypropylene/multiwalled carbon nanotubes (PP/MWNTs) nanocomposites were prepared by a melt compounding process. The morphology and nonisothermal crystallization of these nanocomposites were investigated by means of optical microscopy, scanning electron microscopy, and differential scanning calorimetry. Scanning electron microscope micrographs of PP/MWNTs composite showed that the MWNTs were well dispersed in the PP matrix and displayed a clear nucleating effect on PP crystallization. Avrami theory, modified by Jeziorny and Mo's method, was used to analyze the kinetics of the nonisothermal crystallization process. It was found that the addition of MWNTs improved the crystallization rate and increased the peak crystallization temperature of the PP/MWNTs nanocomposites as compared with PP. The results show that the Jeziorny theory and Mo's method successfully describe the nonisothermal crystallization process of PP and PP/MWNTs nanocomposites.     

Synthesis of Poly(butylene terephthalate)/Attapulgite Nanocomposites by In-Situ Polymerization and Its Crystallization Behavior

Poly(butylene terephthalate) (PBT)/attapulgite (AT) nanocomposites were prepared via in-situ polymerization without pre-modification of AT. By this method, PBT chains were successfully grafted onto the surface of AT, which was confirmed by Fourier transform infrared spectroscopy and thermogravimetric analysis. Scanning electron microscope examination indicated the uniform dispersion of AT nanoparticles in PBT matrix. The crystallization behavior of PBT/AT nanocomposites was investigated by X-ray diffraction patterns, differential scanning calorimetry, and step-scan differential scanning calorimetry. The non-isothermal crystallization processes were analyzed with the Avrami, Ozawa, and Mo methods. Crystallization activation energies of the samples were also determined by the Kissinger method. The results indicated that AT could act as a heterogeneous nucleating agent in PBT crystallization and lead to an acceleration of crystallization, while AT also acted as a physical hindrance to retard the transport of polymer chains to the growing crystals.     

Isothermal Crystallization and Melting Behavior of Composites Composed of Poly(L-lactic Acid) and Poly(glycolic Acid) Fibers

Several composites of poly (L-lactic acid) (PLLA) with poly (glycolic acid) (PGA) fibers were prepared. The isothermal crystallization kinetics and melting behavior of PLLA and all of the composites were characterized by using differential scanning calorimetry. The experimental data were processed by using the Avrami equation. The relative parameters, such as the Avrami exponent and half-time crystallization, revealed that PGA fibers had positive effects on the crystallization of PLLA, but these effects had only a minimal dependence on the PGA fiber content. Moreover, at low isothermal crystallization temperatures (85°C～110°C), recrystallization during the heating scan was observed, which could lower the melting point of the samples to a certain extent.     

Cold crystallization from chiral smectic phase

Cold crystallization of liquid crystalline (S)-4’-(1-methylheptyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2-fluorobenzoate (3F7HPhF) was studied in isothermal as well as non-isothermal conditions. For isothermal conditions at temperatures between 233 and 250?K X-ray diffraction and differential scanning calorimetry were used. The crystallization kinetics was described by the Avrami–Avramov model, and the values of Avrami exponent, characteristic time and activation energy were determined. The kinetics of the cold crystallization in non-isothermal conditions for chosen heating rates up to 0.5?K/s was studied by differential scanning calorimetry and analyzed using Ozawa, Mo and Augis–Bennett models. Cold crystallization was found to be three-dimensional and controlled by diffusion both in the isothermal and non-isothermal process, however the activation energy determined in the non-isothermal process is about two times smaller than in the isothermal one.     

Isothermal Crystallization Kinetics and Melting Behavior of a Luminescent Conjugated Polymer,Poly(9,9-Dihexylfluorene-Alt-2,5-Didodecyloxybenzene)

A study of the isothermal crystallization behaviors of poly(9,9-dihexylfluorene-alt-2,5-didodecyloxybenzene) (PF6OC12) was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. The Avrami exponent n ranges from 3.43 to 3.71 for PF6OC12 at crystallization temperatures between 100.0°C and 90.0°C, indicating a three-dimensional spherical crystal growth with homogeneous nucleation in the primary crystallization stage for the isothermal melt crystallization process. In the DSC scan, after the isothermal crystallization, multiple melting behavior was found. The multiple endotherms could be attributed to melting of recrystallized materials produced originally during different crystallization processes. According to the Arrhenius equation, the activation energy was determined to be 211.29 kJmol^?1 for the isothermal melt crystallization of PF6OC12.     

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.     

Study of the Nonisothermal Crystallization Kinetics and Melting Behaviors of Polypropylene Reinforced with Single-Walled Carbon Nanotubes Nanocomposite

Nonisothermal crystallization kinetics of polypropylene (PP) nanocomposite reinforced with 0.5 wt. % single-walled carbon nanotubes (SWNT) was characterized by differential scanning calorimetry at five different cooling and heating rates. The Avrami, Ozawa, and Seo-Kim kinetic models were used to describe the nonisothermal crystallization of the polymer and its nanocomposite. The addition of nano-filler, in general, improved the crystallization rate and increased the peak crystallization temperature of the nanocomposite as compared to PP. The results show that the Avrami and Seo-Kim models are suitable under different cooling rate conditions but that the Ozawa model is inappropriate for the nanocomposite. Equilibrium melting temperatures, derived from the linear Hoffman-Weeks equation, were shown to decrease in the nanocomposite. Additional analysis was performed based on the Thomson-Gibbs, Lauritzen-Hoffman, and Dobreva-Gutzowa theories, which were applied to take into account the lamellar thickness, nucleating agent, and nucleating activity of the nanocomposite in the nonisothermal melt crystallization process.     

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.     

Effect of Organoclay Platelets on Crystallization of Polytrimethylene Terephthalate/Polyethylene-octene-g-maleic Anhydride Blends: Isothermal Crystallization

The blends of poly(trimethylene terephthalate) (PTT) with maleic anhydride-grafted poly(ethylene-octene) (POE-g-MA) and organoclay (OMMT) were prepared by melt-blending. The effects of organoclay platelets on the isothermal crystallization behaviors of PTT/POE-g-MA blend were examined using differential scanning calorimetry. The crystallization kinetics of the primary stage under isothermal conditions could be described by the Avrami equation, with values of the Avrami exponent between 2.01 and 2.81 for all samples. The crystallization rate parameter, K, decreased with increase of melt-crystallization temperature for all samples. The activation energies for isothermal crystallization were determined by the Arrhenius equation.     

Miscibility and Isothermal Crystallization Behavior of Poly (Butylene Succinate-co-Adipate) (PBSA)/Poly (Trimethylene Carbonate) (PTMC) Blends

Poly(butylene succinate-co-adipate) (PBSA)/poly (trimethylene carbonate) (PTMC) blend samples with different weight ratios were prepared by solution blending. The morphologies after isothermal crystallization and in the melt were observed by optical microscopy (OM). Differential scanning calorimetry (DSC) was used to characterize the isothermal crystallization kinetics and melting behaviors. According to the OM image before and after melting, it was found that the blends formed heterogenous morphologies. When the PTMC content was low (20%), PBSA formed the continuous phase, while when the PTMC contents was high (40%), PBSA formed the dispersed phase. The glass transition temperatures (T_g) of the blends were determined by DSC and the differences of the T_g values were smaller than the difference between those of pure PBSA and PTMC. In addition, the equilibrium melting points were depressed in the blends. According to these results, the PBSA/PTMC blends were determined as being partially miscible blends. The crystallization kinetics was investigated according to the Avrami equation. It was found that the incorporation of PTMC did not change the crystallization mechanism of PBSA. However, the crystallization rate decreased with the increase of PTMC contents. The change of crystallization kinetics is related with the existences of amorphous PTMC, the partial miscibility between PLLA and PTMC, and the changes of phase structures.     

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.     

Effect of Organoclay Platelets on Crystallization of PTT/EPDM-g-MA Blends: Nonisothermal Melt Crystallization Kinetics

PTT/EPDM-g-MA (80/20 w/w) nanocomposites were prepared by melt mixing of poly(trimethylene terephthalate) (PTT), ethylene-propylene-diene copolymer grafted with maleic anhydride (EPDM-g-MA), and organoclay. The blend nanocomposites show typical sea-island morphologies. The nonisothermal crystallization kinetics of pure PTT and 80/20 (w/w) PTT/EPDM-g-MA blends with various amounts of the clay were extensively studied by differential scanning calorimetry (DSC). The Avrami, Ozawa, and Mo methods were used to describe the nonisothermal crystallization process of pure PTT and 80/20 (w/w) PTT/EPDM-g-MA blends with various amounts of the clay. Avrami analysis results show that the crystallization rates of 80/20 (w/w) PTT/EPDM-g-MA blends with the clay were faster than those of pure PTT or PTT/EPDM-g-MA blends without clay, which indicates that the clay particles promote crystallization effectively, in agreement with the Mo analysis results. Ozawa analysis can describe the nonisothermal crystallization of pure PTT very well but was rather inapplicable to the 80/20 (w/w) PTT/EPDM-g-MA blends with various amounts of the clay.     

Isothermal Crystallization and Miscibility of Isotactic Polypropylene/Poly(cis-butadiene) Rubber Blends

Isotactic polypropylene/poly(cis-butadiene) rubber (iPP/PcBR) blends were prepared by melt mixing. Isothermal crystallization and miscibility for neat iPP and blends of iPP/PcBR were investigated by differential scanning calorimetry. The presence of PcBR remarkably affected isothermal crystalline behaviors of iPP. An addition of PcBR caused shorter crystallization time and a faster overall crystallization rate, meaning a heterogeneous nucleation effect of PcBR upon crystallization of iPP. For the same sample, the crystallization peak was broader and the supercooling decreased as the crystallization temperature increased. The Avrami equation was suitable to describe the primary isothermal crystallization process of iPP and blends. The addition of PcBR led to an increase of values of the Avrami exponent n, which we suggest was because the blends had a stronger trend of instantaneous three-dimensional growth than neat iPP. The equilibrium melting point depression of the blends was observed, indicating that the blends were partly miscible in the melt.     

The Melting Behavior of Poly(Ethylene Terephthalate)/SiO2 Nanocomposites

The melting and recrystallization behavior of poly(ethylene terephthalate) (PET)/SiO₂ nanocomposites after isothermal crystallization from the melt was studied by step-scan differential scanning calorimetry (SDSC). The influences of SiO₂ contents, crystallization temperature, and crystallization time on the melting process were examined. Two melting endotherms (in the SDSC C_P.A curves, reversible part) and one recrystallization exotherm (in the SDSC C_P.IsoK curves, irreversible part) of PET/SiO₂ nanocomposites after isothermal crystallization were observed during the melting process. All are ascribed to the melting–recrystallization mechanism. The low-temperature endotherm is attributed to the melting of initial crystals formed during the isothermal treatment and the high temperature endotherm to the melting of recrystallization materials. The reason why more recrystallization happened with the increase of SiO₂ content is given and the process of recrystallization was described in detail. The effects of crystal perfection and recrystallization were minimized by increasing crystallization temperature and time.     

ISOTHERMAL CRYSTALLIZATION KINETICS OF BALL-MILLED AMORPHOUS Al-Ni-Zr ALLOY

The isothermal crystallization kinetics of amorphous Al-Ni-Zr alloy produced by mechanical alloying was studied by means of differential scanning calorimetry. Accordiag to Arrhenius equation, the apparent activation energy was calculated. The isothermal crystal-lization kinetics follows Johnson-Mehl-Avrami equation with n=2.00 within 0.15相似文献   

Crystallization Kinetics of Nucleated Polypropylene with Organic Phosphates

The crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP with two organic phosphates, sodium salt (NA7) and triglyceride ester (NA8) of 2,2'-methylene-bis(4,6-di-tert-butylphenyl) phosphoric acid, were investigated by means of a differential scanning calorimeter under isothermal and nonisothermal conditions. During isothermal crystallization, a modified Avrami equation was used to describe the crystallization kinetics. Moreover, kinetics parameters, such as the Avrami exponent, n, the crystallization rate constant, k, and the half-time of crystallization, τ_1/2, are compared. The results showed that a dramatic decrease of the half-time of crystallization, as well as a significant increase of the overall crystallization rate, were observed in the presence of the organic phosphates. During nonisothermal crystallization, the primary crystallization was analyzed using the Ozawa model, leading to similar Avrami exponents for iPP and iPP/NA7, which means simultaneous nucleation with three-dimensional spherulitic growth. However, for iPP/NA8, the Avrami exponent in nonisothermal crystallization is evidently different from that in isothermal crystallization, which would indicate a different mechanism of crystal growth. Adding the nucleating agent to iPP makes the overall crystallization activation energy increase.     

Nonisothermal Crystallization Behavior of Polypropylene Grafted Silane and Styrene Using γ-Ray Irradiation

Polypropylene grafted silane and styrene (named PP-g-Si/St in this article) was successfully prepared by radical graft polymerization initiated by γ-ray irradiation. The influence of total absorbed dose on the graft ratio of vinyltrimethoxysilane onto PP and the melt flow rate (MFR) of the PP-g-Si/St product were studied. The effect of graft ratios of vinyltrimethoxysilane on the melting point and nonisothermal crystallization kinetics of PP-g-Si/St was investigated by the method of differential scanning calorimetry (DSC). With increasing vinyltrimethoxysilane and styrene (used as viscosity modifier and free radical source) grafted on PP, the melting point of PP-g-Si/St became lower. Several different analysis methods, including those of Avrami, Jeziorny, and Mo and colleagues, were employed to describe the nonisothermal crystallization process of the grafted samples. The results indicate that the peak temperature of crystallization of PP-g-Si/St sample was lower than that of virgin PP. Crystallization kinetics revealed that the rates of nucleation and growth were affected differently by the graft ratio of vinyltrimethoxysilane onto PP. The activation energy was calculated on the basis of the method of Kissinger, and the values were 253.6 and 215.7 kJ/mol for virgin PP and PP-g-Si/St, respectively.     

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.     

Confined Crystallization in Polymer Blends: DSC Studies of the Behavior and Kinetics of Isothermal Crystallization of Polypropylene in Poly(cis-butadiene) Rubber Blends

Thermal properties of polypropylene with poly(cis-butadiene) rubber (iPP/PcBR) blends have been measured by differential scanning calorimetry (DSC), and the melting point T_m, crystallization temperature T_c, enthalpy Δ H (melting enthalpies and crystalline enthalpies), and equilibrium melting point T⁰ _m have been measured and calculated. The variation of T_m, T_c, Δ H and T⁰ _m with composition in the blends was discussed, showing that an interaction between phases is present in iPP/PcBR blends. The degree of supercooling characterizing the interaction between two phases in the blends and the crystallizability of the blends which bears a relationship to the composition of the blends was discussed. The kinetics of isothermal crystallization of the crystalline phase in iPP/PcBR blends was studied in terms of the Avrami equation, and the Avrami exponent n and velocity constant K were obtained. The Avrami exponent n is between 3 and 2, meaning that iPP has a thermal nucleation with two dimensional growths. The variation of the Avrami exponent n, velocity constant K, and crystallization rate G bear a relation to the composition of the blends, n increases with increasing content ofPcBR. K also increased with increasing content of PcBR. All of the K for the blends are greater than for pure iPP. The crystallization rate G (t_1/2) depends on the compositions in the blends; all G of the blends are greater than for iPP.