Nonisothermal crystallization and melting behavior of poly(beta-hydroxybutyrate)-poly(vinyl-acetate) blends

Nonisothermal crystallization and melting behavior of poly(P-hydroxybutyrate) (PHB)-poly(vinyl acetate) (PVAc) blends from the melt were investigated by differential scanning calorimetry using various cooling rates. The results show that crystallization of PHB from the melt in the PHB-PVAc blends depends greatly upon cooling rates and blend compositions. For a given composition, the crystallization process begins at higher temperatures when slower scanning rates are used. At a given cooling rate, the presence of PVAc reduces the overall PHB crystallization rate. The Avrami analysis modified by Jeziorny and a new method were used to describe the nonisothermal crystallization process of PHB-PVAc blends very well. The double-melting phenomenon is found to be caused by crystallization during heating in DSC. (C) 1999 John Wiley & Sons, Inc.

Crystallization and intriguing morphologies of compatible mixtures of tetrahydrofuran-methyl methacrylate diblock copolymer with poly(tetrahydrofuran)

The crystallization and unusual crystalline morphologies of compatible mixtures of tetrahydrofuran-methyl methacrylate diblock copolymer with tetrahydrofuran homopolymer were studied. It is shown that the PTHF [poly(tetrahydrofuran)] block of the copolymer cocrystalizes with the PTHF homopolymer in the PTHF microphase of the blend. However, the degree of crystallinity of the PTHF block is always lower than that of the PTHF homopolymer in the PTHF microphase. The crystallizability of the PTHF microphase increases appreciably with increasing PTHF microphase size and PTHF homopolymer weight fraction in the microphase. The morphology study of the blends shows that the crystalline morphology is strongly dependent on blend composition, copolymer composition and PTHF block length, as well as crystallization temperature. When alternating PTHF and PMMA [poly(methyl methacrylate)] lamellae are formed, the macroscopic crystalline morphology could be only observed when the thickness of the PTHF lamellae is large enough (similar to 20 nm). In the blend where PMMA spherical or cylindrical microphases are formed, the crystalline morphology changes dramatically with the change in the PTHF microdomain size and PMMA interdomain distance. Many unusual crystalline morphologies have been observed. A study of the solution-crystallized morphology of the blends at different temperatures shows that the morphology is also strongly dependent on the isothermal crystallization temperature, suggesting that the PMMA microdomains may have different effects on the morphology formation when the blend is crystallized at different rates.

Crystallization behaviour of poly(ether ether ketone)/poly(ether sulfone) block copolymer

The crystallization and melting behaviours of a multiblock copolymer comprising poly(ether ether ketone) (PEEK) and poly(ether sulfone) (PES) blocks whose number average molecular weights <((M)over bar (n)'s)> were 10 000 and 2900, respectively, were studied. The effect of thermal history on crystallization was investigated by wide-angle X-ray diffraction measurement. A differential scanning calorimeter was used to detect the thermal transitions and to monitor the energy evolved during the isothermal crystallization process from the melt. The results suggest that the crystallization of the copolymer becomes more difficult as compared with that of pure PEEK. The equilibrium melting point of the copolymer was found to be 357 degrees C, about 30 degrees C lower than that of pure PEEK. During the isothermal crystallization, relative crystallinity increased with crystallization time, following an Avrami equation with exponent n approximate to 2. The fold surface free energy for the copolymer crystallized from the melt was calculated to be 73 erg cm(-2), about 24 erg cm(-2) higher than that of pure PEEK. Copyright (C) 1996 Elsevier Science Ltd.

FRACTIONATED CRYSTALLIZATION IN POLYOLEFINS-NYLON-6 BLENDS

The crystallization behavior of polyolefins-nylon 6 polymer blends was studied by differential scanning calorimetry (DSC) measurements. In these blends, the crystallization of the minor component often starts with distinctly deeper supercooling than that

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

Numerical model of solid phase transformations governed by nucleation and growth: microstructure development during isothermal crystallization

A simple numerical model which calculates the kinetics of crystallization involving randomly distributed nucleation and isotropic growth is presented. The model can be applied to different thermal histories and no restrictions are imposed on the time and the temperature dependences of the nucleation and growth rates. We also develop an algorithm which evaluates the corresponding emerging grain-size distribution. The algorithm is easy to implement and particularly flexible, making it possible to simulate several experimental conditions. Its simplicity and minimal computer requirements allow high accuracy for two- and three-dimensional growth simulations. The algorithm is applied to explore the grain morphology development during isothermal treatments for several nucleation regimes. In particular, thermal nucleation, preexisting nuclei, and the combination of both nucleation mechanisms are analyzed. For the first two cases, the universal grain-size distribution is obtained. The high accuracy of the model is stated from its comparison to analytical predictions. Finally, the validity of the Kolmogorov-Johnson-Mehl-Avrami model SSSR, is verified for all the cases studied

Thermal Fractionation and isothermal crystallization of polyethylene nanocomposites prepared by in situ polymerization

Nanocomposites of high-density polyethylene (HDPE) and carbon nanotubes (CNT) of different geometries (single wall, double wall, and multiwall; SWNT, DWNT, and MWNT) were prepared by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system. In this work, we have studied the effects of applying the successive self-nucleation and annealing thermal fractionation technique (SSA) to the nanocomposites and have also determined the influence of composition and type of CNT on the isothermal crystallization behavior of the HDPE. SSA results indicate that all types of CNT induce the formation of a population of thicker lamellar crystals that melt at higher temperatures as compared to the crystals formed in neat HDPE prepared under the same catalytic and polymerization conditions and subjected to the same SSA treatment. Furthermore, the peculiar morphology induced by the CNT on the HDPE matrix allows the resolution of thermal fractionation to be much better. The isothermal crystallization results indicated that the strong nucleation effect caused by CNT reduced the supercooling needed for crystallization. The interaction between the HDPE chains and the surface of the CNT is probably very strong as judged by the results obtained, even though it is only physical in nature. When the total crystallinity achieved during isothermal crystallization is considered as a function of CNT content, it was found that a competition between nucleation and topological confinement could account for the results. At low CNT content the crystallinity increases (because of the nucleating effect of CNT on HDPE), however, at higher CNT content there is a dramatic reduction in crystallinity reflecting the increased confinement experienced by the HDPE chains at the interfaces which are extremely large in these nanocomposites. Another consequence of these strong interactions is the remarkable decrease in Avrami index as CNT content increases. When the Avrami index reduces to I or lower, nucleation dominates the overall kinetics as a consequence of confinement effects. Wide-angle X-ray experiments were performed at a high-energy synchrotron source and demonstrated that no change in the orthorhombic unit cell of HDPE occurred during crystallization with or without CNT.

SAXS measurements of the porosity in Cu(II)-doped SnO2 xerogels during crystallization

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Crystallization of fluorindate and fluorogallate glasses

Crystallization of binary InF3-MF2 and GaF3-MF2 (where M = Ba, Sr and Ca) glasses was studied. Characteristic temperatures and kinetic parameters E (activation energy) and n (Avrami exponent) were obtained. Stability against devitrification is discussed in terms of the above cited parameters and also of some others parameters proposed in literature. Optical properties (IR and upconversion emissions) are reported in different crystallized samples containing Nd3+. The main observation is that up conversion emission presents an enhanced sensibility to crystallization when compared to conventional emission. © 1997 Published by Elsevier Science B.V.

Non isothermal study of the devitrification kinetics of fluoroindate glasses

The kinetics of crystallization in an indium fluoride-based glass was studied by a non-isothermal method using differential scanning calorimetry. The experiments led to an Avrami's exponent of 4.6 for solid glass and 2.2 for a powdered sample. The apparent activation energy for crystallization was found to be 130 kJ/mol for solid glass and 354 kJ/mol for the powder. These results express the profound effect of glass particle size on those kinetic parameters, as different crystallization mechanisms take place during sample heating.

Phase behavior, crystallization, and morphology in thermosetting blends of a biodegradable poly(ethylene glycol)-type epoxy resin and poly(ε-caprolactone)

Thermosetting blends of a biodegradable poly(ethylene glycol)-type epoxy resin (PEG-ER) and poly(epsilon-caprolactone) (PCL) were prepared via an in situ curing reaction of poly(ethylene glycol) diglycidyl ether (PEGDGE) and maleic anhydride (MAH) in the presence of PCL. The miscibility, phase behavior, crystallization, and morphology of these blends were investigated. The uncured PCL/PEGDGE blends were miscible, mainly because of the entropic contribution, as the molecular weight of PEGDGE was very low. The crystallization and melting behavior of both PCL and the poly(ethylene glycol) (PEG) segment of PEGDGE were less affected in the uncured PCL/PEGDGE blends because of the very close glass-transition temperatures of PCL and PEGDGE. However, the cured PCL/PEG-ER blends were immiscible and exhibited two separate glass transitions, as revealed by differential scanning calorimetry and dynamic mechanical analysis. There existed two phases in the cured PCL/PEG-ER blends, that is, a PCL-rich phase and a PEG-ER crosslinked phase composed of an MAH-cured PEGDGE network. The crystallization of PCL was slightly enhanced in the cured blends because of the phase-separated nature; meanwhile, the PEG segment was highly restricted in the crosslinked network and was noncrystallizable in the cured blends. The phase structure and morphology of the cured PCL/PEG-ER blends were examined with scanning electron microscopy; a variety of phase morphologies were observed that depended on the blend composition. (C) 2004 Wiley Periodicals, Inc.

Phase behavior, crystallization, and nanostructures in thermoset blends of epoxy resin and amphiphilic star-shaped block copolymers

This article reports thermoset blends of bisphenol A-type epoxy resin (ER) and two amphiphilic four-arm star-shaped diblock copolymers based on hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO). 4,4'-Methylenedianiline (MDA) was used as a curing agent. The first star-shaped diblock copolymer with 70 wt% ethylene oxide (EO), denoted as (PPO-PEO)(4), consists of four PPO-PEO diblock arms with PPO blocks attached on an ethylenediamine core; the second one with 40 wt% EO, denoted as (PEO-PPO)(4), contains four PEO-PPO diblock arms with PEO blocks attached on an ethylenediamine core. The phase behavior, crystallization, and nanoscale structures were investigated by differential scanning calorimetry, transmission electron microscopy, and small-angle X-ray scattering. It was found that the MDA-cured ER/(PPO-PEO)(4) blends are not macroscopically phase-separated over the entire blend composition range. There exist, however, two microphases in the ER/(PPO-PEO)(4) blends. The PPO blocks form a separated microphase, whereas the ER and the PEO blocks, which are miscible, form another microphase. The ER/(PPO-PEO)(4) blends show composition-dependent nanostructures on the order of 10-30 nm. The 80/20 ER/(PPO-PEO)(4) blend displays spherical PPO micelles uniformly dispersed in a continuous ER-rich matrix. The 60/40 ER/(PPO-PEO)(4) blend displays a combined morphology of worm-like micelles and spherical micelles with characteristic of a bicontinuous microphase structure. Macroscopic phase separation took place in the MDA-cured ER/(PEO-PPO)(4) blends. The MDA-cured ER/(PEO-PPO)(4) blends with (PEO-PPO)(4) content up to 50 wt% exhibit phase-separated structures on the order of 0.5-1 mu m. This can be considered to be due to the different EO content and block sequence of the (PEO-PPO)(4) copolymer. (c) 2006 Wiley Periodicals, Inc.

Phase separation, porous structure, and cure kinetics in aliphatic epoxy resin containing hyperbranched polyester

Thermosetting blends of an aliphatic epoxy resin and a hydroxyl-functionalized hyperbranched polymer (HBP), aliphatic hyperbranched polyester Boltorn H40, were prepared using 4,4'-diaminodiphenylmethane (DDM) as the curing agent. The phase behavior and morphology of the DDM-cured epoxy/HBP blends with HBP content up to 40 wt% were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The cured epoxy/HBP blends are immiscible and exhibit two separate glass transitions, as revealed by DMA. The SEM observation showed that there exist two phases in the cured blends, which is an epoxy-rich phase and an HBP-rich phase, which is responsible for the two separate glass transitions. The phase morphology was observed to be dependent on the blend composition. For the blends with HBP content up to 10 wt%, discrete HBP domains are dispersed in the continuous cured epoxy matrix, whereas the cured blend with 40 wt% HBP exhibits a combined morphology of connected globules and bicominuous phase structure. Porous epoxy thermosets with continuous open structures on the order of 100-300 nm were formed after the HBP-rich phase was extracted with solvent from the cured blend with 40 wt% HBP. The DSC study showed that the curing rate is not obviously affected in the epoxy/HBP blends with HBP content up to 40 wt %. The activation energy values obtained are not remarkably changed in the blends; the addition of HBP to epoxy resin thus does not change the mechanism of cure reaction of epoxy resin with DDM. (c) 2006 Wiley Periodicals, Inc.

Insights into the influence of solvent polarity on the crystallization of poly(ethylene oxide) spin-coated thin films via in situ grazing incidence wide-angle X-ray scattering

Controlling polymer thin-film morphology and crystallinity is crucial for a wide range of applications, particularly in thin-film organic electronic devices. In this work, the crystallization behavior of a model polymer, poly(ethylene oxide) (PEO), during spin-coating is studied. PEO films were spun-cast from solvents possessing different polarities (chloroform, THF, and methanol) and probed via in situ grazing incidence wide-angle X-ray scattering. The crystallization behavior was found to follow the solvent polarity order (where chloroform < THF < methanol) rather than the solubility order (where THF > chloroform > methanol). When spun-cast from nonpolar chloroform, crystallization largely followed Avrami kinetics, resulting in the formation of morphologies comprising large spherulites. PEO solutions cast from more polar solvents (THF and methanol) do not form well-defined highly crystalline morphologies and are largely amorphous with the presence of small crystalline regions. The difference in morphological development of PEO spun-cast from polar solvents is attributed to clustering phenomena that inhibit polymer crystallization. This work highlights the importance of considering individual components of polymer solubility, rather than simple total solubility, when designing processing routes for the generation of morphologies with optimum crystallinities or morphologies.