Simultaneous liquid–liquid demixing and crystallization and its effect on the spherulite growth in poly(ethylene terephthalate)/poly(ether imide) blends

Poly(ethylene terephthalate) (PET)/poly(ether imide) (PEI) blends were miscible in the melt, but exhibited simultaneous liquid–liquid phase separation and crystallization over a wide range of temperature and composition. The interplay between these two processes is expected to dominate the morphological formation in the blends. In this study, the phase diagram of PET/PEI blend was determined to evaluate the envelop within which liquid–liquid phase separation was operative with crystallization. A UCST phase diagram below 240°C was identified for this system. The effect of liquid–liquid phase separation on the growth of PET spherulites was studied by small-angle light scattering (SALS). Nonlinear spherulite growths were observed for the blends at higher crystallization temperatures of 210°C and 220°C, while the growths were basically linear below 210°C. The nonlinear growth behaviour was discussed based on the competition between spherulite growth and spinodal decomposition.     

Spherulite morphology and crystallization behavior of poly(trimethylene terephthalate)/poly(ether imide) blends

The spherulite morphology and crystallization behavior of poly(trimethylene terephthalate) (PTT)/poly(ether imide) (PEI) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). Thermal analysis showed that PTT and PEI were miscible in the melt over the entire composition range. The addition of PEI depressed the overall crystallization rate of PTT and affected the texture of spherulites but did not alter the mechanism of crystal growth. When a 50/50 blend was melt-crystallized at 180 °C, the highly birefringent spherulite appeared at the early stage of crystallization (t < 20 min). After longer times, the spherulite of a second form was developed, which exhibited lower birefringence. The SALS results suggested that the observed birefringence change along the radial direction of the spherulite was mainly due to an increase in the orientation fluctuation of the growing crystals as the radius of spherulite increased. The lamellar morphological parameters were evaluated by a one-dimensional correlation function analysis. The amorphous layer thickness showed little dependence on the PEI concentration, indicating that the noncrystallizable PEI component resided primarily in the interfibrillar regions of the growing spherulites.     

Light scattering from polymer spherulites in crystal growth simulation

A three-dimensional simulation is carried out in the growth process of a polymer spherulite and the Hv light scattering intensity is calculated. In the simulation, a crystallite is represented by a thin disk with the optical axis perpendicular to the disk. In a growth step, each crystallite generates new crystallites, the optical axes of which are fluctuated. The simulation results reproduce prominent features observed in the light scattering experiments on polymer spherulites: the scattering intensity that consists of a fourfold-symmetry intensity component and an isotropic intensity component, the scattering angle dependence, the relative magnitude and the development with crystallization time of these intensity components. A new interpretation is proposed for the Hv light scattering intensity from the polymer spherulites in the growth process.     

Crystallization and morphological features of syndiotactic polystyrene induced from glassy state

Spherulitic growth rates and microstructure of syndiotactic polystyrene (sPS) cold-crystallized isothermally at various temperatures, T_c (115–240 °C), have been investigated by small-angle light scattering (SALS), optical microscopy and transmission electron microscopy. The derived activation energy for sPS chain mobility at the crystal growing front is 5.4 kJ/mol, which is relatively lower than that of isotactic polystyrene, 6.5 kJ/mol. In addition, the Hv scattering invariant (Q_Hv) measured by SALS on the crystallized sPS samples displays a pronounced minimum at 150 °C. Despite a wide range of T_c used, however, the sample crystallinity estimated by Fourier transformation infrared spectroscopy remains unchanged. Prior to crystallization, the correlation length derived from the Vv patterns on the basis of Debye–Bueche model is ca. 1.13 μm regardless of T_c used. Interconnected domains with a width of ca. 1.8±0.5 μm are readily observed in all the crystallized samples under phase contrast microscopy and the phase-separated structure is conserved within sPS spherulites whose diameters are increased with increasing T_c.

Based on the above facts, we conclude that the presence of a Q_Hv minimum is ascribed to the resultant events of the two competitive transitions i.e. liquid–solid crystallization, and liquid–liquid demixing resulting from the spinodal decomposition (SD). At lower T_c, the unstable SD transition overwhelms the crystallization. Despite the low chain mobility, the coarsening process driven by the interfacial energies has reached a certain level before crystalline nucleation takes place. At higher T_c, on the other hand, cold crystallization becomes the dominant process due to the enhanced chain mobility, leading to the suppression of ongoing SD coarsening process. At an intermediate T_c range, comparable competition of the phase separation and crystallization prohibits the development of ordered symmetry within spherulites, giving the presence of Q_Hv minimum.     

Synthesis, formation and characterization of ZnTiO3 ceramics

Zinc titanate (ZnTiO₃) powders of perovskite structure were synthesized by conventional solid state reaction using metal oxides. Powders of ZnO and TiO₂ in a molar ratio of 1:1 were mixed in a ball mill and then heated at temperatures from 700 to 1000 °C for various time periods in air. The crystallization temperature of ZnTiO₃ powder was 820 °C, activation energy for crystallization was 327.14 kJ/mol and for grain growth was 48.84 kJ/mol. A transition point was observed when the electrical resistivity was measured versus temperature. Like some ferroelectric materials, a PTCR behavior above the transition temperature was observed with Curie temperature of 5 °C.     

Morphology development and exclusion of noncrystalline polymer during crystallization in PVDF/PMMA blends

Morphology development during the crystallization of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blends was investigated at various crystallization temperatures (T_C) by means of time-resolved light scattering measurements and atomic force microscopy (AFM). A coarse spherulite obtained at a high T_C of 162 °C was found to be developed with a two-step crystallization process. The ordering in the spherulites (Pr) increased with time at the early stages and then decreased at the later stages. The rate of spherulite growth started to decrease when Pr started to decrease. In contrast, in the compact spherulite obtained at a low T_C of 148 °C, Pr decreased monotonously with time while the growth rate was constant. AFM observation revealed that such characteristic crystallization behavior is attributed to the exclusion of PMMA from the crystal growth during the crystallization; i.e., the amount of excluded PMMA becomes larger as the distance from the spherulite center increases and the crystallization temperature rises.     

Reversible secondary crystallization during cooling of polypropylene

Ultra-violet microscopy has been used to follow the distribution of fluorescent additives during the isothermal crystallization and cooling of polypropylene. During cooling from the crystallization temperature there is a flow of additives away from the spherulite centres and into the spherulite boundaries. This demonstrates a marked densification at the centre of the spherulites with less within the bulk and only a small increase at the boundary. This change reverses when the sample is reheated to the crystallization temperature. Thus spherulites which appear to be quite uniform in crystallinity at high temperatures become non-uniform on cooling. The spherulites also become fibrous in appearance when cool. These effects are explained in terms of non-uniform concentrations of poorly crystallizable polymeric species which retard secondary crystallization.     

Crystallization morphology of a thermotropic liquid crystalline polymide

Crystallization morphology of an aromatic thermotropic liquid crystalline polyimide was studied by means of polarized light microscopy. The polyimide was synthesized from 1, 2, 4, 5-benzenetetracarboxylic dianhydride (PMDA) and 1, 3-bis[4'(4′ aminophenoxy) cumyl] benzene (BACB), which exhibited three exothermic peaks at onset temperatures of 292, 279, and 250°C in the DSC cooling curve. The results of polarized light microscopy revealed that the polyimide quenched from 350°C in air exhibited fine structure, frozen liquid crystalline texture. The liquid crystalline texture disappeared when the sample was heated to 298°C. However, the polymer melt still exhibited, to some degrees, birefringence until the temperature reached 340°C, where the polyimide was in the truly isotropic state. Isothermal crystallization experiments were carried out at both the isotropic temperature range and the non-isotropic temperature range. Two types of negative spherulites with lamellar structure and positive needle-like crystals formed from the isotropic melt have been observed. Interestingly, with a further decrease of the crystallization temperature to the liquid crystalline state temperature, the whole field was covered by the liquid crystalline texture. However, if the sample was kept at 286°C for a long time, and then reheated to 305°C to melt the liquid crystalline phase, negative spherulites with loose structures formed from the liquid crystalline state could be observed. Surprisingly, if the sample was kept at 305°C for a period of time, further crystallization could be observed using the spherulites formed at 286°C as nuclei. Composite spherulites were developed if the low-high temperature crystallization process was repeated.     

Crystallization and morphology of reactor-made blends of isotactic polypropylene and ethylene-propylene rubber

The crystallization and morphology of reactor-made blends of isotactic polypropylene (PP) with a large content of ethylene-propylene rubber (EPR) (i.e., > 50%) were investigated. In the blends, PP was found to form spherulites during the crystallization process, with the growth rate constant under isothermal conditions. For crystallization temperatures in the range of 118–152°C, the birefringence of the spherulites varied from negative to positive by decreasing crystallization temperature, while homopolypropylene (homo-PP), the same as used in the blends as a matrix, showed negative spherulites in the whole temperature range investigated (118–152°C). Both the spherulite growth rate and the overall crystallization rate were slower for the blends than for homo-PP. The density of the crystallization nuclei was lower in the blends than in the homo-PP. It was concluded that a large amount of EPR content in the reactor-made blends of PP retards and hinders the crystallization of the matrix. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1007–1014, 1997     

On the fractal character of polymer spherulites: an ultra‐small‐angle X‐ray scattering study of poly[(R)‐3‐hydroxybutyrate]

Ultra‐small‐angle X‐ray scattering (USAXS) and small‐angle X‐ray scattering (SAXS) measurements are presented for poly[(R)‐3‐hydroxybutyrate] (PHB) crystallized at room temperature. The USAXS patterns indicated that the spherulites had a radially orientated fibrillar nanostructure with fractal geometry over a length scale ranging from 12 nm up to at least 300 nm, with a mass fractal dimension of approximately 2.7 in aged samples. The SAXS patterns indicated that the fibrils themselves were built up of bundles of crystalline lamellae separated by layers of disordered material, with a period length of approximately 6 nm. The SAXS measurements during primary crystallization gave an initial fractal dimension of 4 during spherulite growth, due to the sharp phase boundary between the spherulites and the melt. Copyright © 2004 Society of Chemical Industry     

Study on the crystal morphology and melting behavior of isothermally crystallized composites of short carbon fiber and poly(trimethylene terephthalate)

The spherulites of the short carbon fiber(SCF)/poly (trimethylene terephthalate) (PTT) composites formed in limited space at designed temperatures, and their melting behaviors were studied by the polarized optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), respectively. The results suggest that SCF content, isothermal crystallization temperatures, and the film thicknesses influence the crystal morphology of the composites. The dimension of the spherulites is decreased with increasing SCF content, but whether banded or nonbanded spherulites will form in the composites is not dependent on SCF content. However, the crystal morphology of the composites depends strongly on the temperature. When the isothermal crystallization temperatures increase from 180°C to 230°C, the crystal morphology of SCF/PTT composites continuously changes in the following order: nonbanded → banded → nonbanded spherulites. Discontinuous circle lines form in the film when the film thickness increases from 30 to 60 μm. Basing on the SEM observation, it is found that these circle lines are cracks formed due to the constriction difference of the different parts of the spherulites. These cracks are formed when the film is cooled from the isothermal crystallization temperature to the room temperature at a slow cooling rate; while they will disappear gradually at different temperatures in the heating process. The crack will appear/disappear first around the center of the spherulite when the film was cooled/heated. The nontwisted or slightly twisted lamellas will reorganize to form highly twisted lamellas inducing apparent banded texture of the spherulites.     

Method of determining the kinetics of spherulite primary nucleation from the truncation of spherulites

Synopsis An attempt is made to describe mutual truncation between two coinciding spherulites. It is shown that if in a sample the borderlines between spherulites are not straight lines, the spherulites did not arise simultaneously. Using. a mathematical approach it is possible to determine the time distribution of primary nucleation from the shapes of truncated spherulites. This was done for polyoxymethylene samples at two different crystallization temperatures. The superiority of thermal over athermal type of nucleation depends on the crystallization temperature which lead to different spherulite size distributions.     

Effects of organic nucleating agents and zinc oxide nanoparticles on isotactic polypropylene crystallization

Organic nucleating agents and inorganic nanoparticles, as well as their hybrid composites, affect the crystallization temperature and morphology of the monoclinic α-form of isotactic polypropylene (iPP). Techniques such as differential scanning calorimetry, hot-stage optical microscopy with cross polars, wide angle X-ray diffraction, and transmission electron microscopy were employed. Nanoparticles of zinc oxide function as efficient supports for 1,3,5-benzene tricarboxylic-(N-2-methylcyclohexyl)triamine because the temperature at which the maximum rate of iPP crystallization occurs during 10 °C/min cooling from the molten state increases from 111 °C for the pure polymer to 125 °C at low concentrations of this hybrid nucleating agent. In the absence of zinc oxide, 0.06 wt% of this aliphatic triamine recrystallizes near 165 °C and increases the crystallization temperature of iPP by 7 °C, relative to the pure polymer. Fluorinated aromatic triamines, such as 1,3,5-benzene tricaboxylic-(N-4-fluorophenyl)triamine, are weak nucleating agents that reduce spherulite size in isotactic polypropylene but only increase the crystallization temperature marginally when the polymer is cooled from the molten state. Both micro- and nanoparticles of zinc oxide reduce spherulite size in isotactic polypropylene, but smaller spherulites are observed when the inorganic nanoparticles exhibit dimensions on the order of 40-150 nm relative to micron-size particles. In contrast, 0.06 wt% of the aliphatic triamine in iPP yields a distorted birefringent texture under cross polars that is not spherulitic. Non-spherulitic birefringent textures in iPP are also observed when the aliphatic triamine nucleating agent is coated onto micro- or nanoparticles of zinc oxide. This study demonstrates that the nonisothermal crystallization temperature of isotactic polypropylene increases by an additional 7 °C when an aliphatic triamine is distributed efficiently within the polymeric matrix by coating this nucleating agent onto zinc oxide nanoparticles.     

Growth process of homogeneously and heterogeneously nucleated spherulites as observed by atomic force microscopy

A poly(bisphenol A octane ether) (BA-C8) was synthesized. The isothermal spherulitic growth process was studied in situ using atomic force microscopy (AFM) at room temperature. For spherulites formed by homogeneous nucleation, the growth process includes the birth of a primary nucleus, the development of a founding lamella and the growth of the founding lamella into a spherulite. An embryo below a critical size is unstable. A stable embryo grows into a founding lamella. There is only one founding lamella in each spherulite. All other lamellae originate from this founding lamella. Two eyes can be seen at the center of a spherulite. For spherulites formed through heterogeneous nucleation, many lamellae grow at the nucleus surface and propagate outward radially. The spherulites acquire spherical symmetry at the early stage of crystallization. No eyes are found for this kind of spherulites.     

Influence of silicon doping on the nanomorphology and surface chemistry of a wood-based carbon molecular sieve

Carbon–silica molecular sieves were prepared by carbonization of Scotch fir (Pinus sylvestris) after impregnation with aqueous waterglass (Na_xSi_yO_z, where x, y and z may take a range of values). Compared to Si-free samples, doping significantly modifies the structure that forms during the carbonization process. For carbonization temperatures between 600 °C and 1000 °C, doped samples shrink less than undoped samples, indicating increased mechanical strength. The specific surface area and pore volume develop in a combined self-activation and chemical vapour deposition (CVD) process. Nevertheless, the presence of the sodium silicates limits self-activation and thus reduces the porosity. Doping drastically reduces the specific surface area, measured both by gas adsorption and small angle X-ray scattering. The latter technique demonstrates that in both doped and undoped samples the specific surface area is isotropic. X-ray photoelectron spectroscopy (XPS) reveals that the spatial distribution of Na and Si atoms within the samples are not identical. The open honeycomb structure, conserved during the heat treatment from the original wood, provides easy access for gas adsorption and separation applications. The ratios of the microporous diffusion time constants of N₂ and O₂ from frequency response (FR) measurements gave separation factors 3.0, 4.3, 2.7 and 1.3 for samples prepared at 600 °C, 700 °C, 800 °C and 900 °C, respectively.     

Crystallization behavior of star-shaped poly(ethylene oxide) with cubic silsesquioxane (CSSQ) core

The crystallization behavior of well-defined star-shaped cubic silsesquioxane-poly(ethylene oxide) (CSSQ-PEO) and linear PEO were studied in terms of differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). It was found in DSC analysis that the glass transition temperature (T_g) and the crystallization temperature (T_c) of CSSQ-PEO are different from those of linear PEO. The presence of CSSQ in PEO reduced the overall crystallization growth rate. This effect can be ascribed to the reduction of the mobility of the PEO crystallites in the presence of CSSQ and the star structure of the polymer. The Ozawa method is qualitatively satisfactory for describing the nonisothermal crystallizations of linear PEO and CSSQ-PEO. The presence of CSSQ leads to the diffusion- and nucleation-controlled mechanisms in the crystallization process of CSSQ-PEO whilst only the nucleation-controlled mechanism was observed in the case of linear PEO. The apparent activation energy required for crystallization was calculated using the Kissinger method. The isothermal crystallization morphology of PEO and CSSQ-PEO were also examined by cross-polarizing optical microscopy (CPOM). The CPOM images indicated the spherulite growth is slower in CSSQ-PEO as compared to linear PEO. It was also investigated that more number of PEO spherulites in CSSQ-PEO were observed, which sizes are markedly smaller than the spherulites developed in linear PEO. Wide-angle X-ray scattering (WAXS) studies showed that the crystallization peaks for linear PEO and CSSQ-PEO appeared at different temperature revealing the crystallization process and crystal growth rate are different from each other. However, no significant distortion of the crystal structure of PEO was evaluated in the presence of CSSQ.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.     

Cooling and annealing properties of copolymer-type polyacetals and its crystallization behavior

Copolymer-type polyacetals (POMs) that have been cooled at seven different rates from the melt at 180°C to the solid at 23°C show average spherulite diameters from 10 to 25 μm on an etched fracture surface using scanning electron microscopy (SEM). The wide-angle X-ray diffraction (WAXD) of POM displays a degree of crystallinity ranging from 60 to 66% by applying the two-phase model. From studies of mechanical properties, physical properties, and dielectric dissipation factor (tan δ), we found that POM with a faster cooling rate shows looser packing and smaller spherulites on the fracture surface than that with a slower cooling rate. This conclusion is in agreement with the observations made on SEM and WAXD. DSC measurements were used to measure the heat of fusion, melting point, and crystallization temperature of POMs. An equilibrium melting temperature was estimated from the Hoffman–Weeks plot. The overall crystallization kinetics of POMs were analyzed by the Avrami equation. Results for the Avrami exponent n, between 2 and 3, indicate small disklike spherulites following nucleation growth kinetics. Annealing the cooled POM at 150°C results in recrystallization featuring a significant increase in the average diameter of spherulites in SEM.     

Crystallization behavior and spherulite growth rate of isotactic polypropylene in isotactic polypropylene/natural rubber based thermoplastic elastomers

The melting and crystallization behavior of isotactic polypropylene/natural rubber (PP/NR) based thermoplastic elastomers (TPEs) were investigated using differential scanning calorimetry. The samples were scanned at a heating rate of 10°C/min under nitrogen atmosphere. The effects of blend ratio on the melting and crystallization characteristics of the blends were analyzed. Normalized crystallinity is unchanged by the addition of small amount of NR, but as the amount of rubber increases crystallinity increased for the 30/70 NR/PP and lowered for the 50/50 NR/PP blend system. Morphology of the blend was analyzed using scanning electron microscopy (SEM). Blend ratio showed a pronounced influence on the phase morphology of the NR/PP TPEs. As the amount of NR increases more than 50 wt % the system changes from dispersed to cocontinuous structure. Hot‐stage polarizing optical microscopy (POM) was used to study the radial growth of spherulite as a function of blend ratio, cooling rate, and crystallization temperature. Spherulite growth rate is marginally influenced by the rubber inclusions. The spherulite morphology observed under polarized optical microscopy is influenced by the blend morphology. It was found that for the cocontinuous 50/50 blend system, spherulites are much different from the usual appearance under polarized light. Attempts have been made to correlate the crystallization behavior with the morphology of the blend. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008