Average Lamellar Thickness Research Articles

Metallocene-catalyzed ethylene−α-olefin isomeric copolymerization: A perspective from hydrodynamic boundary layer mass transfer and design of MAO anion

This study reports a novel conceptual framework that can be easily experimented to evaluate the effects of hydrodynamic boundary layer mass transfer, methylaluminoxane (MAO) anion design, and comonomer steric hindrance on metallocene-catalyzed ethylene polymerization. This approach was illustrated by conducting homo- and isomeric copolymerization of ethylene with 1-hexene and 4-methyl-1-pentene in the presence of bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, using (i) MAO anion 1 (unsupported [MAOCl2]−) and pseudo-homogeneous reference polymerization, and (ii) MAO anion 2 (supported Si−O−[MAOCl2]−) and in-situ heterogeneous polymerization. The measured polymer morphology, catalyst productivity, molecular weight distribution, and inter-chain composition distribution were related to the locus of polymerization, comonomer effect, in-situ chain transfer process, and micromixing effect, respectively. The peak melting and crystallization temperatures and %crystallinity were mathematically correlated to the parameters of microstructural composition distributions, melt fractionation temperatures, and average lamellar thickness. These relations showed to be insightful. The comonomer-induced enchainment defects and the eventual partial disruption of the crystal lattice were successfully modeled using Flory and Gibbs–Thompson equations. The present methodology can also be applied to study ethylene−α-olefin copolymerization, performed using MAO-activated non-metallocene precatalysts.

Kinetic model of polymer crystallization with the lamellar thickness distribution

Kinetic model of development of lamellar thickness distribution and average lamellar thickness during melt crystallization is proposed. The model bases on the Avrami–Evans and Hoffman–Lauritzen approaches to the crystallization and crystal growth kinetics and involves a thermal thickening model proposed in this paper which accounts for two processes – lamellar doubling transformation and logarithmic thickening of the doubled crystals. In the thickening model, continuity equation for the lamellar thickness distribution is proposed which includes a term responsible for fast creation of doubled crystals during a doubling time introduced as phenomenological parameter and a term which accounts for slow, logarithmic with time thickening subsequent to the doubling transformation. The model concerns melt crystallization in systems where the undercooling in the crystallization and melting processes refers to the same thermodynamic point – the thermodynamic equilibrium melting temperature.Analytical formulae for time-dependent lamellar thickness distribution vs. crystallization time are obtained by solving the continuity equation for the case of isothermal crystallization, as well as for the development of the average lamellar thickness of the doubled and un-doubled crystals during the crystallization time.The model predictions are illustrated by example computations for isothermal crystallization of PE grades characterized by high differences in the crystallization half-time resulting in high differences in the development of lamellar thickness distribution. Relative contributions of doubled and un-doubled crystals are predicted during the conversion and influence of the doubling time and logarithmic thickening parameters is discussed.

Uniaxial stretching of polylactide with different initial crystalline morphologies and temperature effect

Abstract The investigation about the uniaxial stretching behavior of two crystallized poly(L-lactide)s (PLLA) with the same degree of crystallinity but different crystalline morphologies was carried out by two dimensional Wide Angle X-ray Scattering (2D-WAXS) and Differential Scanning Calorimetry (DSC). The original lamellar blocks and the orientation induced fibrillar crystals coexist during stretching for both samples, and a mesophase is detected when the material is drawn at 65 °C. Larger elongation and hence higher crystal orientation can be obtained for the sample with nucleating agent which owns the thinner average lamellar thickness, whereas the lower melting point indicates the fibrillar crystals in this stretched sample are less perfect. Besides, higher draw temperature benefits the unfolding of chains in the original lamellae but can also decrease the chain orientation due to the enhanced chain relaxation. The original lamellar thickness is supposed to be the governing factor which determines the different crystalline structures under strain.

Effects of supported (nBuCp)2ZrCl2 catalyst active center multiplicity on crystallization kinetics of ethylene homo- and copolymers

Two different supported zirconocene, that is, bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, catalysts were synthesized. Each catalyst was used to prepare one ethylene homopolymer and one ethylene–1-hexene copolymer. Catalyst active center multiplicity and polymer crystallization kinetics were modeled. Five separate active center types were predicted, which matched the successive self-nucleation and annealing (SSA) peak temperatures. The predicted crystallinity well matched the differential scanning calorimetric (DSC) values for a single Avrami–Erofeev index, which ranged between 2 and 3 for the polymers experimented. The estimated apparent crystallization activation energy Ea did not vary with cooling rates, relative crystallinity α, and crystallization time or temperature. Therefore, the concept of variable/instantaneous activation energy was not found to hold. Ea linearly increased with the weight average lamellar thickness Lwav DSC-GT; and for each homopolymer, it exceeded that of the corresponding copolymer. Higher Ea, hence slower crystallization, was identified as a pre-requisite to attain higher crystallinity. Crystallization parameters were correlated to polymer backbone parameters, which are influenced by catalyst active center multiplicity.

Comparison of chain structure and morphology of an olefinic blocky copolymer and a Ziegler–Natta‐based ethylene random copolymer

AbstractThe microstructure and morphology of an olefinic blocky copolymer (OBC) and an ethylene‐hexene copolymer prepared by conventional Ziegler‐Natta catalysis (ZNEH) are compared. It is found that these two samples have similar melting temperatures, but the overall comonomer content in OBC is slightly higher. The crystallization temperature and crystallinity of OBC are markedly lower than those of ZNEH. A successive self‐nucleation annealing experiment reveals that OBC has a more uniform distribution of crystal thickness, indicating a more homogeneous composition distribution in its hard blocks. Small‐angle X‐ray scattering (SAXS) results show that the long period of OBC hardly changes with temperature in the low‐temperature range, whereas that of ZNEH increases gradually with temperature due to melting of the less perfect crystals. The average lamellar thickness of crystals is larger for OBC than for ZNEH, but the thickness of the thickest crystals is comparable in the two. The SAXS profiles were analyzed using a one‐dimensional correlation function. The result reveals that the partially ordered interphases in OBC are mainly located at the interface of the crystalline and amorphous phases. In contrast, the interface of the crystalline and amorphous phases in ZNEH is quite sharp and it is inferred that the partially ordered interphases are distributed in the bulk as separated domains. Scattered tiny crystals are formed in ZNEH, but OBC exhibits a macroscopic morphology of large spherulites. It is also observed that more amorphous phases are rejected outside of the lamellar stacks and spherulites in OBC. © 2012 Society of Chemical Industry

SAXS/DSC Analysis of the Lamellar Thickness Distribution on a SSA Thermally Fractionated Model Polyethylene

Successive self-nucleation and annealing (SSA) is applied to thermally fractionate a model hydrogenated polybutadiene. SSA produces discrete and well-separated thermal fractions with distinct melting peaks. The samples are studied by SAXS as a function of temperature. The SAXS profile displays only one scattering peak associated with the interlamellar correlation and is unable to discriminate the multimodal distribution of lamellar thicknesses in the samples. The average lamellar thickness associated with each melting fraction can be estimated from SAXS data at different temperatures. A modified Gibbs-Thomson equation is employed to predict the lamellar thickness from DSC data, giving results consistent with those derived from SAXS.

Fabrication and characterization of electrodeposited antimony telluride crystalline nanowires and nanotubes

Arrays of nanowires and nanotubes of antimony-telluride (Sb2Te3) have been fabricated by an electrodeposition technique. Scanning electron microscopy was employed to characterize the morphology and size of the fabricated Sb2Te3 nanowires and nanotubes. Wavelength dispersive spectroscopy analysis confirmed the composition of the fabricated nanowires and nanotubes. The composition of the nanowires fabricated at a cathodic current density of 10 mA cm−2 and nanotubes fabricated at a cathodic current density of 5.5 mA cm−2 was found to be ∼39% Sb and ∼61% Te (2 : 3 ratio between Sb and Te). The fabricated Sb2Te3 nanowire and nanotube arrays were found to be polycrystalline with no preferred orientation. The average lamellar thickness of the nanowires and nanotube crystallites was determined using the Scherrer equation and found to be ∼36 nm and ∼43 nm, respectively. The measured room temperature Seebeck coefficients for the Sb2Te3 nanowires and nanotubes were +359 µV K−1 and +332 µV K−1, respectively, confirming that the Sb2Te3 nanowires and nanotubes were p-type. The electrical resistance measurements indicated that the resistance of the Sb2Te3 nanowires and nanotubes decreased with increasing temperature, consistent with semiconducting behavior.

Effects of lamellar thickness on misfit dislocation introduction and mechanical properties of γ/α2 nano-lamellar TiAl alloys

Stress-strain behavior of lamellar Ti-38Al-3Zr and -3Nb alloys with average lamellar thickness ranging from 10 to 1000 nm were studied at room temperature. Their yield stresses decrease from a high value of coherent lamellar structures to a low value after introducing misfit dislocations onto lamellar boundaries. Their strain hardening rates increase with decreasing lamellar thickness, and then drop to a low level when the misfit dislocations become absent. There is critical thickness of γ lamellae for introduction of misfit dislocations. The thickness decreases with increasing lattice misfit between the constituent phases.

Simultaneous estimation of the phase content and lamellar thickness in isotactic polypropylene by the simulated annealing of wide‐angle X‐ray scattering data

AbstractWide‐angle X‐ray scattering (WAXS) patterns of isotactic polypropylene (iPP) were analyzed in terms of the ideal WAXS patterns of their individual phases. Analysis was done with the technique of simulated annealing. This analysis is a novel method for simultaneously obtaining the volume fractions of the crystalline and amorphous phases and the average lamellar thickness of the individual crystalline phases. The method is different from traditional methods as it allowed us to obtain a large amount of information in a single step from a single WAXS pattern. This study was limited to a few selected specimens of iPP, but the method expounded could, in principle, be extended to other polymers and polymer blends. The limits of its applicability and possible shortcomings are discussed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Structural characterization of amylose-long chain fatty acid complexes produced via the acidification method

Amylose molecular inclusion complexes, or V-amylose, have been studied as a possible nano-sized delivery system for unsaturated fatty acids. This study aimed to study three different structural levels of V-amylose produced via an acidification method. Molecular attributes were studied using XRD, DSC and 13C CP/MAS NMR, nanostructures using SAXS and AFM, and the microscopic level by SEM and AFM. 13C labeled fatty acids revealed head groups were entrapped in both COO- and COOH forms. SAXS data, showed that conjugated linoleic acid yield particles with the highest values for parameters like average crystalline lamellar thickness ( φ = 0.46) and characteristic particle dimension ( R g = 1011). AFM revealed surface roughness increases from 7.72 ± 4.34 nm to 11.54 ± 6.05 nm during the formation of V-amylose. The insights described contribute to the understanding of V-amylose structure and help establish a model for V-amylose structure which may prospectively be used in the fabrication of a novel delivery system.

Molecular Ordering and Molecular Dynamics in Isotactic-Polypropylene Characterized by Solid State NMR

The order-disorder phenomenon of local packing structures, space heterogeneity, and molecular dynamics and average lamellar thickness, <l>, of the alpha form of isotactic polypropylene (iPP) crystallized at various supercooling temperatures, DeltaT, are investigated by solid-state (SS) NMR and SAXS, respectively. <l> increases with lowering DeltaT, and extrapolations of <l>(-1) versus averaged melting point, <T(m)>, gives an equilibrium melting temperature, T(m)(0) = 457 +/- 4 K. High-power TPPM decoupling with a field strength of 110 kHz extremely improves (13)C high-resolution SS-NMR spectral resolution of the ordered crystalline signals at various DeltaT. A high-resolution (13)C SS-NMR spectrum combined with a conventional spin-lattice relaxation time in the rotating frame (T(1rhoH)) filter easily accesses an order-disorder phenomenon for upward and downward orientations of stems and their packing in the crystalline region. It is found that ordered packing fraction, f(order), increases with lowering DeltaT and reaches a maximum value of 62% at DeltaT = 34 K. The ordering phenomenon of stem packing indicates that chain-folding direction changes from random in the disordered packing to order in the ordered packing along the a sin theta axis under a hypothesis of adjacent re-entry structures. It is also found that f(order) significantly increases prior to enhancement of lamellar thickness. Additionally, annealing experiments indicate that <l> is significantly enhanced after a simultaneous process of partial melting and recrystallization/reorganization into the ordered packing at annealing temperature >/=423 K. Furthermore, the center-bands only detection of exchange (CODEX) NMR method demonstrates that time-kinetic parameters of helical jump motions are highly influenced by DeltaT. These dynamic constraints are interpreted in terms of increment of <l> and packing ordering. Through these new results related to molecular structures and dynamics, roles of polymer chain trajectory and molecular dynamics for the lamellar thickening process are discussed.

Studying different dimensions of amylose–long chain fatty acid complexes: Molecular, nano and micro level characteristics

Recently amylose inclusion complexes, or V-amylose, have drawn much attention as a possible vehicle for the nanoencapsulation of unsaturated fatty acids. This study aimed to study three different structural strata of V-amylose, the molecular attributes using XRD, DSC and 13C CP/MAS NMR and the nanostructures using SAXS. Using these methods it was noted that decreased degree of fatty acid unsaturation induces the formation of more organized and well defined structures. Specifically, calculations based on SAXS data show that regardless of the crystallization temperature saturated SA yields the highest values for parameters like average crystalline lamellar thickness ( φ = 0.26) and characteristic particle dimension ( R g = 9.6). SEM shows this trend extends even into the microscopic level. Overall, this study shows that in the case of long chain fatty acids, increased fatty acid unsaturation impairs the structure of amylose inclusion complexes.

完全層状組織 TiAl 合金における &amp;alpha;2/&amp;gamma; 界面への界面転位の導入と格子ミスフィット変化に及ぼすNb および Zr 添加の影響

Changes in microstructures of α2/γ lamellar boundaries was studied on fully lamellar Ti-38Al-xM (at%, x=0, 3, M=Zr, Nb) alloys over a wide range of average lamellar thickness from 290 to 20 nm. The lamellar structure was formed by isothermal aging and lamellar thickness was controlled by changing aging temperature. The addition of Nb or Zr alters the lattice mismatch between α2 and γ phases. At lamellar boundaries the mismatch is accommodated by elastic deformation and/or by introducing misfit dislocations. The elastic deformation totally accommodates the lattice mismatch (coherent boundary) when γ lamellae are thin enough. Misfit dislocations are introduced to accommodate partly the mismatch when γ lamellar thickness exceeds a critical value λc. Density of the misfit dislocations saturates to an upper limit ds in thick lamellar structures. The critical value of λc decreases and the saturation level ds increases with increasing lattice mismatch. Zr increases and Nb decreases the lattice mismatch.

Effect of Microgravity and Magnetic Field on the Metallic and Crystalline Structure of Magnetostrictive SmFe2 Synthesized by Unidirectional Solidification

The Sm-2Fe molten alloy with 1:2 molar ratio was unidirectionally solidified in both microgravity and normal gravity in concurrence with a magnetic flux (0-0.12 T). The compound SmFe2 was produced by the unidirectional solidification in microgravity with a magnetic flux of 0T and exhibited a lamellar microstructure. The average lamellar thickness was 30 mum and each lamella possessed a <111> crystallographic alignment along major axis aligned in the direction of cooling. Unidirectional solidification in microgravity with a magnetic field of 0.04 T produced crystalline SmFe2 and Fe phases. The microstructure of this product was lamellar with an average lamellar thickness of 17 mum and no crystalline alignment. Unidirectional solidification in microgravity with a magnetic flux ranging from 0.06 to 0.12 T and in normal gravity with a magnetic flux ranging from 0 to 0.12 T produced crystalline Sm2Fe17 and Fe. During unidirectional solidification in microgravity without a magnetic flux, few nucleation sites were formed and rapid crystal growth occurred, consequently forming large-grain SmFe2. The produced SmFe2 had a lamellar structure with a dominant <111> crystallographic alignment in the direction of cooling. Convection in the molten state and where a magnetic flux was present caused homogeneous nucleation, forming Sm2Fe17 with a disordered structure and crystalline alignment coinciding with the formation of the dendritic Fe.

Single Crystals of Chain-Folded Copolyterephthalamides

Crystallization of copolyaramides composed of rigid poly(p-phenylene terephthalamide) segments (containing 3−11 aromatic cycles) and short amine aliphatic chains (5−10 CH2 groups) was studied. Crystallization was induced at room temperature by slow saturation with methanol vapors of sulfuric acid solutions. In electron diffraction, the 3AR6DA, which has monodisperse rigid blocks, exhibits weak but typical single crystal diffraction, with relatively sharp diffraction spots. Other wARnDAs, for which the rigid blocks are polydisperse, show a similar diffraction diagram albeit with lower resolution and considerable peak broadening, indicating that considerable disruption of the crystal phase has taken place. All observed diagrams correspond to the hk0 level, and chains are perpendicular to the lamellar plane. Atomic force microscopy observations show that most crystals are multilamellar, the thickness of each lamella increasing regularly with the number of aromatic groups in the rigid segment. The average lamellar thickness value is in agreement with the expected rigid segment length. It is therefore concluded that the addition of a short aliphatic chain fragment allows chain folding to occur.

Structural study of semi-crystalline blends of poly(vinylidene fluoride) and poly(methyl methacrylate) by means of linear correlation and interface distribution functions

AbstractIn this article small-angle X-ray scattering (SAXS) is used to characterize the structural parameters of semi-crystalline blends of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate). Different blend compositions from 100 to 50 wt.-% of PVDF were investigated. The samples were considered to be isotropic. As two-dimensional SAXS patterns with cylindrical symmetry were examined, a single direction in the SAXS pattern plane was chosen to collect and plot absolute intensities versus the scattering vector. Using the one-dimensional (linear) electron density correlation and interface distribution functions obtained, respectively, from the Fourier-transformed Lorentz-corrected experimental scattering intensity and from the interference function, structural parameters such as the minimal value and the most probable value of the long period, the average lamellar thickness, and the volume crystallinity were determined.

Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions

The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases.

Effect of titania nanoparticles on the morphology of low density polyethylene

AbstractThe role of TiO2 nanoparticle surfaces in affecting the crystalline structure of low‐density polyethylene (LDPE) has been investigated by varying the nanoparticle surface from hydrophilic (as‐received) to less hydrophilic (dried) or more hydrophilic (polar silane treated). Differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WXRD) were used to determine the degree of crystallinity and crystalline structure. The impact of nanoparticle aggregates on the nanometer to micrometer organization of LDPE crystals was studied with atomic force microscopy (AFM) and small‐angle light scattering (SALS). This characterization showed that the presence of the TiO2 nanoparticles, with the various different surface conditions investigated, did not alter the degree of LDPE crystallinity, the unit cell dimensions, the average lamellar thickness, or the average spherulite size. However, the nanoparticles did affect the internal arrangement of intraspherulitic crystalline aggregates by decreasing the relative optic axis orientation of these crystals, usually referred to as internal spherulite disorder. The LDPE filled with the nanoparticles treated with a polar silane (N‐(2‐aminoethyl) 3‐aminopropyl‐trimethoxysilane (AEAPS)) showed the highest internal spherulitic disorder and exhibited the most poorly developed spherulite structure. The combination of SALS with AFM has allowed a detailed characterization of the morphology of the semicrystalline polymer nanocomposites. Information on the internal organization of the spherulites, the size of the nanoparticle aggregates, and the location of the nanoparticle aggregates can be uniquely obtained when both techniques are used. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 488–497, 2005

Effects of lamellar boundary structural change on lamellar size hardening in TiAl alloy

Strengthening by refinement of lamellar thickness was studied at room temperature on dual phase Ti–39.4mol%Al alloy over a wide range of average lamellar thickness λ from 850 to 20 nm. The relation between yield stress σ y and λ was examined, paying special attention to the change in lamellar boundary structure. The γ/ α 2 lamellar boundaries in the alloy are found to be perfectly coherent in thin lamellar structure formed at low aging temperatures. In thick lamellar structure formed at high aging temperatures, misfit dislocations were introduced to relieve the lattice misfit and are found on the lamellar boundaries. Both thin lamellae with coherent boundaries and thick lamellae with dislocated ones are present in a lamellar structure formed at an intermediate aging temperature. The critical thickness of γ lamella for the introduction of misfit dislocations is about 50 nm. The dislocated boundaries render a high resistance to dislocation motion across the boundaries. A Hall–Petch relation holds in the range of λ > 170 nm, and the Hall–Petch slope takes a large value corresponding to the high boundary resistance. The coherent boundaries provide a relatively low resistance. Another σ y– λ correlation typical of the coherent boundary appears in the range of λ < 100 nm. The yield stress saturates to an upper limit of 1 GPa at λ = 70 nm. The transition from the property of dislocated boundary to that of coherent boundary proceeds with an increase in the density of the coherent boundaries within the range of λ = 170–100 nm.

Kinetics of non-isothermal melting and crystallization of gamma-irradiated isotactic polypropylene

Abstract The non-isothermal kinetics of melting and crystallization of gamma-irradiated isotactic polypropylene were investigated by differential scanning calorimetry. At constant temperature the melting rate increased with the dose. During the melting, the average lamellar thickness, the free surface energy and the transport energy through the phase boundary reduced with increasing the dose. The crystallization rate decreased with increasing dose at constant temperature. Mixed α-γ spherulites were grown during crystallization. The non-isothermal crystallization rate expressed, in fact, the rate of secondary nucleation of the lamellae in the spherulites and their growth. Raising the irradiation doses led to a decrease of the crystallization temperature. A critical dose of irradiation of about 200 kGy was found, where the dependences of the calculated kinetic parameters underwent an important change. This change could be successfully connected with the effect of radiation annealing and melting.