Eric T. Hsieh

Isotacticity effect on crystallization and melting in polypropylene fractions: 1. Crystalline structures and thermodynamic property changes

Abstract A set of polypropylene (PP) fractions with similar molecular masses and distributions but different isotacticities have been studied through wide-angle X-ray diffraction, small-angle X-ray scattering and differential scanning calorimetry measurements. The crystal unit-cell parameters, crystallinity, apparent crystal size and lamellar crystal thickness are found to be dependent on crystallization temperature and isotacticity. The equilibrium thermodynamic properties (melting temperature and heat of fusion) for these PP fractions were determined following two extrapolation methods. These fractions can be thought of as stereo-copolymers, with configurational defects along the chains. A uniform inclusion model proposed by Sanchez and Eby can be applied to describe the crystals formed in these fractions. Both equilibrium and non-equilibrium data are discussed.

Isotacticity effect on crystallization and melting in polypropylene fractions: 3. Overall crystallization and melting behaviour

Abstract A set of polypropylene (PP) fractions with similar molecular masses and molecular mass distributions but different isotacticities have been investigated and their overall crystallization and crystal melting behaviours determined by differential scanning calorimetry and time resolved small-angle X-ray scattering experiments. The observations indicate that when the supercooling is high ( ΔT > 48 K), only poor and imperfect crystals can form, which are continuously annealed to more perfect crystals upon heating after isothermal crystallization. On the other hand, below ΔT = 48 K, two different crystal morphologies are recognized with their own crystallization kinetics and thermal stabilities. The Avrami treatment for the overall crystallization kinetics of these PP fractions generally reveals a low Avrami exponent ( n ) compared with the literature data. The overall crystallization and crystal melting behaviour can be correlated to the crystal morphology change with supercooling, in particular to the ‘cross-hatching’ lamellar phenomenon uniquely observed in the case of PP. The isotacticity effect on the overall crystallization processes and melting behaviour are also focused upon.

Crystallization, melting and morphology of syndiotactic polypropylene fractions: 1. Thermodynamic properties, overall crystallization and melting☆

Abstract A series of syndiotactic polypropylene (s-PP) fractions with constant syndiotacticities and different molecular weights have been studied through differential scanning calorimetry (d.s.c.), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering experiments. The molecular weights, molecular weight distributions, syndiotacticities and sequence distributions of this series of fractions have been characterized by gel permeation chromatography, solution nuclear magnetic resonance and Fourier transform infra-red spectroscopy. The equilibrium melting temperature of sufficiently high molecular weight (above 40 000) s-PP with about 94% racemic dyads is 160±1°C, and the heat of fusion is 8.0±0.3 kJ mol −1 . Overall crystallization rates exhibit a molecular weight dependence and a discontinuity with respect to crystallization temperature for the fractions. The temperature at which this discontinuity happens is at an undercooling of ca. 50°C. Based on nucleation theory, this discontinuity may be recognized as a regime III to regime 11 transition. With decreasing undercooling (increasing crystallization temperature) and molecular weight, a doubled crystal unit cell along the b axis becomes increasingly dominant during the crystallization. In this unit cell, opposite handedness of the helical chains exists along both the a and b axes (antichiral packing). Double melting peaks can be observed for all fractions in the high to middle undercooling region (ΔT> 50°C), while only one melting peak can be found in the relatively low undercooling region. Different heating rate experiments after isothermal crystallization in d.s.c. and WAXD indicate that the low-melting crystal may undergo reorganization and melt-recrystallization processes to form the high-melting crystal. During this transformation, doubling of the crystal unit cell along the b axis with an antichiral packing of the chain molecules is obtained.

Effects of the phase-separated melt on crystallization behavior and morphology in short chain branched metallocene polyethylenes

Abstract Some metallocene catalyst synthesized short chain branched polyethylene (SCBPE) samples have been found to possess at least intermolecular heterogeneity in the SCB. As-received SCBPE samples are molecularly homogeneous in the isotropic melt. However, phase separation due to the intermolecular heterogeneity can be found via molecular segregation processes induced by multiple-step isothermal crystallization experiments. When the phase-separated SCBPE samples are reheated above their melting temperatures, the phase-separated (heterogeneous) melt is maintained for an extended period of time (at least 20 h at 150°C). Neither phase mixing in the melt nor significant changes in molecular weight, molecular weight distribution, comonomer content, or sequence have been found during the high-temperature treatment. Comparisons of overall crystallization kinetics and morphology of the SCBPE samples obtained from the homogeneous and heterogeneous melts exhibit substantially different behavior which indicates t...

Intermolecular structural homogeneity of metallocene polyethylene copolymers

Abstract Direct proof of intermolecular compositional inhomogeneity of a metallocene polyethylene copolymer has been obtained using a cross-fractionation (CF) technique. The ethylene/1-hexene copolymer sample was synthesized using a Zr-based homogeneous metallocene catalyst. A solvent gradient fractionation (SGF) applied to the whole polymer material has produced nearly monodispersed molecular weight distribution fractions. The component molecules of a representative SGF fraction are then further sorted out, using a temperature gradient fractionation technique, according to their short-chain branching (SCB) differences. Molecular characterization of the resulting CF fractions reveals the presence of significant intermolecular SCB content differences among the components of the sample material.

Crystallization, melting and morphology of syndiotactic polypropylene fractions: 2. Linear crystal growth rate and crystal morphology

Abstract Linear crystal growth rates of two narrow molecular weight fractions of syndiotactic polypropylene having the same syndiotacticity have been measured using polarized light microscopy over a temperature range of > 20°C. It has been found that a regime III to regime II transition at a supercooling of ∼ 50°C exists. Structure analysis via electron diffraction (ED) experiments indicates that no change of growth planes has been found during this regime transition. Nevertheless, a gradual change of the crystal perfection due to a chain packing change from a crystal incorporated with isochiral packing defects to a majority of cell III structure in this supercooling range has been observed. The validity of the nucleation theory applied to s-PP is discussed. For the crystal morphological study, single crystals with rectangularly faceted lamellae can be grown at high crystallization temperatures (low supercooling) in thin s-PP film samples as observed via transmission electron microscopy. Similar to the results reported by Lovinger and Lotz, the ED patterns show that the long axis of the single lamellar crystal is the b -axis. On decreasing the crystallization temperature, spherulites are developed. Cracks on the lamellar crystals have been observed, and they are always perpendicular to the b -axis. This phenomenon has been explained by invoking the observation that the coefficient of thermal expansion along the b -axis is about one order of magnitude larger than that along the a -axis, as measured via wide-angle X-ray diffraction experiments. However, at high crystallization temperatures, the cracks are found less frequently. This is due to the pure cell III crystal packing that forms at these tempratures leading to the incorporation of fewer isochiral packing defects which promote crack initiation.

Molecular heterogeneity of metallocene short-chain branched polyethylenes and their fractions

A comparison on the molecular heterogeneity of two whole metallocene short-chain branched polyethylenes (SCBPEs) with different hexene comonomer (butyl branching) content and their Cross-Fractionated (CF) fractions has been made. As elucidated by differential scanning calorimetry (DSC) thermal segregation experiments and the subsequent isothermal crystallization kinetics investigations, the whole metallocene SCBPEs were found to possess both inter- and intramolecular heterogeneity. The crystallization kinetics of whole SCBPEs are substantially different for samples with or without a pre-multiple-step annealing treatment. For the CF fractions, the inter-molecular heterogeneity is not evident, but intra-molecular heterogeneity exists. Nevertheless, another self-induced SCBPE fraction, as a model sample, shows good homogeneity both inter- and intramolecularly. The crystal morphology observed via transmission electron microscopy (TEM) shows that multiple-step crystallized whole polymers exhibit large scale (molecularly) phase separation, whereas the CF fractions demonstrate microscopic (segmental) segregation only.

Structural and Morphological Inhomogeneity of Short-Chain Branched Polyethylenes in Multiple-Step Crystallization

Three commercial metallocene-catalyst synthesized short-chain branched polyethylene (SCBPE) samples with similar molecular weights and molecular weight distributions were investigated in terms of their molecular structural (comonomer sequences and compositions) inhomogeneity and crystal morphology. Two of these samples [SCBPE(B1) and SCBPE(B2)] contained different ratios of a butene comonomer (20.7 and 26.8 SCB/1000 carbons, respectively), while the third sample, SCBPE(H), contained a branched hexane comonomer (7.8 SCB/1000 carbons). A linear PE fraction was also investigated for comparison. Differential scanning calorimetry (DSC) results indicate that multiple-step isothermal (thermal segregation) experiments lead to multiple endothermic melting processes in these SCBPEs during heating, a phenomenon that was not observed in the linear PE. Wide-angle X-ray diffraction (WAXD) experiments show that all of these SCBPEs possess an orthorhombic crystal lattice with different crystallinities. Linear coefficients of thermal expansion along both the a- and b-axes of the PE crystals were also determined using WAXD at different temperatures. This lateral lattice expansion is critically associated with the comonomer size and composition ratio of the SCB series. Small-angle X-ray (SAXS) scattering results of the thermal segregated samples obtained during heating required a differential scattering data treatment. It was found that the long period for each isothermal crystallization step was different and increased with increasing temperature. By increasing the comonomer composition at a constant temperature, the long period was also increased, although the thickness of the crystal lamellae decreased. This was due to an increase of the noncrystalline layer thickness between two neighboring lamellae. The crystalline morphology was observed under transmission electron microscopy (TEM). During multiple-step isothermal crystallization, the crystalline morphology exhibited a clearly separated lamellar domain texture. All of the experimental results presented suggest that a phase separation occurs during multiple-step crystallization due to the inhomogeneity that exists in these SCBPE molecular structures. These samples may thus possess an intermolecular heterogeneity in comonomer composition and sequence.

Melt crystalfization and crystal morphology of two low molecular weight linear polyethylene fractions

Abstract Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G b) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17°C (at 117°C and 120°C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122°C and 124°C), at which the cusps of the G b data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential sca...

The effect of an unsymmetric branched branch on the 13C NMR chemical shifts of main chain carbons in polyethylene

The 13C NMR spectra of copolymers of ethylene with 4-methyl-1-hexene and 4-methyl-1-pentene, respectively, were compared. The 4-methyl-1-hexene/ethylene copolymer, which contains an unsymmetric 2-methylbutyl branch, exhibits two distinct 13C NMR peaks for each of the pairwise methylenes spaced one, two, and three carbons from the backbone methine. The chemical shift differences for these pairwise methylenes are 57.4 Hz, 18.7 Hz, and 4.3 Hz, respectively, with chemical shift differences decreasing with increasing distance from the asymmetric carbon. The frequency differences for carbons farther from the branch were not distinguishable. The magnitude of the chemical shift difference also varies with temperature, with the first and second methylene carbon chemical shift differences decreasing with increasing temperature. The third carbon is almost unaffected by temperature variations. In contrast, the 4-methyl-1-pentene/ethylene copolymer exhibits a single peak for each of the pairs of methylenes in the branchs vicinity. This is the first reported observation of a branched branch affecting the chemical shifts of main chain carbons in polyethylene containing short chain branches.