Anqiu Zhang

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.

Isothermal thickening and thinning processes in low molecular weight poly(ethylene oxide) fractions crystallized from the melt: 2. Crystals involving more than one fold

Abstract The existence of a non-integral folding chain (NIF) crystal in a poly (ethylene oxide) fraction with molecular weight of 7100 can be proved by differential scanning calorimetry and time-resolved synchrotron small angle X-ray scattering experiments. At this molecular weight, the fraction can crystallize to a final form comprised of integral folding chain (IF) crystals with extended (n = 0), once-folded (n = 1), twice-folded (n = 2), and even triply-folded (n = 3) chain crystals. The fold length of NIF crystals increases with crystallization temperature. When this fold length is between those of IF (n = 2) and IF (n = 1) crystals, the NIF crystal can be thickened into the IF (n = 1) crystal, or thinned into the IF (n = 2) crystal. In the same way, if the fold length of NIF crystal lies between those of IF (n = 1) and IF (n = 0) crystals, similar isothermal thickening and thinning processes can also be observed. Which process occurs depends upon the thermodynamic driving force between the NIF and IF crystals and upon kinetic effects. Detailed kinetic maps of the fold length changes with time will be discussed.

Primary Nucleation in Polymer Crystallization

Isothermal crystallization of a low- molecular- weight polyethylene oxide) from both isotropic and structured melts at low undercoolings has been investigated via simultaneous measurements of small angle and wide angle X ray scatterings (SAXS and WAXS). An evolution of the SAXS pattern was observed prior to the appearance of crystallinity determined through WAXS. A possible interpretation of the experimental observation is provisted through the nucleation concept: primary nuclei exist in the melt due to localized large- amplitude density fluctuations in the isotropic medium.

Two-photon-excited green emission and its dichroic shift of oriented thin-film CdS on glass formed by laser deposition

The photoluminescence of oriented thin-film CdS on glass formed by laser deposition was investigated employing 200 fs, 1.54 eV laser pulses at room temperature. The ultrafast excitation caused a two-photon absorption process, which results in purely green emission at the band gap. The spectra are fitted very well by the application of the van Roosbroeck–Shockley relation, density of states, and Urbach’s rule demonstrating the intrinsic character of the radiative recombination. It is further shown that the energy position of the emission peak depends on the polarization of the impinging laser beam due to the dichroism of the highly oriented films.

High-performance aromatic polyimide fibers. 6. Structure and morphology changes in compressed BPDA-DMB fibers

Abstract A high-performance aromatic polyimide fiber has been spun from a high molecular weight polyimide synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). The fiber exhibits not only excellent tensile properties and high temperature resistance but also a high compressive strength of 655 MPa. Morphological observations of BPDA-DMB fibers indicate that the fiber shows a skin-core structure and microfibrillar textures. A banded texture can also be found with a spacing of about 2μm, which may be introduced by the liquid crystalline behavior that appears during processing. Compressed BPDADMB fibers form kink and microkink bands over different size scales. The detailed formation mechanism of these banded textures is discussed. The structure parameter changes during compression-including crystal unit cell parameters, apparent crystallinity, crystal and overall orientation, and apparent crystallite sizes-are monitored. It is found that after restr...

The crystal structures and thermal shrinkage properties of aromatic polyimide fibers

Three aromatic polyimides based on 3,3′,4,4′-biphenyl-tetracarboxylic dianhydride (BPDA) and three different diamines 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), 2,2′-dimethyl-4, 4′-diaminophenyl (DMB) or 3,3′-dimethylbenzidine (OTOL) have been synthesized. These polyimides are soluble in hotp-chlorophenol,m-cresol or other phenolic solvents. Fibers have been spun from isotropic solutions using a dry-jet wet spinning method. The as-spun fibers generally exhibit low tensile properties, and can be drawn at elevated temperatures (>380° C) up to a draw ratio of 10 times. Remarkable increases in tensile strength and modulus are achieved after drawing and annealing. The crystal structures of highly drawn fibers were determinedvia wide angle X-ray diffraction (WAXD). The crystal unit cell lattices have been determined to be monoclinic for BPDA-PFMB and triclinic for both BPDA-DMB and BPDA-OTOL. Thermomechanical analysis (TMA) was used to measure thermal shrinkage stress and strain. A selfelongation has been found in the temperature region around 450°C. This phenomenon can be explained as resulting from the structural development in the fibers as evidencedvia WAXD observations.

Unit-cell size change of the thermotropic copolyester-TPA/PHQ/PEHQ under different annealing conditions

Abstract Wide-angle X-ray diffraction (WAXD) experiments show an anomalous change of the crystal unit-cell parameters in the thermotropic copolyester, poly[phenyl- p -phenyleneterephthalate)-co-(1-phenylethyl- p -phenyleneterephthalate)], under different annealing conditions, namely, annealing the fibres with free ends, at fixed length, and under tension. The crystallographic volumes of the crystal unit-cells in the annealed copolyester fibres are larger than those in the as-spun fibres. Density measurements reveal a decrease of the overall density in the annealed fibres. This behaviour is attributed to a partial rearrangement of the large pendant side groups in this copolyester during annealing.

A new semicrystalline, thermoplastic polyimide for carbon fiber-reinforced composites

Abstract A new semicrystalline polyimide (PI-2) has been prepared from 1,3-bis(4-aminophenoxy)-2,2-dimethylpropane and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride. The polymer, which has a glass transition temperature of 230°C and a melting point of 335°C, displays outstanding toughness with a fracture energy of 4·28 KJ/m 2 . The toughness may be associated with the polymers unique morphology, which consists of randomly stacked, ribbon-like lamellae. A reactive precursor approach has been used to prepare carbon-fiber reinforced composites that display interlaminar fracture energies ( G 1C ) as high as 1·81 KJ/m 2 .