Fajun Zhang

Improved thermal fractionation technique for chain structure analysis of ethylene/α-olefin copolymers

In this report, we describe an improved thermal fractionation technique used to characterize the polydispersity of crystalline ethylene sequence length (CESL) of ethylene/alpha -olefin copolymers. After stepwise isothermal crystallization, the crystalline ethylene sequences are sorted into groups by their lengths. The CESLs are estimated using melting points of known hydrocarbons. The content of each group is determined using the calibrated peak area. The statistical terms: the arithmetic mean (L) over bar (n), the weighted mean (L) over bar (w) and the broadness index I = (L) over bar (w)/(L) over bar (n) are used to describe the distribution of CESL. Results show that improved thermal fractionation technique can quantitatively characterize the polydispersity of CESL with a high degree of accuracy.

The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/alpha -olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120 degreesC, a low temperature endotherm is observed at lower temperatures (40-80 degreesC), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm T-m(low) varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature T-c. The T-m(low) also depends on the thermal history before the crystallization at T-c, and an extrapolation of T-m(low) (30.6 degreesC) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30 degreesC. The T-m(low) is nearly equal to T-c, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational. entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.

Observation of twisting growth of branched polyethylene single crystals formed from the melt

The twisting growth of a branched polyethylene single crystal formed from the melt was observed directly by means of transmission electron and atomic force miscroscopy. The surface stress asymmetry arising from the asymmetry of the surface-fold structure and, chain tilting resulted in the twisting growth of the single crystals. The handedness of the twisting lamellae was consistent With the chain-tilting direction. When multilayer lamellae piled up in a thicker film, the lamellar twist would be inevitably causing screw dislocations.

Morphologies of metallocene-catalyzed short chain branched polyethylene single crystals formed from the melt at higher temperature

Metallocene-catalyzed short chain branched polyethylene single crystals, formed from the melt at a higher crystallization temperature of 114°C, were obtained. Highly elongated lamellae were formed, which are different from truncated lozenge or lenticular shaped single crystals formed at a lower crystallization temperature. It was found that there existed a definite line in the lamellae along the longitudinal growth direction and two regions were separated by the definite line. The lateral habits of both the regions were asymmetrical about the b-axis due to the chain tilting, which was the same as that at a lower crystallization temperature. Generally, the highly elongated lamellae were not straight, but curved towards the opposite direction with chain tilting direction due to a series of edge dislocation within a lamella. The inner side of a lamella was serrated and the outer side was smooth due to the lamellar curvature. The thickness of both regions of a lamella was different, the broader region was thicker than the narrower region, which was different from the uniform thickness of the lamellae formed at a lower crystallization temperature. The different thicknesses within a lamella were considered as the result of the initial thickness difference and the impact of isothermal thickening.

Effect of branch content on the transition of crystalline structure and morphology of metallocene-catalyzed branched polyethylene

Transition of crystalline structure and morphology of metallocene-catalyzed butyl branched polyethylene with branch content has been studied. It was found that the long periods of the branched polyethylene were controlled by crystallization conditions for the lower branch content samples and by branch contents for the higher branch content samples. When the branch content increased to a critical value the branched polyethylene had no long period because the crystalline morphology was changed from folded chain crystal to a bundled crystal. The TEM observations supported the results. The transition of the crystalline morphology resulted from the reduction of lamellar thickness with increasing of branch content since the branches were rejected from the lattice. The reduction of lamellar thickness with increasing of branch content also resulted in lattice expansion and decrease of melt temperature of the branched polyethylene.

Lateral habits of single crystals of metallocene-catalyzed low molecular weight short chain branched polyethylene from the melt

The lateral habits of low molecular weight short chain branched polyethylene single crystals from the melt were studied. Three crystallization temperatures (102, 104 and 106°C) were selected for single crystal growth. It was found that the lateral habits of single crystals were asymmetric at all the crystallization temperatures selected. The electron diffraction patterns and tilting series experiments evidenced that there existed chain tilting in all the lamellae. It was the chain tilting that lead to the asymmetry of the growth rate and of lateral habits of the single crystals about the b-axis. The lateral habits substantially changed from the growth at 102°C where the truncated lozenge single crystals formed with straight (110) faces to the growth at 104°C where the lenticular single crystals appeared. This change occurred at 20°C lower than that in a low molecular weight linear polyethylene with the same molecular weight. Furthermore, kinetics theory analysis evidenced that the change of lateral habits from truncated lozenge to lenticular shape resulted from the transition of growth regime. The results were the same as that of high molecular weight linear polyethylene but different to that of low molecular weight linear polyethylene. It may be attributed by the existence of short branched chains.

Study on single crystals of butyl branched polyethylene in the presence of electric field

Single crystal of butyl branched polyethylene with various molecular weight formed from the melt in the presence of electric field was studied. It was found that electric field influenced morphology and structure of the butyl branched polyethylene single crystals formed. The lateral habits of the single crystals were circular shape, which was different from truncated lozenge or lenticular shape single crystals formed from the melt in the absence of electric field. The stems in the single crystals formed in the presence of electric field were perpendicular to the basal plane of the single crystals, which was different from chain tilting in single crystals formed from the melt in the absence of electric field. The electron diffraction patterns showed that the structure of the circular single crystals was a quasi-hexagonal with looser chain packing. This looser chain packing was favorable to thickening growth of single crystals through chain sliding diffusion. The thickness of the single crystals was much larger and depended on molecular weight. It indicated that the single crystal in the presence of electric field should be an extended chain type single crystal.

In situ study of nanostructure and morphological development during the crystal–mesophase transition of poly(di-n-hexylsilane) and poly(di-n-butylsilane) by X-ray and hot-stage AFM

Nanostructure and morphology and their development of poly(di-n-hexylsilane) (PDHS) and poly(di-n-butylsilane) (PDBS) during the crystal-mesophase transition are investigated using small angle X-ray scattering (SAXS), wide angle X-ray diffraction and hot-stage atomic force microscopy. At room temperature, PDHS consists of stacks of lamellae separated by mesophase layers, which can be well accounted using an ideal two-phase model. During the crystal-mesophase transition, obvious morphological changes are observed due to the marked changes in main chain conformation and intermolecular distances between crystalline phase and mesophase. In contrast to PDHS, the lamellae in PDBS barely show anisotropy in dimensions at room temperature. The nonperiodic structure and rather small electronic density fluctuation in PDBS lead to the much weak SAXS. The nonperiodic structure is preserved during the crystal-mesophase transition because of the similarity of main chain conformation and intermolecular distances between crystalline phase and mesophase.