ABSTRACT:Molecular and crystalline structures of linear low-density polyethylenes (LLDPE) were investigated by a series of characterization techniques. Molecular structural characteristics were elucidated by temperature-rising elution fractionation (TREF) and solventgradient elution fractionation (SGEF). A bird's eye view and a contour map of LLD PE obtained by a combination of TREF and size exclusion chromatography exhibited a broad and multimodal chemical composition distribution (CCD), in contrast to a sharp and single CCD of conventional high-pressure low-density polyethylene (HP-LOPE). Short chain branching (SCB) was found to decrease with increase of molecular weight by SGEF technique. Thermal analysis of cross-fractions proved that a characteristic broad endothermic curve of LLDPE is attributable to its broad and multimodal CCD. Then, using DSC results, an indicative index (DI) which expresses the degree of the distribution of lamellar crystal thickness is proposed. DI was found to be sensitive both to CCD and to a kind of SCB. The crystallinity and melting temperature of cross-fractions having comparable molecular weights decrease with increasing comonomer content in the order of octene-1 ""4-methyl-pentene-I > hexene-1 >butene-I. From a statistical approach to the relationship between crystallinity and degree of SCB, the probability of exclusion of a bulky branching such as isobutyl from a crystalline lattice is considered to be twice as large as than that of ethyl branching.KEY WORDS Linear Low-Density Polyethylene (LLDPE) / High-Pressure Low-Density Polyethylene (HP-LDPE) / Structural Distribution / Fractionation / Chemical Composition Distribution / Short Chain Branching / Crystallinity / Melting Temperature / In the last decade, interest has grown greatly in linear low-density polyethylene (LLOPE) manufacturing all over the world and LLOPE has gradually replaced conventional high pressure low-density polyethylene (HP-LOPE) through its superior mechanical and thermal properties. Needless to say, LLOPE has short chain branchings (SCB) derived from comonomer units which are cx-olefins, such as butene-1, hexene-1, octene-1, and 4-methylpentene-l. Thus, the molecular structure of LLOPE should be investigated from view points of average content of comonomer (degree of SCB), monomer sequence distribution along a polymer chain (intramolecular distribution of SCB), and distribution of comonomer among polymers (intermolecular distribution of SCB) besides average molecular weight and molecular weight distribution. Recently, investigations for chemical composition distribution (CCO) of LLOPE which is considered to have an important role in final properties 1 are receiving increasing attention and its inhomogeneity has been elucidated by means of various methods. 1 -7 But relatively little has been reported on the details of CCO and the effects of CCO on super structure. The authors have already reported the superstructure of LLOPE 8 -u and found that LLOPE has wide distribution of the superstructure compared to...

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The morphology of ADMET-synthesized polyethylene with n-butyl branches precisely spaced on every 39th carbon (EH39) was studied in comparison with an ethylene/1-hexene addition copolymer possessing the same branching probability, the goal being to elucidate the effect of the intramolecular sequence length heterogeneity on the lamella crystal thickness and its distribution. EH39 was found to have an orthorhombic crystalline polymorphism, which is normal for commercialized polyethylenes and different from that of the other ADMET polyethylenes with shorter CH 2 spacing (C15, C21). EH39 exhibits a narrow lamella thickness distribution; the average thickness (l c,av. ) corresponds exactly to the space length between two consecutive branches, suggesting the complete exclusion of n-butyl branches from the crystal stem. The average thickness, l c,av. mentioned above is also coincident with that obtained from WAXS and SAXS. On the other hand, the 1-hexene copolymer forms much thicker lamellae and a broader thickness distribution than ADMET polyethylene. Here, the average thickness l c,av. determined by TEM observation of the copolymer is 1.5 times larger than that calculated from the most probable ethylene sequence length obtained from 13 C NMR, or for a theoretical ethylene sequence length distribution, indicating that the lamellae are composed predominantly of the sparsely branched longer ethylene sequences that are statistically included. The intramolecular sequence distribution is considered significant to determine the lamella thickness and thickness distribution for short chain-branched polyethylenes with a narrow intermolecular chemical composition distribution.

SynopsisPolymerization of propylene carbonate was carried out at 12O-18O0C mainly with the use of diethylzinc catalyst. The polymer was a pale-yellow, viscous material of relatively low molecular weight (IOOO-iOOO). From the spectroscopic analysis of the polymer and its hydrolyzed product, the polymer was determined to have the structure CH, CH, CH3 CH, CH,where Y = 0.50, y = 0.25, and z = 0.25. This strongly suggested that the polymerization of propylene carbonate proceeded via 2,7-dimethyl-1,4,6,9-tetraoxaspiro[4,4]nonane (DTN) as an intermediate compound. Hence, DTN was synthesized and polymerized with the use of diethylzinc catalyst. The structure of the polymer thus prepared coincided exactly with that of the polymer from propylene carbonate. From these, a plausible mechanism of the polymerization was developed.

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Lamella thickness distribution (LTD) plays a critical role in determining the mechanical properties of polyethylene. LTD is predominantly governed by the intermolecular chemical composition distribution, but intrachain heterogeneity also results in a broadened LTD. Polyethylene synthesized by acyclic diene metathesis (ADMET) contains pristine microstructures free from inter and intrachain heterogeneity and therefore represent ideal models to investigate these phenomena. The crystalline structures of ADMET polyethylene with ethyl or n-hexyl branches every 21 st backbone carbon (EB21and EO21, respectively) were characterized by transmission electron microscopy (TEM), small X-ray scattering and wide angle X-ray diffraction (SAXS and WAXD), and differential scanning calorimetry (DSC). The samples were crystallized for various periods at temperatures near the DSC crystallization peak temperatures: 10 8C for EB21 and 0 8C for EO21. TEM observation exhibited that EB21 displays straight lamellar crystals with axialitic organization and an average thickness of about 55 Å. This corresponds to twice the ethylene sequence length between branches, suggesting that one lamellar stem spans three branches and includes one ethyl branch within the lamella. The lamella thickness distribution was very narrow compared with that of the cross-fraction of ethylene/1-butene copolymer prepared via Ziegler-Natta polymerization. Similarly it was found from the same characterization methods that EO21 also displays a narrow lamella thickness distribution albeit with thinner lamellae, averaging 25-26Å thick. Judging from this lamella thickness, EO21 is considered to have a lamella stem composed of a single ethylene sequence between two braches, suggesting that the n-hexyl branch is entirely excluded from a crystalline phase.

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