Generation mechanism of short-chain branching distribution in linear low-density polyethylenes

Abstract

Some linear low-density polyethylenes (LLDPEs) have been examined in detail by temperature rising elution fractionation (TREF), size exclusion chromatography (SEC), 13C NMR analysis, differential scanning calorimetry (DSC), and FTIR spectroscopy. From the results, the following have been concluded:(i) LLDPEs manufactured by four different processes have in common a characteristic intermolecular bimodal short-chain branching (SCB) distribution; (ii) the bimodal SCB distribution is caused by two kinds of active sites in Ti-based heterogeneous Ziegler catalysts, identified by the values of the reactivity ratio product (rxr2 = 0.5-0.6 and rxr2 = 1.0, respectively); (iii) one kind of active site, having an alternating character in the copolymerization (identified by rxr2 = 0.5-0.6), gives the higher SCB concentration peak in the bimodal SCB distribution and the lower molecular weight polymer, while the other kind of active site, having a random character (identified by rxr2 = 1.0), gives the lower SCB concentration peak and the higher molecular weight polymer.ABSTRACT: A mathematical representation based on a linear elastic theory is proposed by which one may investigate the dependence of the crystal lattice modulus in the chain direction on molecular orientation and crystallinity. This description indicates that the crystal lattice modulus as measured by X-ray diffraction is different from the intrinsic crystal lattice modulus. However, the numerical calculation indicates that the calculated value is almost independent of the molecular orientation and crystallinity except in the case of a low degree of molecular orientation and low crystallinity, although the calculated Young's modulus is strongly affected by them. Thus it turns out that X-ray diffraction has advantages in measuring the crystal lattice modulus exactly.

Identification of Branches in Low-Density Polyethylenes by Fourier Transform Infrared Spectroscopy

“…More accurate chemical shifts and structures assigned to these signals can be found elsewhere. 30 The concentrations and distribution of various branch types can be determined from the integral intensities of these signals. However, these integral intensities are not considered to give the correct concentrations in the 13 C NMR measurements for the 2 s pulse delay and hence should be corrected using the intensity data for the carbons in ethylenejl-olefin copolymers.…”

“…30 (c) The resonance around 8 ppm assigned to the ethyl branch attached to the quaternary carbon 19 is also considered to be in the 1,3-paired form and splits into two peaks because of differences in the steric and neighboring structures. 30 The broad peak around 8.7 ppm may be assigned to the ethyl branches attached to the quaternary carbon and having some differences with neighboring structures. Thus, by counting the integral intensities due to the ethyl branches, the agreement between 13 C NMR and FTIR total branch concentration is satisfactory (see Table VII).…”

Miscibility of hexene-LLDPE and LDPE blends: influence of branch content and composition distribution

“…The interactions of molecular parameters such as heterogeneity of structure and branch distribution make it very difficult to isolate the effects of BC and CD. The use of metallocene-LLDPE (m-LLDPE) is expected to isolate these interactions due to its intermolecular chain homogeneity [16,17]. Also, m-LLDPE has almost uniform distribution of branches [18].…”

Determination of the compositional heterogeneity of polydisperse polymer samples by the coupling of size-exclusion chromatography and thermal field-flow fractionation

“…Finally, also the cross-fractionation of polyethylene and polypropylene polymers and copolymers by means of temperature rising elution fractionation (TREF) and SEC has been employed in order to study the relationship between branching, tacticity, crystallizability or composition and molecular mass [22][23][24][25][26].…”