FTIR Microanalysis and Phase Behaviour of Ethylene/1‐Hexene Random Copolymers

Abstract

Ethylene/1‐hexene random copolymers with 1‐hexene content in the range of 1–5 mol‐%, synthesised in the presence of new heterogeneous catalyst systems based on bis‐carboxylato and ‐bis‐chloro‐carboxylato titanium chelate complexes, have been characterised by FTIR microspectroscopy (FTIR‐M), DSC calorimetry and X‐ray scattering. The co‐monomer content and sequence distribution in the various samples were determined by means of both FTIR‐M and 13C NMR spectroscopy. The deformation bands of methyl groups in the region of 1 400–1 330 cm−1 were used for the structural analysis of these copolymers. The effect of composition on the crystallinity and phase transitions of copolymers was analysed both in 1 500–1 300 and 760–690 cm−1 frequency ranges as a function of the annealing temperature. A neat variation of the absorbance ratio of methyl band at 1 378 cm−1 was recorded between 110 and 130 °C corresponding to the melting range of the copolymer crystals. The crystallisation behaviour of the copolymers was examined by DSC in dynamic and isothermal conditions; the isothermal kinetics were analysed according to the Avrami model. A marked decrease in the bulk crystallisation rate, accompanied by changes in the nucleation and growth of crystals, was found with an increase in the co‐monomer content. The melting behaviour of isothermally crystallised samples was also investigated and the melting temperatures of the copolymers at equilibrium conditions were related to the composition; the experimental data were consistent with the Flory exclusion model of side branches from the crystalline phase. The lowering of crystal growth rate in the copolymers has been accounted for by an increase in the free energy of formation of critical size nuclei due to the effect of the side branches.magnified image

Precisely and Irregularly Sequenced Ethylene/1-Hexene Copolymers: A Synthesis and Thermal Study

“…The IR spectra for EH5 , EH15 , and EH21 in Figure are dominated by two sets of absorption bands (2900 and 1464 cm −1 ), which are usually observed when the packing is disorganized. While orthorhombic crystals show the characteristic Davidov splitting at 720 cm −1 due to the methylene rocking, , our EH models display a single rocking absorbance at 728 cm −1 , indicating the absence of orthorhombic crystal behavior. Moreover, the two experimental absorption bands at 728 and 1464 cm −1 are characteristic of a highly disordered phase, similar to the pattern observed in previous studies containing precisely spaced methyl, ethyl, and hexyl branches .…”

“…The region from 1400 to 1330 cm −1 is useful for compositional analysis of ethylene/α-olefin copolymers. In this region (Figure a), linear polyethylene EH0 shows a broad absorption band at 1369 cm −1 corresponding to the methylene wagging. , Incorporation of butyl branches along the PE backbone results in formation of a new absorption band at 1378 cm −1 , corresponding to the symmetric deformation of the terminal methyl on the butyl branch. While the band at 1378 cm −1 is very weak for EH-2.5 due to the low comonomer content, materials with higher comonomer contents present more intense bands with areas proportional to the branch content.…”

“…Although X-ray diffraction techniques provide the absolute crystal structure, IR spectroscopy can give an idea of the order or crystal structure for these model polymers. Pracella et al reported the structural characterization of EH copolymers made via chain-growth chemistry . In their report, detailed IR study for EH copolymers facilitated the determination of the comonomer contents in the 1−5 mol % range.…”

Thermal behaviour of polyethylene‐block‐poly(methyl methacrylate) block copolymer: effect of multiple heating and cooling ratesversusmathematical artefact

“…This offers steric hindrance to crystallization, which in turn decreases the overall rate of crystallization. The work of Pracella et al 73 also supports this view.…”

Control of branches distribution in linear PE copolymers using fibrillar nanoclay as support of catalyst system

“…Also, the FTIR spectra have been useful to make a semi-quantitative analysis of the comonomer content in the copolymers. It was correlated the peaks absorbance between 1378 and 1369 cm À 1 that belongs to the symmetric deformation of the methyl end-group of 1-hexene units and the latter belongs to the methylene wagging [28].…”