FTIR Microanalysis and Phase Behaviour of Ethylene/1-Hexene random copolymers

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 C-13 NMR spectroscopy. The deformation bands of methyl groups in the region of 1400-1330 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 1500-1300 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 1378 cm-1 was recorded between 110 and 130 degrees 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