Ethylene-propylene (EP) and ethylene-octene (EO) copolymers polymerized with the aid of homogeneous vanadium and metallocene catalysts were compared by DSC and time-resolved simultaneous SAXS-WAXS-DSC at scanning rates of 10 and 20~ rain -1 using synchrotron radiation. An EP eopolymer with a density of 896 kg m -3 (about 89 mol% ethylene) after compression moulding gave orthorhombic WAXS reflections. The crystallinity as a function of temperature [wC(7)] calculated from these reflections using the two-phase model was in good agreement with we(T) calculated from Cp measurements using DSC. The Cp measurements also enabled calculation of the baseline Cp and the excess Cp. The SAXS measurements revealed a strong change in the long period in cooling and in heating. The SAXS invariant as a function of temperature showed a maximum in both cooling and heating, which could be explained from the opposing influences of the crystallinity and the electron density difference between the two phases. Two EO eopolymers with densities of about 871 kg m -3 (about 87 mol% ethylene) no longer showed any clear WAXS reflections, although DSC and SAXS measurements showed that these eopolymers did crystallize. The similarity between the results led to the conclusion that the copolymers, though based on different catalyst systems -vanadium and metallocene -did not have strongly different sets of propagation probabilities of chain growth during polymerization. On the basis of a Monte Carlo simulation model of crystallization and morphology, based on detailed knowledge of the mierochain structure, the difference between WAXS on the one hand and DSC and SAXS on the other could be explained as being due to loosely packed cryz,tallized ethylene sequences in clusters. These do cause the density and the electron density of the cluster to increase (which is measurable by SAXS) and the enthalpy to decrease (which is measurable by DSC) but the clusters are too small and/or too imperfect to give constructive interference in the case of WAXS. Of an EP copolymer with an even lower ethylene content (about 69 mol%), the crystallization and melting processes could still be readily measured by DSC and SAXS, which proves that these techniques are eminently suitable for investigating the crystallization and melting behaviour of the copolymers studied.

SynopsisHeat capacity measurements with DSC on 1-octene-based very low density polyethylenes (VLDPEs), with densities from 888 to 907 kg/m3, show crystallization between 120 and -60°C and melting between -60 and 130°C. Under the assumption that hardly any octene groups are present in the crystal lattice, the experimental heat capacity values were compared with reference values for purely amorphous and purely crystalline linear polyethylene on the basis of a two-phase model. The enthalpy-based weight crystallinity data as a function of temperature show that the crystallinities at -60°C are about 40% higher than those at room temperature, which vary from 25 to 40% and are in good agreement with volume-based weight crystallinities. The crystallization and melting curves show several peaks. Fractionations by the crystallization/dissolution method and the direct extraction method show that this is due to intermolecular heterogeneity of comonomer incorporation and that VLDPE is a reactor blend of molecules ranging from the uncrystallizable and poorly crystallizable type to the HDPE type.

By means of computer-controlled DSC, heat-capacity measurements were performed on NBS SRM 1484 between --70~ and 250~ in heating as well as cooling. By using stepwise and continuous measuring methods together with enthalpy calculations, an insight was obtained into the intrinsic consistency end the accuracy of the measurements. The reference states of polyethylene within the two-phase model were used to determine the crystallinity as a function of temperature, and to evaluate various other methods employed for the same purpose.For many years the DSC technique has been generally used in the thermal analysis of materials. Computerization has enlarged the possibilities of control and data manipulation, so that not only have measurements and evaluation been speeded up, but accuracy has increased as well. Meanwhile the point has been reached where accuracy is no longer restricted by peripheral apparatus, but only by the calorimeter itself.This will be illustrated with reference to Cp measurements on polyethylene.Although polyethylene has long been a subject of study [1,2], hardly any quantitative measurements are known for the interval from room temperature through the whole of the melting range, the very area to which by far the greater part of the present calorimetric studies relate, and, indeed, the area in which Cp and crystallinity show strong changes.This article presents findings in the range from -70 ~ to 250 ~ measured by step- The results are discussed within the framework given by the so-called reference states of purely amorphous and purely crystalline polyethylene [2].

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