Influence of crystallization conditions on the location of side-chain branches in ethylene copolymers as studied by high-resolution solid-state ¹³C N.M.R.

Abstract

High-resolution solid-state ¹³C N.M.R. experiments have been carried out on a series of Ziegler-Natta ethylene copolymers containing a small amount of side chain branches. Selective observations of the amorphous and crystalline phases of these compounds have shown that a small quantity of methyl branches can be accomodated in the crystalline zones, in a way which is independent of the thermal history of the sample. On the contrary, branches longer than methyl appear to be mainly located in the amorphous and interfacial areas and the composition of each phase depends on the thermal treatment undergone by the sample. In polyethylenes and ethylene copolymers, the accomodation of some side groups in the crystalline phase of the material has been investigated by several techniques. From the analysis of the melting temperature/composition relations in ethylene-type copolymers from diazoalkenes containing varying amounts of either methyl or n-propyl side groups arranged in random sequence distribution, it has been concluded (RICHARDSON et al., 1963) that while small quantities of the methyl side groups are incorporated into the crystalline phase at equilibrium, the larger side groups are excluded. These conclusions appear to be disputed by wide-angle X-ray diffraction studies of ethylene copolymers. The unit cell dimensions are known to expand and distort as the concentrations of either methyl, ethyl, n-propyl or n-butyl side groups are increased (WALTER and REDING 1956, EICHORN 1958, COLE and HOLMES 1960, SWANN 1962, BAKER and MANDELKERN 1966). However these unit cell variations do not necessarily demonstrate the incorporation of the branches into the lattice, as the unit cell dimensions may be affected by the crystallite size (RICHARDSON et al., 1963). They can also be affected by the details of folds, fold surface and the interior defect in crystals (DAVIS et al., 1968). Moreover the results appear generally dependent on the crystallization conditons. A detailed study (BAKER and MANDELKERN, 1966) has shown that in the methyl copolymers, a proportion of the side groups enters the lattice as an equilibrium condition, while, in the n-propyl copolymer containing small concentrations of side groups, the lattice spacings are close to those of the homopolymer. These conclusions can be compared with the interpretation of crystalline lattice dimension data reported by SHIRAYAMA et al. (1972): Branches smaller than C3 would be accomodated in the lattice, whereas branches larger than C4 would be excluded from the lattice, enter the amorphous domain and produce smaller relative side group composition of the fast relaxing (T1(¹³C)) amorphous regions, although it increases the crystallinity of the sample (Table 1). In the same way, the comonomer ¹³C lines can be observed in the spectra taken with the delayed contact pulse sequence (Fig.2) and their relative intensities with respect to the polyethylene main peak do not depend on the thermal treatment (quenching or slow cooling) undergone by the sample. These last results lead to the conclusion that a small number of methyl branches can be accomodated in the crystalline zones of the material independently of the thermal history of the copolymer, even if the treatment increases the crystallinity.