Morphological partitioning of ethylene defects in random propylene-ethylene copolymers

A series of four propylene/ethylene, metallocene-catalyzed random copolymer samples, with ethylene mole fractions ranging from 0.8% to 7.5% and melt crystallization histories of cooling at 1 degrees C/min, were studied by C-13 solid-state NMR techniques. The principal objective of the study was to determine the partitioning of the ethylene "defect" residues within the semicrystalline morphology of these isotactic poly(propylene/ethylene) copolymers: Signals from the crystalline (CR) and the noncrystalline (NC) regions were separated on the basis of contrasting T-1 rho(H) behaviors. Four new resonances, three distinct and one strongly overlapping, were identified in the spectrum of the CR regions. The assignment of these new defect resonances to specific carbons at or near the ethylene defect site was made principally on the basis of quantum mechanical chemical shift calculations. These calculations were performed on two methyl-terminated oligomers of about 6.5 monomers in length with a 3(1) helical backbone conformation, characteristic of the iPP backbone conformation in the CR state. One oligomer was the pure iPP chain, and the other contained one centrally located ethylene repeat unit. Good agreement between the experimental shifts associated with the ethylene defect and the computed shifts supported the assumption that the chain conformation in the CR regions in the vicinity of the ethylene defect remained a 3(1) helix. This good agreement between shifts was obtained when the computed shifts were not used directly, but used in a difference mode. This mode was based on the computed shift differences for corresponding carbons on the two oligomers where these differences were applied to the experimental shifts of the main iPP peaks with the same chemical identity. The assignment of the defect resonances, along with the loss of chemical shift equivalences seen in solution-state spectra, was also rationalized in the context of gamma-gauche and vicinal-gauche. interactions as applied to the 3(1) helical structure. Defect line width differences that parallel the line width differences of the main iPP resonances also aid in assigning the defect resonances to particular types of carbons. Over the range of ethylene concentrations studied herein, the partitioning coefficient, P-CR(eth), given by the ratio of the concentration of ethylene residues in the CR region to the sample-average concentration of ethylene residues, is found to be constant, taking a value of 0.42 with a standard uncertainty of 0.03. On the basis of measurements of the NMR crystallinities, this partitioning translates to a fraction of the total ethylene residues in CR regions ranging from 0.24 to 0.30 and an average concentration of ethylenes in the NC region about twice the overall concentration. We also looked for evidence that the ethylene residues become highly concentrated at the CR/NC interface. While we cannot say whether this is happening on the NC side of the interface, since we cannot identify any NC defect resonances, we can claim that a high concentration of ethylene residues is not found on the CR side near the interface.