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 c rystallization histories of cooling at 1 degrees C/min, were studied by C-1 3 solid-state NMR techniques. The principal objective of the study was to d etermine the partitioning of the ethylene "defect" residues within the semi crystalline morphology of these isotactic poly(propylene/ethylene) copolyme rs: Signals from the crystalline (CR) and the noncrystalline (NC) regions w ere separated on the basis of contrasting T-1 rho(H) behaviors. Four new re sonances, three distinct and one strongly overlapping, were identified in t he spectrum of the CR regions. The assignment of these new defect resonance s to specific carbons at or near the ethylene defect site was made principa lly on the basis of quantum mechanical chemical shift calculations. These c alculations were performed on two methyl-terminated oligomers of about 6.5 monomers in length with a 3(1) helical backbone conformation, characteristi c of the iPP backbone conformation in the CR state. One oligomer was the pu re iPP chain, and the other contained one centrally located ethylene repeat unit. Good agreement between the experimental shifts associated with the e thylene defect and the computed shifts supported the assumption that the ch ain conformation in the CR regions in the vicinity of the ethylene defect r emained 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 car bons on the two oligomers where these differences were applied to the exper imental shifts of the main iPP peaks with the same chemical identity. The a ssignment of the defect resonances, along with the loss of chemical shift e quivalences seen in solution-state spectra, was also rationalized in the co ntext 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 defe ct resonances to particular types of carbons. Over the range of ethylene co ncentrations 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 cons tant, taking a value of 0.42 with a standard uncertainty of 0.03. On the ba sis of measurements of the NMR crystallinities, this partitioning translate s 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 abou t twice the overall concentration. We also looked for evidence that the eth ylene 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 conc entration of ethylene residues is not found on the CR side near the interfa ce.