A carbon-13 NMR method for determining the partitioning of end groups and side branches between the crystalline and noncrystalline regions in polyethylene

Abstract

An ethylene-l-butene linear copolymer (E/B) is shown to give rise to a solid-state 13C spectrum which, when taken with proton-carbon cross polarization (CP), magic-angle spinning, and high-power proton decoupling, contains resolvable resonances from vinyl and methyl end groups as well as ethyl side branches. The signal-to-noise generated after 12-20 h of transient accumulation is sufficient to identify signals of the order of 2 carbons per 10000 backbone carbons. In contrast to the backbone methylene resonance, which shows shifted, separable signals arising from the crystalline and noncrystalline regions, these weak "defect" resonances show corresponding shifts too small to be definitive. Therefore, the problem of determining the partitioning of defects between the crystalline and noncrystalline regions of polyethylene must be approached indirectly. The method for identifying the morphological origin of defect signals is based on the concept that 13C CP signals are proportional to the local spin polarization levels, which, in turn, are kept quite uniform over distances of nearest neighbors because of proton spin diffusion. Thus, defect signal intensities are argued to be proportional to backbone signal intensities within a given morphological region. By isolating the backbone resonances corresponding to the pure crystalline and noncrystalline components, one thereby isolates the defect resonances.The determination of these pure component line shapes was accomplished by changing the preparation of the proton spin states prior to CP in ways which changed the mixture of crystalline and noncrystalline components in the resulting spectra. Since it is not obvious that the CP efficiency of the backbone carbons and the defect carbons in a given region should be the same, the consistency of the CP intensities with intensities expected based on solution 13C NMR results was probed. In this initial study of the E/B slowly cooled from the melt, the vinyl and methyl end-group carbon resonances were found to have enhancement factors very similar to those of the backbone methylene carbons. The crystalline region, having a mass fraction of 0.76, contained 57% of the methyls and 46% of the vinyls. The fraction of crystalline vinyls in this study is substantially larger than that found in solution-grown crystals in other studies. In contrast, the ethyl-branch carbons were not so well-behaved; i.e., their total intensity was not fully accounted for. Taking account of the missing intensity and the limits of sensitivity, we argue that no more than 20% of the ethyl branches resides in the crystal. On the other hand, there was no direct evidence that any crystalline ethyl branches exist. Studies of this question and others will appear in forthcoming papers.

Influence of the Type of Fiber on the Structure and Viscoelastic Relaxations in Composites Based on a Metallocenic Ethylene-1-octene Copolymer

“…Exclusively, the methyl branches are included in the lattice at a substantial degree [13][14][15][16][17] and, in a small proportion, the ethyl ones have been also found in crystalline environments. [18][19][20] Moreover, for high comonomer contents, like in the current case, besides the (110) and (200) orthorhombic reflections, a third crystalline reflection, centered at about 4.53 Å, is observed. The origin of this extra reflection is not clear, since it can be ascribed to either monoclinic or hexagonal crystals.…”

Cocrystallization in random copolymers of poly(β-hydroxybutyrate-co-β-hydroxyvalerate) and its effect on crystalline morphology

“…The 13c NMR technique isolates the spectra corresponding to the crystalline and amorphous phases of each sample, as described previously (9,14). It is based on a simple model that the solid is made up of two phases, crystalline and amorphous, a model that will be tested by the SAXS measurements.…”

“…P(3HB-co-3HV) samples used in this study (14) since the crystalline and amorphous domains are large relative to the monomer size. Moreover, since the shorter T~~~ is approximately 6 ms for these samples, spin diffusion rates imply that overall repeat distances should exceed 6 nm for the experiment to work.…”

“…To generate the spectra 4C and 4D, representing the crystalline and noncrystalline phases, respectively, we generated linear combinations of spectra 4A and 4B. The details of generating the crystalline and amorphous spectra have been outlined in a previous paper (9,14). Separation of spectra is based on differences in molecular mobility between the crystalline and amorphous phases in these samples, which means relaxation takes place more rapidly in the amorphous domain than in the crystalline phase.…”

1H spin‐lattice relaxation and observation of interfacial material in solid polyethylenes

“…With a sufficiently high signallnoise ratio (1000) statistically defensible fits to four exponentials were obtained, which are summarised in Table 1 along with time constants for the (single exponential) LFSLR. ( 5 ) and 1 can therefore also be calculated. These are summarised in Ignoring density differences between regions 1 and 2, f, (the fraction of crystalline material) is given by eqn(7)…”