CP/MAS Quantitative
...e show different polarization degrees, eg . The enhancement factors of CH2 group are not the same in polyethylene (PE) due to the differences of TS1ƒâ and TIS in crystalline and amorphous phase. However, quantitative measurements in CPMAS is desirable for analyze the crystallinity of PE, phase structure of PU and cross¡Vlink point of cross-linked PE etc. There have been many solid-state NMR worker devoted themselves to obtain quantitative measurement of CPMAS, and yet to date we still have not a simple and accepted method. Fu, Hu, and Cross [3] attempted to carry out a quantitative measurements of 15N-£_1- L-histidine through a Lee-Goldburg frequency modulated cross-polarization (LG-FMCP) sequence. The experiment greatly shortened the cross-polarization time for the non-protonated Spins, but has an effect on polarization buildup of the protonated S spins. W.-G. Hu and his co-workers[4] proposed a way combined four sequences to measure the crystallinity in ultradrawn polyethylene fibers. Taking into account possible errors in experiment and caculations, their single crystallinity is so reliable. A common approach for quantitative analyses from CP spectra is to perform variable contact time CP experiments [5]. Although these experiments had saved some time compared with direct-polarization (DP) 13C NMR experiment, the complication of manipulation and calculation decreased its extended use. In the present work, we have developed a method for the quantitative determination of phase construction utilizing 13C spin-diffusion CPMAS NMR. A reliable analysis which agrees very well with the 1H wideline NMR result can be accomplished on sample polyethylene (PE). Theory Enhancement factor[1] is the most important factor under certain cross-polarization time in CP experiments. VanderHart and Khoury had studied on polyethylene and obtained the enhancement factor. They found that the CP enhancement factor varies. For the crystalline regions it is relatively constant at 3.5¡Ó0.2, while in the amorphous regions it is more sample dependent, ranging between 2.4 and 3.0. These values are typical for CP times in the range of 0.7-2.0ms. The dependence on sample prevents us to apply this result directly. However the 1H as well as 13C signals arising from undisturbed Boltzmann equilibrium populations has rigorously quantitative relative intensities, we find a factor between 1H and 13C magnetization spins similar to the conventional enhancement factor. This point is therefore considered below. In polyethylene we suppose that it is a two-phase structure including a crystalline phase and an amorphous phase. The carbon magnetizations according to two phases are and , while the proton magnetizations are and . For a homogeneous phase the rate of polarization transfer between protons and carbons, the relaxation times and the proton spin-diffusion constant are all independent of diffusion time. Hence, for a certain choice of contact time tcp in a CP-MAS experiment, the ratio between the proton magnetization and the carbon magnetization MA will be constant, i.e. ¡F (1) where KC and KA are the ratios between carbon magnetization and proton magnetization. In this equation Mc/ MA can be determined by 13C NMR lineshape decomposition. As has been explained, the ratio of MC/ MA is not quantitative. Anyhow, if KC/KA is attainable, it means we can obtain the ratio of crystalline phase and amorphous phase. Figure 1 demonstrates schematically a spin-diffusion pulse sequence monitored via cross polarization to 13C using the dipolar filter [6,7] pulse sequence. The sequence can be used to refocus the dipolar couplings and chemical shifts for the mobile protons, and then the proton magnetization is r...