Accomplishment of VACSY experimental set-up and its application to investigate molecular orientation distribution of solid-state polymers

Abstract

The macroscopic characteristics of polymer materials, especially liquid crystalline polymer materials, depend significantly on their molecular orientation distribution. Mainly three methods, X-ray diffraction, neutron scattering and NMR, are used to investigate molecular orientation distribution in polymer materials. While X-ray diffraction is suitable for studying the orientation distribution of samples in crystalline state, neutron scattering and NMR are two widely adopted methods for samples in amorphous state. NMR’s unprecedented selectivity makes it the unique experimental tool to investigate the orientation distribution of individual segments in a long molecular chain. Along with the advancements of solid-state NMR technology during the last twenty years, a number of NMR approaches become available to study molecular orientation distribution of solid-state polymers. 2H NMR with line-shape analysis is the most popularly used method, this is mainly due to its good S/N ratio and its simplicity of data analysis. However, this method requires very expensive and time consuming isotope labelling. 1H wide-line NMR and moment analysis approach has also been widely used for studying orientation distribution of weakly order polymer samples, but this method can hardly provide us the orientation information of a specific segment in a long molecular chain. Several 13C NMR approaches, which utilise the orientation dependent chemical shift anisotropy and correlate them with their structural related chemical shift isotropy, have the greatest advantage to investigate the orientation distribution of individual segments in a long molecular chain of un-labelled polymer materials. VACSY as a promising method to re-introduce the Chemical Shift Anisotropy (CSA) under the condition of fast variable angle sample spinning and separate them by their corresponding Chemical Shift Isotropy (CSI) in the second dimension of a 2D NMR correlation spectrum has been selected by us to study the orientation distribution of liquid crystal polymers (LCPs). In this work, a probehead specially designed for the implementation of VACSY experiment is constructed from scratches. On top of other functionalities of a normal CPMAS double resonance probe, the VACSY probe adds the capability for the accurate controlling of the angle between the sample spinning axis and the external magnetic field B0 direction. Much effort has been paid to optimise the double resonance RF circuit for maximum efficiency and the angle control system to achieve an accuracy better than 0.25°. A computer program for VACSY spectra simulation in the case of slow sample spinning is created and successfully applied to simulate the influences on the final CSA line-shape due to insufficient sample spinning speed, the angle mis-setting (between the sample spinning axis and the external magnetic field B0 direction), the number of angle sampling steps, etc. The VACSY simulation result proves to be very useful in selecting the correct experimental parameters. To reduce the phase artefacts due to an incomplete time domain data sampling which are inherent to VACSY experiment, two new VACSY data processing approaches are proposed and successfully applied to process our VACSY experimental data. Comparing with the normal interpolation approach published by Frydman et al, these two new proposals allow the final VACSY spectra to be displayed in phase sensitive mode and the interpolation noise is also reduced to some degree. The VACSY experimental set-up and its corresponding processing software are firstly applied to measure the values of chemical shift tensor elements for well known samples such as Glycine, DMS, HMB and Durene, the measurement results are in good agreement with published values. Then, this VACSY experimental set-up is applied to investigate the orientation distribution behaviour of two polymer liquid crystalline samples: hexa-hexyloxytriphenylene and polyacrylates. The procedure for creating certain orientation distribution in LC samples is: heat the sample over its clearing temperature (Tc) while it is put inside a strong magnetic field (9.4T), wait for equilibrium and then slowly cool it down below its glass transition temperature (Tg) to freeze the orientation distribution within the sample. From 13C VACSY spectra of the LC samples in both isotropic state and oriented state, the orientation distribution is analysed by the method of CSA line-shape fitting approach. For a reliable extraction of orientation distribution through an accurate line-shape analysis approach, fast sample spinning relative to the chemical shift anisotropy is highly desirable. For the hexa-hexyloxytriphenylene sample, the result is compared with the result of 2H NMR line splitting measurements published by D. Goldfarb and Z. Luz. Suggestions for further improvements of VACSY as a method for the study of orientation distribution of solid-state polymers are also given.