Separation of Crystalline and Amorphous Signals in Polymers:T₁-filter Method

Summary

T₁-filter method separates crystalline- and  amorphous-phase signals for such mixed-phase samples as solid polymers, by  utilizing the difference of their T₁ values.
In a polyethylene sample, where the ¹³C T₁ values are longer than 100 s for crystalline-phase and about 1 s for amorphous-phase, the method yields the individual ¹³C signals  separately, as shown in the right figure.

Principles of T₁-filter method

The longitudinal magnetizations are prepared in the +z direction parallel to the external static magnetic field, M^A,C₊(τ), or those in the -z direction antiparallel to the field, M^A,C_–(τ), by applying a 90°pulse to CP-enhanced  transverse magnetizations. The initial magnetizations approach to the equillibrium ones exponentially with the time constants, T^A₁ and T^C₁, for the amorphous- and crystalline-phases, as demonstrated in the lower figure.

The sum and the difference of the longitudinal magnetizations prepared along the ±z directions  behave as depicted in the figures.

The sum of the magnetizations (left) yields almost only the amorphous-phase signals after short-time longitudinal relaxation (τ ≪ T^C₁), whereas  the difference (right) gives only the crystalline-phase signals after long-time relaxation (τ ≫ T^A₁). Thus, the T₁-filter method separates the signals originating from the crystalline and amorphous phases.

Procedure 1

The pulse sequences,

• cpmas_t1-filter_add.jxp
• cpmas_t1-filter_sub.jxp

respectively add and subtract the magnetizations experimentally.

(1)Determine the experimental conditions which yield CPMAS spectra for the sample of interest.
(2)Set the parameter interval as τ, during which the longitudinal magnetizations relax, to be 1-3 s for most polymer samples.
(If the T₁ values for the crystalline and amorphous phases of the samples are determined before the T₁-filter measurements, set interval satisfying the conditions, t ≪ T^C₁ and τ ≫ T^A₁.)
(3)Accumulated signals (multiples of 16) immediately give the spectra exhibiting the crystalline- and amorphous-phase signals.

Procedure 2

If the +z- and –z-prepared signals are acquired and saved, the addition and subtraction data processing yields the target spectra more efficiently than the procedure 1 does: Both the two data contribute to the signal  intensities, improving the S/N ratio by a factor √2.  The pulse sequences,

• cpmas_t1-filter_plus.jxp
• cpmas_t1-filter_minus.jxp

respectively accumulate the +z- and –z-prepared signals.

(1)-(3) Conduct the measurements similar to Procedure1; accumulation  numbers should be multiples of 8. The results may be as shown below.

(4)Open the “Operate on Files” window in the NMR software Delta console window.

(5)Load FID data and input addition or subtraction expression in the  “Operate on Files” window, and process and save the files.

(6)Process the saved data, and then obtain the target spectra.