A Robust Proton NMR Method to Investigate Hard/Soft Ratios, Crystallinity, and Component Mobility in Polymers

Abstract

Summary: An improved proton NMR method for the realtime measurement of the hard/soft ratio or the crystallinity, and the mobile-fraction dynamics, in phase-separated or semicrystalline polymers is presented. It avoids some difficulties associated with earlier approaches and can be applied on high-as well as inexpensive low-field instrumentation. A pulsed mixed magic-sandwich echo is shown to provide near-quantitative refocusing of the rigid contribution to the initial part of the free induction decay. This essentially removes the need to account for signal loss during the receiver dead time, and the method should thus be useful for a variety of applications where the magnetization distribution over differently mobile fractions is to be determined. The overall decay of the mobile signal of a semicrystalline polymer was found to exhibit a significant field dependence, such that the apparent transverse relaxation function of the amorphous part is in a real-time experiment best characterized by a subsequent Carr-Purcell-MeiboomGill pulse train. It is demonstrated to be mainly influenced by mobility, while instrumental effects play a minor role. The mobility of the amorphous fraction depends not only on the overall crystallinity, but also on the crystallization conditions, thus on the nanometer-scale morphology.Isothermal crystallization of sPP monitored on a 20 MHz low-field proton NMR spectrometer using the MSE-CPMG experiment.

Proton NMR spin-diffusion studies of PS-PB block copolymers at low field: two- vs three-phase model and recalibration of spin-diffusion coefficients

“…2,11 The strong dipolar couplings in a rigid phase induce a fast transverse relaxation of the signal with T 2r E20 ms, which is on the order of the NMR receiver dead time (t d E12 ms in our case). This part of the rigid signal has to be acquired by the aid of refocusing pulse sequences, for example, the here-used dipolar time-reversed mixed magic-sandwich echo (MSE) 4,5,12,13 (see Figure 2c). The MSE is basically a multi-spin time-reversing pulse sequence, 12,14 forming an efficient echo of almost the total signal, thus overcoming the receiver dead time and allowing for a complete acquisition of initial FID (see Figure 1a).…”

“…1 The amount of the different components, their mobility and morphology can be studied, where in particular, the latter can be addressed by employing spin-diffusion (SD) techniques. [2][3][4][5] In this way, domain sizes in heterogeneous materials are accessible on a length scale of 0.5-200 nm. 2 Such experiments are based upon the distinction of different phases by their mobility, first in order to quantify their relative amount and second to selectively polarize them.…”

“…Because of its orientation dependence, dynamics has a strong effect on the effective DDC and thus on the SD coefficient and on the shape of the free-induction decay (FID), the latter allowing for a distinction of mobile and more rigid phases. 4,5 One major problem of low-field SD experiments is the rather short T 1 relaxation time in particular of mobile polymer phases, requiring in-depth investigations of the interplay of T 1 relaxation and SD effects, because the former competes with the latter in the same experiment, leading to intensity decay at long SD (mixing) times t diff . The process of SD out of a region of length d with a constant diffusion coefficient D has a characteristic diffusion time…”

Um método de RMN no domínio do tempo para caracterização e identificação de relaxações moleculares em sistemas orgânicos

“…A sequência de pulsos denominada Magic Sandwich Echo (M SE) 29,3,30,31,32 representa uma técnica de RMN capaz de contornar a rápida perda de coerência do sinal FID após um pulso RF de π/2 em virtude da presença de intensas interações dipolares homonucleares entre os 1 H dos sistemas. Devido à uma característica instrumental denominada "tempo morto"do receptor, a qual representa um tempo de espera para o início da aquisição do sinal FID ,em torno de 12 µs, parte do sinal proveniente de regiões rígidas de sólidos orgânicos dominadas pela interação dipolar acaba não sendo adquirida, uma vez que o segundo momento em rede rígida dos sistemas, da ordem de M Hz 2 , acarreta um tempo de decaimento do sinal em torno de 20 µs.…”

“…Assim, torna-se importante explorar formas de quantificação que permitam enfatizar a sensibilidade da técnica aos parâmetros Note que as intensidades diferenças mantêm a dependência com o tempo de correlação do movimento molecular e, portanto, as curvas I dif erenca vs. τ C podem ser utilizadas para sondar processos dinâmicos da mesma forma que as curvas I N DF −M SE vs. τ C ou, equivalmentemente, I N DF −M SE vs. T . O procedimento de subtração descrito acima está ilustrado na figura (30). Na figura (31) estão mostradas curvas I dif erenca vs. T onde a conversão de τ C para T foi realizada usando a função de ativação de Arrhenius.…”