Echoes in spin systems with dipolar interactions

Crystallinity of polyolefins with large side groups by low-field 1H NMR T2 relaxometry: Isotactic Polybutene-1 with form II and I crystals

Phase composition and molecular mobility were studied using ¹H NMR T₂ relaxometry in isotactic polybutene-1 (iPB-1) with two polymorphs - form I and II crystals. Several types of NMR relaxation methods and data analysis were evaluated for determining the most reliable way for studying physical phases in iPB-1. Three-phase model provided the most appropriate description of the phase composition in iPB-1, i.e., a crystal-amorphous interface separates the crystalline and the amorphous phases. Due to complex molecular mobility in iPB-1, the amount of rigid fraction should be considered as NMR crystallinity number. Two types of chain segments are present in the amorphous phase: (1) chain segments with anisotropic mobility due to chain anchoring to crystals and chain entanglements; and (2) highly mobile chain end segments. The polymorphic phase II to I transition causes significant immobilization of polymer chains in the crystalline and the amorphous phases. Molecular weight of iPB-1 largely influences phase composition and molecular mobility in crystalline and amorphous phases.

Power-optimized, time-reversal pulse sequence for a robust recovery of signals from rigid segments using time domain NMR

Time domain NMR (TD-NMR) has been widely used on the analysis of liquids or liquid components in heterogeneous materials such as food, biological tissues, synthetic and bio polymers, oil-bearing rocks, biomasses and cement-based materials. The use of TD-NMR for studying solid and soft mater has been growing in number and variety of applications, mostly for organic systems where the detection of ¹H signals is highly advantageous. However, the strong ¹H-¹H dipolar interactions in solids make the ¹H FID to decay in the same order of the dead time of most commercially available NMR probe heads. Thus, solid echoes are often used for recovering signals from solid components. In this article we reinvestigate the time-reversal solid-echo pulse sequence proposed by Rhim and Kessemeier, seeking for optimal pulse power and timing conditions that maximize its efficiency on recovering ¹H signals from rigid segments. We show that under these optimized conditions, which we denote as Rhim and Kessemeier - Radiofrequency Optimized Solid-Echo (RK-ROSE), the experiment can be more efficient than its most popular counterparts Solid-Echo (SE) and mixed-Magic Sandwich Echoes (mixed-MSE). Our results also suggest that, despite the finite pulse power, with current probe technology the RK-ROSE experiment is potentially able to provide an accurate estimation of rigid components, without relying on an external calibration using multiple standard samples, as usually done in SFC analysis of the FID signal. At last, we demonstrate that RK-ROSE can be adapted as a simple filter to supress signals from mobile segments in heterogeneous materials.

Influence of Molecular Hindered Rotations on NQR Spin Echoes

The influence of the hindered reorientations of CCl3 groups on the decay of NQR spin echo signals has been investigated. In contrast to the well known literature approaches to the solution of this problem we do not use the perturbation theory of Bloembergen, Purcell and Pound. (PACS codes: 76.60.-k; 76.60.Gv; 76.60.Lz)