How to measure absolute P3HT crystallinity via ¹³C CPMAS NMR

Abstract

We outline the details of acquiring quantitative C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance on the most ubiquitous polymer for organic electronic applications, poly(3-hexylthiophene) (P3HT), despite other groups' claims that CPMAS of P3HT is strictly nonquantitative. We lay out the optimal experimental conditions for measuring crystallinity in P3HT, which is a parameter that has proven to be critical in the electrical performance of P3HT-containing organic photovoltaics but remains difficult to measure by scattering/diffraction and optical methods despite considerable efforts. Herein, we overview the spectral acquisition conditions of the two P3HT films with different crystallinities (0.47 and 0.55) and point out that because of the chemical similarity of P3HT to other alkyl side chain, highly conjugated main chain polymers, our protocol could straightforwardly be extended to other organic electronic materials. Variable temperature H NMR results are shown as well, which (i) yield insight into the molecular dynamics of P3HT, (ii) add context for spectral editing techniques as applied to quantifying crystallinity, and (iii) show why T , the H spin-lattice relaxation time in the rotating frame, is a more optimal relaxation filter for distinguishing between crystalline and noncrystalline phases of highly conjugated alkyl side-chain polymers than other relaxation times such as the H spin-spin relaxation time, T , and the spin-lattice relaxation time in the toggling frame, T . A 7 ms T spin lock filter, prior to CPMAS, allows for spectroscopic separation of crystalline and noncrystalline C nuclear magnetic resonance signals. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

Negatively Charged Poly(acrylic acid) Side Chains Grafted onto Poly(3-hexylthiophene): Impact on the Structural Order and Molecular Dynamics

“…The CP/MAS spectrum of the P3HT polymer exhibits two distinct spectral regions: the first at ≈125−140 ppm, assigned to the main chain carbons in the thiophene rings, and the second, at ≈15−33 ppm, attributed to side chain carbons. 39,40,42 The peak at 14.5 ppm is assigned to the CH 3 group. 23,24 The chemical shift value suggests the presence of the side chains with a predominant gauche conformation characteristic of a less ordered phase.…”

Efficient Strategy for Determining the Atomic-Resolution Structure of Micro- and Nanocrystalline Solids within Polymeric Microbeads: Domain-Edited NMR Crystallography

“…Fortunately, solid-state NMR spectroscopy offers a range of experimental techniques enabling selective suppression or enhancement of the required signals. For polymeric composites, the T 1 - and T 2 -filtered experiments, which are sensitive to differences in segmental dynamics, are particularly convenient techniques. − This is given by the fact that local motions in even highly crystalline polymers efficiently reduce 1 H spin–lattice relaxation times T 1 ( 1 H), which then tend to be several seconds only. In contrast, low-molecular-weight crystalline compounds often exhibit much longer relaxation times reaching up to several hundreds of seconds.…”

“…In contrast, low-molecular-weight crystalline compounds often exhibit much longer relaxation times reaching up to several hundreds of seconds. As a variety of different relaxation filters and editing techniques can be applied, − the appropriate experimental conditions enabling efficient suppression of the unwanted signals can often be readily attained.…”

Influence of Concentration of Thiol-Substituted Poly(dimethylsiloxane)s on the Properties, Phases, and Swelling Behaviors of Their Crosslinked Disulfides

“…The temperature was held constant at 25.0 • C. Calibration was completed using KBr and adamantane [49]. For quantitative CP/MAS 13 C NMR measurements, the acquisition conditions were optimized following a procedure developed at NIST [50], and specifically involve the optimization of spin rate, contact time, and recycle delay. This resulted in the selection of spinning rate (4900 Hz), contact time (6000 µs), and recycle delay (5 s), along with a spectral width of 197.21 ppm, ramp.…”