A low-field, 60 MHz, 1H NMR spectrometer yields quantitatively comparable results to 400 MHz spectrometers for the compositional analysis of multicomponent polymer systems.
Low-cost, high-accuracy characterization of polymeric materials is critical for satisfying societal demand for high-quality materials with ultra-specific requirements. Low-field nuclear magnetic resonance (NMR) spectroscopy presents an opportunity to replace costlier or destructive methods while utilizing nondeuterated solvents. Many factors play key roles in the ability of low-field NMR spectroscopy to accurately analyze polymer systems. Sample characteristics such as polymer concentration, composition, and molecular weight all directly affect the capability of low-field spectrometers to accurately determine polymer microstructure compositions. In addition to inherent sample properties affecting instrumental accuracy, many choices concerning instrumental parameters (including number of scans, relaxation delay, spectral width, and points per scan) must be made that impact the quality of the resulting NMR spectra. In this work, we benchmark the capability of a 60-MHz low-field NMR spectrometer for analyzing polymer materials using mixed microstructure polyisoprenes as a model polymer system of interest. The aforementioned critical sample and instrumental variables are varied, and we report on the ability to quantitatively characterize polyisoprene microstructure to within 1-2% of a higher field NMR spectrometer (400 MHz). We anticipate our findings to be generally applicable to other low-field spectrometers of similar field strength and other polymer systems.
Transparent materials with robust mechanical properties are essential for numerous applications and require careful manipulation of polymer chemistry. Here, polyurethane (PU) and acrylic-based copolymers out of styrene were utilized to synthesize transparent PU-acrylic graft-interpenetrating polymer networks (graft-IPNs) for the first time. In these materials, PU imparts greater flexibility, while the acrylic copolymer increases rigidity and glass transition temperature of the graft-IPNs. Kinetics of the graft-IPN synthesis was monitored using Fourier transform infrared spectroscopy and 1 H NMR spectroscopy through the conversion of the isocyanate group. System compatibility, degree of phase separation and material transparency were evaluated using transmission electron microscopy and UV-visible spectroscopy. Overall, higher compatibility is observed at a higher percentage of styrene in the acrylate copolymer. The thermomechanical properties of the IPNs were quantified using dynamic mechanical analysis to assess the effect of the acrylic copolymer content on fracture toughness of the resulting graft-IPNs. The high fracture toughness of the graft-IPNs, coupled with excellent transparency, demonstrates the potential of these systems for high-performance applications.
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