The goal of my research is to develop solid state nuclear magnetic resonance (NMR) methods for understanding the packing structures and molecular dynamics in soft matter. This involves both innovating new NMR methods as well as vetting and demonstrating the utility of newly developed protocols. Solid state NMR can ascertain molecular level details in the absence of long range order, making it a powerful tool in the study of heterogeneous systems like semi-crystalline polymers, polymer blends, and polyelectrolytes. Experimental NMR data play a critical role in filling in the blanks of modern soft matter modeling efforts.
1. Semi-crystalline polymers
Semi-crystalline polymers are used in several commercial applications ranging from LCD TVs to plastic bags. However, the end-use functionality of the semi-crystalline polymer is highly dependent on the crystallization process, which can be controlled by varying the processing. NMR is an excellent tool for probing atomic-level structures and dynamics, making it useful for building mesoscale models of polymer crystals. NMR can ascertain details at the crystal/amorphous interface, the degree of disorder within the bulk of a polymer crystal, and the molecular dynamics that are associated with the disorder and paracrystallinity. We recently showed via 13C NMR that poly(3-hexyl thiophene), a model polymer for organic electronics applications, is disordered with the crystal and that the side chain dynamics and main chain dynamics are cooperative. We also showed that 13C CPMAS can be used for quantifying the crystallinity of this polymer.
2. Polymer miscibility
Solid polymer blends are important in applications ranging from pharmaceuticals, to organic photovoltaic cells, to industrial mechanical polymers. Predicting the miscibility of blends can be painstaking and coming up with predictive formulas is a continued effort by the materials science community at large. While powerful, the classic thermodynamic equation of miscibility for polymer solutions, the Flory-Huggins theory, fails to account for effects of molecular shape and size, chain length, and differences in chain rigidity. More modern methods, such as the simplified lattice cluster theory, have attempted to account for these details, but experimental evidence is critically lacking. Solid-state NMR can be utilized for probing atom-atom proximities, so is powerful for probing structures at A-B interfaces as well as measuring domain sizes in A/B blends. We have recently showed via 1H spin diffusion NMR that nanoscale domain sizes can be measured in poly(3-hexyl thiophene)/fullerene blends, and that the amount of mixed phase can be directly quantified, despite the fact that the blends are semi-crystalline and exhibit composition heterogeneity at multiple length scales (i.e. molecular mixing in mixed phase, crystal size).
3. Molecular dynamics in swollen polymers and polyelectrolytes
Due to its atomic-level specificity, solid state NMR is a powerful method for probing the segmental dynamics in long chain molecules as well as the rotational and translational diffusion of low molar mass species contained with solid polymers. Mesoscale models of dynamics are needed for accurate design in heterogeneous systems such as fuel cells, batteries, and water purification membranes, and NMR can yield critical details of dynamics for these models. NMR relaxation, linesahpe, and exchange experiments are utilized for determining these molecular dynamics parameters, generally on microsecond to millisecond time scales.