Establishing structure/property relationships is the cornerstone of materials science. Solid-state nuclear magnetic resonance (NMR) is a powerful method for measuring material structure at the atomic level, even in heterogeneous solids that exhibit little to no long-range order, like crosslinked polymers, composites, and semi-crystalline polymers. Solid-state NMR can quantify mesophases, measure atomic connectivities and interatomic distances, and identify atom-specific molecular dynamics; all of these measurements play important roles in characterizing new materials. Since the goal of my research is to develop solid state nuclear magnetic resonance (NMR) methods for characterizing new materials, in practice I both innovate new NMR methods as well as demonstrate/vet the utility of newly developed experiments.
1. Polymers for water purification
In order to better design reverse osmosis (RO) membranes for purifying water, it is important to establish clear relationships between the chemical structure, water/ion transport, and water filtration performance in the salt selective layer, which is typically a highly crosslinked polyamide (PA) film. We recently demonstrated a chemical separation method for purifying the PA from thin film composite (TFC) membrane, the purity of which was quantified via 13C CPMAS NMR. Using this process, we were able to cleanly measure the compositions and crosslink densities of four trimesoyl chloride (TMC)/isophthaloyl chloride (IPC)/metaphenylene diamine (MPD)-based TFC membranes, which showed that (1) a 2x decrease in crosslinking causes a 30 % increase in salt passage and (2) the addition of IPC leads to increased polar amine groups that reduce water permeance due to tighter binding of water in the membrane. These results demonstrate that both crosslink density and polarity are important design criteria in RO membranes, and that 13C CPMAS is a powerful method for quantitatively monitoring such values.
2. 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. In my previous project we 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.