Accounting for Paramagnetic Influence in Wetted Surface Area Studies of Particle Suspensions Using Solvent Relaxometry

Abstract

Polymer matrix composites (PMC) are structural materials sustained by the synergy between a reinforcing fiber, also known as filler, and a resin matrix. Since performance relies on interfacial interactions between the PMC components, the surface area of particle fillers is a key design consideration. Here, the characterization of Kevlar® pulps and micropulps, derived from milling pulps, was conducted using a multi-technique approach, focusing on the specific surface area. Conventional techniques used to determine surface area require aggressive outgassing steps that can impact the structure of polymeric particles. Hence, a convenient alternative is explored by deploying time-domain nuclear magnetic resonance (TD-NMR) under conditions close to those experienced in-situ. In this thesis, the correlation between NMR relaxation rates and surface area acquired from BET analysis has been established. The specific surface area of Kevlar® pulps was found to increase by as large as a factor of three upon milling pulp materials (8 – 18 m2 g-1) to micropulps (20 – 24 m2 g-1). No changes were found in the chemical structure of Kevlar® following the particle size reduction, however, trace iron (Fe) was identified in the range 11 - 2,633 ppm, which may perturb the NMR signal. To address the influence of Fe, the surface relaxivity of Kevlar® (0.7 ± 0.1 μm s-1) was determined based on a linear relationship between the Fe content and its perturbation of the relaxation time T2. Lastly, the wetted specific surface area was calculated from the NMR data, yielding values within 20% difference of those determined by BET of freeze-dried samples. The obtained trends agree with foreseen effects of polymer drying and particle size reduction. The presented data indicate the value of determining the surface relaxivity of different materials, enabling a deeper understanding of interfacial interactions, as well as the rapid determination of wetted surface area for a variety of polymeric systems.