Ajay Krishnamurthy

Hyperbranched polymers are commonly used as surface and cure modifiers, along with nanoparticles, of epoxy resins used in medical and infrastructure applications; however, their impact on structure and performance of the epoxy resins is not fully understood. Here, a commercial epoxy-aromatic amine resin (EP-AA) system is blended with off-stoichiometric amounts of hyperbranched amine (HA), and the cure kinetics, structure, polymer dynamics, and mechanical properties of the different compositions are evaluated. Cure kinetic studies indicate that the curing of the hyperbranched amine is nearly complete prior to that of the aromatic amines. At its highest concentration (1% by mass), the hyperbranched amine is found to autocatalyze the epoxy and the aromatic amine cure, but the bulky HA groups slow the diffusion kinetics during the final stages of the cure process. This results in a heterogeneous, cocontinuous network structure consisting of unreacted secondary amines that increase the side-group/pendant contribution and increase the free volume within the polymer network. The restricted cooperative segmental mobility of the polymer network in the 1% by mass HA blend results in poor energy dissipative properties, as evidenced by reduced strength and strain to failure.

Carbon molecular sieve (CMS) membranes prepared by carbonization of polymers containing strongly size-sieving ultramicropores are attractive for high-temperature gas separations. However, polymers need to be carbonized at extremely high temperatures (900° to 1200°C) to achieve sub-3.3 Å ultramicroporous channels for H ₂ /CO ₂ separation, which makes them brittle and impractical for industrial applications. Here, we demonstrate that polymers can be first doped with thermolabile cross-linkers before low-temperature carbonization to retain the polymer processability and achieve superior H ₂ /CO ₂ separation properties. Specifically, polybenzimidazole (PBI) is cross-linked with pyrophosphoric acid (PPA) via H bonding and proton transfer before carbonization at ≤600°C. The synergistic PPA doping and subsequent carbonization of PBI increase H ₂ permeability from 27 to 140 Barrer and H ₂ /CO ₂ selectivity from 15 to 58 at 150°C, superior to state-of-the-art polymeric materials and surpassing Robeson’s upper bound. This study provides a facile and effective way to tailor subnanopore size and porosity in CMS membranes with desirable molecular sieving ability.

Polymer nanocomposites containing self-assembled cellulose nanocrystals (CNCs) are ideal for advanced applications requiring both strength and toughness as the helicoidal structure of the CNCs deflects crack propagation and the polymer matrix dissipates impact energy. However, any adsorbed water layer surrounding the CNCs may compromise the interfacial adhesion between the polymer matrix and the CNCs, thus impacting stress transfer at that interface. Therefore, it is critical to study the role of water at the interface in connecting the polymer dynamics and the resulting mechanical performance of the nanocomposite. Here, we explore the effect of polymer confinement and water content on polymer dynamics in CNC nanocomposites by covalently attaching a fluorogenic water-sensitive dye to poly(diethylene glycol methyl ether methacrylate) (PMEO₂MA), to provide insights into the observed mechanical performance. Utilizing fluorescence lifetime imaging microscopy (FLIM), the lifetime of dye fluorescence decay was measured to probe the polymer chain dynamics of PMEO₂MA in CNC nanocomposite films. The PMEO₂MA chains experienced distinct regions of differing dynamics within Bouligand structures. A correlation was observed between the average fluorescence lifetime and the mechanical performance of CNC films, indicating that polymer chains with high mobility improved the strain and toughness. These studies demonstrated FLIM as a method to investigate polymer dynamics at the nanosecond timescale that can readily be applied to other composite systems.

Flexible unidirectional (UD) composite laminates are commonly being used for ballistic-resistant body armor. These laminates comprise UD layers, each constructed by laminating thin layers of high-performance fibers held in place using very low modulus binder resins, with the fibers in each layer oriented parallel to each other. As these materials are used in body armor, it is important to investigate their long-term reliability, particularly with regard to exposure to temperature and humidity as these are known causes of degradation in other commonly used body armor materials. This work investigates the tensile behavior of a poly(p-phenylene terephthalamide), or PPTA flexible UD laminate aged for up to 150 days at accelerated aging conditions of 70 °C and 76% relative humidity. Tests were performed at three different crosshead displacement rates and three different gauge lengths. The effect of aging on the mechanical properties of the material was observed as less than 10% degradation in tensile strength, with a more significant reduction in longer specimens when tested at slower rates.