Synthesis of Polyimides in Molecular-Scale Confinement for Low-Density Hybrid Nanocomposites

Abstract

In this work, we exploit a confinement-induced molecular synthesis and a resulting bridging mechanism to create confined polyimide thermoset nanocomposites that couple molecular confinement-enhanced toughening with an unprecedented combination of high-temperature properties at low density. We describe a synthesis strategy that involves the infiltration of individual polymer chains through a nanoscale porous network while simultaneous imidization reactions increase the molecular backbone stiffness. In the extreme limit where the confinement length scale is much smaller than the polymer's molecular size, confinement-induced molecular mechanisms give rise to exceptional mechanical properties. We find that polyimide oligomers can undergo cross-linking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (>350 °C).

Controlled Nanoconfinement of Polyimide Networks in Mesoporous γ-Alumina Membranes for the Molecular Separation of Organic Dyes

“…It must be noted that the confinement of a polyimide network in the γ-alumina layer should lead to an enhancement of the physicochemical stability of the polyimide network as shown by Isaacson et al 17 Based on the powders' crystallinity and morphology, one can assume that the PI membrane samples exhibit similar structural characteristics as the polyimide powders. If our assumptions are confirmed, it would mean that by simply varying the reaction time, we can engineer the membrane's micropores and thus enhance the membrane separation performance.…”

“…The opposite approach in which PI networks are confined in an inorganic matrix could also be employed. Following this latter strategy, Isaacson et al 16,17 nanoconfined a PI network in a mesoporous tortuous organosilica matrix. The preparation procedure involved infiltrating polyamic acid oligomers into the porous matrix and subsequent cross-linking of the polymer units.…”

Using Unentangled Oligomers To Toughen Materials

“…Molecular-scale confinement of polymers has generated intense interest because of its ability to induce changes in many polymer properties, including altered molecular dynamics, − changes in scaling behavior, and reduction of interchain entanglements. − This last property of polymer confinement has important implications for the fracture of confined polymers because entanglements enable mechanisms that give high-molecular-mass bulk polymers their high toughness . We have recently demonstrated the effects of polymer confinement on the fracture properties of nanocomposite materials, showing that high-molecular-mass polymers confined at molecular length scales dissipate energy through a confinement-induced molecular bridging mechanism. , Although confinement-induced bridging substantially increased the toughness in these materials, the magnitude of their fracture energy was far lower than that of bulk polymers because of the reduction in entanglements − and suppression of traditional polymer toughening mechanisms such as crazing . We now turn our attention to the toughening behavior of low-molecular-mass oligomers that are confined at molecular length scales.…”

Solvent-free preparation of uniform styrene/maleimide copolymer microspheres from solid poly(styrene-alt-maleic anhydride) microspheres

“…For the coating or filling application that requires long-term chemical stability and resistance against moisture, styrene maleimide copolymers (SMI) are superior to SMA owing to the inertness of their maleimide groups. 15,16 In addition, styrene maleimide copolymers have been recognized as having better thermal stability 17 and mechanical properties 18 than those of SMA.…”