Interplay between the Pore Network Modifications and Dynamics of Confined Polyethyleneimine in Self-Assembled Polyethyleneimine–Silica Nanoparticle Microspheres

The CO2 adsorption performance of porous materials loaded with an amine-bearing functionality is determined by the pore network of porous adsorbents and the dynamics of the adsorbed polymeric chains bearing the amine functionality. Herein, we report the modifications in the pore network of evaporation-induced self-assembled microspheres of silica nanoparticles with varying loadings of polyethyleneimine (PEI) using positron annihilation lifetime spectroscopy (PALS) and small-angle X-ray scattering (SAXS). The dynamics of the loaded PEI in the pore network of microspheres have been investigated using temperature-dependent broadband dielectric spectroscopy. PALS investigation confirms microporosity at two different length scales available at the interparticle regions in pristine silica nanoparticle-based microspheres. With PEI loading through the self-assembly process, the pore size and pore volume accessed by PALS are observed to increase up to 20 wt % loading, followed by a decrease due to modification in the clustering behavior of nanoparticles and filling of pores with excess PEI. These variations are highly consistent with the nonmonotonic jamming of silica nanoparticles, as observed from SAXS analysis, arising due to complex interactions between silica nanoparticles and PEI. Both the segmental and localized relaxations of the adsorbed PEI are observed to slow down compared to bulk PEI due to the interactions with silica nanoparticles akin to polymer nanocomposites. The slowing down of the relaxations is consistent with the free-volume variations of the confined PEI determined using PALS. The segmental relaxation time of the loaded PEI in the microspheres varies nonmonotonically with the PEI content. The observations from complementary investigations unequivocally confirm that the packing behavior of silica nanoparticles in the evaporation-induced self-assembled microspheres is controlled by the PEI content. On the other hand, the dynamics of PEI confined within the pore network of the microsphere are determined by the clustering behavior of silica nanoparticles.