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

The CO2 adsorption performance of porous materialsloadedwith an amine-bearing functionality is determined by the pore networkof porous adsorbents and the dynamics of the adsorbed polymeric chainsbearing the amine functionality. Herein, we report the modificationsin the pore network of evaporation-induced self-assembled microspheresof silica nanoparticles with varying loadings of polyethyleneimine(PEI) using positron annihilation lifetime spectroscopy (PALS) andsmall-angle X-ray scattering (SAXS). The dynamics of the loaded PEIin the pore network of microspheres have been investigated using temperature-dependentbroadband dielectric spectroscopy. PALS investigation confirms microporosityat two different length scales available at the interparticle regionsin pristine silica nanoparticle-based microspheres. With PEI loadingthrough the self-assembly process, the pore size and pore volume accessedby PALS are observed to increase up to 20 wt % loading, followed bya decrease due to modification in the clustering behavior of nanoparticlesand filling of pores with excess PEI. These variations are highlyconsistent with the nonmonotonic jamming of silica nanoparticles,as observed from SAXS analysis, arising due to complex interactionsbetween silica nanoparticles and PEI. Both the segmental and localizedrelaxations of the adsorbed PEI are observed to slow down comparedto bulk PEI due to the interactions with silica nanoparticles akinto polymer nanocomposites. The slowing down of the relaxations isconsistent with the free-volume variations of the confined PEI determinedusing PALS. The segmental relaxation time of the loaded PEI in themicrospheres varies nonmonotonically with the PEI content. The observationsfrom complementary investigations unequivocally confirm that the packingbehavior of silica nanoparticles in the evaporation-induced self-assembledmicrospheres is controlled by the PEI content. On the other hand,the dynamics of PEI confined within the pore network of the microsphereare determined by the clustering behavior of silica nanoparticles.