Positron insight into evolution of pore volume and penetration of the polymer network by n-heptane molecules in mesoporous XAD4

Abstract

The adsorption and desorption of n-heptane on the mesoporous polymer resin Amberlite XAD4 were investigated in situ by positron annihilation lifetime spectroscopy (PALS). This technique allows the monitoring of porosity and subnanometer free volume changes as well as the amount of liquid adsorbate captured within an investigated sorbent, without causing any interference with the course of adsorption/desorption. In consequence, the conducted studies provide microscale insight into the sorption processes of n-heptane (which is a significant component of volatile organic compounds – VOCs) on the polymeric material. The total pore volume decreases parabolically with n-heptane pressure until it reaches zero just below the saturated vapor pressure. Simultaneously, the average pore size increases linearly until it has approximately doubled. However, much faster rates of change in both these parameters occur at relative pressures below 0.05. The PALS results can be properly explained only if the swelling of the polymer skeleton is taken into account during the alkane adsorption process. This is confirmed by long-term pumping, which was required to achieve stabilization of PAL spectra during the final phase of desorption. In addition, the evolution of subnanometer free volumes (located between polymer chains and formed in liquid n-heptane) support this interpretation of the results.