Statistical Evaluation of the Imbibition of Polymers into Porous Materials

Frozen transient imbibition states into two complementary pore models having pore diameters of ~380 nm and hydroxyl-terminated inorganic-oxidic pore walls, controlled porous glass (CPG) and self-ordered porous alumina (AAO), were imaged by phase-contrast X-ray computed tomography (X-ray CT), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy mapping (EDX). The deployment of phase-contrast X-ray CT bring together the advantages of macroscopic averaging methods and microscopy by allowing the algorithmic evaluation of three-dimensional reconstructions of imbibition morphologies to study characteristic features with characteristic length scales in the range from a few tens of μm down to the sub-μm scale, not possible with averaging methods, and with high validity, unattainable with microscopy. CPG contains continuous spongy-tortuous pore systems. AAO containing arrays of isolated straight cylindrical pores is a reference pore model with a tortuosity close to 1. Comparative evaluation of the spatiotemporal imbibition front evolution yields important information on the pore morphology of a probed tortuous matrix like CPG and on the imbibition mechanism. Image analysis methods based on the evaluation of pixel brightness fluctuations were developed to determine imbibition front positions and widths by descriptive statistics. The retardation of the imbibition front movement with respect to AAO reference samples may be used as a descriptor for the geometric tortuosity of a tested porous matrix. The velocity of the imbibition front movements in CPG equaled two-thirds of the velocity of the imbibition front movements in AAO. Moreover, the dynamics of the imbibition front broadening discloses whether porous matrices are dominated by cylindrical neck-like pore segments or by nodes. Independent single-meniscus movements in cylindrical AAO pores result in faster imbibition front broadening than in CPG, in which a morphology dominated by nodes results in slower cooperative imbibition front movements involving several menisci. The results presented here may be relevant to a broad variety of applications including printing, adhesive bonding, oil recovery, water filtration, as well as the optimization of production and properties of engineering, construction and hybrid materials