Thermal recoverability of a polyelectrolyte-modified, nanoporous silica-based system
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A. Han and Y. Qiaoa) Department of Structural Engineering, University of California at San Diego, La Jolla, California 92093-0085 (Received 19 February 2006; accepted 7 June 2006)
The thermal recoverability of a nanoporous silica-based system modified by a cross-linked polyelectrolyte is investigated. At room temperature, as a nominally hydrostatic pressure is applied, the gel matrix can be partially dehydrated. The released water molecules will be forced into the initially energetically unfavorable nanopores and are “locked” there. At an elevated temperature, the infiltration pressure increases slightly, which is contradictory to the experimental data of the unmodified system. More importantly, the defiltration of the confined liquid is significantly promoted, leading to a much higher system recoverability. I. INTRODUCTION
Developing nanostructured energy-absorbing materials has been an active area of study. The basic concept is quite straightforward: if energy dissipation could take place simultaneously across the large interface between different components, the energy absorption efficiency would be ultrahigh. However, very often, the controllability of the interface behavior of a nanomaterial is poor. For instance, with a relatively high filler content, the fracture mode of a silicate nanolayer-reinforced polyamide 6 composite is cleavage, and the majority of nanolayers are still well bonded with the matrix even after the final failure occurs.1,2 That is, the large specific interface area cannot be fully used. To solve this problem, it would be desirable if one of the components is sufficiently “flexible.” Based on this concept, nanoporous energy absorption systems (NEAS) have recently received considerable attention.3–6 As nominally hydrostatic pressure is applied on a system consisting of hydrophobic nanoporous particles immersed in water, pressure-induced infiltration occurs when the capillary effect is overcome. As a result, the large pore surface is exposed to the nonwetting liquid, and a significant amount of mechanical work is converted to the excess solid-liquid interfacial tension, which can be regarded as being dissipated because, in many nanoporous materials, the confined liquid would remain in the initially nonwettable nanopores even after the external pressure is entirely removed. Because of the high specific surface area of the nanoporous material,
II. EXPERIMENTAL
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0287 J. Mater. Res., Vol. 21, No. 9, Sep 2006
http://journals.cambridge.org
which typically ranges from 100–2000 m2/g, the energy absorption efficiency of a NEAS can be much higher than that of conventional protective or damping materials such as polymer foams and Ti-Ni alloys. Further investigations indicate that the structure of a NEAS can be greatly simplified if a polyelectrolyte is used to “solidify” the liquid phase.7,8 Under the ambient pressure, a polyelectrolyte-modified system is solid-like. As the applied pressure exceeds the cri
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