JUAN J. DE PABLO AND PAUL F. NEALEY

Department of Chemical Engineering

University of Wisconsin-Madison

Many materials exhibit size-dependent properties as system dimensions approach the atomic or molecular level. For example, metal and semiconducting nanoclusters with dimensions of a few nanometers exhibit remarkable optical, electrical, mechanical, catalytic, and magnetic properties (Murray et al., 2000). Nanocluster properties differ significantly from corresponding bulk properties; they depend on the quantum-level electronic structure of the ensemble and the ratio of surface to bulk atoms, and they provide the foundation for a wide range of innovative nanotechnologies. Unfortunately, much less is known about the properties of amorphous, polymeric materials in nanoscopic structures. Because the characteristic dimensions typically associated with polymeric molecules are on the order of 5- to 10-nm, it is natural to expect size-dependent properties in polymeric structures with dimensions from 10 to 100 nm.

Evidence for dimension-dependent properties of amorphous polymers has been observed in measurements of the glass transition temperature, T_g. Experiments by several research groups, including ours, report that the T_g of polymer films with nanoscopic dimensions can be significantly different from the corresponding bulk value (Forrest and Dalnoki-Veress, 2001). Based on seminal experiments by Forrest et al. (1996), it is now widely perceived that the T_g of freestanding, ultrathin films is substantially lower than that of the bulk material. Our recent simulations suggest that polymer chains or chain segments near the free surfaces of the films have greater mobility than the polymer in the interior of the film (Torres et al., 2000). We postulate that as the film thickness decreases, the fraction of the film with higher mobility increases, thereby resulting in the observed monotonic decrease with film thickness (Jain and de Pablo, 2000). In contrast, T_g for supported, ultrathin films has been observed to increase or de

Front Matter (R1-R10)

Chemical and Molecular Engineering in the 21st Century (1-28)

Technology for Human Beings (29-52)

The Future of Nuclear Energy (53-90)

Engineering Challenges for Quantum Information Technology (91-108)

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