While during the last decade significant progress has been made toward understanding and controlling the structure and properties of polymers at the nanoscale, major challenges and opportunities, both scientific and technological, remain. Research has largely been devoted to the properties of linear-chain polymer systems; the influence of polymer chain architecture on the properties of thin films has largely remained unexplored. The goal of this proposal is to understand the effect of chain architecture on the miscibility, dynamics, glass transition and aging phenomena in thin film linear/star molecule and star/star molecule mixtures. There are several reasons for studying star polymers. Star molecules suffer a smaller loss in entropy than linear chains upon adsorption at interfaces. In additional to their interfacial properties, the dynamics of star shaped molecules, specifically the longest relaxation time, ?ä, and the viscosity, ?Ø, are slow; ?ä and ?Ø scale exponentially with the degree of polymerization, N, of the chain arm length. Perhaps the most fascinating aspect of star shaped molecules is that for low functionalities, f, and for sufficiently large N, they exhibit properties similar to linear chains. However, for very large f, they exhibit behavior akin to colloids. Guided by theoretical predictions, an experimental program, which exploits tools that include neutrons, electrons, X-rays, dielectric spectroscopy, ellipsometry and mechanical instruments, has been designed to develop an understanding of the influence of confinement and interfacial interactions on miscibility, microstructure, structural instabilities, aging phenomena and dynamics in star shaped molecule based systems (linear/star molecule and star/star molecule mixtures). The properties of these systems promise to be truly fascinating and present intriguing possibilities for new science and applications.

NON-TECHNICAL SUMMARY: Diverse technologies, from coatings to functional materials for organic electronic devices and sensors, rely on the properties and performance of polymer thin films. Research on thin polymer films has primarily been devoted to linear chain polymers; little is understood about the properties of chains of different architectures, such as star shaped polymers. Star shaped molecules, by virtue of their architecture offer a number of potential, and unique, advantages over linear chain systems for certain applications. For example, they show promise for adhesive applications, and potentially for certain battery and sensor applications, as it is possible to take advantage of the multi-arm architecture for attachment of molecules/nanoparticles of varying functionalities. The goal of this proposal is to develop an understanding of the role of chain architecture on properties (miscibility, aging phenomena, glass transition and dynamics) of linear chain/star shaped molecule and of star/star molecule mixtures. This is an interdisciplinary program, involving polymer physics, chemistry, interfacial science, thermodynamics and transport processes. It will include an ethnically diverse group, including under represented minorities, of undergraduates, PhD students and high school students. The program also includes an international collaborator in the area of materials synthesis. Outreach activities of the principal investigator include public lectures at technical meetings, retired groups and participation on various National Research Council studies and boards and technical society committees.

Project Report

(DMR-0906425) Peter F. Green The public is generally aware of a vast range of applications for which polymers are used, which includes coatings, packaging materials and electronics. They are however less aware of applications that rely on the performance of polymer films of nanoscale thicknesses. Such applications include some organic electronic based devices, information storage and sensing applications. The local structure polymer films of thicknesses of a few to tens of nanometers, differs from the bulk due to polymer-interface interactions and to confinement. These effects have far reaching consequences on the physical properties and performance of the film. Various transport properties, miscibility and structural transitions, from melting to the glass transition, are strongly influenced by effects associated with confinement and interfacial interactions. Our understanding of these phenomena has largely been restricted to linear chain polymers. In a series of publications, we showed that star shaped macromolecules of sufficiently large functionalities, f, (number of arms extending from a branch point) and degrees of polymerization per arm, Narm, exhibited behavior substantially different from their linear chain analogs, thereby providing opportunities for new applications and research. We investigated the glass transition temperature, Tg, wetting and physical aging properties of star-shaped macromolecules and showed that are significantly different from their linear chain analogs. Physical aging is associated with the time-dependent changes of the thermodynamic properties of a material due to structural relaxations; it occurs in all glasses. With regard to the wetting properties of polymers, we showed that the equilibrium contact angles and line tensions of macroscopic droplets composed of star-shaped polystyrene (PS) macromolecules, on oxidized silicon substrates (SiOx), are smaller than their linear analogs by up to a factor of five, and by up to an order of magnitude, respectively, based on f and Narm. Theory and simulations, providing a rationale for this observation, reveal that star-shaped macromolecules exhibit significant interfacial activity because they suffer much smaller entropic losses when adsorbed at interfaces compared to their linear chain analogs. This finding has positive consequences with regard to the performance of polymers for applications such as adhesion and other related applications. Our second series of studies were devoted to understanding the role of macromolecular architecture on the glass transition temperature of polystyrene (PS). It is well documented that thin film linear chain polymers, including PS, exhibit film thickness-dependent glass transition temperatures. We showed that the dependence of Tg on film thickness h is a function of f and Narm, for PS star-shaped molecules the same substrate. When Narm is sufficiently large, the behavior is virtually identical that of linear chain PS, on the same substrate (SiOx). However for smaller Narm, and larger f, the dependence changes significantly, increasing with decreasing h. Moreover, the glass transition of the free surface Tg(surf) of films of star-shaped macromolecules of sufficiently high f and small Narm, is larger than the bulk (Fig. 3); Tg decreases gradually into the bulk with decreasing distance. Indeed the Tg is a function of not only the polymer, but also the macromolecular architecture and the substrate. Our studies provide important new insight into the fundamental origins of the thickness dependence of the Tg of thin polymer films. Finally, we showed that constraints imposed by the architecture of the star-shaped macromolecule suppress relaxations responsible for aging (i.e.: reduction in free volume) compared to their linear chain analogs. These findings provide new opportunities for basic and applied research in this field.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Andrew J. Lovinger
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University of Michigan Ann Arbor
Ann Arbor
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