Hiroshi Watanabe

Hiroshi Watanabe

2015 Bingham Medalist

Kyoto University

Kyoto University Weblink

The 2015 Bingham Medal of The Society of Rheology is awarded to Hiroshi Watanabe of the Institute for Chemical Research, Kyoto University, Japan.

In an independent career spanning three decades, Hiroshi Watanabe has made a series of seminal contributions to the molecular-level understanding of the rheology of polymer liquids, and in so doing advanced both theoretical description and experimental practice. Above all, his ability to marry dielectric relaxation and rheometry on model polymers has provided a uniquely detailed look at mechanisms of entangled polymer relaxation, and thereby challenged state-of-the-art theory at every step.

Hiroshi trained under Professor Tadao Kotaka at Osaka University in the Department of Macromolecular Science, earning a Bachelor of Science in 1979, a Masters degree in 1981, and a Ph.D. in 1985. Following custom, as an exceptional graduate student he was promoted to Assistant Professor in the same laboratory. From April 1987 to March 1989 Hiroshi came to the University of Minnesota as a visiting scientist, working with Matt Tirrell. In 1994 he was called to the Institute for Chemical Research, Kyoto University, to become Associate Professor with the legendary rheologist Professor Kunihiro Osaki. He succeeded Osaki as Professor in 2003, and he has remained at the ICR ever since. From 2008-2012 he served as Vice-Director of the entire Institute. His work has been recognized in many ways, including the Research Award from the Society of Rheology, Japan, in 1994, the Award from the Society of Polymer Science, Japan, in 2008 and Fellowship in the American Physical Society (2005). Most recently he received the Award of the Society of Rheology, Japan (2012). Hiroshi has also served as an at-large member of the Executive Committee of The Society of Rheology.

Hiroshi’s first contributions to rheology were in the exploration of the linear and non-linear response of ordered block copolymer solutions (J. Rheol., 1984, 28, 393). These experiments of over thirty years’ vintage were really ground-breaking; they preceded the popularization of this topic that was set off by measurements of the order-disorder transition in the mid-1980s and that continues almost unabated today. It is a theme to which Hiroshi has returned throughout his areer; one may count upwards of 50 papers on block copolymer rheology from his group. Among the other recurring themes in Hiroshi’s overall oeuvre are polymer blends (both miscible and immiscible), model star and comb polymers, and suspensions, gels, micelles, and liquid crystals. In each of these areas he has made substantial, incisive contributions, worthy of international recognition. Yet, they are all superseded by the theme that has signified Hiroshi’s greatest contribution, alluded to above: his unique insights into the mechanisms by which entangled polymers relax stress.

Every polymer rheologist is now familiar with the pioneering reptation concept of Pierre-Gilles de Gennes, and how Masao Doi and Sam Edwards built this into the core of a molecular theory of polymer rheology. Although appealingly simple in conception, it soon became clear that this model had important deficiencies in its original form, and many additional processes have been considered (e.g. contour length fluctuations, constraint release, tube dilation, and orientational coupling, to mention some of the more prevalent). Over the ensuing decades the basic model has undergone considerable refinement, due to an impressive interplay of experiment and theory, to the extent that it is now acknowledged as a signature success of condensed matter theory. Much of the theoretical development has been motivated by the experimental approach that Hiroshi exemplifies: his measurements have been carefully crafted to test important assumptions, at the most fundamental level. This is where the use of dielectric relaxation as a complement to rheometry (and to the linear dynamic moduli in particular) is so powerful.

The ability of the linear stress relaxation modulus G(t) (and, equivalently, G’(ω) and G”(ω)) to test molecular theory is limited by the fact that G(t) is expressed as a sum of exponential decays from the normal modes of chain relaxation, each with a prescribed weighting factor. It is often possible to describe a given data set equally well by different distributions of relaxation times. Dielectric relaxation senses the same mode spectrum, but with significantly different weighting factors, such that it is almost impossible for a model to describe the results of both experiments on the same system by accident. And, by extension, when the model cannot quite describe both experiments, it is clear that there is a deficiency that needs to be corrected. Over 50 years ago, Walter Stockmayer first classified polymers that have a monomeric dipole moment component along the chain axis as “Type A” and pointed out that the dipoles sum to give a net contribution that tracks the end-to-end vector. However, by symmetry, all even-numbered modes do not contribute, so the experimental relaxation spectrum is quite different from that sensed by G(t). Hiroshi is likely the only rheologist with the necessary synthetic, experimental, and theoretical skills to successfully exploit this phenomenon. In order to do so, he selected a model system (cis-1,4-polyisoprene) that is amenable to living anionic polymerization. Then, he had the profound insight that if he allowed monodisperse, monofunctional living anionic chains to be coupled in a well-controlled way, he could make a “two-armed” star from a dielectric point of view; in this “inverted dipole” case, only the even numbered modes contribute (Macromolecules, 1991, 24, 2981; 1993, 26, 5073). Therefore, he could prepare two “rheologically equivalent” polymers, which are dielectrically distinct. In addition to using such tricks to study the fundamentals of entangled polymer dynamics, and especially constraint release and tube dilation (Macromolecules, 2008, 41, 6110), he also used this architectural variation to study the relaxation of individual arms of multi-arm stars (Macromolecules, 2002, 35, 2339) and to probe selectively the relaxation of midblocks of triblock copolymers. In fact, Hiroshi was the first to quantify experimentally the fraction of “bridging” and “looping” in ABA triblocks, by using a combination of sequential polymerization from a monofunctional initiator and controlled coupling (Macromolecules, 1995, 28, 5006) to prepare normal and inverted dipole midblocks.

In the past year alone, Hiroshi has co-authored several important papers dealing with unresolved issues in polymer melts, especially the (persistently controversial) respective roles of chain orientation and chain stretching in start-up or changes of flow. Just as dielectric relaxation gives information that is complementary to the moduli, so too does flow birefringence, especially when contrasting orientation and stretch; this point is the central theme of Hiroshi’s most recent contribution (ACS Macro Letters, 2014, 3, 1183). It is worth noting that the interpretation of the dielectric relaxation results is not trivial from a mathematical point of view. Unlike the rheological case, where essentially all that is needed for each new variant of the model is the spectrum of relaxation times (the eigenvalues), in the dielectric case Hiroshi needed to compute the normal modes themselves (the eigenvectors) and their weighting factors, for each model. As a consequence of this, Hiroshi thought very deeply about the models themselves and has become directly engaged in advancing the theoretical aspects of the problem. As a result, he has demonstrated world-class skill in polymer synthesis, experimental technique, and theoretical analysis.

Hiroshi is now serving as President of the Society of Rheology, Japan, and in this role he is in charge of preparing for the International Congress on Rheology (ICR 2016) that will be held in Kyoto in August, 2016. Rumor has it that the main concern is not the technical program but how to entertain the participants during the typically hot and humid Kyoto summer. The local team in Kyoto has been working on plans for excursions and other activities, which will be soon posted on the ICR 2016 web site (//icr2016.com/). Based on precedent from Monterey ICR2008, however, we may anticipate that karaoke will be an essential ingredient!

Since 2012 Hiroshi has been serving as an Associate Editor for Macromolecules, the first editor to be based in Japan. Fortunately, he reports that this has proven to be a relatively straightforward task, as rheologists are famous for their ability to find agreement on all issues, large and small!

Hiroshi and his wife Kyoko have two children, Naoto and Chiaki. After their son and daughter left for their studies at Kobe University, Hiroshi and Kyoko were enjoying an emptier nest with just their two cats. However, Naoto came home to work as a primary school teacher, and consequently the “excluded volume problem” has resurfaced with a vengeance. Hiroshi is particularly keen on udon noodles, and plans a detailed study of their rheological properties in the future; fracture mechanics will be of particular interest.