Quan Chen

Quan Chen

2021 Metzner Awardee

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences

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Citation

Professor Chen has worked to reveal the molecular processes underlying the rheology of ionomers and miscible polymer blends. By combining theoretical and experimental methods, he has significantly advanced the understanding of these highly complex materials.

Quan Chen, currently Professor of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, is the recipient of the 2021 SOR Arthur Metzner Early Career Award. He has been focusing on the relaxation dynamics in miscible polymer blends and ionomers, and his combination of experiments and theory has revealed clear principles underlying complicated rheological behavior of these systems. His achievements for these materials, published in more than 80 peer-reviewed papers, have been highly appreciated in the worldwide community of rheology. This appreciation has resulted in several awards for him that include the 2016 Distinguished Young Rheologist Award from TA Instruments, the 2019 Young Researcher Award from the Society of Rheology, Japan, and this time the 2021 Society of Rheology Arthur Metzner Early Career Award.

Quan received his BS and MS degrees from Shanghai Jiao Tong University under supervision of C. Zhou in 2003 and 2007, and PhD from Kyoto University under supervision of H. Watanabe in 2011. His PhD work focused on the dynamics in miscible blends. Then, he worked on block copolymer dynamics as a postdoc with H. Watanabe at Kyoto University (2011-2012), and with R. H. Colby at Pennsylvania State University (2012-2015) to start his study of ionomers. Finally, in 2015, he moved to Changchun Institute of Applied Chemistry, Chinese Academy of Sciences as a full professor. Since then, he has been extending his research for ionomers and associative polymers (as explained below in more detail), skillfully guiding his students (7 PhD and 5 MS students in total so far), and actively serving to the worldwide rheological community as an editorial board member of Journal of the Society of Rheology, Japan and Acta Polymerica Sinica, China and as a committee member of Polymer Division, Chinese Chemical Society.

Quan is a thinking experimentalist – not truly a theorist but he does adeptly use models in the literature to interpret his data in interesting and insightful ways and further develop/advance those models for whatever he is studying. Moreover, he thinks of what to try on his own and solves problems clearly and quickly with the aid of his strong background in polymer physics. He started showing this outstanding capability of research when he was in the PhD course and studying component dynamics in miscible blends polyisoprene (PI) and poly(p-tert butyl styrene) (PtBS). PI has the so-called type-A dipole parallel along the chain backbone, but PtBS does not. He focused on this characteristic feature of the PI/PtBS blends to design state-of-the-art experiments combining viscoelastic and dielectric methods, the latter selectively detecting the large-scale motion of PI in long time scales, and successfully separated the motion of PI and PtBS components in the blends (Macromolecules 2008, 41, 8694; ibid. 2011, 44, 1570; Polymer J. 2012, 44, 102). For low molecular weight (low-M) PI/PtBS blends, he found that the concentration fluctuation of PtBS introduce a distribution of relaxation times for the faster PI component. For high-M blends, he revealed that the entanglement length a, being common for PI and PtBS in the blend, is described by a blending rule utilizing the number fractions of respective Kuhn segments as the weighing factors. He related validity of this blending rule to a large asymmetry (large difference in the chain bulkiness) of PI and PtBS. Furthermore, he found that the motion of PI chains in a length scale larger than a is retarded by the high friction component PtBS, and that the Rouse relaxation over the length scale < a masks the entanglement plateau when this retardation becomes significant at low temperatures. These findings added novel aspects to our understanding of the dynamics in miscible blends of entangled polymers.

Quan kept showing his outstanding research capability in his postdoctoral study on ionomers at Pennsylvania State University. In ionomer melts having nonpolar backbones, ions very rarely fully dissociate and instead dissociate from ionic clusters as ion pairs with large dipoles. The time scale of ion pair dissociation for nonpolar ionomers is quite slow and controls the terminal viscoelastic relaxation. This slow relaxation itself has been well recognized for long time, but the correlation between the slow relaxation and the dynamic association/dissociation of the ionic groups (behaving as stickers) was not fully elucidated. For this problem, he examined dielectric behavior of PEO-based ionomers having SO3Na groups and revealed, for the first time, the dielectric relaxation reflecting the sticker dissociation from the cluster (J. Rheol. 2013, 57, 1441). Furthermore, he evaluated the number density of the sticky Rouse segments from the chemical composition of the ionomers to find that the onset frequency for the terminal viscoelastic relaxation agrees with the frequency of the dielectric relaxation reflecting the sticker dissociation. This finding unequivocally proved that the sticker dissociation is the trigger of the slow terminal relaxation of ionomers, which gave a very strong impact to the polymer rheology community.

The ionomers are viscoelastic fluids on long time scales but still behave as gels in short time scales where the ionic groups have not yet dissociated from their clusters, termed reversible gels. Taking this molecular view, Quan theoretically predicted that the gelation occurs when the number of the effective crosslinks (i.e., ionic groups) per chain, p, increases to a critical value pc ≈ 1 (Macromolecules 2015, 48, 1221; ibid. 2016, 49, 3936). He confirmed this prediction from viscoelastic data of Bob Weiss on a series of ionomers having various p. In particular, for sols having p < pc, he confirmed the prediction that an increase of p results in a “Ginzburg transition” from an overlapping sol composed of many short sol chains sharing their pervaded volumes to a non-overlapping sol wherein a few, large sol chains having multiple branches are isolated from each other. For the gels, he confirmed the other prediction that an increase of p leads to the second “Ginzburg transition” from a scarce gel network (with no overlapping between long, scarce strands of the network) to a dense network (wherein short strands are heavily overlapping). These findings of Quan brought a significant progress in our understanding of the ionomer dynamics as well as of the sol-gel transition in a variety of materials.

After he moved to Changchun Institute of Applied Chemistry, Quan further conducted extensive research for ionomers and related materials on the basis of the above finding. He demonstrated that the viscoelastic data of ionomers are well described by the sticky Rouse model utilizing the dielectrically evaluated dissociation frequency as a reciprocal of the time constant τs of the dissociation process in the model (Macromolecules 2016, 49, 9192; J. Rheol. 2017, 61, 1199). He also found that this τs and the relaxation time of the monomeric segment τ0 satisfy a relationship, τs/τ0 ≈ exp(Ea/RT) with Ea = 8-13 kJ/mol. This relationship suggests that the sticker dissociation requires cooperative motion of monomeric segments, which adds a novel aspect to our understanding of the ionomer rheology. Furthermore, on the basis of that relationship, he successfully designed ion-containing model polymers exhibiting various rheological properties well-tuned by the medium polarity (Macromolecules 2017, 50, 963). His perspective article (Soft Matter 2018, 14, 2961), summarizing all those findings, serves as an excellent guide for future research and is highly appreciated in the community of rheology. Very recently, he extended his study to telechelic ionomers to reveal that the ionomers exhibit ductility when flow-induced dissociation of an ionic cluster is followed by quick re-association into other cluster (Macromolecules 2018, 51, 4735). For telechelic ionomers with having more than one ionic group at each end of the chain, he also demonstrated a cooperative feature of the dissociation of those groups (Macromolecules 2019, 52, 2265; ACS Macro Letters 2020, 9, 917).

In summary, the outstanding research ability of Quan Chen has enabled him to reveal novel features of miscible blends and ionomers thereby making remarkable progress in our rheological understanding of these materials. We are fully convinced that he will continue developing new ideas about the dynamics of polymer-based materials, leading the worldwide community of rheology for many years to come.

by Ralph Colby (Pennsylvania State University) and Hiroshi Watanabe (Kyoto University), also appeared in Rheology Bulletin.