Grand Séminaire d'Institut

Vendredi 10 Novembre 2023 à 11h00.

Séminaire double sur la physique de la matière molle

''Peter Olmsted

How does the glass transition temperature in polymer depend on molecular weight?

I present a study of the molecular weight dependence of the glass transition Tg(M), experimentally and with simple modeling. We find three regimes for T_g(M), roughly corresponding to (1) short inflexible oligomers, (2) oligomers with 2-20 or so Kuhn steps, and (3) polymeric molecules. We identify quasi-universal behaviour across wide classes of polymers, and argue that
dynamic facilitation that balances intramolecular and intermolecular cooperatively is necessary in order to describe all three regimes.The coupling of intramolecular degrees of freedom to intermolecular degrees of freedom distinguishes polymer glasses from simpler glass formers such as rigid molecules or colloidal suspensions.

Physical Review X, 12 (2022) 021047, https://doi.org/10.1103/PhysRevX.12.021047

Emanuela Del Gado

Elasticity, rigidity and rheology of soft particulate gels.

Particulate gels include materials we can eat, squeeze or 3D print, from foods to bio-inks to cement hydrates. Designing adaptive functions and performances requires gaining insight into the microstructural origin of the broad range of their rheological responses. Varying gelation processes and conditions can favor or not the development of various types of mechanical heterogeneities, where rigid structures coexist with, or are interspersed in, floppy or softer regions. The presence of such mechanical heterogeneities underlies the complex viscoelastic response of these materials, promotes and enhances stress localization under load, controlling reconfigurability, yielding, stiffening or failure. Recent computer simulations have shed new light onto the hierarchies of timescales and lengthscales involved in these processes, and onto the implications for stress transmission and redistribution.

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