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Interrogating the physics of amorphous materials : Mechanics of materials from rubbery to glassy and from the macroscopic to the nanometric
The mechanics of the materials in the world that surrounds us, at least from the time of Galileo, has fascinated humankind. In the present seminar I will describe our studies of the behavior of amorphous materials with special focus on the use of mechanical measurements to probe fundamental materials physics. Amorphous materials are those without structure, hence those that are above their melting point, or in a super-cooled state, i.e., systems in which crystallization has been avoided during cooling below the melting point. I will do this in three vignettes. First, we will go through a series of mechanical and swelling measurements that permitted us to establish the validity of the Frenkel-Flory-Rehner (FFR) hypothesis. Secondly, we will show novel nano-rheology measurements in which we find that the molecular dynamics is greatly accelerated as one reduces the thickness of polymer films—in one instance we find a reduction of more than 120 K in the glass transition temperature Tg relative to that of the bulk material for a polycarbonate film having a 3 nm thickness, i.e., its Tg was below ambient. We will also examine in this section the spontaneous embedment behavior of small diameter particles (10 nm to 300 nm) into polymer surfaces. These studies have permitted us to challenge the general view that glass transition reductions in nano-confined materials are due solely to surface effects. Finally, we will examine the accumulating evidence that the relaxation times or viscosities of glasses below the glass transition do not diverge as expected from classical theories. Of special interest is the use of a 20 million year old amber to establish upper bounds to the relaxation times of an extremely low fictive temperature glass. The results will be discussed in the context of classic and some new theories of glass-forming systems.