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The surprising mechanical properties of polymer glasses: strengthening the constituent chains can weaken the bulk material

last modified Sep 13, 2017 06:00 PM
Dr Chris Ness, Dr Alessio Zaccone
The surprising mechanical properties of polymer glasses: strengthening the constituent chains can weaken the bulk material

Simulated amorphous polymer

Intuition tells us that if we strengthen the component parts of a material, we will strengthen the material itself. Writing in the Rapid Communications of Physical Review E, Chris Ness, Alessio Zaccone and co-workers Vladimir Palyulin and Rico Milkus from the Statistical Physics Group at CEB successfully demonstrated that this is not always the case. In a close collaboration with Dr Tim Sirk and Dr Robert Elder from the US Army (generously providing funds and computational resources for this project), they studied an example of amorphous polymer by means of molecular dynamics simulations. They showed that stiffening the bending of constituent polymer chains frustrates the packing, and leads to a density reduction that in turn weakens cross-chain interactions. The combined effect of intra-chain strengthening and inter-chain weakening actually leads to an overall non-monotonic variation of the shear elastic modulus. This surprising result means that, under certain circumstances, more rigid polymer chains can lead to weaker bulk materials. The computational results are in agreement with state-of-the-art theory for computing the mechanical properties of disordered solids, based on the framework of nonaffine lattice dynamics developed in the Statistical Physics Group at CEB.

These findings pave the way for an analytic description of amorphous polymers starting from the atomistic level. Amorphous polymers represent an important class of materials, and accurately linking their chemical formulation and their mechanical behaviour is essential for technological applications across industry and defence. Furthermore, the new effect discovered in this study might be much more general and may show up also in the mechanics of other soft materials such as colloidal gels (where bending can be tuned by controlling the adhesion area between colloidal particles) and granular systems where “granular polymers” have recently emerged as a new class of granular materials.

 

(a) snapshot of simulated amorphous polymer; (b) schematic of chains showing cross-chain interaction (blue) and chain bending (red); (c) schematic showing nonmonotonic variation of shear modulus, highlighting the shear modulus minimum at intermediate chain stiffness

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