Connectivity and free-surface effects in polymer glasses

Anna Lappala,Paul W Fenimore,Luke Sefton,Eugene M Terentjev

doi:10.1038/s41598-019-40286-2

Anna Lappala, Paul W Fenimore + Show 2 more

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https://doi.org/10.1038/s41598-019-40286-2

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Abstract

The glass transition is one of the few unsolved problems in condensed matter physics: agreement on the cause of the slowing down of structural relaxation in glass-forming liquids is lacking. Glasses are amorphous solids, which do not possess the long-range crystalline order, yet display arrested dynamics and the shear elastic modulus characteristic of equilibrium elasticity. It has been suggested that due to the influence of intramolecular interactions and chain connectivity, the nature of the glass transition in polymers and in standard glass-formers is fundamentally different. Here, we discuss the role of connectivity in polymer glasses, demonstrating that although covalent bonding promotes glass formation, bonding sequentiality that defines a polymer chain is not critical in the bulk: glassy dynamics is purely a result of the number of connections per particle, independently of how these connections are formed, agreeing with the classical Phillips-Thorpe topological constraint theory. We show that bonding sequentiality does play an important role in the surface effects of the glass, highlighting a major difference between polymeric and colloidal glasses. Further, we identify the heterogenous dynamics of model coarse-grained polymer chains both in ‘bulk’ and near the free surface, and demonstrate characteristic domain patterns in local displacement and connectivity.

Highlights

The glass transition is one of the few unsolved problems in condensed matter physics: agreement on the cause of the slowing down of structural relaxation in glass-forming liquids is lacking
The average number of nearest neighbours in the first coordination shell is given by integrating the static radial distribution function g(r) up to its first minimum; this coordination number for central forces of the face-centered cubic crystal is approximately z* = 12, reflecting the optimal packing of hard spheres, and remains roughly constant as the temperature is lowered below the glass transition temperature Tg3
We study the complex relationship between surface effects and connectivity, shining light on how these phenomena regulate the collective motions in glassy dynamics and demonstrate that in the bulk, the same number of contacts leads to the same glass transition, irrespective of connectivity

Summary

Introduction

The glass transition is one of the few unsolved problems in condensed matter physics: agreement on the cause of the slowing down of structural relaxation in glass-forming liquids is lacking. The average number of nearest neighbours in the first coordination shell is given by integrating the static radial distribution function g(r) up to its first minimum; this coordination number for central forces of the face-centered cubic (fcc) crystal is approximately z* = 12, reflecting the optimal packing of hard spheres, and remains roughly constant as the temperature is lowered below the glass transition temperature Tg3 In this manuscript, we will refer to direct contacts that we define as monomers within 1.12σ (where σ is a dimensionless quantity that characterises distance), just below of the position of LJ potential minimum, making it easier to identify ‘touching’ particles. Berthier et al found direct experimental evidence of a growing dynamic correlation length scale as the glass transition temperature is approached[14]

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