Free volume evolution in the process of epoxy curing and its effect on mechanical properties

Published Date
Polymer
5 August 2016, Vol.97:456–464, doi:10.1016/j.polymer.2016.05.059

• Author 

• Chunyu Li
• Alejandro Strachan ^,

• School of Materials Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47906, USA

Received 3 March 2016. Revised 6 May 2016. Accepted 21 May 2016. Available online 24 May 2016. 

Highlights

• •
  Free volume evolution in the curing process of epoxy thermosets is quantified.
• •
  Nanovoids contributing to the free volume have a fractal surface with a fractal dimension ∼2.37.
• •
  Reduction of free volume tends to increase elastic stiffness and yield strength.

Abstract

We use molecular dynamics simulations to study the evolution of free volume during the process of curing of two thermoset epoxy polymers and correlate its volume fraction with the resulting thermo-mechanical properties. The simulations reveal that while the overall volume of the polymer shrinks during cure, the amount of free volume increases. Interestingly, this increase is due to the appearance of large voids (connected regions of free volume) while the number of small and medium-sized voids decreases during cure. We also find correlations between the location of the voids and the local chemical structure of the polymer. An analysis of the individual nanovoids shows a power-law scaling for over three orders of magnitude in volume (0.1 Å³ to 100 Å³) and a fractal scaling in surface with a fractal dimension ∼2.37. The effect of free volume on mechanical properties is also quantified. A very small reduction in the free volume fraction (∼1%) results in considerable (>10%) stiffening and strengthening.

Graphical abstract

Keywords

• Polymer network
• Free volume
• Thermoset
• Computational characterization
• Molecular dynamics

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• ∗ 
  Corresponding author.

© 2016 Elsevier Ltd. All rights reserved.

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