(383d) Thermal and Mechanical Property Predictions for Conjugated Polymers Using Atomistic Simulations

Paralleling successful application in catalyst design, drug discovery, and protein structure analysis, molecular simulations are emerging as reliable property-prediction tools for synthetic polymers as well. Simulations are not only useful for determining functional properties such as band gaps of Ï?-conjugated polymers, but also for accessing microscopic information that is not easily available from experiments.

In this work, we used atomistic simulations to predict thermophysical and mechanical properties of two p-conjugated polymers, polyacetylene (PA) and poly(para-phenylene vinylene) (PPV) [1]. Although the electron transport and optical behaviors of these polymers are well-characterized, information on properties like elastic modulus and glass transition temperature are uncertain. Glass transition temperature, , the temperature above which the polymer softens considerably undergoing a large decrease in stiffness, and the elastic modulus of the polymer, are useful parameters in the design of devices such as flexible LED displays and solar panels that use conjugated polymers.

The literature values of the elastic modulus of PA and PPV at room temperature are scattered over a range spanning two to three orders of magnitude [1]. Similarly, differences in the reported values of the of PPV are greater than 100 °C. Polymer properties are generally quite dependent on their synthesis and processing history. The dispersity in the lengths of the polymer chains, the variations in polymer architecture caused by branching, chemical defects such as oxidation, the presence of plasticizers or other low molar mass impurities, and even the method of measurement can greatly alter the measured property values. These effects seem to be particularly important in the case of the two conjugated polymers investigated. Molecular simulations, on the other hand, allow one to probe a virtual ensemble of perfectly defect-free identical polymer molecules as a basis for comparison with measurements. Architectural variations and chemical defects can be introduced in a controlled manner, if their effects need to be studied.

We used the variations of specific volume and the potential energy of intermolecular interactions with temperature to identify the glass transition temperature. Properties such as the coefficient of thermal expansion, the torsion angle distributions, and the mean square end-to-end distance, were calculated at different temperatures. Ï?-Conjugation of the sp² hybridized carbon atoms in the PPV chain would be expected to impart planarity to the molecule, but a non-planar structure, consistent with previously reported observations in neutron diffraction experiments [2], was predicted.

Conventional mechanical characterization techniques such as tensile testing stretch the polymer specimen to different lengths and measure the force required to produce a given extension of the specimen. However, polymer chains in the sample can align or orient along the stretching direction. We used the technique of dynamic nanoindentation instead to measure polymer stiffness by applying relatively small oscillatory loads, of micronewton magnitude, on the polymer films using a diamond microindenter [3]. The experimental results were in good agreement with the MD predictions employing the static deformation method [4].

This study demonstrates that atomistic simulations can reliably complement existing tools for the prediction of the properties of solid polymers.

References:

1. Venkatanarayanan, Ramaswamy I., Sitaraman Krishnan, Arvind Sreeram, Philip A. Yuya, Nimitt G. Patel, Adama Tandia, and John B. McLaughlin (2016), â??Simulated Dilatometry and Static Deformation Prediction of Glass Transition and Mechanical Properties of Polyacetylene and Poly(para-phenylene vinylene),â? Macromolecular Theory and Simulations, 25(3), pp. 315â??321.
2. Mao, G., J. E. Fischer, F. E. Karasz, and M. J. Winokur (1993), â??Nonplanarity and Ring Torsion in Poly (pâ?phenylene vinylene). A Neutronâ?Diffraction Study,â? The Journal of Chemical Physics, 98(1), pp. 712â??716.
3. Sreeram, Arvind, Nimitt G. Patel, Ramaswamy I. Venkatanarayanan, John B. McLauglin, Stephan J. DeLuca, Philip A. Yuya, and Sitaraman Krishnan, â??Nanomechanical Properties of Poly(para-phenylene vinylene) Determined Using Quasi-static and Dynamic Nanoindentation,â? Polymer Testing (2014), 37, pp. 86â??93.
4. Theodorou, Doros N. and Ulrich W. Suter (1986), â??Atomistic Modeling of Mechanical Properties of Polymeric Glasses,â? Macromolecules, 19(1), pp. 139â??154.