The elastic modulus and coefficient of thermal expansion (CTE) are fundamental material properties. Hybrid molecular materials, such as molecular glasses that form a covalent network, are assumed to have a symmetric elastic modulus in tension and compression due to the fundamental dependence on interatomic bonding, which is symmetric about zero loading. Similarly, the CTE is assumed to be the same in compression and tension. Such molecular materials are extensively used in thin-film electronic device technologies such as ultra low-<i>k </i>dielectric (ULK) constant layers and antireflective (AR) coating systems. Although the materials we study exist as thin films, the atomistic mechanisms we propose are not a thin-film property, but rather a bulk property that applies to a wide range of materials systems.<br/>In this work, we show that molecular materials can have a marked asymmetric elastic modulus and CTE that is inherently related to their network connectivity, density, and the existence of terminal chemical groups (Fig. 1). We particularly investigate the effects of various terminal groups including methyl, vinyl and phenyl termination on the elastic and thermal expansion asymmetries.  In compression, terminal groups sterically interact to stiffen the network, whereas in tension they interact less and disconnect the network, which leads to the surprising asymmetries. The existence of asymmetric elastic and thermal expansion behavior has fundamental implications for computational approaches to molecular materials modeling and practical implications on the thermomechanical strains and associated elastic stresses in electronic device technologies. We develop a design space to control the degree of elastic asymmetry in molecular materials, a vital step towards understanding their integration into device technologies. <br/>

Effects of network termination on elastic and thermal expansion asymmetries in hybrid molecular materials

253rd National Meeting (2017)

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