My research interests are to develop computational methods to address the fundamental relationships between molecular structure and resulting mechanical properties of organosilicate glasses. Using a new simulated annealing approach, large distortion-free hybrid glass models with well-controlled network connectivity can be generated from a wide range of organosilane precursors. Models in both dense and nanoporous form can be generated. With these model structures, we can simulate the elastic, fracture, and thermal properties of these materials to establish complex structure-property relationships and design new glasses that exhibit outstanding mechanical properties.
We have generated highly accurate models of hybrid glasses and discovered that they can have a marked asymmetric elastic modulus and coefficient of thermal expansion (CTE) that is inherently related to the interaction of terminal chemical groups and is controlled by the material’s density and network connectivity. Terminal chemical groups that limit network connectivity sterically interact in compression; in tension, the terminal groups disconnect the molecular network and explore considerably more volume under elastic deformation, which gives rise to the unexpected asymmetric modulus and CTE. 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. We have also developed a design space to control the degree of elastic asymmetry in molecular materials, a vital step towards understanding their integration into device technologies.
Figure 1. (a) Visual representation of terminal and bridging O in the Et-OCS matrix. (b) The degree of asymmetry, EC / ET, as a function of network connectivity lies between bounding models defined by a minimum stiffening coefficient, Γmin (red), and a maximum stiffening coefficient, Γmax (blue). (c) A range of precursors (Et-OCS(Me), Et-OCS, 1,3,5-benzene, and α-SiO2) are used to generate a density plot of the maximum degree of asymmetry, EC / ET, with respect to the network connectivity and density of the material. (d) The CTE of the Et-OCS matrix as a function of the applied pressure for both thermal expansion (orange) and thermal contraction (blue).
Hybrid organic-inorganic glasses exhibit unique electro-optical properties along with excellent thermal stability. However, their inherently fragile nature remains a fundamental challenge for their integration in nanoscience and energy technologies. We have developed a hyperconnected organosilicate network architecture that provides a novel route to realizing exceptional elastic stiffness, higher than fully dense silica, for hybrid organic-inorganic materials. By using 1,3,5 silyl benzene precursors, silicon atoms can be connected to as many five other silicon nearest neighbors via oxygen or short organic bridges. The exceptional intrinsic stiffness of these hyperconnected glass networks was first demonstrated using our molecular dynamics models. Our models have been verified through synthesis and characterization of a hybrid glass processed from 1,3,5(triethoxysilyl)benzene. Hyperconnected networks also significantly improve the elastic properties of nanoporous glasses, exhibiting a ~500% increase in the bulk modulus compared to existing nanoporous organosilicates.
Figure 2. Fully condensed silicon atoms are 4-fold and 5-fold connected within the silicon network for (a) ethane-bridged glasses and (b) 1,3,5-benzene glasses, respectively. (c) The predicted bulk modulus, K, as a function of the predicted mass density, ρ, for model organosilicate glasses with various organic bridges: ethane, methane, 1,4-benzene, 1,3-benzene, and 1,3,5-benzene. The 1,3,5-benzene hyperconnected glasses have a higher stiffness but significantly less density than the experimental value for amorphous SiO2.
An accurate estimate of fracture bond density is very difficult to obtain for hybrid glasses as they have a complex network topology. Additionally, the crack follows a three-dimensional path through the glass network to avoid regions of high bond density. Existing models of fracture surface bond density are unable to capture this behavior. Using a novel application of graph theory, we can compute the complex 3-D fracture path at the molecular scale and show that fracture energy in brittle hybrid glasses is fundamentally governed by the bond percolation properties of the network. Importantly, we have shown that cracks find pores. The fracture energy, which can be expressed as a critical value of the strain energy release rate, Gc, is proportional to the density of bonds broken during fracture. Thus, since nanoscale pores decrease the density of bonds, the 3-D crack path meanders to find pores in order to break the minimum number of bonds. The fracture height depends on the nanopore morphology.
Figure 3. (a) The 3-D crack path follows the nanoporous morphology. (b) A density plot of the crack surface. (c) The dependence of broken bond density and fracture height on the min-cut height. (d) The broken bond density and fracture height as a function of volume % porosity.