51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference
12 - 15 April 2010, Orlando, Florida

AIAA 2010-2809

Analytical and Computational Micromechanics Analysis of the Effects of Interphase Regions and Orientation on the Effective Coefficient of Thermal Expansion of Carbon Nanotube-Polymer Nanocomposites

G.D. Seidel and S.N. Stephens
Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061-0203, USA

In the present work, analytic and computational micromechanics techniques are applied towards predicting the effective coefficients of thermal expansion of polymer nanocomposites containing aligned and randomly oriented bundles of SWCNTs. For well-dispersed SWCNTs, plane strain composite cylinders analytic micromechanics approaches are applied for determining the coefficients of thermal expansion for aligned nanotube cases, and for determining concentration tensors for randomly oriented well-dispersed nanotube nanocomposites. In computational micromechanics approaches, periodic arrangements of well-dispersed SWCNTs are studied using the commercially available finite element software COMSOL Multiphysics 3.4. Periodic boundary conditions corresponding to axial and transverse constrained uniform temperature increase are applied to determine the corresponding local stress distribution within a given representative volume element (RVE), and subsequently, the components of the concentration tensors. RVEs are constructed with either hollow CNTs (in what would be termed a single step method), or using effective solid nanotubes having transversely isotropic effective properties determined from a composite cylinder approach (i.e., a two-step method), and are observed to yield nearly identical results for effective bundle coefficient of thermal expansion. The influence of the presence of an interphase region on the effective coefficient of thermal expansion is considered in a parametric study in terms of both interphase thickness, elastic properties, and coefficient of thermal expansion. Special emphasis is placed on assessing the impact of interphase percolation on the effective coefficient of thermal expansion. The resulting changes in effective coefficient of thermal expansion due to the presence of interphase regions are then put into context by comparison with an analogous parametric study on the effects of interphase regions on the effective elastic properties and thermal and electrical conductivities of nanocomposites. Finally, the concentration tensors obtained for both well-dispersed and bundled SWCNTs will be used within an orientational averaging framework to obtain the effective coefficient of thermal expansion for nanocomposites having well-dispersed, randomly oriented SWCNTs and SWCNT bundles, thereby allowing for comparison with experimental values provided in the literature.