Thermal diffusivity of elastomers subject to finite biaxial deformation

Polymers have shown changes in their thermal conductivity and diffusivity due to thermo-physical changes induced or by changes in their thermodynamic state such as first and second order changes (melting and glass transition) or by plastic deformations induced during their manufacturing process. Applications for this type of material are diverse and growing with time, which demand a further understanding of their thermal mechanical behavior for a more optimal design. Here, changes in orthogonal thermal diffusivity components due to mechanical preconditioning and biaxial deformation are examined for the first time, in silicone and polyurethane room temperature vulcanized rubber (RTV) and commercial rubber (neoprene and natural gum) sheets at room temperature, using the laser pulse method. Results showed that RTV molded rubber are thermally isotropic. Commercially rubber sheets showed in-plane thermal diffusivity components 12 to 18% and 6 to 10% larger than the out-of-plane component for neoprene and natural gum rubber, respectively, at the native state. This thermal anisotropy of rubber sheets disappeared after the mechanical preconditioning. The effect of equibiaxial deformation in the components of α at 1.0 ≤ λ ≤ 2.0 for RTV specimens and 1.0 ≤ λ ≤ 1.5 for rubber sheets (where λ is the deformed to undeformed length ratio) suggest the existence of two competing effects. The first effect is predominant between the native state and λ ≅ 1.4, causes a reduction of approximately 10% in the in-plane thermal diffusivity with respect their native state. A second effect increases the in-plane thermal diffusivity and is predominant at 1.7 ≤ λ ≤ 2.0. The thermal anisotropy ratios (in-plane/out-of plane thermal diffusivity component) for RTV show a nonlinear increase with the in-plane stretch, with a much steep increase (up to 60 percent at λ = 2.0) at 1.6 < λ ≤ 2.0. The measured thermal anisotropy could be attributed to partial polymer chain crystallization or by polymer chain orientation induced by the applied deformation.