Thermal expansion/contraction is less of a mechanical phenomenon and more of a thermodynamic event. Thermal expansion is the result of increased free-volume that arises from the added vibrational (thermal) energy. The more you heat the polymer, the more it vibrates, creating additional free-volume around each molecule, thus expanding the volume of the part.
Inorganic fillers and reinforcements experience the same phenomenon, but to a lesser degree --- about an order of magnitude less than organic polymers. Thus, adding inorganic fillers will reduce the change in volume (or linear dimension) based on its content in the composition --- not through any reinforcing effect.
Also remember, the coefficient of linear expansion is not linear. The coefficient of expansion is fairly constant up to the Glass Transition Temperature (Tg). It changes significantly in the glass transition region and again in the rubbery plateau.
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