Negative thermal expansion in ultrathin plasma polymerized films

The unusual thermal behavior of ultrathin plasma polymerized polymer films on silicon wafers were investigated. The polymer films were deposited by the PECVD technique in a custom built PECVD reactor and characterized by FTIR, AFM, and XPS. Thermal expansion of the polymer films was studied by measuring the thickness of the films in the course of heating and cooling using ellipsometry, and the thickness value was confirmed by AFM. The plasma-polymerized polymers exhibited an important nonlinear variation of thickness with temperature when cooled. Thermal contraction of the polymer films was observed during heating, as well as negative thermal expansion was found to be reversible.