ANISOTROPIC THERMAL-EXPANSION OF DENSELY CROSS-LINKED ORIENTED POLYMER NETWORKS

Densely cross-linked oriented polymer networks are made by in-situ photopolymerization of oriented liquid-crystalline diacrylates. Freestanding films that exhibited highly anisotropic thermal expansion were prepared. The influence of the molecular structure and type and degree of orientation on the anisotrpic thermal expansion is described. The expansion coefficient of uniaxially oriented networks in the direction of the molecular orientation is slightly positive at temperatures below the glass transition of the networks and becomes negative as soon as this transition sets in. The result is an effectively zero expansion over a wide temperature range. By introducing a molecular helix in the network perpendicular to the film surface, a low thermal expansion is established in two directions in the plane of the film or coating.