Unusual Formation of Smectic A Structure in Cross-Linked Monodomain Elastomer of Main-Chain LC Polyester with 3-Methylpentane Spacer

Abstract

The main-chain BB/PDA-5(3-Me) polyester containing a photo-cross-linkable p-phenylenediacrylate (PDA) moiety was synthesized and cross-linked in the transparent monodomain smectic CA (SCA) state by UV irradiation. The monodomain SCA elastomer contracted to about 60% of the original length on heating up to the isotropic phase and recovered only 5% of the original length at the liquid crystallization on cooling. Irrespective of such a small recovery in the length, the liquid crystal was well formed with a high orientation, but the formed smectic structure was of a smectic A (SA) type with the layer spacing 2 times larger than that of the original SCA phase. By elongation of this contracted elastomer, the SA structure transformed to the original SCA one, showing a quasi-plateau in the stress–strain curve. The connection between the structural transformation and the stress–strain behavior lets us propose an unsusal SA structure in which the polymer chain folds every two repeat units. Such a characteristic folding structure may be caused by the limited mobility of the mesogens in the network.