Unusual formation of smectic a structure in cross-linked monodomain elastomer of main-chain LC polyester with 3-methylpentane spacer

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 (S-CA) state by UV irradiation. The monodomain S-CA 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 (S-A) type with the layer spacing 2 times larger than that of the original S-CA phase. By elongation of this contracted elastomer, the S-A structure transformed to the original S-CA 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 S-A 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.