Bulk and thin film ordering in side-chain liquid-crystalline/amorphous diblock copolymers: the role of chain length

The ordering of a series of side-chain liquid-crystalline/amorphous diblock copolymers with different molecular weights, compositions, and polydispersities is reported. The diblock copolymers comprise polystyrene and poly(methacrylate) bearing methoxy biphenyl mesogenic groups. All samples show a lamellar morphology regardless of their molecular weights, compositions, and polydispersities. Apart from the low-molecular-weight sample, a smectic phase is observed at elevated temperatures. At room temperature, the mesogenic groups partially crystallize. For the sample with the lowest molecular weight the crystalline layers are uncorrelated, and their melting induces the order-to-disorder transition. Samples with relatively long main chains exhibit a three-dimensional ordered crystalline structure with a finite correlation length along the layer normal. This correlation length is longer for the sample with the lowest polydispersity. The mesogenic layers are always perpendicular to the block interfaces. These results can be understood in terms of changes in the conformation of the main chain of the liquid-crystalline block with its length. In thin films, the lamellae orient parallel to the interfaces in an asymmetric fashion with PS at the air interface. At the substrate a thin parallel mesogenic layer is induced. However, this parallel orientation does not propagate into the film interior, and the mesogenic crystalline layers retain their preferred perpendicular orientation with respect to the block lamellae.

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