Microstructure Induced Rigidity of Polysiloxane Yielding Hierarchical Self-Assembly of Alkyl Side Chains

The influence of a backbone microstructure on the side chain crystallization of a comb-like polymer is analyzed systematically using a tailor-made random versus block siloxane copolymer system. While the side alkyl chains of the random siloxane undergo a stepwise order-disorder (OD) transition to form well-ordered orthorhombic structure at low temperature, the packing structure of the alkyl chains pertaining to the block siloxane maintains their original hexagonal lattice up to a temperature of as low as 173 K. The unit lattice ordering of side alkyl chains in the random siloxane polymer is also accompanied by a major restructuring of the backbone conformation ultimately losing out long range ordered structure in the solid state. The OD transitions of side alkyl chains and their dynamic relationship with the backbone conformation are established unambiguously by a combination of temperature dependent small-angle X-ray and wide-angle X-ray scattering techniques. The observed conformational variations in random versus block polymers are explicitly discussed in terms of molecular chain mobility and theory of macromolecular chain conformation.