Precise polyethylene derivatives bearing mesogenic side-chains: delicate self-assembly depending on graft density

Abstract

To achieve desirable properties of side-chain polymers, elaborately manipulating the interplay between the polymer backbone and side-chains is vital, which can be done by adjusting the graft density of side-chains. Here, we report a series of polyethylene derivatives with the aromatic biphenyl side-chains precisely grafted on every 2^nd, 7^th and 15^th carbon (Pns, n = 2, 7, and 15) along the aliphatic backbone, the graft densities of which are nearly 4, 1, and 0.5 side-chains per nanometer, respectively. The self-assembly structures and phase transitions of Pns were investigated using various techniques. We demonstrate that precisely adjusting the distance between two adjacent side-chains, i.e., the side-chain spacing, can drastically alter the local coupling of the backbone and rod-like mesogenic side-chains, leading to different backbone conformations and anisotropic interactions. Compared to P2 that is an ordinary side-chain liquid crystalline polymer (SCLCP) forming a crystal E phase, P7 and P15 exhibit a three-dimensionally (3D) ordered structure K_X and a two-dimensional (2D) rectangular columnar phase Col_R, respectively, which are unprecedented in SCLCPs. Moreover, the phase transition pathway can also be modified remarkably when the graft density is varied.