Precise control of grafting density in periodically grafted amphiphilic copolymers: an alternate strategy to fine-tune the lamellar spacing in the sub-10 nm regime

Abstract

A series of hydrocarbon-rich polyesters were prepared by condensing a long chain diol bearing a centrally located clickable propargyl group, with diacid chlorides of different lengths, ranging from oxalyl chloride to eicosanedioic acid chloride (C2 to C20). Hydrophilic PEG550 segments were then clicked onto the periodically located propargyl groups using the alkyne–azide click reaction; these amphiphilic graft copolymers underwent microphase separation via zigzag folding of the backbone, thereby generating a lamellar morphology, wherein inter-lamellar spacing was fine-tuned in the sub-10 nm level by the choice of the diacid comonomer. In contrast to our previous report (S. Chanda and S. Ramakrishnan, Macromolecules, 2016, 49, 3254–3263), where the size of the amorphous pendant PEG segment was utilized to control the spacing, here the crystallizable HC backbone was varied; the linear variation of the inter-lamellar spacing with the number of methylene units in the dicarboxylic acid permitted us to retrieve an increment of 0.127 nm per methylene unit, which matches remarkably well with the expected increment for an all-trans extended hydrocarbon chain. This observation not only provides strong evidence for the zigzag folded conformation of the polymer backbone within the lamellae but also demonstrates fine-tuning of lamellar dimension at the sub-nanometer level. The modular synthetic strategy presented here provides scope for easy variation of the pendant graft segments and, therefore, could be used to create other interesting periodically spaced functional materials.