Controlling Interlamellar Spacing in Periodically Grafted Amphiphilic Copolymers

We have examined a series of periodically grafted amphiphilic copolymers (PGACs) wherein pendant MPEG segments of varying lengths (TREG, 350, 550, and 750) were grafted at periodic intervals along a long crystallizable alkylene (C-20) backbone. The immiscibility of the alkylene and PEG segments, and the strong propensity of the alkylene segments to crystallize, drive these PGACs to self segregate by folding in a zigzag fashion and subsequently organize into a lamellar morphology, which was evident from DSC, SAXS, WAXS, and AFM studies; the interlamellar spacing was seen to increase linearly with MPEG length. Co-grafted PGACs, prepared by grafting a mixture of two different MPEGs (MPEG 350 and MPEG 750), also exhibited a lamellar morphology; interestingly, the interlamellar spacing was seen to depend only on the total PEG content, while the presence of a mixture of PEG lengths exerted little influence. However, a mixture of two homo-PGACs bearing either pendant MPEG350 or MPEG750 segments underwent macrophase separation, and interlamellar spacings corresponding to each were observed in the SAXS profiles. This study provides a new design for controlling the dimensions of the microphase-separated nanostructures at significantly smaller length scales (sub-10 nm) than is typically possible using block copolymers.