Lamellar mesoscopic organization of supramolecular polymers: a necessary pre-ordering secondary structure

Abstract

Thymine-functionalized telechelic poly(propylene glycol) (PPG) chains of 460–4000 g mol⁻¹ present a long-range ordered lamellar microphase separation due to thymine crystallization. This organization is lost if the difference of polarity between the H-bonding units and the chain is not sufficiently high to induce segregation, as with a poly(ethylene glycol) (PEG) chain. Moreover, the mesoscopic order is gradually lost with the longest PPG chains i.e. with the smallest volume fractions of supramolecular units limiting their clusterization/crystallization. Methylated thymine (MeThy) end-functionalized PPG presents the same lamellar mesoscopic organization, proving that Thy/Thy H-bonding self-association does not drive their crystallization. Infrared spectroscopy evidences that the amide functions linking MeThy motifs to PPG chains interact with each other through H-bonds, forming either a random or an aligned pattern, just like protein secondary structures. On cooling from the melt, disordered H-bonded amides align and pre-order the supramolecular units just prior to the disorder–order transition driven by the MeThy crystallization. The close packing, necessary to induce crystallization, is no longer possible with bulky neo-pentyl functionalized thymines (tBuCH₂Thy), preventing mesoscopic organization. Hence, the pattern formed by H-bonded amide links plays a major role in the long-range organization of the telechelic supramolecular polymer in helping supramolecular units to crystallize. Indeed, replacing the amide by an ester link suppresses their crystallization and thus the mesoscopic order.