Separate crystallization and melting of polymer blocks and hydrogen bonding units in double-crystalline supramolecular polymers

Abstract

End functionalization of oligomers by the multiple H-bonding units is a feasible way to prepare the supramolecular polymers (SMPs). The crystallization of oligomer blocks and H-bonding units can lead to form the complicated double crystalline structure in SMPs; such double crystalline behavior and its correlation with the physical properties are of fundamental importance for the processing and applications of SMPs. Herein, we choose the oligomeric polycaprolactone (PCL) end-functionalized by the self-complementary quadruple H-bonding 2-ureido-4-pyrimidinone (UPy) units (named as U-PCL) as the model double crystalline SMPs and investigate their crystalline structure, crystallization kinetics, thermally induced structural evolution, and structure-property relationships. The U-PCLs display the similar composition-dependent crystallization and melting behavior as the double crystalline block copolymers. The U-PCLs with short, medium-length, and long PCL blocks crystallize in the UPy crystals, UPy/PCL mixed crystals, and PCL crystals, respectively. Melting temperature of UPy crystals decreases and their melting peak disappears as the UPy content of U-PCLs decreases. UPy units and PCL blocks are microphase-separated in the U-PCLs, despite the absence of long-range ordered phase structure. Dual shape memory effects capable of switching at the body temperature were realized based on the double crystalline nature of U-PCLs by using UPy crystals as the physical crosslinkers and PCL blocks as the switching segments.