Controlling the Polymorphic Behaviors of Semicrystalline Polymers with Cyclodextrins

Abstract

We present a review of our initial studies concerning the control of polymorphism in semicrystalline polymers with cyclodextrins (CDs). CDs are cyclic starch oligomers with six (α-CD), seven (β-CD), and eight (γ-CD) α-1,4-linked glucose units possessing bracelet structures with hydrophobic and hydrophilic interiors and exteriors, respectively. They are able to act as hosts to form noncovalent inclusion compounds (ICs) with a large variety of guest molecules, including a wide range of high molecular weight guest polymers. In polymer-CD-ICs, the CD host crystalline lattice consists of hexagonally packed CD stacks with guest polymers occupying the narrow channels (∼0.5−1.0 nm) extending down the interiors of the stacked CDs. As a consequence, the included guest polymers must adopt highly extended conformations and are segregated from neighboring guest polymer chains. When the host CDs are appropriately removed from polymer-CD-ICs, the included guest polymers are forced to coalesce into a pure polymer solid, which has been observed to affect their conformations, morphologies, and, for crystallizable polymers, even their polymorphism. Here we present a comparison of the polymorphism observed in nylon-6, PET, polycarbonate, and the polyolefins, isotactic-polypropylene and -poly-1-butene, coalesced from their CD-ICs with that observed when they are crystallized from their melts and/or solutions. Generally, a higher level of crystallinity and higher melting and melt recrystallization temperatures are observed for the CD-IC coalesced samples, and they also often crystallize into different polymorphs.