Abstract
When non-covalently bonded crystalline inclusion compounds (ICs) are formed by threading the host cyclic starches, cyclodextrins (CDs), onto guest polymer chains, and excess polymer is employed, non-stoichiometric (n-s)-polymer-CD-ICs, with partially uncovered and “dangling” chains result. The crystalline host CD lattice is stable to ~300 °C, and the uncovered, yet constrained, portions of the guest chains emanating from the CD-IC crystal surfaces behave very distinctly from their neat bulk samples. In CD-IC crystals formed with α- and γ-CD hosts, each containing, respectively, six and eight 1,4-α-linked glucose units, the channels constraining the threaded portions of the guest polymer chains are ~0.5 and 1.0 nm in diameter and are separated by ~1.4 and 1.7 nm. This results in dense brushes with ~0.6 and 0.4 chains/nm2 (or 0.8 if two guest chains are included in each γ-CD channel) of the un-included portions of guest polymers emanating from the host CD-IC crystal surfaces. In addition, at least some of the guest chains leaving from a crystalline CD-IC surface re-enter another CD-IC crystal creating a network structure that leads to shape-memory behavior for (n-s)-polymer-CD-ICs. To some extent, (n-s)-polymer-CD-ICs can be considered as dense polymer brushes with chains that are tethered on both ends. Not surprisingly, the behavior of the un-included portions of the guest polymer chains in (n-s)-polymer-CD-ICs are quite different from those of their neat bulk samples, with higher glass-transition and melt crystallization temperatures and crystallinities. Here we additionally compare their behaviors to samples coalesced from their stoichiometric ICs, and more importantly to dense polymer brushes formed by polymer chains chemically bonded to surfaces at only one end. Judging on the basis of their glass-transition, crystallization and melting temperatures, and crystallinities, we generally find the un-included portions of chains in (n-s)-polymer-CD-ICs to be more constrained than those in neat bulk as-received and coalesced samples and in high density brushes. The last observation is likely because many of the un-included chain portions in (n-s)-polymer-CD-ICs are tethered/constrained at both ends, while the chains in their dense brushes are tethered at only one end.
Highlights
Cyclodextrins (CDs) are cyclic starches containing 6(α), 7(β), or 8(γ) 1,4-α-linked glucose units and are well known for their abilities to form non-covalently bonded inclusion compounds (ICs) with a variety of guest molecules, including polymers [1]
Judging on the basis of their glass-transition, crystallization and melting temperatures, and crystallinities, we generally find the un-included portions of Polymers 2014, 6 chains in (n-s)-polymer-CD-ICs to be more constrained than those in neat bulk as-received and coalesced samples and in high density brushes
It was observed that a higher fraction or proportion of the un-included N-6 chain portions “dangling” from the 3:1 (n-s)-α-CD-IC crystals are able to crystallize compared to the neat N-6 melt
Summary
Cyclodextrins (CDs) are cyclic starches containing 6(α), 7(β), or 8(γ) 1,4-α-linked glucose units (see Figure 1) and are well known for their abilities to form non-covalently bonded inclusion compounds (ICs) with a variety of guest molecules, including polymers [1]. More than a decade ago Inoue and co-workers [2] first reported the formation of a non-stoichiometric-polymer-cyclodextrin-inclusion compound [(n-s)-Polymer-CD–IC] (see bottom of Figure 2) between excess guest poly (3-hydroxybutyrate) (PHB) and host α-CD. They observed the un-included portions of the PHB chains to crystallize more rapidly from the melt at significantly higher temperatures than neat bulk PHB. For this reason small quantities of the (n-s)-PHB–α-CD-ICs were used to nucleate the melt-crystallization of neat bulk PHB, and they showed that the resulting semi-crystalline morphology was improved with smaller and more homogeneously dispersed crystalline regions. Vogel et al [3] demonstrated that melt-spun PHB fibers nucleated with (n-s)-PHB–α-CD-IC in this manner evidenced greatly enhanced tensile strengths
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