Non-Stoichiometric Polymer-Cyclodextrin Inclusion Compounds: Constraints Placed on Un-Included Chain Portions Tethered at Both Ends and Their Relation to Polymer Brushes

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/nm 2 (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 OPEN ACCESSPolymers 2014, 6 2167 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.

Aliphatic Polyester Nanofibers Functionalized with Cyclodextrins and Cyclodextrin-Guest Inclusion Complexes

“…Chains dangling out of the CD cavities are also able to undergo crystallization and melting [ 91 ]. In addition, depending on the stoichiometry between the cyclodextrin host and guest polymer, the lengths of dangling polymer chains and their molecular weights, crystallizability ( T c s and T m s) increases significantly, irrespective of the crystalline nature of the dangling polymer [ 50 , 92 , 93 ]. Besides improvements in thermal properties, the additions of such non-stoichiometric ICs to bulk polymers have also resulted in significant increases in elastic modulus values and tensile strength, with drastic reductions in the loss tangent and elongation, indicating significant improvements in the stiffness and elasticity of the composites [ 94 ].…”

Polymers Containing Non-Covalently Bound Cyclodextrins

“…As indicated in Figure 7, the unincluded nylon-6 chains “dangling” from their (n-s) α–CD ICs, as they emerge from ~0.5-nm channels, are separated by ~1.4 nm compared to a pair of closely adjacent nylon-6 chains in each ~1 nm γ–CD IC channel, separated by ~1.7 nm. The differences in the constraints imposed on their unincluded chains “dangling” from their CD IC channels may be, at least in part, the source for the distinct behaviors observed for (n-s) nylon-6–CD ICs made with α–CDs and γ–CDs (more extensive discussion of this topic can be found in [13]).…”

Nanoscale Restructuring of Polymer Materials to Produce Single Polymer Composites and Miscible Blends

“…Polymer chains included in their crystalline ICs, in addition to having conformations and mobilities distinct from their bulk samples [18], may, by the careful removal of the host crystalline lattice, be coalesced into bulk samples, which also behave distinctly from bulk samples obtained from their solutions and melts [15,19,20,21,22]. Such coalesced amorphous and semi-crystalline polymers exhibit elevated glass-transion temperatures (T g s) (see Figure 4 and Figure 5) and enhanced crystallizabilities (see Figure 6), respectively.…”

“…Initially surprising were the observations that the distinct behaviors of bulk polymer samples made by coalescence from their ICs were resistant to long-time annealing at temperatures above their T g s and melting temperature (T m s) [15,20,21,22].…”