Constrained/Directed Crystallization of Nylon-6. I. Nonstoichiometric Inclusion Compounds Formed with Cyclodextrins

Noncovalently bonded crystalline inclusion compounds (ICs) have been formed by threading host cyclic starches, cyclodextrins (CDs), onto guest nylon-6 (N-6) chains. When excess N-6 is employed, nonstoichiometric (n-s)-N-6-CD-ICs, with partially uncovered and “dangling” N-6 chains, result. While the host crystalline CD lattice is stable to ∼300 °C, the uncovered, yet constrained, portions of the N-6 chains emanating from the CD-IC surfaces may crystallize below or be molten above ∼225 °C. We have been studying the constrained crystallization of the unthreaded N-6 chains “dangling” from (n-s)-N-6-CD-ICs, with comparison to bulk N-6 samples, as functions of N-6 molecular weights, lengths of uncovered N-6 chains (N-6:CD stoichiometry), and the CD host used. In the IC channels formed with host α- and γ-CDs containing 6 and 8 glucose units, respectively, single and pairs of side-by-side N-6 chains are threaded and included. In α-CD-ICs, the ∼0.5 nm channels produced by stacked α-CDs are separated by ∼1.4 nm, while in γ-CD-ICs the ∼1 nm channels are ∼1.7 nm apart, with each stacked γ-CD channel including two N-6 chains. N-6 chains in the bulk and in the dense (n-s)-N-6-α-CD-IC brushes, with a protruding chain density of ∼0.60 chains/nm 2, show distinctly different kinetic and thermodynamic crystallization behaviors. The constrained protruding chains in the dense (n-s)-N-6-α-CD-IC brushes do not in fact “dangle”, but are apparently highly extended from the α-CD-IC crystal surfaces. This causes them to crystallize faster and to a much greater extent than those in bulk N-6 melts, and this behavioral distinction is enhanced as the molecular weights/lengths of the unincluded protruding N-6 chains are increased. On the other hand, the N-6 chains protruding from (n-s)-N-6-γ-CD-ICs do not show nearly as large a propensity for crystallization. It is suggested that this may be the result of strong interactions between each pair of N-6 chains included in and emerging from each of the γ-CD-IC channels. Furthermore, when added at low concentrations, the nontoxic, biodegradable (n-s)-N-6-CD-ICs serve as effective nucleating agents for the bulk crystallization of N-6 from the melt. This is a consequence of the ability of the N-6 chains protruding from their (n-s)-CD-ICs to crystallize more rapidly at higher temperatures than bulk N-6 chains when their molten mixture is cooled, thereby providing finely dispersed crystalline nuclei for the subsequent crystallization of the bulk N-6 chains.