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

Abstract

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/nm2, 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.

Glass‐transition temperatures of nanostructured amorphous bulk polymers and their blends

“…This can be attributed to the increased constraint placed on the unincluded "dangling" chain portions by the ordering of the remaining included polymer chain portions that are forced to arrange themselves within the c-CD cavities. 20 A similar trend was observed by Uyar et al 5 following coalescence of PVAc and PMMA from their fully covered stoichiometric inclusion compounds formed with c-CD, where it was observed that the T g s for coalesced PMMA and PVAc both shift to higher temperatures in comparison to their as-received samples. Figure 6 illustrates the high surface densities expected for the un-included chains emerging from the surfaces of (n-s)polymer-CD-IC crystals.…”

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

“…The methods used to form and characterize stoichiometric polymer-ICs made with CD and urea (U) hosts (see Figure 2) and (n-s)-polymer-CD-ICs have been described many times [8][9][10][11][12][13] and will not be repeated here. Instead we briefly describe the techniques and instruments used to probe the un-included portions of the guest chains in (n-s)-CD-ICs and polymer samples coalesced from their stoichiometric CD-and U-ICs.…”

“…We begin by summarizing the formation and behaviors of (n-s)-ICs containing guest nylon-6 samples of various molecular weights and α-and γ-CD hosts [8,9]. Nylon-6 (N-6) samples with M W = 30,000 and 60,000 Da were obtained from BASF (Ludwigshafen, Germany), while those with M W = 300,000 and 600,000 Da were obtained by anionically initiated ring-opening polymerization of ε-caprolactam [15].…”

“…Figure 3. Differential Scanning Calorimetry (DSC) cooling scans from the melts of 600,000 Da bulk N-6 and its 3:1 (n-s)-γ-CD-IC [8].…”

“…Our research group began to examine the formation, behaviors, and uses of (n-s)-polymer-CD-ICs [8][9][10][11][12][13] by forming them with both α-and γ-CD hosts and guest nylon-6 with different molecular weights obtained by anionic ring-opening polymerization of ε-caprolactam [8,9]. Similar to Inoue et al we found (n-s)-nylon-6-CD-ICs to effectively nucleate the melt crystallization of neat bulk nylon-6, leading to improved mechanical properties [9,10].…”