Guest Polymer Chains Research Articles

Enhancing the melt crystallization of polymers, especially slow crystallizing polymers like PLLA and PET

Over 20 years ago we demonstrated that it is possible to control the melt crystallization of polymers via self-nucleation. Non-covalently bonded inclusion compounds (ICs) can be formed with cyclodextrins (CDs) or urea (U) when these small host molecules crystallize around guest polymers and form narrow parallel host channels, where the isolated and stretched guest polymer chains are included. Careful removal of the host crystalline lattice yields a neat coalesced (c) guest polymer sample with less entangled chains that are packed to a higher density than bulk samples of the same polymer obtained from its melt or solutions. Consequently, the reorganized c-polymer samples behave distinctly, with higher glass-transition temperatures for amorphous polymers and enhanced crystallizabilities for semi-crystalline polymers. Remarkably, c-polymer samples retain their distinct reorganized structures and behaviors even after extended periods of melt annealing. When a small amount of the rapidly crystallizable c-polymer is homogeneously dispersed in a large quantity of the same as-received (asr) polymer, and it is melted and then cooled, the c-polymer chains crystallize first and nucleate the crystallization of the asr-polymer chains, so that the entire sample (nuc-polymer) crystallizes at nearly the same temperature as the neat nucleant (c-polymer), including slow melt-crystallizing polymers, such as PET and PLLA.

Controlling the Packing of Metal–Organic Layers by Inclusion of Polymer Guests

The preparation of metal-organic structures with a controlled degree of disorder is currently one of the most promising fields of materials science. Here, we describe the effect of guest polymer chains on the transformation of a metal-organic framework (MOF). Heating a pillared MOF at a controlled temperature resulted in the exclusive removal of the pillar ligands, while the connectivity of the metal-organic square-grid layers was maintained. In the absence of a polymer, 2D-layers rearranged to form a new crystalline phase. In contrast, the presence of a polymer in the MOF inhibited totally the recrystallization, leading to a turbostratic phase with layers threaded and maintained apart by the polymer chains. This work demonstrates a new synthetic approach toward the preparation of anisotropic metal-organic materials with controlled disorder. It also reveals how guests can dramatically modify the conversion of host MOFs, even though no chemical reaction occurs between them.

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

I summarize work conducted in our laboratories over the past 30 years using small host molecules to restructure polymer materials at the nanometer level. Certain small molecules, such as the cyclic starches cyclodextrins (CDs) and urea (U) can form non-covalent crystalline inclusion compounds (ICs) with a range of guest molecules, including many polymers. In polymer-CD- and -U-ICs, guest polymer chains reside in narrow channels created by the host molecule crystals, where they are separated and highly extended. When the host crystalline lattice is carefully removed, the guest polymer chains coalesce into a bulk sample with an organization that is distinct from that normally produced from its melt or from solution. Amorphous regions of such coalesced polymer samples have a greater density, likely with less chain entanglement and more chain alignment. As a consequence, after cooling from their melts, coalesced amorphous polymers show glass-transition temperatures (Tgs) that are elevated above those of samples prepared from their solutions or melts. Upon cooling from their melts, coalesced samples of crystallizable polymers show dramatically-increased abilities to crystallize more rapidly and much closer to their melting temperatures (Tms). These unique behaviors of polymers coalesced from their CD- and U-ICs are unexpectedly resistant to extended annealing above their Tgs and Tms. Taking advantage of this behavior permits us to create polymer materials with unique and improved properties. Among these are amorphous polymers with elevated Tgs and semi-crystalline polymers with finer more uniform morphologies. Improved mechanical properties can be achieved through self-nucleation with small amounts of the same polymer made rapidly crystallizable through coalescence from its CD- or U-IC. This can lead to single polymer composites with as-received polymer matrices and self-nucleated reinforcements. Through simultaneous formation and subsequent coalescence from their common CD–ICs, stable well-mixed blends can be achieved between any two or more polymers, despite their inherent immiscibilities. Such coalesced and well-mixed blends are also resistant to phase segregation when heated for extensive periods well above their Tgs and Tms.

Reorganizing Polymer Chains with Cyclodextrins.

During the past several years, we have been utilizing cyclodextrins (CDs) to nanostructure polymers into bulk samples whose chain organizations, properties, and behaviors are quite distinct from neat bulk samples obtained from their solutions and melts. We first form non-covalently bonded inclusion complexes (ICs) between CD hosts and guest polymers, where the guest chains are highly extended and separately occupy the narrow channels (~0.5–1.0 nm in diameter) formed by the columnar arrangement of CDs in the IC crystals. Careful removal of the host crystalline CD lattice from the polymer-CD-IC crystals leads to coalescence of the guest polymer chains into bulk samples, which we have repeatedly observed to behave distinctly from those produced from their solutions or melts. While amorphous polymers coalesced from their CD-ICs evidence significantly higher glass-transition temperatures, Tgs, polymers that crystallize generally show higher melting and crystallization temperatures (Tms, Tcs), and some-times different crystalline polymorphs, when they are coalesced from their CD-ICs. Formation of CD-ICs containing two or more guest homopolymers or with block copolymers can result in coalesced samples which exhibit intimate mixing between their common homopolymer chains or between the blocks of the copolymer. On a more practically relevant level, the distinct organizations and behaviors observed for polymer samples coalesced from their CD-ICs are found to be stable to extended annealing at temperatures above their Tgs and Tms. We believe this is a consequence of the structural organization of the crystalline polymer-CD-ICs, where the guest polymer chains included in host-IC crystals are separated and confined to occupy the narrow channels formed by the host CDs during IC crystallization. Substantial degrees of the extended and un-entangled natures of the IC-included chains are apparently retained upon coalescence, and are resistant to high temperature annealing. Following the careful removal of the host CD lattice from each randomly oriented IC crystal, the guest polymer chains now occupying a much-reduced volume may be somewhat “nematically” oriented, resulting in a collection of randomly oriented “nematic” regions of largely extended and un-entangled coalesced guest chains. The suggested randomly oriented nematic domain organization of guest polymers might explain why even at high temperatures their transformation to randomly-coiling, interpenetrated, and entangled melts might be difficult. In addition, the behaviors and uses of polymers coalesced from their CD-ICs are briefly described and summarized here, and we attempted to draw conclusions from and relationships between their behaviors and the unique chain organizations and conformations achieved upon coalescence.

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

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.

Restructuring polymers via nanoconfinement and subsequent release

During the past several years my students and I have been utilizing certain small-molecule hosts to create nanostructured polymers. This is accomplished by first forming noncovalently bonded inclusion complexes (ICs) between these small-molecule hosts and guest polymers, followed by the careful removal of the host crystalline lattice to obtain a coalesced bulk polymer. We have repeatedly observed that such coalesced polymer samples behave distinctly from those produced from their solutions or melts. Coalesced amorphous homopolymers exhibit higher glass-transition temperatures, while crystallizable homopolymers coalesced from their ICs display higher melting and crystallization temperatures, and sometimes different crystalline polymorphs. When ICs are formed with block copolymers or with two or more different homopolymers, the resulting coalesced samples can exhibit intimate mixing between the copolymer blocks, or between entire homopolymer chains. Each of the distinct behaviors observed for polymers coalesced from their ICs is a consequence of the structural organization of the polymer–host-ICs. Polymer chains in host-IC crystals are confined to occupy narrow channels (diameter ~0.5–1.0 nm) formed by the small-molecule hosts around the included guest polymers during IC crystallization. This results in the separation and high extension of the included guest polymer chains, which leads, following the careful removal of the host molecule lattice, to unique behaviors for the bulk coalesced polymer samples. Apparently, substantial degrees of the extended and unentangled natures of the IC-included chains are retained upon coalescence. In this review we summarize the behaviors and uses of coalesced polymers, and attempt to draw conclusions on the relationship between their behavior and the organization/structures/conformations of the constituent polymer chains achieved upon coalescence from their ICs.

Behavior of Poly(ε-caprolactone)s (PCLs) Coalesced from Their Stoichiometric Urea Inclusion Compounds and Their Use as Nucleants for Crystallizing PCL Melts: Dependence on PCL Molecular Weights

We have formed noncovalent inclusion compounds (ICs) between guest poly(ε-caprolactone) (PCL) chains, with molecular weights ranging from ∼2000 to 80 000 g/mol, and host urea (U). Upon careful removal of the U host, each of the guest PCL chains were coalesced from their U-IC crystals to produce coalesced samples (c-PCLs). As previously observed for PCL and other polymer guests when coalesced from their ICs formed with host cyclodextrins (CDs), upon cooling from their melts, PCLs coalesced from their U-ICs also show enhanced abilities to crystallize, regardless of their molecular weight. Also consistent with polymer guests, including PCL, that were coalesced from their CD-ICs, c-PCLs obtained from their U-ICs retain their enhanced abilities to crystallize even after spending long times (weeks or more) in the melt. Because, unlike CD hosts, U does not thread over guest polymer chains in their ICs, we conclude that the enhanced ability of c-PCLs to crystallize from their melts upon removal of either host from their ICs is solely a consequence of their coalesced conformations/structures/morphologies, which are stable to prolonged melt-annealing. Furthermore, because c-PCLs with chain lengths well below and well above those corresponding to the entanglement molecular weight of PCL behave similarly, we conclude that their enhanced ability to crystallize from the melt is likely an exclusive consequence of the extended and unentangled arrangement of their coalesced chains. In addition, c-PCLs obtained from their U-ICs are observed to effectively act as self-nucleants, when added in small amounts to as-received PCLs, to produce nuc-PCLs, which like neat c-PCLs exhibit an enhanced ability to melt-crystallize.

Formation of crystalline inclusion compounds of poly (vinyl chloride) of different stereoregularity with γ-cyclodextrin

This work reports the formation and detailed characterization of the γ-cyclodextrin (γ-CD) inclusion compounds (ICs) formed with two poly (vinyl chloride) samples with different isotactic content. The ICs were characterized by X-ray diffraction, solid state 13C-NMR, solution 1H-NMR, FT-infrared, differential scanning calorimetry, and thermogravimetric analysis. Experimental evidence of the inclusion of the guest polymer chains into the narrow channels created by the γ-CD crystalline host lattice has been obtained. Examination of coalesced poly (vinyl chlorides) (PVCs) obtained after the host γ-CD is removed reveals different characteristics specifically for the coalesced PVC sample with higher isotactic content. An increase in Tg was observed by DSC for this PVC. To the contrary, the Tg of the coalesced PVC sample with lower isotactic content is almost the same as that of the as-synthesized sample. Thermogravimetric analysis indicated that coalesced PVC with higher isotactic content acquires a degree of stabilization after modification by threading into and being extracted from its γ-CD IC. The results suggest that an irreversible conformational change takes place when PVC forms ICs with a solid host lattice like γ-CD. The PVC molecules extend and reorganize into a more stable conformation in the IC, consequently improving the properties of the coalesced sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2503–2513, 2007

Morphology and dynamics of the poly(ϵ‐caprolactone)‐b‐poly(L‐lactide) diblock copolymer and its inclusion compound with α‐cyclodextrin: A solid‐state 13C NMR study

AbstractA biodegradable diblock copolymer of poly(ϵ‐caprolactone) (PCL) and poly(L‐lactide) (PLLA) was synthesized and characterized. The inclusion compound (IC) of this copolymer with α‐cyclodextrin (α‐CD) was formed and characterized. Wide‐angle X‐ray diffraction showed that in the IC crystals α‐CDs were packed in the channel mode, which isolated and restricted the individual guest copolymer chains to highly extended conformation. Solid‐state 13C NMR techniques were used to investigate the morphology and dynamics of both the bulk and α‐CD‐IC isolated PCL‐b‐PLLA chains. The conformation of the PCL blocks isolated within the α‐CD cavities was similar to the crystalline conformation of PCL blocks in the bulk copolymer. Spin–lattice relaxation time (T1C) measurements revealed a dramatic difference in the mobilities of the semicrystalline bulk copolymer chains and those isolated in the α‐CD‐IC channels. Carbon‐observed proton spin–lattice relaxation in the rotating frame measurements (T1ρH) showed that the bulk copolymer was phase‐separated, while, in the IC, exchange of proton magnetization through spin‐diffusion between the isolated guest polymer chains and the host α‐CD was not complete. The two‐dimensional solid‐state heteronuclear correlation (HetCor) method was also employed to monitor proton communication in these samples. Intrablock exchange of proton magnetization was observed in both the bulk semicrystalline and IC copolymer samples at short mixing times; however, even at the longest mixing time, interblock proton communication was not observed in either sample. In spite of the physical closeness between the isolated included guest chains and the host α‐CD molecules, efficient proton spin diffusion was not observed between them in the IC. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2086–2096, 2005

Nanostructuring polymers with cyclodextrins

AbstractBulk solid polymer samples formed by the coalescence of guest polymer chains from their inclusion compounds (ICs) formed with host cyclodextrins (CDs) can result in significant reorganization of their phase structures, morphologies, and even chain conformations from those more commonly produced from randomly‐coiled, entangled polymer solutions and melts. When the cyclic host CDs are threaded by polymer chains to form crystalline polymer‐CD‐ICs, the guest polymers become highly extended due to the narrow host CD diameters (∼5, 7, and 9 Å for α‐, β‐, and γ‐CDs) and are segregated from neighboring guest polymer chains by the CD‐IC channel walls. As a consequence, when polymer‐CD‐IC crystals are treated with CD solvents that do not dissolve the guest polymers or are treated with amylase enzymes, the resulting coalesced bulk polymer samples often display properties distinct from those of normally produced bulk samples of the same polymers. In this article the CD‐IC processing of polymers to generate novel polymer microstructures and morphologies are described, to control the phase separation of immiscible blocks in block copolymers, and to form well‐mixed intimate blends of two or more polymers that are normally incompatible. The thermal and temporal stabilities of polymer samples coalesced from their ICs formed with CDs will also be mentioned, and it is suggested that the range of polymer properties can be greatly expanded by their CD‐IC processing. Copyright © 2005 John Wiley & Sons, Ltd.

Nanostructuring High Molecular Weight Isotactic Polyolefins via Processing with γ-Cyclodextrin Inclusion Compounds. Formation and Characterization of Polyolefin−γ-Cyclodextrin Inclusion Compounds

The present paper deals with the formation and detailed characterization of the γ-cyclodextrin (γ-CD) inclusion compounds (ICs) formed with two different high molecular weight isotactic polyolefins, i.e., polypropylene (i-PP) and poly(butene-1) (i-PB). Wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), solid-state 13C NMR, and FT-infrared (FTIR) observations were used to prove the inclusion of the guest polymer chains into the narrow channels provided by the stacks of the doughnut-shape CD molecules. The main aim of polyolefin inclusion into a solid host lattice like γ-CD is to extend and reorganize their conformations, with the hope of improving their commercial properties following their coalescence from their ICs. In the second part of the paper, both coalesced i-PP and i-PB obtained after the host γ-CD is removed reveal different characteristics as compared with the as-received or corresponding control samples.

Controlling the Polymorphic Behaviors of Semicrystalline Polymers with Cyclodextrins

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, ...

Interlamellar Incorporation of Charged Polymer Nanobeads into Sodium Montmorillonite

ABSTRACTComplete delamination of clay in polymer matrix has been strongly prohibited due to strong adhesion of guest polymer chains between hydrophilic clay as well as degradation and desorption of organic materials in the gallery at high temperature. Incorporation of charged nanosized polystyrene beads directly into the gallery of pristine clay through exfoliation-exchange mechanism has been proposed to overcome the drawbacks.Synthesis of polymer nanobeads via emulsifier-free emulsion polymerization allowed to achieving formation of particles of appropriate particle size and surface charge density. Surface characterization, performed with XPS and ToF SIMS, has provided the results on the existence and the nature of the functional groups on the polymer particle surface, which have been found to be in a good compliance.Morphology of polymer-incorporated clay was observed from TEM, FE-SEM images. Study on mechanism of incorporation via XRD, XPS, ToF-SIMS suggested that adsorption of polymer nanobeads through cationic exchange of intergallery cation of clay for onium ion at the surface of polymer nanobead not only improves compatibility of clay with polymer matrix, but, what is essential, dramatically promotes expansion of clay gallery.

Formation of and coalescence from the inclusion complex of a biodegradable block copolymer and alpha-cyclodextrin. 2: A novel way to regulate the biodegradation behavior of biodegradable block copolymers.

A biodegradable block copolymer (PCL-b-PLLA, M(n) = 1.72 x 10(4), M(w)/M(n) = 1.37) of poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA) with very low crystallinity was obtained by forming the inclusion complex between alpha-cyclodextrin molecules and PCL-b-PLLA followed by coalescence of the guest polymer chains. Films of the as-synthesized and coalesced copolymer samples, PCL and PLLA homopolymers of approximately the same chain lengths as the corresponding blocks of PCL-b-PLLA, and a physical blend of PCL/PLLA homopolymers with the same molar composition as PCL-b-PLLA were prepared by melt-compression molding between Teflon plates. Subsequently, the in vitro biodegradation behavior of these films was studied in phosphate buffer solution containing lipase from Rhizopus arrhizus, by means of ultraviolet spectra, attenuated total reflectance FTIR spectra, differential scanning calorimetry, wide-angle X-ray diffraction measurements, and weight loss analysis. PCL segments were found to degrade much faster than PLLA segments, both in the pure state and in copolymer or blend samples. Consistent with our expectation, suppression of the phase separation, as well as a decrease of crystallinity, in the coalesced copolymer sample led to a much faster enzymatic degradation than that of either as-synthesized copolymer or the PCL/PLLA physical blend sample, especially during the early stages of biodegradation. Thus the biodegradation behavior of biodegradable block copolymers, which is of decisive importance in drug delivery and controlled release systems, may be regulated by the novel and convenient means recently reported by us.(1)

NMR observation of the conformations and motions of polymers confined to the narrow channels of their inclusion compounds

AbstractWhen polymers are guests in crystalline inclusion compounds (ICs) formed with small‐molecule hosts, they occupy a unique environment. In a cocrystallization process the small‐molecule host forms a crystalline lattice containing long narrow channels where the guest polymer chains are included. Because of the narrow channel diameter and because neighboring channels are separated by walls formed exclusively from the small‐molecule host lattice, the included polymer chains are highly extended and separated from polymer chains in other IC channels. As a consequence, polymer‐IC crystals provide a unique solid state environment for the included polymer chains and serve as models useful for assessing the contributions made by the inherent behavior of individual polymer chains to the properties of ordered, bulk polymers, which can be obscured by pervasive interactions between their tightly packed polymer chains. In this paper we describe the conformations and motions of polymer chains confined to the narrow channels of the following polymer‐ICs: i. polyethylene and trans‐1, 4‐polybutadiene in their ICs with perhydrotriphenylene, ii. polyepsilon caprolactone and its diblock and triblock copolymers with polybutadiene and poly (ethylene oxide) in their ICs with urea, and iii. nylon‐6 in its ICs with alpha‐, beta‐, and gamma‐cyclodextrins. High resolution, solid state NMR serves as both the conformational (C‐13 chemical shifts) and motional (relaxation times and line shapes) probe. Comparison with identical NMR measurements performed on the bulk homo‐ and copolymer samples permits us to draw several conclusions regarding the relationships between the conformations and motions of polymers and their dependence on their ordered solid state environments.

Polymer Inclusion Compounds: Model Systems for Ordered Bulk Polymer Phases and Starting Materials for Fabricating Polymer-Polymer Molecular Composites

Several small molecules can be cocrystallized with polymers to form inclusion compounds (ICs). Urea, perhydrotriphenylene and the cyclodextrins are examples, and serve to form the host crystalline lattice containing the guest polymer chains in their ICs. The guest polymer chains are confined to narrow, cylindrical channels created by the host, small-molecule lattice, where the polymers are highly extended as a consequence of being squeezed, and are separated from neighbouring polymer chains by the IC channel walls composed exclusively of the small-molecule lattice. The net result is a unique solid-state environment for polymers residing in IC channels, which can be utilized as model systems for ordered, bulk polymer phases. Comparison of the behaviour of polymer chains isolated and extended in IC crystals with the behaviour observed for ordered, bulk phases of polymers is beginning to permit an assessment of contributions made by the inherent, single chain and the cooperative, interchain interactions to the properties of ordered, bulk polymers. It is also possible to release and coalesce polymers from their IC crystals in a manner which leads to their consolidation with a chain-extended morphology. Embedding polymer IC crystals into a carrier polymer, followed by in situ release and coalescence of the included polymers from their IC crystals, offers a means to obtain polymer-polymer composites with unique morphologies. Several such polymer IC-generated composites are described and it is suggested that their unique morphologies might translate into useful, tailorable properties, as well as providing a means for addressing several questions that are fundamental to the behaviour of both phase-separated and homogeneous polymer solids.

FTi.r. investigation of the inclusion compound formed between trans-1,4-polyisoprene and urea

Several small-molecule hosts form inclusion compounds (ICs) with polymers. In these ICs the guest polymer chains are highly extended and isolated from neighbouring polymer chains by the clathrate wall. Only polymers with extended conformations are narrow enough to reside in the narrow channel. An inclusion compound has been formed between urea and trans-1,4-polysoprene (TPI) and investigated using FTi.r. spectroscopy. Infrared bands observed for this IC indicate that the crystal structure of this IC is most likely different from the usual hexagonal crystal structure. It is also suggested that TPI in the channel adopts a conformation which is similar to bulk TPI with β crystal modification.

NMR observations of isolated and stretched polymer chains in their crystalline inclusion compounds formed with small‐molecule host clathrates

AbstractSeveral small‐molecule hosts form clathrates or inclusion compounds (ICs) with polymers. In these polymer‐ICs the guest polymer chains are confined to occupy narrow channels in the crystalline matrix formed by the host. The walls of the IC channels are formed entirely from the molecules of the host, and they serve to create a unique solid‐state environment for the included polymer chains. Each polymer chain included in the narrow, cylindrical IC channels (ca 5.5 Å in diameter) is highly extended and also separated by the host matrix channel walls from neighboring polymer chains. The net result is a solid‐state environment where extended, stretched (as a consequence of being squeezed) polymer chains reside in isolation from their neighbors inside the narrow channels of the crystalline matrix provided by the small‐molecule host. Comparison of the behavior of isolated, stretched polymer chains in their crystalline ICs with observations made on ordered, bulk samples of the same polymer are beginning to provide some measure of the contributions made by the intrinsic nature of a confined polymer chain and the pervasive, cooperative, interchain interactions which can complicate the behavior of bulk polymer samples. Just as dilute polymer solutions at the θ temperature have been effectively used to model disordered, bulk polymer phases (both glasses and melts), polymer‐ICs may be utilized to increase our understanding of the behavior of polymer chains in their ordered, bulk phases as found in crystalline and liquid‐crystalline samples. Solid‐state NMR observations of polymer‐ICs can provide detailed conformational and motional information concerning the included, stretched and isolated polymer chains.

Crystalline polymer inclusion compounds: potential models for the behaviour of polymer chains in their bulk, ordered phases

Several small-molecule hosts form clathrates or inclusion compounds (ICs) with polymers. In these polymer ICs the guest polymer chains are confined to occupy narrow channels in the crystalline matrix formed by the host. The walls of the IC channels are formed entirely from the molecules of the host, and they serve to create a unique solid-state environment for the included polymer chains. Each polymer chain included in the narrow, cylindrical IC channels (ca. 5.5 Å in diameter) is highly extended and also separated from neighbouring polymer chains by the host matrix channel walls. The net result is a solid-state environment where extended, stretched (as a consequence of being squeezed) polymer chains reside in isolation from their neighbours inside the narrow channels of the crystalline matrix provided by the small-molecule host. Comparison of the behaviour of isolated, stretched polymer chains in their crystalline ICs with observations made on ordered, bulk samples of the same polymer are beginning to provide some measure of the contributions made by the intrinsic nature of a confined polymer chain and the pervasive, cooperative, interchain interactions which can complicate the behaviour of bulk polymer samples. In the same way that dilute polymer solutions at the Θ temperature have been effectively used to model disordered, bulk polymer phases (both glasses and melts), polymer ICs may be utilized to increase our understanding of the behaviour of polymer chains in their ordered, bulk phases, such as those found in crystalline and liquid-crystalline samples.