Browsing by Author "Alan E. Tonelli, Committee Chair"

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Controlling the Stereoregularity of Polyacrylonitrile and Its Determination Using Small-Molecule Host Inclusion Compounds
(2006-11-02) Yang, Hyungchol; Wendy E. Krause, Committee Member; Alan E. Tonelli, Committee Chair; Charles M. Balik, Committee Member; Saad A. Khan, Committee Member
This research focuses on synthesizing highly stereoregular polyacrylonitrile (PAN) and determining its tacticity (predominantly isotactic or syndiotactic), utilizing guest monomer (acrylonitrile = AN) host inclusion polymerization. Highly stereoregular PAN, with a meso or racemic diad content ~ 80%, was prepared by γ-ray irradiation of an AN urea canal complex at a low temperature (–78° C). Several essential experimental factors for ensuring the highly stereoregular PAN production were considered. After γ-ray irradiation polymerization, the tacticity of PAN was determined from triad peak intensities of the methine (CH) and nitrile (–C≡N) carbons in the 13C-NMR spectra, assuming Bernoullian statistics. When the AN guest forms an inclusion compound (IC) with urea host, it was expected that there is a structural transformation of urea into the hexagonal crystal lattice structure with a narrow channel diameter (5.25-5.5Å). However, in our FTIR observations run at room temperature, a different type of transformation was detected. AN urea IC before and the PAN urea IC after low temperature γ-ray irradiation polymerization are both large tetragonal structures, which have a larger channel diameter (> 5.5Å) at room temperature. Because these infrared observations were not carried out below -20.8° C, known as the decomposition temperature of the hexagonal IC structure, the fact that the structures of both AN urea IC and PAN urea IC are the large tetragonal does not necessarily prove that during polymerization below –20.8°C the AN urea IC was also the large tetragonal structure. If PAN was polymerized in the hexagonal (or pseudo-hexagonal) urea canal lattice (5.5Å), which provides a more confined environment for its conformation and configuration, it would likely be syndiotactic and adopt the all trans conformer. Because of the flexible nature of urea when it forms inclusion compounds with guest molecules, if AN was polymerized in the large tetragonal (> 5.5.Å) lattice structure of urea, which gives more freedom to PAN during its inclusion polymerization or inclusion compound formation, it could have either an isotactic or a syndiotactic configuration. Because no definitive evidence has been previously reported in the determination of γ-ray irradiated PAN by NMR spectroscopy, an effort to prepare PAN in another molecular host crystalline lattice, α-cyclodextrin (CD), was made. Synthesis of PAN in the columnar structure of AN α-CD–IC is a very promising method to reveal the original tacticity of highly stereoregular (~80%) PAN due to the fact that α-CD has a rigid small diameter (4.9Å) channel cavity, and only syndiotactic PAN in the all trans conformation is likely to be produced. However, γ-ray irradiation of a channel structure AN α-CD IC did not produce any PAN, implying that the AN urea IC that produced stereoregular PAN upon γ-irradiation was likely in a large tetragonal structure. Alternatively, because of the disparity in AN:α-CD and PAN:α-CD stoichiometries [1:1 (experimental) versus 3:1 (expected)], after γ-irradiation initiation of AN α-CD IC, a shortage of AN would result in the α-CD IC channels, possibly interrupting polymerization. By analogy to polypropylene (PP) polymerized in host perhydrotriphenylene (PHTP) IC (d ~ 5Å) and polyvinylchloride (PVC) polymerized in urea canals, which are both found to be syndiotactic, we suggest that stereoregular PAN polymerized in urea canals is also predominantly syndiotactic.
Fabrication of Polymer Materials from Their Cyclodextrin Inclusion Complexes
(2003-10-28) Wei, Min; Alan E. Tonelli, Committee Chair
Inclusion complexed (IC) and coalesced biodegradable poly(ε-caprolactone) (PCL), poly(L-lactic acid) (PLLA), and their diblock copolymer (PCL-b-PLLA) were achieved by forming ICs between host α-cyclodextrin(α-CD) and guest PCL, PLLA, and PCL-b-PLLA, followed by removing the α-CD host with an amylase enzyme. The melting and crystallization behavior of these CD-IC treated polymers are investigated. Both isothermal and nonisothermal crystallization studies demonstrate that the PCL and PLLA blocks in the IC-coalesced samples are more readily and homogeneously crystallized than those in the as-synthesized samples or their physical blend, even though the level of crystallinity in the IC-coalesced diblock copolymer is significantly lower. Moreover, unlike the as-synthesized diblock copolymer, the crystallization of PCL and PLLA blocks in the IC-coalesced diblock copolymer are not influenced by their covalent connection. Poly(ethylene terephthalate) (PET) and bisphenol A polycarbonate (PC) samples have been produced by the coalescence of their segregated, extended chains from the narrow channels of the crystalline inclusion complexes formed between the γ-cyclodextrin (γ-CD) host and PET and PC guests. Experimental observations of PET and PC samples coalesced from their crystalline ICs suggest structures and morphologies that are different from those of samples obtained by ordinary solution and melt processing techniques. PC crystals formed upon the coalescence of highly extended and segregated PC chains from the narrow channels in the CD host lattice are possibly more chain-extended and certainly more stable than chain-folded PC crystals. The coalesced PET melt rapidly recrystallizes during the attempted quench, and so upon reheating, it displays neither a glass transitions temperature (Tg) nor a crystallization exotherm but simply remelts at the as-coalesced melting temperature (Tm). An inclusion complex between nylon-6 and α-cyclodextrin was obtained and we attempted to use the formation and subsequent disassociation of the nylon-6/α-CD inclusion complex to manipulate the properties of nylon-6. Examination of as-received and IC coalesced nylon-6 samples show that dominated α-form crystalline phase of nylon-6 and a great increase in crystallinity are in the coalesced sample. When inherently immiscible polymers are included as guests in the narrow channels of their common inclusion complexes formed with host cyclodextrins and then these polymer-1/polymer-2-CD-IC crystals are coalesced, an intimately mixed blends of the polymers are obtained. Polycarbonate (PC)/poly(methyl methacrylate) (PMMA) blends coalesced from their common γ-CD-ICs are amorphous and generally exhibit single glass transitions at temperature (Tg) between those of pure PC and PMMA. FTIR spectroscopy suggests an intimate mixing of and possible specific interactions between PC and PMMA chains in the coalesced blends. An attempt to achieve an intimate blend between nylon 6 and nylon 66 by forming and dissociating their common α-CD-IC was also made. Experimental results demonstrate that α-cyclodextrin can only host single nylon polymer chains in the IC channels. Spectroscopic results illustrate that there is intimate mixing existing in the IC coalesced blend, but not in their solution cast physical blend.
Modification of Nylon 6 Structure via Nucleation
(2009-08-12) Mohan, Anushree; Bruce Novak, Committee Member; Jan Genzer, Committee Member; Alan E. Tonelli, Committee Chair; Richard Kotek, Committee Co-Chair
For nearly two decades inclusion compounds (ICs) have been formed by threading polymer chains into the cyclic starches, cyclodextrins (CDs). Non-covalently bonded crystalline ICs have been formed by threading CDs, onto guest nylon-6 (N6) chains. When excess N6 is employed, non-stoichiometric (n-s)-N6-CD-ICs with partially uncovered and dangling N6 chains result. We have been studying the constrained crystallization of the N6 chains dangling from (n-s)-N6-CD-ICs in comparison with bulk N6 samples, as a function of N6 molecular weights, lengths of uncovered N6 chains, and the CD host used. While the crystalline CD lattice is stable to ~ 300Ã‚Â° C, the uncovered and dangling, yet constrained, N6 chains may crystallize below, or be molten above ~225Ã‚Â° C. In the IC channels formed with host ÃŽÂ±- and ÃŽÂ³-CDs containing 6 and 8 glucose units, respectively, single and pairs of side-by-side N6 chains can be threaded and included. In the ÃŽÂ±-CD-ICs the ~ 0.5nm channels are separated by ~ 1.4nm, while in ÃŽÂ³-CD-ICs the ~ 1nm channels are ~ 1.7 nm apart, with each ÃŽÂ³-CD channel including two N6 chains. The constrained dangling chains in the dense (n-s)-N6-CD-IC brushes crystallize faster and to a greater extent than those in bulk N6 melts, and this behavior is enhanced as the molecular weights/chain lengths of N6 are increased. Furthermore, when added at low concentrations (n-s)-N6-CD-ICs serve as effective nucleating agents for the bulk crystallization of N6 from the melt. Because of the biodegradable/bioabsorbable nature of CDs, (n-s)-polymer-CD-ICs can provide environmentally favorable, non-toxic nucleants for enhancing the melt crystallization of polymers and improving their properties.
Nanostructuring Polymers with Cyclodextrins
(2005-09-26) Uyar, Tamer; Richard Kotek, Committee Member; C. Maurice Balik, Committee Member; Alan E. Tonelli, Committee Chair; Juan P. Hinestroza, Committee Member
The formation of polymer-cyclodextrin inclusion compounds of polycarbonate (PC), poly(methylmethacrylate) (PMMA) and poly(vinylacetate) (PVAc) guests with host γ-cyclodextrin (γ-CD) have been successfully achieved. Coalesced bulk polymer samples were obtained by removal of γ-CD from their inclusion compounds (ICs). Spectroscopic findings indicated that the chain conformations of the bulk polymers were altered when they were included inside the CD channels and extended chain conformations were retained when coalesced from their ICs. Significant improvements were observed in the thermal transitions for the coalesced polymers, with glass transitions shifted to higher temperatures. Thermal studies reveal that the thermal stabilities of coalesced polymers increased slightly compared to the corresponding as-received polymers and degradation products of the polymers are affected once the polymers chains are included inside the γ-CD-IC cavities. A procedure for the formation of intimate blends of binary and ternary polymer systems; PC/PMMA, PC/PVAc, PMMA/PVAc and PC/PMMA/PVAc was studied. PC/PMMA, PC/PVAc, PMMA/PVAc and PC/PMMA/PVAc were included in γ-CD channels and were then simultaneously coalesced from their common γ-CD-ICs to obtain intimately mixed blends. It was observed that the ratios of polymers in coalesced blends were significantly different than the starting ratios, and PC was found to be preferentially included in γ-CD channels when compared to PMMA or PVAc. Physical mixtures of polymer blends were also prepared by co-precipitation and solution casting methods for comparison. The analysis indicates that the ternary and the binary blends of these polymers achieved by coalescence from their common γ-CD-IC results in a homogeneous polymer blends, possibly with improved properties, whereas co-precipitation and solution cast methods produced phase separated polymer blends. It was shown that coalescence of two or more normally immiscible polymers from their common CD-ICs appears to be an applicable method for obtaining well-mixed, intimate blends. The solid complex of guest styrene included inside the channels of host γ-cyclodextrin (styrene/γ-CDchannel-IC) was formed in order to perform polymerization of styrene in a confined environment (γ-CD channels). Modeling of polystyrenes (PS) with various stereosequences in the narrow cylindrical channels corresponding to those found in γ-CD ICs has been conducted. It was calculated that only isotactic PS stereoisomers can fit into the γ-CD cavity. Thus, based on the modeling of stereoisomeric PSs in narrow γ-CD channels, it was suggested that polystyrene with unusual microstructures might be produced via constrained polymerization of styrene monomer in its γ-CD-IC crystals. The in situ polymerization of styrene inside the narrow channels of its γ-CD-IC crystals was performed in aqueous media. Alternatively, the solid-state polymerization of styrene/ϒ-CD IC has also been carried out by radiation polymerization. It was found that most styrene monomer migrates from the γ-CD channels and polymerizes outside of the channels. Yet, a rotaxaned structure has been obtained where some CD molecules entrapped along the PS chains after the polymerization.