• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 4079
  • 828
  • 404
  • 288
  • 94
  • 94
  • 94
  • 94
  • 94
  • 93
  • 74
  • 59
  • 52
  • 42
  • 42
  • Tagged with
  • 7079
  • 1151
  • 1085
  • 1052
  • 758
  • 752
  • 582
  • 571
  • 524
  • 457
  • 443
  • 386
  • 345
  • 332
  • 280
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Nanoscale scaffolding by folding of monodisperse and sequentially precise poly((alanine-glycine)(3)glutamic acid-glycine(glycine-alanine)(3)glutamic acid-glycine): Biosynthesis and characterization by X-ray diffraction, FTIR and NMR

Parkhe, Ajay Dattatraya 01 January 1994 (has links)
We have designed repetitive polypeptides which would self assemble into a unique three dimensional structure. The de novo design of these polypeptides is based on existing information on protein chain folding. As a first step to designing more complicated scaffolds, we are interested in designing repetitive polypeptides which would self assemble into lamellae of uniform, predetermined thickness. The design of the polypeptide repeating unit has been based in part on work on the structure of silk (and its analogues) and in part on the literature on reverse turns in globular proteins. The polypeptides in this work comprise alternating beta sheet forming segments and turn forming segments with exact periodicity. Repetitive alanyl-glycyl diads are known to form a beta sheet structure. It is anticipated that these would reside in the lamella and define its thickness. The stronger the thermodynamic driving force for the chains to form alternating beta sheets and turns, the more is the probability of obtaining a unique scaffold. In this thesis, DNA sequences encoding the synthesis of repetitive units of A, B, C and D have been synthesized. Polypeptides with ten repeats of A, four repeats of D and three repeats of C were successfully synthesized in E. coli.$$\eqalign{{-}(\rm AlaGly)\sb3GluGly(GlyAla)\sb3GluGly{-}&\qquad{\bf A}\cr{-}(\rm AlaGly)\sb3AspGly(GlyAla)\sb3AspGly{-}&\qquad{\bf B}\cr{-}(\rm AlaGly)\sb3GluGly(AlaGly)\sb3ValGly{-}&\qquad{\bf C}\cr{-}(\rm AlaGly)\sb3GluGly(AlaGly)\sb3MetGly{-}&\qquad{\bf D}\cr}$$ The polypeptide containing ten repeats of A (referred to as A-10 in the Abstract) has been characterized in the solid state by x-ray diffraction, FTIR and NMR. The polypeptide A-10 forms an antiparallel beta sheet structure when stirred in 70% formic acid; electron microscopy shows the morphology of the crystallites to be needle like. The unit cell deduced from the diffraction patterns is similar to those from previous diffraction patterns on silks and silk-like polypeptides. The unit cell which best explains the diffraction data for A-10 is monoclinic with a = 9.56 A, b = 10.68 A and c = 7.0 A with the angle between b and c equal to 80$\sp\circ$. FTIR spectroscopy on crystalline A-10 shows the polypeptide predominantly adopting the antiparallel beta sheet structure. The repetitive polypeptide A-10 and a $\sp{13}$C=O enriched analogue have been synthesized biologically in Escherichia coli. The two analogues have been blended in solution and co-crystallized. FTIR spectra (in the amide I region) have been studied as a function of blend composition. Solid state CPMAS NMR experiments were carried out on the amorphous and crystalline forms of A-10. The data suggests that glutamic acids are in identical environments in the crystalline and amorphous samples. The fast relaxation rates of their alpha, beta and gamma carbons suggest that glutamic acid residues are excluded from the crystalline regions in both the samples. Static solid state NMR experiments have been carried out on oriented mats of crystalline A-10. The orientation of the crystallites has been studied by the analysis of lineshapes. The lineshape analysis is consistent with the crystallites being an imperfect form of an orientation in which the crystallographic b axis (Figure 1.6) is perpendicular to the mat and there is free rotation around b. (Abstract shortened by UMI.)

Synthesis and characterization of thermotropic liquid crystalline polyesters for use in molecular composites

Narayan-Sarathy, Sridevi 01 January 1995 (has links)
A molecular composite can be defined as a system where one polymer reinforces another at a molecular level to give enhanced properties. This concept could be applied to the enhancement of mechanical properties of flexible coil polymers like PET. Molecular level reinforcement of PET with rigid polymers could be achieved by increasing the flexibility of the latter in an effort to increase the entropic mixing and by selection of the rod-like and coil molecule, with specific interactions to increase enthalpy of mixing. Thermotropic liquid crystalline polyesters (LCP) were chosen for the rigid counterpart in our research as these materials exhibit orientation in the direction of draw resulting in unusually high strengths and moduli of the fibers in the draw direction. Also they exhibit lower melt-viscosity values at higher temperatures consequently lowering the processing temperatures of their blends compared to PET. Thus they function as processing-aids in addition to being reinforcing agents. This dissertation work has involved the synthesis of thermotropic liquid crystalline polyesters containing various oxyethylene substituted hydroquinones. The hydroquinone monomers were synthesized, which were then polymerized with terephthaloyl chloride by a solution state polycondensation method. Random and block copolymers containing the mesogenic segments and flexible PET/PBT segments were also synthesized and studied as these LCPs are expected to be miscible with PET by the interaction of the isotropic segments in the melt state. The systems investigated include poly (2-butoxyethoxy-1,4-phenylene terephthalate) (PBEHT), poly (2-ethoxy-1,4-phenylene terephthalate) (PEHT) and the random and block copolymers containing PEHT and PET/PBT segments. Some copolymers containing poly (1,4-phenylene terephthalate) and PET/PBT segments were also investigated. The polymers were mainly characterized by viscometry, thermogravimetry, DSC, NMR and optical microscopy. Especially in the case of the copolymer systems, extensive NMR analysis was done to get both the composition and sequence distribution. Structure-property relationships were drawn from copolymers with varying ratios of the rigid and flexible block sizes. Optimization of reaction conditions like solvent, acid-acceptor, temperature and time, to get high molecular weight polymers formed an important aspect of the research work. Rheological studies were done to study the role of the LCPs as processing-aids in the blends. The presence of the flexible pendant group brings down the transition temperatures of the LCP to the processing temperature range of PET. The PBEHT system was found to be a good processing-aid for PET. The high molecular weight PEHT/BT copolymers obtained by conducting polymerization at high temperatures yielded thin films of good strength and mechanical integrity. The processing parameters of the blends of these polyesters with PET have to be optimized to draw conclusions about the reinforcing ability of the LCPs.

Investigations of the miscibility, crystallization, melting, and deformation behavior of poly(ether ether ketone)/poly(ether imide) blends

Chen, Hsin-Lung 01 January 1994 (has links)
This dissertation reports the studies of the blends of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI). The investigations reported encompass both the thermodynamic and kinetic aspects of this binary blend. The phase behavior of PEEK/PEI blends in the amorphous states was investigated by differential scanning calorimetry (DSC), density measurement, and Fourier-Transform Infrared (FTIR) spectroscopy. Amorphous PEEK/PEI blends were miscible over the whole composition range. The specific volume measurement and the equilibrium melting point depression analysis showed the existence of favorable interaction between PEEK and PEI. The FTIR study suggested that the oxygen lone-pair electrons of the ether groups in PEEK interact favorably with the electron-deficient imide rings in PEI. The coplanarity of the two nearest imide rings in PEI was changed by blending to accommodate the favorable interaction with PEEK. The two-stage crystallization behavior of PEEK and PEEK/PEI blends was studied by thermal mechanical analysis (TMA) and DSC. The two crystallization stages of PEEK were first time distinguished clearly by measuring the thickness change of PEEK films during isothermal crystallization. A crystallization kinetics model considering both primary and secondary crystallization was applied to extract the respective rate constants of the two crystallization stages. The results were discussed in terms of the diffusion mechanisms during PEEK crystallization. The growth of PEEK spherulites from the pure melt and from the blends with PEI was studied by hot stage optical microscopy. The spherulite growth kinetics was analyzed using a modified Lauritzen-Hoffman theory which considers the diluent effect of PEI. The regime III-II transition was observed for PEEK and PEEK/PEI blends. The side surface free energy, fold surface free energy, and the work of chain folding were calculated, and were discussed in terms of the stiffness and bulkiness of PEEK molecules. The semicrystalline morphology and the melting behavior of PEEK/PEI blends were studied by optical microscopy and DSC, respectively. The optical micrographs of PEEK spherulites grown from the blends showed that a significant amount of PEI was rejected to the interfibrillar regions of the PEEK spherulites. The melting study indicated that PEEK crystal reorganization on heating was hindered by blending with PEI, and this was attributed to the incorporation of PEI in the PEEK crystalline interlamellar regions. The amorphous and the crystalline PEEK/PEI blend films were drawn uniaxially by solid-state coextrusion. The glass transition, density, crystallization behavior, and the respective orientations were studied. The T$\sb{\rm g}$s of amorphous PEEK/PEI blends were depressed by drawing. The kinetics of crystallization of drawn PEEK/PEI blends was also reported. The orientation studies by IR dichroism showed that the orientations of both PEEK and PEI were decreased by increasing PEI content in the blends.

Phase behavior, crystallization kinetics and morphologies of poly(epsilon-caprolactone) (PCL)/polycarbonate (PC) blends

Cheung, Yunwa Wilson 01 January 1994 (has links)
Small-angle neutron and X-ray scattering (SANS and SAXS) coupled with thermal analysis, optical microscopy and FTIR have been employed to probe the phase behavior, crystallization kinetics and crystalline morphologies of PCL/PC blends. The composition dependence of the glass transition temperature T$\sb{\rm g}$ exhibited a discontinuity and was critically analyzed using the classical equations of Gordon-Taylor and Fox, and the free volume theory of Kovacs. Results derived from the random phase approximation (RPA) analysis of the SANS profiles measured at 30$\sp\circ$C for the deuterated PC-rich blends and those obtained from the melting point depression analysis of the PCL-rich blends suggested favorable blend interactions. The effects of composition, crystallization temperature and PC crystallinity on the athermal and isothermal crystallization kinetics of PCL in PCL-rich blends have been examined. Combination of the overall crystallization rate measurements and the radial growth rate results unequivocally demonstrated that PC is an effective nucleating medium for PCL crystallization. Evolution of the PCL lamellar growth was monitored by synchrotron SAXS. The interlamellar spacing initially varied with time and then approached a plateau value at the later stage of crystallization. An insertion mechanism is proposed in which PCL is crystallized in the amorphous intralamellar phase of PC. Crystalline morphologies of deuterated PC/PCL blends were studied by SANS and SAXS in the semicrystalline/amorphous state (above T$\sb{\rm m}$ of pure PCL). A two-correlation length model provided an excellent fit for the SANS data over the entire composition range. The long range correlation length ($\sim$10$\sp2$ A) and the short range correlation length ($\sim$10 A) derived from this model were inferred to be associated with the crystalline PC domain and the local cluster in the amorphous phase, respectively. Quantitative SAXS analysis suggested that random mixing of PCL and PC lamellae occurred in the semicrystalline/semicrystalline state. Two distinct regions of incorporation were identified in the semicrystalline/amorphous state. It was found that PCL was rejected from the interlamellar region in the PCL-rich blends. In contrast, PCL was incorporated into the amorphous phase between the crystalline lamellae in the PC-rich blends.

Liquid crystal block oligomers of linear and star topology

Milliken, Kenneth Troy 01 January 1994 (has links)
A series of thermotropic block oligomers of linear and star topologies were synthesized and characterized. The synthesis involved attaching mesogenic 4-(4-butoxybenzoyloxy)benzoyl and 4-(4-butoxybenzoyloxy)benzoyloxy-4-benzoyl groups to linear and star hydroxy terminated poly(butadienes) of low molecular weights and low polydispersities. The effects of topology and weight percent hard segment on melting temperatures, enthalpies of fusion, rheological behavior, and morphological properties were studied. Most of these studies focused mainly on 4-(4-butoxybenzoyloxy) benzoyloxy-4-benzoyl-terminated poly(butadienes). Techniques employed to study these properties included differential scanning calorimetry, thermal gravimetric analysis, solid-state rheology, and X-ray diffraction. Star-block oligomers, in general, displayed greater melting temperatures and enthalpies of fusion than their linear analogs. The highest Tm and $\Delta$Hm were observed for the materials of highest weight percent hard segment. In a crystallization study, melting temperatures increased with increasing crystallization temperature. Stars crystallized more efficiently than their linear analogs. Thermal stability of all compounds synthesized proved greatest for 2-ring end-capped block oligomers and the pentad copolymer. Superior rheological properties were observed, in general, for the stars by solid-state rheometry. From WAXD, linear and star block oligomers showed similar crystalline structures in the microdomains. Microdomain size was obtained by SAXD for linear and star block oligomers of highest hard-block content. Evidence of more defined phase separation was observed for the linear topology.

Synthesis, characterization and degradation of biodegradable polymers

Wu, Bin 01 January 1996 (has links)
Poly($\beta$-hydroxybutyrate), PHB and the related copolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3HB-co-4HB), are naturally occurring polyesters produced by numerous microorganisms which have attracted much attention in the past decade as potentially commercial biodegradable thermoplastics. Part of the goal of this research is to chemically synthesize PHB and poly(3HB-co-4HB) by the ring-opening polymerization and copolymerization of butyrolactones. The first part of the research focused on the optimization of reaction conditions, modification of alumoxane catalysts and understanding of mechanism for the ring-opening polymerization of (R,S) -$\beta$-butyrolactone, (R,S) -$\beta$-BL. The stereoregular polymerization of (R,S) -propylene oxide (PO) was also studied, which shed a light on the polymerization of (R,S) -$\beta$-BL. In the second part of the research, a copolymer, poly(3HB-co-4HB) was synthesized by copolymerizing (R,S) -$\beta$-BL and $\gamma$-BL. Copolymers with 4-HB contents up to 33 mol% could be prepared depending on the initial ratio of $\beta$- and $\gamma$-BL. The copolymers, which were characterized by $\sp1$H-NMR, $\sp{13}$C-NMR, GPC and DSC. NMR analysis, had a random sequence distribution of 3-HB and 4-HB units. Biodegradation tests showed that the biodegradation rates of the copolymers were higher than that of poly(3-HB) and increased with the increasing contents of 4-HB units. In last part of the research, a biodegradable polyethylene was prepared by copolymerizing ethylene with 2,-methylene-1,3-dioxepane (MDP) at approximately 1,000 psi. The copolymers obtained, poly(MDP-co-Ethylene) were characterized by elemental analysis, IR, $\sp1$H-NMR, $\sp{13}$C-NMR, DSC and GPC. The copolymers were found to contain 2-15 mol% ester group. The copolymer underwent acidic methanolysis and its molecular weights decreased by ten times on average after the methanolysis.

Compatibility of semicrystalline/amorphous ionomeric blends

Ng, Chui-Wah Alice 01 January 1995 (has links)
Two semicrystalline/amorphous ionomeric blend systems, the blend of poly(ethyl acrylate-co-4-vinylpyridine) with metal neutralized sulfonated poly(ethylene terephthalate) (EAVP/PET-SO$\sb3$M) and the blend of nylon 4 with lithium neutralized sulfonated polystyrene (N4/SPS-Li), were investigated. The motivation of this work was to use specific interactions between the polymer pair to form miscible or compatible blends for property enhancement. Additionally, particular effort was made to address the competing phenomena between the phase mixing by virtue of the specific interactions and the phase separation possibly induced by the crystallization of the semicrystalline component. For the EAVP/PET-SO$\sb3$M blend, an extensive study was undertaken to investigate the various factors affecting the compatibility behavior and hence the ultimate mechanical properties of the system which include: (1) the effects of stoichiometry of the interacting groups upon the amorphous phase; (2) the effects of stoichiometry of the interacting groups upon the crystalline phase; (3) the effects of functionalization level of the constituent polymers; and (4) the effects of counterions of the ionomer component. Results showed that compatibilization of EAVP/PET-SO$\sb3$M was achieved by virtue of the formation of a metal-pyridine-sulfonate group coordination complex between the polymer pair. However, complete miscibilization was not obtained even at the highest level of functionalization, 10 mol %. The crystallization of the PET-SO$\sb3$M component was found to induce some phase separation, but the system remained compatibilized via specific interactions. On the other hand, the crystallization behavior was affected by the phase mixing, the level of sulfonation and the counterions of the ionomer. Dramatic improvement of the ultimate mechanical properties was accomplished as a result of the favorable interfacial adhesion across the phase boundaries as compared to the immiscible counterparts. For the N4/SPS-Li blend, the composition dependence of the compatibilization was investigated in relation to the effects of crystallinity level of the N4 component and the water content of the system. Results showed that compatibilization was achieved by virtue of the specific interactions between the amide groups and the lithium sulfonate groups. Samples with a high level of crystallinity exhibit consistently higher glass transition temperature (T$\rm \sb{g}$) than those with low crystallinity. Wet samples were found to have a lower T$\rm \sb{g}$ than the dry counterparts due to the plasticization effect.

Modification, characterization and application of hyperbranched polyarylates

Ma, Bodan 01 January 1997 (has links)
Hyperbranched polymers are polymers with highly branched, yet non-crosslinked structures. In this work, the existing laboratory polymerization procedure was scaled up by a suspension method to synthesize hyperbranched poly(5-acetoxy isophthalic acid) of high molecular weight. A method of modifying the residual acyl groups of this polymer with different reagents was also established. Especially, the synthesis of monofunctional etherimide facilitated the compatiblization of the hyperbranched polyarylate with commercial polyetherimide. All modified polymers were characterized by FTIR, $\sp1$H and $\sp{13}$C NMR, elemental analysis and DSC. Through this modification route, hyperbranched polymers with glass transition temperatures ranging from $-$50$\sp\circ$C to 188$\sp\circ$C were prepared. Using fractionation techniques, samples with different molecular weights were obtained. The structural profiles of hyperbranched polyarylate were then investigated. The measurement of the degree of branching indicated that these macromolecules had uniform chemical structures. Solution static light scattering revealed that the hyperbranch polyarylates had very compact structures, the dimension of which remained stable regardless of different polymer-solvent interactions. Light scattering, NMR with LSR (Lanthanide Shift Reagent) as well as molecular simulation showed that molecules such as LSR $\rm(d\sim10$ A) and solvent could penetrate most part of the structure. Thus, these molecules should be treated as hard porous particles with small pore sizes. n$\sp\prime$-Butyl hyperbranched polyarylate and its linear analog poly(1,4-butylene isophthalate) were employed to investigate the effect of hyperbranching topology on blend properties. Under all experimental conditions, including a wide range of annealing temperature, molecular weight and blend composition, the blends were found always immiscible by the observation of glass transitions using DSC. Entropically, the compact nature of the hyperbranched polymer prevented itself from mixing with its linear analog at segmental scale. However, TEM revealed that the domain size of the phase separation was around 400 to 600A, indicating good compatibility. A modified Flory-Huggins theory was introduced to explain the immiscibility. The rheological properties of hyperbranched polymer and its blends presented the most promising aspect for future applications. The relaxation spectrum of hyperbranched polyarylate did not exhibit a plateau zone that is an indication of chain entanglement for linear polymers. This observation was true even for samples with molecular weight over 10$\sp5$. Zero shear viscosity of hyperbranched polyarylate was generally one magnitude lower than that of its linear analog with comparable molecular weight. Furthermore, the viscosities of their blends showed negative deviations from the so-called "log-additivity rule". This finding showed a true opportunity for hyperbranched Folymers to be used as rheology modifiers. Finally, the mechanical property of blends of etherimide modified hyperbranched polyarylate and polyetherimide was investigated. While the tensile modulus of the material was enhanced, the toughness was drastically reduced due to the lack of entanglement between the macromolecules. Possible methods for improving this drawback were proposed.

Aggregation of interacting polyelectrolytes and sequence and conformations of polyampholytes

Srivastava, Devesh 01 January 1997 (has links)
The structure of aggregates formed by oppositely charged polyelectrolytes of the same chain length N and charge density and the kinetics of the formation of these aggregates has been studied using a dynamic Monte Carlo algorithm. The model considers the evolution of chains through local motion of monomers and exhibits the complex behavior associated with the formation of such aggregates. The chains interpenetrate in two distinct steps--the chains first diffuse towards each other and then collapse abruptly to form smaller aggregates. This abrupt collapse is a consequence of cooperativity in the aggregation process. The chains collapse and are considerably smaller in the aggregate than in isolation; the conformational properties of each of the two chains of the same chain length and charge density in the aggregate are the same. The radii of gyration of the chains in the aggregate and of the aggregate scale as $N\sp{1/2}$. Aggregates formed by longer chains initially scale as N. These aggregates continue to reorganize and eventually scale as $N\sp{1/2}$. However, it is possible that for sufficiently long chains, as in real experimental situations, this process of reorganization is hindered by the formation of entanglements etc. The aggregates would then form a non-equilibrium structure and would not scale as $N\sp{1/2}$. The conformation of randomly but specifically sequenced polyampholytes are determined by the interplay of the polyelectrolyte effect and the polyampholyte effect. The relative importance of the two is determined by the location of charges along the chain. Polyampholytes adopt extended conformations like self avoiding walk at high temperatures and collapsed structures at low temperatures. This difference arises from differences in the location of charges along the chain. Differences in the location of charges along the chain also lead to differences in the conformational spectrum and in the energy spectrum of the polyampholytes.

Spectroscopic studies of short/long range ordering in polymers

Chien, Bert 01 January 1997 (has links)
The work presented in this thesis is divided into three chapters. In Chapter 1 a general overview is presented of short/long range ordering in polymers. Chapter 2 then discusses the use of vibrational spectroscopy to probe the short range conformational order present in a series of Langmuir films at the air-water interface. Finally, the use of vibrational spectroscopy to probe the long range conformational order in a bulk polymer, poly($\beta$-hydroxybutyrate) is presented in Chapter 3. Using external reflection infrared spectroscopy, we were able to directly characterize the structure of series of "hairy-rod" polymers at the air-water interface. The overall structure of the films were related to the short range conformational order of long side groups attached to the rigid-rod backbones. The structure of the films were also studied as a function of surface packing density at the air-water interface by the use of a Langmuir trough coupled directly to the external reflection infrared spectroscopy setup. A Raman active longitudinal acoustic mode capable of probing the long range conformational order associated with crystalline regions was discovered for the biodegradable thermoplastic, poly($\beta$-hydroxybutyrate). The dependence of this mode on the fold length of crystalline lamella was clearly established. The annealing behavior of single crystals of poly($\beta$-hydroxybutyrate) was also studied.

Page generated in 0.0676 seconds