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Polymer laboratory: teaching polymer chemistry

Ferreira, Olga; Plasencia, Paula; Afonso, Maria João A.P.S.; Silva, A.; Barreiro, M.F.
Fonte: Pixel Publicador: Pixel
Tipo: Conferência ou Objeto de Conferência
ENG
Relevância na Pesquisa
67.889146%
This work aims to present the results achieved within the science communication project CV/PVI/1386-Laboratório de Polímeros (Polymer Laboratory), financed by Ciência Viva - Agência Nacional para a Cultura Científica e Tecnológica (2006-2008). Several demonstration/experimental kits were developed concerning three main thematics: (1) Polymer concepts; (2) Polymer synthesis and (3) Polymer properties. Additionally, a fourth module entitled “Technological aspects” was also put into practice by taking advantage from the LSRE-IPB installed capacity in the area of Polymer Chemistry. The various modules were designed to reach a wide target public, including students from primary and secondary schools to undergraduate levels, and general society. Polymer architecture (linear, branched and crosslinked) and polymer synthesis procedures (step versus chain growth polymerization) can be demonstrated by using the “Kit-models” to construct simplified molecular models that illustrate those concepts. Synthesis examples include the preparation of linear polymers and networks. In the first case, Nylon 6,6 synthesis is proposed and, in the second one, a polyurethane foam and a PVA polymer slime can be experienced. Among polymer properties...

Polymer matrix sensitizing effect on photoluminescence properties of Eu(3+)-beta-diketonate complex doped into poly-beta-hydroxybutyrate (PHB) in film form

KAI, Jiang; PARRA, Duclerc Fernandes; BRITO, Hermi Felinto
Fonte: ROYAL SOC CHEMISTRY Publicador: ROYAL SOC CHEMISTRY
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
47.635244%
In this work is reported the sensitization effect by polymer matrices on the photoluminescence properties of diaquatris(thenoyltrifluoroacetonate)europium(III), [Eu(tta)(3)(H(2)O)(2)], doped into poly-beta-hydroxybutyrate (PHB) with doping percentage at 1, 3, 5, 7 and 10% (mass) in film form. TGA results indicated that the Eu(3+) complex precursor was immobilized in the polymer matrix by the interaction between the Eu(3+) complex and the oxygen atoms of the PHB polymer when the rare earth complex was incorporated in the polymeric host. The thermal behaviour of these luminescent systems is similar to that of the undoped polymer, however, the T(onset) temperature of decomposition decreases with increase of the complex doping concentration. The emission spectra of the Eu(3+) complex doped PHB films recorded at 298 K exhibited the five characteristic bands arising from the (5)D(0) -> (7)F(J) intraconfigurational transitions (J = 0-4). The fact that the quantum efficiencies eta of the doped film increased significantly revealed that the polymer matrix acts as an efficient co-sensitizer for Eu(3+) luminescent centres and therefore enhances the quantum efficiency of the emitter (5)D(0) level. The luminescence intensity decreases, however...

Tailored SERS substrates obtained with cathodic arc plasma ion implantation of gold nanoparticles into a polymer matrix

Ferreira, Jacqueline; Teixeira, Fernanda S.; Zanatta, Antônio Ricardo; Salvadori, Maria Cecilia Barbosa da Silveira; Gordon, Reuven; Oliveira Junior, Osvaldo Novais de
Fonte: ROYAL SOC CHEMISTRY; CAMBRIDGE Publicador: ROYAL SOC CHEMISTRY; CAMBRIDGE
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
47.597793%
This manuscript reports on the fabrication of plasmonic substrates using cathodic arc plasma ion implantation, in addition to their performance as SERS substrates. The technique allows for the incorporation of a wide layer of metallic nanoparticles into a polymer matrix, such as PMMA. The ability to pattern different structures using the PMMA matrix is one of the main advantages of the fabrication method. This opens up new possibilities for obtaining tailored substrates with enhanced performance for SERS and other surface-enhanced spectroscopies, as well as for exploring the basic physics of patterned metal nanostructures. The architecture of the SERS-active substrate was varied using three adsorption strategies for incorporating a laser dye (rhodamine): alongside the nanoparticles into the polymer matrix, during the polymer cure and within nanoholes lithographed on the polymer. As a proof-of-concept, we obtained the SERS spectra of rhodamine for the three types of substrates. The hypothesis of incorporation of rhodamine molecules into the polymer matrix during the cathodic arc plasma ion implantation was supported by FDTD (Finite-Difference Time-Domain) simulations. In the case of arrays of nanoholes, rhodamine molecules could be adsorbed directly on the gold surface...

Bacteria repelling poly(methylmethacrylate-co-dimethylacrylamide) coatings for biomedical devices† †Electronic supplementary information (ESI) available: Polymer microarray screening, including analysis of bacterial adhesion by fluorescence microscopy and SEM, and chemical composition of bacteria repelling polymers identified in the screen; polymer synthesis and characterisation; preparation of catheter pieces and solvent studies, and details for confocal imaging/analysis. See DOI: 10.1039/c4tb01129e Click here for additional data file.

Venkateswaran, Seshasailam; Wu, Mei; Gwynne, Peter J.; Hardman, Ailsa; Lilienkampf, Annamaria; Pernagallo, Salvatore; Blakely, Garry; Swann, David G.; Gallagher, Maurice P.; Bradley, Mark
Fonte: Royal Society of Chemistry Publicador: Royal Society of Chemistry
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
47.378447%
A polymer microarray screen identified poly(methylmethacrylate-co-dimethylacrylamide) as a bacteria repelling surface, which substantially reduced binding of bacteria on coated catheters.

Polymer Amide as an Early Topology

McGeoch, Julie E. M.; McGeoch, Malcolm W.
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
EN_US
Relevância na Pesquisa
47.63145%
Hydrophobic polymer amide (HPA) could have been one of the first normal density materials to accrete in space. We present ab initio calculations of the energetics of amino acid polymerization via gas phase collisions. The initial hydrogen-bonded di-peptide is sufficiently stable to proceed in many cases via a transition state into a di-peptide with an associated bound water molecule of condensation. The energetics of polymerization are only favorable when the water remains bound. Further polymerization leads to a hydrophobic surface that is phase-separated from, but hydrogen bonded to, a small bulk water complex. The kinetics of the collision and subsequent polymerization are discussed for the low-density conditions of a molecular cloud. This polymer in the gas phase has the properties to make a topology, viz. hydrophobicity allowing phase separation from bulk water, capability to withstand large temperature ranges, versatility of form and charge separation. Its flexible tetrahedral carbon atoms that alternate with more rigid amide groups allow it to deform and reform in hazardous conditions and its density of hydrogen bonds provides adhesion that would support accretion to it of silicon and metal elements to form a stellar dust material.

Unusual molecular architectures in liquid crystal and polymer chemistry

Paraskos, Alexander John, 1969-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 2 v. (333 leaves); 9440291 bytes; 9440092 bytes; application/pdf; application/pdf
ENG
Relevância na Pesquisa
57.62361%
This dissertation details the synthesis and characterization of materials that consist of molecules with unusual shapes. We have pursued this goal into the regimes of both small molecules (liquid crystals) and polymeric materials. In both cases, the ultimate goal is the creation of materials that display unique properties that arise due to molecular organizations and/or interactions driven by the underlying shape of the molecules or subunits. Chapter One is an introduction to the study of liquid crystals and their phases. Chapter Two and Three both describe the synthesis and characterization of bent-core tetracatenar liquid crystals. The focus in Chapter Two is on the effects that changing the bend angle and/or lateral dipole have upon the phase behavior of these compounds. We found that the thiophene-based mesogens with the largest lateral dipoles display the most stable liquid crystalline phases. We believe that this effect is due to the formation of antiparallel dimers within the phases of these compounds. Chapter Three describes the synthesis and phase behavior of thiophene-based liquid crystals with desymmetrized cores. Reduction of the symmetry had the effect of either broadening or narrowing the resulting phases, depending on the types of endgroups attached to the molecular core. Chapter Four details the synthesis and phase-behavior of triphenylene-based dione liquid crystals. These molecules are roughly half-discs in terms of molecular shape...

Discrete polyene oligomer synthesis and related polymer characterization : (Appendix I: Reduced tungsten dimeric compounds)

Hafer, Jillian E. (Jillian Elizabeth)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 99 leaves
ENG
Relevância na Pesquisa
47.365703%
Molybdenum complexes ... were used to synthesize polyene oligomeric compounds with up to 15 conjugated double bonds by reaction with aldehyde compounds in a Wittig-like fashion. The optical properties of highly nr-conjugated systems are investigated due to potential application in photooptical materials. To this end, discrete oligomers have been synthesized in order to study properties of the varying lengths and to what extent solvent and substitution patterns affect these properties. Trends have been established based on solvent polarity and steric bulk at the ester functionality. Extrapolation of the data here yields pertinent information to the polymer field since the oligomers mimic polymeric materials made by RCMP of 1,6-heptadiyne substrates that are fully five-membered ring containing structures with a disubstitution pattern of ester functionalities.; by Jillian E. Hafer.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.; Includes bibliographical references (leaves 96-97).

Two-dimensional polymer synthesis : towards a two-dimensional replicating system for nanostructures; 2-D polymer synthesis : towards a 2-D replicating system for nanostructures

Mosley, David W
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 162 leaves
ENG
Relevância na Pesquisa
47.365703%
The general concept of a replicating monolayer system is introduced as a new method of nanostructure synthesis. One possible implementation of a 2-D replicating system is pursued which uses a diacetylene moiety for cross-linking and amide hydrogen bonding for molecular recognition between replicates and templates. The synthesis of several monomers for amide hydrogen-bonded adlayer formation is described. The assembly and crosslinking of diacetylene monomers on an underlying amide-capped self- assembled monolayer (SAM) was studied on unpatterned thermally evaporated gold films. Raman and Fourier-transform infrared spectroscopies, as well as ellipsometry and contact angle data, indicate that amide hydrogen bonding interactions are sufficient to organize an adlayer of diacetylene-containing molecules on the underlying SAM which can be polymerized with ultraviolet light. In order to obtain gold substrates suitable for cross-linking of bis(diacetylene) monomers, new methods of producing ultraflat gold surfaces were developed.; (cont.) A solid- state bonding technique using only gold was developed, yielding ultraflat gold surfaces, with root-mean-square roughnesses of -0.5 nm, on glass slides which are free of impurities from epoxies or other bonding agents. The patterning and cross-linking of poly(diacetylene) adlayers on ultraflat gold surfaces was investigated by atomic force microscopy (AFM). Soft lithography was suitable for adlayer structures down to about 500 nm. Electron beam lithography for patterning of polymerizable adlayers was demonstrated for the first time. The polymerized adlayer patterns were significantly more difficult to remove from the gold surface than unpolymerized adlayer patterns...

Polymer formation, deactivation, and ethylene selectivity decline in palladium/aluminum oxide catalyzed selective acetylene hydrogenation

LeViness, Stephen Claude
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
47.479683%
The removal of acetylene by selective hydrogenation is a critical step in the purification of ethylene streams for industrial polyethylene production. Hydrogenation of 0.35-1.0% acetylene in ethylene over Pd/Al$sb2$O$sb3$ catalysts is accompanied by the formation of significant amounts of surface oligomers/polymers. During the initial stages of CSTR operations at 40-120$spcirc$C and P = 1 atm these accounted for 8 to 50% of the acetylene consumed. Ethylene selectivities calculated ignoring these polymers are incorrect. Approximately 30% of these products are volatile and soluble in hydrocarbon solvents. They consist of even carbon numbered chains from C$sb8$ to at least C$sb{30}$. Normal paraffins are the major species at each carbon number, although branched paraffins, linear and branched mono-and di-olefins, and alkylbenzenes are also produced. The remainder is nonvolatile and insoluble in any known solvent; its degradation products are consistent with those of polyacetylenes of carbon number greater than 24. A mechanism involving polymerization in regions of low surface hydrogen concentrations and terminated by hydrogenation is proposed. The accumulation of the liquids in catalyst pores imposes diffusion limitations on the acetylene reaction; both the rate of acetylene consumption and its selectivity to ethylene decrease. These effects are reversible upon removing the liquid polymers. The decrease in surface polymer selectivity observed during operation is compensated by an increase in gas phase oligomer selectivity; the total oligomer/polymer selectivity does not change appreciably. In conjunction with previous investigations showing nearly constant ethane production only from acetylene...

Nanoparticles as strengthening agents in polymer systems

Shahid, Naureen
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
47.66455%
Carboxylate-substituted alumina nanoparticles are produced solvent free using mechanical shear. The general nature of this method has been demonstrated for L-lysine-, stearate, and p-hydroxybenzoate-derived materials. The reaction rate and particle size is controlled by a combination of temperature and shear rate. The nanoparticles are spectroscopically equivalent to those reported from aqueous syntheses, however, the average particle size can be decreased and the particle size distribution narrowed depending on the reaction conditions. Lysine and p-hydroxybenzoato alumoxanes have been introduced in carbon fiber reinforced epoxide resin composites. Different preparation conditions have been studied to obtain composite with enhanced performances that are ideal for the motor sports and aerospace industries. A new composite material has been fabricated utilizing surface-modified carboxylate alumoxane nanoparticles and the biodegradable polymer poly(propylene fumarate)/poly(propylene fumarate)-diacrylate (PPF/PPF-DA). For this study, composites were prepared using various functional groups including: a surfactant alumoxane to enhance nanoparticle dispersion into the polymer; an activated-alumoxane to enhance nanoparticle interaction with the polymer matrix; a mixed alumoxane containing both activated and surfactant groups. Nanocomposites prepared with all types of alumoxane...

Mechanical properties of fibers made from single-walled carbon nanotube reinforced thermoplastic polymer composites

McIntosh, Daneesh Olivia
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
47.258525%
Single-walled carbon nanotubes are high strength materials with nano-sized dimensions, which make them attractive as reinforcing materials for polymer matrices. Their nano-sized dimensions however make them difficult to manipulate for macroscopic applications. This thesis focuses on manufacturing and studying nanotube continuous fibers as a means of manipulating these stronger reinforcing materials in thermoplastic polymer matrices to increase the mechanical properties of the composite generated. The thermoplastic polymers used in this thesis are polypropylene and nylon 6,6. This thesis demonstrates the effects of alignment, new chemical functionalizations and increasing weight percentages of the single-walled carbon nanotubes in the nanotube continuous fiber systems. The functionalizations studies include the use of fluorinated single-walled nanotubes, and an attempt to use benzoyl peroxide to initiate covalent bonding of single-walled nanotubes to the polymer in the processing stage of the polypropylene composite systems. (Abstract shortened by UMI.)

Triplet exciton dynamics in conjugated polymer films

Hale, Gregory D.
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
47.374814%
A thorough knowledge of the excited state dynamics in conjugated polymers is necessary for efficient use of these novel materials. Triplet excitons are believed to play an important role in conjugated polymer-based device operation. Triplet excitons are also believed to play a primary role in photo-oxidative degredation of polymer devices. Two-photon photoemission is used to probe triplet exciton relaxation dynamics in pristine conjugated polymer films and triplet-triple annihilation is determined to be an important deexcitation mechanism. Controlled photo-oxidation of conjugated polymer films is studied using two-photon photoemission. The triplet exciton-derived photoemission peak is observed to shift and narrow dramatically upon photo-oxidation. The peak narrowing is attributed to narrowing of the distribution of polymer conjugation lengths in the film. The peak shift is attributed to formation of defects at the ends of the polymer chains which change the average triplet exciton energy.

Protein crystals as templates for materials chemistry

Turner, Mary Ellen
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
47.530215%
This work describes the first use of macromolecular biological crystals as templates for materials synthesis. The macromolecular crystal itself offers unique micro- and mesoporous structure with specific binding sites for molecules. This work utilizes this porous structure for the design of polymer and metal loaded nanocomposites. The crystals are first strengthened with intermolecular covalent cross-links. The sturdy crystals then serve as scaffolds for the infusion of polymers or the growth of metallic nanoparticles. Polymer infused and encased Hen Egg White Lysozyme (HEWL) crystals show reproducible diffraction for many months to a resolution of 4A. Metal nanoparticles can be grown in HEWL crystals using an electroless plating technique in which palladium ions bound to amino acids are subsequently reduced and form a catalytic surface for the deposition of platinum metal. If these nanoparticles are grown in a macromolecular crystal with larger pores, such as Cowpea Mosaic Virus, then the protein structure effectively guides the growth of the nanoparticles resulting in monolithic, highly ordered nanocomposites. These novel patterned materials could find use as x-ray optics, sensors, or x-ray diffraction standards.

Single-Walled Carbon Nanotubes (SWNT) polymer composites & composite fibers

Chen, Zheyi
Fonte: Universidade Rice Publicador: Universidade Rice
Tipo: Thesis; Text Formato: 122 p.; application/pdf
ENG
Relevância na Pesquisa
47.308057%
With their potentially extraordinary mechanical, thermal, and electrical properties, Single-Walled Carbon Nanotubes (SWNT)/polymer composites and composite fibers may be the ultimate building blocks for next generation ultra-light-weight, ultra-high performance structural applications. However, the dispersity and processibility of SWNT in polymer matrices have been a challenge because of the strong van der Waal attraction between individual nanotubes and their chemical inertness. Predicated on oleum's (100% H2SO 4 with excess SO3) ability to intercalate between individual SWNT inside SWNT ropes, two types of reinforcing SWNT with much improved solubility and dispersity in common solvents were developed: supra-roped SWNT (SWNT-R) and soluble, ultra-short (length<60 nm), carboxylated SWNT(US-SWNT). SWNT-R hold much improved dispersity in super acid and other solvents, and can facilitate the processing of SWNT/polylmer composites and composites fibers. US-SWNT exhibit up to 2 wt% solubility in common solvents. The availability of SWNT-R and US-SWNT open the opportunities for forming high performance composites, blends, and copolymers without inhibiting their processibility. Studies on the synthesis, processing, properties, and morphology of SWNT-R or US-SWNT/polymer composites and composite fibers have demonstrated the reinforcement efficacy of these SWNT in typical thermoset...

Controlling the architectures and optical properties of conjugated polymer aggregates and films

Satrijo, Andrew
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 186 p.
ENG
Relevância na Pesquisa
47.91118%
The semiconducting properties of conjugated polymers are finding use in various optoelectronic applications, including chemical sensors and light-emitting diodes. In this thesis, we investigate aggregation in conjugated polymers and how it affects the optical properties of these organic materials. We discuss how aggregation enhances exciton transport properties in fluorescent polymers, thereby increasing the probability of excitons reaching low-energy sites in the polymer. A consequence of this aggregation-enhanced exciton migration is that low-energy defect sites in a conjugated polymer can dramatically alter the polymer's fluorescence properties when it is in an aggregated state. In a poly(pphenylene ethynylene) (PPE) that was previously proposed to form green-emitting excimers, we found that a small concentration of anthryl defects in the polymer emitted green fluorescence that was only noticeable when the polymer was in an aggregated state (otherwise the polymer was fluorescent blue). After elucidating the origin of the green fluorescence, we purposely added more emissive anthryl units into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation.; (cont.) Using this anthryl-doped conjugated polymer...

Designing conjugated polymer-based functional materials via the incorporation of supramolecular complexities

Kwan, Phoebe Hoi-Ying, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 211 leaves
ENG
Relevância na Pesquisa
47.63145%
One of the major goals in molecule engineering is the creation of molecule- or polymer- based devices that mimic the integrated functions of their macroscopic (and often inorganic) counterparts. Because of their unique photophysical and/or redox properties, supramolecular systems and conjugated polymers (CPs) have emerged as the ideal candidates for such applications. This dissertation details the design and synthesis of various CPs with pendant supramolecular complexes. The marriage of these two materials should yield complex and unique properties that are not accessible from single-molecule systems. Chapter One is an introduction on conjugated polymers and their properties. The ease of synthetic modifications, coupled with their unique transport properties make CPs very desirable for the design of sensitive chemosensors. The incorporation of rotaxane moieties to poly(p-phenyleneethynylene)s will be described in Chapter Two. By design, the rotaxane groups behave as molecular recognition elements for the detection of small molecules. In addition, these bulky scaffolds prevent polymer aggregation. We turn our focus to the rotaxane monomers in Chapter Three. We investigate the photoinduced charge transfer interactions between donor and acceptor pairs within the rotaxane scaffold and examine the parameters that facilitate these interactions. Chapter Four describes our effort to encapsulate electroactive polymers using these bulky groups. As expected...

Probing dispersion and re-agglomeration phenomena upon melt-mixing of polymer-functionalized graphite nanoplates

Santos, R. M.; Vilaverde, C.; Cunha, E.; Paiva, M. C.; Covas, J. A.
Fonte: Royal Society of Chemistry Publicador: Royal Society of Chemistry
Tipo: Artigo de Revista Científica
Publicado em 25/09/2015 ENG
Relevância na Pesquisa
47.258525%
A one-step melt-mixing method is proposed to study dispersion and re-agglomeration phenomena of the as-received and functionalized graphite nanoplates in polypropylene melts. Graphite nanoplates were chemically modified via 1,3-dipolar cycloaddition of an azomethine ylide and then grafted with polypropylene-graft-maleic anhydride. The effect of surface functionalization on the dispersion kinetics, nanoparticle re-agglomeration and interface bonding with the polymer is investigated. Nanocomposites with 2 or 10 wt% of as-received and functionalized graphite nanoplates were prepared in a small-scale prototype mixer coupled to a capillary rheometer. Samples were collected along the flow axis and characterized by optical microscopy, scanning electron microscopy and electrical conductivity measurements. The as-received graphite nanoplates tend to re-agglomerate upon stress relaxation of the polymer melt. The covalent attachment of a polymer to the nanoparticle surface enhances the stability of dispersion, delaying the re-agglomeration. Surface modification also improves interfacial interactions and the resulting composites presented improved electrical conductivity.; Project Matepro – Optimizing Materials and Processes, with reference NORTE-07- 0124-FEDER-000037 by the Programa Operacional Regional do Norte (ON.2); Portuguese Foundation for the Science and Technology (FCT) for PEst-C/CTM/LA0025/2013

Ring-polymer approaches to instanton theory

Richardson, Jeremy O.
Fonte: University of Cambridge; Department of Chemistry Publicador: University of Cambridge; Department of Chemistry
Tipo: Thesis; doctoral; PhD
EN
Relevância na Pesquisa
47.398374%
Inspired by the success of the ring-polymer molecular dynamics (RPMD) method, we derive a transition-state-theory version (RPTST) with a dividing surface which is, in general, conical in ring-polymer space. It is explained why this conical form is a good approximation to the optimal dividing surface and therefore why centroid-based quantum transition-state theories are inaccurate for asymmetric barriers at low temperatures. The geometry of the ring-polymer transition state is found to describe a finite-difference approximation to the semi-classical instanton trajectory (a classical periodic orbit of length ?? on the inverted potential). Based on this, a new practical method for locating multidimensional instantons is proposed, by computing saddle points on the ring-polymer surface, and a derivation for the reaction rate constant based on the "ImF" premise using the ring-polymer formalism is shown to be far simpler than in previous instanton approaches based on functional determinants. The resulting expression is based only on the ring-polymer potential at the transition-state and its Hessian, and is applied to evaluate the rate in a number of polyatomic systems. We show that a free-energy version of the ImF instanton theory is related to RPTST and thereby provide an explanation for why RPMD produces accurate results for thermal reaction rates in the deep-tunnelling regime and demonstrate how it can be made more efficient and systematically improved. From this...

Polymer Remodeling Enabled by Covalent Mechanochemistry

Ramirez, Ashley Lauren Black
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Dissertação
Publicado em //2013
Relevância na Pesquisa
57.707466%

Material failure is a ubiquitous problem, and it is known that materials fail at much lower stresses than the theoretical maximum calculated from the number and strength of the individual bonds along the material cross-section. The decreased strength is attributed to inhomogeneous stress distributions under load, thus causing the stress to accumulate at localized regions, initiating microcrack formation and subsequent propagation. In many cases, these initiation and propagation steps involve covalent bond scission.

Over the past decade there has been increased interest in channeling the mechanical forces that typically trigger destructive processes (e.g., chain scission) during use into constructive chemical transformations. In an ideal system, these stress-induced chemical transformations would redistribute load prior to material failure, thus extending material lifetime. In this Dissertation, the work of developing constructive transformations through the response of a small molecule "mechanophore" is discussed.

The gem-dihalocyclopropane mechanophore is capable of undergoing a non-scissile electrocyclic ring opening reaction under molecular scale tensile load. The mechanochemistry is demonstrated both in solution via pulsed ultrasound (Chapter 2) and in the bulk via extrusion and uniaxial tension (Chapter 3). In solution...

Probing Mechanical Activation of Covalent Chemistry in Crosslinked Polymer Gels

Wang, Yifei
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Tese de Doutorado
Publicado em //2013
Relevância na Pesquisa
47.793423%

Toughness, the measure of how much energy a material can absorb before rupture, is an important property of materials. It has been demonstrated that the toughness of a single polymer chain of gem-dihalocyclopropane (gDHC) functionalized polybutadiene (PB) is increased dramatically over PB alone, due to the mechanically triggered electrocyclic ring opening reaction of gDHC into 2,3-dibromoalkenes. This thesis explores whether this molecular mechanical property can also be manifested in bulk material properties. Crosslinked gem-dichlorocyclopropane (gDCC) embedded PB polymers were swollen in various solvents, and the resulting gels were mechanically deformed under tensile stress. Young's modulus and fracture toughness were compared among PBs with gDCC incorporated in the backbones and/or crosslinking positions. The results showed that the incorporation of gDCC does not measurably increase the fracture toughness of the crosslinked polymer gels. Neither NMR nor FT-IR characterization of the post-test samples revealed detectable activation of the gDCC in the crosslinked PB. Further experiments will be focused on optimizing the polymer structure and testing methods to more effectively transfer the macroscopic force to the mechanophore in the material and continuing exploring the correlation between molecular responses and changes in macroscopic properties.

; Thesis