Void-free phenolic networks have been prepared by the reaction of phenolic novolac resins with various diepoxides. The stoichiometric ratio can be adjusted to achieve networks with good mechanical properties while maintaining excellent flame retardance. A series of linear, controlled molecular weight, 2,6-dimethylphenol endcapped cresol novolac resins have been synthesized and characterized. The molecular weight control was achieved by adjusting the stoichiometric ratio of cresol to 2,6-dimethylphenol and using an excess of formaldehyde. A dynamic equilibrium reaction was proposed to occur which allowed the targeted molecular weight to be obtained. A 2000 g/mol ortho-cresol novolac resin was crosslinked by a diepoxide oligomer and by an epoxidized phenolic oligomer in defined weight ratios and the structure-property relationships were investigated. The networks comprised of 60 or 70 weight percent cresol novolac exhibited improved fracture toughness, high glass transition temperatures, low water uptake, and good flame retardance. The molecular weights between crosslinks were also determined for these networks. The stress relaxation moduli were measured as a function of temperature near the glass transition temperatures. Crosslink densities as well as the ability to hydrogen bond affect the glassy moduli of these networks. Rheological measurements indicated that cresol novolac/epoxy mixtures have an increased processing window compared to phenolic novolac/epoxy mixtures. Maleimide functionalities were incorporated into cresol novolac oligomers, and these were crosslinked with bisphenol-A epoxy. The processability of oligomers containing thermally labile maleimides were limited to lower temperatures. However, sufficiently high molecular weight oligomers were necessary to obtain good network mechanical properties. Networks prepared from 1250 g/mol cresol novolac containing maleimide functionilities and epoxy exhibited good network properties and could be processed easily. Latent triphenylphosphine catalysts which are inert at processing temperatures (~140°C) but possess significant catalytic activity at cure temperatures 180-220°C were necessary for efficient composite fabrication using phenolic novolac/epoxy matrix resins. Both sequestered catalyst particles and sizings were investigated for this purpose. Phenolic novolac/epoxy mixtures containing sequestered catalysts exhibited significantly longer processing time windows than those containing free catalysts. The resins also showed accelerated reaction rates in the presence of sequestered catalysts at cure temperatures. Trihexylamine salt of a poly(amic acid) was sized onto reinforcing carbon fibers and the composite properties indicated that fast phenolic novolac/epoxy cure could be achieved in its presence. / Ph. D.

Phenylindane monomers 5(6)-amino-1-(4-aminophenyl)-1,3,3-trimethylindane (DAPI), 5,6-diamino-1-(4-aminophenyl)-1,3,3-trimethylindane (TAPI) and 6-hydroxy-1-(4-hydroxyphenyl)-1,3,3-trimethylindane (DHPI) were synthesized and characterized. DAPI, as well as other diamines, were then utilized in solution step polycondensation with a number of commercially available dianhydrides using either the two-step ester-acid solution imidization or the high temperature solution imidization routes. High molecular weight soluble fully cyclized polyimides were successfully synthesized using a 1:1 molar ratio of dianhydride to diamine. The polyimides were film forming and were characterized by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and selective gas permeation methods, as well as other techniques. The O2 permeation and O2/N2 selectivity values obtained for materials prepared in this thesis are discussed in relation to the concept of an "upper bound", as defined in the literature concerning gas separation membranes. The series of polyimides based on DAPI and several dianhydrides were found to have high glass transition temperatures (247°C-368°C) and very good short-term thermal stability as shown by TGA, despite the partially aliphatic character of DAPI. The 5,5'-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzenefurandione (6FDA)/DAPI system also exhibited low weight loss under nitrogen at 400°C, which was comparable to that of a wholly aromatic polyimide based on 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA)/4,4'-oxydianiline (ODA) which is known to have high thermal stability. In addition, the 6FDA/DAPI polyimides had a refractive index value of 1.571 from which the dielectric constant was calculated, giving an attractively low estimated value of 2.47. The rigid, bulky and isomeric structure of DAPI in the repeat unit imparted film forming characteristics that allowed production of solvent cast membranes which displayed a range of O2 permeability and O2/N2 selectivity characteristics. High O2 permeabilities were observed for polyimides in which the DAPI structure predominated in relation to the overall polymer repeat unit, i.e. in combination with low molar mass dianhydrides. The more flexible dianhydrides afforded a greater degree of molecular freedom and were thought to result in a more tightly packed polymer conformation which decreased the rate of gas penetration through thin films. The DAPI/3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) system showed the best combination of O2 permeability and O2/N2 selectivity values (2.8Ba and 7.3, respectively). Modest variations in the DAPI isomeric ratio did not significantly effect the gas permselectivity properties. High molecular weight polyimides based on DAPI and BTDA were synthesized by three different routes. The ester-acid and thermal imidization methods produced polyimides with the highest Tgs and best thermal stability in air, as compared to the chemical imidization procedure. For example, a Tg increase of 22°C and a 68°C increase in the 5% weight loss were found for the ester-acid imidized DAPI/BTDA polyimide over those found for the chemically imidized version. The higher Tg and 5% weight loss values were attributed to the elimination of residual uncyclized amide acid moieties. Polyimides derived from 6FDA were synthesized by the high temperature solution imidization method. Thin films, cast from NMP, were tough and creasable and afforded high Tg (>295°C) systems with good thermal stability. When combined with rigid diamines, 6FDA contributed to high O2 permeation and moderate O2/N2 selectivity. The high O2 permeability was ascribed to hindered interchain packing attributed to the bulky CF3 groups. The exceptionally high oxygen permeability and O2/N2 selectivity values of the 9,9-bis(4-aminophenyl) fluorene (FDA)/6FDA system, were near the desirable "upper bound" for gas separation membrane materials, while those of 3,7-diamino-2,8-dimethyl-dibenzothiophene-5,5-dioxide (DDBT)/6FDA were actually above the upper bound. High performance polymers based on 4,4'-bis [4-(3,4-dicarboxyphenoxy)]biphenyl dianhydride (BPEDA), 2,2'-bis [4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride (BPADA), 2,2-bis(3-amino-4-methylphenyl)hexafluoroisopropylidene dianhydride (Bis-AT-AF) and 3,7-diamino-2,8-dimethyl-dibenxothiophene-5,5-dioxide (DDBT) were also synthesized in this work. Additionally, they were characterized with regard to molecular weight, glass transition temperature, and thermal stability. Polyimide systems containing hydroxyl moieties in the repeat unit were also investigated. Incorporation of hydroxyl moieties in the repeat unit enhanced chain stiffness via intermolecular hydrogen bonding and showed Tg increases of ~30°C Hydroxyl moieties also decreased the thermal stability values typically observed for polyimides. High O2/N2 selectivity was achieved with all of the 4,4'-diaminobiphenyl-3,3'-diol (HAB) containing polymers. However, these materials also had low O2 permeabilities, which suggested a tightly packed structure, possibly facilitated by hydrogen bonding. In contrast to suggestions in the literature, the comparison between a polyimide having pendant hydroxyl groups and another having the same repeat unit without them did not reveal a significant change in permselectivity behavior. The synthesis, characterization and crosslinking behavior of functional polyimides containing phenol, amine and acetylene moieties are also described. A crosslinking reaction of oligomers containing phenol moieties with a tetrafunctional epoxy resin was achieved 100°C below the "dry" glass transition temperature and was attributed to residual solvent. Utilization of this crosslinking mechanism could allow membrane optimization by investigating the influence of a number of variables, such as the concentration of the phenolic moiety, epoxy weight percent, catalyst concentration and residual solvent content. / Ph. D.

The focus of this dissertation was to investigate the influence of sidechain structure and sidechain content on the morphology and physical properties of perfluorosulfonic acid ionomer (PFSA) membranes. One of the primary objectives was to characterize the thermomechanical relaxations for short sidechain PFSAs developed by 3M and Solvay, as well as a new multi-acid sidechain perfluoroimide acid ionomer (PFIA) from 3M. Partial neutralization experiments played a key role in systematically manipulating the strength of the electrostatic interactions between proton exchange groups on each sidechain, leading to the elucidation of the molecular-level motions associated with multiple thermal relaxations observed by dynamic mechanical analysis (DMA). Particularly, 3M PFSA and Solvay Aquivion lack an observable β-relaxation in the sulfonic acid-form that is observed in the long sidechain PFSA, Nafion. By varying the strength of the physically-crosslinked network through exchanging the proton on the sulfonic acid groups for large counterions, we were able to conclude that the shorter sidechain length and increase in ion content in the 3M PFSA and Solvay Aquivion serves to restrict the mobility of the polymer backbone such that the onset of segmental motions of the main chains is not observed at temperatures below the α-relaxation temperature, where destabilization of the physically crosslinked network occurs. As a complementary technique to DMA for probing the relaxations in PFSAs, we introduced a new pretreatment method for differential scanning calorimetry (DSC) measurements that uncover a thermal transition in H+-form 3M PFSA, Aquivion, and Nafion membranes. This thermal transition was determined to be of the same molecular origin as the dynamic mechanical α-relaxation temperature in H+-form PFSAs, and the β-relaxation temperature in tetrabutylammonium (TBA+)-form PFSAs. The thermomechanical relaxations in multi-acid sidechain 3M PFIA were also investigated. Interestingly, the additional acidic site on PFIA led to unexpected differences in thermal and mechanical properties, including the appearance of a distinct glass transition temperature otherwise not seen in PFSA ionomers. We utilized small-angle X-ray scattering (SAXS) studies to probe the differences in aggregate structure between the PFIA and PFSA membranes in order to uncover the morphological origin of the anomalous thermomechanical behavior in PFIA membranes. Larger aggregate structures for PFIA, compared to PFSA, incorporate intervening fluorocarbon chains within the aggregate, resulting in increased spacing between ions that reduce the collective electrostatic interactions between ions such that the onset of chain mobility occurs at lower temperatures than the α-relaxation for PFSA. The SAXS profiles of PFSAs showed two scattering features resulting from scattering between crystalline domains and ionic domains distributed throughout the polymer matrix. In order to fit the "ionomer peak" to models used for the PFIA and PFSA aggregate structure determination, we presented a method of varying the electron density of the ionic domains by using different alkali metal counterions as a tool to make the intercrystalline feature indistinguishable. This allows for isolation of the ionomer peak for better fits to scattering models without any interference from the intercrystalline peak. Lastly, an investigation of annealing PFSAs of different sidechain structures in the tetramethylammonium (TMA+) counterion form above their α-relaxation showed a profound crystalline-like ordering of the TMA+ counterions within the ionic domains. This ordering is maintained after reacidification and leads to improved proton conductivity, which indicates that this method can be used as a simple processing method for obtaining improved morphologies in proton exchange membranes for fuel cell applications. / Doctor of Philosophy / Hydrogen fuel cells offer an environmentally friendly, high efficiency method for powering vehicles, buildings, and portable electronic devices. At the center of a hydrogen fuel cell is a polymer membrane that contains ionic functionalities, which conduct hydrogen ions (protons) from the anode to the cathode while preventing conduction of electrons. The electrons travel through an external circuit to produce electricity, while the protons travel through the polymer membrane and meet with oxygen on the other side to produce water, the only byproduct of a hydrogen fuel cell. The efficiency of this process relies on the ability of the polymer membrane to conduct protons, and the lifetime of a fuel cell depends on the mechanical stability of this membrane. Perfluorosulfonic acid ionomers are good candidates for use as polymer membranes in hydrogen fuel cells due to their Teflon backbone that provides mechanical stability and their sulfonic acid functionalities that form channels for proton conduction. In this work, we probe the structure-property relationships of different perfluorosulfonic acid ionomers for use as fuel cell membranes. We focus on thermal analysis techniques to develop a fundamental understanding of the effect of chemical structure and sulfonic acid content on the temperature-induced mobility of the polymer chains in these ionomers. This mobility at elevated temperatures can be utilized to rearrange the morphological structure of perfluorosulfonic acid ionomer membranes in order to enhance proton conductivity and mechanical integrity.

In 2012, a new type of lignin, catechyl (C)-lignin was found in the seed coat of vanilla orchid (Vanilla planifolia) and Melocactus cacti, and later in the nutshell of Aleurites moluccana (candlenut). This caffeyl alcohol homopolymer is the exclusive lignin in vanilla seed coat but separated in time and/or location with guaiacyl (G)-lignin in candlenut. Unlike conventional guaiacyl/syringyl (G/S-lignins) with alkyl-aryl ether linkages, intermonomer linkages in C-lignin are connected by benzodioxane linkages which are stiffer than alkyl-aryl ether linkages. C-lignin is unusually stable against acid-catalyzed cleavage. Tissues with C-lignin are expected to exhibit high glass transition temperature (Tg) compared to tissues with G/S/H-lignin. C-lignin also probably shows high crystallinity due to its highly linear-homopolymer structure. The ability of some seed coats/nutshells in angiosperms to synthesize a new type of lignin is another level of lignin evolution. However, the role of C-lignin related to the function of the seed coat is unclear while it exhibits different behaviors to the regular G/S/H-lignin. These points motivated us to conduct cell-wall structure/property studies in the context of plant evolution, using microscopy, X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). Light and electron microscopes were used to identify cell's size and type of intact and macerated vanilla seed coat and candlenut shell. Vanilla seeds are tiny, sized approximately 300μm and the surface is covered with dark-colored seed coat. Candlenut is slightly smaller than walnut, with uneven, hard, dark brown shell covering the nut. Microscopy observations indicated that both seed coat and nutshell are dominated by highly lignified cells, known as sclereids. The types of sclereids in vanilla seed coat and candlenut shell are different; vanilla seed coat has ostoesclereid-type cells, while candlenut shell has macrosclereid-type cells. XRD was used to study tissue with C-lignin crystallinity by comparing diffractograms of vanilla seed coat and candlenut shell to Southern Yellow Pine wood diffractograms. The Southern Yellow Pine wood diffractogram corresponds to a typical native cellulose in higher plants, that is cellulose I allomorph. Diffractogram XRD analysis on vanilla seed coat and candlenut shell shows similarities to Southern Yellow Pine native cellulose, suggesting that cellulose is the contributor for crystallinity in seed coat and nutshell, and this also indicated that tissues with C-lignin is not crystalline. Crystallinities of vanilla seed coat and candlenut shell determined using peak deconvolution methods were about half of Southern Yellow Pine crystallinity. DMA was used to measure Tg in vanilla seed coat and candlenut shell. Measurements were conducted in solvent-submersion mode using organic plasticizers to reduce the Tg to non-damaging temperatures. DMA measurement of vanilla seed coat and candlenut shell is challenging due to specimen size and shape. Specimen preparation for DMA measurement included seed coat purification for vanilla and cutting/milling for candlenut shell followed by specimen saturation in plasticizers. Compressive-torsion DMA was used to allow tiny specimens gripping. Vanilla seed coats exhibited higher glass transition temperature compared to wood, while candlenut shells exhibited various Tgs depending on specimen type/size. / Doctor of Philosophy / Lignin is a complex organic material that constructs higher plant cell walls. Lignin provides stiffness and strength and is the landmark of plant evolution to terrestrial life. Typically, lignin in hardwood/softwood has guaicayl and/syringyl (G/S) units derived from coniferyl/sinapyl alcohols. ln 2012, a new type of lignin, catechyl (C)-lignin, was found in the seed coat of vanilla orchid (Vanilla planifolia) and Melocactus cacti, and later in the nutshell of Aleurites moluccana (candlenut). C-lignin is a caffeyl alcohol homopolymer and is exclusive in vanilla seed coat but coexists with guaiacyl (G)-lignin in candlenut shells. This new type of lignin exhibits different behavior than G/S-lignin. C-lignin is unusually stable against acid-catalyzed hydrolysis. Intermonomer linkage in C-lignin is stiffer than G/S lignin(s); it is likely to have higher glass transition temperature (Tg) than normal lignin. Due to its linearity, tissue with C-lignin is also expected to be highly crystalline. C-lignin's roles are not well known and therefore, these are merit for structure/property studies in the context of plant evolution as bio-inspired new materials. Microscopy, X-ray diffraction (XRD), and dynamic mechanical analysis (DMA) were used to study vanilla seed coat and candlenut shell morphology, crystallinity, and glass transition temperatures (Tg), respectively. It was observed that the two tissues have different types of sclereids, but this is not associated with why vanilla seed coats exhibit only C-lignin while candlenut shells have both C /G-lignins. XRD scans revealed that C-lignin is not crystalline due to similarity of their diffractograms to those of wood. DMA measurements revealed that vanilla seed coat tissues exhibit higher Tg than tissue with G/S lignin as expected, while the Tg candlenut shells varied among specimen type and particle sizes.

A series of random and amorphous copolyesters containing different cycloaliphatic rings within the polymer chains were prepared by melt polycondensaton of difunctional monomers (diesters and diols) in the presence of a catalyst. These polyesters were characterized by nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile tests and/or dynamic mechanical analysis (DMA). The copolyester based on dimethyl bicyclo[2.2.2]octane-1,4-dicarboxylate (DMCD-2) was observed to have a higher Tg, about 115ºC, than the other copolyesters with the same compositions in this study. For copolyesters containing different compositions of dimethyl-1,4-cyclohexane dicarboxylate (DMCD) and DMCD-2, the Tg increased linearly with the increase of DMCD-2 mole content. DMA showed that all of the cycloaliphatic copolyesters had secondary relaxations, resulting from conformational transitions of the cyclohexylene rings. The polyester based on DMCD-3 in the hydrolytic tests underwent the fastest hydrolytic degradation among these samples. A new triptycene diol (TD) was synthesized and incorporated into a series of cycloaliphatic copolyester backbones by melt condensation polymerization. Straight chain aliphatic spacers, including ethylene glycol (EG), 1,4-butanediol (BD) and 1,6-hexanediol (HD), were used as co-diols to explore their effects on polyester properties. An analogous series of non-triptycene copolyesters based on various hydroxyethylated bisphenols were also prepared for comparison. The results revealed that the TD-containing polymers had higher thermal stability and higher Tg's than the corresponding non-TD analogs. For TD-containing copolyesters, the mechanical properties were found to be dependent on the types and compositions of the co-diols. A 1,4-butanediol-based triptycene copolyester was observed to have a significantly increased Tg and modulus while maintaining high elongation at ambient temperature. Furthermore, it was demonstrated that the triptycene polyester exhibited higher Tg and modulus than those containing bisphenol derivatives. However, all of the 1,4-butanediol based copolyesters were brittle and had comparable moduli at low temperatures (-25°C or -40 °C). Melt polycondensation was also used to prepare a series of all-aliphatic block and random copolyesters including the following aliphatic monomers: trans-DMCD, DMCD-2, neopentyl glycol (NPG), diethylene glycol (DEG) and dimethyl succinate (DMS). The polymer compositions were determined by 1H NMR, and the molecular weights were determined using SEC. The polyesters were also characterized by TGA, DSC, DMA and tensile tests. Phase separation was not observed in these block copolyesters. However, the block copolyester containing DMCD-2 and NPG was observed to have a higher Tg than the block copolyester based on trans-DMCD and NPG. In addition, these block copolyesters were found to have better mechanical properties than the corresponding random copolyesters. / Ph. D.

The use of amorphous solid dispersions (ASDs) is an effective and increasingly widely adopted approach for solubility and bioavailability enhancement of hydrophobic drugs. Cellulose derivatives have strong potential as ASD polymers. We demonstrate herein design, synthesis and structure-property relationship characterization of a new series of organo-soluble cellulose omega-carboxyalkanoates for ASDs, by two different synthetic approaches. These carboxyl-containing cellulose mixed-esters possessed relatively high Tg values with sufficient differences versus ambient temperature, useful to prevent drug mobility and crystallization during storage or transport. Screening experiments were utilized to study the impact of ASD polymers including our new family of cellulose Ω-carboxyesters on both nucleation induction time and crystal growth rate of three poorly soluble model drugs from supersaturated solutions. Attributed to relatively rigid structures and bulky substituent groups, cellulose derivatives were more significant crystallization inhibitors compared to the synthetic polymers. The effective cellulose omega-carboxyesters were identified as possessing a similar hydrophobicity to the drug molecule and high number of ionization groups. Among them, cellulose acetate suberate prepared by us was an extraordinary solution crystal growth inhibitor for ritonavir and its formulated solid dispersions provided a substantial 15-fold enhancement of apparent solution concentration vs. the equilibrium solubility of the crystalline drug. To offset the issue of slow drug release from some cellulose omega-carboxyester based formulations, a new class of amphiphilic cellulosic polymers with hydrophilic oligo(ethylene oxide)-containing side chains was developed via versatile synthetic pathways, and the evaluation of these materials alone or by pairwise polymer blends will be performed as ASD matrices for the enhancement of drug solubility and stability. / Ph. D.

Les mousses aqueuses sont des fluides complexes constitués de dispersions concentrées de bulles de gaz dans une solution de tensioactifs. A l'instar d'autres fluides complexes comme les émulsions ou les pâtes, une mousse se comporte comme un solide viscoélastique lorsque la fraction volumique de la phase continue est suffisamment faible pour que l'empilement des bulles soit bloqué. Ses propriétés mécaniques résultent de couplages entre processus se produisant à plusieurs échelles de temps et d'espace : celles des tensioactifs adsorbés aux interfaces liquide-gaz, celles d'une bulle de gaz ou de mouvements collectifs à une échelle mésoscopique. A partir de trois expériences, nous avons mis en évidence l'impact du désordre de leur structure d'une part, et celui des tensioactifs d'autre part, sur les propriétés viscoélastiques des mousses. Nous avons mis au point un rhéomètre oscillatoire qui permet de mesurer la relation contrainte-déformation-fréquence d'une monocouche de bulles confinées entre deux parois planes parallèles tout en contrôlant sa pression osmotique. Nous avons montré que les relaxations de ces mousses de structure modèle sont pilotées par la rhéologie interfaciale de cisaillement que nous avons caractérisée indépendamment. Nous proposons un modèle quantitatif de ce couplage. Dans une deuxième expérience, nous avons sondé la réponse viscoélastique des mousses de structure 3D désordonnées. Nos résultats montrent que selon la rigidité des interfaces, le facteur de perte d'une mousse est décrit par une loi d'échelle en fréquence. Son évolution avec la taille des bulles et la viscosité du liquide permet de déterminer le mécanisme à l'origine de la dissipation. Dans une troisième expérience, Nous avons élaboré des mousses monodisperses de structure 3D ordonnées et de pression osmotique contrôlée. De manière remarquable, la variation de leur facteur de perte en fonction de la fréquence est similaire à celle des mousses désordonnées de même composition chimique. Ces résultats démontrent que le désordre de l'empilement des bulles n'est pas à l'origine des relaxations viscoélastiques linéaires des mousses, comme l'avaient suggéré plusieurs modèles théoriques, et ouvrent la voie à une modélisation quantitative du lien entre la viscoélasticité des interfaces et celle des mousses 3D / Aqueous foams are constituted of concentrated gas bubble dispersions in a surfactant solution. Like other complex fluids, such as emulsions or pastes, foam behaves as a viscoelastic solid if the volume fraction of the continuous phase is sufficiently small for the bubble packing to be jammed. The mechanical properties of the foam are due to couplings between processes at a wide range of time and length scales: The ones of the surfactant molecules that are adsorbed to the gas-liquid interfaces, the ones of the bubbles or collective motions at a mesoscopic scale. On the basis of three experiments, we have evidenced the impact of structural disorder and surfactant properties on foam viscoelasticity. We have constructed an oscillatory rheometer to measure the frequency and strain dependent stress response of a bubble monolayer confined between two parallel plates, subjected to an imposed osmotic pressure. We have shown that the relaxation of these model foams are governed by the interfacial shear rheology which we have probed in independent experiments and, we present a quantitative model of this coupling. In a second experiment, we have probed the viscoelastic response of disordered 3D foams. Our results show that, depending on interfacial rigidity, the mechanical loss factor of a foam is described by a scaling law depending on frequency. Its dependence on bubble size and liquid viscosity helps to determine the origin of the dissipation. In our third experiment, we have produced monodispersed ordered foams, subjected to a controlled osmotic pressure. Remarkably, the frequency scaling of their loss factor is similar to the one of disordered foams of the same chemical composition. These results demonstrate that the linear viscoelastic response of foams is not the consequence of disorder on the bubble scale as suggested by several previous theories, and they thus open the way for quantitative models linking the viscoelasticity of the interfaces to that of 3D foams

Master of Science / Department of Chemical Engineering / Jennifer L. Anthony / A polymer has drastically different physical properties above versus below some characteristic temperature. For this reason, the precise identification of this glass transition temperature, T[subscript]g, is critical in evaluating product feasibility for a given application. The objective of this report is to review the behavior of polymers near their T[subscript]g and assess the capability of predicting T[subscript]g using theoretical and empirical models. It was determined that all polymers begin to undergo structural relaxation at various temperatures both nearly above and below T[subscript]g, and that practical assessment of a single consistent T[subscript]g is successfully performed through consideration of only immediate thermal history and thermodynamic properties. It was found that the best quantitative structure-property relationship (QSPR) models accurately predict T[subscript]g of polymers of theoretically infinite chain length with an average error of less than 20 K or about 6%, while T[subscript]g prediction for shorter polymers must be done by supplementing these T[subscript]g (∞) values with configurational entropy or molecular weight relational models. These latter models were found to be reliable only for polymers of molecular weight greater than about 2,000 g/mol and possessing a T[subscript]g (∞) of less than about 400 K.

Foi preparada uma série de quatro betalaínas com o objetivo de determinar o efeito da metilação do nitrogênio imínico e da presença de uma hidroxila fenólica na posição 3 do anel aromático sobre a estabilidade e propriedades antirradicalares, fotofísicas e redox desta classe de pigmentos vegetais. O estudo destes compostos, chamados de m-betalainofenol, N-metil-m-betalainofenol, fenilbetalaína e N-metil-fenilbetalaína, revelou que os derivados metilados apresentam um deslocamento hipsocrômico sutil dos máximos de absorção e fluorescência em relação aos compostos não metilados. Os deslocamentos de Stokes são maiores em cerca de 4 kJ mol-1 para os derivados metilados e os rendimentos quânticos de fluorescência cerca de três vezes menores. A hidrólise destas betalaínas foi investigada na faixa de pH entre 3 e 7. Todas as betalaínas são mais persistentes em pH = 6 e a metilação da porção imínica aumenta significativamente a estabilidade da betalaína em meio aquoso. A presença da porção fenólica, em comparação a um grupo fenila, não afeta as propriedades fotofísicas dos compostos e tem um efeito menos pronunciado do que o da metilação sobre a estabilidade destes em meio aquoso. O comportamento eletroquímico dos compostos foi estudado por voltametria cíclica, nas mesmas condições de pH. A N-metilação foi novamente mais significativa do que a hidroxilação, provocando aumento de até 200 mV no potencial de pico anódico. O aumento do pH diminuiu o potencial de pico anódico dos quatro compostos, com uma razão entre prótons e elétrons igual a 1 para a maioria dos picos. A capacidade antirradicalar foi quantificada pelo ensaio colorimétrico TEAC baseado na redução de ABTS•+. Os dois derivados N-metilados apresentaram, em média, o mesmo valor de TEAC, apesar de um ser fenólico e o outro não. Já entre os não metilados, que têm TEAC de 2 a 3 unidades inferior à dos outros, a presença do fenol provoca elevação da capacidade antirradicalar. Os resultados sugerem a participação dos elétrons do anel 1,2,3,4-tetraidropiridínico, acoplados ao próton do nitrogênio imínico na ação antirradicalar de betalaínas. / A series of four artificial betalains was prepared in order to determine the effect of imine nitrogen methylation and phenyl hydroxylation (position 3) over stability and antiradical, photophysical and redox properties of this class of natural pigments. The study of m-betalainophenol, N-methyl-m-betalainophenol, phenylbetalain and N-methylbetalain, revealed that the methylated compounds present a small hypsochromic shift of both absorption and fluorescence maxima when compared to the others. The Stokes shifts are around 4 kJ mol-1 higher for methylated betalains, whereas the quantum yields are approximately three times lower. Their hydrolysis was investigated between pH 3 and 7. All compounds are more persistent in pH = 6, and imine methylation increases the overall stability in aqueous medium. The presence of a phenol group, in comparison with a phenyl substituent, has a minor effect on the photophysical properties of betalains and has a less pronounced effect over stability than that of methylation. The electrochemical behavior was studied by cyclic voltammetry, in the same pH range, and is also more significantly affected by methylation, rather than by hydroxylation. Methylation increases anodic peak potential up to 200 mV, and the potential is also much higher in more acidic media for all compounds. The number of protons involved in the electrochemical oxidation is the same as the number of electrons for most peaks The antiradical capacity was quantified using the TEAC assay, and ABTS•+ as radical. The methylated betalains presented, in average, the same TEAC value, although only one of them is phenolic. Among the non methylated, which are 2 to 3 units more efficient than the others, the phenolic one has a greater TEAC. These results suggest a participation of the 1,2,3,4-tetraidropiridinic ring electrons in the oxidation by ABTS•+, coupled to the imine nitrogen proton.

Latex d’Hevea brasiliensis est composé principalement de particules de caoutchouc dont le noyau est constitué de cis-1,4-polyisoprène et la membrane de composés dit non-isoprène. Sa stabilité dans l'arbre est assurée par les charges négatives de la membrane en raison de la présence de protéines et de lipides. Le latex d’Hevea brasiliensis coagule après acidification pour former un gel colloïdal qui est la première étape de sa transformation en caoutchouc naturel.Dans cette thèse, nous avons étudié l’agrégation et la gélification induite par ajout d’un acide et les propriétés des gels obtenus. Pour cela, un latex de caoutchouc naturel commercial constitué par des particules de caoutchouc (1 µm de diamètre), a été utilisée. L'agrégation a été induite par l'hydrolyse d'un composé appelé glucano-delta-lactone (GDL). Cette agrégation a été suivie par différentes techniques basées sur la diffusion de la lumière (turbidimétrie, DWS, SLS). La gélification a été suivie in situ par la rhéologie. Ensuite, les propriétés rhéologiques de gels colloïdaux formés à pH 4 ont été caractérisées dans les régimes linéaires et non-linéaires.La dépendance à la fraction volumique et le pH de l'apparition de l'agrégation ont été observée. La non-redispersion des agrégats montre les interactions fortes entre les particules de caoutchouc. Trois comportements d’état différents (solution stable, la séparation de phase et gel) peuvent être prédits à partir d'un diagramme d’état fraction volumique-pH qui a été établi pendant 5 semaines. L'état de gel présente une structure fractale, caractérisé par une relation de loi de puissance entre le module élastique G' et la fraction volumique. Pour des fortes déformations, une réorganisation irréversible sous contrainte a abouti à un durcissement irréversible du matériau. Nous avons observé par rheo-ultrasound qu'au-dessus de 50% de déformation, une déstructuration homogène est produite dans le gel. / Natural rubber latex is mainly composed by rubber particles whose core is made of cis-1,4-polyisoprene and the shell of non-isoprene compounds. It stability in the tree is provided by the negative charge of the shell due to the presence of proteins and lipids. Natural rubber latex coagulates after acidification to form a colloidal gel that is the first step of its transformation into natural rubber.In this PhD thesis, we studied this acid-induced aggregation and gelation and the properties of the resulting gels. For this, a commercial natural rubber latex composed by rubber particles (1 µm in diameter), was used. Aggregation was induced by hydrolysis of a compound called glucano-delta-lactone (GDL). The aggregation was followed by different techniques based on light scattering (turbidimetry, DWS, SLS). Gelation was followed in situ by rheology. Then, the rheological properties of colloidal gels formed at pH 4 were characterized in the linear and non-linear regimes.Dependence to the volume fraction and the pH of the onset of aggregation was observed. The non-redispersion of aggregates proves the strong interactions between rubber particles. Three different state behaviors (stable solution, phase separation and gel) can be predicted from a state diagram pH-volume fraction that was established over 5 weeks. The gel state exhibits a fractal structure, characterized by a power law relationship between the elastic modulus G’ and the volume fraction. At high deformations, an irreversible reorganization under stress resulted in an irreversible strain hardening of the material. We observed by rheo-ultrasound that above 50% deformation, an homogenous destructuration occurred in the gel.