Asymmetric copolymers: neither block nor random

Les monomères d'un copolymère statistique sont aléatoirement mélangés, tandis que ceux d'un copolymère à bloc sont nettement séparés en sections de compositions différentes. Entre ces deux structures modèles existent des copolymères asymétriques, qui sont définis comme une distribution de monomères au sein de la chaîne qui n'est ni complètement ségrégée comme pour un copolymère à bloc ni statistiquement distribuée de manière indépendante de la position au long de la chaîne comme dans le cas des copolymères statistiques. Ainsi, les propriétés des copolymères asymétriques devraient combiner les caractéristiques des structures à bloc et statistiques. Dans cette étude, des copolymères d'acide acrylique-(acrylate de n-butyle) (AA-n-BA) et diméthylacrylamide-N-isopropylacrylamide (DMA-NIPAM), avec des masses molaires ciblés de 10 kg mol-1 et 20 kg mol-1 , ont été obtenus par polymérisation RAFT en utilisant une synthèse forcée et par étapes. Les deux systèmes de copolymères sont des polymères sensibles aux stimuli : des macromolécules qui subissent des transitions de phase lorsqu'elles subissent de subtils changements des conditions environnementales. Les copolymères P(AA-n-BA) réagissent au pH et les copolymères P(DMA-NIPAM) sont thermosensibles. Lors de cette étude, la composition des copolymères a été fixée (50% AA ou 50% NIPAM), mais la distribution des unités de monomères au sein de la chaîne varie. En effet, des structures à blocs, statistiques, à gradient, asymétriques dibloc et tribloc ont été obtenues dans le but de comparer leurs propriétés physiques et d'auto-assemblage. Les caractéristiques macromoléculaires des copolymères ont été obtenues par spectroscopie de résonance magnétique nucléaire (1H RMN) et chromatographie d'exclusion stérique (SEC). Les copolymères P(AA-n-BA) en solution à différents pH ont été étudiés par diffusion dynamique de la lumière (DLS), microscopie électronique à transmission cryogénique (cryo-TEM) et diffusion de neutrons aux petits angles (SANS) et il a été possible de démontrer les changements de taille et de comportement d'auto-assemblage en fonction du pH des différentes solutions de copolymères. Les résultats ont montré que les copolymères asymétriques P(AA-n-BA) forment des agrégats de morphologie différente selon le pH, par exemple des vésicules à pH 4 ou des micelles et des micelles vermiculaires à pH 5. D'autre part, la morphologie des copolymères à bloc de même composition, n'est pas influencée par les changements de pH. Les copolymères de P(DMA-NIPAM) ont été analysés en solution par DLS, SANS et 1H RMN en fonction de la température. L'évolution de la taille hydrodynamique en fonction de la température a pu être suivie par DLS. La micellisation induite par le changement de température a été analysée par SANS. Enfin, l'effondrement de la structure induit par la température et la perte de mobilité résultante des chaînes polymères ont été suivis à un niveau moléculaire par 1H RMN. Des résultats intéressants ont été obtenus, car les copolymères à bloc de faible masse molaire (Mn = 10 kg mol-1) présentent un comportement similaire au copolymère à gradient de masse molaire plus élevé (Mn = 20 kg mol-1). Ce phénomène a été observé par SANS et 1H RMN, et il a été attribué à la faible longueur du copolymère à bloc : une fraction significative des unités NIPAM dans le copolymère à bloc peuvent être en contact avec le DMA du bloc adjacent, conduisant à un changement progressif de la composition effective du polymère en fonction de la longueur de la chaîne.Block copolymers are made from polymer chains of different chemical composition that are covalently joined via their respective end groups. On the other hand, there are statistical copolymers whose monomers are randomly copolymerized together. Between these structures exist asymmetric copolymers, which are defined as a distribution of monomers within the chain which is neither completely segregated as for a block copolymer nor statistically distributed in a manner that is independent of the position along the chain as in the case of statistical copolymers. Based on the latter, the properties of asymmetric copolymers are expected to combine characteristics of block and statistical structures. In this investigation, acrylic acid-(n-butyl acrylate) (AA-n-BA) copolymers and dimethylacrylamide-N-isopropylacrylamide (DMA-NIPAM) copolymers, with targeted molecular weights of 10 kg mol-1 and 20 kg mol-1, were obtained by RAFT polymerization using forced and stepwise synthesis. Both copolymer systems are stimuli-responsive polymers: macromolecules which undergo phase transitions when they experience subtle changes in the environmental conditions. P(AA-n-BA) copolymers are pH-responsive and P(DMA-NIPAM) copolymers are thermosensitive. The composition of the copolymers was always the same (50% AA or 50% NIPAM), but the distribution of the monomer units within the chain was different. Block, statistical, gradient, asymmetric diblock and triblock structures were obtained with the aim to compare their physical and self-assembly properties. The macromolecular characteristics of copolymers were obtained by nuclear magnetic resonance spectroscopy (1H NMR) and size exclusion chromatography (SEC). P(AA-nBA) copolymers in solution at different pH were studied by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and small angle neutron scattering (SANS) and it was possible to demonstrate the changes in size and self-assembly behavior as a function of pH of the copolymers solutions. The results showed that the P(AA-nBA) asymmetric copolymers form aggregates of different morphology depending on the pH, for example vesicles at pH 4 or micelles and worms at pH 5. On the other hand, the morphology of block copolymers with the same composition, is not influenced by changes in pH. P(DMA-NIPAM) copolymers in solutions were analyzed by DLS, SANS and 1H NMR as a function of temperature. The evolution of hydrodynamic size as a function of temperature could be followed by DLS and the temperature-induced micellization was analyzed by SANS whereas by 1H NMR, the temperature-induced collapse and resulting loss of mobility of the polymer chains could be followed at a molecular level. Interesting results were obtained, since low molar mass block copolymers (Mn = 10 kg mol-1) displayed similar behavior to the corresponding to high molar mass gradient copolymer (Mn = 20 kg mol-1). This phenomenon was observed by SANS and 1H NMR, and it was attributed to the short length scale of the block copolymer, in which the chain is short enough that a significant fraction of the NIPAM units in the block copolymer are strongly affected by the DMA of the adjoining block, leading to a gradual change in the effective composition of the polymer as a function of chain length

Thickness effect on the solvent sensing parameters of carbon black-polymer composites

investigación realizada en proyecto de Red TemáticaStudy of the sensing parameters: sensibility, response and recuperation times to hexane vapors for layers of different thickness of poly(butadiene) + carbon black (CB) composites is presented. The results show that sensibility increases as the CB- weight percent diminishes, being in agreement with reported results by several researches. In this work another variable was studied on the sensing parameters, the thickness. The initial electrical resistance of the studied layers increases until three magnitude orders as diminish theirs thickness and the sensibility in this study increases until one magnitude order as the layer thickness diminishes around 0.25 μm.Programa de Mejoramiento al Profesorado (PROMEP, México), proyecto de red PROMEP 2013-2014

Cellulose-Based Polymer Composite with Carbon Black for Tetrahydrofuran Sensing

This work focused on studying the sensing efficiency of tetrahydrofuran (THF) by composite films made of thin layers of a cellulose-based polymer and carbon black. We analyze the reproducibility, durability, desorption time, and the sensitivity percent as a function of the amount of solvent. Two types of experiments were conducted, (1) progressive sensing test (PST) which consisted of progressively increasing the amount of solvent from 0.1 mL increments up to 1.0 mL and (2) multiple sensing test (MST) where the layers were subjected to consecutive pulses of the same amount of solvent, with a minimum of 0.1 mL and a maximum of 0.4 mL. The response and desorption times were a few seconds, and the sensitivity percent ranged from 1% to 170% and was dependent on the solvent quantity

Synthesis and Characterization of Polyaniline/Magnetite Nanocomposite

trabajo de investigación de Red TemáticaA conducting-electroactive polyaniline/magnetite (PAni/Fe304) nanocomposite was synthesized using anilinium dodecylbenzene sulfonate (S1) as a reactive surfactant. First, S1 allowed magnetite dispersion in the aqueous phase and second, S1 performed as the monomer of polyaniline emeraldine base salt. Electron microscopy suggested core-shell morphology based on S1 amphiphilic character; that is, S1 adsorbed onto the magnetite nanoparticles surface and then was polymerized via an oxidative polymerization forming the shell. The PAni/Fe304 composite exhibited improved thermal stability regarding pure PAni, which was related to the strong interaction between PAni and magnetite. Electrical conductivity, determined by the four-probe method, was in the order of 10-1 and 10-3 S cm-1, respectively, for the pure PAni and the composite. Concerning composite magnetic properties, the decrement in magnetization (σr) and hysteresis (Hc) was attributed to the increment in dipolar magnetic interaction due to the increased separation among magnetite nanoparticles because of the PAni shell.PROMEP-SEP, Programa: Redes Temáticas de Colaboración Académica, "Red de Compuestos Poliméricos, Propiedades y Aplicacione

Effect of size and composition on the second harmonic generation from lithium niobate powders at different excitation wavelengths

INVESTIGACIÓN DESARROLLADA EN COLABORACIÓN DE RED TEMATICA PRODEP /SEP/MEXICOLiNbO3 microcrystalline systems, possessing almost stoichiometric composition, were produced by varying the temperature and time parameters in the annealing processes following a mechanochemical reaction of raw powders. SHG from these samples, detected for every fundamental wavelength in the range 800-1300 nm, and being maximal at a certain wavelength, λmax, for each sample, has been addressed to a random scattering of the induced nonlinear polarizations. Possible tuning of λmax could be ascribed to control of composition and grain size of the sample. Random orientation of the produced nanocrystallites was verified since no dependence for SHG intensity on incident polarization was observed.PAPIIT-UNAM through grant IN117116 Y “RED PRODEP-SEP Compuestos poliméricos propiedades y aplicaciones 2015-2016

Optical properties of carbon nanostructures produced by laser irradiation on chemically modified multi-walledcarbon nanotubes

Apoyo a la Red Temática de Colaboración Académica: Desarrollo de Materiales Compuestos con propiedades Ópticas, Eléctricas , Magnéticas y sus aplicacionesThis research focused on the nanosecond(Nd:YAG-1064 nm) laser pulse effect on the optical and morphological properties of chemically modified multi-walled carbon nanotubes(MWCNT).Two sus- pensions of MWCNT in tetrahydrofuran (THF) were prepared,one was submitted to laser pulses for 10 min while the other (blank) was only mechanically homogenized during the same time. Following the laser irradiation, the suspension acquired a yellow-amber color,in contrast to the black translucent appearance of the blank. UV-vis spectroscopy confirmed this observation, showing the blank a higher absorption. Additionally, photoluminescence measurements exhibited a broad blue-green emission band both in the blank and irradiated suspension when excited at 369 nm, showing the blank a lower intensity. However, a modification in the excitation wave length produced a violet to green tuningin the irradiated suspension, which did not occurin the blank. Lastly, the electron microscopy analysis of the treated nanotubes showed the abundant formation of amorphous carbon, nanocages, and nanotube unzipping, exhibiting the intense surface modification produced by the laser pulse. Nanotube surface modification and the coexistence with the new carbon nanostructures were considered as the conductive conditions for optical properties modification.PROME

Magnetic susceptibility studies of the spin-glass and Verwey transitions in magnetite nanoparticles

Magnetite nanostructured powder samples were synthesized by aging chemical method. Phase, structural, and magnetic properties were characterized. X-ray diffraction patterns showed cubic magnetite pure phase, with average crystallite size, , equal to 40 nm. Susceptibility measurements showed the well-known Verwey transition at a temperature of 90 K. The decrease of Verwey transition temperature, with respect to the one reported in literature (125 K) was attributed to the low average crystallite size. Moreover, the spin-glass like transition was observed at 35 K. Activation energy calculated from susceptibility curves, with values ranging from 6.26 to 6.93 meV, showed a dependence of spin-glass transition on frequency. Finally, hysteresis loops showed that there is not an effect of Verwey transition on magnetic properties. On the other hand, a large increase of coercivity and remanent magnetization at a temperature between 5 and 50 K confirmed the presence of a magnetic transition at low temperatures

Synthesis by wet chemistry and characterization of LiNbO3 nanoparticles

Actually, lithium niobate (LiNbO3) has been used for optical wavelength conversion and ultrafast optical signal processing because of its outstanding rapid nonlinear optical response behavior, low switching power and broad conversion bandwidth. LiNbO3nanoparticles, which belong to the ferroelectric oxide class, were synthesized by chemical reaction with wetchemistry. Their sizedistributionwascenteredaround200 nm. Xray diffraction (XRD) and scanning electron microscopy (SEM) were used to further investigate the quality of the obtained LiNbO3powders.The present work shows thatby employingthis chemical method the correct stoichiometric phasewas obtained. This wascorroborated by XPS (X-Ray Photoelectron Spectroscopy) results. Also, the nanoparticles showed a defined crystallinity and uniform morphology. This way of obtaining nanoparticles is innovative because of its low cost and simple way to reproduce it. It isan important method of increasing the surfacearea, controlling thephase purityand reducing theparticle size distribution. The samples were obtained under low temperature annealing at500, 650 and 800 ºC. Those features can be controlled using variables such temperature, time of synthesis,and calcination. In previous worksit wasfound that hydrothermal methods offer many advantages over conventional ceramic synthesis methods

Synthesis of Hierarchical Dorsal Spine Ag

Silver sulfide hierarchical structures with unique dorsal spine morphology were successfully synthesized on mechanically deformed silver substrates by simple solid-vapor reactions. It has been found that it is possible to change the structures morphology by changing the reagent gas composition. The carbon monoxide (CO) presence in a reactive sulfur atmosphere was found to be the key for growing the dorsal spine structures. In all cases, the Ag2S structures grew on the edge of the silver substrates where high plastic deformation occurred