Fluorescence Lifetime Distributions of Labeled Amorphous Polymers in Bulk

Polystyrene, poly(methylmethacrylate), poly(cyclohexyl methacrylate) and poly(1-butylmethacrylate) were labeled with anthryl groups by copolymerization. Films of labeled polymers as well as blends of them with unlabeled polystyrene samples of different polydispersity and molecular weight, were prepared by solvent casting. The time resolved emission of anthryl groups in those films was measured by Single Photon Counting with front face excitation at the standard 30° incident angle and with much lower incident angles to photoselect chromophores on the polymer-air surface. Fluorescence decays were fitted with bimodal fluorescence lifetime distributions, which were analyzed taking into consideration some polymer characteristics and the compatibility of polymer blends. It was thus concluded that solid-like and liquid-like environments are both in the bulk and in the surface of the film although liquid-like domains are more frequent on the surface than in bulk. Polymer Tᵍ determines the position of the two modes and polydispersity justify the broadness of the modes in the fluorescence lifetime distribution.Financial support from DGI (Spain) under grants BQU2000-0251 and MAT2000-0391-P4-02, is gratefully acknowledged

Development of Cocontinuous Morphologies in Initially Heterogeneous Thermosets Blended with Poly(methyl methacrylate)

Morphology and phase separation process in blends of a network-forming reactive polymer, poly(aminopropylmethylsiloxane) (PAMS), in a poly(methyl methacrylate) (PMMA)-modified epoxy system were studied using optical, epifluorescence and scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical thermal analysis (DMTA). The thermoset system was bisphenol A diglycidyl ether (DGEBA) with different PMMA percentages, 2−10% w/w. Phase separation and reaction advancement were monitored in situ. At the concentration studied, PMMA does not influence the kinetics of the curing process, but it strongly affects the reactive compatibilization between DGEBA and PAMS. The morphology obtained consists of a continuous thermoplastic-rich phase surrounding thermosetting connected polyhedral particles of 5−15 μm. This cocontinuous morphology is observed independently of the percentage of PMMA. Results show that the morphology is strongly influenced by the diffusion and viscosity conditions during reactive compatibilization and phase separation. An increase in PMMA content leads to a decrease in the thermosetting polyhedral particle size. In contrast, an increase in curing temperature leads to bigger sizes. The addition of thermoplastic polymers to initially nonhomogeneous reactive blends is a potential route for generating cocontinuous morphologies irrespective of the thermoplastic contentThe authors would like to ex-press their gratitude to the Epoxsil (MAT2000-0391-P4-02) and Fibrodont (MAT2001-0677-P3) projects for financial support. The authors would also like to thank to Dr. I. Esteban (UNED) for his assistance with the TOM micrographs and to Dr. J. Iruin (UPV) for his assistance with the density measurements

Fluorescence probes the early formation of network at the interface of epoxy-silica nanocomposite during curing

The effect of surface modified silica on the cure behavior at the interface of epoxy-silica nanocomposites has been analyzed monitoring the fluorescence of the dansyl probe located at different distances from the silica surface by means of molecular tethers. FTIR analysis revealed the catalytic role of the surface hydroxyls, which is modulated by the surface modification. Fluorescence results show that network formation and associated phenomena as gelation and vitrification occurs first at the interface. The exchange of branched species throughout the permeable interface spreads its influence to the whole system until vitrification is reached.Authors are gratefully acknowledged to Spanish Ministerio de Economía y Competitividad for financial support under grant MAT2010-17091

Confocal microscopy study of phase morphology evolution in epoxy/polysiloxane thermosets

The morphology of stoichiometric initially immiscible reactive blends of DGEBA epoxy resin and poly(3-aminopropylmethylsiloxane) has been characterized by laser scanning confocal microscopy. Observations were done on samples cured isothermally at different curing temperatures in the range 20–120 °C as well as in situ and in real time at 60 °C. Three different processes were revealed: coalescence, which occurs primarily at very low conversion, diffusion of DGEBA through polysiloxane-rich domains and chemical reaction, which occurs at the interphase between both phases. The interphase-thickness and compositional gradients were characterized by laser scanning confocal microscopy (LSCM). Results show that as curing temperature increases within the studied range, the material becomes more homogeneous although the interphase thickness remains almost constant.The authors would like to acknowledge the financial support from projects Epoxsil (MAT2000-0391-P4-02) and Fibrodont (MAT2001-0677-P3)

Nanoindentation and wear behavior of thermally stable biocompatible polysulfone-alumina nanocomposites

The authors would like to acknowledge funding from the Spanish Ministerio de Economía y Competitividad under grant MAT2014-57557-R. R. Prof. Ozisik would like to acknowledge financial support from the U.S. National Science Foundation (CMMI-1538730). The authors also would like to thank I. Garc´ıa and A. Cervera from Euroortodoncia for their invaluable help in nanocomposite processing

Interfacial characterization of epoxy/silica nanocomposites measured by fluorescence

Artículo publicado en papel: enero 2015 pero disponible online desde 10 noviembre 2014.Fluorescence labeling was used as a tool for the interfacial characterization of nanocomposites. The solvatochromic probe dansyl chloride was employed as interfacial reporter in epoxy/silica nanocomposites. Molecular spacers (organosiloxanes and polyetheramines) of different lengths were used to vary the location of the chromophore at the interface. The steady state and time resolved fluorescent responses reflect a rigid polar interface. Fluorescence changes during heating at a constant rate were analyzed for determining the local glass transition (Tg) at the interface region. The fluorescence results were then compared to the Tg obtained from differential scanning calorimetry and the results showed the existence of a gradient interface of a few nanometers thick having different properties than the bulk matrix. The thickness of this interface is small but its altered dynamics due to strong interactions with the nanofiller spreads its influence throughout the whole matrix.J.C. Cabanelas, C. Antonelli, J. Baselga and B. Serrano gratefully acknowledge Spanish Ministerio de Educación, Cultura y Deporte (MAT2010-17091) for financial support. R. Ozisik acknowledges support from the United States National Science Foundation under Grant No. 1200270

Morphology of phase separated blends of poly(cyclohexyl methacrylate) with poly(vinyl acetate)

Blends of poly(vinyl acetate) (PVAc) and poly(cyclohexyl methacrylate) (PCHMA) labeled by copolymerization with 4-methacryloylamine-4′-nitrostilbene (Sb), with (1-pyrenylmethyl)methacrylate (Py) or with 3-(methacryloylamine)propyl-N-carbazole (Cbz), were prepared by casting dilute solutions in tetrahydrofurane (THF) or chloroform onto silanized glass plates. The resulting films were studied by epifluorescence microscopy, microfluorescence spectroscopy, DSC and optical microscopy. Epifluorescence micrography probes the chemical composition of the different regions in phase separated blends, with black areas corresponding to PVAc rich regions and colored areas corresponding to labeled PCHMA rich regions. The technique also visualizes primary and secondary morphologies, which depend on the composition of the polymer blend and on the casting solvent. Mixtures containing 80 wt % PCHMA show, in general, a bicontinuous primary morphology suggesting a spinodal demixing mechanism. Solvent effects are particularly relevant for 50% and 20% PCHMA samples showing morphologies composed of PCHMA rich domains, in a matrix of solvent-dependent compositions. Samples cast from chloroform are more homogeneous and the matrix is always highly fluorescent. In contrast, the domains of samples cast from THF are heterogeneous in size and shape and the matrix is non-fluorescent, being thus formed by nearly pure PVAc. Small voids are formed in the polymer-air interface. They are submicrometric for THF cast films and disappear with annealing at 122°C. For chloroform cast samples they are much less frequent and appear well ordered, forming a mostly hexatic two dimensional network.This work was supported by DCI (Spain) and EU (Brite-Euram) under grants BQU2000-0251 and BE-97-4672, respectively

Molecular probe technique for determining local thermal transitions: The glass transition at Silica/PMMA nanocomposite interfaces

Local glass transition temperatures (Tg) have been measured in the interfaces of solution blended silica/poly(methyl methacrylate) (PMMA) nanocomposites using florescence spectroscopy and compared with Tg measured by differential scanning calorimetry (DSC). It was found that the two types of measurements yielded significantly different information. Combinations of silanes and poly(propylene glycol)-based molecular spacers bound to fluorophores were covalently linked to the surface of the nanoparticles, allowing for variation of the fluorophore response with respect to the distance from the nanofiller surface. Increases in the bulk Tg from the neat PMMA value were found upon the addition of nanofillers, but were independent of the nanofiller concentration when the filler concentration was above 2% by weight. Furthermore, as the size of the grafted molecular spacer was increased, Tg values were found to decrease and approach Tg of the neat PMMA. Owing to variable conformations of the spacers, an effective distribution of fluorophore-silica distances exists, which influences the fluorophores' response to the transition.This research was supported by the National Science Foundation under Grant No. 0500324. Authors from UC3M would like to acknowledge CICYT for financial support (MAT 2007-63722)

Photoluminescence of Bridged Silsesquioxanes Containing Urea or Urethane Groups with Nanostructures Generated by the Competition between the Rates of Self-Assembly of Organic Domains and the Inorganic Polycondensation

The aim of this study was to investigate the changes produced in the nanostructures and the photoluminescence spectra of bridged silsesquioxanes containing urea or urethane groups, by varying the relative rates between the self-assembly of organic domains and the inorganic polycondensation. Precursors of the bridged silsesquioxanes were 4,4‘-[1,3-phenylenebis(1-methylethylidene)]bis(aniline) and 4,4‘-isopropylidenediphenol, end-capped with 3-isocyanatopropyltriethoxysilane. The inorganic polycondensation was produced using either high or low formic acid concentrations, leading to transparent films with different nanostructures as revealed by FTIR, SAXS, and ²⁹Si NMR spectra. For the bridged silsesquioxanes containing urea groups the self-assembly of organic domains was much faster than the inorganic polycondensation for both formic acid concentrations. However, the arrangement was more regular and the short-range order higher when the rate of inorganic polycondensation was lower. The photoluminescence spectra of the most ordered structures revealed the presence of two main processes:  radiative recombinations in inorganic clusters and photoinduced proton-transfer generating NH₂⁺ and N⁻ defects and their subsequent radiative recombination. In the less-ordered urea-bridged silsesquioxanes a third process was present assigned to a photoinduced proton transfer in H-bonds exhibiting a broad range of strengths. For urethane-bridged silsesquioxanes the driving force for the self-assembly of organic bridges was lower than for urea-bridged silsesquioxanes. When the synthesis was performed with a high formic acid concentration, self-assembled structures were not produced. Instead, large inorganic domains composed of small inorganic clusters were generated. Self-assembly of organic domains took place only when employing low polycondensation rates. For both materials the photoluminescence was mainly due to radiative processes within inorganic clusters and varied significantly with their state of aggregation.The financial support of the National Research Council (CONICET, Argentina), the National Agency for the Promotion of Science and Technology (ANPCyT, Argentina, PICT 14738-03), the University of Mar del Plata, the Grant Agency of the Czech Republic (Project 203/05/2252), and Project Nanoter (Project MAT2004/01347, MEC-DGI, Spain) is gratefully acknowledged. INTEMA and the Institute of Macromolecular Chemistry acknowledge the support of the European Network of Excellence Nanofun-Poly for the diffusion of their research results

gamma-Alumina Modification with Long Chain Carboxylic Acid Surface Nanocrystals for Biocompatible Polysulfone Nanocomposites

High performance polysulfone/γ-alumina biocompatible nanocomposites are reported for the first time and the effects of γ-alumina surface modification are explored. We show that some fatty acids chemisorb over the surface of γ-alumina forming nanosized self-assembled structures. These structures present thermal transitions at high temperatures, 100 ºC higher than the melting temperatures of the pure acids, and are further shifted about 50 ºC in the presence of polysulfone. The chemistry involved in the chemisorption is mild and green meeting the stringent bio sanitary protocols for biocompatible devices. It has been found that the self-assembled structures increase mechanical strength by about 20% despite the foreseeable lack of strong particle matrix interactions, which manifests as small variations in both the glass transition temperature and the Young's modulus. Electron microscopy observation of fractured surfaces has revealed that some acids induce an extended region of influence around the nanoparticles and this fact has been used to explain the enhancement of mechanical strength.This work was supported by granst Nanomod (MAT2010-17091) from the Spanish Ministerio de Economía y Competitividad and NANOBRAK (04-AEC0810-000056/2008), as well as to the INTERFASES consortium (Comunidad Autónoma de Madrid, Spain)