Blends based on a stoichiometric mixture of diglycidyl ether of bisphenol A (DGEBA) and 2,2′-bis(4-amino-cyclohexyl)methane (ACHM) and cyclohexane as low molecular weight solvent were studied. The ...polymerization kinetics data experimentally obtained at different temperatures (30–60°C) were accurate fitted using a mechanistic model. The effect of the solvent over the polymerization rate was analyzed. No influence of the presence of the solvent was found up to 20 wt% of cyclohexane in the sample. The thermodynamic analysis using Flory-Huggins model allowed determination of the initial miscibility of binary blends containing DGEBA/cyclohexane and pseudo-binary mixtures of DGEBA/ACHM/cyclohexane. The presence of ACHM increases the DGEBA/cyclohexane initial solubility. The reaction-induced phase separation was described using a conversion-composition transformation phase diagram. It indicates that phase separation takes place by a nucleation and growth (NG) mechanism. The appropriate selection of composition of the blend and polymerization conditions led to final materials with a desired closed cell porous morphology.
Polymer-dispersed liquid crystals (PDLCs), consisting of a dispersion of LC-rich domains in a polymer matrix, are used in different types of electrooptical devices. Their efficiency can in principle ...be increased if the LC domains exhibit a uniform characteristic size in the range of the wavelength of visible light. In an attempt to generate this type of morphology, a model PDLC system based on a 50 wt % solution of N-4-ethoxybenzylidene-4‘-n-butylaniline (EBBA) in an epoxy monomer (diglycidyl ether of bisphenol A, DGEBA) was analyzed. The polymerization-induced phase separation was performed at 80 °C, using a tertiary amine as initiator (benzyldimethylamine, BDMA). By selecting an initial concentration located close to the critical composition to promote spinodal demixing, co-continuous morphologies were obtained, which were rapidly fixed by gelation. The conversion of epoxy groups (p) was followed by near-infrared spectroscopy (NIR). At p = 0.28, phase separation took place as revealed by transmission optical microscopy (TOM) and by the acceleration observed in the isothermal cure rate. Gelation took place at p = 0.35, soon after the cloud point. Although the primary structure was arrested by gelation, the LC-rich phase was continuously enriched in pure EBBA, as revealed by the increase in T NI with conversion monitored by differential scanning calorimetry (DSC). Co-continuous structures remained unmodified after the storage of PDLCs for several months. The nematic range of the LC-rich phase at p = 1 was comprised between 34 °C (melting point) and T NI = 68 °C. A 57% of the initial LC was present in nematic domains at 40 °C, as determined by the variation of the FTIR absorbance of a characteristic LC peak between isotropic and nematic states. Therefore, a possible route to obtain PDLCs with a uniform characteristic size of LC domains is to start with a composition close to the critical one and select conditions to produce liquid−liquid demixing soon before gelation.
•Two different solvents (toluene and chloroform) were used to prepare Langmuir films of PS-b-P4VP(DO13) copolymers.•For toluene, which is a selective solvent for one of the blocks of the copolymer, a ...higher film-forming ability was observed.•A pronounced effect on the optical storage capability of LS and LB films was observed.•Higher photoinduced birefringence for the films prepared with toluene was developed.•The enhanced properties in toluene are ascribed to the formation of micelles.
Langmuir films of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with an azo dye, 4-4-(phenylazo)-1-naphthylazophenol (Disperse Orange 13, DO13), hydrogen-bonded to the pyridine group of P4VP blocks were investigated. The surface pressure isotherms were considerably different for the copolymer dissolved in a nonselective (chloroform) and selective (toluene) solvents. The Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) films produced with PS-b-P4VP(DO13) copolymers displayed higher optically induced birefringence than the cast films owing to their nanostructured nature. The photoinduced birefringence properties were particularly enhanced when toluene was used owing to the confinement of azo chromophores in micelles consisting of a P4VP core and a PS corona.
An azo prepolymer (TAZ) was synthesized by reaction between Disperse Orange 3 (DO3) and diglycidyl ether of bisphenol A (DGEBA). The resulting TAZ was blended with DGEBA and two different monoamines, ...one aliphatic and one aromatic, benzylamine (BA) and
m-toluidine (MT), to prepare various azo copolymers with different azo contents. In addition, two guest–host systems were prepared: 5.16% by weight of DO3 in DGEBA-BA and in DGEBA-MT polymers. Their reversible optical storage properties were studied and compared. The maximum obtainable birefringence increases linearly with TAZ weight fraction, independently of the power of the writing beam, the environment in which the azo moiety is located, and the bonding between the dye and the matrix. It was found that, in BA-based copolymers, with lower
T
g, the fraction of birefringence conserved after relaxation is smaller and the writing rates are higher than in MT-based copolymers. The film thickness dependence of the birefringence signal is also analyzed.
Charm decays at BaBar Oyanguren, A
Journal of physics. Conference series,
05/2008, Letnik:
110, Številka:
5
Journal Article
Recenzirano
Odprti dostop
In this talk I go over recent results involving charm decays obtained with the data collected by the BaBar experiment. I cover here results on leptonic and semileptonic charm decays, D meson ...branching fractions, Dalitz analyses and charm baryons.
Photocontrol of molecular alignment is an exceptionally-intelligent and useful strategy. It enables us to control optical coefficients, peripheral molecular alignments, surface relief structure, and ...actuation of substances by means of photoirradiation. Epoxy linear polymers with azobenzene and alkyl side groups were synthesized and studied. They showed interesting and variable photo-response upon visible-light irradiation due to trans–cis–trans photoisomerization of the azobenzene units. The influence of physical crosslinks between alkyl chains on optical and mechanical properties was also investigated. Mechanical characterization of the polymers was done by nanoindentation. Light induced reversible or irreversible changes in the polymer after being irradiated with visible light, depend on the polarization states of light used. When using circularly polarized light, reversible mechanical response is observed. Permanent structural changes in films are related to anisotropic alignment of azobenzene molecules (after being illuminated with linear polarized light) and/or physical crosslinks of alkyl chains. This study demonstrated the importance of the polymer architecture and the presence of domains of alkyl chains on the potential photo responsiveness.
•Photoresponsive linear epoxy polymers with widely tunable optical/mechanical properties were obtained.•Light induced reversible or irreversible changes in the polymer after being irradiated with visible light.•When using circularly polarized light, reversible mechanical response is observed.•Permanent structural changes in films are related to anisotropic alignment of azobenzene units and/or to physical crosslinks.
Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on ...diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis 3-chloro 2,6-diethylaniline (MCDEA) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out
in situ, during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.
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