Local dynamics in epoxy coatings containing iron oxide nanoparticles by dielectric relaxation spectroscopy

Nanocomposites of photocurable epoxy resin and epoxy‐modified iron oxide magnetic nanoparticles were analyzed by dielectric relaxation spectroscopy to study the local dynamics at temperatures well below the glass‐transition temperature. Two secondary processes were detected, β and γ processes, but the second one was just detected at lower temperatures in the high‐frequency part of the spectra and moved out of the frequency range at higher temperatures. Data were fitted to the Havriliak–Negami and Arrhenius models, and the obtained parameters were analyzed. Relaxation times of the β secondary relaxation did not change with the nanoparticle content, but the relaxation strength increased. The increase could not be explained when we took into account the molecular origin of the relaxation. The presence of ferromagnetic nanoparticles enhanced the internal field and increased the relaxation strengths. Transmission electron microscopy images showed that the nanoparticles were well dispersed in the matrix, without magnetic agglomerates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Electrically insulating polymeric nanocomposites with enhanced thermal conductivity by visible‐light curing of epoxy–boron nitride nanotube formulations

Epoxy–boron nitride nanotube (BNNT) composites were prepared using visible light through a radical‐induced cationic polymerization method activated by camphorquinone. The fully cured films showed an enhancement of glass transition temperature in the presence of the filler. Electrical characterization showed a slight dielectric constant decrease with BNNT content. Finally, thermal conductivity measured using nano‐flash analysis showed a linear increase in the thermal conductivity of the materials with increasing BNNT content in the photocurable formulations. © 2017 Society of Chemical Industry     

Dielectric monitoring of curing of liquid oligomer‐modified epoxy matrices

Dielectric measurements were performed in ‘real‐time’ at several temperatures to follow polymerization reactions on blends of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin with 4, 4′‐diaminodiphenylmethane (DDM) hardener and different amounts of polyoxypropylenetriamine (POPTA) oligomer. These systems exhibit phase separation induced by molar mass increasing through curing of the resin. Monitoring of phase separation and vitrification (related to the α‐relaxation) was performed by this technique. The results are compared with those for the unmodified resin–hardener mixture. The change of the main α‐relaxation with cure time, cure temperature, and amount of modifier was measured for the mixtures. This change of the main relaxation through curing in the frequency domain was indicative of the cure reaction advancement, because of its dependence on the viscosity of the medium. The change of the ionic conductivity during curing was also analysed, showing its dependence upon cure temperature. © 2001 Society of Chemical Industry     

Molecular dynamics of poly(glycolide‐co‐L‐lactide) copolymer during isothermal cold crystallization

Poly(glycolide‐co‐l ‐lactide) (PGA/PLLA) is a random copolymer with 92 wt % PGA, being the basic resin for Vicryl® suture. Molecular dynamics of PGA/PLLA in its wholly amorphous state and during isothermal cold crystallization at 70 and 80°C have been analyzed. Experimental results were generated over a wide range of frequency and temperature by broad‐band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ = [1/2]πf_max where f_max is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for wholly amorphous and crystallized samples has been analyzed. This behavior has been modeled by Havriliak–Negami, Vogel–Fulcher–Tammann, and Kohlrausch–Williams–Watts equations. The evolution profile of the dynamics (frequency at which the maximum loss appears, f_max) depends on the crystallization temperature, being different at 80°C relatively to 70°C, which could reflect different progress of the spherulitic morphology, as it is shown by the evolution of the morphologies obtained during the crystallization processes, followed by optical microscopy. While the loss maximum (and consequently relaxation times) remains almost unmodified during the crystallization process at 70°C, for the process at 80°C the maximum first moves slightly to higher frequencies (shortening of relaxation times) and at the final stages of crystallization it moves to lower frequencies (increasing the relaxation times). Supporting evidence about the thermal behavior of the polymers has been obtained with DSC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Nanostructure development in polystyrene‐b‐polybutadiene‐b‐poly(methyl methacrylate) (SBM) thin films by atomic force microscopy: Effect of copolymer composition and solvent

Surface morphology development for SBM triblock copolymer thin films has been studied by atomic force microscopy. The effect of copolymer composition and solvent on the final morphology has been investigated. Obtained results indicated that depending on the block ratio (symmetric or asymmetric with minority middle block) and solvent, lamellar, hexagonal, cylindrical, or spheres in lamellae (ls)‐type morphologies can be achieved at film surfaces. The influence of the interaction parameters among blocks and solvents and cohesive energy values of block pairs on the final morphology has been proved. POLYM. ENG. SCI., 58:422–429, 2018. © 2017 Society of Plastics Engineers     

Micro- or nanoseparated phases in thermoset blends of an epoxy resin and PEO-PPO-PEO triblock copolymer

Diaminodiphenylmethane (DDM) curing at several temperatures of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer has been investigated in order to characterize the miscibility and morphological features. Two distinct phases are present for every blends studied except for DGEBA/DDM modified with 10 wt% PEO-PPO-PEO and cured at low temperature. Depending on the curing condition, phase separation takes place at micro or nanoscale due to competition among kinetic and thermodynamic factors. The mechanistic approach used for modeling the curing reactions shows that the formation of epoxy-hydroxyl complex and the auto catalytic process are slightly decreased whilst the noncatalytic process is favoured upon copolymer addition. Modifier addition delays curing process as the influence of both formation of epoxy-hydroxyl complex and catalytic process on reaction rate is higher than the influence of noncatalytic process. A thermodynamic model describing a thermoset/block copolymer considered as only one entity system is proposed. The LCST behaviour allows to elucidate nano or micro separated structures obtained at low and high curing temperatures, respectively.     

Interaction of dendronized polymeric nanocomposites with isomeric cyclohexanediols

Dendronized poly(diethylaminoethyl methacrylamide) (PEAM) was studied in blends containing isomeric cyclic dialcohols. Monomers and polymers were characterized by spectroscopic measurements. The phase behaviors of blends of PEAM with 1,2‐, 1,3‐ and 1,4‐cyclohexanediols (1,2‐CHD, 1,3‐CHD and 1,4‐CHD, respectively) were established. Transparent films of the blends exhibited a single glass‐transition temperature (T_g). Thermogravimetric analysis (TGA) revealed the temperature at which the polymer releases the small molecules. UV‐vis spectra of 1,3‐CHD and 1,4‐CHD derivatives showed an isosbestic point that indicated the association of the alcohols. FT‐IR measurements showed shifts in several absorption bands. The results were analyzed in terms of the side‐chain structure and the interactions involved. AFM measurements revealed differences between the polymer and the blends. Compatibilization of blends of PEAM/CHDs occurred via the formation of hydrogen bonds, although hydrophobic interactions could not be disregarded. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42267.     

Interactions in blends of dendronized polymeric nanocomposites with some common drugs

Poly(diethylaminoethyl methacrylamide) (PEAM), a dendronized polymer, was synthesized according to classical procedures. Monomers and polymers were characterized by spectroscopic measurements. The results obtained were in agreement with the expected chemical structure. The phase behavior of blends of PEAM with diclofenac (DCF), ibuprofen (IBU) , and paracetamol (PCM) were studied by different experimental techniques. FT‐IR, UV‐Vis, DSC , and TGA measurements suggested important interactions between the blended components. Thermogravimetric analysis (TGA) indicated the temperature at which the dendronized polymer released a small molecule. AFM measurements and molecular dynamics (MD) simulations were used to better understand the nature of the interactions and to estimate the distance between the components of the blends to explain the interaction involved. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42450.