M. Salmerón Sánchez

Forced compatibility in poly(methyl acrylate)/poly(methyl methacrylate) sequential interpenetrating polymer networks

The aim of this work is to study the miscibility of poly(methyl acrylate)/poly(methyl methacrylate), (PMA/PMMA), sequential interpenetrating networks, (IPNs), as a function of the crosslink density using dielectric and dynamic-mechanical techniques. The PMA/PMMA system is immiscible and so, for low crosslink densities, phase separation appears, as detected by the occurrence of two clearly differentiated main dynamic-mechanical relaxation processes corresponding to the two components. If crosslink density is high enough, a homogeneous IPN can be obtained, achieving a forced compatibilization of both networks. The IPN crosslinked with 10% ethyleneglycol dimethacrylate shows a single main dynamic-mechanical relaxation process. Only the α main relaxation process appears in the PMA networks within the temperature range (−60 to 200°C) of the experiments conducted in this work. The dielectric relaxation spectrum of PMMA networks shows the secondary β relaxation followed by a small α relaxation partially overlapped with it. In the IPNs, both the main relaxation processes tend to merge into a single one and the dielectric spectrum shows a single peak that mainly corresponds to the secondary relaxation of the PMMA.

Blends of styrene–butadiene–styrene triblock copolymer and isotactic polypropylene: morphology and thermomechanical properties

A set of blends of styrene–butadiene–styrene triblock copolymer (SBS) and isotactic polypropylene (i-PP) in a composition range 0–100 % polypropylene by weight was prepared in a twin screw extruder. The morphology of the blends has been studied by transmission electron microscopy. The blends present phase separation. Dynamic mechanical measurements show an improvement of the mechanical properties of SBS when i-PP is the dispersed phase. This reinforcing effect can be observed even at high temperatures when i-PP is in the rubbery state. The mechanical properties of the blends have been interpreted using Takayanagis block model. The melting and crystallization behaviour of the i-PP in the blends has been studied by differential scanning calorimetry. The fractionated crystallization phenomenon has been observed in the blends where i-PP forms the dispersed phase. The results are consistent with the morphology shown by the blends, in particular, with its phase inversion, which occurs at a composition near to 50% i-PP. © 2000 Society of Chemical Industry

Dielectric relaxation spectrum of poly (ε-caprolactone) networks hydrophilized by copolymerization with 2-hydroxyethyl acrylate

Abstract.The dielectric relaxation spectrum of polycaprolactone (PCL) networks hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) is investigated. PCL is a semicrystalline polyester with a complex relaxation spectrum that includes the main α relaxation and two secondary modes (β, γ) at lower temperatures. The overlapping of the different relaxational modes was split by using several Havriliak-Negami functions. Crosslinking the material modifies the dynamics of the main relaxation process as reflected by the parameters that characterize the Vogel behavior of the process and the dynamic fragility. The incorporation of HEA units in the network results in a material with microphase separation: two α processes are detected, the one corresponding to the PCL chains and the new one associated to nanometric regions that contain different amount of both comonomers. The incorporation of the HEA units in the system involves the presence of a new βsw relaxation due to the link of two side chains by water molecules through hydrogen bonding.

Melting of benzene in a poly(ethyl acrylate) network studied by TMDSC

First order transitions of a solvent in a gel differ from those of the solvent alone. In particular, the existence of either several endothermic peaks, or of a broad one is a common feature. In this respect, melting of a solvent sorbed in a polymer matrix presents a phenomenology similar to that of a semi-crystalline polymer. In this work temperature modulated differential scanning calorimetry (TMDSC) has been applied to study the melting behaviour of benzene in a poly(ethyl acrylate), PEA, network. A strong frequency dependence is found both of the real and imaginary part of the complex heat capacity in the transition regions. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al., developed for the kinetic melting behaviour of crystalline polymers, which seems to be applicable also to the system studied in this work.

Phenomenological theory of structural relaxation based on a thermorheologically complex relaxation time distribution

The aim of this work is to explore the consequences on the kinetics of structural relaxation of considering a glass-forming system to consist of a series of small but macroscopic relaxing regions that evolve independently from each other towards equilibrium in the glassy state. The result of this assumption is a thermorheologically complex model. In this approach each relaxing zone has been assumed to follow the Scherer-Hodge model for structural relaxation (with the small modification of taking a linear dependence of configurational heat capacity with temperature). The model thus developed contains four fitting parameters. A least-squares search routine has been used to find the set of model parameters that fit simultaneously four DSC thermograms in PVAc after different thermal histories. The computersimulated curves are compared with those obtained with Scherer-Hodge model and the model proposed by Gómez and Monleón. The evolution of the relaxation times during cooling or heating scans and also during isothermal annealing below the glass transition has been analysed. It has been shown that the relaxation times distribution narrows in the glassy state with respect to equilibrium. Isothermal annealing causes this distribution to broaden during the process to finally attain in equilibrium the shape defined at temperatures above Tg.

Dielectric study of miscibility in poly(methyl acrylate)-poly(methyl methacrylate) sequential interpenetrating polymer networks

The miscibility of PMA/PMMA interpenetrating polymer networks (IPNs) is studied as a function of the crosslinking density using dielectric and dynamic-mechanical techniques.The miscibility of PMA/PMMA interpenetrating polymer networks (IPNs) is studied as a function of the crosslinking density using dielectric and dynamic-mechanical techniques.