Abstract
The molecular mobility related to the glass transition and secondary relaxations in a hyperbranched polyethyleneimine, HBPEI, and its relaxation behaviour when incorporated into an epoxy resin matrix are investigated by dielectric relaxation spectroscopy (DRS) and dynamic mechanical analysis (DMA). Three systems are analysed: HBPEI, epoxy and an epoxy/HBPEI mixture, denoted ELP. The DRS behaviour is monitored in the ELP system in three stages: prior to curing, during curing, and in the fully cured system. In the stage prior to curing, DRS measurements show three dipolar relaxations: γ, β and α, for all systems (HBPEI, epoxy and ELP). The α-relaxation for the ELP system deviates significantly from that for HBPEI, but superposes on that for the epoxy resin. The fully cured thermoset displays both β- and α-relaxations. In DMA measurements, both α- and β-relaxations are observed in all systems and in both the uncured and fully cured systems, similar to the behaviour identified by DRS.
Highlights
Cured epoxy resins are thermosets exhibiting a crosslinked structure with excellent electrical properties, good chemical and corrosion resistance, low shrinkage, and high tensile strength and modulus
The dielectric spectra for the hyperbranched poly(ethyleneimine) (HBPEI) obtained at a heating rate of 0.5 K/min and over a secondary relaxations in this hyperbranched polymer
The dielectric and dynamic mechanical relaxations in an epoxy system cured with a hyperbranched polyethyleneimine, HBPEI, have been studied in experiments at constant heating rate
Summary
Cured epoxy resins are thermosets exhibiting a crosslinked structure with excellent electrical properties, good chemical and corrosion resistance, low shrinkage, and high tensile strength and modulus. Dendrimers are molecules with a three-dimensional globular shape [4,5,6], in which well-defined highly branched molecules emanate from a central core and have a large number of functional groups at the periphery This molecular architecture endows dendrimers with unusual chemical and physical properties, such as their extraordinarily high functionalities [7], and has led to their use in diverse applications. Poly(propylene imine) dendrimers have been shown to plasticize a thermoplastic, polyvinyl chloride [8], the plasticization effect being considered to occur as a result of additional free volume from the highly branched structure of the dendrimer It is their use as curing agents for thermosets, quite widely reported in the literature recently, which is of particular interest here. Some authors [9,10] have used polyamido-amine dendrimers grafted onto silica as the curing agent in epoxy systems, and report increased thermal stability in comparison with
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