Pregledni rad
Relaxation properties and glass transition by radiation crosslinking of unsaturated polyester resins
Želimir JELČIĆ
; PLIVA-Istraživanje i razvoj d.o.o., Zagreb
Franjo RANOGAJEC
; Institut Ruđer Bošković, Zagreb
Sažetak
Variations in the co-monomer, styrene content in the unsaturated polyester (UP) resin affect the thermal stability and the mechanical properties. These are governed by the phase behaviour of the mixture. Dynamic relaxation measurements by dielectric related methods show an increas in the phase separation of cured resin with an increase of styrene concentration. Dielectric permittivity was measured by stepwise changing of frequency during 60Co γ-radiation curing at a constant temperature or by the simultaneous change of frequency and temperature after irradiation. Variations in the appearance of the glass relaxation due to varying degrees of immobilization of long range segmental motion in the uncrosslinked fraction by the introduction of the crosslinked phase by γ-radiation is made obvious by dielectric relaxation measurements of UP resins in the range of the gel fraction from zero up to 100%. A continuous shift of the glass transition of the resin and a change of the dielectric relaxation time distribution was observed as radiation crosslinking reaction proceeded. The shape of the dielectric spectrum bands indicated a heterogeneous nature of the transition which increased with the increasing degree of curing. In-source radiation curing of UP resins was studied by combining dielectric, thermal and charging current analyses. The transition reaction times corresponding to the maximum of the derivative (dε’/dt) curves, where ε’ is the dielectric constant, regarded as the transition times at the corresponding frequency, are shorter for higher frequencies than for the lower ones. It is seen that the average relaxation time is shifted continuously by the reaction, while the temperature is constant and the frequency is changed as a parameter. Meanwhile the distribution of relaxation times is in-homogeneously broadened and the oscillator strength decreases. The α- and β-relaxations of UP resins shift to higher temperatures as the radiation dose (and crosslink density) increases. The relaxations, however, do not appear to be equally affected by the curing, β-relaxation being almost completely superposed by α-relaxation. The simulation of the
dielectric spectra of the partially radiation crosslinked UP resins by the Geny-Monnerie model can be used for following the phase separation. The fragility index of the UP resins glass transition slightly changes as the styrene concentration increases indicating more fragility. The nuclear magnetic resonance (NMR) spin-lattice T1 and spin-spin T2 relaxation times can be described also by the terms from the Geny-Monnerie model that describes the polymer chain isotropic motion (4-bonds motion, θ) and by the shift from the ideal lattice (3-bonds motion in the ideal lattice, ρ). The thermally stimulated depolarisation currents of radiation and peroxy cured UP resins have been measured in the low temperature range. The study of the decrease of the current intensity as a function of the absorbed dose reveals important differences between the peaks of the spectrum; the low
temperature peaks can be attributed to dipolar volume polarisation, and the high temperature peaks to an ionic space charge polarisation. The shift of the peaks may be indicative of plasticization of the polyester by styrene, and of restriction upon motions of the chain imposed by the crosslinks. The αl-peak is a manifestation of the liquid-liquid (Tll) transition, and here it corresponds to the thermally activated disintegration of phase separated structures, which were well documented in the inherently heterogeneous UP resins. From the Tll/Tg ratio, it may be concluded that the liquid-liquid (Tll) transition corresponds to the characteristic crossover temperature Tc derived in the random walk model. The charging current data above Tll and below Tg are very well described by a simple Arrhenius law with the same (or similar) activation energy Elow suggesting that dynamic scaling continues to apply to the glass, with length ξ independent of temperature. The Tll/Tg ratio, from TSD data, is found to be in the range of 1.06 to 1.21. It could be noticed that as the degree of cure increases the temperature at which maximum occurs shifts to higher values. The extent of cure can be
detected as a systematic change in TSD current, as the height of the TSD peaks decrease by curing. TSD current measurements reveal the presence of high internal electrical fi elds due to space charges accumulated at the phase boundaries of glass fi bres in fi lled UP resins. The compliance curves from thermomechanical measurements on partially radiation cured UP resins show transitions at increasing temperature by increasing the radiation dose, i.e. degree of crosslinking.
Ključne riječi
phase separation; relaxation; styrene; unsaturated polyester(UP) resin
Hrčak ID:
65902
URI
Datum izdavanja:
24.3.2011.
Posjeta: 2.856 *