Complete relaxation map of polypropylene: radiation-induced modification as dielectric probe

Abstract

The molecular relaxation behaviour of isotactic polypropylene (iPP) exposed to gamma radiation in air to various absorbed doses (up to 700 kGy) has been investigated by dielectric loss (tan d) analysis; the polar (mainly carbonyl and hydroperoxide) groups that were introduced by radiation-induced oxidation were considered as tracer groups. All relaxation zones (a, b, c and d in the order of decreasing temperature), between 25 K and melting temperature, were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the iPP samples to radiation. Wide-angle X-ray diffraction was used to investigate radiation-induced changes in the crystalline structure and degree of crystallinity, since the a relaxation is connected with this phase. Infrared spectroscopy and gel measurements were used to determine the changes in the oxidative degradation and the degree of network formation, respectively. Conclusions derived using different methods were compared. This study reveals high dielectric and/or relaxation sensitivity of iPP to gamma radiation. Disappearance of the lowtemperature dielectric (c and d) relaxations together with large changes in intensity, position and activation energy of the dielectric a relaxation are observed with gamma irradiation and are mainly connected with oxidative degradation in iPP structure.

Influence of the presence of medium-soft paraffin wax on the morphology and properties of iPP/silver nanocomposites

“…The difference in conductivity between iPP and iPP/Ag slightly increases with increasing temperature up to this temperature, followed by a decrease at higher temperatures. The non-polar iPP usually shows a +-relaxation, but the introduction of polar groups had previously made the +-relaxation peak disappear [25]. The observed +-relaxation in iPP/Ag therefore confirms that the Ag particles did not improve the polarity of iPP.…”

The effect of poly[styrene-b-(ethylene-co-butylene)-b-styrene] on dielectric, thermal, and morphological characteristics of polypropylene/silica nanocomposites

“…The first relaxation process had similar behavior in nanocomposites as in the references, and the temperature dependence of the relaxation times obeyed the Arrhenius law, also characteristic to α‐relaxation processes. The values of E a (Table ) obtained for PP nanocomposites, between 80 and 90 kJ/mol, are close to the activation energy of the α‐relaxation process of the crystalline PP phase measured by other methods, usually from 90 to 170 kJ/mol . Higher activation energy and lower relaxation time were obtained for C2 as compared to the other two nanocomposites.…”

“…Dielectric spectroscopy can provide quantified insights into the molecular dynamics of polymeric materials and to give valuable information on the interaction between components, the reinforcing effect of nanofillers, or the toughening effect of the elastomer phases in polymer blends . Several dielectric studies on the molecular relaxation behavior of PP are available . Although oxidation and chain scission introduce polar groups in PP, dielectric spectroscopy of this nonpolar polymer is less commonly used as a characterization tool.…”

“…Nevertheless, the addition of low concentration of dielectric probe (polar rigid‐rod‐like chromophores) allowed the dielectric study of glass transition dynamics, crystallization, and melting of PP . Recently, a complete relaxation map of gamma‐radiated PP, obtained by dielectric measurements as a function of temperature, has been reported . Two relaxation processes, α‐relaxation at high temperature and β‐relaxation related to glass‐rubber transition ( T g ), were observed in irradiated samples.…”

“…Information regarding the frequency range of the main relaxation processes in PP and PP modified with MA‐PP in the published literature is even scarce. Pandis et al have shown that PP without additives exhibits no detectable dielectric relaxation mechanisms and ε′ remains constant from 10 −2 to 10 6 Hz, but Suljovrujic has detected a β relaxation process, related to the glass transition, close to the room temperature in the isochronal dielectric spectrum of isotactic PP at 1 MHz. Few works related to dielectric properties of PP blends and nanocomposites have been reported .…”