Structural aspects of the radiation modification of the dielectric properties of polyolefins

Abstract

1 A wide variety of supermolecular structure elements in polyolefins, in particular, low-density polyethylene (LDPE), high-density polyethylene (HDPE), and isotactic polypropylene (PP), provides prerequisites to the target-oriented and efficient modification of their structure and electric properties by γ -or electron-beam irradiation [1,2]. It is well known [3][4][5] that polyethylene (PE) is a preferably crosslinkable polymer, whereas PP is prone to oxidative degradation. Changes in the electric and dielectric properties upon radiation modification strongly depend on polymer crystallization conditions and irradiation mode [2,6]. Certain doses of γ -radiation on PE and PP lead to an increase in the electric strength E st [4][5][6][7] and to changes in the dielectric loss tangent , permittivity ε [8], electric conductivity [9], charge state [10], and mechanical strength [1,11]. These changes in the dielectric properties of polymers are primarily associated with crosslinking, oxidation, degradation, and reversible and irreversible processes. Note that the supermolecular structure and crystallization conditions of a polymer can affect the kinetics and direction of these processes. In this case, it is of importance to refine the roles of the crystalline phase and the size of crystallites, spherulites, and individual elements of the polymer supermolecular structure. For example, according to published data [12], treelike tracks branched under exposure to an electric field developed in crosslinked PE along the lamellar structure, whereas the development of a fine spherulitic structure with a high crystallinity in PP improved the electret properties of the polymer [13]. However, the possible influence of transient species (ions, radicals, and geminate and bound pairs) and bulk and polarization effects on the electrophysical, service, and strength properties of PE and PP were not discussed in the liter-1 E-mail: arifm50@rambler.ru δ tan ature. Of the performance characteristics, the electric strength E st and the mechanical strength σ m of polymers and compositions are the most important. At high fields (close to prebreakdown fields) in PE, the processes of strength losses are preceded by free-radical states [14]. Free radicals as electron traps affect radiation-induced electric conductivity [9,15]. At the same time, taking into account structure relaxation, elementary ion-radical reactions, etc., in polyolefins during γ -irradiation could reveal other conceivable reasons for the effect of electric strengthening and changes in the parameters ε and . The mechanisms of the radiation-induced oxidative degradation of PP [14] and the role of intermolecular and intramolecular crosslinks, excited and ionic states, posteffects, and radical formation in the radiation modification of structures and properties are not clearly understood. In combined effects on polymers, synergistic [16] and antagonistic effects [17] were not taken into consideration. The study of these problems in the structure-property-technology aspect is of gre...

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

“…Changes in the intensity of dielectric b and a relaxations with gamma irradiation are related to radiation-induced oxidation. The radiation-induced increase in the amount of carbonyl, hydroperoxide and other polar groups in apolar iPP causes modification of the dielectric properties due to the increase in polymer polarity [19,33,34,[48][49][50]. Moreover, it is known that polar groups generate shallow traps and enhance charge injection [5,51,52].…”

Study of polymer-piezoceramic composites by the radio-thermo-luminescence method

“…Furthermore, the effectiveness of the electroactive properties of polymer composites (PC) formation is also influenced by interphase phenomena, volume charge generation, etc. [4,5]. In particular, the polarized state is shown to lead to recapture of charges stabilized on the traps, which results in a decrease of the dielectric losses in PE and PP electric films [2][3][4][5].…”

“…[4,5]. In particular, the polarized state is shown to lead to recapture of charges stabilized on the traps, which results in a decrease of the dielectric losses in PE and PP electric films [2][3][4][5]. At the same time, the role of the structural and nonequilibrium processes in the formation of the electroactive properties of composites has not been studied sufficiently well.…”

“…Note that radiation-technological issues related to the production of radioelectrets and photoelectrets of PC, as well as related to the radiation and structural modification of their electroactive properties, are becoming of special importance. Study of PC relaxation properties by the method of radio-thermo-luminescence (RTL) will allow one to reveal some particularities of the electroactive properties acquisition by polymer materials [3][4][5][6][7][8] and to determine points of excess charges localization on traps of various nature. The RTL method was used in [8] to study the existence of structural transition temperatures in polymer composites based on BaTiO 3 .…”

Dielectric properties of polymer composites with TlInSe2 semiconductor filler

“…It is known that the ternary semiconductor compound TlInSe 2 exhibits high coefficients of photo-and tensosensitivity, as well as switching and memory effects [10,12,13]. Composites on its basis also exhibit good electret properties [7].…”