Elsevier

Polymer

Volume 138, 28 February 2018, Pages 113-123
Polymer

Morphologies and dielectric properties of epoxy thermosets containing poly(N-vinylcarbazole), fullerene-C60 and their charge transfer complex nanophases

https://doi.org/10.1016/j.polymer.2018.01.057Get rights and content

Highlights

  • Both PEO-C60 and PEO-b-PVK diblock copolymer were synthesized.

  • The nanocomposites involving epoxy, PVK and C60 were prepared with the amphiphiles.

  • Dielectric properties were greatly affected by PVK-C60 charge transfer complexation.

Abstract

Both poly(ethylene oxide)-block-poly(N-vinylcarbazole) (PEO-b-PVK) diblock copolymer and fullerene-capped poly(ethylene oxide) (PEO-C60) were synthesized in this work. The former was obtained via reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthate process whereas the latter via click reaction of azido-capped poly(ethylene oxide) with fullerene-C60. Both of the amphiphiles were incorporated into epoxy to obtain the nanostructured thermosets. It was found that the PVK-C60 charge transfer complexes were formed in the ternary thermosetting blends of epoxy, PEO-b-PVK and PEO-C60. The nanostructured epoxy thermosets containing PVK or/and fullerene displayed the enhanced dielectric constants whereas the dielectric loss remained almost unchanged. Nonetheless, the dielectric constants were significantly decreased while both PEO-b-PVK and PEO-C60 were simultaneously incorporated into the thermosets. The depression in dielectric constant is attributable to the formation of the charge transfer complexes of PVK with fullerene-C60. It was found that the dielectric constants can be modulated with the contents of the PVK-C60 charge transfer complexes.

Introduction

Thermosets such as epoxy, phenolic and maleicimide have been applied as matrix of composites, high performance adhesives and electronic encapsulation materials due to their excellent mechanical strength, chemical resistance and processing properties [[1], [2], [3]]. Of these thermosets, epoxy polymers feature prominently in structural and electronic materials [4,5]. Depending to the assignment of the materials, epoxy must be modified to exhibit different dielectric properties. For the use as encapsulation dielectric in high-speed digital circuits, low dielectric constants together with low dielectric loss are required. The reduction in dielectric constants is favorable to the increase of information transfer rate and to the decrease of crosstalk; the miniaturization and integration of the devices can thus be achieved [6]. For the use for electric charge storage, transportation and heat dissipation, nonetheless, it is necessary to modulate the materials to have higher dielectric constants [7,8]. Therefore, it is important to control the dielectric properties of epoxy thermosets according to the assignment of the materials.

By introducing conductive fillers at the concentrations below percolation thresholds, dielectric constants of epoxy can be significantly enhanced. The conductive domains in materials can significantly promote the extent of polarization. The conductive fillers can be metallic powders [8,9], low dimensional carbon materials [10,11] and conducting polymers [12,13]. In addition, dielectrics with high relative permittivity can also be employed toward this end [[14], [15], [16], [17], [18], [19]]. In marked contrast to the strategy to enhance dielectric constants, investigators have explored a variety of approaches to decrease dielectric constants of epoxy to meet the application of the materials as electric encapsulation materials for embedded passives. In principle, dielectric constants of the materials can be lowered via the incorporation of the components with low dielectric constants. Taking advantage of the lowest dielectric constant, air would be introduced into materials via the formation of highly porous structures [20]. Recently, it is reported that the dielectric constants of epoxy can be lowered via the preparation of the organic-inorganic nanocomposites containing polyhedral oligomeric silsesquioxanes (POSS) [21,22]. The roles of POSS in the nanocomposites are: i) the carrier of porosity owing to their hollow structures; ii) the nanoreinforcement on epoxy matrix, with which the mobility of dipoles in epoxy network is significantly restricted.

Poly(N-vinylcarbazole) (PVK) is a typical photoconductive polymer and can be employed as organic photorefractive and photovoltaic materials and for polymeric light emitting diodes [23,24]. This polymer would display the self-association via the interactions of the π-conjugated electrons of carbazole groups. It is found that the so-called “sandwich” and “second” excimers are formed owing to the stacking of carbazole groups [25,26]; the former are from the fully overlapped structure of neighboring carbazole chromophores in a totally elipsed conformation of the isotactic sequence whereas the latter is due to the partially overlapped structure of the syndiotactic sequence [25,26]. It is the π-conjugated electrons of PVK that can significantly promote the electron polarization in epoxy and thus enhance the dielectric constants of the thermosets [27]. Fullerenes are a class of highly symmetric cage-shaped molecules and they are only composed of π-conjugated carbon atoms. Since the discovery of fullerenes in 1990s, ones have been exploring various possibilities that such a carbon nanostructure is applied in a variety of fields [[28], [29], [30], [31], [32]]. In the past years, there has been ample literature to report the modification of epoxy by the use of fullerenes [[33], [34], [35], [36], [37]]. Most of these previous reports deal with the improvement of mechanical toughness of epoxy thermosets with fullerenes. However, the dielectric properties of the modified epoxy thermosets were scarcely reported. Notably, Pikhurov et al. [38] investigated the molecular dynamics in epoxy thermosets containing fullerene-C60 by means of dielectric spectroscopy. It was found that the dielectric constant was significantly increased by c.a. 67% with the loading of a very small amount of C60 (∼0.08%). It should be pointed out that the goal of the previous report [38] is only to interpret the mechanism of toughness improvement in the composites by means of dielectric spectroscopy. Nonetheless, the mechanism of dielectric constant enhancement has not been elucidated in depth.

It has been realized that the charge transfer complexation can be formed between PVK and C60 and that the charge transfer complexes are ion-radical pairs [[28], [29], [30],[39], [40], [41], [42], [43], [44]]. The charge transfer complexation has been exploited to enhance photoconductive response of the nanocomposites [[39], [40], [41], [42], [43], [44]]. In this work, we explored to modulate the dielectric properties of epoxy thermosets via the formation of the PVK-C60 charge transfer complexes. First, we synthesized a poly(ethylene oxide)-block-poly(N-vinylcarbazole) (PEO-b-PVK) diblock copolymer and a C60-capped poly(ethylene oxide) (denoted PEO-C60). Thereafter, the PEO-b-PVK or/and PEO-C60 was incorporated into epoxy to obtain the nanostructured thermosets. The purpose of this work is two-fold: i) to explore to modulate dielectric constant of epoxy thermosets via formation of PVK, C60 nanophases and ii) to investigate the effect of the charge transfer complexation between PVK and C60 on the dielectric properties. To the best of our knowledge, there is the first report on the effect of PVK-C60 charge transfer complexation on epoxy thermosets. In this work, the formation of the nanophases of PVK, fullerene and their charge transfer complexes in the thermosets was then investigated by means of transmission electron microscopy and small angle X-ray scattering; the dielectric behavior of the materials was addressed on the basis of the morphologies of the nanocomposites.

Section snippets

Materials

N-Vinylcarbazole (NVK) (98%) was purchased from TCI Reagent Co., China; it was purified by recrystallization twice from methanol. 2,2-Azobisisobutylnitrile (AIBN) was of chemically pure grade, purchased from Shanghai Reagent Co., China, and it was re-crystallized from ethanol twice before use. Poly(ethylene oxide) monomethyl ether (PEO5000) was purchased from Fluka Co., Germany and it had a quoted molecular weight of Mn = 5000. Diglycidyl ether of bisphenol A (DGEBA) was supplied by Shanghai

Synthesis of PEO-b-PVK diblock copolymer and fullerene-capped PEO

The route of synthesis for poly(ethylene oxide)-block-poly(vinyl carbazole) (PEO-b-PVK) diblock copolymer is depicted in Scheme 1. First, the macromolecular chain transfer agent was prepared via the esterification reaction of PEO monomethyl ether (PEO5000) with xanthogenic acid (XA). Second, the radical polymerization of N-vinylcarbazole (NVK) was mediated with the macromolecular chain transfer agent (i.e., PEO-CTA) and a PEO-b-PVK diblock copolymer was thus obtained. The 1H NMR spectra of the

Conclusions

In this work, we successfully synthesized a poly(ethylene oxide)-block-poly(N-vinyl carbazole) (PEO-b-PVK) diblock copolymer and a fullerene-capped poly(ethylene oxide) (PEO-C60). The former was synthesized successfully via sequential reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthate process (RAFT/MADIX) and the latter was prepared via click reaction of azido-capped poly(ethylene oxide) with fullerene-C60. Both of the amphiphiles were

Acknowledgment

The financial supports from Natural Science Foundation of China (No. 51133003, 21274091 and 21774078) were gratefully acknowledged. The authors thank the Shanghai Synchrotron Radiation Facility for the support under the projects of Nos. 10sr0260 & 10sr0126.

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