Surface Modification of BaTiO 3 Inclusions in Polydicyclopentadiene Nanocomposites for Energy Storage Sean P. Culver, Christopher W. Beier, Jessica P. Rafson, Richard L. Brutchey Department of Chemistry, University of Southern California, Los Angeles, California 90089 S. P. Culver and C. W. Beier contributed equally to this work. Correspondence to: R. L. Brutchey (E - mail: brutchey@usc.edu) ABSTRACT: A new nanocomposite system displaying high breakdown strength, improved permittivity, low dielectric loss, and high thermal stability is presented. Free-standing nanocomposite films were prepared via a solvent-free in-situ polymerization technique whereby 5 vol % BaTiO 3 (BT) nanocrystals with tailored surface chemistry were dispersed in dicyclopentadiene (DCPD) prior to ini- tiation of ring opening metathesis polymerization by a second generation Grubbs catalyst. The relative permittivity was enhanced from 1.7 in the neat poly(DCPD) film to a maximum of 2.4 in the composite, while the dielectric loss tangent was minimized below 0.7%. Surface modification of the BT nanocrystals mitigated reduction in breakdown strength of the resulting nanocomposites such that only a 13% reduction in breakdown strength was observed relative to the neat polymer films. V C 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40290. KEYWORDS: composites; nanoparticles; nanocrystals; ring-opening metathesis; surfaces; interfaces Received 13 November 2013; accepted 12 December 2013 DOI: 10.1002/app.40290 INTRODUCTION Recently, there has been significant interest in the development of capacitors that can meet energy storage needs that require high breakdown strength, low dielectric loss, and high thermal stability. By using a nanocomposite approach, it has become possible to take advantage of the solution processability and high breakdown strength of polymers combined with the high permittivity of inor- ganic nanoparticles toward achieving this goal. Among the avail- able inorganic filler materials for nanocomposite capacitors, BaTiO 3 (BT) has become prevalent because of its high dielectric constant. 1,2 As a result, the addition of BT nanocrystals into poly- mer matrices has been shown to systematically increase composite permittivity. The main drawback of using inorganic fillers is that their inclusion, even at low volume loadings (5 vol %), often leads to a cataclysmic reduction of the nanocomposite breakdown strength, lessening any benefit achieved from increased permittiv- ity in terms of energy storage capability. 3–6 Polydicyclopentadiene (pDCPD) is a crosslinked thermoset poly- mer that can be prepared via ring opening metathesis polymer- ization of dicyclopentadiene (DCPD) with Grubbs catalyst. 7–9 Polymerization occurs within the neat monomer (i.e., under solvent-free conditions) and is initiated by very low catalyst load- ings (<1 wt %). pDCPD has found widespread application as a result of its high thermal stability and chemical resistance, low water uptake, and excellent mechanical properties. 8,10 In compari- son to the well-known thermoplastics polypropylene and polysty- rene, pDCPD displays similar permittivity but far better thermal stability (i.e., up to 500  C), thereby extending its use to a variety of high temperature applications. 11,12 While several studies have explored the physical and mechanical attributes of various pDCPD nanocomposites, 13–15 there has been very little study on investigating the dielectric properties. Yin et al. recently published the first known study on the dielectric properties of pDCPD- based nanocomposites, using fumed silica inclusions (10 wt % loading). 12 In their study, neat pDCPD films exhibited break- down strengths as high as 750 V lm 21 with very low dielectric loss (tan d 5 0.5%). Although the pDCPD/SiO 2 composites dem- onstrated improvements in the relative permittivity and corona resistance under an AC bias relative to the neat polymer, the DC breakdown strengths were not reported. To date there have been no studies on incorporating BT inclusions into pDCPD nanocomposites. Herein we investigate the dielectric properties of a model pDCPD/BT nanocomposite. Nanocrystals of BT were surface modified with 10-undecenoic acid to affect the nanocrystal–polymer interface and mitigate losses in breakdown strength. It should be noted that while several studies have probed the effects of phosphonic acid modified inclusions, 4,6 the utility of carboxylic acid ligands has received far less attention. A distinct ligand-dependent effect on the measured breakdown strength was Additional Supporting Information may be found in the online version of this article. V C 2013 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2014, DOI: 10.1002/APP.40290 40290 (1 of 5)