APPLIED PHYSICS LETTERS 98, 062901 共2011兲
Cross-linking effect on dielectric properties of polypropylene thin films and applications in electric energy storage Xuepei Yuan and T. C. Mike Chunga兲 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
共Received 7 December 2010; accepted 13 January 2011; published online 7 February 2011兲 A family of cross-linked polypropylene 共x-PP兲 thin film dielectrics is systematically studied to understand the cross-linking effect on the dielectric properties. Evidently, the butylstyrene 共BSt兲 cross-linkers increase both the dielectric constant 共兲 and breakdown strength 共E兲, without increasing energy loss. An x-PP dielectric, with 3.65 mol % BSt cross-linkers, exhibits a ⬃ 3, which is independent of a wide range of temperatures and frequencies, slim D-E hysteresis loops, high breakdown strength 共E = 650 MV/ m兲, narrow breakdown distribution, and reliable energy storage capacity ⬎5 J / cm3 共double that of state-of-the-art biaxially oriented polypropylene capacitors兲, without showing any increase in energy loss. © 2011 American Institute of Physics. 关doi:10.1063/1.3552710兴 Capacitors1–3 store energy in the form of an electrostatic field. Opposite to batteries, which have high energy density and low power density, capacitors usually exhibit high power density but very low energy density. The inherent scientific challenge is to increase the energy density of the capacitor, which is governed by the dielectric material4–8 that separates the opposite static charges on two electrode surfaces. Stateof-the-art metallized biaxially oriented polypropylene 共BOPP兲 film capacitors,9,10 with a dielectric constant = 2.2, thickness d = 10 m, and an applied electric field E = 500 MV/ m, can offer an energy density of 2.4 J / cm3 and exhibit very small energy loss during the chargingdischarging cycles. Based on the linear energy density 共J / cm3兲 = 21 oE2 共o = 8.85⫻ 10−12 F / m兲,11,12 the desirable high energy density dielectric material shall exhibit high dielectric constant 共兲, high breakdown strength 共E兲, and very low energy loss during the charging-discharging cycles. In our previous paper,13 we focused on the increase of the value by synthesis of poly共propylene-co-undecen-11-ol兲 copolymer containing flexible OH polar groups that spontaneously form OH dimmers by H-bonding. This polar group structure shows a significant increase in the dielectric constant to about 4.6 共more than twice of BOPP兲 and a linear reversible charge storage behavior with a high releasing energy density ⬎7 J / cm3 共two to three times of BOPP兲 after an applied electric field of E = 600 MV/ m, without showing any significant increase of energy loss. It is very interesting to investigate other structure features that could increase breakdown strength 共E兲 that has an exponential effect to the energy density. In this letter, we have synthesized a family of thermally cross-linkable isotactic poly共propylene-co-p共3-butenyl兲styrene兲14 共PP-BSt兲 copolymers 共I兲, as illustrated in Scheme 1, for a systematic study of the cross-linking effect on the dielectric properties of isotactic polypropylene. The PP-BSt copolymers 共I兲, containing a long chain branched 共LCB兲 structure and some pending styrene moieties, are completely soluble in xylene at elevated tempera-
tures. The solution case thin films 共II兲 show active crosslinking activity at 220 ° C by engaging in an interchain cycloaddition between pendent styrene units15 to obtain cross-linked polypropylene 共x-PP兲 thin film dielectrics 共III兲 共thickness of ⬃10 m兲, without forming any by-product. Impurity has a detrimental effect to the film stability under high electric fields.16 The resulting x-PP films 共III兲 were subjected to a vigorous solvent extraction to remove the soluble fraction that was not fully cross-linked into the network structure. Evidently, the cross-linking reaction was very effective 共⬎80% gel content兲 for all three high molecular weight PP-BSt copolymers 共I兲 in Table I, with the x-PP-3 sample reaching 98% gel content. It is interesting to note that the combination of the LCB structure and pendent styrene cross-linkers in the PP copolymer offers desirable structure features in the preparation of PP thin film dielectrics with high breakdown strength and high energy density 共discussed later兲. See the supplementary material17 for detailed experimental procedures. Figure S2 in the supplemental material17 compares their differential scanning calorimetry 共DSC兲 curves during the second heating-cooling cycle. The presence of LCB and cross-linking structures in x-PP films slightly reduces melting and crystallization temperatures and the overall crystallinity. The single melting and crystallization peak—with monochromatic reductions of both peak temperatures and crystallinity versus the incorporated BSt content—also implies the homogeneity of the x-PP molecular structures. Overall, the three x-PP films still show good crystallinity, high melting temperature, excellent mechanical strength, and high thin film dimensional stability at elevated temperatures 共⬎220 ° C兲 that is very important in capacitor applications.
a兲
Electronic mail:
[email protected].
0003-6951/2011/98共6兲/062901/3/$30.00
PP
(I)
( CH2-CH)x ( CH2-CH)y ( CH2)2 ( CH2)2
CH=CH2
( CH2)2 PP
solution-casting
LCB PP-BSt copolymer thin film (thickness ~10 mm) (II) cross-linking 220o C x-PP thin film dielectric (thickness ~10 mm)
(III)
SCHEME 1. Preparation of x-PP thin film dielectrics. 98, 062901-1
© 2011 American Institute of Physics
062901-2
Appl. Phys. Lett. 98, 062901 共2011兲
X. Yuan and T. C. M. Chung
TABLE I. Summary of polymer structure, and dielectric properties of x-PP copolymers and the corresponding PP homopolymer. Polymer structure
Dielectric properties
Sample
Molecule weight 共kg/mol兲a
BSt content 共mol %兲 b
Gel content 共wt %兲
Dielectric constant 共25 ° C, 1 kHz兲
Energy density 共J / cm3兲 c
␣ valued 共62.3%兲 共MV/m兲
 valued
PP x-PP-1 x-PP-2 x-PP-3
672 516 459 402
0 0.64 2.97 3.65
0 80.0 91.1 98.1
2.27 2.38 2.73 2.97
2.02 2.36 2.65 2.81
551.4 595.8 617.7 645.1
14.1 16.9 23.7 42.1
a
Molecular weight was determined by intrinsic viscosity of polymer/decalin solution at 135 ° C. The pendent styrene groups were determined by 1H NMR. Energy density was estimated from the discharge P-E curve after applying 500 MV/m electric field. d ␣ and  values were obtained from Weibull distribution curve of electric breakdown strength. b c
Figure 1共a兲 compares dielectric constant 共兲 profiles of the same set of x-PP thin films with a thickness of ⬃10 m, over a wide range of temperatures 共from 0 to 120 ° C兲 and frequencies 共from 100 Hz to 1 MHz兲. It is a pleasant surprise for us to observe the systematic increase of the value 共polarizability兲 with a level proportional to the BSt content. In addition, we see that all x-PP dielectric profiles resemble the BOPP profile, with a dielectric constant 共兲 that is independent over a wide range of frequencies 共between 100 Hz and 1 MHz兲 and temperatures 共between ⫺20 and 100 ° C兲. These overlapped and flat dielectric constant lines imply a fast polarization response for all x-PP copolymers, even under a relatively low electric field condition. The value of x-PP-3 with 3.65 mol % of the BSt content reaches = 3, which is significantly higher than the values 共 = 2.2兲 of BOPP. The BSt groups clearly contribute to the polarizability of the x-PP copolymer, which may be originated from the induced -electronic polarization of aromatic groups that are added to the existing -electronic polarization 共CH3 – CH groups兲 in the PP chain.
FIG. 1. 共Color online兲 共a兲 Dielectric constants 共vs frequency and temperature兲 and 共b兲 D-E hysteresis loops 共charge displacement vs the applied electric field兲 for PP, x-PP-1, x-PP-2, and x-PP-3 thin film dielectrics.
Figure 1共b兲 compares D-E 共charge displacement versus electric field兲 loops of the same set of PP and three x-PP thin film dielectrics, with an applied electric field at 500 MV/m. A dc electric field was applied across each x-PP film with an initial amplitude of 100 MV/m, which was then increased by 100 MV/m intervals until reaching 500 MV/m. As expected, the PP displays a linear D-E loop with very low hysteresis, indicating a constant dielectric constant 共 = 2.2 estimated from slope兲. More interestingly, all the x-PP copolymers also exhibit similar linear and slim D-E loops; the slope of the D-E loop increases with the BSt content, consistent with the dielectric results in Fig. 1共a兲. In addition, it remains constant over a wide range of applied electric fields, up to E = 500 MV/ m. The combination of the dielectric constant 共versus temperature and frequency兲 and polarization loops 共versus applied electric field兲 clearly demonstrates the positive effect of the BSt cross-linking feature to the PP dielectric, with increasing electronic polarizability and energy storage capacity while maintaining low energy loss: 
F共x兲 = 1 − e−共x/␣兲 .
共1兲
As discussed, the other key factor to increasing energy density is the breakdown strength 共E兲, which has an exponential effect to total energy storage. It is reasonable to think that the cross-linking 共network兲 feature that increases mechanical strength and thermal stability may also increase the electric strength. The stable network can dramatically reduce the chain mobility that creates free volume 共or even defects兲 under high applied electric fields. In fact, it is common to use cross-linked polyethylene as an insulation layer for high voltage cables.18–20 The same sets of PP and x-PP films in Fig. 1 were directly applied with an electric field to study their breakdown strengths. Every polymer film was examined in various spots over ten times, and the experimental results were fitted into the Weibull distribution F共x兲,21–23 as illustrated in Eq. 共1兲, wherein the ␣-parameter is the scale parameter and refers to the 63.2% probability for the failure to occur 关for x = ␣, F共x兲 = 0.632兴, and the -parameter is the shape parameter and describes the form of the distribution. Table I summarizes the experimental results of a comparative set of x-PP film dielectrics 共III兲 that were prepared from a set of high molecular weight PP-BSt copolymers 共I兲 having 0, 0.64, 2.97, and 3.65 mol % of the incorporated BSt 共crosslinker兲 units, respectively. Figure 2共a兲 compares breakdown strength between three x-PP copolymers and the corresponding linear PP polymer. Figure 2共b兲 shows their Weibull distributions with the estimated ␣ and  values. Evidently, the cross-linking feature
062901-3
Appl. Phys. Lett. 98, 062901 共2011兲
X. Yuan and T. C. M. Chung
strength.26 It is also worth noting that the stable network structure may be particularly advantageous in PP system that displays only the electronic polarization mechanism, with fast and constant dielectric responses. There is no need for PP chain to move during the electronic polarization. However, the polymer chain or/and polar crystal motions are essential for the ferroelectric polymer systems, such as polyvinylidene fluoride 共PVDF兲 copolymers,5,6 having the polar group orientation mechanism. In conclusion, the BSt cross-linkers systematically increase both the dielectric constant 共兲 and breakdown strength 共E兲 of x-PP dielectrics, narrowing breakdown distribution, without showing detectable increase in energy loss. An x-PP thin film dielectric having 3.65 mol % BSt units exhibits a ⬃ 3—which is independent of a wide range of temperatures 共between ⫺20 and 100 ° C兲 and frequencies 共between 100 Hz and 1 MHz兲, linear slim D-E loops, high breakdown strength 共E = 645 MV/ m兲, narrow breakdown distribution 共 = 42兲, and reliable energy storage capacity ⬎5 J / cm3 共double that of state-of-the-art BOPP capacitors兲. This work was supported by the Office of Naval Research 共Grant No. 00014-99-1-0443兲. FIG. 2. 共Color online兲 共a兲 Breakdown strength and 共b兲 Weibull distribution for PP, x-PP-1, x-PP-2, and x-PP-3 thin film dielectrics.
has a significant effect to the breakdown strength and breakdown distribution—higher cross-linking density, higher breakdown strength 共␣ value兲, and narrower distribution 共 value兲. The x-PP-3 thin film, prepared from the sample containing 2.09 mol % styrene moieties, shows a breakdown strength between 620 and 670 MV/m with an ␣ value = 645 MV/ m, which is almost the same as those of the BOPP films that are carefully conditioned 共through stretching and annealing兲 to increase chain orientation and crystallinity and to reduce defects.24 In addition, the x-PP-3 film exhibits a very narrow breakdown distribution with an exceptionally high  value of 42, indicating excellent dielectric reliability—a very important quality in capacitor applications.25 The combination of high dielectric constant 共 ⬃ 3兲, relatively high breakdown strength 共␣ = 645 MV/ m兲, and low energy loss in x-PP-3 dielectric film offers a reliable energy density ⬎5 J / cm3, significantly higher than the 2 – 3 J / cm3 typically shown in BOPP capacitors. Usually, the solution-cased films are prone to defects due to solvent evaporation. It was also reported that the breakdown strength of PP is proportional to its crystallinity.26 In this case, it is intriguing to note that our solution-cased x-PP films decrease their crystallinity but increase their breakdown strength 共Fig. 2兲. The fundamental reason for increasing breakdown strength may be originated from the control of thin film morphology 共chain motion兲 under high applied electric fields, which can be accomplished by either crosslinking or high crystallinity. The local chain or/and polar crystal motions in the polymer thin film may create free volume 共or even defects兲 that are harmful to breakdown
J. Ho, T. R. Jow, and S. Boggs, IEEE Electr. Insul. Mag. 共USA兲 26, 20 共2010兲. 2 W. J. Sarjeant, J. Zirnheld, and F. W. MacDougall, IEEE Trans. Plasma Sci. 26, 1368 共1998兲. 3 C. W. Reed and S. W. Cichanowski, IEEE Trans. Dielectr. Electr. Insul. 1, 904 共1994兲. 4 J. H. Tortai, N. Bonifaci, A. Denat, and C. Trassy, J. Appl. Phys. 97, 053304 共2005兲. 5 Z. M. Wang, Z. C. Zhang, and T. C. Chung, Macromolecules 39, 4268 共2006兲. 6 Z. C. Zhang and T. C. Chung, Macromolecules 40, 9391 共2007兲. 7 L. A. An, S. A. Boggs, and J. P. Calame, IEEE Electr. Insul. Mag. 共USA兲 24, 5 共2008兲. 8 J. C. Coburn and R. H. Boyd, Macromolecules 19, 2238 共1986兲. 9 P. Michalczyk and M. Bramoulle, IEEE Trans. Magn. 39, 362 共2003兲. 10 H. M. Banford, R. A. Fouracre, A. Faucitano, A. Buttafava, and F. Martinotti, IEEE Trans. Dielectr. Electr. Insul. 3, 594 共1996兲. 11 M. Rabuffi and G. Picci, IEEE Trans. Plasma Sci. 30, 1939 共2002兲. 12 G. Picci and M. Rabuffi, IEEE Trans. Plasma Sci. 28, 1603 共2000兲. 13 X. Yuan, Y. Matsuyama, and T. C. Chung, Macromolecules 43, 4011 共2010兲. 14 J. Langston, R. Colby, F. Shimizu, T. Suzuki, M. Aoki, and T. C. Chung, Macromolecules 40, 2712 共2007兲. 15 F. R. Mayo, J. Am. Chem. Soc. 75, 6133 共1953兲. 16 J. L. Nash, Polym. Eng. Sci. 28, 862 共1988兲. 17 See supplementary material at http://dx.doi.org/10.1063/1.3552710 for sample preparation, DSC curves, and D–E hysteresis loops. 18 S. J. Laihonen, U. Gafvert, T. Schutte, and U. W. Gedde, IEEE Trans. Dielectr. Electr. Insul. 14, 275 共2007兲. 19 H. Mitsui, F. Hosoi, and T. Kagiya, Polym. J. 共Tokyo, Jpn.兲 3, 108 共1972兲. 20 J. Barton and M. Lazar, J. Polym. Sci., Part C: Polym. Symp. 16, 361 共1967兲. 21 W. Weibull, ASME Trans. J. Appl. Mech. 18, 293 共1951兲. 22 J. C. Fothergill, IEEE Trans. Electr. Insul. 25, 489 共1990兲. 23 H. Goshima, N. Hayakawa, M. Hikita, H. Okubo, and K. Uchida, IEEE Trans. Dielectr. Electr. Insul. 2, 385 共1995兲. 24 A. Savolainen, Polym. Eng. Sci. 30, 1258 共1990兲. 25 C. C. Xu, J. Ho, and S. A. Boggs, IEEE Electr. Insul. Mag. 共USA兲 24, 30 共2008兲. 26 J. Artbauer, J. Phys. D: Appl. Phys. 29, 446 共1996兲. 1
