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Functionalized Polypropylene Dielectrics: Applications in Electric Energy Storage

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Wordcount: 5563 words Published: 8th Feb 2020

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Abstract : Biaxial-oriented polypropylene (BOPP) films are right now utilized as dielectrics in cutting edge capacitors that show numerous focal points, for example, high breakdown strength and minimized energy loss , however a restricted energy density (<2 J/cm3). This paper audits a portion of exploratory outcomes in functionalization of polypropylene with the goal to build its electric energy density and retaining every single alluring property. A group of polypropylene copolymers (f-PP) with different moieties, for example, OH, O-Si(CH3)3, long chain branching, and crosslinked structure, have been methodically blended and concentrated to investigate their dielectric properties (for example dielectric constant, breakdown strength, dielectric loss, polarization under different temperatures and electric fields). Clearly, a high atomic weight poly(propylene-co-hexen-6-ol) copolymer (PP-OH) containing 4.2 mol% of polar OH group demonstrates a dielectric constant (ε) of about 4.6 (twice of BOPP)— which is autonomous on a wide scope of temperatures and frequencies and high breakdown strength > 600 MV/m. The PP-OH dielectric exhibits a direct reversible storage with high energy density> 7 J/cm3 (multiple times of BOPP) after application of electric field at E = 600 MV/m, without demonstrating huge increment in loss of energy and leftover polarization at zero electric field. Then again, a crosslinked polypropylene (x-PP) displays a ε ~ 3, which is free of a wide scope of temperatures and frequencies, high breakdown strength (E = 650 MV/m), thin polarization loops and dependable storage capacity > 5 J/cm3 (twice of cutting edge BOPP capacitors), without demonstrating any expansion in loss of energy.

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Introduction & Background: Energy stockpiling has for quite some time been an experimentally testing and mechanically significant zone. A successful electric storage innovation that may exist today could take out the majority of our worries on increasingly atomic or coal-terminated electric generators and grow the use of clean energy sources (wind, solar and so forth.) continuously, just as viably address related natural and monetary issues [1]. Capacitors are latent electronic parts that store energy as an electrostatic field. In their most straightforward structure, capacitors comprise of two leading plates (positive and negative terminals) isolated by a protecting material called the dielectric—which can be air, ceramic, polymers, and so on [2,3,4]. Inverse to batteries, which possess high energy density and low power density, capacitors typically display maximum power density however minimum energy density. The logical test is to build the energy density of the capacitor, which is represented by the dielectric material. Ceramic capacitors produced using layers of ceramic dielectric material, for example, lead zirconate titanate, have been used to manufacture high energy density capacitors [5,6]. Notwithstanding, there are impediments with the matter of a moderately low breakdown voltage and unrecoverable damage in the structure of capacitors. They should be structured minimalistically with essentially decreased energy density in the gadget level. Then again, the metallized polymer film capacitors have pulled in a lot of consideration because of their alluring properties, for example, light weight, ease, and phenomenal processibility for framing thin film with an expansive surface zone[7,8]. They likewise show adaptability and strength under pressure, and the capacity to be bundled into an attractive setup. Some polymer capacitors dependent on semicrystalline thermoplastics, for example, polyester, polycarbonate, polypropylene show self-recuperation after a cut, which just leads in a slow loss of capacitance with the goal that they can be worked close to the breakdown voltage [9]. The  metallized polymer film capacitors, utilizing biaxial arranged polypropylene (BOPP) slim film , show detectably high dielectric quality (breakdown electric field > 600 MV/m) and oneself recuperating marvel. In spite of the low dielectric constant (ε = 2.2), BOPP film delivers fair and dependable discharging energy density in the scope of 2-3 J/cm3 and shows no energy loss amid the charging-releasing cycles [10,11].

Objectives: The scientific test here is to find a polymer dielectric that shows both high energy densities while displaying low energy loss. Based on the relation Energy Density= 12ԑE^2

, where ε is the dielectric constant that stays steady all through the applied electric field (E). The alluring dielectric material will display high dielectric constant (ε), high breakdown quality (E), and exceptionally low loss of energy amid the charging-releasing cycles [12, 13, 14]. At the end of the day main concept is that the polarization-depolarization cycle is completely reversible (low hysteresis). The dielectric constant (ε) in the polymer is contributed by a blend of incited electronic polarization (σ and π electrons) and segmental movement (including dipole orientation); the former is quick and exceedingly reversible, however the latter is moderate and ordinarily not totally reversible in the capacitor application time scale (milliseconds)[16,17,18]. The moderate randomization of the poled polar groups for the most part causes expansive hysteresis in the D-E circle (charge displacement versus applied electric field), bringing about higher loss of energy that altogether constrains the capacitor applications. Then again, the breakdown quality is emphatically subject to the film quality that is related with polymer sub-atomic weight, morphology (i.e., crystallinity and so on.) and imperfections [19, 20].

It is experimentally fascinating to see how it is conceivable to expand the dielectric properties (high ε) in PP polymer without changing great polarization reversibility (slight D-E circles) and high breakdown strength (E) so as to accomplish higher energy density in PP-based capacitors. In this paper, we efficiently explore a group of isotactic polypropylene copolymers (f-PP) that contain different – (CH2)n-CH3 (I), – (CH2)n-O-Si(CH3)3 (II), and – (CH2)n-OH (III) side groups. These synthetically changed PP polymers were incorporated and manufactured into slim films for similar dielectric and electric energy storage studies. The real goal here is to connect the impacts of the side-chain gatherings (comonomer units) and the related morphological change to their dielectric and capacitor properties.

Experiment, Result & Discussion (For f-PP polymers): The f-PP copolymers were set up by the copolymerization of propylene with different comonomers, utilizing both homogeneous and heterogeneous Ziegler-Natta catalysts [15]. Both borane and silane functional groups were known to be steady in Ziegler-Natta catalysis systems [21, 22] and the subsequent borane-and silane-containing PP copolymers were further interconverted into the comparing Hydroxyl (OH) containing polymers. True to form, the comonomer consolidation decreases PP crystallinity and size of crystals, particularly for the poly (propylene-co-1-decene) co-polymers (I). With the increase in addition of 1-decene, there’s a proportional decline in melting temperature (Tm), Heat of fusion (ΔHm), and crystallization temperature (Tc).

  Table 1. Summary of Poly (propylene-co-decene) and Poly (propylene-co-undecenol) Copolymers

runa polymer [Y]b(mol %) Mv (×103 g/mol) Tm (°C)

ΔHm (J/g)

Tc (°C)

Tg (°C)

control 1

PP-1

0

32

134.6

80.9

95.5

control 2

PP-2

0

961

160.9

78.0

104.9

A-1

PP-D-1

1.1

25

127.7

74.0

83.2

-21.7

A-2

PP-D-2

3.8

16

123.7

60.1

65.7

-18.4

A-3

PP-D-3

12.4

12

106.6

48.0

45.8

-13.7

B-1

PP-D-4

0.5

891

151.5

70.6

99.3

-44.5

B-2

PP-D-5

1.4

902

147.93

59.3

95.6

-45.0

C-1

PP-OH-1

0.7

490

156.7

66.1

106.5

-3.1

C-2

PP-OH-2

1.8

592

156.6

65.4

104.9

-2.3

C-3

PP-OH-3

4.2

392

156.2

65.2

102.0

0.73

aRuns in set A: 5 umol Et(Ind)2ZrCl2 catalyst/5 mL MAO (10 wt % in toluene), 75 mL toluene, 40 °C, 120 psi of propylene gas. Runs in Sets B and C:

0.200 g of TiCl3. AA, 5 mL of Al(Et)2Cl (10 wt % in toluene), 75 mL toluene, 60 °C, propylene gas pressure: 120 psi for set B and 30 psi for set C. b[Y] indicates the comonomer content (mol %) in the copolymers.

Be that as it may, in PP-OH (III) copolymers (set C), both Tm and ΔHm values demonstrate an underlying decrease and afterward along these lines level off at the higher comonomer consolidation. Moreover, both PP-OH-1 and PP-OH-2 show higher crystallization temperature (Tc) than the PP homopolymer, demonstrating that the OH groups may likewise encourage the crystallization procedure. It is additionally intriguing to take note of that because of low comonomer reactivity for the undecenyloxytrimethylsilane comonomer in the heterogeneous Ziegler-Natta catalyst, the subsequent PP-OH copolymer (III) may have a decreased atomic structure, with the OH-containing side-chain units amassed toward one side of the copolymer primary chain[23,24]. In this manner, the expansion of comonomer content has less impact on the PP chain crystallization.

 Both side chains marginally increment the PP ε value (polarizability) with the dimension relative to the comonomer content (represented in Figure 1), which are joined by the increment of frequency and temperature reliance.

Figure 1. Dielectric constants (vs frequency and temperature)   for

(a) PP, (b) PP-D-2 with 3.8 mol % 1-decene comonomer units, and

(c) PP-OSi (CH3)3 with 4.2 mol % undecenyloxytrimethylsilane comonomer units.

The expansion in polarizability is because of expanded chain adaptability, since diminished crystallinity (examined previously) takes into consideration progressively segmental movement in shapeless phase, which builds dipole arrangement along the direction of the electric field. Be that as it may, under high-frequency conditions, this generally moderate segmental movement is essentially constrained. The ε value relatively diminishes with the increment in frequency, which turns out to be practically steady (ε = 3) at 1 M Hz recurrence. This diminished dielectric action may just be from the fast prompted electronic polarization system. Then again, the ε value additionally consistently diminished with expanding temperature, particularly in the low-frequency region. The atomic arrangement, by electric field, might be hindered because of the expansion of general sub-atomic movement from the expanded temperature and decreased crystallinity.

Figure 2. Dielectric constants (vs frequency and temperature)   for

(a) PP and three PP-OH copolymers containing (b) 0.7, (c) 1.8, and

(d) 4.2 mol % OH content.

 The dielectric value consistently increments relatively with the OH content (represented in figure 2). The ε estimation of PP-OH-3 with 4.2 mol % of the OH comonomer content reaches about 4.6 (twice that of PP), which is altogether higher than the estimations of PP-D (I) and PP-OSi(CH3)3 (II) with comparable comonomer contents; especially in the high-frequency range. The OH bunches plainly add to the polarizability of the PP-OH (III) copolymer, which might have begun from the instigated electronic polarization of OH gatherings or nearby dipole introduction of OH bunches along the direction of electric field . It is a charming astonishment for us to watch all PP-OH dielectric profiles taking after the PP profile, with a dielectric steady (ε) that is autonomous over a wide scope of frequencies (somewhere in the range of 100 and 1 M Hz) and temperatures (between – 20 and 100 °C). These covered and level dielectric steady lines infer a quick polarization reaction for the PP-OH (III) copolymer, even under a moderately low electric field condition. The interchain H-bondings between OH bunches offer high crystallinity (Table 1) yet additionally a system structure that gives portion security (fixed status and reversibility) notwithstanding when the temperature ascends to 100 °C.

 

Figure 3.  DE hysteresis loops vs the applied electric field (top) for

(a) PP and three PP-OH copolymers containing (b) 0.7, (c) 1.8, and

(d) 4.2 mol % OH content and (bottom) PP-OSi(CH3)3 copolymer with

4.2 mol % undecenyloxytrimethylsilane comonomer units.

Figure 3 compares DEloops of PP and three PP-OH (III) copolymers.

As expected, the PP shows a direct D-E circle with low hysteresis, showing a consistent dielectric value (ε = 2.2) and totally reversible polarization-depolarization over the entire applied electric fields. Most strikingly, all the PP-OH copolymers additionally display comparable direct and thin D-E circles. Predictable with the dielectric results in Figure 2, the dielectric value increments with the OH content in PP-OH copolymers. What’s more, it keeps up steady over a wide scope of applied electric fields, up to E = 600 MV/m. The charge relocation of PP-OH-3 achieves 0.024 C/m2 at 600 MV/m, which is twofold that of PP under the equivalent connected electric field. Obviously, the dielectric loss keeps up little, with even the PP-OH copolymers displaying altogether higher dielectric activities. Then again, the expansive hysterises in the D-E circles were seen in the comparing PP-OSi(CH3)3 (II) copolymer (Figure 3, base), which increments with the connected electric field. The pending side chains containing OSi(CH3)3 bunches reduce crystallinity and polarization reversibility (sub-atomic structure) under the connected electric field. Amid the depolarization cycle, the poled gatherings and additionally chain fragments can’t be totally randomized in the time size of the procedure. Generally, the blend of the dielectric consistent (versus temperature and recurrence) and polarization circles (versus connected electric field) results obviously shows the special commitment of OH polar gatherings in PP-OH dielectric flimsy film; this expands polarizability as well as gives a stable PP-OH structure and morphology under a wide scope of connected electric fields and raised temperatures. This polar PP-OH polymer will have a physical system structure for performing quick polarization-depolarization cycles with low hysteresis and low energy loss. Figure 4 compares the energy density of the same four polymers, including PP and three PP-OH (III) copolymers containing 0, 0.7, 1.8, and 4.2 mol % OH comonomer units.

Figure4. Releasing energy density for (a) PP and three PP-OH copolymers containing (b) 0.7, (c) 1.8, and (d) 4.2 mol % OH content.

The energy density is assessed from the releasing cycle in Figure 3, which unmistakably increases with the OH content and exponentially increments with the connected electric field (E). At the connected electric field E = 600 MV/m, the energy density for PP-OH-3 achieves 7.42 J/cc; more than twofold that appeared in BOPP capacitors. Above all, the expansion of enegy density does not cause an expansion in energy loss (the region encased by the charging-releasing cycle), which stays extremely low (like PP) for all PP-OH copolymers

Experiment, Result & Discussion (For x-PP polymers):

It is exceptionally fascinating to investigate other structure includes that could build separate quality (E) that has an exponential impact to the vitality energy density. In the second experiment, the researchers integrated a group of thermally cross-linkable isotactic poly(propylene-co-p-(3-butenyl)styrene),  (PP-BSt) copolymers (I),  for an efficient investigation of the cross-connecting effect on the dielectric properties of isotactic polypropylene.

TABLE 2. Summary of polymer structure, and dielectric properties of x-PP copolymers and the corresponding PP homopolymer.

Polymer structure Dielectric properties

Molecule weight

BSt content

Gel content

Dielectric constant

Energy density

a valued (62.3%)

Sample

(kg/mol)a

(mol %) b

(wt %)

(25 ° C, 1 kHz)

(J / cm3) c

(MV/m) þvalued

PP

672

0

0

2.27

2.02

551.4

14.1

x-PP-1

516

0.64

80.0

2.38

2.36

595.8

16.9

x-PP-2

459

2.97

91.1

2.73

2.65

617.7

23.7

x-PP-3

402

3.65

98.1

2.97

2.81

645.1

42.1

aMolecular weight was determined by intrinsic viscosity of polymer/decalin solution at 135 ° C.

bThe pendent styrene groups were determined by 1H NMR.

cEnergy density was estimated from the discharge P-E curve after applying 500 MV/m electric field.

da and þ values were obtained from Weibull distribution curve of electric breakdown strength.

The PP-BSt copolymers (I), containing a long chain branched (LCB) structure and some pending styrene moieties, are totally solvent in xylene at raised temperatures. The thin films (II) show dynamic cross-connecting action at 220 ° C by taking part in an interchain cycloaddition between swinging styrene units to get cross-connected polypropylene (x-PP) meager film dielectrics (III) (thickness of ~10 µm), without framing any result. Debasement has an impeding impact to the film security under high electric fields. The subsequent x-PP films (III) were subjected to a fiery dissolvable extraction to expel the solvent portion that was not completely cross-connected into the system structure. The nearness of LCB and cross-connecting structures in x-PP films marginally decreases softening, crystallization temperatures and the general crystallinity. In general, the three x-PP films still show great crystallinity, high dissolving temperature, amazing mechanical quality, and high  film dimensional soundness at raised temperatures (>220 ° C) that is significant in capacitor applications (Table 2 above).

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It is a wonderful shock for us to watch the deliberate increment of the ԑ (polarizability) with a dimension relative to the BSt content. Furthermore, we see that all x-PP dielectric profiles look like the BOPP profile, with a dielectric steady (s) that is independent over a wide scope of frequencies (between 100 Hz and 1 MHz) and temperatures (between — 20 and 100 ° C). These covered and level dielectric steady lines suggest a quick polarization reaction for all x-PP copolymers, even under a moderately low electric field condition. The s estimation of x-PP-3 with 3.65 mol % of the BSt content spans (ԑ= 3), which is essentially higher than the qualities (ԑ= 2.2) of BOPP. The BSt bunches plainly add to the polarizability of the x-PP copolymer, which might be started from the prompted g-electronic polarization of fragrant gatherings that are added to the current o-electronic polarization (CH3– CH gatherings) in the PP chain.

A dc electric field was connected over every x-PP film with an underlying sufficiency of 100 MV/m, which was then expanded by 100 MV/m interims until achieving 500 MV/m (Figure 5 below). True to form, the PP shows a straight D-E circle with exceptionally low hysteresis, demonstrating a steady dielectric constant (ԑ= 2.2 ). All the more strangely, all the x-PP copolymers additionally show comparative straight and thin D-E circles; the incline of the D-E circle increments with the BSt content, steady with the dielectric results in Fig. 5(a). What’s more, it stays consistent over a wide scope of applied electric fields, up to 500 V/m. The blend of the dielectric consistent (versus temperature and recurrence) and polarization circles (versus connected electric field) plainly shows the beneficial outcome of the BSt cross-connecting highlight to the PP dielectric, with expanding electronic polarizability and energy stockpiling limit while keeping up low loss of energy.

FIG. 5. (a) Dielectric constants (vs frequency and temperature) and (b) D-E hysteresis loops (charge displacement vs the applied electric field) for PP, x-PP-1, x-PP-2, and x-PP-3 thin film dielectrics.

Conclusion:

The studies efficiently analyze the structure-property relationship of PP copolymers containing different comonomer units. Both PP-OH and x-PP copolymers having a system structure show reversible polarization with low energy loss and thin breakdown appropriation. Their dielectric constant keeps up steady over a wide scope of temperatures (between – 20˚C and 100˚C), frequencies (somewhere in the range of 100 and 1 M Hz), and applied electric fields (> 600 MV/m). The PP-OH (having 4.2 mol% OH content) based thin film capacitor shows a straight reversible charge storage limit with high discharging energy density > 7 J/cm3 (2- 3 times of BOPP) after a connected electric field at E = 600 MV/m, without displaying huge increment in loss of energy and leftover polarization at zero electric field.

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