Controlled functionalization of poly(4-methyl-1-pentene) films for high energy storage applications

Article abstract of DOI:10.1039/c5ta09949h

A new family of poly(4-methyl-1-pentene) ionomer [PMP-(NH₃)_xA-y] (x = 1, 2, 3 and A = Cl^-, SO₄^2-, PO₄^3-, y = NH₃ content) modified (NH₃⁺)_xA^x- ionic groups has been synthesized. The ionomers were synthesised using either a traditional Ziegler-Natta or a metallocene catalyst for the copolymerisation of 4-methyl-1-pentene and bis(trimethylsilyl)amino-1-hexene. A systematic study was conducted on the effect of the subsequent work-up procedures that can prevent undesirable side reactions during the synthesis of the [PMP-(NH₃)_xA-y] ionomers. The resulting PMP-based copolymers were carefully monitored by a combination of nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), mechanical properties, dielectric properties, and electric displacement-electric field (D-E) hysteresis loop measurements. Our results reveal that the [PMP-(NH₃)_xA-y] ionomer films show a significantly enhanced dielectric constant (～5) and higher breakdown field (～612 MV m^-1) as compared with pure PMP films. Additionally, these PMP-based films show good frequency and temperature stabilities (up to 160 °C). A reliable energy storage capacity above 7 J cm^-3 can be obtained, and is twice the energy storage capacity of state-of-the-art biaxially oriented polypropylene films, which can be attractive for technological applications for energy storage devices.

Full text of DOI:10.1039/c5ta09949h

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Journal of Materials Chemistry A
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Controlled Functionalization of Poly(4-methyl-1-pentene) Films
for High Energy Storage Applications
Min Zhang,a Lin Zhang,*b Meng Zhu,c Yiguang Wang,*c Nanwen Li,d Zhijie Zhang,e Quan Chen,e
Linan An,f Yuanhua Lin*g and Cewen Nan g
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A new family of poly(4-methyl-1-pentene) ionomers [PMP-(NH3)xA-y] (x = 1, 2, 3 and A = Clˉ, SO42ˉ, PO43ˉ, y = NH3
content) modified (NH3+)xAxˉ ionic groups has been synthesized. The ionomers were synthesised using either a traditional
Ziegler-Natta or a metallocene catalyst for the copolymerisation of 4-methyl-1-pentene and bis(trimethylsilyl)amino-1-
hexene. A systematic study was conducted on the effect of the subsequent work-up procedures that can prevent
undesirable side reactions during the synthesis of the [PMP-(NH3)xA-y] ionomers. The resulting PMP-based copolymers
were carefully monitored by a combination of nuclear magnetic resonance (NMR), gel permeation chromatography (GPC),
differential scanning calorimetry (DSC), mechanical property, dielectric properties, and electric displacement-electric field
(D-E) hysteresis loop measurements. Our results reveal that the [PMP-(NH3)xA-y] ionomer films show a significantly
enhanced dielectric constant (~5) and higher breakdown field (~ 612 MV/m) as compared with pure PMP films. Additionally,
these PMP-based films show good frequency and temperature stabilities (up to 160 °C). A reliable energy storage capacity
above 7 J/cm3 can be obtained, and is twice the energy storage capacity of state-of-the-art biaxially oriented polypropylene
films, which can be attractive for technological applications for the energy storage devices.
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weight and flexible energy storage devices, which makes polymer
films the material of choice for the next generation of high pulse
capacitors.14,15 However, polymer film capacitors generally suffer
Introduction
Dielectric materials with a high electric energy density and a low
dielectric loss play a very important role in modern electrical power
systems, e.g., hybrid electric vehicles, medical defibrillators, filters
and switched-mode power supplies.1−5 Compared to batteries and
supercapacitors, dielectric capacitors offer many unique
advantages, e.g., fast charge and discharge characteristics, a high
power density, a high output voltage, and a wide operating
temperature range.6−11 Compared to other dielectric capacitors,
polymer film capacitors are more attractive for the applications
mentioned above due to their higher breakdown strength and a
more graceful failure mechanism.3,12,13 In addition, the
development of dielectric-polymer-based capacitors has been
motivated by the increasing demand for compact, low-cost, light-
from either a low dielectric constant or an unacceptably large
energy loss.3,16−19 For example, although biaxially oriented
polypropylene (BOPP) exhibits a high breakdown strength (Eb
>
600 MV/m) and an inherently low energy loss (tan δ ~ 0.0002),
BOPP-based thin film capacitors can only offer an energy density in
the range from 2 to 3 J/cm3 at Eb = 600 MV/m due to the material's
low dielectric constant (εr = 2.2).
In general, the energy density Ue of a dielectric is defined as:
(1)
U = ∫ E ⋅dD
e
where
E is the applied electric field and D is the electric
displacement. Therefore, a high Ue can not only be achieved by
increasing E but also by increasing D. In the past few decades,
poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers have
attracted much attention because of their relatively high
displacement (~10 µC/cm2).20−29 As a result of strong C
−F dipoles
a.College of Materials Science and Engineering, Beijing Institute of
Petrochemical Technology, Beijing, 102617, China.
and the spontaneous orientation of dipoles in the crystalline
phases, such ferroelectric polymers possess a higher dielectric
constant (12–15 at 1 kHz) at room temperature than other known
polymers.20,30,31 It was demonstrated that the incorporation of
bulky monomers such as chlorotrifluoroethylene (CTFE) and
hexafluoropropylene (HFP) into the PVDF main chain yields slimmer
polarization loops and higher discharge energy densities (Ue > 25
J/cm3) at high electric fields (> 600 MV/m).4,22,24 Utilizing PVDF-
b.Materials Research and Education Center, Auburn University, Auburn, AL
36849, USA. Email: lzz0002@auburn.edu.
c. State Key Laboratory of Solidification Processing, Northwestern Polytechnical
University, Xi’an, 710072, China. Email: wangyiguang@nwpu.edu.cn.
d.State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Science, Taiyuan, 030001, China.
e. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China.
f. Department of Materials Science and Engineering, Advanced Materials
Processing and Analysis Center, University of Central Florida, Orlando, FL
32816, USA.
g.School of Materials Science and Engineering, State Key Lab of New Ceramics
and Fine Processing, Tsinghua University, Beijing 100084, China. Email:
linyh@mail.tsinghua.edu.cn.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
based copolymers as
a representative uniaxially stretched
ferroelectric polymer material, the energy density could be
improved by an order of magnitude compared to BOPP.4
Unfortunately, the presence of highly coupled dipole interactions
and large ferroelectric domains in the ferroelectric polymers results
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in a high energy barrier for a switch of the dipole direction and a polymer films was explored. The modified PMP polymers showed a
strong polarization hysteresis, which in turn leads to a high high energy density (> 7 J/cm3) at 600 MV/m, a high charge-
dielectric loss.32−34
discharge energy efficiency (> 93%) and could withstand
Recently, the grafting of polystyrene or poly(methacrylate temperatures of up to 160 °C, which makes them attractive for a
ether) side chains onto PVDF copolymers has been reported to potential application in high-energy density capacitors.
result in the formation of insulating layers around the polar crystal
domains, which significantly decreased the dielectric constant and
Experimental Section
loss and increased Eb. Eventually, energy densities of up to 17.5
J/cm3 could be achieved.35−39 Recently, Wang et al.40 proposed a
simple and scalable cross-linking method for the synthesis of cross-
linked PVDF, which resulted in a higher dielectric constant, a lower
dielectric and conductive loss, and a higher breakdown strength.
Nevertheless, experimental results showed that these modifications
to PVDF-based copolymers and terpolymers cannot fully suppress
the polarization coupling in these ferroelectric polymers, especially
at high electric fields (> 10 MV/m) Therefore, some research groups
have turned to linear dielectrics. For linear dielectrics, the energy
density can be expressed as:
Materials
All O2- and moisture-sensitive manipulations were performed under
argon atmosphere in
a glovebox. 4-Methyl-1-pentene was
purchased from Tokyo Chemical Industry (Shanghai) and distilled
over CaH2 under reduced pressure before use. Triethylaluminium
(TEA, 1.3 M in heptane), diethylaluminium chloride (AlEt2Cl, 0.9 M
in toluene), aluminium-activated titanium(III) chloride (TiCl3·AA) and
calcium hydride were purchased from J&K Scientific Ltd. All other
chemicals, including methanol, sodium hydroxide, toluene,
hydrochloric acid, sulphuric acid and phosphoric acid were of
analytical grade and purchased from Sigma-Aldrich and used as
received. Toluene was distilled over a sodium/potassium alloy with
benzophenone as indicator under argon atmosphere. 6-
Ue
=1/ 2
ε
0
ε
E2
r
(2)
where ε0 is the vacuum permittivity (8.85×10ˉ12 F/m) and εr is the
relative dielectric constant. Compared with ferroelectric polymers,
the dielectric properties of non-ferroelectric polymers are more
stable over a large frequency and temperature range, and such
polymers, e.g., poly(ether ether ketone) (PEEK), poly(phthalazinone
ether ketone) (PPEK), and polypropylene with polar side groups (PP-
OH copolymers) exhibit a very low dielectric loss and higher
breakdown strengths.41−44 For example, Zhang et al.45,46 reported on
the synthesis of an amorphous and glass phase aromatic
polythiourea with a high breakdown strength of up to 1.1 GV/m an
energy density above 24 J/cm3 and a low energy loss below 150 °C.
The addition of a small amount (2−6 mol.%) of −OH groups to
isotactic polypropylene (PP) can significantly increase the material's
dielectric constant while maintaining a relatively low dielectric loss
after stretching, which was also confirmed by simulations.43,44
Besides increasing the energy density and reducing the energy
loss, thermal management is another approach that allows for the
application of dielectric polymers at elevated temperatures. The
best commercially available dielectric polymer BOPP can only be
used at temperatures below 105 °C.3 Therefore, it is of great
interest to study the dielectric properties of novel ion-containing
Bis(trimethylsilyl)amino-1-hexene was prepared according to
procedure described in literature.47,48
a
Copolymerisation of 4-methyl-1-pentene/bis(trimethylsilyl)amino-
1-hexene
For comparison, 4-methyl-1-pentene/bis(trimethylsilyl)amino-1-
hexene was synthesised using either a Ziegler-Natta catalyst or a
metallocene catalyst. In the first case, in a typical copolymerisation
reaction, 50 mL of toluene and 10 mL of 4-methyl-1-pentene were
poured into a Parr 450 mL stainless autoclave equipped with a
mechanical stirrer in the glovebox. After removal from the
glovebox, a certain amount of bis(trimethylsilyl)amino-1-hexene
was injected into the reactor, heated and left at a constant
temperature of 60 °C under stirring and argon gas protection. Either
about 0.2 g of TiCl3·AA and 5.0 mL of AlEt2Cl or 0.2 g of TiCl3·AA and
5.0 mL of AlEt3 were mixed together and stirred for 30 min in the
glovebox at room temperature to activate the catalyst, and then
injected into the reactor under rapid stirring to initiate the
copolymerisation reaction. After 30 min at 60 °C, the reaction
solution was quenched with tetrahydrofuran (THF), filtered and
extensively washed with THF to remove residual monomers. The
isolated
copolymer was dried under vacuum at room temperature for 12 h.
poly(4-methyl-1-pentene) (PMP) copolymers with high melting Alternatively, the reaction solution was directly quenched with a
4-methyl-1-pentene/bis(trimethylsilyl)amino-1-hexene
methanolic hydrogen chloride solution to convert the incorporated
–N(SiMe3)2 groups to the stable –NH3+Cl– groups. In the second
case, when using a metallocene catalyst, a similar procedure was
used, except that the catalyst system was changed to rac-Me2Si[2-
Me-4-Ph(Ind)]2ZrCl2 (5×10–6 M) and d-MAO (0.35 g, Al/Zr = 1000).
The reaction temperature was 25 °C and the reaction time was
60 min. After the reaction, the products were washed with THF and
methanol three times before drying.
temperatures above 220 °C. It is believed that ion-containing
polymers are not suitable for achieving a high Ue and a high Eb due
to the significant conduction loss at a high electric field. However,
we believe that Eb is more strongly affected by the quality of the
films than the ion-induced conductive loss. Furthermore, ion-
containing polymers are more suitable for film stretching due to the
enhancement of the melt viscosity caused by the ionic association
and the improved mechanical properties.
In this study, a family of poly(4-methyl-1-pentene) copolymers
Conversion of the bis(trimethylsilyl)-amine groups into functional
PMP-(NH3)xA-y containing various –NH3+Clˉ, –(NH3)2SO42ˉ and – groups
(NH3)3PO43ˉ side groups were systematically investigated. These
chemically modified PMP polymers were prepared and then
fabricated into thin films for comparative dielectric and electric
energy storage experiments. The combined effect of the addition of
the side-chain groups (comonomer units) and the subsequent thin
film fabrication step on the dielectric properties of the resulting
The bis(trimethylsilyl)-amine (–N(SiMe3)2) groups in the poly(4-
methyl-1-pentene) (PMP) copolymers were further converted to
the desired –NH3+Cl– and –NH2 groups. To this end, about 3 g of the
4-methyl-1-pentene/bis(trimethylsilyl)amino-1-hexene copolymer
was suspended in 50 mL THF at 50 °C. Then, 20 ml of a 2 M
methanolic hydrogen chloride solution was added dropwise. The
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+
mixture was stirred for 4 h at 50 °C, and the resulting PMP-NH3 Cl– cantilever (TESP tip) of 320 kHz resonant frequency from JPK was
polymer was collected by filtration and washed with deionized used. X-ray diffraction (XRD) analysis was carried out with a Bruker
water several times before drying at 60 °C in vacuum for 24 h. For D8 advance (Germany). The wavelength of the X-rays was 1.542 Å
the preparation of the PMP-NH2 polymer, after the deprotection (Cu Kα radiation, 40 kV and 100 mA), and the scanning rate was 4o
reaction with the methanolic hydrogen chloride solution, the min–1. All mechanical properties (tensile strength, tensile modulus,
polymer solution was poured into 100 ml of a 1 M methanolic elongation at break, etc.) were measured according to the ASTM D-
NaOH solution. The resulting polymer was collected by filtration 1708 test method. The dog bone-shaped specimens with an overall
and washed with a 1 M aqueous ammonia solution and water size of 38 mm × 15 mm (5 mm × 22 mm in the gauge area) were-die
under nitrogen atmosphere. After drying at 50 °C in vacuum cut, and then the mechanical tests were performed on an Instron
overnight, the resulting PMP-NH2 copolymer was obtained with 5866 universal testing machine using a load cell of 100 N and a
quantitative yield. The amino group reactions also occur in the constant cross head speed of 1 mm/min. At least 5 samples of each
polymer film. Thus, the PMP-NH2 copolymer can be moulded into a type were studied in order to minimize possible errors. For the
film first, and then converted into PMP-NH3+Cl–, PMP-(NH3)2SO42– or electrical measurements, gold electrodes with
PMP-(NH3)3PO4 films by submerging the PMP-NH2 film into a 2 M approx. 80 nm were sputtered onto both surfaces of the film using
methanolic hydrogen chloride solution, a 2 M methanolic sulphuric a JEOL JFC-1600 auto fine coater (Japan). The dielectric properties
a thickness of
3–
acid solution or
respectively.
a
2 M methanolic phosphoric acid solution, were then measured using an Agilent (4294A) precision impedance
analyser in the frequency range from 100 Hz to 1 MHz at 1 V, and in
a temperature range from -20 to 160 °C. The D-E hysteresis loops
Film fabrication and processing
were recorded at room temperature on a Premiere II ferroelectric
test system at a frequency of 10 Hz.
The PMP-NH2-y films were prepared by hot pressing in vacuum at
240 °C and at a pressure of 24.000 psi to remove defects, resulting
in an average film thickness in the range from 40 to 50 μm. These
films were placed in beakers, and then a 2 M methanolic hydrogen
chloride solution, a 2 M methanolic sulfuric acid solution or a 2 M
methanolic phosphoric acid solution was added under stirring to
Results and discussion
Synthesis and characterization of PMP-(NH3)xA-y copolymers
As illustrated in Scheme 1, the PMP-N(SiMe3)2-y copolymers were
synthesized by copolymerisation of 4-methyl-1-pentene with
bis(trimethylsilyl)amino-1-hexene comonomers (Figure S1), using
either a homogeneous metallocene catalyst or a heterogeneous
Ziegler-Natta catalyst. Recently, Chung et al.47,48 reported on the
preparation of amino-functionalized polyolefins from silane-
protected α,ω-aminoolefin via a metallocene catalyst-mediated
copolymerisation process, which resulted in the formation of
obtain the corresponding PMP-NH3 Cl–-y, PMP-(NH3)2SO42–-y or
+
PMP-(NH3)3PO43–-y polymer films, respectively. After completion of
the reactions, the films were washed with deionized water to
remove excess acid and then dried overnight at 60 °C, followed by
another drying step in vacuum at 70 °C for 8 h. The stretched films
were eventually produced by uniaxially stretching the obtained
films at 130 °C and at an extension rate of 10mm·min–1 to an
extension ratio of about 600%, resulting in a final thickness of
approx. 11 µm.
random PE-NH3+Clˉ and PP-NH3+Clˉ ionomers. In contrast,
a
heterogeneous Ziegler-Natta catalyst is almost incapable of
incorporating the bulky α,ω-bis(trimethyl)aminoolefin during
Sample characterization
All high-temperature 1H NMR spectra were recorded on a Bruker propylene copolymerisation probably due to the large difference in
AM-300 nuclear magnetic resonance (NMR) spectrometer in a
1,1,2,2-tetrachloroethane-d2 atmosphere at 110 °C. The polymer
molecular weights were measured by gel permeation
chromatography (GPC) on a PL-GPC 220 High Temperature GPC/SEC
System equipped with four PLgel Mixed-A (20 µm) columns. The
oven temperature was set to 150 °C and the temperatures of the
autosampler’s hot and warm zones were 135 and 130 °C,
respectively. The solvent 1,2,4-trichlorobenzene (TCB) containing
~200 ppm tris(2,4-di-tert-butylphenyl) phosphite (Irgafos 168) was
purged with nitrogen. The flow rate was adjusted to 1.0 mL/min
and the injection volume was 200 µL. A test specimen with a
concentration of 2 mg/mL was prepared by dissolving the sample in
the N2-purged and preheated TCB (containing 200 ppm Irgafos 168)
for 2.5 h at 160 °C with gentle agitation. The melting temperatures
of the polymers were obtained by differential scanning calorimetry
(DSC) using a Perkin-Elmer DSC-7 instrument controller with a
heating rate of 20 °C/min. Fourier transform infrared spectroscopy
both size between the two comonomers and their reactivity ratios.
Compared to the highly reactive propylene monomers, 4-methyl-1-
pentene may be a more promising candidate for copolymerisation
with bis(trimethylsilyl)amino-1-hexene via a Ziegler-Natta catalyst.
(FTIR) was performed on
a PE-1710 spectrometer in the
wavenumber range from 4000 to 400 cm–1 with a 4 cm–1 resolution.
64 scans were conducted on each of the polymer thin films. Atomic-
force microscopy (AFM) micrographs were recorded with
a
bioatomic force microscopy (Bio-AFM). AFM tapping-mode height
profiles were acquired with a JPK Instruments AG multimode
NanoWizard (Germany). The instrument was equipped with a
NanoWizard scanner. For tapping-mode AFM, a commercial Si
Scheme 1 Illustration of the synthesis of PMP-NH2-y and PMP-
(NH3)xA-y copolymers (A = Clˉ, SO42ˉ or PO43ˉ; y = NH2 content;
x = 1, 2, 3) via a catalytic copolymerisation process.
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The resulting PMP-N(SiMe3)2-y copolymer, with the pendant
silane-protected amino groups, can be directly deprotected using a obtained
HCl/methanol solution during the sample work-up process to form pentene/bis(trimethylsilyl)amino-1-hexene
Table 1 compares selected properties of the reaction products
through two parallel sets of 4-methyl-1-
copolymerisation
stable PMP-NH3+Clˉ-y ionomers, which can be subsequently reactions using three different catalyst systems, i.e., the two
neutralized with an NaOH aqueous solution under an inert heterogeneous Ziegler-Natta catalysts TiCl3·AA/Et2AlCl and
atmosphere to obtain PMP-NH2-y. The reactive amino groups in the TiCl3·AA/Et3Al and the homogeneous metallocene catalyst rac-CH2-
PMP-NH2-y copolymers can react with various acids, including bis(3-tert-butyl-indene)ZrCl2/d-MAO. Overall, all copolymerisation
hydrochloric acid, sulphuric acid and phosphoric acid to synthesise reactions produced copolymers with an incorporation ratio in the
corresponding PMP ionomers, denoted as PMP-(NH3)xA-y (A = Clˉ, range from 1.9 to 5.1, indicating that both catalytic systems possess
SO42ˉ and PO43ˉ; x = 1, 2, 3).
suitable comparative comonomer reactivity. In the Ziegler-Natta
Table 1 Comparison of selected properties of the reaction product obtained through the 4-methyl-1-pentene/6-bis(trimethylsilyl)amino-
1-hexene copolymerisation for different reaction conditions.
Reaction conditions
Copolymerisation results
comonomerb
[comonomer]c
Mw
Tg
Tm
ΔHe
(J/g)
59.8
51.4
60.1
49.3
48.9
48.4
110.2
̶
d
e
e
Polymer
yield (g)
runa
A-1
A-2
A-3
A-4
A-5
A-6
B-1
B-2
B-3
co-catalyst
Et2AlCl
Et2AlCl
Et3Al
(mL)
0
(mol%)
(kg/mol)
204
̶
PDId
5.35
̶
(oC)
51.2
33.2
52.3
31.3
30.8
30.5
–
(oC)
234.8
223.4
235.3
224.9
224.5
224.1
156.8
̶
3.8
2.7
3.5
2.9
2.5
1.9
4.7
3.6
2.7
0
̶
0.5
0
0
211
189
168
159
210
166
154
5.67
6.06
5.88
5.95
2.34
2.18
2.45
Et3Al
0.5
1.0
2.0
0
1.9
3.1
4.5
0
Et3Al
Et3Al
dMAO
dMAO
dMAO
0.5
2.4
̶
1.0
5.1
̶
̶
̶
aCatalyst used in set A: TiCl3·AA (reaction temperature: 60 °C, reaction time: 30 min); set B: rac-CH2-bis(3-tert-butyl-indene)ZrCl2
(reaction temperature: 25 °C, reaction time: 60 min). b10 mL of 4-methyl-1-pentene and 6-bis(trimethylsilyl)amino-1-hexene added as
c
comonomer. Comonomer content (mol.%) in the copolymers as determined by 1H NMR spectroscopy at 110 °C in 1,1,2,2-
tetrachloroethane-d2. d The weight-average molecular weights and the polydispersity index were determined by GPC at 150 °C in 1,2,4-
trichlorobenzene vs. narrow polystyrene standards. d Determined by DSC.
catalyst-mediated copolymerisation reactions, it is essential to use
the Et3Al co-catalyst. The use of Et2AlCl causes an aversive effect in
the formation of the completely insoluble PMP-N(SiMe3)2-y
copolymer. The Et2AlCl may in situ activate TiCl3·AA due to the
formation of a trace amount of HCl, which may break the reactive
N−Si bonds in the pendent –N(SiMe3)2 groups of the PMP-
N(SiMe3)2-y copolymer, which then engage in an intermolecular
dimerization reaction, resulting in the formation of a cross-linked
polymer network, according to Chung’s report.47
When using the traditional Ziegler-Natta catalyst TiCl3·AA/Et3Al,
soluble PMP copolymers with a high molecular weight (Mw > 145
kg/mol), a typical molecular weight distribution (4.68–6.55) and > 4
mol% –N(SiMe3)2 comonomer units are produced, which is
sufficient to investigate the effect of different NH2-transferred
ionomer groups on the dielectric and capacitor properties, as
shown in Figure S2. Unfortunately, the homogeneous metallocene
system produces amorphous PMP copolymers with
a higher
comonomer content and a narrower molecular weight distribution
(Figure S3), which is not ideal for a potential application in
capacitors due to the poor mechanical properties, which are not
suitable for the film pressing and the subsequent film stretching
process. In particular, for the PMP-N(SiMe3)2-y copolymers obtained
using the TiCl3·AA/Et3Al Ziegler-Natta catalyst, both the Tm and ΔHm
values show an initial reduction and then subsequently level off at a
Fig. 1 1H NMR spectra obtained for (a) PMP-N(SiMe3)2-4.5 and
(b) PMP-NH2-4.5 in 1,1,2,2-tetrachloroethane-d2 at 110 °C.
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4.5 mol% NH2-containing side chains into PMP backbone leads to
weaker XRD peaks compared to PMP homopolymer due to the
disruption of crystallinity by incorporated short side chains located
in amorphous phase. Of particular, it’s expected that PMP-NH2-4.5
shows sharper XRD peak (higher crystallinity) than those of PMP-
based ionomers, which can be ascribed to the fact that the
incorporation of ion-pairs into polymer short side chains may
absorb water and swell to increase amorphous contents, with
crystalline phase keeping intact.
higher comonomer content, as shown in Table 1 and Figure S4,
which is similar to results previously reported in literature.43
The 1H NMR spectra of a typical pair of PMP copolymers before
and after the silane group deprotection reaction is shown Figure 1.
The combination of the complete disappearance of the sharp N–Si–
(CH3)3 peak at 0.21 ppm and the coexistence of a –CH2–NH2 peak at
2.77 ppm indicates a successful deprotection. The NH2 group
content in the copolymer was determined by relating the peak
intensity to the number of protons in the N–Si–(CH3)3 group of the
copolymer or the CH2–NH2 group of the copolymer. In Figure S5,
two absorption peaks at around 3400 and 1640 cm−1, which
indicate the presence of amine groups in PMP-NH2.
Mechanical properties of PMP-(NH3)xA-y copolymers
The effect of ionic association (physical cross-linking) on the
mechanical properties of the PMP copolymers is investigated, and
AFM observations for the microstructure of PMP-based ionomer
membranes were performed and shown in Figure S8. As shown in
Figure S8, the dark areas represent hydrophilic (ionic) domains, and
the brighter areas represent hydrophobic domains. The phase
surface image exhibits hydrophilic–hydrophobic phase separation,
spreading as sea-islands structure, with small ionic domains size
The optimal way to investigate a copolymerisation reaction is to
measure the reactivity ratio of the comonomers. To obtain
meaningful results, a series of experiments were conducted by
varying the monomer feed ratio (ρ
pentene]/[bis(trimethylsilyl)amino-1-hexene]
=
[4-methyl-1-
=
[M1]/[M2]) and
comparing the resulting copolymer composition (R = d[M1]/d[M2])
at a low monomer conversion (< 5%) required to maintain a
constant comonomer mole ratio ([M1]/[M2]). In the Supporting
Information, Tables S1 and S2 summarize the results of two sets of
experiments of 4-methyl-1-pentene/bis(trimethylsilyl)amino-1-
hexene copolymerisation that were conducted using either a
ranging from 5–60 nm for PMP-NH3 Cl–-4.5. As to PMP-(NH3)2SO4
+
2–
-
4.5, the phase surface image becomes not average as that of PMP-
2–
NH3+Cl–-4.5, probably due to physical cross-linking effect by SO4
groups. In addition, PMP-(NH3)3PO43–-4.5 shows not so clear phase
separation compared to those of PMP-NH3+Cl–-4.5 and PMP-
(NH3)2SO42–-4.5, mainly due to stronger cross-linking. Table 2
summarizes the results of the tensile strength, tensile modulus and
elongation at break (%) measurements performed at 25 °C. All
specimens were prepared and tested under identical operating
conditions, and the measurements were repeated several times to
obtain the average values and the ranges of deviation. The PMP-
NH3+Clˉ-4.5 ionomer showed a lower tensile strength and a lower
Young’s modulus but was more ductile with a higher elongation at
break compared to the pure PMP homopolymer. In contrast, the
PMP-(NH3)3-PO43ˉ-4.5 ionomer exhibited a remarkably improved
tensile strength and Young’s modulus as well as a higher elongation
at break in comparison with the pure PMP film.
heterogeneous TiCl3·AA/Et3Al Ziegler-Natta catalyst or
a
homogeneous rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/dMAO metallocene
catalyst, respectively. The reactivity ratios for 4-methyl-1-pentene
(r1 = k11/k12) and bis(trimethylsilyl)amino-1-hexene (r2 = k22/k21
)
were estimated using the Fineman-Ross equation. Both plots in
Figure S6 yield straight lines with slope r1 and intercept r2. For the
linear slope in Figure S6 (a), r1 = 2.09 and r2 = 0.11, with r1 × r2
=
0.23 (not far from unity), indicating a relatively good random
copolymerisation reaction with a slightly higher 4-methyl-1-pentene
reactivity in the rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/dMAO-mediated
homogenous
4-methyl-1-pentene/bis(trimethylsilyl)amino-1-
hexene copolymerisation reactions. In other words, the 4-methyl-1-
pentene/bis(trimethylsilyl)amino-1-hexene copolymers obtained in
runs B1–B3 (Table 1) are mostly random copolymers. As discussed
later, they also showed a narrow molecular weight distribution. On Table 2 Summary of mechanical properties of the PMP ionomers
the
other
hand,
the
similar
linear
4-methyl-1-
Tensile
strength
Young’s
modulus
Elongation at
break
Sample
pentene/bis(trimethylsilyl)amino-1-hexene copolymerisation plot in
Figure S6 (b) yields r1 = 4.14 and r2 = 0.34 with r1 × r2 = 1.41 (not far
from unity), indicating a strong tendency of a consecutive 4-methyl-
1-pentene insertion when using a heterogeneous Ziegler-Natta
catalyst system (TiCl3·AA/Et3Al). Meanwhile, due to the much higher
4-methyl-1-pentene reactivity in the copolymerisation reactions
observed for the runs A1-6 (Table 1), the PMP-N(SiMe3)2-y
(MPa)
(MPa)
(%)
PMP
25±5
15±2
13.8±1.6
23.8±0.8
26.5±1.5
1155±105
895±45
775±35
1048±45
1215±55
28±6
45±9
58±7
53±9
38±6
PMP-NH2-4.5
PMP-Clˉ-4.5
PMP-SO42--4.5
PMP-PO43--4.5
copolymers showed
a
tapered molecular structure with
incorporated side-chain units concentrated at one end of the
copolymer main chain. Therefore, an increase of the comonomer
content has little effect on the PMP chain crystallization, which is
consistent with the DSC results presented in Table 1. Similar results
were also reported for PP-OH copolymers.43
It is interesting to note that all samples prepared by Ziegler-Natta
catalyst, including PMP homopolymer, copolymer with 4.5 mol%
NH2-comonomer units and PMP-based ionomers (PMP-NH3Cl-4.5,
PMP-(NH3)2SO42--4.5 and PMP-(NH3)3PO43--4.5) exhibit sharp XRD
peaks and maintain semicrystalline (Figure S7). The introduction of
Dielectric response under low electric fields
Figure 2 systematically compares the dielectric constant (εr) and
dielectric loss (tan δ) profiles obtained for pure PMP and three
different PMP-NH3 Cl–-y ionomers containing 1.9 (run A-4), 3.1 (run
+
+
A-5) and 4.5 mol% (run A-6) NH3 Cl– comonomer units, respectively,
recorded over a frequency range from 100 to 1 MHz at room
temperature. The dielectric constant of the pure PMP polymer film
is around 2.15, accompanied by a low dielectric loss of 3×10–4, both
of which are frequency independent.
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+
the dielectric loss for PMP-NH3 Cl–-4.5, PMP-(NH3)2SO42–-4.5 and
PMP-(NH3)3PO43–-4.5. A surprisingly low dielectric loss of 0.0081
was observed for PMP-(NH3)3PO43–-4.5 compared to PMP-NH3+Cl–-
4.5 and PMP-(NH3)2SO42–-4.5. The AC conductivity was calculated
from dielectric properties which are shown in Figure S9 according to
literatures49−51. The conductivity of PMP PMP-(NH3)xA-4.5 ionomers
with different anion groups is smaller than 10-5 S/m from 100 Hz to
1 MHz and shows a linear relationship with frequency. It indicates
that these polymers show good dielectric properties with very low
conductivity.
Fig. 2 Frequency dependence at room temperature of (a) the
dielectric constant and (b) the dielectric loss of PMP for
different amounts of Cl–.
+
The dielectric constant increases proportionally with the NH3 Cl–
+
content, reaching 3.8 for PMP- NH3 Cl–-4.5 (Figure 2(a)), which was
accompanied by a higher frequency dependency. The increase in εr
+
is probably due to the strong ionic polarization of the NH3 Cl–
functional groups and a much weaker electron polarization along
the direction of the applied electric field. At high frequencies, the
relatively slow segmental motion of the polymer chains is
significantly inhibited, and therefore εr monotonically decreases
with the frequency. In Figure 2(b), the dielectric loss increases from
3×10–4 to 0.014 for the PMP-based polymers due to the loss in
Fig. 3 Frequency dependence of (a) the dielectric constant and (b)
the dielectric loss of PMP-NH3+Cl–-4.5, PMP-(NH3)2SO42–-4.5 and
PMP-(NH3)3PO43–-4.5 at room temperature.
+
electric conductivity caused by the incorporation of the NH3 Cl–
+
groups. However, the dielectric loss obtained for PMP-NH3 Cl–-4.5 is
still much lower than the dielectric loss of many known dielectric
polymers, e.g., PVDF.40
The weak-field dielectric properties of the PMP-(NH3)xA-4.5
2–
3–
ionomers with different anion groups A, i.e., Cl–, SO4 and PO4
,
were characterized as a function of the frequency. As shown in
Figure 3(a), the εr value observed for PMP-(NH3)3PO43–-4.5 was as
high as 5.1 (more than twice the εr of PP), which is slightly higher
+
2–
than the values obtained for PMP-NH3 Cl–-4.5 and PMP-(NH3)2SO4
-
4.5 with the same NH3+-containing comonomer content, especially
3–
.
Scheme 2 Ionic association in PMP: (a) Cl–, (b) SO42–, and (c) PO4
3–
in the high frequency range, probably due to the fact that the PO4
ion has more negative charges and therefore a stronger ionic
polarization. Furthermore, the dielectric constant of the PMP-
(NH3)3PO43–-4.5 polymer film is less dependent on the frequency
The improved dielectric properties of PMP-(NH3)3PO43–-4.5 are
attributed to the ionic association mechanism illustrated in Scheme
2. For PMP-NH3+Cl–-4.5, because Cl– only has one negative charge,
there is a high probability that the Cl- ion cannot connect with
+
compared with the PMP-NH3 Cl–-4.5 and PMP-(NH3)2SO42–-4.5
polymer films. Figure 3(b) illustrates the frequency dependence of
6 | J. Name., 2012, 00, 1-3
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another positive charge carrier located on the inter- or intra-
Figure 5 compares the D-E loops obtained for pure PMP
polymer chain, and, as a result, is just randomly distributed in the polymer and PMP ionomers film samples with different comonomer
polymer matrix based on the polymer phase separation mechanism. units and a thickness of ~10 µm after stretching measured at the
3–
In contrast, the bulkier and heavier PO4 with three negative same breakdown field. For each sample, a DC electric field was
+
charges requires three NH3 ions to form saturated and stable ion- applied across the polymer film with initial amplitude of 100 MV/m,
pairs, which helps to build a stronger network structure resulting in which was then increased gradually at intervals of 100 MV/m until
a
better segment stability (immobility and reversibility), and reaching 600 MV/m. For the pure PMP polymer, a linear D-E loop
therefore the ionomer exhibits a lower dielectric loss under an with negligible hysteresis was observed, which indicates a constant
applied electric field in the frequency range from 100 Hz to 1 MHz. dielectric constant of approx. 2.2 (calculated from the slope) and a
These results are in good agreement with the results of the completely reversible polarization-depolarization for the applied
mechanical property measurements listed in Table 2.
electric fields. Fortunately, for the PMP ionomers with different
comonomer units, similar linear and slim loops were obtained, and
the slope of the D-E loops was found to increase with the dielectric
constant. For the PMP-(NH3)3PO43–-4.5 polymers, the charge
displacement could reach 0.027 C/m2 at 600 MV/m, which is two
times higher than the charge displacement observed for pure PMP
at the same electric field.
Fig. 4 Temperature dependence of the dielectric constant for (a)
PMP, (b) PMP-NH3+Cl–-4.5, (c) PMP-(NH3)2SO42–-4.5, and (d) PMP-
(NH3)3PO43–-4.5 in the temperature range from -20 to 160 °C.
Fig. 5 D-E hysteresis loops obtained for different applied electric
fields for (a) PMP and three different PMP-A-4.5 copolymers
containing (b) Clˉ, (c) SO42– and (d) PO43– ions, respectively.
The temperature dependence of dielectric constants of pure
PMP polymer and the PMP ionomers samples over a temperature
range from -20 to 160 °C is shown in Figure 4 The selected
frequencies care from 100 Hz to 1 MHz. The dielectric profiles
recorded for all of the PMP-based ionomers resemble the PMP
profile, with the dielectric constant not depending on the
temperature. The curves almost overlap and are rather flat at room
temperature, which is not only in good agreement with the results
shown in Figure 2 and Figure 3, but also indicates a fast polarization
response for the PMP copolymers in the temperature range from –
20 to 160 °C. In fact, most of the current polymer-based capacitors,
e.g., BOPP, can only be operated at temperatures below 105 °C due
to the significant change of the material's dielectric and mechanical
properties at higher temperatures. For PMP-(NH3)3PO43–-4.5, the
dielectric constant was found to be considerably stable with respect
to the temperature for temperatures up to 160 °C, which indicates
that this class of polymers is a promising candidate for high-
temperature capacitor applications. Meanwhile, the dielectric
constants showed no abrupt peaks at the glass transition
temperatures of the corresponding polymers or similar phenomena,
as reported for PP-OH by Chung et al.43
Most interestingly, the PMP-(NH3)3PO43–-4.5 film sample
showed a slimmer loop compared to the other ionomer films,
indicating a lower hysteresis and faster polarization reversibility.
When comparing with the PMP-(NH3)2SO42–-4.5 film, the results
obtained for both the dielectric constant (vs frequency and
temperature) and the polarization loops (vs applied electric field)
clearly indicate the unique contribution of the (NH3)3PO43– group to
the properties of the PMP-(NH3)3PO43–-4.5 dielectric thin film; the
group not only increases the polarization but also provides a stable
structure and morphology over a wide range of applied electric
fields and at elevated temperatures. This PMP-(NH3)3PO43–-4.5 film
may exhibit
a physical network structure which allows fast
polarization-depolarization cycles with a limited hysteresis and a
low energy loss.
The introduction of ionic functional groups into polymer chains
usually results in a higher conduction loss under an applied electric
field. Mechanical stretching is generally regarded as a common
means to improve the mechanical properties, reduce the dielectric
loss and enhance the breakdown strength of polymer films mainly
based on the reduction of free volume during film stretching
process. Fortunately, the mechanical stretching process may benefit
from the increased melt viscosity caused by the addition of ionic
groups to the polymer chains and the resulting formation of
D-E hysteresis behavior under high electric fields
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physical cross-links. In case of PMP-(NH3)3PO43–-4.5, the higher dielectric constant and lower loss. The energy density
combination of the incorporation of ionic groups and a mechanical obtained for the PMP-(NH3)3PO43–-4.5 polymer film is 7.4 J/cm3,
stretching process proved successful. The dielectric loss observed which is more than twice the energy density observed for the pure
for PMP-(NH3)3PO43–-4.5 continuously decreased as the stretching PMP polymer and commercial BOPP capacitors, and also higher
ratio was further increased even at higher electric fields, as than values reported in literature for other polymer films.43-48 Most
illustrated in Figure 6.
importantly, the increase in energy density does not cause a
significant increase in energy loss, which was calculated from the
area enclosed by the charging-discharging cycle. The η calculated
for the pure PMP polymer is around 97%, whereas the η calculated
for the PMP-(NH3)3PO43–-4.5 polymer film only decreased to 93%
under an electric field of 600 MV/m, which indicates that the
introduction of an ionic functional group can significantly improve
the energy density while at the same time maintaining a high
charge-discharge energy efficiency.
Fig. 7 (a) Energy density and (b) energy efficiency η of the PMP-
Fig. 6 Unipolar D-E loops obtained for the PMP-(NH3)3PO43–-4.5 NH3+Cl–-4.5, PMP-(NH3)2SO42–-4.5 and PMP-(NH3)3PO43–-4.5 films
films: (a) after the hot pressing process (thickness: about 25 µm), compared with PMP (at 10Hz).
(b) after uniaxial stretching to 250% of the original length
(thickness: about 15 µm) and (c) after uniaxial stretching to 600% of
The four different types of polymer film, PMP, PMP-NH3+Cl–-4.5,
the original length (thickness: about 9 µm).
PMP-(NH3)2SO42–-4.5 and PMP-(NH3)3PO43–-4.5 were tested at
different spots more than ten times, and the experimental results
were fitted by a Weibull distribution function F(x),52−54
Energy storage capability
β
The energy densities and charge-discharge energy efficiencies η
calculated from the D-E loops are shown in Figure 7. The energy
density clearly exponentially increases with the applied electric field.
The PMP-(NH3)3PO43–-4.5 ionomer shows a higher energy density
under the same electric field than the other copolymers due to its
α
)
F(x) =1− e−( x /
(3)
where α is the scale parameter and refers to a 63.2% probability of
failure [for x = α, F(x) = 0.632], and β denotes the shape parameter
and describes the form of the distribution. Figure 8 shows the
Weibull distributions with the estimated α and β values. Obviously,
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the ionic cross-linking has a significant effect on the breakdown
strength and the breakdown distribution – the higher the cross-
linking density, the higher the breakdown strength (α), and the
more narrow the distribution (β). The PMP-(NH3)3PO43–-4.5 thin film,
prepared by stretching after the hot pressing process, shows a
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 51532003 and 21404084), the
Fundamental Research Funds for the Central Universities (Grant No.
B040307), the State Key Laboratory of Solidification Processing in
NWPU (Grand No. 107-QP-2014) and the 111 Project (Grant No.
B08040).
breakdown strength between 540 and 660 MV/m with
α =
612 MV/m, which is almost the same value obtained for BOPP films
that are carefully conditioned (through stretching and annealing) to
increase the chain orientation and crystallization and to reduce
defects. In addition, the PMP-(NH3)3PO43–-4.5 film exhibits a very
narrow breakdown distribution with an exceptionally high β value
of 34, indicating an excellent dielectric reliability – a very important
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