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. 2020 Jun 23;5(26):16255–16262. doi: 10.1021/acsomega.0c02084

Relationship between the Relative Dielectric Constant and the Monomer Sequence of Acrylonitrile in Rubber

Ryosuke Matsuno †,‡, Yuusaku Takagaki §, Takamasa Ito §, Hitoshi Yoshikawa §, Shigeaki Takamatsu §, Atsushi Takahara ‡,*
PMCID: PMC7346247  PMID: 32656448

Abstract

graphic file with name ao0c02084_0009.jpg

Acrylonitrile-butadiene rubbers (NBRs) have a lower glass transition temperature (Tg) and a higher dielectric constant than other rubbers. To understand how a low Tg and a high dielectric constant are compatible, we focused on the acrylonitrile (AN) monomer sequence in rubber and synthesized random and alternating copolymers to evaluate the effect of the sequence. The AN monomer sequence dependence of the relative dielectric constant was investigated by the C–N stretching vibration of the nitrile group through Fourier transform infrared spectroscopy and internal rotation potential energy measurements around the C–C bond within the nitrile group based on dimer model calculations. The alternating copolymers, including NBR, showed a higher dielectric constant than random copolymers. The alternating copolymer shifted from ∼2243 cm–1 for polyAN to ∼2236 cm–1 for NBRs, while the random copolymer only shifted to ∼2239 cm–1. The peak of the C–N stretching vibration was correlated with the AN sequence. The sequence dependence of the shift can be explained by the C–N bond length calculation. The internal rotation potential energy between gauche and trans of the NBR model was the lowest, indicating that the NBR main chain is flexible and that AN in the main chain rotates easily. Therefore, NBR has a high dielectric constant and a low Tg because of the presence of an alternating sequence and the flexibility of the NBR main chain.

Introduction

Dielectric polymers that include a polar group in the main chain, such as poly(vinylidene fluoride) (PVDF),1−4 poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)),1,5−7 poly(vinyl fluoride-trifluoroethylene) (P(VF-TrFE)),8 poly(vinylidenecyanide-vinylacetate) (P(VDCN-VAc)),9 and poly(acrylonitrile-allylcyanide),10 show a high dielectric constant above the glass transition temperature (Tg). Among these materials, the alternating copolymer P(VDCN-VAc) has the highest relative dielectric constant of 120 over 170 °C.9 The unusually high dielectric constant of P(VDCN-VAc) is attributed to the collaborative dipole orientation of a locally ordered polar structure consisting of trans-rich sequences.11 However, the dielectric constant at room temperature is almost five because of the freezing of the micro-Brownian motion below Tg. On the other hand, the polar group can be freely oriented and polarized along the electric field above Tg, overcoming the dipole–dipole interaction. Theoretically, the melting point (Tm) is expressed as the ratio between the change in enthalpy (ΔHm) and the change in entropy (ΔSm), expressed as eq 1

graphic file with name ao0c02084_m001.jpg 1

and the proportional relationship between Tm and Tg is Tg/Tm ≃ 0.50–0.67.12 Later, Boyer modified the relationship and reported that there are three regions of relationship between Tg and Tm.13 Region A (Tg/Tm ≃ 0.50) contains unsubstituted polymers, region B (Tg/Tm ≃ 0.67) contains the majority of the vinyl, vinylidene, and condensation polymers, and region C (Tg/Tm ≃ 0.72–0.95) includes poly(α-olefins) with long side chains. In addition, Hodge reported a summary of developments in enthalpy relaxation in amorphous materials associated with the glass transition,14 demonstrating that Tg also correlates with the enthalpy change.

From eq 1 and the relationship between Tg and Tm, when a polar or functional group capable of forming hydrogen bonds exists, ΔHm is high because the intermolecular force is large. When the polymer is rigid, ΔSm is small because the conformational change is not distinct before and after melting. Conversely, when the chain is flexible, ΔSm is high. In the case of a copolymer, according to Flory’s theory, the enthalpy change at Tm, the Tm of the random copolymer, and the molar fraction of the crystalline component are closely related.15 Regarding Tg, the Gordon–Taylor,16 Mandelkern,17 and Fox18 equations based on the free volume theory have been reported. Moreover, the Gibbs–Dimarzio19 and Barton20 equations focus on bond rotation. Although the concept of the equations is different, the Gordon–Taylor, Mandelkern, and Gibbs–Dimarzio equations are isomorphic. From eq 1 and the abovementioned theories, it can be observed that the introduction of a polar group is not suitable for lowering the Tm and Tg because of the large magnitude of the melting enthalpy. Conversely, when the dipole number density is decreased by reducing the number of polar groups, the Tg of the polymer decreases because of the weak interaction between the dipoles, and simultaneously, the relative dielectric constant also decreases. Because the Tg and the relative dielectric constant of the polymer are in a proportional relationship, it is difficult to synthesize a polymer with both a low Tg and a high dielectric constant, that is, high-dielectric rubbers. Nevertheless, acrylonitrile-butadiene rubber (NBR) and hydrogenated NBR (HNBR) have a high dielectric constant of 10 or more at room temperature and a low Tg of less than 0 °C21 compared to other rubbers. The copolymerization reactivity ratios of butadiene (BD) and acrylonitrile (AN) in NBR, rBD = 0.1–0.452 and rAN = 0.046–0.1, have been reported.22 The product rBDrAN is almost zero; this indicates that the majority of NBR sequences consist of alternating sequences. From the examples of P(VDCN-VAc) and NBR, the monomer sequence of the polar group AN in a polymer is crucial for achieving a high dielectric constant.

For this study, we focused on the C–N stretching vibration of the nitrile group and the internal rotation energy around the C–C bond within the nitrile group, which seems to be related to the monomer sequence of the polar group in the polymer and the relative dielectric constant. The vibration and rotational motions of polar groups are affected by various intermolecular interactions in the surrounding environment. Therefore, the stretching vibration of the CN group in the polymer is considered to be sequence-dependent. Vibrational spectroscopy is useful for detecting slight shifts or differences in the absorption intensities of stretching vibrations of specific functional groups. As a consequence of molecular interactions arising from chemical or physical effects (hydrogen bonding or dipole association), shifts in the absorption peak are observed. In the polymer field, vibrational spectroscopy has been widely conducted to analyze the chain configuration,23,24 chain conformation,25−27 and crystallization kinetics.28,29 For example, complex formation between the nitrile group of NBR and metal cations has been reported.30−32 In addition to the peak at 2240 cm–1, corresponding to the symmetric stretching vibration mode of the nitrile group of NBR, a new shoulder appeared at 2270 cm–1 and became stronger with increasing amounts of Li after the addition of Li salts. For rubber, the flexibility of the main chain is also important, as described above. A C–C bond in the main chain does not show free rotation; however, it is stable in gauche and trans conformations. When the internal rotation barrier potential between the gauche and trans conformations is low, ΔSm becomes large because of the increase in possible conformations.

Thus, polymers with low internal rotation energy barriers have low Tg values from eq 1 and the relationship between Tg and Tm. The internal rotation energy around the bond is considered to be a factor contributing to the low Tg of NBR.

In this study, we evaluated the relationship between the relative dielectric constant and the AN sequence in rubber. To evaluate the effect of the AN sequence on the relative dielectric constant, alternating and random copolymers including AN were synthesized. We investigated the relative dielectric constant and the peak of the CN stretching vibration of the polymers. To analyze the CN peak shift, the C–N bond length was evaluated. The internal rotation potential of the dimer model was also calculated to investigate the flexibility.

Results and Discussion

Synthesis and Properties of Copolymers

Figure 1 shows the chemical structure of copolymers containing AN. Tables 1–4 summarize the composition and characteristics of the copolymers. In P(VBE-AN), more than 50% of AN was inserted into the copolymer even when the amount of AN in the feed was small because butyl vinyl ether (VBE) does not undergo radical homopolymerization. The copolymerization reactivity ratios of VBE and AN in NBR were estimated to be rVBE = 0.031 and rAN = 0.57, respectively, from the alternating evaluation (Figures S2 and S3). This indicates that P(VBE-AN) is an alternating copolymer. The Tg of P(VBE-AN) was higher than 0 °C, and the dielectric constant was 10 or higher. The increased insertion of butyl acrylate (BA) reduced the Tg of P(VBE-AN-BA) to below 0 °C; however, the relative dielectric constants also decreased to less than 10. Random copolymers, P(BA or OA-AN), had a low Tg and showed low dielectric constants of less than 10. These results show that the sequence of AN in rubber clearly affects the dielectric constant and Tg.

Figure 1.

Figure 1

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Chemical structure of copolymers containing AN.

Table 1. Properties of P(VBE-AN) Copolymers.

  monomer ratio VBE–AN
      
sample in feed in polymer Tg (°C) peak frequency of C–N stretching vibration (cm–1) relative dielectric constant at 100 Hz
P(VBE49-AN51) emulsion 90:10 49:51 7.3 2238.0 10.69
P(VBE39-AN61) emulsion 70:30 39:61 3.6 2239.9 11.85
P(VBE42-AN58) solution 70.30 42.58 15.9 2239.0 10.87
P(VBK33-AN67) bulk 50:50 33:67 34.1 2239.9 12.37
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Table 4. Properties of HXNBR, NBRs, and PAN.

sample product name AN mol % Tg (°C) peak frequency of C–N stretching vibration (cm–1) relative dielectric constant at 100 Hz
HXNBR Therban XT 8889 33.8 –20.0 2236.1 14.90
NBRI Nipol DN302 27.5 –39.3 2237.3 12.15
NBR2 Nipol DN2850 28.0 –38.6 2237.0 14.60
NBR3 Nipol DN3350 33.0 –28.5 2237.0 14.05
PAN   100.0   2242.8  
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Table 2. Properties of P(VBE-AN-BA) Copolymers.

  monomer ratio VBE–AN–BA
      
sample in feed in polymer Tg (°C) peak frequency of C–N stretching vibration (cm–1) relative dielectric constant at 100 Hz
P(VBE32-AN63-BA5) bulk 50:45:5 32:63:5 27.0 2239.9 4.92
P(VBE29-AN61-BA10) bulk 50:40:10 29:61:10 18.2 2239.0 11.82
P(VBE22-AN58-BA20) bulk 50:30:20 22:58:20 –1.6 2239.0 8.06
P(VBE30-AN55-BA15) bulk 67:20:13 30:55:15 –1.6 2239.0 6.60
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Table 3. Properties of P(BA or OA-AN) Copolymers.

  monomer ratio BA or OA–AN
      
sample in feed in polymer Tg (°C) peak frequency of C–N stretching vibration (cm–1) relative dielectric constant at 100 Hz
P(BA62-AN38) bulk 70:30 62:38 –29.7 2239.9 7.50
P(BA45-AN55) bulk 60:40 45:55 –3.3 2239.9 7.90
P(BA39-AN61) bulk 50:50 39:61 5.4 2240.9 7.35
P(BA84-AN16) solution 73:27 84:16 –20.7 2239.9 7.80
P(BA94-AN6) solution 85:15 94:6 –35.5 2239.9 6.50
P(BA98-AN2) solution 95:5 98:2 –47.3 2239.0 5.20
P(OA55-AN45) bulk 60:40 55:45 –31.2 2239.9 6.70
P(OA47-AN53) bulk 50:50 47:53 –13.8 2240.9 5.97
P(OA99-AN1) solution 95:5 99:1 –57.7 2239.0 4.30
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C–N Stretching Vibration Relationship with AN mol % and the Relative Dielectric Constant

Figure 2 shows the Fourier transform infrared (FT-IR) spectra of the copolymers in the C–N stretching vibration range. The wavenumbers corresponding to the spectra maxima (peak frequencies) of the C–N stretching vibration are listed in Tables 1–4. A shift to a lower wavenumber was observed from 2242.8 cm–1 for polyacrylonitrile (PAN) to 2236.1 cm–1 for hydrogenated carboxylated NBR (HXNBR). The peak frequencies of the other polymers were within this range. Figure 3 shows a plot of the AN mol % in the polymer and the peak frequency of the C–N stretching vibration. It can be seen that the peak frequency shift is different between random and alternating copolymers. The alternating copolymers, P(VBE-AN), P(VBE-AN-BA), and NBRs, tend to shift on the straight line connecting PAN and HXNBR as the AN mol % decreased. On the other hand, the random copolymers, P(BA-AN) and P(OA-AN), negligibly shifted as the AN mol % decreased. Figure 4 shows a plot of the relative dielectric constant of the copolymers and the peak frequency of the C–N stretching vibration. The dielectric constant also has a correlation with the peak frequency. The random copolymers, P(BA-AN) and P(OA-AN), have a small peak shift, and the dielectric constant does not exceed 10, whereas the alternating copolymers, P(VBE-AN), P(VBE-AN-BA), and NBR, shift toward lower wavenumbers, and the dielectric constants exceed 10 or more. From these two relationships, the environment surrounding the C–N bond is obviously dependent on the monomer sequence. The frequency shift is usually caused by the effects of adjacent functional groups. Figure 5 shows the chemical structures containing a nitrile group and the peak frequencies of the C–N stretching vibration of small molecules and polymers. In the case of small molecules, the peak frequency differed depending on the adjacent group. For AN and methacrylonitrile with adjacent unsaturated bonds, the peak frequency of the nitrile was observed at 2231 cm–1, which is lower than the peak frequency of NBR. For valeronitrile, propionitrile, butyronitrile, and acetonitrile with adjacent saturated linear alkyl bonds, the peak frequencies were observed at 2247, 2248, 2253, and 2254 cm–1, respectively, which are higher than that of PAN. The peak frequency of allyl cyanide was observed to be the highest at 2263 cm–1. The branch structure of 2-methylbutyronitrile resembled AN in PAN, and the peak maxima was 2241 cm–1. The peak frequency of small molecules is shifted by 31 cm–1 due to the effect of the adjacent group. Compared with this shift, the shift to a lower wavenumber of the polymers containing AN is only about 7 cm–1. It can be understood from the structural similarity that the peak frequency of PAN (2242.8 cm–1) is located between the branch structure of 2-methylbutyronitrile (2241 cm–1) and isobutyronitrile (2246 cm–1). However, because the CN group in the polymer does not have a double bond at the adjacent α-position unlike AN, the influence of the adjacent structure cannot be the origin of the lower wavenumber shift. In principle, the shift could be attributed to the change in the bond length.

Figure 2.

Figure 2

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FT-IR spectra of the copolymers in the C–N stretching vibration range. (a) P(VBE-AN) and PAN, (b) P(VBE-AN-BA) and PAN, (c) P(BA or OA-AN), and (d) NBRs and PAN.

Figure 3.

Figure 3

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Plot of AN mol % in the polymer and peak frequency of the C–N stretching vibration.

Figure 4.

Figure 4

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Plot of the relative dielectric constant of the polymers and the peak frequency of the C–N stretching vibration.

Figure 5.

Figure 5

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Chemical structures of small molecules and polymers containing nitrile ordered by the peak frequency of the C–N stretching vibration. The peak wavenumbers of the small molecules were taken from ref (33).

Analysis of the C–N Bond Length

To analyze the C–N peak shift, the C–N bond length of several models was evaluated. From the fundamentals of IR spectroscopy, the vibrational potential energy of a diatomic molecule is derived from the model of the harmonic oscillator, and then the frequency is calculated from the potential energy as follows

graphic file with name ao0c02084_m002.jpg 2

where ν is the frequency (equivalent to the wavenumber), k is a constant factor characteristic of a spring (equivalent to the bond strength), and μ is the reduced mass.

From eq 2, focusing on the C–N stretching vibration because the reduced mass μ is common, k is the main factor governing the peak frequency. When k is small, ν becomes small. A smaller k indicates a weaker bond strength and a longer bond length. Therefore, a shift in the wavenumber relates to the bond length. For coordination bonds, a shift to low wavenumbers has been reported to correlate with the bond length.34,35Figure 6 shows a plot of the C–N peak frequency and the C–N bond length of small molecules containing CN. It can be seen that the wavenumber of the peak and the bond length are correlated. As the bond length increases, the wavenumber decreases. Figure 7 shows a plot of the number of carbon atoms between AN monomers (carbon number) and the C–N bond length of AN–AN, AN–(CH2)n–AN (n = 1, 2, 3, and 4), AN–(BA)m–AN (m = 1, 2, and 3), AN–VBE–AN, and AN–BD–AN models (Figure S4). AN–AN has a carbon number of 1 and a C–N bond length of 1.1612 Å. As the CH2, BA, or VBE monomer was inserted between AN groups, the C–N bond length increased. Upon insertion of CH2, which is a model of HNBR, the C–N bond length reached between 1.1621 and 1.1622 Å. This length corresponds to the 2230–2240 cm–1 wavenumber in Figure 6. The C–N bond length was longer than that of BA at the same carbon number. The C–N bond length of VBE was slightly longer than that of BA at the same carbon number. The C–N bond length of the NBR model, AN–BD–AN, was almost the same as that after CH2 insertion and also longer than that of BA at a carbon number of 5. From the results of C–N bond length evaluation, the observed shift to lower wavenumbers was consistent with the lengthening of the C–N bond and the shift order was NBR > VBE > BA. This explains why P(BA-AN) shifted only up to 2239.0 cm–1, while NBR shifted to 2236.1 cm–1.

Figure 6.

Figure 6

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Plot of the C–N peak frequency and the bond length of small molecules containing CN.

Figure 7.

Figure 7

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Plot of the number of carbon atoms between AN monomers and the C–N bond length of AN–AN, AN–(CH2)n–AN (n = 1, 2, 3, and 4), AN–(BA)m–AN (m = 1, 2, and 3), AN–VBE–AN, and AN–BD–AN models. When n or m increases by 1, the number of carbon atoms increases by 2.

Internal Rotation Potential of the Dimer Model

Figure 8 shows the internal rotation potential energy curves and Newman projections of the eclipsed, gauche, and trans conformations in dimer models. All models showed the most stable trans (t) conformation around 180°, gauche (g) conformation around 60°, and gauche′ (g′) conformation around 300°. The results show that the energy barriers of the HNBR model, AN–CH2, in its trans–gauche transitions (18.6 kJ mol–1) were smaller than those of other models, which indicates an easier transition between the two conformations. The trans–gauche transition energy of the AN–AN, AN–VBE, and AN–BA dimer models was 26.2, 22.2, and 20.4 kJ mol–1, respectively. This transition from 60 to 180° was brought about by the barrier energy that developed when the CN and other functional groups approached each other. The AN–AN model had the highest barrier energy due to the dipole–dipole interaction. The trans–gauche′ transition energy and local energies of the HNBR model at g and g′ also showed the lowest values. Because NBR is an alternating copolymer, the AN–CH2 sequence occupies the majority, and the ratio of the AN–AN sequence is low. On the other hand, in the case of the random copolymer P(BA-AN), both the AN–AN and BA–AN sequences require a higher energy to rotate compared to the AN–CH2 sequence. Therefore, the main chain in HNBR can rotate easily, indicating that ΔSm is large due to a high degree of freedom rotation. From the above results, the flexibility of the NBR main chain is correlated with the low Tg.

Figure 8.

Figure 8

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Internal rotation potential energy curves and Newman projections of the eclipsed, gauche, and trans conformations in the dimer models.

Conclusions

We evaluated the relationship between the relative dielectric constant and the AN monomer sequence in rubber to explain the low Tg and the high dielectric constant for NBR. For different AN sequences in rubber, we synthesized random and partially alternating copolymers. FT-IR measurements determined that the peak frequency of the C–N stretching vibration was correlated with the monomer sequence and the relative dielectric constant. The alternating copolymer shifted to a lower wavenumber from ∼2243 cm–1 for PAN to ∼2236 cm–1 for the NBRs, while the random copolymer only shifted to ∼2239 cm–1. The shift to a lower wavenumber could be explained by the C–N bond length calculation. As the bond length increased, the wavenumber decreased. Thus, the AN sequence reflects this C–N stretching vibration shift. From the internal rotation potential energy calculations of the dimer models, the HNBR model showed the lowest energy of the calculated models, indicating that AN in the NBR could easily rotate. These analyses confirm the importance of the alternating sequence of the AN monomer for attaining both a high dielectric constant and a low Tg.

Experimental Section

Materials

VBE, BA, and octyl acylate (OA) were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Methanol, N,N-dimethylformamide (DMF), methyl ethyl ketone (MEK), and 2,2′-azobis(isobutyronitrile) (AIBN) were purchased from FUJIFILM Wako Pure Chemical Corporation (Japan). NALOACTY CL-120, used as an emulsifier, was purchased from SANYO Chemical Industries, Ltd. (Japan). HXNBR (Therban XT 8889; CN: 33.8 mol %, COOH: 3.8 mol %, and residual double bond content: 3.5 mol %) was obtained from LANXESS (USA). NBR samples (Nipol DN302 with 27.5% AN, DN2850 with 28.0% AN, and DN3350 with 33.0% AN) were obtained from ZEON Corporation (Japan). PET3811 obtained from Lintec Corporation (Japan) was used as a releasable film. As a finger contact prevention film for the electrode, a PET film (Diafoil, Mitsubishi Chemical Corporation, Japan) was used.

Synthesis of Copolymers Containing AN

To synthesize copolymers with various components of the AN sequence, determination of the e value of each monomer is important. Generally, the alternating copolymer is synthesized by the combination of an electron-withdrawing (e value is positive) monomer and an electron-donating (e value is negative) monomer. Here, the synthesis was performed using a combination of AN (e: +1.23) and BA (e: +0.85) or OA (e: +2.01) for random copolymerization and VBE (e: −1.50)22 for alternating copolymerization. Because the P(VBE-AN) copolymer is an alternating copolymer, P(VBE-AN-BA) was synthesized to partially disarrange the sequence of AN and reduce the Tg. PAN was synthesized as a standard sample.

P(VBE-AN)

For emulsion polymerization, 150 mL of distilled water and 2.0 g of CL-120 were added to a three-neck flask and mixed with a stirring blade. Prescribed weights of AN and VBE were added to the solution. After purging with nitrogen for 30 min, 3.0 g of PSA was added, and the mixture was heated under reflux at 75 °C for 7 h under a nitrogen atmosphere. After reprecipitation and washing were carried out using methanol, the residue was dried under reduced pressure. The polymer was immersed in MEK, and the components soluble in MEK were extracted. The solvent was removed under reduced pressure.

For solution polymerization, the prescribed amount of AN and VBE was placed in a round-bottom flask and dissolved in tetrahydrofuran (THF) (150 mL). After purging with nitrogen for 30 min, 0.1 mol % of AIBN was added, and the mixture was heated under reflux at 75 °C for 6 h under a nitrogen atmosphere. After reprecipitation and washing with methanol, the residue was dried under reduced pressure.

For bulk polymerization, the prescribed weights of AN and VBE were added to a round-bottom flask. After purging with nitrogen for 30 min, 0.1 mol % of AIBN was added, and the mixture was heated under reflux at 45 °C under a nitrogen atmosphere and stirred until the solution solidified or for 24 h if solidification did not occur (Caution: continued heating after solidification produced smoke). After washing with methanol, the residue was dried under reduced pressure. The composition was confirmed by 1H NMR spectroscopy (400 MHz, CDCl3): 2.7–3.0 ppm (−CH(CN)– for AN, 1H) and 3.3–3.7 ppm (−CH(OCH2C3H7)– for VBA, 3H).

To increase the molecular weight of P(VBE-AN), emulsion polymerization was performed. However, the molecular weights were not significantly different from those of solution polymerization (see Table S1).

P(VBE-AN-BA)

P(VBE-AN-BA) was synthesized using the same procedure as described for the bulk polymerization of P(VBE-AN), except that BA was added. The composition was confirmed by 1H NMR spectroscopy (400 MHz, CDCl3): 2.7–3.0 ppm (for AN, 1H), 3.3–3.7 ppm (for VBA, 3H), and 4.2 ppm (−C(=O)OCH2C3H7 for BA, 2H).

P(BA-AN) and P(OA-AN)

P(BA or OA-AN) was synthesized using the same procedure as described for the bulk and solution polymerization of P(VBE-AN), except that VBE was replaced with BA or OA, and THF was replaced with DMF. The composition was confirmed by 1H NMR spectroscopy (400 MHz, CDCl3): 2.7–3.0 ppm (for AN, 1H) and 4.2 ppm (for BA or −C(=O)OCH2C7H15 for OA, 2H).

PAN

PAN was synthesized using the same procedure as described for the solution polymerization of P(VBE-AN), except that only AN was used, and THF was replaced with DMF.

Preparation of the Copolymer Film for Evaluation of the Relative Dielectric Constant

To prepare the polymer film, the polymer was dissolved in acetylacetone at a concentration of 12 wt %. After degassing, the solution was applied onto a releasable film, slid using a bar coater, dried, and annealed at 150 °C for 60 min.

Sample Preparation for the Measurement of Relative Dielectric Constants

A scheme of the sample preparation process for the measurement of relative dielectric constants is shown in Figure S1.

Characterization

The 1H NMR spectra were obtained using an AVANCE III 400 (Bruker, Germany) and a JNM-LA400 (JEOL Ltd., Japan). Differential scanning calorimetric thermograms were obtained using a DSC6220, SII EXSTAR 6000 (Seiko Instruments Inc., Japan) from −100 to 150 °C at a heating rate of 10 °C min–1 under a nitrogen atmosphere. FT-IR spectra were measured using a FT/IR-4200 type A with ATR PRO450-S (JASCO Corporation, Japan). The data were acquired with a resolution of 1 cm–1. The relative dielectric constant from 20 to 300 Hz at 5 V was measured by an LCR METER E4980AL (Keysight Technologies, Inc., USA).

Calculation of the C–N Bond Length and the Internal Rotation Potential

In order to analyze the shift of the C–N stretching vibration, we calculated the CN bond length of small molecules containing CN and evaluated the change in the CN bond length when a monomer was inserted between the AN monomer. Density functional theory (DFT) calculations at the B3LYP/6-31G+(d) level was performed using the Gaussian09 suite. As a calculation model, the geometries of AN–AN, AN–(CH2)n–AN (n = 1, 2, 3, and 4), AN–(BA)m–AN (m = 1, 2, and 3), AN–VBE–AN, and AN–BD–AN were optimized, and the CN bond lengths were evaluated. To calculate the internal rotation potential, DFT calculations at the level of B3PW91/6-31G(d) were used.36−38 The dimer model geometries were optimized. After the center C–C of CH3CH(CN)–(CH2R) was fixed, the dihedral angle was rotated every 30°, and the corresponding energies were optimized. Each energy was normalized based on the most stable state.

Acknowledgments

This research was supported by the Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP, AS2525027M) from Japan Science and Technology Agency (JST). We are grateful to Kazunobu Hashimoto of the former Sumitomo Riko Company for contribution to the establishment of research system and Sachie Inoue of Kyushu University for great help in the FT-IR measurement. NMR measurements were supported by Evaluation Center of Materials Properties and Function, Institute for Materials Chemistry and Engineering, Kyushu University. This work was the result of using research equipment shared in the MEXT Project for promoting public utilization of advanced research infrastructure (program for supporting introduction of the new sharing system) grant number JPMXS0422300120.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02084.

  • Sample preparation for the measurement of relative dielectric constants, time dependence of the residual monomer ratio of AN and VBE, plot of the in feed ratio and in polymer ratio in each composition of VBE:AN, dimer model for the calculation of the C–N bond length, and the weight average molecular weight of copolymers (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02084_si_001.pdf (371.1KB, pdf)

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