2023 Volume 6 Issue 6 (Published 31 August 2024)

Open Access

INEOS OPEN, 2023, 6 (6), 181–189  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io2327a

Impact of the Sequence Control and Phase Separation
on the Properties of Single-Ion Conducting Block Copolymers

Abstract

Block copolymers with an AB-block, consisting of a random copolymer of an anionic monomer bearing Li⁺ and poly(ethylene glycol) methyl ether methacrylate, and a C-block based on poly(2-indanyl methacrylate), were synthesized by the RAFT-polymerization technique. The variation in molecular weights (62–71 kDa) and mass ratio between the ionic (AB) and neutral (C) blocks (M_AB/M_C = 1.92–2.18) allowed for obtaining the copolymers that combine ionic conductivity of up to 6×10^–7 S/cm (25 °C) and enhanced mechanical (viscoelastic) properties. This combination was gained owing to microphase separation caused by the incompatibility of AB and C blocks. The AFM study indicated the formation of lamellar structures.

Key words:single-ion conductor, ion conductivity, solid electrolyte, block copolymers, microphase separation.

 

Introduction

The rapid development of portable electronics and electric transport requires compact and powerful power sources [1, 2]. A promising route for increasing the specific characteristics of lithium power sources is the improvement of batteries with a lithium metal anode, since they should potentially have a high theoretical specific capacity up to 3860 mAh/g [3]. The high chemical activity of lithium does not allow the use in such devices of either liquid electrolytes in the form of solutions of lithium salts (LiPF₆, LiBF₄, etc.) in organic solvents (propylene carbonate, ethylene carbonate, etc.) or polymer gels filled with these solutions [4]. The alternatives are ceramic electrolytes [5, 6], mixtures of a polar polymer, such as poly(ethylene oxide), with lithium salts [7], or single-ion conducting polymers (SICPs), which represent polyanions with free Li⁺ cations [1, 8]. The mentioned solid electrolytes are chemically stable towards the alkali metal and can prevent the growth of dendrites from the surface of the metal electrode [9].

SICPs have additional advantages over two-ion-conducting solid electrolytes: they feature a charge transfer coefficient (t_Li+) close to unity, which allows for avoiding the concentration polarization caused by the accumulation of anions at the electrolyte/electrode interface [10]. The concentration polarization increases the resistance of the electrolyte and leads to the growth of lithium dendrites, which, in turn, increases the possibility of short circuit followed by fire or even explosion. In batteries based on SICPs, ions do not accumulate near the electrodes during charging and discharging, but are distributed evenly over the volume, which suppresses the growth of lithium dendrites [8]. In addition, replacing flammable and toxic liquid electrolytes in commercial lithium-ion batteries with single-ion conducting polymers can significantly improve the safety of the resulting devices [11] and make them more flexible and miniature.

The main disadvantage of SICPs is the low ionic conductivity (10^–7–10^–5 S/cm at 20 °C). Moreover, the highest conductivity is characteristic of ionic polymers with a glass transition temperature T_g < r.t., which are mechanically unstable [12]. To combine conductivity with the required mechanical properties of the material, it is possible to use block copolymers, in which one of the blocks contains charge carriers, while the second one improves the system strength [13, 14]. It is known that incompatibility of the blocks can lead to microphase separation [15, 16]. The formation of micro- and nanosized domains (cylinders, lamellas, gyroids) gives rise to the channels with a high concentration of charge carriers in a solid matrix, which facilitates an increase in the ionic conductivity [17, 18].

The goals of this study were as follows: 1) to obtain a new bicyclic methacrylate monomer containing condensed benzene and cyclopentane rings and to synthesize a homopolymer based on it that would feature a high glass transition temperature (T_g); 2) to accomplish controlled synthesis of anionic block copolymers of the AB-C and C-AB types with different block forming sequences; 3) to study the properties of the resulting block copolymers, namely, ionic conductivity, heat resistance and thermal stability, and morphology. The ionic block (AB) was a random copolymer of an anionic monomer bearing free to move Li⁺ cations and poly(ethylene glycol) methyl ether methacrylate, while the C-block was obtained by the RAFT polymerization of 2-indanyl methacrylate.

Results and discussion

Synthesis of 2-indanyl methacrylate (IndM)

Using cationic [17, 18] and anionic [19–21] polyelectrolytes as examples, it was shown that microphase separation leads to an increase in the ionic conductivity upon transition from a homopolymer or random copolymer to a block copolymer if domains (with lamellar or bicontinuous morphology) with a high concentration of charge carriers are formed in the latter. To create conditions that would facilitate the separation, it is necessary to select the polymers that are poorly compatible with each other. The introduction of charges into one of the blocks results in increasing of the segregation forces compared to neutral block copolymers due to additional interactions and electrostatic correlation effects. As we have shown earlier [19–21], the use of non-polar monomers with aromatic rings, such as phenyl methacrylate, phenylmethyl methacrylate, and 2-phenylethyl methacrylate, in combination with polar ionic monomers provides effective separation of the ionic and neutral phases at the microlevel. In this case, the greater the difference in the glass transition temperatures of two blocks, the greater the probability of microphase separation.

It is known [22] that the presence of cyclic fragments in the main (skeletal) or side chain leads to an increase in the rigidity and the production of carbon-chain polymers with high T_g values. Thus, for poly(1-indanyl methacrylate) and poly(5-indanyl methacrylate), containing cyclic structures, the values of T_g are equal to 65 and 83 °C, respectively [23]. In view of this, in this work, 2-indanyl methacrylate was chosen as the monomer for the synthesis of the neutral C-block (Scheme 1), the homopolymer based on which should demonstrate higher heat resistance than the analogs poly(1-indanyl methacrylate) and poly(5-indanyl methacrylate). For this purpose, 2-indanyl methacrylate (IndM) was synthesized by the nucleophilic substitution between indanol-2 and methacryloyl chloride (Scheme 1).

Scheme 1. Synthesis of 2-indanyl methacrylate, IndM.

The monomer was obtained in high yield (>90%) as a white crystalline powder, which melts in the range of 67–70 °C.

The ¹H NMR spectrum of the main product (Fig. 1) shows, along with the signals of protons at the double bond (1 and 2, δ = 6.08 and 5.55 ppm), the signals of protons of the aromatic (6, δ = 7.21–7.26 ppm) and aliphatic (4 and 5, δ = 5.61, 3.38, and 3.07 ppm) rings.

Figure 1. ¹H NMR spectrum of IndM in CDCl₃.

The results of the ¹³C NMR spectroscopic study also confirmed the structure of the monomer (Fig. 2).

Figure 2. ¹³C NMR spectrum of IndM in CDCl₃.

The IR spectrum of IndM (Fig. 3) contains the intensive absorption bands characteristic of the carbonyl (1702 cm^–1, C=O) and ester (1165 cm^–1, ‒C(O)‒O‒) groups, as well as a low-intensity band at 1631 cm^–1, which is characteristic of the C=C double bond of the methacrylate group. Of note are also a band at 754 cm^–1, attributed to the bending vibrations of aromatic ring C‒H bonds, and an intensive band at 1010 cm^–1, caused by the stretching vibrations of the aliphatic ring C‒H bonds.

Figure 3. IR spectra of IndM, PolyIndM, and PolyIndM-b-I.

Synthesis of block copolymers

According to the previously obtained results [19], in AB-C block copolymers with an ionic AB block, which represents a random copolymer of 1ithium 1-[3-(methacryloyloxy)pro-pylsulfonyl]-(trifluoromethanesulfonyl)-imide (LiM) and poly(ethylene glycol)methyl ether methacrylate (PEGM), with a molecular weight of 60–80 kDa and a molecular weight ratio of the ionic AB and neutral C blocks (M_AB/M_C) of ~2, the favorable conditions for microphase separation arise and the maximum values of ionic conductivity are observed. In the present study, we used not only a new cycloaromatic methacrylate monomer (IndM) as a basis for the neutral C block but also two synthetic approaches differing in the sequence of formation of the AB and C blocks (Scheme 2).

Scheme 2. Synthetic pathways for the preparation of ion-conducting AB-C and C-AB block copolymers.

The molecular weight characteristics and some properties of PolyIndM homopolymer, PolyI random copolymer, and PolyIndM-b-I and PolyI-b-IndM block copolymers, depicted in Scheme 2, are given in Table 1.

Table 1. Selected properties of the polymers under investigation

Polymer (composition)	MNMR,a kDa	Мn,b kDa	Mw/Mnb	MAB/MCc	Tg1,d °C	Tg2,d °C	Tf,d °C	Tonset,e °C	σDC, (S/cm) at 25°C	Morphologyg (domain length, nm)
PolyIndM (Ind95.3)	19.55	1.53	1.91	–	104	–	157	210	–	–h
PolyI LiM16.3-r-PEGM81.5	46.70	48.69	1.17	–	–50	–	–1	170	4.1×10–7	–h
PolyIndM-b-I Ind95.3-b-(LiM15-r-PEGM75)	62.20	46.24	1.23	2.18	–57	83	182	170 (220) f	1.5×10–7	Lamellar 31.1±1.2
PolyI-b-IndM (LiM16.3-r-PEGM81.5)-b-Ind120	70.99	59.45	1.23	1.92	–54	88	169	160	1.6×10–7	Lamellar 39.9±1.3

According to the first approach, the homopolymer (PolyIndM) was first synthesized by the RAFT polymerization of IndM in the presence of 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid used as a chain transfer agent (CTA) (Scheme 2, Ia). This CTA was chosen since it is well suited for the controlled polymerization of methacrylate monomers [24, 25]. After the process completion, the samples were taken from the reaction mass to determine the molecular weight characteristics by gel permeation chromatography (GPC) and to establish the monomer conversion (q) by ¹H NMR spectroscopy. The conversion was calculated from the ratio of the integral intensity of the proton signals at the double bond in the residual monomer (Fig. 4, 1 at δ = 5.76 ppm) to the total integral intensity of four protons located in the aromatic rings of the monomer and polymer (Fig. 4, 6 + 6' at δ = 6.88–6.95 ppm). Thus, after 15 h of polymerization, the proportion of the monomer in the reaction mixture was 0.19; therefore, the conversion of IndM was q = 1–0.19 = 0.81 or 81%.

Figure 4. ¹H NMR spectrum (CDCl₃) of the reaction solution after 15 h of IndM polymerization in DMF (q = 81%).

The molecular weight of PolyIndM was determined using equation (1):

,

(1)

where [М]_o and [CTA]_o are the molar concentrations of the monomer and CTA in the initial reaction solution, q is the monomer conversion (determined based on the ¹H NMR spectrum), M_CTA is the molecular weight of CTA, and M_M is the molecular weight of the monomer.

The value of M_{NMR C} was found to be 19.55 kDa. For comparison, the number-average molecular weight M_n and the molecular weight distribution (M_w/M_n) were determined by GPC (Fig. 5). The molecular weight of PolyIndM (M_n = 1.53 kDa) found by GPC does not coincide with the value of M_{NMR C}. This is due to the difference in the structures of the polymer under study and the PMMA standards used to calibrate the chromatographic column. Despite this, the monomodal form of the GPC curve of PolyIndM (Fig. 5, black line) indicates that the reaction proceeds via the RAFT polymerization mechanism. The isolated homopolymer PolyIndM is colored pink, which is due to the presence of a chromophore group in CTA. The color does not disappear upon polymer reprecipitation, which also indirectly evidences the implementation of the RAFT polymerization mechanism.

Figure 5. GPC curves of PolyIndM and PolyIndM-b-I.

The data of elemental analysis, NMR and IR spectroscopy (Figs. 3, 6, and 7) fully confirmed the structure and purity of PolyIndM. When comparing the ¹H NMR spectra of the initial monomer and homopolymer, it is evident that the signals of protons at the double C=C bond (Fig. 1, 1 and 2 at δ = 6.08 and 5.55 ppm) and the singlet from the CH₃ group (Fig. 1, 3 at δ = 1.94 ppm) disappear, and in the region of δ = 1.89–0.93 ppm, the signals of the CH₃ and CH₂ groups of the polymer chain appear (Fig. 6, 1, 2, 3). In the ¹³C NMR spectrum, the transition from an individual compound to a macromolecule has the strongest effect on the position of the signals from the carbon nuclei at the double bond (Fig. 2, 1 and 3 at δ = 125.52 and 136.47 ppm). In the macromolecule, they are located in the main polymer chain and are significantly shifted upfield (Fig. 7, 1 and 3 at δ = 45.08 and 53.91 ppm).

Figure 6. ¹H NMR spectrum of PolyIndM in CDCl₃.

Figure 7. ¹³C NMR spectrum of PolyIndM in CDCl₃.

The appearance of the second signal from the carbon nucleus of the methyl group at position 2 (Fig. 7, δ = 18.68 and 17.4 ppm) is associated with the possibility of addition of a monomer unit to the growing macroradical not only in a head-to-tail mode, but also, for example, in head-to-head or tail-to-tail modes.

In the IR spectrum (Fig. 3), a broadening and increase in the intensity of the characteristic bands corresponding to the vibrations of –C(O)‒O‒C‒ (1142 cm^–1) and C=O (1720 cm^–1) moieties are observed. The absorption band at 867 cm^–1 was attributed to symmetric C–O–C vibrations of the ether groups. At the same time, the spectrum of the homopolymer completely lacks the band at 1631 cm^–1, which is caused by the C=C stretches of the methacrylate group of the monomer.

The thermal properties of the homopolymer were studied (Table 1). Figure 8 shows the thermogram of PolyIndM obtained by thermal mechanical analysis (TMA) in an inert medium. A transition from the glassy state to a rubbery one occurs at T_g = 104 °C, which is a fairly high value for methacrylate polymers and was the condition necessary for the most pronounced microphase separation in the resulting ionic block copolymers. A rubbery state of PolyIndM is weakly expressed, presumably due to its relatively low molecular weight. A transition to a viscous flow state begins at the flow temperature T_f = 157 °C. According to the TGA data (Fig. 9, red line), the temperature of the onset of polymer destruction (T_onset) in air is 210 °C.

Figure 8. TMA curve of PolyIndM.

Figure 9. TGA curves of PolyIndM and PolyIndM-b-I.

Then PolyIndM with M_{NMR C} = 19.55 kDa was used as a macro-CTA in the process of random RAFT copolymerization of PEGM and LiM to form the C-AB block copolymer PolyIndM-b-I (Scheme 2, IIa). The copolymerization of LiM with a monomer that has ethylene oxide units –СН₂СН₂О– in the side chain promotes an increase in the mobility of Li⁺ due to the interaction of the cation with oxygen atoms (the Lewis base) and alternation of solvation and desolvation processes under conditions of high mobility of polymer chains [7, 26, 27].

The total conversion of LiM and PEGM monomers was determined by analyzing the ¹H NMR spectrum of the reaction mixture. DMF was used as an internal standard. The ratio of the integral intensities of the signals at 7.84 ppm from DMF (H–C(O)–) and 5.82 ppm from the proton at the double bond of the monomers (CH₂=C(CH₃)–) was compared with the ratio of the corresponding intensities in the spectrum of the initial reaction mixture. The conversion determined in this way was 87%.

The molecular weight of the ionic block was also determined using ¹H NMR spectroscopy and calculated by equation (2):

,

(2)

where [LiM]_o, [PEGM]_o and [macro − CTA]_o are the concentrations of monomers and macro-CTA in the initial reaction solution, q is the conversion, M_LiM, M_PEGM, M_{NMR AB} are the molecular weights of LiM, PEGM, and the ionic block, respectively.

The value of M_{NMR AB} = 42.65 kDa obtained in the experiment was in good agreement with the value of the specified or calculated molecular weight (M_{calc AB} = 19.55×2 = 39.10 kDa). Thus, the experimental ratio of the molecular weights of the ionic and neutral blocks М_AB/М_C was 2.18, which was also close to the proposed value.

The molecular weight of the C-AB block copolymer, which was equal to 62.20 kDa, was calculated using equation (3):

,

(3)

where M_{NMR C} and  M_{NMR AB} are the molecular weights of the neutral (macro-CTA) and ionic blocks.

The GPC curve of the block copolymer PolyIndM-b-I (Fig. 5, red line) shows both a significant shift of the block copolymer band relative to macro-CTA (PolyIndM) towards higher molecular weights and a monomodal curve. The narrow molecular weight distribution (M_w/M_n = 1.23) indicates the implementation of the RAFT polymerization mechanism.

Among the tested volatile solvents, diethyl ether was suitable for precipitating the block copolymer, since C-AB was dissolved in other common solvents (acetone, methanol, dichloromethane). However, the ionic monomer LiM is not soluble in diethyl ether. Due to this, dialysis against distilled water was used to purify PolyIndM-b-I from impurities of unreacted LiM and PEGM. Its structure was confirmed by the ¹H NMR spectroscopic data (Fig. 10). The ¹H NMR spectrum showed the signals of protons at -CH₂- and СН₃- groups in the polymer chain (Fig. 10a, 1, 2, 3 at δ = 0.90–1.85 ppm) and lacked the signals in the range from 5.2 to 6.2 ppm that could be attributed to the СН₂=С(СН₃)- protons from LiM and PEGM monomers, which were observed in the spectrum of the initial reaction solution (Fig. 10b). The IR spectrum (Fig. 3) of the C-AB block copolymer showed the absorption band associated with the ‒C(O)‒O‒ vibrations (1142 cm^–1) overlapped with a broadened and more intense band of the asymmetric stretching vibrations of the C‒O‒C ether group (1099 cm^–1) due to the appearance of a large number of ethylene oxide units in the composition of the attached ionic block. The band at 858 cm^–1 corresponds to the symmetric stretching vibrations of the C‒O‒C group. An absorption band in the region of 1350 cm^–1 confirms the asymmetric stretching vibrations of the SO₂ groups found in the –SO₂–N^‒–SO₂CF₃ anions of the LiM monomer units.

Figure 10. ¹H NMR spectrum of PolyIndM-b-I in CDCl₃ (a) and a fragment of the ¹H NMR spectrum of the initial reaction solution during the synthesis of PolyIndM-b-I (in a mixture of CDCl₃ and DMF) (b).

Figure 11 shows the thermogram of PolyIndM-b-I, where two glass transition temperatures and flow temperature are marked. Thus, the curve shows two transitions from a glassy state to a rubbery one for the ionic (T_g1 = –57 °C) and non-ionic (T_g2 = 83 °C) blocks. The transition to a viscous flow state occurs at T_f = 182 °C. Due to the higher molecular weight, a rubbery state of this sample is expressed somewhat more strongly than that of the homopolymer (Fig. 8).

Figure 11. TMA curves of PolyIndM-b-I and PolyI-b-IndM.

According to the results of TGA studies, the temperature of the onset of destruction of the C-AB block copolymer PolyIndM-b-I in air is 170 °C (Fig. 9). This value is lower than that of the homopolymer PolyIndM, possibly due to the presence of ethylene oxide units in the ionic block and their decomposition at lower temperatures.

The sequence of block formation in the synthesis of the AB-C block copolymer was different. PolyI-b-IndM was formed by the polymerization of IndM using the ionic block as macro-CTA (Scheme 2, IIb). For this purpose, at the first stage, the statistical copolymer LiM_m-r-PEGM_k (PolyI) with the k/m ratio equal to 5 was synthesized according to the published procedure [19] (Scheme 2, Ib). Its molecular weight and thermal characteristics are shown in Table 1.

At the second stage, the RAFT polymerization of IndM was carried out in the presence of PolyI. The monomer conversion was calculated according to the procedure described above for PolyIndM (Fig. 4). In the case of PolyI-b-IndM, it was 83%. The molecular weight of the added neutral block M_{NMR C} was determined using the simplified equation (1'):

,

(1')

where [M]_o and [CTA]_o are the molar concentrations of the monomer and CTA in the initial reaction solution, q is the monomer conversion (determined from the ¹H NMR spectrum).

The molecular weight of the neutral block was M_{NMR C}  = 24.29 kDa. This value almost perfectly coincided with the specified or calculated molecular weight (M_{calc C} = 46.70:2 = 23.35 kDa); therefore, the ratio of the molecular weights of the ionic and neutral blocks М_AB/М_C = 1.92 turned out to be close to the expected one (М_AB/М_C ~ 2).

The value of M_NMR (70.99 kDa) for the AB-C block copolymer PolyI-b-IndM was determined using equation (3):

    

It exceeds the number-average molecular weight determined by GPC (M_n = 59.45 kDa). The distinctive features of the controlled process, namely, the narrow M_w/M_n value equal to 1.23 and a shift of the block copolymer band relative to macro-CTA to the region of higher molar masses, are demonstrated in Fig. 12. A small shoulder in the region of high molecular weights on the GPC curve is associated with the bimolecular termination of macroradicals, which can occur at high conversion degrees.

Figure 12. GPC curves of PolyI and PolyI-b-IndM.

The АВ-С block copolymer PolyI-b-IndM was obtained in 74% yield as a rubbery mass of pink color. The color is due to the CTA fragments included in the resulting block copolymer, which also confirms that the reaction proceeded according to the RAFT polymerization mechanism.

The IR spectrum of the АВ-С block copolymer PolyI-b-IndM was completely identical to the previously presented spectrum of its C-АВ analog PolyIndM-b-I (Fig. 3). When comparing their ¹H NMR spectra (Figs. 10 and 13), it is evident that they are identical in the number of signals and their positions, but differ slightly in the ratio of the integral intensities of the signals responsible for the ionic and neutral blocks.

Based on the ¹H NMR spectroscopic data, the k:m:n ratios of three types of units were determined (Scheme 2): 1:0.2:1.18 (PolyIndM-b-I, Fig. 10) and 1:0.2:1.43 (PolyI-b-IndM, Fig. 13). These ratios coincide with the values determined based on the conversion: 1:0.2:1.25 and 1:0.2:1.45.

Figure 13. ¹H NMR spectrum of PolyI-b-IndM in CDCl₃.

PolyIndM-b-I and PolyI-b-IndM exhibit similar thermal characteristics (Table 1). For example, the shapes of the TMA thermograms and the temperatures of the transitions from a glassy state to a rubbery state (Т_g1 and Т_g2) and from a rubbery state to a viscous flow state (T_f) indicate similar heat resistances of the samples (Fig. 11, Table 1).

At the final stage of our work, the ionic conductivity of the samples placed between stainless steel electrodes was measured by impedance spectroscopy [28]. For PolyIndM-b-I and PolyI-b-IndM, almost identical values of ionic conductivity were obtained at 25 °C: 1.6×10^–7 and 1.5×10^–7 S/cm, respectively. Earlier, in the works of our research group [19–21] and Elabd et al. [17, 29–31], it was noted that high conductivity in ionic block copolymers is achieved as a result of the formation of a system of ion-conducting channels, in which charge carriers are concentrated. The ionic conductivity obtained (σ >10^–7 S/cm at r.t.) exceeds the conductivities of some block copolymers from the SICPs class, known from the literature [32] and synthesized by us earlier [21, 33]. To explain these relatively high ionic conductivity values, the resulting block copolymers were studied by atomic force microscopy (AFM) (Table 1). In Fig. 14, the microphase separation of the lamellar morphology is clearly visible in both samples. The contrast in the effective Young's modulus maps (Fig. 14, b and d) indicates a significant difference in the mechanical properties of the rigid neutral phase (light bands) and the soft ionic phase (dark bands), which is a consequence of their significantly different glass transition temperatures.

Figure 14. AFM images of PolyI-b-IndM (a, b) and PolyIndM-b-I (c, d) films: topography (a, c) and phase (b, d). The sizes were 1×1 μm².

Hence, the results of the study of PolyIndM-b-I and PolyI-b-IndM by IR and ¹H NMR spectroscopy, TMA and TGA, impedance spectroscopy and AFM showed that these block copolymers are identical both in structure and properties. The determined values of ionic conductivity do not depend on the sequence of block addition. They approach the ionic conductivity of the initial statistical copolymer (PolyI), but the content of ionic units in the macromolecule is lower than in PolyI.

Experimental section

Materials

Indanol-2 (98%, TCI), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CTA) (>97%, Aldrich), poly(ethylene glycol)methyl ether methacrylate (PEGM) (M_n = 500, Aldrich), dichloromethane (reagent grade, Komponent-Reaktiv), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99%, Acros), methanol (reagent grade, Komponent-Reaktiv), dimethylformamide (DMF, 99.8 %, Panreac), diethyl ether (99%, reagent grade, Komponent-Reaktiv), Al₂O₃ (activated, basic, Brockmann I, Aldrich) were purchased from commercial sources and used without further purification. Methacryloyl chloride (97%, ABCR) and triethylamine (99%, ABCR) (over KOH pellets) were distilled prior to use. 2,2'-Azobisisobutyronitrile (AIBN) (98%, Aldrich) was recrystallized from methanol. Ionic monomer lithium 1-[3-(methacryloyloxy)propylsulfonyl]-(trifluoromethanesulfonyl)-imide (LiM) was synthesized according to the previously described procedure [34]. The random copolymer of LiM and PEGM was obtained in accordance with previously published method [19]. The Spectra/Por 1(Spectrum labs) dialysis tubing with MWCO 6–8 kDa was used for polymer dialysis.

Methods

The molecular weight characteristics of the polymers were determined by GPC in 0.1 M Li(CF₃SO₂)₂N solution in DMF at 50 °C and a flow rate of 1.0 mL/min. A LC-20AD liquid chromatograph (Shimadzu Corporation) equipped with a RID-20A refractometric detector (Agilent), a Plgel 5 μm MIXED-D column, 7.5×300 mm (Agilent) and a Plgel 5 μm guard column, 7.5×50 mm (Agilent) was used for the analysis. Poly(methyl methacrylate) standards (EasiVial PM, Agilent Technologies with M_p = 550–1558×10³) were used to perform calibration. The dielectric measurements were performed on a Novocontrol Broadband Dielectric Spectrometer using an Alpha analyzer and a Quatro temperature controller. Dried polymer samples were placed between stainless steel electrodes and studied in the frequency range of 50–107 Hz at 25 °C. The conductivity value s_DC was determined as the value in the plateau region (constant values) on the frequency dependences of the real part s'(f) of the complex conductivity according to equation (4):

,

(4)

where Z* is the measured complex impedance of the sample, d and S are the sample thickness and the electrode area, respectively. The ¹H, ¹³C, ¹⁹F, and ⁷Li NMR spectra were recorded on a Bruker AMX-400 instrument in CDCl₃ solution, using the signal of residual protons of the deuterated solvent as an internal standard in the ¹H NMR spectra, and CHCl₂F (Freon 21) and LiNO₃ as external standards for the ¹⁹F and ⁷Li NMR spectra, respectively. The IR spectra were recorded on a Tensor 37 FT-IR spectrometer (Bruker, Germany) with a resolution of 2 cm^–1 using KBr pellets or on a Vertex 70v spectrometer (Bruker, Germany) with a resolution of 4 cm^–1 and a GladiATR attachment with a Pike diamond element. Atomic force microscopy images were obtained with an MFP-3D Infinity microscope (Asylum Instruments/Oxford Instruments, United Kingdom) in the tapping mode (30–35 °C, in air). AC160TS-R3 (Olympus, Japan) cantilevers were applied with a stiffness of 26 N·m^–1 and resonance frequency of 300 KHz. The images were recorded in the so-called soft tapping mode, to avoid deformation and indentation of the polymer surface by the tip. The domains periodicity was evaluated on averaged Power Density Spectrum (PSD) generated from phase shift channel on 3 different 1×1 μm² images. All the images were collected with the maximum available number of pixels (512) in each direction. On each image, two profiles were taken, and for each, the distance over ten consecutive periods was recorded. The general procedure for the preparation of the samples for AFM was as follows: borosilicate glass coverslips (22×22 mm, thickness no. 1 (0.13–0.16 mm), free from streaks, bubbles and striations (Epredia, Netherlands) from hydrolytic class I were rinsed with acetone, then with dichloromethane and dried with air flow. The solution of block copolymer in anhydrous DMF with a concentration of 100 mg/mL was prepared at r.t. under an inert atmosphere. The solution was filtered through a 0.22 mm syringe filter and cast at 22 °C onto a glass coverslip placed on the leveled hotplate, whereupon the surface of the hotplate was heated to 80 °C. An inverted glass funnel with the neck filled with cotton was then placed over top of the glass slide in order to ensure gradual evaporation (over the course of hours), thus enabling reorganization of the films to achieve (near) equilibrium morphologies. Finally, the obtained films on the glass coverslips were transferred into the vacuum bell and dried at 80 °C/1 mbar for 24 h. The thermal mechanical analysis of the polymer samples was performed under an inert atmosphere (He) using a DIL 402 select Expedis dilatometer (NETZSCH, Germany) with a constant load of 0.3 N at a heating rate of 5 °C/min in the range of −100 to 220 °C. The heat distortion or fusion temperature (T_f) was determined as a temperature at which a noticeable deformation under the applied load and scanning/heating rate was observed. The thermal stability of the polymers was assessed by thermogravimetric analysis on a DTG-60H synchronous thermal analyzer (Shimadzu, Japan) upon heating in air at a rate of 5 deg/min.

Syntheses

Synthesis of 2-indanyl methacrylate (IndM). Triethylamine (8.25 mL, 59.33 mmol) was added to a solution of indanol-2 (6.634 g, 49.44 mmol) in dichloromethane (20 ml). The reaction mixture was cooled to ~1 °C. A solution of methacryloyl chloride (6.203 g, 59.33 mmol) in 35 mL of CH₂Cl₂ was added dropwise through a dropping funnel under stirring. The mixture was stirred at room temperature under an argon stream for 24 h. The precipitate was filtered off and washed with CH₂Cl₂. The light-brown mother liquor was transferred to a separatory funnel. The organic layer was washed with 10% aq. HCl (2×50 mL), distilled water (2×50 mL), 5% aq. sodium bicarbonate (2×50 mL) and again with distilled water (3×50 mL) until pH = 6. MgSO₄ was added to the solution and stirred for 4 h. MgSO₄ was filtered off, and the solvent was removed at 25 °C/10 mmHg. The resulting light-yellow powder (8.800 g) was dissolved in 200 mL of CH₂Cl₂ and passed through a column filled with aluminum oxide. The solvent was removed at 25 °C/10 mmHg to give 2-indanyl methacrylate as a white powder, which was dried for 36 h at 25 °C/1 mmHg. Yield: 8.210 g (93%). Mp: 67–70 °С.

Anal. Cacld for С₁₃H₁₄O₂ (202.25): C, 77.20; Н, 6.98. Found: С, 77.13; Н, 7.02%. IR (ν/cm^–1, KBr): 2964(w) (ν_C–H), 2906(vw), 1702(s) (ν_C=O), 1631(w) (ν_C=C), 1475(w), 1451(m) (δ_C–H), 1294(m), 1259(m), 1165(s) (ν_{–С(О)–О–}), 1084(s), 1010(vs) (ν_C–H, Ind), 949(s), 817(s), 792(s), 754(vs) (δ_C–H, Ar), 659(s) (δ_C–H, Ar), 420(m). ¹H NMR (400.13 MHz, CDCl₃): δ 7.23–7.21 (m, 4H, CH, Ar); 6.08 (s, 1H, CH₂=C(CH₃)–); 5.61 (t, 1H, –O–CH(CH₂–)–CH₂–); 5.55 (s, 1H, CH₂=C(CH₃)–); 3.43–3.37 (d, 2H, –O–CH(CHH–)–CHH–); 3.11–3.06 (d, 2H, –O–CH(CHH–)₂); 1.94 (s, 3H, CH₂=C(CH₃)–) ppm.

Syntheses of poly(2-indanyl methacrylate) homopolymer (PolyIndM). IndM (0.707 g, 3.495 mmol), CTA (0.0083 g, 29.73 μmol), and AIBN (0.975 mg, 5.90 μmol) were dissolved in 2.101 g of DMF. The reaction solution was transferred to a tube, degassed by three freeze-pump-thaw cycles. The sealed tube was kept at 60 °C for 15 h, cooled with liquid nitrogen, warmed to room temperature, and opened. A sample was taken from the reaction mass for the analysis by GPC and ¹H NMR (to determine the monomer conversion). The remaining reaction solution was precipitated in a 10-fold excess of methanol to isolate the polymer. The precipitate was purified by extraction with methanol in a Soxhlet apparatus and dried for 24 h at 25 °C/10 mmHg and 36 h at 25 °C/1 mmHg. The target product was obtained as a pink powder. Yield: 0.550 g (78%). M_n = 1.53 kDa, M_w/M_n = 1.91. Conversion q = 81%. M_{NMR C}= 19.55 kDa.

Anal. Calcd for C_1251.64H_1345.92O_192.56NS₂ (19549): С, 76.90; Н, 6.94. Found: С, 76.95; Н, 7.09%. IR (ν/cm^–1, KBr): 2948(w), 1720(s) (ν_C=O), 1481(m), 1460(m) (δ_C–H), 1389(w), 1241(m), 1142(vs) (ν_{–С(О)–О–С–}), 1008(s) (ν_C–H, Ind), 963(m), 867(m) (ν_{s–С–О–С}), 737(vs) (δ_C–H, Ar), 455(w), 415(m). ¹H NMR (400.13 MHz, CDCl₃): δ 7.20 (m, 4H, CH, Ar); 5.40 (m, 1H, –O–CH(CH₂–)–CH₂–); 3.25 (m, 2H, –O–CH(CHH–)–CHH–); 3.00 (m, 2H, –O–CH(CHH–)–CHH–); 1.89–0.93 (m, 5H, –CH₂–C(CH₃)=) ppm. ¹³C NMR (100.61 MHz, CDCl₃): δ 177.2 (C=O); 140.0; 126.9; 124.6; 76.1–75.9; 53.9; 45.1; 38.8–38.6; 18.7; 17.4–17.3 ppm.

Syntheses of the С-АВ block copolymer (PolyIndM-b-I). PolyIndM (0.233 g, 11.90 μmol), LiM (0.071 g, 0.2048 mmol), PEGM (0.513 g, 1.0268 mmol), and AIBN (0.40 mg, 2.44 μmol) were dissolved in 2.455 g of DMF. The mixing, polymerization, and sampling for analysis (¹H NMR and GPC) were carried out according to the procedure described for PolyIndM. Then the reaction mixture was dissolved in 20 mL of water, poured into a dialysis bag, and the block copolymer was purified by dialysis against distilled water. The block copolymer solution was lyophilized, and resulting PolyIndM-b-I was dried at 25°C/1 mmHg for 24 h with a P₂O₅ trap built into the vacuum line. Yield: 0.640 g (79%). M_n = 46.24 kDa, M_w/M_n = 1.23. Conversion q = 87%. M_NMR = 62.2 kDa.

IR (ν/cm^–1, KBr): 2869(m) (ν_C–H), 1723(s) (ν_C=O), 1482(w), 1450(m) (δ_C–H), 1350(w) (ν_{as SO2}), 1325(w), 1245(m), 1099(vs) (ν_{–С–О–С–}), 1026(s), 1009(s) (ν_C–H, Ind), 948(s), 858(s) (ν_{s С–О–С}), 742(vs) (δ_C–H, Ar), 513(m), 416(m). ¹H NMR (400.13 MHz, CDCl₃): δ 7.16 (m, 5Н, CH, Ar); 5.37 (s, 1.25H, –O–CH(CH₂ )–CH₂–); 4.1 (m, 2.4H, –C(O)–O–CH₂– in LiM and PEGM); 3.66–3.58 (m, 34H, –CH₂–O–CH₂–CH₂–O– in PEGM); 3.40 (s, 3H, –O–CH₃ in PEGM); 3.22 (m, 2.9H, –CH₂–CH₂–SO₂–N–SO₂CF₃ in LiM and –O–CH(CHН–)–СНН– in IndМ); 2.97 (m, 2.5H, –O–CH(CHН)–СНН– in IndМ); 2.23–0.90 (m, 12.65H, –CH₂–SO₂–N–SO₂CF₃ in LiM and –CH₂–C(CH₃)–) ppm. ¹⁹F NMR (376.50 MHz, CDCl₃): δ –78.06 (s, CF₃) ppm. ⁷Li NMR (155.50 MHz, CDCl₃): δ –0.69 (s, Li) ppm.

Syntheses of the АВ-С block copolymer (PolyI-b-IndM). IndM (0.334 g, 1.6504 mmol), PolyI (0.533 g, 11.42 μmol), and AIBN (0.39 mg, 2.38 μmol) were dissolved in 2.622 g of DMF. The reaction mixture was prepared, polymerized for 22 h, and sampled for analysis (¹H NMR and GPC) according to the procedure described for PolyIndM. The reaction mixture was precipitated in diethyl ether, the product was washed with diethyl ether, and dried at 25 °C/10 mmHg for 24 h. The resulting pink rubbery mass was dissolved in acetone and reprecipitated in diethyl ether. The product was washed, dried at 25 °C/10 mmHg for 24 h, then at 25 °C/1 mmHg for 36 h. Yield: 0.640 g (74%). M_n = 59.45 kDa, M_w/M_n = 1.23. Conversion q = 83%. M_NMR = 70.99 kDa.

IR (ν/cm^–1, KBr): 2868(m) (ν_C–H), 1723(s) (ν_C=O), 1483(w), 1450(m) (δ_C–H), 1350(w) (ν_{as SO2}), 1325(w), 1245(m), 1099(vs) (ν_{as–С–О–С–}), 1026(s), 1008(s) (ν_C–H, Ind), 948(s), 856(s) (ν_{s С–О–С}), 742(vs) (δ_C–H, Ar), 513(m), 417(m). ¹H NMR (400.13 MHz, CDCl₃): δ 7.17 (m, 5.8Н, CH, Ar); 5.37 (m, 1.45H, –O–CH(CH₂–)–CH₂–); 4.1 (m, 2.4H, –C(O)–O–CH₂– in LiM and PEGM); 3.85–3.58 (m, 34H, –CH₂–O–CH₂–CH₂–O– in PEGM); 3.40 (s, 3H, –O–CH₃ in PEGM); 3.22 (m, 3.3H, –CH₂–CH₂–SO₂–N–SO₂CF₃ in LiM and –O–CH(CHН–)–СНН– in Ind); 2.97 (m, 2.9H, –O–CH(CHН–)–СНН– in Ind); 2.23–0.90 (m, 13.65H, –CH₂–SO₂–N–SO₂CF₃ in LiM and –CH₂–C(CH₃)–) ppm. ¹⁹F NMR (376.50 MHz, CDCl₃): δ –78.07 (s, CF₃) ppm. ⁷Li NMR (155.50 MHz, CDCl₃): δ –0.72 (s, Li) ppm.

Conclusions

New ion-conducting АВ-С and С-АВ block copolymers were obtained by the RAFT copolymerization of lithium 1-[3-(methacryloyloxy)propylsulfonyl]-(trifluoromethanesulfonyl)-imide, poly(ethylene glycol)methyl ether methacrylate, and 2-indanyl methacrylate. The study of the properties of the block copolymers PolyIndM-b-I (АВ-С) and PolyI-b-IndM (С-АВ) showed that they are similar in structure, heat resistance and thermal stability, ionic conductivity and morphology type, regardless of the order of formation of the ionic and neutral blocks. The incompatibility of the blocks and a significant difference in their glass transition temperatures contribute to the formation of lamellas with the interplanar distances d = 31.1 ± 1.2 and 39.9 ± 1.3 nm, respectively. Such self-assembly into a lamellar morphology can explain the high values of ionic conductivity (up to 1.6×10^–7 S/cm at 25 °C) by the formation of ion-conducting channels with a high concentration of Li charge carriers. As a result, the ionic conductivity of the block copolymers is almost identical to the conductivity of statistical PolyI, despite the lower concentration of lithium cations. At the same time, a combination of the ionic statistical copolymer, which is a cold flowing rubber at r.t., and the heat-resistant neutral homopolymer in one macromolecule leads to the formation of polyelectrolytes that significantly exceed PolyI in mechanical strength. Such a combination of the properties and the possibility of obtaining thin films, when applied to various substrates, meets the requirements for polyelectrolytes for solid-state lithium metal batteries. Various synthetic routes used to form ion-conducting solid polymer electrolytes may be useful in the synthesis of triple block or star-shaped block copolymers.

Acknowledgements

This work was supported by the Russian Science Foundation (project no. 21-13-00173). The structures and purity of the resulting compounds were studied with financial support from the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00277-24-00) using the equipment of the Center for Molecular Composition Studies of INEOS RAS.

The authors are grateful to I. A. Malyshkina for measuring the ionic conductivity of the block copolymers and P. Grysan for studying the block copolymers by AFM.

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