Regioregularity and Molecular Weight Effects in Redox Active Poly(3

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Regioregularity and Molecular Weight Effects in Redox Active Poly(3hexylthiophene)-block-poly(ethylene oxide) Electrode Binders Hyosung An, Xiaoyi Li, Kendall A Smith, Yanpu Zhang, Rafael Verduzco, and Jodie L. Lutkenhaus ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00886 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Applied Energy Materials

Regioregularity and Molecular Weight Effects in Redox Active Poly(3hexylthiophene) block poly(ethylene oxide) Electrode Binders Hyosung An,1 Xiaoyi Li,2 Kendall A. Smith,2 Yanpu Zhang,1 Rafael Verduzco,*,2,3 and Jodie L. Lutkenhaus*,1,4 1

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station,

Texas 77843, USA. *E-mail: [email protected] 2

Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas

77005, USA. *E-mail: [email protected] 3

Department of Materials Science and NanoEngineering, Rice University, Houston, Texas

77005, USA 4Department

of Materials Science and Engineering, Texas A&M University, College Station,

Texas 77843, USA KEYWORDS poly (3-hexylthiophene), block copolymer, poly(ethylene oxide), electroactive polymers, regioregularity, molecular weight

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ABSTRACT Simultaneous electron- and ion-conducting polymeric binders for battery electrodes offer a multifunctional alternative to commonly used poly(vinylidene fluoride). One example is poly(3hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO), which conducts electrons and ions in the P3HT and PEO domains, respectively. Notably, P3HT stores charge by doping and dedoping, which further adds to overall capacity of the battery electrode. Conjugated P3HT has been extensively studied for various solid state applications (e.g., photovoltaic cells, field-effect transistors, and light-emitting diodes), where the performance is strongly affected by regioregularity and molecular weight. However, in electrochemical systems such as in batteries, the effects of regioregularity and molecular weight on the charge storage performance are not understood for P3HT-b-PEO. Here, by comparing different P3HT-b-PEO block copolymers of varying P3HT regioregularity (86-97%) and molecular weight (8-19 kg mol-1), we demonstrate a strong correlation between regioregularity and molecular weight with electrochemical properties (i.e., capacity, and redox potential). We show that the charge storage capacity of P3HT-b-PEO significantly increases with increasing regioregularity. The changes in capacity and redox potential are attributed to P3HT’s backbone conformation, planarity, and chain packing. This points to the importance of these two parameters in the design of simultaneous electron- and ionconducting polymer binders for battery electrodes.


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Introduction Despite the recognized importance of regioregularity1-9 and molecular weight10-17 on the solid state electronic performance of conjugated polymers such as poly(3-hexylthiophene) (P3HT), a detailed understanding of these effects on P3HT’s energy storage capability is lacking. In solid state applications (photovoltaic cells,16,

18-20

and field-effect transistors1,

17, 21-22

), P3HT has

demonstrated considerable promise because of the manipulation of the self-organization of polymer chains into semicrystalline domains.23 For example, charge-carrier mobility increases with P3HT molecular weight (6×10-3 cm2 V-1s-1 for 30 kg mol-1 vs. 4×10-6 cm2V-1s-1 for 4 kg mol-1)10 and regioregularity (5×10-2 cm2V-1s-1 for 96% vs. 2×10-5 cm2V-1s-1 for 70%).1 With the rise of P3HT-based binders in battery electrodes,24-30 a deeper understanding of the influence of molecular weight and regioregularity on the redox activity this polymer is required.

P3HT stores charge by a pseudocapacitive mechanism (Figure 1a), which is synonymous with the reversible doping of the polythiophene backbone. As P3HT is oxidized, the polymer loses electrons and becomes positively charged. In an electrochemical cell P3HT oxidation and reduction (doping and de-doping) occur reversibly to act as a rechargeable electrode for a secondary battery or pseudocapacitor. Sivaraman and coworkers31 examined the effect of regioregularity for P3HT homopolymers on electrochemical performance using three P3HT homopolymers having various regioregularities (58, 70, and 96%) and molecular weights (15.6, 32.8, 25.1 kg mol-1, respectively). They showed that high regioregularity (96% vs. 58%) enhances specific capacitance (134.5 F/g vs. 71.8 F/g). In that study, molecular weight was not systematically investigated.


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As binders for battery electrodes, block copolymers bearing a conjugated P3HT block and an ion-conducting block (i.e., polyethylene oxide (PEO), polyvinylpyridine) are particularly interesting due to the simultaneous conduction of electrons and ions in the respective phasesegregated domains.24,

26, 32-39

Balsara and coworkers demonstrated P3HT-b-PEO block

copolymers with simultaneous conduction of electrons and ions (approximately 10-5 S/cm and 10-4 S/cm, respectively).35 This binder (regioregularity of >95% and P3HT molecular weight of 11.6 kg mol-1 and 6.0 kg mol-1) was successfully used in a LiFePO4 cathode.24, 26 Recently, our group reported a P3HT-b-PEO micellar aggregate ― having a PEO-shell and P3HT-core ― as a conductive polymeric binder for V2O5 lithium-ion batteries.36-37 As an added benefit, P3HT-bPEO is not only conductive but also redox active. The P3HT-b-PEO micellar aggregate exhibited a pseudocapacitive behavior but the relationship between P3HT molecular weight or regioregularity with energy storage properties (e.g., capacity and redox potentials) was not examined. Understanding this relationship is important considering the increasing application of conducting polymer binders and redox-active polymer binders in battery systems.

Here, we investigate the effects of P3HT regioregularity and molecular weight on the electrochemical properties of P3HT-b-PEO. We compare four different P3HT-b-PEO block copolymers that have identical PEO molecular weights to isolate the influence of the P3HT block. These polymer binders are examined alone to avoid the overlapping contribution from the otherwise majority electroactive material (i.e., V2O5, LiFePO4). The nature of the micellar aggregate in water is explored, as is the thermal properties of the powder. This information is connected to the crystallinity and ordering of the P3HT block using X-ray diffraction (XRD). UV-Vis absorption of cast P3HT-b-PEO micellar aggregates allows for extraction of the exciton


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bandwidth. Together, these results provide a view of the organization of the P3HT domain and the planarization of the P3HT backbone in the micellar aggregate. We correlate these observations to the redox properties of P3HT block, and discuss which factors most strongly influence P3HT capacity and redox activity in the block copolymer.

Figure 1. P3HT redox reaction and regioregularity. (a) P3HT gives up an electron upon oxidation and becomes balanced by an anion; in this case, the anion is bis(trifluoromethane)sulfonimide (TFSI-). Schematic diagrams of (b) regioregular and (c) regiorandom P3HT.

Experimental section Materials Isopropyl magnesium chloride with lithium chloride complex, Ni(PPh3)4, 4-chloro-3methylphenol, 1,3-bis(diphenylphosphino)propane, tetra-n-butylammonium fluoride (TBAF) (1.0 M in THF), 5-hexynoic acid, 4-dimethylaminopyridine (DMAP), N,N,N',N'',N''pentamethyldiethylenetriamine

(PMDETA),

3-hexylthiophene,

dichloromethane,

p-

toluenesulfonyl chloride, azidotrimethylsilane, ethylene carbonate, imidazole, copper (I) bromide, tert-butyldimethylsilane, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and propylene carbonate were purchased from Sigma-Aldrich. N-(3-dimethylaminopropyl)-N’-


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ethylcarbodiimide hydrochloride (EDC) was purchased from TCI America. Lithium ribbon was purchased from Alfa Aesar. All chemicals were used as received, unless stated otherwise. 316 stainless steel coin spacers (15.8 mm diameter × 0.5 mm thickness) were purchased from MTI Corporation. Water was purified to 18.2 MΩ-cm (Milli-Q, Millipore). Celgard 3501 polypropylene separator was obtained from Celgard.

Synthesis of P3HT-b-PEO block copolymers 2,5-dibromo-3-hexylthiophene was dissolved in anhydrous THF and the solution was stirred in an ice bath for 15 minutes. A solution of isopropyl magnesium chloride with LiCl (1.3 M) in THF was added, and the mixture was stirred for another 2 h at 0 oC. Next, 40 mL of THF was added to the reaction flask followed by a solution containing the functionalized linker and catalyst. This catalyst solution was made by dissolving Ni(PPh3)4 and 2-(4-chloro-3methylphenoxy)ethyl tosylate in 1 mL anhydrous THF inside a glove-box. After it was stirred overnight, this solution was added to the reaction mixture in ice bath directly to produce the regiorandom

P3HT.

On

the other hand,

for the more regioregular P3HT,

1,3-

bis(diphenylphosphino)propane (dppp) was dissolved in 1 mL of dry THF and added to the catalyst solution to stir for additional 2 h, before being added to the reaction mixture. The final polymer product was recovered by precipitation in ethanol and dried under vacuum. P3HT-bPEO was then synthesized by click chemistry of azide-functionalized P3HT and alkynefunctionalized PEO. A more detailed, step-by-step description to the overall procedure can be found in our previous paper.40


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Preparation and characterization of polymer dispersions P3HT-b-PEO dispersions were made by sonicating the polymer and LiTFSI in Milli-Q water at 0 o

C to make a 1 mg of polymer + salt/mL of stock solution. The molar ratio of ethylene oxide

units to Li+ was 0.085, which is an optimum concentration.41 Hydrodynamic diameter was determined using dynamic light scattering (DLS) (ZetasizerNano ZS90, Malvern) at 25 °C. For DLS, the P3HT-b-PEO dispersions were diluted to 0.05 mg/mL. Atomic force microscopy (AFM) measurements were carried out using a Bruker Dimension Icon AFM under tapping mode. UV-vis spectra were measured using a Hitachi U-4100 UV-Vis-NIR spectrophotometer (341-F). P3HT-b-PEO morphologies were investigated using scanning electron microscopy (SEM, JEOL JSM-7500F). For AFM, UV-vis spectroscopy, and SEM, about 0.1 - 0.3 mg of P3HT-b-PEO dispersions were drop-cast on glass (1 cm × 1 cm), air-dried for 3 h, and then annealed at 90 °C for 12 h under vacuum. XRD was carried out using a Rigaku Ultima II vertical θ-θ powder diffractometer using Cu Kα radiation (λ = 1.5418 Å) with Bragg-Brentano parafocusing optics. The operating power was 40 kV and 40 mA. The 2-theta angle was varied from 3° to 30°. Differential scanning calorimetry (DSC, Q200, TA Instruments) was performed on approximately 2 mg of P3HT-b-PEOs using a heat-cool method. Dried samples were loaded into a Tzero aluminium pan (TA Instruments) and annealed at 90 °C for 12 h under vacuum. The dried samples were heated up to 250 °C from 25 °C at a rate of 10 °C/min followed by cooling at the same rate for three heat–cool cycles.

Electrode preparation 316 stainless steel coins (15.8 mm diameter × 0.5 mm thickness) were cleaned via sonication for 15 min each in water, isopropanol, and acetone followed by purging with nitrogen gas and


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drying at 75 °C. P3HT-b-PEO dispersions (1 mg/mL) were drop-cast onto cleaned stainless steel coins by depositing about 0.30 mg of P3HT-b-PEO onto the surface and then air drying and annealing at 90 °C for 12 hours under vacuum.

Cell assembly and measurement Electrochemical measurements were performed using two-electrode cells (Tomcell Japan Co., Ltd.) assembled in a water-free, oxygen-free, argon-filled glovebox (MBraun) using lithium metal anodes. 1.0 M LiTFSI in propylene carbonate was used as the electrolyte and Celgard 3501 was used as the separator. Cyclic voltammetry and galvanostatic charge-discharge measurements were performed using a Solartron 1470E.

Results and discussion Synthesis Four P3HT-b-PEO block copolymers with different P3HT regioregularities (86 ‒ 97%) and P3HT molecular weights (8‒19 kg mol-1) were synthesized, while the PEO block was kept the same (7 kg mol-1), using an approach based on our prior report.40 The regioregularity, molecular weights, and dispersity are listed in Table 1. Hereafter, P3HT-b-PEO having regioregularity of XX% and P3HT number average molecular weight of YY kg mol-1 will be called “P3HT-b-PEO (XX%/YY kg mol-1)”. Highly regioregular block copolymers were synthesized with the addition of 1,3-bis(diphenylphosphino)propane (dppp) during P3HT polymerization, which made the reaction more favorable towards a head-to-tail configuration (Figure 1b-c). The three P3HT-bPEOs (93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol-1) were more regioregular (Figure S1-S4). Less regioregular P3HT-b-PEO (86%/8 kg mol-1) was synthesized using a very


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similar approach, but without the use of dppp in the catalyst solution. The molecular weight of P3HT was controlled mostly by varying the reaction time. However, due to the lack of dppp in the synthesis of less regioregular P3HT, the reaction was more uncontrolled and resulted in a low degree of polymerization (8 kg mol-1). We expect all four P3HTs do not strongly entangle as their molecular weights are lower than 25 kg mol-1 or 35 kg mol-1.42-43

Table 1. Characteristics of P3HT-b-PEO Block Copolymers. Regioregularitya (%)

Mn,P3HTb (kg mol-1)

Mn,PEOb (kg mol-1)

Đc

P3HT-b-PEO (86%/8 kg mol-1)

86

8

7

1.38


93

12

7

1.47


94

19

7

1.12


97

14

7

1.37

Polymer

a

Determined using 1H-NMR. bMn = number-average molecular weight, determined using 1H-

NMR. cĐ = dispersity of the P3HT polymer determined by gel permeation chromatography with polystyrene standards.

P3HT-b-PEO dispersions in water were prepared by sonication, (Figure 2a and 2b). The P3HTb-PEO dispersions were stable for over 6 months, after which large aggregates precipitated. The dispersion’s stability is due to a micellization and aggregation process, in which hydrophilic PEO forms a corona around a hydrophobic P3HT core.36 DLS revealed that the average dimeters of P3HT-b-PEO 93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol-1 were similar (274 nm, 258 nm, and 266 nm, respectively), whereas the average dimeter of P3HT-b-PEO 86%/8 kg mol-


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was smaller (189 nm) (Figure 2c). The trends in size were consistent with AFM analysis of

drop-cast dispersions (Figure S5).

Figure 2. P3HT-b-PEO structure and properties as dispersed in water. (a) Schematic of P3HT-bPEO micellar aggregate preparation. (b) P3HT-b-PEO dispersions at a concentration of 0.05 mg/mL with LiTFSI (the molar ratio of Li+ to PEO repeat units was 0.085) in water. Roman numerals (i, ii, iii, and iv) denote P3HT-b-PEO 86%/8 kg mol-1, 93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol-1, respectively. (c) Size distribution profiles for the P3HT-b-PEO dispersions in water obtained using DLS.

Figure 3. Thermal properties of P3HT-b-PEO powders. (a) The second heating DSC scan of


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P3HT-b-PEO powders at a scan rate of 10 oC/min. Melting temperatures (Tm) and enthalpies of fusion (∆H) as a function of (b) P3HT regioregularity and (c) P3HT molecular weight (Mn).

The thermal properties of P3HT-b-PEO powders as a function of regioregularity and molecular weight of the P3HT block were investigated using DSC (Figure 3). Tm increased as molecular weight increased and was not a strong function of regioregularity within the range studied. The molecular weight dependence of Tm is consistent with prior work.44 For the enthalpy of fusion (∆H) ― taken as the area under the melting peak, ∆H increased with increasing molecular weight, and there was no strong trend with regioregularity within the range studied. Snyder et al. reported the heat of fusion (∆H°) for 100% crystalline P3HT as 49 J/g. The P3HT crystallinity was then determined from ∆H/∆H°, Table 2. In general, the P3HT crystallinity was low (4.315.4%), indicating that quasi-ordered or amorphous regions make up the majority portion in the P3HT core.

Table 2. Characteristics of the P3HT-b-PEO micellar aggregates.

Polymer

∆H (J/gP3HT)

Crystallinity, ∆H/∆H° (%)

Band gapa (eV)

Cathodic peak (V vs. Li/Li+)

Discharge capacityb (mAh/gP3HT)

Electrochemical doping level

P3HT-b-PEO (86%/8 kg mol1 )

2.1

4.3

1.9

3.4

9.4 ± 1.2

0.06 ± 0.01


4.3

8.8

1.8

3.5

23.2 ± 4.5

0.15 ± 0.03

P3HT-b-PEO (94%/19 kg

7.6

15.4

1.8

2.8

24.1 ± 5.3

0.15 ± 0.03


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mol-1) P3HT-b-PEO (97%/14 kg mol-1) a

4.9

10.1

1.7

2.8, 3.4

37.3 ± 5.6

0.23 ± 0.04

Calculated from UV-vis absorption spectra, Figure 4a. bMeasured at 1 C-rate.

Figure 4. UV Vis spectroscopy of P3HT-b-PEOs and electronic properties. (a) Representative UV-vis absorption spectra of P3HT-b-PEO cast from water onto glass. The spectra are normalized to the 0-1 peak intensity. (b) A schematic diagram of P3HT chain packing and charge transport (intrachain vs. interchain). The ratio of the 0-0 and 0-1 peak absorbances (A0-0/A0-1) and exciton bandwidth (W) as a function of (c) regioregularity and (d) molecular weight.

Figure 4a shows UV-vis absorption spectra of various P3HT-b-PEO block copolymers cast from water onto glass. These spectra resembled those of typical P3HT homopolymers.18 λmax decreased as regioregularity decreased. This blue-shifted absorption originates from steric hindrance caused by the head-to-head configuration that induces twisting of the backbone,


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disordered structure, and decreasing π-orbital overlap.45 Interestingly, P3HT-b-PEO 93%/12 kg mol-1 and 94%/19 kg mol-1 showed different λmax values (515 nm vs 557 nm respectively) despite their similar regioregularities. This difference is associated with the P3HT molecular weight, in which lower P3HT molecular weight results in more disordered structures and shorterconjugated segments.46 On the other hand, band gap (Table 2) increased as regioregularity decreased, and no trend in molecular weight was observed.

To further understand P3HT chain conformation in the block copolymer, HJ-aggregate analysis was used.47-48 The HJ-aggregate model clarifies the competitive effects of interchain (H-like aggregate) vs. intrachain (J-like aggregate) transitions (Figure 4b), which are influenced by the polymer chain conformation; thus, the HJ-aggregate model allows for insight into both charge transport and polymer chain conformation. Typically, J-like aggregates are desirable because intrachain charge transport (along the polymer backbone) is two orders of magnitude faster than interchain charge transport (through the π-π packing direction).1,

49-50

Generally, P3HT

aggregates show three distinct absorption peaks at 612 nm, 562 nm, and 512 nm, indicating intrachain (0-0) and interchain transitions (0-1 and 0-2), respectively.51-54 The intensity ratio of A0−0/A0−1 identifies the intrachain and interchain transitions:47-48 ⁄ = (1 − 0.24 ⁄ )⁄(1 + 0.073 ⁄ )

(1)

where A0-0/A0-1 is the intensity ratio of the 0-0 and 0-1 peak absorptions, W is the exciton bandwidth (the nearest-neighbor interchain Coulombic coupling), and Ep is the C=C stretching mode energy (0.18 eV).48, 55 Intrachain transition (J-like aggregate) exhibits A0−0/A0−1 slightly larger than 1, while interchain transition (H-like aggregate) shows A0−0/A0−1 in the region of ∼


13


0.5−0.8.27,

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A0−0/A0−1 increased as regioregularity increased (Figure 4c), whereas molecular

weight yielded no trend (Figure 4d). The highest regioregularity P3HT-b-PEO (97%/14 kg mol1

) showed an intrachain transition (J-like aggregate), whereas the other P3HT-b-PEOs had

interchain transition (H-like aggregates).

We next determined the exciton bandwidth W using eq. (1). Exciton bandwidth decreased as regioregularity increased; however, no distinct trend in molecular weight was observed, Figure 4c and 4d. This implies that the higher regioregularity promotes intrachain transport due to the longer-range order (planarity). Conversely, in the lower regioregularity P3HT-b-PEOs, the twisted backbone interrupts intrachain transport, showing higher W.56-57 These results suggest that regioregularity has strong impact on chain conformation in micellar aggregates of P3HT-bPEO.


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Figure 5. XRD analysis of P3HT-b-PEOs. (a) XRD patterns of P3HT-b-PEO cast from water. (b) A schematic diagram of crystalline P3HT. da-a and dπ-π are lamellar and π-π stacking spacings, respectively. da-a and dπ-π are parallel and perpendicular to the chain backbone. (c) da-a and dπ-π as a function of (c) regioregularity and (d) Mn,P3HT. (e) Paracrystallinity as a function of (e) regioregularity and (f) Mn,P3HT.

The influence of regioregularity and molecular weight on the structure of the P3HT-b-PEOs was demonstrated using XRD. Figure 5a showed peaks characteristic of P3HT.18 The (100) and (010) peaks were observed at diffraction angles 2θ of 5.3o and 23.3o, respectively, indicating the lamellar spacing (da-a) and π-π stacking distances (dπ-π) (Figure 5b). Whereas regioregularity had no trend in chain packing distance, molecular weight had a slight trend (Figure 5c-d): as Mn,P3HT increased, dπ-π decreased. From XRD analysis, the paracrystallinity parameter was estimated to determine the degree of structural disorder (in an imperfect crystallite). The paracrystallinity parameter, g, is a measure of the statistical deviation of local static lattice fluctuations normalized by the average lattice spacing distance:23, 58-59 g ≈ ∆ ! ⁄2"

(2)

where ∆q is the full width at half maximum (FWHM) of the diffraction peak and dhkl is the interplanar spacing. For reference, g = 0% for a perfect crystal, g = 5% for a moderately paracrystalline lattice, and g = 10-15% for a strongly disordered lattice (e.g., amorphous silicon dioxide glass).58-59 Here, g increased slightly with increasing molecular weight; regioregularity, however, had no trend in g. The determined g values (ga-a ~ 13-16% and gπ-π ~ 9-11%) suggest that the P3HT domains in the micellar aggregates are largely disordered (Figure 5e-f). Thus, the


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P3HT core domains are more accurately described as “disordered polymer aggregates” rather than as “polymer crystals”. Together, DSC, XRD and UV-vis spectroscopy give representative insights into various structures of the P3HT-b-PEO micellar aggregates. DSC indicated that a minor portion of the P3HT in the P3HT-b-PEOs was 4.3-15.4 wt% crystalline and that the majority was quasi-ordered or even amorphous. XRD revealed that P3HT crystallites in the micellar aggregates were not perfectly ordered but largely disordered. UV-Vis spectroscopy indicated the backbone conformation, in which the P3HT backbone planarity was greatest for the most regioregular samples. This raises the question as to how order (or disorder) and P3HT chain conformation ultimately impact energy storage properties in P3HT-b-PEO micellar aggregates.

Figure 6. Cyclic voltammetry of various P3HT-b-PEOs in a two-electrode coin cell configuration. Cyclic voltammograms of (a) 86%/8 kg mol-1, (b) 93%/12 kg mol-1, (c) 94%/19 kg mol-1, and (d) 97%/14 kg mol-1 P3HT-b-PEOs. The sample masses were about 1.5-1.6


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mg/cm2. Lithium metal and 1 M LiTFSI/propylene carbonate were used as the anode and electrolyte, respectively. All currents were based on the mass of the P3HT block.

Cyclic voltammetry was used to evaluate the redox doping reaction of the P3HT block in the P3HT-b-PEO block copolymer (Figure 6). The low regioregularity P3HT-b-PEO 86%/8 kg mol1

had a weak redox peak and low electrochemical activity; higher regioregularity P3HT-b-PEOs

(93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol-1) showed distinct redox peaks. The highest regioregular P3HT-b-PEO (97%/14 kg mol-1) showed two cathodic peaks in the reduction cycle at 2.8 V and 3.4 V, whereas the other P3HT-b-PEOs showed only one cathodic peak (Table 2). The two cathodic peaks indicate that coexistence of different conjugation lengths.46, 60-61 The cathodic peak located at lower potential (2.8 V) originates from the reduction of highly ordered, polycrystalline structures of higher conjugation length, whereas the more anodic peak (3.4 V) results from the reduction of less ordered, shorter conjugated segments. Interestingly, P3HT-b-PEO 93%/12 kg mol-1 and 94%/19 kg mol-1 had similar regioregularities, but displayed different cathodic peaks (3.5 V vs 2.8 V respectively). This difference is attributed to the P3HT molecular weight, in which lower molecular weight results in shorter-conjugated segments46 such that a more positive potential is required for doping. This is supported by the blue-shifted λmax in the UV-vis study (Figure 4a) and suggests that both regioregularity and molecular weight have significant impact on their redox properties of P3HT block.


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Figure 7. Energy storage performance for various P3HT-b-PEOs in a two-electrode coin cell configuration. (a) Initial charge-discharge profiles of P3HT-b-PEO block copolymers at 1 C-rate. The double-headed arrows (blue) indicate the hysteresis at 3.0 V. The color code in (a) also applies to (b) and (c). (b) Charge-discharge behaviour at 1 C-rate. (c) Representative cycling properties at various C-rates. Discharge capacities as a function of (d) P3HT regioregularity and (e) P3HT Mn, P3HT for 1C-rate. (f) Discharge capacities as a function of both regioregularity and Mn,P3HT. All capacities were based on the P3HT block mass. At least three samples were tested in each case.

To evaluate the regioregularity and molecular weight effects on capacity, we conducted galvanostatic charge–discharge cycling of P3HT-b-PEOs in a lithium half-cell. Figure 7a-b shows galvanostatic charge–discharge profiles for P3HT-b-PEO block copolymers at 1 C-rate, which is the current required to discharge the theoretical capacity of the cell in one hour. The specific capacities for P3HT-b-PEOs 86%/8 kg mol-1, 93%/12 kg mol-1, 94%/19 kg mol-1, and


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97%/14 kg mol-1 were 10, 19, 19, and 40 mAh/gP3HT, respectively. The calculated specific capacitances were 19, 35, 33, and 67 F/g, respectively (Figure S6). We also examined the hysteresis to study the reversibility of the redox reaction, illustrated in Figure 7a by the doubleheaded blue arrow. The lowest regioregularity P3HT-b-PEO (86%/8 kg mol-1) showed the highest hysteresis (0.99 V); the highest regioregularity (97%) showed the lowest hysteresis (0.78 V). The reduced hysteresis is beneficial to electrode performance and efficiency, demonstrating enhanced reversibility.37,

62-63

Figure 7c shows the cycling behaviour at various C-rates or

currents. Each sample was analyzed for 10 cycles at 1 C, followed by 10 cycles at 5 C and 10 C, and finally 100 cycles at 1 C. The capacity retention was greatest for the highest regioregularity. The capacity retention for P3HT-b-PEO 86%/8 kg mol-1, 93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol-1 was 78, 87, 87, and 91% after 130 cycles, respectively. In general, the highest regioregular P3HT-b-PEO resulted in highest discharge capacity at all C-rates.

To summarize the findings, the specific capacities at 1C are plotted as functions of P3HT regioregularity and molecular weight (Figure 7d-f). These emphasize that the specific capacity increased as P3HT regioregularity increased (Figure 7d) and that P3HT molecular weight had no discernible trend (Figure 7e). For lower regioregular P3HT, the twisted chain conformation leads to charge localization and charge carrier trapping,64 resulting in poor doping efficiency and low capacity. Conversely, in highly regioregular P3HT, the planar backbone conformation leads to delocalized polarons along and reduces charge carrier trapping,64 resulting in high doping efficiency and high capacity. Notably, the P3HT-b-PEO binder exhibiting the highest capacity (97%/14 kg mol-1) had been successfully used as a binder for highly flexible V2O5 cathodes in an earlier study.36-37


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Finally, the reversible doping level of the P3HT block was calculated as the ratio of the experimental capacity to the theoretical capacity (i.e., 159 mAh/gP3HT), Table 2. The doping levels for P3HT-b-PEO 86%/8 kg mol-1, 93%/12 kg mol-1, 94%/19 kg mol-1, and 97%/14 kg mol1

were 0.06, 0.15, 0.15, and 0.23, respectively. The highest doping level (0.23) achieved in this

study was comparable with doping levels reported elsewhere for poly(thiophene)s and poly(alkylthiophene).65-68 Other conjugated polymers such as poly(aniline) show higher doping levels (0.3~0.5).69 Therefore, increasing the doping level is still challenging and future studies should explore how to increase the reversible poly(thiophene) doping level.

Conclusions We successfully demonstrated trends between P3HT regioregularity and molecular weight with electrochemical properties of P3HT-b-PEO. We compared four different P3HT-b-PEOs of varying P3HT regioregularity (86-97%) and molecular weight (8-19 kg mol-1) while PEO block was kept the same (7 kg mol-1). All P3HT-b-PEOs assembled into micellar aggregate in water. The P3HT domains had low crystallinity (4.3-15.4%) and were largely disordered, confirmed by XRD and DSC. In UV-vis spectroscopy, lower P3HT molecular weight showed more disordered structure. Further, we investigated charge transport and polymer chain conformation using an HJ-aggregate model. The highest regioregularity P3HT-b-PEO (97%/14 kg mol-1) displayed an intrachain transition (J-like aggregate) and more planar backbone, whereas the other P3HT-bPEOs had an interchain transition (H-like aggregate) and less planar backbone. These changes in P3HT’s backbone conformation and chain packing had direct impact in charge storage capacity and redox potential. The capacity of P3HT-b-PEO significantly increased with increasing


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regioregularity. This finding points to the importance of regioregularity and molecular weight in the rational design of conducting polymer binders and redox-active polymer binders in battery systems. It is noted that due to simultaneous changes in both regioregularity and molecular weight it was difficult to quantify each influence on electrochemical performance in block copolymers. Further work is thus required to further explore the regioregularity-molecular weight space for P3HT-b-PEO polymer binder for energy storage. Looking toward improved P3HT-b-PEO electrode binders, one should consider increasing the regioregularity and the molecular weight of the P3HT block, as these will respectively enhance intrachain transport and reduce the redox potential.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. NMR characterization of P3HT-b-PEO block copolymers, AFM data, and additional electrochemical data. (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. Funding Sources


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This material is based upon work supported by the National Science Foundation under Grant No. # 1604666 and # 1604682. ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant No. # 1604666 and # 1604682. H.A. thanks Kwanjeong Educational Foundation. We thank Dr. Mustafa Akbulut for dynamic light scattering access. We also thank the TAMU Materials Characterization Facility. REFERENCES (1) Sirringhaus, H.; Brown, P.; Friend, R.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B.; Spiering, A.; Janssen, R. A.; Meijer, E.; Herwig, P., Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685-688. (2) Aiyar, A. R.; Hong, J.-I.; Reichmanis, E., Regioregularity and Intrachain Ordering: impact on the Nanostructure and Charge Transport in Two-Dimensional Assemblies of Poly(3hexylthiophene). Chem. Mater. 2012, 24, 2845-2853. (3) Poelking, C.; Andrienko, D., Effect of Polymorphism, Regioregularity and Paracrystallinity on Charge Transport in Poly(3-hexylthiophene)[P3HT] Nanofibers. Macromolecules 2013, 46, 8941-8956. (4) Guo, J.; Ohkita, H.; Benten, H.; Ito, S., Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154-6164. (5) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney, M. F.; Fréchet, J. M., The Influence of Poly(3-hexylthiophene) Regioregularity on Fullerene-Composite Solar Cell Performance. J. Am. Chem. Soc. 2008, 130, 16324-16329. (6) Kajiya, D.; Ozawa, S.; Koganezawa, T.; Saitow, K.-i., Enhancement of Out-of-Plane Mobility in P3HT Film by Rubbing: Aggregation and Planarity Enhanced with Low Regioregularity. J. Phys. Chem. C 2015, 119, 7987-7995. (7) Lee, M.; Jeon, H.; Jang, M.; Yang, H., A Physicochemical Approach Toward Extending Conjugation and the Ordering of Solution-Processable Semiconducting Polymers. ACS Appl. Mater. Interfaces 2016, 8, 4819-4827. (8) Mazzio, K. A.; Rice, A. H.; Durban, M. M.; Luscombe, C. K., Effect of Regioregularity on Charge Transport and Structural and Excitonic Coherence in Poly(3-hexylthiophene) Nanowires. J. Phys. Chem. C 2015, 119, 14911-14918. (9) Kim, J.-S.; Kim, J.-H.; Lee, W.; Yu, H.; Kim, H. J.; Song, I.; Shin, M.; Oh, J. H.; Jeong, U.; Kim, T.-S., Tuning Mechanical and Optoelectrical Properties of Poly(3-hexylthiophene) through Systematic Regioregularity Control. Macromolecules 2015, 48, 4339-4346.


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Figure 1 83x65mm (300 x 300 DPI)


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Regioregularity and Molecular Weight Effects in Redox Active Poly(3

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