ORIGINAL PAPER

Preparation and ionic conductivity of composite polymerelectrolytes based on hyperbranched star polymer

Shitong Ren & Tao Zheng & Qian Zhou & Liaoyun Zhang &

Huayi Li

Received: 6 September 2013 /Revised: 3 December 2013 /Accepted: 27 December 2013 /Published online: 27 February 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Hyperbranched star polymer HBPS-(PPEGMA)xwas synthesized by atom transfer radical polymerization(ATRP) using hyperbranched polystyrene (HBPS) asmacroinitiator and poly(ethylene glycol) methyl ether meth-acrylate (PEGMA) as monomer. The structure of the preparedhyperbranched star polymer was characterized by 1H NMR,ATR-FTIR, and GPC. Polymer electrolytes based onHBPS-(PPEGMA)x, lithium salt, and/or nano-TiO2 were pre-pared. The influences of lithium salt concentration and type,nano-TiO2 content, and size on ionic conductivity of theobtained polymer electrolytes were investigated. The resultsshowed that the low crystallinity of the prepared polymerelectrolyte was caused by the interaction between lithium saltand polymer. The addition of TiO2 into HBPS-(PPEGMA)x/LiTFSI improved the ionic conductivity at low temperature.The prepared composite polymer electrolyte showed thehighest ionic conductivity of 9×10−5 S cm−1 at 30 °C whenthe content of TiO2 was 15 wt% and the size of TiO2 was20 nm.

Keywords Hyperbranched star polymer . Polymerelectrolyte . Nano-TiO2

. Ionic conductivity

Introduction

Compared with conventional organic liquid electrolyte, poly-mer electrolyte has many advantages such as easy processing,flexible design, light weight, nontoxicity, and good stability,which shows broad application prospects in fuel cells, dye-sensitized solar battery, lithium secondary battery, and electricvehicles [1–5]. Since the 1970s, polymer electrolytes based onlinear polyoxyethylene (PEO) have been extensively studied.However, room temperature ionic conductivity of PEO-basedpolymer electrolyte is low (10−7 S cm−1) due to the highdegree of crystallization of linear PEO. As a result, manyefforts have been made to suppress crystallization of linearPEO and great progress has been made [6–10].

Hyperbranched polymer and star polymer are consideredthe most suitable matrix materials for polymer electrolytebecause of the existence of many branching points whichhinder the formation of crystals. Many researches showed thatpolymer electrolytes based on hyperbranched polymer or starpolymer had much higher room temperature conductivity thanlinear PEO-based electrolyte, and their conductivity at roomtemperature can reach 10−5 S cm−1 [11–17]. Besides, accord-ing to the literature, the addition of inorganic nanoparticlesinto the linear PEO-based polymer electrolyte not only in-creases the room temperature ionic conductivity of linearPEO-based polymer electrolyte, but also improves its mechan-ical properties, ionic transport number, and interface stability.Various nanoparticles such as SiO2, TiO2, Al2O3, and BaTiO3

have been used to prepare linear PEO-based composite poly-mer electrolytes [18–23]. However, star PEO-based compos-ite polymer electrolyte is rarely reported [24].

In th i s a r t i c l e , hype rb ranched s t a r po lymerHBPS-(PPEGMA)x was firstly synthesized by combinationof self-condensing vinyl polymerization (SCVP) and atomtransfer radical polymerization (ATRP), then composite poly-mer electrolytes were prepared by mixing the prepared

S. Ren : T. Zheng :Q. Zhou : L. Zhang (*)College of Chemistry and Chemical Engineering, University ofChinese Academy of Sciences, Beijing 100049, Chinae-mail: zhangly@ucas.ac.cn

H. Li (*)Beijing National Laboratory for Molecular Science, Key Laboratoryof Engineering Plastics, Institute of Chemistry, Chinese Academy ofSciences, Beijing 100190, Chinae-mail: lihuayi@iccas.ac.cn

Ionics (2014) 20:1225–1234DOI 10.1007/s11581-013-1061-4

hyperbranched star polymers, lithium salts, and/or nano-TiO2.The influences of lithium salt concentration and type, nano-TiO2 content and size on ionic conductivity of compositepolymer electrolyte were studied.

Experimental

Materials

Copper(I) chloride (CuCl, 99 %, Beijing Yili Fine ChemicalsCo., Ltd.) was used after washing with acetic acid, methanol,and ether, respectively; p-chloromethylstyrene (CMS, 90 %,Aldrich) and styrene (St) were distilled from CaH2 underreduced pressure and subsequently passed through a columnof neutral alumina. Lithium bis(trifluoromethanesulfonimide)(LiTFSI, 99 %, Aladdin) was dried under vacuum at 80 °C for24 h before use; chlorobenzene and toluene were dried withCaH2 and distilled under reduced pressure before use. 2,2-Bipyridine (bpy, 99.5 %, Sinopharm Chemical Reagent Co.,Ltd.), poly(ethylene glycol) methyl ether methacrylate(PEGMA, Mn=950 g mol−1, Aldrich), oxalic acid dihydrate(H2C2O42H2O, Beijing ChemicalWorks), boric acid (H3BO3,Sinopharm Chemical Reagent Co., Ltd.), and lithium hydrox-ide monohydrate (LiOHH2O, Sinopharm Chemical ReagentCo., Ltd.) were used without further purification. Nano-TiO2

(10, 20, and 30 nm, 99.9 %, Beijing Boyu Tech NewMaterialTechnology Co., Ltd.) and other reagents were used asreceived.

Synthesis of hyperbranched polystyrene HBPS

Hyperbranched polystyrene (HBPS) was synthesized by usinga similar procedure as reported [25]. The procedure wasbriefly described as follows: bpy 0.659 g (4.2 mmol), CMS3.0 mL (21.1 mmol), styrene 2.4 mL (21.1 mmol), and chlo-robenzene 8.0 mL were added to a dry round-bottomed flaskwith rubber septum and magnetic stir bar. The solution wasdegassed by two freeze-pump-thaw cycles. Afterwards, CuCl0.209 g (2.1 mmol) was added and another freeze-pump-thawcycle was conducted. The flask was then immersed into apreheated oil bath at 120 °C. After 4 h, the reaction was cooledto room temperature, opened to air, and diluted with tetrahy-drofuran (THF). The reaction mixture was passed through aneutral alumina column. Subsequently, the solution was con-centrated through rotary evaporation and precipitated intomethanol. The obtained polymer was dried overnight undervacuum at 60 °C (yield 93%). Elemental analyses: C 79.20%,H 6.63 %, Cl 14.17 %; degree of branching: DB=0.22;molecular weight of HBPS determined by GPC: Mn=2,600 g mol−1, PDI=3.55.

Synthesis of hyperbranched star polymer HBPS-(PPEGMA)x

The prepared HBPS 0.1 g (0.4 mmol –Cl), bpy 0.0625 g(0.4 mmol), PEGMA 5.0 g (5.26 mmol), and 5 mL toluenewere added to a dry round-bottomed flask with rubber septumand magnetic stir bar. The solution was degassed by twofreeze-pump-thawcycles.Afterwards, CuCl 0.02 g (0.2mmol)was added and another freeze-pump-thaw cycle was conduct-ed. The flask was then immersed into a preheated oil bath at110 °C. After 8 h, the reaction was cooled to room tempera-ture, opened to air, and diluted with THF. The reactionmixturewas passed through a neutral alumina column to remove thecopper residues. Subsequently, the solution was concentratedthrough rotary evaporation and precipitated into ether threetimes. The obtained polymer was dried overnight under vac-uum at room temperature (yield 50 %). The molecular weightof the star polymer determined by GPC was Mn=11,200 g mol−1, PDI=1.60.

Synthesis of lithium bis(oxalato) borate LiBOB

LiBOB was synthesized according to literature [26]. Theprocedure was briefly described as follows: Oxalic aciddihydrate, lithium hydroxide monohydrate, and boric acidwere ground thoroughly at a mole ratio of 2:1:1 in a mortarfor 1 h. Then, the obtained mixture was heated at 240 °C for6 h in air. After cooling to room temperature, the obtainedcrude product was recrystallized in the boiled tetrahydrofuran-diethyl ether mixture (in a volume ratio of 1:1). The recrys-tallized salt was dried in a vacuum oven at 50 °C for 24 h. Thesynthetic route of LiBOB and the structure of BOB−1 areshown in Fig. 1.

Preparation of polymer electrolytes

The prepared hyperbranched star polymer, lithium salt, and/ornano-TiO2 were added to THF, stirring with high speed todisperse the nanoparticles. THF was evaporated at room tem-perature. Then, the polymer electrolyte was transferred into avacuum oven for 24 h at 80 °C to remove the residual THF.

Characterizations

Nuclear magnetic resonance (1H NMR) was recorded on aBruker Avance 400 spectrometer at room temperature with

Fig. 1 Synthetic route of LiBOB and the structure of BOB−1

1226 Ionics (2014) 20:1225–1234

tetramethylsilane (TMS) as internal standard anddeuterochloroform (CDCl3) as solvent. Fourier transform in-frared (FTIR) analyses were carried out using the attenuatedtotal reflectance (ATR) method on a Perkin-Elmer system2000 infrared spectrum analyzer. Gel permeation chromatog-raphy (GPC) measurements were performed at room temper-ature using a PL GPC50 instrument equipped with a differen-tial refractometer with THF as eluent at a flow rate of1.0 mL min−1. X-ray diffraction (XRD) measurements werecarried out on a Shimadzu-6000 X-ray diffractometer withCuKα radiation at a scanning rate of 5° min−1 in the 2θ rangeof 5–60°. Elemental analysis was determined using an ele-mental analyzer Carlo Erba 1106. Differential scanning calo-rimetry (DSC) measurements were carried out on a Q2000instrument in the temperature range from −90 to 150 °C at aheating or cooling rate of 10 °C min−1, and the data werecollected on the second heating cycle. Thermogravimetricanalysis (TGA) was performed under nitrogen atmosphereon a Perkin-Elmer TGA 7 series instrument from 50 to650 °C at a heating rate of 20 °C min−1. A polarizing micro-scope of OLYMPUS BX 51 was used to give information onthe crystal structure and the microstructure of the preparedpolymer electrolytes.

The impedance of the polymer electrolytes was measuredby electrochemical impedance spectroscopy using a SolartronSI 1287 electrochemical interface and Solartron 1255B fre-quency response analyzer, with a frequency range of 10 Hz–1,000 kHz, AC amplitude of 10 mV, and a temperature rangeof 20–80 °C. Temperature conductivity plots were obtained byplacing the electrochemical cell in an oven set at measuringtemperature. The ionic conductivity was calculated from thefollowing equation:

σ ¼ l ARb

Here, σ is the ionic conductivity, l is the distance betweenthe two stainless electrodes, A is the area of the polymerelectrolyte contacted with the stainless electrodes, and Rb isthe bulk resistance, respectively.

Results and discussion

Synthesis of hyperbranched star polymer

The synthesis route of hyperbranched star polymerHBPS-(PPEGMA)x is shown in Fig. 2. Hyperbranched poly-styrene HBPS was synthesized by atom transfer radical self-condensing vinyl polymerization (ATR-SCVP) of styrene (St)and p-chloromethylstyrene (CMS) with CuCl as catalyst and2,2-bipyridine (bpy) as ligand. Themole ratio of St/CMS/CuCl/bpy in feed was 10:10:1:2. The degree of branching of HBPS

Fig. 2 Synthetic route of HBPS-(PPEGMA)x

Table 1 Polymer electrolytes based on hyperbranched star polymer

Samples Salt content (EO/Li) TiO2 content (wt%)

1 10 0

2 20 0

3 30 0

4 40 0

5 50 0

6 20 5

7 20 10

8 20 15

9 20 20

10 20 25

11 20 15a

12 20 15b

a The size of this TiO2 was 10 nmb The size of this TiO2 was 30 nm; the size of TiO2 in the other sampleswas all 20 nm

Fig. 3 1H NMR spectra of HBPS (a) and HBPS-(PPEGMA)x (b)

Ionics (2014) 20:1225–1234 1227

was 0.22, which was calculated from 1H NMR as reported byliterature [25]. The prepared HBPS had many active benzylchlorides which could initiate the ATRP. Hyperbranched starpolymer HBPS-(PPEGMA)x was prepared using HBPS asmacroinitiator and PEGMA as monomer. CuCl and bpy werealso used as catalyst and ligand, respectively. The mole ratio of

PEGMA/–Cl in HBPS/CuCl/bpy in feed was 26:2:1:2. Themo lecu l a r we igh t o f HBPS- (PPEGMA) x (Mn=11,200 g mol−1) was much higher than that of HBPS (Mn=2,600 g mol−1), indicating that HBPS containing chlorineatoms successfully initiated the ATRP of PEGMA. There were11 PEGMA units in each arm on average according to 1HNMR. Interestingly, the PDI of HBPS-(PPEGMA)x waslowered so strongly from 3.55 of HBPS to 1.60. There werethree possible reasons to explain this change. Firstly, the HBPShad high PDI, meaning the existence of HBPS molecules withquite different molecular weights. When these HBPS mole-cules initiate the polymerization of PEGMA, there may be adifference between HBPS with different molecular weights. Atthe beginning of the reaction, HBPS with high molecularweight has more Cl atoms, some of which cannot initiate thepolymerization of PEGMA because of steric hindrance. On thecontrary, the Cl atoms in HBPS with low molecular weight

Fig. 4 ATR-FTIR spectra of (a) HBPS and (b) HBPS-(PPEGMA)x

Fig. 5 ATR-FTIR spectrum (a) and XRD pattern (b) of the preparedLiBOB

Fig. 6 Plot of conductivity and EO/Li at 30 °C for HBPS-(PPEGMA)x/LiTFSI

Fig. 7 DSC curves of HBPS-(PPEGMA)x/LiTFSI with different EO/Li

1228 Ionics (2014) 20:1225–1234

suffer small steric hindrance, most of which can initiate thepolymerization. Secondly, with the growth of the arms, intra-molecular coupling which terminates the growth of the arms iseasier to happen for star polymers with more arms than thosewith fewer arms. Thirdly, intermolecular coupling also happenswith the arms growing longer. All these factors contribute tolowering the PDI of the resulting star polymer.

In this paper, we chose PEGMA of 950 g mol−1 as mono-mer because our previous study showed that polymer electro-lyte prepared from PEGMA950 had higher ionic conductivity[27]. Polymer electrolytes prepared by complexingHBPS-(PPEGMA)x, lithium salt, and/or nanoparticles arelisted in Table 1.

Structural characterization of hyperbranched star polymer

Figure 3 shows the 1H NMR spectra of HBPS andHBPS-(PPEGMA)x. The peaks in HBPS spectrum (Fig. 3a)

were attributed as follows: 7.4–6.4 (–C6H5,–C6H4–), 5.67,5.17 (CH2=CH–), 4.80 (–CHCl–), and 4.55 (–CH2Cl). Thepeaks at 4.80 and 4.55 proved the hyperbranched structure ofthe prepared HBPS. Degree of branching was calculated fromthe areas of the two peaks as reported in the literature [25].F i g u r e 3 b s h ow s t h e 1H NMR s p e c t r um o fHBPS-(PPEGMA)x. Both of the peaks corresponding to theHBPS core (δ=7.4–6.6) and PPEGMA arms (δ=4.2–3.1, 1.1–0.7) appeared. In addition, the signals of –CHCl– and –CH2Clin HBPS disappeared, indicating that all the –Cl in HBPSinitiated the ATRP of PEGMA.

Figure 4 is the ATR-FTIR spectra of HBPS andHBPS-(PPEGMA)x. As can be seen from Fig. 4(b), peaks at1,730 and 1,100 cm−1 were attributed to the –C=O stretchingvibration absorption and –C–O–C– stretching vibration ab-sorption of PEGMA units, respectively. The appearance ofpeaks at 698 and 749 cm−1 indicated the existence of theHBPS core. The above results of 1H NMR and ATR-FTIRspectra proved that HBPS-(PPEGMA)x was successfullysynthesized.

Structural characterization of LiBOB

Figure 5 shows the ATR-FTIR and XRD spectra of the pre-pared LiBOB. The results of ATR-FTIR and XRD were ingood agreement with literature data [26, 28], and no otherpeaks were detected, indicating that LiBOB with high puritywas successfully synthesized.

Ionic conductivity

Influence of lithium salt content

In order to optimize ionic conductivity, we prepared polymerelectrolytes HBPS-(PPEGMA)x/LiTFSI with different EO/Li(mole ratio of EO units and lithium salt). Figure 6 shows theionic conductivity of HBPS-(PPEGMA)x/LiTFSI with

Fig. 8 ATR-FTIR spectra of HBPS-(PPEGMA)x/LiTFSI with differentEO/Li

Fig. 9 Polarizing microscopephotos of HBPS-(PPEGMA)x/LiTFSI with different EO/Li at20 °C (a∞; b50; c40; d30; e20; f10)

Ionics (2014) 20:1225–1234 1229

different EO/Li at 30 °C. With the increase of lithium content,the ionic conductivity of the polymer electrolytes increasedfirst and then deceased. A maximum value of 7×10−5 S cm−1

appeared when EO/Li was 20. This was because lithium salt inthe polymer matrix was easy to dissociate when the content oflithium salt was low. With the increase of the lithium saltcontent, the number of free ions increased and ionic conduc-tivity improved accordingly. Ionic conductivity reached amaximum value when the lithium salt content was increasedto a certain extent. Continuing to increase the lithium saltcontent, free ions aggregated together, and the number ofcarriers decreased.

Figure 7 shows the DSC curves of HBPS-(PPEGMA)x/LiTFSI with different lithium salt contents. With increasinglithium salt content, melting temperature (Tm) and crystalmelting enthalpy (ΔHm) of the prepared polymer electrolytesdecreased because of disturbance of the regular structure ofthe polymer HBPS-(PPEGMA)xdue to the presence of lithiumsalt, while glass transition temperature (Tg) of the polymerelectrolytes increased gradually owing to the interaction of thelithium salt and PEO chains. The melting peak disappearedcompletely at EO/Li=20. These results could explain thechange of ionic conductivity of the polymer electrolyte withlithium salt concentration. When lithium salt content was low,the polymer electrolyte had a high degree of crystallinity andfew carriers. Because the existence of a large amount ofcrystals hindered the transport of ions, ionic conductivity ofthe polymer electrolyte was low. When lithium salt contentwas high, ion aggregations and physical crosslink made Tgincreased significantly, which limited the motion of PEOchains, and the ionic conductivity decreased accordingly.Consequently, ionic conductivity of the polymer electrolytereached a maximum at proper EO/Li (EO/Li=20).

F i g u r e 8 g i v e s t h e ATR - FT IR s p e c t r a o fHBPS-(PPEGMA)x/LiTFSI with different EO/Li. After dop-ing of lithium salt, –C–O–C– absorption peak of

HBPS-(PPEGMA)x moved from 1,100 cm−1 towards lowerwavenumbers, which became more obvious with the increaseof lithium salt content. The absorption peaks at 1,241 and1,280 cm−1 gradually decreased with increasing lithium saltcontent, indicating that the crystallinity of the polymer elec-trolytes deceased gradually. When EO/Li was 20, these twopeaks almost disappeared completely. According to literaturereports [29, 30], the peak at 1,351 cm−1 represented theexistence of free ion TFSI−1, while its shoulder peak at lowerwavenumbers corresponded to ion aggregates. When EO/Li≥40, the relative intensity of TFSI−1 peak increased with thedecrease of EO/Li. Further increasing lithium content, therelative intensity of TFSI−1 peak decreased gradually, indicat-ing that ion aggregates gradually increased with the increaseof lithium salt content. As a result, a maximum ionic conduc-tivity of the polymer electrolytes was observed with changingof the content of lithium salt.

Figure 9 shows the polarizing microscope photos ofHBPS-(PPEGMA)x/LiTFSI with different EO/Li. It was ob-vious that the prepared hyperbranched star polymer exhibitedtypical spherulites. After doping LiTFSI, the structure ofspherulites was damaged, and the crystal area significantlydecreased with the increase of LiTFSI content. When EO/Li≤

Fig. 10 Temperature dependence of conductivity for polymer electro-lytes with different lithium salts

Fig. 11 Plots of conductivity and the content of TiO2 at differenttemperatures

Table 2 DSC result of HBPS-(PPEGMA)x/LiTFSI/TiO2 with differentTiO2 contents

TiO2 content(wt%)

Salt content(EO/Li)

Tg (°C) Tm (°C) ΔHm

(J g−1)

0 20 −52.0 – 0

5 20 −52.3 – 0

10 20 −52.1 23.7 0.5

15 20 −52.2 25.8 2.4

20 20 −52.1 24.0 0.1

25 20 −52.5 25.5 9.6

1230 Ionics (2014) 20:1225–1234

20, the prepared polymer electrolytes existed mainly in amor-phous phase. Besides, when EO/Li was 40~10,continuousamorphous phase was formed, and relatively higher ionicconductivity was achieved (Fig. 6).

Influence of lithium salt type

Lithium salt is a necessary component in polymer electrolyte.Different lithium salts have an important effect on ionic con-ductivity of polymer electrolyte. Our previous research provedthat polymer electrolyte using LiTFSI as salt had much higherionic conductivity than that using LiClO4 as lithium, becauseLiTFSI with larger anion can dissociate easily [27]. Here, wecompared the ionic conductivity of polymer electrolytes

prepared from LiTFSI and LiBOB. Both of them have rela-tively large anion and dissociate easily. Figure 10 shows thetemperature dependence of conductivity for polymer electro-lytes using LiTFSI and LiBOB as lithium salts, respectively. Itwas obvious that ionic conductivity of the polymer electro-lytes increased significantly as the temperature increased.Both of the two polymer electrolytes had high room temper-ature ionic conductivity (~7×10−5 S cm−1 at 30 °C) which wasalso attributed to the larger anion and easy dissociation oflithium salts. Besides, the polymer electrolyte using LiBOB assalt exhibited higher ionic conductivity at high temperature(~9×10−4 S cm−1 at 80 °C).

Influence of nano-TiO2 content

According to the above results that HBPS-(PPEGMA)x/LiTFSI had the highest room temperature ionic conductivitywhen EO/Li was 20, we prepared composite polymer electro-lytes HBPS-(PPEGMA)x/LiTFSI/TiO2 with different TiO2

contents and the same EO/Li of 20. Figure 11 shows the plotsof conductivity and the content of TiO2 at different tempera-tures. Ionic conductivity of polymer electrolytes at low tem-perature (≤40 °C) increased to some extent after adding nano-particles into HBPS-(PPEGMA)x/LiTFSI. Besides, ionic con-ductivity of polymer electrolytes increased first and then de-creased with the increase of TiO2 content. This change ofconductivity with the content of nanoparticles in polymerelectrolyte was in accord with literature reports [31–33].Two factors were responsible for this change. On the onehand, nanoparticles dispersed easily when the content ofTiO2 was low. The interface layers of high ionic conductivityformed between the polymer and TiO2 increased with theincrease of TiO2 content, and ionic conductivity of polymer

Fig. 12 DSC curves of HBPS-(PPEGMA)x and HBPS-(PPEGMA)x/TiO2

Fig. 13 Temperature dependenceof ionic conductivity ofHBPS-(PPEGMA)x/LiTFSI/TiO2

with different TiO2 sizes

Ionics (2014) 20:1225–1234 1231

electrolytes increased accordingly. On the other hand, nano-particles aggregated together when the TiO2 content was high,which led to the decrease of interface layers and low ionicconductivity. As a result, the highest ionic conductivity ap-peared at appropriate TiO2 content (15 wt%).

However, the increase of ionic conductivity of the preparedcomposite polymer electrolyte was not as obvious as the resultreported in literatures for composite polymer electrolyte basedon linear PEO [31–33]. This was attributed to thehyperbranched star structure of HBPS-(PPEGMA)x whichinhibited the crystallization of polymer. According to litera-ture reports [34, 35], nanoparticles added into the linear PEO-based polymer electrolyte prohibited the formation of crystals,which facilitated the increase of ionic conductivity. In thisarticle, the results of DSC and polarizing microscope(Figs. 7 and 9) showed that polymer electrolyteHBPS-(PPEGMA)x/LiTFSI was amorphous when EO/Liwas 20. In other words, the addition of nanoparticles couldnot decrease the degree of crystallization anymore. As a result,the increase of ionic conductivity for the composite polymerelectrolyte based on hyperbranched star polymer was not asobvious as that of the polymer electrolyte based on linearPEO.

Interestingly, the ionic conductivity at high temperature(>40 °C) decreased after the addition of nano-TiO2 intoHBPS-(PPEGMA)x/LiTFSI. This may be because polymerchains moved more vigorously. The increase of ionic conduc-tivity caused by the interaction of polymer and nanoparticleswas relatively weakened. Meanwhile, the nanoparticle itselfprevents the transport of ions. Therefore, compared withHBPS-(PPEGMA)x/LiTFSI, the ionic conductivity of thepolymer electrolyte decreased at high temperature whennano-TiO2 was added.

In order to study the effect of nano-TiO2 on polymerelectrolyte, we compared the DSC results of composite poly-mer electrolytes with different TiO2 contents (Table 2). Tg ofthese polymer electrolytes changed little after the addition of

nano-TiO2 into HBPS-(PPEGMA)x/LiTFSI, and all of themhad low Tg around −52 °C. In addition, small crystal meltingpeaks appeared for composite polymer electrolytes, indicatingthat nanoparticles played a role of a nucleating agent to someextent [36].

In addit ion, we compared the DSC results ofHBPS-(PPEGMA)x and HBPS-(PPEGMA)x/TiO2 to furtherinvestigate the effect of nano-TiO2 on HBPS-(PPEGMA)x(Fig. 12). It was obvious that the degree of crystallization ofHBPS-(PPEGMA)x decreased slightly after adding nano-TiO2 . ΔHm ju s t ch anged f r om 86 . 7 J g − 1 o fHBPS-(PPEGMA)x to 84.8 J g−1 of HBPS-(PPEGMA)x/TiO2. Combining this with the results of Figs. 7 and 9, itwas concluded that the decrease of the degree of crystalliza-tion was caused by the interaction of polymer and lithium saltinstead of polymer and nanoparticles.

Influence of nano-TiO2 size

To investigate the effect of nano-TiO2 size on ionic conduc-tivity of the polymer electrolyte, we prepared composite poly-mer electrolyte with TiO2 of three different sizes (EO/Li=20,TiO2 content=15 wt%). Figure 13 shows the temperaturedependence of ionic conductivity of HBPS-(PPEGMA)x/LiTFSI/TiO2 with TiO2 of different sizes. The result showedthat composite polymer electrolyte with TiO2 of 20 nm had thehighest ionic conductivity (9×10−5 S cm−1 at 30 °C). Thisphenomenon did not agree with the results of most literaturereports [37, 38].

In order to explain the above phenomenon, DSC andpolarizing microscope measurements were conducted to fur-ther explore the effect of nanoparticles on composite polymerelectrolyte. The DSC results of composite polymer electrolytewith different TiO2 sizes are listed in Table 3. Small crystalmelting peak appeared after the addition of TiO2. In thepolarizing microscope photos of these composite polymerelectrolytes (Fig. 14), crystals were observed clearly, and more

Table 3 DSC result ofHBPS-(PPEGMA)x/LiTFSI/TiO2

with different TiO2 sizes

Sizes of TiO2 (nm) Salt content (EO/Li) Tg (°C) Tm (°C) ΔHm (J g−1)

10 20 −51.7 25.1 0.8

20 20 −52.2 25.8 2.4

30 20 −47.1 25.1 1.8

Fig. 14 Polarizing microscopephotos of HBPS-(PPEGMA)x/LiTFSI/TiO2 with different TiO2

sizes (at 20 °C): a10 nm, b20 nm,and c 30 nm

1232 Ionics (2014) 20:1225–1234

spherulites were found when the size of nano-TiO2 was10 nm. According to the above results, two reasons wereprobably responsible for the change of ionic conductivity ofthe composite polymer electrolyte with TiO2 size: (a) nano-particles with small size can formmore interface layers of highconductivity because of high specific surface area; and (b)nanoparticles with small size can play a better role of anucleating agent. The two factors made the prepared compos-ite polymer electrolyte with nano-TiO2 of 20 nm to show thehighest ionic conductivity.

Conclusion

Novel composite polymer electrolytes composed of star poly-mer HBPS-(PPEGMA)x, lithium salt, and/or nano-TiO2 wereprepared. The influences of lithium salt concentration, nano-TiO2 content, and size on ionic conductivity of the obtainedpolymer electrolytes were investigated. The results showedthat ionic conductivity of the prepared star polymer electro-lytes was significantly higher than that of linear PEO-basedpolymer electrolyte because of low crystallinity. It was inter-esting to find that crystals increased when nano-TiO2 wasadded into the polymer electrolytes. Low crystallinity of thepolymer electrolytes was mainly caused by the interactionbetween the polymer matrix and lithium salt, while nano-TiO2 played a role of a nucleating agent to some extent.HBPS-(PPEGMA)x/LiTFSI showed the highest ionic conduc-tivity when EO/Li was 20. When TiO2 was added intoHBPS-(PPEGMA)x/LiTFSI, the ionic conductivity of thecomposite polymer electrolyte was improved at low tempera-ture. When TiO2 content was 15 wt% and the size of TiO2 was20 nm, the prepared polymer electrolyte showed the highestionic conductivity of 9×10−5 S cm−1 at 30 °C.

Acknowledgments The authors gratefully acknowledge the financialsupport provided by the National Natural Science Foundation of China(nos. 51073170 and 50703044).

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