Complicated phase behavior and ionic conductivities
of PVP-co-PMMA-based polymer electrolytes
Chun-Yi Chiu, Ying-Jie Yen, Shiao-Wei Kuo, Hsien-Wei Chen, Feng-Chih Chang
*
Institute of Applied Chemistry, National Chiao-Tung University, Hsin-Chu 30043, Taiwan Received 15 June 2006; received in revised form 13 December 2006; accepted 28 December 2006
Available online 12 January 2007
Abstract
We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate the interactions that occur within complexes of poly-(vinylpyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) and lithium perchlorate (LiClO4) as well as these systems’ phase behavior and ionic
conductivities. The presence of MMA moieties in the PVP-co-PMMA random copolymer has an inert diluent effect that reduces the degree of self-association of the PVP molecules and causes a negative deviation in the glass transition temperature (Tg). In the binary LiClO4/PVP blends,
the presence of a small amount of LiClO4reduces the strong dipoleedipole interactions within PVP and leads to a lowerTg. Further addition of
LiClO4increasesTgas a result of ionedipole interactions between LiClO4and PVP. In LiClO4/PVP-co-PMMA blend systems, for which the
three individual systemsdthe PVP-co-PMMA copolymer and the LiClO4/PVP and LiClO4/PMMA blendsdare miscible at all compositional
ratios, a phase-separated loop exists at certain compositions due to a complicated series of interactions among the LiClO4, PVP and PMMA
units. The PMMA-rich component in the PVP-co-PMMA copolymer tends to be excluded, and this phenomenon results in phase separation. At a LiClO4 content of 20 wt% salt, the maximum ionic conductivity occurred for a LiClO4/VP57 blend (i.e., 57 mol% VP units in the
PVP-co-PMMA copolymer).
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Polymer electrolyte; Ionic conductivity; Phase behavior
1. Introduction
Polymer electrolytes are compounds formed through the dissolution of salts into polar and high-molecular weight macromolecules that can interact strongly with cations. The proposed use of solvent-free polymer electrolytes for high-energy density batteries and other solid-state electrochemical devices [1e5] has spurred considerable interest in the ion-transport properties of these materials. For most potential applications, it is desirable that the solid polymer electrolytes display a reasonable conductivity (ca. 104S cm1), dimen-sional stability, processability, and flexibility under ambient conditions. Great progress has been made over the last 20 years
in increasing the level of ionic conductivity that polymer elec-trolytes exhibit [6]. In recent years, however, the levels of ionic conductivity have been limited to a ceiling of ca. 104S cm1 at room temperature, despite innovations in the design of flexible polymers and the use of salts containing asymmetric anions capable of suppressing crystallinity. Unfor-tunately, such levels of ionic conductivity are insufficient for many lithium battery applications[7,8]. It seems necessary for us to change our way of thinking concerning how to opti-mize and further increase the ionic conductivity. We must determine the fundamental interactions within polymer elec-trolytes if we are to better understand the ion-transport mech-anism and, hence, improve their performance.
Poly(vinylpyrrolidone) (PVP) is amorphous and exhibits a high value ofTgbecause of the presence of its rigid
pyrroli-done groups. PVP’s tertiary amide carbonyl groups possess marked Lewis base character such that they form a variety * Corresponding author. Tel./fax:þ886 3 5131512.
E-mail address:[email protected](F.-C. Chang).
0032-3861/$ - see front matterÓ 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.12.059
of complexes with a range of inorganic salts [9]. Unfortu-nately, the high Tg feature of LiePVP polymer electrolytes
tends to limit the mobility of the ions and results in poor ionic conductivity.
On the other hand, gel-type polymer electrolytes based on poly(methyl methacrylate) (PMMA) [10,11] have been pro-posed for use in lithium batteries mainly because of their ben-eficial effects on the stabilization of the lithiumeelectrode interface[12]. The reasonable conductivity achieved by such plasticizer films is, however, offset by PMMA’s relatively poor mechanical properties at a high concentration of the plas-ticizer. Furthermore, the interactions between Liþcations and PMMA are significantly weaker than those of other polymer matrixes, such as poly(ethylene oxide) and PVP; therefore, PMMA is less able to dissociate the lithium salt, which further weakens its cation transporting function. The LiePMMA polymer electrolyte in its all-solid state also exhibits a low ionic conductivity.
Because PVP and PMMA possess complementary advan-tages for their use as polymer electrolytes, we became interested in studying polymer electrolytes formed by incorpo-rating lithium perchlorate into mixtures of PVP and PMMA. Here, we chose to use a PVP-co-PMMA random copolymer, synthesized through free radical polymerization, as a replace-ment since the random copolymer has higher miscibility behavior than that of blend system. It seemed reasonable to expect that a gel-type polymer electrolyte based on PVP-co-PMMA may not only sustain the mechanical properties of a PMMA-based gel polymer electrolyte but also increase the dissolution of the lithium salt because of the strong polarity of the PVP units[9]. To the best of our knowledge, no previous studies have been reported describing the influences that the miscibility behavior and the interaction mechanism have on the ionic conductivity in polymer electrolytes comprising LiClO4
and random copolymers. In this study, we employed differen-tial scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, and alternating current (ac) impedance techniques to investigate the interactions within and related conductivities of all solid-state polymer electrolytes formed from LiClO4/PVP-co-PMMA blend systems.
2. Experimental 2.1. Materials
Benzene, pyridine, azobisisobutyronitrile (AIBN), N-vinyl-2-pyrrolidone (VP), methyl methacrylate (MMA), and lithium perchlorate (LiClO4) were purchased from Aldrich Chemical
Co. AIBN was purified through recrystallization from ethanol. Benzene and pyridine were fractionally distilled over CaH2.
The monomers MMA and VP were purified through vacuum distillation over CaH2, the fractions were collected at room
temperature and 50C, respectively. LiClO4 was dried in a
vacuum oven at 80C for 24 h and stored in a desiccator prior to use.
2.2. Synthesis of poly(N-vinyl-2-pyrrolidone-co-methyl methacrylate) (PVP-co-PMMA) copolymers
Solution copolymerization of N-vinyl-2-pyrrolidone with methyl methacrylate was performed in benzene at 80C under a nitrogen atmosphere in a glass reaction flask equipped with a condenser (Scheme 1). AIBN (3 wt% with respect to mono-mers) was employed as an initiator for free radical polymeri-zation. To determine the reactivity ratio, the sample of the copolymer was taken from the reaction flask during the early stages of copolymerization when the degree of conversion was low (between 4 and 9%)[13]. The mixture was stirred for ca. 24 h and then poured into excess ethyl ether with vigorous agitation to precipitate and purify the product. The filtered polymer product was dried until it reached a constant weight.
2.3. Characterization
The molecular weights and polydispersities of the synthe-sized copolymers were determined through gel permeation chromatography (GPC) at 50C using N,N-dimethylforma-mide (DMF) as the eluent and polystyrene standards for calibration of the molecular weight. The composition of the copolymer was further ascertained by means of1H NMR spectroscopy and elementary analysis (EA).
The 1H NMR spectrum of the copolymer was recorded in deuterated chloroform (CDCl3) solution at 25C using a
Varian UNITY INOVA-400 NMR spectrometer. EA was per-formed in an oxidative atmosphere at 1021C using a Heraeus CHN-O Rapid Elementary Analyzer. The copolymer composi-tions of VP and MMA correspond to repeating units of C6H9NO and C5H8O2, respectively. The VP content (mol%)
was determined using Eq. (1); it is based on the content of C and N atoms[14]. VPðmol%Þ ¼ 30N 7C 6N 100 ð1Þ N O H2C H2C O O CH3 CH3 CH3 CH3 * * N x O O O y + benzene, 80~90 °C
methyl methacrylate (MMA)
Poly(vinyl pyrrolidone-co-methyl methacrylate) AIBN
N-vinyl-2-pyrrolidone (VP)
2.4. Sample preparation
LiClO4/PVP-co-PMMA polymer electrolytes in various
blend compositions were prepared through solution casting. The desired amounts of PVP-co-PMMA and LiClO4were
dis-solved in pyridine and stirred continuously for 24 h at 60C. The solution was cast onto a Teflon dish and maintained at 50C for an additional 24 h to remove the solvent; the sample was then dried under vacuum at 80C for two days. To pre-vent its contact with air or moisture, the polymer electrolyte film was transferred into a glove box under a nitrogen atmosphere.
2.5. Differential scanning calorimetry (DSC)
Thermal analyses were performed using a DSC instrument (DuPont TA 2010). The instrument was calibrated with indium standards and the analyses were conducted under a nitrogen flow rate of ca. 40 mL/min. The sample was heated sequen-tially from 30 to 200C for the first scan, maintained at 200C for 10 min, cooled rapidly to 0C, and then reheated to 300C. The glass transition temperature (Tg) was obtained
as the inflection point of the heat capacity jump recorded at a scan rate of 20C/min.
2.6. Fourier transform infrared (FTIR) spectroscopy
Infrared spectra of the polymer films were recorded at 120C using a conventional KBr disk method. All polymer films were prepared under a N2 atmosphere. The pyridine
solution was cast onto a KBr disk, from which the solvent was evaporated under vacuum at 80C for 48 h. All films used in this study were sufficiently thin to obey the BeereLambert law. FTIR spectra were recorded over the range 4000e 400 cm1using a Nicolet AVATAR 320 FTIR spectrophotom-eter (Nicolet Instruments, Madison, WI); 32 scans were collected at a spectral resolution of 1 cm1.
2.7. Conductivity measurements
The frequency-dependent impedance properties (from 10 to 10 Hz) of the polymer complexes were measured using an Autolab designed by Eco Chemie. For conductivity measure-ments, the sample was pressed into disks having thicknesses varying from 0.50 to 0.15 mm. The disks were loaded into a sealed conductivity cell between stainless-steel blocking electrodes and the impedance responses were measured at temperatures from 30 to 100C.
3. Results and discussion
3.1. PVP-co-PMMA copolymer characterization
Copolymerization of N-vinyl-2-pyrrolidone with methyl methacrylate was performed at 80C using AIBN as the initi-ator (Scheme 1). A series of copolymers were prepared at var-ious VP and MMA monomer concentrations. The VP content (mol%) in the copolymer was determined through 1H NMR spectroscopy and EA;Table 1summarizes the results. Because traces of water present in PVP-co-PMMA copolymers may have led to overestimation of the integration of the PVP sig-nals in the 1H NMR spectra, which would result in (false) slightly higher PVP readings in the products than in the feeds, for greater accuracy it was necessary to perform EA. The abundances of N and C atoms determined from EA were ap-plied to calculate the VP content using Eq. (1), i.e., because of the presence of moisture, the H atom content of the poly-mers was not taken into account in calculating the VP content. From a comparison of the results obtained from the EA and1H NMR spectroscopic data, the EA results were quite reproduc-ible, regardless of the presence of moisture. Indeed, the VP contents determined from the 1H NMR spectra were usually 5e10% greater than those determined through EA. Thus, EA was a more suitable technique for quantification of the VP content in the PVP-co-PMMA copolymers and, therefore, in Table 1
PVP-co-PMMA copolymer compositional and molecular-weight dataa
Polymerb Monomer feed (mol%) Polymer composition (mol%) Mne(g/mol) Mw/Mne Tgf(C)
EAc NMRd
VP MMA VP MMA VP MMA
PVP 100 0 100 0 100 0 18 200 2.33 181 VP79 78.3 21.7 78.8 21.2 80.8 19.2 18 700 2.40 161 VP57 57.5 42.5 57.0 43.0 62.6 37.4 23 000 2.23 144 VP47 47.4 52.6 46.8 53.2 52.9 47.1 17 000 2.29 139 VP39 37.5 62.5 38.5 61.5 40.7 59.3 20 300 2.52 134 VP19 18.4 81.6 19.4 80.6 18.7 81.3 22 100 2.00 123 PMMA 0 100 0 100 0 100 25 700 1.76 121 a
Polymerization conditions: initiator¼ AIBN; solvent ¼ benzene; temperature ¼ 80C.
b
Labeling based on VP content in the PVP-co-PMMA copolymers obtained from EA.
c
Calculated from the elementary analyzer using Eq.(1).
d
Obtained form the1H NMR spectra.
e
Mn: number-average molecular weight,Mw: weight-average molecular weight, andMw/Mnwere all determined by GPC using polystyrene standards and DMF
as the eluent.
f
the following discussion the sample codes for these copoly-mers are based on the VP contents obtained through EA.
Table 1lists the monomer feed ratios and resultant
copoly-mer compositions from which we calculated the reactivity ratios (r1andr2) using the methodology of Kelen and Tu¨do¨s
[15e17]. All polymerizations were performed in benzene,
un-der the conditions described inSection 2, and terminated at monomer conversions below 10% to minimize any errors due to changes in the feed ratios. The values ofr1andr2represent
the ratios of the homo- and cross-propagation rate constants for each monomer (i.e.,r1¼ k11/k12andr2¼ k22/k21, where k
is the rate constant). The KeleneTu¨do¨s equation is given by Eq.(2): h¼r1þ r2 a xr2 a ð2Þ where h¼ G aþ F and x¼ F aþ F
The values ofF and G can be obtained from the quantities x andy, where x is the ratio of molar concentration of monomers 1 and 2 (M1/M2) andy is the mole ratio of these monomers in
the copolymer (dM1/dM2); namely,F¼ x2/y and G¼ x( y 1)/
y. The value of a is defined by the expression a¼ (FmFM)1/2,
whereFmandFMare the lowest and highest values ofF
ob-tained from the experimental data. By plotting the values of h versus x, we obtain the values of r2andr1from the intercept
and slope.Fig. 1displays the results from which we calculated the values ofrPVPandrPMMAas 0.97 and 0.94, respectively. In
a previous paper[18], it was proposed that copolymerization behavior be termed ‘‘ideal’’ when the product of the two reac-tivity ratios is unity (i.e., r1r2¼ 1). Moreover, when r1¼
r2¼ 1, the two monomers display equal reactivities toward
both propagating species. In this case, the copolymer compo-sition is the same as the comonomer feed with a random distribution of these two monomers along the copolymer chain; such behavior is referred to as random or Bernoullian growth.
In this study, our PVP-co-PMMA copolymers were synthe-sized through free radical polymerization in essentially a ran-dom manner, with a slight tendency toward the formation of an ideal copolymer (rPVP rPMMA¼ 0.91).
Table 1lists the values ofTgof our samples. Several
empir-ical and semi-empirempir-ical equations have been suggested for pre-dicting the dependence ofTgon the copolymer composition.
In this study, we believed that the most suitable equation for a weakly interacting system would be the GordoneTaylor equation[19,20]:
Tg¼
w1Tg1þ kw2Tg2
w1þ kw2
ð3Þ wherew1andw2are the weight fractions of the components,
Tg1andTg2represent the corresponding glass transition
tem-peratures, andk is the fitting constant. Fig. 2 displays plots ofTgversus the PVP weight fractions of the PVP-co-PMMA
copolymers. We obtained a value ofk of 0.45 through a nonlin-ear least-squares ‘‘best fit’’ analysis. This low value ofk im-plies that weak interactions exist between the VP and MMA units in these copolymers[21]. The even distribution of MMA moieties means that these units play the role of an inert diluent to hinder the self-association of the PVP moieties. Moreover, at a low PVP content (20 mol%), the diluent plays the domi-nant role and results in a relatively lower value ofTg. In
con-trast, dipoleedipole interactions between the PVP units are dominant at higher PVP content; they result in higher values of Tg. The Fox and GordoneTaylor equations are generally
recognized to hold true for miscible blends or copolymers in which only weak intermolecular interactions exist. We found it very useful to employ these equations to analyze the compo-sitional behavior of our PVP-co-PMMA copolymers. The neg-ative deviation from the Fox equation that we observed in this study suggests that one of the components in this PVP-co-PMMA random copolymer system possessed a stronger self-association (e.g., to form dimers or multimers) than did the other [22,23]. As a result, the values of Tg for the
PVP-co-PMMA samples having higher PVP contents displayed
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 y = -1.09 + 2.06 x R2 = 0.994 rPVP = 0.97 rPMMA = 0.94
Fig. 1. KeleneTu¨do¨s plot for PVP-co-PMMA copolymers.
0.0 0.2 0.4 0.6 0.8 1.0 390 400 410 420 430 440 450 460 Tg (K) PVP Weight Fraction Gordon-Taylor prediction, k = 0.45 Fox rule prediction
Experimental data PVP-co-PMMA copolymer
relatively smaller deviations from either of the equations than did those having lower PVP contents[21,24,25].
Fig. 3displays scale-expanded infrared spectra, recorded at
120C and plotted in the range 1800e1630 cm1, for neat PVP, neat PMMA, and a series of PVP-co-PMMA copoly-mers. The absorption peaks at ca. 1680 and 1730 cm1 are assigned to the carbonyl vibration bands of PVP and PMMA, respectively. Interestingly, we found that the maximum of the carbonyl vibration band of PVP shifted slightly toward higher frequency when the MMA moiety was incorporated into the PVP. This finding implies that the strength of the dipolee dipole interactions of the PVP units was reduced, or that they were eliminated completely, as a result of the MMA moieties acting as inert diluents[26]. Therefore, the behavior observed in the IR spectra is consistent with the DSC results (Fig. 2), i.e., the diluent effect is responsible for the observed decrease in the values ofTgof the PVP-co-PMMA copolymers.
3.2. LiClO4/PVP and LiClO4/PMMA binary blends
It has been reported that the properties of polymer/salt mix-tures can change dramatically as a result of ionic aggregation
[27e29]. We performed thermal analyses first to determine
whether the properties of LiClO4/PVP and LiClO4/PMMA
blends were affected by the addition of lithium perchlorate.
Fig. 4 displays DSC thermographs of LiClO4/PMMA and
LiClO4/PVP blends containing various LiClO4 salt contents.
Fig. 4a indicates that the maximum Tg increment was ca.
145C for PMMA containing 20 wt% LiClO4. Ionic
interac-tions or ionic cluster formation in the amorphous region of the ionomer usually resembles physical cross-linking. The mo-bility of a polymer chain is restrained through such physical cross-linking and, thus, it leads to higher glass transition tem-peratures relative to those of the mother polymer. Generally, the value of Tg increases gradually upon the addition of the
salt because of the increased number of ionepolymer and ioneion interactions; a maximum glass transition temperature is usually achieved at a certain content of LiClO4. Excess
LiClO4tends to self-aggregate, which causes the value ofTg
of LiClO4/PMMA blends to decrease through a dilution effect
[30,31]. It has been demonstrated quite clearly that most
poly-mer/salt blendsdsuch as those involving poly(ethylene oxide) (PEO)/lithium [32], poly(2-ethyl-2-oxazoline) (PEtOx)/silver [33], and poly(4-vinylpyridine) (P4VP)/zinc [28]dexhibit similar trends as those DSC phenomena described above.
The LiClO4/PVP blend, however, presents (Fig. 4b) a novel
behavior for its variation in glass transition temperature upon increasing the LiClO4salt. The addition of a low amount of
LiClO4(5 wt%) actually causes a decrease in the value ofTg
of PVP. This phenomenon is unusual when compared with the results obtained previously for other polymer/salt blends. Relative to PMMA, PVP is a more water-soluble polymer because the tertiary amide carbonyl groups exhibit stronger Lewis basic character (dipole moment: ca. 4 D) [34,35]. Therefore, it seems reasonable to expect that PVP possesses a higher value of Tg (181C) primarily because of its strong
intermolecular dipoleedipole interactions, even though other factors may also contribute. The presence of LiClO4at a
rela-tive low concentration reduces the degree of self-association of the PVP units and results in lower values ofTg[25]because
the electron donor groups of PVP interact, through ionedipole interaction, with the electron acceptor species (i.e., the Liþ cations). As a result, two competitive interactions exist: self-association of PVP through dipoleedipole interactions and ionedipole interactions between Liþcations and the carbonyl groups of PVP. Although the mobility of a polymer chain tends to be restrained upon the addition of LiClO4, which
re-sults in higher values ofTg, for PVP this effect is offset by the
reduction in the strength of the dipoleedipole interaction units, which results in the lower values of Tgat low LiClO4
concentrations. As indicated inFig. 4b, when the LiClO4
con-tent increased from 0 to 30 wt%, the value of Tg increased
from 181 to 191C. This temperature increment (DTg¼
10C) is less than those that we observed previously for LiClO4/PEO (DTg¼ ca. 35C) and ZnClO4/P4VP (DTg¼ ca.
140C) blend systems[28,32]. Further addition of LiClO4to
the LiClO4/PVP blend system (from 30 to 40 wt%) led to the
value of Tg increasing dramatically up to its maximum at
233C. When we increased the LiClO4 content to 50 wt%,
however, the value of Tg decreased. According to these
1800 1750 1700 1650 VP 79 VP 57 VP 47 VP 39 Absorbance (a.u.) Wavenumber (cm-1) pure PMMA VP 19 pure PVP PVP-co-PMMA
Fig. 3. IR spectra (1800e1630 cm1), recorded at 120C, of neat PVP, neat PMMA, and a series of PVP-co-PMMA copolymers containing various PVP contents.
observations, the optimal ionedipole interaction (strongest en-hancement ofTg) occurred at a LiClO4-to-PVP ratio of 40/60.
An excess LiClO4content (>40 wt%) resulted in salt
aggrega-tion and, thus, a lower value ofTg, because of an increasing
interchain distance at concentrations above the optimized lithium salt content[33].
We performed FTIR spectroscopic characterizations to fur-ther understand the mechanism through which the variations in Tg occur for the LiClO4/PMMA and LiClO4/PVP blend
sys-tems. Monitoring of the carbonyl stretching bands as a function of the blend composition is the method employed most fre-quently for quantifying the relative fractions of free and bonded carbonyl sites within PMMA or PVP polymer chains.
Fig. 5 presents the carbonyl stretching absorptions, ranging
from 1800 to 1550 cm1, in the IR spectra recorded at 120C of LiClO4/PVP blends containing various LiClO4
contents. The stretching band of the ‘‘free’’ carbonyl groups of uncomplexed PVP appears at 1680 cm1; this band is asym-metric and significantly broader than those of other noncom-plexed carbonyl units [35e37]. For example, Painter and co-workers[38,39]demonstrated that the real ‘‘free’’ or ‘‘un-perturbed’’ carbonyl stretching band of the model compound ethyl pyrrolidone (EPr) occurs at ca. 1708 cm1. The carbonyl stretching band of PVP obviously broadens and shifts to a lower frequency (ca. 1680 cm1) because of the strong self-association of PVP molecules (i.e., through dipoleedipole inter-actions between its pyrrolidone groups). Thus, there are very few truly ‘‘free’’ carbonyl units in the PVP; the carbonyl band observed in the FTIR spectrum of PVP actually consists of bands from a large number of associated species (multimers).
As the LiClO4 content increased, the carbonyl bands in
Fig. 5 broaden gradually, and a new band appears at ca.
1654 cm1as a result of coordination between the Liþcation and the oxygen atom on the carbonyl group of PVP. A distinct third band appears at an even lower frequency (ca. 1630 cm1) when the LiClO4 content is above 20 wt%. The lower
fre-quency of this band suggests that an even stronger interaction occurs involving the carbonyl groups of PVP. In addition, its relative intensity increases upon increasing the LiClO4
con-tent. Thus, we believe that the carbonyl absorption band at ca. 1630 cm1 corresponds to the carbonyl groups of PPV that interact simultaneously with several Liþ cations. To further elucidate the effect that the LiClO4 content has on
the charge environment surrounding the carbonyl groups of PVP, in Fig. 6 we display the relative fractions of ‘‘uncom-plexed’’ and ‘‘com‘‘uncom-plexed’’ C]O sites obtained through de-composing C]O stretching band into two or three Gaussian peaks [38,40]; Table 2 summarizes the results. We assign
1800 1750 1700 1650 1600 1550 Absorbance (a.u.) Wavenumber (cm-1) pure PMMA 5 wt% 10 wt% 20 wt% 30 wt% 40 wt% 50 wt% LiClO4 content from 5 to 50 wt% LiClO4/PVP blends
Fig. 5. IR spectra (C]O stretching region), recorded at 120C, of LiClO4/
PVP blends containing varying LiClO4contents.
30 60 90 120 150 180 210 60 90 120 150 180 210 240 270 neat PMMA LiClO4 content LiClO4 content neat PVP Temperature (°C)
Heat Flow (endo, )
LiClO4/PMMA LiClO4/PVP
5 wt% 10 wt% 15 wt% 20 wt% 30 wt% 40 wt% 10 wt% 20 wt% 30 wt% 40 wt% 50 wt% (a) (b)
the carbonyl stretching bands located at 1680, 1654, and 1630 cm1 to the ‘‘uncomplexed,’’ ‘‘complexed I,’’ and ‘‘complexed II’’ C]O units, respectively. The relative inten-sity of the ‘‘uncomplexed’’ C]O band decreased upon in-creasing the LiClO4 content. Moreover, the initial addition
of 5 wt% LiClO4 caused the peak position of the
‘‘uncom-plexed’’ C]O band to shift slightly to higher frequency (from 1680 to 1682 cm1), presumably because of a reduction in the number of dipoleedipole interactions between the PVP
units.Fig. 7provides a summary of the fractional area versus the LiClO4 content for the ‘‘uncomplexed’’ and two
plexed’’ C]O bands. The relative fractions of both the ‘‘com-plexed I’’ and the ‘‘com‘‘com-plexed II’’ C]O bands increased upon increasing the LiClO4content up to 40 wt%, but after that the
relative fraction of the ‘‘complexed I’’ C]O band began to decrease while that of the ‘‘complexed II’’ C]O band contin-ued to increase. That is to say, complex I transformed gradually into complex II when the concentration of LiClO4
increased. This finding provides evidence that the formation of complex II is more favorable at higher concentrations of LiClO4. Furthermore, it is evident that not all of the added
Liþcations associate with the carbonyl groups to form poly-meresalt complexes: some ‘‘uncomplexed’’ C]O groups remain even at excessively high LiClO4concentrations. This
result indicates that the Liþ cations in the blend are in the equilibrium state which is between binding to their ClO4
counterion and to the C]O groups. Accordingly, these results are consistent with the phenomenon we observed from the DSC analyses. 1800 1750 1700 1650 1600 1550 Pure PVP 5 wt% 10 wt% 20 wt% 30 wt% 40 wt% 50 wt% LiClO4 content LiClO4/PVP Wavenumber (cm-1) Absorbance (a.u.)
Fig. 6. Deconvolution of IR spectra (carbonyl stretching region: 1800e 1550 cm1), recorded at 120C, of LiClO
4/PVP blends containing various
LiClO4contents.
Table 2
Curve-fitting results of infrared spectra of C]O group stretching region recorded at 120C for the LiClO4/PVP and LiClO4/PMMA blends with various LiClO4
salt contents
Polymer LiClO4, wt% Free C]O Primary complexed C]O Secondary complexed C]O
n, cm1 w1/2, cm1 Af, % n, cm1 w1/2, cm1 Af, % n, cm1 w1/2, cm1 Af, % PVP 0 1680 33 100 5 1682 34 72 1655 33 28 10 1681 32 71 1655 31 29 20 1681 34 58 1654 31 35 1630 39 7 30 1680 32 46 1654 31 43 1631 40 11 40 1681 32 37 1654 32 43 1632 39 20 50 1678 32 30 1654 31 38 1625 40 32 PMMA 0 1731 22 100 10 1730 22 81 1706 31 19 20 1730 21 68 1705 32 32 30 1730 21 63 1705 32 37 40 1729 22 60 1704 32 40 0 10 20 30 40 50 60 0 20 40 60 80 100
Area Fraction of C=O (%)
LiClO4 Content (wt%)
LiClO4/PVP Blend
"un-complexed" C=O "complexed I" C=O "complexed II" C=O
Fig. 7. Effect that the LiClO4concentration has on the signals of the ‘‘free’’
To further clarify the complicated series of interactions that occur within these blended LiClO4/PVP systems, inFig. 8we
propose three modes of association. Type A describes the di-poleedipole interaction between the carbonyl groups of neat PVP. These interactions are disturbed or diminished when the ionedipole interactions (type B) occur between the Liþions and the carbonyl groups of PVP at a low LiClO4 content.
Type C indicates the situation that occurs when an excess of LiClO4causes the salt to aggregate.
Fig. 9 displays expanded FTIR spectra (from 1800 to
1650 cm1) recorded at 120C from a series of LiClO4/
C C C C N N N N C C C C O C O O O C C C C C C CC N N N N C C C C C O O O O C C C C C C C N N N N O O O O C C C C C N N N N O O O O C C C C N N O O Li+ Li+ Li+ : dipole-dipole interaction : ion-dipole interaction Type A Type B Type C C C C N N N N O O O O C C C C C N N N N N O O O O O C C C C C N N N N N O O O O O Li+ LiClO4 aggregation LiClO4 aggregation Li+ Li+ ClO4 -ClO4 -ClO4 -ClO4 -ClO4 -Li+ Li + ClO4 -+ LiClO4 + LiClO4
Fig. 8. Proposed association schemes for polymer electrolytes based on LiClO4and PVP.
1800 1750 1700 1650 Absorbance (a.u.) Wavenumber (cm-1) LiClO4/PMMA LiClO4 content from 0 to 40 wt%
Fig. 9. IR spectra (C]O stretching region), recorded at 120C, of LiClO4/
PMMA blends containing varying LiClO4contents.
1800 1750 1700 1650 1600 1550 pure VP79 10 wt% 30 wt% 40 wt% LiClO4 content Wavenumber (cm-1) Absorbance (a.u.)
Fig. 10. Deconvolution of IR spectra (carbonyl stretching region: 1800e 1525 cm1), recorded at 120C, of LiClO4/VP79 blends containing various
PMMA blends. The stretching band of the free carbonyl group of the uncomplexed PMMA appears at 1730 cm1. When the LiClO4 salt is added, a shoulder appears at ca. 1700 cm1,
corresponding to C]O groups involved in ionedipole interac-tions with Liþions. It is clear inFig. 9that the relative inten-sity of this shoulder peak increases upon increasing the LiClO4
concentration.Table 2 lists the results of curve fitting of the carbonyl group stretching bands of LiClO4/PVP and LiClO4/
PMMA. The association between LiClO4 and PVP is more
preferable than that between LiClO4 and PMMA.
Further-more, a band for a type-II complex, such as that which ap-peared in the spectra of the LiClO4/PVP blend system at
high LiClO4concentrations, was absent in the spectra of the
LiClO4/PMMA blends. We interpret these differences to the
stronger dipole moments of the functional groups of PVP rel-ative to those of PMMA. From a comparison with the DSC analyses, it appears that LiClO4tended to aggregate in the
Li-ClO4/PMMA blends at a lower salt concentration (ca. 30 wt%)
than that found in the LiClO4/PVP blends (ca. 50 wt%).
3.3. Blends of LiClO4and PVP-co-PMMA copolymers
Fig. 10displays the results of deconvolution of the carbonyl
stretching band region (1800e1525 cm1) of the IR spectra, recorded at 120C, of the LiClO4/VP79 (i.e., PVP-co-PMMA
containing 79 mol% of VP units) blend. We concentrated our
attention on the unperturbed bands at 1680 and 1730 cm1 for PVP and PMMA, respectively. As we mentioned above, the presence of a small amount of LiClO4 (10 wt%) in
PVP results in a new band at ca. 1654 cm1, which we assign to the signal of the ‘‘complexed I’’ C]O groups of PVP. When the LiClO4 content is increased to 30 wt%, a band
for the ‘‘complexed II’’ C]O groups of PVP appears at ca. 1630 cm1. Furthermore, a band appears for the ‘‘complexed’’ C]O groups of PMMA when the LiClO4 content is further
increased to 40 wt% (Fig. 9).Fig. 10indicates that competi-tion exists between the carbonyl groups of PVP and PMMA for their coordination to the Liþ cations. At a low LiClO4
content, Liþ ions interact selectively with the PVP units only. When the LiClO4 concentration is increased, the Liþ
ions begin to interact with both the PVP and PMMA units. Thus, it is clear that the Liþ ions interact preferably with the PVP units over the PMMA ones. All of the carbonyl stretching signals of the LiClO4/PVP-co-PMMA blends are
clearly split into several bands that can be fitted well to Gaussian functions. For brevity, the results of the subsequent curve fitting are summarized in Table 3, which indicates that the relative intensity of the ‘‘uncomplexed’’ C]O groups of both the VP and MMA units decreases upon increasing the LiClO4 content; a similar trend occurred for the individual
LiClO4/PVP and LiClO4/PMMA binary blends. From a
com-parison between LiClO4/PVP (30/70) and LiClO4/VP57 (20/80)
Table 3
Curve-fitting results of IR spectra of C]O group stretching region recorded at 120C for the LiClO4/PVP-co-PMMA blends with various LiClO4contents
Copolymers LiClO4
content, wt%
Carbonyl group in VP unit Carbonyl group in MMA unit
‘‘Uncomplexed’’ C]O ‘‘Complexed I’’ C]O ‘‘Complexed II’’ C]O ‘‘Uncomplexed’’ C]O ‘‘Complexed’’ C]O n, cm1 w1/2, cm1 Au, % n, cm1 w1/2, cm1 AcI, % n, cm1 w1/2, cm1 AcII, % n, cm1 w1/2, cm1 Au, % n, cm1 w1/2, cm1 Ac, % VP79 0 1682 35 100 1729 28 100 10 1684 34 60 1654 34 40 1728 27 100 20 1681 33 59 1654 33 41 1727 28 100 30 1681 32 43 1654 30 42 1630 39 12 1726 28 100 40 1681 31 34 1655 31 43 1628 39 23 1726 27 62 1704 31 38 VP57 0 1680 34 100 1729 28 100 10 1680 34 59 1655 32 41 1728 27 100 20 1680 33 46 1655 31 45 1629 38 9 1728 27 100 30 1680 32 35 1655 32 52 1629 40 13 1728 27 100 40 1681 32 29 1654 30 53 1627 40 18 1729 27 75 1705 30 25 VP47 0 1680 33 100 1729 28 100 10 1680 33 53 1655 32 42 1628 40 5 1728 27 100 20 1681 33 45 1654 32 45 1628 39 10 1729 27 81 1705 30 19 30 1680 32 42 1654 31 45 1629 40 13 1730 28 75 1705 30 25 40 1680 32 21 1655 30 61 1626 41 18 1728 27 58 1705 31 42 VP39 0 1680 33 100 1729 27 100 10 1680 32 59 1655 31 41 1729 27 100 20 1680 33 46 1655 31 47 1626 39 7 1730 28 75 1705 30 25 30 1681 32 42 1654 30 46 1626 39 12 1730 27 72 1705 30 28 40 1681 32 34 1654 30 44 1626 40 22 1730 27 61 1705 31 39 VP19 0 1680 33 100 1730 27 100 10 1680 32 41 1655 32 46 1628 40 13 1730 28 95 1705 30 5 20 1680 32 32 1655 32 51 1628 40 17 1731 28 72 1705 30 28 30 1681 32 9 1655 31 68 1627 39 23 1731 28 68 1706 31 32 40 1655 30 74 1626 39 26 1729 27 66 1704 31 34
systems, both of which possess the same molar ratio of Liþ ions to VP units, the area fraction of the ‘‘uncomplexed’’ C]O groups of the VP units in the latter blend system is lower than that of the former blend system. Because the MMA units in the PVP-co-PMMA copolymer play an inert diluent role to decrease the degree of self-association of the VP units, a lower LiClO4 content is necessary for the
LiClO4/PVP-co-PMMA blend to attain the same degree of
coordination between the Liþions and the VP units.
DSC analysis is one of the most convenient methods for determining miscibility in polymer blends. In this study we measured values of Tg of our blends to identify their phase
behavior. Fig. 11 displays the conventional second-run DSC thermograms that we obtained for various LiClO4
/PVP-co-PMMA blends; we identified either one or two glass transi-tions in each blend. The existence of a single glass transition strongly suggests that a blend is fully miscible and has a homogenous phase. In contrast, a blend containing two glass transitions is considered to be immiscible or phase separated.
Fig. 12 presents the resulting phase diagram of the LiClO4/
PVP-co-PMMA blend at room temperature. Interestingly, even though each individual systemdthe PVP-co-PMMA copolymer and the LiClO4/PVP and LiClO4/PMMA blendsd
is miscible at all compositions, phase-separated loop exists
30 60 90 120 150 180 210 240 270 Temperature (°C) 30 60 90 120 150 180 210 240 270 Temperature (°C) 30 60 90 120 150 180 210 240 270 Temperature (°C) 30 60 90 120 150 180 210 240 270 Temperature (°C) 30 60 90 120 150 180 210 240 270 Temperature (°C) pure VP79 5 wt% 10 wt% 15 wt% pure VP57 5 wt% 10 wt% 15 wt% 20 wt% 20 wt% 30 wt% 40 wt% 30 wt% 40 wt% 50 wt% Heat F low (E ndo, ) Heat F low (E ndo, ) Heat F low (E ndo, ) Heat F low (E ndo, ) Heat F low (E ndo, )
LiClO4/VP47 LiClO4/VP39 LiClO4/VP19
LiClO4/VP57 LiClO4/VP79 LiClO4 content pure VP47 5 wt% 10 wt% 15 wt% 20 wt% 30 wt% 40 wt% LiClO4 content pure VP39 5 wt% 10 wt% 15 wt% 20 wt% 30 wt% 40 wt% LiClO4 content pure VP19 5 wt% 10 wt% 15 wt% 20 wt% 30 wt% 40 wt% LiClO4 content (a) (b) (c) (d) (c)
for certain compositions within the LiClO4/PVP-co-PMMA
blend system. The Dc effect, i.e., the discrepancy between the interaction parameters c of the third component with re-spect to polymers 1 and 2, plays an important role in dictating the phase behavior of these blends. The Dc effect results from nonequivalent strengths of interaction between the different component pairs[41e43]. Zeman and Patterson[44] demon-strated that the Dc effect strongly promoted the phase separa-tion in ternary systems. Phase-separated loop occurs in systems that feature specific interactions when there is an ‘‘at-traction’’ between the different covalently bonded monomers of the copolymers. From our FTIR spectroscopic analyses
(Table 3), we found that LiClO4has a much greater preference
for coordinating with PVP than with PMMA. Therefore, the addition of the third component, LiClO4, tends to exclude
the PMMA component in the LiClO4/PVP-co-PMMA mixture
such that it becomes immiscible. Moreover, the regions of phase-separated loop appear when the LiClO4content is low
and/or the PVP component in the PVP-co-PMMA copolymer is relatively high. Nevertheless, when the LiClO4
concentra-tion is 30 wt%, the blends exhibit full miscibility regardless of the composition of the PVP-co-PMMA copolymer; enough LiClO4is available to interact with both the PVP and PMMA
units simultaneously and, thus, the LiClO4/PVP-co-PMMA
blends become miscible. Fig. 12 indicates that a closed phase-separated loop exists in the phase diagram of the Li-ClO4/PVP-co-PMMA blends as a result of the complicated
set of interactions between the LiClO4, PVP, and PMMA
units.
Table 4 lists the values of Tg of LiClO4/PVP-co-PMMA
blends containing various LiClO4 contents. The lower value
ofTg, observed at ca. 123C, is close to that of neat PMMA
and, therefore, it can be assigned to the glass transition of PMMA-rich domains. Likewise, we attribute the higher value of Tg (>180C) to the glass transition of the PVP-rich
do-mains. Its presence implies that the PMMA phase is excluded from the mixture upon adding LiClO4. In addition, the value of
Tg of the PVP-rich domain in the LiClO4/PVP-co-PMMA
blend is higher than that observed for the binary blend of LiClO4and PVP containing the same LiClO4concentration.
Because the presence of MMA moieties tends to reduce the degree of self-association of the PVP components, the Liþ cations are better able to coordinate with the PVP units; this phenomenon results in the higher value of Tg. These results
are consistent with those from our FTIR spectroscopic analy-ses. It is interesting that a random copolymer, PVP-co-PMMA, possessing covalently bonded monomers, undergoes phase separation upon doping the LiClO4salt at certain
concentra-tions. Fig. 13 provides a schematic illustration of how we believe the phase separation process occurs in the LiClO4/
PVP-co-PMMA blends. 3.4. Ionic conductivity
Fig. 14displays a plot of the room-temperature
conductiv-ity versus the VP content in the copolymer for LiClO4
/PVP-co-PMMA blends containing a fixed LiClO4content (20 wt%).
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 miscible phase-separated LiClO4 PMMA PVP
Fig. 12. Ternary phase diagram for the LiClO4/PVP-co-PMMA system. Open
circles represent miscible blends and full circles represent immiscible blends.
Table 4
Tgs of LiClO4/PVP-co-PMMA blends containing various LiClO4contents
PVP-co-PMMA copolymers LiClO4content, wt% Tg,C
VP79 0 160 5 126, 162 10 126, 190 15 133, 197 20 154, 205 30 189 40 181 50 167 VP57 0 144 5 120, 182 10 126, 202 15 129, 208 20 151, 198 30 161 40 166 VP47 0 139 5 118, 194 10 121, 204 15 133, 209 20 150 30 179 40 156 VP39 0 134 5 119, 182 10 123, 205 15 137 20 152 30 169 40 159 VP19 0 123 5 125 10 129 15 139 20 145 30 159 40 123
This plot indicates that the polymer electrolyte comprising LiClO4 and VP39 exhibits the maximum ionic conductivity
at room temperature. The conductivity (s) behavior can be interpreted using the following equation[7,40,45]:
s¼X
i
nizimi ð4Þ
whereni,zi, and mirefer to the concentration of the charge
car-rier, the ionic charge on the charge carcar-rier, and the mobility of the charge carrier, respectively. In this study, the ionic charge (zi) is the same for all blend systems; thus, the resulting ionic
conductivity depends only onniand mi.
We expected that the concentration of charge carriers (ni)
would be related to the fraction of ‘‘free’’ ClO4anions that
dissociated from the salt.Fig. 15displays IR spectra, recorded at 120C, highlighting the region of the n(ClO4) internal
vibration modes (660e600 cm1) for LiClO4/PVP-co-PMMA
blends containing a constant LiClO4content (20 wt%). Within
this region, the absorptions at ca. 624 and 636 cm1 corre-spond to the signals of the free anions and the contact ion pairs, respectively[46,47]. To clarify the charge environments of the ClO4anions, we quantified the relative fractions of the
free anion by decomposing the n(ClO4) internal vibration
mode into two Gaussian peaks.Table 5lists the relative frac-tional areas and the locations of the related adsorption bands. The relative fraction of the ‘‘free’’ ClO4anions increased
ini-tially upon increasing the VP molar ratio of the PVP-co-PMMA copolymer, but it decreased when the VP molar ratio was over 60% (VP57). It is understandable that the incorpora-tion of PVP moieties into PMMA tends to increase the relative fraction of the ‘‘free’’ ClO4ions because the dipole moments
of the pyrrolidone groups within the PVP molecules are strong enough to dissociate the LiClO4 salt into its Liþand ClO4
ions. Shriver and Spindler [9] demonstrated, however, that the addition of Liþ, to form LiePVP complexes, induces the N atoms within PVP to become quasi-cationic through elec-tron resonance. These quasi-cationic N atoms of PVP attract
Li+ Li + Li+ Li+ Li+ Li+ VP unit MMA unit
Fig. 13. Proposed schematic illustration of the phase separation occurring in LiClO4/PVP-co-PMMA blends.
0 20 40 60 80 100 -9.5 -9.0 -8.5 -8.0 -7.5 -7.0 -6.5 Log σ (S cm -1)
VP content in copolymer (mol%)
LiClO4/PVP-co-PMMA (20/80)
Fig. 14. Plots of the ionic conductivity at 30C of LiClO4/PVP-co-PMMA
blends versus the VP content of the PVP-co-PMMA copolymer.
660 650 640 630 620 610 600 (d) VP57 (c) VP39 (b) VP19 Absorbance (a.u.) Wavenumber (cm-1) LiClO4/PVP-co-PMMA
(a) pure PMMA
(f) pure PVP (e) VP79
Fig. 15. IR spectra [n(ClO4) internal vibration modes] of LiClO4
the anions through coulombic interactions. Therefore, because the ClO4 anions can interact with both the Liþ cations and
the quasi-cationic N atoms of PVP, there is a decrease in the relative fraction of ‘‘free’’ anions.
On the other hand, it is reasonable to assume for solid-state electrolyte systems that the mobility of the charge carrier (mi)
is related to the mobility of the polymer matrix (i.e., it depends on the value ofTg). For our system, however, the value of mi
may be neglected because the values of Tg of the
PVP-co-PMMA copolymers are all above room temperature. When considering the combination of effects that the values of ni
and mi have on the ionic conductivity, it is reasonable that
the maximum ionic conductivity occurred for the LiClO4/
VP57 (20/80) system.
4. Conclusions
We have used DSC, FTIR spectroscopy, and ac impedance techniques to investigate in detail the miscibility behavior, interaction mechanisms, and ionic conductivities of polymer electrolytes comprising LiClO4and PVP-co-PMMA random
copolymers. Although PVP/PMMA blends are immiscible [48], a single glass transition occurs for their corresponding copolymers, i.e., these copolymers are miscible. Furthermore, the MMA units in PVP play the role of an inert diluent that reduces the degree of self-association of the PVP units and, thus, causes negative deviation in the values ofTg. For the
Li-ClO4/PVP binary blend, we observed an unusual phenomenon:
the addition of a small amount of LiClO4actually reduced the
strength of the dipoleedipole interactions between PVP units and resulted in a decrease in the value ofTg. A subsequent
in-crease in the LiClO4content led to an increase in the value of
Tgof the PVP. It is interesting that the LiClO4/PVP-co-PMMA
polymer electrolyte exhibits an immiscibility loop in the phase diagram, even though each of its three individual systemsdthe PVP-co-PMMA copolymer and the LiClO4/PVP and LiClO4/
PMMA blendsdis miscible at every composition. Phase sep-aration appears to occur through PMMA-rich domains being excluded from LiClO4/PVP-co-PMMA blends. From a
combi-nation of the effects that the values ofniand mi have on the
ionic conductivity, the maximum conductivity occurred for the blend having the composition LiClO4/VP57 (20/80).
Acknowledgments
This research was supported financially by the National Science Council, Taiwan, under Contract No. NSC-95-2216-E-009-001 and Ministry of Education ‘‘Aim for the Top University’’ program (MOEATU program).
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![Fig. 15. IR spectra [n(ClO 4 ) internal vibration modes] of LiClO 4 /PVP-co-](https://thumb-ap.123doks.com/thumbv2/9libinfo/7527793.119508/12.892.60.423.807.1077/fig-spectra-clo-internal-vibration-modes-liclo-pvp.webp)
