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Ind. Eng. Chem. Res. 2000, 39, 72-78
Composite Electrolytes Comprising Polytetramethylene/ Polypropylene Glycol-Based Waterborne Polyurethanes and Polyethylene Oxide via a Mixture Design Approach Ten-Chin Wen,* Hwang-Shin Tseng, and Tsung-Tien Cheng Department of Chemical Engineering, National Cheng Kung University, Tainan, 70101 Taiwan
The composite electrolytes (CEs) were prepared by impregnating the ternary composites comprising polypropylene glycol-based waterborne polyurethane (denoted as WPU(PPG)), polytetramethylene glycol-based waterborne polyurethane (denote as WPU(PTMG), and polyethylene oxide (PEO) with LiClO4/PC. The data of the swollen weight (Sw) and the roomtemperature conductivity (σ25) for CEs were fitted as empirical regression equations by using mixture design. These empirical equations were used to construct contour plots, facilitating comparisons of synergistic/antagonistic effects among the mixed polymers. The contour plots show that the maximum Sw (64.3%) appears at point X3 (PEO 95% and WPU(PPG) 5%), while the maximum σ25 (≈10-3 S/cm) appears in a wide region (WPU(PPG) ) 5-43%, WPU(PTMG) < 37%, and PEO > 27%). Differential scanning calorimetry (DSC) results showed immiscibility between WPU(PTMG) and PEO and partial immiscibility between WPU(PPG) and WPU(PTMG)/ PEO. A polarized micrograph (PM) of the composites was also examined for PEO spherulites. The contour plot results of Sw and σ25 can be explained by the interactions among polymers on the basis of their molecular structures, also evidenced by both DSC and PM results. Introduction The polymer electrolyte has attracted a lot of attention for the opportunity to produce a safe, flexible, and thin plastic battery. Polyethylene oxide (PEO) was reported as the host matrix with a variety of dissolved inorganic salts,1-9 suffered from the disadvantage of their low-room-temperature conductivity due to the formation of a partially crystalline polymer-salt complex. To increase the room-temperature conductivity, several copolymers consisting of -(-C-C-O-)n- segments, such as poly(methoxyethoxyethoxyphosphazene) (MEEP),10-13 were synthesized. Consequently, the roomtemperature conductivity of the corresponding polymer electrolyte was improved to ≈10-5 S cm-1. An alternative approach is to prepare a polymer electrolyte by blending PEO with other polymers and salts,14-15 also known as composite electrolytes (CEs). Although blends of polymers and lithium salts are easily prepared, conductivities do not exceed ≈10-5 S cm-1 because of the low ion mobility, resulting from the low degree of freedom of the polymer chains. Polar solvents, such as propylene carbonate (PC) and ethylene carbonate (EC), were added to the polymer matrix to form gel electrolytes16-19 for raising the roomtemperature conductivity to ≈10-3 S cm-1. In that case, Sw always serves as a matrix while the polar solvent plays a role in dissolving salt and swelling a polymer. Accordingly, PVDF20,21 and PAN-based22-24 gel electrolytes were extensively investigated and reported to possess both good conductivity and strength. In our laboratory, waterborne polyurethane (denoted as WPU) had been synthesized25-27 and used as a polymer matrix of gel electrolytes.28-33 In these elec* To whom correspondence should be addressed. Tel.: 8866-2385487. Fax: 886-6-2344496. E-mail: tcwen@mail. ncku.edu.tw.
trolytes, polyethylene glycol-based WPU (denoted as WPU(PEG)) exhibited good conductivity but poor dimensional stability at high temperature and high swollen weight. Polytetramethylene glycol-based WPU (denoted as WPU(PTMG)) displayed good mechanical strength but poor conductivity due to its low swollen capability. On the other hand, PEO was used as an absorbent to increase the swollen capability. Accordingly, WPU(PTMG)-PEO34 electrolytes were reported to possess a good room-temperature conductivity. In light of the copolymer of PVDF and HFP,35 polypropylene glycol-based WPU (denoted as WPU(PPG)) was investigated36 because the side chain -(-C(C)-C-O)n- was thought to contribute to the swollen capability and conductivity. Therefore, WPU(PPG) was used to be incorporated in WPU(PTMG)-PEO binaries by employing a mixture design approach. This systematical study would provide statistical regression equations uand contour plots for understanding the role in ternary polymers for CE. The mixture design method assumes that the properties of the composite electrolyte are a function of its component (x1, x2, x3), compositions. This relationship can be expressed as η ) f (x1, x2, x3), where the variables x1, x2, and x3 represent the weight proportions of WPU(PPG), WPU(PTMG), and PEO, respectively, in the composite film. In this study, a design matrix with 16 experiments and a forward stepwise regression procedure were employed to achieve a statistically significant regression equation. The regression models were then plotted as the contour diagrams of room-temperature conductivity (σ25) and swollen weight (Sw) versus composition, which facilitated straightforward interpretations of the conductivities of binary and ternary systems. Because WPU(PTMG) and PEO are not suitable being used as the matrix of a polymer electrolyte alone, 5% WPU(PEG) was set as the lower limit of the composition for all composite films. Thus, the design matrix of the
10.1021/ie990373c CCC: $19.00 © 2000 American Chemical Society Published on Web 12/10/1999
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 73 Table 1. Design Matrix and Experiment Results of WPU(PPG)-WPU(PTMG)-PEO Composite Electrolytes pseudocomponenta real compositiona runs sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
A B C D E F G H I J K L M N O P
x1
x2
x3
y1
y2
y3
1 0 0 1/ 3 1/ 2 0 1/ 2 2/ 3 1/ 6 1/ 6 3/ 4 1/ 4 3/ 4 1/ 4 0 0
0 1 0 1/ 3 1/ 2 1/ 2 0 1/ 6 2/ 3 1/ 6 1/ 4 3/ 4 0 0 3/ 4 1/ 4
0 0 1 1/ 3 0 1/ 2 1/ 2 1/ 6 1/ 6 2/ 3 0 0 1/ 4 3/ 4 1/ 4 3/ 4
1.00 0.05 0.05 0.37 0.53 0.05 0.53 0.68 0.21 0.21 0.76 0.29 0.76 0.29 0.05 0.05
0.00 0.95 0.00 0.32 0.48 0.48 0.00 0.16 0.63 0.16 0.24 0.71 0.00 0.00 0.71 0.24
0.00 0.00 0.95 0.32 0.00 0.48 0.48 0.16 0.16 0.63 0.00 0.00 0.24 0.71 0.24 0.71
Sw σ25 (wt %) (mS cm-1) 50 18.18 64.29 48.15 41.18 54.17 57.69 56 51.72 55.56 47.83 21.43 55.17 56.25 36.84 51.85
0.447 0.00007 3.538 0.792 0.022 0.443 0.752 1.126 0.543 1.228 0.326 0.0035 0.648 1.124 0.342 1.375
a The subscripts 1, 2, and 3 represent WPU(PPG), WPU(PTMG), and PEO, respectively.
pseudo-components (x1, x2, x3) as well as the real compositions (y1, y2, y3) is listed in Table 1, in which the real compositions are calculated from the pseudocomponents (x1, x2, x3) with the equations shown as follows:
y1 ) 0.95x1 + 0.05; y2 ) 0.95x2; y3 ) 0.95x3 The details can be referred to in our previous paper.37 Experimental Section Materials. PPG (Mw ) 1000, Showa Chemical Co.) or PTMG (Mw ) 1000, Aldrich Co.) used as a soft segment was dried at 75 °C in a vacuum oven for 72 h; isophorone diisocyanate (IPDI) (Lancaster Chemical Co.) and dimethylol propionic acid (DMPA) (Aldrich Co.) used as hard segments were vacuum-dried at 80 °C for 24 h. PC (Merck Co.) was distilled at low pressure and stored over 3-Å molecular sieves before use. Acetone (Tedia Co.) and N-methyl-2-pyrrolidinone (NMP) (Aldrich Co.) were immersed in 3-Å molecular sieves for more than a week before use. Di-n-butyltin(IV) dilaurate (DBTDL) (Wako Co.), ethylenediamine (EDA) (Merck Co.), butane sultone (BS) (Aldrich Co.), lithium hydroxide (LiOH) (Merck Co.), and LiClO4 (Aldrich Co.) were used without further treatment. Synthesis of Waterborne Polyurethane. The WPU(PPG) dispersion was prepared through our modified acetone process 25,26 by polyaddition of isophorone diisocyanate (IPDI) to polypropylene glycol(PPG) and dimethylolpropionic acid (DMPA), followed by neutralization of pendant COOH with LiOH. The preparation of the chain extender was similar to that in our previous paper expect for the use of LiOH instead of NaOH. WPU(PTMG) dispersion was prepared through the same process except for the use of PTMG instead of PPG. Preparation of the Composite Electrolytes. The prepared WPU(PPG) and WPU(PTMG) dispersions were mixed with PEO (Mw ) 4 × 106, Aldrich Co.) aqueous solution in the desired ratio according to the design matrix in Table 1 to form the uniform solutions. Consequently, the solutions were poured onto a Teflon disk to cast film, then dried under vacuum at 100 °C overnight, and stored in an argon-filled drybox (Vacuum Atmosphere Co., Hawthorne, CA). The WPU(PPG)-
Table 2. Analysis of Variance for the Fit of Sw of CEsa source
degrees of freedom
sum of squares
mean square
F value
model error total
3 13 16
38 563.851 37 574.830 33 39 138.681 70
12 854.617 12 44.217 72
290.712
a
R2 ) 0.9853; Radj2 ) 0.9819.
WPU(PTMG)-PEO composite electrolytes were prepared by dipping the dried composites in 1 M LiClO4/ PC solution for 3 min. To investigate the effect of the solution content, the swollen weight (Sw) of WPU(PPG)WPU(PTMG)-PEO films in LiClO4/PC was measured. Conductivity Measurements. CEs were calculated by AC impedance analysis. The impedance analysis of the polymer electrolyte was performed by using a CM300 EIS system (Gamry Instruments, Inc., Warminster, PA) with an SR810 DSP lock-in amplifier (Standford Research System, Inc., Sunnyvale, CA) under an oscillation potential of 10 mV from 100 to 1 kHz. The electrochemical cell was constructed as the composite electrolyte sandwiched between two blocks of stainless steel. The cell was sealed with an O-ring in a tube and covered with a jacket for heating/cooling water circulation. The temperature of the cells was controlled using a water thermostat (HAAKE D8&G) and calibrated using a PT-100. Polarizing Microscope. A Nikon Optiphot2-POL polarizing microscope equipped with a Nikon NFX-35 camera (Nikon, Japan) was used to identify the morphology of the composite films. Samples were prepared by dropping the uniform solution with the polymer composition as shown in Table 1 on a microscope cover slip and drying in the atmosphere and then under vacuum for several days. Thermal Measurements. Thermal analysis of composite electrolytes was carried out using a differential scanning calorimeter (TA 2010, New Castle, DE) with a heating rate of 10 °C min-1. Samples were taken from the dried film and sealed in aluminum capsules. Results and Discussion The conductivity (σ) of CE is supposed to be a function of the content of the liquid electrolyte which is in terms of the swollen weight:
Sw )
weight of liquid electrolyte (g) × 100% (1) weight of CE (g)
To investigate the dependence of Sw and σ on a polymer compositional diagram, a mixture design approach was employed. In this case, the swelling process was controlled for 3 min to obtain Sw and σ data which were listed in Table 1. AC Impedance. To demonstrate the reliability of σ data, experiments for obtaining σ were discussed first. AC (alternating current) impedance of all samples sandwiched between stainless steel (SS) electrodes was performed to obtain the Nyquist plots with an equivalent circuit as shown in Figure 1. The profile shows a straight line in a high-frequency region and could be simulated by an equivalent circuit of charge-transfer resistance (Rct) and double-layer capacitance (Cd) parallel to each other and bulk resistance (Rb) in series. Rb can be obtained from the intercept of the straight line with the real axis. For presentation of the data by the conductivity (σ) value, Rb obtained from impedance
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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 Table 3. Analysis of Variance for the Fit of σ of CEs at 25 °Ca source
degrees of freedom
sum of squares
mean square
F value
model error total
6 10 16
20.521 39 0.453 23 20.974 62
3.420 23 0.054 32
750 462
a
Figure 1. Nyquist plot of SS/CE(sample(A))/SS at 25 °C. CE film comprised of 50% WPU(PPG) and 50% 1 M LiClO4/PC. The CE film is 260-µm thick and 0.785 cm2 in area. Impedance frequency range: 100-1 kHz.
R2 ) 0.9784; Radj2 ) 0.9654.
this reason that increasing the PEO content in composites increases both the Sw and σ. Unfortunately, the composite containing more PEO exhibits worse degree of strength. Therefore, ternary composites are required to compromise their conductivity and strength. Regression Analysis. All Sw and σ25 (the subscript 25 indicates the temperature in °C) data are listed together with the design matrix in Table 1 and fitted with a forward stepwise regression procedure to achieve statistically significant regression equations via the test statistics, F and Radj2 (F ) MSR/MSE and Radj2 ) 1 (SSE × SST/[(N - P) × (N - 1)]) in the analysis of variance in Tables 2 and 3 for Sw and σ25, respectively. MSR is the mean square of regression obtained by dividing the sum of squares of regression with the degree of freedom and MSE represents the mean-square error from the analysis of variance. If the calculated F value is greater than that in the F table at a specified probability level [F(P - 1, ν, 1 - R)], a statistically significant regression model is obtained. The P, ν, and R indicate the number of experiments, the degrees of freedom of error, and the probability level in this model, respectively. Radj2 is the adjusted correlation coefficient (R2) whose value close to 1 means a perfect fit to the experimental data (0.9821 for Sw and 0.9654 for σ25). Thus, the regression equations on the basis of Sw and σ25 are obtained through the aforementioned statistics as
Sw ) 52.845x1 + 26.098x2 + 64.935x3
Figure 2. Typical Arrhenius plot of conductivity for CEs: sample C, b; sample G, 2; sample A, [; sample L, 1; sample B, 9.
studies is transferred by using σ ) (1/Rb)(l/A), where l and A represent the film thickness and surface area of the electrode, respectively. Arrhenius Plots. Although σ data in Table 1 was for 25 °C only, those for various temperatures were also taken in our experiments. Accordingly, typical Arrhenius plots for samples C (Sw ) 64.29%), G (Sw ) 57.69%), A (Sw ) 50%), L (Sw ) 21.43%), and B (Sw ) 18.18%) are only drawn in Figure 2 because most plots are close to each other, making no sense. The temperature plots of log σ show all straight lines, implying two possible reasons: (i) higher Sw renders higher σ; (ii) more PEO renders higher σ. The activation energies (Ea) are 29.30, 28.71, 26.77, 36.14, and 70.40 kJ/mol for samples C, G, A, L, and B, respectively. It is clear that Ea decreases with increasing Sw. As for samples C, G, and A, the magnitude of Ea has an opposite order because the PEO content of these samples is in a decreasing order, sample C > sample G > sample A, and the polarity of the polymer also decreases with decreasing PEO content. The polarity of the polymer provides the absorption capability of the liquid electrolyte as well as the interaction between them. It is for
(2)
σ25 ) 0.399x1 + 3.372x3 - 4.675x2x3 - 4.921x1x3 + 21.068x1x2x3 + 6.315x1x3(x1 - x3) (3) where x1, x2, and x3 represent the pseudo-components of WPU(PPG), WPU(PTMG), and PEO, respectively, in ternary composites. Contour Plots. To facilitate straightforward examination of the dependence of Sw and σ25 on the polymer compositional diagram, the contour lines of Sw (-‚-) and σ25 (s) were respectively plotted by using eqs 2 and 3 on Figure 3. First, an examination of Sw contour lines reveals the minimum Sw at point X2 (95% WPU(PTMG) and 5% WPU(PPG)). Along lines X2-X3 (WPU(PTMG)PEO binary) or X2-X1 (WPU(PTMG)-(WPU(PPG)), Sw is proportionally increased with increasing PEO/ WPU(PPG) composition. Along line X1-X3 (WPU(PPG)PEO), Sw also increases with increasing PEO, indicating that PEO plays the best role in absorbing LiClO4/PC. Accordingly, the maximum Sw occurs at point X3 (95% PEO and 5% WPU(PPG)). In addition, the contours in the ternary region display parallel straight lines, indicating the linear relation between Sw and the polymer composition. The above results can be explained from the viewpoint of the molecular structure of WPU(PTMG), WPU(PPG), and PEO. In polyurethane, the phase separation between hard and soft segments always occurs. The soft
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 75
Figure 3. Contour plots of Sw (%) and σ (mS cm-1) vs the polymer composition for ternary WPU(PPG)-WPU(PTMG)-PEO CEs. Note: (-‚-) for Sw; (s) for σ; X1 represents WPU(PPG); X2 represents WPU(PTMG); X3 represents PEO.
Figure 5. DSC thermograms of CEs: (a) PEO; (b) WPU(PTMG)+PEO (1:1); (c) WPU(PPG)+PEO (1:1).
Figure 4. Typical DSC thermograms of CEs: (1) PEO; (2) WPU(PPG); (3) WPU(PTMG); (4) PEO+WPU(PPG) (1:1); (5) PEO+WPU(PTMG) (1:1); (6) WPU(PTMG)+WPU(PPG) (1:1).
segment in WPU plays an important role in absorbing LiClO4/PC. The soft segment, -(C-C-C-C-O-)-, in WPU(PTMG) possesses the lower polarity than the soft segment, -(-C(C)-C-O-)-, in WPU(PPG). Thus, WPU(PPG) has better capacity than WPU(PTMG) for absorbing LiClO4/PC. In comparison with both WPU(PTMG) and WPU(PPG), PEO possesses the best absorption capability for LiClO4/PC because of the highest polarity of its repeat unit, -(C-C-O-)-. As for the parallel straight lines in the ternary region, it implies that the blending of WPU(PPG), WPU(PTMG), and PEO has no synergistic/antagonistic effects on Sw. After the discussion of Sw, it is desirable to examine σ25 contour lines. The minimum σ25 occurs at point X2 (95% WPU(PTMG) and 5% WPU(PPG)), this being consistent with Sw. Along line X2-X1, σ25 increases slightly with increasing WPU(PPG). On line X2-X3, σ25 increased by an order of magnitude with increasing
PEO. On the other hand, σ25 increases moderately with increasing PEO on line X1-X3. Ultimately, the maximum σ25 occurs at point X3 (95% PEO and 5% WPU(PPG)). Basically, the conductivity of CEs is considered to be a proportional function of Sw because more Sw provided more ions and solvent for the ionic conductivity. Hence, the trend of the contour line for σ25 should be similar to that for Sw. An examination of Figure 3 reveals that the trend of contour lines of σ25 does not completely follow that of Sw, being attributable to the complicated interaction of polymer molecules. In this case, the influencing factors of σ25 might be two: (i) polymer composition of CE and (ii) Sw in CE. To quantitatively clarify these two factors, the following procedure might be taken by combining eqs 2 and 3. Because the relation between σ25 and Sw is not easily quantified via contour lines, eqs 4 and 5 are derived by letting x3 ) 0 and x2 ) 1 - x1 in eqs 2 and 3 to quantitatively examine the interaction between WPU(PPG) and WPU(PTMG).
Sw(x3)0) ) 26.756x1 + 26.098
(4)
σ25(x3)0) ) 0.399x1
(5)
Equations 4 and 5 show that both Sw and σ25 are the linear functions of WPU(PPG) composition. The constant in eq 4, 26.098, means point X2 (WPU(PTMG) 95% and WPU(PPG) 5%) possesses 26.098% LiClO4/PC, while this point has a very low conductivity, being consistent with eq 5 without the constant term. It can be reasonably explained by the fact that point X2 possesses 95% WPU(PTMG) which behaves as an inert phase in the absorption of LiClO4/PC. In this composition, the active phase, only 5% WPU(PPG), might not form a continuous phase where ions can move fast because of the absorption of the liquid electrolyte. Furthermore, σ25 can be in terms of Sw by combining eqs 4 and 5 as
σ25 ) 0.015Sw - 0.39
(6)
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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
Figure 6. Typical polarizing micrographs of CEs: (a) sample N; (b) sample J; (c) sample P; (d) sample G; (e) sample D; (f) sample F.
Equation 6 validates the assertion that the increase in σ25 is mainly contributed by the increase in Sw, which is absorbed by WPU(PPG) in this case. The negligible conductivity occurs as Sw< 26%, being attributable to the discontinuous WPU(PPG) phase in the WPU(PTMG) matrix. The conducting path might be disconnected by the continuous WPU(PTMG) phase, resulting in a negligible conductivity. Although all compositions of line X2-X3 possess 5% WPU(PPG), they can be simply considered as binaries because of the constant WPU(PPG) composition. Accordingly, eqs 2 and 3 can be reduced to eqs 7 and 8 by
letting x1 ) 0 and x2 ) 1 - x3.
Sw(x1)0) ) 38.837x3 + 26.098
(7)
σ25(x1)0) ) 4.675x32 - 1.348x3
(8)
σ25(x1)0) ) 0.00308(Sw - 18.715)2 - 1.938 (9) Note that eq 8 is the quadratic form of x3, implying that σ25 can be expressed in a quadratic form of Sw as in eq 9. It is interesting that σ25 equals zero as Sw ) 43.79%, also implying that σ25 would be negligible as Sw
1, σ25 increases sharply because of the continuous PEO phase. It might be explained by DSC and PM results later. To examine the interaction in detail between WPU(PPG) and PEO, by employing a similar procedure, Sw and σ25 are expressed in eqs 10 and 11 by letting x2 ) 0 and x3 ) 1 - x1 in eqs 2 and 3.
Sw(x2)0) ) -12.15x1 + 64.935
(10)
σ25(x2)0) ) -12.63x13 + 23.86x12 - 14.569x1 + 3.372 (11) From eq 10, Sw is increased by decreasing WPU(PPG). Equation 11 is a cubic form of x1, it being complicated for analysis. Accordingly, there is no further analysis in WPU(PPG)-PEO binaries. Unfortunately, the above mathematical procedure cannot be easily applied in the ternary region. A careful examination of Figure 3 reveals two regions separated by the line Sw ) 50%. In region Sw < 50%, along contour lines of Sw, the peak σ appears in ternaries, implying that the interaction among polymers does influence σ. In region Sw > 50%, the maximum σ appears in binaries (the bottom line), indicating that σ is significantly determined by the PEO content. To investigate the interaction among polymers, the experiments of DSC and PM were performed. DSC and PM Results. According to the above discussion, the interactions among polymers are supposed to be important for explaining Sw and σ25 contours. To clarify the interaction, the DSC thermograms of PEO, WPU(PPG), WPU(PTMG), WPU(PTMG)-PEO, WPU(PPG)-PEO, and WPU(PTMG)-WPU(PPG) are run and shown in Figures 4and 5 for Tg and Tm, respectively. In Figure 4, curves (1), (2), and (3) show Tg’s -57.64, -38.47, and -64.33 °C for PEO, WPU(PPG), and WPU(PTMG), respectively. Curves (4), (5), and (6) demonstrate two Tg’s for binaries, being used to depict the miscibility. First, an examination of Figure 4 reveals two Tg’s, -55.63 and -38.70 °C for PEO and WPU(PPG), respectively. In comparison with curves (1) and (2), Tg of WPU(PPG) shifts slightly and that of PEO stays the same, meaning that these two polymers are partially miscible in a small degree. Curve (5) shows two Tg’s, -64.77 and -57.73 °C for WPU(PTMG) and PEO, respectively, being the same as curves (1) and (3). This is ascribable to the fact that they are immiscible but compatible in the aqueous solution. Finally, an examination of curve (6) also indicates two Tg’s, -63.15 and -39.67 °C, being attributable to the slightly partial miscibility between WPU(PTMG) and WPU(PPG). In conclusion, there exists a small interaction or no interaction among these three polymers. The reason might be that Sw is a linear function of them and shows linear contour lines. As for Tm, because both WPU(PTMG) and WPU(PPG) are amorphous, the melting process occurs only for PEO spherulites. Accordingly, Figure 5 shows Tm’s 65.17, 61.73, and 63.67 °C for PEO, WPU(PTMG)PEO, and WPU(PPG)-PEO, respectively. From the above discussion of Tg, PEO is immiscible with WPU(PTMG) and partially miscible with WPU(PPG), having shifts for Tm of its composites. It might also imply that the state of PEO spherulites is influenced by its counterparts. Meanwhile, PEO was reported to be responsible for the majorly ionic conduction.34 Ac-
cordingly, the distribution and morphology of PEO spherulites might be a good index for explaining the σ25 contours. To further explain the reason that σ25 does not completely follow Sw, PM experiments of six samples were performed. Figure 6 shows PM results of samples N, J, P, G, D, and F from Table 1 in (a), (b), (c), (d), (e), and (f), respectively. An examination of Figures 6a, 6b, and 6c reveals that their PEO spherulites look similar because PEO in these samples is the major content (75% for samples N and P, 66% for sample J), rendering a continuous PEO phase which is responsible for the ionic conduction.34 Accordingly, their conductivities are supposed to be close together. As discussed in Tg, the immiscibility between PEO and WPU(PTMG) renders more PEO channels for ionic conduction than the composites with the addition of WPU(PPG). Although no further evidence can be obtained, Figure 6c shows more solidity and compactability than Figures 6a and 6b, implying more PEO channels for ionic conduction. The values of σ25 are 1.124, 1.228, and 1.375 mS cm-1 for samples N(a), J(b), and P(c) which possess Sw’s that are 56.25%, 55.56%, and 51.85%, respectively. It is consistent with our discussion of σ25 contours and Tg. An examination of Figures 6d, 6e, and 6f reveals that the addition of WPU(PPG) blurs the border of PEO spherulites because of the partial miscibility between WPU(PPG) and PEO/WPU(PTMG). WPU(PPG) seems to play the role of an interfacial promoter to distribute PEO into a WPU(PTMG) matrix, rendering more PEO phase for ionic conduction. The values of σ25 are 0.752, 0.792, and 0.443 mS cm-1 for samples G(d), D(e), and F(f) which possess Sw’s that are 57.69%, 48.15%, and 54.17%, respectively. It is interesting that sample D in Figure 6e possesses the least value of Sw but the largest value of σ25, validating our assertion that, in region Sw < 50%, WPU(PPG) plays a useful role for increasing conductivity. In conclusion, WPU(PPG) plays the role of an interfacial promoter between PEO and WPU(PTMG). Accordingly, the addition of WPU(PPG) increases the conductivity as PEO is the minor content while it presents the negative effect as PEO is a continuous phase. Conclusion Using mixture design strategy, Sw, and σ25 in the full compositional range of WPU(PPG)-WPU(PTMG)-PEO is successfully modeled with a limited number of experiments. The coefficients of the regression models represented as contour plots are extremely useful in studying the effects of the Sw composition on Sw and σ25. DSC and PM are useful in proving the interactions among polymers. In conclusion, WPU(PTMG) provides the mechanical strength; PEO acts as the absorbent of LiClO4/PC; and WPU(PPG) plays the role of an interfacial promoter which is useful in increasing σ as PEO is the minor content. Accordingly, the addition of WPU(PPG) in WPU(PTMG)-PEO (minor PEO) provides the robust characteristics which can be found within the region between 50% and 45% of Sw. The conductivity approximates 10-3 S cm-1 at 25 °C while the mechanical strength is still very good because of the containing of 50% WPU(PTMG). Acknowledgment The financial support for this work by the National Science Council of Taiwan under Contract NSC 88-2622E006-008 is gratefully acknowledged.
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Received for review June 1, 1999 Revised manuscript received August 30, 1999 Accepted October 21, 1999 IE990373C
