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Kinetics of Michael addition polymerizations of N,N'-bismaleimide-4,4'-diphenylmethane with barbituric acid.

INTRODUCTION

Polymers originating from N, N'-bismaleimide-4, 4'-diphenylmethane (abbreviated as BMI) show a hyper-branched or highly crosslinked network structure, depending on the extent of conversion. These materials possess very high glass-transition temperatures and are characterized by excellent mechanical properties, chemical resistance, thermal stability, and attractive performance/cost ratio that have several useful applications such as aerospace and electronics industries. With the reactive bismaleimide groups (two terminal -C=C-groups), BMI can polymerize with active hydrogen atom-containing species such as polyamines via the Michael addition reaction mechanism (1-6). It was reported that free radical polymerization of BMI can be initiated by barbituric acid (BTA) (7) or 2,2'-azobisisobutyronitrile (AIBN) (8). Furthermore, anionic polymerization of BMI initiated by nano-[Na.sup.+]/[TiO.sub.2] was reported (9). The pioneering work of Pan et al. (7) studied the polymerizations of BMI with BTA and characterized the resultant hyper-branched polymers. The electron spin resonance spectra of BTA at different temperatures (363-443 K) confirmed the presence of free radicals and the concentration of free radicals increased with increasing temperature. In our previous work (10), the effects of solvent basicity on the polymerizations of BMI with BTA were studied. The results illustrated the greatly enhanced formation of the three-dimensional crosslinked network structure during polymerization by the nitrogen-containing cyclic solvents such as N-methy1-2-pyrrolidone (NMP). By contrast, the polymerizations of BMI with BTA in a cyclic solvent in the absence of nitrogen atoms such as [gamma]-butyrolactone eventually resulted in nil gel content. It was concluded that the higher the solvent basicity, the larger the amount of insoluble polymer species formed. A tentative polymerization mechanism that took into consideration the formation of a ketone radical pair between BTA and BMI and the subsequent initiation, propagation and termination reactions was proposed to qualitatively describe the experimental results. It was postulated that the nitrogen-containing cyclic solvents were capable of participating in the ketone radical pair formation process, thereby increasing the extent of polymer crosslinking reactions. However, the potential Michael addition reactions cannot be ruled out because each BTA molecule contains two >NH groups and one >[CH.sub.2] group and the active hydrogen atoms of these functional groups may take part in the polymer reactions of BMI with BTA via the Michael addition reaction mechanism.

Recently, the authors used hydroquinone (HQ), an extremely effective inhibitor that captured free radicals, as a molecular probe to study the reaction mechanisms and kinetics involved in the polymerizations of BMI with BTA (11), (12). It was shown that the apparent overall heat of reaction obtained from the non-isothermal polymerizations of BMI with BTA with different molar ratios of BTA/BMI first decreased rapidly and then leveled off as the concentration of HQ was increased. In addition, the extent of reduction in the apparent overall heat of reaction decreased with increasing molar ratio of BTA/BMI. These results indicated that free radical polymerization contributed significantly to the polymerizations of BMI with BTA and it became more important as the mole fraction of BTA was decreased. [.sup.13]C NMR and 13C NMR characterization of the linear polymer prepared by the polymerization of the model compound N-phenylmaleimide with BTA further supported the coexistence of the Michael addition reaction and free radical polymerization mechanisms. The objective of this work was to focus on the investigation of the kinetics of Michael addition polymerization of BMI/BTA (2/1 (mol/mol)) in NMP at different temperatures with the aid of adding sufficient HQ to completely suppress the free radical polymerization. This approach is capable of decoupling the rather complicated competitive Michael addition and free radical polymerization mechanisms, thereby leading to the true key kinetic parameters such as the reaction rate constants and activation energy for the Michael addition polymerization.

EXPERIMENTAL

Materials

The reagents used in this work include N, N'-bismaleimide-4, 4'-diphenylmethane (BMI, Beil, 95%), barbituric acid (BTA, Merk, 99+%), N-methyl-2-pyrrolidone (NMP, Sigma, 99%), dimethyl sulfoxide-d6 (DMSO-d6, Aldrich, 99.9%), hydroquinone (HQ, Acros, 99%) and toluene (Acros, 99+%). All chemicals were used as received.

Polymerization Kinetics and Characterization

The isothermal polymerization of BMI with BTA was carried out in a Tzero hermetic pan in the differential scanning calorimeter (DSC, TA Instruments Q20). The nitrogen flow rate was set at 50 mL [min.sup.-1]. Both the initial concentrations of the -C=C-associated with BMI ([[BMI.sub.c=clo]) and the active hydrogen atoms of the two >NH groups and one > [CH.sub.2] group associated with BTA ([[[BTA].sub.H].sub.o]) in NMP solution were kept constant at 2 M (i.e., BMI/BTA = 2/1 (mol/mol)). The total solids content of the resultant BMI/ BTA polymer in NMP solution (7 [+ or -] 1 mg) was kept constant at 25.6% without taking into consideration HQ throughout this work. The conversion (X) was defined as

X = [DELTA][H.sub.t]/[DELTA]H (1)

where [DELTA][H.sub.t] is the integral area under the heat flow versus time (t) curve from t = 0 to t. The parameter [DELTA]H in the denominator represents the apparent overall heat of reaction obtained from the isothermal polymerization of BMI with BTA in the absence of HQ at 423 K (the highest reaction temperature used in this work that is lower than the boiling point (477 K) of NMP to avoid the contamination of the sample cell) over a period of 1 h that is long enough to achieve a satisfactory base line. This method may well overestimate the conversion of the -C=C-associated with BMI provided that the polymerization was incomplete.

The addition of sufficient HQ to completely quench the free radical polymerization might consume some active hydrogen atoms of the two >NH groups and one > [CH.sub.2] group associated with BTA during polymerization. To quantitatively determine this potential side reaction effect, 20 mL of a BTA in NMP solution containing a prescribed amount of HQ ([[[BTA].sub.H].sub.o] = 2 M, BTA/HQ = 1/1 (w/w)) was charged into a 100-mL three-necked flask in a thermostatic oil bath at 403 K, reacted with magnetic mixing for I h and then cooled to room temperature. This was followed by the precipitation of the product by an excess of toluene. The product was then filtered and dried in a vacuum oven at 333 K for 24 h. The molecular structure of the resultant powder was characterized by [.sup.1]H NMR (Bruker Avance, 500 MHz). DMSO-d6 was used as the solvent in 11-1 NMR measurements and tetramethylsilane was used as the internal standard. Figure I a shows the [.sup.1]H NMR result of BTA, in which the characteristic peaks at [delta] = 3.46 and 11.09 ppm correspond to the > [CH.sub.2] group and the > NH group, respectively (13). Thus, a calibration curve was established by plotting the integral area of the characteristic peak of > NH at [delta] = 11.09 ppm (or that of > [CH.sub.2] at [delta] = 3.46 ppm) versus the concentration of BTA in the absence of the HQ treatment (Fig. 2). It should be noted that Fig. 2 represents the calibration curve (Integral area = 273.12 x (BTA/ DMSO-D6), coefficient of determination ([R.sup.2]) = 0.9975) for either the > NH group or the >CH2 group since one BTA molecule possesses exactly the same number of hydrogen atoms (2) for both the > NH and > [CH.sup.2] groups.

RESULTS AND DISCUSSION

Development of Michael Addition Polymerization Kinetics Model

Scheme 1 illustrates (a) the reaction mechanism involved in the Michael addition of the -C=C-group of BMI with the active hydrogen atom of the >[CH.sub.2] group of BTA and (b) the reaction mechanism involved in the aza-Michael addition of the -C=C-group of BMI with the active hydrogen atom of the >NH group of BTA (14). Taking Scheme la as an example, the Michael addition starts with the deprotonation of BTA (nucleophile) by base or solvent such as NMP with a relatively high proton affinity (923.4 kJ [mot.sup.-1] (15)) used in this work (B:) to result in carbanion stabilized by its electron-withdrawing groups ([BTA.sup.[??]]). This is followed by the conjugate addition reaction of this nucleophile (BTA9) with the electrophilic BMI to form BTA-[BMI.sup.[??]]. Finally, proton abstraction from protonated base or solvent (H-[B.sup.[??]]) by BTA-[BMI.sup.[??]] occurs to form the desired adduct. It should be noted that additional base is generally not required due to the presence of two secondary amine groups of BTA, which act as both nucleophile and base (Scheme lb). The reaction rate laws derived from the elementary reaction steps in Scheme 1 a and b, respectively, are shown as follows (14):

[R.sub.M,CH] = {[k.sub.2][K.sub.eq]([B:]/[H - [B.sup.[??])}[[BTA.sub.CH]][[BMI.sub.c=c]] = [k.sub.M.CH] [[BTA.sub.CH][[BMI.sub.C=C]] (2)

[R.sub.M,NH] = [k.sub.1][[BTA.sub.NH]][[BMI.sub.C=C]] = [k.sub.M,NH] [[BTA.sub.NH]][[BMI.sub.C=C]] (3)

[R.sub.M] = [R.sub.M,CH] + [R.sub.M,NH] = ([k.sub.M,CH][[BTA.sub.CH]] + [k.sub.M,NH][[BTA.sub.NH]])[BMI.sub.C=C] (4)

where [k.sub.1] and [k.sub.2] are the reaction rate constants, [K.sub.eq] the equilibrium constant, and [B:] and [[H-B.sup.[??]] the molar concentrations of B: and [H-B.sup.[??]], respectively, and these parameters are associated with the relevant elementary reaction steps in Scheme 1. [R.sub.M,NH] and [R.sub.M.NH] represent the Michael addition polymerization rates corresponding to the polymerization of the -C=C-group of BMI with the >[CH.sub.2] group and the >NH group of BTA, respectively, and RM the total Michael addition polymerization rate. The kinetic parameters [k.sub.M,CH] and [k.sub.M,NH] represent the reaction rate constants, and [[BTA.sub.CH]], [[BTA.sub.NH]] and [[BMI.sub.C=C]] the molar concentrations of the active hydrogen atom of >[CH.sub.2], the active hydrogen atom of >NH and the -C=C-of BMI, respectively.

With an additional assumption that both the active hydrogen atoms of the >[CH.sub.2] and >NH groups of BTA exhibit exactly the same reactivity toward the -C=C-of BMI (i.e., [k.sub.M,CH] = [k.sub.M,NH = [k.sub.M]), Eq. 4 can be simplified as

RM = -d[[BMI.sub.C=C]]/dt = [k.sub.M] [[BTA.sub.H]] [[BMI.sub.C=C]] = [k.sub.M] [[[BMI.sub.C=C]].sub.0.sup.2](1 - [X.sub.m])([theta] - [X.sub.M]) (5)

where [[BTA.sub.H] = [[BTA.sub.CH] + [[BTA.sub.NH]], [[[BMI.sub.C=C]].sub.0] = the initial concentration of the -C=C- of BMI, [theta] = [[[BTA.sub.H]].sub.0]/[[[BMI.sub.C=C]].sub.0], [[[BTA.sub.H]].sub.0] = the initial concentration of the active hydrogen atoms of the >[CH.sub.2] and >NH groups of BTA, and [X.sub.M] = the fractional conversion of the -C=C-of BMI at t. Integration of Eq. 5 gives

In{([theta] - [X.sub.M])/[[theta](1 - [X.sub.M])]} = ([[[BTA.sub.H].sub.0] - [[[BMI.sub.C=C]].sub.0])[k.sub.M]t (6)

The activation energy ([E.sub.a]) can be determined by the following Arrhenius equation:

In [k.sub.M] = In A - [E.sub.a]/(RT) (7)

where A is the frequency factor, R the gas constant (8.314 J [mol.sub.1]) and T the absolute temperature.

Michael Addition Polymerization Kinetics

First, the average total heat flow data (based on at least duplicate DSC measurements), obtained from the isothermal polymerizations of BMI/BTA (2/1 (mol/mol)) at 423 K over a period of 1 h, as a function of the amount of HQ are shown in Fig. 3. It is shown that the total heat flow first decreases rapidly and then levels off as the quantity of HQ is increased. This trend corresponds to the influence of the amount of HQ on the extent of free radical polymerization of BMT with BTA. The weight ratio of HQ/BTA equal to 1.0 was shown large enough to extinguish the free radical polymerization and, therefore, it was chosen for studying the BMI/BTA Michael addition polymerization kinetics hereinafter. Furthermore, the contribution of the Michael addition polymerization is estimated to be about 69% (= 55.50/80.78 X 100%). The value of the apparent overall heat of reaction ([DELTA]H) for the polymerizations of BMI/BTA (2/1 (mol/mol)) in the absence of HQ is 80.78 [+ or -] 1.71 J [g.sup.-1] based on three measurements. With the knowledge of [DELTA]H in combination with the representative heat flow versus t data at different temperatures (Fig. 4a), the isothermal polymerization kinetics data (i.e., [X.sub.M] versus t data) thus obtained are shown in Fig. 4b. A general feature of these kinetic data is that, at a particular temperature, the rate of polymerization first increases rapidly to a maximum and then decreases toward the end of polymerization (Fig. 4a). Furthermore, the polymerization rate (see the slope of the [X.sub.M] versus t data during the early stage of polymerization in Fig. 4b) increases with increasing temperature. Nevertheless, the limiting conversion occurring during the latter stage of polymerization seems independent of the temperature, which will be discussed later.

The initial composition of the reaction mixture ([theta]) must be determined before the computer simulation for the isothermal Michael addition polymerizations of BMI with BTA can be carried out. Figure lb shows the [.sup.1]H NMR spectrum of the BTA treated by HQ (HQ/BTA = 1/1 (w/w)) at 403 K over a period of 1 h. With the integral areas of the characteristic peaks a BTA/DMSO-D6 (0.0336/1 (w/w)) at 11.10 ppm (>NH) and 3.46 ppm (>CH2) in combination with the calibration curve in Fig. 2, the effect of HQ on [[BTA.sub.H]].sub.0] can be quantitatively determined, and the results summarized in Table 1. It can be concluded that this amount of HQ only consumes the active hydrogen atoms of >[CH.sub.2] to some extent (10.68%), but it has no influence on the >NH group. Thus, the value of 0 can be estimated as follows: [theta] = [[[BTA.sub.H]].sub.0]/[[[BMI.sub.C=C]].sub.0] = 1 x [2 X (100% - 10.68%) + 2 x (100% - 0%)]/(2 x 2) = 0.9466. In addition, these [.sup.1]H NMR data also provide some clue to the initiation reactions involved in the free radical polymerization of BMI with BTA, but this subject is beyond the scope of this study.

TABLE 1. [.sup.1]H NMR results for the structure of BTA treated
with HQ (HQ/BTA = 1/1 (w/w)) at 403 K for 1 h.

                                    Integral area
                                 >[CH.sub.2]   >NH
[.sup.1]H NMR experiment (a)            8.20  9.18
[.sup.1]H NMR calibration (a)           9.18  9.18
H consumed (%)                         10.68     0

(a.) BTA/DMSO-d6 (w/w) = 0.0336.


According to Eq. 6, plotting the data of In{([theta] - [X.sub.M])/[[theta](1 - [X.sub.M])]} as a function of t should result in a straight line with a slope equal to ([[[BTA.sub.H]].sub.0] - [[[BMI.sub.C=C]].sub.0]) [k.sub.M], as illustrated in Fig. 5. Note that the experimental data in duplicate (see each pair of open and closed symbols in Fig. 5) before the limiting conversion was achieved were used to perform the linear regression process. The reproducibility of the dual experiments carried out at a particular temperature is reasonably good, as shown by the data of the slope obtained from the least-squares best-fitted straight line and [k.sub.M] (= s1ope/([[BTA.sub.H]].sub.0] - [[[BMI.sub.C=C]].sub.0]) in Table 2. Furthermore, the relatively high values of [R.sup.2] (in the range 0.9782-0.9976) associated with those least-squares best-fitted straight lines in Fig. 5 confirm the validity of the kinetic model developed in this work. The data of [k.sub.M] as a function of T can be expressed in Arrhenius form (Eq. 7, [R.sup.2] = 0.9944), as shown in Fig. 6. The value of [E.sub.a] (= -slope X R) for the Michael addition polymerizations of BMI/BTA (2/1 (mol/mol)) is estimated to be 36.1 kJ [mol.sup.-1], which is quite comparable to that (43 kJ [mol.sup.-1]) obtained from the Michael addition polymerizations of BMI with 4,4'-diaminodiphenylmethane (16). In addition, these two values of [E.sub.a] are much lower than that (76.3 kJ [mol.sup.-1]) of the polymerizations of BMI/BTA (2/1 (mol/mol)) in the absence of HQ reported in our previous work (11). This is because the competitive Michael addition and free radical polymerization mechanisms are operative simultaneously in the latter BMI/BTA polymerization system.

TABLE 2. Reproducibility of the Michael addition polymerizations
of BMI/BTA (2/1 (mol/mol)) in the presence of HQ (HQ/BTA = 1/1
(w/w)) and their reaction rate constants at different temperatures.

T(K)  Slope (x [10.sup.3]  [K.sub.m] (x [10.sup.2] L
            [min.sup.-1])               [mol.sup.-1]
              ([R.sup.2])              [min.sup.-1])

383        -2.09 (0.9976)        2.44 [+ or -] 0.345

           -2.78 (0.9914)

393         -3.16(0.9782)        3.37 [+ or -] 0.205

           -3.57 (0.9867)

403         -3.62(0.9791)        4.08 [+ or -] 0.455

           -4.53 (0.9897)

413        -5.64 (0.9968)        5.69 [+ or -] 0.005

           -5.73 (0.9905)

423        -6.70 (0.9910)        7.18 [+ or -] 0.485

           -7.66 (0.9944)

The slope of the least-squares best-fitted straight line for the
data of In{([theta] - [X.sub.M])/[[theta](1 - [X.sub.M])]} as a
function of t The numeric value in the parenthesis is the
coefficient of determination.


Diffusion-Controlled Polymerization Kinetics

As mentioned above, a relatively stationary limiting conversion (68.6% [+ or -] 0.8%) independent of the reaction temperature was achieved for the series of BMI/BTA (2/1 (mol/mol)) polymerizations beyond about 60% conversion (Fig. 4b). The limiting conversion is closely related to the diffusion-controlled polymer reactions during the latter stage of polymerization (17). Under the circumstance, the crosslinking density and, consequently, the viscosity increase continuously with the progress of the Michael addition polymerization of the difunctional BMI with the tetrafunctional BTA. Beyond a certain critical conversion, the mobility of the residual -C=C-of BMI and the >NH and >[CH.sub.2] groups of BTA attached to the gigantic polymer network structure becomes diffusion-controlled. Thus, the reaction rate constant [k.sub.M] is greatly reduced, thereby leading to a significant reduction in the polymerization rate. In general, the lower the reaction temperature, the higher the viscosity of the polymer solution is. As a result, the polymerization system carried out at a lower temperature exhibits stronger diffusion-controlled polymer reactions, thereby leading to a more retarded polymerization rate. Furthermore, the decreased reaction rate constant with temperature results in a slower rate of polymerization at a lower temperature. All these factors suggest that the value of limiting conversion should increase with increasing polymerization temperature. Indeed, such a behavior was observed for the polymerizations of BMI/BTA (2/1 (mol/ mol)) governed by a mixed mode of Michael addition/free radical polymerization mechanisms, as illustrated in our previous work (11). However, this is not the case for the polymerizations of BMI/BTA (2/1 (mol/mol)) with HQ/BTA = 1/1 (w/w) investigated in the present study.

To further illustrate the effect of the diffusion-controlled polymer reactions, the [X.sub.M] versus t data obtained from the Michael addition polymerizations of BMI with BTA at 383, 403, and 423 K taken from Fig. 4b are shown in Fig. 7. Note that the dashed line represents the model prediction without taking into consideration the diffusion-controlled polymerization mechanism (see Eq. 6). It is shown that the [X.sub.M] data start to deviate from the model prediction at a [X.sub.M] value of about 0.6 and the deviation increases significantly with increasing temperature. This result implies that the crosslinking density of the three-dimensional network structure (or the degree of the diffusion-controlled Michael addition polymerization) increases rapidly as the reaction temperature is increased from 383 to 423 K. This postulation is confirmed by the rheological data listed in Table 3. The Michael addition polymerizations of BMI/BTA (2/1 (mol/mol)) with HQ/ BTA = 1/1 (w/w) at different temperatures were carried out in a 100-mL three-necked flask with magnetic mixing in a thermostatic oil bath for 1 h. The viscosity data of the polymer solutions were determined by a Brookfield LVT viscometer (spindle LV #3, 60 rpm). It is shown that the viscosity of the BMI/BTA polymerization system increases rapidly with increasing temperature. This trend reflects the greatly increased polymer molecular weight (or crosslinking density) as the reaction temperature is increased.

TABLE 3. Rheological data for the Michael addition
polymerizations of BMI/BTA (2/1 (mol/mol)) in the
presence of HQ (HQ/BTA = 1/1 (w/w)) at different
temperatures for 1 h.

T    [l.sub.g](mm)   [eta] (T)  [eta] (298
(K)            (a)   (ops) (b)    K) (cps)
                                       (c)

383             --         140         140

393             --         180         400

403             --         400  [infinity]
                                     (gel)

413             31  [infinity]  [infinity]
                         (gel)       (gel)

423             25  [infinity]  [infinity]
                         (gel)       (gel)

(a.) The time when the reaction medium gelled.

(b.) The viscosity determined immediately after the end
of polymerization at temperature T.

(c.) The viscosity determined immediately after the
temperature being cooled to 298 K.


In an attempt to quantitatively describe the diffusion-controlled Michael addition polymerizations of BMI/13TA (2/1 (mol/mol)) in the presence of sufficient HQ to fully quench the free radical polymerization, the following equation developed by Chem and Poehlein [17] was adopted in this study.

[k.sub.M,D] = [k.sub.M] exp[-C([X.sub.M] - [X.sub.M,c])] (8)

where [k.sub.M,D] and [k.sub.M] (Eq. 7, A = 2053.5 L [mol.sup.l] [K.sup.l], [E.sub.a] = 36.1 kJ [mol.sup.1]) are the diffusion-controlled and the reaction-controlled Michael addition polymerization rate constants, respectively, C an adjustable constant that is the characteristic of the diffusion-controlled Michael addition polymerization, and [X.sub.M,c] the critical fractional conversion at which point the diffusion-controlled polymer reactions start to occur during the latter stage of polymerization. In this work, a value of 0.6 is assigned to [X.sub.M,c] based on the experimental results. The computer modeling results with C equal to 12.26 are shown by the solid lines in Fig. 7, which indicates that Eq. 8 is capable of predicting the limiting conversion behavior involved in the Michael addition polymerizations of BMI with BTA. The calculated values of [k.sub.M] or [k.sub.M,D] as a function of 44 are shown in Fig. 8. Beyond [X.sub.M,c] it is the rapidly decreased [k.sub.M,D] from dramatically different positions in the reaction-controlled region all the way down to comparable levels at the end of polymerization that is responsible for the observed constant limiting conversion achieved in the diffusion-controlled Michael addition polymerizations of BMI with BTA carried out in the temperature range of 383-423 K.

CONCLUSIONS The isothermal Michael addition polymerizations of N, N'-bismaleimide-4, 4'-diphenylmethane (BMI) and barbituric acid (BTA) with BMI/BTA = 2/1 (mol/mol) in 1-methyl-2-pyrrolidone were investigated independently in this work. This was achieved by the complete suppress of free radical polymer reactions via the addition of a sufficient amount of hydroquinone. A mechanistic model was developed to adequately predict the polymerization kinetics before a critical conversion (ca. 60%), at which point the diffusion-controlled polymer reactions became the predominant factor during the latter stage of polymerization. Based on the kinetic model, the Michael addition polymerization rate constants in the range of 2.44 X [10.sup.-2] - 7.18 x [10.sup.-2] L [mol.sup.-1] [min.sup.-1] were obtained, and the activation energy of 36.1 kJ [mor.sup.-1] determined in the temperature range 383-423 K. Beyond the critical conversion, a relatively stationary limiting conversion (68.6% [+ or -] 0.8%) independent of the reaction temperature was achieved. A diffusion-controlled model taken from the literature was capable of predicting the limiting conversion data obtained from the Michael addition polymerizations of BMI with BTA.

Correspondence to: C. S. Chem; e-mail: cschem@mail.ntust.edu.tw

Contract grant sponsor: National Science Council, Taiwan.

DOI 10.1002/pen.23248

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

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Fu-En Yu, (1) Jung-Mu Hsu, (2) Jing-Pin Pan, (2) Tsung-Hsiung Wang, (2) Chorng-Shyan Chern (1)

(1) Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

(2) Division of Energy Storage Materials & Technology Research, Materials and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31015, Taiwan
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