Mechanical Line

Macromolecules ****, **, ****-****

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Solid-State NMR Investigation of Block Scheme 1. Chemical Structure of Polymers Used To

Prepare Ionically Conductive Copolymers

Copolymer Electrolyte Dynamics

D. J. Harris,*, T. J. Bonagamba,,,

K. Schmidt-Rohr,*,, P. P. Soo,

D. R. Sadoway, and A. M. Mayes

Polymer Science and Engineering Department,

University of Massachusetts, Amherst, Massachusetts 01003;

Universidade de Sao Paulo, Instituto de F sica de Sao

Carlos, Sao Carlos, Sao Paulo, Brazil, Caixa Postal 369,

Table 1. Composition (Volume:Volume) and Molecular

13560-970; Department of Chemistry and Ames Laboratory,

Weight (g/mol) of Polymers Studied by Solid-State NMR;

Iowa State University, Ames, Iowa 50011; and

the Approximate Glass Transition Temperature of the

Department of Materials Science and Engineering,

Poly(n-alkyl methacrylate) Component Is Also Listed for

Massachusetts Institute of Technology, 77 Massachusetts

Reference

Ave., Cambridge, Massachusetts 02139

Received April 23, 2001 polymer comp (v:v) MW (K) PDI Tg (K)

Introduction POEM 100 1.3

POEM-b-PLMA 53:47 65 1.1 238

Recent trends in energy technology have driven

POEM-b-PBMA 50:50 77 1.2 313

considerable interest in solid polymer electrolytes. The POEM-b-PMMA 51:49 52 1.1 373

electrolytic properties of lithium salt-doped poly(ethyl-

ene oxide), PEO, make this polymer and its derivatives, (M 475 g/mol) (Polysciences) (referred to herein as oligo-

including noncrystallizable comb-shaped polymers, fa- oxyethylene methacrylate, OEM), lauryl methacrylate (Ald-

vored candidates for polymer electrolytes.1,2 Some of the rich), LMA, butyl methacrylate (Aldrich), BMA, or methyl

primary applications for polymer electrolyte technology methacrylate (Aldrich), MMA, were prepared by anionic

include high energy density lithium batteries and elec- synthesis in tetrahydrofuran (THF) using diphenylmethylpo-

trochromic devices.3 To improve mechanical properties tassium as initiator. The structures of these polymers are

shown in Scheme 1. The syntheses and characterization of the

while retaining high ionic conductivity, block copolymer

resulting copolymers have been described previously.7 The

electrolytes have been investigated.2,4-7 The microphase

molecular weight characteristics and compositions of the

separation in those block copolymer electrolytes confers

polymer electrolyte materials are listed in Table 1. The table

solidlike mechanical properties to the material at also lists the approximate glass transition temperature of pure

macroscopic scales even when both polymer blocks poly(n-alkyl methacrylate) polymers.

reside above their respective glass-transition tempera- To obtain doped samples, the block copolymer and LiCF3-

ture (Tg) values. A study of electrolytes composed of a SO3 (lithium triflate) were first dried in a vacuum oven at 70

comb-shaped poly(oligo-oxyethylene methacrylate) block, C for several days. The materials were then transferred into

POEM, and a poly(n-alkyl methacrylate) block showed an inert atmosphere, dissolved in anhydrous THF, and solution

that a low-Tg nonconductive phase results in higher cast into a glass dish. The relative amounts of polymer and

salt were determined by the desired stoichiometric ratio of

conductivity than copolymers with a high-Tg noncon-

ethylene oxide units [EO] to Li+, in this case 8:1. The samples

ductive phase.7

were dried under vacuum prior to experiments.

To confirm and better understand this effect of Tg of

Sample Characterization. Solid-state NMR experiments

the nonconductive phase on electrolyte conductivity in

were performed at 1H, 7Li, and 13C frequencies of 300.13, 116,

this family of block copolymers, we have studied these

and 75.5 MHz, respectively. 1H line width and rotating-frame

systems by solid-state nuclear magnetic resonance relaxation time, T1F, and 7Li line width were measured using

(NMR) line width and relaxation measurements. The a Bruker DSX-300 spectrometer. The 1H line width and T1F

1H rotating-frame relaxation times T 1

1F and H line values were measured on undoped copolymers except POEM-

7Li line widths,

widths of the POEM block, as well as b-PMMA, which does not separate into two domains without

were determined for several nonconductive blocks from addition of LiCF3SO3. The 1H experiments were performed in

the n-alkyl methacrylate family, with Tg values span- a 7 mm variable-temperature magic-angle-spinning (MAS)

ning the range -35 to 100 C. Also, differences in dy- probe at spinning rates of 1 kHz. The 7Li line widths were

determined in a 5 mm diameter coil of a static variable-

namics along the side chain in the comb-shaped conduc-

temperature probe. Typical 90 pulse lengths were 4.0 s for

tive POEM block were investigated by wide-line separa-

the 1H experiments and 3.0 s for the 7Li experiments. The

tion (WISE) NMR experiments with a spin-diffusion

spin lock field for the T1F experiment corresponded to 62 kHz.

mixing time.8,9 The results of these NMR experiments

Reproducible temperature determination is paramount for

are compared to the previously reported ac impedance measuring the small differences in dynamics between the

spectroscopy and DSC measurements. copolymers. The experiments were conducted with a constant

flow of N2 gas through the liquid nitrogen cooled heat

Experimental Section

exchanger and a Bruker temperature controller. The liquid

Materials. Polymers and block copolymers from poly- nitrogen dewar was maintained fully filled. The temperature

(ethylene glycol) methyl ether methacrylate macromonomer was calibrated with methanol twice, and the deviation between

the two calibrations was less than 1 K at all temperatures.

University of Massachusetts. To assess whether the different relaxation rates observed

Universidade de Sao Paulo.

for the poly(ethylene oxide), PEO, side chain is a result of the

Iowa State University.

comb architecture, WISE experiments9 with a spin-diffusion

Massachusetts Institute of Technology.

mixing time were performed at 263 K using a Bruker MSL-

* To whom correspondence should be addressed. K.S.-R.: Tel

300 spectrometer. In the t1 dimension, 48 slices with incre-

515-***-****; Fax 515-***-****; e-mail *****@*******.***.

ments of 10 s were acquired. The carbon and proton 90 pulse

D.J.H.: Tel 781-***-****; Fax 781-***-****; e-mail douglas.

lengths were 2.8 s and 3.6 s, respectively. A cross-polariza-

******@********.***.

10.1021/ma0107049 CCC: $22.00 2002 American Chemical Society

Published on Web 04/16/2002

Macromolecules, Vol. 35, No. 9, 2002 Notes 3773

Figure 2. Temperature dependence of the 1H line width of

Figure 1. Temperature dependence of the 7Li line width in

the peak at 3.8 ppm (PEO side chain) in POEM, POEM-b-

LiCF3SO3-doped POEM-b-PLMA, POEM-b-PBMA, and POEM-

PLMA, and POEM-b-PBMA copolymers. The samples were

b-PMMA copolymers. The Tg s of PLMA, PBMA, and PMMA

measured with a spinning rate of 1 kHz. The line width

are 238, 313, and 373 K, respectively. The uncertainty in

temperature is (1 K. increases with decreasing temperature as the rates of the

segmental motions approach rates below 105/s. The uncertainty

in temperature is (1 K.

tion time of 0.1 ms and a mixing time of 0.1 ms were used in

one experiment, and a longer cross-polarization of 0.5 ms was

used for the second WISE experiment with a mixing time of

100 ms. The signal acquisition time was 8 ms.

Results and Discussion

The 7Li line width measurements show a small, but

measurable, difference between LiCF3SO3-doped block

copolymer electrolytes with different nonconductive

phases, as shown in Figure 1. The segmental motion of

the polymer chains averages orientation-dependent

interactions of the observed nucleus with its environ-

ment. A steep change in the 7Li line width occurs when

Figure 3. Temperature dependence of the 1H T1F relaxation

the motional rate exceeds the frequency width of the

rigid limit spectrum ( 6 kHz). The 7Li line width constants of the peak at 3.8 ppm (PEO side chain) in POEM,

POEM-b-PLMA, and POEM-b-PBMA copolymers. Two T1F

measurements display a shift toward higher tempera-

values are reported for each sample, obtained by fitting the

tures as the Tg of the poly(alkyl methacrylate) block observed decay of the magnetization with the sum of two

increases; the PLMA, PBMA, and PMMA blocks exhibit exponential functions. T1F increases as the motional rate

exceeds B1 ) 4 105/s. The uncertainty in temperature is

glass transitions at 238, 313, and 373 K, respectively.

(1 K.

The 7Li results are in agreement with the ac impedance

spectroscopy measurements, which also show a higher

ionic conductivity and lower activation energy for simple exponential decay, a biexponential function with

POEM-b-PLMA.7 The specific conductivities for doped both long and short T1F values was used to fit the

POEM, POEM-b-PLMA, POEM-b-PBMA, and POEM- relaxation data. The resulting pairs of values are shown

b-PMMA ([EO]:[Li+] ) 20:1) at 30 C were reported to in Figure 3. The short T1F values are nearly independent

be 14, 3.5, 2.0, and 1.6 10-6 S/cm, respectively.7 of temperature for all samples.

(Effects including tortuosity and lower volume fraction According to fundamental relaxation theory, T1F

reaches a minimum for motional rates near 2 62

of conducting phase must be considered when comparing

kHz.10 In the polymers studied here, the long T1F

the homopolymer to the copolymer.) The conductivity

curves of POEM-b-PBMA and POEM-b-PMMA are relaxation component is from segments that move with

shifted toward higher temperatures relative to the rates exceeding the minimum. Thus, a longer T1F

POEM-b-PLMA conductivity curve by approximately 4 corresponds to faster mobility. Comparison of the be-

and 6 K, respectively. These differences are similar to havior of the long T1F component for the various samples

the shifts of 3 and 5 K observed in the 7Li line width shows a trend consistent with the 1H and 7Li line width

experiments. This difference is greater than the uncer- measurements: the PBMA phase decreases the mobility

tainty in temperature, (1 K. in the POEM phase. The T1F behaviors of the POEM-

Measurements of the 1H line width can provide b-PLMA copolymer and the POEM homopolymer at

information on segmental mobility of the polymer. As temperatures greater than the Tg of PLMA are similar.

the motional rate exceeds 105/s, the line width drops This shows that the previously observed difference in

from its rigid-limit value of 50 kHz to hundreds of ionic conductivity is at least partially due to slower

hertz in the melt. 1H line width measurements, shown dynamics of segments and ions in the POEM phase.

in Figure 2, were performed on the undoped copolymers Note that these differences in POEM mobility between

and on pure POEM. A shift in the line width transition all samples as shown by the three types of NMR

is observed in this comparison as well. The differences experiments, although significant, are very small.

between the POEM curve and the POEM-b-PLMA and The disparity between the short and long T1F compo-

POEM-b-PBMA curves are 2 and 5 K, respectively. nents indicates that the chain dynamics is quite com-

To complement the 1H line width data, 1H T1F plex. To explain the persistence of a short T1F component

measurements were also conducted on the undoped over a wide temperature range, it is reasonable to

samples. Since the rotating-frame relaxation data for assume that a gradient of mobility exists along the OEM

the protons of the PEO side chain could not be fit by a side chains and that it slowly shifts toward the less

3774 Notes Macromolecules, Vol. 35, No. 9, 2002

than the domains of the diblock copolymer. This finding

is consistent with the hypothesis that the free ends of

the side chains exhibit fast dynamics relative to the

backbone.

Conclusions

1H and 7Li NMR have shown that the enhanced

conductivity of diblock copolymer electrolytes with a

low-Tg nonconductive phase is at least partially due to

faster chain dynamics in the conductive phase. A higher

Tg nonconductive block shifts the observed dynamics

curves toward a higher temperature; this shift is small

(approximately 5 K) but significant. For a secondary

block whose Tg is comparable to that of POEM, mobility

in the POEM domain is roughly equivalent to that in

POEM homopolymer, despite the ordering of the block

copolymer. This result suggests the intriguing possibil-

ity that the dynamics within the ion-conducting POEM

domain might actually be enhanced above that of POEM

homopolymer by choosing a secondary block whose glass

Figure 4. 2D WISE spectra of POEM-b-PLMA at 263 K with

transition resides substantially below that of POEM.

spin-diffusion mixing and cross-polarization times of (a) 100

s, 100 s and (b) 100 ms, 500 s, respectively. At short CP Investigations to address this hypothesis are currently

and spin diffusion times, the signal of the more rigid units is underway.

observed selectively. The more mobile (narrow peak) compo- A second observation, suggested by the biexponential

nent of the PEO side chain transfers magnetization to the more

behavior of the 1H T1F relaxation curves and confirmed

rigid component during the 100 ms spin-diffusion mixing time.

by WISE experiments with and without 1H spin-

diffusion effects, is that the chain dynamics in the PEO

mobile backbone as the temperature is increased. Thus,

side chain is inhomogeneous. The ends of the short

a fraction of OEM segments would always move with a

chains have much faster motional rates than units near

rate near B1, i.e., at the T1F minimum. While the 1H

the backbone.

line of this slow-moving component must be rather

broad, through 1H spin diffusion its magnetization can

Acknowledgment. Financial support was provided

nevertheless become detectable in the narrow signal

by the Arnold and Mabel Beckman Foundation, the

component of the more mobile segments. The similar

MRSEC program of the National Science Foundation

biexponential behavior of the T1F relaxation curves for

under Awards DMR 98-08941 (A.M.M., D.R.S.) and 98-

the POEM homopolymer shows that this gradient of

09365 (K.S.R.), and the Office of Naval Research under

motion is not due to the diblock structure.

Contract N00014-99-0565. T.J.B. acknowledges the

To confirm the hypothesis that the dynamic hetero-

partial support of Fundacao de Amparo a Pesquisa do

`

geneity is due to a gradient of mobility in the POEM

Estado de Sao Paulo (FAPESP). The authors acknowl-

side chain, 2D WISE experiments were conducted on

edge E. R. deAzevedo for assistance with the NMR

the POEM-b-PLMA sample at 263 K. The WISE tech-

measurements.

nique allows determination of 1H line width of specific

components in inhomogeneous systems. The pulse se- References and Notes

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limit on the size of the dynamic inhomogeneity is shorter MA0107049

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