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On the interaction between the potassium—GIC and unsaturated hydrocarbons
St,u,JIhlTflRIIETflll£ mSTf ILS ELSEVIER
Synthetic Metals 96 (1998) 229-233
On the interaction between the potassium-GIC and unsaturated hydrocarbons H. Shioyama *, K. Tatsumi, N. Iwashita, K. Fujita, Y. Sawada Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 28 May i998; accepted 29 May 1998
Abstract KCs and KC24 were allowed to react with several unsaturated hydrocarbons. After contact with isoprene, styrene or 1,3-butadiene, potassiumgraphite intercalation compounds (GICs) expanded slowly along the c-axis direction. Molecules of unsaturated hydrocarbons are considered to be introduced into the interlayer spacing of GICs and polymerized progressively. When acrylonitrile, 1-butene or isobutene is used as the unsaturated hydrocarbon, the interaction between GICs and hydrocarbons is quite different. Monomer molecules are polymerized only in the vicinity of the sample edge, which enhances the stability of potassium--GICs in water. Two other kinds of interaction between alkali metalGICs and unsaturated hydrocarbons, i.e. intercalation with and without oligomerization, are also reported elsewhere by several authors. We classified the interaction into four categories mainly according to the reactivity of unsaturated hydrocarbon for polymerization. © 1998 Elsevier Science S.A. All rights reserved. Ke}words: Graphite intercalation compounds; Chemical treatment; Unsaturated hydrocarbons; Interlayer spacing
1. Introduction The chemical reaction in the interlayer spacing of graphite has been a subject of interest during recent years. Because environmental factors of the chemical reaction are peculiar, i.e. the reaction is undergone in a two-dimensional opening where the strong influence from the graphite layer is expected, novel reaction products are thus expected. In the previous paper we reported the reduction of metal chloride in the interlayer [ 1-3] and reviewed reactions of metal chloride in the interlayer [4]. It is also known that some organic molecules, which are co-intercalated into graphite intercalation compounds (GICs) with alkali metals, react in the interlayer spacing of graphite [ 5 ]. Especially when unsaturated hydrocarbons are co-intercalated, they are found to be polymerized within the interlayer spacing of the alkali metal-GICs. For a few organic molecules such as benzene [6-8] or ethylene [9,10] the polymerization process and the product have been studied in detail. The polymerization occurs after intercalation, and the stoichiometry of organic molecule/carbon at the end of the polymerization is almost similar to that at the end of the intercalation. * Cor~esDndiag author.
The present work describes the interaction between potassium-GICs, KC8 and KC24, and several kinds of unsaturated hydrocarbons. Some hydrocarbons are introduced into the interlayer spacing of potassium-GICs to be polymerized, and others improve the stability of GICs in the ambient air or in water without changing their appearance.
2. Experimental Slabs (5 × 1 ×0.25 mm, about 2.5 mg) of Union Carbide HOPG were used as the starting material. For preparation of KC8 and KC24 the conventional two-bulb method was employed. The reaction tube of Pyrex glass was sealed under vacuum and placed in a two-zone furnace for 4 days. When the temperatures were controlled at Tg = 280°C for the graphite and Ti =250°C for the intercalate, KCs was obtained. KC24 was synthesized when the temperatures were set at Tg=410°C and Ti= 250°C. A sample of KC8 or KCz4 was transferred to a reaction chamber of Pyrex glass in vacuo through a glass break-seal, to be allowed to react with unsaturated hydrocarbons. The liquid hydrocarbons, isoprene, styrene and acrylonitrile, were dehydrated with molecular sieve 4 A, (Merck) and were purified by vacuum distillation. The reaction of potassium-GICs with the vapour of liquid hydrocarbons was carried out at
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All fights reserved.
PIIS0379-6779(98)00098-8
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[4. Shioyama et aL / Synthetic Metals 96 (1998) 229-233
room temperature. The gaseous hydrocarbons, t ,3-butadiene, 1-butene and isobutene, were purchased from Iwatani Industrial Gases (purity: 99.0% up) and were used without further purification. The gas pressure in the reaction chamber was kept constant at 67 kPa, and potassium--GICs were allowed to react at room temperature. The reaction products were analysed by X-ray powder diffractometry (Rigaku, LINT-1500) and thermogravimetry in a nitrogen stream (MAC Science, TG-DTA 2000).
.,= o
.==
3. Results
Several tens of minutes after contact with the vapour of isoprene, KC24 was found to have expanded along the c-axis direction. The expansion proceeded slowly for several hours until the source of about 1 g isoprene was exhausted. The product of a black colour resembles exfoliated graphite in shape; however, gaps characteristic of exfoliated graphite cannot be found. The bulk density of the product is not as low as for exfoliated graphite but almost identical to the commercially available polyisoprene. Molecules of isoprene are considered to be introduced into the interlayer spacing of KC24and potymerized progressively. As shown in Fig. 1, the product stopped up a glass tube with a diameter of 1 cm which was used for a reaction chamber. The product was elastic, like commercially available polyisoprene. When the same procedure was applied for KCs, a similar product was also obtained. In such a case the rate of this reaction was slower than that in the reaction system of KC24, i.e. it took a few days for the KC8 sample to absorb about 1 g of isoprene. X-ray powder diffraction patterns of the products from KC8 and KC24 are shown in Fig. 2(b) and (c), respectively. No diffraction line for graphite or GIC was observed, and the broad peak with a shoulder resembles the pattern of the control polyisoprene shown in Fig. 2(a). Commercially available polyisoprene was used as the control. As shown in Fig. 3, the curves of thermogravimetry (TG) and differential thermal analysis (DTA) with respect to the products from the reaction of isoprene with KCs and KC24
Fig. 1. The productfrom the reactionof isoprene with KC24.
20
40
60
80
20(CuK~) deg. Fig. 2. X-ray powder diffractionpatterns of (a) control polyisoprene,~nd products fromthe reactionof isoprenewith (b) KC8 and (c) KC24.
(a)
100 .
.
.
.
.
.
- oi
°
-)
g
~1oo
, , ,
1
oi---> looi
260 a~ 6~ 860 looo
T (oc) Fig. 3. TG and DTA curvesof (a) controlpolyisoprene,and productsfrom the reaction of isoprene with (b) KC8 and (c) KC24in a nitrogen stream. Heating rate, 200°C/h. are quite similar to those for the control polyisoprene. The polymers were vaporized or decomposed to be completely removed at about 400°C and the residue originated from the HOPG. In Fig. 4, X-ray diffraction patterns of the product from the reaction of isoprene with KC24 after heat-treatment at 500 and 1000°C are shown. After heat-treatment at 500°C, very weak diffraction lines of graphite were observed, which means that the graphite layers are not ordered even if the polyisoprene is removed. The sample heat-treated at 1000°C gives relatively strong diffraction lines of graphite; however, the intensity is less than one hundredth of that of pristine HOPG.
231
H. Shioyama et al. / Synthetic Metals 96 (I998) 229-233
Table i Chemical analysis data of the products from the reaction of KCa4 with some monomers
g
Monomer ]
,
,
i
i
,
i
~ (b) .i
't
'
i
Observed Isoprene 0.039 Styrene 0.037 1,3-Butadiene 0.049
~ I
20
40
I
60
Similar products were obtained when KCs or KC24 was allowed to react with the vapour of styrene or the gas of 1,3butadiene. X-ray powder patterns and the TG data of the products are quite similar to those for the control polymer, in the same manner as the case of isoprene. Examples of X-ray powder patterns and TG data are shown in Figs. 5 and 6, respectively. Table 1 shows the results of chemical analysis of the products from the reaction of KC24 with some monomers. The observed values were obtained for samples synthesized from
i
I
i
1
i
I
1
(b)
20
40
C Calculated
Observed
Calculated
0.075 0.055 0.078
88.3 90.7 86.5
88.2 92.2 88.8
8o
28(CuK~z) deg. Fig. 4. X-ray powder diffraction patterns of the product from the reaction of isoprene with KCz4 after heat-treatment (a) at 500°C and (b) at 1000°C.
•=
Content (wt.%) K
r
60
80
about 2.5 mg of HOPG and about 1 g of monomer. The calculated data were obtained with the exact amount of HOPG which derives the theoretical amount of potassium and the exact amount of product, which lead to the theoretical amount of the responsible monomer. Although the value depends on the amount of starting material and is individual for each run, it should be emphasized that a large portion of potassium is kept in the reaction product. When acrylonitrile, 1-butene or isobutene is used as the unsaturated hydrocarbon, the interaction with potassiumGICs is quite different. Even if KCs or KC24 was treated with such a hydrocarbon for more than 1 month, the sample did not absorb the hydrocarbon and therefore no expansion could be detected. For example, the X-ray powder diffraction pattern suggests the coexistence of stage-2 and stage-3 structures of potassium-GIC in the sample, which was obtained after the treatment of KC24 with 1-butene, and no other phase was observed. This sample proved to be relatively stable in water for a long time; after the immersion in water for 11 days, the sample decomposed to stage-4 potassium-GIC. Only half of the potassium atoms were expelled from the interlayer of graphite, and this kind of high stability is exceptional for potassium-GICs. Similar high stability was also found for the combination of potassium---GIC and acrylonitrite or isobutene.
2e (CuK~) de~. Fig. 5. X-ray powder diffraction patterns of products from the reaction of
KCs with (a) styreneand (b) 1,3-butadiene.
4. Discussion
0
,_.
g
so
(a) t
<
s0
100 0
200 400 600 800 1000
T
(°C)
Fig. 6. TG and DTA curves of products from the reaction of KC8 with (a)
sq/rene and (b) 1,3-buta~ieuein a nitrogenstream.Heatingrate,200°C/h.
The expansion of potassium--GICs depends on the introduction of the unsaturated hydrocarbon into the interlayer spacing of potassium-GIC. The introduced hydrocarbon should be strongly influenced by graphite layers and potassium atoms. The reaction scheme for the polymerization of styrene initiated by aromatic radical-anions such as sodium naphthalene is shown in Fig. 7 [ 11 ]. The naphthalene anionradical transfers an electron to a monomer such as styrene to form the styryl radical-anion, and then anionic propagation occurs at both carbanion ends of the styryl dianion. The introduced monomer of isoprene, styrene and 1,3-butadiene is considered to be under the influence of the negatively charged graphite layer of potassium-GICs, and the polymerization starts by the same mechanism described above. Such anionic polymerization takes place under the condition where there
232
H. Shioyama et aL /Synthetic Metals 96 (1998) 229-233
+
CO
CH2= C H C6H5
2
'CH2-CH- N +
~
Na+-CH-CH2-CH2
I
I
C6H5
I
C 6 H~
Na+ -CH - C H 2- C H 2- CI-I-Na+ I
I
C6 H5
-CH-Na+
l
C6 H5
•C H 2- C H - N+
C6 H5
( n + m ) C H 2= C H +
C6H5
C6H5
+ ['-CH-CH2-fCH-CH2 CH -Crrr- CH2-CH- 1 r t -t +," i ,m C+H, C+H, C+H, C+H,J Fig. 7. The reaction scheme for the polymerization of styrene [ 11 ].
is no effective termination reaction, i.e. propagation occurs with complete consumption of monomer to form living polymers. When the product obtained by the reaction of KC24 with isoprene taken out from the reaction chamber was washed with water and then allowed to react again with isoprene in a new reaction chamber, an increase in the weight of the sample was observed. This supports the fact that the hydrocarbon is polymerized by the anionic mechanism. To elucidate the initial step of the expansion of potassiumGICs, the absorption of 1,3-butadiene into KC2+ was stopped at the early step. The absorption elicited 70% augmentation in the weight of sample, and a slight expansion was observed only at the edge of the slab. The broad peak for polybutadiene and a series of 00l lines of the stage-3 potassium--GIC were observed in the X-ray powder diffraction pattern of the sample. These show that absorbed monomer molecules do not result in the formation of a ternary potassium-monomerGIC, but that the polymerization starts prior to a thorough diffusion of monomer in the interlayer of graphite. Furthermore, this preference of polymerization to the diffusion in the interlayer might give a clue to explain the fact that the expansion of KCs is observed even if most organic molecules are reported not to be co-intercalated into KC8. Taking into account the mechanism of polymerization, we propose a schematic diagram of the GIC-initiated polymerization, which is shown in Fig. 8. This schematic model of the product suggests that the stacked graphite layers are drawn apart, which gives no diffraction line of graphite or GIC. The potassium-GIC is considered to be consumed for the polymerization as an initiator and to be dispersed in the product. When this product is heat-treated at 500°C, all the polymer is volatilized and graphite layers remain. However, graphite layers are not stacked back in a similar manner to the starting
graphite
K-G t C
- /'" ~ K~=~ ~ ~v K
Fig. 8. Schematic diagram showing the GIC-initiated polymerization: (K) potassium; (l-q) unsaturated hydrocarbon.
HOPG at this temperature. It is easy to accept that the crystallinity of graphite becomes higher if the sample is treated at higher temperature. We have tried to obtain the degree of polymerization for the product. A brief calculation suggests that it is about 3800 if 1 g of isoprene is polymerized with a number of radical anions in a slab of 2.5 mg KC24.Here we assumed that one electron transfers from each potassium atom to the graphite layer in KC24. The second group of unsaturated hydrocarbons, acrylonitrile, 1-butene and isobutene, cannot be introduced into KCs or KCe4 at all, and hence no expansion was found. Monomer molecules are polymerized only at the surface of the GIC edge, which stabilizes the GIC to water. At the peripheral part of potassium--GICs, electrons are also transferred from the potassium to graphite layer, and the polymerization starts in the vicinity of the GIC edge with the mechanism shown in Fig. 7. In the literature two other categories for the interaction of alkali metat-GICs with unsaturated hydrocarbons are reported. They are listed in Table 2 together with two cate-
233
H. Shioyama et al. / Synthetic Metals 96 (1998) 229-233
Table 2 Interaction of alkali metal-GICs with unsaturated hydrocarbons Category
Host GIC
Unsaturated hydrocarbon
Refs. and remarks
(A) Expansion of GIC
KC24
isoprene, styrene, 1,3-butadiene
this work
(B) Co-intercalationwith subsequent oligomerization
KC24 CSC24 CsC24
benzene ethylene ethylene acrylonitrile
[7], at 60°C [ 12] [ 9], at R.T. [ 13]
(C) Co-intercalationwithout oligomerization
KCz~ CsC24 CsC24
benzene propyrene ethylene
[7], at R.T. [ 14] [9], at - 79"C
(D) No introduction of hydrocarbon into GIC
KC~
acrylonitrile, I-butene, isobutene
this work
KC24
gories discussed in this paper. The classification is firstly made as to whether the hydrocarbon is introduced into the GIC (Categories A, B and C) or not (Category D ) . When the reactivity of introduced monomer for polymerization is low, nothing happens on the monomer, i.e. the monomer is easily removed by evacuation (Category C). When the reactivity is moderate, the monomer is oligomerized in the interlayer (Category B ) . It is worth remarking that ethylene in CsCz4 which is oligomerized at room temperature has a higher reactivity than benzene in KC24 which begins to oligomerize at 60°C. The introduction of monomer for categories B and C could be expressed in the word co-intercalation, because the products are ternary GICs with characteristic c-axis repeat distances. The expansion of GICs is supposed to be observed if the reactivity of monomer is high enough (Category A ) . One possibility to explain that hydrocarbons of category D are not introduced into KC24 is their high reactivity. In other words, if the reactivity for polymerization is too high, the monomer is supposed to be p o l y m e r i z e d j u s t at the peripheral part of the interlayer as soon as it is introduced, which might prevent the introduction of the majority of monomer into the inner part of KC2,~. To examine this explanation, KC24 was allowed to react with 1-butene or isobutene at a pressure of 1.3 kPa for 1 month. No introduction, however, was observed, even if the amount of monomer to be poly-
merized at the edge of the GIC was small. This result would not support the explanation described above.
References [ 1] H. Shioyama, H. Sakakihara, N. Iwashita, K. Tatsumi, Y. Sawada, J. Mater. Sci. Lett. 13 (1994) 1056. [2] H. Shioyama, H. Sakakihara, H. Enomoto, N. Iwashita, K. Tatsumi, Y. Sawada, J. Mater. Sci. Lett. I5 (1996) 453. [3] H. Shioyama, H. Sakakihara, J. Chem. Soc. Jpn. (1996) 673. [4] H. Shioyama, Tanso 178 (1997) 128. [5] J. Jegoudez, C. Mazieres, R. Setton, Synth. Met. 7 (1983) 85. [6] S. Matsuzaki, M. Sano, Chem. Phys. Lett. 115 (1985) 424. [7] S. Matsuzaki, M. Taniguchi, M. Sano, Synth. Met. 16 (1986) 343. [8] H. Shioyama, Y. Wakukawa, Y. Sawada, Mol. Cryst. Liq. Cryst., in press. [9] Y. Takahashi, K. Oi, T. TerN, N. Akuzawa, Carbon 29 (1991) 283. [10] H. Pilliere, Y. Takahashi, T. Yoneoka, T. Otosaka, N. Akuzawa, Synth. Met. 59 (1993) 191. [ 11] G. Odian, Principles of PoIymerization, Wiley, New York, 3rd edn., 1991, pp. 400-403. [ 12] H. Takahashi, Y. Takahashi, N. Akuzawa, Pro<:.22rid Ann. Meet. of the Japanese Carbon Society, Nagasaki, Japan, 1995, Paper 1A03, p. 8. [13] Y. Takahashi, H. Takahashi, Y. Ishikawa, N. Akuzawa, Proc. 23rd Ann. Meet. of the Japanese Carbon Society, Ibaragi, Japan, i996, Paper 2A07, p. 136. [ 14] Y. Takahashi, Tanso 160 (1993) 301.
Synthetic Metals 96 (1998) 229-233
On the interaction between the potassium-GIC and unsaturated hydrocarbons H. Shioyama *, K. Tatsumi, N. Iwashita, K. Fujita, Y. Sawada Osaka National Research Institute, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan Received 28 May i998; accepted 29 May 1998
Abstract KCs and KC24 were allowed to react with several unsaturated hydrocarbons. After contact with isoprene, styrene or 1,3-butadiene, potassiumgraphite intercalation compounds (GICs) expanded slowly along the c-axis direction. Molecules of unsaturated hydrocarbons are considered to be introduced into the interlayer spacing of GICs and polymerized progressively. When acrylonitrile, 1-butene or isobutene is used as the unsaturated hydrocarbon, the interaction between GICs and hydrocarbons is quite different. Monomer molecules are polymerized only in the vicinity of the sample edge, which enhances the stability of potassium--GICs in water. Two other kinds of interaction between alkali metalGICs and unsaturated hydrocarbons, i.e. intercalation with and without oligomerization, are also reported elsewhere by several authors. We classified the interaction into four categories mainly according to the reactivity of unsaturated hydrocarbon for polymerization. © 1998 Elsevier Science S.A. All rights reserved. Ke}words: Graphite intercalation compounds; Chemical treatment; Unsaturated hydrocarbons; Interlayer spacing
1. Introduction The chemical reaction in the interlayer spacing of graphite has been a subject of interest during recent years. Because environmental factors of the chemical reaction are peculiar, i.e. the reaction is undergone in a two-dimensional opening where the strong influence from the graphite layer is expected, novel reaction products are thus expected. In the previous paper we reported the reduction of metal chloride in the interlayer [ 1-3] and reviewed reactions of metal chloride in the interlayer [4]. It is also known that some organic molecules, which are co-intercalated into graphite intercalation compounds (GICs) with alkali metals, react in the interlayer spacing of graphite [ 5 ]. Especially when unsaturated hydrocarbons are co-intercalated, they are found to be polymerized within the interlayer spacing of the alkali metal-GICs. For a few organic molecules such as benzene [6-8] or ethylene [9,10] the polymerization process and the product have been studied in detail. The polymerization occurs after intercalation, and the stoichiometry of organic molecule/carbon at the end of the polymerization is almost similar to that at the end of the intercalation. * Cor~esDndiag author.
The present work describes the interaction between potassium-GICs, KC8 and KC24, and several kinds of unsaturated hydrocarbons. Some hydrocarbons are introduced into the interlayer spacing of potassium-GICs to be polymerized, and others improve the stability of GICs in the ambient air or in water without changing their appearance.
2. Experimental Slabs (5 × 1 ×0.25 mm, about 2.5 mg) of Union Carbide HOPG were used as the starting material. For preparation of KC8 and KC24 the conventional two-bulb method was employed. The reaction tube of Pyrex glass was sealed under vacuum and placed in a two-zone furnace for 4 days. When the temperatures were controlled at Tg = 280°C for the graphite and Ti =250°C for the intercalate, KCs was obtained. KC24 was synthesized when the temperatures were set at Tg=410°C and Ti= 250°C. A sample of KC8 or KCz4 was transferred to a reaction chamber of Pyrex glass in vacuo through a glass break-seal, to be allowed to react with unsaturated hydrocarbons. The liquid hydrocarbons, isoprene, styrene and acrylonitrile, were dehydrated with molecular sieve 4 A, (Merck) and were purified by vacuum distillation. The reaction of potassium-GICs with the vapour of liquid hydrocarbons was carried out at
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All fights reserved.
PIIS0379-6779(98)00098-8
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[4. Shioyama et aL / Synthetic Metals 96 (1998) 229-233
room temperature. The gaseous hydrocarbons, t ,3-butadiene, 1-butene and isobutene, were purchased from Iwatani Industrial Gases (purity: 99.0% up) and were used without further purification. The gas pressure in the reaction chamber was kept constant at 67 kPa, and potassium--GICs were allowed to react at room temperature. The reaction products were analysed by X-ray powder diffractometry (Rigaku, LINT-1500) and thermogravimetry in a nitrogen stream (MAC Science, TG-DTA 2000).
.,= o
.==
3. Results
Several tens of minutes after contact with the vapour of isoprene, KC24 was found to have expanded along the c-axis direction. The expansion proceeded slowly for several hours until the source of about 1 g isoprene was exhausted. The product of a black colour resembles exfoliated graphite in shape; however, gaps characteristic of exfoliated graphite cannot be found. The bulk density of the product is not as low as for exfoliated graphite but almost identical to the commercially available polyisoprene. Molecules of isoprene are considered to be introduced into the interlayer spacing of KC24and potymerized progressively. As shown in Fig. 1, the product stopped up a glass tube with a diameter of 1 cm which was used for a reaction chamber. The product was elastic, like commercially available polyisoprene. When the same procedure was applied for KCs, a similar product was also obtained. In such a case the rate of this reaction was slower than that in the reaction system of KC24, i.e. it took a few days for the KC8 sample to absorb about 1 g of isoprene. X-ray powder diffraction patterns of the products from KC8 and KC24 are shown in Fig. 2(b) and (c), respectively. No diffraction line for graphite or GIC was observed, and the broad peak with a shoulder resembles the pattern of the control polyisoprene shown in Fig. 2(a). Commercially available polyisoprene was used as the control. As shown in Fig. 3, the curves of thermogravimetry (TG) and differential thermal analysis (DTA) with respect to the products from the reaction of isoprene with KCs and KC24
Fig. 1. The productfrom the reactionof isoprene with KC24.
20
40
60
80
20(CuK~) deg. Fig. 2. X-ray powder diffractionpatterns of (a) control polyisoprene,~nd products fromthe reactionof isoprenewith (b) KC8 and (c) KC24.
(a)
100 .
.
.
.
.
.
- oi
°
-)
g
~1oo
, , ,
1
oi---> looi
260 a~ 6~ 860 looo
T (oc) Fig. 3. TG and DTA curvesof (a) controlpolyisoprene,and productsfrom the reaction of isoprene with (b) KC8 and (c) KC24in a nitrogen stream. Heating rate, 200°C/h. are quite similar to those for the control polyisoprene. The polymers were vaporized or decomposed to be completely removed at about 400°C and the residue originated from the HOPG. In Fig. 4, X-ray diffraction patterns of the product from the reaction of isoprene with KC24 after heat-treatment at 500 and 1000°C are shown. After heat-treatment at 500°C, very weak diffraction lines of graphite were observed, which means that the graphite layers are not ordered even if the polyisoprene is removed. The sample heat-treated at 1000°C gives relatively strong diffraction lines of graphite; however, the intensity is less than one hundredth of that of pristine HOPG.
231
H. Shioyama et al. / Synthetic Metals 96 (I998) 229-233
Table i Chemical analysis data of the products from the reaction of KCa4 with some monomers
g
Monomer ]
,
,
i
i
,
i
~ (b) .i
't
'
i
Observed Isoprene 0.039 Styrene 0.037 1,3-Butadiene 0.049
~ I
20
40
I
60
Similar products were obtained when KCs or KC24 was allowed to react with the vapour of styrene or the gas of 1,3butadiene. X-ray powder patterns and the TG data of the products are quite similar to those for the control polymer, in the same manner as the case of isoprene. Examples of X-ray powder patterns and TG data are shown in Figs. 5 and 6, respectively. Table 1 shows the results of chemical analysis of the products from the reaction of KC24 with some monomers. The observed values were obtained for samples synthesized from
i
I
i
1
i
I
1
(b)
20
40
C Calculated
Observed
Calculated
0.075 0.055 0.078
88.3 90.7 86.5
88.2 92.2 88.8
8o
28(CuK~z) deg. Fig. 4. X-ray powder diffraction patterns of the product from the reaction of isoprene with KCz4 after heat-treatment (a) at 500°C and (b) at 1000°C.
•=
Content (wt.%) K
r
60
80
about 2.5 mg of HOPG and about 1 g of monomer. The calculated data were obtained with the exact amount of HOPG which derives the theoretical amount of potassium and the exact amount of product, which lead to the theoretical amount of the responsible monomer. Although the value depends on the amount of starting material and is individual for each run, it should be emphasized that a large portion of potassium is kept in the reaction product. When acrylonitrile, 1-butene or isobutene is used as the unsaturated hydrocarbon, the interaction with potassiumGICs is quite different. Even if KCs or KC24 was treated with such a hydrocarbon for more than 1 month, the sample did not absorb the hydrocarbon and therefore no expansion could be detected. For example, the X-ray powder diffraction pattern suggests the coexistence of stage-2 and stage-3 structures of potassium-GIC in the sample, which was obtained after the treatment of KC24 with 1-butene, and no other phase was observed. This sample proved to be relatively stable in water for a long time; after the immersion in water for 11 days, the sample decomposed to stage-4 potassium-GIC. Only half of the potassium atoms were expelled from the interlayer of graphite, and this kind of high stability is exceptional for potassium-GICs. Similar high stability was also found for the combination of potassium---GIC and acrylonitrite or isobutene.
2e (CuK~) de~. Fig. 5. X-ray powder diffraction patterns of products from the reaction of
KCs with (a) styreneand (b) 1,3-butadiene.
4. Discussion
0
,_.
g
so
(a) t
<
s0
100 0
200 400 600 800 1000
T
(°C)
Fig. 6. TG and DTA curves of products from the reaction of KC8 with (a)
sq/rene and (b) 1,3-buta~ieuein a nitrogenstream.Heatingrate,200°C/h.
The expansion of potassium--GICs depends on the introduction of the unsaturated hydrocarbon into the interlayer spacing of potassium-GIC. The introduced hydrocarbon should be strongly influenced by graphite layers and potassium atoms. The reaction scheme for the polymerization of styrene initiated by aromatic radical-anions such as sodium naphthalene is shown in Fig. 7 [ 11 ]. The naphthalene anionradical transfers an electron to a monomer such as styrene to form the styryl radical-anion, and then anionic propagation occurs at both carbanion ends of the styryl dianion. The introduced monomer of isoprene, styrene and 1,3-butadiene is considered to be under the influence of the negatively charged graphite layer of potassium-GICs, and the polymerization starts by the same mechanism described above. Such anionic polymerization takes place under the condition where there
232
H. Shioyama et aL /Synthetic Metals 96 (1998) 229-233
+
CO
CH2= C H C6H5
2
'CH2-CH- N +
~
Na+-CH-CH2-CH2
I
I
C6H5
I
C 6 H~
Na+ -CH - C H 2- C H 2- CI-I-Na+ I
I
C6 H5
-CH-Na+
l
C6 H5
•C H 2- C H - N+
C6 H5
( n + m ) C H 2= C H +
C6H5
C6H5
+ ['-CH-CH2-fCH-CH2 CH -Crrr- CH2-CH- 1 r t -t +," i ,m C+H, C+H, C+H, C+H,J Fig. 7. The reaction scheme for the polymerization of styrene [ 11 ].
is no effective termination reaction, i.e. propagation occurs with complete consumption of monomer to form living polymers. When the product obtained by the reaction of KC24 with isoprene taken out from the reaction chamber was washed with water and then allowed to react again with isoprene in a new reaction chamber, an increase in the weight of the sample was observed. This supports the fact that the hydrocarbon is polymerized by the anionic mechanism. To elucidate the initial step of the expansion of potassiumGICs, the absorption of 1,3-butadiene into KC2+ was stopped at the early step. The absorption elicited 70% augmentation in the weight of sample, and a slight expansion was observed only at the edge of the slab. The broad peak for polybutadiene and a series of 00l lines of the stage-3 potassium--GIC were observed in the X-ray powder diffraction pattern of the sample. These show that absorbed monomer molecules do not result in the formation of a ternary potassium-monomerGIC, but that the polymerization starts prior to a thorough diffusion of monomer in the interlayer of graphite. Furthermore, this preference of polymerization to the diffusion in the interlayer might give a clue to explain the fact that the expansion of KCs is observed even if most organic molecules are reported not to be co-intercalated into KC8. Taking into account the mechanism of polymerization, we propose a schematic diagram of the GIC-initiated polymerization, which is shown in Fig. 8. This schematic model of the product suggests that the stacked graphite layers are drawn apart, which gives no diffraction line of graphite or GIC. The potassium-GIC is considered to be consumed for the polymerization as an initiator and to be dispersed in the product. When this product is heat-treated at 500°C, all the polymer is volatilized and graphite layers remain. However, graphite layers are not stacked back in a similar manner to the starting
graphite
K-G t C
- /'" ~ K~=~ ~ ~v K
Fig. 8. Schematic diagram showing the GIC-initiated polymerization: (K) potassium; (l-q) unsaturated hydrocarbon.
HOPG at this temperature. It is easy to accept that the crystallinity of graphite becomes higher if the sample is treated at higher temperature. We have tried to obtain the degree of polymerization for the product. A brief calculation suggests that it is about 3800 if 1 g of isoprene is polymerized with a number of radical anions in a slab of 2.5 mg KC24.Here we assumed that one electron transfers from each potassium atom to the graphite layer in KC24. The second group of unsaturated hydrocarbons, acrylonitrile, 1-butene and isobutene, cannot be introduced into KCs or KCe4 at all, and hence no expansion was found. Monomer molecules are polymerized only at the surface of the GIC edge, which stabilizes the GIC to water. At the peripheral part of potassium--GICs, electrons are also transferred from the potassium to graphite layer, and the polymerization starts in the vicinity of the GIC edge with the mechanism shown in Fig. 7. In the literature two other categories for the interaction of alkali metat-GICs with unsaturated hydrocarbons are reported. They are listed in Table 2 together with two cate-
233
H. Shioyama et al. / Synthetic Metals 96 (1998) 229-233
Table 2 Interaction of alkali metal-GICs with unsaturated hydrocarbons Category
Host GIC
Unsaturated hydrocarbon
Refs. and remarks
(A) Expansion of GIC
KC24
isoprene, styrene, 1,3-butadiene
this work
(B) Co-intercalationwith subsequent oligomerization
KC24 CSC24 CsC24
benzene ethylene ethylene acrylonitrile
[7], at 60°C [ 12] [ 9], at R.T. [ 13]
(C) Co-intercalationwithout oligomerization
KCz~ CsC24 CsC24
benzene propyrene ethylene
[7], at R.T. [ 14] [9], at - 79"C
(D) No introduction of hydrocarbon into GIC
KC~
acrylonitrile, I-butene, isobutene
this work
KC24
gories discussed in this paper. The classification is firstly made as to whether the hydrocarbon is introduced into the GIC (Categories A, B and C) or not (Category D ) . When the reactivity of introduced monomer for polymerization is low, nothing happens on the monomer, i.e. the monomer is easily removed by evacuation (Category C). When the reactivity is moderate, the monomer is oligomerized in the interlayer (Category B ) . It is worth remarking that ethylene in CsCz4 which is oligomerized at room temperature has a higher reactivity than benzene in KC24 which begins to oligomerize at 60°C. The introduction of monomer for categories B and C could be expressed in the word co-intercalation, because the products are ternary GICs with characteristic c-axis repeat distances. The expansion of GICs is supposed to be observed if the reactivity of monomer is high enough (Category A ) . One possibility to explain that hydrocarbons of category D are not introduced into KC24 is their high reactivity. In other words, if the reactivity for polymerization is too high, the monomer is supposed to be p o l y m e r i z e d j u s t at the peripheral part of the interlayer as soon as it is introduced, which might prevent the introduction of the majority of monomer into the inner part of KC2,~. To examine this explanation, KC24 was allowed to react with 1-butene or isobutene at a pressure of 1.3 kPa for 1 month. No introduction, however, was observed, even if the amount of monomer to be poly-
merized at the edge of the GIC was small. This result would not support the explanation described above.
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