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Thermal properties of fullerene (C60) containing poly(alkyl methacrylate)s
Thermochimica Acta 557 (2013) 55–60
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal properties of fullerene (C60 ) containing poly(alkyl methacrylate)s Rashmi Katiyar a , Dibyendu S. Bag b,∗ , Indira Nigam a a b
Department of Plastic Technology, H.B. Technological Institute, Kanpur 208 002, India Defence Materials and Stores Research and Development Establishment (DMSRDE), Kanpur 208 013, India
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 27 January 2013 Accepted 28 January 2013 Available online 16 February 2013 Keywords: Fullerene containing polymers Glass transition temperature Thermal degradation
a b s t r a c t In this investigation, thermal properties of fullerene (C60 ) containing poly(alkyl methacrylate)s have been studied. The glass transition temperatures (Tg ) and thermal stability of C60 containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA) were more than that of the corresponding virgin polymers and the values increased with the increasing fullerene content in such polymers. Chemically linked bulky fullerene imparts chain rigidity and hence restricts segmental motion causing higher Tg values for C60 containing polymers than that of the polymers without fullerene. Thermal degradation of C60 containing poly(alkyl methacrylate)s is hindered prominently due to the presence of fullerene which scavenges the macroradicals formed during polymer degradation. The mechanism of thermal degradation of C60 containing poly(alkyl methacrylate)s is, therefore, associated mainly with three types of reactions: (a) end chain scission, (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals obtained in thermal degradation process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The chemistry of fullerene (C60 ) has opened up an avenue to new research of materials and its applications since the discovery of fullerene in 1985 [1–3]. Fullerene possesses a wide range of attractive properties such as super conductivity, ferromagnetism, mechanical strength, thermal stability etc. But utilization of such unique properties of fullerene has been largely hampered because of the poor processability of fullerene. Hence incorporation of fullerene into polymer systems has been recognized as a simple means of combining the unique properties of fullerene with macromolecular characteristics such as good processability, flexibility and mechanical strength, thermal stability [4–7]. As a result, synthesis of fullerene containing polymers has attracted much interest among polymer and material scientists. The suitably designed fullerene containing polymers would be well processable and these may also exhibit novel properties which are superior to those of the parent forms and thus generates new fullerene based specialty materials [8–11]. There have been several experimental studies on the thermal stability of fullerene (C60 ) including its decomposition kinetics to understand the insights into the chemistry of fullerene [12,13]. It was also observed that solid C60 decomposes into amorphous carbon upon heat treatment beyond 993 K for 24 h [13]. Fullerene additive also influences the thermodegradation
∗ Corresponding author. Tel.: +91 9839634983. E-mail address: ds [email protected] (D.S. Bag). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.01.027
of acrylic polymers by improving the thermal stability [14,15]. There are also reports that fullerene (C60 ) retards the thermal depolymerization of poly(methyl methacrylate) (PMMA) and autocatalytic thermal dehydrochlorination of poly(vinyl chloride) (PVC) at elevated temperatures and also improves the thermal stability of other fullerene based polymeric systems [16–20]. However, there are no detailed reports of describing the thermal degradation of fullerene containing polymers and the mechanism associated with such thermal decomposition reactions and the effect of fullerene in such degradation process. In our earlier communications, we described the synthesis and characterization of fullerene (C60 ) containing poly(methyl methacrylate) and poly(n-butyl methacrylate) [21,22]. The kinetic aspects of polymerization in the presence and absence of fullerene have been studied. It was also observed that fullerene acts as an effective radical scavenger in the polymerization of vinyl monomers thereby decreasing the rate of polymerization and increasing the activation energy. In this paper, we report the thermal properties of such fullerene containing polymers i.e., fullerene (C60 ) containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA). Thermal properties of fullerene containing polymers have been studied using DSC, TGA and DTGA analyses. These C60 containing poly(alkyl methacrylate)s (i.e., FMMA and FBMA) were studied for their glass transition temperatures (Tg ), thermal degradation and the estimation of C60 content in such polymers from the char residues at TGA thermograms. The mechanism of thermal degradation of fullerene containing polymers has also been elucidated.
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Scheme 1. Schematic representation of synthesis of fullerene containing poly(alkyl methacrylate)s.
2. Experimental 2.1. Materials Monomers, methyl methacrylate (MMA) and n-butyl methacrylate (n-BMA) (E. Merck, India) were purified by the standard method [23]. Solvents (o-dichlorobenzene and dioxan) were used as received. Tetraphenylcyclopentadiene (Acros, UK), p-toluene sulphonyl hydrazide (Aldrich, USA), Cu(II) bishexafluoroacetyl acetonato (Merck, Germany), fullerene (99.9 + %, Lancaster, UK) were used as received. Bismuthonium ylide was prepared by the method of Lloyd [24]. 2.2. Synthesis of C60 containing poly(alkyl methacrylate)s The polymerizations of alkyl methacrylates (MMA and n-BMA) in the presence of fullerene (C60 ) with different molar ratios were carried out using bismuthoniumylide as an initiator in dioxan medium under an inert atmosphere of nitrogen for 4 h at 70 ◦ C as per our reported procedure [21,22]. The concentration of fullerene was varied in feed composition and hence fullerene containing polymers of varied fullerene content were obtained. The initiator concentration for polymerization was taken as 1 wt% of the monomer. The fullerene containing polymers i.e., FMMA and FBMA (Scheme 1) thus formed in each case was precipitated by using acidified methanol and dried to constant weight. The samples (FMMA and FBMA) were purified by giving appropriate solvent treatment to remove the homopolymers and finally again dried to constant weight. The purified C60 containing polymers were characterized. 2.3. Thermal characterization
a heating rate of 20 ◦ C/min under N2 atmosphere. The fullerene contents in the samples were estimated from the char residue at 500 ◦ C. 3. Results and discussion Fullerene (C60 ) containing poly(alkyl methacrylate)s were synthesized by polymerizations of alkyl methacrylates (MMA and n-BMA) in the presence of fullerene (C60 ) using bismuthoniumylide as an initiator under nitrogen atmosphere for 4 h at 70 ◦ C. The concentration of fullerene in feed composition was taken differently in different polymerization systems and hence fullerene containing polymers of varied fullerene content were obtained (Table 1). The molecular weights of fullerene containing poly(methyl methacrylate) (FMMA) were in the range of ¯ n = 1.34 − 1.50 × 105 and M ¯ w = 2.55 − 2.80 × 105 , whereas that M of fullerene containing poly(n-butyl methacrylate) (FBMA) were: ¯ n = 1.12 − 1.66 × 104 and M ¯ w = 3.25 − 4.14 × 104 [21,22]. In M this investigation, thermal properties of such fullerene containing polymers i.e., FMMA and FBMA were studied using DSC, TGA and DTGA analyses. The amount of fullerene actually incorporated into a polymer was determined from the char residue in the TGA thermogram of the fullerene containing polymer. 3.1. Glass transition temperature of fullerene (C60 ) containing poly(alkyl methacrylate)s The DSC curves of C60 containing poly(methyl methacrylate) (FMMA) having different fullerene content are shown in Fig. 1 and the results are given in Table 1. The glass transition temperature in PMMA is reported to be 109 ◦ C [25]. Therefore, it is clear from the
The purified fullerene (C60 ) containing polymer samples (FMMA and FBMA) were taken for thermal characterization. The thermal properties of such fullerene (C60 ) containing poly(alkyl methacrylate)s were investigated by differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermogravimetry (DTG). 2.3.1. Differential scanning calorimetry (DSC) DSC curves of the samples were obtained using a DSC Q200 differential scanning calorimeter (TA Instruments) with a heating rate of 10 ◦ C/min under N2 atmosphere. The glass transition temperatures of the samples were measured following a thermal cycle from ambient to highest temperature set for the sample and back to the ambient temperature and then again to that temperature; all at 10 ◦ C/min. The first cycle was used to dry the samples. The results given are from the second heating cycle. The Tg values were obtained at the inflection point of the jump of heat capacity. 2.3.2. Thermogravimetry (TG) TGA and DTGA thermograms of the samples were taken from TGA/SDTA 851 Thermogravimetric Analyzer (Mettler Toledo) at
Fig. 1. DSC thermograms of FMMA polymer samples.
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Table 1 Effect of fullerene (C60 ) on the glass transition and thermal degradation of fullerene containing poly(alkyl methacrylate)s. Sample code
Feed composition: amount of C60 /monomer/ initiator a
Glass transition temperatures (Tg ◦ C)
Onset of decomposition (◦ C)
Char residue at 500 ◦ C (wt%)
Conc. of fullerene (wt%) calculated by TGA analysis
PMMA FMMA-1 FMMA-II FMMA-III
0/1000/10 30/1000/100 100/1000/10 150/1000/10
109b 124.4 130.1 143.4
270 275 280 290
0.4 1.6 6.3 8.5
0 1.2 5.9 8.1
PBMA FBMA-I FBMA-II FBMA-III
0/1000/10 10/1000/10 25/1000/10 40/1000/10
55c 75.0 80.0 82.4
240 250 265 280
1.2 1.6 2.0 3.2
0 0.4 0.8 2.0
a b c
All values are in mg, solvent: dioxane, temperature: 70 ◦ C and time: 4 h. Data from [25]. Data from [26].
table that the glass transition temperature (Tg ) of fullerene containing polymers are more than that of virgin PMMA. The glass transition temperature also increases from 124.4 ◦ C to 143.4 ◦ C with the increasing fullerene content in the polymers (Table 1). The bulky fullerene (C60 ) chemically bonded to the polymer chain imparts rigidity to the polymer molecules and thereby restricts the segmental movements of the polymer chains. The restriction on the segmental motion of the polymers is low when C60 content is low but the restriction increases with increasing C60 content. Hence glass transition temperature also increases with the increase of C60 content in the polymers. The effect of fullerene content on the glass transition temperature was also studied for the C60 containing poly(n-butyl methacrylate) (FBMA) polymers. The DSC curves of C60 containing polymers (FBMA) are shown in Fig. 2. The glass transition temperature of virgin PBMA is reported to be 55 ◦ C [26] which is much lower than that of virgin PMMA. This may occur due to the fact that the bulky pendant n-butyl group present in PBMA limits the close packing of the polymer chains together. Thus the polymer chains are away from each other thereby giving more free volume. Moreover, the pendant n-butyl group (n-C4 H9 ) is flexible. These factors facilitate easy segmental movements of the virgin PBMA chains resulting in low Tg value than that of virgin PMMA. The incorporation of fullerene in poly(n-butyl methacrylate) also leads
Fig. 2. DSC thermograms of FBMA polymer samples.
to an increase in the glass transition temperature of the fullerene containing polymers because of imparting chain rigidity due the fullerene (C60 ) moiety (Table 1). The glass transition temperature also increases with the increase in fullerene concentration in the polymers. Thus it increases from 75 ◦ C (in polymers containing 0.4 wt% C60 ) to 82.4 ◦ C (in polymers with 2.0 wt% C60 ). Hence in both the cases, it is observed that incorporation of fullerene in the polymers increases the glass transition temperature which continues to increase further with the increasing fullerene content in the polymers. 3.2. Thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s Fullerene containing poly(alkyl methacrylate)s (FMMA and FBMA) were studied using thermogravimetry (TG) and differential thermogravimetry (DTG) analyses to understand the thermal stability and effect of fullerene on the thermal stability of such polymers. Figs. 3 and 4 represent TGA curves of fullerene containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA) polymers respectively. The onset temperatures of thermal degradation of fullerene containing poly(alkyl methacrylate) polymers (FMMA and FBMA) are higher than that of the corresponding virgin PMMA and PBMA polymers (Table 1). Again onset of thermal degradation increases from 275 ◦ C to 290 ◦ C when
Fig. 3. TGA curves of FMMA samples (a) FMMA-III; (b) FMMA-II; (c) FMMA-I; (d) PMMA.
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Fig. 4. TGA curves of FBMA samples (a) FBMA-III; (b) FBMA-II; (c) FBMA-I; (d) PBMA (inset: the magnified TGA curves of the samples from 500 ◦ C to 800 ◦ C).
Fig. 5. DTGA curves of FMMA samples (a) FMMA-III; (b) FMMA-II; (c) FMMA-I; (d) PMMA.
fullerene content in the polymers increases from 1.6 wt% to 8.1 wt%; while it increases from 250 to 280 ◦ C with increasing fullerene content from 0.4 wt% to 2.0 wt% (Table 1). Incorporation of fullerene in the poly(alkyl methacrylate) polymers increases the onset temperature of degradation as fullerene is an effective radical scavenger. It captures the macroradicals formed during the degradation process and retards the thermal degradation of the polymers. The char residue at 500 ◦ C is observed to be more for the fullerene containing polymers having more fullerene content than that of the virgin polymer and polymers with low fullerene content (Table 1). This may be because of no degradation of fullerene at this temperature. Poly(alkyl methacrylate)s e.g., PMMA and PBMA usually completely degrades out above 450 ◦ C leaving almost no residue. On the other hand, C60 undergoes no weight loss up to 500 ◦ C [27]. 3.3. Determination of C60 content in fullerene containing polymers by TGA analysis The char residue data of fullerene containing poly(alkyl methacrylate)s (FMMA and FBMA) was used to determine the fullerene content in the polymers. Fullerene undergoes minor or no weight loss at 500 ◦ C [27] while poly(alkyl methacrylate)s almost completely degrade out at 500 ◦ C [25,28]. The weight percent of C60 incorporated in the fullerene containing polymers was determined from the remaining char residue data at 500 ◦ C obtained after eliminating the char residue due to virgin polymers (polymers without fullerene) at the same temperature [29]. The following equation was used to calculate the C60 content in the polymers: Cf (wt%) = CRf − CRP where, Cf , concentration of fullerene (wt%) incorporated in the polymers; CRf , char residue (wt%) of fullerene containing polymers at 500 ◦ C and CRp , char residue (wt%) of the polymers without fullerene at 500 ◦ C. The results thus obtained are given in Table 1. The values of fullerene content in the fullerene containing polymers calculated by this method are lower than taken in feed which is obvious. The DTGA curves of fullerene containing poly(alkyl methacrylate)s i.e., FMMA and FBMA are shown in Figs. 5 and 6. It is clearly observed from these curves that all these fullerene containing polymers are undergone two-staged thermal degradation. The peak temperatures of thermal degradation of fullerene containing
Fig. 6. DTGA curves of FBMA samples (a) FBMA-III; (b) FBMA-II; (c) FBMA-I; (d) PBMA.
polymers are more than that of the corresponding virgin polymers and it increases with increasing fullerene content in both the stages of decomposition. The extent of increase of decomposition temperature with respect to the decomposition of virgin polymers (i.e., T1(max) and T2(max) ) for both the stages of decomposition is given in Table 2. The increase of first stage degradation temperature T1(max) is marginal and is observed to have slight improvement with increasing fullerene content in the polymers. However, the second Table 2 Effect of fullerene (C60 ) on the thermal decomposition of fullerene containing poly(alkyl methacrylate)s. Sample code
T1(max) (◦ C)
T1(max) (◦ C)
T2(max) (◦ C)
T2(max) (◦ C)
PMMA FMMA-I FMMA-II FMMA-III
307 308 310 313
0 1 3 6
380 399 403 407
0 19 23 27
PBMA FBMA-I FBMA-II FBMA-III
291 294 295 297
0 3 4 8
345 349 394 398
0 4 49 53
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Scheme 2. Schematic representation of mechanism of thermal degradation of C60 containing poly(alkyl methacrylate)s.
step degradation temperature, T2(max) increases significantly with increasing fullerene content in the polymers. Fullerene is well known as a radical scavenger [21,22]. As soon as a macroradical is formed in the degradation, fullerene captures the macroradical and stabilizes the system. The above observation may be due to the fact that fullerene moiety acted as an efficient radical scavenger that captured the macroradicals formed in the first stage of thermal degradation of polymers. Hence fullerene retards the process of thermal degradation in the second stage thereby prominently increasing T2(max) . Hence, T2(max) is much higher than that of T1(max) (Table 2). 3.4. Mechanism of thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s The mechanism of thermal degradation of fullerene containing poly(alkyl methacrylate)s is depicted in Scheme 2. Both
poly(methyl methacrylate) (PMMA) and poly(n-butyl methacrylate) (PBMA) have undergone two-staged thermal degradation which has been clearly observed in their DTGA curves (Figs. 5 and 6). The first stage is associated with the end chain scission and the second stage with the random chain scission. However, fullerene captures the macroradicals formed in the first stage of thermal degradation effectively and that is why the value of T2(max) is much more than that of T1(max) (Table 2). Hence the thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s is associated with main three types of reactions: (a) end chain scission (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals formed during thermal degradation. The first stage i.e., (a) end chain scission involves the breaking of a small unit or group at the end of the chain resulting in the formation of an active macroradical (I) and an allyl radical (II) (Scheme 2). The second stage i.e., (b) random chain scission is the breaking of a main chain bond at a random position to produce radicals (III)
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and an unsaturated moiety (IV). The radical (III) formed by random chain scission then unzips to yield monomer (VI) and a radical (V) which finally produces a polyene structure known as char (X) [30]. Depolymerization to monomer as a result of thermal degradation has been well reported in PMMA and PBMA [31–33]. However, the inhibiting influence of fullerene (C60 ) on thermal degradation of poly(alkyl methacrylate)s is connected with the interaction of fullerene with the macroradical (I) formed. Fullerene retards the process of thermal degradation, particularly the second stage of degradation and increases the onset temperature of degradation thereby providing thermal stability to the polymers. Fullerene captures the macroradical (I) formed in the first stage of thermal degradation and forms an adduct (VII). The adduct (VII) thus formed then abstracts H. from another polymer chain to form products like (VIII) and (IX) which is then converted into polyene structure (char) by the abstraction of alkoxy-carbonyl side group. 4. Conclusion The glass transition temperatures (Tg ) of fullerene (C60 ) containing poly(alkyl methacrylate)s i.e., C60 containing poly(methyl methacrylate) (FMMA) and C60 containing poly(n-butyl methacrylate) (FBMA) are observed to be more than that of the corresponding virgin polymers. Again the Tg increases with the increase of fullerene content in the polymers. The bulky fullerene moiety chemically linked in the polymers imparts chain rigidity and hence restrict the segmental motion of the polymer chains causing high Tg values. Incorporation of fullerene in the polymers increases the onset temperature of polymer degradation/decomposition and also temperature of maximum weight loss due to radical scavenging effect of fullerene. Char residue at 500 ◦ C also increases with increasing fullerene content. The char residue data at 500 ◦ C was used to determine the C60 content in the fullerene containing polymers. The values of fullerene content determined from TGA analysis are lower than the feed which is obvious. The thermal degradation of fullerene containing polymers involves two-staged degradation and is associated with three types of reactions: (a) end chain scission (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals produced during thermal degradation. In the absence of fullerene, the macroradical formed in the degradation process unzips to yield the monomer. However, the unzipping process is inhibited in the presence of fullerene as the macroradicals formed during degradation are captured by the fullerene and forming adducts. Thus fullerene acts as an effective stabilizer to increase the thermal stability of the poly(alkyl methacrylate)s. Acknowledgements The Director of H.B.T.I. Kanpur and the Director of DMSRDE Kanpur are gratefully acknowledged for providing the necessary facilities and support required to carry out the research work. References [1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60 : Buckminsterfullerene, Nature 318 (1985) 162. [2] D.J.D. Moet, P. de Bruyn, J.D. Kotlarski, P.W.M. Blom, Enhanced efficiency in double junction polymer: fullerene solar cells, Org. Electron. 11 (11) (2010) 1821–1827. [3] V.A. Kostyanovsky, D.K. Susarova, A.S. Peregudov, P.A. Troshin, Polymerizable fullerene based material for organic solar cells, Thin Solid Films 519 (2011) 4119–4122.
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal properties of fullerene (C60 ) containing poly(alkyl methacrylate)s Rashmi Katiyar a , Dibyendu S. Bag b,∗ , Indira Nigam a a b
Department of Plastic Technology, H.B. Technological Institute, Kanpur 208 002, India Defence Materials and Stores Research and Development Establishment (DMSRDE), Kanpur 208 013, India
a r t i c l e
i n f o
Article history: Received 24 July 2012 Received in revised form 27 January 2013 Accepted 28 January 2013 Available online 16 February 2013 Keywords: Fullerene containing polymers Glass transition temperature Thermal degradation
a b s t r a c t In this investigation, thermal properties of fullerene (C60 ) containing poly(alkyl methacrylate)s have been studied. The glass transition temperatures (Tg ) and thermal stability of C60 containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA) were more than that of the corresponding virgin polymers and the values increased with the increasing fullerene content in such polymers. Chemically linked bulky fullerene imparts chain rigidity and hence restricts segmental motion causing higher Tg values for C60 containing polymers than that of the polymers without fullerene. Thermal degradation of C60 containing poly(alkyl methacrylate)s is hindered prominently due to the presence of fullerene which scavenges the macroradicals formed during polymer degradation. The mechanism of thermal degradation of C60 containing poly(alkyl methacrylate)s is, therefore, associated mainly with three types of reactions: (a) end chain scission, (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals obtained in thermal degradation process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The chemistry of fullerene (C60 ) has opened up an avenue to new research of materials and its applications since the discovery of fullerene in 1985 [1–3]. Fullerene possesses a wide range of attractive properties such as super conductivity, ferromagnetism, mechanical strength, thermal stability etc. But utilization of such unique properties of fullerene has been largely hampered because of the poor processability of fullerene. Hence incorporation of fullerene into polymer systems has been recognized as a simple means of combining the unique properties of fullerene with macromolecular characteristics such as good processability, flexibility and mechanical strength, thermal stability [4–7]. As a result, synthesis of fullerene containing polymers has attracted much interest among polymer and material scientists. The suitably designed fullerene containing polymers would be well processable and these may also exhibit novel properties which are superior to those of the parent forms and thus generates new fullerene based specialty materials [8–11]. There have been several experimental studies on the thermal stability of fullerene (C60 ) including its decomposition kinetics to understand the insights into the chemistry of fullerene [12,13]. It was also observed that solid C60 decomposes into amorphous carbon upon heat treatment beyond 993 K for 24 h [13]. Fullerene additive also influences the thermodegradation
∗ Corresponding author. Tel.: +91 9839634983. E-mail address: ds [email protected] (D.S. Bag). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.01.027
of acrylic polymers by improving the thermal stability [14,15]. There are also reports that fullerene (C60 ) retards the thermal depolymerization of poly(methyl methacrylate) (PMMA) and autocatalytic thermal dehydrochlorination of poly(vinyl chloride) (PVC) at elevated temperatures and also improves the thermal stability of other fullerene based polymeric systems [16–20]. However, there are no detailed reports of describing the thermal degradation of fullerene containing polymers and the mechanism associated with such thermal decomposition reactions and the effect of fullerene in such degradation process. In our earlier communications, we described the synthesis and characterization of fullerene (C60 ) containing poly(methyl methacrylate) and poly(n-butyl methacrylate) [21,22]. The kinetic aspects of polymerization in the presence and absence of fullerene have been studied. It was also observed that fullerene acts as an effective radical scavenger in the polymerization of vinyl monomers thereby decreasing the rate of polymerization and increasing the activation energy. In this paper, we report the thermal properties of such fullerene containing polymers i.e., fullerene (C60 ) containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA). Thermal properties of fullerene containing polymers have been studied using DSC, TGA and DTGA analyses. These C60 containing poly(alkyl methacrylate)s (i.e., FMMA and FBMA) were studied for their glass transition temperatures (Tg ), thermal degradation and the estimation of C60 content in such polymers from the char residues at TGA thermograms. The mechanism of thermal degradation of fullerene containing polymers has also been elucidated.
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Scheme 1. Schematic representation of synthesis of fullerene containing poly(alkyl methacrylate)s.
2. Experimental 2.1. Materials Monomers, methyl methacrylate (MMA) and n-butyl methacrylate (n-BMA) (E. Merck, India) were purified by the standard method [23]. Solvents (o-dichlorobenzene and dioxan) were used as received. Tetraphenylcyclopentadiene (Acros, UK), p-toluene sulphonyl hydrazide (Aldrich, USA), Cu(II) bishexafluoroacetyl acetonato (Merck, Germany), fullerene (99.9 + %, Lancaster, UK) were used as received. Bismuthonium ylide was prepared by the method of Lloyd [24]. 2.2. Synthesis of C60 containing poly(alkyl methacrylate)s The polymerizations of alkyl methacrylates (MMA and n-BMA) in the presence of fullerene (C60 ) with different molar ratios were carried out using bismuthoniumylide as an initiator in dioxan medium under an inert atmosphere of nitrogen for 4 h at 70 ◦ C as per our reported procedure [21,22]. The concentration of fullerene was varied in feed composition and hence fullerene containing polymers of varied fullerene content were obtained. The initiator concentration for polymerization was taken as 1 wt% of the monomer. The fullerene containing polymers i.e., FMMA and FBMA (Scheme 1) thus formed in each case was precipitated by using acidified methanol and dried to constant weight. The samples (FMMA and FBMA) were purified by giving appropriate solvent treatment to remove the homopolymers and finally again dried to constant weight. The purified C60 containing polymers were characterized. 2.3. Thermal characterization
a heating rate of 20 ◦ C/min under N2 atmosphere. The fullerene contents in the samples were estimated from the char residue at 500 ◦ C. 3. Results and discussion Fullerene (C60 ) containing poly(alkyl methacrylate)s were synthesized by polymerizations of alkyl methacrylates (MMA and n-BMA) in the presence of fullerene (C60 ) using bismuthoniumylide as an initiator under nitrogen atmosphere for 4 h at 70 ◦ C. The concentration of fullerene in feed composition was taken differently in different polymerization systems and hence fullerene containing polymers of varied fullerene content were obtained (Table 1). The molecular weights of fullerene containing poly(methyl methacrylate) (FMMA) were in the range of ¯ n = 1.34 − 1.50 × 105 and M ¯ w = 2.55 − 2.80 × 105 , whereas that M of fullerene containing poly(n-butyl methacrylate) (FBMA) were: ¯ n = 1.12 − 1.66 × 104 and M ¯ w = 3.25 − 4.14 × 104 [21,22]. In M this investigation, thermal properties of such fullerene containing polymers i.e., FMMA and FBMA were studied using DSC, TGA and DTGA analyses. The amount of fullerene actually incorporated into a polymer was determined from the char residue in the TGA thermogram of the fullerene containing polymer. 3.1. Glass transition temperature of fullerene (C60 ) containing poly(alkyl methacrylate)s The DSC curves of C60 containing poly(methyl methacrylate) (FMMA) having different fullerene content are shown in Fig. 1 and the results are given in Table 1. The glass transition temperature in PMMA is reported to be 109 ◦ C [25]. Therefore, it is clear from the
The purified fullerene (C60 ) containing polymer samples (FMMA and FBMA) were taken for thermal characterization. The thermal properties of such fullerene (C60 ) containing poly(alkyl methacrylate)s were investigated by differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermogravimetry (DTG). 2.3.1. Differential scanning calorimetry (DSC) DSC curves of the samples were obtained using a DSC Q200 differential scanning calorimeter (TA Instruments) with a heating rate of 10 ◦ C/min under N2 atmosphere. The glass transition temperatures of the samples were measured following a thermal cycle from ambient to highest temperature set for the sample and back to the ambient temperature and then again to that temperature; all at 10 ◦ C/min. The first cycle was used to dry the samples. The results given are from the second heating cycle. The Tg values were obtained at the inflection point of the jump of heat capacity. 2.3.2. Thermogravimetry (TG) TGA and DTGA thermograms of the samples were taken from TGA/SDTA 851 Thermogravimetric Analyzer (Mettler Toledo) at
Fig. 1. DSC thermograms of FMMA polymer samples.
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Table 1 Effect of fullerene (C60 ) on the glass transition and thermal degradation of fullerene containing poly(alkyl methacrylate)s. Sample code
Feed composition: amount of C60 /monomer/ initiator a
Glass transition temperatures (Tg ◦ C)
Onset of decomposition (◦ C)
Char residue at 500 ◦ C (wt%)
Conc. of fullerene (wt%) calculated by TGA analysis
PMMA FMMA-1 FMMA-II FMMA-III
0/1000/10 30/1000/100 100/1000/10 150/1000/10
109b 124.4 130.1 143.4
270 275 280 290
0.4 1.6 6.3 8.5
0 1.2 5.9 8.1
PBMA FBMA-I FBMA-II FBMA-III
0/1000/10 10/1000/10 25/1000/10 40/1000/10
55c 75.0 80.0 82.4
240 250 265 280
1.2 1.6 2.0 3.2
0 0.4 0.8 2.0
a b c
All values are in mg, solvent: dioxane, temperature: 70 ◦ C and time: 4 h. Data from [25]. Data from [26].
table that the glass transition temperature (Tg ) of fullerene containing polymers are more than that of virgin PMMA. The glass transition temperature also increases from 124.4 ◦ C to 143.4 ◦ C with the increasing fullerene content in the polymers (Table 1). The bulky fullerene (C60 ) chemically bonded to the polymer chain imparts rigidity to the polymer molecules and thereby restricts the segmental movements of the polymer chains. The restriction on the segmental motion of the polymers is low when C60 content is low but the restriction increases with increasing C60 content. Hence glass transition temperature also increases with the increase of C60 content in the polymers. The effect of fullerene content on the glass transition temperature was also studied for the C60 containing poly(n-butyl methacrylate) (FBMA) polymers. The DSC curves of C60 containing polymers (FBMA) are shown in Fig. 2. The glass transition temperature of virgin PBMA is reported to be 55 ◦ C [26] which is much lower than that of virgin PMMA. This may occur due to the fact that the bulky pendant n-butyl group present in PBMA limits the close packing of the polymer chains together. Thus the polymer chains are away from each other thereby giving more free volume. Moreover, the pendant n-butyl group (n-C4 H9 ) is flexible. These factors facilitate easy segmental movements of the virgin PBMA chains resulting in low Tg value than that of virgin PMMA. The incorporation of fullerene in poly(n-butyl methacrylate) also leads
Fig. 2. DSC thermograms of FBMA polymer samples.
to an increase in the glass transition temperature of the fullerene containing polymers because of imparting chain rigidity due the fullerene (C60 ) moiety (Table 1). The glass transition temperature also increases with the increase in fullerene concentration in the polymers. Thus it increases from 75 ◦ C (in polymers containing 0.4 wt% C60 ) to 82.4 ◦ C (in polymers with 2.0 wt% C60 ). Hence in both the cases, it is observed that incorporation of fullerene in the polymers increases the glass transition temperature which continues to increase further with the increasing fullerene content in the polymers. 3.2. Thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s Fullerene containing poly(alkyl methacrylate)s (FMMA and FBMA) were studied using thermogravimetry (TG) and differential thermogravimetry (DTG) analyses to understand the thermal stability and effect of fullerene on the thermal stability of such polymers. Figs. 3 and 4 represent TGA curves of fullerene containing poly(methyl methacrylate) (FMMA) and poly(n-butyl methacrylate) (FBMA) polymers respectively. The onset temperatures of thermal degradation of fullerene containing poly(alkyl methacrylate) polymers (FMMA and FBMA) are higher than that of the corresponding virgin PMMA and PBMA polymers (Table 1). Again onset of thermal degradation increases from 275 ◦ C to 290 ◦ C when
Fig. 3. TGA curves of FMMA samples (a) FMMA-III; (b) FMMA-II; (c) FMMA-I; (d) PMMA.
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Fig. 4. TGA curves of FBMA samples (a) FBMA-III; (b) FBMA-II; (c) FBMA-I; (d) PBMA (inset: the magnified TGA curves of the samples from 500 ◦ C to 800 ◦ C).
Fig. 5. DTGA curves of FMMA samples (a) FMMA-III; (b) FMMA-II; (c) FMMA-I; (d) PMMA.
fullerene content in the polymers increases from 1.6 wt% to 8.1 wt%; while it increases from 250 to 280 ◦ C with increasing fullerene content from 0.4 wt% to 2.0 wt% (Table 1). Incorporation of fullerene in the poly(alkyl methacrylate) polymers increases the onset temperature of degradation as fullerene is an effective radical scavenger. It captures the macroradicals formed during the degradation process and retards the thermal degradation of the polymers. The char residue at 500 ◦ C is observed to be more for the fullerene containing polymers having more fullerene content than that of the virgin polymer and polymers with low fullerene content (Table 1). This may be because of no degradation of fullerene at this temperature. Poly(alkyl methacrylate)s e.g., PMMA and PBMA usually completely degrades out above 450 ◦ C leaving almost no residue. On the other hand, C60 undergoes no weight loss up to 500 ◦ C [27]. 3.3. Determination of C60 content in fullerene containing polymers by TGA analysis The char residue data of fullerene containing poly(alkyl methacrylate)s (FMMA and FBMA) was used to determine the fullerene content in the polymers. Fullerene undergoes minor or no weight loss at 500 ◦ C [27] while poly(alkyl methacrylate)s almost completely degrade out at 500 ◦ C [25,28]. The weight percent of C60 incorporated in the fullerene containing polymers was determined from the remaining char residue data at 500 ◦ C obtained after eliminating the char residue due to virgin polymers (polymers without fullerene) at the same temperature [29]. The following equation was used to calculate the C60 content in the polymers: Cf (wt%) = CRf − CRP where, Cf , concentration of fullerene (wt%) incorporated in the polymers; CRf , char residue (wt%) of fullerene containing polymers at 500 ◦ C and CRp , char residue (wt%) of the polymers without fullerene at 500 ◦ C. The results thus obtained are given in Table 1. The values of fullerene content in the fullerene containing polymers calculated by this method are lower than taken in feed which is obvious. The DTGA curves of fullerene containing poly(alkyl methacrylate)s i.e., FMMA and FBMA are shown in Figs. 5 and 6. It is clearly observed from these curves that all these fullerene containing polymers are undergone two-staged thermal degradation. The peak temperatures of thermal degradation of fullerene containing
Fig. 6. DTGA curves of FBMA samples (a) FBMA-III; (b) FBMA-II; (c) FBMA-I; (d) PBMA.
polymers are more than that of the corresponding virgin polymers and it increases with increasing fullerene content in both the stages of decomposition. The extent of increase of decomposition temperature with respect to the decomposition of virgin polymers (i.e., T1(max) and T2(max) ) for both the stages of decomposition is given in Table 2. The increase of first stage degradation temperature T1(max) is marginal and is observed to have slight improvement with increasing fullerene content in the polymers. However, the second Table 2 Effect of fullerene (C60 ) on the thermal decomposition of fullerene containing poly(alkyl methacrylate)s. Sample code
T1(max) (◦ C)
T1(max) (◦ C)
T2(max) (◦ C)
T2(max) (◦ C)
PMMA FMMA-I FMMA-II FMMA-III
307 308 310 313
0 1 3 6
380 399 403 407
0 19 23 27
PBMA FBMA-I FBMA-II FBMA-III
291 294 295 297
0 3 4 8
345 349 394 398
0 4 49 53
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Scheme 2. Schematic representation of mechanism of thermal degradation of C60 containing poly(alkyl methacrylate)s.
step degradation temperature, T2(max) increases significantly with increasing fullerene content in the polymers. Fullerene is well known as a radical scavenger [21,22]. As soon as a macroradical is formed in the degradation, fullerene captures the macroradical and stabilizes the system. The above observation may be due to the fact that fullerene moiety acted as an efficient radical scavenger that captured the macroradicals formed in the first stage of thermal degradation of polymers. Hence fullerene retards the process of thermal degradation in the second stage thereby prominently increasing T2(max) . Hence, T2(max) is much higher than that of T1(max) (Table 2). 3.4. Mechanism of thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s The mechanism of thermal degradation of fullerene containing poly(alkyl methacrylate)s is depicted in Scheme 2. Both
poly(methyl methacrylate) (PMMA) and poly(n-butyl methacrylate) (PBMA) have undergone two-staged thermal degradation which has been clearly observed in their DTGA curves (Figs. 5 and 6). The first stage is associated with the end chain scission and the second stage with the random chain scission. However, fullerene captures the macroradicals formed in the first stage of thermal degradation effectively and that is why the value of T2(max) is much more than that of T1(max) (Table 2). Hence the thermal degradation of fullerene (C60 ) containing poly(alkyl methacrylate)s is associated with main three types of reactions: (a) end chain scission (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals formed during thermal degradation. The first stage i.e., (a) end chain scission involves the breaking of a small unit or group at the end of the chain resulting in the formation of an active macroradical (I) and an allyl radical (II) (Scheme 2). The second stage i.e., (b) random chain scission is the breaking of a main chain bond at a random position to produce radicals (III)
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and an unsaturated moiety (IV). The radical (III) formed by random chain scission then unzips to yield monomer (VI) and a radical (V) which finally produces a polyene structure known as char (X) [30]. Depolymerization to monomer as a result of thermal degradation has been well reported in PMMA and PBMA [31–33]. However, the inhibiting influence of fullerene (C60 ) on thermal degradation of poly(alkyl methacrylate)s is connected with the interaction of fullerene with the macroradical (I) formed. Fullerene retards the process of thermal degradation, particularly the second stage of degradation and increases the onset temperature of degradation thereby providing thermal stability to the polymers. Fullerene captures the macroradical (I) formed in the first stage of thermal degradation and forms an adduct (VII). The adduct (VII) thus formed then abstracts H. from another polymer chain to form products like (VIII) and (IX) which is then converted into polyene structure (char) by the abstraction of alkoxy-carbonyl side group. 4. Conclusion The glass transition temperatures (Tg ) of fullerene (C60 ) containing poly(alkyl methacrylate)s i.e., C60 containing poly(methyl methacrylate) (FMMA) and C60 containing poly(n-butyl methacrylate) (FBMA) are observed to be more than that of the corresponding virgin polymers. Again the Tg increases with the increase of fullerene content in the polymers. The bulky fullerene moiety chemically linked in the polymers imparts chain rigidity and hence restrict the segmental motion of the polymer chains causing high Tg values. Incorporation of fullerene in the polymers increases the onset temperature of polymer degradation/decomposition and also temperature of maximum weight loss due to radical scavenging effect of fullerene. Char residue at 500 ◦ C also increases with increasing fullerene content. The char residue data at 500 ◦ C was used to determine the C60 content in the fullerene containing polymers. The values of fullerene content determined from TGA analysis are lower than the feed which is obvious. The thermal degradation of fullerene containing polymers involves two-staged degradation and is associated with three types of reactions: (a) end chain scission (b) random chain scission and (c) reaction of fullerene (C60 ) with macroradicals produced during thermal degradation. In the absence of fullerene, the macroradical formed in the degradation process unzips to yield the monomer. However, the unzipping process is inhibited in the presence of fullerene as the macroradicals formed during degradation are captured by the fullerene and forming adducts. Thus fullerene acts as an effective stabilizer to increase the thermal stability of the poly(alkyl methacrylate)s. Acknowledgements The Director of H.B.T.I. Kanpur and the Director of DMSRDE Kanpur are gratefully acknowledged for providing the necessary facilities and support required to carry out the research work. References [1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60 : Buckminsterfullerene, Nature 318 (1985) 162. [2] D.J.D. Moet, P. de Bruyn, J.D. Kotlarski, P.W.M. Blom, Enhanced efficiency in double junction polymer: fullerene solar cells, Org. Electron. 11 (11) (2010) 1821–1827. [3] V.A. Kostyanovsky, D.K. Susarova, A.S. Peregudov, P.A. Troshin, Polymerizable fullerene based material for organic solar cells, Thin Solid Films 519 (2011) 4119–4122.
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