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Journal of Nanomaterials
Volume聽2012聽(2012), Article ID聽352937, 8 pages
Research Article

Cast Nanostructured Films of Poly(methyl methacrylate-b-butyl acrylate)/Carbon Nanotubes: Influence of Poly(butyl acrylate) Content on Film Evaporation Rate, Morphology, and Electrical Resistance

Polymer Processing Department, Centro de Investigaci贸n en Qu铆mica Aplicada (CIQA), Boulevard Enrique Reyna, No. 140, Colonia San Jos茅 de los Cerritos, 25294 Saltillo, COAH, Mexico

Received 28 February 2012; Accepted 24 March 2012

Academic Editor: Sevan P.聽Davtyan

Copyright 漏 2012 F. Soriano-Corral et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Nanocomposites of poly(methyl methacrylate-b-butyl acrylate)/multiwalled carbon nanotubes were prepared from different copolymers synthesized by RITP technique using iodine functionalized poly(methyl methacrylate) as macrochain transfer agent to obtain block copolymers with butyl acrylate as comonomer in a sequential copolymerization. Poly(butyl acrylate) contents of 7, 20, and 30鈥墂t% were attained in each copolymer. These copolymers were used to prepare nanostructured films by casting process, using chloroform as solvent, and carboxyl functionalized MWCNT at 0.4, 0.6, 0.8, 1.0, and 1.2鈥墂t%. During the film preparation, the absolute drying rate (饾憗) was calculated with respect to the poly(butyl acrylate) and MWCNT composition. For copolymers containing 7 and 20鈥墂t% of poly(butyl acrylate) the 饾憗 values slightly decrease with the MWCNT concentration, while for the suspension prepared with the copolymer at 30鈥墂t% of poly(butyl acrylate) the 饾憗 values decrease drastically down to 50% approximately. The MWCNT content at the percolation threshold point was found to be 0.8鈥墂t%, for all nanostructured films. The dispersion of MWCNT within the polymer matrix decreased with increasing the poly(butyl acrylate) composition, but it did not affect the electrical properties, which is assumed to be due to induction of the bridging effect and the MWCNT preference to locate into the poly(methyl methacrylate) phase.

1. Introduction

Synthesis of carbon nanotubes (CNTs) was firstly reported by Iijima [1] in the 1990s. CNT present a cylindrical structure and they can be single walled (SWCNT) or multi walled (MWCNT) with an average diameter oscillating between 1 and 50鈥塶m. The incorporation of SWCNT and/or MWCNT in polymeric and/or ceramic materials can result in an improvement of some specific properties with respect to the virgin materials. Properties like mechanical performance in polypropylene, polycarbonate [2], and polyethylene terephthalate (PET) [3]; crystallization rate in poly (L-lactide) [4]; surface electric conductivity in polydimethylsiloxane [5] and PET [6]; and heat transfer in ethylene-vinyl acetate copolymer (EVA) [7] can be increased when CNTs are efficiently incorporated. Concentrations of 0.5鈥2鈥墂t% of CNT provoke electric conductivity in several polymers [2, 5, 6]. Chemical surface modification of CNT is frequently carried out through the addition of hydroxyl or carboxylic acid functionalities allowing a stronger physical interaction between CNT and the polar polymeric matrices. This functionalization will facilitate the CNT dispersion and reduce the amount required to produce a given improvement in properties. Khosla and Gray [6] reported that the percolation threshold鈥攆or resistivity鈥攊n polydimethylsiloxane/CNT compounds, occurred at 2鈥墂t% when using nonmodified CNT, whereas it occurred at 1.5鈥墂t% when using carboxyl-modified CNT. In a study of PET/CNT compounds, Cruz-Delgado et al. [5] reported conductivity values of 11012鈥塖/cm when using 1鈥墂t% of hydroxyl modified CNT, whereas it was cero when using nonmodified CNT. This was attributed to the much better dispersion when using modified CNT. In all cases, however, the addition of CNT into a polymer matrix will increase the modulus, which could be a problem in applications where flexibility and comfort are needed, such as in electrostatic dissipative (ESD) coatings on textile fibers. It is at this point where design of polymer matrices (nature and structure) becomes an important factor to consider, in order to obtain a polymer nanocomposite with desirable properties such as a flexibility and conductivity. The choice of the polymerization method, to obtain homopolymers or copolymers, becomes of great relevance.

Reversible deactivation radical polymerization (RDRP) techniques provide novel routes to synthesize well-defined low-polydispersity block copolymers and other types of complex architectures [810]. The most popular RDRP techniques are nitroxide-mediated polymerization (NMP) [11], atom transfer radical polymerization (ATRP) [12], and reversible addition-fragmentation chain transfer polymerization (RAFT) [13]. A newer and simpler technique, known as reverse iodine transfer polymerization (RITP) [14, 15], relies on the use of molecular iodine to control (and confer functionality) the polymerization. The low cost of molecular iodine and the simplicity of the technique (the chain transfer agents are synthesized in situ at the beginning of the polymerization) are enormous advantages over other techniques. Synthesis of different types of block copolymers using RITP have been reported [1618] demonstrating the efficiency of this technique, regulated by a degenerative transfer mechanism. The controlled synthesis of poly(methyl methacrylate) (PMMA) by RITP in toluene has been reported by Lacroix-Desmazes and coworkers [1921], attaining molecular weights between 5000 and 20000鈥塯/mol.

Here we report the synthesis of three different materials of iodine-functionalized PMMA by RITP, targeting higher molecular weights than those previously reported [19]. Synthesis of block copolymers using the PMMA as macro-chain transfer agents and butyl acrylate are also reported. Thereafter, we studied the effect of poly(butyl acrylate) (PBuA) content on the drying rate, morphology, and the electric properties of nanocomposite films constituted of the copolymer poly(methyl methacrylate-b-butyl acrylate) and carboxyl-modified carbon nanotubes using chloroform (CHCl3) as solvent.

2. Experimental

2.1. Materials

Methyl methacrylate (MMA) and n-butyl acrylate (BuA) were purified by vacuum distillation before use. 2,2鈥-azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. Toluene was distilled before use and molecular iodine (I2) was used as received. All the aforementioned substances were from Aldrich, while the carboxyl-modified MWCNTs, with 0.5鈥3鈥墂t% of COOH, average diameter of 30鈥50鈥塶m, and length of 15鈥20鈥m, were from AlphaNano Technology Co. LTD.

2.2. Synthesis of Copolymers

The procedure for the copolymers synthesis is exemplified following experiment number 1 from Table 1; 45鈥塯 (0.45鈥塵ol) of MMA, 45鈥塯 (0.489鈥塵ol) of toluene, 0.090鈥塯 (0.354鈥塵mol) of I2, and 0.110鈥塯 (0.670鈥塵mol) of AIBN were introduced in a round flask. The mixture was fluxed with argon for 30鈥塵inutes, after which the flask was placed in an oil bath at 70掳C. The polymerization was conducted for 12 hours in the dark, with magnetic stirring and under argon atmosphere. Conversion (X) was determined via 1H NMR analysis on a crude sample of iodine functionalized PMMA and molecular weight distribution was determined by size exclusion chromatography (SEC). Results obtained were 饾憢PMMA=0.85, MnPMMA鈥=鈥44500鈥塯/mol, and Mw/MnPMMA鈥=鈥1.4 (see Table 2 for the complete results). In the flask containing the crude sample (PMMA), 5鈥塯 (0.039鈥塵ol) of BuA, 5鈥塯 (0.054鈥塵ol) of toluene, and 0.042鈥塯 (0.257鈥塵ol) of AIBN were added. In all cases, an [AIBN]/[PMMA]鈥=鈥0.3 was used. Again, the mixture was fluxed with argon for 30鈥塵inutes and the flask was placed in an oil bath at 70掳C. The polymerization was conducted for 12鈥塰ours in the dark, with magnetic stirring and under argon atmosphere, resulting in a poly(methyl methacrylate)-b-poly(butyl acrylate-co-methyl methacrylate). Conversion (X) was determined via 1H NMR analysis on a crude sample of copolymer and molecular weight distribution was determined by size exclusion chromatography (SEC). Results obtained were XPBuA鈥=鈥0.70, MnCopol鈥=鈥48,000鈥塯/mol, and Mw/MnCopol鈥=鈥1.6 (see Table 2 for the complete results). Experiments 2 and 3 were conducted in a similar way, adjusting the monomer mass ratio.

Table 1: Reaction conditions for the synthesis of macrochain transfer agents and block copolymers by RITP.
Table 2: Results of the synthesis of macrochains transfer agents and block copolymers by RITP.
2.3. Characterization of Copolymers

Molecular weights of polymer samples were determined by size exclusion chromatography (SEC) using a Hewlett-Packard instrument (HPLC series 1100) equipped with a refractive index detector. A PLGel-mixed column was used. Calibration was carried out with polystyrene standards and THF (HPLC grade from Sigma-Aldrich) was used as eluent at a flow rate of 1鈥塵L/min.

1H nuclear magnetic resonance (NMR) spectra of polymer samples were obtained with a JEOL Eclipse-300鈥塎Hz spectrometer, using CDCl3.

Morphology of the copolymers was observed in a TITAN transmission electron microscope (TEM). Previously, the compression-molded samples were cut cryogenically at 鈭32掳C and a cut rate of 2鈥塵m/min, in slices of 50鈥塶m thick using an LEICA ultra-microtome, after which the samples were stained using ruthenium tetraoxide (RuO4) vapors to contrast the PBuA phase in the copolymers [22].

Thermal properties of copolymers were analyzed using a V4.3A TA Instruments differential scanning calorimeter (DSC) from 鈭50 to 150掳C at 5掳C/min.

2.4. Nanostructured Films Preparation and Characterization

0.5鈥塯 of the synthesized copolymers was dissolved in 20鈥塵L of chloroform and carboxyl-modified MWCNTs at 0.4, 0.6, 0.8, and 1鈥墂t% were added to form colloidal suspensions. The mixing of colloidal suspensions was made by means of ultrasound at a frequency of 40鈥塳Hz for 1 minute at room temperature (25掳C 卤 2). The colloidal suspensions were deposited on Pyrex glass coverslips, from Aldrich, in an analytical balance and the drying rate (饾憗) was evaluated according to (1):饾憗=饾惪饾憼饾惔饾憫饾懁饾憫饾憽,(1)

where 饾憗 is the drying rate; 饾惪饾憼 is the dry mass; 饾惔 is the evaporation area; 饾憫饾懁/饾憫饾憽 is the mass loss through evaporation as a function of time [23].

The storage modulus of the neat copolymers and nanostructured films was evaluated by means of DMA Q800 dynamical mechanical analyzer in strain mode at a frequency of 1鈥塇z, from 鈭50 to 150掳C at 5掳C/min.

The surface electrical resistance of the prepared films was measured using an ACL 390 Staticide surface resistance meter, and the percolation threshold concentration of MWCNT was established.

To analyze the dispersion of MWCNT in the copolymers matrices, a JEOL scanning electron microscope (SEM) was used.

3. Results and Discussion

3.1. Synthesis of Copolymers

In the RITP mechanism the I2 reacts with fragments of the initiator or with low molecular weight species to generate iodinated chain transfer agents. The consumption of monomer in this step is low and it is called the inhibition period. Once all molecular iodine is consumed, the polymerization period takes place. Three experiments were performed by RITP to produce PMMA with different molecular weights. Polymerizations were carried out in toluene (50鈥墂t%), using AIBN as initiator. A molar ratio AIBN/I2鈥=鈥1.9 was used, which is common in an RITP experiment. Table 2 shows the obtained results for the synthesis section. During the PMMA syntheses high conversions were observed and polydispersity indexes around 1.6 were similar to results reported for RITP of MMA in solution [19]. Conversions close to 0.90 are advantageous to avoid a purification step and continue directly with the formation of the second block since small amount of residual monomer should not severely modify the properties of the second PBuA block. According to;Mn=(gofmonomer)(conversion)2molesofI2+饾憖饾憡chain-ends,(2)

the expected Mn of PMMA in experiment 1 was 54000鈥塯/mol, while the experimental value was determined as 44500鈥塯/mol based on a polystyrene calibration curve. This difference between the experimental and theoretical Mn is because of the high targeted molecular weight. An adequately controlled RDRP technique is guaranteed when the targeted molecular weight is below 20000鈥塯/mol. However, considering the high molecular weights required for this work, this insufficiently adequate control was disregarded. Theoretical Mn for experiments 2 and 3 (synthesis of PMMA) was 44000 and 32000鈥塯/mol, respectively.

Sequential step block copolymerizations of BuA as monomer- and iodine-functionalized PMMA as macrochain transfer agent were carried out. Thereafter, the prepared PMMAs were used for the synthesis of block copolymers of poly(methyl methacrylate)-b-poly(butyl acrylate-co-methyl methacrylate). In experiments 1, 2, and 3, the Mn was increased by growing the second block. In experiment 1, for example, the Mn attained after the synthesis of the diblock copolymer was 48000鈥塯/mol, whereas the Mn after the MMA homopolymerization was 44500鈥塯/mol. Theoretical Mn for three different copolymers adjusted acceptably with the experimental values; in experiment 1, for example, it was calculated to be 56000鈥塯/mol according to;Mn=(gBuAconversion)+(gMMAconversion)molesofPMMA+MnofPMMA(experimental).(3)

Polydispersity indexes increased slightly after the block copolymerization which is typical in an RITP experiment due to the accumulation of dead chains that is unavoidable in any RDRP technique. Figure 1 shows the SEC curves of the macrochain transfer agent (PMMA) and the resulting diblock copolymer in experiment 1. Both chromatograms show a monomodal distribution but the copolymer presents a shift toward higher molecular weights region, confirming that most of the growing PMMA chains remain in the living stage and take part in the formation of the diblock copolymer. A small fraction of PMMA with low molecular weight remains inactive during the copolymerization.

Figure 1: SEC analysis of a PMMA synthesized by RITP and a block copolymer synthesized by sequential copolymerization with BuA.

Composition of copolymers was determinate by 1H NMR resulting in 93, 80, and 70鈥墂t% of PMMA in experiments 1, 2, and 3, respectively; the rest for 100鈥墂t%, in each case, is constituted by PBuA. Most compositions give glassy properties to these materials, except the one with 70鈥墂t% of PMMA. The presence of two shifted glass transition temperatures (饾憞饾憯) reinforces the statement of these being block copolymers. 饾憞g of pure PBuA is expected at 鈭52掳C; however the large shift to around 0掳C observed in our materials can be explained considering the portion of PMMA in the second block (soft segment), in all cases (see Table 3). Comparing the experimental results with those calculated using the Fox equation [24] (4), a high deviation is observed as shown in Table 3, especially for the copolymers with 20 and 30鈥墂t% of PBuA; however, according to Brostow et al. [25], this deviation is normal and it is explained because of the immiscibility of the components of soft segments; that is, the higher the miscibility, the closer the predicted 饾憞饾憯 to the experimental 饾憞饾憯:1饾憞饾憯=饾惞1饾憞饾憯,1+1饾惞1饾憞饾憯,2,(4)

Table 3: Parameters used to calculate prediction of 饾憞饾憯 of soft segments by the Fox equation.

where 饾憞饾憯 is the soft segment 饾憞饾憯 prediction; F1 is the PBuA fraction in the soft segment; 饾憞饾憯,1 is the 饾憞饾憯 of PBuA homopolymer; 饾憞饾憯,2 is the 饾憞饾憯 of PMMA homopolymer.

To determine the effect of PBuA content on the mechanical properties of the diblock copolymers, DMA analysis was done. The storage modulus (饾惛) at 鈭20掳C decreases from 4428, 3846, and 2739鈥塎Pa for copolymers with 7, 20, and 30鈥墂t% of PBuA content, respectively, as expected; this behavior is due to the rubbery nature of PBuA.

On the other hand, Figure 2(a) presents the variation of tan饾浛(饾惛/饾惛) with temperature for the three diblock copolymers. The ones with 30 and 20鈥墂t% of PBuA show two transitions whereas the one with 7鈥墂t% shows only one transition. Those transitions occur at 33 卤 3 and 100掳C and correspond to the P(BuA-co-MMA) block (soft segment) and to the PMMA block (hard segment) in the copolymer, respectively. All transitions shift to high temperatures with respect to the PBuA homopolymer (鈭52掳C) and remain constant for the PMMA (100掳C); this behavior is explained due to the sequential formation of the second block P(BuA-co-MMA) that represents the soft segment rich in PBuA but still containing PMMA. It was also observed that an increase in the PBuA content causes an increase in the area under the tan饾浛 curve that corresponds to the soft segment.

Figure 2: (a) tan饾浛(饾惛/饾惛) for the synthesized copolymers, (b) TEM image for the copolymer with 20鈥墂t% of PBuA, and (c) with 30鈥墂t% of PBuA (scale bar at 10鈥塶m).

Figures 2(b) and 2(c) show the micrographs by TEM of the copolymers with 20 and 30鈥墂t% of PBuA. In both cases, the presence of a PBuA-stained phase [22] (darker phase in the images) was observed, but the copolymer with 30鈥墂t% of PBuA in Figure 2(c) presented a higher volume fraction of rubbery phase than the one with 20鈥墂t% of PBuA in Figure 2(b). This coincides with the observed areas under the tan饾浛 curve, at ca. 饾憞饾憯 of the soft segments, which is greater for the sample with 30鈥墂t% of PBuA.

3.2. Nanostructured Films

Taking the synthesized copolymers, colloidal suspensions were prepared with鈥0.4鈥1.2鈥墂t% of carboxyl functionalized MWCNT. These colloidal suspensions were deposited on glass cover slits and the drying rate (饾憗) was evaluated. Figure 3(a) shows the weight loss of films prepared from the solution of CHCl3/bock copolymer with 7鈥墂t% of PBuA, where the 饾憫饾懁/饾憫饾憽 was determined from the linear function of weight loss with time. In this case, 饾憗 was 0.203鈥塯/min路cm2. Similarly, the 饾憗 values were calculated for the other colloidal suspensions. Figure 3(b) shows the 饾憗 values for the CHCl3/block copolymer/MWCNT solutions with 7, 20, and 30鈥墂t% of PBuA with鈥0.4鈥1.2鈥墂t% of functionalized MWCNT, as a function of MWCNT concentration.

Figure 3: (a) The lost weight as a function of time for the solution CHCl3/block copolymer with 7鈥墂t% of PBuA and (b) 饾憗 values for the suspensions CHCl3/block copolymers (with 7, 20, and 30鈥墂t% of PBuA)/MWCNT.

When comparing the pure copolymers, the solvent evaporation or film drying rate (饾憗) of all three copolymer compositions studied is similar at low CNT contents; however, as the CNT content increases, 饾憗 decreases, especially for those copolymers with 30鈥墂t% of PBuA.

In all three cases however, the drying rate (饾憗) decreases with increasing MWCNT content. This decrease is more pronounced for those compositions with 7 and 30鈥墂t% of PBuA.

3.3. Nanocomposites Electric Resistance

Figure 4 shows the variation of the electrical resistance with respect to the MWCNT content, for the copolymer with 7鈥墂t% of PBuA. A drastic decrease in the resistance from 11012 to 1106鈥塐hms is observed at 0.8鈥墂t% of MWCNT, which would indicate the concentration at the percolation threshold for this compound. Similar behavior was observed for the copolymers with 20 and 30鈥墂t% of PBuA that reached a resistance of 1106鈥塐hms at the same content of MWCNT.

Figure 4: Electrical resistance of the nanostructured films as a function of the MWCNT content.
3.4. Dynamic-Mechanic Properties of Nanostructures Films

Figure 5 shows the DMA analysis for the block copolymers with 0.8鈥墂t% of MWCNT, which is the concentration at the percolation threshold. As expected the storage modulus of the copolymers with MWCNT, at 鈭20掳C, increases from 3121 to 4331 and finally to 5358鈥塎Pa, for the copolymers at 30, 20, and 7鈥墂t% of PBuA, respectively. This result can be attributed to the decreasing content of the elastomeric PBuA content in the copolymer at constant MWCNT content. Comparing these 饾惛 values with those corresponding to the pure copolymers, the storage modulus increases from 11 to 17% as the PBuA content decreases.

Figure 5: DMA analysis for the block copolymers/MWCNT nanocomposites with 8鈥墂t% of MWCNT.

Considering, the intended application of the nanocomposites to be as a coating for textiles, the variation in modulus can be regulated by the PBuA content in the synthesized poly(methyl methacrylate)-b-poly(butyl acrylate-co-methyl methacrylate) copolymers. For example, the incorporation of 0.8鈥墂t% of MWCNT in a block copolymer with a concentration of 30鈥墂t% of PBuA can result in a lower storage modulus (3639鈥塎Pa), in respect of a block copolymer with 20鈥墂t% of PBuA (4041鈥塎Pa). On the other hand, apparently there is a decrease in 饾憞饾憯 (from Figure 2(a)) from 30鈥35掳C, for the pure copolymers, to 21鈥20掳C for the nanocomposites at 7 and 20鈥墂t% of PBuA, respectively. This behavior is attributed to the nanocomposites preparation method (casting), which allows a slow arrangement of the polymer chains so that the PBuA rich block, in the copolymer, will present more interaction with itself, resulting in a larger dispersed phase. Apparently, it tends to behave as a polymer blend; however, those transitions are still far from the values for the PBuA (ca. 鈭50 0掳C).

Figure 6 presents the SEM micrographs of the nanocomposites prepared with the three different copolymers at 0.8鈥墂t% of MWCNT. In all three cases, a good homogeneous distribution and dispersion is apparent. However, after a thorough analysis, a clear tendency to agglomerate can be observed (dots 1, 2, and 3 in Figures 6(b) and 6(c)). This tendency seems to increase with increasing PBuA content in the copolymers. This can be attributed to poor compatibility between the copolymers and the CNT, which becomes poorer with increasing the PBuA content.

Figure 6: SEM images of block copolymers/MWCNT nanocomposites with 8鈥墂t% of nanotubes with (a) 7, (b) 20, and (c) 30鈥墂t% of PBuA.

Also, the presence of two distinctive phases can be observed in copolymers with 7 and 30鈥墂t% of PBuA in Figures 6(a) and 6(c). In the first case, we have a dispersed discrete phase as droplets with an average diameter of 0.5鈥塵m, whereas in the second case, no distinction can be observed between the two components, which could indicate that at 30鈥墂t% of PBuA, there is a type of cocontinuity of phases.

In this sense, it appears that the MWCNTs have a preference to locate at the PMMA pure block and/or in the interface, which causes a well distribution but poor dispersion in the copolymers with higher PBuA content. It results in the formation of tridimensional network of MWCNT (by bridging effect) [26] which favors the drastic decrease in the surface resistance at 8鈥墂t% of MWCNT.

4. Conclusions

Randomized block copolymers of poly(methyl methacrylate)-b-poly(butyl acrylate-co-methyl methacrylate) were successfully synthesized by the RITP technique, which showed, in terms of dynamic-mechanic properties, low storage modulus with the PBuA content increase. It was observed that an increase in the PBuA content results in a decrease in the drying rate (饾憗), of the different colloidal suspensions. In terms of electrical resistance for the nanocomposites, the PBuA content did not show a significant effect. On the other hand, the dispersion and distribution of MWCNT into the copolymer matrix were confirmed by SEM. An increase in the PBuA content causes more interaction between the nanotubes themselves, presenting a good distribution but a poor dispersion which induces the percolation threshold.


The authors would like to thank Francisco Zendejo, Jose L贸pez Rivera, and Guadalupe Mendez for their technical support in the preparation and characterization of colloidal suspensions and nanostructured films and Judith Cabello for her help in 1H NMR analysis. The authors also thank CONACYT for its financial support to carry out this study through projects 84424 and 146970.


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