Multiwalled Carbon Nanotube Nanocomposites: Effect of the Tacticity

Article pubs.acs.org/JPCC

Poly(vinyl chloride)/Multiwalled Carbon Nanotube Nanocomposites: Effect of the Tacticity Distribution on the Polymer/Nanofiller Interface Horacio J. Salavagione,* Gary Ellis, and Gerardo Martínez Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain S Supporting Information *

ABSTRACT: The preparation and characterization of nanocomposites of poly(vinyl chloride) (PVC) of different tacticity with alkyl-modified multiwalled nanotubes (eMWNTs) is reported. Evidence from FTIR and thermal degradation measurements suggests that at the polymer−nanofiller interface specific isotactic conformations in the PVC chains are the preferred sites for the accommodation of eMWNTs through halogen bonding interactions, CO···Cl−C. The differences between the IR intensity ratio I614/I635 (related to the syndio/ isotactic distribution) observed for the nanocomposite and the polymer increase as the isotactic content increases as a result of interaction with the nanotubes, which reduces the distribution of the isotactic conformation in the polymer matrix. Further, the improvements in thermal stability of the nanocomposites with respect to the pure polymer increase with the isotactic content since the eMWNT restricts mobility of the labile g+ttg− conformation especially located at the end of long isotactic sequences.

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INTRODUCTION The use of carbon nanotubes (CNTs) in polymer nanocomposites (PNCs) has been intensely explored since the discovery in 1991.1 Nowadays a consensus exits on the crucial role of both the dispersion of CNTs in the polymer matrix and the interfacial interactions between both components. Clearly, in the absence of good dispersion of the tubes almost all of the important advantages of combining nanotubes with polymers will be negated. A common strategy is to maximize dispersion and debundling of CNTs via processing, avoiding some negative side-effects (such as reduction in length, cost, etc.), and then controlling, if possible, the amount of reaggregation/ rebundling that occurs during subsequent processing steps.2 The intermolecular interaction between the polymer matrix and the CNTs can be achieved either by covalent attachment of polymers onto CNTs3 or through noncovalent molecular association such as charge transfer, adsorption via π−π interaction4 or nonspecific interactions between carbon− hydrogen groups and π systems.5 The control of the formation of noncovalent interactions in polymer mixtures or nanocomposites is dominated by the accessibility to the functional groups that can form intermolecular interactions.6 Recently some interest has been generated in elucidating how polymer chain connectivity and chain conformation as well as the size and shape of the nanoparticles can modulate the formation of intermolecular interactions and directly influence the dispersion of the filler in the resulting nanocomposite.7−9 Furthermore, polymer confinement in nanocomposites is also drawing a great deal of attention in view of the changes in chain conformation and mobility in confined geometries.10 Numer© 2012 American Chemical Society

ous studies have shown that improved interactions between a polymer matrix and the reinforcing phase lead to improved mechanical properties, and recently different studies have examined how the tacticity of the chain influences the properties of the polymer nanocomposites.11−13 Studies on the effect of the tacticity on the interface of the nanocomposites represent only a small portion of the numerous studies on CNT-based polymer nanocomposites. However, it is possible that this feature of the polymer may play an important role in controlling the CNT/polymer interface, and some efforts to understand this are required. Kovalchuk et al. synthesized isotactic-polypropylene (iPP) and syndiotactic-polypropylene (sPP)/multiwall carbon nanotube (MWCNT) nanocomposites via in situ polymerization to improve compatibility between the PP and filler particles. A considerable enhancement in Young’s modulus of iPP and sPP (between 25−66%) took place even at low MWCNT loadings (below 0.5 wt %), and a substantial retardation effect on the thermooxidative degradation was observed, where the temperature of maximum weight loss rate rose by ∼52 °C upon incorporating only 1.4 wt % MWCNTs.12 In another study, semicrystalline syndiotactic polystyrene (sPS) and amorphous atactic polystyrene (aPS) composites with carbon nanocapsule (CNC) and carbon nanotube (CNT) fillers were prepared to study the effects of matrix tacticity, as well as filler aspect ratio, on the rheological and electrical Received: May 24, 2012 Revised: July 20, 2012 Published: August 2, 2012 18256

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maintain a constant flow of ∼10% O3 in oxygen to the sample at a pressure lower than 2.5 psi. Typically, 300 mg samples were first dispersed in 120 mL of NMP by extensive sonication. The reaction mixture was stirred vigorously during ozonolysis, which was carried out for 4 h to create the expected ozonide intermediate. The reaction mixture was then flushed with O2 at the end of ozonolysis to eliminate any remaining O3. The expected primary ozonides generated were cleaved by addition of ∼20 mL of 50% H2O2 in aqueous solution at 70 °C for 4 h. The H2O2-cleaved tubes were then filtered over a 0.2 μm polycarbonate membrane and washed with a large excess of 50% HCl solution, followed by copious amounts of deionized water. The filtered MWNTs were dried overnight at 80 °C. Using this procedure the carboxylic species (−COOH) predominate over those of alcohols (−COH) and aldehydes/ ketones (CO).16 A final concentration of acidic species (active sites for esterification) of 16 mmol g−1 was determined by back-titration.17 Ester-Functionalized MWNTs (eMWNTs). The esterification procedure employed for the MWNTs was as follows: 200 mg of ozonized MWNTs were suspended in DMF (50 mL) and sonicated for 30 min. Then a solution of 1-octadecylalcohol (1 g, 3.7 mmol), DCC (5 g, 24.2 mmol), and DMAP (0.35 g, 2.9 mmol) in DMF (50 mL) was added, and the resulting mixture was stirred at 40 °C for 3 d. After cooling, the solid was repeatedly treated with ethanol and the washed solid filtered through a membrane (pore size 0.2 μm). The concentration of remaining −COOH groups after esterification, determined by back-titration,14,17 corresponds to an esterification yield of 35%. This degree of modification is adequate because it facilitates good nanotube dispersion while retaining a high density of carboxyl groups capable of forming halogen bonds with PVC. In fact, it is interesting to point out that the ester functionalized MWNTs, denominated eMWNTs, were soluble in THF, CH, and DMF among other solvents. Preparation of Nanocomposites. The nanocomposites with 5 wt % of nanotubes were prepared by mixing appropriate amounts of the different PVC samples and eMWNTs. The samples were denominated P1C5, P2C5, and P3C5 for nanocomposites of P1, P2, and P3, respectively. Blends of PVC and PCL with 10 wt % of PCL were prepared by mixing appropriate amounts of each component. Polymer composite films were fabricated by the solution casting technique, whereby a solution of polymer with dispersed nanotubes was evaporated to give a composite film. Characterization. The FTIR measurements were carried out with a Perkin-Elmer Spectrum One FTIR spectrometer equipped with a deuterated triglycine sulfate detector (DTGS) using a resolution of 4 cm−1. The samples were thoroughly mixed with KBr and pressed into pellet form. Because of the hydroscopic behavior of KBr, the sample cell was purged with desiccated air. The tacticity of parent PVC samples was obtained from 13C NMR decoupled spectra (methine region) obtained at 80 °C using a Varian Unity instrument operating at 125 MHz in dioxane-d8 solutions. The spectral width was 2500 MHz and a pulse repetition rate of 3 s and 16 000 data points were used. The relative peak intensities of the spectra were measured from the integrated peak areas, calculated by means of an electronic integrator. To obtain glass transition temperatures (Tg), differential scanning calorimetric scans were conducted on 8−10 mg samples with a Mettler TA4000 calorimeter previously

properties of PS composites. Owing to the pronounced nucleating effects of the CNTs, crystalline sPS composites exhibit a conductivity threshold 4-fold higher than their amorphous aPS counterparts, albeit their rheological thresholds are similar.13 In recent work on nanocomposites of PVC with MWNTs the predominant role of cyclohexanone (CH) in the dispersion of MWNT modified with long-alkyl chains (eMWNT) was demonstrated.14 It was shown that PVC facilitated the dispersion of eMWNTs through a polymer/solvent synergetic effect. In fact the results highlighted the existence of a cooperative effect between eMWNT, PVC and CH via van der Waals interactions, hydrogen bonding and halogen bonding that was responsible for the improved solubility and properties of the parent PVC. The results were explained in accordance with the specific interactions between PVC and plasticisers, i.e., dioctyl phthalate and a number of different polyesters in compatible blends such as poly(caprolactone), PCL. In the present work we have employed the same functionalized MWNTs to prepare nanocomposites with PVCs of different tacticity with the objective to study the characteristics of the interactions existing between the eMWNT and the polymeric matrix and to elucidate their molecular nature.

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EXPERIMENTAL SECTION Materials. The PVC samples were prepared at 90 °C (sample P1) and 0 °C by a bulk polymerization process with 2,2′-azobisisobutyronitrile as an initiator, following a previously reported procedure.15 The polymer obtained at 0 °C was extracted with dioxane (DI) stirring at 50 °C for 24 h in a thermostatic vessel under nitrogen. After separation of the insoluble fractions by centrifugation, the soluble fraction was precipitated with methanol and then washed and dried at 50 °C (sample P3). Although the soluble fraction is rich in low molecular weight isotactic polymers, the insoluble part mainly contains high molecular weights fractions of syndiotactic polymers. Sample P2 is a commercial PVC (Atochem, Spain) obtained by bulk polymerization at 70 °C; the polymerization process was stopped at a conversion of 62%. The average molecular weights of all samples were determined in cyclohexanone (CH) at 34 °C with a Knauer membrane osmometer. MWNTs (length: 1−10 μm, average outside diameter of 13 nm, no amorphous carbon, bulk density: 150 g/L, and purity: >95%) prepared via chemical vapor deposition (CVD) were a generous gift from Bayer (Baytubes C 150P). Poly(εcaprolactone) (PCL), N,N′-dicyclohexylcarbodiimide (DCC), 4-(dimethylamino) pyridine (DMAP, 99%), and 1-octadecyl alcohol (95%) were purchased from Aldrich and used as received. Tetrahydrofuran (THF, Aldrich) was distilled under nitrogen with aluminum lithium hydride for the removal of peroxides immediately before use. DI was refluxed over sodium and distilled from aluminum lithium hydride. CH (Aldrich), Nmethylpyrrolidone (NMP, Aldrich), dimethylformamide (DMF, Aldrich), and hexamethylphosphoric triamine (HMPT, Aldrich) were purified by fractional distillation under nitrogen. Carboxylated MWNTs (MWNT-COOH). The MWNTs were furnished with carboxylic groups by using a previously reported treatment.14 Oxygenated surface groups were generated by ozonolysis employing a Fisher model 500 ozonolysis apparatus at room temperature. In this instrument the flow of O2 to the arc discharge is kept below 2 psi. Ozone was formed in a 180 W discharge with the pressure adjusted to 18257

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calibrated with an indium standard. A heating rate of 10 °C/ min was employed, the samples were scanned twice, and the Tg was taken as the midpoint between the intersections from the glassy state to the liquid state of the second scan. The reproducibility of duplicate runs of samples with well-defined Tg values was better than ±0.2 °C. The thermal stability of the individual polymers and the nanocomposites was analyzed using a Mettler TA4000/TG50 thermogravimetric analyzer (TGA) on 5−10 mg samples. Isothermal experiments were carried out at 160 °C for 1 h in a nitrogen atmosphere. UV−visible absorption spectra of degraded samples in HMPT solutions (of ca. 0.4 mg mL−1) were measured using a Perkin-Elmer UV−vis Lambda 16 spectrometer. The dynamic mechanical performance of the polymers was studied using a Mettler DMA 861 dynamic mechanical analyzer. Rectangular shaped samples of ∼19.5 × 4 × 0.5 mm3 were mounted in a large tension clamp. The measurements were performed in the tensile mode at frequencies of 1 Hz, in the temperature range between −100 and +110 °C, at a heating rate of 2 °C/min. A dynamic force of 6 N was used oscillating at fixed frequency and amplitude of 30 μm.

heptad content was found to be markedly lower as the isotactic triad content decreases, which means that the content of isotactic sequences equal to or longer than one heptad falls as the overall isotactic content decreases. Consequently, the contents of the mmr tetrad and its fraction in the g+ttg‑tt conformation will decrease in a similar manner. Although the difference in both the mmmr content and rmmmrx/mmmmrx ratio (Table 1) for the samples used in this work is small, it is significant enough for an adequate analysis of the influence of the stereoregularity of the chain in its interaction with eMWNT and, consequently, in the properties of the PVC nanocomposites, which is the final purpose of our study. The crystallinity of PVC is considered to depend on the content of tactic sequences, in particular the syndiotactic. However, the method of preparation from solution is an inhibiting factor and in this case the development of crystallinity of the PVC samples will be limited. Indeed DSC measurements confirm no evidence of crystallinity. The PVC nanocomposites were prepared following the same protocol as in the preliminary work.14 Regardless of the route taken to improve MWNT dispersion; the extent of intermolecular interaction within the nanocomposite must be addressed. In order to identify the nature of these interactions a blend of PVC and PCL, a polymer containing one carboxylic (CO) group in its molecular repeat structure, was used as model. In the case of PVC the frequencies of the C−Cl stretching vibrations have been extensively studied by infrared spectroscopy and shown to be very sensitive to the local conformation, characterized by the H or C atoms that are in a trans position to each chlorine atom.20−23 The corresponding IR bands are assigned as SHH and SCH modes, where SHH means that the two substituents trans to the Cl atom across both neighboring C−C bonds are hydrogen atoms, whereas SHC means that of the two trans atoms one is hydrogen and the other is carbon.24 It should be noted that the all trans chain sequences and the local gauche conformation are related, as a general rule, to chlorines of the SHH and SCH type respectively, and also the occurrence of the most likely local chain conformation in PVC is determined by the tacticity. Thus, from the IR spectra some valuable information can be obtained on the type and content of local chain conformations, and on the syndiotactic or isotactic distribution. Different infrared studies of plasticized PVC and polyesterPVC blends, especially the PCL/PVC system, have indicated that specific interactions exist between the two components that could be responsible for the apparent compatibility of these blend systems in the amorphous state.25−28 In particular, Tabb and Koenig showed that the plasticizer solvates the amorphous chain segments possibly by complex formation of the type CO···Cl−C.29 With the purpose of providing further evidence of the type of interaction and the nature of the polymer segments that take part in those interactions, we explored the efficiency of some conformations of isotactic sequences in nanocomposites of PVC and eMWNT in the solid state. With this in mind we analyzed the corresponding IR spectra of eMWNT/PVC nanocomposites in order to detect possible microstructural changes with respect to the parent PVC to elucidate the role of the microstructure of the polymer in the interactions. Taking into account that in the PCL/PVC blend the debate is based on interactions involving the carbonyl bond of PCL with the αhydrogen or with the carbon-chloride bond of PVC, we turn

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RESULTS AND DISCUSSION PVC is a well-known atactic polymer with a stereochemical composition that can be manipulated over a narrow range by undertaking the polymerization at different temperatures. Accordingly, from polymerizations at 90, 70, and 0 °C PVC samples of different tacticity were obtained:18 samples P1, P2, and P3, respectively. In order that the samples were adequate for 13C NMR experiments, an extraction with DI was undertaken for the sample synthesized at 0 °C. From these dioxane-soluble fractions, composed of low molecular weight chains, solution NMR spectra could be obtained with excellent resolution allowing an adequate quantification of the tacticity content. The results from a comprehensive characterization of the samples are listed in Table 1. Since variations in the Table 1. Characteristics of the Polymers Used in This Study sample

Mn × 103

Prra

Prma

Pmma

ρb

Prmmmrx/Pmmmmrxc

P1 P2 P3

21.5 44.0 54.3

0.292 0.305 0.353

0.498 0.497 0.495

0.210 0.198 0.152

1.00 1.00 0.97

1.08 1.14 1.39

a Probability of syndio, hetero and isotactic triads. bPersitence ratio [ref 19]. cRelation of the probability of the heptads rmmmrx and mmmmrx.

molecular weight characteristics are only associated to the changes in the temperature of polymerization the influence of the microstructure of PVC on the phenomenon induced by nanotube loading can be disregarded. It was observed that the isotactic contents of the samples studied were lower as the temperature of the polymerization decreased. Table 1 includes the persistence ratio, ρ, defined by Reinmöller and Fox as the ratio between the normalized intensity of isotactic dyads and the conditional probability of a syndiotactic placement on an isotactic chain end.19 It is apparent that samples P1 and P2 exhibit Bernoullian behavior (ρ = 1), whereas P3 departs slightly from Bernoullian behavior, indicating a small change in the steric rearrangement. Furthermore, the resolution of the spectra from the 13C NMR permits the observation of the changes in rmmmrx and mmmmrx (x = m or r) heptad content as the isotactic content decreases.18 The mmmmrx (x = m or r) 18258

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our attention to polymer nanocomposites in this work, and have used the PCL/PVC system as a comparative model. It is known that the carbonyl band of PCL shifts to lower frequency upon forming a compatible blend with PVC, however, the variations that occur in the C−Cl stretching region of PVC in the blends have been little considered.26,29 Assignments of the IR bands corresponding to the C−Cl stretching region are well documented in the literature, as is the fact that the C−Cl stretching vibrations in this highly overlapped spectral region between 550 and 750 cm−1 are very sensitive to chain conformations.24 The peak at the lower frequency side of the band profile contains contributions from principally syndiotactic straight chain short trans segments, SHH′ at around 614 cm−1, and long trans segments, SHH at around 605 cm−1, whereas the broad and overlapped band at around 633 cm−1, albeit containing some information from syndiotactic conformations, has contributions from SHH′ and SHH of isotactic chain conformations.30−32 At higher frequency, although the assignments are disputed, the broad band between 660 and 720 cm−1 also contains a mixture of syndiotactic, isotactic, and heterotactic conformational bands, with a certain predominance of the isotactic conformations at 681 and 698 cm−1. The distribution of these conformational bands is detailed in the Supporting Information (Figure S2). The relative intensities of the 614/635 and 614/692 cm−1 bands may be employed to provide an internal comparison of the relative amount of syndiotactic and isotactic conformations in these systems. Also, other band ratios such as 1428/1434 cm−1 have been habitually employed to estimate the relative tacticity of PVC samples.33 Figure 1 shows the IR spectra of the polymers P1, P2, and P3 in the spectral range between 550 and 750 cm−1, where the

Table 2. IR Band Ratios for the Samples Studied sample

I614/I635

I614/I692

I1427/I1434

P1 P1C5A P2 P2C5A P3 P3C5A P1/PCL

1.13 1.25 1.16 1.28 1.20 1.28 1.17

1.61 1.99 1.78 1.99 1.96 2.23 1.77

1.01 1.09 1.05 1.10 1.12 1.18

low transmittance of the nanotubes and some slight intensity differences throughout the spectra. Figure 2 shows the comparisons in the 550 and 750 cm−1 region, corresponding to the νC−Cl vibrations, between the spectra of the parent polymers and those of their corresponding nanocomposites with 5 wt % eMWNT, and that of a sample corresponding to a PVC/PCL blend where the polymer P1 was employed. Overall, the most salient changes observed are a decrease in the relative intensity of (i) the ∼635 cm−1 band region and (ii) the ∼692 cm−1 band region, due to the involvement of the isotactic structures in the interaction with the eMWNT. The variations in relative intensity between the parent polymers also vary in degree depending on the level of isotacticity in the parent polymer. If we consider the I614/I635 band ratio, an increase of around 11% is found in the case of P1, and almost 10% in the case of P2. These results are similar to those observed in our previous study on nanocomposites of P214 and are in agreement with previous reports on the molecular interactions of PVC with some solvents, i.e., CH, esters, dioctylphalate, polyesters, and PCL,34 where it was demonstrated that the CO···Cl−C interaction involved a fraction of the mmr structures, more specifically those with g+ttg‑ conformations. In addition, the blend PCL/PVC also displays the same variations in this part of the spectrum, Figure 2a. A possible explanation of the changes in the C−Cl vibration in our system, not observed in previous work26,29 is that the polymer used (P1) is a PVC with a high content in isotacticity, and thus in the g+ttg‑ conformation. Interestingly, the differences between the nanocomposite and the parent PVC are similar to those of the sample P2 (Figure 2b). These results show that the interactions rest upon small changes in the conformationally sensitive C−Cl stretching vibration clearly suggesting that the chlorine atoms of PVC are involved in the interaction. In addition it must be emphasized that these specific conformations, mainly the g+ttg‑, imply local discontinuity of the terminal isotactic sequence that changes the spatial arrangement of the chain and may introduce a higher local free volume and rotational motion, thereby influencing both the interchain and the intrachain interactions and as a consequence the thermal35 and physical properties of the polymer.36 In the case of the nanocomposite with lower isotactic triads content (sample P3, Table 1) the differences between P3 and P3C5 are smaller and an increase of less than 7% in the I614/I635 band ratio is perceived (Figure 2c). Based on the aforementioned results we propose that the interaction between the blend PVC/PCL and between the PVC and eMWNT is of a local conformational nature where the specific conformation g+ttg‑ at the mmmr termini of isotactic sequences preferentially participate. In order to investigate the consequences of this observation on the eMWCNT/PVC interphase, some bulk materials properties were examined.

Figure 1. Infrared spectra of PVC of different tacticity. (Spectra normalized to the intensity of the 614 cm−1 band over the observed region).

different isotactic content is clearly observable in the spectra, with P1 containing the highest concentration of isotactic sequences and P3 the lowest. The relative intensity values from the 614/635 and 614/692 cm−1 band ratios along with that of 1428/1434 cm−1 are provided in Table 2 and, as expected, these are in good agreement with the NMR data. The infrared spectra of the nanocomposites studied were found to be quite similar to those of the corresponding parent PVC spectra (Figure S1), except for high baseline due to the 18259

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Figure 2. Comparison of the IR spectra of the parent PVC polymers of varying tacticity with its respective nanocomposite with 5 wt % of eMWNTs (Spectra normalized to the intensity of the 614 cm−1 band over the observed region).

In the SEM images of cryo-fractured samples of the eMWNTs in the distinct matrices, Figure 3, a good dispersion

Table 3. Thermal Parameters Including Glass Transition Temperature (Tg), Degradation Rate, and Degree of Stabilization for the Distinct PVC Samples and Its Nanocomposites with 5 wt % of eMWNTsa sample

Tg/°C

degradation rate ×103, % min−1

P1 P1C5 P2 P2C5 P3 P3C5 P1/PCL

78.1 82.0 83.0 86.5 91.5 90.5

2.40 1.70 0.90 0.74 3.00 2.85 1.50

stabilization,% 28 18 5 38

a

Values of degradation for the blend of P1 with 10 wt % of PCL are also shown for comparison.

Figure 3. SEM images on the cryo-fractured samples of P2 (A) and the nanocomposites with 5 wt % of eMWNTs P1C5 (B), P2C5 (C), and P3C5 (D). Scale bar corresponds to 2 μm in all cases.

polyenes.31 Therefore, an important observation to emphasize here is that the stabilization in the nanocomposites increases as the isotactic mmr concentration increases, providing further evidence that the distribution and concentration of the different conformations determines the strength of the interface, which strengthens as the overall isotactic content increases. The main reason of this stabilization may lie in the formation of halogen bonds (type CO···Cl−C), since the stabilization in the PVC/ PCL model system is the highest measured in this study. Furthermore, it is well documented that the thermal degradation of PVC leads to the formation of polyene sequences as a result of the sequential elimination of hydrogen chloride. These polyene sequences have been characterized by ultraviolet/visible spectroscopy, assuming that the observed spectrum is the sum of the overlapping spectra of a range of polyenes of different conjugated sequence lengths, Figure 4. Consequently, the average length of the polyenes in equally degraded polymers should decrease when the highest stability is attained, and this effect occurs in a parallel way to that of stabilization by interaction of eMWNTs with PVC. Figure 4 shows a higher concentration of polyenes of 7−9 double bonds (430−450 nm) relative to both the longer and shorter polyenes especially for P1, which is associated to an easy initiation process, while P3 has a polyene distribution somewhat broader due to the influence of syndiotactic sequences on the propagation step.35 When comparing each polymer with its respective nanocomposite some stabilization is observed in all cases, but the characteristics of the UV−visible curves are quite different. The maximum stabilization is achieved for the nanocomposite P1C5, where the signal of the polyenes (of any length) is very difficult to observe. In the case of polymer P2, the nanocomposite (P2C5) shows lower concentration of all polyenes detected, whereas in the P3 its nanocomposite (P3C5) seems to be more stable as the concentration of

of the filler was observed in all cases. However, in the case of sample P3C5 various nanotubes were seen to be projecting from the plane of section (enlarged view in Figure S3). Although this phenomenon arises during cryo-fracture of the samples, it is related to the strength of the interaction between the polymer and the nanotube which, based on the higher frequency of debonded tubes observed in this sample, appears to be weaker in the case of this polymer, in agreement with the FTIR data. This implies improved embedding of the eMWNTs in polymers with higher isotactic content due to the greater strength of the interface. A simple manner to test whether the interaction between the eMWNT and the PVC occurs preferably in the labile chlorine atoms of isotactic sequences is to study the early stages of the thermal dcomposition of the nanocomposites, since the initiation of degradation is directly related to these chlorine atoms. In previous work we found significant stabilization of the P2C5 with respect to the parent P2 by isothermal treatment at 160 °C for 1 h in nitrogen atmosphere (which represent ∼0.5− 1.0% degradation) and this was attributed to interactions between groups in the eMWNTs with labile chlorine atoms in PVC.14 Table 3 shows the decomposition rate values observed for the parent polymers and the different nanocomposites. First, we observed that stabilization for the parent polymers follows the order P2 > P1 > P3. It must be emphasized that this behavior is the typical of polymers of different tacticity, as demonstrated in previous work where it was explained that the degradation mechanism of PVC involves two basic steps due to initiation through labile structures, and propagation or build-up of 18260

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with higher isotactic content, P2C5. While the changes at low temperature are associated to local motions, those at temperatures above Tg are due to cooperative movements. In the case of sample P2, the number of definite isotactic sequences to preferentially accommodate eMWNTs is slightly higher and therefore the number of segments moving cooperatively is also higher. Although these preliminary results may support the hypothesis of preferential interactions with isotactic segments of the PVC chains, it is clear that more work is required. A deeper study of the confinement effect caused by MWNTs on the dynamics of the nanocomposites and to analyze whether specific conformations of isotactic triads enhance the chain segments motion at molecular level in the polymer and the nanocomposites is currently underway.

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CONCLUSIONS All the PVC polymers studied produce nanocomposites with well-dispersed eMWNTs. The intermolecular eMWNTs/PVC interactions are of a complex nature as they involve specific halogen bonding as well as nonspecific interactions whose magnitude depends on the tacticity distribution of the polymer. A higher number of specific CO···Cl−C interactions in more isotactic polymers leads to improved stability and higher rigidity. The improvements in the thermal stability of nanocomposites of more isotactic PVC samples arises from the formation of halogen bonding between carboxylic groups in eMWNTs with chlorine atoms at isotactic conformations that inhibits the initiation stage of the degradation. The present study provides a deeper view at a molecular level of the nanofiller/polymer interphase and gives some idea of the type of interactions involved, emphasizing the importance of tacticity in the strength and extent of the interface. It suggests that future studies on carbon nanostructures-based polymer nanocomposites should consider not only the nature of nanofillers but also the conformation of the polymer chains in order to to be able to explain variations in materials properties.

Figure 4. UV−visible spectra showing the distribution of polyenes after isothermal treatment at 160 °C for 1 h. (A) Comparison among the distinct PVC samples, (B) comparison of P1 with P1C5 and P1PCL, (C) comparison of P2 with P2C5, and (D) comparison of P3 with P3C5.

polyenes of shorter length increases in detriment to the longer polyenes in the pure polymer. In summary, the mmmr structure adopting g+ttg‑ conformation at the end of isotactic sequences provides an easy initiation of the thermal degradation, and the specific interaction between the respective g+ttg‑ conformation and the eMWNT would certainly produce a restriction on the mobility of the labile mmr structure and lead to an improvement of the stability of the polymer. The effects of nanofillers on polymer dynamics are related to the strength of the polymer/filler interfaces, the nature of the filler and the morphology in terms of filler dispersion and spacing between filler particles.37 In this study we expected that the changes in the dynamics of the polymer are also related to the concentration of segments of specific tacticity in terms of higher active interfacial surface. The values of Tg obtained by DSC (listed in Table 3) show that this parameter is related to the tacticity of the polymer in both the parent samples and the nanocomposites. It can be clearly seen that the Tg increases as the isotacticity decreases, showing the expected behavior of PVC.38 In addition, for the nanocomposites the larger the content of isotactic sequences of the pure polymer the higher the shift of the Tg of the nanocomposites with respect to the parent polymer, manifesting a stronger effect of the filler on the more isotactic polymers. Therefore, it seems to be clear that the interfacial strength can be associated with the concentration of discrete isotactic sequences, while interactions of the eMWNTs with atactic sequences or those tending to syndiotactic must not be discarded. Finally, with a view to assess whether this molecular level effect extends to macroscopic properties, as a first approximation we studied the dynamic mechanical properties. Unfortunately, the sample with the highest degree of isotacticity, P1 (a priori the most significant) was extremely brittle and was impossible to introduce into the equipment without damage. However, the changes in the storage modulus for the other samples when passing from the pure polymers to the nanocomposites are similar (Figure S4). In both cases the glassy modulus increases slightly for the nanocomposites with respect to the pure polymer (∼30%), whereas in the rubbery state the increase in modulus is slightly higher for the sample

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ASSOCIATED CONTENT

S Supporting Information *

Full range FTIR spectra of all samples, spectral deconvolution for the C−Cl modes, enlarged view of SEM images and variation of storage modulus with the temperature. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +34-912587432. Fax: +34-915644853. E-mail: horacio@ ictp.csic.es. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Spanish Ministry of Science and Innovation, MICINN in projects MAT-2009-09335 and MAT2010-21070-C02-01 is gratefully acknowledged. H.J.S. thanks MICINN for a Ramón y Cajal Senior Research Fellowship.

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REFERENCES

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