Use of Inorganic Fullerene-like WS2 to Produce New High

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Use of Inorganic Fullerene-like WS2 to Produce New High-Performance Polyphenylene Sulfide Nanocomposites: Role of the Nanoparticle Concentration Mohammed Naffakh,*,† Carlos Marco,† Maria´n A. Go´mez,† Julio Go´mez-Herrero,‡ and Ignacio Jime´nez§ Departamento de Fı´sica e Ingenierı´a de Polı´meros, Instituto de Ciencia y Tecnologı´a de Polı´meros, CSIC, c/Juan de la CierVa, 3, 28006, Madrid, Spain, Departamento de Fı´sica de la Materia Condensada, UniVersidad Autónoma de Madrid, 28049, Madrid, Spain, Instituto de Ciencia de Materiales de Madrid, CSIC, Campus de Cantoblanco, 28049 Madrid, Spain ReceiVed: January 19, 2009; ReVised Manuscript ReceiVed: May 19, 2009

The use of tungsten disulfide (WS2) nanoparticles offers the opportunity to produce novel and advanced polymer-based nanocomposite materials via melt blending. The developed materials, based on the highperformance engineering thermoplastic polyphenylene sulfide (PPS), display a unique nanostructure on variation of the nanoparticle concentration, as confirmed by time-resolved synchrotron X-ray diffraction. The coldcrystallization kinetics and morphology of PPS chains under confined conditions in the nanocomposite, as determined by differential scanning calorimetry (DSC) and atomic force microscopy (AFM), also manifest a dependence on the IF-WS2 concentration which are unexpected for polymer nanocomposites. The addition of IF-WS2 with concentrations greater than or equal to 0.5 wt % of IF-WS2 remarkably improves the mechanical performance of PPS with an increase in the storage modulus of 40-75%. Introduction Polymer nanocomposites have attracted considerable attention in recent years. This interest is promoted by the promise of unprecedented performance, design flexibility, and lower unit and life-cycle costs. However, the development of polymer nanocomposites with dispersed nanofillers has, with some notable exceptions, been largely hindered by poor miscibility, dispersion, and interfacial strength, which have prevented such nanocomposites from attaining their full potential. New strategies such as the dispersion of inorganic fullerene-like materials (IF), discussed here, are required to break through the performance ceiling of current nanocomposites and to create a basis for future advances. The extraordinary properties of IF-like tungsten disulfide (IF-WS2),1,2 such as high modulus and low friction coefficient, attributed to their small size, closed structure, and chemical inertness, make them outstanding candidates to incorporate into polymer materials. These nanoparticles have improved the thermal and mechanical properties of one of the most important commodity thermoplastics such as isotactic polypropylene (iPP).3,4 In particular, the efficiency of the use of tungsten disulfide (WS2) nanoparticles as potential candidates for polymer tribological applications has recently been demonstrated for epoxy resin, polyacetal, and PEEK.5,6 Polyphenylene sulfide (PPS) is an important engineering plastic that exhibits outstanding physical and chemical properties. It has been widely used in various market segments such as electrical, electronic, automotive, industrial, and chemical sectors because of its excellent thermal stability, chemical resistance, flame resistance, electrical insulation, antiaging, and precision moldability.7,8 To further extend its applications for engineering uses, many microscaled and nanoscaled fillers, such * To whom correspondence should be addressed. E-mail: mnaffakh@ ictp.csic.es. † Instituto de Ciencia y Tecnologı´a de Polı´meros. ‡ Universidad Autoı´noma de Madrid. § CSIC, Campus de Cantoblanco.

Figure 1. TGA curves of nanocomposites obtained at 20 °C/min in oxygen atmosphere.

as short glass fiber,9 metal10 and its oxide/sulfide,11 nanoclay,12,13 carbon nanofiber,14 nano-SiOx,15,16 and expanded graphite,17 have been successfully compounded with PPS. The crystallization of isotropic polymers by the heating above the glass transition temperature (Tg) is denominated coldcrystallization. Unlike melt crystallization, in which the motion of polymer chains can be carried out entirely via molecular reptation,18 the polymer chains in the rubber state complete the corresponding conformational rearrangement via cooperative segmental movements.19 As a result, the crystal structure and morphology obtained from cold-crystallization is expected to differ from that obtained by melt crystallization. In our previous study,20 two endothermic peaks were related to the melting of the isothermally crystallized PPS crystals with different sizes and degree of perfection and were influenced by the crystallization temperature as well as the IF-WS2 content. The analysis of isothermal crystallization revealed that the incorporation of the IF-WS2 significantly altered the crystallization behavior of PPS, in a way that strongly depended on composition. A drastic

10.1021/jp902700x CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

New Polyphenylene Sulfide Nanocomposites change from retardation to promotion of crystallization was observed, which was supported by the variation of the fold surface free energy value (σe) calculated for PPS chains as a function of IF-WS2 content. It well-known that the molecular motion of semicrystalline polymers above Tg involve interactions between amorphous and crystalline regions. The formation and development of crystalline regions inevitably limits the motion of polymer chains in the amorphous region. An investigation of the coldcrystallization behavior of amorphous samples would contribute to a further understanding of the kinetic behavior of polymer chains under confined conditions. The evaluation of the kinetics of cold-crystallization has also considerable practical significance in many technological processes (e.g., extrusion, melt spinning processes, injection blow molding, thermoforming, etc.), where premature crystallization hinders the forming stage and thus is one of the main consequences of processing faults. The goal of this study was to prepare new PPS nanocomposites with well-dispersed IF-WS2, and to study the consequences of such dispersed IF-WS2 on the thermal, morphological, and mechanical properties of PPS matrix. The understanding of the role of the IF-WS2 concentration on the cold-crystallization kinetics of PPS nanocomposites would lead to the ability to design nanocomposites with adequate control over desired properties. Our motivation for preparing such nanocomposites is that efficient dispersion of IF-WS2 nanoparticles into polymer matrices by melt-mixing is the most simple and effective method from both an economic and industrial perspective. Experimental Section Preparation of Nanocomposites. PPS (Fortron 02054P4) was supplied in pellet form by Ticona. The IF-WS2 nanoparticles (NanoLubTM), were provided by Nanomaterials (in Israel, ApNano Materials in USA). The thermoplastic polymer was dried at 100 °C for 14 h before use to minimize the effects of moisture. Several concentrations of IF-WS2 (0.1-8 wt %) were introduced in the PPS matrix by melt-mixing using a Haake Rheocord 90 system operated at 320 °C with a rotor speed of 150 rpm for 20 min. Sample Characterization. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) were undertaken to analyze the thermal, mechanical, and structural characteristics of the nanocomposites.3 Thermal stability was measured on a Mettler TA4000/TG-50 thermobalance under an oxidative atmosphere. The crystallization and melting behavior of nanocomposites quenched from the melt in liquid nitrogen (melt-quenched) were investigated by differential scanning calorimetry using a Mettler TA4000/DSC30 operating under a nitrogen flow. The dynamicmechanical performance was studied using a dynamic mechanical analyzer (Mettler DMA861) operating in the tensile mode (sample size ) 19.5 × 4 × 1 mm3). Simultaneous SAXS/WAXS experiments using synchrotron radiation were performed at the A2 beamline of the HASYLAB synchrotron facility (DESY, Hamburg). Atomic force microscopy (AFM) images were obtained on a Nanotec Electro´nica system21 using Nanosensortips (nominal radii ) 30 nm, spring constant ) 40 N/m, resonance frequency ) 285 kHz) operating in the tapping mode in air under atmospheric conditions. The AFM samples were prepared as thin films on silicon substrates. Only the IF-WS2 sample for AFM imaging was prepared using mica substrate

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Figure 2. DSC thermograms of nanocomposites obtained during heating at 10 °C/min.

containing 0.1 wt % nanoparticles mixed in acetone by sonication for 10 min. Results and Discussion As nanofillers generally affect the thermal stability of nanocomposites, thermogravimetric analysis (TGA) of the PPS/IFWS2 nanocomposites was carried out to establish the limiting processing conditions, and the thermograms are shown in Figure 1. Pure PPS started to lose weight at about 478 °C and was completely decomposed at 565 °C. The addition of IF-WS2 did not influence the initial degradation temperature of PPS. However, the residual fraction of degradation of PPS increased as the IF-WS2 concentration was increased (e.g., around 50% for the nanocomposite with 8 wt % of IF-WS2 at 565 °C). This implies that the IF-WS2 induces a stabilization of the PPS during the final stages of the degradation process in the PPS/IF-WS2 nanocomposites. Apart from the thermal stability, one of the major issues in thermal analysis is the study of the crystallization characteristics of the polymer matrix in the nanocomposite, especially the role of reinforcing particles in the process because the macroscopic properties of the nanocomposites strongly depend on this. In general, the crystallization kinetics of PPS was found to be significantly enhanced by the presence of microscaled or nanoscaled fillers.8,11-16 However, in the case of PPS/IF-WS2 nanocomposites, the DSC scans of the cold-crystallized nanocomposites obtained during the heating of the melt-quenched samples (Figure 2) show four facts that must be considered. First, PPS is an engineering thermoplastic polymer that presents high-temperature resistance with a glass transition temperature around 89 °C and a melting point at 285 °C. Second, the difference between the overall enthalpy change measured from the endotherm and from the exotherm is nonzero, indicating that it is not possible to quench the PPS to a perfectly amorphous state. Third, only one prominent exotherm was observed, indicating that for all nanocomposites the cold-crystallization process took place from a single homogeneous phase. Finally, the cold-crystallization rate of PPS presented an unusual behavior depending on the IF-WS2 concentration. A drastic change from retardation to promotion of the crystallization was observed depending on the nanoparticle content (i.e., increase or decrease of the crystallization temperature), whereas the change in the melting temperature of the nanocomposites with respect to neat PPS was negligible. On the basis of the experimental results and data analysis by combining techniques able to assess the morphology, crystallinity, thermal properties of the crystals, and relaxation

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Figure 3. WAXS diffractograms obtained at room temperature from nanocomposite films quenched from the melt in liquid nitrogen.

Figure 5. Liu plots of the cold-crystallization of (a) PPS and (b) PPS/ IF-WS2 (1 wt %).

Figure 4. Variation of the degree of conversion (x) versus time for cold-crystallization of PPS/IF-WS2 (2 wt %) nanocomposite.

behavior (which will be discussed later), the following scenario may be described. The addition of IF-WS2 has two opposite effects on the crystallization behavior of PPS depending on the nanoparticle content. On the one hand, the IF-WS2 hinders the nucleation of PPS, and on the other hand the IF-WS2 promotes the mobility and diffusion of PPS chains, which results in a faster crystallization rate than that observed for neat PPS. To understand the crystallization behavior of the nanocomposites, first it is necessary to analyze the crystallization of pure PPS. As shown in Figure 2, the PPS undergoes a cold-crystallization process during heating at a high enough temperature above the glass transition temperature where the crystallizable polymer chains possess enough segmental mobility to crystallize. Taking into account the difference between the crystallinity measured under the endotherm (i.e., 43%) and under the exotherm (i.e., 66%), the crystallinity at room temperature is around 23% if the crystalline perfection and reorganization processes during heating are assumed to be negligible. Thus, the presence of the PPS crystals at room temperature act as autonucleating agents for PPS crystallization. For the PPS nanocomposites, the addition of a small concentration of IF-WS2 (i.e., less than or equal to 0.1 wt %) may disrupt the autonucleation process of PPS crystals, which could induce an increase in the cold-crystallization temperature of PPS without variation of the crystallinity (i.e., around 43%). This implies the absence of a nucleating role of IF-WS2 in the crystallization of PPS, which can also confirmed by the WAXS diffracto-

grams presented in Figure 3. The two diffraction peaks at 2θ angles of 18.5° and 20.5° corresponding to the (110) and (200) planes of the orthorhombic structure of PPS22 appear to be present only at higher concentrations of IF-WS2. However, the further addition of IF-WS2 caused a significant reduction of the cold-crystallization temperature of PPS as well as the corresponding crystallinity (Figure 2). To understand the experimental data obtained for the coldcrystallization behavior of PPS/IF-WS2 nanocomposites, we obtained various DSC curves at different heating rates to determine the kinetic parameters (i.e., rate constant, crystalline transformation, activation energy, nucleation activity, etc.). The physicochemical changes during an exothermic event in DSC are complex and involve multistep processes occurring simultaneously at different rates. It is well-known that the crystallization from the melt-state involves simultaneous nucleation and growth during the initial stage, whereas from the region nearer to the crystallization peak the process is mainly dominated by growth, that is nucleation if it at all exists becomes negligible from the region nearer to the crystallization peak and the process becomes isokinetic. For crystallization from the glassy state, which is governed by chain mobility rather than by nucleation, the diffusion rate of the very entangled longer chains is much smaller than that for the low molecular weight chains, leading to a reduced crystallization rate. The dynamics of the cold-crystallization behavior is presented in Figure 4, which shows an example of the evolution of the crystalline transformation or conversion (x) of a PPS nanocomposite with 2 wt % of IF-WS2 at different heating rates. The conversion curves shifted over a longer time with decreasing heating rate, suggesting that the diffusion of PPS becomes very difficult for cold-crystallization. The kinetics of crystallization can be described using an alternative model recently proposed by Lui et al.23 who

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TABLE 1: Values of r and f(T) Versus Conversion (x) Based on the Liu Model for PPS/IF-WS2 Nanocomposites IF-WS2 content (%) x (%) Xx (%)a 0

0.05

0.1

0.5

1

2

4

8

10 30 50 70 90 10 30 50 70 90 10 30 50 70 90 10 30 50 70 90 10 30 50 70 90 10 30 50 70 90 10 30 50 70 90 10 30 50 70 90

4.3 12.9 21.5 30.1 38.7 4.2 12.6 21.0 29.4 37.8 4.1 12.3 20.5 28.7 36.9 3.4 10.2 17.0 23.8 30.6 4.0 12.0 20.0 28.0 36.0 1.9 5.7 9.5 13.3 17.1 1.9 5.7 9.5 13.3 17.1 1.1 3.3 5.5 7.7 9.9

R

f(T)

1.10 1.14 1.17 1.19 1.23 1.10 1.12 1.14 1.18 1.21 1.18 1.18 1.20 1.22 1.22 1.26 1.28 1.26 1.28 1.29 1.15 1.15 1.17 1.21 1.27 1.27 1.26 1.28 1.32 1.31 1.28 1.27 1.29 1.34 1.37 1.23 1.25 1.29 1.31 1.27

118.10 130.32 141.64 155.76 202.72 121.58 128.95 139.30 154.78 180.64 143.48 150.22 158.57 171.74 188.03 154.38 169.97 170.19 187.96 210.00 126.56 131.25 140.09 155.89 191.45 148.21 154.93 169.81 193.20 214.99 152.84 156.63 171.30 196.88 236.00 136.22 151.02 169.25 185.14 193.18

∆E NA (%)b (kJ/mol)c 1.00

88.1

1.27

112.4

1.28

106.6

0.65

55.9

0.77

69.9

0.74

57.7

0.87

68.0

0.82

62.3

ln φ ) ln f(T) - R ln t

(1)

where f(T) ) [k′(T)/k]1/m, referring to the value of cooling rate chosen at unit crystallization time, when the system has a certain degree of crystallinity, R is the ratio of the Avrami exponents to Ozawa exponents (i.e., R ) n/m), and φ is the heating rate. Plotting ln φ versus ln t at a given degree of conversion yielded a linear representation, as shown in Figure 5. This indicates that the Lui model provided a satisfactory description of the cold-crystallization for PPS/IF-WS2 nanocomposites. The kinetic parameters, ln f(T) and R, which are derived from the slope and the intercept of those lines, are listed in Table 1. The values of f(T) systematically increased with the conversion, indicating that, at unit crystallization time, a higher heating rate should be used to obtained a higher conversion but the values of R are almost constant, at about 1.1-1.2. A similar trend was observed for PPS/IF-WS2 nanocomposites. However, in the case of the values of f(T) obtained for PPS nanocomposites, the results indicate that no relationship can be drawn between the f(T) values for PPS and the concentration of IF-WS2. For the nanocomposites with a higher concentration of IF-WS2 (e.g., 8 wt %), f(T) reaches the same values as those for PPS even for very low crystallinities developed during the cold-crystallization (i.e., Xx,8 wt % ) 1.1-9.9% and Xx,PPS ) 4.3-38.7%). This fact suggests that the cold-crystallization behavior of PPS is controlled by the crystallinity developed at room temperature, due to the highly efficient nucleating effect of IF-WS2, after the quenching process (explanation of Figure 2) rather than

a Cold-crystalinity calculated at conversion x. b Nucleation activity calculated using Dobreva and Gutzow equation. c Effective energy barrier calculated using Kissinger’s equation.

Figure 7. Kissinger plots for evaluating effective energy barrier of PPS/IF-WS2 nanocomposites.

Figure 6. Dobreva plots for evaluating nucleation activity of IF-WS2 in PPS/IF-WS2 nanocomposites.

combined the Avrami and Ozawa models.24,25 The final expression of their model can be written as follows:

Figure 8. Long period values obtained during heating at 10 °C/ min from nanocomposite films quenched from the melt in liquid nitrogen.

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NA )

B* B

(4)

where φ is the heating rate, A is a constant, and ∆Tp is the difference between the melting temperature and the temperature corresponding to the peak temperature of the DSC coldcrystallization curves. B is a parameter that can be calculated from the following equation:

B)

2 ωσ3Vm 0 3nkBTm ∆Sm2

(5)

where ω is a geometrical factor, σ is the specific energy, Vm is the molar volume of the crystallizing substance, n is the Avrami exponent, ∆Sm is the entropy of melting, kB is the Boltzmann constant, and Tm0 is the infinite crystal melting temperature. Therefore, the nucleation activity NA was simply calculated by the ratio of the slopes of log φ versus 1 ) ∆T2 with and without IF-WS2 filler using eqs 2 and 3. The linear plots of these representations for PPS and PPS/IF-WS2 nanocomposites are shown in Figure 6. From the slopes of these lines, the values of B and B* can be calculated for PPS and the nanocomposites respectively, and the results of all PPS/IF-WS2 nanocomposites

Figure 9. AFM images of neat PPS and IF-WS2 nanoparticles: (a) melt-quenched PPS, (b) melt-crystallized PPS obtained at room temperature after fast cooling, (c) melt-crystallized PPS obtained at room temperature after slow cooling, and (d) high magnification image of (c) and (e) neat IF-WS2 nanoparticles.

the concentration of IF-WS2 in the PPS nanocomposites at the cold-crystallization process. For these nanocomposites, it is difficult to analyze the cold-crystallization process with a single equation because there are many parameters that must be considered simultaneously in the Lui model (i.e., autonucleation process, crystallinity developed before the coldcrystallization, etc.). Hereafter, the nucleation activity and effective energy barrier of crystallization will be presented to understand the unique dependence of the crystallization rate of PPS with composition. Dobreva and Gutzow26,27 suggested a simple method for calculating the nucleation activity for foreign substrates. It is known that the nucleation activity (NA) decreases with the addition of fillers. If the foreign substrate is extremely active, NA approaches 0, whereas for inert particles, NA approaches 1. For homogeneous nucleation, the heating rates can be written as:

ln φ ) A -

B ∆T2p

(2)

whereas, for the heterogeneous case,

ln φ ) A -

B* ∆T2p

(3)

Figure 10. AFM images of melt-crystallized nanocomposites obtained at (a,b) faster and (c,d) slower cooling rates.

New Polyphenylene Sulfide Nanocomposites are listed in Table 1. It can be seen that the values of nucleation activity for PPS/IF-WS2 (0.05 wt %) and PPS/IF-WS2 (0.1 wt %) are close to 1, implying that IF-WS2 is not nucleating the PPS matrix. However, increasing in the concentration of IFWS2 provoked a reduction in the values of NA, indicating that IF-WS2 was acting effectively as a nucleating agent in the PPS matrix. This unique dependence of the nucleating activity of IF-WS2 on the PPS matrix justifies the variation the coldcrystallization temperature of PPS with the content of IF-WS2 (Figure 2). On the other hand, the crystallization activation energy, or effective energy barrier ∆E, can be used to estimate the growth ability of the chain segments. The higher ∆E, the more difficult

J. Phys. Chem. B, Vol. 113, No. 30, 2009 10109 is the transport of macromolecular segments to the growing surface. Considering the variation of the peak temperature Tp with the heating rate φ, ∆E could be derived from the Kissinger equation through eq 6:28

()

ln

φ ∆E ) constant RTP T2p

(6)

where R is the universal gas constant, the rest of the parameters being described previously. Plots of ln(φ/Tp2) versus 1 ) Tp are shown in Figure 7 and the values of ∆E all PPS/IF-WS2

Figure 11. Dynamic mechanical analysis of nanocomposites: (a) storage modulus versus temperature, (b) value of storage modulus obtained at room temperature versus IF-WS2 concentrations, and (c) loss tangent versus temperature.

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nanocomposites are listed in Table 1. It can be seen that, in the nanocomposites with lower concentrations of IF-WS2 (i.e., less than or equal to 0.1 wt %), the values of ∆E observed were higher than that of PPS. This implies that the transport of the macromolecular segments to the growing surface of PPS in the nanocomposite is difficult and crystallization is slower because of the inactive nucleating role of IF-WS2 nanoparticles. However, the increase in the concentration of IF-WS2 caused a decrease of ∆E, which facilitated the crystallization of the molecular chains of PPS and increased the crystallization rates due to the nucleation activity of the IF-WS2. These observations on the nucleation activity and effective energy barrier of cold-crystallization are quite common to the general understanding of the role of nanofillers toward polymer crystallization.3,4,11-16 In the same way, the results of the crystal long period are in perfect agreement with the DSC results. As an example, Figure 8 presents the temperature evolution of the long period (L) of PPS and its nanocomposites obtained from the scattering maxima of real-time SAXS measurements using synchrotron radiation. It can be found that the change from retardation to promotion of crystallization of PPS is associated with the variation of L with IF-WS2 content. The L values also suggest that IF-WS2 is absent from the interlamellar spaces of the PPS crystals. In all cases, the PPS nanocomposites experiment crystalline reorganization and perfection during heating, which leads to the increase of the long period (L) until melting. It is well-known that the morphology of the nanocomposites, mainly related with the dispersion of the nanoparticles in the polymer matrix, is one of the most important criteria for a successful preparation of polymer nanocomposites. For this purpose, we have used atomic force microscopy (AFM) to perform a detailed study of the morphology at the nanometer scale. Figure 9 shows the topographic AFM images of the individual components of the nanocomposites (i.e neat PPS and IF-WS2 nanoparticles). From the observation of neat PPS, it is clear that the thermal history has a significant effect on the morphology. When PPS was rapidly cooled from the melt-state, lamellar crystals were observed (part b of Figure 9), whereas the well-defined spherulite texture was developed (part c of Figure 9) for the sample obtained after slow cooling (the box in this figure clearly shows stacks of lamellae). No traces of crystalline morphology could be detected when the molten films were quenched in liquid nitrogen (part a of Figure 9). In the case of the AFM image of IF-WS2 nanoparticles presented in part d of Figure 9, the size of the quasi-spherical IF-WS2 particles was observed to be very uniform over the whole examined area, with an average size of around 50 nm. The estimated value calculated from the AFM image was lower than that obtained for the as-received particles analyzed by SEM.3 The most significant change of the morphology of PPS was observed when the IF-WS2 nanoparticles were added to the polymer matrix. Figure 10 shows direct observation of the dependence of the crystallization of PPS on the IF-WS2 concentration, already evidenced by DSC and SAXS. Indeed, the appearance of the lamellar crystals and the spherulitic texture for nanocomposite with 0.1 wt % of IF-WS2 is direct proof of the absence of the nucleation effect of IF-WS2 on the crystallization of PPS, which is in good agreement with previous DSC results. At this concentration, IF-WS2 nanoparticles cannot be observed easily because they tend to be covered by the polymer. However, an increase in the IF-WS2 content may disrupt the crystalline superstructure of the PPS due the large increase in the crystallization rate of PPS (parts b and d of Figure 10). In this case, the resolution of the AFM images was insufficient to

Naffakh et al. disclose internal details of the morphology of PPS, and the images obtained reveal only the individually dispersed IF-WS2 nanoparticles. The dynamic mechanical properties of the nanocomposites were also studied and corrected for the morphology of the materials. The dynamic mechanical technique is more sensitive than DSC, providing a deeper insight to relaxation processes associated with the complex behavior of these nanocomposites. Figure 11 shows the effect of the nanoparticle content on the storage modulus and loss tangent (tan δ) of PPS. Clearly, the addition of IF-WS2 generated an increase in the storage modulus (part a of Figure 11). A remarkable improvement in the dynamic mechanical performance of PPS, between 40-75%, was observed for the nanocomposites with high concentrations of IFWS2 (part b of Figure 11). This suggests that the acceleration of the crystallization rate of PPS due to the nucleation effect of the IF-WS2 favors the formation of small imperfect crystals, which can provide excellent rigidity to the crystalline PPS.29 Moreover, the presence of IF-WS2 provoked a more complex behavior in the loss tangent spectra, and various relaxation processes could be observed, as shown in part c of Figure 11. As mentioned in the DSC study, on the one hand, the presence of 0.1 wt % of IF-WS2 reduced the crystallization rate of PPS, contributing to the formation of more amorphous phase. As a result, the glass transition temperature (i.e., the first relaxation peak) decreased and a second relaxation peak appeared, which may contribute to the cold-crystallization process. As the temperature increased, a third broad relaxation peak was observed, which may be related to the reorganization of the crystals on heating. On the other hand, the change from retardation to promotion of the crystallization of PPS with increasing IF-WS2 content was also evidenced by the increase in the glass transition temperature of PPS (e.g., nanocomposie with 1 wt % of IF-WS2). Conclusions Tungsten disulfide (WS2) nanoparticles have proved to be effective in breaking the performance ceiling of current polymer nanocomposites. In particular, desired thermal behavior and morphology of high-performance engineering thermoplastics such as polyphenylene sulfide (PPS) can be achieved by controlling the concentration of IF-WS2 nanoparticles. A drastic change from crystallization retardation to crystallization promotion was observed as a function of the nanoparticle content. On the basis of the experimental and theoretical analyses, the study of the cold-crystallization kinetics provides a picture describing the transformation of PPS molecules from the nonordered state to the ordered state. The nucleation activity and effective energy barrier confirmed the unique dependence of the cold-crystallization behavior of PPS on composition. In the same way, the bottom-up morphology evidenced by AFM analysis was a direct manifestation of the unique composition-dependent crystallization behavior of PPS. Furthemore, the reorganization and perfection of the PPS crystals in the nanocomposites during heating were also supported by real-time measurements using synchrotron X-ray diffraction. Finally, in this study we have shown that these well-dispered nanoparticles are a potential candidate to improve the mechanical properties of PPS. Acknowledgment. Dr. M. Naffakh thanks the CSIC for a postdoctoral contract (I3PDR-6-02), financed by the European Social Fund. This research was also supported by the European Union sixth Framework Program (FOREMOST project, NMP3CT-2005-515840), the European Commission for the X-ray

New Polyphenylene Sulfide Nanocomposites synchrotron experiments performed at the Soft Condensed Matter A2 beamline at HASYLAB (DESY-Hamburg, I-20060118 EC), and the Spanish MICINN for national projects (NAN200409183-C10-02 and MAT2006-13167-C01). The authors would like to thank Dr. S. Funari for his technical assistance in the synchrotron experiments. References and Notes (1) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (2) Zhu, Y. Q.; Sekine, T.; Li, Y. H.; Wang, W. X.; Fay, M. W.; Edwards, H.; Brown, P. D.; Fleischer, N.; Tenne, R. AdV. Mater. 2005, 17, 1500. (3) Naffakh, M.; Martı´n, Z.; Fanegas, N.; Marco, C.; Go´mez, M. A.; Jime´nez, I. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 2309. (4) Naffakh, M.; Martı´n, Z.; Marco, C.; Go´mez, M. A.; Jime´nez, I. Thermochim. Acta 2008, 472, 11. (5) Rapoport, L.; Nepomnyashchy, O.; Verdyan, A.; Popovitz-Biro, R.; Volovik, Y.; Ittah, B.; Tenne, R. AdV. Eng. Mat. 2004, 6, 44. (6) Hou, X.; Shan, C. X.; Choy, K. L. Surf. Coat. Technol. 2008, 202, 2287. (7) Brady, D. G. J. Appl. Polym. Sci. Appl. Polym. Symp. 1981, 36, 23. (8) Cheng, S. Z. D.; Wunderlich, B. Macromolecules 1987, 20, 2802. (9) Shangankuli, V. L.; Jog, J. P.; Nadkani, V. M. J. Appl. Polym. Sci. 1988, 36, 335. (10) Choa, M. H.; Bahadura, S.; Anderegg, J. W. Tribol. Int. 2006, 39, 1436.

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