Blends of polypropylene and ethylene octene comonomer with conducting fillers: influence of state of dispersion of conducting fillers on electrical conductivity.INTRODUCTION
In recent years conducting fillers (viz., multiwall carbon nanotubes (MWNT), carbon black (CB) etc.) incorporated polymer blends have received immense interest in both academic and industrial fields due to the possibility to achieve conducting polymer composites utilizing unique blends morphology associated with the blend compositions during melt-mixing. In this connection co-continuous morphology provides a unique opportunity to confine conducting fillers in one of the phases or at the interface in which conducting fillers establish "network-like" structure leading to geometrical contacts between the filler particles. This has been demonstrated as "double percolation" in the literature and the restriction of the filler network depends primarily on thermodynamic factors associated with the surface energy differences between the filler and the polymer phase and kinetic factors involving melt-mixing parameters. In addition, melt viscosity ratio also plays an important role in confining filler specifically in one of the phases. The influence of thermodynamic factor, kinetic factor, and the melt viscosity ratio in confining the conducting filler in one of the phases has been demonstrated successfully in PP/HDPE with CB (1), (2), PP/PA6 with CB (3), (4), PP/ABS with MWNT (5), PA6/ABS with MWNT (6), PC/PE with MWNT (7). However, in few cases it has been observed that one of the factors (viz., wetting parameters, associated with surface energy difference) may dominate over the other (melt viscosity ratio or kinetic factors) in deciding the localization of the filler specifically in one of the phases.
Blends of polypropylene/ethylene octene comonomer (PP/EOC) have significant commercial importance due to the better processability observed when compared with PP/EPDM (ethylene propylene diene monomer) blends where either EOC or EPDM are used to improve the impact resistance of PP. Substantial amount of work has been carried out where impact modification of PP homopolymer as well as copolymer has been achieved with EOC (8-12). In case of PP/EOC blends, it has been found that the interfacial tension is found to decrease as octene content of EOC is increased, e.g., interfacial tension of 1.5 [+ or -] 0.16 dyne [cm.sup.-1] at an initial octene level of 9% decreased to 0.56 [+ or -] 0.07 dyne [cm.sup.-1] at an octene content of 24% which manifests a better compatibility between PP/EOC blends at higher octene content (13).
Even if there are substantial literatures available in connection with morphology and impact toughness of PP/EOC blends; however, conducting fillers incorporated PP/EOC blends have not received much attention. It is to be noted that conducting filler based PP/EOC blends may fulfill the requirements of electronic packaging materials (e.g., electronic housing) due to the combination of electrical conductivity (either conducting or dissipating) and mechanical properties.
The work has been undertaken in PP/EOC blends with conducting fillers like CB and MWNT to understand the role of morphology in controlling the electrical conductivity of the respective composites. The state of dispersion and subsequent "network-like" structure formation of the fillers is evaluated through morphological analysis. Overall, structure property relationship studies are conducted in conducting filler incorporated PP/EOC blends.
Materials and Specimen Preparation
Polypropylene (PP) was obtained from Reliance Industries Ltd (H200MA) with melt flow index (MFI) of 23 (230[degrees]C/2.16 kg load) (here after assigned as 23P). A commercial grade of polyolefin elastomer Engage 8150 (EOC, with octene content of 39 wt%) was obtained from DuPont Dow Elastomers, having MFI 0.5 (190[degrees]C/2.16 kg load). CB (grade: Vulcan XC72) was obtained from Cabot India Ltd. The particle size of CB used is in between 100 and 200 nm. CCVD synthesized MWNT were obtained from Nanocyl CA Belgium (NC-3100, L/D: [10.sup.2] to [10.sup.3], purity >95%).
Neat PP/EOC blends and with CB/MWNT were prepared by melt-mixing in a conical twin-screw micro-compounder (Micro 5, DSM Research, Netherlands) at 240[degrees]C with a rotational speed of 150 rpm. All blend components were predried in vacuum oven at 80[degrees]C for at least 24 hr. During melt mixing a two step sequential mixing was adopted wherein PP was initially compounded for 10 min (Step 1) followed by EOC for 5 min (Step 2) in case of PP/EOC binary blends in which EOC content was varied from 20 to 55 wt%. Similar sequential mixing protocol was adopted for PP/EOC blends with either CB or MWNT. In blends with either CB or MWNT, PP and the filler was initially melt mixed for 10 min followed by EOC for 5 min. CB was varied from 0 to 20 wt% and MWNT was varied from 0 to 5 wt% in the respective PP/EOC blends. The following notations were adopted for the composites; for PP/EOC blends with CB:23PXEYCBZ and for PP/EOC blends with MWNT:23PXEYNTZ (where 23P indicates PP with 23 MFI, E represents EOC and X, Y, Z represents the amount of PP, EOC and the filler in weight percent). Injection molded samples (according to ASTM D 638, Type V) were prepared using mini-injection molding machine from DSM Research Netherlands. The injection-molding parameters maintained for all the compositions were injection pressure 3 bar, melt temperature 240[degrees]C, mold temperature 60[degrees]C, holding time of 60 sec and cooling time of 2-3 min.
Thermo Gravimetric Analysis. Thermal stability of PP/EOC blends, PP/EOC blends with CB/MWNT along with pure PP and EOC was studied using SDTQ 600 of TA Instruments in the temperature range between room temperature and 600[degrees]C at [N.sub.2] atmosphere.
Scanning Electron Microscopy. Morphological features of binary neat PP/EOC blends along with the state of dispersion or preferential localization of either CB or MWNT in PP/EOC blends was studied by scanning electron microscopy (SEM). Extrudate samples were cryofractured in liquid nitrogen and etched in n-heptane at 50-60[degrees]C to selectively remove EOC. The etched surface was gold sputtered to avoid the charging of the sample. These samples were then observed under SEM using Hitachi S3400N. Three micrographs of different magnifications taking into considerations of around 200 etched holes were considered to evaluate the number average diameter ([D.sub.n]) and weight average diameter ([D.sub.w]). The number average diameter ([D.sub.n]) and the weight-average diameter ([D.sub.w]) of the droplets were calculated using the Eqs. 1 and 2:
[D.sub.n] = ([summation][n.sub.i][d.sub.i])/([summation][n.sub.i]) (1)
[D.sub.w] = ([summation][n.sub.i][d.sub.i.sup.2])/([summation][n.sub.i][d.sub.i]) (2)
where [n.sub.i] indicates the number of droplets having [d.sub.i] as their diameter.
AC Conductivity. The ac electrical conductivity measurements were performed on the injection molded samples (across the thickness) in the frequency range between [10.sup.-1] and [10.sup.6] Hz using Alpha high resolution analyzer coupled to a Novocontrol interface (broad band dielectric converter). The samples were placed between the two gold electrodes that were pressed together with a screw. The dc conductivity of the samples was determined from the ac conductivity plots in the region of low-frequency plateau by fitting power law equation [Eq. 3]
[[sigma].sub.ac] = [[sigma].sub.dc] + A[[omega].sup.n], 0<n<1 (3)
Melt Rheology. Rheological measurements were performed on PP/EOC blends with MWNT composites using Rheo Stress 300 rheometer from Thermo Haake at 240[degrees]C under nitrogen atmosphere with parallel plate geometry (plate diameter of 35 mm, gap of 1-2 mm). Frequency sweeps were carried out between 0.01 and 100 rad/sec. The stresses used were chosen to be within the linear viscoelastic range.
RESULTS AND DISCUSSION
Thermo Gravimetric Analysis
Thermo gravimetric analysis (TGA) was carried out to study the thermal stability of PP/EOC blends, PP/EOC blends with CB/MWNT along with pure PP and EOC. It can be seen in Fig. 1 that EOC is stable up to a higher temperature (401[degrees]C) when compared with PP which is stable up to 332[degrees]C and the thermal stability of blends is found to be more than PP but lower than EOC. Table 1 enlists the blends compositions and their degradation temperatures. Figure 1 exhibits the thermal stability of the composites of PP/EOC with MWNT and CB. The composites with CB and MWNT are stable even above 400[degrees]C. It is evident from the figure that in presence of fillers the onset of thermal degradation temperature is significantly enhanced which is also reported for various filled systems (14). The increase in thermal stability of PP/EOC blends in presence of fillers may be attributed to both the excellent thermal stability of CNT and their interactions with the polymer matrix.
[FIGURE 1 OMITTED]
TABLE 1. Sample codes and the compositions of the PP/EOC blends. Sample No. Sample code PP (wt%) EOC (wt%) Degradation temperature ([degrees]C) 1 23 PP 100 0 332 2 EOC 0 100 401 3 23P80E20 80 20 379 4 23P70E30 70 30 363 5 23P60E40 60 40 -- 6 23P55E45 55 45 -- 7 23P50E50 50 50 -- 8 23P45E55 45 55 386
Figure 2 shows matrix-dispersed droplet type of morphology in 80/20 and 70/30 PP/EOC blends (Fig. 2a and b) in which the average droplet size of EOC increased with increase in EOC content (see Table 2). The increase in average droplet size is due to the coalescence phenomenon of the EOC droplets possibly due to the high interfacial tension between PP and EOC. At 40 wt% EOC level, one can find partial co-continuous structure along with elongated droplets of EOC (figure not shown here), whereas at 45 wt% EOC level, blends exhibit fully developed co-continuous structure. However, one can even observe mixed morphology of partial co-continuous structure along with elongated fibrillar morphology at 50/50 (figure not shown here) and 45/55 PP/EOC blends. This observation is perhaps related to the increase in higher viscous EOC component in the PP/EOC blends. It is to be noted that the width of co-continuous range observed in PP/EOC blends may be governed mainly by melt-viscosity ratio. In this context phase morphology development for the PP/EOC blend system melt-mixed at 180[degrees]C with EOC content varying from 10 to 90 wt% has been reported (15) and co-continuity has been observed with PP level of 50-60 wt% and it persisted till 30 wt% of EOC. However, PP used in that work had a MFI of 1.1 when compared with the present work where MFI of PP is 23, which indicates much lower melt viscosity.
[FIGURE 2 OMITTED]
TABLE 2. Sample codes with number average ([D.sub.n]) and weight average diameter ([D.sub.w]) of the composites. Sample No. Sample code [D.sub.n] ([mu]m) [D.sub.w] ([mu]m) 1 23P80E20 1.53 1.66 2 23P70E30 1.61 1.72 3 23P80E20CB10 0.98 0.93 4 23P80E20NT1 1.73 1.85 5 23P80E20NT3 0.82 0.86 6 23P80E20NT5 0.65 0.81
McNally et al. (16) studied PP/EOC blend system in connection with impact modification of PP and subsequent morphology development by varying EOC content from 0 to 30 wt%. As the concentration of EOC was increased it was observed that the dispersed phase transformed from spherical domains to a more elongated domains, the tendency to form co-continuous morphology was noticed even at 25 wt% EOC level.
It is generally accepted that the torque value during melt mixing is indicative of variation in melt viscosity as a function of processing parameters and constituent components. Figure 3 exhibits the variation of the torque value (observed at the end of 15 min during melt mixing) of PP/EOC blends with increase in CB content. A similar observation is also noticed in case of PP/EOC blends with MWNT (figure not shown here). The observation suggests that the filler enhances the melt viscosity thereby increases the torque values for all the blend compositions with increase in filler content. Overall torque values are higher for blends with higher EOC content due to the higher melt viscosity of EOC phase.
[FIGURE 3 OMITTED]
Figures 4 and 5 exhibit the representative SEM micrographs of the morphological analysis carried out to understand the role of CB and MWNT in influencing the phase morphology of 80/20 PP/EOC and 55/45 PP/EOC blends respectively wherein CB content was varied from 10 to 20 wt% and MWNT level was varied from 1 to 5 wt%. These two compositions were chosen as representative compositions for the matrix-dispersed phase morphology (80/20) and co-continuous morphology (55/45). It is evident that on increasing CB content the domain size of EOC is found to decrease significantly in PP/EOC blends, whereas in the case of MWNT, domain size of EOC increased at low MWNT content followed by a decrease at higher MWNT level (see Table 2). It is worth mentioning that at higher CB content (20 wt%) the domain size of the EOC phase decreased drastically and hence it was difficult to evaluate the average droplet size of the EOC phase. This observation can be attributed to the compatibilization action of CB as well as MWNT in suppressing coalescence. The action of compatibilization is presumably due to kinetic barriers between the droplets provided by either CB particles or MWNT. At this point it is to be noted that at low MWNT level (1 wt%), domain size and size distribution of EOC is found to increase when compared with 80/20 PP/EOC blends in which MWNT possibly could not act as a compatibilizer. Further, the compatibilizing action of CB particles once again manifested in 55/45 PP/EOC blends wherein one can find finer ligaments of co-continuous structures which remain almost unaltered while varying CB content from 10 to 20 wt%. In addition, the 55/45 PP/EOC blends with 5 wt% MWNT show comparatively finer co-continuous structure when compared with 1 wt% MWNT. In brief, the compatibilization action of either CB particles or MWNT is observed in PP/EOC blends irrespective of blend morphology. A similar compatibilizing action of MWNT is also found in PP/ABS blends (5).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Figure 6 exhibits aggregated CB particles or MWNT network in the EOC phase in 80/20 and 55/45 PP/EOC blends, it seems both CB and MWNT migrated to the EOC phase during melt-mixing. However, solution experiment and AC electrical conductivity results (discussed in the subsequent section) indicated the formation of network like structure of CB particles or MWNT beyond 10 wt% CB content and 2 wt% of MWNT in the respective composites wherein the CB/MWNT are found to be in the PP phase as well. It is well reported that the interfacial tension between PP and EOC in PP/EOC blends is found to decrease as the octene content of EOC increases (13). In our case the high octene content in the EOC (39 wt%) suggests that the interfacial energy difference between PP/MWNT and between EOC/MWNT is close enough which further leads to migration of MWNT to the EOC phase during melt mixing though the melt viscosity of EOC phase is significantly higher than the PP phase. Hence, the thermodynamic aspect dominates over the melt viscosity factor in governing the overall state of dispersion of the filler in PP/EOC blends. Similar results have been reported for PP/HDPE/CB system where CB has been found to localize in the higher viscous HDPE phase (1).
[FIGURE 6 OMITTED]
The fillers are generally known to migrate to the phase with a lower melt viscosity. In contrary, we observed aggregation of the fillers in the higher viscous EOC phase despite the processing conditions employed may be due to the fact that both CB and MWNT have a strong affinity toward EOC phase. As evident from Figure 6, MWNT are found to bridge between the phases because of their larger length dimension (about 10 [micro]m) when compared with the phase dimensions of the blends.
Electrical Conductivity of PP/EOC Blends With CB and MWNT
Figure 7a and b show the variation in AC electrical conductivity of PP/EOC + CB composites with varying EOC content in which CB loading was fixed at 15 and 20 wt%, respectively. The bulk electrical conductivity of PP and EOC increases with increase in frequency as expected for an insulating material (not shown here). In addition, all the compositions exhibit insulating behavior at 7.5 and 10 wt% CB level (not shown here). As observed from Figure 7a, 45/55, 50/50, and 60/40 PP/EOC blends with 15 wt% CB showed insulating behavior. 55/45 blends exhibit a frequency independent plateau ([10.sup.-7] S [cm.sup.-1]) up to a critical frequency ([[omega].sub.c]) above which the conductivity dispersion is observed. Both 70/30 and 80/20 blends exhibit a frequency independent plateau over the entire measured frequency range indicating the formation of percolative network like structure of the filler. Interestingly, blends with EOC content 20 and 30 wt% showed frequency independent plateau over the entire measured frequency range at similar CB content. At 20 wt% CB (Fig. 7b) all the composites show conducting behavior although there is a difference of about three orders of magnitude of the conductivity value of blends with co-continuous morphology of 45 wt% EOC and 50 wt% EOC. The electrical conductivity of the PP/EOC blends with CB and MWNT is highest for 80/20 composition and it steadily decreases as PP content decreases in the blends. The percolation threshold for the PP/EOC + CB system seems to lie between 10 and 15 wt% of CB for the 80/20 and 70/30 blend compositions whereas it increases to 15-20 wt% for blends with EOC content higher than 30 wt%. The percolation threshold is found between 2-3 wt% for PP/EOC + MWNT system. The electrical conductivity of 60/40 composition is found to be lower than 55/45 composition for all CB loadings. Similar observations were found in case of MWNT (Fig. 7c and d) as well but the lower values of conductivity in case of 60/40 composition of PP/EOC has been found to be consistent for 4 and 5 wt% MWNT when compared with 55/45 blends. The observation regarding electrical conductivity results can be better understood by the morphological observation where it is evident that both CB and MWNT tend to migrate to the EOC phase despite its high melt viscosity and the processing conditions where PP and the conducting filler are melt mixed initially for 10 min. followed by the addition of EOC and mixing continued for another 5 min. However, EOC seems to be facilitating a high degree of aggregation of the fillers leading to poor dispersion and hence decrease in conductivity as EOC content increases in the blends. The aggregation of filler may also be the reason for the increased electrical percolation threshold leading to the increased concentration of filler required to form a three dimensional network like structure to form a conducting pathway throughout the bulk of the composite. Electrical conductivity data suggest that CB and MWNT are present in both the phases and not selective toward PP phase which has a lower melt viscosity which has been confirmed by performing solution experiments (results not shown here). At this point, it is to be noted that "double percolation" phenomenon associated with co-continuous morphology is not found to be applicable in PP/EOC blends of this specific set of MFI of PP and EOC and with the processing parameters employed during melt mixing. It is expected that with the help of a suitable modifier and compatibilizers one can restrict MWNT or CB particles in specifically one of the phases or at the interface in PP/EOC blends provided a suitable grade of PP (with higher MFI) could be utilized. In context to "double-percolation" it is to be pointed out at this stage that a percolation threshold ~0.4-0.5 wt% MWNT has been reported in 55/45 PP/ABS blends which is much lower than the percolation threshold of the individual components (5).
[FIGURE 7 OMITTED]
Melt Rheological Response of PP/EOC Blends With MWNT
The melt rheological response during frequency sweep for PP/EOC blends with MWNT along with neat PP and EOC is shown in Figures 8 and 9. It is evident that the melt-viscosity of EOC is found to be much higher when compared with that of PP phase (Fig. 8a). Further, the melt-viscosity of 80/20 blends of PP/EOC and 80/20 PP/EOC blends with 0.5 wt% MWNT are found to be comparable to that of neat PP. 80/20 PP/EOC blends with 3 wt% MWNT exhibit a linear decrease in [eta]* with increasing frequency manifesting the existence of yield stress along with a strong shear thinning behavior. It is reported that the effect of MWNT is pronounced particularly in the low frequency range and this effect diminishes with increasing frequency presumably due to shear thinning behavior (17), (18). Similar observations are found in 55/45 PP/EOC blends (Fig. 9a). A dramatic increase in the storage modulus (G') with increasing MWNT content can be seen for both 80/20 and 55/45 PP/EOC blends (Fig. 8b and 9b). At low frequencies the PP chain is relaxed and exhibit typical terminal behavior with the scaling properties of G' ~ [[omega].sup.2] and G" ~ [[omega].sup.0.91]. With addition of MWNT this terminal behavior disappears and the dependency at low frequencies weakens gradually resulting in a plateau in G' versus frequency curves. This observation is consistent in both 80/20 and 55/45 PP/EOC blends with 3 wt% MWNT (Fig. 8b and 9b) where a plateau in the low frequency regime in G' and a pronounced shear thinning behavior is noticeable in the melt-viscosity indicating the transition from "liquid-like" to solid-like" elastic behavior. The power law dependence of G' on frequency weakens monotonically with increasing MWNT concentration indicating "network-like" structure formation.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Morphological analysis of PP/EOC blends revealed the formation of matrix-dispersed droplet type of morphology up to 30 wt% EOC level in PP/EOC blends, where as 55/45 PP/EOC blends showed fully developed co-continuous structure. EOC content higher than 45 wt% led to the formation of mixed morphology consisting of droplet/fibrillar features along with co-continuous structures. CB and MWNT were observed to show compatibilizing action above certain content, resulting in decrease in the domain size. Further, CB as well as MWNT were found to be aggregated in the EOC phase irrespective of blends compositions and could be related to the lower surface energy difference between EOC and fillers.
The electrical conductivity of the PP/EOC blends with CB and MWNT was found to be highest for 80/20 composition and was found to decrease as EOC content increased in the blends. The percolation threshold for the PP/EOC + CB system lied between 10 and 15 wt% of CB for the 80/20 and 70/30 blend compositions whereas it increased to 15-20 wt% for blends with EOC content higher than 30 wt%. The percolation threshold was found to be between 2 and 3 wt% for PP/EOC + MWNT system. Further all blend compositions with CB content of 20 wt% showed an electrical conductivity independent of complete investigated frequency range (0.1-[10.sup.6] Hz). The highest electrical conductivity of 8 S [cm.sup.[-1]] was observed for 80/20 PP/EOC blends with 20 wt% CB. Maximum electrical conductivity achieved with 5 wt% MWNT was observed to be around [10.sup.-3] S [cm.sup.[-1]] for 80/20 PP/EOC blends. The melt rheological response of PP/EOC blends indicated a strong shear thinning behavior with increasing MWNT content and exhibited a dramatic increase in storage modulus.
The authors thank Mr. S. Bose and Dr. S. L. Kamath (MEMS, IIT Bombay) for their help and experimental support.
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Sheleena Hom, (1) Arup R. Bhattacharyya, (1) Rupesh A. Khare, (1) Ajit R. Kulkarni, (1) Madhumita Saroop, (2) Amit Biswas (2)
(1) Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
(2) Materials and Engineering Research, Polymer Research and Technology Centre, Reliance Industries Limited, Swastik Mill Compound, V. N. Purav Marg, Chembur, Mumbai 400071, India
Correspondence to: Arup R. Bhattacharyya; e-mail: firstname.lastname@example.org
Contract grant sponsor: Reliance Industries Limited, India; contract grant number: 06SP018.
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2009 Society of Plastics Engineers