Formation of new electric double percolation via carbon black induced co-continuous like morphology

Abstract

A new electric double percolation was realized via carbon black self-networking induced co-continuous like morphology (composed of disconnected PU clusters and bands) in polylactide/poly(ether)urethane (PLA/PU) blends. As a result, a simultaneous improvement in electrical conductivity and impact toughness of the blends without compromising strength and modulus has been achieved.

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Simultaneously adding ductile elastomer and rigid fillers into polymers has been intensively investigated.^1–8 Heretofore, numerous studies have proved that the key point is to regulate the phase morphology, especially the distribution of fillers in the matrix blends.^4–6 Depending on the distribution of fillers in the polymer/elastomer/filler ternary system, at least three structures have been reported in the literatures. That is: separated dispersion structure in which elastomer and filler are dispersed in the matrix separately (as shown in Fig. 1a), core–shell structure in which filler is included in the elastomer phase (as shown in Fig. 1b) and filler-network structure in which the elastomer particles are surrounded by the filler network (as shown in Fig. 1c). It has been found that separated dispersion structure contributes to reinforcement in strength and modulus, while core–shell structure promotes to improvement in toughness.^4–7 While filler-network structure can lead to a large enhancement in impact toughness with strength and modulus well maintained.⁸ Therefore, depending on the requirements, one can control mechanical properties over a wide range via designing the phase morphology of polymer/elastomer/filler ternary composites. It would be more interesting if a conductive filler is used, not only good stiffness-toughness balance but also excellent conductivity will be obtained via controlling the morphology of elastomer and the distribution of fillers in the polymer/elastomer/filler ternary composite.

Fig. 1 Schematic of common phase morphologies of polymer/elastomer/filler ternary system.

Forming the double percolated structure, i.e. the percolation of fillers in the fillers-rich phase and the continuity of this phase in the polymer/elastomer blend, is a facile and effective way to obtain conductive polymer composites (CPCs) with very low filler content.^9–11 Unfortunately, the relative high content of elastomer which is needed to realize double percolation usually leads to a significant decrease in strength and modulus.¹¹ Therefore, it is still a great challenge of achieving double percolation at both low content of elastomer and filler content in polymer/elastomer/conductive filler ternary composites.

In our previous study,¹² a unique silica (SiO₂) nanoparticles self-networking induced co-continuous like morphology (composed of discontinuous PU clusters and bands) in PLA/PU/SiO₂ ternary composites was first reported (as shown in Fig. 1d). The formation of this structure is mainly attributed to the strong affinity of SiO₂ with PU and self-networking of SiO₂ nanoparticles, which resulting in a change of droplet phase morphology to co-continuous like morphology. This structure could promote the percolation of the tress fields around PU particles and lead to a remarkable improvement in impact toughness of PLA/PU/SiO₂ ternary composites without sacrificing its strength and modulus. In addition, it has been reported that the conductive particles do not need to directly interact with each other to form a conductive path since the electrons can pass across the thin polymer layers between adjacent conductive particles by tunnelling effect.^13,14 Therefore, it is logical to expect that by the formation of conductive fillers filled co-continuous like morphology in elastomer-toughened polymers, an optimal combination of good stiffness-toughness balance and low percolation threshold could be realized. In this work, the carbon black (CB) nanoparticles with the similar self-networking capability but also conductivity were introduced into PLA/PU blends by melt blending. We expect that CB self-networking could induce the formation of co-continuous like morphology in PLA/PU blends and obtain a new type of double percolated CPC with balanced mechanical properties.

PLA/PU/CB ternary composites were prepared by melt blending in a internal mixer at 190 °C and 60 rpm for 5 min. The weight ratio between PLA and PU was 85 : 15 and the weight fraction of CB was varied from 0 to 10 phr. Then, the obtained mixtures were compression molding at 190 °C for 5 min under 10 MPa to gain the standard specimens for mechanical and electrical testing [experimental details for the materials, sample preparation and characterization are provided in the ESI†].

Fig. 2a–d shows the SEM images of PLA/15PU with various amounts of CB nanoparticles. The PLA/15PU binary blend exhibits a typical sea-island morphology, e.g. discrete PU spherical domains are uniformly dispersed in the PLA matrix (Fig. 2a). With the addition of low content CB (e.g. 1 phr) into the blend, most CB nanoparticles are found to be selectively located in the PU phase but the morphology is almost unchanged except for a slightly increase in PU domain size (Fig. 2b). However, further increasing the content of CB to 2 phr, some independent PU droplets aggregate together to form elongated PU particles (as highlighted by red circles in Fig. 2c). Once the content of CB is raised to 3 phr, as expected, most PU droplets fuse together and form co-continuous like structure, suggesting that 3 phr CB nanoparticles can induce the morphological change from the sea-island morphology to the co-continuous like morphology (as highlighted by red circles in Fig. 2d). To further check this observation, TEM experimental was conducted. From Fig. 2d′, it can be clearly seen that the co-continuous like morphology is composed of disconnected PU clusters and bands, which is quite different from the common co-continuous morphology, where the PU phase is 100% continuous. These observations are well consistent with the results of our previous work in PLA/PU/SiO₂ system.¹² The formation of such special co-continuous like morphology has been proven to be closely associated with the self-networking of nanoparticles. During melt blending, the nanoparticles with the self-networking capability tend to self-aggregate to form a 3D-network in polymer melts and drove adjacent PU droplets approach each other to form co-continuous like morphology in PLA matrix.¹²

Fig. 2 SEM images of the cryofractured surfaces of (a) PLA/15PU, (b) PLA/15PU/1CB, (c) PLA/15PU/2CB, (d) PLA/15PU/3CB and TEM image of (d′) PLA/15PU/3CB.

Fig. 3 presents the impact toughness of PLA and PLA/15PU blend with varying CB concentrations. Although no obvious toughening effect on PLA matrix can be obtained by only introducing PU or CB nanoparticles, the addition of CB nanoparticles leads to a large enhancement in the impact toughness of PLA/15PU blend. But, it should be noted that there is an optimum CB content of toughening effect. For example, the impact strength of PLLA/15PU blend is improved remarkably from 8.12 to 31.80 kJ m⁻² as the CB content is increased from 0 to 3 phr. However, the impact toughness decreases significantly to 2.92 kJ m⁻² when further increasing CB content up to 10 phr. It has been demonstrated that the largely improved impact toughness is mainly attributed to CB nanoparticles self-networking induced special co-continuous like morphology.¹² This structure could promote the percolation of the stress fields around PU particles because the elastomer (PU) particles appear to have greater volume fraction when the filler (CB) is selectively inside them, thus increasing impact strength due to shear banding of the matrix (PLA) at the interface. More importantly, when the impact toughness are significantly enhanced in rubber-toughed polymers, the co-continuous morphology often leads a evident deterioration in strength and modulus,^15,16 however, the co-continuous like morphology can well maintain them (Table 1) because this phase structure consists of discrete PU clusters and bands and the elastomer phase (PU) is stiffened by the filler (CB).^6,7

Fig. 3 Notched Izod impact strength of neat PLA and PLA/15PU blend with various amounts of CB.

Table 1 Mechanical properties of neat PLA and PLA/15PU blend with various amounts of CB.

Samples	Yield strength (MPa)	Young's modulus (MPa)	Elongation at break (%)	Notched impact strength (kJ m^−2)
PLA	66.55 ± 1.49	2270.14 ± 21.18	4.42 ± 0.35	2.73 ± 0.26
PLA/3CB	69.49 ± 0.48	2565.3 ± 23.17	4.29 ± 0.32	3.78 ± 0.56
PLA/15PU	43.01 ± 0.92	1969.58 ± 18.23	276.78 ± 12.71	8.12 ± 0.94
PLA/15PU/1CB	39.59 ± 0.69	1727.95 ± 50.48	292.44 ± 11.69	27.44 ± 0.56
PLA/15PU/2CB	39.51 ± 2.40	1786.81 ± 25.38	297.31 ± 16.95	30.57 ± 1.26
PLA/15PU/3CB	37.55 ± 0.78	1697.39 ± 26.45	253.1 ± 14.22	31.8 ± 1.89
PLA/15PU/5CB	41.74 ± 3.61	1640.31 ± 8.62	238.38 ± 9.8	23.9 ± 0.59

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Fig. 4 displays the electrical conductivity of neat PLA and PLA/15PU blend with various amounts of CB. For PLA/CB composites, only when the content of CB is higher than 3 phr, the relatively higher electrical conductivity can be attained. However, for PLA/15PU/CB ternary composites, a largely improved electrical conductivity can be obtained at a relative lower CB content, i.e. 2.5 phr. Moreover, except for CB content, the electrical conductivity of PLA/PU/CB composites is largely determined by the phase morphology. For the PLA/15PU/2CB composites with the elongated PU particles morphology, in which the weight percentage of CB to PU is comparable to that of PU/11.76CB binary composite, this content of CB is obviously higher than the reported percolation threshold of CB in neat PU matrix,¹⁷ but its electrical conductivity is only 5.03 × 10⁻¹² S m⁻¹. It is widely accepted that the electrical conductivity of conductive filler filled polymer blend is determined by the double percolation, e.g. the percolation of fillers in the fillers-rich phase and the continuity of this phase in the polymer blend.^18–20 Therefore, it is reasonable to deduce that the continuity of PU phase of elongated PU particles morphology is not enough to percolate in PLA/15PU/2CB composite. However, when the co-continuous like structure is achieved by increasing the CB content to 3 phr, the CPCs conductivity improves remarkably to 5.78 × 10⁻³ S m⁻¹, nearly 9 orders of magnitude improvement. This implies that the double percolation can also be realized in co-continuous like structure although the CB filled PU phase is discontinuous, which is significantly different from the usually thought of the conductive fillers-rich phase need to be 100% continuous to achieve double percolation.^18–20The double percolation mechanism in PLA/15PU/3CB composites with CB induced co-continuous like morphology can be described by the schematic illustration shown in Fig. 5. Although the co-continuous like morphology is composed of discontinuous PU clusters and bands (Fig. 1d and d′), double percolation can also prevail through the matrix. That may be because SEM and TEM observations are only displays of two-dimensional plane and they can not reflect the PU phase continuity in three-dimensional space. Due to the largely improvement in electrical conductivity by the formation of co-continuous like morphology, it is logical to deduce that the structural continuity of co-continuous like structure is high enough to form the conductive paths in three-dimensional space despite its discontinuity in two dimensional plane. Furthermore, the conductive paths are composed of two types of CB nanoparticles, i.e. continuous CB nanoparticles located in PU phase and discontinuous CB nanoparticles concentrated at neighbouring PU particles interface. For the former type, electrons can transport through directly contacted CB nanoparticles, while for the latter type, electrons can pass across the thin PLA layers between the neighbouring CB nanoparticles when the size of layers are smaller than the tunnelling distance (TEM images of Fig. 5). By the combination of two types of CB nanoparticles, the integrated conductive paths prevail through the whole matrix.

Fig. 4 Electrical conductivity of neat PLA and PLA/15PU blend with various amounts of CB.

Fig. 5 Schematic of double percolation mechanism in PLA/15PU/3CB ternary composites.

Conclusions

In this work, a small amount of conductive fillers CB was introduced into PLA/PU blends and a change of droplet phase morphology to co-continuous like morphology was observed. It could be understood due to the strong affinity between PU and CB nanoparticles and the self-networking effect of CB. The prepared ternary composites exhibit much improved impact toughness and better conductivity compared with PLA/PU binary blends. Our work provides a new pathway for the simple fabrication of double-percolated CPCs with both good stiffness-toughness balance and excellent conductivity.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 50903048, 51121001).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental methods. See DOI: 10.1039/c4ra06836j

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