Elsevier

European Polymer Journal

Volume 47, Issue 11, November 2011, Pages 2078-2086
European Polymer Journal

Macromolecular Nanotechnology
Electrical conductivity of PUR/MWCNT nanocomposites in the molten state, during crystallization and in the solid state

https://doi.org/10.1016/j.eurpolymj.2011.07.026Get rights and content

Abstract

The electrical conductivity of a nanocomposite constituted of multiwalled carbon nanotubes (MWCNT) dispersed in a semicrystalline polyurethane matrix, is investigated during cooling from the melt to the solid state. The same percolation threshold, ϕc = 0.85 wt.%, is obtained in the molten and in the solid state, although the exponent t of the percolation equation is significantly higher in the solid state. A remarkable increase of the conductivity during crystallization is observed for nanocomposites of MWCNT content above ϕc, but for contents below ϕc the conductivity decreases. Combined conductivity and PVT results, lead us to discard the hypothesis of an increase of the density of the conductive network (associated with volume shrinkage) as being the cause of the conductivity enhancement during crystallization. Instead, the analysis of the parameters of the percolation equation for the molten and the solid state, suggests a transition from a less effective conductive network to a more performing three dimensional network.

Highlights

► Conductivity of molten and crystallized CNT/polyurethane nanocomposites is studied. ► Same electrical percolation threshold is obtained in the molten and solid state. ► Conductivity increases remarkably during crystallization from the melt. ► A small volume shrinkage during crystallization cannot justify conductivity increase. ► A transition to a more performing network takes place during crystallization process.

Introduction

In spite of the considerable amount of papers about electrical properties of polymer/carbon nanotubes (CNT) nanocomposites, relatively few works refer to the electrical conductivity in the molten state and the subsequent effect of crystallization on conductivity for nanocomposites which contain a semi-crystalline matrix [1], [2], [3], [4], [5], [6], [7]. The published works can be divided in two groups: papers which report a conductivity decrease during crystallization [1], [2], [3] and papers which reveal a conductivity enhancement as the polymer crystallizes [4], [5], [6], [7]. According to Alig et al. [1], the effect of cooling polypropylene (PP)/multiwalled carbon nanotubes (MWCNT) nanocomposite melts to temperatures below crystallization, is a significant decrease in the electrical conductivity. More recently Deng et al. [2] have also noticed a conductivity decrease during the crystallization of a PP matrix in nanocomposites of this polymer and MWCNTs and carbon black. This conductivity diminution is explained by a reduction of the amorphous phase (which exhibits ionic conductivity) on expense of the crystalline phase [1]. This interpretation is consistent with the percolation picture offered in a previous paper of Alig et al. [8] to explain the reduction of the conductivity of a neat semicrystalline polymer (not containing CNTs), poly(ethylene glycol), during crystallization. A different picture of the effect of crystallization on conducting nanoparticles is given by Pang et al. [3] for an electrically conducting polyethylene/graphene nanocomposite. Based partially on an early work of Meyer [9] about conductivity of polymer/carbon black composites, the authors consider that as the sample is cooled to crystallization temperature, graphene nanosheets are ejected from the crystalline phase and only remain in the amorphous phase. This weakens the graphene conductive paths and the electrical conductivity decreases. On their part, Li et al. [5] state that crystallization during cooling of CNT polymer nanocomposites has two opposite effects on electrical conductivity: (a) conductivity decrease, compatible with the growth of the crystallites, which expels the conductive CNTs that penetrate the crystalline region, breaking the conductive pathways. (b) Conductivity enhancement due to an increase of the density of the conductive network, associated to the volume shrinkage when the sample crystallizes. As alleged by the authors, for their polyethylene/CNT nanocomposites the effect of shrinkage during crystallization surpasses that of CNT expulsion and a sharp increase of electrical conductivity is noticed at the crystallization temperature, Tc = 120 °C. The same trend is observed by Lim et al. [4] who report a higher electrical conductivity in ethylene-co-vinylacetate (EVA)/MWCNT nanocomposites for samples slowly cooled to the crystallization temperature, as compared with samples submitted to fast cooling. Similarly to Li et al. [5], the authors associate this conductivity enhancement to the shrinking linked to the crystallization of the sample (which is larger in slowly cooled sample) that induces a shorter distance between MWCNTs. A different hypothesis is posed by Jeon et al. [6] to justify the observed conductivity enhancement with increasing the crystallinity of polypropylene in PP/single-walled carbon nanotubes (SWCNT) nanocomposites. As discussed by these authors, for crystallinities above 35% the fast nucleation and growth of crystals entraps the SWCNT network between the semicrystalline structure reducing the driving force of nanotubes to curl and twist. This gives rise to a more connected SWCNT network that leads to higher electrical conductivities than for less crystalline matrices (below 35%). An opposite trend is, however, observed for dispersions of MWCNTs and graphite powder in a PP matrix [10]: using a lower crystalline PP matrix leads to higher conductivity, apparently because more continuous electrical paths are available.

The concept of “segregated network”, introduced by Kusy [11] in the context of electrically conductive composites composed of a polymer matrix and metal powders, and employed to explain the low percolation of three phase composites [12], [13], [14], [15], [16], can also be adapted to explain the increase in the electrical conductivity observed during crystallization. This is based on the assumption of a distribution of the conductive particles in amorphous regions, around the crystalline regions [7]. Therefore, the constitution of electrical paths is favored as crystallization progress and the amorphous regions are reduced.

The contradictory results found in the literature so far, stimulates the interest of the research on this topic, which, moreover, is of significant relevance from an applied point of view. In this paper the electrical conductivity of a nanocomposite constituted of multiwalled carbon nanotubes (MWCNT) dispersed in a polyurethane matrix (which contains a large amount of polycaprolactone), is investigated, offering novel results with respect to previous papers on melt mixed PUR/CNT nanocomposites [17], [18], [19], [20], [21], [22], [23], [24], [25]. Within this framework we have recently studied the immediate adhesion of these nanocomposites in the molten state [26] as well as the thermal and rheological properties in the liquid and solid states [27]. As far as we know, these are the only papers reporting on this nanocomposite which offers potential applications as an electrically conducting hot melt adhesive.

Section snippets

Materials

The investigated polyurethane is a thermoplastic polyurethane (PUR) sample produced by Merquinsa (Spain) as PB121, with a melting temperature of Tm = 54 °C and employed as a Hot Melt adhesive. The hard segment is a diphenylmethane diisocyanate (MDI) and a chain extending short chain diol, 1,4-butanediol, and the soft segment is constituted by a long chain diol, ε-polycaprolactone. The composition deduced from NMR is 10% hard segment and 90% soft segment.

Multiwall carbon nanotubes (MWCNT), supplied

Electrical and rheological results in the molten state

Fig. 1 shows the real part of the electrical conductivity σ′ as a function of frequency for neat PUR and PUR/MWCNT dispersions at a temperature of 100 °C. This temperature is well above the melting temperature of the investigated samples, Tm = 54 °C, which is not affected by MWCNT content [27], so actually the data reflect the electrical conductivity of our samples in the liquid state. These results contribute to fill the relative absence of studies on the conductivity of polymer/CNT systems in the

Conclusions

For PUR/MWCNT dispersions in the molten state, the percolation threshold determined through rheological data is higher than the electrical percolation threshold. This result leads to pose the hypothesis of an electron hopping/tunneling mechanism for the electrical conductivity of the PUR/MWCNT nanocomposites, which is confirmed, since the dependence ln σαp−1/3 of the conductivity on MWCNT weight fraction is accomplished.

The analysis of the evolution of the electrical conductivity, during

Acknowledgements

Financial support through MICIN MAT 2010-16-171 Project (Spanish Government) and GIC IT-441-10 (Basque Government) is acknowledged, and one of us, Maite Landa acknowledges a Thesis Grant from Basque Government.

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