Rheology and electrical conductivity of nanocomposites obtained dispersing carbon nanotubes in polypropylene and irradiated polypropylene matrices

Resumen

PP/MWCNT nanocomposites were elaborated using a single melt-mixing dispersing method.Non-modified carbon nanotubes were employed and no compatibilizers were added. Initial agglomerateswere disintegrated during melt mixing process and microscale MWCNT aggregates were formed.Keeping nanocomposites in a quiescent state at 180 ºC, more effective interactions amongPP-MWCNTs were constituted, as a result of the formation of the final MWCNT network formed by thesecondary agglomeration. This rearrangement gave rise to a very significant electrical conductivityincrease, which was particularly large in the case of 2 wt. % of MWCNT nanocomposite: The conductivitypassed from 10-11 S/cm at the initial time to 10-4 S/cm after an hour. Actually, the addition of MWCNTsinto the PP matrix caused a huge increase of the electrical conductivity and the most loaded PP/MWCNTnanocomposite showed a conductivity of 0.01 S/cm, within the range of the conductivities of siliconsemiconductors.Rheological characterisation allowed analysing the interactions of polypropylene chains withMWCNTs in the molten state, by means of small amplitude shear flow experiments and uniaxialelongational flow measurements. Carbon nanotubes hindered PP chains motion, resulting in a suppressionof the terminal viscoelastic zone. A rheological percolation behaviour was determined, with a percolationthreshold at fc = 1 wt. % of MWCNTs. According to uniaxial elongational results, MWCNTs increaseduniaxial elongational viscosity, but PP-MWCN interactions were not able to produce strain-hardeningresponse.The sole addition of MWCNTs into PP did not improve the processing characteristics in extensionalflow processes, therefore, electron-beam irradiation (EB) was used to induce long-chain branching (LCB)in PP matrix, and so produce strain hardening. Thus, two alternative routes were followed to achieve anelectrically conductive polypropylene with a strain-hardening behaviour: I) After PP/MWCNTnanocomposites were elaborated, they were irradiated. II) PP/MWCNT nanocomposites were fabricatedusing irradiated polypropylene matrices.Irradiated PP/MWCNT nanocomposites showed an electrical conductivity rise of nine orders ofmagnitude; from approximately 10-11 to 0.01 S/cm, adding up to 8 wt. % of MWCNTs into the PP matrix.In addition, these nanocomposites presented strain-hardening behaviour, even though thestrain-hardening decreased with MWCNT concentration. Neither oscillatory shear flow measurements norsize exclusion chromatography (GPC/SEC) could confirm the existence of LCB in irradiated PP/MWCNTnanocomposites. Oscillatory measurements were not able to separate the effect of MWCNT addition fromthe presence of LCB. Size exclusion chromatography combined with light scattering could not be used toevaluate the number of LCBs, since the presence of nanotubes tends to block the column set. Thedetection of strain-hardening, carried out by uniaxial elongational tests, was the only way to effectivelyconfirm the presence of LCB.This proposed route opens a new way to obtain electrically conductive PP based nanocompositesthat can be processed using extrusion blowing, blow moulding and others, which are discarded for linearpolypropylenes and nanocomposites based on them.PP/MWCNT nanocomposites elaborated with irradiated polypropylene matrices did not showstrain-hardening response. Long-chain branches formed in PP matrices by irradiation process were notstrong enough to stand the high shear rates generated during the mixing process. But, regarding themicrostructure, TEM analysis showed that the dispersion state of MWCNTs was considerably improved,with respect to nanocomposites obtained with non-irradiated PP and irradiated PP/MWCNTnanocomposites. Therefore, interactions between MWCNTs and PP chains, as well as MWCNT-MWCNTinteractions, were improved, and consequently, nanocomposites elaborated with irradiated PP matricesshowed higher electrical conductivity as compared with the other nanocomposites considered in thisThesis. Even more, for the most loaded nanocomposite, PP80kGy/8%MWCNT, an electrical conductivity of3.6·100 S/m was measured, which constitutes the higher conductivity ever reported for PP/MWCNTnanocomposites, as far as I know. The application of the equation of the statistical theory of percolationallowed demonstrating that the rheological, as well as the electrical, percolation thresholds were bothreduced in nanocomposites elaborated with irradiated PP. For instance, the PP/MWCNT nanocompositesfabricated with PP irradiated at 80 kGy presented rheological and electrical percolation thresholds of 0.3and 1.7 wt. % of MWCNT, respectively. This result, together with the outcomes explained above, opens anew route to take advantage of the recycling of polypropylenes which are employed in food packagingand for medical purposes, since these polymers should be irradiated to be sterilised.