For the commercial success of polymer - multiwalled carbon nanotube (MWNT) composites the production of these materials on industrial scale by melt processing is of significant importance. The complete dispersion of primary MWNT agglomerates in a polymer melt is difficult to achieve, making it an important and challenging technological problem. Hence, it is necessary to understand the process of MWNT agglomerate dispersion in a polymer melt.
Based on an intensive literature research on mechanisms and influencing factors on dispersion of other agglomerated nanostructured fillers (e.g. carbon black), the main dispersion steps were evaluated and investigated concerning the agglomerated MWNT.Consequently, systematic investigations were performed to study the effect of the melt infiltration on MWNT agglomerate dispersion and to analyse the corresponding main dispersion mechanisms, namely rupture and erosion. The states of MWNT agglomerate dispersion were assessed by quantifying the agglomerate area ratio and particle size distribution using image analysis of optical transmission micrographs. Additionally, the composite’s electrical resistivity was determined. In the prevailing study, polycarbonates (PC) varying in molecular weight were used to produce composites containing 1 wt% MWNT (Baytubes C150HP) as model systems and a discontinuous microcompounder was applied as melt mixing device. The agglomerate structure of the used MWNT material made them especially suitable for the reported investigations.
The step of melt infiltration into the primary nanotube agglomerates plays a crucial role for their dispersion in the PC melt. During melt mixing when low shear rates were applied, better state of MWNT dispersion was obtained in high viscosity matrices because applied shear stresses were high. On the contrary, if high shear rates were applied, similar states of MWNT dispersion were obtained in low and high viscosity matrices although significantly lower shear stresses were applied in the low viscosity matrix as compared to the high viscosity matrix. The results indicate that if the applied shear stress values are compared, with increasing matrix viscosity the agglomerate dispersion gets worsen. This is attributed to the fact that low viscosity matrices can infiltrate relatively faster than high viscosity matrices into the agglomerate making them weaker and reducing the agglomerate strength. Thus, at sufficient shear rates MWNT agglomerates disperse relatively faster in low viscosity matrix. This illustrates a balance between the counteracting effects of viscosity on agglomerate infiltration and agglomerate dispersion.
Additionally, the effect of matrix molecular weight on the size of un-dispersed MWNT agglomerates was investigated. Under similar conditions of applied shear stress, the composites based on low molecular weight matrix showed smaller sized un-dispersed primary agglomerates as compared to composites with higher molecular weight matrices. This again highlights the role of matrix infiltration as the first step of dispersion.
Following the step of melt infiltration, agglomerate size gets reduced due to the dispersion mechanisms. To analyse the corresponding contributions of different dispersion mechanisms (rupture and erosion), the kinetics of MWNT agglomerate dispersion was investigated. If high mixing speeds are employed dispersion is quite fast and needs less time as compared to low mixing speed. A model is proposed to estimate the fractions of rupture and erosion mechanisms during agglomerate dispersion based on the kinetic study in the discontinuous mixer. Under the employed experimental conditions, at high mixing speeds, the dispersion was found to be governed by rupture dominant mechanism, whereas at low mixing speeds the dispersion was controlled by both mechanisms.
As far as electrical resistivity is concerned, for a given content of MWNT as the state of dispersion improves, the resistivity values decrease significantly but only up to a plateau value. The composites produced using low viscosity matrices have lower resistivity values as compared to high viscosity matrices. Additionally, composites were prepared using additives, whereas the additives were found to be useful for improving filler dispersion and electrical conductivity.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:25949 |
| Date | 15 July 2011 |
| Creators | Kasaliwal, Gaurav |
| Contributors | Heinrich, Gert, Pötschke, Petra, Radusch, Hans Joachim, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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