THERMOPHYSICAL AND RHEOLOGICAL PROPERTIES OF POLYMER NANOCOMPOSITE AND IONIC LIQUIDS

Date

2015-05-12

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Abstract

The viscoelastic bulk modulus K(t) is an important material property for high pressure polymer processing and design of structural components. However, in spite of its importance, experimental investigations of K(t) have not been widely performed due to experimental difficulty. Recent model predictions indicate that polymer nanocomposites may have lower bulk moduli compared to the neat polymer. These results, if verified experimentally, suggest that nanocomposites could be used as the matrix to mitigate residual stresses in macroscopic composite applications. In addition, one fundamental motivation for measuring K(t) is to resolve the literature debate on the underlying molecular origins of the bulk and shear moduli.

In this work, we perform K(t) and pressure-volume-temperature (PVT) measurements on a silica nanoparticle-filled polystyrene nanocomposite using a custom-build pressurizable dilatometer. We also measure the viscoelastic shear modulus and study the glass transition behavior. Comparison of the bulk and shear moduli between the nanocomposite and neat polystyrene show that the absolute values of the limiting moduli in glassy and rubbery states are higher for the nanocomposite, in agreement with the hydrodynamic theory of reinforcement and mixing rules. The presence of nanoparticles does not appear to significantly change the polymer dynamics associated with glass transition, except to slightly increase the glass transition temperature (Tg). Furthermore, consistent with our previous work as well as Plazek's earlier findings, we suggest that underlying molecular mechanisms for the bulk and shear responses are similar at short times, although there might be a difference in the effectiveness of these local modes. On the other hand, at long times, the chain mechanisms that available to the shear response are not available to the bulk response.

As a material class, ionic liquids possess attractive properties and have a wide range of potential uses. The great number of possible ion combinations allows for tailoring ionic liquids with specific properties. In this work, the structure-property relationships of a series of imidazolium-cation ionic liquids with a fixed anion are investigated. In particular, the effects of aromatic substituents versus alkyl substituents on the imidazolium cation on the thermophysical and rheological properties are studied.

The findings from the work on ionic liquids include that densities decrease linearly with increasing temperature, with values well predicted using the group contribution method. The Tg follows a systematic trend, increasing as substituent flexibility decreases. Short-term thermal stability is not strongly dependent on different substituents on the cation. Rheological responses are very similar from glass to terminal region for all of the ionic liquids. The temperature dependence of viscosity and shift factors for reducing mechanical responses follows the classic Vogel-Fulcher-Tamman or Williams-Landel-Ferry relationship. The fragility index is found to be higher for the aromatic-substituted ionic liquids compared to those with saturated alkyl substituents, suggesting lower ionicity for ionic liquids with aromatic functionality.

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Keywords

Bulk modulus, Dilatometry, Nanocomposite, Rheology, Pressure-volume-temperature behavior, Ionic liquids, Structure-property relationships, Thermal analysis

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