Thermal and rheological characterization of polystyrene and polystyrene grafted silica nanocomposites



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Polymer chain grafting on nanoparticles has been shown to minimize nanoparticle aggregation in nanocomposite systems. Recent advances in nanocomposite research have demonstrated the ability to use only polymer grafted nanoparticles as the nanocomposite system, resulting in matrix-free polymer grafted nanocomposites.

This dissertation presents the thermal and rheological study of two matrix-free polystyrene-grafted silica nanocomposites. Polystyrene chains of 35 and 112 kg/mol are grafted onto silica nanoparticles and properties are compared with analogous neat polymers, both in enthalpic and in volumetric space, using differential scanning calorimetry and rheometry. An approximately 1 to 2 K increase in the glass transition temperatures (Tg) of the nanocomposites are observed as compared to those of the neat polymers of equivalent molecular weight. An approximately 4−7% reduction of the absolute heat capacity is observed in the glassy and liquid states along with a reduction in ΔCp at Tg of 15% from which an immobilized glassy layer of approximately 2 nm is determined on the nanoparticles' surfaces. A shifting, of approximately one decade, of the dynamic storage (G') and loss modulus (G'') curves toward lower frequencies is observed consistent with the increase in Tg. Terminal flow behavior is not observed for the nanocomposites in the experimental window, A 7 % reduction of the rubbery plateau moduli (GNo) for the 35 kg/mol nanocomposite is detected, whereas a two and a half-fold increase is detected by for the 112 kg/mol nanocomposite. A 4 % increase in the glassy modulus is detected consistent with hydrodynamic reinforcement. Two different contributions are hypothesized to govern the magnitudes of the rubbery modulus of the nanocomposite systems: hydrodynamic reinforcement and a change in the effective entanglement density originating from corona interpenetration with the silica nanoparticles acting as physical entanglement points.

In addition, the glass transition of the interfacial immobile glassy layer around the nanoparticles of the 35 kg/mol system is studied using ultrafast flash scanning calorimetry (Flash DSC) by heating the polystyrene sample up to 300 °C at ultrafast rates to avoid degradation in an attempt to mobilize the glassy layer. No visible step change in the absolute heat capacity data is detected indicating that the glassy layer remains immobile even at high temperature. More interestingly, the absolute heat capacity data demonstrates an interesting flattening at temperatures above 250 °C which suggested to not be an artifact of the Flash DSC measurements since the same results are obtained at heating rates from 100 to 2000 K/s.

The last part of my dissertation describes the development of a novel capillary dilatometer based on the theory of cylindrical capacitor. A capacitor circuit is built by using the dilatometer fluid mercury (Hg) as the inner electrode, by coating the exterior of the capillary with silver to be used as the outer electrode and by utilizing the glass capillary itself to work as the dielectric medium. The volume change of a sample placed inside the dilatometer is reflected by a change in the Hg height and the corresponding capacitance value. An Andeen-Hagerling ultra precision capacitance bridge is used to measure capacitance response as a function of temperature from which the volume response is calculated using a model of the system. The dilatometer calibration is performed using mercury, quartz and stainless steel standards. The average absolute error in specific volume is found to be 1.9 x 10-5 cm3/g, 1.7 x 10-4 cm3/g and 2.8 x 10-4 cm3/g for mercury, quartz, and stainless steel, respectively as compared to corresponding literature values. The efficacy of the dilatometer is tested by studying the cooling rate dependence of glass transition of neat polystyrene and data are found to be consistent with the literature. In addition to the improved resolution and sensitivity over a higher temperature range, the new setup has advantages of ease of use and elimination of moving parts over prior experimental setups.

Embargo status: Restricted until 06/2022. To request the author grant access, click on the PDF link to the left.



Polymer Nanocomposite, DSC, Rheology, Glass Transition, Absolute Heat Capacity, Immobile Layer, Dilatometry