Tuning The Properties Of Soft Materials Through Extreme Nanoconfinement

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Doctor of Philosophy (PhD)
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Physical Chemistry
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Wang, Haonan

Polymers and small molecules under nanoconfinement can show significantly deviated properties from bulk. While the effect of nanoconfinement has been studied for several decades, the origin of the deviation is yet to be investigated and different people can observe very different effects in similar systems. Capillary Rise Infiltration (CaRI) enables the fabrication of polymer-infiltrated nanoparticle (NP) films in which extreme nanoconfinement can be achieved. In this thesis, we first use CaRI films as the model system to study the glass transition of soft materials under extreme nanoconfinement. In-situ spectroscopic ellipsometry with a temperature-control stage is used to monitor the glass transition process. We show that in CaRI films the Tg of confined polystyrene (PS), which interacts weakly with SiO2 NPs, significantly increases with decreasing pore size such that the Tg increases by up to 50 K in 11 nm NP packings, while Tg is close to the bulk Tg in 100 nm NP packings. Such effect on Tg is almost independent of molecular weight. For Poly(2-vinylpyridine)(P2VP)/SiO2 CaRI films, a ~100 K increase in Tg can be achieved with a huge gradient of dynamics. We then studied the thermal degradation of polymers in CaRI films. We demonstrate that as the NP size is decreased, increasing the degree of confinement, the isothermal degradation time of PS is dramatically increased due to the slowed dynamics and diffusion in CaRI films. In addition to the slowed diffusion, the small pore size can suppress the carbonization reactions. The competition of these two effects results in a non-monotonic trend of the thermal degradation of some polymers. Finally, we characterized the molecular conformation of a small molecule glass N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) to investigate the origin of the different confinement effects in nanoporous media with different pore geometries. We show that TPD has a more planer conformation in CaRI films with concave pores while no change in conformation is observed when TPD is confined in controlled pore glass (CPG) with convex pores. The results indicate that pore geometry is an essential factor in Tg nanoconfinement effects and highlight the role of intramolecular degrees of freedom in the glass transition, which have not been extensively explored.

Zahra Fakhraai
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