Reorientational dynamics of methyl methacrylate and nanoconfinement effects studied by optical Kerr effect spectroscopy

Date

2014-12

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Abstract

Methyl methacrylate (MMA) is the object in our research due to its wide application in plastics industry main in the form of poly methyl methacrylate (PMMA). As studies have found that the confined condition has effect on the polymerization of MMA, 1,2 here we focus on the nanoconfinement effect on the monomer of MMA. This thesis covers studies using optical Kerr effect (OKE) spectroscopy on liquids and liquids confined in nanopores. OKE spectroscopy provides a way to understand the reorientational dynamics of molecules of liquids. In a small restricted area, the motions (vibration, rotation) of molecules will be different than in the bulk. In this study, silica sol-gel glasses were applied to provide the confined environment for molecules of liquids because of their large surface-to-volume ratios, high pore connectivity and narrow pore size distributions. From OKE spectroscopy studies, we found that the reorientations of molecules tend to be slower in confined conditions and the reorientation time increases when the nanoconfinement effect becomes larger. Analyzing the relaxation of liquids in sol-gel glasses with the two-state dynamic model, 3 the portion of molecules on the pore surface increases with the increase of the nanoconfinement effect and while at the same time, the portion of bulk molecules decreases. In the temperature dependent study of dynamics of MMA (methyl methacrylate), when the temperature is increased, the energy of molecules will increase and which corresponds to the faster motion of molecules. Therefore in the OKE spectroscopy study, the reorientations of molecules become faster as the temperature increases. The behavior of reorientational time follows the temperature and viscosity dependence of Debye-Stokes-Einstein (DSE) equation, , , where τ0 here is the free rotation time of MMA molecules, τROT is the single molecule rotational correlation time, η is the viscosity of the liquid, VH is the hydrodynamic volume, kB is the Boltzmann’s constant, and T is the temperature, f is the collective factor that is greater than 1. However, the hydrodynamic volume VH obtained is smaller than the van der Waals volume Vvdw.

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Keywords

Nanoconfinement effect, Dynamics, Optical Kerr effect spectroscopy, Liquid

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