Calorimetry And Fluorescence Studies On The Glass Transition Of Polymer Thin Films And The Diffusion Of Small Molecules

The properties of polymer thin films are often different from the corresponding bulk materials.Understanding the origin of changes in the thermodynamic and kinetic properties of thin films is important not only for the application of materials,but also for understanding the nature of glass formation.The most commonly used indicator to quantify the dynamic changes of thin films is the glass transition temperature Tg.Many studies have focused on how the substrate,film thickness,molecular weight,or physical aging affects the apparent Tg of thin films.Due to the extremely small mass of the nanoconfined polymers,it is difficult to distinguish the signal of the sample itself from the background noise,so few characterization methods can study the micro-structure of the confined polymers.The work of this thesis is mainly to use the methods of calorimetry and fluorescent dyes to study the glass transition of confined polymers,supplemented by the non-radiative energy transfer(FRET)method to characterize the interchain proximity of polymers,and further study the diffusion process of volatility organic molecules(VOC)in polymer films.Firstly,we used an AC chip calorimeter to study the effect of cooling rate on the glass transition temperature of polystyrene thin films.We found that the glass transition of polystyrene thin films is closely related to molecular weight.For high molecular weight polystyrene thin films,the kinetic Tg(Tg,dyn)has almost no dependence on the cooling rate,but for low molecular weight polystyrene thin films,the Tg,dyn changed significantly with the change of the cooling rate.As the cooling rate increased from 0.1K/min to liquid nitrogen quenching(approximately 100 K/s),the Tg,dyn of 47 nm and 68 nm polystyrene thin films decreased by about 3K,and for polystyrene thin films with a thickness of 15 nm,the Tg,dyn decreased by about 7K.We think it is due to the excess free volume formed at the end of the low molecular weight polystyrene chain,and the change in the polymer chain packing density of the low molecular weight film is greater when the same cooling rate change occurs.In addition,we found decoupling phenomena between Tg,dyn and thermodynamic Tg.Then,we investigated the glass transition temperature of polystyrene and bare silica nanoparticle composites by using a fluorescent dye method.We found that with the increase of attractive interactions,the Tg of both the bulk composites materials and the film gradually increased.For PS/Si O2-PTS nanocomposites film,Tg is increased by 7K compared to pure PS bulk.Through the FRET method,we found that the interchain proximity of PS/Si O2-PTS nanocomposites film was the lowest,on the contrary.For the nanocomposites film systems we studied,the changes in glass transition temperature,interchain proximity and fragility were consistent.Lastly,we used calorimetry to track the desorption process of toluene in polystyrene thin films.By correlating the voltage signal of the calorimeter with the film mass,we found that,below the bulk Tg temperature,the desorption rate of toluene was independent of temperature,and in line with Fick's law of diffusion.We prepared twoand three-layer stacked thin films with various structures.Through the average distance between carbazole and anthracenemethanol was characterized by FRET method,the swelling process of toluene in polystyrene films was indirectly tracked.We found that swelling of polystyrene is a relatively slow process in the direction perpendicular to the film plane,and the solvent annealing takes longer time to bring the film to equilibrium.And as the interface between the films increases,the annealing time required also increases significantly.