Segmental Relaxation Behavior Of Polymer Chains Constrained In Micelles On The Micelle Film Surface

With the advances in nanotechnology, the glass transition and dynamics for polymer molecules in a confined geometry have attracted great attentions. Since nanomaterials have a large surface to volume ratio, surface dynamic strongly influences the fundamental thermodynamic properties of a material. The studies on the surface dynamic of polymer are of significant importance to understand the mechanism of the molecular dynamic in confined polymer system. This thesis present an experimental investigation of the segmental relaxation behavior of polystyrene chains in the cores of collapsed dry micelles tethered on the micelle film surface by a poly(acrylic acid) corona. The main conclusions are summarized as follows:(1)The segmental relaxation of PS chains in the micelle core on the micelle film surface was affected significantly by the molecular weight of the PS block and the density of the micelle core. The Tonset increased with increasing density of the PS core in the micelles. For the block copolymers with the molecular weight of their PS block larger than the critical entanglement molecular weight, the ΔTonset has a stronger denpendence on the ρ/ρmin compared with the block copolymers with the molecular weight of their PS block lower than the critical entanglement molecular weight. This means that the increase in core density of both PS370-PAA111 and PS283-PAA111 micelles is more favorable for increasing Tonset compared with that of the PS70-PAA25 and PS84-PAA67 micelles. This difference in the relationship between ΔTonset and ρ/ρmin may be attributed to the packing mode of the PS chains in the micelle core. When the molecular weight of the PS block was lower than its critical entanglement molecular weight, the increase in the density of the micelle core prepared from dioxane solution with high water content was attributed only to close packing of the PS chains. When the molecular weight of the PS block exceeded its critical entanglement molecular weight, the increase in the density of the micelle core was attributed to both close packing and enhanced entanglement density of the PS chains.(2)The length of the corona-forming block(PAA) have a significant influence on the dynamic of polystyrene chains in the cores of collapsed dry micelles on the micelle film surface. For PSm-b-PAAn with a constant length of PS block, as the length of corona-forming blocks increases, ΔTonset shows a stronger dependence on ρ/ρmin. That means the movement of PS IV segments is further inhibited in the micellar core with increasing the length of PAA block. The spherical micelles system, which consists of a core of PS block surrounded by a thin shell of PAA block, can be considered as nanoparticles system. This phenomenon is attributed to free surface effect on the dynamic of this kind of particle system. Tg perturbations originating at the free surface lead to a distribution of Tgs away from the interface. The influence of the free surface effects on the dynamic of polystyrene chains in the cores gradually weaken with the increasing of the corona thickness. Therefore, the dynamic of PS chains inside the cores will be weakened.(3)The morphologies of micelles have a significant impact on the segmental dynamic of polystyrene chains in the cores of collapsed dry micelles tethered on the micelle film surface. The segmental relaxation times of PS chains in the rod micellar cores are more than 2 orders of magnitude shorter than those of PS chains in the sphere micellar cores. By measuring the surface rearrangement kinetics of these films, the onset temperature of rearrangement(Tonset) and the activation barrier for relaxation(Ea) of surface PS chains are determined. It is found that the Tonset and Ea of the rod micelle films are 50℃ and 145 k J/mol, respectively, which are lower than those of the sphere micelle films(60℃ and 183 k J/mol). These results reveal that PS chains in the rod micellar cores can be highly mobile relative to PS chains in the sphere micellar cores. According to the mechanism of morphological transitions, the degree of stretching of PS chains in the rod micelles is lower than those in the sphere micelles. This suggests that PS chains in the rod micelles have more number of possible conformations. The increasing of entropy in the rod micelle system can lead to enhanced dynamic of PS chains in the cores.(4)We investigated the segmental relaxation behavior of polystyrene chains in the core of single micelle in situ by atom force microscopy(AFM). It was found that the changing of the single micelle’s height with temperature is similar to the deformation-temperature curve of the amorphous polymer. When the temperature is low, the height of polymer micelles almost unchanged. When the temperature rises to a certain value(defined as Tfirst onset), the height of the micelles begin to decline. With the increasing of temperature, in a certain range of temperature, the micellar height remains unchanged with temperature and forms a plateau. As the temperature is further increased to a certain value(defined as Tsecond onset), the height of single polymer micelles decreased sharply. It was found that the Tfirst onset increased with increasing density of the PS core in the micelles. The molecular weight of the PS block has a significant influence on the curves of temperature deformation of single polymer micelles. When the molecular weight of the PS block exceeded its critical entanglement molecular weight, the curves of temperature deformation of single polymer micelles have the plateau. When the molecular weight of the PS block was lower than its critical entanglement molecular weight, the plateau and the Tsecond onset of the curve disappeared. These results suggest that the Tfirst onset is related to the glass transition temperature(Tg) of PS in core, the plateau is related to chain entanglement and the Tsecond onset is related to the viscos flow temperature(Tf) of PS in core.