Preparation Of Poly(Octadecyl Methacrylate) Molecular Brush And Study On Its Crystallization Behavior

With the development of nanotechnology,the study of polymers has transferred from macroscopic materials to nanoscopic ones.When the thickness of the polymer film is reduced to the size of the polymer chain,its crystallization behavior deviates from the bulk,and however such behavior is difficult to be assessed due to the nanoscale volume probed.As compared to the conventional polymer films,polymer brush has a controlled chain conformation and aggregation state.The chain conformation of molecular brush can be controlled by the graft density,which makes it an ideal model to study the crystallization of ultrathin films.However,due to the limitation in approach to regulate grafting density and the characterization methods,how the graft density affect the crystallization behavior of the polymer brush was not completely clarified yet.In this thesis,we used surface-initiated atom transfer radical polymerization(SI-ATRP),combined with a"de-initiator"method,to prepare poly(octadecyl methacrylate)(PODMA)brushes on a flat silicon substrate with different grafting densities.The crystalline and melting behavior of the PODMA molecular brushes were studied by ellipsometry,atomic force microscopy(AFM)and grazing incidence X-ray diffraction(GI-XRD).The main conclusions are as follows:1)The density of initiator on silica substrate surface was regulated by the"de-initiator"method,which allow cutting off part of initiators from substrate.PODMA molecular brushes with different thickness and various grafting densities were successfully prepared by surface-initiated atom transfer radical polymerization on the silicon surface.2)The ellipsometry was used to measure the melting point of the PODMA brush.The melting point of the PODMA molecular brush is determined by the temperature corresponding to the jumping of the elliptic angle in temperature versus elliptic angle curves.Combined with the in-situ heating technique,the changes of crystalline morphology on PODMA brush surface with temperature was observed by atomic force microscope.The modulus of PODMA molecular brush at different temperature was also studied by mechanical measurement mode based on AFM.The reliability of measuring the melting point of the polymer molecular brush by the ellipsometry is further confirmed by the temperature dependence of crystalline morphology and modulus using AFM.By investigating the melting points as function of thickness and grafting density of PODMA brushes using ellipsometry,it was found that the melting point of PODMA molecular brush increased with the increase of film thickness and graft density.3)The crystal structure and chain packing of PODMA brush was further studied by AFM and GI-XRD.As the thickness of the brush increased,the crystalline morphology of molecular brush transformed from the sparse fibrous lamellae into the tightly packed 2-dimensional spherulites.When the thickness of polymer molecular brushes is fixed,polymer brushes with lower density are more likely to form spherulitic structures.Crystal structure studies using GI-XRD imply that the side chains of the low-density PODMA molecular brush formed a layered structure with a period of 3.0 nm.The side chains of medium density polymer molecular brush formed layered structures with a period of 2.6 nm,consisting of rod-like octadecyl groups tilted ca.30°with respective to the substrate.The side chains of high-density PODMA molecular brush could not form a layer structure but could only closely pack side-by-side.4)As the grafting density increased,the crystal structure of PODMA brush undergone a transition of sparse layered to tightly packed hexagonal structure.The distance between main chains of the low-density PODMA brush was larger than the rod-like octadecyl groups,and the side chains had sufficient space to form a layer.Although the medium-density polymer brush could formed layered structure,the period became short and rod-like octadecyl groups tilted for a little crowded of molecular chains.As for the high-density of polymer brush,the distance between the molecular chains was too narrow to form a layered structure.Under the combined effect of space and confinement,the main chains of high-density polymer brush were stretched straight,and the side chains extended outward to be closely packed side-by-side.So the granular crystals of high-density brush appeared under atomic force microscope.Further more,as the grafting density increased,the interaction of polymer chains transformed from intramolecular folding into intermolecular synergy,and the intramolecular synergy provided lower conformational entropy.According to the change formula of the melting point T_m=△H/△S,the polymer brush with lower conformational entropy possessed higher melting point.Therefore,the higher melting point of the polymer brush increased with the graft density.