Effect Of Chain Architecture On The Glass Transition Behavior Of Polystyrene Brushes

With the advances in nanotechnology,the physical properties and performances of polymer materials in the nanometer scale already attract a great interest in many fields of science and technology.The glass transition behavior of nanoconfined polymer film will deviate from its bulk when the thickness reduced to the scale of the polymer chain.And the reason why the glass transition behavior of polymer films deviates from the bulk remains unclear.The conformation of polymer chains is closely related to its glass transition behavior.Therefore,it is of great significance to investigate the influence of chain conformation on the glass transition behavior of polymer thin films,which is important for the preparation of high performance materials.Polymer brushes have controllable aggregation morphology compared with conventional polymer films,in which the transition from collapse to extended brush conformation can be achieved by adjusting the grafting density.Excellent structure controllability of polymer brushes provides an ideal model to study the relationship between confined polymer structure and glass transition behavior.In this thesis,polystyrene brushes with different grafting densities and molecular weights were prepared by surface initiated atom transfer radical polymerization?SI-ATRP?.The molecular weights and molecular weight distributions of the PS brushes were directly measured by cutting off the covalent bond between the PS chains and the substrate with the purpose of obtaining their precise architecture.The glass transition temperatures of PS brushes with different structures were studied by ellipsometry.The relationship among brush molecular weight,grafting density,brush thickness and their Tgs was established for the first time.The main conclusions of this paper are as follows:?1?A new method for the effective control of the grafting density of polymer brushes,called "degrafting initiator" method,was developed.The substrate surfaces with various initiator densities were obtained by controlling the reaction time,which is based on the principle of Si-O bonds being cleaved by TBAF.The characterization of XPS found Br/O ratios decreased linearly with the cleaving time,indicating that this method was very effective to control initiator density.?2?Polystyrene brushes with various grafting densities and chain lengths on silicon were prepared by the surface-initiated atom transfer radical polymerization?SI-ATRP?.It is obvious that the growth of the polymer brushes was well controlled,leading to a linear increase in thickness vs polymerization time for various initiator density samples in which the slope decreased with decreasing Br/O ratio.Furthermore,the molecular weight and molecular weight distribution of PS brushes were determined by using TBAF to separate the chain from the substrate.The results showthat the grafting density of the prepared PS brush increases from 0.08 nm^-2 to 0.61 nm^-2,achieving a transition from low to high grafting density.At the same time,the thickness of the brushes shows a good linear relationship with the corresponding molecular weight for every grafting density.It shows that the polymerization process is controllable,and the decrease of grafting density leads to the decrease of polymer chain extension.And the relationship between the thickness in the good solvent and the grafting density of the PS brush was determined by ellipsometry.It is necessary to emphasize that ?_p?0.32 nm^-2 should be a transition point of chain configuration from so-called mushroom to so-called brush conformation.?3?The effects of grafting densities??_p?,molecular weights?Mn?and thicknesses of dry polystyrene?PS?brushes on their glass transition temperature(T_g^brush)were investigated by spectroscopic ellipsometry.The results show that T_g^brush strongly depends on the grafting density and molecular weights of the PS brushes:i)The T_g^brush of the PS brushes with ?_p>0.30 increases with decreasing Mn?or brush thickness?and it is mainly dominated by entropic effects,in which the graft chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy.When the molecular weight is high enough,?T_g=0.The number of conformations of the chains increases accordingly as the molecular weight increases.The effect of entropic and the free surface are mutually offsetting,causing Tgbrusn is equal to the bulk.?)When the grafting density of the brushes was relatively low(?_p<0.30 nm^-2),the free surface effect predominates over the entropic effect,causing T_g^brush suppression with molecular weight or brush thickness?brush thickness decreased with decreasing molecular weight?,since the distance between grafting points became large and a "mushroom" conformation was formed,with the coil dimensions similar to those of ungrafted?free?chains.iii)The effect of one chain end anchored and the chain structures are mutually offsetting,causing the molecular weight to have little impact on Tg at intermediate density PS brushes(?_p = 0.30 nm^-2).The Tgbrusb of the PS brushes with ?_p>0.30 is mainly dominated by entropic effects.The Tgbrusn of PS brushes with ?_p<0.30 is mainly dominated by surface effects.The turning point of grafting density from entropic effect dominant Tgbrusn to surface effect dominant Tgbrusn was about 0.32 nm^-2,at which point the architecture of the polystyrene chains changed from "mushroom" regime into "brush" regime.