Preparation Of The High-Density Polymer Brush And Its Glass Transition Behavior

With the development of nanotechnology,the chemical and physical properties of thin polymer films have received more and more attentions.The properties of the polymer film such as viscosity,modulus and glass transition temperature deviate from the bulk,when the film thickness was reduced to the nanometer scale.Polymer brush is a special nano-constrained thin film,in which the chain conformation can be precisely controlled by the grafting density???.Increasing the grafting density is the key to improve the physical properties of polymer brushes.So,it is of great significance to study the glass transition and molecular motion of ultra-high density polymer brushes in order to achieve the better prediction and regulation of the structure and properties of polymer brushes,and as well it is meaningful in clarifying the influence of molecular chain conformation on the molecular motion of polymers.To this end,we herein explored the glass transition and molecular motion behavior of the polymer brush with an ultra-high grafting density.In this thesis,the polystyrene?PS?brush was used as the model system.We investigated?a?the method to prepare the ultra-high density PS brush and?b?the glass transition and molecular mobility of the prepared ultra-high density brushes.The main conclusions obtained are summarized as follows:?1?Generally,it has been reported that the graft density???of PS brush prepared by a SI-ATRP is about 0.6-0.8 nm^-2,which is far lower than the theoretical maximum?of PS brush(i.e.,1.45 nm^-2).This indicates that the?of PS brush can be further improved.Herein,we proposed to use the surface-initiated atom transfer radical polymerization?SI-ATRP?to prepare the polymer brushes.The?of the PS brushes was further improve by optimizing the density and efficiency of the initiators and properties of solvents during polymerization.Long chain?docos-21-en-1-ol?molecules with vinyl and hydroxyl groups on the chain ends were prepared by Grignard reaction using 11-bromo-1-undecene and 11-bromo-1-undecanol.Then,the benzyl bromide group?i.e.,initiator of SI-ATRP?was introduced at one end through the esterification reaction between hydroxyl groups on docos-21-en-1-ol and 2-bromo-2-phenylacetic acid.Trichlorosilane is then added to another end of the docos-21-en-1-ol by an addition reaction between vinyl and trichlorosilane,and finally a long alkyl chain silane coupling agent molecule?i.e.,22-?trichlorosilyl?docosyl 2-bromo-2-phenylacetate;BPASiCl?containing an benzyl bromide group with high initiating activity(activation rate constant,k=5.3×10³ M^-1 s^-1)was successfully synthesized.The above reaction process and chemical structure of BPASiCl were characterized and verified by NMR.Subsequently,BPASiCl was attached onto the surface of the silicon substrate by a condensation reaction between trichlorosilane and Si-OH on silicon substrate.Van der Waals interaction between long alkyl chains promotes the formation of BPASiCl self-assembled monolayers.The results from atomic force microscopy?AFM?and sum frequency vibrational spectroscopy?SFG?revealed that the BPASiCl molecules were regularly arranged on the surface of the substrate to form an ordered self-assembled monolayer.Following this,the SI-ATRP polymerization of styrene was initiated by the benzyl bromide group on the substrate surface to prepare PS brushes.Toluene,cyclohexane and n-hexane with different solubilities to PS were selected as solvents to study the solvent effect.As a result,it was found that the?of the PS brushes depends on the solvent property.When the polymerization was carried out in n-hexane?i.e.,bad solvent of PS?,the PS brushes with?of 0.69 nm^-2 were obtained.When cyclohexane?i.e.,?48?solvent of PS?was used,the PS brushes with?of 0.95 nm^-2 were prepared.Particularly,when toluene?i.e,good solvent of PS?was used,the?of PS brush can be reached as high as 1.23 nm^-2,which is much higher than the?of PS brush reported in most literatures and is close to the theoretical maximal density of 1.45 nm^-2.This means that the solubility of solvents play an important role in adjusting the grafting density of polymer brushes.The greater solubility of the chain in the solvent,which associated with a more stretched chains conformation,results in smaller steric hindrance of the chain in the brushes,thereby increasing the?of the PS brush.This method for preparing the ultra-high density polymer brushes can be applied to other types of polymer,and therefore has wide applications.?2?The glass transition behavior of ultra-high density PS brush was studied by ellipsometry.The T_g of ultra-high density PS brush increased with the decrease of film thickness,and such increase trend becomes more significant with the increase of?.When PS brush density was 1.23nm^-2,the T_g of the 10 nm thick PS brush was about 35 degrees higher than that of its bulk,and is75 degrees higher than that of PS thin film of the same thickness.In addition,the thermal expansion coefficient of PS brush decreased with the decrease of film thickness,and the reduction became more significant with the increase of?.When the?was 1.23 nm^-2,the expansion coefficient of PS brush with a thickness of 10 nm was about 2?10^-4 K^-1 lower than the bulk value,which is about9?10^-4 K^-1 lower than the value of the PS film at the same thickness.These results indicate that increasing the molecular chain orientation of the grafted polymer can greatly improve its thermal stability,which provides a new method and idea for preparing high stable functional polymer materials.We believe that the high degree of chain extension causes a decrease in conformational entropy of chains,resulting in a suppression of the chain mobility and an increase in T_g.In addition,the free volume between the molecular chains dominates the thermal expansion behavior of the polymer brush.The free volume reduced with increasing the grafting density of the polymer brush.The reduced free volume decreases the space for chain movement,and so,the reduced thermal expansion coefficient can be resulted.Overall,the results not only provide a new method for preparing ultra-high graft density polymer brushes,but also provide new insights for understanding the glass transition and thermal stability behavior of highly oriented molecular chains.