The Study On The Formation,electrodissolution And Hydrothermal Treatment Of PEOX/PAA Hydrogen-bonded Polymer Complex Film

Poly（2-oxazoline）s（POX）are a class of polymers with the amide group in side chain and are structural isomers of polypeptides.They can be synthesized by the living cationic ring-opening polymerization of 2-subsitutied-2-oxazolines.Recently,POX has been considered to be the ideal multifunctional polymeric scaffolds.The POX has the characteristics of biocompatibility,thermosensitivity and antifouling which have greatly attracted the interest of scientists for their applications in biomedical,life science and smart materials.Nowadays,the researches on POX are focus on the synthesis,drug molecular carriers,thermoresponsive behavior and antifouling behavior.However,these studies are limited to the block copolymers,homopolymers and conjugates.The studies of interfacial complexes about POX are relatively less.The layer-by-layer（LbL）technology can realize the interfacial recombination of polymers well,and the interfacial composite system with hydrogen bonding as the driving force needs further discussion in basic theory and applications.Therefore,In this thesis,we studied the interfacial complexation of poly（2-oxazoline）s hydrogen boned film and the factors which influence the assembly process.In addition,combining the unique properties of POX,the decomposition behaviors of the POX film under electrolytic bath and hydrothermal environment have also been investigated,which can provide a theoretical basis for its applications in the field of drug controlled release and surface modification.Exploring the LbL assembly process of POX hydrogen bonded film is the basic for subsequent applications.Poly（2-oxazoline）s hydrogen bonded polymer thin film are prepared by Layer-by-Layer（LbL）assembly technology using poly（2-ethyl-2-oxazoline）（PEOX）as hydrogen bond acceptor and poly（acrylic acid）（PAA）as hydrogen bond donor.The Fourier transform infrared spectroscopy（FTIR）proved that the driving force of the film is assembled by hydrogen bonding interactions.The growth process of film thickness is characterized by the Fabry-Pérot fingerprint peaks or the characteristic absorption peak intensities on the ultraviolet visible（UV-vis）spectra.The experimental results show that the film exhibits exponential growth at early stage and transfers to linear growth after 10 assembly cycles.The film growth should be related with polymer chain diffusion in the assembled film and the assembling solution.Many factors influence the film growth process.Increasing the temperature and the assembly time will accelerate the film growth,because it is beneficial to the diffusion behavior of polymer chains and making more chains involve in hydrogen bond interaction.While increasing the rinsing time is helpful to produce a smooth film,which is due to the full rearrangement between the polymer chains.pH value affects the hydrogen bond between PEOX and PAA.Increasing the pH value is not conducive to the assembly of PEOX/PAA thin films.The hydrogen bond will be destroyed at high pH value.PEOX and PAA can be LbL assembled to prepare thin film below pH 4.5 while complete dissolution needs a pH value higher than 7.0.This difference behavior is related to the dynamics.The concentration gradient of the assembly liquid is the driving force for the polymer chain diffusing.Two experimental schemes are designed to study the influence of concentration:（1）keeping the concentration ratio unchanged,but concentration of PEOX and PAA both increases;（2）fixing the total concentration,but the concentration ratio between PEOX and PAA varies.Experiment（1）shows that the film becomes thicker with the increment of concentration.However,the growth trend will slow down with the continuous increase of concentration until finally it remains unchanged.Meanwhile,the FTIR spectra show that the component ratio of PEOX and PAA in the film keeps constant.Correspondingly,experiment（2）shows that when the concentration of PEOX and PAA is closer,the film thickness is larger.FTIR spectra show that the component ratio of PEOX and PAA in film is related with the concentration ratio of assembly liquid.With the increment of concentration[EOX]/[AA]ratio,the composition of PEOX in film also increased.Therefore,the component ratio of the film can be controlled by changing the concentration ratio of assembly liquid.Besides,both molecular weight and molecular weight ratio have effects on the growth of thin film.When assembly cycle number is lower than 10,the film thickness exhibits exponential growth,PEOX or PAA with higher molecular weight will produce thick film;when assembly cycle number is higher than 10,the film thickness shows linear growth,PEOX and PAA that have obvious molecular weight ratio show faster growth.Therefore,studying the LbL assembly process of hydrogen bonded PEOX/PAA film is not only helpful to understand the film internal structure and molecular interactions,but also to obtain the PEOX/PAA film which meets the experimental requirements by controlling the parameters in the process.These provide a basis for our follow-up experiments.The biocompatibility of POX is beneficial to the application of drug controlled-release.Therefore,it is of great significance to study the controllable dissociation of POX hydrogen bonded composite films.The disassembly behavior of hydrogen bonded PEOX/PAA thin film is studied by an electrochemical stimulus.We report a new method to trigger the dissolution of film which is deposited on conductive indium tin oxide（ITO）glass near the positive electrode in electrolytic bath.The film thickness and topography changes before and after electrolysis are detected by UV-vis spectra and optical microscope,respectively.A uniform film can be obtained and the film thickness is decreased,which indicates that the positive electrode in electrolytic bath can dissociate the hydrogen bonded thin film.Through the analysis of Fourier transform infrared attenuated total reflection（FTIR-ATR）and cyclic voltammetry（CV）,it is shown that the dissociation of hydrogen bonding film is not the redox reaction of PEOX or PAA in the film,but the pH gradient range formed near the anode.Near the positive electrode,the hydroxyl ions（OH^-）moved to the electrode and the hydrogen ions（H⁺）produced by electrolysis of water form a pH value gradient range which resulting in the closer to electrode,the lower the pH value.Hence,the higher pH value induces the dissolution of hydrogen bonded film.Then,comparing the film disintegrated near the positive electrode and near the negative electrode,we have further proved that the pH value gradient range makes a big contribution to the dissolution process and results in different morphology and thickness of multilayers.The electrolytic dissociation speed of the film on positive electrode is slower than that on negative electrode.After electrolysis,a uniform and flat transparent film can be obtained near the positive electrode,while the negative electrode electrolysis film exhibits cloudy and porous.Here,the results are related to the different way of film dissolution rather than the disassembled speed.Two different pH gradient ranges induce different disassembled way that on the positive electrode the film dissolves from outside to inside,on the negative electrode the film dissolves from inside to outside.In addition,the factors that influence the film dissolution near the positive electrode have been studied,such as the electrolyte pH value,the applied electrical potential and the electrolyte type.The pH value affects the stability of hydrogen bond and property of pH gradient range,so its influence must be taken into account.Experimental results show that the lower pH value is beneficial for the dissolution of film which is beyond our expectations.This is because the lower pH value can lead the ionic strength in solution to be higher and higher ionic strength allows the electrical conductivity of the solution to be better.Rapid electrolysis of water is able to generate more H⁺ions on the surface of positive electrode and move more OH^-ions towards the electrode.These two activities let the pH gradient range be able to achieve much stronger acidic and alkaline,which promotes the film dissolution.The applied electrical potential influences the current in the electrolytic bath:higher electrical potential corresponds to higher current and leads to faster dissociation speed.The effects of three different electrolytes on the dissociation behavior of the films are studied.It is found that the external ions such as Cl^-,SO₄^2-and PO₄^3-do not affect the dissociation behavior.The electrodissolution of thin film is mainly related to the pH gradient range after hydrolysis.Due to the low pH value or high voltage,the hydrolysis near the electrode makes the region become strong acidic,which is not conducive to the stability of the ITO layer and then destroys the film.Therefore,we draw a conclusion that the proper condition for film dissolution is electrolyte pH=4,applied potential 4 V,and the current is about 25μA.Therefore,the factors such as the electrolysis time,electrolyte pH value,applied electrical potential and the different electrode could be used to control the electrodissolution of hydrogen bonded film,which provides a theoretical basis for the field of electrical stimulation.The dewetting microstructure of polymer films is an important research direction of "bottom-up"patterning technology.Meanwhile,the introduction of POX polymer into the hydrogen bond film provides more possibilities in the field of dewetting.We studied the dewetting behavior of PEOX/PAA thin film in solid-liquid interface with hydrothermal treatment.The typical surface morphology of dewetting process such as microporous,irregular cellular structure and droplets are detected by the observation of optical microscope and atomic force microscope（AFM）.Meanwhile,the changes of the Fabry-Pérot fingerprint peaks or the characteristic absorption peak intensities on the ultraviolet visible（UV-vis）spectra further prove that the film surface has been dewetted.Microscopic Raman and IR imaging demonstrated that the position of carbonyl expansion vibration peak in PEOX and PAA molecules is not changed,which indicates that the hydrogen bond in film still exists and the two components is not separated during the dewetting process.In addition,the Raman spectra of the pores formed by the dewetting are basically the same as that of the substrate,indicating that there is no residual PEOX or PAA polymer in the hole.The micro-infrared spectroscope can be used to characterize the dewetting droplets.The designed area is scanned using IR intensity signal at 1724 cm^-1（the position of carbonyl stretching vibration peak of PAA）and 1617 cm^-1（the position of carbonyl stretching vibration peak of PEOX）separately.The images of intensities at 1724 cm^-11 and 1617 cm^-1 are almost the same,which indicates that the droplets formed by dewetting are still composed by PEOX and PAA and their hydrogen bond still exists.According to the size and the distribution of micropore formed by dewetting,the mechanism of hydrogen bonded film dewetts with hydrothermal treatment is conforming to the thermal nucleation dewetting.The water molecules in hydrothermal environment promote the motion of molecular segments,which makes holes appear on the film and then dewetts.The dewetting mechanism is related with film thickness.As the film thickness increases from 50 nm to2400 nm,the dewetting speed will become slow.This is because the bigger the film thickness is,the bigger the viscosity is.The water molecules are difficult to diffuse into the thin film and the motion of the chain segment is not obvious.The restricted motion of chain segment is the fundamental reason for the slower dewetting speed.Furthermore,the dewetting behavior of film thickness 50 nm is observed.It is found that the dewetting mechanism is similar to that of spinodal dewetting,which indicates that with the film thickness increasing from 50nm to 2400nm,the dewetting mechanism transfers from spinodal to nucleation.In addition,increasing the hydrothermal temperature will accelerate the movement of molecular chains in films which is favorable for dewetting.The change of the pH value of the hydrothermal solution will affect the hydrogen bond interaction among the molecules in the film,and the hydrothermal solution with high pH value is more beneficial to the film dewetting.Moreover,the droplet size obtained by hydrothermal solution of pH 5.0 is smaller than that of pH 3.0,and the arrangement is more closely.Therefore,we can get dewetting droplet patterns with different sizes and distributions by controlling the pH value of hydrothermal solution.