Study On Synthesis And Self-assembly Of Stimulii-responsive Polyamides

Up to now,block copolymers have been the main materials for the macromolecular self-assembly.Compared with the block copolymers,the polycondensates are one of the most important polymer materials,too.However,only a few studies have been performed on the solution self-assembly of polycondensates.Polyamide is one of the most widely applied polycondensates,which is usually prepared through the polycondensation of diamine and diacid.The strong inter-molecular hydrogen bonds can render the self-assemblies of polyamides with extraordinary stability and environmental stimuli-responsive ability.In addition,the self-assembly of polyamides might display the crystallization-driven “living self-assembly” based on the crystallinity of polyamides.Moreover,the functional moieties can be easily introduced into the polyamide molecular chains through the simple copolycondensation.Therefore,the polyamides have been expected to be a promising macromolecular self-assembly building block.Unfortunately,the investigations on the self-assembly of polyamides have seldom been reported.Based on this fact,herein,we synthesized a series of amphiphilic copolyamides and endowed the self-assemblies of polyamides with stimuli responsibility through the introduction of stimuli-responsive units in the copolycondensation process.Furthermore,we systematically probed the self-assembly behaviors and potential applications of as-prepared amphiphilic polyamides in aqueous solution and disclosed the mechanism.The main research contents and conclusions are as follows:First,we synthesized a novel multiblock copolyamides(feed ratio : 0.4:1).We denoted the synthesized polyamides as MBCPA.The multiblock structures of as-prepared copolyamides were characterized using FT-IR,1H NMR,TGA and DSC measurements.DLS,AFM and TEM results showed that the prepared multiblock copolyamides could self-assemble into vesicles with the thickness of 4.5 nm.DPD simulation and TEM measurement show that the multiblock copolyamide vesicles should be generated through the mechanisms of “micelle-membrane-vesicles”.In addition,it can also be seen that the polymer chains displayed three kinds of packing behaviors in the vesicle membrane,namely the folding chains,hanging chains and spanned chains.Furthermore,the fluorescence emission spectra and TEM image suggested that the vesicles should be disrupted under an external ultrasound stimulus.And there was almost a 50% of nile red released from the vesicles after 14 min of ultrasonic irradiation.The micro differential scanning calorimetry(Micro-DSC)and infrared spectroscopy measurements of the MBCPA vesicle aqueous solution both indicated that the intermolecular hydrogen bonds inside the MBCPA vesicles were broken after ultrasonic irradiation.This might be the origin of the ultrasound stimuli-responsive behaviour of the MBCPA vesicles.Second,the pH-responsive polyamides(PAPIP.26)was synthesized by the polycondensation of ?,?-dicarboxyl poly(ethylene glycol)(PEG segment)and pH-responsive 1,4-Bis(3-aminopropyl)piperazine(PIP segment).The chemical structures of as-prepared polyamides were ascertained by FT-IR,1H NMR,GPC measurements.Herein,a direct hydration method was used to induce the self-assembly of PAPIP.26 by putting polymers into deionized water with a concentration of 0.6 mg/mL.SEM results showed the prepared polyamides could self-assemble into giant elliptic platelets more than 20 ?m in axial length.AFM results indicated that the platelets are around 500 nm in thickness,indicating a multilayer structure.We observed the intermediates during the hierarchical self-assembly process of PAPIP.26 in different concentration aqueous solution via SEM and AFM measurements.The results show that the giant platelet was obtained through the lay-by-layer stacking process from small frustum pyramid platelets.And the whole self-assmbly process is very similar to the “installation art” process.Interestingly,with increasing solution concentration to 1 mg/mL,the giant elliptic platelets further stacked together into multi-horned superstructures,such as four-horned,six-horned,the starfish or star anise-like super-structures.Based on the WXRD and SAXS results,we proposed a molecular folding model of PAPIP.26 inside the giant elliptic platelet.The polymer will fold into an ordered layer structure with a thickness of 12.32 nm,which includes two extended PEG chains with a length of 5.2 nm each and three tilting PIP units with a length of 0.64 nm each.The polymer chain will fold back after reaching the layer thickness through the bending of the PEG chain at the end.The layer structure is fixed due to the crystallization of piperazine and PEO units.Interestingly,when the pH value of self-assembly solution(c = 0.6 mg/mL)were adjusted to 0.5,1.0,1.5 and 2.0,wheat-like,gyrus-like,frog spawn-like and rectangle platelet-like superstructures were observed,respectively.Third,the temperature-responsive polyamides(APA)were synthesized by the polycondensation of hexanedioic acid and temperature-responsive poly(propylene glycol)bis(2-aminopropyl ether).The polyamide structures of as-prepared polyamides were confirmed by FT-IR and GPC measurements.Micro-DSC and UV results showed a LCST of 33 oC for APA.TEM and AFM results showed that the as-prepared APA should self-assemble into normal fibers at room temperature(c = 1 mg/mL).However,the fibers transformed into helical fibers when the temperature was increased to 60 oC.And the average helix pitch was 35 nm.Based on the study of temperature-responsive polamides,we synthesized a temperature and light dual responsive polyamide throug the polycondensation between ?,?-Dicarboxyl Poly(Ethylene Glycol)and 4,4-Azodianiline.We denote the prepared polyamides as PAAzo.26.The chemical structures of PAAzo.26 were studied by FT-IR,1H NMR,GPC measurements.TEM results showed that the PAAzo.26 self-assemblies underwent an irregular micelles-to-spherical micelles transformation in the heating process.Moreover,the PAAzo.26 micelles could transform into vesciles containing unimolecular micelles after 4 h UV irradiation.And the origin of morphology transformation was that the UV light could induce the cis-trans isomerism of azo group according to the UV-VIS results.In addition,fluorescent spectra suggested that the PAAzo.26 self-assembies displayed an aggregation-induced emission behavior.And the prepared PAAzo.26 self-assembies showed a high stability against photobleaching.Importantly,PAAzo.26 also possessed an upconversion property,that is,green fluorescence can be obtained through the excitation of near-infrared light.