Study On The Change Of Waterborne Polyurethane Particle During Film Formation By FRET Technology

Polyurethane is a kind of material,which is widely used,with excellent properties such as the wear resistance,the weather resistance,the great ability of film forming and adhesion.Then the waterborne polyurethane reduces VOC to a very low standard,replacing organic solvent with water,and it has excellent properties which are not inferior to solvent-based polyurethane.It is gradually replacing solvent-based polyurethane as a new green material widely used in coatings,adhesives,leather,textiles and other fields.Since the development of emulsification technology in 1970s,waterborne polyurethane has made great progress.At present,there is no good detection method for particle change and its diffusion depth in the process of waterborne polyurethane film formation.In this paper,fluorescent chain extender was synthesized with Pyrenebutyric acid,as the starting material.The labeled fluorescent waterborne polyurethane dispersions with concentration gradient were prepared by controlling the amount of Pyrenebutyric acid.In order to explore the factors affecting the film formation of waterborne polyurethane,the non-fluorescent labeled waterborne polyurethanes with different particle size,with degree of crosslinking and with different soft segment content were also synthesized.Fluorescent chain extenders were characterized by 13C NMR and LC-MS.The dispersions and films of the fluorescent labeled waterborne polyurethane and non-fluorescent labeled waterborne polyurethane were characterized by Fourier transform infrared spectroscopy(IR),Malvern particle size analyzer and steady-state fluorescence spectroscopy.According to the principle of fluorescence resonance energy transfer(FRET)and the steady-state fluorescence spectrum,the relationship between monomeric-state pyrene(IM)and aggregation-state pyrene(IA)fluorescence and the concentration of pyrene CPy were analyzed.The working curve,CPy�IA/IM,was fitted,and the mathematical relationship between the amount of change with particle size and concentration of pyrene before and after film formation was deduced,and we can calculate the diameter of the maximum range of diffusion(D)and the diffuse depth(?R)after the film formation of waterborne polyurethane.At the same time,the film-forming behavior of waterborne polyurethane particles was explored.The main contents of this paper are as follows:(1)Synthesis and characterization of the labeled fluorescent waterborne polyurethane containing pyrene ring4-Pyren-1-y l-butyric acid 2,3-dihydroxy-propyl ester,the fluorescent small-molecule chain extender,was synthesized with pyrenebutyric acid and glycidol which are raw materials and characterized.Ester-based carbon peaks,pyrene-ring carbon peaks and other carbon atom peaks appeared at ?172.8 ppm,?123.2 ppm-136.2 ppm,respectively.The strong signal peak of molecular appeared at 363 m/z of LCMS spectrum.the labeled fluorescent waterborne polyurethane with a gradient of concentration of pyrene was synthesized by adding 4-Pyren-1-yl-butyric acid 2,3-dihydroxy-propyl ester in proportion of 1%,2%,3%,4%and 5%in the formulation.After film formation,the waterborne polyurethane was characterized by infrared spectroscopy.Carbamate bonds,ester bonds and pyrene rings were found near 3500-3250cm-1,1750-1690cm-1 and 880cm-1 respectively,which indicated that pyrene rings had been successfully added into waterborne polyurethane molecular chains.In addition,the particle size of the dispersions was measured.The results show that the particle size of all the labeled fluorescent samples is mainly concentrated at about 40 nm,and the distribution is narrow,which is at the same particle size level.The sample WPUD5 is selected as the research object of this paper.(2)Establishment of mathematical relationship between particle size and pyrene concentration of the labeled fluorescent waterborne polyurethaneAccording to the principle of FRET,when pyrene is stimulated by ultraviolet light,it will produce monomer fluorescence(IM).When it meets a ground-state pyrene,it will form aggregates.The energy is transferred to the ground state pyrene by resonance,which excites the pyrene and generates the aggregated-state fluorescence(IA).The integral values of 398-403 nm for monomeric-state pyrene(IM)and 450-470 nm for aggregation-state pyrene(IA).The overlap of adjacent peaks can be minimized in these two ranges.IA/IM values can reflect the proportion relationship between the two forms of pyrene:the larger the value,the more aggregated pyrene,the fewer monomer pyrene,and vice versa,the fewer aggregated pyrene and the more monomer pyrene.The change of conditions can make pyrene change between monomer and aggregates:at high concentration,the probability of the encounter between excited pyrene and ground-state pyrene increases,then the aggregated pyrene forms easily,and the monomeric-state pyrene decreases;at low concentration,the probability of encounter between excited pyrene and ground pyrene decreases,and pyrene tends to exist as monomer,so the IA/IM value can be used to judge the concentration of pyrene in the area.Then through the steady-state fluorescence measurement of labeled samples WPUD1-WPUD5,the peak-to-integral ratio IA/IM of pyrene aggregates and monomer was calculated,and the working curve of Cpy-IA/IM was fitted:IA/IM ?0.33255 Cpy+0.14037.Thus the CPy of any sample could be calculated by IA/IM value.According to the volume formula of sphere and the volume concentration formula,the formula of the diameter of the maximum range of diffusion is deduced D=(?)D0,and the diffuse depth after film formation can also be calculated:?R=1/2(D-D0).(3)Study on the factors influencing the film formation of waterborne polyurethaneThe factors affecting the formation of waterborne polyurethane films are investigated from four aspects:particle size,the degre of crosslinking,soft segment content and film forming temperature.WPUD6-WPUD10,WPUD10-WPUD15 and WPUD16-WPUD20 are synthesized.The labeled fluorescent sample WPUD5 is evenly mixed with the above three series of unlabeled samples according to the mass of 1:10,and the steady-state fluorescence of the films is measured.In addition,films of WPUD5 and WPUD10 are formed at 20?,35 ?,50?,65 ? and 80 ?.The IA/IM values of each film are calculated by fluorescence spectra,and the working line is used to calculate the concentration of pyrene(CPy).Then we can calculate the diameter of the maximum range of diffusion D and the diffuse depth ?R by using the formula we deduced.The results of film formation can be analyzed from D and ?R values.The larger the D and AR values,the larger the deformation of the labeled particles,the larger the diffusion range,the better the film formation effect,conversely,the worse the film formation effect.The experimental results show that the increase of particle size,degreed of crosslinking and soft segment content of unlabeled waterborne polyurethane can improve the film-forming effect for the unlabeled waterborne polyurethane and the labeled waterborne polyurethane,and the increase of film-forming temperature can also promote the film-forming effect.