Effects Of Interaction And Stability Of Conjugated Polymer Solutions On Films' Condensed State Structure

As a class of polymer materials with excellent optoelectronic properties,conjugated polymers are mainly used in the field of optoelectronics and have developed rapidly in recent years.At present,the molecular design and device preparation process of optoelectronic conjugated polymers with excellent properties have been widely explored.Studies on their solution behavior and film condensed state structure are rarely reported due to limited research methods.However,its solution behavior has a significant impact on the later device preparation due to the change of solution behavior induced by internal factors or external factors.In the kinetic process from solution to film,the molecular chain of conjugated polymer undergoes a slow self-assembly process from single chain to aggregation.The solution behavior of conjugated polymer mainly includes single chain conformation,chain aggregation size,chain packing density,etc.From these aspects,the mechanism of the behavior change of the conjugated polymer solution can be clearly revealed,and its influence on the condensed state structure of films can be further understood.Starting from the behavior of polymer solution,the study of its solution behavior and film condensed state structure has important theoretical guiding significance for the preparation of devices and practical applications of the entire conjugated polymer.The stability of conjugated polymer materials in applications is crucial to the development of optoelectronic devices with superior performance,but little research has been done on them today.And most of the research is from the perspective of the post-processing of conjugated polymer films and device preparation,but the stability of the conjugated polymer solution itself is ignored.The stability of conjugated polymer solutions is crucial,not only for the preparation of optoelectronic thin films with superior performance,but also for the application of optoelectronic devices in real life.PPV-based conjugated polymers have very poor solubility due to their rigid conjugated backbone structure.Researchers have begun to synthesize various grafted PPV-based conjugated polymers to improve this shortcoming.Poly[2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylvinylene](MEH-PPV)is an orange red light material with excellent photoelectric properties,which plays an important role in full-color display.However,as one of the representatives of PPV derivatives,after grafting a long alkoxy side chain,its self-solubility is increased and its stability is also reduced.Under the catalysis of light and oxygen,MEH-PPV solution itself will undergo degradation and chain scission,which further hinders its application in practical production.Therefore,the improvement of the stable performance of MEH-PPV is crucial to expand its application prospects.In this paper,the classical conjugated polymer MEH-PPV was selected to explore the effects of its solvent properties and solution stability on the condensed state structure of the films.The thesis is mainly divided into the following two parts:In the first part,solvent properties are crucial for in-depth insights into the solution behavior of conjugated polymer chains,the physical nature of film condensed state structure formation,and optoelectronic device performance.We investigated the effect of the synergistic effect of solvent and external electric field on the solution behavior of the conjugated polymer MEH-PPV and its film condensed state structure based on the second virial coefficient(A₂).The study found that although MEH-PPV solutions in different solvents(toluene,chlorobenzene,chloroform and tetrahydrofuran)could respond to the regulation of the external electric field,the degree of response changed significantly with the change of solvent.In addition to this,the degree of order of the condensed matter structures in the films prepared from the precursor solutions is also significantly different.The fundamental reason why MEH-PPV solutions with different solvents have different degrees of response to the regulation of the external electric field is that the degree of interaction between the solute and the solvent is different,and this degree of interaction can be quantitatively described by the second virial coefficient(A₂).The larger the second virial coefficient(A₂),the stronger the interaction between the solvent and the solute in the solution,and the greater the response to the external electric field.In addition,under the control of an external electric field,chain-like aggregates with different size distributions were formed in MEH-PPV solutions in different solvents,and large-area ordered structures were formed.This ordered structure can not only be efficiently transferred into the films prepared from the precursor solutions,but also be beneficial to improve the carrier mobility of optoelectronic films.In this research,a new method of directly applying an external electric field to the solution to regulate the solution behavior is employed,and the root cause of the difference in the interaction between the solute and the solvent is revealed.This study reveals the essential reason for the influence of solvent on solution behavior,it has important theoretical guiding significance not only for deeply realizing and understanding the physical nature of the solution behavior of conjugated polymers,but also for the regulation of the order of the condensed state structure from solution to film and the structure-activity relationship between the construction of optoelectronic devices.In the second part,in order to improve the stability of MEH-PPV solution and increase its utilization in practical production,the solution stability was studied.Not only new methods are proposed to improve the stability,but also the effect of the improved solution stability on the film stability is explored.We found that the MEH-PPV dilute solution will degrade and break the chain in the natural environment,and the color of the solution changes from orange-red,orange-yellow,yellow,yellow-green,to green.From the dynamic light scattering results,it can be found that the chain aggregation size in the dilute solution of MEH-PPV is also gradually decreasing.From the structural characterization by FT-IR,the results indicated the change of its side chain structure.We made a concentration gradient of MEH-PPV solution from dilute solution to concentrated solution,applying the method of external electric field to improve the stability of MEH-PPV solutions with different concentrations has produced significant effects.After applying external electric field induction,both the dilute solution and the concentrated solution,the chain aggregation size and chain packing density increase,and the formed aggregates can exist stably for a certain period of time.After that,we selected a concentrated solution of MEH-PPV and prepared films by spin coating after applying an external electric field.It was found that the luminescence intensity and ordered structure of the films prepared from the MEH-PPV precursor solution induced by an external electric field were improved.The results demonstrate that the stability of the precursor solution has a crucial influence on the stability of the film.The simple and convenient method of effectively utilizing the external electric field can not only improve the stability of MEH-PPV solution,but also improve the stability of its film.This method is also applicable to a series of conjugated polymer with the same properties as MEH-PPV.This research is not only conducive to further understanding the physical nature of optoelectronic performance enhancement,but also has practical guiding significance for improving the stability of conjugated polymer solutions and films to promote the application of multicolor large-area optoelectronic devices.