Controlling Polyfluorene(PFs) Chain Solution Behaviors And Its Effect On Film Condensed Structures

Organic light emitting diodes(OLEDs)have been widely used in various fields.Polyfluorene(PFs)has promoted the development of OLEDs as blue-light materials.Especially,poly(9,9-dioctylfluorene)(PFO or PF8)is a classic hairy-rod conjugated polymer with polymorphism characteristic,due to a variety of single chain conformations formed by the rotation of C-C bonds between fluorene units on PFO backbone,which provides a model for studying the physical essential of structure-property relationships.The common are?and?conformation.In particular,?conformation has a more coplanar geometry and longer effective conjugated length.Correspondingly,?phase in film is conducive to improving carrier mobility.In addition,?phase has a narrower energy band gap and larger stimulated emission cross section,its laser threshold is several orders of magnitude lower than that of PFO amorphous glass state,hence,PFO is also expected to be used as an electrically pumped organic lasing material.In addition,poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(n-alkoxy)-9,9diphenylfluoren-2,7-diyl](PODPF)which can form more planar conformation similar to PFO,and(poly(9-(4-(octyloxy)-phenyl)-2,7-fluoren-9-ol)(PPFOH),which can achieve multi-color and larger-area electroluminescence,have become excellent optoelectronic materials.Therefore,many studies still always have concentrated on PFs conjugated polymers.Conjugated polymers can be dissolved in common organic solvents,hence the advantage is to prepare optoelectronic thin film devices by solution processing.It's well known that optimizing film condensed state structures is the key to improving OLED performance,but current researches have always ignored that controlling conjugated polymer chain behaviors in the film-forming precursor solution is also an effective approach,but just concentrated on the film post-treatment methods.In the dynamic process from solution state to film state,conjugated polymer chains undergo a slow self-assembly process from single chain to aggregation,so conjugated polymer chain solution behaviors mainly include single-chain conformation,chain aggregation size,chain packing density,etc.The factors affecting conjugated polymer chain solution behaviors can be divided into internal intrinsic factors,such as polymer structure,solvent properties and solution concentration,as well external environmental factors such as temperature and external electric field.The single-chain conformation of conjugated polymer is an intrinsic cause that affects aggregation structure by changing the way of chain arranging and packing,which also strongly depends on the structure of side chain and the nature properties of solvent medias.External electric field and temperature will provide external field forces to drive polymer chain conformation transformation.Especially,external electric field as a directional field can induce conjugated polymer chain to self-assemble into ordered structures,but it hasn't been applied to optoelectronic conjugated polymer solution so far.In particular,?conformation content could directly affect the order degree of aggregation structure,and up to the device efficiency.Therefore,it is of great theoretical and practical significance to study controlling conjugated polymer chain solution behaviors and its effect on films condensed state structures.The thesis work is divided into the following three parts:In the first part,we selected poly(9,9di-hexlyfluorene)(PF6),poly(9,9-diheptylfluorene)(PF7),poly(9,9-dioctylfluorene)(PF8),poly(9,9-dinonylfluorene)(PF9),and poly(9,9-didecylfluorene)(PF10),with linear alkyl side chains of different carbon numbers as the research samples,and explored PFs chain solution behaviors,?conformation and its stability with side chain lengths change during solution dynamic process.Interestingly,the shape parameter(Rg/Rh)(ie,the ratio of the radius of gyration(Rg)to the hydrodynamic radius(Rh))of PFs single chain in pure toluene solution showed odd-even effect as the side chain length increased,and the Rg/Rh of PFs single chain with even carbon atoms in side chain was larger than that of PFs single chain with even carbon atoms in side chain,which indicated that PFs single chains with even carbon atoms numbers in side chain were more rigid than that with odd carbon atoms numbers.PF8,PF9 and PF10 chains could induce?conformation by adding different percentage of poor solvent ethanol to PFs toluene solutions.The highest?conformation content was about 42%,but the characteristic peaks in the UV-vis spectra were different and located at 437,427 and 428 nm,respectively.PF8 chains were the easiest to form?conformation because they formed the smallest size but tight aggregation in solution reflected by the smaller Rh size distribution and larger static fractal dimension(d_f).It was a good proof that high chain packing density in aggregation was more conducive to the formation of?conformation.Once?conformation content reached the saturation value of 42%in the toluene mixed solution with 80%ethanol,it wouldn't change even after the solution was placed for 21 days.These conclusions were of great significance for understanding of the dynamics process from solution state to film state based on conjugated polymer condensed matter physics to regulate film condensed state structures and to realize the fabrication of optoelectronic devices with high carrier mobility,stability and efficiency.In the second part,we focused on the effect of solvent aromaticity on PFO chain solution behaviors and film condensed state structures.We explored the dynamic evolution of PFO chain solution behaviors in aromatic solvents(CB and Tol)and non-aromatic solvents(THF and CF).The solution concentration changed from the dilute solution range to the concentrated solution range,in detailed,it was from 0.005 mg/mL to mg/mL.Firstly,effect of solvent aromaticity on PFO chain conformation transformation was studied.Due to the larger exclude volume derived from the?-?interaction between PFO backbone and aromatic solvents,PFO single chains in aromatic solvents CB and Tol were more rigid than those in non-aromatic solvents THF and CF,and adopted more extended chain geometry with longer effective conjugated length,which was consistent with the conclusion about PFO single-chain conformation obtained by static light scattering.Therefore,PFO chains were easier to form?conformation in aromatic solvents.Secondly,effects of solvent aromaticity on PFO chain aggregation size and chain packing density were discussed.PFO chains began to aggregate when solution concentration was 1 mg/mL.but PFO chain aggregation size in aromatic solvents CB and Tol were larger,meanwhile the chain packing density were smaller.Then,the influence of solvent aromaticity on film condensed state structure was studied.It was found that ordered fiber structures were easily formed in films prepared from aromatic solvents CB and Tol,while circular and short rod-like amorphous structures were formed from non-aromatic solvents THF and CF,respectively.Finally,the relationship of solvent aromaticity and the dissolution mechanism of PFO chain was revealed.This is conducive to deeply understand the physical nature of self-assemble process and optoelectronic properties,so that film condensed state structure can be well regulated and it was favor of efficient devices fabrication.In the third part,solution behaviors of different polar polyfluorene(PFs)chains were regulated by applying external electric field across solutions of polar solvent tetrahydrofuran(THF)and film condensed state structure was further optimized.This was the first time that external electric field was applied to optoelectronic conjugated polymer solution.Polyfluorene(PFs)chain all became more planar geometry and chain order degree increased,but the weakest polar PFO chains changed the most,the polar PPFOH chains only slightly changed,the polar PODPF chain didn't changed at all.Especially for the weakest polar PFO,whether in dilute solution(0.1 mg/mL)or concentrated solution(5 mg/mL),the planarity and effective conjugation length of the weakest polar PFO chains all increased,and PFO chain conformation always transformed towards?conformation.These findings subverted the conventional insight that weakly polar polymer chains were not easily susceptible to external electric field and only polar conjugated polymers could be dramatically affected.Especially in the 5mg/mL concentrated PFO solution,?conformation content increased significantly by the simple physical method of directly applying external electric field across PFO solution.PFO chains were orientally drove to self-assemble and formed ordered structures with large chain packing density.?-phase crystal was formed directly in films.This is difficult for other?-phase crystal preparation methods.Surprisingly,the long-wavelength green band emission of PPFOH chains was also suppressed,which has been an obstacle hindering the practical application of PPFOH.Therefore,the key to effective utilizing external electric field is to increase the dielectric constant gap between polymer chains and solvent molecule,rather than simply changing the polymer chain polarity.The above conclusions were also suitable to other conjugated polymer systems.This research is not only helpful for further understanding of the physical nature of enhancing optoelectronic devices performance based on polymer condensed matter physics,but also for controlling condensed state structure of conjugated polymers to achieve chains orderly packing and promote the application of multi-color,high-carrier mobility and large area photovoltaic devices...