Control Of The Chain Conformation Of Conjugated Polymer Poly(9,9-dioctylfluorene) And Research On Its Aggregation Structure In Solution

Polyfluorene (PF) is one kind of extremely promising conjugated polymermaterials, it can be well dissolved by organic solvent and easily processed underrelatively low temperature. In this paper, our research system is the PF derivativewith linear alkyl chain of eight carbon, namely poly(9,9-dioctylfluorene)(PFO). Inrecent years, conjugated polymer PFO has attracted much attention because of itsexcellent photophysical and optoelectronic characteristics, especially its specialchain conformation β-conformation. The β-conformation has a large intrachaintorsion angle and shows a coplanar conformation, which results in an extended PFOchain conjugation length and increased conformational order. From PFO solution tofilm, the structural characteristics of β-conformation are beneficial in enhancing thecharge carrier mobility and efficiency of PFO optoelectronic devices. Moreover,PFO/β-conformation is considered as a potential system for electrically pumpedorganic lasing.Based on the concept of―soft matter‖in polymer condensed matter physics,conjugated polymer PFO has strong response with weak stimulation. In this paper,using different external fields (solvent field and temperature field), the relationshipbetween β-conformation and interchain aggregation structure is researched, and the proportion of β-conformation in PFO solution is well controlled. Also, we haveexplored the formation mechanism of β-conformation in solution. They are allsignificant and instructive to fabricate the optoelectronic devices from solutions tofilms with high carrier mobility, high efficiency and good stability in the future.Moreover, β-conformation is just transformed from the α-conformation of PFO insolution by increasing the intrachain torsion angle of α-conformation, which meansthat α-conformation is the source of β-conformation. Hence, the research on theα-conformation of PFO in solution is very critical. In this work, the conformationalcharacteristics of the α-conformation of PFO in solution are studied by the scalinglaw, self-similarity and light scattering.In the first work, two different forms of solvent field (solvation effect anddesolvation effect) are used to control the proportion of β-conformation in PFOsolution. It is found that both solvation effect and desolvation effect can enhance theproportion of β-conformation by changing the aggregation structure in solution. Theproportion of β-conformation can be enhanced up to40%by the solvation effect andbecome a metastable state; and the desolvation effect can enhance the the proportionof β-conformation remarkably by18percentage units. In the process, the proportionof β-conformation is closely related to the aggregation degrees of PFO chains.According to the relevant reports, the concepts of mesoscopic aggregates andmacroscopic aggregates are first proposed by us and they are well proved by thefiltration experiment. The mesoscopic aggregates are mainly composed ofβ-conformation of PFO and are beneficial to enhance the proportion ofβ-conformation, while the macroscopic aggregates are composed of bothβ-conformation and α-conformation and can make the proportion of β-conformationnot only much higher but also stable. Also, compared with the previoussemi-quantitative calculation method, by combining Lambert-Beer’s law withtheoretical calculation (DFT and TD-DFT) in this work, we proposed a new method to quantitatively calculate the the proportion of β-conformation, which is applicableto both PFO solution and films.In the second work, the conformational characteristics of the α-conformation(individual locally separated chain) of PFO in solution were studied by the followingthree points: Mark-Houwink exponent a, fractal dimension D and form factor Rg/Rh.These parameters represent polymer viscosity, scaling law and self-similarity, andtopological structure respectively. The conclusions drawn from them agree well witheach other. That is the α-conformation of PFO has semi-rigid, relatively loose andextended chain conformations in dilute solution. In the process of using a to studythe α-conformation, the PFO precipitated samples with narrow molecular weightdistribution are gained, which is not easy for conjugated polymer and makes ourresult more reliable. The mathematical relationship between the a and D of polymerchain in dilute solution is got by us, which is from the derivation of Flory-Foxequation, and it is proved by the classic polymer polystyrene (PS). Also, the result ofD for the α-conformation of PFO can be considered as a further study, based on theprevious reports about the fractal behavior of chains aggregation of conjugatedpolymers. Moreover, it is found that the Rg/Rhof PFO solution increase undertemperature field, indicating that α-conformation is loose and extended, which issignificant and instructive to explore the transformation process and mechanismbetween the α-conformation and β-conformation in PFO solution in our third work.In the third work, solvent field is used to research the transformation mechanismfrom the α-conformation to β-conformation, namely the formation mechanism ofβ-conformation. Temperature field is used to research the transformation mechanismfrom the β-conformation to α-conformation. It is concluded that for the formationmechanism of β-conformation: firstly, PFO interchain attraction becomes bigger,leading to a shorter distance between PFO chains and the formation of aggregationstructure. Secondly, the aggregation structure further strengthens PFO interchaininteraction, which leads to a coplanar chain conformation and makes the α-conformation transform into β-conformation, namely the aggregation structureinduces the formation of β-conformation. The transformation mechanism from theβ-conformation to α-conformation: the increase of temperature enhances the chaoticdegree of PFO solution system, the conformational entropy become larger, makingthe β-conformation transform into α-conformation.(It is because β-conformation isone kind of relatively ordered and oriented conformation and has smallerconformational entropy than α-conformation.) Thus in this process, firstly, theβ-conformation transforms into α-conformation, which decreases the PFO interchainattraction and make the aggregation structure loose, then it gradually transforms intothe transition state structure and finally disaggregates to individual locally separatedchains. Furthermore, in the heating process, we find there is small proportiondifference of β-conformation between two adjacent temperatures, and the rates ofboth two transformation stages have good linear correlations (aggregation structureand transition state structure). It indicates that temperature field can be considered asa good method to control the proportion of β-conformation in PFO solution, and itcan help us get the desired proportion of β-conformation.