Phase Behavior Of Semi-Rigid Conjugated Polymer Solution

The photoelectronic properties of organic conjugated polymers depend not only on the molecular parameters of the materials(molecular weight,regionregularity,degree of branching,etc.),but also on the complex phase transition characteristics during the solution processing of the films.The fundamental theory describing crystallization and phase transition of semi-rigid conjugated polymers is still lacking in contrast to the conventional flexible alkane polymers.During the solution processing of conjugated polymers,the intermediate steps that possibly involved in crystallization are still in wide debates.It will make big sense to illuminate the chain disorder-order conformation transition,liquid-liquid phase separation,crystallization nucleation processes in solution.In addition,whether a liquid crystalline state exists prior to semi-rigid conjugated chains crystallization is interesting and very valuable for the possibility to prepare highly ordering polymer films.Therefore,deep insights to these basic physical processes of crystallization are significant to achieve fine control of the multi-scale structural morphologies of polymer films as well as to understand the solution processing-structure-perbormance relationship for conjugated polymers.Employing the model conjugated polymer,poly(3-hexylthiophene)(P3HT)with specific molecular parameters,this thesis investigated mainly the variation of chain conformation and aggregation state in toluene solution with respect to temperature and concentration of the solution.To reveal the chain conformation and crystallization in solution,we used multiple experimental techniques including the wide and small angel X-ray scattering(WAXS/SAXS),Differential Scanning Calorimetry(DSC),UV-vis absorption spectroscopy(UV-vis),Photoluminescence(PL)and birefringence microscope.Moreover,P3HT based OFET tests were carried out to demonstrate to some degree the processing-structure-performance relationship.In this work,we not only devised a set of home-made solution capillary setup for X-ray scattering measurement,but also established a series of methods and some basic data processing ways to study the chain conformation and crystallization of conjugated polymers in solutioa The main results in this thesis are as follows:1)The order-disorder transitions of poly(3-hexylthiophene)(P3HT)/toluene solution during cooling and heating processes were investigated by using combined in-situ synchrotron radiation SAXS/WAXS,Ultrasensitive Differential Scanning Calorimetry(US-DSC)and UV-vis absorption Unexpectedly,a chain collapse transition occurs prior to the onset of crystallization during cooling,which assists the subsequent crystallization.Starting from solutions with un-collapsed chain,collapsed chain and P3HT crystallites,the charge transport properties of P3HT-based OFETs devices show a large performance difference.Notably,chain collapsed solution results in OFETs with a higher conductivity than that of un-collapsed solution.ii)The effects of concentration on the chain conformation and the disorder-order transition of P3HT/toluene solution at room temperature are investigated by using photoluminescence spectroscopy(PL)and UV-vis absorption spectroscopy combined with synchrotron radiation X-ray scattering technique and Atomic Force Microscope(AFM).The results show that in the dilute solution(0.1-5mg/mL),the P3HT chain maintain independent random coil,and the interaction between the chains is weak.At the critical concentration(?5mg/mL),the random coiled chain collapses,thus the increased intrachain ?-? interaction leads to local ordering as rod-like segments.When the concentration is higher than the critical concentration,the entanglement of the polymer chains become more pronounced,and the interchain ?-? stacking dominates,leading to the growth of the local ordered aggregates.The portion of the local segment ordering is found to positively correlate with the concentration,while the chain configuration does not change significantly any more.This concentration dependence of the regional chain disorder-order transition in P3HT/toluene solution is further verified to exert great influence on the final crystalline morphologies of the spin-casted films.