Design And Synthesis Of Single Pyrene-Capped Polymer And Its Application In The Preparation Of Graphene And Polymer/Graphene Nanocomposite

As a new member of the carbon nanomaterial family, graphene, owing to possessing a variety of outstanding physical properties within a single material system, provides a new choice for preparation of high-performance polymer/graphene nanocomposite. To actualize this kind of nanocomposite, however, some challenges have to be confronted, such as mass production of high-quality graphene in an affordable way, uniform dispersion of graphene in polymer matrix, and formation of strong polymer-graphene interfacial interaction.To address these issues, a so-called dispersant/stabilizer assisted liquid-phase exfoliation (LPE) method has been developed in this work for production of graphene. In addition to retaining the merit of common LPE of being capable of mass producing high-quality graphene, this method can reasonably resolve the problems existed in common LPE such as only several solvents applicable for LPE, long period of sonication, and poor stability of graphene dispersion, Single pyrene-capped polymer (sPyPM) is opted as the dispersant/stabilizer based on the following considerations. First, a strong intermolecular interaction can be formed between graphene and pyrene species, whereas formation of strong interaction is the principal requisite for an eligible dispersant/stabilizer. Second, relative to pyrene-containing small molecule, the corresponding polymer, especially pyrene-capped polymer (PyPM), is a more efficient dispersant/stabilizer because its long chain provides more sites for solvation and sufficient spatial volume to repel graphene sheets from each other for stabilization. Third, the polymer featured as each chain end-capped by one pyrene group (sPyPM) is preferable since it can present the optimal dispersing performance resulting from maximization of the number of attached polymer chains on the graphene surface. Fourth, sPyPM can be used in the polymer/graphene nanocomposite for improving both dispersing quality of graphene in polymer matrix and polymer-graphene interfacial interaction.Free-radical polymerization (FRP), which can be performed using many simple methods under the mild condition and has the ability to execute the polymerization of nearly all vinyl monomers, is selected for synthesis of sPyPM. Although little attention to date has been paid to the direct synthesis of PyPM, the fact that the initiator residue locates at the chain end clearly suggests that it is feasible for FRP to realize such a subject, provided that the desirable pyrene group is safely included in the initiator. Once some appropriate measure, such as increasing polymerization temperature and/or adding chain transfer agent, is further taken to suppress the combination termination of active chain ends, it is also possible for FRP to synthesize sPyPM.With these considerations in mind, the thesis work is carried out from four aspects:1) molecular design and synthesis of pyrene-containing free radical initiator (PyFRI),2) synthesis of sPyPM by free radical polymerization,3) production of graphene by sPyPM assisted LPE, and d) preparation of polymer/graphene nanocomposite. The main results already achieved include:1. By reaction of 4,4’-Azobis(4-cyanopentanoic acid) with 1-pyrenemethylamine or 1-pyrenebutanol, a new type of PyFRI (PyFRI-A or PyFRI-B has been successfully synthesized through the amidation or the esterification.2. FRP of styrene or methyl methacrylate (MMA) can be initiated by PyFRI-A. In the polymerization of styrene, chain termination occurs mainly by combination of two active chain ends, and the combination can be significantly suppressed or even eliminated by addition of chain transfer agent of 1-dodecanethiol (DDT). Single pyrene-capped polystyrene (sPyPS) with the desired number-average molecular weight (Mn) could be controllably synthesized by fine-tuning the proportion of styrene, PyFRI-A, and DDT. In the polymerization of MMA, chain termination dominantly occurs by radical disproportionation when the polymerization temperature is no lower than 85℃. Single pyrene-capped poly(methyl methacrylate) (sPyPMMA) with the desired Mn could be controllably synthesized by fine-tuning the proportion of MMA and PyFRI-A at 85℃.3. In the presence of dispersant/stabilizer of sPyPM, graphene could be produced by LEP in chloroform, a low boiling point and poor solvent for common LPE. When sPyPS and sPyPMMA are used as the dispersant/stabilizer, respectively, the production amount of graphene realized by one mole of pyrene unit can reach 5965 and 5892 g mol-1.4. In the presence of dispersant/stabilizer of sPyPM, graphene produced by common LPE could be re-dispersed in chloroform, a low boiling point and poor solvent for pristine graphene. When sPyPS and sPyPMMA are used as the dispersant/stabilizer, respectively, the amounts of graphene dispersed by one mole of pyrene unit can reach 18.6×104 and 8.7×104g mol-1.5. In the presence of sPyPS, PS-based nanocomposite with 1.0 wt% graphene could realize 57% improvement in tensile strength,48% improvement in tensile modulus, and 27℃ lag in initial decomposition temperature (Tinit) as compared with blank PS, and the percolation threshold of electrical conductivity (mc) for PS-based nanocomposite is 0.73 wt%. In the presence of sPyPMMA, PMMA-based nanocomposite with 1.0 wt% graphene could realized 66% improvement in tensile strength,76% improvement in tensile modulus, and 30℃ lag in Tinir as compared with blank PMMA, and mc for PMMA-based nanocomposite is 0.47 wt%.In summary, to reach the final object of achieving high-performance polymer/ graphene nanocomposite, the present work developed an economical method for mass production of high-quality graphene, i.e. sPyPM assisted LPE method. The decisive link of the proposal is to design and synthesize a new pyrene-containing initiator. Thereafter, diverse sPyPM can be synthesized by a classic free-radical polymerization. Such a concept that the molecular design of dispersant/stabilizer is guided by specific application of graphene will contribute a new clew for realizing the seamless connection between graphene production and graphene application.