Preparation Of Functionalized Graphene Sheet For High-Performance Polymer Materials

Graphene sheet (GS) is a two-dimensional carbon material, which is composed of monolayers of carbon atoms tightly packed into hexagnol symmetry. Because of its extraordinary mechanical, thermal and electrical properties, GS has become a hot topic in material science. Up to now, several methods have been proposed to prepare GS. Wherein, the reduction of graphene oxide sheet (GOS) has been widely used because it enables the high-yield, large-scale production of GS at low cost. Both GS and its precursor, GOS, are ideal fillers for polymer composites because of their extremely high aspect ratio and excellent comprehensive performances. However, the incorporation of GS into polymers still faces several challenges that are needed to be solved. For example, how to prevent the agglomeration of GS in polymer matrices; how to optimize the interfacial properties between the nanofiller and polymer hosts. In this regard, we focused our studies on the introduction of functional groups into GS to improve its interfacial interactions with polymer matrices, and used the functionalized GS to improve the properties of polymers, fabricating high-performance polymer materials. The main contents of this dissertation lie as follows:(1) Though the oxidation/intercalation of graphite and subsequent ultrasound exfoliation of the oxidative product, we realized the preparation of single-layer GOS. Then, the GOS was reduced using hydrazine, yielding GS. During the process of obtaining GS powders from their suspension, we firstly employed the lyophilization technology to remove the solvent. The lyophilized GS powders are extremely light, loosely packed and could be easily redispersed in polar organic media.(2) We covalently functionalized GS and GOS using their reactive oxygen groups (mainly, carboxyl and epoxy) to enhance their surface activity, improve their dispersion in some specific organic solvents and facilitate the fabrication of high-performance polymer composites. This field includes (a) grafting long alkyl chains onto GS through an amidation reaction to enhance its lipophilicity;(b) linking polystyrene (PS) and poly(styrene-b-ethylene-co-butylene-b-styrene)(SEBS) on alkyne-functionalized GOS via click chemistry;(c) grafting maleated polypropylene (MAPP) onto GOS through the linkage of amine groups on GOS and subsequent bonding of MAPP onto amine-terminated GOS. (3) GOS was noncovalently functionalized with aromatic materials through the π-π stacking using its conjugated basal plane. The aromatic materials include (a) rare-earth complex and (b) polyphenylene oxide (PPO). The GOS decorated with rare-earth complex can emit bright red light when excited with UV radiation because GOS do not quench the luminescence of rare-earth complex. The PPO-attached GOS demonstrates the strong interaction between GOS and aromatic polymers.(4) We used the solution blending method to fabricate high-performance polymer composites that are based on functionalized GS. This field includes the following four aspects,(a) Taking the characteristics of lyophilized GS powders to be easily redispersed in polar organic solvents, we blended them with poly(lactic acid)(PLA) in DMF, yielding PLA/GS composites. The introduction of GS into PLA can improve its mechanical strength and thermal stability,(b) Because the alkyl-functionalized GS exhibits enhanced lipophilicity, i.e., improved compatibility with polyolefins, we fabricated PP/alkyl-functionalized GS composites. Compared with unmodified GS, the alkyl-functionalized GS can be dispersed in PP host more homogeneously, and thus enhance the properties of PP more effectively,(c) We used the SEBS-click GOS to reinforce PS. When the filler loading reached2.0wt%, the Young’s modulus and tensile strength of PS increased by73%and78%, respectively. Such a remarkable reinforcement demonstrates the favorable adhesions between filler and matrix, which allow the efficient load transfer between these two phases,(d) We compounded rare-earth complex-decorated GOS with poly(vinyl chloride)(PVC), obtaining luminescent PVC films. Moreover, compared with neat PVC films, the filled PVC films have higher mechanical strength and glass transition temperature.(5) By taking the advantage of the unique amphiphilic structure of functionalized GS, we firstly proposed that functionalized GS could compatibilize immiscible polymer blends, and developed functionalized GS-based multifunctional compatibilizers. This field includes the following two aspects,(a) We used GOS to compatibilize immiscible polyamide (PA)/PPO blends. GOS can adsorb PPO on its basal plane through π-π stacking, while form strong hydrogen bonding with PA, thus acting as coupling agent for PA and PPO and compatibilizing them. The addition of GOS into PA/PPO blends can not only improve their compatibility but also enhance their mechanical strength and thermal stability, indicating that GOS is preferable to traditional copolymer compatibilizers.(b) We used MAPP-grafted GOS (PP-g-GOS) to compatibilize immiscible PP/PPO blends. On the one hand, PP-g-GOS can interact with PPO via π-π stacking. On the other hand, PP-g-GOS exhibits a good compatibility with PP. As a result, it can serve as compatibilizer for PP/PPO blends. In the system of PP/PPO/PP-g-GOS, PP-g-GOS and PPO have a synergy in enhancing the flame retardancy of PP matrix. Therefore, this work presents a potential strategy to design halogen-free flame-retardant polymeric materials with good melt processability and low cost.