Academic literature on the topic 'Polymer graphite composites'

Polymer composites formed from layered fillers with high surface volume ratio show enhanced reinforcement. Graphite oxide is a high modulus material that can be separated into thin layers with high surface area. The aim of this study is to prepare polymer layered graphite oxide composites using functionalised polyolefin to enhance compatibility with various forms of layered graphite oxide in varying concentration. Functionalised polyolefins reinforced with layered graphite oxides and expanded graphite oxides were prepared using solution blending and melt blending methods. Three different mixing methods with varying shear intensity were employed to prepare polymer layered graphite oxide composites. The crystalline structure, thermal and mechanical properties of the prepared polymer layered graphite oxide composites was studied. Oxidised graphite prepared from the Staudenmaier method and its exfoliated form were dispersed in poly(ethylene-co-methyl acrylate-co-acrylic acid) (EMAA) via solution blending to prepare EMAA layered composites. The thermal stability was determined using thermogravimetric analysis. The EMAA layered composites showed higher thermal stability in comparison with pure EMAA. The mechanical properties of these EMAA layered composites were determined through dynamic mechanical analysis. Shear modulus, yield stress and storage modulus of EMAA in the presence of graphite oxide fillers decreased. A solution blending method was used to prepare poly(propylene-grafted-maleic anhydride) layered expanded graphite oxide composites (PPMA-EGO). Two types of PPMA-EGO were prepared using different mixing methods - low and high shear were employed. The effects of preparative mixing methods on the PPMA-EGO properties were investigated. The mechanical properties of PPMA-EGO obtained from dynamic mechanical analysis indicated that EGO had a reinforcing effect on the elastic behaviour of PPMA-EGO. This is due to strong interfacial adhesion between PPMA and EGO as a result of hydrogen bonding. The elastic behaviour of PPMA-EGO was affected by the surface area of graphite flakes. Low sheared PPMA-EGO elastic behaviour was found to be higher compared with that of high sheared PPMA-EGO. A melt blending method was used to prepare PPMA-EGO with varying EGO concentration. The interconnected network structure of EGO in the PPMA-EGO was not observed as shown by its scanning electron microscopy images. Thermogravimetric analysis of PPMA-EGO indicates increased decomposition temperature of the PPMA matrix. Dynamic mechanical analysis showed enhanced storage modulus of PPMA-EGO. The maximum elastic modulus of PPMA-EGO was observed at 3 %wt of EGO. The electrical conductivity of PPMA-EGO was measured only for EGO concentrations above 2 %wt. The EGO concentration was found to be the most critical factor in the enhancement of the electrical conductivity of PPMA-EGO. Wide angle X-ray diffraction analysis of all polymer layered graphite oxide composites revealed no change in interlayer spacing of graphite layers, indicating the absence of EMAA intercalation in the graphite layers. The crystallisation temperature and crystallinity of all polymer layered graphite oxide composites were determined using differential scanning calorimetry. The results indicated that graphite oxide and expanded graphite oxides acted as nucleating agents in inducing the crystallisation of functionalised polyolefin in the layered composites. However, the degree of crystallinity of functionalised polyolefin decreased in the layered composites.

Thesis (PhD)--Stellenbosch University, 2012.<br>ENGLISH ABSTRACT: New methods are described for the synthesis of polymer/graphite nanocomposites using the miniemulsion polymerization process. Natural graphite was functionalized by oxidation to produce graphite oxide (GO) nanosheets. Poly(styrene-co-butyl acrylate) (poly(St-co-BA)) nanocomposite latices containing GO nanosheets were successfully synthesized using miniemulsion as a one-step nano-incorporation technique. The approach followed included expanding the GO nanosheets in situ during the miniemulsification step and then polymerizing the monomers in the presence of these expanded nanosheets. Styrene (St) and butyl acrylate (BA) were mixed with GO and then emulsified in the presence of a surfactant and a hydrophobe to afford pre-miniemulsion latex particles. The stable pre-miniemulsions were then polymerized to yield poly(St-co-BA)/GO nanocomposite latices. The polymerization proceeded with relatively high monomer conversion and produced stable nanocomposite latex particles. The nanocomposites exhibited mainly an intercalated morphology, irrespective of the percentage of GO filler loading. The synthesis of exfoliated polymer nanocomposites made with modified GO is described. GO was modified with a surfmer (reactive surfactant), 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), which widened the gap between the GO nanosheets and facilitated monomer intercalation between its nanogalleries. The AMPS-modified GO was used for the synthesis of poly(St-co-BA)/GO nanocomposite latices using a similar miniemulsion procedure. The obtained nanocomposites had exfoliated morphologies and the GO nanosheets were largely exfoliated (about 2–5 nm thick) in the resultant films obtained from the synthesized nanocomposite latices. The synthesized nanocomposites had enhanced thermal and mechanical properties compared to pure polymer as a result of the presence of AMPSmodified GO. Furthermore, the nanocomposites made with AMPS-modified GO had better thermal and mechanical properties than the unmodified GO. The mechanical properties of the nanocomposites depended on the AMPS-modified GO loading in the nanocomposites. The synthesis of polystyrene/GO (PS-GO) nanocomposites using the reversible additionfragmentation chain transfer (RAFT) mediated polymerization method is also described. The GO was synthesized and immobilized with a RAFT agent to afford RAFT-functionalized GO nanosheets. The RAFT-immobilized GO was used for the synthesis of PS nanocomposites in a controlled manner using miniemulsion polymerization. The molar mass and dispersity of the PS in the nanocomposites depended on the amount of RAFT-grafted GO in the system, in accordance with the features of the RAFT-mediated polymerization. X-ray diffraction and transmission electron microscopy analyses revealed that the nanocomposites had exfoliated morphology, even at relatively high GO content. The thermal stability and mechanical properties of the PS-GO nanocomposites were better than those of the neat PS polymer. Furthermore, the mechanical properties were dependent on the modified-GO content (i.e., the amount of RAFT-grafted GO). The hydrophobicity and barrier properties of the resulting films prepared from the synthesized poly(St-co-BA)/GO nanocomposite latices to water and water vapor were also investigated. The hydrophobicity of the synthesized nanocomposite films was determined using contact angle measurements. The water permeability was determined by measuring the moisture vapor transmission rate of the films. The GO in the nanocomposites was reduced to its original form (i.e., graphite), and the barrier properties of the obtained nanocomposite films were determined and compared to films containing the unmodified GO (as-prepared GO). Results showed that reduction of GO had a significant impact on the water affinity of the resultant films prepared from the synthesized nanocomposite latices. The presence of reduced-GO (RGO) instead of unmodified GO in the miniemulsion formulation significantly improved the hydrophobicity and barrier properties of the final films to water. However, the barrier properties of the nanocomposites were unaffected by the amount of RGO in the nanocomposites.<br>AFRIKAANSE OPSOMMING: Nuwe metodes is beskryf vir die sintese van polimeer/grafiet nanosamestellings deur gebruik te maak van die miniemulsie polimerisasieproses. Natuurlike grafiet is gefunksionaliseer dmv oksidasie om grafietoksied (GO) nanovelle te vorm. Polistireen-ko-butielakrilaat (poli[St-ko- BA]) nanosamestellinglatekse wat GO nanovelle bevat is suksesvol gesintetiseer deur gebruik te maak van miniemulsie polimerisasie as ‘n een-stap nano-insluitingstegniek. Die benadering wat gevolg is het die uitbreiding van die GO nanovelle, in situ, gedurende die miniemulsifiseringstap behels, gevolg deur die polimerisasie van die monomere in die teenwoordigheid van hierdie uitgebreide nanovelle. Stireen (St) en butielakrilaat (BA) is met GO gemeng en daarna emulgeer in die teenwoordigheid van ‘n seepmiddel (surfactant) en ‘n hidrofoob om pre-miniemulsielateksdeeltjies te lewer. Die stabiele pre-miniemulsies is gepolimeriseer om poli(St-ko-BA)/GO nanosamestellinglatekse te vorm. Die polimerisasie het met redelike hoë monomeeromskakeling verloop en het stabiele nanosamestellinglateksdeeltjies gelewer. Hierdie nanosamestellings het hoofsaaklik geïnterkaleerde morfologie, onafhanklik van die persentasie GO vullers, getoon. Die sintese van afgeskilferde polimeernanosamestellings berei met gewysigde GO is beskryf. GO is gewysig met ‘n ‘surfmer’ (reaktiewe seepmiddel), 2-akrielamido-2-metiel-1- propaansulfoonsuur (AMPS), wat die gapings tussen die GO nanovelle vergroot het en die monomeer interkalering tusssen sy nanogange fasiliteer. Die AMPS-gewysigde GO is gebruik vir die sintese van poli(St-ko-BA)/GO nanosamestellinglatekse deur gebruik te maak van ‘n soortgelyke miniemulsie prosedure. Die nanosamestelling sό verkry het ‘n afgeskilferde morfologie getoon en die GO nanovelle was grootendeels afgeskilfer (ongeveer 2–5 nm dik) in die films wat berei is van die gesintetiseerde nanosamestellinglatekse. Laasgenoemde het verhoogde termiese en meganiese eienskappe gehad in vergelyking met die suiwer polimeer, as gevolg van die teenwoordigheid van die AMPS-gewysigde-GO. Die meganiese eienskappe van die nanosamestellings hang af van persentasie AMPS-gewysigde GO vullers in die nanosamestellings. Die sintese van PSt/GO nanosamestellings dmv die omkeerbare-addisie-fragmentasieoordrag- (OAFO-, Eng. RAFT-) bemiddelde polimerisasie metode is ook beskryf. Die GO is berei en geïmmobiliseer met ‘n RAFT verbinding om GO nanovelle met RAFT funksionaliteit te lewer. Die RAFT-geïmmobiliseerde GO is gebruik vir die sintese van PSt nanosamestellings in ‘n gekontrolleerde manier mbv miniemulsie polimerisasie. Die molêre massa en dispersie van die PSt in die nanosamestellings hang af van die hoeveelheid RAFTgeënte GO in die sisteem, in ooreenstmming met die kenmerke van RAFT-bemiddelde polimerisasie. X-straaldiffraksie en transmissie-elektronmikroskopie analises het bewys dat die nanosamestellings, selfs by relatiewe hoë GO inhoud, ‘n afgeskilferde morfologie gehad het. Die termiese stabiliteit en meganiese eienskappe van die PSt-GO nanosamestellings was beter as dié van die suiwer PSt polimeer. Verder was die meganiese eienskappe afhanklik van die gewysigde-GO-inhoud (dws, die hoeveelheid RAFT-geënte-GO). Die hidrofobisiteit en spereienskappe van die films berei vanaf die gesintetiseerde poli(St-ko- BA)/GO nanosamestellinglatekse teenoor water en waterdamp is ook ondersoek. Die hidrofobisiteit is ondersoek deur gebruik te maak van kontakhoekmeting. Die waterdeurlaatbaarheid is bepaal deur die waterdampoordragtempo van die films te bepaal. Die GO in die nanosamestellings is gereduseer tot sy eenvoudigste vorm (grafiet) en die spereienskappe van die nanosamestellingfilms is bepaal en vergelyk met die films wat die ongewysigde GO bevat het. Resultate het getoon dat reduksie van GO ‘n groot invloed gehad het op die wateraffiniteit van die films wat berei is vanaf die gesintetiseerde nanosamestellinglatekse. Die teenwoordigheid van die gereduseerde-GO (RGO) in plaas van die onveranderde GO in die miniemulsie formulasie het die hidrofobisiteit en spereienskappe van die finale films, teenoor water, baie verbeter. Die spereienskappe van die nanosamestellings is egter nie beïnvloed deur die hoeveelheid RGO in die nanosamestellings nie.