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Polymer-halloysite Nanocomposites With Novel Interfacial Structure

Posted on:2011-08-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:M X LiuFull Text:PDF
GTID:1101360308964613Subject:Materials science
Abstract/Summary:PDF Full Text Request
Incorporating nanopartcicles into polymers for preparing hybrids or nanocomposites has attracted extensive interesting in the recent years for their unique structure and unusual properties which could not be achieved by conventional composites. Natural occurred halloysite nanotubes (HNTs) offer great opportunities for fabricating polymer nanocomposites with promising performance which may be attributed to their high strength, unique morphology and low cost. In the present work, the interfacial structures of polymer/HNTs systems are designed according to the characters of HNTs. The interfacial interactions between HNTs and the component of composites and their effect on the structure and properties of the composites are studied in detail.2,5-bis(2-benzoxazolyl) thiophene (BBT), capable of donating electrons, is selected as the interfacial modifier for polypropylene (PP)/HNTs systems. The electron transfer interactions between HNTs and BBT are confirmed. The mechanical properties and the unique morphology of the nanocomposites are examined. Formation of fibrils of BBT in presence of HNTs is found in the nanocomposites. The chemical composition of the fibrils in the nanocomposites is found to be composied of largely BBT and a small amount of HNTs. The formation mechanism of BBT fibrils are elucidated to be the strong interactions between BBT and HNTs under the melt shearing. PP crystal could epitaxially grow on the BBT and hybrid fibril substrates, indicating the nucleating ability of BBT. The nanocomposites with BBT show significantly increased tensile and flexural properties which are attributed to the changed crystallization behavior by the hybrid fibrils.The electron transferring interactions between HNTs and 2,2'-(1,2-ethenediyldi- 4,1-phenylene) bisbenzoxazole (EPB) is utilized to fabricate PP/HNTs composite with improved mechanical properties. The electron transferring can take place between EPB and HNTs during heating. Based on this mechanism, large aggregates HNTs in tens of micrometers are formed within the composites in presence of EPB during processing. The composites with EPB show a substantially increased strength and modulus compared with those of the control sample. The reinforcement mechanism of EPB incorporated PP/HNTs composites is elucidated to the combination of the stress transferring by the well dispersed HNTs and the energy dissipation by cracking the HNTs aggregates during fracture process. Different organic conjugate molecule has a different effect on the reinforcement of the PP/HNTs composites.Clay-philic benzothiazole sulfide, capable of donating electrons, is grafted onto PP backbones when N-cyclohexyl-2-benzothiazole sulfonamide (CBS) is selected as the compatibilizer for PP/HNTs composites. CBS decomposes at elevated temperature and yields benzothiazole sulfide radical, which reacts with the PP polymeric free radicals generated during the processing of the composites. On the other hand, the benzothiazole group of CBS is reactive to HNTs via electron transferring. The compatibilization between HNTs and PP is thus realized via interfacial grafting and electron transferring mechanism.The dispersion of HNTs and the interfacial bonding are enhanced substantially in the compatibilized composites. The significantly improved mechanical properties and thermal properties for benzothiazole sulfide compatibilized PP/HNTs composites are correlated to the enhanced interfacial property.The nucleating ability of HNTs towards isotactic polypropylene (iPP) was investigated by different methods. HNTs are identified to have dual nucleating ability forα-iPP andβ-iPP under appropriate kinetics conditions. The composite with 20 phr of HNTs is found to have highest content ofβ-iPP. Under non-isothermal crystallization, the content ofβ-iPP increases with decreasing of the cooling rate. The maximumβ-crystal content is obtained at cooling rate of 2.5oC/min. The supermolecular structure of theβ-iPP is identified asβ-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization theβ-crystal can be formed in the temperature range of 115~140oC. The content ofβ-crystal reaches the maximum value at crystallization temperature of 135oC. Theβ-nucleation ability of HNTs for iPP is correlated to the matching dimension between c-axis of the iPP and the layer spacing of HNTs.HNTs are further incorporated into PP for tailoring the surface microstructures of the composites prepared by solution casting. HNTs act as heterogeneous nuclei for PP, which leads to the change of phase separation process during drying of the composites and consequently the microstructures of composite surfaces. Micro-papilla like hybrid spherulites with nanostructures are formed on the PP/HNTs composite surfaces. These rough surfaces demonstrate superhydrophobicity with a water contact angle higher than 160o and sliding angle less than 10o. The spherulites size, surface roughness, and wetting property of PP can be tuned by HNTs. HNTs can significantly improve the anti-oxidation degradation behavior of the composites which is attributed to the well-dispersed HNTs and the improved interfacial interactions by the nucleation effect. This work provides an alternative routine for preparing polymer superhydrophobic surfaces via heterogeneous nucleation and is an example of using hydrophilic inorganic as a component of polymer composites for superhydrophobic surface applications.HNTs are co-cured with epoxy/cyanate ester resin to form organic-inorganic hybrids. The coefficient of thermal expansion (CTE) of the hybrids with low HNTs concentration is found to be substantially lower than that of the plain cured resin. The moduli of the hybrids at glassy state and rubbery state are significantly higher than those for the plain cured resin. The dispersion of HNTs in the resin matrix is uniformly. The interfacial reactions between the HNTs and cyanate ester are revealed by the results of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The substantially increased properties of the hybrids are attributed to the covalently bonding between the nanotubes and the matrix. Silane modified HNTs can increase the mechanical properties, char yield and dimensional stability of epoxy resin.The polyvinyl alcohol (PVA)/HNTs solutions were prepared through casting and coagulating the PVA/HNTs solution.The particle size and distribution of HNTs in the PVA/HNTs solution are independent of the ratio between HNTs and PVA. The aggregation of HNTs takes place during the drying process of the as cast film. Compared with the film by coagulation method, the HNTs in the as cast film show less profound effect on the nucleation of the crystallization of the PVA. The glass transition temperature (Tg) of the composite film decreases with HNTs loading and the aggregation process shows practically no effect on the Tg. Inclusion of HNTs greatly depresses the decomposition of the PVA backbone. Transparent PVA/HNTs nanocomposite films were also prepared via solution casting and glutaraldehyde crosslinking. The dispersion of HNTs in the composites is uniform. The mechanical properties of PVA were significantly enhanced by the incorporated HNTs. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and DSC illustrated the stain-induced crystallization behavior of these composites.
Keywords/Search Tags:Polypropylene, halloysite, Nanocomposite, interfacial structure, electron transfer, crystallization
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