Highly Uniform And Porous Polyurea Microspheres:Preparation,Characterization And Applications

Polyureas?PU?are known for long as important engineering materials thanks to their high chemical,mechanical,thermal shock and abrasion resistance,good flexibility,and water repellency etc.Their common applications have been limited to protective coating materials for different structural materials.PU are commonly synthesized either through condensation polymerization using diisocyanate?NCO?and diamines or through the reaction of NCO groups with water,turning itself into amine by release of CO₂.The in-situ formed amine groups copolymerize quickly with NCO groups,leading also to PU formation.Obviously,this later process of PU synthesis is advantageous because the reactions involve only NCO monomers and water without need of any other chemical.Through this process,fabrication of solid PU microspheres without porous structure and porous PU materials without definite form or shape are reported as elaborated in this dissertation.The sensational features and cost-effective fabrication of PU have inspired the researcher to develop their new morphologies for their novel applications such as support for catalysts.It is obvious that solid microspheres are of low specific surface area and the amount of immobilized substance on their surface are limited,or inaccessible as catalytic active site if incorporated in the microspheres;whereas for porous materials with irregular forms,it makes their recovery difficult due to their irregular forms combined with their size difference.It is therefore highly desired and interesting to syntheses porous and uniform PU microspheres.This has been a challenging task,limited by the nature of the process of PU synthesis,i.e.precipitation polymerization.Face to these challenges,in this work,a simple and easy microfluidics device is designed,which turns PU synthesis from precipitation polymerization to interfacial polymerization,and is used to fabricate highly uniform and porous PU microspheres?PPM?by interfacial polymerization of toluene diisocyanate?TDI?with water.PPM are used to immobilize palladium?Pd?nanoparticles to get hybrid composite of Pd and PPM,Pd@PPM.Formation mechanism of PPM and their composite microspheres Pd@PPM is studied and their structure is characterized.Pd@PPM are used as catalyst for environmental remediation.The main contents of this dissertation are as follows:?1?Highly uniform PU microspheres are prepared by interfacial polymerization of TDI in water via microfluidics approach.In this process,TDI droplets were generated by injecting TDI in a flowing aqueous polyvinyl alcohol?PVA?solution inside a silicone tube leading to a batch reactor.The polymerization started in the tube while flowing to the reactor and completed in the batch reactor.Through this process,highly uniform PPM,with size between200?m and 500?m,are prepared.The influence of various process parameters on PPM formation are studied and optimized such as flow rate of aqueous and TDI phase,PVA concentration in aqueous phase,polymerization time and temperature.Results demonstrate that the presence of PVA in the aqueous flow is necessary to obtain uniform PPM and the size of PPM is readily adjustable by changing the process parameters and polymerization conditions.?2?It is well-known that owing to their high specific surface area and low density,porous polymer materials are of great importance as supports for transition-metals nanoparticles?NPs?for their easy recovery and reusability.In this study,we have reported two different approaches to immobilize Pd on PPM.In first approach,one-step process was used to immobilize Pd on PPM to get hybrid composite microspheres of PPM and Pd,Pd@PPM1.In this process,Pd?OAc?₂ was dissolved in TDI by ultra-sonication,and the Pd?OAc?₂ solution in TDI was introduced into the aqueous flow in the microfluidic tubing system.At the end of the polymerization uniform microspheres of Pd and PPM?Pd@PPM1?were obtained.In second approach,a facile two-step protocol was reported to obtain composite microspheres?Pd@PPM2?of PPM and Pd:highly uniform PPM were prepared first by interfacial polymerization of TDI in water through a microfluidic device,followed by Pd immobilization on their surface.The hybrid composites Pd@PPM1 and Pd@PPM2 are characterized by OM,SEM,NMR,XPS,FTIR,XRD,EDS,BET,ICP-OES,CO pulse chemisorption analysis,and temperature-programmed reduction.The combined results of these characterization techniques revealed that Pd was immobilized on PPM surface through a coordination interaction with urea ligand and composite microspheres were highly monodispersed with porous structure.?3?The composite microspheres Pd@PPM1 and Pd@PPM2 were used as catalysts for the degradation of methylene blue?MB?,methyl orange?MO?and rhodamine B?RhB?dyes in the presence of NaBH₄.Both the catalysts were demonstrated very high performance and good reusability for dyes degradation.However,Pd@PPM2 exhibited high catalytic activity for dyes degradation in comparison with Pd@PPM1 due to larger number of active sites availability on its surface.Monodispersity,efficient dye degradation,easy recovery,and remarkable reusability make Pd@PPM2 a promising catalyst.?4?Moreover,the catalytic performance of Pd@PPM1 and Pd@PPM2 was evaluated in the reduction of 4-nitrophenol?4-NP?to 4-aminophenol?4-AP?by using NaBH₄ as the reducer.The effect of the reaction parameters are studied and optimized such as reaction temperature,pH,amount of catalyst and amount of Na BH₄ on the reduction of 4-NP to obtained the kinetic data.In all the catalytic processes,Pd@PPM2 out-performed Pd@PPM1,thanks to their higher specific surface area of immobilized Pd-NPs and a larger number of active sites on the surface.The catalysts were shown to have sustainable reusability in dyes degradation.In contrast,the catalytic activity was obviously deteriorated in its reuse in 4-NP reduction,owing to the formation of a complex between Pd-NPs and the reduction product of4-NP,which resulted in the Pd active sites being covered by the complex,leading to the decrease in their catalytic performance.Their good reusability in dyes degradation confirmed that the decrease in catalytic activity was limited only to the processes where 4-AP and similar amine containing compounds were involved.The high uniformity,easy recovery,high catalytic activity and sustainable reusability make Pd@PPM2 a great catalyst with good potential for applications in different fields.?5?In addition,Pd-NPs were also immobilized on the surface of highly porous PU?PPU?particles to get hybrid composite Pd@PPU with surface area of 143.07 m²/g.Pd@PPU was used as a catalyst for the reduction of carcinogenic hexavalent chromium(Cr⁶⁺)to benign trivalent chromium(Cr³⁺).The reaction parameters are studied and optimized to obtain the kinetic data.Pd@PPU was shown remarkable catalytic performance with turnover frequency value of 3.40 mol_Cr6+/mol_Pd/min which is higher in comparison to commercial Pd@C and other supported heterogeneous catalysts for the same reaction at room temperature.