Design, Preparation And Application Of Polyolefins-modified Nanoparticles

Four different types of polyolefins,including pyrene-end-capped polyethylene (PPE),hyperbranched polyethylene(HBPE),linear short-branched polyethylene(LBPE) and polystyrene(PS),were covalently/noncovalently grafted onto the surface of multi-walled carbon nanotubes(MWNT),organized mesoporous silica(OMS),and nano-sized titanium dioxide(nm TiO₂),respectively,to promote their applications as functional nanomaterials.The structure of the modified nanoparticles was characterized and their dispersibility in different system was investigated.The surface of MWNT was noncovalently modified with a range of narrow-distributed pyrene-end-capped polyethylenes(PPE)to improve its dispersibility in solvent.A pyrene group was first introduced into an acetonitrile Pd-Diimine complex, a typical late-transition-metal catalyst used for olefin coordination polymerization,and then a series of PPE samples were synthesized via "living" ethylene polymerization with the obtained pyrene-functionalized Pd-Diimine as a catalyst at a temperature of 5℃and an ethylene pressure of 400 psi.The functionalization of MWNT with PPE was subsequently carried out by ultrasonication in tetrahydrofuran(THF),heptane,and toluene,respectively.The structure of the prepared PPE samples,the interactions between PPE and MWNT,and the dispersibility of MWNT in solvent were characterized through GPC,GPC-LLS,¹HNMR,UV-Vis,Fluorescent spectra,TGA,FT-IR,TEM,WAXRD etc.The influence of various factors on dispersibility of MWNT was also discussed including end-group type of polyethylene,molecular weight of PPE, and solvent used etc.It was confirmed that each polyethylene chain is functionalized with an end-capped pyrene group and the pyrene-end-capped polyethylenes have controllable average molecular weight with narrow moleculhear weight distribution (MWD)of 1.01-1.16 and higher short-branch density of 86-90/1000 C.The stronger noncovalent interaction between PPE and MWNT was confirmed,which was attributed to bothÏ€-Ï€stacking and CH-Ï€interaction in THF,whereas mainlyÏ€-Ï€stacking in heptane.A few PPE chains had been noncovalently grafted to the surface of MWNT via above noncovalent interactions between PPE and MWNT,which led to a higher dispersibility of MWNT both in THF(up to 812.9 mg/L)and in heptane(up to 230.8 mg/L). To further enhance its dispersibility in solvent,the surface of MWNT was noncovalently functionalized using HBPE,which could be conveniently synthesized from commercially abundant ethylene via one-step chain walking polymerization.The HBPE was first synthesized by ethylene polymerization with the acetonitrile Pd-Diimine complex as a catalyst at a temperature of 35℃and an ethylene pressure of 1 atm(≈15 psi),and then the surface modification of MWNT with HBPE was conducted by ultrasonication in THF,chloroform,heptane,and toluene,respectively.The dispersibility of MWNT in solvent and the interactions between HBPE and MWNT were characterized through a series of methods,including TEM,HRTEM,¹HNMR,TGA,FT-IR,UV-Vis,WAXRD etc.The influence of solvent type,HBPE chain topology,and mass ratio of HBPE to MWNT on MWNT dispersibility was discussed via a theoretical model.It was found that the HBPE could be steadily grafted to the surface of MWNT both in THF and chloroform by means of the stronger noncovalent nonspecific CH-Ï€interaction between HBPE and MWNT.The HBPE was found to effectively exfoliate MWNT bundles to form stable MWNT dispersions both in THF and chloroform at surprisingly high concentrations(up to 919 mg/L in THF and 1235 mg/L in chloroform).It was also found that solvent type had a notable influence on MWNT dispersibility,which increases according to the following sequence:toluene or heptane＜THF＜chloroform.To obtain inorganic/organic hybrid mesoporous materials,the surface of two kinds of organized mesoporous silica,SBA15 and MSUF,was functionalized with a linear short-branched polyethylene(LBPE) via surface-initiated "living" coordination polymerization technique.First the surface of SBA15/MSUF silica was treated with 3-acryloxypropyltrichlorosilane,as a coupling agent,to introduce acryloyl group,and then the acetonitrile Pd-Diimine catalyst was covalently immobilized onto pore surface of SBA15/MSUF silica by reacting with the surface-bonded acryloyl group.The obtained SBA15/MSUF silica-supported Pd-Diimine catalyst(Pd-SBA15/Pd-MSUF) was subsequently used to catalyze ethylene polymerization at a temperature of 5℃and an ethylene pressure of 400 psi to covalently graft LBPE chains from the pore surface of SBA15/MSUF silica.The structure of SBA15/MSUF silica,before and after modification,was characterized using TGA,FTIR,ICP-MS,nitrogen adsorption-desorption testing,and DSC,respectively.The controllability of above polymerization was also evaluated.It was found that the acetonitrile Pd-Diimine catalyst could be homogeneously and covalently immobilized on the pore surface of SBA15/MSUF silica by using the coupling agent,and the resulting catalyst-functionalized SBA15/MSUF silica(Pd-SBA15/Pd-MSUF)still retained organized porous structure,in which the thickness of catalyst layer was 0.66 nm and 0.95 nm for Pd-SBA15 and Pd-MSUF silica,respectively.The LBPE chains can be covalently bonded to the pore surface of SBA15/MSUF silica through the ethylene polymerization catalyzed by Pd-SBA15/ Pd-MSUF.The thickness of homogeneous LBPE layer could be adjusted by controlling polymerization time to give the LBPE-functionalized SBA15/MSUF silica(PE-SBA15/PE-MSUF)with accessible porosity.It was found that PE20 min-MSUF silica,a sample taken at 20 min during ethylene polymerization catalyzed by the Pd-MSUF silica,exhibited typical mesoporous structure with a pore volume of 0.57 cm³/g and a BET of 141.54 m²/g.To improve the dispersibility of nm TiO₂ in polypropylene(PP),polystyrene was grafted to the surface of nm TiO₂ and the in situ compatibilization between the resulting polystyrene-grafted nm TiO₂(PS-g-TiO₂)and polypropylene(PP)was carried out by means of Friedel-Crafts(FC)alkylation reaction.A dispersion polymerization of styrene (St)was first conducted in the presence of nm TiO₂ particles modified with a silane coupling agent,3-acryloxypropyltrichlorosilane(MPS),to prepare the PS-encapsulated nm TiO₂ microspheres(PS@TiO₂),and then the PS-grafted nm TiO₂ was obtained by purifying the resulting PS@TiO₂ microspheres with toluene to remover free PS.The prepared PS@TiO₂ microspheres and PS-g-TiO₂ particles were subsequently added into PP,respectively,along with FC catalyst(AlCl₃/St)of different concentration by melting blending process to give a series of PP nanocomposites.The structure of modified nm TiO₂ was characterized through TGA,FTIR,XPS,TEM,EA,and its dispersibility in PP matrix,and the resistance to UV aging of corresponding PP nanocomposites were compared using TEM,SEM,TGA,DSC,and UV artificially accelerating aging testing, etc.It was revealed that a few PS chains were covalently linked to the surface of nm TiO₂ particles.For the PP/PS@TiO₂ system,nm TiO₂ particles were found to exist selectively within PS phase of 100-120 nm due to the poor compatibilization between PP matrix and grafted PS.The dispersibility of PS-g-TiO₂ particles in PP could be further improved by using FC catalyst.For the PP/PS-g-TiO₂/AlCl₃/St system,when AlCl₃ concentration reached 1.0 wt%,the PS-g-TiO₂ particles were dispersed homogeneously within the whole PP matrix in nanoscale,with a considerably enhanced interfacial adhesion.Surface modification of nm TiO₂ by grafting with PS,as well as in situ compatibilization using FC catalyst,can impart the corresponding PP nanocomposite with highly enhanced resistance to UV aging and better thermal stability.