Design And Preparation Of POSS-based Hybrid Capillary Monoliths

Monolithic column has the advantages of facile preparation,high permeability,fast mass transfer,devisable surface properties and frit less design.The use of capillary monolithic column in micro-column separation can reduce solvent and sample consumption.These do motivate further research,developing novel monolithic materials' preparation and their applications.Generally,monolithic columns can be classified into three categories:polymer-based monolith,silica-based monolith and hybrid monolith.Compared with silica-based monolith prepared by sol-gel method,polymer-based monolith is of easy preparation,stable to pH,suitable for separation of macro-biomolecules,and there are various functional monomers available to be used.But it is easy to swell in organic solvents,which will shorten its lifetime.In order to avoid complex and time-consuming preparation process,and get the advantages of polymer-based and silica-based monoliths,hybrid material such as functionized POSS was chosen as a monomer to prepare hybrid monolith directly by means of organic polymerization.Chosing functionized POSS as monomers to prepare monoliths has the following advantages:1)simpler preparation process and more helpful to improve the separation reproducibility when compared with the traditional method combining polycondensation and addition polymerization,in which tetraalkoxysilane,active trialkoxysilane and monomer are used;2)no remained silicon hydroxyl residuals that cause peak tailing of basic compounds;3)highly ordered polymer structure due to the regular structure and fixed size of POSS;4)better heat and oxidation resistences as well as mechanical and pH stabilities because functionized POSS itself is a nano-hybrid material at molecular level;5)more conducive to polymerization and additional chemical modifications owing to the eight same active sites in functionized POSS unit.Most of the POSS-based hydrophobic monolithic columns prepared in recent years were used in the reversed phase mode,and could be used in other separation modes that depend on the nature of the functional groups introduced by post-modification.However,post-modification is time-consuming and frequently leads to poor reproducibility.In addition,the amount of functional groups introduced by post-modification is less than that using a functionized POSS by a one-pot approach.The commonly used organic polymerization is free radical chain polymerization.Its fast polymerization rate makes the reaction process hard to control,resulting in irregular microstructure,lower permeability and efficiency and poor reproducibility.In order to obtain the monolithic columns with homogenous structure and available for efficient separations in various separation modes,step-growth polymerizations based one-pot synthetic method have been studied during my MSc study.This dissertation has five parts described as follows:1.The current state of monolithic columns research were reviewed,especially focusing on the strategies for the preparation of monolithic columns via different kinds of polymeration methods,and the correlations between preparation conditions and stationary phase structure.Then my research proposal was presented.2.Using pentaerythritol tetrakis(3-mercaptopropionate)(PETMP)and octa vinyl POSS(OVS)as monomers,OVS-PETMP hybrid monolithic column was prepared via thiol-ene reaction.Choice of the solvents to prepare monolith was investigated in detail.Additionally,this reaction was also used to modify POSS.The results obtained suggested that the concentration of the monomers used was biggest in a ternary solvents medium containing THF/MeOH/PEG,and a typical monolithic column could be obtained.The resulting monolith exhibited high permeability(B0= 3.15 × 10-6m2 using water as mobile phase),good mechanical stability(good linearity between backpressure and flow rate of the mobile phase,correlation coefficient R2>0.999)and thermal stability(decomposition temperature T>340 ?).However,restricted by the OVS solubility and PETMP oxidation properties,the resulting monolith presented low efficiency separation in reversed phase mode although it had a well-organized structure.The plate height was only 42.09 ?m for toluene and the corresponding ?CH2 was 1.345.It also had a strong interaction with polycyclic aromatic hydrocarbons but couldn't make them selectively separated.Moreover,thiol-ene reaction was used in the modification of POSS,but the production rate was extremely low.Further modification process was hard to follow and purification hard to process.Under this circumstance,I didn't go further to directly prepare monolith and modify POSS using thiol-ene reaction.3.Using octa epoxy POSS(EP-POSS),octa phenyl amino POSS(PAM-POSS)and aminopropylisooctyl POSS(IAM-POSS)as monomers,PAM-EP-POSS monolithic column and PAM-EP-LAM-POSS hybrid column were prepared.Effects of the composition and concentration of monomers as well as the solvent composition on morphology and separation performance were systematically investigated.Both the monoliths obtained had homogenous structure,good mechanical stability(good linearity between backpressure and flow rate of the mobile phase,correlation coefficient R2>0.993),thermal stability(decomposition temperature T>400 ?)and high permeability(B0 = 2.22 × 10-11 m2 using water as mobile phase).They both could be used in reversed phase mode.Meanwhile,hydrophilic PAM-EP-POSS monolith could be obtained by adjusting the monomer proportion.Considering the bad distribution of the internal pore size,performance of PAM-EP-IAM-POSS monolith was not as good as PAM-EP-POSS monolith.Alkyl benzenes and alkyl benzene ketones were separated efficiently on PAM-EP-POSS column in reversed phase mode.The plate height reached 6.42?m for toluene.PAM-EP-POSS monolith also was applied to the separation of compounds.Separation of amino acids and organic carboxylic acids with aqueous mobile phase containing 0.1%TFA were realized.Under the condition of 40%ACN,it was selective for the phenol isomers.While with 80%ACN,it presented a typical HILIC behavior in the case of the four small molecular probes(toluene,DMF,formamide and thiourea),their corresponding theoretical plate heights were 9.70 ?m,7.46 ?m,7.98 ?m and 8.78 ?m,respectively.Without addition of other salts,inorganic anions(IO3-,BrO3-,I-and Br-)could be separated with 20%ACN containing 0.1%TFA in weak anion exchange mode.All these results indicated that by using epoxy-amine reaction and highly active POSS we could obtain the monoliths of homogeneous structure,and they could be used in various separation modes with high efficiency.4.Using Boc-protected amino carboxylic acids to modify octa(3-aminopropyl)silsesquioxane hydrochloride(OAS),we obtained L-phenylalanine-POSS(POSS-PA)for the preparation of chiral monolith,and different length alkyl-modified POSS(POSS-C6 and POSS-C12)for preparing the monoliths of different polarity.A preliminary investigation of the solvents composition for the chiral monolith preparation was performed,finding that the solvents remarkably affected the morphology of the obtained stationary phase.Although porous crosslinked monolith could be obtained when MeOH/DMF/PEG(50/40/10 by weight)was employed,its through-pore size was larger than 5 ?m.Such a big through-pore size was not favorable to the retention of analytes.Clearly,further optimization of the preparation conditions will be needed.5.Achievements of this dissertation were summarized,and the remaining defects as well as future work were also proposed.