materials Article Preparation of Porous Poly(Styrene-Divinylbenzene) Microspheres and Their Modification with Diazoresin for Mix-Mode HPLC Separations Bing Yu 1,2, Tao Xu 1, Hailin Cong 1,2,*, Qiaohong Peng 1 and Muhammad Usman 1 1 Institute of Biomedical Materials and Engineering, College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, China; [email protected] (B.Y.); [email protected] (T.X.); [email protected] (Q.P.); [email protected] (M.U.) 2 Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China * Correspondence: [email protected]; Tel.: +86-532-85953995; Fax: +86-532-85955529 Academic Editor: Dominik Brühwiler Received: 8 February 2017; Accepted: 20 April 2017; Published: 22 April 2017 Abstract: By using the two-step activated swelling method, monodisperse porous poly(styrene- divinylbenzene) (P(S-DVB)) microparticles were successfully synthesized. The influence of porogens, swelling temperatures and crosslinking agents on the porosity of porous microparticles was carefully investigated. Porous P(S-DVB) microparticles were used as a packing material for high performance liquid chromatography (HPLC). Several benzene analogues were effectively separated in a stainless-steel column as short as 75 mm due to the high specific surface area of the porous microparticles. Porous P(S-DVB) microparticles were further sulfonated and subsequently modified with diazoresin (DR) via electrostatic self-assembly and UV (ultraviolet) radiation. After treatment with UV light, the ionic bonding between sulfonated P(S-DVB) and DR was converted into covalent bonding through a unique photochemistry reaction of DR. Depending on the chemical structure of DR and mobile phase composition, the DR-modified P(S-DVB) stationary phase performed different separation mechanisms, including reversed phase (RP) and hydrophilic interactions. Therefore, baseline separations of benzene analogues and organic acids were achieved by using the DR-modified P(S-DVB) particles as packing materials in HPLC. According to the π–π interactional difference between carbon rings of fullerenes and benzene rings of DR, C60 and C70 were also well separated in the HPLC column packed with DR-modified P(S-DVB) particles. Keywords: polystyrene; seeded polymerization; macroporous structures; diazoresin; high performance liquid chromatography 1. Introduction In the last few decades, a lot of investigation has been done on the synthesis and properties of porous P(S-DVB) beads. The activated swelling method, developed by Ugelstad et al. was utilized for the synthesis of monodisperse porous particles in the range of 1–20 µm [1–3]. Recently, a lot of research has been conducted to study the different properties of monodisperse polystyrene microspheres with porous structure [4–11], since they have high specific surface area, tunable pore morphology, mechanical stability and strong adsorption [12–14]. The widely-used techniques for the synthesis of porous P(S-DVB) beads include activated swelling [15], seeded emulsion polymerization [16], precipitation polymerization [17], template imprinting [18,19] and membrane techniques [20,21]. Due to lower back pressure, a more regular flow regime in the column and higher resolutions, which result from their excellent permeability and high surface area, the monodisperse Materials 2017, 10, 440; doi:10.3390/ma10040440 www.mdpi.com/journal/materials Materials 2017, 10, 440 2 of 13 MaterialsMaterials 2017 2017, 10, 10, 440, 440 2 of2 of 14 14 microspheres with porous structures have been widely used in chromatographic columns as packing DueDue to to lower lower back back pressure, pressure, a amore more regular regular flow flow regime regime in in the the column column and and higher higher resolutions, resolutions, materials [22–27]. For example, Unsal et al. [28] investigated the effect of the monomer/seed latex ratio whichwhich resultresult fromfrom theirtheir excellentexcellent permeabilitypermeability andand highhigh surfacesurface area,area, thethe monodispersemonodisperse for themicrospheresmicrospheres chromatographic with with porous performanceporous structures structures of monodisperse have have been been widely widely porous used used particles in in chromatographic chromatographic produced by a modifiedcolumns columns as seededas polymerizationpackingpacking materials materials method. [22 [22–27 Fr–27].é chet].For For example, etexample, al. [29 Unsal– Unsal32] exploredet et al. al. [28 [28] investigated the] investigated impact ofthe the different effect effect of of the surface the monomer/seed monomer/seed chemistries of monodisperselatexlatex ratio ratio for macroporous for the the chromatographic chromatographic particles onperformance performance the chromatographic of of monodisperse monodisperse behavior. porous porous particles particles produced produced by by a a modifiedInmodified this paper, seeded seeded we polymerization polymerization synthesized method. monodispersemethod. Fr Fréchetéchet porouset et al. al. [ 29[29 P(S-DVB)–32–32] ]explored explored microspheres the the impact impact byof of different utilizingdifferent the activatedsurfacesurface swelling chemistries chemistries technique of of monodisperse monodisperse and investigated macroporous macroporous the particles particles effect on ofon the the chromatographic chromatographic porogen ratio, behavior. behavior. temperature and crosslinkingInIn this agents.this paper, paper, The we we poroussynthesized synthesized P(S-DVB) monodisperse monodisperse microparticles porous porous wereP(S P(S-DVB)-DVB) further microspheres microspheres sulfonated by by with utilizing utilizing concentrated the the sulfuricactivatedactivated acid swelling and swelling subsequently technique technique and modifiedand investigated investigated with the diazoresin(DR)viathe effect effect of of the the porogen porogen electrostatic ratio, ratio, temperature temperature self-assembly and and and crosslinkingcrosslinking agents. agents. The The porous porous P(S P(S-DVB)-DVB) microparticles microparticles were were further further sulfonated sulfonated with with concentrated concentrated UV (ultraviolet) radiation. These porous microspheres were applied as column packing material sulfuricsulfuric acid acid and and subsequently subsequently modified modified with with diazoresin( diazoresin(DRDR)via)via electrostatic electrostatic self self-assembly-assembly and and UV UV in mix-mode HPLC analysis of benzene analogs, organic acids and fullerenes. (ultraviolet)(ultraviolet) radiation. radiation. These These porous porous microspheres microspheres were were applied applied as as column column packing packing material material in in mixmix-mode-mode HPLC HPLC analysis analysis of of benzene benzene analogs, analogs, organic organic acids acids and and fullerenes. fullerenes. 2. Results and Discussion 2.2. Results Results and and Discussion Discussion 2.1. Synthesis of Porous P(S-DVB) Microparticles 2As.1.2.1. illustratedSynthesis Synthesis of of Porous in Porous Figure P(S P(S1-DVB),-DVB) the synthesisMicroparticles Microparticles process of monodisperse porous polymer microparticles was basedAsAs onillustrated illustrated monodisperse in in Figure Figure linear1, 1, the the synthesis polystyrene(PS)synthesis process process of of seedmonodisperse monodisperse particles, porous porous undergoing polymer polymer microparticles porogen microparticles swelling, monomerwwasas based based and crosslinkeron on monodisperse monodisperse swelling, linear linear polymerization polystyrene( polystyrene(PSPS) and)seed seed extraction.particles, particles, undergoing undergoing Monodisperse porogen porogen PS seedswelling, swelling, particles preparedmonomermonomer by dispersion and and crosslinker crosslinker polymerization swelling, swelling, poly poly havemerizationmerization an average and and extraction. extraction. diameter Monodisperse Monodisperse of 2.7 µm, andPS PS seed theseed particles coefficient particles of deviationpreparedprepared (CV) by by is dispersion onlydispersion about polymerization polymerization 1.9% (Figure 2have). have The an an monodispersity average average diameter diameter was of of 2.7 inherited2.7 µm, µm, and and when the the coefficient porouscoefficient polymer of of microparticlesdeviationdeviation (CV) were(CV) is prepared.isonly only about about Dibutyl 1.9% 1.9% ( Figure( phthalateFigure 2). 2). The (DBP)The monodispersity monodispersity is a seed swelling was was inherited inherited activator when when and porous porogenporous is polymerpolymer microparticles microparticles were were prepared. prepared. D Dibutylibutyl phthalate phthalate (DBP) (DBP) i si sa aseed seed swelling swelling activator activator and and a classical plasticizer. Toluene as a porogen is an excellent solvent of polystyrene. These two organic porogenporogen is is a aclassical classical plasticizer. plasticizer. Toluene Toluene as as a aporogen porogen is is an an excellent excellent solvent solvent of of polystyrene. polystyrene. These These solvents can be diffused into PS seed particles from their emulsion. After being swollen with twotwo organic organic solvents solvents can can be be diffused diffused into into PS PS seed seed particle particles sfro fromm their their emulsion. emulsion. After After being being swollen swollen porogen,withwith porogen, monomer, porogen, monomer, monomer, crosslinker crosslinker crosslinker and initiator,