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School of Chemical Engineering, Hanoi Univ

Co Viet Street, Hanoi, Vietnam. E-mail: hoa

Cite this: RSC Adv., 2014, 4, 8283

Received 11th October 2013Accepted 3rd December 2013

DOI: 10.1039/c3ra45747h

www.rsc.org/advances

This journal is © The Royal Society of C

Fast synthesis of an inorganic–organic blockcopolymer in a droplet-based microreactor

Phan Huy Hoang* and Le Quang Dien

In this report, we used a non-lithographic embedded template method to fabricate a polyvinyl silane (Kion)

microfluidic device. The device possesses a good solvent resistance, thermal stability and air-

impermeability. The device was used for the synthesis of an inorganic–organic block copolymer through

the generation and merging of reactant droplets. The SP-b-PMMA block copolymer was synthesized

successfully by the atom transfer free radical polymerization (ATRP) process with a high conversion and

narrow polydispersity in a very short reaction time, about ten minutes. The results obtained using the

droplet-based microreactor were much better than those obtained with a macroscale batch reactor.

Moreover, the as-synthesized SP-b-PMMA block copolymer was used to generate a highly ordered self-

assembled ceramic pattern hence demonstrating the high quality of the block copolymer with superior

molecular weight distribution control.

Introduction

The advantages of controlled radical polymerization techniquesresult from the combination of living polymerization tech-niques, such as anionic or cationic polymerizations, with therobust reaction conditions employed in free-radical polymeri-zations.1,2 Atom transfer radical polymerization (ATRP) is one ofthe controlled radical polymerization techniques based onreversible deactivation.3–5 The process can be applied to a widerange of monomers, and numerous well-dened complexpolymer architectures have been prepared using ATRP.6–8 It isimportant to note that precise temperature control is essentialfor carrying out a free radical copolymerization in a highlycontrolled manner, because free radical polymerization reac-tions are usually highly exothermic. Therefore, copolymeriza-tion reactions in conventional macroscale bath reactors oensuffer from inefficient heat removal and a lack of homogeneityof the reactor temperature, which eventually give rise to a lowlevel of molecular weight distribution control.9 In addition,polymerization rates in ATRP are typically not high, thusrequiring long residence times to reach a practical yield.Another contributing factor is the long lifetime of the polymerchains in ATRP that, when combined with the residence timedistribution, signicantly broaden the molecular weight distri-bution of the resulting polymer.1,10

Recently, microuidic syntheses compartmentalized withindroplets11–13 have received signicant interest because they areexpected to make an innovative and revolutionary change tochemical syntheses by virtue of their advantages over

ersity of Science & Technology, No.1 Dai

ng.phanhuy@hust.edu.vn

hemistry 2014

conventional macroscale batch reactors. These advantagesinclude effective mass transfer and heat transfer, fast mixingand precise residence time control.14–18 Among these inherentadvantages of microreactors, the efficient heat transfer seems tobe one of the most important features for free radical copoly-merization reactions. In copolymerization reactions in macro-scale batch reactors, the heat removal capacity oen becomes alimiting factor. Therefore, the advantage of a copolymerizationreaction in a microuidic reactor is obvious.9,16 It is hypothe-sized that the droplet synthesis may further facilitate the heattransfer and mixing between the reactants for radical poly-merization reactions, leading to a very fast reaction with a lowmolecular weight distribution.

Moreover, fabricating well-dened nanostructures fornanopatterning using the self-assembly of block copolymers isof great interest because of the tunable dimensions and theprecise tunability of the shape, size and chemical propertieswith a exible, simple, and low-cost process.19–21 However, theblock copolymers used in self-assembly studies and lithographyapplications have focused on organic–organic diblock copoly-mers. The general properties of organic polymers are notsufficient for use in applications that subject them to harshenvironments; they are required to show tolerance to hightemperatures, resistance to corrosion, and have tribologicalproperties.22 Therefore, there is clearly a continuous demand forthe development of a fabrication process for ceramic structureson the micro- or nanoscale.

To date there have been no reports using a droplet-basedmicrouidic reactor for the synthesis of inorganic–organicblock copolymers from monomers. Thus, here we report therst demonstration of a droplet-based microuidic method forthe copolymerization of a solution phase through the ATRP

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process to produce a well dened inorganic–organic poly-hydrido vinylsilazane-b-polymethylmethacrylate (SP-b-PMMA)block copolymer with a narrow polydispersity. The SP-b-PMMAblock copolymer was successfully synthesized with a highconversion and low polydispersity (PDI) in a short reaction time.Furthermore, the as-synthesized diblock copolymer has beenshown to be a suitable candidate for the simple and directgeneration of a highly ordered self-assembled ceramic micro-structure thus demonstrating the high quality of the blockcopolymer with superior molecular weight distribution control.

ExperimentalFabrication of the microuidic device

A solvent-resistant and thermally stable polyvinyl silane (Kion)microuidic device with varying channel sizes was fabricatedusing a pre-ceramic polymer (polyvinyl silane, HTT-1800, KionCorporation, Charlotte, NC). The device was made using amodied scaffold method without the use of a photolithog-raphy master, as developed by our own group (Fig. 1).16,23 Thetemplate frameworks were assembled using commerciallyavailable tubes of different sizes. All the tubes were purchasedfrom Upchurch Scientic. For this case, the main microchannelhad a diameter of about 500 mm (equal to the outer diameter ofthe tube). HTT-1800 was mixed with 0.5% dicumyl peroxide(thermal initiator, Sigma Chemicals, St. Louis, MO) and thiswas initially cured at 100 �C for 3 hours followed by subsequentcuring at 180 �C for 2 hours with a temperature ramp of 1 �Cmin�1.

Fig. 1 Schematic illustration of the fabrication process for the solventresistant microfluidic droplet generator with different channel widths.(A) The PDMS mold, (B) the PFA and PEEK assembly were set into thePDMS mold, and the polyvinyl silane (Kion) precursor was poured overand cured to make the Kion channel. (C) Kion microfluidic device afterremoving the templates.

8284 | RSC Adv., 2014, 4, 8283–8288

Synthesis of the SP-b-PMMA block-copolymer

For the synthesis of the inorganic–organic block copolymer SP-b-PMMA, polyhydrido vinylsilazane (SP) was modied and usedas a macroinitiator. The rst block of the copolymer, SP, wasmodied to attach a Br group at the end of the polymer chain. Asolution of SP in diphenyl ether (1 g/4 ml, ow rate 1.75–17.5 mLmin�1) and a solution of CuBr + 4,40-di-5-nonyl-2,20-bipyridine(dNBpy) + initiator (0.1 mmol and 0.05 mmol, respectively) indiphenyl ether (ow rate 1.75–17.5 mL min�1) were introducedseparately into a double T-junction microuidic device. The SPsolution and the solution of (CuBr + dNBpy + initiator) indiphenyl ether formed the dispersed phase. The continuousphase, uorocarbon oil (FC oil, 3 M) was injected from thehorizontal inlet at a ow rate of Qc mL min�1 (ow rate 10.5–105mL min�1). The SP solution was forced into the continuousphase at the rst T-junction of the channel at a ow rate ofQ1 mLmin�1 using a syringe pump (PHD 2000, Harvard Instruments)to initially form the rst droplet. The solution of (CuBr + dNBpy+ initiator) in diphenyl ether was forced into the continuousphase at the second T-junction of the channel at a ow rate ofQ2

mL min�1 to form the second droplet. This merged immediatelywith the rst droplet in the main horizontal channel to producea larger droplet. Each resulting droplet contained the reactionmixture of SP, CuBr, dNBpy and the initiator, and this is called adroplet microreactor. Each droplet reactor then owed in thePFA tube (ID 508 mm), which was immersed in a silicon oil bathat 90 �C, with different delay loop lengths (45–180 cm) of thePFA tube. At the outlet of the rst PFA tube, the modied SPwas continuously used as a macroinitiator to synthesize theSP-b-PMMA block copolymer through a reaction with themethylmethacrylate monomer in the second step of a consec-utive step-wise process. Similarly, the droplet of the modied SPmerged with the droplet of methylmethacrylate monomer (neat,0.02 mol) forced at a ow rate of Q3 mL min�1 (1.75–17.5 mLmin�1) in the microuidic device and the resulting droplet owin the PFA tube was maintained at 90 �C. The experiments werecarried out at different ow rates of dispersed and continuousphases but the ratio of the ow rates between the dispersed andcontinuous phases was maintained at Q1:Q2:Q3:Qc ¼ 1 : 1 : 1 : 6,which allowed easy control of the generation and merging ofdroplets. At the outlet of the PFA tube, the product was isolatedby diluting in THF and adding excess n-hexane to causeprecipitation. The process is shown in Fig. 2. Finally, the as-synthesized SP-b-PMMA block copolymer was characterized by1H-NMR spectroscopy and GPC.

To compare with the droplet-based microreactor, a bulkphase copolymerization was carried out in a ask at 90 �C for 20h with magnetic stirring at 700–1000 rpm for the mixing of thereactant mixture.

Self-assembly of the SP-b-PMMA block-copolymer

To investigate the microphase segregation of the bulk phase ofthe mesoporous ceramic material, a thin lm of the meso-porous SP-b-PMMA pattern was fabricated by a thermalannealing method. For the self-assembly, 1 wt% of the diblockcopolymer was dissolved in THF (anhydrous 99.9%) to make a

This journal is © The Royal Society of Chemistry 2014

Fig. 2 Generation and merging of the reactant droplets for SP-b-PMMA copolymerization: (A) schematic diagram of the microfluidicdroplet generator for the inorganic–organic SP-b-PMMA blockcopolymer synthesis, (B) optical image of the reactant droplets in aKion microfluidic device for the modification of SP to get the modifiedSP macroinitiator, (C) optical image of the reactant droplets in a Kionmicrofluidic device for the SP-b-PMMA synthesis step. The scale bar is500 mm.

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solution. Aer stirring, for uniform coating of the layers, anydust or particles and/or bubbles were removed using 0.2 mmlters. SP-b-PMMA lms of approximately 0.5 mm thicknesswere cast over 3–4 days in a 100 mm-diameter Teon disk. Thethick cast lms were self-assembled by a thermal annealingmethod without a thermal initiator under an Ar atmosphere at180 �C for 20 h and then pyrolyzed at temperatures up to1200 �C at a heating rate of 1 �C min�1 in an air atmosphere toconvert them into a ceramic bulk powder type of product.

Characterization1H-Nuclear magnetic resonance (NMR) spectroscopy was per-formed in CDCl3 on a Bruker DMX600 instrument with a 7788Hz spectral width, a relaxation delay of 1.0 s, and a pulse widthof 30�. The molecular weight distribution of the synthesizedpolymers was examined by gel permeation chromatography(GPC) using aWaters 515 HPLC isocratic pump equipped with aWaters 2414 Refractive Index detector and Waters styragelcolumns (HR 1, 2, 3, 4, 5 E). THF (ow rate of 1.0 mLmin�1) wasused as the solvent and polystyrene (Shodex standard) was usedas a standard for universal calibration. High-resolution trans-mission electron microscopy (HRTEM) images were obtainedusing a JEM 2100F, JEOL, Japan, operating at 200 kV. Thescanning electron microscopy (SEM) was performed using aJSM-7000F, JEOL, Japan. The degree of polymerization wascalculated from the NMR spectra using the signals from thevinyl groups of the polymer and monomer. The particle size ofthe self-assembled nanoparticles in an aqueous medium was

This journal is © The Royal Society of Chemistry 2014

measured by dynamic light scattering (DLS) using a Part III laserparticles analyzer (Photal Otsuka Electronic, Japan) at aconcentration of 1 mg mL�1. The small angle X-ray diffraction(SA-XRD) patterns were recorded on a MX Labo powderdiffractometer using Cu-Ka radiation (40 kV, 20 mA) at a scanrate of 1.0 min�1 over the range 0.5–7.0 (2q). The pyrolyzedsamples were crushed into ne powders and dispersed onto athin holey-carbon support lm. The surface area and N2

adsorption–desorption isotherms were measured at 77 K on aMicromeritics (ASAP 2010, USA) using the Brunauer–Emmett–Teller (BET) method.

Results and discussionFabrication of the polyvinyl silane (Kion) microuidic device

Fig. 1 shows the fabrication process. Fig. 1(A) shows the PDMSmold used for making the framework that was assembled usingcommercially available tubes of different sizes. Viscous poly-vinyl silane was poured into the framework (Fig. 1(B)), followedby initial thermal curing at 100 �C for 3 hours and thensubsequent curing at 180 �C for 2 hours with a temperatureramp of 1 �C min�1.20 Aer curing, the template was simplyremoved from the polymer matrix to make the channel(Fig. 1(C)), so called the Kion microchannel. The advantage ofthis method is the simple and cost-effective approach for thefabrication of a monolithic, microuidic device with variouschannel dimensions through a single step using low surfaceenergy templates that can be removed easily from the matrix.13

This embedded template method enables the fabrication of amonolithic Kion microuidic device within a short timespanwith minimal or no fabrication facilities.

Moreover, a Kion microuidic channel possesses solventresistance, thermal stability and air-impermeable properties. Itis known that the ATRP process requires robust reactionconditions with trace impurities such as air and water.1 Thusthe Kion channel, which can resist most organic solvents andair, was used due to the easy swelling of PDMS in most organicsolvents and the air-permeability of PDMS.

Synthesis of the inorganic–organic block-copolymer SP-b-PMMA in the droplet microreactor

We used the “T-junction” geometry of the microuidic device todesign an assembly type of microuidic system for our experi-ments as previous reported.16 We assembled two microuidicdevices for the continuous two step copolymerization reactionin the droplet-based microreactor. In addition, the microuidicsystem was assembled using a combination of lab-on-a-chipparts (microuidic devices) for generating and merging thedroplet and a PFA capillary tube (45–180 cm, ID 508 mm) forcontrolling the resident time at elevated temperatures. Both thedispersed and continuous phases were injected into themicrouidic device using syringe pumps (PHD 2000, HarvardInstruments, Holliston, MA). The continuous phase, uoro-carbon oil, was injected from the horizontal inlet with a owrate of Qc mL min�1. The reactants solution was forced into thecontinuous phase at the “T-junction” of the channels with a

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ow rate of Q1 mL min�1, Q2 mL min�1 and Q3 mL min�1 to formthe droplets, respectively. The rst two kinds of droplet mergedtogether immediately in the main horizontal channel of the rstdevice and moved along the PFA loop to reach the desiredtemperature in the desired resident time. Then it came to thesecond device and merged with the third droplet to form an“n-reactor” droplet of nanoliter scale that contained themixtureof reactants owing in the channel of the second microuidicdevice. Subsequently, this “n-reactor” droplet owed into thePFA tube kept in the 90 �C oil bath, where the block copolymerreaction took place. The copolymerization in the droplet-basedmicroreactor was compared with the copolymerization in thebulk phase.

The experiments were carried out with different ow rates ofthe dispersed and continuous phases but at the same ow rateratio between the dispersed and continuous phasesQ1 : Q2 : Q3 : Qc ¼ 1 : 1 : 1 : 6, which allowed easy control of thegeneration andmerging of the droplets.16,17 It has been reportedthat a change of ow rate affects not only the residence time butalso other factors such as the degree of mixing, the nature of theow, and changes in the size and shape of the droplet.24 Herein,we summarize the results of the copolymerization reaction interms of different residence times and the same residence timebut at different ow rates.

The mixture of reactants in the droplet was conned to theone nanoliter-scale and the droplet kept moving in the tubeaccording to the residence time. This may generate signicantheat transfer during the exothermic living radical polymeriza-tion reaction and efficient mixing between the monomer, theATRP agent and the initiator.16–18,24 It is most likely thatthe droplet-basedmicroreactor performed rapid consumption ofthe initial ATRP agent and fast equilibration of the dormant andactive species, presumably to promote polymerization of thevinyl group.9,16 This induced the copolymerization reaction,which occurred very fast and was almost nished within theresidence time of several minutes (Table 1) and also led to ahigher molecular weight. In particular, Table 1 shows that thecopolymerization reaction in the droplet-based microreactorwith different delay loop lengths of PFA tube was achieved with avery high conversion and a high molecular weight. These valueswere higher than those in the macroscale batch reactor (bulk

Table 1 Conversions for the block copolymerization reactions with diff

Entry Total reaction time (min)c

Droplet-based microreact

Flow rate, Qc,mL m�1 ha

1b 20 (10) 10.5 97.2 15 (10) 21 93.3 12.5 (10) 42 90.4 11 (10) 105 88.5 15 (5) 10.5 94.6 10 (5) 21 91.7 6 (5) 105 84.

a Conversion to SP-b-PMMA, measured by 1H-NMR spectroscopy. b This smacroinitiator (modication of SP).

8286 | RSC Adv., 2014, 4, 8283–8288

reaction). Surprisingly, aer a total reaction time of 20 minutes(entry 1), the conversion to SP-b-PMMA was almost complete(�98%), which was much faster than the bulk reaction.

Interestingly, the polydispersity index (PDI) for the copoly-merization reaction in the droplet-based microreactor wasmuchsmaller than that obtained from the polymerization in the bulkreaction. A smaller local deviation of the temperature in themicroreactor also seems to be responsible for a narrowermolecular weight distribution. In the case of the copolymeriza-tion reaction in the microuidic reactor, a signicant heattransfer to the reactant mixture, efficient chaotic advectionmixing and a good contact between the reactants inside thenanoliter-scale droplet induces the homogeneous temperatureand a better homogenization of methylmethacrylate and the SPmacroinitiator in each droplet and also in all of the droplets.16

Consequently this uniformity in the chemical composition andthe temperature allowed for a higher incorporation of the MMAmonomer to form a PMMA chain and higher incorporation ofPMMA chain with SP unit. Thus longer copolymer chains wereformed in a short reaction time and yielded the copolymer with avery narrow molecular weight distribution (i.e. a very small PDI).

Moreover, the modication of SP also inuenced the blockcopolymerization reaction of SP-b-PMMA. As seen in Table 1,using the SPmacroinitiator obtained over a longer reaction time(10 min) for the diblock copolymer synthesis led to a higherquality of copolymer with a higher molecular weight and a lowerPDI compared to using the SP macroinitiator obtained over 5min. This feature can be attributed to the higher concentrationof SP macroinitiator obtained over a longer reaction time. Thiswould induce a good contact between the SP macroinitiator andthe MMA monomer under the chaotic advection inside thenanoliter-scale droplet, and lead to a fast reaction to form thediblock copolymer SP-b-PMMA with a higher molecular weightand narrower molecular weight distribution. The use of the SPmacroinitiator obtained from a 10min reaction followed by a 10min block copolymerization in the droplet microreactorproduced the highest quality SP-b-PMMA diblock copolymerwith a MW of 25 075 and a PDI of 1.11. This is presumably dueto the enhanced homogeneous temperature distributionwithout hot spots and a better homogenization of the reactantsin the nanoliter-scale droplet. Therefore, it can be concluded

erent reaction times

or

Bulk reactionfor 20 h(%) MW PDI

65 25 075 1.11 ha: 72.6% MW: 13 975 PDI: 1.1988 18 659 1.1466 15 683 1.1758 11 940 1.1816 22 750 1.1307 16 348 1.1673 10 784 1.19

ample is named sample N1. c () reaction time taken for synthesis of SP

This journal is © The Royal Society of Chemistry 2014

Fig. 3 Characterization of the mesoporous ceramic structure obtained from the SP-b-PMMA (MW: 25 075, PDI: 1.11) template, after pyrolysis at1200 �C in air: (A) TEM images with scale bar 20 nm, (B) small angle X-ray diffraction patterns, and (C) N2 adsorption–desorption isotherms andthe pore size distribution (inset).

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that the droplet-based microreactor is a reliable, efficientmethod for the synthesis of a well-dened block copolymer.

Formation of self-assembled well-dened ceramicnanostructures from the SP-b-PMMA block copolymer

As a demonstration of the high quality of the SP-b-PMMA blockcopolymer, the as-synthesized copolymer (sample N1, MW:25 075, PDI: 1.11) was used to generate self-assembled well-dened nanostructures. It is well-known that the self-assembly ofblock copolymers is driven by microphase separation behavior.The chemically distinct macromolecular blocks covalently linkedwithin block copolymer chains undergo spontaneous segrega-tion into dense, periodic nanoscale domains.25 The self-assemblyof well-dened block copolymers can be used to construct arange of ordered nanostructures in a wide range of morphol-ogies, including spheres, cylinders, bicontinuous structures,lamellae, vesicles, and many other complex or hierarchicalassemblies, depending on the block lengths and the ratios aswell as the chemical composition.26,27 Furthermore, the self-assembly process can be further controlled by selecting a non-solvent and/or using solvent mixtures.28,29

The functionalized SP-b-PMMA diblock copolymer couldself-assemble to form ceramic nanostructures at a hightemperature. The organic PMMA block was pyrolyzed to form

This journal is © The Royal Society of Chemistry 2014

pores while the inorganic SP block was transformed to form aceramic wall. A well-ordered mesoporous ceramic structure wasobtained with a pore diameter of �8 nm (Fig. 3) aer pyrolysisat 1200 �C in air. It has been noted that the relative volumefraction of the macromolecular blocks determines the shape ofthe nanodomains (nanospheres or nanocylinders or nano-lamellae), and the overall molecular weight of the blockcopolymer chain determines the characteristic size of thenanopatterns.25 We proposed that the functionalized SP-b-PMMA block copolymer with a suitable volume fraction of theSP block acts as a precursor for the mesoporous ceramic, inaddition to the intrinsic self-assembly. In particular, the largearea of highly ordered hexagonal structures is shown in theTEM image in Fig. 3A. The results indicate that the well-orderedhexagonal arrays of mesoporous structures were formed usingthe diblock copolymer SP-b-PMMA. This is in excellent agree-ment with the results from the small-angle X-ray diffraction(SA-XRD) patterns (Fig. 3B) of the self-assembled polymeric lmwith sharp peaks at 2q ¼ 0.8� (aer annealing at 180 �C) and1.16� (aer pyrolysis at 1200 �C), which indicates that a hexag-onally packed cylindrical morphology was formed.

The porosity characteristics of the sample calcined at1200 �C in an air atmosphere were investigated by measuringthe N2 adsorption–desorption isotherm (Fig. 3C). TheBrunauer–Emmertt–Teller (BET) analysis of this isotherm

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conrmed the presence of a highly mesoporous structure with apore volume close to 0.68 cm3 g�1, a high specic surface area of628 m2 g�1, and a narrow pore size distribution of 8.4 nm (asdetermined by the BJH desorption pore distribution method).This is in good agreement with the TEM and SA-XRD results.The observed results clearly demonstrate that using a dropletmicrouidic reactor is a reliable route for preparing high qualityblock copolymers that can meet the requirements for the self-assembly of uniform nanostructures.

Conclusions

A droplet-based microreactor was used to synthesize an inor-ganic–organic block copolymer, where the reagents for theATRP process were dispensed in monodisperse nanoliterdroplets within a microchannel. The SP-b-PMMA block copol-ymer was synthesized successfully with a higher conversion andnarrower polydispersity in a very short reaction time, approxi-mately 20 minutes, compared to the block copolymer producedusing the macroscale batch reactor. It can be concluded thatdroplet-based microreactors are quite effective for copolymeri-zation reactions. The heat transfer ability and the extremelyeffective mixing in the droplet-based microreactor are the mostoutstanding factors compared to conventional macroscalebatch reactors. Self-assembled well-dened ceramic nano-structures were examined as an application of the as-synthe-sized SP-b-PMMA that was obtained from the blockcopolymerization reaction in the droplet-based microuidicreactor. Ordered mesoporous ceramic nanostructures with aregular framework, a high BET surface area of 628 m2 g�1 andan average mesopore size of 8.4 nm were prepared from the as-synthesized inorganic–organic block copolymer.

Moreover, this work also shows the usefulness of anembedded template method to fabricate a monolithic solvent-resistant, thermally stable and air-impermeable polyvinyl silane(Kion) microuidic device within a short time span with few orno fabrication facilities. Further, the assembled dropletmicrouidic system composed of a lab-on-a-chip part for thedroplet generator and an external capillary tube was used for therst time to synthesize an inorganic–organic block copolymerusing a living radical copolymerization technique.

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