Tailoring Pore Size, Structure, and Morphology of Hierarchical

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Tailoring Pore Size, Structure, and Morphology of Hierarchical Mesoporous Silica Using Diblock and Pentablock Copolymer Templates Gyudong Lee, Eunji Choi, Shu Yang, and Eun-Bum Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00277 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry

Tailoring Pore Size, Structure, and Morphology of Hierarchical Mesoporous Silica Using Diblock and Pentablock Copolymer Templates Gyudong Lee,† Eunji Choi,† Shu Yang,*,‡ and Eun‐Bum Cho*,† †

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 01811, Korea

‡

Department of Materials Science and Engineering, University of Pennsylvania, PA 08826, United State

ABSTRACT: Mesoporous materials of tailored pore size, structure, and morphology are of interests for a wide range of applications. It is important to develop synthetic methods that will allow for easy processing and facile structure modifica‐ tion. Here, we present the preparation of hierarchically structured bimodal mesoporous silicas using water soluble poly(lac‐ tic acid‐co‐glycolic acid)‐b‐poly(ethylene oxide) (PLGA‐b‐PEO) diblock copolymer and a poly(lactic acid‐co‐glycolic acid)‐ b‐poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide)‐b‐poly(lactic acid‐co‐glycolic acid) (PLGA‐b‐PEO‐ b‐PPO‐b‐PEO‐b‐PLGA) pentablock copolymer as templates. The block copolymers were synthesized through a step‐growth polymerization method using a commercial Pluronic F68 macro‐initiator. Mesoporous silica samples were obtained by sol‐ gel chemistry in acidic aqueous solutions. Hexagonally (p6mm) ordered mesoporous silica particles were obtained in the presence of a PLGA‐PEO diblock copolymer and exhibited bimodal pore size distributions in the range of 2‐9 nm. Core‐ shell type mesoporous silica particles were obtained in the presence of PLGA‐PEO‐PPO‐PEO‐PLGA pentablock copolymer and exhibited large pore diameter up to 20 nm with distinct bimodal pore size distributions. The pore size increased when using longer pentablock copolymer template in strong acid. The physicochemical properties were investigated using small‐ angle X‐ray scattering (SAXS), nitrogen adsorption‐desorption, transmission electron microscope (TEM), solid‐state 29Si nuclear magnetic resonance (NMR), and scanning electron microscope (SEM), respectively.

INTRODUCTION Ordered mesoporous materials with diameter in the range of 2 – 50 nm templated from liquid crystalline sur‐ factant micelles were first reported in 1992 to overcome disadvantage of conventional zeolites and molecular sieves that have pore diameters less than 2 nm for more facile conversion of heavy petroleum oil.1,2 Since then, ordered mesoporous materials have received increasing attention because of their high surface area, tailorable ordered mesostructures, tunable uniform mesopore size, and ver‐ satile modification of framework compositions. As a result, mesoporous materials have been studied for a wide range of applications including adsorption,3,4 seaparation,5,6 cat‐ alyst,7,8 drug delivery,9,10 and energy conversion and stor‐ age.11,12 Nevertheless, low molecular weight surfactants as well as block copolymers as a single template have limit to obtain the uniform pore size over 10 nm especially in mesoporous silica. The physical and chemical control of pore size and pore structure13 still remains one of key issues for various applications of mesoporous silicas. Commercial Pluronic triblock copolymers14 and poly(ethylene oxide) (PEO)‐ based amphiphilic block copolymers have been identified as useful soft templates to increase pore size in mesopore regime.15‐17 Thus, larger mesopores with a variety of pore morphologies using block copolymer templates have been demonstrated due to their facile self‐assembly and tunable length scales.18,19 Organic swelling agents such as 1,3,5‐tri‐ methylbenzene (TMB) were also found as valuable addi‐ tives to further enlarge the mesopore size greater than 10

nm.14,20‐22 Meanwhile, mesoporous materials with multi‐ modal pores are of great potentials to enhance the perfor‐ mance in catalysis, sorption, separation, and biomedical areas based on unique and adjustable pore structure.23‐26 Until now, various templating approaches have been tried to prepare hierarchically bimodal mesoporous materials especially by using a dual‐templating approach or post‐ treatments of mesoporous silica materials.27‐37 Generally, Pluronic triblock copolymers, constituting of poly(eth‐ ylene oxide) (PEO) and poly (propylene oxide) (PPO), with additional surfactants have been widely used to pre‐ pare bimodal mesoporous silicas. In the case of using non‐ Pluronic block copolymers, more hydrophobic block chains located at the one or both ends of block copolymer usually make them insoluble in water and the mesoporous materials should be prepared by evaporation‐induced self‐ assembly (EISA) process with non‐polar solvents for cross‐ linking and condensation of silicate precursors. Wiesner et al. firstly reported the preparation of mesostructured sil‐ ica‐aluminas using poly(isoprene‐b‐ethylene oxide) (PI‐b‐ PEO) diblock copolymer templates via EISA process.16 Un‐ til now, a variety of PEO‐based diblock copolymers, includ‐ ing poly(styrene‐b‐ethylene oxide) (PS‐b‐PEO),38 poly(ca‐ prolactone‐b‐ethylene oxide) (PCL‐b‐PEO),39 poly(lactide‐ b‐ethylene oxide) (PLA‐b‐PEO),40 poly(methyl methacryl‐ rate‐b‐ethylene oxide) (PMMA‐b‐PEO),37 and poly(eth‐ ylene‐co‐butylene)‐b‐ethylene oxide) (Kraton‐liquid‐b‐ PEO) (KLE)34 have been studied using EISA strategy for synthesis of ordered mesoporous silica with large and tun‐ able pore sizes. However, aforementioned diblock copoly‐

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mer templates generally lead to unimodal mesoporous ma‐ terials during EISA process since thermodynamically it is difficult to obtain hierarchical micellar systems from a ho‐ mogeneous mixture of a single block copolymer in a start‐ ing good solvent.41 Thus, different types of ABC triblock co‐ polymers, such as poly(ethylene‐alt‐propylene‐b‐ethylene oxide‐b‐n‐hexyl methacrylate)42 and polyisoprene‐b‐poly‐ styrene‐b‐poly(propylene carbonate)43 have been investi‐ gated to make new complex nanostructures with tunable length scales. Recently, ABC type triblock copolymer, poly(ethylene‐b‐ethylene oxide‐b‐ε‐caprolactone) (PE‐b‐ PEO‐b‐PCL), as a single template was used to prepare hi‐ erarchical mesoporous materials using EISA process, sug‐ gesting the two hydrophobic blocks, PE and PCL, formed different mesopores separately.44 However, there was no report to prepare multimodal mesoporous materials sys‐ tematically from solution state not using EISA process. It was known that a typical BAB type Pluronic triblock copol‐ ymer, with small difference of solubility parameter be‐ tween hydrophilic (B) block and hydrophobic (A) block, generally makes uniform polymer micelles easily in solu‐ tion/solid states. On the contrary, it is expected that AB diblock copolymers and ABA type triblock copolymers with hydrophobic block at the both ends of polymer chain can be assembled faster even in more diluted solutions which can prepare hierarchical size distribution of mi‐ celles. Furthermore, in the case of ABA type triblock copol‐ ymer in solution, its more rigid hydrophobic chains in mi‐ celle core and some of loop type hydrophilic chains can prohibit unimer chains from the new insertion into the mi‐ celles more tightly than a typical BAB type Pluronic triblock copolymer. Therefore, it is possible to prepare hi‐ erarchical micellar system in aqueous solution by adjusting the solubility of block copolymer chains and pH using AB, ABA, and CBABC type block copolymers with hydrophobic A and more hydrophobic C blocks. It can be expected that it is possible to prepare multimodal mesoporous silica ma‐ terials by finding multimodal micellar system experimen‐ tally in dilute solution (i.e. aforementioned kinetically‐ and sterically‐driven multimodal micelle system) during the cooperative sol‐gel reaction using block copolymers with sufficient hydrophobic segments. Herein, we synthesized PEO‐based water soluble diblock, poly(lactic acid‐co‐glycolic acid)‐b‐poly(ethylene oxide) (PLGA‐b‐PEO), and CBABC type pentablock copolymers, poly(lactic acid‐co‐glycolic acid)‐b‐poly(ethylene oxide)‐b‐ poly(propylene oxide)‐b‐poly(ethylene oxide)‐b‐poly(lac‐ tic acid‐co‐glycolic acid) (PLGA‐b‐PEO‐b‐PPO‐b‐PEO‐b‐ PLGA), in which hydrophobic chains are located at the one and both ends of polymer chain, as templates for prepara‐ tion of hierarchical mesoporous silica. Hydrophobicity and crystallinity of poly(lactic acid) can be suppressed by ran‐ domly linking glycolide and D,L‐lactide monomers with‐ out stereoregularity, which is very important issue to make the segment more hydrophobic as well as soluble in wa‐ ter.45,46 As a result, we obtained a PLGA‐b‐PEO diblock co‐ polymer with weight fractions of PLGA segments around 0.3 using methoxy poly(ethylene glycole) as a macro‐initi‐ ator and by adjusting amounts of glycolide and D,L‐lactide

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monomers. Also, PLGA segments were linked chemically at both ends of Pluronic F68 (EO75PO30EO75) triblock co‐ polymer as a macro‐initiator to obtain PLGA‐b‐PEO‐b‐ PPO‐b‐PEO‐b‐PLGA pentablock copolymers. Molecular weights were adjusted around 12,000 ~ 16,000 g/mol with weight fractions of PLGA segments around 0.3‐0.5 to be soluble in water. In this study, we report the synthesis and characterization of hierarchically bimodal mesoporous sil‐ icas templated from each diblock copolymer and pen‐ tablock copolymers without addition of swelling agents in acidic aqueous solutions. We demonstrate the variation of pore size by using more hydrophobic AB and CBABC type block copolymer templates originated from geometric and chemical structures which is different from a typical BAB type Pluronic triblock copolymer template. To the best of our knowledge, this is the first report to fabricate well‐de‐ fined hierarchical mesoporous silica samples using CBABC type pentablock copolymers as a single template in acidic aqueous solutions not by using EISA process. EXPERIMENTAL SECTION Materials. The diblock copolymer of PLGA‐b‐PEO was synthesized using 3,6‐dimethyl‐1,4‐dioxane‐2,5‐dione (D,L‐lactide (DLLA), ~ 98%), 1,4‐dioxane‐2,5‐dione (gly‐ colide, ≥ 99%), and poly(ethylene glycol) methyl ether (MPEG, Mn ~ 2,000 g/mol) supported by tin(II) 2‐ethylhex‐ anoate (Sn(Oct)2, 92.5 ‐ 100%) as a catalyst in anhydrous toluene (99.8%) solvent. The pentablock copolymers of PLGA‐b‐PEO‐b‐PPO‐b‐PEO‐b‐PLGA were synthesized us‐ ing Pluronic F68 triblock copolymer as macroinitiator, to‐ gether with D,L‐lactide, and glycolide. Pluronic F68, com‐ posed of 30 propylene oxide units and 75 ethylene oxide units at both ends, has molecular weight of 8,500 g/mol. Ethyl acetate was used as a solvent to recrystallize D,L‐lac‐ tide, and glycolide monomers before use. Diethyl ether with low boiling point was used as a non‐solvent for sepa‐ ration of polymer product. Mesoporous silica materials were prepared using tetraethylorthosilicate (TEOS), hy‐ drochloric acid (HCl, ~ 37.0%), iron(III) chloride hexahy‐ drate (FeCl3∙6H2O, Ka = 6.3 × 10‐3), acetic acid (CH3COOH, Ka = 1.8 × 10‐5), and ethanol in distilled water. All chemicals were purchased from Aldrich. Synthesis of Diblock and Pentablock Copolymers. D,L‐lactide (DLLA) and glycolide were recrystallized in ethyl acetate solvent and stored at ‐2 oC before use. A PLGA‐PEO diblock copolymer (DLGE23 in Table 1) was synthesized by ring opening polymerization of cyclic D,L‐ lactide and glycolide monomers using a Sn(Oct)2 as cata‐ lyst, followed by in‐situ reaction with mono‐functional poly(ethylene glycol) methyl ether (MPEG). Detailed ex‐ perimental procedures for DLGE23 diblock copolymer are as follows: First, a three neck round‐bottom 1L flask with a reflux condenser and a solvent trap was purged under ar‐ gon gas flow for at least 3 h to remove the moisture inside the flask. 70 g of MPEG and 750 mL of toluene were mixed in the flask by stirring for 3 h at 120 oC under argon flow. After cooling the reaction flask to room temperature, 100 mL of solvent was removed via a solvent trap connected with the flask to remove remaining water trapped inside


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Table 1. Characterization of PLGA‐PEO Diblock and PLGA‐PEO‐PPO‐PEO‐PLGA Pentablock Copolymersa 1H‐NMR

GPC No. of repeating unit Mn Mw Mn Molecular Formula wLG PDI (g/mol) (g/mol) (g/mol) PO EO LA GA DLGE23 0 34.9 6.4 2.2 (LA6GA2)EO35 2,138 0.275 3,300 3,660 1.11 PLGF68‐12 30 150 48.0 16.5 (LA24GA8)EO75PO30EO75(LA24GA8) 12,700 0.345 10,720 14,090 1.32 PLGF68‐16 30 150 89.9 23.0 (LA45GA12)EO75PO30EO75(LA45GA12) 16,100 0.483 11,520 17,060 1.48 a Repeating units: PO = propylene oxide, EO = ethylene oxide, LA = lactic acid, and GA = glycolic acid; molar mass (g/mol) was calculated from molar mass of each unit: PO = 58, EO = 44, LA = 72, and GA = 58; wLG: weight fraction of poly(lactic acid‐co‐ glycolic acid) block in total chain; Mn is the number‐average molecular weight and Mw is the weight‐average molecular weight; PDI = polydispersity index of Mw/Mn. Sample

MPEG. Then, 24 g of D,L‐lactide, 6 g of glycolide, and 0.3 g of Sn(Oct)2 were added into the flask under argon flow. Af‐ ter further stirring for 24 h at 120 oC under argon flow, the flask was cooled to room temperature. The block copoly‐ mers were precipitated in 4,500 mL of diethyl ether in a 5 L beaker for 3 d. The final diblock copolymers as white powders were obtained after filtering using a suction flask and drying for 7 d under vacuum. PLGA‐PEO‐PPO‐PEO‐PLGA pentablock copolymers (PLGF68‐12 and PLGF68‐16 in Table 1) were synthesized by initiating polymerization of D,L‐lactide and glycolide from both ends of Pluronic F68 triblock copolymer. A typical polymerization procedure of PLGF68‐12 pentablock copol‐ ymer is similar to that of dibock copolymer as follows: Af‐ ter purging a vacant three neck round‐bottom 1L flask for 3 h under argon flow, 63.2 g of Pluronic F68 and 750 mL of toluene were mixed in the flask by stirring for 3 h at 120 oC maintaining argon flow. After cooling the reaction flask to room temperature, 100 mL of solvent was removed. Then, 29.4 g of D,L‐lactide, 7.4 g of glycolide, and 0.37 g of Sn(Oct)2 were added into the flask under argon flow. After further reaction for 24 h at 120 oC under argon flow, the flask was cooled to room temperature. The final pen‐ tablock copolymer powders were obtained after aforemen‐ tioned precipitating in diethyl ether, filtering, and drying processes. Characterization of Diblock and Pentablock Copoly‐ mers. The chemical structures and compositions of the di‐ block and pentablock copolymers were confirmed by 1 HNMR performed on Varian Unity (INOVA) 500 MHz sys‐ tem at Korea Basic Science Institute (KBSI) Daegu center. The copolymers were dissolved in CDCl3. Chemical shifts were calibrated using tetramethylsilane (TMS) as refer‐ ence. The relative molecular weights of diblock and pentablock copolymers were characterized by EcoSEC HLC 8320 GPC system (Tosoh Co.) equipped with a differential refractom‐ eter at Korea Polymer Testing and Research Institute (KOPTRI). The block copolymer samples were dissolved in tetrahydrofuran (THF) to prepare 3 mg/mL solution and filtered with PTFE syringe filter with pore diameter of 0.45 μm. 10 μL solution was injected into the GPC column with a flow rate of 0.35 mL/min at 40 oC. The molecular weights were obtained using narrowly dispersed polystyrene as standards (the molecular weights of each PS standard used in calibration were 590, 2,600, 10,000, 18,000, 96,000,

Table 2. Experimental Conditions for Mesoporous Sil‐ ica Samples Prepared in this Study sample

template co‐solvent

acid

Cacida

MSD‐1

DLGE23

none

HCl

1 M

MSD‐2

DLGE23

ethanol

HCl

1 M

MSD‐3

DLGE23

none

HCl

1 M

MSP‐1

PLGF68‐12 ethanol

HCl

2 M

MSP‐2

PLGF68‐16 ethanol

HCl

2 M

MSP‐3

PLGF68‐16 ethanol FeCl3∙6H2O

MSP‐4

PLGF68‐16 ethanol

Fe/Sib = 5

CH3COOH AA/Sic = 5

Notation: aCacid = acid concentration, bFe/Si = 5: molar ratio of FeCl3∙6H2O/TEOS in the initial reactants, cAA/Si = 5: molar ratio of CH3COOH/TEOS in the initial reactants.

190,000, 705,000, and 1,090,000 g/mol, respectively.; see Supporting Information Figure S1). Thermogravimetric (TG) analysis was performed on a TA Q50 instrument. TG profiles were recorded up to 800 oC with a heating rate of 5 oC/min in flowing nitrogen of 100 mL/min as well as in flowing mixed gas composed of air of 40 mL and nitrogen of 60 mL. Sol‐Gel Synthesis of Mesoporous Silica Samples. 1) Mesoporous silica in the presence of a DLGE23 diblock co‐ polymer (samples denoted as MSD‐1~3 in Table 2): 0.5 g of DLGE23 polymer template was dissolved completely in a mixture of 3.5 g of HCl (~37%) and 31.5 g of distilled water by stirring at 30 oC for 2 h. Next, 2.6 g of TEOS was added and stirred for about 30 min to obtain the precipitates. Then, the solid precipitate was aged at a quiescent condi‐ tion of 30 oC for 20 h, followed by additional aging at 100 o C for 24 h in a convection oven. The solid precipitate was stirred with the mixture of 54 mL of ethanol and 6 mL of HCl at 80 oC for 24 h, followed by filtration with an ade‐ quate amount of acetone and ethanol using a suction flask and membrane filter papers. Final solid samples were ob‐ tained after drying at 80 oC for 24 h in a convection oven. MSD‐2 sample (Table 2) was prepared with addition of 2 mL of ethanol and slightly smaller amount of distilled wa‐ ter at the same 1 M of HCl solution. MSD‐3 sample was pre‐ pared with larger amount of TEOS precursors at the same acidic condition with MSD‐1.


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Table 3. Physicochemical Analysis of Mesoporous Silica Samples Prepared in this Studya DKJS (nm)

Vbimodal (cm3/g)

sample

SBET (m2/g)

Vt (cm3/g)

Vmeso (cm3/g)

Vmicro (cm3/g)

large

d (nm)

a (nm)

small

large

small

MSD‐1

704

0.4534

0.3644

0.0375

2.36

8.08

0.2750

0.0699

10.70

12.35

MSD‐2

687

0.4988

0.4443

0.0250

2.88

8.90

0.3686

0.0569

11.42

13.18

MSD‐3

578

0.3817

0.3245

0.0231

2.87

7.70

0.2671

0.0526

10.70

12.35

MSP‐1

546

0.4373

0.4005

0.0254

3.25

13.87

0.2338

0.1466

17.92

n.d.

MSP‐2

698

0.6247

0.5245

0.0377

2.90

19.68

0.2668

0.2413

22.36

n.d.

MSP‐3

570

0.7387

0.6486

0.0155

5.27

12.19

0.3157

0.2725

17.57

n.d.

MSP‐4

811

0.9378

0.8707

0.0274

4.01

11.84

0.3910

0.4380

17.13

n.d.

aNotation: S BET = BET specific surface area; Vt = total pore volume measured at relative pressure (P/P0) of 0.99, Vmeso = mesopore volume calculated by integration of the PSD curve in pore diameter range of 2 nm – 50 nm, Vmicro = micropore volume calculated by integration of the PSD curve in pore diameter range of 0 – 2 nm, DKJS = pore diameters calculated at the small and large peak maximums of PSD by KJS method, Vbimodal = partial pore volumes calculated by integration of the PSD curve representing the small and large maximums, respectively, d = Bragg’s d‐spacing (= 2π/q*, q* is the q‐value at the maximum of SAXS peak), a = unit cell parameter (= 2d100/√3 for p6mm).

2) Mesoporous silica in the presence of a PLGF68‐12 pen‐ tablock copolymer (sample denoted as MSP‐1 in Table 2): 0.5 g of PLGF68‐12 polymer template was dissolved com‐ pletely in a mixture of 1 mL of ethanol, 7 g of HCl (~37%), and 27.0 g of distilled water by stirring at 30 ℃ for 2 h. Then, 2.0 g of TEOS was added and stirred for about 1 h to obtain precipitate. 3) Mesoporous silica in the presence of a PLGF68‐16 pen‐ tablock copolymer (sample denoted as MSP‐2~4 in Table 2): 0.5 g of PLGF68‐16 polymer template was dissolved completely in a mixture of 6 mL of ethanol, 11 g of HCl (~37%), and 38.0 g of distilled water by stirring at 30 ℃ for 2 h. Then, 2.1 g of TEOS was added and stirred for about 1 h to obtain precipitate. MSP‐3 and MSP‐4 samples were prepared with an ade‐ quate amount of FeCl3∙6H2O (i.e. molar ratio of Fe/TEOS = 5) and acetic acid (i.e. molar ratio of acetic acid/TEOS = 5) instead of strong acidic conditions such as 2 M HCl solu‐ tion used when preparing MSP‐2. The aging and filtration steps of synthesis methods 2) and 3) proceeded at the same procedure as done in method 1). Characterization of Mesoporous Silica Samples. The nanostructures of mesoporous silica samples were ana‐ lyzed by synchrotron small angle X‐ray scattering (SAXS), nitrogen adsorption‐desorption experiments, and trans‐ mission electron microscopy (TEM). The morphologies were analyzed by scanning electron microscopy (SEM). Chemical structure and composition was confirmed by solid state 29Si cross polarization (CP)‐MAS NMR. The SAXS experiments were performed using the 3C and 4C beamlines of Pohang radiation accelerator laboratory with synchrotron radiation (E = 10.5199 keV, λ = 1.1785 Å). The distance from sample to detector was fixed both at 1 m and 3 m to obtain peak position more exactly at very small angles. Each powder sample was placed in a sample holder made of copper alloy and both sides were protected with Kapton tape and then measured in room temperature.

Exposure time to obtain SAXS patterns was in the range of 0.1 ~ 5 s per sample. The nitrogen adsorption‐desorption isotherms were measured at ‐198 oC with a Micromeritics 2420 analyzer. The samples were pretreated for over 2 h at 500 oC in vac‐ uum to remove other gases in the sample. The BET (Brunauer‐Emmet‐Teller) specific surface area (SBET) was calculated from adsorption isotherms with relative pres‐ sures between 0.05 and 0.2. Single point pore volume (Vt) was obtained from the amount of nitrogen adsorbed at a relative pressure of 0.99. Pore size distributions (PSD) were calculated from adsorption isotherms using Barrett‐ Joyner‐Halenda (BJH) method with Kruk‐Jaroniec‐Sayari (KJS) correction for cylindrical mesopores.47 The pore vol‐ ume for bimodal pores was obtained by the integration of PSD curve for each peak. The physicochemical properties obtained by SAXS and nitrogen adsorption‐desorption were summarized in Table 3. TEM images were obtained using a FEI TECNAI G2 F30 ST transmission electron microscope operated at an accel‐ erating voltage of 200 kV. The specimens were prepared with sonication for 2 h dispersing in acetone followed by dropping onto a porous carbon film on a meshed copper grid. SEM images were obtained using a TESCAN VEGA3 in‐ strument at 20.0 kV. Solid‐state 29Si CP‐MAS NMR spectra were obtained using a Bruker AVANCE II + (400 MHz) instrument equipped with a 4 mm magic angle spinning (MAS) probe at the Seoul West Center, Korea Basic Science Institute. The ex‐ periments were performed at a spinning rate of 6 kHz, a delay time of 3 s, a contact time of 2 ms, and a radio fre‐ quency of 79.488 MHz. Chemical shifts were calibrated us‐ ing tetramethylsilane (TMS) as an internal reference.


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The Journal of Physical Chemistry RESULTS AND DISCUSSION

Characterization of Diblock and Pentablock Copoly‐ mers. A PLGA‐PEO diblock copolymer and two kinds of PLGA‐F68‐PLGA block copolymers with different block lengths were synthesized by step‐growth polymerization of cyclic D,L‐lactide and glycolide monomers. Their molecu‐ lar weights and compositions are summarized in Table 1. Molecular weight of PLGA block was controlled by the amount of D,L‐lactide and glycolide monomers to adjust its volume fraction around as 0.3 for diblock copolymers. The volume fractions of PLGA blocks in two pentablock copolymers were adjusted to be around 0.3 and 0.5, respec‐ tively. Figure 1(a) shows the chemical structures of diblock and pentablock copolymers and corresponding protons shown in 1HNMR. Figure 1(b) shows 1H NMR spectra of a DLGE23 diblock copolymer (spectrum A) and PLGF68‐12 and PLGF68‐16 pentablock copolymers (spectrum B and C). The degree of polymerizations (m, n) of PLGA block in a poly{(lactic acid)m‐co‐(glycolic acid)n}‐b‐poly(ethylene ox‐ ide)35 diblock copolymer were obtained by calculating the peak intensity ratios of methyl (f, ‐O‐CH(CH3)‐CO‐, δ = 1.45‐1.79) and methylene (d, ‐OCH2‐CO‐, δ = 4.55‐5.00) protons of lactic acid (LA) and glycolic acid (GA) repeating units, respectively.45,46,48,49 The final degree of polymeriza‐ tion for m and n was calibrated on the basis of the number of PEO unit (b, ‐O(CH2CH2O)35‐, δ = 3.50‐3.75) and meth‐ oxy group at the end of PEO chain (a, ‐O(CH2CH2O)35‐CH3, δ = 3.35).45,46,48,49 The small resonance peak at 4.20‐4.40 ppm is attributed to methylene protons (c, ‐CO‐O‐CH2‐ CH2‐O‐) in PEO block linked with ester group in LA or GA as shown in Figure 1(b‐A).45,46,48,49 The other two resonance peaks appeared at 2.50‐2.70 and 5.05‐5.45 ppm correspond to protons in hydroxyl group and C‐H bond in LA or GA units, respectively, which were used as supporting peaks to calculate the final repeating units. Through 1H NMR anal‐ ysis, the number‐average molecular weight (Mn) of DLGE23 diblock copolymer was determined as 2,138 g/mol and the weight fraction of PLGA block was obtained as 0.275, as listed in Table 1. The degree of polymerizations (m, n) of PLGA block in poly{(lactic acid)m‐co‐(glycolic acid)n}‐b‐poly(ethylene oxide)75‐b‐poly (propylene ox‐ ide)30‐b‐poly(ethylene oxide)75‐b‐poly{(lactic acid)m‐co‐ (glycolic acid)n} pentablock copolymer were also calcu‐ lated by the similar method to the aforementioned diblock copolymer. The final numbers (m, n) of repeating units were calibrated on the basis of the number of PPO unit (a, ‐OCH2‐CH(CH3)‐, δ = 1.05‐1.25) in the F68 triblock copoly‐ mer. The small peak at 4.20‐4.40 ppm was also found both in Figure 1(b‐B) and Figure 1(b‐C), which clearly indicates the chemical linkage is formed successfully between PEO block in F68 and LA or GA unit. From the 1H NMR results, each number of repeating units, Mn, and the weight frac‐ tions of PLGA blocks for two kinds of pentablock copoly‐ mers were calculated and summarized in Table 1. Gel permeation chromatography (GPC) analysis was per‐ formed to obtain the number‐ (Mn) and the weight‐average molecular weights (Mw) of diblock and pentablock copoly‐ mers. Figure 2(a) shows very narrow GPC distribution curves with elution time for DLGE23 diblock copolymer

Figure 1. (a) Molecular structures and respective positions of each H atom for diblock (PLGA‐PEO) and pentablock (PLGA‐ PEO‐PPO‐PEO‐PLGA) copolymers synthesized in this study. (b) 1H‐NMR spectra and corresponding resonance peak index for a diblock copolymer (A) and two kinds of pentablock co‐ polymers (B, C), respectively.

and PLGF68‐12 and PLGF68‐16 pentablock copolymers. The Mn of DLGE23 diblock copolymer and PLGF68‐12 and PLGF68‐16 pentablock copolymers was determined as 3,300, 10,720, and 11,520 g/mol, respectively. The Mw was obtained accordingly as 3,660, 14,090, and 17,060, respec‐ tively. Polydispersity indices (PDI) were obtained as 1.11, 1.32, and 1.48, respectively, for DLGE23, PLGF68‐12, and PLGF68‐16 block copolymers. The GPC results are summa‐ rized in Table 1. TG thermograms were used to study the thermal stability of diblock and pentablock copolymers. Figure 2(b) shows the weight percent for DLGE23, PLGF68‐12, and PLGF68‐ 16 block copolymers up to 800 in flowing nitrogen. The thermal degradation was found to be distinct two‐step transition with the first step from 160 to 260 and the second step from 320 to 400 , corresponding to the


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Figure 3. Synchrotron SAXS patterns for mesoporous silica samples prepared with diblock (a) and pentablock copolymer templates (b). Each pattern represents the following samples, respectively: (a‐A) MSD‐1, (a‐B) MSD‐2, (a‐C) MSD‐3, (b‐A) MSP‐1, (b‐B) MSP‐2, (b‐C) MSP‐3, and (b‐D) MSP‐4.

Figure 2. Gel permeation chromatography curve (a) and ther‐ mogravimetric analysis in flowing nitrogen (b) of a diblock (PLGA‐PEO) and two kinds of pentablock (PLGA‐PEO‐PPO‐ PEO‐PLGA) copolymers synthesized in this study.

thermal degradation of PLGA and PEO/PPO chains in co‐ polymers, respectively. There was no weight change up to 160 suggesting most of PLGA chains were covalently bonded to MPEG or F68 chains.49 The remaining weight percent at the end point of the first step (i.e. 300 ) were 72, 66, and 52, respectively, for DLGE23, PLGF68‐12, and PLGF68‐16 block copolymers, nearly identical to the weight percent of PEO and F68 block in each block copol‐ ymer system, which was obtained from 1H NMR spectra, as listed in Table 1. On the other hand, TG and differential TG (DTG) thermograms, obtained in flowing air, did not show distinct two‐step transition for thermal degradation as shown in Figure S2 (see supporting Figure S2). TGA ther‐ mograms strongly suggested the weight fractions of PLGA block can be obtained as the supporting data of 1H NMR analysis and PLGA blocks can be removed selectively. Characterization of Mesoporous Silicas Prepared with Diblock and Pentablock Copolymer Templates. Hydrophilic (B)‐b‐hydrophobic (A) typed diblock copoly‐ mer and CBABC typed pentablock copolymers with more hydrophobic block (C) at the chain ends, were synthesized as soft templates for preparation of mesoporous silica. The sample name and the corresponding experimental condi‐ tions used are listed in Table 2. Figure 3(a‐A, a‐B, a‐C) are SAXS patterns of MSD‐1, MSD‐2, and MSD‐3 samples, re‐ spectively, prepared in the presence of the DLGE23 diblock copolymer template and a TEOS inorganic precursor un‐ der 1 M HCl condition. MSD‐2 was prepared with increased

solubility by addition of ethanol and MSD‐3 was prepared with larger amount of a TEOS precursor compared with MSD‐1 sample. As can be seen in Figure 3(a), SAXS patterns of three MSD samples exhibited p6mm hexagonal struc‐ ture with small high‐order diffraction peaks at the posi‐ tions of √3 and √4 on the basis of Bragg’s peaks. The Bragg’s spacing (d) of each sample (i.e., dsp = 2π/q*) calculated from the first maximum peak (q*) was in the range of 10.70 – 11.42 nm. MSD‐2 sample showed the highest d‐spacing value of 11.42 nm, as indicated in Figure 3(a), while the Bragg’s peak is broader and the high‐order peaks are smaller than the other samples, suggesting that its struc‐ tural integrity is the lowest among three samples. MSD‐3 sample showed the strongest but split high‐order peaks, suggesting that the structural integrity is better than the other samples but the structural regularity is not so good. Accordingly, the unit cell parameter (a = 2d100/√3) of each sample was calculated in the range of 12.35 – 13.18 nm, as listed in Table 3. Also, it was found that MSD mesoporous silica samples prepared in the presence of a DLGE23 di‐ block copolymer template exhibited better SAXS patterns under 1 M HCl condition than the other acidic conditions (data not shown). Figure 3(b‐A) is the SAXS pattern of MSP‐1 prepared in the presence of a PLGF68‐12 pentablock copolymer template under 2 M HCl condition and Figure 3(b‐B, b‐C, and b‐D) are for MSP‐2, MSP‐3, and MSP‐4 samples, respectively, prepared using a PLGF68‐16 pen‐ tablock copolymer template at different acidic conditions. MSP‐2 sample was prepared under a strong acidic condi‐ tion of 2 M HCl solution while MSP‐3 and MSP‐4 samples were prepared with weakly acidic conditions, including molar ratios of FeCl3∙6H2O/TEOS = 5 and acetic acid/TEOS = 5, respectively. All SAXS patterns in Figure 3(b) exhibited a Bragg’s peak and the second diffracted peak at the posi‐ tion of √3 while each peak showed broader FWHM than


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Figure 5. Pore size distributions for mesoporous silica samples prepared with diblock (a) and pentablock copolymer tem‐ plates (b).

Figure 4. Nitrogen adsorption‐desorption isotherms for mes‐ oporous silica samples prepared with diblock (a) and pen‐ tablock copolymer templates (b).

typical diffraction patterns for highly ordered mesoporous structures. However, the Bragg’s spacing values (d) of each sample increased in the range of 17.13 – 22.36 nm, which is nearly twice of the typical values for mesoporous silicas prepared with diblock copolymer templates in solution. As shown in Figure 3(b), MSP‐2 sample prepared under 2M HCl solution showed the highest d‐spacing value of 22.36 nm, while MSP‐3 and MSP‐4 samples prepared under weakly acidic conditions showed the lowest d‐spacing value of 17.23 nm. It is noted that d‐spacing increased un‐ der strongly acidic condition and it decreased under weakly acidic conditions in the presence of pentablock co‐ polymer templates. The smaller d‐spacing value of MSP‐1 sample (17.92 nm) than that of MSP‐2 sample (22.36 nm) at the same acidic condition could be attributed to the length of hydrophobic block. Figure 3(b) represents the characteristic pore‐to‐pore distance can be tailored by var‐ ying the lengths of pentablock copolymers and acidic con‐ ditions. Figure 4(a,b) show the nitrogen adsorption‐desorption isotherms for the aforementioned mesoporous silica sam‐ ples prepared in the presence of diblock copolymer (Figure 4(a)) and pentablock copolymers (Figure 4(b)). As can be seen in Figure 4(a,b), the nitrogen adsorption‐desorption isotherms for all samples showed typical type IV patterns with a H2 hysteresis, indicating significant uptake of nitro‐ gen gas. The hysteresis is attributed to condensation‐evap‐ oration of nitrogen gas determined by capillary force inside mesopores. The significant hysteresis was observed in the

range of 0.4 – 0.8 of relative pressure (P/P0) for MSD sam‐ ples, as shown in Figure 4(a). The BET specific surface ar‐ eas for MSD samples were in the range of 578 – 704 m2/g and the pore volumes were in the range of 0.3817 – 0.4988 cm3/g, as listed in Table 3. MSD‐2 sample showed the high‐ est total pore volume as 0.4988 cm3/g and the mesopore volume as 0.4443 cm3/g which was estimated only in the range of 2 – 50 nm. Figure 4(b) for MSP samples indicate more broad H2 hysteresis in the range of 0.4 – 0.9 of relative pressure (P/P0) than MSD samples and significant gas up‐ take in the range of 0.75 ‐ 0.9. The BET specific surface ar‐ eas for MSP samples were in the range of 546 – 811 m2/g and the pore volumes were in the range of 0.4373 – 0.9378 cm3/g, as listed in Table 3. MSP‐4 sample showed the high‐ est total pore volume as 0.9378 cm3/g and the mesopore volume as 0.8707 cm3/g. In the case of using pentablock copolymer templates, the BET surface area and the pore volume increased at weakly acidic conditions, especially for MSP‐4 sample prepared with acetic acid. Figure 5(a,b) show the pore size distributions (PSD) for MSD (a) and MSP (b) mesoporous silica samples, respec‐ tively, which were obtained by BJH method corrected by KJS method. All PSD curves indicate the similar bimodal distributions in the mesopore region of 2 – 50 nm, as shown in Figure 5 and Table 3. Similar bimodal structure was reported in mesoporous silica samples prepared with poly(ethylene oxide)‐poly(butylene oxide)‐poly(ethylene oxide) (EO39BO47EO39) B50‐6600 triblock copolymer tem‐ plate in which the pore size distribution was adjusted by reaction temperature during sol‐gel process.17 However, the B50‐6600 templated mesoporous silicas did not show large pores greater than 10 nm and smaller pore size distri‐ butions contain micropores beyond mesopore regime.17 It is believed that hierarchical mesopore structures were at‐ tributed to kinetically‐ and sterically driven bimodal mi‐ cellar structure when using longer and more hydrophobic block copolymer chains were used in acidic solutions. Larger mesopores show smaller portion between two PSD maximum peaks for all samples except for MSP‐4 sample by comparing pore volumes obtained by integrating each peak, as listed in Table 3. The volume fraction of larger


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Figure 6. TEM images for MSD‐3 (a‐1,a‐2) and MSP‐2 (b‐1,b‐ 2,b‐3,b‐4) mesoporous silica samples prepared with diblock and pentablock copolymer templates.

micellar structure when using longer and more hydropho‐ bic block copolymer chains were used in acidic solutions. mesopores in MSP‐4 samples were calculated as 0.53 from Table 3. In the case of using a DLGE23 diblock copolymer template, MSD‐2 sample showed the largest pore diameter of 8.90 nm with smaller mesopore of 2.88 nm, as listed in Table 3. Among MSP samples, MSP‐2 showed the largest pore diameter of 19.68 nm with smaller mesopore of 2.90 nm. The pore wall thickness of MSD samples can be ob‐ tained as 4.27 – 4.65 nm by subtracting the larger pore di‐ ameter from the hexagonal unit cell length (a). The wall thickness for MSP samples was estimated in the range of 6 – 8 nm by assuming hexagonal array of mesopores. Overall physicochemical parameters for bimodal mesoporous sam‐ ples prepared in this study are summarized in Table 3. Figure 6(a, b) show TEM images for MSD‐3 and MSP‐2 samples prepared with a DLGE23 diblock and a PLGF68‐16 pentablock copolymer template, respectively. Figure 6(a‐1, a‐2) representing MSD‐3 sample exhibited highly ordered mesopores with p6mm symmetry, corroborating with SAXS patterns shown in Figure 3(a‐C). Figure 6(b‐1, b‐2, b‐ 3, and b‐4) from MSP‐2 exhibit core‐shell type of mesopo rous structures consisting of aggregated and nanometer‐ sized particles with mesopores as the core. Figure 6(b‐4) shows distinct pore size of ~ 20 nm in diameter in core and

Page 8 of 12

Figure 7. SEM images for mesoporous silica samples prepared with diblock (a‐d) and pentablock copolymer templates (e‐h): (a,b) MSD‐2, (c,d) MSD‐3, (e,f) MSP‐1, and (g,h) MSP‐2. Mag‐ nification of left‐side 4 photos (a, c, e, and g) is the same as 1,000 times and the right‐side 4 photos (b, d, f, and h) were taken as 5,000 times.

the shell thickness is ~ 29 ‐ 34 nm, as indicated in the Fig‐ ure. TEM images for MSP‐2 sample also clearly support the information about pores size and structure obtained from SAXS pattern and nitrogen adsorption‐desorption iso‐ therm. Figure 7 shows the SEM images for mesoporous silica samples prepared in this study. As seen in Figure 7(a‐d), silica particles templated from diblock copolymers, MSD‐2 and MSD‐3, exhibit similar spherical shape with diameter in the range of 3 – 8 μm. Particles prepared with pentablock copolymer templates exhibit spherical shape but smaller particle diameter, e.g. 2 ‐ 7 μm for MSP‐1 sample as seen from Figure 7(e,f), and MSP‐2 has quite a large amount of nanoparticle below 1 μm as well as spherical particles with particle diameter of 2 ‐ 7 μm, as shown in Figure 7(g,h). We further investigated the chemical nature of the mes‐ oporous silica by solid state 29Si CP‐MAS NMR. Figure 8(a,b) shows the 29Si CP‐MAS NMR spectra for mesoporous silica samples prepared with diblock (a) and pentablock copolymer templates (b), respectively. All spectra show the Q resonance peaks at ‐91, ‐101, and ‐111 ppm, corresponding to Q2 ((HO)2‐Si‐(OSi)2), Q3 ((HO)‐Si‐(OSi)3), and Q4 (Si‐ (OSi)4), respectively. These peaks are commonly found in


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The Journal of Physical Chemistry mesostructured silica in acidic aqueous solution. It is be‐ lieved that the preparation of bimodal mesoporous silica could be achieved from hierarchical micellar system in aqueous solution by adjusting the solvency of block copol‐ ymer chains and pH. Also, AB, ABA, and CBABC type block copolymers with hydrophobic B and more hydrophobic C blocks can have advantages for preparation of bimodal mesostructure due to their stronger driving force to pre‐ pare kinetically‐ and sterically‐driven multimodal micelles.

Figure 8. Solid‐state 29Si CP‐MAS NMR spectrum for mesopo‐ rous silica samples prepared with diblock (a) and pentablock copolymer templates (b).

mesoporous silica and the Q2 and Q3 peaks are the result of incomplete condensation of silica precursors during sol‐gel reaction. The condensation efficiency can be estimated from the molar fraction of (Q3 + Q4), which were in the range of 80 – 88%, as listed in Table S1 (see supporting in‐ formation Table S1). The results suggest the siloxane bonds in mesoporous materials are well formed in the presence of diblock and pentablock copolymer templates. CONCLUSIONS A PLGA‐PEO diblock (Mn ~ 2,100 Da, wPEO ~ 0.73) and two PLGA‐PEO‐PPO‐PEO‐PLGA pentablock copolymers (Mn ~ 12,700 Da, wPPO+PEO ~ 0.65; Mn ~ 16,100 Da, wPPO+PEO ~ 0.52) were successfully synthesized through a facile step‐growth polymerization method. 1H‐NMR, GPC, and TGA measure‐ ments confirmed the chemical structure and molecular weight of the synthesized block copolymers. Mesoporous silica samples were synthesized in the presence of diblock and pentablock copolymer templates under acidic aqueous conditions. For all the samples, bimodal mesoporous silica samples were obtained by using each single copolymer template, which were confirmed by PSD curves calculated by BJH and KJS methods using nitrogen adsorption branches. Hexagonally (p6mm) ordered mesoporous sili‐ cas were obtained from the diblock copolymer template with bimodal pore size distributions in the range of 2 – 9 nm. In the case of using pentablock copolymer templates, core‐shell type mesoporous silicas were prepared, which also exhibited the bimodal pore size distribution contain‐ ing small pores with diameters around 2.90 – 5.27 nm and large pore diameters up to 20 nm surrounded with thick shell of 34 nm. It is noteworthy that bimodal mesoporous silicas with large pores greater than 10 nm were obtained without using additives even in the weakly acidic condi‐ tions including iron chloride and acetic acid. The pore size increased by using longer pentablock copolymer template and strong acidic conditions. This report clearly demon‐ strated a new fabrication method for hierarchically

The bimodal mesoporous silica can be used as specific ad‐ sorbents for increasing diffusion paths and amount of ad‐ sorbed molecules inside the pores. Also, core‐shell type mesoporous silica materials can be used as support mate‐ rials for controlled release of guest molecules. Further‐ more, it is expected that mesoporous silicas with large mesopores can host larger molecules and the large mole‐ cules can be fixed and perform any chemical reactions in‐ side the mesopores. ASSOCIATED CONTENT Supporting Information. GPC calibration fit using polysty‐ rene standard samples, TGA and DTG thermograms for block copolymers in flowing air, and NMR peak integration for mes‐ oporous materials prepared in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E.‐B. Cho (E‐mail: [email protected], Tel: +82‐2‐970‐ 6729) and S. Yang (E‐mail: [email protected], Tel: +1‐215‐898‐9645). Author Contributions The manuscript was written through E.‐B. Cho and S. Yang’s main contributions. Experimental works were orga‐ nized by E.‐B. Cho and conducted by D. Lee and E. Choi. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT E.‐B. Cho acknowledges supports under the Basic Science Research Program through the National Research Founda‐ tion of Korea funded by the Ministry of Education (NRF‐ 2014R1A1A2059947 and NRF‐2017R1D1A1B03027982). Ex‐ periments at PLS‐II were supported in part by MSICT and POSTECH. REFERENCES (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Syn‐ thesized by a Liquid‐Crystal Template Mechanism. Nature 1992, 359, 710‐712. (2) Beck, J. S.; Vartuli, J. C.; Roth. W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; et al. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Template. J. Am. Chem. Soc. 1992, 114, 10834‐10843.


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