Grafting PMMA Brushes from α-Alumina Nanoparticles via SI-ATRP

Feb 22, 2016 - Hye Sung Kim , Young Ju Son , Wei Mao , Kam W. Leong , and Hyuk Sang Yoo. Nano Letters 2018 18 (1), 314-325. Abstract | Full Text HTML ...
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Grafting PMMA Brushes from α‑Alumina Nanoparticles via SI-ATRP Amir Khabibullin,† Karan Bhangaonkar,† Clare Mahoney,‡ Zhao Lu,‡ Michael Schmitt,‡ Ali Kemal Sekizkardes,§ Michael R. Bockstaller,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, Pennsylvania 15213, United States Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania, 15213, United States § National Energy and Technology Lab (NETL), U.S. Department of Energy, 626 Cochrans Mill Rd, Pittsburgh, Pennsylvania 15129, United States ‡

ABSTRACT: Alumina nanoparticles are widely used as nanofillers for polymer nanocomposites. Among several different polymorphs of alumina, α-alumina has the most desirable combination of physical properties. Hence, the attachment of polymer chains to α-alumina to enhance compatibility in polymeric matrixes is an important goal. However, the chemical inertness and low concentration of surface hydroxyl groups have rendered polymer modification of α-alumina a long-standing challenge. Herein, we report that activation of α-alumina in concentrated or molten NaOH as well as in molten K2S2O7 increased polymer graft density up to 50%, thereby facilitating the synthesis of α-alumina brush particles with uniform grafting density of 0.05 nm−2 that are readily miscible or dispersible in organic solvents or in chemically compatible polymeric hosts. KEYWORDS: SI-ATRP, hybrid materials, α-alumina, polymer brushes, poly(methyl methacrylate), grafting density

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tion, as well as brush grafting density.2,21 Another important advantage of SI-ATRP is its applicability to various substrate surface geometries (i.e., flat surfaces, nanoparticles, inside pores, etc.).2,21,27,28 and various surface compositions, including metals, metal oxides, silicon, organic polymers, natural products, and so on.24,25 In order to introduce polymer brushes via SI-ATRP onto a specific surface, the substrate surface has to be initially modified with a suitable polymerization initiator.24,25 In many cases, the initiator is anchored to the substrate via reaction with surface hydroxyl groups. The combination of thermodynamic stability and surface passivation of α-alumina has rendered its effective surface modification a long-standing challenge that presents a major barrier in realizing α-alumina-based polymer matrix composites. There are only few studies on introducing polymer brushes onto the surface of α-alumina.29,30 Other alumina polymorphs, such as γ- and δ-forms, are more reactive and were efficiently modified with polymer brushes to enable stable nanocomposite materials via SI-ATRP.19,20,31−34 In this contribution, we evaluated several strategies to activate the surface of α-alumina particles. Surface activation was carried out in concentrated or even molten NaOH as well as K2S2O7 to increase the number of surface hydroxyl groups. The activated α-alumina particles were modified with initiators to tether poly(methyl methacrylate) (PMMA) chains via SI-

INTRODUCTION Polymer nanocomposites are widely studied because of their potential utility to modify the properties of polymeric matrixes to meet the requirements for various applications.1−4 Typically a nanocomposite embodies a polymer matrix reinforced with inorganic fillers, such as silica,5−7 metal oxides,1,8−10 clay,11 or metal nanoparticles.8,12 Alumina has attracted particular attention as a filler to augment the properties of polymer materials due to its high refractive index, its high thermal conductivity, and the absence of light absorption in the visible range.13−16 Among the various polymorphs of alumina, the αform, also known as corundum, has the highest density of 4.02 g/cm3 and the highest thermal conductivity of 35 W/K·m.16 Hence, it is of particular interest as an additive to improve thermal transport properties of polymer nanocomposites. A common strategy to enable a (stable) dispersion of particle fillers within polymeric hosts involves the tethering of polymeric chains with a composition compatible with the matrix to the surface of particle fillers.7,17−20 Surface-initiated atom transfer radical polymerization (SIATRP) is one the most robust and widely used techniques for grafting a broad range of polymer brushes from various solid surfaces in a controlled manner.2,21−23 SI-ATRP is of particular utility due to its simple experimental setup with readily available initiators and catalysts that can be used in the presence of a range of solvents under a broad spectrum of reaction conditions.24−26 SI-ATRP allows for precise macromolecular engineering of the grafted polymer brushes with control over all parameters, including brush length, molecular weight distribu© 2016 American Chemical Society

Received: December 17, 2015 Accepted: February 10, 2016 Published: February 22, 2016 5458

DOI: 10.1021/acsami.5b12311 ACS Appl. Mater. Interfaces 2016, 8, 5458−5465

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under vacuum for 12 h. Nitrogen isotherms of a sample were collected by the gas sorption analyzer at 77 K, which was controlled using a liquid nitrogen bath. The BET surface area of a sample was calculated from the slope at relative pressure (P/Po) points of nitrogen between 0.03 and 0.30 with a correlation coefficient (r) of the linear regression around 0.99. Grafting PMMA Brushes from Alumina Surface. The poly(methyl methacrylate) brushes were grafted from the surface of initiator-modified alumina nanoparticles using the following procedure: A clean dry Schlenk flask (10 mL) was charged with 1.5 mg CuCl2, 15 μL PMDETA, 3 μL EBiB, 5 mL anisole, 0.2 mL DMF, initiator-modified alumina nanoparticles (typically 0.15 g), and 4.5 mL methyl methacrylate (MMA) monomer. The flask was sealed and the reaction mixture was bubbled with N2 for 20 min. The reaction mixture was then frozen by placing the Schlenk flask in liquid nitrogen bath, and 4 mg of CuCl powder was added to the flask on top of frozen solution. The flask was then sealed and degassed by nitrogen bubbling for an additional 10 min. The flask was immersed in an oil bath, and the grafting from polymerization was carried out at constant temperature of 70 °C for various periods of time. The reaction mixture was exposed to air to oxidize Cu catalyst and stop the reaction. The polymer grafted particles were separated by centrifugation. The supernatant was passed through alumina column and injected into GPC to measure molecular weight and molecular weight distribution of unattached polymer in solution. The centrifuged particles were washed with THF via sonication and centrifugation to remove all traces of polymer and organic impurities. Then, the particles were redispersed in THF and precipitated by addition to methanol to remove copper salts. After drying in air, the solid particles were characterized by TGA and TEM. Preparation of Dispersions of Pristine and PolymerModified Particles in THF. Three dispersions of alumina particles in THF were prepared at the concentration 10 mg/mL in 1 mL glass vials. The first solution contained pristine α-alumina particles, the second contained PMMA-modified nonactivated particles, and the third had PMMA-modified alumina particles activated in molten NaOH for 60 min. The vials were capped and sonicated in bath sonicator for 30 min. Then, the vials were placed in vibration-free area, and the photographs of dispersions were taken within specified time intervals. Preparation of Dispersions of Pristine and PolymerModified Particles in PMMA Matrix. Three dispersions of alumina particles and linear PMMA in THF were prepared as follows: 1 mg of alumina particles and 9 mg of linear PMMA (Mn = 33 000, Mw/Mn= 1.08) were dissolved in 10 mL of THF in 20 mL glass vials. The first solution contained pristine α-alumina particles, the second contained PMMA-modified nonactivated particles, and the third had PMMAmodified alumina particles activated in molten K2S2O7 for 180 min. The vials were capped, and the solutions were stirred at room temperature for 24 h. Then, the samples were drop-casted onto copper grids for TEM microscopy. After solvent evaporation, the dispersions of alumina particles in PMMA matrix (10 wt % inorganic content) were formed on the grid surface. Calculation of Average Particle Size based on TEM Images. The average sizes of alumina particle dispersed in PMMA matrix (90 wt %) were calculated as follows: 10 representative TEM images of each sample were processed using ImageJ software. The color threshold was applied to isolate alumina particles from the background polymer matrix on the images. The area of each particle on the image was obtained by the Particle Analysis Function in ImageJ. Assuming all the particles were circles, the size of each particle was calculated. The number-average particle size was calculated on the basis of all particle size values obtained from 10 images for each sample.

ATRP. The effect of activating agent and activation time on the grafting density of polymer brushes grown from the alumina surface was studied The dispersibility of pristine, PMMAmodified nonactivated and PMMA-modified activated alumina particles both in a solvent (THF) and a polymer matrix (PMMA) was compared.

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EXPERIMENTAL SECTION

Materials. Methyl methacrylate (MMA, 99%) was purchased from Aldrich and passed through a basic alumina column to remove inhibitor prior to use. Triethylamine (TEA), copper(II) chloride (CuCl2, 99.9%), copper(I) chloride (CuCl, 99%) 2-bromoisobutyryl bromide (2-BiB, 98%), ethyl α-bromoisobutyrate (EBiB, 98%), potassium disulfate (K2S2O7, 99%), 4-(dimethylamino)pyridine (DMAP, 99%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%) were purchased from Aldrich and used as received. Sodium hydroxide pellets (99%) were purchased from Fisher Scientific. A colloidal dispersion of γ−δ-alumina nanoparticles (30% in mineral spirit, average particle diameter 20 nm) was purchased from Alfa Aesar. α-Alumina nanoparticles, a powder with average particle size of 80 nm, were purchased from U.S. Research Nanoparticles, Inc. Instrumentation. Monomer conversion was measured using 1H NMR in CDCl3, in a Bruker Avance 300 MHz spectrometer at room temperature. Molecular weight and molecular weight distribution were determined by GPC in THF as eluent with flow rate 1 mL/min, using PMMA standards. The organic fraction in polymer-modified nanoparticles was measured using TA Instruments TGA 2950 thermogravimetric analyzer. Nanoparticles were dispersed in solvents using Misonix S-4000 probe sonicator and Branson 1800 bath sonicator. Nanoparticles were imaged using transmission electron microscopy (TEM) using a JEOL 2000 EX electron microscope operated at 200 kV. Images were acquired using a Gatan Orius SC600 high-resolution camera. Base-resistant stainless steel crucibles were purchased from Thomas Scientific. Nitrogen sorption measurements and surface area calculations were performed using a high speed gas sorption analyzer (Quantachrome Instruments/NOVA) and UHP grade adsorbates. The surface of alumina particles was studied using XPS (ESCALAB 250Xi X-ray Photoelectron Spectrometer Microprobe, with a 900 mm spot size) and FTIR spectroscopy (PerkinElmer Frontier FTIR spectrometer with germanium crystal attenuated total reflectance (ATR) attachment). Alumina Surface Activation and Initiator Anchoring. Alumina particles were activated by heating 2 g of particles in 100 mL of 2 M NaOH solution or 48 wt % HF solution at 70 °C for 12 h. The particles were then collected via centrifugation and washed with distilled water via three repetitive centrifugation and sonication cycles, and then left to dry in air overnight. Activation of 1 g of particles by 15 mL of saturated (ca. 20 M) aqueous solution of NaOH was carried out in stainless steel cups at 70 °C for 12 h. Activation of 1 g of particles in 10 g of molten NaOH or molten K2S2O7 were each carried out in stainless steel cups at 500 °C for 15, 30, 60, and 180 min. The particles were then suspended in water, collected via centrifugation, and washed via repetitive centrifugation and sonication cycles in 2 M HCl solution, 2 M NaOH solution and distilled water (three times). Then, the particles were left to dry in air overnight. The surface area of pristine and activated alumina particles was characterized using BET method. The particles were then modified with 2-BiB to introduce ATRP initiating sites onto the surface. The modification was carried out by stirring 1 g of particles in 50 mL of dry THF in the presence of 10 mL of TEA, 5 mL of 2-BiB and catalytic amounts of DMAP at room temperature for 12 h. The particles were collected via centrifugation, washed with THF and methanol (three times) via repetitive centrifugation and sonication cycles, and left in air overnight to dry. Measurement of Surface Area of Pristine and Activated Alumina Particles. The surface area was measured using Brunauer− Emmet−Teller (BET) method for pristine alumina particles and particles activated in molten NaOH, as well as particles activated in molten K2S2O7. Prior to porosity measurements, samples (55−170 mg) were placed in a 9 mm bulb cell and degassed at 300 o C and

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RESULTS AND DISCUSSION

Activation of Alumina Surface and Grafting PMMA Brushes. The surface of α-alumina nanoparticles was activated using various agents, including dilute and concentrated solutions of NaOH, molten NaOH and K2S2O7. After alumina 5459

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Scheme 1. Surface Activation with (a) Sodium Hydroxide and (b) Potassium Disulfate and Grafting PMMA Brushes from αAlumina Nanoparticles

Figure 1. (Left) Representative TGA curves of (1) pristine α-alumina nanoparticles, (2) initiator-modified alumina nanoparticles activated in molten K2S2O7, (3) initiator-modified alumina nanoparticles activated in molten NaOH, (4) PMMA-modified alumina without any activation, (5) PMMAmodified alumina activated in molten NaOH for 15 min, and (6) PMMA-modified alumina activated in molten K2S2O7 for 15 min. (Right) GPC traces of free polymer initiated by sacrificial initiator in solution concurrently with polymer grafted from the surface of (blue) alumina nanoparticles with no surface activation, (black) alumina nanoparticles activated in molten NaOH for 15 min, and (red) alumina nanoparticles activated in molten K2S2O7 for 15 min.

Table 1. PMMA-Modified Alumina Nanoparticles Activated by Different Agentsa activation

surface area (BET) (m2/g)

monomer conv. %

wt % organic

Mnb

Dc

graft density (nm−2)

2 M NaOH, 70 °C HF, 48%, 70 °C NaOH, conc 70 °C NaOH, melt, 15 min K2S2O7, melt, 15 min

10.3 15.44 18.1 18.0 19.6 18.6

25 29 25 35 30 32

2.5 4.9 3.5 7.6 6.3 7.1

45 600 56 500 40 500 62 000 46 000 54 700

1.06 1.13 1.08 1.12 1.14 1.1

0.032 0.032 0.028 0.041 0.042 0.042

Reaction conditions: [Al2O3−Br]/[EBiB]/[MMA]/[CuCl2]/[CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C. Reaction time 3 h. bGPC number-average molecular weight of polymer. cDispersity (Mw/Mn). a

aggregation of nonactivated alumina particles. The surface area for activated alumina was in the range 15−20 m2/g, with the average value 18 nm2/g, which is in good agreement with predicted value of 18.75 m2/g. Activation should break the particle aggregates and increase the surface area. Grafting Density of PMMA Brushes on the Alumina Surface. The grafting density of PMMA brushes on alumina surface was calculated using polymer molecular weight obtained from GPC and the organic fraction in polymer-modified particles obtained by TGA. Calculation of grafting densities was performed based on the assumption of similar molecular weights of sacrificial and tethered chains,2,35,36 spherical particle shape and uniform particle size. The latter was set equal to the average particle diameter that was determined by electron imaging (information provided by vendor), that is d = 80 nm for α-alumina and d = 20 nm for γ−δ-alumina. The mass density of α- and γ−δ-alumina was set to 4.02 and 3.5 g/cm3,

activation and surface modification with initiator, PMMA brushes were grafted from the surface of the particles (Scheme 1). Successful grafting from polymerization was confirmed using TGA and GPC data. Representative TGA curves and GPC plots are shown in Figure 1. To calculate the grafting density, we initially estimated the surface area of α-alumina assuming uniform spherical shape with a diameter of 80 nm, according to the supplier U.S. Research Nanomaterials, Inc. The surface area of α-alumina calculated this way was 18.75 m2/g. However, the TEM images (cf. infra) indicated that α-alumina particles have irregular shapes and broad size distribution. Thus, the surface area of both pristine and activated α-alumina particles was directly measured using BET method. The surface area of nonactivated alumina was 10 m2/g, which is smaller than the calculated value assuming uniform spherical shape. This can be due to 5460

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ACS Applied Materials & Interfaces respectively. The grafting density was subsequently calculated as ρS = n/S, where n denoted the average number of chains tethered to one particle (calculated as n = m/M, where m denotes the mass of polymer per particle and M the GPC molecular weight of chains), and S = πd2 is the average surface area per particle. The result was in excellent agreement with values determined on the basis of BET measurements as ρS = ntotal/SBET where ntotal = mtotal/M is the total number of chains contained in per weight of sample and SBET is the total accessible area as determined by BET analysis. The resulting grafting densities are shown in Table 1. The grafting density of PMMA brushes as a function of activation conditions is plotted in Figure 2. The grafting density

3K 2S2 O7 + Al 2O3 → 3K 2SO4 + Al 2(SO4 )3

The resulting alumina particles with aluminum sulfate on the surface underwent hydrolysis under basic conditions (2 M NaOH) to form surface hydroxyl groups. The BET analysis revealed for all three samples near constant values of surface area, thus suggesting that the activation of alumina particles only altered few layers of the outer atomic layers. Unfortunately, quantitative information on the surface composition and in particular the concentration of hydroxyl groups on the surface could not be obtained by standard spectroscopic techniques such as attenuated total reflectance FTIR or XPS due to low signal-to-noise ratio. We interpret the difficulties to be a consequence of the nanoparticulate character of the materials that is generally found to render spectroscopic analysis of surface chemistries difficult. Therefore, our conclusion of the increased number of surface hydroxyl groups rests on the measurement of the grafting density of surfacetethered chains. We note that the characterization of surface functional groups by polymer tethering (and the associated ‘compositional amplification’ of the surface chemical groups) requires the quantitative separation of polymer tethered particles from potential sacrificial homopolymer. Because the latter can be accomplished by a variety of established methods such as repetitive centrifugation (used here) or recrystallization, the surface-amplification process could be a more generally useful technique to characterize surface functionalities in particulate materials. Grafting Density As a Function of Activation Time. The successful activation with molten base or potassium disulfate was carried out for only 15 min, but it led to a 30% increase in grafting density of PMMA brushes. Therefore, it was necessary to determine how the grafting density changed upon increasing the time of activation in molten NaOH or K2S2O7. Samples of α-alumina were activated with molten NaOH for 15, 30, 60, and 180 min respectively, and later, PMMA brushes were grafted from the surface of the activated nanoparticles. The results are summarized in Table 2, and the grafting density as a function of activation time is plotted in Figure 3. The ATRP reaction was carried out for 3 h.

Figure 2. Grafting density of PMMA brushes on α-alumina surface as a function of surface activation by various agents. Activation with molten NaOH and molten K2S2O7 was carried out for 15 min, activation with other agents was carried out for 12 h. Reaction conditions: [Al2O3−Br]/[EBiB]/[MMA]/[CuCl2]/[CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C.

values in the plot can be divided into two clearly identifiable groups. The first group consists of particles with low grafting density, with an average value of ca. 0.03 chain/nm−2. This could be due to low chemical activity of the surface of αalumina particles. The alumina activation in dilute base or concentrated acid significantly increased the surface area but did not increase the number of surface hydroxyl groups. The second group in the plot represents particles with grafting density values ca. 30% times higher than that of the first group. Activation of α-alumina under harsh conditions: saturated base, molten base, or salt at high temperature lead to a significant increase in the concentration of surface hydroxyl groups along with an increase in surface area. Presumably, the larger number of hydroxyl groups on the particle surface resulted from partial dissolution of the alumina surface under harsh conditions. Alumina is an amphoteric substance. It is hypothesized that sodium hydroxide acts as a base, forming sodium aluminates: hydroxyaluminates in the presence of water (saturated NaOH) and aluminates in molten NaOH.

Table 2. PMMA-Modified Alumina Nanoparticles Activated in Molten NaOH for 15, 30, 60, and 180 mina activation time (min)

monomer conv. %

wt % org

15 30 60 180

30 28 19 28

6.3 7.4 11 8.8

Mnb 46 46 98 45

000 500 000 500

Dc

graft density (nm−2)

1.14 1.11 1.17 1.06

0.042 0.051 0.040 0.057

a Reaction conditions: [Al 2 O 3 −Br]/[EBiB]/[MMA]/[CuCl 2 ]/ [CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C. Reaction time 3 h. bGPC number-average molecular weight of polymer. cDispersity (Mw/Mn).

As shown in Figure 3, the grafting density of the PMMA brushes increased slightly as a result of increasing activation time. This behavior could be explained by the assumption that liquid sodium hydroxide slowly dissolves the crystalline αalumina layer by layer. The dissolved alumina was converted to sodium aluminate, while still leaving hydroxyl groups on the substrate surface. However, the average number of available hydroxyl groups on the surface at a given time did increase as a function of activation time. In other words, the longer exposure

6NaOH + Al 2O3 + 3H 2O → 2Na3Al(OH)6 2NaOH + Al 2O3 → 2NaAlO2 + H 2O

The resulting alumina particles with surface sodium aluminate moieties underwent hydrolysis under acidic conditions (2 M HCl) to form surface hydroxyl groups. Molten potassium disulfate acts as a strong acid and reacts with alumina forming aluminum sulfate.37 5461

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Figure 3. Grafting density of PMMA brushes on the surface of α-alumina as a function of activation time in molten NaOH (left) and K2S2O7 (right). The linear fit trend line is shown in red. Reaction conditions: [Al2O3−Br]/[EBiB]/[MMA]/[CuCl2]/[CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C. Reaction time 3 h.

to liquid NaOH resulted in greater dissolution of alumina and in formation of soluble sodium aluminates. At the same time, it created more hydroxyl groups on the surface, and negligibly reduced particle size. Similar behavior was observed for alumina activation in molten potassium disulfate, Table 3. The grafting density of

Table 4. PMMA-Modified Alumina Nanoparticles Activated in Molten K2S2O7 for 180 mina

Table 3. PMMA-Modified Alumina Nanoparticles Activated in Molten K2S2O7 for 15, 30, 60, and 180 mina activation time (min)

monomer conv. %

wt % org

15 30 60 180

32 30 25 29

7.1 5.5 6 6

Mnb 54 45 39 40

700 000 000 000

Dc

graft density (nm−2)

1.10 1.11 1.07 1.09

0.042 0.042 0.053 0.051

reaction time (h)

monomer conv. %

wt % organic

Mnb

Dc

graft density (nm−2)

2 3 4 6 8

22.9 29 35.8 42 40.7

4.8 6 7.3 7.3 10.3

32 400 40 000 48 300 89 000 100 100

1.06 1.09 1.08 1.18 1.20

0.048 0.051 0.0485 0.027 0.033

Reaction conditions: [Al 2 O 3 −Br]/[EBiB]/[MMA]/[CuCl 2 ]/ [CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C. bGPC number-average molecular weight. c Dispersity (Mw/Mn). a

alumina surface remained the same after 6 h or longer. However, with the increase in viscosity and termination, some of the PMMA chains on the alumina surface stopped growing, while others continued to grow. This created polymer-modified particles with bimodal or multimodal molecular weight distribution.38 When grafting density of multimodal polymer brushes was estimated using the method, detailed above, the calculated value was smaller. Thus, the optimal reaction time to introduce uniform polymer brushes for the given conditions was 3−4 h. TEM Images of PMMA-Modified Pristine and Activated Alumina Samples. The average size of α-alumina nanoparticles remained the same before and after activation under such harsh conditions. In other words, longer activation did not lead to particle dissolution and a decrease in size. As can be seen in the TEM image (Figure 4A), the pristine αalumina particles had irregular shapes, and their actual size was greater than the 80 nm, claimed by the supplier and the size distribution was broad. Given the broad size distribution of pristine particles and their irregular shapes, the change in size after activation was difficult to detect by DLS. Furthermore, the pristine particles agglomerated to form large aggregates, making it difficult to distinguish individual particles. TEM images show α-alumina nanoparticles activated in molten K2S2O7 for 15 min (Figure 4B) and 30 min (Figure 4C), as well as the α-alumina nanoparticles activated in molten NaOH for 60 min (Figure 4D). All are similar in size and shape to the pristine particles (Figure 4A). Nevertheless, it can be concluded from the TEM images that activation under harsh conditions did not lead to any significant dissolution that could have changed the particle shape or size. At the same time, activation under such conditions and grafting polymer brushes

Reaction conditions: [Al 2 O 3 −Br]/[EBiB]/[MMA]/[CuCl 2 ]/ [CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C. Reaction time 3 h. bGPC number-average molecular weight of polymer. cDispersity (Mw/Mn). a

PMMA brushes on the alumina surface as a function of time of activation in molten K2S2O7 is plotted in Figure 3 (right). The plot shows that the grafting density of PMMA also increased slightly upon longer exposure to molten K2S2O7. Presumably, molten potassium disulfate reacted with α-alumina in a manner similar to molten sodium hydroxide and formed small amount of alumina sulfate Al 2(SO4 )3 in solution and on the nanoparticles surface. The sulfate moieties on the surface were later converted to hydroxyl groups by treatment with NaOH and HCl solutions. The average number of hydroxyl groups on the surface of alumina particles increased slightly, thus the grafting density showed a small increase with greater activation time. Grafting Density As a Function of Reaction Time. The grafting of PMMA brushes from alumina surface, activated under harsh conditions, was carried out for various periods of time. The results are summarized in Table 4. The PMMA grafting density remained approximately same in the reaction carried out for 2, 3, and 4 h. However, a significant increase in viscosity happened when ATRP was carried out for 6 h and longer. This resulted in termination reactions and a broadening of molecular weight distribution. As a result, the grafting density, calculated by our method for PMMA brushes, appeared to be smaller after 6 h than after only 2−4 h. However, the real grafting density of PMMA brushes did not change. In other words, the number of PMMA chains on 5462

DOI: 10.1021/acsami.5b12311 ACS Appl. Mater. Interfaces 2016, 8, 5458−5465

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ACS Applied Materials & Interfaces

nonactivated particles formed a sediment from THF dispersion within 3 h. The dispersion of particles activated in molten NaOH was stable up to 24 h. The activated alumina particles formed smaller agglomerates, and more densely grafted polymer brushes provided better particle stability in solution. Dispersion of PMMA-Modified Alumina Particles in PMMA Matrix. The polymer-modified alumina particles activated under harsh conditions were also better dispersed in PMMA matrix in comparison to pristine alumina particles (Figure 6). In the images of the particle dispersions, the polymer matrix can be seen as dark gray background. The circular light “bubbles” in the background originated from solvent evaporation and PMMA matrix drying during sample preparation. The TEM images show that PMMA-modified alumina particles, both nonactivated and activated in molten K2S2O7 for 180 min, formed significantly smaller aggregates inside the polymer matrix. Although, activation under harsh conditions did not completely break all the agglomerates, the PMMA-modified activated alumina particles appear to be better dispersed throughout the matrix when compared to pristine alumina particles or nonactivated PMMA-modified particles. To determine the average particle size, we processed the TEM images of pristine alumina particles, PMMA-modified nonactivated alumina, and PMMA-modified alumina activated in molten K2S2O7 for 180 min using ImageJ software. The 10 representative images for each sample were chosen and the particles were assumed to be spherical. The results are summarized in Table 5. The 10% weight fraction of α-alumina in a PMMA matrix corresponds to ca. 3% volume fraction. In thin films, the area fraction of components represents the volume fraction in bulk composites. The area fractions of alumina in the TEM images were in good agreement with the expected value. Thus, the alumina content in the image represented the actual alumina content in the sample. The pristine alumina particles formed the largest aggregates in PMMA matrixca. 400 nm in diameter. This was expected due to surface incompatibility between pristine alumina and PMMA. PMMA-modified nonactivated alumina formed significantly smaller aggregates −120 nm. Activation under harsh condition further reduced the average particle size to 110 nm. This result is in a good agreement with the THF dispersion behavior. Alumina activation under harsh conditions reduced the agglomerate size and ensured better dispersion of particles both in a solvent and in a polymer matrix. Grafting PMMA Brushes from γ−δ-Alumina Nanoparticles. α-Alumina required aggressive surface treatment in order to increase the particles surface area and the grafting density of polymer brushes. However, the other forms of alumina, such as γ−δ-alumina already have more reactive surfaces. The average diameter of the γ−δ-alumina particle was 20 nm according to the supplier. However, as shown in TEM images in Figure 7A, the pristine γ−δ-alumina particle size was not very uniform, although the particles had a regular spherical shape. PMMA-modified γ−δ-alumina nanoparticles were more dispersible, compared to pristine particles (Figure 7B). The size and shape of the particles remain unchanged. Under the same reaction conditions, PMMA brushes were grafted from the surface of γ−δ-alumina nanoparticles without any preliminary surface treatment, and a grafting density of 0.05 chains/nm−2 was obtained. The higher grafting density of PMMA brushes on the surface of alumina was possible because of the larger number of available hydroxyl groups on the surface of γ−δ-

Figure 4. TEM images of (A) PMMA-modified pristine α-alumina nanoparticles and (B) the PMMA-modified α-alumina nanoparticles activated in molten K2S2O7 for 15 min, (C) the PMMA-modified αalumina nanoparticles activated in molten K2S2O7 for 30 min, (D) and the PMMA-modified α-alumina nanoparticles activated in molten NaOH for 60 min. The scale bars are 200 nm in all pictures. Reaction conditions: [Al2O3−Br]/[EBiB]/[MMA]/[CuCl2]/[CuCl]/[PMDETA] = 1/1/1000/0.3/1/2 in anisole (50 vol %), DMF (2 vol %). T = 60 °C.

from the particles reduced the particle agglomeration. This result is in good agreement with 2-fold increase of surface area determined by BET. The grafting density in the range of 0.04−0.05 chains/nm−2 is lower than grafting density from the surface of silica nanoparticles by SI-ATRP.2 However, it is comparable to the grafting density of brushes grafted from more active γ−δalumina (0.05−0.1 chains/nm−2)19,34 and to other inorganic substrates, such as magnesium hydroxide (0.01 chains/nm−2).39 Nevertheless, up to a 50% increase of the grafting density was achieved upon alumina activation under harsh conditions. Stability of Alumina Particle Dispersion in THF. PMMA-modified alumina particles activated under harsh conditions formed more stable dispersions in THF, compared to pristine and PMMA-modified nonactivated particles (Figure 5). Indeed, the majority of pristine and PMMA-modified

Figure 5. Stability of alumina particles dispersed in THF as a function of time. (1) pristine α-alumina particles, (2) PMMA-modified nonactivated alumina particles, and (3) PMMA-modified alumina particles, activated in molten NaOH for 60 min. 5463

DOI: 10.1021/acsami.5b12311 ACS Appl. Mater. Interfaces 2016, 8, 5458−5465

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ACS Applied Materials & Interfaces

Figure 6. Representative original and processed TEM images of alumina particles dispersed in PMMA matrix (10 wt % alumina in matrix). Pristine α-alumina particles: (A) original and (D) processed. PMMA-modified nonactivated alumina particles: (B) original and (E) processed. PMMAmodified alumina particles, activated in molten K2S2O7 for 180 min: (C) original and (F) processed.

α-alumina by ca. 50%, as compared to nonactivated particles or α-alumina particles activated under milder conditions. Increasing the grafting density of polymer brushes proves important for the use of α-alumina particles in various applications. The higher grafting density should prevent aggregation and can provide better dispersion of particles in specific polymer matrixes. The γ−δ-alumina nanoparticles are more reactive, as compared to α-alumina, and PMMA brushes can be grafted from their surface with a density of 0.05 chains/nm−2 without any surface pretreatment.

Table 5. Average Particle Size and Area Fraction on the TEM Image for Pristine, PMMA-Modified Non-activated, PMMA-Modified Activated Alumina in PMMA Matrixa sample description pristine PMMA-modified nonactivated PMMA-modified activated

av particle diameter (nm)

av particle area fraction on the image (%)

392 120

4.2 5.2

110

4.1

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a

All samples contained 10 wt % particles in PMMA matrix (90 wt %, Mn 33 000, D 1.08).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (NSF DMR-1501324) and the U.S. Department of Energy (DoE EE0006702).

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Figure 7. TEM image of (A) pristine and (B) PMMA-modified γ−δalumina nanoparticles. Scale bar is 200 nm.

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CONCLUSIONS The surface of α-alumina particles, the least reactive of all alumina forms, was activated via treatment with molten NaOH or K2S2O7, to enhance the number of surface hydroxyl groups. This led to an increase of the polymer content and of the grafting density of PMMA brushes grafted from the surface of 5464

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