Matrix-free Particle Brush System with Bimodal Molecular Weight

Article pubs.acs.org/Macromolecules

Matrix-free Particle Brush System with Bimodal Molecular Weight Distribution Prepared by SI-ATRP Jiajun Yan,†,§ Tyler Kristufek,†,⊥,§ Michael Schmitt,‡ Zongyu Wang,† Guojun Xie,† Alei Dang,‡,∥ Chin Ming Hui,† Joanna Pietrasik,†,∇ Michael R. Bockstaller,*,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry and ‡Department of Materials Science & Engineering, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ⊥ Department of Chemical Engineering, University of Pittsburgh, Benedum Hall, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States ∥ School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China ∇ Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland ABSTRACT: The modification of the surface of nanoparticles with polymeric chains is ubiquitously used to engineer the physicochemical properties of nanoparticle fillers and to enable new material technologies based on polymer hybrid materials with controlled microstructure. The tethering of particles with polymeric chains of distinct (high and low) degree of polymerization (so-called “bimodal polymer grafts”) has emerged as a particularly interesting strategy to combine the synergistic benefits of dense and sparse polymer grafts (i.e., good control of particle interactions facilitated by densely grafted polymer chains with the high inorganic content characteristic for sparsely grafted systems). In this contribution, surface-initiated atom transfer radical polymerization (SI-ATRP) is demonstrated to be a versatile tool that enables the synthesis of bimodal graft modifications with precise control of the degree of polymerization of the respective graft species. For the particular case of polystyrene-tethered silica particles, it was demonstrated that the presence of even small fractions of “long” chains provided an order-of magnitude increase of the mechanical toughness of particle films that is comparable to values found in densely tethered particle systems only in the limit of high degree of polymerization of tethered chains (and corresponding low inorganic content).

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harness the beneficial properties of the polymer constituent.2 Since steric constraints in densely grafted brush particles increase the entanglement segment length, polymer-like properties of one-component-hybrids are only observed when the tethered chains reach high degree of polymerization (DP). This constraint presents a significant challenge to applications based on one-component-hybrid materials since it limits the attainable inorganic fraction to small values (typically less than 10 wt % for particles of size d = 15 nm). We note that a similar challenge applies when polymer tethering is used to facilitate compatibilization of particles in polymer matrices. For example, an important “compatibility criterion”, first postulated by Leibler, stipulates that compatibility in the absence of particle/matrix interactions increases with the DP of tethered chains.5 According to Leibler’s original argument, compatibility is contingent on the DP of tethered chains (Ng) exceeding those of the matrix (Nm), i.e., Ng > Nm. For practical applications, where DP is typically on the order of 1000, miscibility is hence limited to high molecular weight (MW) tethers.

INTRODUCTION The tethering of polymeric chains to the surface of organic or inorganic particles has emerged as a powerful approach to “engineer” the physicochemical properties of particles and to facilitate the controlled dispersion of particle fillers within polymeric matrices. A major driving force in the development of the field has been the advent of surface-initiated controlled radical polymerization (SI-CRP) techniques that provide fine control of the relevant structural characteristics (such as grafting density and degree of polymerization of tethered chains) of polymer-grafted particles.1 For example, the ability to densely tether particles with polymeric chains to form particle− brush materials has fueled research in novel areas of nanocomposite materials such as one-component hybrid materials, i.e., composite materials that are fabricated by the assembly of brush particles.2 The enhanced level of control of the microstructure and interactions between particle constituents, facilitated by tailoring of the polymeric shell, provided unprecedented control of a range of physical properties (from mechanical to phonon transport).3 Also, it enables the fabrication of nanocomposite materials with enhanced electrical breakdown strength.4 Systematic evaluation of the mechanical properties of particle brush solids by Choi et al. has revealed that entanglement between tethered chains is required to © 2015 American Chemical Society

Received: August 30, 2015 Revised: October 8, 2015 Published: November 9, 2015 8208

DOI: 10.1021/acs.macromol.5b01905 Macromolecules 2015, 48, 8208−8218

Article

Macromolecules

ATRP has been successfully employed to prepare different hybrid nanocomposites.1b,9 A large variety of ATRP initiators and ligands are commercially available.8b,10 In recent years, many new ATRP techniques have been developed that require only ppm of Cu catalyst. They include activator regenerated by electron transfer (ARGET) ATRP,11 initiators for continuous activator regeneration (ICAR) ATRP,12 use of Cu(0) as a reducing agent and supplementary activator (SARA) ATRP,13 and more recently developed electrochemical ATRP,14 photoATRP,15 and metal-free ATRP.16 Moreover, ATRP can be carried out under various conditions, for instance, from many types of inorganic surfaces,9b−d,17 in aqueous media,18 or even from biomolecules.19 Furthermore, ATRP is capable of constructing a wide range of polymer architectures, such as gradient,20 bottlebrush,21 star,22 sequence-controlled23 and block24 copolymers including those by transformation from various different mechanisms.25 Surface-initiated ATRP (SI-ATRP) is an important synthetic technique that allows creation of stable particle−polymer interface, and thus suitable for preparation of polymer composites.26 Herein, two strategies were studied for the preparation of hybrid particles with a bimodal MWD of tethered chains: “extending-from” and “attaching-onto”. “Extending-from” means chain-extension from a brush with partially deactivated chain-ends. On the other hand, “attaching-onto” refers to attaching polymers onto a fraction of activate chain ends. Halide end groups that are amenable to functionalization enable the use of these two strategies in SIATRP. Successful preparation of molecular bottlebrush with bimodal MWD using the “extending-from” strategy via ATRP has been recently demonstrated.27 This process was also applied for the particle−brush composite system in this paper. Conventional composite materials prepared by “graftingfrom” methods, including SI-ATRP, usually require blending of hybrid particles with matrix polymer. Particle interactions with the polymer matrix introduce additional complexity into multicomponent nanocomposite material. Not only the additional interface between matrix and filler, but also the dispersibility of filler in the matrix can compromise the predictability of the final properties of the material. Previously, entanglement of grafted polymer brushes when the DP of the tethered polymers were in semidilute polymer brush (SDPB) regime was studied.2 Quasi-one-component nanocomposite materials were achieved by grafting polystyrene (PS) with DP = 770 from 15 nm silica NPs. The inorganic content of the material reached 10 wt %. Entanglement of polymer brushes was demonstrated by the observation of crazing in TEM images of the hybrid monolayers at high levels of stress.4c The same technique was adopted herein to examine a particle brush material with bimodal MWD which enables attaining a much higher inorganic content in the final flexible tough materials.

The tethering of polymeric chains with bimodal molecular weight distribution (MWD) has emerged as an intriguing concept to alleviate the constraints of dense (unimodal) particle brush materials.6 Thus, it enables the fabrication of polymer nanocomposite materials with unprecedented combinations of enhanced properties, arising from very high inorganic content, fine control of the material microstructure, as well as favorable mechanical strength and processability. The unique opportunities in particle brush systems with bimodal MWD originate in the complementary function of the distinct polymer tethers: densely grafted low MW tethers effectively screen off particle core interactions (that otherwise can lead to the formation of complex and difficult to control superstructures) as well as sparse high MW tethers provide entanglement and polymer-like mechanical characteristics and enhanced processability. Figure 1 illustrates the concept of nanoparticles modified by grafting polymers with bimodal MWD.

Figure 1. Interactions of hybrid particles with bimodal MWD. (a) Polymers with bimodal MWD on hybrid particles via “extending-from” (partial deactivation) strategy and “attaching-onto” (polymer attachment) strategy. (b) Long brushes on hybrid particles promote entanglement while short brushes prevent aggregation without compromising inorganic loading.

Nanoparticles grafted with polymers with bimodal MWD (abbreviated as NP-g-bi-polymer) were first prepared by Benicewicz et al. by reversible addition−fragmentation transfer (RAFT) polymerization and “grafting-onto” technique.7 They facilitated more stable NP dispersion, improved mechanical, thermal and optical properties. For example, the matrix-free transparent ZrO2-g-bi-PDMS system prepared by “graftingonto” has potential application for LED encapsulation.7c However, control of chain ratio was difficult to achieve using these techniques due to lack of control over remaining active sites after the first graft. Control of chain composition and graft characteristics is essential to understand the structure property relation in bimodal system and to identify material compositions with optimal property combinations. Therefore, we investigated here other methodologies for synthesis of NP with bimodal MWD using atom transfer radical polymerization (ATRP). ATRP8 is the most widely used controlled radical polymerization method for hybrid particle synthesis.1a ATRP can polymerize a wide range of monomers, and provides a simple procedure to introduce and retain ATRP functionality on NP.

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EXPERIMENTAL SECTION

Materials. Styrene (S, 99%, Aldrich), butyl acrylate (BA, > 98%, Alfa), acrylonitrile (AN, 99%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich) were passed through a column filled with basic alumina to remove the inhibitor. Anisole (99%, Aldrich Reagent Plus), N,Ndimethylformamide (DMF, > 99.8%, Fisher), tris[2-(dimethylamino)ethyl]amine28 (Me6Tren, 99%, Alfa), tris(2-pyridylmethyl)amine29 (TPMA, 98%, Aldrich), 4,4′-dinonyl-2,2′-bipyridyne (dNbpy, 97%, Aldrich), 2,2′-bipyridine (bpy, > 99%, Aldrich), N,N,N′,N″,N″pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich), (4-butoxy-2,2,6,6-tetramethylpiperidin-1-yl)oxy (bTEMPO, 98.6%, Nufarm), tin(II) 2-ethylhexanoate (Sn(EH)2, ∼ 95%, Aldrich), ethyl 28209


Article

Macromolecules Table 1. Results of Model Syntheses of Polymer with Bimodal Molecular Weight Distribution MWDa reaction condition

high MW

polymer

methodology

1 2 3 4 5 6 7 8 9 10 11

PS PS PBA PS PBA PBA PSAN-b-PMMA PBA PS PS PBA-b-PEO

bTEMPO deactivation bTEMPO deactivation bTEMPO deactivation bTEMPO deactivationd thiophenol substitution benzoic acid substitution chain-end reactivity initiator attachment initiator attachment polymer attachment polymer attachment

0.80

3.7 × 104

0.40 0.50 0.50 0.55 0.61 0.50 0.80 0.50 0.50

6.0 4.0 8.2 2.8 1.1 1.3 8.2 2.8 7.4

Mn

× × × × × × × × ×

low MW Đ

entry

fs,theob

104 104 104 104 105 105 104 104 103

1.71 no bimodal 1.04 1.06 1.11 1.76 1.11 1.04 1.32 1.03 1.12

Mn

Đ

4.4 × 103 1.12 product was separatedc 1.1 × 104 1.07 7.0 × 103 1.04 5.0 × 103 1.10 1.3 × 104 1.10 3.7 × 104 1.20 8.5 × 103 1.29 3.3 × 103 1.05 3.3 × 103 1.06 3.3 × 103 1.06

fs,exp

fs,exp/fs,theo

0.81

1.01

0.49 0.54 0.37 0.37 0.63 1.0e 0.98 0.90 0.27

1.22 1.08 0.74 0.67 1.03 2.00 1.22 1.80 0.54

a After purification. bCalculation of fs,theo: entries 1−6, n(deactivating agent)/n(chain); entry 7, styrene fraction in azeotropic PSAN; entries 8−9, n(“dummy” initiator)/n(alkyne); entries 10−11, 1 − n(alkyne)/n(azide). cBimodal MWD was observed during the reaction, but not in a final product. dIntermediate product separated and purified. eShort chain number was over two magnitudes larger than the long chain number.

SARA ATRP. Initiator, BA, solvent(s), Cu(II) and Me6Tren were mixed thoroughly in a sealed Schlenk flask and the mixture was degassed by bubbling with nitrogen. Then, the mixture was flashfrozen by submersion in liquid nitrogen under continuous nitrogen purge and a piece of copper wire, dimensioning 1 cm × 0.5 mm, was added. Another 10 min of nitrogen purge removed residual air from the flask. The reaction mixture was allowed thaw by immersion in water and stirred at room temperature. The conversion and MW of polymer were monitored by 1H NMR and SEC, respectively. Upon reaching the desired conversion, bTEMPO was injected to the flask in order to quench part of the propagating chains. The activator was spontaneously restored by the copper wire. The reaction was transferred to a 50 °C oil bath and the progression of the polymerization was monitored by 1H NMR and SEC. General Procedures for TEMPO Addition to an ATRP “Grafting from” Reaction with Purification. Initiator, S, CuBr2 and PMDETA were mixed thoroughly in a sealed Schlenk flask. The mixture was degassed by purging with nitrogen. Then, the mixture was flash-frozen over liquid nitrogen under continuous nitrogen purging and CuBr was immediately added. Another 10 min of nitrogen purge removed residual air from the flask. The reaction mixture was allowed to thaw by immersing the flask in water then subsequently transferred into an oil bath. The conversion and MW of polymer were monitored by 1H NMR and SEC, respectively. Upon reaching the desired conversion, bTEMPO was added to quench a fraction of the propagating chains. After another purge-thaw cycle, the reaction was stirred at room temperature for 30 or 120 min to allow full conversion of the TEMPO to an alkoxyamine. The sample was purified by dialysis against methanol. After purification, a similar procedure was conducted to chain extend the uncapped chain ends with S. All hybrid samples were etched with HF before injection into SEC. Procedure for Chain-Extension from Chain-Ends with Dual Reactivity. Poly(styrene-co-acrylonitrile) (PSAN) macroinitiator with azeotropic composition was prepared via normal ATRP with the ratio of reagents: [S] 0 :[AN] 0 :[EBiB] 0 :[CuBr 2 ] 0 :[CuBr] 0 :[bpy] 0 = 200:130:1:0.3:3:6.6. The mixture was diluted with an equal volume of anisole to a total volume of 14.0 mL. The reaction was conducted at 60 °C for 97 h. Then, the PSAN macroinitiator was chain-extended via ARGET ATRP with the ratio of reagents: [MMA]0:[PSAN-Br]0: [CuBr2]0:[Me6Tren]0:[Sn(EH)2] = 200:1:0.07:0.8:0.1. The mixture was again diluted with one equivalent of anisole to reach a total volume of 8.3 mL. The reaction was conducted at 60 °C for 29 h. Set up of the ARGET ATRP reaction was the same as AGET ATRP described above. Chain-End Azide Modification and Cu-catalyzed Azide− Alkyne Cycloaddition (CuAAC). A macroinitiator with bromine chain-end prepared via ATRP was dissolved in 0.33 M NaN3 solution in DMF to provide a concentration of 0.02 M. The mixture was stirred

bromoisobutyrate (EBiB, 98%, Acros), sodium azide (99%, Aldrich) and solvents for polymer purification and analysis: tetrahydrofuran (THF, > 99%; Fisher), methanol (>99.8%, Fisher), hexane (Fluka), toluene (Fluka), 48% hydrofluoric acid aqueous solution(HF, > 99.99%, Aldrich), ammonium hydroxide aqueous solution (NH4OH, 28.0−30.0%, Fisher), anhydrous magnesium sulfate (MgSO4, Fisher), copper(II) bromide (CuBr2, 98%; Acros), copper(I) iodide (CuI, 98%, Alfa) were used as supplied. Silica NPs, 30 wt % solution in methyl isobutyl ketone (MIBK-ST), effective diameter d ≈ 15.8 nm, were kindly donated by Nissan Chemical Corp. and used as received. The tetherable ATRP initiator 1-(chlorodimethylsilyl)propyl-2-bromoisobutyrate and surface modified silica (SiO2−Br) were prepared using previously reported procedures. The surface initiator densities are moderated with “dummy” initiator chlorotrimethylsilane (99%, Aldrich).9a,30 Propargyl 2-bromoisobutyrate and propargyl acetate were synthesized following previous reports.31 Copper(I) bromide (CuBr, 98%, Acros) was washed with glacial acetic acid to remove any soluble oxidized species, filtered, washed twice with anhydrous ethyl ether, dried, and kept under vacuum. Copper wire was cut then washed with 4 M HCl in MeOH and acetone before use. General Procedures for bTEMPO Addition to ATRP without Purification. Normal ATRP. Initiator, S, solvent(s), CuBr2, and PMDETA were mixed thoroughly in a sealed Schlenk flask. The mixture was degassed by bubbling with nitrogen. Then, the mixture was flash-frozen by immersion in liquid nitrogen under continuous nitrogen purge and CuBr was immediately added. Another 10 min of nitrogen purge removed residual air from the flask. The reaction mixture was thawed by immersing the flask in water then subsequently put into an oil bath set at the desired temperature. The conversion and molecular weight (MW) of polymer were monitored by 1H NMR and SEC, respectively. Upon reaching the desired conversion, the reaction was again flash-frozen and bTEMPO, Sn(EH)2 and TPMA were added to the flask to quench a fraction of the propagating chains and regenerate the consumed activators. After another purge-thaw cycle, the reaction was transferred back to the oil bath and the continued polymerization was monitored by 1H NMR and SEC. AGET ATRP. Initiator, S, solvent(s), CuBr2 and dNbpy were mixed thoroughly in a sealed Schlenk flask. Meanwhile, a stock solution of Sn(EH)2 in anisole was prepared. Both of the mixtures were degassed by nitrogen purging, then the Sn(EH)2 solution was injected into the Schlenk flask to activate the catalyst complex and the flask was immediately put into an oil bath. The conversion and MW of polymer were monitored by 1H NMR and SEC, respectively. Upon reaching the desired conversion, bTEMPO, Sn(EH)2 and Me6Tren were injected to quench part of the propagating chains and restore consumed reducing agent. The reaction was continued to be monitored by 1H NMR and SEC. 8210


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by constant load indentation for 10 s by depth control to a depth of 2 μm. Sample toughness was determined by analysis of the geometry of cracks formed at the indenter tip region as described previously.2 Imaging of residual impressions from indentation (at least 10 per sample) was performed using an NT-MDT SolverNEXT AFM in tapping mode with a silicon cantilever (300 kHz resonance frequency, 40 N m−1 force constant) with a sharpened pyramidal tip. Phase images provided the most accurate measure of residual crack lengths.

at room temperature for 24 h and the polymer with modified chainends was precipitated by addition to an excess of methanol to remove salts then filtered. A mixture of 0.22 equiv of propargyl 2-bromoisobutyrate (alkyne initiator) and 0.88 equiv of propargyl acetate (“dummy initiator”) was mixed with azide functionalized polymer in toluene at the concentration of 0.033 M. Next, 1.1 equiv of CuI was added to catalyze the reaction at room temperature. The polymer with modified chain-ends was recovered by precipitation in methanol. The modified macroinitiator was extended with corresponding monomers, following the procedures described above. A similar procedure was used to attach propargyl-functionalized polymer onto the azide-modified chain ends. In the case of PBA-b-PEO (Table 1 entry 11), same equiv of CuBr/PMDETA complex was used instead of CuI. Nucleophilic Chain-End Substitution. The polymers were first prepared via ATRP. Then, they were treated with specified amount of thiophenol or benzoic acid (Table 1, entry 6−7). In the case of thiophenol, 5 equiv of tributylamine (to thiophenol) were added. In the case of benzoic acid, 10 equiv of TEA was added. All reaction mixtures with thiophenol were stirred at room temperature for 18 h while the reaction mixtures with benzoic acid were stirred at room temperature for 48 h. Characterization. Number-average molecular weights (Mn) and MWDs were determined by size exclusion chromatography (SEC). The SEC was conducted with a Waters 515 pump and Waters 410 differential refractometer using PSS columns (Styrogel 105, 103, 102 Å) in THF as an eluent at 35 °C and at a flow rate of 1 mL min−1. Linear PS standards were used for calibration. Conversion was calculated by following the decrease of the monomer peak area relative to the peak areas of the internal standards. 1H NMR spectroscopy used for polymerization monitoring was performed using a Bruker Advance 300 MHz NMR spectroscope with CDCl3 as a solvent. The fraction of short and long brushes were calculated by deconvolution of differential refractive index (dRI) vs elution volume (Ve), in Origin 9.0 assuming both of the polymer signals follow Gaussian distribution.32 Nearly monolayer films of all particle brush systems were prepared by spin-casting of dilute particle solutions (8−10 mg mL−1 in toluene) on the poly(acrylic acid) (PAA) substrate and subsequent thermal annealing in a vacuum for 24 h at T = 120 °C. The films were lifted-off the substrate by water immersion and transferred onto Cu-grids. Both the particle film morphology and its craze formation were imaged by transmission electron microscopy (TEM) using a JEOL EX2000 electron microscope operated at 200 kV. Images were taken by amplitude and phase contrast using a Gatan Orius SC600 highresolution camera. The spatial distribution and radius of particles was analyzed using ImageJ software. The thermogravimetric analysis (TGA) was conducted using a TA Instrument TGA Q50 and the data was analyzed with TA Universal Analysis. The heating procedure involved four steps: (1) jump to 120 °C; (2) hold at 120 °C for 10 min; (3) ramp at a rate of 20 °C/min to 800 °C; (4) hold for 5 min. The TGA plots were normalized to the total weight after holding at 120 °C. The graft density of hybrid particles were calculated by the following equation:

σTGA =

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RESULTS AND DISCUSSION Untethered polymers with a bimodal MWD can be prepared by several techniques: (1) Mixing two unimodal polymers with different chain lengths (“mixing polymer”);33 (2) adding a second initiator to the polymerization mixture after a fraction of monomer was polymerized (“adding initiator”); (3) chainextension from polymers with partially deactivated chain-ends (“extending-from”); (4) attaching polymers onto polymers with a fraction of active chain-ends (“attaching-onto”). Previous work on tethered polymers mainly focused on “adding initiator” or “mixing polymers” approaches, used in the SI-RAFT7a,b or “grafting-onto” technique,6,7c respectively. However, sequential functionalization could lead to aggregation of silica NPs due to insufficient surface coverage during the first step. Also, nanocomposites prepared by “grafting-onto” techniques could lose part of their favorable properties due to lower attainable graft density.34 In addition, control over long/short chain ratio is limited using both techniques. Therefore, to prevent the drawbacks of the first two techniques, “extending-from” and “attaching-onto” were studied using SI-ATRP. SI-ATRP was previously successfully used for grafting monodisperse polymer brushes from various surfaces.1a,35 This surface initiation method is based on monomer reacting with chain ends with an activity similar to ATRP untethered-initiators.1b,9c,36 Therefore, ATRP synthetic methods toward brushes with bimodal MWD were first studied in model systems before extending them to a surface-initiation platform. The developed procedures were then further applied to the fabrication of quasi-one-component hybrid materials based on silica NPs with bimodal MWD using the “grafting-from” approach.4c Model Reactions. Model reactions included ATRP of styrene (S) and n-butyl acrylate (BA) followed by quenching a fraction of chains with either 4-butoxy-TEMPO (bTEMPO) (Table 1 entry 1−4) or nucleophiles (entry 5,6), extending chains with variable chain-end reactivity (S−Br and AN-Br in PSAN system, entry 7) and also attaching new ATRP initiators or alkyne functionalized polymers to a fraction of primary chains containing azide functionality (entry 8−11). For each model reaction, the predicted short chain fraction (fs,theo) is listed. The number-average MWs (Mn), dispersities (Đ), and measured short chain fractions (fs,exp) were calculated by deconvolution of the bimodal MWD. In the column of methodologies, bTEMPO deactivation and thiophenol/benzoic acid substitution refer to partial deactivation of chain ends by atom transfer radical coupling with TEMPO,37 or by nucleophilic substitution, respectively. They will be discussed in detail in the following subsections. Deactivation with bTEMPO and Nucleophilies. Nitroxides are highly efficient radical scavengers. Under ATRP conditions, TEMPO derivatives couple with the alkyl radicals and form dormant alkoxyamines.38 Therefore, a series of experiments were carried out via ATRP techniques with addition of a fraction of quencher relative to the initial ATRP initiator (Table 2).

(1 − fSiO )NAρSiO d 2

2

6fSiO M n 2

(1)

Here f SiO2 is the silica fraction measured by TGA after exclusion of the residue solvent; NA is the Avogadro number; ρSiO2 is the density of silica NPs; d is the average diameter of silica NPs; Mn is the overall number-average MW of polymer brushes. Elastic modulus, hardness, and fracture toughness were measured via nanoindentation using a MTS Nanoindenter XP with a Berkovich tip under displacement control to no more than ∼10% of the nanocrystal film thickness. Experimental data for particle brush samples were obtained from at least 20 indentations per sample, and the standard deviation of the measurements was calculated as experimental error. The displacement rate during the indentation of the particle brush samples was 5 nm s−1 to a maximum load followed 8211


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Figure 2). However, reaction was not complete during 30 min and therefore a longer reaction time (2 h) was subsequently

Scheme 1. Partial Deactivation of PS Chain End with bTEMPOa

a

Ligand (L) and temperature are specified in Table 2.

While monitoring the conversion and MWD with 1H NMR and SEC, 4-butoxy-TEMPO (bTEMPO) was added into the reaction at desired conversion. In entry 1−3 (Table 2), no additional purification step was involved. After bTEMPO addition, the reaction was reactivated by addition of a reducing agent or by the excess reducing agent already present in the reaction mixture. Since a large excess of Cu(I) was required to recover the initial [Cu(I)]/[Cu(II)] ratio in a normal ATRP, both normal ATRP and activator generated by electron transfer (AGET) ATRP reactions were reactivated with additional reducing agent and continued as AGET ATRP (Table 2 entry 1−2). However, without a strongly binding ligand, the rate of polymerization could not recover. Also, at higher temperatures for the AGET ATRP reaction, alkoxyamines could dissociate,39 which can lead to a combination of ATRP and nitroxidemediated polymerization (NMP).39b No bimodal MWD was observed in the final product (Table 1, entry 2). To circumvent these problems, a low ppm procedure, SARA ATRP was chosen, in which Cu(0) served as a reducing agent and supplementary activator.13b,c Since styrene is not an optimal monomer for SARA ATRP,13b BA was studied instead. After bTEMPO addition, the Cu(I) activator was restored by comproportionation of newly formed Cu(II) with Cu(0).40 Therefore, no additional reducing agent was required. However, a long induction period (100 h) was observed at room temperature. Therefore, the temperature was raised to 50 °C. Although less effort was required by omitting the purification step, polymers with slightly broadened MWDs were formed (Table 1 entry 3). An intermediate purification step was introduced to mitigate the side-reactions. The radical coupling reaction was allowed to complete by maintaining the reaction at room temperature for 30 min. The lower temperature largely eliminated the propagation by reducing kp of S to 86 M−1 s−1.41 Polymers with much narrower MWDs were obtained (Table 1 entry 4,

Figure 2. SEC traces of polymer samples from entry 4 (Table 1 and Table 2). Reaction conditions: [EBiB]0:[S]0:[CuBr]0:[CuBr2]0: [PMDETA]0:[bTEMPO]0 = 1:200:0.95:0.05:1:0.5, 50 vol % anisole, 60 °C; TEMPO treatment conducted at room temperature for 0.5 h under nitrogen; chain extension: [PS-Br]0:[S]0:[Sn(EH)2]0:[CuBr2]0: [dNbpy]0: = 1:500:0.45:1:2, 50 vol % anisole, 60 °C. Toluene was used as internal standard.

applied. Thus, this procedure was subsequently used for the preparation of hybrid polymer brushes with bimodal MWD (Table 2 entries 55B and 20B, and discussion in the next sections). Strong nucleophiles, such as thiophenol or benzoic acid, were studied as well to partially deactivate Br chain ends. Thiophenol modification of the chain end resulted in a clean bimodal MWD of extended chains and short chain fraction close to a theoretical value. On the other hand, benzoate was not very efficient at deactivating bromine chain ends. Also the extended chains had much broader MWD (Table 1 entry 5−6). Although typical nucleophilic substitution reactions are nearly quantitative using excess of nucleophile, in this study a substoichiometric ratio was used. As a result, less than quantitative substitution was observed. Chain Extension. Two other methodologies were also evaluated to extend from chain ends with different reactivity. An azeotropic PSAN copolymer has a statistical chain-end composition of S and AN.42 However, the two types of Brchain-ends have very different reactivity in the extension with methyl methacrylate (MMA). When PSAN was extended with

Table 2. Reaction conditions for deactivation with bTEMPO before TEMPO treatment

after TEMPO treatment

entry

monomer

[M]0:[I]0

condition

temp, °C

[M]0:[I]0

condition

temp, °C

equiv of TEMPO

1 2 3 4g 55Bg,i 20Bg,i

S S BA S S S

200 200 200 200 400 400

normala AGETd SARAe normala normala normala

60 90−110e room temp 60 60 60

b b b 500 400 1600

AGETc AGETc SARAf AGETd normala normala

60 110 50 60 60 60

0.8 0.8 0.4 0.5h 0.8j 0.8j

a

Typical normal ATRP condition: [I]0:[CuBr]0:[CuBr2]0:[PMDETA]0 = 1:0.95:0.05:1, 50 vol % anisole, temperature as specified. bNo additional monomer was added. cSn(EH)2 and TPMA/Me6Tren added [I]0:[Sn(EH)2]t:[TPMA/Me6Tren]t = 1:0.45:1. dTypical AGET ATRP condition: [I]0: [Sn(EH)2]0:[CuBr2]0:[dNbpy]0 = 1:0.45:1:2, 50 vol % anisole, temperature as specified. eTypical SARA ATRP condition: [I]0:[CuBr2]0:[Me6Tren]0 = 1:0.01:0.1, copper wire, 1 cm × 0.5 mm, 50 vol % DMSO, temperature as specified. fTemperature was increased to accelerate the reaction. g Purification after TEMPO treatment. hTreated with TEMPO at room temperature for 0.5 h. ipolymerization initiated from silica NPs. jtreated with TEMPO at room temperature for 2 h. 8212


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typical CuAAC catalysts can activate the alkyne-containing ATRP initiator. To avoid the side-reaction, copper(I) iodide without ligand, which cannot activate ATRP, was selected to catalyze the reaction.47 After the polymer chain-ends were modified with azides,45,48 they subsequently were reacted with a known ratio of ATRP initiator and “dummy” initiator. The higher fraction of deactivated short chains than the expected values could result from incomplete CuAAC reaction and unequal reactivity of two alkynes (Table 1, entries 8−9).

Scheme 2. Partial Deactivation of PBA Chain End with Strong Nucleophiles

PMMA via ARGET ATRP, activation and crosspropagation from the S chain-ends was much slower than from AN chainends.42 As a result, AN chain-ends initiated the polymerization of MMA resulting in polymers with the bimodal MWD. The short chain fraction corresponded to the S chain end fraction in the azeotropic copolymer (Table 1 entry 7, Figure 3).

Scheme 4. Partial Deactivation of PS Chain End by Azide Modification Followed by Attachment of Mixture of New Initiator and “Dummy” Initiator

CuAAC was also employed for the “attaching-onto” methodology. Initially, polymerization from the propargyl initiator was examined, but low initiation efficiency was determined.49 Also, a short chain fraction was much higher than the theoretical value (Table 1 entry 10). To enhance the initiation efficiency and activation of polymer chain-end in ATRP, alkyne functionalized PEO were grafted to azide functionalized PBA (entry 11). However, low fraction of short chain was observed which could be due to differences in RI of PEO and PBA. However, the bimodal MWD was clearly observed. Preparation of Hybrid Particles. Among the four strategies used in model reactions, radical coupling with intermediate purification demonstrated the best control of brush composition and MWD (Table 1). Thus, this method was chosen to prepare particle brushes with bimodal MWD. One more model reaction was conducted to verify the feasibility of the methodology applied to SI-ATRP. Since a limited fs,exp/fs,theo value (0.52) was observed after 30 min, reaction time with bTEMPO was extended to 2 h in the following reactions. A longer reaction time allowed bTEMPO to completely react with chain ends (Table 2 entries 55B and 20B). Two hybrid particles with bimodal MWD were prepared with ca. 80% of short chains. In order to achieve sufficient toughness of the one component hybrid material without compromising inorganic content, high molar ratio (0.8) of bTEMPO to chain ends was used to form a larger fraction of short chains. The two resulting samples were referred below as sample 55B and 20B in accordance to their approximate inorganic fraction and bimodal MWD. Three unimodal samples were accordingly designed and prepared using SI-ATRP as reference. Samples 55U and 20U were unimodal samples with similar inorganic fractions and graft densities to samples 55B and 20B, respectively. Another sample, 55UL, with lower graft density, had the same DP as long brushes in sample 55B (Table 3 and Figure 4). All samples were etched with HF before SEC measurement. The DPs and dispersities, Đ, of bimodal samples were calculated from deconvolution of the SEC traces, assuming the differential refractive index (dRI) vs elution

Figure 3. SEC traces of polymer samples from entry 8. Reaction conditions: [EBiB] 0 :[S] 0 :[AN] 0 :[CuBr 2 ] 0 :[CuBr] 0 :[bpy] 0 = 1:200:130:0.3:3:6.6, 50 vol % anisole, 60 °C. Chain extension: [PSAN-Br] 0 :[MMA] 0 :[CuBr 2 ] 0 :[Me 6 Tren] 0 :[Sn(EH) 2 ] 0 = 1:200:0.07:0.8:0.1, 50 vol % anisole, 60 °C. Toluene was used as internal standard.

Scheme 3. Chain Extension of PSAN with MMA

Although this is one of the simplest methods to prepare copolymers with bimodal MWD and it gives good control of the short chain fraction, it limits the procedure to a narrow selection of copolymer types. Also, it is difficult to adjust the fraction of short chains. Nevertheless, interesting hybrid materials could be created based on the miscibility of PSAN with PMMA.43 Initiator or Polymer Attachment. Copper-catalyzed azide− alkyne cycloaddition (CuAAC) click chemistry44 is another highly efficient reaction for partial modification and extension. However, since fractional modification of bromine chain ends by azides cannot be quantitatively controlled (at low concentration of end groups, typically large excess of azides is used),45 the complete azide-modification of chain-ends was followed by a reaction with a mixture of alkyne ATRP initiator and “dummy” initiator.46 One drawback of CuAAC is that 8213


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Macromolecules Scheme 5. Attachment of PEO onto Modified PBA Chain End

Table 3. Degree of Polymerization (DP), Dispersity (Đ), Short Chain Number Fraction ( fs), Graft Density (σ), Inorganic Content (f ino), and Interparticle Distances (dip) of the Two Bimodal Hybrid Samples (55B and 20B) and the Corresponding Unimodal Samples (55U, 55UL, and 20U) sample

DPsa

Đs a

DPla

Đl a

fs,theo

fs,expa

σ(nm−2)b

σs (nm−2)

σl (nm−2)

f ino

dip (nm)c

Tg (°C)

55B 55U 55UL 20B 20U

13 − − 69 −

1.06 − − 1.05 −

170 80 170 790 250

1.46 1.04 1.07 1.10 1.05

0.80 1 1 0.80 1

0.69 − − 0.72 −

0.37 0.34 0.15 0.43 0.45

0.26 − − 0.31 −

0.11 0.34 0.15 0.11 0.45

0.57 0.51 0.53 0.21 0.20

19.3 24.5 29.8 30.9 34.4

± ± ± ± ±

98.1 103.8 100.7 106.4 105.9

0.6 0.5 1.1 0.8 1.2

a Calculated from the deconvoluted SEC peaks. bCalculated from the TGA plot of purified sample following eq 1. cMeasured from TEM image of monolayer. All samples were PS grafted from initiator modified silica nanoparticles (R0 ≈ 7.9 nm).

Figure 4. Overlaid SEC traces (a, toluene as internal standard) and TGA plots (b, initial weight recalibrated to the weight after solvent removal at 120 °C) of the two bimodal hybrid samples and their corresponding unimodal samples. Samples 55B and 20B were prepared as specified in Table 2. [SiO2−Br/SiO2-g-PS−Br]0:[S]0:[CuBr]0:[CuBr2]0:[PMDETA]0 = 1:400/400/400/1600:0.95:0.05:1, 50 vol % anisole, 60 °C. Similar conditions were used to prepare unimodal reference samples: [SiO2−Br]0:[S]0:[CuBr]0:[CuBr2]0:[PMDETA]0 = 1:400/1000/640:0.95:0.05:1, 50 vol % anisole, 60 °C. SiO2−Br: apparent initiator density = 0.40 (55B, 20B, 55U, 20U) or 0.15 (55UL) mmol Br/g (0.56 or 0.21 Br/nm−2) with R0 ≈ 7.9 nm.

volume (Ve) curves were overlapped Gaussian peaks.32 The elution volume was then converted to MW with the SEC calibration curve. The number-average DPs were calculated from the Mn while dispersities were calculated as Đ = Mw/Mn. The short chain fraction was calculated with eq 2. fs ,exp =

sample are narrowly distributed single peaks (Figure 4a). In the TGA plots, the five samples were grouped into two sets. Sample 55B, 55U and 55UL had ca. 55% inorganic while sample 20B and 20U had ca. 20% inorganic (Figure 4b). Material Characterization. The conformation of grafted polymer chains in particle brushes sensitively depends on the architecture of the graft (i.e., graft length, grafting density, and particle size).1a,39a,50 A modified Daoud−Cotton model50,51 for star polymers accurately reflects qualitative trends in the chain conformations divided into two regimes for tethered particles.50a,52 The concentrated particle brush (CPB) regime is near the surface, with significant chain stretching, and the semidilute particle brush (SDPB) regime is away from the surface where reduced spatial density of chains allows for a more relaxed conformation. Choi et al. determined that the fracture toughness, KIC, of particle brush solids depends on the conformation of grafted chains.2 Toughness increased greatly when chain length exceeded the transition length from CPB to

∫s dRIi dVe, i/Mi ∫s + l dRIi dVe, i/Mi

(2)

where dRIi, dVe,i, and Mi are differential refractive index within an infinitesimal change elution volume and its corresponding molecular weight. The graft densities were calculated from TGA inorganic fraction with eq 1. Interparticle distance are calculated from TEM images of monolayers using ImageJ software. The MWD and f ino of the five samples were characterized with SEC and TGA, respectively. Sample 55B (black) and 20B (magenta) displayed bimodal MWD, while the three reference 8214


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Macromolecules

Figure 5. Bright field TEM images of approximate monolayers (a−e), crack formation (f−j), and illustrations of cracks (k−o) of the five samples. All scale bars: 100 nm. Samples were prepared as specified in Figure 4. Specifications are listed in Table 3. Unimodal sample 55U (SiO2-g-PS80, a, f, k): N < Ne ∼ 160, extensive crack propagation. Unimodal sample 55UL (SiO2-g-PS170, b, g, l): above entanglement limit but in CPB regime (DP < 250),4c sharp crack formation. Bimodal sample 55B (SiO2-g-bi-PS13,170, c, h, m): long brushes above entanglement limit and slightly beyond CPBSDPB transition (σl ∼ 0.11 nm−2), plastic deformation. Unimodal sample 20U (SiO2-g-PS250, d, i, n): N > Ne and slightly beyond CPB-SDPB transition, stent-like undulation formation. Bimodal sample 20B (SiO2-g-bi-PS69,790, e, j, o): long brushes in SDPB regime and far above entanglement limit, craze formation. Scale bars = 100 nm.

scaling models have proven very useful, these models cannot easily be extended to bimodal brush systems. However, in the present case, the following argument can be made to relate the structure of bimodal systems to the particle brush model. For a unimodal system of equivalent grafting density, the critical degree of polymerization (Nc) for the CPB-SDPB transition is estimated to Nc = 250, which is greater than the DPs of dense short chains in sample 20B. Thus, it can be expected that chains are strongly stretched (CPB regime) until N = DPs. Given the low grafting density of long chains, we expect segments of length DP − DPs to assume relaxed conformations. Hence we can expect that in the case of the bimodal system discussed in the present work the CPB/SDPB transition occurs approximately at Nc = DPs. Thus, the segment length of relaxed segments (of long chains) can be estimated to DPL − DPs > 500 which is significantly greater that the critical segment length of entanglement formation. Note, the benefit of the bimodal architecture with regard to accessible fraction of inorganic component. Compared to a particle brush sample in a similar system from our previous study, but with unimodal MWD, this sample with bimodal MWD has a twice higher inorganic content!4c As for the unimodal counterpart 20U, a crack with stent-like undulation (Figure 5i) implies that the unimodal brushes, which are slightly beyond the CPB-SDPB transition contributed to marginal interpenetration. However, they do not sufficiently exceed the critical length to promote entanglement and craze formation. This is in agreement with the criterion for entanglement, NSDPB > 2Ne, where NSDPB is the relaxed segment length and Ne is the length of free polymer necessary to form entanglements.2 Another interesting observation is that the dip is significantly lower in the bimodal sample 55B, as compared to the two unimodal samples 55U and 55UL despite similar inorganic content. This is attributed to the fact that both 55U and 55UL have PS brushes within CPB regime,2 whereby the stretching of graft chains greatly increases the interparticle distance. However, a close packing of such systems produces significant interstitial volume. The long brushes in sample 55B will

SDPB, as predicted by the theory. This was predicted to occur at DP = 250 for PS grafted from 15 nm silica with graft density, σ, of 0.4 nm−2 due to the ability of grafted chains to entangle. In Table 3, two samples with bimodal MWD (55B and 20B) with ca. 70% short chains were compared with their unimodal counterparts. The DPs and grafting densities of unimodal brushes 55U, 55UL, and 20U were designed to have a similar inorganic content f ino as 55B and 20B, respectively. Pristine and microscopically cracked areas of approximately monolayered regions of all samples from Table 3 were imaged via TEM (Figure 5a−j). The prepared hybrid particles were drop cast and annealed on poly(acrylic acid) (PAA) substrates.2 The PAA substrates were removed by immersion in water and the approximate monolayers of the hybrid particles were transferred to copper grids for TEM imaging. The presence of microscopic cracking was induced by the lift-off procedure from the water surface. Without cracking, samples with bimodal MWD show uniform particle distribution similar to the unimodal samples owing to highly dense, uniform short graft layer.4c,7a,c Sharp cracks in both unimodal (55U and 55UL) samples indicated brittleness of the samples due to the lack of brush entanglement (Figure 5f,g).2,4c On the other hand, plastic deformation was observed in the TEM of bimodal sample 55B (Figure 5h). This is due to the lower grafting density of DP = 170 brushes (0.11 nm−2) in sample 55B as compared to 0.15 nm−2 in sample 55UL. Lower grafting density promotes more relaxed chain conformations and thus more efficient entanglement behavior.50b,51 This agreed with previous study that chains have higher brush conformation entropy at lower grafting density.2 However, the DP was not large enough to form well-defined entanglements. Bimodal sample 20B with 28% of DP = 790 long brushes and with 72% of DP = 69 short brushes demonstrated clear crazing formation (Figure 5j), due to the long graft chains greatly exceeding the critical segment length for entanglements. Here a comment should be made regarding the interpretation of chain conformational transitions in bimodal as compared to unimodal particle brush systems. While for the latter Daoud−Cotton type 8215


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Macromolecules preferentially fill the interstitial volume, but due to a small fraction of long chains, they do not significantly increase interparticle distance. In the other case, similar dip was observed in 20B and 20U despite significantly different graft architectures. While the grafted chains in sample 20B are predominately stretched, increasing the interparticle distance, the very long brushes in 20B overfill the interstitial volume and cause a significant increase in interparticle distance above values predicted based solely on the short brushes. Nanoindentation was performed on samples 20B and 20U to quantify the effect of the bimodal MWD on the mechanical properties of particle brush assemblies. Characteristic force displacement curves for the two samples are presented in Figure 6. No pile-up or sink-in was observed (Figure 6 inset),

for the bimodal sample 20B was calculated to be KIC/KICo = 1.23 ± 0.1 thus exceeding the respective value for the high MW reference polymer (KICo). The increase of fracture toughness at equivalent inorganic content is remarkable and significantly surpasses the corresponding values that were reported previously for high MW unimodal densely grafted particle systems.2 We interpret the pronounced increase of fracture toughness as a consequence of the reduced “steric constraints” on long chains in bimodal architectures that promote chain relaxation and entanglement formation. Since the fracture toughness in amorphous polymers is directly related to the entanglement density, its pronounced increase suggests that bimodal architectures are more effective in facilitating chain entanglements than dense brushes even in the limit of high MW tethered chains. Furthermore, for particle brushes, the interactions between entanglement points should be of longer range (due to the particle junction) higher fracture toughness as compared to linear polymer analogues is expected. This conclusion of relaxed chain conformations in bimodal systems is supported by measurements of the glass transition temperature in 20B (Tg ∼ 106 °C) that is approximately equal to the glass transition temperature of the reference homopolymer.

■

CONCLUSION Polymers with bimodal MWD were prepared by ATRP via “extending-from” and “attaching-onto” methods. The “extending-from” method employing atom transfer radical coupling reaction for partial deactivation of chain-ends controlled the fraction of long chains and MWD of the resulting polymers. This robust method was applied to create brushes with bimodal MWD by stepwise treatment of polymer chain ends with bTEMPO. The morphology of the resulting hybrid particles with brushes with bimodal MWD were characterized by TEM of monolayered films. In contrast to unimodal samples with similar inorganic content, the craze formation in bimodal samples demonstrated the effectiveness of brushes with bimodal MWD in promoting interparticle chain entanglement. Matrix-free monolayer films with unprecedented high inorganic content exhibited craze formation. Thus, grafting polymer brushes with bimodal MWD is a promising strategy for preparation of strong and tough matrix-free hybrid material with predictable properties.

Figure 6. Characteristic load−displacement curves for sample 20B (black) and 20U (gray) showing similar response (similar to Young’s modulus and hardness). Inset shows indent of sample 20B with cracks emanating from indent tip. Scale bar is 5 μm.

allowing for the standard Oliver and Pharr analysis to be applied to calculate Young’s modulus and hardness.53 The results reveal comparable values for both Young’s modulus E and hardness H in case of the bimodal sample 20B (E ∼ 5.06 GPa, H ∼ 0.186 GPa) and unimodal sample 20U (E ∼ 4.77 GPa, H ∼ 0.145 GPa) despite the significant differences in graft architecture. This confirms earlier results on dense particle brush systems that revealed only a weak dependence of the elastic modulus on the DP of tethered nonoligomeric chains. This was interpreted as a consequence of dispersion interactions dominating the response of the material in the limit of small strain.2 The fracture toughness of film from materials with bimodal MWD was calculated by evaluation of the residual indents by AFM with the following equation:52 ⎛ l ⎞2/3⎛ E ⎞1/2 ⎛ P ⎞ ⎟ KIC = 1.073x v ⎜ ⎟ ⎜ ⎟ ⎜ max ⎝ a ⎠ ⎝ H ⎠ ⎝ c 3/2 ⎠

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.R.B.). *E-mail: [email protected] (K.M.). Author Contributions §

J.Y. and T.K. contributed equally to the work.

(3)

Notes

The authors declare no competing financial interest.

■

In eq 3, Pmax represents the pressure at maximum load, l is the center-to-corner distance of the residual indent, a is length of the crack from corner to tip, c is the total crack length (a + l), and xv is a constant related to the indenter geometry (0.015 for a Berkovich tip). We note that for the unimodal system 20U, the fracture toughness could not be determined due to brittleness that led to extensive cracking of films that occurred during indentation. This is consistent with previous reports of the small fracture toughness in particles grafted with unimodal short chain brushes with analogous characteristics.2 In contrast to the unimodal system, bimodal particle brush materials formed robust films. The normalized fracture toughness (KIC)

ACKNOWLEDGMENTS We acknowledge Nufarm for their generous donation of bTEMPO, Nissan Chemical for their generous donation of silica NPs, and Rachel Ferebee for her help on the characterization of an early sample. M.S. acknowledges the John and Clare Bertucci Graduate Fellowship. A.D. acknowledges support received from the China Scholarship Council. K.M. and M.R.B. acknowledge support by the National Science Foundation (via Grants DMR 1501324, DMR 1436219, and DMR-1410845) as well as the Department of Energy (via 8216


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Macromolecules Grant DE-EE0006702). J.P. acknowledges the financial support from the National Science Center (Grant DEC-2012/04/M/ ST5/00805 and Grant UMO-2014/14/A/ST5/00204).

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