Effect of the micelle opening in self-assembled amphiphilic block

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Effect of the micelle opening in self-assembled amphiphilic block copolymer films on the infiltration of inorganic precursors. Yunlong She, Jihyung Lee, Byeongdu Lee, Benjamin T. Diroll, Thomas Scharf, Elena V. Shevchenko, and Diana Berman Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04039 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Effect of the micelle opening in self-assembled amphiphilic block copolymer films on the infiltration of inorganic precursors.

Yunlong She1, Jihyung Lee1, Byeongdu Lee2, Benjamin Diroll3, Thomas Scharf1, Elena V. Shevchenko3*, Diana Berman1* 1

Materials Science and Engineering Department and Advanced Materials and Manufacturing Processes Institute,

University of North Texas, 1155 Union Circle, Denton, TX 76203 2

Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439

3

Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439

Corresponding

authors:

Elena

Shevchenko

at

[email protected]

[email protected]

ACS Paragon Plus Environment

and

Diana

Berman

at

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Abstract: Infiltration of the polymer templates with inorganic precursors using the selective vapor-phase infiltration approach, or sequential infiltration synthesis (SIS), allows the design of materials with advanced properties. Swelling of the block copolymer (BCP) templates enables the additional control of the structure, porosity, and thickness of the composite or inorganic materials. Here, we use the highly precise quartz crystal microbalance (QCM) technique to investigate quantitatively the effect of the micelle opening by swelling and inorganic precursor infiltrating on the evolution of porosity in amphiphilic BCPs. We show that swelling of the polystyrene-blockpoly-4-vinyl pyridine (PS-b-P4VP) BCP in ethanol at 75°C occurs rapidly and results in a stable polymer structure in 30 minutes. By using an alumina model system, we found that swelling enables access to all available polar domains of the PS-b-P4VP film leading to increase in the SISinfiltrated alumina mass as compared to the non-swelled BCP layer. Our results demonstrate, that swelling of the 110 nm thick BCP template results in the formation of 192 nm thick alumina films with two times larger alumina mass and four times larger effective pore volume than in case of the non-swelled sample. In the case of thicker polymer template, the difference due to swelling becomes even more substantial because the fraction of accessible polymer is increased much more than in thin films. Our findings provide important insights into the mechanism of the infiltration of the inorganic precursors into swelled and non-swelled, spin-coated BCP templates enabling the design of highly porous materials thick ceramic films by SIS.

Keywords: swelling, nanoporous materials, quartz crystal microbalance, alumina, porosity accessibility, poly vinyl pyridine, sequential infiltration synthesis.


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Introduction Self-assembly of polymer, biopolymer, or BCP templates[1] and their infiltration with inorganic precursors[2] has been lately recognized as a powerful platform to create highly functional materials. Infiltration of the spin-coated polydimethylsiloxane (PDMS) with alumina significantly improved the resolution of the structures obtained by photolithography [3, 4]. Infiltration of the polyurethane and polyimides foams with the organometallic precursor trimethylaluminum (TMA) enabled the deposition of silane-based oleophilic compounds converting the cheap polymer foam into the material with superior oil adsorbing properties [5]. Infiltration of the titanium or aluminum into spider dragline silks resulted in greatly improved toughness of the resulting silks [6]. Selective infiltration [7, 8] of the self-assembled BCP template allows patterning of nanosized inorganic materials [2, 9], as well as deposition of uniform, highly porous conformal coatings that are of a great interest for a wide range of applications, from sensors and capacitors, to filtering membranes, and to biomedical implants and antireflective coatings [1014]. Atomic layer deposition (ALD) [15] is an efficient technique to deliver the inorganic gas phase precursors into the polymer template [16]. Recently, the ALD processing conditions were adapted to more effectively infiltrate the BCPs; the modified process was named the sequential infiltration synthesis (SIS) [17-19]. SIS is based on diffusion-controlled penetration and subsequent chemisorption of inorganic precursor molecules from a gas phase inside a polymer template. It was shown that the infiltration of the inorganic precursors into the polar polymer or polar domains of the BCP occurs through a weakly-bound intermediate and its kinetics is self-limited [20]. The exposure of the BCP containing polar (e.g. poly (methyl methacrylate) or poly(vinyl pyridine)) and nonpolar (e.g. polystyrene) domains to inorganic precursors (e.g. TMA) results in the selective growth of alumina inside the polar part of the BCP while the nonpolar polystyrene part remained uncoated [2, 9, 21, 22]. Following SIS, the polymer matrix is removed via thermal annealing, UVozone or oxygen plasma treatment. As a result, the porosity and composition of the synthesized inorganic material replicate the self-assembled nanostructures of the BCP [23], where the inorganic precursors can diffuse. In typical SIS, the diffusion of the inorganic precursors has limitations in terms of depth of the vapor penetration inside the polymer [3, 11, 20, 21] that prevents the growth of thick coatings or membranes. The limitation can be overcome by the introduction of additional porous channels in


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the BCP template. Swelling of the BCP is an effective approach for introducing porosity in the polymers [24-26]. When dissolved in a nonpolar solvent, BCPs form the micelles in which the hydrophilic chains assemble as cores surrounded by the corona of dissolved hydrophobic chains. In spin-casted films, the coronae of the neighboring micelles merge forming a continuous hydrophobic film with the embedded hydrophilic domains [25]. Exposure of the spin-coated BCP films to polar solvents leads to the opening of the micelles as a result of their swelling caused by the polar solvent molecules diffused through the hydrophobic coronae [27]. The interface between the non-polar matrix and polar nanodomains has been found to play an important role in the diffusion/reaction behavior of the infiltrated inorganic precursors [22]. Since the BCP swelling significantly modifies the interface between polar and nonpolar domains, this process is likely to affect significantly the diffusion and reaction behavior of the gas phase inorganic precursors in the swelled BCP template. In our previous studies, we demonstrated that swelling enables the synthesis of the films with up to 10 μm thickness [28]. While atomic force microscopic (AFM) and small angle X-ray scattering (SAXS) studies satisfactorily explain the modification of the self-assembled BCPs [19, 24, 25, 29-32], there is no understanding on the effect of the BCP swelling on the infiltration of the modified BCP template by the inorganic precursors. Understanding this is important for the further development of the synthesis of highly porous structures for the design of advanced catalytic systems, antireflective coatings and structures for water purification and sensing applications. Here we present the results of the detailed study that provides the quantitative information about the opening of micelles in BCPs upon swelling and its effect on the infiltration of the swelled BCP templates with inorganic precursors, as well as the effect of the UV exposure on the BCP removal after the infiltration.

Experimental Procedure Synthesis of the samples: The samples on the QCM were prepared using a polystyrene-b-poly-4vinyl pyridine (PS-b-P4VP) block-copolymer template, with the molecular weights of the nonpolar domain (PS) and polar domain (P4VP) equal to 75k g/mol and 25k g/mol, correspondingly. The polymer powders were purchased from Polymer Source Inc. The 3 wt% and 5 wt% toluene solutions of PS-b-P4VP, filtered from agglomerated clusters through the 0.2 μm pore size syringe filters (Fisher Scientific), were spin-coated at 3500 rpm on the surface of AT-cut (oscillating in a shear mode) QCMs with titanium-coated electrodes. Thicknesses of resulting polymer films were


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110 nm and 243 nm correspondingly. Prior to spin coating, the BCP solutions were filtered through polytetrafluoroethylene (PTFE) filters with 200 nm large pores to remove non-dissolved polymer agglomerates. After spin-coating, the QCM samples were baked at 90oC for 10 minutes to ensure full evaporation of the toluene and improve adhesion of the film to the substrate. During swelling, samples were immersed in ethanol for 1 hour at 75 oC. The beaker with ethanol was covered to ensure uniformity of the temperature monitored with an immersed in ethanol probe. SIS of alumina, as a model material for the presented study, in the polymers was performed using the Cambridge Nanotech Inc ALD system at 90⁰C. The temperature was lower than for traditional ALD process in order to prevent modification of the polymer. A QCM substrate coated with a polymer film was loaded on a stainless steel tray and kept at 20 sccm nitrogen flow for 30 minutes prior to the deposition. Five cycles of SIS were performed using the following recipe. In each cycle, first, 10 mTorr of the TMA precursor was administered with 20 sccm nitrogen flow into the reactor for 100 s. After that, the excess of the reactant was evacuated for 20 s and 10 mTorr of H2O was applied for 100 s followed by purging of nitrogen (100 sccm) for 300s to remove notinfiltrated byproducts. The polymer removal after the SIS was performed using the UV ozone cleaner (UVOCST16x16 OES, 254 nm UV wavelength) for 24 hours at room temperature to avoid heatproduced densification of the porous alumina. Additionally, in the case of traditional oxidative thermal annealing used before, the high-temperature conditions (≥450⁰C) are destructive for the QCM substrate. During the UV ozone treatment process, the QCM resonant frequency was intermittently monitored to confirm that the polymer removal process is complete and no further changes to the samples occur. Schematic of the whole synthesis process can be found in Supplementary Info (Figure S1). QCM measurements: QCM technique was used for the quantitative analysis of the porosity evolution events. Previous studies indicate QCM as a sensitive technique for non-disruptive insitu analysis of the materials modifications [33, 34]. Titanium-coated AT-cut crystals (1 inch in diameter) with 5MHz base resonant frequency were purchased from Fil-Tech. The resonant frequency and mechanical resistance of the QCM oscillations were monitored using SRS QCM 200 system. In the classic approach for evaluating QCM frequency change under applied load, caused by the deposited mass is [35, 36]:


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𝛥𝑓 = − )

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&'( ∆𝑚 *+, -,

(Eq.1)

In the case of immersing the QCM in a viscous medium, the change in the frequency is [37]:

Df = - f 0

3/ 2

r Lh L pµ q r q

(Eq.2)

where f0 is the fundamental frequency of the QCM, ρq= 2.648 g/cm3 is the density of quartz, µq= 2.947 × 1011 g∙cm-1s-2 is the shear modulus of quartz, ρL, and ηL are the density and viscosity of the liquid, respectively. In case of QCM immersed in water, ρL = 0.9982 g/cm3 and ηL = 0.01 g/(cm∙s). In case of QCM immersed in ethanol, ρL = 0.789 g/cm3 and ηL = 0.007 g/(cm∙s). Based on the density and viscosity values the expected frequency shift upon immersing in water is 700Hz. In the case of the water penetration inside the pores, the resonant frequency change is modified by the added mass of the water entrapped inside the pores[26]: ∆𝑓 = −𝑓

12 + 5 &3 4 4 6-, +,

−)

&' ( *-, +,

∆𝑚

(Eq.3)

Therefore, the porosity volume inside the formed structures can be determined from the additional frequency change observed when immersing porous structures in water and ethanol environments. The mechanical resistance of the QCM oscillations, measured in Ohms is another measure for the surface changes. The resistance caused by the viscous environment is quantitatively added to the oscillator circuit to sustain stable QCM oscillations [38]. For the QCM with ideally smooth surface immersed in liquid, the mechanical resistance can be calculated as [39]:

DR = (2 fLu )

4pfr Lh L

µq rq

(Eq.4)

where Lu is inductance for the dry resonator. In the case of the water environment, the expected mechanical resistance value is 320 Ohms. All QCM measurements were performed under ambient conditions. The QCM measurements in water were used for quantitative monitoring of the micelles opening and pore generation process.


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Characterization: The thickness changes during the stages of infiltration were further characterized using an FEI Nova Scanning Electron Microscope (SEM). For that, 10nm thick gold film was e-beam evaporated on the surface of the samples to avoid charging effects and improve the resolution of the SEM images. The thickness and porosity of the films were further confirmed using Horiba Ellipsometer analysis. Surface morphology analysis of the samples was performed with Bruker Multimode Atomic Force Microscope (AFM). AFM images were acquired in a taping mode (0.5 Hz scanning rate) using the silicon tips. Transmission electron microscopy (TEM) was conducted using the JEOL 2100F instrument. The samples for TEM analysis were prepared by drop casting of the alumina scratched from the substrate and suspended in acetone to deposit on to the TEM grids. Chemical modification of the polymers during swelling and SIS were evaluated using a Nicolet 6700 Fourier Transformation Infrared spectrometer (FTIR) with 1000−4000 cm−1 spectral range. To analyze the dissolution of the polymer in ethanol during swelling, the ethanol used for swelling was drop-casted on the silicon substrate and dried in nitrogen gas flow.

Results and Discussion Previously, we have shown that the swelling of the BCP template in ethanol prior to SIS facilitated the infiltration of the polar domains with inorganic precursors and allowed an efficient increase in the thickness and porosity of the resulting inorganic highly porous films [14, 26, 28]. Indeed, the swelling of the 110 nm thick PS-b-P4VP template at 75 oC in the ethanol followed by 5 cycles of SIS and polymer removal by UV ozone treatment results in the formation of highly porous 190 nm thick alumina film (Figure 1a). The TEM analysis on the fragments of the deposited porous alumina scratched from the substrate indicates the presence of entangled channels. Such channels are formed during the TMA infiltration of the P4VP micelles fully opened during PS-bP4VP swelling in the ethanol [25].


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Figure 1. (a) SEM and (b) TEM images of the nanoporous alumina films produced via swelling-assisted SIS. The 110 nm thick PS-b-P4VP was swelled for 1 h in ethanol followed by 5 SIS cycles for deposition of alumina and UV ozone exposure for polymer removal. The TEM image in (b) is taken out of focus to highlight the random orientation and hollow structure of the alumina, while the inset presents an in-focus magnified view of individual alumina hollow structure.

In order to obtain the insights into micelle opening and optimize the swelling regime of PS-bP4VP BCP, we used the QCM technique. The titanium-coated AT-cut crystals with deposited 110 nm thick PS-b-P4VP films were immersed in ethanol at different temperatures. Increase in the polymer porosity associated with the penetration of the solvent into the pores caused the change in the resonant frequency and mechanical resistance of the QCM oscillations (Figure 2). Notably, the major swelling process proceeds very rapidly, within the first five minutes indicating the fast P4VP micelles opening that is in agreement with the AFM studies reported earlier for PS-b-P2VP BCPs [25]. We attribute such a change to the rapid absorption of the ethanol inside the polymer with a continuous increase in absolute value of the frequency change during the expansion process of the polystyrene matrix. The largest shift in the frequency is observed in case of swelling the samples in ethanol at 75 °C (Figure 2). Observed in Figure 2b increase in mechanical resistance of the QCM oscillations (as compared to the non-coated QCM) for the polymer during swelling is attributed to viscous nature of the swollen polymer partially decoupling from the oscillation frequency. Swelling of the PS75k-b-P4VP25k in ethanol at 75 °C was found to modify the polymer template the most and, hence, such swelling regime was selected for all further experiments in this study.


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Figure 2. Changes in (a) resonant frequency and (b) mechanical resistance of PS75k-b-P4VP25k polymer film (110 nm original thickness) during swelling in ethanol at different temperatures. The results for the swelling of the block copolymer are compared to uncoated QCM upon immersion in ethanol at 75°C. Mechanical resistance increase suggests more viscous nature of the swelled polymers and confirms no detachment of the film from the surface. Inset represent magnified view for initial 300s of immersion.

Analysis of the QCM data indicates that the weight of the 110 nm thick PS75k-b-P4VP25k is ~63 μg where ~16 μg belongs to P4VP. The ethanol absorbing capacity of such films during the swelling in ethanol at 75 °C, estimated from the difference in the QCM frequency change before and after swelling, is found to be ~32 μg. In turn, the weight of the BCP film dried after swelling is estimated as ~56 μg. This observation points out to the mass reduction in the PS75k-b-P4VP25k film by ~7 μg due to, probably, partial dissolution of the BCP during the micelle opening.


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Figure 3. Summary on mass changes measured by the QCM during the polymer swelling, 5 cycles of SIS and polymer removal (all changes in the frequency are normalized by the frequency of a clean QCM): (a) for non-swelled and swelled 110 nm thick PS75k-b-P4VP25k films and swelled 243 nm thick PS75k-b-P4VP25k films; (b) for 110 nm thick P4VP and PS polymer films deposited on the QCM. Swelling of pure P4VP results in full detachment of the polymer during swelling and therefore cannot be monitored.

Infiltration of the non-swelled and swelled 110nm thick PS75k-b-P4VP25k templates with alumina precursors during 5 SIS cycles resulted in the mass increase by ~ 14 μg and ~36 μg, respectively. While in the case of non-swelled PS75k-b-P4VP25k templates infiltration of only a top layer of the BCP film takes place, the porous channels developed during the swelling provide effective pathways for the delivery of the alumina precursors from the gas phase resulting in ~2.6 more efficient infiltration of the BCP template. Note, that the SIS process does not induce any deformation of the swelled BCP template (Supplementary Figure S2). It is worth noting that for both non-swelled and swelled samples, the final masses of the material after the UV ozone treatment are larger than infiltrated 14 μg and 36 μg of alumina. This difference is attributed to the fact that the UV ozone treatment oxidizes the titanium coated QCM leading to small increase in the mass of the crystals. In case of the P4VP and PS coated QCMs, the attributed to the oxidation mass increase is very little due to more uniform protective alumina layer than in case of the BCPproduced porous alumina film. Further exposure of the QCM to the prolonged UV ozone cleaning or oxygen plasma has no effect on the final mass. Energy-dispersive spectroscopy analysis of the films before and after UV ozone treatment indicates no detectable presence of carbon in the final nanoporous alumina samples (Supplementary Figure S3).


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Similar trends in terms of the mass loss of the BCP during swelling and presence of the remnant carbon species are observed for the thicker (243 nm) films of PS75k-b-P4VP25k deposited on the QCM and swelled. Both thin (110 nm) and thick (243 nm) BCP films infiltrate the same amount of alumina precursors per weight of the P4VP weight in each sample. Note, that in the case of thermal oxidative annealing (≥ 450 oC), the complete removal of carbon can be achieved [14]; however, such high temperatures destroy the QCM and initiate reconstruction of alumina. Our control QCM experiments performed on pure P4VP and PS polymers demonstrate that pure P4VP adsorbs ~24 μg of the alumina, which is higher than for non-swelled polymer (due to higher presence of the P4VP domains in the polymer), but lower than for swelled polymer (due to inaccessibility of the polymer film underneath the top layer) (Figure 3b). Meanwhile, almost negligible change in the mass is observed during swelling and infiltration of the pure polystyrene. The mass of the sample during infiltration increased by 3.7 μg only. We attribute this change to the growth of the alumina on the surface of the PS[22]. The FTIR measurements (Figure 4) indicate that swelling increases absorption for CH bending of CH3 and CH2 compounds at 1580, 1500, 1450, and 1410 cm-1 [40], which are the reactive sites during alumina infiltration process. As it was reported before, in case of the P4VP polymer, CHx compounds are the major sites for alumina growth [28]. In order to understand the origin of the mass decrease of the PS75k-b-P4VP25k templates during the swelling we collected the ethanol used for swelling and analyzed its composition by FTIR. For that, we evaporated the ethanol used for swelling on the silicon substrate. Figure 4b demonstrate that sample obtained by evaporation of the ethanol used for swelling demonstrates the absorption features characteristic for P4VP, PS, and PS75k-b-P4VP25k. This observation confirms that the observed decrease in the mass during the swelling step is attributed to partial dissolution of the PS-b-P4VP molecules. In the case of thick and thin PS75k-b-P4VP25k films, the mass losses during the swelling are similar that indicates that dissolution takes place mainly from the surface layer.


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Figure 4. FTIR data on (a) non-swelled and swelled PS75k-b-P4VP25k templates during their infiltration with alumina precursors and (b) comparison of FTIR spectra of PS, P4VP and PS75k-b-P4VP25k with FTIR spectrum of the sample prepared by evaporation of ethanol solution used for swelling.

To study the evolution of the porosity in swelled BCP template, its infiltration with inorganic precursors by the SIS, and the final inorganic structure, we monitored the frequency and resistance change of the QCM oscillations of all samples in water. Water was chosen as a media since swelling of the PS75k-b-P4VP25k in water is significantly slower as compared to ethanol [25]. As a result, no micelle opening, and pore formation take places during the first minutes after immersion of the QCM modified with BCP into deionized (DI) water. Therefore, the switching of the media from ethanol to water enables the comparative study on the porosity of the samples undergoing and not undergoing swelling. All samples prepared using non-swelled PS75k-b-P4VP25k polymer films demonstrate similar behavior in the frequency change during their immersion in DI water, though alumina films after polymer removal by UV ozone demonstrate larger frequency change due to the added mass of the water entrapped in the pores formed from PS removal (Figure 5a). The PS75k-b-P4VP25k film swelled in ethanol demonstrates a very large frequency change, accompanied by an increase in the mechanical resistance originated from the viscous nature of the polymer. However, infiltration of the polymer with alumina suppresses viscous behavior of the polymer and only porous channels remained open after infiltration initiate the water trapping effect. The UV ozone treatment performed for the polymer removal generates the additional porosity.


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Previously, we have demonstrated that alumina structures obtained by the SIS are highly hydrophilic and water easily penetrates inside the pores [26]. As a result, we can estimate the porosity from the amount of water penetrated inside the structures. Analysis of the data presented in Figure 5a and Figure 5b demonstrates that swelling of 110 nm thick PS75k-b-P4VP25k template results in more than 4 times increase in porosity (Figure 5e).

Figure 5. QCM analysis of the porosity for the non-swelled and swelled PS-b-P4VP polymer films (110nm thickness of initial polymer) during different stages of swelling, SIS, and polymer removal. Changes in the resonant frequency (a) and (c) resistance for the sample prepared using non-swelled and (b) and (d) swelled 110 nm thick polymer templates. (e) Summary of the effective pore volume values when neglecting the decoupling of the polymer due to visco-elastic nature. The effective pore volume values were calculated using Eq. 3. Note that viscous behavior of the swelled polymer film before infiltration of the alumina precursors significantly contributes to the QCM oscillations. Therefore, the actual volume of the porosity of the polymer immediately after swelling is smaller.


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We also monitored with the SEM the changes in the polymer thickness and morphology during BCP swelling, its infiltration with alumina precursors and polymer removal (Figure 6). In this study, we e-beam evaporated ~10 nm of the gold layer on top of the samples to make the surface conductive for the SEM measurements. The initial thickness of the 3% coated polymer film is measured to be ~110 nm that is in good agreement with the thickness estimated from the mass increase effect on the frequency change after spin-coating of the PS75k-b-P4VP25k film (~117 nm). SEM data show that the swelling of the PS75k-b-P4VP25k film leads to ~2 times increase in the polymer thickness. Infiltration of the swollen BCP structure during SIS increased its mechanical stability accompanied by the small thickness reduction associated with the increase in the temperature (up to 90 °C SIS processing temperature). The changes in the samples’ thickness during different stages of the SIS are almost the same for thin (110 nm) and thick (243 nm) BCP films. These data suggest complete infiltration with the alumina of the polar domains through the whole thickness of the PS75k-b-P4VP25k.

Figure 6. AFM surface topography (a-d) and SEM thickness analysis (at 52o) (e-l) of PS75k-b-P4VP25k films during different stages of swelling and infiltration for 110 nm and 243 thick PS75k-b-P4VP25k BCP templates, (e-f) and (i-j) respectively.


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Conclusions We conducted a quantitative study of the swelling effect on the SIS- infiltration of the BCP template by inorganic precursors by the QCM. Our results demonstrated that swelling of the 110nm thick PS75k-b-P4VP25k film in ethanol results in up to 4 times increase of its effective porosity volume that, in turn, leads to 2 times increase of the film thickness. We show that swelling of the polymer progresses rapidly and results in a stable structure after 30 minutes at 75°C ethanol. Swelling does not create additional polar groups in the polymer, but rather enables access to all available polar domains in the polymer enabling the more efficient infiltration of the alumina precursors. Understanding of the micelles opening upon swelling of the BCP and its impact on the infiltration of the BCP templates with the inorganic precursors is important for the predictive design of the filtering membranes, antireflective coatings, sensors, and catalytic supports.

Supporting Information Available Figure S1: Schematic of the swelling and infiltration process. Figure S2: Effect of the heating on the porosity changes in the swelled polymer. Figure S3. Energy-dispersive x-ray spectroscopy analysis of the alumina infiltrated BCP.

Acknowledgments This work was performed in part at the University of North Texas’ Materials Research Facility. Support from Advanced Materials and Manufacturing Processes Institute (AMMPI) at the University of North Texas is acknowledged. The authors appreciate the support from A. Voevodin in the AFM work. Work at the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH-11357.


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TOC Graphic


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Table of Contents Graphic 82x44mm (96 x 96 DPI)


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Effect of the micelle opening in self-assembled amphiphilic block

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