Integrated Compositional and Nanomechanical Analysis of a

Sep 30, 2014 - It should be borne in mind that while the analysis depth for XPS is ... Thus, XPS gives a view of chemical composition on a microscale ...
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Integrated Compositional and Nanomechanical Analysis of a Polyurethane Surface Modified with a Fluorous Oxetane SiliceousNetwork Hybrid Sithara S. Nair,† Eric J. McCullough,† Vamsi K. Yadavalli,* and Kenneth J. Wynne* Department of Chemical and Life Science Engineering, Virginia Commonwealth University 601 West Main Street, Richmond, Virginia 23284, United States S Supporting Information *

ABSTRACT: Investigating the surface characteristics of heterogeneous polymer systems is important for understanding how to better tailor surfaces and engineering specific reactions and desirable properties. Here we report on the surface properties for a blend consisting of a major component, a linear polyurethane or thermoplastic elastomer (TPU), and a minor component that is a hybrid network. The hybrid network consists of a fluorous polyoxetane soft block and a hydrolysis/condensation inorganic (HyCoin) network. Phase separation during coating formation results in surface concentration of the minor fluorous hybrid domain. The TPU is H12MDI/BD(50)-PTMO-1000 derived from bis(cyclohexylmethylene)diisocyanate and butane diol (50 wt %) and poly(tetramethylene oxide). Surface modification results from a novel network-forming hybrid composed of poly(trifluoroethoxymethyl−methyl oxetane) diol) (3F) as the fluorous moiety end-capped with 3-isocyanatopropylriethoxysilane and bis(triethoxysilyl)ethane (BTESE) as a siliceous stabilizer. We use an integrated approach that combines elemental analysis of the near surface via X-ray photoelectron microscopy with surface mapping using atomic force microscopy that presents topographical and phase imaging along with nanomechanical properties. Overall, this versatile, high-resolution approach enabled unique insight into surface composition and morphology that led to a model of heterogeneous surfaces containing a range of constituents and properties.

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INTRODUCTION Polyurethanes form a versatile class of polymers with diverse properties based on a wide range of compositions. They find use as surface coatings, sealants, adhesives, paints, elastomers, insulating, and cushioning foams and are often employed as biomaterials due to good biocompatibility.4−6 Polyurethanes are composed of hydrogen-bonded hard segments that confer strength and low glass-transition (Tg) soft segments for elasticity. Low molecular weights for hard and soft segments give rise to nanoscale phase separation and optical transparency. Compositional possibilities are multiplied in that polyurethane or polyurethane−urea linkages depend on whether the chain extender is a diol or a diamine. Although a plethora of polyurethanes and polyurethane ureas are available as commodity polymers, laboratory-synthesized compositions are often employed so that composition can be correlated with engineered surface and bulk morphologies.7−9 Because the response of a polymer surface to external stimuli is controlled by the outermost layer, modifications to the surface on the nanoscale may be used to tune macroscale interactions. Understanding the effects of a modifier on surface characteristics is an important step in tailoring surfaces to elicit specific interactions and desirable properties. For polyurethanes, surface modification may be accomplished by adding a surface-active © 2014 American Chemical Society

species to the feed during synthesis. For example, small amounts of hydroxyl-terminated perfluoropolyether (PFPE) (0.1, 0.25, 0.5, and 1 mol %) added during polyurethane synthesis resulted in increased hydrophobicity.10 Polydimethylsiloxane chain ends concentrate at the nanosurface (∼1 nm) and confer hydrophobicity and biodurability.11 An alternative involves blending in a small quantity of a polyurethane with a surface active soft block to give a desired surface function.7 Both polydimethylsiloxane12 and fluorous11,13−16 soft blocks have been employed. Our group recently studied a blend of an MDI-based polyurethane MDI-BD(36)-PTMO(2200) with 2 wt % MDI/BD(29)-P(3F-7100). Using X-ray photoelectron spectroscopy (XPS), the surface elemental composition was indistinguishable from that calculated for the neat fluorous polyurethane.7 Instead of using polyurethane with a fluorous soft block, the present investigation focuses on a hybrid approach for polyurethane surface modification. The pioneering work of Saegusa and Chujo led to development of the field of hybrid polymeric materials.17 The hydrolysis/condensation of silicon Received: August 12, 2014 Revised: September 26, 2014 Published: September 30, 2014 12986

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Scheme 1. End-Capping Reaction of Purified 3F-5.2 Diol 1 and 3-Isocyanatopropyltriethoxysilane 2 Giving the Modifier, ES-3F 3, and Subsequent Hybrid Coating Preparation

alkoxides to a cross-linking inorganic −SiOx-component or “sol gel chemistry” often improves mechanical and thermal properties of the resulting nanocomposites.18−25 The conceptual beginning for hybrid surface modification was the discovery of nanoscale and mesoscale concentration for the hybrid domain in a polyurethane hybrid.26 These optically transparent heterogeneous blends were composed of a linear polyurethane having a fluorinated “3F” polyoxetane soft block and a hybrid domain composed of the same 3F soft block and a moisture-cured siliceous component. A surprising result from ATR-IR spectroscopy was mesosurface (1000 nm) concentration of the hybrid component and concomitant depletion of bulk linear polyurethane U-3F. Given facile moisture cure for silicon alkoxides and hybrid domain surface concentration,26 the same approach was tried for 3F polyurethane surface modification described herein. Given hydrolysis, condensation, and inorganic network formation, the term “HyCoin” is useful to represent concisely network formation chemistry.27−29 To employ HyCoin for 3F surface modification, we end-capped a poly(trifluoroethoxymethyl−methyl oxetane) diol with a triethoxysilane to yield the modifier ES-3F (Scheme 1). Inclusion of (triethoxysilyl)ethane (BTESE) provides an additional volume fraction of siliceous domain. The aliphatic polyurethane H12MDI-BD(50)-PTMO-1000 or simply U50 was chosen for surface modification, where the hard block (50 wt %) is derived from 4,4′-methylenedicyclohexyl diisocyanate and butane diol, while the soft block is polytetramethylene oxide. Here we develop a multiscale approach for the characterization of this hybrid polymer by correlating elemental nanosurface chemical analysis obtained via XPS, with nanoscale surface topography and nanomechanical analysis obtained via atomic force microscopy (AFM). Both survey spectra and highresolution C 1s spectra contribute to understanding chemical composition and bulk/modifier and hybrid modifier phase separation. AFM is widely used for visualizing the micro- and nanoscale surface topography of materials.30−34 A number of morphological investigations of polyurethanes at the nanoscale

have been reported using conventional AFM.35−40 AFM provides high-resolution imaging and can simultaneously characterize the surface mechanically and chemically.41,42 The capability to measure viscoelastic, friction, and adhesive interaction forces at nano- and piconewton sensitivity provides unique mechanical insight into the surface. Using nanoindentation, we report on nanoscale topography and mechanical properties obtained simultaneously and rapidly. The overlay of this data allows the creation of a “surface map” that uncovers a deeper, quantitative characterization of the surface. As such, nanoindentation mapping provides an unambiguous identification of the origin of contributions to phase separated nano- and microdomains. Previous AFM studies on blends of a base polyurethane with a polyurethane having a fluorous soft block have revealed nanoand microscale phase separation.7 Here polyurethane modified by a new hybrid method is characterized by mapping nanosurface mechanical properties at different depths. The objective is to report on this novel hybrid surface-modified polyurethane as well as to present a new approach for characterization. We draw correlations between elemental analysis and nanoscale features such as height and phase and mechanical properties. By integrating this data with surface composition, a new way is described to investigate surface modification. We show how conditions for probing these nanomechanical properties reveal different behaviors at nanoscale depths. These studies are important for guiding the development of the new hybrid method for surface modification.

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EXPERIMENTAL SECTION

Materials. PolyFOx diol PF-6320, which is designated as 3F-5.2 based on Mn (number-average molecular weight), was provided by OMNOVA Solutions, Inc. (Fairlawn, OH). 3-Isocyanatopropyltriethoxysilane (IPTS) (SII 6455) and bis(triethoxysilyl)ethane (SIB 1817, BTESE) were purchased from Gelest (Morrisville, PA). Methylene bis(4-cyclohexylisocyanate) (HMDI), poly(tetramethylene oxide) (PTMO), and dibutyltin diacetate (DBDTA) were purchased from Aldrich. Tetrahydrofuran, 99.6% (for analysis ACS, stabilized 12987

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with BHT), and 1,4-butanediol (BD) was obtained from Acros Organics. 3F Hybrid Polyurethane Blends. The purification of commercially available poly(trifluoroethoxymethoxy-methyl)oxetane (3F) diol was carried out by liquid−liquid extraction as described in detail previously.43 Briefly, as-received 3F diols are extracted 8−10 times with hexane to yield 3F-5.2 (Mn 5.2 kDa) and a polydispersity of 1.26. To synthesize the base polyurethane, HMDI-BD(50)/PTMO-1000 (U50), 50 wt % hard block, we used the “soft segment first” method.44 Purification was done by dissolution in THF and precipitation into water. After precipitation, U50 was vacuum-filtered and dried under vacuum at 60 °C for 48 h. The hybrid modifier, designated 3F−Si(OEt)3 or ES-3F was prepared in a single step described in Scheme 1. In a typical example, 5 g 3F-5.2 diol (1) in 10 mL of THF was added dropwise to 0.58 mL of 3-isocyanatopropyltriethoxysilane (2) in 15 mL of THF (molar ratio of 1:2) and 0.04 g of DBDTA catalyst. The solution was held at 40 °C under a dry nitrogen purge for 4 h. The formation of the hybrid was achieved by adding the modifier ES-3F (3) in THF with a 10 wt % base U50 (5) solution also in THF. Modifier concentrations of 0.2, 0.5, and 1 wt % were generated (Scheme 1). To aid cross-linking and increase the volume fraction of siliceous domain, we also added 2 wt % bis(triethoxysilyl ethane) (BTESE) (4). This low BTESE feed minimizes shrinkage and generation of volatiles from HyCoin cure. The reactants were stirred for a minimum of 30 min. Microscope slides were drip-coated (one side), held at ambient temperature for 2 to 3 h, overnight at 60 °C, and finally 120 °C for 2 h. Final coating thickness was ∼60 μm. Modified polyurethane compositions are represented as U50-(3Fx)-y-SiO1.5; U50 is HMDI-BD(50)-PTMO-1000, and 3F represents the hybrid modifier (Scheme 1). HyCoin composition is complex but represented as SiO1.5 derived from ES-3F alkoxy-Si(OC2H5)3 end groups and BTESE (CH2CH2)[Si(OC2H5)3]2. As discussed herein, x and y are weight percent estimated to remain after HyCoin cure (Scheme S1, Supporting Information). The molecular weight of 3F is several times higher than that of the end groups so that the contribution to −SiO1.5 after HyCoin network formation from ES-3F end groups (C(O)NH(CH2)3SiO1.5) is small. Thus, the wt % of ES-3F is little changed from the feed. However, a considerable weight loss (∼67% lost as ethanol, Scheme S1, Supporting Information) occurs in the formation of Si2C2H4O3 from BTESE. Designations in figures and elsewhere are sometimes based on the x/y wt % in the final composition after HyCoin formation, that is, U50-(3F-0.2)-0.74-SiO1.5, (0.2/0.74), U50-(3F-0.5)-0.74-SiO1.5 (0.5/ 0.74), and U50-(3F-1.0)-0.74-SiO1.5 (1/0.74). X-ray Photoelectron Spectrometry. Measurements were carried out with a Thermo Fisher Scientific ESCALAB 250 instrument. Analysis utilized monochromatic Al Kα X-rays with an X-ray spot size of 500 μm and a takeoff angle of 90°. Pass energy for survey spectra was 150 eV. Pressure in the analytical chamber during spectral acquisition was maintained at 2 × 10−8 Torr, while an argon electrostatic flood source affected charge neutralization. Cured samples were cut and attached to the surface of a silicon wafer using carbon tape. Data were analyzed with the Thermo Avantage software (v4.40). Imaging and Nanomechanical Measurements. Surface topography and mechanical properties were determined from AFM imaging and nanoindentation using an MFP 3D instrument (Asylum Research, Santa Barbara, CA). Images and indentation profiles for each sample were taken over a 25 μm2 area of the surface using a Multi75DLC tip (Innovative Solutions, Bulgaria) with a nominal spring constant of 3.54 N/m. Spring constants were measured prior to each experiment using the thermal fluctuation method.45,46 Data analysis was performed using custom routines written in Igor Pro (WaveMetrics, Oswego, OR). Surface topographic images were collected using noncontact mode imaging in air. Three different 5 × 5 μm areas of the polymer surface were analyzed for each of the compositions. Indentation profiles were collected after surface topographic imaging by selecting >20 discrete points on the surface per sample. The indentation trigger mode was set to a relative force deflection of 1 nN, after which indentation force−distance profiles

were collected by indenting 10 nm into the surface at a rate of 10 nm/ s. Indentation for modulus maps was performed in a constant force mode, wherein the AFM tip was lowered into the surface at a constant rate of 400 nm/s until a maximum force was reached, typically 50 nN. Surface maps for estimating mechanical properties were collected as sequential n × n arrays of force curves over an area of interest. Processing was performed using a custom program written in Igor Pro software. Briefly, the program analyzes each force curve to determine the point of contact with the surface. After the contact point has been established, the program then separates the loading and unloading portions of the force distance (F−D) curve, determines the force of adhesion, and fits the data with the contact model. It then checks that the fit is statistically relevant and ignores any data that is not. If the fit is determined to be good, material properties are then calculated based on the model. 3D maps showing spatial distribution were created by overlaying the modulus data map on the 3D topography data. Data Analysis/Contact Model. The most widely used model for interpreting indentation data of polymers has been the Hertz model using Sneddon’s modified Hertz equation when conical tips are employed (eq 1)47 F=

Esurface 2 tan(α)δ 2 π (1 − νsurface )

(1)

where F is the indent force, ν is the Poisson’s ratio, E is the elastic modulus, α is the cone half angle of the indenter tip, and δ is the depth of the indent. For this work, the force−distance curves were analyzed using this model.

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RESULTS Preparation of Hybrid 3F-Modified U50. Hydrolysis, condensation, and inorganic network formation (HyCoin) for silicon alkoxides and hybrid domain surface concentration reported by our group26 led to the present investigation of a new method for surface modification. For HyCoin, 3F diol polyoxetane end groups were converted to −Si(OEt)3 using a method similar to that previously reported.29,48 As shown in Scheme 1, the end-capping of purified 3F-5.2 diol 1 and 3isocyanatopropyltriethoxysilane 2 gives the modifier, ES-3F 3. Hybrid coatings designated U-(3F-x)-y-SiO1.5 were prepared by adding aliquots of the ES-3F solution and BTESE to a solution of base polyurethane U50, followed by dip or drip coating, removal of solvent, and initial cure at 25 °C for 2 to 3 h, overnight at 60 °C, and finally 120 °C for 2 h. Coating thickness was ∼60 μm. All coatings were optically transparent (Figure 1). The wt % siliceous domain Si2C2H4O3 from BTESE designated “-SiO1.5” was constant (0.74), but three levels of fluorous modifier were investigated: x = 0.2, 0.5, and 1.0 wt %.

Figure 1. Optically transparent cast films of U50-(3F-0.2)-0.74-SiO1.5 (top) and U50-(3F-1.0)-0.74-SiO1.5 (bottom). 12988

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calculated values for U-(3F-1.0)-0.74-SiO1.5 compared with the modifier alone. These trends are further reflected in the nanomechanical measurements presented in the next section. High-Resolution C 1s Spectrum. Figure 2 shows C 1s XPS spectra and curve fitting for U50-(3F-0.2)-0.74-SiO1.5 and U50-

Results from XPS chemical characterization of the nanosurface are first discussed, followed by nanomechanical characterization. X-ray Photoelectron Spectroscopy. Nanosurface Elemental Composition. XPS atom % F, O, C, N, and Si for U(3F)-SiO1.5 coatings are listed in Table 1. The 90° takeoff angle Table 1. XPS Results for U50-(3F-x)-y-SiO1.5 Coatings atom percent (%) (observed)

a

designation

C

U50-(3f-0.2)-0.74-Sio1.5 U50-(3f-0.5)-0.74-Sio1.5 U50-(3f-1.0)-0.74-Sio1.5

60.2 53.1 56.8

(3F-0.2)-0.74-SiO1.5 (3F-0.5)-0.74-SiO1.5 (3F-1.0)-0.74-SiO1.5 3F soft block 3F + SiO1.5 end groupsa SiO1.5 (Si2C2H4O3)b U50

51.6 54.9 55.8 58.3 57.6 28.6 78.9

F

O

Si

10.3 19.8 8.6 16.8 22.1 6.8 18.1 19.8 4 atom percent (%) (calculated) 18.8 21.5 22.4 25 23.7

22.8 19.9 18.6 16.7 17.5 42.8 17.5

N 0 1.0 1.0

6.4 3.2 2.7

0.4 0.5 0.5

0.6 28.6 0

0.6 3.5

ES-3F after HyCoin. bBTESE after HyCoin.

employed corresponds to a penetration depth of ∼5 nm.49 At least two to three areas on each coating were used for analysis. For peak deconvolution, PeakFit version 4.12 was used for high-resolution C1s spectra assuming 100% Gaussian peaks. The shift of the binding energy was corrected using the hydrocarbon C 1s binding energy at 284.8 eV as an internal standard. To guide the discussion concerning nanosurface elemental composition, we calculated atom% for the following: U-(3F-0.2)-0.74-SiO1.5, U-(3F-0.5)-0.74-SiO1.5, U-(3F-1.0)0.74-SiO1.5, 3F soft block alone, and U. In addition, 3F-SiO1.5 (the designated composition from ES-3F) and Si2C2H4O from BTESE are listed, both after HyCoin. High fluorine atom% was found for U-(3F-x)-y-SiO1.5 compositions confirming hybrid surface modification. The trends for atom% C, F, O, Si, and N with increasing modifier wt % follow the calculated values for the 0.2, 0.5, and 1.0 wt % modified polyurethane coatings. Atom% C decreases and F increases with increasing modifier wt % approaching the calculated values for 3F-SiO1.5 after ES-3F HyCoin. For the U(3F-x)-0.74SiO1.5 coatings, F atom % was somewhat lower than calculated, while C and O atom % were closer to the calculated values. A systematic trend is not observed for atom% O but is in the range calculated for the respective modified compositions. Although near the limits of detection, 1 atom % N is close to that calculated for the modifier rather than that for the bulk polyurethane U50 (3.5 atom %), suggesting minimal phase mixing of the hybrid domain with the bulk. Minimal phase mixing is also supported by the much lower at % C for U-(3Fx)-0.74SiO1.5. Interestingly, the observed Si atom % for all three coatings was at least ∼50% higher than calculated. This finding suggests that in addition to phase mixing of the siliceous network from chain ends and BTESE, there is enhanced nanosurface concentration of the siliceous domain. The nanosurface concentration of the siliceous domain is especially evident for U-(3F-0.2)-0.74-SiO1.5, which has high Si but low F. However, increasing F atom% and decreasing Si atom% with increased modifier content results in a fairly close approach to the

Figure 2. High resolution C 1s XPS spectrum for (A) U50-(3F-0.2)0.74-SiO1.5 and (B) U50-(3F-1.0)-0.74-SiO1.5: a, CF3; b, −O−C−CF2; c, C−O; d, aliphatic C.

(3F-1.0)-0.74-SiO1.5. Table 2 lists binding energies and assignments along with peak areas. With reference to the 3F structure shown in Figure 2, peak a is assigned to CF3 at 293.1/ 292.6 eV, and peak b at 287.1 eV has the same relative area as a and is therefore assigned to O−CH2−CF3. The observed b/a ratio is >1:1, which may be due in part to superposition of C O from modifier end groups with −O−CH2CF3 as the carbonyl carbon BE is ∼287.6 to 287.8 eV.3 Peak c at 285.9/286.5 eV is assigned to ether carbons C−O−C. Peak d at 284.8/284.4 eV is assigned to the aliphatic −CH3 oxetane carbon and to −CH2CH2−Si from the Si2C2H4O3 siliceous domain formed from BTESE 3 after cure. The area percent for d is high for the lowest 0.2 wt % modifier (67%) but decreases to 43.7% for 1 wt % as peaks for a−c increase in intensity. This result provides additional evidence for phase mixing of siliceous components from chain ends and BTESE after HyCoin, which was previously discussed. In summary, XPS analysis indicates phase separation of the modifier domain from the bulk polyurethane, even at 0.2 wt %. That is, the base polyurethane is absent from the nanosurface. Second, the near-surface composition consists of phaseseparated 3F and siliceous domains. It should be borne in mind that while the analysis depth for XPS is ∼5 nm, the XPS spot size is ∼400 μm in diameter. Thus, XPS gives a view of chemical composition on a microscale width but nanoscale depth that is complementary to the nanoscale topological and mechanical analysis provided by AFM that is discussed in the next section. Surface Topography and Phase. For all modified compositions, a clear and uniform pattern of dome-like shapes was observed to rise from a relatively flat background surface 12989

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Table 2. Comparison of C 1s Peak Areas for U50-(3F-0.2)-0.74-SiO1.5 and U50-(3F-1.0)-0.74-SiO1.5 Coatings U50-(3F-0.2)-0.74-SiO1.5

a

U50-(3F-1.0)-0.74-SiO1.5

carbon (C)

BE lit., eVa

BE, obs., eV

area %

BE, obs., eV

area %

CF3 −O−CH2CF3 C−O C−C

292.6−294.4 287.1 286.0−286.5 284.8

293.1 287.9 286.5 284.8

4 6 23 67

292.5 287.1 285.9 284.4

8.4 9.8 38.1 43.7

Binding energies obtained from literature.1−3

Figure 3. Characterization of U50-(3F-0.2)-0.74-SiO1.5 (a,d,g), U50-(3F-0.5)-0.74-SiO1.5) (b,e,h), and U50-(3F-1.0)-0.74-SiO1.5 (c,f,i). The 3D surface topography (a−c) was generated from TM-AFM imaging, the mechanical maps (d−f) were collected via force−volume mapping and displayed by overlaying the colored mechanical map on the topography data also collected during the FV scan, and the phase images (g−i) were collected during TM-AFM imaging.

Figure 4. Histograms of the distribution of mechanical properties on sample surfaces (a) U50-(3F-1.0)-0.74-SiO1.5, (b) U50-(3F-0.5)-0.74-SiO1.5, and (c) U50-(3F-0.2)-0.74-SiO1.5.

the nanosurface to ∼5 nm. Further on, the detection of U50 bulk at greater depths is discussed. Features vary in size among coating compositions ranging from 20−30 nm in height and 1 to 2 μm wide for U50-(3F-

(Figure 3). This is due to phase separation during the HyCoin process, whereby the fluorous domains aggregate and selectively concentrate at the polymer−air boundary to reduce surface energy. That is, the U50 concentration is negligible at 12990

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Figure 5. Depth profile analysis. The same area of U50-(3F-1.0)-0.74-SiO1.5 was indented in constant force mode, while the maximum force was varied for each image from (a) 5, (b) 10, (c) 15, (d) 20, (e) 50, to (f) 75 nN.

concentrated at the surface. Thus, the nanoscale mechanical measurements do not probe the bulk polyurethane that is found deeper. The near-surface moduli may best be used for comparison between areas and are not considered absolute values. From histograms of the surface moduli, there are two distinct regions of contrasting material properties (Figure 4). The raised features cover 15−25% of the surface so that modulus data in the histograms are dominated by less stiff background, which also had a much narrower distribution of mechanical properties. In comparison, the raised features occupied a small fraction of the surface and spanned a wider range of mechanical properties. Depth Profile Analysis Maps. As previously noted, a common strategy for mechanically characterizing surfaces uses the phase signal in AFM imaging. This mode combines imaging with viscoelastic and adhesive properties. While phase contrast arises from material properties, it also includes contributions from topometric differences. That is, phase is a measure of the energy dissipation involved in the contact between the tip and the sample, which depends upon several factors including viscoelastic response, adhesion, and contact area. As the contact area is dependent on roughness, the phase image also contains such topographic contributions with the result that interpretation of contrast in phase images may be uncertain. Even in favorable cases with samples having minimal rugosity, understanding contributions of individual factors to the phase shift is not simple. A more quantitative surface analysis is therefore needed for better characterizing the material at the surface and subsurface levels. To quantitatively elucidate the mechanical differences in these domains, we used nanoindentation to characterize the nanomechanical properties. From surface topography and phase previously discussed, it is clear that there are two distinct

1.0)-0.74-SiO1.5 and U50-(3F-0.2)-0.74-SiO1.5 to 10−15 nm high and 0.5 μm wide for U50-(3F-0.5)-0.74-SiO1.5 (Figure S1, Supporting Information). Repeated imaging showed some heterogeneity among feature sizes for different areas on the same samples. In all cases, phase images indicated that the raised features were also of higher phase value than the flat background surface, so that tapping mode imaging with phase could be used as a qualitative tool to estimate mechanical properties. Nanomechanical Characterization of Surfaces. Nanomechanical characterization was carried out to elucidate further nanoscale phase separation of the hybrid modifier and the polyurethane. Also, nanoscale phase separation within the hybrid domain was revealed. The coating samples were all indented with the same constant maximum force, and differences between each sample were inferred from this standardized indention setting. Because the samples were of similar composition, it was assumed that proportionally similar changes would occur on the depth scale found for U50-(3F-1.0)-0.74-SiO1.5. The calculated moduli revealed that the raised features had both a higher phase and moduli compared with the background surface (Figure 3). The moduli of the raised features varied from 3 to 13 times greater than the surrounding surface (Figure 4). The raised features of sample U50-(3F-0.2)-0.74-SiO1.5 had an average modulus of 6.36 ± 2.5 GPa, while the background surface had a modulus of only 1.06 ± 0.15 GPa. U50-(3F-0.5)-0.74-SiO1.5 features were 854 ± 670 MPa with a background of 65 ± 41 MPa, and U50(3F-1.0)-0.74-SiO1.5 had raised features of 1.39 ± 0.53 GPa and background of 180 ± 29 MPa. In comparison, it was found using an MTS System uniaxial tensile tester that neat U50 (not modified) had a bulk modulus of 10.1 MPa.50 This difference from the background moduli found through nanoindentation agrees with XPS, showing that a 3F hybrid composition is 12991

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Figure 6. Model depicting the modified polyurethane depth profile as well as the layered nanostructure of the raised surface-concentrated features.

raised features. The two distinct regions observed from this study suggests that there may be a very near-surface concentrated layer of a softer material with an underlying hard and more elastic siliceous object (Figure S2, Supporting Information).

areas for all polymer samples: raised features surrounded by a relatively flat field. A series of modulus maps was created at the same location using progressively increasing maximum force (Figure 5). Each map was created by indenting to a constant maximum force starting at 5 nN and going to 75 nN. Figure 5 shows that at the lowest indentation force (5 nN) and indent depth (∼2 nm), the modulus of the flat background area is ∼1.5 GPa. As the indentation force increases (10 nN, ∼5 nm), the modulus decreases sharply at first and then more slowly. At the maximum indentation force (75 nN, 20 nm) the modulus is in the vicinity of 0.2 GPa or less. This agrees with XPS analysis that showed that the harder 3F-SiO1.5 modifier is concentrated at the outermost surface. As the material is probed deeper beyond the range of XPS detection, more of the softer base polyurethane is being contacted (∼0.1 GPa). This is comparable to the bulk modulus of U50, which was found to be 10.1 MPa using an MTS system uniaxial tensile tester.50 The decrease in modulus at probe depths greater than a few nanometers suggests there is a fairly sharp phase separation between the bulk polyurethane and the 3F-SiO1.5 domain. On the basis of these data, a model that describes the composition and morphology derived from integrating XPS and AFM is shown in Figure 6. For the raised features, Figure 5 shows that at the lowest indentation force (5 nN) and indent depth (1 to 2 nm) the modulus is about four times higher (∼6 GPa) than the surrounding flat domain. As the indentation force increases (10 nN, ∼4 nm) the modulus decreases to ∼3 GPa. Surprisingly, as indentation force increases from 20 to 75 nN the depth of penetration levels off to ∼5 nm and the modulus increases back to ∼6 GPa. This result suggests a layered nanostructure having an outermost, higher modulus SiO1.5 rich domain followed by a lower modulus 3F rich domain and a third higher modulus SiO1.5-rich domain. This layered structure is illustrated for the raised features in Figure 6. As the surface of the raised domains was indented deeper, their size did not increase. This may reflect the resistance to indentation due to the higher modulus SiO1.5-rich composition. Repeated indentation maps demonstrated that indentation was nondestructive. Multiple maps were collected in the same area with no deformation such as material pile up or permanent pits. This result testifies to the robust character of the siliceous-rich

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DISCUSSION On the basis of the discovery of nanoscale and mesoscale concentration for the hybrid domain in a polyurethane hybrid,26 a new hybrid blend approach to surface modification has been developed using a major component, a linear polyurethane or thermoplastic elastomer (TPU), and a minor component that is a hybrid network. The hybrid network consists of a fluorous polyoxetane soft block and a hydrolysis/ condensation inorganic (HyCoin) network. Phase separation during coating formation results in surface concentration of the minor fluorous hybrid domain. The TPU is H12MDI/BD(50)PTMO-1000 derived from bis(cyclohexylmethylene)-diisocyanate and butane diol (50 wt %) and poly(tetramethylene oxide). Surface modification results from a novel networkforming hybrid composed of poly(trifluoroethoxymethyl− methyl oxetane) diol) (3F) as the fluorous moiety end-capped with 3-isocyanatopropylriethoxysilane and bis(triethoxysilyl)ethane (BTESE) as a siliceous stabilizer. Nanoindentation has been commonly used as to quantify mechanical properties for a range of surfaces including inorganic materials, polymers, and biomaterials and particularly suited to complex systems having several constituents or heterogeneous domains.51−53 Applicability is evidenced by nanomechanical mapping of polymer blends,54 block copolymers,55,56 and polymer surfaces modified by grafting.57,58 However, the collection of combined topographical and mechanical data still remains a technical challenge. For example, because AFM tips are used for indentation they gradually experience wear and become blunted or damaged. Additionally, increasing resolution or scan area causes the number of indentations required for mapping to grow very large for data processing. For example, a 100 × 100 μm2 map with 100 × 100 curves (points 1 μm apart) would result in 10 000 total force curves. To facilitate modulus mapping of the hybrid modified polyurethanes described herein, we employed wear-resistant, hard-diamond-like carbon tips, and a custom 12992

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so much of the three-dimensional depth structure of the material, and once the tip is indenting through multiple layers of materials, the resulting calculated modulus will not provide a good assessment of what material is there. In these maps, and especially from the depth profile series (Figure 5), it does not appear that any subsurface features were encountered and that if anything some very shallow uniform surface layer may have been present.

program was created to automate and rapidly process force− distance data. The nanomechanical maps point to a surface composed of two very different domains. XPS analysis is consistent with a nanosurface composed of 3F copolyoxetane and siliceous components. From nanomechanical analysis, it is clear that the raised features, which are both stiffer and more elastic, are composed primarily of a domain rich in the siliceous component of the fluorous modifier. These siliceous-rich regions are glass-like and elastic, while the 3F polyoxetanerich surrounding regions are more compliant. The difference in modulus for these features is larger than that which has been observed for base polyurethane hard-block and soft-block regions,37 which is again consistent with evidence from XPS that the bulk polyurethane is absent from the nanosurface. Thus, to obtain a depth profile analysis in terms of elemental composition, angle-resolved XPS could be a useful tool. Because the fluorinated species migrate to the air−polymer interface to minimize interfacial surface energy,59−66 it is hypothesized that the 3F soft block drives surface phase separation followed by, or contemporaneous with, HyCoin network formation that stabilizes the surface composition by virtue of the inorganic −SiO1.5 network. The results from nanoindentation mapping and uniaxial tensile testing show a surface composed of hard, elastic raised features and a softer “background” surface with a yet softer bulk modulus. The XPS results confirm the aggregation of fluorous domains on the near-surface region; furthermore, they provide an important insight into the possibility of partial phase mixing, with the siliceous network being present at the meso-surface as well as at the outermost region to an extent. This leads to a model of the polyurethane system with a topmost nanoscale layer composed of siliceous-rich raised features, a surrounding area that is relatively 3F-rich, a mesoscale 3F-silieous domain, and (by default) a bulk composed of U50. Because the modulus of the features and background vary between samples, it is clear that composition alone influences how these observed harder nanostructures form. Further work will be needed to fully understand this relationship. Moduli presented here are estimated from contact models that have inherent assumptions and approximations. However, experimental conditions are kept the same for each sample, and all areas on each sample have similar tip interactions such as adhesion. The tip geometry is also monitored to be the same across the experiments. Finally, viscoelastic effects in polymers generally result in different moduli calculated via modeling when indent parameters such as indent speed are changed, so it is important that the indentations were performed at a constant rate. With the same assumptions, moduli values are useful as a means of comparison for the relative differences in mechanical properties. The spatial limit of resolution for indentation is determined by the area deformed and the elastic recovery of the material. There are numerous methods to calculate the contact area depending on the contact model used,47,51,67,68 but the area is always dependent on the depth of the indent. The nanomechanical maps provided here were probed at successive areas well beyond all estimates for the contacted area and thus that limiting resolution because the features on the surfaces were of significant size that higher resolution maps would not provide any more meaningful detail while requiring more time and resources. Similarly the materials were not probed as deep as is possible; due to the nature of indentation we can only deduce

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CONCLUSIONS We have demonstrated a new hybrid approach to surface modification of polyurethanes and characterization of modifier compositions from 0.2 to 1 wt %. Using XPS analysis, TMAFM, and nanomechanical mapping by nanoindentation, a precise elemental and topographical profile of this hybrid polymer was obtained. XPS analysis demonstrated phase separation of the bulk polyurethane and modifier domain, with the latter concentrated at the outermost surface. Elemental composition and C 1s spectrum analyses revealed partial phase mixing with the siliceous network, with little to no intrusion of the bulk polyurethane probed to ∼5 nm. A positive correlation of higher moduli with increased phase was observed, which allows for the possibility of inferring mechanical properties from the quicker and simpler noncontact AFM imaging. Further studies are needed to determine the reliability of the correlation between these characterization methods. From the depth profile analysis, it was evident that there exists a layered nanostructure within the raised features consisting of the hard siliceous-rich domains and the softer fluorous domains. The nanoscale material property resolution demonstrated here could not have been discovered by any other means. The insight gained provides a unique strategy for improved nanoscale control in designing tailored surfaces for a wide range of applications.

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ASSOCIATED CONTENT

S Supporting Information *

Mass balance calculations on HyCoin cure and representative height profiles and sample indent curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V.K.Y.). *E-mail: [email protected] (K.J.W.). Author Contributions †

S.S.N. and E.J.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (DMR-0802452 and DMR-1206259) for support of this research. REFERENCES

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