Shape evolution control of phase-separated colloidal nanoparticles

Abstract

These studies illustrate controlled shape synthesis of two distinct phase-separated copolymers within one colloidal nanoparticle which consists of poly(methylmethacrylate) (p-MMA)/n-butylacrylate (nBA) and poly(nBA)/pentafluorostyrene (p-PFS) phases. Using sequential free radical polymerization, particle morphologies ranging from acorn to ellipsoidal, core–shell, and spherical were produced by adjusting the glass transition temperature (T_g) via compositional gradients during copolymerization. These studies show for the first time that in order to achieve desirable asymmetric particle shapes, the T_g as well as interfacial surface tension between two copolymers within one nanoparticle should be maintained during and after polymerization.

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Introduction

Mother Nature has mastered orchestrated growth as well as stimuli-responsiveness of biological species, which often inspired developments of new materials that mimic shapes and adoptability observed in the natural world. Many observations stimulated not only novel physico-chemical concepts manifested by an ongoing quest for novel stimuli-responsive nanomaterials,¹ but also challenged scientists and engineers to create controllable nano-scale shapes. Examples of such assemblies are multiphase nanoparticles,^2,3 cocklebur⁴ or hollow nanoparticles,⁵ non-spherical heterogeneous particles as well as other nano-objects with a high aspect ratio, such as nanotubes, nanorods,^6–8 or cilia-like surfaces.⁹ Although numerous past studies have shown that creating none-spherical nano-objects often requires the use of templates, one would like to monitor and follow shape developments by controlling chemico-physical processes. Polymerization reactions, if designed properly, may provide such opportunities.

While many advantages of reversible addition chain transfer polymerization (RAFT),¹⁰ atom transfer radical polymerization (ATRP)¹¹ as well as nitroxide mediated polymerization (NMP)¹² significantly contributed to advances in a precise control of molecular weight as well as the placement of monomer sequences or side chains, making heterogeneous shapes at nano- or micro-scales represents another level of difficulties. One could argue that precise synthetic methods alone may not be sufficient in developing orchestrated networks, especially when larger scale heterogeneous entities resembling or interacting with biological systems are sought. Emulsion polymerization represents an example that typically generates a fairly broad range of molecular weight distributions using batch¹³ or semi-continuous synthesis,³ but the shape and particle morphology controls may be achieved by designing suitable synthetic conditions. Furthermore, one can take advantage of a multi-stage synthesis,^14,15 and produce various nano-scale objects manifested by already existing technologies in photolithographic,^16,17 internal phase separation^18,19 or coaxial jet extrusion²⁰ processes. Although colloidal synthesis has found many applications, it can be affected by other adverse factors.^21,3 These include solubility and reactivity of monomers, control of surface tension by dispersing species, and polymerization conditions, just to name a few. To draw the analogy from biological processes which typically involve many components and conditions required to achieve desirable species, the particle shape control may not be trivial unless all components and conditions satisfy inter- and intra molecular interactions as well as chemico-physical reaction conditions.

Recent examples of synthesis of acorn-shape colloidal particles illustrated that heterogeneous colloidal synthesis can lead to stable water-dispersible particles³ in which the choice of monomers and dispersing components dictates synthetic conditions for F-containing low surface tension colloidal nanoparticles. These studies have also shown that the glass transition temperature (T_g) development during and after polymerization plays a key role in the shape development and particle stability. In an effort to develop heterogeneous colloidal particles with controllable and desirable shapes, these studies focus on synthesis of two-phase colloidal particles which show for the first time how compositional changes during synthesis and the T_g adjustments facilitate conditions for shape developments of multiphase copolymer nanoparticles.

Experimental section

Methyl methacrylate (MMA), n-butyl-acrylate (n-BA), pentafluorostyrene (PFS) and dioctylsulfosuccinate (SDOSS) were purchased from Aldrich Chemical Co. 2,2′-Azobis[2-2(2-imidazolin-2-yl) propane] dihydrochloride (VA-44) initiator was purchased from Wako Pure Chemicals Ind. Ldt. Bio-Rad AG^®502-X8 cation-anion resin was purchased from Bio-Rad Laboratories, Inc. Polytetrafluoroethylene (PTFE) substrate was purchased from Electron Microscopy Sciences, Inc.

p-nBA/PFS and p-MMA/nBA primary colloidal dispersions were synthesized using a semi-continuous process outlined elsewhere,^22,23 and adapted for small scale polymerization. The reaction flask was placed in a water bath set at 50 °C, purged with N₂ gas, charged with 30ml of DDI water under continuous stirring at 350 rpm, whereby SDOSS surfactant was utilized. Table 1 provides ratios of colloidal dispersions prepared for the purpose of these studies. SDOSS was dissolved in water followed by the addition of monomers in the given ratios to produce a semi-stable pre-emulsion. The pre-emulsion was fed continuously over the period of 4 h while the initiator solution was added for 4.5 h. Upon completion of the initiator feed, polymerization was continued for another 10 h. The second step involved copolymerizing nBA and PFS monomers on the seed (MMA/nBA) particles in the given ratios listed in Table 1.To completely remove ionic species from the system, resin exchange experiments were performed on colloidal solutions utilizing Bio-Rad^® AG 501-X8 cation-anion resins. 2 g of activated resin was placed in a beaker, followed by the addition of a 5 mL colloidal dispersion, 15 mL DDI water, stirring overnight, and filtering.

Table 1 Composition, particle size and MMA/nBA ratio of seed P100/0, P80/20, P60/40, P55/45, P50/50, P45/55-S₂, P40/6, P20/80, P0/100 nanoparticle as well as composition of P100/0-S₂, P80/20-S₂, P60/40-S₂, P55/45-S₂, P50/50-S₂, P45/55-S₂, P40/6-S₂, P20/80-S₂, P0/100-S₂

Composition (w/w%)	P100/0		P80/20		P60/40		P55/45		P50/50		P45/55		P40/60		P20/80	P0/100
	Seed	S2	Seed	S2	Seed	S2	Seed	S2	Seed	S2	Seed	S2	Seed	S2	Seed	S2	Seed	S2
Seed		50		50		50		50		50		50		50		50		50
MMA	40		32		24		22		20		18		16		8		—
nBA	--	10.9	8	10.9	16	10.9	18	10.9	20	10.9	22	10.9	24	10.9	32	10.9	40	10.9
PFS	--	10.9	--	10.9	--	10.9	--	10.9	--	10.9	--	10.9	--	10.9	--	10.9	--	10.9
DDI	30	0.66	30	0.66	30	0.66	30	0.66	30	0.66	30	0.66	30	0.66	30	0.66	30	0.66
SDOSS	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4	1.22	27.4
VA-44	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04	0.08	0.04
Particle Size (nm)	65	90	80	99	89	108	98	112	104	115	110	120	115	125	125	133	130	143
Exp. MMA/nBA ratio (w/w %)*	100/0		77/23		65/35		57/43		52/48		46/54		41/59		22/78		0/100

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Particle size analysis was performed using a Malvern Zetasizer Nanoseries Nano-ZS (Malvern Instrument) equipped with a 633 nm laser, at a constant backscattering angle of 173° at 25 °C. The hydrodynamic diameter was calculated using the Stokes–Einstein relation.²⁴ Morphologies of the resulting colloidal particles were investigated using a JEOL TEM-2100 transmission electron microscope (TEM) operated at 200 kV. The samples were placed in a dilution of 1 : 10000 on a Formvar/Carbon Copper grid (EMS) with 300 Mesh.

Atomic force microscopy (AFM) measurements were conducted on a Nanoscope IIIa Dimension 3000 scanning probe microscope (Digital Instruments). A silica probe with a 125 mm long silicacantilever, a nominal force constant of 40 Nm⁻¹, and resonance frequency of 275 kHz and a scan rate of 1 Hz were used in a tapping mode, thus allowing surface topography and roughness measurements.

Static and dynamic contact angle measurements were conducted using a single drop technique using a ramé-hart goniometer coupled with a DROP image data analysis software. 10 μl drops were placed onto a flat surface and an image of the drop was captured. The surface energy was calculated utilizing Fowkes²⁵ and Owens–Wendt²⁶ method by measuring the contact angle between p-MMA/nBAand p-nBA/PFS film surface and distilled de-ionized water (DDI) as well as methylene iodide. The glass transition temperature (T_g) was measured using differential scanning calorimetry (DSC) (TA instruments Q100) in the temperature range of −55 °C–200 °C at a heating rate of 10 °C/min. Grazing-angle attenuated total reflectance Fourier transform infrared (GATR FT-IR) spectroscopy measurements were conducted at the film-air (F-A) interfaces using a BioRad FTS 6000 FT-IR single beam spectrometer at a 4 cm⁻¹ resolution. The surfaces were analyzed using a 2 mm Ge-Crystal with a 65° angle maintaining constant contact pressure between the crystal and the specimens. All spectra were recorded for spectral distortions using software for the Urban-Huang algorithm.²⁷¹³C-NMR spectra were acquired on a Varian ^UNITYINOVA spectrometer operating at a frequency of 125 MHz for carbon and using a standard 5 mm two channel probe. For quantitative analysis recycle delay of 30 s was used. In addition, the proton decoupler was gated off during the recycle period in order to suppress signal enhancement due to NOE. Spectral acquisition was performed at 90 °C to narrow the peak width, and integrated peak areas of the alkyl methyl carbons of MMA and nBa were used to determine copolymer compositions. Free induction decays (FIDs) were zero-filled to 128k and an exponential line broadening of 10 Hz was applied prior to Fourier transformation. Base lines were corrected using linear prediction and polynomial fitting.

Coalescence of one particle layer thick films was conducted by spin-coating depositing of P100/0-S₂, P50/50-S₂, and P0/100-S₂ colloidal dispersions on polytetrafluoroethylene (PTFE) and silica (SiO₂) substrates. Such liquid films were allowed to dry overnight under ambient conditions. An approximate film thickness of coalesced films was 70 nm.

Results and discussion

Particle morphology control

Fig. 1-a, A through I, illustrates TEM images of colloidal nanoparticles prepared by sequential polymerization of seed copolymers of n-butyl acrylate (nBA) and methyl methacrylate (MMA), followed by copolymerization of p-nBA/PFS at 50/50 ratio. The MMA/nBA feed ratio of the seed was adjusted from 100 to 0 wt%, in 5–20% increments. Table 1 provides the summary of compositions utilized for the purpose of these studies. The actual MMA/nBA ratio of the copolymer was determined using ¹³C-NMR spectroscopy, as outlined in the Supporting Documents of Fig. S-1.† The TEM images shown in Fig. 1 illustrate that each particle consists of two phases: p-MMA/nBA (lighter) and p-nBA/PFS (darker), the latter being darker due to a higher election density of the fluorinated component of the copolymer. As seen, such prepared colloidal particles consist of p-MMA/nBA and p-nBA/PFS phases, and the shape of each phase changes as a function of the MMA/nBA ratio. Fig. 1 also shows that when MMA/nBA ≈ 1, acorn shape particles are produced (Image E), but when MMA/nBA > 1 (Images A–D), instead of a hemispherical shape, an inverse acorn is obtained. In contrast, when MMA/nBA < 1 (Images F-I), initially asymmetric particles assume symmetric core–shell morphologies. It should be noted that as MMA/nBA≪1, phase inversions occur, and the initial p-MMA/nBA core becomes shell, whereas the p-nBA/PFS phase resides as core.

Fig. 1 TEM micrographs of colloidal particles consisting of (A) P100/0-S2, (B) P80/20-S2,(C)(P60/40-S2, (D) P55/45- S2,(E) P55/50-S2, (F) P45/55-S2, (G) P40/60-S2, (H) P20/80-S2, (I) P0/100-S2. Table 1 provides a compositional content.

In view of these data there are several factors that influence the shape of colloidal particles. While reaction conditions such as temperature, reactivity ratios of monomers, and monomer solubility, along with the ability of dispersing agents to lower the surface tension²⁸ play an essential role during polymerization, another aspect is the shape development and its stability during polymerization. After all, it is well established⁴ that when the low T_gnBA monomer is initially polymerized, followed by polymerization of a high T_g monomer, such as styrene (Sty) or MMA, the core–shell morphology is produced. However, regardless of the polymerization order, the higher T_gpolymer core will be always surrounded by a softer shell. This behavior is attributed to the low T_gpolymer phase inversion during the synthesis. As illustrated in the TEM images shown in Fig. 1, p-MMA/nBA/p-nBA/PFS particles not only maintain their shapes, which are attributed to significant temperature differences between the reaction temperature (T) and the T_g of the growing particles,^2,29 but also retain p-MMA/nBA copolymer composition.

Postponing temporarily the influence of the T_g development during polymerization, let us consider the effect of the interfacial surface tension between p-MMA/nBA and p-nBA/PFS phases on the particle stability. To analytically determine the influence of the interfacial surface tension effect, it is not only necessary to eliminate surface active components and ionic species in the dispersion, but also to determine the surface tension between the two phases within one particle. For that reason colloidal dispersions were subjected to a resin exchange,³ allowing the separation of ionic species from colloidal dispersions. Such “disinfected” dispersions were coalesced and the contact angle measurements were conducted on the following film surfaces: p-nBA/PFS, p-nBA, p-MMA, and p-MMA/nBA copolymers. Using static contact angle data summarized in Table 2, we utilized Young's equation³⁰W_A = γ_LV (1 + cosθ), (where: γ_LV, γ_SV are liquid–vapor and solid vapor interfacial tensions, respectively, and θ is the contact angle) to determine polar (γ^p_SV) and disperse (γ^d_SV) contributions to the surface energies. The following relationship, W_A = 2[{γ^d_SV * γ^d_SV}^½₊ {γ^p_SV* γ^p_SV}^1/2],^25,26 was used and the γ^p and γ^d values of coalesced films are listed in Table 2. In turn, these values were utilized to determine interfacial surface tension between the two phases within one particle using the following relationship:³¹

where: γ₁₂ is the interfacial tension between the two colloidal particles γ_{p-MMA/nBA//p-nBA/PFS}, γ_i is the surface tension of the separated particles, γ₁ = g_p-MMA-nBA, γ₂ = g_p-nBA/ PFS, and γ_i^d,γ_i^p are dispersion and polar components of γ_i. The results are summarized in Table S-1 of Supporting Documents. To illustrate the influence of the interfacial tension Fig. 2 depicts the γ_{p-MMA/nBA//p-nBA/PFS} plotted as a function of the p-MMA content of the seed. The interfacial surface tension energies ranging from 15.2 mN/m γ_{(p-MMA/nBA)//p-nBA/PFS} (seed polymer P100/0) to 1.7 mN/m for γ_{(p-MMA/nBA//p-nBA/PFS} (seed polymer P0/100) correspond to the phase separated domains formation within one particle. The acorn shape is formed when the contact area between the two phases is 30.15*10⁻³ nm², thus being energetically most favorable until γ_{p(MMA/nBA//p-nBA/PFS} reaches 9.2 mN/m for MMA/nBA = 1. Further decrease of the interfacial surface tension energy may lead to the increase of the contact area up to 91.02 × 10⁻³ nm² when γ_{(p(MMA/nBA//p-nBA/PFS} = 1.7 mN/m (for a seed polymer P0/100), thus facilitating suitable conditions for obtaining concentric core–shell morphologies.

Table 2 Results of static consstact angle measurements and calculations of surface tension films of coalesced particles and the resulting roughness (R_RMS) as a function of composition

Copolymer Composition	RRMS	Static Contact Angle [°]		γ [mN/m]	γ^dsv [mN/m]	γ^psv [mN/m]
	[nm]	Water	Methylene Iodide
P100/0	21.2	51	43	56.31	38.07	18.24
P80/20	15.21	56	46	52.46	36.47	15.99
P60/40	9.21	60	51	48.53	33.71	14.82
P55/45	5.21	63	53	46.12	32.59	13.53
P50/50	3.34	65	54	44.63	32.02	12.61
P45/55	2.9	69	57	41.35	30.30	11.05
P40/60	3.25	73	61	37.78	28.00	9.78
P20/80	2.7	78	65	31.78	25.70	6.08
P0/100	1,08	82	69	27.41	23.43	3.98
p-nBA/PFS 50/50 S2	2.5	103	78	19.82	18.53	1.29

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Fig. 2 Interfacial surface tension of γ_{(p-MMA/nBA//p-nBA/PFS)} plotted as a function of p-MMA content of the seed.

With these considerations in mind let us go back to the T_g effect. Using p-nBA (P0/100) (T_g = −49 °C) as seed or shell, the same core–shell morphologies were produced. This is demonstrated in Fig. 3, A and A′, where dark areas in the TEM images are due to p-nBA/PFS core (higher electron density) and lighter areas represent the p-MMA/nBA phase. However, when two phases exhibit similar T_g values which are above the synthesis temperature, both phases exhibit controllable flow at the reaction temperature of 50 °C, thus retaining their morphologies. As shown in Fig. 3, B and B′, for a 50/50 p-MMA/nBA (T_g = 16 °C) and p-nBA/PFS (T_g = 20 °C), the acorn-shape morphologies are stable, and further increase of the T_g to 110 °C (P100/0) also results in stable inverted acorn morphologies (Fig. 3 C and C′). Using p-MMA as the seed, the p-nBA/PFS phase facilitates sufficient flow leading to inverse acorn-shape morphologies. However, starting with the p-nBA/PFS core, the p-MMA/nBA shell with a content of 100% MMA is unable to flow under these reaction conditions. In summary, the interfacial surface tension and the T_g control during polymerization play a major role in the particle shape developments and their maintenance during and after synthesis.

Fig. 3 TEM micrographs of colloidal particles of P0/100-S₂(A), S₁-P0/100(A′), P50/50-S₂ (B), S₁-P50/50 (B′), P100/0-S₂, (C) S₁-P100/0 (C′).

To examine the role of the T_g of the seed copolymer under given reaction temperature (T) conditions, the values of |T-T_g|/T_g (|ΔT|/T_g) were tabulated in Table 3 and plotted as a function of the copolymer seed composition. This is shown in Fig. 4 which illustrates that when the reaction temperature is set at 30 °C, the minimum is obtained at MMA/nBA = 1 ratio (curve A), but increasing the temperature to 50 °C shifts the minimum to 0.65 p-MMA content (curve B). At this point, the mass flow of the seed as well the shell are minimized due to the high interfacial surface tension giving hemispherical morphologies. However, when the reaction temperature is set at 100 °C, no minimum is observed (curve C), which is attributed to the flow of the copolymer significantly above its T_g.

Table 3 Summary of the T_g, |T_g-T|, and (T_gSeed–T_gShell) values as a function of nanoparticle composition

Copolymer Composition	Tg (°C)	|Tg-T| (°C)	TgSeed -TgShell (°C)
P100/0	100.6	50.6	80.6
P80/20	77.7	27.7	57.7
P60/40	55.7	5.7	35.7
P55/45	39.8	10.2	19.8
P50/50	23.0	26.9	3
P45/55	19	48.1	−1
P40/60	−13.5	63.5	−33.5
P20/80	−33.31	83.3	−53.31
P0/100	−49.2	99.2	−69.2
nBA/PFS 50/50 S2	20	30

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Fig. 4 |ΔT|/T_g plotted as a function of p-MMA (w/w %) content at T = 30 °C (●) (A), T = 50 °C (■) (B), T = 100 °C (▲) (C).

In an effort to determine the influence of the interfacial particle surface tension, Fig. 5 illustrates the relationship between |ΔT|/T_g, the MMA (w/w%) content in p-MMA/nBA copolymer, and the interfacial surface tension. These data show that when T > T_g, the polymer flow is enhanced, and core–shell particles are obtained, regardless of the p-MMA content. These conditions are marked in Fig. 5, as AB zone. Furthermore, regardless of the order of the low T_g component polymerization, the core–shell morphologies consisting of a hard-core and soft shell are obtained as long as T≫T_g. When T < T_g, the system is highly sensitive to compositional changes and temperature variations, where anisotropic morphologies (for example, acorn) can be produced, which is labeled as zone BC in Fig. 5. In contrast, hemispherical morphologies at the ΔT/T_g minimum will dominate the region marked C in Fig. 5. Further increase of the MMA content leads to inversed acorn shape particles obtained under the high values of the interfacial tension energy (11–15 mN/m) conditions represented by the CD zone in Fig. 5. Within the CD zone, the T_g of the core and the shell as well as the order of polymerization play a significant role. Specifically, for two copolymers with T_g1 and T_g2 glass transition temperatures, for T_g1 > T_g2 and T_g2 < T < T_g1, inverse acorn shape particles will be produced, but when T_g1 < T_g2 and T_g1 < T ≪ T_g2, phase separation within the particle will occur, leading to two-sphere morphologies shown in Fig. 3B.

Fig. 5 |ΔT|/T_g plotted as a function of interfacial surface tension and the p-MMA content (w/w %).

Coalescence of controllable shape morphologies

The presence of two-phase copolymer colloidal particles will also influence their coalescence. Fig. 6 illustrates assembled monolayer particle films obtained by the coalescence of P100/0-S₂ (A), P50/50-S₂ (B), P0/100-S₂ (C) particles, where P100/0-S₂ and P50/50-S₂ exhibit a phase separation, and P0/100-S₂ is the phase separated p-nBA/PFS copolymer surrounded by p-nBA. In an effort to determine the influence of the surface energy of acorn shape particles on coalescence on substrates with significantly different surface energies, we coalesced one-particle layer thick films (approx. thickness: 70 nm) on polytetrafluoroetylene (PTFE) (=18.5 mN/m) and SiO₂ (γ = 78 mN/m)³⁰ substrates. Upon coalescence, the contact angle measurements (using water) were performed and are summarized in Table 4. For P100/0-S₂, P50/50-S₂ and P0/100-S₂, the p-nBA/PFS rich phase of the acorn particle stratifies near the film-substrate (F-S) interface on the low surface tension PTFE substrate (18.5 mN/m). In contrast, the same particles coalesced on the higher surface energy SiO₂ substrate resulted in the p-MMA rich phase which aligns itself toward the F-S interface, while the p-nBA/PFS phase is oriented outwards. However, slightly higher hydrophilic p-MMA leads to the decrease of the static contact angle to θ_S = 90° (θ_S = 95°) for P50/50-S2 particles on SiO₂. Due to P0/100-S₂ core–shell morphologies and the enhanced hydrophobic character of p-nBA, the static contact angle increased to 97° on SiO₂. These observations were also confirmed by GATR FT-IR and AFM analysis shown in Figures S-2A and S-2B of the Supporting Documents. In summary, these data show that non-spherical, dual phase heterogeneous particle morphologies may provide means for the particle orientation during coalescence which are driven by the surface energy of the substrate. Earlier studies conducted on stratification during coalescence also identified that the substrate surface energy plays a significant role on the alignment near the interfacial regions.³²

Fig. 6 TEM images of coalesced particles of P100/0-S₂ (A), P50/50-S₂ (B) and P0/100-S₂ (C) particles showing a phase separation near the film-air (F-A) interface.

Table 4 Static, advancing and receding contact angle measurements of P100/0-S2, P50/50-S2, P0/100-S2 deposited on SiO₂ and PTFE substrates. Water contact angle [°] of blank SiO₂ and PTFE are 36° and 120° respectively.^a

Copolymer→	P100/0-S2			P50/50-S2			P0/100-S2
Substrate↓	θS°	θA°	θR°	θS°	θA°	θR°	θS°	θA°	θR°
a θS - static water contact angle [°] after coating with particles; θA - advancing water contact angle [°] after coating with particles, θR – receding water contact angle [°] after coating with particles.
SiO2	90	94	90	95	101	88	97	98	91
PTFE	72	79	68	68	74	62	87	88	86

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Conclusions

These studies show for the first time that the synthesis of two distinct phase-separated copolymers within one colloidal nanoparticle allows morphology control by compositional and interfacial adjustments. Spectroscopic and morphological analysis combined with the contact angle measurements revealed that in an effort to create stable heterogeneous two-phase particle morphologies it is essential to control the copolymer composition that influences glass transition temperature and subsequently interfacial surface tension. The relationship between the ΔT/T_g and the copolymer composition as well as the interfacial particle surface energy shows that in order to achieve desirable morphologies, energetically favorable conditions must be met. Coalescence of the dual phase nanoparticles can be controlled by the substrate surface tension. When two phase copolymer nanoparticles coalesce on a high surface tension substrate, the p-PFS phase expresses itself near the film–air interface, whereas for low surface energy substrates, the p-PFS phase dominates the film–substrate interfacial regions. For symmetrical core–shell particle morphologies, there is no influence of the substrate surface tension.

Acknowledgements

This work was partially supported by the NSF MRSEC program DMR 0213883 as well as Major Research Instrumentation program DMR 0215873.

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Footnote

† Electronic supplementary information (ESI) available: Further images and tables. See DOI: 10.1039/c0py00220h

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