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Towards Achieving Highly Ordered Fluorinated Surfaces of Spin-coated Polymer Thin Films by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions Biao Zuo, Cheng Li, Yawei Li, Wenhao Qian, Xiuyun Ye, Li Zhang, and Xinping Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00309 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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Towards Achieving Highly Ordered Fluorinated Surfaces of Spin-coated Polymer Thin Films by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions Biao Zuo, Cheng Li, Yawei Li, Wenhao Qian, Xiuyun Ye, Li Zhang, Xinping Wang* Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China

Abstract: Thin polymer films with well-assembled fluorinated groups on their surfaces are not easily achieved via spin-coating film-fabrication methods, because the solution solidifies very rapidly during spin-coating, which hinders the fluorinated moieties from segregating and organizing on the film surface. In this contribution, we have proposed a comprehensive strategy towards achieving well-ordered fluorinated thin films surfaces by optimizing the molecular organization at air/liquid interface of the film-formation solutions. To validate such a route, poly(methyl methacrylate) (PMMA) end-capped with several 2-perfluorooctylethyl methacrylate (FMA) units was employed as model polymer for investigations. The air/solution interfacial structures were optimized by systematically changing the polymer chain structures and properties of the casting solvents. It was found that the polymers which form loosely associated aggregates (e.g. FMA1-ec-PMMA65-ec-FMA1) and a solvent with better solubility to FMA while having not too low surface tension (i.e. toluene) can combine to produce solutions with well-assembled FMA at the interfaces. By spin-coating the solutions with well-organized interfaces, an ultra-thin film with perfluorinated groups that were highly oriented toward the film surface were readily achieved, exhibiting surface energies as low as 7.2 mJ/m2, which is among the lowest 1

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one reported so far for the spin-coated thin films, and a very high F/C ratio (i.e. 0.98).

1. INTRODUCTION Fluoropolymers have attracted great attention due to their many unique properties, including low friction, low-adhesion, self-cleaning, and antibacterial effects, which mainly originate from the ordered fluorine moieties at the film surface. 1 - 8 Regularly aligned closest-hexagonal-packed -CF3 groups result in a surface with the lowest critical surface tension ever discovered (~ 6 mN/m).9,10 Improvement of the molecular ordering and packing efficiency of the fluorinated groups at the surface are of vital importance in designing thin film materials with superior interfacial performance. Generally, two film preparation methods, namely, spin-coating and solvent-casting, are utilized to prepare the fluorinated polymer films, and different surface structures are mostly produced using the different film-formation methods. 11 - 14 When the solvent-casting method is employed, a well-organized fluorinated surface is normally acquired, because the fluorinated moieties assembling in the solution have a relatively long time in which to disassemble and segregate onto the surface due to the slow evaporation of the solvents.15-19 In a series of works, it was found that a specific polymer architecture of a nonfluorinated chain end-capped with a few fluorinated units is very efficient in achieving surface migration of the fluorinated units, especially when solvent-casting is applied to fabricate the films.15-22 This is most likely due to the high mobility of the chain terminal groups and less entropy penalty for surface segregation of the chain ends.15-22 It was reported that the surface energy of the solvent-cast films of poly(acrylates) end-capped with merely one 2-perfluorooctylethyl methacrylate (FMA) unit can be effectively reduced to a value 2


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close to that of the surface constituted by perfectly ordered -CF3 groups (6.0 mN/m).15-19 This means that the fluorinated chain ends overwhelmingly segregate onto the film surface and become closely packed in the course of film solidification by slow evaporation of the solvent. Unlike the solvent-casting method, with spin-coated films the fluorinated units are not so efficient in surface segregation and the packing densities of the fluorinated component are far lower than those films prepared by solvent-casting.12,16, 23 , 24 Spin-coating is a versatile technology generally used in nanomaterials science to fabricate flat and smooth ultra-thin coatings of large areas with highly controllable and reproducible film thickness.25-27 The surface of the thin coating is definitely important in the related applications. During the spin-coating process, the solvent quickly evaporates and the film vitrifies within a few seconds,28-31 resulting in the freeze-in of the aggregated chain conformation and therefore prevention of the segregation of the fluorinated moieties onto the surface. Such “rapid-freezing effects” are also to be encountered in the electrospinning process, in which the fluorine units are totally depleted on the fiber surface.32 Consequently, such a situation poses challenges in preparing ultra-thin coatings with ordered fluorinated surfaces using the spin-coating method. Keeping in mind the aforementioned difficulty, we herein developed a comprehensive strategy to improve the molecular packing and ordering of fluorinated groups on the spin-coated film surface. Previously, we have revealed that the spin-coating film surface of a fluorinated polymer was mainly dominated by the original structure at the air interface of the corresponding film-formation solution.33 Such mechanism implies that the well-ordered fluorinated surfaces of thin polymer films could be achieved by optimizing the interfacial structure of the casting solution. 3


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In this work, we will illustrate the excellent ability of such strategy in controlling the surfaces structures of the spin-coated films. A poly(methyl methacrylate) (PMMA) end-capped with several 2-perfluorooctylethyl methacrylate (FMA) units, the optimal chain structures for surface enrichment,15-22,34 with well-defined architectures (i.e. AB and ABA type), were employed as model polymers in the investigation. The interfacial structures of the solutions were controlled by changing the fluorinated copolymer chains structures and the properties of the solvents, and characterized by surface tension measurements and sum frequency generation vibrational spectroscopy (SFG). The molecular aggregation within the solution was probed using dynamic light scattering (DLS). It was found that the polymer which forms loosely associated polymer aggregates in solutions and a solvent with better solubility to FMA while having not too low surface tension can combine to produce solutions with well-assembled FMA at interfaces. Meanwhile, the one-to-one relationship between the surface structure of the films and the interfacial structure of the film-formation solutions was verified to be applicable for the fluorinated copolymers with a large variety of chain structure and in the various types of solvents. Under an optimized condition of a triblock copolymer with short middle chains and very few FMA end units (FMA1-ec-PMMA65-ec-FMA1) and a solvent of toluene, in which the solution interface was occupied by the highly ordered fluorinated side chains, an ultra-thin film with closely packed fluorinated groups on the surfaces were readily obtained by spin-coating the solutions, exhibiting extremely low surface energies of 7.2 mJ/m2 and very high F/C ratio (i.e. 0.98).

2. EXPERIMENTAL SECTION 2.1 Polymers. Two types of linear copolymers (Figure 1), consisting of poly 4


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(methyl methacrylate) (PMMA) end-capped with 2-perfluorooctylethyl methacrylate (FMA) on their single and double terminals were synthesized using atom transfer radical polymerization, as described in our previous publications.15-19 The copolymer chain structures were checked by fluorine element analysis, gel permeation chromatography and infrared spectroscopy (results not shown). The physical parameters of the fluorinated copolymers are listed in Table 1. Table 1. Characteristics of the fluorinated copolymers used in this study.

Polymer a

Mw/Mn b

WF (%) c

FMA (mol %)

CMC (g/L) e

PMMA150-ec-FMA1

1.18

1.47

0.46

0.89

PMMA230-ec-FMA1

1.22

1.51

0.48

0.56

PMMA350-ec-FMA1

1.25

0.64

0.20

0.53

PMMA430-ec-FMA1

1.25

0.60

0.19

0.76

PMMA672-ec-FMA1

1.30

0.62

0.19

0.61

PMMA920-ec-FMA1

1.27

0.31

0.10

0.77

PMMA430-ec-FMA2

1.24

1.47

0.46

0.73

PMMA430-ec-FMA4

1.21

3.07

0.99

0.65

PMMA430-ec-FMA7

1.28

5.03

1.67

0.55

PMMA430-ec-FMA22

1.30

12.99

4.87

0.37

FMA1-ec-PMMA65-ec-FMA1

1.15

9.10

3.27

6.05


1.24

3.92

1.28

4.94


1.25

2.93

0.94

4.85


1.29

1.74

0.55

5.61


1.29

1.49

0.47

5.01


1.30

1.77

0.36

1.43


1.24

0.76

0.24

1.10


1.24

3.26

1.03

4.81


1.27

6.35

2.03

3.80


1.27

8.30

2.89

3.18


1.20

13.66

4.55

1.02

FMA17-ec-PMMA424-ec-FMA17 1.20 18.37 7.54 0.87 a: Calculated by both fluorine element analysis and GPC results (see Supporting Information for

5


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more details) and the degree of polymerization of MMA and FMA are rounded to the nearest; b: determined by GPC and calibrated by polystyrene standards; c: obtained by fluorine element analysis; d: critical micelle concentration of copolymers in cyclohexanone, estimated by the surface tension data (error: ± 0.2 g/L).

2.2 Film Formation. The fluorinated copolymers were dissolved in solvents (i.e. cyclohexanone, toluene, trifluorotoluene) to make solutions of various concentrations. Spin-coating was employed to fabricate the thin films. Droplets of solution at a specific concentration (higher than the CMC of the copolymers) were deposited onto glass slides mounted on the spin-coater, and then the coater, together with the slides, rotate at varying speed (i.e. 2000 – 6000 rpm) for 30 s to allow rapid evaporation of solvents, leading to glassy ultra-thin films with thickness of 42 ± 2 nm. The prepared films were dried in vacuum at 40 °C for 72 h. Note that, the fluorinated components in the films are unable to migrate toward the free surface at such low temperature.23,24 (a)

(b)

Figure 1. Chemical structures of (a) PMMAx-ec-FMAn and (b) FMAn-ec-PMMAx-ec-FMAn.

2.3 Characterization of the Air/solution Interfacial Structures. Surface tensions of the fluorinated solutions were measured at 25 oC on a DCA-322 surface tensiometer (Cahn Instruments, USA) using a technique based on the Wilhelmy balance principle. Five parallels were measured and the average for the measurements was calculated from the parallels. The critical micelles concentration (CMC) values 6


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were estimated based on the concentration dependence of the surface tension. Sum frequency generation (SFG) vibrational spectra of the interfaces of the solutions were obtained using a custom-designed SFG spectrometer (EKSPLA, Lithuania); the method has been described in previous papers13,35-37. Briefly, the visible input beam (i.e. 0.532 µm) was generated by frequency doubling of the fundamental output from an EKSPLA Nd:YAG laser. The IR beam, tunable between 1000 and 4300 cm-1 was obtained from an optical parametric generation/amplification/difference frequency generation (OPG/OPA/DFG) system based on LBO and AgGaS2 crystals. Both VIS and IR beams were guided to the sample surface or interface. The incident angles of the visible and the IR beam were 60 and 55 degrees respectively with respect to the surface normal. SFG spectra with an ssp polarization combination (i.e. an s-polarized sum frequency output, an s-polarized visible, and a p-polarized infrared) and ppp polarization combination (i.e. an p-polarized sum frequency output, an p-polarized visible input, and a p-polarized infrared input) were collected in our experiments. Molecular aggregations of the copolymers in cyclohexanone solution were measured by dynamic light scattering with a 90 Plus NanoParticle Size Distribution Analyser (Brookhaven Instruments Ltd., UK). The concentration of the polymer solutions was 0.01 g/ml.

2.4 Characterization of the Film Surfaces. The contact angle (θ) on the sample was measured with a drop shape analysis system (KRÜSS BmbH Co., Germany) based on the sessile droplet method, in a temperature and humidity controlled room (25 °C, 60% relative humidity). The contact angle is the average of the values obtained from at least 8 different points on the sample surface. Surface energies of the fluorinated thin films (γs) were estimated using the Owens and Wendt theory38 (eq. 1) 7


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from the static contact angles of water and diiodomethane on the film surfaces. (γ sd γ ld )1/2 + (γ spγ lp )1/ 2 = γ l (cos θ + 1) / 2

(1)

where γ s is the surface free energy of the films, γ l is the surface free energy of liquid. The superscript d and p represent the contribution from dispersion force and polar force, respectively. Angle-resolved X-ray photoelectron spectroscopy (XPS, PHI5000C ESCA System) with a Mg Kα X-ray source (1253.6 eV) was applied to characterize the surfaces of the fluorinated thin films. The X-ray gun was operated at a power of 250 W, and the high voltage was kept at 140 kV. All survey and high-resolution spectra were calibrated by the peak of the C-C bond at 285 eV. Three take-off angles: 30, 60, and 90 degrees were adopted to collect the spectra, corresponding to information depths of 4.5, 7.8 and 9.0 nm, respectively23,39,40.

3. RESULTS AND DISCUSSION 3.1 Air/liquid Interfacial Structures of the Cyclohexanone Solution of Fluorinated Copolymers with Different Chains Structures. The equilibrium surface tensions of the cyclohexanone solution of the fluorinated copolymers with various

chain

structures

(i.e.

PMMAx-ec-FMA1,

PMMA424-ec-FMAn

and

FMA1-ec-PMMAx-ec-FMA1, FMAn-ec-PMMA424-ec-FMAn) are summarized in Figure 2. It is shown that the equilibrium surface tensions of PMMAx-ec-FMA1 and PMMA424-ec-FMAn cyclohexanone solutions always remain at 23.5 mN/m (i.e. the value of PMMA homopolymer33, 41 ) and are not influenced by the chemical compositions

of

the

copolymers.

For

FMA1-ec-PMMAx-ec-FMA1

and

FMAn-ec-PMMA424-ec-FMAn, when the degree of polymerization (DP) of PMMA exceeds 540 (x ≥ 540) and the number of FMA is larger than 10 (n ≥ 10), the 8


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copolymers exhibit the same surface tension as that of PMMA homopolymer (i.e. 23.5 mN/m), while if the DP of PMMA is reduced to below 540 and n becomes smaller than

10,

the

surface

tension

at

equilibrium

gradually

declines.

For

FMA1-ec-PMMA65-ec-FMA1, the surface tension was reduced to 17.5 mN/m. 26 PMMAx-ec-FMA1

(a)

FMA1-ec-PMMAx-ec-FMA1

24

22

20

18

16

Equilibrium surface tension (mN/m)

26

Equilibrium surface tension (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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FMAn-ec-PMMA430-ec-FMAn 24

22

20

18

0

200

400

600

800

1000

Degree of polymerization of PMMA (x value)

PMMA430-ec-FMAn

(b)

0

5

10

15

20

25

The Number of FMA units (n value)

Figure 2. (a) Equilibrium surface tension of PMMAx-ec-FMA1 and FMA1-ec-PMMAx-ec-FMA1 cyclohexanone solutions as functions of DP of PMMA, and (b) equilibrium surface tension of PMMA430-ec-FMAn and FMAn-ec-PMMA424-ec-FMAn solutions as functions of the number of FMA units. The equilibrium surface tensions were collected at concentration higher than CMC.

The interfacial structures of cyclohexanone solutions were then directly examined by SFG, which has a sub-monolayer sensitivity42-44 (Figure 3, 4). The SFG spectra of the solvents (i.e. cyclohexanone) (Figure 3a) are dominated by two intensive peaks arising from symmetric stretching (2865 cm-1) and Fermi resonance (2945 cm-1) of the methylene groups in the ssp spectra and by a single peak from a -CH2 Fermi resonance (2945 cm-1) in the ppp spectra.45,46 This spectral feature means that the cyclohexanone molecules are oriented at the air/liquid interface. When PMMA was added into the cyclohexanone to form PMMA solutions, new peaks at 2910 and 2950 cm−1, originating from the Fermi vibrations of CH2 and the symmetric stretching of the ester methyl groups (-OCH3) of PMMA, respectively, dominate the ssp spectra. Meanwhile, a peak at 2965 cm-1, possibly from antisymmetric stretching 9


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of the α-CH3 of PMMA was observed in the ppp spectra.33,47-49 This indicates that the PMMA chains segregate onto the cyclohexanone interface. In the case of solutions of PMMAx-ec-FMA1 (Figure 3a) and PMMA424-ec-FMAn (Figure 4a), it is apparent that the SFG spectra have very similar features with those of the PMMA homopolymer, regardless of the length of the PMMA chains and the number of FMA units. This suggests that the air/liquid interface of PMMAx-ec-FMA1 and PMMA424-ec-FMAn cyclohexanone solutions was almost completely occupied by the segments of PMMA, leading to the surface tension almost equal to that of PMMA homopolymer (Figure 2).

(a) ssp

(a) ppp

cyclohexanone

(b) ssp cyclohexanone 2965 cm-1

2910 cm-1 2950 cm-1 PMMA

SFG Intensity (a.u.)

SFG Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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x = 920 x = 672 x = 430

PMMA

2910 cm-1

2950 cm-1

PMMA x = 830

x = 830

x = 920

x = 540

x = 672

x = 424

x = 540 x = 424 x = 430

x = 360

x = 350

x = 360

x = 210

x = 230

x = 210

x = 150

2850

2900

2950 -1

Wavenumber (cm )

x = 154

x = 154

x = 230

2800

2965 cm-1

PMMA

x = 350

x = 150

(b) ppp

2930 cm-1

x = 65

3000 2800 2850 2900 2950 3000 3050 -1

2800

Wavenumber (cm )

x = 65 2955 cm-1

2850

2900

2950 -1

Wavenumber (cm )

3000 2800 2850 2900 2950 3000 3050 -1

Wavenumber (cm )

Figure 3. ssp and ppp SFG spectra of (a) PMMAx-ec-FMA1 and (b) FMA1-ec-PMMAx-ec-FMA1 cyclohexanone solutions. (Concentration: 0.01 g/mL)

SFG spectra of the fluorinated copolymers with ABA type chain architectures (FMAn-ec-PMMAx-ec-FMAn) are displayed in Figures 3b and 4b. The spectra of FMA1-ec-PMMAx-ec-FMA1 (x ≥ 540) (Figure 3b) and FMAn-ec-PMMA424-ec-FMAn (n ≥ 10) (Figure 4b) are analogical to that of the PMMA homopolymer. Upon decreasing DP of PMMA (x < 540) and reducing the number of FMA units (n < 10), two additional dominant peaks at 2930 and 2955 cm-1 were observed in the ssp spectra, and the original peak at 2965cm-1 from α-CH3 of PMMA in the ppp spectra totally disappear (Figure 3b, 4b). On the basis of previous works,15,33,41 the peaks at 10


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2930 and 2955 cm-1 are most likely caused by the symmetric stretch of the -CH2 bonded to the oxygen atom in the ester groups of FMA and another -CH2 close to the perfluoroalkyl side chains [C8F17], respectively. Therefore, the appearance of the peaks at 2930 and 2955 cm-1 (in ssp spectra) is indicative of the occurrence of surface segregation of the FMA, and the disappearance of 2965 cm-1 peaks (in ppp spectra) suggests the depletion of PMMA segments at the solution interface. A possible explanation for these observations in the SFG spectra is that the air/solution interface was entirely covered by PMMA segments when n and x are large (i.e. x ≥ 540, n ≥ 10), and the FMA were then effectively adsorbed at the solution interface and became increasingly well-oriented with decreasing the DP of PMMA and the amount of FMA units. The peak intensities at 2930 cm-1 in the ssp spectra of FMAn-ec-PMMA424-ecFMAn cyclohexanone solutions were plotted against the number of FMA units (n) (Figure S1). Compared with the surface tension vs. n curves (Figure 2b), it is apparent that the decrement in surface tension is particularly associated with the increase of the SFG intensity at 2930 cm-1, which further indicates that the surface tension decrement is due to the increase of the surface segregation and orderly arrangement of fluorinated side chains at interface. For the sample with x = 65 and n = 1, the spectra closely resemble those from films of PFMA homopolymers33, indicating that the solution interface was occupied by highly ordered FMA.

11


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(a) ssp

2910 cm-1 2950 cm

-1

(a) ppp

PMMA

PMMA

n = 22

n = 22

n=7

n=7

2965 cm-1

(b) ssp

2910 cm-1

2950 cm-1

(b) ppp

2965 cm-1

PMMA n = 17

SFG Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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n = 17 n = 10 n = 10 n=6 n=6

n=4

n=4 n=4

n=2

n=2

n=1

n=1

n=4

n=2

n=2 2930 cm-1

n=1

n=1 2955 cm-1

2800

2850

2900

2950 -1

Wavenumber (cm )

3000 2800 2850 2900 2950 3000 3050 -1

Wavenumber (cm )

2800

2850

2900

2950 -1

Wavenumber (cm )

3000 2800 2850 2900 2950 3000 3050 -1

Wavenumber (cm )

Figure 4. ssp and ppp SFG spectra of (a) FMAn-ec-PMMA424 and (b) FMAn-ec-PMMA424ec-FMAn cyclohexanone solutions. (Concentration : 0.01 g/mL)

Briefly, the interfacial structure analyses based on surface tension and SFG data reveal that the air/solution interfaces of all the AB type copolymers (e.g. PMMAx-ec-FMA1 and PMMA424-ec-FMAn) and FMAn-ec-PMMAx-ec-FMAn with x ≥ 540 and n ≥ 10 were covered by the PMMA segments. However, surface enrichment of fluorinated moieties is particularly accentuated when FMAn-ec-PMMAx-ec-FMAn (x < 540, n < 10) with shorter middle segments and fewer fluorine units are employed. The perfluoroalkyl side groups [C8F17] covering the solution interfaces become increasingly ordered with the decrement of the length of PMMA and the number of FMA units. The interfacial structures of PMMAx-ec-FMAn and FMAn-ec-PMMAx-ecFMAn are shown schematically in Scheme 1.

12


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: FMA

: PMMA

(a) Stable micelles

: PMMA-ec-FMA : FMA1-ec-PMMA-ec-FMA1

Segregation

(b)

Loosely associated micelles

Stable micelles

Unimer

Increasing molecular weight of PMMA

Scheme 1. Schematic illustration of the air/liquid interfacial structure of the cyclohexanone solution of fluorinated copolymers, micellization in the solution and possible pathway for the interface

segregation

of

fluorinated

end-capped

chains

(a:

PMMA-ec-FMA;

b:

FMA1-ec-PMMAx-ec-FMA1).

The formation of the interfacial structure of the amphiphilic block copolymer solutions is related to the stability of the macromolecular micelles in the solutions.50-56 In this case, cyclohexanone preferentially dissolve PMMA, whereas FMA is insoluble, and as a consequence the fluorinated polymers aggregate to form micelles. Table 1 provides critical micelle concentration (CMC), a key parameter characterizing the thermodynamic stability of micelles, of the fluorinated copolymers. Obviously, the surface inactive fluorinated copolymers exhibit low CMC values (i.e. 0.37 ~ 0.89 g/L for PMMAx-ec-FMAn; 0.87 ~ 1.43 g/L for FMAn-ec- PMMAx-ec-FMAn with x ≥ 540, n ≥ 10). The samples of FMAn-ec-PMMAx-ec-FMAn (x < 540, n < 10), in which the FMA units voluminously segregate onto the solution interface, always associate with apparently higher CMC values (i.e. 3.80 ~ 6.05 g/L). According to micellization theory, the higher the value of the CMC, the less thermodynamically stable are the 13


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micelles.57-59 Thus, from the presented CMC data, it seems reasonable to infer that the molecular aggregates of FMAn-ec-PMMAx-ec-FMAn (x < 540, n < 10) are less stable than the other copolymers, in which the FMA units are surface inactive. PMMA430-ec-FMA22

PMMA920-ec-FMA1

PMMA672-ec-FMA1

Intensity (a.u.)

PMMA430-ec-FMA7

PMMA430-ec-FMA1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PMMA350-ec-FMA1

PMMA430-ec-FMA4

PMMA430-ec-FMA2

PMMA230-ec-FMA1

PMMA430-ec-FMA1

PMMA150-ec-FMA1

0

100

200

300

400 500

600

Hydrodynamic diameter (nm)

0

100

200

300

400

500

600


Figure 5. Dynamic light scattering data for (a) PMMAx-ec-FMA1 and (b) PMMA430-ec-FMAn cyclohexanone solutions. (Concentration: 0.01 g/mL).

The micelle formation and their kinetic stability for the fluorinated copolymers were qualitatively characterized by dynamic light scattering (DLS), with the results shown in Figures 5 and 6. Stable micelles with average hydrodynamic diameters ranging from 150 to 360 nm were formed in the solution of PMMAx-ec-FMAn (Figure 5). Moreover, both the diameter and size distribution of the micelles become smaller with decreasing DP of PMMA (i.e. x value) and increasing the number of FMA units (i.e. n value). Notably, there exist two populations of aggregates in the solution of FMAn-ec-PMMAx-ec-FMAn with x < 540 and n < 10 (Figure 6). Unimers appear (i.e. 10 ~ 35 nm in diameter) in the solution in addition to the polymer micelles with large size (i.e. 250 ~ 400 nm in diameter). The unimers, generally formed by slight 14


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association of one or several chains, are highly unstable and would easily be disassembled from time to time.12,60,61 The DLS results are consistent with the CMC data, both indicating the quite unstable nature of the aggregates formed by chains of FMAn-ec-PMMAx-ec-FMAn (x < 540, n < 10) (Scheme 1). An entropic effect can explain the different association behavior of the end-capped copolymers. For the ABA type copolymers, the middle chains (e.g. PMMA) must be looped with their two ends (e.g. FMA) taking part in the same micellar core, leading to large configuration loss.62-65 Such entropy loss would definitely inhibit the macromolecular chains from associating, and accordingly reduce the stability of the micelles. Similarly, since the conformations of the loop reduce with the decrement of molecular weight, the micelles are therefore further unstable for FMAn-ec-PMMAx-ec-FMAn with shorter PMMA segments (Scheme 1). (a)



(b)


Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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FMA1-ec-PMMA424-ec-FMA1 FMA6-ec-PMMA424-ec-FMA6

FMA1-ec-PMMA360-ec-FMA1 FMA4-ec-PMMA424-ec-FMA4

FMA1-ec-PMMA210-ec-FMA1 FMA2-ec-PMMA424-ec-FMA2 FMA1-ec-PMMA154-ec-FMA1

0


FMA1-ec-PMMA 100 200 300 400 500 1600 65-ec-FMA


0

100 200 300 400 500 600


0

100

200

300

400

500

600


Figure 6. Dynamic light scattering data for (a) FMA1-ec-PMMAx-ec-FMA1 and (b) FMAn-ecPMMA430-ec-FMAn cyclohexanone solutions. (Concentration: 0.01 g/mL)

From the above discussion, the relationship between the interfacial structure and 15


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the molecular association behavior of the fluorinated copolymers in the solution could be deduced. It was shown that the micelle stability plays an important role. For PMMAx-ec-FMAn and FMAn-ec-PMMAx-ec-FMAn (x ≥ 540, n ≥ 10), the polymer chains associate to form stable micelles with a dense-packed FMA core, due to the large interfacial energy between cyclohexanone and FMA. The micelles are very stable, such that they cannot be disassembled even if a large centrifugal shear force is applied during spin-coating (Figure S2).24 In this scenario, the FMA moieties are severely constrained inside the micelle core, and as a result, the solution surface is occupied by the corona component of the macromolecular micelles (i.e. PMMA) (Scheme 1). On the contrary, for FMAn-ec-PMMAx-ec-FMAn with x < 540 and n < 10, the fluorinated chains are less favorable for micelle formation. The chains only aggregate as loose micelles and unimers (Scheme 1). The fast chains exchange kinetics of the unstable aggregates make it feasible for the FMA buried in the micellar core to be released and segregate to the interface under the driving force of minimizing the interfacial energy. The ability for the surface segregation of FMA apparently becomes increasingly stronger for the copolymers with shorter middle chains (i.e. PMMA) and fewer FMA units, due to the decrement of the micelles stability. Such result is a good supplement to our previous finding on the mechanism for surface segregation, which suggests that increasing FMA content would promote migration of the polymer to the air/solution interface by increasing the magnitude of |∆H|. Here, it is worthy to note that such simple consideration on the basis of the thermodynamic for surface segregation (i.e. ∆G = ∆H - T∆S) holds only when the micelles is not so stable. When more stable micelles generated in solution, the stability of the aggregates dominates the surface segregation of FMA, as shown in the current work. Scheme 1 shows the proposed mechanism for the formation of the 16


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air/solution interfacial structure of the fluorinated end-capped copolymers.

3.2 Air/solution Interfacial Structures of the Fluorinated Copolymers in Different Solvents. Besides the structures of copolymers, the properties of solvent (e.g. solubility parameters and surface tension) also play important roles on the formation of interfacial structures of the fluorinated copolymer solutions. The other two solvents (i.e. toluene and trifluorotoluene) were employed in the studies. The solubility parameters (δ) of cyclohexanone, toluene and trifluorotoluene are 9.9, 8.9 and 8.3 cal1/2·cm-3/2,66 respectively, corresponding to the order of solubility toward FMA:

trifluorotoluene

>

toluene

>

cyclohexanone.

The

DLS

data

of

FMA1-ec-PMMA65-ec-FMA1 in toluene and trifluorotoluene solution are shown in Figure S3. As can be seen, the fluorinated chains only associate as unimers in trifluorotoluene, and a considerable fraction of unimers, together with loose micelles, was found in toluene solutions (Figure S3). Combining the DLS results of the cyclohexanone solution (Figure 6), it could be concluded that the stability of the aggregates of FMA1-ec-PMMA65-ec-FMA1 in the solutions follows the order: trifluorotoluene < toluene < cyclohexanone, which is in the reverse order of solubility of FMA in the solvents. The SFG spectra of the FMA1-ec-PMMA65-ec-FMA1 trifluorotoluene, toluene and cyclohexanone solutions are shown in Figure 7 and Figure 3. It is shown that the fluorinated side chains are segregated onto the interface of toluene and cyclohexanone solutions. While, compared with the cyclohexanone solutions, the fluorinated groups are more closely packed and better oriented in toluene solutions, as manifested by the complete disappearance of the resonant peaks from toluene67 (2920, 3060 cm-1) and appearance of two clear and separated peaks from CH2 groups in fluorinated side 17


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chains (i.e. 2930, 2955 cm-1) in the SFG spectra (Figure 7). However, surprisingly, the SFG spectrum of trifluorotoluene solution was almost the same as that of the pure trifluorotoluene, both of which are dominated by resonant peaks from the phenyl rings (3045, 3075 cm-1) of trifluorotoluene, indicating that there are not any fluorinated chains resided at the solution interface. The SFG data is consistent with the equilibrium surface tensions of the trifluorotoluene (20.7 mN/m), toluene (16.5 mN/m) and cyclohexanone (17.6 mN/m) solutions of FMA1-ec-PMMA65-ec-FMA1. The fluorinated side chains are organized most orderly at the interface of the toluene solution, therefore exhibiting the lowest surface tension among the three solvents. Since the interface was completely covered by trifluorotoluene, the surface tension of trifluorotoluene solution of fluorinated copolymer (20.7 mN/m) is almost the same with that of the pure trifluorotoluene (21.2 mN/m). ssp

3075 cm-1

ppp

3045 cm-1

3075 cm-1

Trifluorotoluene solution

SFG intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Trifluorotoluene 2955 cm-1 2930 cm-1 Toluene solution 3020 cm-1

2920 cm-1 Toluene

2800 2850

3060 cm-1

2900

2950

3000

3050 3100 2800

-1

Wavenumber (cm )

2850

2900

2950

3000

-1

3050

3100

Wavenumber (cm )

Figure 7. SFG spectra of the air/liquid interface of toluene and trifluorotoluene and the corresponding FMA1-ec-PMMA65-ec-FMA1 solutions. (Concentration: 0.01 g/mL)

Taking into account the above analysis, the effect of solvents on the formation of interfacial structures can be obtained. The solubility toward FMA and the surface tension of the solvents should be paid much attention to. Due to the formation of 18


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loosely associated micelles in solutions, a solvent with better solubility with the core-forming component (i.e. FMA) would improve the degree of segregation and self-assembly of FMA at interface, and therefore the fluorinated groups of copolymers in toluene solution is more order than that in cyclohexanone solution. In the meanwhile, the differences in surface tensions of the fluorinated moieties and solvents determine the thermodynamic driven force for the fluorinated units to be segregated and replace the solvent molecules at interface. The higher the surface tension of solvent is, the stronger tendency in surface segregation the fluorinated units will have. The surface tension of cyclohexanone, toluene and trifluorotoluene are 32.4, 28.2, and 21.2 mN/m, respectively, and that of FMA bearing long fluorinated side chains was predicted to be less than 15 mN/m. It is obvious that the surface tension of trifluorotoluene is closest to that of the FMA, leading to small driven force for surface segregation, and thereof the FMA is unable to enrich onto the solutions interface, as evidenced by the surface tension and SFG data. Based on the above analysis, it is reasonable to claim that toluene is the optimal solvent which can mostly promote the surface segregation and well-assembly of FMA at interface. Consequently, the results of all our investigations in section 3.1 and 3.2 demonstrate that the sample with optimal chain structure (i.e. FMA1-ec-PMMA65-ec-FMA1) for surface segregation and a toluene solvent with better solubility to FMA and not too low surface tension can work together to produce a solution with fluorinated groups well-assembled at interface, showing very low surface tension (i.e. 16.5 mN/m).

3.3 Achieving Highly Ordered Fluorinated Spin-Coated Film Surfaces by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions. The surface properties and structures of ultra-thin films which are spin-coated from 19


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the solutions of fluorinated copolymers with various chains structures and different solvents at concentrations higher than the corresponding CMC values were studied. Figure 8 shows the surface energies and F/C ratios (determined by XPS) of the surfaces of the prepared thin films. Remarkably, the surface energies and F/C ratios of the surfaces of the thin films were linearly correlated with the equilibrium surface tension of their corresponding casting solutions (Figure 8). The lower the surface tension that the film-formation solution has, the more the FMA can enrich the film surface. The linear correlation shown in Figure 8 clearly implies that the surfaces of the thin films are essentially determined by the orderly packing of molecular at the casting solutions interface, if spin-coating was applied to make the films. This result extends the linear correlation between the surface of the spin-coated film and interface of the casting solution found in our previous work33 to large variety of fluorinated polymers with various chains structures and solutions with different solvents. Thereby, we can propose that the surface structure of the spin-coated films can be reasonably optimized by the air/liquid structure of the corresponding casting solutions. 60 1.0

2

50 0.8 40 0.6

30

0.4

20

0.2

10 0 16

F/C ratio of films

Surface energy of films (mJ/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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17

18

19

20

21

22

23

0.0 24

Surface tension of solution (mN/m)

Figure 8. Surface energies and F/C ratios of the surface of fluorinated ultra-thin films prepared by spin-coating of PMMAx-ec-FMAn and FMAn-ec-PMMAx-ec-FMAn solutions with various surface tensions. 20


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When FMA1-ec-PMMA65-ec-FMA1 toluene solution with the lowest equilibrium surface tension of 16.5 mN/m was used, ultra-thin films with highly ordered fluorinated groups on the surface were readily obtained. It was shown that the surface energies of the spin-coated films of FMA1-ec-PMMA65-ec-FMA1 are as low as 7.2 mJ/m2, very close to the ideal surface composed of closely hexagonal packed -CF3 groups (~ 6 mN/m)9,10. As far as we know, the surface energy (i.e. 7.2 mJ/m2) is the lowest one among the reported values for the fluorinated spin-coated thin films. The F/C atomic ratio at the top surface reaches as high as 0.98, close to the maximum value the solvent-cast film has (~ 1.0)15-19, and increased significantly from the inner bulk to the surface (Figure 9b). To further investigate the orientational state of the perfluorinated groups, high-resolution resolved XPS spectra in the C1s region collected at various takeoff angles were recorded (Figure 9a). The curve-fitted peaks at 293.9 eV and 291.7 eV are assignable to –CF3 and –CF2 moieties, respectively.20,68 The ratio CF3/CF2 is indicative of the orientation of the perfluoroalkyl groups of FMA at the film surfaces. The theoretical ratio of CF3/CF2 is 0.142, which reflects the situation of a film surface saturated with perfluorinated groups, all of which are lying parallel on the surface. Ratios above 0.14 indicate that the fluorinated chains are oriented perpendicularly to the surface.11,16,17,20 It is obvious that the CF3/CF2 ratio reaches 0.19 at the information depth of 4.5 nm, suggesting that the perfluorinated groups are almost perpendicularly assembled at the free surface; therefore, a surface with exposed -CF3 groups was formed. As well, the CF3/CF2 ratio decreases from 0.19 to 0.056 with increasing information depth from 4.5 to 9.0 nm, demonstrating the strong enrichment and ordered arrangement of the fluorinated groups on the outermost surface. 21


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1.4

-CF2

(a)

(b) -CF3

30°

0.20

1.2 0.16 1.0

CF3/CF2

0.12

F/C ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

60°

0.08

0.6 0.04

90°

280

0.4 285

290

295

Binding energy (eV)

300

4

5

6

7

8

9

0.00

Information depth (nm)

Figure 9. (a) High-resolution C1s spectra at different take-off angles (30, 60 and 90°) and (b) plots of ratios of F/C and CF3/CF2 vs. information depth for the fluorinated thin films prepared by spin-coating the FMA1-ec-PMMA65-ec-FMA1 toluene solutions.

Finally, it is worth mentioning that although the poly(perfluoroalkyl acrylate)s with long fluoroalkyl side chains (-CnF2n+1, n ≥ 8; e.g. FMA) have excellent surface properties due to the well-assembly and crystallization of side chains at the surface, such material is harmful to human body owing to the strong resistance to degradation and accumulation in organisms69,70, and thus was banned for producing commercial products. Currently, extensive research is focused on developing nontoxic alternative fluorinated materials, such as polymer with a short perfluoroalkyl chain, however, due to the lack of crystallization of the short side chains, the fluorinated groups at surface is less regular-packed, resulting in relatively inferior surface properties of films.2,71-73 Our proposed strategy to enhance the ordering of fluorinated groups on film surface is not restricted to the PFMA, but applicable for all the fluorinated materials. Therefore, the current investigation itself is meaningful for producing well-ordered fluorinated surface using fluorinated polymer with short perfluoroalkyl chain and also for 22


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developing the high-performance environment-friendly fluorinated materials as substitutes for the traditional fluorine-containing polymers.

4. CONCLUSIONS Polymer thin films with well-aligned fluorinated groups on the surface have broad-reaching applications in nanodevice manufacturing. Spin-coating, a highly versatile film preparation technique, is generally utilized for such films. However, the rapid transition from a solution to the dry films caused by solvent evaporation during spin-coating prevents the fluorinated components from being enriched and assembling on the film surface, thereby posing challenges in preparing nanometer coatings with ordered fluorinated surfaces using spin-coating. In this work, we have shown that the ordering of fluorinated groups at spin-coated thin film surfaces can be efficiently enhanced by optimizing the molecular packing of fluorinated groups at the air/liquid interface of the casting solutions. A series of fluorinated end-capped PMMA (i.e. FMAn-ec-PMMAx and FMAn-ec-PMMAx-ec-FMAn) and various solvents were applied to optimize the organization and self-assembly of fluorinated side chains of FMA at interface of the polymer solutions. It was found that the coverage and packing efficiency of the fluorinated groups on the interface were mainly determined by the stability of the formed micelles in the solutions, and as well the surface tension of the solvents. In the case when stable micelles formed, FMA was strongly constrained in the compact micellar core, resulting in solution interfaces essentially being absent of fluorinated components. When the loose aggregates are generated in solution, the fast chain exchange kinetics of the unstable aggregates permit the FMA buried in the micellar core to be released and segregate onto the solution interface. In addition to the micelles stability, a solvent with not too low surface tension is also required to 23


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form the oriented fluorinated interface. Based on the above designing principles, we found that the sample of FMA1-ec-PMMA65-ec-FMA1, which has proven to be the optimal chains structures for surface segregations among the investigated polymers, and a toluene solvent can combine to produce a solution with the fluorinated groups well-assembled at the interface, thus showing very low surface tension (i.e. 16.5 mN/m). The degree of ordering of perfluorinated groups at the surface of the spin-coated ultra-thin films were linearly scaled with the arranging regularity of the fluorinated moieties at the air/liquid interface of all our investigated solutions. Under the optimized condition of FMA1-ec-PMMA65-ec-FMA1 toluene solution (i.e. surface tension: 16.5 mN/m), in which the fluorinated side chains of FMA are orderly self-assembled at interface, a polymer thin film with the surface covered by closely and regularly packed perfluorinated groups was accordingly procured by spin-coating the solution, showing a surface energy as low as 7.2 mJ/m2 and a F/C ratio as high as 0.98. The results of this study demonstrate that ultra-thin polymer films with highly ordered fluorinated groups on their surfaces can be strategically designed by carefully manipulating the interfaces of the film-formation solutions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Estimation of the amount of FMA units in the copolymers; peak intensity at 2930 cm-1 in the SFG ssp spectra collected from interface of the FMAn-ec-PMMA424-ec-FMAn cyclohexanone solutions as functions of n values; AFM images of the spin-coated films of PMMA430-ec-FMA22 and FMA17-ec-PMMA424-ec-FMA17 cyclohexanone solutions; DLS data of toluene and trifluorotoluene solutions of FMA1-ec-PMMA65- ec-FMA1. 24


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AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] or [email protected]

Notes The authors declare no competing ﬁnancial interest

ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation of China (21504081, 21674100), Natural Science Foundation of Zhejiang Province (LQ16B040001), Science Foundation of Education Department of Zhejiang Province (Y201534629).

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For Table of Contents (TOC):

Towards Achieving Highly Ordered Fluorinated Surfaces of Spin-coated Polymer Thin Films by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions Biao Zuo, Cheng Li, Yawei Li, Wenhao Qian, Xiuyun Ye, Li Zhang, Xinping Wang* Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China

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Towards Achieving Highly Ordered Fluorinated ... - ACS Publications

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