surface structure of spin-coated fluorinated polymers films dominated by corresponding...

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Article

Surface Structure of Spin-Coated Fluorinated Polymers Films Dominatedby Corresponding Film-Formation Solution / Air Interface Structure

Hua-Gang Ni, XueHua Li, YanYan Hu, Biao Zuo, ZeLiang Zhao,JuPing Yang, DaXiang Yuan, XiuYun Ye, and Xinping Wang

J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Oct 2012

Downloaded from http://pubs.acs.org on October 29, 2012

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1

Surface Structure of Spin-Coated Fluorinated

Polymers Films Dominated by Corresponding

Film-Formation Solution / Air Interface Structure

Huagang Ni, XueHua Li, YanYan Hu, Biao Zuo, ZeLiang Zhao, JuPing Yang, DaXiang Yuan,

XiuYun Ye, XinPing Wang*

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing

Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China

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ABSTRACT: In this paper, the relationship between the surface structures of spin-coated

fluorinated polymer films and their corresponding film-formation solution/air interface

structures was investigated. Film-forming poly (n-alkyl methacrylate) end-capped with

2-perfluorooctylethyl methacrylate (FMA) (PFMAy-ec-PnAMAx-ec-PFMAy) was synthesized

via a controlled/living atom-transfer radical polymerization (ATRP) technique. The structures

both at solution interface and on the spin-coated film surface for these polymers were studied

by X-ray photoelectron spectroscopy (XPS), sum frequency spectroscopy (SFG), and surface

tension measurements. The results showed that with increasing polymerization degree of

PnAMA, the fluorinated moieties in PFMAy-ec-PnAMAx-ec-PFMAy adsorbed at the

solution/air interface were gradually completely replaced by PnAMA segments, resulting in

an increase in corresponding solution surface tension until it was equal to that of poly

(n-alkyl methacrylate) solution. Additionally, it was observed for the first time that the

surface F/C ratios of spin-coated films decreased linearly with increasing surface tension of

the corresponding film-formation polymer solution. Overall, the results indicate that the

ultimate surface composition of spin-coated films of these fluorinated methacrylates was

mainly dominated by their corresponding film-formation solution/air interfacial structure.

This work provides a fundamental understanding of the formation of film surface structures

from fluorinated polymer solution to the resulting solid film.

Keywords: spin-coated film; surface structure on the film; end-capped poly (n-alkyl

methacrylate); fluorinated poly (n-alkyl methacrylate); film-formation solution/air interface

structure

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1. Introduction

Fluorinated polymers have unique surface characteristics that are exploited in many

fields,1-3

including microelectronics, antifogging, antifouling, and medical applications. Their

unique properties are attributed to their low surface free energy caused by the enrichment of

fluorine moieties at the surface layers. Understanding the correlation between the structure

and the properties of the surface of a material and the tuning of appropriate chemical-physical

properties at the molecular level may lead to novel developments in a number of fields where

interfacial interactions are important. In such cases, operating within a few nanometers of a

surface is often critical. Therefore, considerable attention has focused on understanding and

controlling the surface chemical structures of fluorinated polymer materials. It was reported

that the chain shapes or conformations of polymers in solution could affect the structures and

properties of the polymers in the bulk.4, 5

The fluorine moieties are insoluble and collapse in

common solvents, with fluorinated copolymers usually forming unimers, micelles and other

aggregates in solution, with fluorinated segments packed together in soluble segments. As the

solvent evaporates, the macromolecules begin to self-assemble into diversified ordered

structures on the surface during film formation. When a methyl methacrylate (MMA) and

2-perfluorooctylethyl methacrylate (FMA) block copolymer (PMMA-b-PFMA) film was

prepared from its micelle solution with a PMMA block as the micelle corona and PFMA

block as the micelle core, the segregation of the fluorinated moieties was affected greatly by

micelle stability and by the evaporation rate of the solvent. 6-9

The rapid solvent evaporation

rate plus the good micelle stability resulted in the formation of a micelle-like aggregate

structure at the film surface, on which the PMMA coronas were partly exposed to the

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surface.6-8

The nature of the solvent also affects the surface segregation and packing of the

functional groups at the uppermost surface.10,11

A relatively perfect close-packed and

well-ordered structure of the perfluoroalkyl side chains at the surface of end-capped

poly(methyl methacrylate) film was formed when the film was cast from a benzotrifluoride

solution, as opposed to being cast from cyclohexanone and toluene solutions.10

A result

contrary to that observed for perfluorinated block polymers,7 was obtained when our group

investigated the relationship between film-forming methods and the solid surface structure of

random copolymers composed of butyl methacrylate (BMA) and dodecafluorheptyl

methylacrylate (DFHMA).12,13

In addition, de Gennes 14,15

found that the surface morphology

of the final spin cast polymer films was, with the exception of the solvent vapor pressure,

affected primarily by the molecules occupying the air/solution interface. It was also observed

that the difference between the surface tension of the solvent, γs and the surface tension of the

polymer, γp influenced the structure of the air/polymer solution interface. Nishino et al 7

investigated the effects of film-forming conditions on the surface properties and structures of

diblock copolymers with perfluoroalkyl side chains, and found that the unimers localized at

the air-solution interface produced different surface fluorine concentrations and surface free

energies for cast and spin-coated films. It is thus clear that the arrangement and orientation of

the macromolecules occupying the air/solution interface can directly influence the resulting

polymer films during film formation from solution to solidification. Therefore, it is very

important to understand the relationship between the surface structure of the film and the

corresponding air/solution interfacial structure during the solution- to-film process.

Surface tension measurement, surface pressure(π)-area measurement, X-ray and neutron

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reflectivity(XR and NR), scanning tunneling microscopy(STM), atomic force microscopy

(AFM), and Brewster angle microscopy(BAM) have all been found to be very useful

techniques for studying the structure at the air/liquid interface. 16-26

Sum frequency generation

(SFG) spectroscopy has been shown to be a promising technique for the investigation of

monolayers of simple surfactants at the air/water interface 27-29

as well as the chain

orientation at the air/polymer interface and water/ polymer interface. 12,30-34

Kim et al 35

found

with the aid of vibrational sum frequency spectroscopy that the orientations of the methyl

groups and the molecular configurations of PDMS chains at the air/water interface showed an

apparent difference with increasing surface concentration and proposed a new set of models

for the chain conformation of PDMS monolayers at the air/water interface. There are a few

reports on the air/solution interface structures of fluorinated copolymers, which are concerned

with the molecular chain structure and solvent nature, among other phenomena. The

monolayer behavior of poly((perfluorohexyl)ethyl methacrylate)-b-poly(ethylene oxide)-b-

poly((perfluorohexyl) ethyl methacrylate) (PFMA-b-PEO-b-PFMA) at the air-water interface

was investigated using surface pressure-area and XR measurements. It was found that the

orientations of whole PFMA chains were vertically rearranged from the horizontal at the

air-water interface for water-insoluble species with increasing surface pressure. For

water-soluble species with a very low amount of PFMA, no enrichment at the air-water

interface was observed. 26

It was previously reported 36

that the surface activity of poly(butyl

methacrylates) (PBMA) end-capped with 2-perfluorooctylethyl methacrylate (FMA) units in

toluene solution is related to the degree of polymerization of PBMA and the number of FMA

units at the chain end. There was no fluorinated component adsorbed at the solution surface

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for this polymer when the number of FMA units was more than 4. However, as far as we

know, there are few reports discussing the influence of the film-formation solution/air

interfacial structures on the surface properties of the resulting film. The study of surface

structure formation during the solidification of polymer solutions is extremely challenging.

In this paper, poly(n-alkyl methacrylate) end-capped with 2-perfluorooctylethyl

methacrylate (FMA) (PFMAy-ec-PnAMAx-ec-PFMAy) was synthesized via a controlled/

living atom-transfer radical polymerization (ATRP) technique. The surface properties of the

spin-coated films and the structure at air/corresponding film-formation solution interfaces

were investigated using X-ray photoelectron spectroscopy (XPS), sum frequency

spectroscopy (SFG), and surface tension measurements. The ultimate aim is to understand

how the surface structures of fluorinated copolymer films prepared by spin-coating are

affected by the corresponding film-formation solution/air interfacial structure.

2. EXPERIMENTAL SECTION

2.1 Materials. Methyl methacrylate (MMA) and n-butyl methacrylate (BMA) (Shanghai

Reagent Co., China), 2-perfluorooctylethyl methacrylate (FMA) (Aldrich Chemical Co.,

USA) were purified prior to use as previously described 9,10

and stored in an air-free flask in

the freezer. CuBr, diethyl 2,6-dibromoheptanedioate (DEtBr), and

N,N’,N’,N”,N”-pentamethyl -diethylenetriamine (PMDETA) were purchased from Aldrich

Chemical Co., USA and used as received. Toluene was purified in the usual manner and dried

by refluxing over sodium and then distilled prior to use. Other reagent grade chemicals were

purchased from Shanghai Reagent Co. China and used without further purification.

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2.2. Polymer Synthesis and Characterization. The macroinitiators Br-PMMAx-Br and

Br-PBMAx-Br with various molecular weights were prepared by atom transfer radical

polymerization according to our previous reports. 36-39

Macroinitiator Br-PMMAx-Br was

prepared by typical reaction in a three-neck round-bottom flask equipped with a stopcock and

a magnetic stirring bar. Prior to use, the flask was vacuumed and back-filled with dry

nitrogen several times. CuBr, N,N’,N’,N”,N”- pentamethyl-diethylenetriamine (PMDETA),

diethyl 2,6-dibromoheptanedioate (DEtBr), methyl methacrylate (MMA), and toluene were

added sequentially under a nitrogen atmosphere with the molar ratios of

[MMA]/[DEtBr]/[CuBr]/[PMDETA] standing at A:1:1:2, where the value of A was 120, 150,

270, 340, 385 and 650, respectively. The reaction mixture was then kept at 70ºC for 6h. After

polymerization, the mixture was precipitated into methanol, and the resulting product was

filtered and dried in vacuum for 12h. The product was then dissolved into tetrahydrofuran

(THF), and the resulting solution was passed through an alumina column to remove the

catalyst. The solution was precipitated into methanol and poly(methyl methacrylate)

(Br-PMMAx-Br) product was filtered and dried in vacuum, with a resulting yield of 70%. The

molecular weights of Br-PMMAx-Br were determined to be 15.4, 21.0, 36.0, 42.4, 54.0 and

83.0×103 g/mol respectively by gel permeation chromatography (GPC). Similarly, the

macroinitiators Br-PBMAx-Br with various degrees of polymerization were synthesized by

ATRP. The corresponding degrees of polymerization (DP) of these macroinitiators were 85,

200, 300, 500 and 711.

PFMAy-ec-PMMAx-ec-PFMAy and PFMAy-ec-PBMAx-ec-PFMAy were synthesized by

ATRP with Br-PMMAx-Br or Br-PBMAx-Br as macroinitiator. The macroinitiator,

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Br-PMMAx-Br or Br-PBMAx-Br, CuBr, and PMDETA were introduced into the flask, and

the flask was then vacuumed and back filled with dry nitrogen several times, followed by an

addition of benzotrifluoride to the flask. Once the solution became homogeneous,

2-perfluorooctylethyl methacrylate (FMA) was added into the solution with the molar ratios

of [Br-PMMAx-Br] or [Br-PBMAx-Br]/[CuBr]/[PMDETA]/[FMA] of 1:1:2:3 or 1:1:2:6

respectively. Polymerization was performed at 110ºC under a nitrogen blanket for 10 h. After

polymerization, the mixture was diluted with THF and passed through an Al2O3 column. The

reaction solution was filtered and then precipitated into methanol. The resulting product was

dried in vacuum at room temperature. The chemical structures and characteristics of the final

product polymers are shown in Scheme 1 and Table 1.

2.3 Film formation. The PFMA1-ec-PMMAx-ec-PFMA1 and PFMA3-ec-PBMAx-ec-

PFMA3 polymers were dissolved in either cyclohexanone or toluene to make 4 wt%

solutions. The films were prepared by spin-coating at 1000 rpm for 30s onto clean glass

slides, and dried in air at 25 ºC for 24h and then in vacuum at 40 ºC for 48h. The thickness of

the resulting films, evaluated by ellipsometry (M-50, JASCO Co., Ltd.), was 220 ± 15nm.

2.4 Characterization. The molecular weight and polydispersity of the polymers were

determined by gel permeation chromatography (GPC) using a Waters 515 gel permeation

chromatograph equipped with Waters Styragel HR 3 and Waters Styragel HR4 columns with

THF as eluent at 35ºC at a flow rate of 0.8 mL/min. The GPC chromatograms were calibrated

against standard polystyrene samples. FTIR spectra of the polymers were obtained on a

Nicolet Avatar 370 Fourier Transform Infrared (FTIR) spectrometer. The percentage of

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fluorine in the copolymers was obtained through fluorine elemental analysis using the

ignition method.

Contact angles (θ) of water were measured by the Sessile drop method at room

temperature and ambient humidity with a Krüss (Hamburg, Germany) DSA-10 contact angle

goniometer. The reported θ values were the averages of at least eight measurements taken

within 10-20s of applying each drop of liquid. The experimental errors when measuring the θ

values were evaluated to be less than ±2°, thus indicating that the results were sufficiently

accurate.

The X-ray photoelectron spectroscopy (XPS) experiments were performed using a

PHI5000C ESCA System with an Mg Kα X-ray source (1253.6 eV). The X-ray gun was

operated at a power of 250W and the high voltage was kept at 140 kV with takeoff angle of

30°. The chamber pressure during analysis was about 1×10-8

Torr. All spectra were calibrated

by the C1s peak of the C-C bond at 284.6 eV.

The surface tension was measured at 25°C on a DCA-322 surface tensiometer (Cahn

Instruments, USA) using a technique based on the Wilhelmy balance principle with

cyclohexanone or toluene as solvent. Five parallels for each concentration were measured and

the average and standard deviations were calculated from these parallels.

Sum frequency generation (SFG) vibrational spectra were obtained by a custom-designed

EKSPLA SFG spectrometer, which has been described in detail by various researchers.40-44

Briefly, the visible input beam at 0.532 µm was generated by frequency doubling a part of the

fundamental output from an EKSPLA Nd:YAG laser. The IR beam, tunable between 1000

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and 4300 cm−1

(with a line width <6 cm−1

) was obtained from an optical parametric

generation/amplification/difference frequency generation (OPG/OPA/DFG) system based on

LBO and AgGaS2 crystals, which were pumped by the second harmonic and the fundamental

output of the laser. Both beams had a pulse width of ~30 ps, a repetition rate of 50 Hz, and a

typical beam diameter of ~0.5 mm at the sample surface or interface. The incident angles of

the visible beam and the IR beam were 60° and 55°, and their energies at the sample surface

were ~230 and ~130 µJ, respectively. In this study, SFG spectra with ssp and ppp

polarization combination (s or p-polarized sum frequency output, s or p-polarized visible

input, and p-polarized infrared input) were collected.

3. RESULTS AND DISCUSSION

3.1. End-capped polymethacrylates synthesis and characterization. The poly (methyl

methacrylates) or poly (butyl methacrylate) end-capped with several FMA units, i.e.,

PFMAy-ec-PMMAx-ec-PFMAy or PFMAy-ec-PBMAx-ec-PFMAy were prepared by ATRP

using the corresponding macroinitiators Br-PMMAx-Br (x=154, 210, 360, 424, 540, 830) or

Br-PBMAx-Br (x=85, 200, 300, 500, 711), respectively. The targeted numbers of FMA units

(i.e., the n values) were an average of one or three. Since the content of FMA was very low in

the polymers, the molecular weight difference between macroinitiator and the corresponding

fluorinated polymer could not be determined by GPC. The low FMA content and the intrinsic

association of the FMA units also made it difficult to use 1H NMR to determine the

fluorinated content. Thus, fluorine elemental analysis was employed to measure fluorinated

monomer content. The number of FMA units (y) in PFMAy-ec-PMMA(or PBMA)x-ec-

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PFMAy was calculated respectively according to the following equation:

WF (%)=(17×19×2y)/ (Mx, nAMA+532×2y) (1)

where y is the number of FMA units. Each fluorinated monomer FMA contains 17

fluorine atoms, and the fluorine atomic weight is 19. Mx, nAMA is the molecular weight of the

PMMA or PBMA measured by GPC. The molecular weight of fluorinated monomer FMA is

532. Table 1 shows the chemical structure of all the copolymers used in this study.

The fluorinated polymethacrylates synthesized by ATRP were characterized by FT-IR.

Figure 1 shows the FT-IR spectra of PFMA1-ec-PMMA154-ec-PFMA1, Br-PMMA154-Br,

PFMA3-ec-PBMA200-ec-PFMA3 and Br-PBMA200-Br. The absorbance around 1200–1270

cm-1

for C–F stretching vibration bands overlapped with the C–O stretching vibration band

around 1270–990 cm-1

. Two medium bands at 660 and 708 cm-1

were readily observed,

which resulted from a combination of rocking and wagging vibrations of CF2 groups.45

These

observations suggest that the FMA unit was introduced into both copolymers.

3.2 Air/solution interfacial structure and property of PFMA1-ec-PMMAx-ec-PFMA1

and PFMA3-ec-PBMAx-ec-PFMA3 solutions. The equilibrium surface tension was

employed to evaluate the surface activity of the fluorinated polymers. Surface activity of

PFMA1-ec-PMMAx-ec-PFMA1 and PFMA3-ec-PBMAx-ec-PFMA3 in solution was

determined by the Whihelmy plate method.12

In Figure 2, the surface tension (γ) was plotted

against PFMA1-ec-PMMAx-ec-PFMA1 and PFMA3-ec-PBMAx-ec-PFMA3 content in

cyclohexanone or toluene solution. Similar to homopolymers, the surface tensions of PMMA

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in cyclohexanone and PBMA in toluene solutions decreased with increasing polymer

concentration. When the concentration was 0.01g/mL, their surface tensions were 23.5 and

25.2 mN/m, respectively. In the same manner as conventional surfactants and other non-ionic

polymeric surfactants, all these fluorinated polymers show a declining surface tension with

increasing polymer concentration, and then reach an equilibrium value. The surface tensions

of these two series of end-capped polymers at the higher concentrations exhibited similar

tendencies, with surface tension increasing with increasing polymerization degree (DP) of

PMMA or PBMA. The relationship between the polymerization degree of poly(n-alkyl

methacrylate) in PFMAy-ec-PnAMAx-ec-PFMAy and the corresponding solution surface

tension at a concentration of 0.01g/mL is shown in Figure 3. For PFMA1-ec-PMMAx-

ec-PFMA1, the surface tension values increased from 18.9 to 23.5mN/m when the

polymerization degree of PMMA increased from 154 to 540. When the DP was above 540,

the corresponding surface tension was the same as that of PMMA homopolymer solution.

Similarly, the surface tension for PFMA3-ec-PBMAx-ec-PFMA3 increased in approximately

linear fashion, from 19.3 to 25.0 mN/m, with increasing DP of PBMA. When the DP of

PBMA was 711, the corresponding surface tension was close to that of PBMA homopolymer

solution. From these surface tension measurements, it was found that the surface activity of

poly(methacrylate) end-capped with fluorinated methacrylate units eventually disappeared as

the polymerization degree of poly(n-alkyl methacrylate) in PFMAy-ec-PnAMAx-ec-PFMAy

increased.

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According to the Gibbs adsorption isotherm equation, the data depicted in the γ-C curves

can be employed for calculating the maximum excess surface concentration (Γmax), and the

minimum occupied surface area of one molecule (Amin) 46

max

1

lnRT c

γ− ∂Γ = •

∂ (2)

where R is the gas constant, T is the absolute temperature and ∂γ/∂lnc is the slope of the

γ-lnc plot. The occupied surface area, Amin, (in nm2) is given by:

20

min

max

10

A

AN

=Γ

(3)

where NA is Avogadro’s number. The Γ and Amin values of various polymers in solution as

calculated from Eqs. 2 and 3 are listed in Table 2.

It was found that with increasing polymerization degree of PnAMA, the excess surface

concentration of the fluorinated end-capped polymers gradually decreased while the surface

area occupied by one molecule increased, for both PFMA3-ec-PBMAx-ec-PFMA3 toluene

solution and PFMA1-ec-PMMAx-ec-PFMA1 cyclohexanone solution. However, the excess

surface concentration for the PFMA1-ec-PMMAx-ec-PFMA1 cyclohexanone solution was

much higher than that for the PFMA3-ec-PBMAx-ec-PFMA3 toluene solution. Since the

fluorinated moieties are distributed at either of the two ends of the polymer chains, the whole

chain may lie at the air/solution interface when the polymerization degree of PnAMA in the

end-capped polymer is relatively high. This may be the main reason for the observed

decrease in excess surface concentration of the polymer and the increase in surface area

occupied by one molecule with increasing polymerization degree of PnAMA. The results

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shown in Table 2 strongly indicate that the PFMA3-ec-PBMAx-ec-PFMA3 chain packing at

the toluene solution surface was much looser than that of PFMA1-ec-PMMAx-ec-PFMA1 at

the cyclohexanone solution surface. From this result, we can conclude that the

PFMA3-ec-PBMAx-ec-PFMA3 chains in the toluene solution are much more flexible than the

PFMA1-ec-PMMAx-ec-PFMA1 chains in the cyclohexanone solution, which may be

associated with an expanded PFMA3-ec-PBMAx-ec-PFMA3 chain conformation in the

toluene solution.

Recently, sum frequency generation (SFG) vibrational spectroscopy has been applied to

study the structure of copolymers at the air/solution interface, providing further

understanding of interfacial polymer structures. 12, 30-36

In order to obtain a molecular level

understanding, the surface/interface-sensitive SFG vibrational spectroscopy technique was

employed to investigate the molecular structures of these two series of fluorinated polymers

at their corresponding air/solution interfaces. Figure 4 shows the SFG spectra of a

PFMA1-ec-PMMAx-ec-PFMA1 cyclohexanone solution/air interface in the range of

2800-3100 cm-1

, which corresponds to the C–H stretching vibrations. As shown in Figure 4,

the SFG spectrum of the pure cyclohexanone/air interface is dominated by the symmetric

stretch and the Fermi resonance of methylene groups at 2865 and 2945 cm-1

in ssp

polarization combinations, and the Fermi resonance of methylene groups at 2945 cm-1

in ppp

polarization combinations.30,47

For homopolymer PMMA cyclohexanone solution, the new

peaks at 2910 and 2950 cm-1

, which are respectively assigned to vibrations from CH2 in the

backbone of MMA unit and the symmetric stretching vibration of the ester methyl

groups(-OCH3) 48,49

, were observed in the SFG spectra of ssp polarization combinations. At

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the same time, a new peak at 2965 cm-1

assigned to the antisymmetric stretching vibration of

α-CH3 50

appears in the SFG spectra of ppp polarization combinations. The SFG spectra and

the corresponding surface tensions of PMMA homopolymer at different concentrations are

shown in Fig. S1 and S2 (Supporting Information). It was found the air/solution interface was

completely occupied by PMMA segments when the surface tension reached 23.5mN/m. For

PFMA1-ec-PMMAx- ec-PFMA1, when the DP of PMMA was lower than 360, two new peaks

appeared at 2930 and 2955 cm-1

in the SFG spectra shown in Figure 4, which are assigned to

asymmetric stretching vibrations of -CH2- in the backbone of the FMA unit and the

asymmetric stretching vibration of the -CH2- connected to the perfluoroalkyl group [C8F17],

respectively.9,13

This was further confirmed by SFG spectra of poly(2-perfluorooctylethyl

methacrylate) (PFMA) homopolymer film shown in Figure 4. Since it is very hard to find a

suitable solvent to dissolve PFMA, we were required to use PFMA film to obtain SFG

spectra. Simultaneously, no peak was observed in the SFG spectra of ppp polarization

combination. With increasing DP of PMMA, the peaks at 2930 and 2955cm-1

disappeared

and the peaks at 2910 and 2950 cm-1

appeared in the SFG spectra of ssp polarization

combination. The peak at 2965cm-1

attributed to the –CH3 group in PMMA accordingly

appeared in SFG spectra of the ppp polarization combination. A possible explanation for

these observations is that the PFMA segments were adsorbed at the air/solution interface, and

PFMA1-ec-PMMAx-ec-PFMA1 thus possessed increased surface activity when the length of

the PMMA block was short. The air/solution interface was then gradually completely

occupied by PMMA segments with increasing DP of PMMA, and the enrichment of

fluorinated moieties at the air/solution interface lessened and finally disappeared. The results

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shown in the SFG spectra are in agreement with those of their corresponding solution surface

tension measurements.

The SFG spectra from the air/toluene solution interface of PBMA and

PFMA3-ec-PBMAx-ec-PFMA3 at the concentration of 0.01g/mL are presented in Figure 5. As

shown in Figure 5, the SFG spectra of the toluene/air interface are dominated by the

symmetric stretch of CH3 at 2925 cm-1

and the symmetric v2 stretch from the phenyl group at

3060 cm-1

in ssp polarization combinations.48, 51

The peak at 3030cm-1

was assigned to v20b

stretch from phenyl in the SFG spectra of ppp polarization combinations. For the PBMA

homopolymer toluene solution at the concentration of 0.01g/ml, the new peaks observed at

2910 cm-1

assigned to vibrations from CH2 in the backbone of BMA unit 48

were found in the

ssp combination. At the same time, the peaks at 2925 and 3060 cm-1

became very weak. The

peak at 2965cm-1

assigned to the antisymmetric stretching vibration of α-CH3 of PBMA 50

accordingly appeared in the ppp combination SFG spectra. In a manner similar to that of the

PMMA cyclohexanone solution, the surface tension of the PBMA homopolymer toluene

solution decreased to about 25.0 mN/m with increasing concentration, which was attributed

to the air/toluene solution interface being occupied by PBMA segments (as shown in Fig. S3

and S4, Supporting Information). The SFG spectra at the air/solution interface of

PFMA3-ec-PBMAm-ec-PFMA3 with different DP values of PBMA at certain concentration

points are presented in Figure 5. For PFMA3-ec-PBMAx-ec-PFMA3, when the DP of PBMA

was less than 200, two new peaks at 2930 and 2955 cm-1

appeared, which are related to the

PFMA component.9,13

Simultaneously, no peak was found in the SFG spectra of the ppp

polarization combination. However, with increase of DP of PBMA to about 500, the peaks at

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2930 cm-1

disappeared in the SFG spectra of ssp polarization combination. When the DP of

PBMA was 711, its SFG spectrum was almost the same as that of pure PBMA solution, and

their surface tensions were also the same. It is thus possible that the PFMA segments were

adsorbed at the air/solution interface when the length of the PBMA block in PFMA3-ec-

PBMAx-ec-PFMA3 was short. With increasing DP of PBMA, the orientation of the

fluorinated moieties was accordingly altered, and the PFMA segment gradually disappeared

at the toluene/air interface. Furthermore, the PBMA segments in PFMA3-ec-PBMAx-ec-

PFMA3 dominated the toluene/air interface. All the SFG results are in accordance with the

surface tension measurements. Similar results were reported 52

for the case when the

hydrophilic block in poly(2-(dimethylamino)ethyl methacrylate-block-n-butyl methacrylate)

copolymer was the larger block and hence dominated the space occupied at the air-water

interface, and the hydrophobic block had little effect on the surface tension. The reason why

the fluorinated component in PFMAy-ec-PnAMAx-ec-PFMAy was not adsorbed at the

air/solution interface when the polymerization degree of PnAMA was large may be attributed

to the molecular aggregates in solution. Work on this will be carried out in the future and will

be published in a subsequent paper.

3.3 Surface structure and properties of PFMAy-ec-PnAMAx-ec-PFMAy spin-coated

films prepared by corresponding polymer solutions. Spin-coating is a very popular

method for preparing various films and investigating their surface structure or corresponding

surface performance. The surface wettability is very sensitive to the structure at the outermost

polymer surface. The water contact angles of PFMAy-ec-PnAMAx-ec-PFMAy spin-coated

films were studied and the results are shown in Fig. S5 (Supporting Information). The water

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contact angles for the spin-coated films of PFMA1-ec-PMMAx-ec-PFMA1 decreased steadily

from 112° to 78° with increasing DP of the PMMA block, which was close to that of

homopolymer PMMA.9 Similarly, the water contact angles for PFMA3-ec-PBMAx-ec-

PFMA3 spin-coated films also decreased linearly from 115° to 93° with increasing DP of the

PBMA block.

It is well-known that the wetting behaviors of polymer film surfaces are determined by

their chemical structures and physical roughness.53, 54

The surface roughness of those films

obtained by solution spin-coating was investigated by AFM. The results show that all the

films possessed a very smooth and flat surface, with RMS roughness values less than 10.0 nm,

indicating that the effect of physical roughness is negligible. 54

Therefore, the difference in

contact angles shown above should be solely associated with the surface chemical structures

of the copolymer films. X-ray photoelectron spectroscopy (XPS) is a useful technique for

examining the enrichment behavior of the fluorinated groups, 7 due to its ability to provide

information about elemental composition and chemical bonding of the outer few nanometers

of polymer films in a quantitative way. The typical XPS wide-scan spectra of the spin-coated

films of fluoroalkyl methacrylate end-capped polymer recorded at a takeoff angle of 30° are

shown in Figure 6. It is observed that the F1s peak occurs at about 689 eV, while O1s displays

a peak at 531 eV, and C1s has a peak at 285 eV, respectively.7 The results show that the

content of fluorine on the surface decreased with increasing DP of the PnAMA block

regardless of whether the polymer was PFMA1-ec-PMMAx-ec-PFMA1 or PFMA3-ec-PBMAx

-ec-PFMA3. As shown in Figure 7, the surface F/C ratio for PFMA1-ec-PMMAx-ec-PFMA1

decreases with DP of the PMMA block increasing from 0.48 to 0.037. Similarly, the F/C

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ratios of the PFMA3-ec-PBMAx-ec-PFMA3 film surfaces follow the same trend.

In order to control the surface structure of materials, the influence of the corresponding

air/solution interface structure should be taken into consideration.7,12-15 It was reported that

the unimers localized at the air/solution interface can affect the surface fluorine enrichment

and surface free energy of cast and spin-coated films of diblock copolymers with

perfluoroalkyl side chains.7

In our previous paper12,13

, it was found that the chain

conformations and migration of fluorinated random copolymers in the solution and at the

solution/air interface play a very important role in the surface structure of the resulting solid

films, determining entropic or enthalpic influences in the surface segregation of fluorinated

moieties on random fluorinated polyacrylate films during film formation. Figure 8 shows the

surface F/C ratio of PFMAy-ec-PnAMAx-ec-PFMAy spin-coated films as a function of the

surface tension of the corresponding polymer solutions. The results show that when the

solution surface tension increased from 18.9 to 23.5mN/m, the corresponding surface F/C

ratio of PFMA1-ec-PMMAx-ec-PFMA1 spin-coated films decreased linearly from 0.48 to

0.037. Simultaneously, the surface F/C ratio of PFMA3-ec-PBMAx-ec-PFMA3 films also

decreased linearly from 0.59 to 0.09 with increase of surface tension from 19.3 to 25.0 mN/m.

However, it is apparent that the surface F/C ratio for PFMA3-ec-PBMAx-ec-PFMA3

spin-coated films is higher than that of the PFMA1-ec-PMMAx-ec-PFMA1 films when the

corresponding solution surface tension is approximately the same.

Taking into account the discussion above, we propose an explanation for the orientation

of the perfluoroalkyl groups on the solution surface in the present study, based on a

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thermodynamic analysis. The migration of perfluoroalkyl groups to the solution/air interface

would lead to ∆H<0 and ∆S<0 as a result of the polymer chains becoming increasingly

ordered. When the concentration of fluorinated moieties in the copolymer chain was low and

the length of PnAMA was large, migration of perfluoroalkyl groups to the solution/air

interface resulted in |T∆S| > |∆H|, and accordingly, ∆G>0. As a result, the perfluoroalkyl

groups become buried in the close-packed random-coil chain, as shown in Scheme 2 (B),

which results in an increase in solution surface tension. A similar result was also observed 52

for poly(2-(dimethylamino)ethyl methacrylate-block-n-butyl methacrylate) copolymers-when

the hydrophilic block was the larger block, it dominated the space occupied at the air/water

interface. When the content of fluorinated moieties in the copolymer chains was high and the

length of PnAMA was short, the migration of perfluoroalkyl groups to the solution/air

interface resulted in |T∆S|<|∆H|. Therefore, it is reasonable to assume that the perfluoroalkyl

groups would migrate to the solution/air interface with close-packed random-coil chains, as

shown in Scheme 2 (A), which results in a decreasing of the solution surface tension.

According to excess surface concentration (Гmax) and area occupied per polymer

molecule, the chain conformation of PFMA3-ec-PBMAx-ec-PFMA3 in toluene was more

extended than that of PFMA1-ec-PMMAx-ec-PFMA1 in cyclohexanone. Therefore, although

the migration of perfluoroalkyl groups in PFMA3-ec-PBMAx-ec-PFMA3 to the interface leads

to ∆S<0, its ∆S value decreased compared to that of PFMA1-ec-PMMAx-ec-PFMA1 in

toluene. It is reasonable to assume that the perfluoroalkyl groups in PFMA3-ec-PBMAx-

ec-PFMA3 would migrate to the toluene solution/air interface even if the FMA content is low.

The combination of the effect of the expanded chain structure and the interaction between the

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polymer and the solvent may lead to an easier migration of perfluoroalkyl groups in

PFMA3-ec-PBMAx-ec-PFMA3 to the interface and also promote a relatively ordered

orientation and conformation.

A question accordingly arises as to why there exists the linear relationship between the

surface F/C ratios of PFMAy-ec-PnAMAx-ec-PFMAy spin-coated films and the surface

tension of the corresponding casting solution. A plausible answer to this question may come

from consideration of the chain conformations at the interface shown in Scheme 2. Since

spin-coated films are formed by the very rapid evaporation of solvent under a high rate of

spinning of the substrate, the chain conformations in the corresponding casting solution are

frozen in a non-equilibrium state, resulting in a linear relationship between the F/C ratios at

the film surface and the surface tension of the corresponding solution. The surface structure

of the film is mainly controlled by the corresponding solution interfacial structure. The high

content of fluorinated groups at the interface of the casting solution, which lowers its surface

tension, will produce a higher F/C ratio on the surface of the resulting spin-coated film. Since

the chain conformation of PFMA3-ec-PBMAx-ec-PFMA3 in toluene is more extended in

toluene than that of PFMA1-ec-PMMAx-ec-PFMA1 in cyclohexanone, this chain extension

would weaken the entropy effect of the chains in the former polymer. At the same time, it

was reported previously 39

that the high flexibility of poly (n-alkyl methacrylate), as reflected

by the lower Tg, also enhanced the segregation of poly (n-alkyl methacrylate) end-capped

2-perfluorooctylethyl methacrylate (PAMAx-ec-PFMAy). The Tg values of PMMA and

PBMA were about 105°C and 20°C, respectively.55,56

Thus, the preceding discussion explains

the reason why the fluorinated moieties in PFMA3-ec-PBMAx-ec-PFMA3 can more easily

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segregate to the film surface than those in the PFMA1-ec-PMMAx-ec-PFMA1 system, as

shown in Figure 8. To the best of our knowledge, this is the first report concerning the linear

relationship between the solution surface tension of end-capped fluorinated polymers and the

corresponding surface structures of the resulting spin-coated films.

4. CONCLUSIONS

The surface properties of polymer materials are determined by their surface structures and

have a great influence on their ultimate applications. Therefore, considerable attention has

focused on controlling the surface chemical structures of such materials. However, the

formation mechanism of the surface chemical structures at the molecular level, which may

follow assorted pathways according to the particular thermodynamics and dynamics, remains

largely unstudied and poorly understood, despite the immense importance for practical

applications, especially when compared to the vast theoretical and empirical literature

available exclusively on bulk structures. The aim of the present study is to therefore obtain an

enhanced understanding of the formation mechanism of the surface chemical structures on

fluorinated polymer films during the process from casting solution to solid film formation.

Poly(n-alkyl methacrylate) end-capped with 2-perfluorooctylethyl methacrylate (FMA)

(PFMAy-ec-PnAMAx-ec-PFMAy) was synthesized via a controlled/living atom-transfer

radical polymerization (ATRP) technique. The surface properties of the resulting spin-coated

films and the corresponding film-formation solution/air interfacial structure of these polymers

were investigated by a combination of X-ray photoelectron spectroscopy (XPS), sum

frequency spectroscopy (SFG), and surface tension measurements. It was found that the

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fluorinated moieties in PFMAy-ec-PnAMAx-ec-PFMAy adsorbed at their solution/air

interface were gradually completely replaced by PnAMA segments with increasing

polymerization degree of PnAMA. At the same time, an interesting and previously

unreported result noted that there is a linear relationship between the surface F/C ratios of

spin-coated films and their corresponding polymer solution surface tension. In this first-time

observation, the surface F/C ratios of spin-coated films decreased with increasing surface

tension values of fluorinated polymer solution used for film-formation. The results indicate

that the ultimate surface composition of spin-coated films of these end-capped fluorinated

methacrylates is mainly dominated by the corresponding film-formation solution/air

interfacial structure. This study highlights the importance of the air/liquid interface structure

on the formation of the corresponding surface structure during polymer solution

solidification.

■ ASSOCIATED CONTENT

Supporting Information

The surface tensions of both PMMA and PBMA homopolymers with different

concentrations, SFG spectra collected at air/solution interface for PMMA and PBMA in

various concentrations, and water contact angle on the spin-coated film surface as a function

of the length of PnAMA block. This material is available free of charge via the Internet at

http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author

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*Tel/fax: +86-571-8684-3600. Email: [email protected] or [email protected]

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We are thankful for support from the National Natural Science Foundation of China

(NSFC, No.21174134, No.51003097 and No. 20874089) and the Natural Science Foundation

of Zhejiang Province (Grant No. Z4100463).

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Table 1. Characteristics of PFMAy-ec-PnAMAx-ec-PFMAy polymers.

Code Polymer Structure Mn×10-3 a)

Mw/Mn b)

WF c)(%)

FMA

d) (mol%)

PMMA154-ec-2F1 PFMA1-ec-PMMA154-ec-PFMA1 16.5 1.24 3.92 1.28






PBMA85-ec-2F3 PFMA3-ec-PBMA85-ec-PFMA3 15.3 1.28 12.70 6.60





a) Calculated from PFMAy-ec-PnAMAx-ec-PFMAy. b) Determined by GPC, and calibrated by polystyrene

standards. c) WF represents fluorine content obtained from fluorine elemental analysis. d) Calculated from

the equation: WF (%)=(17×19×2y)/ (Mx, nAMA+532×2y)

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Table 2. Excess surface concentration (Гmax) and area occupied per polymer molecule (Amin)

of PFMAy-ec-PnAMAx-ec-PFMAy

Code

Гmax

(µmol×m-2

) Amin (nm2) Code

Гmax

(µmol×m-2

) Amin (nm2)

PMMA154-ec-2F1 (a) 0.999 1.66 PBMA85-ec-2F3

(b) 0.66 2.51


(b) 0.42 3.89


(b) 0.41 4.05


(b) 0.25 6.63


(b) 0.17 9.54

PMMA830-ec-2F1 (a) 0.418 3.97

(a) Measured in cyclohexanone solution at 298K. (b) Measured in toluene solution at 298K

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Figure Captions

Figure 1. Transmission FT-IR spectra of PFMA1-ec-PMMA154-ec-PFMA1 (a),

Br-PMMA154-Br (b), PFMA3-ec-PBMA200-ec-PFMA3 (c) and Br-PBMA200-Br (d).

Figure 2. Concentration dependence of surface tension for solutions of PFMA1-ec-PMMAx

-ec-PFMA1 (A) and PFMA3-ec-PBMAx-ec-PFMA3 (B) polymers.

Figure 3. Surface tension as a function of DP of PnAMA block in PFMA1-ec-PMMAx -ec-

PFMA1 (○) in cyclohexanone and PFMA3-ec-PBMAx-ec-PFMA3 (▲) in toluene.

Concentration: 0.01g/mL

Figure 4. SFG spectra of (A) ssp and (B) ppp polarization combinations collected at the

air/cyclohexanone solution interface for PFMA1-ec-PMMAx-ec-PFMA1 solution.

Concentration: 0.01g/mL. The dots are experimental data and the lines are fitted results. The

SFG spectra for PFMA (◊) shown at the bottom were obtained from

poly(2-perfluorooctylethyl methacrylate) homopolymer film prepared by hot-pressing.

Figure 5. SFG spectra of ssp (A) and ppp (B) polarization combinations obtained from the

air/toluene solution interface for PFMA3-ec-PBMAx-ec-PFMA3. Concentration: 0.01g/mL.

Figure 6. XPS spectra of the PFMA1-ec-PMMAx-ec-PFMA1 (A) and PFMA3-ec-PBMAx-ec-

PFMA3 (B) spin-coated films recorded at a takeoff angle of 30°.

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Figure 7. F/C ratios at the surface of spin-coated films as a function of the length of the

PnAMA block in PFMA3-ec-PBMAx-ec-PFMA3 (▲) and PFMA1-ec-PMMAx-ec-PFMA1(○).

Figure 8. Relationship between surface tension of film-formation polymer solutions and

surface F/C ratio of corresponding PFMA1-ec-PMMAx-ec-PFMA1(▲) and

PFMA3-ec-PBMAx-ec-PFMA3(●) spin-coated films.

Scheme 1. Synthetic route to PFMAy-ec-PnAMAx-ec-PFMAy polymers.

Scheme 2. Schematic representation of the chain behavior of PFMAy-ec-PnAMAx-

ec-PFMAy with (A) short and (B) long PnAMA blocks during film formation.

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Figure 1. Transmission FT-IR spectra of PFMA1-ec-PMMA154-ec-PFMA1 (a),

Br-PMMA154-Br (b), PFMA3-ec-PBMA200-ec-PFMA3 (c) and Br-PBMA200-Br (d).

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Figure 2. Concentration dependence of surface tension for solutions of PFMA1-ec-PMMAx

-ec-PFMA1 (A) and PFMA3-ec-PBMAx-ec-PFMA3 (B) polymers.

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Figure 3. Surface tension as a function of DP of PnAMA block in PFMA1-ec-PMMAx -ec-

PFMA1 (○) in cyclohexanone and PFMA3-ec-PBMAx-ec-PFMA3 (▲) in toluene.

Concentration: 0.01g/mL

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Figure 4. SFG spectra of (A) ssp and (B) ppp polarization combinations collected at the

air/cyclohexanone solution interface for PFMA1-ec-PMMAx-ec-PFMA1 solution.

Concentration: 0.01g/mL. The dots are experimental data and the lines are fitted results. The

SFG spectra for PFMA (◊) shown at the bottom were obtained from

poly(2-perfluorooctylethyl methacrylate) homopolymer film prepared by hot-pressing.

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Figure 5. SFG spectra of ssp (A) and ppp (B) polarization combinations obtained from the

air/toluene solution interface for PFMA3-ec-PBMAx-ec-PFMA3. Concentration: 0.01g/mL.

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Figure 6. XPS spectra of the PFMA1-ec-PMMAx-ec-PFMA1 (A) and PFMA3-ec-PBMAx-ec-

PFMA3 (B) spin-coated films recorded at a takeoff angle of 30°.

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Figure 7. F/C ratios at the surface of spin-coated films as a function of the length of the

PnAMA block in PFMA3-ec-PBMAx-ec-PFMA3 (▲) and PFMA1-ec-PMMAx-ec-PFMA1(○).

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Figure 8. Relationship between surface tension of film-formation polymer solutions and

surface F/C ratio of corresponding PFMA1-ec-PMMAx-ec-PFMA1(▲) and

PFMA3-ec-PBMAx-ec-PFMA3(●) spin-coated films.

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Scheme 1. Synthetic route to PFMAy-ec-PnAMAx-ec-PFMAy polymers.

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Scheme 2. Schematic representation of the chain behavior of PFMAy-ec-PnAMAx-

ec-PFMAy with (A) short and (B) long PnAMA blocks during film formation.

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Table of Contents Only

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surface structure of spin-coated fluorinated polymers films dominated by corresponding...

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