Surface Characterization of Block Copolymers with Water-soluble Block by using Sum-Frequency Generation Spectroscopy∗

A series of amphiphilic block copolymers containing poly[oligo(ethylene glycol) methacrylate] segments and the corresponding poly[methyl ethers of oligo(ethylene glycol) methacrylate] block copolymer surfaces are investigated by using sum-frequency generation (SFG) vibrational spectroscopy. The terminal moiety on the oligo(ethylene glycol) side chain played an important role in determining the surface structure of the block copolymer films. Due to the amphiphilic character of the methoxy oligo(ethylene glycol) side chain, the PME3MA block exposes the terminal methyl group to the surface and reduces the surface tension in air or vacuum, and it is possible that configurational entropy may also favor the presence of side chains at the surface. The molecular orientation of the block copolymer at the surface is modified due to the multilayered structures of the block copolymers. [DOI: 10.1380/ejssnt.2006.515]


I. INTRODUCTION
Water-soluble polymers have been attracting considerable attention in the biomedical field [1].By attaching a water-soluble polymer to a solid surface, the elastic repulsion of the polymer coils, which swollen in water, can be utilized to hinder the underlaying hydrophobic surface from adhering of proteins, platelets and cells.Various techniques have been developed to modify polymer surfaces that will not activate blood coagulation.Among these techniques, the approach involving the use of a copolymer composed of hydrophilic and hydrophobic units is very promising.Recently, we discovered a block copolymer of polystyrene (PS) and 2-[2-(2methoxyethoxy) ethoxy]ethyl methacrylate (PME3MA) mixed with PS spontaneously exposes the PME3MA block, which is soluble in water, to the surface in air or vacuum [2].Furthermore, this block copolymer surface exhibits excellent resistance toward protein, adsorption, and cell and platelet adhesion without preincubation in water [3].
Sum-frequency generation (SFG) spectroscopy is a second-order nonlinear optical technique in which two laser beams overlap in space and time at the probed surface or interface [4].One of beams has a tunable infrared frequency ω 1 and the other is characterized by a fixed visible frequency ω 2 .The SFG signal is detected at the visible frequency ω 3 = ω 1 + ω 2 .A vibrational fingerprint of the surface is obtained by scanning the infrared laser frequency and recording the signal at frequency ω 3 .A res-onant enhancement in the SFG signal intensity is detected whenever the infrared frequency matches the molecular vibration frequency.Due to its surface sensitivity, SFG is a powerful tool for studying surfaces and interfaces.Recently, SFG has been applied to surface study [5], the determination of the alignment of surface polymeric chains [6,7], the study of the chemical composition of a surface, and for processing induced molecular changes at the interface [8].

II. EXPERIMENTAL
The block copolymers used in this study were synthesized using anionic polymerization, which has been described previously [9].The samples are listed in Table I and Fig. 1.Thin films of ca.300 nm were prepared by spin-casting of toluene solutions of the block copolymers or dPS-PME3MA/PS mixtures on a glass plate.The films were annealed at 140  ether (M n = 2000) was purchased from Aldrich.The PEG dimethyl ether films were prepared by spin-casting a toluene solution on a glass plate.
In IR-visible SFG experiments, a mode-locked Nd:YAG laser (PL2143D, EKSPLA) at 1064 nm with a pulse width of 20 ps and a repetition rate of 10 Hz was employed as a master light source.A tunable IR beam in the range of 1000 to 4300 cm −1 was generated by an AgGaS 2 crystal by difference frequency mixing of the fundamental of the Nd:YAG laser with the output of an optical parametric oscillator/amplifier (OPO/OPA) system.The OPO/OPA system comprised a LiB 3 O 5 crystal, which was pumped by the third harmonic of the Nd:YAG laser.The visible and IR beams were overlapped at a sample surface with the incidence angles of 70 • and 50 • , respectively.The pulse energy and the spot size of the tunable IR were 0.3 mJ and 0.5 mm (in diameter), respectively.The spectral resolution of the IR beam was 6 cm −1 .The SFG spectra were recorded with various polarization combinations of the SFG, the incident visible light pulses, and the incident IR pulses with respect to the incident plane.The spectra were abbreviated in the following order: ssp for s-polarized SFG, s-polarized visible light, and p-polarized IR.The detailed experimental setup has been described previously [10].

A. PS-PME3MA surface
Vibrationally resonant SFG has emerged as a powerful tool for the study of the molecular structure of various surfaces and interfaces.SFG spectra with ssp, ppp, and sps polarization combinations of homo-PS-PME3MA thin film appear in the 2700-3200 cm −1 region in Fig. 2(a).For comparison, we also show the SFG spectra of the PEG dimethyl ether thin film in Fig. 2(b).The ssp SFG spectrum of PEG dimethyl ether is identical to the published results [11].The ssp SFG spectrum of PEG dimethyl ether in Fig. 2(b) shows a strong symmetric C-H stretch peak corresponding to the end group OCH 3 at 2820 cm −1 .The peak at 2870 cm −1 is due to the symmetric stretching of the OCH 2 backbone.The strong peak at 2820 cm −1 clearly shows that most of the PEG surface is covered with the hydrophobic methoxy end group and the backbone covers the remaining surface.The hydrophobic methoxy end group tends to segregate to the surface due to the lower surface energy.Although the amount of end groups in PEG dimethyl ether is about 3 wt%, they segregate to the surface and are clearly detected by SFG.However, it should be noted that the PEG dimethyl ether surface is not completely covered with the hydrophobic end groups because the peak originating from the backbone is also present.Therefore, a small amount of the backbone appears at the PEG surface.
In the case of the PS-PME3MA block copolymer, the SFG spectra exhibit seven vibrational modes comprising C-H stretches, which are both Raman and infrared active.The peaks observed at 2820 and 2965 cm −1 originate from the symmetric and asymmetric stretching of the terminal OCH 3 group, respectively [11].The peak at 2860 cm −1 is due to the symmetric stretching of the OCH 2 side chain [13].It is noted that the presence of OCH 2 peaks is an evidence of gauche conformation of the EO side chains unlike the oligo EO chains attached to a surface [14].The assignments of the peaks at 2883 and 2915 cm −1 are not well established: these peaks may be attributed to the symmetric and asymmetric stretching of the CH 2 backbone.The peak at 2950 cm −1 represents the Fermi resonance, which originates from the interaction between CH 2 symmetric stretching and the overtones of the antisymmetric bending modes [11,12].The broad and weak peak at around 3065 cm −1 represents the overlap of the ν 20b and ν 2 vibrational modes of the phenyl side group of PS [15].In general, the lower surface energy block covers the surface exclusively and reduces the surface energy of the system.Since PME3MA is a hydrophilic polymer, it was expected that the hydrophobic PS segments would cover the surface of the block copolymer in order to reduce the surface energy.However, the peaks derived from PME3MA are clearly observed by SFG.This behavior is due to the segregation of the hydrophobic methoxy end group to the surface, which reduces the surface tension.Although the amount of methyl group in the PS-PME3MA block copolymer is about 6 wt%, the SFG spectra of PS-PME3MA clearly show that the terminal methyl groups of PME3MA aggregate at the surface, as in the case of PEG dimethyl ether.The PME3MA blocks are spontaneously enriched at the surface when the surface is prepared in vacuum in order to minimize the surface free energy.Since this segregation process is thermodynamic in nature, the surface remains stable for long periods in a hydrophobic environment.
A quantitative comparison of the peaks corresponding to s-OCH 3 at 2820 cm −1 can reveal the orientation of the terminal methyl groups.The quantitative analysis technique is described in literature [2].The polar tilt angle between OCH 3 and the surface normal is about 20 • when a δ-function distribution is assumed.A similar value is obtained from the SFG spectra of the PEG dimethyl ether surface, thereby indicating that the terminal methyl groups tend to stand nearly normal to the surface and expose the methyl termini to the surface.It should be noted that the surface of the PS-PME3MA film is not completely covered with the PME3MA blocks because a phenyl peak at 3065 cm −1 is also present.This point will be subsequently discussed in this report.

B. Effect of the methyl termination of the side chain
The surface compositions of multi-component systems are not always identical to bulk compositions.While PS-PTEGMA and PS-PME3MA differ only in the addition of the CH 3 group at the termini of the side chain, this difference in the chemical structure completely changes the surface composition.The methacrylates with the EO side chains without methyl termini are known to stay away from the surface [16].From the XPS measurements, it was revealed that the PS-PDEGMA and PS-PTEGMA surfaces are completely covered with PS segments [17].In order to investigate the effect of the methyl termini of the side chain, the ssp SFG spectra of PS-PME3MA, PS-PDEGMA, and PS-PTEGMA surfaces are shown in Fig. 4. The SFG spectra of the PS-PDEGMA and PS-PTEGMA surfaces are very similar.The peaks at 2883 and 2915 cm −1 originate from the symmetric and asymmetric stretching of the CH 2 backbone, as described in the previous section.The peaks derived from the aromatic C-H vibrational stretching associated with the phenyl side groups are clearly detected.The phenyl side groups of http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp)PS exhibit five vibrational modes: these modes appear at 3024, 3035, 3055, 3067, and 3078 cm −1 corresponding to ν 20a , ν 7b , ν 7a , ν 2 , and ν 20b vibrational modes, respectively [20].The most prominent ν 2 peak at 3067 cm −1 indicates that the phenyl rings are more or less tilted toward the surface plane [21].Since no OCH 2 stretching signal can be detected and strong peaks derived from the phenyl groups are clearly observed, the PS-PDEGMA and PS-PTEGMA surfaces are completely covered with the PS segment.In contrast, the peak at 3067 cm −1 due to the phenyl ring is considerably weaker in the case of the SFG spectrum of PS-PME3MA.Although the intensities of the SFG signal depend significantly on the orientation of the surface functional group, the polarization experiments suggest that the surface composition of the PS segment in PS-PME3MA is consideravbly smaller than that in the PS-PTEGMA and PS-PDEGMA surfaces.
In the case of the SFG spectrum of pure PS film, the peaks due to the CH 2 backbone are very weak or almost invisible [20,21].Remarkably, the SFG spectra of PS-PDEGMA and PS-PTEGMA exhibit peaks derived from the CH 2 backbone.A spin-coated polystyrene film is macroscopically amorphous, and its surface is covered with the phenyl side ring.The backbone may be buried under the surface or it may show less ordering or both; however, it can assume a flexible orientation.In contrast, it is known that a block copolymer forms a layered structure that is induced by the difference in the surface energies of the blocks.From small-angle X-ray scattering measurements, it was found that PS-PDEMGA shows a strongly segregated lamellar structure with sharp interfaces, while PS-PTEGMA shows weakly segregated lamellar or disordered structures [17].It has been shown that the interactions of the blocks with the interfaces induce a near perfect orientation of the lamellar microdomains parallel to the film surface, thereby producing a multilayered structure.In the case of these amphiphilic block copolymers, the lamellar thicknesses for PS-PDEMGA and PS-PTEGMA are estimated to be about 22.8 and 19 nm, respectively [17].Such lamellar structures force the modification of the molecular orientation of the PS backbone and the limitation of chain flexibility.As a result, the PS backbone assumes a more ordered structure at the surface and the interface.Thus, we detect the SFG signals obtained from the CH 2 backbone of the PS-covered block copolymer surfaces.

C. Effect of EO chain length
In order to understand the surface segregation of the oligo EO units in detail, we measured the SFG spectra of three different block copolymers with different number of EO units.The PMEMA segment has a single EO unit in the side chain, and it is water insoluble.The PME2MA and PME3MA segments are water-soluble polymers with LCSTs of 26 • C and 52 • C, respectively [9].In Fig. 3, we show the ssp SFG spectra of PS-PMEMA, PS-PME2MA, and PS-PME3MA block copolymers.The spectral features in these figures are not very different from each other, which indicates that these block polymer surfaces show an almost similar structure, and most of these surfaces are covered with hydrophilic blocks.The peak strengths of s-CH 3 are not very different from each other, while the amount of terminal CH 3 groups in PS-PMEMA, PS-PME2MA, and PS-PME3MA are 10%, 8.4%, and 6%, respectively.This result indicates that the presence of methyl termination in the side chain plays an important role in the surface segregation of the hydrophilic block, irrespective of the number of EO units.
However, in the case of PS-PMEMA, the peak at 2860 cm −1 is very weak when compared with those of the other two polymers, while the other peaks remain almost unchanged.We believe that this behavior is due to the different glass transition temperature (T g ) effects.T g of PMEMA, PME2MA, and PME3MA were 30, −40, and −47 • C, respectively.The low T g of PME2MA and PME3MA indicate that their molecules are quite mobile at room temperature; this is believed to play a crucial role in surface morphology.T g of PMEMA is slightly higher than room temperature; thus, its side chains become ordered.
It should be noted that the surfaces of these block copolymers are not completely covered with the hydrophilic blocks, as mentioned in the preceding section, because the phenyl peak is observed in all block copolymers.Hence, if the surfaces are completely covered with hydrophilic segments, where do the SFG signals of the PS ring originate from?As mentioned above, even in a disordered phase, a block copolymer forms a layered structure that is induced by the difference in the surface energies of the blocks [18].Is it possible to detect the buried lamellar interfaces between the PME3MA layer and the buried PS layer?This issue is addressed by SFG studies of thin polymer/polymer interfaces.In our previous report, the interaction between PME3MA and PS is screened by the presence of the methyl end groups; therefore, the interface between PME3MA and PS appears weak in the neutron reflectivity measurements and the small-angle Xray scattering measurements [2,17].Z. Chen, et al. reported that the SFG signals generated from the ultrathin polymer/polymer interface were considerably weaker than those from the polymer/air interface [19].Furthermore, since the typical lamellar thickness is about one order of magnitude smaller than the coherent length of the SFG, the SFG signal vanishes due to the inversion symmetry of the each layer.Thus, the SFG signal from PS does not originate from the buried interface between PME3MA and PS.The appearance of the PS segment at the surface may be attributed to the fact that the macroscopic SFG data includes the inhomogeneity of the sample.The driving force for the surface segregation of PS-PME3MA does not appear to be very strong, and the surface does not form dense polymer brushes in air [2].Therefore, PS-PME3MA may not have a homogeneous composition at the surface.Thus, we conclude that the miniscule amounts of PS segments appear on the surface in the SFG sampling area (0.4 × 0.4 mm).
In our previous report, we found that PS-PME3MA was highly resistant to protein and cell adsorption [3].However, SFG experiments revealed that its surface is covered with the hydrophobic methyl end groups and a small amount of PS.Although the PS surface has poor blood compatibility, it is expected that the PME3MA components at the surface will swell considerably as soon as they come into contact with water, and the residual PS component will be buried under the surface layer.Thus, the PS-PME3MA surface can achieve the excellent resistance to bio-related materials in a hydrophilic (water) environment, while its surface contains a small amount of PS segments in a hydrophobic environment.

D. Segregation of dPS-PME3MA in mixtures with PS
In order to conduct a more rigorous test of hydrophilic surface segregation in PS-PME3MA, we blend a small amount of PS-PME3MA with homo-PS so as to alter the surface properties of PS.In this experiment, we use dPS-PME3MA [polystyrene units are fully deuterated] instead of PS-PME3MA in order to identify the SFG signal of homo-PS.In Fig. 5, we show the SFG spectra with ssp, ppp, and sps polarization combinations of homo-PS mixed with 10% dPS-PME3MA.For 10% dPS-PME3MA, the segregated PME3MA blocks dominate the spectra while the phenyl groups of homo-PS are absent.We also measured the aromatic C-D stretch region of the mixture of 10% dPS-PME3MA and homo-PS: no significant peaks were observed in these measurements.The SFG spectra clearly show that the surface is completely covered with PME3MA units, while the amount of CH 3 end groups is less than 1 wt%.The comparison between Fig. 2(a) and Fig. 5 reveals that the SFG intensity of s-OCH 3 is considerably weaker than that of the homo-PS-PME3MA film.Moreover, the SFG signal intensities of the peak at 2910 cm −1 are large in the case of the homo-PS-PME3MA film, while they are very weak in the case of the 10

IV. CONCLUSION
SFG has been applied to study the PS-PME3MA block copolymer surface and the families of hydrophilichydrophobic block copolymers.The terminal moiety on the oligo(ethylene glycol) side chain played an important role in determining the surface structure of the block copolymer films.The SFG measurements show that the small amount of methyl end groups of oligo(ethylene glycol) side chains with lower surface tensions tends to segregate to the polymer-air interface.Owing to the amphiphilic nature of the PME3MA blocks, the PME3MArich surface rapidly reconfigures its structure in order to minimize its surface free energy in ambient atmosphere.Due to the multilayered structures of the block copolymers, the molecular orientation of the polymer backbone changes drastically; this can be detected by SFG.Although the PS-PME3MA surface is not completely covered with hydrophilic segments, the PME3MA block on the surface swells rapidly as soon as it comes into contact with the surrounding water, and it hinders the underlaying hydrophobic PS blocks from adhering of proteins, platelets and cells.

FIG. 5 :
FIG. 5: (a) SFG spectra with ssp, ppp, and sps polarization combinations of the mixture of PS+dPS-PME3MA.(b) Schematic morphologies of thin films of block copolymer and polymer blend.

TABLE I :
Samples of block copolymers.a) Mn was determined by end-group analysis using 1 H NMR. b) Mw/Mn was determined by SEC (size exclusion chromatography) calibration using PMMA standards in THF.