Nano-Ordered Surface Morphologies by Stereocomplexation of the

Department of Chemistry, University of Memphis, Memphis, Tennessee 38152, United States. Langmuir , 2014, 30 (46), pp 14030–14038. DOI: 10.1021/ ...
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Nano-Ordered Surface Morphologies by Stereocomplexation of the Enantiomeric Polylactide Chains: Specific Interactions of SurfaceImmobilized Poly(D‑lactide) and Poly(ethylene glycol)-Poly(L‑lactide) Block Copolymers Maho Nakajima,† Hajime Nakajima,† Tomoko Fujiwara,‡ Yoshiharu Kimura,*,† and Sono Sasaki† †

Department of Biobased Materials Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Department of Chemistry, University of Memphis, Memphis, Tennessee 38152, United States

‡

S Supporting Information *

ABSTRACT: Both AB diblock and ABA triblock copolymers consisting of poly(L-lactide) (PLLA: A) and poly(ethylene glycol) (PEG: B) were deposited on a silicon surface on which poly(D-lactide) (PDLA) had been preimmobilized. The deposit of the diblock copolymer (PLLA-PEG) formed band structures similar to those observed when the same copolymer was directly deposited on the silicon surface. In contrast, the deposit of the triblock copolymer (PLLA-PEG-PLLA) formed many particulates scattering over the surface. When the PLLA-PEG deposit was subjected to water-soaking, the original band morphology was completely replaced by the particulate morphology that was identical to that of the PLLA-PEG-PLLA deposit. Their FT-IR analyses revealed that both copolymers had been bound through the stereocomplex (sc) formation between the preimmobilized PDLA chains and the PLLA blocks of the copolymers. Grazingincidence small-angle X-ray scattering (GISAXS) also supported these surface morphologies. It was therefore evident that hydrophilic PEG chains can be immobilized on the PDLA-preimmobilized surface by the sc formation.

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INTRODUCTION Surface modification has extensively been studied until now because of its important roles in the real usages of various materials, both organic and inorganic. Particularly, in the biomedical application, the materials’ surfaces contacting with cells and tissues must have proper structure and properties, for example, roughness, wettability, adhesion, and so on, to provide excellent biocompatibility and biofunctions. In the ordinary surface modification of such biomaterials, functional molecules are immobilized on surface to impart specific properties. A variety of macromolecular modifiers are also effectively immobilized for the surface functionalization. For example, derivatives of poly(ethylene glycol) (PEG) are immobilized for providing a hydrophilic surface that is antithrombogenic and cell-repellent,1,2 while collagen and gelatin are for attaining specifically bioreactive surfaces.3,4 Polylactides (PLAs) and their copolymers have also been examined as surface modifiers for excellent biocompatibility that is suitable for tissue engineering, bone curing, and drug delivery.5−8 Previously, Domb and Slager immobilized poly(D-lactide) (PDLA) chains on a surface pretreated with an aminoalkylsiloxane derivative and further deposited enantiomeric poly(L-lactide) (PLLA) chains to form stereocomplex crystals on the surface.9−11 They also demonstrated that the peptide molecules such as L-leuprolide can be immobilized through hetero-stereocomplexation with the surface-immobilized PDLA. Very recently, we reported that a © 2014 American Chemical Society

monofunctional ethoxydimethylsilyl-terminated PDLA (SiPDLA: shown by the chemical structure below) can homogeneously be immobilized on a plasma-treated surface of silicon wafer through single Si−O−Si bond12 and that the surface-immobilized PDLA chains can bind free PLLA and PDLA molecules to form specific surface morphologies with homochiral (hc) and stereocomplex (sc) crystallizations, respectively.13 Particularly, the formation of a single crystal array of the PDLA/PLLA complex strongly suggested a possibility of biding various PLLA block copolymers, and by which the copolymerized macromolecular chains can readily be immobilized on the surface for modification.

In the present study, therefore, we examine surface-binding of block copolymers of PLLA and PEG on the PDLAimmobilized surface. Herewith, we use AB diblock (PLLAPEG) and ABA triblock (PLLA-PEG-PLLA) copolymers for the binding (Scheme 1) and analyze the changes in the surface Received: August 17, 2014 Revised: October 2, 2014 Published: November 3, 2014 14030

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Then, a portion of A-PDLA (3.0 g) was dissolved in CH2Cl2 (40 mL) and reacted with acetic anhydride (8.0 mmol) in the presence of pyridine (8.0 mmol) at room temperature to obtain the terminalacetylated A-PDLA (abbreviated as A-ac-PDLA). Then, A-ac-PDLA (1.0 g) was reacted with EDMS (8.0 mmol) in CH2Cl2 (20 mL) containing Pt-DTS (0.01 mL) at 50 °C for 12 h with refluxing under a nitrogen atmosphere. Finally, the reaction system was thoroughly evaporated to obtain the Si-PDLA as a residue, which was preserved in a freezer under a nitrogen atmosphere. Here, Si-PDLA having Mn of 4500 Da was prepared. Its structure was supported by 1H NMR spectroscopy in our previous report.12 Si-PDLA (quantitative recovery, Mn = 6300 Da, Mw/Mn = 1.30), 1H NMR (CDCl3): δ (ppm) = 5.17 (q, CH of the lactate unit), 4.28 (m, CH2−O), 3.63 (q, CH3−CH2−O), 3.62 (t, O−CH2−CH2), 3.41 (t, −CH2−O−), 2.13 (s, −OCCH3), 1.59 (d, CH3 of the lactate unit), 1.19 (t, CH3−CH2−O), 0.58 (m, Si−CH2), 0.12 (d, Si−CH3). Comparison of the remaining olefinic proton signals (δ = 5.80 and 5.15) of A-ac-PDLA and the Si−CH2 signal (δ = 0.58 ppm) of SiPDLA revealed that the terminal hydrosilylation had reached 96% in conversion. Synthesis of Block Copolymers of PLLA and PEG. Both PLLAPEG and PLLA-PEG-PLLA were prepared by the conventional ROPs of L-lactide which were initiated by Me-PEG and PEG, respectively, in the presence of Sn(Oct)2 as the catalyst.14,15 Both polymerizations were conducted at 120 °C for 2 h. The resultant copolymers were dissolved in CH2Cl2 and reprecipitated into an excess amount of diethyl ether. Their sequence structures and molecular weights were confirmed by 1H NMR and GPC. The exactly same procedures were used for preparing PDLA-PEG and PDLA-PEG-PDLA with D-lactide as the monomer. PLLA-PEG (quantitative recovery, Mn = 12 900 Da, Mw/Mn = 1.08) and PDLA-PEG (quantitative recovery, Mn = 13 500 Da, Mw/Mn = 1.09), 1H NMR (CDCl3): δ (ppm) = 5.17 (q, CH of the PLA units), 4.26−4.34 (m, COOCH2− of the oxyethylene units connecting with the PLA block and CH−OH of the PLA block terminal), 3.64 (s, O− CH2−CH2−O of the MePEG block), 3.38 (s, O−CH3 of the MePEG terminal), 1.59 (d, CH3 of the PLA units). PLLA-PEG-PLLA (quantitative recovery, Mn = 16 300 Da, Mw/Mn = 1.27), PDLA-PEG-PDLA (quantitative recovery, Mn = 16 900 Da, Mw/Mn = 1.22), 1H NMR (CDCl3): δ (ppm) = 5.17 (q, CH of the lactate unit), 4.26−4.34 (m, COOCH2− of the oxyethylene units connecting with the PLA blocks and CH−OH of the PLA block terminals), 3.64 (s, O−CH2−CH2−O of the PEG block), 1.59 (d, CH3 of the PLA units). The PLLA/PEG and PDLA/PEG compositions of the copolymers determined from the integral ratios of the oxyethylene signal (δ = 3.64 ppm) and the lactate signal (δ = 5.17 ppm) were almost identical to those estimated from the feed ratio. Table 1 summarizes the characteristics of PLLA-PEG, PLLA-PEG-PLLA, PDLA-PEG, and PDLA-PEG-PDLA as well as Si-PDLA. The characteristics of the copolymers are summarized in Table S1 (Supporting Information) including those of a polymer blend of PLLA-PEG and PDLA-PEG samples. Measurements. 1H NMR spectra (600 MHz) were obtained by using an ARX-600 spectrometer (Bruker Biospin, Germany) in CDCl3 containing 0.03 vol % tetramethylsilane (TMS) as the internal standard. Gel permeation chromatography (GPC) was conducted on a Shimadzu (Kyoto, Japan) analyzer system composed of an LC-20AD

Scheme 1. Surface Binding of PEG-PLLA and PLLA-PEGPLLA Block Copolymers on the PDLA-Immobilized Silicon Surface

morphology and properties by using atomic force microscopy (AFM), Fourier-transform infrared (FT-IR) spectroscopies, and grazing-incidence small-angle X-ray scattering (GISAXS) to disclose a unique surface structure formed by the segmental sc crystallization between the preimmobilized PDLA chains and the PLLA chains of the block copolymers.

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EXPERIMENTAL SECTION

Materials. D- and L-Lactide were supplied by Corbion Purac (Gorinchem, The Netherlands). Ethylene glycol monoallyl ether (EGMAE) was purchased from Tokyo Chemical Industries, Ltd. (Tokyo, Japan). Ethoxydimethylsilane (EDMS) and a solution of platinum (0)-1,3-divinyl-1,1,3,3-tetramethyl-disiloxane complex (PtDTS) in xylene (ca. 2 wt % in Pt concentration) were purchased from Gelest (Morrisville, PA) and Sigma-Aldrich (St. Louis, MO), respectively. Poly(ethylene glycol) monomethyl ether (MePEG) having number-average (Mn) and weight-average (Mw) molecular weights of 7300 and 7470 Da, respectively, and poly(ethylene glycol) (PEG) having Mn of 4460 Da and Mw of 4580 Da were purchased from Sigma-Aldrich and Nacalai Tesque (Kyoto, Japan), respectively. Other chemicals (tin octoate, acetic anhydride, and pyridine) and solvents (toluene, dichloromethane (CH2Cl2), acetonitrile, and diethyl ether) were commercially supplied and distilled before use. A piece of silicon wafer (size 1 cm2) was treated with oxygen plasma using a plasma generator, YHS-360 100 V (Sakigake-Semiconductor Co., Ltd., Kyoto, Japan), for 30 s just before use. This generator is known to homogeneously oxidize the top surface only. Sakigake-Semiconductor reported that the depth of the oxidation reaches 3 nm at maximum. Synthesis of Si-PDLA. The Si-PDLA was successfully prepared by following our previous report (see Scheme S1 in the Supporting Information).12 In short, an allyl-terminated PDLA having 5000 Da and Mw/Mn = 1.04 (abbreviated as A-PDLA) was first prepared by the ordinary ring-opening polymerization (ROP) of D-lactide with EGMAE and tin octoate as the initiator and catalyst, respectively.

Table 1. Characteristics of Si-PDLA and the PLA-PEG Block Copolymers

a

sample

chain lengthsa

Mn (NMR) (103 Da)

Mn (GPC) (103 Da)

Mw (GPC) (103 Da)

Mw/Mnb

Si-PDLA PLLA-PEG PLLA-PEG-PLLA PDLA-PEG PDLA-PEG-PDLA

4.5 k 4.6k-5.2k 4.0k-3.2k-4.0k 4.6k-5.2k 4.0k-3.2k-4.0k

4.5 9.8 11.2 9.8 11.2

6.30 12.9 16.3 13.5 16.9

8.19 14.0 20.7 14.7 20.6

1.30 1.08 1.27 1.09 1.22

Determined by 1H NMR. bDetermined by GPC (eluent: 1,3-dioxolane at 45 °C). 14031

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Figure 1. AFM images of the silicon surfaces obtained after (a) the first deposit of Si-PDLA (4.5k) and the second deposits of (b) PLLA-PEG (4.6k5.2k) and (c) PLLA-PEG-PLLA (4.0k-3.2k-4.0k). predetermined amount of Si-PDLA sample was dissolved in CH2Cl2 in a concentration of 0.02 g/mL. A plasma-treated silicon wafer (size: 1 cm2) was then immersed in this solution for 1 h and dried at room temperature with the wafer surface kept vertical to the horizontal plane. After the drying, the wafer was washed with CH2Cl2 several times (the first deposit). Subsequently, a wafer having the first deposit was immersed in solution (0.02 g/mL) of PLLA-PEG, PLLA-PEGPLLA, PDLA-PEG, or PDLA-PEG-PDLA in the same way as the first deposit. The wafer was then washed with acetonitrile several times and thoroughly dried in air (the second deposit). The second deposits on the wafer were heat-treated at 80 °C in air for 3 h for annealing. Their water soaking was conducted by submerging the wafers into water overnight and gently dried in a clean bench at room temperature for another overnight. The first and second deposits obtained were subjected to various analyses.

pump, an RID-10A refractive index detector, and an LCsolution data processing system. A set of two identical TSKgel GMHHR-M columns (7.8 mm I.D. × 300 mm; Tosoh, Tokyo, Japan) of a fixed pore size (5 μm in diameter) was used. The eluent was 1,3-dioxolane flowing at a rate of 0.75 mL/min at 45 °C. The Mn and Mw values were calibrated with polystyrene standards. FT-IR spectra were measured on a FTIR8200PC spectrometer (Shimadzu) in a wavenumber range from 1900 to 800 cm−1 where silicon wafer is transparent with a resolution of 8 cm−1 at room temperature. AFM was conducted in dynamic force mode (tapping mode) using a Nanoscope IIIa scanning probe microscope (Veeco Instruments, Plainview, NY). A commercially available silicon tip with a spring constant of 25−35 N/m and a single beam cantilever of 125 μm long was used in a resonance frequency range from 260 to 410 kHz. Ellipsometry was conducted on a DVA-FL ellipsometer (Mizojiri Optical Co., Ltd, Tokyo, Japan). He−Ne laser of 632.8 nm in wavelength was injected at an incident angle of 70°. The following parameters were used to determine the layer thickness: where n0, n1, ns, and ks denote air, n0 = 1; silicon oxide, and silane- and polymer-derived layers, n1 = 1.46; silicon substrate, ns = 3.88 and ks = 0.02, respectively. Three to five measurements were done on different spots of each sample, and the data were averaged to determine the layer thickness. GISAXS measurements using synchrotron radiation were performed at the BL-45XU beamline of SPring-8 (RIKEN SPring-8 Center, Hyogo, Japan). The wavelength of the incident beam (λ) was 0.1 nm. Scattering patterns from samples prepared on Si wafers were collected using a PILATUS 300k detector (Rigaku Co., Tokyo, Japan) whose size was 195 pixels in height by 1475 pixels in width with a pixel size of 172 × 172 μm2. The sample-to-detector distance in the measurements was 2055 mm. The grazing angle of the incident X-ray to the sample surface, αi, was 0.11°, which was slightly higher than the critical angle of total reflection of the sample. The magnitude of the scattering vector, q, is defined by q = 2π/d, where d denotes the spacing between scatters. The x, y, and z components of q in the rectangular coordinate system were defined as qx = (2π/λ) cos(αf) sin(2θf)−cos(αi), qy = (2π/λ) sin(2θf) cos(αf), and qz = (2π/ λ)(sin(αi) + sin(αf)), where the x and y coordinates are the horizontal axes and the z coordinate was the vertical axis. The optical plane of the direct X-ray was the x−z plane. The 2θf and αf denote components of the takeoff scattering angle in the x−y plane and along the z axis. The q was calculated by using an equation of q = (qx2 + qy2 + qz2)1/2. Surface Immobilization. All procedures were carried out in a clean room. All apparatuses such as vials, Pasteur pipets, and volumetric cylinders were cleaned by dipping in conc-H2SO4 for 3 h, rinsed with Milli-Q water several times, and dried before use. A

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RESULTS AND DISCUSSION Surface Binding of PLLA-PEG and PLLA-PEG-PLLA on the PDLA-Immobilized Silicon Surface. A silicon wafer was first dipped in a solution of Si-PDLA having a Mn of c.a. 4.5 kDa (the first deposit) for surface immobilization of PDLA chains by siloxane bond formation with the surface silanol groups as reported previously.12,13 Figure 1a shows a typical AFM image of the resultant PDLA-immobilized surface of the first deposit, exhibiting a homogeneous scattering of particulate deposits of PDLA chains. The average thickness of the deposit layer was 3.7 nm with bumpiness less than 0.7 nm. This surface morphology, formed by the aggregation of the terminally anchored PDLA chains, was identical to that reported in our previous study.13 This surface-modified silicon wafer was then submerged into a CH2Cl2 solution of each of PLLA-PEG (4.6k5.2k in the block sequence) and PLLA-PEG-PLLA (4.0k-3.2k4.0k in the block sequence) (the second deposit). The AFM images of the surfaces obtained after the second deposition are shown in Figure 1b and c, which exhibit completely different morphologies from each other. On the surface treated with PLLA-PEG (Figure 1b), many band structures having an average width of 30 nm and an average length of about 70 nm are found, and the polymer layer thickness has increased (6.2 nm) with larger surface bumpiness (8.3 nm) displaying. In some places, the band structures are aligned in parallel with 14032

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Figure 2. AFM height images of the surfaces of the second deposits of (a) PLLA-PEG and (b) PLLA-PEG-PLLA after annealing (at 80 °C for 3 h) and of (c) PLLA-PEG and (d) PLLA-PEG-PLLA after water soaking.

completely different from the one observed when the same copolymer was directly deposited on a silicon wafer in which lamellar band structures were formed with narrower gap in cross-linked shape.16 These different morphologies suggest that the di- and triblock copolymers, consisting of the PLLA block of a similar length, interact with the preimmobilized PDLA chains in different ways to form the different aggregation structures on surface. We also examined the second deposition of PDLA-PEG and PDLA-PEG-PDLA on the first deposit of Si-PDLA. However, the second deposits formed by the interaction through homochiral complexation of the PDLA chains were found out to be much weaker, and most of the second deposits were washed out during the washing procedure with acetonitrile. The AFM analysis of the resultant surfaces, therefore, showed no specific structure, being similar to that (Figure 1a) of the first deposit of Si-PDLA (see Figures S1 and S2 in the Supporting Information). This fact suggests that the sc crystallization between the preimmobilized PDLA and the

spaces ranging from 35 to 60 nm as shown by white bidirectional arrows. This morphology is almost identical to the lamellar structure previously observed when a PLLA-PEG diblock copolymer was directly cast on a silicon wafer surface.16 It has been shown that the band structure of the PLLA-PEG block copolymer consists of crystal lamellae of PLLA blocks that is separated from other lamellae by the PEG blocks. The band structure has a nanometer size in diameter and likely aligns on the surface. In the ordinary amorphous block copolymers in which two immiscible polymer chains are coupled by a chemical bond and occupy equal volumes, a lamellar phase is formed. In the PLLA-PEG block copolymer, consisting of semicrystalline segments, it can be formed with crystallization of the PLLA blocks that are phase-separated along with the crystallization of PEG segments (vide infra). On the other hand, the surface treated with PLLA-PEGPLLA (Figure 1c) is covered with many spherical particulates having a similar size (an average diameter = 30 nm). The thickness of the polymer layer is as thick as 5.2 nm with slightly increased surface bumpiness (6.3 nm). This morphology is 14033

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Figure 3. FT-IR spectra of (a) the first deposit of Si-PDLA, second deposits of (b) PLLA-PEG and (e) PLLA-PEG-PLLA, the second deposits of (c) PLLA-PEG and (f) PLLA-PEG-PLLA after the annealing, and the second deposits of (d) PLLA-PEG and (g) PLLA-PEG-PLLA after the water soaking in the wavenumber ranges of (i) 1800−1720 cm−1, (ii) 1320−1240 cm−1, and (iii) 980−820 cm−1. (The triangles (△) represent the absorptions assigned to crystalline PEG (1280, 963, 947,844 cm−1). The absorption at 1268 cm−1 is sensitive to the amorphous phase of PLLA (or PDLA).)

surface as in the case of the second deposit of PLLA-PEGPLLA in which such a stable particulate morphology was already established even without passing the water soaking. Structure Analyses of the Surface-Bound Copolymers. Figure 3a, b, and e compares the IR spectra (in through-view) of (a) the first deposit of Si-PDLA (4.5k) and the following second deposits of (b) PLLA-PEG (4.6k-5.2k) and (e) PLLAPEG-PLLA (4.0k-3.2k-4.0k) in the wavenumber ranges of (i) 1800-1720 cm−1 (carbonyl stretching), (ii) 1320−1240 cm−1 (C−O−C stretching), and (iii) 980−820 cm−1 (backbone vibrations). The first deposit of Si-PDLA (a) exhibits absorptions at 1759 and 955 cm−1, corresponding to the coil conformation or amorphous state of the PDLA chains.18,20 The shoulder peak around 1752 cm−1 suggests the presence of a nematic state of the chains in small ratio.21 In the second deposit of PLLA-PEG-PLLA (e), a carbonyl absorption is shown at 1755 cm−1 that is 4 cm−1 lower in wavenumber than that of the first deposit (a).22 The two relatively strong absorptions (iii, e) shown at 953 and 915 cm−1 can be assigned to the coil and 31 helix conformations, respectively.23 The latter absorption (915 cm−1) appears in a wavenumber 7 cm−1 higher than that reported for the ordinary 31 helical conformation, probably because of the short macromolecular chains and the block copolymer structure with PEG. These observations strongly support the sc formation between the preimmobilized PDLA chains and the PLLA block chains of the triblock copolymer that has promoted the particulate formation on the surface. In the second deposit of PLLA-PEG (b), in return, the carbonyl absorption appears at 1752 cm−1 (i), being slightly lower in wavenumber than that of the second deposit of PLLAPEG-PLLA (e). In the backbone vibration (iii) a strong absorption is detected at 953 cm−1 together with a very weak absorption at 915 cm−1. The latter absorption at 915 cm−1 is indicative of partial sc formation,23 while the other absorptions are attributed to the nematic state of the PLLA chains.21 It is therefore revealed that the sc crystallization between the preimmobilized PDLA and the PLLA blocks is responsible for the surface binding of the diblock copolymers. However, the observation of the stronger nematic peak suggests that many of the PLLA block chains have escaped from the sc formation to

PLLA blocks of the copolymers is essential for surface-binding of PEG segments and creating specific surface morphologies. Effects of Heat Treatment and Water Soaking on the Morphology. The second deposits of PLLA-PEG and PLLAPEG-PLLA were then annealed at 80 °C for 3 h and subjected to AFM. Since this annealing temperature of 80 °C was enough higher than the melting temperature (Tm) of PEG17 and close to the cold crystallization temperature (Tc) of PLLA18 and scPLA,19 the second deposits were considered to reorganize their block chains and change their morphologies into thermodynamically more stable ones. Figure 2a and b compares the AFM height images of the second deposits after the annealing. In the case of the second deposit of PLLA-PEG (Figure 2a), the original band aggregations have been connected with each other to constitute longer bands in network form. The average length of the bands has increased to 300 nm, which is 4 times longer than that observed before the annealing. It is therefore concluded that the band formation is promoted in the second deposit of PLLA-PEG probably because the one-end anchored PEG block chains have more easily interacted with each other to help the copolymer chains phase-separate and promote the band aggregation. In the second deposit of PLLA-PEG-PLLA (Figures 2b), in return, no change has been observed even after the annealing. The two PLLA blocks connecting to the PEG blocks may have decreased the inter-PEG interaction and more easily come into the stereocomplexation with the preimmobilized PDLA to form the stable particulate aggregation. The second deposits of PLLA-PEG and PLLA-PEG-PLLA were then submerged into water overnight and gently dried in a clean bench at room temperature for overnight. Figures 2c and 2d show the AFM images obtained after the water soaking of the second deposits of PLLA-PEG and PLLA-PEG-PLLA, respectively. Evidently, the former shows a dramatic change in the morphology, whereas the latter shows an intrinsically identical morphology with the one obtained before the water treatment. In the case of PLLA-PEG, the inter-PEG interaction that guided the formation of band structure has diminished by absorbing water, resulting in promotion of the sc formation of the PLLA chains with the preimmobilized PDLA on surface. In consequence, the particulate structure has appeared on the 14034

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Figure 4. GISAXS patterns from (a) the first deposit of Si-PDLA, (b) the second deposit of PLLA-PEG after annealing, and (c) the second deposit of PLLA-PEG-PLLA after annealing as well as (d) one-dimensional SAXS profiles in the qy plane for the three samples of (a) square, (b) circle, and (c) triangle.

reported that the nematic form is readily transformed into the α-form without passing an α′-form by annealing.21 Therefore, similar crystal transformation has occurred also in the present case of PLLA-PEG, enhancing the hc crystallization with the formation of the lamellar structure. On the other hand, it is evident that no spectrum change has occurred in the second deposit of PLLA-PEG-PLLA (e) before and (f) after the annealing. The aforementioned formation of crystalline PEG in the second deposit of PLLA-PEG strongly suggests that the PLLA blocks are allowed to phase-separate from the PEG blocks and enter into homochiral crystallization to form the lamellar structure. The PEG blocks of PLLA-PEG-PLLA, flanked with two PLLA blocks, should have less freedom and remain in an amorphous state. Therefore, the PEG block may not inhibit the interaction of the enantiomeric chains, and the particulate form is spontaneously generated with sc crystallization to minimize the surface tension. Figure 3d and g compares the IR spectra of the second deposits of PLLA-PEG and PLLA-PEG-PLLA after the water soaking. Evidently, the water-soaked deposit of PLLA-PEG shows a spectrum identical to that of the second deposit of PLLA-PEG-PLLA (e), suggesting that the PLLA blocks have totally entered into the sc-crystallization with the preimmobilized PDLA chains. This result supports the idea that the

enter into self-aggregation or homochiral (hc) crystallization with formation of the lamellar structures of band form. This different behavior of PLLA-PEG may be attributed to the relatively higher inter-PEG interaction than the PLLA-PDLA interaction when drying. Since the PEG chains of PLLA-PEG should have higher mobility enough to induce their own crystallization and mixing with the PLLA blocks, the phase separation of PLLA and PEG is likely induced to allow the inter-PLLA interaction more easily than the enantiomeric interaction with PDLA chains when drying, resulting in the formation of the lamellar structure. The PEG blocks of PLLAPEG-PLLA, in contrast, are flanked with two PLLA blocks and ought to have less freedom as to self-aggregate and retard the interaction of the enantiomeric chains on surface. As shown in Figure 3c, the annealed second deposit of PLLA-PEG shows a carbonyl absorption at 1759 cm−1 (i), being completely different in wavenumber in comparison with that observed before the annealing. In the backbone vibration (iii), a strong absorption is detected at 955 cm−1 together with a weak absorption at 921 cm−1 that are reasonably attributed to α-crystals of PLLA.20 The former absorption overlaps with two absorptions at 963 and 947 cm−1 that can be assigned to the crystalline PEG.24−26 The shoulder peak around 1749 cm−1 (i) also supports the α-crystal structure of PLLA.20 It has been 14035

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In other words, the particulate structures are randomly scattered on the surface. Instead of the direct analysis of the deposits by DSC and WAXD, which was difficult, a blend film of the PLLA-PEG (4.6k/5.2k) and PDLA-PEG (4.6k/5.2k) having similar block lengths was prepared by casting method and analyzed by DSC. As shown in Table S1 in the Supporting Information, Tm of the blend was about 30 °C higher than that of a single PLLA-PEG (4.6k/5.2k), indicating the formation of the sc crystals in the blend film. This result is a support for the mechanism that the sc crystallization between the preimmobilized PDLA and the PLLA blocks of the PLLA-PEG block copolymers is responsible for the surface binding of the block copolymers. Water Contact Angle on the Surface. Table 2 summarizes the equilibrium water contact angle (θe) of various

particulate formation is attributed to the sc formation on surface. In the deposit of PLLA-PEG-PLLA, the same spectrum is shown even after the water soaking because of the stable scformation retained. Comparing the spectra in the wavenumber ranges (ii) from 1320 to 1240 cm−1 and (iii) from 980 to 820 cm−1, it is revealed that only the annealed sample of PLLA-PEG deposit exhibits clear absorptions at 1280, 963, 947, and 844 cm−1 that are attributable to the α-crystal form of PEG.24−26 The weak absorption at 844 cm−1 shown by the second deposit of PLLA-PEG is also indicative of presence of PEG crystals. Therefore, the crystallization of PEG is considered to have caused the phase separation of the diblock copolymer, while such crystallization has not been observed in the triblock copolymer for which the sc crystallization is preferentially promoted. Note that the absorption at 1268 cm−1 that is sensitive to the amorphous phase of PLLA (or PDLA) is not exhibited in the respective spectra except the initial PDLAimmobilized sample.20,23 As discussed above, in the annealing process of the second deposit of PLLA-PEG, the one-end anchored PEG block chains are allowed to melt and more easily interact with each other to help the copolymer chains phaseseparate and promote the band aggregation. When the second deposit of PLLA-PEG is water-soaked, the inter-PEG interaction that has guided the formation of the band structure is diminished by absorbing water, resulting in promotion of the sc formation between the PLLA chains and the preimmobilized PDLA on surface. In consequence, the particulate structure appears on the surface as in the case of depositing PLLA-PEGPLLA in which such a stable particulate morphology has already been established even without passing the water soaking. To reveal the difference in surface morphology between the second deposits of PLLA-PEG and PLLA-PEG-PLLA, a GISAXS study was conducted. Figure 4 shows typical GISAXS patterns for the representative samples having different surface morphologies. As shown in Figure 4a, the first deposit of SiPDLA, showing the small particulate deposits, does not exhibit a clear GISAXS pattern, probably because of the deficient amount and amorphous nature of the PDLA chains preimmobilized on the surface. The annealed second deposit of PLLA-PEG (Figure 4b), in return, shows two rodlike scattering peaks in much stronger intensity than the above one from the first deposit. These two peaks are shown in the qz axis. This pattern is attributed to the perpendicular orientation of the lamellar. The average lamellar spacing, dlamellae, in the parallel direction of the surface was calculated from the qy value at the peak top of the in-plane intensity profile along the Yoneda peak as plotted by the circles in Figure 4d.27,28 The dlamellae was then calculated to be 49.5 nm (dlamellae = 2π/qy; qy = 0.127 nm−1), which is almost correspondent to the average space of 35−60 nm between the band structures observed in Figure 2a. This result is a clear evidence for the perpendicular lamellae formed by the phase separation of PEG and PLLA blocks. On the other hand, in Figure 4c measured for the annealed second deposit of PLLA-PEG-PLLA, only one rodlike peak is observed along the qz axis without any specific or characteristic scattering. This pattern is attributable either to an ordered structure having a longer dimension than the above lamellar structure or to a disordered state. The in-plane profile represented by the triangles in Figure 4d shows no peaks of the Bragg reflection. Therefore, in the present case, having the particulate morphology, the pattern implies a disordered state.

Table 2. Equilibrium Water Contact Angles of Various Surfaces surface PLLA film silicon wafer plasma-treated silicon wafer (control) PDLA (4.5k)-immobilized (first deposit) second deposit of PLLA-PEG (4.6k-5.2k) after annealing after water soaking second deposit of PLLA-PEG-PLLA (4.0k-3.2k-4.0k) after annealing after water soaking PEG (5.0k)-immobilized acetal-terminated PEG-PLLA methoxy-terminated PEG-PLLA tethered PEG brushes a b

contact angle θe (deg) 64 80 2 63 62 65 58 62 63 63 30−24 79−48 66−49 72−62

ref a b b b b b b b b b 30 29 30 31

Commercially available film (supplied by Unitika Ltd., Osaka, Japan). Present paper.

surfaces having the first deposit and the second deposits of diand triblock copolymers as compared with those of the original silicon wafer and the related surfaces previously reported.29−31 Interestingly, no difference is shown between the first deposit (PDLA layer) and the two second deposits (PEG-involving layer), being intermediate values indicating between hydrophobic and hydrophilic in nature. Previously, Otsuka et al. reported that the θe value of acetal-terminated PEG-PLLA diblock copolymers decreases with increasing the block chain length of PEG and falls into ca. 48° for the copolymer having a composition of PEG/PLLA = 5.0k/4.6k from 79° for the copolymer consisting of a short PEG block (PEG/PLLA = 0.65k/11.5k).29 Later, Mert et al. reported that the θe value of methoxy-terminated PEG-PLLA block copolymers decreases from 66° to 49° with increasing the PEG composition from PEG/PLLA = 2k/8k to 5k/8k.30 The former θe value is very close to that of a PLLA homopolymer (70°), suggesting that the methylene units of PEG chains mostly prefers to remain at the surface rather than inside, probably because of the stronger interaction of the ether oxygen atoms with PLLA units. Ostaci et al. reported that the θe value becomes about 62° for the tethered PEG brushes grafted to alkyne-functionalized silicon surface because the methoxy chain ends of the brushes lower the hydrophilic character of PEG.31 When the surface density of the PEG brushes was not high enough, the θe was reported to 14036

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stereocomplexation among the enantiomeric PLLA and PDLA segments has been clarified in a qualitative manner, giving a clue for understanding the binding mechanism of PEG segments through the sterecomplex formation.

be as high as 72°. On the other hand, Papra et al. reported that a similar PEG graft layer on an oxidized silicon wafer showed their advancing contact angles between 36° and 39°.32 In this case, the layer thickness of PEG was as thin as 1−1.5 nm, and the influence of the oxidized silicon surface ought to manifest. All these former data combined, it is evident that the θe value of the PEG-modified surface is strongly affected by the concentration of PEG chains. In the present deposit of PEG−PLLA copolymers having a composition of PEG/PLLA = 5.2k/4.6k (diblock) or 3.2k/8.0k (triblock), the surface concentration of PEG is even lowered to 36 (diblock) to 20 wt % (triblock) by the sc formation with the PDLA (4.5k) chains tethered to the silicon surface. Therefore, the PEG chains cannot cover the whole surface, making the θe value higher than that of the PEG-immobilized film. The medium θe values for the both deposits may therefore be comparable to or even relatively lower than the ordinary θe values of the PEG-PLLA copolymers. This fact suggests that the PEG block chains have successfully been immobilized on the surface through the formation of sc crystals between the PEG-PLLA copolymers and the PDLA-tethered surface in a mushroom regime.31 The the θe value is not so different even after the heat treatment and water soaking of the second deposit of PLLAPEG-PLLA, while it only slightly increases and decreases after the heat treatment and water soaking of the second deposit of PLLA-PEG, respectively. In the latter case, the heat treatment has promoted the crystallization of the PEG and PLLA blocks to cause the increase in the θe value, while the water soaking has broken the inter-PEG interaction to slightly decrease the θe value.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic Route to Si-PDLA, Thermal properties of Si-PDLA and PLLA-PEG block copolymers, and AFM height images of the surfaces of the second deposits of PDLA-PEG and PDLAPEG-PDLA. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Kyoto Environmental Nanotechnology Cluster from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Sakigake Semiconductor and Corbion Purac are highly acknowledged for their support and helpful discussion.

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REFERENCES

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CONCLUSIONS Both PLLA-PEG diblock and PLLA-PEG-PLLA triblock copolymers were deposited on a PDLA-immobilized silicon surface. The deposit of PLLA-PEG formed the band structures similar to the one observed when the same copolymer was directly cast on a silicon surface. This morphology was found to grow and stabilize after the annealing at 80 °C. In contrast, the deposit of PLLA-PEG-PLLA formed many particulates scattering over the surface, and this morphology did not change even after the annealing or water soaking. A great morphology change was found out when the deposit of PLLAPEG was subjected to water-soaking; the original band morphology was completely replaced by the particulate morphology that was similar to that of PLLA-PEG-PLLA deposit. Their FT-IR analyses revealed that both copolymers had been bound through the enantiomeric chain interaction or sc formation of the PLLA blocks and the preimmobilized PDLA chains on surface. In the case of PLLA-PEG, however, many of the copolymer chains were allowed to enter phase separation of PLLA and PEG because of the easy crystallization behavior of one-end anchored PEG having higher mobility and the following α-crystallization of PLLA. The water-soaking diminished that inter-PEG interaction to promote the surface sc crystallization with particulate formation. The PEG blocks of PLLA-PEG-PLLA, flanked with two PLLA blocks, did not retard the surface sc formation to readily allow the particulate formation. GISAXS patterns also supported the respective surface morphologies. It was therefore evident that the PEG blocks are successfully immobilized on the surface through the sc formation of the PLLA blocks of the copolymers with the preimmobilized PDLA chains that guides the formation of particular morphology. From these data, the process of surface 14037

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