Crystallization-Driven Surface Segregation and Surface Structures in

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Crystallization-Driven Surface Segregation and Surface Structures in Poly(L‑lactide)-block-Poly(ethylene glycol) Copolymer Thick Films Jingjing Yang,†,‡ Yongri Liang,*,† and Charles C. Han*,† †

State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, The Beijing National Laboratory for Molecular Sciences, and Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In this work, we used poly(L-lactide)-blockpoly(ethylene glycol) (PLLA-b-PEG) copolymer thick films to investigate the effect of crystallization on surface segregation, surface crystal orientation, and morphology by attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR), reflection optical microscopy (ROM), and twodimensional grazing incident wide-angle X-ray scattering (2D GIWAXS) methods. ATR-FTIR results indicated that the surface fraction of PLLA block increased from 0.48 to 0.79 when Tc,PLLA increased from 70 to 110 °C. Polarized ATRFTIR and 2D GIWAXS results indicated that PLLA crystal lamellae preferentially oriented parallel to the film surface with increasing Tc,PLLA. The surface crystallinity of PLLA was almost independent of Tc,PLLA, while the surface crystallinity of PEG decreased with increasing Tc,PLLA. On the basis of surface crystal orientation and crystallization kinetics, we suggested that the excess of PLLA component at the surface was mainly dominated by a coupling effect of crystallization behavior and surface segregation.

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chemical composition. One example that Chan and co-workers8 have studied is the surface chemical composition and morphology in the copolymer [(BA-C8)18-(6FBA)]n. They f o u n d t h a t t h e l ow su r f a c e e n e r g y u n i t [ 4 , 4′(hexafluoroisopropylidene)diphenol, 6FBA], which is inserted between 18 units of the high surface energy component (bisphenol A-co-octane, BA-C8), migrates away from the surface as the copolymer crystallizes, although the surface of the copolymer is enriched with 6FBA units in the amorphous state. When crystallization occurs, the low surface energy units have to reside at the folding face of the edge-on lamellae instead of freely segregating to the surface, which is different from the blend case mentioned above. Due to crystallization, the reduction in enthalpy of the polymer system can overcome the increase in surface energy caused by the movement of 6FBA to the bulk and the decrease in entropy due to demixing. Those results clearly demonstrate that crystallization can be a dominant driving force for component distribution on the surface. Nevertheless, interplay between surface segregation and crystallization in multicomponent polymer systems is far from being understood. Poly(L-lactide)-block-poly(ethylene glycol) (PLLA-b-PEG) copolymer is an amphiphilic crystalline block copolymer and

INTRODUCTION Surface properties of biopolymer materials, which are determined by surface composition and structures, are very important in biomedical applications.1−5 The surface composition and structure of polymers are very different with corresponding bulk due to surface effects. In the case of multicomponent polymer systems, including polymer blends and block copolymers, the surface effects often lead to preferential enrichment or surface segregation of lowest surface free energy of component on the polymer/air interface. However, almost all biopolymer blends or block copolymers can be crystallized. In that case, the surface composition formation is more complex due to crystallization effect on surface segregation. Up to now, only a few research works have been reported the effect of crystallization on surface segregation in polymer blends or block copolymers. For example, Clark et al.6 studied the effects of crystallinity and molecular weight dependence on the surface composition of poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends. They found that the surface composition of PCL/PVC blends is governed by a combination of molecular weight of PVC and degree of crystallinity of PCL. Cheung et al.7 also studied the surface composition of PCL/ PVC blends. Their results demonstrated that the surface chemical composition was controlled by surface morphology driven by crystallization of PCL. In block copolymer systems, there is still little knowledge on the crystallization effect on the © XXXX American Chemical Society

Received: October 28, 2013 Revised: December 12, 2013

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has potential applications in many biomedical fields such as drug delivery, surgery, and scaffolds.9−11 Introduction of PEG block can not only improve the physical properties of PLLA but also adjust the hydrophilicity/hydrophobicity balance and the interaction with surrounding cells. It has been reported that the presence of PEG chains in the surface could have a suppressive effect on biodegradation and adsorption of peptides.1 Therefore, it is worth controlling the surface content of PEG in PLLA-b-PEG copolymer. In our previous works,12,13 we investigated the crystallization behavior of PLLA-b-PEG copolymer bulk on the multilength scale and also investigated the effect of surface segregation on crystallization behavior of PLLA in PLLA-b-PEG thin film. We found that surface wetting can induce enrichment and preferential chain arrangement of PLLA blocks on the PLLA-b-PEG thin film surface, and the crystallization behavior of PLLA was significantly influenced by prior wetting time. In this study, we focused on understanding the effect of PLLA crystallization on surface segregation and structures of PLLA-b-PEG copolymer thick film. The surface composition and structure were characterized with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), twodimensional grazing incidence wide-angle X-ray scattering (2D GIWAXS), reflection optical microscopy (ROM), and atomic force microscopy (AFM) methods. We found that surface composition, surface crystal orientation, morphology, and crystallinity of PLLA-b-PEG thick films were significantly influenced by the crystallization kinetics.

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GS10642) accessory with diamond ATR crystal (refractive index is 2.4) was used to obtain ATR-FTIR spectra. The KRS-5 wired polarizer was used to measure polarized ATR-FTIR spectra for characterization of molecular orientation. Reflection Optical Microscopy. The surface morphology of annealed PLLA5K-b-PEG5K films was observed by optical microscope (Nikon E600POL) with reflection mode at room temperature. Images were recorded by a C-5050 Zoom camera. Atomic Force Microscopy. Atomic force microscopy (AFM) images of the annealed PLLA5K-b-PEG5K thick films were recorded by a commercial scanning probe Nanoscope multimode IV8 (Bruker Co.) operating in the tapping mode. A silicon cantilever tip was used for measurements of AFM images. The resonance frequency was 300 kHz and the scan rate was 20 μm/s. The scanning density was 512 lines/ frame. Two-Dimensional Grazing Incidence Wide-Angle X-ray Scattering. Two-dimensional grazing incidence wide-angle X-ray scattering (2D GIWAXS) was performed at room temperature on the BX14B1 beamline in the Shanghai synchrotron radiation facility (SSRF), China. The incident angle αi was set as 0.18° [the critical angle of silicon substrate is 0.2° (λ = 0.1381 nm),14 and the critical angle of PLLA was about 0.15°15 (λ = 0.154 nm)] and the exposure time was 20 s. A Mar345 (3450 × 3450) detector was used to collect 2D GIWAXS patterns. The wavelength of the incident X-ray was 0.124 nm, and sample to detector distance (SDD) was 273.7 mm. Silicon powder was used as standard material to calibrate the scattering angle. The air scattering was subtracted before analysis of 2D WAXS profiles. Crystallinity was calculated by Xw = Acry/(Acry + Aamor), where Xw is the mass crystallinity and Acry and Aamor are the peak areas of diffraction peaks contributed by crystalline and amorphous phases in the sample, respectively. A more detailed calculation procedure for crystallinity can be found in the Supporting Information.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Effects of Crystallization Temperature on Surface Morphology. The surface morphology of crystallized PLLA5Kb-PEG5K was observed by reflection optical microscopy (ROM; Figure 1) and atomic force microscopy (AFM; Figure 2). As

Materials and Characterization. The PLLA-b-PEG copolymer sample was provided by Ji’nan Daigang Co, Ltd., in China. The polydispersity index (PDI) of this PLLA-b-PEG copolymer was 1.25, and the number-average molecular weights (Mn) of PLLA and PEG were 4900 and 5000, respectively. More detailed information on PLLA-b-PEG can be found in the Supporting Information and our previous work.12 In this work, PLLA5K-b-PEG5K is used to represent this PLLA-b-PEG copolymer. The melting s of PLLA and PEG in the PLLA5K-b-PEG5K copolymer were measured as 130 and 52.5 °C, respectively, by differential scanning calorimetry (DSC) with a heating rate of 2 °C/min. The glass transition of PLLA-b-PEG was measured by DSC as about −44 °C. In order to calibrate the fraction of PLLA in PLLA5K-b-PEG5K by FTIR, PLLA-b-PEG copolymers with Mn = 2500 for PLLA block and Mn = 5000 for PEG block (PLLA2.5K-b-PEG5K) and Mn = 10 000 for PLLA block and Mn = 5000 for PEG block (PLLA10K-b-PEG5K) were also used. Sample Preparation and Annealing. PLLA5K-b-PEG5K films were obtained by casting PLLA5K-b-PEG5K/chloroform solution (3% weight fraction) onto glass slides or KBr plates (for T-FTIR measurement). The thickness of the as-cast PLLA5K-b-PEG5K films was about 30 μm. PLLA5K-b-PEG5K samples were crystallized by a two-step crystallization process using a Linkam hot stage (LTS350). At the first step, PLLA5K-b-PEG5K samples were preheated at 180 °C for 5 min to eliminate thermal history and then rapidly quenched to a certain crystallization temperature of PLLA, Tc,PLLA (between the melting temperatures of PLLA and PEG; that is, Tm,PLLA > Tc,PLLA > Tm,PEG), to isothermally crystallize for 6 h under nitrogen atmosphere. Subsequently, the samples were quickly quenched to 30 °C (below Tm,PEG) for isothermal crystallization for 2 h in order to achieve crystallization of PEG. Fourier Transform Infrared Spectroscopy. Attenuated total reflection and transmission Fourier transform infrared spectroscopy (ATR- and T-FTIR) were used to characterize chemical composition of PLLA5K-b-PEG5K films at surface and in the bulk, respectively. The spectra of ATR- and T-FTIR were recorded on a Bruker Tensor 27 system with 2 cm−1 resolution. The golden gate ATR (Specac

Figure 1. Reflection optical microscopy images of PLLA5K-b-PEG5K thick films crystallized at different PLLA crystallization temperatures, Tc,PLLA = (a) 70, (b) 90, (c) 100, and (d) 110 °C, and then crystallized at 30 °C for 2 h.

shown in Figure 1, the spherulite size was increased and the compactness of fibrillar crystals was decreased with increasing Tc,PLLA. These results indicate that the surface morphology of PLLA5K-b-PEG5K thick films was greatly affected by Tc,PLLA, on the micrometer and submicrometer length scales. However, the fine crystalline morphology features are difficult to observe by ROM due to limited contrast and resolution. Detailed surface crystalline morphologies of crystallized PLLA5K-b-PEG5K thick films were observed by AFM as shown in Figure 2. It was clear that a rough surface containing B

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Figure 2. (a−d) Atomic force microscopy height and (a′−d′) phase images of PLLA5K-b-PEG5K samples crystallized at different PLLA crystallization temperatures, Tc,PLLA = (a) 70, (b) 90, (c) 100, and (d) 110 °C, and then crystallized at 30 °C for 2 h.

valleys and ridges was formed at the PLLA5K-b-PEG5K film surface, where the height difference between ridges and valleys is about 1000−1600 nm. In the ridges, grainlike packed crystal lamellae (i.e., “edge-on”-like oriented lamellae) were formed when samples crystallized below 100 °C Tc,PLLA, and pyramidlike packed lamellae (i.e., “flat-on”-like oriented lamellae) were formed when samples crystallized above 100 °C Tc,PLLA. Surface Crystal Orientation. The detailed crystal orientation and crystallinities of PLLA and PEG in PLLA5K-bPEG5K thick films at the surface were determined by polarized ATR-FTIR and 2D GIWAXS methods. ATR-FTIR spectroscopy is a highly sensitive surface characterization technique. The sample absorption in ATR-FTIR spectrum is proportional to the integration (convolution) of the product between the evanescent field amplitude and the imaginary part of the complex dielectric constant of the absorbing material. Accordingly, the spectral contributions from molecules at various depths to the total absorption in an ATR spectrum are not the same, due to decay characteristic of the evanescent field. Quantitative analysis of molecular orientation by polarized ATR-FTIR is more complex than that by polarized T-FTIR, due to more complex experimental procedure and irreproducible optical contact problem.16 For example, to obtain threedimensional orientation information with polarized ATR-FTIR, one set of four ATR-FTIR spectra, which can be obtained by rotating the sample and the polarizer by 90°, are required. Furthermore, irreproducible optical contact between the ATR crystal and the sample during the reclamping process has to be solved. The set of p and s polarization ATR-FTIR spectra can be obtained without rotating sample direction or relamping sample during measurements; that is, measurement of the set of p and s polarization ATR-FTIR spectra does not change the optical contact between sample and ATR crystal. According to polarized ATR-FTIR experimental geometry, we know that the s polarization ATR-FTIR spectrum contains orientation information only from the in-plane direction of the sample, and the p polarization ATR-FTIR spectrum contains orientation information from both normal and in-plane directions of the sample. Therefore, the absorbance ratio of selected infrared bands from the set of p and s polarization ATR-FTIR spectra, D = Ap/ As (where Ap and As are absorbance values of the selected band

obtained from the set of p and s polarization ATR-FTIR spectra, respectively), can be employed for semiquantitative characterization of molecular orientation of PLLA and PEG in crystalline phase. It is well-known that the absorbance of a specific band obtained from the polarized ATR-FTIR spectra is affected not only by the molecular orientation and quality of the optical contact between sample and ATR crystal but also by the polarization direction of the incident infrared (IR) radiation. Usually the effective thickness (or effective penetration) for the p wave (de,p) is twice that for the s wave (de,s,) under 45° incidence angle and ideal optical contact between ATR crystal and sample.17 It means that the D value (=Ap/As) should be 2.0 for an isotropic sample, when the sample and ATR crystal have perfect optical contact. However, practically, the D value is always smaller than 2.0 due to nonideal optical contact between the sample and ATR crystal. In the case of anisotropic sample with ideal optical contact, we can deduce that the D value for parallel (∥) band (if the angle between chain axis and transition dipole moment is assumed to be 0°) or for perpendicular (⊥) band (if the angle between chain axis and transition dipole moment is assumed to be 90°) should become smaller or larger than 2.0, if the chains are oriented perpendicular to the normal direction of film surface. Oppositely, the D value for ∥ band or for ⊥ band should become larger or smaller than 2.0 if the chains are oriented parallel to the normal direction of the film surface. Figure 3 shows sets of (A) p and (B) s polarization ATRFTIR spectra of crystallized PLLA5K-b-PEG5K thick films with different Tc,PLLA. As shown in Figure 3, the relative intensity of bands at 737 and 756 cm−1 in the set of s and p polarization ATR-FTIR spectra were significantly influenced by crystallization temperature. Furthermore, the relative intensity of 963 cm−1 in the set of s and p polarization ATR-FTIR spectra was also significantly changed by crystallization temperature. Since the band at 737 cm−1 belongs to A polarization mode (∥ band, if the angle between transition dipole moment and chain axis is assumed to be close to 0°) and the band at 756 cm−1 belongs to E1 polarization mode (⊥ band, if the angle between transition dipole moment and chain axis is assumed to be close to 90°) of PLLA crystalline phase,18 the relative intensity changes of those peaks in s or p polarization spectra are relative to the PLLA crystal orientation on the surface. Similarly, relative intensity C

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decreased and increased with increasing Tc,PLLA, respectively. Furthermore, the D value of 737 cm−1 was larger than 2.0 at Tc,PLLA above 90 °C. This means that PLLA lamellae were gradually oriented perpendicular to the sample surface with increasing Tc,PLLA. Even though the relative intensity of the peak at 963 cm−1 decreased with increasing Tc,PLLA, D value of 963 cm−1 was above 2.0 at whole Tc,PLLA and increased with increasing Tc,PLLA. This implies that chains in the PEG crystalline phase oriented perpendicular to the surface and the degree of orientation increased with increasing Tc,PLLA. Surface crystal structure and orientation were characterized by 2D GIWAXS. The 2D GIWAXS method can be used to probe about tens of nanometers to a few micrometers depth from sample free surface, which is dependent on the incidence angle and reflective index of sample.21 In this work, tens of nanometers depth from sample free surface can be probed by 2D GIWAXS method since the incidence angle was close to the critical angle. Figure 5 shows the 1D and 2D GIWAXS data of

Figure 3. (A) p polarization and (B) s polarization ATR-FTIR spectra of PLLA5K-b-PEG5K thick films crystallized at different Tc,PLLA, (a) 70, (b) 80 (c) 90, (d) 100, and (e) 110 °C, and then at 30 °C.

changes of the band at 963 cm−1 in s or p polarization spectra are relative to the PEG crystal orientation on the sample surface, since the infrared band at 963 cm−1 belongs to the CH2 rocking vibration mode (parallel mode, if the angle between transition dipole moment and chain axis is assumed to be close to 0°)19,20 and is sensitive to PEG crystalline phase. For example, in the case of uniaxially drawn PLLA sample, the ∥ band of 737 cm−1 shows intensive and weak absorbance in the parallel and perpendicular spectra of T-FTIR, respectively, whereas the ⊥ band of 756 cm−1 shows weak and intensive absorbance in parallel and perpendicular spectra of T-FTIR. Figure 4 shows the plots of D values of 737, 756, and 963 cm−1 bands versus Tc,PLLA, respectively. At 70 °C Tc,PLLA, the D values of ⊥ band (756 cm−1) and ∥ band (737 cm−1) were 1.52 and 1.79, respectively. The D values of 756 and 737 cm−1 were

Figure 5. (A) 1D GIWAXS profiles and (B) 2D GIWAXS patterns of PLLA5K-b-PEG5K samples crystallized at different Tc,PLLA, (a) 70 and (b) 110 °C, and then at 30 °C. (C) Transmission WAXS profiles of (c) crystallized PLLA homopolymer and (d) PEO homopolymer, where the PEO homopolymer was completely crystallized at 30 °C and the PLLA homopolymer was completely crystallized at 120 °C.

crystallized PLLA5K-b-PEG5K thick films. The PLLA5K-b-PEG5K samples for 2D GIWAXS measurements were crystallized at Tc,PLLA (70 and 110 °C) and at 30 °C by a two-step crystallization process. The 1D transmission WAXD profiles of PLLA and PEG homopolymers are displayed in Figure 5C. On the basis of WAXS profiles of PLLA and PEO homopolymers (Figure 5C), we identified the diffraction peaks of 1D GIWAXS profiles as labeled in Figure 5A. In Figure 5A, the diffraction peak at 13.4° can be attributed to (200)/(110) crystal planes of PLLA α-form crystal, and the diffraction peak at 18.6° can be attributed to (1̅24)/(112)/(032) crystal planes of PEG crystal.22 The α-form PLLA crystal, which is usually obtained above 120 °C crystallization temperature from melt crystallization of PLLA homopolymer, has a 103 helix chain conformation and an orthorhombic unit cell with parameters of a = 1.06 nm, b = 0.61 nm, and c = 2.88 nm.23 Recently, α′form PLLA crystalline modification has been proposed for the

Figure 4. Absorbance ratio D of the infrared bands of 737, 756, and 963 cm−1 plotted against Tc,PLLA. D

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Figure 6. (A) T-FTIR and (B) ATR-FTIR spectra of PLLA5K-b-PEG5K samples with different Tc,PLLA in the infrared spectrum regions (1) 3100− 1600 cm−1 and (2) 1000−700 cm−1. Tc,PLLA = (a) 70, (b) 80, (c) 90, (d) 100, and (e) 110 °C.

Figure 7. (A) Plot of weight fraction of PLLA blocks in PLLA5K-b-PEG5K copolymers versus R = ACO/AC−H. (B) Ratios R = ACO/AC−H (squares) and R = A871/A843 (circles) obtained from T-FTIR (open symbols) and ATR-FTIR (solid symbols) of PLLA5K-b-PEG5K samples with different Tc,PLLA. (C) PLLA weight fraction of PLLA5K-b-PEG5K films with different Tc,PLLA obtained by ATR-FTIR.

formed PLLA α-form crystals and PEG crystals on the surface after crystallization at Tc,PLLA and at 30 °C by a two-step crystallization process. Furthermore, the surface crystal structure of PLLA in the PLLA-b-PEG thick films was independent of crystallization temperature. We used the diffraction peak at 18.6° (labeled by the arrow in Figure 5B) to determine the surface orientation of PEG

PLLA homopolymer samples cold- or melt-crystallized at low crystallization temperature (Tc < 100 °C).24,25 The chain conformation and crystal unit cell of α′-form PLLA crystal are similar to those of α-form PLLA crystal. But the packing of side groups in the helical chains of the α′-form crystal is less ordered and looser than that of the α-form crystal. Therefore, the GIWAXS results indicate that the PLLA5K-b-PEG5K samples E

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when Tc,PLLA increased from 70 to 110 °C. Correspondingly, the PLLA fraction ( f PLLA) increased from 0.48 to 0.79 (increased 65%), when Tc,PLLA increased from 70 to 110 °C, as shown in Figure 7C. This indicates that the surface PLLA fraction ( f PLLA) of PLLA5K-b-PEG5K thick films increased with increasing Tc,PLLA, even though the corresponding bulk PLLA fraction of PLLA5K-b-PEG5K thick films was almost independent of Tc,PLLA. We also determined the surface content of PLLA in PLLA5K-b-PEG5K by X-ray photoelectron spectroscopy. The XPS method is sensitive to less than 10 nm of penetration depth from the sample free surface. The XPS results showed that the relative content of carbons from O−CO groups (289 eV) of PLLA in PLLA5K-b-PEG5K increased from 3.4% to 5.5% (increased 62%) when Tc,PLLA increased from 70 to 100 °C. However, the XPS results may be influenced by surface roughness and errors from experiment and data analysis. Nevertheless, the XPS results also provided a similar trend as ATR-FTIR; that is, the surface fraction of PLLA increased with increasing Tc,PLLA. Our results indicate that the ATR-FTIR method is very sensitive to surface composition of PLLA5K-bPEG5K even though the penetration depth of ATR-FTIR (about 2000 nm) is much larger than the long period of PLLA crystal lamella (about 23 nm) in PLLA5K-b-PEG5K. Surface Crystallinities of PLLA and PEG in PLLA5K-bPEG5K Thick Films. The absorbance ratio of peaks at 871 and 843 cm−1 (i.e., R = A871/A843) was used to estimate relative surface crystallinity of PEG in PLLA5K-b-PEG5K. The infrared band at 871 cm−1, which can be assigned to the C−COO stretching vibration mode of PLLA, is sensitive to the crystalline phase of PLLA,24,25 and the infrared band at 843 cm−1, which can be assigned to a combination of rocking vibration of the CH2 and the COC stretching vibration modes from PEG, is sensitive to the crystalline phase.26 As shown in Figure 7B, the R = A871/A843 values from T-FTIR spectra of PLLA5K-b-PEG5K (○; R = A871/A843 ≈ 0.5) were almost independent of Tc,PLLA, while the R = A871/A843 values from ATR-FTIR spectra of PLLA 5K-b-PEG5K (●) increased significantly with increasing Tc,PLLA. For example, the R = A871/A843 values from ATR-FTIR spectra of PLLA5K-b-PEG5K increased from 0.67 to 1.71 when Tc,PLLA increased from 70 to 110 °C. This indicates that the relative crystallinity of PEG at the surface decreased significantly with increasing Tc,PLLA. Surface crystallinities of PLLA and PEG in PLLA5K-b-PEG5K films can also be determined by the 1D GIWAXS method. Surface crystallinity of PEG was calculated as 0.11 and 0.07 for Tc,PLLA = 70 and 110 °C, respectively. The surface crystallinity of PLLA was calculated as 0.28 and 0.30 for Tc,PLLA = 70 and 110 °C, respectively. In contrast with surface crystallinity, bulk crystallinities of PLLA and PEG in PLLA5K-b-PEG5K were almost independent of Tc,PLLA and were calculated as about 0.26 and 0.28, respectively, as shown in Figure S5 in the Supporting Information. This means that the surface crystallinity of PEG decreased significantly with increasing Tc,PLLA, while the surface crystallinity of PLLA was almost independent of Tc,PLLA. Interplay between Crystallization and Surface Segregation. The ATR-FTIR and 2D GIWAXS results indicated that the surface composition and crystallinity of PEG in PLLA5K-b-PEG5K decreased significantly with increasing Tc,PLLA. Here we raise a question: why is the surface composition dependent on crystallization temperature? In our previous work, we found that the PLLA blocks in PLLA5K-b-PEG5K copolymer are preferentially enriched or segregated on the surface at molten state due to its lower surface energy.13 The

crystal. The diffraction peak at 18.6° showed an arc shape in the 2D GIWAXS pattern, and the intensity along the out-of-plane direction was much higher than that along the in-plane direction in the PLLA5K-b-PEG5K thick film with Tc,PLLA = 110 °C. The result indicates that PEG crystals in the PLLA5K-bPEG5K thick film with Tc,PLLA = 110 °C adopted an anisotropic orientation at the surface. Quantitative analysis of crystal orientation was difficult to obtain from this peak because the peak at 18.6° is attributed to a superposition of many crystal planes, such as (1̅24), (112), and (032). However, it is clear that the relative degree of surface orientation of PEG increased with increasing Tc,PLLA as shown in the 2D GIWAXS patterns (Figure 5B). The PLLA crystal diffraction peaks (for example, the peaks at 13.4°) showed almost ring shapes in the 2D GIWAXS patterns. This indicated that the PLLA crystals were weakly oriented at the sample surface. The degree of crystal orientation observed by 2D GIWAXS is smaller than that observed by polarized ATR-FTIR results. This may be due to different probing depths between polarized ATR-FTIR and 2D GIWAXS methods. Effect of Crystallization Temperature on Surface Chemical Composition. In order to compare the chemical composition of PLLA5K-b-PEG5K samples at the surface and in the bulk, ATR- and T-FTIR methods were used. Figure 6 shows T- and ATR-FTIR spectra of crystallized PLLA5K-bPEG5K samples with different Tc,PLLA in different infrared regions. In the spectrum of PLLA-b-PEG copolymer, peaks in the region 1856−1670 cm−1 are from PLLA blocks, and peaks in the region 3040−2660 cm−1 are from both PLLA and PEG blocks. The peaks at 1856−1670 cm−1 can be assigned to -C O stretching vibration modes, and the peaks at 3040−2660 cm−1 can be assigned to C−H stretching vibration modes. Moreover, the peaks at 963 and 843 cm−1 are sensitive to the crystalline phase of PEG, while the peaks at 871, 756, and 737 cm−1 are sensitive to the crystalline phase of PLLA. In the TFTIR spectra, the relative intensities of peaks from PLLA and PEG in crystallized PLLA5K-b-PEG5K samples were almost independent of Tc,PLLA, as shown in Figure 6A. For example, the band at 871 cm−1 from PLLA and the band at 843 cm−1 from PEG were almost independent of Tc,PLLA. Those results also demonstrated that the crystallinities of PLLA and PEG in the bulk PLLA5K-b-PEG5K were almost independent of Tc,PLLA. However, in the ATR-FTIR spectra, for example, the band at 843 cm−1 decreased significantly with increasing Tc,PLLA as shown in Figure 6B2. The fraction of PLLA in PLLA5K-b-PEG5K was estimated by the peak area ratio of two sets of vibration bands, where one set of vibration bands was selected from the 1856−1670 cm−1 region contributed by -CO stretching vibrations and the other set of vibration bands was selected from the 3040−2660 cm−1 region contributed by C−H stretching vibrations. The peak area ratio of the two sets of vibration bands is shown as R = ACO/AC−H. We demonstrated that R = ACO/AC−H has a linear relationship with fraction of PLLA as shown in Figure 7A. This means that R = ACO/AC−H is dependent on mass fraction of PLLA in PLLA-b-PEG copolymers. Figure 7B shows Tc,PLLAdependent values of R = ACO/AC−H obtained from T- and ATR-FTIR spectra of crystallized PLLA-b-PEG samples. The values for T-FTIR (□; R = ACO/AC−H ≈ 0.71) were almost independent of Tc,PLLA as expected, whereas those for ATRFTIR (■) increased significantly with increasing Tc,PLLA, as shown in Figure 7B. The R = ACO/AC−H of PLLA5K-b-PEG5K by ATR-FTIR increased from 0.79 to 1.34 (■; increased 69%) F

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were mainly composed of crystallized PLLA lamellae and the valleys were mainly composed of crystallized PEG lamellae and amorphous PLLA-b-PEG. The ATR-FTIR results indicated that the surface PLLA content of crystallized PLLA5K-b-PEG5K thick films increased with increasing Tc,PLLA. For example, the surface fraction of PLLA increased from 0.48 to 0.79 when Tc,PLLA increased from 70 to 110 °C. The polarized ATR-FTIR and 2D GIWAXS results indicated that surface crystallinity of PLLA was almost independent of Tc,PLLA, while surface crystallinity of PEG decreased with increasing Tc,PLLA. The surface crystal orientation of PLLA and PEG tended to orient parallel to the film surface with increasing Tc,PLLA. On the basis of analysis of the Tc,PLLA-dependent lamella orientation, crystallinity, and crystallization kinetics, we suggested that the surface excess of PLLA was mainly dominated by crystal orientation and crystallization kinetics of PLLA. Our results indicated that the surface composition and structure of crystalline block copolymers can be simply regulated by the kinetics pathway.

surface segregation is mainly driven by the free energy difference between PLLA and PEG blocks in PLLA5K-bPEG5K thin film. Surface segregation should increase with increasing temperature due to enhanced chain mobility (or diffusion). However, surface segregation can be disturbed by the crystallization process due to crystallization-induced solidification. If the crystal growth rate is fast, amorphous regions (or blanks) should be quickly occupied by crystals. Meanwhile, surface segregation should be suppressed by the solidification process. In the case of PLLA 5K -b-PEG 5K copolymer, the crystal growth rate of PLLA decreased significantly as Tc,PLLA increased, as shown in Figure 8. For

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional text and equations, six figures, and one table showing (1) characterization of PLLA-b-PEG copolymer, (2) PEG and PLLA crystallinities of PLLA5K-b-PEG5K samples with different Tc,PLLA by WAXS, (3) effective thickness in ATR-FTIR measurements, and (4) XPS results of PLLA-b-PEG thick films. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Tc,PLLA-dependent crystal growth rate, G, of PLLA in PLLA5K-b-PEG5K copolymer.

Corresponding Authors

example, the PLLA crystal growth rate was 11.0 and 3.3 μm/ min for Tc,PLLA = 70 and 110 °C , respectively. The chain diffusion and crystal growth rate of PLLA increased and decreased, respectively, with increasing Tc,PLLA. Accordingly, we can deduce that surface segregation of PLLA blocks on the surface should be enhanced as Tc,PLLA increases. The orientation of crystallized PLLA lamellae is another important factor to influence the surface composition of PLLA5K-b-PEG5K films, due to alternating arrangements of PLLA and PEG lamellae in the crystallized PLLA lamellae. The sample surface should be almost occupied by PLLA if the crystallized PLLA lamellae are oriented parallel to the sample surface (i.e., flat-on lamella orientation), whereas the sample surface should be occupied by both PLLA and PEG if the crystallized PLLA lamellae are oriented perpendicular to the sample surface (i.e., edge-on lamella orientation). Therefore, the surface PLLA fraction should be much larger in flat-on than in edge-on lamellar orientation of PLLA. The 2D GIWAXS, AFM, and polarized ATR-FTIR results indicated that the lamellar orientation of PLLA tended to orient parallel to the sample surface as Tc,PLLA increased. Therefore, we suggest that the excess of PLLA component at surface was mainly dominated by the PLLA crystal lamellar orientation and the crystallization kinetics of PLLA.

*E-mail [email protected]; telephone +86 10 82618089; fax +86 10 62521519. *[email protected]; telephone +86 10 82618089; fax +86 10 62521519. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the project sponsored by National Natural Science Foundation of China (No. 21004070 and No. 50930003) and Joint research fund (No. 21111140381). The synchrotron 2D GIWAXS experiments were supported by 14B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) in China.

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CONCLUSIONS Effects of PLLA crystallization on surface composition, morphology, and crystal orientation of PLLA5K-b-PEG5K thick films were investigated with ATR-FTIR, polarized ATR-FTIR, 2D GIWAXS, ROM, and AFM methods. The AFM results indicated that crystallization of PLLA induced rough surface morphology containing valleys and ridges, where the ridges G

dx.doi.org/10.1021/la4041387 | Langmuir XXXX, XXX, XXX−XXX

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