Article pubs.acs.org/Langmuir
In Situ Study of the Breakout Crystallization in the Poly(butadiene)block-Poly(ε-caprolactone) Thin Film Peng Zhang, Haiying Huang, Derong Yan, and Tianbai He* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Graduate University of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *
ABSTRACT: Despite its wide occurrence in soft confined block co-polymers, breakout crystallization remains poorly understood and is difficult to control. In this work, thin films of cylinder-forming poly(butadiene)-block-poly(ε-caprolactone) (PB-b-PCL) diblock co-polymers, with PCL being the minority block, have been chosen as the study subject. We demonstrate a new route to study the breakout crystallization by obtaining the microphase separation structure within terraced lamellae first and then in situ tracking down the lamellar coalescence, resulting from the development of the crystal growth front. We find that the crystal growth front has sucked materials from the surrounding amorphous lamellae, which lead to the decrease of the lamellar zones and coalescence of the microphase separation structure. Dividing the breakout crystallization into parallel breakout and vertical breakout, we illustrate that it is the crystallization-driven molecular diffusion that make the molecules overcome the topography constraint and grow into large-scale spherulite. Moreover, the results show that the polymer microphase separation structure has a significant influence on the crystal nucleation and greatly retarded the crystal growth rate. With a well-designed microphase separation structure within terraces and an easily tunable atomic force microscopy in situ imaging technique, an intensive study of the breakout crystallization and concomitant microdomain coalescence has been offered. experimental results. Vasilev et al.17 and Nojima et al.18 have proposed that the breakout occurs only when the crystallization driving force is large enough to overcome the energy barrier, resulting from the amorphous surroundings. According to direct imaging results, Hobbs et al. demonstrated that breakout was induced by the dissociation of crystalline blocks out of the microdomains and following diffusion into the crystal growth front.19 In addition, by Monte Carlo simulation, Qian et al. attributed the breakout to the postgrowth crystal thickening.1 However, how the molecular chains overcome constraints from the topography and microphase separation tendency and grow into a large-scale crystal is still unknown. To intensify the understanding on the breakout crystallization, in our opinion, high-resolution in situ imaging results, including both the development of the crystal (nucleation and growth) and the coalescence of the microdomains, are very important. On the other hand, for the breakout crystallization in thin film, considering the thin film geometry, choosing a suitable surface-sensitive technique is essential; atomic force microscopy (AFM)11,17,19−23 and X-ray reflectometry13 are generally applied. In recent work, Papadakis et al. have applied an X-ray technique successfully to characterize the breakout crystallization in cylinder-forming poly(isoprene)-block-poly(ethylene oxide) (PI-b-PEO) diblock co-polymers, with PEO
1. INTRODUCTION In a soft confined diblock co-polymer (order−disorder transition temperature > crystallization temperature > glass transition temperature of the amorphous block), the melt mesophase is generally destroyed when one block crystallizes; i.e., breakout crystallization occurs,1−10 and crystallization creates an entirely new structure, bearing little resemblance to that present in the melt. Breakout crystallization of a thin film is, in turn, a more complex phenomenon because it presents added complexities stemming from the change of crystallization morphology11,12 and the effect of the surface on the nucleation mode.13−15 Although great progress have been made in the past few decades, breakout crystallization is still poorly understood; for example, how the microdomain coalesce proceeds during breakout crystallization and how the polymer chain overcomes the topography constraint and grows into a large-scale crystal remain elusive.16 It is generally accepted that, when crystallization occurs, the microdomain structure will change from original nanometer length scale domains (e.g., spheres, cylinders, and lamellae) to lamellae containing alternating amorphous and crystalline layers. For instance, in cylindrical poly(butadiene)-blockpoly(ε-caprolactone) (PB-b-PCL), with PCL as the minority block, it is found that the PCL crystals have broken out of the microphase separation structure and formed a lamellar structure consisting of a crystalline PCL layer and an amorphous PB layer.2,4 Moreover, different explanations for the breakout mechanism have been raised on the basis of the respective © 2012 American Chemical Society
Received: May 13, 2011 Revised: March 16, 2012 Published: March 17, 2012 6419
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being the minority block.13 In contrast to X-ray techniques, AFM imaging exhibits a number of promising characteristics. First, it offers real-space, with nanometer resolution, information of the structure change. Second, the evolution of the focused structure can be tracked in real time because we can control the scanning zone by adjusting the scanning position and size. Third, it can offer contrast between the melt and crystals. These attractive properties enable the AFM technique especially suitable for us to detect the breakout crystallization in a thin film. The present study is unique in reporting direct observation of the breakout crystallization process in the terraced lamellar structure, among which we choose PB-b-PCL, which is a typical soft confined diblock co-polymer, as the study subject.2−5,7,8,18,24,25 Terraced structures containing a cylinder-forming microphase separation structure were obtained and evaluated with optical microscopy and AFM at elevated temperatures. High-resolution real-space information on the development of the crystal growth front and the coalescence of the terraced microphase separation structure was investigated at the designed crystallization temperature. The topography information indicates that the development of the crystal growth front would destroy the surrounding amorphous microdomain structures. The inherent breakout crystallization has been attributed to the molecular diffusion from the amorphous zone to the crystal growth front. In addition, to further understand the breakout crystallization, the nucleation mode and crystal growth kinetics have also been elucidated.
Polymer thin films were obtained by spin-casting a toluene solution (20 mg/mL) onto the substrate. The as-cast thin films were set in a vacuum environment at room temperature for 24 h to remove the remaining solvent. The thickness was evaluated with AFM by determining the height of the cross-sections (blade carefully scratched), and the thickness was ca. 98 nm. Besides, as observed in the thinner sample (cast from 5.0 mg/mL toluene solution), there was a wetting layer with a thickness of ca. 10.5 nm near the substrate. 2.3. Characterization. 2.3.1. X-ray Scattering Measurement. The bulk sample structure was detected by in-house small-angle X-ray scattering (SAXS). The SAXS instrument was equipped with a 1.3 kW X-ray generator operated at 40 kV and 35 mA (Nanostar, Bruker Co., Ltd.) and a two-dimensional positionsensitive proportional counter. The Cu Kα line (λ = 0.154 nm) was used. The intensity profile was output as the plot of the scattering intensity (I) versus the scattering vector, q. The SAXS profiles were corrected for the background scattering. 2.3.2. X-ray Photoelectron Spectroscopy (XPS) Analysis. The XPS measurement was performed in a Thermo ESCALAB 250 spectrometer by means of monochromatic Al Kα radiation, without additional charge compensation. The surface composition was obtained from the measurement of the areas of the C 1s and O 1s peaks. The measurement was performed at 20.0 eV pass energy in constant analyzer energy (CAE) mode, at a photoemission angle of 0° (i.e., normal to the sample surface). The binding energies of the photoelectrons were correlated by the aliphatic hydrocarbon C 1s peak at 284.6 eV. The temperature heating and cooling rate was 5 °C/min. 2.3.3. Tapping Mode AFM. A commercial scanning probe microscope (Agilent 5500 AFM, Santa Clara, CA) equipped with a temperature controller (Lakeshore 332) was used to capture images at a set temperature. Because the film thickness, ca. 100 nm, was below the 500 nm limit found by Schönherr et al.21 for a PEO thin film, it was assumed that the temperature of the sample equaled the temperature read on the hot-stage controller. Samples were first imaged at room temperature. Then, the AFM probe was disengaged before the melting of the polymer film set at 100 °C for 40 min. Next, the temperature was cooled at 5 °C/min to 60 °C, at which the microphase separation structure was detected. The isothermal crystallization experiment was conducted by annealing the sample with a hot stage (Linkam, U.K.) at 100 °C for 40 min and then quickly transferred to the standard heat plate of AFM set at 32 °C. Height and phase images were collected simultaneously. The scanning rates varied from 0.6 to 1.5 Hz depending upon the scanning zone size (from 95 × 95 to 5 × 5 μm). The free oscillation amplitude was set at 2.0 V, and the amplitude set point was between 72 and 85% to track the surface without disturbing nucleation or crystal growth.21 Growth rates were measured by AFM on height or phase images, depending upon which can offer the best contrast between the crystal and the surrounding melt. In details, sequential AFM results were analyzed with PicoView 1.6/Pico Image (software provided by the manufacturer), and the radius of the minimum circle around the crystalline pattern was measured. At least 5 trial and error circles for each image were applied, and the average value was accepted. 2.3.4. Optical Microscopy. The samples were placed onto an enclosed hot plate (Linkam, U.K.), under a Leica-D2500P optical microscope. The thermal treatment procedure was performed by increasing the temperature from room temperature to 100 °C at 30 °C/min and then isothermal annealing at 100 °C. The structure was captured with a charge-coupled device (CCD) camera. No polarization or phase contrast was applied. The image contrast is due to the interference of the reflected light at the polymer−air interface. For the interference colors, the darker the color, the thicker the film.
2. EXPERIMENTAL SECTION 2.1. Materials. An asymmetric PB-b-PCL diblock co-polymer was obtained from Polymer Source, Inc., Canada. The molecular weight and molecular-weight distribution of the diblock co-polymer were characterized by gel permeation chromatography (GPC). The detailed molecular character is listed in Table 1. The melting point and degree
Table 1. Characteristics of the PB-b-PCL Diblock Co-polymer Mn (kg/mol) number of monomers, N volume fraction, f a polydispersity
PB
PCL
13 241 67.4
7.9 69 32.6 1.1
Calculated from f PB = (wPB/ρPB)/[wPB/ρPB + (1 − wPB)/ρPCL], where f PB, wPB, ρPB, and ρPCL indicate the volume content of PB, the weight content of PB, the density of PB, and the density of PCL, respectively. Here, ρPCL = 1.20 g/cm3 and ρPB = 0.94 g/cm3 were applied. a
of crystallinity were determined using a TA Q 100 at a heating rate of 10 °C/min. The differential scanning calorimetry (DSC) cooling curve from the melt did not show any crystallization exothermic enthalpy and neither did the subsequent heating curve, which indicated that the crystallization did not occur during the cooling scan at 10 °C/min from the melt. However, when the sample was stored at room temperature (25 °C) for 3 days, there was a dominant melting endotherm with a peak temperature at around 49.5 °C, as shown in the DSC heating curve. 2.2. Sample Preparation. The single-crystal silicon wafers were supplied by the Shanghai Institute of Ceramics, China. They were cut into strips of about 12 × 12 mm and then treated with “Piranha” solution, a mixture of 70 vol % concentrated sulfuric acid and 30 vol % hydrogen peroxide, for about 30 min at 120 °C to generate a clean, hydrophilic oxide surface. The substrate was then rinsed with a large volume of distilled water and then purged with dry nitrogen flow.
3. RESULTS AND DISCUSSION 3.1. Microstructures of PB-b-PCL before and after Crystallization. The microdomain structures of PB-b-PCL in the melt and crystalline states were measured by SAXS at 70 °C (which lies above the melting temperature of PCL, Tm = 49.5 °C) and 30 °C, respectively. As shown in Figure 1, the 6420
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Figure 1. Temperature-resolved SAXS profiles of PB-b-PCL obtained before and after crystallization. The temperature has changed from 70 °C (above the melting temperature of PCL) to 30 °C (crystalline temperature).
crystalline PB-b-PCL shows well-defined lattice peaks with relative positions (i.e., 1:2:3) that are relevant to lamellar morphology. In contrast, the PB-b-PCL melt does not show a high order scattering peak because of the weak segregation ability of the sample.18 However, the sharp diffraction at q = 0.34 indicates that the PB-b-PCL melt should display an extremely regular microdomain structure. To remove the influence of crystallization, we observed the thin-film morphology just after thermal annealing. The TEM result shows that PB-b-PCL forms cylindrical structures, with the PCL block being the cylinder phase (see Figure S1 of the Supporting Information). In addition, the periodicity values of amorphous and crystalline PB-b-PCL, calculated from the primary peak position (q) via L = 2π/q, are 18.7 and 32.4 nm, respectively. XPS as a generally applied method could characterize the surface chemical composition in 3−10 nm from the top of the surface. Figure 2 shows XPS spectra of PB-b-PCL thin films annealed under different temperatures. It is found that PB-bPCL shows two dominant peaks, centered at 284.6 eV (Figure 2a) and 531.65 eV (Figure 2b), corresponding to the C 1s and O 1s peaks, respectively.26 Because the PCL contains oxygen, the relative ratio of the peak areas of O 1s and C 1s was calculated to express the change of the surface component with the temperature. As a result, the content of PCL has decreased from 11.7 to 7.7% with the temperature changed from 100 to 25 °C. This phenomenon could be attributed to the following reasons: First, in the melt, the PCL molecules tend to diffuse to the polymer−air interface in view of the lower molecularweight effect,27,28 which would overcome the surface tension differences of the main chains because of the conformational enthalpic and entropic effects.29 Second, in the solid state, crystallization and corresponding kinetics could be the main driving forces,30 which result in a PB layer at the polymer−air interface. Meanwhile, in the solid state, the surface energy of PB (31 mJ/m2 at 20 °C31) is lower than that of PCL (34.1 mJ/m2 at 25 °C32); hence, it is natural to understand that the PB molecules preferentially concentrate at the polymer−air interface to minimize the surface free energy. Furthermore, as reflected from the AFM (see Figure S2a of the Supporting Information) and TEM (see Figure S2b of the Supporting Information) results, the crystalline PB-b-PCL shows lamella-based dendrite morphology. The selected area electron diffraction (SAED) pattern (see inset of Figure S2b of the Supporting Information)
Figure 2. XPS spectra of PB-b-PCL thin films annealed for 30 min at 100 and 25 °C, respectively: (a) C 1s and (b) O 1s.
proves that PCL had crystallized, although it did not form a single crystal. 3.2. In Situ Observation of the Breakout Crystallization. 3.2.1. Microphase Separation Structure within Terraces. The temperature-dependent structure evolution of the PB-b-PCL thin film has been real-time-observed with an optical microscope. Figure 3a shows typical results of the asprepared PB-b-PCL spherulite morphology. The thin-film morphology changed into terraces after the sample was annealed at 100 °C for 10 min (Figure 3b). When the annealing time was prolonged to 40 min, as shown in Figure 3c, a typical island− hole structure is found. The formation of terraces or an island− hole structure could be attributed to the results of mismatching between the film thickness and bulk periodicity values and dewetting. When the sample was cooled to room temperature, secondary crystalline morphology is offered in Figure 3d. In comparison of Figure 3d to Figure 3a, spherulite has also been found, which implies that crystalline morphology has replaced the microphase-separated structure again. Figure 4a displays an AFM height image of the thermal annealed PB-b-PCL sample before crystallization, which shows a terraced structure. The corresponding cross-section line profile (Figure 4b) shows that the terrace depth lies between 19.4 and 22.0 nm, which agree well with the microdomain periodicity obtained from SAXS (i.e., 18.7 nm at 70 °C). The terrace structure formation that can be attributed to the film thickness is incommensurate with the bulk layer spacing. Then, with the submission to the constraint of the conservation of material, top layers broke up into islands and holes and the film thickness must have an integral numbers of the microphase periodicity to achieve the minimum of the free energy.22,23,33−35 6421
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Figure 3. Typical optical micrographs of the PB-b-PCL thin film: (a) as-prepared, (b) annealed at 100 °C for 10 min, (c) annealed at 100 °C for 40 min, and (d) secondary crystalline morphology.
Therefore, by thermal annealing, we could obtain cylinderwithin-terraced lamellae. Furthermore, the morphology transition between crystalline spherulite and amorphous cylinder-withinterraced lamellae inspired us to make an intensive study of the breakout crystallization, as illustrated in the following sections. 3.2.2. In Situ Observation of the Crystal Growth Process. In this section, the development of the crystal (nucleation and growth) and the coalescence of the microdomains have been given attention. The crystallization temperature was set at 32 °C to readily track down the crystal growth process with AFM, which including fast nucleation and slow crystal growth. Sequentially, scanned AFM images with an interval of ca. 11 min were collected, and the typical AFM images are offered in Figure 5. Either a phase image or height image is selectively chosen to give the most obvious contrast between the crystal and the surrounding environment. An overview of the crystallization-resulted morphology change, terraced structure has been observed before crystallization takes place (Figure 5a), and then the nucleus forms in the terraced lamellae (Figure 5b), which gradually grows into dendrites/spherulite (Figure 5e) with an increase of the crystallization time. It should be noted that amorphous PB-b-PCL was too soft to observe in situ the “initial crystallization”. As shown in Figure 5, there is no detectable crystallization observed until isothermal annealing at 52 min (Figure 5b). When the isothermal annealing time was increased to 100 min, we observe a crystal eye structure at the center of the spherulite (Figure 5c). Although Lei et al.20 have suggested that lamellar sheaf and crystal eye
Figure 4. (a) AFM height image of the thermal annealed sample (set at 60 °C and isothermal annealing for 34 min). (b) Height fluctuation along the cross-sectional line, as shown in panel a. 6422
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Figure 5. Typical AFM images of the crystal growth, collected at 32 °C at different crystalline times: (a) 16 min, (b) 52 min, (c) 100 min (the inset is the 3 times magnified image), (d) 486 min (the square outlined zone corresponds to the scanning zone in Figure 6), and (e) 872 min (a−c, phase images; d−e, height images). (f) Time-resolved variation of the diameter of PCL spherulite and its fitting curve. We set the time when the sample was quenched to 32 °C to be 0 min, with the crystal size of 0 μm.
block is surface nucleation. It is also in accordance with the aggregation of the PCL block at the polymer−air interface of the melt, as proven by the XPS result in section 3.1, because the nucleation rate is closely related to the amount of the crystalline component.14,15 On the other hand, to quantitatively evaluate the crystal growth, we set the time when the sample was quenched to 32 °C as 0 min, and the plot of spherulite radius versus time is presented in Figure 5f. We find that the spherulite growth rate is in a linear mode (the coefficient of determination calculator R2 = 0.997 74) and the rate is ca. 0.08 μm/min. By comparison, the growth rate value is just two-thousandths of that of the PCL homopolymer crystallized at 30 °C (as reported by Chen et al.36).
were the typical features of homogeneous nucleation based on the crystallization of poly(bisphenol A octane ether) (BA-C8) at a supercooling value ΔT = 53.3 °C, here, we believe that the situation is different, and the reason lies in two aspects. First, as reported by Hsu et al.,2 the necessary ΔT for homogeneous nucleation of PCL was ca. 96 °C. However, in the present study, ΔT = 17.5 °C is much lower than 96 °C, which suggests that the nucleation of the PCL block may obey other modes. Second, in the weakly segregated block co-polymer thin film, interfaces have great influence on the thin-film crystallization; hence, surface nucleation is more prone to occur.19,20 Therefore, we infer that the primary nucleation of the PCL 6423
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Figure 6. Typical AFM height images in terms of the crystal growth front development, which were taken at different crystallization times: (a) t0 min, (b) t0 + 33 min, and (c) t0 + 78 min. t0 was the moment at which we obtained the first scanning of the outlined square zone depicted in Figure 5d. (a′−c′) Corresponding 3D images of panels a−c, among which the amorphous zone, crystalline zone I, and crystalline zone II have been pointed out.
This indicates that the crystallization of the PCL block has been significantly retarded by the microphase separation morphology. To give a thorough study of breakout crystallization, sequential AFM height images centered on the crystal growth front (square zone in Figure 5d) were recorded. Detailed experimental results are offered in Figure S3 of the Supporting Information, and typical AFM height images are shown in Figure 6. It is clear that crystal growth front has gradually extended into the terraced lamellar zone (panels b and b′ of Figure 6). In the end,
the lamella-based dendrites have replaced the cylinder-withinterraces structure (panels c and c′ of Figure 6). It should be noted that there are three zones that can be distinguished in terms of the crystal growth front development. They correspond to the amorphous zone, crystalline zone I, and crystalline zone II, as shown in 3D images. To compare the height change of these zones induced by crystallization, the upper-most points of these zones are considered. In addition, to judge if crystallization has taken place, the thickness of the lamella lying just before the 6424
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upper-most points of the crystalline zone II and amorphous zone, as depicted in Figure 6a′, while Figure 7b shows the intercept changes between the upper-most points of the crystalline zone II and crystalline zone I, as depicted in Figure 6c′. When the results in Figure 7a are compared to those in Figure 7b, one observes that the lines have opposite trends; namely, the height intercept between the crystalline zone I and amorphous zone is gradually increasing (Figure 7a), whereas the height intercept between the crystalline zone II and crystalline zone I is gradually diminishing with time (Figure 7b). Because the material transfers from amorphous lamellae to the crystal growth front, it is reasonable that the amorphous terrace structure will sink, while the crystalline zone I will lift. In the meantime, both parallel breakout and vertical breakout can contribute to the height lifting of the crystalline zone; hence, the diminishing intercept between crystalline zones I and II that can be attributed to the height lifting rate of the crystalline zone I is faster than that of the crystalline zone II. As reflected in the AFM results (Figure 6), the parallel breakout leads material transfer more readily to reach the crystalline zone I rather than the crystalline zone II. Thus, both parallel breakout and vertical breakout contribute to the lifting of the crystalline zone I, while only vertical breakout plays a role in the lifting of the crystalline zone II. Figure 8a displays the morphology feature resulting from the vertical breakout in the thinner films (cast from 5.0 mg/mL
crystal growth fronts has been evaluated with cross-sectional analysis. It is found that the values of lamellar thickness, i.e., 20.6 nm (inset of Figure 6a) and 19.6 nm (inset of Figure 6b), are close to the amorphous microphase periodicity (19.4−22.0 nm) mentioned in section 3.1. It indicates that crystallization does not occur in the focused amorphous zone, as depicted in panels a and b of Figure 6. Therefore, we can ascertain that, in the front of the crystal growth front, the size of the lamellar zone has gradually decreased (see Figure S3 of the Supporting Information) without changing the lamellar height. Furthermore, it seems that the materials from the amorphous layers have been sucked into the crystal growth front. Because the crystal growth has little influence on the amorphous layers, the material transfer should be attributed to molecular diffusion. An analogous material transfer mechanism has also been demonstrated in the breakout crystallization of block co-polymers by Hobbs et al.19 Meanwhile, the crystallization driving force has been wellrecognized as the impetus for the molecular diffusion.2,17,19 Considering that the molecular diffusion and the corresponding breakout crystallization can take place in two directions that are parallel and vertical to the substrate, to distinguish them, they are named as parallel breakout and vertical breakout, respectively. Figure 7a shows the intercept changes between the
Figure 8. Vertical breakout feature as reflected in the thinner (5.0 mg/mL) PB-b-PCL film. (a) AFM height image. (b) Height fluctuation along the cross-sectional line as shown in panel a. The sample was crystallized at 20 ± 1 °C for 180.5 h after thermal annealed at 100 °C for 40 min.
solution). As reflected in the cross-sectional result (Figure 8b), there is no depletion zone (resulting from parallel molecular diffusion) observed in the crystal growth front because the decrease of the film thickness will weaken molecule diffusion.37 Although the step height has changed from 18.0 nm in the amorphous state to 36.3 nm after crystallization, the molecules
Figure 7. Time-resolved height intercept changes between chosen domains. (a) Height intercept change between the upper-most points of the crystalline zone II and amorphous zone, as depicted in Figure 6a′. (b) Height intercept change between the upper-most points of the crystalline zone II and crystalline zone I, as depicted in Figure 6c′. 6425
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do not grow into large-scale spherulite and are confined in the microphase separation structure in the in-plane direction. In addition, it should be noted that Figure 8a was taken after isothermally crystallized at 20 ± 1 °C for 180.5 h, which confirms that the vertical breakout rate is very slow. Then, we can infer that parallel-breakout-induced in-plane material transfer is the main driving force for the formation of large-scale spherulite. Figure 9 shows a schematic drawing of the breakout crystallization
*Telephone: +86-431-85262123. Fax: +86-431-85262126. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Fajun Zhang at the Institut für Angewandte Physik, Universität Tübingen, and Dr. Shuichi Nojima at the Department of Polymer Chemistry, Tokyo Institute of Technology, for helpful discussions. This work is supported by the National Natural Science Foundation of China (21074135).
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REFERENCES
(1) Qian, Y.; Cai, T.; Hu, W. B. Breakout and breakdown induced by crystallization in cylinder-forming diblock copolymers. Macromolecules 2008, 41 (20), 7625−7629. (2) Hsu, J.-Y.; Hsieh, I. F.; Nandan, B.; Chiu, F.-C.; Chen, J.-H.; Jeng, U. S.; Chen, H.-L. Crystallization kinetics and crystallization-induced morphological formation in the blends of poly(ε-caprolactone)-blockpolybutadiene and polybutadiene homopolymer. Macromolecules 2007, 40 (14), 5014−5022. (3) Nojima, S.; Hashizume, K.; Rohadi, A.; Sasaki, S. Crystallization of ε-caprolactone blocks within a crosslinked microdomain structure of poly(ε-caprolactone)-block-polybutadiene. Polymer 1997, 38 (11), 2711−2718. (4) Nojima, S.; Kato, K.; Yamamoto, S.; Ashida, T. Crystallization of block copolymers. 1. Small-angle X-ray scattering study of an εcaprolactone−butadiene diblock copolymer. Macromolecules 1992, 25 (8), 2237−2242. (5) Nojima, S.; Nakano, H.; Ashida, T. Crystallization behavior of a microphase-separated diblock copolymer. Polymer 1993, 34 (19), 4168−4170. (6) Koo, C. M.; Hillmyer, M. A.; Bates, F. S. Structure and properties of semicrystalline-rubbery multiblock copolymers. Macromolecules 2006, 39 (2), 667−677. (7) Akaba, M.; Nojima, S. Temperature dependence of crystallization behavior in a phase-separated blend of poly(ε-caprolactone) homopolymer and poly(ε-caprolactone)-block-polybutadiene copolymer. Polym. J. 2006, 38 (6), 559−566. (8) Tanimoto, S.; Ito, K.; Sasaki, S.; Takeshita, H.; Nojima, S. Crystallization process in binary blends of poly(ε-caprolactone)-blockpolybutadiene copolymers. Polym. J. 2002, 34 (8), 593−600. (9) Xu, J. T.; Turner, S. C.; Fairclough, J. P. A; Mai, S. M.; Ryan, A. J.; Chaibundit, C.; Booth, C. Morphological confinement on crystallization in blends of poly(oxyethylene-block-oxybutylene) and poly(oxybutylene). Macromolecules 2002, 35 (9), 3614−3621. (10) Lammertink, R.; Hempenius, M.; Vancso, G. Morphology and crystallization of thin films of asymmetric organic−organometallic diblock copolymers of isoprene and ferrocenyldimethylsilane. Langmuir 2000, 16 (15), 6245−6252. (11) Zhang, F.; Huang, H.; Hu, Z.; Chen, Y.; He, T. Crystallization of weakly segregated poly(styrene-b-ε-caprolactone) diblock copolymer in thin films. Langmuir 2003, 19 (24), 10100−10108. (12) Hong, S.; MacKnight, W. J.; Russell, T. P.; Gido, S. P. Structural evolution of multilayered, crystalline−amorphous diblock copolymer thin films. Macromolecules 2001, 34 (9), 2876−2883. (13) Papadakis, C. M.; Darko, C.; Di, Z.; Troll, K.; Metwalli, E.; Timmann, A.; Reiter, G.; Förster, S. Surface-induced breakout crystallization in cylinder-forming P(I-b-EO) diblock copolymer thin films. Eur. Phys. J. E: Soft Matter Biol. Phys. 2011, 34 (1), 7. (14) Müller, A. J.; Balsamo, V.; Arnal, M. L. Nucleation and crystallization in diblock and triblock copolymers. Adv. Polym. Sci. 2005, 190, 1−63.
in the PB-b-PCL thin film. It is apparent that the crystal (dendrite consisting of alternating layers of amorphous and crystalline) has replaced the amorphous cylinder-withinterraces structure and the periodicity has changed from ca. 21 to 32.4 nm after crystallization.
4. CONCLUSION On the basis of in situ recording the developments of the wellseparated amorphous microphase separation structure and crystal growth front, we demonstrate an intensive study of the breakout crystallization, namely, how the polymer chains overcome the topography constraint and grow into large-scale crystals. It is found that the crystallization-resulted material transport is achieved through molecular diffusion. When the molecular diffusion directions are divided into parallel and vertical to the substrate, some conclusions are obtained. On one hand, it is the molecular dissociation from the microphase domain that results in the coalescence of the microphase separation structure. On the other hand, the dissociated molecules diffuse to the crystal growth front through parallel molecular diffusion, which brings enough material to facilitate the growth of large-scale crystals, i.e., spherulite. Moreover, as shown in the thinner film (Figure 8), where the parallel molecular diffusion apparently weakened, the vertical diffusion takes dominance and the microphase separation structure has changed into lamellae parallel to the substrate after crystallization. Furthermore, with illustrations of the surface nucleation and greatly retarded crystal growth, we attest the fundamental aspects of crystallization and its effects on the morphology features.
ASSOCIATED CONTENT
S Supporting Information *
TEM result of the amorphous PB-b-PCL (5.0 film (Figure S1), AFM and TEM results morphology (Figure S2), and sequential AFM breakout (Figure S3). This material is available via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Figure 9. AFM images, showing the breakout crystallization in the PBb-PCL thin film. D indicates the periodicity value of the microphase separation structures, which was obtained from SAXS.
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mg/mL) thin of crystalline results during free of charge 6426
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dx.doi.org/10.1021/la300439h | Langmuir 2012, 28, 6419−6427
