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Chinese Journal of Polymer Science Vol. 29, No. 3, (2011), 325-335 Chinese Journal of Polymer Science

? Chinese Chemical Society

Institute of Chemistry, CAS

Springer-Verlag Berlin Heidelberg 2011 EFFECT OF MOLECULAR WEIGHT AND FILM THICKNESS ON THE

CRYSTALLIZATION AND MICROPHASE SEPARATION IN POLYSTYRENE-

BLOCK-POLY(L-LACTIC ACID) THIN FILMS AT THE EARLY STAGE* Yu-han Wei a, Cai-yuan Pan b, Bin-yao Li a, Xin-hong Yu a** and Yan-chun Han a

a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of

Sciences, Graduate School of the Chinese Academy of Sciences,Changchun 130022, China

b Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China

Abstract We investigated the effects of molecular weight and film thickness on the crystallization and microphase separation in semicrystalline block copolymer polystyrene-block-poly(L-lactic acid) (PS-b-PLLA) thin films, at the early stage of film evolution (when T g < T < T ODT) by in situ hot stage atomic force microscopy. For PS-b-PLLA 1 copolymer which had lower molecular weight and higher PLLA fraction, diffusion-controlled break-out crystallization started easily. For PS-b-PLLA 2 with higher molecular weight, crystallization in nanometer scales occurs in local area. After melting of the two copolymer films, islands were observed at the film surface: PS-b-PLLA 1 film was in a disordered phase mixed state while PS-b-PLLA 2 film formed phase-separated lamellar structure paralleling to the substrate. Crystallization-melting and van der Waals forces drove the island formation in PS-b-PLLA 1 film. Film thickness affected the crystallization rate. Crystals grew very slowly in much thinner film of PS-b-PLLA 1 and remained almost unchanged at long time annealing. The incompatibility between PS and PLLA blocks drove the film fluctuation which subsequently evolved into spinodal-like morphology.

Keywords: Crystallization; Microphase separation; Molecular weight; Film thickness; Early stage.

INTRODUCTION

Block copolymers, which contain at least one crystallizable block, show unique properties because of the presence of the crystallizable block. Thus, much attention has been concentrated on the semicrystalline block copolymer for its fundamental scientific interests[1-7]. Coexistence of microphase separation provoked by the immiscibility of different blocks and crystalline behavior complicates the self-assembly of the semicrystalline block polymer[8-11], which has been widely investigated. It is discovered that the final morphology of a semicrystalline block copolymer is strongly dependent upon the interplay between microphase separation and crystallization. The interplay is basically determined by the order-disorder transition temperature (T ODT), the crystalline temperature of the crystallizable block (T c), the glass transition temperature of the amorphous block (T g) and the crystallization rate. Although the bulk properties of semicrystalline block copolymers have been extensively studied in the past years, the phase behavior in thin films is less known[12-16].

Phase behavior of semicrystalline block copolymers in thin films is inclined to deviate from that of the bulk due to confinement of the substrate[14-16]. Schultz[16] addressed the effect of specimen thickness on crystallization rate, and found that the overall crystallization rate on bulk specimens was higher than that of thin films, the ratio * This work was financially supported by the National Natural Science Foundation of China (Nos. 20621401, 50773080, 20834005) and the Ministry of Science and Technology of China (No. 2009CB930603).

** Corresponding author: Xin-hong Yu (于新红), E-mail: xhyu@https://www.360docs.net/doc/021455999.html,

Received April 13, 2010; Revised June 12, 2010; Accepted June 22, 2010

doi: 10.1007/s10118-011-1040-z

Y.H. Wei et al. 326

of the film thickness to the growth velocity was found to determine the crystallzation kinetics, and thin films usually exhibited abnormal low Avrami exponent. Additionally, interfaces influence the structure and the orientation of the phases in diblock copolymer thin films. One block preferentially interacts with an interface (free surface or substrate) and form parallel orientation structure below ODT. For the symmetric diblock copolymers, this alignment often leads to a “thickness quantization” of the thin film. When the film thickness is not commensurate to the equilibrium lamellar spacing, either holes or islands are nucleated on the film surface to adjust the local film thickness to the preferred quantized values. Green et al.[15] found that for asymmetric polyethylene-b-poly(styrene-r-ethylene-r butene) (E-b-SEB) diblock copolymer, the film instability also drove formation of holes, islands and lamellae.

However, much work on the crystalline behavior at the later stage of the film evolution (when microphase separation has occurred) has been studied, few work about crystalline behavior at the early stage (T g < T < T ODT) is carried out[17-20]. At the temperature above T g but below T m, the molecular chains start to relax so that microphase separation and crystallization happen simultaneously, the crystallization under this circumstance is a relaxation-controlled process[19]. Previous work in our group has investigated effects of crystallization of PLLA blocks on the surface orientation and film decomposition, interplay between microphase separation and crystallization upon annealing at the early stage of film evolution[17-19]. The primary interest of the present work is to study the effect of molecular weight and film thickness on the crystallization and microphase separation of PS-b-PLLA copolymer films at the early stage. Here, morphological evolution of two PS-b-PLLA copolymers with different molecular weight is followed in real-time and different crystalline nucleation is observed. Formation of the islands is followed and the molecular weight determines the microphase separation in the molten state. Decrease of the film thickness greatly slows down the crystalline growth and the film fluctuation evolves into spinodal-like structure at the temperature above T g PS.

EXPERIMENTAL

Synthesis and Characterization of the Block Copolymer

The block copolymer, polystyrene-b-poly(L-lactic acid) (PS-b-PLLA), was synthesized by combination of atom transfer radical polymerization (ATRP) of styrene monomers and ring-opening polymerization (ROP) of L-lactide[21]. Two kinds of PS-b-PLLA block copolymers with different molecular weights and volume fractions were selected in this experiment. The molecular weight and polydispersity were determined by 1H-NMR and gel permeation chromatography (GPC, waters-410), respectively. PS-b-PLLA 1 has a number-average molecular weight of 8.89 × 103 (2.86 × 103-b-6.03 × 103), with the polydispersity 1.15. PS-b-PLLA 2 has a number-average molecular weight of 19800 (10400-b-9400), with the polydispersity less than 1.2. The volume fraction of the PLLA block for PS-b-PLLA 1 is 0.68 and 0.42 for PS-b-PLLA 2, thus, PS-b-PLLA 1 is almost asymmetric and PS-b-PLLA 2 is approximately symmetric. T g of the two blocks and T m were determined by a Perkin-Elmer Diamond differential scanning calorimeter at 10 K/min. For PS-b-PLLA 1, two glass transitions at 59?C (for PLLA block) and 89?C (for PS block), melting at 167?C were obtained[16]. For PS-b-PLLA 2, the T g of PLLA and PS blocks is 55?C and 90?C, respectively. The T m of PLLA is 155?C[17-19].

According to the results obtained by Zalusky et al.[22] and the mean-field theory[23], the Flory-Huggins interaction parameter (χ) between the PS and PLLA blocks can be expressed by χ(T) = 98.1/T - 0.112, where T is the Kelvin temperature. The order-disorder transition temperature (T ODT) of PS-b-PLLA 1 and 2 was calculated to be about 149°C and 349°C, respectively[24]. The phase-separated strength of PS-b-PLLA 2 is always larger than that of PS-b-PLLA 1 under the experimental condition.

Sample Preparation

The two copolymers were dissolved in tetrahydrofuran (THF) solution with the concentration of 5 g/L at room temperature, then stored over 24 h till their complete dissolution. The concentration of 1 g/L for PS-b-PLLA 1 was also prepared to investigate the effect of film thickness on the film evolution. Then they were spin-coated onto the freshly cleaned silicon wafers (cleaned with 7/3 (V/V) solution of 98% H2SO4/30% H2O2 at boiling state

Crystallization and Microphase Separation of PS-b-PLLA 327 for 30 min, rinsed with deionized water after cooling, then dried by nitrogen atmosphere). The residual solvent

in the thin films was removed in a vacuum oven at room temperature for more than 24 h.

Characterization

The film thickness was characterized by spectroscopic ellipsometry (UVISEL, Jobin Yvon, France).

A commercial atomic force microscope (SPA300HV/SPI3800N, Seiko Instruments, Inc. Japan) was used to detect the surface morphology of the thin film. A silicon cantilever (Olympus, Japan) with an integrated conical tip at the apex worked for tapping mode AFM scan. The nominal spring constant is 2 N/m and the resonant frequency ca. 70 kHz. The radius of curvature of the tip was about 40 nm, as evaluated by scanning over very sharp needle (radius < 10 nm) array (NT-MDT, Russia). For in situ AFM scan, a hot stage integrated to the AFM system was used to change the sample temperature from 25?C to 180?C. The temperature was monitored by a pair of thermal couples which was calibrated by using standard gallium, indium and tin samples. The accuracy is ±2?C. The morphology evolution was recorded with tapping mode AFM scan at a specific temperature.

The X-ray photoelectron spectroscopy (XPS) was measured with a Thermo ESCALAB 250 instrument at room temperature by using an Al K Alpha (hv = 1200 eV) at 15 kV and 20 mA.

WAXD spectra were obtained with a Rigaku D/max 2500 V PC X-ray diffractometer (Japan) with a Cu KR source working at 40 kV and 200 mA.

RESULTS

Effect of the Molecular Weight on the Morphological Evolution of PS-b-PLLA Thin Films

Morphological evolution of PS-b-PLLA 1 film with lower molecular weight

Morphological evolution of PS-b-PLLA 1 film obtained from 5 g/L THF solution (the film thickness is about 22.8 nm) is displayed in Fig. 1. Considering the polymer-solvent interaction parameter (χP-S)[25] at room temperature (25°C), χPS-THF is about 0.34 and χPLLA-THF is about 0.42, Δχ is about 0.08. THF is a good solvent for PS and PLLA but a little selective to PS. PLLA blocks will crystallize and some large crystalline aggregates appear in the initial spin-coated PS-b-PLLA thin film (shown in Fig. 1a). As increasing the temperature to 65°C (above T g of PLLA), rapid crystallization induced by the crystalline aggregates takes place (Figs. 1b, 1c). The crystallization formed at this temperature should be dendritic crystal, see from their appearance (compact pattern with few branches around). With the increasing of the temperature, some edge-on crystals emerge around the dendritic crystals and soon spread the whole film surface (Fig. 1d). Crystallization breaks out easily for the presence of the crystalline aggregates in the initial film, which likely depress the activation energy of crystallization and induce the heterogeneous nucleation of the copolymer.

As the temperature was elevated to 150°C (approaching T m of PLLA), some depressions appear at the film surface which is possibly caused by the melting of the PLLA (Fig. 1e). To our surprise, the depressions almost appear at the dendritic crystalline area and there is a sphere left in the center, eye-like morphology is observed (indicated by the arrow in Fig. 1e). Figure 1(e) displays the coexistence of dendritic crystals and eye-like morphology as the temperature was elevated. Continuous increasing the temperature to 165°C results in disappearance of the spheres in the center of the eye-like structure, only “eye socket” structure is left (Fig. 1g). Then the islands nucleate from the “eye socket” rim structure (indicated by the circles in Figs. 1f, 1g), the flatter area of the film melt first and fuse into the “eye socket” rim structure, then the “eye socket” rim structure fuse to form the island (Figs. 1g, 1h). Prolonging the annealing time, the line tension drives the coarsening of the islands and some small islands merge into larger ones (Figs. 1h, 1i), the phenomenon is similar to that formed by symmetric diblock copolymer.

Y.H. Wei et al. 328

Fig. 1 AFM height images of morphological evolution with temperature of PS-b-PLLA 1 film spin

coated from 5 g/L THF solution: (a) 25°C, (b, c) 65°C, (d) 117°C, (e) 150°C, (f) 153°C, (g) 165°C

and (h, i) 170°C (the inserted image in (f) is the respective 3D image.)

The WAXD spectra of the PS-b-PLLA 1 film at different temperatures are displayed in Fig. 2. Characteristic crystalline peak of PLLA is found at 2θ≈ 16.5° and remains almost unchanged with elevating the temperature. The weak intensity of the crystalline peaks is possibly caused by such thin film thickness.

Crystallization and Microphase Separation of PS-b-PLLA 329

Fig. 2 WAXD spectra of PS-b-PLLA 1 film at different temperatures: (a) 73°C, (b) 130°C and (c) 150°C

Fig. 3 AFM height images of morphological evolution with temperature of PS-b-PLLA 2 films spin coated

from 5 g/L THF solution: (a) 25°C, (b) 93°C, (c, d) 131°C, (e) 150°C and (f) 160°C (inserted images in (a, b)

are AFM phase images, respectively.)

Morphological evolution of PS-b-PLLA 2 film with higher molecular weight

As we follow the evolution of PS-b-PLLA 2 film by in situ hot stage AFM, we observe different morphological evolution. Figure 3(a) displays the surface morphology of PS-b-PLLA 2 film spin-coated from 5 g/L THF solution (the film thickness is about 33.1 nm). There are also some large crystalline aggregates in the pristine spin-coated film, which is very similar to the morphology of PS-b-PLLA 1 film. The film surface is in a

Y.H. Wei et al. 330disordered state (inserted image in Fig. 3a) except the crystalline aggregates. Crystallization does not take place until the temperature increases to 93°C which is above T g of PS. Crystalline particles with diameter of several hundreds of nanometers appear at the film surface, the phase image displays higher phase contrast (the inserted image in Fig. 3b) which means the crystallization of PLLA [18]. With the increasing of the temperature to 131°C, crystalline particles of PLLA have covered the whole film surface (Fig. 3c). Prolonging the annealing time at 131°C, some cracks appear at the film surface (Fig. 3d) which are possibly caused by the lateral contraction of the film during the crystalline process.

The film starts to melt and the depressions appear at the film surface as the temperature increases to 150°C (Fig. 3e). The film melts completely and islands appear until the temperature increases to 160?C (Fig. 3f). The coarsening of the island induced by the line tension is investigated below (Fig. 4). The height of the island is measured by the AFM height image and the corresponding cross-section line scan profile. The edge of the island becomes smooth with elevating the temperature. The height of the island increases from ca. 15.3 nm (Fig. 4a) to ca. 19.1 nm (Fig. 4b) with the increasing of the temperature, approaching the lamellar period 20 nm (L 0)[17-19].

Z 1 (nm) Z 2 (nm) ?Z (nm) Distance (nm)

φ (?) Z 1 (nm) Z 2 (nm) ?Z (nm) Distance (nm) φ (?) 27.93265 43.23442 15.30177 4148.921 0.21131345.4274164.5132819.08587 4903.256 0.223022

Fig. 4 Coarsening of the island in PS-b -PLLA 2 film as the temperature increases from 160°C (a) to 170°C (b)

Effect of the Film Thickness on the Morphological Evolution of the PS-b-PLLA 1 Film

It is well known that the film thickness is an important parameter in controlling the surface morphology of the thin film. In this section, we adjust the film thickness by decreasing the solution concentration. As we spin coat the PS-b -PLLA 1 film from 1 g/L THF solution (the film thickness is about 8.7 nm), spheres and nanorods (indicated by the arrows in Fig. 5a) appear at the film surface. As the concentration of THF solution decreases, the difference between χPS-THF and χPLLA-THF is likely to increase, leading to a slight increase in the solvent selectivity [24]. Thus, morphological transition from spheres to cylinders happens which leads to the formation of the nanorods in the PS-b -PLLA thin film [26, 27].

As the temperature increases to 97°C (Fig. 5b), the morphology of the film nearly does not change, spinodal-like morphology is observed at the film surface until the temperature increases to 106°C (Fig. 5c). As the temperature increases above T g PS , the free surface fluctuates and the incompatibility between PS and PLLA induces spinodal decomposition, but the chains move very slowly due to the thickness confinement. The crystalline aggregates remain no change. Then we investigate the isothermal crystallization of the PS-b -PLLA thin film at 106°C. The growth of the spheres and nanorods is very slow and the spinodal-like patterns are

Crystallization and Microphase Separation of PS-b-PLLA 331 obvious after 1 h (Fig. 5d, the inserted image displays the amplified morphology). As the temperature approaching T m of the PLLA, the crystalline aggregates depress and the film melts gradually (Fig. 5e), and finally fuses into the underlying pattern (Fig. 5f). Simultaneously, the spinodal pattern breaks up[28] and islands has not formed in annealing time (Figs. 5e, 5f).

Fig. 5 AFM height images of morphological evolution with temperature of PS-b-PLLA 1 film spin

coated from 1 g/L THF solution: (a) 25°C, (b) 97°C, (c, d) 106°C, (e) 161°C and (f) 173°C

DISCUSSION

Seen from the above experimental results, PS-b-PLLA copolymer films demonstrate the distinctive morphological evolution at the early stage (T g < T < T ODT) of the film evolution with varying the molecular weight and the film thickness. Compared to PS-b-PLLA 1 film, crystallization in PS-b-PLLA 2 film does not take place until the temperature increases to 93°C which is above T g of PS. There are no flat-on or edge-on dendritic crystals formed in PS-b-PLLA 2 film as in PS-b-PLLA 1 film. It is possible because that the phase separation of PS-b-PLLA 2 is higher and the volume fraction of PLLA is lower, resulting in stronger phase-separated strength and weaker crystallizability of PLLA blocks. The cold crystallization from the vitrified film is a relaxation-controlled process which is very different from the crystallization from the melt, the polymer chains should be sufficiently activated before they start to nucleate and fold[29]. Only as the temperature increases to T g of PS, both PS and PLLA can move simultaneously and chain-folding will occur. According to the theory of crystal growth, dendrites and compact seaweed morphologies could appear in the crystal growth in a diffusion field[30]. Crystallization is a complex process that contains molecular diffusion process from the liquid phase to the interface and surface kinetic process. Many theoretical studies carried out through computer simulations indicated that, the molecular diffusion process from the liquid phase to the interface was the major factor in

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determining the pattern formation of a crystalline process in the nonequilibrium conditions[31-33]. Since the molecular weight of PS-b-PLLA 2 is higher, the chains mobility is relatively slow and the crystallization occurs

in the local area. So the crystallization is confined in particles with nanometer scale and break-out dendritic crystals are not observed. While in PS-b-PLLA 1 film with lower molecular weight, the volume fraction of PLLA is 0.68 and the major PLLA blocks will be mobile above T g PLLA, simultaneously, the reorganization of the minor PS draught by the major PLLA blocks will occur and diffusion-controlled dendritic crystals easily break out at 65°C.

After the two copolymer films melt, the island morphology forms at the film surface. If the blocks have preferentially affinity to the substrate and the free surface, the molecular chains tend to orient normal to the substrate and form lamellar structure paralleling to the substrate in the molten state at T < T ODT[34, 35]. In our case, the surface tension of PLLA is 60.89 mN/m, and that of PS is 39.4 mN/m (at 20°C)[17-19]. In order to minimize the surface energy, the polar PLLA blocks preferentially wet the SiO x surface, and the PS blocks wet the free surface. Then we characterize the surface composition of two copolymer films before and after melting by XPS. Figure 6(a) displays the C 1s spectra of the initial PS-b-PLLA 1 film and the molten film, the C 1s peak at 288.8 eV corresponds to the carbonyl group. The relative intensity of the carbonyl peak remains no change for the initial and molten film, which means the PLLA still stays at the film surface after the melting of the film. The phase separation between PS and PLLA does not happen because PS-b-PLLA 1 film is in a disordered phase mixed state above T ODT. While for PS-b-PLLA 2 film, the relative intensity of the carbonyl peak decreases a lot after the film melt (Fig. 6b), which indicates that the PLLA segregates to the internal of the film and the top layer is mainly composed of PS after the melting of the film. It is possible that PS at the top layer is very thin and PLLA at the underlayer can be detected slightly, which results in the emergence of the weak carbonyl peak

in the molten film. Typical parallel lamellar structure forms in the molten film of PS-b-PLLA 2. According to above discussion that parallel lamellar structure forms in the molten film of PS-b-PLLA 2, the chain stretching

in the lamellar structure with increase of the temperature will lead to a slightly increase of the island height displayed in Fig. 4.

Fig. 6 XPS spectra of the initial and molten films: (a) PS-b-PLLA 1 and (b) PS-b-PLLA 2 The formation of the island morphology in PS-b-PLLA 1 film is very interesting. Depression appears as the temperature approaches T m of PLLA and eye-like morphology is observed. It is noted that the changes of morphology during the melting or the annealing process usually start at the defective or thermally instable sites

in crystals[36]. The dendritic crystals formed at lower crystalline temperature (65°C) is possibly instable crystalline structure due to the rapid chain-folding and there are many defects inside, they start to melt and depress as the temperature approaches T m of PLLA. Melting and subsequent reorganization (e.g. chain mobility and reoriganization) happened and constructed the surrounded “eye socket” structure (Fig. 1f and the inserted image). The shape of the “eye socket” structure is not round but a little irregular. The spheres in the “eye” center

Crystallization and Microphase Separation of PS-b-PLLA 333 should be the crystalline aggregates in the initial spin-coated film, which perhaps has much stable crystalline structure as spin-coated from THF solution and cannot melt at this temperature[17]. Dewetting of the thin film initiates from the spheres as temperature approaches the melting temperature of PLLA and forms the “eye socket” structure analogous to the dewetted holes. As the temperature increases to 165°C, the spheres in the center of the eye-like structure disappear, only “eye socket” structure is left which can be considered to be the rim accumulated at the hole periphery[37, 38] (Fig. 1g). Green et al.[37, 38] reported that holes surrounded by elevated outer rims will grow being driven by interfacial and hydrodynamic forces until the rims impinge, consequently, forming liquid ribbons that break up into droplets due to a Rayleigh instability. But in our case, the islands nucleate from the “eye socket” rim structure (indicated by the circles in Figs. 1f, 1g), the flatter areas of the film melt first and fuse into the “eye socket” rim structure, then the “eye socket” rim structures fuse and connect to form the island (Figs. 1g, 1h). The formation mechanics of the islands is also different from that in previous reports: the spinodal pattern breaks up and islands form[28]. The driving force of the island formation in this experiment is likely to be van der Waals (dispersion) forces. van der Waals (dispersion) forces always tend to drive the molecules from thinner to thicker parts of the film[39], the molecules will migrate from the flatter film to the “eye socket” rim structure (as shown in Fig. 1g) because of melting and islands form.

The confinement of the substrate owing to the film thickness alters the transport property of molecular chains, which thus influences the crystallization of the semicrystalline block copolymers. The glass transition temperature increases with the decreasing of the film thickness as a result of attractive interfacial interaction between the molecular chains and the substrate[40], the diffusion of the chains in thin film is largely depressed for the confinement of substrate[41, 42], so the mobility and rearrangement of the two blocks become difficult. Schultz[16] addressed the effect of specimen thickness on crystallization rate, and found that the overall crystallization rate on bulk specimens was higher than those of thin film, the ratio of the film thickness to the growth velocity was found to determine the crystalline kinetics, and thin films usually exhibited abnormal low Avrami exponent. In this experiment, the crystalline rate decreases a lot for the confinement in such thin films, the crystalline aggregates grow very slow as the temperature is above T g PS and remain almost unchanged after long time annealing. Simultaneously, the incompatibility between PS and PLLA blocks will induce microphase separation, and spinodal-like morphology is observed at the surface of the film. Compared to the thicker film obtained from 5 g/L THF solution where break-out crystallization easily occurs and the islands form in molten state, the thinner film decreases the chain mobility a lot and local fluctuation evolves into spinodal-like morphology. Green et al.[43] showed that the topography of the film exhibited a hierarchy of patterns depending on the film thickness above the ODT. Films of thickness h < 3.5 nm dewet the silica wafers and form spinodal-like patterns. When the film thickness 3.5 nm< h <7 nm, discrete holes are observed randomly throughout the film. When the film thickness 7 nm < h < 19 nm, interconnected spinodal-like patterns appear throughout the surface and finally evolve into droplets.

CONCLUSIONS

In summary, the effects of the molecular weight and film thickness on the crystallization and microphase separation of the PS-b-PLLA thin film at the early stage are investigated by in situ hot stage atomic force microscope. Molecular weight of the PS-b-PLLA determines the phase-separated strength which subsequently affects the crystalline evolution with elevating the temperature. For PS-b-PLLA 1 with lower molecular weight and higher PLLA fraction, the phase-separated strength is weaker and the crystallizability is stronger. As the temperature increases above T g of PLLA, molecular chains easily move and diffusion-controlled dendritic crystallization breaks out easily. For PS-b-PLLA 2, the phase-separated strength is relatively stronger. Crystallization does not start until the temperature increases to T g of PS when both of PS and PLLA can move simultaneously. What’s more, the molecular chains move slowly and crystallization is confined in local particles with nanometer scales, no break-out crystals are observed. After melting of the two copolymer films, islands are observed at the film surface. PS-b-PLLA 1 film is in a disordered phase mixed state because the T is above T ODT while PS-b-PLLA 2 film forms phase-separated lamellar structure after melt because of the higher phase-separated strength. The island formation in PS-b-PLLA 1 film is driven by the melting and van der Waals

Y.H. Wei et al. 334

forces.

As we decrease the film thickness by decreasing the solution concentration of PS-b-PLLA 1, spheres and nanorods are observed at the film surface. No break-out crystallization is induced by the crystalline aggregates because of the thickness confinement, and the crystalline aggregates remain almost unchanged with elevating the temperature. The thinner film decreases the chain mobility and local fluctuation of the film evolves into spinodal-like morphology.

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