Branched Crystalline Patterns of Poly(Îµ-caprolactone) - American

Article pubs.acs.org/JPCB

Branched Crystalline Patterns of Poly(ε-caprolactone) and Poly(4hydroxystyrene) Blends Thin Films Chunyue Hou, Tianbo Yang, Xiaoli Sun, Zhongjie Ren, Huihui Li,* and Shouke Yan* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: The chain organization of poly(ε-caprolactone) (PCL) in its blend with poly(4-hydroxystyrene) (PVPh) in thin films (130 ± 10 nm) has been revealed by grazing incident infrared (GIIR) spectroscopy. It can be found that PCL chains orient preferentially in the surface-normal direction and crystallization occurs simultaneously. The morphology of the PCL/PVPh blends films can be identified by optical microscopy (OM). When crystallized at 35 °C, the blends film shows a seaweed-like structure and becomes more open with increasing PVPh content. In contrast, when crystallized at higher temperatures, i.e., 40 and 45 °C, dendrites with apparent crystallographically favored branches can be observed. This characteristic morphology indicates that the diffusionlimited aggregation (DLA) process controls the crystal growth in the blends films. The detailed lamellar structure can be revealed by the height images of atomic force microscopy (AFM), i.e., the crystalline branches are composed of overlayered flat-on lamellae. The branch width has been found to be dependent on the supercooling and PVPh content. This result differs greatly from pure PCL, in which case the crystal patterns controlled by DLA process developed in ultrathin film or monolayers of several nanometers. In the PCL/PVPh blends case, the strong intermolecular interactions and the dilution effect of PVPh should contribute to these results. That is to say, the mobility of PCL chains can be retarded and diffusion of them to the crystal growth front slows down greatly, even though the film thickness is far more than the lamellar thickness of PCL.

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crystalline anisotropy.14 In the case of PCL ultrathin film, the morphology is compact seaweeds in the film thickness of 15 nm, and will change to fractal dendrites when the film thickness decreased to 6 nm.2 The special morphology of ultrathin polymer films has been attributed to different growth mechanism from that in the bulk phase, i.e., a diffusion-limited aggregation (DLA) process,15 and the DLA process is thought to play role when the polymer films are equal or less than the lamellar thickness. Most of the studies of thin and ultrathin films are, as we know, mainly about pure polymers. Since most of the polymeric materials often contain more than one component, the structural and morphological controlling of polymer blends in bulk have been investigated for a long time. The crystalline modification, structure, and morphologies of crystalline component can be modified greatly by blending with amorphous ones. Taken into account the increasing application in many fields, for example, the blends of conjugated polymers in light-emitting devices (LED)16 and the active layer in solar cells,17,18 polymer blends in thin and ultrathin films also deserve further investigations,19−22 since it is possible to tune the properties of these devices via changing its microstructure. Moreover, the kinetics of chain organizing process may be in great relation with the resultant morphology and patterns in thin and ultrathin polymer films. For example, it has been

INTRODUCTION With the wide application of polymer materials in thin and ultrathin films a fundamental understanding of the chain organization, such as their conformation, orientation and crystalline structure, have attracted great interest. It has been proposed that the crystalline morphology, structures and kinetics of thin and ultrathin films can be changed greatly compared with those in bulk phase. For example, the crystalline morphology of polycaprolactone (PCL) thin films has been found to have dendrites, axialites, branched edge-on lamellae and truncated lozenge shape structure, depending on crystallization temperature.1−4 The results of Frank and Schö nherr show that poly(ethylene oxide) (PEO) films crystallize with lamellae oriented flat-on, and the crystallization rates were significantly reduced for film thicknesses below ∼100 nm, whereas crystallization occurred preferentially in the form of edge-on lamellae for PEO films in thickness of ∼1 μm and thicker.5,6 In another study of thin films of poly(di-nhexylsilanes) on different substrates, Frank et al. have also proposed that the molecular orientation, the crystallization kinetics, and the degree of crystallinity are all closely related with film thickness.7,8 That is to say, the polymer chains prefer to orient along the normal direction of the film surface with decrease of film thickness. In the case of ultrathin film and monolayers, the morphologies are even more typical due to extremely constrained states, i.e., morphology can be named as dendrite if it has apparent geometrical order, otherwise, it can be called seaweed.9−13 The transition between seaweeds and dendrites can be realized by changing supercooling and © 2015 American Chemical Society

Received: October 11, 2015 Revised: December 14, 2015 Published: December 29, 2015 222

DOI: 10.1021/acs.jpcb.5b09960 J. Phys. Chem. B 2016, 120, 222−230

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The Journal of Physical Chemistry B found that the chain orients parallel with the film surface with a naphthalene ring perpendicular to the film plane for poly(pentamethylene 2,6-naphthalate) (PEN) cold crystalline process. Moreover, the chain orientation occurs during the induction period, and subsequently locally ordered chain pack into the crystalline phase.23 In another study of molecular orientation of epitaxial crystallization process of PCL on highly oriented polyethylene (PE) substrate, the parallel alignment of PCL chains along the chain direction of PE occurs in its molten state, and the ordered PCL chains can form crystals during the cooling process.24 In both cases, the chain orientation develops before the occurrence of crystallization process. It should be noted, however, that the films of the two cases are more than several micrometers and even reach several hundred micrometers. That is to say, to probe the organizing process of chains in polymer thin films of submicrometers is essential for optimizing their performance. Thus, in this study, crystalline patterns and molecular chain organizing process of PCL/ poly(vinylphenol) (PVPh) blends thin films (100−150 nm) have been revealed. The chain orientation process of PCL can be followed due to its slowed crystalline rate via blending with PVPh. In addition, the miscibility of PVPh/PCL blends is attributed to intermolecular hydrogen bonds.25,26 Thus, the PVPh/PCL blends should be a model system to reveal the influence of strong intermolecular interactions on crystalline patterns in thin and ultrathin polymer blends films.

coadded for each IR spectral measurement to ensure a high signal-to-noise ratio. For the GIR-IR measurement, a 80 degree specular reflectance accessory (Spectra-Tech, FT80 RAS) was employed, together with a PIKE Technologies infrared polarizer. The transmission infrared (TIR) spectra of thick film (several micrometers) of PCL/PVPh (70:30 wt %) blends film were also measured for comparison. The blend film was first melted at 90 °C for 5 min and then cooled to 25 °C for isothermal crystallization and measured by GIR-IR simultaneously.

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RESULTS AND DISCUSSION FT-IR Spectroscopy. As we know, when film thickness is approaching the lamellar thickness of the crystalline polymers, the molecular chains prefer to orient in the normal direction of substrate surface. In the present case, the blend film thickness is 130 ± 10 nm, and the orientation of PCL chain segments in the blend film can be measured by GIR-IR. According to the selection rules of GIR-IR, the changes of band intensity may appear in GIR-IR spectrum compared with those of transmission spectrum (isotropic state) if the preferential orientation existed in the film.27,28 Figure 1 shows the GIR-IR spectrum of

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EXPERIMENTAL SECTION Materials and Sample Preparation. PCL (Mw ∼ 14 000, Mn ∼ 10 000) and PVPh (Mw ∼ 11 000) were purchased from Sigma-Aldrich Com. Both pure PCL and PCL/PVPh blends were dissolved in tetrahydrofuran (THF) for at least 12 h, at room temperature yielding a 1 wt % solution. Thin films were prepared by spin-coating from solutions with different PCL weight fraction onto cleaned silicon wafer using a spin-coater at 3000 rpm for 40 s. The silicon wafer was first cleaned in isopropyl alcohol using ultrasonic waves to remove any organic contamination, then rinsed with deionized water and dried before use. The spin-coated samples were annealed at room temperature in vacuum oven for 12 h to remove the residual solvent. The thickness of the as-prepared samples was verified by atomic force microscopy (AFM) and the obtained films are in thickness of 130 ± 10 nm. These thin films were melted at 90 °C to remove thermal history, and isothermal crystallization was carried out on a Linkam hot stage at preset temperatures. Optical Microscopy (OM). The optical microscopic images were obtained by using the Axioskop 40A Pol optical microscope (Carl Zeiss). Reflection mode with the analyzer was applied to obtain clearer pictures. Atomic Force Microscopy (AFM). Detailed morphology and structures of the crystals were captured by the Agilent Technologies 5500 atomic force microscope. Both height and amplitude images were recorded simultaneously in tapping mode with a silicon cantilever having a spring constant of 20− 30 N/m and a resonating frequency of 320−350 kHz, and the scanning rates varied from 2 to 5 μm/s. Fourier Transform Infrared (FTIR) Spectra. The PCL/ PVPh (70:30 wt %) blends solution was spin-coated on golden substrates for GIR-IR analysis. Grazing incidence reflection infrared (GIR-IR) spectra were measured with a 2 cm−1 resolution using a Spectrum 100 FT-IR spectrometer (PerkinElmer) with a liquid nitrogen cooled mercury− cadmium−telluride (MCT) detector. A total of 32 scans were

Figure 1. Normalized TIR and GIR-IR spectrum of PCL/PVPh (70:30 wt %) blends film crystallized at 25 °C after melting at 90 °C for 5 min.

PVPh/PCL blend film after crystallization at 25 °C in the region 1800−900 cm−1, and the transmission IR spectrum of the blend in bulk (thick film of several micrometers) is also present. The band intensity was normalized in order to compare the band intensity change ratio between the two spectra. By comparing the GIR-IR spectrum with the transmission spectrum, it can be clearly seen that the band shifts to high frequency for several wavenumbers and has an intensity increase of some bands at 1297, 1247, and 1196 cm−1, accompanying an intensity decrease of band at 1727 cm−1. As to the band assignments of PCL in the literature, the bands located at 1727, 1297, 1247, and 1196 cm−1 are the conformational sensitive bands, and relevant to the crystallization of PCL. The band at 1727 and 1747 cm−1 has been assigned to carbonyl stretching and asymmetric COC stretching in PCL crystalline phase, respectively.24 Moreover, the transition moment of vibrators of bands at 1247 cm−1 are considered to be parallel to C−C chain skeleton, whereas that 223


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The Journal of Physical Chemistry B of 1727 cm−1are intersect to the C−C chain axis. Thus, the intensity increase of band at 1247 cm−1 and decrease at 1727 cm−1 in the GIR-IR spectrum may indicate the PCL chain segment perpendicular to the film plane. Indeed, the carbonyl stretching frequency appeared mainly at 1737 cm−1 in the melt, and will shift to low frequency at about 1727 cm−1 during the latter crystalline process of PCL.29−31 Therefore, the chain organization process in the blends thin film can be revealed by following intensity changes of the bands at 1247 and 1737 cm−1. Figure 2 presents the change of band intensity at 1247,

shaped curve, indicating that the PCL chain segments has no orientation at the first 20 min, then they are oriented preferentially along the surface-normal direction rapidly in the following 40 min and keep nearly constant in the end. On the basis of the GIR-IR spectroscopic results, the chain anisotropy of PCL in the blends film can be identified. Optical Microscopy. In order to get the morphology of the blends film directly, the micrographs of the blends films of different compositions crystallized at different temperatures were captured by OM. The morphology present in Figure 3a shows spherulitic organization with irregularly branched radial arms, which should be the dark areas in the micrograph. The outlines of radial arms can be discerned easily due to the loose structure, and the arms are separated by the noncrystallizable materials. The details of the morphology will be illustrated by AFM height image in the latter part and the difference from spherulites can be revealed. This morphology should be called seaweed according to the classification proposed by Brener et al.9 Because of the confusion about the name, this morphology is sometimes called dense branching morphology (DBM).10 The diameter of the seaweed can reach more than several hundred micrometers. From Figure 3b, it can be seen clearly that the seaweed become coarser as they develop. The central part of the seaweed is composed of rather thin growth arms and the growth arms become wider along the radial direction. From Figure 3c, one can note that the morphology changed greatly and a highly branched pattern can be discerned. The seaweed propagates along the radial direction with randomly branched structure, with fine and dense branches in the central part, whereas open and ordered structure appear in the outmost regions. This different morphology from central to outer part looks like a transition from seaweed to dendrites. Consequently, the radial arms can be discerned clearly near the periphery of the seaweed. Figure 4 illustrates the OM micrographs of PCL/PVPh blends films melt-crystallized at 40 °C with different PVPh content. It is apparent that the morphology of the crystals differs greatly from that shown in Figure 3. Different from the irregular branches of the seaweed present in Figure 3, the main trunk and the parallel arranged side branches can be discerned in the crystals shown in Figure 4a. This indicates the present dendrites with obvious geometrical order. The profile of each dendritic structure can be revealed clearly by OM micrographs since the borders are covered by noncrystalline materials. As to the inner part of the dendrite, it seems compact and one cannot distinguish the border area between branches. With higher PVPh content, as show in Figure 4b, the dendrites can be revealed more clearly, which may be attributed to the more open inner structure. In addition, the border area has no great change and the size of the dendrites decreases with PVPh content. The influence of PVPh on PCL crystallization can be even greater, as shown in Figure 4c, and one can see that the dendrites are composed of six main trunks and the side branches in parallel from the main trunks. In addition, large areas of the film are covered by the noncrystalline materials. One may argue that this area may be not covered by PVPh/ PCL blends at all. Indeed, there are some regions of Si wafer that are not covered by PVPh/PCL blends due to dewetting and one can see the exposed substrate surface in Figure 3 and 4. Figure 5 presents the OM micrographs of the blends films melt-crystallized at 45 °C. The branched patterns with apparent geometrical order can be observed clearly. With increasing PVPh content from 10 to 20 wt %, the dendritic pattern, with

Figure 2. GIR-IR absorbance (a) and absorbance ratio (b) of selected bands (1247 and 1737 cm−1) of PCL/PVPh (70:30 wt %) blends film (130 ± 10 nm) as a function of crystalline time.

1737 cm−1and the change of ratio (A1247/A1737) with time during the isothermally crystallization process, respectively. It can be seen that the intensity of band at 1247 cm−1 is nearly constant at the first 18 min, and then changes rapidly during the following 40 min, and ultimately remains nearly constant. An opposite trend can be observed about the intensity change of band at 1737 cm−1 in the crystallization process. The increasing intensity of band at 1247 cm−1, accompanying the decreasing of band at 1737 cm−1 may indicate the PCL chain segments orient along the surface-normal direction. The exact quantitative value of the absorbance ratio of the two bands (A1247/A1737) can be used to estimate the orientation of PCL chain segments in the blends film. One can see that the band intensity ratio between 1247 and 1737 cm−1 also shows an “S” 224


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Figure 3. OM micrographs of PCL/PVPh blends films of different compositions of (a) 90:10, (b) 80:20, and (c) 70:30 wt % crystallized at 35 °C on silicon wafer (scale bar = 25 μm).

Figure 4. OM micrographs of PCL/PVPh blends films of different compositions of (a) 90:10, (b) 80:20, and (c) 70:30 wt % crystallized at 40 °C on silicon wafer. The scale bars in parts a and b represent 50 μm, and that in part c represents 100 μm.

decreased dimension and branches density, can be observed clearly (Figure 5, parts a and b). One may note that the average size of the dendrites decreases with PVPh content, and the border area covered with noncrystalline melt increases, resulting in more open inner structure of the dendrites (Figure 5b).The main trunks and side branches can, thus, be distinguished easily. If PVPh content is 30 wt %, the

morphology of the crystals change even greatly (Figure 5c). At first, one can see several large dendrites spread on the substrate. Most of these dendrites have only six main trunks and are separated by the region scattered with a large number of crystals that cannot be distinguished clearly by OM. Moreover, the dimension of these dendrites reduces greatly 225


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overgrowth anymore in the film thickness of 15 nm and below. Moreover, one can note that with an increase of temperature and PVPh content, the overall shape of the dendrite is approaching the outline of PCL single crystal with hexagonal shape. The OM results may indicate that the crystalline patters of PCL/PVPh blends film may be controlled by the diffusion process of molecular chains. In addition, from the inner structure and the envelope of the crystal revealed by OM, a transition from seaweed to dendrite can be realized by an increase of temperature or PVPh content. When crystallized at 35 °C, the morphology of the blends film is seaweed and shows more open structure with PVPh content. In contrast, when crystallized at higher temperature, 40 and 45 °C, the dendrite with apparent crystallographically favored branches can be observed. Moreover, the branch density and outline of dendrites change with PVPh content. In the literature, polymer crystallization controlled by DLA process can often be observed in ultrathin film of pure polymers, such as, PCl, PEO, isotactic polystyrene (iPS) and so on. By using pseudodewetting technique, for example, a PEO monolayer can be obtained and the fingerlike crystalline patterns of the monolayer were revealed by Reiter.11−13,32 On the basis of the studies on pure PCL by Prud’home et al., when the film thickness is 15 nm or thinner, a dendritic morphology with a one layer lamella can be found.2,3 In the crystallization process, the growth front of crystals will absorb relatively pure material from the melt and the impurities will be rejected simultaneously. If the film thickness is less or equal to lamellae thickness, the transport of the crystalline molecules is restricted and there will be a depletion region in the growth front. Consequently, the instability of the growth front will result in fingered or branched patterns.2 Thus, for pure polymer, the diffusion controlled growth plays role in films as thin as lamellar thickness and the dendrite or seaweed are both composed of one layer flat-on lamellae. In the PVPh/PCL blends case, though the films thickness (130 ± 10 nm) are far more than PCL lamellar thickness, the dendritic patterns of the morphology indicates characteristics of DLA growth process. Thus, it is interesting to disclose the detailed structure of dendrites on lamellae level. Moreover, in the DLA growth process, branch width is an important parameter that determined by the diffusion kinetics. Thus, AFM was employed to reveal more information on the lamellar scale. Atomic Force Microscopy. Figure 6a shows the AFM height image of pure PCL crystallized at 35 °C, in which characteristic spherulites can be captured. The spherulites consist of radiating arrays of fine crystalline fibers. Figure 6b shows the AFM height image of PCL/PVPh (90:10 wt %) blends film crystallized at 35 °C. One can note the difference between the seaweed and spherulite clearly. The radial arms of the seaweed are, in fact, composed of a large quantity of smallsized flat-on crystals in contrast to the radial branched edge-on lamellae in spherulites. On the basis of the results of GIR-IR, these small- sized crystals should be lamellae seen flat-on with oriented PCL chains along the surface-normal direction. This morphology can also be found in pure PCL at film thickness of 30 nm in the literature.2 In addition, the seaweed are divided into several parts by some grooves along radial direction, which may account for the radial arms revealed by OM micrographs. The overgrowth of the seaweed stacks tightly and the border regions can be revealed clearly by the height image.

Figure 5. OM micrographs of PCL/PVPh blends films of different compositions of (a) 90:10, (b) 80:20, and (c) 70:30 wt % crystallized at 45 °C on silicon wafer (scale bar = 100 μm).

compared with that shown in Figure 5, parts a and b. One may also note that these dendrites have nearly hexagonal shape. In the work of Prudʼhomme, the crystalline morphology of PCL in thin film was investigated.2 They found that if crystallized at low supercooling, hexagonal shaped basal lamellae with overgrows are present when film thickness is 30 nm and thicker. The hexagonal shaped lamellae distorted with no 226


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Figure 6. AFM height images of (a) pure PCL and (b) PCL/PVPh (90:10 wt %) blends film crystallized for 45 h at 35 °C (scale bar = 10 μm).

In the dendrites related to DLA growth process, branch width is controlled by chain diffusion and relavent to crystallization temperature in pure polymer case. In the blends case, it can be imaged that the content of the noncrystalline polymers will also influence the chain diffusion kinetics of the crystalline component and result in different characteristic branch width. Therefore, the influence of blends ratio and crystallization temperature on the morphology was revealed by AFM. Figure 7a illustrates the morphology corresponding to the periphery region of the seaweed shown in Figure 3c. One can see that the branches observed by OM are composed of a large amount of overlapped small-sized lamellae. If compared with the morphology shown in Figure 6b, it presents much more the characteristics of dendrites than seaweeds. With an increase of crystallization temperature, as shown in Figure 7b, the average size of the overgrowth increases. Moreover, one may note that the width of the side branch increases obviously with crystallization temperature. At the same time, the dendrites became more open. More close observation can find a depletion region in the growth front of the basal lamellae. At higher crystallization temperature, as shown in Figure 7c, one can see that the branch is also composed of the basal lamellae with the biggest size and the overlapped small-sized lamellae. Moreover, it should be noted that most of the overgrowth has a hexagonal shape. As we know, the outline of the dendrite prepared at this condition has been revealed, by OM, to be nearly hexagonal. The branch width increases

Figure 7. AFM height images (50 × 50 μm2) of PCL/PVPh (70:30 wt %) blends films crystallized at (a) 35, (b) 40, and (c) 45 °C for 45, 50, and 55 h, respectively (scale bar = 10 μm). The white arrow indicates the overgrowth from basal lamella defect.

obviously with crystallization temperature. In addition, the main trunk can be revealed to have much more overgrowth than the side branch, which may account for the darker contrast of them than the side one from OM micrographs. Most of the overgrowth lamellae should be nucleated by screw dislocations; the substrate defects, however, can also contribute to the overgrowth of lamellae (indicated by a white arrow). Figure 8 shows the magnified AFM height image of the PCL/PVPh (70:30 wt %) films crystallized at 35, 40, and 45 °C 227


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large amount of tiny-sized lamellae decorated on the border of basal big sized lamella. The overgrowth of the lamellae is, in main, attributed to the screw dislocation. Figure 9 presents AFM height images of PCL/PVPh films crystallized at 45 °C for 50 h. At lower PVPh content (10 wt

Figure 8. AFM height images (10 × 10 μm2) of PCL/PVPh (70:30 wt %) blends films crystallized at (a) 35, (b) 40, and (c) 45 °C for 45, 50, and 55 h, respectively (scale bar = 1 μm).

for different times. At first, one can see the branches shown in Figure 8a is also composed of overlapped small-sized lamellae. The depletion region between branches can also be revealed from the height image. The average size of the lamellae increases with temperature and the overgrowths of the lamellae through screw dislocation are apparent (Figure 8b). Indeed, the screw dislocation can also be detected, though not very clearly, in Figure 8a. Figure 8c shows the AFM height image of blends film crystallized at 45 °C. The size of the lamellae and branch width increase greatly. In addition, it can be detected that a

Figure 9. AFM height images (50 × 50 μm2) of PCL/PVPh blends films with different blend ratio (a) 90:10, (b) 80:20, and (c) 70:30 wt % crystallized at 45 °C for 50 h (scale bar = 10 μm) .

%), the crystal shows dense structure and it cannot distinguish one branch from another. With an increase of PVPh content, the dendritic structure is apparent, and a large number of side branches are spaced in parallel with the main trunk, indicating the crystallographic order of the dendrite. The size of overgrow 228


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PMMA case, the seaweed to dendrite transition can be realized by increasing the content of the noncrystalline PVPh.

lamellae decreases with top-layers, which may be ascribed to the decreased amount of melt available. The dendritic growth of PCL can be revealed more clearly when the content of PVPh is 30 wt % (Figure 9c). One can see that the size of the basal lamellae is larger than that shown in Figure 9b, and it should be noticed that the overgrowth lamellae has nearly hexagonal shape compared with the round shape present in Figure 9b. From the AFM results, it can be concluded that the dendrites can develop in PCL/PVPh blends film whereas spherulites can grow from pure PCL film of same thickness. Moreover, the main truck and side branches of dendrites are composed of overlapped flat-on lamellae. Thus, the growth of PCL in thin film may be influence greatly by blending with PVPh. Indeed, microstructures of melt-miscible, crystalline/amorphous polymer blends in bulk have been well documented. It has been found that the intermolecular interactions will influence the segregation length of the amorphous component greatly, i.e., segregation of the amorphous component will be dependent on the glass transition temperature in polymers having weak intermolecular interactions, whereas the strong intermolecular interactions resulted in significantly reduced crystal growth rates and promoted diluents segregation over greater length scales.33 For example, it has been established that PCL and PVPh blends are melt-miscible due to intermolecular hydrogen bonds between two components. The PVPh has been revealed to be extralamellar segregation which should be attributed to intermolecular hydrogen bonds and the resulted slow growth rate of PCL.25,26,34,35 So, it may be reasonable that the strong intermolecular interactions also influence the kinetics of chain deposition to the crystal front and result in different morphology in PVPh/PCL thin films from that in the pure PCL case. The branch of the dendrite is composed of stacked lamellae, whereas the dendrite of pure PCL is only in thickness of several nanometers. It can be imaged that in the PVPh/PCL blends case, the basal lamellae will form first and be separated from each other by the impurities. Because of the availability of enough polymer melt, the nucleation and growth on the basal lamellae develop and propagate layer by layer. In addition, in the DLA growth process, the parameter, δ = D/G, can be used to measure the branch width. In the theory of spherulites growth brought forward by Keith and Padden, δ is a measure of the lateral dimensions of the fibers in the spherulites, in which case D is the diffusion coefficient for impurity in the melt and G is the radial growth rate of a spherulite. In the PVPh/PCL blends case, δ can also be associated with the width of dendritic branches. Since the Tg of PVPh is more than 150 °C, the increase of temperature from 35 to 45 °C will not alter the molecular mobility greatly, that is to say, the G is rather constant. On the other hand, the growth rate of PCL will decrease greatly with a decrease of crystallization temperature. Consequently, the branch width will decrease with temperature. The same trend can be found in the blends films containing different PVPh content, i.e., the branch width increases with PVPh content. The decrease of PCL growth rate with PVPh content can account for the increase of branch width. The transition from seaweed to dendrite in PCL/PVPh blend case can take place by an increase of temperature. This result is in agreement with the morphology diagram proposed by Brener, in which they proposed that morphology transition can be tuned by varying the supercooling, the anisotropy or by varying them together.9 This transition can often be observed in pure polymers and PEO/PMMA blends case.20,36 Just as in PEO/

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CONCLUSIONS The morphology of PVPh/PCL blend thin films has been revealed to present seaweed and dendrite patterns with PCL chain segments along the surface-normal direction, and this differs greatly from the spherulites in pure PCL in the same film thickness. This morphology should be controlled by the DLA growth process in the blends and the branch width can be found to be dependent on crystallization temperature and PVPh concentration. The strong intermolecular hydrogen bonding may retard the diffusion of PCL chain segment to the growth front and result in a decrease of growth rate of crystals. Consequently, according to the diffusion length determined by δ = D/G, the branch width will increase with crystallization and PVPh content. Moreover, the main trunk and side branches of the dendrite are revealed to be composed of overlapped flat-on lamellae, with less size than the basal lamellae.

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AUTHOR INFORMATION

Corresponding Authors

*(H.L.) E-mail: [email protected]. *(S.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of NSFC (Nos. 21174014, 21374007, 51221002, and 21434002) and New Century Excellent Talents in University (NCET-13-0648) is gratefully acknowledged.

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