Tuning Radial Lamellar Packing and Orientation into Diverse Ring

Feb 17, 2014 - ... into Diverse Ring-Banded Spherulites: Effects of Structural Feature and ... of tuning radial lamellar organization via controlled e...
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Tuning Radial Lamellar Packing and Orientation into Diverse RingBanded Spherulites: Effects of Structural Feature and Crystallization Condition Yiguo Li,†,‡,∥ Haiying Huang,† Zongbao Wang,*,§ and Tianbai He*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, China § Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China ∥ Key Laboratory of Marine New Materials and Related Technology, Zhejiang Key Laboratory of Marine Materials and Protection Technology, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China S Supporting Information *

ABSTRACT: Spherulite morphologies of a highly asymmetrical double crystallizable poly(ε-caprolactone-b-ethylene oxide) diblock copolymer in solution-cast films were explored from a unified standpoint of tuning radial lamellar organization via controlled evaporation. Besides Maltese cross spherulites, three kinds of ring-banded spherulites that display non- and half-birefringent concentric ringed features as well as extinction banding were first encountered in the same polymer. Structural analyses based on atomic force microscopy, transmission electron microscopy, and grazing incidence X-ray technique revealed that concentric ringed spherulites possess the nature of a rhythmic variation of the radial lamellar packing and extinction banded spherulites have the origin of a periodic change of the radial lamellar orientation. Morphological transitions among different kinds of spherulites were achieved by altering the drying condition. PEO segment crystallized at low temperatures even if it is being confined by PCL lamellae. Combined with poly(ε-caprolactone) and poly(ethylene adipate), the influences of structural feature and crystallization condition on radial lamellar organization of polymer spherulites were discussed. These present findings are encouraged to enhance our understanding and then governing generation of expected polymer crystal morphology for special material performance.

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INTRODUCTION Exploring crystal morphologies of semicrystalline polymers is of both scientific and technological interest and importance because of their dramatic impact on material performance.1−4 Spherulites, as the most common morphology, can be encountered when polymers crystallized from disordered melt or concentrated solution. Specially, ring-banded spherulites have received extensive attention due to their attractive feature. The widely accepted explanation for the appearance of extinction banding in polarized light is the periodic twisting of lamellae along the radial direction of spherulites.5−8 Lotz and Cheng extensively analyzed the cause(s) of lamellar twisting in a comprehensive review, and they believed that unbalanced surface stresses from structural features are the mechanical origin for lamellar twisting.9 Yet, in thin films, in addition to extinction banding, concentric rings that possess sharp contrast in both polarized and unpolarized light,10,11 or even appear directly opposite optical features with extinction banding, i.e., poor contrast under polarized light and clear appearance in unpolarized light,12−15 were also observed in two-dimensional polymer spherulites. Actually, these concentric rings are a © 2014 American Chemical Society

reflection of periodic change of thickness stemming from rhythmic-crystallization-induced structural discontinuity. Ringbanded spherulites with the discrete structure also occurred in some polymer blends.16−18 Despite extensive explorations over the past six decades, the structure and then formation mechanism of ring-banded spherulites remain controversial.19−23 It is now recognized that flexible polymer crystalline aggregates possess the unique structural feature of being composed of crystalline lamellae that are arranged and organized together.24−26 For polymer spherulites, a large amount of lamellae develop simultaneously along the radial direction, implying that their morphological features are mainly determined by radial lamellar organization, more accurately orientation and packing. Therefore, one can distinguish two distinctly different types of ring-banded spherulites. In the first case so-called classical banded spherulite, the extinction banding originates from a periodic Received: December 17, 2013 Revised: January 24, 2014 Published: February 17, 2014 1783

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logical transitions among various spherulites and the crystallization of PEO block were also analyzed. Finally, we discussed the effects of structural feature and crystallization condition on radial lamellar organization of polymer spherulites.

change of radial lamellar orientation. In the second case we named nonclassical ringed spherulite, in which concentric rings with a rhythmic variation of thickness can be in fact considered as a reflection of discrete radial lamellar packing. The former has been observed in pure polymers, polymer blends, and copolymers, but the latter has only occurred in thin films of limited polymers. Clearly, ring-banded spherulites resulting from the structural discontinuity in some polymer blends can also be assigned to discrete radial packing of the crystalline specie. Unexpectedly, hitherto the two kinds of ring-banded spherulites are not reported in the same polymer. Concomitantly, the effects of structural feature and crystallization condition on radial lamellar packing and orientation and then formation of different ring-banded spherulites are quite unclear. Theoretically, by combining and coupling of changes in radial lamellar packing and orientation, even lamellae grow along the invariant axis direction, one can encounter six kinds of spherulite morphologies in thin films of the same polymer, among them four types can appear ring-banded features, as depicted in Figure 1. Moreover, change of growth axis can lead

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EXPERIMENTAL SECTION

Materials and Sample Preparation. A highly asymmetric double crystallizable poly(ε-caprolactone-b-ethylene oxide) diblock copolymer (P(CL24.5KEO5.0K)) with number-average molecular weights of 24.5 kg mol−1 (MnPCL) and 5.0 kg mol−1 (MnPEO) (ωPEO = 0.17) and a PDI of 1.3 was purchased from Polymer Source, Inc. and used as received. PCL10.0K, PCL42.5K, PCL84.4K, and PEA10.0K were also employed for comparison. The subscript in abbreviation of each polymer indicates its molecular weight. Spherulite morphologies of P(CL24.5KEO5.0K) were modulated by the controllable-evaporation-induced crystallization method. 10 μL of P(CL24.5KEO5.0K)/toluene solution was cast onto silicon wafers placed on a stage lodged inside a container. An average solvent evaporation rate, Re, of 1.50 × 10−4 mL h−1 was gained at 20 °C in a cylindrical container with radius and height of 1.0 and 2.5 cm when covered the lid and added 200 μL solvent. Under this environment, non- and halfbirefringent concentric ringed spherulites (NBCRS and HBCRS) were produced from 5 and 10 mg mL−1 solution, respectively. In-situ experiments were conducted using another volumetric flask (radius and height of 2.0 and 2.5 cm) that was covered with a piece of glass immediately after dropping, which led to an increase of Re to ca. 3.70 × 10−4 mL h−1. Both NBCRS and HBCRS appeared again. Classical extinction banded spherulites (CEBS) were formed at 0 °C from 10 mg mL−1 solution with Re of 4.68 × 10−3 mL h−1 that was achieved by drying solution-cast films inside the volumetric flask (radius and height of 1.0 and 2.5 cm) covered with its lid. Maltese cross spherulites (MCS) occurred with free evaporation at both 0 and 20 °C. Equipment. Polarized optical microscopy (POM) observations were conducted on a Carl Zeiss A2m microscope equipped with a CCD camera under reflectance mode, and a λ compensator was applied to detect the birefringence of various spherulites. Atomic force microscope (AFM) (PICOSCAN SPM, Molecular Imaging Inc., now Agilent 5500AFM/SPM System (Agilent Technologies)) was employed to examine surface morphologies with tapping mode using silicon cantilevers (Nanosensors, PPP-NCL). Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) experiments were performed upon a JEOL JEM-1011 working at an accelerating voltage of 100 kV. Grazing incidence wide-angle X-ray diffraction (GIWAXD) data were collected by the D8 Discovery X-ray diffractometer (Bruker, Germany) (λ = 1.54 Å) and synchrotron radiation at BL16B1 (λ = 1.24 Å) at Shanghai Synchrotron Radiation Facility (SSRF).

Figure 1. Schematic representation of possible cross-sectional radial lamellar organization for polymer spherulites formed in thin films. The change of radial growth axis may lead to more spherulite morphologies.

to more spherulite morphologies,27 and alteration of crystalline subjects, like polymer blends, results in more complex ringbanded spherulite patterns.26 It is expected that diverse ringbanded spherulites can be produced in the same polymer under favorable conditions. Recently, nonbirefringent concentric ringed spherulite with invariant orientation and half-birefringent one with varied orientation but both with discrete radial lamellar packing were occurred in poly(ε-caprolactone) (PCL) solution-cast films,28,29 and we proved the important role of evaporation rate on radial lamellar packing.30,31 By choosing a appropriate poly(ethylene adipate) (PEA), which lamellae are more easily twisted from its structural feature, a nested ringbanded spherulite was obtained.32 In the present work, we further explored the spherulite morphologies of a highly asymmetrical poly(ε-caprolactone-bethylene oxide) diblock copolymer from a standpoint of tuning lamellar organization by controlling evaporation of solutioncast films in an attempt to unveil possible factors that govern radial lamellar packing and orientation and then appearance of different ring-banded patterns in polymer thin films. Block copolymer was selected for the excluded second block during crystallization process can enhance the twisting of lamellar crystals.33,34 We first systematically investigated the microstructure of spherulites developed at varied conditions. To our knowledge, it is the first time that three kinds of ring-banded spherulites were encountered in the same polymer. Morpho-

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RESULTS AND DISCUSSION Bulk Behavior of P(CL24.5KEO5.0K). PCL and PEO have the near crystallization temperature (Tc) and melting point (Tm), while both Tc and Tm of P(CLEO) diblock copolymers depend on relative content. Figure 2 shows DSC cooling and subsequent heating scans performed at 5 °C min−1 for P(CL24.5KEO5.0K). It has been reported that PEO block cannot crystallize at room condition when its weight fraction is lower than 20% and that the major component crystallizes first.35 Hence, the Tc and Tm for PCL segment in P(CL24.5KEO5.0K) are 38.7 and 57.4 °C, and those of PEO block are −5.3 and 42.0 °C, respectively. However, the crystallization of P(CLEO) diblock copolymers from solution is expected to be different, for which is determined not only by the relative composition but also by their solubility in the used solvent. Emergence of Three P(CL24.5KEO5.0K) Ring-Banded Spherulites. With the change of drying conditions, three ring-banded spherulites were encountered in P(CL24.5KEO5.0K)/ toluene solution-cast films (Figure 3). Differences in optical 1784

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added to further identify birefringence of the three ring-banded spherulites (Figure 3d−f). NBCRS showing strong and weak pink rings again supports its zero birefringence. HBCBS and CEBS both exhibit an alternative zero and negative birefringent characters. Furthermore, the latter two are negative spherulites due to the characteristic interference colors in different quadrants.36 Figure 4 exhibits AFM topographies and corresponding height profiles of the three P(CL24.5KEO5.0K) ring-banded spherulites. NBCRS and HBCRS appear similar features. First, they both display an evident periodic pattern in topography (Figure 4a,b), and ring periodicities are ca. 6.5 and 23.5 μm, respectively. Concomitantly, both height profiles reflect a rhythmic variation of thicknesses along the radial direction, and height differences are ca. 200 and 1000 nm, respectively, close to the film thickness. The other striking feature is that the thickness increases slowly toward ridge but reduces sharply into valley. These features indicate that the two concentric rings derive from rhythmic crystallization leading to the discrete radial lamellar packing. A comparison of the height profile and POM image in Figures 4a and 3a of NBCRS with those of HBCRS (Figures 4b and 3b) implies that the distinctions in periodicity and thickness seem to be the reason for the generation of birefringence in ridges of HBCRS. In contrast to concentric ringed spherulites, the banding in CEBS is quite irregular and hard to distinguish (Figure 4c). Although there is height fluctuation, the amplitudes are in the range of 300−400 nm, much smaller than the film thickness. In other words, the radial lamellar packing in CEBS should be continued, meaning that the periodic banded pattern originates from the change of radial lamellar orientation. Lamellar Organization in P(CL24.5KEO5.0K) Ring-Banded Spherulites. Differences in morphological features are indicative of varied lamellar organization. Detailed microstructures are further examined by AFM. Figure 5 exhibits enlarged phase pictures of NBCRS. It is apparent that most of lamellae stop to grow in valley, again confirming that the lamellar packing along radial direction is discrete. Meanwhile,

Figure 2. DSC scans of P(CL24.5KEO5.0K) in the first cooling and second heating processes (scan rate: 5 °C min−1).

characteristics and other features of the three ring-banded spherulites are evident from POM photos. As exhibited in Figure 3a, the first kind of spherulite consists of concentric or pseudoconcentric alternative bright and dark rings with equal width but poor contrast and without birefringence, so we can define them as nonbirefringent concentric ringed spherulites (NBCRS). With the exception of Maltese cross, the second regular concentric pattern appearing sharp contrast is composed of alternative unequal birefringent bright and nonbirefringent dark rings (Figure 3b). With respect to NBCRS, we can call them as half-birefringent concentric ringed spherulites (HBCRS). Apparently, the third class of spherulite displays the same optical property as classical extinction banded spherulites (CEBS, Figure 3c). The situation is altered when they were viewed through unpolarized light (Figure S1). Both NBCRS and HBCRS display a clear contrast, while the banding in CEBS becomes poorly observable or even disappears, further illustrating their different optical features. In addition, both NBCRS and HBCRS possess a nearly invariant ring periodicity, and the value is ca. 6 ± 1 and 24 ± 2 μm, respectively, while band spacing of CEBS reduces with radial distance and within range of 10−20 μm. A λ compensator is

Figure 3. POM (a−c) photographs revealing the optical properties of three P(CL24.5KEO5.0K) ring-banded spherulites formed in solution-cast films at varied drying conditions. (d−f) The corresponding images after adding a λ compensator. The former two concentric ringed spherulites were developed under the same Re of 1.50 × 10−4 mL h−1 at 20 °C but from (a) 5 and (b) 10 mg mL−1 solutions, respectively, while the third extinction banded spherulites were emerged upon a rapider Re of 4.68 × 10−3 mL h−1 from 10 mg mL−1 solution at 0 °C. 1785

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Figure 4. AFM topographies and corresponding height profiles along the white arrow of the three P(CL24.5KEO5.0K) ring-banded spherulites (a: NBCRS; b: HBCRS; c: CEBS). The white arrow in Figures 4−8 denotes the radial direction.

Figure 5. Enlarged AFM phase microphotographs depicting the discrete radial lamellar packing and nearly uniform flat-on surface orientation in NBCRS.

Figure 6. Enlarged AFM phase images revealing the discrete lamellar packing and gradually changed surface orientation along the radial direction in HBCRS. Selected-area pictures (b, c, d, e, and f) are indicated by the black, white, green, red, and yellow squares in (a), respectively.

packing and valleys consist of flat-on lamellae resembling NBCRS, as can be seen from the lower-left corner of Figure 6a and upper-right in Figure 6f. But orientation in ridge is quite distinct. Major lamellae in Figure 6b (located with black square in Figure 6a) are flat-on. With the extending of ridge, lamellae become slightly and subsequently seriously tilted (Figure 6c).

all lamellae on surfaces of both valley and ridge are arranged as flat-on or pseudo flat-on orientation. Change of surface lamellar organization in HBCRS, relative to NBCRS, is expected. A phase image of one period HBCRS is presented in Figure 6a, and five selected areas along the radius are given in Figure 6b−f. HBCRS has discrete radial lamellar 1786

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Figure 7. Enlarged AFM pictures reflecting the continuous twisted lamellar orientation along the radius of CEBS.

This feature is accentuated with increasing thickness (Figure 6d). Eventually, stacks of edge-on lamellae are observed (Figure 6e,f). Again, the growth of edge-on lamellae on ridge stops abruptly and then quickly changes to evolution of valley with flat-on lamellae. It is now convincing concluded that concentric ringed patterns in both NBCRS and HBCRS are a reflection of rhythmic variation of thickness that is attributed to rhythmic crystallization leading to the discrete packing of lamellae along the radius of spherulites. To proceed further in analysis and for comparison, surface morphology of CEBS is also detected. As illustrated in Figure 7, the periodic change of surface orientation is discernible. With careful inspection, it is evident that lamellar orientations shift from flat-on in the concave region, through tilt and eventually to edge-on on the convex surface (Figure 7d). Enlarged images allow for more distinct discrimination that the concave zone consists of stacks of flat-on lamellae (Figure 7b,c). Meanwhile, Figure 7e further depicts the transition of lamellar orientations from tilt, through edge-on, then again to tilt in the convex area. Edge-on lamellae in the center region of convex surface are observable in a more magnified micrograph (Figure 7f). These results fully confirmed that the appearance of extinction banding in CEBS derives from the twisted orientation of lamellae along the radial growth direction of spherulites. AFM observations emphasize surface organization that unveils origins of the three periodic patterns, but the crystallographic orientation and inner organization remain unclear. Figure 8 shows TEM and SAED patterns of NBCRS and CEBS (obtained from 5 mg mL−1 solution). All diffractions can be indexed to the orthorhombic structure of PCL.37 Strong (hk0) diffractions in SAED patterns of both valley and ridge regions denote that in NBCRS the molecular chains (c-axis) in whole film are parallel to the film normal with a- and b-axes in the film plane (Figure 8a). (200) and (020) diffractions coincide with the tangential and radial directions, implicating that the crystallographic a- and b-axes correspond to the tangential and radial directions, respectively. Owing to the large thickness in ridges of HBCRS, SAED is difficult to use to analyze chain orientation, but the valley exhibits the similar SAED pattern with NBCRS (not shown), illustrating that the radius in HBCRS is also along the b-axis. In CEBS, concave zones display typical (hk0) diffractions with (200) in tangent,

Figure 8. TEM bright images and SAED patterns of (a) NBCRS and (b) CEBS in P(CL24.5KEO5.0K) solution-cast films. The white circles denote the lower surfaces (the valley of NBCRS and the concave of CEBS).

verifying the tangent and radius of a- and b-axes, respectively. Namely, the three P(CL24.5KEO5.0K) ring-banded spherulites share the same radial direction of the crystallographic b-axis of PCL, identical with that of PCL Maltese cross spherulites (MCS).24 GIWAXD is a powerful tool to explore the molecular and lamellar orientations in thin films. The X-ray penetration depth increases with the incident angle, allowing for determination of orientations within different thickness.38 Dependences of lamellar orientation on incident angle in three P(CL24.5KEO5.0K) ring-banded spherulites are elucidated by out-of-plane GIWAXD (Figure 9). The invariant appearance of (002) and (004) diffractions in all incident angles fully convinces that NBCRS is composed of discrete stakes of flat-on lamellae (Figure 9a). The emergence of (004), (102) and (201), and (200) diffractions implicates that flat-on, tilted, and edge-on lamellae are all encountered in HBCRS (Figure 9b). As incident angle is enlarged, the relative strength of (004) decreases and that of (200) increases, proving that the amount of flat-on lamellae increases and that of edge-on lamellae reduces toward substrate. Namely, NBCRS is composed of discrete stakes of random oriented lamellae. Lamellar orientation in CEBS is seem to be independent of thickness due to rather poorly distinguishable diffractions and the nearly constant relative strength in all incident angles (Figure 9c). It should be mentioned that the characteristic diffraction peak of 1787

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Figure 9. Out-of-plane GIWAXD data of (a) NBCRS, (b) HBCRS, and (c) CEBS at varied incident angles.

0 and 20 °C, respectively, prove the crystallization of PEO stem at low temperature (Figure 9). Specially, after being cooled, characteristic diffractions of PEO also appear in NBCRS (Figure S3). This means that PEO chains can crystallize even if it is being confined by PCL lamellae. Similar behavior was also reported in P(CL24.0KEO5.8K).41 Furthermore, under low humidity with the same drying environment as that of Figure 3b, PEO segment can even dominate P(CL24.5KEO5.0K) crystallization, which leads to continued dendritic crystals rather than concentric ringed spherulites (Figure 11a). It is

PEO crystals is also present in CEBS (the point is discussed subsequently).39 Tunable Transition among P(CL24.5KEO5.0K) Spherulites. Based on above analyses, it is evident that as crystallization condition is altered, a sharp change in lamellar organization along the radial direction of P(CL24.5KEO5.0K) spherulites takes place, which leads to the emergence of three ring-banded structures. By further controlling condition, morphological transitions are tunable. First, the change from NBCRS to HBCRS relies on film thickness, which can be proved by slanting substrate that results in a gradient film. As illustrated in Figure 10a,b, as the film is thickened, the

Figure 11. OM (a) and POM (b) pictures of P(CL24.5KEO5.0K) crystals formed by drying a solution-cast film at a low humidity (ca. 15%). The drying environments for (a) and (b) are the same as that of Figures 3b and 3c, respectively.

Figure 10. OM (a) and POM (b−d) micrographs depicting morphological transitions of P(CL24.5KEO5.0K) spherulites: (a, b) NBCRS → HBCRS, (c) HBCRS → MCS, and (d) CEBS → MCS. (e) Schematic illustration of radial lamellar organization for different spherulites and the corresponding transition conditions.

apparent from Figure S4 that although PCL often nucleate first, the evolution of PEO-dominated crystals wraps around central spherulites due to its quite slower nucleation but much faster growth.42 Likewise, for case of CEBS, similar morphological behavior also takes place at low humidity, as demonstrated in Figure 11b and Figure S5. The generation of PEO-dominated crystals should be attributed to the enough chain length of PEO for crystallization occurring and the decrease of its solubility in toluene with reducing humidity. This unusual phenomenon illustrates powerfully that both kinds of P(CL24.5KEO5.0K) ringbanded spherulites merely appear when the PCL segment dominates the crystallization process. Effect of Structural Feature and Crystallization Condition on Radial Lamellar Organization. To our knowledge, it is the first time that two fundamentally different kinds of ring-banded spherulites that derive from the periodic change of radial lamellar packing and orientation respectively are observed in the same polymer. The morphology of any crystals is determined not only by the molecular structure but also by the crystallization condition and the process of that growth. Yet, key factors that determine the radial lamellar organization and then formation of various ring-banded spherulites remain obscure. In our whole research, three polymers and five samples, PCL10.0K, PCL42.5K, PCL84.4K, P(CL24.5KEO5.0K), and PEA10.0K, were chosen to explore the impact of structural feature, and varied drying environments and temperatures were employed to examine the influence of crystallization condition. PCL is

nonbirefringence becomes poorly evolved. Instead, gradually enhanced birefringent ringed patterns develop. Second, once exposing the growing HBCRS from the confined evaporation to free drying, the rhythmic growth no longer happens, and Maltese cross spherulite (MCS) replaces, implying the occurrence of the transition in radial lamellar packing from discrete to continuous (Figure 10c). Lowing temperature to 0 °C, MCS again form (Figure S2). Finally, by drying a solutioncast film inside volumetric flask, the random radial orientation in MCS becomes periodic twisted, and thus CEBS appears. Since it is difficult to modulate evaporation rate from low to high, we display the opposite process, i.e., transformation of CEBS to MCS (Figure 10d), which can be achieved by opening the covered lid. Radial lamellar organizations of above four types of spherulites and corresponding transition conditions are schematic illustrated in Figure 10e. Crystallization of PEO Block in P(CL24.5K EO5.0K). Essentially, in P(CL24.5KEO5.0K), both NBCRS and HBCRS possess perfectly same morphological characters as those of PCL10.0K,28−30 and CEBS exhibits similar features with that of PCL84.4K.40 It is reasonable that PCL block dominates the P(CL24.5KEO5.0K) crystallization at above conditions. The presence and absence of (120) diffraction of PEO crystals at 1788

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Figure 12. In-situ OM images revealing the evolution processes of P(CL24.5KEO5.0K) concentric ringed spherulites formed from solution-casting films with varied concentrations of (a) 5, (b) 10, and (c) 20 mg mL−1 at Re of 3.70 × 10−4 mL h−1. The average radial growth rates and ring periodicities are indicated in the upper right corner of each picture.

imposing on the twisting of P(CL24.5KEO5.0K) lamellae, for in which CEBS is accessible at quite limited conditions. This issue reminds us to approach the impact of PEO segment in the development of different P(CL24.5KEO5.0K) spherulites. As mentioned above, PEO stems crystallize first at low humidity, which gives rise to disappearance of both concentric ringed spherulites and extinction bands (Figure 11). Moreover, the introduction of PEO suppresses growth rate of concentric ringed spherulites. Under the identical evaporating circumstance, the average radial rates for cases in 5, 10, and 20 mg mL−1 P(CL24.5KEO5.0K) solutions are 107, 66, and 30 nm s−1 (Figure 12), while those of PCL10.0K are 266, 181, and 65 nm s−1, respectively.30 Although the enhancement in viscosity with increasing PCL length should also be responsible for the decrease in spherulite growth, the evolution rate for 10 mg mL−1 PCL42.5K solution dried at the unvaried conditions is measured to be of 85 nm s−1,31 supporting the depressing effect of PEO stems imposed on P(CL24.5KEO5.0K) crystallization. This point was also reported in development of diblock P(CLEO) spherulites from melt crystallization.45 Specifically, PCL10.0K and P(CL24.5KEO5.0K) concentric ringed spherulites produced at the same condition possess similar growth process and approximate ring periodicity (Figure S7), reflecting a slight impact of PEO on lamellar packing. Owing to the inconsistent formation conditions for PCL and P(CL24.5KEO5.0K) extinction banded spherulites and the coupling effect of increasing molecular weight and introducing of PEO, the influence of PEO block on lamellar twisting is difficult to quantitatively analyze. Meanwhile, rapid drying causes the coincidental crystallization of PCL and PEO segments, which results in formation of irregular spherulites differing from both cases of PCL and PEO homopolymer (Figure S8). However, at the same drying condition, tilt and even edge-on lamellae were formed in P(CL24.5KEO5.0K), while quite uniform flat-on oriented lamellae were developed in both PCL10.0K and PCL42.5K, as revealed in Figure S7. Hence, it is the introduction of PEO block rather than the increase of molecular weight that has the dominate role to induce the change in lamellar orientation of P(CL24.5KEO5.0K). Essentially, discrete packing of radial lamellae results from rhythmic crystallization that is driven by material depletion. Material deficit originates from the long-range transport of crystalline species during crystallization process. Herein, concentric ringed spherulites can only evolve in low molecular weight PCL10.0K, P(CL24.5KEO5.0K), and PEA10.0K upon slow evaporation, and increase in both drying rate and PCL molecular weight results in compact spherulites with continued lamellar packing. Low molecular weight affords the good mobility of crystalline medium, and slow drying provides enough time for occurrence of the long-range transport. It has certified that suitable time scale and good medium mobility are

known to only crystallize in orthorhombic structure, and no chain tilt has been reported.37 The disorder tends to increase with molecular weight.6 Introduction of second block can further accentuate uneven folded surfaces.33,34 In PEA monoclinic unit cell the molecular stem is oblique to the lamellar normal, and double-banded spherulites with very small banding spacing are easily observed.43,44 The disorder and asymmetry on opposite lamellar surfaces are therefore expected to enhance gradually from PCL to P(CLEO) and then to PEA. Changes of dying environment and temperature have a strong impact on solvent evaporation rate that determines the supersaturation of solution-cast films, then the nucleation and growth rate, and eventually polymer spherulite morphology. Consequently, concentric ringed spherulites with a rhythmic variation of the radial lamellar packing were encountered in PCL10.0K,28−30 P(CL24.5KEO5.0K), and PEA10.0K,32 and extinction banded spherulites with periodically twisted lamellae along the radial growth direction were observed in PCL42.5K and PCL84.4K,40 P(CL24.5KEO5.0K), and PEA10.0K.32 It is first necessary to consider the effect of solvent impinging on structural evolution of P(CL 24.5K EO 5.0K ) spherulite morphologies. Resembling that of PCL10.0K concentric ringed spherulites with discretely packed lamellae,30 the periodic pattern can only occur upon slow evaporation, and ring periodicity increases with solution concentration (Figure 12). Again, evaporation-driven flow becomes a common occurrence in the growing spherulite front (Figure 12), and spherulite growth follows a nonlinear manner, as illustrated in Figure S6. Specially, a comparison of Figure 3a,b or 4a,b with Figure 12a,b demonstrates dramatically that a 2.5 times increase in average Re from 1.50 × 10−4 to 3.70 × 10−4 mL h−1 reduces nearly onethird of the average ring periodicity (case a: from 6.5 to 4.2 μm; case b: from 23.5 to 16.5 μm). The faster evaporation means the smaller solvent partial pressure, so ring periodicity enlarges with decreasing solvent partial pressure, which can be attributed to the coupling of extending time scale and decreasing viscosity because of the slower solvent withdrawing and lower supersaturation. This point has been convincingly supported by the fact in PCL10.0K that as the solvent evaporation gradually enhances, ring spacing becomes continually reduced and eventually the periodic feature disappears.31 For CEBS, it has been noted that in PCL84.4K the uneven solvent extraction upon opposite lamellar surfaces can strongly enhance twisting frequency, relative to melt crystallization, but band spacing has a slight dependence on Re. For example, a ca. 26 times increase of Re from 3.7 × 10−4 to 9.7 × 10−3 mL h−1, and concomitantly a 28 times acceleration in growth rate from 650 to 22 nm s−1, merely halves band spacing.40 Comparatively speaking, solvent and its escape have a more profound on radial lamellar packing rather than lamellar twisting. Unfortunately, there is not enough data to uncover the role of solvent 1789

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Figure 13. POM pictures showing effect of initial solution concentration on PEA10.0K nested ring-banded spherulites. The insets are enlarged inner double-banded patterns. Concentration: (a) 5, (b) 10, and (c) 20 mg mL−1.

aliphatic polyesters of ethylene glycol that crystallizing in orthorhombic structure do not exhibit extinction banding and that crystallizing in monoclinic structure all can show similar banded spherulites. Third, extinction banded spherulites occur in only miscible polymer blends, and the band spacing reduces with increasing amorphous content,24 for the excluding of amorphous blend can enhance the disorder. Fourth, extinction banded spherulites easily happen in block copolymer. Likewise, increase of the second block ratio leads to the decrease in twisting frequency.34 In other words, structural feature is the primary factor for lamellar twisting, and the crystallization condition that can intensify the disorder of folded surfaces drives the increase in twisting frequency, namely the decrease of band spacing. The above analyses demonstrate the dominate role of structural feature on radial lamellar twisting and the significant influence of crystallization condition on radial lamellar packing of spherulites, respectively. Meanwhile, the structural feature here, mainly molecular weight, holds an important impact on chain mobility that affect the emergence of long-range material transport and then discrete lamellar packing, and the crystallization condition also has a secondary effect on lamellar twisting frequency. This is further proved by the effect of initial solution concentration having on PEA nested ring-banded spherulite (Figure 13). As the concentration is enlarged, ring periodicity of discrete lamellar packing increases, while band spacing of twisting holds constant. So far the expected six kinds of spherulites are all obtained (Figure 14).

the two key factors to trigger the occurrence of long-range motion30 and that a gradually enhanced evaporation lead to a continued decrease in ring periodicity and finally the discreteto-continuous transition of radial lamellar packing.31 In solution, the existence of solvent first enhances the mobility of crystalline subject and then enlarges the volume contraction after crystallization, benefiting for rhythmic crystal growth. For melt, excepting special cases of isotactic polystyrene (iPS) and poly(bisphenol A hexane ether) that have extraordinarily slow crystal growth rate (1.7 nm s−1 in iPS), discrete lamellar packing caused ringed spherulites are seldom happened in homopolymer.13−15 Solvent annealing also intensify the polymer mobility that gives rise to concentric ringed spherulites of low molecular weight tetraaniline-b-poly( L-lactide).46 Recently, both NBCRS and HBCRS were also reported in thin films of poly(L-lactic acid) (PLLA11.0K).11 Addition of the second component further enlarges the martial deficit, so that rhythmic crystallization appears to take place more easily in polymer blends.16−18 On the other hand, it is generally accepted that lamellar twisting has the origin of unbalanced stresses on opposite lamellar surfaces. CEBS cannot form in PCL10.0K, whereas it can appear in PCL42.5K, PCL84.4K, P(CL24.5KEO5.0K), and PEA10.0K. In the case of PCL, it is suggest that the disposition of ester groups already be staggered within unit cell, which is expected to cause slight unbalanced stresses that lead to emergence of irregular extinction banding with large spacing in the range of 50−100 μm.24,37 For solution-casting films, combination of the increase in both chain length and growth rate and uneven solvent withdrawing on opposite lamellar surfaces strong enhances the unequal stresses that drive occurrence of smallspaced extinction bands in high molecular weight PCL42.5K and PCL84.4K.40 When PCL segment crystallizes first, the excluding and then dangling PEO chains significantly intensify the unequal stresses that induce the easy twisting of lamellae in low molecular weight P(CL24.5KEO5.0K). Similar behavior was also elucidated in PS-b-PLLA diblock copolymer.33 Actually, asymmetric structural factors appear to be a permanent presence in polymers that can form extinction banded spherulites with twisted lamellae, and the conditions that can enlarge surface disorders increase the twisting frequency.6,9 First, despite absence of fixed relationships between the molecular chirality and twist handedness, which itself imposes an uneven feature upon crystalline polymer lamellae.9 Second, chain tilt is a factor in all cases involving extinction banding with small spacing of achiral homopolymer.6 For orthorhombic polyethylene5 and poly(α-vinylidene fluoride),47 molecular stems are oblique to the lamellar normal in banded spherulites, while in unit cell of low symmetry such chain tilt is mandated even if fold surfaces have indices (001).6 Typical examples are

Figure 14. POM images of the six kinds of spherulites expected in Figure 1 and their major morphological features. 1790

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ABBREVIATIONS MCS, Maltese cross spherulite; NBCRS, nonbirefringent concentric ringed spherulite; HBCRS, half-birefringent concentric ringed spherulite; CEBS, classical extinction banded spherulite.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S8: OM images of NBCRS, HBCRS, and CEBS; POM pictures of MCS observed from PCL10.0K and P(CL24.5KEO5.0K); 2D GIWAXD pattern of P(CL24.5KEO5.0K) NBCRS after cooling; OM photos of growth process of P(CL24.5KEO5.0K) at low humidity; POM morphologies of P(CL24.5KEO5.0K) and PEO10.0K formed at low humidity under 0 °C; nonlinear growth of P(CL24.5KEO5.0K) concentric ringed spherulite; comparison of PCL10.0K; P(CL24.5KEO5.0K) and PCL42.5K morphologies formed under the same confined condition; POM images of P(CL24.5KEO5.0K) evolved upon faster evaporation at 0 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21004073, 21104079, and 21274148) and China Postdoctoral Science Foundation (2013M541801). The authors are grateful for experimental support to this work by Shanghai Synchrotron Radiation Facility (SSRF).

In summary, we investigated spherulite morphologies of a highly asymmetrical double crystallizable P(CL24.5KEO5.0K) in solution-cast films from a unified standpoint of tuning radial lamellar organization by controlling solvent evaporation. With the change of radial lamellar organization, MCS and three kinds of ring-banded spherulite, NBCRS, HBCRS, and CEBS, were encountered in the same polymer. MCS has continued lamellar packing and random orientation, NBCRS is composed of discrete stacks of flat-on lamellae, HBCRS possesses the nature of discrete packing of random oriented lamellae, CEBS exhibits the origin of continued twisting of lamellar orientation, and the four classes of spherulites share the same radial growth direction of the crystallographic b-axis of PCL. PCL block dominated the crystallization of P(CL24.5KEO5.0K) when spherulites developed, and PEO stem crystallized at low temperature even it has been confined by PCL lamellae. While at low humidity PEO-dominated dendritic crystals were observed. Tunable morphological transitions from NBCRS to HBCRS, then to MCS, and eventually to CEBS were achieved by varying drying condition. Combining PCL, PEA with relevant literatures demonstrated dramatically the dominate role of structural feature on lamellar twisting and the significant influence of crystallization condition on lamellar packing along the radial direction of spherulites. Meanwhile, molecular weight holds an important impact on chain mobility that affect the occurrence of long-range material transport and then discrete lamellar packing, and crystallization condition also has a secondary effect on lamellar twisting frequency. To our knowledge, it is the first time that two fundamentally different kinds of ring-banded spherulites that derive from the periodic change of radial lamellar packing and orientation respectively are observed in the same polymer. These presented findings enhance our understanding for the formation of different ringbanded spherulites of polymers and also provide a unique perspective to modulate generation of the fixed morphology for expected material performance.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Z.W.). *E-mail [email protected] (T.H.). Notes

The authors declare no competing financial interest. 1791

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