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Manipulating poly(lactic acid) surface morphology by solvent-induced crystallization
Applied Surface Science 261 (2012) 528–535
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Manipulating poly(lactic acid) surface morphology by solvent-induced crystallization Jian Gao, Lingyan Duan, Guanghui Yang, Qin Zhang, Mingbo Yang, Qiang Fu ∗ College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 13 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: Poly(lactic acid) Solvent-induced crystallization Morphology Surface structure
a b s t r a c t Here, we report some unique crystalline morphologies of poly(lactic acid) (PLA) via organic solventinduced crystallization. It was revealed that the surface morphology of PLA can be fine tuned by simply varying the volume ratio of a mixed solvent (acetone/ethanol). By increasing the ethanol content in the mixed solvent, we observed a morphological evolution of PLA surface from spherulite to shish–kebab and bamboo-cage-like structure. It was also interesting to find that the initial surface structure of PLA plays an important role to determine the final solvent-induced crystalline morphology. This work provides a new method for manipulating PLA crystal morphology through a simple solvent-induced crystallization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Morphological control of polymer crystallization has always been an important and fundamental research topic in polymer physics [1–3]. In general, control of polymer crystal morphology can be realized by varying external conditions, such as temperature, flow field, solvent and nucleating agents [4–7]. For morphological control, one of the simple ways is the solvent-induced crystallization, which is usually applied to the small molecule system [8–10]. However, for polymers, studies on their crystal morphology controlling by solvent-induced crystallization are rather limited so far, although solvent-induced crystallization is a common phenomenon which has been paid an increased attention [11–13]. It is often described as follows: the solvent molecules swell the polymer and then increase the chain mobility, allowing crystallization to occur even at room temperature. Over the past few decades, an enormous amount of information has been collected which indicates that some crystallizable polymers, for instance, poly(carbonate of bisphenol A) (PC), poly(ethylene terephthalate) (PET), poly(ether ether ketone) (PEEK), aromatic polyimide and syndiotactic polystyrene, show peculiar interactions with organic solvent in their solid state and undergo solvent-induced crystallization [14–18]. By absorbing organic solvent, the crystallization rate of these polymers can be greatly enhanced due to the plasticization effect. It is also found that the degree of polymer–solvent interaction plays an important role in the solvent-induced
∗ Corresponding author. Tel.: +86 28 85461795; fax: +86 28 85461795. E-mail address: [email protected] (Q. Fu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.050
crystallization. Generally, the stronger the polymer–solvent interaction is, the higher crystallinity could be induced. Well-known as eco-friendly polyester, which can be produced from renewable resources such as corn, poly(lactic acid) (PLA) has been widely studied due to its excellent mechanical property, biodegradability and biocompatibility [19,20]. As usually, PLA is difficult to crystallize thermally from its melt due to its relatively low crystallization rate. Thus enhancement of PLA crystallization and study on the PLA crystal morphologies are attractive due to their close relationship with the physical performance of PLA. So far just a few investigations have been published for the solvent-induced crystallization of PLA. Naga et al. have shown that PLA can undergo a process of solvent-induced crystallization from various organic solvents at room temperature, and demonstrates that acetone was the most effective solvent to crystallize amorphous PLA [21]. This can be understood from the fact that the solubility parameter of acetone is very close to that of PLA. In a recent publication, Marubayashi and co-workers revealed that PLA can form crystalline complex (form) with specific organic solvents below room temperature [22]. Nevertheless, all reported work so far has been devoted to the use of single solvent when inducing PLA crystallization. Moreover, precise control of PLA crystal morphology cannot be realized using a single solvent to the best of our knowledge. We note that the degree of polymer–solvent interaction mainly depends on the type of solvent, which affects the diffusion rate of solvent into polymer matrix and the polymer chain mobility. Thus, our interest is whether such interaction could be continuously adjustable using a mixed solvent, which plays a crucial role in solvent-induced crystallization. This can be done through the mixing of very strong solvent (for example, acetone) and very weak solvent (for example, ethanol). In this
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Fig. 1. Fabrication procedure of the crystalline PLA sheet with mixed solvent (acetone/ethanol). (A) The hot-pressed surface. (B) The fractured surface.
sense, the overall aim of this report is to manipulate PLA crystal morphologies via changing the ratio of a mixed solvent. In this work, we use a specific fractured surface of amorphous PLA as template, attempting to manipulate its crystal morphology with the aid of mixed solvent acetone/ethanol. As a result, various crystal morphologies including spherulite, shish–kebab, and bamboo-cage-like structure have been successfully observed for the first time using SEM. On the basis of experimental data, the mechanism of crystal structural formation and evolution is discussed. 2. Experimental 2.1. Materials and sample preparation The PLA, supplied by Haizheng Biomaterial Co. (Zhejiang, China), had a Mw of 1.59 × 105 g/mol and Mw /Mn of 2.75, respectively. All solvents were purchased from Aldrich–Sigma and used without further purification. Uniform amorphous PLA sheets (thickness = 2 mm) for the following characterization were prepared by quenching them into ice water from the compression molded melt. Then sheets were quick-fractured (Fig. 1) at room temperature. 2.2. Solvent-induced crystallization of PLA Briefly, acetone and ethanol, which are miscible at all concentrations, were first mixed in various ratios (v/v) (acetone/ethanol = 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80, 10/90, 0/100, respectively). The fractured sheets were subsequently immersed in the mixed solvent for different durations at room temperature, then taken out of solvent, and finally blown dry with slowly nitrogen flow to allow all solvents to evaporate. Experiments were repeated for each situation in order to clarify the universality of the results in the later section. 2.3. Characterization The morphologies of the PLA surfaces were investigated using an Inspect F field-emission scanning electron microscopy (SEM) (FEI Company, USA) with 5 kV accelerating voltage. Before SEM characterization, the fractured surfaces were coated with a thin layer of gold. X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert pro MPD apparatus. A conventional Cu K␣ ( = 0.154 nm,
Fig. 2. XRD patterns of PLA treated with and without solvent. (a) Different solvent ratio (acetone/ethanol) with immersion time of 50 min, (b) different immersion time with constant solvent ratio (acetone/ethanol = 50/50).
reflection mode) X-ray tube at a voltage of 40 kV and a filament current of 40 mA was used to obtain the XRD spectra. The scanning was conducted at a 2 range between 5◦ and 40◦ and a scanning rate of 5◦ /min. 3. Results and discussion 3.1. Solvent-induced crystallization of PLA In view of the different microstructure between the hot-pressed surface (position A in Fig. 1) and fractured surface (position B in Fig. 1), fractured surface of PLA is chosen as special research objective. As mentioned in Section 1, Naga et al. revealed that acetone was the most effective organic solvent to accelerate PLA crystallization [21]. As expected, transparent PLA sheet becomes opaque after the treatment with acetone-containing solvent, suggesting a formation of a thin crystalline layer (see supporting information Fig. S1). Generally, the loss of transparency of amorphous polymer indicates the occurrence of its crystallization [23]. This is also confirmed by XRD, as shown in Fig. 2a. In this study, X-ray would indeed penetrate a few hundred micrometers into the sample, and the XRD results would indicate solvent-induced crystallization in the total irradiated volume. Although the penetration depth is greater than
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the thickness of the surface crystalline layer, the indication of the existence of surface crystalline is enough. For both the cases of asquenched PLA and PLA treated with absolute ethanol, XRD patterns give very broad bread peak without any sharp crystalline peaks, which indicates that the ethanol is not able to induce the crystallization of PLA. Whereas, as shown in these patterns, two sharp peaks appear for the samples treated with solvent containing acetone, and no obvious change is observed with the change of acetone/ethanol ratio, revealing that PLA crystallization is caused and determined by acetone. To investigate time dependence of the mixed solvent on PLA crystallization, the XRD patterns of the samples with different immersion time are shown in Fig. 2b. From Fig. 2b, one can see that the sample with immersion time of only 5 s exhibits obvious crystallization peaks in comparison with that of as-quenched PLA, which indicates that well crystallized PLA can be obtained just in a few seconds. Roughly speaking, the solvent-induced crystallization here occurs very rapidly and may last only several seconds. Therefore, to ensure the complete growth of the lamellar crystalline, 50 min was used as a unified immersion time for the samples in the later discussion. However, it is difficult to recognize the crystal
morphologies from their XRD patterns. Hence, the crystal morphology of PLA was investigated via SEM, and the result is presented as follows. 3.2. Crystal morphological evolution of PLA From the over view images in Fig. 3 (Fig. 1 position B, all immersion time is 50 min), PLA crystals with varying structural features have been observed by adjusting the solvent ratio. It is found that the crystal morphology can be fine tuned by increasing the ethanol content in acetone/ethanol mixed solvent. In acetone, lots of small particles are observed with uniform morphology and an average diameter about 7 m, as shown in Fig. 3a. In fact, the enlarged view of a single particle (see the insert in Fig. 3a) indicates that the small particle appears to be chrysanthemums-like structure, with petals stretching outwards from their core. It is described as “chrysanthemums-like spherulite” in this paper. However, in acetone-rich (in which acetone contents in the solvent mixtures are higher than 50%) mixed solvent (for example acetone/ethanol = 60/40 in Fig. 3b), the surface seems to be composed
Fig. 3. SEM images showing PLA surface topography from position B in Fig. 1. The images are representative of the morphologies observed for the samples subjected to different solvent ratio for 50 min. (a) Acetone/ethanol = 100/0, (b) acetone/ethanol = 70/30, (c) acetone/ethanol = 50/50, (d) acetone/ethanol = 30/70, (e) acetone/ethanol = 0/100. All the inserts are the magnified images of the regions which are indicated by the rectangles.
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Fig. 4. SEM images of typical shish–kebab structures at a constant solvent ratio (a–f) acetone/ethanol = 50/50. (g and h) Acetone/ethanol = 30/70.
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of many leaf-like crystals covering the chrysanthemums-like spherulites, which may be formed by twisted lamellae. Indeed, the content of leaf-like crystals show an increasing trend when ethanol content increases (see supporting information Fig. S2). Surprisingly, visible morphological change is observed when the solvent ratio reaches to 50/50 (acetone/ethanol). From Fig. 3c, it seems that a great many “carpenterworm” with length ranging from several microns to dozens of microns can clearly be seen. At higher magnification (see the inset), “carpenterworm” is actually the typical shish–kebab structure which is mainly a result of shear or extension during melt processing. Meanwhile, an interesting intergrowth phenomenon of shish–kebabs and spherulites is found throughout the view, more structural details can be seen in supporting information Fig. S3a. Comparing the morphology of these shperulites (Supporting information Fig. S3b) with that of chrysanthemums-like spherulites, one can see they are quite different in structure. Through a careful observation, spherulite here is well known as bundled spherulite which is usually formed in thermal-induced crystallization. With further change of acetone/ethanol ratio to ethanol-rich system (for instance, acetone/ethanol = 40/60, 30/70, 20/80, and 10/90, respectively), imperfect shish–kebab structures can also be clearly seen lying in the matrix (Fig. 3d), and the appearance of shish–kebab becomes fuzzy with increasing ethanol (see supporting information Fig. S4). However, no crystals of any type are found in the sample treated with absolute ethanol, as shown in Fig. 3e. Additionally, it is found that fuzzy images of shish–kebab structure can be obtained when the sample is subjected to acetone/water ratio of 70/30, which is displayed in supporting information Fig. S5. This further indicates that acetone play a crucial role in the solvent-induced crystallization of PLA. And the interaction between PLA and acetone can be weakened by a poor solvent.
Interestingly, an easy formation of PLA shish–kebab structure is obtained through this simple way, which has not previously been reported for solvent-induced crystallization of PLA. To further ascertain the detailed information of as-obtained shish–kebab structure, some higher magnification SEM images of representative PLA shish–kebab are displayed (acetone/ethanol ratio of 50/50 (Fig. 4a–f) and 30/70 (Fig. 4g and h)). Generally, shish–kebab consists of a central fibril (shish) and disk-shape folded chain lamellae (kebab) decorated perpendicularly on the shish [24]. One can easily identify that a large amount of shish–kebabs exist in the image, as shown in Fig. 4a. Moreover, some shish–kebab crystals connect by kebabs and penetrate into each other, forming an interlocking state. From the clear image in Fig. 4b, PLA fibril serves as the shish and its lamellae forms kebabs, in which some kebabs are perpendicular to the long axis of shish, others are oblique to it. In addition, it is noted that there are mainly two morphological features of shish–kebab observed from the fractured surface: interlocked and twisted shish–kebab. Besides interlocked shish–kebab structures in Fig. 4a, b and e, straight (Fig. 4c) and twisted (Fig. 4d) shish–kebab structures also exist in the sample, depending on the initial amorphous state of PLA, which will be discussed in a later section. For the sample immersed in acetone/ethanol of 30/70, shish–kebab can be observed mainly embedding in the matrix. Such embedded shish–kebab, actually imperfect shish–kebab, seems to exhibit longer kebabs compared with that of completely imperfect shish–kebab, indicating a different growth condition. According to the SEM images, there is also a common situation, that is, as shown in Fig. 4f, some “carpenterworms” seem to be shish–kebab structures without shish. In this case, they are a series of spherulites arranging shoulder to shoulder, actually formed by protuberant lamellar structures which is observed on the surface of the top layer of spherulites. Particularly, it is truly amazing to find that
Fig. 5. SEM images of some sophisticated crystal structures observed in some regions at a constant acetone/ethanol ratio of 50/50. The inset is the image with higher magnification, indicated by the white dashed circle.
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more sophisticated crystal structures were formed in special shapes (Fig. 5). In general, these shapes are similar to bamboo-cage, confirmed by Fig. 5c. That is, the kebab-like crystals are decorating around the long axis, knitting together in bamboo-cage shape. Meanwhile, bead curtain morphology can also be seen in the edge of the fractured surface, as shown in Fig. 5d. In comparison with the hot-pressed surface (position A in Fig. 1), however, such structures including spherulites and shish–kebabs are clearly absent regardless of acetone content, even though a layer of crystal fragments can be found (for instance, image shown in supporting information Fig. S6). As will be later discussed, the increased level of morphological complexity of crystal reflects the changes of surface microstructure introduced by the fracture process. 3.3. Mechanism analysis Based on the nucleation-growth mechanism of crystallization, nucleation is often the decisive step in crystallization process [25]. To control the crystal morphology in this study, initial microstructure of the fractured surface may be of great importance, which can serve as crystallization precursor before the solvent-induced crystallization. From the comparison of Fig. 6a–c, it is obvious that the shish–kebab originates from the mechanically generated fibrils. It is clear that kebabs arranges around the fibril from the root (roots are indicated by the white dashed ellipses). Fracture process is actually regarded as a local phase transition, which leads to the formation of craze and fibrils, which is reported by Juska and co-worker [26]. Concretely, under local high shear stress, some of the amorphous PLA chains undergo a rapid extension to form a fibrous state, and then fibrils are fixed instantly. With the aid of SEM, fibrils, exposed and embedded, with different size can be observed on all the fractured surfaces, as shown in Fig. 6a. In addition, there are some fibrils which are embedded in the matrix. During the subsequent solvent-induced crystallization, fibrils must play a crucial role, and solvent serves as driving force for the formation of the final crystal morphology. Since the solubility parameter of acetone is similar to that of PLA, a swelling and dissolving effect occurs with the permeation of acetone into amorphous PLA. As a result, this effect causes the reduction of glass transition temperature (Tg ) and crystallization temperature (Tc ). This means that the increased polymer chains mobility favors the formation of crystalline domains. During this process, polymer–solvent interactions may have strong impact on the solvent-induced crystallization. When the interactions are strong enough to restrain the Tg below the exposure temperature, crystallization becomes kinetically favorable [27]. In our present study, solubility of solvent can be effectively controlled by adjusting the ratio of mixed solvent of acetone and ethanol. Based on the above results and analysis, the subsequently solvent-induced crystallization is described as follows: when the solvent encounters the glassy PLA, the acetone molecules start to disrupt the intermolecular cohesive forces between some PLA chains from the unstressed region, enhancing chain mobility [28–30]. The equilibrium dissolution can be reached within a short time. For better explanation, a formation mechanism is described schematically as shown in Fig. 7. If the solvent is strong enough (for instance in Fig. 7, pure acetone), one can imagine that all the initial solid structure including fibrils can be dissolved and destroyed. Upon the subsequent removal from the solvent bath, the solvent desorption occurs, and then a large number of spherulites are produced with the formation of initial nuclei, which is similar to the crystallization from melt. Similar to the literature [23], Tg increases when the solvent desorption occurs, effectively “supercooling” the swollen polymer, then inducing crystallization, initiating the nucleation and growth of spherulite. Furthermore, it is known that the peripheral part of the fibril is of lower stability than the central parts [31]. Therefore, the structure within the fibril is possibly less
Fig. 6. The comparison of the fracture surfaces with and without solvent treatment. (a) Fresh fracture surface treated without solvent, (b and c) fracture surface treated with acetone/ethanol of 30/70.
accessible to the solvent. When such PLA is exposed to a mixed solvent (acetone/ethanol = 50/50 in Fig. 7, lower degree of swelling and solubility), the smaller fibrils must be less stable toward the solvent and are consequently dissolved, allowing the formation of bundled spherulites and shish–kebab-like crystals (Fig. 4f). Nevertheless, larger fibril is only partially dissolved, and thus the remaining partially-dissolved fibril can serves as shish to induce the decoration of kebabs, as also shown in Fig. 4. The kebabs may be formed
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Fig. 7. Schematic illustration of the morphological evolution for the PLA fracture surfaces via different solvent inducement: ethanol, acetone/ethanol = 50/50 and acetone, respectively. (1) Chrysanthemums-like spherulite, (2) bundled spherulite, (3) shish–kebab crystal.
by the crystallization of partially dissolved chains of the fibril. However, no crystallization occurs when PLA is treated with the pure ethanol, as suggested in Fig. 7. In this paper, this proposed mechanism may be the most probable physical origin which gives insight into the formation of PLA crystals.
4. Conclusion In summary, mixed solvent of acetone and ethanol has been used for the solvent-induced crystallization of PLA fractured surface. It is found that solvents have strong influence on the resulting crystal morphology of PLA. In other words, appropriately chosen solubility differentials of two solvents can lead to pronounced morphology changes. In this paper, upon changing the volume ratio of the mixed solvent, crystals including chrysanthemums-like spherulite, bundled spherulite, shish–kebab and bamboo-cage-like crystal are formed via solvent-induced crystallization. Especially, perfect PLA shish–kebab crystal is easily obtained and directly observed in some ethanol-rich mixed solvents, which is often invisible through melt processing. And interestingly, initial structure is also very helpful for the formation of the final PLA crystal morphology except solvent ratio. Based on large amounts of SEM measurements, a possible schematic mechanism concerning the change of crystal morphology has been established. Such a study can result in an improved perspective on the relationship between growth condition and final morphology of PLA, which may be of great importance in the fundamental aspects of crystallization science.
Acknowledgment We express our sincere thanks to the National Natural Science Foundation of China (NSFC) for financial support (21034005, 51121001). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.apsusc.2012.08.050. References [1] H.H. Li, S.K. Yan, Surface-induced polymer crystallization and the resultant structures and morphologies, Macromolecules 44 (2011) 417–428. [2] T. Kongkhlang, K. Tashiro, M. Kotaki, S. Chirachanchai, Electrospinning as a new technique to control the crystal morphology and molecular orientation of polyoxymethylene nanofibers, Journal of the American Chemical Society 130 (2008) 15460–15466. [3] F. Luo, C.Z. Geng, K. Wang, H. Deng, F. Chen, Q. Fu, B. Na, New understanding in tuning toughness of -polypropylene: the role of -nucleated crystalline morphology, Macromolecules 42 (2009) 9325–9331. [4] B. Zhang, J.B. Chen, X.L. Zhang, C.Y. Shen, Crystal morphology and structure of the -form of isotactic polypropylene under supercooled extrusion, Journal of Applied Polymer Science 120 (2011) 3255–3264. [5] E. Koscher, R. Fulchiron, Influence of shear on polypropylene crystallization: morphology development and kinetics, Polymer 43 (2002) 6931–6942. [6] W.T. Xu, L.G. Li, H.W. Tang, H. Li, X.L. Zhao, X.N. Yang, Solvent-induced crystallization of poly(3-dodecylthiophene): morphology and kinetics, Journal of Physical Chemistry B 115 (2011) 6412–6420. [7] S.E. Wang, T.X. Liu, Z.S. Mo, J.Z. Wang, F. Xu, Z.W. Wu, The crystal structure and drawing induced polymorphism in poly(ary1 ether ketone)s. 3. Crystallization
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Manipulating poly(lactic acid) surface morphology by solvent-induced crystallization Jian Gao, Lingyan Duan, Guanghui Yang, Qin Zhang, Mingbo Yang, Qiang Fu ∗ College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 13 August 2012 Accepted 15 August 2012 Available online 23 August 2012 Keywords: Poly(lactic acid) Solvent-induced crystallization Morphology Surface structure
a b s t r a c t Here, we report some unique crystalline morphologies of poly(lactic acid) (PLA) via organic solventinduced crystallization. It was revealed that the surface morphology of PLA can be fine tuned by simply varying the volume ratio of a mixed solvent (acetone/ethanol). By increasing the ethanol content in the mixed solvent, we observed a morphological evolution of PLA surface from spherulite to shish–kebab and bamboo-cage-like structure. It was also interesting to find that the initial surface structure of PLA plays an important role to determine the final solvent-induced crystalline morphology. This work provides a new method for manipulating PLA crystal morphology through a simple solvent-induced crystallization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Morphological control of polymer crystallization has always been an important and fundamental research topic in polymer physics [1–3]. In general, control of polymer crystal morphology can be realized by varying external conditions, such as temperature, flow field, solvent and nucleating agents [4–7]. For morphological control, one of the simple ways is the solvent-induced crystallization, which is usually applied to the small molecule system [8–10]. However, for polymers, studies on their crystal morphology controlling by solvent-induced crystallization are rather limited so far, although solvent-induced crystallization is a common phenomenon which has been paid an increased attention [11–13]. It is often described as follows: the solvent molecules swell the polymer and then increase the chain mobility, allowing crystallization to occur even at room temperature. Over the past few decades, an enormous amount of information has been collected which indicates that some crystallizable polymers, for instance, poly(carbonate of bisphenol A) (PC), poly(ethylene terephthalate) (PET), poly(ether ether ketone) (PEEK), aromatic polyimide and syndiotactic polystyrene, show peculiar interactions with organic solvent in their solid state and undergo solvent-induced crystallization [14–18]. By absorbing organic solvent, the crystallization rate of these polymers can be greatly enhanced due to the plasticization effect. It is also found that the degree of polymer–solvent interaction plays an important role in the solvent-induced
∗ Corresponding author. Tel.: +86 28 85461795; fax: +86 28 85461795. E-mail address: [email protected] (Q. Fu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.050
crystallization. Generally, the stronger the polymer–solvent interaction is, the higher crystallinity could be induced. Well-known as eco-friendly polyester, which can be produced from renewable resources such as corn, poly(lactic acid) (PLA) has been widely studied due to its excellent mechanical property, biodegradability and biocompatibility [19,20]. As usually, PLA is difficult to crystallize thermally from its melt due to its relatively low crystallization rate. Thus enhancement of PLA crystallization and study on the PLA crystal morphologies are attractive due to their close relationship with the physical performance of PLA. So far just a few investigations have been published for the solvent-induced crystallization of PLA. Naga et al. have shown that PLA can undergo a process of solvent-induced crystallization from various organic solvents at room temperature, and demonstrates that acetone was the most effective solvent to crystallize amorphous PLA [21]. This can be understood from the fact that the solubility parameter of acetone is very close to that of PLA. In a recent publication, Marubayashi and co-workers revealed that PLA can form crystalline complex (form) with specific organic solvents below room temperature [22]. Nevertheless, all reported work so far has been devoted to the use of single solvent when inducing PLA crystallization. Moreover, precise control of PLA crystal morphology cannot be realized using a single solvent to the best of our knowledge. We note that the degree of polymer–solvent interaction mainly depends on the type of solvent, which affects the diffusion rate of solvent into polymer matrix and the polymer chain mobility. Thus, our interest is whether such interaction could be continuously adjustable using a mixed solvent, which plays a crucial role in solvent-induced crystallization. This can be done through the mixing of very strong solvent (for example, acetone) and very weak solvent (for example, ethanol). In this
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Fig. 1. Fabrication procedure of the crystalline PLA sheet with mixed solvent (acetone/ethanol). (A) The hot-pressed surface. (B) The fractured surface.
sense, the overall aim of this report is to manipulate PLA crystal morphologies via changing the ratio of a mixed solvent. In this work, we use a specific fractured surface of amorphous PLA as template, attempting to manipulate its crystal morphology with the aid of mixed solvent acetone/ethanol. As a result, various crystal morphologies including spherulite, shish–kebab, and bamboo-cage-like structure have been successfully observed for the first time using SEM. On the basis of experimental data, the mechanism of crystal structural formation and evolution is discussed. 2. Experimental 2.1. Materials and sample preparation The PLA, supplied by Haizheng Biomaterial Co. (Zhejiang, China), had a Mw of 1.59 × 105 g/mol and Mw /Mn of 2.75, respectively. All solvents were purchased from Aldrich–Sigma and used without further purification. Uniform amorphous PLA sheets (thickness = 2 mm) for the following characterization were prepared by quenching them into ice water from the compression molded melt. Then sheets were quick-fractured (Fig. 1) at room temperature. 2.2. Solvent-induced crystallization of PLA Briefly, acetone and ethanol, which are miscible at all concentrations, were first mixed in various ratios (v/v) (acetone/ethanol = 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80, 10/90, 0/100, respectively). The fractured sheets were subsequently immersed in the mixed solvent for different durations at room temperature, then taken out of solvent, and finally blown dry with slowly nitrogen flow to allow all solvents to evaporate. Experiments were repeated for each situation in order to clarify the universality of the results in the later section. 2.3. Characterization The morphologies of the PLA surfaces were investigated using an Inspect F field-emission scanning electron microscopy (SEM) (FEI Company, USA) with 5 kV accelerating voltage. Before SEM characterization, the fractured surfaces were coated with a thin layer of gold. X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert pro MPD apparatus. A conventional Cu K␣ ( = 0.154 nm,
Fig. 2. XRD patterns of PLA treated with and without solvent. (a) Different solvent ratio (acetone/ethanol) with immersion time of 50 min, (b) different immersion time with constant solvent ratio (acetone/ethanol = 50/50).
reflection mode) X-ray tube at a voltage of 40 kV and a filament current of 40 mA was used to obtain the XRD spectra. The scanning was conducted at a 2 range between 5◦ and 40◦ and a scanning rate of 5◦ /min. 3. Results and discussion 3.1. Solvent-induced crystallization of PLA In view of the different microstructure between the hot-pressed surface (position A in Fig. 1) and fractured surface (position B in Fig. 1), fractured surface of PLA is chosen as special research objective. As mentioned in Section 1, Naga et al. revealed that acetone was the most effective organic solvent to accelerate PLA crystallization [21]. As expected, transparent PLA sheet becomes opaque after the treatment with acetone-containing solvent, suggesting a formation of a thin crystalline layer (see supporting information Fig. S1). Generally, the loss of transparency of amorphous polymer indicates the occurrence of its crystallization [23]. This is also confirmed by XRD, as shown in Fig. 2a. In this study, X-ray would indeed penetrate a few hundred micrometers into the sample, and the XRD results would indicate solvent-induced crystallization in the total irradiated volume. Although the penetration depth is greater than
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the thickness of the surface crystalline layer, the indication of the existence of surface crystalline is enough. For both the cases of asquenched PLA and PLA treated with absolute ethanol, XRD patterns give very broad bread peak without any sharp crystalline peaks, which indicates that the ethanol is not able to induce the crystallization of PLA. Whereas, as shown in these patterns, two sharp peaks appear for the samples treated with solvent containing acetone, and no obvious change is observed with the change of acetone/ethanol ratio, revealing that PLA crystallization is caused and determined by acetone. To investigate time dependence of the mixed solvent on PLA crystallization, the XRD patterns of the samples with different immersion time are shown in Fig. 2b. From Fig. 2b, one can see that the sample with immersion time of only 5 s exhibits obvious crystallization peaks in comparison with that of as-quenched PLA, which indicates that well crystallized PLA can be obtained just in a few seconds. Roughly speaking, the solvent-induced crystallization here occurs very rapidly and may last only several seconds. Therefore, to ensure the complete growth of the lamellar crystalline, 50 min was used as a unified immersion time for the samples in the later discussion. However, it is difficult to recognize the crystal
morphologies from their XRD patterns. Hence, the crystal morphology of PLA was investigated via SEM, and the result is presented as follows. 3.2. Crystal morphological evolution of PLA From the over view images in Fig. 3 (Fig. 1 position B, all immersion time is 50 min), PLA crystals with varying structural features have been observed by adjusting the solvent ratio. It is found that the crystal morphology can be fine tuned by increasing the ethanol content in acetone/ethanol mixed solvent. In acetone, lots of small particles are observed with uniform morphology and an average diameter about 7 m, as shown in Fig. 3a. In fact, the enlarged view of a single particle (see the insert in Fig. 3a) indicates that the small particle appears to be chrysanthemums-like structure, with petals stretching outwards from their core. It is described as “chrysanthemums-like spherulite” in this paper. However, in acetone-rich (in which acetone contents in the solvent mixtures are higher than 50%) mixed solvent (for example acetone/ethanol = 60/40 in Fig. 3b), the surface seems to be composed
Fig. 3. SEM images showing PLA surface topography from position B in Fig. 1. The images are representative of the morphologies observed for the samples subjected to different solvent ratio for 50 min. (a) Acetone/ethanol = 100/0, (b) acetone/ethanol = 70/30, (c) acetone/ethanol = 50/50, (d) acetone/ethanol = 30/70, (e) acetone/ethanol = 0/100. All the inserts are the magnified images of the regions which are indicated by the rectangles.
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Fig. 4. SEM images of typical shish–kebab structures at a constant solvent ratio (a–f) acetone/ethanol = 50/50. (g and h) Acetone/ethanol = 30/70.
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of many leaf-like crystals covering the chrysanthemums-like spherulites, which may be formed by twisted lamellae. Indeed, the content of leaf-like crystals show an increasing trend when ethanol content increases (see supporting information Fig. S2). Surprisingly, visible morphological change is observed when the solvent ratio reaches to 50/50 (acetone/ethanol). From Fig. 3c, it seems that a great many “carpenterworm” with length ranging from several microns to dozens of microns can clearly be seen. At higher magnification (see the inset), “carpenterworm” is actually the typical shish–kebab structure which is mainly a result of shear or extension during melt processing. Meanwhile, an interesting intergrowth phenomenon of shish–kebabs and spherulites is found throughout the view, more structural details can be seen in supporting information Fig. S3a. Comparing the morphology of these shperulites (Supporting information Fig. S3b) with that of chrysanthemums-like spherulites, one can see they are quite different in structure. Through a careful observation, spherulite here is well known as bundled spherulite which is usually formed in thermal-induced crystallization. With further change of acetone/ethanol ratio to ethanol-rich system (for instance, acetone/ethanol = 40/60, 30/70, 20/80, and 10/90, respectively), imperfect shish–kebab structures can also be clearly seen lying in the matrix (Fig. 3d), and the appearance of shish–kebab becomes fuzzy with increasing ethanol (see supporting information Fig. S4). However, no crystals of any type are found in the sample treated with absolute ethanol, as shown in Fig. 3e. Additionally, it is found that fuzzy images of shish–kebab structure can be obtained when the sample is subjected to acetone/water ratio of 70/30, which is displayed in supporting information Fig. S5. This further indicates that acetone play a crucial role in the solvent-induced crystallization of PLA. And the interaction between PLA and acetone can be weakened by a poor solvent.
Interestingly, an easy formation of PLA shish–kebab structure is obtained through this simple way, which has not previously been reported for solvent-induced crystallization of PLA. To further ascertain the detailed information of as-obtained shish–kebab structure, some higher magnification SEM images of representative PLA shish–kebab are displayed (acetone/ethanol ratio of 50/50 (Fig. 4a–f) and 30/70 (Fig. 4g and h)). Generally, shish–kebab consists of a central fibril (shish) and disk-shape folded chain lamellae (kebab) decorated perpendicularly on the shish [24]. One can easily identify that a large amount of shish–kebabs exist in the image, as shown in Fig. 4a. Moreover, some shish–kebab crystals connect by kebabs and penetrate into each other, forming an interlocking state. From the clear image in Fig. 4b, PLA fibril serves as the shish and its lamellae forms kebabs, in which some kebabs are perpendicular to the long axis of shish, others are oblique to it. In addition, it is noted that there are mainly two morphological features of shish–kebab observed from the fractured surface: interlocked and twisted shish–kebab. Besides interlocked shish–kebab structures in Fig. 4a, b and e, straight (Fig. 4c) and twisted (Fig. 4d) shish–kebab structures also exist in the sample, depending on the initial amorphous state of PLA, which will be discussed in a later section. For the sample immersed in acetone/ethanol of 30/70, shish–kebab can be observed mainly embedding in the matrix. Such embedded shish–kebab, actually imperfect shish–kebab, seems to exhibit longer kebabs compared with that of completely imperfect shish–kebab, indicating a different growth condition. According to the SEM images, there is also a common situation, that is, as shown in Fig. 4f, some “carpenterworms” seem to be shish–kebab structures without shish. In this case, they are a series of spherulites arranging shoulder to shoulder, actually formed by protuberant lamellar structures which is observed on the surface of the top layer of spherulites. Particularly, it is truly amazing to find that
Fig. 5. SEM images of some sophisticated crystal structures observed in some regions at a constant acetone/ethanol ratio of 50/50. The inset is the image with higher magnification, indicated by the white dashed circle.
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more sophisticated crystal structures were formed in special shapes (Fig. 5). In general, these shapes are similar to bamboo-cage, confirmed by Fig. 5c. That is, the kebab-like crystals are decorating around the long axis, knitting together in bamboo-cage shape. Meanwhile, bead curtain morphology can also be seen in the edge of the fractured surface, as shown in Fig. 5d. In comparison with the hot-pressed surface (position A in Fig. 1), however, such structures including spherulites and shish–kebabs are clearly absent regardless of acetone content, even though a layer of crystal fragments can be found (for instance, image shown in supporting information Fig. S6). As will be later discussed, the increased level of morphological complexity of crystal reflects the changes of surface microstructure introduced by the fracture process. 3.3. Mechanism analysis Based on the nucleation-growth mechanism of crystallization, nucleation is often the decisive step in crystallization process [25]. To control the crystal morphology in this study, initial microstructure of the fractured surface may be of great importance, which can serve as crystallization precursor before the solvent-induced crystallization. From the comparison of Fig. 6a–c, it is obvious that the shish–kebab originates from the mechanically generated fibrils. It is clear that kebabs arranges around the fibril from the root (roots are indicated by the white dashed ellipses). Fracture process is actually regarded as a local phase transition, which leads to the formation of craze and fibrils, which is reported by Juska and co-worker [26]. Concretely, under local high shear stress, some of the amorphous PLA chains undergo a rapid extension to form a fibrous state, and then fibrils are fixed instantly. With the aid of SEM, fibrils, exposed and embedded, with different size can be observed on all the fractured surfaces, as shown in Fig. 6a. In addition, there are some fibrils which are embedded in the matrix. During the subsequent solvent-induced crystallization, fibrils must play a crucial role, and solvent serves as driving force for the formation of the final crystal morphology. Since the solubility parameter of acetone is similar to that of PLA, a swelling and dissolving effect occurs with the permeation of acetone into amorphous PLA. As a result, this effect causes the reduction of glass transition temperature (Tg ) and crystallization temperature (Tc ). This means that the increased polymer chains mobility favors the formation of crystalline domains. During this process, polymer–solvent interactions may have strong impact on the solvent-induced crystallization. When the interactions are strong enough to restrain the Tg below the exposure temperature, crystallization becomes kinetically favorable [27]. In our present study, solubility of solvent can be effectively controlled by adjusting the ratio of mixed solvent of acetone and ethanol. Based on the above results and analysis, the subsequently solvent-induced crystallization is described as follows: when the solvent encounters the glassy PLA, the acetone molecules start to disrupt the intermolecular cohesive forces between some PLA chains from the unstressed region, enhancing chain mobility [28–30]. The equilibrium dissolution can be reached within a short time. For better explanation, a formation mechanism is described schematically as shown in Fig. 7. If the solvent is strong enough (for instance in Fig. 7, pure acetone), one can imagine that all the initial solid structure including fibrils can be dissolved and destroyed. Upon the subsequent removal from the solvent bath, the solvent desorption occurs, and then a large number of spherulites are produced with the formation of initial nuclei, which is similar to the crystallization from melt. Similar to the literature [23], Tg increases when the solvent desorption occurs, effectively “supercooling” the swollen polymer, then inducing crystallization, initiating the nucleation and growth of spherulite. Furthermore, it is known that the peripheral part of the fibril is of lower stability than the central parts [31]. Therefore, the structure within the fibril is possibly less
Fig. 6. The comparison of the fracture surfaces with and without solvent treatment. (a) Fresh fracture surface treated without solvent, (b and c) fracture surface treated with acetone/ethanol of 30/70.
accessible to the solvent. When such PLA is exposed to a mixed solvent (acetone/ethanol = 50/50 in Fig. 7, lower degree of swelling and solubility), the smaller fibrils must be less stable toward the solvent and are consequently dissolved, allowing the formation of bundled spherulites and shish–kebab-like crystals (Fig. 4f). Nevertheless, larger fibril is only partially dissolved, and thus the remaining partially-dissolved fibril can serves as shish to induce the decoration of kebabs, as also shown in Fig. 4. The kebabs may be formed
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Fig. 7. Schematic illustration of the morphological evolution for the PLA fracture surfaces via different solvent inducement: ethanol, acetone/ethanol = 50/50 and acetone, respectively. (1) Chrysanthemums-like spherulite, (2) bundled spherulite, (3) shish–kebab crystal.
by the crystallization of partially dissolved chains of the fibril. However, no crystallization occurs when PLA is treated with the pure ethanol, as suggested in Fig. 7. In this paper, this proposed mechanism may be the most probable physical origin which gives insight into the formation of PLA crystals.
4. Conclusion In summary, mixed solvent of acetone and ethanol has been used for the solvent-induced crystallization of PLA fractured surface. It is found that solvents have strong influence on the resulting crystal morphology of PLA. In other words, appropriately chosen solubility differentials of two solvents can lead to pronounced morphology changes. In this paper, upon changing the volume ratio of the mixed solvent, crystals including chrysanthemums-like spherulite, bundled spherulite, shish–kebab and bamboo-cage-like crystal are formed via solvent-induced crystallization. Especially, perfect PLA shish–kebab crystal is easily obtained and directly observed in some ethanol-rich mixed solvents, which is often invisible through melt processing. And interestingly, initial structure is also very helpful for the formation of the final PLA crystal morphology except solvent ratio. Based on large amounts of SEM measurements, a possible schematic mechanism concerning the change of crystal morphology has been established. Such a study can result in an improved perspective on the relationship between growth condition and final morphology of PLA, which may be of great importance in the fundamental aspects of crystallization science.
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