Article pubs.acs.org/JPCB
Solvent-Induced Crystallization Behaviors of PLLA Ultrathin Films Investigated by RAIR Spectroscopy and AFM Measurements Ningjing Wu,* Shuguo Lang, Hong Zhang, Meichun Ding, and Jianming Zhang Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology, Qingdao City 266042, People’s Republic of China ABSTRACT: The crystallization of poly(L-lactide acid) (PLLA) ultrathin films induced by different solvents was investigated using reflection− absorption infrared (RAIR) spectroscopy and atomic force microscopy (AFM). Irregular PLLA dendrite lamellae grew in the flat-on orientation with dichloromethane solvent before being redissolved after longer induction times owing to the strong interaction between the PLLA segments and solvent molecules. Faster formation of PLLA spherulites was induced with acetone than with dichloromethane, and these remained unchanged with increasing induction time because of the polarity difference between the PLLA segments and acetone molecules. PLLA ultrathin films could not be induced to crystallize using chloroform because of the very strong interactions between the chloroform (CHCl3) molecules and PLLA amorphous chains, which caused the CHCl3 solvent molecules to rapidly permeate the PLLA random coils and dissolve the amorphous chains. These phenomena are attributed to solvent-specific competition between solventinduced crystallization and dissolution effects in PLLA ultrathin films, which ultimately leads to the higher degree of crystallinity obtained with acetone than with dichloromethane. form).27 Marubayashi et al.27 investigated the formation of crystalline complexes of PLLA and specific solvent molecules using wide-angle X-ray diffraction (WAXD). They concluded that PLLA forms a crystalline complex (ε-form) with specific five-membered ring compounds including cyclopentanone (CPO), 1,3-dioxolane (DOL), γ-butyrolactone (GBL), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) below room temperature and that the ε-to-α transition occurred with solvent desorption. They also investigated various kinds of crystal-to-crystal transitions relating to PLLA cocrystallized with low-molecular weight compounds (CO2 and ε-solvents) using WAXD and Fourier transform infrared spectroscopy (FTIR). These results revealed that the guest-induced transition behaviors of noncomplex PLLA crystals were strongly influenced by the order of the noncomplex crystals (α, α′, and α″) as well as the kinds of guests.28 In a previous paper, we investigated the molecular orientation and crystallization dynamics of PLLA ultrathin films at various temperatures using in situ reflection−absorption infrared (RAIR) spectroscopy. We found that the annealing temperature and thickness of the thin films has a significant effect on the crystallization kinetics and lamellar orientation of PLLA thin films.29 Because of the slow crystallization rate and low crystallinity of PLLA film, solvent induction could be an effective means of improving the degree of crystallinity in a PLLA film. Naga et
1. INTRODUCTION In recent years, polymer thin films have attracted widespread interest for applications in biomaterials, microelectronics fabrication, liquid crystal displays, photoresists for photolithography, and antireflection coatings.1,2 The confinement of polymer ultrathin films (thickness dichloromethane > chloroform. Taking the slope of the curve representing the degree of crystallinity as a function of the induction time in Figure 9 as the crystallization rate, the same ordering of the solvents is obtained, namely, acetone > dichloromethane > chloroform.
In the initial diffusion stage, the solvent molecules diffuse into and interact with the PLLA random coil chains to activate the motion of the PLLA segments. Because the interaction between chloroform molecules and PLLA amorphous chains is very strong, the chloroform molecules diffuse rapidly into the PLLA thin film, interact with PLLA random coils, and dissolve the amorphous chains, such that chloroform cannot induce the formation of crystalline PLLA. In a second induction stage, the solvent molecules promote PLLA segments to arrange into short regular helical chains and long crystal lattices. The formation of PLLA irregular dendrite crystals is induced by dichloromethane, where the dendrite lamellae grow preferentially in the flat-on orientation. Acetone provokes the PLLA segments to form spherulites in less than 10 min. This rapid induction of crystallization by the solvent leads to the distortion and contortion of the PLLA lamella, such that they present different orientations in the PLLA spherulites. With increasing induction time, the solvent molecules continue to diffuse into the PLLA crystal lattice and interact with PLLA segments. Because of the strong interaction between the PLLA segments and dichloromethane molecules, the PLLA dendrite crystals redissolve in dichloromethane. However, the PLLA spherulites remain unchanged because of the weak interaction between the acetone molecules and PLLA segments. These phenomena are attributed to solvent-specific competition between solventinduced crystallization and dissolution effects in PLLA ultrathin films.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; ningjing_wu@qust. edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21104038). Figure 9. Relative degree of crystallinity changes of PLLA ultrathin films induced by different solvents with induction time.
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4. CONCLUSIONS On the basis of the results described above regarding the changes in the RAIR spectra of PLLA ultrathin films crystallized using different solvents, the schematic of the aggregation and structural evolution of PLLA ultrathin film shown in Figure 9 is proposed (see Scheme 2). In particular, the results suggest that the solvent-induced crystallization process of PLLA ultrathin films is divided into several stages. Scheme 2. Aggregation Structural Evolution Process of PLLA Thin Films Induced by Different Solvents
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dx.doi.org/10.1021/jp506840e | J. Phys. Chem. B 2014, 118, 12652−12659
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