Shaoyong
Huang
a,
Hongfei
Li
*b,
Huiying
Wen
b,
Donghong
Yu
c,
Shichun
Jiang
*d,
Gao
Li
a,
Xuesi
Chen
a and
Lijia
An
*b
aKey Laboratory of Polymer Eco-materials, Changchun 130022, PR China
bState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China. E-mail: hfli@ciac.ac.cn; ljan@ciac.ac.cn
cDepartment of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, DK-9000, Aalborg, Denmark
dSchool of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China. E-mail: scjiang@tju.edu.cn
First published on 24th October 2013
The crystalline morphology and structure of poly(L-lactide) (PLLA) in a PLLA film–chloroform system were investigated by means of wide angle X-ray diffraction (WAXD), polarized optical microscopy (POM) and atomic force microscopy (AFM). Birefringent and nonbirefringent ring banded supra-structures with radial periodic variation of thickness were obtained, which were induced by micro-evaporation of solvents and concentration gradient of PLLA. The ring banded morphologies consisted of multilayer lamellar crystals, which is a manifestation of alternating ridge and valley bands of periodic variation of thicknesses along the radial direction. The formation of the ring banded supra-structures is associated with diffusion and crystal growth induced periodic variation of concentration gradient, which is attributed to diffusion-related rhythmic growth and the competition between diffusion of polymer segments and growth of crystal lamellae. The mechanism of such banded-ring formation was explored on the basis of rhythmic growth resulting from non-linear diffusion.
PLLA is a biodegradable semi-crystalline polymer widely investigated from a biomedical application and an environmentally degradable plastic view point. Its application and properties are strongly related to its crystal structure and morphology.26–30 Crystal structure and morphology are determined not only by thermodynamics but also by kinetics of crystallization. To achieve a better understanding of the crystallization process and the resulting final structure of polymers, a model system was employed by controlling kinetic factors. The formation of crystal structure and morphology is determined by two competitive factors: the migration of polymer chains to the crystal growth front and the rate of crystal growth, which can be modulated in solution crystallization by controlling rate of solvent evaporation. Thus, the development of crystalline structure and morphology can also be modulated in this way.31
In this paper, we investigated the crystalline morphology and structure of PLLA in a PLLA–chloroform system by controlling the kinetic process. Birefringent and nonbirefringent ring banded supra-structures were obtained by solvent micro-evaporation. The ring banded supra-structures are different from the classical ring banded spherulites grown from melts. The morphologies and structures were studied by wide angle X-ray diffraction (WAXD), polarized optical microscopy (POM) and atomic force microscopy (AFM). The development of the morphologies was explored on the basis of a rhythmic growth mechanism.
After solvent evaporation, the films were dried in vacuum for 24 hours and then quenched to 30 °C from complete melts at 200 °C, leading to disordered PLLA in films. Each disordered PLLA film was sealed in a cylinder which was placed in a thermostatic oven at 30 °C under atmospheric pressure. The sample preparation has been shown in a previous work.32
Nucleation of PLLA occurred because of concentration fluctuation of PLLA which was associated with the variation of concentration gradient and the non-linear kinetics of solvent evaporation. Then, crystal lamellae of PLLA grew around nuclei from the PLLA–chloroform system, which was driven by solvent evaporation, as shown in Fig. 1(b). Meanwhile, nucleation and crystal growth depleted the disordered PLLA segments, and then the adjacent PLLA segments diffused to the growth point, resulting in a concentration gradient of PLLA chains along the radial direction. The temperature and pressure in the thermostatic oven were constant during the whole process, and the phase equilibrium of the solvent was maintained. Consequently, the concentration of amorphous PLLA phases in the whole system was kept approximately constant. The decrease in the concentration of PLLA phases due to crystal growth can be compensated by the depletion of the amount of solvent associated with slow evaporation. After crystallization, the samples were taken out for further measurements.
Solvent evaporation and the variation of concentration gradient in the system result in fluctuation of local concentration of PLLA chains, and then nuclei are obtained for further crystal growth. The diameter (D) of the ring banded spherulites increases from ca. 50 μm to ca. 100 μm when film thickness decreases from ca. 25 μm to ca. 15 μm, due to the decrease in the number of nuclei per unit volume. For melt crystallization of PLLA blends or copolymers, large ring banded spherulites with diameters ranging from 300 μm to 1500 μm could be formed within 1 hour.3–5,7,26,34 The widths of bright and dark bands grown from melts are ~50 μm and ~30 μm, respectively.3–5,7,26,32,35 The alternating bright and dark bands in spherulites grown from melt are associated with lamellae twisting in crystal packing. The widths of the bands increased on addition of amorphous components in blends or copolymers, because the amorphous phases, e.g. PEG, PEO, PHB, are dispersed among crystal lamellae of PLLA. The widths of the dark bands of ring banded spherulites induced by solvent evaporation are ~5.0 μm, which are much smaller than those of ring banded spherulites grown from melts. Moreover, the dark bands in Fig. 2 are discontinuous, which are different from those of classic ring banded spherulites.
The widths of the bands of ring banded spherulites induced by solvent evaporation are much smaller than those grown from melts. Several factors might be responsible for the differences between the two kinds of ring banded spherulites. First, the crystal growth rate is much lower in the PLLA–solvent system than that in melt crystallization. Second, the temperature (30 °C) is much lower, while the crystallization temperatures in melt crystallization are often 100 °C and above.26–31 Third, the driving force of solvent evaporation for crystal growth is much weaker than that for super-cooling in melt crystallization, and the imbalance of stress of crystal lamellae was strengthened due to the concentration gradient of PLLA. Fourth, solvent molecules are excluded from folded chains of PLLA, and the bands only consisted of PLLA folded chains, without PEO and PEG components. Moreover, the formation of the ring banded morphology may take a new growth mechanism, which is different from the lamellar twisting of classic ring banded spherulite.11,17,18 The birefringent ring banded morphology in Fig. 2 might have originated from a rhythmic crystal growth resulting from non-linear diffusion.20
Fig. 3 shows the one dimensional WAXD patterns of thick PLLA films (tens of microns) induced by chloroform evaporation at 30 °C for 30 days. Typical reflections for the α-crystal of PLLA, including (2 0 0)/(1 1 0) and (2 0 3) reflections,4–6,27–29 were pronounced, which indicated that the morphologies in Fig. 2 were stacked with α-crystals of PLLA. The nucleation and crystal growth of PLLA were induced by the variation of concentration gradient and solvent evaporation, and the crystallization process has been mentioned in the experimental part. The intensities of crystalline peaks are quite weak compared to those of amorphous peaks, indicating that the lamellar packing is not as compact as that of the PLLA crystal structure grown from melt and solution, and also the overall crystallization rate (G) is much slower than those crystallized from melt and solution, due to the much different temperatures used for these two systems.7,24,29,35–38 The relatively slow G could be ascribed to the adsorption of solvent molecules into PLLA films, the long inducement time of nucleation and the weak driving force for crystal growth, from a kinetics points of view. These have been confirmed by the small number of spherulites per unit volume and the small size of spherulite as revealed in Fig. 2.
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Fig. 3 WAXD patterns of PLLA ring banded spherulites collected at different angles in films with thicknesses of (a) 25.0 ± 5.0 μm and (b) 15.0 ± 3.0 μm. |
The sizes of the supra-structures in Fig. 4 ranged from 200 to 400 μm, which were much larger than those of birefringent ring banded spherulites in Fig. 2. Moreover, as shown in Fig. 4, all of the birefringent spherulites were in the center, and the nonbirefringent concentric ring bands developed around the birefringent spherulites. The boundaries of the concentric supra-structures are straight as suggested in Fig. 4(b), and the whole micrograph of Fig. 4(b) was divided into four parts on the basis of the straight boundary lines, and three parts were labeled as A, B, and C. Each part includes a birefringent spherulite in the center and nonbirefringent concentric ring bands around. The concentric ring bands in the outer part are discontinuous in each band, caused by non-linear diffusion-induced rhythmic growth associated with variation of concentration gradient of PLLA chains and fluctuation of concentration of polymer chains. The development of the crystal structures and the concentric ring banded morphologies are predominated by two competitive kinetic processes, the diffusion of polymer chains towards the growth front and the rate of growth.22–24,42,43
As labeled by arrows in Fig. 4(b), concentric ring bands were observed. The thickness of the film was 10.0 ± 3.0 μm, which was thinner than those in Fig. 2, indicating that the amount of PLLA chains per volume for each nucleus was smaller than those in Fig. 2. Consequently, it was difficult to form continuous spherulitic morphologies, due to insufficient polymer chains for further crystal growth. A new morphology of spherulitic supra-structure in the center and concentric ring bands in the outer part was then generated as shown in Fig. 4, which could grow with rhythmic growth mode.20,42 The spherulites in the center were similar to those formed in thicker films in Fig. 2, indicating that the crystal growth started in the center, because there were sufficient polymer chains for crystal growth in the initial stage. The concentric ring bands in the outer part were discontinuous, due to variation of concentration gradient formed by diffusion, crystal growth and fluctuation of polymer chains.22–24 The thickness of the boundary lines between the two supra-structures, which were labeled by lines in Fig. 4(b), was the thinnest, because the amount of polymer chains per unit volume was the smallest due to the diffusion of polymer chains to the growth fronts.
Fig. 5 shows the X-ray diffraction pattern of the PLLA film corresponding to that in Fig. 4. As shown in Fig. 4, concentric ring banded supra-structures were obtained under micro-evaporation. The typical reflections of the PLLA crystal, including (1 0 3)/(0 0 4) and (2 0 0)/(1 1 0) reflections, indicated that the ordered supra-structures were multilayer ensembles of PLLA crystals. However, the reflection peaks were much weaker, which suggested that the packing of the crystal structure was quite loose, and the degree of crystallization was very low, which was less than 10%.
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Fig. 5 WAXD pattern of PLLA concentric ring banded supra-structures grown from solution under solvent micro-evaporation. The film thickness of the PLLA film is 10.0 ± 3.0 μm. |
The nonbirefringent concentric ring bands shown in Fig. 4 were further investigated by AFM. The typical AFM topography images have been reported in our recent work.32 Alternating ridge and valley bands are evident. Lamellar twisting of birefringent ring banded spherulites was not observed, which was the reason for the nonbirefringent optical property.13–17 As presented in Fig. 4, in each selected domain, besides a birefringent spherulite in the center, nonbirefringent ring bands further grew around it. In the first ridge band, the packing of lamellar crystals takes an edge-on mode combined with the flat-on mode due to sufficient supply of polymer chains for growth fronts, and then screw dislocations and lamellae tilt may take place. As a result, the concentric ring banded structure may probably take a similar initiation mode for crystal growth as that for the birefringent spherulite in Fig. 2 and 4.22,23
Fig. 6 clearly reveals the morphological details of the outer ring banded structures. From Fig. 6(a), it can be seen that lamellar crystals grow with the flat-on mode in the valley bands. Instead, tilted and even edge-on lamellae can be seen in proximity to the ridges in Fig. 6(b), which suggests a similar initiation mode for new layers of lamellar crystals in valleys and ridges in the ring banded morphologies. The packing of crystal lamellae takes a flat-on mode, which is different from the growth and packing of crystal lamellae in spherulites. As a result, the bands formed are nonbirefringent.
The two kinds of ring banded morphologies with different optical properties and their correlations with lamellar orientations are illustrated in Fig. 7.15,24,42 The micro-structure of alternating bright and dark bands in birefringent ring banded spherulites is demonstrated in Fig. 7(a). In such spherulites, lamellar growth and packing take flat-on and edge-on modes. The orientation of lamellar crystal changes gradually in the radial direction by lamellar twisting, and the thickness of the bright band includes two different parts, the radial orientation part and the twisting part as shown in Fig. 7(a). While in nonbirefringent ring banded morphology, lamellar growth and packing predominately take a flat-on mode. As a result, the ridge band includes only the radial orientation part as proven in Fig. 7(b). Moreover, the orientation of lamellae in dark bands is perpendicular to that in bright ones, and thus alternating bright and dark optical properties occur in birefringent ring banded spherulites in Fig. 7(a). However, both ridge and valley bands of the concentric ring banded morphology as shown in Fig. 7(b) are arranged by lamellae in a radial direction. As a result, it is nonbirefringent, and alternating ridge and valley bands with different thicknesses are the origin of the concentric ring bands.
The nonbirefringent concentric ring banded morphology is different from classical ring banded spherulites, and its formation is attributed to a nonlinear diffusion-induced rhythmic growth process.20,24,43 The concentric ring banded structures reflect the competition between diffusion flux of polymer chains in solution (J) and the crystal growth velocity (V). The competition can be illustrated by the amount of polymer chains per unit volume diffused to the crystal growth front (δ), where δ = J/V.24,42 The development of alternating ridge and valley bands results from periodic changes of δ along the growth direction. The periodic imbalance of supply (J) and demand (V) of polymer chains results in alternating ring banded structures with periodic variation of thicknesses. When δ is sufficient, a thick ridge band is generated. When δ is insufficient, a thin valley band is formed.
The influence of fluctuation of polymer concentration on the development of structure and morphology should be considered, when the amount of polymer chains diffused into a new growth front was not large enough. In solutions formed in films of tens of microns in Fig. 2 and 4, polymer chains are sufficient for crystal growth in the initial stage of crystallization. The diffusion flux of polymer chains (J) increases a little due to a slight increase in concentration gradient.24,43 As a result, the thicknesses of ridge and valley bands are kept nearly constant. In solutions formed in films of several microns, the influence of fluctuation of polymer concentration is evitable. The diffusion flux of polymer chains (J) increases with the increase in concentration gradient. The growth velocity (V) can be considered as invariable during the growth process. Therefore, the thickness and the band spacing of the initial ridge bands are larger than those of the outer ridge bands.
The variation of concentration is balanced via the decrease of polymer chains by crystal growth and the decrease of solvent by evaporation. Generally, the concentration of the whole solution (C0) is kept approximately constant during the whole process. The growth velocity is faster, and the depletion of polymer chains due to crystal growth cannot be balanced by diffusion of polymer chains adjacent to the growth front. Consequently, the concentration of polymer chains at the growth front (CG) sharply decreases. The development of the structure and morphology is predominated by diffusion of polymer chains and periodic variation of concentration gradient (G). We denote with l the diffusion length of polymer chains to the growth front along the radial growth direction. Accordingly, the concentration gradient G along the growth direction can be calculated as G = (C0 − CG)/l. Then, the diffusion flux of polymer chains (J) from the solution to the growth front is obtained according to Fick's first diffusion law, J = DG = D(C0 − CG)/l, where D is the diffusion coefficient of polymer chains in solution. Thus, the amount of polymer chains per unit volume diffused to the growth front δ can be calculated by δ = J/V = D(C0 − CG)/lV.
Then, the development of the concentric ring banded morphologies with periodic variation of thickness can be illustrated by schematics in Fig. 8. In the initial growth stage of the ridge band, the concentration at the growth front sharply decreases to zero, due to consumption of polymer chains by fast crystal growth. At the same time, the diffusion length l reaches its minimum, as shown in Fig. 8(a). Thus, the concentration gradient G reaches the maximum, G = C0/lmin. As has been previously discussed, the values of diffusion coefficient D and crystal growth velocity V are kept nearly constant during the whole growth process. Therefore, the amount of polymer chains per unit volume diffused to the growth front, δ = J/V = DC0/lminV, is the largest. Consequently, there are sufficient polymer chains at the growth front for further crystal growth. The ridge band in the concentric ring banded morphologies is formed as crystal lamellae growing upward and forward as shown in Fig. 8(b).
The consumption of polymer chains results in rapid decrease of δ. In order to supply more polymer chains, polymer chains from neighboring regions need to cover longer distance to the growth front, which makes l increase. Meanwhile, the increase in diffusion length (l) reduces the concentration gradient along the growth direction (G). Then, the amount of polymer chains is not enough for further crystal growth with constant thickness, and the lamellar crystals in the ridge band cannot grow any longer upward. As a result, the thickness of the ridge band reaches its maximum. As diffusion length (l) increases and concentration gradient (G) decreases, δ becomes smaller and smaller. The amount of polymer chains for further crystal growth becomes fewer and fewer, which cannot supply sufficient polymer chains for further growth with constant thickness, and the thickness of multilayers packed with lamellar crystals rapidly decreases. When concentration gradient G reaches its minimum, growth of most crystal lamellae terminates, and the layers of crystal lamellae stop thickening. Consequently, the valley band is obtained as shown in Fig. 8(c).
The decrease in thickness in the valley band reduces the consumption of polymer chains. The diffusion of polymer chains from neighboring regions gradually balanced the consumption of polymer chains, and the concentration gradient (G) restores to the initial stage. As concentration gradient (G) gradually increases and diffusion length (l) decreases, the amount of polymer chains diffused to the new growth front (δ) becomes sufficient for fast crystal growth. The thickness of the crystal layers increases, and another ridge band will be formed, as shown in Fig. 8(d).
Accordingly, the concentric ring banded spherulitic morphologies with alternating ridge and valley structures in the radial growth direction are produced through a repetition of the rhythmic growth process. The periodic variation of concentration gradient is responsible for the periodic variation of thickness in the concentric ring banded morphologies. The concentration gradient is much smaller than that in melt; thus, the diffusion length (l) of polymer chains in solution is much larger than that in melt. Consequently, the crystallization rate is much slower than that in melt, and the values of thicknesses of ridges and valleys, and band spacing are smaller than those in ring banded spherulites obtained in melt crystallization.
The development of the concentric ring banded morphologies is predominated by a periodic competition between diffusion and growth induced rhythmic growth mechanism, which is associated with periodic variation of concentration gradient in the growth direction. The parameters can be characterized as in the following equation: δ = J/V = D(C0 − CG)/lV. Alternating ridge and valley bands in the ring banded morphologies grow step by step from the center to the outside with periodic variation of concentration gradient.
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