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Heating and Annealing Induced Structural Reorganization and Embrittlement of Solution-Crystallized Poly(L‑lactic acid) Pengju Pan,* Lili Han, Guorong Shan, and Yongzhong Bao State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China S Supporting Information *
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annealing-induced crystalline structural reorganization were discussed. It is found that the solution-crystallized PLLA undergoes crystalline ordering, embrittlement, and stiffening upon subsequent annealing and heating; this would be important to control the microstructure and physical characteristics of semicrystalline polymers in solution processing. Because chloroform is the most common solvent of PLLA and can be easily evaporated in solution crystallization, it is selected as a model solvent of PLLA here. For the solution crystallization, a chloroform solution of PLLA (20 g/L) was cast onto a Petri dish and the solvent was allowed to evaporate at 23 °C. The cast film was further dried in vacuo (∼50 Pa) at 25 °C for 72 h. Small amount of solvent (∼8.5 wt %) was still remained in the dried PLLA film, because the residual solvent in PLLA is difficult to remove after drying below Tg. Since the equilibrium water uptake in PLLA was extremely low (ca. 0.5− 1.0 wt %),17 the residual solvent in solution-crystallized PLLA was mainly organic solvent. The annealing and heating induced structural rearrangement of solution-crystallized PLLA was investigated by wide-angle Xray diffraction (WAXD) and small-angle X-ray scattering (SAXS). As shown in Figure 1a, the Bragg angles of two major reflection peaks (110/200 and 203) are unvaried with annealing at different temperatures for 1 h (Figure 1a). For both the solution-crystallized and annealed samples, the 110/ 200 diffraction peak locates at 2θ ≈ 17.0° and two weak reflections (016 and 206) are present at around 2θ = 25.0°. The characteristic diffraction of α′(δ) crystals at 2θ ≈ 24.5° is
oly(lactic acid) (PLA) is a well-known biobased thermoplastics that can be synthesized from the renewable resources.1 It has been widely used in the biomedical and commodity applications for substitution of the conventional oilbased thermoplastics. The most popular stereoisomer of PLA, poly(L-lactic acid) (PLLA), can crystallize into several crystal polymorphs such as α,2 α′(δ),3−7 α″,8 β,9 γ,10 and ε11 forms with changing the crystallization conditions. Previous studies have showed that the β-form PLLA has a higher tensile strength and modulus than its α counterpart12 and the α-form PLLA possesses a higher Young’s modulus and a better barrier property to water vapor than its α′ counterpart.13 Therefore, investigation on the relationships between crystalline structure and processing condition is of fundamental importance for tailoring the physical performances of PLLA. Solvent and supercritical fluid have been frequently used in the polymer processing such as wet spinning, dry spinning, and solution casting. Because solvent can enhance the segmental mobility and depress the glass transition temperature (Tg) of polymer chains, semicrystalline polymers can crystallize from the solution state even at a low temperature, with the gradual evaporation and desorption of solvent. Solvent is an important factor for the crystallization kinetics, crystalline structure, and crystal transition of PLA.8,11,14,15 It has been reported that PLLA forms new polymorphs (e.g., α″, ε-form) when it is crystallized from the high-pressure CO28 and some specific organic solvents such as tetrahydrofuran (THF) and N,Ndimethylformamide (DMF).11 These unique structural characteristics offer PLLA the improved properties such as larger elongation-at-break16 and higher transparency.8 Because it is difficult to examine the structural changes of polymers in solution crystallization process, the crystallization behavior of polymers from solution as well as the structural characteristics of solution-crystallized polymers are far from well understood. Furthermore, due to the solvent−polymer interactions, solvent may be absorbed by the polymers and is thus difficult to be completely removed in solution processing. The solvent (e.g., water) can disrupt the intermolecular interactions between polymer chains, causing the enhanced free volume and segmental mobility, possible transition of crystalline structure,17 and variation of physical properties.18−20 Therefore, it is essential to understand how the solvent influences the crystalline structure of polymer, which could alter the macroscopic properties. In this study, we investigated the structural rearrangement and change of mechanical properties for solution-crystallized PLLA upon heating and annealing. The mechanisms involved in the heating and © XXXX American Chemical Society
Figure 1. (a) WAXD profiles and (b) crystallite size of solutioncrystallized PLLA after annealing at different temperatures for 1 h. Received: September 22, 2014
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dx.doi.org/10.1021/ma501956f | Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) IR spectra and their second derivatives in 1820−1700 cm−1 range for solution-crystallized PLLA after annealing at different temperatures for 1 h. Temperature-dependent intensities of (b) crystalline- and (c) amorphous-sensitive bands for solution-crystallized PLLA collected in the heating process. The corresponding spectra of amorphous and α-form PLLAs were included in panel a for comparison.
absent.3 Besides, the exothermic peak prior to the major melting peak, which assigns to the α′(δ)-to-α phase transition,3−6 is not detected in the DSC heating curve (Figure S1). Therefore, it is concluded that the α crystals, but not the α′(δ) or α″ crystals,3,8 are generated in the solution-crystallized and further annealed PLLAs. In the solution crystallization, the polymer chains have sufficient mobility and will tend to reach an equilibrium state, which would be favorable to form the thermodynamic stable α crystals, rather than the kineticcontrolled α′(δ) crystals. This is also consistent with the report of Asai et al. that PLLA does not form complex crystal with chloroform.11 As shown in Figure 1a, the intensities of diffraction peaks increase significantly after annealing. Several weak reflections (e.g., 010 and 015 planes) become more pronounced and sharper with annealing. Assuming that the crystals are prefect, the crystallite sizes of 110/200 (D110/200) and 203 planes (D203) were estimated from the half widths of diffraction peak according to the Scherrer’s equation.21 The crystallite size increases with the annealing temperature, corresponding to the sharpening of diffraction peaks (Figure 1b). After annealing at 160 °C for 1 h, D110/200 and D203 increase from 16.0 and 13.2 nm to 25.7 and 19.5 nm, respectively. As shown in the temperature-variable WAXD patterns, the intensities of 110/200 and 203 planes change little during heating at a temperature below Tg (∼55 °C), while they increase gradually in the temperature range of 60−175 °C (panels a and b of Figure S2). At a temperature higher than 175 °C, the intensities decrease drastically because of the melting of crystallites. The crystallite size increases gradually upon heating (panel c of Figure S1). When the temperature is increased to above 160 °C, the changes of crystallite size become more pronounced, because the structural reorganization are accel-
erated at a temperature close to melting point. The remarkable increase of diffraction intensity and sharpening of diffraction peaks suggest enhanced crystallinity, crystalline ordering, and perfection upon annealing and heating. This is consistent with the DSC results, which show that the melting enthalpy of solution-crystallized PLLA increases remarkably after annealing at a temperature above 80 °C (Figure S1). As seen from the Lorentz-corrected SAXS profiles (Iq2 ∼ q) of solution-crystallized PLLA (Figure S3), no discernible peak is observed at a temperature lower than 90 °C, which might be due to the less regular laminar structure of crystalline phase and the wide distribution of amorphous and crystalline laminar thickness. As the temperature is increased from 100 to 140 °C, a scattering peak is observed at around q = 0.37 nm−1, suggesting the ordering of crystalline lamellae upon heating. This is also confirmed by the results of atomic force microscopy (AFM) that regular crystalline lamellae toward the spherulite radial direction are clearly seen after annealing (Figure S4). This scattering peak corresponds to a long spacing of 17.0 nm (2π/q), which is smaller than that of PLLA melt-crystallized at 143 °C (22 nm).22 This suggests that, even after annealing, the solution-crystallized PLLA has thinner crystalline regions than the melt-crystallized one. When the sample is heated to a temperature above 150 °C, the scattering peak shifts to the low q gradually, due to the lamellar thickening occurred prior to the melting of crystallites.22,23 At 180 °C, the long spacing of PLLA is increased to 26.7 nm, which is much larger than that measured at 100−140 °C (17.0 nm). In the FTIR spectra (Figures 2a and S5), no spectral splitting is observed in the solution-crystallized PLLA that is unannealed or annealed below 80 °C, similar to the amorphous sample. However, as the annealing temperature is increased to above 80 °C, quite a few new splitting components such as 1749 cm−1 of B
dx.doi.org/10.1021/ma501956f | Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. 13C CP-MAS NMR spectra for (a) carbonyl, (b) methine, and (c) methyl carbons of solution-crystallized PLLA after annealing at different temperatures for 1 h.
Figure 4. (a) Stress vs strain curves of solution-crystallized PLLA after annealing at different temperatures for 1 h. Plots of (b) tensile strength, (c) modulus, and (d) elongation-at-break as a function of annealing temperature.
ν(CO), 3005 cm−1 of νas(CH3), 2964 cm−1 of νs(CH3), 1443 cm−1 of δas(CH3), 1222 cm−1 of νas(COC) + ras(CH3), 1106 cm−1 of νs(COC), and 1053 cm−1 of ν(C−CH3) (Table S1) are present in the IR spectra. These splitting components, which can be distinctly identified from the second derivatives of original IR spectra, become clearer as the annealing temperature increases. The band splitting, characteristic of the α-form PLLA, is originated from the intermolecular dipole−dipole interactions.24−26 The absence of IR band splitting in solutioncrystallized PLLA indicates the weak intermolecular interactions, and disordered, less regular chain packing in its crystallites. Therefore, it is considered that, upon heating, the rearrangement of lateral packing mode of molecular chains occurs in the crystalline phase. This results in the more dense, ordered chain packing, and stronger intermolecular dipole− dipole interactions in the crystal lattice. From this viewpoint,
this annealing-induced structural reorganization in solutioncrystallized PLLA is somewhat similar to the α′(δ)-to-α phase transition of PLLA occurred in annealing.27 On basis of the temperature-dependent IR spectra (Figure S6), the change of band intensity in the heating process was evaluated. IR bands can be divided into two groups, that is, crystalline and amorphous-sensitive bands. The variation trends of crystalline- and amorphous-sensitive band intensities are opposite with temperature (panels b and c of Figure 2). Three features should be addressed in panels b and c of Figure 2. First, the intensity changes little with heating at a temperature below Tg (∼60 °C). Second, as the sample is heated to 80−100 °C, the intensities of crystalline-sensitive bands increase. The most remarkable change can be observed for the 921 cm−1 band, which is a pure crystalline band and associated with r(CH3) + ν(C−COO) vibrational mode for the 103 helical chains packed C
dx.doi.org/10.1021/ma501956f | Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. (a) Temperature-dependent storage modulus and (b) tan δ curve of solution-crystallized PLLA after annealing at different temperatures for 1 h.
in the crystalline phase of α-form PLLA. The significant increase of crystalline band intensity and decrease of the amorphous band intensity further demonstrate the ordering, perfection of crystalline structure and enhancement of crystallinity upon heating. Third, at the temperature range of 100−160 °C, the band intensity changes little with heating. Because of the crystallite melting, the intensities of crystalline bands decrease and those of the amorphous bands increase with heating to the melting temperature. In the 13C CP-MAS NMR spectra (Figure 3), each carbon of solution-crystallized PLLA, exhibits a broad resonance and several less resolved splitting peaks (e.g., P2, P6, P7, P9). The resonance splitting is a character of α-form PLLA,24,28−30 which has been correlated to the crystallographically inequivalent sites in polymer chain resulted by the intermolecular dipolar interactions and lateral chain packing within the crystal lattice.24 However, the resonance splitting in solution-crystallized PLLA is much less significant than that observed in the α-form PLLA.24,29 This is consistent with the aforementioned WAXD and IR results and demonstrates the weaker intermolecular dipolar interactions, looser and less ordered chain packing in the solution-crystallized PLLA. It is considered that the possible interactions between solvent and PLLA, e.g., the dipole−dipole interactions between C−H band of chloroform and CO, C− H bands of PLLA, may disrupt the dipolar interactions between PLLA chains, which may induce the less ordered chain packing in the crystalline lattice.17 After annealing at 80−140 °C, the splitting peaks (e.g., P1, P2, P6, P7, P9) become more marked and their intensities increase, indicating that the more perfect α crystals of PLLA are generated. Therefore, the annealinginduced solvent desorption and structural rearrangement lead to the enhanced intermolecular interactions and more ordered chain packing in PLLA crystalline phase. Because of the structural reorganization, it is expected the thermal treatment can influence the optical and mechanical properties of solution-crystallized PLLA. Quenched PLLA is transparent, characteristic of the amorphous material. The solution-crystallized PLLA is translucent and its transparency changes little after annealing at 40−80 °C. After annealing at a temperature above 100 °C, a decrease of transparency is seen, ascribed to the enhanced scattering of visible light for the larger crystallites or spherulites formed in annealing (Figure S7). The polarized optical microscopy (POM) results have confirmed that the spherulite size increases and Maltese cross patterns in
spherulites become more pronounced after annealing (Figure S8). The crystalline and physically aged amorphous PLLAs are typically brittle and have a tensile strength of >40 MPa, a Young’s modulus of >1.4 GPa, and an elongation-at-break of
