Article pubs.acs.org/Macromolecules
Thermally Stabilized Poly(lactide)s Stereocomplex with Bio-Based Aromatic Groups at Both Initiating and Terminating Chain Ends Hiroharu Ajiro,†,‡ Yi-Ju Hsiao,† Hang Thi Tran,§ Tomoko Fujiwara,∥ and Mitsuru Akashi*,†,‡ †
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan The Center for Advanced Medical Engineering and Informatics, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan § Faculty of Chemical Technology, Viet Tri University of Industry, Ministry of Industry and Trade, Tien Kien, Lam Thao, Phu Tho, Viet Nam ∥ Department of Chemistry, The University of Memphis, Memphis, Tennessee 38152, United States ‡
S Supporting Information *
ABSTRACT: In order to improve the thermal stability of polylactides (PLA), conjugation approaches have been applied to both the terminal and initiating chain end of PLA. We selected benzyl alcohol as the initiator to introduce an aromatic group at one end. The terminal hydroxyl group of the resultant PLAs was conjugated with 3,4-diacetoxycinnamic acid (DACA). The modified polymers showed a dramatic improvement in their thermal decomposing temperature (T10) from 326 to 355 °C. The stereocomplexation of the modified PLAs could be achieved by a variety of molecular weight (Mn) combination. The melting temperature (Tm) increased after stereocomplexation with any size of polymer mixture, but the best improvements on thermal properties in both Tm and T10 were obtained by a mixture of specific Mn ranges of enantiomeric PLAs (Mn = 11 400 and 9600). In contrast, the longer and shorter Mn discouraged the simultaneous improvements in the Tm and T10, suggesting that a balance of chain end groups and stereocomplexation efficiency was important.
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PLA under acid and alkaline conditions.19 They insisted that protonation into −OH groups and the subsequent chain-end scission started the degradation under low pH, whereas the intramolecular cyclization reaction accelerated the degradation under high pH conditions. They concluded that the −OH groups played a crucial role in the degradation process under both conditions. Similarly, Nishida and co-workers investigated chain-endmodified PLAs with Ca salt,20 acetyl group,21 and Snalkoxide.22 They discussed that the downshift of degradationonset temperature from 280 °C (Mn = 94 200) to 220 °C (Mn = 138 000) which occurred after treatment with the Ca salt was due to the production of alkoxide groups on the hydroxyl end group,20 whereas the higher shift of the degradation-onset temperature from 260 °C (Mn = 181 000) to 300 °C (Mn = 165 000) for the PLA treated with anhydrous acetic acid was mainly due to the removal of the Sn catalyst during the treatment.22 Such reports imply that the design of the chainend structure and molecular weight of PLA are important to improve their properties against thermal decomposition.
INTRODUCTION Polylactides (PLAs) have been widely used as bioabsorbable and biodegradable materials, from medical surgery devices1 to the packaging containers.2 There are numerous studies on PLA, including thermal and mechanical property modifications.3−5 The thermal properties of such useful PLA materials have relied on the development of polymerization techniques that improve molecular weights and structural regularity.6−11 Although nanocomposite12−15 and blend16,17 approaches can improve the thermal and mechanical properties of materials, the structural control of polymers represents the essential path to develop these chemical and physical properties. In regard to improving the thermal degradation temperature, the chemical modification of the end group(s) of the PLA polymer chain is one of the more successful approaches. For example, Kim and co-workers reported that 4-armed PLA with −OH-terminated groups (the number-average of molecular weight: Mn = 31 700) improved its degradation temperature when the −OH groups were replaced with −COOH, −NH2, and −Cl groups by thermogravimetric analysis.18 They also stated that the poorer polarity of −NH2 and −Cl groups increased the thermal resistance against polymer decomposition as compared to PLAs with −OH groups. In the same year, de Jong and co-workers proposed the degradation mechanisms of © XXXX American Chemical Society
Received: April 5, 2013 Revised: June 18, 2013
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Furthermore, other chain-end modifications were also applied to functionalize PLAs, such as electroactivity using aniline trimer-capped PLA,23 a controlled surface immobilization with silyl group termination,24 and an efficient dispersion of carbon nanotube by polymerization initiation from the wall of the carbon nanotube.25 On the other hand, it is known that the melting temperature (Tm) of polymers is primarily controlled by the polymer main backbone structure rather than the chain-end groups. An improvement in the Tm of polymers is indispensable for various industrial applications. In the biomedical applications of PLA, for example, it has been reported that steam sterilization (high steam pressure, 135 °C) and dry heat sterilization (160−190 °C) have shown disadvantages such as deformation and degradation of the PLA copolymers.26,27 Ikada and co-workers discovered stereocomplex formation from optically active poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) and found that the Tm increased from 180 to 230 °C after stereocomplexation, as evidenced by differential scanning calorimetry (DSC) and X-ray diffraction (XRD).28 The improved Tm value would enable the PLA to maintain its material shape in actual applications. The structure and physical properties of the PLLA/PDLA stereocomplex have been well-studied29 with many approaches such as infrared spectra,30 photomicrographs,31 and molecular simulations with powder diffraction32 as well as DSC and XRD. It is noteworthy that the thermal stability of PLA was enhanced by the van der Waals interaction of the stereocomplex formation.33 The polymer−polymer interaction in the stereocomplex caused a different helical packing from the homopolymer (PLLA or PDLA) and could lead to higher hydrolytic stability and more acidic hydrolysis products.34 Several PLA architectures have been applied to form a stereocomplex. For example, triblock copolymers,35 branched PLAs on the polyglycidol,36 star-shaped PLAs,37 and 3-armed PLAs38 have formed stereocomplex, resulting in an improved Tms. Among them, the stereoblock PLAs showed excellent efficiency in stereocomplex formation.39−41 Recently, the stereocomplex formation conditions were also improved,42 and it has been reported that PLAs can be immobilized onto a flat surface.43 In our group, PLAs have been employed for thin film coating approaches, such as physical adsorption onto substrates by layer-by-layer assembly44,45 and the inkjet system.46 Previously, we reported improved thermal properties for synthetic biodegradable polymers using the hyperbranched architecture, which was obtained from 4-hydroxycarboxylic acid (4HCA) and 3,4-dihydroxycinnamic acid (DHCA) as monomers. This polymer showed high melting points between 220 and 260 °C and a high thermally decomposing temperature (T10: 10% weight loss temperature) between 290 and 320 °C.47 Inspired by natural aromatic compounds, we conjugated 3,4-diacetoxycinnamic acid (DACA), which is a derivative of 4HCA, with the hydroxyl group at the chain end of PLA48 and other polymers,49 and achieved dramatic improvements in the T10. Recently, we introduced natural aromatic moieties into PLA as both terminal and initiating groups in order to stabilize it thermally.50 In this study, we investigated the thermal properties of stereocomplexed PLAs with natural aromatic compounds conjugated at both ends as terminal and initiating groups (Figure 1). The stereocomplex was prepared as precipitates from a mixed solution. In order to observe the effects of the
Figure 1. Chemical structures of polylactides with the conjugated chain end groups.
chain-end modification, three kinds of molecular weights were selected, and the effects of the molecular weights on the Tm and T10 related to the amount of chain end groups were discussed.
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EXPERIMENTAL SECTION
Materials. L-Lactide (LLA; Musashino Chemical Laboratory, Ltd., Japan) and D-lactide (DLA; Musashino Chemical Laboratory, Ltd., Japan) were recrystallized from ethyl acetate and then dried in vacuo at room temperature for 24 h. Benzyl alcohol (Tokyo Chemical Industry, Ltd., Japan) was distilled with CaH2. Thionyl chloride (SOCl2), 3,4dihydroxycinnamic acid (DHCA), and acetic anhydride (Ac2O) were used without purification. Measurements. The Mn values of the PLAs were determined by size exclusion chromatography (SEC). A JASCO Chem NAV system was used with polystyrene standards at 40 °C, equipped with PU-2080, AS-2055, CO-2065, and RI-2031. Two commercial columns (TSKgel SuperH4000 and TSKgel GMHXL) were connected in series, and tetrahydrofuran (THF) was used as the eluent. The 1H NMR spectra were collected with a NMR spectrometer (JEOL FX400) at 400 and 600 MHz. Attenuated total reflection (ATR) infrared (IR) spectra were obtained with a Spectrum 100FT-IR spectrometer (PerkinElmer). The interferogram was coadded four times and Fouriertransformed at a resolution of 4 cm−1. The X-ray diffraction (XRD) patterns were taken with a Rigaku RINT2000. Cu Kα (λ = 0.154 nm) was used as the X-ray source, and the instrument was operated at 40 kV and 200 mA with a Ni filter (Rigaku ultra X18). The samples were examined at 2θ = 5°−35° at a scan rate of 0.5°/min. Differential scanning calorimetry (DSC) was performed with a SEIKO Instruments EXSTAR 6000 series and DSC 6120 under a nitrogen atmosphere. The heating and cooling rates were 10 °C/min, and the second heating was monitored. Thermogravimetry analysis (TGA) was performed with a TG/DTA 6200 instrument under a nitrogen atmosphere. The heating rate was 10 °C/min. Optical rotation was measured by digital polarimeter DIP-181 (JASCO) with 1 mg/mL at a wavelength of 589 nm in CHCl3 at 25 °C. Syntheses of Poly(L-lactide) (PLLAb) and Poly(D-lactide) (PDLAb). PLLAb and PDLAb were prepared according to the literature.50 The typical procedure is as follows. In a round-bottom flask, the required amount of LLA was dissolved in toluene under a N2 atmosphere, and then the required amounts of benzyl alcohol and SnOct2 were combined and heated at 120 °C for 2 h. After the reaction, the product was dissolved in chloroform and purified by reprecipitation over methanol twice. Syntheses of DACA Conjugated-PLLAb (DACA-PLLAb) and DACA Conjugated-PDLAb (DACA-PDLAb). DACA-PLLAb and DACAPDLAb were prepared according to the literature.50 The typical procedure is as follows. In a round-bottom flask, DHCA (10 g, 55.5 mmol) was dissolved in 20 mL of dry pyridine under a N2 atmosphere at 0 °C. After 30 min, 30 mL of acetic anhydride (318 mmol) was added and stirred for 30 min at 0 °C. Next, the reaction mixture was B
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heated at 130 °C for 5 h. After the reaction, the solvent was evaporated, and the product was recrystallized from toluene. The obtained compound was washed by 0.1 N HCl repeatedly. The yield was 50%. In a round-bottom flask, DACA (0.264 g, 1 mmol) was dissolved in 0.6 mL of dichloromethane. Next, 0.5 mL of SOCl2 and 0.79 μL of dimethylformamide (DMF) were added and heated at 60 °C for 7 h. The reaction mixture including 3,4-diacetoxycinnamoyl chloride (DACC) was directly transferred to the next reaction without further purification. PLLAb (0.525 g) was dissolved into 1.875 mL of dichloromethane and 0.094 mL of pyridine. The mixture was then introduced into the DACC (0.23 g, 0.83 mmol) at 0 °C and stirred for 1.5 h. The reaction mixture was then stirred at room temperature for another 24 h. After the reaction, the product was washed by 0.1 N HCl repeatedly and reprecipitated in ethanol. Stereocomplex Preparation. The corresponding PLLAb and PDLAb were dissolved in acetonitrile at 5 mg/mL at room temperature. Then, they were mixed and placed for 48 h until the precipitation generated. The precipitation was filtrated and washed by acetonitrile to recover in this study.
rotation [α] of the polylactides in Table 1 shows the adequate values to form stereocomplex.51,52 Table 2 lists the series of DACA-PLLAb and DACA-PDLAb used in this study. Starting from the PLLAb and PDLAb in Table 1, the corresponding DACA-PLAs were then obtained. The conjugation of DACA with the PLAs proceeded quantitatively, and the molecular weight differences among the three Mn levels were maintained despite the slightly broadened polydispersity (PDI) (Table 2, entries 1 and 2) and the small increases in the Mn (Table 2, entries 3, 4, and 6). The specific optical rotation [α] of the DACA conjugated polylactides decreased the values in Table 2, probably not due to the racemization but due to the large moiety of DACA groups in the polymer. To confirm the chain-end structure of the polymers, the proton NMR spectra were collected. The 1H NMR spectra of PLLAb1, PLLAb2, and PLLAb3 are shown in Figure 2. As the
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RESULTS AND DISCUSSION Table 1 lists the series of PLLAb and PDLAb used in this study. By changing the molar ratio of the benzyl alcohol initiator Table 1. PLA Properties in This Studya entry
sample ID
Mn (×103)
PDI
Tmc (°C)
T10d (°C)
1 2 3 4 5 6
PLLAb1 PDLAb1 PLLAb2 PDLAb2 PLLAb3 PDLAb3
35.8 26.2 8.5b 8.3b 2.8 2.1
1.13 1.21 1.19b 1.20b 1.50 1.44
165 162 168b 161b 126 124
243 233 237b 230b 237 222
[α]D25 (deg) −154 +153 −148 +152 −148 +148
± ± ± ± ± ±
2 4 2 3 2 3
a
PLA was prepared by ring-open polymerization with each lactide. Reference 50. cDetermined by DSC in the second heating scan. d Determined by TGA. b
against the L-lactide monomer, the three molecular weights of PLLAb were obtained; PLLAb1 (Mn = 35 800), PLLAb2 (Mn = 8500), and PLLAb3 (Mn = 2800) (Table 1, entries 1, 3, and 5). Similarly, PDLAb1 (Mn = 26 200), PDLAb2 (Mn = 8300), and PDLAb3 (Mn = 2100) were also prepared (Table 1, entries 2, 4, and 6). We used these PLAs to investigate the molecular weights effects on stereocomplexation and their thermal properties because those values have significant intervals. Most of the molecular weights estimated by 1H NMR spectroscopy were slightly lower than those of SEC. Tsuji and Ikada reported that exclusive formation of the stereocomplex was not possible if the D- and L-polymers contain the opposite monomer unit higher than 3 mol %, which corresponded to an optical purity of 94% ([α]D25 < −147 for PLLA and [α]D25 > +147 for PDLA). The specific optical
Figure 2. 1H NMR spectra of the enlarged drawing of PLLAb3 (a), PLLAb2 (b), and PLLAb1 (c). 1H NMR spectra of PLLAb3 (d), PLLAb2 (e), and PLLAb1 (f) (CDCl3, room temperature, in 400 MHz).
molecular weight decreases (from f to d), the peak intensities around 7.4−7.3 ppm appeared more strongly, corresponding to the initiating group of benzyl alcohol. Benzyl alcohol is known to be one of the components in natural fragrances such as roses. We believe this is a biologically friendly modification when the carboxylic acid moieties of the PLAs are protected by the initiating polymerization with benzyl alcohol, together with the protection of the hydroxyl groups by the natural aromatic
Table 2. DACA-PLA Properties in This Studya
a
entry
sample ID
Mn (×103)
PDI
Tmc (°C)
T10d (°C)
1 2 3 4 5 6
DACA-PLLAb1 DACA-PDLAb1 DACA-PLLAb2 DACA-PDLAb2 DACA-PLLAb3 DACA-PDLAb3
21.3 25.3 11.4b 9.6b 2.1 2.5
1.65 1.59 1.16b 1.12b 1.47 1.44
167 168 169b 158b 124 124
343 346 355b 353b 326 326
[α]D25 (deg) −141 +150 −131 +135 −111 +114
± ± ± ± ± ±
3 1 1 2 1 2
DACA-PLA was prepared by polymer reaction with the corresponding PLA. bReference 50. cDetermined by DSC in the second heating scan. Determined by TGA.
d
C
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then filtered and washed by acetonitrile. The structural analysis of the stereocomplexed PLAs was performed by XRD. The crystallinity was estimated by the heat of fusion by a second scan using DSC and varied from 13.0% to 44.4%. The smaller Mn addition tended to attain high crystallinity, but the Tm values did not always result in higher values. One reason could be that the larger amount of chain end groups also affected the Tm. We might postulate the possibility of effective stereocomplexation and higher crystallinity, tuning the proper chain end groups in the near future. This idea is supported by the fact that the [2 + 2] cycloaddition reaction can be induced by UV irradiation.48 Figure 4 shows the XRD patterns of the series of SCb, which are composed of PLLAb and PDLAb with various Mn values
compounds. The same protection with benzyl groups in PDLAb was also confirmed (Supporting Information, Figure S1). The 1H NMR spectra of the DACA-PLAs are shown in Figure 3. Additional peaks around the aromatic regions were
Figure 3. 1H NMR spectra of the enlarged drawing of DACA-PLLAb3 (a), DACA-PLLAb2 (b), and DACA-PLLAb1 (c). 1H NMR spectra of DACA-PLLAb3 (d), DACA-PLLAb2 (e), and DACA-PLLAb1 (f) (CDCl3, room temperature, in 400 MHz).
clearly observed in DACA-PLLAb1, DACA-PLLAb2, and DACA-PLLAb3, especially including the distinct peaks of the double bond in the DACA groups at 7.7 and 6.4 ppm. The acetyl groups of DACA also appeared at 2.2 ppm, and those peak intensities based on the chain-end groups increased when the molecular weight decreased as well. The same increase was observed in the series of DACA-PDLAb (Figure S2). In addition, the FT-IR/ATR spectra of the PLAs were analyzed to confirm the introduction of the DACA groups into the PLA. The additional peaks at 1640 cm−1 were observed, which were based on the vibration of the CC bond (Figures S7 and S8). Next, we examined stereocomplex formation with various combinations as shown in Table 3. At first, PLLAb and PDLAb were dissolved in acetonitrile at 5 mg/mL at room temperature, respectively. Next, they were mixed and incubated for 48 h until the precipitation occurred (Figure S9). The precipitate was
Figure 4. XRD patterns of PLLAb2 (a), SCb1 (b), SCb2 (c), SCb3 (d), SCb4 (e), and SCb5 (f).
along with the XRD pattern of the PLLAb2 homopolymer for comparison. All of the SCb samples had diminished homopolymer crystal peaks at 2θ = 15, 16, and 18.5 as shown in Figure 4a, and the peaks at 2θ = 12, 21, and 24 were observed corresponding to the stereocomplex structure (Figure 4b−f). Similarly, the series of DACA-SCb which are composed of DACA-PLLAb and DACA-PDLAb of various Mn is shown in Figure 5, along with the XRD pattern of DACA- PDLAb2 as a comparison for the homopolymer. All of the mixed samples
Table 3. PLA and DACA-PLA Stereocomplex Properties Initiated by Benzyl Alcohola L-isomer
D-isomer
entry
sample ID
sample ID
Mn (×10 )
sample ID
Mn (×103)
Tmc (°C)
T10d (°C)
ΔH (J/g)
χe (%)
1 2 3 4 5 6 7 8 9 10
SCb1 SCb2 SCb3 SCb4 SCb5 DACA-SCb1 DACA-SCb2 DACA-SCb3 DACA-SCb4 DACA-SCb5
PLLAb1 PLLAb2 PLLAb3 PLLAb1 PLLAb3 DACA-PLLAb1 DACA-PLLAb2 DACA-PLLAb3 DACA-PLLAb1 DACA-PLLAb3
35.8 8.5 2.8 35.8 2.8 21.3 11.4 2.1 21.3 2.1
PDLAb1 PDLAb2 PDLAb3 PDLAb3 PDLAb1 DACA-PDLAb1 DACA-PDLAb2 DACA-PDLAb3 DACA-PDLAb3 DACA-PDLAb1
26.2 8.3 2.1 2.1 26.2 25.3 9.6 2.5 2.5 25.3
230 230b 194 207 207 205 224b 195 200 200
286 313b 282 271 273 358 359b 350 287 343
18.5 47.1 46.5 49.9 55.7 47.6 38.4 45.7 26.3 63.0
13.0 33.2 32.7 35.1 39.2 33.5 27.0 32.2 18.5 44.4
3
a Stereocomplexation was achived by the mixing of acetonitile solution of L- and D-isomers. bReference 50. cDetermined by DSC in the second heating scan. dDetermined by TGA. eχ was calculated on the basis of the specific heats of fusion of stereocomplex crystals (142 J/g).
D
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were thus analyzed, and were confirmed as a stereocomplex structures.
Figure 7. DSC analyses of DACA-PDLAb1 (a), DACA-PDLAb2 (b), and DACA-PDLAb3 (c). The second heating scan with 10 °C/min.
with DACA. Interestingly, the DACA-PDLAb series, which are PLAs with aromatic groups at both initiating and terminating chain ends, showed additional exothermic peaks at around 100 °C under the same analytical conditions as the PDLAb series. Since they are second scans, the incomplete crystallization during the first cooling process would cause the crystallization peaks at the second scan. The presence of DACA and benzyl groups at both chain ends could give impair crystallization during the cooling process, possibly allowing more flexible polymer conformations. This interpretation is also supported by the results of the stereocomplex in Figures 8 and 9.
Figure 5. XRD patterns of DACA-PDLAb2 (a), DACA-SCb1 (b), DACA-SCb2 (c), DACA-SCb3 (d), DACA-SCb4 (e), and DACASCb5 (f).
The thermal properties of these PLA samples were investigated. As a control, the Mn effects on PLLAb and PDLAb against the Tm were examined. The Tm of PLLA has been reported within a range of Mn values from 5 × 102 to 7.5 × 104, which increases with higher Mn up to 180 °C.53,54 Figure 6 shows the DSC traces of the PDLAb series: PDLAb1 (Mn =
Figure 8. DSC analyses of SCb1 (a), SCb2 (b), SCb3 (c), SCb4 (d), and SCb5 (e). The second heating scan with 10 °C/min.
Figure 6. DSC analyses of PDLAb1 (a), PDLAb2 (b), and PDLAb3 (c). The second heating scan with 10 °C/min.
The DSC traces of SCb series stereocomplex are shown in Figure 8. The SCb1 with the highest Mn combination showed its Tm at 172 and 230 °C (Figure 8a), implying the existence of homopolymer crystallization, whereas the SCb2 with a medium Mn combination showed its Tm at only 224 °C such that stereocomplexation would occur predominantly (Figure 8b). As expected, the SCb3 with the smallest Mn combination showed a much smaller Tm at 194 °C (Figure 8c) than SCb1 and SCb2, and this low Tm improved after combination with the large Mn (Figure 8d,e). SCb4 and SCb5 showed lower Tm values (207 and 207 °C) than SCb3, but homopolymer crystallization was not observed. On the other hand, the DSC traces of the DACA-SCb series stereocomolex, which are conjugated with natural aromatic
26 200), PDLAb2 (Mn = 8300), and PDLAb3 (Mn = 2100). The Tm of PDLAb1 (162 °C) and PDLAb2 (161 °C) ranged within reasonable values, and the smallest PDLAb3 (124 °C) indicated a low molecular weight effect, related to the chain-end groups. The same results were obtained as per the PLLAb series (Table 1, entries 1, 3, and 5). Figure 7 shows the DSC traces of DACA-PDLAb1 (Mn = 25 300), DACA-PDLAb2 (Mn = 9600), and DACA-PDLAb3 (Mn = 2500). The Tm values of these polymers were detected at 168, 158, and 124 °C, which showed the same tendency as the PDLAb series in Figure 6. The slight decrease from 161 °C (PDLAb2) to 158 °C (DACA-PDLAb2) might indicate the effect of the additional chain-end groups at the terminal groups E
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Figure 10. TGA charts of stereocomplex: (a) DACA-SCb2, (b) DACA-SCb1, (c) DACA-SCb3, (d) DACA-SCb5, (e) SCb2, (f) SCb1, (g) DACA-SCb4, (h) SCb3, (i) SCb5, and (j) SCb4. Figure 9. DSC analyses of DACA-SCb1 (a), DACA-SCb2 (b), DACASCb3 (c), DACA-SCb4 (d), and DACA-SCb5 (e). The second heating scan with 10 °C/min.
DACA-SCb4, and DACA-SCb5). Although only the DACASCb4 including the low Mn showed lower T10 values than SCb2 and SCb1, most of the DACA-conjugated stereocomplex showed high T10 values in the range of 343 and 359 °C. Even DACA-SCb4 (T10 = 287 °C) still had higher values than before DACA conjugation: SCb4 (T10 = 271 °C), PLLAb1 (T10 = 243 °C), and PDLAb3 (T10 = 222 °C). However, the best T10 was observed for DACA-SCb1 (T10 = 358 °C) and DACASCb2 (T10 = 359 °C), which were composed of Mn over 9600. Since the Tm of DACA-SCb2 (Tm = 224 °C) was apparently superior to that of DACA-SCb1 (Tm = 205 °C), a moderate range of Mn values of the PLAs should be required for the simultaneous improvement of Tm and T10.
groups at both chain ends, are shown in Figure 9. The Tm of the homopolymer was not observed in DACA-SCb1 (Figure 9a), which was observed in the corresponding SCb1 (Figure 8a). This is probably because the stereocomplexation of PLA with aromatic groups at both chain ends slows to form homopolymer crystallization, as mentioned in Figures 7 and 8, which allows the organization of the PLLA and PDLA chains for favored stereocomplex formation. However, it was observed that there was a decrease in the Tm for DACA-SCb1 to 204 °C versus the Tm of the complete stereocomplex, which also supported imperfect crystallization due to the larger Mn (Figure 9a). However, the Tm value of DACA-SCb2 was maintained at 224 °C without any homopolymer melting point in spite of the slight decrease as compared to SCb2 (Figure 9b). Other than that, the same decrease in the Tm was observed in DACA-SCb3, DACA-SCb4, and DACA-SCb5 due to the low molecular weight effect (Figure 9c−e). Overall, the conjugation at both chain ends would allow stereocomplexation more preferably than homopolymer crystallization, and the double amount of chain ends could generate a lower Tm in the case of low Mn values. Finally, the decomposition temperature values, T10, were examined. T10 comparisons with constant Mn values around 8000 have been previously reported50 (Figure S11). As listed in Tables 1−3, such distinct T10 increases by DACA introduction were also recognized among PLLAb1 (T10 = 243 °C) vs DACA-PLLAb1 (T10 = 343 °C), PDLAb1 (T10 = 233 °C) vs DACA-PDLAb1 (T10 = 346 °C), and SCb1 (T10 = 286 °C) vs DACA-SCb1 (T10 = 358 °C) using Mn values around 30 000 (Figure S10) as well as among PLLAb3 (T10 = 237 °C) vs DACA-PLLAb3 (T10 = 326 °C), PDLAb3 (T10 = 222 °C) vs DACA-PDLAb3 (T10 = 326 °C), and SCb3 (T10 = 282 °C) vs DACA-SCb3 (T10 = 350 °C) using Mn values around 2000 (Figure S12). Thus, it was revealed that the simultaneous modification of Tm and T10 values was possible with any Mn values. Furthermore, we compared the T10 values among the stereocomplex as shown in Figure 10. The stereocomplex samples included the low Mn (SCb3, SCb4, and SCb5), which hampered the dramatic increase in the T10. This tendency was also recognized in the series of DACA-PLAs (DACA-SCb3,
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CONCLUSION A series of PLAs with natural aromatic groups at both terminal and initiating chain ends were synthesized with various Mn combinations. These stereocomplex showed simultaneous improvements in both Tm and T10 regardless of the Mn. The best Tm and T10 values were obtained by DACA-SCb2, composed of DACA-PLLAb2 (Mn = 11 400) plus DACAPDLAb2 (Mn = 9600). These results showed that a lower amount of chain-end groups would be desirable to improve the Tm and that DACA conjugation at the hydroxyl groups of the PLAs would be desirable to improve the T10. The present results showed that it was possible to control the thermal properties represented by the Tm and T10 values using natural aromatic compounds at both terminal and initiating chain ends groups.
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ASSOCIATED CONTENT
S Supporting Information *
1 H NMR spectra of PDLAb and DACA-PDLAb; FT-IR/ATR spectra of PLLAb, DACA-PLLAb, PDLAb, and DACA-PDLAb; photos of PLA solutions in acetonitrile and stereocomplex precipitation; TGA charts compared among PLAs, with the similar molecular weights. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax +81-6-6879-7359; Tel +81-6-6879-7356; e-mail akashi@ chem.eng.osaka-u.ac.jp (M.A.). Notes
The authors declare no competing financial interest. F
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (S) from the Ministry of Education, Culture, Sports, Science and Technology (23225004). This work was also supported in part by the MEXT project, “Creating Hybrid Organs of the future” at Osaka University. We acknowledge Drs. T. Kida, M. Matsusaki, and T. Akagi for the fruitful discussions.
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dx.doi.org/10.1021/ma400709j | Macromolecules XXXX, XXX, XXX−XXX
