ISSN 1995-0780, Nanotechnologies in Russia, 2017, Vol. 12, Nos. 3–4, pp. 193–198. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.V. Izraylit, N.G. Sedush, A.V. Bakirov, S.N. Chvalun, 2017, published in Rossiiskie Nanotekhnologii, 2017, Vol. 12, Nos. 3–4.
Synthesis of Nanocomposite Based on Poly(Lactic Acid) and Montmorillonite by In Situ Polycondensation1 V. V. Izraylita*, N. G. Sedusha, A. V. Bakirova,b, and S. N. Chvaluna,b aNational
bEnikolopov
Research Centre “Kurchatov Institute,” Moscow, 123182 Russia Institute of Synthetic Polymer Materials, Russian Academy of Sciences, Moscow, 117393 Russia *e-mail:
[email protected] Received November 23, 2016; in final form, December 13, 2016
Abstract—In the present work, a method of synthesis of polymer nanocomposite based on poly(lactic acid) and layered alumosilicate montmorillonite by in situ polycondensation is described. At reaction temperatures of 160–200°С and catalyst concentration from 0 up to 5 ppt, the samples of composite materials containing 0.5–3 wt % of montmorillonite were synthesized. The composite material is characterized by an exfoliated structure of alumosilicate and a complex of enhanced physicochemical properties. DOI: 10.1134/S1995078017020070
INRODUCTION Polylactic acid (PLA) is one of the most popular biodegradable plastics produced on an industrial scale. Over the last decade, PLA is drawing attention as an ecofriendly substitute for the production of packaging and various single-use articles. There are two ways to synthesize PLA: polymerization of the lactic acid dimer lactide and direct polycondensation of the lactic acid (LA). The polymerization method is preferred in the industrial process of PLA production; however, this multistage approach makes the product more expensive than traditional polymer materials produced at large scale [1, 2]. Polycondensation of LA is an alternative one-step method of PLA synthesis. Due to water generated as a byproduct of the reaction and the complexity of its removal at later stages of the reaction due to the high viscosity of the reaction medium, the equilibrium cannot be shifted towards high molecular weight PLA. This results in low molecular weight and mechanical properties of the polymer [3]. To increase the molecular weight of poly(lactic acid), the reaction is carried out at reduced pressure [4], in solid state [5], or in the presence of chain-extending agents [6]. However, even high-molecular weight PLA is characterized by lower mechanical and barrier properties compared to polyolefins [7]. To improve the barrier, mechanical, and thermal properties of the material, composites containing various nanosized fillers are produced. Layered silicates with a high ratio of linear sizes are the most interesting high-aspect-ratio anisometric fillers [8, 9]. The most popular of them is 1 The article was translated by the authors.
alumosilicate montmorillonite (MMT), a three-layer clay mineral 1 nm thick, 50–500 nm broad, and 200– 800 nm long. The most proficient increase of material properties is observed when the nanoparticles are dispersed evenly, maximizing the interaction surface between the filler and the polymer matrix. The MMT high specific surface area (800–1400 m2 g–1) leads to a proficient increase of barrier, mechanical, and thermal properties after the addition of only several weight percents of the filler [10]. There are three ways to introduce MMT into the PLA matrix: the mixing of polymer and filler in solution [11] or in melt [10] and in situ polymerization— lactide polymerization in the presence of MMT [12]. The first two methods allow obtaining an intercalated or partially exfoliated structure of filler in the polymer. The third method includes the synthesis of polymer in the galleries of MMT, resulting in a composite with the highest degree of exfoliation. The first successful example of intercalative polymerization of nylon-6 was reported in [13]. An attempt to carry out polycondensation of the LA in the presence of MMT was described earlier [14]. Molecules of LA could attach to OH groups of MMT producing a Si–O–C bond inside the galleries. The further growth of the polymer chains proceeds on segments attached to MMT and prevents MMT pallets from aggregation. However, the authors fail to achieve high molecular weight (MW) PLA, which is crucial for practical application of the material. Moreover, they used microwave irradiation, which is hardly implemented at industrial scales. We propose to synthesize a PLA/MMT masterbatch which could be blended with high MW PLA. To achieve such a highly filled exfoliated composite
193
194
IZRAYLIT et al.
material, MMT should be exfoliated before running the in situ polymerization. The aim of the present research was to develop the method of synthesis of composite material based on PLA and layered silicate MMT by in situ polycondensation. The effect of temperature on the rate of growth of the polymer chains, structure, and properties of the resulting composites was studied.
Mw, Da Without catalyst
12 000 10 000
1 ppt 2 ppt 3.5 ppt 5 ppt
(a)
8000 6000 4000
EXPERIMENTAL
2000
An aqueous 90% solution of L-lactic acid (2-hydroxypropionic acid) was purchased from Merck (Germany) and used as received. A stannous octoate catalyst was purchased from Aldrich (Germany). Sodium-modified layered alumosilicate montmorillonite Cloisite Na+ was purchased from Southern Clay (United States). Two types of reaction mixtures of LA and MMT were prepared. In the first case, a certain amount of MMT (0.5–3 wt %) without any preliminary processing was mixed with 90% LA. In the second case, MMT was first exfoliated with water, resulting in a stable water suspension. Then 90% LA was added to this suspension. Water was removed at 80°C at normal pressure. The process was stopped when the concentration of water in the suspension reached 10%. Thus, the reaction mixture of the second type was a 0.5–1 wt % suspension of exfoliated MMT in 90% aqueous LA. Polycondensation was conducted in a cone-shaped glass flask under the flow of nitrogen at 10 mL min–1. At the first step, water (including water formed as a result of low MW oligomers synthesis) was removed at 180°C for 3 h. Then, the temperature was changed and 0–5 ppt of the catalyst was added and the polycondensation lasted for another 8 h. Polycondensation without MMT was conducted in a similar manner. To study MW distribution during the reaction, probes were taken every 2 h. The weight-average molecular weight (Mw) of samples was measured by gel-permeation chromatography using Knauer chromatographic complex with tetrahydrofuran (THF) as the eluent under f low rate of 1 mL min–1 and temperature 40°C; the MW is reported against polystyrene standards. The samples of composites were dissolved in THF and centrifuged to separate the grafted PLA from the pure one. The molecular weight of PLA in the supernatant was measured. The structure of MMT was investigated by smallangle X-ray scattering using X-ray diffractometer Hecus S3-Micropix (CuKα-radiation, λ = 1.542 Å). A Pilatus 100K 2D detector was used. Maximum voltage and current on Xenocs Genix generator were 50 kV and 1 mA, respectively. Slits in the Kratky collimator were set to 0.1 mm. The camera was vacuumed to pressure 3 × 10–2 mm Hg. The exposure was varied from 600 to 2500 s.
0
2
4 6 Reaction time, h
Mw, Da 12 000 10 000
8
(b)
160 C 180 C 200 C 220 C
8000 6000 4000 2000 0
2
4 6 Reaction time, h
8
Fig. 1. Mw of PLA during polycondensation. Influence of the catalyst concentration at the reaction temperature 180°С (a) and influence of the reaction temperature at constant catalyst concentration 2 ppt (b).
The FTIR spectrum of samples was measured by a Nicolet iS FTIR spectrometer. The sample was diluted in methylene chloride and centrifuged. The supernatant was removed and the precipitate was dissolved again. The procedure was repeated 15 times. The final precipitate was dried in a vacuum oven and studied with FTIR. Thermophysical properties of the nanocomposites were studied using a PerkinElmer DSC-7 calorimeter in the temperature range of 0–160°C and heating rate of 10°C min–1; the sample weight was about 20 mg. The calorimeter was calibrated against the indium standard. Thermogravimetric curves were obtained on a Mettler TG-50 of thermoanalytical complex Mettler TA 3000 in nitrogen flow. The temperature range was 25–450°C and the heating rate was 10°C min–1. RESULTS AND DISCUSSION A series of LA polycondensation experiments were conducted without the addition of MMT in order to
NANOTECHNOLOGIES IN RUSSIA
Vol. 12
Nos. 3–4
2017
SYNTHESIS OF NANOCOMPOSITE BASED ON POLY(LACTIC ACID)
Mw, Da 8000
6000
Intensity 10 000
Without catalyst
1 ppt 2 ppt 5 ppt
(a) 2% type-1 reaction mixture 1% type-2 reaction mixture 1% water suspension
(a) 1000
4000
100
2000
10 1
0
195
2
Mw, Da 8000 160 C 180 C 200 C
4 6 Reaction time, h
8
2
3
4
5
6
7
8
Intensity 1% Nanocomposite based on type 1 1% Nanocomposite based on type 2
(b) 1000
(b)
6000 100 4000 10
2000
1 0
2
4 6 Reaction time, h
8
Fig. 2. Mw of PLA during polycondensation of the type-2 reaction mixture (concentration of MMT 1%). Influence of the catalyst concentration at the reaction temperature of 180°С (a) and influence of the reaction temperature at constant catalyst concentration 2 ppt (b).
find the optimal conditions of reaction. The dependence of Mw over reaction time at 180°C and various Sn(Oct)2 concentrations is presented in Fig. 1a. The addition of 1 ppt of the catalyst barely affects the reaction rate and the resulting Mw = 5.5 kDa. The further increase in the concentration of the catalyst leads to higher Mw increase rates. PLA with Mw = 9.5 kDa can be synthesized at 5 ppt of Sn(Oct)2 in 8 h. At the concentration of Sn(Oct)2 2 ppt, a significant increase in the reaction rate is observed with an increase in the reaction temperature from 180 to 200°C (Fig. 1b). This can be associated to a decrease in viscosity of the reaction medium, which promotes water removal. The MMT concentration increase (in the whole range of studied concentrations) in reaction mixtures of both types at the reaction temperature 180°C and Sn(Oct)2 concentration 2 ppt does not affect the resulting Mw. It is important to notice that the MW disNANOTECHNOLOGIES IN RUSSIA
Vol. 12
Nos. 3–4
2
3
4
5
6
7
8
Fig. 3. (Color online) SAXS curves of the type-1 reaction mixture with 2% MMT, the type-2 reaction mixture with 1% MMT and 1% water suspension of MMT (a) and of the composite samples with 1% MMT (b).
tribution of only unbounded PLA is discussed. At the same time, the degree of polymerization for PLA chains grafted to MMT could be sufficiently higher due to the additional catalytic effect of the filler. The catalyst concentration increase (T = 180°C, [ММТ] = 1%) in polycondensation of the type 2 reaction mixture has a very low effect on the reaction rate (Fig. 2a). The reason for this might be the sorption of the catalyst on the surface of MMT and the decrease in its effective concentration. The reaction temperature increase for the type-2 reaction mixture ([SnOct2] = 2 ppt, [ММТ] = 1%) accelerates the reaction (Fig. 2b). Unlike the polycondensation of pure LA, the temperature increase from 160 to 180°C in the reaction with a type-2 mixture significantly affects the reaction rate. Thus, Mw of PLA produced by in situ polycondensation depends on the reaction temperature, the reaction time, and the catalyst concentration. The latter has more influence on the polycondensation of the pure LA. The introduction of the long MMT plates can prevent water removal and the sorption of the cat2017
196
IZRAYLIT et al.
Optical density Nanocomposite MMT
0.6
0.4
0.2 3800
0 4000
3500
3600
3400
3000
2500
2000
1500
1000
500
Wave number, cm1 Fig. 4. (Color online) FTIR spectra of 1% PLA-MMT exfoliated nanocomposite and pure MMT.
Specific heat flow, J/(g °C) Sample 1 (3% MMT, type 1) Sample 2 (1% MMT, type 1) Sample 3 (1.5% MMT, type 2) Sample 4 (1% MMT, type 2)
−1.0
−1.5
−2.0
50
100 Temperature, °C
150
Fig. 5. (Color online) DSC curves of nanocomposite samples with various MMT concentrations.
alyst on the surface of MMT, which leads to a decrease in its effective concentration. The optimal synthesis values for the Sn(Oct)2 concentration and the reaction temperature and time are 2 ppt, 180°С, and 8 h, respectively. At these reaction conditions, the composites were synthesized. The discussion of their structure and properties are presented further.
Exfoliated nanocomposites are the most interesting polymer composite materials for practical application. That is why the study of structural properties of produced materials is very important. SAXS curves of the reaction mixtures and polymer composites are depicted in Fig. 3. A maximum on the scattering curve of the type 1 reaction mixture with 2 wt %. MMT (Fig. 3a) corresponds to periodic structures of the lay-
NANOTECHNOLOGIES IN RUSSIA
Vol. 12
Nos. 3–4
2017
SYNTHESIS OF NANOCOMPOSITE BASED ON POLY(LACTIC ACID)
197
Thermal properties of nanocomposite samples MMT No. concentration, % 1 2 3 4
3 1 1.5 1
Reaction mixture type
Tg, °С
Тc, °С
Tm, °С
1 1 2 2
40 39 36 35
– 122 – 109
– 136, 143 – 127, 134
ered alumosilicate with d spacing of 17 Å, which is higher than 11.2 Å in dry MMT. The difference proves the intercalated structure of MMT in the type-1 reaction mixture. This could happen due to the sorption of water or LA on the surface of MMT pellets inside the galleries. The scattering curve of the type-2 reaction mixture has no maxima, indicating the exfoliated structure of MMT. The reflex on the scattering curve of the composite sample synthesized from the type-1 reaction mixture (Fig. 3b) corresponds to d spacing of 20.5 Å, which is considerably higher than the one in the reaction mixture, 17 Å. This result proves that polymer-chain propagation occurs in the MMT galleries, leading to an increase in d spacing and probably partial exfoliation. As one can see on the scattering curve of the type-2 nanocomposite (Fig. 3b), it has an exfoliated structure, which is crucial for achieving an enhanced barrier and mechanical and thermal properties when compared to intercalated systems [9]. Covalent bonding of PLA to MMT in the nanocomposite, which was synthesized from the type 2
Melt enthalpy, Degree of crystallinity, % J g–1 – 41 – 25
0 34 0 27
reaction mixture with 1 wt % MMT, was proven by FTIR analysis. The comparison of FTIR spectra of the exfoliated nanocomposite after extraction of unbounded PLA (MMT load before extraction was 1 wt %) and dry MMT is shown in Fig. 4. The band at 2995 and 2942 cm–1, which correspond to the CH and CH2 groups, and the band at 1754 cm–1, which corresponds to the CO bond, are specific for PLA and cannot be found in the pure MMT. Moreover, there are shifts in the 3625 cm–1 band of the OH groups and the 1047 cm–1 band of the Si–O–Si bonds of the MMT. This data indicates the formation of covalent bonds between PLA and MMT in the same manner as in [14]. The thermal properties of the nanocomposite samples, which were synthesized at 180°С, 2 ppt of Sn(Oct)2, and various concentrations of MMT from both types of reaction mixtures, were studied by DSC. The curves of second heating are depicted in Fig. 5; the thermal transition points are listed in the table. The glass transition (35–40°С) and melting (127– 143°С) temperatures of the nanocomposites are lower
Residual weight, % PLA
100
3% Nanocomposite
90
1% Nanocomposite
80 70 60 50 40 30 20 10 0 100
200 300 Temperature, °C
400
Fig. 6. (Color online) TGA curves of the PLA sample and nanocomposites of type 1 and 2 with 3 and 1% of MMT, respectively. NANOTECHNOLOGIES IN RUSSIA
Vol. 12
Nos. 3–4
2017
198
IZRAYLIT et al.
when compared to 65 and 180°C, respectively, for industrial PLA. This can be attributed to the low Mw. Earlier it was shown that the increase in Mw from 3.5 to 41.5 kDa leads to a rise in the glass transition temperature of PLA from 27 to 44°C [15]. The degree of crystallinity was calculated based on theoretical melting enthalpy of the 100% crystalline PLA—93 J g–1 [16]. Lower transition temperatures (Tg is 5°C lower and Tm is 10°C lower) and lower degree of crystallinity of the exfoliated samples compared to the intercalated ones can be associated with the specific behavior of macromolecules inside the limited space of MMT galleries. There, the diffusion rate of polymer chains is decreased and the mobility of the polymer molecules is significantly hindered. MMT could also affect the crystallization behavior of PLA. Crystalline domains of composite materials with 1% MMT are presented by small and defective crystallites with low melting temperature. Synthesized composites with MMT concentrations of 1.5 and 3% in exfoliated and intercalated materials, respectively, are characterized by fully amorphous structure. The TGA curves of the composites, which were synthesized at 180°C and 2 ppt of the catalyst from both types of the reaction mixtures, are shown in Fig. 6. The MMT concentration in the type-1 composite was 3 wt %. The MMT concentration in the type-2 composite was 1 wt %. For comparison, the TGA curve of the pure PLA is shown as well. The TGA curves of the composites are nearly identical. The destruction temperature is 230°C for both samples, while it is lower for pure PLA—220°C. Thus, both intercalated and exfoliated composites are characterized by improved thermal stability. This can be due to the slower diffusion of the low-molecularweight decomposition products, which is caused by the presence of the high aspect ratio filler. This is also indicated at higher barrier properties of the synthesized nanocomposites compared to pure PLA. CONCLUSIONS A method of synthesis of exfoliated nanocomposites based on PLA and MMT by in situ polycondensation was developed. It was observed that the reaction temperature had the most effect on the polycondensation due to its influence on the viscosity of the medium and the rate of the byproduct water removal. It was shown that, depending on the structure of MMT in the initial reaction mixture, an intercalated or an exfoliated nanocomposite with improved thermal properties can be synthesized. The covalent-bond formation between PLA an MMT was proved by FTIR spectroscopy. The synthesized nanocomposites are prospective as masterbatches for the production of biodegradable packaging with improved barrier and thermal properties.
ACKNOWLEDGMENTS This research was supported by the Ministry of Education and Science of Russia (project RFMEFI57714X0037). REFERENCES 1. D. A. Garlotta, “Literature review of poly(lactic acid),” J. Polym. Environ. 9, 63–83 (2002). 2. O. I. Bogdanova et al., “Polylactide, a biodegradable biocompatible polymer based on vegetable raw materials,” Ekol. Prom-st' Rossii, No. 5, 15–23 (2010). 3. R. Auras, et al., Poly(Lactic Acid) (Wiley, Hoboken, NJ, USA, 2010). 4. D. K. Yoo, D. Kim, and D. S. Lee, “Reaction kinetics for the synthesis of oligomeric poly(lactic acid),” Macromol. Res. 13, 68–72 (2005). 5. S. I. Moon et al., “Melt/solid polycondensation of l-lactic acid: an alternative route to poly(l-lactic acid) with high molecular weight,” Polymer 42, 5059–5062 (2001). 6. K. Hiltunen, J. V. Seppala, and M. Harkonen, “Lactic acid based poly(ester-urethanes): Use of hydroxyl terminated prepolymer in urethane synthesis,” J. Appl. Polym. Sci. 63, 1091–1100 (1997). 7. D. Ferrari, “High barrier PLA films for flexible packaging,” in Proceedings of the AIMCAL Fall Technical Conference, 2007. 8. P. Bordes, E. Pollet, and L. Averous, “Nano-biocomposites: Biodegradable polyester/nanoclay systems,” Prog. Polym. Sci. 34, 125–155 (2009). 9. S. Sinha Ray and M. Okamoto, “Polymer/layered silicate nanocomposites: A review from preparation to processing,” Prog. Polym. Sci. 28, 1539–1641 (2003). 10. N. Ogata et al., “Structure and thermal/mechanical properties of poly(l-lactide)-clay blend,” J. Polym. Sci., Part B: Polym. Phys. 35, 389–396 (1997). 11. S. Sinha Ray et al., “New polylactide/layered silicate nanocomposites. 1. Preparation, characterization, and properties,” Macromol. 35, 3104–3110 (2002). 12. M.-A. Paul et al., “(Plasticized) polylactide/(organo-)clay nanocomposites by in situ intercalative polymerization,” Macromol. Chem. Phys. 206, 484–498 (2005). 13. Y. Kojima et al., “Synthesis of nylon 6-clay hybrid by montmorillonite intercalated with ε-caprolactam,” J. Polym. Sci., Part A: Polym. Chem. 31, 983–986 (1993). 14. H. L. Cao, P. Wang, and Y. Li, “Preparation of poly(lactic acid)/Na-montmorillonite nanocomposite by microwave-assisted in situ melt polycondensation,” Macromol. Res. 18, 1129–1132 (2010). 15. M. Omelczuk and J. McGinity, “The influence of polymer glass transition temperature and molecular weight on drug release from tablets containing poly (DL-lactic acid),” Pharm. Res. 9, 26–32 (1992). 16. E. Fischer, H. Sterzel, and G. Wegner, “Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reactions,” Colloid Polym. Sci. 990, 980–990 (1973).
NANOTECHNOLOGIES IN RUSSIA
Vol. 12
Nos. 3–4
2017
