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Abstract We prepared a PMMA-PLLA-PMMA triblock copolymer using ATRP. We analyzed the structure and properties of the copolymer with IR, GPC, 1HNMR, thermogravimetric (TGA), and differential scanning calorimetry (DSC). We improved the thermal stability by modifying the samples: the initial temperature of thermal decomposition increased from 244 ËšC to 350 ËšC and the Tg increased from 58 ËšC to 107 ËšC. We employed a new wash-extraction method to remove Cu from the ATRP. The method achieved satisfactory results. The Kinetic plot for the ATRP of MMA with PLLA initiation shows a linear increase of reaction time with ln ([M] 0/ [M]). The results of our study indicate that it is possible to achieve grafted chains with well-defined molecular weights, and block copolymers with narrowed PDI.
PLA is a kind of aliphatic polyester that is polymerized by lactic acid. It is biologically degradable, chemically inert, and bio-compatible. PLA can be obtained from sustainable biological resources and is innocuous. Moreover, its performance can be modulated through copolymerization with other monomers. Thus, it is an important bio-degradable universal polymer material. PLA can be used as a wrapper, fiber, and thermoplastic material, and in biomedicine. However, the glass transition temperature (Tg) of PLA is too low (60 ËšC) to meet the requirements for wide application. Studies regarding PLA modification for specific uses have intensified in recent years in an attempt to overcome the drawback of thermal instability. Copolymerization is one of the principal methods. PMMA is an important polymer material for its chemical stability, mechanical performance, and relatively high thermal decomposition and glass transition temperatures. In addition, its processability, weatherability, and electrical insulating properties are also remarkable. In this study, we set out to improve the thermal stability of PLA by copolymerizing PMMA with PLA. Rohman G. reported a method for preparing PMMA-PLA hydrolysis oligoester by interpenetrating polymer networks. Robert M. reported using the ATRP method and a bpy(CH2O-PLA-OC(O) C(CH3)2 Br)2 initiator to prepare a bpy(PLA-PMMA)2 copolymer. Tianqi Liu used RO-PLA-Br and HO-PLA-Br as initiator with the ATRP method to prepare a PLA-b-PMMA diblock copolymer. However, no study has successfully improved the thermal stability of PLA-PMMA.
ATRP is a new polymerization process that consists of three components: a halide initiator, a transition metal salt, and an electron donor (RX/MtnXn/Lm). Compared with other polymerization methods, ATRP can reduce the copolymer's weight and improve the controllability of the reaction. Consequently, it has become an important method of polymer structure design. In addition, compared with living ionic polymerization, ATRP has better controllability and can be applied to most monomers under milder reaction conditions. Thus, is has considerable potential for use in industry. In particular, ATRP can copolymerize polycondensate when modifying the structure for specific uses.
We started by improving the thermal stability of PLA. Using LLA as the raw material, we added 1,4-butylene glycol. Through ring opening polymerization, we obtained PLA (HO-PLLA-OH) with a special molecular weight. Then, we prepared the Br-PLLA-Br initiator from PLA with two hydroxide radicals. Through ATRP's reaction with MMA, we obtained a PMMA-PLLA-PMMA copolymer with a controllable molecular weight. We then examined the thermal properties of this block copolymer.
1.2.1 Preparation of the polylactide
We added 1.0 g purified LLA, 1,4-butylene glycol (0.025 %, mol%), catalyst Sn(Oct)2 (0.1 %, mol %) to a 10 mL ampule. After 30 min of vacuum pumping we welded, cut, and sealed the mixture with an alcohol burner. We heated the mixture for 2 h in a130 ËšC oven. Following the reaction, we removed the ampule, scraped it, and placed the product and glass fragments into a clean conical beaker. We then added 20 mL chloroform. After the product dissolved, we filtered out the glass fragments. We condensed the filtration to 5 mL, then precipitated the white polymer in 50 mL alcohol. We filtered out the HO-PLLA-OH product and vacuum dried it to a constant weight.
1.2.2 Preparation of the Br-PLLA-Br initiator
We dissolved 10 g HO-PLLA-OH(Mn=22500, GPC) in 40 mL methylene dichloride (DCM) and added pyridine. We put the reaction bulb in an ice bath, then added α-bromopropionyl bromide (dissolved in DCM, HO-PLLA-OH¼šPyridine¼šα-bromo-propionyl bromide = 1:5:10, mol) dropwise, stirred the mixture for 2 h at 0 â„ƒ, and then stirred it for 48 h at room temperature. We filtered out the pyridinium, washed it with DCM, collected the filtrate, precipitated it with methanol after concentration to obtain the Br-PLLA-Br, and vacuum dried it to a constant weight.
1.2.3 Preparation of the PMMA-PLLA-PMMA copolymer
We added Br-PLLA-Br, cuprous chloride (CuCl), 2,2- bipyridyl (bpy), MMA (1:4:8.5:2000, mol), vacuum pumped repeatedly, sealed the ampule, and put it in an 80 ËšC oven. After a certain amount of time, we removed the ampule, scraped it, and put the product and glass fragments into a clean conical beaker. We added DCM to dissolve the product and filtered out the glass fragments. We concentrated the product under reduced pressure, added 30 mL alcohol to precipitate the product, washed it five times until the wash solution became colorless, collected the product, and vacuum dried it to a constant weight to obtain the crude product PMMA-PLLA-PMMA.
Fig. 2.1 shows the IR spectrogram of the PMMA (I), PLLA (II), and PMMA-PLLA-PMMA (III). The figure shows that after copolymerization, the typical PMMA-α-methyl characteristic peak in 1385-1388 cm-1 and methylene peak in 2950 cm-1 is strengthened. These results indicate that MMA is connected to PLLA. In the IR spectrogram of (III), the 1761 cm-1 and 1732 cm-1 peaks indicate there is a copolymer (1760cm-1 is PLLA's characteristic peak, and 1730 cm-1 is ester's characteristic peak of PMMA).
Fig. 2.2 is the 1H-NMR spectrogram of PLLA and the PMMA-PLLA-PMMA copolymer. δ1.5 and 5.1 correspond to methyl's (a) and methylene's (b) hydrogen peaks, respectively. Since in PMMA and α-methyls with different configurations have different chemical shifts, we thus have syndiotactic methyl (δ0.78 [d1]) and isotactic methyl (δ0.95 [d2]). In addition, the δ1.7 (c) and δ3.5 (e) hydrogen peaks belong to the -CH2- and hydrogen in carbomethoxy, respectively. Through area integration of peak a and we can obtain the mol ratio of the PLLA and PMMA blocks. We can then obtain the block molecular weight of the copolymer.
2.2.1 Removal of Cu from crude PMMA-PLLA-PMMA
Al2O3 chromatographic column, and precipitation and dissolution are common methods for removing catalyst Cu from ATRP products. Although the Al2O3 column is effective in removing Cu with a low residual quality of 5.55 mg/kg, Al2O3 has a strong adsorption effect on PLLA and its copolymer. Thus, the productive rate of PMMA-PLLA-PMMA is very low (52.14 %). In this paper, we used the filter wash/extraction method to remove copper ions from products. After extracting them five times, the products contained nearly no Cu (residual quality = 0 mg/kg), with a productive rate of > 85 %.
2.3.2 Effect of PMMA-PLLA-PMMA polymerization reaction time
As shown in Fig 2.1, we sealed the ampule in order to prepare the Br-PLLA-Br (Mn = 22500, GPC) initiator for reaction with MMA. Fig. 2.3 shows the kinetics curve of the initiation reaction. There is a linear relationship between ln([M] 0/ [M]) and time. Thus, the reaction rate and monomer concentration satisfy the requirements of first order kinetics.
Fig. 2.3 Kinetic plot for the ATRP of MMA with PLLA initiation.
According to Fig. 2.4, molecular weight increases with the conversion rate, while the Mw/Mn remains constant, and narrowly distributed. Thus, the polymer system follows the characteristics of the ATRP polymerization possess that is Controlled/living characteristics.
2.3 Thermal properties of the copolymer
Using DSC, we studied the PMMA copolymer chain length's effect on the melting point, glass transition temperature, and degree of crystallinity. The results are shown in Table 2.1.
Table 2.1 DSC data for PLLA, PS, and the copolymer.
The copolymer's Tg increased from 58 ËšC to 107 ËšC when we increased the PMMA block (Table 2.1). Although PMMA's molecule weight in the copolymer was 2.5 times the PMMA homopolymer's molecular weight, the Tg of the copolymer never reached the Tg of the homopolymer (Tg = 124 ËšC) because of the effect of the PLLA chain. In addition, the PLLA and PMMA blocks are highly compatible, and we observed no phase disengagement with the PMMA chain ranging from 0 to 46500. The crystal peak and melting peak of the copolymer both disappeared when we increased the PMMA block because of the polymer's melting point and crystallinity. Thus, the copolymer transitioned from partial crystallization to an amorphous state. Increasing the copolymer chain causes the chain to intertwine. On the other hand, randomizing the PMMA block damages the typical arrangement of the PLLA, decreasing its degree of crystallinity.
To study PLMA-PLLA-PMMA's thermal stability, we performed TGA on a copolymer with a different molecular weight.
Table 2.2 Parameters of the TGA curve for PLLA and PMMA-PLLA-PMMA.
At 250 ËšC, the pyrolysis of PLLA (line 1) caused it to undergo continuous heat weight loss (Fig. 2.5). When the initial decomposition temperature and weight retention rate decrease from 50 % to 10%, the copolymer's thermal property was better than of PLLA. Following copolymerization, the initial decomposition temperature increased from 243 ËšC to 350 ËšC. We observed that even a short PMMA block (Mn = 790) caused an increase in the initial decomposition temperature of 100 ËšC. Thus, different PMMA block chain lengths do not effect the initial decomposition temperature and weight retention rate.
In this study, we reacted a Br-PLLA-Br initiator with MMA using the ATRP copolymerization process to obtain a PMMA-PLLA-PMMA block copolymer. We also conducted thermal property analysis using DSC and TGA. Our results indicate that compared with PLLA, the copolymer's thermal properties improved greatly. The initial decomposition temperature increased from 244 ËšC to 350 ËšC and the Tg increased from 58 ËšC to 107 ËšC. This ATRP reaction is a first order kinetics process, indicating that this reaction has controllability and narrow distribution of products.