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Synthesis and characterization of polylactide–poly(methyl methacrylate) copolymer by combining of ROP and AGET ATRP
Journal of Industrial and Engineering Chemistry 18 (2012) 993–1000
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Synthesis and characterization of polylactide–poly(methyl methacrylate) copolymer by combining of ROP and AGET ATRP Chantiga Choochottiros, Eunha Park, In-Joo Chin * Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2011 Accepted 7 November 2011 Available online 4 February 2012
Block copolymers of polylactide (PLA) and poly(methyl methacrylate) (PLA–PMMA) were synthesized by the combination of ring-opening polymerization (ROP) and activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP), where PLA was prepared as macroinitiator with active bromo end group (PLA–Br). Tin octoate (Sn(oct)2) and benzyl alcohol were applied as the initiation system for ROP of lactide. During AGET ATRP, copper (II) chloride (CuCl2) with N,N,N0 ,N00 ,N00 pentamethyl-diethylenetriamine (PMDETA) was used as the catalyst system including Sn(oct)2 as reducing agent. At the feed ratio [PLA–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA] of 1/1/9.6/0.45/100, the mole fraction of the PMMA block was 0.6 as determined by 1H NMR. Thermal stability of PLA was enhanced by incorporating of PMMA as block copolymers. In addition, blend between of PLA and PLA– PMMA copolymer was investigated and 5 phr of PLA–PMMA showed optimum condition to decrease in Young’s modulus and increase in impact strength. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polylactide Poly(methyl methacrylate) AGET ATRP Block copolymer
1. Introduction Polylactide (PLA) is thermoplastic polyester which is obtained from renewable resources. PLA is a crystalline thermoplastic with clear and transparent appearance. In addition, PLA has relatively high strength and modulus, biocompatibility, and biodegradability. Therefore, PLA is a good candidate in both of the industrial packaging field and the biocompatible/bioabsorbable medical device market. Limitations of PLA are brittleness, insufficient impact strength and low thermal stability. In order to improve its properties, various studies have been conducted by blending with non-biodegradable resins [1], preparing as copolymers, or as functionalized polymers [2]. For example, when PLA was blended with a rubbery polymer in order to improve impact strength, in spite of an increase in toughness, the blend showed varying degrees of success [3,4]. In addition, transparency of PLA was diminished, and blending capacity was limited by compatibility/ miscibility between the rubbery polymer and PLA [5]. Chemical modification of polymer by preparing as copolymer can attract much interest due to the controllability of the structural architecture, its properties, and molecular weight which depend on the molecular composition of copolymer. Many studies of PLA copolymers were reported using various methods, for example, (i)
* Corresponding author. Tel.: +82 32 860 7480; fax: +82 32 860 5178. E-mail address: [email protected] (I.-J. Chin).
two-step method by combining PLA and PEG to obtain PLA–PEG amphiphilic block copolymer for drug carriers [6], and (ii) by introducing a reactive functional monomer on the TPO chain before it was reacted with PLA in order to enhance miscibility of PLA/TPO blends [7]. In view of the foregoing, PLA was modified mainly to improve structural or mechanical properties but its clarity was rarely of concern. To retain transparency of PLA, PLA and PMMA blends were prepared by mixing with impact modifier which consists of methyl methacrylate unit as shell layer and the polymer having alkyl acrylate unit as core [8]. Cygan and Brake [9] reported an improvement in impact strength of PLA by using methyl methacrylate–butadiene–styrene (MBS) copolymer as impact modifier. Transparency of the compound was controlled by proper balance between the impact modifier and PLA. PMMA or acrylic polymers are good candidates in maintaining transparency of the compound. Fujii et al. [8] investigated blending of PLA and PMMA with core–shell type of acrylic copolymer as impact modifier. The results showed an improvement in impact strength including transparency and heat resistance. In order to improve simultaneously the impact property and clarity of the compound, diblock copolymers of PLA and PMMA were designed. In the past, the preparation of PLA–PMMA copolymer was carried out through functionalization of PLA with the end group of active methyl methacrylate (MMA) monomer and subsequent polymerization by free radical initiator [10] or atom transfer radical polymerization (ATRP) [11]. Recently, ATRP has
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.153
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Scheme 1. Synthesis of PLA–PMMA copolymer.
gained rapidly increasing popularity due to the relatively mild reaction conditions, availability of plenty of monomers, initiators, catalysts, specific functionalities and various architectures [12]. However, the typical catalyst of ATRP, which is a transition metal complex, can easily be oxidized to the higher oxidation state. Therefore, ATRP needs some requirements such as special handling procedure under inert atmosphere, where oxygen or other oxidizing agents should be removed. Matyjaszewski and Jakubowski [13] presented a new procedure by using initiator with a radically transferrable atom or group, catalyst complex, and reducing agent. This method is named as activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP). In this study, we report an alternative route for the synthesis of diblock copolymer of PLA and PMMA through the combination of ring opening polymerization (ROP) and activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP), where PLA was modified as macroinitiator for AGET ATRP (Scheme 1). 2. Experimental 2.1. Materials L-Lactide was supplied by Cheil Industries in Korea and recrystallized from diethyl ether and vacuum dried prior to use. CuBr (99.999%, Aldrich) was used as received. N,N,N0 ,N00 ,N00 pentamethyl-diethylenetriamine (PMDETA) was purchased from Tokyo Chemical Industry Corp. (Japan). 2-Bromoisobutyryl bromide (98%), tin (II) 2-ethylhexanoate, ammonium bicarbonate (minimum 99%), and 4-tert-butylbenzyl alcohol (BBA, 98%) were purchased from Aldrich. Methyl methacrylate (99%, Aldrich) was purified following the procedure reported in the literature [14]. Polylactide (PLA) pellets for compounding were a kind gift from NatureWorks LLC, USA.
2.2. Instruments and equipments 1
H NMR and 13C NMR spectra were obtained with a Varian Inova 400 at 400 MHz under ambient temperature, using chloroform-d and tetramethylsilane (TMS) as the corresponding solvent and the internal chemical shift standard, respectively. Fourier Transform Infrared (FTIR) spectra were obtained from a Bruker Equinox 55 spectrometer. The number-average molecular weight (Mn) of the obtained polymer was measured by gel permeation
chromatography (GPC) analysis on a Water Breeze HPLC System. Thermogravimetric analysis (TGA) was performed with a TA instruments Q50 thermogravimetric analyzer in the range of ambient temperature to 700 8C at a heating rate of 10 8C/min, under nitrogen flow. Differential scanning calorimetry (DSC) was conducted using Perkin Elmer, Jade DSC, where nitrogen gas was purged into the DSC cell with a flow rate of 19.8 mL/min. Measurements were carried out by using 2–4 mg of samples in sealed aluminum pan. The samples were first heated from 25 8C to 175 8C at the heating rate of 10 8C/min and were annealed for 5 min at this temperature to erase previous thermal history, followed by cooling to 25 8C with the same rate. The second heating was subsequently made to 175 8C at the heating rate of 10 8C/min. In case of PLA–PMMA copolymers, the maximum temperature was 240 8C. The thermograms of the first and second DSC heating run were both recorded. The glass-transition temperature (Tg) was taken as the temperature at the midpoint of the corresponding heat-capacity jump in the second heating run. The melting temperature (Tm) of each sample was determined from the maximum of the endothermic peaks, and cold crystallization temperature of each sample was determined from the maximum of the exothermic peaks in the second heating run. Young’s modulus was measured under uniaxial elongation at room temperature in accordance with the ASTM D638 standard using a UTM from Hounsfield Test Equipment. Each sample had a dog-bone shape, and the average of at least five measurements was reported. Notched Izod test (CEAST, code 65451000) of the blends was performed in accordance with the ASTM D256 standard at room temperature. 2.3. Ring-opening polymerization of lactide L-Lactide (3 g, 21 mmol) was weighed into a round bottom flask, where toluene (10 mL) was added. The reaction vessel was immersed in oil bath with controlled temperature at 120 8C, and the reaction was set up as reflux reaction under nitrogen atmosphere. After benzyl alcohol (0.21 mmol) was added to lactide, the initiator (0.21 mmol) was added in the form of a 1 M solution in toluene. After 24 h, the reaction was stopped by precipitation in cold methanol, and the products were dried in vacuum. FTIR (KBr, cm1): 3440 (OH stretch), 2947 (C–H stretch), 1769 (C5 5O stretch), 1454 (C–H bend), 1213–1000 (C–(C5 5O)–O stretch). 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.15 (H of methine), 4.3 (H of methine end group), 1.54 (CH3).
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2.4. Polylactide macroinitiator (PLA–Br)
2.6. Blending of PLA and PLA–PMMA copolymer
PLA was prepared as macroinitiator for copolymerization with MMA monomer by following the method of Zhao et al. [15]. In a 100 mL three-neck round-bottom flask, PLA (1 g, 0.08 mmol) was dissolved in 25 mL of methylene chloride. Triethylamine (0.39 mL, 2.8 mmol) was added to the solution, and the mixture was stirred under N2 and cool to 0 8C. Then, 2-bromoisobutyryl bromide (0.35 mL, 2.8 mmol) in methylene chloride (5 mL) was added dropwise via a funnel for over 30 min. The reaction mixture was further stirred at room temperature overnight, and it was washed with the solution of 3% NH4HCO3 and water. The combined organic layer was precipitated in methanol and dried in vacuum. 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.14 (H of methine), 4.4 (H of methine end group), 1.94 and 1.97 (H of C(Br)(CH3)2), 1.58 (H of methyl in PLA), 1.3 (H of methyl end group).
PLA[b]–PMMA copolymers (run 6) with 1, 3, 5, and 10 phr were blended with PLA by solution blending. For example, 1 phr of PLA[b]–PMMA (run 6) (0.4 g) was dissolved in 50 g of chloroform and mixed with 40 g of PLA in 750 g of chloroform to obtain the mixed solution. The mixed solution was stirred for 2 h, and the film was cast by evaporating the solvent in a conventional oven at 60 8C for a day and in vacuum oven at 60 8C for 2 days.
2.5. Synthesis of block copolymer PLA–PMMA via AGET ATRP The block copolymer PLA–PMMA was prepared via AGET ATRP of MMA monomer with CuCl2/PMDETA as the catalyst system and PLA–Br macroinitiator including Sn(oct)2 as reducing agent (Scheme 1(c)). PLA–Br (0.3 g, 0.04 mmol), CuCl2 (5.38 mg, 0.04 mmol), 2 M of Sn(oct)2 (90 ml, 0.018 mmol), PMDETA (80.2 ml, 0.38 mmol), and MMA (400.48 mg, 4 mmol) were added in a 100 mL three-neck round bottom flask, where toluene (1 mL) was used as solvent. The mixture was heated at 90 8C under N2 for 7 h. After cooling, the reaction mixture was diluted with chloroform and then precipitated in methanol, followed by filtration and drying in vacuum. 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.14 (H of methine in PLA), 3.59 (H of CH3 in PMMA), 1.8–2.0 (H of methane in PMMA, overlapped with H of CH3 in macroinitiator), 0.84 and 1.0 (H of methyl in PMMA). 13 C NMR in CDCl3 (d, ppm): 169.5 (C of carbonyl in PLA and PMMA), 125 and 128 (C of benzyl end group), 69 (C of methine in PLA), 51.7 (C of –OCH3 in PMMA), 44.7 (C of quaternary carbon in PMMA), 31.2 (C of CH2–Br end chain), and 16.6 (C of methyl in PLA and PMMA).
3. Results and discussion 3.1. Synthesis of PLA via ring opening polymerization PLA is conventionally prepared by anionic or coordination– insertion ring-opening polymerization of lactide monomer initiated by nucleophiles such as alcohols. Tin 2-ethylhexanoate (tin octoate, or Sn(oct)2) is a highly efficient catalyst with low risk of racemization [16]. In this work, two systems of ring-opening polymerization (ROP) of lactide, i.e. with and without solvent, were carried out including tin octoate and butyl benzyl alcohol as initiator and coinitiator, respectively. Yields of the obtained PLA from the solution system (PLA[s]) and the bulk system (PLA[b]) were 81% and 92%, respectively. FT IR and 1H NMR were applied to confirm the ROP of lactide. In Fig. 1, FT IR of PLA, both of PLA[s] and PLA[b], showed characteristic peaks of C–(C5 5O)–O stretching at 1213–1000 cm1, C5 5O stretching at 1769 cm1, and a broad peak of hydroxyl end group at 3440 cm1. In addition, an absence of the peak at wavenumber of 935 cm1, which is assigned to COO ring breathing mode of lactide monomer, supported the completion of ROP [16]. Characteristic peaks of protons of PLA are presented in Fig. 2. Protons of methine (H-a) and methyl (H-b) groups of lactide monomer at 5.00 and 1.54 ppm were shifted to 5.15 (H-c) and 1.54 (H-d) ppm, respectively (Fig. 2). PLA has an aromatic ring as an end chain which is obtained from butyl benzyl alcohol, and 1H NMR shows proton peaks around 7.4 (H-e) ppm. The degree of PLA polymerization was calculated using the quantitative analysis of 1 H NMR spectra. The integral peak of H-d at 1.54 ppm attributable to the methyl proton of PLA and that of H-f at 4.3 ppm attributable to the methane proton at the chain end of PLA was peaks of
Fig. 1. FT IR spectra of (A) lactide and (B) PLA.
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Fig. 2. 1H NMR spectra of (A) lactide and (B) PLA.
interest. The degree of polymerization (DP) of PLA was calculated using the following equation: DP ¼
IH-d =3 IH-f =1
(1)
DP of PLA[s] and PLA[b] were 41 and 31, respectively. Also, the number-average molecular weight (Mn) of PLA[s] and PLA[b] were 7560 g/mol (PdI 1.15) and 4873 (PdI 1.34), which were measured by GPC using polystyrene as standard for calibration. 3.2. Synthesis of macroinitiator (PLA–Br) As mentioned in the previous section, synthesized PLA in this study had an aromatic ring at one end and a hydroxyl group at the other end. An active hydroxyl group was chosen to react with 2bromoisobutyryl bromide in order to yield PLA–Br macroinitiator. FT IR analysis showed a decrease in intensity of the hydroxyl group at 3440 cm1 as a result of bromination (data was not shown). 1H NMR spectrum of PLA–Br (Fig. 3(A)) shows new proton peaks at 1.94 and 1.97 ppm (H-g) corresponding to methyl proton of 2-bromoisobutyrates. In addition, the degradation temperature of PLA–Br was delayed to 277 8C, compared to PLA (196 8C), which will be discussed later. This implies the presence of the halide group at the chain end [17]. 3.3. Copolymerization of MMA by AGET ATRP The third step was the preparation of PLA–PMMA copolymer by AGET ATRP, which is shown in Scheme 1(c). The reaction was carried out as a homogeneous system, as PLA–Br was soluble in toluene, a conventional solvent for ATRP. Both of PLA[s]–Br and PLA[b]–Br were copolymerized with MMA by the same manner. Successful copolymerization of PLA[s]–PMMA and PLA[b]–PMMA were confirmed by 1H NMR and 13C NMR analysis. Characteristic protons of PMMA, which belong to –OCH3, –CH2–, and –CH3, appeared at 3.59 (H-h), 1.8–2.0 (H-i), and 0.84 and 1.0 (H-j) ppm, respectively, as shown in the 1H NMR spectrum of PLA[s]–PMMA (Fig. 3(B)).
Furthermore, resonances of the –C–CH3 (H-j) in the PMMA block reveal twin peaks at 0.8–1.1 ppm, which are attributed to the random and syndiotactic structure of PMMA [18]. 13C NMR spectrum (Fig. 4) shows characteristic carbon peaks of the PMMA block at 51.7 ppm (–OCH3) and 44.7 ppm (–C–), carbon peak from the PLA block at 69 ppm (–CH–), and the overlap peaks of PLA and PMMA blocks at 169.5 ppm (C5 5O) and 16.6 ppm (–CH3). In addition, quantitative analysis of 1H NMR spectrum was performed to determine the mole ratio of the PMMA unit and the PLA unit. The integral ratio of the signals originated from the PMMA block at 3.59 (H-h) and from the PLA–Br segment at 5.15 ppm (H-c) were used as follows: f MMA ¼
ðIH-h ð100=3Þ ½IH-h =ð100=3Þ þ IH-c ð72=1Þ
(2)
In this experiment, two different PLA macroinitiators were used; one was obtained from the solution-ROP (Table 1, runs 1–5) and the other from the bulk-ROP (Table 1, runs 6–8), as mentioned in the previous section. However, both of macroinitiators showed similar molecular weights (PLA[s]–Br = 7716 g/mol and PLA[b]–Br = 6629 g/ mol). In all cases, homogeneous reactions were carried out under nitrogen atmosphere, and thus the copolymer was obtained by precipitation in methanol. The effect of different reaction conditions in preparing the PLA–PMMA copolymer was investigated. Table 1 shows molecular weight obtained from GPC measurement and the mole ratios of PLA and PMMA in the PLA–PMMA copolymers. When the feed molar ratio of the MMA monomer increased from 100 to 400, there was a decrease in fPMMA. However, PLA[b] showed higher mole fraction of PMMA than PLA[s], even with the same feed molar ratio (Table 1, runs 1 and 6). The effect of the transition metal complex, e.g. CuCl2/PMDETA, was important to the grafting of PMMA as well. When the molar feed ratio of the transition metal complex was reduced to one half of the macroinitiator, fPMMA was significantly decreased (Table 1, runs 4 and 5). As a result, the molar feed ratio of [PLA[b]–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA] as 1/ 1/9.6/0.45/100 produced highest fPMMA of 0.6 (Table 1, run 6). Hence, the relationship between the reaction time and the mole fraction of
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Fig. 3. 1H NMR spectra of (A) PLA–Br and (B) PLA–PMMA copolymer.
PMMA was also investigated for the reaction in Table 1 (run 6). Fig. 5 shows linear relationships of reaction time (from 5 to 50 min) versus fPMMA and reaction time versus Mn which increased from 0.12 to 0.60 and 7749 to 10035 g/mol, respectively. 3.4. Glass transition temperature and melting temperature of PLA– PMMA copolymers As the mole fraction of PMMA in the PLA–PMMA copolymer between PLA[s] and PLA[b] macroinitiator is different, the structure
of the PLA–PMMA copolymer was investigated in detail. DSC analysis of PLA[s] and PLA[b] was performed by scanning from 25 8C to 175 8C in order to determine the glass transition temperature. According to the literature amorphous PLA exhibits Tg in the range of 50–60 8C [19]. Fig. 6 shows the second heat scans of PLA[s] and PLA[b]. While PLA[s] showed a Tg at 54.5 8C (Fig. 6(A)), the Tg of PLA[b] was not detected during the second heat scan (Fig. 6(B)). This implies the solvent effect during the ROP reaction on the crystal structure of PLA. Percent crystallinity of PLA was calculated by using the following equation: % crystallinity ¼ 100
CH (PLA)
O C (PLA, PMMA)
CH3 (PLA, PMMA)
DH m / f D Hm
(3)
where DHm is the measured heat of fusion, f is the weight fraction / of PLA (in this case, f = 1) and DHm is the enthalpy of fusion for a
CDCl3 880
0.7 0.7
CH 2 Br
0.6 0.6
10000
chain end
770
PLA, chain end
C
9000
660
0.4 0.4 0.3 0.3
550
0.2 0.2
(PMMA)
8000
440
0.1 0.1
0.0
180
160 140 120
Fig. 4.
13
100 80 δ (ppm)
60
40
C NMR spectrum of PLA–PMMA copolymer.
Tg(°C)
OCH3 (PMMA)
fPMMA
Mn(g/mol)
0.5 0.5
20
0
10
20 30 40 reaction time (min)
50
330 60
Fig. 5. Molar fractions of PMMA (*), Mn (&), and the glass transition temperature (*) as a function of the reaction time for PLA[b]–PMMA copolymers (run 6).
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Table 1 AGET ATRP of MMA with CuCl2/PMDETA catalyst system in toluene at 90 8C with reaction time of 50 min.
a
Run
PLA
[PLA–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA]
fPMMA
fPLA
1 2 3 4 5
PLA[s]
1/1/9.6/0.45/100 1/1/9.6/0.45/100a 1/1/9.6/0.45/200 1/0.5/4.8/0.22/100 1/0.5/4.8/0.22/200
0.13 0.08 0.09 0.04 0.06
0.87 0.92 0.91 0.96 0.94
6 7 8
PLA[b]
1/1/9.6/0.45/100 1/1/9.6/0.45/200 1/1/9.6/0.45/400
0.60 0.30 0.17
0.40 0.70 0.83
Mn
Mw
PDI
8686 6734 8266 6970 7788
12,574 9499 11,231 9489 10,263
1.45 1.41 1.36 1.36 1.32
10,035 8029 5845
13,400 11,156 7891
1.34 1.39 1.35
Reaction temperature was 120 8C.
fraction of PMMA increased. The fact that the Tg of the PLA[b]– PMMA copolymer increases with the polymerization time indicates an increase of the amorphous PMMA segment proportional to the mole fraction of PMMA, as determined by 1H NMR. The Tg of PLA[s]–PMMA showed a tendency to approach the Tg of syndiotactic PMMA which was reported as 100.9 8C [24]. 3.5. Thermal stability Thermal degradation of PLA[s] and PLA[b] were investigated by TGA in Fig. 8. Both PLA[s] and PLA[b] showed single step degradation and the onset temperature was 196 8C and 210 8C, respectively. Thermal stability of PMMA is known to be superior to that of PLA, so the thermal stability of the PLA–PMMA block copolymers was expected to be improved compared to the neat PLA. In Fig. 8(A), PLA[s]–PMMA showed an increase in the thermal stability compared to PLA[s], with two-step degradation. The onset of the first step occurred ca. 264 8C and the onset of the second step occurred ca. 361 8C. The result was similar to the degradation behavior of amphiphilic block copolymers of MMA with PEO. Krishnan and Srinivasan referred to the first and second step of degradation of PMMA-co-PEO corresponding to the chain-end initiation from the vinylidene ends and random scission within the polymer chain, respectively [25]. In the case of PLA[b]–PMMA, TGA thermogram (Fig 8(B)) shows three steps of degradation. The onset of the first step of the weight loss was slightly decreased to around 173 8C with only ca. 10% weight loss, and the second step of the weight loss occurred around 255 8C with about 50% weight loss. The onset of the last step occurred around 352 8C. Borman et al. [26] and Colombani et al. [27] suggested that the C–Br bond at the
heat flow (endo up)
heat flow (endo up)
crystal having infinite crystal thickness (94 J/g for PLLA) [20]. The crystallinity of PLA[s] and PLA[b] was calculated to be 62 and 72%, respectively. This information also supports the solvent effect on the packing structure of PLA. Introduction of the amorphous PMMA block into PLA may interrupt the regularity of the arrangements of PLA chains, thereby reducing the crystallinity and decreasing in brittleness [21]. Fig. 6(C) shows a Tg of PLA[s]–PMMA at 55.3 8C. The Tg was not significantly varied when PMMA was introduced to PLA[s]. Considering the melting temperature of PLA[s] and PLA[s]–PMMA, PLA[s] has one peak at 158 8C, while PLA[s]–PMMA has double peaks; smaller peak at 151 8C and larger peak at 155 8C (Fig. 6(C)). The lower endotherm (occurring at lower temperature) is due to the melting of original crystals and the higher temperature endotherm is due to the melting of crystals that were recrystallized during heating after melting of the original crystal [22]. It was difficult to determine the Tg of PLA[b]–PMMA by scanning from 25 8C to 175 8C, therefore the temperature range was changed to 60 8C to 175 8C. Fig. 7 shows Tg of PLA[b] and PLA[b]–PMMA (run 6) at 43 8C and 69 8C, respectively. It is clear that the amorphous PMMA increased the Tg of PLA–PMMA block copolymer. In addition, both of PLA[s]–PMMA and PLA[b]–PMMA showed single Tg, which refers to miscibility between the PLA and PMMA segment. Zhang et al. [23] also reported that the PLLA/PMMA blend had only one Tg which occurred during crystalline phase melt, and that two components mixed together. In addition, the Tg of PLA[b]–PMMA (run 6) was also investigating as a function of the mole fraction of PMMA. In Fig. 5 the Tg (open circle) shows an increase of Tg with the reaction time. In other words, the Tg of PLA[b]–PMMA increased as the mole
(C)
(B)
(A)
40
(B)
(A)
60
80 100 120 temperature (°C)
140
160
Fig. 6. DSC second heat scans of (A) PLA[s], (B) PLA[b], and (C) PLA[s]–PMMA.
-40
-20
0
20 40 60 temperature (°C)
80
100
120
Fig. 7. Glass transition temperature of (A) PLA[b] and (B) PLA[b]–PMMA (run 6) obtained from the second heat scan.
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100
999
240
(A) Young’s modulus (MPa)
weight (wt%)
80 60 40
20
160
80
0 100
200
300 400 temperature ( C)
500
600
neat PLA
1 phr
3 phr
5 phr
10 phr
sample
1000
(B)
808 weight (wt%)
0
Fig. 9. Young’s modulus of the PLA/PLA[b]–PMMA copolymer blend containing different amount of the PLA[b]–PMMA (run 6) copolymer.
606
linkages at ca. 170 8C, (2) unsaturated end groups at ca. 250 8C, and (3) saturated chains at above 330 8C [28].
404
3.6. Mechanical testing
202
0 100
200
300 400 temperature ( C)
500
600
Fig. 8. TGA thermograms of PLA (. . .. . .), PLA–Br (——), and PLA–PMMA (—); (A) PLA[s] and (B) PLA[b].
terminal chain end of the low molecular weight PMMA yielded low thermal stability. The other probability may be due to the characteristic degradation of PMMA which obtained by free radical polymerization. PMMA usually exhibits three-step degradation, that consists of the degradation of (1) head-to-head
PLA[b]–PMMA (run 6) was blended with PLA resin to measure mechanical properties of the blend. A series of copolymers with 1, 3, 5, and 10 phr of PLA[b]–PMMA (run 6) were prepared. Young’s modulus of the blend was plotted with the block copolymer composition, as shown in Fig. 9. Young’s modulus of neat PLA is ca. 176 MPa. As the content of the PLA[b]–PMMA copolymer (run 6) increased, Young’s modulus decreased. This might be due to originality of PMMA has Tg about 100 8C so results show insignificantly decreased in Young’s modulus. Young’s modulus of the blend containing PLA[b]–PMMA (run 6) at 5 and 10 phr decreased to ca. 100 MPa. Impact strength of the blend was also examined using notched Izod impact testing. Neat PLA showed impact strength of 0.112 kgf cm/cm. Blend of PLA with various compositions of the PLA[b]–PMMA copolymer (run 6) had impact strength of 0.109 0.006, 0.115 0.002, 0.135 0.017, and 0.119 0.006 for 1,
Fig. 10. Photographs of (A) neat PLA, (B) blend of PLA and PLA[b]–PMMA copolymer (run 6), and (C) blend of PLA and butadiene–PLA triblock copolymer.
1000
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3, 5, and 10 phr of the copolymer, respectively. Similarly to Young’s modulus, impact strength did not significantly increase, because PMMA is a relatively hard polymer. Overall, 5 phr of PLA[b]–PMMA (run 6) seemed to be the optimum composition for the blend. Fig. 10 shows comparative transparency of dumbbell-shape specimens. Neat PLA appears transparent (Fig. 10(A)). By the naked eyes, transparency of the blend of PLA and PLA[b]–PMMA copolymer (Fig. 10(B)) was comparable to the neat PLA. When PLA was blended with butadiene–PLA triblock copolymer that was prepared previously in our laboratory to enhance impact property of PLA [29], the blend lost transparency (Fig. 10(C)). 4. Conclusions PLA–PMMA copolymers were successfully prepared by combination of ROP and AGET ATRP. The effect of the solvent during ROP of lactide was studied. PLA[b] showed the highest mole fraction of the MMA segments of 0.6, when [PLA–Br]/[CuCl2]/[PMDETA]/ [Sn(oct)2]/[MMA] was 1/1/9.6/0.45. A single peak of Tg in DSC referred to the miscibility between PLA and PMMA. An increase in thermal stability of the PLA–PMMA copolymer was observed compared to neat PLA. The blend between PLA and the PLA[b]– PMMA copolymer was prepared, and Young’s modulus and impact strength were investigated. At 5 phr of the PLA[b]–PMMA copolymer the blend appeared to show optimum properties by presenting reduced Young’s modulus and increased in impact strength. Moreover, transparency of PLA was maintained upon blending with the copolymer. Acknowledgments The financial support of this work by the Brain Korea 21 project in 2010 and Cheil Industries is greatly appreciated.
References [1] I. Noda, M.M. Satkowski, A.E. Dowrey, C. Marcott, Macromol. Biosci. 4 (2004) 269. [2] (a) C.S. Wu, H.T. Liao, Polymer 46 (2005) 10017; (b) P. Sarazin, G. Li, W.J. Orts, B.D. Favis, Polymer 49 (2008) 599; (c) E.J. Choi, B. Son, T.S. Hwang, E. Hwang, J. Ind. Eng. Chem. (2011). [3] Z.Z. Yu, Y.C. Ou, G.H. Hu, J. Appl. Polym. Sci. 69 (1998) 1711. [4] O.K. Muratog˘lu, A.S. Argon, R.E. Cohen, M. Weinberg, Polymer 36 (1995) 4771. [5] M. Baiardo, G. Frisoni, M. Scandola, M. Rimelen, D. Lips, K. Ruffieux, E. Wintermantel, J. Appl. Polym. Sci. 90 (2003) 1731. [6] H. Zhang, H. Xia, J. Wang, Y. Li, J. Control. Release 139 (2009) 31. [7] C.H. Ho, C.H. Wang, C.I. Lin, Y.D. Lee, Polymer 49 (2008) 3902. [8] S. Fujii, T. Matsumoto, K. Ueda, T. Yano, US patent, US20090018237, 2009. [9] C. Cygan, J.M. Brake, WIPO. WO2008/063988 A2, 2008. [10] J.L. Eguiburu, M. Fernandez-Berridi, Polymer 37 (1996) 3615. [11] H. Shinoda, K. Matyjaszewski, Macromolecules 34 (2001) 6243. [12] (a) R. Patel, S.J. Im, Y.T. Ko, J.H. Kim, J. Ind. Eng. Chem. 15 (2009) 299; (b) J.T. Park, J.A. Seo, S.H. Ahn, J.H. Kim, S.W. Kang, J. Ind. Eng. Chem. 16 (2010) 517. [13] K. Matyjaszewski, W. Jakubowski, Macromolecules 38 (2005) 4139. [14] D.D. Perrin, W.L.F. Armarego, Purification of Polymers, 3rd ed., Pergamon Press, Great Britain, 1988. [15] Y. Zhao, X. Shuai, C. Chen, F. Xi, Macromolecules 37 (2004) 8854. [16] B. Braun, J.R. Dorgan, S.F. Dec, Macromolecules 39 (2006) 9302. [17] S.H. Lee, S.H. Kim, Y.K. Han, Y.H. Kim, J. Polym. Sci. A: Polym. Chem. 39 (2001) 973. [18] K. Tharanikkarasu, H. Verma, W. Jang, S.K. Lee, J. Seo, S. Baek, H. Han, J. Appl. Polym. Sci. 108 (2008) 1538. [19] Z. Kulinski, E. Piorkowska, Polymer 46 (2005) 10290. [20] K.S. Anderson, M.A. Hillmyer, Polymer 47 (2006) 2030. [21] W. Wang, W. Ren, L. Jiang, Y. Dan, J. Appl. Polym. Sci. 118 (2010) 2379. [22] H. Chen, M. Pyda, P. Cebe, Thermochim. Acta 492 (2009) 61. [23] G. Zhang, J. Zhang, S. Wang, D. Shen, J. Polym. Sci. B: Polym. Phys. 41 (2003) 23. [24] K. Ute, N. Miyatake, K. Hatada, Polymer 36 (1995) 1415. [25] R. Krishnan, K.S.V. Srinivasan, J. Appl. Polym. Sci. 97 (2005) 989. [26] C.D. Borman, A.T. Jackson, A. Bunn, A.L. Cutter, D.J. Irvine, Polymer 41 (2000) 6015. [27] D. Colombani, M. Steenbock, M. Klapper, K. Mu¨llen, Macromol. Rapid Commun. 18 (1997) 243. [28] J. Norman, S.C. Moratti, A.T. Slark, D.J. Irvine, A.T. Jackson, Macromolecules 35 (2002) 8954. [29] N. Kim, Y.S. Yun, J. Lee, C. Choochottiros, H. Pyo, I. Chin, H. Jin, Macromol. Res. 19 (2011) 943.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Synthesis and characterization of polylactide–poly(methyl methacrylate) copolymer by combining of ROP and AGET ATRP Chantiga Choochottiros, Eunha Park, In-Joo Chin * Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 June 2011 Accepted 7 November 2011 Available online 4 February 2012
Block copolymers of polylactide (PLA) and poly(methyl methacrylate) (PLA–PMMA) were synthesized by the combination of ring-opening polymerization (ROP) and activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP), where PLA was prepared as macroinitiator with active bromo end group (PLA–Br). Tin octoate (Sn(oct)2) and benzyl alcohol were applied as the initiation system for ROP of lactide. During AGET ATRP, copper (II) chloride (CuCl2) with N,N,N0 ,N00 ,N00 pentamethyl-diethylenetriamine (PMDETA) was used as the catalyst system including Sn(oct)2 as reducing agent. At the feed ratio [PLA–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA] of 1/1/9.6/0.45/100, the mole fraction of the PMMA block was 0.6 as determined by 1H NMR. Thermal stability of PLA was enhanced by incorporating of PMMA as block copolymers. In addition, blend between of PLA and PLA– PMMA copolymer was investigated and 5 phr of PLA–PMMA showed optimum condition to decrease in Young’s modulus and increase in impact strength. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Polylactide Poly(methyl methacrylate) AGET ATRP Block copolymer
1. Introduction Polylactide (PLA) is thermoplastic polyester which is obtained from renewable resources. PLA is a crystalline thermoplastic with clear and transparent appearance. In addition, PLA has relatively high strength and modulus, biocompatibility, and biodegradability. Therefore, PLA is a good candidate in both of the industrial packaging field and the biocompatible/bioabsorbable medical device market. Limitations of PLA are brittleness, insufficient impact strength and low thermal stability. In order to improve its properties, various studies have been conducted by blending with non-biodegradable resins [1], preparing as copolymers, or as functionalized polymers [2]. For example, when PLA was blended with a rubbery polymer in order to improve impact strength, in spite of an increase in toughness, the blend showed varying degrees of success [3,4]. In addition, transparency of PLA was diminished, and blending capacity was limited by compatibility/ miscibility between the rubbery polymer and PLA [5]. Chemical modification of polymer by preparing as copolymer can attract much interest due to the controllability of the structural architecture, its properties, and molecular weight which depend on the molecular composition of copolymer. Many studies of PLA copolymers were reported using various methods, for example, (i)
* Corresponding author. Tel.: +82 32 860 7480; fax: +82 32 860 5178. E-mail address: [email protected] (I.-J. Chin).
two-step method by combining PLA and PEG to obtain PLA–PEG amphiphilic block copolymer for drug carriers [6], and (ii) by introducing a reactive functional monomer on the TPO chain before it was reacted with PLA in order to enhance miscibility of PLA/TPO blends [7]. In view of the foregoing, PLA was modified mainly to improve structural or mechanical properties but its clarity was rarely of concern. To retain transparency of PLA, PLA and PMMA blends were prepared by mixing with impact modifier which consists of methyl methacrylate unit as shell layer and the polymer having alkyl acrylate unit as core [8]. Cygan and Brake [9] reported an improvement in impact strength of PLA by using methyl methacrylate–butadiene–styrene (MBS) copolymer as impact modifier. Transparency of the compound was controlled by proper balance between the impact modifier and PLA. PMMA or acrylic polymers are good candidates in maintaining transparency of the compound. Fujii et al. [8] investigated blending of PLA and PMMA with core–shell type of acrylic copolymer as impact modifier. The results showed an improvement in impact strength including transparency and heat resistance. In order to improve simultaneously the impact property and clarity of the compound, diblock copolymers of PLA and PMMA were designed. In the past, the preparation of PLA–PMMA copolymer was carried out through functionalization of PLA with the end group of active methyl methacrylate (MMA) monomer and subsequent polymerization by free radical initiator [10] or atom transfer radical polymerization (ATRP) [11]. Recently, ATRP has
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.153
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Scheme 1. Synthesis of PLA–PMMA copolymer.
gained rapidly increasing popularity due to the relatively mild reaction conditions, availability of plenty of monomers, initiators, catalysts, specific functionalities and various architectures [12]. However, the typical catalyst of ATRP, which is a transition metal complex, can easily be oxidized to the higher oxidation state. Therefore, ATRP needs some requirements such as special handling procedure under inert atmosphere, where oxygen or other oxidizing agents should be removed. Matyjaszewski and Jakubowski [13] presented a new procedure by using initiator with a radically transferrable atom or group, catalyst complex, and reducing agent. This method is named as activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP). In this study, we report an alternative route for the synthesis of diblock copolymer of PLA and PMMA through the combination of ring opening polymerization (ROP) and activator generated by electron transfer for atom transfer radical polymerization (AGET ATRP), where PLA was modified as macroinitiator for AGET ATRP (Scheme 1). 2. Experimental 2.1. Materials L-Lactide was supplied by Cheil Industries in Korea and recrystallized from diethyl ether and vacuum dried prior to use. CuBr (99.999%, Aldrich) was used as received. N,N,N0 ,N00 ,N00 pentamethyl-diethylenetriamine (PMDETA) was purchased from Tokyo Chemical Industry Corp. (Japan). 2-Bromoisobutyryl bromide (98%), tin (II) 2-ethylhexanoate, ammonium bicarbonate (minimum 99%), and 4-tert-butylbenzyl alcohol (BBA, 98%) were purchased from Aldrich. Methyl methacrylate (99%, Aldrich) was purified following the procedure reported in the literature [14]. Polylactide (PLA) pellets for compounding were a kind gift from NatureWorks LLC, USA.
2.2. Instruments and equipments 1
H NMR and 13C NMR spectra were obtained with a Varian Inova 400 at 400 MHz under ambient temperature, using chloroform-d and tetramethylsilane (TMS) as the corresponding solvent and the internal chemical shift standard, respectively. Fourier Transform Infrared (FTIR) spectra were obtained from a Bruker Equinox 55 spectrometer. The number-average molecular weight (Mn) of the obtained polymer was measured by gel permeation
chromatography (GPC) analysis on a Water Breeze HPLC System. Thermogravimetric analysis (TGA) was performed with a TA instruments Q50 thermogravimetric analyzer in the range of ambient temperature to 700 8C at a heating rate of 10 8C/min, under nitrogen flow. Differential scanning calorimetry (DSC) was conducted using Perkin Elmer, Jade DSC, where nitrogen gas was purged into the DSC cell with a flow rate of 19.8 mL/min. Measurements were carried out by using 2–4 mg of samples in sealed aluminum pan. The samples were first heated from 25 8C to 175 8C at the heating rate of 10 8C/min and were annealed for 5 min at this temperature to erase previous thermal history, followed by cooling to 25 8C with the same rate. The second heating was subsequently made to 175 8C at the heating rate of 10 8C/min. In case of PLA–PMMA copolymers, the maximum temperature was 240 8C. The thermograms of the first and second DSC heating run were both recorded. The glass-transition temperature (Tg) was taken as the temperature at the midpoint of the corresponding heat-capacity jump in the second heating run. The melting temperature (Tm) of each sample was determined from the maximum of the endothermic peaks, and cold crystallization temperature of each sample was determined from the maximum of the exothermic peaks in the second heating run. Young’s modulus was measured under uniaxial elongation at room temperature in accordance with the ASTM D638 standard using a UTM from Hounsfield Test Equipment. Each sample had a dog-bone shape, and the average of at least five measurements was reported. Notched Izod test (CEAST, code 65451000) of the blends was performed in accordance with the ASTM D256 standard at room temperature. 2.3. Ring-opening polymerization of lactide L-Lactide (3 g, 21 mmol) was weighed into a round bottom flask, where toluene (10 mL) was added. The reaction vessel was immersed in oil bath with controlled temperature at 120 8C, and the reaction was set up as reflux reaction under nitrogen atmosphere. After benzyl alcohol (0.21 mmol) was added to lactide, the initiator (0.21 mmol) was added in the form of a 1 M solution in toluene. After 24 h, the reaction was stopped by precipitation in cold methanol, and the products were dried in vacuum. FTIR (KBr, cm1): 3440 (OH stretch), 2947 (C–H stretch), 1769 (C5 5O stretch), 1454 (C–H bend), 1213–1000 (C–(C5 5O)–O stretch). 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.15 (H of methine), 4.3 (H of methine end group), 1.54 (CH3).
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2.4. Polylactide macroinitiator (PLA–Br)
2.6. Blending of PLA and PLA–PMMA copolymer
PLA was prepared as macroinitiator for copolymerization with MMA monomer by following the method of Zhao et al. [15]. In a 100 mL three-neck round-bottom flask, PLA (1 g, 0.08 mmol) was dissolved in 25 mL of methylene chloride. Triethylamine (0.39 mL, 2.8 mmol) was added to the solution, and the mixture was stirred under N2 and cool to 0 8C. Then, 2-bromoisobutyryl bromide (0.35 mL, 2.8 mmol) in methylene chloride (5 mL) was added dropwise via a funnel for over 30 min. The reaction mixture was further stirred at room temperature overnight, and it was washed with the solution of 3% NH4HCO3 and water. The combined organic layer was precipitated in methanol and dried in vacuum. 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.14 (H of methine), 4.4 (H of methine end group), 1.94 and 1.97 (H of C(Br)(CH3)2), 1.58 (H of methyl in PLA), 1.3 (H of methyl end group).
PLA[b]–PMMA copolymers (run 6) with 1, 3, 5, and 10 phr were blended with PLA by solution blending. For example, 1 phr of PLA[b]–PMMA (run 6) (0.4 g) was dissolved in 50 g of chloroform and mixed with 40 g of PLA in 750 g of chloroform to obtain the mixed solution. The mixed solution was stirred for 2 h, and the film was cast by evaporating the solvent in a conventional oven at 60 8C for a day and in vacuum oven at 60 8C for 2 days.
2.5. Synthesis of block copolymer PLA–PMMA via AGET ATRP The block copolymer PLA–PMMA was prepared via AGET ATRP of MMA monomer with CuCl2/PMDETA as the catalyst system and PLA–Br macroinitiator including Sn(oct)2 as reducing agent (Scheme 1(c)). PLA–Br (0.3 g, 0.04 mmol), CuCl2 (5.38 mg, 0.04 mmol), 2 M of Sn(oct)2 (90 ml, 0.018 mmol), PMDETA (80.2 ml, 0.38 mmol), and MMA (400.48 mg, 4 mmol) were added in a 100 mL three-neck round bottom flask, where toluene (1 mL) was used as solvent. The mixture was heated at 90 8C under N2 for 7 h. After cooling, the reaction mixture was diluted with chloroform and then precipitated in methanol, followed by filtration and drying in vacuum. 1 H NMR in CDCl3 (d, ppm): 7.4 (H of benzyl end group), 5.14 (H of methine in PLA), 3.59 (H of CH3 in PMMA), 1.8–2.0 (H of methane in PMMA, overlapped with H of CH3 in macroinitiator), 0.84 and 1.0 (H of methyl in PMMA). 13 C NMR in CDCl3 (d, ppm): 169.5 (C of carbonyl in PLA and PMMA), 125 and 128 (C of benzyl end group), 69 (C of methine in PLA), 51.7 (C of –OCH3 in PMMA), 44.7 (C of quaternary carbon in PMMA), 31.2 (C of CH2–Br end chain), and 16.6 (C of methyl in PLA and PMMA).
3. Results and discussion 3.1. Synthesis of PLA via ring opening polymerization PLA is conventionally prepared by anionic or coordination– insertion ring-opening polymerization of lactide monomer initiated by nucleophiles such as alcohols. Tin 2-ethylhexanoate (tin octoate, or Sn(oct)2) is a highly efficient catalyst with low risk of racemization [16]. In this work, two systems of ring-opening polymerization (ROP) of lactide, i.e. with and without solvent, were carried out including tin octoate and butyl benzyl alcohol as initiator and coinitiator, respectively. Yields of the obtained PLA from the solution system (PLA[s]) and the bulk system (PLA[b]) were 81% and 92%, respectively. FT IR and 1H NMR were applied to confirm the ROP of lactide. In Fig. 1, FT IR of PLA, both of PLA[s] and PLA[b], showed characteristic peaks of C–(C5 5O)–O stretching at 1213–1000 cm1, C5 5O stretching at 1769 cm1, and a broad peak of hydroxyl end group at 3440 cm1. In addition, an absence of the peak at wavenumber of 935 cm1, which is assigned to COO ring breathing mode of lactide monomer, supported the completion of ROP [16]. Characteristic peaks of protons of PLA are presented in Fig. 2. Protons of methine (H-a) and methyl (H-b) groups of lactide monomer at 5.00 and 1.54 ppm were shifted to 5.15 (H-c) and 1.54 (H-d) ppm, respectively (Fig. 2). PLA has an aromatic ring as an end chain which is obtained from butyl benzyl alcohol, and 1H NMR shows proton peaks around 7.4 (H-e) ppm. The degree of PLA polymerization was calculated using the quantitative analysis of 1 H NMR spectra. The integral peak of H-d at 1.54 ppm attributable to the methyl proton of PLA and that of H-f at 4.3 ppm attributable to the methane proton at the chain end of PLA was peaks of
Fig. 1. FT IR spectra of (A) lactide and (B) PLA.
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Fig. 2. 1H NMR spectra of (A) lactide and (B) PLA.
interest. The degree of polymerization (DP) of PLA was calculated using the following equation: DP ¼
IH-d =3 IH-f =1
(1)
DP of PLA[s] and PLA[b] were 41 and 31, respectively. Also, the number-average molecular weight (Mn) of PLA[s] and PLA[b] were 7560 g/mol (PdI 1.15) and 4873 (PdI 1.34), which were measured by GPC using polystyrene as standard for calibration. 3.2. Synthesis of macroinitiator (PLA–Br) As mentioned in the previous section, synthesized PLA in this study had an aromatic ring at one end and a hydroxyl group at the other end. An active hydroxyl group was chosen to react with 2bromoisobutyryl bromide in order to yield PLA–Br macroinitiator. FT IR analysis showed a decrease in intensity of the hydroxyl group at 3440 cm1 as a result of bromination (data was not shown). 1H NMR spectrum of PLA–Br (Fig. 3(A)) shows new proton peaks at 1.94 and 1.97 ppm (H-g) corresponding to methyl proton of 2-bromoisobutyrates. In addition, the degradation temperature of PLA–Br was delayed to 277 8C, compared to PLA (196 8C), which will be discussed later. This implies the presence of the halide group at the chain end [17]. 3.3. Copolymerization of MMA by AGET ATRP The third step was the preparation of PLA–PMMA copolymer by AGET ATRP, which is shown in Scheme 1(c). The reaction was carried out as a homogeneous system, as PLA–Br was soluble in toluene, a conventional solvent for ATRP. Both of PLA[s]–Br and PLA[b]–Br were copolymerized with MMA by the same manner. Successful copolymerization of PLA[s]–PMMA and PLA[b]–PMMA were confirmed by 1H NMR and 13C NMR analysis. Characteristic protons of PMMA, which belong to –OCH3, –CH2–, and –CH3, appeared at 3.59 (H-h), 1.8–2.0 (H-i), and 0.84 and 1.0 (H-j) ppm, respectively, as shown in the 1H NMR spectrum of PLA[s]–PMMA (Fig. 3(B)).
Furthermore, resonances of the –C–CH3 (H-j) in the PMMA block reveal twin peaks at 0.8–1.1 ppm, which are attributed to the random and syndiotactic structure of PMMA [18]. 13C NMR spectrum (Fig. 4) shows characteristic carbon peaks of the PMMA block at 51.7 ppm (–OCH3) and 44.7 ppm (–C–), carbon peak from the PLA block at 69 ppm (–CH–), and the overlap peaks of PLA and PMMA blocks at 169.5 ppm (C5 5O) and 16.6 ppm (–CH3). In addition, quantitative analysis of 1H NMR spectrum was performed to determine the mole ratio of the PMMA unit and the PLA unit. The integral ratio of the signals originated from the PMMA block at 3.59 (H-h) and from the PLA–Br segment at 5.15 ppm (H-c) were used as follows: f MMA ¼
ðIH-h ð100=3Þ ½IH-h =ð100=3Þ þ IH-c ð72=1Þ
(2)
In this experiment, two different PLA macroinitiators were used; one was obtained from the solution-ROP (Table 1, runs 1–5) and the other from the bulk-ROP (Table 1, runs 6–8), as mentioned in the previous section. However, both of macroinitiators showed similar molecular weights (PLA[s]–Br = 7716 g/mol and PLA[b]–Br = 6629 g/ mol). In all cases, homogeneous reactions were carried out under nitrogen atmosphere, and thus the copolymer was obtained by precipitation in methanol. The effect of different reaction conditions in preparing the PLA–PMMA copolymer was investigated. Table 1 shows molecular weight obtained from GPC measurement and the mole ratios of PLA and PMMA in the PLA–PMMA copolymers. When the feed molar ratio of the MMA monomer increased from 100 to 400, there was a decrease in fPMMA. However, PLA[b] showed higher mole fraction of PMMA than PLA[s], even with the same feed molar ratio (Table 1, runs 1 and 6). The effect of the transition metal complex, e.g. CuCl2/PMDETA, was important to the grafting of PMMA as well. When the molar feed ratio of the transition metal complex was reduced to one half of the macroinitiator, fPMMA was significantly decreased (Table 1, runs 4 and 5). As a result, the molar feed ratio of [PLA[b]–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA] as 1/ 1/9.6/0.45/100 produced highest fPMMA of 0.6 (Table 1, run 6). Hence, the relationship between the reaction time and the mole fraction of
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Fig. 3. 1H NMR spectra of (A) PLA–Br and (B) PLA–PMMA copolymer.
PMMA was also investigated for the reaction in Table 1 (run 6). Fig. 5 shows linear relationships of reaction time (from 5 to 50 min) versus fPMMA and reaction time versus Mn which increased from 0.12 to 0.60 and 7749 to 10035 g/mol, respectively. 3.4. Glass transition temperature and melting temperature of PLA– PMMA copolymers As the mole fraction of PMMA in the PLA–PMMA copolymer between PLA[s] and PLA[b] macroinitiator is different, the structure
of the PLA–PMMA copolymer was investigated in detail. DSC analysis of PLA[s] and PLA[b] was performed by scanning from 25 8C to 175 8C in order to determine the glass transition temperature. According to the literature amorphous PLA exhibits Tg in the range of 50–60 8C [19]. Fig. 6 shows the second heat scans of PLA[s] and PLA[b]. While PLA[s] showed a Tg at 54.5 8C (Fig. 6(A)), the Tg of PLA[b] was not detected during the second heat scan (Fig. 6(B)). This implies the solvent effect during the ROP reaction on the crystal structure of PLA. Percent crystallinity of PLA was calculated by using the following equation: % crystallinity ¼ 100
CH (PLA)
O C (PLA, PMMA)
CH3 (PLA, PMMA)
DH m / f D Hm
(3)
where DHm is the measured heat of fusion, f is the weight fraction / of PLA (in this case, f = 1) and DHm is the enthalpy of fusion for a
CDCl3 880
0.7 0.7
CH 2 Br
0.6 0.6
10000
chain end
770
PLA, chain end
C
9000
660
0.4 0.4 0.3 0.3
550
0.2 0.2
(PMMA)
8000
440
0.1 0.1
0.0
180
160 140 120
Fig. 4.
13
100 80 δ (ppm)
60
40
C NMR spectrum of PLA–PMMA copolymer.
Tg(°C)
OCH3 (PMMA)
fPMMA
Mn(g/mol)
0.5 0.5
20
0
10
20 30 40 reaction time (min)
50
330 60
Fig. 5. Molar fractions of PMMA (*), Mn (&), and the glass transition temperature (*) as a function of the reaction time for PLA[b]–PMMA copolymers (run 6).
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Table 1 AGET ATRP of MMA with CuCl2/PMDETA catalyst system in toluene at 90 8C with reaction time of 50 min.
a
Run
PLA
[PLA–Br]/[CuCl2]/[PMDETA]/[Sn(oct)2]/[MMA]
fPMMA
fPLA
1 2 3 4 5
PLA[s]
1/1/9.6/0.45/100 1/1/9.6/0.45/100a 1/1/9.6/0.45/200 1/0.5/4.8/0.22/100 1/0.5/4.8/0.22/200
0.13 0.08 0.09 0.04 0.06
0.87 0.92 0.91 0.96 0.94
6 7 8
PLA[b]
1/1/9.6/0.45/100 1/1/9.6/0.45/200 1/1/9.6/0.45/400
0.60 0.30 0.17
0.40 0.70 0.83
Mn
Mw
PDI
8686 6734 8266 6970 7788
12,574 9499 11,231 9489 10,263
1.45 1.41 1.36 1.36 1.32
10,035 8029 5845
13,400 11,156 7891
1.34 1.39 1.35
Reaction temperature was 120 8C.
fraction of PMMA increased. The fact that the Tg of the PLA[b]– PMMA copolymer increases with the polymerization time indicates an increase of the amorphous PMMA segment proportional to the mole fraction of PMMA, as determined by 1H NMR. The Tg of PLA[s]–PMMA showed a tendency to approach the Tg of syndiotactic PMMA which was reported as 100.9 8C [24]. 3.5. Thermal stability Thermal degradation of PLA[s] and PLA[b] were investigated by TGA in Fig. 8. Both PLA[s] and PLA[b] showed single step degradation and the onset temperature was 196 8C and 210 8C, respectively. Thermal stability of PMMA is known to be superior to that of PLA, so the thermal stability of the PLA–PMMA block copolymers was expected to be improved compared to the neat PLA. In Fig. 8(A), PLA[s]–PMMA showed an increase in the thermal stability compared to PLA[s], with two-step degradation. The onset of the first step occurred ca. 264 8C and the onset of the second step occurred ca. 361 8C. The result was similar to the degradation behavior of amphiphilic block copolymers of MMA with PEO. Krishnan and Srinivasan referred to the first and second step of degradation of PMMA-co-PEO corresponding to the chain-end initiation from the vinylidene ends and random scission within the polymer chain, respectively [25]. In the case of PLA[b]–PMMA, TGA thermogram (Fig 8(B)) shows three steps of degradation. The onset of the first step of the weight loss was slightly decreased to around 173 8C with only ca. 10% weight loss, and the second step of the weight loss occurred around 255 8C with about 50% weight loss. The onset of the last step occurred around 352 8C. Borman et al. [26] and Colombani et al. [27] suggested that the C–Br bond at the
heat flow (endo up)
heat flow (endo up)
crystal having infinite crystal thickness (94 J/g for PLLA) [20]. The crystallinity of PLA[s] and PLA[b] was calculated to be 62 and 72%, respectively. This information also supports the solvent effect on the packing structure of PLA. Introduction of the amorphous PMMA block into PLA may interrupt the regularity of the arrangements of PLA chains, thereby reducing the crystallinity and decreasing in brittleness [21]. Fig. 6(C) shows a Tg of PLA[s]–PMMA at 55.3 8C. The Tg was not significantly varied when PMMA was introduced to PLA[s]. Considering the melting temperature of PLA[s] and PLA[s]–PMMA, PLA[s] has one peak at 158 8C, while PLA[s]–PMMA has double peaks; smaller peak at 151 8C and larger peak at 155 8C (Fig. 6(C)). The lower endotherm (occurring at lower temperature) is due to the melting of original crystals and the higher temperature endotherm is due to the melting of crystals that were recrystallized during heating after melting of the original crystal [22]. It was difficult to determine the Tg of PLA[b]–PMMA by scanning from 25 8C to 175 8C, therefore the temperature range was changed to 60 8C to 175 8C. Fig. 7 shows Tg of PLA[b] and PLA[b]–PMMA (run 6) at 43 8C and 69 8C, respectively. It is clear that the amorphous PMMA increased the Tg of PLA–PMMA block copolymer. In addition, both of PLA[s]–PMMA and PLA[b]–PMMA showed single Tg, which refers to miscibility between the PLA and PMMA segment. Zhang et al. [23] also reported that the PLLA/PMMA blend had only one Tg which occurred during crystalline phase melt, and that two components mixed together. In addition, the Tg of PLA[b]–PMMA (run 6) was also investigating as a function of the mole fraction of PMMA. In Fig. 5 the Tg (open circle) shows an increase of Tg with the reaction time. In other words, the Tg of PLA[b]–PMMA increased as the mole
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Fig. 6. DSC second heat scans of (A) PLA[s], (B) PLA[b], and (C) PLA[s]–PMMA.
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Fig. 7. Glass transition temperature of (A) PLA[b] and (B) PLA[b]–PMMA (run 6) obtained from the second heat scan.
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linkages at ca. 170 8C, (2) unsaturated end groups at ca. 250 8C, and (3) saturated chains at above 330 8C [28].
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Fig. 8. TGA thermograms of PLA (. . .. . .), PLA–Br (——), and PLA–PMMA (—); (A) PLA[s] and (B) PLA[b].
terminal chain end of the low molecular weight PMMA yielded low thermal stability. The other probability may be due to the characteristic degradation of PMMA which obtained by free radical polymerization. PMMA usually exhibits three-step degradation, that consists of the degradation of (1) head-to-head
PLA[b]–PMMA (run 6) was blended with PLA resin to measure mechanical properties of the blend. A series of copolymers with 1, 3, 5, and 10 phr of PLA[b]–PMMA (run 6) were prepared. Young’s modulus of the blend was plotted with the block copolymer composition, as shown in Fig. 9. Young’s modulus of neat PLA is ca. 176 MPa. As the content of the PLA[b]–PMMA copolymer (run 6) increased, Young’s modulus decreased. This might be due to originality of PMMA has Tg about 100 8C so results show insignificantly decreased in Young’s modulus. Young’s modulus of the blend containing PLA[b]–PMMA (run 6) at 5 and 10 phr decreased to ca. 100 MPa. Impact strength of the blend was also examined using notched Izod impact testing. Neat PLA showed impact strength of 0.112 kgf cm/cm. Blend of PLA with various compositions of the PLA[b]–PMMA copolymer (run 6) had impact strength of 0.109 0.006, 0.115 0.002, 0.135 0.017, and 0.119 0.006 for 1,
Fig. 10. Photographs of (A) neat PLA, (B) blend of PLA and PLA[b]–PMMA copolymer (run 6), and (C) blend of PLA and butadiene–PLA triblock copolymer.
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3, 5, and 10 phr of the copolymer, respectively. Similarly to Young’s modulus, impact strength did not significantly increase, because PMMA is a relatively hard polymer. Overall, 5 phr of PLA[b]–PMMA (run 6) seemed to be the optimum composition for the blend. Fig. 10 shows comparative transparency of dumbbell-shape specimens. Neat PLA appears transparent (Fig. 10(A)). By the naked eyes, transparency of the blend of PLA and PLA[b]–PMMA copolymer (Fig. 10(B)) was comparable to the neat PLA. When PLA was blended with butadiene–PLA triblock copolymer that was prepared previously in our laboratory to enhance impact property of PLA [29], the blend lost transparency (Fig. 10(C)). 4. Conclusions PLA–PMMA copolymers were successfully prepared by combination of ROP and AGET ATRP. The effect of the solvent during ROP of lactide was studied. PLA[b] showed the highest mole fraction of the MMA segments of 0.6, when [PLA–Br]/[CuCl2]/[PMDETA]/ [Sn(oct)2]/[MMA] was 1/1/9.6/0.45. A single peak of Tg in DSC referred to the miscibility between PLA and PMMA. An increase in thermal stability of the PLA–PMMA copolymer was observed compared to neat PLA. The blend between PLA and the PLA[b]– PMMA copolymer was prepared, and Young’s modulus and impact strength were investigated. At 5 phr of the PLA[b]–PMMA copolymer the blend appeared to show optimum properties by presenting reduced Young’s modulus and increased in impact strength. Moreover, transparency of PLA was maintained upon blending with the copolymer. Acknowledgments The financial support of this work by the Brain Korea 21 project in 2010 and Cheil Industries is greatly appreciated.
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