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Synthesis and characterization of isosorbide based polycarbonates
Polymer 179 (2019) 121685
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Synthesis and characterization of isosorbide based polycarbonates 1
1
Sugil Yum , Hoyeon Kim , Yongsok Seo
T
*
RIAM, School of Materials Science and Engineering, College of Engineering, Seoul National University, Kwanakro-1, Kwanak-gu, Seoul, 08826, South Korea
H I GH L IG H T S
series of copolycarbonates of ISB, hydrogenated bisphenol A (HBPA), and diphenyl carbonate (DPC) were synthesized by melt polycondensation. • AHigh molar mass (M = 73000–103000 g/mol) ISB/HBPA copolycarbonates were obtained with NaOMe catalyst. • The inclusion of flexible and environmentally friendly HBPA resulted in a reduction in Young's modulus, thus, a more softer polycarbonate but an increase in • elongation at break as HBPA content increased. • For the ISB/HBPA 95:5 sample demonstrates the possibility of a softer but mechanically comparable polycarbonate production. w
A R T I C LE I N FO
A B S T R A C T
Keywords: Isosorbide polymer Soft polycarbonate Melt polycondensation Eco-friendly copolycarbonate Hydrogenated bisphenol A
Despite of its non-toxicity and eco-friendliness, isosorbide (ISB) polycarbonate is difficult to use solely due to its excessive brittleness. To overcome this drawback, it is necessary to prepare high-molecular-weight ISB-based copolycarbonates with flexible diol monomers. In this research, a series of copolycarbonates of ISB, hydrogenated bisphenol A (HBPA), and diphenyl carbonate (DPC) were synthesized by melt polycondensation process using sodium methoxide (NaOMe) as a catalyst. Using NaOMe catalyst, high molecular weight ISB/HBPA copolycarbonates (Mw = 73,000–103,000 g/mol) were obtained. The incorporation of thermally stable and flexible HBPA improved the thermal stability of the copolycarbonates and the flexibility of the polymer chains to demonstrate the possibility of producing soft polycarbonates which are more environmentally friendly and can overcome the brittleness of the ISB polycarbonates.
1. Introduction Polycarbonate is widely used in many industrial sectors due to its excellent physical properties such as high thermal stability, impact resistance, and transparency. Bisphenol A (BPA) was mainly used as a representative monomer for polycarbonate because of its rigidity and transparency [1–3]. However, BPA causes serious problem for the human body as it acts as an endocrine disruptor when leached [4–7]. In addition, BPA is a petroleum-based monomer, which has a negative effect on the environment [8]. Thus, it is necessary to replace BPA with an alternative of low toxicity and environmental friendliness. Isosorbide (ISB,1,4:3,6-dianhydro-D-sorbitol) is a well-known biobased and renewable replacement for BPA [8–10]. ISB can be synthesized by dehydration of D-sorbitol from starch [10–13]. ISB has the advantages of non-toxicity, bio-degradability, high rigidity and thermal stability [8–10,14]. Because of these advantages, several researchers have attempted to synthesize ISB polycarbonates [15–17]. However,
ISB polycarbonates are not suitable for commercial use because of their excessive brittleness [18]. Thus, it is necessary to improve the flexibility of ISB polycarbonates. In order to improve the flexibility of ISB polycarbonates, it has been attempted to copolymerize ISB and other flexible monomers via melt polycondensation [18–20]. Flexible diol monomers such as BPA, cyclohexanedimethanol (CHDM), and aliphatic diols were copolymerized with ISB using various catalysts and dimethyl carbonate (DMC) or diphenyl carbonate (DPC) as a carbonate source [18–20]. Although these processes are more eco-friendly compared to solution process using phosgene, it is difficult to obtain a high-molecular weight polycarbonate [18,20,21]. This is due to the low reactivity of the secondary hydroxyl groups of ISB under melt polycondensation condition [22]. Therefore, it is necessary to increase the reactivity of the hydroxyl groups of ISB using an appropriate catalyst to obtain flexible and high molecular weight ISB-based copolycarbonates. In this work, we synthesized ISB copolycarbonates via melt
*
Corresponding author. E-mail address: [email protected] (Y. Seo). 1 These authors contributed equally. https://doi.org/10.1016/j.polymer.2019.121685 Received 21 March 2019; Received in revised form 15 June 2019; Accepted 27 July 2019 Available online 29 July 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Chemical structures of (a) ISB, (b) HPBA and (c) DPC.
stirring under N2 atmosphere. The temperature was held for 5 min. Thereafter, the temperature was raised to 183 °C while the pressure in the flask was lowered by a vacuum pump to < 0.1 mm Hg for 1 h to distill off the byproduct, phenol. Subsequently, the temperature was gradually raised to 240°C-290 °C. The final temperature was lowered with the HBPA content, from 290 °C for the ISB polycarbonate sample to 240 °C for the ISB-HBPA 85/15 sample due to the difference in the viscosity of the reaction mixtures. The polymerization was carried out for 4 h at the final temperature. After completion of the reaction, the synthesized polymer was cooled to room temperature, dissolved in chloroform, and precipitated by pouring the chloroform solution into ethanol. Then, the precipitated polymer was collected by vacuum filtration. The final polymer product was dried in a vacuum oven at 100 °C for 24 h. 2.3. Characterization The Fourier transform infrared (FT-IR) analysis of the polymers was performed using Nicolet S10 FTIR Spectrometer equipped with an attenuated total reflection (ATR) accessory. 1H nuclear magnetic resonance spectroscopy (NMR) measurements were conducted using JEOL JNM LA-400 spectrometer. Molecular weights and molecular weight distributions of the polymers were measured by gel permeation chromatography (GPC). The measurements were done by Thermo Ultimate 3000 equipped with a KF-806L column using N, N-dimethylformamide (DMF) as the mobile phase. Number-average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) were determined against linear polystyrene standards. Thermogravimetric analysis (TGA) of the samples were done with Mettler Toledo TGA DSC 1. The experiments were carried out at a heating rate of 10 °C/min from 25 °C to 500 °C in N2 atmosphere. To measure the glass transition temperature (Tg), differential scanning calorimetry (DSC) analysis was carried out using Mettler Toledo DSC823e. The measurements were carried out at a temperature range from 25 °C to 250 °C at a heating rate of 10 °C/min in N2 atmosphere. To exclude thermal history of the polymers, second heating data were used for the analysis. Tensile tests of the polymers were conducted using a universal testing machine. Prior to the measurements, the polymers were slowly heated to 270 °C, and then pulled out to obtain fibril-shaped samples with the tip of a long needle. The diameter of the fibril-shaped samples was about 20–60 μ m. Then, the fibril-shaped samples with a gauge length of 10 mm were attached to a paper tip. The measurements were carried out with 500 g load cell, at the pulling speed of 10 mm/min. All the measurements were averages of at least seven measurements.
Fig. 2. FT-IR spectra of ISB-HBPA copolycarbonates.
polycondensation using 4,4′-4,4′-isoprolylidenedicyclohexanol (hydrogenated bisphenol A, HBPA) as the comonomer, DPC as the carbonate source and sodium methoxide (NaOMe) as the catalyst. The synthesized copolycarbonates showed a very high molecular weight and a narrow molecular weight distribution. In addition, we investigated the effect of HBPA on the thermal and mechanical properties of ISB PCs. When the proper ratio of HBPA to ISB polycarbonate was incorporated, the thermal stability as well as the flexibility of ISB polycarbonate were improved without deteriorating the mechanical properties. 2. Experimental section 2.1. Materials Isosorbide (1,4:3,6-dianhydro-D-sorbitol (ISB, 98%)) was purchased from Aldrich chemical Co. It was recrystallized in acetone before use. Hydrogenated bisphenol A (4,4′-isoprolylidenedicyclohexanol (HBPA)) was purchased from Tokyo chemical industry and used as received. Diphenyl carbonate (DPC) and Sodium methoxide (NaOMe) were purchased from Aldrich chemical Co. and used as received. The chemical structures of ISB, HBPA, and DPC are shown in Fig. 1. 2.2. Synthesis of ISB/PBPA copolycarbonates
3. Results and discussion
The synthesis of ISB-based homo and copolymers was carried out with four different ISB/HBPA molar ratios (100/0, 95/5, 90/10, 85/ 15). The total amount of monomer was 21 mmol. The amount of DPC and NaOMe catalyst was the same for all experiments. First, ISB, HBPA, DPC and NaOMe were put into a 100 ml 3-neck round bottom flask equipped with a mechanical stirrer and a short path distillation head connected to a vacuum pump. The flask was immersed in a silicone oil bath, then the temperature was increased to 160 °C with mechanical
3.1. Synthesis of ISB/HPBA copolycarbonates 4,4′-isoprolylidenedicyclohexanol (hydrogenated bisphenol A, HBPA) is a diol co-monomer derived from the hydrogenation of bisphenol A. It has been used with epoxy resin to fabricate a light-emitting diode encapsulant due to its flexibility, thermal stability and long2
Polymer 179 (2019) 121685
S. Yum, et al.
Fig. 3. 1H NMR spectra of (a) 100/0 and (b) 85/15.
All samples exhibited absorption bands at 1740–1760 cm−1 corresponding to C]O stretching of carbonyl bond, and 1240–1260 cm−1 corresponding to C–O–C stretching of ether bond. Two absorptions near 2930 cm−1 and 2870 cm−1 are peaks for the asymmetric and the symmetric stretching of aliphatic C–H. As the HBPA ratio increases, the relative intensity of 2930 cm−1 and 2870 cm−1 peaks become stronger, which indicates more HBPA monomers were incorporated into the copolycarbonate. Fig. 3 shows 1H NMR spectra of ISB homo-polycarbonate and ISB/ HBPA copolycarbonate. In the 1H spectrum of the ISB polycarbonate, the peaks of the ISB moiety at 3.9–5.2 ppm were assigned to the hydrogens of isosorbide moiety. In 1H NMR spectrum of ISB-HBPA copolycarbonates, the additional peaks of HBPA moiety at 0.9–2.1 ppm were assigned to the hydrogens of the HBPA moiety. The hydrogen peaks of HBPA moiety in Fig. 3(a) overlapped that of hydrogen atom 1, 6 of the ISB moiety. Molar percentages of incorporated ISB in copolycarbonates were calculated from the integral intensity ratio of the HBPA peaks over the isosorbide peak. The results show that both the
Table 1 Molecular weights and PDI of ISB-HBPA copolycarbonates. ISB/HBPA
Mn (g/mol)
Mw (g/mol)
PDI
ISB/HBPAa
100/0 (ISB) 95/5 90/10 85/15
73,000 66,000 57,000 52,000
103,000 96,000 83,000 73000
1.41 1.46 1.47 1.40
– 95.8/4.2 90.5/9.5 86.0/14.0
a
The composition ratio determined by 1H NMR results.
term color stability. In addition, HBPA is not an endocrine disruptor due to the absence of benzene ring. Poly(ISB-co-HBPA carbonates) were synthesized via melt polycondensation using two different diol monomers and a carbonate monomer (DPC). The transesterification was enhanced by a metal catalyst (NaOMe). The chemical structures and the diol composition of ISB/HBPA copolycarbonates were characterized by FT-IR spectroscopy and 1H NMR spectroscopy. FT-IR spectra for the copolycabonates are shown in Fig. 2.
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transesterification process, the protons of hydroxyl groups of ISB can be easily detached because of the strong nucleophilicity of methoxide anion [20]. As a result, alkoxide anions are formed. Simultaneously, sodium ions interact with the carbonyl oxygen of DPC, thereby polarizing DPC. The formation of alkoxide anions with strong nucleophilic characters makes it possible to attack the carbonyl carbon of the polarized DPC to produce phenol [7,20]. The polycondensation mechanism is similar to transesterification reaction. Meanwhile, increasing the reaction temperature or the reaction times lead to increased molecular weights of the polycarbonates. However, the thermal stability of ISB at higher temperature sets the limit of the reaction temperature (< 270 °C). The optimum reaction temperature and reaction time were determined by trial and error to obtain the high molecular weight copolycarbonates. Although the increase in the reaction temperature causes detrimental effects on polycarbonates, some properties of ISB-HBPA copolycarbonates did not significantly deteriorate as described below. Using the strong nucleophilic methoxide anion, the reactions were carried out at very high rate and at high reaction temperature to produce high molecular weight ISBHBPA copolycarbonates. The molecular weight distributions of ISBHPBA copolycarbonates were relatively narrow, with a PDI in the range of 1.40–1.47. The theoretical PDI of step polymerization is 1 + p, where p is extent of reaction [7]. The narrow molecular weight distributions (lower than 2) indicate that the melt condensation reaction during polycondensation was not completed due to the high viscosity of polycarbonate melts. When the melt viscosity exceeded the maximum torque of mechanical stirrer, the polymerization was stopped.
Table 2 Decomposition temperature and glass transition temperatures of ISB-HBPA copolycarbonates. ISB/HBPA
Td,5% (°C)
Tg (°C)
100/0 95/5 90/10 85/15
351.6 361.0 385.6 394.8
173.5 163.5 161.7 157.8
Table 3 Mechanical properties of ISB-HBPA copolycarbonates. ISB/HBPA
Tensile Strength (MPa)
Young's Modulus (GPa)
Elongation at Break (%)
Toughness (MPa)
100/0 95/5 90/10 85/15
185 ± 10 151 ± 4 84 ± 8 83 ± 4
1.35 0.88 0.65 0.40
4.4 5.5 4.8 5.4
8.4 8.7 4.1 4.4
± ± ± ±
0.08 0.05 0.10 0.01
± ± ± ±
0.2 0.5 0.3 0.1
± ± ± ±
0.3 0.4 0.3 0.2
isosorbide group and the HBPA group were completely incorporated in the copolycarbonates. However, the ratio was slightly lower than the molar percentage of the initial mixture due to the evaporation of HBPA monomers during melt condensation (see Table 3). 3.2. Molecular weights and molecular weight distributions
3.3. Thermal properties
The number average molecular weight, weight average molecular weight, and polydispersity index (Mn, Mw, and PDI) of ISB-HBPA copolycarbonates obtained from GPC are presented in Table 1. As HBPA content increased, Mn gradually decreased, from 73,000 g/mol for ISB homopolymer to 52,000 g/mol for ISB-HBPA 85/15 copolycarbonate. Mw also showed a similar tendency, with a maximum of 103,000 g/mol for ISB homopolymer and a minimum of 73,000 g/mol for ISB-HBPA 85/15 copolycarbonate. The molar mass reduction of copolycarbonates is due to the lower reactivity of HBPA compared to isosorbide. However, the molecular weights of ISB-HBPA copolycarbonates were higher than other ISB-based copolycarbonates synthesized by the melt polycondensation method [18–20]. The synthesis of high molecular weight ISB polycarbonate under melt condensation conditions remains difficult due to low reactivity of ISB and high melt viscosity. The high molar mass of ISB/HBPA copolycarbonates is attributed to the increase in reactivity of ISB hydroxyl groups with the strong metal catalyst sodium methoxide (NaOMe). NaOMe is known as a highly basic catalyst, and has been used to synthesize high molecular weight linear aliphatic polycarbonates [23]. The catalyst which possesses a stronger electrophilic capability improves the reactivity of hydroxyl groups of ISB and HBPA. During the
Thermal stability of ISB-HBPA copolycarbonates was checked by TGA. TGA curves for ISB-HBPA copolycarbonates are summarized in Table 2. From TGA curves in Fig. 4(a), decomposition temperature (Td,5%), of each sample was determined at 5% weight loss. All samples were stable up to approximately 350 °C and the Td,5% values of the polymers gradually increased with HBPA content. This is due to the thermal stability of HBPA, attributed to the stable alicyclic structure [24]. Fig. 4(b) presents the second heating curves for ISB-HBPA copolycarbonates. All curves showed no melting point, which means that all polymers were amorphous. The Tg values of the polymers were determined from the midpoint of transition temperature range. The Tg of the previously reported ISB homo-polycarbonate varies from 120 to 176 °C [18,19,25]. In general, the glass transition temperature is varied by the difference in molecular weight [7]. The Tg of synthesized ISB polycarbonates was similar to that of polymers synthesized by solution polymerization, due to the high molecular weight of the synthesized ISB polycarbonates [19]. The Tg values of copolycarbonates gradually decreased with increasing HBPA composition. HBPA is a more flexible
Fig. 4. (a) TGA curves and (b) DSC curves of ISB-HBPA copolycarbonates. 4
Polymer 179 (2019) 121685
S. Yum, et al.
Fig. 5. Mechanical properties of copolycarbonates. (a) Tensile strength and elongation at break and (b) Young's modulus and toughness.
4. Conclusions
compared to rigid ISB, which reduces the stiffness of the whole polymer as shown later in the mechanical properties.
A series of high molecular weight ISB-HBPA copolycarbonates with four different ISB-HBPA ratios were successfully synthesized by melt transesterification and polycondensation using a metal oxide catalyst (NaOMe). Using a highly basic NaOMe catalyst, high molar mass copolycarbonates were obtained of which Mw is between 73,000 and 103,000 g/mol. This is attributed to the strong nucleophilic character of methoxide anion, which increases the reactivity of the hydroxyl groups of ISB and efficient removal of the byproduct in a high vacuum. The thermal and mechanical properties of ISB/HBPA copolycarbonates were changed with ISB-HBPA ratio. The thermal stability was improved with HBPA content due to the addition of the thermally stable alicyclic structure of HBPA. As the content of flexible HBPA increased, the glass transition temperature (Tg), tensile strength and Young's modulus of the copolycarbonates decreased, while elongation at break increased. Among the synthesized copolycarbonates, ISB-HBPA 95/5 copolycarbonate showed a comparable toughness of 8.7 MPa to ISB (8.4 MPa) due to the higher elongation despite the lower modulus. The comparable mechanical properties of ISB and ISB/HBPA copolycabonates are attributed to the inclusion of the soft HBPA moiety and the high molecular weight of all polymers. The results prove experimentally that ISB-based high molar mass copolycarbonates can be synthesized via melt polycondensation of ISB and HBPA using NaOMe as a catalyst. In addition, the comparable mechanical properties confirm the possibility of producing soft polycarbonates which are more environmentally friendly and can overcome the brittleness of the ISB polycarbonates.
3.4. Mechanical properties The mechanical properties of ISB-HBPA copolycarbonates were investigated by tensile measurements of fibril-shaped copolycarbonate samples. Tensile strength, Young's modulus, elongation at break and toughness of ISB-HBPA copolycarbonates are shown in Fig. 5 and their numerical values are reiterated in Table 3 for clarity. ISB homopolymer exhibited a highest tensile strength value (185 MPa) and the tensile strength values of ISB-HBPA copolycarbonates gradually decreased with the HBPA content. Young's modulus also showed analogous trend with the tensile strength. In contrast, the elongation at break of the copolycarbonates increased from 4.4% to 5.5% (for ISB/HBPA 95/5 sample) with the HBPA addition. These results indicate that the flexibility of ISB-HBPA copolycarbonates was improved while the rigidity of the polymers decreased with the introduction of flexible HBPA to rigid ISB polycarbonate. The toughness was slightly increased from 8.4 MPa for ISB polycarbonate to 8.7 MPa for ISB/HBPA 95/5 sample, then decreased for ISB/HBPA 90/10 and ISB/HBPA 85/15 copolymers (Table 3) due to the softness of the HBPA (thus, reduction of the modulus). In terms of toughness, incorporation of 5 mol % of HBPA into the copolycarbonates gave the best mechanical properties comparable to that of ISB polycarbonate. The mechanical properties of ISB homo-polycarbonate are consistent with those already reported in the literature [19,26]. ISB polycarbonates generally show brittle behavior due to the high stiffness of ISB moieties. Feng et al. observed that preparation of ISB polycarbonate was infeasible due to its high stiffness and low molecular weight of 28,500 g/mol [26]. Lee et al. prepared ISB polycarbonates, and achieved a tensile strength of 84 MPa and elongation at break of 9% with molecular weight of 67,000 g/mol [19]. Though direct comparison of the mechanical properties with these results is difficult due to the different specimen shape, our results show that the HBPA copolymer can outperform or at least be a par with the previous reported ISB PCs in terms of the toughness. Though its modulus is lower than the ISB polycarbonate, 95/5 copolymer exhibits more softness, higher elongation at break than that of ISB PC due to the higher molecular weight and inclusion of soft HBPA. The molecular weights of the copolymers and the ISB homo polymer are larger than previously reported polycarbonates which also contributes to good mechanical properties and flexibility. It should be emphasized that the removal of the byproduct (phenol) was the key to obtain the high molar mass polymers.
Acknowledgements The authors would like to thank the support from the Korea National Research Foundation [BK21PLUS SNU Materials Division for Education Creative Global Leaders], the Korea Research Institute of Chemical Technology (KRICT, Basic Project) and the Institute of Engineering Research at Seoul National University for the DSC measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121685. References [1] D.G. Legrand, J.T. Bendler, Handbook of Polycarbonate Science and Technology, Marcel Dekker, New York, 1999. [2] J.E. Biles, T.P. McNeal, T.H. Begley, H.C. Hollifield, Determination of bisphenol-A in reusable polycarbonate food-contact plastics and migration to food-simulating
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Synthesis and characterization of isosorbide based polycarbonates 1
1
Sugil Yum , Hoyeon Kim , Yongsok Seo
T
*
RIAM, School of Materials Science and Engineering, College of Engineering, Seoul National University, Kwanakro-1, Kwanak-gu, Seoul, 08826, South Korea
H I GH L IG H T S
series of copolycarbonates of ISB, hydrogenated bisphenol A (HBPA), and diphenyl carbonate (DPC) were synthesized by melt polycondensation. • AHigh molar mass (M = 73000–103000 g/mol) ISB/HBPA copolycarbonates were obtained with NaOMe catalyst. • The inclusion of flexible and environmentally friendly HBPA resulted in a reduction in Young's modulus, thus, a more softer polycarbonate but an increase in • elongation at break as HBPA content increased. • For the ISB/HBPA 95:5 sample demonstrates the possibility of a softer but mechanically comparable polycarbonate production. w
A R T I C LE I N FO
A B S T R A C T
Keywords: Isosorbide polymer Soft polycarbonate Melt polycondensation Eco-friendly copolycarbonate Hydrogenated bisphenol A
Despite of its non-toxicity and eco-friendliness, isosorbide (ISB) polycarbonate is difficult to use solely due to its excessive brittleness. To overcome this drawback, it is necessary to prepare high-molecular-weight ISB-based copolycarbonates with flexible diol monomers. In this research, a series of copolycarbonates of ISB, hydrogenated bisphenol A (HBPA), and diphenyl carbonate (DPC) were synthesized by melt polycondensation process using sodium methoxide (NaOMe) as a catalyst. Using NaOMe catalyst, high molecular weight ISB/HBPA copolycarbonates (Mw = 73,000–103,000 g/mol) were obtained. The incorporation of thermally stable and flexible HBPA improved the thermal stability of the copolycarbonates and the flexibility of the polymer chains to demonstrate the possibility of producing soft polycarbonates which are more environmentally friendly and can overcome the brittleness of the ISB polycarbonates.
1. Introduction Polycarbonate is widely used in many industrial sectors due to its excellent physical properties such as high thermal stability, impact resistance, and transparency. Bisphenol A (BPA) was mainly used as a representative monomer for polycarbonate because of its rigidity and transparency [1–3]. However, BPA causes serious problem for the human body as it acts as an endocrine disruptor when leached [4–7]. In addition, BPA is a petroleum-based monomer, which has a negative effect on the environment [8]. Thus, it is necessary to replace BPA with an alternative of low toxicity and environmental friendliness. Isosorbide (ISB,1,4:3,6-dianhydro-D-sorbitol) is a well-known biobased and renewable replacement for BPA [8–10]. ISB can be synthesized by dehydration of D-sorbitol from starch [10–13]. ISB has the advantages of non-toxicity, bio-degradability, high rigidity and thermal stability [8–10,14]. Because of these advantages, several researchers have attempted to synthesize ISB polycarbonates [15–17]. However,
ISB polycarbonates are not suitable for commercial use because of their excessive brittleness [18]. Thus, it is necessary to improve the flexibility of ISB polycarbonates. In order to improve the flexibility of ISB polycarbonates, it has been attempted to copolymerize ISB and other flexible monomers via melt polycondensation [18–20]. Flexible diol monomers such as BPA, cyclohexanedimethanol (CHDM), and aliphatic diols were copolymerized with ISB using various catalysts and dimethyl carbonate (DMC) or diphenyl carbonate (DPC) as a carbonate source [18–20]. Although these processes are more eco-friendly compared to solution process using phosgene, it is difficult to obtain a high-molecular weight polycarbonate [18,20,21]. This is due to the low reactivity of the secondary hydroxyl groups of ISB under melt polycondensation condition [22]. Therefore, it is necessary to increase the reactivity of the hydroxyl groups of ISB using an appropriate catalyst to obtain flexible and high molecular weight ISB-based copolycarbonates. In this work, we synthesized ISB copolycarbonates via melt
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Corresponding author. E-mail address: [email protected] (Y. Seo). 1 These authors contributed equally. https://doi.org/10.1016/j.polymer.2019.121685 Received 21 March 2019; Received in revised form 15 June 2019; Accepted 27 July 2019 Available online 29 July 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Chemical structures of (a) ISB, (b) HPBA and (c) DPC.
stirring under N2 atmosphere. The temperature was held for 5 min. Thereafter, the temperature was raised to 183 °C while the pressure in the flask was lowered by a vacuum pump to < 0.1 mm Hg for 1 h to distill off the byproduct, phenol. Subsequently, the temperature was gradually raised to 240°C-290 °C. The final temperature was lowered with the HBPA content, from 290 °C for the ISB polycarbonate sample to 240 °C for the ISB-HBPA 85/15 sample due to the difference in the viscosity of the reaction mixtures. The polymerization was carried out for 4 h at the final temperature. After completion of the reaction, the synthesized polymer was cooled to room temperature, dissolved in chloroform, and precipitated by pouring the chloroform solution into ethanol. Then, the precipitated polymer was collected by vacuum filtration. The final polymer product was dried in a vacuum oven at 100 °C for 24 h. 2.3. Characterization The Fourier transform infrared (FT-IR) analysis of the polymers was performed using Nicolet S10 FTIR Spectrometer equipped with an attenuated total reflection (ATR) accessory. 1H nuclear magnetic resonance spectroscopy (NMR) measurements were conducted using JEOL JNM LA-400 spectrometer. Molecular weights and molecular weight distributions of the polymers were measured by gel permeation chromatography (GPC). The measurements were done by Thermo Ultimate 3000 equipped with a KF-806L column using N, N-dimethylformamide (DMF) as the mobile phase. Number-average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) were determined against linear polystyrene standards. Thermogravimetric analysis (TGA) of the samples were done with Mettler Toledo TGA DSC 1. The experiments were carried out at a heating rate of 10 °C/min from 25 °C to 500 °C in N2 atmosphere. To measure the glass transition temperature (Tg), differential scanning calorimetry (DSC) analysis was carried out using Mettler Toledo DSC823e. The measurements were carried out at a temperature range from 25 °C to 250 °C at a heating rate of 10 °C/min in N2 atmosphere. To exclude thermal history of the polymers, second heating data were used for the analysis. Tensile tests of the polymers were conducted using a universal testing machine. Prior to the measurements, the polymers were slowly heated to 270 °C, and then pulled out to obtain fibril-shaped samples with the tip of a long needle. The diameter of the fibril-shaped samples was about 20–60 μ m. Then, the fibril-shaped samples with a gauge length of 10 mm were attached to a paper tip. The measurements were carried out with 500 g load cell, at the pulling speed of 10 mm/min. All the measurements were averages of at least seven measurements.
Fig. 2. FT-IR spectra of ISB-HBPA copolycarbonates.
polycondensation using 4,4′-4,4′-isoprolylidenedicyclohexanol (hydrogenated bisphenol A, HBPA) as the comonomer, DPC as the carbonate source and sodium methoxide (NaOMe) as the catalyst. The synthesized copolycarbonates showed a very high molecular weight and a narrow molecular weight distribution. In addition, we investigated the effect of HBPA on the thermal and mechanical properties of ISB PCs. When the proper ratio of HBPA to ISB polycarbonate was incorporated, the thermal stability as well as the flexibility of ISB polycarbonate were improved without deteriorating the mechanical properties. 2. Experimental section 2.1. Materials Isosorbide (1,4:3,6-dianhydro-D-sorbitol (ISB, 98%)) was purchased from Aldrich chemical Co. It was recrystallized in acetone before use. Hydrogenated bisphenol A (4,4′-isoprolylidenedicyclohexanol (HBPA)) was purchased from Tokyo chemical industry and used as received. Diphenyl carbonate (DPC) and Sodium methoxide (NaOMe) were purchased from Aldrich chemical Co. and used as received. The chemical structures of ISB, HBPA, and DPC are shown in Fig. 1. 2.2. Synthesis of ISB/PBPA copolycarbonates
3. Results and discussion
The synthesis of ISB-based homo and copolymers was carried out with four different ISB/HBPA molar ratios (100/0, 95/5, 90/10, 85/ 15). The total amount of monomer was 21 mmol. The amount of DPC and NaOMe catalyst was the same for all experiments. First, ISB, HBPA, DPC and NaOMe were put into a 100 ml 3-neck round bottom flask equipped with a mechanical stirrer and a short path distillation head connected to a vacuum pump. The flask was immersed in a silicone oil bath, then the temperature was increased to 160 °C with mechanical
3.1. Synthesis of ISB/HPBA copolycarbonates 4,4′-isoprolylidenedicyclohexanol (hydrogenated bisphenol A, HBPA) is a diol co-monomer derived from the hydrogenation of bisphenol A. It has been used with epoxy resin to fabricate a light-emitting diode encapsulant due to its flexibility, thermal stability and long2
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Fig. 3. 1H NMR spectra of (a) 100/0 and (b) 85/15.
All samples exhibited absorption bands at 1740–1760 cm−1 corresponding to C]O stretching of carbonyl bond, and 1240–1260 cm−1 corresponding to C–O–C stretching of ether bond. Two absorptions near 2930 cm−1 and 2870 cm−1 are peaks for the asymmetric and the symmetric stretching of aliphatic C–H. As the HBPA ratio increases, the relative intensity of 2930 cm−1 and 2870 cm−1 peaks become stronger, which indicates more HBPA monomers were incorporated into the copolycarbonate. Fig. 3 shows 1H NMR spectra of ISB homo-polycarbonate and ISB/ HBPA copolycarbonate. In the 1H spectrum of the ISB polycarbonate, the peaks of the ISB moiety at 3.9–5.2 ppm were assigned to the hydrogens of isosorbide moiety. In 1H NMR spectrum of ISB-HBPA copolycarbonates, the additional peaks of HBPA moiety at 0.9–2.1 ppm were assigned to the hydrogens of the HBPA moiety. The hydrogen peaks of HBPA moiety in Fig. 3(a) overlapped that of hydrogen atom 1, 6 of the ISB moiety. Molar percentages of incorporated ISB in copolycarbonates were calculated from the integral intensity ratio of the HBPA peaks over the isosorbide peak. The results show that both the
Table 1 Molecular weights and PDI of ISB-HBPA copolycarbonates. ISB/HBPA
Mn (g/mol)
Mw (g/mol)
PDI
ISB/HBPAa
100/0 (ISB) 95/5 90/10 85/15
73,000 66,000 57,000 52,000
103,000 96,000 83,000 73000
1.41 1.46 1.47 1.40
– 95.8/4.2 90.5/9.5 86.0/14.0
a
The composition ratio determined by 1H NMR results.
term color stability. In addition, HBPA is not an endocrine disruptor due to the absence of benzene ring. Poly(ISB-co-HBPA carbonates) were synthesized via melt polycondensation using two different diol monomers and a carbonate monomer (DPC). The transesterification was enhanced by a metal catalyst (NaOMe). The chemical structures and the diol composition of ISB/HBPA copolycarbonates were characterized by FT-IR spectroscopy and 1H NMR spectroscopy. FT-IR spectra for the copolycabonates are shown in Fig. 2.
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transesterification process, the protons of hydroxyl groups of ISB can be easily detached because of the strong nucleophilicity of methoxide anion [20]. As a result, alkoxide anions are formed. Simultaneously, sodium ions interact with the carbonyl oxygen of DPC, thereby polarizing DPC. The formation of alkoxide anions with strong nucleophilic characters makes it possible to attack the carbonyl carbon of the polarized DPC to produce phenol [7,20]. The polycondensation mechanism is similar to transesterification reaction. Meanwhile, increasing the reaction temperature or the reaction times lead to increased molecular weights of the polycarbonates. However, the thermal stability of ISB at higher temperature sets the limit of the reaction temperature (< 270 °C). The optimum reaction temperature and reaction time were determined by trial and error to obtain the high molecular weight copolycarbonates. Although the increase in the reaction temperature causes detrimental effects on polycarbonates, some properties of ISB-HBPA copolycarbonates did not significantly deteriorate as described below. Using the strong nucleophilic methoxide anion, the reactions were carried out at very high rate and at high reaction temperature to produce high molecular weight ISBHBPA copolycarbonates. The molecular weight distributions of ISBHPBA copolycarbonates were relatively narrow, with a PDI in the range of 1.40–1.47. The theoretical PDI of step polymerization is 1 + p, where p is extent of reaction [7]. The narrow molecular weight distributions (lower than 2) indicate that the melt condensation reaction during polycondensation was not completed due to the high viscosity of polycarbonate melts. When the melt viscosity exceeded the maximum torque of mechanical stirrer, the polymerization was stopped.
Table 2 Decomposition temperature and glass transition temperatures of ISB-HBPA copolycarbonates. ISB/HBPA
Td,5% (°C)
Tg (°C)
100/0 95/5 90/10 85/15
351.6 361.0 385.6 394.8
173.5 163.5 161.7 157.8
Table 3 Mechanical properties of ISB-HBPA copolycarbonates. ISB/HBPA
Tensile Strength (MPa)
Young's Modulus (GPa)
Elongation at Break (%)
Toughness (MPa)
100/0 95/5 90/10 85/15
185 ± 10 151 ± 4 84 ± 8 83 ± 4
1.35 0.88 0.65 0.40
4.4 5.5 4.8 5.4
8.4 8.7 4.1 4.4
± ± ± ±
0.08 0.05 0.10 0.01
± ± ± ±
0.2 0.5 0.3 0.1
± ± ± ±
0.3 0.4 0.3 0.2
isosorbide group and the HBPA group were completely incorporated in the copolycarbonates. However, the ratio was slightly lower than the molar percentage of the initial mixture due to the evaporation of HBPA monomers during melt condensation (see Table 3). 3.2. Molecular weights and molecular weight distributions
3.3. Thermal properties
The number average molecular weight, weight average molecular weight, and polydispersity index (Mn, Mw, and PDI) of ISB-HBPA copolycarbonates obtained from GPC are presented in Table 1. As HBPA content increased, Mn gradually decreased, from 73,000 g/mol for ISB homopolymer to 52,000 g/mol for ISB-HBPA 85/15 copolycarbonate. Mw also showed a similar tendency, with a maximum of 103,000 g/mol for ISB homopolymer and a minimum of 73,000 g/mol for ISB-HBPA 85/15 copolycarbonate. The molar mass reduction of copolycarbonates is due to the lower reactivity of HBPA compared to isosorbide. However, the molecular weights of ISB-HBPA copolycarbonates were higher than other ISB-based copolycarbonates synthesized by the melt polycondensation method [18–20]. The synthesis of high molecular weight ISB polycarbonate under melt condensation conditions remains difficult due to low reactivity of ISB and high melt viscosity. The high molar mass of ISB/HBPA copolycarbonates is attributed to the increase in reactivity of ISB hydroxyl groups with the strong metal catalyst sodium methoxide (NaOMe). NaOMe is known as a highly basic catalyst, and has been used to synthesize high molecular weight linear aliphatic polycarbonates [23]. The catalyst which possesses a stronger electrophilic capability improves the reactivity of hydroxyl groups of ISB and HBPA. During the
Thermal stability of ISB-HBPA copolycarbonates was checked by TGA. TGA curves for ISB-HBPA copolycarbonates are summarized in Table 2. From TGA curves in Fig. 4(a), decomposition temperature (Td,5%), of each sample was determined at 5% weight loss. All samples were stable up to approximately 350 °C and the Td,5% values of the polymers gradually increased with HBPA content. This is due to the thermal stability of HBPA, attributed to the stable alicyclic structure [24]. Fig. 4(b) presents the second heating curves for ISB-HBPA copolycarbonates. All curves showed no melting point, which means that all polymers were amorphous. The Tg values of the polymers were determined from the midpoint of transition temperature range. The Tg of the previously reported ISB homo-polycarbonate varies from 120 to 176 °C [18,19,25]. In general, the glass transition temperature is varied by the difference in molecular weight [7]. The Tg of synthesized ISB polycarbonates was similar to that of polymers synthesized by solution polymerization, due to the high molecular weight of the synthesized ISB polycarbonates [19]. The Tg values of copolycarbonates gradually decreased with increasing HBPA composition. HBPA is a more flexible
Fig. 4. (a) TGA curves and (b) DSC curves of ISB-HBPA copolycarbonates. 4
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Fig. 5. Mechanical properties of copolycarbonates. (a) Tensile strength and elongation at break and (b) Young's modulus and toughness.
4. Conclusions
compared to rigid ISB, which reduces the stiffness of the whole polymer as shown later in the mechanical properties.
A series of high molecular weight ISB-HBPA copolycarbonates with four different ISB-HBPA ratios were successfully synthesized by melt transesterification and polycondensation using a metal oxide catalyst (NaOMe). Using a highly basic NaOMe catalyst, high molar mass copolycarbonates were obtained of which Mw is between 73,000 and 103,000 g/mol. This is attributed to the strong nucleophilic character of methoxide anion, which increases the reactivity of the hydroxyl groups of ISB and efficient removal of the byproduct in a high vacuum. The thermal and mechanical properties of ISB/HBPA copolycarbonates were changed with ISB-HBPA ratio. The thermal stability was improved with HBPA content due to the addition of the thermally stable alicyclic structure of HBPA. As the content of flexible HBPA increased, the glass transition temperature (Tg), tensile strength and Young's modulus of the copolycarbonates decreased, while elongation at break increased. Among the synthesized copolycarbonates, ISB-HBPA 95/5 copolycarbonate showed a comparable toughness of 8.7 MPa to ISB (8.4 MPa) due to the higher elongation despite the lower modulus. The comparable mechanical properties of ISB and ISB/HBPA copolycabonates are attributed to the inclusion of the soft HBPA moiety and the high molecular weight of all polymers. The results prove experimentally that ISB-based high molar mass copolycarbonates can be synthesized via melt polycondensation of ISB and HBPA using NaOMe as a catalyst. In addition, the comparable mechanical properties confirm the possibility of producing soft polycarbonates which are more environmentally friendly and can overcome the brittleness of the ISB polycarbonates.
3.4. Mechanical properties The mechanical properties of ISB-HBPA copolycarbonates were investigated by tensile measurements of fibril-shaped copolycarbonate samples. Tensile strength, Young's modulus, elongation at break and toughness of ISB-HBPA copolycarbonates are shown in Fig. 5 and their numerical values are reiterated in Table 3 for clarity. ISB homopolymer exhibited a highest tensile strength value (185 MPa) and the tensile strength values of ISB-HBPA copolycarbonates gradually decreased with the HBPA content. Young's modulus also showed analogous trend with the tensile strength. In contrast, the elongation at break of the copolycarbonates increased from 4.4% to 5.5% (for ISB/HBPA 95/5 sample) with the HBPA addition. These results indicate that the flexibility of ISB-HBPA copolycarbonates was improved while the rigidity of the polymers decreased with the introduction of flexible HBPA to rigid ISB polycarbonate. The toughness was slightly increased from 8.4 MPa for ISB polycarbonate to 8.7 MPa for ISB/HBPA 95/5 sample, then decreased for ISB/HBPA 90/10 and ISB/HBPA 85/15 copolymers (Table 3) due to the softness of the HBPA (thus, reduction of the modulus). In terms of toughness, incorporation of 5 mol % of HBPA into the copolycarbonates gave the best mechanical properties comparable to that of ISB polycarbonate. The mechanical properties of ISB homo-polycarbonate are consistent with those already reported in the literature [19,26]. ISB polycarbonates generally show brittle behavior due to the high stiffness of ISB moieties. Feng et al. observed that preparation of ISB polycarbonate was infeasible due to its high stiffness and low molecular weight of 28,500 g/mol [26]. Lee et al. prepared ISB polycarbonates, and achieved a tensile strength of 84 MPa and elongation at break of 9% with molecular weight of 67,000 g/mol [19]. Though direct comparison of the mechanical properties with these results is difficult due to the different specimen shape, our results show that the HBPA copolymer can outperform or at least be a par with the previous reported ISB PCs in terms of the toughness. Though its modulus is lower than the ISB polycarbonate, 95/5 copolymer exhibits more softness, higher elongation at break than that of ISB PC due to the higher molecular weight and inclusion of soft HBPA. The molecular weights of the copolymers and the ISB homo polymer are larger than previously reported polycarbonates which also contributes to good mechanical properties and flexibility. It should be emphasized that the removal of the byproduct (phenol) was the key to obtain the high molar mass polymers.
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