Poly(arylene ether)s are a type of super engineering plastic (SEP), which is used in various industries owing to its characteristics of high thermal stability (continuous service temperature ≥ 150 °C), low dielectric constant, high chemical resistance, and structural flexibility by ether bonds between the aromatic moieties [1
]. Nucleophilic aromatic substitution (SN
Ar), a typical method used for the synthesis of arylene ether-type polymers, involves the formation of an ether bond by substituting the leaving group, activated by the para
-positioned electron withdrawing group, with a nucleophilic monomer, such as phenoxide [7
]. Commercial polysulfone (PSU) polymers based on such a reaction can be obtained by the polymerization of bisphenol-A (BPA) and aromatic dihalogen monomers in a polar aprotic solvent containing alkali salt, and moisture that disturbs the reaction can be removed by an azeotropic solvent, such as toluene or benzene [9
Because BPA is considered an environmental hormone disruptor, there is some controversy about its negative health effects. The long list of environmental and health issues has motivated the search for sustainable plastics that are partially or entirely derived from biomass feedstocks, replacing petrochemicals. Accordingly, there have been much research and development efforts in the plastics industry to find substitutes for BPA for materials in which BPA is primarily used, such as polycarbonate and epoxy materials [11
]. Isosorbide (ISB) is an anhydrosugar alcohol prepared by a dehydration reaction from sorbitol, which is a hydrogenated sugar that can be obtained by the reduction of biomass glucose, and ISB is the most promising environmentally friendly substitute for BPA [14
Polymers sourced from biomass have been studied extensively and commercialized as they can replace petrochemical plastic while being environmentally friendly and allowing sustainable growth [15
]. Various types of polymers have been studied, from the most commonly known polylactic acid made from lactic acid [17
] to aliphatic polymers, such as polybutylene succinate [20
], and other polymers made from cyclic monomers, such as ISB, 2,5-furandicarboxylic acid, terpene, and lignin [15
Recently, interest has shifted from commodity polymers to high-value-added engineering plastics, and accordingly, studies on the fabrication of thermally stable polymers that use ISB as the starting material, such as polyester and polycarbonate, have increased [31
]. ISB has properties that are more attractive than those of BPA, and in particular, it is known to offer better superior mechanical properties, as well as optical properties. In addition, there have been reports of ISB monomers being used for the synthesis of poly(arylene ether)s, a type of SEP [38
]. However, the results of such efforts have shown the inability of surpassing a molecular weight (MW) of 10,000 due to the difficulty of moisture control and low reactivity of ISB.
Therefore, there is ongoing research to find commercial, thermally recyclable, and sustainable SEPs, also called high-performance plastics, that utilize bio-derived monomers, e.g., ISB, instead of restricted petrochemicals, e.g., BPA. In the present study, a phase-transfer catalyst was used to polymerize ISB-based poly(arylene ether ketone)s with a high MW without removing moisture from the polymerization constituents. Through an experiment with various controlled parameters, such as the type of halogen monomer, polymerization solvent, time, and temperature, the effect on MW was identified. The biomass contents and thermal degradation stability were compared against commercial PSU. Solution cast transparent films and melt injection molded specimens were prepared to investigate the transparency, mechanical strength, and thermal dimensional stability.
2. Materials and Methods
Among the reagents used in the reaction and analysis, ISB was procured from Roquette Frères (Lestrem, France) and used after recrystallization in acetone, 4,4′-difluorobenzophenone (FBP, 99%) and 4,4′-dichlorobenzophenone (CBP, 99%) were purchased from TCI (Tokyo, Japan) and used after recrystallization in methanol. Potassium carbonate (K2CO3, 99%) was purchased from Sigma-Aldrich Corp. (St. Louis, MO, USA) and used after grinding into fine particles and drying in a vacuum oven together with phosphorus pentoxide, 18-crown-6 (18C6, 99%), dimethyl sulfoxide (DMSO, 99%), N,N-dimethlacetamide (DMAc, 99%), 1-methyl-2-pyrrolidinone (NMP, 99%), sulfolane (99%), toluene (99.5%), acetic acid (99%), chloroform (HPLC, 99.9%), and polyethylene glycol (PEG, 400 g/mol) were purchased from Aldrich. Methanol was purchased from Daejung Chemical (Gyeonggi-do, Korea), Commercial PSU was procured from BASF (Ludwigshafen, Germany). All chemicals were used without further purification unless stated.
2.2. Polymerization of ISB-Based Poly(arylene ether ketone)s
The polymerization experiment for IK-110 utilizing a phase-transfer catalyst is described as an example. After setting up a mechanical stirrer and Dean–Stark trap on a 100-mL three-neck round-bottom flask, ISB (3.00 g, 20.5 mmol), FBP (4.47 g, 20.5 mmol), K2CO3 (3.55 g, 25.7 mmol), 18C6 (0.271 g, 1.02 mmol), and DMSO (20.4 mL, 37 wt/v% to the monomer content) were added to the flask. The reactor was set to stir for 24 h at 155 °C under a nitrogen atmosphere to carry out the polymerization process. Upon completion of the polymerization process, DMSO (20 mL) was used to dilute the contents. After cooling to room temperature, the contents were precipitated in a distilled water/methanol mixture (1 L, 50/50 vol%) containing acetic acid (10 mL). To remove any residual salt, the filtered precipitate was redissolved in DMAc solvent, after which it was reprecipitated. The precipitate was washed with distilled water and methanol and then vacuum-oven-dried for 24 h at 80 °C. Product yield (percent per theoretical yield): 6.46 g (96%), Mw: 110,200 g/mol, PDI: 1.68, 1H NMR (chloroform-d, 300 MHz, ppm): δ 7.83–7.80, δ 7.08–7.01, δ 5.11–5.08, δ 4.97–4.90, δ 4.72–4.70, δ 4.28–4.11.
The polymerization of IK-72 utilizing toluene as an azeotropic solvent and without a phase-transfer catalyst was carried out by the same process as IK-110 described above, except toluene (5 mL) was added to the polymerization medium instead of 18C6. Prior to starting the actual polymerization process, moisture was removed from the reactants by azeotropic distillation at 120 °C for 2 h. Product yield: 6.39 g (95%), Mw: 72,200 g/mol, PDI: 1.73.
summarizes the experimental parameters for the polymerization of other polymers.
The chemical structures of the polymer and biomass monomer repeating unit-based content by weight were measured using a 300-MHz nuclear magnetic resonance (NMR) spectrometer (Bruker Avance, Billerica, MA, USA). The biocarbon content of the polymer was measured by accelerator mass spectroscopy (AMS, IonPlus, Dietikon, Switzerland). The MW of the polymer was measured by gel permeation chromatography (GPC). The MW relative to standard polystyrene was calculated through an experiment using chloroform as the elution solution at 40 °C in ACQUITY APC XT columns (Waters Corp., Milford, MA, USA). The glass transition temperature of the polymer was measured by differential scanning calorimetry (DSC, Q2000, TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere within the range of 30–250 °C with a temperature variation rate of 10 °C/min. The thermal degradation stability of the polymer was determined by a thermogravimetric analyzer (TGA, Pyris 1, PerkinElmer Inc., Waltham, MA, USA), measuring the sample weight reduction and decomposition temperature by increasing the temperature by 10 °C/min under a nitrogen atmosphere.
Polymer films were fabricated by casting DMAc solution (10 wt%) in an aluminum dish, followed by drying for two days at room temperature, and then drying for an additional two days in a 100 °C convection oven. The transparency of the polymer film was measured by a UV/vis spectrometer (UV-2600, Shimadzu Corp., Kyoto, Japan). The scratch resistance of the polymer film was determined by a pencil hardness test in accordance with ASTM Standard D3360-00. The films hot-pressed under 100 bar at 200 °C for 5 min were subjected to tensile measurements using a universal testing machine made by Instron (Norwood, MA, USA) with a drawing rate of 10 mm/min. The test specimens were cut into a dumbbell shape, with a length, width, and thickness of 63.50 mm, 3.18 mm, and 100–120 μm, respectively. The fabrication process for the melt injection specimens was as follows. After dissolving the polymer (20 g) and PEG (2 g) in DMAc (200 ml), the mixture was dried for two days in a 100 °C convection oven. After grinding the dried mixture specimen, Haake™ Minijet (Thermo Scientific, Waltham, MA, USA) was used for injection molding into a dumbbell shape under a cylinder temperature of 200 °C, mold temperature of 160 °C, injection pressure of 500 bar, and filling time of 15 s. A tensile strength test on the injection specimens was performed at a drawing rate of 50 mm/min. The coefficient of thermal expansion (CTE) of the polymer film was measured by thermomechanical analysis (TMA, TA Instruments) under a nitrogen atmosphere with a probe force of 20 mN and heating rate of 10 °C/min. The film used for TMA was 15-mm wide, 4-mm long, and 70-μm thick. As a simple experiment for measuring the dimensional stability of the polymer film after exposure to heat, a heat gun (BOSCH, GHG 630 DCE, Gerlingen, Germany) was set-up, and film specimens (1-cm wide, 2.2-cm long, and 100-μm thick) with a dangled 10-g weight were exposed to a temperature of 200 °C from the heat gun at the same distance. After 2 min of heat exposure, changes in outer appearance of the film specimens, and stretch amount from the original length were observed.