Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Microwave-Assisted Synthesis of Sucrose Polyurethanes and Their Semi-interpenetrating Polymer Networks with Polycaprolactone and Soybean Oil Atanu Biswas,*,† Sanghoon Kim,† Analía Gómez,†,‡ Megan Buttrum,† Veera Boddu,† and Huai N. Cheng*,§ †
National Center for Agricultural Utilization Research, USDA Agricultural Research Service, 1815 North University Street, Peoria, Illinois 61604, United States ‡ Chemical Engineering Department, Escola Politécnica, University of São Paulo, São Paulo, SP, Brazil § Southern Regional Research Center, USDA Agricultural Research Service, 1100 Robert E. Lee Blvd., New Orleans, Louisiana 70124, United States S Supporting Information *
ABSTRACT: Because of the current interest in sustainability, environmental stewardship, and green chemistry, there has been a lot of interest in using agrobased raw materials for the design of polymeric materials. One of the promising biorenewable materials is sucrose, which is inexpensive and widely available. In this work we have carried out the synthesis of sucrose−toluene diisocyanate-based polyurethane through microwave-assisted reactions. Comparisons of conventional heat versus microwave reactions have been made. Microwave-assisted synthesis has been found to significantly decrease the reaction time and save energy relative to conventional heat. The sucrose polyurethane has turned out to be a suitable matrix to prepare semi-interpenetrating polymer networks (semi-IPNs) involving a second material. Two examples shown in this work are the semi-IPNs of sucrose polyurethane with polycaprolactone and soybean oil. Characterization of the polymers has been conducted with 13C NMR, FT-IR, sizeexclusion chromatography, and thermal analysis.
1. INTRODUCTION Because of the current interest in sustainability, environmental stewardship, and green chemistry, there has been a lot of interest in using agro-based raw materials for polymer design and synthesis.1−3 An often used approach is to start with agricultural products or derivatives and then employ various polymerization methods to produce new biobased materials; for example, new polymeric materials have been made from triglycerides or epoxidized derivatives,4−9 or from amino acids or their analogues.10−13 In our laboratories, one of our synthetic approaches is to make polyurethanes from polysaccharides and their derivatives.14−16 In particular, we have found the microwave-assisted technology to be helpful in making polyurethanes; the process is efficient, saves time, and minimizes energy consumption. Thus, we have previously developed microwave-assisted procedures for the synthesis of polyurethanes from starch and maltodextrin,14 cyclodextrins,15 and xylan.16 In the literature, microwave-assisted technology has also been found to be useful in a variety of polymerization reactions; several excellent review articles are available.17−19 Currently one of the most available products of commerce is sucrose. Global production for sucrose during the marketing year 2017/18 is a record 180 million tons.20 It is also inexpensive, selling currently at about 15 cents per lb.21 Many products have been developed from sucrose, notably sucrose polyesters as fat substitutes22,23 and surfactants.24,25 However, © XXXX American Chemical Society
there have been only a few reports of sucrose polyurethanes in journal publications. Jhurry and Deffieux26 carried out the synthesis of sucrose polyurethanes with two different methods: the first method involved the conversion of sucrose to a diol, which was then reacted to form the polyurethane, and the second method entailed the reaction of sucrose directly with a diisocyanate. In the latter case, the NMR data showed that all the hydroxy groups on sucrose reacted with the diisocyanate. Ganta et al.27 reacted pentane diisocyanate with sucrose and water to produce a microtextured spongy urethane matrix, which might be used in tissue engineering. Kizuka and Inoue28 made polyurethane elastomer, using 4,4′-diphenylmethane diisocyanate, polyether polyol (or polyester polyol), and sucrose as a cross-linker. A semi-interpenetrating polymer network (semi-IPN) is a polymeric material comprising one or more polymer networks and one or more linear or branched polymers characterized by the penetration on a molecular scale of at least one of the networks by at least some of the linear or branched macromolecules.29 Semi-IPN is a good way to produce new biobased polymeric materials, where monomers are polyReceived: Revised: Accepted: Published: A
September 29, 2017 February 8, 2018 February 14, 2018 February 14, 2018 DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
three times with ethanol and then heated at 70−80 °C in a vacuum oven overnight. Microwave-assisted syntheses of sucrose polyurethane and the semi-IPN were carried out on the Biotage microwave reactor. A typical procedure entailed the addition of a stir bar, 5.5 mL of anhydrous DMF, and 1.0 g of sucrose into a 10−20 mL Biotage vial. (For semi-IPN synthesis, PCL or SBO was added at the same time to the reaction mixture.) The mixture was stirred at room temperature (or 60 °C for the IPN involving SBO); thereupon, a variable amount of TDI (0.3−0.5 g) was added. The Biotage reactor was programmed for 30 s prestirring and then heating to 145 °C at a predetermined time (typically 3−10 min). The resulting polyurethane was transferred into a beaker containing ethanol, and worked up in the same manner. 2.3. Polymer Characterization. For NMR analysis, each sucrose polyurethane sample was dissolved in DMSO-d6 in a NMR tube (at 10% or higher concentration). The sample was heated overnight at 60 °C if needed to ensure dissolution. The 1 H and 13C NMR spectra were acquired on a Bruker DRX 400 spectrometer (Karlsruhe, Germany) at ambient temperature using standard operating conditions. The 1H and 13C chemical shifts were referenced to tetramethylsilane at 0 ppm. FT-IR spectra were obtained on a Nicolet iS10 spectrometer (Thermo Scientific Inc., Waltham, MA, USA) equipped with a Smart Orbit single bounce ATR accessory with a diamond crystal. For each spectrum, 32 scans were collected at room temperature at a spectral resolution of 4 cm−1 between 600 and 4000 cm−1 using a DTGS detector and KBr beam splitter. Data were processed with the Omnics software (version 9.2.98). Thermogravimetric analysis (TGA) was performed using a TA Instruments Q500 TGA (TA Instruments, New Castle, DE, USA). Each sample (∼5 mg) was weighed into a tared, open platinum TGA pan. The samples were analyzed in a nitrogen atmosphere by heating at 10 °C/min up to 900 °C. The onset temperature of the decomposition of each sample was measured using TA Instruments Universal Analysis software program. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q2000 DSC (TA Instruments, New Castle, DE, USA). Samples (∼3 mg) were weighed into an aluminum DSC pan, which was then hermetically sealed. Each sample was analyzed under the nitrogen atmosphere; it was equilibrated at 0 °C and heated at 10 °C/min up to 150 °C. The glass transition (Tg) of each sample was measured by taking the initial temperature of the slope change. Size-exclusion chromatography (SEC) was performed using a Shimadzu Prominence LC system (Shimadzu, Kyoto, Japan) equipped with refractive index (RI) and diode-array (UV) detectors. DMSO was used as the mobile phase as well as solvent for the samples. A 0.5% solution of each sample was prepared in DMSO. The sample was filtered using a 0.45 μm syringe filter, then 50 μL of sample was injected onto the SEC at a flow rate of 0.5 mL/min using a Phenogel 5 μ Linear(2) SEC column (Phenomenex, Torrance, CA) at 60 °C. Dextran standards were used for molecular weight calibration. The tensile strength (MPa), Young’s modulus (MPa), and elongation at break (%) of each sample were determined via an Instron Model 4201 universal testing machine (Norwood, MA) using the ASTM method D638 type V.33 Samples were molded into dog-bone specimens about 1.5−1.8 mm thick. The initial grip distance was 25.4 mm, and the grips were separated at a speed of 10 mm/min, using a 1 kN load cell. Samples were
merized in the presence of an existing biopolymer. In this way, more diverse properties can be achieved with the resulting material. Some examples of semi-IPNs involving polyurethanes have previously appeared.30,31 In this work, we have used a microwave-assisted methodology to make polyurethanes from sucrose and toluene diisocyanate (TDI). Comparisons have been made with products from both conventional and microwave heating, and they appear to have similar chemical structures. In addition, we have successfully incorporated other biodegradable materials (polycaprolactone and soybean oil) into the polyurethane matrix as semi-IPNs.
2. EXPERIMENTAL SECTION 2.1. Materials. Samples of sucrose and 2-propanol were obtained from Fisher Scientific (Pittsburgh, PA, USA). Tolylene-2,4-diisocyanate (TDI) was acquired from Fluka through Fisher Scientific; the specification sheet indicated that it consisted primarily of 2,4 isomer and only 4% 2,6 isomer. Anhydrous N,N-dimethylformamide (DMF) and phenyl isocyanate (PI) came from Sigma-Aldrich (Milwaukee, WI, USA). Polycaprolactone (PCL) was acquired from Polysciences, Inc. (Warrington, PA, USA). Soybean oil (SBO) was purchased from a local grocery store. Dimethyl sulfoxide-d6 (DMSO-d6) was obtained from Cambridge Isotope Laboratories (Andover, MA, USA). Absolute ethanol was procured from Decon Laboratories, Inc. (King of Prussia, PA, USA). 2.2. Synthetic Procedures. The reaction of phenyl isocyanate with sucrose was carried out with both conventional and microwave heating. For conventional heating, 1 g of sucrose was dried in an oven at 80 °C for at least 1 h and then dissolved in 5.5 mL of DMF, together with 0.12−0.61 g of phenyl isocyanate. The reaction mixture was heated at 145 °C for 20 min in a heating and stirring module (Reacti-therm, from Thermo Fisher, Pittsburgh, PA, USA) with feedback control of the temperature. For microwave heating, the same reaction mixture was added to a vial on a microwave reactor (Biotage Initiator Microwave Synthesis Systems, from Biotage AB, Uppsala, Sweden). It was then heated with microwaves at 145 °C for 6 min. For both conventional and microwave heating, after the reaction was completed, the product was cooled and DMF was removed via a Kugelrohr distillation apparatus with the help of a vacuum pump and trap. For conventional thermal synthesis of sucrose polyurethanes, typically we placed 1.0 g of sucrose, variable amounts of TDI (0.1−0.5 g), and 5.5 mL of DMF in the Reacti-therm heating and stirring module. The reacting mixture was heated at 145 °C for 20 min. After the reaction, the product was cooled and recovered by washing three times with ethanol or isopropanol and then heated at 70−80 °C in a vacuum oven overnight. For conventional thermal synthesis of sucrose polyurethane semi-IPN with PCL, 1.0 g of sucrose, 0.5 or 1.0 g of PCL, and 5.5 mL of DMF were placed in the reaction vessel in the Reactitherm module. Then 0.3 or 0.4 g of TDI was added and the mixture heated at 145 °C with stirring for 20 min. As for SBO, it was reported earlier that SBO was not miscible in DMF at room temperature.32 We first tried pyridine as a solvent, but we discovered that SBO was miscible with DMF at 60 °C. The same procedure for PCL was then used for the semi-IPN synthesis involving SBO, by first mixing sucrose, SBO, and DMF at 60 °C, adding TDI, and then heating at 145 °C to carry out the reaction. The workup procedure for both semiIPNs was the same: the product was recovered by washing B
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research conditioned before testing for at least 40 h at 50 ± 5% relative humidity and 23 ± 2 °C.
Table 1. Synthesis of Polyurethane with Different Sucrose/ TDI Stoichiometrya
3. RESULTS AND DISCUSSION 3.1. Model Reaction of Sucrose with Phenyl Isocyanate. To gain a better understanding of sucrose/isocyanate reaction, we carried out the reaction of sucrose with phenyl isocyanate (Scheme 1). Thus, sucrose was reacted with phenyl
sample
TDI (g)
heating mode
heating time (min)
product yield (g)
yield (%)
A1 A2 A3 A4 A5 M1 M2 M3
0.121 0.243 0.364 0.485 0.607 0.364 0.364 0.364
conventional conventional conventional conventional conventional microwave microwave microwave
20 20 20 20 20 3 6 10
0.113 0.208 0.523 1.061 1.797 0.431 0.639 0.616
10 17 47 76 76 32 47 45
Scheme 1. Reaction between Phenyl Isocyanate and Sucrose
a
All reactions were done with 1 g of sucrose and 5.5 mL of DMF at 145 °C.
isocyanate at different levels using both conventional and microwave heating processes at 145 °C. The samples prepared and the reaction conditions are summarized in Table S1 in the Supporting Information. The 13C NMR spectra for both conventional and microwave heating processes looked the same; only the spectrum of the microwave product is shown in Figure S1 in the Supporting Information. Assignments of major peaks were accomplished through empirical additivity rules34,35 and comparison of similar compounds in the literature.16,36 Because the reaction could produce many different sucrose carbamate products (e.g., eight 1:1 sucrose/phenyl isocyanate adducts, and 28 1:2 sucrose/ phenyl isocyanate adducts), it would be difficult to sort out the peaks of these adducts. For simplicity, we monitored the decrease of intensities of the unreacted sucrose peaks. The data are shown in Table S2. All eight C−OH peaks of unreacted sucrose showed approximately the same relative intensities for each sample in the series. Thus, there was no observable difference in the reactivity of the alcohols (primary or secondary) on sucrose under the reaction conditions employed. Furthermore, there was no observable difference between the reactivity of sucrose under conventional and microwave heating processes. Basically the same products were obtained. 3.2. Sucrose Polyurethane Synthesis via Conventional Heat. The polyurethanes derived from sucrose−diisocyanate were first prepared via conventional heat (Scheme 2). In a
The molecular weight distributions of the five conventionalheat products in Table 1 are given in Figure 1. Thus, samples
Figure 1. Size exclusion chromatography curves for five samples of sucrose polyurethanes with increasing levels of TDI used in synthesis. Samples 1−5 are samples A1, A2, A3, A4, and A5 as shown in Table 1.
A1 and A2 had relatively low and narrow molecular weight distributions with Mn ∼ 3000, and Mw ∼ 4500. Sample A3 (made with a higher TDI level) had a broader molecular weight distribution, with Mn ∼ 6000, and Mw > 10 000. Samples A4 and A5 (with higher TDI levels) had even broader molecular weight distributions, indicating the presence of more extensive cross-linking. The above five samples were also studied by TGA (Figure 2). The initial weight loss at ca. 40−80 °C was due to loss of water. All five samples A1−A5 started to show rapid weight loss at around 200 °C due mostly to the decomposition of sucrose.37,38 At temperatures above 270 °C the polyurethane started to degrade.14 The maxima on the differential TGA curves increased slightly from 238 to 250 °C as the molecular weight increased. Because sucrose by itself decomposed around 180 °C,37,38 the formation of the urethane bond apparently improved the thermal stability of the products; this result is consistent with prior reports in the literature.14,16 The DSC data for the same five samples are shown in Figure 3. Samples A1 and A2 showed distinct and broad glass transition temperatures (Tg’s), but the corresponding Tg’s for samples A3, A4, and A5 were weak and difficult to detect, probably reflecting the presence of cross-linked structures.
Scheme 2. Reaction between TDI and Sucrose
preliminary study, TDI was observed to react satisfactorily with sucrose in DMF at 145 °C in 20 min. Because the stoichiometry of diisocyanate and sucrose was important to the molecular weight and the properties of the product, a study was made of the conventional heat reaction of sucrose with different TDI concentrations (Table 1, first five entries). The percentage yield varied from 10% at a low TDI level to 76% at the highest TDI level. The products obtained at low TDI levels were soft and pliable; those at the high TDI levels were hard and crumbly. Sample A3 with 0.364 g of TDI (or possibly sample A4 with 0.485 g of TDI) seemed to be an optimal formulation for future development. All the samples were soluble in DMSO, albeit with different viscosities. C
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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The 13C NMR spectra of two samples made with the same sucrose/TDI ratio but with conventional and microwave heating modes are shown in Figure 4a,b (samples AR and MR). The spectral features were somewhat complex, but detailed assignments were achieved (as shown in Figure 4a) with the help of the spectra of reference compounds and related structures in the literature14−16 and from empirical shift additivity rules.34,35 The urethane carbon resonated at 155 ppm. The toluene moiety of the polymer had peaks at 136 and 138 (carbons 2 and 4), 131 ppm (carbon 6), 127 ppm (carbon 1), 115 ppm (carbons 3 and 5), and 18 ppm (methyl). The sucrose carbons resonated at 60−110 ppm. Except for the presence of ethanol (from the workup procedure), the two spectra of samples made via conventional and microwave heat (Figure 4a,b, respectively) were very similar, indicating that the same chemical structure was obtained. For additional information, the FT-IR spectra for the same two sucrose polyurethane samples made via conventional heat and microwave are shown in Figure 5a,b. The y-axis on the spectra represents the spectral intensity. Spectra of pure aqueous solution of sucrose showed absorbance bands at 928, 1007, 1054, 1123, 1362, and 1427 cm−1, with two distinctive peaks at 1049 and 994 cm−1 being most characteristic of the presence of the sucrose moiety.39−41 The 1049 cm−1 peak was due to CO stretching and COH bending modes, and the 994 cm−1 peak was related to the glycosidic linkage in sucrose.39,41 The urethane peaks could be found at 1726 cm−1 (free CO of urethane), 1704 cm−1 (hydrogen-bonded CO of urethane), 1530 cm−1 (N−H of urethane),14,42−44 and 1240 cm −1 (CO of urethane). 43 The peak at 1600 cm −1 corresponded to CH of the toluene moiety.14 The two characteristic sucrose peaks at 1049 and 994 cm−1 and the urethane peaks at 1730, 1600, and 1530 cm−1 are labeled in Figure 5a. The spectral features for the two samples made with conventional heat (Figure 5a) and microwave heat (Figure 5b) were the same, indicating that the products made via the two processes had similar chemical structures. 3.4. Semi-Interpenetrating Polymer Networks. Because sucrose has multiple hydroxy groups, the polyurethanes resulting from it are brittle. We surmise that a semipenetrating polymer network involving sucrose polyurethane and a suitable biobased material may be attractive. Two materials have been attempted as proof of principle. Polycaprolactone (PCL) is a biodegradable polyester with a low melting point (ca. 60 °C) and a low Tg (ca. −60 °C). This polymer is often used as an additive for resins to improve their processing characteristics and their end-use properties (e.g., impact resistance).45,46 Soybean oil (SBO) is a well-known and inexpensive commodity that has been widely utilized as raw material for a variety of polymeric materials.4−8 As it turned out, it was relatively easy to incorporate PCL into sucrose polyurethane matrix as a semi-IPN because PCL is compatible with sucrose and its polyurethane. Thus, PCL was added to sucrose at two levels, which were then polymerized at two levels of TDI (Table 2). The results showed that the semiIPN yields varied from 50 to 90%. In general, a higher level of TDI in the polyurethane synthesis tended to retain PCL more effectively. The yields were approximately the same whether we used conventional heat or microwave. Again, microwaveassisted reaction took less time, about one-quarter of the reaction time. All the products were firm solids. In comparison to the case for PCL, the incorporation of SBO into the sucrose polyurethane matrix was more difficult because
Figure 2. TGA and differential TGA data for polyurethane products from sucrose and TDI with increasing levels of TDI: samples A1 (red), A2 (blue), A3 (green), A4 (gray), A5 (black).
Figure 3. DSC data for polyurethane products from sucrose and TDI with increasing levels of TDI: samples A1 (red), A2 (blue), A3 (green), A4 (orange), A5 (black).
From the change in the slopes of the curves, the following Tg values could be estimated: A1 (45 °C), A2 (75 °C), A3 (85 °C), A4 (96 °C), and A5 (98 °C). The increasing Tg with increasing TDI/sucrose ratio in the resulting polyurethanes indicated increasing chain stiffness with chain growth and crosslinking. 3.3. Microwave-Assisted Sucrose Polyurethane Synthesis. We next carried out the same sucrose−TDI reaction using a microwave-assisted methodology. The same weight ratio of TDI/sucrose as sample A3 was used and the microwave energy was applied at 3, 6, and 10 min (Table 1, entries 6−8). At 3 min, the yield was somewhat low (32%), but at 6−10 min, the yields were the same as the value obtained for the conventional heat at 20 min (45−47%). The fact that the microwave reaction can accomplish the same reaction at a shorter time is of interest. In this way, microwave-assisted reaction can save time or energy and represents a “greener” method for synthesis. D
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. 13C NMR spectra of (a) sample AR (sucrose−TDI product from conventional heat), (b) sample MR (sucrose−TDI product from microwave), (c) sample Q3 (sucrose polyurethane semi-IPN with PCL), and (d) sample T3 (sucrose polyurethane semi-IPN with SBO). T = toluene moiety, S = sucrose moiety, B = SBO, P = PCL, D = DMSO-d6, E = ethanol, F = DMF; the subscripts refer to carbon numbers as shown in the structures below.
PCL, the major IR peaks47 occurred at 2949, 2865, 1727, and 1170 cm−1. For SBO, the more distinctive IR peaks48 could be found at 2931, 2853, 1743, 1463, and 1161 cm−1. Some of these peaks overlapped with those of sucrose polyurethane, but most of them could be discerned. These peaks are labeled in Figures 5c,d. Thus, the FT-IR data also confirm the presence of sucrose polyurethane, PCL, and SBO in these semi-IPN samples. For further characterization, a comparison of TGA curves of sucrose polyurethane with the two semi-IPN samples is shown in Figure 6. For sample MR, the TGA curve was similar to those shown in Figure 2. The TGA curve for the semi-IPN involving sucrose polyurethane and PCL reflected the degradation of both polymeric components; thus, it showed the initial sucrose degradation at 210 °C, the slope change and the incipient polyurethane degradation at 270 °C, and the degradation of PCL at around 400 °C.49 The TGA curve for the semi-IPN involving sucrose polyurethane and SBO could be likewise interpreted; the sucrose moiety started to degraded at 210 °C, and then SBO started to degrade at around 250
of the lower molecular weight of SBO. The same reactions gave only 13−46% yields (Table 3). The products were soft and gellike, but not liquids. The 13C NMR spectra of the semi-IPNs containing PCL and SBO are shown in Figure 4c,d. The spectra were similar to that of the sucrose−TDI product, with the exception of new peaks corresponding to PCL and SBO. Spectral assignments were made on the basis of reference spectra and shift additivity rules34,35 and are noted directly on the spectra in Figure 4cd. In particular, PCL had peaks at 173 ppm (carbon 1), 64 ppm (carbon 6), 35 ppm (carbon 2), 29 ppm (carbon 5), 27 ppm (carbon 3), and 26 ppm (carbon 4). Soybean oil had the carboxyl peak at 173 ppm, olefin peaks at around 130 ppm, glycerate peaks at 67 and 63 ppm, and aliphatic peaks at 14−35 ppm. Thus, the semi-IPN products shown in Figure 4c,d definitely contain PCL and SBO, respectively. The FT-IR spectra for the same semi-IPN materials are shown in Figure 5c,d. The spectra match well with the spectra of sucrose polyurethane and the starting PCL and SBO. For E
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. TGA data in nitrogen for microwave-assisted sucrose polyurethane samples: sample MR (TDI-sucrose reaction product); sample Q3 (sucrose polyurethane semi-IPN with PCL); and sample T3 (sucrose polyurethane semi-IPN with SBO).
Figure 5. FT-IR spectra of (a) sample AR (sucrose−TDI product from conventional heat), (b) sample MR (sucrose−TDI product from microwave), (c) sample Q3 (sucrose polyurethane semi-IPN with PCL), and (d) sample T3 (sucrose polyurethane semi-IPN with SBO).
polyurethane by itself was very brittle, and the compression molded dog-bone samples all broke on the grips of the tensile tester before measurements could be made. The semi-IPN with 50% soybean oil was still somewhat brittle; it gave the following measurements: Young’s modulus 13 MPa, tensile strength at break 1 MPa, and elongation at break 14%. The semi-IPN with 50% polycaprolactone gave much better mechanical properties: Young’s modulus 298 MPa, tensile strength at break 14 MPa, and elongation at break 13%. The latter values compared favorably with the mechanical properties of polycaprolactone by itself, where the tensile strength was reported to range from 10.5 to 16.1 MPa and modulus ranged from 343.9 to 364.3 MPa.51 Thus, the mechanical properties of sucrose polyurethane can be improved by using the semi-IPN approach to incorporate a second polymer that has the desired attributes. 3.5. Comments on Microwave-Assisted Reaction Technology. It is encouraging that for the synthesis of both sucrose polyurethane and their semi-IPNs, the microwaveassisted technology can reduce reaction time and save energy. Since 1986, there has been an increasing use of microwave technology in chemical and polymer reactions.17−19,52 Although the benefits of microwave-assisted reactions are known, there is still some debate concerning the detailed mechanism.17,52,53 Two effects are most often cited: the thermal microwave effect (where microwaves are a more efficient way to heat up the reaction mixture) and specific microwave effects (where unexplained enhancement in reactivity or selectivity is observed). At the practical level, what is important is whether the use of microwave heating shows any advantages over conventional heating for a given reaction. It is useful therefore to compare the reaction in question via both modes of heating and to document the benefits of microwave-assisted reactions (as we have done for sucrose polyurethanes in this work). Such reactions are usually conducted on the laboratory bench using microwave reactors. If commercial scale manufacturing is contemplated, the scale-up of such syntheses is often nontrivial.54 Nonetheless, in recent years there has been a lot of progress made in developing large-scale microwave equipment for manufacturing, and several companies are currently either supplying such equipment or actively working on it.54−57
Table 2. Sucrose Polyurethane Semi-IPN with Polycaprolactone (PCL)a sample no.
TDI (g)
PCL (g)
heating mode
heating time (min)
product yield
yield (%)
AR P1 P2 P3 P4 MR Q1 Q2 Q3 Q4
0.364 0.364 0.485 0.364 0.485 0.364 0.364 0.485 0.364 0.485
0 0.5 0.5 1.0 1.0 0 0.5 0.5 1.0 1.0
conventional conventional conventional conventional conventional microwave microwave microwave microwave microwave
20 20 20 20 20 5 5 5 5 5
0.66 0.88 1.37 1.53 1.97 0.68 1.16 1.27 1.75 2.17
51 49 72 67 82 53 64 67 76 90
All reactions done at 145 °C on 1 g of sucrose in 5.5 mL of DMF, with variable amounts of PCL and TDI.
a
Table 3. Sucrose Polyurethane Semi-IPN with Soybean Oil (SBO)a sample no.
TDI (g)
SBO (g)
heating mode
heating time (min)
product yield
yield (%)
AR S1 S2 S3 S4 MR T1 T2 T3 T4
0.364 0.364 0.485 0.364 0.485 0.364 0.364 0.485 0.364 0.485
0 0.5 0.5 1.0 1.0 0 0.5 0.5 1.0 1.0
conventional conventional conventional conventional conventional microwave microwave microwave microwave microwave
20 20 20 20 20 5 5 5 5 5
0.66 0.49 0.68 0.39 0.56 0.68 0.36 0.88 0.29 0.56
51 27 36 17 23 53 20 46 13 23
All reactions were done at 145 °C on 1 g of sucrose in 5.5 mL of DMF, with variable amounts of SBO and TDI.
a
°C,50 almost coinciding with the degradation of the polyurethane. A preliminary study of the mechanical properties of sucrose polyurethane and the two semi-IPNs was carried out. The samples were made with a sucrose/TDI/X weight ratio of 1.0/ 0.4/0.5, where X is PCL or SBO in the semi-IPNs. Sucrose F
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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4. CONCLUSIONS We have been interested in green polymer chemistry for many years, and this work is part of our overall research thrust. The idea is to use a benign reaction (heat only, no hazardous organic solvent, no metal catalyst) that can be carried out quickly (microwave), using agro-based, sustainable, or biodegradable materials (sucrose, polycaprolactone, and soybean oil) to produce new polymeric products. In the first part of this work, we have shown that it is possible to use microwaveassisted synthetic methodology to produce sucrose polyurethanes, thereby saving energy and reaction time. The TDI/ sucrose ratio needs to be customized to provide the molecular weight and thermal properties needed for a given application. The chemical structures of the sucrose polyurethanes produced from conventional heat and from microwave are similar. In the second part of this work, we have demonstrated that the sucrose polyurethane is a convenient matrix to produce semiIPNs. As proof of principle, we have incorporated PCL and SBO into sucrose polyurethanes. A problem with sucrose polyurethanes is that it is brittle and has limited applications. By incorporation of PCL as a semi-IPN, the mechanical properties of the sucrose polyurethane are notably improved. Other sucrose polyurethane semi-IPN’s involving different polymers may be made in a similar fashion to impart desirable properties; in this way, we can potentially expand the application areas for sucrose polyurethanes.
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REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04059. Reaction parameters for the reaction between sucrose and phenyl isocyanate (Table S1), reaction of phenyl isocyanate with sucrose (Scheme S1), sucrose structure and its numbering scheme (Scheme S2), 13C NMR spectrum of sample SM-1 (Figure S1), observed intensities of 13C NMR peaks of unreacted sucrose and calculated fraction of primary alcohols (Table S2) (PDF)
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Article
AUTHOR INFORMATION
Corresponding Authors
*A. Biswas. E-mail:
[email protected]. Tel: 1-309-6816406. *H. N. Cheng. E-mail:
[email protected]. Tel: 1-504-2864450. ORCID
Atanu Biswas: 0000-0002-8112-1968 Huai N. Cheng: 0000-0001-8647-057X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge Karl Vermillion for supplying NMR data, Jason Adkins for SEC and thermal data, Daniel Knetzer for graphics, and Kelly Utt for help with IR data processing. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. G
DOI: 10.1021/acs.iecr.7b04059 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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