Synthesis and Characterization of Polyurethane Networks Derived

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Synthesis and Characterization of Polyurethane Networks Derived from Soybean Oil-Based Cyclic Carbonates and Bio-Derivable Diamines Satyabrata Samanta, Sermadurai Selvakumar, James Bahr, Dona Suranga Wickramaratne, Mukund P Sibi, and Bret J. Chisholm ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01409 • Publication Date (Web): 10 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Sustainable Chemistry & Engineering

Synthesis and Characterization of Polyurethane Networks Derived from Soybean Oil-Based Cyclic Carbonates and Bio-Derivable Diamines

Satyabrata Samanta,1 Sermadurai Selvakumar,2 James Bahr,3 Dona Suranga Wickramaratne,1 Mukund Sibi,2,4 Bret J. Chisholm1,4*

1

Department of Coatings and Polymeric Materials 1735 NDSU Research Park Drive North

North Dakota State University, Fargo, ND 58102

2

Department of Chemistry and Biochemistry 1231 Albrecht Blvd

North Dakota State University, Fargo, ND 58102

3

Research and Creative Activity

1735 NDSU Research Park Drive North North Dakota State University, Fargo, ND 58102

4

Materials and Nanotechnology Program 1301 Administration Avenue

North Dakota State University, Fargo, ND 58102 *To Whom Correspondence Should be Addressed: [email protected] 1 ACS Paragon Plus Environment


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ABSTRACT Non-isocyanate polyurethane (NIPU) thermoset networks were produced from a novel soybean oil-derived poly(vinyl ether) (i.e. poly[(2-vinyoxy)ethyl soyate]) possessing cyclic carbonate functional groups in the fatty acid ester side chains of the polymer. Three different linear aliphatic diamines, namely, 1,6-hexamethylenediamine, 1,9-nonanediamine, and 1,13tridecanediamine, were used to crosslink the cyclic carbonate-functional poly[(2-vinyoxy)ethyl soyate] [C-poly(2-VOES)]. All three of these diamines can be readily obtained from renewable resources. For comparison purposes, analogous NIPU networks were produced using cyclic carbonate-functional soybean oil (CSBO) in place of the C-poly(2-VOES).

The chemical,

thermal, viscoelastic, and mechanical properties of the six NIPU networks were characterized. With regard to the chemical nature of the soy-based, carbonate-functional component, it was found that the polymeric nature of C-poly(2-VOES) resulted in very different NIPU properties compared to analogous crosslinked networks based on CSBO. While the CSBO-based NIPU networks exhibited lower Young’s moduli and ductile behavior, the networks based on C-poly(2VOES) showed significantly higher Young’s moduli and brittle behavior.

In addition,

measurements using dynamic mechanical analysis showed significantly high crosslink densities for the networks based on C-poly(2-VOES), which can be attributed to much higher number of methine carbon atoms per molecule in C-poly(2-VOES) as compared to CSBO. In addition to the crosslinks resulting from the reaction of the amine groups of the crosslinker with the cyclic carbonate groups of the soy-based carbonate-functional materials, these methine carbon atoms serve as crosslinks in the NIPU networks. The higher crosslink densities achieved with the use

2 ACS Paragon Plus Environment

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of C-poly(2-VOES) explain the thermal and mechanical property differences observed between networks based on the two different soy-based carbonate-functional materials. With regard to the influence of the diamines on NIPU network properties, as expected, increasing the chain length of the diamine crosslinker decreased crosslink density, which, in general, resulted in decreases in Young’s moduli and glass transition temperature.

Key words: polyurethane, soybean oil, non-isocyanate polyurethane, cyclic carbonate, vinyl ether

INTRODUCTION Polyurethanes (PUs) are one of the most important classes of polymeric materials used in today’s society. The utility of PUs largely stem from the nature of the intermolecular hydrogen bonding that occurs between urethane groups.1 The breaking and reforming of hydrogen bonds provides a mechanism for the dissipation of mechanical energy into heat, which inhibits the covalent bond scission that can lead to fracture. As a result, the urethane group provides exceptional toughness, abrasion resistance, impact resistance, and durability to a polymeric material. Depending on the choice of monomers, prepolymers, crosslinkers, etc., PU-based materials can range from very soft, flexible materials to very hard, stiff materials. As a result of the wide range of desirable properties that can be achieved by incorporating urethane linkages into a polymeric material, a plethora of applications are enabled, such as soft and rigid foams, high-performance coatings, adhesives, encapsulants, castings, and sealants.2 Historically, PUs have been derived from di- and/or multi-functional isocyanate compounds and di- and/or multi-functional hydroxy compounds.

Unfortunately, both the



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production of isocyanates and the use of isocyanates represent a significant health and environmental hazard. Commercial production of isocyanates generally involves the reaction of primary amine-functional compounds with phosgene, which is a colorless, highly reactive, highly toxic gas. Exposure to phosgene can cause severe respiratory problems, eye and skin irritation/burning, and even death.3 With regard to isocyanates, their high reactivity represents a human health risk, since they can readily react with the nucleophilic functional groups found in the biomacromolecules that make-up the body. hydroxyl, carboxyl, and thiol groups.

These functional groups include amine,

Thus, human exposure to isocyanates is highly

undesirable. Common isocyanates, such as 4,4’-methylenediphenyl diisocyanate and toluidene diisocyanate, can enter the body through inhalation, skin (open wound), eye contact, or by mouth.4,5 In addition, most low molecular weight diisocyanates are sensitizers, which means that a sensitized person can undergo a strong allergic reaction to these compounds even when exposed to very minor amounts of the compounds. The level of exposure leading to sensitization varies, but, for some individuals, it may only occur after years of repeated exposure to the diisocyanate. As a result of these issues, the Occupational Safety and Health Administration has put forth regulations about the hazards and exposure limits of isocyanates. Another issue with isocyanate-functional materials is their sensitivity to moisture. Isocyanates can react with water at ambient temperature to produce an amine and carbon dioxide. The amine can then react with another isocyanate to produce a urea linkage. While this feature has been utilized to produce one component, moisture-curable materials, it can be problematic for the shelf/storage stability of isocyanates. Considering the discussion above, there is a strong need for the development of PU thermoplastics and thermosets without the use of isocyanates. There are a number of different


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polycondensation methods that can be used to produce PUs without the use of isocyanates. For example, the reaction of diamines with dialkylcarbonates has been used to produce dicarbamates that were subsequently used to produce PUs by transurethanization with a diol.6 Similarly, selfcondensation of bishydroxyalkylcarbamates has been used to produce PUs.7 The method that has been receiving the most attention in recent years involves the reaction of cyclic carbonates with primary amines.8-12 Five, six, and seven-membered cyclic carbonates have been used to produce urethanes by reactions with amines.13-17 Due to ring strain, it has been shown that relative reaction rate increases with increasing cyclic carbonate ring size.13,17,18 While the six and seven-membered cyclic carbonates provide higher reactivity, their synthesis typically involves hazardous reagents, such as ethyl chloroformate and triphosgene. In contrast, fivemembered cyclic carbonates can be produced by the reaction of carbon dioxide with an epoxide. Thus, the generation of cyclic carbonate-functional materials by this route can be considered environmentally favorable, since it enables the fixation of the greenhouse gas, carbon dioxide.17 It has been shown that cyclic carbonate materials for the production of non-isocyanate PUs (NIPUs) can be readily produced from unsaturated renewable materials, such as plant oil triglycerides and limonene.20 For these renewable materials, the unsaturation present in the molecule can be easily oxidized to epoxy groups and then CO2 inserted into the oxirane ring to produce the five-membered cyclic carbonate. Plant oils, such as soybean oil (SBO), linseed oil, sunflower oil, and rapeseed oil, are relatively inexpensive, non-toxic, biodegradable, and can be readily functionalized with five-membered cyclic carbonate groups. As a result, their use for the production of thermoset NIPUs has been investigated.21-26 Since the double bonds present in plant oils are internal double bonds, and thus disubstituted, they are less reactive than monosubstituted cyclic carbonates. As a result, elevated temperatures and relatively long reaction



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times are typically needed to obtain high conversions. Using linear aliphatic diamines to react with plant oil-based cyclic carbonates, the reaction conditions typically used to produce NIPU thermosets are 70 to 100 °C for up to 15 hours.10 Figure 1 shows the conversion of a plant oil triglyceride to a cyclic carbonate-functional compound and its subsequent reaction with a primary diamine to produce a crosslinked network with urethane linkages. As shown in Figure 1, the product from the reaction of an amine group with a cyclic carbonate group yields a urethane group as well as a hydroxyl group located on the β-carbon atom adjacent to the urethane group. These hydroxyl groups can form hydrogen bonds with the urethane carbonyl group. As a result of the production of this hydroxyl group, NIPUs generally have higher water uptake, improved resistance to organic solvents, and higher strength and stiffness compared to urethane groups generated through the reaction of an isocyanate and a hydroxyl group.27


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CO2



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Figure 1. A schematic for the conversion of a plant oil triglyceride to a cyclic carbonatefunctional compound and its subsequent reaction with a primary diamine to produce a crosslinked network with urethane linkages.

The authors have developed a plant oil-based polymer technology that enables linear polymers to be produced that possess fatty acid ester pendent groups.28-39 As illustrated in Figure 2, the plant oil polymer technology involves the production of a vinyl ether monomer from a plant oil by base-catalyzed transesterification.38 This vinyl ether monomer is then converted to homopolymers and copolymers using cationic polymerization. It has been shown that the use of an appropriate cationic polymerization system can provide a living polymerization of the plant oil-based monomer and homopolymers that possess narrow molecular weight distributions.39 In addition, all of the unsaturation derived from the plant oil is retained in the polymer. Compared to the parent plant oil, a poly(vinyl ether) derived from the plant oil possesses a much higher number of double bonds per molecule. If the double bonds are used for crosslinking, either directly or through derivatization to other functional groups, this feature results in the gel-point being reached at a much lower degree of functional group conversion. As described by Alam,38 the gel-point decreases exponentially with the degree of polymerization (DP).


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+

KOH

+

excess

Cationic Polymerization

Figure 2. Schematic illustrating the monomer and homopolymer synthesis for the plant oil-based polymer technology developed by Chisholm and coworkers.38,39 A vinyl ether monomer is produced from the base-catalyzed transesterification of 2-(vinyloxy)ethanol a plant oil triglyceride. This vinyl ether monomer is then converted to homopolymers and copolymers using cationic polymerization.

The objective of the research described in this paper was to prepare and characterize NIPUs that can be readily produced from biobased starting material and to compare the properties of these NIPUs to similar NIPUs derived from carbonate-functional plant oil-based poly(vinyl ethers). All of the carbonate-functional components were derived from SBO, while two of the long chain diamines utilized can be produced from two different plant oil-derived fatty acids.

The two long chain diamines are 1,9-nonanediamine (NDA) and 1,13-

tridecanediamine (TDA), which can be synthesized from oleic and erucic acid, respectively.40 9 ACS Paragon Plus Environment


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Oleic acid is a major component of many common plant oil triglycerides, such as high oleic SBO, canola oil, olive oil, and peanut oil.

Erucic acid can be obtained from plant oil

triglycerides, such as rapeseed oil and crambe oil.

The short chain diamine, 1,6-

hexamethylenediamine, can be produced from renewable resources including biomass derived ethanol,41 succinic acid,42 hexane,43 furfural,44 glycerol,45 levulinic acid,44 adipic acid,46 lactic acid,47 and 5-(hydroxymethyl)furfural.48

EXPERIMENTAL

Materials and chemicals. Epoxidized soybean oil (ESBO) was purchased from Arkema (Vikoflex®7170) and the epoxy equivalent weight was determined to be 245 g/mole using American Oil Chemist’s Society (AOCS) Method, AOCS Cd-9-57.

Tetrabutylammonium

bromide (TBAB, 98% purity), HMDA (98% purity), and anhydrous magnesium sulfate (99.5% purity) were obtained from Sigma-Aldrich. NDA (98% purity) and toluene were purchased from Alfa Aesar and BDH Chemicals, respectively.

All reagents were used without further

purification. Poly[2-(vinyloxy)ethyl soyate] [poly(2-VOES)] and its epoxidized derivative [E-poly(2VOES)] were synthesized as described by Alam and Chisholm.38 Epoxy equivalent weight for E-poly(2-VOES) was determined to be 255 g/mole using the method described in AOCS Cd-957. 1,13-Tridecanediamine (TDA) was synthesized according to the procedure described by He et al.49


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Procedure for producing carbonate-functional materials. Epoxy groups present in ESBO and E-poly(2-VOES) were converted to cyclic carbonate groups using supercritical carbon dioxide and TBAB as a catalyst. Reactions were carried out in a Series 4584 six-liter high pressure reactor from Parr Instrument Company equipped with overhead stirring, inlet ports, vacuum pump, and an electric heating mantle. For the production of cyclic carbonate-functional soybean oil (CSBO), the reactor was charged with 400 g of ESBO and 24 g of TBAB and then sealed by bolting on the reactor head. Air was then removed from the sealed vessel by reducing the pressure below 0.05 mm Hg. Next, the vessel was connected to a 22.7 kg cylinder of liquid carbon dioxide (PraxAir) and approximately 900 g of CO2 transferred to the vessel through a port in the reactor that was equipped with a shut-off valve. The reaction mixture was then heated to 140 °C over a six h period, while stirring at 200 rpm. The pressure generated at 140 °C was about 12.4 MPa. After reaching 140 °C, the reaction was allowed to continue over a 48 h period before allowing the vessel to slowly cool to room temperature under continuous stirring. After reaching room temperature, the CO2 was slowly bled-off. 1.5 L of ethyl acetate was added to the vessel to dissolve the material and the solution washed once with deionized water and then twice with brine solution. The washed ethyl acetate solution was then dried with magnesium sulfate before isolating the CSBO by removing the ethyl acetate using a rotary evaporator. For the E-poly(2-VOES), it was found that the procedure used for the production of CSBO resulted in gelation during the reaction. This issue was overcome by using a solution of E-poly(2-VOES) as opposed to the neat material and stopping the carbonation reaction before reaching essentially complete conversion. 160 g of E-poly(2-VOES) and 9 g of TBAB was dissolved in 400 g of toluene and the solution charged to the reaction vessel. Due to the volatility of the toluene, the reactor was immersed in a bucket of ice water while stirring at 200



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rpm to chill the vessel and the toluene solution for 15 min prior to the evacuation of air from the vessel. Once the air had been removed from the vessel, the rest of the procedure to produce carbonate-functional poly(2-VOES) [C-poly(2-VOES)] was the same as that described for the production of CSBO. The carbonate equivalent weight for CSBO and C-poly(2-VOES) was determined to be 289 g/mole and 299 g/mole, respectively. Carbonate equivalent weight was calculated from the difference in epoxy content before and after carbonation and assuming that all of the epoxy groups consumed during the carbonation reaction were due to carbonate formation. In addition, integration of peaks associated with carbonate protons in the proton nuclear magnetic resonance (1H NMR) spectra of CSBO and C-poly(2-VOES) corroborated the carbonate equivalent weights determined by the epoxy titration method.

Procedure for the production of NIPU crosslinked networks. Crosslinked NIPU networks based on CSBO and C-poly(2-VOES) were produced by reaction with the diamines, HMDA, NDA, and TDA. Table 1 provides a description of the reaction mixtures used to produce the crosslinked networks. Since amine groups can react with both carbonate groups and epoxy groups, the amount of amine crosslinker used to produce each NIPU was based on both the concentration cyclic carbonate groups and any unreacted epoxy groups. The concentration of amine crosslinker was used at a 1.0/1.0 mole/mole ratio of the sum of carbonate and epoxy groups to amine groups. For CSBO, 0.92 mole percent of epoxy groups remained after the carbonation reaction. However, to avoid gelation during the carbonation of E-poly(2-VOES), the carbonation reaction was carried out to 85.3 percent conversion of epoxy groups. Thus, 14.7 mole percent of the original epoxy groups present in E-poly(2-VOES) remained in the C-poly(2-


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VOES). For the CSBO and C-poly(2-VOES) samples produced for study, the equivalent weight of the sum of carbonate and epoxy groups was 289 and 293 g/mole, respectively. Mixing of the reaction mixtures was achieved using a FlackTek mixer operating at 3500 rpm for one min. Each liquid composition was cast over Teflon®-laminated glass panels using a drawdown bar with a 200 µm gap. Panels were heated at 100 °C in a forced-air for 16 h to promote crosslinking. After curing, free film specimens were obtained by peeling films from the Teflon® substrate.

Table 1. The composition of the reaction mixtures used to produce the crosslinked networks. The concentration of diamine crosslinker used for each reaction corresponded to a 1.0/1.0 mole/mole ratio of the sum of carbonate and epoxy groups present in CSBO or C-poly(2-VOES) to amine groups present in the diamine. Coating ID CSBO-HMDA CSBO-NDA CSBO-TDA C-poly(2-VOES)-HMDA C-poly(2-VOES)-NDA C-poly(2-VOES)-TDA

CSBO (g) 25 25 25 -

C-poly(2-VOES) (g) 20 20 20

HMDA (g) 4.83 3.86 -

NDA (g) 6.58 5.26 -

TDA (g) 8.91 7.13

Toluene (mL) 12 15 20 12 15 20

Analytical methods and instrumentation. The epoxy equivalent weight for CSBO and Cpoly(2-VOES) was determined according the procedure described in AOCS Cd-9-57. A JEOLECA 400 NMR spectrometer (400 MHz) equipped with an autosampler was used to generate 1H NMR spectra. Data acquisition was completed using 16 scans in CDCl3 as the solvent. Infrared spectra of liquid specimens were obtained with a Nicolet 6700 FTIR spectrometer. The liquid



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specimens were prepared by coating a thin layer of liquid on a KBR plate. Spectra were recorded using 64 scans and 4 cm-1 resolution with a data spacing of 0.964 cm-1. For cured free films, FTIR spectra were obtained using a Bruker Optics Vertex 70 FTIR spectrometer and the attenuated total reflectance (ATR) technique, which employed the use of a germanium ATR crystal. The gel content of crosslinked NIPU networks was determined using free film samples and Soxhlet extraction. Free film specimens weighing approximately one gram were accurately weighed, placed in a pre-weighed cellulose extraction thimble (obtained from Whatman®), and the sample-containing thimble placed in the Soxhlet extractor. Extraction was done for 24 h and one cycle of thimble filling and draining took about 8 to 10 min. After 24 h of solvent extraction, the weight of sample remaining was determined gravimetrically by first drying the thimble and sample before weighing. Viscoelastic properties of crosslinked networks were obtained using dynamic mechanical analysis (DMA) and a Q800 Dynamic Mechanical Analyzer from TA Instruments.

The

measurements were carried out as a function of temperature using a heating rate of 5 °C/min, frequency of 1.0 Hz, and strain amplitude of 0.02%. Mechanical properties were obtained from “dumb bell”-shaped free-film specimens using an Instron 5545 Tensile Tester fitted with a 100 N load cell and the procedure outlined in ASTM D 638–5. The displacement rate of the movable clamp was set as 1.0 mm/min. The data reported was the average of 5 replicate measurements. Gelation of the NIPU networks was rheologically characterized using an ARES Rheometer from TA Instruments. Liquid samples were placed in between a cone and plate and heated to 100 °C. Once the temperature had reached 100 °C, measurements were made as a function of time using an oscillation frequency of 10 rad/s and strain of 0.5%.


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The thermal degradation behavior of NIPU networks was determined in an air atmosphere using a Q500 Thermogravimetric Analyzer from TA Instruments.

Sample

specimens with weights between 12 and 25 mg were heated to 800 °C using a heating rate of 10 °C/min and sample weight monitored as a function of temperature.

RESULTS AND DISCUSSION Synthesis and characterization of cyclic carbonate derivatives of SBO and poly(2-VOES). Poly(2-VOES) is a novel poly(vinyl ether) that possesses an SBO-derived fatty acid ester pendent group in every repeat unit. Figure 3 shows the chemical structure of poly(2-VOES) as well as its epoxidized derivative [i.e. E-poly(2-VOES)] and carbonate-functional derivative [i.e. C-poly(2-VOES)]. The synthesis of poly(2-VOES) using living cationic polymerization has been previously described in detail elsewhere as has the synthesis of E-poly(2-VOES).38,39 The synthesis of CSBO and C-poly(2-VOES) from the epoxy-functional precursors was characterized using 1H NMR and FTIR spectroscopy and an epoxy titration method.



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a)

SBO

54%

poly(2-VOES)

23% 11% 7% 4%

b)

ESBO

56%

E-poly(2-VOES)

23% 9% 8% 4%

c)

CSBO

57%

C-poly(2-VOES)

23% 8%

8% 3%


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Figure 3. Chemical structures for: a) SBO and poly(2-VOES); b) ESBO and E-poly(2-VOES); and c) CSBO and C-poly(2-VOES). The percentages for the unfunctionalized and functionalized (i.e. epoxy-functional and carbonate-functional) fatty chains are weight percentages based on the typical distribution of fatty chains for SBO and not experimentally determined.50

Figure 4 displays the FTIR spectrum that shows successful conversion of the epoxy groups of ESBO to cyclic carbonate groups. As shown in Figure 4a, the FTIR spectrum of the commercially available ESBO exhibits the characteristics peaks for the ester carbonyl at 1744 cm-1 and the epoxy groups at 847 cm-1 and 824 cm-1. Reaction with CO2 results in the formation of a new carbonyl peak at 1802 cm-1 and a new C-O stretching absorbance at 1054 cm-1 associated with the cyclic carbonate groups and loss of the bands associated with the epoxy groups (Figure 4b).25 Successful conversion of the epoxy groups of ESBO to carbonate groups was also confirmed using 1H NMR. For the spectrum of CSBO, peaks associated with oxirane protons (δ = 2.65 ppm and δ = 3.10 ppm) were absent, while peaks attributed to protons of the 1,3-dioxolan-2-one rings (δ = 4.15 ppm and δ = 5.00 ppm) were present.21,51,52 In addition to FTIR and 1H NMR, an epoxy titration method was used to determine the percentage of epoxy groups that remained after conversion to cyclic carbonate groups. The percentage of epoxy groups remaining for CSBO was 0.92 mole percent.



1802 cm-1 1742 cm-1

(a)

(b)

1054 cm-1

CSBO

1744 cm-1

ESBO 847 cm-1

4000

3600

3200

2800

2400

2000

1600 -1

1200

800

824 cm-1

CSBO

Absorbance (A.U.)

Absorbance (A.U.)

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824 cm-1

847 cm-1

ESBO

900

880

860

840

820

800

-1

Wavelength (cm )

Wavelength (cm )

Figure 4. FTIR spectra for ESBO and CSBO.

It was found that the carbonation procedure used to produce CSBO resulted in gelation for E-poly(2-VOES). As mentioned in the Introduction section of this document, the much higher number of functional groups per molecule associated with poly(2-VOES) and its derivatives results in the gel-point being reached at a much lower extent of functional group conversion than SBO or its analogous derivatives.38 Considering the proposed mechanism for the conversion of epoxy groups to cyclic carbonate groups using CO2 and TBAB as the catalyst, a possible side reaction could be epoxy polymerization, which would explain the formation of an insoluble gel for E-poly(2-VOES).53

To successfully produce a soluble cyclic carbonate-

functional version of poly(2-VOES) [i.e. C-poly(2-VOES)], E-poly(2-VOES) was diluted with toluene prior to the carbonation reaction and the extent of carbonation was reduced significantly compared to CSBO.


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The C-poly(2-VOES) produced was characterized using FTIR and epoxy titration. Figure 5 displays the FTIR spectra that shows conversion of the epoxy groups of E-poly(2VOES) to cyclic carbonate groups. As shown in Figure 5, the cyclic carbonate carbonyl peak at 1802 cm-1 for C-poly(2-VOES) and the C-O stretching absorbance at 1054 cm-1 can be easily seen confirming successful production of cyclic carbonate groups. Also, evidence exists for the presence of unreacted epoxy groups in the C-poly(2-VOES) (peaks at 847 cm-1 and 824 cm-1). By titration for epoxy groups, it was determined that 14.7 mole percent of the epoxy groups present in E-poly(2-VOES) remained after the carbonation reaction. The FTIR spectrum for Cpoly(2-VOES) also shows a broad peak centered at 3493 cm-1 indicating the presence of hydroxyl groups, which is most likely due to epoxy ring-opening reactions during the carbonation reaction.53

1802 cm-1 1736 cm-1

C-poly(2-VOES)

Absorbance (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1054 cm-1

824 cm-1 847 cm-1

3493 cm-1

1736 cm-1

E-poly(2-VOES)

4000

847 cm-1 824 cm-1

3600

3200

2800

2400

2000

1600 -1

1200

800

Wavelength (cm )



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Figure 5. FTIR spectra for E-poly(2-VOES) and C-poly(2-VOES).

Chemical characterization of NIPU crosslinked networks. Six different NIPU networks were produced using the CSBO and C-poly(2-VOES) produced and the three different diamines of interest, namely, HMDA, NDA, and TDA.

Since the reactivity of disubstituted cyclic

carbonates with amines is quite slow, crosslinking was done at elevated temperature. The crosslinking conditions utilized were 100 °C for 16 hours. Under these conditions, it is possible that other reactions could take place in addition to the formation of the urethane by reaction of the amine groups with the cyclic carbonate groups. For example, Javni and coworkers22,23 have shown that the amines can also react with the ester groups present in CSBO to produce amides. As a result, the networks produced were characterized using FTIR. Figure 6 shows the FTIR spectrum obtained for crosslinked networks derived from CSBO and HMDA and, for comparison purposes, the spectrum for CSBO. By casual observation of the carbonyl regions of the two spectra, it can be seen that reaction of CSBO with HMDA results in dramatic changes in this region. While the spectrum for CSBO shows two distinct bands corresponding to the carbonyl of the carbonate and ester groups at 1802 cm-1 and 1742 cm-1, respectively, the carbonyl region for the CSBO-HMDA crosslinked network appears to display four different bands. The carbonate carbonyl band at 1802 cm-1 is almost eliminated, while a shoulder to the ester peak carbonyl peak at 1736 cm-1 is observed at 1713 cm-1. This peak is due to the urethane carbonyl formed by ring-opening of the cyclic carbonate groups with the amine groups of HMDA. The less intense band at 1664 cm-1 suggests amide formation by aminolysis of some ester groups. The band at 1614 cm-1 can be attributed to bending of N-H bonds. The region of the spectrum for CSBO-HMDA between 3,000 and 4,000 cm-1 is also consistent with the introduction of N-H 20 ACS Paragon Plus Environment

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bonds through the observation of a diffuse band centered at 3217 cm-1, which is not present in the spectrum of CSBO. This band can be attributed to stretching of N-H bonds. Also, the hydroxyl band, which has a peak at 3494 cm-1

for CSBO, is shifted to lower wavenumbers

causing it to merge with the N-H stretching band. This shift in the hydroxyl band to lower wavenumbers upon the reaction of CSBO with an amine has been observed by others and can be attributed to hydrogen bonding interactions within the NIPU network.23,24,54 As shown in Figure 7, the same bands are present in the FTIR spectrum obtained for the crosslinked network derived from C-poly(2-VOES) and HMDA.

CSBO-HMDA 1713 cm-1 1736

Absorbance (A.U.)

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3217 cm-1

cm-1

1664 cm-1

1614 cm-1

1802 cm-1

1802 cm-1

CSBO

1742 cm-1

3508 cm-1

4000

3600

3200

2800

2400

2000

1600 -1

1200

800

Wavelength (cm )

Figure 6. FTIR spectra for CSBO and the CSBO-HMDA crosslinked network.



C-poly(2VOES)-HMDA 1716 cm-1 1615 cm-1 1660 cm-1 1735 cm-1

Absorbance (A.U.)

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3258 cm-1 1802 cm-1

1802 cm-1

C-poly(2VOES)

1736 cm-1

3494 cm-1

4000

3600

3200

2800

2400

2000

1600 -1

1200

800

Wavelength (cm ) Figure 7.

FTIR spectra for C-poly(2VOES) and the C-poly(2VOES)-HMDA crosslinked

network.

Characterization of crosslinked network formation. As mentioned in the Introduction, one of the major differences between a plant oil-derived poly(vinyl ether) and the triglyceride used to produce the poly(vinyl ether) is the much lower degree of functional-group conversion needed to reach the gel-point in a thermoset system. To demonstrate this for the carbonated derivative produced with the study, the viscosity of the mixtures used to produce the CSBO-HMDA crosslinked network and the C-poly(2-VOES)-HMDA crosslinked network was monitored as a function of time at the cure temperature of 100 °C. The viscosity was monitored long enough to 22 ACS Paragon Plus Environment

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observe the onset of gelation. As shown in Figure 8, a sharp increase in viscosity was observed after approximately 400 to 500 s of residence time for the CSBO-HMDA mixture, while Cpoly(2-VOES)-HMDA mixture appeared to increase substantially in viscosity before data points could even be collected. The poly(2-VOES)-HMDA mixture reached 4,000 Pa-s in 361 seconds, while the CSBO-HMDA mixture required 978 seconds to reach 4,000 Pa-s.

5000 CSBO-HMDA C-poly(2-VOES)-HMDA

Complex Viscosity (Pa-s)

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4000

3000

2000

1000

0 0

200

400

600

800

1000

Time (s) Figure 8. Complex viscosity as a function to time at 100 °C for the CSBO-HMDA mixture and the C-poly(2-VOES)-HMDA mixture. The increase in viscosity indicates that the extent of functional group conversion is approaching the gel-point.



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Solvent extraction experiments were conducted to confirm that the curing conditions utilized for the production of the six crosslinked NIPU networks was sufficient to obtain relatively high extents of crosslinking. Figure S1 shows that the weight percent of toluene insoluble material obtained after 24 hours was 88 weight percent or higher, indicating relatively high functional group conversion for all six crosslinked networks. With regard to the influence of the amine crosslinker, a general trend was observed with the gel content decreasing with increasing amine crosslinker chain length. To understand the nature of the soluble component from these networks, the extract from the CSBO-TDA network was characterized using FTIR and 1H NMR.

The results indicated that the soluble component was largely fatty acid residues

from the soy-component of the networks. There was no indication of amine crosslinker or amine crosslinker reaction products such as amides or urethanes in the extract. Thus, all of the amine was fully incorporated into the crosslinked network. The production of fatty acid residues suggests that some hydrolysis of fatty acid esters chains occurred either during the curing process or during the Soxhlet extraction process. The trend observed with respect to the gel content decreasing with increasing diamine crosslinker chain length may be the result of the efficiency of the extraction process increasing with decreasing crosslink density of the NIPU network. Variations in crosslink density are discussed in greater detail in the next section of this document.

Viscoelastic Properties. The viscoelastic properties of free film specimens were characterized using DMA by keeping the frequency constant and varying temperature. Figures S2a and S2b show data obtained from the tangent delta response of films derived from CSBO and C-poly(2VOES), respectively. For the films based on CSBO, the glass transition temperature (Tg) 24 ACS Paragon Plus Environment

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produced when HMDA was used as the diamine crosslinker was substantially higher than for the films based on either NDA or TDA as the crosslinker. As shown in Table 2, the Tg for the CSBO-HMDA network was 21 °C and 23 °C higher than the CSBO-NDA network and CSBOTDA network, respectively. The higher Tg obtained with HMDA as the crosslinker can be attributed to the lower average molecular weight between crosslinks resulting from the shorter chain length of HMDA as compared to NDA and TDA. The Tgs of the CSBO-NDA and CSBOTDA networks were similar indicating that increasing the number of carbon atoms in the diamine from 9 to 13 did not significantly influence the nature of the cooperative segmental motions that generate free volume. Similarly, the Tgs of the C-poly(2-VOES)-NDA network and the Cpoly(2-VOES)-TDA network were essentially the same. However, compared to the analogs based on CSBO as the carbonate-functional component, these C-poly(2-VOES)-based networks gave Tgs that were significantly higher. This result is consistent with the higher crosslink densities that result from the polymeric nature of C-poly(2-VOES). For poly(2-VOES), the precursor to C-poly(2-VOES), the methine carbon in each repeat unit of the polymer backbone can serve as a crosslink in a crosslinked network. Thus, the higher the molecular weight of the poly(2-VOES) – the higher the number of these methine carbon atoms per g of material. Figure S3 shows how the number of methine carbon atoms per g of material varies with the degree of polymerization (DP) for poly(2-VOES). SBO has just one methine carbon atom per molecule and the number of methine carbon atoms per g of material is 1.087 x 10-3 moles/g. As shown in Figure S3, the number of methine carbons per g of material increases rapidly as the DP is increased from 2 to 20 and then starts to level off as the DP is increased beyond 20.



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Variations in crosslink density resulting from the use of C-poly(2-VOES) in place of CSBO can be readily observed from the storage moduli data shown in Figure S4. According the theory of rubber elasticity,55 the relationship between the rubbery plateau modulus from DMA and crosslink density is given by: ʋe = E’/3RT

(1)

where E’ is the tensile storage modulus obtained in the rubbery plateau, T is temperature in °K corresponding to the storage modulus value, and R is the gas constant. Using this relationship, the crosslink density of the six different NIPU networks was determined and the values are listed in Table 2. The E’ value used for the calculation was taken at a temperature 30 °C above the Tg as determined from the peak maximum of the tangent delta curve. For a given amine crosslinker, the use of C-poly(2-VOES) as the carbonate-functional component provided a crosslink density that was more twice that of the analogous CSBO-based network. As mentioned previously, this can be attributed to the methine carbon atoms present in the polymer backbone of C-poly(2VOES).

Table 2. The Tgs and crosslink densities obtained for the NIPU networks derived from CSBO, C-poly(2-VOES), and the diamines, HMDA, NDA, and TDA. Both Tgs and crosslink densities were obtained from DMA data. Tgs were obtained from the peak maxima of tangent delta curves, while crosslink densities were obtained from storage moduli data in the rubbery plateau region.


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NIPU Network CSBO-HMDA CSBO-NDA CSBO-TDA C-poly(2-VOES)-HMDA C-poly(2-VOES)-NDA C-poly(2-VOES)-TDA

Tg (°C) 2 8 6 26 14 15

Crosslink density (mol/m3) 447 439 335 1,310 1,004 700

MECHANICAL AND THERMAL PROPERTIES The mechanical properties of the six NIPU networks were characterized by measuring stress as function of strain using the tensile test. Representative engineering stress-strain curves are shown in Figure S5. The stress-strain data displayed in Figure S5 shows a very large difference in behavior based on the chemical composition of the carbonate-functional, soy-based component. All of the networks based on CSBO displayed ductile behavior, while all the networks based on C-poly(2-VOES) displayed brittle behavior. The brittle behavior observed with the C-poly(2-VOES)-based materials can be attributed to their relatively high crosslink densities. Crosslinks inhibit polymer chain molecular mobility and chain disentanglements. Thus, a relatively high crosslink density can lead to brittle behavior with the stress increasing linearly with strain until fracture. The networks based on CSBO showed slight yielding below 25 % elongation and, for samples CSBO-HMDA and CSBO-TDA, strain hardening above about 75% elongation. Table 3 provides values for Young’s modulus, ultimate stress (i.e. tensile strength), and elongation at break. As shown in Table 3, the Young’s moduli of the C-poly(2-VOES)-based networks decreased with increasing chain length of the diamine crosslinker. This result is



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consistent with expectations based on the differences in crosslink density resulting from differences in the diamine crosslinker chain length. Increasing the chain length of the diamine crosslinker decreases network crosslink density. In addition, for a given amine crosslinker, the use of C-poly(2-VOES) provided a significantly higher Young’s modulus, but lower elongation at break than the corresponding CSBO-based network. This difference can be attributed to the higher crosslink density that results from the use of C-poly(2-VOES). The NIPU network, CSBO-HMDA, was the toughest network providing the highest ultimate stress and elongation at break. Compared to the other CSBO-based networks, the CSBO-HMDA network possesses the highest urethane group concentration, which may have enabled the enhanced toughness for this network. It is well known that intermolecular hydrogen bonding between urethane groups enhances toughness by converting mechanical energy into heat by the breaking and forming of hydrogen bonds.1

Table 3. Mechanical property data obtained from tensile testing for the six NIPUs. Sample ID CSBO-HMDA CSBO-NDA CSBO-TDA C-poly(2-VOES)-HMDA C-poly(2-VOES)-NDA C-poly(2-VOES)-TDA

Young's modulus (MPa) 7.2 ± 0.9 3.6 ± 0.8 3.7 ± 0.8 17.0 ± 1 13.0 ± 0.9 5.8 ± 0.5

Ultimate stress (MPa) 6.7 ± 0.5 3.4 ± 0.8 3.2 ± 0.2 1.25 ± 0.2 3.7 ± 0.4 1.8 ± 0.4

Elongation at Break (%) 156 ± 12 132 ± 12 128 ± 8 13 ± 3 27 ± 4 28 ± 5

The thermal stability of the NIPU networks was characterized using TGA in an air atmosphere. Figure S6 shows weight retention as a function of temperature, while Table 4 shows the temperature corresponding to different levels of weight loss. As shown in Figure S6 28 ACS Paragon Plus Environment

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and Table 4, the networks based on C-poly(2-VOES) as the carbonate-functional component possessed somewhat higher thermal stability than analogous networks based on CSBO.

Table 4. The temperatures associated with 5, 25, and 50% weight loss determined using TGA and an air atmosphere. Sample ID CSBO-HMDA CSBO-NDA CSBO-TDA C-poly(2-VOES)-HMDA C-poly(2-VOES)-NDA C-poly(2-VOES)-TDA

Temp. @ 5% wt. loss (°C) 255 252 258 262 262 270

Temp. @ 25% wt. loss (°C) 317 328 331 359 335 377

Temp. @ 50% wt. loss (°C) 384 382 386 400 407 433

CONCLUSION From the results obtained from the study, it can be easily seen that the polymeric nature of Cpoly(2-VOES) results in very different behavior of NIPU networks as compared to the lower molecular weight CSBO. For a given diamine crosslinker, the use of C-poly(2-VOES) results in a much higher crosslink density, which can be attributed to the higher number of methine carbon atoms per molecule for C-poly(2-VOES) as compared to CSBO. These methine carbon atoms act as additional crosslinks in the NIPU network beyond those formed by reaction of the diamine crosslinker with the cyclic carbonate groups. In addition, the higher number of cyclic carbonate groups per molecule for C-poly(2-VOES) as compared to CSBO results in the gel-point of the network being reached at a lower degree of urethane formation.



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With regard to mechanical properties, the higher NIPU crosslink densities achieved with the use of C-poly(2-VOES) resulted in relatively high Young’s moduli, but brittle behavior. In contrast, the lower NIPU crosslink densities associated with the use of CSBO resulted in ductile behavior. As expected, the higher crosslink densities obtained with the C-poly(2-VOES)-based NIPU networks generally resulted in higher Tgs. With regard to the effect of the chain length of the linear aliphatic diamine crosslinkers, the NIPU networks based on the shortest diamine, HMDA, provided the highest Tgs and Young’s moduli than the analogous networks based on the longer diamines, NDA and TDA. These results can be attributed to the higher crosslink densities (i.e. lower molecular weight between crosslinks) enabled by the use of HMDA. Overall, the CSBO-HMDA NIPU network provided the toughest, highest strength NIPU. This result may be due to the fact that this NIPU network also contains the highest overall urethane content. As discussed in the Introduction, urethane groups enable desirable mechanical properties by dissipating mechanical stress into heat via the breaking and reforming of hydrogen bonds. Since HMDA, NDA and TDA can be obtained from renewable resources, the NIPU networks derived from these diamines and CSBO have the potential to be 100 percent biobased.

ACKNOWLEDGEMENTS The authors thank the National Science Foundation EPSCoR Program (grant IIA-1355466) and the Department of Energy (grant DE-FG36-08GO88160) for financial support.

SUPPORTING INFORMATION


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Illustration of NIPU network properties - Figures S1, S2, S3, S4, S5, and S6

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(38) Alam, S.; Chisholm, B. J. Coatings derived from novel, soybean oil-based polymers produced using carbocationic polymerization. J. Coating Tech. Res., 2011, 8, 671-683. (39) Chernykh, A.; Alam, S.; Jayasooriya, A.; Bahr, B.; Chisholm, B. J. Living carbocationic polymerization of a vinyl ether monomer derived from soybean oil, 2-(vinyloxy)ethyl soyate. Green Chem., 2013, 15, 1834-1838. (40) Nieschlag, H. J.; Rothfus, J. A.; Sohns, V. E. Nylon-1313 from Brassylic Acid. Ind. Chem., Prod. Res. Dev., 1977, 16, 101-107. (41) Bhattacharyya, S. K.; Ganguly, N. D. One-step catalytic conversion of ethanol to butadiene in the fixed bed. II Binary- and ternary-oxide catalysts. J. Appl. Chem., 1962, 12, 105-110. (42) Minh, D. P.; Besson, M.; Pinel, C.; Fuertes, P.; Petitjean, C. Aqueous-Phase Hydrogenation of Biomass-Based Succinic Acid to 1,4-Butanediol Over Supported Bimetallic Catalysts. Top. Catal., 2010, 53, 1270-1273. (43) Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Renewable Alkanes by Aqueous-Phase Reforming of Biomass-Derived Oxygenates. Angew. Chem., Int. Ed., 2004, 43, 1549-1551. (44) Hachihama, Y.; Hayashi, I. The preparation of polyamides containing heterocyclic groups from furfural and levulinic acid. Die Makromol. Chem., 1954, 13, 201-209. (45) Pearlman, P. S.; Chen, C.; Botes, A. L. Methods of making nylon intermediates from glycerol. US Patent 20130210090 A1, Invista, 2013. (46) Boussie, R. T.; Dias, L. E.; Fresco, M. Z. Murphy, V. J. Production of Adipic Acid and Derivatives from Carbohydrate-Containing Materials. US Patent 20100317822 A1, Rennovia, 2010. (47) Datta, R.; Tsai, S. P.; Bonsignore, P.; Moon, S. H.; Franck, J. R. Technological and economic potential of poly(lactic acid) and lactic acid derivatives. FEMS Microbiol. Rev., 1995, 16, 221-231. (48) Sanborn, A. J.; Bloom, P. D. Conversion of 2,5-(hydroxymethyl)furaldehyde to industrial derivatives, purification of the derivatives, and industrial uses therefor. US Patent 7,432,382, Archer-Daniels-Midland Company, 2008. (49) He, J.; Samanta, S.; Selvakumar, S.; Lattimer, J.; Ulven, C.; Sibi, M.; Bahr, J.; Chisholm, B. J. Polyamides based on the renewable monomer, 1,13-tridecane diamine I: synthesis and characterization of nylon 13,T. Green Mater., 2013, 1, 114-124. (50) Solomon, D. H. The Chemistry of Organic Film Formers, Robert E. Krieger, Malabar, FL, 1982.


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For Table of Contents Use Only Synthesis and Characterization of Polyurethane Networks Derived from Soybean OilBased Cyclic Carbonates and Bio-Derivable Diamines Satyabrata Samanta, Sermadurai Selvakumar, James Bahr, Dona Suranga Wickramaratne, Mukund Sibi, and Bret J. Chisholm Synopsis: The process for producing a novel carbonate-functional poly(vinyl ether) derived from soybean oil. The materials described in the manuscript can be largely derived from renewable resources, which contributes to environmental sustainability.


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Synthesis and Characterization of Polyurethane Networks Derived

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