Lubricating and Waxy Esters. 6. Synthesis and Physical Properties of

Dec 1, 2014 - ABSTRACT: A fatty aliphatic “Jojoba-like” ester, didec-9-enyl octadec-9-enedioate, was synthesized by Steglish esterification,...
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Lubricating and Waxy Esters. 6. Synthesis and Physical Properties of (E)‑Didec-9-enyl Octadec-9-enedioate and Branched Derivatives Shaojun Li,† Laziz Bouzidi,† and Suresh S. Narine* Trent Centre for Biomaterials Research Departments of Physics & Astronomy and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada S Supporting Information *

ABSTRACT: A fatty aliphatic “Jojoba-like” ester, didec-9-enyl octadec-9-enedioate, was synthesized by Steglish esterification, and C3-branched derivatives were prepared from its epoxide by a solvent-free epoxide ring-opening and one-pot normal condensation reaction. The thermal stability, phase transition behavior, solid fat content, and flow behavior were investigated using thermogravimetric analysis, differential scanning calorimetry, p-NMR, and rotational rheometry, respectively. These properties were predictably varied as a function of branching, explained by the combined effects of mass, hydroxyl groups, and geometric steric hindrances imposed by the protuberant branches. The compounds demonstrated high thermal stability (>230 °C), competitive flow characteristics (210−773 cP at 40 °C and 31−66 cP at 100 °C) and superior low-temperature performance properties (−27 to −70 °C) suitable for exploitation in various applications such as lubricants, cosmetics, and pharmaceuticals.

1. INTRODUCTION Remarkable advances in organic synthesis targeted at plant oils and their derivatives have been made in recent years.1 A large array of methods is now available to transform the fatty acids by reactions at the carboxyl group or to selectively functionalize the alkyl chains of fatty compounds, particularly on the carbon− carbon double bond.2,3 Oxidation reactions, C−C bond-forming additions, activation of unreactive C−H bonds, and metathesis are rapidly becoming routine and essential tools in oleochemistry.1 As a result, novel environment-friendly materials suitable for a variety of industrial applications, such as fuels and lubricants, have been achieved.4−7 The introduction of branches into the fatty acids (FA) of vegetable oils triacylglycerols (TAGs) is of particular interest as it has been demonstrated to notably affect their functionality.8−10 Branching is in fact the most common approach employed for lowering the melting point and improving the oxidative stability and the viscosity of ester materials.7−9,11,12 Several methods have been developed to introduce branches into unsaturated fatty acids. Skeletal isomerization with large pore zeolite catalysts was used to prepare methyl and ethyl branched unsaturated fatty acids.11,13 Bromination of methyl oleate in the allylic position followed by treatment with organocuprate reagents was used to produce novel branched-chain derivatives.14 Epoxidation reactions are also interesting as the resulting epoxide rings are highly reactive, allowing straightforward transformations.15 Furthermore, epoxidation can be carried out under moderate conditions, such as acid-free conditions, and with different catalysts that are easy to remove by filtration.16 Epoxidation followed by ring-opening esterification is a synthesis route that has received special attention for preparing branched compounds. The esterification of the hydroxyl groups obtained from epoxidation is a common method for preparing multibranched compounds of fatty esters.17,18 Epoxidized fatty acids as well as their branched derivatives were evaluated for lubricant applications.19,20 For example, it was © 2014 American Chemical Society

reported that epoxidized soybean oil can be a potential source of lubricant formulations suitable in high-temperature applications.21 A series of ester and ether branched products were also prepared from oleic acid and investigated for their lowtemperature performance, oxidation stability, and lubricity behavior.19,22 The derivatives were found to have improved low-temperature properties and significant potential as lubricant base oils. Our research group at the Trent Centre for Biomaterials Research (TCBR) is conducting a series of investigations of jojoba-like ester (JLE) model systems and their pure branched derivatives in order to establish the structure−function relationships that govern this important family of compounds. Select pure monoester and diester compounds were synthesized, and their phase behavior was studied comprehensively using a battery of techniques including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), rheometry, and PLM. The monoesters that are most prevalent in jojoba oil23 and monoesters incorporating either 9decenoic acid and/or 9-decenol and oleic acid and/or oleoyl alcohol24 were synthesized, and their crystal structure, thermal stability, thermal transition behavior, and flow behavior were investigated in detail. The effects of chain length and symmetry on these properties were evaluated and contrasted. Pure multibranched polyol derivatives of these JLE were prepared using a solvent-free and catalyst-free epoxide ring-opening esterification followed by a condensation esterification.12,25 Their phase behavior was investigated and related to the number and relative position of the branches and hydroxyl groups, as well as the symmetry of the structures.12,25 A series of novel glycolderived aliphatic diesters were prepared from diols and fatty acids Received: Revised: Accepted: Published: 20044

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with varying carbon numbers.2,8 Their chemical and thermal stabilities and weight-loss kinetics were examined and related to the number of methylene units between the ester groups. Unbranched terminally functionalized linear diol, diacid, and diisocyanates have also been produced from oleic acid via a diester precursor.2 These compounds are important monomers for the manufacture of biodegradable thermoplastic polyurethanes and polyesters. This approach consisting of targeted custom-engineered structural modification at the fatty acid and/or alcohol chain level of ester compounds has been particularly successful in improving our fundamental understanding of the phase behavior of monoesters and diesters and structure−function relationships. The present study reports on the synthesis and physical properties of (E)-didec-9-enyl octadec-9-enedioate (labeled H), and its 3-, 4-, 5- and 6-branched derivatives (labeled H3, H4, H5, and H6, respectively). H is a diester with both terminal and internal double bonds. It can be obtained economically and relatively safely as a component of self-metathesis of dec-9-enyl dec-9-enoate.24 H was prepared from 1, 18-octadec-9-enedioic acid and 9-decen-1-ol using Steglich esterification,26 and its branched derivatives were prepared from its epoxide (EoH) via ring-opening esterification by carboxylic acid and condensation esterification. The generalized structure of the synthesized compounds is presented in Scheme 1, and nomenclature, IUPAC names, and other relevant structural details are provided in Table S1 in the Supporting Information.

and 85% acetonitile (ACN) for propionic acid derivatives and a mixture of 50% ethyl acetate and 50% ACN for 9-decenoic acid derivatives. 2.2.3. Electrospray Ionization Mass Spectrometry. Mass spectra were acquired on a triple quadrupole mass spectrometer equipped with a Z-electrospray ionization source (ESI) (Micromass Quattro LC, Micromass, Altrincham, U.K.). The samples were introduced by infusion through a Harvard syringe pump (Harvard Apparatus, Holliston, MA, U.S.A.) at a low rate of 10 μL/min. 2.2.4. Thermogravimetric Analysis. TGA was carried out in triplicate on a TGA Q500 (TA Instruments, DE). Approximately 8.0−15.0 mg of fully melted and homogeneously mixed sample was loaded into the open TGA platinum pan. The sample was equilibrated at 25 °C and heated to 600 °C at a constant rate of 10 °C/min. The TGA measurements were performed under dry nitrogen of 40 mL/min for balance purge flow and 60 mL/min for sample purge flow. 2.2.5. Differential Scanning Calorimetry. DSC analysis was carried out on a Q200 model (TA Instruments, New Castle, DE, U.S.A.) equipped with a refrigerated cooling system (RCS 90, TA Instruments) under a nitrogen flow of 50 mL/min. The sample (5.0−6.0 ± 0.1 mg) in a hermetically sealed aluminum pan was cooled from the melt (50 °C) to −90 °C and subsequently reheated to 70 °C at the same constant rate of 3.0 °C/min to obtain the crystallization and melting profiles, respectively. TA Universal Analysis software was used to analyze the data and extract the main characteristics of the peaks. The measurement temperatures are reported to a certainty of better than ±0.5 °C. 2.2.6. Solid Fat Content Determination. The solid fat content (SFC) measurements were performed on a Bruker Minispec mq 20 p-NMR spectrometer (Milton, ON, Canada) equipped with a combined high- and low-temperature probe supplied with N2. The temperature was controlled with Bruker’s BVT3000 temperature controller to better than ±0.1 °C. The temperature was calibrated with highly unsaturated canola oil and a type K thermocouple (TRP-K, Omega, Stamford, CT, U.S.A.) immersed in the oil. Approximately 0.57 ± 0.05 mL of fully melted sample was quickly pipetted into the bottom portion of the NMR tube. Bruker’s minispec V2.58, rev. 12 and minispec plus V1.1, rev. 05 software were used to collect SFC data as a function of time and temperature. The SFC values are reported as the ratio of the NMR signal intensity of the solid part to the total detected NMR signal in percent (labeled as SFC%). 2.2.7. Flow Behavior and Viscosity Measurements. Flow characteristics and viscosity of the sample were measured on a computer-controlled AR2000ex (TA Instruments, DE, U.S.A.) using a standard 40 mm 2° steel cone geometry (SIN 511406.901, TA Instruments) under an air bearing pressure of 27 psi. Temperature control was achieved by Peltier Plate (AR Series, TA Instruments) with an accuracy of 0.1 °C. The circulating fluid heat exchange medium was provided by a temperature-controlled circulating water bath (Julabo F25, Allentown, PA, U.S.A.). The shear rate−shear stress experiments were performed with increasing and decreasing shear rate using the continuous ramp procedure. The shear rate range was optimized for each measurement temperature to take into account the lowest torque accessible (10 μNm) and the maximum angular velocity suggested by the supplier (40 rad/ s). Duration was 10 min in the log mode and sampling was 20 points per decade. The procedure was performed with decreasing as well increasing temperatures in 5 °C steps and 5 min equilibration between temperatures. The viscosity versus

Scheme 1. Generalized Structure of (E)-Didec-9-enyl Octadec-9-enedioate (H) and Its Branched Derivativesa

a

R1−R6 are specified in Table S1 in the Supporting Information.

2. EXPERIMENTAL SECTION 2.1. Materials. Oleic acid (90%), 9-decen-1-ol, propionic acid, formic acid, chloroform, dichloromethane, N,N′-dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), hydrogen peroxide, and Grubbs second-generation catalyst were purchased from Sigma-Aldrich. Hexanes and ethyl acetate were purchased from ACP Chemical Int. (Montreal, Quebec, Canada). The materials were used without further treatment. 2.2. Analytical Methods. 2.2.1. Nuclear Magnetic Resonance. 1D 1H NMR and 1D 13C NMR spectra were recorded on a Bruker Avance III 400 spectrometer (Bruker BioSpin MRI GmbH, Karlsruhe, Germany) at a frequency of 400 and 100 MHz respectively, using a 5 mm broadband observe (BBO) probe. The 1H NMR spectra were acquired at 25 °C over a 16 ppm spectral window with a 1 s recycle delay and 32 transients. The 13C NMR spectra were acquired at 25 °C over a 240 ppm spectral window with a 0.2 s recycle delay and 2048 transients. Both 1H NMR and 13C NMR spectra were Fourier transformed, phase corrected, and baseline corrected. Window functions were not applied prior to Fourier transformation. Chemical shifts were referenced relative to residual solvent peaks. 2.2.2. High-Pressure Liquid Chromatography. HPLC was performed on a Waters Alliance system (e2695 HPLC, Milford, MA, U.S.A.) fitted with an evaporative light scattering detector (Waters ELSD 2424) and a C18 column (5 μm 4.6 × 150 mm). The mobile phase run at 1 mL/min was a mixture of 15% water 20045

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Scheme 2. Synthesis of (E)-Didec-9-enyl Octadec-9-enedioatea

a

The double bond on the alkyl chain of the diacid has a trans-geometry28.

dicarboxylic acid, 1,18-octadec-9-enedioic acid was produced by self-metathesis of oleic acid using Grubbs catalyst, following a previously reported procedure.28 The synthesis route for the preparation of the branched derivatives of H is presented in Scheme 3. The epoxide of H (EoH) was prepared with peroxyacid as the catalyst and CH2Cl2 as the solvent, as reported in a previous study.23 The branched compounds were achieved by a two-step reaction in one pot without the need of either solvent or catalyst, as shown in Scheme 3. H3, the compound with 3-branches, was prepared by introducing carboxylic acid (propionic acid) to the epoxide with a ring-opening esterification at 95 °C. In this reaction, carboxylic acid served as both reactant and solvent. The 3branched compound (H3) was further reacted with carboxylic acid by a normal condensation esterification to give the compounds with more branches (H4, H5, and H6). In this case, a higher reaction temperature of 120 °C was required. Note that at this temperature, the water produced in the reactions was removed. In the condensation esterification for preparing H4 and H5, the hydroxyl groups are available for attack by the carboxylic acid in three optional positions: two at the end of the chain, i.e., the terminal (−CH(R)CH2(OH)) and the nonterminal (−CH(OH)CH2(R)), and one, the (−CH(R)CH(OH)−), in the middle of the chain. Because of geometric steric hindrances, the terminal hydroxyl groups at the end of the chain would be attacked more easily than the other to form a branched derivative having one acryl branch in the middle of the chain (isomer I, in Scheme 3) rather than a branched derivative with two branches in the middle of the chain (isomer II, in Scheme 3). 2.3.1. Synthesis of 1,18-Octadec-9-enedioic Acid. Oleic acid (76 g) was transferred into a 250 mL three-necked round bottomed flask and stirred at 45 °C under nitrogen gas for 30 min. Grubbs catalyst second generation (85 mg) was added and the reaction mixture stirred at 45 °C for ∼5 min. The diacid began to precipitate from the reaction mixture. The reaction was kept at this temperature for 24 h and then quenched with ethyl vinyl ether (15 mL). The excess ether was removed under reduced pressure. The residue was purified by recrystallization from ethyl acetate and hexanes (1:2) to give 29.75 g of product as a white solid. 2.3.2. Synthesis of the Base Diester (E)-Didec-9-enyl Octadec-9-enedioate (H). To a solution of 1,18-octadec-9enedioic acid (15.6 g, 50 mmol) and 9-decen-1-ol (23.4 g, 150 mmol) in CHCl3 (100 mL) at ∼0 °C, was added DMAP (12.2 g, 100 mmol), followed by slow addition of DCC (22.7 g, 110

temperature data were collected using the constant temperature rate method. The sample was quickly heated to 110 °C and equilibrated at this temperature for 5 min then cooled at a constant rate (1.0 or 3.0 °C/min). The shear rate of 200 s−1 was chosen as it was common to the Newtonian region of the compounds. All other measurement conditions were kept constant. Sampling points were recorded every 1 °C. The two methods yielded measured viscosities in good agreement within the experimental uncertainty. Note that in this investigation, because of the experimental limitations imposed to optimize the shear rate range for torque (see above), it is not possible to state with confidence whether the materials display limiting viscosities in the limits of γ̇ → ∞. The shear rate−shear stress curves were fitted with the Herschel−Bulkley equation (eq 1), a model commonly used to describe the general behavior of materials characterized by a yield stress τ = τ0 + Kγ ṅ

(1)

where γ̇ denotes the shear stress, τ0 is the yield stress below which there is no flow, K the consistency index, and n the power index. n depends on constitutive properties of the material. For Newtonian fluids, n = 1; for shear thickening fluids, n > 1; and for shear thinning fluids, n < 1. The power law equation (eq 2) is a valid model for materials without yield stress τ = Kγ ṅ

(2)

For Newtonian fluids, n = 1 and K = η is the fluid viscosity. The experimental viscosity−temperature data were analyzed using a generalized form of the van Velzen expression (GvVE, eq 3) ⎛ 1 ⎞ ln(η) = A⎜ −1 + m ⎟ ⎝ T ⎠

(3)

The GvVE fits yield physically meaningful paramaters: its parameter A relates directly to the magnitude of the viscosity of the liquid, and its exponent m is related to the complexity of the molecule, similar to the parameters of the Andrade and generalized Andrade models.27 2.3. Synthesis of H and its Branched Derivatives. (E)Didec-9-enyl octadec-9-enedioate (H), was prepared from derivatives of oleic acid, i.e., 1,18-octadec-9-enedioic acid and 9-decen-1-ol, by Steglich esterification,26 with 4-dimethylaminopyridine (DMAP) as catalyst and dicyclohexylcarbodiimide (DCC) as condensing agent (Scheme 2). The unsaturated 20046

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Scheme 3. Synthesis of Branched Derivatives of (E)-Didec-9-enyl Octadec-9-enedioate (H)a

a

R: C2H5. The branched compounds are referred to as H3 (3-branched), H4 (4-branched), H5 (5-branched), and H6 (6-branched).

proceeded at room temperature with vigorous stirring for 48 h. After removal of the water phase, more CH2Cl2 (10 mL) was added to the organic phase, then washed sequentially with water (2 × 20 mL), saturated aqueous NaHCO3 (2 × 10 mL), and brine (2 × 20 mL), and then dried on Na2SO4, filtered, and concentrated. The residue was purified by column chromatography with ethyl acetate: hexanes (1:4) to give 2.1 g of product as a white solid. 2.3.4. Synthesis of Branched Derivatives of H Using Propionic Acid. To a solution of the epoxidation products

mmol). The reaction mixture was allowed to warm to room temperature and kept overnight. The mixture was filtered to remove the solid. The filtrate was concentrated with a rotary evaporator. The residue was purified by flash chromatography using hexanes:ethyl acetate (40:1) to give 28 g of product as a colorless oil. 2.3.3. Epoxidation of (E)-Didec-9-enyl Octadec-9-enedioate (EoH). To a stirred solution of ester (2.7 g, 4.56 mmol) and formic acid (2.2 g, 9 mmol) in CH2Cl2 (3 mL) at 4 °C, was slowly added H2O2 (30%) (3.4 g, 6.6 mmol). The reaction 20047

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estimated ratio of terminal and nonterminal hydroxyl derivatives in H4 was also about 1:3. The weak peak at δ ∼ 4.95 ppm observed in the 1H NMR of H4 suggests the presence of a small amount of isomer H4-II containing −CH (R) CH(R) CH2−. The peak at ∼4.95 ppm in 1H NMR of the 5-branched derivatives (H5) indicates the presence of -CH (R) CH(R) CH2−. The amount of -CH(OH)CH(R)CH2− estimated based on the peak at δ 3.64 ppm is about 0.87 protons. The ratio of −CH(R) CH(R) CH2− in H5-II and CH2(R) CH(R) in H5-I is ∼13:87. 3.1.2. 13C NMR Analysis. In 13C NMR, the terminal CH3CH2C(O)− was presented at δ ∼174.8 ppm, and the other carbonyl carbons were at δ ∼174.1 ppm. The carbon at δ 77.9 ppm is assigned to −CHCH(R)CH2−, at δ 75.6 ppm to CH2CH(R)−, at δ 73.5 ppm to−CHCH(R)CH2−, at δ 70.19 ppm to CH2(R)CH−, at δ 68.84 ppm to CH2(R)CH−, at δ 65.0 ppm to CH2CH(R)−, and at δ 64.5 ppm to −CH2CH2O−. CH2(R)CH(R)− was presented at δ 65.2 ppm, CH2(R) CH(R)− at δ 71.5 ppm, and −CH(R)CH(R)CH2− at δ 74.1 ppm. The 13C NMR chemical shift at δ 65.0 ppm presented by H3 and H4 indicates the presence of terminal hydroxyl groups. The occurrence of the chemical shift at δ = 74.1 ppm in 13C NMR of H4 indicates the presence of the H4-II isomer. The chemical shifts at δ = 68.8, 74.1, and 70.1 ppm in the 13C NMR of H5 prove the presence of both H5-I and H5-II. No peak at δ = 65.0 ppm was presented in H5, indicating that it did not have terminal hydroxyl derivatives. 3.1.3. HPLC Analysis. HPLC curves of H and its branched derivatives are shown in Figure 1. The corresponding HPLC retention times are listed in Table S2 in the Supporting Information. As can be seen in Figure 1, two HPLC peaks were presented in H4 and H5. In both cases, the first and largest peak (P1 in Figure 1b,c) is associated with isomer I, and the second (P2 in Figure 1b,c) is associated with isomer II of each branched derivative. The amount of H4-II estimated from the relative area of its associated HPLC peak is less than 2%, corroborating the amount estimated from the weak peak at δ ∼ 4.95 ppm of -CH (R) CH(R) CH2− observed in the 1H NMR of H4. The ratio of H5II to H5-I estimated from the relative areas of their corresponding associated HPLC peaks is 11:89, a value that is very close to the ratio of ∼13:87 obtained from 1H NMR. 3.2. Thermal Stability. The derivative of TGA (DTG) curves and onset temperature of degradation of H and its branched derivatives as measured at 5% and 10% weight loss (T5% and T10%, respectively) are shown in panels a and b of Figure 2, respectively. The corresponding TGA and DTG characteristic data are provided in Table 2. The TGA data revealed that the thermal stability of the present ester compounds is affected by branching, OH group content, and the neighboring environment of the ester groups. As shown in the TGA curves and more manifestly in the DTG curves (Figure 2a), the decomposition of the base ester H and its branched derivatives spanned from ∼230 to 490 °C via complex and different processes of decomposition linked to their key structural elements, such as OH and ester groups. The thermal stability of H, as indicated by the onset temperature of degradation (Figure 2b), was measurably improved by branching. As can be seen in Figure 2b, T5% increased by practically 7 °C per branch added. The DTG of the base ester presented three peaks indicative of three main mechanisms of degradation. The first degradation step signaled by the first DTG peak (centered at TD1 = 340 °C)

above (1.6 g, 2.5 mmol), was added 6.1 g of propionic acid. The reaction was carried out under N2 protection and heated to 95 °C for the preparation of H3 and H4 and 120 °C for the preparation of H4, H5, and H6. The reaction was stirred at the prescribed temperature for 26 h. The resulting product was poured into 10 mL of water and extracted with ethyl acetate (2 × 10 mL). The organic phase was washed sequentially with water (2 × 10 mL), saturated aqueous NaHCO3 (2 × 10 mL), and brine (2 × 20 mL) and then dried on Na2SO4. After filtration, the filtrate was concentrated. The residue was purified by column chromatography with ethyl acetate:hexanes (1:1 for H3, 1:2 for H4, 1:3 for H5, and 1:4 for H6).

3. RESULTS AND DISCUSSION 3.1. Chemical Characterization and Identification of the Compounds. 1H NMR and 13C NMR spectra of (E)-didec9-enyl octadec-9-enedioate (H) and its branched derivatives (H3, H4, H5, and H6) are provided in Figures S1 and S2 in the Supporting Information, respectively. Relevant 1H NMR in CDCl3, 13C NMR in CDCl3, and MS data are provided in Table S2 in the Supporting Information. Yield and purity of the compounds are provided in Table 1. The 1H NMR and 13C NMR data were consistent with the structures proposed. Table 1. Yield and Purity of Synthesized Compoundsa sample

yield (%)

purity (%)

1, 18-octadec-9-enedioic acid H epoxide of H (EoH)

72 71 78

>95 99 99

H3 H4 H5 H6

at 95 °C 38 44 12 −

at 120 °C − 9 48 38

99 99 96 99

a

H3, H4, H5, and H6 are the branched derivatives of (E)-didec-9-enyl octadec-9-enedioate (H).

3.1.1. 1H NMR Analysis. The chemical shifts of −OCH2−, −CH2CH2C(O)−, CH3CH2C(O)−, −CH2−, and −CH3 in branches were presented at δ 4.02, 2.24, 2.39, 1.60−1. 2, and 1.18−1.10 ppm, respectively, in all of the compounds. The −OCHCHO− in nonterminal epoxide rings was presented at δ 2.88 ppm, the −OCH2CHO− in terminal epoxide rings at δ 2.88 ppm, and −OCH2CHO− at 2.72 and 2.43 ppm. As reported previously,25 the chemical shifts at δ 4.12−4.10 and 3.94−3.90 ppm are assigned to CH2(OCOC2H5 (R))CH(OH)CH2− in nonterminal hydroxyl branched derivatives, δ 3.80 ppm to CH2 (R)CH(OH)CH2−, δ 4.80 ppm to −CH (R) CH(OH)CH2−, and δ ∼3.60 ppm to −CH (R) CH (OH)CH2−. (OH)CH2CH(R)CH2− in terminal hydroxyl branched derivatives are presented at δ 3.70−3.60 ppm, and (OH)CH2CH(R)CH2− at δ 4.91 ppm. On the basis of the amount of protons, the ratio between terminal and nonterminal hydroxyl groups in the 3-branched derivative was about 1:3. Incidentally, a similar terminal-to-nonterminal hydroxyl groups ratio was also found in the 3-branched derivatives of octadec-9-enyl dec-9enoate.25 CH2(R)CH(R)CH2− in 4-, 5-, and 6-branched compounds is presented at δ 5.08−5.02 ppm, CH2(R)CH(R)CH2− at δ 4.21− 4.18 and 4.03−3.98 ppm, and -CH (R)CH(R)CH2− at δ 4.98− 4.95 ppm. The presence of 1H NMR peaks at δ ∼4.90 ppm indicates that H4 also contains (OH)CH2CH(R)CH2−. The 20048

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Figure 1. HPLC spectra of branched derivatives of H: (a) H3, (b) H4, (c) H5, and (d) H6.

Figure 2. (a) Derivative of TGA (DTG) curves and (b) onset temperature of degradation determined at 5 and 10% weight loss (T5% and T10%, respectively) of H and its branched derivatives.

Table 2. Temperature of Degradation at 5 and 10% Weight Loss (Td5% and Td10% in °C) and DTG Peak Temperatures (TD in °C) of H and its Branched Derivatives.a step I H0 H3 H4 H5 H6 a

step II

step III

TR

TD

WL

TR

TD

WL

230−370 260−310 249−292

340 290 300

39 7 4

370−440 310−375 295−379 250−429 250−436

421 357 368 368 369

81 48 52 83 80

TR 372−422 379−432

TD 387 388 390b 390b

step IV WL 80 86

TR

TD

WL

440−480 422−284 432−481 429−488 436−483

446 443 443 448 444

17 19 13 7 8

TR: temperature range (°C); WL, weight loss (%). bShoulder peak temperature.

spanned from 230 to 370 °C and involved ∼40% weight loss. It is related to breakage of the ester bonds and the loss of fatty acid

carbon chains.29 In addition, evaporation during the early stages cannot be ruled out as indicated by the asymmetry of this DTG 20049

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Figure 3. DSC (a) cooling and (b) heating thermograms of H and its branched derivatives. (c) Characteristic temperatures of crystallization versus number of branches of H and its branched derivatives.

Table 3. DSC Data for H and its Branched Compoundsa Cooling H H3 H4 H5 H6

H H3 H4 H5 H6

TCOn

TC1

18.76 ± 1.10 −26.71 ± 0.10 −33.73 ± 2.94

19.23 ± 0.81 −33.22 ± 0.09

−49.7 ± 0.8

TC2

TCOff

−43.73 ± 0.12 −43.0 ± 1.10

17.19 ± 0.80 −46.14 ± 0.34 −50.7 ± 1.40

−51.9 ± 0.20 Heating

−54.9 ± 0.30

TM On

TM1

TM2

TM Off

22.27 ± 0.12 −37.84 ± 0.39 −63.98 ± 0.11

23.79 ± 0.20 −5.45 ± 0.02 −25.76 ± 0.57

−15.85 ± 0.23 −59.13 ± 0.19

24.94 ± 0.40 14.46 ± 0.26 −17.63 ± 0.23

Tg −57.70 −64.14 ± 0.29 −68.17 ± 0.14 −72.01 ± 0.26 Tg −57.62 ± 0.16 −61.64 ± 0.09 −65.39 ± 0.14 −68.64 ± 0.29

ΔHC 202 ± 5 29 ± 3 0.6 ± 0.5 0.4 ± 0.1 ΔHM 195 ± 5 30.9 ± 2.7 1.0 ± 0.5

onset temperature; TC1,M1, TC2,M2, peak temperatures; TC,M Off , offset temperature; Tg, glass transition temperature; ΔHC,M, enthalpy. Index C, crystallization; index M, melting. Temperatures are in °C and enthalpy in joules per gram.

a C,M TOn ,

step at 440−470 °C is related to the decomposition of fragments of ester groups and others with higher decomposition temperatures.30 3.3. Thermal Transition Behavior. Typical cooling and heating thermograms of the purified compounds obtained at a cooling rate of 3.0 K/min are shown in panels a and b of Figure 3, respectively. The related crystallization, melting, and glass transition data are summarized in Table 3. As can be seen in Figure 3a,b, H displayed one intense exotherm upon cooling and one endotherm upon heating with total enthalpies that were approximately the same (Table 3). The spans of crystallization and melting of H were also very close, indicating a single phase and suggesting reversibility in the transformation. The thermal behavior of the branched derivatives of H depended on the number of branches in a relatively simple manner. In fact, the crystallization was reduced dramatically with the addition of three branches and was completely suppressed with higher branching (see H4, H5, and H6 thermograms in Figure 3a). H3 presented clear exotherms at ∼−27 and −33 °C, possibly attributable to the crystallization of its constituent isomers, and a glass transition at ∼−58 °C. Its enthalpy of

peak. The last two steps from 370 to 440 °C (peak at TD2 = 421 and 456 °C) are associated with the decomposition of the fragments of ester groups with higher decomposition temperatures and the decomposition of fragments of the compound due to the breaking of stronger bonds, such as carbon−carbon bonds.30 The DTG profiles revealed four main decomposition steps for H3, H4, and H5 and three for H6. The first step of degradation of H3, H4, and H5, recognizable by the prominent DTG shoulder at 260−320 °C (arrow in Figure 2a), involved 7−10% weight loss and is associated with the loss of their hydroxyl groups. As can be seen in Figure 2a, the magnitude and extent of this shoulder as well as related weight loss, decreased from H3 to H4 and H5 and did not show for H6, as expected from their decreasing OH groups content. The signal appearing most prominently at ∼360−370 °C in the DTG of the branched derivatives of H are associated with the loss of fatty acids and the breakage of the ester bonds.29 This process involved ∼60% of weight loss. As suggested by the decrease of the relative intensity of the second peak from H3 to H6, the split in two peaks (360 and 388 °C) was related to differences in degradation between the structural elements on the branches and those on the backbone. The last 20050

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Scheme 4. Isomers of H3: (a) Symmetrical with OH Internal at Both Ends, (b) Asymmetrical with OH Internal at One End and External at the Other, and (c) Symmetrical with OH External at Both Ends. OCOR: H5C2COO

Figure 4. SFC versus temperature curves of H and its branched derivatives during (a) heating and (b) cooling. (c) Induction (Tind) and solidification (Tsolid) temperatures obtained at the onset and offset of the SFC, respectively, and DSC onset of crystallization (TonC).

Its overall symmetry about the center of the molecule may have made it easier to pack very efficiently under melt mediation. On the other hand, the multiple endothermic events presented by H3 can be related to the melting of polymorphs of its asymmetrical constituting isomers, in terms of the terminal OH or terminal acyl group (Scheme 4). The polymorphic activity weakened dramatically with the addition of five and six branches. Although some very weak and broad recrystallization was observed in H5, it occurred at higher temperatures than for H4. No polymorphic activity was recorded in H6. This indicates that the amorphous phases of H5 and H6 compounds were significantly different from those of H4 and H3 in terms of shortrange order. The differences in short-range order can also be appreciated through the enthalpies of relaxation of the glass transition. The broad and weak endotherms appearing above 0 °C in the heating profiles of H4 and H5 can be related to moderate-to-weak hydrogen bonding.31 3.4. Solid Fat Content of H and Its Branched Derivatives. The SFC (%) versus temperature curves obtained during cooling and heating (3.0 K/min) of H and its branched compounds are shown in panels a and b of Figure 4, respectively. The induction (Tind) and solidification (Tsolid) temperatures obtained at the onset and offset of the SFC, respectively, are

crystallization was less than a sixth of that of its base ester. The cooling profiles of H4 and H6 presented a very small crystallization signal at very low temperature (−43 and −55 °C, respectively), with enthalpy of less than 1 J/g. The presence of such small exotherms in H4 and H6 but not in H5 may be related to their isomer compositions. A very prominent glass transition was observed at ∼64−72 °C for all the branched compounds (Table 3). Although relatively close (12 °C difference between Tg of H3 and H6), Tg of the branched compounds scaled predictively with the number of branches (Figure 3c) and with the number of OH groups. The heating profiles of the branched compounds indicate a relatively complex transformation behavior that included crystallizations mediated by melt, particularly strong in H4 (Figure 3b). Also, the glass transitions observed upon heating were particularly pronounced and Tg values were similar to those observed during cooling. H3 presented the most complex polymorphism, with multiple melting steps and recrystallizations following the glass transition. H4 recrystallized strongly (peak at −40 °C in Figure 3b) after its glass transition relaxation then melted quite simply, indicating that its glassy state was very unstable. The H4-I isomer which was the main component of H4 was probably the cause of its relatively simple melting behavior. 20051

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Figure 5. (a) Shear stress−shear rate curves obtained at selected temperatures for H. Shear stress−shear rate curves of the branched compounds obtained at (b) 0 °C, (b) 40 °C, and (c) 95 °C. Solid lines are fits to the Bulkley−Hershel model (eq 1).

as branches were added to H but well above the onset of crystallization of H3, H4, and H6 by an estimated 20 °C (Figure 4c), indicating that these compounds are structural gels. The expanded solidification process of the branched compounds shown by SFC indicates major mass and heat transfer limitations and reveals a large temperature window for the coexistence of the liquid and amorphous phase (100% final SFC was achieved after ∼38 ± 1 °C, Figure 4c). The SFC versus temperature heating cycles (Figure 4b) corresponded well with its cooling counterpart. The SFC versus temperature curve of H also presented two segments corresponding to the melting of the two phases that were detected during cooling. The branched derivatives of H presented one single sigmoid corresponding to a straight liquefying of the solid phase. The polymorphic transformations observed in the DSC heating thermogram of the branched compounds did not show in their SFC counterpart, indicating that the polymorphic transformations, if any, involved only small enough amounts of material to be overwhelmed by the large mass of liquid. The branched compound remained liquid above 3 °C and solidified fully only below −33 °C for the least branched, H3. The temperature span of melting matched the span of solidification, indicating homogeneously organized plastic fats with a predictable solidification behavior and very good reversibility. These characteristics can be very advantageous in practical applications. The compounds, although starting to gel at somewhat moderate temperatures (3 to −14 °C), remain viscous liquids at temperatures (−35 to −51 °C) low enough to be exploited in applications requiring low-temperature performance such as lubricants, waxes, greases, cosmetics, and pharmaceuticals.

presented in Figure 4c together with the DSC onset of crystallization. The solidification process as unveiled by the SFC is discussed in terms of the modified Avrami model, which takes into consideration the variances within the growth curve.32 In this model, each step is characterized by a constant growth rate and is described by an Avrami equation (a sigmoid) with an Avrami constant and Avrami exponent, applicable to nucleation, growth, and dimensionality of the solidifying molecules over that step. The SFC versus temperature curves of H displayed two very distinct segments, indicating that it solidified via two main steps. The first segment during which ∼95% SFC was achieved can be associated with the exotherm observed in its DSC cooling cycle, and the second can be tentatively related to the formation of an amorphous phase at low temperature. Contrary to that obtained with the base ester, the SFC of the branched compounds of H presented a simplified solidification process, characterized by a single sigmoid during cooling and heating (Figure 4a,b), usually associated with an overall unique solidification mode followed by straight melting. Although some crystal phase was formed in H3, its SFC trace was similar to that of the other branched derivatives, suggesting that the processes of crystal and amorphous phase formation were close. This corroborates the DSC that indicated a weakly organized crystal phase for H3, closer to an amorphous than a well-ordered structure. Note that H presented, if it is of any indication in terms of crystal phase content and degree of order and homogeneity, an enthalpy of crystallization of more than six times, and a fwhm of more than 20 times that of H3. H4, H5, and H6, which did show little or no crystal phase in the DSC, started to gel and achieved 100% SFC at relatively low temperatures (Tind and Tsolid in Figure 4c). Tind decreased linearly 20052

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Figure 6. (a) Power index n of the Herschel−Bulkley (eq 1) versus temperature curves of H and its branched derivatives. Lines are fits with exponential rise to a maximum function (eq 4). (b) Characteristic temperature T0 of eq 4 (n = n0 + a(1 − exp(−T/T0))) as a function of number of branches. Dashed line is a fit of the data to a straight line.

Figure 7. (a) Viscosity versus temperature curves of H and its branched derivatives. (b) Viscosity of H (open symbols) and its branched derivatives (filled symbols) measured at 25 and 40 °C as a function of number of hydroxyl groups. (c) Parameter A and (d) exponent m of the generalized van Velzen equation (GvVE, eq 3).

As illustrated in Figure 5b−d, the flow of H3, H4, H5, and H6 varied progressively with increasing temperature from shear thinning, where the apparent viscosity decreased with increasing shear rate, to Newtonian. This behavior is well-reflected by the values of the Herschel−Bulkley power index (n), which ranged from ∼0.76 to ∼1.01 ± 0.01. In fact, n versus temperature data of all the branched derivatives fitted an exponential rise to a maximum function (eq 4, lines in Figure 6a).

3.5. Rheology of H and Its Branched Derivatives. 3.5.1. Shear Stress−Shear Rate and Flow Behavior. Shear stress−shear rate curves obtained at selected temperatures for H are presented in Figure 5a, and those obtained at 0, 40, and 95 °C, which are typical of the flow behavior at low, moderate, and high temperature, are presented in panels b, c, and d of Figure 5, respectively. Fits to the Herschel−Bulkley model (eq 1) are included in the figures (solid lines in Figure 5a−d). The application of eq 1 to share rate−shear stress data of H and its branched derivatives (R2 > 0.9999) generated a yield stress of less than 0.0025 ± 0.0008 Pa at all temperatures used, a value which is below the sensitivity limits of our equipment, indicating that these fluids show no yield stress. The power index values (n) obtained for H using eq 1 as well as the power law model (eq 2), is 1.000 ± 0.001, indicating a Newtonian behavior.

⎛ ⎛ T ⎞⎞ n = n0 + a⎜⎜1 − exp⎜ − ⎟⎟⎟ ⎝ T0 ⎠⎠ ⎝

(4)

Furthermore, the characteristic temperature T0 of eq 4 varied linearly with the number of branches (Figure 6b), substantiating the progressive change of the flow with further branching toward a Newtonian type. This combined effect of hydroxyls and 20053

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4. CONCLUSIONS The ratio of isomers with terminal acyl to terminal OH group was about 1:3 in H3 and H4. Isomer I, containing only one acyl branch in the middle double bond position, was the main component of the isomeric mixtures, with more than 98% in H4 and 90% in H5. The thermal stability of the base ester, already quite high, was significantly improved by branching. The onset temperature of degradation increased by approximately 7 °C per branch added. The thermal transformation behavior was profoundly affected by branching. Adding three branches at the end of the hydrocarbon chain of H was very effective in depressing the onset of crystallization and mitigating its scope, but did not suppress polymorphic activity. Crystallization was completely suppressed in the 5- and 6-branched derivatives of H. Glass transitions were observed at temperatures well below the final SFC observed, indicating that these compounds present a gel-like structure over a large low-temperature range. The branched derivatives of H were shown to be relatively simple fluids with no yield stress but a flow behavior that depended strongly and predictively on temperature and shear rates. Their flow varied progressively with increasing temperature from shear thinning to Newtonian, indicating a gradual change in the intermolecular interactions favored by the flexible structure of the diester hydrocarbon chains that provides adapted van der Waals contributions to ensure adjusted resistance to flow under shear. The variations in flow behavior could be explained by the combined effect of hydroxyls and geometric steric hindrances imposed by the protuberant branches. At any given temperature, viscosity was highest for H3, followed by H4, H5, then H6 and finally the base ester, reflecting a strong correlation with the number of hydroxyl groups. As indicated with H and H6, although viscosity was affected primarily by the amount of OH groups, the branches through increased mass and stearic hindrances were demonstrated to add to the bulkiness of the molecule and to contribute measurably to viscosity. The branched derivatives of the diester H form plastic gels with predictable glass transition and solidification behavior. They remain viscous liquids at temperatures that are sufficiently low for them to be exploited well in applications such as lubricants and waxes.

geometric steric hindrances imposed by the protuberant branches explains the gradual change of the flow from Newtonian for the base ester to a more pronounced shear thinning for H3, H4, H5, and finally H6. The variations in flow behavior observed from the least to the most branched compound indicate a gradual change in the intermolecular interactions. This may be explained by the flexible structure of the diester hydrocarbon chains that provide adapted van der Waals contributions to ensure a continuous mass transfer under shear. 3.5.2. Viscosity versus Temperature. Viscosity versus temperature of H and its branched derivatives presented exponential curves typical of hydrocarbon liquids (Figure 7a). At any given temperature, viscosity of H3 was higher than that of H4, which in turn was higher than that of H5 and H6, a trend that is consistent with their decreasing OH content (no OH groups in H6, and 1, 2, and 3 OH groups in H5, H4 and H3, respectively). As expected, the differences in viscosity between any two compounds decreased exponentially with increasing temperature. The viscosity of the base ester was lower than that of H6, which does not have any OH group, also indicating that branching had a measurable effect on viscosity. As illustrated in Figure 7b with two temperatures (25 and 40 °C), the viscosity increased exponentially with the number of hydroxyl groups. The application of the generalized form of the van velzen expression (eq 3) to the experimental viscosity versus temperature data yielded good fits for all the compounds (R2 > 0.999979; solid lines in Figure 7a). The residuals determined for the region of temperatures where the compounds were liquid were less than 1% and in some cases less than 0.2%. The fit parameters A and exponent m are presented in panels c and d of Figure 7, respectively. Parameter A captured the variation of viscosity among the branched derivatives. One can notice first that A was highest for H3, followed linearly by H4, H5, then H6 and finally the base ester, reflecting the scale of viscosity from the most to the least viscous compound. The parameter A scaled linearly with the number of hydroxyl groups in the branched compounds, indicating a strong correlation between them (Figure 7c). The value of A, through the compounds without OH groups (H and H6 in Figure 7c), also captured the effect of branching on viscosity. Parameter A showed that although viscosity was affected primarily by the amount of OH groups, the branches add to the bulkiness of the molecule, be it through mass or stearic hindrances, and contribute measurably to viscosity. The exponent m is related to the activation energy and its magnitude is an indication of the difficulty of transferring molecules through the liquid matrix. The values obtained for m decreased from ∼3.3 for H3 to ∼3.6 for H6, indicating that although measurable, the differences in the activation energy of the branched derivatives of H is not dramatic. The exponent m of the base ester is relatively much lower (m = 2.8, Figure 7d), suggesting complex relationships between the different structural elements (OH groups, protuberant branches, mass, etc.) that are most relevant to the rheology of these compounds. It appears that m is much more sensitive to the structural complexity related to the dangling chains than the OH groups. However, although it elucidated some aspects of the contribution to viscosity of branching and hydroxyl groups, the phenomenological nature of this parameter obviously did not clarify the relative contribution of particular structural features or their mutual interactions.

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ASSOCIATED CONTENT

* Supporting Information S

Structure, IUPAC names, and nomenclature of synthesized compounds; 1H NMR in CDCl3, 13C NMR in CDCl3, MS, and HPLC data of synthesized compounds; 1H NMR spectra of H, its epoxide, and its branched derivatives; and 13C NMR spectra of H and its branched derivatives. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 1-705-748-1011, ext 6105. Fax: 1-705-748-1652. E-mail: [email protected]. Author Contributions †

These authors (S.L. and L.B.) contributed equally to this work.

Notes

The authors declare no competing financial interest. 20054

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ACKNOWLEDGMENTS The financial support of Elevance Renewable Sciences, NSERC, Grain Farmers of Ontario, GPA-EDC, Industry Canada, and Trent University is gratefully acknowledged.

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