Structure and Properties of Triolein-Based Polyurethane Networks

Biomacromolecules 2002, 3, 1048-1056

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Structure and Properties of Triolein-Based Polyurethane Networks Alisa Zlatanic´ ,† Zoran S. Petrovic´ ,*,† and Karel Dusˇ ek‡ Kansas Polymer Research Center, Pittsburg State University, 1501 South Joplin Street, Pittsburg, Kansas 66762, and Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, 162 06 Prague, Czech Republic Received April 11, 2002; Revised Manuscript Received June 13, 2002

Polyurethane networks based on vegetable oils have very heterogeneous composition, and it is difficult to find a close correlation between their structure and properties. To establish benchmark structure-properties relationships, we have prepared model polyurethane networks based on triolein and 4,4′-diphenylmethane diisocyanate (MDI). Cross-linking in the middle of fatty acid chains leaves significant parts of the triglyceride as dangling chains. To examine their effect on properties, we have synthesized another polyurethane network using triolein without dangling chains (removed by metathesis). The structure of polyols was studied in detail since it affects the structure of polyurethane networks. The network structure was analyzed from swelling and mechanical measurements and by applying network and rubber elasticity theories. The crosslinking density in both networks was found to be close to theoretical. The triolein-based model network displayed modulus (around 6 MPa), tensile strength (8.7 MPa), and elongation at break (136%), characteristic of hard rubbers. Glass transition temperatures of the networks from triolein and its metathesis analogue were 25 and 31.5 °C, respectively. Introduction There is a growing worldwide interest in the development of vegetable oil based polyurethanes. This interest is economically driven because vegetable oils are relatively inexpensive and a renewable resource. In addition, due to the hydrophobic nature of triglycerides, vegetable oils produce polyurethanes which have excellent chemical and physical properties such as enhanced hydrolytic and thermal stability. Vegetable oils are triglycerides of predominantly unsaturated fatty acids. The fatty acid composition of triglycerides varies not only from oil to oil but also within the same oil.1 Vegetable oils are chemically relatively unreactive and must be functionalized to serve as components for polymeric materials. Hydroxyl groups are usually introduced at the position of double bonds to prepare polyols for polyurethanes. Elucidating the structure of polyurethanes derived from naturally occurring triglycerides is complicated by the chemical diversity inherent within these molecules. To try and gain insight into the structure and properties of vegetable oil based polyurethanes, we have synthesized polyurethanes from triolein. Triolein is a triglyceride of oleic acid (Chart 1). To use triolein as the starting material for polyurethane synthesis, multiple hydroxyl functionality is required.2 We have introduced a single OH group per double bond, and since triolein has three double bonds, our polyol is a triol. * To whom correspondence may be addressed. Tel: 620-235-4928. Fax: 620-235-4919. E-mail: [email protected]. † Pittsburg State University. ‡ Academy of Sciences of the Czech Republic.

Chart 1

Because double bonds are in the middle of the unsaturated fatty acid chains, the hydroxyl groups will also be located in the middle of the chains. Part of the resulting polyurethane network will remain elastically inactive even at full conversion of functional groups. The elastically inactive regions belong to the category of dangling chains. Dangling chains represent imperfections in the network structure. They do not support stress when the sample is loaded. Dangling chains act as plasticizers that reduce polymer rigidity and improve polymer flexibility.3-12 By definition, dangling chains are part of the infinite structure (the gel), that is composed of units only singly connected to the gel structure (i.e., having only one bond with infinite continuation). Dangling chains in the networks may result from several factors including incomplete cross-linking reactions, stoichiometric imbalances,3-8 or the presence of monofunctional monomers.9,10 They may also be introduced into the polymer network due to the chemical structure of the monomer. For example, in our synthesis of polyurethanes from triolein, dangling chains are still present even under stoichiometric conditions and assuming full conversion of reactive groups, negligible functionality distribution, and absence of cyclization. The

10.1021/bm020046f CCC: $22.00 © 2002 American Chemical Society Published on Web 07/19/2002

Structure of Triolein-Based Polyurethanes

Biomacromolecules, Vol. 3, No. 5, 2002 1049

Scheme 1

Chart 2

length of dangling chains depends on the position of the double bonds in the fatty acid chains. In the case of the triolein-based polyurethanes, the dangling chains are 8 or 9 carbon atoms long. The introduction of hydroxyl groups into triolein was accomplished by a two-step method: epoxidation of the triglyceride, followed by epoxy ring opening with methanol in the presence of an acid catalyst. Assuming 100% conversion of double bonds to OH groups, the resulting polyol would have the structure shown in Chart 2. Triolein-based polyol, after cross-linking with MDI, should produce a polymer network with a precisely defined structure. The elastically active network chain will consist of two parts from the polyol and one MDI unit. In a perfect network the molecular weight between cross-links (Mc) including dangling chains should be 935. The length of the polyol part of the network chain will vary slightly depending on whether the OH substitution was at carbon 9 or 10 in the fatty acid chain and depending on whether the middle or terminal branch of the polyol molecule is incorporated in the network chain. For these same reasons, the elastically inactive part of the triolein-based polymer network should be uniform in length. In the ideal case of 100% conversion, the polyurethane networks derived from triolein should contain 29 wt % of dangling chains. To examine the effect of dangling chains on network properties, a new polyol was prepared from modified triolein by metathesis (called metathesized triolein in this text). Cross-metathesis of triolein was carried out by reacting triolein with ethylene in the presence of bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride (Grubbs catalyst). Main products of the metathesis should be a modified triolein with terminal (9,10) double bonds and 1-decene (Scheme 1). The derivitized triglyceride was then epoxidized, followed by ring opening with methanol

to obtain a polyol with terminal OH groups. A polyurethane network without dangling chains was prepared with this polyol and MDI. The whole process of preparing this polyol and polyurethane is fairly complex and expensive. Unfortunately, the sample size was sufficient only for Tg, density, and swelling measurements. The triolein-based polyols were characterized by titration (OH number determination), FTIR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and rheological methods. Cross-linking density of the polyurethane networks was estimated from the equilibrium modulus obtained from stress relaxation experiments. DSC, thermomechanical analysis (TMA), dielectric analysis (DEA), dynamic mechanical analysis (DMA), and mechanical methods were used to study the physical and mechanical properties of the polyurethanes. Experimental Section Materials. Triolein (99%+) was purchased from NUCHEK PREP, Inc. Hydrogen peroxide was purchased from Sigma as a 30% w/w solution. The anion-exchange resin, Amberlite IR-120, was supplied by Supelco. Tetrafluoroboric acid (48% solution in water), aluminum oxide, activated, neutral, Brockmann I, and methyltrioxorhenium(VII) were purchased from Aldrich. Bis(tricyclohexylphosphine)benzylidine ruthenium(IV) dichloride (Grubbs catalyst) was supplied by Strem Chemicals. Toluene (HPLC grade), methanol (99%), glacial acetic acid (certified A.C.S. Plus), ethyl acetate (HPLC grade), methylene chloride (HPLC-GC/MS grade), anhydrous sodium sulfite, and anhydrous sodium sulfate were purchased from Fisher Scientific. The cation-exchange resin, Lewatite MP-64, was purchased from Bayer. MDI, Rubinate 44, was obtained from ICI and further purified by distillation under reduced pressure. Methods. The hydroxyl values of the polyols were determined using a modified ASTM titration method (D 1957-86) where 2-propanol is used instead of ethanol in the procedure. The number average molecular weight of polyols was determined using a vapor pressure osmometer (Osmomat 070) from UIC Inc. The measurements were carried out at 60 °C in toluene with benzil as the calibration standard. A Waters gel permeation chromatograph, consisting of the 510 pump, a 410 differential refractometer, and Millenium

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software, was used for determining molecular weight distribution. The flow rate of tetrahydrofuran eluent was 1 mL/ min at room temperature. Three Styragel HR columns from Waters, covering a molecular weight range from 102 to 106, were used. Calibration was performed using nine polystyrene standards. The polyol viscosity was measured on the Rheometric SR500 dynamic stress rheometer in a stress-controlled mode, between parallel plates of 25 mm in diameter and a gap of 0.20 mm. A Thermal Analyst system from TA Instruments, consisting of a Controller 3100 with DSC 2910 module, TMA 2940 module, and dielectric analyzer DEA 2070 module, was used to study the glass transition and dielectric properties of the triolein-based polyols and polyurethanes. Dynamic mechanical tests were carried out on a DMA 2980 Dynamic Mechanical Analyzer from TA Instruments. The storage modulus (E′), loss modulus (E′′), and tan δ were recorded from -80 to 100 °C, at the heating rate of 2 °C/ min and a frequency of 10 Hz. A Q-Test 2 tensile machine (from MTS) was used for the determination of equilibrium modulus from stress relaxation experiments. The samples were 6 mm wide, 2 mm thick, and 50 mm long. A constant temperature of 35 °C during relaxation was maintained using warm air. Tensile properties were measured on the same tensile machine using 50 mm long samples at the extension rate of 50 mm/min. Synthesis of Polyols. Polyols were synthesized in two steps: epoxidation of the oil followed by ring opening of the epoxy groups to obtain the polyol. Epoxidation of Triolein. The epoxidation of triolein was carried out for 12 h in toluene, with peracetic acid formed “in situ” from acetic acid and hydrogen peroxide, at 60 °C, in the presence of the ion-exchange resin Amberlite IR-120 as the catalyst.13 The molar ratio of double bonds:acetic acid: hydrogen peroxide used was 1:0.5:1.5. Conversion of double bonds to epoxy groups was 100% as determined by measuring epoxy oxygen content. Metathesis of Triolein. Cross-metathesis of triolein with ethylene to obtain metathesized triolein was carried out in a Parr mini reactor (450 mL) under a pressure of 10 MPa. The reactor was initially charged with 50 g of triolein, 0.2 g of Grubbs catalyst, and ethylene. Temperature was raised to 50 °C, and the reaction was run for 1 h. The content was discharged into a round-bottom flask, and low molecular components (1-decene) were removed by applying high vacuum at 50 °C for 1 h. The oil was then returned to the reactor, an additional 0.2 g of catalyst was added, and the process was repeated 3 times. The final product was purified by passing the diluted solution in toluene through the column filled with activated aluminum oxide, but the catalyst was not completely removed. Epoxidation of Metathesized Triolein. The epoxidation of metathesized triolein was carried out in methylene chloride at room temperature using methyltrioxorhenium(VII) (MTO) catalyst and hydrogen peroxide. Since MTO also catalyzes the oxirane ring opening,14,15 pyridine was added as an acid scavenger. The hydrogen peroxide/double bonds molar ratio was 1.5. It has been shown16 that an MTO concentration of

Zlatanic´ et al. Table 1. Properties of Polyols Based on Triolein and Metathesized Triolein metathesized triolein-based triolein-based polyol polyol conversion of double bonds to OH, % hydroxyl number, mg of KOH/g equivalent molecular weight number-average mol wt (VPO) exptl density at 23 °C, g/cm3 calcd density (20 °C), g/cm3 viscosity at 27 °C, Pa‚s

96 157 357 1123 0.9729 1.0525 1.7

97 234 240 756 1.0335 1.3

0.5 mol % and a pyridine concentration of 6.0 mol %, related to the double bonds, gave the highest epoxy yield. The epoxy oxygen content of our product was 6.9% (93% conversion of double bonds to epoxy groups). Conversion of the Epoxidized Oil to Polyol. The polyols were synthesized from epoxidized oil by an oxirane ring opening with boiling methanol in the presence of the tetrafluoroboric acid catalyst. The molar ratio of epoxy groups to methanol was 1:9. The concentration of the catalyst was 0.05% and 0.25% of the total weight of the reaction mixture, for the epoxidized triolein and epoxidized metathesized triolein, respectively. Epoxidized oil was then added to boiling methanol and the catalyst. The reaction mixture was kept boiling for 35 min. An ion-exchange resin, Lewatite MP-64, was added to neutralize the catalyst. After removal of the resin by filtration, the solvent was removed on a rotary evaporator under low vacuum, followed by high vacuum at 60 °C. The final triolein-based triol had an OH number of 157 mg of KOH/g, (theoretical value is 163.5 mg of KOH/ g), a viscosity of 1.7 Pa‚s at 27 °C and a number-average molecular weight determined by vapor pressure osmometry (VPO) as 1123. Metathesized triolein-based polyol had the OH number of 234 mg of KOH/g (the theoretical value for the ideal structure would be 242 mg of KOH/g), a viscosity of 1.3 Pa‚s and a VPO number-average molecular weight of 756 (the ideal structure would have Mn ) 692). The conversion of double bonds to OH groups (97%) is higher than expected from the conversion to epoxy groups (93%) which is an intermediate step. This inconsistency could be generated by remaining methanol in the polyol or the presence of residual catalysts in the polyol. Synthesis of Polyurethanes. Triolein-based polyurethanes were synthesized by reacting the polyols with MDI. The polyol and MDI were preheated separately to 60 °C and then mixed and poured into molds preheated to 100 °C. Curing was carried out at 100 °C overnight. The density of the triolein-based polyurethane was 1.061 g/cm3 and 1.156 g/cm3 for metathesized triolein-based polyurethane. Results and Discussion Characterization of Polyols. Properties of the polyols are presented in Table 1. The conversion of double bonds to hydroxyls for the triolein-based polyol was 96%. The loss of functional groups occurred only during hydroxylation (conversion of double bonds to epoxy groups during the epoxidation reaction was 100%). The side reactions in the



Table 2. Calculated First and Second Moment Average Functionality and Corresponding Hydroxyl Content of Polyol from Triolein dimer functionality

fn

f2

OH no. mg of KOH/g

4 5

3.07 3.14

3.09 3.22

156.3 160.0

Figure 1. GPC chromatograms of triolein- and metathesized trioleinbased polyols.

hydroxylation stage were primarily due to polymerization (i.e., the formation of dimers and trimers through the epoxy ring opening by newly formed OH groups). GPC chromatogram of triolein-based polyol (Figure 1) shows that the content of dimers was 13 wt %. The amount of each species determined from GPC is taken to be proportional to the area under the peaks, assuming there is no change of the refractive index with molecular weight. This assumption is reasonable since there is no major change in chemical composition. The difference between dn/dc values for monomer and dimer species in THF calculated from group contribution method according to Goedhart17 was 1.1%, i.e., within the experimental error of GPC measurements. The value of the number-average molecular weight of the triolein based polyol, determined by VPO, was 1123. This value is higher than the theoretical Mn of 1029 due to the presence of dimers. The number-average molecular weight of the triolein polyol with 13% of dimers was calculated to be 1100. This is in close agreement with the VPO result. Functionality of the Triolein-Based Polyol. The numberaverage functionality, fn, calculated from the hydroxyl number (157 mg of KOH/g) and VPO molecular weight (1123) was 3.14. The incomplete double bond to hydroxyl conversion (96%) and formation of dimers are responsible for the observed functionality distribution. Though there is a possibility of ether formation from two hydroxyl groups, dimerization primarily results from the reaction of a hydroxyl with an epoxy group, forming an ether bond and a new OH group. Thus, the functionality of the dimer is either 4 or 5, and the number fraction of the dimer is approximately 7 mol %. The number fraction of the triol monomer is 93 mol %. From these values the number (first moment-) average functionality, fn, and second-moment-average functionality, f2, defined by eqs 1 and 2 are calculated.

∑nifi f2 ) ∑nifi2/∑nifi fn )

(1) (2)

Here ni is a number (molar) fraction of molecules of functionality fi. For the two cases of 7 mol % dimer of functionality 4 or 5, the calculated functionality averages are displayed in Table 2. The functionality distribution and its effect on network formation will be studied in detail later.

Figure 2. GPC chromatogram of the metathesized triolein. The numbers above peaks represent weight fraction in the sample.

Here, we will consider both values when discussing the experimental results on gelation and concentration of elastically active network chains. In the absence of cyclization, assuming equal reactivity of all functional groups of the polyol cross-linked with a bifunctional diisocyanate with groups of equal and independent reactivity, f2 is related to the critical conversion at the gel point, Rc, by Rc )

( ) 1 f2 - 1

1/2

(3)

For the above values of f2, the critical conversions Rc were calculated to be 0.692 and 0.671, respectively. The experimental value was 0.735. The difference is apparently due to cyclization. Some of the bonds are engaged in closing cycles (intramolecular bonds); only intermolecular bonds are active in branching. Thus, the conversion of functional groups at the gel point, Rc, can be considered composed of inter- and intramolecular contributions, (Rc)inter and (Rc)intra, respectively Rc ) (Rc)inter + (Rc)intra

(4)

The relative extent of cyclization, (Rc)intra/Rc, has the values 0.059 and 0.087, assuming four and five functional dimers, respectively. These values are reasonable but are somewhat higher than those for networks of telechelic polymers7 that have longer chains between branch points. The numberaverage molecular weight of metathesized triolein-based polyol was 756. This value is higher than the calculated value of Mn ) 692, assuming complete removal of the terminal parts of the alkyl chains. The starting metathesized triolein displayed a polymodal distribution of molecular weights arising from the incomplete removal of the terminal chains (Figure 2). The content of the totally metathesized triolein did not go above 50 wt % after repeated metathesis reactions.

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Figure 3. DSC curves of triolein and metathesized triolein-based polyols.

The content of species with two and one terminal chain cut from triglyceride molecules was 20 and 3.2 wt %, respectively, while the amount of unchanged triolein was 13.2 wt %. The lowest molecular weight GPC peak (10 wt %) probably arises from the C20 hydrocarbon chains, which are the side products during metathesis and could not be removed completely from the system by high vacuum. The density of the polyols is closely related to their structure. The density of the triolein based polyol was calculated to be 1.0525 g cm-3 using the group contribution method,17 which is based on the additivity of molar volumes of the chemical groups present in the material. We used the values of the group contributions reported by Fedors.17 The experimentally determined polyol functionality was included in the calculation. The experimental density, determined at 23 °C using a Paar DMA5000 densitometer, was 0.9729 g cm-3. The physical properties of the polyols are related to the structural order and the physical state. Figure 3 compares DSC thermograms of the two polyols. Both polyols showed a glasslike transition with end points at -42.6 °C (trioleinbased polyol) and -48.5 °C (metathesized triolein-based polyol). The lower Tg of the metathesized polyol results from its lower molecular weight. Stress-Strain Behavior and Cross-Linking Density of Triolein-Based Polyurethane. Equilibrium moduli were obtained from the stress relaxation experiments at 35 °C (10 °C above Tg) in the uniaxial extension mode after 30 min, when no appreciable changes in the retractive force took place.18 The samples were successively subjected to extensions of 10, 20, 30, 40, and 50%. The results, expressed as reduced stress (or Mooney stress) [f*] ) f/(λ - λ-2) vs reciprocal extension ratio, 1/λ, fit well with a straight line (Figure 4) in the experimental range of 1/λ, giving values of 1.680 and 0.703 MPa for the reduced stress at 1/λ ) 1 and 0, respectively. The phenomenological Mooney-Rivlin relationship [f*] ) 2C1 + 2C2/λ

(5)

where 2C1 and 2C2 are empirical constants independent of λ, was used to process the data. According to the current rubber-elasticity theories, the constants 2C1 and 2C2 characterize the cross-linking density and the level of interchain interaction. The junction-fluctuation theory of Flory (JFF theory)11,18-20 was used for the analysis of polymer cross-linking density. From 2C1, an

Figure 4. Reduced stress vs reciprocal deformation for the trioleinbased polyurethane network.

extrapolated value of modulus at high deformations (phantom network model limit, free fluctuations of cross-links), one can calculate the concentration of elastically active network chains (EANCs), νe, or number-average molecular weight of EANCs, Mc, if the sample density, F, and the absolute temperature, T, are known (eq 6)

(

2C1 ) 1 -

2 2 F ν RT ) 1 RT f e f Mc

)

(

)

(6)

In eq 6, R is the universal gas constant and f is the functionality of cross-links. On the other hand, for the affine limit (junction fluctuations are suppressed due to interchain interactions) eq 7 applies 2C1 + 2C2 ) νeRT )

F RT Mc

(7)

In our case, the ratio of (2C1)/(2C1 + 2C2) was found to be 1/2.34. The concentration of EANCs, calculated from (2C1) for the phantom network model was 8.24 × 0-4 mol/cm3 (average molecular weight of network chains Mc ) 1287). For the affine model, using the (2C1 + 2C2) value, the concentration of EANCs was 6.43 × 10-4 mol/cm3 or Mc ) 1555. These values are to be compared with the value expected for an ideal structure (trifunctional polyols, full conversion, no cycles) and for the systems containing the dimer. For the ideal trifunctional structure, the following considerations (Figure 5) are used: each f-functional unit contributes to f EANCs (contribution to the number of EANCs f/2) by its weight (M(polyol)) and f/2 diisocyanate molecules contribution (f/2)M(diisocyanate). Therefore Mc )

M(polyol) + (f/2)M(diisocyanate) f/2

(8)

For f ) 3, Mc is 935 and the concentration of EANCs, νe, is 1.13 × 10-3 mol/cm3. In other words, there is a 3/2 contribution from each trifunctional polyol to the number of EANCs. A tetrafunctional dimer contributes by 2(3/2) ) 6/2 and a pentafunctional trimer by 3(3/2) ) 9/2 to the number of EANCs. The values of νe calculated for the ideal systems as well as for systems containing dimers are compared with the experimental values in Table 3.



Figure 6. Master curve for the storage modulus of the triolein-based polyurethane at the reference temperature of 30 °C. Figure 5. Model structure unit of the triolein-based polyurethane network. Table 3. Experimentala and Calculatedb Values of the Concentration of Elastically Active Network Chains, νe, in Triolein Polyurethanes νe, mmol/cm3 calcd 100% trifunctional calcd 7 mol % tetrafunctional dimer calcd 7 mol % pentafunctional dimer exptl (phantom network) exptl (affine network)

1.54 1.54 1.65 0.824 0.643

a Determined from equilibrium modulus. b Calculated from the composition, full conversion of functional groups and absence of elastically inactive cycles.

It is seen that the experimental concentrations of EANCs are lower than the calculated ones assuming only intermolecular reaction and 100% conversion of functional groups. Similarly, as in the case of the shift of the critical conversion, one can estimate the fraction wasted in rings by comparing the calculated and experimentally determined concentrations of EANCs. However, there is a difference in defining the inactive cyclic structures. Up to the gel point, the closing of any cyclic structure is considered as an intramolecular reaction. In the case of EANCs, only bonds closing so-called elastically inactiVe cycles are considered.7,21 The values of ∆νe,rel(intra) ) (νe(id) - ∆νe(exp))/(νe(id) vary between 0.45 and 0.55 depending on whether the phantom or affine models and tetra- or pentafunctional dimer are considered. The theoretical treatment of network formation for these systems offering the dependence of νe on conversion is outlined elsewhere.22 From this dependence, one can estimate the fraction of bonds lost in elastically inactive cycles. Within the range of ∆νe,rel(intra), the fraction of bonds closing elastically inactive cycles is estimated to vary between 0.05 and 0.09, which is in agreement with values found from the shift of the critical conversion at the gel point. Generally, it should be remembered that the concentration of EANCs is very sensitive to various kinds of network imperfections.7 Dynamic mechanical Analysis of Triolein-Based Polyurethane. The time-temperature superposition principle makes it possible to characterize the viscoelastic properties of materials at various temperatures over an experimentally inaccessible time or frequency range. In general, the timetemperature superposition principle assumes that by changing the temperature, the complete relaxation spectrum is affected to the same degree.

We carried out multiple isothermal oscillation experiments in the uniaxial extension mode, over the frequency range from 0.1-100 Hz. The set of isothermal curves (from -80 to 140 °C) of storage modulus, E′, versus frequency were superimposed to generate a master curve at the reference temperature of 30 °C (5 °C above Tg from DSC). Also, a slight vertical shift was necessary to compensate for an inherent change in modulus brought about by a change in temperature. The master curve for the triolein-based polyurethane network is displayed in Figure 6. The extended frequency range obtained by the superposition is 10-10 to 1010 Hz. The value of the dynamic modulus at very low frequencies can be treated as the equilibrium modulus (G′ ) E′/3 ) 1.56 MPa). Thus, the concentration of the EANCs in the network was calculated to be 5.99 × 104 mol/cm3 (Mc ) 1770). This value was comparable to that obtained from (2C1 + 2C2) values in stress-relaxation measurements at low strains. Swelling of Networks and Sol Fraction. The swelling of the triolein-based polyurethane in toluene at room temperature was much higher (95%) than that of polyurethanes from metathesis-based polyol (40%). The swelling values observed were surprising considering the similarity between the two polymer structures. The sol fraction for each polymer was determined after multiple extractions with toluene. The content of the soluble part (Ws), calculated from the difference in weight of the dry sample before (w1) and after (w2) extraction (eq 9) was only 0.35% in the triolein polyurethane and 2.51% for the metathesized polyol based polyurethane. This indicates less perfect cross-linking in the metathesized polyurethane network, although it is possible that the residual low molecular components from the metathesis could contribute to the higher sol fraction. Ws ) (w1 - w2)/w1

(9)

Molecular weight of EANCs can be extracted from swelling measurements provided the polymer-solvent interaction parameter is known. According to the Flory-Rehner theory, the concentration of EANCs, νe, or molecular weight of EANCs, is given by23 1/νe ) Mc/F2 )

[-V1(AΦ21/3 - 2BΦ2/f)] [ln(1 - Φ2) + Φ2 + χ12Φ22]

(10)

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Figure 7. DSC curves of triolein and metathesized-based polyurethanes cross-linked with MDI. Table 4. Glass Transition Temperatures and Mechanical Properties of the Polyurethane Based on Triolein and Metathesized Triolein triolein-based polyurethane DSC Tg, °C TE′′max,a °C (at 10 Hz) tensile strength, MPa elongation at break, % storage modulus at rubbery plateau from E′ vs temperature curve, MPa from master curve, MPa

25 35.6 8.7 136

metatesized triolein-based polyurethane 32 33.8b

6.17 4.53

a

Temperature of the maximum on the loss modulus-temperature curve from DMA. b Obtained with a sample containing bubbles.

where F2 is the density of the dry polymer, V1 is the molar volume of the solvent, φ2 is the volume fraction of the polymer in the swollen sample, f is the functionality of the network branch points, and χ12 is the polymer-solvent interaction parameter. The factors A and B within the JFF theory have the following limits: A ) (f - 2)/f, B ) 0 (phantom network); A ) 1, B ) 1 (affine network).

The swelling equation can be used for determination of the interaction parameter if the value of νe, obtained from the stress-strain measurement, is known. For the network derived from triolein, χ12 evaluated from equilibrium swelling and using the equilibrium modulus was found to be 0.64 for the phantom network model and 0.58 for the affine network model. Glass Transition Temperature of the Triolein- and Metathesized Triolein-Based Polyurethanes. DSC curves of the polyurethanes are shown in Figure 7. The glass transition of a polymer network is affected by its crosslinking density and chemical structure. The Tg of the two polyurethanes determined by different methods are given in Table 4. Tg values of 25 and 31.5 °C were determined for the triolein- and metathesized triolein-based polyurethanes, respectively. The difference in Tg values may arise from several factors including: (a) lower cross-linking density of the triolein-based network, (b) the plasticizing effect of dangling chains in the triolein-based network, and (c) differences in composition of the networks (i.e., the higher content of MDI in metathesis polyurethanes (35 vs 27 wt %), which increases the stiffness of the network structure). Changes in the storage (E′) and loss (E′′) moduli with temperature, obtained from DMA on the triolein- and metathesized triolein-based polyurethane samples, carried out at the frequency of 10 Hz, are shown in Figure 8. Temperatures where E′′ exhibits a maximum (TE′′max) were 35.6 and 33.8 °C for the triolein-based and metathesized triolein-based polymers, respectively. Unfortunately, the sample from metathesized triolein contained bubbles, which was reflected in a lower storage modulus, and it was shown here to illustrate the shape of the curve. Both samples show β-transition on E′′-temperature curves at about -65 °C, which was also found in polyurethanes based on the polyols from several vegetable oils. It is known that the magnitude of the storage modulus affects the position of the maximum on the loss curve, which explains lower TE′′max of the

Figure 8. Storage (E′) and loss (E′′) moduli versus temperature of triolein- and metathesized triolein-based polyurethanes.


Figure 9. Stress-strain curve for the triolein-based polyurethane at 25 °C.

metathesized polyurethane, although Tg of this sample measured by DSC was higher. TMA results corroborated the DSC findings, although the shape of the thermal expansion curves was different and the precise positions of Tg were less certain. The value of the storage modulus, E′ (6.17 MPa), at the rubbery plateau (100 oC) was also used to estimate crosslinking density of the polymer network. The ν value obtained from DMA was 6.63 × 10-3 mol/cm3 (or Mc ) 1600). The obtained value for ν is in very good agreement with the results from the stress-relaxation and dynamic mechanical measurements (master curve), probably because at 100 oC, relaxation is fast and the dynamic modulus is close to the equilibrium modulus. Mechanical Properties of Model Polyurethane Networks. Tensile properties of the triolein-based polyurethane network are shown in Table 4. Stress-strain behavior (Figure 9) of the triolein-based polyurethane is considerably different from the behavior of the samples based on soybean oil from our previous study.24 While soybean oil-based polyurethanes


exhibited strengths of 40-50 MPa, the triolein-based sample had the tensile strength of only 8.7 MPa, but higher elongation at break (136%). The main reason for the lower mechanical strength is the low glass transition temperature of the polyurethane based on the triolein, which is close to the testing temperature of 25 °C. Triolein-based polyurethane was tested in its leathery region, and the same reason explains much higher elongation at break than that found in glassy polyurethanes based on soybean oil. Dielectric Properties of Polyurethane Based on Triolein. Triolein-based polyurethane is composed of polar urethane and ester groups and nonpolar hydrocarbon chains, which is reflected in moderate permittivity of 3 below Tg and 6.8 (1 Hz) to 5.4 (1000 Hz) above Tg. The loss factor vs temperature curves, Figure 10, show R transitions between 50 and 90 °C depending on frequency, and a β transition at about -50 °C for f ) 1000 Hz. The origin of the β transition was not studied. It may be associated with water as in other types of polyurethanes,25 or it has the same origin in the main chain motions, as observed in dynamic mechanical measurements. The shift of the maximum, T, on the loss factor-temperature curves with frequency, f, was used for calculation of the activation energy, E, of the glass transition ln f ) ln f0 - E/RT

(11)

The activation energy of the polyurethane based on triolein was 180 kJ/mol. The same value of activation energy of the glass transition was obtained for most of the soybean oilbased polyurethanes in the previous study.24 On the other hand, soft segments with Mn ) 2000 in segmented polyurethanes based on polypropylene oxide showed a 40 kJ/mol lower value of glass transition activation energy. This reflects lower flexibility of the hydrocarbon chains compared to polyethers. This is also observed in viscosity measurements,

Figure 10. Loss factor of the triolein-based polyurethane vs temperature at different frequencies.

1056


which show higher viscosity for vegetable oil-based polyols than for polypropylene oxide polyols of the same molecular weight. Conclusions Two polyols, based on triolein and metathesized triolein, were synthesized, and the corresponding polyurethane networks were prepared by reacting polyols with MDI. Removing dangling chains decreases the viscosity and Tg of polyols. These model polyurethane networks showed that dangling chains present in the triglyceride structures act as plasticizers, decreasing Tg by about 6 °C. Removing dangling chains increases the solvent resistance of resulting polyurethanes (lowers their swelling). Triolein-based polyurethane has a moderate cross-linking density, characterized by the EANCs concentration of νe between 6.43 × 10-4 mol/cm3 (Mc ) 1555) and 8.24 × 10-4 mol/cm3 (Mc ) 1287). This polyurethane displayed a relatively low glass transition temperature of 25 °C and a typical behavior of a hard rubber at room temperature, with a tensile strength of 8.7 MPa and elongation at break of 136%. These properties are similar to the ones for polyurethanes from castor oil, which has similar functionality. Higher functionality of polyols and thus higher cross-linking density is required if these polyurethanes are going to be used as engineering materials. The relatively high activation energy of the glass transition of 180 kJ/mol for the polyurethane based on triolein, determined by dielectric analysis, suggests a lower mobility of the fatty acid chains than in the standard (polyoxypropylene-based) polyurethane networks of the same cross-linking density. Acknowledgment. This material is based upon work supported by the Cooperative State Research, Education and Extension Service, U.S. Department of Agriculture, under Agreement No. 99-35504-7873. References and Notes (1) Guo, A.; Cho, Y.; Petrovic, Z. S. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3900-3910.

Zlatanic´ et al. (2) Hofer, R.; Daute, P.; Grutzmacher, R.; Westfechtel, A. J. Coat. Technol. 1997, 69, 65-72. (3) Dusek, K. Macromolecules 1984, 17, 716. (4) Dusek, K.; Ilavsky, M. ACS Symp. Ser. 1982, No. 193, 403. (5) Ilavsky, M.; Dusˇek, K. Polymer 1982, 24, 981. (6) Dusek, K.; Ilavsky, M. Prog. Colloid Polym. Sci. 1989, 80, 26. (7) Dusek, K. In Networks from Telechelic Polymers. Synthesis and Application; Goethals, E. J., Ed.; CRC Press: Boca Raton, FL, 1989; pp 289-360. (8) Dusek, K. Material Science and Technology. In Processing of Polymers; Meijer, H. E. H., Ed.; Wiley-VCH: New York, 1997. (9) Fedderly, J. J.; Lee, G. F.; Lee, J. D.; Hartmann, B.; Dusˇek, K.; Dusˇkova´-Smrckova´, M.; Sˇ omva´rsky, J. J. Rheol. 2000, 44, 961972. (10) Fedderly, J. J.; Lee, G. F.; Lee, J. D.; Hartmann, B.; Dusˇek, K.; Sˇomva´rsky, J.; Dusˇkova´-Smrckova´, M. Macromol. Symp. 1999, 148, 1. (11) Erman, B.; Mark, J. E. Structure and Properties of Rubberlike Networks; Oxford University Press: New York, 1997. (12) Lee, Y. L.; Sung, P. H.; Liu, H. T.; Chou, L. C.; Ku, W. H. J. Appl. Polym. Sci. 1993, 49, 1013-1018. (13) Crivello, J. V.; Narayan, R. Chem. Mater. 1992, 4, 692-699. (14) Piazza, G. J. In Recent DeVelopments in the Synthesis of Fatty Acid DeriVatiVes; Derksen, J. T. P., Ed.; AOCS Press: Champaign, IL, 1999; Vol. Chapter 11, pp 182-195. (15) Rudolph, J.; Reddy, K. L.; Chiang, J. P.; Sharpless, K. B. J. Am. Chem. Soc. 1997, 119, 6189-6190. (16) Refvik, M. D.; Larock, R. C. J. Am. Oil Chem. Soc. 1999, 76, 99102. (17) van Krevelen, D. W. Properties of Polymers, 3 ed.; Elsevier: New York, 1990. (18) Mark, J. E.; Erman, B. Rubberlike Elasticity. A Molecular Primer; Wiley: New York, 1988. (19) Ferry, J. D. Viscoelastic Properties of Polymers, 3 ed.; Wiley: New York, 1980. (20) Langley, N. R.; Polmanteer, K. E. J. Polym. Sci.: Polym. Phys. Ed. 1974, 12, 1023-1034. (21) Dusˇek, K.; Vojta, V. Br. Polym. J.l 1977, 9, 164-171. (22) Dusˇek, K.; Dusˇkova´-Smrckova´, M.; Zlatanic, A.; Petrovic, Z. S. In Book of Abstracts; 223rd National Meeting of the American Chemical Society: American Chemical Society: Washington, DC, 2002; Vol. 86, pp 381-382. (23) Dusˇek, K.; Prins, W. AdV. Polym. Sci. 1969, 6, 1. (24) Petrovic, Z. S.; Guo, A.; Zhang, W. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4062-4069. (25) Hedvig, P. Dielectric Spectroscopy of Polymers; Akademiai Kiado: Budapest, 1977.

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Structure and Properties of Triolein-Based Polyurethane Networks

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