Abstract Methods to increase the yield of copolymer versus previous synthetic methods were explored and developed. When this proved successful, the copolymerization was more thoroughly investigated in terms of kinetics and overall energy generated. In addition, coatings were made and evaluated in terms of cure energies and film properties such as solvent resistance, hardness, gloss and flexibility. In most cases these properties were excellent. Solubility parameters and surface smoothness were calculated or measured. The solubility parameters showed that the polymer had potential for miscibility with a wide range of materials, including many epoxies. Atomic force microscopy revealed films with a high level of smoothness and excellent miscibility with the crosslinkers used confirming the solubility parameter calculations. Keywords: Copolymer; Coatings; Characterization; Maleic anhydride; Pamolyn 380; Thermoset films
1. Introduction Previous work1 on attempting to obtain the Diels-Alder adduct of maleic anhydride and Pamolyn 380 (a conjugated linoleic acid) resulted in moderate yield (40-60%) of a polymeric species and very little Diels-Alder adduct (<10%). The polymer proved to be a copolymer of maleic anhydride and Pamolyn 380. The copolymer yielded films with many excellent properties. However, the yield of copolymer was not optimized and the copolymerization itself could be better characterized. In addition, a broader range of uses for the copolymer needed to be explored. Therefore, we launched this investigation. As previously stated, the initial work on Pamolyn 380 and maleic anhydride was an effort to produce the Diels-Alder adduct:
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Scheme 1 Projected Synthesis of Diels-Alder Adduct
However, instead of a relatively high yield of Diels-Alder adduct, we obtained significant quantities of relatively high molecular weight polymer. Excellent work by Hall et.al. (1982) and Hall (1983) showed that this result was somewhat uncommon but not rare. They showed that in ˜30-33% of the cases they investigated, polymerization occurred, with very little Diels-Alder adduct being obtained. Hall and coworkers as well as Smith and Kharas (1993) attribute this behavior to the two molecules having just the right polarity differences. Trumbo et. al. (2001) confirmed through NMR that the polymer had a 1:1 composition that was regularly alternating in terms of morphology as shown below.
Scheme 2 Copolymerization of Pamolyn 380 and Maleic Anhydride
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Trumbo’s work also showed that films of this copolymer obtained excellent solvent resistance, gloss and flexibility. However, in order to increase the possible commercial viability of this copolymer we decided to investigate the copolymerization of these monomers more thoroughly with the idea of increasing the polymer yield and understanding some of the particulars of the polymerization. The first step, then, was to add a known free radical initiator and monitor the results. If this worked i.e. the yield increased, we would then repeat some of the film studies done in earlier work and expand the area where these films could be useful.
2. Experimental All the chemicals used in this work were obtained from commercial sources and were used without further purification. Molecular weights were measured using a Waters Alliance GPC equipped with a Waters 2695 separations module and a Waters RI detector. Actual values for the molecular weights were obtained from a polystyrene calibration curve. Nuclear magnetic resonance spectra were recorded on DMSO-d6 solutions of material at ambient temperature using a Bruker AVANCE 300 FT NMR. Infrared spectra were recorded on thin films of material using a BIO-RAD FTS-6000 FTIR. Glass transition temperatures and reaction exotherms were determined using a TA-Instruments Q2000 DSC at several different heating rates. Thermal stabilities were measured using a TA-Instruments TGA Q50 at a heating rate of 10˚C/min. Atomic Force Microscopy was done on surfaces and crosssectioned areas of certain cured samples using a Multi-Mode (TM) Atomic Microscope and Nanoscope IIIa Controller in tapping mode. The samples were prepared using a Leica Ultracut UCT Ultramicrotome. Acid numbers were determined by titration using a Metrorohm 716 DMS Titrino titrater. Film properties were measured using the appropriate ASTM methods. Rheological properties were measured using an AR2000 Advanced Rheometer. Polymer Synthesis A 1 liter flask equipped with a mechanical stirrer, reflux condenser, temperature sensing thermocouple and a N2 inlet was charged with 460 g (1.6 mol) of Pamolyn 380 (Eastman). Stirring and a slow flow of N2 was started and 156.8 (1.6 mol) of maleic anhydride was added. The temperature of the reaction mixture was raised to 100˚C and AIBN (0.64 g) was added all at once. The temperature of the reaction mixture then rose rapidly (3-4 min), to 159-160˚C. The heating mantle was lowered and the reaction mixture was allowed to cool to 50˚C over the course of 90 minutes and was diluted with 250 ml of xylene to aid quantitative removal of the reaction mixture from the flask. The reaction mixture was further diluted with 500 ml of methylene chloride and the copolymer was isolated by adding this solution to an eight-fold excess of diethyl ether. The copolymer was purified by two reprecipitations from methylene chloride into diethyl ether. The copolymer was dried to a constant weight at 40˚C in a vacuum oven and conversion was determined gravimetrically. Film Preparation The copolymer (25 g) was dissolved in 30 g of methyl ethyl ketone (MEK). The desired crosslinker was added in an amount that ensured a 1:1 mole ratio of functional groups. N,N-dimethylaminobenzyl amine was added as a catalyst. Films were made by drawing 3 or 4 g of this solution over Bonderite 1000 steel panels with a #3 Bird bar (dry film thickness = 25-30 µm). The panels were placed in a forced air oven at 150˚C for arbitrary lengths of time. The panels were
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removed from the oven and allowed to cool to ambient temperature and the films were assessed for property development.
3. Results and Discussion In our previous work, it was initially surprising that a polymeric species was obtained instead of the Diels-Alder adduct. As previously stated, a search of the literature revealed that this was not an unknown phenomenon. However, having established that copolymer was the primary product and that the polymer possessed some interesting properties, we set out to investigate the polymerization more completely. The yields of polymer obtained in our initial work were 40-60 wt%. Since these materials were all synthesized without using an initiator, we decided to first investigate whether or not the yield of polymer would be improved through the use of a free radical initiator. This would also help to confirm that the polymerization proceeds by a free radical mechanism. The initiator we chose was 2,2′-azo bis(2-methylpropionitrile, AIBN). The initiator was added at the 0.25-0.30 wt% level. This caused a notable rise in temperature when added to the reaction mixture. The presence of an initiator also resulted in yields of 89-93 wt% of polymer, a significant improvement versus the initiator-free process. The molecular weights obtained using polystyrene standards for comparison were MN = 26,000 – 32,000 D; MW = 62,000 – 66,000 D and Mz = 145,000 – 151,000 with an average dispersity of 2.37. The Tg was 30.0˚C. A typical GPC chromatogram is shown in Figure 1.
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Assuming that the structure of the copolymer is that shown in Scheme 2, these numbers indicate a DP of 70-86 based on MN and 165-176 based on MW. In order to further prove the assumption of polymeric structure, 1H-nmr, 13C-nmr and infrared spectra were recorded. The 1H-nmr spectrum is shown in Figure 2 with peak assignments given in the Figure.
Fig. 2. 300 MHz 1H-NMR Spectrum of Pamolyn 380-MAH Copolymer The assignments were made by consulting the literature Hall (1983) and Beauchamp and Marquez (1997) and by using an in-house computer spectral simulation program Allaway (2007). The 1H resonances show that both the Pamolyn 380 and maleic anhydride are presented, this is confirmed by the 13C spectrum in Figure 3, Hall (1983) and Colthrup, et. al. (1990).
-CH2-CH2-C-OH
|| O
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Fig. 3. 75 MHz 13 C-NMR Spectrum of Pamolyn 380-MAH Copolymer Integration of the resonance at 3.43 ppm (1H-nmr) compared to the integration of the remaining resonances in the 1H-nmr spectrum shows that the ratio of MAH/Pamolyn 380 in the copolymer is 1:1. The IR spectrum (Figure 4) also confirms the presence of both species (MAH and Pamolyn 380) in the copolymers, but more importantly, the absorbance at 1855 cm-1 and 1775 cm-1 show that the maleic anhydride ring is intact.
Carbonyls from
Pamolyn
380
Carbonyls
from
MAH
CH2 CH2
from MAH
CH2
from Pamolyn 380
CH3 From
Pamolyn 380
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Fig. 4. IR-Spectrum of Pamolyn 380-MAH Copolymer
This is important when considering future modifications and crosslinking of this copolymer as it establishes the conditions and crosslinking moieties that may be used with the copolymer. This is also true of the acid number of the copolymers, AN = 470 (theoretical AN based on the expected structure = 462). The acid number is measured by a potentiometric titration. It is calculated by assuming a regularly alternating structures, and knowing the molecular weight. The yields how many moles of MAH should be present in a given quantity of polymer and, subsequently how many milligrams of KOH would be necessary to neutralize the two acid groups from the MAH. Having shown that the material produced has the expected morphology we made simple coatings formulations and evaluated them in films over steel panels. The results are summarized in Table 1.
a. MEK DR = Methyl ethyl ketone double rubs b. EPLO = Epoxidized linseed oil
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c. ERGE = Emsys 22 (cyclohexane dimethyl diglycidyl ether) d. EDGE = Ethylene glycol diglycidyl ether e. NPGDE = Neopentyl glycol diglycidyl ether
In order to begin to understand how this polymerization proceeds we characterized the polymerizations and crosslinking reactions more thoroughly. We used DSC to measure the heat of polymerization which was 70 ± 2 J/g. To calculate the kinetics of polymerization we again used the DSC at heating rates of 2-15˚C/min and the Kissinger equation, Kissinger (1957) and Budrugeac and Segal (2007). The Kissinger equation requires DSC scans at different scan rates then uses the following equation to derive kinetics parameters.
ln (B/Tp2) = 2n (AR/Ea) – (Ea/RTp)
Equation 1
Kissinger Equation
where β is the heating rate, n is the order of the reaction and R is the gas constant. At different heating rates, peak temperatures will shift. At the peak temperatures the rate of reaction is maximum. By plotting ln (β/Tp2 ) as a function of 1/Tp (where Tp is the peak temperature (the energy of activation Ea and the pre-exponential factor A can be obtained. Such a plot along with the DSC traces is shown in Figure 5.
(A)
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(B)
Fig. 5. A) DSC traces at various heating rates for Pamolyn 380 – MAH Copolymer; B) Kissinger Plot from the DSC Data
The Arrhenius equation can be then used to calculate k.
k = A –Ea/RT
Equation 2 Arrhenius Equation
This yields the following values: A = 2.0 x 106, Ea = 56 kJ/mol and k = 1.66 x 10-3 at 50˚C and k = 2.74 x 10-2 at 100˚C. The reaction seems to be first order in both reactants, as the maximum rate in conversion/hour was obtained at equal mol quantities of reactants an excess of one over the other caused a rate decrease as the excess was acting as a diluent. It is possible that the reaction is other than first order, further work is necessary. However, if a change transfer complex is found first then equal mole concentrations would be necessary for maximum rate. Having obtained an idea of the kinetics of the polymerization, we wanted to test the limits of thermal stability of the polymer. To do this we obtained TGA data as shown in Figure 6.
Ln(β/Tp2)
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Fig. 6. TGA trace for the Pamolyn 380 – MAH Copolymer
This data shows that, under N2, the polymer is stable up to 340-345˚C. This is comparable to other polymers formed by chain addition mechanisms, Gaylord, et. al. (1983). In order to aid in the formulation of the coatings, we calculated the solubility parameters for the copolymer using the approach of Hansen ( ) which is a group contribution model based on three parameters; the dispersion (o) polar (p) and hydrogen bonding (HB) interaction parameters. In order to calculate the total solubility parameter () equation three is used: 2 = d + 2p +2HB (3) The values arrived at (using the group calculations) are summarized in Table 2.
This tells us that the copolymer is polar enough to be miscible with the materials we planned to use as crosslinkers. As the data in Table 1 shows, the crosslinkers chosen did have good miscibility with
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the copolymer. The data indeed shows that the films have good to excellent solvent resistance, surprisingly high hardness given that some of the values for the forward and reverse impact-resistances are quite high. The softer and less chemically resistant films are those obtained using an epoxidized linseed oil. This is not surprising given the nature of the triglyceride oils and that the degree of epoxidation of this oil is 4-5. In order to more fully characterize the films, crosslink densities were estimated by measuring the storage modulus using the rheometer. The crosslink densities were then calculated using equation 4, Treloar (1975).
VC = E’/RT (4) Where VC = crosslinking density, E’ = elastic modulus, R = gas constant and T = temperature. The most crosslinked system was that obtained by using EPON 828, with an averaged crosslink density of 12.50 mol/liter. The cyclohexane diethanol diglycidal ether (GE-22) had the lowest value at 0.58 mol/l. All the crosslink densities and moduli are listed in Table 3. The surprising fact is how large the crosslink density is for the epoxidized linseed oil, 2.93 mol/l, given the relative softness of these films. However, as previously stated, epoxidized linseed oil has a large component of aliphatic carbons. These gave high flexibility to systems containing them. Logically, much higher crosslink densities would then be needed to significantly increase film hardness. The cure kinetics of each system were evaluated through DSC as described above. Since crosslinking proceeds virtually the same reaction mechanism in all cases, the values for k are the same, 1.0 ± .02 x 10-4. The Tg’s of the cured systems are summarized in Table 3. The Tg’s are all somewhat higher than the Tg of the copolymer (C) with the more functional or higher Tg crosslinkers yielding films having the greater cured film Tg’s. The EPLO and the EGDE are softer materials and/or the crosslink densities are insufficient for higher Tg cured films to be obtained.
Table 3 Modulei and Crosslink Densities
E′ (MPa)
E′ (Pa)
Vc
(mol/mꞈ3)
Vc
(x links/mꞈ3)
mol xl/L
EPON Cure 144.80 464.40 1.25E+04 7.53E+27 12.50
ELO 27.38 375.15 2.93E+03 1.76E+27 2.93
NPGDGE Cure 42.55 385.15 4.43E+03 2.67E+27 4.43
EGDGE 10.04 348.17 1.16E+03 6.96305E+26 1.16
GE-22 4.83 367.57 5.27E+02 3.17296E+26 0.53
Table 4 Glass Transition Temperaturea
Crosslinker Cured Film Tg (˚C)
EPON 828 85.3
EPLO 24.3
NPGDGE 32.5
EGDE 24.6
GE-22 79.5
a. Tg’s after 60 minutes at 150˚C
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The hardest (EPON 828) and softest films were selected for AFM studies to obtain an idea of what the surfaces of the films were like. (Figures 7-12) These show the surfaces and cross sections of the films. Figures 7-10 show the EPON 828 crosslinked films, while 11-14 show the EPLO crosslinked films. Figures 5-8 are the cross sections of the films also. While Figure 9 presents a surface profile. The phase images (on the right of Figure 7-14) have darker areas, which correspond to softer regions of the films, while the higher areas correspond to the harder regions of the films. But the images do not show significant phase separations confirming that the Pamolyn 380-MAH copolymer is very compatible with the chosen crosslinkers and that crosslinked films have been obtained. The date confirms that the EPLO films relative softness is not due to incompatibility, but occurs for the reasons stated above.
Figs. 7 and 8. Surface. (EPON 828) crosslinked film. Tapping AFM Height (left) and Phase (right) images at 2µm x 2µm (top) and 500 nm x 500 nm (bottom).
7)
8)
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Figs. 9 and 10. X-section. (EPON 828) Tapping AFM Height (left) and Phase (right) images at 2µm x 2µm (top) and 500 nm x 500 nm (bottom).
Score lines are artifacts of microtoming and should be ignored.
10)
9)
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Figs. 11 and 12. Surface. (ELO crosslinked film). Tapping AFM Height (left) and Phase (right) images at 2µm x 2µm (top) and 500 nm x 500 nm (bottom)
scan areas. Z-scale = 10 nm
12)
11)
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Figs. 13 and 14. X-section. Tapping AFM Height (left) and Phase (right) images at 2µm x 2µm (top) and 500 nm x 500 nm (bottom).
Score lines in 2µm images due to artifacts of microtoming and should be ignored.
14)
13)
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4. Conclusion We have synthesized a Pamolyn 380-MAH copolymer using more optimum conditions than previous workers. We have obtained high yields of relatively high molecular weight species. We characterized the polymerization more completely and discovered and reported the rate of polymerization and the heat of polymerization. All of this information can be used to design and optimize manufacturing conditions and equipment. We want to emphasize that this polymer is predominately based on bio-renewable material (82% based on carbon content). In addition, the material can be used to make films with high levels of solvent resistance, high gloss, reasonable hardnesses and flexibility; all desirable film properties. Because the polymer is soluble in other functional natural oils, materials that are zero or near zero VOC can be formulated. Also because of this miscibility, the copolymer was discovered to have good harrier properties to various fluids. These results were incomplete at the time of this writing, but will be repeated in a subsequent polymerization.
