Novel Conjugated Linseed OilStyreneDivinylbenzene Copolymers

Biomacromolecules 2005, 6, 797-806

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Novel Conjugated Linseed Oil-Styrene-Divinylbenzene Copolymers Prepared by Thermal Polymerization. 1. Effect of Monomer Concentration on the Structure and Properties Patit P. Kundu† and Richard C. Larock* Contribution from the Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received September 13, 2004; Revised Manuscript Received December 21, 2004

A variety of novel opaque, white polymers ranging from rubbery materials to tough and rigid plastics have been prepared by the thermal polymerization at 85-160 °C of varying amounts of 87% conjugated linseed oil, styrene, and divinylbenzene. Gelation of the reactants typically occurs at temperatures higher than 120 °C, and fully cured thermosets are obtained after postcuring at 160 °C. The fully cured thermosets have been determined by Soxhlet extraction to contain approximately 35-85% cross-linked materials. The microcomposition of these polymers, as determined by 1H NMR spectroscopy, indicates that the crosslinked materials are composed of both soft oily and hard aromatic phases. After solvent extraction, the insoluble materials exhibit nanopores well distributed throughout the polymer matrixes. Dynamic mechanical analysis of these polymers indicates that they are phase separated with a soft rubbery phase having a sharp glass transition temperature of -50 °C and a hard brittle plastic phase with a broadened glass transition temperature of 70-120 °C. These polymers possess cross-link densities of 0.15-2.41 × 104 mol/m3, compressive Young’s moduli of 12-438 MPa, and compressive strengths of 2-27 MPa. These materials are thermally stable below 350 °C and exhibit a major thermal degradation of 72-90% at 493-500 °C. Introduction Linseed oil, a typical fatty acid ester triglyceride, is composed of about 53% linolenic acid, 18% oleic acid, 15% linoleic acid, 6% palmitic acid, and 6% stearic acid.1 Linseed oil is traditionally used as a drying oil for surface coating applications. To make it a superior drying oil in terms of film properties, different olefinic monomers, such as styrene, have been copolymerized with linseed oil.2-5 In 1955, a German patent6 described the copolymerization of about 50 wt % linseed oil with styrene in the presence of 1% di-tertbutyl peroxide. Waterman et al.7 studied the copolymerization of linseed oil and styrene in the gaseous phase. In 1961, a British patent8 reported the copolymerization of 50% linseed oil with styrene in the presence of traces of iodine at 260 °C under a N2 atmosphere. Without iodine, styrene homopolymerizes to a turbid product. Fedeli et al.9 reported that the thermal polymerization of linseed oil at 260 °C afforded various oligomers. Recently, it has been reported10 that the reaction of styrene with drying oils depends on the concentration of the conjugated double bonds present in the drying oil. Although drying oils have been mainly used as surface coatings, Larock et al. have successfully prepared useful bioplastics from tung oil by thermal copolymerization,11 and from soybean,12,13 tung,14 and corn15 oils by cationic copolymerization with varying amounts of styrene and divinylbenzene. The present work aims at developing novel bio-

plastics by the thermal copolymerization of linseed oil, styrene, and divinylbenzene. Experimental Section Materials. The linseed oil used in this study was 87% conjugated (Alnor Oil Company, Inc., Valley Stream, NY). The highly conjugated linseed oil was chosen for its higher reactivity toward thermal polymerization over normal linseed oil. Styrene (ST) and divinylbenzene (DVB) (80 mol % DVB and 20 mol % ethylvinylbenzene) purchased from Aldrich Chemical Co. (Milwaukee, MI) were used as received. Polymer Preparation and Nomenclature. The polymeric materials have been prepared by heating the desired mixture of conjugated linseed oil, ST, and DVB in a glass vial. To get a uniform mixture, the vial was shaken vigorously before heating. The reaction mixture was then heated for a given time at the appropriate temperatures, usually 2 h at 80 °C, followed by 24 h at 120 °C and then 24 h at 160 °C. The yields of the resulting bulk polymers are essentially quantitative. The nomenclature adopted in this work for the polymer samples is based on the original composition of the reactants. For example, C87LIN50-ST20-DVB30 represents a polymer prepared from 50 wt % linseed oil (87% conjugated), 20 wt % ST, and 30 wt % DVB. Since the amount of ethylvinylbenzene present in the DVB is minimal, it is omitted from the nomenclature to avoid confusion. Characterization

* To whom correspondence should be addressed. [email protected]. † On leave from SLIET, Longowal 148106 India.

E-mail:

Soxhlet Extraction. Soxhlet extraction has been used to help characterize the structures of the linseed oil bulk

10.1021/bm049429z CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005

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polymers. A 3.0 g sample of the bulk polymer was extracted for 24 h with 100 mL of refluxing methylene chloride using a Soxhlet extractor. After extraction, the resulting solution was concentrated by rotary evaporation and subsequent vacuum-drying. The insoluble solid was dried under vacuum for several hours before weighing. 1 H Nuclear Magnetic Resonance (1H NMR). The extracted substances, as well as the conjugated linseed oil, ST, and DVB, were dissolved in CDCl3 containing a small amount of tetramethylsilane (TMS). The solution was scanned with a Varian Unity NMR spectrometer at 400 MHz. A total of 32 scans were averaged to obtain the final data. Dynamic Mechanical Analysis (DMA). The dynamic mechanical properties of the bulk polymers were conducted by using a Perkin-Elmer dynamic mechanical analyzer DMA Pyris-7e in a three-point bending mode with a 100 mN static force and a 110 mN dynamic force. A rectangular specimen was prepared by machining the cylindrical product (obtained from heating in a vial) to specimens of 2 mm thickness and 5 mm depth, and the span-to-depth ratio was maintained at approximately 4. Each specimen was first cooled under liquid nitrogen to ca. -120 °C and then heated at 3 °C/min and a frequency of 1 Hz under helium. The viscoelastic properties, i.e., storage modulus E′ and mechanical loss factor (damping) tan δ, were recorded as a function of temperature. The glass transition temperature Tg of the polymer was obtained from the peak of the loss tangent plot. The cross-link densities were determined from the rubbery modulus plateau based on the theory of rubber elasticity16,17 E′ ) 3νeRT

(1)

where E′ is the storage modulus (Young’s) of cross-linked polymer in the plateau region, R is the universal gas constant (8.314 J mol-1 K-1), and T is the absolute temperature (K). Since the three point bending mode was used for all dynamic measurements, the storage modulus obtained is a flexural modulus, which is related to Young’s modulus as follows:18,19 E′ ) 12D′(1 - υ2)/h3

(2)

where D′ is the flexural storage modulus per unit width, υ is Poisson’s ratio, and h is the thickness of the specimen. For simplicity and ease of comparison with the previously published results,11 eq 1 has been used to calculate the crosslink density, assuming that E′ and D′ are equal. Compressive Mechanical Testing. The compressive mechanical tests have been performed according to the ASTM-D695M-91 specification using an Instron universal testing machine (model-4502) at a cross-head speed of 1 mm/min. Cylindrical specimens of 10 mm diameter and 21 mm height were used for testing. Five specimens were tested for each sample. The moduli of elasticity (Young’s modulus, E) in compression were obtained from the initial slope of the stress-strain curves. The compressive strengths correspond to the stress at which the cylindrical specimen breaks. The ultimate compressive strength (σb) and compres-

sion at break (b) of the polymers were obtained from the break point of the samples in the compressive tests. The toughness of the polymer, which is the fracture energy per unit volume of the specimen, was obtained from the area under the corresponding compressive stress-strain curve. Scanning Electron Microscopy (SEM). The polymeric surfaces were mounted on a graphite sample holder and were sputter coated under vacuum with gold. The coated samples were examined using a JEOL 840A scanning electron microscope at a 20 kV accelerating voltage and ∼0.5 nA of beam current. The working distance between the microscope and the sample was 39 mm; 0.005 nA of the beam current was used for imaging, which was digitized at 1024 × 1024 pixels using the IXRF system. Point averaging has been used to reduce noise. An IXRF X-ray analyzer with quantitative X-ray and digital imaging capabilities fitted with an energy dissipative X-ray detector [EDX of Kevex Quantum (tm) Si(Li) light-element X-ray detector] was used at a 40° takeoff angle for elemental mapping. Approximately 2000 X-ray counts per second were used during a ∼60 s acquisition time; ∼145 eV resolutions at 5.9 keV were used for 200 × 200 pixels during X-ray mapping. Live Spectral Mapping collects data very quickly in a frame-averaging mode. An entire field of view is presented within seconds and repeatedly updated until the map is stopped manually or the preset value has been reached. Thermogravimetric Analysis (TGA). A Perkin-Elmer thermogravimetric analyzer (model Pyris 7e) was used to test thermal stability of the samples. The change in weight loss of the samples was measured by heating in air (20 mL/min) from 30 to 650 °C at the programmed rate of 20 °C/min. Results & Discussion Thermal Polymerization of Conjugated Linseed Oil, ST, and DVB. Nine samples (S1-S9) have been prepared by the thermal polymerization of 87% conjugated linseed oil, ST, and DVB in various proportions as reported in Table 1. It has been reported10 that nonconjugated oils during their heating with styrene generally act as transfer agents for polymerization of styrene leading to the formation of homopolymers. On the other hand, conjugated oils are capable of thermal copolymerization with styrene and hence devoid of phase separation between the triglyceride oil and the aromatic phase. This is the reason for choosing a highly (87%) conjugated linseed oil. The thermal polymerization of natural triglyceride oils, such as heat-bodied soybean oil and linseed oil, apparently proceeds by a Diels-Alder reaction with the formation of intermolecular bonds at 230 °C and subsequent cross-linking at about 300 °C.20 The thermal polymerization of ST has been investigated since the first half of the last century. The most-widely accepted mechanism of initiation in the thermal polymerization of ST is outlined in Scheme 1. The transient Diels-Alder dimer 1 reacts with ST to produce the initial radicals 2 and 3, which are responsible for initiation of the thermal polymerization of ST.21,22 It is reported that the thermal polymerization of ST occurs at temperatures as low

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Copolymers Prepared by Thermal Polymerization

Table 1. Compositions of Crosslinked Conjugated Linseed Oil-ST-DVB Copolymers from Soxhlet Extraction and 1H NMR Spectroscopic Results soluble extractible compositionsa

Soxhlet results

sample no.

polymer

wt % of oil

wt % of ST+DVB

solubleb (wt %)

insolublec (wt %)

S1 S2 S3 S4 S5 S6 S7 S8 S9

C87LIN30-ST28-DVB42 C87LIN40-ST24-DVB36 C87LIN50-ST20-DVB30 C87LIN60-ST12-DVB28 C87LIN70-ST08-DVB22 C87LIN50-ST00-DVB50 C87LIN50-ST10-DVB40 C87LIN50-ST20-DVB30 C87LIN50-ST30-DVB20

96 97 95 95 97 97 95 95 95

4 3 5 5 3 3 5 5 5

22 (21+1) 30 (29+1) 34 (32+2) 53 (50+3) 65 (63+2) 15 (14.6+0.4) 18 (17+1) 34 (32+2) 37 (35+2)

78 (9+69) 70 (11+59) 66 (18+48) 47 (10+37) 35 (7+28) 85 (35.4+49.6) 82 (33+49) 66 (18+48) 63 (15+48)

a Microcomposition of the extracted soluble materials calculated from the 1H NMR integrals of the glyceride CH peak at 4.2 ppm and aryl CH peak at 2 7 ppm. b The data in the parentheses have been calculated directly from the wt % of oil and percent aromatics in the soluble extract. The first number c in the parentheses is the percent oil content and the second number is the wt % of aromatic materials. The data in the parentheses have been calculated indirectly from the wt % of oil and aromatics in the soluble extract as the total mass of the insoluble and soluble portions is held constant. The first number in the parentheses is the percent oil content and the second number is the wt % of aromatic materials.

Scheme 1. Mechanism of Initiation in the Thermal Polymerization of Styrene

as 100 °C. Although DVB also readily undergoes thermal polymerization under conditions similar to those of ST, the mechanism of its thermal polymerization has apparently not been studied or reported. It is observed that the conjugated (87%) linseed oil employed in this study is oligomerized on heating and becomes more viscous at 160 °C and above. No cross-linked polymers have been identified at 160 °C, but on standing for several days at room temperature, this oil forms an upper solid surface. This indicates that the conjugated linseed oil polymerizes in the presence of oxygen. Thus, the formation of a cross-linked homopolymer from conjugated linseed oil is possible depending on the conditions employed, namely the thickness of the surface layer, the temperature and the time of reaction with surface oxygen, etc. To get a viable thermoplastic of considerable strength within some stipulated time, it is required that one mix linseed oil with some more thermally active reactants, like ST and DVB. The activation temperature for the thermal polymerization of ST or DVB is much lower than that of linseed oil. Both ST and DVB undergo thermal polymerization at 120 °C, resulting in hard rigid plastics. Based on the above findings, the mixture of conjugated linseed oil, ST, and DVB has been heated at 80 °C for 1 h, and then 120 °C for 24 h, followed by 160 °C for 24 h. During the heating at 120 °C, all of the reactant mixtures (S1-S9 of Table 1) solidified within 2 h of heating. Thus, a reasonable mechanism for initiation may involve formation of the initial radicals 2 and 3 from the ST. These radicals subsequently attack the CdC bonds of ST, DVB, and the conjugated linseed oil. It has been reported

that radicals generated from ST or any other radical generators have no impact on the triglyceride oil if it is not sufficiently conjugated.10 After postcuring at 160 °C, almost all of the starting materials have been converted into crosslinked polymers. It is expected that radical 3 generated as shown in Scheme 1 would form relatively stable radicals by adding to either DVB or the conjugated linseed oil and, thus, form two polymeric phases, one consisting predominantly of a copolymer of ST and DVB and the other phase being the conjugated linseed oil-ST copolymer. When only DVB is added to the linseed oil (S6), the DVB presumably serves as the initiator in much the same fashion as the ST shown in Scheme 1. In that case, there is a strong possibility of forming predominantly a homopolymer of DVB with pendant vinylic double bonds and an insignificant amount of conjugated linseed oil-DVB copolymer. However, during prolonged heating, the ST-DVB copolymer or DVB homopolymer chains get cross-linked at the pendant vinylic double bonds and the rest of the pendant vinylic double bonds react with the many unreacted double bonds in the conjugated linseed oil, leading to the formation of a cross-linked polymer having two distinctly separated soft and hard phases. The soft phase is comprised mainly of conjugated linseed oil copolymer, whereas the hard phase consists primarily of ST-DVB copolymer or DVB homopolymer. It has been reported previously11 that the conjugated triene in tung oil forms only one phase during its copolymerization with ST and DVB. Since the presently employed linseed oil is not fully conjugated (87%) and consists of only a diene, not a triene, the above proposed two microphases make sense and this is confirmed later as noted in the discussion of the dynamic mechanical properties of the samples. There is a strong possibility of seeing phase separation (macroheterogeneity) between these two phases leading to some fault or defect like cracks in the bulk and surface when relatively few dangling vinylic double bonds from the ST-DVB copolymer are available for grafting with the double bonds of the fatty acid chains of the conjugated linseed oil. This condition prevails when very low amounts of DVB (less than 20%) are present in the mixture. Thus, the sample S9, containing the lowest amount of DVB (20%), exhibits no visible phase separation (copolymer of ST and DVB).

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Figure 1.

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NMR spectra of ST, DVB, conjugated linseed oil, and extract from S1 (C87LIN30-ST28-DVB42).

Apparently, the aromatic comonomers (ST and DVB) play a dual role in the reaction of the 87% conjugated linseed oil systems: (1) lowering the initiation temperature for the thermal polymerization of the conjugated linseed oil and (2) stiffening and strengthening the cross-linked polymer chains and thus making the resulting fully cured polymers more viable as structural materials. To determine the appropriate stoichiometries for the formation of viable bioplastics, first a 50:50 composition of conjugated linseed oil and ST were heated. This resulted in a bulk opaque rubbery material with cracks and several white solid beads appeared above the bulk solid. White solids could be seen in the cracked areas of the bulk material. When 5% DVB was added (the total concentration of ST plus DVB remains constant at 50%), cracks in the bulk material were reduced. When 10% DVB was employed, the bulk product still had lots of cracks with solid white particles appearing above the bulk material. Cracks in the bulk material and on the surface totally disappeared, but small amounts of a white solid still separated out in the upper portion of the vial, when the DVB concentration reached 20%. A material with no cracks or separated solids was obtained at 30% or more DVB content. As noted earlier, there is a strong possibility for agglomerating soft and hard phases forming clusters among themselves, leading to macrophase separation, when the DVB concentration is not sufficiently high. The macrophase separation induces cracks in the product along with separation of white cross-linked ST-DVB copolymer. With 30% DVB content (S3 of Table 1), both soft and hard phases are still present in the product, but the formation of an oil grafted ST-DVB copolymer prevents the formation of clusters. Thus, macroheterogeneity reduces to microheterogeneity,

which is indicated by the opaque product (the microheterogeneity is confirmed later by dynamic mechanical analysis). Based upon the above results, different concentrations of conjugated linseed oil were added to a 2:3 mixture of ST and DVB. It is observed that for a conjugated linseed oil concentration of 30-50% (S1-S3), the samples are free of cracks or separation of any white solid. But when the conjugated linseed oil concentration is greater than 50%, cracks are observed along with separation of a white solid. The greater overall DVB content results in the formation of higher cross-linked copolymers of conjugated linseed oil and further reduces the possibility of macrophase separation. For conjugated linseed oil concentrations of greater than 50%, the same 2:3 ratio of ST and DVB cannot be employed as it leads to a weak product, full of cracks with separation of visible white solids. Thus, a higher content of DVB is required in samples S4 and S5 as reported in Table 1. Microstructure of Conjugated Linseed Oil-ST-DVB Polymer Soxhlet Extraction. Soxhlet extraction of the polymer samples has been used to determine the percent insoluble cross-linked material. The Soxhlet extraction results are shown in Table 1. On increasing the conjugated linseed oil content from 30 to 70% (S1-S5), the insoluble materials obtained after extraction decrease from 78% to 35%, whereas the soluble extract increases from 22% to 65%. These results indicate that there is a decrease in the cross-link density of the polymers obtained on increasing the conjugated linseed oil content in the polymer and the decrease is more substantial beyond 50 wt. % of oil. With a decrease in the DVB


concentration in the samples S6-S9, a decrease in insoluble materials and an increase in the soluble extracts are observed. With substitution of 10% of the DVB by ST (S6 and S7), the reduction in insoluble materials and hence cross-link density is minimal, but as more DVB is substituted by ST (S8 and S9), the reduction is more substantial. 1 H NMR Spectroscopic Characterization. The 1H NMR spectra for ST, DVB, conjugated linseed oil, and the soluble extract from S1 (C87LIN30-ST28-DVB42) are shown in Figure 1. The soluble extracts from the other samples (S2-S9) are very similar to those of S1 and thus only S1 is shown. The peak at 2.8 ppm is ascribed to the methylene (CH2) protons between the two CdC double bonds of linseed oil, indicating that there is approximately 13% of regular linseed oil present in the conjugated linseed oil. The presence of a similar peak in the divinylbenzene used is indicative of the existence of the methylene protons in ethylvinylbenzene, which is present to the extent of about 20% in the DVB mixture. The vinylic protons of conjugated linseed oil, styrene, and DVB are evident at 5.2-6.9 ppm. For the 1H NMR spectra of the soluble extracts, the vinylic protons of the aromatic and conjugated linseed oil components overlap partially. The peak present at 4.1-4.4 ppm in both S1 (and S2-S9, not shown) and the conjugated linseed oil is attributed to the methylene protons (CH2) of the glyceride unit. This is a characteristic peak for the conjugated linseed oil. The small peak at 7.26 ppm in all of the 1H NMR spectra comes from the solvent, CDCl3. The solvent peak overlaps with the aromatic protons (at 7.1-7.8 ppm) of styrene, DVB, and oligomeric versions of these materials from the extraction of samples S1-S9. Thus, the aromatic proton peaks are distinctive for styrene, DVB, and copolymers of these materials (provided one eliminates the solvent peak). From the peak area under the aromatic protons and the glyceride methylene protons, one can calculate the molar ratio of the aromatic and oil components present in the soluble extracts of samples S1-S9. The weight % of the oil and the aromatic components in the soluble extracts as calculated from the 1 H NMR peak areas are reported in Table 1. Once the wt % of the oil and aromatic components is known, one can easily calculate the wt % of the aromatic and oil components in the insoluble fractions corresponding to samples S1-S9, since the total of the insoluble and soluble fractions is constant. From Table 1, the wt % of the conjugated linseed oil in the soluble extract varies from 97 to 95, whereas the wt % of the aromatic component varies from 3 to 5. The values in parentheses from the Soxhlet results of Table 1 indicate the detailed microcomposition of the polymers (S1-S9). The soluble component plasticizes the cross-linked insoluble component. So, it is the insoluble composition which will mainly determine the properties of the polymer specimens. As the conjugated linseed oil content increases from 30% to 70% (S1-S5), the aromatic component of the insoluble materials decreases, whereas the oil component increases up to 50% conjugated linseed oil (S3), beyond which it decreases. With a decrease in DVB content from 50% to 20% (S6-S9), the oil components of the insoluble fractions show a substantial decrease. This supports our hypothesis that the conjugated linseed oil is grafted to the polymer chains of


Figure 2. SEM micrographs of the surfaces of S1 after solvent extraction (C87LIN30-ST28-DVB42) and S3 (C87LIN50-ST20DVB30).

the DVB homopolymer (for S6) or ST-DVB copolymer during prolonged heating. In considering the soluble extract, a decrease in the DVB content brings about an increase in both the aromatic and oil components, although the increase in the aromatic content is quite minimal. Scanning Electron Microscopy of the Insoluble Extract. It is important to know the morphological behavior of the samples after the soluble liquid extraction. The SEM micrographs of all of the extracted samples (S1-S9) are very similar and show that these materials are porous in nature and the pores are evenly distributed throughout the whole matrix. Two representative micrographs for the insoluble portions of the samples S1 and S3 are shown in Figure 2. It is observed that the pores are very small in size (approx 50 nm). With a reduction in the aromatic content, the material S3 becomes more porous than S1.

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Figure 3. (A) EDS spectrum of S5 (C87LIN60-ST12-DVB28) compared to that of S1 (C87LIN30-ST28-DVB42) before solvent extraction. (B) EDS spectra of S5 (C87LIN60-ST12-DVB28) before and after solvent extraction.

From the energy-dispersive spectra (EDS) presented in Figure 3, the distribution of elemental oxygen is compared before and after extraction of the samples with 30% (S1) and 70% (S5) conjugated linseed oil. Before extraction (upper spectra, 3A), the difference in the intensity of the elemental oxygen of S1 and S5 indicates that S5 has a much higher linseed oil content than S1, which is in fact expected from their composition (30% vs 70%). The intensity of the oxygen is significantly reduced after the extraction (lower spectra, 3B), indicating the presence of less of the oily phase in the extracted sample. Characterization of the Conjugated Linseed Oil-ST-DVB Polymers Dynamic Mechanical Analysis. Figure 4 shows the variation of tan δ and the storage modulus with temperature

with a variation in the conjugated linseed oil concentration in the conjugated linseed-ST-DVB polymers. The glass transition temperatures obtained from the tan δ peaks and the cross-link densities calculated from the plateau storage moduli (eq 1) are listed in Table 2. From the tan δ plots of Figure 4, the presence of two peaks or a hump in all of the samples indicates that there is clearly phase separation between the soft oily phase and the hard aromatic phase. The peak or hump due to the oil appears around -50 °C, indicating its soft and rubbery character, whereas the peak or hump, which appears around 75 °C, corresponds to a sample with a hard aromatic phase. The low-temperature peak is sharpest for the 70 wt % conjugated linseed oil polymer (S5) and the sharpness of the peak decreases with decreasing conjugated linseed oil content. For the 30 wt % conjugated linseed oil polymer (S1), the low-temperature peak reduces to a hump. The sharpness of the high-



Figure 4. Variation of tan δ and flexural storage modulus with temperature for S1-S5.

temperature peak for all of these samples exhibits a trend similar to that of the low-temperature peak. As the aromatic content in the samples increases (S5-S1), both the low and high-temperature peaks broaden, indicating decreasing amounts of the unreacted oil (soluble extract) and increasing amounts of the hard aromatic phase in the insoluble fractions (see the amounts of soluble and insoluble materials reported in Table 1). With an increase in the conjugated linseed oil content in these samples (S1-S5), the percent of unreacted oil (as determined by the soluble content reported in Table 1) increases, allowing the materials to plasticize and the cross-linked polymer to soften, causing a sharpening of the tan δ peaks. In considering the storage modulus plots of these polymers, it is observed that the storage modulus at the high temperature (>50 °C) is higher for the samples containing higher amounts of aromatic materials and it decreases with the decrease in aromatic content. This is clearly due to the higher

aromatic content in the insoluble portion (see S1-S5 of Table 1), since the storage modulus is a manifestation of the crosslink density and hence the stiffness. The decrease in crosslink density as reported in Table 2 (calculated from the storage modulus, Equation 1) with increase in the conjugated linseed oil content also corroborates the insoluble content reported in Table 1. Since the aromatic component is relatively stiff, it is expected that the same increase in storage modulus with increase in aromatic content of the samples at high temperature (>50 °C) should be observed for the low temperature (less than -100 °C) modulus of the samples. However, contrary to this expectation, S3 with 50 wt % conjugated linseed oil shows the highest modulus, followed by S4, S1, S5, and S2. Below the glass transition temperature of the soft phase, the whole system (soft and hard phases) becomes brittle and hard and thus shows a higher modulus. From the microcomposition of the insoluble material (Table 1), it is found that S3 has the highest content (18%) of crosslinked oily phase, followed by S2 (11%), S4 and S1 (9%), and S5 (7%). Thus, it is not surprising that S3 exhibits the highest modulus. The low-temperature storage modulus for the other samples is not explainable from the microcomposition. The variations of tan δ and storage modulus with temperature for differing amounts of DVB are shown in Figure 5. The glass transition temperatures and cross-link densities of the samples are listed in Table 2. The tan δ plots exhibit two peaks, one at around -50 °C and another at about 120 °C, corresponding to the respective glass transition temperatures of the soft oily phase and the harder aromatic phase. The peaks are the sharpest for the lowest amount of DVB (20%) and broaden with an increase in the DVB content of the polymers. For S6 and S7 (DVB 50% and 40%), the broadening effect is so dominant that the high-temperature peaks disappear and generate humps. Since an increase in the DVB content affords an increase in insoluble materials (Table 1), it is expected that the storage modulus values will follow the same pattern at high temperatures (>150 °C). However, contrary to expectations, S3 (30% DVB) shows a higher modulus than S2. The crosslink densities for the samples S6-S9, as calculated from the plateau modulus, show the same trend (Table 2) as those of the storage modulus. The reason is not clear at this time. The storage modulus at low temperature (less than -100

Table 2. Dynamic Mechanical, Mechanical, and Thermal Properties of Crosslinked Linseed Oil-ST-DVB Copolymers

sample no. (polymer composition) S1(C87LIN30-ST28-DVB42) S2(C87LIN40-ST24-DVB36) S3(C87LIN50-ST20-DVB30)b S4(C87LIN60-ST12-DVB28) S5(C87LIN70-ST08-DVB22) S6(C87LIN50- ST00-DVB50) S7(C87LIN50-ST10-DVB40) S8(C87LIN50-ST20-DVB30)b S9(C87LIN50-ST30-DVB20) a

dynamic mechanical results νe Tg (°C) (104 mol/m3) hump & 120 -50&72 -49&74 -49& 76 -49 &77 -49 & hump -50 & hump -49 &74 -50 &120

2.41 1.16 0.94 0.50 0.15 1.32 0.87 0.94 0.37

TGA results, °C (wt. % loss)a

T1

T2

compr Str (MPa)

503 (81) 491 (85) 493 (88) 493 (88) 493 (88) 484 (72) 505 (84) 493 (88) 490 (90)

637 (16) 615 (13) 597 (10) 597 (10) 597 (10) 604 (26) 642 (15) 597 (10) 604 (9)

26.6 16.4 13.3 5.6 2.0 12.2 9.3 13.3 11.02

tensile mechanical properties under compression compr at break yield str Young’s modulus (%) (MPa) (MPa) 8.2 9.7 14.9 9.8 16.0 8.4 6.8 14.9 23.56

21.5 16.4 9.9 5.6 0.2 12.2 9.3 9.9 7.39

438.1 237.1 121.8 58.1 11.7 157.6 145.5 121.8 64.5

Along with stage 1 and stage 2 degradation, the rest of the mass is evaporated in the initial stage. b S3 and S8 are the same compounds.

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Figure 6. Variation of stress with strain in compression for S1-S5.

Figure 5. Variation of tan δ and flexural storage modulus with temperature for S6-S9.

°C) shows a clear trend. The polymer with the lowest insoluble content (S9) has the highest modulus. There is a decrease in the modulus as the insoluble content increases (see S6-S9 of Table 1). During the discussion of the variation of conjugated linseed oil content, it was mentioned that both the soft and hard phases are responsible for the storage modulus at low temperature. The soft phase consists of the insoluble oily phase (cross-linked) and a soluble oligomeric phase. From Table 1, it is observed that the soluble oil component is the highest for S9, followed by S8, S7, and S6. As both the soft cross-linked and non-cross-linked oily phases become brittle below the glass transition temperature, there is a resulting increase in storage modulus. Thus, a decrease in the storage modulus with an increase in the DVB content at low temperature (less than -100 °C) is reasonable. Compressive Mechanical Testing. Figure 6 shows the variation of stress with strain in compression for different concentrations of conjugated linseed oil (S1-S5). The ultimate properties (compressive strength and compression at break), yield stress and Young’s modulus, are listed in Table 2. Young’s modulus, which is a manifestation of the stiffness of the material, is measured from the initial slope of the stress-strain plot. From Figure 6, the steep nature of the plot for the sample with 30 wt % conjugated linseed oil indicates that the Young’s modulus is relatively high. The steepness and hence Young’s modulus decreases with an increase in conjugated linseed oil content (S1-S5 of Table 2). The yield point is defined as a distinct maximum or the region of strong curvature approaching zero slope in the stress-strain plot.23 It is observed that the polymers with

Figure 7. Variation of stress with strain in compression for S6-S9.

the higher aromatic content have higher yield stress (first yield), which decreases with a decrease in the aromatic content. The compressive strength (the stress at failure) follows the same trend as that of the yield stress with variation of the conjugated linseed oil content. The compression at break is low for samples with high aromatic content and it increases with an increase in the conjugated linseed oil content of the polymer, except for S4 (60 wt % conjugated linseed oil). The toughness of the material is the area under the stressstrain curve. The area under the curve is proportional to the integral of the force over the distance the polymer stretches or is compressed before breaking area ∝

∫F(L) dL

This integral is the work (energy) required to break the sample. The toughness is a measure of the energy a sample can absorb before it breaks. From the area under the plots, it is found that the polymers with higher aromatic content are stiffer and tougher, whereas polymers with more than 60 wt % conjugated linseed oil (S4 and S5) are neither stiff nor tough.



Figure 9. Thermogravimetric weight loss with temperature for S1-S5.

Figure 8. SEM micrographs of mechanically fractured surfaces of S1 (C87LIN30-ST28-DVB42) and S5 (C87LIN60-ST12-DVB28).

The variations of stress with strain in compression for polymer samples with differing DVB content (S6-S9) are shown in Figure 7, and all of the mechanical properties (compressive strength and compression at break, yield stress and Young’s modulus) are listed in Table 2. A higher DVB content results in higher insoluble content (Table 1) and hence a higher cross-link density. This increases the stiffness of the material. This is evident from the higher yield stress for the sample with 50% DVB (S6). The yield stress decreases with a decrease in the DVB content, except for S8 (30% DVB). This is probably due to the increase in the insoluble fraction in this sample (Table 1). The compressive strength is the highest for S8, followed by S9, S6, and S7. The highly plasticized (higher content of soluble and insoluble oily phase from Table 1) samples S8 and S9 can elongate more and thus can withstand higher force at failure.

Morphology of Fractured Surfaces. The morphologies of the mechanically fractured surfaces of S1 and S5 as seen under a scanning electron microscope (SEM) are shown in Figure 8. The samples have been selected so as to compare samples with lower and higher amounts of conjugated linseed oil (S1 and S5). With lower conjugated linseed oil content (S1), the sample is brittle. Due to its brittle nature, no definite failure direction is observed and the fracture surfaces are deep. With higher conjugated linseed oil content (S5), the sample is rubbery in character, and cracks are propagated in the lateral direction of the force leading to a definite ridge pattern. Thermogravimetric Analysis. The variation of weight loss (%) with temperature for samples with different conjugated linseed oil concentrations (S1-S5) is plotted in Figure 9, and the results are tabulated in Table 2. All of the samples are pretty stable at 100 °C, beyond which low molecular weight soluble components start to evaporate. The major degradation (first stage) occurs at around 350 °C, and the degradation process results in char formation at around 500 °C with weight loss of 81-86%. It is observed that the amount of char is higher for the highly cross-linked material S1 and decreases with increasing conjugated linseed oil content. At the second stage of degradation, the char oxidizes to volatiles (carbon dioxide and water vapor), and all the samples completely burn off at 650 °C. The variations of weight loss with temperature for samples with different concentrations of DVB (S6-S9) are plotted in Figure 10 and the results are listed in Table 2. Two stage degradations starting at 350 and 500 °C, similar to those for S1-S5, are observed. The sample with 50% DVB (S6) is highly cross-linked and thus affords more char formation after completion of the first stage degradation. The char content decreases with a decrease in DVB concentration (S7-S9). The second stage degradation process is completed at 650 °C with total burn off. Conclusions A variety of novel polymers are synthesized by the thermal polymerization of 87% conjugated linseed oil, ST, and DVB.

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350 °C and exhibit a major thermal degradation with a maximum degradation rate of 72-90% at 493-500 °C. Acknowledgment. This project was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, Grant No. 200335504-12874. P.P.K. is thankful to Dr. Perminus Mungara of the Department of Food Science and Human Nutrition for extending help during testing of the mechanical and thermal properties. Discussions with Mr. Warren E. Straszheim of the Materials Research Laboratory regarding SEM results are also gratefully acknowledged. References and Notes

Figure 10. Thermogravimetric weight loss with temperature for S6-S9.

The thermal polymerization is performed in the temperature range of 85-160 °C with variations in the concentrations of conjugated linseed oil, ST, and DVB. Gelation of the reactants starts at 120 °C, and fully cured thermosets are obtained after postcuring at 160 °C. By solvent extraction, it is observed that the resulting polymers contain approximately 35-85% cross-linked materials. The microcomposition of these polymers obtained by 1H NMR spectroscopic analysis of the soluble materials indicates that the cross-linked materials are composed of both soft oily and hard aromatic phases. The SEM morphology of these polymers after their extraction is porous at the nanolevel, indicating well distributed soluble materials throughout these polymer matrixes. The new bulk polymeric materials are white in color and opaque in character. The dynamic mechanical analyses of these polymers indicate that they are phase separated as two separate glass transition temperatures appear; there is a soft rubbery phase with a sharp glass transition temperature of -50 °C and a hard brittle plastic phase with a broadened glass transition temperature of 70-120 °C. These polymers possess cross-link densities of 0.15-2.41 × 104 mol/m3, compressive Young’s modulus of 12-438 MPa, and compressive strengths of 2-27 MPa. Thus, different types of polymers ranging from soft rubbers to hard plastics have been prepared from conjugated linseed oil, ST, and DVB. These polymers are thermally stable below

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BM049429Z

Novel Conjugated Linseed OilStyreneDivinylbenzene Copolymers

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