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Phase-separation dominating mechanical properties of a novel tung-oil-based thermosetting polymer
Industrial Crops and Products 43 (2013) 677–683
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Phase-separation dominating mechanical properties of a novel tung-oil-based thermosetting polymer Chengguo Liu a,b,∗∗ , Yan Dai a , Chengshuang Wang c , Hongfeng Xie c , Yonghong Zhou a,∗ , Xiaoyu Lin a , Liyun Zhang a a Institute of Chemical Industry of Forestry Products, CAF; National Engineering Lab. for Biomass Chemical Utilization; Key and Lab. on Forest Chemical Engineering, SFA, Key Lab. of Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China b Institute of Forest New Technology, CAF, Beijing 100091, China c Key Laboratory for Mesoscopic Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e
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Article history: Received 3 March 2012 Received in revised form 18 July 2012 Accepted 20 July 2012 Keywords: Tung oil Thermosets Phase separation Structure–property relationship Reactivity ratio
a b s t r a c t This paper focuses on the structure–property relationship of a tung-oil-based thermosetting polymer, which was obtained by curing a newly developed tung-oil-based monomer (TOPERMA) with different styrene contents. Phase separation was first observed from transparency of the polymer matrixes and further studied by scanning electron microscopy (SEM). The separation resulted from the incompatibility of maleinated oil-based resins and styrene, and might be influenced by curing temperature and reactivity ratios of the reactive monomers. By dynamic mechanical analysis (DMA) storage modulus, glass transition temperature, and crosslink density of the bio-based polymer materials were investigated. The two effects of phase separation and crosslink density were used to correlate the microstructure factors with the obtained thermo-mechanical and mechanical properties of the tung-oil-based resins. It was found that the phase-separation effect was the dominating factor affecting the mechanical properties rather than other factors. The matrix of TOPERMA with 33% styrene exhibited good stiffness–toughness balance due to the minimum extent of phase separation it had. This developed eco-friendly bio-based polymer shows potential structural application as sheeting molding compounds. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polymer materials derived from natural resources have gained increased attention in recent years due to the threats of uncertain petroleum supply in the future and environmental pollution. As one of the most widely applied renewable resources, plant oils are suitable starting materials for polymers because of their low cost, the rich chemistry that their triglyceride structure provides, and their potential biodegradability (Guner et al., 2006; Meier et al., 2007). Among various plant oil-based polymers, unsaturated polyester resins (UPEs) have been successfully employed in many diverse applications such as adhesives, coatings, foams, and structural plastics – a relatively new area (Raqueza et al., 2010; Biermann et al., 2011). These thermosetting plastics are mainly prepared by two methods. One is to mix oils or derivatized oils with general-purpose UPEs. For example, Haq et al. (2009, 2011)
∗ Corresponding author. ∗∗ Corresponding author at: Institute of Chemical Industry of Forestry Products, CAF, Nanjing 210042, China. Tel.: +86 25 85482520; fax: +86 25 85482777. E-mail addresses: [email protected] (C. Liu), [email protected] (Y. Zhou). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.07.072
prepared bio-based resins by a partial substitution of UPE with epoxidized-soybean oil or -methyl linseedate, and reinforced them with natural fibers, nanoclays or layered silicates; Miyagawa et al. (2006, 2007) used UPEs containing epoxidized methyl-linseedate or -soyate to prepare novel bio-based resins; Ghorui et al. (2011) used maleated castor oil as biomodifier in UPE/fly ash composites. Another approach is to introduce polymerizable functional groups onto the triglyceride structure by using reactive sites available. Wool and co-authors have developed a broad range of chemical routes to utilize plant oils to make polymers and composite materials that can be used in structural applications (Eren et al., 2003; Lu et al., 2005; Wool and Sun, 2005; Can et al., 2006a). Both methods are widely adopted in preparing bio-based polymer materials. In our previous work, based on the second method, an unsaturated polyester-like resin was obtained by functionalizing tung oil triglyceride molecules in two basic steps: alcoholysis to produce tung oil pentaerythritol alcoholysis product and then reaction with maleic anhydride to produce tung oil pentaerythritol glyceride maleates (TOPERMA) (Liu et al., 2012). By a structural analysis of IR, 1 HNMR, and electrospray ionization-mass spectrometry (ESI-MS), it was found that adduct products had formed in the TOPERMA product due to the Diels–Alder (D–A) reaction between maleic anhydride and the tung oil conjugated triene. The product was
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Fig. 1. Chemical structures of tung oil pentaerythritol glyceride maleates (TOPERMA) mainly composed of (a) non Diels–Alder (D–A) adducts, (b) ˛-type D–A adducts and (c) ˇ-type D–A adduct.
further blended with styrene, the reactive co-monomer, and cured via a free-radical polymerization to give a crosslinked thermosetting polymer. With the optimization of reaction conditions in the above two steps, the polymer matrix with a ratio of 67:33 TOPERMA to styrene by weight got a flexural strength of 46.2 MPa, which was in the middle of the strengths of the similar soybean- and castoroil-based (SOPERMA and COPERMA) polymers (Can et al., 2006b). To study the structure–property relationship of this novel bio-based polymer, mechanical properties of the tung-oil-based polymers with different styrene contents are presented in this study. It was observed that the transparency of both the TOPERMA–styrene solutions and matrixes, which is related to phase separation, varied with different styrene concentrations. Thus, the effect of phase separation on the mechanical properties of the bio-based polymers is also investigated. Scanning electron microscopy (SEM) was employed to produce a surface morphology and dynamic mechanical analysis (DMA) was conducted to study the thermo-mechanical properties. 2. Materials and methods
2.2. Curing of the tung-oil-based resin The TOPERMA product was blended with a specified amount of styrene by stirring under nitrogen gas (N2 ) atmosphere for 1 h at 70–80 ◦ C. After that the resin was degassed by pumping for 10 min, and then mixed with the initiator at a 2% of the resin weight. At last the resin was poured into molds. The filled molds were placed in an oven at room temperature. The temperature was increased to 120 ◦ C at a rate of 5 ◦ C/min. The resin was cured at this temperature for 3 h and postcured at 150 ◦ C for 1 h. The cured samples were polished slightly to avoid a surface defect in the following mechanical tests. Four polymer samples containing different ratios of TOPERMA to styrene by weight were prepared. The ratios of TOPERMA monomer and styrene were 80:20, 70:30, 67:33, and 60:40 by weight, thus the TOPERMA–styrene polymers were designated as TOPERMA80-ST20, TOPERMA70-ST30, TOPERMA67-ST33, and TOPERMA60-ST40, respectively. All the experiment series revealed phase separation, because the reaction mixtures indicated turbidity in solution at room temperature. All the samples were cured according to the same procedure.
2.1. Materials 2.3. Characterization Tung-oil-based monomer used in this work is shown in Fig. 1. This monomer corresponded to the alcoholysis reaction of tung oil with pentaerythritol, followed by maleination of the alcoholysis product with maleic anhydride. The details of synthesis and characterization of the resin were presented and discussed in a previous publication (Liu et al., 2012). Styrene (≥98%) was obtained from Chengdu Kelong Chemical Reagent Co. Ltd (China). The initiator tert-butyl peroxy benzoate (≥98%) was obtained from Aladdin Chemistry Co. Ltd (Shanghai, China).
Tensile and flexural tests of the polymer samples were evaluated using a SANS7 CMT-4304 Universal tester (Shenzhen Xinsansi Jiliang Intrustument Co. Ltd, China). Impact tests of the samples were performed in a XJJY-5 impact tester (Chengde Xinguo Intrustument Co. Ltd, China). All the above tests followed the procedure specified in the GBT 2567-2008 Standard. At least six specimens of each polymer sample were tested and five results were selected to calculate the mean values for these mechanical
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3. Results and discussion 3.1. Phenomenon of phase separation by transparency and SEM analysis
Fig. 2. Polymer matrixes of the TOPERMA resin with (A) 20%, (B) 30%, (C) 33% and (D) 40% styrene concentration by weight for flexural test.
tests. Dumbbell specimens with a size of 50 mm × 10 mm × 4 mm at the narrow middle part were conducted for the tensile tests at a constant draw speed of 5.0 mm/min. Cuboid specimens with a size of 100 mm × 15 mm × 4 mm (Fig. 2) were performed for the flexural tests at a constant crosshead speed of 10 mm/min. Cuboid specimens with a size of 80 mm × 10 mm × 4 mm were performed for the impact tests. Dynamic mechanical analysis (DMA) (DMA+450, 01dBMetravib) was employed for dynamic mechanical analysis of the bio-based materials. The measurements were taken under tension mode from −50 to 200 ◦ C at a heating rate of 2 ◦ C/min using a frequency of 1 Hz. The morphology of the samples was detected by scanning electric microscope (SEM) (S-4800, Hitachi). The surface of the fractured samples after completion of the tensile tests was coated with a gold film prior to SEM observation.
It was mentioned that the solutions of TOPERMA with different styrene contents before curing had showed phase separation, which were also reported in the SOPERMA–styrene resins (Can et al., 2006a). Fig. 2 shows the transparency of the four polymer matrixes after curing. According to the visibility of Chinese characters under the matrix, four kinds of typical transparency were demonstrated: (A) opaque for TOPERMA80-ST20, (B) quasi-opaque for TOPERMA70-ST30, (C) transparent for TOPERMA67-ST33, (D) half-transparent for TOPERMA60-ST40, respectively. The transparency is directly related to the phase separation of polymer samples (Miyagawa et al., 2005, 2006, 2007; Haq et al., 2011). In other words, the lack of transparency is the result of the phase separation. Hence, the order of the phase-separation extent can be simply listed as (A) > (B) > (D) > (C). To understand the phase-separation mechanism in the microscale, the failure-surface morphologies of the bio-based polymer samples were observed by SEM. Fig. 3 demonstrates the failure-surface morphologies after completion of the tensile tests. It was seen that the surface became less rough as the styrene concentration increased from 20 to 40%. This may result from a growing solubilization of the maleinated glycerides in styrene as the styrene concentration increases (Can et al., 2006b). The TOPERMA80-ST20 polymer matrix showed a roughest surface with a large number of small resin pieces compared to the other three matrixes. The surfaces of TOPERMA67-ST33 and TOPERMA60-ST40 samples were generally flat and featureless, which suggested that the behavior of the bio-based materials was elastic and the crack propagated in a planar manner under the tensile loading. The resins containing 20–30% styrene seemed to have a different fracturing manner with the resins containing 33–40% styrene, according to the straight and non-straight fracture curves on their surfaces. On the other hand,
Fig. 3. SEM micrographs of the tensile fracture surfaces of the bio-based polymers with different styrene contents.
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Fig. 4. Dynamic mechanical analysis in (a) storage modulus and (b) damping parameter for the four tung-oil-based polymers.
phase separation was observed in the polymer matrixes. The second phase seemed to be present in craters and holes, which had also been observed by other researchers (Mehta et al., 2004; Miyagawa et al., 2005, 2006, 2007; Das et al., 2011). They appeared in different sizes, but had the same content (Mehta et al., 2004), which may contain TOPERMA (the maleates hardly homopolymerize (Lewis and Mayo, 1948; Ishizu and Shen, 1999; Can et al., 2006a)), polystyrene, and oil molecules. From the SEM images (a), (b) and (d), it was seen that the craters and holes in the second phase became less dense and smaller in size as the increase of styrene concentration, while SEM image (c) showed an uniform feature and a planar direction without the second phase. It indicated that the four samples had an order of the phase separation extent as (c) < (d) < (b) < (a), which agreed well with the transparency analysis. The phase separation in solution or matrix can be attributed to the incompatibility of the maleinated plant oil-based resins and styrene. It was found that the acid number of the TOPERMA was above 200 mg KOH/g (Liu et al., 2012), which is much higher than general-purpose UPEs (about 30 mg KOH/g). Thus, the maleate half esters has a much higher polar compared to styrene, leading to the incompatibility between them. Similar results for the mixtures of SOPERMA and styrene before curing were reported by Can et al. (2006a). As increasing in styrene content, the phase separation extent should decrease gradually due to a better solubilization of the malinated glycerides in styrene. However, after curing, why did the TOPERMA67-ST33 matrix show the minimum extent of phase separation? One possible effect for this is that the increase of temperature during curing will accelerate the diffusion of TOPERMA and styrene, thus improve the collision chance possibility between them. This fact was indirectly supported by that the average size of the second phase after curing (about 0.5–2 m) was smaller than that of the non-uniform droplets in the equal-styrene-content solutions before curing (Can et al., 2006a). Another effect that may result in this minimum extent of phase separation is the reactivity ratios of reactive monomers. As we know, each propagation reaction has a characteristic rate constant, Kmn , where the first subscript refers to the active center and the second refers to the monomer. Defining the propagation rate constant ratio k11 /k12 and k22 /k21 as r1 and r2 , respectively, one finally obtains (Mayo and Lewis, 1944; Ishizu and Shen, 1999) d[M1 ] [M1 ] r1 [M1 ] + [M2 ] · = [M2 ] r2 [M2 ] + [M1 ] d[M2 ]
(1)
where [M1 ]/[M2 ] and d[M1 ]/d[M2 ] denote the initial feed ratio of monomers and the mole fraction ratio of monomer units in the copolymer, respectively. In this paper styrene and TOPERMA were designated to the M1 monomer and the M2 macromonomer, respectively. When d[M1 ]/d[M2 ] = [M1 ]/[M2 ], an azeotropic copolymerization between them is founded. At this moment Eq. (1) can be written as r2 − 1 [M1 ] = r1 − 1 [M2 ]
(2)
The reactivity ratios of styrene and monoethyl maleate were reported as r1 = 0.13 and r2 = 0.035, respectively (Lewis and Mayo, 1948). If these values are utilized for the TOPERMA–styrene system, the molar ratio [M1 ]/[M2 ] is about 1.11, i.e. the weight fraction of styrene is about 32%. This value is almost equal to the styrene content in the TOPERMA67-ST33 feed. Normally, when the feed ratio of two monomers reaches the ratio for an azeotropic copolymerization, the resulting polymer has a tendency to form a homogeneous system (Hull and Kennedy, 2001; Li et al., 2011). Otherwise matrixes appear as complex heterogeneous systems composed of styrene-rich phase (hard domain) and oil-rich phase (soft domain) (Andjelkovic et al., 2005). Hence, the matrix TOPERMA67-ST33 tends to be a homogeneous crosslinked polymer, despite that the incompatibility between the two monomers will lead to phase separation. 3.2. Dynamic mechanical analysis Fig. 4(a) shows temperature dependence of the storage modulus (E ) for the TOPERMA polymers. A distinct feature for the four triglyceride-based resins was that these polymers showed a very broad transition from glassy to rubbery state. Another feature was that they showed a skew glassy region, unlike most common thermosetting polymers with a distinct glassy region in which modulus is independent on temperature. This broad glass transition behavior mainly results from three effects. First, TOPERMA is a complicated mixture containing many kinds of maleate species as shown in our previous work (Liu et al., 2012). All of these species can copolymerize with styrene, thus forming a network with complicated components. Second, the phase separation results in higher glass-transition-temperautre (Tg ) styrene-rich and lower Tg TOPERMA-rich regions in the polymer matrix. Third, the plasticizing effect of the fatty acid chains present in the TOPERMA
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Table 1 Density (d), glass transition temperature (Tg ), storage modulus (E ), crosslink density (e ), and effective molar mass between crosslinks (Mc ) of the tung-oil-based thermosetting polymers. Sample ida
db (g/cm3 )
Tg c (◦ C)
E at 25 ◦ C (MPa)
E at 180 ◦ C (MPa)
e (mol/m3 )
Mc (g/mol)
TOPERMA80-ST20 TOPERMA70-ST30 TOPERMA67-ST33 TOPERMA60-ST40
1.11 1.11 1.12 1.13
65, 107 133 121 144
765 1360 1290 1740
16.8 24.2 20.1 24.3
1483 2139 1775 2145
748 518 631 527
a b c
The thermosetting polymers were designated as TOPERMAm-STn (where m and n represent weight percents (%) of TOPERMA and styrene in the matrixes, respectively). Calculated by the ratio of weight to volume using the matrix samples for flexural tests. Determined from the tan ı curves in Fig. 4(b).
monomer may also result in the broad glass transition. As we know, the transition from the glassy to the rubbery state broadens significantly with the addition of small amounts of plasticizers to polymer (Wool and Sun, 2005). At room temperature, both the four polymers were already in transition from the glassy region to the rubbery plateau. The storage modulus values at room temperature and in the rubbery region (at 180 ◦ C) showed similar changes as the styrene concentration increased, as shown in Table 1. This may be closely related to the cross-link density. Based on the kinetic theory of rubber elasticity, the experimental crosslink density of all copolymers (e ), namely the average number of crosslinks per unit volume, can be determined from the rubbery moduli (at 180 ◦ C) using the following equation (Lu et al., 2005; Miyagawa et al., 2005; Can et al., 2006b) E = 3e RT =
3dRT Mc
(3)
where E represents the storage modulus of the crosslinked copolymer in the rubbery plateau region, R is the gas constant, T is the absolute temperature, d is the density of the polymer, and Mc is the effective molar mass between crosslinks. As shown in Table 1, the values of crosslink density for these bio-polymers were 1483–2145 mol/m3 , which corresponded to crosslink molar masses 755–522 g/mol. These crosslink-density values from 20% to 40% styrene concentration were a little higher than that of SOPERMA–styrene polymers (1050–1550 mol/m3 ), but lower than that of COPERMA–styrene polymers (1700–4500 mol/m3 ) (Can et al., 2006b). The possible reason for this lies in that the fatty acid chains in the SOPERMA monomer do not participate in polymerization, while these chains present in the COPERMA monomer are functionalized and all incorporate in polymerization; for the TOPERMA monomer, the fatty acid chains only have some conjugated trienes left which partially participate in polymerization. It was observed that the two storage modulus values increased as the crosslink densities increased (Table 1). The following question is why the crosslink density has a complex change at increasing styrene concentrations. This may result from the phase separation mentioned above, because the solutions of TOPERMA-styrene had showed phase separation before crosslinking. The second phase dispersed in the matrix may act as microscale crosslinks. Therefore, the TOPERMA70-ST30 and TOPERMA60-ST40 polymers got a higher crosslink density than the TOPERMA67-ST33 polymer. The storage modulus of TOPERMA67-ST33 below 0 ◦ C showed highest values among them may also be caused by the lack of phase separation. Fig. 4(b) shows temperature dependence of the loss factor (tan ı), in which the tan ı curves also exhibited very broad peaks. It was obvious that tan ı curves for TOPERMA80-ST20 and TOPERMA70-ST30 polymers showed two glass transitions due to phase separation. This revealed two glass transition temperatures at the tan ı peaks: one was of the TOPERMA-rich region at about 65 ◦ C; the other was of the styrene-rich region in the range of 107–144 ◦ C. This two-glass-transition phenomenon was similar to
the previous reported result (Li and Larock, 2003; Wool and Sun, 2005; Andjelkovic et al., 2005; Sharma et al., 2010). As the styrene contents increased, the lower transition peak decreased in intensity relative to the individual higher transition peak, which indicated a content decrease of TOPERMA-rich phase and a content increase of styrene-rich phase. The possible effect for this decrease can be attributed to that the increase of styrene content makes a better solubilization of TOPERMA in styrene, which will lead to the formation of a less heterogeneous system. The tan ı is a sensitive indication of crosslinking. As crosslink density increases, the tan ı maximum shifts to higher temperatures, the peak broadens and the tan ı values decrease (Can et al., 2006b). As a result, the TOPERMA67-ST33 polymer with a lower crosslink density had a higher tan ı peak than the TOPERMA70-ST30 and TOPERMA60-ST40 polymers. Although the TOPERMA80-ST20 had the lowest crosslink density, it had a lowest content of styrene-rich phase, thus the tan ı-peak value of this phase was not the largest one among them. The glass transition temperatures of these polymers are also a measure of the internal friction and correlates to the crosslink density (Fox and Loshaek, 1955). As shown in Fig. 5, the glass transition temperatures of the styrene-rich region for these new polymers exhibit a linear fit with the crosslink density, which is following the theory of Fox and Loshaek (1955) Tg = TgL + KFL · e
(4)
where TgL is the glass transition temperature for an infinite straight chain polymer and KFL is a universal constant (Lu et al., 2005). The linear fit of the experimental data gives a value of 18.5 ◦ C and 0.0479 m3 /mol for TgL and KFL , respectively. The correlation coefficient of the fitting curve is above 0.96.
Fig. 5. The glass transition temperature (Tg ) in the styrene-rich region for the four tung-oil-based polymers follows a linear fit with the crosslink density (e ).
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Fig. 6. Tensile and flexural properties in (a) strength and (b) modulus of the four tung-oil-based polymers with different styrene concentrations.
3.3. Effect of styrene concentration on mechanical properties To find out the appropriate styrene usage, it is necessary to investigate the effect of co-monomer styrene concentration on the mechanical properties of the tung-oil-based polymer. It was observed that the mechanical properties of TOPERMA polymers (Fig. 6) varied with the styrene concentration in a different manner with the changes of SOPERMA and COPERMA polymers as reported by Can et al. (2006b). For example, the flexural strength and modulus of the TOPERMA polymers had a maximum value at the styrene content 33%, while the flexural strength and modulus of both the SOPERMA and COPERMA polymers increased with the increase of styrene content. Can et al. had explained these changes using the crosslink densities of them, however, after a careful analysis, we found that the variation of crosslink density for them did not match the property change. It seemed that some other important factors affecting the mechanical properties were neglected. By the transparency and SEM analysis, we believe that the effect of phase separation is the exact most important one among them. Hence, the mechanical properties for the four resins will be explored with consideration of the effects of phase separation and crosslink density in this study. Fig. 6 shows the changes in tensile strength and modulus, flexural strength and modulus of the four resins with different styrene contents. These mechanical properties reflect stiffness of a polymer matrix. It was seen that all these properties had a similar trend: increasing with the increase of styrene content until at 33% styrene, then decreasing after this point. This trend was similar to the change of phase separation for the four resins, but did not agree with the change of crosslink density. In other words, the phase-separation seems to have a more pronounced effect on the stiffness of the polymers, rather than the crosslink density. It was known to us that the introduce of a second rubber phase like oil-rich phase improved the toughness but sacrificed the stiffness of a polymer matrix, while the increase of crosslinkdensity values showed an opposite effect (Mehta et al., 2004; Lu et al., 2005; Miyagawa et al., 2005; Can et al., 2006a,b; Haq et al., 2011). Due to the absence of phase separation, the TOPERMA67ST33 matrix got the best stiffness in the four resins, although it has a moderate crosslink density (Table 1). Hence, it was obvious that the effect of phase separation dominated the tensile and flexural properties of the four resins, rather than the effect of
crosslink density. The change in impact strength of the four resins was shown in Fig. 7. The impact strength reflects toughness of a polymer matrix. The TOPERMA70-ST30 and TOPERMA60-ST40 polymers had higher impact strengths than the TOPERMA67-ST33 polymer, although they got higher crosslink densities. This was because the rubber TOPERMA-rich phase absorbed the impact energy and thus improved the impact strength of the two polymers (Mehta et al., 2004; Miyagawa et al., 2005). The TOPERMA80-ST20 matrix had the lowest crosslink density and the highest extent of phase separation, but showed a worst toughness, which may be caused by the lowest reaction conversions (Can et al., 2006a) and the increase of plasticizing effect by too much TOPERMA content (Can et al., 2006b). All the results of the impact-strength analysis also indicated that the effect of phase separation played a more important role than the effect of crosslink density. Overall, the phase-separation effect in the TOPERMA polymer matrixes was the dominating factor affecting the mechanical properties rather than the crosslink-density effect. Besides, the TOPERMA with 33% styrene content provided a synergistic stiffness–toughness performance balance.
Fig. 7. Impact strength of the four tung-oil-based polymers with different styrene concentrations.
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4. Conclusions (1) In this study, the simple binary mixture of TOPERMA–styrene was chosen to study the effect of phase separation on the mechanical properties. The mixtures with different styrene contents were cured by a same procedure. By doing these the effect of curing procedure on the mechanical properties of the bio-based materials were eliminated effectively. Therefore, the left other effects, such as the phase separation and the crosslink density, on the mechanical properties can be analyzed easily. (2) The phase separation in the TOPERMA–styrene solution or matrix can be attributed to the incompatibility of the plantoil-based monomer and styrene, which may be affected by the curing temperature and the reactivity ratios of reactive monomers. (3) It was found that the phase-separation effect was the dominating factor affecting the mechanical properties rather than other structural factors for the TOPERMA polymer matrixes. The crosslink densities of the matrixes may also be influenced by the phase separation, because the TOPERMA–styrene resin had showed phase separation before curing. (4) The polymer matrix of TOPERMA with 33% styrene by weight showed optimal performance with balanced stiffness– toughness performance in the four polymers. This promising material from renewable resources can be a potential substitution for petroleum products when used as sheet molding compounds. (5) The structure–property analysis by combining the phaseseparation and the crosslink-density effects organically may also be referenced by analogous thermosetting polymer systems such as epoxy resins, polyester resins, phenolic resins, and so on. Acknowledgements The authors are grateful to the financial support from Chinese Academy of Forest for the special fund of fundamental research (No. CAFINT2011C02). Dr. Long Yin and Dr. Sunjie Ye from Nanjing University (China) were also appreciated for their language help. References Andjelkovic, D.D., Valverde, M., Henna, P., Li, F., Larock, R.C., 2005. Novel thermosets prepared by cationic copolymerization of various vegetable oils-synthesis and their structure–property relationships. Polymer 46, 9674–9685. Biermann, U., Bornscheuer, U., Meier, M.A.R., Metzger, J.O., Schafer, H.J., 2011. Oils and fats as renewable raw materials in chemistry. Angew. Chem. Int. Ed. 50, 3854–3871. Can, E., Wool, R.P., Küsefoglu, S., 2006a. Soybean and castor oil based monomers: synthesis and copolymerization with styrene. J. Appl. Polym. Sci. 102, 2433–2447. Can, E., Wool, R.P., Kusefoglu, S., 2006b. Soybean- and castor-oil-based thermosetting polymers: mechanical properties. J. Appl. Polym. Sci. 102, 1497–1506.
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Das, K., Ray, D., Chitrita, B., Bandyopadhyay, N.R., Mohanty, A.K., Misra, M., 2011. Novel materials from unsaturated polyester resin/styrene/tung oil blends with high impact strengths and enhanced mechnical properties. J. Appl. Polym. Sci. 119, 2174–2182. Eren, T., Kusefoglu, S.H., Wool, R.P., 2003. Polymerization of maleic anhydridemodified plant oils with polyols. J. Appl. Polym. Sci. 90, 197–202. Fox, T.G., Loshaek, S., 1955. Influence of molecular weight and degree of crosslinking on the specific volume and glass temperature of polymers. J. Polym. Sci. 15 (80), 371–390. Ghorui, S., Bandyopadhyay, N.R., Ray, D., Sengupta, S., Kar, T., 2011. Use of maleated castor oil as biomodifier in unsaturated polyester resin/fly ash composities. Ind. Crop. Prod. 34, 893–899. Guner, F.S., Yagcı, Y., Erciyes, A.T., 2006. Polymers from triglyceride oils. Prog. Polym. Sci. 31, 633–670. Haq, M., Burgueno, R., Mohanty, A.K., Misra, M., 2009. Bio-based unsaturated polyester/layered silicate nanocomposites: characterization and thermophysical properties. Compos. Part A 40, 540–547. Haq, M., Burgueno, R., Mohanty, A.K., Misra, M., 2011. Bio-based polymer nanocomposites from UPE/EML blends and nanoclay: development, experimental characterization and limits to synergistic performance. Compos. Part A 42, 41–49. Hull, D.L., Kennedy, J.P., 2001. Synthesis, characterization, and crosslinking of novel stars comprising eight poly(isobutylene–azeotropic-styrene) copolymer arms with allyl or hydroxyl termini. I. Living azeotropic copolymerization of isobutylene and styrene. J. Polym. Sci. A: Polym. Chem. 39, 1515–1524. Ishizu, K., Shen, X., 1999. Radical copolymerization reactivity of maleateterminated poly(ethylene glycol) with vinylbenzyl-terminated polystyrene macromonomers. Polymer 40, 3251–3254. Lewis, F.M., Mayo, F.R., 1948. Copolymerization. IX. A comparison of some cis and trans isomers. J. Am. Chem. Soc. 70, 1533–1536. Li, F., Larock, R.C., 2003. Synthesis, structure and properties of new tung oil–styrene–divinylbenzene copolymers prepared by thermal polymerization. Biomacromolecules 4, 1018–1025. Li, J.W., Yi, L.M., Lin, H.M., Hou, R.G., 2011. Synthesis of poly(tert-butyl methacrylate)-graft-poly(dimethylsiloxane) graft copolymers via reversible addition–fragmentation chain transfer polymerization. J. Polym. Sci. A: Polym. Chem. 49, 1483–1493. Liu, C.G., Yang, X.H., Cui, J.F., Zhou, Y.H., Hu, L.H., Zhang, M., Liu, H.J., 2012. Tung oil based monomer for thermosetting polymers: synthesis, characterization, and copolymerization with styrene. Bioresources 7 (1), 447–463. Lu, J., Khot, S., Wool, R.P., 2005. New sheet molding compound resins from soybean oil. I. Synthesis and characterization. Polymer 46 (1), 71–80. Mayo, F.R., Lewis, F.M., 1944. Copolymerization. I. A basis for comparing the behavior of monomers in copolymerization; the copolymerization of styrene and methyl methacrylate. J. Am. Chem. Soc. 66, 1594–1601. Mehta, G., Mohanty, A.K., Misra, M., Drzal, L.T., 2004. Biobased resin as a toughening agent for biocomposites. Green Chem. 6, 254–258. Meier, M.A.R., Metzgerb, J.O., Schubert, U.S., 2007. Plant oil renewable resources as green alternatives in polymer science. Chem. Soc. Rev. 36, 1788–1802. Miyagawa, H., Misra, M., Drzal, L.T., Mohanty, A.K., 2005. Fracture toughness and impact strength of anhydride-cured biobased epoxy. Polym. Eng. Sci. 45, 487–495. Miyagawa, H., Mohanty, A.K., Burgueno, R., Drzal, L.Y., Misra, M., 2006. Development of biobased unsaturated polyester containing functionalized linseed oil. Ind. Eng. Chem. Res. 45 (3), 1014–1018. Miyagawa, H., Mohanty, A.K., Burgueno, R., Drzal, L.Y., Misra, M., 2007. Novel biobased resins from blends of functionalized soybean oil and unsaturated polyester resin. J. Polym. Sci. B: Polym. Phys. 45 (6), 698–704. Raqueza, J.M., Deleglise, M., Lacrampea, M.F., Krawczak, P., 2010. Thermosetting (bio) materials derived from renewable resources: a critical review. Prog. Polym. Sci. 35, 487–509. Sharma, V., Banait, J.S., Larock, R.C., Kundu, P.P., 2010. Synthesis and characterization of linseed oil-based nanocomposites. Polym. Compos. 31, 630–637. Wool, R.P., Sun, X.S., 2005. Bio-based Polymers and Composites. Elsevier, Burlington.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Phase-separation dominating mechanical properties of a novel tung-oil-based thermosetting polymer Chengguo Liu a,b,∗∗ , Yan Dai a , Chengshuang Wang c , Hongfeng Xie c , Yonghong Zhou a,∗ , Xiaoyu Lin a , Liyun Zhang a a Institute of Chemical Industry of Forestry Products, CAF; National Engineering Lab. for Biomass Chemical Utilization; Key and Lab. on Forest Chemical Engineering, SFA, Key Lab. of Biomass Energy and Material, Jiangsu Province, Nanjing 210042, China b Institute of Forest New Technology, CAF, Beijing 100091, China c Key Laboratory for Mesoscopic Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e
i n f o
Article history: Received 3 March 2012 Received in revised form 18 July 2012 Accepted 20 July 2012 Keywords: Tung oil Thermosets Phase separation Structure–property relationship Reactivity ratio
a b s t r a c t This paper focuses on the structure–property relationship of a tung-oil-based thermosetting polymer, which was obtained by curing a newly developed tung-oil-based monomer (TOPERMA) with different styrene contents. Phase separation was first observed from transparency of the polymer matrixes and further studied by scanning electron microscopy (SEM). The separation resulted from the incompatibility of maleinated oil-based resins and styrene, and might be influenced by curing temperature and reactivity ratios of the reactive monomers. By dynamic mechanical analysis (DMA) storage modulus, glass transition temperature, and crosslink density of the bio-based polymer materials were investigated. The two effects of phase separation and crosslink density were used to correlate the microstructure factors with the obtained thermo-mechanical and mechanical properties of the tung-oil-based resins. It was found that the phase-separation effect was the dominating factor affecting the mechanical properties rather than other factors. The matrix of TOPERMA with 33% styrene exhibited good stiffness–toughness balance due to the minimum extent of phase separation it had. This developed eco-friendly bio-based polymer shows potential structural application as sheeting molding compounds. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polymer materials derived from natural resources have gained increased attention in recent years due to the threats of uncertain petroleum supply in the future and environmental pollution. As one of the most widely applied renewable resources, plant oils are suitable starting materials for polymers because of their low cost, the rich chemistry that their triglyceride structure provides, and their potential biodegradability (Guner et al., 2006; Meier et al., 2007). Among various plant oil-based polymers, unsaturated polyester resins (UPEs) have been successfully employed in many diverse applications such as adhesives, coatings, foams, and structural plastics – a relatively new area (Raqueza et al., 2010; Biermann et al., 2011). These thermosetting plastics are mainly prepared by two methods. One is to mix oils or derivatized oils with general-purpose UPEs. For example, Haq et al. (2009, 2011)
∗ Corresponding author. ∗∗ Corresponding author at: Institute of Chemical Industry of Forestry Products, CAF, Nanjing 210042, China. Tel.: +86 25 85482520; fax: +86 25 85482777. E-mail addresses: [email protected] (C. Liu), [email protected] (Y. Zhou). 0926-6690/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2012.07.072
prepared bio-based resins by a partial substitution of UPE with epoxidized-soybean oil or -methyl linseedate, and reinforced them with natural fibers, nanoclays or layered silicates; Miyagawa et al. (2006, 2007) used UPEs containing epoxidized methyl-linseedate or -soyate to prepare novel bio-based resins; Ghorui et al. (2011) used maleated castor oil as biomodifier in UPE/fly ash composites. Another approach is to introduce polymerizable functional groups onto the triglyceride structure by using reactive sites available. Wool and co-authors have developed a broad range of chemical routes to utilize plant oils to make polymers and composite materials that can be used in structural applications (Eren et al., 2003; Lu et al., 2005; Wool and Sun, 2005; Can et al., 2006a). Both methods are widely adopted in preparing bio-based polymer materials. In our previous work, based on the second method, an unsaturated polyester-like resin was obtained by functionalizing tung oil triglyceride molecules in two basic steps: alcoholysis to produce tung oil pentaerythritol alcoholysis product and then reaction with maleic anhydride to produce tung oil pentaerythritol glyceride maleates (TOPERMA) (Liu et al., 2012). By a structural analysis of IR, 1 HNMR, and electrospray ionization-mass spectrometry (ESI-MS), it was found that adduct products had formed in the TOPERMA product due to the Diels–Alder (D–A) reaction between maleic anhydride and the tung oil conjugated triene. The product was
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Fig. 1. Chemical structures of tung oil pentaerythritol glyceride maleates (TOPERMA) mainly composed of (a) non Diels–Alder (D–A) adducts, (b) ˛-type D–A adducts and (c) ˇ-type D–A adduct.
further blended with styrene, the reactive co-monomer, and cured via a free-radical polymerization to give a crosslinked thermosetting polymer. With the optimization of reaction conditions in the above two steps, the polymer matrix with a ratio of 67:33 TOPERMA to styrene by weight got a flexural strength of 46.2 MPa, which was in the middle of the strengths of the similar soybean- and castoroil-based (SOPERMA and COPERMA) polymers (Can et al., 2006b). To study the structure–property relationship of this novel bio-based polymer, mechanical properties of the tung-oil-based polymers with different styrene contents are presented in this study. It was observed that the transparency of both the TOPERMA–styrene solutions and matrixes, which is related to phase separation, varied with different styrene concentrations. Thus, the effect of phase separation on the mechanical properties of the bio-based polymers is also investigated. Scanning electron microscopy (SEM) was employed to produce a surface morphology and dynamic mechanical analysis (DMA) was conducted to study the thermo-mechanical properties. 2. Materials and methods
2.2. Curing of the tung-oil-based resin The TOPERMA product was blended with a specified amount of styrene by stirring under nitrogen gas (N2 ) atmosphere for 1 h at 70–80 ◦ C. After that the resin was degassed by pumping for 10 min, and then mixed with the initiator at a 2% of the resin weight. At last the resin was poured into molds. The filled molds were placed in an oven at room temperature. The temperature was increased to 120 ◦ C at a rate of 5 ◦ C/min. The resin was cured at this temperature for 3 h and postcured at 150 ◦ C for 1 h. The cured samples were polished slightly to avoid a surface defect in the following mechanical tests. Four polymer samples containing different ratios of TOPERMA to styrene by weight were prepared. The ratios of TOPERMA monomer and styrene were 80:20, 70:30, 67:33, and 60:40 by weight, thus the TOPERMA–styrene polymers were designated as TOPERMA80-ST20, TOPERMA70-ST30, TOPERMA67-ST33, and TOPERMA60-ST40, respectively. All the experiment series revealed phase separation, because the reaction mixtures indicated turbidity in solution at room temperature. All the samples were cured according to the same procedure.
2.1. Materials 2.3. Characterization Tung-oil-based monomer used in this work is shown in Fig. 1. This monomer corresponded to the alcoholysis reaction of tung oil with pentaerythritol, followed by maleination of the alcoholysis product with maleic anhydride. The details of synthesis and characterization of the resin were presented and discussed in a previous publication (Liu et al., 2012). Styrene (≥98%) was obtained from Chengdu Kelong Chemical Reagent Co. Ltd (China). The initiator tert-butyl peroxy benzoate (≥98%) was obtained from Aladdin Chemistry Co. Ltd (Shanghai, China).
Tensile and flexural tests of the polymer samples were evaluated using a SANS7 CMT-4304 Universal tester (Shenzhen Xinsansi Jiliang Intrustument Co. Ltd, China). Impact tests of the samples were performed in a XJJY-5 impact tester (Chengde Xinguo Intrustument Co. Ltd, China). All the above tests followed the procedure specified in the GBT 2567-2008 Standard. At least six specimens of each polymer sample were tested and five results were selected to calculate the mean values for these mechanical
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3. Results and discussion 3.1. Phenomenon of phase separation by transparency and SEM analysis
Fig. 2. Polymer matrixes of the TOPERMA resin with (A) 20%, (B) 30%, (C) 33% and (D) 40% styrene concentration by weight for flexural test.
tests. Dumbbell specimens with a size of 50 mm × 10 mm × 4 mm at the narrow middle part were conducted for the tensile tests at a constant draw speed of 5.0 mm/min. Cuboid specimens with a size of 100 mm × 15 mm × 4 mm (Fig. 2) were performed for the flexural tests at a constant crosshead speed of 10 mm/min. Cuboid specimens with a size of 80 mm × 10 mm × 4 mm were performed for the impact tests. Dynamic mechanical analysis (DMA) (DMA+450, 01dBMetravib) was employed for dynamic mechanical analysis of the bio-based materials. The measurements were taken under tension mode from −50 to 200 ◦ C at a heating rate of 2 ◦ C/min using a frequency of 1 Hz. The morphology of the samples was detected by scanning electric microscope (SEM) (S-4800, Hitachi). The surface of the fractured samples after completion of the tensile tests was coated with a gold film prior to SEM observation.
It was mentioned that the solutions of TOPERMA with different styrene contents before curing had showed phase separation, which were also reported in the SOPERMA–styrene resins (Can et al., 2006a). Fig. 2 shows the transparency of the four polymer matrixes after curing. According to the visibility of Chinese characters under the matrix, four kinds of typical transparency were demonstrated: (A) opaque for TOPERMA80-ST20, (B) quasi-opaque for TOPERMA70-ST30, (C) transparent for TOPERMA67-ST33, (D) half-transparent for TOPERMA60-ST40, respectively. The transparency is directly related to the phase separation of polymer samples (Miyagawa et al., 2005, 2006, 2007; Haq et al., 2011). In other words, the lack of transparency is the result of the phase separation. Hence, the order of the phase-separation extent can be simply listed as (A) > (B) > (D) > (C). To understand the phase-separation mechanism in the microscale, the failure-surface morphologies of the bio-based polymer samples were observed by SEM. Fig. 3 demonstrates the failure-surface morphologies after completion of the tensile tests. It was seen that the surface became less rough as the styrene concentration increased from 20 to 40%. This may result from a growing solubilization of the maleinated glycerides in styrene as the styrene concentration increases (Can et al., 2006b). The TOPERMA80-ST20 polymer matrix showed a roughest surface with a large number of small resin pieces compared to the other three matrixes. The surfaces of TOPERMA67-ST33 and TOPERMA60-ST40 samples were generally flat and featureless, which suggested that the behavior of the bio-based materials was elastic and the crack propagated in a planar manner under the tensile loading. The resins containing 20–30% styrene seemed to have a different fracturing manner with the resins containing 33–40% styrene, according to the straight and non-straight fracture curves on their surfaces. On the other hand,
Fig. 3. SEM micrographs of the tensile fracture surfaces of the bio-based polymers with different styrene contents.
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Fig. 4. Dynamic mechanical analysis in (a) storage modulus and (b) damping parameter for the four tung-oil-based polymers.
phase separation was observed in the polymer matrixes. The second phase seemed to be present in craters and holes, which had also been observed by other researchers (Mehta et al., 2004; Miyagawa et al., 2005, 2006, 2007; Das et al., 2011). They appeared in different sizes, but had the same content (Mehta et al., 2004), which may contain TOPERMA (the maleates hardly homopolymerize (Lewis and Mayo, 1948; Ishizu and Shen, 1999; Can et al., 2006a)), polystyrene, and oil molecules. From the SEM images (a), (b) and (d), it was seen that the craters and holes in the second phase became less dense and smaller in size as the increase of styrene concentration, while SEM image (c) showed an uniform feature and a planar direction without the second phase. It indicated that the four samples had an order of the phase separation extent as (c) < (d) < (b) < (a), which agreed well with the transparency analysis. The phase separation in solution or matrix can be attributed to the incompatibility of the maleinated plant oil-based resins and styrene. It was found that the acid number of the TOPERMA was above 200 mg KOH/g (Liu et al., 2012), which is much higher than general-purpose UPEs (about 30 mg KOH/g). Thus, the maleate half esters has a much higher polar compared to styrene, leading to the incompatibility between them. Similar results for the mixtures of SOPERMA and styrene before curing were reported by Can et al. (2006a). As increasing in styrene content, the phase separation extent should decrease gradually due to a better solubilization of the malinated glycerides in styrene. However, after curing, why did the TOPERMA67-ST33 matrix show the minimum extent of phase separation? One possible effect for this is that the increase of temperature during curing will accelerate the diffusion of TOPERMA and styrene, thus improve the collision chance possibility between them. This fact was indirectly supported by that the average size of the second phase after curing (about 0.5–2 m) was smaller than that of the non-uniform droplets in the equal-styrene-content solutions before curing (Can et al., 2006a). Another effect that may result in this minimum extent of phase separation is the reactivity ratios of reactive monomers. As we know, each propagation reaction has a characteristic rate constant, Kmn , where the first subscript refers to the active center and the second refers to the monomer. Defining the propagation rate constant ratio k11 /k12 and k22 /k21 as r1 and r2 , respectively, one finally obtains (Mayo and Lewis, 1944; Ishizu and Shen, 1999) d[M1 ] [M1 ] r1 [M1 ] + [M2 ] · = [M2 ] r2 [M2 ] + [M1 ] d[M2 ]
(1)
where [M1 ]/[M2 ] and d[M1 ]/d[M2 ] denote the initial feed ratio of monomers and the mole fraction ratio of monomer units in the copolymer, respectively. In this paper styrene and TOPERMA were designated to the M1 monomer and the M2 macromonomer, respectively. When d[M1 ]/d[M2 ] = [M1 ]/[M2 ], an azeotropic copolymerization between them is founded. At this moment Eq. (1) can be written as r2 − 1 [M1 ] = r1 − 1 [M2 ]
(2)
The reactivity ratios of styrene and monoethyl maleate were reported as r1 = 0.13 and r2 = 0.035, respectively (Lewis and Mayo, 1948). If these values are utilized for the TOPERMA–styrene system, the molar ratio [M1 ]/[M2 ] is about 1.11, i.e. the weight fraction of styrene is about 32%. This value is almost equal to the styrene content in the TOPERMA67-ST33 feed. Normally, when the feed ratio of two monomers reaches the ratio for an azeotropic copolymerization, the resulting polymer has a tendency to form a homogeneous system (Hull and Kennedy, 2001; Li et al., 2011). Otherwise matrixes appear as complex heterogeneous systems composed of styrene-rich phase (hard domain) and oil-rich phase (soft domain) (Andjelkovic et al., 2005). Hence, the matrix TOPERMA67-ST33 tends to be a homogeneous crosslinked polymer, despite that the incompatibility between the two monomers will lead to phase separation. 3.2. Dynamic mechanical analysis Fig. 4(a) shows temperature dependence of the storage modulus (E ) for the TOPERMA polymers. A distinct feature for the four triglyceride-based resins was that these polymers showed a very broad transition from glassy to rubbery state. Another feature was that they showed a skew glassy region, unlike most common thermosetting polymers with a distinct glassy region in which modulus is independent on temperature. This broad glass transition behavior mainly results from three effects. First, TOPERMA is a complicated mixture containing many kinds of maleate species as shown in our previous work (Liu et al., 2012). All of these species can copolymerize with styrene, thus forming a network with complicated components. Second, the phase separation results in higher glass-transition-temperautre (Tg ) styrene-rich and lower Tg TOPERMA-rich regions in the polymer matrix. Third, the plasticizing effect of the fatty acid chains present in the TOPERMA
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Table 1 Density (d), glass transition temperature (Tg ), storage modulus (E ), crosslink density (e ), and effective molar mass between crosslinks (Mc ) of the tung-oil-based thermosetting polymers. Sample ida
db (g/cm3 )
Tg c (◦ C)
E at 25 ◦ C (MPa)
E at 180 ◦ C (MPa)
e (mol/m3 )
Mc (g/mol)
TOPERMA80-ST20 TOPERMA70-ST30 TOPERMA67-ST33 TOPERMA60-ST40
1.11 1.11 1.12 1.13
65, 107 133 121 144
765 1360 1290 1740
16.8 24.2 20.1 24.3
1483 2139 1775 2145
748 518 631 527
a b c
The thermosetting polymers were designated as TOPERMAm-STn (where m and n represent weight percents (%) of TOPERMA and styrene in the matrixes, respectively). Calculated by the ratio of weight to volume using the matrix samples for flexural tests. Determined from the tan ı curves in Fig. 4(b).
monomer may also result in the broad glass transition. As we know, the transition from the glassy to the rubbery state broadens significantly with the addition of small amounts of plasticizers to polymer (Wool and Sun, 2005). At room temperature, both the four polymers were already in transition from the glassy region to the rubbery plateau. The storage modulus values at room temperature and in the rubbery region (at 180 ◦ C) showed similar changes as the styrene concentration increased, as shown in Table 1. This may be closely related to the cross-link density. Based on the kinetic theory of rubber elasticity, the experimental crosslink density of all copolymers (e ), namely the average number of crosslinks per unit volume, can be determined from the rubbery moduli (at 180 ◦ C) using the following equation (Lu et al., 2005; Miyagawa et al., 2005; Can et al., 2006b) E = 3e RT =
3dRT Mc
(3)
where E represents the storage modulus of the crosslinked copolymer in the rubbery plateau region, R is the gas constant, T is the absolute temperature, d is the density of the polymer, and Mc is the effective molar mass between crosslinks. As shown in Table 1, the values of crosslink density for these bio-polymers were 1483–2145 mol/m3 , which corresponded to crosslink molar masses 755–522 g/mol. These crosslink-density values from 20% to 40% styrene concentration were a little higher than that of SOPERMA–styrene polymers (1050–1550 mol/m3 ), but lower than that of COPERMA–styrene polymers (1700–4500 mol/m3 ) (Can et al., 2006b). The possible reason for this lies in that the fatty acid chains in the SOPERMA monomer do not participate in polymerization, while these chains present in the COPERMA monomer are functionalized and all incorporate in polymerization; for the TOPERMA monomer, the fatty acid chains only have some conjugated trienes left which partially participate in polymerization. It was observed that the two storage modulus values increased as the crosslink densities increased (Table 1). The following question is why the crosslink density has a complex change at increasing styrene concentrations. This may result from the phase separation mentioned above, because the solutions of TOPERMA-styrene had showed phase separation before crosslinking. The second phase dispersed in the matrix may act as microscale crosslinks. Therefore, the TOPERMA70-ST30 and TOPERMA60-ST40 polymers got a higher crosslink density than the TOPERMA67-ST33 polymer. The storage modulus of TOPERMA67-ST33 below 0 ◦ C showed highest values among them may also be caused by the lack of phase separation. Fig. 4(b) shows temperature dependence of the loss factor (tan ı), in which the tan ı curves also exhibited very broad peaks. It was obvious that tan ı curves for TOPERMA80-ST20 and TOPERMA70-ST30 polymers showed two glass transitions due to phase separation. This revealed two glass transition temperatures at the tan ı peaks: one was of the TOPERMA-rich region at about 65 ◦ C; the other was of the styrene-rich region in the range of 107–144 ◦ C. This two-glass-transition phenomenon was similar to
the previous reported result (Li and Larock, 2003; Wool and Sun, 2005; Andjelkovic et al., 2005; Sharma et al., 2010). As the styrene contents increased, the lower transition peak decreased in intensity relative to the individual higher transition peak, which indicated a content decrease of TOPERMA-rich phase and a content increase of styrene-rich phase. The possible effect for this decrease can be attributed to that the increase of styrene content makes a better solubilization of TOPERMA in styrene, which will lead to the formation of a less heterogeneous system. The tan ı is a sensitive indication of crosslinking. As crosslink density increases, the tan ı maximum shifts to higher temperatures, the peak broadens and the tan ı values decrease (Can et al., 2006b). As a result, the TOPERMA67-ST33 polymer with a lower crosslink density had a higher tan ı peak than the TOPERMA70-ST30 and TOPERMA60-ST40 polymers. Although the TOPERMA80-ST20 had the lowest crosslink density, it had a lowest content of styrene-rich phase, thus the tan ı-peak value of this phase was not the largest one among them. The glass transition temperatures of these polymers are also a measure of the internal friction and correlates to the crosslink density (Fox and Loshaek, 1955). As shown in Fig. 5, the glass transition temperatures of the styrene-rich region for these new polymers exhibit a linear fit with the crosslink density, which is following the theory of Fox and Loshaek (1955) Tg = TgL + KFL · e
(4)
where TgL is the glass transition temperature for an infinite straight chain polymer and KFL is a universal constant (Lu et al., 2005). The linear fit of the experimental data gives a value of 18.5 ◦ C and 0.0479 m3 /mol for TgL and KFL , respectively. The correlation coefficient of the fitting curve is above 0.96.
Fig. 5. The glass transition temperature (Tg ) in the styrene-rich region for the four tung-oil-based polymers follows a linear fit with the crosslink density (e ).
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Fig. 6. Tensile and flexural properties in (a) strength and (b) modulus of the four tung-oil-based polymers with different styrene concentrations.
3.3. Effect of styrene concentration on mechanical properties To find out the appropriate styrene usage, it is necessary to investigate the effect of co-monomer styrene concentration on the mechanical properties of the tung-oil-based polymer. It was observed that the mechanical properties of TOPERMA polymers (Fig. 6) varied with the styrene concentration in a different manner with the changes of SOPERMA and COPERMA polymers as reported by Can et al. (2006b). For example, the flexural strength and modulus of the TOPERMA polymers had a maximum value at the styrene content 33%, while the flexural strength and modulus of both the SOPERMA and COPERMA polymers increased with the increase of styrene content. Can et al. had explained these changes using the crosslink densities of them, however, after a careful analysis, we found that the variation of crosslink density for them did not match the property change. It seemed that some other important factors affecting the mechanical properties were neglected. By the transparency and SEM analysis, we believe that the effect of phase separation is the exact most important one among them. Hence, the mechanical properties for the four resins will be explored with consideration of the effects of phase separation and crosslink density in this study. Fig. 6 shows the changes in tensile strength and modulus, flexural strength and modulus of the four resins with different styrene contents. These mechanical properties reflect stiffness of a polymer matrix. It was seen that all these properties had a similar trend: increasing with the increase of styrene content until at 33% styrene, then decreasing after this point. This trend was similar to the change of phase separation for the four resins, but did not agree with the change of crosslink density. In other words, the phase-separation seems to have a more pronounced effect on the stiffness of the polymers, rather than the crosslink density. It was known to us that the introduce of a second rubber phase like oil-rich phase improved the toughness but sacrificed the stiffness of a polymer matrix, while the increase of crosslinkdensity values showed an opposite effect (Mehta et al., 2004; Lu et al., 2005; Miyagawa et al., 2005; Can et al., 2006a,b; Haq et al., 2011). Due to the absence of phase separation, the TOPERMA67ST33 matrix got the best stiffness in the four resins, although it has a moderate crosslink density (Table 1). Hence, it was obvious that the effect of phase separation dominated the tensile and flexural properties of the four resins, rather than the effect of
crosslink density. The change in impact strength of the four resins was shown in Fig. 7. The impact strength reflects toughness of a polymer matrix. The TOPERMA70-ST30 and TOPERMA60-ST40 polymers had higher impact strengths than the TOPERMA67-ST33 polymer, although they got higher crosslink densities. This was because the rubber TOPERMA-rich phase absorbed the impact energy and thus improved the impact strength of the two polymers (Mehta et al., 2004; Miyagawa et al., 2005). The TOPERMA80-ST20 matrix had the lowest crosslink density and the highest extent of phase separation, but showed a worst toughness, which may be caused by the lowest reaction conversions (Can et al., 2006a) and the increase of plasticizing effect by too much TOPERMA content (Can et al., 2006b). All the results of the impact-strength analysis also indicated that the effect of phase separation played a more important role than the effect of crosslink density. Overall, the phase-separation effect in the TOPERMA polymer matrixes was the dominating factor affecting the mechanical properties rather than the crosslink-density effect. Besides, the TOPERMA with 33% styrene content provided a synergistic stiffness–toughness performance balance.
Fig. 7. Impact strength of the four tung-oil-based polymers with different styrene concentrations.
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4. Conclusions (1) In this study, the simple binary mixture of TOPERMA–styrene was chosen to study the effect of phase separation on the mechanical properties. The mixtures with different styrene contents were cured by a same procedure. By doing these the effect of curing procedure on the mechanical properties of the bio-based materials were eliminated effectively. Therefore, the left other effects, such as the phase separation and the crosslink density, on the mechanical properties can be analyzed easily. (2) The phase separation in the TOPERMA–styrene solution or matrix can be attributed to the incompatibility of the plantoil-based monomer and styrene, which may be affected by the curing temperature and the reactivity ratios of reactive monomers. (3) It was found that the phase-separation effect was the dominating factor affecting the mechanical properties rather than other structural factors for the TOPERMA polymer matrixes. The crosslink densities of the matrixes may also be influenced by the phase separation, because the TOPERMA–styrene resin had showed phase separation before curing. (4) The polymer matrix of TOPERMA with 33% styrene by weight showed optimal performance with balanced stiffness– toughness performance in the four polymers. This promising material from renewable resources can be a potential substitution for petroleum products when used as sheet molding compounds. (5) The structure–property analysis by combining the phaseseparation and the crosslink-density effects organically may also be referenced by analogous thermosetting polymer systems such as epoxy resins, polyester resins, phenolic resins, and so on. Acknowledgements The authors are grateful to the financial support from Chinese Academy of Forest for the special fund of fundamental research (No. CAFINT2011C02). Dr. Long Yin and Dr. Sunjie Ye from Nanjing University (China) were also appreciated for their language help. References Andjelkovic, D.D., Valverde, M., Henna, P., Li, F., Larock, R.C., 2005. Novel thermosets prepared by cationic copolymerization of various vegetable oils-synthesis and their structure–property relationships. Polymer 46, 9674–9685. Biermann, U., Bornscheuer, U., Meier, M.A.R., Metzger, J.O., Schafer, H.J., 2011. Oils and fats as renewable raw materials in chemistry. Angew. Chem. Int. Ed. 50, 3854–3871. Can, E., Wool, R.P., Küsefoglu, S., 2006a. Soybean and castor oil based monomers: synthesis and copolymerization with styrene. J. Appl. Polym. Sci. 102, 2433–2447. Can, E., Wool, R.P., Kusefoglu, S., 2006b. Soybean- and castor-oil-based thermosetting polymers: mechanical properties. J. Appl. Polym. Sci. 102, 1497–1506.
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Das, K., Ray, D., Chitrita, B., Bandyopadhyay, N.R., Mohanty, A.K., Misra, M., 2011. Novel materials from unsaturated polyester resin/styrene/tung oil blends with high impact strengths and enhanced mechnical properties. J. Appl. Polym. Sci. 119, 2174–2182. Eren, T., Kusefoglu, S.H., Wool, R.P., 2003. Polymerization of maleic anhydridemodified plant oils with polyols. J. Appl. Polym. Sci. 90, 197–202. Fox, T.G., Loshaek, S., 1955. Influence of molecular weight and degree of crosslinking on the specific volume and glass temperature of polymers. J. Polym. Sci. 15 (80), 371–390. Ghorui, S., Bandyopadhyay, N.R., Ray, D., Sengupta, S., Kar, T., 2011. Use of maleated castor oil as biomodifier in unsaturated polyester resin/fly ash composities. Ind. Crop. Prod. 34, 893–899. Guner, F.S., Yagcı, Y., Erciyes, A.T., 2006. Polymers from triglyceride oils. Prog. Polym. Sci. 31, 633–670. Haq, M., Burgueno, R., Mohanty, A.K., Misra, M., 2009. Bio-based unsaturated polyester/layered silicate nanocomposites: characterization and thermophysical properties. Compos. Part A 40, 540–547. Haq, M., Burgueno, R., Mohanty, A.K., Misra, M., 2011. Bio-based polymer nanocomposites from UPE/EML blends and nanoclay: development, experimental characterization and limits to synergistic performance. Compos. Part A 42, 41–49. Hull, D.L., Kennedy, J.P., 2001. Synthesis, characterization, and crosslinking of novel stars comprising eight poly(isobutylene–azeotropic-styrene) copolymer arms with allyl or hydroxyl termini. I. Living azeotropic copolymerization of isobutylene and styrene. J. Polym. Sci. A: Polym. Chem. 39, 1515–1524. Ishizu, K., Shen, X., 1999. Radical copolymerization reactivity of maleateterminated poly(ethylene glycol) with vinylbenzyl-terminated polystyrene macromonomers. Polymer 40, 3251–3254. Lewis, F.M., Mayo, F.R., 1948. Copolymerization. IX. A comparison of some cis and trans isomers. J. Am. Chem. Soc. 70, 1533–1536. Li, F., Larock, R.C., 2003. Synthesis, structure and properties of new tung oil–styrene–divinylbenzene copolymers prepared by thermal polymerization. Biomacromolecules 4, 1018–1025. Li, J.W., Yi, L.M., Lin, H.M., Hou, R.G., 2011. Synthesis of poly(tert-butyl methacrylate)-graft-poly(dimethylsiloxane) graft copolymers via reversible addition–fragmentation chain transfer polymerization. J. Polym. Sci. A: Polym. Chem. 49, 1483–1493. Liu, C.G., Yang, X.H., Cui, J.F., Zhou, Y.H., Hu, L.H., Zhang, M., Liu, H.J., 2012. Tung oil based monomer for thermosetting polymers: synthesis, characterization, and copolymerization with styrene. Bioresources 7 (1), 447–463. Lu, J., Khot, S., Wool, R.P., 2005. New sheet molding compound resins from soybean oil. I. Synthesis and characterization. Polymer 46 (1), 71–80. Mayo, F.R., Lewis, F.M., 1944. Copolymerization. I. A basis for comparing the behavior of monomers in copolymerization; the copolymerization of styrene and methyl methacrylate. J. Am. Chem. Soc. 66, 1594–1601. Mehta, G., Mohanty, A.K., Misra, M., Drzal, L.T., 2004. Biobased resin as a toughening agent for biocomposites. Green Chem. 6, 254–258. Meier, M.A.R., Metzgerb, J.O., Schubert, U.S., 2007. Plant oil renewable resources as green alternatives in polymer science. Chem. Soc. Rev. 36, 1788–1802. Miyagawa, H., Misra, M., Drzal, L.T., Mohanty, A.K., 2005. Fracture toughness and impact strength of anhydride-cured biobased epoxy. Polym. Eng. Sci. 45, 487–495. Miyagawa, H., Mohanty, A.K., Burgueno, R., Drzal, L.Y., Misra, M., 2006. Development of biobased unsaturated polyester containing functionalized linseed oil. Ind. Eng. Chem. Res. 45 (3), 1014–1018. Miyagawa, H., Mohanty, A.K., Burgueno, R., Drzal, L.Y., Misra, M., 2007. Novel biobased resins from blends of functionalized soybean oil and unsaturated polyester resin. J. Polym. Sci. B: Polym. Phys. 45 (6), 698–704. Raqueza, J.M., Deleglise, M., Lacrampea, M.F., Krawczak, P., 2010. Thermosetting (bio) materials derived from renewable resources: a critical review. Prog. Polym. Sci. 35, 487–509. Sharma, V., Banait, J.S., Larock, R.C., Kundu, P.P., 2010. Synthesis and characterization of linseed oil-based nanocomposites. Polym. Compos. 31, 630–637. Wool, R.P., Sun, X.S., 2005. Bio-based Polymers and Composites. Elsevier, Burlington.