Sequential Interpenetrating Polymer Networks Produced from

Biomacromolecules 2008, 9, 2221–2229

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Sequential Interpenetrating Polymer Networks Produced from Vegetable Oil Based Polyurethane and Poly(methyl methacrylate) Xiaohua Kong and Suresh S. Narine* Alberta Lipid Utilization Program, Department of Agricultural Food and Nutritional Science, 4-10 Agriculture/Forestry Centre, University of Alberta, Edmonton, Alberta, T6G 2P5 Received March 31, 2008; Revised Manuscript Received May 26, 2008

Sequential interpenetrating polymer networks (IPNs) were prepared using polyurethane produced from a canola oil based polyol with primary terminal functional groups and poly(methyl methacrylate) (PMMA). The properties of the material were studied and compared to the IPNs made from commercial castor oil using dynamic mechanical analysis, differential scanning calorimetry, as well as tensile measurements. The morphology of the IPNs was investigated using scanning electron microscopy and transmission electron microscopy. The chemical diversity of the starting materials allowed the evaluation of the effects of dangling chains and graftings on the properties of the IPNs. The polymerization process of canola oil based IPNs was accelerated because of the utilization of polyol with primary functional groups, which efficiently lessened the effect of dangling chains and yielded a higher degree of phase mixing. The mechanical properties of canola oil based IPNs containing more than 75 wt % PMMA were comparable to the corresponding castor oil based IPNs; both were superior to those of the constituent polymers due to the finely divided rubber and plastic combination structures in these IPNs. However, when PMMA content was less than 65 wt %, canola oil based IPNs exhibited a typical mechanical behavior of rigid plastics, whereas castor oil based IPNs showed a typical mechanical behavior of soft rubber. It is proposed that these new IPN materials with high performance prepared from alternative renewable resources can prove to be valuable substitutes for existing materials in various applications.

Introduction Interpenetrating polymer networks (IPNs) are a kind of polymer system prepared by polymerizing a monomer and its cross-linker in the presence of another cross-linked polymer network with different composition. Such polymer systems can be prepared sequentially, in which one network is formed and swollen in the second set of monomer reagent, or they can be prepared by reacting all of the constituents simultaneously.1,2 IPNs typically consist of flexible elastomers and one or more rigid, high modulus component. From a morphological point of view, all IPNs could be divided into ideal, partly interpenetrating, and phase-separated. An ideal IPN is a system with a molecular level of mixing of constituent networks. Particularly, it is impossible to obtain such a system due to the thermodynamic incompatibility of the constituent components. Similar to many other polymer-polymer systems, IPNs have a tendency to show phase separation due to the low entropy of mixing. However, the extent of phase separation is restricted because interpenetration plays a significant role in enhancing the intermixing of the polymer components through a physical interlocking. IPNs were first prepared and studied in some detail by Millar in 1960.3 Since then, significantly worldwide research has been devoted to this field, which has undoubtedly proven the economic, scientific and commercial importance of IPNs. Among them, the main contribution was made by these teams, namely those of Sperling,4–10 Frisch and Klempner,11–16 Hourston,17–23 and Lipatov.24–26 They established that the detailed morphology and the mechanical properties of the * To whom correspondence should be addressed. Tel.: 1-780-492-9081. Fax: 1-780-492-7174. E-mail: [email protected].

materials obtained depend on reaction conditions, reaction kinetics and phase behavior. However, most of the polymer couples used to prepare IPNs are derived from petroleum. Current worldwide interests in the replacement of petrochemical derivatives with renewable material in the production of valuable polymers are quite significant from both social and environmental viewpoints. There is an urgent need to reduce carbon footprints, shorten life cycles, and to find alternative starting materials to synthesize new monomers and then produce polymers with comparable properties. Vegetable oils, possessing a triglyceride structure with unsaturated fatty acid chains, have been considered as a readily available candidate for the production of biobased polymeric materials. Polymers produced from vegetable oils and their derivatives have excellent chemical and physical properties, including enhanced hydrolytic and thermal stability.27–33 Since the late 1970s, studies of IPNs from natural products have attracted the attention of many research groups. Yenwo and Sperling10 performed pioneering research on this aspect by synthesizing IPNs with sulfur-cross-linked castor oil, followed by swelling with a plastic forming monomer, such as acrylate, plus cross-linker. They have reported that such IPNs prepared behave like reinforced elastomers with low polystyrene content or toughened plastics with high polystyrene content. During the past few decades, a considerable amount of work has been carried out on IPNs based on castor oil compositions with various acrylics.34–38 It has been found that these combinations exhibited somewhat inferior properties as compared to those of the constituent component materials due to the greater interpenetration of the two component phases. Many other functionalized vegetable oils, such as vernonia and lesquerella, have also been used to prepare IPNs.4,39 However, the double

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bonds in the fatty acid chains of these vegetable oils are subject to attack by the free radicals from the vinyl polymerization, resulting in the formation of long grafts within this type of IPNs.40 These intersystem chemical grafts would yield materials different from IPNs where only physical entanglement is thought to exist. Additionally, the location of the hydroxyl groups of these vegetable oils is in the middle of the fatty acid chains. The pendant chains act as dangling chains when these polyols are cross-linked, resulting in significant steric hindrance to further cross-linking. This prolongs the polymerization process and significantly restricts industrial applications. Significant novelty and potential for industrial relevance therefore exist in employing novel polyols with terminal hydroxyl groups and the absence of double bonds on the fatty acid chains, in the formation of IPNs. Recently, with the advent of new types of polyol with terminal functional groups from vegetable oils using ozonolysis and hydrogenation technology, the effect of pendant chains has been significantly reduced.32,41 This type of polyol has been used successfully to produce polyurethane (PUR) which had better thermo-mechanical and mechanical properties than the corresponding PUR made from commercially available biobased polyol.31,32 Furthermore, among the fatty acid chains of this polyol, all the unsaturated groups were either converted to functional groups or to saturated bonds by the hydrogenation process. In this manner, the possibility of grafting which existed in the previous vegetable oil based IPNs was removed.42 In this paper, we report on the properties of sequential IPNs prepared using polyol with terminal functional groups synthesized from canola oil and poly(methyl methacrylate) (PMMA) and compared to IPNs made from commercial castor oil and poly(methyl metharylate). The thermal properties of the IPNs were studied using dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) techniques. The morphology of the IPNs was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that canola oil based polyol offered different characteristics from castor oil, both in terms of the chemistry involved and the physical properties and phase behavior of the IPNs.

Experimental Section Materials. Canola oil used in this study was a sample of “100% pure canola” supplied by Canbra Foods Limited. Canola oil based polyol with terminal primary hydroxyl groups was synthesized using ozonolysis and hydrogenation based technology and the procedure was reported in detail elsewhere.27,41 The hydroxyl number of the polyol was 237 mg KOH/g, as determined according to the ASTM D195786. The polyol contained 60.18 ( 1.16 wt %, 26.00 ( 0.48 wt %, and 4.72 ( 0.03 wt % of triol, diol, and mono-ol, respectively.43 The rest, about 9 wt % compound, are saturated triacylglycerols (TAGs). Unrefined crude castor oil was obtained from CasChem Company. (NJ, U.S.A.). The chemical structures of canola oil based polyol and castor oil are shown in Figure 1. Aliphatic 1,6-hexamethylene diisocyanate (HDI) based polyisocyanate (Desmodur N-3200) was sourced from Bayer Corporation, Pittsburgh, PA. Its functionality was 2.6 and equivalent weight was 183, as provided by the supplier. Benzoyl peroxide (BPO), 97%, and ethylene glycol dimethacrylate (EGDM), 98%, were obtained from Aldrich Chemical (U.S.A.). Methyl methacrylate (MMA, 99%) was purchased from Acros Organics (U.S.A.). BPO was recrystallized from methanol prior to use. MMA was washed twice with 5% aqueous NaOH and twice with distilled H2O, dried with CaCl2, filtered, kept at 5 °C over MgSO4 for 24 h, and distilled at 100-101 °C.

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Figure 1. Representative chemical structures of canola oil based polyol and castor oil.

PUR and PMMA Synthesis. The PUR prepolymer was prepared by fixing the molar ratio of the isocyanate (NCO) group to the OH group (NCO/OH) of 1.8/1.0. A suitable amount of polyol and HDI were weighed in a plastic container, mixed thoroughly at room temperature for 20 min in the case of canola PUR, whereas at 45 °C for 60 min in the case of castor PUR. Both prepolymer were isolated as thick syrup and were immediately used for IPNs synthesis. Pure PUR specimens were also prepared using the same procedure followed by post curing at 50 °C for 24 h. Pure PMMA specimen was prepared in the following procedure: a suitable amount of MMA, 1% of EGDM cross-linker, and 0.5% of BPO initiator was stirred at room temperature for 5 min to form homogeneous solution, followed by raising the temperature to 60 °C. After stirring for 60 min, the solution was poured into a Teflon mold and was kept at 60 °C for 24 h and then at 120 °C for 4 h before demolding. Sequential IPNs Synthesis. Sequential IPNs were synthesized by changing the weight ratio between PUR and PMMA from 100:0 to 45:55, 35:65, 25:75, and 0:100. PUR prepolymer was poured in different mass proportions into a round-bottom flask. To this, a suitable amount of MMA, 1% of EGDM cross-linker, and 0.5% of BPO initiator were added. The mixture was stirred at room temperature for 5 min to form homogeneous solution. Then the temperature was raised to 50 °C to initiate the radical polymerization of MMA along with added crosslinker. After stirring for 30 min in the case of canola oil based IPNs, and 60 min in the case of castor oil based IPNs, the solution was poured into a Teflon mold kept in a preheated air circulating oven maintained at 60 °C. It was kept at this temperature for 24 h and then at 120 °C for 4 h before demolding. The nomenclature used for these IPNs is listed in Table 1. CN-PUR, CS-PUR, and PMMA represent pure polyurethane produced from canola oil based polyol, castor oil, and pure poly(methyl methacrylate), respectively. The IPNs are identified as follows: the number represents the weight percentage of PUR in the IPNs while CN- and CS- stand for the source of PUR. Curing Measurements. Rheological analyses were performed with an AR2000 Advanced Rheometer (TA Instruments, U.S.A.), equipped with the Environmental Test Chamber (ETC). Because of the thermosetting nature of the material, 25 mm disposable plates were used as the test geometry. Isothermal time sweeps were run at a constant temperature of 60 °C to investigate the storage modulus (G′) and loss modulus (G′′) change. Attempts to measure gel point for IPN samples

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Table 1. Variable Manipulation and Tg of All the IPNsa DMA PUR content (wt %)

PMMA content (wt %)

0 15 25 35 45 100 15 25 35 45 100

100 85 75 65 55 0 85 75 65 55 0

PMMA CS-PUR15%-PMMA CS-PUR25%-PMMA CS-PUR35%-PMMA CS-PUR45%-PMMA CS-PUR CN-PUR15%-PMMA CN-PUR25%-PMMA CN-PUR35%-PMMA CN-PUR45%-PMMA CN-PUR a

Tg1 (°C) 26 ( 1 28 ( 1 26 ( 1 14 ( 1 15 ( 1 56 ( 1 39 ( 1 31 ( 1 23 ( 1

DSC

Tg2 (°C)

Tg3 (°C)

106 ( 1 95 ( 1

134 ( 1 122 ( 1 117 ( 1 125 ( 2 130 ( 1

109 ( 1 90 ( 1

133 ( 1 124 ( 1 128 ( 2 130 ( 1

Tg1 (°C) 5(1 1(1 2(1 -11 ( 1 -11 ( 1 25 ( 1 35 ( 1 27 ( 0 23 ( 1 17 ( 1

Tg2 (°C) 115 ( 1 115 ( 1 113 ( 1 117 ( 2 120 ( 1 118 ( 0 123 ( 0 124 ( 1 127 ( 0

Determined by DMA and DSC. Errors are standard deviations; n ) 3.

using this method did not work because evaporation of the MMA monomer occurred during the period of measurement. Therefore, only PUR samples prepared by following the above-mentioned procedure were reported. Degree of Conversion. Degree of conversion measurements were carried out on a MDSC Q100 (TA Instruments, U.S.A.), equipped with a refrigerated cooling system. The procedure was described as follows: the samples were transferred to DSC chamber immediately after the stirring procedure, heated to 60 °C, and held isothermally until DSC could not detect any heat flow. The degree of conversion was calculated according to the following equation:

Rt )

∆Ht ∆H

(1)

where Rt is the degree of conversion of reaction, ∆Ht is the heat generated up to time t, and ∆H is the total exothermic heat involved in the reaction. The glass transition temperature (Tg) of IPN samples was measured using the following DSC procedures: the samples were heated at a rate of 10 °C/min from 25 to 160 °C to erase thermal history, cooled down to -60 °C at a cooling rate of 5 °C/min, and then heated again to 160 °C at a heating rate of 10 °C/min. The second heating stage was selected to be analyzed for the collection of heating data. All the procedures were performed under a dry nitrogen gas atmosphere. Three identical specimens were tested and the results averaged. The reported errors are the subsequent standard deviations. DMA Measurements. DMA measurements were carried out on a DMA Q800 (TA Instruments, U.S.A.) equipped with a liquid nitrogen cooling apparatus, in the single cantilever mode, with a constant heating rate of 1 °C/min from -60 °C to +150 °C. The size of the samples was 18 × 7 × 2 mm. The measurements were performed following ASTM E1640-99 standard at a fixed frequency of 1 Hz and a fixed oscillation displacement of 0.015 mm. Three identical specimens prepared by cutting the material out of a polymer sheet were tested and the results were averaged. The reported errors are the subsequent standard deviations. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The microstructure of the obtained IPN samples was examined using a scanning electron microscope (Philips XL30 ESEM LaBB6B, manufactured by FEI Company, Oregon, U.S.A.) and TEM (Philips Morgagni 268, manufactured by FEI Company, Oregon, U.S.A.), respectively. The SEM was equipped with a gaseous secondary electron detector within a gaseous environment and a partial vapor pressure of 1.2 mbar. The sample for SEM observation was broken by hand to reveal a fracture surface and was coated with gold before subsequent microscopy observation. The sample for TEM was microtomed in a Reichert-Jung Ultracut E ultramicrotome using a diamond knife and was exposed to osmium tetroxide vapor at room temperature for 1 week before observation to selectively stain the double bond of the castor oil PUR phase.

Figure 2. Changes of storage modulus (G′) and loss modulus (G′′) with time at 60 °C for (a) canola oil based polyurethane (CN-PUR) and (b) castor oil based polyurethane (CS-PUR).

Mechanical Properties. The tensile property of the IPNs was conducted on an Instron 4202 according to ASTM D638 standard. Dumbbell-shaped specimens were cut out from the IPNs using an ASTM D638 type V cutter. The cross-head speed was 20 mm/min, with a load cell of 50 Kgf. At least five identical specimens prepared by cutting the material out of a polymer sheet were tested and the results were averaged. The reported errors are the subsequent standard deviations.

Results and Discussion The gelation time for the two pure PUR was measured by monitoring the evolution of storage modulus (G′) and loss modulus (G′′) with time at 60 °C, as shown in Figure 2a,b, respectively. The point at which the two curves intersect is typically taken as the gel point of the system and is the point at which the system begins cross-linking. The gel time is 9.3 min

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Figure 3. The conversion-time profiles of interpenetrating polymer networks (IPNs) with different polyurethane (PUR) content (a) canola oil based IPNs and (b) castor oil based IPNs

for CN-PUR and 210 min for CS-PUR, indicating that the former sample reacts much faster than the latter one. This was attributed to the difference in polyol structure. Canola oil based polyol contained only primary terminal hydroxyl groups located at carbon 9, whereas castor oil contained secondary functional hydroxyl groups all located in the middle of the fatty acid chains, which resulted in significantly higher steric hindrance to crosslinking. The conversion-time profiles of canola and castor oil based IPNs with different PUR content are shown in Figure 3a,b, respectively. It is not surprising that the reaction rates of the canola oil based IPNs are faster than those of castor oil based IPNs, which can be seen from the slopes of these curves. Meanwhile, the change of reaction rates of IPNs with the concentration of PUR content is significant: a higher PUR content shifts the maximum rate toward smaller conversion ratios, which is more pronounced in the case of castor oil based IPNs. This could be explained by considering the formation process of sequential IPNs. In the IPN mixture, PUR network was swollen in the initial monomers required for the synthesis of PMMA network. This stage of equilibrium swelling can be considered as meeting the requirements of thermodynamic equilibrium. In the course of the reaction, the thermodynamic incompatibility of growing networks appeared at a certain value of conversion degree. Because of this, diffusion led by phase separation would occur, and then PMMA network would start to form. In the initial stage, the formation rate of the PUR network was higher than the formation rate of PMMA. This is

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due to the induction period of the free radical polymerization of PMMA, which could be accelerated by the heat released during the formation of PUR network. For the IPNs containing more PUR compound, the conversion rates were faster since more heat was generated. Nevertheless, after the reaction reached a certain point, the Trommsdorff effects44 would cause an important viscosity increase of the reaction medium, the propagation of polymerization would then slow down, and the reaction rate would be dominated by curing process of PUR network again. For IPNs composed of CS-PUR, this was a rather long process since castor oil contained secondary functional hydroxyl groups. This resulted in CS-PUR45%-PMMA conversion proceeding at a very slow rate until fully converted, as shown in Figure 3b. Recall that castor oil consists of TAGs molecules with carbon-carbon double bonds in the fatty acid chains. During polymerization of PMMA, grafting can occur by chain transfer from the propagating free radical to the PUR network. However, the double bonds of castor oil react much less readily when compared with the acrylic radical, this might be another reason leading to a reduction of conversion rate. The occurrence of grafting polymerization in the PUR and acrylic based IPNs has been confirmed by several researchers using solid-state 13C NMR.40,45 They found that the ratio between the carbon in the carbon-carbon double bonds in the fatty acid and the carbon in the urethane groups was decreased with the increase of PMMA content, implying that some of the carbon-carbon double bonds in the fatty acid chains of PUR have been reacted and copolymerized with PMMA. Similar grafting reactions should have been occurred in the castor oil based PUR and PMMA system, even though experimental confirmation is lacking in this instance. Grafting could somewhat enhance the compatibility between PUR and PMMA, however, phase separations are still observed for this type of IPNs prepared, which will be discussed in detail later. The phase behavior of a sequential IPN depends on the thermodynamic compatibility of the constituent polymers and the cross-link density (mainly that of the first network) as well. Cross-link density of a polymer network can be described by the following equation according to the theory of rubber elasticity:46

E′ ) νeRT 3

(2)

where E′ is the storage modulus, R is the gas constant, νe is the cross-link density, and T is the absolute temperature. E′ of pure PUR network was determined by measuring isothermal oscillation as a function of frequency in the Tg region and then superposed into respective master curves using the timetemperature superposition principle.47 The values of νe obtained are 9.3 × 102 M/m3 and 6.9 × 102 M/m3 for CN-PUR and CS-PUR networks, respectively. In terms of IPNs, the formation of two distinct polymer networks in the same reaction medium is more complicated. The exact cross-link density of PUR network in IPNs might be different from the pure compound. However, the trend should be the same: the cross-link density of PUR in canola oil based IPNs should be higher than that in castor oil based IPNs with the same composition. The differences in cross-link density, combined with the kinetics of polymerization as mentioned previously, resulted in different phase behavior being observed for these two types of IPNs. Storage modulus (E′) of CS-PUR25%-PMMA and CNPUR25%-PMMA as a function of temperature are shown in Figure 4 as an example. In the case of CS-PUR25%-PMMA,



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Figure 4. Storage modulus (E′) as a function of temperature for canola oil based and castor oil based interpenetrating polymer networks (IPNs) with 25 wt % of polyurethane (PUR) content.

a two-step decrease of the storage modulus, E′, in the temperature region from -50 to 50 °C and from 20 to 150 °C is observed. The first rapid drop is attributed to the glass transition of PUR phase and the second to the glass transition of PMMA phase. In the case of CN-PUR25%-PMMA, only one broad transition is observed in the temperature range of 20-100 °C. This is an indication that the lower degree of phase separation was obtained for the CN-PUR25%-PMMA sample. This could be explained by considering the cross-link density of the first formed PUR networks and conversion rate difference of these two IPNs as discussed previously. For CS-PUR25%-PMMA, the cross-link density of CS-PUR network is lower, during the polymerization of the MMA absorbed in the PUR network, the growing PMMA network push apart the CS-PUR chains and form a phase with no or little presence of CS-PUR segments. On the contrary, for CN-PUR25%-PMMA, the cross-link density of CN-PUR network is higher, the space allowed for the growth of the MMA absorbed in the PUR network is smaller. The thermodynamic equilibrium criterion leads to the PMMA segments tending to separate from the CN-PUR network, but the domain size of the PMMA network will be smaller and a part of the PMMA chains are forced to interpenetrate into the CN-PUR networks (forced compatibility). Additionally, the extent of phase separation is determined by the rate at which the system achieves the state of incompatibility. During the formation of IPNs, PUR behaves like a viscous medium in which the MMA polymerization proceeds; the initial rates increase as the reaction medium becomes more viscous till a certain value of conversion degree at which the thermodynamic incompatibility of growing networks appears, the phases begin to separate. For CN-PUR25%-PMMA, the CN-PUR component reacted very fast because of the primary structure of canola oil based polyol, which accelerates the rate at which the state of thermodynamic incompatibility is reached, leading to a lower degree of phase separation. However, for CS-PUR25%-PMMA, the CS-PUR component reacted comparatively very slowly, which allowed significant diffusion (caused by phase separation) to occur before MMA components started to polymerize, resulting in a high degree of phase separation. This result could be further confirmed by analyzing tan δ spectra. The dependences of tan δ on temperature for canola oil and castor oil based IPNs with various PUR/PMMA ratios are shown in Figure 5a,b, respectively. Tan δ curves of pure PUR and

Figure 5. The dependences of tan δ on temperature for interpenetrating polymer networks (IPNs) with different polyurethane (PUR) content (a) canola oil based IPNs, and (b) castor oil based IPNs. The curves were shifted vertically to discriminate peaks. The values in the bracket aretheshiftingdegreeofindividualsamples:PMMA(0.7),PUR15%-PMMA (0.4), PUR25%-PMMA (0.25), PUR35%-PMMA (0.15).

PMMA sample are also presented in each figure as a reference. One sharp peak corresponding to the glass transition was observed for both pure PUR samples. Two peaks were observed for pure PMMA. The first peak located at 31 °C is associated with side-chain motion of CH3- and ester groups attached to the main chain. The second peak at 134 °C is assigned to the R-transition, that is, glass transition of the main chain of PMMA. These values are similar to those reported for highly cross-linked PMMA from DMA studies.48 In the case of canola oil based IPNs, the following features were observed: (1) for CNPUR15%-PMMA, the second and third transition peaks at higher temperature were shifted inward; (2) for CNPUR25%-PMMA and CN-PUR35%-PMMA, the second and third transition peaks were combined into one broad peak, whereas another peak at lower temperature appeared, which was corresponding to the R-transition of PUR; and (3) for CN-PUR45%-PMMA, three distinct transition peaks were observed. However, different features were obtained for castor oil based IPNs: (1) only one transition peak at higher temperature was present for CS-PUR15%-PMMA; (2) this peak started to split into two peaks when the PUR content increased to 35 wt %, and another well-resolved transition peak at lower temperature was also observed. The three transition peaks are assigned to PUR-rich phase, interphase, and PMMA-rich phase, respectively. The interphase transition is associated with the

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Figure 6. Scanning electron micrographs of (a) CN-PUR15%-PMMA, (b) CN-PUR25%-PMMA, (c) CN-PUR35%-PMMA, (d) CN-PUR45%-PMMA, (e) CS-PUR15%-PMMA, (f) CS-PUR25%-PMMA, (g) CS-PUR35%-PMMA, and (h) CS-PUR45%-PMMA.

beginning of molecular motion of chains at the interface. The corresponding Tgs of these phases determined from these tan δ curves are summarized in Table 1. For CN-PUR25%-PMMA and CS-PUR25%-PMMA, the divergence between the two Tgs, corresponding to PUR-rich phase and PMMA-rich phase, is 68 and 89 °C, respectively, which demonstrates the smallest divergence when compared with the other IPNs with different

components. This indicates that the highest degree of phase mixing was obtained for the sample with 25 wt % of PUR content in both series of IPNs. Nevertheless, the extent of phase mixing for castor oil based IPNs is much lower, which could be further justified from the broadness of tan δ spectra. By comparing the tan δ curves, presented in Figure 5, it is obvious that the transitions of canola oil based IPNs are broader than



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Figure 7. Transmission electron micrographs of (a) CS-PUR15%-PMMA, (b) CS-PUR25%-PMMA, (c) CS-PUR35%-PMMA, and (d) CS-PUR45%-PMMA.

those of castor oil based IPNs, implying again that a modest degree of molecular mixing exists in the former IPNs. This phenomenon becomes more prominent with the increasing of PUR content. It is worth mentioning that the broadening of tan δ spectra of canola oil based IPNs provide potential opportunity for this type of materials to be used as sound and vibration damping polymers over wide temperature ranges.49 It should be pointed out that the existence of interphases is common in multicomponent polymer systems, which is due to the incomplete mixing of the two polymers.2 In terms of IPN, even though “permanent” entanglements produced by interpenetration could reduce the extent of phase separation, it is impossible to achieve complete phase mixing. Lipatov et al.24 first observed interphase in sequential PUR and polystyrene IPN systems by using broad-line NMR spectroscopy and inverse gas chromatography techniques. For canola oil based IPNs, the existence of interphases is due to the higher cross-link density and the faster reaction rate of CN-PUR component, which effectively prevent diffusion (caused by phase separation) of MMA components. In other words, it is a result of the balance between thermodynamics and kinetics because thermodynamic equilibrium would be kinetically limited. For castor oil based IPNs, the most likely reason for the existence of interphases is due to grafting occurring between CS-PUR and PMMA, which could chemically restrain phase separation. This is easy to understand by considering the formation process of sequential IPNs as mentioned previously. Once CS-PUR reaches its gel point, the free energy of the system might become positive, diffusion resulted from phase separation might occur due to the thermodynamic equilibrium criterion and lead to the formation of CS-PUR and PMMA domains. During this process, only those carbon-carbon double bonds existing on the domain

boundary could be attacked by the free radicals propagated by the polymerization of MMA. In other words, grafting could most likely occur on the domain border, therefore, the interphases might be mainly consisted of the grafted CS-PUR and PMMA molecules. Nevertheless, this interphase transition was not observed with the DSC measurement (not shown). Tgs of these IPNs determined by DSC are listed in Table 1. Different from DMA, only two Tgs corresponding to PUR-rich phase and PMMA-rich phase were obtained. The values are slightly lower than those determined from DMA, but the trend is the same: with the increasing of PUR content, the Tgs of PUR-rich phase and PMMA-rich phase in the IPNs are shifted outward. Such differences are very common when considering the nature of these two technologies. DSC measures the change in heat capacity when chains go from the glassy to the nonglassy stage, whereas DMA measures the change in mechanical response of these chains,45 which may be influenced to a much greater extent by phase continuity.18 Thus, DMA is more sensitive to measure glass transition than DSC. The morphology of the IPNs is revealed by SEM (Figure 6) to a limited extent, constrained by the resolution of the instrument. In the case of canola oil based IPNs, two phases was observed in all the micrographs, indicating phase separation. In the case of castor oil based IPNs, the domain was difficult to resolve even at high magnifications, implying that the domains did not exhibit a sharp contrast with the matrix. This might be due to the reason that graftings occurred on the domain border as mentioned previously. The morphology of castor oil based IPNs was further investigated by TEM (Figure 7). Osmium tetroxide50 is known to stain preferentially the double bonds of the CS-PUR phase,

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and permanent entanglement introduced by the interpenetrating structure increased the cross-link density of the IPNs. Nevertheless, the deformation behavior of the IPNs depended strongly on the PUR source and PMMA content. Both series of IPNs displayed characteristics typical of rigid and tough plastics with similar tensile strength when the PMMA content was more than 75 wt %, due to the finely divided rubber and plastic combination structures in these IPNs. However, canola oil based IPNs still exhibited a typical mechanical property of rigid plastics with a yield point, while castor oil based IPNs started to behave as a soft rubber with long elongation when PMMA content was less than 65 wt %. This is a result of the large amount of dangling chains in the castor oil based IPNs, which are imperfections in the final polymer networks, that is, they do not support stress when the network is under load. In contrast, the effect of dangling chains in the canola oil based IPNs is significantly diminished due to the utilization of canola oil based polyol with primary functional groups.

Conclusions

Figure 8. Stress vs strain curves for interpenetrating polymer networks (IPNs) with different polyurethane (PUR) content (a) canola oil based IPNs and (b) castor oil based IPNs

thus, the darker regions in the micrographs consist of PUR or have a high PUR content. For CS-PUR15%-PMMA and CS-PUR25%-PMMA IPNs, PMMA was present as the continuous phase with globules of PUR dispersed within it. The domain size of CS-PUR25%-PMMA IPN is larger than that ofCS-PUR15%-PMMAIPN.InthecaseofCS-PUR35%-PMMA and CS-PUR45%-PMMA IPNs, the PUR domains were comparatively large and interconnected, suggesting that phase inversion occurred, which is in agreement with the DMA data. For canola oil based IPNs, there are no double bond or benzene ring which are suitable for staining. Therefore, even the two phases elucidated by SEM could not be distinguished under TEM. The stress versus strain curves with various PMMA content for the two series of IPNs are shown in Figure 8a,b, respectively. Tensile strength of pure CN-PUR is 36 ( 1 MPa, whereas that of pure CS-PUR is 7 ( 1 MPa. The tensile strength increased, whereas elongation at break decreased with a corresponding increase in the PMMA content in both series of IPNs. For example, the tensile strength was increased to 77 ( 2 MPa in the case of canola oil based IPNs and to 73 ( 2 MPa in the case of castor oil based IPNs when 85 wt % of PMMA was added in the IPNs. A yield point with yield strength 74 ( 1 MPa for CN-PUR25%-PMMA and 72 ( 1 MPa for CS-PUR25%-PMMA was displayed for the IPNs containing 75 wt % of PMMA. The improved mechanical properties of IPNs, observed in Figure 8, could result from the fact that physical interlocking

A variety of new IPNs prepared from canola oil based polyol with terminal functional groups and PMMA have been successfully synthesized. The physical properties of these novel IPNs are compared to those IPNs made from commercial castor oil. The polymerization process is accelerated, and extent of phase mixing for canola oil based IPNs is enhanced, due to the nature of the polyol used. For the IPNs containing more than 75 wt % PMMA, the mechanical properties of canola oil based IPNs are comparable to castor oil based IPNs both exhibit better mechanical properties than those of its constituent polymer networks. With the decreasing of PMMA content in IPNs, castor oil based IPNs start to behave as a soft rubber with long elongation, while canola oil based IPNs still behave as a rigid plastics with an obvious yield point. Furthermore, these new IPN material cover a broad spectrum of useful properties, such as sound and vibration damping, and can prove to be valuable substitutes for existing materials. Thus, this work provides an alternative way of utilizing renewable resources to prepare environmentally friendly IPNs with high performance for various applications. Acknowledgment. The financial support of Bunge Oils, NSERC, Alberta Canola Producers Commission, Alberta Agricultural Research Institute, and Alberta Crop Industry Development Fund are gratefully acknowledged.

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