Physical Properties of Sequential Interpenetrating Polymer Networks

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Biomacromolecules 2008, 9, 1424–1433

Physical Properties of Sequential Interpenetrating Polymer Networks Produced from Canola 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, Canada Received February 11, 2008; Revised Manuscript Received February 27, 2008

Sequential interpenetrating polymer networks (IPNs) were prepared using polyurethane (PUR) synthesized from canola oil-based polyol with terminal primary functional groups and poly(methyl methacrylate) (PMMA). The properties of the material were evaluated by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and modulated differential scanning calorimetry (MDSC), as well as tensile properties measurements. The morphology of the IPNs was investigated using scanning electron microscopy (SEM) and MDSC. A fivephase morphology, that is, sol phase, PUR-rich phase, PUR-rich interphase, PMMA-rich interphase, and PMMArich phase, was observed for all the IPNs by applying a new quantitative method based on the measurement of the differential of reversing heat capacity versus temperature from MDSC, although not confirmed by SEM, most likely due to resolution restrictions. NCO/OH molar ratios (cross-linking density) and compositional variations of PUR/PMMA both affected the thermal properties and phase behaviors of the IPNs. Higher degrees of mixing occurred for the IPN with higher NCO/OH molar ratio (2.0/1.0) at PUR concentration of 25 wt %, whereas for the IPN with lower NCO/OH molar ratio (1.6/1.0), higher degrees of mixing occurred at PUR concentration of 35 wt %. The mechanical properties of the IPNs were superior to those of the constituent polymers due to the finely divided rubber and plastic combination structures in these IPNs.

Introduction Polymeric materials are traditionally industrially produced from petroleum-based monomers. The growing demand for such materials has increased our dependence on petroleum sourced crude oil. With the reality of depleting oil resources and the increasing social emphasis on issues concerning the environment and waste disposal, researchers have sought different ways and technologies to viably produce polymers from renewable resources.1–11 Natural vegetable oils, as a source of raw materials, have been utilized in polymer synthesis via the attachment of functional groups such as hydroxyl, epoxy, or carboxyl groups introduced onto the fatty acid chains of the vegetable oils.1–11 Moreover, polymers produced from vegetable oil and their derivatives have excellent hydrolytic stability and thermo-mechanical properties due to the hydrophobic nature of triglycerides.11 Polymer blends have received considerable attention in both industrial and academic areas because of the simplicity and effectiveness of mixing two different polymers to obtain potential new materials. Interpenetrating polymer network (IPN) materials, a kind of polymer blend held together by permanent entanglement between two or more distinctly cross-linked polymers, have created increasing interest due to their excellent properties brought about by the interlocking of the polymer chains.12 There are many kinds of IPNs. Sequential IPNs and simultaneous interpenetrating networks (SINs) are the two most important types. Sequential IPNs starts with the synthesis of a cross-linked polymer I. Monomer II along with its cross-linker are swollen into polymer I and polymerized in a sequential mode. SINs begin with a mutual solution of both monomers * To whom correspondence should be addressed. Tel.: 1-780-492-9081. Fax: 1-780-492-7174. E-mail: [email protected].

plus their respective cross-linkers, which are then polymerized simultaneously via noninterfering reactions.12 In the past few decades, Sperling et al.,1,13–17 Frisch et al.,18–24 Hourston et al.,25–31 and Lipatov et al.32–36 have performed seminal research on IPNs, which have undoubtedly proven their technological importance in areas such as coatings, sound and vibration damping polymers, and so on. Since the late 1970s, the synthesis and characterization of IPN from natural products have attracted the attention of many research groups. The first study on this aspect can be traced back to 1977, the pioneering work by Sperling and co-workers. They have reported that the IPNs prepared from castor oil-based polyurethanes and styrene monomers behave like tough elastomers or reinforced plastics, depending on their composition.13,17 Suthar and co-workers37–39 have studied a series of IPNs based on castor oil with acrylic. They demonstrated that the IPNs thus obtained were elastomers and exhibited good mechanical properties. However, grafting undoubtedly occurred within this type of IPN, because castor oil is an unsaturated material and subject to attack by the free radicals from the vinyl polymerization.40 These intersystem chemical grafts would yield materials different from IPNs where only physical entanglement is thought to exist. Recently, a new type of polyol with terminal functional groups has been produced from vegetable oil by using ozonolysis and hydrogenation technology.41 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 that existed in the earlier castor oil-based IPNs was removed. 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.5,7,8,42

10.1021/bm8001478 CCC: $40.75  2008 American Chemical Society Published on Web 04/15/2008

Polymer Networks from Canola Oil-Based Products Scheme 1. Chemical Reaction Procedure of Polyol from Triolein

Biomacromolecules, Vol. 9, No. 5, 2008

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polyol + diisocyanate f MMA monomer, initiator, crosslinker

PUR prepolymer 98 IPN

In this paper, we report on the properties of sequential IPNs prepared using novel polyol with terminal functional groups synthesized from canola oil and poly (methyl methacrylate) (PMMA). The thermal properties of the IPNs were studied using dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) techniques. The phase behavior of the IPNs was analyzed by applying a quantitative method using the differential of heat capacity with temperature, dCp/dT, from modulated differential scanning calorimetry (MDSC), and by SEM. The effects of NCO/OH molar ratios (cross-linking density) as well as compositional variations of PUR and PMMA are discussed. It was found that this type of 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. Polyol was synthesized using ozonolysis and hydrogenation-based technology from canola oil and the procedure was reported in detail elsewhere.5,41 The chemical reaction procedure using triolein as an example is illustrated in Scheme 1. The hydroxyl number of the polyol was 237 mg KOH/g, as determined according to the ASTM D1957-86. The polyol contained about 60, 26, 5, and 9% (on a mass basis) of triol, diol, mono-ol, and saturated triacylglycerols (TAGs), respectively.5 Aliphatic 1,6-hexamethylene diisocyanate (HDI, 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. PUR and PMMA Synthesis. The PUR prepolymer were prepared using two formulations by fixing the molar ratio of the isocyanate (NCO) group to the OH group (NCO/OH) of 1.6/1.0 and 2.0/1.0. A suitable amount of polyol and HDI were weighed in a plastic container and mixed thoroughly for 20 min at room temperature. This prepolymer was isolated as thick syrup and part of it was immediately used for IPN synthesis. The other part was transferred to the oven, which was maintained at 50 °C post-curing for 24 h to achieve pure PUR specimens. 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. Two series of sequential IPNs were synthesized in terms of molar ratio variations of NCO/OH as well as compositional variations of PUR and PMMA. The general scheme was as follows:

As listed in Table 1, series 1, NCO/OH molar ratio was kept at 1.6/ 1.0; series 2, NCO/OH molar ratio was kept at 2.0/1.0; in the meantime, PUR content was varied from 15 to 45 wt % by increasing every 10 wt %. 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 was 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 cross-linker. After stirring for 30 min, 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. It should be noted here that, although the temperature of mixing for the IPNs were kept at 50 °C while the pure PMMA sample was formed at 60 °C, the PUR component is formed under exothermic conditions so that in the formation of the IPNs, the temperature of the reactions is indeed higher than 50 °C. FTIR. The FTIR spectra were recorded on a Nicolet Magna 750 FTIR, equipped with an MCT-A detector and a Nicolet Nic-Plan IR microscope used in transmission mode. The spectra were recorded in the range 650–4000 cm–1, with a nominal resolution of 4 cm–1. A background spectrum was first collected before each absorbance spectrum. A total of 128 interferograms were coadded before Fourier transformation using the Nicolet Omnic software. Curing Measurements. Rheological analyses were performed with an AR2000 Advanced Rheometer (TA Instrument, 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 50 °C to investigate the storage modulus (G′) and loss modulus (G′′) change. Due to the vigorous free radical reactions, which occurred during the period of transferring IPN solution from flask to ETC, it is difficult to perform the test for IPN samples. Therefore, only PUR samples prepared by following the above-mentioned procedure were measured at this point. DSC and MDSC Measurements. Conventional DSC and MDSC measurements were carried out on a MDSC Q100 (TA Instruments, U.S.A.), equipped with a refrigerated cooling system. The conventional DSC procedure was described as follows: 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. The MDSC procedure was as follows: a modulation amplitude of 1 °C, a modulation period of 60 s and a heating rate of 3 °C/min were used. The temperature range was -50 to +150 °C. All the procedures were performed under a dry nitrogen gas atmosphere. Three identical specimens were tested, and the results were averaged. The reported errors are the subsequent standard deviations. DMA Measurements. DMA measurements were carried out on a DMA Q800 (TA Instrument, 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 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. 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

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Table 1. Variable Manipulation and Tg of All the IPNs Determined by DMAa NCO/OH series 1

Series 2

a

S1-Pure-PMMA S1-PUR15%-PMMA S1-PUR25%-PMMA S1-PUR35%-PMMA S1-PUR45%-PMMA S1-Pure-PUR S2-Pure-PMMA S2-PUR15%-PMMA S2-PUR25%-PMMA S2-PUR35%-PMMA S2-PUR45%-PMMA S2-Pure-PUR

1.6 1.6 1.6 1.6 1.6 2.0 2.0 2.0 2.0 2.0

PUR content (wt %)

PMMA content (wt %)

0 15 25 35 45 100 0 15 25 35 45 100

100 85 75 65 55 0 100 85 75 65 55 0

Tg 1(°C)

Tg 2(°C)

Tg 3(°C)

52 ( 1 50 ( 1 36 ( 1 26 ( 1 23 ( 1

113 ( 1 93 ( 1

134 ( 1 130 ( 1 128 ( 1 131 ( 2 135 ( 1

52 ( 2 51 ( 1 41 ( 1 34 ( 1 23 ( 1

107 ( 1 112 ( 2 94 ( 1 88 ( 1

134 ( 1 131 ( 1 128 ( 1 129 ( 2 132 ( 1

Errors are standard deviations; n ) 3.

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. Scanning Electron Microscopy (SEM). The microstructure of the obtained IPN samples was examined using a Scanning Electron Microscope (Philips XL30 ESEM LaB6 manufactured by FEI Company, Oregon, U.S.A.). The SEM was equipped with a Gaseous Secondary Electron Detector (GSED), within a gaseous environment and a partial vapor pressure of 1.2 mbar. The sample chamber vacuum was around 9.4 × 10–5 mbar. The sample was broken by hand to reveal a fracture surface and was coated with gold before subsequent microscopy observation.

Results and Discussion Evidence for Interpenetration. FTIR spectra of PUR showed the characteristic absorption band at 1740 cm–1 and 3400 cm–1, corresponding to urethane and amide(-NH stretching), respectively. As the prepolymer is isocyanated terminated, an intense band at 2270 cm–1 due to NCO is observed. FTIR spectra of IPNs showed all the bands corresponding to PUR and PMMA networks and do not show appearance of additional bands, therefore, ruling out the possibility of any chemical interaction between the component networks. Glass Transition Behavior. DMA and DSC are the two common techniques that were applied to determine glass transition temperature (Tg) and, therefore, polymer–polymer miscibility of polymer blends. However, the position of the Tg, the broadness of the transition interval, and even the very possibility of detecting the existence of Tg experimentally are affected by the size of the domain. It was reported33 that the domain dimensions at which the glass transition is comparatively detected by DMA, DSC, and electron microscopy has a lower limit of 15 nm. The dependences of tan δ on temperature for the IPNs with a fixed NCO/OH molar ratio 1.6/1.0 and 2.0/1.0, but with various PUR/PMMA ratios, are shown in Figure 1a and b, respectively. Tan δ curves of pure PUR and PMMA sample are also presented in each figure as a reference. One sharp peak that corresponds to the glass transition was observed for both pure PUR samples. Two peaks were observed for pure PMMA. The first peak at 31 °C corresponds to secondary (β) transition caused by CH3 and ester side groups on the polymer chain. The second peak located at 134 °C corresponds to the R-transition, that is, glass transition of the main chain of PMMA. These values are in agreement with those reported for cross-linked PMMA from DMA studies.43 In the case of IPNs, three transition peaks were observed for most of the samples, with

Figure 1. The dependences of tan δ on temperature for two series IPNs with (a) NCO/OH molar ratio 1.6/1.0 and (b) NCO/OH molar ratio 2.0/1.0. The curves were shifted vertically to discriminate peaks. The values in the bracket are the shifting degree of individual samples: PMMA (0.7), PUR15%-PMMA (0.4), PUR25%-PMMA (0.3), PUR35%PMMA (0.15), and PUR (0.25).

the exception of S1-PUR25%-PMMA and S1-PUR35%-PMMA. The three transition peaks are assigned to PUR-rich phase, interphase, and PMMA-rich phase, respectively. The corresponding Tgs of these phases were determined from the tan δ curves and were summarized in Table 1. Figure 2 illustrates the relationships of Tgs of each phase with PUR content of these two series IPNs. The Tg of the PUR-rich phase decreased as the PUR content in the IPN increased. Different features were obtained for the two series IPNs when the PUR content was increased to 25 and 35 wt %. In the case of series 1 IPNs (see

Polymer Networks from Canola Oil-Based Products

Figure 2. Relationships of Tgs determined by DMA of each phase with PUR content of two series IPNs.

Figure 3. DSC curves of series 1 IPNs (NCO/OH molar ratio 1.6/ 1.0). The curves were shifted vertically to discriminate transitions.

Figure 1a), the second and third transition peaks were combined into one broad peak, whereas in the case of series 2 IPNs, three well-resolved transition peaks (see Figure 1b) were always observed. It is worth mentioning that the existence of interphases is common in multicomponent polymer systems, which is due to the incomplete mixing of the two polymers.12 The interphase transition results from molecular motion of chains at the interface, and the transition region is an area of the interphase interaction caused by common intermolecular forces; in particular, it is a result of local diffusion. Shifts of Tg and weight fraction are the two common parameters used to determine the degree of phase mixing. The details are discussed later. Nevertheless, this interphase transition was not observed with the conventional DSC measurement. The DSC curves of series 1 IPNs are shown in Figure 3 as an example. It is obvious that only two glass transitions corresponding to PUR-rich phase and PMMA-rich phase were obtained. To illustrate this difference between conventional DSC and DMA measurements, the nature of these two technologies should be mentioned. Conventional DSC often provides only a weak indication of Tg because it has relatively poor sensitivity. The identification of Tg is also influenced by any additional phenomenon, which occurs near the Tg process.44 DMA is very sensitive to the glass transition because the mechanical properties of polymer such as modulus and damping, measured by DMA usually undergo large changes during the glass transition.45 It provides the most credible


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information and is therefore well-suited to measure Tg. Nevertheless, DMA traces are frequency dependent. Generally, after applying frequency shifts based on the WLF equation,45 Tg determined by DMA is in agreement with the Tg value determined by DSC. Recently, with the invention of MDSC, a new quantitative method based on the measurement of the differential of reversing heat capacity versus temperature, dCp/dT, has been developed.30 This is due to the fact that MDSC provides not only the total heat capacity signal, but also its kinetic components. The total heat capacity is divided into reversing heat capacity and nonreversing heat capacity. The value of heat capacity at glass transition can be determined independent of any enthalpic effects. Meanwhile, the differential manipulation of reversing heat capacity also increases its sensitivity, therefore, the Tg determined by this method has a high resolution compared to the above-mentioned other two techniques. It has been proved that this method could be a useful alternative or a powerful complement to NMR, scattering and direct nonradiative energytransfer methods in the study of degree of phase mixing in polymer blends and IPNs.31 Another advantage of this method is the possibility of determining weight faction of each compound in multicomponent polymer materials.26,30,31 The dCp/dT versus temperature plots for pure PMMA, S1PUR15%-PMMA, S1-PUR25%-PMMA, S1-PUR35%-PMMA, S1-PUR45%-PMMA, and pure PUR are shown in Figure 4a-f, respectively. Dashed lines are the peak deconvolution results. Similar to the DMA results discussed previously, there are two transition peaks in pure PMMA. However, two well-resolved peaks at 5 and 25 °C are obtained for the pure PUR, which are different from the DMA result where only one transition peak was observed. The peak located at 5 °C corresponds to the sol phase in PUR. The existence of the sol phase is due to the fact that there was about 5% of mono-ol and 9% of saturated TAGs in the synthesized polyol, as reported in our previous paper.5 These types of “inert” material would disrupt the network formation and would form the sol fraction in the PUR networks. In terms of IPNs, a five-phase morphology was observed. From the lowest to highest temperature, these correspond to a sol phase (phase 1), a PUR-rich phase (phase 2), a PUR-rich interphase (phase 3), a PMMA-rich interphase (phase 4), and a PMMArich phase (phase 5). Similar results have been reported by Hourston and his co-workers for a PUR-polystyrene IPN system.30 The observation of the sol phase and resolution of interphase suggest that the dCp/dT versus temperature signal of MDSC seems to be somewhat more sensitive than DMA and conventional DSC. It could be used to determine those phases whose domain sizes are smaller than 15 nm. Furthermore, the weight fraction of each phase could be determined from the area under the peak. The Tgs and weight fractions of each phase of all the IPNs are listed in Table 2. The Tg values were about 10–20 °C lower than corresponding values determined by DMA (Table 1), which indicated again that the measurement principle of these techniques are different. MDSC measurement may be sensitive to a difference in heat capacity and material weight fraction, whereas DMA may to a much greater extent be influenced by phase continuity.26 IPN Phase Behavior. The phase behavior of a sequential IPN depends on the thermodynamic compatibility of the constituent polymers, the cross-linking density (mainly that of the first network), kinetics of polymerization, and so on. PUR/PMMA IPN comprises two components with different solubility parameters, 19.5 (J/cm3)1/2 for PMMA46 and 18.1 (J/cm3)1/2 for PUR (calculated from cohesive energy densities and the volume contribution

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Figure 4. dCp/dT vs temperature curves of (a) pure PMMA, (b) S1-PUR15%-PMMA, (c) S1-PUR25%-PMMA, (d) S1-PUR35%-PMMA, (e) S1-PUR45%-PMMA, and (f) pure PUR. The dash lines are peak deconvolution results.

of atomic groups using group contributions theory47), implying that this is an incompatible system. Therefore, these IPNs are a result of forced compatibility, resulting from the cross-linking network, which prevents vigorous phase separation and leads to a microheterogeneous structure of the resulting sequential IPNs.16 In other words, the cross-linking density is the domain factor for determining phase behavior of sequential IPN. Cross-linking density of a polymer network can be described by the following equation according to the theory of rubber elasticity:48

E′ ) νe RT 3

(1)

where E′ is the storage modulus, R is the gas constant, νe is the

cross-linking 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, then superposed into respective master curves using the time–temperature superposition principle.45 The values of νe obtained are 8.9 × 102M/m3 and 9.9 × 102 M/m3, for PUR with NCO/ OH molar ratios of 1.6/1.0 and 2.0/1.0, respectively. The difference of cross-linking density, combined with the kinetics of polymerization, resulted in different phase behavior for series 1 and 2 IPNs. PUR/PMMA sequential IPNs are obtained by a two-step process, consisting in the diffusion of monomers into a preformed network, with a heterogeneous swelling of the



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Table 2. PhaseTg and Weight Fraction of all the IPNs Determined by MDSCa phase 1

series 1

series 2

a

S1-Pure-PMMA S1-PUR15%-PMMA S1-PUR25%-PMMA S1-PUR35%-PMMA S1-PUR45%-PMMA S1-Pure-PUR S2-Pure-PMMA S2-PUR15%-PMMA S2-PUR25%-PMMA S2-PUR35%-PMMA S2-PUR45%-PMMA S2-Pure-PUR

phase 2

phase 3

Tg (°C)

weight fraction (wt %)

Tg (°C)


Tg (°C)


-9 ( 1 -2 ( 1 -4 ( 2 -5 ( 1 5(1

4(2 7(2 6(2 6(3 28 ( 1

23 ( 1 29 ( 1 32 ( 1 26 ( 1 24 ( 2

24 ( 2 30 ( 1 35 ( 2 46 ( 2 71 ( 1

75 ( 3 68 ( 1 75 ( 1 63 ( 2

-3 ( 2 2(1 2(2 1(1 9(1

3(1 8(2 7(2 8(1 30 ( 2

32 ( 2 42 ( 1 35 ( 1 36 ( 2 29 ( 1

30 ( 2 36 ( 1 34 ( 2 44 ( 2 69 ( 1

80 ( 1 95 ( 2 75 ( 1 73 ( 3

phase 4

phase 5

Tg (°C)


Tg(°C)


25 ( 2 9(1 18 ( 1 8(1

107 ( 1 101 ( 1 111 ( 2 102 ( 1

25 ( 1 17 ( 1 15 ( 1 13 ( 2

115 ( 1 119 ( 1 120 ( 1 121 ( 2 124 ( 1

97 ( 1 22 ( 1 38 ( 1 17 ( 2 26 ( 1

18 ( 2 19 ( 2 14 ( 1 8(1

104 ( 1 113 ( 2 104 ( 1 102 ( 2

20 ( 1 17 ( 1 15 ( 1 14 ( 2

115 ( 1 117 ( 1 122 ( 1 120 ( 1 123 ( 2

97 ( 1 29 ( 1 20 ( 2 29 ( 1 26 ( 1

Errors are standard deviations; n ) 3.

Figure 6. Variation of interphase weight fraction (the total weight fraction of PUR-rich interphase and PMMA-rich interphase) vs PUR content of the two series IPNs.

Figure 5. Variation of Tgs of each phase (apart from phase 1) determined by MDSC with PUR content: (a) series 1 IPNs (NCO/OH molar ratio 1.6/1.0) and (b) series 2 IPNs (NCO/OH molar ratio 2.0/ 1.0).

primary network, likely leading to several domains (phases) having different segment concentration at the microstructural level as listed in Table 2. The relationships of Tgs of each phase (apart from phase 1) with PUR content of series 1 and 2 IPNs are illustrated in Figure 5a and b, respectively. Weight fraction of interphase (the total weight fraction of PUR-rich interphase and PMMA-rich interphase) versus PUR content of the two series IPNs are shown in Figure 6. As seen in Figures 5 and 6, the general trend of the shift of Tg of the interphase and the variation of the weight fraction of the interphase are similar for

both series of IPNs, but some substantial differences existed. For the S1-PUR35%-PMMA and S2-PUR25%-PMMA IPNs, the Tgs of phases 3 and 4 were shifted to higher temperatures, and the weight fractions of interphases of these two samples were also much higher, with the exception of the IPNs containing 15 wt % of PUR. In other words, for IPN with NCO/ OH molar ratio 1.6/1.0, the highest degree of phase mixing was obtained for the sample with 35 wt % of PUR content, while for the IPN with NCO/OH molar ratio 2.0/1.0, the highest degree of phase mixing was obtained for that with 25 wt % of PUR content. This could be explained by considering the formation process of sequential IPNs. In the first stage of sequential IPN formation, a swelling of the PUR network occurs 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. After the transition of the system from the one-phase state into the metastable and unstable stages, the stable chain entanglements (physical or chemical) do not allow the full separation of network fragments and the system remains in a state of forced compatibility. For the IPN with NCO/OH molar ratio 2.0/1.0, the cross-linking density of first formed PUR network is higher, therefore, it requires less PUR content to attain acceptable compatibility. The effects of forced compatibility in IPNs are of great importance for understanding their phase behavior. Forced compatibility leads to an additional contribution to the free

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Figure 7. Changes of storage modulus (G′) and loss modulus (G′′) with time at 50 °C for PUR (a) NCO/OH molar ratio 1.6/1.0 and (b) NCO/OH molar ratio 2.0/1.0.

energy of the system. As a result, the system is stabilized when the thermodynamic driving force for phase separation is balanced by the forces from entanglements of network fragments. On the other hand, the final structure of the IPN is also determined by the time at which the system loses its mobility due to cross-linking. Namely, the reaction kinetics, the composition, and the diffusion are the most essential parameters, which control IPN structure and phase behavior as well.12 As mentioned previously, during formation of sequential IPNs, the initial reaction system is a one-phase system. In the course of reaction, the thermodynamic incompatibility of growing networks appears at a certain value of conversion degree. Because of this, the extent of phase separation is determined by the rate with which the system achieves the state of immiscibility. Therefore, the gelation time of first formed PUR network becomes the controlling factor of phase separation, because the free energy of the system might turn to positive at the gelation point. The gelation time was measured by monitoring the evolution of storage modulus (G′) and loss modulus (G′′) with time at 50 °C, as shown in Figure 7a,b for the two pure PUR with NCO/OH molar ratio 1.6/1.0 and 2.0/1.0, 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.49 The gel time is 540 s for PUR with NCO/OH molar ratio 1.6/1.0 and 610 s for that with NCO/OH molar ratio 2.0/1.0, indicating that the former sample reacts faster than the latter. In terms of IPNs, the kinetics of these two distinct polymer networks in the same reaction medium is more complicated. PUR behaves like a viscous medium in which the MMA polymerization proceeds; the initial rates increase as the reaction medium becomes more viscous

Kong and Narine

till a certain value of conversion degree at which the thermodynamic incompatibility of growing networks appears and phase starts to separate. Morphology of IPNs. The morphology of the IPNs is revealed by the electron micrographs (Figure 8) to a limited extent, constrained by the resolution of the instrument. In all the micrographs, the PUR was present as the continuous phase with large globules of PMMA dispersed within it, indicating phase separation. The domains of the dispersed phase were clearly irregular, rather than spherical in shape. It is interesting to note that the phase separation between two polymers appeared to be more pronounced with the increasing of PMMA content. However, it is worth mentioning that SEM is not a very precise technique to determine domain sizes smaller than a few tens of nanometers. It should also be noted that morphological classification does not allow one to distinguish the real level of mixing due to the different scale of electron microscopy measurements (not molecular scale). Besides, morphology gives no answer as to the phase composition of the structures revealed through electron microscopy. Mechanical Properties of IPNs. The stress versus strain curves with various PUR content for the two series of IPNs are shown in Figure 9a and b, respectively. In some cases, the tensile properties improved dramatically when suitable amounts of PUR were added. The following examples illustrate this point. Tensile strength decreased from 77 ( 1 MPa to 68 ( 2 MPa when 15 wt % of PUR was added in the IPNs, and Young’s modulus decreased as well. A yield point with yield strength 67 ( 1 MPa in the case of S1-PUR25%-PMMA IPN and 75 ( 1 MPa in the case of S2-PUR25%-PMMA IPN was displayed when the PUR content was increased to 25 wt %. Then the yield strength dropped drastically when the PUR content was beyond 35 wt %. Different tensile properties were achieved when PUR content was increased to 45 wt %; S1-PUR45%-PMMA IPN (with NCO/OH molar ratio 1.6/1.0) behaves as a hard rubber with long elongation, while S2-PUR45%-PMMA IPN (with NCO/OH molar ratio 2.0/1.0) has a typical mechanical property of rigid plastics with a yield point. This is not surprising as Tg (25 °C as exhibited in Table 2) of the PUR-rich phase for the S1-PUR45%-PMMA IPN coincides with the measuring temperature (25 °C), whereas Tg (35 °C) of the corresponding phase for the S2-PUR45%-PMMA IPN is higher than the measuring temperature. Similar results were observed for the corresponding two pure PUR systems and similar reason should apply, that is, Tg of PUR with NCO/OH molar ratio 1.6/1.0 is 22 °C and that with 2.0/1.0 is 30 °C (by MDSC). As shown in Figure 9a,b, the original PMMA was rather brittle, and the PUR was rubbery but fairly weak, while the mechanical properties of IPNs are superior to those of its constituent polymers. This is due to the finely divided rubber and plastic combination structures of the IPNs, as shown in Figure 8. Furthermore, higher NCO/OH molar ratio enhanced the probability of reaction with polyol to generate urethane linkages after these components have been diluted in PMMA. Higher yield strength was achieved for the IPNs with the same PUR/PMMA composition but higher NCO/OH molar ratio. In the meantime, longer elongation at break was obtained for the IPNs with the same PUR/PMMA composition but lower NCO/ OH molar ratio. However, the details of structure versus composition relation and effects on mechanical properties of this new type of IPN are still unknown. To this end, our group is currently engaged in further studies of the IPNs made from vegetable oil-based polyol with terminal functional groups investigating their various properties.



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Figure 8. Scanning electron micrographs of (a) S1-PUR15%-PMMA, (b) S1-PUR25%-PMMA, (c) S1-PUR35%-PMMA, (d) S1-PUR45%-PMMA, (e) S2-PUR15%-PMMA, (f) S2-PUR25%-PMMA, (g) S2-PUR35%-PMMA, and (h) S2-PUR45%-PMMA.

Conclusions IPNs have been prepared using polyol with terminal primary functional groups synthesized from canola oil and PMMA. The phase behavior of the IPNs has been investigated

using a new quantitative method based on the measurement of the differential of reversing heat capacity versus temperature, dCp/dT, from MDSC. A five-phase morphology, that is, sol phase, PUR-rich phase, PUR-rich interphase, PMMA-

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Figure 9. Stress vs strain curves for two series IPNs with (a) NCO/ OH molar ratio 1.6/1.0 and (b) NCO/OH molar ratio 2.0/1.0.

rich interphase, and PMMA-rich phase have been detected in all the IPN samples. The thermal properties and phase behaviors were influenced by NCO/OH molar ratios (i.e., cross-linking density of PUR network) as well as PUR/ PMMA compositional variations. The lower PUR component (25 wt %) with higher NCO/OH molar ratio (2.0/1.0) appeared already favorable for generating IPN with a higher degree of mixing. In contrast, 35 wt % of PUR component with lower NCO/OH molar ratio (1.6/1.0) was required to generate IPN with a higher degree of mixing. The mechanical properties of the IPNs were superior to those of the constituent polymers as a result of the finely divided rubber and plastic combination structures of the IPNs. 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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Physical Properties of Sequential Interpenetrating Polymer Networks

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