Relationship between the Morphology of Nanostructured Unsaturated

Article pubs.acs.org/JPCC

Relationship between the Morphology of Nanostructured Unsaturated Polyesters Modified with PEO‑b‑PPO‑b‑PEO Triblock Copolymer and Their Optical and Mechanical Properties Daniel H. Builes,†,‡ Hugo Hernandez,‡ Iñaki Mondragon,† and Agnieszka Tercjak*,† †

Group ‘Materials + Technologies’, Polytechnic School, Dpto. Ingeniería Química y M. Ambiente, Universidad País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), Pza. Europa 1, 20018 Donostia-San Sebastián, Spain ‡ Department of Innovation, Andercol S.A. Autopista Norte 95-84, Medellín, Colombia S Supporting Information *

ABSTRACT: Nanostructured thermosets were achieved by mixing an unsaturated polyester (UP) resin with an amphiphilic poly[(ethylene oxide)b-(propylene oxide)-b-(ethylene oxide)] block copolymer (EPE). Differential scanning calorimetry and dynamic light scattering were used to study the miscibility and molecular dynamics of nonreactive mixtures. Obtained results indicated that the formation of the nanostructured thermosets followed a self-assembly mechanism. Atomic force microscopy was used to study the morphology of the thermosets. It was found that mixtures cured at 25 °C nanostructured with smaller domains had higher transparency if compared to the mixtures cured at 35 °C. The mechanical properties of nanostructured thermosets showed that UP resin was significantly toughened by addition of the EPE. Results indicated that, for an EPE content of 15 wt %, the critical stress intensity factor, KIc, of the mixture increased ca. 40%, if compared to the neat UP thermoset.

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INTRODUCTION Thermosetting resins usually show poor fracture toughness mainly due to the brittleness derived from the high crosslinking density.1,2 To overcome this limitation, they are mixed with other components such as fibers or thermoplastics.3,4 These additives usually reduce costs and improve mechanical properties; however, they simultaneously cause a decrease in the optical transparency of thermosetting based materials. Nanostructured thermosetting matrixes with high transparency5−8 and improved mechanical properties9−12 can be obtained by mixing the thermosetting precursor with block copolymers (BCPs), which are able to phase separate within a nanometric scale. Consequently, many researchers have focused their interest on toughening thermosetting systems by this method due to the potential applications where optical (i.e., bulk and surface appearance) and mechanical properties are required simultaneously which could also be utilized in applications such as adhesives13 and functional coatings.14,15 In the case of unsaturated polyester (UP) resins, interest in their advantages as related to chemical resistance, high strength and modulus, and easy processability under various conditions16,17 has been growing since they began to be commercialized in 1941.18 Recently, nanostructures of these resins7,8,10,19 achieved by self-assembly or polymerization induced phase separation (PIPS) of the BCP have been widely reported. It has been shown that the phase behavior and microdomain structures of thermosetting matrixes modified © 2013 American Chemical Society

with BCPs are determined by curing kinetics, BCP content, and the ratio between blocks.12,20 The family of poly[(ethylene oxide)-b-(propylene oxide)-b(ethylene oxide)] triblock copolymers (PEO-b-PPO-b-PEO) is one of the most studied,21 mainly in mixtures with water, epoxy,11,12,22,23 or UP resins.19,24 In the case of aqueous solutions, the capability of PEO-b-PPO-b-PEO to form microstructures was deeply investigated: at temperatures lower than 15 °C, the blocks are miscible with water, and near 25 °C, water becomes a selective solvent25,26 due to the water-immiscible PPO block form micelles stabilized by watermiscible PEO blocks.21,27−31 At higher temperature, PEO blocks lose their miscibility with water, leading to macrophase separation of BCP in a low critical solution temperature (LCST).21,31,32 An analogous phenomena was shown for mixtures of this family of BCPs with nonreactive UP resins; viz., the miscibility is mainly controlled by hydrogen boning interactions, the phase separation temperature of the PPO block from nonreactive UP resin is lower than the phase separation temperature of the PEO blocks, and LCST behavior is also presented.24,33 In our previous work,33 a commercially available UP resin was modified with a PEO-b-PPO-b-PEO triblock copolymer. In Received: October 2, 2012 Revised: January 18, 2013 Published: January 23, 2013 3563

dx.doi.org/10.1021/jp3097617 | J. Phys. Chem. C 2013, 117, 3563−3571

The Journal of Physical Chemistry C

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UP/EPE nonreactive mixtures were performed in a PerkinElmer DSC-7 calorimeter under a helium flow of 10 mL min−1 as purge gas. Samples of approximately 15 mg were placed in 40 μL sealed aluminum pans. The samples were first heated from 25 to 60 °C at 20 °C min−1. Then, the samples were cooled from 60 to −95 °C at a rate of 1 °C min−1. A second heating scan was performed from −95 to 60 °C at a rate of 10 °C min−1. For the UPol/EPE mixtures, DSC measurements were carried out using a Mettler Toledo DSC 822. Nitrogen was used as a purge gas (10 mL min−1). Measurements were performed in open aluminum pans containing a sample weight of approximately 10 mg. The samples were heated from −60 to 40 °C at 10 °C min−1. Temperature was calibrated by using an indium standard. The maxima of the transition peaks were taken as the melting temperature, Tm. The middle point of the slope change of the heat capacity plot was taken as the glass transition temperature, Tg. Dynamic Light Scattering (DLS). The measurements were performed in a Brookhaven BI-200SM goniometer with a 9000AT correlator. A light beam from a He−Ne laser (Mini L30, wavelength λ = 637 nm, 10 mW) directed to a pot with a glass vat with a refractive index matching liquid surrounding the scattering cell and thermostatted at 25 °C was used. The scattered light intensity was measured at angles of 45, 90, and 135° with respect to the incident beam. A total of 256 ratio spaced delay channels were used with a sampling time of 20 μs covering a delay time range from 20 μs to 200 ms. In order to avoid afterpulsing, the first correlation time channel was discarded during data analysis. Each measurement was carried out for 10 min and repeated several times. The measured quantity in DSL experiments is the fluctuation in the function of time of the scattered light intensity, I(t), which is expressed in terms of the normalized intensity correlation function, g2(t). The information about particle size is contained in the relaxation or decay rate of the fluctuations, Γ (i.e., rapid diffusion of small particles leads to fast decay, while slow fluctuations result from the motions of larger particles).35,34 Γ is defined as τ−1 = Dq2, where τ is the decay time, D is the diffusion coefficient, q = (4πn/λ) sin(θ/2) is the magnitude of the scattering wave vector, n is the refractive index of the medium, λ is the wavelength of the laser in a vacuum, and θ is the scattering angle. The decay rate distributions, calculated from the inverse Laplace transform of the measured intensity correlation function, were analyzed using the CONTIN regularization procedure. The samples of nonreactive UP/EPE mixtures were prepared following the protocol explained above after filtering the UP resin in two steps. The first step was through an Albet FVC filter, and the next through a Durapore Membrane 0.45 μm HV filter. Prior to the measurements, the samples were kept for about 48 h at room temperature which led to a considerable reduction of dust. Atomic Force Microscopy (AFM). The morphology of cured mixtures was analyzed using atomic force microscopy with a scanning probe microscope (SPM) (NanoScope IIIa Multimode from Digital Instruments, Veeco Instruments, Inc.) in tapping mode (TM-AFM). One beam cantilever (125 μm) with a silicon probe (curvature nominal radius of 5−10 nm) was used. Samples were cut using an ultramicrotome Leica Ultracut R with a diamond blade. UV−vis Measurements. UV−vis transmittance spectra of 1 mm thickness sheets of UP/EPE cured mixtures were obtained using a spectrophotometer (Shimadzu UV-3600) in

that work, the temperature−BCP content phase diagram of nonreactive UP resin/BCP mixtures was reported. Furthermore, partial miscibility of PEO blocks and phase separation of the PPO central block from the cross-linked UP-rich phase was proved for mixtures precured at 35 and 60 °C, leading to microand macrophase separation, respectively. Taking into account the behavior of this family of BCPs in aqueous solutions, it seems that significant changes in self-assembled microstructures of systems with nonreactive UP resins, and consequently in the final morphology of thermosetting systems, could be reached if the curing process begins at temperatures lower than 35 °C. In order to investigate the last temperature dependency on miscibility, in the present work, the relationship between morphology, transparency, and the mechanical properties of the thermosetting system using different PEO-b-PPO-b-PEO contents were investigated using two different curing cycles with precuring at 25 and 35 °C. Differential scanning calorimetry (DSC) and dynamic light scattering (DLS) were used to study liquid nonreactive mixtures in order to better understand the miscibility behavior and the mechanism of the morphology formation of the investigated systems. The relationship between the final morphologies and the transparency of the designed thermosetting system was investigated using atomic force microscopy (AFM) and ultraviolet−visible spectroscopy (UV−vis). In addition, the flexural modulus, E, critical stress intensity factor, KIc, and the critical strain energy release rate, GIc, were studied to discuss the mechanical properties of cured mixtures.

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EXPERIMENTAL SECTION Materials and Chemicals. Linear PEO-b-PPO-b-PEO triblock copolymer with structure E20P69E20 (E = ethylene oxide and P = propylene oxide, the subscripts indicate the number of repeated units) named here as EPE, with a number average molecular weight (Mn) of 5750 g mol−1, was purchased from Aldrich Chemical, Inc. A commercial orthophthalic UP resin with 36 wt % styrene (St) with the trade name Crystalan 860 manufactured by Andercol S.A. was used as the thermosetting precursor. Methyl ethyl ketone peroxide (MEKP) with the trade name Peroxan ME50L as polymerization initiator and cobalt n-octoate in solvent mixture-6% (OCo) as accelerator promoter were supplied by Hegardt S.L. Blending Protocol. UP resin was preaccelerated mixing it with 0.3 phr OCo, i.e., 0.3 g of OCo per 100 g of UP resin. UP/ EPE nonreactive mixtures were prepared mixing UP resin with an adequate EPE content by stirring with a magnetic mixer until a homogeneous liquid was obtained. The UP/EPE mixtures were designed taking into account the EPE content (wt %) followed by the name of the block copolymer; e.g., the mixture named 25% EPE contained 25 wt % EPE and 75 wt % UP resin. UP oligomers, UPol, and UPol/EPE mixtures were obtained by evaporation of St from thin films of UP/EPE mixtures for 2 weeks in a vacuum stove at 25 °C. The reacting mixtures were prepared adding 1.5 phr MEKP, i.e., 1.5 g of MEKP per 100 g of UP resin. A mold with two flat glasses separated by a U-shaped PTFE sheet was used. Two isothermal curing cycles were carried out in a forced convection oven. In the first cycle, the mixtures were precured at 25 °C for 24 h, followed by 12 h at 35 °C, 3 h at 85 °C, and 1 h at 170 °C. In the second cycle, the mixture was precured at 35 °C during 48 h, followed by 3 h at 85 °C and 5 h at 170 °C. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry measurements of EPE/St mixtures and 3564



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the spectral range between 200 and 800 nm and performed at 25 °C. Mechanical Properties. Three-point bending and fracture toughness tests were performed following ASTM D 790-10 and ASTM D 5045-99 standards, respectively, in a universal testing machine MTS model Insight 10 provided with a 250 N load cell. Rectangular samples of 12.7 × 1.0 × 40 mm3 using a length span of 16 mm were tested at a crosshead rate of 0.4 mm min−1 for flexural tests. Flexural modulus was determined from the slope of the load−displacement curve. The fracture toughness was evaluated in terms of the critical stress-intensity factor, KIc, and the critical strain energy release rate, GIc. An approximate estimation of KIc values was obtained from a three-point bending test performed on single edge notched specimens (SENB). GIc was obtained directly from the energy derived from integration of the load−displacement curve up to the same load point as used for KIc. Rectangular samples of 6 × 1.5 × 30 mm3, with 2.7 mm V-shaped notches and microcracked were tested at a crosshead rate of 10 mm min−1 and length span of 23 mm. At least five measurements were carried out per mixture.

Table 1. Thermal Transitions for Non-Reactive UP Resin/ BCP Mixtures during Second Heating Scan UP/EPE

Tg‑UP (°C)

100/0 95/5 85/15 75/25 65/35 50/50 40/60 0/100

−62 −62 −59 −56 −54 b b

Tg‑EPE (°C) b −87 −84 −82 −80 −77 −63

Tg‑UPola (°C) −17 −18 −18 −25 −34

a

Values of Tg reported in this column correspond to UPol/EPE mixtures which were obtained evaporating St from the UP/EPE mixtures with the compositions of the first column. bNot detected.

case of 50% EPE and 60% EPE mixtures, corresponding melting peaks were observed due to the strong influence of crystalline PEO blocks. The melting peak of these mixtures shifted to lower temperatures (25 and 19 °C, respectively) if compared with the Tm of PEO blocks in EPE. This behavior can be attributed to the partial miscibility of PEO blocks with UP resin.38 Mixtures containing less than 35 wt % EPE did not show any detectable endothermic transitions, indicating higher miscibility between PEO blocks and UP resin, which can be related to the formation of intermolecular hydrogen bonds between the ether oxygen of PEO blocks and the hydroxyl groups of UPol.8,39 For 35% EPE, 25% EPE, and 15% EPE mixtures, two Tg’s were detected, one associated with UP resin, Tg‑UP, and the other with EPE, Tg‑EPE. In these mixtures, the change of EPE leads to shifts in both thermal transitions. The decrease of EPE content resulted in the shift of Tg‑EPE of mixtures to lower temperatures if compared with Tg of EPE. In the case of 5% EPE, Tg‑EPE was not distinguished due to the low amount of segregated phase or to the fact that this transition occurs at temperatures below the measurement range. Simultaneously, the Tg‑UP of mixtures shifted to higher values if compared with the Tg‑UP of UP resin. In the case of 50% EPE and 60% EPE mixtures, this transition was not detected possibly due to an overlapping of the melting peak of the PEO blocks. Here, one can ask two questions. The first one, why the Tg‑UP of the mixtures did not tend to be between the Tg of EPE and the Tg of UP resin. The second one is why did the Tg‑EPE of mixtures decrease with the decrease of the EPE content. The last phenomena could be better understood taking into account the interaction parameters of the ternary system UPol/St/EPE. Applying the Hoftyzer and Van Krevelen method,40 the calculated interaction parameters for UPol-St, UPol-PPO, and PPO-St were χUPol‑St = 2.85, χUPol‑PPO = 4.1, and χPPO‑St = 0.36, respectively. Those values indicate high compatibility between PPO and St and a very low one between UPol and PPO, which is in good agreement with the results reported in the literature.24,33,41 Thus, Tg‑UP could be associated with a phase composed of UPol, St, and PEO (UPol-rich phase), whereas Tg‑EPE could be associated with a separated phase composed of PPO and St (PPO-rich phase). Consequently, the changes in both Tg‑UP and Tg‑EPE can be related to a distribution of St between the UPol-rich phase and the PPO-rich phase. In order to verify the effect of St on Tg, DSC of both UPol/ EPE and EPE/St mixtures was carried out (for simplicity, they were not included). In the case of UPol/EPE mixtures, the neat UPol exhibited a Tg‑UPol at −17 °C (see Table 1), whose value

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RESULTS AND DISCUSSION Nonreactive UP/EPE mixtures with the ratio between components from 100/0 to 40/60 were investigated. Visually, all of these liquid mixtures were homogeneous and transparent at room temperature, indicating that no phase separation occurred at the scale of wavelength of visible light. In order to study the miscibility of these mixtures, a DSC study was carried out. Figure 1 shows the thermal transitions of nonreactive neat UP resin, pure EPE, and nonreactive UP/EPE mixtures during

Figure 1. DSC thermograms of nonreactive neat UP resin, pure EPE, and UP/EPE nonreactive mixtures from 5 to 60 wt % of EPE during the second heating scan. The lines (|) denote the midpoints of the transitions.

the second heating scan (cooling scans were not shown here for brevity). Table 1 summarizes the values of detected Tg’s. The Tg of pure EPE was around −63 °C, and the melting point, that corresponds to PEO blocks, was detected around 39 °C. These transitions are in good agreement with those reported in the literature.36,37 The neat nonreactive UP resin showed a single Tg at around −62 °C. Once EPE was added to the UP resin, investigated mixtures displayed changes in Tm and Tg’s. This fact could be related to partial miscibility between EPE and UP resin. In the 3565



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be related to DSC results, where a phase separation was detected in a transparent mixture. This confirms the formation of self-assembled micelles in nonreactive UP/EPE mixtures at length scales below the wavelength of visual light. These structures consist of a PPO-rich phase surrounded by an UPrich phase. Under preparation conditions, homogeneous and transparent cured mixtures were achieved, indicating the lack of macrophase separation in all designed thermosetting systems. The morphology of the fully cured mixtures was investigated by means of TM-AFM. As is shown in Figure 3, the morphology of the neat UP cured resin and UP/EPE cured mixtures contains 5−50 wt %,

decreased with the increasing of the EPE content. This phenomenon can be related to the high miscibility between PEO and UPol, as has been reported elsewhere.38 Comparing the values of Tg‑UPol with Tg‑UP, it is notorious that the latter shifts to higher values if decreasing St, i.e., St dissolved the UPrich phase and decreased its Tg. In addition, DSC thermograms of EPE/St mixtures containing 10, 20, and 30 wt % St displayed Tg’s equal to −78, −83, and −88 °C, respectively. Then, as occurred with the UP-rich phase, St dissolved EPE which can be the reason for the reduction of Tg‑EPE of UP/EPE mixtures. Taking the differences in miscibility between UPol, St, and blocks of EPE into account, the Tg‑UP of the investigated mixtures shown in Figure 1 could be attributable to the UP-rich phase, the Tg‑EPE to an immiscible PPO-rich phase, and the changes of both Tg’s could be attributable to a distribution of St among these two phases. Furthermore, the visual appearance of the mixtures at temperatures from 25 to −90 °C was followed, and no changes in transparency were detected. In order to investigate the ability of EPE to the self-assembly in its mixtures with UP resin, the dynamics of nonreactive mixtures varying EPE content from 1 to 5 wt % was monitored by DLS at 25 °C. Figure 2 shows the normalized intensity correlation function, g2(t), obtained for the neat UP resin and the 1% EPE, 3% EPE, and 5% EPE mixtures.

Figure 2. Intensity correlation function versus time at a scattering angle of 90° for neat UP resin (rhombus) and 1% EPE (squares), 3% EPE (triangles), and 5% EPE (circles) mixtures. Only each third data point is shown.

A comparison of the intensity correlation functions reveals that the mixtures shifted to lower relaxation times if compared with neat UP resin. The tendency of the correlation functions to shift to lower relaxations or decay times, τ, is indicative of a new regimen with faster dynamics.29,42,43 This phenomenon suggests that smaller structures were generated in the mixtures when EPE was added. Decay time distributions corresponding to 5% EPE at 135, 90, and 45° displayed a coincidence of the modes at ca. τq2 = 7 × 1012 s m−2, which suggests a diffusive character.29,35,44,45 Indeed, the ability of EPE to self-assemble in aqueous solutions44 forming structures such as spherelike, wormlike, very long wormlike micelles,46 vesicles,30 and multilamellar vesicles,47 viz., “onions”, has been well established. Some of these structures depend on BCP content and temperature;48 e.g., changes from spherelike to wormlike morphology were obtained with the increase of temperature.21,31,49,50 Besides, it has been reported elsewhere28,30,51,52 that the last structures presented instability. DLS results could

Figure 3. AFM phase images (1 μm × 1 μm) of cured: (a) neat UP; (b) 5% EPE; (c) 15% EPE, (d) 25% EPE, and (e) 50% EPE. The insets at the top of each image correspond to the digital image of transparency of a sheet of 1 mm thickness. The insets at the bottom correspond to 200 nm × 200 nm AFM images.

confirming that all investigated mixtures reached microphase separated structures. The optical transparency of 1 mm thickness sheets is seen in the digital images as insets at the top of each image. The neat UP resin reached the typical spherelike morphology generated by the heterogeneous curing process. For the 5% EPE mixture, the spherelike nanostructure of the microphase separated EPE from the UP-rich matrix was clearly distinguished (see Figure 3b). From analysis of the morphology generated in investigated systems, one can 3566



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elsewhere for PEO-b-PPO-b-PEO triblock copolymers mixed with fully cured UP19,33 and epoxy thermosets.11,12,22,23,37 Figure 4 summarizes the proposed process of the structure

conclude that the increase of EPE content caused a progress in the morphology from spherelike to wormlike nanostructures with a progressive growth of length domains. Similar evolution in morphology has been observed in other systems53−55 and explained via the combination of many mechanisms such as the coalescence and elongation−distortion of the spherical micelles. As can be seen in Figure 3c−e, the mixtures with EPE content higher than 15 wt % revealed the coexistence of wormlike structures with rather more complex structures encasing another phase surrounded by phase separated domains which were harder than the phases of EPE blocks (see the arrows in the insets at the bottom of Figure 3c−e). The two types of these complex structures were better visualized in the 50% EPE mixture. If observed in detail inside of the wormlike domains (see the inset at the bottom of Figure 3e indicated by the arrow pointing upward), long, narrowed, and almost parallel structures were clearly distinguished. These structures could be generated by the fraction of St that remained in the PPO-rich phase (see the DSC analysis). Thus, it could be suggested that the homopolymerization of that St and its phase separation from the PPO-rich phase was due to the polymerization induced phase separation (PIPS) that took place. This fits with the facts observed in our previous work,33 where a UP/PPO cured mixture achieved similar structures inside the macroseparated domains, and where UP/EPE cured mixtures displayed a Tg‑EPE almost with the same value of the Tg of neat EPE. In addition, as can be seen in Figure 3e (see the inset at the bottom indicated by the arrow pointing downward), closed interconnected wormlike structures, which were probably encasing a cross-linked UP-rich network within, were also easily distinguished. This phenomenon is strongly corroborated with the fact that the enclosed structure presented the typical spherelike nanostructure of the UP-rich matrix. The observed interconnected wormlike structures resembled the interconnected wormlike structures reported in the literature for other thermosetting systems.5,53,56 The instability of the structures of these BCPs in aqueous solutions could also be a property of UP/EPE nonreactive mixtures, which would be one of the reasons for the very irregular microstructures achieved in UP/EPE cured mixtures. The anomalous micellization due to polydispersity, fractions of PPO and PEO homopolymers, and PEO-b-PPO diblock copolymer content in commercial PEO-b-PPO-b-PEO triblock copolymers57−59 could also provoke irregularities in UP/EPE mixtures. Furthermore, in the case of epoxy mixtures, PEO blocks lose their miscibility from the matrix during curing37 which has been proposed elsewhere54 as one of the reasons for the deformation from spherelike to wormlike domains in epoxy mixtures modified with a PEO-b-PEP block copolymer. This loss of miscibility also occurs in UP systems mixed with BCPs composed of PEO blocks,19 and similarly could have an effect on the deformation of nanodomains. Another explanation of the irregular nanostructures was presented in our previous work.33 A more detailed investigation of the mechanisms of structuration is recommended including variables such as the curing rate which is a key parameter for industrial applications. Taking the above into account and the results obtained by DSC and DLS, one can conclude that nanostructured UP/EPE cured mixtures followed a self-assembly mechanism rather than PIPS. Moreover, it could be deduced that the nanodomains were mainly composed of PPO surrounded by PEO partially mixed with a cross-linked UP-rich network, as reported

Figure 4. Schematic description of the structures achieved in UP/EPE mixtures: (a) nonreactive system before and after adding EPE; (b) changes of morphology of cured mixtures upon increase of EPE content.

formation of UP/EPE mixtures by means of a schematic representation. Figure 4a shows the distribution of the components after the self-assembling process in nonreactive mixtures, and Figure 4b the evolution of domain morphology after curing upon increasing EPE content. Comparison between the AFM analysis of the investigated mixtures precured at 25 °C (Figure 3) and these mixtures precured at 35 °C published by us elsewhere33 allowed us to conclude that a lower curing temperature led to smaller and more defined nanostructures. Thus, precuring at those temperatures did not provoke almost any change in diameter of wormlike domains with an increase of EPE content. On the contrary, the length and quantity of wormlike domains increased drastically, e.g., for 35% EPE and 50% EPE, precuring at 35 or at 25 °C changed the diameter of wormlike domains from 16 to 8 nm and from 20 to 12 nm, respectively. The changes in function of the curing temperature confirmed the strong temperature dependence on the miscibility in UP/EPE mixtures. Similar effects with temperature have been reported in the case of PEO-b-PPO-b-PEO in aqueous solutions, where changes to more disordered structures occurred with increasing temperature due to the dehydration of PEO blocks in the corona of micelles.21,25,31 This behavior is expected in both UP/BCP and aqueous solution/BCP systems, since their miscibility is driven mainly by hydrogen bonding interactions. Indeed, similarly an increase of domain sizes with the increasing of the curing temperature for thermosetting systems based on epoxy resin modified with PEO-b-PPO-b-PEO has been reported elsewhere.11,60 3567



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Figure 5. Regular transmittance in UV−vis spectra of neat UP cured resin and UP/EPE cured mixtures: (a) precured at 25 °C and (b) precured at 35 °C.

the size of the scattering entities increases, so does the total scattered light.64 The mechanical properties of the mixtures were investigated using the flexural modulus, E, and fracture toughness by means of the critical stress intensity factor, KIc, and the critical strain energy release rate, GIc. Figure 6 shows values of E, KIc, and GIc for the cured mixtures as a function of EPE content. It is worth noting that E

In order to examine practical application concerning the effects of EPE content and curing temperature on the optical transparency of UP/EPE cured mixtures, UV−vis measurements at wavelengths from 200 to 800 nm were performed. Figure 5 shows the regular light transmittance of UV−vis spectra for neat UP and 15% EPE, 25% EPE, and 50% EPE mixtures precured at 25 °C (Figure 5a) and at 35 °C (Figure 5b). First of all, it should be pointed out that the investigated UP/ EPE mixtures exhibited optical transparency, as confirmed by their visual appearance and transmittance levels. As can be clearly observed, light transmittance of mixtures decreased with an increase of both EPE content and curing temperature. Similar effects on transmittance with additive content of composites has been reported in the literature.61−63 Table 2 shows the values of transmittance at a wavelength of 700 nm of UP/EPE mixtures at the several EPE contents precured at 25 and 35 °C. Table 2. Transmittance at a Wavelength of 700 nm for Several UP/EPE Cured Mixtures transmittance (%) UP/EPE

precured at 25 °C

precured at 35 °C

100/0 85/15 75/25 50/50

91 87 84 61

90 86 79 54

Figure 6. Flexural modulus, E (--Δ--), critical stress intensity factor, KIc (−○−), and critical strain energy release rate, GIc (J m−2), as a function of EPE content in UP/EPE mixtures precured at 25 °C.

Neat UP precured at 25 °C exhibited a transmittance of approximately 91%, and the 15% EPE and 25% EPE mixtures revealed transmittances very close to this value (see Table 2), whereas for the 50% EPE mixture the reduction in transmittance was more notorious although remained transparent. On the other hand, the UP/EPE mixtures precured at 25 °C presented higher transmittances than the systems precured at 35 °C for all investigated mixtures. As known from results obtained by using the AFM, this increase of the curing temperature led to the increasing of domain sizes for the same EPE content. The last phenomenon can be explained taking into account that extinction of light could occur on the surface and in the bulk of the matter due to absorption and scattering effects.15 Moreover, the scattering of light in the bulk is caused by refractive index fluctuations or heterogeneities, as can be seen by the microphase separated domains. Consequently, as

decreased continuously with the increase of EPE content. This fact could be attributed to the plasticization effect of the EPE due to the partial miscibility between PEO blocks and the cross-linked UP-rich network. Messori et al. reported a similar effect on E of a UP resin modified with poly(ε-caprolactone-bperfluoroethers) block copolymer.65 In addition, based on the morphology analysis, one can expect a reduction of E due to a decrease of cross-linking density of an UP-rich network generated by the fraction of St that homopolymerized in a PPO-rich phase. On the other hand, UP resins were clearly toughened by the incorporation of EPE. Some authors have reported thermosetting systems based on UP resins toughened by the addition of modifiers such as BCP,10,65 homopolymers,3,17,66 reactive low profile additives,67 and functional elastomers.68 As shown in Figure 6, the fracture toughness of 3568



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also thank the Spanish Ministry of Economy and Competitiveness for the project MAT2012-31675. Moreover, we are grateful to the Macrobehavior-Mesostructure-Nanotechnology SGIker unit of the UPV/EHU. Dedicated to the memory of Prof. Iñaki Mondragon.

mixtures increased with the increase of EPE content up to 15 wt %. The 5% EPE mixture shows an improvement of fracture toughness of around 30% in comparison with the cured UP, only 10% lower than the maximum improvement reached by the 15% EPE mixture. The improvement of toughness at 5 wt % EPE is comparable with those reported in the literature for UP/plasticizer systems3 and UP/BCP systems.10 One of the reasons for the toughness improvement of the 5% EPE mixture could be attributed to the high dispersion of the phase separated domains (see Figure 3b). The dependence of dispersion, size, and shape of the microstructure on toughness of the cured mixtures has been reported in the literature.2,5,9,69−71

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(1) Pascault, J. P.; Sautereau, H.; Verdu, J.; Williams, R. J. J. Thermosetting Polymers; Marcel Dekker: New York, USA, 2002. (2) Ratna, D. Handbook of Thermoset Resins; iSmithers Rapra Technology: Shrewsbury, U.K., 2009. (3) Bakar, M.; Djaider, F. Effect of Plasticizers Content on the Mechanical Properties of Unsaturated Polyester Resin. J. Thermoplast. Compos. Mater. 2007, 20, 53−64. (4) Serré, C.; Vayer, M.; Erre, R.; Ollive, C. Morphology of the Outermost Layer of a Compression Molded Composite Based on Unsaturated Polyester Resin. J. Mater. Sci. 1999, 34, 4203−4208. (5) Ocando, C.; Tercjak, A.; Martín, M. D.; Ramos, J. A.; Campo, M.; Mondragon, I. Morphology Development in Thermosetting Mixtures through the Variation on Chemical Functionalization Degree of Poly(Styrene-b-Butadiene) Diblock Copolymer Modifiers. Thermomechanical Properties. Macromolecules 2009, 42, 6215−6224. (6) Ocando, C.; Tercjak, A.; Serrano, E.; Ramos, J. A.; CoronaGalván, S.; Parellada, M. D.; Fernández-Berridi, M. J.; Mondragon, I. Micro- and Macrophase Separation of Thermosetting Systems Modified with Epoxidized Styrene-block-Butadiene-block-Styrene Linear Triblock Copolymers and Their Influence on Final Mechanical Properties. Polym. Int. 2008, 57, 1333−1342. (7) Mahaling, R. N.; Vayer, M.; Guillot, S.; Sinturel, C. Oriented Array of Polyethylene-block-Poly(ethylene oxide) Nanoplatelets in Unsaturated Polyesters Cross-Linked Coatings. Eur. Polym. J. 2011, 47, 2277−2282. (8) Sinturel, C.; Vayer, M.; Erre, R.; Amenitsch, H. Nanostructured Polymers Obtained from Polyethylene-block-Poly(ethylene oxide) Block Copolymer in Unsaturated Polyester. Macromolecules 2007, 40, 2532−2538. (9) Dean, J. M.; Grubbs, R. B.; Saad, W.; Cook, R. F.; Bates, F. S. Mechanical Properties of Block Copolymer Vesicle and Micelle Modified Epoxies. J. Polym. Sci., Polym. Phys. 2003, 41, 2444−2456. (10) Serrano, E.; Gerard, P.; Lortie, F.; Pascault, J. P.; Portinha, D. Nanostructuration of Unsaturated Polyester by All-Acrylic Block Copolymers, 1-Use of High-Molecular-Weight Block Copolymers. Macromol. Mater. Eng. 2008, 293, 820−827. (11) Larrañaga, M.; Serrano, E.; Martin, M. D.; Tercjak, A.; Kortaberria, G.; de la Caba, K.; Riccardi, C.; Mondragon, I. Mechanical Properties-Morphology Relationships in Nano-/Microstructured Epoxy Matrices Modified with PEO-PPO-PEO Block Copolymers. Polym. Int. 2007, 56, 1392−1403. (12) Zheng, S. In Epoxy Polymers. New Materials and Innovations; Pascault, J. P., Williams, J. J., Eds.; Wiley-VCH: Weinheim, Germany, 2010. (13) Veselovsky, R. A.; Kestelman, V. N. Adhesion of Polymers; McGraw-Hill: New York, USA, 2002. (14) Bentley, J. In Paint and Surface Coatings-Theory and Practice, 2nd ed.; Lambourne, R., Strivens, T. A., Eds.; Woodhead Publishing Ltd.: Cambridge, U.K., 1999. (15) Boyard, N.; Serre, C.; Vayer, M. A Physical Approach to Define a Class a Surface in Polymer Thermosetting Composite Materials. J. Appl. Polym. Sci. 2007, 103, 451−461. (16) Sharmila, R. J.; Premkumar, S.; Alagar, M. Toughened Polyester Matrices for Advanced Composites. J. Appl. Polym. Sci. 2007, 103, 167−177. (17) Suspene, L.; Yang, Y. S.; Pascault, J.-P. In Toughened Plastics 1. Science and engineering; Riew, K., Kinloch, A. J., Eds.; American Chemical Society: Washington, DC, USA, 1993. (18) Gündüz, G. In Advanced Polymer Processing Operations; Cheremisinoff, N. P., Ed.; Noyes Publications: Westwood, NJ, 1998.

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CONCLUSIONS Novel information about nanostructuration of an UP resin using an amphiphilic PEO-b-PPO-b-PEO triblock copolymer (EPE) was provided. The interaction parameters and thermal and dynamic analysis of the ternary UPol/St/EPE system permitted us to prove that the microphase separation of the central PPO block of EPE took place before the network formation. This phenomenon is related to the fact that the UP resin acted as a selective solvent for the EPE and consequently the designed materials followed a self-assembly mechanism. The morphology of cured mixtures strongly depended on the miscibility of components. It was also proved that curing at 25 °C led to a well-defined structure, smaller microphase separated domains, and higher transparency in the UV−vis spectra than the mixtures cured at 35 °C. Furthermore, a role of St outside of the UP-rich phase was revealed. Due to the miscibility of St and PPO, a fraction of St remained inside the micelles and consequently microphase separation followed the polymerization induced phase separation (PIPS) mechanism generating domains inside the PPO-rich phase. Fracture toughness measurements revealed a maximum value of KIc for the mixture modified with 15 wt % EPE, which improved the toughness more than 40% compared to neat UP resin.

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ASSOCIATED CONTENT

S Supporting Information *

DSC thermograms of EPE/St mixtures containing 10, 20, and 30 wt % St, DSC thermograms of UPol and UPol/EPE mixtures, and intensity weighted decay time distributions for 5% EPE mixture plotted with respect to q2τ at 135, 90, and 45°. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 943 017 169. Fax: +34 943 017 130. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.H.B. is gratefully acknowledged to Andercol S.A (Colombia) and Group ‘Materials + Technologies’ of the University of the Basque Country for their support. A.T. acknowledges MICINN for Ramón y Cajal program (RYC-2010-05592). Financial support from Basque Country Government in the frame of ETORTEK NANOIKER (IE11-304) and SAIOTEK2012 (SPE12UN106) is gratefully acknowledged. The authors wish to 3569



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Relationship between the Morphology of Nanostructured Unsaturated

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