Article pubs.acs.org/IECR
Reaction-Induced Phase Separation and Thermomechanical Properties in Epoxidized Styrene-block-butadiene-block-styrene Triblock Copolymer Modified Epoxy/DDM System Sajeev Martin George,*,†,‡ Debora Puglia,§ Jose M. Kenny,§ Jyotishkumar Parameswaranpillai,*,∥ and Sabu Thomas*,†,⊥,# †
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam, Kerala 686560, India Department of Chemistry, St. Thomas College, Pala, Kottayam, Kerala 686574, India § Materials Engineering Centre, University of Perugia, Local Pentima Bassa, 21, 05100 Terni, Italy ∥ Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, Kerala 682022, India ⊥ International and Interuniversity Centre for Nanoscience & Nanotechnology, Mahatma Gandhi University, Priyadarshini Hills, Kottayam, Kerala 686560, India # Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selongor, Malaysia ‡
S Supporting Information *
ABSTRACT: Styrene-block-butadiene-block-styrene (SBS) triblock copolymer epoxidized with 47 mol % degree of epoxidation (eSBS47) by hydrogen peroxide in a water/dichloroethane biphasic system was blended with epoxy based on diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diaminodiphenylmethane (DDM) as a curing agent. The amounts of eSBS in the blends were 10 and 20 wt %. The evolution of the glass transition temperatures (Tg) of the cured blends at different cure times was analyzed using differential scanning calorimetry (DSC) to understand the thermal behavior of epoxy system under dynamic conditions in the presence of eSBS. Transmission electron microscopy (TEM) analysis revealed core−shell nanodomains of eSBS dispersed in the epoxy matrix. The relationship between rheology and phase separation was carefully explored. Dynamic mechanical analysis (DMA) validated the nanophase-separated structure of the eSBS47-modified epoxy system. Upon addition of eSBS47 to the epoxy system, the fracture toughness of the nanostructured thermosets was improved, and the thermal stability was retained, but the dimensional stability was slightly decreased.
1. INTRODUCTION One of the most commonly used thermosetting polymer, epoxy resin (diglycidyl ether of bisphenol A, DGEBA), is employed in industrial applications in the automotive, aerospace, and electronics industries as an adhesive and structural material, because of its good mechanical and thermal properties, good adhesion to other substrates, good chemical and electrical resistance properties, and so on.1 These specific properties are due to its cross-linked structure. Nevertheless, cured epoxy resins are highly brittle and have poor fracture toughness because of their extreme cross-linked structures when compared to other engineering thermoplastic polymers. To improve the toughness of epoxy resins, different modifiers can be considered to broaden the final application of this type of thermosetting resin. Epoxy resins are commonly modified with elastomers, which usually leads to a loss in modulus and thermal properties.2−4 Engineering thermoplastics such as poly(ether sulfone) (PES) and poly(acrylonitrile-butadienestyrene) (ABS) can be used to replace elastomers with the aim of improving the toughness of the resulting materials.5−11 Recently, block copolymers have been used as modifiers to toughen epoxy resins.12−14 One of the most important properties of block copolymers is the ability to self-assemble into different nanoscale structures.15−17 Nanoscale morpholo© 2014 American Chemical Society
gies such as spherical, wormlike, and vesiclelike structures are formed before or during the curing itself. One of the feasible pathways for generating nanostructures is the use of amphiphilic block copolymers, where one of the blocks is miscible with epoxy resin.14,18−20 Another concept is the chemical modification of one of the blocks to improve the compatibility of the block copolymer in the thermosetting matrix.21 One of the first studies on the formation of nanostructures in epoxy resin modified with block copolymers was reported by Hillmyer et al. in 1997.22 They observed the formation of nanostructures in DGEBA-type epoxy resins, by using poly(ethylene oxide)-block-poly(ethyl ethylene) (PEO− PEE) and poly(ethylene oxide)-block-poly(ethylene propylene) (PEO−PEP) diblock copolymers as modifiers. Later, several studies were carried out by different scientists using reactive and nonreactive diblock and triblock copolymers.12−17 Ordered and disordered nanostructures are formed in the thermosetting matrix prior to the curing reaction itself, and these nanostructures are further fixed with the subsequent curing Received: Revised: Accepted: Published: 6941
December 5, 2013 March 25, 2014 April 8, 2014 April 8, 2014 dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
Table 1. Characteristics of Unmodified and Epoxidized SBS block copolymer
degree of epoxidationa (mol %)
polystyreneb (wt %)
polybutadieneb (wt %)
epoxidized PBb (wt %)
Tg PB/epPB (°C)
SBS eSBS47
0 47
31 31
69 29
0 40
−87 −38
a Mole percentage of epoxidized polybutadiene units measured by solid-state 1H NMR analysis. bWeight percentages of polystyrene, polybutadiene, and epoxidized PB measured by solid-state 1H NMR analysis.
nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) analyses. 2.2. Sample Preparation. Epoxy/eSBS47 blends containing 10 and 20 wt % eSBS47 were obtained by the solvent casting method using tetrahydrofuran (THF) as the solvent. Specifically, block copolymer and epoxy resin were dissolved in THF to obtain a homogeneous mixture, after which the mixture was heated at 80 °C in an oil bath to completely remove the solvent. Next, the temperature was increased to 90 °C, and a stoichiometric amount of the DDM hardener was added to the mixture. The mixtures were then mechanically mixed was for 5−10 min using a magnetic stirrer, poured into a silicone mold at room temperature, and kept in a vacuum oven for 24 h for the removal of air bubbles and solvent. The curing reaction was performed at 50 °C for 2 h, 80 °C for 3 h, and 140 °C for 3 h in an oven, after which the samples were allowed to cool slowly to room temperature. For DSC and rheological analysis, freshly prepared mixtures were used for curing analysis. On the other hand, for other studies, completely cured samples were used.
reaction. The formation of nanostructures such as spherical, vesiclelike, and wormlike micelles in thermosetting matrix due to the presence of an epoxyphobic block in the block copolymer could follow mainly two mechanisms, namely, selfassembly or reaction-induced microphase separation.23−35 In this study, epoxy/4,4′-diaminodiphenylmethane (DDM) systems modified with styrene-block-butadiene-block-styrene (SBS) triblock copolymer epoxidized to a 47 mol % degree of epoxidation (denoted as eSBS47) were analyzed using differential scanning calorimetry (DSC), rheology, transmission electron microscopy (TEM), dynamic mechanical analysis (DMA), universal testing machine (UTM), thermomechanical analysis (TMA), and thermogravimetric analysis (TGA). The eSBS47 used in our study was a mixture of unepoxidized SBS (SBS) and epoxidized SBS (eSBS). This means that the epoxy/ eSBS47 mixture was heterogeneous; that is, unepoxidized SBS was present in the homogeneous epoxy/eSBS phase (eSBS is miscible with the epoxy prepolymer, unlike SBS). The initially homogeneous epoxy/eSBS phase undergoes phase separation.36,37 Different weight percentages of eSBS47 were used to modify the epoxy system. Recently, Mondragon and co-workers epoxidized the butadiene block of a diblock copolymer (styrene−butadiene) and a triblock copolymer (styrene-blockbutadiene-block-styrene) and poly(styrene-block-isoprene-blockstyrene) and isoprene block of poly (styrene-block-isopreneblock-styrene) developed nanostructured thermosets.38−42 However, the effects of the epoxidized triblock copolymer (eSBS47) on the rheological, viscoelastic, thermal, dimensional, and fracture toughness properties in the eSBS47/epoxy system cured with DDM have never been reported. The present investigation focused on the plasticizing action of eSBS47 in the epoxy matrix. The cure kinetics, morphology, rheology, thermal properties, dimensional stability, and fracture toughness of blends of eSBS47-modified thermosets are discussed in the present article.
3. CHARACTERIZATION 3.1. Differential Scanning Calorimetry. To determine the evolution of the glass transition temperature (Tg) and the heat flow (ΔH), DSC analysis was performed using a PerkinElmer Diamond DSC system. Samples of about 5−10 mg were place in an aluminum pan and heated from −30 to 300 °C at a heating rate of 10 °C/min. For kinetic studies, the epoxy/ eSBS47/DDM mixtures were cured in an air oven for periods of 0, 15, 30, 60, 90, 120, 150, 180, and 360 min at 90 °C prior to DSC studies. 3.2. Rheological Analysis. The rheological characterization of the system was carried out using an ARES Rheometric Scientific model dynamic viscosity spectrometer. Steel parallel plates with a diameter of 12.5 mm were used throughout and moved with the help of a computer with specially designed software. A transducer measured the viscous response of the material under the form of a torque exerted by the fluid on the upper plate. A frequency of 1 Hz and 0.5% strain were used for the measurements. Freshly prepared samples (about 5−10 mg) were placed in the parallel plates, and the analysis was done isothermally at different temperatures (90, 110, and 130 °C). 3.3. Dynamic Mechanical Thermal Analysis. Dynamic mechanical tests were carried out on a Perkin-Elmer DMA 8000 dynamic mechanical thermal analyzer (DMTA) equipped with a liquid-nitrogen apparatus operating in three-point bending mode. The frequency used was 1 Hz, and a selected heating rate of 2 °C/min was employed . The specimen dimensions were 50 × 5 × 2 mm3, cut from cured samples. Temperature dynamic scans were carried out from −90 to 250 °C. 3.4. Transmission Electron Microscopy. A Leica Ultracut ultramicrotome was used for the preparation of thin sections of about 70 nm, cut with a diamond knife at room temperature
2. EXPERIMENTAL SECTION 2.1. Materials. Diglycidyl ether of bisphenol A (DGEBA), namely, Araldite LY556 with an epoxide equivalent weight of 186 g/equiv, was obtained from Huntsman. The diamine curing agent 4,4′-diaminodiphenylmethane (DDM) was purchased from Fluka Co, and tetrahydrofuran (99%) was supplied by Sigma-Aldrich. Styrene-block-butadiene-block-styrene triblock (SBS) copolymer supplied by Kraton (KRATON D-1101, Mw = 150000 g·mol−1, 31 wt % styrene) was used as a modifier. The synthesis of epoxidized SBS was done using hydrogen peroxide in the presence of an in situ prepared catalyst system in a water/dichloroethane biphasic system. The reaction procedure for the epoxidation reaction is given in the literature.43,44 Different epoxidation degrees can be obtained by varying the time (2, 4, and 6 h) of epoxidation. Here, we considered epoxidized SBS only with the highest epoxidation degree of 47 mol % (eSBS47) observed after 6 h of epoxidation because it provides the best properties.36 The characteristics of eSBS47 are given in Table 1. The results were obtained by 1H 6942
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
Table 2. Evolution of Tg and ΔH for Epoxy/DDM (Neat Epoxy) and the Systems Containing 10 and 20 wt % eSBS47Modified Epoxy Blends at Different Cure Times
and deposited on copper grids. After treatment in RuO4 vapor, the stained samples were examined in a JEOL JEM-2100 transmission electron microscope at an acceleration voltage of 100 kV in both bright-field and dark-field modes. 3.5. Fracture Toughness Measurements. The singleedge-notched three-point bending test was selected for the evaluation of fracture toughness of the sample. Tests were done using a Lloyd Instruments LR30K universal testing machine and tested according to standard method ASTM D-5045 with a crosshead speed of 10 mm/min. The critical stress intensity factors (KIC) were evaluated according to the equation KIC = (Fmax /BW 1/2)f (a /W )
cure time (min) 0 15 30 60 90 120 150 180 360
(1)
where Fmax is the maximum load at failure, B is the thickness of the specimen, W is the width of the specimen, a is the crack length, and f(a/W) is the expression (given in ASTM standard D-5045) in which the geometry of the sample is considered. Samples in the form of 35 × 8 × 4 mm3 parallelepiped bars were considered, with central V-shaped notches of around 4 mm and a razor-sharp crack tip initiated with a fresh razor blade. At least five successful measurements were made, to obtain significant average values. 3.6. Thermogravimetric Analysis. Thermogravimetric analyses were carried out in a Seiko SII Instruments Exstar 6000 thermogravimetric analyzer under a nitrogen atmosphere at temperatures ranging from 30 to 900 °C. A heating rate of 10 °C min−1 and cured samples of 5−10 mg were used in each experiment. 3.7. Thermomechanical Analysis. The thermomechanical properties of neat epoxy and epoxy blends were measured using a Perkin-Elmer thermomechanical analyzer. The cured samples were scanned from room temperature to 200 °C at a heating rate of 10 °C/min. Rectangular specimens were used for the analysis.
0 15 30 60 90 120 150 180 360 0 15 30 60 90 120 150 180 360
4. RESULTS AND DISCUSSION 4.1. Differential Scanning Calorimetry. The evolution of glass transition temperature (Tg) and heat of the reaction (ΔH) of epoxy/eSBS47/DDM blends at different cure times was analyzed by dynamic DSC measurements. The DSC profile remained the same irrespective of the blend composition, with
Tg (°C) Neat Epoxy −16 −10 −2 10 95 104 104 104 108 10 wt % eSBS47 −22 −17 −4 58 73 82 86 88 88 20 wt % eSBS47 −24 −20 −11 16 29 67 71 82 97
ΔH (J g−1) −565.81 −462.32 −371.17 −346.18 − − − − − −489.13 −414.26 −410.22 −117.29 − − − − − −499.80 −447.95 −394.70 −240.95 −95.64 − − − −
a maximum reaction rate at around 150 °C. The evolution of Tg and the heat of reaction (ΔH) of the 10 wt % eSBS47-modified epoxy system is shown in Figure 1 and is a representative of all of the blends studied. The Tg values of the epoxy phase in epoxy/DDM and epoxy/eSBS47/DDM blends can be evaluated by a small change in the baseline. Tg was taken as the initial point of the heat capacity change. As can be observed from Figure 1, Tg increased, whereas the area of the exothermal peak, corresponding to ΔH, decreased as a function of cure time, as a result of the epoxy/amine reaction.45 The data obtained from the thermograms are shown in Table 2. In the case of neat epoxy or blends cured in the oven for the shortest times, an exothermic peak was observed during the DSC scans, due to network formation occurring during the epoxy/amine reaction. This indicates that the curing reaction was not completed in the oven; rather, some reactive sites still remained in the system. A broad exothermic peak appeared in the in situ prepared blend (t = 0) during the DSC analysis. The exothermic peak area decreased as the cure time increased, and essentially no exothermic reaction could be observed when the cure time reached 90 min. This does not indicate that all of the entire epoxide groups and amino hydrogen atoms were involved in network formation; however, the mobility of some of the reactive sites might have been frozen, causing polymerization to stop. Recently, Garate et al. observed an exothermic peak for the epoxidized poly(styrene-b-isoprene-bstyrene) (SIS)/hardener reaction other than the epoxy/amine
Figure 1. Evolution of Tg and residual heat of the reaction (first heating) for 10 wt % eSBS47-modified epoxy blends as a function of curing time at 90 °C. 6943
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
Figure 2. HR-TEM images of (a) neat epoxy and (b) 10 and (c) 20 wt % eSBS47 blends.
Figure 4. Profiles of (a) storage modulus and (b) tan δ for neat epoxy and eSBS47-modified epoxy blends as functions of temperature.
4.2. Morphology of Thermosets. The phase separation is induced by the decrease in the entropic contribution to free energy of mixing due to the epoxy/amine reaction. The morphology of the prepared blends was studied by highresolution transmission electron microscopy (TEM). The TEM image (Figure 2a) of the neat epoxy system appears flat, showing a homogeneous morphology. Panels b and c of Figure 2 show blends of the epoxy system containing 10 and 20 wt % eSBS47 block copolymers, respectively. Irrespective of the blend concentration, the epoxy phase represents the continuous phase, in which eSBS47 block copolymer domains with an average diameter of around 15 nm in size are dispersed; the dark outer layers of the core−shell domains can be ascribed to the unepoxidized PB phase, whereas the bright inner core can be assigned to the phase-separated polystyrene (PS) blocks, as illustrated in Figure 2b,c.36,46,47 With increasing eSBS47
Figure 3. (a) Changes in the complex viscosities of neat epoxy and eSBS47-modified epoxy blends at 130 °C. (b) Storage and loss modulus curves of neat epoxy and eSBS47-modified epoxy blends at 130 °C.
reaction,40 but this behavior was not observed in our experiments.37 This means that epoxidized SBS and unepoxidized SBS should have limited reactivity with the NH group in DDM, which could be due to the limited epoxidation degree (47%) of SBS compared with the epoxidation degree used in the work by Garate et al., who epoxidized the SIS to 100%. Therefore, the stoichiometry is not affected for the curing reaction between DGEBA and DDM. 6944
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
stages of the epoxy/amine reaction, the viscosity value was low, and as the cure time increased, there was an increase in viscosity. However, for the nanostructured thermosets, before the sudden increase in viscosity, a fluctuation occurred. This fluctuation was due to the phase separation of the initially miscible eSBS47 phase.36,38 This result suggests that the eSBS phase separated before gelation and that later nanostructures were fixed by a cross-linking reaction. The increase in viscosity for the nanostructured thermosets at the initial stages of the cure reaction implies the profound influence of nanophase separation on the viscosity and, hence, the cure kinetics. Moreover, it is understandable that the reaction rate was decreased by the addition of eSBS47. This result is in accordance with our previous publication.37 For a better understanding of the phase-separation process, we determined the rheological parameters log G′ and log G″ as functions of cure time for the neat epoxy system and the 20 wt % eSBS47-modified epoxy blend, as shown in Figure 3b. For the neat epoxy system, during the initial stages of the epoxy/ amine reaction, log G′ was less than log G″, representing the viscous nature (liquid) of the material. After the initial stages of the reaction, both log G′ and log G″ increased with the growing epoxy chains. This was followed by a crossover point or chemical gelation point, where log G′ equaled log G″; at this point, the system acted as both elastic and viscous phases, storing (elastic phase) and dissipating (viscous phase) equal amounts of energy.48 After the crossover point, the epoxy behaved as a solid material as a result of the formation of a three-dimensional cross-linked epoxy network. However, the rheological profile of the eSBS-modified epoxy blend followed a different trend. For the eSBS47-modified epoxy blend, log G′ and log G″ started to increase much before the chemical gelation. In other words, the crossover point shifted to a shorter time for the eSBS47-modified epoxy blend. This means that the crossover for the blend systems is due not to chemical gelation but to physical gelation because of nanophase separation, thus confirming that the formation of nanostructures in the eSBS47modified epoxy blends occurs through reaction-induced microphase separation (RIPS).38 It is important to mention that both log G′ and log G″ crossed over only once for the blend systems, that is, the chemical gelation for these blends might be overlapped by the physical gelation process. The physical gelation is due to the formation of more elastic eSBS47 in the nanoscale (phase separation) and, hence, has a profound effect on the modulus and viscosity of the epoxy resin. 4.4. Dynamic Mechanical Characteristics of the eSBS47-Modified Epoxy System. The dynamic mechanical analyses of neat epoxy and eSBS47-modified epoxy blends were carried out using a DMTA instrument. Figure 4 shows the storage modulus and tan δ values of the blend systems plotted against temperature. The storage modulus (G′) versus temperature curves of the neat cured epoxy resin and nanostructured epoxy thermosets with 10 and 20 wt % eSBS47 triblock copolymer are shown in Figure 4a at 0.1-Hz frequency. The storage modulus (G′) values of the blends are lower than those of the neat epoxy system because of the presence of the less stiff eSBS47 block copolymer. The block copolymer can plasticize the epoxy matrix because of its increased miscibility with the epoxy phase and, hence, the decreased modulus of the blends.51 The cross-linked epoxy phase exhibited a well-defined α transition centered at 130 °C and a weak relaxation at around 40 °C called the ω-relaxation peak due to the lower cross-link density in the epoxy network.
Figure 5. Variation with eSBS47 content of Tg of the epoxy-rich phase from the tan δ profile.
Figure 6. Fracture toughness of eSBS47-modified epoxy blends.
content, the sizes of the core−shell nanodomains remained constant, but the domains were more densely packed with more uniformity, meaning that the interparticle domain distance decreased. Because epoxidized PB is miscible with the epoxy, it can be considered that the growth of the PS domains will be confined to the nanometer scale. 4.3. Rheological Behavior during Nanophase Separation. A close relationship between phase separation and rheology exists for the thermosetting resins. We investigated the evolution of the complex viscosity, storage modulus (G′), and loss modulus (G″) at different temperatures, namely, 90, 110, and 130 °C. The results showed similar trends at all experimental temperatures; therefore, a representative rheological profile obtained at 130 °C is shown here. The change in morphology upon the formation and destruction of the thermoplastic phase results in sudden changes in viscosity and modulus, for example, that can be used as criteria for the identification of phase separation.48−50 Figure 3a shows the evolution of the complex viscosity during isothermal tests on epoxy samples modified with 0, 10, and 20 wt % eSBS47 at 130 °C. For the neat epoxy system (see Figure 3a), during the early 6945
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
Figure 7. SEM micrographs of (a) neat epoxy and (b,c) 10 and (d,e) 20 wt % eSBS47.
one at around −65 °C, called β relaxation, attributed to the motion of glycidyl units in the epoxy network53 and a second at around 65 °C, called the ω-relaxation peak, due to the lower cross-link density in the epoxy network.54 The variation of Tg of the epoxy-rich phase from the tan δ profile as a function of the eSBS47 content is shown in Figure 5. The compatibility was further confirmed by the depression in Tg of the epoxy-rich phase.55 Moreover, the width of the tan δ peak for the nanostructured thermosets was significantly greater than that of the control epoxy. The broadening can be attributed to the enrichment of eSBS47 in the epoxy matrix. The epoxy matrix was effectively plasticized by eSBS47 chains and exhibited a lower Tg value than the control epoxy. In addition, the height of the tan δ peak (segmental mobility) of the epoxy phase was suppressed upon the addition of eSBS47, indicating that the eSBS47 subchains remained miscible with the epoxy matrix. The compatibility can be considered to be due to the hydrogen bonding between the hydroxyl groups of
On the other hand, the nanostructured blends exhibited three transitions; that is, in addition to the α and ω transitions of the control epoxy system, a new minor transition at around 90 °C assignable to PS nanodomains was observed. The new minor transition is associated with the phase separation of miscible PS blocks. The tan δ profile reveals a sharp relaxation peak centered at around 150 °C, observed in all of the blend systems (Figure 4b), that can be ascribed to the glass transition temperature (Tg) of the amine-cured epoxy resin. A broadening of the main peak was observed for the nanostructured blends and was slightly overlapped with the Tg of the PS phase. The Tg value of of the neat epoxy system was found to be 10−20 °C higher than those of the 10 and 20 wt % eSBS47 systems. The decrease in Tg demonstrates that the eSBS47 subchains were effectively interpenetrated into the cross-linked epoxy matrix at the segmental level.52 In addition to the main relaxation peak, there were two other relaxation peaks of very low amplitude, 6946
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
at the nanoscale and the plasticization of epoxidized polybutadiene units in the epoxy. The morphology of the blends, modifier amount, and interfacial adhesion between the phases represent determinant factors in the variation of the fracture toughness.10 Figure 7a reveals the fracture surface of the unmodified epoxy, with typically large cracks freely and regularly propagated and oriented in the direction of loading, as commonly observed in brittle systems. On the other hand, the fracture surfaces of the epoxy blends (Figure 7b,d) exhibit significantly smooth smaller cracks, typical for ductile materials. For a better understanding of the morphology, we obtained magnified images of the fracture surfaces of the blends with 10 and 20 wt % eSBS47, as shown in panels c and e, respectively, of Figure 7. The identification of nanodomains, due to the hydrogenbonding interactions between the hydroxyl groups of the growing epoxy and the epoxy groups of the epoxidized butadiene, is quite difficult using SEM micrographs (Figure 7c,e), because of the small dimensions; therefore, it can be concluded that a good adhesion between the matrix and the nanodispersed block copolymer domains was obtained.37 According to this observation, the applied load was more effectively transferred to the nanostructured domains from the cross-linked epoxy phase.56,57 Nanocracks in all directions and deviating from its original plane were observed in field-emission SEM micrographs, resulting in an increased crack surface and a consequent increase of the toughness value.10,29 In addition, improvement of the fracture toughness in the epoxy/eSBS47 blend systems could also be related to the ductility generated in the epoxy matrix, due to the plasticizing action of the eSBS47 block copolymer, especially for the 20 wt % eSBS47 blend system, whose surface looked like that of a rubbery material, validating the DMA profiles. Thus, the increase in fracture toughness is due to the effects of different phenomena acting concurrently in the polymer matrix during the application of the applied load. 4.6. Thermal Degradation Stability. The thermal stability of the blends was studied by TGA. Figure 8 shows that a singlestep weight loss occurred in the case of the neat epoxy and the nanostructured blends containing 10 and 20 wt % eSBS47modified epoxy systems. There was no weight loss or release of any molecules from the host during heating, and all of the blends were found to be stable up to 350 °C (water molecules would be released, if any). The weight loss above 350 °C was due the breakdown of polymer. The temperature of the maximum rate of decomposition (Tpeak) was taken as the peak value in the DTG curves. Blends of 0, 10, and 20 wt % eSBS47 in the epoxy/DDM system were found to have Tpeak values of 377.2, 379.0, and 383.0 °C, respectively; no dramatic change in thermal stability was observed for the blends. This result clearly indicates that the thermal stability of neat cross-linked epoxy was retained upon the addition of eSBS47. 4.7. Thermal Expansion Behavior. The thermal expansion behavior was studied by TMA. Figure 9 shows the thermograms of the eSBS47-modified epoxy systems. From the thermograms, it can be seen that the dimensional changes with respect to temperature for the eSBS47-modified blends were greater than that of the neat epoxy system over the complete temperature range. As seen in our previous publications, this increase in the change in dimension with respect to temperature is due to molecular vibrations, which increase with temperature.11,58 In the case of the eSBS47-modified epoxy blends, these increased molecular vibrations could be due
Figure 8. DTG thermograms of neat epoxy and eSBS47-modified epoxy blends.
Figure 9. Dimensional change vs temperature profiles for neat epoxy and eSBS47-modified epoxy blends.
Table 3. Coefficients of Thermal Expansion (CTE) for Neat Epoxy and eSBS47-Modified Epoxy Blends below and above Their Glass Transition Temperatures sample
CTE below Tg
CTE above Tg
neat epoxy 10 wt % eSBS 20 wt % eSBS
92 × 10−6/°C 93 × 10−6/°C 133 × 10−6/°C
190 × 10−6/°C 175 × 10−6/°C 187 × 10−6/°C
the growing epoxy thermoset and the epoxy groups of the epoxidized butadiene. This was demonstrated in our earlier study.37 4.5. Fracture Toughness. The fracture toughness values of the eSBS47-modified epoxy systems are reported in Figure 6. The critical stress intensity value of the neat epoxy system (KIC) gradually increased with increasing concentration of epoxidized copolymer. In the case of the 20 wt % eSBS47modified epoxy, this value reached 1.74 MPa·m1/2, much higher than that of the neat epoxy, and this behavior can be justified considering the concurring self-assembly of block copolymers 6947
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
Figure 10. Schematic of the mechanism for the drop in dimension change near Tg.
fracture toughness of the thermosets showed an excellent improvement upon the addition of eSBS47.
to the plasticizing effect imparted by the eSBS47 chains; hence, the intermolecular distance increases and results in increased thermal expansion for the blends. The coefficients of thermal expansion (CTE) for the blends below and above Tg are reported in Table 3. In the case of the neat epoxy system, a drop in the dimension change near Tg was observed. This phenomenon was recently observed in epoxy/multiwalled carbon nanotube (MWCNT) composites cured with diaminodiphenylsulfone (DDS).58 It was learned that the drop in dimension change near Tg is due to relaxation of nonequilibrium states of the cured samples that are frozen below Tg. This means that, when the sample temperature nears Tg, the motion capacity is high enough to unfreeze the frozen chains, so that the vacant spaces will be lost, resulting in a drop in dimension change; this is schematically represented in Figure 10a,b. It is important to mention that the inserted eSBS47 or the associated mixing limits the freezing of such local sites in the nanostructured thermosets and is schematically represented in Figure 10c,d.
■
ASSOCIATED CONTENT
* Supporting Information S
Peak areas, peak heights, and Tg values calculated for the epoxy/eSBS47 blends from the tan δ profiles. SEM micrographs of epoxy blends with 10 wt % eSBS39. Storage and loss modulus curves of neat epoxy and epoxy blend systems at 130 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S.M.G.). *E-mail:
[email protected] (J.P). *E-mail:
[email protected] (S.T.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank Huntsmann for its kind supply of chemicals and Nanomission of the Department of Science and Technology, New Delhi, India, for financial support. J.P. acknowledges the Department of Science and Technology, Government of India, for financial support under an INSPIRE Faculty Fellowship (IFA-CH-16). The present work was performed under the MIUR program Italy−India (2007) Grants for Young Researchers in the field of nanoscience and nanotechnology.
5. CONCLUSIONS Epoxidized SBS with an epoxidation degree of 47 mol % (eSBS47) was synthesized. The eSBS47 was used as a modifier in the curing of epoxy with diaminodiphenylmethane (DDM) curing agent. The addition of eSBS47 to the thermosetting matrix permitted nanophase (core−shell nanodomains) morphologies to be obtained. Network formation and the cross-linking reaction occurring during the curing reaction were analyzed in terms of the evolution of Tg and the heat of reaction (ΔH). As the cure time increased, the heat of reaction decreased, which indicates vitrification by network formation and reduced mobility of the chains. TEM investigations of epoxy blends revealed core−shell nanodomains dispersed in the epoxy matrix that were more densely packed for higherconcentration blends. The phase separation was closely related to the rheological parameters. Epoxy blends exhibit physical gelation due to the elastic effect of the generated nanophase structures. Dynamic mechanical analysis revealed the plasticizing nature of eSBS47 in the thermosetting resins. The thermal and dimensional stability of the epoxy system used was retained upon the addition of eSBS47 block copolymer. However, the
■
REFERENCES
(1) Pascault, J. P.; Williams, R. J. J. Epoxy Polymers: New Materials and Innovations; Wiley-VCH: Weinheim, Germany, 2010. (2) Drake, R. S.; Siebert, A. R. Elastomer-modified epoxy resins for structural applications. SAMPE Q. 1975, 6, 11. (3) Kinloch, A. J.; Shaw, S. J.; Tod, D. A.; Hunston, D. L. Deformation and fracture behaviour of a rubber-toughened epoxy: 1. Microstructure and fracture studies. Polymer 1983, 24, 1341. (4) Pearson, R. A.; Yee, A. F. Toughening mechanisms in elastomermodified epoxies. Part 2. Microscopy studies. J. Mater. Sci. 1986, 21, 2475. 6948
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
(25) Guo, Q.; Thomann, R.; Gronski, W.; Thurn-Albrecht, T. Phase Behavior, Crystallization, and Hierarchical Nanostructures in SelfOrganized Thermoset Blends of Epoxy Resin and Amphiphilic Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) Triblock Copolymers. Macromolecules 2002, 35, 3133. (26) Ritzenthaler, S.; Court, F.; David, L.; Girard-Reydet, E.; Leibler, L.; Pascault, J. P. ABC Triblock Copolymers/Epoxy−Diamine Blends. 2. Parameters Controlling the Morphologies and Properties. Macromolecules 2003, 36, 118. (27) Rebizant, V.; Abetz, V.; Tournilhac, F.; Court, F.; Leibler, L. Reactive Tetrablock Copolymers Containing Glycidyl Methacrylate. Synthesis and Morphology Control in Epoxy−Amine Networks. Macromolecules 2003, 36, 9889. (28) Dean, J. M.; Verghese, N. E.; Pham, H. Q.; Bates, F. S. Nanostructure Toughened Epoxy Resins. Macromolecules 2003, 36, 9267. (29) Rebizant, V.; Venet, A.-S.; Tournilhac, F.; Girard-Reydet, E.; Navarro, C.; Pascault, J. P.; Leibler, L. Chemistry and Mechanical Properties of Epoxy-Based Thermosets Reinforced by Reactive and Nonreactive SBMX Block Copolymers. Macromolecules 2004, 37, 8017. (30) Wu, J.; Thio, Y. S.; Bates, F. S. Structure and properties of PBO−PEO diblock copolymer modified epoxy. J. Polym. Sci. B: Polym. Phys. 2005, 43, 1950. (31) Zucchi, I. A.; Galante, M. J.; Williams, R. J. J. Comparison of morphologies and mechanical properties of crosslinked epoxies modified by polystyrene and poly(methyl methacrylate) or by the corresponding block copolymer polystyrene-b-poly(methyl methacrylate). Polymer 2005, 46, 2603. (32) Thio, Y. S.; Wu, J.; Bates, F. S. Epoxy Toughening Using Low Molecular Weight Poly(hexylene oxide)−Poly(ethylene oxide) Diblock Copolymers. Macromolecules 2006, 39, 7187. (33) Maiez-Tribut, S.; Pascault, J. P.; Soule, E. R.; Borrajo, J.; Williams, R. J. J. Nanostructured Epoxies Based on the Self-Assembly of Block Copolymers: A New Miscible Block That Can Be Tailored to Different Epoxy Formulations. Macromolecules 2007, 40, 1268. (34) 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. (35) Ocando, C.; Serrano, E.; Tercjak, A.; Pena, C.; Kortaberria, G.; Calberg, C.; Grignard, B.; Jerome, R.; Carrasco, P. M.; Mecerreyes, D.; Mondragon, I. Structure and Properties of a Semifluorinated Diblock Copolymer Modified Epoxy Blend. Macromolecules 2007, 40, 4068. (36) George, S. M.; Puglia, D.; Kenny, J. M.; Causin, V.; Parameswaranpillai, J.; Thomas, S. Morphological and Mechanical Characterization of Nanostructured Thermosets from Epoxy and Styrene-block-Butadiene-block-Styrene Triblock Copolymer. Ind. Eng. Chem. Res. 2013, 52, 9121. (37) George, S. M.; Puglia, D.; Kenny, J. M.; Jyotishkumar, P.; Thomas, S. Cure kinetics and thermal stability of micro and nanostructured thermosetting blends of epoxy resin and epoxidized styrene-block-butadiene-block-styrene triblock copolymer systems. Polym. Eng. Sci. 2012, 52, 2336. (38) Serrano, E.; Tercjak, A.; Ocando, C.; Larrañaga, M.; Parellada, M. D.; Galván, S. C.; Mecerreyes, D.; Zafeiropoulos, N. E.; Stamm, M.; Mondragon, I. Curing Behavior and Final Properties of Nanostructured Thermosetting Systems Modified with Epoxidized Styrene−Butadiene Linear Diblock Copolymers. Macromol. Chem. Phys. 2007, 208, 2281. (39) Ramos, J. A.; Espósito, L. H.; Fernández, R.; Zalakain, I.; Goyanes, S.; Avgeropoulos, A.; Zafeiropoulos, N. E.; Kortaberria, G.; Mondragon, I. Block Copolymer Concentration Gradient and Solvent Effects on Nanostructuring of Thin Epoxy Coatings Modified with Epoxidized Styrene−Butadiene−Styrene Block Copolymers. Macromolecules 2012, 45, 1483. (40) Garate, H.; Mondragon, I.; D’Accorso, N. B.; Goyanes, S. Exploring Microphase Separation Behavior of Epoxidized Poly-
(5) Bucknall, C. B.; Partridge, I. K. Phase separation in epoxy resins containing polyethersulphone. Polymer 1983, 24, 639. (6) Kinloch, A. J.; Yuen, M. L.; Jenkins, S. D. Thermoplastictoughened epoxy polymers. J. Mater. Sci. 1994, 29, 3781. (7) Brooker, R. D.; Kinloch, A. J.; Taylor, A. C. The Morphology and Fracture Properties of Thermoplastic-Toughened Epoxy Polymers. J. Adhes. 2010, 86, 726. (8) Hameed, N.; Sreekumar, P. A.; Thomas, P. S.; Jyotishkumar, P.; Thomas, S. Mechanical properties of poly(styrene-co-acrylonitrile)modified epoxy resin/glass fiber composites. J. Appl. Polym. Sci. 2008, 110, 3431. (9) Zheng, S.; Gao, R.; Li, J.; Guo, Q. Epoxy resin containing the poly(sily ether): Preparation, morphology, and mechanical properties. J. Appl. Polym. Sci. 2003, 89, 505. (10) Jyotishkumar, P.; Pionteck, J.; Haßler, R.; George, S. M.; Cvelbar, U.; Thomas, S. Studies on Stress Relaxation and Thermomechanical Properties of Poly(acrylonitrile-butadiene-styrene) Modified Epoxy−Amine Systems. Ind. Eng. Chem. Res. 2011, 50, 4432. (11) Jyotishkumar, P.; Abraham, E.; George, S. M.; Elias, E.; Pionteck, J.; Moldenaers, P.; Thomas, S. Preparation and properties of MWCNTs/poly(acrylonitrile-styrene-butadiene)/epoxy hybrid composites. J. Appl. Polym. Sci. 2013, 127, 3093. (12) Yang, X.; Yi, F.; Xin, Z.; Zheng, S. Morphology and mechanical properties of nanostructured blends of epoxy resin with poly(εcaprolactone)-block-poly(butadiene-co-acrylonitrile)-block-poly(ε-caprolactone) triblock copolymer. Polymer 2009, 50, 4089. (13) Liu, J.; Sue, H.-J.; Thompson, Z. J.; Bates, F. S.; Dettloff, M.; Jacob, G.; Verghese, N.; Pham, H. Strain rate effect on toughening of nano-sized PEP−PEO block copolymer modified epoxy. Acta Mater. 2009, 57, 2691. (14) Liu, J.; Thompson, Z. J.; Sue, H.-J.; Bates, F. S.; Hillmyer, M. A.; Dettloff, M.; Jacob, G.; Verghese, N.; Pham, H. Toughening of Epoxies with Block Copolymer Micelles of Wormlike Morphology. Macromolecules 2010, 43, 7238. (15) Lodge, T. P. Block Copolymers: Past Successes and Future Challenges. Macromol. Chem. Phys. 2003, 204, 265. (16) Park, C.; Yoon, J.; Thomas, E. L. Enabling Nanotechnology with Self Assembled Block Copolymer Patterns. Polymer 2003, 44, 6725. (17) Bhatia, S. R.; Mourchid, A.; Joanicot, M. Block Copolymer Assembly to Control Fluid Rheology. Curr. Opin. Colloid Interface Sci. 2001, 6, 471. (18) Yi, F.; Zheng, S.; Liu, T. Nanostructures and Surface Hydrophobilicity of Self-Assembled Thermosets Involving Epoxy Resin and Poly(2,2,2-trifluoroethyl acrylate)-block-poly(ethylene oxide) Amphiphilic Diblock Copolymer. J. Phys. Chem. B 2009, 113, 1857. (19) Hu, D.; Zhang, C.; Yu, R.; Wang, L.; Zheng, S. Self-organized thermosets involving epoxy and poly(ε-caprolactone)-block-poly(ethylene-co-ethylethylene)-block-poly(ε-caprolactone) amphiphilic triblock copolymer. Polymer 2010, 51, 6047. (20) Tercjak, A.; Larrañaga, M.; Martin, M. D.; Mondragon, I. Thermally reversible nanostructured thermosetting blends modified with poly(ethylene-b-ethylene oxide) diblock copolymer. J. Therm. Anal. Calorim. 2006, 86, 663. (21) Ocando, C.; Tercjak, A.; Martin, 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. (22) Hillmyer, M. A.; Lipic, P. M.; Hajduk, D. A.; Almdal, K.; Bates, F. S. Self-Assembly and Polymerization of Epoxy Resin−Amphiphilic Block Copolymer Nanocomposites. J. Am. Chem. Soc. 1997, 119, 2749. (23) Lipic, P. M.; Bates, F. S.; Hillmyer, M. A. Nanostructured Thermosets from Self-Assembled Amphiphilic Block Copolymer/ Epoxy Resin Mixtures. J. Am. Chem. Soc. 1998, 120, 8963. (24) Kosonen, H.; Ruokolainen, J.; Nyholm, P.; Ikkala, O. SelfOrganized Thermosets: Blends of Hexamethyltetramine Cured Novolac with Poly(2-vinylpyridine)-block-poly(isoprene). Macromolecules 2001, 34, 3046. 6949
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
Industrial & Engineering Chemistry Research
Article
and its nanostructured blends with epoxy resin. Polymer 2008, 49, 3318. (58) Jyotishkumar, P.; Logakis, E.; George, S. M.; Pionteck, J.; Haussler, L.; Haßler, R.; Pissis, P.; Thomas, S. Preparation and properties of multiwalled carbon nanotube/epoxy-amine composites. J. Appl. Polym. Sci. 2013, 127, 3063.
(styrene-b-isoprene-b-styrene) Block Copolymer Inside Thin Epoxy Coatings. Macromolecules 2013, 46, 2182. (41) Ocando, C.; Fernández, R.; Tercjak, A.; Mondragon, I.; Eceiza, A. Nanostructured Thermoplastic Elastomers Based on SBS Triblock Copolymer Stiffening with Low Contents of Epoxy System. Morphological Behavior and Mechanical Properties. Macromolecules 2013, 46, 3444. (42) Serrano, E.; Tercjak, A.; Kortaberria, G.; Pomposo, J. A.; Mecerreyes, D.; Zafeiropoulos, N. E.; Stamm, M.; Mondragon, I. Nanostructured Thermosetting Systems by Modification with Epoxidized Styrene−Butadiene Star Block Copolymers. Effect of Epoxidation Degree. Macromolecules 2006, 39, 2254. (43) Grubbs, R. B.; Dean, J. M.; Broz, M. E.; Bates, F. S. Reactive Block Copolymers for Modification of Thermosetting Epoxy. Macromolecules 2000, 33, 9522. (44) Serrano, E.; Larranaga, M.; Remiro, P. M.; Mondragon, I.; Carrasco, P. M.; Pomposo, J. A.; Mecerreyes, D. Synthesis and Characterization of Epoxidized Styrene−Butadiene Block Copolymers as Templates for Nanostructured Thermosets. Macromol. Chem. Phys. 2004, 205, 987. (45) Lapprand, A.; Arribas, C.; Salom, C.; Masegosa, R. M.; Prolongo, M. G. Epoxy resins modified with poly(vinyl acetate). J. Mater. Process. Technol. 2003, 143, 827. (46) Wu, S.; Guo, Q.; Peng, S.; Hameed, N.; Kraska, M.; Stuhn, B.; Mai, Y. W. Toughening Epoxy Thermosets with Block Ionomer Complexes: A Nanostructure−Mechanical Property Correlation. Macromolecules 2012, 45, 3829. (47) Heck, B.; Arends, P.; Ganter, M.; Kressler, J.; Stühn, B. SAXS and TEM Studies on Poly(styrene)-block-poly(ethene-co-but-1-ene)block-poly(styrene) in Bulk and at Various Interfaces. Macromolecules 1997, 30, 4559. (48) Jyotishkumar, P.; Moldenaers, P.; George, S. M.; Thomas, S. Viscoelastic effects in thermoplastic poly(styrene-acrylonitrile)-modified epoxy−DDM system during reaction induced phase separation. Soft Matter 2012, 8, 7452. (49) Liu, Y.; Zhong, X. H.; Zhan, G. Z.; Yu, Y. F.; Jin, J. Y. Effect of Mesoscopic Fillers on the Polymerization Induced Viscoelastic Phase Separation at Near- and Off-Critical Compositions. J. Phys. Chem. B 2012, 116, 3671. (50) Jyotishkumar, P.; Pionteck, J.; Ozdilek, C.; Moldenaers, P.; Thomas, S. Rheology and pressure−volume−temperature behavior of the thermoplastic poly(acrylonitrile-butadiene-styrene)-modified epoxy-DDS system during reaction induced phase separation. Soft Matter 2011, 7, 7248. (51) Xu, Z.; Zheng, S. Morphology and thermomechanical properties of nanostructured thermosetting blends of epoxy resin and poly(εcaprolactone)-block-polydimethylsiloxane-block-poly(ε-caprolactone) triblock copolymer. Polymer 2007, 48, 6134. (52) Meng, F.; Zheng, S.; Li, H.; Liang, Q.; Liu, T. Formation of Ordered Nanostructures in Epoxy Thermosets: A Mechanism of Reaction-Induced Microphase Separation. Macromolecules 2006, 39, 5072. (53) Guo, Q.; Figueiredo, P.; Thomann, R.; Gronski, W. Phase behavior, morphology and interfacial structure in thermoset/thermoplastic elastomer blends of poly(propylene glycol)-type epoxy resin and polystyrene-b-polybutadiene. Polymer 2001, 42, 10101. (54) Francis, B.; Thomas, S.; Asari, G. V.; Ramaswamy, R.; Jose, S.; Rao, V. L. Synthesis of hydroxyl-terminated poly(ether ether ketone) with pendent tert-butyl groups and its use as a toughener for epoxy resins. J. Polym. Sci. B: Polym. Phys. 2006, 44, 541. (55) Xu, Z.; Zeng, S. Reaction Induced Microphase Separation in Epoxy Thermosets Containing Poly(ε-caprolactone)-block-poly(nbutyl acrylate) Diblock Copolymer. Macromolecules 2007, 40, 2548. (56) Yi, F.; Yu, R.; Zheng, S.; Li, X. Nanostructured thermosets from epoxy and poly(2,2,2-trifluoroethyl acrylate)-block-poly(glycidyl methacrylate) diblock copolymer: Demixing of reactive blocks and thermomechanical properties. Polymer 2011, 52, 5669. (57) Gong, W.; Zeng, K.; Wang, L.; Zheng, S. Poly(hydroxyether of bisphenol A)-block-polydimethylsiloxane alternating block copolymer 6950
dx.doi.org/10.1021/ie404124b | Ind. Eng. Chem. Res. 2014, 53, 6941−6950
