Thermoset Blends Compatibilized with an

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Nanostructured Thermoset/Thermoset Blends Compatibilized with an Amphiphilic Block Copolymer Brian J. Rohde,† Tyler E. Culp,‡ Enrique D. Gomez,‡ Jan Ilavsky,§ Ramanan Krishnamoorti,*,† and Megan L. Robertson*,† †

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77004, United States Department of Chemical Engineering and the Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Advanced Photon Source, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, Illinois 60439, United States

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ABSTRACT: Blends of thermoset polymers offer an avenue to combine the mechanical properties of complementary high glass transition temperature systems. An epoxy resin with high tensile strength and modulus, composed of the diglycidyl ether of bisphenol A cured with an anhydride, was combined with polydicyclopentadiene, cured via ring-opening metathesis polymerization, to improve the toughness. An amphiphilic block copolymer, poly(1,4-butadiene-b-ethylene oxide), where 1,4-polybutadiene has a strong affinity for polydicyclopentadiene and poly(ethylene oxide) has a strong affinity for the epoxy resin, was added to control phase separation and manipulate the morphology of the thermoset blend, analogous to compatibilization in thermoplastic blends. A systematic study of the influence of block copolymer loading and blend composition on the structural morphology was performed, using a combination of ultrasmall-angle X-ray scattering (USAXS), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). As the block copolymer content was increased, the thermoset/ thermoset blend morphology transitioned from a phase-separated surface fractal type morphology to finer dispersed spherical domains, until a critical particle size was reached at which blend mechanical properties were optimum. The resultant mechanical properties for select compatibilized blends showed a strong positive influence of the morphology on the fracture properties while maintaining the well-behaved tensile properties observed in the uncompatibilized system.

1. INTRODUCTION Toughening of brittle thermoset materials, such as epoxy resins, is of considerable interest to industries that use fiber-reinforced polymer composites to increase the material life span and expand applicability.1−7 While recent developments of mechanical reinforcement through the application of graphene,8−10 carbon black,11,12 and carbon nanotubes13−18 have attracted a new wave of interest, the full promise of such systems has yet to be realized due to cost, processing limitations, and poor interfacial adhesion between the dispersed phase and the thermoset matrix. As a result, traditional rubber toughening19−22 and toughening via block copolymers23−28 have continued to evolve despite their own limitations with maintaining the thermal and mechanical properties of the thermoset. Despite the differences in various toughening techniques, one common need is precise morphological control in polymer blends due to the direct relationship between the underlying morphology and the ultimate properties of the blend.29−38 For dispersed spherical domains, such as those that typically persist in rubber, block copolymer, or rigid particle toughened blends, it has been shown that there is a direct influence of particle size20,30,31,37,39 and extent of interparticle correlation30,35,40−42 on the ultimate © XXXX American Chemical Society

mechanical properties of the blend. Prior studies indicate that an optimum domain size, typically around 200 nm, results in the largest improvement of toughening with minimum negative impact on the tensile properties of the material.30,31,43 Previously we presented a study of the structure and properties of sequentially cured blends of an epoxy resin and polydicyclopentadiene (PDCPD) that possessed complementary mechanical properties in an attempt to avoid drawbacks of traditional toughening methods.44,45 Despite the formation of a hierarchical phase-separated structure and microheterogeneous surface fractal type morphology, the ultimate mechanical properties were well behaved and tunable across a wide range of compositions, providing a thermoset/thermoset blend analogue to more traditionally studied thermoplastic/thermoplastic or thermoset/thermoplastic blends. Considering the documented efficacy of diblock copolymer addition to the controlled phase separation in immiscible thermoplastic blends,38,46−51 diblock copolymer addition to thermoset/ Received: September 24, 2018 Revised: December 22, 2018

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DOI: 10.1021/acs.macromol.8b02061 Macromolecules XXXX, XXX, XXX−XXX

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Preparation of 1,4-Polybutadiene and PDCPD Blends. On a 10 g basis, blends of 1,4-polybutadiene with DCPD of various compositions were prepared in a 20 mL scintillation vial. G2 (1.5 × 10−3 M loading) was added, and the mixture was cured for 1 h at 30 °C, 2 h at 70 °C, 5 h at 100 °C, 4 h at 160 °C, and 2 h at 200 °C in a convection oven. Preparation of Poly(ethylene oxide) and DGEBA Blends. On a 10 g basis, poly(ethylene oxide) was mixed with DGEBA oligomer to achieve compositions of 50, 70, and 90 wt % DGEBA in a 20 mL scintillation vial. The solutions were solvent cast by dissolving in dichloromethane and vacuum drying overnight at room temperature and then heated at 80 °C for several hours in vacuo to remove any residual solvent. Preparation of Compatibilized Epoxy Resin and PDCPD Blends. For blends containing epoxy resin, PDCPD, and PB−PEO, the mass of each component was precalculated on a 1 mL basis for USAXS/SAXS measurements and 50 mL basis for mechanical testing. For the epoxy resin components, the NMA curing agent and K54 catalyst were added at concentrations of 95.3 and 1 phr, respectively (relative to the DGEBA monomer). When only considering the DGEBA, NMA, and K54 content in the blend, this is equivalent to 50.9 wt % of DGEBA, 48.5 wt % of NMA, and 0.6 wt % of K54. The densities of the master batches of uncompatibilized blends were used to determine the required weight of PB−PEO in each blend. Once calculated, the necessary amount of DGEBA was added to a 2 mL vial, and the appropriate amount of PB−PEO was added. 0.5 mL of dichloromethane was added to the vial (to properly disperse the PB−PEO in the DGEBA), and the vial was sealed and placed on a vortex mixer overnight to dissolve the PB−PEO. Vials containing DGEBA and PB− PEO were then placed in a vacuum oven overnight at room temperature to remove the dichloromethane. NMA and DCPD were then added to the dried DBEGA/PB−PEO blend by weight and volume, respectively, according to the precalculated values for the blend. The vials containing DGEBA, NMA, DCPD, and PB−PEO were closed and placed in a foam holder (12 vials at a time) placed on the rod of the Cole-Parmer Stir-Pak heavy duty mixer (impeller removed). The configuration was adjusted such that the vials were nearly horizontal. The setup was placed on a low speed setting that was ∼1 revolution per second and left overnight to incorporate all the components. After sufficient mixing of DGEBA, NMA, DCPD, and PB−PEO, the epoxy resin catalyst, K54, was added to the mixture, and the mixing step was repeated. After incorporation of K54, G2 was added to the mixture at 1.5 × 10−3 M and mixed by hand using a glass stir rod. G2 particulates were dispersed by repeatedly drawing the solution up into a 1 mL syringe and then expelling it back into the vial. The blends were placed in a centrifuge (Beckman Coulter Allegra X-22 centrifuge) for 10 s at 4000 rpm after mixing to remove bubbles and transferred to (a) small aluminum pans 1 mm in thickness and 8 mm in diameter for USAXS/SAXS measurements or (b) tensile and fracture testing molds. All samples were cured for 1 h at 30 °C, 2 h at 70 °C, 5 h at 100 °C, 4 h at 160 °C, and 2 h at 200 °C in a convection oven. After curing, the samples were removed from the pans or molds. Under these curing conditions, thermal degradation of the polymer was not significant.44 The 100E-5.0 sample (containing epoxy resin and 5.0 wt % PB−PEO) was prepared similarly, with the omission of DCPD and G2. The 0E-5.0 sample (containing PDPCD and 5 wt % PB−PEO) was prepared on a 1 mL basis for USAXS/SAXS measurements. PB−PEO was added directly to the DCPD monomer and held at 50 °C overnight until the PEO block melted and the block copolymer became well dissolved. The sample was then vigorously mixed on a vortex mixer while warm. G2 was added to the system at 1.5 × 10−3 M at 30 °C, and then the mixture was centrifuged for 10 s at 4000 rpm after mixing to remove bubbles and transferred to small aluminum pans 1 mm in thickness and 8 mm in diameter for USAXS/SAXS measurements The samples were cured for 1 h at 30 °C, 2 h at 70 °C, 5 h at 100 °C, 4 h at 160 °C, and 2 h at 200 °C in a convection oven. After curing, the samples were removed from the pans or molds. 2.3. Characterization. X-ray Scattering. Measurements for ultrasmall-angle X-ray scattering (USAXS) and small-angle X-ray scattering (SAXS) were performed at the Advance Photon Source (APS) at Argonne National Lab on beamline 9ID-C at 21 keV

thermoset blends is anticipated to have considerable impact on the ultimate morphology and mechanical properties. Additionally, the epoxy resin/PDCPD system could be considered curein-place, in which the rigid dispersed phase is formed in situ, potentially avoiding the viscosity consequences on processing normally associated with particle addition and rubber toughening. Here, we present the unique use of a diblock copolymer in controlling the phase separation in thermoset/thermoset blends with disparate kinetics and reaction mechanisms such that the two systems form nanoscale structures that are bridged to the matrix via diblock copolymers. The thermoset blend components were selected to be mechanically complementary such that one component possessed high strength and stiffness (epoxy resin) and the other was more tough and ductile (PDCPD) but possessed a high Tg. The diblock copolymer consists of symmetric blocks of 1,4-polybutadiene and poly(ethylene oxide), chosen for the anticipated compatibility of the olefinrich polybutadiene with similarly olefin-rich PDCPD, and the documented compatibility of poly(ethylene oxide) with epoxy resin.24,28,52,53 Symmetric diblock copolymers have been shown to be more effective at reducing the interfacial tension than asymmetric diblock copolymers, resulting in more efficient reduction in domain size with respect to compatibilizer loading.38,50,54−57 Furthermore, the polybutadiene may also participate in the ROMP reaction as a chain transfer agent, enabling reactive or in situ compatibilization. The influence of block copolymer loading and the ratio of epoxy to PDCPD in the blend were examined to manipulate the dispersed phase size and interparticle correlations in compatibilized blends. In this fashion, the morphology of this class of immiscible thermoset blends was tailored from the nano- to micrometer length scales and from dilute to correlated regimes of dispersed phases. This study provides new insight into tailoring thermoset−thermoset blend morphology through the addition of carefully designed block copolymers, resulting in precise control of morphological features and related blend mechanical properties.

2. EXPERIMENTAL METHODS 2.1. Materials. The diglycidyl ether of bisphenol A (DGEBA) was supplied by Dow Chemical in the form of Dow Epoxy Resin (D.E.R.) 331 (the fraction of DGEBA molecules that are prepolymerized and possess an extra hydroxyl-containing midgroup, n = 0.15). Nadic methyl anhydride (NMA, >95% purity) and dicyclopentadiene (DCPD) with butylated hydroxytoluene as a stabilizer (>96% purity) were purchased from Sigma-Aldrich and used as received. The secondgeneration Grubbs catalyst (G2) was purchased from Sigma-Aldrich and stored under an ultrapure nitrogen environment. The G2 catalyst was freeze-dried in benzene before use. An epoxy resin catalyst, 2,4,6tri(dimethylaminomethyl)phenol, was supplied by Air Products as Ancamine K54 (termed K54 in this paper) and used as received. 1,4Polybutadiene (PB3k; Mn = 3 kg/mol), poly(ethylene oxide) (PEO2k; Mn = 2 kg/mol), and short chain DGEBA oligomer (DGEBA4k; Mn = 4 kg/mol) were purchased from Sigma-Aldrich and used as received. Poly(1,4-butadiene-b-ethylene oxide) (PB−PEO) block copolymers were purchased from Polymer Source and used as received. 2.2. Sample Preparation. Blend Nomenclature. Blends of epoxy resin and polydicyclopentadiene (PDCPD) are labeled as xxE-y.y, where xx corresponds to the vol % of epoxy resin (consisting of DGEBA, NMA, and K54) and y.y corresponds to the wt % of block copolymer. For example, blend 30E-5.0 contained a 30:70 (volume ratio) mixture of epoxy resin:PDCPD, and the blend contained 5.0 wt % block copolymer. Blend 50E is simply a 50/50 blend of epoxy resin and PDCPD (without block copolymer). B

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metal file, and then a fresh razor blade, cooled in liquid nitrogen, was gently tapped on the sample to produce a natural crack. Specimens were then tested in a Instron Model 5966 universal tester in compression mode at 10 mm/min under three-point bend geometry. The critical stress intensity factor, KIC, was calculated from the peak load, Pq, as described in the ASTM D5045 standard.

corresponding to an X-ray wavelength of 0.5904 Å (for blends 95E-5.0, 90E-5.0, 80E-5.0, and 70E-5.0) and at 24 keV corresponding to an X-ray wavelength of 0.5106 Å (for blends 50E-5.0 through 50E-0.1, 30E-5.0, and 20E-5.0). This beamline employs Bonse−Hart-type double-crystal optics to extend the scattering vector q range of SAXS down to 0.0001 Å−1. The beam size was approximately 2.0 mm × 1.0 mm. A more detailed description of the instrument can be found in prior publications.58,59 The data were slit smeared with a slit length of 0.025164 Å−1. The sample thickness was determined using a micrometer. Instrumental background scattering was determined using an empty cell (no Kapton film was used). USAXS data were reduced using the Indra package within Igor, and reduced data were fitted using a spherical model and Unified model with the Irena software package for Igor.60−64 To explore the melt phase behavior of the neat PB−PEO block copolymer and blend 100E-5.0, an in-house benchtop Rigaku S-MAX 3000 SAXS instrument was employed, equipped with CMF optic incident beam (λ = 0.154 nm), three pinhole collimation, and a 2dimensional multiwire area detector with 1024 × 1024 pixels. The sample-to-detector distance was 3 m. The recorded scattered intensity distributions were integrated over the azimuthal angle and are presented as a function of the scattering vector. To melt the neat block copolymer at 80 °C, a Linkam HFSX350 stage was used. Differential Scanning Calorimetry (DSC). DSC was performed on a TA Instruments Q2000 calorimeter calibrated with an indium standard. The testing procedure followed various heat−cool−heat cycles at 10 °C/min under 50 mL/min nitrogen flow. To prevent PEO crystallization in the blends of PEO and DGEBA, a cooling rate of 100 °C/min was used. Gel Permeation Chromatography (GPC). GPC was conducted on a Viscotek GPCmax instrument with two ResiPore columns, using stabilized THF (OmniSolv, HPLC grade) as the mobile phase. The instrument operated at a temperature of 30 °C and a flow rate of 1 mL/ min. The injection volume was 100 μL. Triple detection was employed for characterization of number-average molecular weight (Mn) and dispersity (Đ). Samples were prepared for GPC by dissolving in stabilized THF at a concentration of 1−2 mg/mL. The refractive index increment (dn/dc) of the PB−PEO block copolymer was determined from three separate samples to be 0.091± 0.002 mL/g. Nuclear Magnetic Resonance (NMR). Proton NMR (1H NMR) spectroscopy experiments were performed on JEOL ECA-500 instruments using deuterated chloroform (99.96 atom % D, Cambridge Isotope Laboratories) as the solvent. Chemical shifts of 1H were referenced to the residual protiated solvent proton resonance (7.26 ppm). Transmission Electron Microscopy (TEM). TEM was performed using two instruments: a FEI Tecnai G2 20 TWIN with LaB6 gun at an accelerating voltage of 200 kV (for blends 50E-0.25, 50E-0.5, and 30E5.0) and a FEI Talos F200X with a FEG electron source at 200 kV (to achieve better resolution on blends 50E-5.0 and 70E-5.0). Samples were stained with osmium tetraoxide (OsO4) to enhance contrast between domains after using a Leica ultramicrotome UC6 to obtain sections ∼90 nm thick. Particle size and distribution analysis were conducted using ImageJ. Tensile Testing. Tensile testing was performed at room temperature using an Instron Model 5966 universal tester with a 2 kN load cell at a speed of 10 mm/min. Dogbone-shaped testing bars (following ASTM D638,65 bar type 5, thickness 3.2 mm) were prepared by pouring unreacted blends in an aluminum mold and following the curing schedule outlined previously. Pneumatic grips (maximum 2 kN) were used to affix the sample in the testing frame, at a compressed air pressure of 90 psi.65 Five test specimens were used for testing per blend. Fracture Toughness Testing. Fracture toughness specimens were molded following the ASTM D5045 standard and single-edge notched bending geometry (SENB).66 For the neat epoxy resin and all blends, a bar geometry of 1.225 cm × 0.615 cm × 5.37 cm was used that ensured plane strain criteria was met. For neat DCPD, a bar geometry of 2.475 cm × 1.190 cm × 10.75 cm was used to accommodate the higher fracture toughness and lower tensile strength when considering plane strain criteria. A Chevron notch was created in each sample using a

3. RESULTS AND DISCUSSION 3.1. Block Copolymer Selection for Compatibilization of Epoxy Resin/PDCPD Blends. Our prior work demonstrated the presence of hierarchical phase-separated structures and a microheterogeneous surface fractal type morphology in uncompatibilized epoxy resin/PDCPD blends, which exhibited well-behaved mechanical properties that were tunable across a wide range of compositions.44,45 To design an effective block copolymer for compatibilization of these immiscible blends, components of the block copolymer were chosen to exhibit selective miscibility with the epoxy resin and PDCPD phases. 1,4-Polybutadiene (PB) was identified as a potentially PDCPDphilic block. Additionally, there is the possibility that PB may act as a chain transfer agent for the ROMP reaction, resulting in reactive or in situ compatibilization of the PDCPD/epoxy resin domains.67 We do note, however, that the limited mobility of PB (due to its presence at the domain interface), low concentration of PB relative to DCPD monomer, and significantly slower rate of cross metathesis relative to ROMP limit the significance of this effect. Based on prior literature studies, poly(ethylene oxide) (PEO) was identified as an epoxy-philic block.24,27,28,53,68,69 To confirm the miscibility of the PB and PEO blocks with PDCPD and epoxy, respectively, the miscibilities of binary mixtures of PB3k/PDCPD and PEO2k/ DGEBA4k were explored. Mixtures of PB3k and PDCPD, cured with G2, remained optically clear. DSC analysis of the PB3k/ PDCPD blends showed a single Tg, that closely followed the Flory−Fox Tg prediction (Figure 1a). We note that the compatibility of PDCPD and PB3k may have been enhanced by the presence of chain transfer to the PB3k polymer during the ROMP of DCPD to prepare this blend. Similarly, mixtures of PEO2k with DGEBA4k exhibited a single Tg at all mixture combinations (Figure 1b), and the observed Tg’s were significantly depressed compared to the values anticipated by the application of the Flory−Fox prediction. Though there are differences between the DGEBA4k molecular structure and that found in the anhydride-cured epoxy network, these results are in line with literature studies demonstrating the compatibility of PEO with epoxy resins.24,28,52,53 On the basis of these results, we concluded that the PB−PEO block copolymer is an appropriate choice of compatibilizer for the epoxy resin/PDCPD blends. 3.2. Block Copolymer Characterization. A combination of GPC and NMR analyses (Figures S1 and S2) was employed to characterize the PB−PEO block copolymers. The PB−PEO characteristics are summarized in Table 1. The PB−PEO block copolymer exhibited a unimodal molecular weight distribution, and the PB block contained high 1,4-content (93%). Each block’s molecular weight was well above its entanglement molecular weight, Me (Me,PB = 2.9 kg/mol70 and Me,PEO = 1.7 g/ mol70 ), to ensure proper interfacial adhesion through entanglements.71,72 The peak melting (Tm) and crystallization (Tc) temperatures of the PEO block in the PB−PEO block copolymer were determined from DSC to be 61 and 42 °C, respectively. The PB−PEO melt structure was characterized at 80 °C using SAXS, shown in Figure 2. Higher order peak C

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Figure 2. SAXS data for melt state PB−PEO at 80 °C. Arrows indicate expected peak positions for a lamellar morphology (due to phase symmetry, even-numbered higher order peaks are diminished).

Figure 1. Glass transitions (Tg) of (a) PB3k/PDCPD and (b) PEO2k/ DGEBA4k blends. Solid squares denote the experimental measurement from DSC. Black solid curves represent a linear fit between neat components Tg’s, while the dashed red curves show the fit to the Flory− Fox equation to the data. All samples exhibited a single Tg.

Figure 3. Scattering intensity I as a function of the magnitude of the scattering wave vector q obtained for blend 50E compatibilized with 0.1 and 0.25 wt % of block copolymer. The data were shifted along the intensity axis for clarity (a factor of 10 for 50E-0.1). Solid curves indicate fits of eq 1 employing the form factor for spheres (interparticle correlations were neglected).

Table 1. Characteristics of PB−PEO Diblock Copolymer Mn,PB blocka (kg/mol)

Mn,PEO blocka (kg/mol)

1,4 contentb

PB vol fractionb

Tcc (°C)

Tmc (°C)

Đd

9.2

10.8

93%

0.52

42

61

1.14

a

Determined from the PB volume fraction (characterized through NMR) and the total Mn obtained from GPC (20.0 kg/mol). b Determined from 1H NMR analysis (Figure S1). cDetermined from DSC analysis (Figure S3). dDetermined from GPC analysis (Figure S2).

yielded microheterogeneous samples with lateral length scales beyond 2 μm and cross-sectional sizes on the order of 50 nm.44 From Figure 3 it is clear that the addition of block copolymer results in smaller structures that were measurable in the USAXS region (