Block Copolymer Modified Epoxies: The Effect of Resin Chemistry on Morphology Amelia Labak and Raymond A. Pearson Lehigh University Bethlehem, PA, 18015, USA [email protected], [email protected] Introduction An epoxy matrix is inherently brittle. In order to provide a more versatile material, these epoxies are often toughened with a rubber. In this rubber toughened system, an additive, such as a diblock copolymer, is included in the matrix to provide different toughening mechanisms. These mechanisms include localized shear yielding, plastic void growth, and crack bridging. [1] The type and amount of the additive has a clear impact on the properties of the system. Some of these particle variations in a consistent epoxy system have been investigated. [2,3] Diblock copolymers self assemble in a resin matrix due to the fact that one of the blocks is miscible in the matrix and the other is not. In addition to the rubber particle addition, variations in the epoxy matrix also have an impact on the fracture behavior of the toughened systems. It is necessary for there to be sufficient molecular weight between crosslinks in order to facilitate shear yielding of the epoxy matrix. For epoxy systems toughened by self-assembling block copolymers, the size and morphology of the rubber particles are controlled by the block length and the polarity of the miscible block [4,7] Similarly, controlling the chemical structure of the chain extender in order to create a more polar resin system should have a similar effect on rubber particle size and morphology. Therefore, the purpose of this research is to investigate the effect of the structure of the epoxy system on the morphology of the diblock copolymers and, ultimately, the fracture behavior the toughened epoxy systems. Experimental Materials In this study, an epoxy system of diglyidyl ether of bisphenol A (DGEBA) was cured using m-phenylene diamine, mPDA, (Acros Organics), aniline (Alfa Aesar), and hydroquinone (Alfa Aesar). The DGEBA had the designation of D.E.R. (Dow Epoxy Resin) 331 and was provided by Sigma-Aldrith. The epoxy resin has a molecular weight of 374 g/mol. The curing agent (mPDA), chain extender (aniline), and accelerator (hydroquinone) were added to the epoxy resin in different amounts of mole ratios. Nanostrength D51N (NM27), a poly(butyl acrylate-co- methacylate) block copolymer, was provided by Arkema in order to rubber toughen the resin matrix. Six resin matrices will be investigated to determine the role of the polarity of the chain extender on the morphology of the diblock copolymer and the mechanical properties of the epoxy systems. The stoichiometric ratios of these resin matrices can be seen in Table 1 below. Additionally, each of these systems will be toughened with 10 phr of NM 27. These systems will establish the effect of varying the crosslink density and the polarity of the resin matrix on the rubber toughened system. Increasing the ratio of aniline to mPDA will decrease the crosslink density. Furthermore, as hydroquinone is more polar than aniline, increasing the amount of hydroquinone will increase the polarity of the system. Table 1. Stoichiometric Mole Ratios of the Systems Mole Ratios System DGEBA mPDA Aniline Hydroquinone A 3 1 1 0 B 4 1 2 0 C 5 1 3 0 D 4 1 3/2 1/2 E 4 1 5/3 1/3 F 4 1 7/4 1/4 Table 2. Theoretical Crosslink Densities of the Systems System Theoretical Molecular Weight Between Crosslinks, Mnc (g/mol) A 1326 B 1794 C 2262 D 1802 E 1799 F 1798 It can be seen in Table 2 that he systems which include hydroquinone (D, E, and F) should have comparable crosslink densities to system B which involves a ratio of 2 aniline (chain extender) to 1 mPDA (curing agent). The slight variation is due to the fact that the hydroquinone has a slightly higher molecular weight than aniline. In these systems, hydroquinone replaces 1/4th, 1/6th, and 1/8th of the aniline for systems D, E, and F, respectively. Processing Initially, a master batch of 15 phr NM27 was made by adding NM27 into DER 331. It was mechanically mixed at 80ºC for 2 hours. Next this mixture was mechanically mixed at 150ºC under a vacuum for 1 hour. Finally, this master batch was then diluted to 10 phr following a similar method (80ºC for 2 hours, then 150ºC under a vacuum for 2 hours). The master batch mixture was cured using the various systems discussed above in Table 1. The mPDA and hydroquinone were provided in a solid state and the aniline was in a liquid state. In DGEBA, mPDA, and aniline systems (systems A, B, and C), the materials were combined at 80°C under a vacuum and mechanically mixed for 10 minutes. This heat and mechanical mixing dissolves and disperses the mPDA. For the systems that contain hydroquinone (systems D, E, and F), the aniline and hydroquinone were initially added into the master batch and mechanically mixed under a vacuum for half an hour. After the solid particles of the hydroquinone were incorporated, the mPDA was added and mixed under a vacuum at 80°C for 10 minutes. Immediately afterwards, the mixture was cast into a 6mm-thick pre-heated Teflon coated aluminum mold and cured for 12 hours at 50°C followed by a post-cure for 3 hours at 130°C. The samples are then cooled to room temperature while remaining in the oven. Fracture Toughness Testing sectional area was exposed to a uniaxial compression with a crosshead speed of 1 mm/min and a 30 kN load cell. The yield stress average values were derived from a minimum of seven samples. Glass Transition Temperature Testing TA Instruments DSC, Series Q2000, is used in order to determine the glass transition temperature, Tg. Samples are placed in aluminum hermetical sealed pans. The sample size ranges from 10-20 mg. In order to determine the T g, the sample is subjected to a heat-cool-heat cycle. The first heat ramp is to remove any uncured portions or effects of physical aging. Then, the T g is found from the midpoint of the step change of the second heat ramp according to ASTM D-7426. Dynamic Mechanical Analysis (DMA) Samples (50mm x 12.75mm x 2.5mm) are tested by the TA Instruments Advanced Rheometric Expansion System (ARES). The samples are run with a .2% strain rate until a temperature 15°C above the Tg. Then the strain rate is increased to .6% until 200°C. The storage and loss modulus, G’ and G”, respectively, and the tan(delta) are recorded. The Tg is determined from this measurement according to the peak of the loss modulus (ASTM E-1640). The shear modulus, Gc, which is used for crosslink density calculations, is recorded as the storage modulus when it reaches a constant value, at 50°C above Tg. The crosslink density is then calculated using (1) The fracture toughness of the modified epoxies is measured according to the ASTM D5045-99 test method. The critical stress intensity factor, KIC, is determined using single-edge-notched, three-point-bend (SEN-3PB) specimens with dimensions of 75.6 mm x 12.7 mm x 6.0mm. In order to create the pre-cracks in the samples, a two step process is performed. First, the samples are notched with a jewelers saw. Next, a razor blade that has been in nitrogen is hammered into the notch in order to propagate a crack. In order to ensure that the average crack length is within the allowable percentage of the sample (45%-55%), the crack length is averaged from three points across the fracture surface using calipers. The SEN-3PB test is performed using a screw-driven universal testing machine (Model 5567 Instron) equipped with a 500 N load cell at a crosshead speed of 1 mm/min in compression mode. The KIC average values are derived from a minimum of five samples to provide accuracy. Compressive Yield Stress Testing The compressive yield stress of modified epoxies is measured according to the ASTM D790 test method. Samples with dimensions of 6 mm x 6 mm x 12 mm are tested according to this method using a screw-driven universal testing machine (Instron Model 5567). The square cross- where Mnc is the molecular weight between crosslinks, q is the front factor (usually one), and ρ is the density at the temperature. [4] Transmission Optical Microscopy (TOM) TOM samples are made from the fractured surface of the SEN-3PB by manually grinding the sample until it has a thickness between 80-120 μm. This thin sample is then looked at under an optical microscope using two modes. First, bright field mode is used to illuminate the particle cavitation. Second, the crossed polars is used to show the birefringence that arises from the shear plasticity. Scanning Electrical Microscopy (SEM) Samples for the SEM are made by mounting the fractures surface from the SEN-3PB samples on an aluminum stub and sputter coating them for 30 seconds with iridium (~5 nm) in order to prevent charging. A Hitachi 4300 low voltage SEM is used with an accelerating voltage of 5kV. Both the fast fracture and the stress whitened zone of the fracture surface is analyzed in order to determine the toughening mechanism of the system. Bright Field Scanning Transmission Electrical Mi- croscopy (BF-STEM) Samples from each system are cryo-microtomed and OsO4 stained at the University of Massachusetts Medical School. They are then investigated using a Hitachi S4300SE/N at 30kV. A standard STEM-IN-SEM technique is used involving a specimen holder in order to produce a bright field image of one plane of the stained specimen. These images have a phase-contrast so that the dispersion and morphology of the diblock copolymers can be observed. [5] Results and Discussion Previously, research was conducted to determine the properties of neat resin systems involving mPDA, aniline, and hydroquinone. [6] When comparing the stoichiometric rario seen in system B from Table 1 in neat and rubber toughened systems, it can be seen that there is a difference in the properties (Table 3). As expected, the addition of the rubber toughening diblock copolymer increases the fracture tougheness, yield stress, and glass transition temperature. Table 3. Comparison for Rubber Toughened and Neat Systems Sample KIC Yield Stress Tg (MPa-m1/2) (MPa) (°C) Neat 0.68 ± 0.03 114.99 ± 97 ± 0.35 0.79 Rubber 1.48 ± 0.08 116.07 ± 113 ± 0.35 Toughened 0.55 Conclusions By adding a diblock copolymer into a lightly crosslinked epoxy resin there is an increase in the fracture toughness of the system. From past research it was seen that varying the resin systems also has an effect on the physical and mechanical properties, therefore, it can be extrapolated that these various resin systems, when rubber toughened, will also show a range in properties. Future Work Moving forward, the effect of the variety in the resin systems on the diblock copolymer in the rubber toughened system will be investigated. In the “Materials” section the systems that will be analyzed are described. In the “Experimental” section the various tests that will be used to quantify these differences are outlined. Acknowledgements The research was partially supported for a research grant from the Semiconductor Research Corporation (SRC Task No. 2251.001). References 1. H.R. Azimi, R.A. Pearson and R.W. Hertzberg, J Mat Sci Lett, 13, 1994, 1460. 2. Lauren Bacigalupo. PhD Thesis; Fracture Behavior of Nano-Scale Rubber-Modified Epoxies, Lehigh University, 2013. 3. R. Bagheri, R. A. Pearson, Polym., 31, 1996, 4529. 2571. 4. R.A. Pearson and A.F. Yee, J Mat Sci, 24, 1989, 5. A. Bogner, P.H. Jouneau, G. Thollet, D. Basset, and C. Gauthier, Micron, 38, 2007,, 390. 6. A. Labak and R.A. Pearson, Modifying the Network of an Aromatic Amine-Cured Epoxy, Adhesion Society Annual Meeting, Daytona, Fl., 2012. 7. H. Kishi, Y. Kunimitsu, et al, Polym., 52, 2011, 760.
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