Co-continuous network polymers using epoxy monolith for the design of tough materials

High-performance polymer materials that can exhibit distinguished mechanical properties have been developed based on material design considering energy dissipation by sacrificial bond dissociation. We now propose co-continuous network polymers (CNPs) for the design of tough polymer materials. CNP is a new composite material fabricated by filling the three-dimensionally continuous pores of a hard epoxy monolith with any cross-linked polymer having a low glass transition temperature (Tg). The structure and mechanical properties of the CNPs containing epoxy resins, thiol-ene thermosets, and polyacrylates as the low-Tg components were investigated by differential scanning calorimetry, dynamic mechanical analysis, tensile tests as well as scanning electron microscopic observations and non-destructive 3D X-ray imaging in order to clarify a mechanism for exhibiting an excellent strength and toughness. It has been demonstrated that the mechanical properties and fractural behavior of the CNPs significantly depend on the network structure of the filler polymers, and that a simultaneous high strength and toughness are achieved via the sacrificial fracture mechanism of epoxy-based hard materials with co-continuous network structures.


Experimental Methods
General Procedure. The SEM observation was performed using VE-9800 (Keyence Corporation, Ltd., Osaka, Japan) with an acceleration voltage of 1.0 kV and Au vapor deposition or JSM-IT100 (JEOL, Ltd., Tokyo, Japan) with an acceleration voltage of 10 kV and Os vapor deposition. The differential scanning calorimetry (DSC) measurement was carried out using DSC-60 (Shimadzu Corporation, Kyoto, Japan) at the heating rate of 10 °C/min in a nitrogen stream. The T g was determined from a trace during the second heating process. DMA was carried out using similar test pieces and DMS 6100 (Seiko Instruments, Inc., Tokyo, Japan). The conditions were a dual cantilever mode at the heating rate of 2 °C/min.
Sinusoidal strains with an amplitude of 10 μm at 1 Hz were applied. The T g was determined as the peak temperature of the tanδ curves. The tensile test was carried out and the tensile rate of 1.0 mm/min using an Autograph AGSX 5 kN (Shimadzu Corporation, Ltd., Kyoto, Japan) at room temperature. The size of the test pieces was 10 mm × 40 mm. The thickness was determined using a Peacock dial thickness gauge (Ozaki Mfg. Corporation, Ltd., Osaka, Japan). The sample number (N) was 2−5 for the tensile measurements except for the single measurement for CNP-T6/A6 in Table 2. The modulus was determined from the initial slope of the stress−strain curve in the range of elongation of 0.05%−0.25%. The cycle number was 5−10 for the cyclic tensile test.
X-ray Imaging. The 3D X-ray imaging was carried out using an X-ray microscope CT nano3DX, Rigaku Corporation, Tokyo, Japan. The sample was placed on a 2-axis goniometer stage. A Cu target (K α , λ = 0.15418 nm, 8.048 keV) was chosen to visualize the fine structure of the epoxy monoliths. The tube voltage and current were set to 40 kV and 30 mA, respectively. The field of view (FOV) for the camera was 0.66 mm x 0.66 mm and effective pixel was 0.62 μm. The goniometer stage was rotated 180 degrees and 600 projection images were taken. The scan time was 15−60 min. The FDK (Feldkamp-Davis-Kress) algorithm was then used to reconstruct the tomograms.
Poly(ethylene glycol) diacrylate (P2, n = 9) was purchased from Tokyo Chemical Industry Corporation, Ltd., Tokyo, Japan, and used as received. P6 was commercially available as SA-TE60 (the number of functional groups, 6; the number-average molecular weight, 3400) from Sakamoto Yakuhin Kogyo Corporation, Ltd., Osaka, Japan, and used without further purification. Benzoyl peroxide (BPO) as a radical initiator was purchased from Nacalai Tesque, Inc., Kyoto, Japan, and used as received. The Pyrex glass and Al plates were purchased from the AsOne Corporation, Osaka, Japan, and used after cleaning with acetone.
The mixed paste was spread on the glass or Al plate to the desired thickness followed by thermal curing using an oven at 120 or 130 °C (VOC-210SD, Tokyo Rikakikai (EYELA) Corporation, Ltd., Tokyo, Japan) for 60-90 min. The temperature measurements of the reaction samples and the glass and Al plates were conducted using a multi-channel USB data logger (TC-08, Pico Technology, Cambridgeshire, U.K.) with a K-type thermocouple. The thermocouple was fixed with Kapton tape. After curing, the samples were washed with ionexchanged water by ultrasonics for 5 min to remove the PEG200, stored in ion-exchanged water overnight, then dried in vacuo for 2 h at room temperature.

Synthesis of the CNPs.
To the pores of the monolith sheets, an epoxy resin (E2 or E3) and a diamine curing agent (BACM) at the ratio of 2[NH 2 ]/[epoxy] (γ value) = 1.0 were penetrated under reduced pressure, then heated in an oven at 120 °C for 60 min to obtain the epoxy/epoxy-type CNPs. Similarly, the thiol-ene CNPs were prepared at 160 °C for 2 h using a mixture of polyfunctional thiol and acrylate. For the synthesis of the polyacrylate CNPs, acrylate monomers penetrated into the pores of the monolith were polymerized in the presence of BPO as the radical initiator at 90 °C for 60 min. The residual free monomer was less than 2% after curing of the polyacrylate systems based on the results of an extraction experiment. a Based on tanδ peak temperature of DMA measured at the frequency of 1 Hz and the heating rate of 2 o C/min. The values in parentheses indicate the T g values determined by DSC from the second heating process at the heating rate of 10 o C/min. b The crosslinking density (n) was calculated using the following equation: n (mol/cm 3 ) = E'/(2(1 + μ)RT. E' is the elasticity (in Pa), R is the gas constant, T is the absolute temperature, and μ is the Poisson ratio, which was assumed to be 0.5.