One-Pot Synthesis and Characterization of Novel Shape-Memory Poly(ε-Caprolactone) Based Polyurethane-Epoxy Co-networks with Diels–Alder Couplings

Abstract

Abstract:The present work aimed at the preparation and investigation of different epoxypolyurethane (EP-PU) co-networks. The EP-PU co-networks were obtained by applying two different synthetic strategies, in which the coupling element, the Diels-Alder (DA) adduct, was prepared previously or formed "in situ" in the reaction between furan functionalized polyurethane and furfuryl amine-diglycidyl ether bisphenol-A oligomers (FA_DGEBA). For the synthesis of these EP-PU networks, poly(ε-caprolactone)-diol (PCD, M n = 2 kg/mol) and poly(ε-caprolactone) (PCL) with different molecular weights (M n = 10, 25 and 50 kg/mol) and 1,6-hexamethylenediisocyanate (HDI) were used. The EP-PU co-networks were characterized by Attenuated Total Reflectance Fourier-Transform Infrared spectroscopy (AT-FT-IR), differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA). Scanning electron microscopy (SEM) was applied to assess the morphology of the EP-PU samples. It was demonstrated that the stress-strain curves for the EP-PUs could be interpreted based on the Standard Linear Solid (SLS) model. The DMA traces of some EP-PUs (depending on the composition and the synthetic method) revealed a plateau-like region above the melting temperature (T m ) of PCL confirming the presence of cross-linked structure. This feature predicted shape memory (SM) behavior for these EP-PU samples. Indeed, very good shape fixity and moderate shape recovery were obtained. The shape recovery processes of these EP-PU samples were described using double exponential decay functions.

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“…The description of the σ−ε relationship was based on the standard linear solid (SLS) viscoelastic model. As it was shown earlier in uniaxial tensile mode and at constant strain rate (d ε /dt) “the equation of motion” can be given by Equation (9) [20,21]. σ+a1dεdtdσdε=a2ε+a3dεdt where a 1 , a 2 and a 3 are the parameters that contain the moduli of springs and the viscosity of the dashpot in the constitutive SLS model.…”

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“…To this end, for specimen FCB-EPX-I, the Equation (2) should be revised to Equation (3) which includes an extra term regarding the contribution of the jacket to the axial capacity of the specimen. According to compressive tests on epoxy specimens by Yekani Fard [37], the measured compressive strain of epoxy at peak load is 0.005, whereas the strain of epoxy at ultimate is 0.03. This means the stress-strain curve of epoxy in compression is not a linear curve until failure.…”

“…This means the stress-strain curve of epoxy in compression is not a linear curve until failure. A softening stage starting strains at 0.005 exists when epoxy is under compressive load (e.g., Yekani Fard [37]). Meanwhile, the compressive tests by Ogendal [38] on PVC specimens shows that the strain at peak and ultimate are similar values (linear stress-strain behaviour in PVC till failure).…”

“…Meanwhile, at a strain of 0.0052, epoxy has only slightly passed its strength of 86.2 MPa and to some extent entered its softening stage. Based on the studies by Yekani Fard [37], the strength of epoxy at the compressive strain of 0.0052 can be approximated equal to 86.0 MPa (rather than its strength at the pick of 86.2 MPa). In addition, considering the linear behaviour of the PVC material until failure, the stress in a PVC jacket can be calculated using a strain of 0.0052 and used in Equation (3) as the stress of jacket σ j to calculate the FCB-EPX-I specimen P theory as follows:…”

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“…This Special Issue also reports the description of novel shape-memory epoxy-polyurethane co-networks by means of Diels–Alder couplings, i.e., one of the most intriguing and attractive areas of research on self-healing and recyclable polymer structures [17]. In this case, the (reversibly) crosslinked epoxy resin was rendered more flexible and ductile thanks to the interpenetrating network composed of polyurethane.…”