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Interpenetrating polymer networks (IPN) are model polymeric systems for vibration damping applications owing to their unique viscoelastic properties. They are characterized by presence of static frozen inhomogeneities at molecular scale and are dynamically heterogeneous at a segmental level. These fundamental and characteristic features of IPNs need to be characterized in order to deduce their structure–macroscopic property correlations. In the present study, we report sequential IPNs prepared from a polyether diamine cross-linked epoxy and triethylene glycol dimethacrylate (TEGDM) cross-linked poly­(methyl methacrylate) (PMMA). Positron annihilation lifetime spectroscopy (PALS) studies revealed a decrease in free volume hole sizes with increased PMMA content in the IPNs, implying interpenetration of polymer chains at a molecular level. Dynamic mechanical analysis measurements were carried out to get insight into the structural relaxations and viscoelastic properties of the IPNs. The IPNs exhibited dissipation factor (tan δ) values >0.3 over a broad temperature (∼10–120 °C) and frequency range (20–20000 Hz), which qualify them as efficient vibration dampers. Stress strain profiles of the IPNs evolved from an elastic to a glassy response with strain-hardening characteristics as the PMMA content was increased in the IPNs. Simultaneous strengthening, stiffening, and toughening of the epoxy matrix were observed with increased PMMA content in the networks. The IPNs were also characterized for liquid water and toluene sorption characteristics to obtain mechanistic insights into the transport properties of the synthesized IPNs. Increasing water uptake and decreasing toluene sorption characteristics were observed with increased PMMA content in the IPNs. The free volume size plausibly governs the water transport properties, while toluene sorption in the IPNs could be influenced by its thermodynamic interaction with the networks. In both cases, non-Fickian sorption kinetics was observed. We posit that the present studies and results provide the basis for designing and characterizing vibration damping networks for practical applications.

Morphology of random ionomers has been a topic of fundamental research interest for decades. However, structural underpinning and hence modulation of the ionic phase morphology in this special class of polymers still remains challenging and topical. In the present work, we investigate the local structure and morphology of the ionic aggregates of carboxylated nitrile rubber (XNBR) cross-linked ionically (I-X series) by a mixed system of covalent and ionic curatives (M-X series) using Fourier transform infrared (FTIR) and small-angle Xray scattering (SAXS) techniques. FTIR investigations revealed that curatives induced significantly disparate and heterogeneous nature of the ionic and coordination complexes of the XNBR vulcanizates, when cured with varied contents of ZnO and combination of sulfur and ZnO. Besides the formation of zinc carboxylate ionic phase, in both modes of crosslinking, the nitrile group of XNBR was found to be coordinated with ZnO, which has not been hitherto addressed in the literature. The critical role of stearic acid (SA) as a processing aid or organic activator, in facilitation of ionic and coordination complex formation in XNBR vulcanizates in different modes of crosslinking, has been identified and delineated. SAXS studies revealed a single characteristic ionic peak at a scattering vector (q = 4π/λ sinθ/2) value of 2.0−2.1 nm −1 for I-X series and M-X series of networks prepared without SA. In striking contrast, the networks prepared with SA showed higher order reflections with four well-resolved ionic peaks at relative q positions of 1:1.440 ± 0.024:2.00 ± 0.01:3.006 ± 0.036 for I-X series and 1:1.376 ± 0.007:2.010 ± 0.001:3.00 ± 0.02 for M-X series, implying that the ionic aggregates self-assemble into a lamellar morphology, albeit with some deviations. The sharp upturn in I(q) at low q values of the networks could be fitted to a power law dependence, I(q) = q −d , with the d values assuming either mass fractal (d = 2.5 to 3) or surface fractal (d = 3.5 to 4.0) dimensions, depending on the mode of crosslinking. The SAXS and FTIR results establish that the ionic phase morphology of XNBR vulcanizates is not consistent with that of conventional random ionomers and comprise both ordered structures as well as heterogeneous mixture of ionic and coordinated complexes which are evidently of fractal constructs. We posit that these findings will provide new insights in establishing the structure−property correlations of ionomeric elastomers.

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The advent of dynamic covalent bond chemistry (DCBC) has sparked renewed research interest in reprocessability of thermosetting polymers. However, combining reprocessability with stimuli-responsiveness remains a challenge for futuristic applications. Usually, a combination of complementary DCBCs is employed to integrate reprocessability with shape memory as well as self-healing (SH) attributes in a single polymeric system. In the present work, we report two-component poly(urethane-urea) (PUU) networks, with varied hard segment (HS) concentrations, that exhibit quadruple shape memory (QSM), efficacious vibration damping, thermally activated intrinsic SH as well as multiple cycles of thermal reprocessability. The component A is an isocyanate capped bi-soft segment blend of polybutadiene diol and polypropylene glycol, while the second component comprises an aromatic diamine chain extender solubilized in an oligomeric polyoxypropylene triol. The networks characterized through FTIR as well as small- and wide-angle X-ray scattering (SAXS and WAXD) revealed a phase-separated morphology with extensive hydrogen-bonding (H-bonding) interactions in the HS domains of the PUU networks. Dynamic mechanical analysis (DMA) measurements revealed broad dissipation factor (tan δ) vs temperature and frequency profiles of the networks, which could be leveraged for efficacious passive vibration damping applications. A dissociative mechanism of urethane and urea bond exchange, evidenced from temperature-dependent FTIR studies enabling facile stress relaxation, is proposed to be the mechanistic origin of the reprocessability and SH of the networks. A broad dissipation factor (tan δ) vs temperature profiles endowed the networks with QSM characteristics. We posit that the study is relevant for expanding the scope of DCBCs for development of reprocessable thermosetting polyurethanes with multiple smart functionalities.