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The hydrolysis of thermoplastic poly(ester urethane) (PEU) is convoluted by its block copolymer phase structure and competing hydrolytic sensitivities of multiple functional groups. The exact pathways for water ingress, water interaction with the material and ultimately the kinetics and order of functional group hydrolysis remain to be refined. Additional diagnostics are needed to enable deeper insight and deconvolution of material changes. In combination with GPC results, a promising analytical technique – two-dimensional correlation spectroscopy (2D-COS) – has been reviewed and applied to analyze FTIR spectra of hydrolyzed PEUs aged under various conditions, such as exposure time, temperature, and relative humidity.more » 2D-COS allows the complex role of water with distinct intermediate steps to be established, plus it emphasizes the initial stages of PEU hydrolysis at more susceptible functional groups. As a complication for the raw material, ATR IR detected some talc on the surface of commercial PEU beads and pressed sheets thereof, which can interfere with water ingress and thereby retards PEU hydrolysis, particularly in its natural form or moderate aging at lower temperatures (e.g., below the melting point of PEU). As aging temperature increases above the melting temperature, even traces of water trapped inside the PEU are sufficient to initiate the hydrolysis, which then progresses strongly with increasing temperatures. Feedback from 2D-COS analysis confirms that PEU hydrolysis starts at esters in the soft-segments before those in the urethane linkage become susceptible. Only when the molecular weight of PEU is below a critical molar mass (M_c) will the hydrolysis occur in parallel in the hard-segments since protective morphological phase structures are then absent. The current observations demonstrate unexpected behavior that may result from 'unknown' additives in polymer degradation, the temporal and group-specific hydrolysis of PEU as a function of locally available water molecules, the order of reactivity of susceptible functional groups, and the importance of changes in molecular weight coupled with the phase structure of the polymer.« less

The use of self–assembling, pre–polymer materials in 3D printing is rare, due to difficulties of facilitating printing with low molecular weight species and preserving their reactivity and/or functions on the macroscale. Akin to 3D printing of small molecules, examples of extrusion–based printing of pre–polymer thermosets are uncommon, arising from their limited rheological tuneability and slow reactions kinetics. The direct ink write (DIW) 3D printing of a two–part resin, Epon 828 and Jeffamine D230, using a self–assembly approach is reported. Through the addition of self–assembling, ureidopyrimidinone–modified Jeffamine D230 and nanoclay filler, suitable viscoelastic properties are obtained, enabling 3D printing of themore » epoxy–amine pre–polymer resin. A significant increase in viscosity is observed, with an infinite shear rate viscosity of approximately two orders of magnitude higher than control resins, in addition to, an increase in yield strength and thixotropic behavior. As a result, printing of simple geometries is demonstrated with parts showing excellent interlayer adhesion, unachievable using control resins.« less

Previous observation of EPDM and FEPM materials aged in thermo-oxidative and thermo-oxidative plus hydrolytic environments revealed an unusual trend: the degradation and disintegration of these polymers in the former case but the ability to maintain mechanical performance and shape in the latter. No abnormalities were observed in the chemical (oxidation rates, FTIR spectra, solvent uptake, gel content, and weight loss vs. temperature) or physical (modulus profile) measurements that could explain these empirically observed aging differences. A second controlled aging test was conducted to verify this trend using only EPDM. Once again it was shown that thermo-oxidative conditions appear to causemore » more degradative damage (enhanced embrittlement) than observed for the combined thermo-oxidative plus hydrolytic environments. From these data we conclude that water may favorably interfere with normal thermo-oxidative degradation processes. This interference may occur via some combination of chemical and physical property changes in the presence of steam such as: oxidation rate and O₂ permeability changes, additional sensitivity to hydrolytic damage, and/or mechanistic changes in relation to pH and hydroperoxide formation.« less

Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. In this paper, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into themore » backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temperature as well as unique stress relaxation behavior above its glass transition in the fully cured state. The relaxation experiments exhibit features characteristic of a physical relaxation process. Furthermore, the cure stress is observed to vanish precipitously upon deliberate introduction of network defects through an increasing imbalance of epoxy and amine functional groups. Finally, we postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.« less