Solid state nuclear magnetic resonance (NMR) spectroscopy and small-to wide-angle X-ray scattering (SWAXS) methods were used to characterize the heterogeneous dynamics and polymer domain structure in rubber modified thermoset materials containing the diglycidyl ether of bisphenol A (DGEBA) epoxy resin and a mixture of Jeffamine reactive rubber and 4,4-diaminodicyclohexylmethane (PACM) amine curing agent. The polymer chain dynamics and morphologies as a function of the PACM/Jeffamine ratio were determined. Using dipolar-filtered NMR experiments, the resulting networks are shown to be composed of mobile and rigid regions that are separated on nanometer length scales, along with a dynamically immobilized interface region. Proton NMR spin diffusion experiments measured the dimensions of the mobile phase to range between 9 and 66 nm and varied with the relative PACM concentration. Solid state 13C magic angle spinning NMR experiments show that the highly mobile phase is composed entirely of the dynamically flexible polyether chains of the Jeffamine rubber, the immobilized interface region is a mixture of DGEBA, PACM, and the Jeffamine rubber, with the PACM cross-linked to DGEBA predominantly residing in the rigid phase. The SWAXS results showed compositional nanophase separation spanning the 11–77 nm range. These measurements of the nanoscale compositional and dynamic heterogeneity provide molecular level insight into the very broad and controllable glass transition temperature distributions observed for these highly cross-linked polymer networks.
Once limited to chain-growth polymerizations, fine control over polymerization-induced phase separation (PIPS) has recently been demonstrated in rubber-toughened thermoset materials formed through step-growth polymerizations. The domain length scales of these thermoset materials can be elegantly tuned by utilizing a binary mixture of curing agents (CAs) that individually yield disparate morphologies. Importantly, varying the composition of the binary mixture affects characteristics of the materials such as glass transition temperature and tensile behavior. Here, we establish a full phase diagram of PIPS in a rubber-toughened epoxy system tuned by a binary CA mixture to provide a robust framework of phase behaviour. X-Ray scattering in situ and post-PIPS is employed to elucidate the PIPS mechanism whereby an initial polymerization-induced compositional fluctuation causes nanoscale phase separation of rubber and epoxy components prior to local chain crosslinking and potential macrophase separation. We further demonstrate the universality of this approach by alternatively employing binary epoxy or binary rubber mixtures to achieve broad variations in morphology and glass transitions.
The development of chemistry is reported to implement selective dual-wavelength olefin metathesis polymerization for continuous additive manufacturing (AM). A resin formulation based on dicyclopentadiene is produced using a latent olefin metathesis catalyst, various photosensitizers (PSs) and photobase generators (PBGs) to achieve efficient initiation at one wavelength (e.g., blue light) and fast catalyst decomposition and polymerization deactivation at a second (e.g., UV-light). This process enables 2D stereolithographic (SLA) printing, either using photomasks or patterned, collimated light. Importantly, the same process is readily adapted for 3D continuous AM, with printing rates of 36 mm h–1 for patterned light and up to 180 mm h–1 using un-patterned, high intensity light.
Polymerization-induced phase separation enables fine control over thermoset network morphologies, yielding heterogeneous structures with domain sizes tunable over 1-100 nm. However, the controlled chain-growth polymerization techniques exclusively employed to regulate the morphology at these length scales are unsuitable for a majority of thermoset materials typically formed through step-growth mechanisms. By varying the composition of a binary curing agent mixture in a classic rubber-toughened epoxy thermoset, where the two curing agents are selected based on disparate compatibility with the rubber, we demonstrate facile tunability over morphology through a single compositional parameter. Indeed, this method yields morphologies spanning the nano-scale to the macro-scale, controlled by the relative reactivities and thermodynamic compatibility of the network components. We further demonstrate a profound connection between chain dynamics and microstructure in these materials, with the tunable morphology enabling exquisite variations in glass transition. In addition, previously unattainable control over tensile mechanical properties is realized, including atypical increase of elongation at failure while maintaining the modulus and ultimate strength.
This communication describes a novel series of linear and crosslinked polyurethanes (PUs) and their selective depolymerization under mild conditions. Two unique polyols are synthesized bearing unsaturated units in a configuration designed to favor ring-closing metathesis (RCM) to five- and six-membered cycloalkenes. These polyols are co-polymerized with toluene diisocyanate to generate linear PUs and trifunctional hexamethylene- and diphenylmethane-based isocyanates to generate crosslinked PUs. The polyol design is such that the RCM reaction cleaves the backbone of the polymer chain. Upon exposure to dilute solutions of Grubbs’ catalyst under ambient conditions, the PUs are rapidly depolymerized to low molecular weight, soluble products bearing vinyl and cycloalkene functionalities. These functionalities enable further re-polymerization by traditional strategies for polymerization of double bonds. It is anticipated that this general approach can be expanded to develop a range of chemically recyclable condensation polymers that are readily depolymerized by orthogonal metathesis chemistry.
A series of networks is introduced with systematically varied network heterogeneity and high overall values of average glass transition temperature (Tg), based on polymerization of rigid acrylate and aromatic thiol monomers. The curing behavior, chain dynamics, and microstructure of these networks were investigated through a combination of dynamic mechanical analysis and infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and x-ray scattering, respectively. The maximum Tg achieved during cure can be related to the breadth of the mechanical loss tangent, as others have previously suggested, as well as the temperature dependence of the chain dynamics in the network as monitored by 1H NMR. In addition, the microstructures of the networks are characterized by periodic, fractal microgels with characteristic length scales of ca. 20–40 nm. Intriguingly, this structural motif persists in the more homogeneous networks exhibiting comparatively narrow glass transitions and chain dynamics, indicating that dynamically homogeneous networks can still exhibit significant compositional heterogeneity at the mesoscale.
NMR spectroscopy continues to provide important molecular level details of dynamics in different polymer materials, ranging from rubbers to highly crosslinked composites. It has been argued that thermoset polymers containing dynamic and chemical heterogeneities can be fully cured at temperatures well below the final glass transition temperature (Tg). In this paper, we described the use of static solid-state 1H NMR spectroscopy to measure the activation of different chain dynamics as a function of temperature. Near Tg, increasing polymer segmental chain fluctuations lead to dynamic averaging of the local homonuclear proton-proton (1H-1H) dipolar couplings, as reflected in the reduction of the NMR line shape second moment (M2) when motions are faster than the magnitude of the dipolar coupling. In general, for polymer systems, distributions in the dynamic correlation times are commonly expected. To help identify the limitations and pitfalls of M2 analyses, the impact of activation energy or, equivalently, correlation time distributions, on the analysis of 1H NMR M2 temperature variations is explored. It is shown by using normalized reference curves that the distributions in dynamic activation energies can be measured from the M2 temperature behavior. An example of the M2 analysis for a series of thermosetting polymers with systematically varied dynamic heterogeneity is presented and discussed.
We report a novel approach whereby cross-linked polybutadiene (PB) networks can be depolymerized in situ based on thermally activated alkene metathesis. A commercially available latent Ru catalyst, HeatMet, was compared to the common second-generation Hoveyda-Grubbs catalyst, HG2, in the metathetic depolymerization of PB. HeatMet was found to possess exceptional stability and negligible activity toward PB under ambient conditions, in solution and in bulk. This enabled cross-linked networks to be prepared containing homogeneously distributed Ru catalyst. The dynamic mechanical properties of networks containing HeatMet and cross-linked using alcohol-isocyanate or thiol-ene chemistry were evaluated during cross-linking and post-cross-linking under isothermal and nonisothermal heating. In both cases, above minimum catalyst loadings ranging from 0.004 to 0.024 mol %, the networks exhibited rapid degelation into a soluble oil upon heating to 100 °C. At these temperatures, extensive depolymerization of the PB segments through ring-closing metathesis of 1,4/1,2 diads by the activated HeatMet introduced network defects in significantly greater proportion than the original number of cross-links. The in situ depolymerization of cross-linked PB networks through latent catalysis, as described here, may enable facile disposal and recycling of PB encapsulants and adhesives, among other applications.
This report catalogues the results of a project exploring the incorporation of organometallic compounds into thermosetting polymers as a means to reduce their residual stress. Various syntheses of polymerizable ferro cene derivatives were attempted with mixed success. Ultimately, a diamine derivative of ferrocene was used as a curing agen t for a commercial epoxy resin, where it was found to give similar cure kinetics and mechanical properties in comparison to conventional curing agents. T he ferrocen e - based material is uniquely able to relax stress above the glass transition, leading to reduced cure stress. We propose that this behavior arises from the fluxional capacity of ferrocene. In support of this notion, nuclear magnetic resonance spectroscopy indicates a substantial increase in chain flexibility in the ferrocene - containing network. Although t he utilization of fluxionality is a novel approach to stress management in epoxy thermosets, it is anticipated to have greater impact in radical - cured ther mosets and linear polymers.
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. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the 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. We postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.
Preparation of sodium zirconium silicate phosphate (NaSICon), Na1+xZr2SixP3–xO12 (0.25 ≤ x ≤ 1.0), thin films has been investigated via a chemical solution approach on platinized silicon substrates. Increasing the silicon content resulted in a reduction in the crystallite size and a reduction in the measured ionic conductivity. Processing temperature was also found to affect microstructure and ionic conductivity with higher processing temperatures resulting in larger crystallite sizes and higher ionic conductivities. The highest room temperature sodium ion conductivity was measured for an x = 0.25 composition at 2.3 × 10–5 S/cm. In conclusion, the decreasing ionic conductivity trends with increasing silicon content and decreasing processing temperature are consistent with grain boundary and defect scattering of conducting ions.
Fiber Bragg gratings (FBGs) are well-suited for embedded sensing of interfacial phenomena in materials systems, due to the sensitivity of their spectral response to locally non-uniform strain fields. Over the last 15 years, FBGs have been successfully employed to sense delamination at interfaces, with a clear emphasis on planar events induced by transverse cracks in fiber-reinforced plastic laminates. We have built upon this work by utilizing FBGs to detect circular delamination events at the interface between epoxy films and alumina substrates. Two different delamination processes are examined, based on stress relief induced by indentation of the epoxy film or by cooling to low temperature. We have characterized the spectral response pre-and post-delamination for both simple and chirped FBGs as a function of delamination size. We show that delamination is readily detected by the evolution of a non-uniform strain distribution along the fiber axis that persists after the stressing condition is removed. These residual strain distributions differ substantially between the delamination processes, with indentation and cooling producing predominantly tensile and compressive strain, respectively, that are well-captured by Gaussian profiles. More importantly, we observe a strong correlation between spectrally-derived measurements, such as spectral widths, and delamination size. Our results further highlight the unique capabilities of FBGs as diagnostic tools for sensing delamination in materials systems.
Deboronation is observed during the reductive amination of formylphenylboronic acid (FPBA) to the amine termini and side chains of peptides. This deboronation is sensitive to the isomerism of the boronic acid (BA), with ortho-FPBA yielding complete deboronation in the preparation of an N-terminally-modified dipeptide. The observed behavior is also clearly mediated by the chemical identity of the amine substrate. These results reveal a previously undocumented subtlety of BA functionalization and highlight the importance of thorough spectroscopic characterization in the preparation of peptide and small molecule BAs.
Modification of the dipeptide of phenylalanine, FF, with a boronic acid (BA) functionality imparts unique aqueous self-assembly behavior that responds to multiple stimuli. Changes in pH and ionic strength are used to trigger hydrogelation via the formation of nanoribbon networks. Thus, we show for the first time that the binding of polyols to the BA functionality can modulate a peptide between its assembled and disassembled states.
The secondary structure of peptides in the presence of interacting additives is an important topic of study, having implications in the application of peptide science to a broad range of modern technologies. Surfactants constitute a class of biologically relevant compounds that are known to influence both peptide conformation and aggregation or assembly. We have characterized the secondary structure of a linear nonapeptide composed of a hydrophobic alanine/phenylalanine core flanked by hydrophilic acid/amine units. We show that the anionic surfactant sodium dodecyl sulfate (SDS) induces the formation of β-sheets and macroscopic gelation in this otherwise unstructured peptide. Through comparison to related additives, we propose that SDS-induced secondary structure formation is the result of amphiphilicity created by electrostatic binding of SDS to the peptide. In addition, we demonstrate a novel utility of surfactants in manipulating and stabilizing peptide nanostructures. SDS is used to simultaneously induce secondary structure in a peptide and to inhibit the activity of a model enzyme, resulting in a peptide hydrogel that is impervious to enzymatic degradation. These results complement our understanding of the behavior of peptides in the presence of interacting secondary molecules and provide new potential pathways for programmable organization of peptides by the addition of such components.