16 research outputs found
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Hierarchical Sticker and Sticky Chain Dynamics in Self-Healing Butyl Rubber Ionomers
We present a detailed comparison of the microscopic dynamics and the macroscopic mechanical behavior of novel butyl rubber ionomers with tunable dynamics of sparse sticky imidazole-based sidegroups that form clusters of about 20 units separated by essentially unperturbed chains. This material platform shows promise for application as self-healing elastomers. Size and thermal stability of the ionic clusters were probed by small-angle X-ray scattering, and the chain and sticker dynamics were studied by a combination of broadband dielectric spectroscopy (BDS) and advanced NMR methods. The results are correlated with the rheological behavior characterized by dynamic-mechanical analysis (DMA). While the NMR-detected chain relaxation and DMA results agree quantitatively and confirm relevant aspects of the sticky-reptation picture on a microscopic level, we stress and explain that apparent master curves are of limited use for such a comparison. The cluster-related relaxation time detected by BDS is much shorter than the elastic chain relaxation time, although the weak conductivity does follow the latter. The systematic trends across the sample series suggest that all relaxations are dominated by a cluster-related activation barrier, but also that the BDS-based cluster relaxation does not seem to be directly associated with the effective sticker lifetime. Nonlinear stress-strain experiments demonstrate a reduction of sticker lifetime on stretching and that the stored stress and the elastic recovery depend on the deformation rate. © 2019 American Chemical Society
Recommended from our members
Hierarchical Sticker and Sticky Chain Dynamics in Self-Healing Butyl Rubber Ionomers
We present a detailed comparison of the microscopic dynamics and the macroscopic mechanical behavior of novel butyl rubber ionomers with tunable dynamics of sparse sticky imidazole-based sidegroups that form clusters of about 20 units separated by essentially unperturbed chains. This material platform shows promise for application as self-healing elastomers. Size and thermal stability of the ionic clusters were probed by small-angle X-ray scattering, and the chain and sticker dynamics were studied by a combination of broadband dielectric spectroscopy (BDS) and advanced NMR methods. The results are correlated with the rheological behavior characterized by dynamic-mechanical analysis (DMA). While the NMR-detected chain relaxation and DMA results agree quantitatively and confirm relevant aspects of the sticky-reptation picture on a microscopic level, we stress and explain that apparent master curves are of limited use for such a comparison. The cluster-related relaxation time detected by BDS is much shorter than the elastic chain relaxation time, although the weak conductivity does follow the latter. The systematic trends across the sample series suggest that all relaxations are dominated by a cluster-related activation barrier, but also that the BDS-based cluster relaxation does not seem to be directly associated with the effective sticker lifetime. Nonlinear stress-strain experiments demonstrate a reduction of sticker lifetime on stretching and that the stored stress and the elastic recovery depend on the deformation rate. © 2019 American Chemical Society
Recommended from our members
Hierarchical Sticker and Sticky Chain Dynamics in Self-Healing Butyl Rubber Ionomers
We present a detailed comparison of the microscopic dynamics and the macroscopic mechanical behavior of novel butyl rubber ionomers with tunable dynamics of sparse sticky imidazole-based sidegroups that form clusters of about 20 units separated by essentially unperturbed chains. This material platform shows promise for application as self-healing elastomers. Size and thermal stability of the ionic clusters were probed by small-angle X-ray scattering, and the chain and sticker dynamics were studied by a combination of broadband dielectric spectroscopy (BDS) and advanced NMR methods. The results are correlated with the rheological behavior characterized by dynamic-mechanical analysis (DMA). While the NMR-detected chain relaxation and DMA results agree quantitatively and confirm relevant aspects of the sticky-reptation picture on a microscopic level, we stress and explain that apparent master curves are of limited use for such a comparison. The cluster-related relaxation time detected by BDS is much shorter than the elastic chain relaxation time, although the weak conductivity does follow the latter. The systematic trends across the sample series suggest that all relaxations are dominated by a cluster-related activation barrier, but also that the BDS-based cluster relaxation does not seem to be directly associated with the effective sticker lifetime. Nonlinear stress-strain experiments demonstrate a reduction of sticker lifetime on stretching and that the stored stress and the elastic recovery depend on the deformation rate. © 2019 American Chemical Society
Recommended from our members
Hierarchical Sticker and Sticky Chain Dynamics in Self-Healing Butyl Rubber Ionomers
We present a detailed comparison of the microscopic dynamics and the macroscopic mechanical behavior of novel butyl rubber ionomers with tunable dynamics of sparse sticky imidazole-based sidegroups that form clusters of about 20 units separated by essentially unperturbed chains. This material platform shows promise for application as self-healing elastomers. Size and thermal stability of the ionic clusters were probed by small-angle X-ray scattering, and the chain and sticker dynamics were studied by a combination of broadband dielectric spectroscopy (BDS) and advanced NMR methods. The results are correlated with the rheological behavior characterized by dynamic-mechanical analysis (DMA). While the NMR-detected chain relaxation and DMA results agree quantitatively and confirm relevant aspects of the sticky-reptation picture on a microscopic level, we stress and explain that apparent master curves are of limited use for such a comparison. The cluster-related relaxation time detected by BDS is much shorter than the elastic chain relaxation time, although the weak conductivity does follow the latter. The systematic trends across the sample series suggest that all relaxations are dominated by a cluster-related activation barrier, but also that the BDS-based cluster relaxation does not seem to be directly associated with the effective sticker lifetime. Nonlinear stress-strain experiments demonstrate a reduction of sticker lifetime on stretching and that the stored stress and the elastic recovery depend on the deformation rate. © 2019 American Chemical Society
Complex Morphology of the Intermediate Phase in Block Copolymers and Semicrystalline Polymers As Revealed by <sup>1</sup>H NMR Spin Diffusion Experiments
Nanostructured multiphase polymers
exhibiting a mobile and a rigid
phase also contain a phase of intermediate mobility that is usually
assumed to be a continuous, uninterrupted interphase layer. This assumption
is contrary to recent molecular-resolution micrographs and contradicts
results from NMR spin diffusion experiments, all of which suggest
a nontrivial interface structure. In this contribution, we reconsider
our previous <sup>1</sup>H NMR spin diffusion data sets (Roos Colloid. Polym. Sci. 2014, 292, 1825) and perform optimized 2D and 3D numerical spin diffusion calculations
to characterize the basic intermediate-phase morphological pattern,
thus overcoming previous inconsistencies in data fitting. For the
diblock copolymer poly(butadiene)-poly(styrene), PS<i>-<i>b</i>-</i>PB, we demonstrate that the interphase region
comprises nanometer-size intermixed immobile, intermediate and mobile
subregions. In contrast, for the semicrystalline polymer poly(ε-caprolactone),
PCL, the spin diffusion data are best reproduced by an intermediate
phase that is fully embedded within the rigid phase, which is attributed
to an imperfect crystalline structure. For both samples, the new findings
reveal a complex discontinuous, dynamically inhomogeneous structure
of the intermediate phase
Entanglement Effects in Elastomers: Macroscopic vs Microscopic Properties
This
Perspective highlights how entanglement effects on rubber elasticity
can be unveiled by a combination of different macroscopic and microscopic
methods, taking advantage of new developments in proton low-field
NMR spectroscopy as applied to bulk and swollen rubbers. Specifically,
the application of a powerful yet routinely applicable double-quantum
method, combined with a back-extrapolation procedure over results
measured at different degrees of swelling, allows one to characterize
the recently introduced “phantom reference network”
state, which only reflects contributions of actual cross-links and
topologically trapped entanglements. We further present an assessment
of the qualitative yet popular Mooney–Rivlin analysis of mechanical
data, where the influence of entanglement contributions on the fitted,
purely empirical parameters <i>C</i><sub>1</sub> and <i>C</i><sub>2</sub> is reconsidered in the context of different
tube models of rubber elasticity. We also review the impact of
entanglements on results of equilibrium swelling experiments and address
the validity of the common Flory–Rehner approach, where we
stress its qualitative nature and the need to use NMR observables
for a correct estimation of the relevant volume fractions. We discuss
semiquantitative estimations of the cross-link density from these
macroscopic experiments with its microscopic determination by NMR
on the example of lowly cross-linked synthetic and natural poly(isoprene)
rubber prepared by a novel UV-based curing protocol of dried latex
based upon thiol–ene chemistry, which in contrast to previously
studied thermally peroxide-cured natural rubber contain only small
amounts of short-chain defects. We find that the entanglement effects
in these samples can best be described by the Heinrich–Straube
tube model with positive scaling exponent ν > 0.3 as well
as by the slip-link model of Ball et al./Edwards–Vilgis with
a slip parameter η > 0.1. A comparison with literature results
demonstrates that these findings are not universal in that the apparent
entanglement contribution depends significantly on the sample (in)homogeneity,
i.e., of the NMR-determined fraction of inelastic defects and spatial
cross-linking inhomogeneities. This means that conclusions on the
validity or invalidity of specific tube theories cannot be drawn without
careful consideration of the network microstructure
Influence of Chain Topology on Polymer Dynamics and Crystallization. Investigation of Linear and Cyclic Poly(ε-caprolactone)s by <sup>1</sup>H Solid-State NMR Methods
We report on the investigation of cyclic and comparable linear poly(ε-caprolactone)s (PεCL) with molecular weight between 50 and 80 kg/mol with regard to chain mobility in the melt and crystallinity using low-field solid-state <sup>1</sup>H NMR. Our results from NMR Hahn echo and more advanced multiquantum measurements demonstrate a higher segmental mobility of cyclics in the melt as compared to their linear counterparts. Rheological experiments indicate that the cyclics are less viscous than the linear analogues by about a factor of 2, confirming the NMR results. FID component analysis shows higher crystallinities of the cyclic samples by some percent under the condition of isothermal crystallization at 48 °C, suggesting that due to their enhanced overall mobility in the melt, the cyclics reach a more perfect morphology leading to higher crystallinity. We compare this finding with results from DSC measurements obtained under identical conditions and critically evaluate the applicability of polymer crystallinity determination from nonisothermal crystallization investigations by DSC. We further highlight the use of nucleating agents to investigate the particular effect of crystal growth on (nonisothermal) crystallization, separated from the influence of nucleation. These experiments indicate a faster crystal growth for cyclic samples
Large-Scale Diffusion of Entangled Polymers along Nanochannels
Changes in large-scale polymer diffusivity
along interfaces, arising
from transient surface contacts at the nanometer scale, are not well
understood. Using proton pulsed-gradient NMR, we here study the equilibrium
micrometer-scale self-diffusion of poly(butadiene) chains along ∼100
μm long, 20 and 60 nm wide channels in alumina, which is a system
without confinement-related changes in segmental relaxation time.
Unlike previous reports on nonequilibrium start-up diffusion normal
to an interface or into particulate nanocomposites, we find a reduction
of the diffusivity that appears to depend only upon the pore diameter
but not on the molecular weight in a range between 2 and 24 kg/mol.
We rationalize this by a simple volume-average model for the monomeric
friction coefficient, which suggests a 10-fold surface-enhanced friction
on the scale of a single molecular layer. Further support is provided
by applying our model to the analysis of published data on large-scale
diffusion in thin films