Effect of temperature, rate, and molecular weight on the failure behavior of soft block copoly(ether-ester) thermoplastic elastomers.

Thermoplastic elastomers (TPEs) based on multiblock copolymers are an important class of engineering polymers. They are widely used in many applications where flexibility and durability are required and are seen as a sustainable (recyclable) alternative to thermoset rubbers. While their high-temperature mechanical behavior has received recent interest, few studies have explored their fracture and fatigue behavior. Understanding how the temperature and rate-dependence of the deformation behavior at both a local and global scale influences the fatigue resistance and failure behavior is critical when designing with these materials. In this study, the failure behavior in tensile, fracture, and fatigue of well-characterized, industrially relevant, model block copoly(ether-ester) based TPEEs were evaluated over a wide range of temperatures, deformation rates, and molecular weights. Small changes in temperature or rate are shown to result in a sharp transition between a highly deformable and notch resistant response, to a more brittle and strongly notch-sensitive response. This behavior surprisingly manifests itself as a threshold strain below which the cracks do not propagate in fatigue and increasing deformation rates decreases the materials toughness in fracture tests, whereas in tensile tests the opposite is observed. The change from homogenous to inhomogeneous stress fields for tensile and fracture experiments coupled with the viscoelasticity and strain-dependent morphology of TPEs explains why a different rate dependency is observed. Strain and stress delocalization is key to achieve high toughness. Digital Image Correlation is used to measure the size and time dependence of the process zone. Comparison with micromechanical models developed for soft, elastic, and tough double network gels highlights the dominance of high strain properties for toughness and explains the strong molecular weight dependence. However, to understand the rate dependence, the characteristic times for stress transfer from the crack tip and the time to nucleate failure must be compared. The results presented in this study demonstrate the complex effect of loading conditions on the intrinsic failure mechanisms of the TPE material, and provide a first attempt at rationalizing that behavior.

Ionic aggregate structure in ionomer melts: effect of molecular architecture on aggregates and the ionomer peak.

We perform a comprehensive set of coarse-grained molecular dynamics simulations of ionomer melts with varying polymer architectures and compare the results to experiments in order to understand ionic aggregation on a molecular level. The model ionomers contain periodically or randomly spaced charged beads, placed either within or pendant to the polymer backbone, with the counterions treated explicitly. The ionic aggregate structure was determined as a function of the spacing of charged beads and also depends on whether the charged beads are in the polymer backbone or pendant to the backbone. The low wavevector ionomer peak in the counterion scattering is observed for all systems, and it is sharpest for ionomers with periodically spaced pendant charged beads with a large spacing between charged beads. Changing to a random or a shorter spacing moves the peak to lower wavevector. We present new experimental X-ray scattering data on Na(+)-neutralized poly(ethylene-co-acrylic acid) ionomers that show the same two trends in the ionomer peak, for similarly structured ionomers. The order within and between aggregates, and how this relates to various models used to fit the ionomer peak, is quantified and discussed.

Disordered spheres with extensive overlap in projection: image simulation and analysis.

This article simulates highly overlapped projections of spherical particles that are distributed randomly in space. The size and number of the features in the projections are examined as well as how these features change with particle size and concentration. First, there are discernable features in projection even when particles overlap extensively, and the size of these discernable features is the expected size of an individual particle. Second, the number of features increases with specimen thickness at a rate of t(0.543) when the specimen thickness is below a critical value and becomes independent of specimen thickness at higher thicknesses. A criterion is established for the critical thickness based on particle size and particle volume fraction. When the specimen thickness is known and smaller than the critical thickness, a single representative transmission electron microscopy (TEM) (or scanning TEM) image exhibiting extensive particle overlap can be used to determine the size and number density of the spherical particles.

Nanoscale morphology in precisely sequenced poly(ethylene-co-acrylic acid) zinc ionomers.

The morphology of a series of linear poly(ethylene-co-acrylic acid) zinc-neutralized ionomers with either precisely or randomly spaced acid groups was investigated using X-ray scattering, differential scanning calorimetry (DSC), and scanning transmission electron microscopy (STEM). Scattering from semicrystalline, precise ionomers has contributions from acid layers associated with the crystallites and ionic aggregates dispersed in the amorphous phase. The precisely controlled acid spacing in these ionomers reduces the polydispersity in the aggregate correlation length and yields more intense, well-defined scattering peaks. Remarkably, the ionic aggregates in an amorphous, precise ionomer with 22 mol % acid and 66% neutralization adopt a cubic lattice; this is the first report of ionic aggregate self-assembly onto a lattice in an ionomer with an all-carbon backbone. Aggregate size is insensitive to acid content or neutralization level. As the acid content increases from 9.5 to 22 mol % at approximately 75% neutralization, the number density of aggregates increases by approximately 5 times, suggesting that the ionic aggregates become less ionic with increasing acid content.