Influence of α′‐/α‐crystal polymorphism on properties of poly(l‐lactic acid)

Crystallization of biodegradable and biobased polyesters: Polymorphism, cocrystallization, and structure-property relationship

The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 106 K s-1, allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation in polymers is discussed in the framework of the classical nucleation theory. The majority of our observations are consistent with the theory. The discrepancies may guide further research, particularly experiments to progress theoretical development. Progress in the understanding of homogeneous nucleation is much needed, since most of the modelling approaches dealing with polymer crystallization exclusively consider homogeneous nucleation. This is also the basis for advancing theoretical approaches to the much more complex phenomena governing heterogeneous nucleation.

Homogeneous crystal nucleation in polymers

Nanocalorimetry: Door opened for in situ material characterization under extreme non-equilibrium conditions

Crystal reorganization of poly (butylene terephthalate)

The effect of supercooling of the melt on the semicrystalline morphology of PA 66

Over the last two decades, it has become increasingly unarguable that the addition of a small amount of nanoparticles to elastomers can lead to a drastic enhancement of their elastic response not only at small deformations but, more importantly, at large deformations. Yet, because of the experimental difficulties of conducting direct quantitative measurements of mechanical properties at the length scale of the nanoparticles together with the mathematical challenges associated with the analysis of large deformations in the presence of nanoscale heterogeneities, the precise mechanisms responsible for such an enhancement have remained unresolved. This paper reports a combined experimental/theoretical investigation aimed at revealing and quantifying the precise mechanisms behind the enhanced elastic properties of a prototypical class of elastomer nanoparticulate composites: polydimethylsiloxane (PDMS) filled with an isotropic distribution of TiO2 nanoparticles. The synthesized composites exhibit drastically enhanced stress-stretch responses, featuring up to about a 10-fold increase with respect to the response of the unfilled PDMS elastomer, over the entire spectrum of small and large deformations considered. Inter alia, it is found that the “bulk” PDMS elastomer — i.e., the regions of the PDMS elastomer not immediately surrounding the nanoparticle aggregates formed during the synthesis process — is softer than the unfilled PDMS elastomer, while the “interphasial” PDMS elastomer surrounding the aggregates is significantly stiffer. The latter mechanism is found to rule over the former and to constitute the dominant mechanism behind the drastic enhancements in the macroscopic elastic properties of the composites.

https://par.nsf.gov/servlets/purl/10092702

Tim Tighe

Papers

The effect of supercooling of the melt on the semicrystalline morphology of PA 66

Extreme enhancement of the nonlinear elastic response of elastomer nanoparticulate composites via interphases

Modeling avian eggshell microstructure to predict ontogenetic age and reveal patterns of human-avifauna interaction