Mapping the Mechanical Properties of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Banded Spherulites by Nanoindentation

Nanoindentation provides clear evidence that spherulite banding can be associated with a continuous modulation of mechanical properties from the more compliant peaks to the stiffer valleys. The structural arrangement in polymer-banded spherulites has intrigued scientists for many decades, and the debate has been recently intensified with the advent of new experimental evidence. The present paper approaches this issue by exploring the local mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-ringed spherulites via nanoindentation and discussing the confidence of the results. It was found that storage modulus and hardness across the banding morphology can be described as a sequence of regular oscillations with a periodicity that exactly matches the one observed using optical and atomic force microscopy. Results are consistent with the model of regular twisting of the lamellae, with flat-on arrangement in the low regions and edge-on lamellae in the crests.

Biaxially Oriented Polyethylene Films by Compression of Injection Moldings

The tensile strength, σ, of biaxially oriented polyethylene (PE) films with various degrees of biaxial stretching has been measured. The dependence of σ in the two principal directions x and y is investigated as a function of two parameters: the biaxial draw ratio (average draw ratio of the film) and the degree of biaxiality (strain distribution along the principal directions). Both parameters are related to the final applied pressure PF. The influence of initial orientation and anisometry of the injection molded precursors on the final σx and σy values along the principal directions is studied. Results are discussed in the light of the deformation mechanism which transforms the original uniaxial fiber structure of the moldings into the biaxial fibrillar structure of the final films. Direct evidence of this transformation is provided by small angle X-ray scattering (SAXS).

Biaxially-oriented Polyethylene Films by Compression of Injected Moldings

Biaxially oriented PE films were prepared by compression of uniaxially oriented elongational-flow injection-molded precursors. The films so prepared, having different extents of biaxial stretching were compared on the basis of two derived functions, biaxial draw ratio (ƛ) and degree of biaxiality (B). The former gives a quantitative measure of the average draw ratio of the film surface. The latter describes the strain distribution along the principal directions of the film plane. It is shown that ƛ and B depend on the final applied pressure and on the type of pressing surfaces used. The dependence of ƛ and B on the initial orientation and on the initial anisometry of the precursors is also discussed in the light of structural arguments.

Simultaneous birefringence, small- and wide-angle X-ray scattering to detect precursors and characterize morphology development during flow-induced crystallization of polymers

An experimental configuration that combines the powerful capabilities of a short-term shearing apparatus with simultaneous optical and X-ray scattering techniques is demonstrated, connecting the earliest events that occur during shear-induced crystallization of a polymer melt with the subsequent kinetics and morphology development. Oriented precursors are at the heart of the great effects that flow can produce on polymer crystallization (strongly enhanced kinetics and formation of highly oriented crystallites), and their creation is highly dependent on material properties and the level of stress applied. The sensitivity of rheo-optics enables the detection of these dilute shear-induced precursors as they form during flow, before X-ray techniques are able to reveal them. Then, as crystallization occurs from these precursors, X-ray scattering allows detailed quantification of the characteristics and kinetics of growth of the crystallites nucleated by the flow-induced precursors. This simultaneous combination of techniques allows unambiguous correlation between the early events that occur during shear and the evolution of crystallization after flow has stopped, eliminating uncertainties that result from the extreme sensitivity of flow-induced crystallization to small changes in the imposed stress and the material. Experimental data on a bimodal blend of isotactic polypropylenes are presented.