Strength and toughness of crystalline polymer systems

Cavitation during deformation of semicrystalline polymers

Formation of β-modification of isotactic polypropylene in its late stage of crystallization

Positron annihilation lifetime spectroscopy (PALS) was conducted to follow the evolution of free volume during crystallization of PLLA at 100 °C. A finite lifetime distribution of three components was used to fit the PALS spectra. The analysis of the longest lifetime component (τ3) indicates that the free volume distribution evolves during crystallization by increasing the number of holes yet decreasing their size; moreover, the free volume fraction increased during crystallization. Following the evolution of the shortest components a correlation was found with crystalline and amorphous phase contents present in PLLA. The first component (τ1) was assigned to positron annihilation in occupied zones of the crystalline phase whereas the second component (τ2) was attributed to annihilation by different amorphous arrangements. A model for the supramolecular arrangement of PLLA chains was devised in terms of free volume enlargement for annealed samples. According to this model transformations occur in mobile am...

A PALS Contribution to the Supramolecular Structure of Poly(l-lactide)

The morphology and confined crystallization behavior of poly(butylene succinate) (PBS) in miscible poly(vinylidene fluoride) (PVDF)/PBS blends has been studied using differential scanning calorimetry (DSC) and optical and atomic force microscopy (OM and AFM). It was found that PBS crystal lamellae nucleated and grew confined inside the matrix of PVDF spherulites. Crystallized PBS domains grow with an ellipsoidal outline within PVDF spherulites formed at a relatively high PVDF crystallization temperature (T(c,PVDF)), while circular domains, engulfing several PVDF spherulites, are seen when growing in the PVDF spherulites created at lower T(c,PVDF). The growth kinetics of PBS confined in the PVDF matrix was investigated under various conditions. The growth rate of PBS (G(PBS)) increases with decreasing crystallization temperature and increasing PBS content under a given PVDF crystallization temperature (T(c,VDF)). For T(c,PVDF) above 145 °C, G(PBS) decreases with T(c,PVDF) for both 50:50 and 30:70 PVDF/PBS blends. However, for T(c,PVDF) below 145 °C, 50:50 and 30:70 PVDF/PBS blends exhibit the opposite G(PBS) trend; that is, G(PBS) for the 50:50 blend decreases with decreasing T(c,PVDF), while for the 30:70 PVDF/PBS blend G(PBS) increases with decreasing T(c,PVDF). It is shown that this behavior cannot be associated with the effect of crossing the boundary of smaller PVDF spherulites formed at a lower temperature. Rather, the behavior appears to be related to the interleaving growth of PBS lamellae among PVDF lamellae or between bundles of PVDF lamellae (fibrils), as in situ AFM observation shows. It is found that the interconnectedness of the molten pockets within the PVDF spherulites, which depends on the PVDF crystallization temperature, is an important factor determining the growth kinetics of PBS confined within the PVDF scaffold.

Confined Growth of Poly(butylene succinate) in Its Miscible Blends with Poly(vinylidene fluoride): Morphology and Growth Kinetics

Ultrasonic emission was found to accompany several physical processes involving stress release, for example, microfracture in solids1 or cavitation and bubble collapse in liquids2. We here report the first observation of acoustic emission during crystallization. Acoustic emission was found to occur during the spherulitic crystallization of polymers from melt. The source of acoustic waves was identified as an abrupt stress release in regions of the melt occluded by spherulites during crystallization. The stress buildup in occluded areas is a result of density changes during crystallization. When the level of stress approaches the limit of the melt cohesion cavitation occurs, the stress is released and sound emitted. Because crystallization, in the form of aggregates growing simultaneously from many nuclei, always leads to the formation of occluded areas, and because the density of the melt or glass from which these aggregates grow is always lower than the crystallized matter, stress always arises. Depending on its level, the stress is either released by cavitation or remains frozen in the material. Thus acoustic emission is also expected to accompany crystallization in other, non-polymeric materials.

Acoustic emission during polymer crystallization

Ultrasonic emission was found to accompany spherulitic crystallization of polymers from the melt. The source of acoustic waves was identified as an abrupt negative pressure release in regions of melt occluded by spherulites (weak spots) in the course of crystallization. The buildup of a negative pressure in occluded areas is a result of a density change during crystallization. When the level of stress approaches the limit related to melt cohesion, cavitation occurs, the stress is released, and acoustic wave is emitted. On the basis of a statistical approach to the description of spherulitic structure formation, it was predicted that most acoustic events should occur at a high degree of conversion—close to the end of crystallization. The ultrasonic emission was found during isothermal crystallization of isotactic polypropylene and poly(methylene oxide). In the case of Nylon 6, acoustic emission was attributed to environmental stress cracking and degradation. No acoustic events were recorded during crystallization of high-density polyethylene and during similar thermal treatment of a series of noncrystallizable polymers. Addition of a nucleating agent to crystallizable polymers drastically reduces the acoustic emission during crystallization, which indicates that the spherulite size is one of the most important parameters. Depending on the size and geometry of weak spots, melt properties, and mechanical properties of spherulites, the stress in weak spots is either released by cavitation or remains frozen in the material. It was found that the time distribution of acoustic emission is located between the time distributions of the formation of weak spots and the formation of quadruple boundary points between spherulites.

Acoustic emission during crystallization of polymers

The finite element method is applied to contours (measured by interferometry) of craze zones in front of stationary crack tips in polymethylmethacrylate (PMMA) in order to compute the stress distribution. The stress along the craze zone has its maximum at the crack tip, being about twice that predicted by the Dugdale model; and it decreases at first sharply and then more gradually, being almost constant as the craze tip is approached. Stress distributions, in a growing craze at the still stationary crack tip, at various times can be combined in a normalized curve. Craze stresses relax with time as the craze continues to propagate in a stable manner. When the specimen is unloaded, the craze stress is compressive near the crack tip, but is tensile at the craze tip and for most of the craze length. Analysis of measurements made during a loading cycle establishes minimum stress intensity levels at which no fibrils are in compression. The levels are different for increasing and decreasing loads, and it is found the craze stresses are tensile for most of the cycle. Throughout the cycle, there is an excellent correlation between the average craze stress and the theoretical stress predicted by the Dugdale model.

L. Koenczoel

Papers

Acoustic emission during polymer crystallization

Acoustic emission during crystallization of polymers

Micromechanics of a craze zone at the tip of a stationary crack