Amorphous fracture behavior

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

ANALYSIS OF THE FRACTURE BEHAVIOR OF AMORPHOUS SEMI-AROMATIC POLYAMIDES... 

Analysis of the Fracture Behavior of Amorphous Semi-Aromatic Polyamides... 

When the polymer has the potential to reach the three-dimensional order (12), that is, the crystalline state, the fracture behavior becomes more complex. It is well known that the distance between the macromolecular chain segments in the crystal are shorter than those located in the amorphous phase. Hence, when the material is required to dissipate any kind of energy, this would be preferably dissipated through the amorphous phase and, even more, through the zone where the chain segments are significantly constrained, as it is an amorphous/crystal interphase. 

More recently there has been a strong interest in the deformation and fracture behavior of plastics under large hydrostatic pressures [31—32]. One should expect — and one observes — that the rigidity of a polymer increases with pressure. Sauer et al. [32] report that a pressure of 3.5 kbar raises the initial Young s moduli of amorphous thermoplastics (PC, PI, PSU, PVC, CA) by a factor of 1.2 to 1.9, that of crystalline polymers by 1.4 (POM) to 7.5 (PUR). Despite the increased rigidity, ductile fracture occurs. The effects are not yet understood in all generality. Following the two major review articles on this subject by Radcliffe [31] and Sauer and Pae [32] the Coulomb criterion corresponds best to most pressure-yield experiments. 

With the exception of PC, amorphous, non-oriented polymers did not produce measurable amounts of broken segments when subjected to tension. As has been shown in previous paragraphs, large axial stresses capable of chain scission in amorphous polymers can only be transmitted into the chain by friction of slipping chains requiring strong intermolecular interactions. In addition, macroscopic fracture occurs before a widespread chain overloading and scission occurs, which is opposite to the behavior of semicrystalline polymers. 

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

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