ABSTRACT:The commercial grade of isotactic polypropylene was modified by a specific ␤-nucleating agent in a broad concentration range. The supermolecular structure of the specimens prepared by injection molding was characterized by X-ray scattering and correlated with mechanical behavior. It was found that at a critical nucleant concentration of 0.03 wt % the content of the ␤-modification virtually reaches a saturation level. With further addition of the nucleant, the ␤-phase content increases only slightly. The long period passes through a distinct maximum at the same nucleant concentration. This singularity in structure remarkably correlates with a minimum of the yield stress and maxima of strain at break and fracture toughness. Such general behavior is also reflected in the correlation between the ␤-phase concentration and fracture toughness profiles along the injection-molded bars. It is suggested that in the critically nucleated material an optimum thickness of the amorphous interlayer with connecting chains between the ␤-crystallites is established, rendering the material the highest possible ductility and toughness.

To clarify the problem of undesired crystallization of amorphous aluminosilicate inorganic polymers (AIPs) accompanied by the loss of mechanical properties, we synthesized two types of chemically identical AIPs that differ in durability under hydrothermal conditions. Whereas phase-stable AIPs remain amorphous, phase-unstable AIP systems undergo extensive crystallization with the formation of high fractions of chabasite and zeolite P. The application of 27 Al MQ/MAS and { 1 H}-REDOR-27 Al MQ/MAS NMR spectroscopies, combined with the recently developed biaxial shearing transformations, revealed a two-component character of the prepared AIPs. The prevailing fraction of [AlO 4 ] − species (amorphous phase) is charge-balanced by Na + counterions and exhibits considerable distribution of 27 Al chemical shifts induced by the variation of Al−O−Si valence angles. In contrast, the minor fraction of [AlO 4 ] − tetrahedra, which can be attributed to the partially ordered domains of aluminosilicate networks, shows a broad distribution of quadrupolar parameters that result from variability in the chemical nature of the counterions. A comparison of the prepared AIPs revealed that the partially ordered domains of the phase-unstable AIPs contained a considerably larger amount of [AlO 4 ] − species charge-balanced by H + . Therefore, we assume that the destabilization of AIPs is associated with the presence of bridging hydroxyl groups (Si−OH + −Al, Brønsted-acid sites) that induce breaking of Si−O−Al bonds. The resulting decrease in network density can induce a temporal release of extraframework Al species, their subsequent rearrangement, and the formation of a crystalline phase. The process of crystallization is supported by the higher mobility of proton species as revealed by a range 1 H-receptive MAS NMR experiments. In contrast, the factors that stabilize amorphous nature of aluminosilicate frameworks are the presence of four-coordinate extraframework Al species, such as Al(OH) 3 + or AlOH 2+ −H 2 O, and tight incorporation of proton fraction into the inorganic matrix.

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