Isotactic polypropylene (iPP) samples with different molecular weights were used to investigate the relationship between cavitation processes triggered at small- and large-strain regimes. The cavitation triggered at large strains was observed in all deformation cases regardless of the occurrence of cavitation activated at small strain. The initiation of the large-strain-cavitation process required a critical stress, which was mainly determined by the molecular weight of iPP, because it was a result of the disentanglement of the highly oriented amorphous network initiated by the breakage of interfibrillar load-bearing tie chains. In general, iPP with higher molecular weight required a higher critical stress to trigger this cavitation and presented more intensive whitening. However, the intensity of cavitation activated at small strains decreased with increasing iPP molecular weight. The cavities generated at small strain are therefore supposed to have no contribution to the cavitation initiated at large strain.

Second generation dendrons with six long alkyl groups and six benzyl groups as peripheral groups were modified into cis-5-norbornene-exo-2,3-dicarboxylicanhydride to prepare dendronized monomers.

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GP et al (2019) Die geometry induced heterogeneous morphology of polypropylene inside the die during die-drawing process. Polymer Testing. 74: 104-112. Rights

The molecular weight (M w)-dependent formation of mesophase and the α phase of isotactic polypropylene (iPP) was explored by utilizing in situ synchrotron microfocus X-ray scattering measurements combined with a portable fast-scanning chip calorimetry (FSC) technique. A phase diagram illustrating the relationship between the lamellar long spacing (d ac) and crystallization as well as melting temperatures of three iPP samples with different molecular weights suggests that a common crystallization line was shared by the mesophase of all three samples, whereas three different crystallization lines were observed for these iPP samples. Furthermore, unlike in the case of the α phase where the melting lines were significantly different from their crystallization lines, the melting line of the mesophase was found to overlap with its crystallization line. The results indicate that the crystallization of the α phase satisfied the multistage crystallization model in which an intermediate layer is suggested to be generated prior to the formation of native crystallites. The M w-independent formation of the mesophase demonstrated that the nucleus size did not rely on the M w of iPP because the final sizes of the mesophase were close to the initial nuclei sizes. The larger difference of the lamellar long spacing presented in the α phase of iPP with different M w indicated that the growth from nuclei to crystallites was significantly influenced by the M w.

Orientation direction dependency of whitening activated at large strains was studied using four pre-oriented isotactic polypropylene (iPP) samples with different molecular weights stretched along different directions with respect to the pre-orientation (0°, 45°, and 90°) by means of in situ wide-, small-, and ultra-small-angle X-ray scattering techniques. A macroscopic fracture of iPP materials was also observed following the stress whitening at large strains. These two associated processes in pre-oriented iPP samples at elevated temperatures were found to be governed by not only the molecular weight of iPP but also the pre-orientation direction. For a certain pre-orientation direction of iPP, both the critical stress of cavitation induced-whitening and failure stress increased with increasing molecular weight. For one given molecular weight, the pre-oriented iPP showed the smallest critical stress for whitening and failure stress along the pre-orientation direction (0°) while the samples displayed larger values for the same behaviors when stretched at 45° or 90° with respect to the pre-orientation direction. Such behavior suggested that oriented amorphous networks, with different mechanical strengths, can be generated during the second deformation processes in these pre-oriented iPP samples. The evolution of inter-fibrillar tie chains in highly oriented amorphous networks was considered as the main factor controlling the response of the inner network to the external stress since the cavitation-induced whitening activated at large strains was caused by the failure of load bearing inter-fibrillar tie chains in the oriented amorphous network.

The crystallization behavior of polybutene-1 (PB-1) into its I′, II, and III polymorphs, under supercritical carbon dioxide (CO 2 ), has been investigated by means of in situ synchrotron X-ray diffraction, wide angle X-ray diffraction mapping, and differential scanning calorimetry techniques. At low CO 2 pressures, crystals of stable forms I′ and III as well as metastable form II are all observed, but form II crystals transform into form I crystals quickly after their formation. With increasing CO 2 pressure, the direct crystallization of both form II and form III is suppressed. The suppressed growth of form II at higher CO 2 pressures is due to its highest nucleation barrier, owing to its large critical nucleus size. Consequently, there is competition between nucleation of forms I′ and III, which possess smaller nuclei sizes under supercritical fluids (SCFs), as is ascribed to the solvent effect of SCFs. At a CO 2 pressure of 10 MPa, form III is observed, being randomly dispersed in the system. It is proposed that random chain coils are transformed into a 4/1 helix conformation, thereby promoting the growth of form III. With further increasing the SCF pressure, more random coils develop into helices, while short helices extend into longer ones. Here, form III with a "shorter" and "wider" chain conformation is no longer the most favorable conformation for the well-developed long helices. As a result, the growth of form I′ overtakes the growth of form III. This lays the framework for control of crystal polymorphism in PB-1 using a simple, environmentally friendly supercritical fluid approach.