Yusuke Hiejima

We have investigated the mechanical strength of sideweld sealing for biaxially oriented polypropylene bags. The tensile strength of the seal of the biaxially oriented polypropylene films showed a smaller deviation with increasing crystallinity in the sealed zone. Because the distribution of the crystallinity indicated sufficient homogeneity within the sealed zone at the microscopic level, the smaller deviation of the sealing strength with increasing crystallinity is ascribed to the formation of macroscopic defects under stretching. The defects around the sealed zone were visualized by photoelasticity observation under tensile stretching. We observed fluctuation of the strain near the sealed zone, accompanied by wavy patterns owing to the troughs induced by the shrinkage stress perpendicular to the stretching direction. Cross‐sectional observation of the sealed zone in the sideweld sealed specimen after the tensile test suggested that the fracture was initiated by ductile fracture. The subsequent fracture process became more brittle for specimens with lower strength, presumably because of the higher crystallinity. We also confirmed that the crystallinity can be controlled by the cooling rate in a rapid‐cooling experiment mimicking the air cooling in the bag forming process, where the crystallinity was significantly reduced by about 15%.

The mechanical performance of poly(methyl methacrylate) (PMMA) is highly sensitive to moisture absorption, which induces plasticization and softening. In this study, we investigated the ductilization mechanism of PMMA by incorporating various metal salts with different cations (Li⁺ and Mg²⁺) and controlling water absorption through hygroscopic interactions. A nonequilibrium constitutive model is introduced, in which localized water domains around salt-rich regions gradually diffuse into the PMMA matrix during tensile deformation. The stress–strain behavior is described by combining rigid (dry) and soft (hydrated) matrix components, connected through an internal kinetic variable governed by the strain-dependent diffusion rate. The model successfully reproduces experimental tensile data and captures the transition from brittle to ductile behavior as a function of the moisture content. Notably, Mg salts exhibit stronger water binding and slower moisture redistribution than Li salts, resulting in distinct mechanical responses. These findings provide a mechanistic framework for tailoring the ductility of hygroscopic polymer systems via ion–water–polymer interactions.

Small molecules are absorbed selectively in the amorphous regions in semicrystalline polymers, except for some specific polymeric systems forming host‐guest cocrystals such as syndiotactic polystyrene. The crystalline density of isotactic poly(4‐methyl‐1‐pentene) (P4MP1) is substantially lower than those of analogs, and it is even slightly lower than the amorphous density. This study is investigated the absorption/desorption behaviors of short alkanes in P4MP1 in situ, and found that the alkanes are mainly absorbed in its amorphous region. Infrared spectroscopy measurements reveal that the P4MP1 film with low crystallinity absorbs higher amount of decane than the high crystallinity film does. Also it is found by small‐angle X‐ray scattering (SAXS) that, when hexane is dropped onto the P4MP1 as‐cast film, a lamellar long‐spacing peak appears at 0.16 nm⁻¹ which disappears with the spontaneous desorption of hexane. Because the pristine P4MP1 films give no obvious SAXS peak due to the close density of crystalline and amorphous regions, the appearance of the SAXS peak indicates that an X‐ray contrast is obtained by the preferential absorption to amorphous region. The disappearance of the peak is thus reasonably understood by the loss of the contrast due to desorption.

In this work, two types of solid paraffins (i.e., linear and branched) were added to high-density polyethylene (HDPE) to investigate their effects on the dynamic viscoelasticity and tensile properties of HDPE. The linear and branched paraffins exhibited high and low crystallizability, respectively. The spherulitic structure and crystalline lattice of HDPE are almost independent of the addition of these solid paraffins. The linear paraffin in the HDPE blends exhibited a melting point at 70 °C in addition to the melting point of HDPE, whereas the branched paraffins showed no melting point in the HDPE blend. Furthermore, the dynamic mechanical spectra of the HDPE/paraffin blends exhibited a novel relaxation between −50 °C and 0 °C, which was absent in HDPE. Adding linear paraffin toughened the stress–strain behavior of HDPE by forming crystallized domains in the HDPE matrix. In contrast, branched paraffins with lower crystallizability compared to linear paraffin softened the stress–strain behavior of HDPE by incorporating them into its amorphous layer. The mechanical properties of polyethylene-based polymeric materials were found to be controlled by selectively adding solid paraffins with different structural architectures and crystallinities.