Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) microcellular foams using a melt memory effect as bubble nucleation sites

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) foams were prepared by a foam injection molding (FIM) process using nitrogen (N2) as a physical blowing agent. PHBH is a marine degradable polymer known for its heat treatment weakness. Gel permeation chromatography and rheological measurements showed that the molecular weight was reduced when the polymer was processed at a temperature higher than 160°C; however, it had a prominent melt memory of crystallization at a temperature lower than 170°C. The melt memory effect increased the crystallization temperature and provided crystal nucleation sites, which became bubble nucleation sites in the foaming process. Microcellular foams, whose cell size is smaller than 30 μm and cell density is higher than 3 × 10⁸ cells cm⁻³, were prepared using the melt memory effect while suppressing the thermal decomposition by tuning the injected polymer's temperature (melting temperature) and the core-back timing. The mechanical properties of the resulting foams were evaluated, indicating that a high ductility PHBH microcellular foam was obtained at a melting temperature of 150°C

Orientational mapping augmented sub-wavelength hyper-spectral imaging of silk

Molecular alignment underpins optical, mechanical, and thermal properties of materials, however, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber. Here, we show orientation mapping of the internal silk fiber structure via polarisation-dependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk. Microtomed lateral slices of silk fibers, which may avoid possible artefacts that affect spectroscopic measurements with fibers of an elliptical cross sections were used in the study. Amorphisation of silk by ultra-short laser single-pulse exposure is demonstrated

Microcellular foam of styrene-isobutylene-styrene copolymer with N₂ using polypropylene as a crystallization nucleating and shrinkage reducing agent

Styrene-isobutylene-styrene copolymer (SIBS) is a thermoplastic elastomer with excellent chemical stability, biocompatibility, and low gas permeability. SIBS is a good candidate with a high melt viscosity and a high storage modulus to develop new lightweight elastomeric products. Foam injection molding with core-back operation is an efficient method to prepare SIBS foams. However, it is challenging to prepare a microcellular foam from neat SIBS by melt processing, such as foam extrusion or foam injection molding, because the hard segments cannot play a role in bubble nucleation sites in the molten state. Furthermore, a significant degree of shrinkage occurs after foaming. By introducing a semicrystalline polymer such as polypropylene (PP), the foamability can be improved in foam injection molding processes. By adjusting the foaming temperature to the crystallization temperature of PP, PP crystals provide bubble nucleation sites and increase the viscosity to suppress bubble growth. Microcellular foams with high cell density and small cell size were achieved at 10⁸ cells/cm³ and approximately 13 μm. PP can also impede the shrinkage of SIBS foams

Millefeuille-like cellular structures of biopolymer blend foams prepared by the foam injection molding technique

Microcellular foams with unique cellular structures were prepared from poly(lactic acid) (PLA) and poly(butylene succinate-co-adipate) (PBSA) blends using a foam injection molding technique with a core-back operation. PLA and PBSA are biopolymers that are partially miscible with each other. When these biopolymers were blended, several blend morphologies appeared that were found to be dependent on the blend ratio. The blend morphologies and thermal and rheological properties of polymer blends with PLA/PBSA ratios of 100/0, 30/70, 50/50, 70/30, and 0/100 were observed, and the cellular structures of their foams were investigated. While most of the polymer blends showed sea and island structures, a polymer blend with a PLA/PBSA ratio of 50/50 showed a layered structure for the two polymers in which the continuous phases of both polymers were elongated along the flow direction used for injection molding. By utilizing the unique blend morphology and the difference in the viscosities of the two polymers, a millefeuille-like microcellular structure was created from a 50/50 blend. Foams with millefeuille-like cellular structures show unique anisotropic mechanical properties, and this study reveals a method for preparing foams of this type

Improvement of the surface quality of foam injection molded products from a material property perspective

Microcellular injection molding is an attractive method. However, their surface imperfections have been a major problem hindering wide industrial applications. Several methods have been proposed to improve the surface appearance of foams. In this study, we proposed a method to improve the surface appearance of polypropylene (PP) foams from the material property perspective, especially with regard to crystallization and viscosity. The basic idea of the surface improvement is to reduce the size of bubbles generated at the flow front, delay the solidification behavior of the polymer at the mold interface, squeeze the bubbles existing at the mold–polymer interface, and redissolve the bubbles into the polymer by holding pressure. Blending a low-modulus PP delays the crystallization of the polymers at the skin layer and solidification, taking enough time to squeeze the bubbles smaller. A sorbitol-based gelling agent, bis-O-([4 methylphenyl]methylene)-D-Glucitol, was used to increase the viscosity at a low strain rate to reduce the size of the bubbles generated at the flow front during the filling stage. The foam injection molding experiments demonstrated that the proposed method effectively improved the surface appearance of the foams. In particular, the surface appearance of the foams became almost equivalent to that of solid samples using low-modulus PP

Effect of Cellulose Nanofiber (CNF) Surface Treatment on Cellular Structures and Mechanical Properties of Polypropylene/CNF Nanocomposite Foams via Core-Back Foam Injection Molding

Herein, lightweight nanocomposite foams with expansion ratios ranging from 2⁻10-fold were fabricated using an isotactic polypropylene (iPP) matrix and cellulose nanofiber (CNF) as the reinforcing agent via core-back foam injection molding (FIM). Both the native and modified CNFs, including the different degrees of substitution (DS) of 0.2 and 0.4, were melt-prepared and used for producing the polypropylene (PP)/CNF composites. Foaming results revealed that the addition of CNF greatly improved the foamability of PP, reaching 2⁻3 orders of magnitude increases in cell density, in comparison to those of the neat iPP foams. Moreover, tensile test results showed that the incorporation of CNF increased the tensile modulus and yield stress of both solid and 2-fold foamed PP, and a greater reinforcing effect was achieved in composites containing modified CNF. In the compression test, PP/CNF composite foams prepared with a DS of 0.4 exhibited dramatic improvements in mechanical performance for 10-fold foams, in comparison to iPP, with increases in the elastic modulus and collapse stress of PP foams of 486% and 468%, respectively. These results demonstrate that CNF is extraordinarily helpful in enhancing the foamability of PP and reinforcing PP foams, which has importance for the development of lightweight polymer composite foams containing a natural nanofiber

New evaluation method for the curing degree of rubber and its nanocomposites using ATR-FTIR spectroscopy

A nondestructive method of evaluating the curing degree (crosslinking density) of cured rubbers and their nanocomposites based on attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was proposed and applied to fluorine-based rubber (FKM), in which triallyl-isocyanurate (TAIC) was employed as a curing agent. ATR-FTIR spectroscopy demonstrated that the CO band in TAIC at 1699 cm−1 decreased in intensity and broadened as the curing reaction progressed. A calibration model relating the crosslinking density in the FKM with the full width at half maximum (FWHM) of the band of the CO bond was developed. The model showed good applicability to both FKM and FKM nanocomposites with various nanofillers, including single- and multi-walled carbon nanotubes (CNTs), carbon black and silica particles. Interestingly, when the fillers, especially CNTs, were added to the rubber, the FWHM was more sensitive to the change in the crosslinking density than the change in the Young's modulus