Effect of dissolved CO2 on the crystallization behavior of linear and branched PLA

Multiple actions of poly(ethylene octene) grafted with glycidyl methacrylate on the performance of poly(lactic acid)

Poly(ethylene octene) grafted with glycidyl methacrylate (POE-g-GMA) was employed to improve the rheological and thermal properties, toughness, and foaming behaviors of poly(lactic acid) (PLA) through a chain extension effect.

Preparation of Desirable Porous Cell Structure Polylactide/Wood Flour Composite Foams Assisted by Chain Extender

Polylactide (PLA)/wood flour composite foam were prepared through a batch foaming process. The effect of the chain extender on the crystallization behavior and dynamic rheological properties of the PLA/wood flour composites were investigated as well as the crystal structure and cell morphology of the composite foams. The incorporation of the chain extender enhanced the complex viscosity and storage modulus of PLA/wood flour composites, indicating the improved melt elasticity. The chain extender also led to a decreased crystallization rate and final crystallinity of PLA/wood flour composites. With an increasing chain extender content, a finer and more uniform cell structure was formed, and the expansion ratio of PLA/wood flour composite foams was much higher than without the chain extender. Compared to the unfoamed composites, the crystallinity of the foamed PLA/wood flour composites was improved and the crystal was loosely packed. However, the new crystalline form was not evident.

Effect of solubilised carbon dioxide at low partial pressure on crystallisation behaviour, microstructure and texture of anhydrous milk fat

The crystallisation and melting behaviour, fat polymorphs, microstructure and texture of anhydrous milk fat (AMF) was investigated in the presence of dissolved CO₂ (0-2000ppm) under two crystallising conditions (non-isothermal versus isothermal). CO₂ was found to induce higher onset crystallisation temperature during cooling from 35 to 5°C at 0.5°Cmin^-1. X-ray scattering analysis showed that, in the presence of dissolved CO_2, this rapid crystallisation caused the formation of unstable, α polymorph fat crystals. For milk fat crystallised under isothermal condition at 25°C for 48h, dissolved CO₂ improved solid fat content, slightly depressed melting temperature and exhibited a sharper melting peak. Microstructure of AMF visualised by Polarised light microscopy of crystallised AMF showed that increasing dissolved CO₂ concentration was associated with smaller crystal size and greater crystal number. The bulk properties of the fat appeared to mirror the microstructural differences, in that the texture of CO₂-treated AMF was harder under isothermal condition but became softer than untreated AMF under cooling condition. The results of this study are of significance in understanding how CO₂ treatment might be used to modulate the crystallisation behaviour of milkfat and thereby the structural development and physical functionality of fat-containing dairy products.

Improved expansion ratio and heat resistance of microcellular poly(L-lactide) foam via in-situ formation of stereocomplex crystallites

It is critical to broaden the applications of poly(L-lactic acid) foams by improving heat resistance properties. The stereocomplex crystallites that are formed by melt blending of poly(L-lactic acid)/polylactide possess high melting point of about 220℃ and thus exhibit high heat resistance; therefore, the introduction of stereocomplex crystallites tends to improve the thermal stability of poly(lactic acid) foam. Unfortunately, using the solid-state foaming method, it was found that the expansion ratio of the obtained poly(lactic acid) foams was compromised with the value of 1.7 times once the stereocomplex crystallites were formed during the sample saturation stage. In this study, by applying a high compression molding temperature of 230℃, the as-prepared poly(L-lactic acid) and poly(L-lactic acid)/polylactide blends were amorphous. After being CO2 saturated at a mild condition, the specimens were foamed at 90–160℃. The wide-angle X-ray diffraction profiles presented that the stereocomplex crystallites and PLA homocrystals were in-situ generated during the foaming process. It is observed that the in-situ formed stereocomplex crystallites could act as the physical cross-linking agent to stabilize the nucleated bubbles and suppress cell coalescence, resulting in the increased expansion ratio (with value of about 23.6–25.6 times) and cell density, especially at high foaming temperatures and extended foaming time. Furthermore, the in-situ formed stereocomplex crystallites during the foaming increased the heat resistance performance of poly(L-lactic acid) foams. This novel crystallization control method helps us to find a balance point in preparing poly(L-lactic acid) foam with high expansion ratio, well-defined cell structure and high heat resistance performance.

From micro/nano structured isotactic polypropylene to a multifunctional low-density nanoporous medium

A homogeneous low-density nano-porous medium of isotactic polypropylene (iPP) with a low thermal conductivity was fabricated using supercritical carbon dioxide (scCO₂).

Rheological properties and crystallization behavior of comb-like graft poly(l-lactide): influences of graft length and graft density

Radius growth rate of spherulites (G) versus crystallization temperature (T_c) for graft PLLA with different graft density and graft length.

Study of volume swelling and interfacial tension of the polystyrene-carbon dioxide-dimethyl ether system

We investigated the interaction of blended carbon dioxide (CO2) and dimethyl ether (DME) with polystyrene (PS) through volume swelling and interfacial tension. The experiments were carried out over a temperature range of 423-483 K, and the pressure was varied from 6.89 MPa to 20.68 MPa. With an incremental concentration of DME in the blend, the volume swelling increased while the interfacial tension between the PS/blend gas mixture and the blend gas decreased. The validity of the Simha-Somcynsky (SS) equation of state (EOS) for the ternary system was established by comparing experimentally measured volume swelling to that obtained via SS-EOS.

A polymer visualization system with accurate heating and cooling control and high-speed imaging

A visualization system to observe crystal and bubble formation in polymers under high temperature and pressure has been developed. Using this system, polymer can be subjected to a programmable thermal treatment to simulate the process in high pressure differential scanning calorimetry (HPDSC). With a high-temperature/high-pressure view-cell unit, this system enables in situ observation of crystal formation in semi-crystalline polymers to complement thermal analyses with HPDSC. The high-speed recording capability of the camera not only allows detailed recording of crystal formation, it also enables in situ capture of plastic foaming processes with a high temporal resolution. To demonstrate the system’s capability, crystal formation and foaming processes of polypropylene/carbon dioxide systems were examined. It was observed that crystals nucleated and grew into spherulites, and they grew at faster rates as temperature decreased. This observation agrees with the crystallinity measurement obtained with the HPDSC. Cell nucleation first occurred at crystals’ boundaries due to CO₂ exclusion from crystal growth fronts. Subsequently, cells were nucleated around the existing ones due to tensile stresses generated in the constrained amorphous regions between networks of crystals.

Rheology, thermal properties, and foaming behavior of high d-content polylactic acid/cellulose nanofiber composites

Cellulose nanofibers (CNFs) substantially improved the foam structure of a high d-content PLA by enhancing its melt strength and crystallization kinetics and serving as effective heterogeneous cell-nucleating agents.

Effects of saturation temperature/pressure on melting behavior and cell structure of expanded polypropylene bead

The expanded polypropylene beads with microcellular structure were prepared by an autoclave-based batch foaming process using non-supercritical CO2 as the foaming agent. Herein, smaller polypropylene micropellets (∼0.6 mm) were obtained from three kinds of polypropylene resin by underwater micro-pelletizer system. The results from differential scanning calorimetry indicated that the double melting peaks of beads moved to higher temperature with increasing saturation temperature and pressure. The kind of comonomer did not exert noticeable influence on the double melting behaviors. The X-ray diffraction of expanded polypropylene bead revealed that characteristic peaks of α-type crystal did not change compared with that of original polypropylene micropellets. The relationship between cell morphology and saturation temperature/pressure of expanded polypropylene beads are discussed preliminarily by scanning electron microscope.