Liquid–liquid phase separation in a polyethylene blend monitored by crystallization kinetics and crystal-decorated phase morphologies

CRYSTAF, DSC and SAXS Study of the Co-Crystallization, Phase Separation and Lamellar Packing of the Blends with Different Polyethylenes

The crystallization of polyethylene (PE) blends is a highly complex process, owing to the significant differences in crystallizability of the various PE components and the varying PE sequence distributions resulting from short- or long-chain branching. In this study, we examined both the resins and their blends through crystallization analysis fractionation (CRYSTAF) to understand the PE sequence distribution and differential scanning calorimetry (DSC) to investigate the non-isothermal crystallization behavior of the bulk materials. Small-angle X-ray scattering (SAXS) was utilized to study the crystal packing structure. The results showed that the PE molecules in the blends crystallize at different rates during cooling, resulting in a complicated crystallization behavior characterized by nucleation, co-crystallization, and fractionation. We compared these behaviors to those of reference immiscible blends and found that the extent of the differences is related to the disparity in crystallizability between components. Furthermore, the lamellar packing of the blends is closely associated with their crystallization behaviors, and the crystalline structure varies significantly depending on the components' compositions. Specifically, the lamellar packing of the HDPE/LLDPE and HDPE/LDPE blends is similar to that of the HDPE component owing to its strong crystallizability, while the lamellar packing of the LLDPE/LDPE blend is approximately an average of the two neat components.

Research on Microstructure and Mechanical Properties of Nylon6/Basalt Fiber/High-Density Polyethylene Composites

As a representative polyolefin, high-density polyethylene (HDPE) has become one of the most commonly used commercial plastics with a wide range of applications in the world. However, its applications are limited due to poor mechanical properties. Hence, it is indispensable to develop composites with improved mechanical properties to overcome this disadvantage. In our work, basalt fiber (BF) and polyamide 6 (PA6)-reinforced HDPE composites were prepared. The effects of adding fiber, organic filler, and polar component maleic anhydride (MAH) on the microstructural characteristics of composites were investigated. Microstructural characterization evidenced that the binary-dispersed phase (PA6/BF) possesses a core-shell structure in which the component PA6 encapsulates the component BF, and the extent of encapsulation declines with the increase of MAH addition. It has been confirmed by scanning electron microscopy (SEM) observation that the microstructure is related to the interfacial tension of components. The effects of multicomponents on the crystallization behavior of composites were studied. The differential scanning calorimeter (DSC) analysis exhibited a significant change in the HDPE microstructure. Results showed that, as nucleating agents, PA6 and BF improve the crystallization rate in the cooling process. Furthermore, the rheological behavior of multicomponent composites was studied. With the increase of MAH, a clear improvement of complex viscosity and storage modulus was observed, of which the mechanism has been discussed in detail. The relationship between microstructure and heat resistance of composites was studied by a thermal deformation test under static fore. It is confirmed that the thermally conductive fiber BF and other components can form a thermally conductive network and channels, thus improving the heat resistance. It can become a composite material, which is suitable for special environments.

Crystallization, Structures, and Properties of Different Polyolefins with Similar Grafting Degree of Maleic Anhydride

Maleic anhydride (MAH) grafting to different polyolefins with similar grafting degree can have different effects on crystallization, crystal structure, and mechanical and thermal properties. The grafting leads to a smaller crystal size, less ordered lamellar structure, and a shorter long period for HDPE, LLDPE, and PP. The grafting makes PP lamellar packing less ordered the most and almost no effect to LLDPE. The grafting does not have that much impact on the crystallization ability of the HDPE, LLDPE, and HDPE/PP blend, but appreciably reduces the crystalline ability of PP-g-MAH, due to a dramatical drop in its molecular weight during the grafting process. As a result, the grafting makes PP a very brittle material with a lowered average melting point than the corresponding neat PP, but the grafting has almost no effect on elongation at break for LLDPE and some effect on HDPE (decreased by one-third). However, the PP degradation due to MAH grafting can be avoided in the presence of PE component, i.e., making the grafting of PP and PE at the same time with HDPE/PP blend. The grafted HDPE/PP blend shows a significantly improved compatibility, which leads to overall appreciably better mechanical properties than the neat HDPE/PP blend.

An investigation into the role of polymeric carriers on crystal growth within amorphous solid dispersion systems

Using phase diagrams derived from Flory-Huggins theory, we defined the thermodynamic state of amorphous felodipine within three different polymeric carriers. Variation in the solubility and miscibility of felodipine within different polymeric materials (using F-H theory) has been identified and used to select the most suitable polymeric carriers for the production of amorphous drug-polymer solid dispersions. With this information, amorphous felodipine solid dispersions were manufactured using three different polymeric materials (HPMCAS-HF, Soluplus, and PVPK15) at predefined drug loadings, and the crystal growth rates of felodipine from these solid dispersions were investigated. Crystallization of amorphous felodipine was studied using Raman spectral imaging and polarized light microscopy. Using this data, we examined the correlation among several characteristics of solid dispersions to the crystal growth rate of felodipine. An exponential relationship was found to exist between drug loading and crystal growth rate. Moreover, crystal growth within all selected amorphous drug-polymer solid dispersion systems were viscosity dependent (η(-ξ)). The exponent, ξ, was estimated to be 1.36 at a temperature of 80 °C. Values of ξ exceeding 1 may indicate strong viscosity dependent crystal growth in the amorphous drug-polymer solid dispersion systems. We argue that the elevated exponent value (ξ > 1) is a result of drug-polymer mixing which leads to a less fragile amorphous drug-polymer solid dispersion system. All systems investigated displayed an upper critical solution temperature, and the solid-liquid boundary was always higher than the spinodal decomposition curve. Furthermore, for PVP-FD amorphous dispersions at drug loadings exceeding 0.6 volume ratio, the mechanism of phase separation within the metastable zone was found to be driven by nucleation and growth rather than liquid-liquid separation.

Liquid–liquid phase separation and its effect on the crystallization in polylactic acid/poly(ethylene glycol) blends

Phase diagram of PLA/PEG determined from rheology and DSC data.