Morphologies and Crystallization Behaviors in Melt-Miscible Crystalline/Crystalline Blends with Close Melting Temperatures but Different Crystallization Kinetics

Double Spherulite Formation via Two-Step Crystallization in PTT/PET Blends

We investigated the crystallization kinetics and morphology evolution of miscible crystalline/crystalline blends of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) (PET) during isothermal melt crystallization. The integrated light-scattering intensity and the spherulite size increased gradually and then steeply as crystallization progressed in 70/30 PTT/PET at 215 °C, indicating the two-step crystallization behavior. The compact PET spherulite grew in the first step, and the dendritic PTT spherulite grew in the second step, forming the double spherulite consisting of a PET component in the inner region and a PTT one in the outer region. The spherulite size of PET increased nonlinearly with time, suggesting the exclusion of PTT from the crystal growth front. Atomic force microscopy (AFM) observation revealed that the PTT fibrils were interfiled within the PET spherulite in the inner region and continued outward to the outer region consisting of the PTT spherulite. These results suggest that the excluded PTT crystallizes into fibrils by interfiling crystallization within the inner PET spherulite, and then the interfiled PTT fibrils continue to grow outward to form the outer dendritic PTT spherulite after the spherulite growth of PET stops due to the excluded PTT at the growth front.

Influence of temperature dependence on the structural characteristics of polyoxymethylene/poly(lactic acid) blends by injection molding

In this research, different strategies to modify the structure of polymer blends were investigated with the objective of adjusting the composition of polyoxymethylene (POM) and poly(lactic acid) (PLA) under typical processing conditions. POM shows a good balance of mechanical and thermal properties. However, this polymer is obtained from petrochemical sources, and in some markets, environmentally friendly materials are important. Blending POM with PLA preserves the advantages of POM while ensuring the bio-based content of PLA. POM/PLA blends were prepared by an injection molding process with various injection speeds of 10, 50, 100, and 1000 mm/s to ensure high ductility. Mechanical property analysis showed that the PLA content and processing temperatures are highly effective in modifying the stiffness of POM/PLA blends. The effect of crystallization on POM/PLA blends was assessed by varying the annealing time. High-magnification scanning electron microscopy images revealed that the gaps between fibrillar regions represent the growth direction of the PLA phase before it was removed. This was evidence for the effect of PLA on the crystallization of POM. The crystal size and crystalline volume also affected the structural characteristics of POM/PLA blends.

Crystalline Characteristics, Mechanical Properties, Thermal Degradation Kinetics and Hydration Behavior of Biodegradable Fibers Melt-Spun from Polyoxymethylene/Poly(l-lactic acid) Blends

A series of polyoxymethylene (POM)/poly(l-lactic acid) (PLLA) blends were prepared by melt extrusion, and their spinnability was confirmed by rheological characterizations, successive self-nucleation, and annealing thermal fractionation analysis. The bicomponent fibers were prepared by means of the melt-spinning and post-drawing technologies using the above-obtained blends, and their morphology, crystalline orientation characteristics, mechanical performance, hydration behavior, and thermal degradation kinetics were studied extensively. The bicomponent fibers exhibited a uniform diameter distribution and compact texture at the ultimate draw ratio. Although the presence of PLLA reduced the crystallinity of the POM domain in the bicomponent fibers, the post-drawing process promoted the crystalline orientation of lamellar folded-chain crystallites due to the stress-induced crystallization effect and enhanced the crystallinity of the POM domain accordingly. As a result, the bicomponent fibers achieved the relatively high tensile strength of 791 MPa. The bicomponent fibers exhibited a partial hydration capability in both acid and alkali media and therefore could meet the requirement for serving as a type of biodegradable fibers. The introduction of PLLA slightly reduced the thermo-oxidative aging property and thermal stability of the bicomponent fibers. Such a combination of two polymers shortened the thermal lifetime of the bicomponent fibers, which could facilitate their natural degradation for ecological and sustainable applications.

Development of Polyoxymethylene/Polylactide Blends for a Potentially Biodegradable Material: Crystallization Kinetics, Lifespan Prediction, and Enzymatic Degradation Behavior

This paper reported the development of polyoxymethylene (POM)/polylactide (PLA) blends for a potentially biodegradable material. A series of POM/PLA blends at different weight ratios were prepared by melt extrusion with a twin-screw extruder, and their mechanical properties, crystallization behavior and kinetics, thermal degradation kinetics and stability, lifespan prediction and enzymatic degradation behavior were investigated extensively. POM and PLA were found to be partially miscible in the melt state at low temperature and become phase-separated at elevated temperatures, and their blends exhibited a typical lower critical solution temperature behavior. There were two distinct glass transition temperatures (Tg) observed for POM/PLA blends at any mass ratios when quenched from the homogeneous state, and both POM and PLA domains showed an apparent depression in their respective Tg's in the blends. Owing to the partial miscibility between two domains, the tensile strength and impact toughness of POM/PLA blends gradually decreased with an increase of PLA content, but their flexural strength and modulus presented an increasing trend with PLA content. The studies on non-isothermal and isothermal crystallization behaviors of the blends indicated that the crystallization rates of the blends decreased continually with increasing the PLA content, confirming that the crystallization of POM domain was controlled by the molecular-confined mechanism. The introduction of PLA into POM not only led to a slight increase of thermal stability of POM domain at low PLA contents but also shortened the lifespan of the blends, favoring the natural degradation of the blends. The POM/PLA blends exhibited an improvement in partially biodegradable performance with an increase of PLA content and their mass loss reached up to 25.3 wt % at the end of 48-h enzymatic degradation when 50 wt % of PLA was incorporated.

Responsive Ionogel Surface with Renewable Antibiofouling Properties

The synthesis of ionogels with a responsive, self-replenishing surface for combating biofouling is described. Ionogels are prepared by infiltrating poly(vinylidene fluoride-co-hexafluoropropylene) with binary mixtures of ionic liquids (IL): 1-octadecyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([C₁₈ C₁ im][NTf₂ ], melting point T_m = 55 °C) and 1-hexyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([C₆ C₁ im][NTf₂ ], T_m = -9 °C). The IL mixtures release spontaneously from the gel matrix and eventually crystallize on the surface. This leads to self-replenishment of the surface of ionogels even after mechanical damage. The incorporation of [C₆ C₁ im][NTf₂ ] provides the antimicrobial efficacy of ionogels while the crystals of [C₁₈ C₁ im][NTf₂ ] serve as a skeleton maintaining [C₆ C₁ im][NTf₂ ] on the surface. By heating, the ionogel surface transforms from solid to liquid-infused state-the removal of biofilms/bacteria developed under a long time of colonization is facilitated. The antimicrobial efficacy is maintained even after several cycles of biofilm formation and detachment. This work provides an opportunity to apply ionogels as functional coatings with renewable antibiofouling properties.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure and morphology from the bulk when they are confined in the nanoscale environments; this may dramatically influence the physical performances of the resulting materials. Therefore, investigations on the crystalline and spherulitic morphology of semicrystalline polymers in confined systems are essential from both scientific and technological viewpoints; significant progresses have been achieved in this field in recent years. In this article, we will review the recent research progresses on the crystalline and spherulitic morphology of polymers crystallized in the nanoscale confined environments. According to the types of confined systems, crystalline, spherulitic morphology and morphological evolution of semicrystalline polymers in the ultrathin films, miscible polymer blends and block copolymers will be summarized and reviewed.

Fabrication of Superhydrophobic Surfaces with Controllable Electrical Conductivity and Water Adhesion

A facile and versatile strategy for fabricating superhydrophobic surfaces with controllable electrical conductivity and water adhesion is reported. "Vine-on-fence"-structured and cerebral cortex-like superhydrophobic surfaces are constructed by filtering a suspension of multiwalled carbon nanotubes (MWCNTs), using polyoxymethylene nonwovens as the filter paper. The nonwovens with micro- and nanoporous two-tier structures act as the skeleton, introducing a microscale structure. The MWCNTs act as nanoscale structures, creating hierarchical surface roughness. The surface topography and the electrical conductivity of the superhydrophobic surfaces are controlled by varying the MWCNT loading. The vine-on-fence-structured surfaces exhibit "sticky" superhydrophobicity with high water adhesion. The cerebral cortex-like surfaces exhibit self-cleaning properties with low water adhesion. The as-prepared superhydrophobic surfaces are chemically resistant to acidic and alkaline environments of pH 2-12. They therefore have potential in applications such as droplet-based microreactors and thin-film microextraction. These findings aid our understanding of the role that surface topography plays in the design and fabrication of superhydrophobic surfaces with different water-adhesion properties.

Deformation-induced structure evolution of poly(butylene terephthalate)/poly(carbonate) blends during uniaxial stretching

Phase separation occurs during deformation for all the PBT/PC samples. At large strains, microfibril slippage plays a leading role in the macroscopic strain.