Factors influencing the formation of elongated morphologies in immiscible polymer blends during melt processing

Effect of the Elongational Flow on the Morphology and Properties of Polymer Systems: A Brief Review

Polymer-processing operations with dominating elongational flow have a great relevance, especially in several relevant industrial applications. Film blowing, fiber spinning and foaming are some examples in which the polymer melt is subjected to elongational flow during processing. To gain a thorough knowledge of the material-processing behavior, the evaluation of the rheological properties of the polymers experiencing this kind of flow is fundamental. This paper reviews the main achievements regarding the processing-structure-properties relationships of polymer-based materials processed through different operations with dominating elongational flow. In particular, after a brief discussion on the theoretical features associated with the elongational flow and the differences with other flow regimes, the attention is focused on the rheological properties in elongation of the most industrially relevant polymers. Finally, the evolution of the morphology of homogeneous polymers, as well as of multiphase polymer-based systems, such as blends and micro- and nano-composites, subjected to the elongational flow is discussed, highlighting the potential and the unique characteristics of the processing operations based on elongation flow, as compared to their shear-dominated counterparts.

Relationship between the Processing, Structure, and Properties of Microfibrillar Composites

The relationship between processing, morphology, and properties of polymeric materials has been the subject of numerous studies of academic and industrial research. Finding an answer to this question might result in guidelines on how to design polymeric materials. Microfibrillar composites (MFCs) are an interesting class of polymer-polymer composites. The advantage of the MFC concept lies in developing in situ microfibrils by which a perfect homogeneous distribution of the reinforcement in the matrix can be achieved. Their potentially excellent mechanical properties are strongly dependent on the aspect ratio of the fibrils, which is developed through a three-stage production process: melt blending, fibrillation, and isotropization. During melt blending, the polymers undergo different morphological changes, such as a breakup and coalescence of the droplets, which play a crucial role in defining the microstructure. During processing, various parameters may affect the morphology of the MFCs, which must be taken into account. Besides the processing parameters, the microstructure of the composite is dependent on the composition ratio of the blend and viscosity of the components, as well as the dispersion and distribution of the microfibrils. The objective here is to outline this importance and bring together an overview of the processing-structure-property relationship for MFCs.

Rheology⁻Microstructure Relationships in Melt-Processed Polylactide/Poly(vinylidene Fluoride) Blends

In this study, small amplitude oscillatory shear tests are applied to investigate the rheological responses of polylactide/poly(vinylidene fluoride) (PLA/PVDF) blends and to correlate their viscoelastic properties with the morphological evolutions during processing. Although the analysis of the elastic moduli reveals some changes as a function of blend composition and processing time, the weighted relaxation spectra are shown to be more useful in detecting changes. The analysis demonstrates that when PVDF, i.e., the more viscous phase, is the matrix, the blend relaxes cooperatively and only a single relaxation peak is observed. By contrast, blends with highly concentrated morphologies do not fully relax, showing instead an upward increasing trend at longer times. This outcome is attributed to the broad distribution of highly concentrated droplets with a high probability of droplet⁻droplet contacts. Dynamic mechanical analysis (DMA) reveals that crystalline segmental motions attributed to the α-relaxation of PVDF at around 100 °C are restricted by the highly concentrated morphology of the 50/50 PLA/PVDF blend processed for 10 min. Relaxation analyses of the blends via dynamic oscillatory shear tests and DMA are shown to be powerful tools for investigating small microstructural changes in immiscible polymer blends.

Poly(lactic acid)-Based in Situ Microfibrillar Composites with Enhanced Crystallization Kinetics, Mechanical Properties, Rheological Behavior, and Foaming Ability

Melt blending is one of the most promising techniques for eliminating poly(lactic acid)'s (PLA) numerous drawbacks. However, success in a typical melt blending process is usually achieved through the inclusion of high concentrations of a second polymeric phase which can compromise PLA's green nature. In a pioneering study, we introduce the production of in situ microfibrillar PLA/polyamide-6 (PA6) blends as a cost-effective and efficient technique for improving PLA's properties while minimizing the required PA6 content. Predominantly biobased products, with only 3 wt % of in situ generated PA6 microfibrils (diameter ≈200 nm), were shown to have dramatically improved crystallization kinetics, mechanical properties, melt elasticity and strength, and foaming-ability compared with PLA. Crucially, the microfibrillar blends were produced using an environmentally friendly and cost-effective process. Both of these qualities are essential in guarantying the viability of the proposed technique for overcoming the obstacles associated with the vast commercialization of PLA.