Modern composites are ubiquitous in many parts of modern societies such as:
Interface plays a pivotal part in the final mechanical properties of composites. There are two key methods which have been introduced to fortify the interface between reinforcement and matrix:
- Incorporating a third component as compatibilizer
- Morphological development
Morphological development of polymer blends is another method promoting the interfacial adhesion of two incompatible polymers.
Various morphologies such as droplet1, platelet2, coral3, and fibrillar4 have been introduced over the last ten years. The aim of this is to promote the mechanical properties of polymer blend systems. The formed morphology is under control of various parameters such as molecular weight, viscosity ratio, chemical structure, and processing parameters.3 Therefore, it is expected that variation in the aforementioned parameters causes adjustments to the final formed morphology.
Over the last ten years, microfibrillar architecture has emerged as a new variety of micro structure in which two incompatible polymers form an in situ-reinforced blend by formation of fibrillar morphology.4 To achieve the fibrillar morphology, three steps should be followed:
- Blending of two incompatible blends with different melting temperatures
- Cold drawing at temperatures below the Tm of dispersed phase
- Isotropization at temperatures below the Tm of dispersed phase
Numerous methods, such as injection molding5, compression molding,6 and fiber spinning7 are applied in order to fibrillate the dispersed phase. The spinning process inserts higher elongational stress compared to the other methods, which may facilitate the fibrillation mechanism and droplet.
Laboratory Mixing Extruder (LME)
Designed by Dynisco, Laboratory Mixing Extruder (LME) has been proven as a device with the capability of spinning the polymer melt at different temperatures.7,8
The LME’s temperature profile is controlled in rotor and die zones. The rotor is capable of rotating at a number of speeds, plus a take-up device can be coupled to draw the polymer melt as monofilament. The LME has the ability to not only function as a spinning device, but also to fibrillate the dispersed phase in polymer blends of [Polypropylene (PP)/Poly amide 6 (PA6)] and [Polypropylene (PP)/ Polytrimethylene Terephthalate (PTT)] (schematic 1).7,8
Schematic 1: The LME setup and its components to produce fibrillary reinforced monofilaments.
Cryogenically fractured surfaces of as-extruded samples and fibers of PP/PTT are represented in Figure 1. As shown, a-b, as-extruded samples exhibit a droplet-matrix morphology whose microstructures become coarser after increasing the PTT content. The mean diameters of droplets in as-extruded samples of 6 wt.% and 10 wt.% were 1.26±0.7 μm, and 1.9±0.9 μm respectively.
The microstructures of fibers in figure 1 c-d show fibrillar entities which are located along the fiber axis. The mean diameters of fibrils throughout the fibers were 0.53±0.1 μm, and 0.9±0.4 μm respectively. As shown, the LME has the capability to spin monofilaments containing fibrillar entities of PTT.
Figure 1 SEM images. (a), and (b): fractured surfaces of as-extruded samples with 6wt.% and 10wt.% of PTT respectively. (c), and (d): etched surfaces of one spun blend filament containing 6wt.% and 10wt.% of PTT respectively. The scale bar is 10 μm.
Rheological approach was carried out to examine the various steps of fibrillation and gain a better understanding about the droplet deformation mechanism. The measurement temperature was kept at 195 °C which is below the Tm of PTT, this was to keep elongated inclusions intact.
Figure 2 shows dynamic viscoelastic responses of as-extruded blends and fibers at 195 °C. By increasing the PTT content, storage moduli of as-extruded samples increased. This enhancement can be credited to interfacial tension, volume fraction, and size of the dispersed phase.
Fibers represented higher magnitudes along with non-terminal trend when compared with as-extruded samples at a low frequency. The appearance of secondary plateau was also seen for PP/PA6 blend system in which PA6 droplets formed fibrillated texture throughout the PP matrix.7
This non-terminal behavior was due to the presence of fibrillated droplets. The growth of the physical fibrillar network (PFN) is shown by enhancement in storage modulus and widening of the secondary plateau at low frequency region.8
Figure 2 Linear viscoelastic responses of as-extruded samples and fibers containing 6wt% and 10wt% of PTT at 195°C. (a) dynamic storage modulus, (b) complex viscosity.
The flow behaviors of the fibril-fortified fibers can be observed in η*-ω graphs (figure 2-b). By increasing the PTT content, the complex viscosity of as-extruded samples at low frequencies was enhanced.
Interestingly, the flow behaviors of the fibers exhibited a pronounced increase in complex viscosity in the form of viscosity upturn, together with quicker shift to power-law region when increasing the PTT content and as a result of fibril maturation. Yield behavior of fibril-concentrated PP matrix can be attributed to the observed upturn of complex viscosity at low frequencies.
Conclusion and Outlook
Fibrillar morphology, as one of the most efficient morphologies, permits the blend systems to obviate the aggregation issue of nano-fillers in nanocomposites. The processing conditions are optimized and the LME proved itself as a spinning tool to fibrillate the droplet phase in the blend system of PP/PTT.
It was discovered that terminal behavior is replaced with non-terminal trend upon formation of fibrillar entities. Furthermore, the fibril growth can be reflected by enhancement of the secondary plateau along with broadening of this region which can be considered as the transformation of droplet phase into fibril.
Fibrillar morphology has the ability to enhance the mechanical strength significantly, so it would be of interest to trace the behavior of secondary plateau by adjusting the kind of dispersed phase, and based on the individual molecular characteristic of each polymer, the typical behavior of the secondary plateau may vary. In addition, the capability of the LME to spin nanocomposite fibers is still being studied.
The author also thanks Prof. Dr. J. Schmidt and Mr. S. Pieper for rheological facilities. Also author appreciates Nadine Buitkamp for SEM measurements. Supplying PP and PTT by UNIPETROL (Mr. Martin Malíček), and RTP DEUTSCHLAND GmbH (Mr. Waldemar Müller) is gratefully appreciated. Mr. Fabrizio Ranieri and Ms. Liliana Orban are appreciated for giving the opportunity to work with the LME (Dynisco Co., Heilbronn, BW, Germany).
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- R. Hajiraissi, Y. Jahani, and T. Hallmann: ‘Investigation of rheology and morphology to follow physical fibrillar network evolution through fiber spinning of PP/PA6 blend fiber’, Polym Eng Sci, 2017, 65, 107, doi: 10.1002/pen.24686.
- R. Hajiraissi and Y. Jahani: ‘Non-terminal behavior as a finger print to follow droplet deformation’, Adv Polym Technol, 2017, 298, 9, doi: 10.1002/adv.21810.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
For more information on this source, please visit Dynisco.