Preparation And Properties Of Polymer Blends With Highly-oriented Ribbons

Polymer blending is a very important approach to produce new polymer materials with high performances. Their final properties strongly depend on the morphology of the dispersed phase. Compared with a conventional sphere-matrix or fibrillar-matrix blend, a polymer blend with a ribbon-like dispersed phase has the potential to significantly widen the application range from mechanical enhancement to gas barrier, excellent optical performance, shape memory material, and electrical conductivity, which is very significant for the preparation of novel polymer blends. However, it has been a challege to effectively control the formation of ribbon-structure dispersed phase via an approach of polymer blending.In this work, a polymer blend with high-oriented and ribbon-like structures was successfully prepared by tape extrusion. The formation mechanism of ribbon-like structure during tape extusion and the morphology evolution of the obtained ribbon-containing blend in the quiescent annealing were systematically studied. The influences of draw ratio, compression ratio, viscosity ratio and compatibilizer on formation of ribbon-like structure were investigated. Moreover, the optical properties of this ribbon-containing polymer blend and its application in foaming area were also discussed. The detailed results are as follows:1. In the process of tape extrusion, the formation of ribbon-like structure mainly results from the joint effects of stretching force and squeezing force. The stretching force can drive the deformation and coalescence of the spherical dispersed phase so that the morphology of the dispersed phase is gradually evolved into rotational ellipsoidal, rod-like and fibrillar shapes. Then, those formed fibers are squeezed perpendicularly to the extrusion direction, and form highly-oriented ribbons. As the draw ratio and compression ratio increases, the deformation of dispersed phase increases, which results in thinner and narrower ribbons. Meanwhile, the tensile strength along extrusion direction becomes stronger. Furthermore, it was found that a viscosity ratio (<1) of the dispresed phase and the matrix facilitates the deformation and coalescence of dispersed phase to form highly-oriented ribbons. The introduction of graft copolymer as a compatibilizer can promote the formation of ribbon structure because the backbone of graft copolymer prefers to run with the matrix while the grafting chain would like to stay with the dispersed phase, in other words, the copolymer worked as a vinculum to transfer the shear stress from the PS matrix to the dispersed PA6 phase. However, a relatively high compatibilizer concentration could hinder the formation of ribbon structure because coalescence of dispersed phase is restrained.2. Upon the quiescent annealing, the ribbon-like structure in polymer blend at a low PP ribbon concentration (10% and 20%) can shrink into fibers first, then break into rods and spheres, as a inverse process of tape extrusion. Moreover, the diameter of final spherical partices is similar to that in the unstretched blends. When PP concentration increases to 35%, the initial dispersed phase/matrix structure in the unstretched blends becomes a co-continuous structure after tape extrusion and following quiescent annealing. Moreover, the continuity maintains about 90%. This may be an approach to broad the cocontinuous range of polymer blend. For the blend with a compatibilizer, the morphology change is brought forward due to a larger deformation during tape extrusion. It should be noted that at a low compatibilizer concentration, the final spherical particles after quiescent annealing do not exhibit a single distribution but a bi-modal distribution, which may result from a gradient compatibilizer distribution on the interface formed via tape extrusion.3. Foams of ribbon-matrix blends were prepared by supercritical carbon oxide (sCO2) foaming technology. The foaming temperature (100～120 ℃) is lower than the melting temperature of dispersed phase PA6. As a resut, the dispersed phase is solid and can act as a heterogenous nucleation agent. Due to the strong heterogeneous nucleation effect and significant increase in melt strength from solid dispersed phase, foams with smaller cell sizes and higher cell densities can be obtained. The highly-oriented ribbon can directionally restrict the cell growth to produce oriented cells. Interestingly, the obtained foams show an obvious anisotropy in mechanical property, as the compression modulus along the ribbon orientation is 4 times as much as that in the vertical direction. This is mainly because ribbons in the former situation can work as a support in the foam. When carbon black was selectively dispersed in PA6 ribbons, the compression strength further increases because of the stronger support effect. Comparing with the spherical dispersed phase, the ribbons can increase the bending strength by a factor of 9.4. The thickness of dispersed PP ribbons in the PS matrix was successfully controlled below 100 nm by adjusting the ratio of two polymers in the blend and processing parameters, such as feeding speed, stretching speed and so on. Due to the unique hierarchical structure, this nanoribbon-containing film shows interesting structural color effect. Vivid colorful strips can be seen from the film in sunlight. When the incident light was introduced at 45°, the reflectivity peaks appear in the ultraviolet region (250 nm) and visible region (aroud 600 nm and 400-500 nm). Moreover, chatoyancy and light polarization effects of this nanoribbon-containing film are observed as well.