Blends of Semicrystalline Polymers

The second example of semicrystalline polymer blends involved the SEM imaging of a mixture of two different types of polyethylene, the surfaces of which were 

The SEM images showed that the minority HD PE phase existed as isolated islands scattered in the matrix of the EOC. The HDPE lamellae appeared bright in the SEM images and extended to a length of several hundred nanometers. 

The thin sections for i, lower- (b) magnification TEM images of HDPE/EOC (40/60) blend, ne investigations were stained with RUO4 vapor [59]. 

Correct sample preparation is also crucial when studying the morphology of semicrystalline polymer blends using TEM. Typically, the blend is stained with ruthenium tetroxide (RUO4) vapor which, by virtue of its rapid difiusivity in the loosely packed amorphous regions of the sample, is deposited preferentially in the amorphous part of the blend. Consequently, in the TEM images the amorphous phase will appear dark and the crystalline phase bright. 

One quite reliable microscopic technique that requires no special sample preparation is that of AFM [22,62,63]. In particular, the phase-imaging mode in tapping mode AFM delivers important information regarding the structure and properties of heterogeneous semicrystalline polymer blends. 

Depending on the mixing conditions, polymers having a similar chemical structure (like HDPE, LDPE, and LLDPE) can show separated phases or mixing on the macromolecular level up to cocrystallization. Often used are also blends of the different types of PE with PP, PA, and other semicrystalline polymers. Because of the broad application of PE and PP, after use they are blended in recycled polymer combinations, with the main problem of reduced compatibility. 

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SMP based on miscible blends of semicrystalline polymer/amorphous polymer was reported by the Mather research group, which included semicrystalline polymer/amorphous polymer such as polylactide (PLA)/poly vinylacetate (PVAc) blend [21,22], poly(vinylidene fluoride) (PVDF)/PVAc blend [23], and PVDF/polymethyl methacrylate (PMMA) blend [23]. These polymer blends are completely miscible at all compositions with a single, sharp glass transition temperature, while crystallization of PLA or PVDF is partially maintained and the degree of crystallinity, which controls the rubbery stiffness and the elasticity, can be tuned by the blend ratios. Tg of the blends are the critical temperatures for triggering shape recovery, while the crystalline phase of the semicrystalline PLA and PVDF serves well as a physical cross-linking site for elastic deformation above Tg, while still below T ,. 

In immiscible blends, the t-T principle does not hold. For immiscible amorphous blends, it was postulated that two processes must be taken into account the t-T superposition and the aging time (Maurer et al. 1985). On the other hand, in immiscible blends, at the test temperature, the polymeric components are at different distances from their respective glass transition temperatures, T — Tgi T — Tg2. In blends of semicrystalline polymers, such as PE/PP, the superposition is limited to the molten state, within a narrow, high temperature range (Dumoulin 1988). 

Rathi, P. (2009) Theoretical Modeling of Morphology Development in Blends of Semicrystalline Polymers undergoing Photopolymerization, PhD Dissertation, University of Akron. 

As a route for improving the melt-elongational properties of semicrystalline polymers, Siripurapu et al. [7] proposed the blending of amorphous and semicrystalline blends of PS and PVDF nevertheless, their approach showed only limited success. In contrast, Reichelt et al. [29] successfully developed blends of HMS-PP and PP-fe-PE block copolymers. As could be shown, the melt strength increases with the HMS-PP content, while blends rich in HMS-PP also show the lowest densities. 

Al-Saigh, Z. Y. Chen, P., "Characterization of Semicrystalline Polymers by Inverse Gas Chromatography. 2. A Blend of Poly(Vinylidene Fluoride) and Poly(Ethyl Methacrylate)," Macromolecules, 24, 3788 (1991). 

This review has illustrated various properties of multiphase polymer systems obtained from computer simulation. Three modeling techniques - atomistic, coarse-grained, and atomistic-continuum modeling - are applied to miscibility of homopolymer/copolymer and homopolymer/homopolymer blends, compat-ibilizing effect of block copolymers, and mechanical properties of semicrystalline polymers, respectively. 

The remainder of this chapter will focus on the thermal conductivities of amorphous polymers (or the amorphous phase, in the case of semicrystalline polymers). See Chapter 20 for a discussion of methods for the prediction of the thermal conductivities of heterogeneous materials (such as blends and composites) in the much broader context of the prediction of both the thermoelastic and the transport properties of such materials. 

The immiscible blends with <5 wt% of either component were introduced first (e.g., POM with PA or PARA) before adequate methods of compatibiUza-tion were developed. Owing to the crystalline nature of these resins, the blends should also be impact modified. It is noteworthy that in blends of semicrystalline resins the total crystallinity tends to increase [Nadkami and Jog, 1991]. The compatibihzation and impact modification are often accomplished using a multi-polymer. For example, POM/PA-66 blends have been modified by adding either an ethylene-methylacrylate copolymer (EMAC), PEG [Kohan, 1982], or a melamine-derivative dispersant [Tsukahara and Niino, 1992, 1994]. 

The addition of a second non-crystallizable component to a crystallizable matrix can cause drastic variations of important morphological and structural parameters of the semicrystalline phase, such as the shape, size, regularity of sphemlites and intersphemlitic boundary regions, lateral dimensions of the lamellae, etc. These factors may greatly influence the mechanical behavior and, in particular, the fracture mechanisms, and thus are of great importance, especially when the toughening of semicrystalline polymer blends is considered. 

Incorporation of semicrystalline polymer into an amorphous resin to improve solvent and chemical resistance (e.g., in blends of PC with PEST). 

According to Stein et al. (84), the diffusion range of the noncrystallizable rubbery chains during the crystallization of crystallizable chains for miscible blends with semicrystalline polymer matrix can be estimated using the parameter 3p (72) as follows ... 

Dell Eiba M., Groeninckx G., Maglio G., Malinconico M., Migliozzi A., Immisdble polymer blends of semicrystalline biocompatible components Hrermril properties and phase morphology analysis of PLLA/PCL blends. Polymer, 42(18), 2001, 7831-7840. 

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