Introduction
From a commercial point of view, semicrystalline polymers are of prime importance. Among the four mostly used commodity plastics (PE, PS, PVC and PP), only PS is completely amorphous.
The three semicrystalline polymers account for the largest volume of the commercial polymer blends. A majority of the polymer blends contains at least one crystalline component. Most polymer blends are immiscible.
The immiscible semicrystalline polymer blends may be classified in terms of:
1-Crystalline/crystalline systems in which both components are crystallizable, and
2-Crystalline/amorphous systems in which only one component can crystallize, being either the matrix or the dispersed phase.
The properties of the finished articles made from immiscible blends are governed by the morphology created as a result of the interplay of processing conditions and inherent polymer characteristics, including crystallizability.
Therefore, a scientific understanding of the crystallization behavior in immiscible polymer blends is necessary for the effective manipulation and control of properties by compounding and processing of these blends.
In table below, an overview is given of a number of important immiscible crystallizable blend systems.
Factors Affecting the Crystallization Behavior of Immiscible Polymer Blends:
The discussion on the crystallization behavior of neat polymers would be expected to be applicable to immiscible polymer blends, where the crystallization takes place within domains of nearly neat component, largely unaffected by the presence of other polymers.
However, although both phases are physically separated, they can exert a profound influence on each other. The presence of the second component can disturb the normal crystallization process, thus influencing crystallization kinetics, spherulite growth rate, semicrystalline
morphology, etc.
?Important factors are:
1- Molecular structure and molecular mass of the components
2- Blend composition
3- Type and degree of dispersion of the phases in T the melt state
4- Phase interactions (nature of the interface, migration of nuclei, etc.)
5- Melt history (Tmelt , tmelt , etc.),
6- Crystallization conditions (for example Tc, cooling rate, etc.),
7- Physical crystallization conditions (surrounded by melt or solidified material).
These factors influence the crystalline morphology development, resulting in changes of crystallization parameters such as:
1- Nucleation density (N)
2- Spherulite growth rate (G)
3- Overall crystallization rate (K)
4- Total degree of crystallinity (Xc)
5- Semicrystalline morphology, i.e., shape, size and texture of the spherulites, interspherulitic boundaries, etc.
To discuss these topics in a systematic way, a distinction will be made between three main blend categories, namely:
1. Blends with a crystallizable matrix and an amorphous dispersed phase,
2. Blends with an amorphous matrix and a crystallizable dispersed phase,
3. Blends containing two crystallizable components
1-Blends with a Crystallizable Matrix & an Amorphous Dispersed Phase:
In immiscible blends, the phases are separated in the molten state, before crystallization of the matrix starts. The dispersed amorphous phase is assumed to be homogeneously distributed in the melt in droplet-like domains.
1.1 Nucleation Behavior of the Crystallizable Matrix
When a crystallizable component forms the matrix phase in a polymer blend, nucleation can occur via heterogeneous nucleation by heterogeneities in a similar way as in the pure component.
The heterogeneities, available in the melt, can be residual catalysts, fillers, impurities, crystalline residues (due to incomplete melting), etc.
Since in immiscible blends the phases are physically separated, the same heterogeneities that nucleate the homopolymer at Tc,pure may nucleate the crystallizable matrix. As a result, the crystallization temperature, Tc, of the blend during cooling from the melt will in general not differ that much from the Tc of the pure component.
Some general principles governing the crystallization behavior of homopolymers also remain valid for immiscible polymer blends in which the crystallizable component forms the continuous phase.
? The premelting temperature, Tmelt, may have a profound influence on the crystallization temperature of the matrix, Tc , during cooling from the melt .
The higher the temperature at which the blend is kept in the melt prior to crystallization, the less residual crystalline parts (otherwise leading to self-seeded nucleation) remain in
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|