Amorphous/crystalline polymer blends

Table 3.26. Influence of compatibilization on the crystallization behavior of the dispersed phase in amorphous /crystalline polymer blends...

Polymer blends containing a crystallizable component have attracted many scientists, both from basic research and applied research laboratories. This is probably due to the fact that the majority of commercially used thermoplastic blends and alloys contain at least one crystallizable material [5]. In order to obtain the desired product properties, it is often very important to control the crystallization process. For instance, in certain applications it is useful to have amorphous polyester (e.g., PET, as a package material), whereas for other applications a higher degree of crystallinity is necessary (e.g., as a fiber material). In amorphous/crystalline polymer blends the crystallization behavior is often strongly influenced by the amorphous component. Usually, the crystallization rate of the crystalline polymer is reduced by the amorphous polymer. In most systems this is caused by an increase... 

Zhang, X., Takegoshi, K., and Hikichi, K. (1992) Composition dependence of the miscibility and phase structure of amorphous/crystalline polymer blends as studied by high-resolution solid-state carbon-13 NMR spectroscopy. Macromolecules, 25 (9), 2336-2340. 

Fig. 10. Schematic representation of segregation of amorphous components in partially crystalline polymer blends depicting the location of residual crystallisable polymer and non-crystallisable polymer in the interlamellar and interfibrillar regions within spherulites and interspherulitic locations. Solid lines represent the crystallisable component and dotted lines the non-crystallisable component taken from [60]...

Blends comprised of amorphous, low Tg polymers are of primary interest for elastomeric type applications, of which the large tire market commands considerable interest. This section will consider blends of elastomeric polymers, generally low Tg, amorphous blends. In specific cases, low modulus, crystalline polymer blends (such as ethylene copolymers) with other elastomeric materials will be included. Also blends containing crystalline polymer, where the primary component of the blend is the elastomeric component and the blend is considered an elastomeric material, will be discussed. Specifically, dynamic vulcanized blends such as polypropylene/ethylene-propylene rubber blends will be included in this section. 

Up to now, the study about phase-separation theory is mainly focused on amorphous/amorphous polymer blend and crystalline/amorphous polymer blend models. There is still lack of systemically theoretical guidance and support for crystalline/crystalline polymer blend models. Therefore, the establishment of the phase-separation theory... 

Figure 10.2 Crystallization temperature ranges for crystalline/amorphous miscible polymer blends (A amorphous polymer, B crystalline polymer) as a function of volmne fraction of the crystalline component. Tg, glass transition temperature of blend TgA < (sohd Une), (dashed line). T, ...

Matkar et al. have hypothesized what would happen to crystalline blend phase diagrams if one relaxes the last assumption of the Floty diluent theory of crystalline polymer solutions, namely, the complete rejection of polymeric solvent from the crystalline phase [66, 67]. In addition, Xu et al. have developed a new theory for a binary crystalline polymer blends based on a combination of liquid-liquid phase separation and solid-liquid phase transition by taking into consideration the coupling interaction between the solid crystal and amorphous liquid phase [71]. 

Volume 1 is devoted to fundamental principles of polymer blends and is divided into eight chapters. These chapters cover the basic thermodynamic principles defining the miscible, immiscible, or compatible nature of amorphous, semi-crystal-hne and Uquid crystalline polymer blends, and temperature and composition dependent phase separation in polymer blends. They are detailed below and build upon each other. 

As Carfagna et al. [61] suggested, the addition of a mesophasic polymer to an amorphous matrix can lead to different results depending on the properties of the liquid crystalline polymer and its amount. If a small amount of the filler compatible with the matrix is added, only plasticization effect can be expected and the dimensional stability of the blend would be reduced. Addition of PET-PHB60 to polycarbonate reduced the dimensionality of the composite, i.e., it increased the shrinkage [42]. This behavior was ascribed to the very low... 

The main limitation of birefringence is that it only provides an averaged orientation value without any discrimination between amorphous and crystalline phases, or between the components in polymer blends, copolymers, and... 

Non-crystalline polymers or copolymers can also be used to generate fibers with relatively low softening temperatures. Such fibers can be blended with regular fibers, e.g. staples, and bonded together by applying sufficient heat to melt the low-temperature component. Such fibers need not be exotic. The use of undrawn, amorphous fibers suffices for many such purposes, for example, bonded nonwo-ven webs formed from a mix of drawn and undrawn PET staple fibers. Without crystalline structure, the undrawn fibers will soften and become tacky at relatively low temperatures, so providing bond sites. 

The value of the modulus and the shape of the modulus curve allow deductions concerning not only the state of aggregation but also the structure of polymers. Thus, by means of torsion-oscillation measurements, one can determine the proportions of amorphous and crystalline regions, crosslinking and chemical non-uniformity, and can distinguish random copolymers from block copolymers. This procedure is also very suitable for the investigation of plasticized or filled polymers, as well as for the characterization of mixtures of different polymers (polymer blends). 

It is important to mention that the structure/properties relationships which will be discussed in the following section are valid for many polymer classes and not only for one specific macromolecule. In addition, the properties of polymers are influenced by the morphology of the liquid or solid state. For example, they can be amorphous or crystalline and the crystalline shape can be varied. Multiphase compositions like block copolymers and polymer blends exhibit very often unusual meso- and nano-morphologies. But in contrast to the synthesis of a special chemical structure, the controlled modification of the morphology is mostly much more difficult and results and rules found with one polymer are often not transferable to a second polymer. 

Firstly Though a high softening temperature is attractive, we should realise that the polymer has to undergo processing operations, which are carried out, for an amorphous polymer, well above Tg, and for a crystalline polymer, above Tm- At these temperatures the polymer should be sufficiently chemically stable (for i.a. that reason PPE is being blended with PS). 

One method of reducing crystallinity in PEO-based systems is to synthesize polymers in which the lengths of the oxyethylene sequences are relatively short, such as through copolymerization. The most notable hnear copolymer of this type is oxymethylene-linked poly(oxyethylene), commonly called amorphous PEO, or aPEO for short. Other notable polymer electrolytes are based upon polysiloxanes and polyphosphazenes. Polymer blends have also been used for these applications, such as PEO and poly (methyl methacrylate), PMMA. The general performance characteristics of the polymer electrolytes are to have ionic conductivities in the range of cm) or (S/cm). 

Thermal Properties. A typical dsc thermogram of an HPL/PVA blend (Fig. 4) shows a single Tg and Tm (10). Differences in the shape of the melting endotherms of PVA(96), (88), and (75) can be attributed to different degrees of crystallinity in the three polymers. Changes in crystalline structure of polymer blends usually result from polymer-polymer interactions in the amorphous phase. Such interactions result in a reduction of crystallinity, thereby reducing the enthalphy of the phase change (16,17). The observed reductions in melt endotherm area of HPL blends with PVA (> 0) may therefore indicate the existence of polymer-polymer interactions between the two types of macromolecules. 

The blend of poly(bisphenol A carbonate)-(poly(caprolactone) PC-PCL is particularly unusual in that both polymers are capable of crystallization and FT-IR has been used to study the state of order in these blends as a function of the method of preparation 254,255). In this case, PCL is a macromolecular plasticizer for PC. The PCL becomes progressively less crystalline as the concentration of PC increases. PC is amorphous if the blend is cast from methylene chloride but semicrystalline if cast from tetrahydrofuran. When PC in the pure form is exposed to acetone, it will not crystallize, but in the blend, exposure of acetone causes the PC to crystallize which emphasizes the additional mobility of the PC in the blend. 

It is interesting that the PEMA-PVdF blends are amorphous up to at least 50 wt % PVdF even though the Tg of the latter is 24°C. The crystallization of PVdF observed in the analogous PMMA blend does not occur under the same conditions with PEMA—PVdF. This suggests that there is a specific interaction between the fluoropolymer and the methacrylate polymer which is sufficient to "dissolve PVdF in the PMMA and PEMA, and that this specific interaction is superimposed on the conventional diluent-crystalline polymer interactions. The complexity of the rate processes involved with high molecular weight systems arising from molecular mobility makes it impossible to elucidate the nature of... 

Mixtures of poly(vinylidene fluoride) with poly (methyl methacrylate) and with poly (ethyl methacrylate) form compatible blends. As evidence of compatibility, single glass transition temperatures are observed for the mixtures, and transparency is observed over a broad range of composition. These criteria, in combination, are acceptable evidence for true molecular intermixing (1, 19). These systems are particularly interesting in view of Bohns (1) review, in which he concludes that a compatible mixture of one crystalline polymer with any other polymer is unlikely except in the remotely possible case of mixed crystal formation. In the present case, the crystalline PVdF is effectively dissolved into the amorphous methacrylate polymer melt, and the dissolved, now amorphous, PVdF behaves as a plasticizer for the glassy methacrylate polymers. 

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