Polymer semi-miscible

It is known that PLA forms miscible blends with polymers such as PEG [53]. PLA and PEG are miscible with each other when the PLA fraction is below 50 per cent [53]. The PLA/PEG blend consists of two semi-miscible crystalline phases dispersed in an amorphous PLA matrix. PHB/PLA blends are miscible over the whole range of composition. The elastic modulus, stress at yield, and stress at break decrease, whereas the elongation at break increases, with increasing polyhydroxybutyrate (PHB) content [54]. Both PLA/PGA and PLA/PCL blends give immiscible components [55], the latter being susceptible to compatibilization with P(LA-co-CL) copolymers or other coupling agents. 

The amount of the degradable impact modifier required to realize the desired film properties depends upon the degree of miscibility of the modifier in the polylactide. The properties of polymer blends are strongly dependent upon the degree of compatibility and/or miscibility of the components. Only a few blends are truly miscible at the microscopic level while most are semi-miscible. Typically, the amount of modifier in the film ranges from about 10 to about 25% by weight of the film. 

Characterization methods for analyzing blends may be divided into two types single-phase and multi-phase. Evidently, the number of single-phase systems is limited to amorphous polymers with miscible additives. The semi-crys-taUine polymers (such as PA-6 or PET) are suspension of the crystalline phase in a vitreous or molten phase, thus subjected to the same analysis of phase formation and evolution with stress and temperature as that of an immiscible blend. The characterization of single-phase systems focuses on the individual macromolecules, their configuration, conformation, molecular weight and its distribution, as well as on properties directly related to the molecular mass and constitution, namely, stability, thermodynamic interactions, rheology, etc. 

So, the PVA/poly(sodium styrene sulphonate) [PSSNa] blend was obtained by casting aqueous solution of polymers mixture (PVA with Mw= 124,000-186,000 and HD=99% and PSSNa with Mw= 70,000). The resulted films were crosslinked with 1,2-dibromethane in gaseous phase. A semi-interpenetrating network (SIPN) in which polyelectrolyte (PSSNa) chains are trapped inside a based PVA network was obtained [44], A totally miscible blend with a very good film clarity and high mechanical resistance [44] resulted. 

Polymer blend of two or more different semi-crystalline polymers that are miscible in the crystalline state as well as in the molten state. 

Melting Point Depression. A more quantitative evaluation of the relationships existing between lignin structure and blend miscibility is possible through the Tm depression observed in these materials. For semi-crystalline blend systems, such as these, the polymer-polymer interaction parameter, B , can be determined through the following simplified expression (15) ... 

As shown in a previous section, fluorinated nadimides exhibit the best thermo-oxidative behavior. NR 150 linear polymers are prepared by reaction of HFDE and a mixture of para- and raefa-phenylenediamine. A semi-IPN was prepared by addition of a 150 °C staged PMR-15 to a solution of the NR 150 precursor. The solution was used for the composite manufacture. After curing at 250 °C, the DSC diagram showed two peaks in agreement with a non-miscible system. After curing, the Glc of the blend is higher than that of pure PMR-15 (Table 8) [119]. 

The role of the miscibility of semi-IPN components on the mechanical properties has been discussed. The linear bisnadimide was a benzhydrol bisnadimide (Fig. 33). Three polyimides prepared from the same diamine and three different dianhydrides (Fig. 37) were used as linear components. The blends were cured up to 300 °C in a similar fashion to the bisnadimide alone. The results for the blend containing 20% by weight of linear polymers are summarized in Table 9. The non-miscible character of the components gives a phase segregation leading to the best toughness [121]. 

M. Muller (1999) Miscibility behavior and single chain properties in polymer blends a bond fluctuation model study. Macromol. Theory Simul. 8, pp. 343-374 M. Muller and K. Binder (1995) Computer-simulation of asymmetric polymer mixtures. Macrrmolecules 28, pp. 1825-1834 ibid. (1994) An algorithm for the semi-grand-canonical simulation of asymmetric polymer mixtures. Computer Phys. Comm. 84, pp. 173-185... 

For miscible case, the parameter k = AC j/ACp -However, it was found that relaxing this condition transforms the relation into a semi-empirical one, valid for either miscible or for immiscible systems. The dependence is symmetrical — it must be valid when the indices are exchanged. Thus, the miscibility requires that k = 1/k = 1. Larger is the difference between k and 1/k, larger is the immiscibUity of the system. The dependence should not be used for strongly associating polymer blends where T may reach values higher than these observed for either component. These... 

Deheuw. G.. Groeninckx. G., and Reynaers, H., Miscibility, crystallization and melting behaviour, and semi-crystalline morphology of binary blends of polycaprolacione with polyjhydroxy ether of bisphcnol A), Polymer. 462 489 (1989). 

The following semi-empirical equation has been found by Utracki and Jukes to describe the variation of Tg for some pairs of polymers that are miscible over the whole composition range ... 

Poly(8-caprolactone) (PCL), on the other hand, is a semi-ciystaUine biodegradable polymer. This material is commonly used as plasticizer. The P(3HB)/ PCL blend exhibited partial miscibility, however, the elongation at break was increased about 81 % in comparison with pure P(3HB) [43]. 

Li et al. synthesized a PMMA-PEG semi-IPN by radical polymerization and cross-linking of PMMA in the presence of linear PEG, which exhibits two independent shape memory effects at two transition temperatures, the of the PEG crystal and the Tg of the semi-IPN [39]. In the IPN, a single Tg appeared due to the miscibility of the amorphous phase of the two polymers. Based on a reversible order-disorder transition of the crystals below and above the of PEG, and the large difference in storage modulus below and above the Tg of the semi-IPN, the polymer has a recovery ratio of 91 and 99%, respectively For the shape-memory behavior at the of PEG crystals, the fixing phase was the PMMA network and the reversible phase was PEG crystals. For the shape memory behavior at the Tg of the semi-IPNs, the fixing phase was the chemical cross-linked point, while the reversible phase was the PMMA-PEG complex phase. 

The advantage of using the miscibility parameter lies in its direct correlation with the solubility parameters of the polymer and the plasticizer and in the possibility it offers to find some semi-empiiieal relationships with compatibility that differ from those in Eq. [6.35]. 

At present, the most common way to gain information about the phase behavior of polymer systems is first to obtain experimental data about the temperature and composition dependence of phase transitions and then to interpret them by applying thermodynamic principles. Following this semi-empirical method, the observation of glass transition(s) is commonly used to study miscibility in amorphous and semicrystalline mixtures. 

It has been known since the early days that behavior of the aromatic polyamides (aramids) depends critically on the type of isomeric substitutions -para-substimtimis result in crystalline, while mefa-substitutions in amoiphous polymers (Kwolek et al. 1962). Similarly, the two aramids poly(m-xylene adipamide) and poly (hexamethylene isophthalamide), MXD6 and PA-61, respectively, show different miscibility, e.g., with aliphatic polyamides. Clearly, blind application of the segmental interaction strategy to aromatic or semi-aromatic polyamides leads to conflicts. However, the problem can be resolved considering p- and m-substituted phenyl as two different statistical segments (Ellis 1995). This idea is indeed evident in the segmental contributions listed in Table 2.14. 

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