Polymer blends with liquid-crystalline components

Polymer Blends with Liquid-Crystalline Components, 314... 

Section 10.6 is dedicated to blends containing liquid-crystalline components. The mesophase behavior of liquid crystals (LCs) and liquid crystal polymers (LCPs), as well as the crystallization processes and the superstructure of blends of crystallizable polymers with LCPs and blends of LCPs, is described. 

POLYMER BLENDS WITH LIQUID-CRYSTALLINE COMPONENTS... 

Here (])i = (]) and <[>2 = (1 — 4>) volume fractions of the crystalline component and amorphous component, respectively r and ri represent the numbers of statistical segments of the polymer chain of crystalline and amorphous constituents, respectively, with Xaa = Xfh = A+B/T, where the FH interaction parameter represents the amorphous-amorphous interaction and A and B are constants. Let us consider a situation where a crystalline polymer is mixed with an amorphous polymer, that is, polymeric solvent. The chemical potential of the liquid phase can be calculated from the Flory-Huggins theory of liquid-liquid mixing for polymer blends as follows ... 

The purpose of the present work is to extend the above studies to blend systems consisting of two components, each of which is capable of forming a liquid crystalline phase in the melt. Such blends become of increasing importance with the recently reported finding (6) that it is possible to observe synergisms in mechanical properties in these systems. Our previous studies in this area have suggested that the theories of traditional polymer blend systems (7.8) are applicable to these blends. This paper is a further study of the applicability of these concepts. 

Very little work has appeared in the literature which deals with blends in which the component materials can cocrystallize. It is generally believed (16.17) that a requirement for cocrystallization is that there must be a close matching of the polymer chain conformations and of crystalline dimensions. Also, some level of miscibility should exist between the two polymers and the crstallization kinetics cannot be very different. Certainly, in the case of liquid crystalline polymers, in general, these requirements would be expected to be met. Some of our recent work (8) has suggested, however, that not all liquid crystal polymers do cocrystallize. The present work suggests that in certain cases it may be possible to achieve this effect. 

The formation of miscible rubber blends slows the rate of crystallization (Runt and Martynowicz, 1985 Keith and Padden, 1964) when one of the components is crystallizable. This phenomenon accounts for data that show lower heats of fusion that correlate to the extent of phase homogeneity (Ghijsels, 1977) in elastomer blends. Additionally, the melting behavior of a polymer can be changed in a miscible blend. The stability of the liquid state by formation of a miscible blend reduces the relative thermal stability of the crystalline state and lowers the equilibrium melting point (Nishi and Wang, 1975 Rim and Runt, 1520). This depression in melting point is small for a miscible blend with only dispersive interactions between the components. 

Blends were prepared with cellulose or silk as soon as a common solvent was available [63, 69-71]. Recently, ionic liquids were used. The solvent l-ethyl-3-methyl-imidazolium acetate completely dissolves raw crustacean shells allowing to recover high purity chitin powder or films and fibres by direct spinning [72]. Films of poly(e-caprolactone) (PCL) blends with a-chitin and chitosan were produced. They are completely biodegradable and the crystallinity of PCL is suppressed in the blends due to hydrogen bond interaction between PCL and polysaccharides [73]. Blends were also realized with poly (3-hydroxybutyric acid) (PHB) and chitin or chitosan. They show faster biodegradation than the pure-state component polymers [74,75]. 

When dealing with crystallizable miscible polymer blends containing a noncrystallizable component, some refinements had to be made. Some modifications were proposed by Alfonso and Russell (1986) and by Cimmino et al. (1989) for blends in which the amorphous component is segregated into the interlamellar region (see also Sect. 3.2.2.1). First, the chemical potential of the liquid phase might be altered by the specific interactions that are often responsible for the miscibility of polymers (Olabisi et al. 1979). Such interactions may change the free energy required to form a critical nucleus as well as the mobility of both the crystalline and amorphous components. Second, the noncrystallizable component has to... 

Thermotropic, main-chain, liquid-crystalline polymers (LCPs) have attracted considerable attention as a result of their high stiffness and mechanical properties. There has been interest in combining the LCPs with other materials. In one area, LCPs are used, in relatively low concentration, to reinforce less-stiff materials. In another case, a second component is used as a solvent to increase the mobility in the LCP and form lyotropic liquid-crystalline materials. There are now two reports, from Kricheldorf s group, of blends of PCL with liquid-crystalline polyesters [156,157]. 

It was also remarked that the formation of a lyotropic blend from PCL and 27c, two isotropic components, was particularly notable. It was suggested that, as formation of a liquid-crystalline phase is strongly dependent on free-volume fraction, the combination of PCL chains aligned with rigid-rod polymer chains can reduce the overall free volume, induce nematic order and stiffen the aggregate [157]. 

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