Structure-crystallinity relationships

The melting points of a series of poly (a-olefin) crystals were studied. All of the polymers were isotactic and had chains substituents of different bulkiness. The results are listed below. Use Eq. (2.12) to derive a relationship between the melting point, T, and the enthalpy and entropy of fusion. A/// and A5/, respectively. Use this relationship, plus what you know about polymer crystallinity and structure from Chapter 1, to rationalize the trend in melting point. 

Asrar. J.. Thomas, O., Zhou, Q. and Blumstein, A. Thermotropic liquid crystalline polyesters Structure property relationship. Proc. 28th Macromol. Symp. IUPAC, U. Mass., Amherst, MA, p. 797, 1982... 

Structure of Polymers Kinetics of Polymerization Property-Molecular Weight Relationships Interchain and Intrachain Forces Crystalline-Amorphous Structures Transitions... 

Un-cross-linked semicrystalline poly vinyl alcohol) hydrogels were prepared by solvolysis of the corresponding vinyl trifluoroacetate polymers and copolymers. The relationships between polymer crystallinity, hydrogel structure, and mechanical properties in the subject hydrogels were examined. Evidence was presented that comonomers acted to disrupt crystal structure and increase water content. The effects of copolymer structure on surface characteristics important to biomedical applications were examined, and the importance of hydrogel nonionic character was demonstrated through protein binding studies. 

Fig. 109. Schematic illustration of phase relationships, structural transitions and anomalies appearing in the nonstoichiometric range of 123-0,. First row EM investigations of Beyers et al. (1989). Second row XRD and NPD on poly- and single-crystalline samples, after Plakhty et al. (1994, 1995). Third row Hard X-ray single-crystal refinements of superstructures, after von Zimmermann et al. (1999). Fourth and fifth rows under normal and higher pressures, measured in equilibrium samples. Sixth to tenth rows phase relationships deduced from lattice parameters of slowly cooled samples (preparation methods described in sect. 3.1.2.2). Eleventh row EXAFS results after Rohler et al. (1997a,b, 1998).

In studies aiming at understanding the relationships between the structure of the carbohydrate amphiphiles and their liquid crystalline properties, structural variations can concern the number of hydro)q l groups, the position of the hydrophobic chain on the sugar backbone, the... 

One purpose of the present investigation was to obtain some additional information on structure-crystallinity relationships in this family of polymers by the preparation of stereoregular isotactic polyesters from a single asymmetric isomer of the chiral monomer that is, from an optically-active a,a-disubstituted-s-propiolactone. Because the polymerization reaction mechanism operates through scission of the alkyl-oxygen bond and does not involve bond reorganizations at the asymmetric center, it was fully expected that polymerization of the optically-active monomer... 

Biological membranes provide the essential barrier between cells and the organelles of which cells are composed. Cellular membranes are complicated extensive biomolecular sheetlike structures, mostly fonned by lipid molecules held together by cooperative nonco-valent interactions. A membrane is not a static structure, but rather a complex dynamical two-dimensional liquid crystalline fluid mosaic of oriented proteins and lipids. A number of experimental approaches can be used to investigate and characterize biological membranes. However, the complexity of membranes is such that experimental data remain very difficult to interpret at the microscopic level. In recent years, computational studies of membranes based on detailed atomic models, as summarized in Chapter 21, have greatly increased the ability to interpret experimental data, yielding a much-improved picture of the structure and dynamics of lipid bilayers and the relationship of those properties to membrane function [21]. 

MW and MWD are very significant parameters in determining the end use performance of polymers. However, difficulty arises in ascertaining the structural properties relationship, especially for the crystalline polymers, due to the interdependent variables, i.e., crystallinity, orientation, crystal structure, processing conditions, etc., which are influenced by MW and MWD of the material. The presence of chain branches and their distribution in PE cause further complications in establishing this correlation. 

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

Oxysalt bonded cements are formed by acid-base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homo-geneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex. However, as will be described in the succeeding parts of this chapter, in many cases there is enough knowledge about these cements to enable their properties and limitations to be generally understood. 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

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