Semicrystalline state

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Hence polysaccharides have been viewed as a potential renewable source of nanosized reinforcement. Being naturally found in a semicrystalline state, aqueous acids can be employed to hydrolyze the amorphous sections of the polymer. As a result the crystalline sections of these polysaccharides are released, resulting in individual monocrystalline nanoparticles [13]. The concept of reinforced polymer materials with polysaccharide nanofillers has known rapid advances leading to development of a new class of materials called Bionanocomposites, which successfully integrates the two concepts of biocomposites and nanometer sized materials. The first part of the chapter deals with the synthesis of polysaccharide nanoparticles and their performance as reinforcing agents in bionanocomposites. 

The present discussion of physical structure and properties is intended to serve merely as a basis for appraising the characteristics of various polymers here surveyed. The nature of the semicrystalline state in polymers and its influence on their physical properties will be dealt with in greater detail in a later chapter. 

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

As has been emphasized previously (IJ), the level of crystallinity is not the major determinant of the linewidth in the semicrystalline state. Rather the supermolecular structure or morphology is a major factor in governing the magnitude of the linewidth. Structural factors and crystallization conditions under which low density (branched) polyethylene forms... 

The fluidity of membranes primarily depends on their lipid composition and on temperature. At a specific transition temperature, membranes pass from a semicrystalline state to a more fluid state. The double bonds in the alkyl chains of unsaturated acyl residues in the membrane lipids disturb the semicrystalline state. The higher the proportion of unsaturated lipids present, therefore, the lower the transition temperature. The cholesterol content also influences membrane fluidity. While cholesterol increases the fluidity of semicrystalline, closely-packed membranes, it stabilizes fluid membranes that contain a high proportion of unsaturated lipids. 

NMR proton relaxation times for water in living tissues are found to be shorter than in ordinary water, and the lines correspondingly broader. Some workers have interpreted this fact as indicating that water in living cells may be in a semicrystalline state, rather than a liquid state the... 

Estimate the solubility and the heat of solution (sorption) of oxygen in poly(ethylene terephthalate) (PETP), both in the quenched amorphous glassy state and in the semicrystalline state (xc = 0.45). 

The curves are referred to as melting curves, because the region over which the absorbance increases reflects the collapse (or melting) of the highly organized, semicrystalline state of double-helical DNA. The temperature at which 50 percent melting has occurred is called the melting temperature, or Tm. 

The first effect implies that the difference in order between the semicrystalline state and the mesophase is smaller than the difference between the mesophase and the isotropic melt. This indicates that the mesophases involved are highly otdered and the high values of the enthalpies and entropies may also be taken as another indication supporting this assumption. The photomicrograph in Figure 6 clearly shows that the mesophase is highly ordered. The mosaic domain is characterized by a uniform... 

The second effect, the well-known odd-even dependence, may be explained in terms of different degrees of order for polymers having odd or even numbers of methylene units in the spacer. The overall order in the semicrystalline state and in the mesophase is larger in the even-numbered samples than in the odd-numbered. 

A variety of homopolypeptides were synthesized by the N-carboxy a-amino acid anhydride (NCA), activated ester, or azide methods 5 except for deuterium labelled samples and polyglycines. The physical state of these samples is in general a semicrystalline state. Poly(glycine) (MW = 5200) was purchased from Sigma Chemical Company. 

In many publications the electron density difference is addressed as Ap (r) = p (r) — (p (r))y. Exercise Compute the average electron density (p)y of a sample from pure poly(ethylene terephtha-late) (PET) with a mass density of 1.38 g/cm. The chemical formula of PET is C10H8O4. Because PET is most probably in the semicrystalline state, it makes sense to stress that the computed electron density is a volume average ()y. 

Another use of Raman spectroscopy for quantitative analysis is the determination of percent crystallinity in polymers. Both the frequency and intensity of peaks can shift on going from the amorphous to the semicrystalline state for polymers. The percent crystallinity can be calculated with the help of chemometrics software. 

Upon isothermal cooling at 25 C from the melt state(120°C) as shown in Figure 5(c), some recrystallization of the room-temperature-irradiated sample was observed to occur within the former persisting spheruiitic pattern. On the other hand, the unirradiated control sample underwent the usual transformation from an isotropic liquid to a spheruiitic semicrystalline state. In a striking contrast to the samples irradiated at a temperature below the melting point, the formation of the spheruiitic order was observed to be hindered for the case of 80°C-irradiated sample. Therefore, these microscopy results clearly emphasize the importance of the morphological state at the time of EB irradiation. 

In conclusion, the control of surface microstructure, including the size and distribution of crystalline and amorphous phases, is a promising innovative concept for the molecular design of excellent antithrombogenicity materials with semicrystalline states. 

Starch, a polysaccharide, is naturally found in a semicrystalline state and aqueous acids can be employed to hydrolyze the amorphous section with the consequent release of the crystalline sections resulting in individual monocrystalline nanoparticles. The particles obtained display platelets or granular form [1]. They consist of crystalline nanoplatelets about 6-8 nm thick with a length of 20-40 nm and a width of 15-30 nm. Piyada et al. 

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