Chain stiffness physical properties

Few sets of K,a parameters were given in references 17 and 22 K and a are parameters which may depends on the fine structure i.e. rhamnose content, DE... when they play on the stiffness of the chain. Nevertheless, it seems that the viscometry must be used in carefully defined conditions to avoid aggregation which often surestimates the viscosity in that conditions, [ti] is related to the physical properties of the solution (tickening properties) but not directly to My. 

Physical properties of polymers, including solubility, are related to the strength of covalent bonds, stiffness of the segments in the polymer backbone, amount of crystallinity or amorphousness, and intermolecular forces between the polymer chains. The strength of the intermolecular forces is directly related to the CED, which is the molar energy of vaporization per unit volume. Since intermolecular attractions of solvent and solute must be overcome when a solute (here the polymer) dissolves, CED values may be used to predict solubility. 

Stretching denotes a monoaxial or biaxial mechanical stress of a molded article close to the glass transition temperature. This leads to a controlled orientation of the molecular chains in the direction of stretching and thus to a substantial change in some physical properties. Fibers and foils made of synthetic polymers gain their optimal properties only by this mechanical post-treatment. Stability, stiffness, and dimensional stability of fibers, for example, increase nearly proportionally with the stretch ratio, whereas stretchability decreases. In practice, the stretch ratio is between 1 2 and 1 6, depending on the polymer material and the desired properties. 

The influence of temperature and strain rate can be well represented by Eyring s law physical aging leads to an increase of the yield stress and a decrease of ductility the yield stress increases with hydrostatic pressure, and decreases with plasticization effect. Furthermore, it has been demonstrated that constant strain rate. Structure-property relationships display similar trends e.g., chain stiffness through a Tg increase and yielding is favored by the existence of mechanically active relaxations due to local molecular motions (fi relaxation). 

PHB (or poly-3-hydroxybutyrate (P(3HB))) is the most common type of PHA produced and is an example of a short chain length homopolymer produced by A. eutropbus. PHB has poor physical properties for commercial use, as it is stiff, brittle and hard to process. This has led to an increased interest to produce heteropolymers with improved qualities. 

The best-known physically robust method for calculating the conformational properties of polymer chains is Rory s rotational isomeric state (RIS) theory. RIS has been applied to many polymers over several decades. See Honeycutt [12] for a concise recent review. However, there are technical difficulties preventing the routine and easy application of RIS in a reliable manner to polymers with complex repeat unit structures, and especially to polymers containing rings along the chain backbone. As techniques for the atomistic simulation of polymers have evolved, the calculation of conformational properties by atomistic simulations has become an attractive and increasingly feasible alternative. The RIS Metropolis Monte Carlo method of Honeycutt [13] (see Bicerano et al [14,15] for some applications) enables the direct estimation of Coo, lp and Rg via atomistic simulations. It also calculates a value for [r ] indirectly, as a "derived" property, in terms of the properties which it estimates directly. These calculated values are useful as semi-quantitative predictors of the actual [rj] of a polymer, subject to the limitation that they only take the effects of intrinsic chain stiffness into account but neglect the possible (and often relatively secondary) effects of the polymer-solvent interactions. 

Gomez, C. et al.. Physical and structural properties of barley (1-3), (l-4)-P-D-glucan. Part II. Viscosity, chain stiffness and macromolecular dimensions, Carbohydr. Polym., 32,17, 1997. 

Up to this point we have confined ourselves to ideally flexible chains. Thus, the theories developed on the models of such chains (for example, the spring-bead chain) should no longer be adequate for polymers whose chemical stmcture suggests considerable stiffness of the chain backbone. Many cheiin models may be used to formulate a theory of stiff or semi-flexible polymers in solution, but the most frequently adopted is the wormlike chain mentioned in Section 1.3 of Chapter 1 it is sometimes called the KP chain. This physical model was introduced long ago by Kratky and Porod [1] to represent cellulosic polymers. However, significant progress in the study of its dilute solution properties, static and dynamic, has occurred in the last two decades. 

However the constant effort of polyimide suppliers Is to provide materials which will flow during fabrication of parts below 400 F, but which will retain structural stiffness in use of that part at 550 F. The polymer chain stiffness which affords 550 F physicals wrecks havoc with 350-400 F processing. Even low molecular weight prepolymers which can build during cure to higher molecular weight crossllnked structures often do not flow below 400°F unless a species is chosen which sacrifices 550°F thermo-physical properties. 

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