Mixtures enthalpic part

A similar series of experiments were carried out on mixtures of deuterated polybutadiene (PBD) and hydrogenous polyisoprene (PIH). The PBD had a content of 1,2-units of 12-28% and the PIH had a 3,4-unit content of 7-15%. For a fixed temperature, % was observed to decrease as the content of 1,2-units in PBD increased and to increase as the content of 3,4-units in the PIH increased. Although the change in % with microstructure is small ( 10" ), it was pointed out that the net interaction parameter is % multiplied by the degree of polymerisation, and hence small changes in % can still produce significant effects. Furthermore, changes in microstructure influence the en-tropic part of % (i.e. a in the empirical expression for y) more than the enthalpic part b). 

Although a modelling of a liquid polymer mixture on a lattice may first look rather artificial, it makes sense because it retains the important aspects of both the entropic and enthalpic part of A mix- In recent years, lattice models have gained a renewed importance as a concept which is suitable for computer simulations. Numerical investigations make it possible to check and assess the validity range of the Flory-Huggins treatment. In fact, limitations exist and, as analytical calculations are difficult, simulations are very helpful and important. We shall present one example in a later section. 

Although a modeling of a liquid pol3uner mixture on a lattice may at first look rather artificial, it makes sense because it retains the important aspects of both the entropic and enthalpic parts of Af/mix- In recent years, lattice... 

The enthalpic part of the FH parameters appears very much influenced by the isotope effect, e.g., whether the PB(l,2) or PB(l,4) is deuterated or not. If one considers a PB copolymer mixture of only protonated components, using the van der Waals interaction parameters in Table 3 and setting... 

Referring to the analysis of kinetic barriers that contribute to the differential rates, outlined above, it would appear that enantioselectivity may not reside in any single part of the kinetic scheme. In fact, analysis of the distribution of entropic and enthalpic contributions to the enantioselectivity of CaLB in mixtures of organic solvents showed that the gradual replacement of one organic solvent with the other may lead to a switch from one state to another [145]. More information on the response of individual steps in the kinetic scheme to a change of solvent is needed to arrive at a conclusive answer. 

Fig. 15 Enthalpic and entropic parts of FH parameters for the three dPB/PS blends versus pressure. In all cases a decrease with pressure is found. The lowest numbers are found for the PB copolymer mixtures which saturates above 100 MPa...

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