Homopolymers molecular weight dependence

In any application of a copolymer the rate of formation of the product, its molecular weight, and the uniformity of its composition during manufacture are also important considerations. While the composition of a copolymer depends only on the relative rates of the various propagation steps, the rate of formation and the molecular weight depend on the initiation and termination rates as well. We shall not discuss these points in any detail, but merely indicate that the situation parallels the presentation of these items for homopolymers as given in Chap. 6. The following can be shown ... 

Most of the data presented in this chapter were obtained by applying RPA to small angle neutron scattering profiles from homogeneous homopolymer blends. This approach was pioneered by Herkt-Maetzky and Schelten, Murray et al., and Hadziioannou and Stein [20-22]. The composition and molecular weight dependence of x thus obtained has been studied in some systems. While x is generally... 

In the tabulation, data are presented mainly for common homopolymers. When available, molecular weights are also given, but no major attempt has been made to present molecular weight dependencies. Additionally, a few illustrations of specific copolymers have been included as examples of copolymer behavior. 

Experimentally, the effect of end-chain association is evidenced by the lower shift of the LCST line and its inverted molecular-weight dependence. Experimental values of the LCST and maximum temperature (collapse transition) recorded in the endotherm by differential scanning calorimetry (DSC) for solutions of telechelic C18-PNIPAMs of various concentrations [37] are presented in Figure 10.13(b) for various molecular weight of polymers. The data of the homopolymer are also presented as reference data. 

For the disordered state, the segmental a-relaxation with relaxation r and the normal mode relaxation with relaxation time r show a pronounced dependence on the molecular weight of the PI block (see Figure 26). ° This behavior can be discussed in an analogous way to miscible blends. For the ordered state, the molecular weight dependence is similar to what can be expected for the corresponding PI homopolymer. ... 

Hu et al. [48] studied the addition of PS-h-PDMS diblock copolymer to the PS/PDMS blend. A maximum interfacial tension reduction of 82% was achieved at a critical concentration of 0.002% diblock added to the PDMS phase. At a fixed PS homopolymer molecular weight, the reduction in interfacial tension increases with increasing the molecular weight of PDMS homopolymer. Moreover, the degree of interfacial tension reduction was found to depend on the homopolymer the diblock is mixed with when the copolymer was mixed into the PS phase, the interfacial tension reduction was much less than that when the copolymer was blended into the PDMS phase. This behavior suggested that the polymer blend interface may act as a kinetic trap that limits the attainment of global equilibrium in these systems. 

In this complex morphology, the copolymer molecules form monolayers that divide space nearly equally, in which case they must be reasonably flexible. This constraint may, depending on the value of x between the A and B moieties, require the use of relatively low molecular weight copolymers [128]. Since the translational entropy of the homopolymer molecules is responsible for preventing monolayer attraction (and the onset of macrophase separation) and is predicted to vary inversely with homopolymer molecular weight, it again follows that a must be much less than unity for a BpE to be stabilized. With these... 

In methacrylic ester polymers, the glass-transition temperature, is influenced primarily by the nature of the alcohol group as can be seen in Table 1. Below the the polymers are hard, brittle, and glass-like above the they are relatively soft, flexible, and mbbery. At even higher temperatures, depending on molecular weight, they flow and are tacky. Table 1 also contains typical values for the density, solubiHty parameter, and refractive index for various methacrylic homopolymers. 

Crystallinity and Density. Crystallinity and density of HDPE resins are derivative parameters both depend primarily on the extent of short-chain branching in polymer chains and, to a lesser degree, on molecular weight. The density range for HDPE resins is between 0.960 and 0.941 g/cm. In spite of the fact that UHMWPE is a completely nonbranched ethylene homopolymer, due to its very high molecular weight, it crystallines poorly and has a density of 0.93 g/cm. ... 

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