Diblock molecular weight effect

Retsos et al. [55, 56] made an attempt to provide a semiquantitative analysis of the interfacial activity of block copolymers at the polymer-polymer interface the emphasis was on understanding the nonmonotonic dependence of the interfacial tension reduction on diblock molecular weight as well as the effects of macromo-lecular architecture and composition when graft copolymers were utilized as additives. The attempt was based on a modification of the analysis of Leibler [75], where the possibility of micellar formation was also taken into account. The thermodynamic equilibrium under consideration was, thus, that between copolymer chains adsorbed at the interface, chains homogeneously distributed in the bulk homopolymers, and chains at micelles formed within the homopolymer phases. 

The effect of blending an AB diblock copolymer with an A-type homopolymer has been the subject of many research activities. On a theoretical basis the subject was investigated e.g. by Whitmore and Noolandi [172] and Mat-sen [173]. If a diblock exhibiting lamellae morphology is blended with a homopolymer of high molecular weight, macrophase separation between the... 

Similar observations were made on the higher molecular weight PEP-PEE diblock, which undergoes an ODT at 473 K. Figure 6.11 presents the initial slopes obtained at 473 and 533 K. The RPA predicted collective mode was calculated on the basis of the effective Flory-Huggins parameter and structural... 

The mechanical and thermal properties of a range of poly(ethylene)/ poly(ethylene-propylene) (PE/PEP) copolymers have been examined by Mohajer et al. (1982). They studied the effect of variation of composition and copolymer architecture on the polymer properties by synthesizing a range of PE-PEP-PE and PEP-PE-PEP triblocks and PE-PF.P diblocks with high molecular weights (M > 200 kg mol (.The crystallinity, density and melting enthalpy for all copolymers were found to be linearly dependent on the PE content, indicating microphase separation of PE and rubbery PEP in the solid state. The... 

The spatial distribution of homopolymer within lamellar domains formed by the same PS-PB-PS triblock blended with homopolymer has been determined (Kimishima et al. 1995). Blends with PS or PAMS with molecular weights lower than that of the PS in the diblock were investigated using SAXS and TEM. It was found that both PS and PAMS were solubilized in the PS microdomains however, the PAMS was not uniformly distributed, tending to be localized at the domain centres. This localization was diminished on increasing the temperature. In contrast, the PS homopolymer was uniformly distributed. The localization of the PAMS was ascribed to repulsive interactions between PS chains and the PAMS homopolymer, this effect decreasing with increasing temperature. 

Roe and co-workers (Nojima et al. 1990 Rigby and Roe 1984,1986 Roe 1986) have used SAXS to characterize micelles formed by PS-PB diblocks at low concentrations in blends with low-molecular-weight PB. Micellar dimensions and association numbers were determined for symmetric and asymmetric diblocks (Rigby and Roe 1984,1986). The effective hard sphere radius, the core radius and volume fraction of hard spheres were determined using the Percus-Yevick model (Rigby and Roe 1986). These results were compared (Roe 1986) to the predictions of the theory of Leibler et al (1983). The theory qualitatively reproduced the observed trend for the cmc to increase with temperature for blends containing a particular diblock. The cmc was found to decrease at a fixed temperature... 

Fig. 6.9 Schematic showing the effect of addition of low-molecular-weight homopolymer on block copolymer chain configuration (Hasegawa and Hashimoto 1996). (a) A symmetric diblock forms a lamellar phase, (b) On addition of homopolymer, swelling induced by solubilized homopolymer causes stretching of the corresponding block chain/and or contraction of the other block, resulting in a decrease in conformational entropy, (c) Alternatively, a curved interface is formed to attain a uniform packing density.

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