Polydisperse blends

As a consequence it is obvious that polymer dispersity will have an influence on surface segregation. Smaller chains in the samples will migrate at the interfaces [62]. Tanaka et al. used scanning force microscopy in order to investigate the surface molecular motion of PS films. It was revealed that the surface was in a glass/rubber transition state at 293 K due to the surface segregation of the lower molecular weight chains of a polydisperse blend (compared with 373 K in the bulk) [63]. 

All the above results for polydisperse blends are consistent with a blob model first suggested by de Gennes. In the long chain, we group the monomers into subunits with a radius equal to the radius of the smaller chains (Fig. 1). The excluded volume Gab is the total volume occupied by these blobs. Each blob having Ra° monomers the mutual virial coefficient is... 

The effects of composition distribution on the morphology of PS-fc-P2VP diblock copolymers were investigated by Matsushita et al. [160]. They produced PS- -P2VP samples with various composition distributions but with constant average composition by blending. If the polydispersity indices of each block were lower than 1.7, the expected lamellar domains were detected (Fig. 49). 

Polydispersity of the molecular weight is not so well described by the DE approach, even qualitatively. In reptation theory the blend of two compatible homopolymers A and B of different molecular weights is given by... 

Dextran Blend 52,050 32,850 1.58 Blend (1 part T-70 and 1 part T-40) used as polydisperse calibratioi standard. 

The blend of T-70 and T-40 dextran materials was utilized as a polydisperse calibration standard for the linear calibration method and the T-40 dextran standard was used as a sample for evaluation. Concentrations of 0.15% W/V were injected for each dextran material chromatographed. 

Dextran polymers were used to evaluate the utility of the linear, polydisperse calibration method for water-soluble polymer characterization. A blend of T-40 and T-70 dextran standards was used as a polydisperse calibration standard. Table VIII displays the report from the linear calibration method using this standard. Nine Iterations of the search algorithm were required for convergence to the true and Mn values of the standard. As can be seen in the report, the elution volume profile of the standard contained 72 area/time slices upon which calibration calculations were based. The slice width was set at 10 seconds/siIce. Figure 5 shows a plot of the calibration curve generated from the linear calibration method utilizing the dextran standard,... 

Other important pitfalls lie again in the low selectivity of SEC, which does not allow identifying small amounts of the macromolecular admixtures that is the minor components of polymer blends. The bell-shaped chromatograms with a broad base and a slim upper part are often erroneously proclaimed to signalize the narrow molar mass distribution of sample. On the other hand, the accumulation peaks due to presence of macromolecules excluded from the packing pores (Section 16.8.1) are interpreted as the sign of sample bimodality. The absolute detectors may also contribute to erroneous conclusions concerning sample polydispersity (Section 16.8.1). 

Morphological structures and properties of a series of poly(ethyl acrylate)/clay nanocomposites prepared by the two distinctively different techniques of in situ ATRP and solution blending were studied by Datta et al. [79]. Tailor-made PNCs with predictable molecular weights and narrow polydispersity indices were prepared at different clay loadings. WAXD and studies revealed that the in situ approach is the better option because it provided an exfoliated morphology. By contrast, conventional solution blending led only to interlayer expansion of the clay gallery. 

Studies on both binary blends and whole polymers of moderate polydispersity have established that t]0 at constant friction factor Co depends on the weight-average molecular weight alone (75,76,181, 209) ... 

Processability can be improved by blending mPE with LDPE, which exhibits an excellent processability. Efforts are being made to increase the polydispersity of mPE by use of two or... 

Gel permeation chromatography of polystyrene separated from blend samples indicates that high-molecular-weight polymer is formed. Polystyrene removed from a 43 wt % polystyrene/HDPE specimen exhibited Mw = 628 K with polydispersity index, PDI = 2.7. The corresponding polystyrene formed outside this specimen exhibited Mw = 48 K with PDI = 3.1. 

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