Narrow molecular weight fractions

Nonionic surfactants, including EO-PO block copolymers, may be readily separated from anionic surfactants by a simple batch ion exchange method [21] analytical separation of EO-PO copolymers from other nonionic surfactants is possible by thin-layer chromatography (TLC) [22,23] and paper chromatography [24], and EO-PO copolymers may themselves be separated into narrow molecular weight fractions on a preparative scale by gel permeation chromatography (GPC) [25]. 

To illustrate the advantages of the technique, we apply it to narrow molecular weight fractions of low molecular weight polystyrene. 

Figure 2. Measured fracture energy (Glc) vs. molecular weight (distribution peak) for narrow molecular weight fractions of...

In general, synthetic polymers exhibit a wide molecular weight distribution. Their separation Into narrower molecular weight fractions is a fairly difficult task. Distillation Is not an effective separation technique for these materials because of their low vapor pressures. The solubility of different fractions of a parent polymer In conventional liquid solvents are normally too high which makes the liquid extraction a nonselective technique for these materials. Fractionation of polymers by using supercritical fluids presents unique advantages and has attracted the attention of several investigators in the past few years (1-5). 

As the next step in the evaluation of supercritical fluid fractionation of the polysiloxane polymers, fractions are being converted into block copolymers for the purpose of determining the properties that derive from the narrow molecular weight fractions. 

Hocquet, S. et al.. Lamellar and crystalline layer thickness of single crystals of narrow molecular weight fractions of linear polyethylene, Macromolecules 35 (13), 5025-5033, 2002. 

Chu and co-workers (IJO, 111) have studied the photon correlation function of light scattered by solutions of narrow molecular weight fractions of PS (Mn = 397,000) in cyclohexane near the critical demixing temperature Tc- Their data fit Equations 82 and 83 with y = 1.26 0.08, y = 0.77. The correlation lengths were in the range of 100-250A, and Vs = 0.62, vr = 0.58. For (T — Tc) < 0.1°, deviations from the critical exponent concept were discovered which may result from macro-molecular polydispersity or configurational fluctuations 111)-... 

The following data are for a narrow molecular weight fraction of poly(methyl methacrylate) in acetone... 

Garvey, M. J.. Tadros, T. F. and Vincent B., A comparison of the volume occupied by macromolecules in the ad.sorbed state and in bulk solution. Adsorption of narrow molecular weight fractions of poly(vinyl alcohol) at the polystyrene/water interface, J. Colloid Interface Sci., 49, 57-68 (1974). 

Most of the measurements on diffusivities of compounds in the liquid phase, in catalyst pores, have been made with small molecules. Recently Chantong and Massoth [26] measured diffusivities of non-metallic porphyrin molecules. Baltus and Anderson [27] made diffusivity measurements using narrow molecular weight fractions of a petroleum residuum. In their work tetrahydrofuran asphaltenes were separated into five fractions by gel permeation chromatography. The polystyrene equivalent average molecular weights they reported were 3000, 6000, 12000, 24000, and 48000. 

Figure 38 Effect of crystallization temperature on the thickness of lamellae of a narrow molecular weight fraction M = 70,000) of high density polyethylene crystallized from the melt. (From Ref. 109.)...

Broad range of products of varying MW and EO/PO ratios preparative separation into narrow molecular weight fractions Sephadex LH-20, 2.2 x 115 cm 50 50 2-Pr0H/H20 or 80 20 2-PrOH/ -butanol RI 284... 

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