High molar mass samples

Similar considerations apply to best volume flow rates for samples of different molar mass. For high molar mass samples, flow rates should be reduced to avoid shearing the macromolecule in the column. Moreover, a reduced flow rate is necessary because the diffusion coefficients of large molecules will get pretty small. This means that the macromolecule will pass by a pore in the packing material without having the time to enter it, if the linear flow rate is too high. 

Graziano and Mackley [36] reported a very extensive study on oscillatory sheared, nematic, longitudinal PLCs based on a series of random 50/50 copolyesters of chlorophenylene terephthalate and bis-phenoxyethane carboxylate. They observed a variety of textures in the polarized microscope and also noticed a pronounced difference between low and high molar mass samples. 

Very high molar mass samples, typically with molar mass of 10 g mol or greater, form so-called random lamellar structures. The entanglement effect is so extensive that the crystalline mass becomes very low and regular lamellar stacking is absent in these samples. 

Melt-grown crystals have the same shape as the solution-grown single crystals in most respects. They are lamella-shaped with a thickness-to-width ratio of 0.01-0.001. A typical feature of melt-crystallized samples, shown in Fig. 7.27, is the crystal stack. The lamellae of the stack are almost parallel and the amorphous component is located in the space between the crystal lamellae. The number of lamellae in a stack varies considerably between different samples. Linear polyethylenes of low or intermediate molar mass form stacks with a great many lamellae, whereas branched and/or high molar mass samples exhibit stacks containing only a few crystal lamellae (Fig. 7.27). 

The total enthalpy method yields crystallinity values in agreement with X-ray and density estimates. Significant deviation was found by Mandelkem (1965) for very high molar mass samples of linear polyethylene. The differences were attributed to the presence of a third, intermediate phase, with mobility in between that of the crystalline and the amorphous components. A problematic case is samples with low thermal stability. Polyvinylchloride is one example. An additional problem with this polymer is also its low crystallinity (5-10%). 

Except for very high molar mass samples, viscosities of dilute polymer solutions are Newtonian, and the relations between the polymer molar mass and its viscosity are established in this context. 

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