Polymer characterization weight-average

Polymer Characterization. Number average molecular weights (M ) of polymers were determined using an automatic membrane osmometer (Shell Development Design) in toluene solution at 35.5 C. Membranes (Sartorious MembranfiIter, SM 11539) were cautiously conditioned from ethanol into methyl ethyl ketone and finally toluene. Weight... 

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

Polymer solutions are often characterized by their high viscosities compared to solutions of nonpolymeric solutes at similar mass concentrations. This is due to the mechanical entanglements formed between polymer chains. In fact, where entanglements dominate flow, the (zero-shear) viscosity of polymer melts and solutions varies with the 3.4 power of weight-average molecular weight. 

In the past three decades, industrial polymerization research and development aimed at controlling average polymer properties such as molecular weight averages, melt flow index and copolymer composition. These properties were modeled using either first principle models or empirical models represented by differential equations or statistical model equations. However, recent advances in polymerization chemistry, polymerization catalysis, polymer characterization techniques, and computational tools are making the molecular level design and control of polymer microstructure a reality. 

With this chapter we have tried to provide an overview of experimental techniques for determining molecular weight averages and molecular weight distributions. All methods discussed here have their specific advantages and weaknesses and differ very much in their complexity. The choice of the best method strongly depends on polymer properties, the information needed for a particular purpose, and on the available resources. Yet another aspect in the analytical characterization of polymers may be speed. 

It should be noted that the product of a step polymerization is a mixture of polymer molecules of different molecular weights. The molecular weight distribution is characterized by the number-average and weight-average degrees of polymerization, X and Xw> respectively, defined by... 

A given polymer is characterized by its number-average molecular weight (Mn), together with its weight-average molecular weight (Mw), which can both be obtained by analytical techniques. 

The concentrated solution viscosity measurement yields the weight-average degree of association of active chain ends rather than the more conventional number-average (mole fraction) value. However, the calculation of the equilibrium constant for association, K, can be accomplished if Mw and the heterogeneity index of the polymer sample are known. The latter parameter can be determined via postpolymerization characterization. 

Recently, Siu et al. [139] studied the effect of comonomer composition on the formation of the mesoglobular phase of amphiphilic copolymer chains in dilute solutions. The copolymer used was made of monomers, N,N-diethylacrylamide (DEA) and N,N-dimethylacrylamide (DMA). like PNI-PAM, PDEA is also a thermally sensitive polymer with a similar LCST, but PDMA remains water-soluble in the temperature range (< 60 °C) studied. At room temperature, copolymers made of DMA and DEA are hydrophilic, but become amphiphilic at temperatures higher than 32 °C. Before the association study, each P(DEA-co-DMA) copolymer was characterized by laser light scattering to determine its weight average molar mass (Mw) and its chain size ( Rg) and (R )). The copolymer solutions (6.0 x 10 A g/mL) were clarified with a 0.45 xm Millipore Millex-LCR filter to remove dust before the LLS measurement. 

Similar to the above discussed processes, the photoinitiation of the copolymerization between cyclohexene and AN in the presence of pyromellitic dianhydride or phthalic anhydride is based on the sequence of PET and proton transfer [30]. Consequently, the copolymerization rate with the former acceptor (Rp = 1.6x 10-4moll-1 s-1) is higher than this of the latter (Rp = 1.4x 10-4moll-1 s-1 [AN] = 4.5 mol l-1, 30°C). Interestingly, the average-molar weights of the alternating copolymers lie between 1000-2000 g mol- The reason for this very small value is possibly an efficient primary radical termination due to the formation of the two radicals IV and V see Eq. (4). Exact polymer characterization data are not available, so far. 

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