Average comonomer composition

Looking again at Figure 2.7, it will be remembered that resins such as LLDPE made with heterogeneous Ziegler-Natta catalysts and tandem reactors have a rather complex relation between MWD and average comonomer composition. Because this relation has such an important impact on the mechanical properties of polyolefins, an FTIR detector is often added to the GPC (GPC/RI-FTIR) to measure comonomer fraction as a function of molecular weight. 

Stockmayer s bivariate distribution is an analytical expression describing the weight distribution of the kinetic chain length and the chemical composition for linear binary copolymers. This distribution quantifies the deviation from the average comonomer composition and molecular weight due to the statistical nature of copolymerization. 

In the case of SEC/FTIR characterization, a method for recovering the full distribution from the average comonomer composition was proposed based on a polymerization kinetics model and the deconvolution of the MWD [43]. It was found, however, that both SEC/FTIR and P-Tref/light scattering techniques generally lose part of the information on the bivariate distribution of molecular weight and comonomer composition [44]. 

Use of Average Comonomer Composition Parameters Benoit provided a useful alteruative representation of in terms of mass weighted moments of the copolymer composition distribution. Generalizing his method to N comonomers, F. is expressed as ... 

The transition enthalpies of the s- and p-fractions obtained from the feed with a comonomer molar ratio of 85 15 were equal to 6 and 7 J/g, respectively, i.e. the values are very close. This, therefore, can be indicative of almost the same average length of oligoNVCl blocks. Moreover, as we have already stressed, the fractions also had virtually the same final comonomer composition. However, since the solution properties of these fractions are drastically different, one can draw the conclusion that this is apparently due to a specific distribution of hydrophobic and hydrophilic residues along the polymer chains. In turn, because of all the properties that are exhibited by the s-fraction, this fraction can be considered to be a protein-like copolymer [27]. 

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. 

Many analytical techniques are available for the determination of the primary molecular structure of a block copolymer and its average chemical composition. Among them spectroscopic techniques used for low molecular weight compounds are the most powerful and most widely employed. NMR can provide both qualitative and quantitative information with respect to comonomer composition and stereochemical configuration of polymeric molecules (136,137). The IR technique provides information on chemical, structural, and conformational aspects of polymeric chains (138). Because of the inherently high sensitivity of UV spectroscopy the technique is often utilized for the identification and quantitative determination of comonomers in block copolymers. 

Fig. 8 Broadening of the chemical composition distribution due to increasing average comonomer content as predicted by Stockmayer s distribution, Eq. 7...

Both the average instantaneons composition and cumnlative mole fraction of each comonomer along the polymer chains computed by the authors from continuous knowledge of the comonomer concentrations allow for a rich variety of comonomer and initiator schemes to be tried in order to control the types of profiles formed. 

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