Short Chain-Branched Macromolecules

Kleintjens, L. A. Koningsveld, R. Gordon, M., "Liquid-Liquid Phase Separation in Multicomponent Polymer Systems. 18. Effect of Short-Chain Branching," Macromolecules, 13, 303 (1980). 

The determination of the viscosity law in GPC-viscometry is even more important for branched polymers. Branches reduce the sizes of a macromolecule, including its hydrodynamic volume H. This size reduction is reflected by the changes in the shape and position of the viscosity law plot for a branched polymer. Short-chain branches usually do not change the linearity and slope of the Mark-Houwink plot and just decrease the value of parameter K, whereas the long-chain branches cause bending of the corresponding plot. 

McCord, E.F. Shaw, W.H., Jr. Hutchinson, R.A. Short-chain branching structures in ethylene copolymers prepared by high-pressure free-radical polymerization an NMR analysis. Macromolecules 1997, 30, 246-256. 

LCT (originally developed for di-block copolymers) was found to be particularly useful to explain miscibility of polyolefin blends where the two resins differ in the type and size of short chain branching. The stractural units of a polymer with two carbons in the main chain can be written as PE = (CH,-CH,), PP = [CH,-CH (CHj)], poly-2-butene (P2B) = [CH (CH3)-CH (CHj)], PIB = [CH -C (CHj) ]jj, poly(4,4-dimethyl 1-butene) (PDMB) = [CH -CH (C Hg)], etc. Three structural parameters (ratio of end to interior groups) have been used to distinguish PO structure r, p, and q. Their values for the model macromolecules discussed above are listed in Table 2.7. 

HAN Han, S.J., Lohse, D.J., Radosz, M., and Sperling, L.H., Short chain branching effect on the cloud-point pressures of ethylene copolymers in subcritical and supercritical TproTpans, Macromolecules, 31, 2533, 1998. 

Strongly short-chain-branched polymers can often be spectroscopically or chemically determined. However, with long-chain branching, the number of branch points is very small compared to the number of chain-links. Consequently, the existence and extent of long chain branching is mostly estimated from the dimensions of the macromolecule in solution. A long-chain-branched macromolecule, namely, exhibits smaller molecular dimensions than the unbranched macromolecule with the same molar mass, as can be clearly seen in Figure 2-4. 

Baig, C., Alexiadis, O., and Mavrantzas, V. G. 2010. Advanced Monte Carlo algorithm for the atomistic simulation of short- and long-chain branched polymers Implementation for model h-shaped, aSaaS multiarm (pom-pom), and short-chain branched polyethylene melts. Macromolecules, 43(2) 986-1002. 

Ramos, J., Peristeras, L. D., and Theodorou, D. N., 2007. Monte Carlo simulation of short chain branched polyolefins in the molten state. Macromolecules,... 

FIGURE 2.3 Schematic representation of linear polymers, short- and long-chain branching (brush) polymers, star-type polymers, and cross-linked polymer chains. The dendrimer structure on the right is an extreme form of highly short-chain branched macromolecules. 

The SCBDI = wt% of macromolecules having a comonomer content within 50 % of the median total molar comonomer content, calculated from TREF (temperature rising elution fractionation) data. The elastic, substantially linear C2-Cg copolymer has 0.01 < LCB/IOOOC < 3, M /Mn = 1.5-2.5, 2 < SCB (CH3/IOOOC) < 30), and short-chain branch distribution index SCBDI > 50 %. The homogeneously branched copolymer may he produced as described in C. T. Elston (DuPont Canada Ltd.) patent. Films produced from the bimodal MWD new copolymers show good impact and tensile properties... 

K.F. Freed, J. Dudowicz, Influence of short chain branching rai the miscibility of binary polymer blends application to polyolefin mixtures. Macromolecules 29(2), 625-636 (1996)... 

T. Usami, Y. Gotoh, S. Takayama, Generation mechanism of short-chain branching distribution in linear low-density polyethylenes. Macromolecules 19(11), 2722-2726 (1986)... 

Topology. The skeletal or overall configurational structure of a macromolecule is called its topological structure. Polymer molecules can be linear, contain some level of short chain branches and can have multiple or occasional long chain-branches. Some level of the latter, which can be stars, combs or networks, shown in Fig. 1, establishes what the topology will be. 

Krishnaswamy RK, Yang Q, Femandez-BaUester L, Komfield JA Effect of the distribution of short-chain branches on crystallization kinetics and mechanical properties of high-density polyethylene. Macromolecules 41(5) 1693—1704, 2008. 

A brief review is presented on techniques for the analysis of polyolefins and additives in polyolefins. Techniques considered include high-temperature GPC combined with FTIR spectroscopy for the analysis of chemical composition as a function of molar mass, crystallisation fiactionation for the analysis of short-chain branching in LLDPE and of polyolefin blends and pyrolysis-gas chromatography-mass spectrometry for the determination of additives, such as antioxidants, in polyolefins. 13 refs. (3rd Annual UNESCO School lUPAC Conference on Macromolecules and Materials Science, Stellenbosch, South Africa, 2000)... 

In addition to linear polyphosphazenes with one type of side group, as shown in 3.1, other molecular architectures have also been assembled. These include polyphosphazenes in which two or more different side groups, R1 and R2, are arrayed along the chain in random, regular, or block distributions (3.2-3.4). Other species exist with short phosphazene branches linked to phosphorus atoms in the main chain (3.5,3.6). Also available are macromolecules in which carbon or sulfur replace some of the phosphorus atoms in... 

Short- and long-chain branching can be introduced into polymers. Extreme forms are cross-linked structures, networks, and gels or even perfect dendrimers (Fig. 2.3). These macromolecules have then a branched... 

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