Molecular weight distribution properties

Structure. The physical properties of LDPE depend on the molecular weight, the molecular weight distribution, as well as the frequency and distribution of long- and short-chain branching (2). 

Phenolic resin substantially increases open time and peel strength of the formulation (80). For example, higher methylol and methylene ether contents of the resin improves peel strength and elevated temperature resistance. Adhesive properties are also influenced by the molecular weight distribution of the phenoHc low molecular weight reduces adhesion (82). 

Fatty Acid Process. When free fatty acids are used instead of oil as the starting component, the alcoholysis step is avoided. AH of the ingredients can therefore be charged into the reactor to start a batch. The reactants are heated together, under agitation and an inert gas blanket, until the desired endpoint is reached. Alkyds prepared by the fatty acid process have narrower molecular weight distribution and give films with better dynamic mechanical properties (34). 

Model Networks. Constmction of model networks allows development of quantitative stmcture property relationships and provide the abiUty to test the accuracy of the theories of mbber elasticity (251—254). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link functionahty, and controlled molecular weight distribution of cross-linked chains. Sihcones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cure model network consists of an a, CO-polydimethylsiloxanediol, tetraethoxysilane (or alkyltrimethoxysilane), and a tin-cure catalyst (255). A typical addition-cure model is composed of a, ffl-vinylpolydimethylsiloxane, tetrakis(dimethylsiloxy)silane, and a platinum-cure catalyst (256—258). 

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

The equivalent weight distribution of natural petroleum sulfonates depends on the boiling range of the aromatic components in the feedstock, but generally consists of a broad continuum of molecular weight components (139). For many appHcations it is precisely this property of derived petroleum sulfonates that provides the unique properties, such as emulsification. Conversely, most oil-soluble synthetic sulfonates have much more limited components and molecular weight distribution. 

HDPE melts at about 135°C, is over 90% crystalline, and is quite linear, with more than 100 ethylene units per side chain. It is harder and more rigid than low density polyethylene and has a higher melting point, tensile strength, and heat-defiection temperature. The molecular weight distribution can be varied considerably with consequent changes in properties. Typically, polymers of high density polyethylene are more difficult to process than those of low density polyethylene. 

---