The Mark-Houwink equation:[a] = K -G t relates the intrinsic viscosity [ q ] of a polymer to its viscosity average molecular weight m. through the empirical constants K and a. mv is the most easily measured molecular weight average, if the constants for the particular polymer-solvent system are known. These constants are also needed for universal calibration in size exclusion chromatography (SEC)'-3). A range of other solution properties can be calculated readily when the appropriate Mark-Houwink constants are available4).The standard procedure for measuring the coefficients K and a requires the use of fractionated polymer samples, whose molecular weights have been measured by some independent method, such as light scattering or membrane osmometry. A log-log plot of corresponding values of intrinsic viscosity and molecular weight yields K and a, for the molecular range of the particular fractions. This procedure comprises fractionation, molecular weight analyses and intrinsic viscosity measurements and is evidently unattractive if alternative solutions can be found.Several alternative methods have been recommended, based on the SEC behaviour of whole polymers'-'). The procedures that have already been published appear to be reliable and efficient. Other variations are undoubtedly being studied in laboratories around the world.This communication describes a particularly simple procedure for measuring Mark-Houwink constants using SEC along with a low angle laser light scattering detector (LALLS). The method is illustrated by application to polypropylene and polyethylene. These are difficult polymers to fractionate and handle, because true solutions are obtained only at relatively high temperatures after special dissolution procedures8lo).The method described relies on the SEC fractionating the polymer such that the contents of the detector cell at any instant can be assumed to be monodisperse. It is necessary that the polymer constitution be independent of molecular weight. Erroneous results may be obtained, if the polymer varies in long chain branching or copolymer composition across the molecular weight distribution. Makromol. Chem., Rapid Commun. 6, No. 4, April 1985 0173-2803/85/$01 .OO

No abstract

Measurement of the molecular weight distributions of copolymers by size exclusion chromatography (SEC) presents problems because the elution volume of any species may depend on its composition as well as its molecular weight. Also, the response of the usual concentration detectors may also be influenced by the copolymer composition as well as its concentration. These problems arise when the copolymer composition may vary with molecular size. Conventional SEC techniques are suitable for copolymers with invariant compositions. This article describes and illustrates a method for measuring molecular weight distributions of copolymers. In many cases, the variation of copolymer composition with molecular weight can also be determined. The technique uses three detectors: (a) an evaporative detector (ED) to measure the concentration, Δc, of the eluting species; (b) a differential refractive index detector (DRI) to measure the refractive index difference, Δn, between the solution and solvent at any given elution volume; and (c) a low‐angle laser light scattering (LALLS) detector that measures the corresponding molecular weight of the eluting solutes. This latter measurement is possible because the appropriate values of Δn/Δc are available from the outputs of the other two detectors. For LALLS measurements of molecular weight all the species in the detector cell at any instant must have the same composition or, at least not have Δn/Δc that varies with composition. The method is illustrated with data from ethylene‐propylene and ethylene‐propylene‐diene copolymers.

SynopsisThe highest temperature at which molecular weight distributions can be characterized by current techniques is 145°C. Supermolecular aggregates may exist at this temperature in polypropylene solutions in trichlorobenzene and other solvents. Dissolution procedures at higher temperatures are ineffective in this case because of the limited thermal stability of polypropylene. Aggregate-free solutions can be prepared, however, by controlling storage times at 145°C in mixtures with added stabilizers. Low angle laser light scattering measurements can be used to determine when true solutions have been produced. This occurs when measured second virial coefficients agree with values predicteccfor theM, measured in the light scab tering experiment. GPCLALLS measurements of M, and M, provide similar information about the effects of storage time on dissolution of aggregates and polymer degradation.

SynopsisMolecular weights of National Bureau of Standards SRM 1476 polyethylene have been reported by six laboratories. The measured values are in remarkably good agreement and all show t h a t g w from SECLALLS analyses is significantly lower than the same average determined by LALLS on the whole polymer itself. This is shown to be due to the presence of high molecular weight species which become too diluted on passage through the SEC columns to be observed in the LALLS detector.The resulting error in ?@, , and higher averages may vary from slight to very serious, depending on the molecular weight distribution of the particular polyethylene. A procedure is described to detect the presence of such high molecular weight species.