Molecular Weight Distribution MWD

Monitoring the molecular weight distribution or its averages from a batch or continuous polymer reactor in real time would be desirable. Monitoring and feedback control of polymerizations can provide fimdamental benefits for improved quality. 

Traditionally, gel permeation chromatography (GPC) or size-exclusion chromatography (SEC) has been used to determine MWD [8, 13]. In GPC/SEC, polymer solutions are injected into one or more columns in series, packed with porous particles. The packing has small pores and during elution the polymer molecules may 

This can be used to calibrate the GPC/SEC, which allows the evaluation of average molecular weights and molecular weight distributions. 

For on-line purposes, the viscosity measures previously mentioned (see Section 12.2.7) have been used as a proxy for molecular weight averages in on-line control. Some vendors are commercializing more rapid GPC/SEC instruments for on-line control, with certain instruments already available. 

Probability that a given group has reacted = fractional extent of reaction (p). 

In a polymer consisting of x monomer residues, the number of ester linkages = (x - 1) 

Therefore, the probability that the molecule in question is composed of exactly X units = P) = fh (5.44) 

Here Nx is the number of molecules which are x-mers and N is the total number of molecules at the extent of reaction p. However, N = Na(l-p), which is the same as Eq. (5.43) with /av = 2. 

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The term HDPE embraces a large variety of products differing predominandy in molecular weight, molecular weight distribution (MWD), and crystallinity. 

The large number of commodity and specialty resias collectively known as LLPDE are in fact made up of various resias, each different from the other in the type and content of a-olefin in the copolymer, compositional and branching uniformity, crystallinity and density, and molecular weight and molecular weight distribution (MWD). 

The width of molecular weight distribution (MWD) is usually represented by the ratio of the weight—average and the number—average molecular weights, MJM. In iadustry, MWD is often represented by the value of the melt flow ratio (MER), which is calculated as a ratio of two melt indexes measured at two melt pressures that differ by a factor of 10. Most commodity-grade LLDPE resias have a narrow MWD, with the MJM ratios of 2.5—4.5 and MER values in the 20—35 range. However, LLDPE resias produced with chromium oxide-based catalysts have a broad MWD, with M.Jof 10—35 and MER of 80-200. 

For commercial grades of unfractionated PVP prepared by similar means (presumed to exhibit similar molecular weight distribution (MWD) and degree of branching), the following regression formula can be employed (71) ... 

The bulk viscosity control parameter for CSM, as with other elastomers, is molecular weight M and molecular-weight distribution (MWD). Mooney viscosity for CSM is determined by selection of the polyethylene precursor. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

The main problem of determination of molecular weight distribution (MWD) of dextrans (polysachaiides which ai e used as active substances for infusion medicines) is low robustness of the existing method. It means that obtained results are strongly dependent on controlled and uncontrolled pai ameters of chromatographic system standai d substances for calibration loading on columns etc. It has been shoved on practical examples. 

The primary molecular parameters affecting the processing and ultimate properties of PEs are type, content, and distribution of chain branching, molecular weight (MW), and molecular weight distribution (MWD). 

As there are many different plastics, a number of techniques for defining and quantifying their characteristics exist. As an example molecular weight distribution (MWD) is an indication of the relative proportions of molecules of different weights and lengths. In turn MWD relates to processing characteristics that directly relate to product performances (Chapter 8). 

The molecular weights and molecular weight distributions (MWD) of phenolic oligomers have been evaluated using gel permeation chromatography (GPC),23,24 NMR spectroscopy,25 vapor pressure osmometry (VPO),26 intrinsic viscosity,27 and more recently matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).28... 

Our ultimate objective is to produce automatically with laboratory-scale reactors polymers with pre-defined molecular characteristics in reasonable amounts for test purposes. Whatever control is exercised over the chemistry of a polymerization to introduce novel structural features into polymer chains, the final molecular weight distribution (MWD) of the product is always of importance hence attention has been given to... 

Mathematical models of the reaction system were developed which enabled prediction of the molecular weight distribution (MWD). Direct and indirect methods were used, but only distributions obtained from moments are described here. Due to the stiffness of the model equations an improved numerical integrator was developed, in order to solve the equations in a reasonable time scale. 

The molecular weight distribution (MWD) is of vital importance for polymers of all types. It determines the ease of manufacture, the ease of fabrication, and the end-use properties of the polymer. A proper kinetic description of a polymerization requires determination of the molecular weight distribution of the polymer in addition to the usual concepts of conversion and selectivity. 

In general, the use of temperature programming to achieve only a reduction in batch time is not always practical. Besides being difficult, if not impossible, to cany out on a large scale, it can seriouly affect the quality of the polylmer produced. For example, wide ranges in polymerzation temperature lead to broad molecular weight distributions (MWD) which may be undesirable. 

As early as 1952, Flory [5, 6] pointed out that the polycondensation of AB -type monomers will result in soluble highly branched polymers and he calculated the molecular weight distribution (MWD) and its averages using a statistical derivation. Ill-defined branched polycondensates were reported even earlier [7,8]. In 1972, Baker et al. reported the polycondensation of polyhydrox-ymonocarboxylic acids, (OH)nR-COOH, where n is an integer from two to six [ 9]. In 1982, Kricheldorf et al. [ 10] pubhshed the cocondensation of AB and AB2 monomers to form branched polyesters. However, only after Kim and Webster published the synthesis of pure hyperbranched polyarylenes from an AB2 monomer in 1988 [11-13], this class of polymers became a topic of intensive research by many groups. A multitude of hyperbranched polymers synthesized via polycondensation of AB2 monomers have been reported, and many reviews have been published [1,2,14-16]. 

Size exclusion chromatography (SEC) separates molecules of a polymer sample on the basis of hydrodynamic volume. When the chromatograph is equipped only with a concentration-sensitive detector, i.e. conventional SEC, a molecular weight distribution (MWD) can be obtained from the chromatogram only through use of a calibration function relating molecular weight and elution volume V (2). 

In this study, four Styragel columns were utilized one column had a nominal porosity rating of 10, two colvtmns of 10, and the fourth column of 10 A. The refractometer was maintained at 37°C. A 5 ml syphon was used to monitor a solvent flow rate of 1 ml/min. The instrviment was run at the highest sensitivity setting because the refractive index difference between our solvent and polymer was only moderate and because a number of samples analyzed had a broad molecular weight distribution (MWD). 

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