MOLECULAR WEIGHT AND BRANCHING

Traditional methods for measuring molecular weight date from the dawn of modem physical chemistry in the 19th century. These relied on so-called colligative property measurements (see box opposite), such as boiling point elevation and freezing 

43 When you synthesize a block copolymer do you end up with pure product or a mixture of block copolymer and homopolymers How would you know  

Colligative means depending on the collection and in dilute solution properties such as  

Vapor Pressure Freezing Point Boiling Point Osmotic Pressure 

TABLE 12-1 Major Methods of Measuring Molecular Weight 

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Table 2. Effects of Molecular Weight and Branching on Mechanical Properties of Polyethylenes...

Inhalation of aerosols or heated vapors may result in irritation of the nose, throat, and upper respiratory system. Lower molecular weight and branched-chain amines are more volatile and can cause irritation if inhaled. Volatile amines are easily recognized by their unpleasant, fishy odor. 

The method outlined above for characterizing branched polymers will hereafter be referred to as the molecular weight and branching distribution (MWBD) method. In the following sections, its application to the long chain branching in polyvinyl acetate and high pressure low density polyethylene will be demonstrated. 

Under conditions of low radical initiation, Graessely (21) has shown that the following set of equations describes the molecular weight and branching development in the bulk polymerization of vinyl acetate ... 

The whole polymer Mfj, Mj, B, and X values obtained using the MWBD method and e = 0.75 for these three LDPE resins are shown in Table IV. The molecular weights and branching parameters are in agreement with expectations b ed on the levels of conve sioji. With increasing conversion M remains constant whereas Bj, and X increase. 

Figure 10 On-line determination of molecular weight and branching by light scattering. (A) Molecular weight as a function of elution volume. (B) Dependence of branching index, gM, on molecular weight. (From Yu, L. P. and Rollings, J. E., /. Appl. Polym. Sci., 33, 1909, 1987. With permission.)...

Mourey, T. H. and Coll, H., Size exclusion chromatography with two-angle laser light-scattering (SEC-TALLS) of high-molecular-weight and branched polymers, /. Appl. Polym. Sci., 56, 65, 1995. 

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

The general structure of linear low density polyethylene is shown in Fig. 18.2 c). Linear low density resins are copolymers of ethylene and 1-alkenes principally 1-butene, 1-hexene, and 1-octene. Comonomer levels range from approximately 2 to 8 mole %. This family of polyethylene is widely known as LLDPE. Linear low density polyethylenes are polydisperse with regard to molecular weight and branch distribution. 

Moreover, alcohol functionalities have been introduced into the polynor-bornene (PNB) backbone by copolymerization of norbornene with a few percent of 5-acetate norbornene and subsequent acetate reduction. After transformation of the pendant hydroxyl functions into diethyl aluminum alkoxides, sCL has been ring opening polymerized (Scheme 31). Owing to the controlled/ liv-ing character of both polymerization processes the isolated poly(NB- -CL) graft copolymers were characterized by well-defined composition, controlled molecular weight and branching density, and narrow MWD (PDI=1.2-1.4) [92]. 

Other analysis methods dependent on multiple detectors can be implemented using this automated system. Two methods under development are the use of a continuous viscometer detector with a refractive index detector to yield absolute molecular weight and branching, utilizing the universal calibration curve concept (4), and the use of a UV or IR detector with the refractive index detector to measure compositional distribution as a function of molecular weight. 

Polyethylenes, PE, have been characterized by SEC since the mid-sixties and frequent problems with polystyrene gel columns have been reported ( ). The low density PE, LDPE, because of complexity of the molecular weight and branching distributions,... 

L = TPP low molecular weight and branched L = TOTP high molecular weight and linear... 

Recent Trends in the Determination of Molecular Weights and Branching in Elastomers... 

Iedema, P.D. and Hoefsloot, H.C.J. (2001) Synthesis of branched polymer architectures from molecular weight and branching distributions for radical polymerisation with long-chain branching, accounting for topology-controlled random scission. Macromol. Theor. Simul., 10, 855. 

Bahary, W. S. (1973). Recent trends in the determination of molecular weights and branching in elastomers. In Polymer Molecular Weight Methods, Gould, R. F. (Ed.), pp. 85-97. Am. Chem. Soc., Washington, DC. 

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