Polymer, branched Linear

Figure 24 (a) Classification of different units in a hyperbranched polymer, branched, linear,... 

Figure C2.1.2. Polymers witli linear and nonlinear chain architectures. The nonlinear polymers can have branched chains. Short chains of oligomers can be grafted to tire main chain. The chains may fonn a. stor-like stmcture. The chains can be cross-linked and fonn a network.

Silicone Resins. Sihcone resins are an unusual class of organosdoxane polymers. Unlike linear poly(siloxanes), the typical siUcone resin has a highly branched molecular stmcture. The most unique, and perhaps most usehil, characteristics of these materials are their solubiUty in organic solvents and apparent miscibility in other polymers, including siUcones. The incongmity between solubiUty and three-dimensional stmcture is caused by low molecular weight < 10, 000 g/mol) and broad polydispersivity of most sihcone resins. 

Both polymers are linear with a flexible chain backbone and are thus both thermoplastic. Both the structures shown Figure 19.4) are regular and since there is no question of tacticity arising both polymers are capable of crystallisation. In the case of both materials polymerisation conditions may lead to structures which slightly impede crystallisation with the polyethylenes this is due to a branching mechanism, whilst with the polyacetals this may be due to copolymerisation. 

Following their introduction in 1953, Ziegler-Natta catalysts revolutionized the field of polymer chemistry because of two advantages the resultant polymers are linear, with practically no chain branching, and they are stereochemical ly controllable. Isotactic, syndiotactic, and atactic forms can all be produced, depending on the catalyst system used. 

Recently, Brich and coworkers (40) reported the synthesis of lactide/glycolide polymers branched with different polyols. Polyvinyl-alcohol and dextran acetate were used to afford polymers exhibiting degradation profiles significantly different from that of linear poly-lactides. The biphasic release profile often observed with the linear polyesters was smoothened somewhat to a monophasic profile. Further, the overall degradation rate is accelerated. It was speculated that these polymers can potentially afford more uniform drug release kinetics. This potential has not yet been fully demonstrated. 

Fig. 12. Molecular weight dependences of the normalized chain relaxation time, tJTs, for linear polymers ( ), branched fractions ( ), and branched feed polymer (+). (Reproduced with permission from [88]. Copyright 2001 American Chemical Society.)...

This remarkably simple treatment suffers one serious deficiency the value of remains quantitatively undefined. More or less intuitively it has been suggested by various investigators that should increase as the root-mean-square end-to-end distance /for a linear chain, or, more generally, as the root-mean-square distance /s2 q beads from the center of any polymer molecule, linear or branched. Accepting this postulate unquestioningly, we should then have/o proportional to and [rj] proportional to These conclusions happen to... 

The hydrodynamic volume separation mechanism of SEC, along with the different molecular size/weight relationships of branched and linear polymers of identical chemical composition, can be exploited with the SEC/LALLS method to gain information about polymer branching. In the studies described in this paper both conventional SEC and SEC/LALLS are used to obtain data about branching in samples of poly(vinyl acetate) (PVA) and polychloro-prene (PCP). 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

Experimental and analytical studies over the past 25-30 years revealed that microgels are intramolecularly crosslinked macromolecules, which represent a new class of polymers besides linear and branched macromolecules and crosslinked polymers of macroscopic dimensions. In some ways microgels may be considered as a transition from molecules to larger polymer particles or macroscopic polymer materials. 

Other dilute solution properties depend also on LCB. For example, the second virial coefficient (A2) is reduced due to LCB. However, near the Flory 0 temperature, where A2 = 0 for linear polymers, branched polymers are observed to have apparent positive values of A2 [35]. This is now understood to be due to a more important contribution of the third virial coefficient near the 0 point in branched than in linear polymers. As a consequence, the experimental 0 temperature, defined as the temperature where A2 = 0 is lower in branched than in linear polymers [36, 37]. Branched polymers have also been found to have a wider miscibility range than linear polymers [38], As a consequence, high MW highly branched polymers will tend to coprecipitate with lower MW more lightly branched or linear polymers in solvent/non-solvent fractionation experiments. This makes fractionation according to the extent of branching less effective. 

Water solubility is determined by polymer structure (linear, branched, etc.), concentration and placement of charged species [ionomeric (cationic or anionic) or amphoteric (cationic and anioiuc)], hydrophihc/hydrophobic substituents, and hydrogen bonding, to name the more commonly encountered factors. In general, polymer water solubility requires polar functional... 

These polymers contain linear chains having some branches, e.g., low density polythene. These are depicted as follows ... 

Branching Parameter g from. SEC/LALLS. The effect of polymer branching upon the dilute solution configuration of polymers is conveniently expressed as the ratio of intrinsic viscosities of branched and linear polymers of the same chemical composition and molecular weight (35), i.e.. 

The polymers thus formed are very heat-resistant. Polyphenylsilsesquioxane, for example, starts to decompose above 600 °C. At 900 °C only loss of phenyl groups occurs, the silicon-oxygen framework of the macromolecule remaining intact . This sharply differs the above polymers from linear and branched polyphenylsilses-quioxanes. 

Blends of PC, ABS, and MMBS are useful to form articles with good impact and low gloss. The articles produced are useful as automotive components, bottles, and tool housings. A mixture of randomly branched carbonate polymers and linear carbonate polymers has been suggested (19). MMBS acts also as a melt strength enhancing agent. 

These heterogeneities, which can be called elementary , can be superimposed one on the other, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- and bifunctional, etc. In order to characterize in an ideal way a telechelic polymer with respect to its subsequent transformation, it is necessary to know a set of functions (fj(M), the molecular weight distributions within each heterogeneity type. Clearly, it is very difficult in a general case to solve this characterization problem. 

The concept of a unique hydrodynamic volume for all rodlike polymers was derived from examination of the Mark-Houwink constants, K and a, of the equation [rj ] = KMa. Macromolecules with values of a greater than unity are commonly accepted to be stiff or rigid rods. However, it was also found that such molecules (even for values of a less than unity) obey a relation illustrated by close concordance with the curve in Fig. lb (13) flexible, branched or otherwise irregular polymers, on the other hand, show dispersion around the upper part of the curve. The straight line curve in Fig. lb implies that the constants K and a are not independent parameters for the regular macromolecules to which they apply. Poly (a- and polyQJ-phenylethyl isocyanide) fall on this line the former has a value of a > 1 while the latter has a value a < 1 (14) both polymers give linear concentration dependence of reduced specific viscosity for fractionated samples... 

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