Properties, molecular long-chain

Ortho ester linkages can also be used to improve biodegradation properties in long chain ethoxylates or block copolymers. It has been shown that a conventional PEG-PPG copolymer with a molecular weight of 2200 biodegrades to only 3% in 28 days. However, if an equivalent molecule is built up from PEG 350 and PPG 400, connected by ortho ester links, it will reach 62% biodegradation within 28 days and thus be classified as readily biodegradable [64]. 

The motion of polymers in concentrated solution and bulk is of major theoretical and practical concern. For example, the strong dependence of zero-shear viscosity on molecular weight (approximately the 3.4 power) and the marked decrease of viscosity 1) with shear rate y not only bespeak some of the unusual properties of long-chain molecules but also are of essential importance in virtually every processing operation. Yet the reasons for these unusual behaviors have become clear only recently. The reptation con-... 

The brief review of the basic crystallite structure has centered on the nature of the lamellar habit. There are several points that should be emphasized. The molecular basis for the formation of lamellae can be developed in a natural way, based on the conformational properties of long chains. It is not necessary to invoke any inherent propensity of chains to fold or to arbitrarily draw upon monomeric nucleation theory. The structure of the equilibrium interphase has been quantitatively described. Although the number of chains that return to the crystallite from which they emanate is relatively large, the number that return in adjacent positions is small. These equilibrium considerations will be tempered in general by kinetic factors and by the melt structure at high molecular weights. The amount of adjacent reentry will be further reduced while the residual melt structure will enhance the value of the interfacial free energy. 

Comonomers can be used to create a variety of polymer stmctures that can impart desirable properties. For example, even higher molecular weight PPS polymers can be produced by the copolymerization of a tri- or tetrafunctional comonomer (18). The resultant polymer molecules can have long-chain branching, which can be used to tailor the rheological response of the polymer to the appHcation. 

The presence of long chain branches in low density polyethylene (LDPE) accounts for the difference in properties e.g. higher melt strength, greater toughness for the same average molecular weight) between LDPE and linear low density polyethylene (LLDPE, made by coordination polymerization). 

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