Uncross-Linked Polymers of High Molecular Weight

UNCROSS-LINKED POLYMERS OF HIGH MOLECULAR WEIGHT 

The extensive stress relaxation experiments of Tobolsky on uncross-linked polymers of high molecular weight similarly showed two stages of relaxation (like those in Examples III and IV of Fig. 2-2) with a period in the time scale where the stress relaxed very slowly, leading to the concept of an entangled network structure. 

The essential feature of these observations is that there are two sets of relaxation or retardation times with very different magnitudes, represented by the two maxima 

Normalized logarithmic recoverable creep compliance plot for Rouse theory (I) and for polystyrene of high molecular weight (II) (Example III of Fig. 2-1). 

A quite different type of observation which leads also to the concept of an entanglement network is the dependence of viscosity on molecular weight in undiluted polymers or at constant concentration in concentrated solutions, as advanced by Bueche. This is illustrated in Fig. 10-10 for fractions of polystyrene. At low molecular weights, rjo increases only slightly more rapidly than directly proportional to Af, and its magnitude is actually predicted by the Rouse theory, in accordance with the principle of Bueche. Thus, from equations 4 and 6, rjo is given by 

---

The condition of swelling equilibrium can be calculated by means of two theoretical approaches. It is assumed that the chemical potential of mixing for a network is the same as the chemical potential of mixing an uncross-linked polymer of high molecular weight and of the same structure as the network polymer. The mixing term can be described by means of the Flory-Huggins (FH) equation. The calculation of the elastic deformation term is based on the rubber elasticity theory (RET). 

C. UNCROSS-LINKED POLYMERS OF HIGH MOLECULAR WEIGHT... 

It has already been pointed out in Chapter 13, Section Al, that maxima in the loss compliance and the retardation spectrum are characteristic of network structures as predicted from the Rouse theory, suitably modified, in Fig. 10-7. Such maxima appear in moderately cross-linked polymers as well as in uncross-linked polymers of high molecular weight, and their shapes are remarkably similar. In Fig. 14-3, J" is compared for styrene-butadiene copolymer without cross-links and cross-linked to a value of Gg = 7.3 X 10 dynes/cm, characteristic of a well-vulcanized soft rubber e.g., curve VI of Figs. 2-1 to 2-8). The curves are both close in shape to that predicted by the Rouse-Mooney theory for a most probable distribution of network strands, i.e., curve D of Fig. 10-7, as already evident from Fig. 13-1. 

E.g., uncross-linked polymers of high molecular weight and their concentrated solutions in the terminal zone. 

Give several possible ways of telling the difference between an uncross-linked polymer of very high molecular weight and one that has a very low degree of cross-linking. 

Viscoelasticity is perhaps the most ubiquitous characteristic of high molecular weight polymers at temperatures above Tg. Here we consider the implications of coupling rubber elasticity and mesomorphism via synthesizing covalent networks from conventional elastomers (siloxanes, isoprenes, etc.) and typical MLC mesogenic cores such networks are thermotropic PLCs. At low levels of crosslink densities in the PLC network, there is no appreciable change in the transition temperatures (Tg, Td, etc.) from those of the uncross-linked PLC (and its ancestral MLC) [68]. Below Td, rather modest mechanical deformations (extension ratio A < 1.5) may convert an initial random and disclination-ridden texture into a... 

In practically all cases, composite functions are utilized, obtained by reduction of data from experiments at various temperatures as described in the preceding chapter. Most of the data refer to uncross-linked polymers of quite high molecular weight, but since moderate cross-linking affects the transition zone only to a minor degree, a few cross-linked systems are included. Polymers of low molecular weight are discussed in Section D. 

What will radiation during a test do to the stress relaxation of an elastomeric material if the radiation brings about chain scission Compare a cross-linked polymer with a high-molecular-weight uncross-linked one. 

MAJOR APPLICATIONS POE is a new family of ethylene a-olefin copolymers produced using metallocene catalyst. The uncross-linked polymers referred to in this chapter are known to have only moderate elastomeric recovery properties (up to 96%). These copolymers are characterized by a narrow molecular weight distribution (MWD) (M /Mn = 2-2.5) and homogeneous comonomer distribution.The control of chain microstructure by the use of metallocene catalyst makes it possible to produce poly(a-olefin) copolymers with considerably lower density, which has not been possible before using the conventional Ziegler-Natta catalyst. Some of the highly branched ethylene copolymers presented in the entry on Polyethylene, metallocene linear low-density, in this handbook may be closely related. 

---