Chain architecture ratio

Polymers can be made that contain more than one type of repeat unit. For example, the R group on the asymmetric carbon in Figure 1.56 could be chlorine in some of the monomer units and fluorine in the rest. Such polymers are called copolymers. The ratio of the two types of monomers can vary from 0 to 1, and there can be more than two types of monomers in a copolymer. The presence of more than one type of repeat unit opens up many possibilities for variation in the structure of the polymer, or chain architecture as it is sometimes called. We will not describe the myriad of possible variations and the important consequences in terms of polymer physical properties, but here merely categorize copolymers in some broad, structural terms. 

This method applies for whatever polymer concentration. In practice, it is preferable to use high concentrations in order to increase the signal-to-noise ratio and therefore minimize counting time. However, it can also be applied to semidilute or even dilute solutions where Zimm plots are useful. It also applies not only to linear polymers but also to any form of chain architecture and to deuterated/protonated mixtures in non-solvent matrices such as polymer blends or polymer networks provided that changing the deuterated fraction does not change the homogeneous nature of the mixture (i.e., no change to the chain conformations and interactions). 

Table 6.4 presents a typical formulation for emulsion polymerization. The principal monomers present low water solubihty and the ratio between hard (leading to high Tg polymers) and soft (leading to low Tg polymers) monomers is chosen to achieve the Tg required for the application. Minor functional monomers provide some special characteristics, such as improved latex stability and adhesion. Crosslinking agents and chain transfer agents (CTAs) are used to control the chain architecture and the MWD of the polymer. 

The main molecular characteristics of the 4-arm comb star polystyrene are collected in Table 21.2. The Mw Ri(app)ls ratio, which is related to the volume contraction due to branching is 0.19 for the star comb versus 0.29 for the linear comb of same structure. These very low values agree with the highly branched chain architecture of the comb polymers on which is superposed the volume contraction related to the comb star architecture. The radius of gyration (/ g) and hydrodynamic radius (/ h) of the star comb on the one hand are very close to each other as are and of the corresponding one-branch linear comb polymer. They are equal to 45 nm, in THE at 25 °C for the star comb M = 9 000 000 g/mol) and 25 nm for the similar linear comb (M = 2 700 000 g/mol) in the same conditions. 

One of the most sophisticated architectures prepared by this method has the general formula (AB)2(BA)2, where A and B are polystyrene and polyisoprene chains, respectively two arms are linked by the styrene block, the other two arms are linked via the isoprene block to the central core. Moreover, the ratio of inner and outer blocks has been kept constant [65],... 

The nematic phase (N) is the least ordered, and hence the most fluid liquid crystal phase. The order in this type of LC phases is based on a rigid and anisometric (in most cases rod-shaped or disc-shaped) molecular architecture. Such molecules tend to minimize the excluded volume between them, and this leads to long range orientational order. For rod-like molecules the ratio between molecular length and its broadness determines the stability of the nematic phase with respect to the isotropic liquid state and the stability rises with increase of this ratio. In most cases the rigid cores are combined with flexible chains, typically alkyl chains, which hinder crystallization and in this way retain fluidity despite of the onset of order. 

A nonideal network may be obtained as in the previous case but using different nonstoichiometric molar ratios or arresting the polymerization at different conversions, to modify the structure. In these cases, the presence of a sol fraction and dangling chains will introduce an additional plasticization effect, surimposed on the new architecture (Vallo et al., 1993). 

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