Homopolymer conformation effect

Isomerism is a term covering different molecular conformation effects. We will not speak here again on the type of information that can be gained on the alternating or block structure of copolymers, as was explained above for an ethylene-tetrafluoroethylene copolymer. For homopolymers, we will distinguish various categories ... 

Chauve et al. [253] utilized the same technique to examine the reinforcing effects of cellulose whiskers in EVA copolymer nanocomposites. It was shown that larger energy is needed to separate polar EVA copolymers from cellulose than for the nonpolar ethylene homopolymer. The elastomeric properties in the presence of spherical nanoparticles were studied by Sen et al. [254] utilizing Monte Carlo simulations on polypropylene matrix. They found that the presence of the nanofillers, due to their effect on chain conformation, significantly affected the elastomeric properties of nanocomposites. 

Conformational energies are calculated for chain segments in poly(vlnyl bromide) (PVB) homopolymer and the copolymers of vinyl bromide (VBS and ethylene (E), PEVB. Semlempirical potential functions are used to account for the nonbonded van der Waals and electrostatic Interactions. RIS models are developed for PVB and PEVB from the calculated conformational energies. Dimensions and dipole moments are calculated for PVB and PEVB using their RIS models, where the effects of stereosequence and comonomer sequence are explicitly considered. It is concluded from the calculated dimensions and dipole moments that the dipole moments are most sensitive to the microstructure of PVB homopolymers and PEVB copolymers and may provide an experimental means for their structural characterization. 

Fig. 6.9 Schematic showing the effect of addition of low-molecular-weight homopolymer on block copolymer chain configuration (Hasegawa and Hashimoto 1996). (a) A symmetric diblock forms a lamellar phase, (b) On addition of homopolymer, swelling induced by solubilized homopolymer causes stretching of the corresponding block chain/and or contraction of the other block, resulting in a decrease in conformational entropy, (c) Alternatively, a curved interface is formed to attain a uniform packing density.

A block copolymer is expected to be superior to a graft copolymer in stabilizing dispersions of one polymer in another because there will be fewer conformational restraints to the penetration of each segment type into the homopolymer with which it is compatible. Similarly, diblock copolymers might be more effective than triblock copolymers, for the same reason, although tri- and multiblock copolymers may confer other advantages on the blend because of the different mechanical properties of these copolymers. 

Homopolymers Simple polymer chains that can adsorb on the surface and entropic repnlsion stabilizes the particles. Branched polymers usually adopt more complicated conformations which results in a further entropy increase and more effective stabilization. 

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