Homopolymer, conformation

The efficiency of the copolymers, either block or graft, acting as the compatibilizer depends on the structure of the copolymers. One of the primary requirements to get maximum efficiency is that the copolymer should be located, preferentially at the blend interface (Figs. 2a, b, and c). There are three possible conformations, as shown in the figure. Many researchers [10-12] found that the actual conformation is neither fully extended nor flat (Fig. 2c). A portion of the copolymers penetrates into the corresponding homopolymer and the rest re-... 

Before analyzing in detail the conformational behaviour of y9-peptides, it is instructive to look back into the origins and the context of this discovery. The possi-bihty that a peptide chain consisting exclusively of y9-amino acid residues may adopt a defined secondary structure was raised in a long series of studies which began some 40 years ago, on y9-amino acid homopolymers (nylon-3 type polymers), such as poly(/9-alanine) 3 [14, 15], poly(y9-aminobutanoic acid) 4 [16-18], poly(a-dialkyl-/9-aminopropanoic acid) 5 ]19], poly(y9-L-aspartic acid) 6 ]20, 21], and poly-(a-alkyl-/9-L-aspartate) 7 [22-36] (Fig. 2.1). 

Fig. 2.1 Examples of / -amino acid homopolymers (nylon-3 type polymers) for which conformational studies have been reported...

The conformational preferences of mixed /9-peptides containing both /9 - and /9 -amino acid residues in their sequence differ markedly from that of the corresponding homopolymers consisting exclusively of /9 - or /9 -amino acid residues. Several types of mixed /9-peptides have been investigated including block peptides constructed with triads of /9 -amino acid residues and triads of /9 -amino acid residues (e.g. 93) [104,161], as well as alternating peptides of jf lff type (e.g., 72,... 

Relationships between dilute solution viscosity and MW have been determined for many hyperbranched systems and the Mark-Houwink constant typically varies between 0.5 and 0.2, depending on the DB. In contrast, the exponent is typically in the region of 0.6-0.8 for linear homopolymers in a good solvent with a random coil conformation. The contraction factors [84], g=< g >branched/ <-Rg >iinear. =[ l]branched/[ l]iinear. are another Way of cxprcssing the compact structure of branched polymers. Experimentally, g is computed from the intrinsic viscosity ratio at constant MW. The contraction factor can be expressed as the averaged value over the MWD or as a continuous fraction of MW. 

A235 with vespQct to A254 nearly conforms to that expected from homopolymer mixtures. 

A less obvious explanation is that the observed residual structure is not due to attractive interactions, but rather to repulsive ones. The steric repulsion between atoms forced to partially overlap is a dominant, if not the dominant, force in all of chemistry. These highly local interactions are known to be important in polymer conformations (Flory, 1969 Ramachandran and Sasisekharan, 1968). For homopolymers or simple alternating polymers, they can often be safely neglected by assuming they confer no net directionality to the chain. Polypeptide chains, however, are chiral and support specific sequences of 20 differently shaped... 

The PPII conformation, as suggested by its name, was originally observed in homopolymers of proline (Isemura et al., 1968 Tiffany and... 

Copolyesters of poly(3HB-co-3HV) have approximately the same degree of crystallinity as the homopolymer PHB and all copolymers show similar conformation characteristics as those observed for PHB [24,26,65]. They show a minimum in their melting point versus composition curve at a 3HV content of approximately 40 mol%. The apparent ability of the two different monomeric units to cocrystallize might result from the fact that the copolymers are prone to show isodimorphic behavior [21, 26, 66-70]. However, the considerable reduction of the heat of fusion upon 3HV inclusion, as reported by Bluhm et al. 

Unfortunately, data on the temperature-dependent solution behaviour of these fractions are not available to date, although it will be of considerable interest to compare, e.g., HS-DSC and NMR results for the bound and unbound fractions of poly(NiPAAm-co-NVIAz) over the temperature range characteristic of the conformational and phase transitions of NiPAAm homopolymers and copolymers. 

Its derivation implies a succession of two formal procedures. First, it is necessary to color the homopolymer globule units marking every z -th unit by color of/ with the probability waj(rl) which coincides with the ratio of concentration of units Ma, at point rl to the overall concentration of all units at this point. As a result of such a coloring, the joint distribution for configurations and conformations of proteinlike heteropolymers is obtained. Integration of this distribution over coordinates of all units results in the desired molecular-structure distribution (Eq. 23). 

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