Chain unattached

Any real network must contain terminal chains bound at one end to a cross-linkage and terminated at the other by the end ( free end O of a primary molecule. One of these is indicated by chain AB in Fig. 92, a. Terminal chains, unlike the internal chains discussed above, are subject to no permanent restraint by deformation their configurations may be temporarily altered during the deformation process, but rearrangements proceeding from the unattached chain end will in time re-... 

For the single labelled arm the limiting behavior of Q(Q)/Q3 at small and large Q(z 1 and z > 1) agrees with that of the full star. Thereby, the crossover of Q from Q3- to the Q2-dependence is much more gentle than that observed for unattached linear PDMS chains [128] or in the case of the 4-arm PIP stars [150] (see Fig. 52). 

T divided by the viscosity of the solvent r s. For n-octane this number is 837 K/cP at T = 323 K. The results of the fitting process are all below this theoretical value. This is not surprising, since even in the case of dilute solutions of unattached linear chains, the theoretical values are never reached (see Sect. 5.1.2). In addition the experimental T/r s values differ considerably for the different labelling conditions and the different partial structure factors. Nevertheless, it is interesting to note that T/r s for the fully labelled stars is within experimental error the arithmetic mean of the corresponding core and shell values. 

Fig. 27. Molar mass dependence of [rj] for a fractionated comb macromolecule. The fractionation was made with a SEC/LALLS/VISC set-up. The comb macromolecule consists oi a polyimidazole backbone prepared by free radical polymerization. The imidazol side groups acted in a melt with phenylglycidylether and phthalic anhydride as multifunctional initiator for the anionic growth of polyester chains. The straight lines correspond to the behavior of unattached polyester chains and the comb polymers at low and high molar masses respectively [136]...

This section on the contraction factor may be concluded with an example of a comb macromolecule [136]. Due to the route of preparing this comb, unattached side chains also occurred in the system. Figure 27 shows the result of the molar mass dependence of [rj] which was obtained from a SEC/LALLS/VISC fractionation. One observes at low molar masses a straight line with an exponent of 11 =0 JO that coincides with the exponents of linear side chains. It follows a... 

In ideal random crosslinking polymerization or crosslinking of existing chains, the reactivity of a group is not influenced by the state of other groups all free functionalities, whether attached or unattached to the tree, are assumed to be of the same reactivity. For example, the molecular weight distribution in a branched polymer does not depend on the ratio of rate constants for formation and scission of bonds, but only on the extent of reaction. Combinatorial statistics can be applied in this case, but use of the p.g.f. simplifies the mathematics considerably. 

Block copolymers consist of chemically distinct polymer chains that are tethered together to form a single macromolecule. If the individual blocks are immiscible when they are unattached, phase separation will also normally occur in the case of the copolymer, with morphologies that depend on the relative composition of the separate block species, and their manner of attachment (diblocks, triblocks, stars, etc.). This is a result of the physical connection of the blocks, which prevents them from separating over distances greater than the contour lengths of the respective blocks. The result is a microphase separation with adjacent domains that are richer in either of the chemical species. 

This mean-field effect is also clearly demonstrated by the observed orientation of free (unattached) chains chemically identical to network chains, which are introduced during or after network formation and free to diffuse in it (Figure 15.6) [20, 38, 39]. According... 

Their hydrophobic/hydrophilic content seems to be just right for applications in cancer and gene therapies. Such nanospheres are prepared by dispersing the methylene chloride solution of the copolymer in water and allowing the solvent to evaporate [38]. By attaching biotin to the free hydroxy groups and complexa-tion with avidin, cell-specific delivery may be attained.NMR studies of such systems [39] revealed that the flexibility and mobility of the thus attached PEG chains is similar to that of the unattached PEG molecules dissolved in water. Re-... 

These features are typical of surfaces on which the polymer is able to adsorb with a weak surface density Z. We have not been able up to now to quantify the surface density of adsorbed chains, because of sensitivity problems the total area of the drop of liquid deposited on the prism is too small to allow the use of the X ray reflectivity technique to measure the thickness of the adsorbed layer remaining on the surface after washing all the unattached chains with a good... 

Path length fluctuations (breathing or tube leakage) will compete with pure reptation in the case of unattached linear chains, although the effect will die cwt fta- suffidentty long chains. Suppose all chains occupy N steps initially. For pure reptation the fraction of still surviving steps at position j, measured from the center of the initial path (j = o, 1, 2,—, N/2), is... 

Relaxation of dilute spedes (star or linear) in monodisperse matrices (star or linear) can also be worked out with Eq. 98, using F(t) appropriate to the dilute spedes and R(t) for the matrix. Preluninary results indicate that long-arm molecule relaxations can be controUed over wide ranges by the choice of chain length for a linear polymer matrix. On the other hand, relaxations of linear chains in a star matrix should be less affected by matrix chain structure. Their behavior should move from the homologous linear melt behavior when in the matrix is of the order of ra for the linear chains to behavior as unattached linear chains in a network when r , > ra. The latter prediction seems inconsistent with recent experimental lesults where linear chain relaxation rates were found to be the same in the homologous melt and in a star matrix with > Ta. This may indicate some fundamental problem with the equation suggeted to estimate (Eq. 85). 

Figure 1. Hemoprotein showing the large globin polypeptide chain, with one imidazole group (proximal) attached to Fe in heme and opposite it the unattached (distal) imidazole in the hydrophobic pocket.

Similarly, a variety of two, three, and multiblock olefin polymers [(P-E)jj, P-EP-P, P-E-P, E-EP] have been reported. However, in most cases the nature of Ziegler-Natta catalysis (low chain lifetimes, chain transfer from Ti to A1 or Zn, thermal dieout, different reactive sites, etc.) is such that the polymer produced is often a mixture of homopolymer, unattached copolymers, and two or three segment block polymers. It is clear that the larger the number of blocks desired, the more difficult it is to obtain a... 

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