Polyethylene dendrites

Figure 5.58 represents a polyethylene dendrite still floating in the solvent out of which it was grown (causing the reduced contrast). It reveals that dendrites do not grow flat, as the snow flakes of Fig. 5.1, but splay apart from a common center, the crystal nucleus. Only on settling on the microscope slide is the regular nature of the dendrite shown as seen in Fig. 5.56. Similar three-dimensional shapes are also found...

Fig. 18. Polyethylene dendrites grown from 0.05% solution in toluene at 70°C. They were observed under interference microscopy (135).

Figure 3.2 Polyethylene dendrite formed on cooling a 0.1% xylene solution. The large primary growth arms are along the crystallographic a direction (10 o clock) and b direction (1 o clock). Secondary growth arms are along b and a, respectively. There is faint evidence of tertiary arms growing from the secondary arms. Optical micrograph from GeU and Reneker [3] with permission from John WUey Sons, Inc.

Figure 3.27 Transmission electron micrograph of the edge of a polyethylene dendrite grown at 67°C from 0.01% xylene solution. Notice the channels or shts leading inward from reentrant comers. At two of these (A and arrow) giant screw dislocations have formed. Three other rather irregular growth spirals are evident. From Keith and Chen [43] with permission from Elsevier.

Wunderlich and Sullivan [53] investigated polyethylene dendrites formed below 80°C by cooling toluene solutions at a rate of ca. 5°C/min. The most dilute solutions gave dendrites with primary growth arms along a... 

Figure 3.36 Interference micrograph of the side view of a six-arm polyethylene dendrite similar to that in Figure 3.35, but suspended in a mounting medium to prevent collapse. The width of the micrograph is 155 pm. Note the pronounced splaying of the primary growth arms. The feathery features are secondary arms. From Wunderlich and Sullivan [53] with permission from John Wiley Sons, Inc.

Figure 13 Polyethylene dendrite crystallized from solution at low temperature...

Polystyrene/polyethylene oxide dendrimers were prepared by ATRP using tri- and tetra (bromomethyl) benzene as the initiators [207]. Each bromine end-group of the resulting stars was transformed first to two - OH groups and subsequently to potassium alcholate, as shown in Scheme 114. These - OK sites served to initiate the anionic polymerization of EO. The synthesized dendritic copolymers were found to display monomodal and narrow molecular weight distribution. 

If combs represent one extreme of the topological family of branched polymers, then another extreme is given by the case of dendritic polymers, which retain a branched structure at all timescales. The study of tree-like branched architectures is also motivated by the important commercial low density polyethylene (LDPE), which has remarkable rheological properties making it suitable for many processing operations [3]. 

The synthesis of the branched core of the lipid headgroups [24, 45] proceeds in the same manner as that of multiple antigenic peptides (MAPs) [50, 51 ] or polyethylene glycol-dendritic oligo-lysine block copolymers [52]. It starts from ornithine... 

Newkome et al. were the first to synthesise symmetrical, quater-directionaF cascade molecules with a carbon scaffold bearing 36 terminal carboxyl groups -all at an equal distance from the neopentyl core (Fig. 6.20a). The carboxyls were converted into the corresponding ammonium and tetramethylammonium car-boxylates. Synthesis of these dendritic unimolecular micelles with hydrophobic core and hydrophilic shell was accomplished up to the fourth generation by coupling of a dendritic hypercore (constructed from 4,4-bis(4 -hydroxyphenyl)-pentanol monomer) and PEG mesylate (PEG = polyethylene glycol). Dyes such... 

For polymer chemists it is interesting to know how well-known linear polymers can be linked with dendritic architectures and what the supramolecular consequences of this approach might be. Combination of dendrimers with linear polymers in hybrid linear-dendritic block copolymers has been employed to achieve particular self-assembly effects. Block copolymers with a linear polyethylene oxide block and dendritic polybenzylether block form large micellar structures in solution that depend on the size (i.e., the generation) of the dendritic block [10]. Amphiphilic block copolymers have been prepared by the combination of a linear, apolar polystyrene chain with a polar, hydrophilic poly(propylene imine) dendrimer [11] as well as PEO with Boc-substituted poly-a, -L-lysine dendrimers, respectively [12]. Such block copolymers form large spherical and cylindrical micelles in solution and have been described as superamphi-philes and hydra-amphiphiles , respectively. 

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