Poly , molecular shapes

For all three types of dendrimers described above, a flattened, disk-like conformation was observed for the higher generations. However, the molecular shape at the air-water interface is also intimately associated with the polarity, and hence the type of dendrimer used. In case of the polypropylene imine) and PAMAM dendrimers the hydrophilic cores interact with the sub-phase and hence these dendrimers assume an oblate shape for all generations. The poly(benzyl ether) dendrimers, on the other hand, are hydrophobic and want to minimize contact with the water surface. This property results in a conformational shape change from ellipsoidal, for the lower generations, to oblate for the higher generations [46]. 

Takafuji, M., Fukui, M., Ansarian, H.R., Derakhshan, M., Shundo, A., and Ihara, H., Conformational effect of silica-supported poly(octadecyl acrylate) on molecular-shape selectivity of polycyclic aromatic hydrocarbons in RP-HPLC, Anal. Sci., 20, 1681, 2004. 

Mannisto M, Vanderkerken S, Toncheva V et al (2002) Structure-activity relationships of poly(L-lysines) effects of pegylation and molecular shape on physicochemical and biological properties in gene delivery. J Control Release 83(1) 169—182... 

Under certain conditions, rather well-defined ring and honeycomb patterns can be formed by solution casting of thin films. Recently Nolte described the formation of isolated rings with diameters of several micrometers formed by stacked porphyrin molecules [287], Phase segregation or eruption of gas bubbles was assumed to be responsible for the peculiar assembly of the porphyrin wheels . However, the regular shape of the solute molecules was also considered to be of crucial importance, Another example is the honeycomb pattern of star shaped poly(styrene)-block-poly(-p-phenylene) films described by Francois [288], In this case, the peculiar molecular shape was emphasized to be of crucial importance besides the proper choice of solvent and evaporation conditions [289],... 

K.W. Chooi, A.I. Gray, L. Tetley, Y. Fan, I.F. Uchegbu, The molecular shape of poly(propylenimine) dendrimer amphiphiles has a profound effect on their self assembly, Langmuir 26 (4) (2010) 2301-2316. 

Diara, H. Goto, Y. Sakurai, T. Takafuji, M. Sagawa, T. Nagaoka, S. Enhanced molecular-shape selectivity for polyaromatic hydrocarbons through isotropic-to-crystalline phase transition of poly(octadecyl acrylate). Chem. Lett. 2001, 72,1252. 

The data in Fig. 4 refer to linear polyethylene samples with less broad distributions since they were obtained by fractionation of a commercial Marlex 50 type polyethylene into 10 fractions. The shape of the curves indicates that some of them would intersect, if extrapolated. This points to considerable residual poly-molecularity in the fractions so that identification of minima with critical points is not permitted. Nakajima et al. [35, 36] nevertheless analyzed the data assuming the identification to be allowed and, using the Shultz-Flory method [45], derived values for the critical temperatures for solutions of polyethylene of infinite molar mass in -alkanes. This specific temperature is known as the theta temperature, . Nakajima et al. s values, listed in Table 4, agree within 5 K with those... 

Fignie 5.28 Molecular shapes of poly(diethyl cw,cw-muco-nate) and the corresponding monomers in the crystal lattice [130]. The space group symmetry (P2i/c) between them is preserved during the photoinduced solid-state polymerization process. (See color insert.)... 

The general physical properties are as a rule independent of whether the particles are mono- or poly-molecular on the other hand they will often be found dependent to a high degree on the shape of the. particles, discussed in the previous paragraph ... 

Silicone polymers have a long history as stationary phases in gas-liquid (partition) chromatography (GC). Thus poly(dimethylsiloxane) and methylphenylsilicones provide the useful SE and OV series of phases. More recently, small-molecule LC phases and their blends with OV materials have been evaluated. The occurrence of ordering (e.g. nematic or smectic) in a stationary phase is itself beneficial but, in addition, different separation criteria seem to apply with LC phases. Whereas relative volatility of analyte components dominates separations on ordinary SE or OV columns, molecular shape becomes significant when LC phases are employed (e.g. in the resolution of the isomeric ben-zo[a]pyrene and benzo[e]pyrene). ... 

There are a number of polymers which in fact cannot be melt processed because of their high molecular weights. These include PTFE, very high molecular weight polyethylene and most grades of cast poly(methyl methacrylate). In such cases shaping in the rubbery phase is usually the best alternative. 

Another limitation is related to the fact that synthetic poly(amino acids) have rather unfavorable material properties. For instance, most synthetic poly (amino acids) derived from a single amino acid are insoluble, high-melting materials that cannot be processed into shaped objects by conventional fabrication techniques. The often undesirable tendency to absorb a significant amount of water when exposed to an aqueous environment is another common property of many poly (amino acids) (7). Finally, high molecular weight poly-(amino acids) are best prepared via N-carboxyanhydrides which are expensive to make. Hence poly(amino acids) are comparatively costly polymers, even if they are derived from inexpensive amino acids (8). 

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