Monodisperse molecules

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

Usually the adjustment of properties is carried out by using mixtures of high molecular weight polydispersed telechelic prepolymers and simple low molecular weight molecules. Recently, we produced a review about the prepolymers mentioned above [8] which we will not describe in this paper. This work concerns the synthesis of low molecular weight monodispersed molecules, their properties and eventually their applications. 

Vogtle s early work on cascade molecules was followed by the synthesis of highly branched polyols in the laboratory of Newcome (Fig. 3) [62], Since the first polyol structures reminded him of small trees, they were named arboroles (lat. arbor - tree). However, it has been the Greek-derived term dendrimer (8ev5pov - tree, pspcx - particle), which has become the most popular term for highly branched monodisperse molecules with fractal character, as first used in a patent... 

In Fig. 5 we report the advancing and receding contact angles of ionic-nonionic mixtures at supramicellar concentration. A similar behaviour is observed for monodisperse molecules such as CnDMPO and CiEj but not for Triton X-100. Again this can be related to the presence of smaller size molecules in this commercial surfactant. This progressive adsorption phenomenon is clearly evidenced by Fig. 6a and 6b where the variation of L-S area fraction, fl, at each cycle is reported for SDS, CTAB and their mixtures with non-ionic species. 

Molecules composed of hydrophilic as well as of hydrophobic parts are called amphiphilic, soaps are simple examples. In strongly polar (e.g., water) or apolar solvents (e.g., alkanes) their solubility as monodisperse molecules is small. The contact of the respective lyophobic moieties to the solvent can be avoided by accumulation at the interfaces of the solution, see Figure 14.1. The interfacial properties of solutions are determined distinctly by dissolved amphiphilic matter which for that reason is termed surfactant (surface active agent). Typical examples of ionic (cationic or anionic), zwitter-ionic, and nonionic surfactants of low molecular weight are sketched in Figure 14.2. 

In this experiment, the LLDPEs were simulated by hydrogenated (or deuterated) polybutadienes, because they can be prepared as monodisperse molecules (with a ratio of weight- and number-averaged molecular weights, Mw/M < 1.1) and a homogenous branch distribution within the chains. Such studies are therefore unaffected by polydispersity effects, either in the branch content or molecular... 

Assuming monodisperse molecules with radius of gyration Rg and molecular weight M, one has for the scattered light intensity I ... 

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

While this example shows an extreme difference in the two molecular weight averages, the other extreme is where all of the molecules have the same size, i.e. they are said to be monodisperse. In tbis case the two averages will have the same value. 

This equation is based on experience with liquid chromatography of low molecular weight samples displaying single peaks. Its application for the GPC of polymers, however, contains a disadvantage, as it mixes two inseparable properties the retention difference for the separation and the peak width for the contrary effect of band broadening. Such a procedure is acceptable if both effects are accessible for an experimental examination. For the GPC experiment, we do not possess polymer standards, consisting of molecules that are truly monodisperse. Therefore, we cannot determine the real peak width necessary for a reliable and reproducible peak resolution R,. This equation then is not qualified for a sufficient characterization of a GPC column. 

According to the concepts, given in the paper [7], a significant difference between the values of yield stress of equiconcentrated dispersions of mono- and polydisperse polymers and the effect of molecular weight of monodisperse polymers on the value of yield stress is connected with the specific adsorption on the surface of filler particles of shorter molecules, so that for polydisperse polymers (irrespective of their average molecular weight) this is the layer of the same molecules. At the same time, upon a transition to a number of monodisperse polymers, properties of the adsorption layer become different. 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

The sequential growth and branching involved in the preparation of dendrimers had been considered by Flory many years before they were actually prepared. Flory developed a sound understanding of the kind of processes that would occur in the self-polymerization of a molecule of the type ABj most of which have been shown to be correct by the relatively recent experimental studies. In particular, the existence of a limit to growth was predicted. This limit has become known as the starburst limit, and is the reason for the highly monodisperse nature of fully developed dendrimers. 

The chains share the same chemical environment and grow at the same average rate. If the final chain length is large, all the molecules will have more or less the same size and the resulting pol5Tner will be approximately monodisperse. Furthermore, if a second monomer is added to the system after the first has been depleted, a block copol5Tner can be formed. 

In the transmission electron microscopy (TEM) images, the starch nanoplatelets (SNPs) are believed to aggregate as a result of hydrogen bond interactions due to the surface hydroxyl groups [13] (Fig. lA). Blocking these interactions by relatively large molecular weight molecules obviously improves the individualization of the nanoparticles. The acetylated starch and cellulose nanoparticles (SAcNPs and CelAcNPs) appeared more individualized and monodispersed than their unmodified counterparts with a size of about 50 nm (Fig. IB C). 

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