Single macromolecules

It has been outlined by several authors that the single macromolecule may be irreversibly bound because of the large number of weakly interacting segments. The first papers on the construction of polymer-coated silica adsorbents involved the physical adsorption of water-soluble polymers. Polyethylene oxides [28, 29] and poly-/V-vinylpyrrolidone [30] are examples of the stationary phases of this type. 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

Because the single macromolecules in the considered structures have a same periodical conformation, for a length of the chains which in the ideal case tends to infinity, the difference between the entropies of an ideally ordered structure and a disordered one is small. 

Another important sub-case, of disorder in macromolecular crystals, corresponds to the statistical occurrence of two specific orientations only, at well defined positions in a 3-D lattice, of a group of macromolecules or of each single macromolecule. 

In order to understand polymer solution behaviour, the samples have to be characterised with respect to their molecular configuration, their molar mass and polydispersity, the polymer concentration and the shear rate. Classical techniques of polymer characterisation (light scattering, viscometry, ultracentrifugation, etc.) yield information on the solution structure and conformation of single macromolecules, as well as on the thermodynamic interactions with the solvent. In technical concentrations the behaviour of the dissolved polymer is more complicated because additional intramolecular and intermolecular interactions between polymer segments appear. 

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

This impossibility of reducing a complex process to single macromolecules explains the co-existence of different levels of explanation in biologists molecular descriptions. This does not mean that the nature of the molecular components is of no importance, nor that the complex functions originate only from the rules of assembly of the different macromolecular components. The organization of living beings is based both on the precise nature of the molecular components and on the way that these molecular components are assembled. 

It is clear that the combination of different architectures and the precise localization of functionalities within a single macromolecule provide unique opportunities for the control of molecular shape as well as molecular, optical, and electronic properties. A significant hurdle that still remains today is the relatively demanding multistep process used to prepare dendrons and hybrids. This, in turn, translates into limited availability but, as high added-value applications emerge, it is clear that current, as well as yet-to-be-developed, syntheses will be used to prepare specialty materials that benefit from the unique properties derived from the combination of dendritic and linear architectures. 

Finally, single macromolecules, because of their one-dimensional character, offer the promise of sequential side group coding, information storage, and template function in the manner that is well known in biological polymers (Figure 1). 

Fig, 9. (a) Optical trapping (b) The use of an optical trap to estimate extensions and forces in single macromolecules. Adapted from Bennink [82], For details see text. 

The prevalence of one of the antipodes in each single macromolecule might in principle be due to the interaction of the growing chain with the monomer molecules, admitting a high stereospecificity of the system (30,128,129). 

This review article describes progress made in scanning force microscopy of polymers during the last 5 years including fundamental principles of SFM and recent developments in instrumentation relevant to polymer systems. It focuses on the analytical capabilities of SFM techniques in areas of research where they give the most unique and valuable information not accessible by other methods. These include (i) quantitative characterisation of material properties and structure manipulation on the nanometer scale, and (ii) visualisation and probing of single macromolecules. 

Keywords scanning force microscopy, polymers, single macromolecules, thin films... 

Fig. 2 A schematic view of an experimental setup for measuring the dynamical variable r(t). A single macromolecule is subjected to a train of short pulses with a repetition rate kex. r(tj) is defined as the time interval between photoexcitation of the donor at time tj and the emission of a fluorescence photon by either the donor (green) or the acceptor (red).

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. 

To consider the behaviour of a single macromolecule in the solution, the interaction of the atoms of the macromolecule with the atoms of solvent molecules has to be taken into account, apart from the interactions between the different parts of the macromolecule. To find the distribution function for the chain co-ordinates, one ought to consider N+1 big particles of chain interacting with each other and each with small particles of solvent. One can anticipate that after eliminating the co-ordinates of the small particles in the... 

The methods used by Karl Freed with associates (Chang and Freed 1993 Tang et al. 1995 Guenza and Freed 1996 Rostov and Freed 1997) for calculation time-correlation functions of single macromolecules can be apparently useful in a more complicated case of many interacting macromolecules. 

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