Physical Architecture

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

EG-LC was successfully applied to many polymer separations, especially those aimed at the characterization of polymer blends and statistical copolymers [20,22,23,25-29,31,32,210-216]. Sato et al. [216] have demonstrated potential of EG-LC in the separation of chemically identical polymers according to their physical architecture. 

These materials, however, as a rule exhibit rather broad chemical composition distribution. Block copolymers may contain important amounts of parent homopolymer(s) [232,244,269], In any case, it is to be kept in mind that practically all calibration materials contain the end groups that differ in the chemical composition, size, and in the enthalpic interactivity from the mers forming the main chain. In some cases, also the entire physical architecture of the apparently identical calibration materials and analyzed polymers may differ substantially. The typical example is the difference in stereoregularity of poly(methyl and ethyl methacrylate)s while the size of the isotactic macromolecules in solution is similar to their syndiotactic pendants of the same molar mass, their enthalpic interactivity and retention in LC CC may differ remarkably [258,259]. 

The variability can be accounted for largely by the diffusion barriers of the different fibre types and by Na+ channel density. For example, the presence of a Schwann cell and myelin sheath poses a considerable barrier to the diffusion of local anaesthetic to the interior of the cell. There is in vitro evidence to indicate that all desheathed nerves require a similar minimum concentration of local anaesthetic to induce block irrespective of fibre type. A consequence of the physical architecture of a mixed nerve is that access of the drug to the outer fibres is easier than access to fibres at the core. It is for this reason that the onset of proximal analgesia of the limb precedes distal analgesia with a brachial plexus block. 

The Technical Infrastructure and Network Specihcation shall document the logical and physical architecture of the infrastructure components to be installed. The Technical Specification should define ... 

One should also consider the physical architectural components of the system. Would it be appropriate to undertake separate safety analyses on the processing, storage and network components of the system For example, suppose a hospital... 

The polymeric substances can be classified according to different qnalities. The most appropriate classification parameters include molecular characteristics of polymers, such as above presented molar mass, and further chemical stracture, as well as physical architecture of macromolecules ... 

Homopolymers. Homopolymers are formed by one single kind of low-molecular species, called monomers. Homopolymers may exhibit various physical architectures of macromolecules linear, cyclic, branched, comblike, star-shaped, ladder-like, and so on. 

Chemical stractuie and physical architecture of maciomolecules may exhibit large intercormected variabihty, which is reflected in the secondary molecular characteristics. Differences in the secondary molectrlar characteristics may even appear within a group of polymers possessing the same overall chemical stracture or architecture. For example, some block copolymers may possess the same overall chemical stracture but stereoregularity of their chains can be different. It is evident that the actual properties of macromolecules may be extremely intricated if the effects of two or even all three basic molecular characteristics are combined. 

MEAN (AVERAGE) VALUES OF CHEMICAL STRUCTURE AND PHYSICAL ARCHITECTURE OF POLYMERS... 

Important data on physical architecture of crystallable polymers can be obtained from temperature rising elution fractionation, TREF and crystallization fractionation, CRYSTAF. 

Coupled and two-dimensional methods of liquid chromatography with detectors that monitor physical architecture of eluted macromolecules. 

As indicated, molar mass both average and dispersity belong to the most important molecular characteristic of synthetic polymers. Molar mass of macromolecules affects their solubility and size in solution and to some extent also their interactivity. For both the synthesists and the technologists, also chemical stracture and physical architecture of macromolecules are highly important becanse they either confirm successful synthesis or markedly affect the end-nse properties of polymers. Similar to molar mass, the latter molecular characteristics impact the interactivity of macromolecules and partially also their size in solution. Development of methods for assessment of dispersities in chemical stracture and pltysical architecture of macromolecules is still only in a rather initial stage. 

From its principle, SEC suffers from the limited separation selectivity because the retention volumes of samples are restricted by F and F values (Figure 3(a) and 12). This drawback prevents application of SEC to quantitative characterization of numerons complex polymer systems that contain macromolecules of distinct chemical stmcture or physical architecture possessing similar molecular sizes (compare Figure 16). In turn, low sample capacity and often insufficient sensitivity of detection makes it impossible to identify and characterize the minor components of complex polymer systems that are present in a matrix of a major constituent in the amonnt below abont 10% - even if molar masses of minor and major constitnent differ substantially. Still, SEC is applied in many laboratories for just the above- mentioned pnrposes. To solve the latter analytical challenges, coupled and two-dimensional methods of polymer HPLC are to be employed (see sections 11.8 and 11.9). 

To suppress the effect of certain molecular characteristic on sample retention volume so that the resulting chromatogram reflects mainly or even exclnsively other molecular characteristic(s) of sample. In practice, it is usually attempted to partially or fully suppress the influence of polymer molar mass. In this instance, the coupling of LC retention mechanisms may allow assessment of chemical structure or physical architecture of a complex polymer irrespective of its molar mass average and dispersity. Under favorable conditions, also the constituents of a complex polymer system with similar molar masses can be discriminated and molar mass of one constituent determined. For example, in the case of a two-component polymer system, the molar mass effect can be suppressed selectively for one constituent so that it elutes in a completely different retention volume compared with the retention volume pertaining to SEC. In some cases, the... 

The presently most popular approach to two-dimensional polymer HPLC avails partial or preferably full suppression of the molar mass effect in the Id column so that the complex polymer or complex polymer system is separated mainly or even exclusively according to chemical structure or physical architecture of macromolecules occuring in sample. Appropriate coupled methods of polymer HPLC are to be applied to this purpose (compare section 11.8). In the 2d separation column - it is usually SEC - the fractions from the Id column are further discriminated according to their molecular size. In other words, fractions obtained in the first-dimension column are separated in the self-existent second-dimension column, which applies distinct separation mechanism(s). Only exceptionally SEC... 

The enterprise shall apply the SEP, described in Clause 6, to each subsystem for the purpose of generating subsystem fimctional and physical architectures. Specific activities to be accomplished are fisted in Table 3. 

In order to properly guide tissue regeneration, a biomaterial ought to satisfy the structural requirements of the native tissue. For each type of tissue and application, the implant must have the correct physical architecture with the appropriate mechanical and chemical properties. Table 16.3 provides examples of approaches that utilize these properties for tissue regeneration. 

The challenge in this field is to control both the physical architecture and chemical reactivity of the film so as to promote selected electron transfer reactions while inhibiting others. With polymer-modified electrodes (PMEs), the electrode is conferred with the molecular selectivity and specificity that is lacking at a conventional pristine electrode. For example, poly(4-vinyl)pyridine and poly(N-vinyl)imi-dazole can be functionalized with osmium and ruthenium polypyridyl complexes. These synthetic macromolecules act as useful model systems for... 

The successful use of ruthenium-coated porous Ti electrodes coupled with the benefits of adopting back-flow configuration [43] is strongly suggestive that there are combinations of physical architecture, flow configurations and electrode functionalization with good operational capability stiU awaiting discovery. 

---