Flexibility of polymers

A criticism often aimed at the use of extrinsic fluorescent probes is the possible local perturbation induced by the probe itself on the microenvironment to be probed. There are indeed several cases of systems perturbed by fluorescent probes. However, it should be emphasized that many examples of results consistent with those obtained by other techniques can be found in the literature (transition temperature in lipid bilayer, flexibility of polymer chains, etc.). To minimize the perturbation, attention must be paid to the size and shape of the probe with respect to the probed region. 

This investigation shows that it is indeed possible to study the flexibility of polymer chains in polymer matrices by means of excimer-forming probes and that the rotational mobility of these probes reflect the glass transition relaxation phenomena of the polymer host matrix, in agreement with the appropriate WLF equation. 

Plasticizers help the flexibility of polymers and are chosen so that they do not dissolve the polymer but rather allow segmental mobility. Through experience, it is found that the solubility parameter differences between the plasticizer and polymer should be less than 1.8 H for there to be compatibility between the plasticizer and polymer. 

Table 5.2 lists polymers and their tendency toward crystallinity. Yield stress and strength, and hardness increase with an increase in crystallinity as does elastic modulus and stiffness. Physical factors that increase crystallinity, such as slower cooling and annealing, also tend to increase the stiffness, hardness, and modulus of a polymeric material. Thus polymers with at least some degree of crystallinity are denser, stiffer, and stronger than amorphous polymers. However, the amorphous region contributes to the toughness and flexibility of polymers. 

Changes in the flexibility of polymer coils owing to concentration - variation may effect the entropy. Huggins [21] introduced two corrections for the athermal entropy of mixing, that take into account the influence a second polymer has on the stiffness of the other polymer... 

As has been depicted in Fig. 1, various conformations are possible for adsorbed polymers, depending on polymer-polymer, polymer-solvent, and polymer-interface interactions and the flexibility of polymers. To determine experimentally the conformation of adsorbed polymers only adsorption isotherm data are insufficient. The average thickness of the adsorbed polymer layer, the segment density distribution in this layer, the fraction of adsorbed segments, and the fraction of surface sites occupied by adsorbed segments must be measured. Recently, several unique techniques have become available to measure these quantities. 

Two conflicting theoretical views concerning the flexibility of polymer chains and the role of the volume effect and the draining effect on fry] are discussed in the literature polymer chains of typical flexibility such as vinyl polymer chains, and a large value of Ip] can be interpreted in terms of the excluded volume effect (view point A) polymer chains are semi- or inflexible and their large unperturbed chain dimension is mainly responsible for a large [ry] (view point B). The former has its foundation on the two parameter theory 110. Untill 1977 these inconsistencies constituted one of the most outstanding problems yet unsolved in the science of polymer solutions. 

During the past few years the microhardness technique has frequently been applied to the characterization of super-hard-surfaced polymers obtained by ion implantation and to plasma-deposited hard amorphous carbon films (Balta Calleja Fakirov, 1997). These products represent an entirely new class of materials that are lightweight and have the flexibility of polymers combined with a surface microhardness and wear resistance greater than those of metallic alloys (Lee et al., 1996). 

The estimated equation of state parameters seem to have some physical significance e.g., they seem to be proportional to the polymer molecular weight. Moreover, it seems that the estimated b values represent a measure of the flexibility of polymer molecules for many polymers and for different approaches, the parameter b is closely related to the van der Waals volume. Generally, the performance of these cubic equation of state in correlating volumetric (PVT) data, although it varies from model to model, is satifactory considering the simplicity of the approach. 

The thermal energy of the molecular environment provides the energy required to overcome the rotational energy barrier. Consequently, the shape (flexibility) of a polymer molecule is temperature dependent. At sufficiently high temperatures, the polymer chain constantly wiggles, assuming a myriad of random coil conformations. As we shall see later, the flexibility of polymer molecules, which is a function of substituents on the backbone, has a strong influence on polymer properties. 

For radiation grafting, the stabilization of radicals in the polymer films is very important. Due to the flexibility of polymer chains, rearrangements and chemical interactions are possible over longer distance, in particular above the glass transition temperature. To lower the probability of radical recombination, exposed polymer substrates can be stored at low temperatures to reduce the chain mobility inside the polymer. Temperatures of -80°C are usually sufficient to stabilize the radicals over weeks to months. 

In the second approach, the new constructed polymer joint is assumed of lower strength than bonded materials (Fig. 7), to assure lack of damage appearance in joined structural elements. Flexibility of polymer introduces higher deformability and thus increases amount of energy needed for destructing of the joint, which is a sum of structure and polymer deformation energy. 

Fluorescence anisotropy studies are popular in biological and biochemical research of lipid membranes [16-18], proteins [19-22], etc. and also in polymer science. They have been performed for monitoring the conformations and flexibility of polymer chains in dilute, semidilute and concentrated solutions [23-27], in polymer melts and blends [28-31], and also for studying polymer self-assembly [32-34]. Nowadays, steady-state and time-resolved fluorescence anisotropy are currently used methods in polymer chemistry. 

F. Fabulyak, in Molecular Flexibility of Polymers in the Border Layers, Naukova Dumka, Kiev, p. 144 (1983). 

The use of polymers in medicine is steadily growing. The synthetic and processing flexibility of polymers continue to permit polymers to be applied in a broad range of medical, biological, and implant applications. Creative polymer synthesis continues to expand the functionality and tunability of polymers for medical applications. There are now excellent biomedical polymers available to address general needs in medidne (the subject of this chapter). Opportunities that present themselves for enhanced or improved biomedical polymers are in the following areas ... 

The semi-flexibility of polymer chains due to the hindered internal rotation is revealed by the correction from the contribution of the internal rotation in the mean-square end-to-end distance, as... 

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