Polymer segment concept

Polymer segment concept and glass transition temperature... 

The same effect happens inside a random flight chain where the close proximity of the polymer segments offers mutual screening from the bulk flow field. The idea of a chain being non-drained was first considered by Debye Bueche who introduced the concept of a shielding length defined as [46] ... 

The bundle model of polymer crystallization will be discussed first. This mean-field approach describes the metastable configurational equilibrium of the undercooled polymer chain in solution or in the melt, and we will summarize and update previous work of ours [7-9]. The concept of the bundle, i.e. an aggregate of a few parallel polymer segments or stems connected by folds (see Fig. 1 see also [10]) and stabilized by attractive crystal-like interac-... 

At the segment level, the cooperative motions of polymer segments that allow chain movement are the key determinant of the dynamic behavior of a polymer under mechanical stress. The concept of glass transition rests on these segmental motions, which refiect the steric inhibition of backbone conformational relaxation. The impact of backbone conformation on electronic and optical properties is becoming better understood. 

Over the past decade or so, these remarkable achievements by nature have been recognized by the polymer science community. This has led to an increased interest in the use of biological concepts to synthesize polymers or to control the structure and properties of synthetic polymers. Of particular interest are peptide hybrid polymers. Combining peptide and synthetic polymer segments into a single macromolecule offers interesting possibilities to synergize the properties of the individual components and to compatibilize bio- and synthetic systems. 

A completely fresh and different approach to polymer solutions is being studied in at least two places under the auspices of the Paint Research Institute. Some years ago, Prausnitz (25) reported on polymer segment-interaction theory to predict phase separation. This was based on a lead by Wilson (24) who proposed a local volume fraction concept or domain theory for explaining deviations from ideality. More recently, Prausnitz (27) reported on further work involving anomalous viscosities in high solid polymer solutions. High viscosities may occur when solvent mixtures are used which have values near the extreme ends of the solu-... 

Thus, if two polymers are compatible - SINGLE PHASE - there will be one Tg given by equation (1). If two polymers are in-coapatible - TWO PHASES - there will be two Tg s - those of the individual polymers. These concepts are demonstrated in Figure 1 which shows blends of the hard segment (MDI-DPG) in soft segments (MDI/Poly G 53-56 and MDI/Poly G 55-56 ). 

Since overall diffusion is governed by the diffusion of chain segments, the overall diffusion coefficient, D, is expected to be inversely proportional to the relaxation time of polymer segments [108], which enables a model based on the free volume concept and a description similar to the Williams-Landel-Ferry (WLF) equation [109-112] ... 

Many attempts have been made to improve the UNIFAC-frf model which cannot be listed here. A comprehensive review was given by Fried et al. An innovative method to combine the free-volume contribution within a corrected Flory-Huggins combinatorial entropy and the UNIFAC concept was found by Elbro et al. and improved by Kontogeorgis et al. These authors take into account the free-volume dissimilarity by assuming different van der Waals hard-core volumes (again from Bondi s tables ) for the solvent and the polymer segments... 

The concept of free volume in a polymer is an extension of the ideas of Cohen and Turnbull [141], first used to describe the self-diffusion in a liquid of hard spheres. Such theories suggest that the permeant diffuses by a cooperative movement between the permeant and the polymer segments, from one hole to the other within the polymer. The creation of a hole is caused by fluctuations of local density due to thermal motion. Based on the concept of the redistribution of free volume to represent the thermodynamic diffusion coefficient [142], and the standard reference state for free volume [143], Stem and Fang [144] interpreted their permeability data for nonporous membranes, and Fang, Stem, and Frisch [ 145] extended the theory to include the case of permeation of gas and liquid mixtures. 

One important feature of a polymeric material is its glass transition. The mobility of the polymer segments involved in the glass relaxation is described by the WLF equation using the free-volume concept ... 

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