Polymer structure modification solvents

It was shown (8,9) that the pretreatment of PET yarns with certain strongly interacting solvents can lead not only to swelling but also to irreversible modifications of polymers structure. The basis of structural modification during the DMF treatment of PET is solvent-induced crystallization which occurs while the PET structure is swollen by DMF. At low treatment temperatures (i.e., 50-100°C, Table I), only small crystallites are formed and after removal of the solvent the swollen structure cannot be supported by the small crystallites and consequently collapses. 

Normally synthetic reactions for modification of these natural polymers have been conducted heterogeneously. In the absence of acceptable solvents, characterization of starting materials is difficult and reaction yields are often low due to unfavorable kinetics. Only in those cases in which the substituted products were soluble, have polymer structures been readily identifiable by instrumental analysis.. . ... 

The characteristics of pore structure in polymers is a key parameter in the study of diffusion in polymers. Pore sizes ranging from 0.1 to 1.0 pm (macroporous) are much larger than the pore sizes of diffusing solute molecules, and thus the diffusant molecules do not face a significant hurdle to diffuse through polymers comprising the solvent-filled pores. Thus, a minor modification of the values determined by the hydrodynamic theory or its empirical equations can be made to take into account the fraction of void volume in polymers (i.e., porosity, e), the crookedness of pores (i.e., tortuosity, x), and the affinity of solutes to polymers (i.e., partition coefficient, K). The effective diffusion coefficient, De, in the solvent-filled polymer pores is expressed by ... 

The properties of a variety of surfactants (36-41), novel cationic phases (42-44), mixed micelles (45-49), microemulsions (50), vesicles (51-55), liposomes (56, 57), and synthetic polymers (58-65) have all be screened by LSER. Micelle structural modifications by differing head groups (66) and spacers (67,68), chain lengths (69), and counterions (70), as well as the use of deuterated water buffers (71) and the addition of cyclodextrins (72) and organic solvents (73) to the micellar medium, have also been characterized by LSER studies. 

Photodegradation of polymers (photoageing), involving chain scission and/or cross-linking, occurs by exposure to solar or artificial radiation and causes structural modifications, usually accompanied by a dramatic deterioration of the physical and mechanical properties of the polymer.954 Typically, radical intermediates are formed upon excitation, which initiate subsequent (dark) degradation. The presence of other species, such as oxygen, water, organic solvents or additives, and also mechanical stress and heat, may enhance the efficiency of these processes. 

Theory. The relationship of the chemical aspects of complexatlon reactions to the performance of facilitated transport membranes Is discussed by Koval and Reyes (108). They describe a procedure which can be used to predict and optimize the facilitated transport of gases, Including measurement of the appropriate equilibrium, transport, and kinetic parameters and structural modification of the carrier to Improve the performance of the membrane. Examples of this procedure and carrier modification are given for derivatives of Fe(II) tetralmlne complexes which reversibly bind CO In nitrile solvents (118). Experimental challenges In the measurement of the appropriate properties for other membrane configurations such as reactive Ion exchange membranes and reactive polymer membranes are also discussed. 

It was observed that the TBAPP-based polyimides derived from dianhydrides BPDA and BTDA were soluble in polar aprotic solvents. The high solubility of TBAPP-based polyimides was possibly governed by the structural modification through the incorporation of the flexible isopropylidene, increasing the overall chain flexibility, and methyl substituted arylene ether groups into the polyimide structure. It has been verified that polyimide derived from diamine DAM3 (Table 2.4) had less solubility than that derived from diamine TBAPP. This result clearly demonstrated that the tetramethyl substituents on the phenylene unit effectively improved the polymer solubility. It has been reported that methyl... 

The structural modification of the cyclic 1,3-diene system of Phase I afforded novel noncyclic bis(1,3-diene)s of varying reactivity in the Diels-Alder reaction with bismaleimides. Low to high molecular weight polyimides were prepared from the noncyclic bis(1,3-diene) systems and bis(4-maleimidyl-phenyl)methane, which exhibited better solubility in organic solvents than the Phase I system. Moderate thermal stability was observed by TGA for these polymers. 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

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