Glass transition temperatures different polymeric systems

With continuous development of systems for controlled drug release, new materials are being used whose influence on peptide stability must be carefully examined. Thus, the model hexapeptide Val-Tyr-Pro-Asn-Gly-Ala (Fig. 6.30) embedded in poly (vinyl alcohol) and poly(vinyl pyrrolidone) matrices had rates of deamidation that increased with increasing water content or water activity, and, hence, with decreasing glass transition temperature (Tg). However, the degradation behavior in the two polymers differed so that chemical reactivity could not be predicted from water content, water activity, or T% alone. Furthermore, the hexapeptide was less stable in such hydrated polymeric matrices than in aqueous buffer or lyophilized polymer-free powders [132],... 

Summary In this chapter, a discussion of the viscoelastic properties of selected polymeric materials is performed. The basic concepts of viscoelasticity, dealing with the fact that polymers above glass-transition temperature exhibit high entropic elasticity, are described at beginner level. The analysis of stress-strain for some polymeric materials is shortly described. Dielectric and dynamic mechanical behavior of aliphatic, cyclic saturated and aromatic substituted poly(methacrylate)s is well explained. An interesting approach of the relaxational processes is presented under the experience of the authors in these polymeric systems. The viscoelastic behavior of poly(itaconate)s with mono- and disubstitutions and the effect of the substituents and the functional groups is extensively discussed. The behavior of viscoelastic behavior of different poly(thiocarbonate)s is also analyzed. 

The differences in the polymerization kinetics and colloidal behavior of polymerization systems based on monomers of different polarity may be illustrated (Bakaeva et al., 1966 Yeliseyeva and Bakaeva, 1968) by the polymerization of the model monomers, methyl acrylate and butyl methacrylate, at various concentrations of sodium alkylsulfonate (C,5H3 S03Na). The fact that the solubility of the monomers in water differs by two orders of magnitude (5.2 and 0,08/ , respectively) was used as a criterion of polarity. An additional advantage to comparing these two monomers is that their polymers have rather close glass transition temperatures which is important for coalescence of particles at later stages of polymerization. 

Novel sulfonated and carboxylated ionomers having "blocky" structures were synthesized via two completely different methods. Sulfonated ionomers were prepared by a fairly complex emulsion copolymerization of n-butyl acrylate and sulfonated styrene (Na or K salt) using a water soluble initiator system. Carboxylated ionomers were obtained by the hydrolysis of styrene-isobutyl-methacrylate block copolymers which have been produced by carefully controlled living anionic polymerization. Characterization of these materials showed the formation of novel ionomeric structures with dramatic improvements in the modulus-temperature behavior and also, in some cases, the stress-strain properties. However no change was observed in the glass transition temperature (DSC) of the ionomers when compared with their non-ionic counterparts, which is a strong indication of the formation of blocky structures. 

Figure 9 Correlation of the segmental relaxation time ts measured in dilute solution and the glass transition temperature of the dense system. The data were taken from Adachi, K. Dielectric Spectroscopy of Polymeric Materials, American Chemical Society Washington, DC, p 261 and represent different chain structures. The line is a linear regression to the data.

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