Segmental solubility

The insolubility of peptides is not limited to the solution phase and can occur during peptide synthesis on the solid phase where it manifests itself as the difficult sequence problem. On-resin peptide aggregation and poor peptide segment solubility are due to the formation of (3-sheet-like structures. Difficult sequences contain predominantly hydrophobic residues that consistently show incomplete acylation and a characteristic shrinkage of the peptide-resin during synthesis.111 Reversible backbone protection was introduced into Fmoc/ tBu SPPS after the study of synthesis failure during the assembly of difficult sequences1121 and is now routinely used to circumvent synthesis failure in SPPS. 

It should also be noted that the stability of the distinct mesophases can be quite different. It seems that there is a significant effect of molecular shape and topology, stabilizing SmA phases in the system 41/43 and Colhex phases in the system 35/37. In addition, the mesophase stability is often reduced close to the transition to another mesophase (see Fig. 15). Hence, the order-disorder temperatures can only be roughly estimated based on segmental solubility parameters [24, 25]. 

Melt viscosity is critically affected by the difference in segmental solubility parameters, that is, by the interaction parameter between blocks. The deviation from normal behavior is dependent on the composition and block structure of the particular block polymer examined. It is most dramatic when the block polymer is capable of forming a three-dimensional network in the bulk state and the different polymer segments have high mutual incompatibility as evidenced by diverse solubility parameters. 

Two-phase mixtures of polymers differ from classic ll colloid systems mainly by a transition layer that exists between the system components and is of special significance. The formation of such a layer in mixtures of linear polymers is governed by the kinetic factors of the retarded process of phase sep u ation, by the collid-chemic ll mechanism of formation, or by the adsorption interaction as well as by the segmental solubility [103,104], In mixtures of crosslinked polymers its formation is governed also by the conditions of synthesis. Note also that in the thermodynamic llly nonequilibrium mixtures of polymers in the two-phase systems, the processes of segmental solubility usually have time to reach completion while the macromolecules do not move inside the high-viscosity medium, which ensures the stability of the structure and its mechanical properties [103]. 

The host-guest results, along with the segment solubility results of Narita et al. (48, 49) and Blackmeer et al. (52), suggested that the difficult peptide problem might be eliminated by the introduction of a reversibly A -alkylated amino acid residue prior to aggregation. 

These results make it possible to assume that the melting heat and the degree of crystallinity of PHB decrease in EPC-enriched blends due to the mutual segmental solubility of the pol5mers [4] and due to the appearance of an extended interfacial layer. Also note that the degree of crystallinity... 

Two important classes are cellulose esters (qv) and cellulose ethers (qv). Cellulose esters are not water soluble and are not discussed here cellulose ethers are an important segment of water-soluble polymers. 

The role of activators in the mechanism of vulcanization is as follows. The soluble zinc salt forms a complex with the accelerator and sulfur. This complex then reacts with a diene elastomer to form a mbber—sulfur—accelerator cross-link cursor while also Hberating the zinc ion. The final step involves completion of the sulfur cross-link to another mbber diene segment (18). 

Some commercial durable antistatic finishes have been Hsted in Table 3 (98). Early patents suggest that amino resins (qv) can impart both antisHp and antistatic properties to nylon, acryUc, and polyester fabrics. CycHc polyurethanes, water-soluble amine salts cross-linked with styrene, and water-soluble amine salts of sulfonated polystyrene have been claimed to confer durable antistatic protection. Later patents included dibydroxyethyl sulfone [2580-77-0] hydroxyalkylated cellulose or starch, poly(vinyl alcohol) [9002-86-2] cross-linked with dimethylolethylene urea, chlorotria2ine derivatives, and epoxy-based products. Other patents claim the use of various acryUc polymers and copolymers. Essentially, durable antistats are polyelectrolytes, and the majority of usehil products involve variations of cross-linked polyamines containing polyethoxy segments (92,99—101). 

If natural rubber is treated with proton donors a product is formed which has the same empirical formula. (CjHjj), and is soluble in hydrocarbon solvents but which has a higher density, is inelastic and whose unsaturation is only 51% that of natural rubber. It is believed that intramolecular ring formation occurs to give products containing the segments shown in Figure 30.5. Known as cyclised rubber it may be prepared by treating rubber, on a mill, in solvent or in a latex with materials such as sulphuric acid or stannic chloride. 

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