Reversible gels of synthetic polymers: Phase relations, formation mechanisms and structure

Abstract

The complex problem of thermoreversible gelation of solutions of synthetic polymers is discussed. Several possible mechanisms, based on supramolecular structure formation in solution, are discussed. The gelation of polyethylene solutions illustrates the structure formation through lamellar, folded chain crystallization. Gelation of solutions of stereoisomers of polystyrene and poly(methyl methacrylate) is more complex and two mechanisms can are proposed. The complex temperature‐concentration phase diagrams of syndiotactic polystyrene/solvent systems illustrate the competition between gelation by crystallization and the formation of elastic gels. The influence of chain conformation, participation of the solvent in the structure formation and the influence of solvent quality is discussed. Insight in the formation mechanism of elastic gels is obtained through the study of the gelation of solutions of syndiotactic poly(methyl methacrylate).

Gelation Mechanism of Syndiotactic Polystyrene in Bromoform

“…The phase behavior of sPS in bromoform is very similar to the one observed with most other good solvents for polystyrene. 21,22,25,26 The temperature-concentration diagram is characteristic for the formation of a polymer-solvent compound so that we can propose that bromoform is intercalated between the polymer chains in the T 2 G 2 conformation. Although the overall view of the temperature-concentration diagram is similar to that in other solvents, there exist some differences, notably in the melting and gelation temperature.…”

“…The resulting temperature-concentration diagram is very similar to the one obtained with the system sPS/ chloroform and sPS in good aromatic solvents. 21,22,25,26 The shape of such a temperature-concentration diagram is characteristic for the formation of an incongruently melting polymer-solvent compound. From comparison with these other systems one can conclude that in the low concentration range only the δ-phase, responsible for the gel formation, is formed during cooling.…”

“…Evidence for this compound formation could be found in temperature-concentration diagrams in good solvents where the melting of the δ-phase shows the characteristics of an incongruently melting compound. [21][22][23][24][25][26] The mechanism of gelation can be studied in the most efficient way in good solvents. At low polymer content only the δ-phase will be formed with no interference with the crystallization into the T 4 zigzag phase.…”

A Thermosensitive Poly(organophosphazene) Gel

“…Elemental analysis was carried out with a Fisons 1108 CHNS Microanalizer and Polyscan 61E ICP. 1 H NMR measurements were made with a Varian Gemini-300 spectrometer operating at 300 MHz in the Fourier transform mode. Proton-decoupled 31 P NMR spectra were measured with the same spectrometer operating at 121.4 MHz using triphenyl phosphate as an external standard.…”

“…Hydrogels usually have three-dimensional networks that are formed by covalent bonding or by physical association between polymer segments in aqueous solution. The cross-links of physical networks do not occur at points on the chain as do covalent cross-links but involve physical junction zones. , Thermoassociative polymers showing a reversible phase transition form physical gels owing to physical association between the polymer and solvent molecules or between polymer segments in an aqueous solution in response to temperature. In the case of neutral polymers, the physical network is usually considered to be formed by hydrophobic interaction between polymer chains such as alkyl, perfluoroalkyl, or aromatic fragments in aqueous solution.…”