Thermo-reversible

Lee T.A.T., Cooper A., Apkarian R.P., and Conticello V.P. Thermo-reversible self-assembly of nano-particles derived from elastin-mimetic polypeptides. Advanced Materials, 12, 1105, 2000. 

The linkage between two chains can also be ionic. Thus the copolymer between ethylene and methacrylic acid (MA) (up to 15% MA), made by free radical polymerisation, yields a polymer with pendant carboxyl groups. Neutralisation with zinc ions gives a crosslinked, thermo-reversible polymer (Surlyn ). The resulting polymer (ionomer) has limited properties, although it is the favoured material for the outer covering of golf balls. 

Gelling agents under appropriate conditions self-associate to produce a three-dimensional structure. Some gelling, as with gelatine, is thermo-reversible while others, such as with high methoxyl pectin, is irreversible. Apart from the effects on the texture of the product an irreversible gelling agent is more of a problem in the factory since it cannot readily be recycled. 

Beginn developed Percec-type dendrimers, which are known to form supramolecu-lar channels, with polymerizable acrylate groups in order to obtain ion-permeable membranes [97-99]. First, the dendron 78 (Scheme 40) was dissolved in a polymerizable acrylate mixture that does not shrink on polymerization. The second step was the thermo-reversible gelation of the acrylate mixture, which was followed by the last step, polymerization to fix the supramolecular channel structure (Scheme 40). In the first experiments, compounds with only one polymerizable group were used but it turned out that the gelating properties were not sufficient [100, 101] so threefold modified 78 had to be developed. 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Physical ageing or "age stiffening" - is a thermo-reversible process, that occurs in all glassy materials and affects their properties primarily by changing the relaxation times, all of them in the same way. 

Thermo-reversible has the following meaning if an amorphous polymer is heated to above Tg, it readily reaches thermodynamic equilibrium by definition the sample has then "forgotten" its history, any previous ageing it may have undergone below Tg having been erased. In other words it is completely rejuvenated. Ageing therefore is a thermo-reversible process to which one and the same sample can be subjected an arbitrary number of times. It has just to be retreated each time to the same temperature above Tg. 

The above-mentioned examples belong to the captivating field of covalent reversible polymers. This field includes examples like reversible polymerization of cyclic oligomers from poly (ethylene terephthalate) [62], thermo-reversible polymers based on Diels-Alder and retro-Diels-Alder reaction (see, for example [63-69]), reversible photo-polymerization in the solid state of molecules... 

Teramoto N, Arai Y, Shibata M (2006) Thermo-reversible Diels-Alder polymerization of difurfurylidene trehalose and bismaleimides. Carbohydr Polym 64 78-84... 

Nowadays, it is an accepted mechanistic model [5, 6] that the photolysis step (which proceeds under thermo-reversible CO insertion) leads to species best described as chromium ketene complexes of type 7 (Scheme 2). Indeed, these intermediates exhibit a ketene-like reactivity they undergo [2 + 2] cycloaddition reactions with olefins, imines and enol ethers, whereas reaction with nucleophiles leads to carboxylic acid derivatives. 

Nunes, S. P. Wolf, B. A. Jeberien, H. E., "On the Cooccurrence of Demixing and Thermo-reversible Gelation of Polymer Solutions. 2. Thermodynamic Background," Macromolecules, 20, 1948 (1987). 

Nishinari, K., Koide, S., and Ogino, K. 1985. On the temperature dependence of elasticity of thermo-reversible gels. J. de Physique 46 793-797. 

Copoly(ester ester)s belong to the family of thermoplastic elastomers (TPEs) and consist in general of thermo-reversible hard and elastic soft domains [11]. The copoly(ester ester) used here consists of 60% poly(butylene terephthalate), 35% poly(butylene adipate) and 5% 4,4 -methylenebis(phenyl isocyanate), and shows domain sizes of about 20 nm [12]. The material possesses a rubber plateau between the glass transition temperature of the mixed amorphous PBA/PBT phase (the PBT phase is semi-crystalline) at about -30°C and the melting point of the PBT at about 220°C. Due to the vulnerability of the amorphous PBA/PBT soft domains towards water attack [13] the PBT/PBA copoly(ester ester) is used here to study the existence of ESC of a chemical rather than a physical nature. For the sake of clarity it should be emphasized that no additives have been used in the copoly(ester ester) described here. 

In contrast, low-methoxy (LM) pectins gel in the presence of divalent cations, especially calcium, by the egg box mechanism proposed for alginates. Moreover, calcium pectinate gels prepared at neutral pH are heat stable, whereas acidic pH gels are thermo-reversible. Gel strength depends on the extent of esterification (levels from 30% to 50% are optimal), the distribution of ester groups on the chain, and the average molecular weight. LM pectins have been used traditionally in antidiarrheal formulations with kaolin. HM pectins were evaluated in controlled release matrix formulations. Pectin microspheres were reported to improve ophthalmic bioavailability of piroxicam in rabbits compared with commercial piroxicam eye drops. ... 

Yun M, Choi H, Jung J, Kim C. Development of a thermo-reversible insulin liquid suppository with bioavailability enhancement. Int J Pharm 1999 189(2) 137-145. 

Recently a polyrotaxane composed of multiple methylated a-CD rings threaded on a high molecular weight PEO chain and end-capped by bulky adamantyl groups was synthesized (Fig. 17) and investigated in terms of its thermo-reversible sol-gel transition and hydrogel formation [87],... 

Physical hydrogels were formed induced by self-assembled water-insoluble and crystalline polypseudorotaxane domains which act as physical crosslinking points. Such physical hydrogels include the polypseudorotaxane systems of a-CD threading on PEO or its copolymers, a-CD threaded on PEI, PL, or their copolymers, and even 3-CD or y-CD threaded PPO or PEI copolymers. The thermo-reversible and thixotropic properties of these supramolecular hydrogels have inspired their applications as injectable drug delivery systems. Physical hydrogels induced by physical interaction of threaded CD molecules in polyrotaxanes were also developed. 

Recently the kinetic treatment of a photoreversible reaction with additional thermal reaction has been applied to thermo-reversible photo-chromism with degradation in the case of triphenylimidazolyl dimer reactions [170]. Fit techniques allow the simultaneous determination of quantum yields and absorption coefficients. In the case of a-3,5-dimethoxyphenyl-5,5-dimethylfulgide even photo-equilibria with two superimposed thermal reaction steps have been evaluated by formal integration [171]. 

The ability of these light-sensitive groups to form crosslinks between polymer chains under the Influence of UV light has been described in paragrah 5. The rate constants of the thermo-reversion for the slow Isomer at 298 K are given in Table 7 for various irradiation times. Also in Table 7 the fractions of the slow Isomer in the polymer film are given. These values could... 

Scheme 16. Thermo-reversible Photoisomerization of Cyclohexa-2,4-dienones [72]...

Chen G, Hoffman AS. Preparation and properties of thermo-reversible, phase-separating enzyme-oligo(Al-isopropylacryla-mide) conjugates. Bioconj Chem 1993 4 509-514. 

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