Temperature dependence liquid/polymer gels

One type of gel which has been extensively investigated in relation to pharmaceutical applications is that formed by certain PEO-PPO-PEO block copolymers (153). These systems are particularly interesting since even a concentrated polymer solution is quite low-viscous in nature at low temperature, whereas a very abrupt gelation (liquid crystal formation (25, 187, 188)) occurs on increasing the temperature. The precise value of the transition temperature depends on the polymer molecular weight, composition and concentration, the concentration and nature of the drug, etc., but by... 

The ESR working window in which the ESR line shapes are sensitive to rotational reorientation depends on the anisotropy of the A- and g-tensors of the radical for nitroxides, the range for Xp, is 10 -10 s, which means that the technique is applicable to polymer solutions, polymer gels, and solid polymers at temperatures close to or above Tg. At the X-band ESR frequency ( 9 GHz), the rotational dynamics in most liquids is usually sufficiently fast, that is, Xp,Aco 1 [where Am is a measure of the magnitude of the orientation-dependent part of the spin Hamiltonian ffj(t)] such motions (Xjj < lO s) fall within the motional narrowing (fast-motional) regime. For slower dynamics (Xp,Am > 1), more complicated slow-motional spectra are observed. ... 

Because the physical and chemical properties of a number of injectable polymer hydrogels are pH and temperature dependent, a new copolymer of poly (A-isopropylacrylamide)-g-methylcellulose (PNIPAAm-g-MC) has been explored as a 3-D scaffold for AC regeneration. In this system, PNIPAAm has a lower critical solution temperature (LCST) of approximately 33°C and undergoes a liquid-to-gel reversible phenomenon while also having the added advantage of an LCST very close... 

Fig. 9.8 (a) Temperature dependence of ionic conductivity of the ionic liquid gel polymer electrolyte film. Inset The corresponding log (a T ) vs 1/(T - To) plot, (b) Cyclic voltammetry curves of EDLC cells at different scan rates. On the graph, there is a picture of a transparent and free-standing gel polymer electrolyte film, (c) Variation in the capacitance values of the EDLC cells [110]... 

Another behaviour different from that of the typical three types was shown by a gel polymer electrolyte nanocomposite consisting of PVDF-HFP with a mixture of EC and PC as the liquid electrolyte, the ion salt Mg(C104)2, and the nanosized filler magnesium oxide, MgO. The conductivity temperature dependence followed the relation log a vs. 7" , with curves that were linear up to 60 °C, and thereafter no conductivity enhancement was observed. This is typical conductivity behaviour for gel polymer electrolyte nanocomposite films. ... 

The solidity of gel electrolytes results from chain entanglements. At high temperatures they flow like liquids, but on cooling they show a small increase in the shear modulus at temperatures well above T. This is the liquid-to-rubber transition. The values of shear modulus and viscosity for rubbery solids are considerably lower than those for glass forming liquids at an equivalent structural relaxation time. The local or microscopic viscosity relaxation time of the rubbery material, which is reflected in the 7], obeys a VTF equation with a pre-exponential factor equivalent to that for small-molecule liquids. Above the liquid-to-rubber transition, the VTF equation is also obeyed but the pre-exponential term for viscosity is much larger than is typical for small-molecule liquids and is dependent on the polymer molecular weight. 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

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