Polymers gelation

M. Gnyba, M. Keranen, A. Maaninen, J. Suhonen, M. Jedrzejewska-Szczerska, B.B. Kosmowski and R Wierzba, Raman system for on-line monitoring and optimisation of hybrid polymer gelation, Opto-Electron. Rev., 13, 9-17... 

The percolation transition can be described in space of any dimension. Examples of two-dimensional percolation are deluge, forest fire, spreading of a contagious disease in an orchard, and gelation of a polymer at an air-water interface. Examples of three-dimensional percolation are substitutional alloys and bulk polymer gelation. A problem analogous to one-dimensional percolation is the condensation polymerization of bifunctional monomers described in Section 1.6.2. 

Formation of condensation structures is the reason for gelation of solutions of various natural and synthetic polymers. Gelation may be accompanied by conformational changes of macromolecules, which occur in the case of gelling of gelatin and other biopolymers, or in the course of chemical reactions. For instance, according to Vlodavets, partial acetalization of polyvinyl alcohol with formaldehyde in acidic medium under the conditions of supersaturation yields fibers of polyvinyl formals which further undergo coalescence and form a network with properties similar to those of leather (and artificial leather substitute). 

Two sets of keys open the way to understanding solidification. One is the basics of phase equilibria - liquid-vapor, liquid-liquid, and liquid-solid - of reaction equilibria, polymer gelation and vitrification, and colloidal transitions. The other is heat and mass transport processes reaction, transformation, and shrinkage kinetics and stress phenomena in polymeric systems and polymer-particulate composites. Engineering approximations... 

Since these phenolic-rich oils are complex mixtures of more than 100 distinct compounds, they are difficult to study. We selected a series of well-defined model compounds that can emulate the behavior of the phenolic-rich oils. The two reaction characteristics of interest are the inherent reactivity of these substituted phenolics with formaldehyde and their assembly into crosslinked polymers (gelation). 

Thus having proved the necessity of the presence of the carbonyl group we tried next to demonstrate the crucial role the pendant methyl groups play in gelation. We went about this task by irradiating bisphenol-A ether ketone polymer under an identical experimental set up used for tetramethyl bisphenol-A ether ketone polymer gelation. 

Zhang, S. Yang, S. Lan, J. Yang, S. You, J. Helical nonracemic tubular coordination polymer gelators from simple achiral molecules. Chem. Commun. 2008, 6170-6172. 

A discussion of polymer gelation could proceed by many different routes depending on the particular aspect of the gel that was to be considered. Polymer gels could be categorized by their chemical structure, their application or properties, the origin Of the polymer, and so on. The approach adopted here is based on the mechanisms for gelation. 

Phase separation in polymer solutions and polymer blends has been greatly studied from both the theoretical and experimental aspect. The observation that turbidity sometimes accompanies polymer gelation has led to the suggestion, which can be traced back to Hardy, that phase separation could be the mechanism for gelation of many polymers. A liquid-liquid or a liquid-solid transition may be involved. Although the latter includes polymer crystallization from solution, this is usually considered as a separate gelation mechanism. 

In the context of mixed polymer gelation, it is worth mentioning the observation that the addition of 1% isotactic polystyrene to a 15% solution of atactic polystyrene at room temperature results in the formation of a weak gel under conditions in which neither of the polymers would gel alone. 

ForX> 5/4, cross-linking predominates over chain scissions. Polymer gelation occurs when -> 0, i.e. when ... 

It is worth noting that the scaling law holds also for homo sPMMA polymer gelation in o-xylene (sample G16, Table III), the scaling exponent A is quite the same as for block copolymer, although a higher concentration is needed due to the weaker association of homo sPMMA. 

The analogy between small and giant vesicles is also reflected in the case of shapes of vesicles with encapsulated polymers. Gelation of a drug and counterion into fibrilar structures in SUV [63], as well as polymerization of encapsulated monomers in GUV [64] resulted in very similar vesicle elongation into oval shapes. 

Fig. 3. Illustration of polymer gelation with crosslinker (dots) connecting polymer chains (lines) a) before gel point, isolated polymers increasing molecular weight b) at critical gel point, network reaches percolation threshold c) final gel network with defects.

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