Crosslinking of polymer chains

Note 1 In both network-forming polymerization and the crosslinking of polymer chains, the gel point is expressed as an extent of chemical reaction (c.f, gel time). 

Figure 34.25 Nanoparticles formed by intramolecular crosslinking of polymer chains, using noncovalent interactions. (Reprinted with permission from Ref. [99] 2009,...

Both these reactions cause the crosslinking of polymer chains. 

The second step of gelation was apparently thought to be a relatively rapid part of the overall gelation reaction in early studies > , However, other work indicated that the attachment of metal ions to the polymer and the subsequent crosslinking of polymer chains was not Instantaneous, and was affected by temperature by an Arrhenius relationships ... 

Chapter 2 Section 4 General Theory of Gel Preparation Table 2 Chemical crosslinking of polymer chains. 

Figure 2. Crosslinking of polymer chains to form hydrogels.

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

It must be crosslinkedor vulcanized. Crosslinking is the chemical joining together of polymer chains, usually by sulfur bonds at random positions, to make a three-dimensional network (see Figure A). 

Schematic of polymer chains randomly placed in space, with crosslinks also placed randomly, but frequently averaging every 5,000-10,000 g/mol along the chains.

The connection between polymer chemistry and ceramic science is found in the ways in which linear macromolecules can be converted into giant ultrastructure systems, in which the whole solid material comprises one giant molecule. This transformation can be accomplished in two ways—first by the formation of covalent, ionic, or coordinate crosslinks between polymer chains, and second, by the introduction of crystalline order. In the second approach, strong van der Waals forces within the crystalline domains confer rigidity and strength not unlike that found when covalent crosslinks are present. 

The size and shape of polymer chains joined in a crosslinked matrix can be measured in a small angle neutron scattering (SANS) experiment. This is a-chieved by labelling a small fraction of the prepolymer with deuterium to contrast strongly with the ordinary hydrogenous substance. The deformation of the polymer chains upon swelling or stretching of the network can also be determined and the results compared with predictions from the theory of rubber elasticity. 

Another possible explanation is that singlet O2 somehow leads to crosslinking. The reactions of O2 have been extensively studied (34), and do not appear relevant to these copolymers. The only functionality that could conceivably react with singlet O2 is a vinyl chain termination, which could produce a hydroperoxide that might then participate in crosslinking. However, in a study of free radical polymerized PMMA (35), the maximum fraction of polymer chains with vinyl ends was found to be 0.36, for bulk polymerized material in benzene solution the fraction was 0-3%. This result, plus the fact that the insolubilization occurs immediately during photolysis at room temperature, makes it very unlikely that such hydroperoxides are involved. 

The neutral polymer radicals which are produced also often undergo further reactions, which can result in chemical changes in the polymer. These reactions may include crosslinking or scission of polymer chains, formation of small molecule products, changes in the stereochemistry of the polymer chains, changes in the crystallinity of the polymer or a variety of other chemical and physical processes. 

Thus, polymethacrylic acid undergoes net scission on gamma radiolysis with a G-value of approximately 4, while polyacrylic acid, on the other hand, undergoes net crosslinking with a G-value of approximately 1.2 (7). Crosslinking in polyacrylic acid is favourable because of the formation of main chain radicals. These can react to form crosslinks between polymer chains. 

Number-average molar mass of polymer chains between two adjacent crosslinks or junction points in a polymer network. 

BB] is the concentration of BB, and [B] = 2[BB], The number of crosslinks is simply the number of BB monomer molecules in which both B double bonds are reacted, that is, //[BB. The number of polymer chains is the total number of A and B double bonds reacted divided by the degree of polymerization, ([A] + [B])ppCw (the weight-average degree of polymerization is employed for reasons previously described in Sec. 2-10). The critical extent of reaction at the gel point pc occurs when the number of crosslinks per chain is and thus is given by... 

Several theories have been proposed to calculate the molecular weight between crosslinks in a hydrogel membrane. Probably the most widely used of these theories is that of Flory and Rehner [5]. This theory deals with neutral polymer networks and assumes a Gaussian distribution of polymer chains and tetrafunctional crosslinking within the polymer network. 

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