Swelling of crosslinked polymers

The values of parameters Xn and X23 were calculated from the Flory-Rehner equation, using data on swelling of crosslinked polymer in individual solvents, the values Xn X23 > and %i23 were estimated from Eqs. [6.2.3-6.2.5). Then equilibrium swelling of elastomer in SL was calculated from the equation similar to Flory-Rehner equation, considering the mix-... 

Figure 10 illustrates the result of a swelling of crosslinked polymer particles. In this case 2 ym particles of styrene-divinyl benzene copolymer (5 % divinylbenzene) were first swollen with dioctyl adipate (20 % by volume) and then in the second step with chlorobenzene (125 times the volume of the polymer). It is seen that the cores all seem to lie at the side of the droplets. However, this position is an artefact in the way that it apparently is caused by rotational movements of the droplets under the microscope. 

The swelling of crosslinked polymer melts is just a limited dissolution. The only difference is that the effective molecular-weight for interaction with the solvent is (Mj.) that of the chains between crosslinks. Hence minimization of swelling is achieved by the choice of polymer and solvent chemistry that minimizes mutual compatibility. 

YH Bae, T Okano, SW Kim. Temperature dependence of swelling of crosslinked poly(N,N-alkyl substituted acrylamides) in water. J Polym Sci B 28 923-936, 1990. 

Gee [41] has shown that the same is normally true for swellability of crosslinked polymers, i.e. the magnitude of swelling exhibited by a polymer with 8pol in a liquid with 8]iq increases as (Spol — S,Jq) approaches zero. Such correlations have been used to establish the solubility parameters of many polymers and these data are also collected in extensive tables [27, 28]. 

In the Theoretical Considerations (Sect. 3.2.1), of polymer swelling, it was stated that the number, as, of adsorbed molecules per accessible monomer unit of crosslinked polymer at liquid-saturation (Eq. 15) can be calculated directly from the observed swelling power, C, as defined in Eq. 14. It follows, therefore, that Eq. 14 can be re-written in the form ... 

On the other hand, if crosslinking is too high, a sufficient swelling of the polymer is impossible as the polymer chains are too tightly bonded and therefore the entry of substrate molecules into the polymer becomes more and more restricted. As a consequence, a significant decrease in activity is observed [38, 43]. 

The situation is a little different in the case of a crosslinked ASE with an anionic surfactant such as sodium lauryl sulfate (SLS), as shown in Figure 5.7. The overall shapes of the curves at the different SLS contents are similar to that of the aqueous solution. However, the curves are shifted downwards, illustrating a loss in efficiency of the polymer. This, however, is not a surfactant effect, but an electrolyte effect, showing how the ionic strength of the matrix is reducing the swelling of the polymer and reducing its efficiency. 

As polymerization proceeds inside the monomer droplets, polymer will precipitate as a separate phase. The droplets th i constitute a two-phase syston, namely precipitated polymer swollen with monomer and Y-compound, and a continuous phase consisting of monomer, Y-compound and traces of polyma. Such situations, which bear some resemblance to cases who-e precipitation due to crosslinking takes place, have been treated by Ugelstad et al. [86]. An inq)or-tant conclusion of these calculations, which was also verified eiqperimaitally, was that whether a network of crosslinked polymer is formed within the miniemulsion droplets, or the droplets consist of two phases due to precipitation of polymer, the swelling capacity is maintained because the swelling is determined by the presence of Y-compound in the monomer-rich phase. 

Determining the solubility parameters of crosslinked polymers uses method based on their maximum swelling in binary solvents of variable composition. The two components of the binary solvent are selected in such a manner that the solubihty parameter of a polymer occupies position intermediate between the parameters of the components. The method is based on the assumption that the maximum swelhng of the polymer occurs when the solubility parameters of mixture and polymer are equal. There should be a maximum on the plot of swelling ratio versus binary mixture composition. 

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