Polymers hypercrosslinked

Hypercrosslinked polymers (HCPs) which are obtained by polymerization of monomers with crosslinking molecules to give a highly microporous material (Figure 2.6) [11]. 

In contrast to the above discussion, theoretical treatments favour the model of the molecular micelle [195, 207]. At least an analogue to the latter has been realized recently in some dendrimers, arborols (Fig. 35) and hypercrosslinked polymers, replacing the self-organization of hydrophobic moieties by covalent bonding [41-45],... 

Another distinguishing feature of hypercrosslinked polymers is that they swell with the non-solvent, ethanol, to almost the same extent, but at a somewhat slower rate. Probably toluene, by readily solvating polystyrene chains, acts as a plasticizer, whereas ethanol cannot facilitate conformational rearrangement to the same extent. 

Figure 7.21 shows micrographs taken in polarized light for dry beads of the initial styrene-0.5% DVB copolymer (Fig. 7.21a) and hypercrosslinked polymer on its base (Fig. 7.21b). Both the polymers are transparent, though the hypercrosslinked one is a highly porous material with a significant free volume and apparent specific surface area as large as 1300 m /g. If now one crosses the polaroids, the dry bead of the isotropic gel-type polymer remains transparent (Fig. 7.21c), while the hypercrosslinked polymer reveals its inner stains and optical anisotropy in the form of a clear Maltese cross (Fig. 7.21d).

Accordingly, it is legitimate to assume that in the course of sweUing with toluene in a dry bead of a hypercrosslinked polymer the initial stress of shrinkage should be replaced by a final stress of extension. Indeed, as a droplet of toluene falls on the dry bead and the latter starts to swell, one can observe in the microscope the cross disappearing for a while at the moment the solvent reaches the bead center, and, then emerging again, in the form shown in Fig. 7.22. Here, it is important to emphasize that the transition of... 

Figure 7.26 Sorption isotherm of nitrogen at 77 K on the hypercrosslinked polymer prepared by post-crosslinking styrene-0.3% DVB copolymer with monochlorodimethyl ether to 100%. Measured by Micromeritics Instrument Corporation.

In contrast to the main tendencies known for truly macroporous copolymers, the dilution of initial linear polystyrene solutions was observed to have no dramatic influence on the apparent inner surface area of hypercrosslinked polymers. The surface area rather shows a tendency to decrease with dilution, contrary to the pore volume (Table 7.9). 

The replacement of one good solvent, EDC, for another good solvent, nitrobenzene or ethylene tetrachloride, used to synthesize hypercrosslinked polymers, exerts no influence upon the apparent inner surface area of the resulting networks. However, the synthesis of hypercrosslinked... 

The porous structure of hypercrosslinked polymers is rather stable and tolerates heating at 150°C for at least 1 h, that is, well above the glass transition point of polystyrene. After heating, the surface area does not change markedly (Table 7.11). 

Similarly, treatment of polymers with various solvents followed by drying at elevated temperatures (Table 7.12) will also not affect noticeably the porous structure of the hypercrosslinked polymers. On the other... 

Table 7.11 Heat resistance of hypercrosslinked polymers prepared in ethylene dichloride [133]...

As opposed to the apparent specific surface area, pore volume of hypercrosslinked polymers is strongly dependent on the conditions of their synthesis. Tables 7.9 and 7.10 demonstrate that the pore volume noticeably increases with increasing dilution of the initial polystyrene solution, both in EDC and cyclohexane. 

Table 7.13 Comparison of apparent inner specific surface area (mVg) measured by low-temperature sorption of nitrogen and argon on hypercrosslinked polymers prepared by crosslinking a linear polystyrene having a molecular weight of 3 x 10 Da, in EDC solution with MCDE and p-XDC...

In spite of the different types of crosslinking agents, the degree of bridging, and the concentration of polystyrene chains in the initial solution, hypercrosslinked polymers exhibit narrow pore-size distribution the diameter of the majority of the pores is about 10 A. The exclusion limit of the polymeric gels is rather small polystyrene standards with coils of50-60 A in diameter in chloroform solution (molecular weight of 12,000-16,000 Da) are completely excluded from the polymer phase. It should be emphasized. 

Note that the homogeneous structure of swollen polymers crosslinked with flexible bis-(p,p-chloromethylphenyl)butane-l,4 bridges (which are non-porous when in dry state) is very similar to that of swollen typical nanopor-ous hypercrosslinked polymers. 

In conclusion, it should be pointed out that none of the physicochemical techniques discussed above permits the direct measurement of the elements of the polymeric materials porous structure we measure the properties of the systems where the polymers interact with certain test substances (nitrogen, mercury, water, polystyrene standards, ions, etc.), and not the dimensions of the pores or other supramolecular elements of the material. Therefore, the evaluation of the surface area and diameters of pores available to the molecules of these substances must be considered as indirect methods of examining the porous structure. Because of this, all calculations are based on assuming certain models of the structure of the material and accepting certain assumptions as to the mechanism of interaction between the material and test molecules. Only transmittance, scanning, and, in particular, atomic force microscopy can be considered as direct methods of measuring dimensions and distances. However, up to now the last technique has not been appHed to microporous hypercrosslinked polymers. 

Thus, electron microscopy did not reveal any visible pores in porous hypercrosslinked materials obtained from linear polystyrene or styrene copolymers in a good solvent. Consequently, the size of these pores is smaller than the resolution of electron microscopy. Also, this technique did not reveal any supramolecular constructions, the appearance of which could be attributed to microphase separation. Hence, hypercrosslinked polymers prepared in thermodynamically good solvents, such as EDC, have to be considered as singlephase materials. (As will be shown in Section 7, microphase separation can occur during hypercrossHnking in a poorer solvent, cyclohexane.)... 

The behavior of hypercrosslinked polymers under uniaxial compression and/or heating was studied using a technique that was specially developed... 

The maximum deformation AD of this 100% hypercrosslinked polymer achieves the value as high as 30% when the temperature approaches 300°C. Although this deformation is characteristic of rubber-like elasticity (Fig. 7.38, plots 1 and 2), no typical plateau can be observed (plot 3). This statement is also valid for the whole set of plots obtained at varying loadings in the interval from 10 to 450 g per bead (Fig. 7.39). AH plots have a flat... 

Thus, we can conclude that the shrinking of the hypercrosslinked polymers at temperatures in the range of250-300°C is caused by oxidation, with the rupture of a certain portion of the stressed carbon—carbon bonds. Under the permanent tendency for reducing the free volume of the material, the scission of the most stressed bonds leads unavoidably to the formation of a more compact structure and to the decrease of the beads volume and their inner specific surface area. The rupture of bonds can then be followed by their partial recombination or termination with oxygen. The latter process seems to proceed to a lower extent, since no noticeable weight change of the air-exposed samples was observed. 

Interestingly, the presence of residual chlorine in the hypercrosslinked polymer enhances the yield of the final carbonizate (plot 1 in Fig. 7.48). It was found that the presence of carboxyl or sulfonic substituents in the aromatic rings of the polymer also enhances the yield of final carbons [207]. In fact it has been known [208] that sulfbnated styrene—DVB copolymers can be used for the preparation of carbonaceous adsorbing materials. However, because of the ehmination of heavy sulfonic substituents (over 40% of the material weight), the final yield of carbons is by no means hi er than in the... 

Figure 7.52 Deswelling of various polymers initially swollen with toluene. Macroporous polymers (1) polydivinylbenzene, (4) styrene-15% DVB copolymer. Hypercrosslinked polymers (2) styrene-0.6% DVB copolymer crosslinked to 200% with MCDE, (5) linear polystyrene crosslinked to 200% with XDC. (3) Gel-type non-porous styrene-2.7% DVB copolymer. (After [159]).

Hypercrosslinked Polymers - A Novel Class of Polymeric Materials... 

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