Polymer bimodal size distribution

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

Polymer latex particles[169] in the range from 20 to 400 nm (or larger) with different surface functionalities can be employed as templates for the synthesis of macroporous materials. The route of templating polymer dispersions is complementary to the synthesis in lyotropic liquid-crystalline phases, leading to a bimodal size distribution of the pores. 

One can discern for PTMSN the two broad minima in the curve AHjy that correspond to the values of Vc of 426cmVmol (u-Ct) and 754cmVmol (n-Cia). The presence of the two minima is rather unexpected because in most cases only one minimum was observed in the curves AHj[Vc). The only exception so far was amorphous Teflon AF2400, where also two minima, though flat and broad, could be noted [19], Here we have apparently another manifestation of bimodal size distribution of free volume in glassy polymers that attracts now a keen interest [33], The error bars shown in Figure 3.6 seem to support the assumption of bimodal size distribution in this polymer. 

Poslinski et al. [72] undertook the study of the influence of a bimodal size distribution of glass spheres on the rheology of filled polymer systems as already discussed in section 6.4. The bimodal size distribution was prepared by mixing together various vol% of tiie smaller 15 )un spheres with the larger 78 pm spheres given in Table 6.3. Various bimodal mixtures were compounded into a polybutene grade 24 polymer obtained from Petrochemical Division of the Chevron Company, at total solids concentrations up to 60% by volume. 

Moreover, it will be especially interesting to test these results also by molecular modeling. The creation of well equilibrated, realistic amorphous packing models for PTMSP and the polymers having similar high FV is the major prerequisite to determine if there is, in fact, a bimodal size distribution of FV elements, whether the FV elements form an infinite or, at least, large, in the microscopic scale, cluster, and, therefore, why PTMSP and similar materials are so permeable. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Moreover, the size of the polyacrolein blocks depends on the ratio of the propagation rate constant kpr to the transfer rate constant hm (kpr/hm). This ratio may depend on the nature but also on the complexatlon of the living ends. Indeed, an evolution of the complexatlon state exists for the (1,4) living end and may involve a variation of (kpr/hm) (1,4) during the propagation. The previous mechanism can explain the bimodal weight distributions obtained for both homopolyacrolelns and block polymers. 

Microcellular foaming, bimodal cell size distributions, and high open-celled contents of molecular composites of HT-polymers were reported by Sun et al. [33], investigating blends of a rod-like polymer polybenzimidazole with an aminated PSU and poly(phenyl sulfone) by using carbon dioxide as a blowing agent. The complex foaming behavior was related to phase separation within the otherwise... 

Several approaches towards the synthesis of hierarchical meso- and macro-porous materials have been described. For instance, a mixture that comprised a block co-polymer and polymer latex spheres was utilized to obtain large pore silicas with a bimodal pore size distribution [84]. Rather than pre-organizing latex spheres into an ordered structure they were instead mixed with block-copolymer precursor sols and the resulting structures were disordered. A similar approach that utilized a latex colloidal crystal template was used to assemble a macroporous crystal with amesoporous silica framework [67]. 

Figure 3-22. Cumulative pore volume distribution of different HPLC columns indicating monomodal pore size distribution for polymer monolith and bimodal distributions for both packed particulate silica and silica monolith columns. (Reprinted from reference 90, with permission.)...

M. Antonietti, B. Berton, C. Goltner, and H.P. Hentze, Synthesis of Mesoporous Silica with Large Pores and Bimodal Pore Size Distribution hy Templating of Polymer Latices. Adv. Mater., 1998, 10, 154-159. 

Based on the gas permeation measurement results on a variety of silica membranes, we have also reported similar findings [7]. We reported that the pore size distribution of sol-gel silica membranes appear to be bimodal with majority pores with sizes around 0.3 nm and some pores with much larger sizes formed due to the opaqueness of silica polymer clusters. Figure 16.5 shows the permeation results of silica membranes. The four membranes used in the study had been tailored with different porosity values. In spite of the differences in porosity values between the membranes, all the membranes showed ability to separate He from other molecules but the ability to separate between molecules of sizes greater than 0.3 nm was poor. Figure 16.6 shows a schematic representation of the separation behavior exhibited by silica membranes in comparison to a zeolite ZSM-5 membrane [7, 16, 36]. 

Antonietti M, Berton B, Goltner C, Hentze HP (1998) Synthesis of mesoporous silica with large pores and bimodal pore size distribution by templating of polymer latices. Adv Mater 10 154... 

Graphical differentiation of the Kj D function demonstrated that imlike the hypercrosslinked networks obtained in EDC, the networks prepared in cyclohexane display bimodal pore size distribution. In addition to the pores of the same size as those in the polymers obtained in EDC, these polymers have larger pores with diameters of up to 350 and 250 A for the samples with degrees of crosslinking of 40 and 100%, respectively. The formation of such large pores appears to be a s p of phase separation during network preparation. Indeed, these polymers do not look transparent any more. 

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