Formation of Porous Polymers

Polymers with broad molecular weight distributions or with specially constructed molecular weight distributions can be extracted of low molecular weight oligomers to form a substrate with a different morphology. We illustrate this concept using isotactic polypropylene. 

The polymer discussed is composed of an artificially assembled distribution 

The synthetically prepared polypropylene mixture is composed of 50% parent polymer and 50% extract. The substrate shape that is tested to examine the feasibility of the concept is a thin sheet. To form the sheet, a few grams of the synthetic mixture is pressed between heated platens maintained at 170°C. The thickness of the pressed preform is 4 mil. Depending upon the amount of polymer used, the sheets obtained were 3-5 in in diameter. 

The pieces for extraction were preforms cut into small sections one was maintained for SEM examination, the others were extracted at a variety of conditions. The extraction system is a variation of the flow-through system shown in figure 4.1. 

The tests on porous polymer formation described in this section are quite rudimentary. Certainly more need to be performed before such factors as pore size and size distribution can be controlled or predicted. Examination of swelling, plasticization, and melting phenomena in a view cell determination of molecular weight distributions of the extract and substrate (and of the parent polymer) and investigations of other thermoplastic polymers are just a few areas for study. In spite of the rudimentary nature of the experiments, the feasibility tests have established that the concept is reduced to practice, at least 

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C/W emulsions may be used as templates in the formation of porous polymers, as shown in Figure 5 (52). Polymerization takes plaee in the aqueous phase eontinuous channels between the CO2 droplets. The CO2 is vented and the water is removed to form a porous polymer. The median pore diameter on the order of 1 pm reflects the size of the original CO2 droplets. Polymer foams may be used as adsorbents, as substrates for catalysts, and as scaffolds in biomedical engineering. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

The viscosity or resistance to flow increases as the number of repeat units increases, but physical properties, such as surface tension and density, remain about the same after a DP of about 25. The liquid surface tension is lower than the critical surface tension of wetting, resulting in the polymer spreading over its own absorbed films. The forces of attraction between polysiloxane films are low resulting in the formation of porous films that allow oxygen and nitrogen to readily pass though, but not water. Thus, semipermeable membranes, films, have been developed that allow divers to breath air under water for short periods. 

Popcorn polymers are hard, brittle, highly crosslinked porous masses, named as such because of their physical appearance. The formation of popcorn polymers in industrial polymerization processes is highly undesirable. The formation can be suppressed by suitable crosslinking inhibitors. However, in order to avoid the formation on the walls of a reactor that are mainly in contact with the gas phase, volatile crosslinking inhibitors must be chosen. Examples for volatile crosslinking inhibitors are nitric oxide and sulfur dioxide (15,16). 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Cross-linking of 23b by ethylene dimethacrylate (EDMA) leads to formation of porous monolithic sulfobetaine polymers [258]. Alternatively, grafting of the internal surfaces of porous poly(trimethylpropane trimethacrylate) (polyTRIM) by DMAPS provides grafted monoliths. Both synthesis routes yielded polymers capable of interacting reversibly with proteins in aqueous solutions. The SEM pictures show the copolymerized monolith poly(DMAPS-co-EDMA) comprised of spherical units with average diameter approximately... 

The third route is defined as substractive (lUPAC), in that certain elements of an original structure are selectively removed to create pores. Examples include the formation of porous metal oxides by thermal decomposition of hydroxides, of porous glasses by chemical etching, of activated carbons by controlled pyrolysis, of ceramic foam membranes by burning off a polymer (e.g. polyurethane), of alumina by anodic oxidation of aluminium to give oriented cylindrical pores with a narrow size distribution. 

It is concluded that, for the most part, catalysis in the porous crystalline zeolites occurs within the intracrystalline voids. However, some role must be allocated to the external surface, especially at higher temperatures, to explain the formation of isobutylene polymers over chabazite 19), the dehydrohalogenation of ter<-butyl chloride over 5A 20), and the detection of small amounts of triphenylbenzene and triphenylphenanthrene in the condensation of acetophenone over hydrogen Y (HY) zeolite 21). Table I shows the external surface areas of... 

A theoretical approach to the formation of porous polymeric membranes is demonstrated through the phase separation phenomena of polymer solutions. 

The theory presented here for the formation of porous polymeric membranes will also be applicable to the formation of dense membranes. In the latter membrane formation, the solution at stage (b) In Figure 2 (or at most between stages (b) and (c)) Is dipped Into a nonsolvent bath and rapid coagulation and gelation occur. If this is true, the theory predicts the existence of heteogenelty on order of some tens of nanometres, even In the dense polymer membrane. 

Many aspects of the formation of symmetric or asymmetric membranes can be rationalized by applying the basic thermodynamic and kinetic relations of phase separation. There are, however, other parameters-such as surface tension, polymer relaxation, sol and gel structures-which are not directly related to the thermodynamics of phase separation but which will have a strong effect on membrane structures and properties. A mathematical treatment of the formation of porous structures is difficult. But many aspects of membrane structures and the effect of various preparation parameters Can be qualitatively interpreted. 

Partap S, Hebb AK, Rehman I et al (2007) Formation of porous natural-synthetic polymer composites using emulsion templating and supercritical fluid assisted impregnation. Polym Bull 58 849-860... 

Often these polymers are insoluble, although in several cases this can be overcome by adding bulky substituents. Several coordination polymers are stable only in the solid crystalline state. Properties so far described are electrical conductivity, photoactivity, non-linear optical behavior, liquid crystallinity and the formation of porous networks. 

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