Comb stabilizer

An alternative (and perhaps more efficient) polymeric snrfactant is the amphipathic graft copolymer consisting of a polymeric backbone B (polystyrene or polymethylmethacrylate) and several A chains (teeth) snch as polyethylene oxide [4], This graft copolymer is sometimes referred to as a comb stabilizer. This copolymer is nsnally prepared by grafting a macromonomer such methoxy polyethylene oxide methacrylate with polymethylmethacrylate. The grafting onto technique has also been nsed to synthesize polystyrene-polyethylene oxide graft copolymers. 

Polymeric surfactants can be homopolymers, random amphiphilic copolymers, and of the A-B (diblock), A-B-A (triblock), and BA (graft) types [23]. The A chain is referred to as the stabilizing chain (soluble in the medium), and the B chain is referred to as the anchor chain (insoluble in the medium with strong affinity to the surface). The simplest type of a polymeric surfactant is a homopolymer, such as PEO and poly(vinyl pyrrolidone) (PVP). Homopolymers are not the most suitable surfactants and it is better to use polymers with some groups that have affinity to the surface. The most employed copolymers are random amphiphilic copolymers, like poly(vinyl alcohol), diblocks of polystyrene-block-poly(vinyl alcohol) (PS-b-PVA), poly(ethylene oxide)-block-polystyrene (PEO-b-PS), and triblocks of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO, Pluronic) (PPO resides at the hydrophobic surface, leaving the two PEO chains dangling inaqueous solution), andpoly(ethyleneoxide)-block-polystyrene-block-poly(ethylene oxide) (PEO-b-PS-b-PEO). The graft copolymer is referred to as a comb stabilizer Atlox 4913,... 

Anotlier model system consists of polymetliylmetliacrylate (PMMA) latex, stabilized in organic solvents by a comb polymer, consisting of a PMMA backbone witli poly-12-hydroxystearic acid (PHSA) chains attached to it [10]. The PHSA chains fonn a steric stabilization layer at tire surface (see section C2.6.4). Such particles can approach tire hard-sphere model very well [111. 

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

The combinational contribution to AG,n for PMMA particles stabilized by PIB in 2-methylbutane is shown plotted as a function of temperature in Figure 3(a). The values of the parameters used in Equations 2 and 3 were u = 8 x 10- g cm-, a = 300 nm, >2 = 1.09 cm g- and V = 116.4 cnr mole- . The thickness of the steric barrier,L, was taken to be 25 nm and the particle separation, do, was fixed at 30 nm. It can be seen from Figure 3(a) that AGj (comb) is a positive quantity that becomes more positive as the temperature increases, indicating that in the absence of other contributions to AG, the particle would become more stable with increasing temperature. In the above calculation, we have assumed that the S function, Equation 3, remains invariant with temperature, which is incorrect. 

The feasibility of the fabrication of comb like fluorocarbonpolymer LB films has been shown. These films can be deposited onto different kinds of substrates as y-type layers by the usual LB technique. In this case the deposition procedure is much simpler than the one for polyimide LB films, but the temperature, chemical and mechanical stability, and dielectricproperties of the fluorocarbonpolymers are not inferior to those of polyimides. The fluorocarbons are more hydrophobic than ordinary hydrocarbons, hence shorter hydrophobic chains can be used and thinner monolayers can be prepared (the PFHA-AA LB monolayer thickness investigated was 16.5 x l(L8cm). 

TABLE V Relative Stability of Oxytetracycline in Packed Comb Cells (micrograms OTC/ml)... 

Ziemniak, S. Jones, M.E. Combs, K.E.S. (1995) Magnetite solubility and phase stability in alkaline media at elevated temperatures. J. Solut. Chem. 24 837—877... 

The graft copolymers were already used for preparation and stabilization of polymer particles by Barrett [1]. He synthesized a poly(12-hydrostearic acid) macromonomer with a methacrylate end group. This macromonomer was copolymerized with MMA to obtain a preformed comb-graft copolymer, which was successfully used as stabilizer in nonaqueous dispersions of MMA. 

This model was used in dispersion polymerization to predict the size of polymer particles stabilized through grafting on hydrophilic polymers such as PVPo. It provides a reasonable description of, for example, PVPo-stabilized polymerization of styrene in polar solvents. The present model does not apply to other types of dispersion polymerization where grafted comb or block copolymer stabilizers are active. The key controlling parameters in this model are the availability of graft and the minimum and maximum coverage, Qmin and Qmax. 

The subtle differences between sugar molecules which cause such dramatic effects (Le. Dulcitol verses sorbitol) are possibly caused by adverse interactions either with water or the amino acids of the protein. It is possible that under certain conditions the position of hydroxyl groups causes strong binding of water and leads to confirma-tional distortion of the protein rather than stabilization. Monsan and Combes (13) have suggested that this is due to excessive binding to the stabilizer thus disrupting the protein surface/water interactions. As yet we have no evidence to further elucidate this theory. 

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