Steric stability

Interestingly, this behavior of the reaction mixture can be prevented by employing another principle of particle stabilization steric protection. Inclusion of pegylated comonomer (PEG-AEPD) into the reaction mixture did enable the formation of nonaggregating DNA particles. It also caused the particles to form worm -like structures (as judged by transmission electron microscopy) that have previously been observed with DNA complexes formed from block copolymers of PEL and PEG [98]. 

Steric Stabilization. Steric stabilization was a term first introduced by Heller to explain how adsorbed polyethylene oxide polymers increased the salt concentration required for flocculation of negatively charged aqueous suspensions.(6) Heller s systems were stabilized by both mechanisms, as are most commercial dispersions today, aqueous and non-aqueous. Much of the more recent literature on steric stabilizers has been preoccupied with solubility requirements, for the solubility of polymers is a delicate matter and very sensitive to temperature and solvent... 

Scheme4.59. Ring opening of epoxides by nucleophilic attack or acid or base-mediated rearrangement. R = carbocation-stabilizing, sterically demanding group.

Often in ceramic processing, where the surface potential is small or the double layer thickness is thin, the electrostatic repulsion is not sufficient to stabilize the colloidal suspension against coagulation. As a result another form of stabilization is needed—steric stabilization. Steric stabilization has been reviewed by two recent books, one by Napper [27] and the other by Sato and Rudi [26]. The following presentation draws heavily fiium both these books. 

Tile 4-position is attacked rather than the tertiary carbon atom 2-position as would be expected from radical stabilization steric hindrance favors the 4 osition in relation to the 3-posiiion (electronically stabUized for radical or cation formation). 

A suspension is a dispersion of particles within a solvent (usually a low-molar-mass liquid). Thermodynamics (Brownian motion and collisions) favours the clumping of small particles, and this can be increased by flow. However, particles over 1 pm tend to settle under gravity, unless stability measures have been considered (matching the density of the particle to that of the medium, increasing the Brownian/gravitational force ratio, electrostatic stabilization, steric stabilization). Other complications can occur in the dynamics of suspensions, such as particle migration across streamlines, particle inertial effects and wall slip (Larson, 1999). 

Several factors influence whether a reaction will occur by an Sn1 or Sn2 mechanism carbocation stability, steric effects, strength of nucleophile, and the solvent. Tertiary halides tend to react by the SN1 process because they can form the relatively stable tertiary carbocations and because the presence of three large alkyl groups sterically discourages attack by the nucleophile on the carbon-halogen bond. The Sn2 reaction is favored for primary halides because it does not involve a carbocation intermediate (primary carbocations are unstable) and because primary halides do not offer as much steric hindrance to attack by a nucleophile as do the more bulky tertiary halides. Strong nucleophiles favor the Sn2 mechanism and polar solvents promote SN1 reactions. 

This mechanism has been used to separate isomers, but contrary to the above argument, the reverse order of Rv values has also appeared in the literature [17— 23], The differences in Rv values for cis and trans isomers may originate in the differences in dipole moments, solubilities, ion-pair formation between the complex cation and the anion present in the species, stability, steric hindrance, adsorp-tivity and symmetry of the complexes. All these physical differences allow the use of the solubility of complexes to separate their isomers. 

Reversible flocculation of aqueous dispersions stabilized sterically with POE nonionics can be accomplished by increasing the temperature. With increase in temperature, the hydrogen bonding of the POE chains to water is reduced and the chains tend to aggregate, flocculating the dispersion. Upon reducing the temperature, the chains hydrate again and the particles redisperse. 

Describe the two paradigms of colloidal stabilization, steric and electrostatic, and provide an example of each. 

There are at present two different mechanisms whereby polymer chains can impart colloid stability steric stabilization and depletion stabilization. 

A) Steric stabilization. Steric stabilization of colloidal particles is imparted by macromolecules that are attached (e.g. by grafting or by physical adsorption) to the surfaces of the particles. This is represented schematically in Fig. 1.5. Over recent years there have been an impressive number of reviews of steric stabilization to which the reader is referred (Ottewill, 1969 Napper, 1970c Medalia, 1971 Napper and Hunter, 1972 Ottewill, 1973 Vincent, 1973a Vincent, 1973c, 1974 Barrett, 1975 Napper and Hunter, 1975 Napper, 1977 Parfitt and Peacock, 1978 Ottewill, 1980 Tadros, 1980 Laible and Hamann, 1980 Sato and Ruch, 1980 Vincent and Whittington, 1981 Tadros, 1982). These reviews show clearly the rapid expansion in our... 

It should further be noted that any agreement between theory and experiment presented in this section may be somewhat fortuitous in that the dispersions studied experimentally were sterically stabilized. Steric layers, unless very thin, can influence profoundly the quantitative effects of free polymer. This is vividly illustrated by the exponents reported by de Hek and... 

Polycarbodlmlde Polymers containing-N=C=N- linkages in the main chain, typically formed by catalyzed polycondensation of polyisocyanates. They are used to prepare open-celled foams with superior thermal stability. Sterically hindered polycarbodimides are used as hydrolytic stabilizers for polyester-based urethane elastomers. 

In addition to electrostatic stabilization, steric stabilization was found to be an important factor in the microemulsion polymerization of MMA. A comparative stability study was performed on MMA polymerization in tetradecylammonium bromide (TTAB) or stearyltrimethylammonium chloride (STAC) microemulsions [128]. A strong increase in particle size was observed above 50% conversion for the former system, whereas the size remained rather constant up to 98% conversion for the latter. Similarly, ftp increased from 1.75 to 35.6 when the conversion went from 61% to 93% for TTAB systems but remained around 1.4 for STAC systems. These results give clear evidence of the occurrence of particle... 

PP for textile applications must be stabilized. Sterically hindered amines and phenol derivatives are most often used as light and thermal stabilizers. 

CoUoidally stable polyplexes are PEC systems in the transition range to phase separation, exhibiting an observable light scattering or Tyndall effect [65], These systems can be stable because of electrostatic stabilization, steric stabilization, or a combinatimi of both called electrosteric stabilization. 

Proteins act in a similar way to polymeric stabilizers (steric stabilization). However, molecules with compact structures may precipitate to form small particles that accumulate at the oil/water interface. These particles stabilize the emulsions (sometimes referred to as Pickering emulsions) by a different mechanism. As a result of the partial wetting of the particles by the water and the oil, they remain at the interface. The equUibrium location at the interface provides the stability, since their displacement into the dispersed phase (during coalescence) results in an increase in the wetting energy. 

Keywords Antioxidants, dehydrating agents, PBT, PET, phosphate stabilizers, phosphite antioxidants, stabilizers, sterically hindered carbodi-imides, sterically hindered phenols, thermal degradation, thermo-oxidative degradation, thermo-oxidative stability. 

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