Weak polybase

This conclusion is based on a high char yield and similar rates of mass loss at 300°C for untreated and boric acid treated samples. Boric acid samples also had much higher AH+ s and, consequently, higher Ea s. Our results suggest that certain thermally-stable, weak polybasic acids which can complex with polysaccharides may provide fire-resistant properties to lignocellulosics. The results and conclusions were strongly influenced by the technique used to analyze the TGA data. 

Oppositely charged polyelectrolytes interact with each other to form polyelectrolyte complexes in solution, the possible combinations including strong poly-acids-strong polybases, strong polyadds-weak polybases, weak polyadds-strong polybases, weak polyadds-weak polybases, or polyampholytes. Consid-... 

In the system of a weak polyaeid and a weak polybase, whose degrees of dissociation depend on the pH of the solution, the composition of the obtained complex is as follows ... 

Fig. 8. pH Dependence of the com-plexation ability of the combination of a weak polyadd with a weak polybase. 

Under acidic conditions, weak polybases (P2VP or PEI) are almost all protonated but a weak polyacid (PMAA) is scarcely dissociated. Thus, the complex of PMAA with the proton-accepting polymers (PEO or PVPo), which may form hydrogen bonds with PMAA, is preferentially formed. 

At neutral pH, both a weak polyacid and a weak polybase are partially ionized, resulting in the formation of the polyelectrolyte complexes. 

Some papers60-61 have been devoted to phase separation of polyionic complexes from partially furated (PVA-S) and aminoacetylated (PVA-AAC)poly(vinyl alcohol) in aqueous salt solutions. The separation liquid-liquid or complex coacervation occurs at a definite value of the charge density on the macromolecule. From the concentration dependence of the reduced viscosity of the initial components PVA-S, PVA-AAc and their equivalent mixture in water it follows that the viscosity of the components noticeably increases with dilution, and the curve of the equivalent mixture is concentration independent. This fact confirms the formation of the neutral polymer salt, due to electrostatic interactions of PVA-S (strong polyadd) and PVA-AAc (weak polybase). 

A large number of macromolecules complementary to PMAA, namely polyvinylpyrrolidone, polyvinylpyridine, polyacrylamide, poly(vinyl alcohol), poly(ethylene oxide), oligoethylenimine, poly(sodium styrene sulfonate), polycations of the integral type ionen (2X) were used as P2 and P3. The pH of the media strongly influences the studied reactions of complex formation. For example, in PVPy + PVP + PMAA or OEI + PEO + PMAA systems in the add region, where weak polybases are completely protonized and PMAA does practically not dissodate, complexes with hydrogen bonds (PMAA-PVP or PMAA-PEO) are formed. In neutral medium weak polybases are partially ionizated and polyelectrolyte complexes (PMAA-PVPy, PMAA-OEI) are generated. In the alkaline medium formation of complexes has not been observed. 

The exchange reactions between salts of polymer adds (bases) and weak polybases (polyadds) in aqueous solutions are accompanied by considerable pH changes and also by the appearance of turbidity, particularly if the components are mixed in equivalent quantities. The copredpitation of polymeric adds and polybases was described first by Fuoss and Sadek This behavior of the mixture of two oppositely charged polyelectrolytes can be explained by the formation of a polyelectrolyte complex, this reaction being accompanied by elimination of a low-molecular weight acid or base. Thus, the exchange reaction between poly(acrylic add) and pdly(4-vinyl-ethylpyridinium bromide) was shown by potentiometry and turbidimetry to result in the precipitation of an insoluble macromolecular product, i.e. the ionic comj ex, and... 

The collapse of a polymer gel in response to a change in environment can be scaled down to the single chain level. A layer of polymer chains grafted to a surface forms a polymer brush, for which the collapse transition can be nicely observed using neutron reflectivity. A pore lined with a responsive polymer brush will form a selective valve for example if the grafted polymer is a weak polybase, in aqueous acidic conditions the brush will be charged and will expand to close the pore, while in basic conditions the brush will be neutral. This principle has been used to create a selective membrane, which shows greatly reduced permeability in acidic conditions. 

In case the host chain is a weak polyacid or weak polybase, similar effects can be induced by changes of the pH value, suppressing the ionization of the single stranded blocks. For example, a PEC between Na-poly(meth-... 

Weak polybasic and polyacidic drugs exhibit even more complex pH-rate profiles. For example, a weak base with three ionizable groups has three ionized forms in addition to the un-ionized form, the degradation of each species being potentially catalyzed by hydronium ion, hydroxide ion, and water, Therefore, the apparent rate constant, kobs, could theoretically contain up to 12 terms, and the drug would exhibit a complex pH-rateprofile, as illustrated in Fig. 52.355 356... 

Furthermore, polymers such as PDMAEMA combine a weak polybase character with thermoresponsive properties at high pH and low temperatures PDMAEMA is not ionized, but, nevertheless, is soluble in water. An increase in temperature, however, leads to an increase in the hydrophobicity of the monomers and, at T > LCST, the unionized polymers collapse, lose solubility, and precipitate from aqueous solution [47]. 

An important parameter that characterizes the polyelectrolyte behaviour of this weak polybase is its intrinsic pK, pKo, which is the pK-value extrapolated to zero charge. According to Katchalsky, the apparent value of pK, pKa, is defined by [68]... 

The stoichiometry of PECs of weak polyacids and weak polybases is pH dependent, because of the variation of their dissociation degree with pH. The composition of the PEC is then given by ... 

It is noteworthy that deviations from universal calibration have been observed on PW gel for weak polybases, namely PVA and PEI (62). At neutral or basic pH, such polymers may display hydrophobic character. The hydrophobicity of PW gel is indicated by the elution behavior of aliphatic alcohols, and the influence of methanol, as co-eluant, on the retention of those solutes (64). 

PDMAEMA is a weak polybase with a pKa of about 7 in aqueous solution, which is significantly higher than the pKa of 4-5 that was found for the surface-grafted PDMAEMA [54,55]. This difference can be explained by the fact that the surface-bound chains are located in close proximity to each other, and electrostatic repulsions... 

They found similar behavior for the acid salts of most strong and weak polybasic acids. The primary alkali salts exhibited 1-1 electrolyte behavior, the secondary salts could be treated as 1-2 electrolytes and the ternary salts as 1-3. The potassium, sodium and ammonium salts of HsPOi, and H3ASO1, therefore fit into the Meissner plot. They also claimed that monosodium salts of succinic and malonic acids were treatable as 1-1 electrolytes the disodium salts of fumaric and maleic acid and the bisulfates could be treated as 1-2 electrolytes. 

A weak polybase is fully charged at low pH and uncharged at high pH, whereas weak polyacids are fully charged at high pH and uncharged at low pH. The exact pH at which weak polyelectrolytes become fully charged of course depends on the chemical nature of the polymer. 

Amphiphilic diblock copolymers undergo a self-assembly micellar process in solvents that are selective for one of the blocks [100]. By choosing selective conditions for each block, conventional micelles and so-called inverse micelles can be formed. Examples of the so-called schizophrenic micelles were reported [101]. In this case hydrophilic AB diblock copolymers can form micelles in an aqueous solution, in which the A block forms the inner core and inverted micelles (with the B block forming the iimer core) [102]. A diblock copolymer with two weak polybases, (poly-[2-(N-morphoUno)ethyl methacrylate-i)Iock-2- and (diethyl amino)ethyl methacrylate) (PMEMA-block-DEAEMA), forms stable micelles with DEAEMA cores by adjusting the pH value of the solution. The formation of inverted micelles (MEMA core) was achieved by a salting out effect by adding electrolytes to the aqueous solution. 

The transition from regime I to regime III has also been monitored by changing the charge density of the weak polyacid NaPMA or of the weak polybase poly(L-lysine) [225]. 

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