This work documents a sharp adsorption cutoff for a cationic polymer (poly(dimethylamino ethyl methacrylate)) adsorbing onto a negative surface (silica) from buffered aqueous solution, as a function of ionic strength. In this system, electrostatic attractions drive adsorption, without contribution from chemical or hydrophobic interactions between the polymer and the surface. In the limit of dense surface charge and sparse polymer charge, a small increase in monovalent salt concentration (less than a factor of 2) completely eliminates polymer adsorption. At conditions where the polymer charge is more dense and the surface charge less dense, added salt still diminishes adsorption, albeit more gradually. The sharpness of the salt-induced polymer adsorption cutoff can be tuned through the polymer charge density. Comparisons with existing theory agree in the limit of the low-charge sharp-cut behavior. The more gradual transition at high polymer charge density is not accurately predicted.

This paper examines the desorption and self-exchange dynamics of polyelectrolyte chains on an oppositely charged surface, in the limit of weak charge, such that the polymer is readily displaced by added ions. The model system adhering to this physically limiting behavior is poly[(dimethylamino)ethyl methacrylate] [PDMAEMA] adsorbing on silica from aqueous solution at elevated pH. Desorption kinetics follow expectations:  a single-exponential decay consistent with a first-order desorption model, with an activation energy for chain removal that adheres reasonably to the Muthukumar treatment and quantitatively to the Dobrynin−Rubinstein model. Self-exchange kinetics, however, present a surprise:  They are quantitatively identical to desorption, with the exchange process being completely controlled by chain release from the surface. The single-exponential form for self-exchange is atypical, and the quantitative similarity with desorption suggests that, with weakly charged chains, approximately 3−5 segment−surface contacts make up a minimum dynamic unit for adhesion.

The redox initiator, cumene hydroperoxide/ tetraethylenepentamine (CHPO/TEPA), was used to initiate the emulsion polymerizations of styrene and methyl methacrylate (MMA). The hydrophobic CHPO acting as oxidizer would interact with the hydrophilic TEPA employed as the reducer at the particle-water interface where the vinyl monomer is present. The kinetics of the styrene and MMA polymerizations were studied under a temperature range of 30-70°C. The polymerization rate, the concentration of monomer in the particles, the average number of latex particles and the number of radicals per particle were investigated to evaluate the propagation rate constant during interval II of the polymerization. With increasing temperature, the number of latex particles increases while the monomer concentration in the particles slightly decreases. The observed number of radicals per particle, calculated from the number average volume of the particles and the relation between the entry rate coefficients of free radicals into the particles and the exit rate coefficients of free radicals from the particle, was found to be approximately 0.47 and 0.5 in case of styrene and MMA polymerization, respectively. The calculation shows that the propagation rate constant, k p = 6.84 × 10 7 e −37.65/RT and k p =3.4×10 7 e −31.05/RT L/mol s for styrene and MMA polymerizations over the temperature range studied, respectively.

The possibility of prevulcanizing natural rubber latex via ultraviolet (UV) irradiation was investigated using 2-hydroxy-2-methyl-1-phenylpropanone as the photoinitiator and 1,9-bis­(acryloyloxy)­nonane as the coagent. Effects of the following process variables were assessed on the tensile strength and the cross-link density of thin films prepared from the prevulcanized natural rubber (PVNR) latex: the duration of UV irradiation, the distance between the light source and the latex being irradiated, the mixing time of the latex with the coagent and the phtoinitiator, the depth of the latex pool, and the mass ratio of the photoinitiator to the coagent used. The PVNR films produced under optimal conditions had nearly 4-fold the tensile strength of the film produced without UV irradiation. Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance spectroscopy were used to assess the UV reactions, and a mechanism was proposed for UV-induced prevulcanization with the above identified photoinitiator and coagent.

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