Bis(diphenylphosphino)alkanes quantitatively react with excess 1-bromododecane to prepare novel phosphonium gemini surfactants with spacer lengths ranging from 2 to 4 methylenes (12-2/3/4-12P). Dodecyltriphenylphosphonium bromide (DTPP), a monomeric surfactant analog, was readily water soluble, however, in sharp contrast, phosphonium gemini surfactants were poorly soluble in water due to two hydrophobic tails and relatively hydrophobic cationic head groups containing phenyl substituents. Isothermal titration calorimetry did not reveal a measurable critical micelle concentration for the 12-2-12P phosphonium gemini surfactant in water at 25 °C. Subsequent studies in 50/50 v/v water-methanol at 25 °C showed a CMC of 1.0 mM for 12-2-12P. All phosphonium gemini surfactants effectively complexed nucleic acids, but failed to deliver nucleic acids in vitro to HeLa cells. The solution behavior of phosphonium gemini surfactants was investigated in chloroform, which is an organic solvent where reverse micellar structures are favored. Solution rheology in chloroform explored the solution behavior of the phosphonium gemini surfactants compared to DTPP. The 12-2-12P and 12-3-12P gemini surfactants were successfully electrospun from chloroform to generate uniform fibers while 12-4-12P gemini surfactant and DTPP only electrosprayed to form droplets.

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The large amplitude oscillatory shear (LAOS) and steady shear behavior of star‐shaped SSBR/silica 60 phr (21 vol%) compounds with various filler surface areas was measured and simulated. An SBR gum and SBR compounds containing four different silicas with surface areas of 55, 135, 160, and 195 m2/g were utilized. Rheological behavior indicated clear correlation with surface area. LAOS tests showed an increase in dynamic moduli, shear stress, and higher order harmonic contributions with surface area. Elastic and viscous Lissajous figures showed significant distortion at intermediate and higher strain amplitudes. Additionally, ratios of third and fifth order stress harmonics to the first stress harmonic (I3/1 and I5/1, respectively) showed a ''bump'' at intermediate strain amplitudes for the three highest surface area compounds. With regards to steady shear, all materials showed strong shear thinning behavior, and an increase in shear viscosity with surface area. The Cox‐Merz rule was shown to be valid for the SBR gum but not for the filled compounds. However, the complex viscosity as a function of shear rate amplitude at various frequencies at high strain amplitudes and the steady shear viscosity as a function of shear rate coincided. This correlation, referred to as the Philippoff approach, has important ramifications for the rubber industry, providing quick data for predicting processing behavior. The Simhambhatla‐Leonov model was successfully employed to simulate rheological behavior for the SBR gum and the lowest surface area silica compound, but the model yielded mixed results for the higher surface area silica compounds.

The nonlinear stress relaxation behavior after a step shear strain of star‐shaped SSBR/silica compounds containing 21 vol% filler of various surface areas was measured and simulated using constitutive equations. A styrene‐butadiene rubber (SBR) gum and SBR filled with silica having BET surface areas of 55, 135, 160, and 195 m2/g were used. Relaxation modulus behavior of the filled compounds was found to be dependent on surface area. Specifically, stress relaxation tests indicated that an increase in surface area led to increase in values of relaxation moduli in both the linear and nonlinear regimes. The time‐dependent relaxation modulus exhibited a plateau at long times of relaxation in compounds containing silica of high surface area. Additionally, good time‐strain superpositions were achieved for all samples at intermediate times of relaxation, and the strain‐dependent damping function decreased with filler surface area. The constitutive equations proposed by Leonov and Simhambhatla and Leonov, modified to include multimodal relaxation of the particle network, were used to predict the time evolution of the relaxation modulus in the nonlinear regime for all samples. The simulations provided good results for the SBR gum for all tested strain levels. Also, in the compounds filled with silica, both models satisfactorily described the experimental observation in the nonlinear regime at low strain levels. However, at higher strain levels, due to a possible slip effect, the simulations overpredicted measured values of the relaxation moduli, thus leading to only qualitative predictions of the observed behavior. It is also possible that neither model accurately captured the floc rupture kinetics of these complex rubber compounds.