Asymmetric polymers in bicontinuous microemulsions and their accretion to the bending of the membrane

Abstract

After linear hydrophilic polymers with a short hydrophobic sticker have been proven to be highly suitable as efficiency boosters in microemulsions, a whole set of asymmetric polymers was studied in more detail. Sticker polymers with two hydrophilic polymeric arms, hydrophobic polymers with ionic stickers, and a Y-shaped tri-arm-polymer were studied for their high surface activity. While low asymmetry of the Y-shaped polymer resulted in very similar dependence as a diblock copolymer, the highly asymmetric polymers showed a renormalized behavior of the bending rigidity coming from preferential accretion of the polymer at the membrane. The experimental evidence for the accretion of the polymer according to the preferential membrane bending is reported here for the first time. Furthermore, the isolated, diffuse counter ion clouds of the ionic sticker polymers act in a way similar to that of a polymer anchored at the membrane. All asymmetric polymers are highly suitable as efficiency boosters in microemulsions.

Appendix A

The PEP-(PEO)₂ star-block copolymer was prepared by a five-step process. A detailed reaction scheme is presented in Online Resource 1. The first step of the synthesis includes the anionic polymerization of isoprene using t-butyllithium as initiator and benzene as polymerization solvent. The living polyisoprene (PI) chains were endcapped with trichloromethylsilane. Thereby, a large excess of the chlorosilane linking agent was used such that multiple substitution could be essentially avoided. Thus, the resulting PI polymer still carried two remaining chloro functions in terminal position, PI-Si(Me)-Cl₂. In a subsequent second step the difunctional PI-Si(Me)-Cl₂ was reacted with a small excess of the organolithium compound 3-(t-butyldimethylsilyloxy)-1-propyllithium. This material, usually used as an OH-protected functionalized initiator for anionic polymerization, was prepared according to a procedure described by Schwindeman [33]. The reaction with the difunctional PI-Si(Me)-Cl₂ yielded the corresponding PI-Si(Me)-(3-(t-butyldimethylsilyloxy-1-propyl))₂ which was isolated by precipitation in methanol. In the third step, the OH-functionalized PI was converted to poly(ethylene-alt-propylene) (PEP) by catalytic hydrogenation using an unreduced Pd on Bariumsulfate catalyst in heptane at 80 °C. In a fourth step, the difunctionalized PEP polymer was deprotected from the t-butyldimethylsilyloxy-groups. This was achieved by adding a slight stoichiometric excess of 1.25 m hydrochloric acid to a polymer solution in THF and stirring at room temperature for more than 4 h. The deprotected PI-Si(Me)-(3-hydroxy-1-propyl)₂ was again isolated by precipitating in methanol. The polymer was dried in high vacuum for several days in order remove all volatile protic impurities. Finally, in the fifth step, the hydroxy groups of the functionalized PEP polymer were transformed to the corresponding potassium analog PI-Si(Me)-(3-(KO-1-propyl))₂. This was achieved by titration with freshly prepared potassium naphthalenide solution in THF. The potassium compound was then taken as macroinitiator for the polymerization of ethylene oxide. The polymerization was performed at 50 °C for 24 h and then terminated with acetic acid yielding terminal OH groups. The resulting PEP-(PEO)₂ star-block copolymer was precipitated in acetone at −20 °C and finally freeze-dried from benzene in high vacuum. The final PEP(PEO)₂ star-block copolymer as well as the precursors were characterized by size exclusion chromatography (SEC) and ¹H-NMR. SEC-chromatograms were obtained with a PL GPC 220 from Polymer Labs equipped with a refractive index detector. Three Polypore (Polymer Labs) columns with a continuous pore size distribution were used with a 85/15 mixture of tetrahydrofuran and N,N-dimethylacetamide as eluent at 50 °C. Molecular weight distributions were calculated either from PEO-calibration (PEP(PEO)₂) or PS-calibration (PI and PEP). ¹H-NMR spectra were recorded with a Varian Inova 400-MHz spectrometer. All samples were measured at 295 K in CDCl₃ with 5-mm PFG Auto X DB Probe. A small sample of the uncapped PI polymer was removed as reference material for separate characterization. The number average molecular weight, Mn = 5,240 g/mol, was calculated from the NMR spectrum taking the nine protons from the t-butyl initiator group as internal standard. The molecular weight distribution, M _w/M _n, was smaller than 1.03 as calculated from SEC data using PS calibration. SEC-chromatograms of functionalized and unfunctionalized PI are shown in Online Resource 2, corresponding ¹H-NMR spectra in Online Resource 3. From the NMR spectra, by comparing the integrals of the t-butyl group of the initiator with those of the protecting group, one can conclude that the functionalization is close to 100 %. SEC data, however, reveal a small fraction of a polymer with higher molecular weight, most likely due to a double substitution of living PI chains to the crosslinking agent. The molecular weight is increased by the protecting group as can be seen from the slight shift of the SEC curve. The corresponding number average molecular weight is calculated to be Mn = 5630 g/mol. The completeness of the hydrogenation was verified by the disappearance of vinyl-protons in the ¹H-NMR-spectrum, which further indicates that the functional groups remained unaffected by the saturation process (Online Resource 4). The deprotection from the t-butyldimethylsilyl-group was monitored by ¹H-NMR by following the disappearance of the 12 protons for the four methyl-groups at 0.03 ppm (see Fig. S3). Complete deprotection was achieved after 4 h. SEC-data of the PEP precursor after deprotection ((3-hydroxy-1-propyl)₂-Si(Me)-PEP) and of the final product (PEP(PEO)₂) are shown in Online Resource 5. The SEC data of PEP-(PEO)₂ revealed a narrow molecular weight distribution of M _w/M _n<1.06. Small fractions of high and low molecular weight byproducts (≈5 %) additionally appear in the chromatograms. For the purpose of this work, however, these small amounts of impurities were neglected and the origin not further explored. The ¹H-NMR -spectrum of the final product is shown in Online Resource 6. Molecular weights of the PEO chains were calculated from the integrals of the PEO-methylene protons at ≈3.5 to 3.9 ppm relative to the aliphatic signals between 0.7 and 1.5 ppm of the PEP polymer with known molecular weight. Thus, the PEO number average molecular weight was determined to be Mn = 14250 g/mol (7125 g/mol/PEO-chain) which gives an overall M _n of ≈19.8 kg/mol for the PEP(PEO)₂ star-block copolymer.