Effect of Acid–Base Interactions on Conformation of Adsorbed Polymer Chains

Abstract

Sum frequency generation spectroscopy (SFG) and attenuated-total-reflection IR (ATR-IR) were used to investigate polymer adsorption on solid surfaces in CCl 4 (neutral), CHCl 3 (acidic), and acetone (basic) solvents. Fowkes showed that the adsorbed amount of the polymer from acidic and basic solvents is less than that from a neutral solvent (Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3−7). Here, we show that besides the differences in adsorbed amount, chains adsorbed from an acidic solvent adopted a flat conformation with a much smaller ratio of segments of loops and tails to trains compared to those adsorbed from a neutral solvent. Sapphire (Al 2 O 3 ) surfaces were saturated by train segments at 1.3 × 10 −5 volume fraction for both CCl 4 and CHCl 3 solutions, with a large fraction of the surface sites occupied by the PMMA segments, which was different from what was expected based on Fowkes' experiment. In contrast, PMMA segments were not able to replace acetone molecules from the surface in a time period of 2 h. Surface interaction parameters alone were unable to predict the differences in conformation of chains adsorbed from acidic or neutral solvents.

Hygroscopic compounds in spider aggregate glue remove interfacial water to maintain adhesion in humid conditions

“…The amount of red-shift is related to the interaction strength according to the Badger–Bauer equation; the higher the shift, the stronger the interaction strength. The calculated interaction strength between pristine aggregate glue and sapphire O-H groups (Supplementary Table 1 ) is comparable to the strongest interaction between carbonyl groups and sapphire surface O–H groups reported in literature 29 , 42 . We also observe C–H stretching vibrations in the 2800–3050 cm −1 region.…”

“…The blue-shift in D 2 O peaks could be related to absence of hygroscopic LMMCs in washed aggregate glue. Along with D 2 O peaks, we observe the shifted sapphire O–H peak at ~3550 cm −1 29 , 42 . The SFG spectra for the pristine and washed aggregate glue are similar except for subtle differences in the 2200–2800 cm −1 .…”

“…When sapphire O–H groups are in contact with air (air/sapphire interface), a sharp peak at ~3720 cm −1 is observed for the sapphire hydroxyl groups. However, when sapphire O–H groups come in contact with other molecules such as poly(methyl methacrylate) or polystyrene, the O–H peak becomes broader and shifts to lower wavenumbers due to acid–base interactions between the sapphire O–H groups and the other molecules in contact 29 , 42 . The amount of red-shift is related to the interaction strength according to the Badger–Bauer equation; the higher the shift, the stronger the interaction strength.…”

“…Figure 4a, b show SFG spectra for pristine aggregate glue/sapphire at low and high humidity in PPP and SSP polarizations, respectively. The SFG spectrum for pristine aggregate glue at low humidity in PPP polarization consists of low coordination water peaks 3 , 34 – 36 and the broad shifted sapphire O–H peak 29 , 42 . At high humidity, the SFG spectrum for pristine aggregate glue consists of peaks at ~2600, ~2730 cm −1 and the shifted sapphire O–H peak at ~3550 cm −1 .…”

“…4a ), representative of bonding between the pristine aggregate glue and sapphire substrate, persists at high humidity. The interaction energy calculated using the Badger–Bauer equation changes minimally with increasing humidity (Supplementary Table 1 ) 29 , 42 , thus suggesting that the bonds between the pristine aggregate glue and sapphire are preserved at high humidity.

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