Conformation of End-Tethered PNIPAM Chains in Water and in Acetone by Neutron Reflectivity

Abstract

Poly(N-isopropylacrylamide) is perhaps the most well-known member of the class of temperature responsive polymers. It has a lower critical solution temperature (LCST) in water at about 32 °C. This very sharp transition (∼5 °C) is attributed to alterations in the hydrogen-bonding interactions of the amide group. In this work we investigated the conformation of end-tethered PNIPAM chains at the interface of silicon with D 2O and d-acetone using neutron reflection. End-tethered PNIPAM layers were prepared utilizing the interaction between COOH-terminated PNIPAM and OH-terminated selfassembled monolayers ("method A") and also by polymerizing N-isopropylacrylamide monomers from the silicon surface ("method B"). Reflectivity data from the protonated layers in deuterated water were obtained using a liquid cell over a range of temperature from 10 to 55 °C. For method A, PNIPAM molecular weights of 33K and 220K were examined. In D 2O, we observed very limited change in the conformation of the tethered chains as the temperature increased through the LCST. No conformational change was detected for the lowest molecular weight PNIPAM-COOH sample (33K). Only a slight change in conformation with temperature was detected for the higher molecular weight sample (220K) and the sample from method B. The profiles indicated that the chains were well hydrated above 32 °C, consistent with the observation of very low receding water contact angles. On the other hand, a significant change in the segment concentration profiles occurred when D 2O was replaced by d-acetone. Bilayer profiles were observed for all samples in D2O. By contrast, in d-acetone the profiles were composed of a single monotonically decaying layer. Surprisingly, the profiles were more contracted in d-acetone than in D2O despite the fact that PNIPAM dissolves in d-acetone but solutions of PNIPAM in D2O are cloudy above the LCST. This observation, as well as the lack of conformational change with temperature for these low surface density brushes in D2O, is explained on the basis of a concentration-dependent Flory χ parameter.

Modeling lower critical solution temperature behavior of associating polymer brushes with classical density functional theory

“…Leckband et al 18,19 conducted the surface force and water contact angle measurement for PNIPAAM brushes. In agreement with Yim et al, [8][9][10][11] they found that the temperature-driven collapse transition for PNIPAAM brushes only happened at the higher grafting density and molecular weight. Bittrich et al 21 used ellipsometry to measure the thickness of PNIPAAM brushes and observed similar behavior.…”

“…In addition, the swelling ratio increases monotonically from about 1.6 to 1.9 with increasing N A from 50 to 200, which is partially consistent with Mendez et al 's SCFT results 27 and Yim et al's experiments. [8][9][10] The partially collapsed structure is not found at N A = 200. However, it could reappear at even higher molecular weight.…”

“…12 The effect of grafting density on the swelling ratio is partially consistent with the viewpoint that intermediate grafting density and high molecular weight gives maximum swelling ratio change with temperature, which was shown by Mendez et al's SCFT results 27 and Yim et al's experiments. [8][9][10] At even higher grafting density ρ g σ 2 = 0.2, due to appearance of partial-collapsed structure, the swelling ratio curve first has a discontinuity at 285 K (from extended to partial-collapsed), and then continuously decreases until a small step at 299 K (from partial-collapsed to extended).…”

“…It is noteworthy that the PNIPAAM brushes may act differently as compared to bulk aqueous solutions since they are constrained by one end to a surface. The dependence of PNIPAAM brush conformation change on different molecular weight and grafting density has been extensively investigated by using a number of experimental techniques including neutron reflectivity (NR), [8][9][10][11] atomic force microscopy (AFM), [12][13][14][15] quartz crystal microbalance measurements (QCM), 12,13,16,17 surface forces, [18][19][20] ellipsometry, 21 water contact angle measurement, 18,19 and surface plasmon resonance (SPR). 22 Generally, the molecular weight and a) Author to whom correspondence should be addressed.…”

“…For example, Yim et al [8][9][10][11] performed neutron reflectivity to obtain the monomer concentration profile of PNIPAAM brushes at a variety of molecular weight and grafting density. A distinct phase transition for the polymer brushes was found only at higher grafting density and molecular weight.…”