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Infrared Dichroism, Chain Flattening, and the Bound Fraction Histogram in Adsorbed Poly(Methy1 Methacrylate) Layers

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Infrared Dichroism, Chain Flattening, and the Bound Fraction Histogram in Adsorbed Poly(Methy1 Methacrylate) Layers Macromolecules 1995,28, 6915-6925 6915 Infrared Dichroism, Chain Flattening, and the Bound Fraction Histogram in Adsorbed Poly(methy1 methacrylate) Layers Peter Frantz and Steve Granick* Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 Received March 7,1995; Revised Manuscript Received July 12, 1995@ ABSTRACT: We examine the distribution of adsorption substates, in contrast to their average. The distribution of infrared dichroism and bound fraction were determined in a model system (poly(methy1 methacrylate)adsorbed chiefly by hydrogen bonding onto oxidized silicon from CC14). The method was infrared spectroscopy in attenuated total reflection (FTIR-ATR). From the surface excess (r),dichroic ratio (D,), and fraction of carbonyl groups bound to the surface @), a broad distributionof conformations was found to prevail, from severely flattened (p % 0.5 and D, 1for the asymmetric methyl stretch) to nearly solution-like (p = 0.1 and D, % 1 for the asymmetric methyl stretch). The extent of flattening depended mainly on the number of adsorption sites available to each chain it the time it deposited to the surface; chains that adsorbed later, finding fewer and fewer surface sites available, then became attached by fewer and fewer segments. This in turn implied that chains which arrived initially had the center- of-mass closer to the solid surface than those which arrived later. Conformational rearrangements were not observed on the experimental time scale of 3 h; this allowed determination of the distribution of conformational substates by subtraction of sequentially-acquired infrared spectra. The broad and asymmetric distribution of conformational substates appeared to originate, predictably, from the piecemeal process by which the surface was coated. It presents analogies with issues of random sequential adsorption. Introduction standing of how kinetic and conformational properties The last two decades have seen rapid progress in are related. understanding macromolecular interfacial properties. Previous work on these questions took the approach Advances in theory, experiment, and simulation have of probing structure through its kinetic response. In constructed an exhaustive description of the equilibrium pioneering experiments by Pefferkorn, Varoqui, and co- composition and structure of an adsorbed layer based workers,loJ1 experiments were conceived to study the upon the relative free energies of ad~orption.l-~How- adsorption of tritium-labeled polystyrene (M,= 3.6 x ever, a common admonition, which has echoed through- lo5)onto silica beads from dilute carbon tetrachloride. out these works, reminds us that deviations from this They first mapped out a peculiar adsorption isotherm equilibrium picture will arise when the adsorbed layer with two distinct regions. It was postulated that the loses its m~bility.~?~~~Steric constraints and tenacious first region, a plateau for solution concentrations less segment-surface interactions may hinder relaxation than roughly 10 yg-mL-l, indicated that the polystyrene from nonequilibrium conformations. The potentially was tightly bound in an almost flat conformation. The profound influence of a “glassy” sublayer on the com- second region, in which the adsorbed mass grew with position, structure, and dynamics has been ~tressed.~ increasing concentration, indicated growth in the num- The term “glassy” has come to be synonymous with a ber of segments contained in loops and tails. Thus, it persistent nonequilibrium state. Indeed, it is evident was possible to produce adsorbed layers with a variable in principle that a path to the state of lowest energy fraction of segments bound to the surface. might not be available. The second experiment sought to determine the We distinguish three time scales in the development intrinsic self-mobility of the polymers composing these of an adsorbed layer: For diffision to an initially bare structurally distinct layers by finding, in each case, the surface, for attachment to the surface, and for equilibra- mean time that a chain occupied the surface. This was tion of the adsorbed chains. The rate of each individual done by measuring the rates of mass flux into the process is intimately coupled to that of the others. The solution during displacement by an isotopically distin- composition and structure of the layer depend sensi- guishable molecule. This exchange was much slower tively on the relative kinetics of these processes, as well for the tightly bound layer, occurring with a character- as the traditional thermodynamic quantities (for ex- istic time of ~20h. They concluded that because of the ample: the segment-surface interaction enthalpy, xs, restricted mobility, the layer was essentially frozen in the solution concentration, &, and the degree of polym- a metastable state. erization, n).l While the influence of the thermody- Additional evidence of long-lived nonequilibrium con- namic quantities is well understood, less is definitively formational states has been accumulating in our known about kinetic properties. The primary motiva- laborat~ry.~-~J~-~~We have investigated the interfacial tion for this study is to investigate the behavior properties of a variety of model polymer/solvent systems of a model system which, due to its limited surface adsorbed to a single solid oxidized surface. Surface m0bility,6-~is expected to be restricted from structural excess (r),kinetics of adsorption and desorption, bound equilibration via the third of the time scales alluded to fraction @), and the segment-surface interaction en- above. Ultimately, we hope to contribute to the under- thalpy (xs)have been some of the parameters directly accessible to us. From these measured kinetic and @ Abstract published in Advance ACS Abstracts, September 1, thermodynamic quantities, we have inferred changes in 1995. the composition and structure of adsorbed layers and 0024-9297/95/2228-6915$09.00/00 1995 American Chemical Society 6916 Frantz and Granick Macromolecules, Vol. 28, No. 20, 1995 the dynamics which precipitate this evolution from one SOLUTION SOLUTION state to another. " \ OUT But all of the above studies were based on indirect 'r I1 inferences as to surface structure; kinetics, not struc- ture, was actually measured. In this paper we present THERMOSTATTED direct evidence of persistent nonequilibrated structures STEEL CELL nl within an adsorbed layer. Measurements of the mean segmental orientation, Da,combined with knowledge of the bound fraction, show that the first chains on the surface lay in a predominantly flattened conformation while the later chains arrived progressively more iso- Figure 1. Schematic illustration of the infrared flow cell tropic. This entropically unfavorable situation was apparatus. The infrared beam sustains multiple internal observed to persist throughout the duration of our reflections as it travels the length of a silicon prism. At each experiment (3 h), resisting the conformational equilibra- internal reflection, an evanescent wave penetrates to a depth of a few microns and suffers losses due to the excitation of tion found in less tenaciously bound po1ymers.l2J3 vibrational modes of the material in the surface region. The Finally, from spectral subtraction, estimates were made flow cell is designed to produce laminar flow. The polymer of the shape of the conformational distribution. solution is exposed to the top surface of the ATR prism. Experimental Section Table 1. Characterization of the Poly(methy1 methacrylates) (Suppliers' data) Sample Cell for ATR. The surface excess, bound fraction, and dichroic ratio of poly(methy1 methacrylate) sample M, MdMn source (PMMA) adsorbed to oxidized silicon were monitored in protio 107 000 1.10 Polymer Laboratories situ during adsorption from dilute solution in carbon 1 300 000 1.06 Polymer Laboratories deuterio 90000 Polymer Standards Services, tetrachloride (CCl4). Infrared spectra were collected Mainz, Germany using a Bio-Rad FTS-60 Fourier transform infrared spectrometer (FTIR) equipped with a broad-band mer- Figure 1 shows a schematic diagram of the stainless cury-cadmium-telluride (MCT) detector. Experiments steel, thermostated flow cell. A special point of design were performed in the mode of attenuated total reflec- was a specially-manufactured O-ring, composed of Viton tion (ATR). Related work was described by Flournoy (alternatively, polyisoprene) encased by Teflon. A tight and Shaffers,17and a similar apparatus was applied to seal was necessary to prevent leakage of liquid. This the study of adsorbed polymer by Van Alstenl8 and by was usually provided by an aluminum plate clamped Sung.lg The ATR element was mounted inside a stain- on the back, separated from the ATR plate by a silvered less steel jacket thermostated at 30 "C. Viton gasket. Silver, which reflected the IR evanescent The ATR cell, designed to provide laminar flow up to wave, was needed to avoid otherwise-huge IR absor- very high shear rates, was placed in a compartment bances in the 1700 cm-l region. external to the FTIR spectrometer. Reasons to work Cell Preparation. The protocol for cleaning the cell outside the standard sample compartment were es- and crystal may be found in ref 13, and preparation of sentially 3-fold. Most importantly, the volume of the the oxidized silicon surface in refs 13 and 20. The latter standard compartment was too small for needed acces- treatment has been shownz0to reproducibly cultivate sories. Second, the capability was desired to change the an oxidized
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