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42995-AC7
Interfacial Structure and Fluctuations in Block Copolymer Brushes: Tailoring Switchable Surfaces
The overall objective is to elucidate the structures of diblock copolymer brushes (DCBs) and changes in structure with changes in environment. DCBs have garnered considerable attention due to their stimuli-responsive behavior. Their surface properties can be changed from those characteristic of one polymer block to those characteristic of the other polymer block by treating them with a selective solvent. Neither the mechanism of the rearrangement or the structure of the “as-deposited” and “rearranged” state has been determined. BCBs of polystyrene-b-poly(methyl acrylate) (PS-b-PMA), poly(methyl acrylate)-b-polystyrene) (PMA-b-PS), poly(methyl acrylate)-b-poly(n-butyl acrylate) (PMA-b-PnBA) and polystyrene-b-poly(n-butyl acrylate) (PS-b-PnBA) were synthesized with atom transfer radical polymerization using the “grafting from” technique. Using this technique, we obtained very high grafting densities (~0.6 chains/nm2) which ensure that the BCBs are in the “dense brush” regime and the polymer chains are strongly stretched.
Recently, we have resolved the internal structure of ultrathin BCBs using neutron reflectivity and grazing incidence small angle scattering. NR studies show that the internal brush structure depends strongly on the synthesis sequence of the polymer blocks and the value of χN, where χ is the segment exchange interaction parameter and N is the length of the polymer chain. For the thinnest films a structural model consisting of two layers with a smooth interfacial gradient is consistent with the data. For thicker DCBs of sufficiently asymmetric composition a third layer must be included in the structural model. This is consistent with our observation that when these brushes are studied with Grazing Incidence Small Angle X-ray Scattering (GISAXS), peaks in the two-dimensional scattering pattern indicate the presence of an in-plane ordering, with nearest neighbor correlation. Thus, the layer in the middle of the brush seen by NR is one that is not laterally uniform, but rather contains domains of some sort. Initial analysis of the structure factor of these peaks indicate these domains are 3-dimensional, but do not extend all the way from the substrate to the surface of the brush. After a DCB has been treated in acetone and dried, these peaks indicating in-plane correlation remain and Bragg rods appear at values of qy = ±0.196 nm-1 in the GISAXS data, revealing that a new ordering of some sort in the plane of the sample has appeared. GISAXS measurements at various incident angles and comparison with AFM images confirm that this is a 2-D structure at or near the surface.
More recently, DCBs swollen with different solvent vapors have been measured in order to gain clues to the unswollen structure as well as the process of ‘rearrangement'. Samples were held in a saturated vapor of dichloromethane or ethanol for 8-10 hours before being probed with NR. Preliminary results on DCBs in which the block tethered to the substrate is PS and the second block is PnBA or PtBA show that a three layer structural model is suitable for both dry and vapor swollen samples. Estimates of the solubility parameters for the various components lead one to expect that dichloromethane is a better solvent for all the polymers (PS, PtBA, PnBA) than is ethanol. This is consistent with the more prominent swelling of the DCBs in dichloromethane vapor. Also, the PS block swells more in both solvent vapors than do the PtBA or PnBA blocks.
The brushes do not switch when in contact with solvent vapor. Therefore in-situ measurements in which the DCB was in contact with a liquid solvent were run to resolve the mechanism of the surface rearrangement and the internal structure after rearrangement. Measurements were made with acetone, cyclohexane, and dichloromethane. Swelling of the films adjacent to liquid was much higher than that in solvent vapor. For example, increase in thickness with contact with liquid dichloromethane was three times that in contact with dichloromethane vapor. Ongoing analysis of the data for brushes next to liquid and after contact with liquid should provide information on the mechanism of surface rearrangement and the brush structure before and after rearrangement.
Observations of interface/interface correlation in these DCBs has also hinted that the dynamics of the surface height fluctuations on these brushes are strongly changed by the fact that the chains are tethered. Therefore the surface height fluctuations on analogous densely grafted brushes in which the chains are nearly monodisperse homopolymers have been studied using x-ray photon correlation spectroscopy (XPCS). Surface fluctuations of wavelengths from 620 to 3100 nm on annealed brushes of either polystyrene and poly(n-butyl acrylate) are “frozen” on the time and length scales probed by XPCS even at temperatures more than 130 °C above the glass transition temperature of films of corresponding untethered chains. No relaxation of the surface is seen for a time window of 0.1 to 1000 seconds. This alteration of surface dynamics by tethering has implications for wetting, friction, and adhesion.
