Unique Difference in Transition Temperature of Two Similar Fluorinated Side Chain Polymers Forming Hexatic Smectic Phase: Poly{2-(perfluorooctyl)ethyl acrylate} and Poly{2-(perfluorooctyl)ethyl vinyl ether}

Observation of molecular motions in polymer thin films by laboratory grazing incidence diffracted X-ray blinking

Research on polymer surfaces has shown that the mobilities of polymer chains, which affect the aggregation state and thus the physical properties of the material, differ between the surface and bulk. However, the mobilities of the surface polymers have not been fully characterized. Therefore, we propose a time-resolved method for evaluating surface mobility. This measurement scheme is called grazing incidence diffracted X-ray blinking (GI-DXB) and can be used to evaluate the molecular motions occurring at polymer surfaces by continuously measuring X-ray diffraction patterns near the total reflection angle over small time periods. In this study, the crystallized polymer poly{2-(perfluorooctyl)ethyl acrylate}(PC8FA) was measured. The decay constants, which are indexes of molecular motions, were calculated to be 3.98 × 10−3 s−1 for the fluoroalkyl groups in the side chains observed along the in-plane direction and 3.36 × 10−3 s−1 for the lamellar structure observed along the out-of-plane direction when 2000 diffraction profiles of 500 ms were recorded and the incident angle was 0.07°. In contrast, transmission DXB indicated decay constants of 2.63 × 10−3 s−1 for the side chains and 2.87 × 10−3 s−1 for the lamellar structures. These results suggested that the PC8FA surface is mobile, because a larger decay constant indicates a higher mobility. GI-DXB can be used to measure surface dynamics. The authors contend that GI-DXB is a highly versatile tool because it allows the evaluation of local motions with a laboratory X-ray system, and these motions cannot be detected by conventional surface analyses. This measurement scheme may facilitate the development of high-performance polymers and discovery of new physical properties.

Water modulates the lamellar structure and interlayer correlation of poly(perfluorooctyl acrylate) films: a specular and off-specular neutron scattering study

Comb-like polymers with pendant-like perfluorocarbon side chains self-assemble into smectic lamellae and have been extensively used as water-repellent, hydrophobic coating materials characterized by large water contact angles (θ > 120°). As poly(perfluorooctyl acrylate) films are “apparently hydrophobic” (θ > 120°), the interaction of such materials and water molecules has been largely overlooked. To unravel the molecular-level interactions between water and apparently hydrophobic polymers, specular and off-specular neutron scattering experiments were conducted at defined osmotic pressure ΠH2O. The poly{2-[(perfluorooctylethyl)carbamate]ethyl} acrylate (PFAUr-C8), which had a carbamate linker, transitioned to another lamellar phase at 89 °C. At T = 25 °C; the lamellar periodicity of PFAUr-C8 slightly increased with decreasing osmotic pressure, while the vertical correlation length increased. However, the poly[(perfluorooctyl)ethyl] acrylate (PFA-C8) that did not contain a carbamate linker directly transitioned to a disordered phase at 84 °C. The lamellar periodicity of PFA-C8 was largely independent of the osmotic pressure, suggesting that PFA-C8 was poorly hydrated. Remarkably, the vertical correlation length decreased with decreasing osmotic pressure. Because hydration facilitated by the linker modulated the smectic lamellae of the poly(perfluoroalkyl acrylate), water molecules could be used to optimize the self-assembly of apparently hydrophobic liquid crystalline polymers.

Interlayer structure and self-healing in suspensions of brush-stabilized nanoplatelets with smectic order

We have investigated the rheology of an uncured epoxy fluid containing high aspect ratio (length/thickness ≈ 160) α-zirconium phosphate (ZrP) nanoplatelets with smectic order. The nanoplatelets were exfoliated into monocrystalline sheets with uniform thickness using a monoamine-terminated oligomer. The oligomers were densely grafted to the plate surfaces and behave as a molecular brush. Suspensions containing ∼ 2 vol.% ZrP and above show liquid crystalline order with scattering peaks characteristic of a smectic (layered) mesophase. At much higher loading, ∼ 4 vol.% ZrP, there is a sharp transition in visual appearance, steady shear rheology, and linear and non-linear viscoelasticity that is attributed to the reversible interdigitation of oligomer chains between closely spaced layers. The oligomers are proposed to serve as inter-lamellar bridges that store elastic stresses for intermediate rates of deformation, but are able to relax on longer time scales. Under steady shearing conditions, the smectic suspensions with "overlapped" microstructure show a discontinuous flow curve characteristic of shear banding that is attributed to the dynamic pull-out of oligomer chains from the overlap region. At high shear rates, the limiting viscosity of the concentrated suspensions is on the same order of magnitude as the unfilled suspending fluid. When the rate of deformation is reduced below a critical time scale, the original network strength, and corresponding microstructure, is recovered through a passive self-healing process. The unique combination of concentration-dependent yield stress, low post-yield viscosity, and self-healing is potentially useful for various applications in the liquid state, and desirable for scalable processing of nanocomposite materials for structural applications.