A Single-Component Inimer Containing Cross-Linkable Ultrathin Polymer Coating for Dense Polymer Brush Growth

We have developed a highly versatile universal approach to grow polymer brushes from a variety of substrates with high grafting density by using a single-component system. We describe a random copolymer which consists of an inimer, <i>p</i>-(2-bromoisobutyloylmethyl)­styrene (BiBMS), copolymerized with glycidyl methacrylate (GMA) synthesized by reversible addition–fragmentation chain-transfer (RAFT) polymerization. Thermal cross-linking created a mat that was stable during long exposure in organic solvent even with sonication or during Soxhlet extraction. The absolute bromine density was determined via X-ray photoelectron spectroscopy (XPS) to be 1.86 ± 0.12 Br atoms/nm³. The ratio of experimental density to calculated absolute initiator density suggests that ∼25% of the bromine is lost during cross-linking. Surface-initiated ATRP (SI-ATRP) was used to grow PMMA brushes on the substrate with sacrificial initiator in solution. The brushes were characterized by ellipsometry, XPS, and atomic force microscopy (AFM) to determine thickness, composition, and homogeneity. By correlating the molecular weight of polymer grown in solution with the brush layer thickness, a high grafting density of 0.80 ± 0.06 chains/nm² was calculated. By synthesizing the copolymer before cross-linking on the substrate, this single-component approach avoids any issues with blend miscibility as might be present for a multicomponent curable mixture, while resulting in high chain density on a range of substrates

Segmental Dynamics of an Isolated Component Polymer Chain in Polymer Blends Near the Glass Transition

The segmental dynamics of a component chain isolated in its blending partner chains is examined using the reorientation of polymer-tethered fluorescent probes near the glass transition. It is found that the temperature dependence of the dynamics of an isolated component follows that of the other component, with a horizontal shift corresponding to the glass transition temperature modification, which may result from a local composition of ≈10% isolated component. On the contrary, the dynamic heterogeneity, another key dynamic feature near the glass transition, shows that the local dynamic environment of an isolated component becomes either as heterogeneous as a more inherently heterogeneous component or more heterogeneous than either. These observations emphasize that not only the chain connectivity but also the dynamic modulation of a component by the other component needs to be addressed in order to understand the segmental dynamics of an isolated component in polymer blends

Electronic Transport and Raman Scattering in Size-Controlled Nanoperforated Graphene

We demonstrate the fabrication and study of the structure–property relationships of large-area (>1 cm²) semiconducting nanoperforated (NP) graphene with tunable constriction width (<i>w</i> = 7.5–14 nm), derived from CVD graphene using block copolymer lithography. Size-tunable constrictions were created while minimizing unintentional doping by using a dual buffer layer pattern-transfer method. An easily removable polymeric layer was sandwiched between an overlying silicon oxide layer and the underlying graphene. Perforation-size was controlled by overetching holes in the oxide prior to pattern transfer into graphene while the polymer protected the graphene from harsh conditions during oxide etching and lift off. The processing materials were removed using relatively mild solvents yielding the clean isolation of NP graphene and thereby facilitating Raman and electrical characterization. We correlate the D to G ratio as a function of <i>w</i> and show three regimes depending on <i>w</i> relative to the characteristic Raman relaxation length. Edge phonon peaks were also observed at 1450 and 1530 cm^–1 in the spectra, without the use of enhancement methods, due to high density of nanoconstricted graphene in the probe area. The resulting NP graphene exhibited semiconducting behavior with increasing ON/OFF conductance modulation with decreasing <i>w</i> at room temperature. The charge transport mobility decreases with increasing top-down reactive ion etching. From these comprehensive studies, we show that both electronic transport and Raman characteristics change in a concerted manner as <i>w</i> shrinks

Light-Driven Reversible Modulation of Doping in Graphene

We report a route to noncovalently latch dipolar molecules on graphene to create stable chromophore/graphene hybrids where molecular transformation can be used as an additional handle to reversibly modulate doping while retaining high mobilities. A light switchable azobenzene chromophore was tethered to the surface of graphene via π–π interactions, leading to p-doping of graphene with an hole concentration of ∼5 × 10¹² cm^–2. As the molecules switch reversibly from trans to cis form the dipole moment changes, and hence the extent of doping, resulting in the modulation of hole concentration up to ∼18% by alternative illumination of UV and white light. Light-driven conductance modulation and control experiments under vacuum clearly attribute the doping modulation to molecular transformations in the organic molecules. With improved sensitivities these “light-gated” transistors open up new ways to enable optical interconnects

Raman Enhancement of a Dipolar Molecule on Graphene

We show a large enhancement in the Raman signal from a highly polarizable molecule attached to single layer graphene. Through spatial mapping of the Raman signal and wavelength-dependent Raman measurements from a dipolar chromophore latched to a graphene/SiO₂ substrate and to a bare SiO₂ substrate, we show that strong electronic coupling in the hybrid structure contributes to the enhancement. The dipolar molecule is a pyrene tethered Disperse Red 1 (DR1P) that noncovalently binds to graphene. Upon comparison of the Raman signal of DR1P on single layer graphene with that on a bare SiO₂/Si substrate, we found that the enhancement factor is in the range 29–69 at 532 nm excitation. As the surface coverage of DR1P on graphene increases, Raman intensity also increases and saturates at a certain concentration. The saturation of the Raman signal intensity at higher DR1P concentrations were accompanied by shifts in the G band and the 2D band of graphene due to p-doping. We further show that the Raman enhancement that occurs on single layer is larger than on few layer graphene. Quantitative analysis on the Raman scattering cross section of DR1P on graphene shows a higher Raman scattering cross section compared to that in solution confirming a strong electronic coupling. A series of all-electron ab initio calculations using density functional theory (DFT) modeled the noncovalent binding of DR1P on a large graphene fragment where the pyrene tether is interacting with the graphene fragment via π–π stacking interactions. The DR1P molecule has occupied energy levels that are close to the Fermi level of graphene, and these interact strongly with the semimetallic nature of graphene. As a consequence, in complete contrast to the isolated DR1P molecule, our time-dependent DFT calculations show that the orbital energies and densities for DR1P are significantly modified by the graphene substrate

Bulk and Thin Film Morphological Behavior of Broad Dispersity Poly(styrene-<i>b-</i>methyl methacrylate) Diblock Copolymers

We describe the morphological implications of broad molecular weight dispersity on the bulk and thin film self-assembly behavior of seven model poly­(styrene-<i>block</i>-methyl methacrylate) (SM) diblock copolymers. Derived from sequential nitroxide-mediated polymerizations, these unimodal diblock copolymers are comprised of narrow dispersity S blocks (<i>Đ</i> ≤ 1.14) and broad dispersity M blocks (<i>Đ</i> ∼ 1.7) with total molecular weights <i>M</i>_n,total = 29.2–42.9 kg/mol and M volume fractions <i>f</i>_M = 0.35–0.63. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) analyses demonstrate that these diblock copolymers microphase separate into lamellar and cylindrical morphologies with substantially larger microdomain spacings at lower overall molecular weights as compared to their narrow dispersity analogues. The observed microphase-separated melt stabilization is also accompanied by a substantial shift in the lamellar phase composition window to higher values of <i>f</i>_M. In thin films, these polydisperse copolymers form perpendicularly oriented morphologies with modest degrees of lateral order on substrates functionalized with P­(S-<i>ran</i>-MMA) neutral polymer brush layers

Post-Fabrication Placement of Arbitrary Chemical Functionality on Microphase-Separated Thin Films of Amine-Reactive Block Copolymers

We report an approach to the post-fabrication placement of chemical functionality on microphase-separated thin films of a reactive block copolymer. Our approach makes use of an azlactone-containing block copolymer that microphase separates into domains of perpendicularly-oriented lamellae. These thin films present nanoscale patterns of amine-reactive groups (reactive stripes) that serve as handles for the immobilization of primary amine-containing functionality. We demonstrate that arbitrary chemical functionality can be installed by treatment with aqueous solutions under mild conditions that do not perturb underlying microphase-separated patterns dictated by the structure of the reactive block copolymer. This post-fabrication approach provides a basis for the development of modular approaches to the design of microphase-separated block copolymer thin films and access to coatings with patterned chemical domains and surface properties that would be difficult to prepare by the self-assembly and processing of functionally complex block copolymers

Functionalization of Single-Wall Carbon Nanotubes with Chromophores of Opposite Internal Dipole Orientation

We report the functionalization of carbon nanotubes with two azobenzene-based chromophores with large internal dipole moments and opposite dipole orientations. The molecules are attached to the nanotubes noncovalently via a pyrene tether. A combination of characterization techniques shows uniform molecular coverage on the nanotubes, with minimal aggregation of excess chromophores on the substrate. The large on/off ratios and the subthreshold swings of the nanotube-based field-effect transistors (FETs) are preserved after functionalization, and different shifts in threshold voltage are observed for each chromophore. Ab initio calculations verify the properties of the synthesized chromophores and indicate very small charge transfer, confirming a strong, noncovalent functionalization

A Dual Functional Layer for Block Copolymer Self-Assembly and the Growth of Nanopatterned Polymer Brushes

We present a versatile method for fabricating nanopatterned polymer brushes using a cross-linked thin film made from a random copolymer consisting of an inimer (<i>p</i>-(2-bromoisobutyloylmethyl)­styrene), styrene, and glycidyl methacrylate (GMA). The amount of inimer was held constant at 20 or 30% while the relative amount of styrene to GMA was varied to induce perpendicular domain orientation in an overlying P­(S-<i>b</i>-MMA) block copolymer (BCP) film for lamellar and cylindrical morphologies. A cylinder forming BCP blend with PMMA homopolymer was assembled to create a perpendicular hexagonal array of cylinders, which allowed access to a nanoporous template without the loss of initiator functionality. Surface-initiated ATRP of 2-hydroxyethyl methacrylate was conducted through the pores to generate a dense array of nanopatterned brushes. Alternatively, gold was deposited into the nanopores, and brushes were grown around the dots after removal of the template. This is the first example of combining the chemistry of nonpreferential surfaces with surface-initiated growth of polymer chains

Rational Design of a Block Copolymer with a High Interaction Parameter

A series of poly­(4-<i>tert</i>-butylstyrene-<i>block</i>-2-vinylpyridine) [P­(tBuSt-<i>b</i>-2VP)] block copolymers (BCPs) with varying volume fractions, molecular weights, and narrow dispersities were synthesized from the commercially available monomers by sequential living anionic polymerization. The copolymers were thoroughly characterized by ¹H NMR spectroscopy, size exclusion chromatography, thermal gravimetric analysis, and differential scanning calorimetry (DSC). To examine the effect of the <i>tert</i>-butyl group on the effective interaction parameter (χ_eff) relative to poly­(styrene-<i>block</i>-2-vinylpyridine) [(P­(S-<i>b</i>-2VP)], the self-assembly of symmetric copolymers was studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy. Order-to-disorder transitions (ODTs) were identified by both DSC and SAXS on five copolymers, to define the equation χ_eff(<i>T</i>) = (67.9 ± 1.3)/<i>T</i> – (0.0502 ± 0.0029), which shows a higher enthalpic contribution to χ_eff than P­(S-<i>b</i>-2VP) and approximately 1.5 times larger χ_eff. This enables a minimum full pitch of 9.6 nm for the symmetric copolymers. Asymmetric copolymers were also examined for bulk self-assembly by SAXS and TEM, exploring both P2VP and PtBuSt cylindrical phases with diameters as small as 6 nm. Feasibility of thin film assembly by thermal annealing was demonstrated for a P2VP cylinder forming BCPs to yield parallel cylinders that were seeded with Pt ions and etched to yield Pt nanowires with diameters as small as 5.8 nm