Formation Mechanism of High-Density, Flattened Polymer Nanolayers Adsorbed on Planar Solids

Thermal annealing is one of the most indispensable polymer fabrication processes and plays essential roles in controlling morphologies and properties of polymeric materials. We here report that thermal annealing also facilitates polymer adsorption from the melt on planar silicon (Si) substrates, resulting in the formation of a high-density polymer nanolayer with flattened chain confirmations. Three different homopolymers (polystyrene, poly­(2-vinylpyridine), and poly­(methyl methacrylate)), which have similar inherent stiffness and bulk glass transition temperature (<i>T</i>_g), but have different affinities with Si substrates, were chosen as models. Spin-cast films (∼50 nm in thickness) with the three polymers were prepared on cleaned Si substrates and then placed in a vacuum oven set at a temperature far above the bulk <i>T</i>_g. In order to monitor the polymer adsorption process at the solid-polymer melt interface during thermal annealing, we used the protocol that combines vitrification of the annealed films (via rapid quench to room temperature) and subsequent intensive solvent leaching (to remove nonadsorbed chains). The detailed structures of the residual films (i.e., flattened layers with 2–3 nm in thickness) were characterized by using X-ray reflectivity and atomic force microscopy. As a result, we found that the film thicknesses of the flattened layers for the three different polymers increase as a power-law of annealing time before reaching the “quasiequilibrium” state where the film growth is saturated. We have also revealed that the final thickness of the flattened layer at the quasiequilibrium state increases with increasing the solid-segment interaction, while the kinetics becomes more sluggish. The observed formation kinetics corresponds to a “zipping-down” process of the transient flattened chains on planar solids in order to further increase the number of solid/segment points, which is the driving force for flattening so as to overcome the conformational entropy loss in the total free energy

Flattening Process of Polymer Chains Irreversibly Adsorbed on a Solid

We report the structural relaxation process of irreversibly adsorbed polymer chains via thermal annealing that lie flat on a solid (“flattened chains”). Amorphous polystyrene and quartz, which together constitute a weakly attractive system, was used as a model where the local chain conformations of the flattened chains were investigated by sum frequency generation spectroscopy (SFG). Two different film preparation processes (i.e., spin coating and dip coating methods) were utilized to create different initial chain conformations. The spin-coated and dip-coated PS thin films were annealed at a temperature far above the bulk glass transition temperature to reach the “quasiequilibrium” state and subsequently rinsed with chloroform to uncover the buried flattened chains. The SFG results revealed that the backbone chains (constituted of CH and CH₂ groups) of the flattened PS chains preferentially orient to the weakly interactive substrate surface via thermal annealing regardless of the initial chain conformations, while the orientation of the phenyl rings becomes randomized. We postulate that increasing the number of surface-segmental contacts (i.e., enthalpic gain) is the driving force for the flattening process of the polymer chains, even onto a weakly interactive solid to overcome the conformational entropy loss in the total free energy

Nanostructures and Dynamics of Macromolecules Bound to Attractive Filler Surfaces

We report in situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called “bound polymer layer (BPL)”) in a good solvent. The BPL on the CB fillers was extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. The results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of ≈ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called “breathing mode” and is generalized with the thickness of the swollen BPL

Novel Effects of Compressed CO₂ Molecules on Structural Ordering and Charge Transport in Conjugated Poly(3-hexylthiophene) Thin Films

We report the effects of compressed CO₂ molecules as a novel plasticization agent for poly­(3-hexylthiophene) (P3HT)-conjugated polymer thin films. In situ neutron reflectivity experiments demonstrated the excess sorption of CO₂ molecules in the P3HT thin films (about 40 nm in thickness) at low pressure (<i>P</i> = 8.2 MPa) under the isothermal condition of <i>T</i> = 36 °C, which is far below the polymer bulk melting point. The results proved that these CO₂ molecules accelerated the crystallization process of the polymer on the basis of ex situ grazing incidence X-ray diffraction measurements after drying the films via rapid depressurization to atmospheric pressure: both the out-of-plane lamellar ordering of the backbone chains and the intraplane π–π stacking of the side chains were significantly improved, when compared with those in the control P3HT films subjected to conventional thermal annealing (at <i>T</i> = 170 °C). Electrical measurements elucidated that the CO₂-annealed P3HT thin films exhibited enhanced charge carrier mobility along with decreased background charge carrier concentration and trap density compared with those in the thermally annealed counterpart. This is attributed to the CO₂-induced increase in polymer chain mobility that can drive the detrapping of molecular oxygen and healing of conformational defects in the polymer thin film. Given the universality of the excess sorption of CO₂ regardless of the type of polymers, the present findings suggest that CO₂ annealing near the critical point can be useful as a robust processing strategy for improving the structural and electrical characteristics of other semiconducting conjugated polymers and related systems such as polymer:fullerene bulk heterojunction films

Relaxor Ferroelectric Behavior from Strong Physical Pinning in a Poly(vinylidene fluoride-<i>co</i>-trifluoroethylene-<i>co</i>-chlorotrifluoroethylene) Random Terpolymer

Relaxor Ferroelectric Behavior from Strong Physical Pinning in a Poly(vinylidene fluoride-<i>co</i>-trifluoroethylene-<i>co</i>-chlorotrifluoroethylene) Random Terpolyme

Locally Favored Two-Dimensional Structures of Block Copolymer Melts on Nonneutral Surfaces

Self-assembly of block copolymers (BCPs) into arrays of well-defined nanoscopic structures has attracted extensive academic and industrial interests over the past several decades. In contrast to the bulk where phase behavior is controlled by the segmental interaction parameter, the total number of segments in BCPs and volume fraction, the morphologies and orientations of BCP thin films can also be strongly influenced by the substrate surface energy/chemistry effect (considered as a “substrate field”). Here, we report the formation of locally favored structures where all constituent blocks coexist side-by-side on nonneutral solid surfaces irrespective of their chain architectures, microdomain structures, and interfacial energetics. The experimental results using a suite of surface-sensitive techniques intriguingly demonstrate that individual preferred blocks and nonpreferred blocks lie flat on the substrate surface and form a two-dimensional percolating network structure as a whole. The large numbers of solid-segment contacts, which overcome a loss in the conformational entropy of the polymer chains, prevent the structure relaxing to its equilibrium state (i.e., forming microdomain structures) even in a (good) solvent atmosphere. Our results provide direct experimental evidence of the long-lived, nonequilibrium structures of BCPs and may point to a new perspective on the self-assembly of BCP melts in contact with impenetrable solids

Composite Poly(vinylidene fluoride)/Polystyrene Latex Particles for Confined Crystallization in 180 nm Nanospheres via Emulsifier-Free Batch Seeded Emulsion Polymerization

Recently, nanoconfined poly­(vinylidene fluoride) (PVDF) and its random copolymers have attracted substantial attention in research. In addition to the drastic change in crystallization kinetics, major interest lies in crystal orientation and polymorphism in order to understand whether enhanced piezoelectric and ferroelectric properties can be achieved. For example, PVDF has been two-dimensionally (2D) confined in cylindrical nanopores of anodic aluminum oxide (AAO) with various pore diameters. The crystal <i>c</i>-axis becomes perpendicular to the cylinder axes, which favors dipole switching in the impregnated AAO membrane. However, no polar phases have been obtained from 2D confinement even down to 35 nm pores after melt recrystallization. In this work, we realized three-dimensionally (3D) confined crystallization of PVDF in 180 nm nanospheres by employing a facile emulsifier-free batch seeded emulsion polymerization to prepare PVDF@polystyrene (PS) core–shell particles. Influences of polymerization temperature, PVDF/styrene feed ratio, and polymerization time were systematically investigated to achieve completely wrapping of PS onto PVDF particles and avoid the formation of Janus particles. Exclusive confined PVDF crystallization was observed in these core–shell composite particles. Intriguingly, after melt recrystallization, polar β/γ phases, instead of the kinetically favored α phase, were resulted from 3D confinement in 180 nm nanospheres. We attributed this to the ultrafast crystallization rate during homogeneously nucleated PVDF crystallization. For the first time, we reported that 3D confinement was more effective than 2D confinement in producing polar crystalline phases for PVDF

Phase Behavior of Alkyne-Functionalized Styrenic Block Copolymer/Cobalt Carbonyl Adducts and <i>in Situ</i> Formation of Magnetic Nanoparticles by Thermolysis

A series of polystyrene-<i>block</i>-poly­(4-(phenyl­ethynyl)­styrene) (PS-<i>b</i>-PPES) diblock copolymers with a range of compositions were prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization. Block copolymer/cobalt carbonyl adducts (PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i>) were subsequently prepared by reaction of Co₂(CO)₈ with the alkyne groups of the PPES block. Phase behavior of the block copolymer/cobalt carbonyl adducts (PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i>, 8% ≤ wt % PS ≤ 68%) was studied by small-angle X-ray scattering and transmission electron microscopy (TEM). As the composition of PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> copolymers was shifted from PS as the majority block to PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> as the majority block, the morphology was observed to shift from lamellar with larger PS domains to cylindrical with PS as the minority component and then to spherical with PS as the minority component. These observations have been used to map out a partial phase diagram for PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> diblock copolymers. Heating of PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> samples at relatively low temperatures (120 °C) results in the formation of nanoparticles containing crystalline cobalt and cobalt oxide domains within the PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> regions as characterized by TEM, X-ray diffraction (XRD), and X-ray scattering

Effect of CO₂ on a Mobility Gradient of Polymer Chains near an Impenetrable Solid

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO₂) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3<i>R</i>_g (<i>R</i>_g is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO₂ and subsequent solvent leaching, we reveal the presence of the CO₂-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vicinity of the solid substrate even in the presence of the effective plasticizer