Microscopic Origins of the Nonlinear Behavior of Particle-Filled Rubber Probed with Dynamic Strain XPCS

The underlying microscopic response of filler networks in reinforced rubber to dynamic strain is not well understood due to the experimental difficulty of directly measuring filler network behavior in samples undergoing dynamic strain. This difficulty can be overcome with in situ X-ray photon correlation spectroscopy (XPCS) measurements. The contrast between the silica filler and the rubber matrix for X-ray scattering allows us to isolate the filler network behavior from the overall response of the rubber. This in situ XPCS technique probes the microscopic breakdown and reforming of the filler network structure, which are responsible for the nonlinear dependence of modulus on strain, known in the rubber science community as the Payne effect. These microscopic changes in the filler network structure have consequences for the macroscopic material performance, especially for the fuel efficiency of tire tread compounds. Here, we elucidate the behavior with in situ dynamic strain XPCS experiments on industrially relevant, vulcanized rubbers filled (13 vol %) with novel air-milled silica of ultrahigh-surface area (UHSA) (250 m2/g). The addition of a silane coupling agent to rubber containing this silica causes an unexpected and counterintuitive increase in the Payne effect and decrease in energy dissipation. For this rubber, we observe a nearly two-fold enhancement of the storage modulus and virtually equivalent loss tangent compared to a rubber containing a coupling agent and conventional silica. Interpretation of our in situ XPCS results simultaneously with interpretation of traditional dynamic mechanical analysis (DMA) strain sweep experiments reveals that the debonding or yielding of bridged bound rubber layers is key to understanding the behavior of rubber formulations containing the silane coupling agent and high-surface area silica. These results demonstrate that the combination of XPCS and DMA is a powerful method for unraveling the microscale filler response to strain which dictates the dynamic mechanical properties of reinforced soft matter composites. With this combination of techniques, we have elucidated the great promise of UHSA silica when used in concert with a silane coupling agent in filled rubber. Such composites simultaneously exhibit large moduli and low hysteresis under dynamic strain

Dynamics of Surface Fluctuations on Macrocyclic Melts

A hydrodynamic continuum theory (HCT) of thermally stimulated capillary waves describing surface fluctuations of linear polystyrene melts is found to describe surface fluctuations of sufficiently thick films of unentangled cyclic polystyrene. However, for cyclic polystyrene (CPS) films thinner than 10<i>R</i>_g, the surface fluctuations are slower than expected from the HCT universal scaling, revealing a confinement effect active over length scales much larger than <i>R</i>_g. Surface fluctuations of CPS films can be slower than those of films of linear polystyrene analogues, due to differences between the glass transition temperatures, <i>T</i>_g, of the linear and cyclic chains. The temperature dependences of the surface fluctuations match those of bulk viscosities, suggesting that whole chain dynamics dictate the surface height fluctuation dynamics at temperatures 25–60 °C above <i>T</i>_g. When normalized surface relaxation rates of thicker films are plotted as a function of <i>T</i>/<i>T</i>_g, a universal temperature behavior is observed for linear and cyclic chains

Manipulation of Polymer/Polymer Interface Width from Nonequilibrium Deposition

We demonstrate, using neutron reflectivity, that the width of a nonequilibrium interface between an organo-soluble aromatic polyimide film and triacetate cellulose (TAC) support film created by spin-coating or solution-casting can be broadened in a controllable way using a “swelling agent” in the deposition process. In a favorable case, the adhesion, as measured by T-peel tests, can be increased by a factor of 7 by adjustment of the solvent composition. The morphologies of the TAC fractured surfaces after peeling tests measured by AFM reveal that broadening of the interfacial width causes an interconnected network in the interface, leading to a sharp increase in the interfacial adhesion. Differences in the chemistry (solubility) of the materials being deposited do make a difference in the effectiveness of this strategy of using a “swelling agent”. For one polyimide, a 3-fold increase in adhesion can be obtained by optimizing the deposition temperature, but this approach for improving adhesion is less effective than that of adding “swelling agent”. The formation of robust interfaces of this type is important because of the critical roles that multilayer films containing polymers with special properties and tailored structures play in applications as diverse as computer displays, photovoltaic devices, and polymeric electronics. The “swelling agent” strategy makes it possible to produce polymer multilayer structures in a cost-effective way with roll-to-roll mass production using direct solution coating

Prolonged Blinking with TERS Probes

Tip-enhanced Raman spectroscopy (TERS), an emerging technique combining scanning probe microscopy (SPM) and Raman spectroscopy, provides the sensitivity and selectivity necessary for chemical imaging with nanoscale resolution. We demonstrate that coating the fragile plasmonic structures used to enhance the electric field with a protective layer improves significantly its mechanical, chemical, and thermal stability without loss of the extreme optical enhancement that appears as a “blinking” phenomenon. This observation provides important information on the mechanism of blinking. The fact that TERS blinking from a polymer layer is preserved when the analyte is separated from the plasmonic nanostructure by a 3 nm thick coating proves that chemical enhancement and charge transfer do not play a significant role in the blinking. Rather, thermally activated diffusion appears to be the dominant mechanism. While extreme enhancement persists only for minutes for an unprotected silver-coated tip, it persists for tens of hours with plasmonic structures coated by an ultrathin layer of alumina (Al2O3), opening broader opportunities for the study of blinking and for chemical imaging and sensing based on plasmonic structures

Interface Roughness Correlation in Diblock Copolymer Brushes Synthesized by Atom Transfer Radical Polymerization

Interface Roughness Correlation in Diblock Copolymer Brushes Synthesized by Atom Transfer Radical Polymerizatio

Polymer Film Surface Fluctuation Dynamics in the Limit of Very Dense Branching

The surface fluctuation dynamics of melt films of densely branched comb polystyrene of thickness greater than 55 nm and at temperatures 23–58 °C above the bulk <i>T</i>_g can be rationalized using the hydrodynamic continuum theory (HCT) known to describe melts of unentangled linear and cyclic chains. Film viscosities (η_XPCS) inferred from fits of the HCT to X-ray photon correlation spectroscopy (XPCS) data are the same as those measured in bulk rheometry (η_bulk) for three combs. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, η_XPCS > η_bulk. These discrepancies are much smaller than those seen for less densely branched polystyrenes. We conjecture that the smaller magnitude of η_XPCS – η_bulk for the densely grafted combs is due to a lack of interpenetration of the side chains when branching is most dense. Both <i>T</i>_g,bulk and the specific chain architecture play key roles in determining the surface fluctuations

Surface Fluctuations of Polymer Brushes Swollen in Good Solvent Vapor

Swollen polymer brushes are found in various applications including biomedical coatings where the brush provides stability against harsh environmental conditions and mediates interactions with the surroundings. The surface height fluctuations of planar polystyrene brushes (0.04–0.61 chains/nm2) highly swollen in toluene vapor are so strongly slowed by the tethering of the chains that they are unobservable in the current experimental window of length and time. This is the case despite the fact that the segmental dynamics of the brush chains should be very fast due to the substantial plasticization by the solvent. With respect to thermally stimulated fluctuations, the surfaces of these swollen brushes are solidlike on time scales and length scales pertinent to many practical applications

Precision Synthesis of ω‑Branch, End-Functionalized Comb Polystyrenes Using Living Anionic Polymerization and Thiol–Ene “Click” Chemistry

A combination of living anionic polymerization and thiol–ene “click” chemistry provides an efficient and convenient method for synthesis of well-defined comb polystyrenes with precisely controlled architecture details and a wide selection of functionalities. ω-(p-Vinylbenzyl)­polystyrene macromonomer was synthesized by sec-butyllithium-initiated polymerization of styrene followed by termination with 4-vinylbenzyl chloride (VBC). For the synthesis of α-4-pentenyl-ω-(p-vinylbenzyl)­polystyrene macromonomer, an unsaturated initiator, 4-pentenyllithium, was used followed by termination with VBC. To ensure successful living anionic polymerization of macromonomers, impurities present in the macromonomers and glass reactors were readily removed by titration with excess sec-butyllithium initiator right before initiation, resulting in polymacromonomers with controlled Mn (74 000, 130 000 g/mol) and narrow Mw/Mn. Living anionic copolymerization of mixtures of both types of macromonomers yielded a well-defined comb-shape precursor with controlled fractions of ω-vinyl branch end groups, which were subsequently subjected to facile and efficient functionalization by photoinitiated thiol–ene “click” reactions with diverse functional groups (−OH, −CO2H, and −C8F17). Characterization by NMR, SEC, and MALDI-TOF mass spectrometry established their chemical structures and chain-end functionalities, which indicates precisely defined comb polystyrenes with controlled degrees of functionalization

Probing Surface Concentration of Cyclic/Linear Blend Films Using Surface Layer MALDI-TOF Mass Spectrometry

Surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-TOF MS) is a powerful new surface sensitive technique to quantify the surface concentration of multicomponent polymer films with enrichment of one component at the surface. Its capabilities are demonstrated for the novel case of a blend of cyclic polystyrene with linear polystyrene, in which we find the composition of linear chains enriched at the surface after annealing, contrary to the expectation of a self-consistent field theory. The probing depth was confirmed to be monomolecular, which for these short chains is less than 2 nm, even though material at a much greater depth is removed by the analysis