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Polymer networks generally consist of an ensemble of single chains. However, understanding how chain conformation affects the structure and properties of polymer networks remains a challenge for optimizing their functionality. Here, we present the fabrication and comparative study of a polymer network composed of collapsed self-entangled chains (intrachain entangled network) and a standard polymer network in which random-coil chains are entangled with each other (interchain entangled network). For poly(methyl methacrylate) thin films composed of these networks, we coupled solvent vapor swelling and single-molecule tracking techniques to examine the anomalies in the dynamics of a small-molecular probe included in the system. We demonstrate that when compared to the interchain entangled network the intrachain one exhibits a more substantial structural heterogeneity, particularly under highly crowded conditions. This network also exhibits physical compactness, which keeps the heterogeneous network structure frozen over time and impedes network plasticization through solvent uptake by the film.

The rotational dynamics of fluorescent probes of different sizes in glass-forming materials were examined to correlate the time distribution and length scale of the dynamic heterogeneity (ξhet). As the size of the probe increased, the temperature dependence of the rotation correlation time (τc) shifted to longer times, and from this shift, the length scale associated with the glass transition (ξα) was estimated through the Debye–Stokes–Einstein (DSE) relationship and the length scale of the probe (ξsDFT) estimated from quantum mechanical calculations. The estimated ξα values roughly matched with ξhet obtained from calorimetric analysis but were considerably smaller than those deduced from 4D NMR, boson peak, and four-point dynamic susceptibility measurements but with a similar trend of decrease in the length scale upon the increase in the stretching exponent (β) of the system. Because β of the glass formers represents the time distribution of the system, and τc is related to the weighted average of the distribution, the length-scale distribution of the glass transition can be deduced by adopting the DSE relationship and assuming ξα is the weighted average of this distribution at the glass transition temperature. In such a case, the upper bound of the length scale and trend matches the experimentally obtained ξhet from 4D NMR, boson peak, and four-point dynamic susceptibility measurements. Furthermore, at a given temperature, as the probe size increased, the β value reported by the probe increased, whereas the temperature dependence of β, which strongly correlates with the fragility of the system, was independent of the probe size.

We employed imaging rotational fluorescence correlation microscopy to directly observe the segmental dynamics of the polymer near the polymer–substrate interface using a probe-tethered polymer brush buried in bulk polymer films. The probe-tethered polymers grafted onto the polymer–substrate interface provide spatial selectivity to probe only the segmental dynamics of polymer near the polymer–substrate interface. The perturbation caused by the grafted polymer chains on the segmental dynamics of the polymer film near the interface was minimized by reducing the molecular weight of the polymer brush compared with the critical entanglement molecular weight of the polymer and grafted with a low grafting density (≈0.1 chains/nm²) using a “grafting-to” method, which allows precisely controlled and fully characterized polymer brush structures. In addition, the molecular weight of the brush polymer was further controlled to ≈10 kg/mol with the root-mean-square end-to-end distance of ≈5 nm to probe dynamics within 10 nm of the polymer–substrate interface. Polystyrene and poly(methyl methacrylate) films carrying rather unfavorable and similar surface energies, respectively, with the silane-modified silicon wafer substrates were investigated. The polymer dynamics near the interface characterized by the degrees of non-Arrhenius temperature dependence and nonexponential relaxation were not altered in both systems compared with those of the bulk, suggesting that the polymer dynamics is not sensitive to moderate differences in enthalpic interaction between the polymer and the substrate.

How tethered probes report dynamics of host polymers near the glass transition was investigated by changing the length of the flexible linkers and the number of tethering points via imaging rotational fluorescence correlation microscopy and compared with free probes of different sizes. The results show that tethering did not alter the temperature-dependence of polymer dynamics and the shape of the correlation decay reported by the probe; however, the rotation slowed down up to ≈1 decade when both ends of the probe were restricted with short alkyl chain linkers. Upon comparison with the bigger free probe, the mechanism of the slowdown was attributed to the restricted motion upon tethering for tethered probes compared to averaging over different regions of the dynamic heterogeneity for the bigger probe. If the size of the probe was comparable to that of the dynamic heterogeneity of the system, tethered probes accurately report dynamics relevant to glass transition, regardless of tethering conditions.

We demonstrate an approach to examine the local segmental dynamics of a polymer near the interface of an inorganic filler by observing the rotational dynamics of the fluorescent probe at the chain ends of polymer brushes grafted onto the surface of the filler particles. Localization of the fluorescent probe was realized by designing and synthesizing fluorophore-tethered polystyrene (PS) brushes anchored on the surface of silica particles of controlled sizes. Fluorophore-functionalized telechelic PS with an azide functionality at the other chain end was achieved via a combination of atom transfer radical polymerization and post-polymerization modification. The azide-bearing PS chains were tethered to alkyne-functionalized particles via copper-catalyzed cycloaddition reaction. The molecular weight of the grafted polymer chains was controlled to be less than the critical entanglement molecular weight, and the chain density was controlled to be low enough so that the observed dynamics was not perturbed by the polymer brush conformation and brush-matrix polymer entanglement. The polymer dynamics near the surface of the particles at low concentrations was closely examined in the bulk film geometry by employing imaging rotational fluorescence correlation microscopy (irFCM). The observed polymer dynamics near the interface were not altered in the inorganic/polymer composite geometry when the surface did not have favorable interaction with the matrix polymer. The presented rational design of the chemical route and examination of local dynamics highlight a feasible approach to construct material systems with high complexities towards a deeper understanding of composite materials.

This study examined the kinetics of copolymerization of two different simple model monomers, methyl methacrylate (MMA), and ethyl methacrylate (EMA), via Cu(0)-mediated reversible deactivation radical polymerization (RDRP), where the reactivities of MMA and EMA are expected to be nearly equal, and therefore, random copolymerization is favored. In these kinetic studies, the apparent propagation rate constants and induction periods with variations in the feed ratio and polymerization temperature were estimated. The reactivity ratios determined based on the kinetic studies were close to unity. In addition, the reactivities of MMA and EMA radicals to both monomers were evaluated by determining the thermodynamic parameters of the activation processes; the reactions of active species with two different monomers experience slightly different enthalpic barriers but almost zero entropic contributions, strongly suggesting that the copolymerization provides an almost, but not perfectly, random sequence and providing a clear chemical picture of the propagation reaction that occurs during copolymerization. The glass transition studies of synthesized P(MMA-r-EMA) highlight the significance of the kinetic information to predict the glass transition temperature in a copolymer system realized by Cu(0)-mediated RDRP. The current study provides a deeper understanding of the fundamental aspects and a tool to gain insight into copolymerization via RDRP, which are ultimately correlated with various physical and chemical properties. Copolymerization kinetics of methyl methacrylate and ethyl methacrylate by Cu(0)-mediated reversible deactivation radical polymerization was thoroughly explored. The copolymerization follows pseudo-first order kinetics in a highly controlled manner. Detailed kinetic studies revealed that two different monomers experience different enthalpic barriers in propagation, which strongly suggest that the copolymerization exhibits not perfectly, but nearly random composition. Further studies on glass transition and chain extension highlight the significance of these fundamental studies to predict thermal properties and to realize complex polymer architecture.

We examined the glass transition behaviors of a series of homopolymers and random copolymers, poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(methyl methacrylate-random-ethyl methacrylate) (P(MMA-r-EMA)), in films deposited by two conventionally employed methods, spin-coating and the Langmuir-Blodgett (LB) method, exhibiting uncontrolled and controlled chain conformations at the molecular level, using spectroscopic ellipsometry measurements. The processing parameters of the LB method (surface pressure, deposition rate, and number of deposited layers) were controlled and the films were compared with those made by spin-coating to gain a deeper understanding of how the glass transition temperature is affected by the processing conditions. Our results strongly suggest that the glass transition temperature of polymer films follows typical thermal behaviors of corresponding bulk samples, even though they are deposited using the LB method. The bulk glass transition temperature (Tg) was controlled to further investigate how the deposition temperature affects the thermal properties of polymer films by adjusting the composition of P(MMA-r-EMA) random copolymers. The molecular layer by layer deposition using the LB method at temperatures ranging from 0.76 Tg to 0.87 Tg, where the small molecule glass films by physical vapor deposition have extraordinary high density, demonstrated glass transition behavior identical to that of the bulk samples. The thermal stability of the polymer films deposited using the LB method could not be compared to that of the small molecule films with similar deposition temperatures relative to Tg, as the inevitable trapping of water molecules during LB deposition resulted in low density and high apparent fictive temperature of LB deposited films.

To study the mechanism and formation of gamma ray–irradiated porous polymeric microspheres, a polymerization reaction was seeded with particles and pores of various sizes. The porous particles were formed via phase separation derived from the modification of an un-grafted polystyrene (PS). The morphology of the porous sulfonated polystyrene/poly(methyl methacrylate) (SPS/PMMA) microspheres was controlled by varying the degree of swelling and the duration of sulfonation. The procedure was initiated by swelling SPS microspheres with MMA, which possessed similar solubility parameters to PS. Water permeated the SPS microspheres via osmosis and produced a sizeable water phase, which formed pores after drying. A sufficient swelling period was required to stabilize the permeated MMA and water. Furthermore, hydrophilic groups within the particles contributed to the degree of water and monomer that was absorbed by the particles. As a result, various micro-sized porous particles were fabricated and pore sizes could be controlled by adjusting the sulfonyl groups.

Graphene is one of the most promising materials for post-silicon electronics and has outstanding physical and electronic properties. In particular, its unique 2D sp2-hybridized networks of carbon atoms arranged in a honeycomb lattice make graphene potential for exceptional electronic quality. However, in order to use graphene in possible applications such as photodetector, photovoltaics, sensors, organic light-emitting diodes, organic thin-film transistors, supercapacitor, and catalytic applications, it is essential to precisely modulate its electronic properties, i.e. doping. In this review, we present various strategies for engineering the Fermi level in graphene, including heteroatom substitution, molecular adsorption, introducing functional molecules for external stimuli responsiveness. We anticipate that the current review provides a concise information on the methods to probe doping level, effective doping approaches, and achievable doping type and charge carrier concentration ranges so that an appropriate doping approach can be readily designed.

We demonstrate the synthesis of poly(methyl methacrylate) (PMMA) and poly(styrene-b-methyl methacrylate) (P(S-b-MMA)) brushes on crosslinked random copolymer thin films, compositionally varied poly(styrene-r-glycidyl methacrylate) (P(S-r-GMA)), which can be further functionalized with a molecule featuring an initiator group upon crosslinking to form highly stable thin films. With careful optimizations, PMMA brushes were successfully grown from the surfaces of initiator functionalized P(S-r-GMA) via surface-initiated atom transfer radical polymerization. The grafting densities of the PMMA and P(S-b-MMA) brushes were effectively controlled to be in different density regimes by controlling the composition of P(S-r-GMA) and post-crosslinking functionalization methods. Synthesized BCP brushes were stable upon repetitive washing and thermal annealing processes even at high grafting density, highlighting that the outstanding stability of crosslinked P(S-r-GMA) thin films enables close examination of the morphology of thermally annealed P(S-b-MMA) brushes in different grafting density regimes.

Fibrillar type I collagen-based hydrogels are commonly used in tissue engineering and as matrices for biophysical studies. Mechanical and structural properties of these gels are known to be governed by the conditions under which fibrillogenesis occurs, exhibiting variation as a function of protein concentration, temperature, pH, and ionic strength. Deeper understanding of how macroscopic structure affects viscoelastic properties of collagen gels over the course of fibrillogenesis provides fundamental insight into biopolymer gel properties and promises enhanced control over the properties of such gels. Here, we investigate type I collagen fibrillogenesis using confocal rheology—simultaneous confocal reflectance microscopy, confocal fluorescence microscopy, and rheology. The multimodal approach allows direct comparison of how viscoelastic properties track the structural evolution of the gel on fiber and network length scales. Quantitative assessment and comparison of each imaging modality and the simultaneously collected rheological measurements show that the presence of a system-spanning structure occurs at a time similar to rheological determinants of gelation. Although this and some rheological measures are consistent with critical gelation through percolation, additional rheological and structural properties of the gel are found to be inconsistent with this theory. This study clarifies how structure sets viscoelasticity during collagen fibrillogenesis and more broadly highlights the utility of multimodal measurements as critical test-beds for theoretical descriptions of complex systems.

The spatial arrangement of nanoparticles (NPs) within thin polymer films may influence their properties such as the glass transition temperature. Questions regarding what may affect the spatial arrangement of NPs, however, still remain unanswered at a molecular level. In this work, we perform molecular dynamics simulations for a free-standing thin polymer film with a single NP. We find from simulations that depending on the NP size and the inter-particle interaction between the NP and polymers, one may control the spatial arrangement of the NP. When the interaction between the NP and polymers is sufficiently attractive (repulsive), the NP is likely to be placed at the center (at the surface) of the thin film in equilibrium. Interestingly, for a moderate interaction between the NP and polymers, the first-order transition occurs in the spatial arrangement of the NP as one increases the NP size: a small NP prefers the surface of the polymer film whereas a large NP prefers the center. Such a first-order transition is corroborated by calculating the free energy of the NP as a function of the position and can be understood in terms of a sixth-order Landau free energy. More interestingly, the diffusion of the NP also changes drastically due to the first-order transition in the spatial arrangement. The NP diffusion is enhanced drastically (more than expected in bulk polymer melts) as the NP is shifted to the polymer film surface.

In a polymeric material near its glass transition temperature, segmental dynamics of a given spatial region may differ considerably from that of neighboring regions without apparent structural origin, analogous to the supercooled liquid state of low molecular weight glass formers. Given that the supercooled liquid state is a (metastable) equilibrium state, spatial variations in dynamics are expected to average out in time, consistent with ergodicity of the system. By probing the rotations of fluorescent guest molecules, local segmental dynamics of polystyrene was scrutinized molecule by molecule. Two perylene dicarboximide dyes were investigated as potential reporters, and one of these was found to report a substantial proportion of the dynamic heterogeneity of the host polystyrene. Using this probe, we demonstrate that the polystyrene system is ergodic and characterize time scales over which molecules experience changes to their dynamics. We identify a characteristic time scale of exchange much longer th...

Heterogeneity in relaxation rates is a key feature of supercooled liquids. It implies the existence of a rate-exchange process to restore ergodicity, but the experimental characterization of that exchange has been incomplete and controversial. Here, a recently proposed, ensemble-based analysis is applied to single-molecule dichroism data to extract a detailed correlation function for rotational-rate exchange. A large, late phase is $8.7\ifmmode\pm\else\textpm\fi{}0.3$ times slower than the probe rotation and 22 times the alpha-relaxation time, and it has its own exchange-rate dispersion. A small, early phase tracks the initial rotational decay. We propose that the late phase is due to molecules in the core of spatial regions of correlated rates and that the early phase is due to molecules on the boundaries. The results imply that multiple processes and spatial fields are involved in the primary relaxation in supercooled liquids.

We demonstrate effective functionalization chemistry for cellulose nanofiber modification using thiol functionality. Electrospun cellulose acetate nanofibers were deacetylated to obtain cellulose nanofibers, which were modified further to incorporate thiol on their surface by the esterification of hydroxyl groups with 3,3'-dithiodipropionic acid and further reductive cleavage of the disulfide bond. The thiol functionality was highly versatile to bring simple and efficient chemical reactions to attain (i) Ag nanoparticle-adsorbed cellulose nanofibers by Ag ion reduction at surface, (ii) various amine (primary amine and quaternary amine) functionalized cellulose nanofibers by Michael addition, and (iii) complex polymer functionalized cellulose nanofibers by a radical-based thiol-ene reaction, under mild conditions, i.e. in any reaction media, at room temperature, and under ambient atmosphere, evidenced by a variety of characterization methods including a quantitative analysis with X-ray photoemission spectroscopy. These scalable thiol-based chemistries should offer a new generation of well-tailored cellulose nanofiber materials with complex inorganic, organic, and polymeric functionalities, potentially expanding to functionalized surfaces of other carbohydrate-based materials to achieve the desired properties.

The relationship between photostability and conformation of 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) conjugated polymers was studied via excitation polarization modulation depth (M) measurements. Upon partial photobleaching, M distributions of collapsed, highly ordered MEH-PPV molecules shifted toward lower values. Conversely, M distributions of MEH-PPV molecules with random coil conformations moved toward higher values after partial photobleaching. Monte Carlo simulations of randomly distributed dipole moments along polymer chains subjected to partial photobleaching revealed that a statistical effect leads to an increase in peak M value. Decreases in M values seen experimentally in the population of MEH-PPV molecules with high M values, however, are due to conformation-dependent photostability within single MEH-PPV polymers. We show that, while folded MEH-PPV molecules are relatively more photostable than extended MEH-PPV molecules in an ensemble, extended portions of particular molecules are more photostable than folded domains within single MEH-PPV molecules.

Significance Supercooled liquids are believed to exhibit spatially heterogeneous dynamics, where molecular mobility within a given spatial region may differ from that of a neighboring region, potentially by orders of magnitude. If supercooled liquids are ergodic, such that the spatial average of all regions with distinct dynamics equals the time average of a given region, these regions of distinct dynamics must interchange over time. With an appropriate probe, similar in size and mobility to the host, single-molecule measurements can provide direct access to these spatial and temporal variations. Here, such a probe is used, revealing how relaxation dynamics are distributed in time and space and directly demonstrating ergodicity of a prototypical glass former down to the glass transition temperature.

To thoroughly elucidate how molecular conformation and photophysical properties of conjugated polymers (CPs) are related requires simultaneous probing of both. Previous efforts used fluorescence imaging with one nanometer accuracy (FIONA) to image CPs, which allowed simultaneous estimation of molecular conformation and probing of fluorescence intensity decay. We show that calculating the molecular radius of gyration for putative folded and unfolded poly(2-methoxy-5-(2'-ethylhexyloxy)1,4-phenylenevinylene) (MEH-PPV) molecules using FIONA underestimates molecular extension by averaging over emitters during localization. In contrast, employing algorithms based on single molecule high resolution imaging with photobleaching (SHRImP), including an approach we term all-frames SHRImP, allows localization of individual emitters. SHRImP processing corroborates that compact MEH-PPV molecules have distinct photophysical properties from extended ones. Estimated radii of gyration for isolated 168 kDa MEH-PPV molecules immobilized in polystyrene and exhibiting either stepwise or continuous intensity decays are found to be 12.6 and 25.3 nm, respectively, while the distance between exciton recombination sites is estimated to be ∼10 nm independent of molecular conformation.

Temperature-ramping anisotropy measurements were used to probe the molecular mobility of fluorescent probes in polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) films supported upon silicon wafers with native oxide coatings. All polymer films showed evidence of high mobility at the free surface. The fraction of a film with high mobility was characterized as a mobile surface layer thickness, which increased with temperature. The mobile surface layer thickness for supported films of polystyrene and poly(methyl methacrylate) reasonably matched that previously deduced from freestanding films of these polymers; for poly(methyl methacrylate), enhanced mobility extends about 4 nm into the film from the free surface at Tg. For supported polystyrene and poly(methyl methacrylate) films, the results are consistent with no decrease in mobility near the solid substrate but do not eliminate this possibility. On the other hand, the mobility of supported poly(2-vinyl pyridine) thin films provides some evidence for slower-than-bulk relaxation near the solid substrate.

A photobleaching technique has been used to measure the reorientation of dilute fluorescent probes in free-standing polymer films from 7 nm to 20 μm thick. Similar to our previous report on free-standing polystyrene films, two subsets of probes with distinct dynamics were observed in free-standing poly(methyl methacrylate) films. The more mobile subset of probes is consistently interpreted in terms of a surface layer, and the thickness of the mobile surface layer is shown to increase with temperature. Poly(methyl methacrylate) has a mobile surface layer thickness of 4 nm at the bulk Tg. Similar measurements on free-standing poly(2-vinylpyridine) and poly(4-tert-butylstyrene) films indicate mobile surface layer thicknesses from 7 to 10 nm at their bulk Tg values, similar to the 7 nm value previously reported for polystyrene. In contrast to the other four polymers, free-standing poly(α-methylstyrene) films show no evidence of a mobile surface layer. We interpret these results in terms of contributions from ...

Using extrinsic probes to study a host system relies on the probes’ ability to accurately report the host properties under study. Probes have long been used to characterize dynamic heterogeneity, the phenomenon in which a liquid near its glass transition exhibits distinct dynamics as a function of time and position, with molecules within nanometers of each other exhibiting dynamics that may vary by orders of magnitude. The spatial and temporal characteristics of dynamic heterogeneity demand the selection of probes using stringent criteria on their size and dynamics. In this report, we study the dynamic heterogeneity of the prototypical molecular glass former o-terphenyl by investigating single molecule rotation of two perylene dicarboximide probe molecules that differ in size and comparing this to results obtained previously with the probe BODIPY268. It is found that a probe’s ability to accurately report dynamic heterogeneity in o-terphenyl depends on whether the reported distribution of dynamics overlaps with the intrinsic dynamics of the host, which is naturally related to the width of the intrinsic dynamics and the magnitude of dynamical shift in probe dynamics relative to the host. We show that a probe that rotates ≈15 times more slowly than the intrinsic dynamics of the host o-terphenyl senses the slowest ≈5% of the full dynamic heterogeneity whereas one that rotates ≈65 times more slowly than the host fails to report dynamic heterogeneity of the host.

We induced a terpyridine moiety into a norbornene-based polymer to demonstrate its self-healing property, without an external stimulus, such as light, heat, or healing agent, using metal&ndash, ligand interactions. We synthesized terpyridine incorporated norbornene-based polymers using a ring-opening metathesis polymerization. The sol state of diluted polymer solutions was converted into supramolecular assembled gels, through the addition of transition metal ions (Ni2+, Co2+, Fe2+, and Zn2+). In particular, a supramolecular complex gel with Zn2+, which is a metal with a lower binding affinity, demonstrated fast self-healing properties, without any additional external stimuli, and its mechanical properties were completely recovered.

Polymeric systems close to their glass transition temperature are known to exhibit heterogeneous dynamics that evolve both over time and space, comparable to the dynamics of small molecule glass formers. It remains unclear how temperature influences the degree of heterogeneous dynamics in such systems. In the following report, a fluorescent perylene dicarboximide probe molecule that reflects the full breadth of heterogeneity of the host was used to examine the temperature dependence of the dynamic heterogeneity lifetime in polystyrene at several temperatures ranging from the glass transition to 10 K above this temperature via single molecule microscopy. Contrary to prior reports, no apparent temperature dependence of time scales associated with dynamic heterogeneity was detected; indeed, the probe molecules report characteristic dynamic heterogeneity lifetimes 100-300 times the average alpha-relaxation time (τα) of the polystyrene host at all temperatures studied.

An optical photobleaching method has been used to measure the segmental dynamics of a poly(methyl methacrylate) (PMMA) glass during uniaxial creep deformation at temperatures between T g - 9 K and T g - 20 K. Up to 1000-fold increases in mobility are observed during deformation, supporting the view that enhanced segmental mobility allows flow in polymer glasses. Although the Eyring model describes this mobility enhancement well at low stress, it fails to capture the dramatic mobility enhancement after flow onset, where in addition the shape of the relaxation time distribution narrows significantly. Regions of lower mobility accelerate their dynamics more in response to an external stress than do regions of high mobility. Thus, local environments in the sample become more dynamically homogeneous during flow.

A photobleaching method was used to measure the reorientation of dilute probes in liquid o-terphenyl near a crystal growth front. Near the glass-transition temperature Tg, mobility in the supercooled liquid was enhanced within ∼10 μm of the crystal growth front, by as much as a factor of 4. This enhanced mobility appears to be caused by tension created in the sample as a result of the density difference between the supercooled liquid and crystal. The maximum observed mobility enhancement corresponds to a tension of about −8 MPa, close to the cavitation limit for liquid o-terphenyl. Whereas the observed mobility near the growing crystal is not large enough to explain the extraordinary fast crystal growth observed near Tg in o-terphenyl and some other low-molecular-weight glassformers, these observations suggest that cavitation or fracture plays a key role in releasing tension and allowing fast crystal growth to occur at a steady rate.

Surface pressure (π)–area (A) isotherms of hydrophobically modified polyethylene oxide (HEUR) at the air–water interface was examined. Conformational transitions between pancake, mushroom, and brush states of the hydrophilic backbone influence the intermolecular interaction between the hydrophobic chains. We choose relatively long (18 carbons) hydrophobic ends, which have large hydrophobic interactions, and investigate the main chain effect by change in the length of the hydrophilic PEO chain. At high surface concentration region, the temperature coefficient of surface pressure, dπ/dT, was larger by increasing the portion of the hydrophobicity. This indicates an increase in surface energy and a decrease in surface enthalpy at high surface concentrations. As alkyl chains on both sides of HEURs are anchored at the air–water interface, restriction caused by the alkyl chain would be smaller for the long PEO chain, but the larger for the short PEO chain length.

Monitoring single molecule probe rotations is an increasingly common approach to studying dynamics of complex systems, including supercooled liquids. Even with advances in fluorophore design and detector sensitivity, such measurements typically exhibit low signal to noise and signal to background ratios. Here, we simulated and analyzed orthogonally decomposed fluorescence signals of single molecules undergoing rotational diffusion in a manner that mimics experimentally collected data of probes in small molecule supercooled liquids. The effects of noise, background, and trajectory length were explicitly considered, as were the effects of data processing approaches that may limit the impact of noise and background on assessment of environmental dynamics. In many cases, data treatment that attempts to remove noise and background were found to be deleterious. However, for short trajectories below a critical signal to background threshold, a thresholding approach that successfully removed data points associated with noise and spared those associated with signal allowed for assessment of environmental dynamics that was as accurate and precise as would be achieved in the absence of noise.

Crystal growth kinetics and liquid dynamics of 1,2-diphenylcyclopentene (DPCP) and 1,2-diphenylcyclohexene (DPCH) were characterized by optical microscopy and dielectric spectroscopy. These two molecules are structurally homologous and dynamically similar to the well-studied glassformer ortho-terphenyl (OTP). In the supercooled liquid states of DPCP and DPCH, the kinetic component of crystal growth ukin has a power law relationship with the primary structural relaxation time τα, ukin [proportionality] τα(–ξ) (ξ ≈ 0.7), similar to OTP and other fragile liquids. Near the glass transition temperature (Tg), both DPCP and DPCH develop much faster crystal growth via the so-called GC (glass to crystal) mode, again similar to the behavior of OTP. We find that the α-relaxation process apparently controls the onset of GC growth, with GC growth possible only at sufficiently low fluidity. These results support the view that GC crystal growth can only occur in systems where the liquid and crystal exhibit similar local packing arrangements.

Diblock copolymer (A−B) behaviors in quasi-two-dimensional space have been examined where the A block is polystyrene (PS) and the B block is poly(methyl methacrylate) (PMMA). The length of the PMMA block was varied while that of the PS block was kept almost the same. The surface pressure isotherms were determined with the aid of a Langmuir film balance. The surface pressure isotherms exhibited two phase transitions attributed to surface micelle formation at low surface pressure and to further assembly of surface micelles at high surface pressure. Atomic force microscope images are correlated with the aggregates of the chains forming the quasi-two-dimensional surface micelles at low surface pressure. Chains having shorter PMMA block than PS block form the micelles with many block copolymer molecules (ca. 430 molecules) whereas chains having 4 times longer PMMA block than PS block form the micelles with several molecules. High compression led the micelles to have a more compact structure. The new structure ...

To examine possible adsorptive mechanism of polyglutamates on the aluminum surface, ATR-FTIR (attenuated total reflectance-Fourier transform infrared) spectrometer and MO (molecular orbital) calculation were used. Expected C˭O stretching shift due to the chemical binding was not observed even in less-substituted glutamate polymers. ATR studies suggest: (1) the interaction between Al surface and polyglutamates is driven by physical adsorption, and (2) the interaction between the hairy rod polymer and solid substrate is governed by the intercrossing of side chain pendent groups.

Much of the interesting behavior that has been observed in supercooled liquids appears to be related to dynamic heterogeneity, the presence of distinct dynamic environments – with no apparent underlying structural basis – in these systems. To most directly interrogate these environments, proposed to span regions just a few nanometers across, molecular length scale probes are required. Single molecule fluorescent microscopy was introduced to the field a decade ago and has provided strong evidence of dynamic heterogeneity in supercooled systems. However, only more recently has the full set of challenges associated with interpreting results of these experiments been described. With a fuller understanding of these challenges in hand, single molecule measurements can be employed to provide a more precise picture of dynamic heterogeneity in supercooled liquids and other complex systems. In this tutorial review, experimental and data analysis details are presented for the most commonly employed single molecule approach to studying supercooled liquids, the measurement of rotational dynamics of single molecule probes. Guidance is provided in experimental set-up and probe selection, with a focus on choices that affect data interpretation and probe sensitivity to dynamic heterogeneity.

An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with different dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at T(g,DSC) - 25 K (T(g,DSC) is the glass transition temperature of bulk polystyrene). At T(g,DSC) - 5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are interpreted as indicating the presence of a high-mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at T(g,DSC).

A temperature-ramping anisotropy measurement is introduced as an efficient way to study molecular motion in polymer glasses. For these experiments, fluorescent molecules were dispersed in the polymer glass and the reorientation of these dyes was used as a probe of segmental dynamics. For thick samples of polystyrene, poly (4-tert-butyl styrene), and poly(2-vinyl pyridine), temperature-ramping anisotropy measurements have a shape similar to differential scanning calorimetry measurements and nearly the same transition temperature. We present results using different fluorescent molecules and different temperature-ramping rates; such experiments show potential for accessing slow molecular motions considerably below T(g). Temperature-ramping anisotropy measurements were performed on freestanding poly (4-tert-butyl styrene) films of varying thicknesses. The anisotropy decay of a 22 nm film was shifted about 12 K lower in temperature as compared to a bulk sample.

When sufficient force is applied to a glassy polymer, it begins to deform through movement of the polymer chains. We used an optical photobleaching technique to quantitatively measure changes in molecular mobility during the active deformation of a polymer glass [poly(methyl methacrylate)]. Segmental mobility increases by up to a factor of 1000 during uniaxial tensile creep. Although the Eyring model can describe the increase in mobility at low stress, it fails to describe mobility after flow onset. In this regime, mobility is strongly accelerated and the distribution of relaxation times narrows substantially, indicating a more homogeneous ensemble of local environments. At even larger stresses, in the strain-hardening regime, mobility decreases with increasing stress. Consistent with the view that stress-induced mobility allows plastic flow in polymer glasses, we observed a strong correlation between strain rate and segmental mobility during creep.

The reorientation of dye molecules can be used to monitor the segmental dynamics of a polymer melt. We utilize this technique to measure stress-induced mobility in a lightly cross-linked poly(methyl methacrylate) (PMMA) glass during tensile creep deformation. At 377 K (18 K below the glass transition temperature Tg), the mobility increased by a factor of 100 during deformation with a stress of 20 MPa. Generally, the mobility increased as the stress, strain, and strain rate increased. After removing the stress, we observed that the enhanced mobility slowly disappeared during strain recovery. At 377 K, when the stress is lower than 11 MPa, almost no mobility enhancement was observed. Once the stress crossed this threshold value, the mobility dramatically increased.

Monolayers of 2-(3,4,5-(trisdodecyloxyl)phenyl)[1,3,4]oxadiazole based "tripod" dye, P2G, has been studied at the air/water interface with in situ X-ray reflectivity. Compression of the disordered Langmuir-Blodgett monolayer film induces a transition to a unique ordered phase, representing a supramolecular assembly with a unique spatial distribution and orientation of the molecules. At low pressure, the molecules having face-on orientation are interdigitated by the three arms. After first transition in the pi-A isotherm, the molecular conformation is turned into an edge-on orientation, where the molecules are self-assembled into supramolecular structures.