When Does a Molecule Become a Polymer?

The influence of elongation-induced concentration fluctuations on segmental friction in polymer blends

Recent experimental studies have revealed a lack of universality in the extensional behavior of linear polymers, which is not envisioned by classical molecular theories. These surprising findings, particularly the sharp contrast between polymer melts and solutions, have catalyzed the development of new theoretical ideas, including the concept of friction reduction in highly stretched polymer melts. By presenting evidence from rheology and small-angle neutron scattering, this work shows that deformation-induced demixing, which is due to the viscoelastic asymmetry in binary mixtures, contributes to the observed nonuniversality. In the case of polystyrene/oligostyrene blends, demixing increases the effective glass transition temperature of the long chain, leading to an apparent friction enhancement. On the other hand, the opposite case is found for the polystyrene/poly(α-methylstyrene) blend. These results highlight the important influence of deformation-induced concentration fluctuations on polymer segmental friction.

Preserving fast ion dynamics while introducing mechanical rigidity in gelatin-based ionogels

Ionogels are gels containing ions, often an ionic liquid (IL), and a gelling agent. They are promising candidates for applications including batteries, photovoltaics or fuel cells due to their chemical stability and high ionic conductivity. In this work we report on a thermo-irreversible ionic gel prepared from a mixture of the ionic liquid 1-butyl-3-methylimidazolium ([BMIM]) dicyanamide ([DCA]), water and gelatin, which combines the advantages of an ionic liquid with the low cost of gelatin. We use (i) dielectric spectroscopy to monitor the ion transport, (ii) dynamic light scattering techniques to access the reorientational motions of the ions, as well as fluctuations of the gel matrix, and (iii) rheology to determine the shear response from above room temperature down to the glass transition. In this way, we are able to connect the microscopic ion dynamics with the meso- and macroscopic behavior of the gelatin matrix. We show, by comparing our results to those for a IL-water mixture from a previous study, that although some weak additional slow relaxation modes are present in the gel, the overall ion dynamics is hardly changed by the presence of gelatin. The macroscopic mechanical response, as probed by rheology, is however dominated by the gel matrix. This behaviour can be highly useful e.g. in battery gel electrolytes which prevent electrolyte leakage and combine mechanical rigidity and flexibility.

End-Tethered Chains Increase the Local Glass Transition Temperature of Matrix Chains by 45 K Next to Solid Substrates Independent of Chain Length

The local glass transition temperature T_g of pyrene-labeled polystyrene (PS) chains intermixed with end-tethered PS chains grafted to a neutral silica substrate was measured by fluorescence spectroscopy. To isolate the impact of the grafted chains, the films were capped with bulk neat PS layers eliminating competing effects of the free surface. Results demonstrate that end-grafted chains strongly increase the local T_g of matrix chains by ≈45 K relative to bulk T_g, independent of grafted chain molecular weight from M_n = 8.6 to 212 kg/mol and chemical end-group, over a wide range of grafting densities σ = 0.003 to 0.33 chains/nm² spanning the mushroom-to-brush transition regime. The tens-of-degree increase in local T_g resulting from immobilization of the chain ends by covalent bonding in this athermal system suggests a mechanism that substantially increases the local activation energy required for cooperative rearrangements.

Polymer Molecular Weight Determination via Fluorescence Lifetime

Control of polymer molecular weight is critical for tailoring structure-function properties; however, traditional molecular weight characterization techniques have limited ability to determine the molecular weight of polymers in real time without sample removal from the reaction mixture, with spatial resolution, and of insoluble polymers. In this work, a fluorescence lifetime imaging microscopy (FLIM) method was developed that overcomes these limitations. The method is demonstrated with polynorbornene and polydicyclopentadiene, polymers derived from ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The polymer M_w, ranging from 35 to 570 kg/mol as determined by gel-permeation chromatography, was quantitatively correlated with the fluorescence lifetime. The revealed correlation then enabled time-resolved measurement of M_w during an ongoing ROMP reaction, requiring only 1 s per measurement (of a 45 μm × 45 μm polymer sample area), and provided spatial resolution, resulting in simultaneous characterization of polymer morphology. To provide the fluorescence signal, the initial reaction solutions contained a very low doping of a reactive norbornene monomer labeled with fluorescent boron dipyrromethene (BODIPY), such that 1 in every 10⁷ monomers contained a fluorophore. The resulting FLIM visualization method enables the rapid determination of the molecular weights of growing polymers without removal from the reaction mixture and regardless of polymer solubility.

Quadratic Non-Linear Optical Properties of the poly(2,5-bis(but-2-ynyloxy) Benzoate Containing the 2-(ethyl(4-((4-nitrophenyl)buta-1,3-diynyl)phenyl)amino)ethanol) Chromophore

Excellent quadratic non-linear optical (ONL-2) properties of the poly(2,5-bis(but-2-ynyloxy) benzoate, containing a polar diacetylene as a chromophore, were found. According with the Maker fringes method, oriented polymer films showing an order parameter of ∼0.23 can display outstanding and stable Second Harmonic Generation (SHG) effects under off-resonant conditions (SHG-532 nm). Also, the macroscopic non-linear optical (NLO)-coefficients were evaluated under the rod-like molecular approximation, obtaining: χ_zzz⁽²⁾ and χ_zxx⁽²⁾ in the order of 280 ± 10 and 100 ± 10 pm V^-1, respectively. The mechanical and chemical properties, in addition to the large ONL-2 coefficients exhibited by this polymer, make it a promising organic material in the development of optoelectronic/photonic devices.

Impact of the Pendant Group on the Chain Conformation and Bulk Properties of Norbornene Imide-Based Polymers

Three series of well-defined norbornene imide-based polymers with different pendant groups were synthesized to investigate the effect of the pendant group on the polymer conformation in solution and bulk melt properties. Each of these three series was examined by analyzing the polymers' bulk z-average radius of gyration via static light scattering and the polymers' melt viscoelastic properties via oscillatory measurements and differential scanning calorimetry. Sterically bulky pendant wedge groups modestly increase the rodlike conformation of the norbornene-imide polymer, however, the inherent rigidity of the polymer main-chain can still be observed with less bulky substituents. In stark contrast, the different side groups significantly impacted the bulk viscoelastic and thermal properties. By increasing the pendant group size, the chain diameter of the polymer increases and lowers the entanglement modulus. Finally, as the wedge pendant group size increases, the segmental relaxation time and the fragility index of these norbornene-based polymers are decreased.

Optimizing the Grafting Density of Tethered Chains to Alter the Local Glass Transition Temperature of Polystyrene near Silica Substrates: The Advantage of Mushrooms over Brushes

We measured the local glass transition temperature T_g(z) of polystyrene (PS) as a function of distance z from a silica substrate with end-grafted chains using fluorescence, where competing effects from the free surface have been avoided to focus only on the influence of the tethered interface. The local T_g(z) increase next to the chain-grafted substrate is found to exhibit a maximum increase of 49 ± 2 K relative to bulk at an optimum grafting density that corresponds to the mushroom-to-brush transition regime. This perturbation to the local T_g(z) dynamics of the matrix is observed to persist out to a distance z ≈ 100-125 nm for this optimum grafting density before bulk T_g is recovered, a distance comparable to that previously observed by Baglay and Roth [J. Chem. Phys. 2017, 146, 203307] for PS next to the higher-T_g polymer polysulfone.

Effects of Tailored Dispersity on the Self-Assembly of Dimethylsiloxane-Methyl Methacrylate Block Co-Oligomers

The effect of dispersity on block polymer self-assembly was studied in the monodisperse limit using a combination of synthetic chemistry, matrix-assisted laser desorption ionization spectroscopy, and small-angle X-ray scattering. Oligo(methyl methacrylate) (oligoMMA) and oligo(dimethylsiloxane) (oligoDMS) homopolymers were synthesized by conventional polymerization techniques and purified to generate an array of discrete, semidiscrete, and disperse building blocks. Coupling reactions afforded oligo(DMS-MMA) block polymers with precisely tailored molar mass distributions spanning single molecular systems (Đ = 1.0) to low-dispersity mixtures (Đ ≈ 1.05). Discrete materials exhibit a pronounced decrease in domain spacing and sharper scattering reflections relative to disperse analogues. The order-disorder transition temperature (T_ODT) also decreases with increasing dispersity, suggesting stabilization of the disordered phase, presumably due to the strengthening of composition fluctuations at the low molar masses investigated.

The unique role of bond length in the glassy dynamics of colloidal polymers

Bond length is generally not considered as a controllable variable for molecular polymers. Hence, no experimental, simulation or theoretical research, to our knowledge, has examined the influence of bond length on the glassy dynamics of polymers. Recently, a new class of assembling materials called "colloidal polymers" has been synthesized. These colloidal polymers have advantages over molecular polymers in the visibility and flexibility of tuning, for example, the size and shape of the "monomers", the interaction, and the bond length. Dense suspension of colloidal polymers will become a very promising ideal model system for exploring the fundamental problems in the glass transition of chain "molecules". Here, we study the static structure and activated dynamics of hard-sphere colloidal polymers by generalizing the colloidal nonlinear Langevin equation theory to colloidal polymers. Surprisingly, we find that the bond length plays a critical and unique role in many aspects. For instance, the universal relations of the characteristic local lengths and the activated barrier versus the "degree of supercooling", and the structural relaxation versus local vibrational motion are found to be dependent on bond length and independent of chain length and rigidity. We hope that our findings inspire future experimental and simulation research studies on the glassy dynamics of colloidal polymers.

Computing thermomechanical properties of dry homopolymers used as raw materials for formulation of biomedical hydrogels

Different static properties have been calculated with COMPASS force field for polyacrylamide, poly(2-hydroxyethylacrylate) (HEA), poly(2-hydroxyethylmethacrylate) (HEMA), poly(glycidylmethacrylate) (GMA), polyethylene glycol (PEG), and poly(2,2,2-trifluoroethylmethacrylate) (TFEM). For each polymers, the calculated values were averaged on five equilibrated configurations of amorphous cell composed of one atactic chain containing 100 repeat units. The ranking obtained from the densities calculated at 300 K is TFEM > HEA ≈ xpolycrylamide > HEMA ≈ GMA > PEG. Concerning the glass transition temperature we have obtained polyacrylamide > HEMA ≈ GMA ≈ HEA > PEG, and polyacrylamide > HEMA ≈ HEA > GMA ≈ PEG > TFEM for the bulk modulus. The calculated results, when available, have been compared with experimental data coming from literature.

Density Functional Theory of Polymer Structure and Conformations

We present a density functional approach to quantitatively evaluate the microscopic conformations of polymer chains with consideration of the effects of chain stiffness, polymer concentration, and short chain molecules. For polystyrene (PS), poly(ethylene oxide) (PEO), and poly(methyl methacrylate) (PMMA) melts with low-polymerization degree, as chain length increases, they display different stretching ratios and show non-universal scaling exponents due to their different chain stiffnesses. In good solvent, increase of PS concentration induces the decline of gyration radius. For PS blends containing short (m1=1−100) and long (m=100) chains, the expansion of long chains becomes unobvious once m1 is larger than 40, which is also different to the scaling properties of ideal chain blends.

A grazing incident XRD study on the structure of poly(3-hydroxybutyrate) ultrathin films sandwiched between Si wafers and amorphous polymers

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology.

Stable Freestanding Thin Films of Copolymer Melts Far from the Glass Transition

Thin polymer films have attracted attention because of both their broad range of applications and of the fundamental questions they raise regarding the dynamic response of confined polymers. These films are unstable if the temperature is above their glass transition temperature T_g. Here, we describe freestanding thin films of centimetric dimensions made of a comb copolymer melt far from its glass transition that are stable for more than a day. These long lifetimes allowed us to characterize the drainage dynamics and the thickness profile of the films. Stratified regions appear as the film drains. We have evidence that the stability, thinning dynamics, and thickness profile of the films result from structural forces in the melt. Understanding the key mechanisms behind our observations may lead to new developments in polymeric thin films, foams, and emulsions without the use of stabilizing agents.

Are polymers standard glass-forming systems? The role of intramolecular barriers on the glass-transition phenomena of glass-forming polymers

Traditionally, polymer melts have been considered archetypal glass-formers. This has been mainly due to the fact that these systems can easily be obtained as glasses by cooling from the melt, even at low cooling rates. However, the macromolecules, i.e. the structural units of polymer systems in general, are rather different from the standard molecules. They are long objects ('chains') made by repetition of a given chemical motif (monomer) and have intra-macromolecular barriers that limit their flexibility. The influence of these properties on, for instance, the glass-transition temperature of polymers, is a topic that has been widely studied by the polymer community almost from the early times of polymer science. However, in the framework of the glass-community, the relevant influence of intra-macromolecular barriers and chain connectivity on glass-transition phenomena of polymers has started to be recognized only recently. The aim of this review is to give an overview and to critically revise the results reported on this topic over the last years. From these results, it seems to be evident that there are two different mechanisms involved in the dynamic arrest in glass-forming polymers: (i) the intermolecular packing effects, which dominate the dynamic arrest of low molecular weight glass-forming systems; and (ii) the effect of intra-macromolecular barriers combined with chain connectivity. It has also been shown that the mode coupling theory (MCT) is a suitable theoretical framework to discuss these questions. The values found for polymers for the central MCT parameter--the so-called λ-exponent--are of the order of 0.9, clearly higher than the standard values (λ ≈ 0.7) found in systems where the dynamic arrest is mainly driven by packing effects ('standard' glass-formers). Within the MCT, this is a signature of the presence of two competing mechanisms of dynamic arrest, as it has been observed in short-ranged attractive colloids or two component mixtures with dynamic asymmetry. Moreover, recent MD-simulations of a 'bead-spring' polymer model, but including intra-macromolecular potential of different strengths, confirm that the high λ-values found in polymers are due to the effect of intra-macromolecular barriers. Although there are still open questions, these results allow to conclude that there is a fundamental difference between the nature of the glass transition in polymers and in simple (standard) glass-formers.

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.

Low-temperature dynamics in amorphous polymers and low-molecular-weight glasses--what is the difference?

Numerous experiments have shown that the low-temperature dynamics of a wide variety of disordered solids is qualitatively universal. However, most of these results were obtained with ensemble-averaging techniques which hide the local parameters of the dynamic processes. We used single-molecule (SM) spectroscopy for direct observation of the dynamic processes in disordered solids with different internal structure and chemical composition. The surprising result is that the dynamics of low-molecular-weight glasses and short-chain polymers does not follow, on a microscopic level, the current concept of low-temperature glass dynamics. An extra contribution to the dynamics was detected causing irreproducible jumps and drifts of the SM spectra on timescales between milliseconds and minutes. In most matrices consisting of small molecules and oligomers, the spectral dynamics was so fast that SM spectra could hardly or not at all be recorded and only irregular fluorescence flares were observed. These results provide new mechanistic insight into the behavior of glasses in general: At low temperatures, the local dynamics of disordered solids is not universal but depends on the structure and chemical composition of the material.

Glass transitions in nanoscale heated volumes of thin polystyrene films

Glass transitions in confined polystyrene films on a silicon substrate were studied using atomic force microscopy incorporating a thermal tip. Three-dimensional spatial nanoconfinements were achieved by controlling size and boundary conditions of small heated volumes of polymer nanostrands drawn from the polymer surface with the thermal tip, using appropriate loads and temperatures at the tip-polymer contact. Finite element analysis was performed to model mechanical contact and thermal transport, including the effects of contact radius, film thickness, and load on temperature and pressure distributions in the confined volume at the contact. The glass transition temperature (T(g)) was measured by observing the softening of polymers with increasing temperature. The measured surface T(g) exhibited a strong size dependence, while the subsurface T(g) increased with decreasing the distance to the substrate. A large increase in the surface T(g) was observed when the radius of contact was reduced below about 10 nm. The increase in the glass transition temperature at the surface was attributed to the presence of surface and line tension at the nanometer contact, while the enhanced T(g) near the substrate was attributed to the pinning effects that reduces the mobility of the polymer molecules in the film over several hundreds of nanometers away from the polymer-substrate interface.

NMR observation of entangled polymer dynamics: tube model predictions and constraint release

We use a benchtop NMR technique to monitor entangled polymer melt dynamics over 5 decades in time covering regimes II-IV of the tube model. We confirm the familiar molar mass scaling exponents of the regime transitions, but we observe a mass-dependent time scaling exponent describing segmental fluctuations in the constrained-Rouse regime II up to high molecular weights. Local chain motions are thus governed by modes that are much less restricted than predicted by the tube model. Diluting protonated chains in deuterated invisible matrix chains, we prove that the exponent is determined by the matrix molecular weight, suggesting constraint-release effects as the origin.