Conformational transition characterization of glass transition behavior of polymers

The conformational transition behavior of polymer in the amorphous state has been investigated through molecular dynamics simulations across the glass transition temperature (T_g). We find that the conformational transition, a localized and short time dynamics feature, crosses over different barrier heights when the system transforms from the molten state into the glass state and the barrier height in the glass state is markedly lower than that above T_g. In addition to the overall transition behavior, the specific transitions between the rotational isomeric states (RIS) g⁺, t⁻, t⁺ and g⁻ are also investigated in detail. The populations of these specific transitions undergo considerable changes when the temperature decreases; meanwhile, the larger transition rates of the ending torsions get diminished. Besides the rate, the rotation degrees of the dihedrals during the transitions also change their distributions tremendously through T_g, below which most of the larger transition angles (50-100°) were inhibited remaining those sharply around 30°. This possibly explains why below T_g the conformational transition process has a lower effective barrier.     

Dielectric spectroscopy using dielectric probes: a new approach to study glass transition dynamics in immiscible apolar polymer blends

Dielectric relaxation spectroscopy using dielectric probes was applied to study the (glass transition) dynamics in binary blends of isotactic PP, PS and LDPE. The blends were prepared by melt-mixing and doped with 0.5% of the dielectric probe 4,4′-(N,N-dibutylamino)-(E)-nitrostilbene (DBANS) (van den Berg O, Sengers WGF, Jager WF, Picken SJ, Wübbenhorst M. Macromolecules 2004;37:2460. [17]). Due to the selective amplification of the dielectric relaxation processes related to the dynamic glass transition of the polymers, accurate relaxation data were obtained, even for the minor phases. No substantial influence of the blend composition and the blend morphology on the glass transition dynamics was found, indicating that both blend constituents behave like homogeneous bulk materials. The normalised relaxation strength of glass transition processes remained constant, regardless of the blend type and blend composition. This indicates that the probe molecule, DBANS, was equally distributed over the two blend components in all three polymer combinations PE-PP, PE-PS and PP-PS.     

The dynamics of montmorillonite clay dispersion and morphology development in immiscible ethylene-propylene rubber/polypropylene blends

The evolution of morphology during the melt compounding of polypropylene (PP), maleated ethylene-propylene rubber (EPR-g-MAn) and onium-ion exchanged montmorillonite clay (NR₄⁺-MM) is described. Irrespective of the ratio of components, clay partitions into the EPR-g-MAn phase exclusively, with significant amounts of mineral exfoliation occurring in the very early stages of compounding. These changes in filler distribution and dispersion are accompanied by reductions in the size of the dispersed PP phase, as the rate of droplet coalescence falls in response to an elevated EPR-g-MAn matrix viscosity. However, when NR₄⁺-MM is localized in a dispersed EPR-g-MAn phase, coalescence increases as a result of hindered particle break-up.     

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

Miscibility and interactions in poly(n-propyl methacrylate)/poly(vinyl alcohol) blends

Poly(n-propyl methacrylate) (PPMA) is miscible with poly(vinyl alcohol) (PVA) over the whole composition range as shown by the existence of a single glass transition temperature in each blend. The interaction between PPMA and PVA was examined by Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance spectroscopy. The interactions mainly involve the hydroxyl groups of PVA and the carbonyl groups of PPMA. The measurements of proton spin-lattice relaxation time reveal that PPMA and PVA do not mix intimately on a scale of 1-3 nm, but are miscible on a scale of 20-30 nm. A small negative interaction parameter value has been obtained by melting point depression measurement.     

Glass transition and the rigid amorphous phase in semicrystalline blends of bacterial polyhydroxybutyrate PHB with low molecular mass atactic R, S-PHB-diol

The glass transition and the crystallinity of blends of isotactic bacterial PHB and low molecular mass atactic R, S-PHB-diols was investigated by means of differential scanning calorimetry (DSC), temperature-modulated DSC and dielectric spectroscopy. It was found that (i) T_g of crystallized blends is much lower than T_g of quenched blends, (ii) the semi-crystalline blends can only be described with a three-phase model. From the experimental results the amount of the oligomer component in the mobile amorphous as well as in the rigid amorphous phase was determined. It could be shown that the low molecular mass atactic R, S-PHB-diol is enriched in the mobile amorphous phase of the semi-crystalline blends, but 5-15% oligomer remains, however, in the rigid amorphous phase.     

DC electric field effects on Ehrenfest-like relations at the glass transition

There have been numerous studies on the effects of dc and alternating electric fields on phase transitions in small-molecule mixtures and high-molecular weight polymers. Furthermore, textbook examples discuss modifications in the melting temperature of pure materials that are subjected to uniform dc fields. This thermodynamic analysis extends some of these predictions to second-order phase transitions. Upon invoking both volume and entropy continuity via the integral approach to phase equilibrium at second-order transitions, electric field effects on the glass transition are developed that parallel the Ehrenfest equations for the pressure dependence of T_g. Both T_g-field equations predict small changes in the glass transition temperature that scale as the square of the electric field strength. If one equates the dependence of T_g on the magnitude of the electric field via (i) volume continuity and (ii) entropy continuity, it is possible to obtain the electric-field analog of the Prigogine-Defay equality, in which thermophysical properties and discontinuous observables at the zero-field and field-dependent second-order phase transition temperatures are related. When the temperature and pressure dependencies of the relative electric permittivity (i.e., dielectric constant) are neglected in the absence of external fields, one recovers the classic Prigogine-Defay equality (i.e., the lower limit of the Prigogine-Defay ratio) that was developed from a consideration of volume and entropy continuity for the pressure dependence of T_g, by invoking the differential approach to phase equilibrium.     

The glass transition and thermoelastic behavior of epoxy-based nanocomposites: A molecular dynamics study

In this study, the glass transition and thermoelastic properties of cross-linked epoxy-based nanocomposites and their filler-size dependency are investigated through molecular dynamics simulations. In order to verify the size effect of nanoparticles, five different unit cells with different-sized silicon carbide (SiC) nanoparticles are considered under the same volume fraction. By considering a wide range of temperatures in isobaric ensemble simulations, the glass transition temperature is obtained from the specific volume-temperature relationship from the cooling-down simulation. In addition, the coefficient of thermal expansion (CTE) and the elastic stiffness of the nanocomposites at each temperature are predicted and compared with one another. As a result, the glass transition and thermoelastic properties of pure epoxy are found to be improved by embedding the SiC nanoparticles. Especially regarding the CTE and elastic moduli of nanocomposites, the particle-size dependency is clearly observed below and above the glass transition temperature.     

Influence of co-solvent content on electro-deposition behavior of acrylic lattices of different glass transition temperatures

The influence of co-solvent content on the constant-voltage electro-deposition behavior of three acrylic lattices was investigated. It was found that co-solvent effect is system-specific and the type and the extent of the influence depend on the specific features of the latex itself, specially its glass transition temperature, T_g. For a low-T_g high molecular weight latex, the addition of co-solvent makes the electro-coagulates more sticky but it does not show a quick influence on the deposition behavior. In contrast for higher T_g lattices the influence is more pronounced. For medium and high-T_g lattices, the co-solvent was found to have a significant influence on the electro-deposition behavior and the film morphology.     

The effect of glass transition on the desired and undesired agglomeration of amorphous food powders

In the food manufacturing and pharmaceutical industry several agglomeration technologies are applied: fluidised bed agglomeration, steam jet agglomeration, agglomeration during drying and pressure agglomeration like extrusion, roller compaction or tabletting. In addition, caking or sticking of amorphous substances, which is a kind of undesired agglomeration, is frequently observed.Any desired or undesired agglomeration of amorphous substances is dependent on the mechanical properties of the entire particle or the particle surface. Changes in the mechanical properties of the material are linked to changes in moisture and temperature and can be predicted by applying the glass transition concept. Using this concept it is possible to estimate the viscosity and the Young's modulus for a given amorphous substance while knowing their glass transition temperature in dependence on the water content.Knowing the viscosity and the Young's modulus and applying equations derived from the sintering technology or the theory of viscoelasticity it is possible to define suitable conditions for most of the agglomeration processes mentioned above.     

Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

Effect of sepiolite on glass transition temperature and melting behaviour of semicrystalline polymer blends

Summary Thermal analysis of blends poly(vinylidene fluoride) (PVF₂) with poly(methyl methacrylate) (PMMA) or polystyrene (PS) filled with sepiolite was carried out to examine the effects of the filler on properties such as melting behaviour and glass transition temperature. For the compatible PVF₂/PMMA system, the presence of the filler did not cause any substantial changes in the thermal behaviour of the blend. In the non-compatible PVF₂/PS system, some compatibilization is achieved in the blend, indicated by PVF₂ melting point depression as well as by a shift of the glass transition of the homopolymers in the blend.     

Morphology and rheology of immiscible polymer blends filled with silica nanoparticles

The effect of silica nanoparticles on the morphology and the rheological properties of an immiscible polymer blend (polypropylene/polystyrene, PP/PS 70/30) was investigated. Two types of pyrogenic nanosilica were used: a hydrophilic silica with a specific surface area of 200 m²/g and a hydrophobic silica having a specific surface area of 150 m²/g. First, a significant reduction in the PS droplet volume radius, from 3.25 to nearly 1 μm for filled blends with 3 wt% silica, was observed. More interestingly, image analysis of the micrographs proved that the hydrophilic silica tends to confine in the PS phase whereas hydrophobic one was located in the PP phase and at the PP/PS interface (interphase thickness ≈ 100-200 nm). Furthermore, a migration of hydrophilic silica from PP phase toward PS domains was observed.An analysis of the rheological experimental data was based on the framework of the Palierne model, extended to filled immiscible blends. Due to the partition of silica particles in the two phases and its influence on the viscosity ratio, limited cases have been investigated. The rheological data obtained with the hydrophobic silica were more difficult to model since the existence of a thick interphase cannot be taken into account by the model. Finally, the hypothesis that hydrophilic silica is homogeneously dispersed in PS droplets and that hydrophobic silica is dispersed in PP matrix was much closer to the actual situation. It can be then concluded that stabilization mechanism of PP/PS blend by hydrophilic silica is the reduction in the interfacial tension whereas hydrophobic silica acts as a rigid layer preventing the coalescence of PS droplets.     

The viscoelastic behavior of polymer/oligomer blends

The viscoelastic properties of poly(α-methyl styrene), its hexamer, and their athermal blends at various concentrations are studied. Master curves for the dynamic shear responses, G′ and G″, are successfully constructed for both the pure materials and the blends, indicating the validity of the time-temperature superposition principle for these systems. The temperature dependence of the shift factor follows the Vogel-Fulcher behavior over the temperature range studied, and the temperature dependence is slightly weaker for the blends. The rubbery plateau modulus scales with the polymer concentration as ; the terminal relaxation time scales with the polymer concentration as . The shape of the segmental dispersion appears unchanged by concentration, which differs from our calorimetric studies where mixtures show obviously temperature-broadened glass transitions and depressed enthalpy overshoots. The TNM (Tool-Narayanaswamy-Moynihan) model indicates that the change in the temperature dependence is not sufficient to account for the observed calorimetric broadening. We conclude that the temperature broadening of the glass transition for our blends is not due to a broadening of the dynamic spectrum or to changes in its temperature dependence. The possibility that the broadening is due to changes in the non-linearity parameter x in the TNM model is also considered. While the broadening could be due to a decreasing value of x, we found that this same decrease would lead to increasing enthalpy overshoots on heating, contrary to the experimental observations. The combination of the calorimetric results with the rheological measurements further indicates that the fundamental basis of the TNM-type of model of structural kinetics in glasses is potentially wrong.     

Photo-induced deformation behavior depending on the glass transition temperature on the surface of urethane copolymers containing a push-pull type azobenzene moiety

Urethane copolymers containing a push-pull type azobenzene moiety with the same dye content were synthesized to investigate the relationship between photo-induced deformation and molecular mobility. The copolymers exhibit different glass transition temperatures (T_g), from 46 to 143 °C, due to their different main chain structures. An indented nanostructure induced by the optical near field around the polystyrene microspheres and a surface relief grating (SRG) induced by exposure to a two-beam interference pattern were examined using films of copolymers. We found the dependency of the deformation efficiency on T_g was inverted depending on the irradiation power. The deformation depth increased with T_g under high power irradiation in both the indented nanostructure and the SRG forming experiments. In contrast, the deformation depth of the SRG decreased with increasing T_g under low power irradiation. The discovery of this inverted tendency suggests that, in addition to the molecular mobility, we should consider other factors in the deformation mechanism, such as the recovery of deformation, the degree of plasticization, and the thermal effects.     

Pressure influence on the glass transition of polymers and polymer blends

Summary p-V-T measurements have been carried out on a broad series of polymers and polymer blends. The effect of pressure on the glass transition temperature, T_g, and on related properties-increments of the coefficients of expansion, , and compressibility, K, at T_g-has been evaluated critically. The results are discussed in terms of the free volume, the order parameter, and the statistical mechanical theory. It is concluded that neither the simple free volume theory, nor the order parameter theory in the one order assumption predict correctly the observed behaviour. A possible explanation of the phenomena observed is offered assuming an expansion of the free volume, which is independent of the nature of the polymer. For the blends binary contacts between the components are discussed, with respect to interaction energy and to conformational rearrangements.Dedicated to Prof. Dr. W. H. Stockmayer to his 80th birthday with the best wishes     

Morphologies of miscible PCL/PVC blends confined in ultrathin films

Morphologies of ultrathin films (10–60 nm) of miscible poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends have been investigated under isothermal crystallization conditions by real time atomic force microscopy, and electron diffraction techniques. It was found that the morphology and growth rate of PCL/PVC blends strongly depend on the blend composition, crystallization temperature and film thickness. At a film thickness of 30 nm, the truncated lozenge-shape morphology of pure PCL crystals, found when the growth rate is slow, bent with increasing PVC content to form S-shaped or inverted S-shaped crystals, the curvature increasing by lowering the crystallization temperature. Electron diffraction patterns reveal that these crystals are flat-on single crystals with the PCL molecular chains (c axis) in the blends slightly tilted with respect to the lamella normal, while the b direction of the crystal lattice, corresponding to the fast growing direction of the growth front, follows a S line. Upon decreasing the film thickness (<30 nm), the S-shaped or inverted S-shaped crystals transform into four-branch dendritic lamellae.     

Modeling the uniaxial rate and temperature dependent behavior of amorphous and semicrystalline polymers

Strain rate and temperature dependent constitutive equations are proposed for polymer materials based on existing isotropic formulations of viscoplasticity. The proposed formulations are capable of simulating some of the important features of deformation behavior of amorphous and semicrystalline polymers. The materials model is based on the assumption that the evolution of flow stress is dependent on the rate of deformation, temperature, and an appropriate set of internal variables. The proposed theory is capable of modeling yielding, strain softening, and the orientation hardening exhibited by amorphous polymers. It is also possible to model the initial viscoplastic and subsequent nonlinear hardening behavior shown by semicrystalline polymers at large strains. Uniaxial tensile tests with uniform and hourglass specimens are made at temperatures ranging from 23 to 100°C and under various crosshead speeds. Both amorphous polycarbonate and semicrystalline polypropylene sheet materials are tested to characterize the stress and strain behavior of these materials and to determine their appropriate material constants. Load relaxation experiments are also conducted to obtain the necessary material constants describing the rate and temperature dependent flow stress behavior of polypropylene. Simulation results compare favorably against experimental data for these polymer materials.     

Molecular dynamics simulations of the glass transition in polymer melts

Computer simulations of polymer models have contributed strongly to our understanding of the glass transition in polymer melts. The ability of the simulation to provide information on experimentally not directly accessible quantities like the detailed spatial arrangement of the particles allows for stringent tests of theoretical concepts about the glass transition and provides additional insight for the interpretation of experimental data. Comparing coarse-grained simulations of a bead-spring model and chemically realistic simulations of 1,4-polybutadiene the importance of dihedral barriers for the glass transition phenomenon can be elucidated.