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Gregory B. Mckenna - One of the best experts on this subject based on the ideXlab platform.
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Exceptional Property Changes in Ultrathin Films of Polycarbonate: Glass Temperature, Rubbery Stiffening, and Flow
Macromolecules, 2012Co-Authors: Paul A. O'connell, T. Adeniyi Ishola, Jian Wang, Gregory B. MckennaAbstract:The TTU nanobubble inflation method has been used to determine the creep compliance of ultrathin films of polycarbonate over a thickness range of 3–22 nm and a Temperature range from ambient (25 °C) to 112 °C. The compliance from the segmental regime through a stiffened rubbery plateau regime was able to be measured for films of thickness from 4.2 nm and above, and the results show a greatly reduced Glass Temperature and stiffening of the rubbery regime relative to that of the macroscopic material. In the case of the 3 nm film, we found that even at ambient conditions the system is above the enhanced rubbery plateau and is in the terminal flow regime as evidenced by a slope of unity on a double-logarithmic representation of the compliance versus time. The biaxial viscosity of the film at this Temperature is ∼2.7 × 1011 Pa s. We also observed flow behavior in a 9.1 nm thick film that had been heated to 75 °C, i.e., 10 °C above the reduced Glass transition of the thin film. These are the first evidence of p...
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a brief discussion thermodynamic and dynamic fragilities non divergent dynamics and the prigogine defay ratio
Journal of Non-crystalline Solids, 2009Co-Authors: Gregory B. MckennaAbstract:Abstract This article brings together some of the work performed by the present author and collaborators that is related to the Glass transition event and to some of its entropy aspects. The purpose of the work and discussion was motivated by a view that some of the frameworks in which we currently look at Glassy behavior, while potentially useful, may also have limitations that we often do not fully consider. Discussion focuses on isochoric Glass formation paths, thermodynamic and dynamic fragilities and how dynamic fragility in many systems (especially polymers, metals, ionic liquids and hydrogen bonding systems) seems to vary primarily with the Glass transition Temperature itself. This leads to the conclusion that such systems have an apparent activation energy that varies as the square of the Glass Temperature. The work then discusses evidence for a non-diverging relaxation time or viscosity as the Glass Temperature is approached and ends with a discussion of the Prigogine–Defay ratio.
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Effects of freeze-drying on the Glass Temperature of cyclic polystyrenes
Polymer, 2003Co-Authors: Sindee L. Simon, Paul Bernazzani, Gregory B. MckennaAbstract:Abstract The calorimetric Glass Temperature was measured for three cyclic polystyrenes with apparent molecular weights ranging from 4.0×10 3 to 195.5×10 3 g/mol for both bulk material and for samples freeze-dried from dilute solution. Freeze-drying from dilute solution was found to reduce the Glass Temperature by 7–14 K depending on the sample. These T g depressions are 5–12 K greater than those found previously for freeze-dried linear polystyrene. Annealing at 403.2 and 443.2 K (130 and 170 °C) resulted in recovery of the T g back to the bulk value with the time scales depending on both Temperature and the magnitude of the T g reduction; the low apparent activation energy dependence of the recovery of T g precludes its being due to viscous flow.
Hans Adam Schneider - One of the best experts on this subject based on the ideXlab platform.
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Polymer class specificity of the Glass Temperature
Polymer, 2005Co-Authors: Hans Adam SchneiderAbstract:Abstract It is shown that assuming linearity of the polymer class specific correlation between the Glass transition Temperature and the ratio mass to effective number of ‘flexible’ bonds of monomeric (repeat) unit, μ / ρ , it is possible to evaluate by least square fit the ‘flexibility’ factor, ρ , characterizing the share of those simple bonds within the repeat unit responsible for conformational rearrangements of the polymer. The evaluated effective number of simple bonds accounts for the influence of polymer class specific interactions and steric hindrances on the ‘free rotation’ of the simple bonds comprised in the repeat unit. It is shown that the well-known increase of the Glass transition Temperature of polymers bearing longer n -alkylic side chains may be the result of preferred ‘crankshaft’ like motions of neighboring methylenes, hindering the ‘free rotation’ of the simple bonds between the methylenes involved in the ‘crankshaft’. It is also possible to quantify steric hindrances of bulky substituents.
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The Meaning of the Glass Temperature of Random Copolymers and Miscible Polymer Blends
Journal of Thermal Analysis and Calorimetry, 1999Co-Authors: Hans Adam SchneiderAbstract:The Glass transition Temperature of random copolymers and miscible polymer blends exhibits generally a non-additive composition dependence, showing both positive and negative deviations of additivity predicted by 'Gordon-Taylor' like equations, among which the Fox relation represents the simplest additivity rule for the Glass Temperature of these polymeric systems. It is shown that the real T_g vs. composition behaviour of both copolymers and polymer blends can be adapted by a parameterized third order T_g vs. composition equation. The fitting parameter, K_1, of the square concentration term of this equation accounts essentially for the effect of binary hetero-sequences in copolymers and for specific contact hetero-interactions in polymer blends. The fitting parameter of the third order concentration term, K_2, is related exclusively to the effects of heterotriad sequences (copolymers) and conformational entropy changes due to hetero-contact formation (polymer blends), respectively. It is shown that the K_1 parameter correlates roughly with the difference between the solubility parameters of the components.
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Conformational Entropy Contributions to the Glass Temperature of Blends of Miscible Polymers.
Journal of research of the National Institute of Standards and Technology, 1997Co-Authors: Hans Adam SchneiderAbstract:Because of negligible contributions of combinatorial entropy, miscibility of polymers is attributed predominantly to favorable (exothermic) enthalpic effects of mixing, i.e., to strong interactions between the blend components, which have to overcome the cohesive forces acting within the components. Miscibility of amorphous polymers usually is associated with the presence of a single Glass Temperature of the blend. Although stronger hetero-contact interactions are thermodynamically required for polymer miscibility, the majority of miscible binary polymer blends exhibit negative deviations of the Glass Temperature from values predicted by the free volume or flexible bond additivity rules, suggesting a looser packing within those blends. A reasonable explanation assumes that binary hetero-contact formation within the blend may be accompanied by local interchain orientation contributing consequently to conformational entropy changes. The smaller the induced interchain orientation by hetero-contact formation, the larger the mobility in the neighborhood of the contacts and the probability of related conformational entropy changes, causing an equivalent increase of the "free volume" within the blend, i.e., a corresponding decrease of the blend Tg, which finally can be situated below the values predicted by the additivity rules. Vice versa, the corresponding argument will hold for blends with higher interchain orientation induced by intensive exothermic hetero-contact forces.
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Influence of flexible oligo(tetrafluoroethene) segments on the Glass Temperature of poly(aramide)s and poly(amide imide)s
Polymer Bulletin, 1994Co-Authors: Hans Adam Schneider, N. Steinhauser, Rolf MülhauptAbstract:The insertion of oligo(tetraflouroethene) segments into the main chain of poly(aramide)s and poly(amide imimide)s is accompanied by a decrease of the Glass Temperature. The decrease is strictly related to the number of CF2-units and is the same for both poly(aramide)s and poly(amide imide)s. The absolute Glass Temperature depends, however, on the initial structures of the monomeric units. As a consequence ‘mastercurves’ can be constructed by shifting the individual Tg vs. number of CF2-units curves along the Temperature axis. A detailed analysis of the data indicates that besides the Tg-mass/‘flexible’ bond of monomeric unit criterion additional volume and interaction dependant packing effects have to be considered for an accurate prediction of the Glass Temperatures of polymers.
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The Glass Temperature of polymer blends: comparison of both the free volume and the entropy predictions with data
Polymer, 1992Co-Authors: Hans Adam Schneider, Edmund A. Di MarzioAbstract:Abstract Experimental Glass Temperatures for 30 compatible blends have been compared with an equation derived from the hypothesis that the Glass Temperature is determined by conformational entropy changes and with the well known Glass transition Temperature ( T g ) versus composition equation of Fox based on volume additivity. Since these equations neglect interactions, it is not surprising that they fail in predicting at least half the experimental T g data. Nevertheless the approximate equivalence of the predictions of these two equations suggests that the Glass Temperature of an infinite molecular weight polymer is proportional to the mass divided by the number of flexible bonds of the monomer unit. As the interactions are disregarded, the two equations are zero-order treatments and thus they can both be improved.
A. Yang - One of the best experts on this subject based on the ideXlab platform.
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Glass Temperature depression of polymer by use of mixed solvents: A colligative property
Journal of Polymer Science Part B: Polymer Physics, 1996Co-Authors: E. A. Di Marzio, C. Castellano, A. YangAbstract:The entropy theory of Glasses is used to determine the Glass Temperature depression by a multicomponent low molecular weight plasticizer (diluent). The Glass Temperature, T g , is calculated as a function of pressure, P, the mole fractions, m i , of the plasticizers, and the degree of polymerization p. One finds, provided there is no phase separation, that to a good approximation, the initial Glass Temperature depression is a function of the total mole fraction of plasticizer. Moreover, the Glass Temperature depression for small plasticizer molecules is found to be nearly a universal function of the plasticizer mole fraction (it depends on no other plasticizer variable), and to vary inversely as the number of flexible bonds per monomer unit of the polymer. A useful approximation is found, γdT g /dm 1 = -3T g , where m i is the total mole fraction of diluent on a per monomer of polymer basis, and γ is the number of flexible bonds per monomer. Although these results agree with experimental data in the literature, a more definitive experimental test is needed.
George Floudas - One of the best experts on this subject based on the ideXlab platform.
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What Determines the Glass Temperature and dc-Conductivity in Imidazolium-Polymerized Ionic Liquids with a Polythiophene Backbone?
Macromolecules, 2020Co-Authors: Achilleas Pipertzis, George Papamokos, Markus Mühlinghaus, Markus Mezger, Ullrich Scherf, George FloudasAbstract:We report the results of a combined work based on density functional theory (DFT) calculations and experiments of the factors that influence the Glass Temperature, Tg, and the associated ion conduc...
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Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature
ACS Macro Letters, 2018Co-Authors: Mahdis Hesami, George Floudas, Werner Steffen, Hans-juergen Butt, Kaloian KoynovAbstract:We studied experimentally the influence of interfaces on the dynamics in thin polymer films at Temperatures far above the Glass Temperature (Tg + 80 °C). Polyisoprene (PI) was employed as a model system. We examined Glass substrate supported films with thicknesses (d) spanning the range from 10 μm to 10 nm that correspond to d/Rg from 400 to 1, where Rg is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the ...
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interfacial energy and Glass Temperature of polymers confined to nanoporous alumina
Macromolecules, 2016Co-Authors: Stelios Alexandris, Periklis Papadopoulos, Hansjurgen Butt, Martin Steinhart, Georgios Sakellariou, George FloudasAbstract:We report on the effect of interfacial energy on the Glass Temperature, Tg, of several amorphous polymers with various Glass Temperatures and polymer/substrate interactions confined within self-ordered nanoporous alumina (AAO). The polymers studied include poly(phenylmethylsiloxane) (PMPS), poly(vinyl acetate) (PVAc), 1,4-polybutadiene (PB), oligostyrene (PS), and poly(dimethylsiloxane) (PDMS). The segmental dynamics and associated Tg’s are studied by means of dielectric spectroscopy. The interfacial energy for the polymer/substrate interface, γSL, is calculated with Young’s equation whereas the AAO membrane surface energy is obtained by measuring contact angles for several reference liquids. We find that interfacial energy plays a significant role in the segmental dynamics of polymers under confinement within AAO. There is a trend for a decreasing Glass Temperature relative to the bulk with increasing interfacial energy. PDMS exhibits the highest interfacial energy and the highest reduction in Glass temp...
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The Origin of Heterogeneity of Polymer Dynamics near the Glass Temperature As Probed by Defocused Imaging
Macromolecules, 2011Co-Authors: Ania Deres, George Floudas, Klaus Müllen, M. Van Der Auweraer, F. C. De Schryver, Joerg Enderlein, Hiroshi Uji-i, Johan HofkensAbstract:Single molecule defocused wide-field fluorescence microscopy (SMDWM) has been used to monitor the 3D reorientation of single molecules in a thin polymer film (∼300 nm) of monodisperse poly(n-butyl methacrylate) near the Glass Temperature (Tg). Stroboscopic illumination allows for estimating reliable correlation times of single molecule rotational diffusion owing to the drastic lengthening of the observable trajectories. We demonstrate that homogeneity is restored ∼19 K above the Tg determined with calorimetry. The rotational correlation times obtained from SMDWM show similar Temperature dependence as the ones measured with established bulk measurements, such as dielectric spectroscopy and rheology, on the same polymer sample. Single molecular reorientation is coupled to the segmental rather than terminal relaxation of the surrounding polymer matrix. SMDWM revealed that spatial heterogeneity is more pronounced than temporal heterogeneity within the measurement time scale (hours to days), whereas this infor...
C. M. Roland - One of the best experts on this subject based on the ideXlab platform.
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On the Glass Temperature under extreme pressures.
The Journal of chemical physics, 2007Co-Authors: Aleksandra Drozd-rzoska, Sylwester J. Rzoska, Marian Paluch, Attila R. Imre, C. M. RolandAbstract:The application of a modified Simon-Glatzel-type relation [Z. Anorg. Allg. Chem. 178, 309 (1929)] for the pressure evolution of the Glass Temperature is presented, namely, Tg(P)=Tg0[1+ΔP∕(π+Pg0)]1∕bexp[−(ΔP∕c)], where (Tg0,Pg0) are the reference Temperature and pressure, ΔP=P−Pg0, −π is the negative pressure asymptote, b is the power exponent, and c is the damping pressure coefficient. The discussion is based on the experimental Tg(P) data for magmatic silicate melt albite, polymeric liquid crystal P8, and glycerol. The latter data are taken from Cook et al. [J. Chem. Phys. 100, 5178 (1994)] and from the authors’ dielectric relaxation time (τ(P)) measurements, which employs the novel pressure counterpart of the Vogel-Fulcher-Tammann equation: τ(P)=τ0Pexp[DPΔP∕(P0−P)], where ΔP=P−PSL (PSL is the stability limit hidden under negative pressure), P0 is the estimation of the ideal Glass pressure, and DP is the isothermal fragility strength coefficient. Results obtained suggest the hypothetical maximum of the T...
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Dynamics near the Glass Temperature of Low Molecular Weight Cyclic Polystyrene
Macromolecules, 2001Co-Authors: P. G. Santangelo, C. M. Roland, Taihyun Chang, Donghyun Cho, Jacques RooversAbstract:The segmental relaxation properties of a low molecular weight (4.6 kg/mol), cyclic polystyrene (PS) were characterized. The sample was obtained by fractionation using HPLC at the chromatographic critical condition, which yields a ring uncontaminated by its linear precursor. Both the Glass Temperature and the Temperature dependence of the segmental relaxation times for the ring PS were equivalent to the high molecular weight limiting values for the linear polymer. These results are interpreted by considering the configurational mobility of a polymer lacking chain ends.
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Creep of selenium near the Glass Temperature
The Journal of Chemical Physics, 1999Co-Authors: C. M. Roland, P. G. Santangelo, D. J. Plazek, K. M. BernatzAbstract:Creep experiments were carried out on amorphous selenium (Se) at Temperatures in the vicinity of the Glass Temperature. The recoverable compliance lacks a plateau, indicating Se chains are too short to form an entanglement network. The measured compliance function was thermorheological complex, even after subtraction of the Glassy level and normalizing by the steady state compliance. The Temperature dependence determined from the viscosity was in accord with previous viscosity data, although weaker than the near-Arrhenius dependence deduced from the stress relaxation of Se. Based on a comparison to other, small-molecule Glass-formers, the dynamic fragility calculated from the viscosity was larger than expected from Se’s thermodynamic fragility (i.e., steepness of the normalized Kauzmann curve). In contrast, although polypropylene (PP) is substantially more dynamically fragile than Se, PP is less thermodynamic fragile. Thus, when compared to either small-molecule liquids or polymers, Se exhibits a disconnect between dynamic and thermodynamic measures of fragility.