Suchergebnisse

The fate of matter when decreasing the temperature at constant pressure is that of passing from gas to liquid and, subsequently, from liquid to crystal. However, a class of materials can exist in an amorphous phase below the melting temperature. On cooling such materials, a glass is formed; that is, a material with the rigidity of a solid but exhibiting no long-range order. The study of the thermodynamics and dynamics of glass-forming systems is the subject of continuous research. Within the wide variety of glass formers, an important sub-class is represented by glass forming polymers. The presence of chain connectivity and, in some cases, conformational disorder are unfavourable factors from the point of view of crystallization. Furthermore, many of them, such as amorphous thermoplastics, thermosets and rubbers, are widely employed in many applications. In this review, the peculiarities of the thermodynamics and dynamics of glass-forming polymers are discussed, with particular emphasis on those topics currently the subject of debate. In particular, the following aspects will be reviewed in the present work: (i) the connection between the pronounced slowing down of glassy dynamics on cooling towards the glass transition temperature (Tg) and the thermodynamics; and, (ii) the fate of the dynamics and thermodynamics below Tg. Both aspects are reviewed in light of the possible presence of a singularity at a finite temperature with diverging relaxation time and zero configurational entropy. In this context, the specificity of glass-forming polymers is emphasized. ; The author acknowledges the University of the Basque Country and Basque Country Government (Ref. No IT-654-13 (GV)), Depto Educacion, Universidades e investigacion and the Spanish Government (Grant No MAT2012-31088) for their financial support. ; Peer Reviewed

We employ fast scanning calorimetry to assess the thermodynamic state attained after a given cooling rate and the molecular mobility of glassy poly(4-tert-butylstyrene) confined at the micrometer length scale. We show that, for such a large confinement length scale, thermodynamic states with a fictive temperature (Tf) 80 K below the polymer glass transition temperature (Tg) are attained, which allows to bypass the geological timescales required for bulk glasses. Access to such states is promoted by a fast mechanism of equilibration. Importantly, the tremendous Tf decrease takes place while the molecular mobility remains bulklike, indicating marked decoupling between vitrification kinetics and molecular mobility. ; The authors thank the University of the Basque Country and the Basque Country government [Grant No. IT-654-13 (GV)], Departamento de Educación, Universidades e investigación, and the Spanish government [Grant No. MAT2015-63704-P, (MINECO/FEDER, UE)] for their financial support. ; Peer reviewed

We present VITRIFAST, a high throughput optimization procedure to characterize the vitrification kinetics based on calorimetric measurements. By analyzing the temperature dependence of specific heat capacity, the method determines the fictive temperature, Tf , and the enthalpy change during physical aging, ΔH, within only a few seconds. We tested VITRIFAST on the low molecular weight glass-former o - terphenyl (OTP) and on an archetypal glass forming polymer, polystyrene (PS), by analyzing the outcome of two classical sets of experi- ments. By means of fast scanning calorimetry (FSC), we characterized the vitrification kinetics in a wide range of cooling rates and the isothermal physical aging after vitrification at a given rate. In less than 3 minutes, our method could process 18 different calorimetric scans and provided values of Tf and ΔH in excellent agreement with those reported in the literature. VITRIFAST can be employed in the analysis of the temperature dependence of any type of second order derivative of free energy and represents a tremendous advance in the data analysis of calorimetric scans. The method is particularly helpful for fast scanning calorimetry users, considering the extremely large number of heat capacity scans recorded by this technique within a few minutes. ; S. N and A. A. acknowledge financial support from Action Concert Recherche-ULB under project SADI, and the Fonds de la Recherche Scientifique FNRS under Grant T.0184.20 EXOTICAGE. D.C. acknowledges financial support from the project PGC2018-094548-B-I00 (MICINN-Spain and FEDER-UE) and the project IT-1175-19 (Basque Government). ; Peer reviewed

Tuning the thermodynamic state of a material has a tremendous impact on its performance. In the case of polymers placed in proximity of a solid wall, this is possible by annealing above the glass transition temperature, Tg, which induces the formation of an adsorbed layer. Whether heating to higher temperatures would result in desorption, thereby reverting the thermodynamic state of the interface, has so far remained elusive, due to the interference of degradation. Here, we employ fast scanning calorimetry, allowing to investigate the thermodynamics of the interface while heating at 104 K s−1. We show that applying such rate to adsorbed polymer layers permits avoiding degradation and, therefore, we provide clear-cut evidence of desorption of a polymer melt. We found that the enthalpy and temperature of desorption are independent of the annealing temperature, which, in analogy to crystallization/melting, indicates that adsorption/desorption is a first order thermodynamic transition. ; D.C. acknowledges financial support from the project PGC2018-094548-B-I00 (MICINN-Spain and FEDER–UE) and the project IT-1175-19 (Basque Government). S.N. acknowledges financial supports from the Action Concertée Recherche-ULB under project SADI and the Fonds de la Recherche Scientifique FNRS under Grant EXOTICAGE. ; Peer reviewed

7th IDMRCS: Relaxation in Complex Systems ; Understanding why the glass transition temperature (Tg) of polymers deviates substantially from the bulk with nanoscale confinement has been a 20-year mystery. Ever since the observation in the mid-1990s that the Tg values of amorphous polymer thin films are different from their bulk values, efforts to understand this behavior have intensified, and the topic remains the subject of intense research and debate. This is due to the combined scientific and technological implications of size-dependent glassy properties. Here, we discuss an intriguing aspect of the glassy behavior of confined amorphous polymers. As experimentally assessed, the glass transition is a dynamic event mediated by segmental dynamics. Thus, it seems intuitive to expect that a change in Tg due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of Tg for confined polymers based on bulk rules. We highlight past and recent findings that have examined the relationship between Tg and segmental dynamics of confined polymers. Within this context, the decoupling between these two aspects of the glass transition in confinement is emphasized. We discuss these results within the framework of our current understanding of the glass transition as well as efforts to resolve this decoupling. Finally, the anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces in polymer thin films is discussed. ; RDP acknowledge support from the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860), the NSF through a CAREER Award (DMR-1053144) and the AFOSR through a YIP Award (FA9550-12-1-0223). SN acknowledges financial support from the Defay Foundation and the funds FER of the Université Libre de Bruxelles. DC acknowledges the University of the Basque Country and Basque Country Government (Ref. No. IT-654-13 (GV)), Depto Educacion, Universidades e investigacion and Spanish Government (Grant No. MAT2012-31088) for their financial support. ; Peer Reviewed

Homogeneous diethyl phthalate/phenylphthalein-dimethylether (DEP/PDE) mixtures have been investigated by means of broadband dielectric spectroscopy. Contrarily to the widespread view that homogenous binary mixtures should give rise to a single glass transition, the mixture displays two dynamics giving rise to two glass transitions. Such a finding can be rationalized invoking the self-concentration concept that relies on the localized nature of the glass transition phenomenon. In such a way, the analogy with miscible polymer blends, for which this concept has been introduced, is highlighted. A model based on the combination of the Adam-Gibbs (AG) theory of the glass transition and the self-concentration concept resulted to be fully predictive once the only unknown variable, namely, the glass-former specific parameter (α) connecting the characteristic length for the glass transition to the configurational entropy, is extracted applying the model itself to DEP/toluene and DEP/PDE solutions highly concentrated, respectively, in DEP and PDE. The α parameter obtained in such a way allows the precise determination of the most probable relaxation time even for those DEP/PDE mixtures displaying a strong overlap of the dielectric response. The model incorporating the self-concentration concept to the AG theory also provides the characteristic length scale for the glass transition for both DEP and PDE. Such a length scale was found to be on the order of 1–2 nm. This is comparable to that obtained for other glass formers. ; The authors acknowledge the University of the Basque Country and Basque Country Government (Ref. No. IT-436-07), Depto. Educación, Universidades e Investigación and Spanish Minister of Education (Grant No. MAT 2007-63681) for their support. The support of the European Community within the SOFTCOMP program is also acknowledged. ; Peer reviewed

Using a simple mathematical formalism, we show that temperature dependent nonexponential relaxation found in glass-forming liquids and amorphous polymers, often resulting in a decrease in the stretching exponent when decreasing temperature, can be suitably described assuming the combination of an intrinsic stretched response and the existence of temperature independent heterogeneities. The effect of the latter is incorporated by assuming a Gaussian distribution of Vogel temperatures. Comparison with experimental data of a large number of glass formers showed that this approach is able to quasiquantitatively describes the temperature dependence of the stretching exponent using the width of the distribution as the single fitting parameter. According to this approach, the rapidity of the decrease in the stretching exponent with decreasing temperature depends not only on the magnitude of the standard deviation of Vogel temperatures but also on the value of the intrinsic stretching exponent and on the fragility of the glass former. The latter result is able to rationalize, at least partially, the empirical correlation between the fragility and the stretching exponent at Tg. ; The authors acknowledge the University of the Basque Country and Basque Country Government (Ref. No. IT-436-07, Depto. Educación, Universidades e Investigación) and the Spanish Minister of Education (Grant No. MAT 2007-63681) for their support. The support of the European Community within the SOFTCOMP program was also acknowledged. ; Peer reviewed

The dynamics of homogenous polychlorinated biphenyl (PCB54)/toluene mixtures have been investigated by means of broadband dielectric spectroscopy. The mixture presents dynamical heterogeneity, alike miscible polymer blends, which is manifested with the presence of two relaxational processes. The relatively slow one has been attributed to the motion of PCB54 in the mixture, whereas the relatively fast one was related to the toluene dynamics in the mixture. These results have been interpreted according to the self-concentration concept, first introduced to describe the dynamics of miscible polymer blends, which relies on the limited size of the cooperative length scale in glass-forming liquids. The self-concentration concept has been incorporated in the Adam–Gibbs theory of the glass transition relating the characteristic relaxation time and the length scale for structural relaxation to the configurational entropy. This allowed the determination of the cooperative length scale of PCB54 and toluene both in mixture and alone through the fitting of a single parameter, namely, that connecting the cooperative length scale to the configurational entropy. This length scale resulted to be in the range of 1–2nm for all systems. Finally, the out-of-equilibrium dynamics of toluene induced by the selective freezing in of PCB54 has been examined and its relation to the Johari–Goldstein relaxation of pure toluene was critically discussed. ; The authors acknowledge the University of the Basque Country and Basque Country Government (Ref. IT-436-07, Depto. Educación, Universidades e Investigación) and Spanish Minister of Education (MAT 2007-63681) for their support. The support of the European Community within the SoftComp program is also acknowledged. ; Peer reviewed

The occurrence of glass transition is believed to be associated to cooperative motion with a growing length scale with decreasing temperature. We provide a route to calculate the size of cooperatively rearranging regions (CRR) of glass-forming polymers combining the Adam-Gibbs theory of the glass transition with the self-concentration concept. To do so we explore the dynamics of glass-forming polymers in different environments. The material specific parameter α connecting the size of the CRR to the configurational entropy is obtained in this way. Thereby, the size of CRR can be precisely quantified in absolute values. This size results to be in the range 1–3nm at the glass transition temperature depending on the glass-forming polymer. ; The authors acknowledge the University of the Basque Country and Basque Country Government -Contract No. 9/UPV 00206.215-13568/2001 and Spanish Minister of Education -Contract No. MAT 2004-01617 for their support. The support of the European Community within the SoftComp program is also acknowledged. ; Peer reviewed

The mobility of isolated polychlorinated biphenyl (PCB54) in 1,4-polybutadiene (PB) has been investigated by means of broadband dielectric spectroscopy. The aim was to provide new insights about the effect of the environment on the dynamics of PCB54. The authors' results indicate that PCB54 structural dynamics is neither independent of the PB matrix nor slaved to the matrix itself. The authors interpret these results as a consequence of the limited size of cooperatively rearranging regions (CRRs) involved in PCB54 structural relaxation possessing an effective concentration different from the macroscopic one. This implies a non-negligible influence of "self-concentration," already proven for the component segmental dynamics in polymer blends, also in the relaxation of binary mixtures involving low molecular weight glass formers. This allowed the evaluation of the size of CRR, which was about 1nm for PCB54 in PB. This means that the cooperativity extends over the first shell around PCB54 molecules. ; The authors acknowledge the University of the Basque Country and Basque Country Government -9/UPV00206.215-13568/2001 and Spanish Minister of Education MAT 2004-01617 for their support. The support of the European Community with the SoftComp program -Project No. NMP3-CT-2004-502235 is also acknowledged. ; Peer reviewed

The existence of an ideal glass and the resolution to the Kauzmann paradox is a long-standing open question in materials science. To address this problem, we exploit the ability of glasses with large interfacial area to access low energy states. We submit aggregates of spheres of a polymeric glass former to aging well below their glass transition temperature, Tg; and characterize their thermodynamic state by calorimetry, and the vibrational density of state (VDOS) by inelastic neutron scattering (INS). We show that, when aged at appropriate temperatures, glassy spheres attain a thermodynamic state corresponding to an ideal glass in time scales of about one day. In this state, the boson peak, underlying the deviation from the Debye level of the VDOS, is essentially suppressed. Our results are discussed in the framework of the link between the macroscopic thermodynamic state of glasses and their vibrational properties. ; D. C. and J. C. acknowledge financial support from the Project No. PGC2018-094548-B-I00 (MICINN-Spain and FEDER-UE) and the Project No. IT-1175-19 (Basque Government). ; Peer reviewed

Understanding how glasses form, the so-called vitrification, remains a major challenge in materials science. Here, we study vitrification kinetics, in terms of the limiting fictive temperature, and atomic mobility related to the α-relaxation of an Au-based bulk metallic glass former by fast scanning calorimetry. We show that the time scale of the α-relaxation exhibits super-Arrhenius temperature dependence typical of fragile liquids. In contrast, vitrification kinetics displays milder temperature dependence at moderate undercooling, and thereby, vitrification takes place at temperatures lower than those associated to the α-relaxation. This finding challenges the paradigmatic view based on a one-to-one correlation between vitrification, leading to the glass transition, and the α-relaxation. We provide arguments that at moderate to deep undercooling, other atomic motions, which are not involved in the α-relaxation and that originate from the heterogeneous dynamics in metallic glasses, contribute to vitrification. Implications from the viewpoint of glasses fundamental properties are discussed. ; D.C. acknowledges financial support from the project PGC2018-094548-B-I00 (MICINN-Spain and FEDER-UE) and the project IT-1175-19 (Basque Government). I.G. acknowledges financial support from the German Research Foundation (DFG) through grant nos. GA 1721/2-2 and GA 1721/3-1. R.B. and I.G. acknowledge financial support from the German Federation of Industrial Research Associations (AiF/IGF) through project no.17716N and Heraeus Holding GmbH for the gold, palladium, and silver supply. B.R. thanks the CNRS for the PICS 278566 funding. ; Peer reviewed

We have recently proposed [D. Cangialosi et al., J. Chem. Phys. 123, 144908 (2005)] an extension of the Adam-Gibbs [J. Chem. Phys. 43, 139 (1965)] theory, combined with the concept of self-concentration, to describe the temperature dependence of the relaxation time for the component segmental dynamics in miscible polymer blends. Thus, we were able to obtain the dynamics of each component in the blend starting from the knowledge of the dynamic and thermodynamic data of the pure polymers, with a single fitting parameter (α) which had to be obtained from the fitting of the experimental data. In the present work we demonstrate that this model is also suitable to describe the polymer segmental dynamics in concentrated polymer solutions. From this result we have developed a new route for determining the value of the α parameter associated with any given polymer. Once this value is known for the two components of a possible polymer blend, our model for polymer blends dynamics becomes fully predictive. ; This work has been supported in part by the Spanish Ministry of Science and Technology (MICyT) (Project No. MAT 2004/01617) and by the Government of the Basque Country (Project No. 9/UPV 00206.215-13568/2001). The authors want also to thank the support of the SoftComp NoE/NMP3-CT-2004-502235. ; Peer reviewed

We investigate the kinetics of enthalpy recovery in stacked glassy polystyrene (PS) films with thickness from 30 to 95 nm over a wide temperature range below the glass transition temperature (T). We show that the time evolution toward equilibrium exhibits two mechanisms of recovery, in ways analogous to bulk PS. The fast mechanism, allowing partial enthalpy recovery toward equilibrium, displays Arrhenius temperature dependence with low activation energy, whereas the slow mechanism follows pronounced super-Arrhenius temperature dependence. In comparison to bulk PS, the time scales of the two mechanisms of recovery are considerably shorter and decreasing with the film thickness. Scaling of the equilibration times at various thicknesses indicates that the fast mechanism of recovery is compatible with the free volume holes diffusion model. Conversely, the slow mechanism of recovery appears to be accelerated with decreasing thickness more than predicted by the model and, therefore, its description requires additional ingredients. The implications, from both a fundamental and technological viewpoint, of the ability of thin polymer films to densify in relatively short time scales are discussed. ; The authors acknowledge the University of the Basque Country and Basque Country Government (Reference No. IT-654-13 (GV)), Depto. Educacion, Universidades e investigacion, and Spanish Government (Grant No. MAT2015-63704-P, (MINECO/FEDER, UE)) for their financial support. ; Peer Reviewed

We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel-Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics. © 2013 American Physical Society. ; The authors acknowledge the University of the Basque Country and Basque Country Government [Reference No. IT-654-13 (G.V.)], Departamento Educación, Universidades e investigación, and the Spanish Government (Grant No. MAT2012-31088) for their financial support. ; Peer Reviewed