Nonequilibrium dynamics and molecular mobility in polymer glasses: from bulk to 3-D confinement

155 p.El objetivo general de éste trabajo es alcanzar una comprensión profunda de los procesos cinéticos y termodinámicos que influyen en la transición vítrea de materiales amorfos basados en polímeros. La ciencia en el estado vítreo sigue siendo un campo de investigación continua, donde numerosas teorías intentan explicar el origen de la movilidad molecular y la conexión con el fenómeno conocido como la transición vítrea. En la actualidad, numerosos estudios se han focalizado en el confinamiento de materiales poliméricos amorfos, donde el efecto del confinamiento a escala nanométrica ha resultado presentar desviaciones de la temperatura de transición vítrea (Tg) cuando es comparada con al material en estado ¿bulk¿. Para la realización de dicho estudio, se han preparado varios sistemas de polímeros confinados en una y tres dimensiones (1-D, películas delgadas y 3-D, nanoesferas). El presente trabajo pretende extender las investigaciones realizadas en el estudio de la dinámica vítrea de polímeros amorfos en estado ¿bulk¿ y confinados a nivel nanométrico, a través de técnicas calorimétricas.Materials Physics Center-Centro de Física de Materiales (CSIC-UPV/EHU); ULB Université Libre de Bruxelles; Ministerio de Economía, Indutria y Competitivida

Thermodynamics of Thermoplastic Polymers and their Solutions

Glassy organic polymers are technologically important across the gamut of materials applications from structural (hyperbaric windows) to electronic (ionic conductors, surface coatings for printed circuit boards) to environmental (membranes for industrial gas separation). A formal description and understanding of the glass transition temperature is necessary in order to determine the configurational state and hence physical properties of the glass. Moreover, the non-equilibrium glassy state appears to be unstable: volume-relaxation studies of glassy materials have revealed that they undergo slow processes, which attempt to establish equilibrium. These types of retardation/ relaxation phenomena are called physical ageing. As well as pressure and temperature, sorption of a plasticizer may affect in several ways the membrane physical properties. Generally speaking structural rearrangement of the chains is enhanced and, consequently, the glass transition temperature decreases, physical ageing is usually speed up, the membrane is affected by swelling and/or plasticization and even crystallization can be activated. The research work focuses on the investigation of industrial polymers’ glassy – rubbery behaviour due to thermodynamic state variables change (e.g. temperature T, mechanical pressure P and solvent content ) within the polymer matrix. The goal is to obtain a fundamental insight of the sorption process on both macroscopic and microscopic levels. As a result several polymer—penetrant systems have been studied. Different techniques have been implemented to achieve this goal: dilatometry, MTDSC, gravimetry, manometry and in situ FTIR. The instruments used are: a PVT apparatus from GNOMIX®; a MDSC from TA Instruments®; four different handmade systems consisting of a CAHN microbalance from Thermo Fisher Scientific®, a QSM from RUSKA Co.®, a pressure decay system from MKS® and finally a FTIR from Perkin-Elmer®. All data have been modelled with statistical thermodynamic theories and empirical approaches . The study is divided as follows: the first chapter introduces the research goal and fields of application along with the theoretical background for membrane science; the second chapter reports the study conducted on the system PEI—CO2; the third chapter describes the results obtained on the PS—Toluene system; finally in the fourth chapter the results for the PPO—benzene system are given. The order in which these systems are presented is related to the increase of structural modifications as a result of polymer— penetrant interactions

An Off-Lattice Model of the Sanchez-Lacombe Equation of State for Polymers with Finite Flexibility

An extension to the off-lattice Sanchez Lacombe equation of state is proposed by including internal degrees of freedom to account for the finite flexibility of polymer molecules. The extension allows polymer molecules to have two energy levels of flexing, a ground/unflexed state and a degenerate excited/flexed state. The finite flexibility of polymer molecules is characterized by two regression parameters, the energy of excited states and their degeneracy. The flexibility parameters are regressed by using two experimental values: the glass transition temperature of pure polymers at atmospheric pressure and the change in isobaric heat capacity of pure polymers across the glass transition at atmospheric pressure. A method has been outlined to regress these parameters from experimental data. The glass transition temperature versus pressure and the isobaric heat capacity versus temperature predictions of the off-lattice model are compared with a lattice-based model from the literature. The lattice-based model is based on the Gibbs DiMarzio criterion which says that, at the glass transition, the configurational entropy of polymers becomes zero. However, the proposed off-lattice model shows that the Gibbs DiMarzio criterion is a consequence of the artificial lattice. Thus a new criterion is introduced which says that the glass transition occurs at a particular fraction of maximum polymer entropy that also minimizes the degeneracy of the excited state. Experimental data of several pure polymers are utilized to compare the predictions of both models. The proposed off-lattice model is found to be more accurate than the lattice-based model. The model is also employed to predict the glass transition temperature versus pressure behaviour of binary polystyrene/CO2, polycarbonate/CO2 and poly(methyl methacrylate) mixtures. Experimental solubility data is used to regress the binary interaction parameters of the system. The model predicts that the binary polystyrene/CO2 mixture shows depression in glass transition temperature with the increasing pressure of CO2. The prediction is in agreement with the experimental observations and is superior to the lattice-based model. However, for binary polycarbonate/CO2 and poly(methyl methacrylate)/CO2 mixtures that undergo retrograde vitrification, predictions for the present theory are not correct because underlying inconsistencies in the model make regressed values of binary interaction parameters less accurate. Nonetheless, using hand-picked values of binary interaction parameters, retrograde vitrification trend can be obtained

Selected thermodynamic aspects of the influence of pressure on polymer systems

A review with 45 refs. on effects on pressure and temp. on equil. and time-dependent thermodn. thermal properties (e.g., the glass-transition temp.) of polymers. A model, based on cell model, of the dense disordered state, pertinent to chain and small mol. fluids, is discusse

Dynamics and thermodynamics of polymer glasses

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 Reviewe

Physical Aging Behavior of a Glassy Polyether

[Abstract] The present work aims to provide insights on recent findings indicating the presence of multiple equilibration mechanisms in physical aging of glasses. To this aim, we have investigated a glass forming polyether, poly(1-4 cyclohexane di-methanol) (PCDM), by following the evolution of the enthalpic state during physical aging by fast scanning calorimetry (FSC). The main results of our study indicate that physical aging persists at temperatures way below the glass transition temperature and, in a narrow temperature range, is characterized by a two steps evolution of the enthalpic state. Altogether, our results indicate that the simple old-standing view of physical aging as triggered by the α relaxation does not hold true when aging is carried out deep in the glassy state.D.C. research was funded by MICINN-Spain and FEDER-UE, grant PGC2018-094548-B-I00; and the Basque Government, grant IT-1175-19. J.M. thanks MICINN /FEDER for grant Ref. PGC2018-094620-A-I00Eusko Jaurlaritza; IT-1175-1

Coupling of Caged Molecule Dynamics to JG β-Relaxation II: Polymers

At temperatures below the nominal glass transition temperature Tgα, the structural α-relaxation and the Johari-Goldstein (JG) β-relaxation are too slow to contribute to susceptibility measured at frequencies higher than 1 GHz. This is particularly clear in the neighborhood of the secondary glass transition temperature Tgβ, which can be obtained directly by positronium annihilation lifetime spectroscopy (PALS) and adiabatic calorimetry, or deduced from the temperature at which the JG β-relaxation time τβ reaches 1000 s. The fast process at such high frequencies comes from the vibrations and caged molecules dynamics manifested as the nearly constant loss (NCL) in susceptibility measurements, elastic scattering intensity, I(Q, T), or the mean-square-displacement, «u2(T)», in quasielastic neutron scattering experiment. Remarkably, we find for many different glass-formers that the NCL, I, or «u2» measured in the glassy state changes its temperature dependence at temperature THF near Tgβ. In paper I (Capaccioli, S.; et al. J. Phys. Chem. B 2015, 119 (28), 8800-8808) we have made known this property in the case of the polyalcohols and a pharmaceutical glass former, flufenamic acid studied by THz dielectric spectroscopy, and explained it by the coupling of the NCL to the JG β-relaxation, and the density dependence of these processes. In this paper II, we extend the consideration of the high frequency response to broader range from 100 MHz to THz in the glassy state of many polymers observed by quasielastic light scattering, Brillouin scattering, quasielastic neutron scattering, and GHz-THz dielectric relaxation. In all cases, the NCL changes its T-dependence at some temperature, THF, below Tgα, which is approximately the same as Tgβ. The latter is independently determined by PALS, or adiabatic calorimetry, or low frequency dielectric and mechanical spectroscopy. The property, THF Tgβ, had not been pointed out before by others or in any of the quasielastic neutron and light scattering studies of various amorphous polymers and van der Waals small molecular glass-formers over the past three decades. The generality and fundamental importance of this novel property revitalize the data from these previous publications, making it necessary to be reckoned with in any attempt to solve the glass transition problem. In our rationalization, the property arises first from the fact that the JG β-relaxation and the caged dynamics both depends on density and entropy. Second, the JG β-relaxation is the terminator of the caged dynamics, and hence the two processes are inseparable or effectively coupled. Consequently, the occurrence of the secondary glass transition at Tgβ necessarily is accompanied by corresponding change in the temperature dependence of the NCL, I, or «u2» of the fast caged dynamics at THF =Tg