Last-modified: 2010-08-21 (土) 19:40:30 (1832d)
講演者： Dr. Didier R. Long (Laboratory of Polymers and Advanced Materials, UMR 5268 – CNRS/Solvay, 69192 Saint-Fons, France)
題目： Strain hardening of glassy polymers : theory and simulation
日時： 2016年8月16日（火） 14：00から16:00
場所: ウエストウイング７階 数物第2会議室
概要： Over the past twenty years empirical evidence has shown that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of ξ ≃ 3 – 5 nm. A model for the dynamics of non-polar amorphous polymers, based on percolation of slow domains, has been developed and solved by 3D numerical simulations. This so-called PFVD model succeeds in explaining many features observed in glassy polymers: the heterogeneous nature of the dynamics, the violation of the Stokes law observed for small probes, ageing and rejuvenation asymmetry, the shift of the glass transition temperature in thin films. Experiments show that, under applied deformation, polymers undergo yield at deformations of a few percent and stresses of some 10 MPa, followed by a quick drop in stress and plastic flow, namely the strain-softening. Yield behavior is often described through the phenomenological Eyring model, according to which stress reduces free energy barriers. After plastic flow, some polymers of high molecular weight display an increase of stress with increasing strain in the large amplitude regime of deformation. The typical slope, namely hardening modulus GR, is of order 107 – 108 Pa well below Tg. Classical theories involving the entropic response of the rubbery network cannot explain such a high value. GR is also found to increase upon cooling, as well as with strain rate and cross-linking density. We assume that local deformation induces a reduction of mobility, at the scale of dynamical heterogeneities, by orienting monomers in the drawing direction. We assume that consequent strengthening of monomer-monomer interactions results in a local increase of the glass transition temperature. Simulations show hardening moduli GR of order 10 – 100 MPa a few 10 K below Tg. This modulus decreases with temperature. The strain hardening regime is observed from strain amplitudes of about 20%. The model we propose provides a physical description of mechanical properties of glassy polymers with a resolution at the scale of e few nanometers. Simulation results are in agreement with experimental data, such as the elastic modulus (G’∼1 GPa Pa and its temperature dependence), the yield stress and the yield behavior (strain softening), and the strain hardening regime (GR∼10 MPa) with its temperature dependence, and its dependence on reticulation density. Our work opens the way for further developments, such as studying in detail the stabilizing behavior of strain hardening at the nano-scale, or further studies by combining molecular dynamics simulations for probing at the molecular level the strain induced orientation of the monomers, or on the contrary by helping to develop larger scale modelling by e.g. finite element methods.
Ref: Luca Conca PhD thesis, Lyon, 2016
問い合わせ先： 理工学部物理科学科 深尾 浩次 Tel: 077-561-2720
講演者： Dr. Prof. Simone Napolitano (Universite Libre de Bruxelles)
題目： Tailoring properties of polymers confined at the nanoscale level via controlled irreversible adsorption
日時： 2015年11月19日（木） 15：00から16:30
場所: ウエストウイング７階 数物第2会議室
概要： Polymer films prepared by spincoating are intrinsically non-equilibrium systems [1]. Differently than in bulk melts, equilibration is not achieved upon relaxation of the chains on the timescale of the reptation time. Prolonged annealing above the glass transition temperature induces, in fact, the formation of interfacial layers irreversibly adsorbed onto the supporting substrate, which affects several properties of the confined system [2]. Although a large experimental evidence demonstrated a strong correlation between the presence of these adsorbed layers and the deviation from bulk behavior, the knowledge of the mechanisms permitting pinning of monomers onto a solid substrate are not well known in the case of polymer melts. With these considerations in mind, we could think of fabrication methods permitting to finely tune materials properties, based on the control of the adsorption kinetics of polymer chains onto a supporting substrate. To achieve this goal, we started to investigate the physics of irreversible adsorption, in the benchmark case of thin supported polymer films. In this presentation, after reviewing current theoretical models on adsorption of polymer chains, we will present recent experimental results on films of polystyrene on silicon wafers [3] and the outcome of molecular dynamics simulations on a similar model system. In particular, we will show that at short annealing times the adsorbed amount (experimentally determined as the thickness of the irreversibly adsorbed layer) increases linearly with time. This first order reaction mechanism is inhibited at longer annealing times, where the space available for pinning of new chains is strongly limited by previously adsorbed chains. This condition is achieved at a molecular weight independent crossover time, and at adsorbed amount scaling as N1/2, where N is the polymerization degree. In this regime, following models treating adsorption of loops, we show that the growth of the adsorbed amount becomes logarithmic in time. We developed a series of analytical expressions permitting to follow the kinetics of irreversible adsorption, in line with the outcome of our experiments and simulations.
References,
[1] Non-equilibrium Phenomena in Confined Soft Matter, Napolitano (ed.), Springer (2015)
[2] S Napolitano et al. Nature Comm. 2, 260 (2011)
問い合わせ先： 理工学部物理科学科 深尾 浩次 Tel: 077-561-2720
講演者： Prof. G. R. Strobl (Fakultaet fuer Physik, Universitaet Freiburg)
題目： Laws controlling crystallization and melting in bulk polymers
日時： 2013年6月28日（金） 15：00から16:30
場所: ウエストウイング７階 数物第2会議室
概要： Experiments carried out during the last two decades on various polymer systems revealed a number of laws which control polymer crystallization and melting in bulk. They show in particular that the crystal thickness is inversely proportional to the distance to a temperature Tc which is located above the equilibrium melting point Tf and that crystal growth stops already at a temperature Tzg which is below Tf. The observations indicate that the pathway followed in the growth of polymer crystallites includes an intermediate metastable phase. In a model proposed by us a thin array of nanoblocks with mesomorphic inner structure forms between the lateral crystal face and the melt. The repeated block formation at the growth face supported by epitaxial contact forces resembles repeated nucleation steps. As is known since Ostwald's time nucleation processes can be accelerated by a passage through an intermediate mesomorphic, i.e.partially ordered, phase. Due to the high inner mobility in the mesomorphic phase blocks expand up to a critical size, at which they change into the crystalline state. The transitions between melt, mesomorphic blocks and the final lamellar crystallites can be described with the aid of a temperature-thickness phase diagram. Tc and Tzg are identified with the equilibrium temperatures of the hidden transitions between the mesomorphic and the crystalline phase, and between the liquid and the mesomorphic phase, respectively. Comparison of the predictions of the model theory with experimental results from small angle X-ray scattering, optical microscopy and calorimetry yield in addition to the three equilibrium transition temperatures latent heats of transition and surface free energies.
Ref: G. Strobl, Rev.Mod.Phys. 81, 1287 (2009)
問い合わせ先： 理工学部物理科学科 深尾 浩次 Tel: 077-561-2720
講演者： Dr. Didier R. Long (Laboratoire Polym`eres et Mat´eriaux Avanc´es; CNRS/Rhodia; Rhodia Recherches et Technologies F-69192 Saint Fons)
題目： How a Solvent Penetrates a Glassy Polymer and Induces it to Yield and Melt: a Microscopic Model for Case II Diffusion
日時： 2010年8月30日（月） 10：30から12:00
場所: ウエストウイング７階 数物第2会議室
概要： It has been shown over the past fifteen years that the dynamics close to and below the glass transition is strongly heterogeneous: fast domains coexist with domains 4 decades or more slower, the size of these regions being about 3 nm at Tg. We extend a model for describing ageing and rejuvenating in van der Waals liquids [1] to the case of solvent-polymer systems [2,3]. We show that the presence of solvent enhances the heterogeneous nature of the dynamics. We propose a model for calculating a constitutive relation for the dynamics in these systems, and in particular when a solvent is put into contact with a glassy matrix. We propose that the solvent first penetrates relatively fast path in the matrix. Then, the osmotic pressure induces melting of the layer in a time scale that we calculate and which controls the solvent penetration dynamics on larger scale. Our model provides thus a physical interpretation for the parameters of the Thomas-Windle model and allows for their (semi-quantitative) calculation as a function of the solvent, the polymer, and the dynamical state (ageing) of the glassy matrix. Our model allows also for the description of solvent-polymer films drying. Regarding this process, our model provides a physical explanation of why films up to 1 micrometer thick can be almost completely dried in an accessible experimental time, even at temperatures well below the polymer glass transition temperature. Our model opens the way for a microscopic and quantitative description of solvent diffusion through a polymer matrix close to and below the glass transition and should help designing materials with controlled barriers properties.
[1]
Merabia, S.; Long, D. J. Chem. Phys (2006) 125, 234901
[2]
Souche, M.; Long, D. Europhysics Lett. (2007) 77, 48002
[3]
Masnada, E.; Souche M.; Long, D. in preparation (2010)~
問い合わせ先： 理工学部物理科学科 深尾 浩次 Tel: 077-561-2720