Session T45: Polymeric Glasses

Formation of Polymer Glasses Under Stress and Its Influence on Physical Aging

Understanding and controlling the stability and physical aging of polymer glasses is important for many technological applications from gas separation membranes to optical coatings. We investigate the stability of polymer glasses when thermally quenched under different stress conditions. Ellipsometry is used to measure the physical aging rate of polystyrene films supported or transferred onto silicon wafers. We quantify the time-dependent decrease in film thickness that results from the increase in average film density during aging to obtain a physical aging rate. We have observed significant differences between films quenched in a free-standing versus supported state, even though all films were aged in a supported state. Films quenched in a free-standing state exhibit a strong thickness dependence to their physical aging rate at micron length scales, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. In contrast, supported films do not display any film thickness dependence to their aging rate at this large length scale. All available evidence suggests that different stress conditions are the underlying cause of this effect. In order to investigate the role of stress during the vitrification of polymer glasses, we have constructed a unique jig to apply a known stress to free-standing films during the thermal quench.   

Deformation-induced molecular mobility allows polystyrene glasses to flow

Experiments on colloidal glasses show that local rearrangements occur more rapidly during deformation. For polymeric glasses, similar features are expected but cannot be directly imaged. Dye reorientation has been used to measure segmental mobility in polymer glasses during active deformation. In creep deformations, we see a hundred-fold enhancement of mobility occurring in polystyrene glasses lightly cross-linked with 2 and 4 weight percent of divinylbenzene. Qualitatively similar mobility enhancement has been previously reported for lightly cross-linked poly(methyl methacrylate). Data from all three systems superpose on a master plot of the mobility as a function of the local strain rate during creep. Additionally, in the flow regime we see a significant narrowing of the distribution of relaxation times for both the polystyrene and poly(methyl methacrylate) glasses, similar to what has been reported for colloidal systems. Because polystyrene lacks the prominent beta relaxation of poly(methyl methacrylate), we conclude that the changes in mobility during creep deformation are a result of changes in the alpha segmental relaxation time.   

Temperature Divergence of the Dynamics of a PVAc glass: Dielectric vs. Mechanical Behaviors

The dynamics of glass forming liquids as the glass transition is traversed has become of special interest because of the continuing question as to whether or not these dynamics diverge towards an ideal glass transition/Kauzmann temperature or if the apparent Vogel-Fulcher divergence is lost as one goes below the conventional T$_{g}$ but remains in equilibrium. Here we examine the response of a PVAc polymer glass-former using both dielectric and mechanical methods in the vicinity of T$_{g}$. Isothermal measurements were performed to study the aging behavior of PVAc and to assure that the equilibrium state was achieved and for temperatures to as much as 15 $^{o}$C below the T$_{g}$. Surprisingly, we found that the mechanical response took much longer to age into equilibrium than did the dielectric response. Also, although the dielectric and mechanical responses seem to probe the glassy dispersion, the temperature dependence of the time-temperature shift factors obtained from the two methods are different and the dielectric measurement response shows a turnover to Arrhenius behavior rather than a continuation of the VFT divergence at the lowest temperatures tested.   

Determination of effective correlation length in a glassy polymer using electrostatic force microscopy

Various fourth-order correlation functions have been used to study the size of dynamically correlated regions in colloidal glasses and simulated glass forming liquids or polymers. However, measuring these correlation functions in molecular glasses has been limited by the small length scales on which the dynamics occur. Electrostatic force microscopy techniques are employed here to probe dielectric noise in polyvinyl acetate. We analyze fourth-order statistical fluctuations in order to determine spatio-temporal correlation lengths and their temperature dependence near the sample's glass transition. The first harmonic response of an applied AC voltage between the conducting AFM tip and the conducting substrate beneath the thin film polymer sample is proportional to the local electric polarization. Noise in this signal is examined and many hours are recorded at various temperatures in order to improve statistical precision. We employ a variety of statistical analysis techniques ranging from power spectrum analysis to variance of autocorrelation functions in order to find deviations from Gaussian statistics. Super-sharp carbon nanotube EFM tips (nominal radius of 10 nm) are employed to probe smaller effective volumes and thus more easily detect these fluctuations.   

Stable Nanostructured Polymer Films Formed Via Matrix Assisted Pulsed Laser Evaporation

Via typical routes to the vitreous state, the ability to significantly alter the properties of amorphous solids is restricted due to the kinetic nature of the glass transition. In this talk, we show that matrix assisted pulsed laser evaporation (MAPLE) can be used to form ultra-stable and nanostructured glassy polymers with significantly reduced densities, enhanced glass transition temperatures, and superior kinetic stability at high temperatures. Relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, glasses prepared by MAPLE can be 40 percent less dense and have 40 K higher glass transition temperatures. Furthermore, the kinetic stability in the glassy-state can be enhanced by 2-orders-of-magnitude. The unique combination of properties is a result of the glass morphology, i.e., the glassy films are formed by the assembly of nearly spherical-like polymer nanoglobules.   

A Scattering Model for Amorphous, Intrinsically Microporous Polymers

We discuss the development of a scattering model for glassy, porous polymers in which porosity is equivalent to free volume, deriving significant insight from molecular dynamics simulations. Polymers of intrinsic microporosity (PIMs) exhibit high gas permeability and a large concentration of pores smaller than 1 nm; their porosity arises from an unusual chain structure combining rigid segments with sites of contortion, rather than from any templating effect. Although for many porous materials, useful information such as pore sizes and specific surface areas can be extracted from scattering patterns, a successful scattering model for PIMs must account for several unusual features. Simulated structures that reproduce characteristic scattering features are used to test model assumptions, with the ultimate goal of increasing the utility of scattering for studying microporous polymeric membrane processing and physical aging.   

Non-Equilibrium Water-Glassy Polymer Dynamics

For many applications (e.g., medical implants, packaging), an accurate assessment and fundamental understanding of the dynamics of water-glassy polymer interactions is of great interest. In this study, sorption and diffusion of pure water in several glassy polymers films, such as poly(styrene) (PS), poly(methyl methacrylate) (PMMA), poly(lactide) (PLA), were measured over a wide range of vapor activities and temperatures using several experimental techniques, including quartz spring microbalance (QSM), quartz crystal microbalance (QCM), and time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Non-Fickian behavior (diffusion-relaxation phenomena) was observed by all three techniques, while FTIR-ATR spectroscopy also provides information about the distribution of the states of water and water transport mechanisms on a molecular-level. Specifically, the states of water are significantly different in PS compared to PMMA and PLA. Additionally, a purely predictive non-equilibrium lattice fluid (NELF) model was applied to predict the sorption isotherms of water in these glassy polymers.   

Self-consistent Mixing Rule for the Viscoleasticity of Heterogeneous Systems

Heterogeneous systems --like glasses mixtures - can exhibit huge local fluctuations of their viscoelastic modulus. The average viscoelastic modulus is often approximated either as the average modulus, or as the inverse of the average of the inverse of the modulus. Here we test a self-consistent method based on the Olroyd Palierne model for blend viscoelasticity [1]. We test this method on polymer miscible blends, with a very different glass transition temperature. We first deduce from Differential Scanning Calorimetric measurements the distribution of glass transition and thus of local viscoelastic modulus. From that we predict quantitatively the viscoelastic modulus of the polymer blends. It appears thus that the self-consistent averaging for disordered systems is extremely efficient, describing quantitatively systems where the modulus varies locally by at least 3 decades. [1] F. Lequeux, A. Ajdari,``\textit{Averaging rheological quantities in descriptions of soft glassy materials}`` Phys. Rev E 63 R030502 (2001)   

Understanding failure behavior of polymer glasses: a molecular viewpoint

In surveying the vast literature we note that a unified simple picture appears to be lacking to account for all the known facts on failure behavior of polymer glasses. In this work, we first apply the fresh insight we have gained from studying nonlinear extensional rheology of entangled melt to show why a melt-stretched polystyrene turns ductile. We further show that the ductile polycarbonate can also turn brittle upon pre-melt-stretching. Finally, blending oligomeric PC into an originally ductile PC also causes the mixture to become brittle. All these old and new phenomena point to the fact that the strength of the load bearing chain network dictates at a given temperature whether the polymer glass would undergo ductile failure through shear yielding or brittle fracture via crazing. This presentation will provide a description of how the entanglement structure can be altered by melt deformation or the choice of different chemical specificity (that influences the packing length) to affect the strength of polymer glasses. It is this network strength relative to the yield strength associated with the inter-segmental van der Waals interactions that determines how strain localization (shear yielding -- necking vs. crazing) takes place.   

Heterogeneous Dynamics During Creep of Rod Containing Polymer Nanocomposites

Polymer glasses exhibit regions of locally higher or lower mobility leading to heterogeneous dynamics. While heterogeneous dynamics have been examined in some detail in pure polymers, less is known about polymer nanocomposites (PNCs). We have previously studied PNCs, providing descriptions of how particles alter the network of entanglements, measuring local mechanical heterogeneity, demonstrating strain response under uniaxial deformation, and examining crazing and failure under multiaxial deformation. In the present work, we examine dynamic heterogeneity in rod-containing PNCs by performing creep deformation simulations and monitoring several measures of mobility. We are able to directly probe how dynamic heterogeneity evolves during deformation, and explore the origins of molecular mobility in polymer glasses. Examination of the segmental motion of a PNC undergoing creep reveals that the glassy heterogeneity of these systems decreases significantly following the onset of flow. It is found that the more heterogeneous distribution of relaxation times characteristic of PNCs in the bulk remains unaltered regardless of deformation state. It is found that the mobility and heterogeneity of PNCs are less susceptible to change upon deformation than those for the pure polymer.   

Molecular dynamics simulations of highly cross-linked polymer networks: prediction of thermal and mechanical properties

We use all-atom molecular dynamics (MD) simulations to predict the mechanical and thermal properties of thermosetting polymers. Atomistic simulation is a promising tool which can provide detailed structure-property relationships of densely cross-linked polymer networks. In this work we study the thermo-mechanical properties of thermosetting polymers based on amine curing agents and epoxy resins and have focused on the DGEBA/DETDA epoxy system. At first we describe the modeling approach to construction of realistic all-atom models of densely cross-linked polymer matrices. Subsequently, a series of atomistic simulations was carried out to examine the simulation cell size effect as well as the role of cross-linking density and chain length of the resin strands on thermo-mechanical properties at different temperatures. Two different methods were used to deform the polymer networks. Both static and dynamic approaches to calculating the mechanical properties were considered and the thermo-mechanical properties obtained from our simulations were found in reasonable agreement with experimental values.   

Heterogeneity: A Solution to the Mysteries of the Glass transition?

For most phase transitions, dynamic slowdown is accompanied by static structure change. However, in supercooled liquids there is a pronounced dynamic slowdown, i.e. diffusion coefficient, relaxation time and viscosity change 14 orders of magnitude within a small temperature range, without any static structure change. Over the past several decades, extensive research has been performed to understand this dramatic dynamical slowing, i.e., why the glass transition occurs? What exactly is the glass transition? And how molecules move near the glass transition? In the present work, the idea that the large decrease in mobility in supercooled liquids during cooling from above T$_{g}$ occurs due to the increasing length scale of heterogeneous sub-regions, or the Cooperative Rearranging Regions (CRR) proposed by Adam and Gibbs, is evaluated by analyzing both experiment and computer simulation results. Although the existence of microscopic heterogeneous regions is confirmed, the values of the fitting parameters obtained from both VFT and power law fits of the temperature dependent heterogeneity data suggest that the heterogeneity, itself, might not play in the key role in the T$_{g}$ and/or tightly connect with the CRRs.   

The effect of polar interactions on the dynamics in vitreous liquids

It is known that in small molecule and polymeric systems the long-range process like diffusion and chain relaxation decouple from the local structural relaxation temperature dependence as system approaches its T$_{g}$. Recently, it was shown by Sokolov and Scweizer (\textit{Phys. Rev. Lett. }\textbf{2009}, 102, 248301) that these decoupling phenomena seem to have similar underlying mechanism irrespective whether system is a polymeric or a molecular liquid. The degree of decoupling for both polymers and small molecule systems show very similar trend with respect to the fragility of the material. More fragile systems show higher degree of decoupling. However, such behavior was shown only on the example of dynamics in weakly interacting van der Waals systems. On the example of three polymers and one room temperature ionic liquid (RTIL) it is demonstrated that the presence of polar interactions leads to rather steep temperature dependence of large length scale dynamic processes, like chain relaxation and self-diffusion. As a result, the degree of decoupling between large length scale and local dynamic processes in such polar materials is significantly lower than in weakly interacting liquids with comparable fragilities. The microscopic mechanism behind an unusual dynamic behavior seen in polar polymers and RTILs remains unclear. Nevertheless, knowledge of such structure-property relationship can significantly aid in the development of novel materials. Authors are thankful to NSF and (U.S.) DOE, Office of Basic Energy Sciences for the financial support.   

The Twinkling Fractal Theory of the Glass Transition: Applications to Soft Matter

The Twinkling Fractal Theory (TFT) of the glass transition has recently been demonstrated experimentally [J.F. Stanzione et al., J. Non Cryst. Sol., (2011, 357,311]. The hard to-soft matter transition is characterized by the presence of solid fractal clusters with liquid-like pools that are dynamically interchanging via their anharmonic intermolecular potentials with Boltzmann energy populations with a characteristic temperature dependent vibrational density of states g($\omega ) \quad \sim \quad \omega ^{df}$ . The twinkling fractal frequencies $\omega $ cover a range of 10$^{12}$ Hz to 10$^{-10}$Hz and the fractal solid clusters of size R have a lifetime $\tau \quad \sim $ R$^{Df/df}$, where the fractal dimension D$_{f}$ $\approx $ 2.4 and the fracton dimension d$_{f}$ = 4/3. Here we explore its application to a number of soft matter issues. These include (a) Confinement effects on T$_{g}$ reduction in thin films of thickness h, where by virtue of large cluster exclusion, $\Delta $T$_{g} \quad \sim $ 1/h$^{Df/df}$; (b) T$_{g}$ gradients near bulk surfaces, where the smaller clusters on the surface have a faster relaxation time; (c) Effect of twinkling surfaces on cell growth, where at T $\approx $ T$_{g}$ + 20 C, there exists a twinkling fractal range that leads to bell-shaped enhancement of cell growth and chemical up-regulation via the twinkling surfaces ``communicating `` with the cells through their vibrations; and (d) adhesion above and below T$_{g}$ where topological fluctuations associated with g($\omega )$ promotes the development of nano-nails at the interface.   

Nano-indentation of Polycarbonate and Diamine Blends

Nanoindentation of complex surfaces is of great interest from academic and industrial point of view. There are unique properties such as indentation effects resulting in strain softening and strain hardening. There is a differentiation in structure with the depth exhibited with variation of Tg. Hertzian and non-linear deformation models including usage of FEM offer opportunity in analyzing nano-indentation. In polycarbonate, the effective elastic modulus and the hardness decreases as the applied load is increased. As the hold time was increased, the effective elastic modulus and the hardness also decreased. The contact stress increases as the contact strain rate is increased. Presence of diamine(MTBD) in polycarbonate results in making the surface and bulk brittle and acts as an anti-plasticizer by increasing it modulus and reducing yield stress (hardness) and strain to break. Data on modulus and hardness of polycarbonate and blends of diamine as function of depth (strain) and strain rate are presented and compared with those of composites with silica.