Glassy Polymeric Materials Research Articles

Topology-Enabled Simultaneous Enhancement of Mechanical and Healable Properties in Glassy Polymeric Materials Using Larger POSS

Topology-Enabled Simultaneous Enhancement of Mechanical and Healable Properties in Glassy Polymeric Materials Using Larger POSS

A methodology for determining the local mechanical properties of model atomistic glassy polymeric nanostructured materials

We propose a methodology for calculating the distribution of the mechanical properties in model atomistic polymer-based nanostructured systems. The use of atomistic simulations is key in unravelling the fundamental mechanical behavior of composite materials. Most simulations involving the mechanical properties of polymer nanocomposites (PNCs) concern their global (average) properties, which are typically extracted by applying macroscopic strain on the boundaries of the simulation box and calculating the total (global) stress by invoking the Virial formalism over all atoms within the simulation box; thus, extracting the pertinent mechanical properties from the corresponding stress-strain relation.However, in order to probe the distribution of mechanical properties within heterogeneous multi-component polymer-based systems, a detailed computation of stress and strain fields within specific sub-domains is necessary. For example, it is well known for multi-component nanostructured systems, such as PNCs, that the mechanical behavior of the polymer/nanofiller interphases, or interfaces, is crucial for determining the global mechanical properties of the composite materials. Here we propose a new methodology to probe the distribution of mechanical properties by directly computing the (local) stress and strain at the atomic level, and averaging over user-defined subdomains. The workflow of our computational method possesses the following features:•Calculating the stress and strain per atom (or per particle) for nanostructured microscopic (here atomistic) model configurations, under an imposed applied deformation.•Averaging the local, per-atom defined, stress and strain on user-defined subdomains within the nanostructured model system.•Predicting the mechanical properties within the specific subdomains, focusing on polymer/solid interphases.

Tuning the Johari-Goldstein β-Relaxation and Its Separation from α-Relaxation of Poly(n-alkyl methacrylate)s by Small Molecule-bridged Hydrogen Bonds

Introducing small molecule-bridged hydrogen bonds (HBs) between polymer chains has been reported to effectively reduce the inter-chain cooperativity despite of strengthening the intermolecular interaction. Here, a systematic investigation on tuning the Johari–Goldstein β (βJG) relaxation by adding various low-molecular-weight phenols in poly(n-alkyl methacrylate)s is carried out to further clarify the anomalous dynamics. Given these small molecules capable of coupling the motion with pendent groups of host polymers due to forming at least two HBs per molecule, poly(n-alkyl methacrylate) mixtures exhibit rich dynamic changes in the βJG-properties and α, βJG separations. An increased loading of phenols with a small size and strong inter-HB strength (Δυi) clearly benefits for significant retardation and suppression of the βJG-relaxation, narrows the α, βJG separation and converges the βJG-peak with the α-peak, which demonstrates the alleviation of inter-chain topological constraints. However, small molecules with a relatively big size and weak Δυi are found to amplify the magnitude of the α, βJG separation of poly(butyl methacrylate), even though experimental results of changes in α-dispersion and dynamic fragility confirm a reduction of the coupling factor n in all of these hybrids. The counterintuitive phenomenon suggests that the crossover time tc in the Coupling Model is no longer a universal quantity if the inter-chain of polymers is strengthened by HBs. These compelling findings shed vital insights into the HB-induced anomalous dynamics, and provide essential guidance for tailoring the βJG behavior and designing glassy polymeric materials.

Can crosslinking improve both CO2 permeability and plasticization resistance in 6FDA–pBAPS/DABA copolyimides?

In membrane-based CO2 separation, glassy polymeric materials suffer from plasticization phenomenon. To suppress plasticization, cross‐linking is a possible remedy, which however alters the separation performance significantly. This study investigates the effect of crosslinking on the CO2 transport properties and plasticization resistance of 2,2′-bis-(3,4-dicarboxyphenyl) hexafuoropropane dianhydride - bis [4-(4-aminophenoxy) phenyl] sulfone/3,5-diaminobenzoic acid (6FDA-pBAPS/DABA) copolyimide using molecular modeling and simulation tools that integrate quantum level calculations with Molecular Dynamics (MD) and Monte Carlo (MC) simulations. Mimicking the thermal crosslinking procedure, 25, 50, 75, and 100% crosslinked copolyimide structures are constructed for sorption and diffusion simulations and compared with the uncrosslinked polymer. The increased glass transition temperature of the polymers with crosslinking density confirms enhanced rigidity due to crosslinking, which reduces the packing efficiency of polymer chains and leads to higher fractional free volume, hence increasing the CO2 diffusivity. Analyses of CO2-accessible free volume evolution as a function of pressure show increased plasticization resistance with crosslinking. However, radial distribution function analyses indicate that crosslinking reduces the number of preferential sorption sites for CO2 molecules and prevents also the expansion of free volume elements around these sites. Consequently, CO2 solubility of the polymers decreases while diffusivity increases with increasing crosslinking density. This study reveals that the resultant effect of thermal crosslinking on 6FDA-pBAPS/DABA copolyimide is an increase in both CO2 permeability and plasticization resistance, which are the most desirable properties in CO2 separation applications.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers.

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.

Low-Cost, Ambient-Dried, Superhydrophobic, High Strength, Thermally Insulating, and Thermally Resilient Polybenzoxazine Aerogels

A family of ambient-dried polybenzoxazine aerogels is prepared with a facile and scalable process as a high-performance polymeric aerogel with strong and robust thermomechanical properties at elevated temperatures. Those materials are inherently flame-retardant and superhydrophobic over the entire bulk density range (0.24–0.46 g cm–3). In addition, they are mechanically strong with strengths (e.g., 1 MPa at 0.24 g cm–3 at room temperature) higher than those of other high-performance aerogels of similar density, including polyimide and polyamide (Kevlar-like) aerogels as well as polymer-cross-linked X-silica and X-vanadia aerogels, at a significantly lower cost. Furthermore, unlike most other glassy polymeric materials, the maximum strength of the synthesized aerogels occurs at service temperatures slightly higher than room temperature (about 50 °C), which eliminates the possibility of any drop in strength with respect to the room temperature strength up to 150 °C at all densities. At higher temperatures (up to 250 °C), the overall performance of those aerogels is also stable and robust without any significant drop in Young’s modulus or strength levels, which makes them suitable for various industrial applications including high-performance structural and thermal protection applications as an alternative to the significantly more expensive polyimides.

CO2 plasticization effect on glassy polymeric membranes

The effect of CO2 on the mechanical properties of three different glassy polymeric materials, polystyrene (PS), polymethylmethacrylate (PMMA) and Matrimid polyimide (PI), has been investigated by dedicated Dynamic Mechanical Analysis (DMA) at 35 °C on samples equilibrated at different penetrant pressures (0–30 bar). The experimental campaign has been designed to inspect the mechanical properties associated to the so-called plasticization phenomenon, which has been invoked as responsible for the increase of gas permeability versus feed pressure in glassy polymeric membranes. The main aim of the work is to find if any correlation holds between the effects induced on polymer mechanical properties and on gas permeability by the presence of different amounts of penetrant gas.The DMA analysis revealed that CO2 produces a decrease of polymer storage modulus E′ and an enhancement of tan δ factor. Experimental data for both E’ and tan δ vary regularly and monotonically with CO2 content in the membrane, with no indication of the onset of other new phenomena at pressures higher than the “plasticization pressure” associated to permeability data. The mechanical properties show very similar behaviors with CO2 content for all polymers analyzed, notwithstanding their CO2 permeability behaviors are different: decreasing with upstream pressure for PS, increasing for PMMA and nonmonotonous for Matrimid. The results obtained indicate that there is no direct correlation between gas permeation dependence on CO2 content and the changes in the polymer mechanical properties induced by the same CO2 content.

Extreme Toughness Exhibited in Electrospun Polystyrene Fibers

Polystyrene (PS) commonly exhibits brittle behavior and poor mechanical properties due to the presence of structural heterogeneities promoting localized failure. The removal of this localized failure is shown here by processing PS into fibers with a range of diameters using electrospinning. Mechanical properties of individual electrospun fibers were quantified with atomic force microscopy based nanomechanical tensile testing. The resultant stress–strain behavior of PS fibers highlights considerable enhancement of mechanical properties when fiber diameter decreases below 600 nm such that polystyrene toughness increases significantly by over two orders of magnitude compared to the bulk. Consideration of the network properties of polystyrene is used to demonstrate the increase of draw ratio toward a theoretical limit and is potentially applicable to a range of glassy polymeric materials. image

Concepts and experimental protocols towards a molecular level understanding of the mechanical properties of glassy, cross-linked proteins: Application to wheat gluten bioplastics

Linking the mechanical performance of glassy polymeric materials to their molecular characteristics is a major challenge, even more so when proteinaceous raw materials are used with a complex cross-linking chemistry and composition dependent molecular interactions. Here, wheat gluten was separated into gliadin-rich and glutenin-rich fractions. Blends prepared thereof were compression molded (130–150°C, 5–28min) into glassy, brittle materials. Molecular characteristics as well as mechanical properties in bending and compression modes were determined and their mutual relations extensively discussed. The compression data were used to qualitatively predict the flexural strength values. Experimental outcomes deviated from the predictions possibly because of stress concentrating contaminants in glutenin-rich and molecular network interruptions in gliadin-rich materials. Depending on molding conditions and protein composition either only disulfide bonds or also non-disulfide cross-links were present. It is suggested that disulfide based cross-links contribute less to material toughness than other types of covalent cross-links.

Non-equilibrium thermodynamics of glassy polymers: Use of equations of state to predict gas solubility and heat capacity

The use of Non-Equilibrium Thermodynamics for Glassy Polymers (NET-GP) model to predict infinite dilution gas solubility coefficient is revised in this work and its extension to the analysis of apparent constant pressure heat capacity in polymeric materials below the glass transition temperature Tg is developed. Use is made of different Equations of State (EoS), in the class of tangent-hard-spheres-chain theories, to predict the thermal expansion coefficient below Tg, resulting in consistent representations of the Henry's coefficient for gaseous species in the same temperature range, for the case of different versions of Statistical Associating Fluid Theory (SAFT) EoS. With reference to calorimetric properties, the analysis here performed indicates that NET-GP endowed with EoS tuned on melt phase equilibrium properties does not allow for the prediction of the bulk modulus of the glassy polymeric phase, and only qualitative behavior for the apparent heat capacity are reproduced. On the other hand, after the use of experimental data from structural relaxation experiments to evaluate the bulk modulus in glassy state, a satisfactory prediction of the excess heat capacity is obtained within the same framework, for the case of different non-equilibrium conditions. Conclusion are finally drawn for the need to account for additional order parameters in NET-GP approach in order to address the representation of the complex calorimetric behavior exhibited by glassy polymeric materials.

A Macro-Damaged Viscoelastoplastic Model for Thermomechanical and Rate-Dependent Behavior of Glassy Polymers

he primary objective of this paper is to develop a macro-damaged viscoelastoplastic constitutive model to describe large deformation mechanical behavior for glassy polymers at various kinds of experimental conditions. First of all, quasi-static and dynamic tension and compression tests were carried out to obtain stress–strain responses over wide range of rates and temperatures for two glassy polymeric materials, polymethylmethacrylate and polycarbonate. Then a phenomenological macroscopic damage model, covering effects of strain rates, temperatures, and effective strain, is incorporated into the constitutive model in the previous work. Furthermore, two distinct criteria are applied to predict the damage evolution affected by strain rate and temperature at elastic phase and plastic phase, respectively. The validations of the novel developed damaged model are made by the better matches with the testing data compared with the original undamaged model, and excellent agreements with the experimental result in all cases.

Transport and Stability of Laser-Deposited Amorphous Polymer Nanoglobules.

We characterized the transport, i.e., time-of-flight, and nanoscale thermal properties of amorphous polymer nanoglobules fabricated via a laser-deposition technique, Matrix-Assisted Pulsed Laser Deposition (MAPLE). Here, we report the first experimental measurement of the velocity of polymer during MAPLE processing and its connection to nanostructured film formation. A nanoscale dilatometry technique using atomic force microscopy was employed to directly measure the thermal properties of MAPLE-deposited polymer nanoglobules. Similarly to bulk stable polymer glasses deposited by MAPLE, polymer nanoglobules were found to exhibit enhanced thermal stability and low density despite containing only thousands of molecules. By directly connecting the exceptional properties of the nanostructured building blocks to those of bulk stable glasses, we gain insight into the physics of glassy polymeric materials formed via vapor-assisted techniques.

Light upconversion by triplet–triplet annihilation in diphenylanthracene-based copolymers

Low-power light upconversion by triplet–triplet annihilation (TTA-UC) was only recently achieved in glassy materials. Here, a new strategy based on covalent tethering of diphenylanthracene (DPA) emitters to a polymeric backbone is reported. The design aims to optimize the efficiency of this photophysical process in glassy polymeric materials by increasing the emitter content. To that end, DPA molecules were covalently attached to a methacrylate-type monomer and further copolymerized with methylmethacrylate (MMA). Green-to-blue (543 to 440 nm) upconversion was observed at power densities as low as 32 mW cm−2 in films prepared by solution casting and compression molding (co)polymers containing 8–72 wt% of DPA and palladium octaethyl porphyrin (PdOEP) as a sensitizer (0.03–0.7 wt%). The upconversion intensity was studied as a function of DPA and PdOEP contents and the results suggest that upconversion is optimal for DPA and PdOEP weight fractions of 34 and 0.05 wt% respectively.

NELF predictions of a solubility–solubility selectivity upper bound

Non-equilibrium lattice fluid theory (NELF) is used to evaluate the relationship between solubility and solubility selectivity for glassy polymeric materials. The NELF model provides a priori predictions of gas solubility and solubility selectivity for gas penetrants in glassy polymers without adjustable parameters. The results suggest an upper bound exists on solubility and solubility selectivity. The solubility upper bound depends on gas and polymer material properties through NELF model parameters. The polymer–polymer interaction energy determines the location of the upper bound. The upper bound moves in a favorable direction as polymer cohesive energy density increases and is limited by the highest value achievable in real materials. Furthermore the temperature dependence of solubility and solubility selectivity is predicted by the analysis.

Structural heterogeneity in glassy polymeric materials revealed by positron annihilation and other supplementary techniques

AbstractA disagreement between effective sizes of elementary free volumes found from the four component PATFIT analyses of positronium annihilation lifetime spectra and experiments on low temperature sorption for two membrane materials of internal microporosity poly(tri‐methyl‐silyl‐propyne) PTMSP and spiro‐linked benzodioxane polymer PIM‐1 is described and discussed. This disagreement can be minimized essentially using the fifth lifetime component but not the Gaussian distribution of the fourth component. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural heterogeneity in elastic and glassy polymeric materials revealed by positron annihilation and other supplementary techniques

Elastic and glassy polymers are normally considered as homogeneous disordered materials with unimodal (statistical) distribution of elementary free volumes. In this work, we discuss the results of our positron annihilation lifetime (PAL) measurements for a number of elastic and glassy polymers, where observations of the two long-lived ortho-positronium components in the PAL spectra and irregular (nonlinear) variations of annihilation characteristics in the vicinity of glass transition temperature reveal structural heterogeneity of these systems. The conclusion is confirmed by measurements of thermo-stimulated luminescence, thermo-mechanics and mobility of penetrants.

A Study of Probe Characteristics on the Diffusion Process in Materials Containing Acryloxy and Acrylate Units

AbstractWe study the diffusion of an atomic probe in materials used in restorative dentistry. We rely on computer simulation and on a version of transition state theory to access the diffusion regimes of interest. We find that the calculated logD values are in good agreement with reported results in other glassy polymeric materials. We report the effect of atomic mass and probe radius on the diffusion process.

Viscoelastic characterization of polymers using instrumented indentation. I. Quasi‐static testing*

AbstractThe use of instrumented indentation to characterize the mechanical response of polymeric materials was studied. A model based on contact between a rigid probe and a linear viscoelastic material was used to calculate values for the creep compliance and stress relaxation modulus for two glassy polymeric materials, epoxy and poly(methyl methacrylate), and two poly(dimethyl siloxane) (PDMS) elastomers. Results from bulk rheometry studies were used for comparison with the indentation stress relaxation results. For the two glassy polymers, the use of sharp pyramidal tips produced responses that were considerably more compliant (less stiff) than the rheometry values. Additional study of the deformation remaining in epoxy after indentation creep testing as a function of the creep hold time revealed that a large portion of the creep displacement measured was due to postyield flow. Indentation creep measurements of the epoxy with a rounded conical tip also produced nonlinear responses, but the creep compliance values appeared to approach linear viscoelastic values with decreasing creep force. Responses measured for the unfilled PDMS were mainly linear elastic, with the filled PDMS exhibiting some time‐dependent and slight nonlinear responses in both rheometry and indentation measurements. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1794–1811, 2005

Spherical Indentation Creep Following Ramp Loading

ABSTRACTDepth-sensing indentation testing is a common way to characterize the mechanical behavior of stiff, time-independent materials but presents both experimental and analytical challenges for compliant, time-dependent materials. Many of these experimental challenges can be overcome by using a spherical indenter tip with a radius substantially larger than the indentation depth, thus restricting deformation to viscoelastic (and not plastic) modes in glassy polymers and permitting large loads and contact stiffness to be generated in compliant elastomers. Elastic-viscoelastic correspondence was used to generate spherical indenter solutions for a number of indentation testing protocols including creep following loading at a constant rate and a multiple ramp-and-hold protocol to measure creep response at several loads (and depths) within the same test. The ramp-creep solution was recast as a modification to a step-load creep solution with a finite loading rate correction factor that is a dimensionless function of the ratio of experimental ramp time to the material time constant. Creep tests were performed with different loading rates and different peak load levels on glassy and rubbery polymeric materials. Experimental data are fit to the spherical indentation solutions to obtain elastic modulus and time-constants, and good agreement is found between the results and known modulus values. Emphasis is given to the use of multiple experiments (or multiple levels within a single experiment) to test the a priori assumption of linear viscoelastic material behavior used in the modeling.

せん断変形下におけるポリマー材の局所不安定伝ぱ挙動―ポリカーボネートの表面微視構造の形成と成長―

Polymeric materials under large deformation exhibit localized instability propagation, such as neck propagation under tension and shear band propagation under shearing. In this study, formation and growth of micro-shear bands in a glassy polymeric material, polycarbonate (PC) deformed under simple shear, were observed using an Atomic Force Microscope (AFM). In the early stage of shear deformation, the first micro-shear bands along the shearing direction was formed, and they merged together with increase in its density, and then, resulted in the formation of the macro-shear band. In this process, crossing orthogonally to the first micro-shear band, the secondary micro-shear bands were formed. After the applied force reached its maximum, the macro-shear band propagation started in the perpendicular direction to the shearing direction. As the macro-shear band propagated, the rotation of the secondary micro-shear band was observed. In addition, hierarchical structure of the secondary micro-shear band was revealed in region right after the macro-shear band propagation.