Amorphous Bulk Polymers Research Articles

Roles of repeating-unit interactions in the stress relaxation process of bulk amorphous polymers: 2. nonlinear viscoelasticity

Stress relaxation manifests itself as the most fundamental mechanical response of bulk amorphous polymers. We characterized polymer repeating units with four-set interaction parameters separately representing the thermodynamic and kinetic aspects of local intrachain and interchain interactions, and evaluated their relative importance in stress relaxation with linear viscoelasticity at high temperatures by means of kinetic Monte Carlo simulations (DOI: 10.1016/j.polym.2021.123740). Hereby we continued to evaluate their relative importance in the initiation and the deviation extent of nonlinear viscoelasticity at low temperatures. We observed that thermodynamic interchain interactions seem no effective, while the other three local interactions retard the later stage of stress relaxation for nonlinear viscoelasticity, responsible for the KWW indexes of polymer rubbery states around 0.5. Both the kinetic aspects of intrachain and interchain interactions dominate the initiation temperatures of nonlinear viscoelasticity, and their high barriers suppress the deviation extent of nonlinear viscoelasticity towards the strong liquids like metals and ceramics, while their low barriers leave the fragile liquids to more thermodynamically rigid polymers. Our work facilitates a better understanding of structure-property relationship in nonlinear viscoelasticity of bulk amorphous polymers.

Nascent structure memory erased in polymer stretching.

Stretching of semicrystalline polymer materials is fundamentally important in their mechanical performance and industrial processing. By means of dynamic Monte Carlo simulations, we compared the parallel stretching processes between the initially bulk amorphous and semicrystalline polymers at various temperatures. In the early stage of stretching, semicrystalline polymers perform local and global melting-recrystallization behaviors at low and high temperatures, while the memory effects occur upon global melting-recrystallization at middle temperatures. However, the final crystallinities, crystalline bond orientations, chain-folding probabilities, residual stresses, and crystallite morphologies at high enough strains appear as the same at each temperature, irrelevant to the initially amorphous and semicrystalline polymers, indicating that the common post-growth melting-reorganization processes determine the final products. In addition, both final products harvest the highest crystallinities in the middle temperature region because the postgrowth stage yields the vast nuclei followed with less extent of crystal growth in the low temperature region and few nuclei followed with large extent of crystal growth in the high temperature region. Our observations imply that a large enough strain can effectively remove the thermal history of polymers, similar to the thermal treatment at a high enough temperature; therefore, the fracture strength of semicrystalline polymers depends upon their final structures in stretching, not related to their nascent semicrystalline structures.

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

Local Transient Jamming in Stress Relaxation of Bulk Amorphous Polymers

Bulk amorphous polymers become stretched and parallel-aligned under loading stress, and their intermolecular cooperation slows down the subsequent stress relaxation process. By means of dynamic Monte Carlo simulations, we employed the linear viscoelastic Maxwell model for stress relaxation of single polymers and investigated their intermolecular cooperation in the stress relaxation process of stretched and parallel-aligned bulk amorphous polymers. We carried out thermal fluctuation analysis on the reproduced Debye relaxation and Arrhenius fluid behaviors of bulk polymers. We found a transient state with stretch-coil coexistence among polymers in the stress relaxation process. Further structure analysis revealed a scenario of local jamming at the transient state, resulting in an entropy barrier for stretch-coil transition of partial polymers. The microscopic mechanism of intermolecular cooperation appears as unique to polymer stress relaxation, which interprets the hydrodynamic interactions as one of essential factors raising a high viscosity in bulk amorphous polymers. Our simulations set up a platform of molecular modeling in the study of polymer stress relaxation, which brought new insights into polymer dynamics and the related mechanical/rheological properties.

Thermal Transport in Polymers: A Review

Abstract In this article, we review thermal transport in polymers with different morphologies from aligned fibers to bulk amorphous states. We survey early and recent efforts in engineering polymers with high thermal conductivity by fabricating polymers with large-scale molecular alignments. The experimentally realized extremely high thermal conductivity of polymer nanofibers is highlighted, and understanding of thermal transport physics is discussed. We then transition to the discussion of bulk amorphous polymers with an emphasis on the physics of thermal transport and its relation with the conformation of molecular chains in polymers. We also discuss the current understanding of how the chemistry of polymers would influence thermal transport in amorphous polymers and some limited, but important chemistry-structure-property relationships. Lastly, challenges, perspectives, and outlook of this field are presented. We hope this review will inspire more fundamental and applied research in the polymer thermal transport field to advance scientific understanding and engineering applications.

Roles of repeating-unit interactions in the stress relaxation process of bulk amorphous polymers

How the repeating-unit chemistry dominates polymer viscoelasticity remains as unclear. We employed four-set interaction parameters to represent thermodynamic and kinetic aspects of local intrachain and interchain interactions between monomers, which characterize polymer repeating-unit chemistry. We performed kinetic Monte Carlo simulations of Debye stress relaxation in stretched and parallel-aligned bulk amorphous polymers and evaluated the relative importance of each interactions in the diffusion energy derived from their Arrhenius fluid behaviors. The results demonstrated that, under the same strengths, the kinetic aspects of intrachain and interchain interactions raise much higher diffusion energies than their thermodynamic aspects. We discussed the example case of polyethylene. Our work paves the way towards the prediction of the structure-property relationship of the mechanical properties in bulk amorphous polymers.

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers (Part III)

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers (Part III)

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers (Part II)

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers (Part II)

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers

Definitions of Terms Relating to Individual Macromolecules, Macromolecular Assemblies, Polymer Solutions, and Amorphous Bulk Polymers

Phase Transformation and Shock Sensor Response of Additively Manufactured Piezoelectric PVDF

Recently the authors discovered the ability to print electroactive PVDF using a modified FDM printer. This paper discusses the results from investigating the phase change during printing using FTIR and DSC measurements. FTIR and DSC data were collected on the filament before the print, the printed filament without an applied electric field and the printed filament with a 30MV/m field applied during the print. The results show a definitive phase change during the print and indicate that the alpha phase of the PVDF was significantly modified. The FTIR results do not indicate a significant change in the beta phase of the polymer which is the dominant electroactive phase. Therefore it is likely that the bulk amorphous polymer phases have been aligned in the process. The response of the printed and poled PVDF demonstrates piezoelectric behavior when subjected to impulse indicating the potential use of the modified printing process to produce sensors.

Role of Chain Morphology and Stiffness in Thermal Conductivity of Amorphous Polymers.

Designing thermally conductive polymer is of scientific interest and practical importance for applications like thermal interface materials, electronics packing, and plastic heat exchangers. In this work, we study the fundamental relationship between the molecular morphology and thermal conductivity in bulk amorphous polymers. We use polyethylene as a model system and performed systematic parametric study in molecular dynamics simulations. We find that the thermal conductivity is a strong function of the radius of gyration of the molecular chains, which is further correlated to persistence length, an intrinsic property of the molecule that characterizes molecular stiffness. Larger persistence length can lead to more extended chain morphology and thus higher thermal conductivity. Further thermal conductivity decomposition analysis shows that thermal transport through covalent bonds dominates the effective thermal conductivity over other contributions from nonbonded interactions (van der Waals) and translation of molecules disregarding the morphology. As a result, the more extended chains due to larger persistence length provide longer spatial paths for heat to transfer efficiently and thus lead to higher thermal conductivity. In addition, rigid rod-like polymers with very large persistence length tend to spontaneously crystallize and form orientated chains, leading to a thermal conductivity increase by more than 1 order of magnitude. Our results will provide important insights into the design of thermally conductive amorphous polymers.

Crystalline polymers with exceptionally low thermal conductivity studied using molecular dynamics

Semi-crystalline polymers have been shown to have greatly increased thermal conductivity compared to amorphous bulk polymers due to effective heat conduction along the covalent bonds of the backbone. However, the mechanisms governing the intrinsic thermal conductivity of polymers remain largely unexplored as thermal transport has been studied in relatively few polymers. Here, we use molecular dynamics simulations to study heat transport in polynorbornene, a polymer that can be synthesized in semi-crystalline form using solution processing. We find that even perfectly crystalline polynorbornene has an exceptionally low thermal conductivity near the amorphous limit due to extremely strong anharmonic scattering. Our calculations show that this scattering is sufficiently strong to prevent the formation of propagating phonons, with heat being instead carried by non-propagating, delocalized vibrational modes known as diffusons. Our results demonstrate a mechanism for achieving intrinsically low thermal conductivity even in crystalline polymers that may be useful for organic thermoelectrics.

Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)

Abstract This document defines terms relating to the properties of individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers. In the section on polymer solutions and amorphous bulk polymers, general and thermodynamic terms, dilute solutions, phase behaviour, transport properties, scattering methods, and separation methods are considered. The recommendations are a revision and expansion of the IUPAC terminology published in 1989 dealing with individual macromolecules, macromolecular assemblies, and dilute polymer solutions. New terms covering the principal theoretical and experimental developments that have occurred over the intervening years have been introduced. Polyelectrolytes are not included.

Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)

Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)

Interfacial interactions and complex segmental dynamics in systems based on silica-polydimethylsiloxane core–shell nanoparticles: Dielectric and thermal study

Effects of two types of silica particles, with different characteristics of porosity, on segmental mobility of poly(dimethylsiloxane), PDMS, in core–shell nanocomposites with two contents of polymer, 40 and 80 wt%, were studied by employing thermal (DSC) and dielectric (DRS, TSDC) techniques, under different annealing procedures. Filler morphology and polymer–particle interactions were found to affect PDMS crystallinity and segmental mobility. The dielectric techniques allowed the detection of four contributions to the segmental dynamics associated with the glass transition arising, in the order of decreasing mobility, from confined polymer chains in the pores of silica gel (αp relaxation), from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (αc) and from semibound polymer in an interfacial layer close to the nanoparticle surface (α′). For the core–shell nanocomposites with high PDMS content (80 wt%) the interfacial α′ relaxation process is found to have similar characteristics with that in conventional PDMS/silica nanocomposites, for lower PDMS content (40 wt%), however, the interfacial relaxation process exhibits distinct differences. The experimental findings are discussed in terms of models proposed for the description of conformations of polymers adsorbed onto a solid surface.

How Geometric Distortions Scatter Electronic Excitations in Conjugated Macromolecules.

Effects of disorder and exciton-phonon interactions are the major factors controlling photoinduced dynamics and energy-transfer processes in conjugated organic semiconductors, thus defining their electronic functionality. All-atom quantum-chemical simulations are potentially capable of describing such phenomena in complex "soft" organic structures, yet they are frequently computationally restrictive. Here we efficiently characterize how electronic excitations in branched conjugated molecules interact with molecular distortions using the exciton scattering (ES) approach as a fundamental principle combined with effective tight-binding models. Molecule geometry deformations are incorporated to the ES view of electronic excitations by identifying the dependence of the Frenkel-type exciton Hamiltonian parameters on the characteristic geometry parameters. We illustrate our methodology using two examples of intermolecular distortions, bond length alternation and single bond rotation, which constitute vibrational degrees of freedom strongly coupled to the electronic system in a variety of conjugated systems. The effect on excited-state electronic structures has been attributed to localized variation of exciton on-site energies and couplings. As a result, modifications of the entire electronic spectra due to geometric distortions can be efficiently and accurately accounted for with negligible numerical cost. The presented approach can be potentially extended to model electronic structures and photoinduced processes in bulk amorphous polymer materials.

Dielectric and thermal studies of segmental dynamics in silica/PDMS and silica/titania/PDMS nanocomposites

ABSTRACTEffects of silica and silica/titania nanoparticles on glass transition and segmental dynamics of poly(dimethylsiloxane) (PDMS) were studied for composites of a core–shell type using differential scanning calorimetry, thermally stimulated depolarization current, and dielectric relaxation spectroscopy techniques. Strong interactions between the filler and the polymer suppress crystallinity (Tc, Xc) and affect significantly the evolution of the glass transition in the nanocomposites. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (αc relaxation), and from the semi‐bound polymers in an interfacial layer with strongly reduced mobility due to interactions with surface hydroxyls of silica and silica/titania nanoparticles (α′ relaxation). The evolution of surface affected CH3 groups, as well as the degree of interaction of PDMS molecules with surface hydroxyl groups as a function of treatment temperature, was assessed by Fourier transform infrared spectroscopy, thermogravimetry and differential thermal analysis. The effectiveness of silica/PDMS and silica/titania/PDMS nanocomposites as hydrophobic coatings was investigated by static contact angle measurements. It was shown that the presence of titania nanoparticles and adsorbed PDMS promotes the hydrophobic properties of the PDMS coating after treatment in the 80–650°C range. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41154.

Glass‐transition temperatures of nanostructured amorphous bulk polymers and their blends

ABSTRACTNanostructured amorphous bulk polymer samples were produced by processing them with small molecule hosts. Urea (U) and gamma‐cyclodextrin (γ‐CD) were utilized to form crystalline inclusion compounds (ICs) with low and high molecular weight as‐received (asr‐) poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), and their blends as included guests. Upon careful removal of the host crystalline U and γ‐CD lattices, nanostructured coalesced (c‐) bulk PVAc, PMMA, and PVAc/PMMA blend samples were obtained, and their glass‐transition temperatures, Tgs, measured. In addition, non‐stoichiometric (n‐s)‐IC samples of each were formed with γ‐CD as the host. The Tgs of the un‐threaded, un‐included portions of their chains were observed as a function of their degree of inclusion. In all the cases, these nanostructured PVAc and PMMA samples exhibited Tgs elevated above those of their as‐received and solution‐cast samples. Based on their comparison, several conclusions were reached concerning how their molecular weights, the organization of chains in their coalesced samples, and the degree of constraint experienced by un‐included portions of their chains in (n‐s)‐γ‐CD‐IC samples with different stoichiometries affect their chain mobilities and resultant Tgs. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1041–1050

Comparative studies on effects of silica and titania nanoparticles on crystallization and complex segmental dynamics in poly(dimethylsiloxane)

Effects of in situ synthesized silica and titania nanoparticles, 5 and 20–40 nm in diameter, respectively, on glass transition and segmental dynamics of poly(dimethylsiloxane) networks were studied by employing differential scanning calorimetry, thermally stimulated depolarization currents and broadband dielectric relaxation spectroscopy techniques. Strong interactions between the well dispersed fillers and the polymer suppress crystallinity and affect significantly the evolution of the glass transition in the nanocomposites. Next to the α relaxation associated with the glass transition of the bulk amorphous polymer fraction, two more segmental relaxations were recorded, originating from polymer chains restricted between condensed crystal regions (αc-relaxation) and the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α′ relaxation), respectively. Interactions with the polymer were found to be stronger in the case of titania than silica, leading to an estimated interaction length of around 2 nm for silica and at least double for titania nanocomposites.

Dielectric studies of segmental dynamics in poly(dimethylsiloxane)/titania nanocomposites

Differential scanning calorimetry, thermally stimulated depolarization currents and dielectric relaxation spectroscopy techniques, covering together a broad frequency range of 10 −4 to 10 6 Hz, were employed to investigate the effects of in situ synthesized titania nanoparticles on thermal transitions, segmental dynamics and interfacial interactions in poly(dimethylsiloxane)/titania nanocomposites. Titania particles (TiO 2, 20–40 nm in diameter) were prepared and well dispersed into the polymer network through sol–gel technique, aiming at stable and mechanically reinforced systems. The interactions between polymer and fillers were found to be strong, supressing crystallinity and affecting the temperature development of the glass transition. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction ( α relaxation), from polymer chains restricted between condensed crystal regions ( α c relaxation), and from the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface ( α΄ relaxation). The thickness of the interfacial layer was estimated to be in the range of 3–5 nm. Measurements using different thermal protocols proved very effective in analyzing the origin of each relaxation and the respective effects of filler addition.