Linear Amorphous Polymers Research Articles

Sorption–Desorption of Water Vapor by Linear Amorphous Polymers

Sorption–Desorption of Water Vapor by Linear Amorphous Polymers

Thermal Conductivity of Semicrystalline Polymer Networks: Crystallinity or Cross-Linking?

Understanding the heat flow in polymers is at the onset of many developments in designing advanced functional materials. Here, however, amorphous linear polymers usually exhibit a very low thermal conductivity κ, often hindering their broad applications. In this context, two common routes to increase κ are via semicrystallinity and cross-linking. It can therefore be inferred that the combination of these two effects may result in a further increase of κ with respect to the systems where only one of these two effects is important. Using molecular dynamics simulations, we investigate κ in semicrystalline polymer networks. Contrary to a priori understanding, we show that a combination of cross-linking and crystallinity does not always increase κ. Instead, a delicate competition between the lattice periodicity, the cross-linker types, and the bond density dictates the tunability of κ in these complex macromolecular systems. These results are also compared with the existing experiments.

In-situ generation of high performance thiol-conjugated solid polymer electrolytes via reliable thiol-acrylate click chemistry

A kind of high performance thiol-conjugated solid polymer electrolytes is in situ generated via a facile thiol-acrylate Michael addition reaction between ethylene glycol dimethacrylate (EGDMA) and 3,6-dioxa-1,8-octane dithiol (DODT) without UV irradiation or thermal initiation. The resulting amorphous linear polymer P(EGDMA-DODT) has a high average molecular weight of 47,000 Da with a low glass transition temperature of −50 °C. The in-situ generated P(EGDMA-DODT) solid polymer electrolyte exhibits room temperature ionic conductivity up to 2.7 × 10 −5 S cm−1 with a high lithium ion transference number above 0.40 and electrochemical stability window up to 4.3 V vs. Li/Li+. Advantages of longer C–S bond length (1.81 Å), lower electronegativity (2.58) and moderate coordination facilitate the segment motion and contribute to higher room temperature ionic conductivity. It is demonstrated that the all-solid-state LiFePO4/Li cells using the in-situ generated P(EGDMA-DODT) electrolytes delivers considerable specific capacities, rate capability and excellent cyclic performance at room temperature. These results demonstrate that this thiol-acrylate click chemistry without UV irradiation or thermal initiation is a facile and intriguing strategy for in-situ preparing high performance solid polymer electrolytes.

Low-Temperature Features of the Micromechanical Properties of Polystyrene

The microindentation of thin polystyrene plates, which is an amorphous linear thermoplastic polymer with a glass transition temperature Tg ≈ 373 K, was carried out in the 77–295 K temperature range. A decrease in the temperature from 295 K to 140 K generated a two-fold linear increase in microhardness, which is in good agreement with the temperature dependence of the forced elasticity limit of polystyrene measured by other authors during uniaxial compression deformation. At temperatures of T = 77–140 K the local deformation of polystyrene was reversible: during heating to room temperature the impressions on the sample surface disappeared completely. Partial deformation reversibility was observed upon indentation in the temperature range of 140–190 K, but at indentation temperatures T > 190 K the impressions remained. The microstrain reversibility of polystyrene in nitrogen at, and close to, 77 K is due to the effect of the adsorption-active medium on the formation of crazes (fibrillar-porous nanostructures), the glass transition temperature of which is tens of degrees below that of bulk polymer.

PolyCOFs: A New Class of Freestanding Responsive Covalent Organic Framework Membranes with High Mechanical Performance.

Traditional covalent organic frameworks (COFs) are prepared via polymerization based on small molecular monomers. However, the employment of polymers as building blocks to construct COFs has not been reported yet. Herein, we create a new concept of polymer covalent organic frameworks (polyCOFs) formed by linear polymers as structural building blocks, which inherit the merits from both COFs and linear polymers. PolyCOFs represent a new category of porous COF materials that demonstrate good crystallinity and high stability. More importantly, benefiting from the flexibility and processability of a linear polymer, polyCOFs can spontaneously form defect-free, flexible, and freestanding membranes that exhibit excellent mechanical properties and undergo reversible mechanical transformation upon exposure to various organic vapors. For the first time, we demonstrated that polyCOF membranes can be used as artificial muscles to perform various complicated motions (e.g., lifting objects, doing “sit-ups”) triggered by vapors. This study bridges the gap between one-dimensional amorphous linear polymers and crystalline polymer frameworks and paves a new avenue to prepare stimuli-responsive actuators using porous COF materials.

The temperature dependence of the coefficients of thermal expansion of phenolic resin

Cross‐linked polymers, such as phenolic resin and its composites are widely used in industrial practice, but the absence of clear understanding on their thermal expansion behaviors in glassy state limits their applications, especially in the occasions where dimensional accuracy are addressed. The coefficients of thermal expansion (CTEs) of cured phenolic resin and inorganic particles filled phenolic resin at glassy state at different temperatures are analyzed in this article. The results show that the CTEs of glassy cross‐linked polymers change with temperature, and exhibit a strong dependence on the temperature. By modifying the state equation of linear amorphous polymer, the equation that can be used to quantitatively describe the temperature dependence of CTEs of cross‐linked polymers is established. By analyzing the temperature dependence of CTE, an equation is proposed to predict the CTEs of filled phenolic resins with different filler loadings at a specific temperature. The results of this study have contributed to a better understanding of the thermal expansion behaviors of glassy cross‐linked polymers, and are of great significance in industrial practice. POLYM. COMPOS., 146–152, 2016. © 2014 Society of Plastics Engineers

Dynamic Mechanical and Thermal Kinetics of Polyvinyl Acetate Type Emulsions from Vinyl Acetate, N‐hydroxymethyl Acrylamide, and Montmorillonite

ABSTRACTIn this study, we used vinyl acetate (VAc), N‐hydroxymethyl acrylamide (NMA), and montmorillonite (MMT) to prepare polyvinyl acetate (PVAc), PVAc–NMA, PVAc–MMT, and PVAc–NMA–MMT. Dynamic mechanical and thermogravimetric analysis revealed their glass transition, cold crystallization, and pyrolysis kinetics. PVAc–NMA, PVAc–MMT, and PVAc–NMA–MMT can be applied in harsh or low temperature conditions and surroundings. They are homogeneous amorphous linear polymers and all have cold crystallization phenomena. NMA and MMT did not have any significant effect on pyrolysis temperatures, but delayed the thermal degradation processes. The glass transition, cold crystallization, and pyrolysis kinetics were analyzed using Agrawal integral equation. © 2013 Wiley Periodicals, Inc. Adv Polym Technol 2014, 33, 21393; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21393

1,1,2,2-Tetramethyl-3-trimethylsilyl-1,2-disilacyclobutane: synthesis and spontaneous polymerization

Gas-phase dehalogenation of 1,2-bis[chloro(dimethyl)silyl]-2-trimethylsilylethane with alkali metals gave 1,1,2,2-tetramethyl-3-trimethylsilyl-1,2-disilacyclobutane (3). Its spontaneous ring-opening polymerization at room temperature afforded an amorphous linear polymer with Tg = −8.9°C, Mw = 3.47·105–3.85·105, and Mw/Mn = 2.43—2.86. According to spectroscopic data (IR and 1H, 13C, and 29Si NMR), the backbone of the polymer consists of alternating monomer units joined in the “head-to-tail” ([Me2SiCH(SiMe3)CH2SiMe2SiMe2CH-(SiMe3)CH2SiMe2]) and “head-to-head” ways ([Me2SiCH2CH(SiMe3)SiMe2SiMe2CH-(SiMe3)CH2SiMe2]).

Analysis of local properties during a scratch test on a polymeric surface using molecular dynamics simulations

This work demonstrates a possible route to connect a particle (chain) based understanding with continuum mechanical questions about contact mechanics. The bond orientation, chain conformation and stress field of a polymer film were analyzed during scratch tests (tangential contact) using a molecular dynamics (MD) simulation approach. Scratch tests with a conical tip at constant scratching velocity were simulated on linear amorphous polymer surfaces at various temperatures and roughnesses of the tip and for various interactions between the tip and the particles of the polymer chains. The second Legendre polynomial (computed for small domains around the tip) gave the bond orientation inside the polymer film during sliding of the tip. The gyration tensor (layer-resolved in the direction of the polymer film thickness) provided information about the conformation of the polymer chains. These results allowed us to argue in favor of Briscoe's hypothesis (thin film sheared vs. "bulk" compressive behavior) concerning the friction properties of the polymer surfaces. Finally, the first stress measurements of the virial stress tensor (in sub-boxes placed in the MD cell) revealed a complex combination between compressive hydrostatic pressure and shear stress, which may be interpreted as a complex sheared domain at the interface.

A toolbox for parameter‐free predictions of solid‐state properties of monodisperse glassy polymers with frozen‐in molecular orientation

AbstractThis study presents a toolbox for the prediction of birefringence and craze initiation stress in oriented monodisperse linear amorphous polymers. The toolbox is assembled from a previously proposed melt–solid constitutive model that provides the necessary residual stress components required for predictions of birefringence and craze initiation stress. The Likhtman–McLeish theory for linear rheology of entangled polymers is used to generate the low reduced frequency part of the linear viscoelastic spectrum, the only molar mass‐dependent input parameter. All other parameters are obtained by experiment or from literature and can be considered material constants. Toolbox predictions are compared to new experimental data on two grades of linear monodisperse polystyrene (PS) of known molar mass but unknown rheology and to literature data. The toolbox is able to account for the role of molar mass on birefringence and craze initiation stress of PS subjected to supraentanglement orientation processes. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Autoadhesion of Glassy Polymers

AbstractSummary: Thick bulk films of linear amorphous polymers with different chain architecture and molecular weight were brought into contact with themselves in a lap‐shear joint geometry at bonding temperatures (T) below the glass transition temperatures of their bulk (T), at a small contact pressure, in order to form autoadhesive joints. As‐bonded joints were shear‐fractured in tension at ambient temperature, and their lap‐shear strength was measured as a function of T, bonding time and molecular weight. The kinetics of the process of the development of the lap‐shear strength at T < T was investigated, and the molecular mechanisms governing this process were discussed. The quasi‐equilibrium surface glass transition temperatures of the investigated polymers were estimated and compared with the corresponding values of T.

Structure and dynamic mechanical kinetics of polyvinyl acetate–montmorillonite–dioctadecyl dimethyl ammonium bromide

To study the dynamic mechanical kinetics of exfoliated nanocomposites using dynamic mechanical analysis, polyvinyl acetate (PVAc)–montmorillonite (MMT)–dioctadecyl dimethyl ammonium bromide (DOAB) was synthesized through five different processes. Different synthesis processes and MMT-DOAB had little effect on the structure of PVAc-MMT-DOAB. MMT-DOAB lowered the glass transition temperature of PVAc, hence improving its low temperature resistance greatly. PVAc and PVAc-MMT-DOAB were homogeneous amorphous linear polymers and they appeared cold crystallized, and their glass transition and cold crystallization kinetics were analyzed using Agrawal integral equation.

Publisher's Note: Mechanical behavior of linear amorphous polymers: Comparison between molecular dynamics and finite-element simulations [Phys. Rev. E85, 021808 (2012)

Publisher's Note: Mechanical behavior of linear amorphous polymers: Comparison between molecular dynamics and finite-element simulations [Phys. Rev. E<b>85</b>, 021808 (2012)

Electrophysical properties of amorphous polymer films exposed to radiation

The influence of radiation on the dielectric dispersion and the conductivity of the most wide-spread linear amorphous polymers in industry is studied, i.e., polyvinyl chloride, poly(methyl methacrylate), and polystyrene. The initial samples were exposed to radiation at T = 25°C for 45 days; as the source of radiation, 226Ra was used. The nuclei of 226Ra emit alpha rays with an energy of 4.777 MeV and gamma quanta with an energy of 0.188 MeV. Our study confirms that the changes in the real parts of the complex dielectric permittivity and electrical conductivity depend on the type of polymer and the dose of irradiation. The most significant effect of radiation is observed in the case of polystyrene.

Mechanical behavior of linear amorphous polymers: Comparison between molecular dynamics and finite-element simulations

This paper studies the rheology of weakly entangled polymer melts and films in the glassy domain and near the rubbery domain using two different methods: molecular dynamics (MD) and finite element (FE) simulations. In a first step, the uniaxial mechanical behavior of a bulk polymer sample is studied by means of particle-based MD simulations. The results are in good agreement with experimental data, and mechanical properties may be computed from the simulations. This uniaxial mechanical behavior is then implemented in FE simulations using an elasto-viscoelasto-viscoplastic constitutive law in a continuum mechanics (CM) approach. In a second step, the mechanical response of a polymer film during an indentation test is modeled with the MD method and with the FE simulations using the same constitutive law. Good agreement is found between the MD and CM results. This work provides evidence in favor of using MD simulations to investigate the local physics of contact mechanics, since the volume elements studied are representative and thus contain enough information about the microstructure of the polymer model, while surface phenomena (adhesion and surface tension) are naturally included in the MD approach.

Using Agrawal integral equation to study the glass transition and cold crystallization kinetics of exfoliated nano-composites synthesized by vinyl acetate and organic montmorillonite

To study the glass transition and cold crystallization kinetics, the exfoliated nano-composites synthesized by vinyl acetate (VAc) and organic montmorillonite (OMMT) through different processes were investigated by dynamic mechanical analysis. Different synthesis processes and OMMT had significant effect on their dynamic mechanical properties. They brought combined effect on the modulus, loss tangent, and glass transition temperature ( Tg). OMMT could lower PVAc’s Tg, so improved PVAc’s low temperature resistance. PVAc–OMMT was a homogeneous amorphous linear polymer and it appeared the cold crystallization phenomenon. The glass transition and cold crystallization kinetics were first analyzed using the Agrawal integral equation. The kinetic parameters, including the kinetics mechanism function, the linear fit equation, the linearly dependent coefficient, the activation energy, the frequency factor, the mechanism, the reaction order, and the kinetic compensation effect equation, were obtained.

Interdiffusion of polymers with glassy bulk

Mechanical properties developing at the contact zone of two samples of linear amorphous polymers with vitrified bulk have been employed to analyze the process of segmental diffusion of reptating chains across polymer–polymer interfaces. The diffusion coefficients (D) of this process have been estimated for polymers with different chain architecture and investigated with respect to healing temperature (T). It has been shown that logD ∼ 1/T at \( T < T_{\text{g}}^{\text{bulk}} \) (\( T_{\text{g}}^{\text{bulk}} \) is the glass transition temperature of the polymer bulk), which implies that the process of segmental diffusion under healing conditions investigated is a thermally activated Arrhenius-like process describing as D = D0 exp[−Ea/(RT)], where D0 and Ea are the activation energy and the pre-exponential factor, respectively. It has been found that logD0 ∼ Ea, which points to the validity of the so-called kinetic compensation effect for the interdiffusion process at the contact zones of polymers with glassy bulk when an increase in Ea is “compensated” by the corresponding increase in D0.

Molecular dynamics simulations of the scratch test on linear amorphous polymer surfaces: A study of the local friction coefficient

This work presents a mechanical analysis of tangential contact using molecular dynamics simulations. Scratch tests with a conical tip on amorphous polymer surfaces were simulated at various temperatures, scratching velocities, roughnesses of the tip and with various interactions between the tip and the monomers. The local friction coefficient for the different contact conditions (temperature, tip roughness and scratching velocity) was calculated by studying the apparent friction and the contact asymmetry. The results are in good agreement with experimental data from tests on classical polymer surfaces on a microscopic scale.

Development of an Exfoliated Nanocomposite Prepared by Vinyl Acetate and Organic Montmorillonite: Structure, Dynamic Mechanical Properties and Thermal Degradation

Using in-situ emulsion polymerization, the exfoliated nanocomposites were prepared from vinyl acetate (VAc) and organic montmorillonite (OMMT) through five different experimental conditions. The different experimental conditions and OMMT had little effect on the structure of PVAc–OMMT, but significant effect on the dynamic mechanical properties. They produced a combined effect on the modulus, loss tangent and glass transition temperature (T g). OMMT could lower T g of PVAc, and improved PVAc's low temperature resistance. PVAc and PVAc–OMMT all were homogeneous amorphous linear polymers and they all displayed cold crystallization. The different experimental conditions and OMMT had some effects on the thermal degradation. The thermal degradation process was found to consist of eight phases, and those of PVAc and PVAc–OMMT were similar. OMMT had no obvious effect on the thermal degradation temperature, but mainly delayed the thermal degradation process.

Synthesis of 1-ethyl-l,2,2-trimethyl-l,2-disilacyclobutane and its spontaneous polymerization and copolymerization with 1,1,2,2 -tetramethyl-1,2 -disilacyclobutane

Dehalogenation of 1-[(chloro)(ethyl)(methyl)silyl]-2-[chloro(climethyl)silyl]ethane with the alkali metal vapors gave l-ethyl-l,2,2-trimethyl-1,2-disilacyclobutane. Its spontaneous ring-opening polymerization at room temperature afforded an amorphous linear polymer with T g = −75 °C, M w = 454 200–512 800, and M w/Mn = 1.87–2.82. According to NMR data, the backbone of the polymer consists of alternating monomeric units linked in the “head-to-tail” (SiMe 2CH2CH2SiMeEtSiMe2CH2CH2SiMeEt) and “head-to-head” ways (SiMeEtCH2CH2SiMe2SiMe2CH2CH2SiMeEt) in a ratio of 1: 0.96. Spontaneous copolymerization of the above disilacyclobutane with 1,1,2,2-tetramethyl-l,2-disilacyclobutane gave partially crystalline copolymers with different glass transition and melting temperatures, depending on the ratio of the components in the reaction mixture. The compositions of the copolymers were examined by 1H, 13C, and 29Si NMR spectroscopy.