Dielectric properties and molecular mobility of organic/inorganic polymer composites

Microstructure-dielectric properties relationship and molecular mobility of organic/inorganic polymer composites (OIPCs), consisting of polyurethane (PU) and sodium silicate (NaSi), were investigated in this work. Broadband dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) techniques were employed. Our interest was focused on the study of the glass transition mechanism and conductivity relaxation. The influence of the molecular weight of PU and inorganic phase content on the dielectric properties of the composites was of particular interest. Glass transition temperature shifts to higher temperatures with the addition of NaSi. The overall molecular mobility was found to increase in the composites, compared to the pure PU matrix. The results are more intense for the composites based on the PU with low molecular weight.     

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at T_g – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1957–1967     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

Structural aspects of deformation of amorphous polymers

Experimental and theoretical data on the inelastic deformation of amorphous glassy polymers were analyzed. The decisive role of direct structural methods in determination of the deformation mechanism of glassy polymers was established. A new mechanism of deformation and thermally stimulated recovery of strained glassy polymers was considered on the basis of structural data analysis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–6, January, 2005.     

Dynamic Mechanical Characterization of Molecular Motion in a Wide Temperature Range of Various Polymers Containing Propylene Units

The temperature transitions for a series of flexible polymers containing propylene units were studied by dynamic mechanical spectroscopy. It was found that the gradual activation of the local motion of different structural units involved in polymers occurs with increasing temperature. Initially, the rotation of the side groups, such as side methyl groups, is activated and on further heating the main chain structural units show their local motions. It is important that the temperature interval of the appearance of the local motion of each structural unit is almost independent of the presence of other structural units. Accordingly, the polymers investigated can be divided into two groups. The activation of the local motion of the most rigid structural unit determines the glass transition in the first group of polymers. The glass transition of the polymers of the second group takes place at a higher temperature which depends on the content of side methyl groups and the intermolecular interaction. The increased influence of both these factors on the cooperative amorphous motion of polymers of the second group leads to their increased T_g values.This revised version was published online in November 2005 with corrections to the Cover Date.     

Molecular mobility of nitroxide spin probes in glassy polymers. Quasi‐libration model

ESR spectra of three spin probes with different molecular volumes: 2,2,6,6‐tetramethyl‐4‐oxopiperidine‐1‐oxyl, di‐p‐anisylnitroxide, and nitroxide derivative of fullerene in glassy polystyrene, polyvinyl trimethylsilane, and Teflon AF‐2400 were calculated numerically within the model of quasi‐libration motions. Temperature ranges, where the model is capable to reproduce spectra within experimental errors, were defined. It was found that simulation of X‐band ESR spectra allows to determine quasi‐libration amplitudes around molecular axes X and Y with accuracy ～ 3° and around Z axis with accuracy ～ 15–20°. A shape of distribution of quasi‐libration amplitudes was also determined qualitatively by ESR spectra simulations. It was established that the average amplitude of quasi‐libration motion depends on the free volume of each polymer and geometrical molecular volume of a spin probe. Quasi‐libration amplitudes increase as the temperature increases, and reach the value of 40 degrees. We found that upon further temperature increase, quasi‐libration model becomes inapplicable for quantitative numerical spectra simulation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 107–120, 2009     

{\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs20 Calorimetric study of transition phenomena in molecular solids \par }

Summary {\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs20 Principle and technical development of low temperature calorimetry are described. Typical experimental results obtained by our group at Osaka University over the four decades are given. These include phase transitions in equilibrium crystals and glass transitions in non-equilibrium frozen-in disordered solids including crystals. It can be concluded that the glass transitions observed exclusively in liquids so far are just one example of transitions that must be of wide occurrence in solids arising from freezing of relevant degrees of freedom. Interplay between the phase and glass transitions in crystals is discussed in relation to useful dopant that may accelerate some molecular motions that had failed to maintain equilibrium at low temperatures. \par }     

Deformation induced evolution of mobility in PMMA

The stress relaxation response in the glassy state just below T_g was measured for poly(methylmethacrylate) following application of constant strain rate uniaxial tensile deformation at various locations on the stress–strain curve, including the yield and post‐yield region. The macroscopic mobility was determined from analysis of the relaxation response. Up to a factor of 3 decrease in relaxation time was observed with the fastest relaxation occurring in the post‐yield softening region. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Computer simulation study of bulk atactic polystyrene in the vicinity of the glass transition

Molecular dynamics (MD) simulations of bulk atactic polystyrene have been performed in a temperature range from 100 K to 650 K at atmospheric pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers. The long-time asymptotic slope of these dependencies is 0.54 at T>T_g, showing Rouse behavior. Cross-over from motion in the cage to Rouse like dynamics has been studied at T>T_g with a characteristic crossover time follows a power law behavior as a function of T, as predicted by mode-coupling theory (MCT). Local orientational mobility has been studied via the orientational autocorrelation functions, ACFs, (Legendre polynomials of the first and second, order) of both the main-chain and side-group bonds. The relaxation times of the orientational α-relaxation follow the same power law (γ∼2.9) as the characteristic translational diffusion time. Below T>T_g both types of dynamics are described by the same activated law. The ACFs time-distribution functions reveal the existence of activated local rearrangements already above T>T_g.     

Secondary molecular mobility in amorphous ethyl cellulose: Aging effects and degree of co‐operativity

The secondary relaxations in amorphous ethyl cellulose are studied using thermally stimulated depolarization currents (TSDCs) in the temperature region from 165 °C (108 K) up to 145 °C (418 K). The influence of aging on the current peaks of the secondary relaxation is discussed, and it is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. A particular attention is focused on the discussion of the degree of co‐operativity of the motional modes of the secondary relaxation. The conclusion to be driven from the obtained TSDC results is that the aging‐independent motional modes, as well as those that are aging dependent, do not show any appreciable co‐operative character. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 820–829, 2009     

Thermal history and enthalpy relaxation of an interpenetrating network polymer with exceptionally broad relaxation time distribution

Differential scanning calorimetry (DSC) of an interpenetrating network polymer of composition 25% polyurethane–75% poly(methyl methacrylate) shows a slowly increasing heat capacity, instead of the usual glass transition endotherm, whose onset temperature is not clearly discernible. On aging of the polymer at several temperatures between 193 and 333 K, an endothermic peak is observed whose onset is in the vicinity of the respective temperature of aging. The area under these peaks increases with increasing aging time at a fixed temperature. The effects are attributed to a very broad distribution of relaxation times, which may be represented by either a sum of discrete structural relaxation times of local network arrangement or by a nonexponential relaxation function which is equivalent to a distribution of relaxation times. In either view the vitrified state of the polymer can be envisaged as containing local structures whose own T_gs extend over a wide range of temperature. Aging decreases the enthalpy and produces an endothermic region which resembles an increase in C_p on heating because of relaxation of that local structure. The interpretation is supported by simulation of DSC scans in which the distribution of relaxation times is assumed to be exceptionally broad and in which aging introduced at several temperatures over a wide range produces endothermic effects (or regions of DSC scans) qualitatively similar to those observed for the interpenetrating network polymer. © 1994 John Wiley & Sons, Inc.     

Temperature- and hydration-dependence of molecular mobility in seeds

Broad-band ac dielectric relaxation spectroscopy (DRS) and various techniques of thermally stimulated currents (TSC) have been used to investigate molecular mobility in cereal and legume seeds, over wide ranges of water content and temperature. We focused our interest on the detailed study of the interactions between water and seed constituents. The results are quantitatively discussed, using various concepts dictated by the experimental techniques employed and in relation to the protein and carbohydrate contents of the seeds. In addition, the glass transition in the seeds, freezing and melting of water, and the protonic conduction process have been studied in some detail.Financial support through the Greek Ministry of Industry, Energy and Technology (Secretariat of Research and Technology), the Czech Ministry of Education, Youth and Sport (Dept. of International Scientific and Technological Cooperation), and the Empirikos Foundation is gratefully acknowledged.     

Dielectric studies of tetraaryl and triaryl polycarbonates and comparisons with bisphenol A‐polycarbonate

The relative permittivity, loss, and breakdown strength are reported for a commercial sample of bisphenol A‐polycarbonate (comm‐BPA‐PC) and a purified sample of the same polymer (rp‐BPA‐PC) as well as for two new polycarbonates having low molecular cross‐sectional areas, namely a copolymer of tetraaryl polycarbonate and BPA‐PC (TABPA‐BPA‐PC) and a triaryl polycarbonate homopolymer (TriBPA‐PC). The glass transition temperatures of the new polymers are higher than the T_g of BPA‐PC (187 and 191 °C vs. 148 °C). Relative permittivity and loss measurements were carried out from 10 to 10⁵ Hz over a wide temperature range, and results for the α‐ and γ‐relaxation regions are discussed in detail. For the α‐relaxation, the isochronal peak position, T_α, scales approximately with T_g. On the other hand, the peak temperature for the γ‐relaxation is approximately constant, independent of T_g. Also, in contrast to what is observed for α, γ exhibits a strong increase in peak height as temperature/frequency increases and a significant difference is found between Arrhenius plots determined from isochronal and isothermal data analyses. Next, the γ‐relaxation region for comm‐BPA‐PC and associated activation parameters show strong history/purity effects. The activation parameters also depend on the method of data analysis. The results shed light on discrepancies that exist in the literature for BPA‐PC. The shapes of the γ loss peaks and hence glassy‐state motions for all the polymers are very similar. However, the intensities of the TriBPA‐PC and TABPA‐BPA‐PC γ peaks are reduced by an amount that closely matches the reduced volume fraction of carbonate units in the two new polymers. Finally, for comm‐BPA‐PC, the breakdown strength is strongly affected by sample history and this is assumed to be related to volatile components in the material. It is found that the breakdown strengths for TriBPA‐PC and TABPA‐BPA‐PC are relatively close to that for rp‐BPA‐PC with the value for TriBPA‐PC being slightly larger than that for rp‐BPA‐PC or the value usually reported for typical capacitor grade polycarbonate. Finally, it is shown that the real part of the relative permittivity remains relatively constant from low temperatures to T_g. Consequently, based on the dielectric properties, TriBPA‐PC and TABPA‐BPA‐PC should be usable in capacitors to at least 50 °C higher than BPA‐PC. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Thermally Stimulated Current Spectroscopy of Amorphous and Semi-Crystalline Polymers

Thermally Stimulated Current (TSC) spectrometry has been applied to the characterization of polymeric materials. The study of a series of amorphous polymers having different physical structures has shown that the compensation parameters are independent of physical aging; contrarily, the activation enthalpy distribution reflects the evolution of the heterogeneity of the amorphous phase. In copolymers, TSC allows us to identify segregated amorphous phases. In semi-crystalline polymers, with semi-rigid chains, we have shown the existence of an amorphous crystalline interphase characterized by a plateau in the temperature distribution of activation enthalpy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physical aging of epoxy polymers and their composites

Exposure to extended periods of sub‐T_g temperatures causes physical changes in the molecular structure of epoxy resins and epoxy‐based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamical, and physical properties are affected in ways that can compromise the reliability of epoxy‐based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk‐level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability is reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/copolymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

The friction coefficient on polycarbonate as a function of the contact pressure and nanoscale roughness

During dry friction the rheological behavior at the interface between a polymer surface and a rigid indenter is linked to the local pressure and the roughness of the surface of the indenter. This study on polycarbonate reports experimental results obtained at ambient temperature using rigid, spherical glass indenters of various radii. In a first step, a plot of the experimental friction coefficient μ versus the mean contact pressure p_mean showed that below the yield stress (p_mean < 100 MPa) the intrinsic friction coefficient follows a master curve for smooth indenters. In a second step, roughened indenters were prepared by chemical etching, which allowed the monitoring of nanoscale roughness parameters. From 5.5 to 140 nm Rrms, the friction coefficient μ progressively fell to a plastic‐like constant value, indicating that the nanoroughness mediates the friction. These results form the basis for a study of the rheology of confined polymer layers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Influence of molecular architecture on fast and segmental dynamics and the glass transition in polybutadiene

Changes in the fast dynamics of polybutadiene (PB) with molecular weight and molecular architecture have been investigated by light and neutron scattering spectroscopy. Differences observed in the fast dynamics of various molecules correlate with differences seen in the value of the glass‐transition temperature (T_g). The segmental and fast dynamics as well as the value of T_g are dependent on the total molecular weight of the molecule but independent of its architecture. In other words, the dynamics of PB depend on the number of segments in the molecule but do not show a significant dependence on how the segments are connected (molecular topology), even for arm molecular weights commensurate with the entanglement molecular weight. Literature data for the T_g's of highly branched, phenolic‐terminated dendritic poly(benzyl ethers) of various core structures exhibit the same trend. There is no explanation for why the segmental motion appears to be sensitive to the total molecular weight of the molecule but is independent of its architecture. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2431–2439, 2002     

Relationship between the α‐ and β‐relaxation processes in amorphous polymers: Insight from atomistic molecular dynamics simulations of 1,4‐polybutadiene melts and blends

Amorphous polymers exhibit a primary (glass, or α‐) relaxation process and a low‐temperature relaxation process associated with polymer backbone motion usually referred to as the β‐relaxation process. The latter process can be observed below the glass transition temperature of the polymer and usually merges with the α‐relaxation process at temperatures somewhat above the glass transition temperature. While it is widely held that both the α‐relaxation and β‐relaxation processes are engendered by localized (segmental) motions of the polymer backbone, and that there is a strong mechanistic connection between them, the molecular mechanisms of the α‐relaxation and β‐relaxation processes in amorphous polymers are not well understood. Recently, atomistic molecular dynamics simulations of melts and blends of 1,4‐polybutadiene have provided insight into the relationship between the α‐ and β‐relaxation processes in glass‐forming polymers and an improved understanding of their molecular origins. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 627–643, 2007