V. A. Bershtein

New possibilities for the study of deformation kinetics and relaxations in polymers using a laser interferometer

Abstract An original precise technique for measuring creep rate using a laser interferometer has been developed. This makes it possible to obtain information on creep kinetics at any point on the creep curve over wide ranges of temperatures and deformation values. Some regularities in the deformation kinetics of glassy polymers have been found. The relationships obtained between activation energy and activation volume, and cohesion energy confirm the intermolecular origin of the potential barriers to deformation. The possibilities of studying microplasticity in different materials, including brittle materials, have been demonstrated. On this basis, the Creep Rate Spectra (CRS) method is suggested as a low-frequency and high-resolution procedure for application in relaxation spectrometry.

Heterogeneity of segmental dynamics around tg and nanoscale compositional inhomogeneity in polyurethane/methacrylate interpenetrating networks as estimated by creep rate spectroscopy

Segmental relaxations in a series of poly(propylene oxide)-based polyurethane/butyl methacrylate-triethylene glycol dimethacrylate copolymer interpenetrating networks (IPNs) of various compositions, as compared to those in the pure constituent networks, were studied by an original laser-interferometric Creep Rate Spectroscopy (CRS) technique. The spectra, obtained over the range from 150 K to 360 K, confirmed the CRS superiority in resolution to generally utilized relaxation spectrometry techniques and allowed to characterize in detail the heterogeneity of segmental dynamics within or near the extraordinarily broad glass transition range in these IPNs. Up to eight creep rate peaks have been registered which were shown to be associated just with the predicted kinds of segmental motions, cooperative and partly- or noncooperative; thereby, molecular assignments could be done for these peaks. The relative peak contributions to dynamics around Tg as a function of IPN composition were approximately estimated that provided also some information on nanoscale compositional inhomogeneity of the networks.

Dielectric studies of molecular mobility in hybrid polyimide-poly(dimethylsiloxane) networks

Abstract Dielectric techniques, including broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents, and to a lesser extent differential scanning calorimetry and equilibrium water sorption isotherm measurements, were employed to investigate molecular mobility in relation to morphology in polyimide–poly(dimethylsiloxane) hybrid networks (PI–PDMS). NMR measurements on the same samples had indicated that short PDMS chains with both ends chemically bound to the PI phase, form spherical domains of nanometer size. The local, secondary γ relaxation of PI, the primary α relaxation, associated to the glass transition of PDMS, and the interfacial Maxwell–Wagner–Sillars relaxation, related to the microphase-separated morphology, were studied in detail. The results are discussed in terms of nanoscale confinement of both components and of fixed chain ends of the PDMS component. These factors were found to affect considerably the magnitude of γ and the α relaxations, whereas the time scale of the relaxations remains practically unchanged.

Dielectric studies in homogeneous and heterogeneous polyurethane/polycyanurate interpenetrating polymer networks

Abstract Broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents techniques were employed to investigate molecular mobility in relation to morphology in semi-interpenetrating polymer networks (semi-IPNs) of linear polyurethane and polycyanurate networks (PCN) and in full sequential IPNs of crosslinked polyurethane and the same PCN. The semi-IPNs are found to be homogeneous at length scales larger than about 2 nm, whereas heterogeneity is suggested at shorter length scales. The full IPNs are characterized by microphase separation. The results are discussed in terms of the formation of chemical bonds between the components.

Dielectric studies of chain dynamics in homogeneous semi-interpenetrating polymer networks

Semi-interpenetrating polymer networks (semi-IPNs) were prepared from linear polyurethane (PUR) and polycyanurate (PCN) networks. Wide-angle X-ray scattering measurements showed that the IPNs were amorphous, and differential scanning calorimetry and small-angle X-ray scattering measurements suggested that they were macroscopically homogeneous. Here we report the results of detailed studies of the molecular mobility in IPNs with PUR contents greater than or equal to 50% via broadband dielectric relaxation spectroscopy (10−2–109 Hz, 210–420 K) and thermally stimulated depolarization current techniques (77–320 K). Both techniques gave a single α relaxation in the IPNs, shifting to higher temperatures in isochronal plots with increasing PCN content, and provided measures for the glass-transition temperature (Tg) close to and following the calorimetric Tg. The dielectric response in the IPNs was dominated by PUR. The segmental α relaxation, associated with the glass transition and, to a lesser extent, the local secondary β and γ relaxations were analyzed in detail with respect to the timescale, the shape of the response, and the relaxation strength. The α relaxation became broader with increasing PCN content, the broadening being attributed to concentration fluctuations. Fragility decreased in the IPNs in comparison with PUR, the kinetic free volume at Tg increased, and the relaxation strength of the α relaxation, normalized to the same PUR content, increased. The results are discussed in terms of the formation of chemical bonds between the components, as confirmed by IR, and the reduced packing density of PUR chains in the IPNs.

STRUCTURE AND SEGMENTAL DYNAMICS HETEROGENEITY IN HYBRID POLYCYANURATE-POLYURETHANE NETWORKS

Structure and segmental dynamics (at 140–600 K) in a series of hybrid materials with regularly varied composition, based on polycyanurate (PCN) networks and linear polyurethane (PUR), were studied by infrared spectroscopy, small-angle X-ray scattering, differential scanning calorimetry, and laser-inter-ferometric creep rate spectroscopy (CRS) techniques. Hybridization effect via cyanate/urethane group chemical interaction was evidenced in these systems and that led to formation of a completely homogeneous structure on a scale of > 2 nm, irrespective of material composition. At the same time, combined CRS/differential scanning calorimetry analysis indicated the pronounced nanoscale (≤ 2 nm) dynamic heterogeneity within or below the extraordinarily broad glass transition in these single-phase materials. Discrete CRS analysis of constituent motions, associated with the predicted kinds of polyurethane segmental relaxations (cooperative and noncooperative) and with the dynamics in differently cross-linked PCN network sections, has been performed.

Laser-Interferometric Creep Rate Spectroscopy of Polymers

Laser-interferometric creep rate meter (LICRM) and creep rate spectroscopy (CRS), as an original high-resolution method for discrete relaxation spectrometry and thermal analysis, were developed in the authors’ Materials Dynamics Laboratory at Ioffe Physical-Technical Institute of the Russian Academy of Sciences (Saint-Petersburg). In the last few decades they have been successfully applied to solving various problems of polymer physics and materials science, especially being combined with DSC, structural, and other techniques. CRS involves measuring ultra-precisely a creep rate at small tensile or compressive stress, typically much lower than the yield stress, as a function of temperature, over the range from 100 to 800 K. LICRM setup allows one to register precisely creep rates on the basis of deformation increment of 150–300 nm. The survey describes this method and summarizes the results of numerous studies performed with the LICRM setup and CRS technique for different bulk polymeric materials, films, or thin fibers. This approach provided new experimental possibilities superior in resolution and sensitivity compared to the conventional relaxation spectrometry techniques. Among such possibilities are discrete analysis of dynamics; creep on submicro-, micro- and meso-scales; revealing relations between stepwise microplasticity and morphology; kinetic information on creep at any temperature and deformation; polymer dynamics at interfaces; analysis of microplasticity, relaxations, and phase transitions in brittle materials; using creep rate spectra for non-destructive prediction of temperature anomalies in mechanical behavior of materials, etc. Considerable attention has been paid to combined CRS/DSC analysis of the peculiarities of segmental dynamics, nanoscale dynamic, and compositional heterogeneity in different kinds of complex polymer systems and nanocomposites.

Peculiarities of the segmental dynamics in amorphous miscible polymer blends as a consequence of the common nature of α and β relaxations

Abstract Amorphous polystyrene-poly(α-methyl styrene) (PS/PMS) and polysty-rene-poly(vinyl methyl ether) (PS/PVME) miscible blends have been studied in detail by differential scanning calorimetry. A number of segmental dynamics characteristics, including the temperature dependence of motional activation energies over the range from the β relaxation region to the glass transition, the scale of motional unit events, and the degree of intermolecular cooperativity for segmental motion, have been determined. A number of peculiarities of Tg manifestation as well as two types of segmental dynamics heterogeneity in the glass transition have been observed. They could be explained and predicted quantitatively, proceeding from the concept of the common segmental nature of the α and β relaxations in flexible chain polymers, as a consequence of a partial or complete breakdown of intermolecular cooperation (“individualizing”) or segmental motions. A linear correlation between the segmental dynamics activation energy an...

The impact of ultra-low amounts of amino-modified MMT on dynamics and properties of densely cross-linked cyanate ester resins

Thermostable nanocomposites based on densely cross-linked cyanate ester resins (CER), derived from bisphenol E and doped by 0.01 to 5 wt. % amino-functionalized 2D montmorillonite (MMT) nanoparticles, were synthesized and characterized using Fourier transform infrared (FTIR), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDXS), wide-angle X-ray diffraction (WAXD), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), far-infrared (Far-IR), and creep rate spectroscopy (CRS) techniques. It was revealed that ultra-low additives, e.g., 0.025 to 0.1 wt. %, of amino-MMT nanolayers covalently embedded into СER network exerted an anomalously large impact on its dynamics and properties resulting, in particular, in some suppression of dynamics, increasing the onset of glass transition temperature by 30° to 40° and twofold rise of modulus in temperature range from 20°C to 200°C. Contrarily, the effects became negligibly small or even negative at increased amino-MMT contents, especially at 2 and 5 wt. %. That could be explained by TEM/EDXS data displaying predominance of individual amino-MMT nanolayers and their thin (2 to 3 nanolayers) stacks over more thick tactoids (5 to 10 nanolayers) and the large amino-MMT aggregates (100 to 500 nm in thickness) reversing the composite structure produced with increasing of amino-MMT content within CER matrix. The revealed effect of ultra-low amino-MMT content testifies in favor of the idea about the extraordinarily enhanced long-range action of the ‘constrained dynamics’ effect in the case of densely cross-linked polymer networks.PACS82.35.Np Nanoparticles in polymers; 81.05.Qk Reinforced polymers and polymer-based composites; 81.07.Pr Organic-inorganic hybrid nanostructures

Nanostructure and molecular dynamics in rodlike polyimide/flexible-chain polyimide molecular composites

A complete study of structure and molecular dynamics was performed for a series of rigid rodlike polyimide (PIR)/flexible-chain polyimide (PIF) molecular composites, with 0, 20, 40, 50, 60, 80, and 100 wt% of the rigid component. The PIR and PIF were synthesized from 3,3′4,4′-biphenyl-tetracarboxylic dianhydride and 1,4-phenylenediamine or 4,4′-oxydianiline, respectively. Small-angle x-ray scattering, differential scanning calorimetry, dynamic mechanical analysis, laser-interferometric creep rate spectroscopy, dielectric relaxation spectroscopy, and thermally stimulated depolarization currents techniques were used for overall characterization of nanostructure, glass transition, and sub-T g relaxations. The experiments were carried out, on the whole, at temperatures ranging from 100 to 660K and frequencies ranging from 10−2 to 106 Hz. All the experiments indicated pronounced deviations from additivity in both nanostructure and dynamics of the molecular composites. Mixing of the PIR and PIF components led, in particular, to a smaller nanostructure, down to formation of the nanoscale-homogeneous 20PIR/80PIF composite. Changes of the glass transition characteristics in two opposite directions and the arising of large dynamic heterogeneity around T g were observed. The results were readily treated in terms of nanostructural changes, loosening of molecular packing due to confinement of PIF chains between PIR chains as the “rigid walls,” and the constraining influence of the latter on segmental motion in PIF chains adjoining the “rigid walls.”