P. N. Yakushev

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.

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.

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.”

Hybrid Polyurethane-Poly(2-hydroxyethyl methacrylate) Semi-IPN–Silica Nanocomposites: Interfacial Interactions and Glass Transition Dynamics

Polyurethane-poly(2-hydroxyethyl methacrylate) semi-IPN-silica nanocomposites with low content (0.25 and 3 wt%) of differently functionalized 3-D fumed silica nanoparticles were studied using a combined AFM/DSC/CRS approach over the −100 to 160°C range. The pronounced heterogeneity of the PHEMA and PU glass transitions’ dynamics and the effects of considerable suppression of dynamics and increasing elastic properties by silica additives were shown. It was caused by formation of peculiarly cross-linked structures due to “double hybridization,” in particular via selective covalent bonding of the silica surface, functionalized by ‒OH, ‒NH2 or ‒CH˭CH2 groups, with the matrix constituents. The silica dispersion remained unchanged in these nanocomposites; therefore the relationships between interfacial interactions and dynamics/modulus behavior could be followed.

Polycyanurate -Organically Modified Montmorillonite Nanocomposites: Structure -Dynamics- Properties Relationships

Polycyanurate‐modified montmorrilonite (PCN‐MMT) nanocomposites were synthesized by polymerization of dicyanate ester of bisphenol A in the presence of MMT dispersed by ultrasound. Techniques of IR spectroscopy, WAXD, and TEM were applied to study polymerization kinetics and structure of the nanocomposites prepared, whereas their dynamics and thermal/mechanical properties over the −30 to 420°C range were studied by using DSC, laser‐interferometric creep rate spectroscopy (CRS), and dielectric relaxation spectroscopy (DRS) techniques. It was shown that a small amount of MMT additive acts as a catalyst of polymerization and results in the formation of complicated intercalated/exfoliated structures, as well as strongly modifies the dynamics in the PCN network. Pronounced dynamic heterogeneity was observed for PCN/MMT nanocomposites. Along with the main PCN glass transition, two new glass transitions, at much higher and much lower temperatures, were revealed as a consequence of constrained dynamics in matrix interfacial nanolayers and due to incomplete local cross‐linking in the PCN matrix, respectively. In addition, increased sub‐T g mobility was observed in these nanocomposites. A two‐fold rise of modulus of elasticity as well as increasing thermal stability and arising microplasticity at low temperatures, promoting, obviously, improved crack resistance in a brittle PCN network, were found for the PCN‐MMT nanocomposites.

Dynamics, thermal behaviour and elastic properties of thin films of poly(vinyl alcohol) nanocomposites

A combined differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and laser-interferometric creep rate spectroscopy (CRS) approach has been utilised to study the dynamics, thermal behaviour and elastic properties of thin films (0.05–0.10 mm) with poly(vinyl alcohol) (PVA) based nanocomposites at temperatures ranging from 20 to 180–220 °C. 3D amorphous fumed silica (nanoparticles of ∼9 nm in average diameter, specific surface area SBET = 330 m2 g−1, bulk density ρb = 0.045 g cm−3) initial and compacted by mechanochemical activation to ρb = 0.32 g cm−3 (dense nanosilica with a small change in SBET), and exfoliated graphite (oxidised 2D sheets packed in stacks of approximately 200 nm in thickness) were used as nanofillers of the PVA films at content of 1, 10 or 20 wt%. The impact of nanofillers which significantly modified structure, dynamics and other properties of the PVA matrix due to constrained dynamics effects depends on the filler type and concentration. The most detailed, discrete pictures of the dynamics and pronounced dynamics heterogeneity in the glass transition of the composites were shown by the CRS method. At any temperature within the 20–180 °C range, the composite modules changed by an order of magnitude. Up to a 14 °C rise of the glass transition temperature was observed for the nanocomposites. The most dramatic enhancing effect of elastic and creep resistance properties was attained for a composite incorporating 10 wt% of exfoliated graphite.

Relationship of Processing Conditions to Structure and Properties in PMDA‐ODA Polyimide

A combined wide angle X‐ray diffraction (WAXD)/small‐angle X‐ray scattering (SAXS)/scanning electron microscopy (SEM)/density study of structure and morphology was carried out for a large series of pyromellitic dianhydride‐oxydianiline (PMDA‐ODA) polyimide (PI) samples processed using different powder metallurgy techniques. Using a combined DSC/creep rate spectroscopy (CRS)/long‐term creep resistance (LTCR) approach, their molecular dynamics, thermal and elastic properties, and creep resistance in the temperature range from 20 to 470°C were also studied. Both a choice of the method of formation of fine PI particles and the order of applying high pressure relative to high temperature to form the monolithic samples led to the observation of significant property differences. Relationships between the processing conditions, structure, and properties were determined. As a result, the conditions for optimizing certain PMDA‐ODA polyimide properties, especially creep resistance and elastic properties at extreme t...