Larisa M. Egorova

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.

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.”

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.

A comparative study of air-dry and water swollen flax and cotton fibres

Thermal stability and structural characteristics of air-dry and swollen crude flax, bleached flax and cotton fibres and the behaviour of bound water were analysed using thermogravimetry, microscopy, differential scanning calorimetry, low-temperature 1H NMR spectroscopy and cryoporometry methods. Both air-dry and swollen fibres contain strongly (SBW) and weakly (WBW) bound water which differ in their behaviour at temperatures below 273 K. All samples have a higher content of SBW than WBW because of the structural features of natural fibres. The air-dry fibres studied have low porosity and similar inner mesoporous structure. The air-dry fibre samples of both cotton and bleached flax contain mainly SBW located in nanopores with radius R 10 nm. According to cryoporometry, swelling substantially increases the pore volume (by a factor of 20–30) and specific surface area Smeso (two–three times) of fibres in the mesoporous region. The largest changes were observed in cotton fibres, owing to their chemical structure and textural characteristics affected by swelling. In the nanopore range, swelling reduced the specific surface area of nanopores (Snano) in cotton fibres and increased Snano in flax fibres, so that for air-dry samples of all fibres Snano > Smeso but for swollen samples Snano < Smeso.

DSC study of main relaxations in the poly(imide-dimethylsiloxane) block copolymers

Abstract The segmentai dynamics in a series of alternating, phase-separated poly(imidedimethylsiloxane) (PI/PDMS) block copolymers has been studied by differential scanning calorimetry (DSC) over the temperature range of 100–630 K, covering the regions of the glass transition and β-relaxation for soft and rigid blocks. The PI blocks had degrees of polymerization that ranged from 6 to 30, and the PDMS moiety content varied from 10 to 47 mass%. Complex behavior of the low-temperature, PDMS block glass transition characteristics was shown, and the conditions for maintaining the PI domains with high T g, providing thermal stability for these block copolymers, were ascertained. The peculiarities of segmentai dynamics and its heterogeneity observed within the glass transition could be interpreted by using as a basis the concept of the common segmentai nature of the α- and β-transitions in flexible-chain polymers.

Polyurethane–poly(2-hydroxyethyl methacrylate) semi-IPN–nanooxide composites

Two sets of hybrid polyurethane–poly(2-hydroxyethyl methacrylate) semi-interpenetrating polymer network–nanooxide composites with 0.25 or 3 wt% nanosilica or nanoalumina functionalised with OH, NH2 or CHCH2 groups were prepared. A combination of atomic force microscopy, infrared spectroscopy, thermally stimulated depolarisation current measurement, differential scanning calorimetry and creep rate spectroscopy analysis of the nanostructure and properties of the composites was performed. The pronounced dynamic heterogeneity and the strong impact of oxide additives, basically suppression of the dynamics and temperature-dependent increasing modulus of elasticity, were observed. The effects correlated with either interfacial interactions (for silica) or the nanostructure (for alumina). A low oxide content strongly affected the matrix due to the formation of an unusual cross-linked, via double covalent hybridisation of three components, structure of the nanocomposites.