Alexander Fainleib

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.

Relaxation in semi-interpenetrating polymers network of linear polyurethane and heterocyclic polymer networks

Abstract New semi-interpenetrating polymers (semi-IPN) of linear polyurethane (PU) and heterocyclic polymer networks prepared by trimerized dicyanate of Bisphenol-A (TDCE) have been analyzed by dynamic mechanical spectroscopy (DMS) in the 130–550 K temperature and 0.3–30 Hz frequency ranges. Single mechanical α a -relaxations suggest a large-scale homogeneity ascribed to the affinity between TDCE and PU. Below the glass transition temperature, the interpenetration affects the local molecular motions as a consequence of modifications in the mutual local environments of pure components.

Structure–thermal property relationships for polycyanurate–polyurethane linked interpenetrating polymer networks

Chemical structure and thermal-oxidative degradation of sequential IPNs based on crosslinked polyurethane (CPU) and polycyanurate network (PCN) have been studied. As a result of chemical interaction between monomer, dicyanate ester of bisphenol A (DCEBA) and CPU during PCN formation (polycyclotrimerization of DCEBA into CPU matrix), covalent bonding between the networks occurs, leading to creation of linked IPNs (LIPNs). The formation of a new hybrid network (HN) structure has been confirmed. The coexistence of CPU, PCN and HN structures in the LIPNs, confirmed by infrared spectroscopy and by the observation of degradation stages characteristic for each structure, depends on the ratio of the components. Three structures were found for the 90/10 CPU/PCN; HN and PCN were observed in the 70/30 LIPN and the hybrid network was the main structure detected for the 50/50 LIPN.

Structure-Microhardness Relationship in Semi-interpenetrating Polymer Networks

Two series of single-phase, semi-interpenetrating polymer networks (semi-IPNs) based on the same linear polyurethane (LPU) and two different heterocyclic polymer networks (HPNs), respectively, were characterized by the room temperature microhardness H measurement. It is shown that the H value linearly increases with both the mass content of the stiffer component (HPN) and the glass transition temperature of a semi-IPN. The latter dependence can be justified on the assumption that the excess enthalpy of the glass with respect to a hypothetical melt state is a measure of the yield strength of the glassy quasi lattice.

Molecular mobility in semi-IPNs of linear polyurethane and heterocyclic polymer networks

Abstract A study of the thermal and mechanical properties of new semi-interpenetrating polymer networks (IPNs) based on linear polyurethane (PU) and crosslinked trimerized dicyanate (TDC) reveals the existence of structures characterized by the absence of chemical interactions. Two distinct glass transitions are observed in the thermograms, as an indication of the fact that the two polymeric components preserve their molecular structure. The interpenetration affects markedly the glass transition temperatures revealed in the pure components in consequence of modifications in the local environments of the relaxing molecular units in the two phases. The primary and secondary relaxations of these systems show features which can be explained by accounting for the free-volume decrease due to the inclusion of PU in the network of TDC. Below the glass transition two molecular relaxations have been observed which have been ascribed to the secondary relaxation motions characterizing each polymeric component. Both ...

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

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.

Structure-property relationships for cyanurate-containing, full interpenetrating polymer networks

Full sequential interpenetrating polymer networks (seq-IPN) of cross-linked polyurethane (CPU) and heterocyclic polymer networks (HPN) based on thermally cured dicyanic ether of Bisphenol A (DCE) were characterized by small-angle X-ray diffraction, dynamic mechanical analysis, stretching calorimetry and microhardness measurements. Neat CPU was shown to be a microphase-separated system characterized by a regular, three-dimensional macrolattice of network junctions, embedded in uniform-size microdomains of stiff chain fragments which spanned the continuous matrix of soft chain fragments. In contrast, no large-scale structural heterogeneities were detected in the HPN. The X-ray long spacing (L), the degree of microphase segregation (DMS), the α-relaxation temperature and the mechanical properties (elastic modulus and microhardness) were studied as a function of HPN content. Results are explained in the light of a model that discusses the maximum degree of CPV swelling by molten DCE as a function of composition. It is suggested that predominantly chemical interactions between the molten DCE and the stiff chain fragment microdomains, reinforcing primary physical interactions, are responsible for the observed transition at 40% HPN content to a more homogeneous phase morphology of seq-IPNS.