Synthesis of novel phosphorus-containing cyanate esters and their curing reaction with epoxy resin

Novel phosphorus-containing cyanate esters with phosphorus contents between 1-4% (badcyp_x, x=1-4) were synthesized from the addition reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (dopo) and bisphenol A dicyanate (badcy). The structures of synthesized cyanate ester resins were confirmed by NMR and IR spectra. The modified cyanate esters were either self-curing (badcy and badcyp_x series) or curing with epoxy (badcye and badcyp_xe series). Thermal properties of cured cyanate ester were evaluated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermal gravimetric analysis (TGA), dielectric analyzer (DEA) and UL-94 vertical test. It was found that T_g and T_d (5%) decrease markedly with the phosphorus content for the self-curing system, while T_g and T_d (5%) decrease slightly with the increase of cyanate equivalent weight for the cyanate-epoxy curing system. The flame retardancy increases with the phosphorus content and a UL-94 V-0 rating can be achieved for cured badcyp₂-badcyp₄ and badcyp₃e-badcyp₄e. These flame retardant thermosets exhibit low moisture absorption as well as low dielectric constants and dissipation factors. Experimental results indicate that introducing the flame retardant phosphorus element into cyanate ester will not sacrifice its low dielectric characteristic.     

Real-time monitoring of segmental dynamics during crystallization of poly(l(-)-lactide) by simultaneous DRS/SALS technique

A novel experimental set up consisting of dielectric spectroscopy and small angle light scattering (or potentially any other optical technique used in combination) is introduced. The sample is enclosed in a Dielectric-Optical-Shear (DOS) cell capable of precisely controlling temperature and thickness. This method allows for simultaneous in situ, real-time monitoring of the changes in the molecular dynamics and corresponding development of supramolecular structure of a given polymer system during various time-sensitive processes. The DOS cell is utilized to investigate relaxation phenomena related to the glass transition, T_g, of poly(l(−)-lactide) during isothermal crystallization under quiescent, partially constrained conditions. A decrease in T_g was observed in situ via a shift in dipolar relaxation to higher frequency (confirmed calorimetrically, ex situ). The shift was most rapid in the spherulitic impingement stage of crystallization, at approximately 30% overall crystallinity.     

Mobility and glass transition temperature of polymer nanospheres

Recent studies have illustrated a decoupling between cooperative segmental mobility and the glass transition temperature (T_g) of thin polymer films and nanocomposites. Here, we use dielectric spectroscopy to probe the cooperative segmental mobility and capacitive dilatometry to determine the T_g of films of polystyrene nanospheres with diameters (d) less than 400 nm. We find that both capacitive dilatometry and calorimetry revealed nearly identical suppressions in T_g as the size of the nanospheres was reduced. While T_g was impacted by confinement, in the range 130 nm ≤ d ≤ 400 nm, in stark contrast, the cooperative segmental mobility, i.e., the peak position of the α-relaxation process was not. Furthermore, when d ≤ 200 nm, an additional molecular relaxation process, not observed in bulk, was present. We interpret these findings as evidence of a decoupling between T_g and cooperative segmental mobility in nanospheres. That is, the latter may be impacted by confinement under conditions in which the former is not.     

Relaxation behavior of layered double hydroxides filled dangling chain-based polyurethane/polymethyl methacrylate nanocomposites

A series of dangling chain based-polyurethane/poly(methyl methacrylate) (DPU/PMMA) filled with exfoliated layered double hydroxides (LDH) were synthesized by methyl methacrylate in-situ intercalative polymerization. The dangling chains were introduced by using vegetable oils as chain extender. The effect of dangling chain and the contents of LDH on the molecular dynamics of DPU/PMMA was investigated by a combination of dynamic mechanical analysis and broadband dielectric relaxation spectroscopy. Compared with polyurethane/poly(methyl methacrylate) (PU/PMMA) without dangling chain, the glass transition temperature (T_g) of DPU/PMMA shifts to lower temperature and the segmental dynamics becomes faster. A plateau with a high loss factor value above T_g significantly broadens the damping temperature range due to the synergy effect between the dangling chains and LDH layers. In DPU/PMMA/LDH nanocomposites, the α-relaxation associated with the glass transition of the polymer matrix becomes slower with the increase of LDH content, which indicates a restricted molecular mobility in the interfacial regions between polymer and LDH. However, the local relaxations at relatively low temperature remain unaffected by dangling chain or the addition of LDH. When the LDH content increases, Maxwell–Wagner–Sillars (MWS) interfacial polarization process caused by charge accumulation at interfaces becomes faster because of the smaller mean distance d between the exfoliated LDH layers.     

Atomistic molecular simulations of structure and dynamics of crosslinked epoxy resin

Many excellent thermal and mechanical performances of cured epoxy resin products can be related to their specific network structure. In this work, a typical crosslinked epoxy resin was investigated using detailed molecular dynamics (MD) simulations, in a wide temperature range from 250 K to 600 K. A general constant-NPT MD procedure widely used for linear polymers failed to identify the glass transition temperature (T_g) of this crosslinked polymer. This can be attributed to the bigger difference in the time scales and cooling rates between the experiments and simulations, and specially to the highly crosslinked infinite network feature. However, by adopting experimental densities appropriate for the corresponding temperatures, some important structural and dynamic features both below and above T_g were revealed using constant-NVT MD simulations. The polymer system exhibited more local structural features in case of below T_g than above T_g, as suggested by some typical radial distribution functions and torsion angle distributions. Non-bond energy, not any other energy components in the used COMPASS forcefield, played the most important role in glass transition. An abrupt change occurring in the vicinity of T_g was also observed in the plots of the mean squared displacements (MSDs) of the crosslinks against the temperature, indicating the great importance of crosslinks to glass transition. Rotational dynamics of some bonds in epoxy segments were also investigated, which exhibited great diversity along the chains between crosslinks. The reorientation functions of these bond vectors at higher temperatures can be well fitted by Kohlrausch-Williams-Watts (KWW) function.     

Physical crosslinking effects in α,ω-dihydroxy terminated polybutadienes

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320-10?500 g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between T_g and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δ?_α(M_n), the fraction of associated hydroxy groups (f_bond) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity f_bond(M_n) in a modified Fox-Flory approach, the measured T_g(M_n) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.     

Cooperativity in stereoregular PMMAs observed by molecular simulation

The motion correlation along the backbone and between the side-chain and the backbone for the two PMMA chains of opposite tacticity, are investigated using molecular dynamics simulations. According to the coupling model (CM), the index of fragility, which can be considered as a measure of the cooperativity, can be related to the Kohlrausch-Williams-Watts (KWW) exponent. This exponent actually stems from the computation of the correlation time of different bonds inside the polymer chains. From suitably selected bonds, its behavior with temperature can thus unveil cooperativity along the backbone and between the side-chain and the backbone. Compared with experimental data and present theories, these simulation data show that at a relative temperature above the glass transition temperature, T_g, both configurations exhibit the same behavior for the backbone, but a different coupling is clearly observed between the side-chain and the backbone. Such a behavior tends to explain the difference in T_gs between the two configurations in PMMA.     

Dielectric loss of an oligomeric poly(propylene oxide) in the liquid region above Tg

Dynamic dielectric studies of oligomeric poly(propylene oxide) (PPO) of $\text{M}\text{?}{}_{\mathrm{n}}\text{=3034}$, between ?10° and 40°C at 0.1, 1, and 10 KHz, reveal a glass transition and a T >T_g liquid-liquid transition. Analysis of $\text{d}\text{?′}\text{d}\text{T}$ in the liquid region of PPO also indicates the presence of T₁₁. The activation enthalpies for the T_g and T₁₁ transitions have been calculated to be 39 and 18 kCal mol^?1, respectively. The T₁₁ transition in poly(propylene oxide) has been assigned to the motion of the entire polymer molecule.     

Effects of molecular entanglement on molecular dynamics and phase-separation kinetics of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) blends

Poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends with various compositions were prepared through solution casting and melt blending. Two preparation routes, solution casting and melt blending, were used to achieve different degrees of molecular entanglement in the samples with solution casting giving rise to a lower degree of entanglement. Therefore, the effect of molecular entanglement on molecular dynamics and phase-separation kinetics of PMMA/SMA blends was investigated by using broadband dielectric spectroscopy and small-angle laser light scattering (SALLS). Molecular entanglement is found to have a pronounced effect on the α-relaxation process. The glass transition temperature (T_g) is related to the degree of entanglement and a higher degree of entanglement can result in a higher T_g which shifts to a higher temperature after annealing. The relaxation time (τ) of the α-relaxation process is lower for lower degrees of entanglement. Neither the dynamics nor the distribution width of the β-relaxation process is affected by degree of entanglement, regardless of the blend composition. The kinetics of phase-separation by spinodal decomposition (SD) in PMMA/SMA blends are however significantly influenced by the degrees of entanglement with decomposition rate being higher at lower degrees of entanglement.     

Distribution of oil in olefinic thermoplastic elastomer blends

The distribution of processing oil in two olefinic thermoplastic elastomer (OTPE) blends was determined using dielectric spectroscopy. The OPTE blends are blends of dynamically vulcanised EPDM with polypropylene (PP), TPVs, and blends of PP with SEBS. Both blend types contain paraffinic oil, which is present in both the PP and in the elastomer phase. The determination of the actual oil concentration by measuring the reduction in the glass transition temperatures (T_g) is inaccurate using DSC or DMA, because the glass transition dynamics of the two phases overlap. The blends were made sensible for dielectric spectroscopy by the addition of a probe molecule. The oil distribution was determined by modelling of the dielectric loss of the OPTE blends in the T_g regime from the ones of the binary mixtures. The mean value for the oil distribution coefficient was found to be 0.6 for PP/SEBS blends and 0.63 for TPVs.     

Dynamics of concentrated solutions of low molecular weight phenolics and poly(2-vinylpyridine): Role of intermolecular hydrogen bonding

The molecular dynamics of solutions of poly(2-vinylpyridine) (P2VPy) and a series of low molecular weight phenols containing from one to six hydroxyl groups were investigated using broadband dielectric spectroscopy (DRS). Dynamic mechanical analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry, small-angle X-ray scattering and wide-angle X-ray diffraction were employed in a complementary role. Segmental relaxation times for the α processes of all solutions follow expectations from T_gs derived from DSC experiments. For three of the model mixtures at 30 and 50 mol% [i.e., those containing bis (4-hydroxyphenyl) methane, 2,6 dihydroxynaphthalene, and 2,2-methylenebis[6-(2-hydroxy-5-methylbenzyl)-p-cresol] significantly broadened dielectric α relaxation time distributions were observed, indicating dynamic heterogeneity. On the other hand, 4-ethyphenol-P2VPy solutions display dynamic homogeneity. P2VPy with 10 mol% 2,3,3,4,4,5-hexahydroxybenzophenone behaved differently than all mixtures investigated in this study: it displayed a T_g (and T_α) significantly higher than that of the neat components, a small SAXS scattering peak, and an additional dielectric relaxation that we propose originates from Maxwell-Wagner-Sillars interfacial polarization. We propose that this behavior is a result of a phase separation of different types of hydrogen-bonded complexes, one rich in P2VPy and the other involving the type of 2,3,3,4,4,5-hexahydroxybenzophenone hydrogen-bonded structures found in the neat state. Intermolecular hydrogen bonding in all of the P2VPy-phenol mixtures suppresses, in some cases completely, the local P2VPy β relaxation by decreasing the mobility of the pyridine side groups.     

Cooperative behavior of poly(vinyl alcohol) and water as revealed by molecular dynamics simulations

An aqueous poly(vinyl alcohol) (PVA) model has been extensively studied by using the molecular dynamics (MD) simulation method. The employed molecular and force field models are validated against the available data in the literature. In particular, the glass transition temperature (T_g) is determined from the specific volume versus temperature, which compares well with the experimental observations. The diffusion coefficients of water (H₂O) through the PVA matrix follow the Arrhenius equations at both temperature regions separated by T_g, indicating the existence of free and bound water defined by hydrogen bonds (HBs). It has also been confirmed that HBs occur between PVA and H₂O, between PVA and PVA, between H₂O and H₂O, and all of them play the key roles in the glass transition. The local dynamics suggested by the decorrelations of various bond vectors can be well described by the Williams-Landel-Ferry (WLF) equation. This work demonstrates the cooperative behavior of PVA and H₂O which is responsible for the glass transition of the whole binary system.     

Effect of pressure on the segmental dynamics of bisphenol-A-polycarbonate

The segmental dynamics of bisphenol-A-polycarbonate (BPA-PC) are studied as a function of temperature (in the range from 143 to 473 K) and pressure (0.1-300 MPa) within the frequency range from 3 × 10⁻³ to 1 × 10⁶ Hz using dielectric spectroscopy aiming at extracting the more relevant parameter associated with the liquid-to-glass transition. Rheological measurements are also made in the temperature range from 408 to 513 K for comparison. The dynamic results coupled with the equation of state reveal that both density and thermal energy control the segmental dynamics with density being the most important variable in the vicinity of the transition. This is documented by independent estimates of the value of the dynamic ratio E_V^∗/H^∗ (∼0.44). This low value of the dynamic ratio is discussed in terms of the packing irregularities and large monomer volume of BPA-PC. In addition, the pressure coefficient of T_g (dT_g/dP ∼ 0.52 K/MPa) is one of the highest for a polymeric substance.     

Diluent effects on carbonate mobility in bisphenol-A polycarbonate in the solid state

The effect of miscible low molecular weight additives on the mobility of the carbonate group in bisphenol-A polycarbonate (BPAPC) has been studied using n.m.r. and dielectric relaxation experiments in the solid state. Proton-enhanced dipolar-decoupled carbon-13 n.m.r. spectra of BPAPC, isotopically enriched at the carbonate position, are obtained without magic-angle sample spinning. The resolved chemical shift anisotropy allows study of nuclear spin relaxation for the carbonate groups in the polymer that have different orientations relative to the static magnetic field in the laboratory frame. The spin-lattice relaxation time in the rotating frame (T_1?) is measured at a motional-probe frequency of 50 kHz for the undiluted polymer and for BPAPC-diluent blends containing either dibutylphthalate or dinitrobiphenyl. The T_1? exhibits some dependence on orientation in all systems studied. In the blend containing dibutylphthalate (DBP), T_1? is decreased by a factor of two for all orientations of the carbonate group. This implies that DBP substantially increases the spectral density of 50 kHz motions in the carbonate region of the polymer at ambient temperature. In contrast, dinitrobiphenyl does not significantly alter the Fourier component of thermal fluctuations at 50 kHz. Dielectric relaxation measurements at 10 kHz reveal that the primary (T_g) and secondary (β) motional processes in BPAPC are affected by low molecular weight additives. An intermediate relaxation process appears in the temperature interval between the glass transition temperature (T_g) and the sub-T_g β-relaxation (T_β) in the polymer-diluent blends. The n.m.r. spin-lattice relaxation rate in the rotating frame, T^?1_1?, correlates well with the relative magnitude of the dielectric dissipation factor (tan δ_ε) between T_g and T_β.     

Conformational transition characterization of glass transition behavior of polymers

The conformational transition behavior of polymer in the amorphous state has been investigated through molecular dynamics simulations across the glass transition temperature (T_g). We find that the conformational transition, a localized and short time dynamics feature, crosses over different barrier heights when the system transforms from the molten state into the glass state and the barrier height in the glass state is markedly lower than that above T_g. In addition to the overall transition behavior, the specific transitions between the rotational isomeric states (RIS) g⁺, t⁻, t⁺ and g⁻ are also investigated in detail. The populations of these specific transitions undergo considerable changes when the temperature decreases; meanwhile, the larger transition rates of the ending torsions get diminished. Besides the rate, the rotation degrees of the dihedrals during the transitions also change their distributions tremendously through T_g, below which most of the larger transition angles (50-100°) were inhibited remaining those sharply around 30°. This possibly explains why below T_g the conformational transition process has a lower effective barrier.     

Achieving both large transduction coefficient and high Curie temperature of Bi and Fe co-doped PZT piezoelectric ceramics

Achieving both the large transduction coefficient (the product of piezoelectric charge d₃₃ and voltage coefficients g₃₃) and high Curie temperature is very important to improve the power generation performance and their thermal stability of piezoelectric energy harvesters. It is difficult to improve the transduction coefficient of the commercial PZT based piezoelectric ceramics due to the same variation trend of piezoelectric charge coefficient and dielectric constant with chemical modifications. In this work, Bi₂O₃ and Fe₂O₃ co-modified ((Pb_1-xBi_x)((Zr_0.53Ti_0.47)_1-xFe_x)O₃) ceramics were prepared by conventional solid state reaction method, and their dielectric and piezoelectric properties were studied. The piezoelectric charge coefficient d₃₃ increases by Bi and Fe co-modifications due to the enlarged grain size and reduced lattice distortion, while the dielectric constant ε₃₃ deceases mainly owing to the increased micro-pores in grains, leading to the enhancement transduction coefficient d₃₃×g₃₃. The Curie temperature Tc and maximum transduction coefficient d₃₃×g₃₃ are 346 °C and 17169 × 10^?15 m²/N, respectively, which are both higher than those of commercial PZT and PZN-PZT based piezoelectric ceramics. This work provides a new way to enhance the transduction coefficient of PZT based ceramics for piezoelectric energy harvesters used in wide temperature range.     

Understanding the depolarization temperature in (Bi0.5Na0.5)TiO3-based ferroelectrics

Depolarization temperature, or T_d, in ferroelectric materials limits the high temperature operation regime as the long range dipolar ordering could not be maintained beyond T_d during zero field heating. Therefore, the control of T_d has been always drawing the attention of the ferroelectric/piezoelectric community. Some particular issues include: Which point defect is more responsible for the change of T_d? What is the phase equilibrium as a function of T_d? In this paper, our goal is to elaborate the correlation between T_d and defect chemistry in 85% (Bi_0.5Na_0.5)TiO₃-15% BaTiO₃ (BNT-15BT) ferroelectric ceramics. Various experimental tools are employed and cross referenced, including dielectric measurements, thermal stimulated depolarization current, impedance spectroscopy, scanning transmission electron microscopy and in-situ transmission electron microscopy. Collectively this study generates a more insightful understanding of T_d in BNT-based ferroelectrics.     

Glass transition and the rigid amorphous phase in semicrystalline blends of bacterial polyhydroxybutyrate PHB with low molecular mass atactic R, S-PHB-diol

The glass transition and the crystallinity of blends of isotactic bacterial PHB and low molecular mass atactic R, S-PHB-diols was investigated by means of differential scanning calorimetry (DSC), temperature-modulated DSC and dielectric spectroscopy. It was found that (i) T_g of crystallized blends is much lower than T_g of quenched blends, (ii) the semi-crystalline blends can only be described with a three-phase model. From the experimental results the amount of the oligomer component in the mobile amorphous as well as in the rigid amorphous phase was determined. It could be shown that the low molecular mass atactic R, S-PHB-diol is enriched in the mobile amorphous phase of the semi-crystalline blends, but 5-15% oligomer remains, however, in the rigid amorphous phase.     

Thermal properties of poly(lactic acid) fumed silica nanocomposites: Experiments and molecular dynamics simulations

Poly(lactic acid) (PLA) fumed silica nanocomposites were prepared by twin-screw extruder. Thermal properties were investigated by experiments and molecular dynamics simulations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used and 1.34 °C increase of the glass transition temperature (T_g) and 12 °C improvement of thermal stability were observed for PLA–silica nanocomposites as compared to neat PLA. Molecular dynamics simulations (NPT ensemble) were carried out using modified OPLS-AA force field, and T_g and root-mean-square radii of gyration (R_g) were calculated. A good agreement between the simulation results and experiments was obtained.