V. P. Privalko

Influence of cross-linking on the segmental dynamics in model polymer networks

In an attempt to study the specific influence of cross-linking on the α relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10−1 Hz<F<105 Hz frequency range and in the 123 K<T<493 K temperature interval. The α relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the α-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme clearly shows that fragility increases as polymer network develops.In an attempt to study the specific influence of cross-linking on the α relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10−1 Hz<F<105 Hz frequency range and in the 123 K<T<493 K temperature interval. The α relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the α-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme cl...

Influence of chain extenders and chain end groups on properties of segmented polyurethanes. II. Dielectric study

Abstract Thermally stimulated depolarization currents (TSDC) measurements (temperature range 77–300 K) and broadband a.c. dielectric relaxation spectroscopy (frequency range 10 mHz–2 GHz) were employed to investigate molecular mobility and microphase separation in model segmented polyurethanes (SPUs) from oligotetramethylene glycol 1000, 4,4′-diphenylmethane diisocyanate and different chain extenders. The magnitude of the interfacial Maxwell-Wagner-Sillars (MWS) polarization TSDC peak and of d.c. conductivity have proved to sensitively reflect changes of the degree of microphase separation (DMS). The dielectric strength of both the primary and the secondary transition of the soft segments rich microphase are highest for the SPU with the highest DMS, whereas frequency (temperature) position and shape of the response are not significantly affected by DMS.

Molecular mobility and hydration properties of segmented polyurethanes with varying structure of soft‐ and hard‐chain segments

The molecular mobility and hydration properties of model segmented polyurethanes from either poly(propylene glycol) (PPG) or poly(butylene adipate) (PBAD), both of molecular weight 2000 (soft segments), and three different diisocyanates (all-trans 4,4′-dicyclohexylmethane diisocyanate, 100% t,t HMDI; HMDI with 20% of trans isomers, 20% t,t HMDI; and 4,4′-diphenylmethane diisocyanate, MDI) (hard segments) were investigated using differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) measurements, ac dielectric relaxation spectroscopy (ac DRS), equilibrium water-sorption isotherms (ESI), and dynamic water-sorption isotherms (DSI). No effects of the structure and of the amount of the soft segments on the overall degree of microphase separation (DMS) into microphases rich in soft and hard segments, respectively, were observed. On the contrary, DMS depends on the composition of the diisocyanates used and systematically increases in the order MDI, 20% t,t HMDI, 100% t,t HMDI as indicated by DSC, TSDC, and ac DRS. The PPG-based polyurethanes are characterized by larger values of water content at saturation, h, and smaller values of the diffusion coefficient of water, D. h increases with temperature, indicating that the sorption process is endothermic.

Influence of chain extenders and chain end groups on properties of segmented polyurethanes. I. Phase morphology

Abstract Segmented polyurethanes from oligotetramethylene glycol 1000, 4,4′-diphenylmethane diisocyanate and different chain extenders were characterized by specific heat capacity (temperature interval 130–450 K) and small-angle X-ray scattering measurements. A regular macrolattice of uniform-size micro-domains of stiff segments spanning a continuous matrix of soft segments was observed for the sample chain-extended with dihydrazide of isophthalipc acid, DIPA. The distribution of microdomains by sizes remained unchanged, whereas the overall degree of micropahase separation (DMS) increased due to dilution of DIPA and/or blocking of chain ends with crown ether-containing di- and monohydrazides. The distribution of microdomains by sizes broadened, whereas the DMS either remained unchanged or slightly decreased on dilution of DIPA with hydroxyl-containing chain extenders [1,4-di-N-oxy-2,3-bis-(oxymethyl)-quinoxaline and 1,4-butane-diol, respectively].

Polyimide/silica nanocomposites with low values of dielectric permittivity

Polyimide nanocomposites prepared by the in situ generation of crosslinked organosilicon nanophase by sol-gel techniques were investigated by dielectric relaxation spectroscopy, themally stimulated depolarization currents and dynamic mechanical analysis. Two series of samples were investigated with molar mass 5000 and 10000 of the polyimide chains. In both series a non-additive decrease of the dielectric permittivity with increasing amount of filler was observed, indicating a loose inner structure of the spatial aggregates of the organosilicon nanophase. The magnitude of the dielectric γ relaxation of polyimide was found to increase with increasing filler content for the shorter chains, whereas the opposite was observed for the longer ones.

Dielectric studies of molecular mobility and microphase separation in segmented polyurethanes

Abstract This work deals with molecular mobility and microphase separation studies on segmented polyurethanes with different fragments, including crown ethers, as chain extenders and/or chain end groups. The techniques used include thermally stimulated depolarization current (TSDC) in the temperature range −185°–30°C and broadband dielectric relaxation spectroscopy (DRS) in the frequency and temperature ranges of 10 −2 –10 6 Hz and −55°–80°C. The combination of these techniques allowed the secondary γ and β mechanisms, the primary α mechanism, the Maxwell–Wagner–Sillars (MWS) mechanism associated with interfacial polarization and dc conductivity σ dc to be recorded. The results suggest that addition of crown ethers promotes microphase separation. Specific characteristics of the primary α and the MWS relaxations and σ dc were found to systematically change with the degree of microphase separation. They should be further quantified and tested on selected systems.

Synthesis and thermomechanical characterization of polyimides reinforced with the sol?gel derived nanoparticles

Abstract Polyimide-based nanocomposites prepared by the in situ generation of inorganic nanoparticles (silica) through the sol–gel process were characterized by kinetics of water uptake, ther mogravimetry and dynamic mechanical analysis. Silica particles turned out to possess a rather loose inner structure characterized by enhanced water diffusivities andby dynamic elasticity moduli comparable to that of the pristine, glassy PI. Thermal stability and thermomechanical properties of nano composites in the glassy state remained nearly the same as those of the pristine PI, while a significant reinforcement effect was observed for the rubbery PI matrix.

Review: the microhardness of non-crystalline materials

Hardness is defined as a phenomenological measure of resistance of a material to shear stresses under local volume compression. It is shown that this definition may serve as a theoretical basis for existing empirical relationships between the Vickers microhardness HV and the various phenomenological, packing density-sensitive parameters of non-crystalline materials, including among them, the internal pressure, the glass transition temperature Tg, the excess enthalpy, and the free volume fraction at Tg.

Viscoelasticity and flow behavior of irradiation grafted nano-inorganic particle filled polypropylene composites in the melt state

Abstract Nanoparticles (mean size about 7 nm) of the standard pyrogenic Aerosil 1380 (Degussa) pregrafted by γ-irradiation with styrene were melt-compounded with the general purpose isotactic polypropylene homopolymer to prepare four nanocomposites with filler volume contents up to 4.68%. Storage G′(ω) and loss G″(ω) shear moduli in the melt state (measured in the range of linear viscoelasticity at three temperatures in the frequency window spanning about three decades) were treated to derive the relaxation times spectra h(τ) using the nlreg computer program based on Tikhonov’s method of non-linear regularization. The experimental data were interpreted in terms of the tentative model highlighting the structural significance of the ratio of mean thickness of polymer interlayer between neighboring filler particles, 〈L〉, to the mean radius of gyration of a polymer coil, 〈Rg〉. In the range of very low filler loadings characterized by large scaled distances, 〈L〉/〈Rg〉 ≥ 1, all nanocomposites behaved as Newtonian liquids in which the self-diffusion of macromolecular coils was, however, slowed down. The onset of plastic yield phenomenon for a nanocomposite with the filler volume content as low as 4.68% was regarded as the experimental evidence for the shear-resistant, infinite cluster of filler particles coated with polymer boundary interphase when the scaled distance approached the ‘critical’ value, 〈L〉/〈Rg〉 ≤ 1.

Gas transport properties of soluble poly(amide imide)s

The influence of the molecular structure of five soluble poly(amide imide)s (PAI)s on their gas transport properties for carbon dioxide, oxygen, nitrogen, and methane has been studied. Permeabilities, diffusivities, and solubilities were determined by time lag measurements and correlated to chain packing and mobility as well as to polymer gas interaction. The PAIs were characterized by small- and wide-angle X-ray scattering. Molar masses and polymerization degrees were measured by light scattering. Additionally, glass transition temperatures, densities, and persistence lengths were determined. Pressure- and temperature-dependent gas transport measurements have been done. It was found that the permeability is increasing with the diffusion coefficient which can be related to the fractional free volume. PAIs containing cardo diamines show higher diffusivities and permeabilities than poly(amide imide)s containing linear aromatic diamines due to higher fractional free volumes. The solubilities for PAIs containing the same imide compound correlate with the molar cohesive energy density. The exchange of hydrogen to fluorine atoms at one aromatic ring of the diamine increases the fractional free volume and cohesive energy density and, in consequence, the diffusion and solubility coefficient. Arrhenius behavior was observed for temperature dependence and decreasing permeability with increasing pressure.