Comparative dielectric studies of segmental mobility in novel polyurethanes (original) (raw)
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Dielectric and hydration properties of segmental polyurethanes
e-Polymers, 2004
Novel polyurethanes (PUs), based on oligo(oxytetramethylene glycol), 4,4'-diphenylmethanediisocyanate and 1,1-dimethylhydrazine as chain extender, were prepared in a two-step process. The ratio prepolymer (PP) to chain extender (CE) was systematically varied in extreme ranges, from 1:1 to 10:1. Dielectric relaxation spectroscopy and thermally stimulated depolarization currents (TSDC) techniques were employed to investigate molecular dynamics and to conclude on microphase separation (MS). In that respect TSDC was proven to be very powerful, in particular as far as the investigation of the interfacial Maxwell-Wagner-Sillars polarization is concerned. Additional information on micromorphology is obtained from water sorption/diffusion measurements. A part of the results suggest that MS improves with increasing the PP:CE ratio. The whole body of results can be explained if, at the same time, it is assumed that a branched structure is developed for samples out of stoichiometry and branching increases with increasing the PP:CE ratio. Preliminary experiments with solutions of the PUs in organic solvents provide support for that assumption.
Dielectric relaxation spectroscopy in crosslinked polyurethanes based on polymer polyols
European Polymer Journal, 2000
This work deals with molecular mobility and microphase separation studies in blends of crosslinked polyurethanes and styrene-acrylonitrile (SAN), prepared by reactive blending, by means of dierential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS) techniques. The DRS techniques employed include broadband ac techniques, 10 À2 ±10 6 Hz, and thermally stimulated depolarization currents (TSDC) techniques, À180 to 308C. The results suggest, in agreement with each other by the dierent techniques, that SAN promotes the microphase separation between hard segment (HS) microdomains and soft segment (SS) microphase due to SAN±HS interactions, whereas SAN±SS interactions are weak. TSDC thermal sampling techniques show that a compensation law holds for the a relaxation associated with the glass transition of the SS microphase, with the compensation temperature being very close to the glass transition temperature. Analysis of the a relaxation in terms of fragility indicates that the systems under investigation are fragile. #
2011
We revisit molecular mobility and crystallinity in a series of Polyurethanes prepared with polytetramethylene ether glycol (PTMEG) with varying molecular weights as soft component, methylene diisocyanate (MDI) as diisocyanate component and butanediol as chain extender in close comparison to the properties of the respective macrodiols used as soft segments. Modulated differential scanning calorimetry (MDSC) was utilized to study the glass transition and in detail crystallization and melting. Thermally stimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) were used to investigate local dynamics, dynamic glass transition and indirectly, crystallinity and morphology. The dielectric data were evaluated through representation by contour plots in the Arrhenius plane. Glass transition temperature of the diols is practically unaffected by molecular weight due to H-bonding effect, while for the PUs it showed a decreasing dependence due to higher microphase separation. Results are also discussed in terms of fragility. Crystallization of PTMEG becomes more intense and stable with increasing molecular weight, while the reflection of its melting process on its dielectric response was also observed. A weak crystallization process in the Polyurethane with long soft segments was well resolved using MDSC. Evidence for the occurrence of spinodal decomposition before this crystallization was observed by means of DRS.
Microphase separation in ion-containing polyurethanes studied by dielectric measurements
Acta Materialia, 2004
Ac dielectric relaxation measurements, thermally stimulated depolarization currents (TSDC) measurements, and differential scanning calorimetry (DSC) measurements, were employed to investigate the dielectric properties in relation to the microphase separation in blends of an anion containing polyurethane (PU 1 ) and polyaminourethane (PU 2 ). The temperature at which a-and Maxwell-Wagner-Sillars relaxation appear in the TSDC plots and the magnitude of these relaxations are strongly affected by the degree of microphase separation (DMS) of the ionomer blends. The results, of the various techniques, were compared to each other with respect to their characteristic, and indicate that the DMS decreases on addition of the pure component PU 2 to the blends. Analysis of the a-relaxation, in terms of fragility, indicates that the systems under investigation are fragile. Results indicate a correlation between fragility and DMS. Lower phase mixing suggests higher fragility.
Polyurethane-POSS hybrids: Molecular dynamics studies
Polymer, 2010
A series of hybrid polyurethane-POSS materials have been synthesized on the basis of poly(tetramethylene glycol) (Terathane 1400 Ò ) as soft component, 4,4 0 -methylenebis(phenylisocyanate) (MDI) as hard component, and 1,4-butanediol as chain extender. POSS particles properly modified have been tethered on the main chain by substitution of the chain extender to weight fractions up to 10%. AFM measurements indicate the formation of POSS crystallites in the PU matrix, extended structures at low POSS content and more regular, smaller structures at higher POSS content. A detailed investigation of molecular mobility by means of Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Currents (TSDC) and, mainly, Broadband Dielectric Relaxation Spectroscopy (DRS) has been conducted in all samples of the series and in addition in neat Terathane, as reference. Four relaxations have been studied in detail: two secondary relaxations g and b, the segmental a relaxation (dynamic glass transition) and an a 0 relaxation slower than a associated with crystallinity in neat Terathane and with the presence of hard microdomains in the polyurethane and the hybrids. Secondary relaxations remain unaffected by POSS. The glass transition temperature rises by a few degrees and, in consistency with that, segmental dynamics slightly slows down with increasing POSS content. In addition, the dielectric strength of the segmental relaxation decreases with increasing POSS content, suggesting that a fraction of polymer is immobilized, making no contribution to the relaxation. These results are discussed in relation to morphology.
Thermal and dielectric properties of polycarbonatediol polyurethane
Journal of Applied Polymer Science, 2015
The dielectric relaxation behavior of segmented polyurethane has been studied using Broad-Band Dielectric Spectroscopy in the frequency domain, 10 22 to 10 8 Hz, and in the temperature range of 2120 to 140 C. The spectra show three secondary processes (d, c, and b) followed by the a relaxation and conductive processes. The Havriliak-Negami (HN) phenomenological equation was used in order to characterize all the processes. The d, c, and b relaxations are probably associated with (i) local motions of the main chain (ii) motions of the carbonate group in the soft phase and (iii) reorientational motions of water molecules. The microphase separated morphology associated with soft and hard domains is reflected in the dielectric spectra, at high temperatures, by the presence of the Maxwell-Wagner-Sillars (MWS) interfacial polarization process.
2013
Polyhedral Oligomeric Silsesquioxanne particles were tethered by urethane or urea linkages on the hard segments of polyurethane chains with varying segment Mw. Evidence by imaging, thermal and dielectric techniques suggests that tethering of POSS by urethane bond leads to increase of glass transition temperature by a few degrees due to two mechanisms: the direct POSS-chain interactions and the indirect slowing of dynamics by disruption of microphase separation. The direct mechanism is dominant for the materials with long (high Mw) segments and the indirect becomes more important as the Mw of the segments decreases. High polarity of the urea bond counteracts the effect of disruption of microphase separation and enhances an interfacial glass transition mechanism α′, slower than α, leading to significantly higher apparent glass transition temperatures, despite its small effect on time scales of the individual relaxations.
Molecular dynamics of linear and hyperbranched polyurethanes and their blends
Journal of Non-crystalline Solids, 2005
The molecular dynamics of two polyurethanes with different architecture (linear and hyperbranched) and their blends with compositions 80/20 and 65/35 have been studied by dielectric spectroscopy and dynamic mechanical analysis yielding results complementary to those obtained by differential scanning calorimetry. In pure polyurethanes, relaxation phenomena correlated with glass transitions and with local chain motions are observed. The molecular dynamics in the blends is dominated by the linear component. A significant influence of hyperbranched polyurethane in the blends is observed only in the case of the primary relaxation connected with the high-temperature glass transition. At higher temperatures the dielectric spectra are dominated by conductivity.
Dielectric Relaxation of Linear and Cross-Linked Polyurethane
Macromolecules, 2001
The dielectric relaxation is studied of linear and cross-linked polyurethane based on linear and three-armed star poly(propylene oxide), also called poly(propylene glycol). The dielectric relaxation of polyurethane melts and gels is indistinguishable if the density of urethane links is the same. The introduction of urethane links slows down the segmental relaxation of a number of segments adjacent to the links. The influence is much stronger on the dielectric relaxation than on the relaxation of the shear modulus. The relaxation of the end-to-end vector of the PPO precursors is not much influenced by endlinking at least as long as they are not strongly entangled.
Dielectric relaxations of Acrylic-Polyurethane hybrid materials
Polymer, 2015
In this work we present a dielectric relaxation study of Acrilyc/Polyurethane polymers synthesized via miniemulsion photo-polymerization. Three different samples, both parts of the synthesized hybrid latex (SOL and GEL fractions) and a full acrylic sample as reference have been considered. Besides dielectric experiments using broadband Dielectric Spectroscopy (BDS) technique, complementary Differential Scanning Calorimetry measurements have been performed. The thermodynamic characterization outcomes evidence very similar characteristics of the glass transition phenomena among the investigated samples without any signature of melting/crystallization. However BDS experiments show three well resolved dielectric relaxation processes above Tg, two of them evidence marked differences among the samples. A detail study allows us to provide a molecular origin for these three dielectric relaxation processes in connection with the sample characteristics. In particular, our results show that the characterization of these hybrid Acrylic/PU by BDS can provide access to some of the structural features that would ultimately influence the adhesives properties.