Dielectric studies in homogeneous and heterogeneous polyurethane/polycyanurate interpenetrating polymer networks (original) (raw)
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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. #
Journal of Applied Polymer Science, 1998
Traditional and gradient interpenetrating polymer networks (IPNs) of various composition have been prepared on the basis of crosslinked siliceous polyurethane and a copolymer of butylmethacrylate and dimethacrylate triethylene glycol. For various layers of gradient IPNs cut from the surface to the center of the sample detailed investigations by methods of volume dilatometry, vapor sorption and broadband ac dielectric relaxation spectroscopy were carried out. From data of benzene vapor sorption by various layers of gradient IPN the free energy of mixing of the IPN components has been calculated. For all the layers the free energies of mixing are positive and depend on the distance from the sample surface. Each layer consists of two phases with their own transition temperatures. From these data the composition of each phase for various layers has been calculated. The dipolar a mechanism of the polyurethane-rich phase and the ac conductivity mechanism were studied in detail by analyzing the dielectric susceptibility data within the complex permittivity and the modulus formalism.
Journal of Non-Crystalline Solids, 1998
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 a a -relaxations suggest a large-scale homogeneity ascribed to the anity between TDCE and PU. Below the glass transition temperature, the interpenetration aects the local molecular motions as a consequence of modi®cations in the mutual local environments of pure components. Ó
Comparative dielectric studies of segmental mobility in novel polyurethanes
e-Polymers, 2004
Molecular dynamics in selected novel linear/low-branched polyurethanes (PUs), based on oligo(oxytetramethylene glycol), 4,4'-diphenylmethanediisocyanate (MDI) or 2,6-toluenediisocyanate (TDI), and unsymmetrical dimethylhydrazine (I) and a derivative of that (II) as chain extenders (CE), were studied by dielectric techniques. Special attention was paid to the investigation of the α relaxation, associated to the glass transition, by dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC). The TSDC method was used to study the interfacial Maxwell-Wagner-Sillars (MWS) relaxation, related to the accumulation of charges at the interfaces between soft-segment and hardsegment microdomains. The results obtained by DRS and TSDC were in good agreement with each other and in reasonable agreement with results for the microphase separation (MS) obtained by small-angle X-ray scattering and differential scanning calorimetry. TSDC proved to be an attractive complementary technique to DRS for the study of MS in PUs. The results suggest that the position of the MWS band, as well as its separation from the α band, is a good measure of the degree of MS. As regards the PUs studied here, the degree of MS enhances by increasing the mole ratio of CE, and by replacing MDI by TDI or CE I by CE II.
Polymer Engineering & Science, 1999
Semi-interpenetrating polymer networks (semi-IPN) of linear polyurethane (LPU) and heterocyclic polymer network (HPN) were characterized by IR spectroscopy, small-angle X-ray scattering, differential calorimetry with diathermal shells (temperature interval 130-500 K), and dynamic mechanical analysis (temperature interval 150-500 K and frequency range 0.3-30 Hz). The main results obtained may be summarized as follows: 1) The affinity of HPN towards the soft LPU microphase is responsible for the absence of large-scale heterogeneity of semi-IPNs on their base (at least, at low HPN/LPU ratios). 2) Phenomenological analysis of small-scale, Arrhenius-like subglass relaxations (yl and yz in LPU and in HPN, respectively) revealed little interference of HPN with relaxation centers (RCs) of LPU in semi-IPNs, while the local environment of RCs in HPN, on the contrary, strongly depended on semi-IPN composition.
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.
Mechanical behavior of polycyanurate–polyurethane sequential full-interpenetrating polymer networks
Journal of Non-Crystalline Solids, 2002
Differential scanning calorimetry and dynamic mechanical spectroscopy studies have been performed between 120 and 600 K in a new series of sequential full-interpenetrating polymer networks (full-IPNs) of crosslinked polyurethane and polycyanurate network (PCN), based on dicyanate ester of 1,1 0 -bis-4,cyanatophenyl-ethane. The measurements have revealed the existence of two distinct glass transitions in the thermograms and of relaxation losses given by the overlap of two a a -relaxations in the mechanical spectra whose temperature locations change with changing weight ratio of polymeric component. These observations indicate that the morphology of this new class of full-IPNs is based on double-phase structures characterized by weak interchain interactions. The interpenetration affects markedly the glass transition temperatures revealed in the pure components as a consequence of modifications in the local environments of the relaxing molecular units in the two phases. Below the glass transition, two secondary relaxation losses have been observed which have been ascribed to local molecular transitions within each polymeric component. The analysis of their concentration behaviors leads to a conclusion that the interpenetration process affects markedly the local motion of the PCN component.
Polymer, 2007
The molecular relaxation characteristics of rubbery amorphous crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] and poly(propylene glycol) diacrylate [PPGDA] have been investigated using broadband dielectric spectroscopy. Dielectric spectra measured across the sub-glass transition region indicate the emergence of an intermediate ''fast'' relaxation in the highly crosslinked networks that appears to correspond to a subset of segmental motions that are more local and less cooperative as compared to those associated with the glass transition. This process, which is similar to a distinct sub-T g relaxation detected in poly(ethylene oxide) [PEO], may be a general feature in systems with a sufficient level of chemical or physical constraint, as it is observed in the crosslinked networks, crystalline PEO, and PEO-based nanocomposites.
Semi-interpenetrating polymer networks based on polyurethane ionomer/poly(vinyl chloride)
Advances in Polymer Technology, 2004
Semi-interpenetrating polymer networks (semi-IPNs) based on crosslinked polyurethane (PU) and linear polyvinylpyrrolidone (PVP) were synthezised, and their thermal and dynamic mechanical properties and dielectric relaxation behavior were studied to provide insight into their structure, especially according to their composition. The differential scanning calorimetry results showed the glass transitions of the pure components: one glasstransition temperature (T g) for PU and two transitions for PVP. Such glass transitions were also present in the semi-IPNs, whatever their composition. The viscoelastic properties of the semi-IPNs reflected their thermal behavior; it was shown that the semi-IPNs presented three distinct dynamic mechanical relaxations related to these three T g values. Although the temperature position of the PU maximum tan ␦ of the ␣-relaxation was invariable, on the contrary the situation for the two maxima observed for PVP was more complex. Only the maximum of the highest temperature relaxation was shifted to lower temperatures with decreasing PVP content in the semi-IPNs. In this study, we investigated the molecular mobility of the IPNs by means of dielectric relaxation spectroscopy; six relaxation processes were observed and indexed according the increase in the temperature range: the secondary -relaxations related to PU and PVP chains, an ␣-relaxation due to the glass-rubber transition of the PU component, two ␣-relaxations associated to the glass-rubber transitions of the PVP material, and an ionic conductivity relaxation due to the space charge polarization of PU. The temperature position of the ␣-relaxation of PU was invariable in semi-IPNs, as observed dynamic mechanical analysis measurements. However, the upper ␣-relaxation process of PVP shifted to higher temperatures with increasing PVP content in the semi-IPNs. We concluded that the investigated semi-IPNs were two-phase systems with incomplete phase separation and that the content of PVP in the IPNs governed the structure and corresponding properties of such systems through physical interactions.
Soft Matter, 2018
The influence of structural constraints on the relaxation dynamics of three polyurea networks with a varying degree of crosslinking, has been studied by means of a thorough analysis of broadband dielectric spectroscopy measurements. Two different relaxation processes are observed, namely, a fast process involving the soft poly(propylene oxide) chains, and a slower and much broader process associated with the immediate surroundings of the hard crosslinkers. Microphase separation in soft and hard domains characterizes the systems in the presence of hydrogen bonding. In this case, different confinement conditions are explored by varying the soft chain length; overall, so called ''adsorption'' effects dominate. With respect to both cooperativity and the rearrangement energy threshold in fast relaxation, it is found that the enhancement of configurational constraints is similar to cooling, but only on qualitative grounds. An upper bound of the hard domains' interface thickness, in which the slow relaxation is believed to take place, is estimated from the analysis of the fast relaxation in the system characterized by the highest degree of confinement, taking into account the results of the structural analysis. Dropping the hydrogen bonding mechanism, phase separation does not occur anymore and the configurational constraints at the ends of the soft chains are reduced, leaving just those imposed by the rigid crosslinkers. This leads to a significant increase in cooperativity on approaching the glass transition, and to a complex behavior that is thoroughly discussed in comparison with those observed in the micro-segregated systems.