Influence of a metal–polymer interfacial interaction on dielectric relaxation properties of polyurethane

The influence of metal–polymer interaction on dielectric relaxation properties of polyurethane (PU), prepared from poly(oxypropylene)diol and hexamethylendiisocyanate, was investigated by means of dielectric relaxation spectroscopy in the frequency domain on the dependence of the thickness of the PU layer and electrode material (gold and steel). Strong electrostatic part of an adhesion interaction between steel substrate and PU results in changes of the -, β-, γ- and Maxwell–Wagner–Sillars relaxation of the PU, as compared to a gold substrate. The effect of the metal–polymer double layer results in the complication of the cooperative and local motions in the polymer. The influence of metal–polymer interaction on the relaxation parameters becomes pronounced at a thickness of less than 60 μm for the steel substrate. For the gold substrate the relaxation characteristics hardly depend on the thickness.     

Cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) by simultaneous measurements of X-ray scattering and dielectric spectroscopy

The isothermal cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) was investigated by simultaneous small and wide angle X-ray scattering and dielectric spectroscopy (DS). By this experimental approach, simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization, namely about the chain ordering through wide angle X-ray scattering, about the lamellar crystals arrangement by means of small angle X-ray scattering, and about the amorphous phase evolution by means of DS. The results indicate that average mobility of the amorphous phase suffers a discontinuous decrease upon passing from the primary to the secondary crystallization regime. We interpret these results assuming that the restriction to the mobility of the amorphous phase occurs mainly in the amorphous regions between the lamellar stacks and not in the amorphous regions within the lamellar stacks.     

The β relaxation as a probe to follow real-time polymer crystallization in model aliphatic polyesters

The dielectric spectra of two aliphatic polyesters, poly(propylene succinate) and poly(propylene adipate), are reported. For these two model aliphatic polyesters the α and β relaxations appear simultaneously and well resolved in the experimental frequency window. During isothermal crystallization, this fact allows one to use the β relaxation to characterize the crystalline structural development while the α relaxation provides information about the evolution of the amorphous phase dynamics. In this way structural development and dynamics evolution can be characterized by a single experiment during the crystallization process. Additionally, the unambiguous determination, in the amorphous polymers, of the relaxation times for the α and β relaxations further support a close relationship between local and segmental dynamics in polyesters as proposed by the coupling model.     

Dielectric spectroscopy using dielectric probes: a new approach to study glass transition dynamics in immiscible apolar polymer blends

Dielectric relaxation spectroscopy using dielectric probes was applied to study the (glass transition) dynamics in binary blends of isotactic PP, PS and LDPE. The blends were prepared by melt-mixing and doped with 0.5% of the dielectric probe 4,4′-(N,N-dibutylamino)-(E)-nitrostilbene (DBANS) (van den Berg O, Sengers WGF, Jager WF, Picken SJ, Wübbenhorst M. Macromolecules 2004;37:2460. [17]). Due to the selective amplification of the dielectric relaxation processes related to the dynamic glass transition of the polymers, accurate relaxation data were obtained, even for the minor phases. No substantial influence of the blend composition and the blend morphology on the glass transition dynamics was found, indicating that both blend constituents behave like homogeneous bulk materials. The normalised relaxation strength of glass transition processes remained constant, regardless of the blend type and blend composition. This indicates that the probe molecule, DBANS, was equally distributed over the two blend components in all three polymer combinations PE-PP, PE-PS and PP-PS.     

Dielectric relaxation of poly (trimethylene terephthalate) in a broad range of crystallinity

Here we present dielectric relaxation experiments carried out in semicrystalline poly (trimethylene terephthalate) (PTT) samples covering a broad range of crystallinity values. Special attention has been devoted to characterize the two extremes of low and high crystallinity. In particular a high temperature relaxation in the dielectric spectra of highly crystalline PTT, attributed to a Maxwell–Wagner–Sillars process, has been revealed. Moreover, the existence of ordering phenomena during the induction period prior to crystallization has been characterized by dielectric spectroscopy.     

Microviscosity of an ion-conducting polymer probed by fluorescence depolarization and dielectric spectroscopy

The local segmental motions of polypropylene oxide melt with variable amounts of added LiClO₄ salt (0.3-2.2 mol% per repeat unit) were studied using dielectric relaxation spectroscopy and single photon counting methods after two-photon excitation of an unattached fluorescent dye, rhodamine123, embedded at low concentration within the polymer. The lithium ions serve as junctions between polymer chains and increase the bulk viscosity. The measured microviscosity contains information on the local environment of ClO₄⁻ ions.     

Cyclic auto-seeded polythermal preferential crystallization—Effect of impurity accumulation

Preferential crystallization is a crystallization process where two enantiomers are separated into the pure chiral species. Usually the process is operated in a cyclic mode crystallizing the two species in sequel batches with an intermediate addition of fresh racemate. In this cyclic scheme reproducibility is a key issue.     

Transition of the bounded polymer layer to a rigid amorphous phase: A computational and DSC study

The study focuses on the transition of the bounded to solid surfaces polymer layer to a rigid amorphous phase (RAP). Based on previous Monte Carlo (MC) simulation studies on bulk polyethylene (PE), we refine our numerical variable density Self Consistent Field (nSCF) method in order the calculated density in bulk to follow the predictions of the MC simulation. The proposed modification of the Sanchez–Lacombe equation of state allows us to examine thermodynamic aspects of the glass transition. By imposing a glass transition (T_g = 220 K) in the bulk we predict an earlier (during cooling), stronger transition of the bounded layer to a RAP, at ∼350 K. Also at short separation distances we record the appearance of undisturbed polymer bridges. Differential scanning calorimetry (DSC) experiments on polydimethylsiloxane (PDMS) and polyamide 6 (PA6) nanocomposites suggest that although the crystallization can be significantly suppressed by the addition of nanoparticles, the RAP layer may locally equilibrate, above T_g, for long experimental times with the melt phase. Our results support the idea that a significant free energy barrier of entropic origin appears due to the RAP formation, below the melting temperature (T_m) and above the T_g.     

Improved non-ohmic and dielectric properties of Cr3+ doped CaCu3Ti4O12 ceramics prepared by a polymer pyrolysis solution route

CaCu_3-xCr_xTi₄O₁₂ (x?=?0.00–0.20) ceramics were prepared via a polymer pyrolysis solution route. Their dielectric properties were improved by Cr³⁺ doping resulting in an optimal dielectric constant value of 7156 and a low tanδ?value of 0.092 in a sample with x?=?0.08. This might have resulted from a decrease in oxygen vacancies at grain boundaries. XANES spectra confirmed the presence of Cu⁺ ions in all ceramic samples with a decreasing Cu⁺/Cu²⁺ ratio due to an increased content of Cr³⁺ ions. All CaCu_3-xCr_xTi₄O₁₂ ceramics showed nonlinear characteristic with improvement in both the breakdown field (E_b) and its nonlinear coefficient (α). Interestingly, the highest values of α, ～ 114.4, and that of E_b, ～8455.0?±?123.6?V?cm^?1, were obtained in a CaCu_3-xCr_xTi₄O₁₂ sample with x?=?0.08. The improvement of dielectric and nonlinear properties suggests that they originate from a reduction of oxygen vacancies at grain boundaries.     

Solid state 13C‐NMR study of a plasticized PLA/PHB polymer blend

High‐resolution solid‐state ¹³C nuclear magnetic resonance spectra and ¹³C spin–lattice relaxation times T₁(¹³C) are used to characterize the structure of a polymer blend prepared from poly(lactic acid) (85 wt %) and poly(3‐hydroxybutyrate) (15 wt %) and the effect of the plasticizer triacetine on the structure and molecular dynamics of the blend. Single‐pulse and cross‐polarization magic angle spinning ¹³C nuclear magnetic resonance spectra indicate that the nonplasticized polymer blend consists of semicrystalline poly(3‐hydroxybutyrate) domains built into an amorphous poly(lactic acid) matrix. Triacetine supports formation of the crystalline regions within both polymer components in the blend. Spin–lattice relaxation times of carbonyl carbons indicate that the nonplasticized polymer blend consists of noninteracting chains of blended polymers and plasticization of the polymer blend increases the relaxation rate. The glass transition, cold crystallization, and melting processes of the nonplasticized and plasticized blends were also studied using differential scanning calorimetry methods. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46296.     

Multifunctional fiberglass-reinforced PMMA-BaTiO3 structural/dielectric composites

Fiberglass-reinforced polymer composites were investigated for potential use as structural dielectrics in multifunctional capacitors that require simultaneous excellent mechanical properties and good energy storage characteristics. Composites were fabricated employing poly(methyl methacrylate), PMMA, as the structural matrix. While barium titanate (BaTiO₃) nanopowder was added to the composites for its high room temperature dielectric constant, fiberglass was employed to confer high stiffness. A conductive polymer blend of poly (3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS) was used to coat the BaTiO₃ nanoparticles with the purpose of further elevating the dielectric constant of the resultant PMMA-composites. FTIR spectroscopy, TGA and SEM measurements were conducted to prove the successful coating of BaTiO₃ nanoparticles with the PEDOT:PSS blend. TEM measurements revealed a good dispersion of coated nanoparticles throughout the PMMA matrix. The fiberglass-reinforced-PMMA composites containing neat and coated BaTiO₃ were found to exhibit excellent stiffness. In addition, the use of PEDOT:PSS in conjunction with BaTiO₃ was observed to improve the dielectric constant of the composites. Finally, the dielectric constant of the structural composites was found to vary only slightly with temperature.     

Solvent retention in solution-cast films of PMMA: study by dielectric spectroscopy

Conventional PMMA was dissolved in two different solvents (chloroform and toluene). Solid polymeric films were formed after casting of the solutions on a substrate (aluminium) and evaporation of the solvent at room temperature during several days. The dielectric properties of the solid films were studied by dielectric spectroscopy. The evolution of the loss tangent (tanδ) versus temperature was analysed. The results show that the tanδ peak, corresponding to the -transition (glass transition), is shifted towards the lower temperatures, as compared with the bulk polymer obtained by heating press, without any solvent. The magnitude of the shift depends on the type of solvent. These results indicate that, in all cases, solvent molecules are retained and cannot be completely evaporated at room temperature, even after a long period. Solvent molecules are still trapped inside the PMMA films, but their effect on the polymer chains mobility differs in function of the nature of the solvent. Explanations based on acid–base interactions are proposed to try to understand this phenomenon. This work illustrates the interest of dielectric spectroscopy for studying in situ the properties of thin cast polymer films.     

Dynamically confined crystallization in a soft lamellar space constituted by alternating polymer co-brushes

Crystallization kinetics and behavior of PCL side chains in polymer co-brushes constituted with PCL and PEO side chains alternatively attached on poly(styrene-alt-maleimide) backbones have been determined using in-situ FT-IR and DSC methods. Avrami analysis shows the exponent n increasing from one at 10 °C to two at 30 °C, demonstrating confined crystallization of PCL side chains through homogeneous or heterogeneous nucleation. PLM morphological characterization displays typical spherulites of which size is dependent on the crystallization temperature and further AFM visualization shows typical PCL lamellae at 30 °C and broken lamellae at 10 °C embedded within PEO + backbone matrix inside of spherulites. Such lamellar structure explains the confined crystallization with Avrami exponent n ≤ 2. Formation of the broken lamellae can further clarify the reason why Avrami exponent decreases to n ≈ 1 at 10 °C, that is, homogeneous nucleation in the isolated crystals. Dynamically confined crystallization has been proposed based on their special molecular architecture. Comparing to statically confined crystallization, the construction of confined space and the crystallization process were almost synchronous. The formation of spherulites mesoscopically reveals the entire molecule motion and assembly through a pathway of conventional crystalline polymers and the crystallization of PCL side chains in a space constituted by stiff backbones of poly(styrene-alt-maleimide) plus soft PEO layer microscopically reflects a confined character which has been observed in some conventional block copolymers.     

Electric force microscopy of dielectric heterogeneous polymer blends

We report the applicability of Electric Force Microscopy (EFM) for the analysis of thin films of dielectric heterogeneous polymer blends constituted of polymers with both close and significantly different dielectric constants and offer a simple model that enables quantitative analysis of EFM images of such blends.     

Evolution of the coefficient of thermal expansion of a thermosetting polymer during cure reaction

The evolution of the coefficient of thermal expansion (CTE) of a thermosetting polymer during cure reaction is an important parameter for industrial applications such as composite processing since it influences the development of internal stresses in the material. The CTE being almost impossible to measure on a reacting thermoset, we propose to use an indirect method based on the modelling of ionic conductivity by a modified WLF equation, allowing to calculate the evolution of CTE from dielectric spectroscopy measurements. This method is applied to a dicyanate ester thermosetting polymer, leading to encouraging results both qualitatively and quantitatively.     

Low-field NMR studies of polymer crystallization kinetics: Changes in the melt dynamics

The free induction decay in ¹H NMR experiments carried out for crystallizing polymers can be directly decomposed in contributions from crystals, melt-like regions and amorphous regions with a reduced mobility. Here, the results of time-dependent experiments conducted with the aid of a cost-efficient low-field NMR instrument are presented, obtained for sPP, P?CL and P(EcO). Crystallization isotherms are compared with those obtained by X-ray scattering and dilatometry. There are some minor systematic deviations which can be explained and accounted for. For all systems, a large fraction of amorphous chain parts in regions with a reduced mobility is found.     

Flow-induced shish-kebab precursor structures in entangled polymer melts

Flow-induced crystallization has long been an important subject in polymer processing. Varying processing conditions can produce different morphologies, which lead to different properties. Recent studies indicated that the final morphology is in fact dictated by the initial formation of crystallization precursor structures (i.e. shish-kebabs) under flow. In this article, factors that affect the shish-kebab formation in entangled polymer melts are systematically reviewed, including the concept of coil-stretch transition, chain dynamics, critical orientation molecular weight, phase transition during shish and kebab formations. In particular, recent experimental results from in situ rheo-X-ray studies and ex situ microscopic examinations have been presented to illustrate several new findings of flow-induced shish-kebab structures in polymer melts. (1) The shish entity consists of stretched chains (or chain segments) that can be in the amorphous, mesomorphic or crystalline state. (2) The kebab entity mainly arises from the crystallization of coiled chains (or chain segments), which seems to follow a diffusion-control growth process. (3) A shish-kebab structure with multiple shish was seen in the ultra-high molecular weight polyethylene (UHMWPE) precursor. Based on the above results and recent simulation work from other laboratories, a modified molecular mechanism for the shish-kebab formation in entangled melt is presented.     

Molecular dynamics modeling of polymer crystallization from the melt

Molecular pathways to polymer crystallization and the structures of crystal-melt interfaces are investigated by molecular dynamics simulation. We adopt a simplified molecular model for polymethylene-like chains; the chain is made of CH₂-like beads connected by harmonic springs, and the lowest energy conformation is a linear stretched sequence of the beads with slight bending stiffness being imposed. Two molecular systems are considered, one is made of 640 chains of C100 and the other is made of 64 chains of C1000, both being placed between two parallel substrates that represent the growth surfaces of the lamellae growing toward each other. The initial melt kept at a sufficiently high temperature above the melting point is rapidly cooled down to various crystallization temperatures, and the molecular processes of crystallization that follow are investigated. In both systems, we clearly observe the growth of stacked chain-folded lamellae from the substrates. The growing lamellae have a definite tapered shape, and they show marked thickening growth along the chain axis as well as usual growth perpendicular to it. The overall crystallization rate is found to be very sensitive to the crystallization temperature, showing an apparent maximum around 320-330 K for C100. We find that the lamellae do not grow keeping pace with each other but grow in independent rates especially at higher temperatures. We also examine the structures of the lateral growth surfaces and find that the growth surfaces are locally flat and the Kossel mechanism of crystal growth seems to be operative. In addition, the fold surfaces are found to be covered with relatively short chain-folds; at least about 60-70% of the folds are connecting the nearest or the next nearest neighbor crystalline stems. No appreciable bond orientational order is found in the undercooled melt of C100 and C1000.