Dielectric properties of graphene–iron oxide/polyimide films with oriented graphene

The preparation of high‐dielectric‐constant (k) materials is important in the field of electronics. However, how to effectively use the function of fillers to enhance k is still a challenge. In this study, anisotropic graphene (GNS)–iron oxide (Fe₃O₄)/polyimide (PI) nanocomposite films with oriented GNSs were prepared by the in situ polymerization of 4,4′‐oxydianiline and pyromellitic anhydride in the presence of GNS–Fe₃O₄. Films of the precursors were fabricated, and this was followed by stepwise imidization under a magnetic field at a higher temperature to orient the magnetic sheets. The orientation of GNS–Fe₃O₄ and the relationships of the GNS–Fe₃O₄ content and measurement frequency with the dielectric properties of the GNS–Fe₃O₄/PI films were studied in detail. The dielectric property differences of the GNS–Fe₃O₄/PIs with GNS–Fe₃O₄ parallel or perpendicular to the film surface were not obvious, when the content of GNS–Fe₃O₄ was lower than 5 wt %. However, at the percolation threshold, the k values of GNS–Fe₃O₄/PI films with horizontal GNS–Fe₃O₄ were much higher than those of the other two kinds of films at 10³ Hz; this was derived from the contribution of more effective microcapcitors parallel to the film surface. So, making the GNS–Fe₃O₄ parallel to the film surface greatly enhanced k of GNS–Fe₃O₄. However, switching the charges on the large lateral surface of the parallel GNSs with the electric field also caused a higher dielectric loss and the frequency dependence of k and the dielectric loss at low frequency. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43041.     

New polyimide–silica organic–inorganic hybrids

A series of polyimide–silica hybrid films with silica contents up to 30 wt % were successfully prepared by the sol‐gel reaction of tetraethoxysiliane in the presence of poly(amic acid) containing pendent hydroxyl groups. The films were yellow and transparent when the silica content was less than 11 wt %. The chemical structure of the films was characterized by Fourier transform infrared spectroscopy, and the morphology of the films was investigated by scanning electronic microscopy and atomic force microscopy. Thermogravimetric analysis, differential scanning calorimetry, and stress–strain tests were used to measure the performance of the films. The results indicate that the glass‐transition temperatures and decomposition temperatures of the hybrid films increased with increasing silica content, whereas the tensile strength had a maximum with the variety of silica contents. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2210–2214, 2003     

Fluorinated graphene/polyimide nanocomposites for advanced electronic packaging applications

Low dielectric constant and low dissipation factor, superior hydrophobicity, excellent thermal and mechanical property, and good optical performance are desired for advanced electronic packaging of fan-out wafer level package (FO-WLP). An effective approach was introduced to fabricate novel fluorinated graphene/polyimide(FG/PI) nanocomposite films in this paper. FG nanosheets exhibit excellent dispersion in the PI matrix due to their high surface area with some oxygen-containing functional groups, and individual graphitized planar structure. Besides, the effects of the addition of FG on the dielectric, optical, mechanical, thermal properties as well as the hydrophobicity of the films are investigated with controlled amounts of FG. The dielectric constant and loss can be as low as 2.64 (at 10⁶ Hz) and 0.00176 for PI-0.5wt% FG. And with the increase of the loading of FG, the thermal stability (T₅ = 514°C) and mechanical property (tensile modulus = 2.11GPa, tensile strength = 93.23 MPa, elongation at break = 11.60%) has been improved successfully. Besides, the contact angles have been increased from 83° to 92° showing a superior hydrophobicity which is essential for the FO-WLP. Moreover, the incorporation of FG also proved excellent optical performance of 88% transmittance at 550 nm. Therefore, with the excellent comprehensive performance, the as-prepared FG/PI films possess widespread applications in the microelectronics especially in FO-WLP.     

Synthesis and characterization of polyimide–silica nanocomposites using novel fluorine‐modified silica nanoparticles

Polyimide–silica nanocomposites were synthesized with 4,4′‐oxydianiline, 4,4′‐(4,4′‐isopropylidenediphenoxy)bis(phthalic anhydride), and fluorine‐modified silica nanoparticles. Fluorinated precursors such as 4″,4?‐(hexafluoroisopropylidene)bis(4‐phenoxyaniline) (6FBPA) and 4,4′‐(hexafluoroisopropylindene)diphenol (BISAF) were employed to modify the surface of the silica nanoparticles. The microstructures and thermal, mechanical, and dielectric properties of the polyimide–silica nanocomposites were investigated. An improvement in the thermal stability and storage modulus of the polyimide nanocomposites due to the addition of the modified silica nanoparticles was observed. The microstructures of the polyimide–silica nanocomposites containing 6FBPA‐modified silica exhibited more uniformity than those of the nanocomposites containing BISAF‐modified silica. The dielectric constants of the polyimide were considerably reduced by the incorporation of pristine silica or 6FBPA‐modified silica but not BISAF‐modified silica. The addition of a modifier with higher fluorine contents did not ensure a lower dielectric constant. The uniformity of the silica distribution, manipulated by the reactivity of the modifier, played an important role in the reduction of the dielectric constant. Using 6FBPA‐modified silica nanoparticles demonstrated an effective way of synthesizing low‐dielectric‐constant polyimide–silica nanocomposites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 882–890, 2007     

In situ polymerization of polyimide‐based nanocomposites via covalent incorporation of functionalized graphene nanosheets for enhancing mechanical,thermal, and electrical properties

In this study, we report an effective method to fabricate high‐performance polyimide (PI)‐based nanocomposites using 3‐aminopropyltriethoxysilane functionalized graphene oxide (APTSi‐GO) as the reinforcing filler. APTSi‐GO nanosheets exhibit good dispersibility and compatibility with the polymer matrix because of the strong interfacial covalent interactions. PI‐based nanocomposites with different loadings of functionalized graphene nanosheets (FGNS) were prepared by in situ polymerization and thermal imidization. The mechanical performance, thermal stability, and electrical conductivity of the FGNS/PI nanocomposites are significantly improved compared with those of pure PI by adding only a small amount of FGNS. For example, a 79% improvement in the tensile strength and a 132% increase in the tensile modulus are achieved by adding 1.5 wt % FGNS. The electrical and thermal conductivities of 1.5 wt % FGNS/PI are 2.6 × 10^?3 S/m and 0.321 W/m·K, respectively, which are ～10¹⁰ and two times higher than those of pure PI. Furthermore, the incorporation of graphene significantly improves the glass‐transition temperature and thermal stability. The success of this approach provides a good rationale for developing multifunctional and high‐performance PI‐based composite materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42724.     

Relationship between microstructure of lamellar graphene sheets and properties of polyimide/graphene nanocomposites film under different imidization stages

Herein, polyimide/graphene sheets (PI/GS) nanocomposite films with different GS distribution structures have been successfully obtained by controlling the imidization degrees, and the effect of the lamellar structure on the properties of PI film has been investigated. The results show that GS are gradually parallel to the surface of PI nanocomposite film with the increase of the imidization temperature, and 150 °C is the critical temperature, where the imidization rate is the fastest and the lamellar structure begins to form. Furthermore, with the drying temperature increasing, the corresponding thermal, electrical and mechanical properties of PI/GS nanocomposite films are significantly improved compared with that of pure PI films, which are ascribed to both the higher imidization degree and the lamellar GS structure. It is noteworthy that the formation process of the lamellar structure at different imidization stages can be directly observed by scanning electron microscope. Based on these results, a model has been proposed to explain the relationship between the lamellar structure and properties of PI composite film under different imidization stages, and the confinement of the thickness may be the most important factor for the formation of lamellar GS structure. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43575.     

Thermal and mechanical properties of blended polyimide and amine‐functionalized poly(orthosiloxane) composites

Nanocomposite films were prepared through the blending of polyimide (PI) with octaphenyl silsesquioxane (OPS) and an amino‐functionalized analogue, octaaminophenyl silsesquioxane (OAPS), with a solution‐casting method. Although the PI–OPS composites showed visible phase separation at 5 wt %, the PI–OAPS composites were transparent with visible phase separation occurring only at 50 wt % OAPS. The interfacial interactions and homogeneity of the composites were characterized with scanning electron microscopy (SEM) and dynamic mechanical analysis. SEM analysis showed a uniform fracture surface for OAPS composites at concentrations up to 20 wt %. Interestingly, OAPS‐rich particles with sizes of less than 1 μm were formed within the PI matrix for the 50 wt % composite. The PI–OAPS composites showed higher glass‐transition temperatures (T_g's) than the pure PI. The PI–OPS composites showed a T_g lower than that of the pure PI, and this suggested poor interfacial interactions. The slightly enhanced thermal stability of PI–OAPS composites (up to 20 wt %) was attributed to the inherent thermal stability of OAPS at higher temperatures. There were small increases in the modulus and strength for the composites with respect to the base polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Tribological behavior of short‐fiber‐reinforced polyimide composites under dry‐sliding and water‐lubricated conditions

Polyimide composites reinforced with short‐cut fibers such as carbon, glass, and quartz fibers were fabricated by the polymerization of monomer reactants process. The mechanical properties of the composites with different fiber contents were evaluated. The friction and wear properties of the polyimide and its composites were investigated under dry‐sliding and water‐lubricated conditions. The results indicated that the short‐carbon‐fiber‐reinforced polyimide composites had better tensile and flexural strengths and improved tribological properties in comparison with glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. The incorporation of short carbon fibers into the polyimide contributed to decreases in the friction coefficient and wear rate under both dry and water‐lubricated conditions and especially under water lubrication because of the boundary lubrication effect of water. The polyimide and its composites were characterized by plastic deformation, microcracking, and spalling under both dry and water‐lubricated conditions, which were significantly abated under the water‐lubricated condition. The glass and quartz fibers were easily abraded and broken; the broken fibers transferred to the mating metal surface and increased the surface roughness of mating stainless steel, which led to the wear rate increasing for the glass‐fiber‐ and quartz‐fiber‐reinforced polyimide composites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Preparation and properties of (BATB–ODPA) polyimide–clay nanocomposite materials

A series of polymer–clay nanocomposite (PCN) materials consisting of 1,4‐bis(4‐aminophenoxy)‐2‐tert‐butylbenzene–4,4′‐oxydiphthalic anhydride (BATB–ODPA) polyimide (PI) and layered montmorillonite (MMT) clay were successfully prepared by an in situ polymerization reaction through thermal imidization up to 300°C. The synthesized PCN materials were subsequently characterized by Fourier‐Transform infrared (FTIR) spectroscopy, wide‐angle powder X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of material composition on thermal stability, mechanical strength, molecular permeability and optical clarity of bulk PI and PCN materials in the form of membranes were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), molecular permeability analysis (GPA) and ultraviolet‐visible (UV/VIS) transmission spectra, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1072–1079, 2004     

Adhesion and permeability of polyimide–clay nanocomposite films for protective coatings

A polyimide (PI)–clay nanocomposite was prepared from a solution of poly(amic acid), a precursor of 2,2‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]propane dianhydride and p‐phenylenediamine, and dodecylamine–montmorillonite. Fourier transform infrared spectroscopy, thermogravimetric analysis, X‐ray diffraction, and atomic absorption spectroscopy were used to verify the incorporation of the modifying agents into the clay structure and the intercalation of the modified clay into the PI matrix. Both PI and PI–clay films were subsequently prepared by solution casting. The gas permeability, resistivity, and adhesion properties were determined. In the case of gas permeability, only a 3 wt % addition of clay reduced oxygen permeability to less than half that of unfilled PI. Furthermore, this hybrid showed an improvement in electrical resistivity because of the prevention of electrical tree growth by clay particles. More importantly, adhesion between the films and silicon increased with increasing clay content. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2875–2881, 2003     

Silver–polyimide nanocomposite membranes: Macromolecular‐matrix‐mediated metallization of an aromatic,fluorinated polyimide yielding highly reflective films at low metal concentrations

Highly reflective, surface‐metalized, flexible polyimide films were prepared by the incorporation of a soluble silver‐ion complex, (hexafluoroacetylacetonato)silver(I) (AgHFA), into dimethylacetamide solutions of poly(amic acid) prepared from 2,2‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride and 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane. The thermal curing of solution‐cast silver(I)–poly(amic acid) films to 300°C led to cycloimidization of the amic acid with concomitant silver(I) reduction and the formation of a reflective, air‐side‐silvered surface at very low (2 wt % and 0.3 vol %) silver concentrations. The reflective surface evolved only when the cure temperature reached about 275°C, although X‐ray diffraction showed metallic silver in the hybrid film by 200°C. After a maximum specular reflectivity greater than 80% was achieved for the 2 wt % silver film, the specular reflectivity diminished sharply with further heating at a constant temperature of 300°C. Incorporating the AgHFA complex into the soluble, fully imidized form of poly{(1,3‐dihydro‐1,3‐dioxo‐2H‐isoindole‐2,5‐diyl)[2,2,2‐trifluoro‐1‐(trifluoromethyl)ethylidene](1,3‐dihydro‐1,3‐dioxo‐2H‐isoindole‐5,2‐diyl)‐1,4‐phenyleneoxy‐1,4‐phenylene[2,2,2‐trifluoro‐1‐(trifluoromethyl)ethylidene]‐1,4‐phenyleneoxy‐1,4‐phenylene} gave films that were 25% less reflective than those beginning with poly(amic acid). Though highly reflective, the films were not electrically conductive. The metalized membranes were thermally stable and maintained mechanical properties similar to those of the parent polyimide. Transmission electron microscopy revealed an air‐side, near‐surface layer of silver that was about 40 nm thick; the interior of the film had well‐dispersed metal particles with diameters mostly less than 2 nm. The near‐surface silver layer maintained its integrity because of physical entrapment of the metal nanoparticles beneath a thin layer of polyimide; that is, the practical adhesion of the metal layer was good. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2409–2418, 2007     

Influence of structure of amines on the properties of amines‐modified reduced graphene oxide/polyimide composites

Graphene oxide (GO), as an important precursor of graphene, was functionalized using alkyl‐amines with different structure and then reduced to prepare reduced amines grafted graphene oxide (RAGOs) by N₂H₄ · H₂O. The successful chemical amidation reaction between amine groups of alkyl‐amines and carboxyl groups of GO was confirmed by Fourier transform infrared (FTIR), X‐ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA). Then RAGOs/polyimide nanocomposites were prepared via in situ polymerization and thermal curing process with different loadings of RAGOs. The modification of amine chains lead to homogenous dispersion of RAGOs in the composites and it formed strong interfacial adhesion between RAGOs and the polymer matrix. The mechanical and electrical properties of polyimide (PI) were significantly improved by incorporation of a small amount of RAGOs, the influence of structure of amines grafted on RAGOs on the enhancement effects of composites was discussed. The research results indicated that the proper structure of amine could effectively enhance the properties of composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43820.     

Relaxation behavior of conductive carbon black reinforced chlorosulfonated polyethylene composites

Dynamic mechanical analysis and dielectric relaxation spectra of conductive carbon black reinforced chlorosulfonated polyethylene (CSM) composites were used to study their relaxation behavior as a function of temperature and frequency, respectively. A marginal increase in glass transition temperature has been observed upto 30 phr carbon black filled polymer composite, beyond which it decreases, which has been explained on the basis of aggregation of filler particles in the polymer matrix. The strain dependent dynamical parameters were evaluated at dynamic strain amplitudes of 0.1–200%. The nonlinearity in storage modulus increases with increase in filler loading. It can be explained on the basis of filler–polymer interaction and aggregation of the filler particulates. The frequency dependent dynamical mechanical analysis has also been studied at frequency range of 0.1–100 Hz. The variation in real and complex part of impedance with frequency has been studied as a function of filler loading. The effect of filler loading on ac conductivity has been observed as a function of frequency. An increase in conductivity value has been observed with increase in filler loading. This can be explained on the basis of formation of conducting paths between filler particulates. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Semi‐aromatic polyimide/Ag nanocomposite derived from vanillin

The development of bioprecursor polyimide/Ag nanocomposites (PI/Ag NCs) is reported in this investigation. Semiaromatic bioprecursor PI was successfully synthesized through direct polycondensation reaction between aromatic diamine containing pyridine ring and aliphatic dianhydride. Aromatic diamine as a monomer was synthesized using a renewable resource, vanillin. The main attractive aspects of this PI are the renewable origin of the diamine, presence of pyridine and high aromatic rings content, as well as aliphatic content on the polymer backbone. The structure of synthesized monomer and PI were proven by FTIR, and nuclear magnetic resonance. The PI/Ag NCs containing 3, 5, and 7 wt % of Ag nanoparticles (Ag NPs) were prepared through solution technique and the resulting NCs were characterized by Fourier transform infrared spectra, wide angle X‐ray diffraction, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). TEM results showed that the Ag NPs were dispersed homogeneously in the PI matrix on nanoscale. TGA results indicated improving in thermal properties of PI/Ag NCs compared to the neat PI due to the interaction between the PI matrix and the Ag NPs. Antibacterial activity of PI/Ag NCs was tested by the disk diffusion method using Escherichia coli as model strain of gram‐negative bacteria. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44001.     

Enhanced mechanical and anisotropic thermal conductive properties of polyimide nanocomposite films reinforced with hexagonal boron nitride nanosheets

To attain thermally conductive but electrically insulating polymer films, in this study, polyimide (PI) nanocomposite films with 1–30 wt% functionalized hexagonal boron nitride nanosheets (BNNSs) were fabricated via solution casting and following imidization. The microstructures, mechanical and thermal conductive properties of PI/BNNS nanocomposite films were examined by taking account of the relative content, anisotropic orientation, and interfacial interaction of BNNS and PI matrix. The scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry data revealed that BNNSs with hydroxy and amino functional groups have specific molecular interactions with PI matrix and they form stacked aggregates in the nanocomposite films with high BNNS loadings of 10–30 wt%. The tensile mechanical strength/modulus, thermal degradation temperatures, and thermal conductivity of the nanocomposite films were found to be significantly enhanced with increasing the BNNS loadings. For the nanocomposite films with 1–30 wt% BNNS loadings, the in-plane thermal conductivity was measured to be 1.82–2.38 W/mK, which were much higher than the out-of-plane values of 0.35–1.14 W/mK. The significant anisotropic thermal conductivity of the nanocomposite films was found to be owing to the synergistic anisotropic orientation effects of both BNNS and PI matrix. It is noticeable that the in-plane and out-of-plane thermal conductivity values of the nanocomposite film with 30 wt% BNNS were ~1.31 and ~3.35 times higher than those of neat PI film, respectively.     

Preparation and properties of elastic polyimide‐silica composites using silanol sol from water glass

A new approach for obtaining elastic polyimide‐silica composites using a silanol sol prepared from water glass and the imide‐containing elastic polymers (IEPs) with polytetramethyleneoxide (PTMO) soft‐segment was investigated. Methods of increasing the degree of compatibility between the silica phase prepared from a silanol sol and IEPs obtained via elastic polyureas were examined. Elastic polyimide‐silica composites were obtained by the thermal treatment at 200°C for 4 h in vacuo after N‐methyl‐2‐pyrrolidone was evaporated from the IEP precursor solutions to which the silanol tetrahydrofuran solution and Bis(trimethoxysilylpropyl)amine (BisA) were added. The use of BisA possessing a reaction site with a carboxylic acid group on the IEP precursors improved the degree of compatibility between the IEPs and silica. Transparent composites were obtained when the concentration of SiO₂ was below 22 wt %. FT‐IR analyses confirmed that the composite was a segmented hybrid material composed of the PTMO segment, the imide segment, and SiO₂. The silica matrix obtained via silanol sol from water glass and the silica matrix obtained by the conventional sol‐gel process with an alkoxysilane were essentially the same, and there was no significant loss of silicon due to incomplete hydrolysis of the alkoxides when preparing composites via the water glass route, in contrast to the situation that can occur in the case of the sol‐gel route. Dynamic mechanical and thermal analyses suggested that microphase separation between the imide segment and the PTMO segment occurred in the composites and that there was a substantial amount of phase mixing at the same time. The formation of the silica composite had a great influence on the mobility of the segmnets in the phase‐mixing domain. TGA analyses indicated that the formation of the 10 wt % silica hybrid gave a composite with a 50°C higher degradation temperature. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2211–2219, 2004     

Effect of amphiphilic compatibilizers on the filler dispersion and properties of polyethylene—thermally reduced graphene nanocomposites

Compatibilizers of different chemical structures and specifications were used to enhance the filler exfoliation in nanocomposites of polyethylene and thermally reduced graphene prepared by melt mixing route. The mechanical performance of the compatibilized nanocomposites was observed to be better than PE/G nanocomposites due to enhanced extent of filler exfoliation and distribution. Highest increase of 45% in tensile modulus and 13% in peak stress was observed in the composites. Overall, from the mechanical, rheological, thermal, and calorimetric properties, the compatibilizers with best performance were ethylene acrylic acid (EAA) copolymer and chlorinated polyethylene (CPE25). Furthermore, the extent of filler exfoliation was observed to increase with increasing EAA content thus confirming positive interactions between EAA and thermally reduced graphene, though no specific chemical interactions could be detected. The composite properties were observed to reach maximum around 7.5 wt % EAA content, followed by reduced performance due to extensive matrix plasticization. The observed behaviors were a result of interplay of opposing factors like filler exfoliation due to compatibilizer addition and matrix plasticization due to its lower molecular weight, thus the observed optimum comaptibilizer amount was specific to the compatibilizer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42484.     

Bis[4‐(4‐maleimidephen‐oxy)phenyl]propane/N,N‐4,4′‐bismaleimidodiphenylmethyene blend modified with diallyl bisphenol A

In this article, 2,2′‐bis[4‐(4‐maleimidephen‐oxy)phenyl)]propane (BMPP) resin and N,N‐4,4′‐bismaleimidodiphenylmethyene (BDM) resin blends were modified by diallyl bisphenol A (DABPA). The effects of the mole concentration of BMPP on mechanical properties, fracture toughness, and heat resistance of the modified resins were investigated. Scanning electron microscopy was used to study the microstructure of the fractured modified resins. The introduction of BMPP resin improves the fracture toughness and impact strength of the cured resins, whose thermal stabilities are hardly affected. Dynamic mechanical analysis shows that the modified resins can maintain good mechanical properties at 270.0°C, and their glass transition temperatures (T_g) are above 280.0°C. When the mole ratio of BDM : BMPP is 2 : 1(Code 3), the cured resin performs excellent thermal stability and mechanical property. Its T_g is 298°C, and the Charpy impact strength is 20.46 KJ/m². The plane strain critical stress intensity factor (K_IC) is 1.21 MPa·m^0.5 and the plane strain critical strain energy release rate (G_IC) is 295.64 J/m². Compared with that of BDM/DABPA system, the K_IC and G_IC values of Code 3 are improved by 34.07% and 68.10%, respectively, which show that the modified resin presented good fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40395.     

Influence of change in ether structure on the low temperature dielectric relaxation of some poly(ether imide)

Broad band dielectric relaxation spectra are reported on a range of polymers created by varying the ether segment in a series of poly(ether imide)s. Changes in the structure allow the effects of steric constraints on the local conformational dynamics of the polymer chain to be explored. These changes have a significant effect on the glass transition temperatures of these polymers which range from 245 to over 420°C. In contrast, the low temperature dielectric relaxation behavior of these polymers is very similar and is attributed to cooperative local oscillatory—librational motions. Changes in the stereochemistry effect the amplitude, activation energy for the relaxation process, the packing chain density, and values of the high frequency limiting permittivity, ε_∞′. This latter parameter is sensitive to the extent of dipole induced dipole and π‐π electron interactions and is influenced by the packing density. The magnitude of ε_∞′ is a very important parameter in determining the suitability of poly(imide)s for electrical applications. The magnitude of ε_∞′ increases with the density; however, deviations from this general trend are observed when large nonpolar groups inhibit the interaction of neighboring chains. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41191.     

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007