Size Effect of Al2O3 Nanowires on the Molecular Relaxation in Epoxy Composites

The size effect of Al₂O₃ nanowires on the molecular relaxation of an epoxy system was investigated by means of DSC and dielectric relaxation spectroscopy. T_g and activation energy of dielectric relaxation decreased with the incorporation of the nanowires. To explain the observed results, a coarse‐grained confined‐region structure model is proposed, and on the basis of this model, the change in molecular motion arising from the nanowires could be understood. The variations of thermal stability and water absorption for the nanocomposites were also discussed. These original experimental results should be helpful in scientific research and nanocomposite applications.

     

Kinetic Analysis of Isothermal and Nonisothermal Epoxy‐Amine Cures by Model‐Free Isoconversional Methods

Model‐free and model‐fitting kinetic approaches were applied to isothermal and nonisothermal DSC data on epoxy cure. The dependence of the effective activation energy on the extent of cure was determined using integral and differential isoconversional methods. A different shape of the dependence in the kinetically controlled regime (α < 0.4) was found when compared to the dependence typically reported for epoxy‐amine systems. This result was shown to arise from the presence of low amounts of bisphenol A in the present epoxy that are efficient catalysts for epoxy amine reaction. A model‐free method prediction of isothermal cure from isothermal data has also been considered.

     

In‐situ‐Synthetized Silver/Epoxy Nanocomposites: Electrical Characterization by Means of Dielectric Spectroscopy

In situ synthesis of silver/epoxy nanocomposites was achieved by UV‐induced polymerization through a simultaneous photoinduced electron transfer and cationic polymerization processes. The FESEM morphological investigation showed a uniform dispersion of silver nanoparticles within the polymeric matrix having narrow size distribution between 15 and 20 nm. The evolution of dielectric properties with frequency and NP content seem to indicate that the nanocomposites of this study remain below the percolation threshold. The dielectric spectroscopy results are in good agreement with the morphological investigation, which showed the absence of NPs percolative network.

     

Using Molecular Simulation to Explore Unusually Low Moisture Uptake in Amine‐Cured Epoxy Carbon Fiber Reinforced Nanocomposites

Reinforced nanocomposites based on a diamine‐cured diglycidylether of bisphenol A are reported containing a monoamino‐substituted polyhedral oligomeric silsesquioxane (POSS) reagent and polyacrylonitrile fibers. Covalent incorporation of monoamino‐POSS at 2 wt% leads to especially attractive properties, including an increase (20 K) in glass transition temperature (T_g) and thermal stability (increase in char yield of up to 5%). The addition of POSS (2 wt%) in the diglycidylether of bisphenol A (DGEBA) leads to a reduction in moisture uptake of less than 0.30–0.91 wt%, depending on relative humidity (after 6000 h), with little effect on T_g (reduction of 9–11 K compared with 11–22 K in the unmodified DGEBA). Molecular dynamics simulation is used to visualize the cured network structure of these nanocomposites, relating free volume to water uptake. Translation of the properties from neat resin to CFRP is very encouraging with a reduction in the equilibrium moisture absorption of up to 29% in the latter.

     

Modeling the Cure of an Epoxy‐Amine Resin with Bisphenol A as an External Catalyst

The cure kinetics of a DGEBF/DGEBA mixture with a commercial aliphatic amine mixture [poly(propylene) oxide diamine D230 and diethylene triamine (DETA)] was investigated by DSC. Two types of kinetic models based upon the formation of amine‐epoxy‐hydroxyl transition complex without or with fast pre‐equilibria have been tested and discussed. The mechanistic model with pre‐equilibria was based on the existence of two different epoxy‐hydroxyl complexes namely one involving hydroxyl groups of BPA and the other, the hydroxyl groups of polymer chains. All the kinetic and thermodynamic parameters have been evaluated and discussed. The results show that the mechanistic model with pre‐equilibria determined from isothermal cure provide a more reasonable fitting of the polymerization kinetics under programmed heating‐rate modes than Horie's model. Moreover, the former kinetic model allows the prediction of changes in the BPA content of the initial formulation with good accuracy.

     

Electrically Insulated Epoxy Nanocomposites Reinforced with Synergistic Core–Shell SiO2@MWCNTs and Montmorillonite Bifillers

With unique physicochemical properties, multiwalled carbon nanotubes (MWCNTs) have enabled major achievement in polymer composites as reinforcing fillers. Nevertheless, high conductivity of raw MWCNTs (R‐MWCNTs) limits their wider applications in certain fields, which require outstanding thermal conductivity, mechanical, and insulation properties simultaneously. In this article, silica (SiO₂) coated MWCNTs core–shell hybrids (SiO₂@MWCNTs) and organically modified montmorillonite (O‐MMT) are employed to modify epoxy (EP) simultaneously. The epoxy‐clay system is cured by using anhydride curing agent. The impact strength and flexural strength of final nanocomposites are greatly improved. Meanwhile, the final composites remain in high electrical insulation. Compared to mixed acid treated MWCNTs (C‐MWCNTs) (0.5 wt%)/EP nanocomposites, the volume resistivity of the O‐MMT(4 wt%)/SiO₂@MWCNTs(0.5 wt%)/EP nanocomposites increases more than six orders of magnitude. Synergistic toughening effect occurs when using core–shell SiO₂@MWCNTs and MMT bifillers. The electrical insulation is attributed to the suppressed electron transport effect by SiO₂ layer on the CNTs surface, and the blocked conductive CNTs network by the buried 2D structural O‐MMT. The SiO₂@MWCNTs core–shell hybrids also benefit to decrease the dielectric constant and dielectric loss of CNTs/EP composites. This work provides guidance to using CNTs as reinforcement fillers to toughen the polymers for electric insulating applications.     

Bisphosphonic Acid‐Functionalized Water‐Soluble Photoinitiators

The first bisphosphonic acid‐functionalized benzophenone (BP) and thioxanthone (TX) photoinitiators (PIs), BPBP and TXBP, are synthesized as a new class of water‐soluble PIs for free radical polymerization. BPBP shows excellent solubility (28 g L^?1) in water at ambient temperature, compared to TXBP (0.05 g L^?1) and also the commercial PI Irgacure 2959 (5 g L^?1). BPBP and TXBP show UV–vis absorption at ≈260 (ε = 12912) and 405 (ε = 3314) nm in water. Photopolymerization results demonstrate that they can successfully initiate the photopolymerization of poly(ethylene glycol) diacrylate (M_n = 250 D) in the presence of bis‐(4‐tert‐butylphenyl)‐iodonium hexafluorophosphate (Iod). The photochemical mechanisms are investigated by electron‐spin resonance‐spin trapping, fluorescence, and steady‐state photolysis techniques.     

Enthalpy Relaxation in Poly(4‐hydroxystyrene)/Poly(methyl methacrylate) Blends

Summary: The influence of blend composition on enthalpy relaxation behaviour was assessed for miscible blends of poly(4‐hydroxystyrene)/poly(methyl methacrylate) (PHS/PMMA). Values of enthalpy lost (ΔH(T_a, t_a)) were calculated from experimental data plotted against log₁₀(t_a) and modelled using the Cowie‐Ferguson (CF) semi‐empirical model. This gives a set of values for three adjustable parameters, ΔH_∞(T_a), log₁₀(t_c) and β. The blends relaxed more slowly than PMMA, but more quickly and less co‐operatively than PHS. Moreover, the blends released more enthalpy than PMMA, but less than PHS. The enthalpy lost by the fully relaxed glass (ΔH_∞(T_a)) was less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (ΔH_max(T_a)). Infrared spectroscopy was used for assessing the hydrogen bonding interactions in the blends. The ageing results are discussed with reference to the hydrogen bonding interactions.

Dependence of ΔT (□) and ΔC_p (?) on PHS/PMMA blend composition.     

A Facile Route to Bio‐Inspired Supramolecular Oligo(Ethylene Glycol) Catecholates

A particularly facile synthetic route to mussel‐inspired oligo(ethylene glycol) catecholates is described, yielding high purity materials with minimal purification. The oligocatecholates show remarkable thermal and rheological properties for their actual chain length, suggesting the operation of non‐covalent interactions between their l ‐DOPA‐bearing chain ends. This end‐group effect is more pronounced in the shorter oligomers (M _n of ethylene glycol chain of 200 and 400 Da), where the end‐group density is higher. Various surfaces (glass, stainless steel) are modified with the oligocatecholates, using a dip‐coating approach from dilute aqueous solutions. Water contact angle measurements and X‐ray photoelectron spectroscopy confirm the presence of a hydrophilic surface coating layer. The potential of these materials for application as antifouling coatings and viscosity modifiers is highlighted.     

Enthalpy and Volume Relaxation of Core‐Crosslinked Star Polystyrene/Poly(methyl methacrylate) Blends

Blends of linear and core‐crosslinked star (CCS) polymers are prepared. The relaxation and thermal properties of the blends are determined using conventional and modulated‐temperature DSC and modulated‐temperature thermomechanometry. Addition of CCS polymers increase the glass transition temperature while decreasing the enthalpy and the linear coefficient of thermal expansion, suggesting that the hyperbranched polymers restrict matrix chain motions. Isothermal annealing increased the T_g and ΔH and decreased sub‐T_g α due to the reduced free volume and mobility within the polymer films. Volume relaxation during annealing is observed using mT‐TM, while a stretched‐exponential function is utilised to interpret the data.

     

Nucleation‐Driven Kinetics of Poly(ethylene terephthalate) Melting

The kinetics of poly(ethylene terephthalate) melting are studied by DSC and treated in the framework of a nucleation kinetic model. The study focuses on the temperature dependence of the effective activation energy of melting. A theoretical dependence is derived from the model and an experimental one from the Kissinger plot. It is demonstrated that the theoretical dependence satisfactorily predicts the experimental one and fitting the former to the latter can be used for estimating the surface free energy of a nucleus.

     

Branched Base‐Amplifying Oligomers Enhancing UV‐Curing of Epoxy Resins

This paper describes the preparation and the enhancing effect on anionic UV‐curing of epoxy resins of three and four‐armed oligomers bearing 9‐fluorenylmethyl carbamate residues, which displayed the autocatalytic formation of primary amino residues triggered by a photogenerated amine. The branched oligomers were readily synthesized by the Michael addition of the corresponding polythiol with 9‐fluorenylmethyl 2‐acryloyloxyethylcarbamate, which were obtained from 9‐fluorenylmethanol and commercially available 2‐isocyanatoethyl acrylate. Films of poly(glycidyl methacrylate) and a glycidylated novolac resin doped with a photobase generator exhibited boosted photosensitivity leading to photoinsolubilization in the presence of the base‐amplifying oligomers. The more marked effect of the branched oligomers on UV‐curing of epoxy resins was demonstrated by using films of a fluid epoxy, which were converted into hard films after UV exposure and post‐exposure baking, whereas no hardening occurred in the absence of the oligomers.

     

Poly(propylene)‐Layered Silicate Nanocomposites: Gas Permeation Properties and Clay Exfoliation

To monitor the mineral surface coverage and energy, the inorganic cations of two clays (montmorillonite) with different surface area/cation have been exchanged by alkylammonium ions, carrying alkyl chains of different number and length. The prepared OMs were free of unreacted organic ions. With increasing length and number of the alkyl chains, an increase in the basal‐plane spacing (d‐spacing) of the OM was observed. The d‐spacing also increased with increasing CEC of the clay (decreasing available area/cation). The OMs were compounded with PP and their effect on the crystallinity and gas‐barrier properties of the polymer was investigated. The OM had no influence on the degree of crystallinity of PP under the processing conditions used. Oxygen permeation through the composites decreased, depending on the cross‐sectional area of the exchanged organic cation and the CEC of the clay. These parameters control the mineral surface coverage (consequently the surface energy) as well as the tilt angle of the alkyl chains to the mineral surface, and hence the d‐spacing. Increasing the length of the alkyl chains and their number per cation enhanced the d‐spacing, clay exfoliation, and the gas‐barrier properties of the composites. A mixed morphology, consisting of delaminated aluminosilicate layers and OM tactoids of varying thickness was observed but no intercalation took place. The oxygen permeation coefficient of the nanocomposites was found to be a non‐linear function of the volume fraction of the inorganic part of the OM.

     

Synthesis and Characterization of Poly(glycolic acid‐alt‐6‐aminohexanoic acid) and Poly(glycolic acid‐alt‐11‐aminoundecanoic acid)

Summary: Poly(ester amide)s derived from glycolic acid and ω‐amino acid units, such as aminohexanoic or aminoundecanoic acids, are synthesized by a thermal polycondensation reaction that involves the formation of metal halide salts. Polymerization kinetics of different metal salts are studied by isothermal and nonisothermal methods and the corresponding parameters compared. The condensation reaction begins in the solid state for the aminohexanoic derivatives, although a rapid liquefaction is observed. On the other hand, the melting temperatures of the sodium and the potassium chloroacetylaminoundecanoate salts are lower than the reaction temperatures, and consequently polycondensation proceeds fully in the liquefied state. These polymers are characterized by an alternate disposition of ester and amide groups and can be obtained with high molecular weights and short polymerization times. Thermal properties (glass transition and melting temperatures) of the two new polymers are determined and compared. Thermal stability is also investigated; the results indicated that decomposition temperatures were always far from both reaction and polymer fusion temperatures.

DSC heating scans performed at different rates for potassium chloroacetylaminoundecanote.     

Enhanced Performance of Graphene–Epoxy Flexible Capacitors by Means of Ceramic Fillers

Graphene–epoxy flexible capacitors are obtained by graphene–polymer transfer and bonding via UV‐cured epoxy adhesive. Ceramic fillers are dispersed into the epoxy resin with the aim of enhancing the capacitive behavior of the final device. Parallel plate capacitors in which epoxy resin is filled with ceramic nanoparticles demonstrate superior performance, up to two orders of magnitude better than unfilled samples. Zirconia, showing the highest dielectric constant, fails to give a stable output in the whole frequency range, as some competing phenomena occur and reduce the overall polarization of the system. Boehmite appears to be a better choice and gives reasonable performances, better than gibbsite

     

Coil‐to‐Globule Transition of Poly(N‐isopropylacrylamide) in Aqueous Solution: Kinetics in Bulk and Nanopores

Differential scanning calorimetry (DSC) is used to study the kinetics of the coil‐to‐globule transition in aqueous solutions of poly(N‐isopropoylacrylamide) (PNIPAM) prepared in the bulk (3 and 10 wt%) and nanoconfined (10 wt% inside 30 nm silica pores) forms. It is demonstrated that the kinetics can be described in terms of the classical nucleation model. The proposed treatment affords estimating the free‐energy barrier and pre‐exponential factor of the transition. The application of the nucleation model to the DSC data collected for the three systems studied provides physical insights into the effect of increasing the transition temperature due to dilution and nanoconfinement. Dilution appears to raise the free‐energy barrier, whereas nanoconfinement causes a decrease in the pre‐exponential factor.

     

The Initial Molecular Interactions in the Course of Enthalpy Relaxation and Nucleation in Polyethylene Terephthalate (PET) as Monitored by Combined Nanocalorimetry and FTIR Spectroscopy

Fast scanning calorimetry (FSC) and Fourier transform infrared (FTIR) spectroscopy are combined to trace  the evolution of the calorimetric properties and of the molecular interactions in a strongly crystallizing polymer, polyethylene terephthalate (PET). The minute sample amount (≈5 ng) required for FSC allows for the realization of cooling rates up to 50 000 K s⁻¹ and to quench the sample into the amorphous state. By subsequent annealing at varying temperatures T_a and times t_a enthalpy relaxation, (homogeneous) nucleation and crystallization can be monitored with high precision. Determining the difference IR spectra between a quenched and an annealed sample unravels the development of the intra- and inter-molecular interactions in detail; i) as first response taking place during enthalpy relaxation, the far reaching Coulomb interactions between the polar carbonyl (C ═ O) moieties are active; ii) in contrast, the methylene unit (CH₂) and the aromatic ring show a response only, if homogeneous nucleation sets in, while the ester (COC) moiety remains uninfluenced; iii) a hierarchy is observed in of the response of the different molecular moieties; iv) if crystallization comes to play, all molecular units are involved. The results are compared with recently published findings for polyamide 66 demonstrating the high molecular specificity of homogeneous nucleation and crystallization.     

Ordering Behavior of Layered Silicate Nanocomposites with a Cylindrical Triblock Copolymer

Summary: The ordering behavior of the nanocomposites of organically modified montmorillonite (OMMT) with a cylindrical triblock copolymer of polystyrene‐block‐poly(ethylene‐co‐butylene)‐block‐polystyrene (SEBS) has been investigated by temperature‐resolved small‐angle X‐ray scattering (SAXS) and rheometry. X‐Ray diffraction (XRD) confirms that the polymer chains are successfully intercalated with the interlayer gallery of the silicates. The data obtained from the SAXS and rheological measurements show that the addition of OMMT leads to a change in the microphase separation behavior of SEBS in the nanocomposites. The molecular interaction between OMMT and the polystyrene (PS) chains of SEBS decreases the structural perfection of the self‐assembling, phase‐separated domain structure of the nanocomposites. Rheological data exhibit that the order‐order (T_OOT) and order‐disorder transitions (T_ODT) of the SEBS/OMMT nanocomposites decrease with the addition of OMMT. The highest elongation at break is obtained at approximately 2% OMMT and its further addition to the mixture leads to decreases in tensile strength and elongation.

The change in the storage modulus (G′) of a) SEBS, b) S98M2, c) S95M5, and d) S90M10, as a function of temperature in the range of 150 ≤ T (°C) ≤ 260.     

Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Summary: Sequential crystallization of poly(L ‐lactide) (PLLA) followed by poly(ε‐caprolactone) (PCL) in double crystalline PLLA‐b‐PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.‐% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 °C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 °C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 °C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre‐crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers.

Polarized optical micrographs during isothermal crystallization of (a) homo‐PLLA, (b) homo‐PCL, (c) and (d) block copolymer after 30 min at 122 °C and after 15 min at 42 °C.     

Novel Thermosetting Resins Based on 4‐(N‐Maleimido)phenylglycidylether, 4

Summary: A series of polymer‐clay nanocomposites were prepared by the simultaneous in situ curing of 4‐(N‐maleimido)phenylglycidylether/4,4‐diaminodiphenylmethane (MPGE/DDM) with the intercalation/exfoliation of various organo‐modified clays. The dispersion of the clays in the polymer matrix was produced by partial exfoliation and intercalation, as observed with X‐ray diffraction and transmission electronic microscopy. Clay modified with a carboxylic acid‐containing modifier showed enhancements with repesect to clay delamination in nanocomposite preparation. In comparison with the pristine resin, MPGE/DDM‐clay nanocomposites showed improved glass transition temperatures, thermal stability and flame retardance.

Preparation and characterization of polymer‐clay nanocomposites based on a new maleimide‐epoxy hybrid monomer.