Surface modification of cellulose nanowhiskers for application in thermosetting epoxy polymers

Cellulose nanowhiskers (CNWs) were chemically modified with dodecenyl succinic anhydride to obtain hydrophobic CNWs called DCNWs. Surface modification was confirmed by infrared spectroscopy, transmission electron microscopy, and X-ray diffraction. The surface substitution degree determined by X-ray photoelectron spectroscopy was 0.30. Nanocomposites were prepared by incorporating different amounts of DCNWs pre-dispersed in a small amount of acetone into an epoxy matrix. Scanning electron microscope demonstrated that DCNWs dispersed well in the epoxy matrix. A strong interaction was proved between the DCNWs and epoxy matrix, as results of which the nanocomposites exhibited an obvious increase in T _g by about 30 °C, simultaneous increases in tensile strength, Young’s modulus, and strain at break; and an improvement in the hydrothermal properties. Compared with the neat epoxy, the nanocomposite containing 3.5 wt% of DCNWs exhibited an increase in tensile strength by 82 %, Young’s modulus by 21 %, and a strain at break by 198 %.     

Chemical modification of polymers. XII. Control of triboelectric charging properties of polymers by chemical modification

Chemical modification of polymers leads to changes in triboelectric charging properties which are proportional to the degree of conversion. This contrasts to physical mixtures in which the surface energy causes one component to dominate the surface, hence the charging properties. Relationships between molecular structure and triboelectric charging have been deduced from the results. Thus the direction of change of triboelectric charging on chemical modification is governed by the nature of the reaction and the magnitude of the change is governed by the extent of reaction.     

Chemical modification of natural polymers in China

Abstract

This paper introduces the main Chinese research work on the chemical modification of natural polymers including silk, Chinese lacquer, gutta-percha, cellulose, and chitin. The following aspects of this research work are emphasized: research on the mechanism of graft copolymerization of vinyl monomer onto natural polymers, research on overcoming the defects of natural polymers to allow further application by chemical modification, and research on exploring new applications of natural polymers.     

Chemical modification of rolivsans with epoxy resins

New glassy network copolymers exhibiting enhanced heat resistance were prepared by thermochemical transformations of rolivsans modified with small amounts (5 ± 3%) of epoxy resins. The influence of the composition of these copolymers on their thermomechanical properties was evaluated by performing dynamic mechanical and thermal analysis and by studying the temperature dependences of the ultimate strength at three-point bending.     

Chemical and structural modification of polymers by matrix polymerization

The condensation polymerization of urea and formaldehyde in water in the presence of a macromolecular matrix, polyacrylic acid, as well as properties of composites obtained by the matrix polymerization, have been studied. The chemical structure of the urea-formaldehyde polymer was changed by the presence of the matrix. The products of matrix polymerization are composites consisting of a stoichiometric polycomplex of polyacrylic acid with the urea-formaldehyde polymer and an excess of one of the components, depending on the initial composition of the system. Introduction of matrix macromolecules into the reacting system leads to a fibrillized structure of the urea-formaldehyde polymer. The polycomplex, as well as composites with high content of polyacrylic acid, are glasses characterized by high compressive strength and ability to forced elastic deformation. They swell in water. The highly cooperative character of potentiometric titration curves for swollen gels and the quantitative yield in intermolecular amidation indicate an ideal structure of the polycomplex stabilized by hydrogen and ionic-type bonds.     

Synthesis and characterization of novel polythiourethane hardeners for epoxy resins

A novel thiol-terminated polythiourethanes were synthesized from low-molecular-weight di- and multifunctional mercaptans and diisocyanates and employed as curing agent of epoxy resin. The curing reaction of epoxy resin and thermal properties of cured products were investigated with differential scanning calorimetry. Evaluation of climatic ageing resistance was made by the change in mechanical properties. Mechanical studies indicated that the application of polythiourethane has toughening effect and significantly increases ageing resistance of the cured resins. The results of this study indicate that molecular structure and functionality of polythiourethane oligomers are of critical importance in governing the curing mechanism, structure of the network and final properties of epoxy compositions.     

Identification of hardeners in solid epoxy resins by high-resolution 13C-NMR

CPMAS ¹³C-NMR spectra at 75 MHz with spinning side band suppression are presented for a series of epoxy resins cured with a variety of commercially important hardeners. The hardeners can be readily identified from the spectra; the high-field is important for distinguishing between related amine hardeners.     

Interfacial and wetting properties of carbon fiber reinforced epoxy composites with different hardeners by electrical resistance measurement

In this research, interfacial and wetting properties of N,N,N,N-tetraglycidyl-4,4-diaminodiphenylmethane (TGDDM) epoxy resin with two hardeners with different chemical structure were evaluated by electrical resistance (ER) measurement. The heat of reaction of TGDDM epoxy with the two different hardeners, 33 and 44 di-amino di-phenyl sulphone (DDS), was analyzed by differential scanning calorimetry (DSC). The TGDDM epoxy exhibited different mechanical properties with the two different DDS hardeners. Combined ER, wetting measurements and the microdroplet test were used for evaluating the spreading effect and interfacial shear strength (IFSS) of carbon fiber (CF) reinforced TGDDM epoxy composites with these different hardeners. The heat of reaction and mechanical properties of TGDDM/DDS were influenced by the chemical structure and different free volumes of the epoxy resins. The relationships between the ER-wetting results and the IFSS were internally consistent. Ultimately it was demonstrated that ER measurements makes it possible to estimate the interfacial and wetting properties of CF reinforced epoxy composites.     

The effect of different hardeners on the ageing of crosslinked epoxy resins

Polymers based on epoxies are widely used in the industrial production of materials for different applications. The crosslinking is often done with hardeners like aliphatic or aromatic amines, with dicarboxylic acid anhydride or anionically. The resins containing mineral fillers possess similar thermal mechanical properties. On thermal oxidative ageing, however, the systems behave very differently. An account is given of the thermal mechanical behaviour and of the weight loss on ageing above the glass transition temperature. By using semiempirical molecular orbital calculations we have been able to identify a possible reason for the different thermal oxidative stability of epoxy resins crosslinked with different hardeners.     

Curing reaction of biphenyl epoxy resin with different phenolic functional hardeners

The investigation of cure kinetics and relationships between glass transition temperature and conversion of biphenyl epoxy resin (4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl) with different phenolic hardeners was performed by differential scanning calorimeter using an isothermal approach over the temperature range 120–150°C. All kinetic parameters of the curing reaction including the reaction order, activation energy, and rate constant were calculated and reported. The results indicate that the curing reaction of formulations using xylok and dicyclopentadiene type phenolic resins (DCPDP) as hardeners proceeds through a first-order kinetic mechanism, whereas the curing reaction of formulations using phenol novolac as a hardener goes through an autocatalytic kinetic mechanism. The differences of curing reaction with the change of hardener in biphenyl epoxy resin systems were explained with the relationships between T_g and reaction conversion using the DiBenedetto equation. A detailed cure mechanism in biphenyl-type epoxy resin with the different hardeners has been suggested. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 773–783, 1998     

Thermal-oxidative aging of DGEBA/EPN/LMPA epoxy system: Chemical structure and thermal-mechanical properties

The evolvement of chemical structure and thermal-mechanical properties of diglycidyl ether of bisphenol-A and novolac epoxy resin blends cured with low molecular polyamide (DGEBA/EPN/LMPA system) during thermal-oxidative aging were investigated by Attenuated Total Reflectance Fourier Transform Infrared spectrometry (ATR-FTIR) and Dynamic Mechanical Thermal Analysis (DMTA). The results revealed that the chemical reactions during thermal-oxidative aging contained oxidation and chain scission. Some possible chemical reaction processes were given. There was a new compound formed during aging processes and the change of its glass transition temperature (T_g) with aging time followed an exponential law. In addition, the changes of dynamic mechanical behavior of this epoxy system aged at four different temperatures (110 °C, 130 °C, 150 °C, 170 °C) were compared. An empirical formula was obtained through kinetic analysis and this formula can be used to predict the oxidative degree of the surface at different aging temperature.     

Study of photopolymers. XXXIII. Chemical modification of poly(epichlorohydrin) and its application as photosensitive polymers

High Tg. polymers containing both vinyloxy moiety and pendant phthalyl, 2-thiobenzoxazole or 2-thiobenzothiazole groups were synthesized by elimination reaction of poly(epichlorohydrin) with potassium hydroxide followed by substitution reaction of the obtained polymer with potassium phthalimide, potassium 2-thiobenzoxazole or potassium 2-thiobenzothiazole using tetrabutylammonium bromide as a phase transfer catalyst. The polymers containing vinyloxy moiety and pendant phthalyl or 2-thiobenzothiazole groups showed high photochemical reactivity and excellent practical photosensitivity as a positive type resist with photo-generated cationic catalyst such as 2,5-dibutoxy-4-morpholinobenzene diazonium hexafluoroanthimonate. On the other hand, the polymers had high practical photosensitivity and good resolution as a negative type resist with 2,6-di-(4′-azobenzal)-4-methylcyclohexanone as a photosensitizer.     

Chemical modification of polymers. XI. Photoreactive polymers from poly(vinylbenzyl chloride)

Photocrosslinking has been observed in a number of polymers derived by chemical modification of poly(vinylbenzyl chloride). Included are polymers bearing pendant benzophenone, stilbazolium, fluorenone, carbazole, and sulfonylazido moieties. The synthesis and properties of these polymers are described.     

Heterogeneity of structure and mechanical properties of polymers

Specific features of development of micro- and macrofibrils as well as the structure of their interfaces are considered for oriented filaments of high-density polyethylene with different initial supermolecular structures. As evidenced by SAXS, WAXS, EPR, Raman spectroscopy and electron microscopy, the melt-crystallized samples contain a greater amount of tie molecules connecting macro- and microfibrils than the samples crystallized from solution. This hampers slippage of fibrils past each other and does not allow high draw ratios to be achieved. It was found that the density of macrofibrillar ends in the drawn melt-crystallized samples is nearly an order of magnitude greater than that in the drawn samples crystallized from solution. This leads to generation of kink bands (dangerous large-scale defects) and, as a result, the sample, being oriented, fractures long before high draw ratios and a perfect fibrillar structure are reached. The ultraoriented samples produced from solution have a more perfect intrafibrillar structure, and the density of intrabrillar disordered regions is close to that of crystalline ones. Nevertheless, they do contain clusters of defects which limit their mechanical properties. The analysis of the Raman and X-ray data shows that these defects are localized at crystallite boundaries in the long periods. Possible routes for improvement of the parameters of the fibrillar structure and their relation with mechanical properties are discussed.     

Novel N‐methylbenzothiazolium salts as hardeners for epoxy and acrylate monomers

Novel N‐methylbenzothiazolium salts [N‐methyl‐2‐benzylthiobenzothiazolium, N‐methyl‐2‐(4‐nitrobenzylthio)benzothiazolium, N‐methyl‐2‐(1‐ethoxycarbonylethylthio)benzothiazolium, and N‐methyl‐2‐methylthiobenzothiazolium hexafluoroantimonates] were synthesized by the reaction of the corresponding 2‐substituted benzothiazole with dimethylsulfate, followed by anion exchange with KSbF₆. These benzothiazolium salts cationically polymerized an epoxy monomer by photoirradiation. They also polymerized an acrylate monomer via a photoradical process. The use of aromatic compounds such as 2‐ethyl‐9,10‐dimethoxyanthracene as photosensitizers was effective in enhancing the polymerization. These benzothiazolium salts also served as thermal cationic initiators. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3828–3837, 2003     

Properties of materials effect of thiokol on properties of epoxy polymers

The complex character of the effect of the concentration of liquid thiokol of brand, I heat treatment, and water on the complex of physicomechanical, relaxation, and adhesion properties of epoxy polymers was demonstrated.     

Influence of additives on the structure and properties of polymers

Investigations into the transformation of hexagonal into monoclinic modification in isotactic polypropylene (iPP) were carried out. The hexagonal phase () was obtained by crystallization from a melt of iPP samples containing small molecular compounds (additives). A decline in the amount of-modification and changes of dimensions of ordering areas, perpendicular to the chain axes of iPP, was analysed at various temperatures and times of heating.The interdependence between the structure of additives and the amount of-phase occurring and also the stability of this modification during thermal treatment was observed.Obtained results indicated that only some additives play the role of moderating agents for the phase transition process.A preliminary report of this work has been presented at XIII Congress of the International Union of Crystallography (9–18 August 1984).     

Processing,structure and properties of oriented polymers

The fabrication techniques now available for the production of highly oriented polymers are reviewed. These techniques include tensile drawing from both melt-spun and gel-spun polymers, extrusion under pressure from the melt, and hydrostatic extrusion, ram extrusion and die drawing in the solid phase. In addition, lyotropic and thermotropic liquid crystalline polymers offer new routes to very stiff and strong polymers. Following the review of processing methods, an account is given of low strain mechanical behaviour and its relationship to structure, thermal properties (including thermal conductivity and thermal expansion behaviour) and barrier properties (permeability to liquids and gases and solubility).     

Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties

The relationships between the composition, structure, chemistry of the surface, and sensor properties of nanocomposites SnO₂-M _n O _m (M _n O _m = Fe₂O₃, MoO₃, V₂O₅) obtained by chemical precipitation from solutions were analyzed. The relationships between the elemental and phase composition of the nanocomposites and the effect of the composition on the nanostructure and the acidic and oxidation properties of the nanocomposite surface were considered. The modification of the SnO₂ surface by other oxides makes it possible to control the type and density of the acid sites and the oxidation properties of the surface and to enhance the selectivity of the materials in the detection of various gases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1086–1105, June, 2008.     

Interfacial structure and properties in polystyrene/epoxy bilayer films

The interfacial structure and properties of immiscible deuterated polystyrene (dPS)/epoxy bilayer films were investigated with neutron reflectivity as functions of the composition of the epoxy layer, the thickness of the dPS layer, and the annealing time. We have found that the interfacial width and its growth rate depend strongly on the compositions of the epoxy layer but only weakly on the thickness of the dPS layer. The effect of the resin/crosslinker composition on the interfacial width and its growth rate is likely due to the different near‐surface structures that result for different epoxy stoichiometries. For an ultra‐thin dPS film (thickness = 2R_g), the data suggest a slight suppression of the growth of the interfacial width that could be due to confinement effects for the long‐chain molecules such as have been previously reported for a thickness of less than approximately 4R_g, where R_g is the radius of gyration of polymer molecules. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2653–2660, 2002