Study on the Synthesis and Interfacial Interaction Performance of Novel Dodecylamine‐Based Bonding Agents Used for Composite Solid Propellants

An effective pathway was explored to design and select proper bonding agents that could effectively improve the interfacial interactions between bonding agents and solid particles, with three novel synthesized alkyl bonding agents, dodecylamine‐N,N‐di‐2‐hydroxypropyl‐acetate (DIHPA), dodecylamine‐N,N‐di‐2‐hydroxypropyl‐hydroxy‐acetate (DIHPHA) and dodecylamine‐N,N‐di‐2‐hydroxypropyl‐cyano‐acetate (DIHPCA), as examples. Molecular dynamics simulation was applied to compare unit bond energies of these bonding agents with the [110] crystal face of ammonium perchlorate (AP) and the [120] crystal face of hexogen (RDX). The infrared test was used to characterize the interfacial interactions of these bonding agents with AP or RDX. XPS test was applied to calculate the adhesion percentage of the bonding agents on the surface of precoated AP or RDX particles. All of the above results indicated that these three bonding agents have strong interfacial interactions with AP or RDX in the order of DIHPCA>DIHPHA>DIHPA. The prepared three bonding agents were used in HTPB/AP/RDX/Al propellants, and their effects on tensile strength (σ), elongation under maximum tensile strength (ε_m), elongation at breaking point of the propellant (ε_b) and adhesion index (Φ) of the propellant were studied. The results show that the bonding agents improve the mechanical properties of the propellant in the order of DIHPCA>DIHPHA>DIHPA. The methods found from theoretical design, materials synthesis, and mechanistics studies up to practical application show effective guiding significance for choosing the proper bonding agent and improving the interfacial interactions between the solid particles and binder matrix.     

Application of Nitramines Coated with Nitrocellulose in Minimum Signature Isocyanate‐Cured Propellants

Cyclotrimethylenetrinitramine (RDX) coated with nitrocellulose (NC‐RDX) is prepared by an internal solution method and applied in a minimum signature isocyanate‐cured propellant. It was found that RDX was coated or bonded by NC to form NC‐RDX particles; the median particle diameter (d₅₀) and specific surface area of NC‐RDX are in the range from 150 to 240 μm and 0.03 to 0.04 m²⋅g⁻¹, respectively. The NC‐RDX particles could swell in nitrate ester plasticizers with relatively low swelling rate compared with NC added directly in the plasticizers. Different types of ballistic modifiers can be effectively added to NC‐RDX. It was experimentally shown that NC‐RDX can increase the content of NC in the propellant with viscosities in the range from 371 to 394 Pa s and improve the mechanical characteristics of the propellant with maximum tensile strength (σ_m) between 0.48 MPa<σ_m<1.92 MPa, elongation at maximum tensile strength (ε_m) between 28.0%<ε_m<37.3%, and elastic modulus between 3.18 MPa<E<8.68 MPa in the temperature range from −40 to +50 °C.     

Effect of Bonding Agent on the Mechanical Properties of GAP High‐Energy Propellant

Three kinds of bonding agent were chosen to improve the mechanical properties of GAP high‐energy composite propellant based on GAP, BuNENA, HMX, AP, and Al. These bonding agents are N,N ′‐bis(2‐hydroxyethyl) dimethylhydantoin and 1,3,5‐trisubstituted isocyanurates (BA1), cyano‐hydroxylated amines (BA2), and hyperbranched polyether with terminal groups substituted by hydroxyl, cyano and ester functional groups (BA3). To study the interaction between bonding agents and oxidizers, the effect of coating by bonding agents on the characteristic absorption peaks of AP and HMX were first studied by infrared spectroscopy. Then the effect of bonding agents on the adhesion work between oxidizers and binder system were determined. The results showed that BA2 has the strongest interaction with AP, whereas BA1 and BA3 have relatively strong interaction with HMX. The AP grain coated by BA2 has the strongest adhesion work to the binder system, and there is not much difference in the values of adhesion work towards binder system of three coated HMX grains. At last the three bonding agents were added to GAP propellant, which has a theoretical specific impulse of 276.03 s. When the three kinds of bonding agent was used alone, the sense of “dewetting” in propellant was relieved but still existed. The combination of BA2 to BA1 or BA3 improved the adhesions between oxidizers and binder system effectively, and the mechanical properties of GAP propellant reached to δ _m=0.69 MPa, ϵ _b=32.7 %.     

Preparation and Properties of a nRDX‐based Propellant

The incorporation of nano‐scaled cyclotrimethylene trinitramine (nRDX) in nitrocellulose (NC)‐based propellants poses processing problems when following conventional methods. Hence, a new preparation method containing a pre‐dispersion process was developed, by which 30 mass % RDX (290 nm) was incorporated in the propellant. Meanwhile, the corresponding 290 nm, 12.85 μm and 97.76 μm RDX‐based propellants were prepared for comparison using a conventional method. The morphology, structure, ballistic and mechanical properties of the prepared propellants were characterized by scanning electron microscopy (SEM), density analyzer, closed vessel (CV), uniaxial tensile tester and impact tester. The results indicate that the nRDX particles were uniformly dispersed in the NC/NG/TEGDN matrix using the novel method, while agglomerated and recrystallized into large particles with the conventional method. The propellant density increased with decreasing RDX particle size. In particular, the 290 nm RDX‐based propellant exhibited a higher burning rate and lower average pressure exponent (α =0.958) compared to the 12.85 μm RDX‐based propellant (α =1.043). The tensile strength, elongation at break and impact strength of the RDX‐based propellant at −40 °C, 20 °C and 50 °C were dramatically improved by using 290 nm RDX with the novel method.     

Formation and Characterization of Nano‐sized RDX Particles Produced Using the RESS‐AS Process

It has been shown that nano‐sized particles of secondary explosives are less sensitive to impact and can alter the energetic performance of a propellant or explosive. In this work the Rapid Expansion of a Supercritical Solution into an Aqueous Solution (RESS‐AS) process was used to produce nano‐sized RDX (cyclo‐1,3,5‐trimethylene‐2,4,6‐trinitramine) particles. When a saturated supercritical carbon dioxide/RDX solution was expanded into neat water, RDX particles produced from the RESS‐AS process agglomerated quickly and coarsened through Ostwald ripening. However, if the pH level of the suspension was changed to 7, particles were metastably dispersed with a diameter of 30 nm. When the supercritical solution was expanded into air under the same pre‐expansion conditions using the similar RESS process, RDX particles were agglomerated and had an average size of approximately 100 nm. Another advantage of using a liquid receiving solution is the possibility for coating energetic particles with a thin layer of polymer. Dispersed particles were formed by coating the RDX particles with the water soluble polymers polyvinylpyrrolidone (PVP) or polyethylenimine (PEI) in the RESS‐AS process. Both PVP and PEI were used because they have an affinity to the RDX surface. Small and well‐dispersed particles were created for both cases with both PVP and PEI‐coated RDX particles shown to be stable for a year afterward. Several benefits are expected from these small polymer coated RDX particles such as decreased sensitivity, controlled reactivity, and enhanced compatibility with other binders for fabrication of bulk‐sized propellants and/or explosives.     

Combustion Effects of Nitrofulleropyrrolidine on RDX‐CMDB Propellants

The effect of N‐methyl‐2‐(3‐nitrophenyl)pyrrolidino[3′,4′:1,2]fullerene (mNPF) on the decomposition characteristics of hexogen (RDX) was investigated using differential scanning calorimetry (DSC). The results show that mNPF can accelerate the decomposition of RDX, the peak temperature (T_p) of the exothermal decomposition is reduced by 6.4 K, and the corresponding apparent activation energy (E_a) is decreased by 8.7 kJ mol⁻¹. N‐methyl‐2‐(3‐nitrophenyl)pyrrolidino[3′,4′:1,2]fullerene (mNPF), carbon black (CB), and C₆₀ were used as combustion catalysts to improve the combustion performance of a composite modified double‐base propellant containing RDX (RDX‐CMDB). The burning rate experimental results show that mNPF has a stronger catalytic effect than C₆₀ and CB. The magnitude of the effect of the three carbon substances on the enhancement of the burning rate is as follows: mNPF>C₆₀>CB. The catalytic effects of different contents of mNPF on the burning rates of RDX‐CMDB propellants were also studied, and the results show that the burning rates of RDX‐CMDB propellants are improved with increasing mNPF content. The plateau burning rate of a RDX‐CMDB propellant can be increased to 19.6 mm s⁻¹ when 1.0 % mNPF is added, and the corresponding plateau combustion region occurs at 8–22 MPa.     

Production and Characterization of Composite Nano‐RDX by RESS Co‐Precipitation

Nanoscale composites of hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) and polymeric binders were produced by co‐precipitation using rapid expansion of supercritical solutions (RESS). The binders used in this study are poly (vinylidene fluoride‐co‐hexafluoropropylene) (VDF‐HFP₂₂) and polystyrene (PS). The RDX/VDF‐HFP₂₂ and RDX/PS co‐precipitated nanoparticles were characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The average size of produced nanoparticles is ca. 100 nm. TEM analysis of RDX/PS nanocomposite shows a core‐shell structure with RDX as the core material and the shell consisting of the polymeric binder. X‐ray Powder Diffraction (XRPD) analysis indicates polycrystalline structure of RDX in the product with a crystallite size of 42 nm. The content of RDX in the composite particles is in the range of 70–73 % by mass as determined by Gas Chromatography Mass Spectroscopy (GCMS) and by XRPD.     

Properties and Application of a Novel Type of Glycidyl Azide Polymer (GAP)‐Modified Nitrocellulose Powders

GAP‐modified nitrocellulose powders were prepared by an internal solution method and applied in cross‐linked modified double base (XLDB) propellants. It was found that GAP‐modified nitrocellulose powders exhibit high round, no bonding between the particles and excellent fluidity. When the amount of GAP increased from 10.0 % to 30.0 %, the median diameter (d₅₀) of powders decreased from 134.53 μm to 94.54 μm. The thermal decomposition process of GAP appeared also in the GAP‐modified nitrocellulose powders, but the thermal decomposition peak temperatures of  N₃ and the GAP main chain were found to be lower for the 10.0 % and 20.0 % GAP‐modified samples than the corresponding peak temperatures for pure GAP, respectively. The plasticizing properties of GAP‐modified nitrocellulose powders are better than that of pure nitrocellulose powders, and the drop weight impact sensitivity of the modified powders is reduced as the mass ratio of GAP increases. It was experimentally shown that GAP‐modified nitrocellulose powders can improve the mechanical characteristics of the propellant with a maximum tensile strength (σ_m) between 0.36 MPa<σ_m< 1.10 MPa and an elongation at maximum tensile strength (ε_m) between 28.8 %<ε_m<51.8 % at temperatures of −40, +20 and +50 °C.     

Composites Between Alumina and an Ester‐Ether‐Ester Bioresorbable Copolymer

Composites between alumina and the bioresorbable poly(ε‐caprolactone)‐block‐poly(oxyethylene)‐block‐poly(ε‐caprolactone) copolymer were obtained by reacting ε‐caprolactone with preformed poly(ethylene glycol), in the presence of ceramic alumina powder, at 185°C under vacuum. The mechanical properties, tested by compression and flexural strengths and Young's modulus, show that the copolymer interacts poorly with the alumina grains. Both scanning electron and atomic force microscopy show a scarce wettability between alumina and copolymer, as well as the aggregation of alumina micro‐particles into clusters of big size. Both mechanical and morphological tests seem to indicate a stronger interaction between the alumina micro‐particles than between the alumina surface and the reaction mixture during the polymerization, as well as a “compacting effect” by alumina on the forming copolymer. The FT‐IR spectra of the composites show both copolymer and alumina absorption bands. The FT‐IR analysis on the fractions of an extraction with CHCl₃ indicates the presence of traces of poly(ε‐caprolactone), stably linked to alumina. The polymerization of ε‐caprolactone with alumina alone in the same conditions gives poly(ε‐caprolactone), mainly free and in minor part linked to the alumina surface. Two polymerization mechanisms, simultaneously occurring, are proposed. The most relevant result of this work is the lack of chemical inertness of alumina towards ε‐caprolactone, which leads to reconsider also the use of alumina as a biochemically inert material.     

Multiscale Simulation on the Influence of Dimethyl Hydantoin on Mechanical Properties of GAP/RDX Propellants

The influence of dimethyl hydantoin (DMH) on the mechanical properties of GAP/RDX propellant was studied by molecular dynamics (MD) and dissipative particle dynamics (DPD) simulation. The results showed that the binding energies (E_binding) between GAP and different surfaces of RDX were in the order of (010)>(001)>(100). Compared to GAP/RDX, GAP grafted with DMH (GAP‐DMH) exhibits higher binding energies with RDX, and the sequence of E_binding turns to (001)>(010)>(100). Radial distribution simulations demonstrated that GAP‐DMH is more close to the surfaces of RDX, increasing the van der Waals energies between GAP‐DMH and RDX. The stress and strain of GAP‐DMH/RDX excel those of GAP/RDX. DPD simulations showed that GAP‐DMH was able to restrain the agglomeration of RDX, to improve the dispersibility and to enlarge the contact surface with RDX, which also increased the mechanical properties of GAP/RDX propellant.     

In–situ Characterization and Cure Kinetics in NEPE Propellant/ HTPB Liner Interface by Microscopic FT‐IR

The content distribution of chemical groups and the kinetics of curing process in the micro‐region interfaces of nitrate ester plasticized polyether (NEPE) based propellant/hydroxyl‐terminated polybutadiene (HTPB) based liner were studied by in‐situ diffuse reflection FT‐IR spectroscopy. During the curing process, the content of –NCO groups showed little increase in the liner region toward the interface. It rises quickly through the interface layer and is then stable in the region of the propellant layer, while the content of –NH groups gradually increases from liner to propellant. In the micro‐region between liner and propellant, the –C=O decreases rapidly through interface and then has a slight increase in the propellant region. Migration of nitrate esters appears at the interface of the NEPE propellant/liner at early period of curing, and –O–NO₂ decreases from propellant to liner in the bonding interface micro‐region. A study of curing kinetics indicates that the second‐order reaction model can describe the curing reaction in the bonding interface at the early stage of curing process. The order of apparent curing reaction rate constant (k ) of liner (L point), intermediate point (I point) and propellant (P point) in the interface micron‐region is k L > k I > k P at the same curing temperature. The apparent reaction activation energy (E _a) at L, I, and P points are 39.96, 81.49, and 62.51 kJ mol^–1, respectively, based on the Arrhenius equation.     

Synthesis and spectroelectrochemistry of dithieno(3,2‐b:2′,3′‐d)pyrrole derivatives

New π‐conjugated polymers containing dithieno(3,2‐b:2′,3′‐d)pyrrole (DTP) were successfully synthesized via electropolymerization. The effect of structural differences on the electrochemical and optoelectronic properties of the 4‐[4H‐dithieno(3,2‐b:2′,3′‐d)pyrrol‐4‐yl]aniline (DTP–aryl–NH₂), 10‐[4H‐dithiyeno(3,2‐b:2′,3′‐d)pirol‐4‐il]dekan‐1‐amine (DTP–alkyl–NH₂), and 1,10‐bis[4H‐dithieno(3,2‐b:2′,3′‐d)pyrrol‐4‐yl] decane (DTP–alkyl–DTP) were investigated. The corresponding polymers were characterized by cyclic voltammetry, NMR (¹H‐NMR and ¹³C‐NMR), and ultraviolet–visible spectroscopy. Changes in the electronic nature of the functional groups led to variations in the electrochemical properties of the π‐conjugated systems. The electroactive polymer films revealed redox couples and exhibited electrochromic behavior. The replacement of the DTP–alkyl–DTP unit with DTP–aryl–NH₂ and DTP–alkyl–NH₂ resulted in a lower oxidation potential. Both the poly(10‐(4H‐Dithiyeno[3,2‐b:2′,3′‐d]pirol‐4‐il)dekan‐1‐amin) (poly(DTP–alkyl–NH₂)) and poly(1,10‐bis(4H‐dithieno[3,2‐b:2′,3′‐d]pyrrol‐4‐yl) decane) (poly(DTP–alkyl–DTP)) films showed multicolor electrochromism and also fast switching times (<1 s) in the visible and near infrared regions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40701.     

Analysis for Decomposition Characteristics and Piecewise Thermokinetics of Nitramine Modified Double‐base Propellant with High Solid Content

The thermal stability and decomposition characteristic of nitramine modified double‐base propellant (RDX‐CMDB propellant) with high solid content and its components were investigated under dynamic and isothermal conditions by differential scanning calorimetry (DSC). It was found that the mixture of nitrocellulose (NC) and nitroglycerin (NG) had a promote effect on the decomposition of RDX. The activation energy (E ) and the pre‐exponential factor (A ) of two obviously exothermic processes were obtained by Friedman iso‐conversional method. The screening method suggested by ICTAC was used to determine the most probable mechanism functions and kinetic parameters of the two processes which are corresponding to the deceleration model and the autocatalytic model. The theoretical value was consistent with the experiment result.     

Polyglycidyl nitrate (PGN)‐based energetic thermoplastic polyurethane elastomers with bonding functions

PGN‐based ETPUEs were synthesized using mixture of chain extenders including 1, 4‐butanediol and Diethyl Bis(hydroxymethyl)malonate (DBM). Through the special chain extenders DBM, the –COOR was introduced into the energetic thermoplastic polyurethane elastomers (ETPUEs) and further enhances the adhesion between ETPUE and nitramine solid ingredients in propellants. From the analysis, with the percentage of DBM increasing, the work of adhesion (W_a) between nitramine solid ingredients and ETPUEs increased and the maximum stress (σ_m) of ETPUEs decreased on the other hand. In order to test the bonding functions of different ETPUEs, the RDX/ETPUE propellants were prepared and the stress–strain curves of all propellants were tested. The results showed that the ETPUE‐75 with 75% DBM can prevent the dewetting and improve the mechanical properties of propellants. The ETPUE prepared with chain extender including 1, 4‐butanediol and DBM were valuable for application in propellants. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42026.     

Asymmetric Hydrogenation Catalyzed by (S,S)‐R‐BisPast;‐Rh and (R,R)‐R‐MiniPHOS Complexes: Scope,Limitations, and Mechanism

A new class of chiral C₂‐symmetric bis(trialkyl)phosphine ligands has been prepared and used in Rh(I)‐catalyzed asymmetric hydrogenation reactions. The ligands, 1,2‐bis(alkylmethylphosphino)ethanes 1a‐g (abbreviated as BisP*, alkyl = t‐butyl, 1‐adamantyl, 1‐methylcyclohexyl, 1,1‐diethylpropyl, cyclopentyl, cyclohexyl, isopropyl) and 1,2‐bis(alkylmethylphosphino)methanes 2a‐d (abbreviated as MiniPHOS, alkyl = t‐butyl, cyclohexyl, isopropyl, phenyl) are prepared by a simple synthetic approach based on the air‐stable phosphine–boranes. These new ligands give the corresponding Rh(I) complexes, which are effective catalytic precursors for the asymmetric hydrogenation of a representative series of dehydroamino acids and itaconic acid derivatives. Enantioselectivities observed in these hydrogenations are universally high and in many cases exceed 99%. X‐Ray characterization of four precatalysts, study of the pressure effects, deuteration experiments, and characterization of the wide series of intermediates in the catalytic cycle are used for the discussion of the possible correlation between the structure of the catalysts and the outcome of the catalytic asymmetric hydrogenation.     

Probing the compatibility and interaction of energetic binders based on 3,3‐bis(azidomethyl)oxetane with some explosives: thermal,interfacial and simulation studies

Several polymer binders based on 3,3‐bis(azidomethyl)oxetane (BAMO) were studied to explore the compatibility and interaction of the energetic binders with three common energetic oxidants. The compatibilities were studied by differential scanning calorimetry and ratings were obtained according to evaluated standards. The results showed that all the binders based on BAMO had good compatibility with cyclotrimethylenetrinitramine, cyclotetramethylenetetranitroamine and hexanitrohexazaiso‐wurtzitane. The work of adhesion (W_a) between binders and explosives was tested via measurement of contact angle and the results are in the following order: chain‐extended poly(3,3‐bis(azidomethyl)oxetane) (PBAMO) by isophorone diisocyanate (IPDI‐CE) with diethyl bis(hydroxymethyl) malonate (IPDI‐DBM‐CE) > chain‐extended PBAMO by IPDI‐CE > PBAMO. In addition, similar results were found in the binding energies reported by molecular dynamics, and the average values of E_binding for the IPDI‐DBM‐CE system were larger than E_binding for the other systems due to the formation of hydrogen bonds between –COOEt and –NO₂, which improve the bonding abilities. © 2017 Society of Chemical Industry     

Neutral Polymeric Bonding Agents (NPBA) and Their Use in Smokeless Composite Rocket Propellants Based on HMX‐GAP‐BuNENA

Very few efficient bonding agents for use in solid rocket propellants with nitramine filler materials and energetic binder systems are currently available. In this work, we report the synthesis, detailed characterization, and use of neutral polymeric bonding agents (NPBA) in isocyanate‐cured and smokeless composite rocket propellants based on the nitramine octogen (HMX), the energetic binder glycidyl azide polymer (GAP), and the energetic plasticizer N‐butyl‐2‐nitratoethylnitramine (BuNENA). These polymeric bonding agents clearly influenced the viscosity of the uncured propellant mixtures and provided significantly enhanced mechanical properties to the cured propellants, even at low NPBA concentrations (down to 0.001 wt‐% of propellant). A modified NPBA more or less free of hydroxyl functionalities for interactions with isocyanate curing agent provided the same level of mechanical improvement as regular NPBA containing a substantial number of reactive hydroxyl groups. However, some degree of reactivity towards isocyanate is essential for function.     

Evaluation of new 3‐mercaptopropionate thiols for thiol‐ene photopolymerization coatings using experimental design

Three 3‐mercaptopropionate thiols, 1,6‐Hexane bis(3‐mercaptopropionate) (HD‐SH), trans‐1,4‐Cyclohexanedimethyl bis(3‐mercaptopropionate) (CHDM‐SH), and 4,4′‐Isopropylidenedicyclohexane bis(3‐mercaptopropionate) (HBPA‐SH) were formulated with 1,3,5‐triallyl‐1,3,5‐triazine‐2,4,6(1H,3H,5H)‐trione (TATATO) and photoinitiator. The formulations were photopolymerized via thiol‐ene photopolymerization. A ternary experimental design was employed to elucidate the influence the three thiols on the thermomechanical and coatings properties of thiol‐ene photopolymerizable materials. Tensile strength, tensile modulus, elongation‐to‐break, glass transition temperature (T_g), and crosslink density (XLD) were investigated. Coating properties including pencil hardness, pull‐off adhesion, MEK double rubs, and gloss were also investigated. Relative reaction conversion was determined by photo differential scanning calorimeter (PDSC). Thiol‐ene photopolymerizable materials containing HBPA‐SH resulted in improving tensile strength, tensile modulus, T_g, and pencil hardness but lowering of crosslink density and relative conversion. This was attributed to steric and rigidity of the double cycloaliphatic structure. The inclusion of CHDM‐SH into the systems resulted in the synergistic effect on elongation‐to‐break and pull‐off adhesion. The HD‐SH generally resulted in a diminution of thermomechanical and coating properties, but improved the crosslink density. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Comparison of Titanium‐Oxo‐Clusters Derived from Sol‐Gel Precursors with TiO2 Nanoparticles in Drying Oil Based Ceramer Coatings

Hybrid coatings (inorganic‐organic) were prepared using a blown and epoxidized soybean oil as the organic matrix. Both TiO₂ particles and titanium sol‐gel precursors (Titanium tetra‐i‐propoxide, TIP; titanium (di‐i‐propoxide) bis(acetylactonate), TIA) were incorporated into the coating. Three sizes of TiO₂ particles ranging from 32 nm to 500 nm were used for comparison with the metal‐oxo‐clusters. General mechanical coating properties, tensile properties, and viscoelastic properties of the sol‐gel (ceramer) system were evaluated for the coatings, and the sol‐gel derived metal‐oxo‐clusters were found to have higher tensile modulus, storage modulus (E ′), and T_g compared with the TiO₂ particles.     

Effects of surface chemistry of silica particles on secondary structure and tensile properties of silica‐filled rubber systems

Silica particles were treated by silane coupling agents to study the effects of surface chemistry of silica particles on the secondary structure formed by silica particles in styrene–butadiene rubber (SBR). The relation between the size of secondary structure and tensile properties of silica‐filled SBR vulcanizates was also investigated. The water molecules adsorbed on the silica surface enhanced both the reactivity of the coupling agents with silanol groups on the silica surface and the condensation reaction between coupling agents. The averaged size of one agglomerate in the filled vulcanizate (S_agg ) decreased with the increase of the hydrophobic character of silica particles, which were obtained by the introduction of coupling agents with alkyl groups. The initial slope of stress–strain curves for silica‐filled vulcanizates decreased with the decrease of S_agg . Further, a clear Payne effect was observed in all silica‐filled vulcanizates, with this tendency more prominent in the larger S_agg . © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1622–1629, 2002