Effect of Pre-coated Vs. Added Coupling Agents on the Vulcanization of Silica-filled Polyisoprene

Solid-state ¹³C NMR was utilized in analysis of the crosslink structure in silica-filled polyisoprene. Samples containing either a pre-coated coupling agent, a coupling agent added in the mixing process, or polyethylene glycol (PEG) were vulcanized and the type and density of crosslinking determined. The precoated coupling agent sample gave lower %swelling, lower extraction amounts, and a lower T₂ relaxation than the mixed coupling agent, indicating increased crosslinking or increased filler -rubber interaction. The precoated also showed less cis -trans isomerization than the mixed, which may be due to its more uniform coating effect on the silica. The PEG sample yielded a ¹³C spectrum similar to an unfilled sample indicating decreased silica adsorption of vulcanization agents. Neither coupling agent significantly decreased the total sulfurization nor the percent monosulfidic crosslinks from that of a silica-filled sample without coupling agent.     

1H NMR Imaging Study of Molecular Mobility in Silica-Filled,TBBS-Sulfur Vulcanized Cis-Polyisoprene

The interactions of silica in zinc-activated, sulfur-vulcanized cis-1.4 polyisoprene were characterized at the 75% cure state using ¹H NMR imaging spectroscopy. Variables examined included silica loading, mixing conditions, and presence of additives, including a coupling agent and polyethylene glycol. Rheometer curves indicated a decrease in cure rate and cure state as silica was increased. ¹H NMR imaging showed an increase in the T ₂ relaxation times, and a decrease in the proton spin density N(H) as the filler load increases. Mixing conditions did not affect the cure rate, cure state, or the average T ₂ relaxation time; however, the distribution of relaxation times broadened with poor mixing. The presence of a coupling agent increased the cure rate and cure state, as well as decreased the T ₂ relaxation times as compared with samples with the same silica level, but without coupling agent. Polyethylene glycol (PEG) had slightly higher average T ₂ relaxation times, and a slightly broader distribution as compared with the sample without PEG added.     

Surface functionalized colloidal silica particles from an inverse microemulsion sol gel process

Colloidal silica particles are prepared via a sol gel technique carried out in an inverse microemulsion of water in a toluene solution of tetraethoxysilane (TEOS), stabilized by either an anionic surfactant AOT or isopropanol. Functionalized material was obtained using a functional coupling agent (RO)₃Si(CH₂)₃ X, X being a functional group such as methacryloyl, thiol, vinyl, amino group, or a chlorine atom. Functionalization can be carried out either directly via the direct copolycondensation of TEOS and the coupling agent, or in a two-step process involving a core-shell polycondensation of the coupling agent onto preformed silica particles. Kinetic studies of the copolycondensation are carried out using either²⁹Si NMR analysis or liquid chromatography. They show that the consumption of TEOS is more rapid than that of the coupling agent. The materials are characterized both chemically (elemental analysis, FTIR,¹³C and²⁹Si NMR CPMAS analysis), and by their particle size. The silica functionalized with a polymerizable methacryloyl group is encapsulated by a polymer layer in an inverse emulsion polymerization of acrylic acid. After inversion of the emulsion in water, the resulting material is covered with a layer of hydrophobic polymer in a conventional emulsion polymerization.     

Isocyanate-crosslinked silica aerogel monolith with low thermal conductivity and much enhanced mechanical properties: Fabrication and analysis of forming mechanisms

The applications of silica aerogels are restricted due to their intrinsic fragile property. Polymerization of di-isocyanates can be templated onto the mesoporous surface of the –NH₂ group modified silica clusters, resulting in the conformal crosslinked coating on surface of silica clusters. Aminopropyltriethoxysilane (APTES), as the silica co-precursor and amine group modification agent, is involved containing tetramethyl orthosilicate (TMOS) silica precursor, while hexamethylene diisocyanate (HDI) is incorporated as the polymer crosslinking agent. The effects of different amounts of APTES on the physicochemical properties of the resulting crosslinked aerogels are investigated. The results show that the optimized APTES/TMOS volume ratio can be determined at 0.5:1. The resulting optimal crosslinked silica aerogel possesses large BET specific surface area of 150.9 m²/g, low thermal conductivity of 0.037 W/(m·K), and the Young's modulus is as large as 18 MPa under strain of 4.2%, much higher than that in the previously published works. The polymerization reaction mechanism forming the polyurethane chains has also been proposed. In addition, the interactions between silica clusters and polymer chains are studied by molecular mechanics and molecular dynamics. The interactions are mainly dependent on non-bonding energy, and the electrostatic energy has decisive impact on the combination of silica clusters and polymer chains. The density field of C, H, N, O, and Si elements overlaps with each other, indicating that the polymer crosslinked silica aerogel maintains typical three-dimensional porous structure. The N element enriches in the region between silica clusters, further verifying the formation –CONH–(CH₂)₆–CONH- polyurethane chains, which is actually responsible for the much enhanced mechanical property.     

Solid state carbon-13 NMR studies of silica-filled TBBS-accelerated, sulfur vulcanized, cis-1,4 polyisoprene

The network formation of silica-filled, TBBS accelerated sulfur vulcanization of cis-1,4 polyisoprene (Natsyn 2200) was studied by solid state ¹³C NMR spectroscopy, and equilibrium swelling measurements. Samples with varying silica levels at different stages of cure were analyzed. It was observed that silica retards the cure reaction, and results in an overall lower cure state. Silica also has an influence on the vulcanization chemistry causing an enhancement of the cis-trans isomerization, chain scission, and the formation of monosulfidic linkages. It was also determined that the polysulfidic linkages decreased as silica level was increased. Intermolecular crosslinks decreased, while intramolecular structures (including pendant side groups and cyclic sulfur structures) increased. Cross link density, as determined by swelling measurements, decreased as the silica level was increased. Received: 1 October 1997/Revised version: 1 December 1997/Accepted: 2 December 1997     

A Comparative Study on Curing Characteristics,Mechanical Properties,Swelling Behavior,Thermal Stability,and Morphology of Feldspar and Silica in SMR L Vulcanizates

Abstract

Comparison studies on effects of feldspar and silica (Vulcasil C) as a filler in (SMR L grade natural rubber) vulcanizates on curing characteristics, mechanical properties, swelling behavior, thermal analysis, and morphology were examined. The incorporation of both fillers increases the scorch time, t ₂, and cure time, t ₉₀, of SMR L vulcanizates. At a similar filler loading, feldspar exhibited longer t ₂ and t ₉₀ but lower values of maximum torque, M_HR, and torque difference, M_HR–M_L than did silica-filled SMR L vulcanizates. For mechanical properties, both fillers were found to be effective in enhancing the tensile strength (up to 10 phr), tensile modulus, and hardness of the vulcanizates. However, feldspar-filled SMR L vulcanizates showed lower values of mechanical properties than did silica-filled SMR L vulcanizates. Swelling measurement indicates that swelling percentages of both fillers-filled SMR L vulcanizates decrease with increasing filler loading whereas silica shows a lower swelling percentage than feldspar-filled SMR L vulcanizates. Scanning electron microscopy (SEM) on fracture surface of tensile samples showed poor filler–matrix adhesion for both fillers with increasing filler loading in the vulcanizates. However, feldspar-filled SMR L vulcanizates showed poorer filler–matrix adhesion than did silica-filled SMR L vulcanizates. Thermogravimetric analysis (TGA) results indicate that the feldspar-filled SMR L vulcanizates have higher thermal stability than do silica-filled SMR L vulcanizates.     

High resolution solid-state NMR and DSC study of poly(ethylene glycol)-silicate hybrid materials via sol-gel process

Hybrid materials incorporating poly(ethylene glycol) (PEG) with tetraethoxysilane (TEOS) via a sol-gel process were studied for a wide range of compositions of PEG by DSC and high resolution solid-state ¹³C- and ²⁹Si-NMR spectroscopy. The results indicate that the microstructure of the hybrid materials and the crystallization behavior of PEG in hybrids strongly depend on the relative content of PEG. With an increasing content of PEG, the microstructure of hybrid materials changes a lot, from intimate mixing to macrophase separation. It is found that the glass transition temperatures (T_g) (around 373 K) of PEG homogeneously embedded in a silica network are much higher than that (about 223 K) of pure PEG and also much higher in melting temperatures T_m (around 323 K) than PEG crystallites in heterogeneous hybrids. Meanwhile, the lower the PEG content, the more perfect the silica network, and the higher the T_g of PEG embedded in hybrids. An extended-chain structure of PEG was supposed to be responsible for the unusually high T_g of PEG. Homogeneous PEG-TEOS hybrids on a molecular level can be obtained provided that the PEG content in the hybrids is less than 30% by weight. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 139–147, 1998     

Facile and controllable synthesis of hybrid silica nanoparticles densely grafted with poly(ethylene glycol)

We report that a mixture of good and poor solvents greatly enhances the grafting density of nanosized silica grafted with poly(ethylene glycol) (PEG ) by the ‘grafting to’ method. Methoxypolyethylene glycol (MPEG ) (molecular weight 750, 2000 and 4000 g mol^?1) was modified in a controlled manner to prepare epoxide terminated PEG (MPEG‐EO ). Silica nanoparticles were modified with N ‐(2‐aminoethy)‐3‐aminopropylmethyldimethoxysilane through the silanization coupling reaction to obtain a well‐defined siloxane structure. MPEG‐EO was coupled to the modified silica by the reaction of their terminal groups in a mixed solvent (n ‐decane/toluene). The grafting density of MPEG‐EO was found to be controlled by the concentration of MPEG‐EO and the ratio of n ‐decane to toluene in the grafting system. Based on TGA , the maximum grafting density was found to be about 2.8, 1.47 and 0.76 chains nm^?2 for molecular weights of 750, 2000 and 4000, respectively, which is extremely high compared to previous reports. This high grafting density can be explained by the decreased chain dimension of PEG in the presence of the poor solvent. The method can be applied to other nanoparticles and polymers which can greatly enhance the application of SiO₂ nanocomposites. © 2017 Society of Chemical Industry     

Estimation of silica flocculation in SBR/BR compounds reinforced with different silica contents from their rheocurves

The rheocurves of silica-filled styrene–butadiene/polybutadiene rubber (SBR/BR) compounds containing 3-octadecyltriethoxy silane (OTES) and bis-[triethoxysilylpropyl]tetrasulfide (TESPT) were investigated to examine the effects of silica content and silanes on silica flocculation during mixing and cure. SBR/BR compounds without curatives were also prepared to infer the effect of cure on silica flocculation. The maximum torque of the compounds could be deconvoluted to individual source torques such as silica flocculation during mixing and cure, crosslinking of rubber, and coupling between rubber and silica by assuming the independence of silica flocculation from cure and coupling. Torque due to silica flocculation increased with the silica content of the SBR/BR compounds, but its effect was significantly reduced by the addition of OTES or TESPT. TESPT suppressed silica flocculation and facilitated coupling, thus yielding enhanced tensile properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48559.     

Silicon-29 solid-state nuclear magnetic resonance spectroscopy of composite interfaces

The types of structures and bonds that are formed with silicons in the composite interface were studied using ²⁹Si cross-polarization/magic angle sample spinning (CP/MAS) nuclear magnetic resonance spectroscopy (NMR). The change in mobility of silane coupling agent bonded to silica, as compared with bulk hydrolyzed silane coupling agent, can be monitored by the change in line width and the shift of resonances to higher fields, as well as by the change in the silicon-proton cross-polarization time T_SiH. In the silane coupling agent-matrix resin interface, the T_SiH values reflect the change in mobility as a function of the concentration and degree of hydrolysis of the silane coupling agent. It has been demonstrated that quantitative measurements of T_SiH can be used to investigate relative mobilities.     

Effect of polyethylene glycol on the properties of styrene‐butadiene rubber/organoclay nanocomposites filled with silica and carbon black

Polyethyene glycol (PEG) is widely used as a dispersing agent and can also be used to prevent the adsorption of ingredients on the surface of silica. From the XRD results, PEG that was used as the dispersing agent on the SBR/organoclay compound filled with silica and carbon black (CB) was intercalated between the organoclay layer. Additionally, the interactions with the PEG differed depending on whether 3‐aminopropyltriethoxysilane (APTES) or N,N‐dimethyldodecylamine (DDA) were used as clay modifiers. When PEG was added, the T_g of the SBR/silica/APTES‐MMT compound increased through the formation of hydrogen bonds between the ether linkages of PEG and the hydroxyl groups of APTES. For the SBR/silica/DDA‐MMT compound with PEG, slippage occurred between the silicate, and DDA because of the alkyl chain of DDA. The SBR/silica/APTES‐MMT/CB compound with PEG exhibited the highest T_g value and the highest bound rubber content, with high modulus values at 100 and 300%. The SBR/silica/DDA‐MMT/CB compound had the best properties in terms of the wet skid resistance and the rolling resistance. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The Effect of Ethylene Diamine Dilaurate and Silane Coupling Agent on Cure Characteristics,Mechanical, and Thermal Properties of Silica-Filled Natural Rubber Composites

The influence of a new coupling agent, ethylene diamine dilaurate (EDD) and a commercial silane coupling agent, (Si-69) on the cure characteristics, mechanical and morphological properties of silica-filled natural rubber (NR) composites was studied. The results show that scorch time and cure time decreased with an increase in both coupling agents' content, but maximum and minimum torques exhibit the opposite trend. The mechanical properties such as tensile strength and tensile modulus, M100 and M300, increased with increasing both coupling agents' content but at a similar coupling agent content, silica-filled natural rubber composites with Si-69 exhibit better tensile strength (more than 2 phr) and tensile modulus than does EDD. Elongation at break (Eb) of silica-filled natural rubber increased with increasing EDD content but Si-69 exhibits the opposite trend. Scanning electron microscopy (SEM) study of tensile fracture surfaces shows the better tensile strength of silica-filled natural rubber composites with Si-69 and EDD over control composites (without EDD or Si-69). Thermogravimetric analysis (TGA) results indicate that silica-filled NR composites with EDD have higher thermal stability than Si-69. Fourier transform infrared spectra (FTIR) provided an evidence of interaction between EDD and Si-69 with silica in NR composites.     

Structure, dynamics and interactions of complex sol-gel hybrid materials through SSNMR and DSC: Part II, ternary systems based on PE-PEG block copolymer, PHS and silica

This work is the second part of a study aimed at understanding in more depth structure, dynamics, interactions and correlations between morphology and barrier properties against oxygen diffusion of complex PE-PEG/PHS/SiO₂ hybrids prepared through a sol-gel process. Using a combined DSC and solid-state NMR approach, including ¹³C and ²⁹Si experiments and ¹H ultra-fast MAS spectra, the structural, phase and interaction properties of three PE-PEG/PHS/SiO₂ samples with different compositions, exhibiting different barrier performances, have been investigated, also taking into account the results obtained for the simpler one- and two-component systems (Part I). While the structure of the silica domains has been found to be not affected by composition, many differences have been observed concerning the phase and dynamic properties of the organic components (PE and PEG crystallinity and mobility of their amorphous domains) and the inter-component interactions (strength of the hydrogen bonds between PHS and both silica and PEG and PHS/PEG miscibility). In particular peculiar phase and interaction properties of the sample exhibiting the best barrier properties have been identified and characterized.     

Intercalation structure and toughening mechanism of graphene/urea‐formaldehyde nanocomposites prepared via in situ polymerization

Based on the industrialized graphene (GN) product, a series of graphene/urea‐formaldehyde nanocomposites were synthesized via in situ polymerization by incorporation of silicon coupling agent with terminal amino groups (SA) as the compatibilizer. The results showed that addition of SA coupling agent led to much more efficient grafting of UF molecules on the GN surface with high layer thickness by formation of hydrogen bonding, and thus complete exfoliation and uniform dispersion of GN were achieved for the composites. Compared with neat UF, the addition of 1.0 wt% GN resulted in a roughly 25% increase in tensile strength and 12% increase in impact strength; meanwhile the impact fracture surfaces of the composite showed obvious ductile fracture characteristics, indicating the reinforcing and toughening effect of GN on the UF matrix. With increasing GN content, the storage modulus, glass transition temperature and crosslinking density of UF increased, while the tan δ_max decreased, suggesting that a double crosslinking network structure with GN centered crosslinking point and chemical crosslinking point of UF molecular chains formed, leading to improvement in the stiffness of the composites. The present work showed promising potential for developing high performance UF resin on an industrial scale. © 2017 Society of Chemical Industry     

Relationship between curing activation energy and free formaldehyde content in urea-formaldehyde resins

This study investigated the effect on the curing behavior, activation energy (E _a) of the curing reaction, crystalline structure, crosslinking, and free formaldehyde content of the addition of the following scavengers in urea-formaldehyde (UF) resins: medium density fiber board flour, rice husk flour, silica powder, and tannin powder. The scavenger content was 3 and 7?wt% of the UF resin solid content. The curing behavior of UF resins was monitored by differential scanning calorimetry, thermogravimetric analysis, and X-ray crystallography. The curing E _a was correlated to the free formaldehyde content of the scavenger containing UF resins. The thermal stability of the UF resins increased but the curing E _a decreased with increasing scavenger content. After curing, the crystallinity of the UF resins decreased in the presence of scavengers. The unreacted free formaldehyde content was reduced in the tannin powder containing UF resins. The degree of crosslinking affects the formaldehyde emission from wood panels bonded with UF resin. This is especially true for wood panels in service for long periods of time and exposed to high humidity conditions. Once the free formaldehyde which influences considerably the emission has disappeared, the presence of the –CH₂– groups then becomes important. Hence, an increased resin crosslinking indicates a higher concentration of –CH₂– groups present, which may hydrolyze and emit formaldehyde slowly over time.     

Preparation of superhydrophobic nanocomposite fiber membranes by electrospinning poly(vinylidene fluoride)/silane coupling agent modified SiO2 nanoparticles

Superhydrophobic nanocomposite fiber membranes were prepared by blend electrospinning of poly(vinylidene fluoride) (PVDF) mixed with silane coupling agent modified SiO₂ nanoparticles. The nanoparticles were prepared by the sol–gel method, and the average particle diameter was measured by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The effects of the type of silane coupling agent, such as n‐octyltriethoxysilane, vinyltrimethoxysilane (A‐171), and vinyltriethoxysilane (A‐151), and the mass ratio of the modified silica particles and PVDF on the surface wettability of the composite fiber membrane were investigated. The results indicated that the incorporation of silane coupling agent modified silica particles into the PVDF membrane increased the roughness of the surface and formed micro/nano dual‐scale structure compared to the pristine PVDF membrane, which was responsible for the superhydrophobicity and self‐cleaning property of the nanocomposite fiber membranes. The value of water contact angle (CA) increased with the increase of the content of modified SiO₂ nanoparticles in the nanocomposite membrane, ranging from 149.8° to 160.1° as the mass ratio of modified 170 nm SiO₂ with PVDF matrix increased from 0.5:1 to 5:1, indicating the membrane possesses a superhydrophobic surface. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44501.     

Self‐crosslinkable epoxidized natural rubber–silica hybrids

Epoxidized natural rubber (ENR)–silica hybrids without any other additives were prepared by an open‐mill mixing method at room temperature. The curing characteristics, crosslinking density, mechanical properties, and dynamical mechanical properties were investigated. The results indicate that the ENR–silica hybrid materials could be cured with silica as a crosslinking and reinforcing agent. Attenuated total reflection–Fourier transform infrared spectroscopy and solid‐state ¹³C‐NMR spectroscopy exposed the characteristics of the interfacial interaction in the hybrids and confirmed the existence of chemical bonds and hydrogen bonds between the epoxy group and Si? OH. Scanning electron microscopy illustrated a good dispersion of silica in the ENR matrix. Meanwhile, the modulus at 100% elongation of the hybrid reached 9.64 MPa when 100‐phr silica was loaded; a similar trend was observed for the hardness. Finally, our findings might extend the concept of rubber curing and open a new space for making an environmentally friendly rubber composite. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44605.     

Porous poly(ethylene glycol)–polyurethane hydrogels as potential biomaterials

We report the synthesis of porous poly(ethylene glycol)–polyurethane (PEG‐PU) hydrogels using PEG‐4000 as a soft segment and 4,4′‐methylenebis(cyclohexylisocyanate) as a hard segment. The degree of swelling in the hydrogels could be controlled by varying the amount of crosslinking agent, namely 1,2,6‐hexanetriol. Structural characterization of the hydrogels was performed using solid‐state ¹³C NMR and Fourier transform infrared spectroscopy. Wide‐angle X‐ray diffraction studies revealed the existence of crystalline domains of PEG and small‐angle X‐ray scattering studies showed the presence of lamellar microstructures. For generating a porous structure in the hydrogels, cryogenic treatment with lyophilization was used. Scanning electron microscopy and three‐dimensional micro‐computed tomography imaging of the hydrogels indicated the presence of interconnected pores. The mechanical strength of the hydrogels and xerogels was measured using dynamic mechanical analysis. The observed dynamic storage moduli (E′) for the equilibrium swollen and dry gels were found to be 0.15 and 4.2 MPa, respectively. Interestingly, the porous PEG‐PU xerogel also showed E′ of 5.6 MPa indicating a similar mechanical strength upon incorporating porosity into the gel matrix. Finally, preliminary cytocompatibility studies showed the ability of cells to proliferate in the hydrogels. These gels show promise for applications as scaffolds and implants in tissue engineering. © 2014 Society of Chemical Industry     

Temperature‐responsive characteristics of poly(N‐isopropylacrylamide) hydrogels with macroporous structure

Macroporous poly(N‐isopropylacrylamide) (PNIPA) hydrogels were synthesized by free‐radical crosslinking polymerization in aqueous solution from N‐isopropylacrylamide monomer and N,N‐methylenebis (acrylamide) crosslinker using poly(ethylene glycol) (PEG) with three different number‐average molecular weights of 300, 600 and 1000 g mol^?1 as the pore‐forming agent. The influence of the molecular weight and amount of PEG pore‐forming agent on the swelling ratio and network parameters such as polymer–solvent interaction parameter (χ) and crosslinking density (ν_E) of the hydrogels is reported and discussed. Scanning electron micrographs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights and compositions of PEG during polymerization. At a temperature below the volume phase transition temperature, the macroporous hydrogels absorbed larger amounts of water compared to that of conventional PNIPA hydrogels, and showed higher equilibrated swelling ratios in aqueous medium. Particularly, the unique macroporous structure provides numerous water channels for water diffusion in or out of the matrix and, therefore, an improved response rate to external temperature changes during the swelling and deswelling process. These macroporous PNIPA hydrogels may be useful for potential applications in controlled release of macromolecular active agents. Copyright © 2006 Society of Chemical Industry     

Generation of antifouling layers on stainless steel surfaces by plasma‐enhanced crosslinking of polyethylene glycol

Polyethylene glycol (PEG) structures were deposited onto stainless steel (SS) surfaces by spin coating and argon radio frequency (RF)‐plasma mediated crosslinking. Electron spectroscopy for chemical analysis (ESCA) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR) indicated the presence of  CH₂ CH₂ O structure and C C C linkage, as a result of the plasma crosslinking, on PEG‐modified SS surfaces. Scanning electron microscopy (SEM) indicated complete deposition, and water contact angle analysis revealed higher hydrophilicity on PEG‐modified surfaces compared to unmodified SS surfaces. Surface morphology and roughness analysis by atomic force microscopy (AFM) revealed smoother SS surfaces after PEG modification. The evaluation of antifouling ability of the PEG‐modified SS surfaces was carried out. Compared to the unmodified SS, PEG‐modified surfaces showed about 81–96% decrease in Listeria monocytogenes attachment and biofilm formation (p < 0.05). This cold plasma mediated PEG crosslinking provided a promising technique to reduce bacterial contamination on surfaces encountered in food‐processing environments. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 485–497, 2005