Low-weight fractions of graphene and hydroxyapatite enhance mechanics in photocured methacrylate adhesives

In many applications, it is desirable for photocured adhesives to have high-mechanical strength in the cured state, but relatively low viscosity when liquid. This was achieved by adding less than 0.5 wt% hydroxyapatite and graphene to methyl methacrylate with diurethane dimethacrylate (UDMA-MMA). Nanoindentation shows hardness increasing by 30–40% and indentation modulus by >30% compared to UDMA-MMA on its own. Rheometry shows only a small increase in uncured viscosity for the liquid state. The additives affect the optical properties, mobility of free radicals, photocuring, and degree of conversion, the effects of which are seen in Fourier transform infrared and micro-Raman spectra. Thermographic images taken during curing show that the additives impact the photocuring process. In addition, changes in intermolecular bonding are seen in the vibrational spectra when the additives are present. The enhanced mechanical properties are attributed to the observed changes in photocuring and bonding.     

Assessment of crosslink network and network defects of unfilled and filled ethylene‐propylene‐diene terpolymer using solid state nuclear magnetic relaxation spectroscopy

Reinforced rubbers are complex compared to unfilled systems. There are differences in the mechanisms affecting network molecular structure as well as properties of the rubber materials. In this article investigation of crosslink network and untied network defects on a molecular level of unfilled and carbon black filled ethylene‐propylene‐diene terpolymer was carried out using proton solid‐state double‐quantum NMR spectroscopy. The results show that the filled system demonstrates lower cure efficiency in conjunction with more noncoupled network defects than the unfilled one. In addition, the filled system yields the greater spatial heterogeneity because of the localization of the free radicals at the rubber–filler boundary. These strongly influence the mechanical properties of the filled rubber. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44224.     

Influence of filler–rubber interactions on the viscoelastic properties of carbon‐black‐filled rubber compounds

The specific role of filler–rubber interactions in dynamic properties was investigated. Natural rubber compounds, filled with N330 carbon black, were used, and the filler surface was modified through a gas treatment in the solid phase. The effects of this filler surface treatment on the dynamic properties were systematically studied at equal filler dispersion levels. The dynamic properties were assessed for both uncured and vulcanized compounds, and a number of advanced investigative techniques were used to characterize not only the modification of the carbon particle surface by an oxidative treatment but also the structure of the filled rubber compounds. Particular attention was paid to techniques that gave access to the segmental mobility to explain the benefit observed with modified carbon black. A molecular interpretation, based on NMR measurements, was considered that took into account physicochemical parameters. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 577–588, 2004     

Effect of filler geometry on the diffusion and transport behavior of aromatic solvents and commercial oil through nitrile rubber nanocomposites

The diffusion and transport behavior of nitrile rubber nanocomposites was studied with respect to different types of filler and also different types of solvents. The nitrile rubber nanocomposites showed considerable variations in the molecular transport owing to the tortuosity of path, decreased segmental mobility, and difference in particle geometry. As the matrix under consideration is polar, the behavior of the filled systems in aniline was also studied with a view to understand the polar–polar interaction between the filled matrix and the solvent. The oil repellency as a result of filler addition in the matrix was investigated by studying oil uptake of the nanocomposites. In all these investigations, it has been observed that the filler geometry played an important role in controlling the molecular transport through the polymer matrix. The layered silicate‐filled system showed better solvent resistance and hence minimum solvent uptake in polar and nonpolar solvents and better oil repellency followed by titanium dioxide and calcium phosphate filled systems. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Recycling of lignocellulosics filled polypropylene composites. I. Analysis of thermal properties,morphology, and amount of free radicals

Composites of isotactic polypropylene filled with comminuted rapeseed straw are studied. Improvement in interfacial adhesion is achieved by chemical modification of the lignocellulose filler. Composites were subjected to recycling by extrusion. The effect of multiple recycling of the composites on the process of nucleation and crystallization of polypropylene matrix, surface topography, and free radical generation was checked. On the basis of differential scanning calorimetry data, a significant influence of the recycling on nucleation activity of the lignocellulose filler was evidenced. A relation between the filler particle size and multiple recycling was established by observations under a polarization microscope, while scanning electron microscope analyses confirmed the positive effect of chemical modification of rapeseed straw on the interfacial adhesion. The composite structure changes forced by multiple recycling are discussed in the context of free radical generation. Concentration of free radicals in the rapeseed straw samples and composites was measured by the electron paramagnetic resonance spectroscopy to show that it was higher in the systems subjected to multiple recycling. Interestingly, the composites after multiple recycling showing elevated concentration of free radicals are also characterized by higher nucleation activity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41693.     

Theoretical aspects of free radical decay in polyethylene and comparison with experimental results

Results of the study of stability of free radicals using electron spin resonance suggest a close relationship between the glass transition temperature (T_g) and free radical decay. In a detailed study of radicals in polyethylene (PE) [J. Polym. Sci. A2 6 (1968) 1435] in the sixties, it was concluded that the decay of radicals in solid polymers is connected to molecular mobility. In this report on molecular mobility using a Monte Carlo method we show with the example of PE that it is the motion of molecular structures, which, depending on temperature, lead to a transfer of radical centers and thus to the approach of radicals and their decay. Theoretical and experimental decay curves for PE are compared and based on their close correspondence it is concluded how the individual types of motions affect the stability of free radicals.     

Bulk polymerization of methyl methacrylate initiated by high intensity ultrasonic irradiation and ESR study

High‐intensity ultrasound was used to initiate the bulk polymerization of methyl methacrylate. The polymerization rate varied with the sonication time, the intensity of the ultrasound, and the initiator concentration of poly (methyl methacrylate) in the monomer. Electron spin resonance (ESR) spectra, obtained by the spin trapping technique, testified that free radicals were produced during the sonication process, and the concentration of radicals also changed with the sonication condition. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1731–1735, 2002     

Effect of crosslinking on the mobility of PDMS filled with polysilicate nanoparticles: Positron lifetime, rheology and NMR relaxation studies

We study the effect of adding trimethylsilyl-treated polysilicate nanoparticles (Rg ∼ 2.2 nm) to crosslinked poly(dimethylsiloxane) (PDMS) elastomers above the entanglement molecular weight. The results are compared to un-crosslinked PDMS of a similar molecular weight, reported in previous studies and filled with the same polysilicate nanoparticles.Three techniques are used and compared to assess the enhancement or reduction in mobility with addition of filler: positron annihilation lifetime spectroscopy (PALS), rheology and nuclear magnetic resonance (NMR) spin-spin relaxation (T₂) measurements. PALS measurements do not show any clear effect of the filler on the mobility of the chains, as assessed by the size of free volume holes, but reveal a net increase in free volume with temperature increase (from 30 °C to 60 °C). A reduction in the dynamic shear storage modulus (measured at 1 rad s⁻¹) is observed in the filled network relative to the unfilled polymer (from 63 kPa without filler to 44 kPa with 40 w/w% filler), attributed primarily to a partial inhibition of the chemical crosslinking reaction by the particles. The NMR relaxation measurements, instead, show a reinforcement of the polymer network with increasing addition of polysilicate particles, as revealed by the faster T₂ decays at higher filler loadings, caused by increasing polymer bridging and particle flocculation. Similar trends are observed at higher temperatures (up to 80 °C), with a higher overall mobility. The apparent disagreement between rheology and NMR stems from the fact that rheology reflects bulk mobility and is primarily sensitive to chemical crosslinks in the network, while NMR probes segmental dynamics, which are affected by the presence of particles.In un-crosslinked PDMS instead, both rheology and NMR show an initial increase in mobility at low filler content, followed by reinforcement with further particle addition. These results strongly suggest that entanglements and filler-induced packing disruption, rather than free volume, play a major role in polymer dynamics.     

The effects of fillers on the chemorheology of highly filled epoxy resins: I. Effects on cure transitions and kinetics

This work examines the effects of level of silica filler (at 0, 10, 30, 50 wt%) on the gelation and vitrification of a model silica‐filled diglycidyl ether of bisphenol F (DGEBF)/methylenedianiline (MDA) system. An increased filler level is shown to decrease the gelation and vitrification times at low temperatures (below 80 °C). FTIR cure kinetics show that the reaction rates are increased and the activation energies of gelation are reduced at these temperatures, indicating that network formation is made easier. Entropic and catalytic reasons for this phenomenon are discussed. © 2003 Society of Chemical Industry     

Effect of low molecular weight polybutadiene as processing aid on properties of silica‐filled styrene–butadiene rubber compounds

Because silica has strong filler–filler interactions, a silica‐filled rubber compound shows a poor filler dispersion compared to a carbon black‐filled one. Improvement of the filler dispersion in silica‐filled styrene–butadiene rubber (SBR) compounds was studied using low molecular weight polybutadiene (liquid PBD) with the high content of 1,2‐unit. By adding the liquid PBD to the silica‐filled SBR compound, the filler dispersion and flow property are improved. The cure time and cure rate become faster as the 1,2‐unit content of the liquid PBD increases for the compounds containing the liquid PBD. The crosslink density increases linearly with increase in the 1,2‐unit content of the liquid PBD. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3135–3140, 2003     

Kinetic Study of Hydroperoxide Degradation in Edible Oils Using Electron Spin Resonance Spectroscopy

Lipid oxidation is a complex phenomenon involving free radicals which are highly reactive molecular species. The life-time of these radical species is extremely short and their detection is therefore difficult. Several electron spin resonance (ESR) spectroscopy methodologies make it possible to identify, quantify and measure the reactivity of radical species formed during oxidation–reduction reactions. In this study we took advantage of the specificity of ESR spectroscopy to detect radical compounds in order to determine the rate constants of hydroperoxide degradation, a key reaction involved in lipid oxidation. The interaction of 5-doxyl stearic acid and lipid-derived radicals was studied by following the intensity of ESR spectra. A kinetic model was developed to simulate data analysis obtained by ESR and values of rate constants for hydroperoxide degradation were determined at 100 and 110 °C. This quantitative approach of ESR spectroscopy has produced useful information about new rate estimates for hydroperoxide degradation in edible oils.     

Correlated electron spin-resonance and infrared spectroscopic study of the postformation auto-oxidation phenomenon in plasma-polymerized 4-vinyl pyridine films

Plasma polymerization of 4-vinyl pyridine (4-VP) proceeds through a gas-phase free radical mechanism to yield a film that retains much of the organic functionality of the monomer. During the deposition process, free radicals, which have been shown to quickly react with oxygen, are trapped to yield a film with a nascent peroxy radical density of 2.9 × 10¹⁸ spins/gas quantified by electron spin resonance (ESR) spectroscopy. In air at room temperature, peroxy radicals in the film react to produce carbonyl, hydroxyl, and ether structures in the polymer that was monitored using infrared (IR) spectroscopy. The free radical population was found to decay rapidly at first and then reach an apparent steady state after 30 hr. As the spin density decreases, a concomitant growth of vibrational modes associated with oxygen-containing functional groups was observed in the IR spectrum of the film. The relative population of oxygen-containing groups continued to increase even after the free radical population reached steady state. This slow, auto-oxidative effect may be attributed, in part, to free radical centers that are anchored to the polymer chain in regions of high crosslinking. In such regions, limited segmental mobility may limit the rate of radical-radical recombination (termination) proceses relative to oxidative radical-center. © 1996 John Wiley & Sons, Inc.     

Influence of γ-irradiation on proton spin–lattice relaxation of SBR used as antirads and its ESR investigation

Proton spin–lattice relaxation time t₁ was measured on SBR samples with carbon black or kaolin filler using modified linseed oil. The NMR pulse technique at 90MHz was used in the temperature range from 180 to 400 K. The temperature dependence of t₁ indicates that samples filled with carbon black have similar molecular dynamics to the standard unfilled SBR samples. The activation energy for the motion of the main chain for these samples amounts to 16.4kJ/mol. Samples containing linseed oil modified with para-toluidine showed an activation energy of about 14.6kJ/mol and were not affected by γ-irradiation. Values of the minimum relaxation time t^min₁ were increased by γ-irradiation in comparison with a standard SBR sample. ESR measurements carried out at room temperature by means of an X-band spectrometer indicated that unidentified radicals within the rubber were formed during its mastication with vulcanizing additives. The ESR spectra did not change during the vulcanization process. Samples filled with carbon black showed a broadening of the ESR line; this is consistent with the increase in the electrical conductivity.     

Silane-functionalized graphene oxide/epoxy resin nanocomposites: Dielectric and thermal studies

Improved dispersion of graphene oxide (GO) in the epoxy resin, as nanofiller, requires surface modification. Hence, functionalization of GO with small silane (GONSi) and bulky silane moieties (GOSi) has been carried out. Structural confirmation analysis of the prepared GO and modified forms have been performed using different analytical techniques. Cationic photocuring polymerization of pure aliphatic epoxy resin (CE) and loaded samples with GO, GOSi, or GONSi in amounts 0.5 and 1 wt % has been followed by FTIR. Loading of CE showed a passive effect for the modified filler on the conversion of the CE during photocuring, whereas the thermal stability of loaded epoxy resin is enhanced. Dielectric properties investigations revealed that the insulation feature of CE is not seriously reduced by the addition of GO or its modified forms. The secondary relaxation β process originating from the fluctuations of the side functional hydroxyl group can be described by the semi-empirical Cole–Cole function. The dielectric loss values are decreasing in the order GONSi > GOSi > GO > CE. Furthermore, it was found that the activation energy of the dynamic process is related to the conversion and to the ratio of the modified filler. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48253.     

Photopolymerization of poly(melamine-co-formaldehyde) acrylate for dental restorative resins

This paper describes the investigation of photoinitiated polymerization of poly(melamine-co -formaldehyde) acrylate (PMFA) by camphorquinone (CQ) and amines (AMH) by visible light (λ>400 nm). It was shown that as the concentration of CQ and/or AMH increases, the rate of polymerization reaches a maximum and then decreases. The double bond conversion of PMFA was 20–35%, whereas monomer conversion was 90–96%, depending on the polymerization conditions. Addition of inorganic filler up to 70 wt% did not significantly influence the polymerization kinetics. The final hardness of the photocured samples (with 70 wt% filler) was about half that found in a commercial dental restorative composite. The shrinkage of a composite with 70 wt% filler was 2.12%. Dental formulations based on photocuring of PFMA can be considered for clinical applications, after biological and toxicological evaluation. © 1999 Society of Chemical Industry     

Particle clustering in photocurable nanocomposites: Dependence of curing kinetics and viscoelastic properties

The aim of this article is to investigate the effect of nanoparticle clustering on the mobility of nanoparticles in nanocomposites, using spectroscopic methods (Brillouin and Raman). Special attention is paid to the effect of particle clustering on photocuring kinetics. The model system was poly(2‐hydroxyethyl acrylate) filled with fumed nanosilica in concentration range encompassing the percolation threshold. Results obtained from Brillouin spectroscopy show substantial changes in the sound velocity and the attenuation coefficient with increasing filler content. The damping of acoustic waves reaches the maximum at the percolation threshold (～15 wt %), which is related to changes in the mechanism of acoustic wave propagation. The formation of the cocontinuous silica phase strongly affects the curing kinetics of the monomer/silica system: the polymerization rate is the highest at a silica content corresponding to the percolation threshold. These results correlate well with the results of AFM surface roughness analysis. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39895.     

Free radicals in coal and coal conversions. 5. Methodology for the in-situ investigation of free radicals in coal depolymerization under SRC-II preheater/reactor conditions

In a continuing series of publications, a systematic investigation of the free radicals during coal depolymerization under SRC-II preheater/reactor conditions has been made. A uniquely designed electron spin resonance (e.s.r.) cavity allows the monitoring of free radicals and how they are affected by residence time, temperature and pressure changes. This paper summarizes the methodology that has been developed to study systematically free radical formation and behaviour in-situ during coal liquefaction. The methodology involved is to be used extensively in subsequent papers that examine in detail the manifold parameters that affect coal depolymerization. Also, results are given on the dependence of free radicals on temperature, pressure and gases in the presence of tetralin. The free radical concentration can be measured 3–4 min after the initiation of the heating process to an accuracy of ± 20%, while the process variables may cause several-fold changes in the free radical concentration.     

Presence of vacuoles in natural rubber–Cloisite 15A nanocomposites

We studied natural rubber (NR) filled with frequently used organoclay Cloisite 15A using transmission electron microscopy (TEM), cryoporosimetry, and electron spin resonance (ESR) spectroscopy. Quantitative analysis of the TEM micrographs showed a high level of dispersion without the formation of a rigid filler network. The presence of vacuoles was established on the surface of Cloisite 15A; this indicated weak filler–matrix interactions. The mechanism of reinforcement is, therefore, discussed. The volume of vacuoles was found to be proportional to the crosslinking density; this was confirmed with ESR spin‐probe method. The shape of the ESR spectra was highly influenced by the presence of vacuoles. In the NR–Cloisite 10A nanocomposites, vacuoles were absent. The strong interactions implied by this result were confirmed by ESR measurements and are discussed further. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44776.     

Effects of palm ash loading and maleated natural rubber as a coupling agent on the properties of palm‐ash‐filled natural rubber composites

Natural rubber composites were prepared by the incorporation of palm ash at different loadings into a natural rubber matrix with a laboratory‐size two‐roll mill (160 × 320 mm²) maintained at 70 ± 5°C in accordance with the method described by ASTM D 3184–89. A coupling agent, maleated natural rubber (MANR), was used to improve the mechanical properties of the natural rubber composites. The results indicated that the scorch time and cure time decreased with increasing filler loading, whereas the maximum torque exhibited an increasing trend. Increasing the palm ash loading increased the tensile modulus, but the tensile strength, fatigue life, and elongation at break decreased. The rubber–filler interactions of the composites decreased with increasing filler loading. Scanning electron microscopy of the tensile fracture surfaces of the composites and rubber–filler interaction studies showed that the presence of MANR enhanced the interfacial interaction of the palm ash filler and natural rubber matrix. The presence of MANR also enhanced the tensile properties and fatigue life of palm‐ash‐filled natural rubber composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Morphology and failure in nanocomposites. Part I: Structural and mechanical properties

Nanocomposites are expected to exhibit new and improved properties when compared to their microcomposite counterparts. By lowering the particle size to nanodimensions (<100 nm), the special effects in polymer composites appear. In this study we compared the properties of composites filled with micro- and nano-sized calcium carbonate (CaCO₃) filler particles in poly (vinyl acetate) (PVAc) matrix. The morphology of the composite was found to be responsible for the composite properties. The filler nanoparticles are dispersed in the matrix in the form of a 'net-like' structure, contrary to microparticles, which are dispersed as 'islands' in the matrix. The other systems investigated in this study were based on polyacrylate (PA) copolymer matrices filled with layered kaolin filler, which is well suited for creating nanocomposites. Mathematical models were used to quantify the interfacial interactions in the composites under investigation. Improved mechanical properties are obtained where there is a strong interfacial bond between the matrix and the filler. It seems that a key characteristic of the nanocomposites is the formation of a three-dimensional interphase with a significant amount of matrix with restricted chain mobility. The restricted molecular mobility in PVAc/CaCO₃ nanocomposites resulted in changes of relaxation behavior, i.e. in the appearance of a second transition above the T_g but only at large enough loading.