Patent Application
20150031811
1. Field of the Invention
The present invention relates to nanocomposite materials, and particularly to a polymer-clay nanocomposite material that provides a nanocomposite made from poly(styrene-co-ethyl methacrylate) and organo-modified clay by in situ polymerization.
2. Description of the Related Art
Compared to conventional filled polymers, polymer/layered silicate nanocomposites have recently attracted the attention of researchers due to their unique material properties. Specifically, the addition of only a very small amount of clay (typically less than 5 wt %) to a polymeric matrix has a significant impact on the mechanical, thermal, fire and barrier properties of the polymer.
The formation of polymer-based nanocomposites has been achieved by several methods, including in situ polymerization, polymer melting, and solution intercalation/exfoliation. Among these, dispersing in situ polymerization may be the most desirable method for preparing nanocomposites, since the types of nanoparticles and the nature of polymer precursors can vary in a wide range to meet the requirements of the process. In in situ polymerization, the clay is swollen in the monomer for a certain time, depending on the polarity of the monomer molecules and the surface treatment of clay. The monomer migrates into the galleries of the layered silicate so that the polymerization reaction occurs between the intercalated sheets. Long-chain polymers within the clay galleries are thus produced.
Although such in situ techniques have been studied with respect to bulk free radical polymerization, such techniques have not been widely applied to methacrylates. Given the broad and far-ranging applications of methacrylates, it would obviously be desirable to be able to modify and improve their properties through such a process.
Thus, a polymer-clay nanocomposite material solving the aforementioned problems is desired.
The polymer-clay nanocomposite material is a nanocomposite formed from poly(styrene-co-ethyl methacrylate) copolymer and organo-modified clay by in situ polymerization. Nanoparticles of a montmorillonite clay that has been modified with a quaternary ammonium salt are dispersed into a mixture of styrene and ethyl methacrylate monomers to form a mixture, which then undergoes bulk radical polymerization. The poly(styrene-co-ethyl methacrylate) copolymer preferably has a styrene to ethyl methacrylate ratio of about 1:1. Preferably, the organically modified montmorillonite clay forms between 1.0 wt % and 5.0 wt % of the mixture. A free radical initiator, such as benzoyl peroxide, is used to initiate polymerization. The clay nano-filler provides the nanocomposite with improved thermal stability. The poly(styrene-co-ethyl methacrylate) copolymer has the structural form:
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The polymer-clay nanocomposite material is a nanocomposite formed from poly(styrene-co-ethyl methacrylate) copolymer and organo-modified clay by in situ polymerization. Nanoparticles of a montmorillonite clay that has been modified with a quaternary ammonium salt are dispersed into a mixture of styrene and ethyl methacrylate monomers to form a mixture, which then undergoes bulk radical polymerization. The poly(styrene-co-ethyl methacrylate) copolymer preferably has a styrene to ethyl methacrylate ratio of about 1:1. Preferably, the organically modified montmorillonite clay forms between 1.0 wt % and 5.0 wt % of the mixture. A free radical initiator, such as benzoyl peroxide, is used to initiate polymerization. The clay nano-filler provides the nanocomposite with improved thermal stability. The poly(styrene-co-ethyl methacrylate) copolymer has the structural form:
Initially, a monomer mixture of styrene with EMA was prepared by mixing equal amounts of the two monomers. Organo-montmorillonite (OMMT) was then added to 25 g of the monomer mixture. The OMMT was dispersed in the monomer mixture within a 100 mL conical flask by magnetic and ultrasonic agitation. The magnetic agitation was performed for 24 hours, and the supersonic agitation was performed for one hour for each prepared sample. The dispersion of the particles in the monomer mixture was homogeneous, as indicated by a high translucency in the visible region. In the final suspension, 0.03 M benzoyl peroxide (BPO) was added as a free radical initiator, and the mixture was degassed by nitrogen passing.
Two series of the polymer-clay nanocomposites were prepared by in situ free radical bulk polymerization. In the first series, CLOISITE® 15A, an organically-modified montmorillonite clay manufactured by Southern Clay Products, Inc. of Gonzales, Tex. was used in differing relative amounts of 1.0, 3.0 and 5.0 wt %, compared to the monomer mixture. In the second series, the type of the nano-clay used was CLOISITE® 10A, again at relative amounts of 1.0, 3.0 and 5.0 wt % compared to the monomer mixture. The organic modifier in CLOISITE® 15A is a quaternary ammonium salt, viz., a dimethyl, dihydrogenated tallow quaternary ammonium salt, and the organic modifier in CLOISITE® 10A is also a quaternary ammonium salt, viz., a dimethyl, benzyl, hydrogenated tallow quaternary ammonium salt.
In order to study the reaction kinetics, free radical bulk polymerization was carried out in small test-tubes by heating the initial monomer-nanoclay-initiator mixture at 80° C. About 2 mL of the pre-weighted mixtures of monomer with the initiator and each type of CLOISITE® were placed into a series of ten small test tubes. After degassing with nitrogen, these were sealed and placed into a pre-heated bath at 80° C. Each test tube was removed from the bath at pre-specified time intervals and was immediately frozen after the addition of a few drops of hydroquinone in order to stop the reaction. The product was then isolated after dissolving in CH2Cl2 and re-precipitating (recrystallizing) in methanol. A different procedure for the nanocomposite isolation was followed in the last two or three samples of each experiment. Since the reaction was already finished, and the polymer/nanofiller mixture was a solid, the test tubes were broken and the products were obtained as such. In this way, it was ensured that the filler was enclosed into the polymer matrix. Subsequently, all isolated materials were dried to constant weight in a vacuum oven at room temperature. All final samples were weighed and the degree of conversion was estimated gravimetrically.
In order to examine the resultant products, X-ray diffraction (XRD), Fourier-transform infrared (FTIR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and thermogravimetric analysis (TGA) were all used. X-ray diffraction patterns were obtained using an X-ray diffractometer equipped with a CuKa generator (λ=0.1540 nm). Scans were taken in the range of diffraction angle 2θ=1-10°.
The chemical structure of the copolymer-based nanocomposites and the two different types of CLOISITE® were confirmed by recording their infrared (IR) spectra. The FTIR resolution used was 4 cm−1. The recorded wavenumber range was from 4000 to 400 cm−1, and 32 scans were averaged to reduce noise. Thin films were used in each measurement, formed by a hydraulic press.
In order to estimate the glass transition temperature of each nanocomposite prepared, DSC was used. About 10 mg of each sample was weighed, put into a standard sample pan, sealed, and placed in the appropriate position of the calorimeter. Subsequently, the samples were heated to 180° C. at a rate of 10° C. per minute to ensure complete polymerization of the residual monomer. Following this, the samples were cooled to 0° C., and their glass transition temperature was measured by heating again to 180° C. at a rate of 20° C. per minute.
The molecular weight distribution (MWD) and the average molecular weights of the pure copolymers and all nanocomposites were determined by GPC. The gel permeation chromatograph included an isocratic pump, a differential refractive index detector, and three PLgel 5μ MIXED-C columns in series. All samples were dissolved in tetrahydrofuran (THF) at a constant concentration of 1 mg per mL. After filtration, 200 μL of each sample was injected into the chromatograph. The elution solvent was THF at a constant flow rate of 1 mL per minute, and the entire system was kept at a constant temperature of 30° C.
The thermal stability of the samples was measured by thermogravimetric analysis. Samples of about 5 to 8 mg were used. The samples were heated from ambient temperature to 600° C. at a heating rate of 10° C. per minute under nitrogen flow, while the samples of clay were heated up to 800° C. Table 1 below shows the chemical structure of the organic modifiers of the different types of CLOISITE® used, together with their cation exchange capacity (CEC) and the d001 spacing measured from XRD.
where R and HT are hydrogenated tallow (˜65% C18, ˜30% C16, ˜5% C14).
Fourier transform infrared spectroscopy was used for the structural characterization of the polymer-clay (P(S-co-EMA)) copolymer and its nanocomposites. Initially, the formation of copolymers was verified and, subsequently, the presence of the OMMT in the copolymer matrix was identified. IR spectra of neat polystyrene (PS) and poly(ethyl methacrylate) (PEMA) homopolymers compared against that of the P(S-co-EMA) copolymer are shown in
In order to verify the formation of copolymers, certain parts of the spectra were isolated and presented in
Further, the IR spectra of the neat copolymer and its nanocomposites were studied to evaluate the interaction between the nanoclay and the polymer matrix (
The type of nanocomposite formed was checked with XRD. Polymer-clay nanocomposites can be characterized as immiscible (tactoids), intercalated, partially exfoliated or exfoliated. The particular form depends on the clay content, the chemical nature of the organic modifier, and the synthetic method. In general, an exfoliated system is more feasible with lower clay content (about 1 wt %), while an intercalated structure is frequently observed for nanocomposites with higher clay content.
From XRD measurements, the d-spacing for CLOISITE® Na+, CLOISITE® 15A and CLOISITE® 10A were measured as 1.18, 2.98 and 1.85 nm, as shown above in Table 1, which are close to the values reported by the manufacturer. The d-spacing for CLOISITE® 15A and 10A are larger than that of CLOISITE® Na+, indicating that the intercalant definitely intercalates into the silicate layer of MMT.
The XRD diffractograms of pure P(S-EMA) and the nanocomposites with 1, 3 and 5 wt % CLOISITE® 10A, or 1, 3 and 5 wt % CLOISITE® 15A, are shown in
The evolution of conversion with time of the copolymer P(S-EMA) is compared to those of the corresponding two homo-polymers (i.e., polystyrene and polyethyl methacrylate)), as shown in
At this conversion interval, the observed decrease in the termination reaction rate is only gradual. At this stage, the center-of-mass motion of radical chains becomes very slow and any movement of the growing radical site is attributed to the addition of monomer molecules at the chain end. This additional diffusion mechanism is the so-called “reaction diffusion”. The higher the propagation reaction rate value, the more likely is reaction-diffusion to be rate-determining. Finally, at very high conversions (beyond 90%), the reaction rate tends asymptotically to zero and the reaction almost stops before the full consumption of the monomer, thus a glassy state appears, corresponding to the well-known glass-effect. This is attributed to the effect of diffusion-controlled phenomena on the propagation reaction and the reduced mobility of monomer molecules to find a macro-radical and react.
In contrast to PEMA, PS presents kinetics that are mainly kinetically controlled with a slight effect of diffusion-controlled phenomena only at high degrees of conversion. The reaction curve of the P(S-EMA) copolymer more closely resembles that of PS. Thus, the copolymer initially presents a behavior obeying classical kinetics until almost 60% conversion. After this point, an increase in the conversion time curve, characteristic of the auto-acceleration, due to the effect of diffusion-controlled phenomena on the termination reaction, appears. The effect of diffusion-controlled phenomena on the propagation reaction results in final conversion values of less than 100%.
The presence of nanoparticles may influence polymerization kinetics, especially in monomers exhibiting strong effects of diffusion phenomena on the reaction kinetics. These results are attributed to the decreased free-volume of the reacting mixture, as well as the restriction imposed in the diffusion of macro-radicals in space due to the existence of the organic modifiers in the MMT platelets, which consist of large molecules (as seen in Table 1). Therefore, the OMMT platelets with large chemical structures of the modifiers add an extra hindrance in the movement of the macro-radicals in space in order to find one another and react (i.e., terminate), resulting in locally increased radical concentrations. Thus, the presence of OMMT nanoparticles seems to enhance the polymerization rate and slightly shorten the polymerization time to achieve a specific monomer conversion. Further, using a high amount of OMMT (i.e., 5 wt %) it was observed that the ultimate conversion was near 91-92 wt % lower than that of pure PMMA (i.e., 96-97 wt %). This may be attributed to the hindered movement of the small monomer molecules to find a macroradical and react due to the high amount of nano-filler at high monomer conversions. Thus, larger amounts of monomer molecules remain unreacted.
After this point, the reaction rate falls significantly and the curvature of the conversion versus time changes. At this conversion interval (from approximately 60% to 90%), the observed decrease in the termination reaction rate is only gradual. At this stage, the center-of-mass motion of radical chains becomes very slow and any movement of the growing radical site is attributed to the addition of monomer molecules at the chain end. This additional diffusion mechanism is the so-called “reaction diffusion”. The higher the propagation reaction rate value, the more likely is reaction-diffusion to be rate determining. Finally, at very high conversions (beyond 90%), the reaction rate tends asymptotically to zero and the reaction almost stops before the full consumption of the monomer, corresponding to the well-known glass-effect. This is attributed to the effect of diffusion-controlled phenomena on the propagation reaction and the reduced mobility of monomer molecules to find a macro-radical and react.
The full MWD of each sample was measured, and the results are shown in
N
W
Additionally, the variation of the MWD and the molecular weight averages with conversion were also examined. The results for the nanocomposite with 3 wt % CLOISITE® 15A are shown in
The glass transition temperature of the final sample of P(S-EMA) and all nanocomposites was measured using DSC. Indicative results of the amount of heat flow versus temperature obtained for neat P(S-EMA) and the nanocomposites with differing amounts of significantly 15A are shown in
The thermal degradation of the different types of CLOISITE® were also studied, and the TGA curves are shown in
In terms of the OMMT type, it appears that both nano-fillers show almost the same protection to thermal degradation of the material formed. The residual mass of all nanocomposites is in accordance with the amount of OMMT initially loaded (as seen above in Table 3). Further, from the DTGA curves, single peaks are observed in all nanocomposites and the pristine copolymer. This is an indication that degradation mechanism is taking place mainly in one step, both for the pristine copolymer and all nanocomposites, indicating the formation of macromolecular chains without defects in their structure. Thermal stability did not significantly change when the amount of CLOISITE® was increased.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
