Patent Grant
9023230
The present invention relates facile method to synthesize magnetic PNCs with highly dispersed and narrow size distributed NPs. The PNCs have highly thermal stability and unique electrical and dielectric properties.
Over the past few decades, magnetic materials with various shapes and sizes have demonstrated wide potential applications, for examples, in data storage [Wang 2008, Terris 2005], magnetic sensors [Li 2003, Guo 2007, Guo 2008], biomedical [Lee 2007, Lu 2005] (i.e., drug delivery) and pharmaceutical areas [Ban 2005, Gupta 2004], and even the environmental remediation [Zang 2010]. Polymer nanocomposites (PNCs) have been well developed in the last two decades due to the combined advantages of polymers, such as lightweight, easy processability and flexibility, and excellent physiochemical characteristics of the inorganic nanomaterials such as high mechanical strength and excellent electrical, magnetic and optical properties. Therefore, magnetic PNCs have attracted wide interest for their diverse potential applications such as energy storage devices [Kim, P. 2009], electrochromic devices [Zhu 2010], electronics [Zhu 2010 II, Zhu 2010 III], microwave absorbers [Guo 2007 II, Guo 2009], and sensors [Shimada 2007].
The major challenge lying ahead to obtain high performance PNCs comes from the serious agglomeration of the nanomaterials owing to their high surface energy and large specific surface area. Thus, a lot of efforts have been made to tailor their surface property through physical and chemical approaches [Tseng 2006, Tasis 2006, Yang 2007, Guo 2006] to improve the interfacial compatibility between the inorganic fillers and the polymermatrix. To overcome the challenges in dispersing the magnetic nanoparticles (NPs) limited by the magnetically induced agglomeration, techniques including encapsulating the magnetic core with surfactant [Kataby 1997], polymer [Boyer 2009], silica [Lu 2002], and carbon [Zhang 2010] have been reported. However, these well-dispersed NPs can only be limitedly applied in specific polymers with versatile surface functionalities. Right now, most of the current research work on fabricating PNCs starts from the as-prepared NPs and polymers (or monomer) with a direct blending [Zhu 2010 III] or surface initiated polymerization method [Gun 2007 III]. A general method is of great interest to simplify the procedures while maintaining the well dispersed magnetic NPs.
The critical concentration of the fillers within the polymer matrix, where the performance of the PNCs experiences a sharp change, is often called “percolation threshold.” Almost all the physical properties, including viscoelastic, thermal, mechanical and electrical properties are related to the percolation phenomenon. Thus, various methods have been developed to determine the percolation value. Most of the current research efforts concentrate on the carbon based nanomaterials such as carbon nanotubes (CNTs) [Potschke 2002], carbon nanofibers [Zhu 2010 II, Zhu 2010 IV], carbon NPs [Kotsilkova 2005], and graphene [Kim, H. 2009] to enhance the thermal, electrical and mechanical properties of the polymers. Nanoclays [Sun 2009, Hyun 2001] are often used to improve the fire retardant performance. Potschke et al. [Potschke 2004] studied the rheological and dielectric percolation of the multiwalled CNTs/polycarbonate PNCs and found that the rheological percolation (0.5-5 wt %) is strongly dependent on the temperature and the electrical percolation is at about 1 wt %. Sandler et al. [Sandler 2003] reported a ultra low electrical percolation in the CNTs/epoxy PNCs at a loading of 0.0025 wt %. It is well-known that the percolation threshold is also dependent on the filler morphology, spherical particles are relatively difficult to reach percolation as compared to those with larger aspect ratio (like fibers and tubes). Therefore, a relatively higher loading of around 16 vol % from geometrical model [Kirkpatrick 1973, Zallen 1983] was required to reach percolation. Recently, Zhu et al. reported a low electrical percolation at 1.5 vol % with spherical Fe(core)-FeO(shell) structured NPs in epoxy resin using a surface wetting method. [Zhu 2010 111].
The present invention relates to an in-situ method to fabricate magnetic PNCs with highly dispersed and narrow size distributed NPs. No surfactant need be used in the whole process. The thermal stability of the PNCs increased surprisingly by ˜120° C. with various particle loadings (3-12 wt %). In embodiments of the present invention, the composites showed conductive behavior when the NPs loading was higher than 5 wt %. And the dielectric constant was found to reach 100-1000 depending on the frequency, which would be of great interest in super-capacitor applications.
A facile surfactant-free process has been utilized to prepare multifunctional polypropylene (PP) nanocomposites filled with highly dispersed Fe@Fe2O3 core@shell nanoparticles (NPs). Transmission electron microscopy (TEM) observations confirmed the formation of uniform NPs in the PP matrix and the particle size increased with increasing the particle loading. The melt rheology measurements showed an apparent change in the frequency dependent storage modulus (G′), loss modulus (G″), and complex viscosity (η*) particularly at low frequencies. These changes were often related to the filler “percolation threshold,” which has also been verified in the sharp change of electrical resistance and dielectric permittivity of these nanocomposites in higher particle loadings. The continuous decrease in the resistivity with increasing filler loading from 5 wt % to 20 wt % demonstrated the structural transition of the nanocomposites. The monotonic increase in the dielectric permittivity with increasing particle loadings combined with the direct evidence from the TEM observations indicated that the NPs are well separated and uniformly dispersed in the polymer matrix. Thermal gravimetric analysis (TGA) results revealed a surprisingly high enhancement of the thermal stability by ˜120° C. in air due to the oxygen trapping effect of the NPs and the polymer-particle interfacial interaction. The differential scanning calorimetry (DSC) results showed that the crystalline temperature (Tc) of the nanocomposites was reduced by 16-18° C. as compared to that of PP, while the melting temperature (Tm) almost maintained the same. The nanocomposites were found to be soft ferromagnetic at room temperature.
In general, in one aspect, the invention features a method that includes forming a solution that includes a nanoparticle precursor, a polypropylene, and a solvent. The solution is surfactant-free. The method further includes synthesizing the solution to form a magnetic polypropylene nanocomposite. The synthesis is a surfactant-free synthesis.
In general, in another aspect, the invention features a composition that includes polypropylene and nanoparticles. In the composition, the nanoparticles are wrapped by the polypropylene. The composition is a magnetic polypropylene nanocomposite.
There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the invention that will be described hereinafter.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting.
A facile in situ surfactant free method to synthesize magnetic polyolefin polypropylene (PP) nanocomposites has been discovered. In an embodiment of the present invention, magnetic NPs can be produced using Fe(CO)5 as a precursor during the refluxing process in the Fe(CO)5/PP/xylene solution. The as-synthesized NPs are physically wrapped by PP. The composition has unique rheological, electrical, and dielectric percolation behaviors.
In the examples illustrated herein, the following materials were utilized:
Isotactic PP (Total Petrochemicals Inc. USA), (0.9 g/cm3 in density, Mn≈40500, Mw≈155000, melt index≈35 g/min).
Iron(0) pentacarbonyl (iron carbonyl, Fe(CO)5, 99%) (Sigma-Aldrich).
Solvent xylene (laboratory grade, F=0.87 g/cm3) with a boiling temperature ranging from 137 to 145° C. (Fisher Scientific).
All the chemicals were used as received without any treatment.
Fabrication of Polymer Nanocomposites.
In an embodiment of the present invention, PP was initially dissolved in xylene with a weight ratio of 1:10 (20 g: 207 mL) and refluxed at the boiling point (˜140° C.) of xylene for around 2 hour until the PP was completely dissolved. Then different weight (2.17, 3.67, 6.08, 9.54, and 17.48 g) of liquid Fe(CO)5 was injected into the dissolved PP solution to obtain the final PNCs containing 3, 5, 8, 12, and 20 wt % of the NPs (based on pure iron element). The mixture solution turned from transparent to yellow immediately after the addition of Fe(CO)5 and then gradually changed to black during the additional 3 hour refluxing process under the nitrogen protecting conditions, indicating the formation of the NPs. The PNC solution was then cooled down to around 90° C. and then poured onto a large glass plate to allow solvent evaporation overnight. The powder-like products were collected and kept in a vacuum oven at room temperature overnight.
Pure PP powders are also prepared following the above procedures without adding Fe(CO)5 and are termed as p-PP to differentiate from the as-received PP (o-PP). Upon heating, Fe(CO)5 was decomposed to Fe2(CO)9 and Fe3-(CO)12 with a rapid formation of CO, reaching an equilibrium mixture of all the three carbonyls. The Fe3(CO)12 was then decomposed and finally formed the metallic NPs [Smith 1980, Van Wonterghem 1985]. Oxidization took place on the surface and then a core-shell structure was formed after exposure to air.
The desired samples were prepared from PP both the as-received original PP (o-PP) and processed PP (p-PP) and its PNC powders using hot press (Carver 3853-0, USA). Briefly, the dried powders were compressed under a pressure of 10 MPa at 180° C. in a mold at a heating rate of 20° C./min. The compressed composites were held at 180° C. for 20 min and then cooled down to room temperature in the mold while maintaining the applied pressure. Finally, a disk-shaped nanocomposite sample was prepared with a diameter of 25 mm and thickness of 2-3 mm.
Characterization
Fourier transform infrared spectroscopy (FT-IR, Bruker Inc. Tensor 27) with hyperion 1000 ATR microscopy accessory was used to characterize PP and its PNCs over the range of 2500 to 400 cm−1 at a resolution of 4 cm−1. The X-ray diffraction (XRD) analysis with Cu radiation source was carried out with a STA Jupiter 449C (Netzsch) on disk samples with a diameter of 25 mm.
The particle distribution in the PP matrix was examined by a transmission electron microscope (TEM). The samples were stained in RuO4 vapor to harden the surface and then microtomed into a film with a thickness of ˜100 nm, which were observed in a JEOL 2010 TEM at a working voltage of 200 kV. Images were recorded with a Gatan Orius SC 1000 CCD camera. In order to obtain more accurate particle size, magnifications were calibrated using commercial cross-line grating replica and SiC lattice images.[Luo 2006].
The rheological behavior of the PNCs was studied using TA Instruments AR 2000ex Rheometer. An environmental test chamber (ETC) steel parallel-plate geometry (25 mm in diameter) was used to perform the measurement at 200° C., with dynamic oscillation frequency sweeping from 100 to 0.1 Hz in the linear viscoelastic (LVE) range (strain 1%) under a nitrogen atmosphere to prevent the oxidation of PP.
The thermal degradation/stability of the PNCs was studied with a thermogravimetric analysis (TGA, TA Instruments TGA Q-500) from 25 to 600° C. in air and nitrogen atmosphere, respectively, with a flow rate of 60 mL/min and a heating rate of 10° C./min. Differential scanning calorimeter (DSC, TA Instruments Q2000) measurements were carried out from 0 to 250° C. under a nitrogen flow rate of approximately 100 mL/min at a heating rate of 10° C./min.
The volume resistivity was determined by measuring the DC resistance on a disk-shaped sample (diameter, ˜50 mm; thickness, 0.5-1.0 mm). An Agilent 4339B high resistance meter equipped with a resistivity cell (Agilent, 16008B) was used to measure the volume resistivity of each sample after inputting the thickness. This equipment allowed the resistivity measurement up to 1016Ω. The source voltage was set at 0.1 V for all the samples. The reported values represent the mean value of eight measurements with a deviation less than 10%.
The dielectric properties were measured by a LCR meter (Agilent, E4980A) equipped with a dielectric test fixture (Agilent, 16451B) at the frequency of 20 HZ-2 MHz. The PP and PNCs were hot pressed in the form of disk pellets with a diameter of 60 mm and an average thickness of about 0.7 mm.
The magnetic property measurements of the PNCs with various particle loadings were carried out in a 9 T physical properties measurement system (PPMS) by Quantum Design at room temperature.
FT-IR Analysis
Microstructure of PNCs and Electron Diffraction of NPs
The measured particle size (
To obtain the phase structure of the formed NPs, the NPs were characterized by high resolution TEM (HRTEM), as shown in
The percolation (or called threshold), which is essentially important for the prediction and interpretation of the switching physical phenomena, can be observed from the particle-particle interaction within the polymer matrix. Once a network structure of the filler was formed in the composites, the electrical [Zhu 2010 II, Barrau 2003], rheological [Zhu 2010 IV] and mechanical properties [Meincke 2004] experienced a sharp change. The PNCs with the particle loading increasing from 5 to 20 wt % illustrate the structural transition of the NPs within the polymer matrix. For 5 wt % loading, the NPs were loosely embedded in the matrix, though continuous network structure could not be observed, the string-like particle chain began to form,
X-ray Diffraction and Mossbauer Analysis
Despite the simplicity of its chemical component, PP showed a remarkable complexity of crystal structures (phases), which include α, β, and γ phases. Each of the α, β, and γ crystalline form had its own distinctive peaks in the XRD patterns.
In a typical XRD pattern of the R phase PP, the intensity of the first peak (110) was always stronger than that of the second peak (040).[Auriemma 2002]. However, it was not true for the samples containing γ phase. All the samples were characterized by a stronger second peak than that of the first one, which was not surprising considering the same location of the strong peak in γ phase. Because of the high diffraction similarity between R and γ phases in the region of 13-17°, the γ phase was usually determined from the peak at 2θ=20.07° (117), and R phase is identified from the peak 2θ=18.50° (120). [Forester 2001] The other peaks at 2θ=14.06, 16.90, 21.20, and 21.86° corresponded to the 110, 040, 131, and 041 crystalline planes of R-PP, respectively. It was interesting to observe that the peak intensity at 2θ=20.07° decreased with the addition of the NPs. However, the amount of γ-PP showed a surprisingly filler loading independent behavior and a comparable peak intensity at 2θ=20.07° was observed as compared to that of the peak at 2θ=18.50°. In a previous study [Mezghani 1995], the ratio of γ to α was simply calculated from the relative intensity of the unique peaks of γ phase at 2θ=20.07° and a phase at 2θ=18.50°. The relatively low intensity of the peak at 20.07° as compared to that of the peak at 18.50° after the incorporation of the NPs suggested a reduced amount of γ phase during the crystallization process. Earlier studies showed that more PP was observed to be crystallized in the γ phase at a lower cooling rate.[Mezghani 1998]. In this embodiment of the present invention, the conductive fillers within the polymer matrix created a pathway for heat transfer and the cooling rate was much faster for the PNCs as compared to that of pure PP, which was observed in the CNTs suspended in surfactant micelles in water.[Huxtable 2003]. Therefore, a decreased amount of PP in γ phase was observed.
However, no additional peak regarding the NPs was detected in the PNCs as compared with those of pure PP, which is due to the limitation of the XRD whose signals only come from the sample surface. The crystalline structure of the NPs was further identified from the comparison between the experimental and simulated TEM-SAED patterns. Mossbauer analysis was also used to justify the valence of iron in the NPs.
The particle composition is examined by the consistence between the experimental and simulation results based on the SAED data. [Fang 2010]. To be specific, the intensity profile of the SAED pattern was measured using ELD program [Zou 1993] and the background of the intensity profile was subtracted using Reflex module in MS Modeling program.[Fang 2010]. Then the simulated SAED powder patterns were calculated using the standard structures of Fe2O3, Fe3O4, FeO and Fe. As shown in
However, there was still a difference between the experimental curve and the standard simulated Fe2O3 pattern. To further confirm the particle composition, room temperature Mossbauer spectrum analysis was conducted on the PNCs with a particle loading of 20 wt %, as shown in
Melt Rheology
The rheological behaviors of the composite melts are essentially important for industrial nanocomposite processing. Also, the formation of a percolated system can be detected by characterizing the complex viscosity (η*), storage modulus (G′), and loss modulus (G″) as a function of frequency. [Zhu 2010 II, Zhu 2010 IV, Potschke 2004, Mitchell 2002].
similar value of η* at high frequency (10-100 Hz) indicated a polymer melt rather than filler dominated fluid dynamics. However, as the particle loading increased to 5, 8, and 12 wt %, the viscosity curve became linear within the whole frequency range. This phenomenon indicated a filler dominated fluid in the PNCs with a relatively high particle loading. The transition in η* indicated that the PNCs had reached a rheological percolation, at which the NPs form a network structure and greatly impede the motion of the polymer chains. The increase of η* with an increase of particle loadings was primarily due to the significant increase in G′ and G″ (η*=η0′−iη″, where η′=G″/ω, η″=G′/ω, and ω is angular frequency, rad/s). [Shenoy 1999].
At 200° C., the PP chains were fully relaxed and exhibited a typical homopolymer-like terminal behavior, which disappeared with the addition of the NPs. As shown in
The tan 8 is the ratio of G00 to G0, which was used to characterize the damping property of the PNCs. As shown in
G′ versus G″ for the PP and its PNCs is plotted in
Electrical Conductivity
Inset 902 of
Dielectric Properties
As shown in FIG. HA, Pure PP and PNCs with a nanoparticle loading of 3 wt % were observed to exhibit constant ∈′ value. However, the PNCs with particle loadings larger than 3 wt % show a dielectric relaxation behavior (∈′ decreases with increasing frequency). This result was in good agreement with the variation of electrical resistance of the PNCs, where a transition starts from the concentration of 5 wt % (often called “percolation”). It is worth noting that both ∈′ and ∈″ of the PNCs were enhanced significantly by orders of magnitude near the percolation, as shown in
Thermal Properties
In addition, as shown in
To remove the effect of heat history on the obtained heat data, all the samples were first heated to 250° C. with a heating rate of 10° C./min under nitrogen, allow the samples to maintain at 250° C. for 2 min and then cool down to room temperature with a cooling rate of 10° C./min. Immediately after that, the samples were heated again to 250° C. following the same procedures as used for the first heating process.
The DSC curves were recorded using the data collected on the first cooling and second heating processes. The melting temperature (Tm) of PP was not affected by the addition of the NPs. However, the crystalline temperature (Tc) of PP decreased by 16-18° C., where PP was crystallized at 119.8° C. and the PNCs were crystallized at 101-103° C. The lowered Tc was attributed to the strong particlepolymer interaction, which greatly restricted the segmental motion of the polymer chains and inhibited the content of the crystalline structures in the polymer chains. The lowered peak intensity in XRD curves with increasing particle loading together with the crystalline fraction calculated from DSC further confirmed this observation. The broad peak from 125 to 145° C. in pure PP curve was due to the melting of the γ-crystals, while this peak was not observed in the PNCs. This observation was quite consistent with the XRD results that the decreased peak intensity at 20=20.07° (117) corresponded to a lower content of γ-phase PP in the PNCs.
Table 1 above lists the DSC characteristics of PP and its PNCs. The crystalline fraction (Fc) of the polymer within the PNCs was calculated from the following equation:
where ΔH is the enthalpy of fusion (J/g), 209 J/g is the fusion enthalpy for a theoretically 100% crystalline PP [Yuan 2006], and fp is the weight fraction of the polymer. The crystallinities were 43.3, 39.9, 43.6, 42.9, and 36.7% for the PNCs with a particle loading of 0, 3, 5, 8, and 12% respectively. The PNCs exhibited lower Fc as compared to pure PP, except for 5 wt % NPs. The lower Fc of the PNCs was attributed to the fact that the NPs are able to disturb the continuity of the polymer matrix and thus introduce more grain boundaries as well as defects, which were also reported previously in high-density polyethylene/MWCNT composites [Yang 2009, Kodjie 2006] and clay reinforced nylon-6 composites [Fornes 2003].
Magnetic Properties
Accordingly, a surfactant-free in situ method has been found to synthesize multifunctional PNCs with high quality dispersion of NPs. The particle size grows up with the increase of particle loading. Results show that the “rheological percolation” was much lower as compared to the “electrical conductivity percolation” owing to the larger effective radius stemmed from the wrapping polymer chains on the particle surface, which also accounts for the uniform dispersion of the NPs. The sharp change in electrical resistivity and dielectric permittivity of the PNCs show high consistency with each other and both confirm the structural transition starting from 5 wt % particle loading, which was due to the formation of the interparticle network structure (percolation threshold). As a result of the polymer-particle interaction and the oxygen trap of the presence of the metal iron, a significant enhancement in thermal stability of around 120° C. was observed in the PNCs as compared to the p-PP. The magnetic measurement indicated a soft ferromagnetic behavior of these PNCs.
Polypropylene is a low cost polymer which has been widely used in many applications. Adding certain functions into PP would significantly widen its application and increase its value. The synthesis of the present invention is very facile, and can be expanded to many other functional nanoparticles. Multifunctional polypropylenes prepared via this approach have applications in electronic, military, and packaging areas.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. For example, the selection traces could be replaced with an array of conventional silicon switches to address individual cells. Accordingly, other embodiments are within the scope of the following claims. The scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.
The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.
McNally, T.; Potschke, P.; Halley, P.; Murphy, M.; Martin, D.; Bell, S. E. J.; Brennan, G. P.; Bein, D.; Lemoine, P.; Quinn, J. P. Polymer 2005, 46, 8222-8232 (“McNally 2005”).
Zhu, J.; Wei, S.; Ryu, J.; Sun, L.; Luo, Z.; Guo, Z. ACS Appl. Mater. Interfaces 2010, 2, 2100-2107 (“Zhu 2010 III”).
This application claims priority to: provisional U.S. Patent Application Ser. No. 61/458,827, filed on Dec. 3, 2010, entitled “Surfactant-Free Synthesis Of Magnetic Polyolefin Nanocomposites,” which provisional patent application is each commonly assigned to the Assignee of the present invention and is hereby incorporated herein by reference in its entirety for all purposes.
This invention was made with government support under Grant DMR-04-49022 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country |
|---|---|---|
| WO 2009141488 | Nov 2009 | WO |
| WO 2012011016 | Jan 2012 | WO |
| Entry |
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| Number | Date | Country | |
|---|---|---|---|
| 61458827 | Dec 2010 | US |
