FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix

1. FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix J.Typek1, N. Guskos1,2, A. Szymczyk1, D. Petridis3 1Institute of Physics, Szczecin University of Technology, Szczecin, Poland 2Department of Physics, University of Athens, Greece 3NCSR Demokritos, Aghia Paraskevi, Athens, Greece

2. Maghemite – γ-Fe2O3 (iron(III) oxide) • inverse spinel cubic structure • stoichiometric formula (Fe3+)AO2-(Fe3+Fe3+2/3[]1/3)BO2-3 • 8 Fe3+ ions located in tetrahedral sites (A-sites) and 16 Fe3+ ions in octahedral sites (B-sites) • collinear ferrimagnet • antiparallel magnetic sublattices A (4.18 μB) and B (4.41 μB) • TC=590-675ºC

3. PMMA (polymethyl methacrylate) • Polymethyl methacrylate (PMMA) or poly (methyl 2-methylpropenoate) is the synthetic polymer of methyl methacrylate. This thermoplastic and transparent plastic is sold by the tradenames Plexiglas, R-Cast, Perspex, Plazcryl, Limacryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast and Lucite and is commonly called acrylic glass or simply acrylic. The material was developed in 1928 in various laboratories and was brought to market in 1933. • Temperature of the glass transition Tg = 85-105ºC • Melting temperatures 130-140ºC

4. Synthesis of γ-Fe2O3 /PMMA nanocomposite • Procedure: preparation of capped magnetic nanoparticles → exchange of the oleate units by methacrylate units → preparation of γ-Fe2O3/PMMA composite • γ-Fe2O3 nanocrystalline particles with an average size of 10 nm, chemically bonded to the chains • Magnetic nanoparticles capped with oleic acid were prepared by one step method involving partial oxidation of Fe(II) in alkaline solutions by dilute H2O2. The reaction was conducted in the presence of oleic acid and under biphase conditions. • The surface bond oleate groups can be fully exchanged with metacrylate units byrefluxing in ethanol. The exchange reaction ensures the chemical bonding of methacrylate units to the surface of nanoparticles, which in turn, undergo the polymerization with the vinyl groups of the methyl mathacrylate. Incorporation of nanoparticles in the polymer matrix through chemical bonding FMR investigated samples – 5 wt% and 10 wt% γ-Fe2O3

5. DSC study of γ-Fe2O3/PMMA nanocomposite • Tg increases with maghemite content increase →reduced dynamics of polymer chains, hidering segmental motion • cp heat capacity decreases with maghemite content → increase of steric hindrance

6. FMR spectra – temperature dependence 5 wt% Tblock ~ 40 K ? T=150 K PMMA relaxation? Low-temperature range High-temperature range

7. FMR parameters – integrated intensity • FMR integrated intensity Iz ~ (FMR signal amplitude)·(ΔB)2 • Integrated intensity Iz ~ spin susceptibility χ’’ • Iz·T ~(magnetic moment)1/2 5 wt%

8. FMR spectra: γ-Fe2O3 content 5 wt% 10 wt% The difference (in intensity) is observed for T>250 K. It could be attributed to the dipol-dipol magnetic interaction between nanoparticles.

9. FMR spectra - decomposition T=71 K, 5 wt% Narrow (high-field) component → magnetic easy axis  external magnetic field Broad (low-field) component → magnetic easy axis || external magnetic field

10. FMR spectrum decomposition 5 wt%

11. FMR spectrum decomposition Magnetic moment g-factor Linewidth [Gs] Narrow component Broad component 10 wt% Temperature [K] Temperature [K]

12. FMR spectrum decomposition 10 wt% B0 Narrow component (high field) Integrated intensity [arb. units] B0 Broad component (low-field) Temperature [K]

13. Conclusions • Increase in maghemite content → Tglass decreases • Blocking temperature ~40 K • Relaxation in PMMA=150 K • Maghemite content differences seen in FMR above 250 K • FMR spectrum reflects magnetic anisotropy of nanoparticles