Study of Porous Structures and Thermal Properties of Luminescent Polymeric Microspheres Derivatives of Naphthalene-2,7-Diol

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1 531 Study of Porous Structures and Thermal Properties of Luminescent Polymeric Microspheres Derivatives of Naphthalene-2,7-Diol Beata Podkościelna* and Mateusz Lutomski Department of Polymer hemistry, Faculty of hemistry, Maria urie-sklodowska University, pl. Maria urie-sklodowskiej 3, Lublin, Poland. ABSTRAT: We studied the porous structures and thermal properties of spherical shaped cross-linked polymers. These novel polymeric microspheres were synthesized by emulsion suspension polymerization of aromatic tetrafunctional monomer [2,7-(2-hydroxy-3-methacryloyloxypropoxy)naphthalene (2,7- NAF.DM)] with typical, commercially available monomers (styrene, divinylbenze ne and methyl methacrylate). The monomer 2,7-NAF.DM was obtained in the twostep process. In the first step, the epoxy resin was synthesized by treating epichlorohydrin with naphthalene-2,7-diol. In the second step, the obtained epoxy derivatives were subjected to esterification with methacrylic acid. Porous structures of the obtained microspheres in the dry state were studied by nitrogen adsorption desorption measurements. Thermal stabilities and degradation behaviours of the obtained co-polymers were characterized by differential scanning calorimetry and thermogravimetric/derivative thermogravimetry analyses. In addition, we also present the photoluminescent properties of these microspheres. 1. INTRDUTIN Polymeric microspheres have a number of specific applications (e.g. in medicine, for the transport of drugs within the body to the selected tissue; in the construction industry, as additives with special mechanical and thermal properties). These microspheres are also used as additives for paints, varnishes, cosmetics or optical devices. orrea et al. (2014) reported that conjugated polymers are materials that are capable of forming a π-electron conjugation along the polymer backbone. Because of their appealing electrical and optical properties, these polymers are widely used in many technological applications. The use of nanostructured materials makes it possible to miniaturize devices down to the micro-scale and nano-scale levels. onjugated nanostructured polymers are used in sensors, actuators or discrete electronic devices. Huang et al. (2014) presented a new method to study the luminescent properties of Er 3+ /Yb 3+ co-doped oxyfluoride glass ceramic microsphere with the tapered fibre-microsphere coupling system. Tellurite microspheres have the potential to be used in nanoparticle sensing and novel light sources (Ruan et al. 2014). Li et al. (2014) proposed a method to fabricate a microstructure fibre by coating a standard optical fibre with a silica inverse opal through a sol gel co-assembly method, in which the polystyrene colloidal suspension of microspheres and a hydrolyzed silicate precursor were added to the solvent together. A reflectometric ion sensor for potassium based on acrylic microspheres has been reported by Pei and Lee (2014). In their study, a combination of acrylic spheres and optical reflectometry had enhanced the sensing performance of the optical K + ion sensor, as the former provides large *Author to whom all correspondence should be addressed. beatapod@umcs.pl (B. Podkościelna).

2 532 B. Podkościelna;cielna and M. Lutomski/Adsorption Science & Technology Vol. 33 No surface area for better loading capacity and higher reaction rate, whereas the latter provides simple and rapid measurements. The initial attempt to prepare 2,7-(2-hydroxy-3-methacryloyloxypropoxy)naphthalene (2,7-NAF.DM) polymeric microspheres (Podkościelna and Gawdzik 2010) was concerned with the synthesis of a new monomer and study of influence of different diluent compositions on the porous structure. This material was porous and did not show photoluminescence properties. In another separate study, Podkościelna (2014) obtained a luminescent material for the synthesis of dimethacrylate derivatives of naphthalene-2,7-diol and N-vinyl-2-pyrrolidone. The naphthalate- 2,7-diol derivatives are interesting compounds, which have the potential to be used as new optical polymers. 2. EXPERIMENTAL ANALYSIS Styrene (ST), 1,4-divinylbenzene (DVB), methyl methacrylate (MMA), bis(2-ethylhexyl)sulphos uccinate sodium salt (DA, BP) and decan-1-ol were from Sigma-Aldrich (Buchs, Switzerland). α, α'-azoiso-bis-butyronitrile was obtained from Merck (Darmstadt, Germany). Acetone and toluene were obtained from Ph (Gliwice, Poland). The methods involved in the synthesis of 2,7-NAF.DM was presented in Podkościelna and Gawdzik (2010) o-polymerization Approximately 150 ml of redistilled water and 1.4 g of bis(2-ethylhexyl)sulphosuccinate sodium salt were stirred for 1 hour at 80 in a three-necked flask fitted with a stirrer, a water condenser and a thermometer (according to the procedure described in Podkościelna et al. 2006). The solution containing 2,7-NAF.DM and DVB/ST/MMA and 1% wt of α, α'-azoiso-bisbutyronitrile (initiator) and a mixture of pore-forming diluents (toluene and decan-1-ol) were added while stirring the aqueous medium (Table 1). o-polymerization was performed for 18 hours at 80. The co-polymers obtained were filtered off, washed with distilled hot water, dried and extracted in a Soxhlet apparatus (first with acetone, and then with methanol). The chemical structures of monomers used for synthesis of microspheres are shown in Figure haracterization of the Synthesized Polymer Attenuated total reflectance (ATR) spectra were obtained using a Bruker Fourier transform infrared spectroscopy (FTIR) spectrophotometer TENSR 27. The spectra were acquired in the cm 1 TABLE 1. Experimental Parameters of the Syntheses of Microspheres o-polymer Monomers Initiator Toluene Decan-1-ol (g) (ml) 2,7-NAF.DM DVB ,7-NAF.DM ST ,7-NAF.DM MMA ,7-NAF.DM DVB ,7-NAF.DM ST ,7-NAF.DM MMA

3 Study of Porous Structures and Thermal Properties of Luminescent Polymeric 533 Microspheres Derivatives of Naphthalene-2,7-Diol H 2 H 3 H 3 ST H H 2 DVB H MMA H 3 H 3 2,7-NAF.DM Figure 1. hemical structures of monomers used for the synthesis of microspheres. spectral region. Resolution was set as 4 cm 1 and number of scans/spectrum was 32. The porous structure of co-polymers was investigated by nitrogen adsorption at 77 K using an adsorption analyzer ASAP 2405 (Micrometrics Inc., USA). Before performing the measurements, the co-polymers were outgassed at 150 for 2 hours. Specific surface areas were calculated by the standard BET method, assuming that the area of a single nitrogen molecule in the adsorbed state is 16.2 Å 2. Differential scanning calorimetry (DS) thermograms were obtained using a DS Netzsch 204 calorimeter (Netzsch, Günzburg, Germany). All DS measurements were carried out in aluminium pans with a pierced lid (sample weight approximately 5 10 mg) under nitrogen atmosphere (30 ml/minute). Dynamic scans were performed at a heating rate of 10 K/minute in the temperature range of uring characteristics, such as temperature of the peak maximum and final cure temperature, were determined. The beads were examined using an atomic force microscope (AFM), AFM Nanoscope III (Digital Instruments, USA) operating in the contact mode. The images presented in this paper contain data points, which were obtained within a few seconds. 3. RESULTS AND DISUSSIN Experimental parameters of the syntheses are presented in Table 1. The data obtained from the ATR FTIR spectra of the 2,7-NAF.DM co-polymers are presented in Table 2. An analysis of these data confirms the differences in the structure of the monomers used in co-polymerization. In addition, Figure 2 shows the ATR FTIR spectra for all of the co-polymers. In the spectra, strong signals coming from carbonyl ( cm 1 ) and ester species ( cm 1 ) can be observed. The aromatic skeletal absorption is observed at approximately 1480, 1600 and 3050 cm 1, respectively. The strong H deformations ( cm 1 ) in the aromatic unit is clearly visible in the spectra of 2,7-NAF.DM ST8 and 2,7-NAF.DM DVB8 because additional aromatic rings are introduced in the structure of monomers with which these co-polymers were synthesized. The structure of MMA contains ester and carbonyl groups and in the spectrum of 2,7- NAF.DM MMA8, the signal emitted by this group is very strong. The characterization of the porous structure of the 2,7-NAF.DM co-polymers obtained by the nitrogen adsorption desorption method is presented in Table 3. From these data, it can be seen that the co-polymer obtained with DVB has the most developed porous structures. This is a tetrafunctional monomer (having two vinyl groups) that has capabilities to create a more cross-linked structure. ther

4 534 B. Podkościelna;cielna and M. Lutomski/Adsorption Science & Technology Vol. 33 No TABLE 2. ATR FTIR Spectra o-polymer = H stretching skeletal stretching H vibration vibration vibrations asymmetrical (cm 1 ) (cm 1 ) (cm 1 ) stretching vibration (cm 1 ) 2,7-NAF.DM MMA ,7-NAF.DM ST ,7-NAF.DM DVB H 1 Transmitance (%T) ,7-NAF.DM-DVB2 2,7-NAF.DM-ST2 2,7-NAF.DM-MMA2 0.4 Figure 2. ATR FTIR spectra Wave numbers (cm 1 ) TABLE 3. Parameters of the Porous Structure of the Studied o-polymers o-polymer a Specific surface Pore volume Average pore area (m 2 /g) (cm 3 /g) diameter (Å) 2,7-NAF.DM DVB ,7-NAF.DM ST ,7-NAF.DM MMA a o-polymers obtained using 2 ml of toluene and 2 ml of decan-1-ol (pore-forming diluents) are non-porous.

5 Study of Porous Structures and Thermal Properties of Luminescent Polymeric 535 Microspheres Derivatives of Naphthalene-2,7-Diol monomers are difunctional, in which one unit has an aromatic (ST) group and the other has an aliphatic (MMA) character. The chemical structure of these monomers plays a pivotal role in the formation of the porous structure of the microspheres synthesized. The second factor affecting the formation of porous structure is the use of the pore-forming diluents. The co-polymers obtained using a small amount of pore-forming diluents (2,7-NAF.DM DVB2, 2,7-NAF.DM ST2 and 2,7- NAF.DM MMA2) are non-porous. The DS curves for all co-polymers are presented in Figure 3. There are two endothermic effects: the first is associated with the loss of adsorbed water and the second appearing in the range is due to degradation of co-polymers. Figure 4 presents the photos of the polymer samples, which were not exposed to UV light [Figure 4(a)] and exposed to UV light [Figure 4(b)]. These photos clearly show the 0 exo 2,7-NAF.DM-ST2 2,7-NAF.DM-MMA2 2,7-NAF.DM-DVB2 DS (mw/mg) Temperature ( ) Figure 3. DS curves. (a) (b) Figure 4. Photo of the 2,7-NAF.DM DVB2 microspheres. (a) Not exposed to UV light; (b) exposed to UV light.

6 536 B. Podkościelna;cielna and M. Lutomski/Adsorption Science & Technology Vol. 33 No (a) (b) 0.0 Height 1.0 µm 0.0 Height 1.0 µm (c) 0.0 Height 1.0 µm Figure 5. ontact-mode AFM images: (a) 2,7-NAF.DM DVB8; (b) 2,7-NAF.DM ST8; (c) 2,7-NAF.DM MMA8. photoluminescence of the 2,7-NAF.DM DVB2 co-polymers. The photoluminescence property of 2,7-NAF.DM is undoubtedly related to the suitable arrangement of substituents (2,7) in the naphthalene ring. After excitation by UV radiation, the polymer emits yellowish green radiation. The excitation maximum is located at 500 nm, which corresponds to the range of wavelength for the green ( nm) and yellow lights ( nm). The surface texture of the synthesized porous co-polymers is shown in Figure 5 (magnification 135,000 ). The contact-mode AFM images present a fragment of the surface of the studied co-polymers. These photos show that the obtained co-polymers have a differently developed structure. The atomic force microscopy allows to determine the surface roughness. Root mean square of the obtained co-polymers is approximately nm for 2,7-NAF.DM DVB8; nm for 2,7-NAF.DM ST8 and 5.28 nm for 2,7-NAF.DM MMA8. This observation confirms the data obtained by the nitrogen adsorption desorption method. Microspheres with large porosity have a higher roughness. 4. NLUSIN In this study, we synthesized new cross-linked polymers with luminescent properties. The polymeric microspheres were obtained by the emulsion suspension polymerization of aromatic tetrafunctional monomer (2,7-NAF.DM) with commercially available monomers (ST, DVB and MMA). The co-polymer obtained using DVB was found to have the most developed porous

7 Study of Porous Structures and Thermal Properties of Luminescent Polymeric 537 Microspheres Derivatives of Naphthalene-2,7-Diol structure. The chemical structure of the monomers and pore-forming diluents are crucial for the formation of the porous structure of the obtained microspheres. Microspheres with luminescent properties may have potential applications for use in polymer optical fibre technology. AKNWLEDGEMENTS The research leading to these results has received funding from the People Programme (Marie urie Actions) of the European Union s Seventh Framework Programme FP7/ under REA grant agreement no. PIRSES-GA REFERENES orrea, D.S., Medeiros, E.S., liveira, J.E., Paterno, L.G. and Mattoso, L.H.. (2014) J. Nanosci. Nanotechnol. 14, Huang, J., Huang, Y.T., Wu, T.J., Huang, Y., Zhang, P.J. and Guo,.L. (2014) Acta Phys. Sin. 63, Li, X., Tian, J., Lv, Z., Guo, W. and Wang, M. (2014) Mater. Sci. Eng., B. 183, 29. Pei, L.Y. and Lee, Y.H. (2014) Sens. Actuators, B. 191, 719. Podkościelna, B. (2014) J. Therm. Anal. alorim. 116, 785. Podkościelna, B. and Gawdzik, B. (2010) Appl. Surf. Sci. 256, Podkościelna, B., Gawdzik, B. and Bartnicki, A. (2006) J. Polym. Sci., Part A: Polym. hem. 44, Ruan, Y.L., Boyd, K., Ji, H., Francois, A., Ebendorff-Heidepriem, H., Munch, J. and Monro, T.M. (2014) pt. Express. 22,