Abstract
In this article, synthesis and characterization of new crosslinked hybrid microspheres are presented. This material was obtained by the emulsion-suspension polymerization of 1,4-divinylbenzene and trimethoxyvinylsilane. In the case of introducing biologically active functional groups on the surface of microspheres, methacrylamide as a third monomer was added. Chemical structures of all new materials were confirmed by the spectroscopic method (attenuated total reflectance-Fourier transform infrared spectroscopy) and elemental analysis. The influence of comonomer structure on the physico-chemical properties of the hybrid microspheres was investigated. Porous structures in a dry state were studied by means of nitrogen adsorption–desorption measurements. Thermal stabilities and degradation behavior of the obtained copolymers were characterized by the use of thermogravimetric/derivative thermo-gravimetric analysis.
Introduction
Amides are widely found in biologically active compounds. In nature, this group can be found in proteins, peptides and many amide-bond containing biomolecules. Peptides and proteins belong to the group of polyamides, and in their case, the term peptide bond instead of amide bond is used. Amides are of great interest due to their high stability, polarity and their conformational diversity (Pattabiraman and Bode, 2011). Various amide-derived synthetic compounds with a broad range of applications in biotechnology, agriculture, medicine and chemical industry, e.g. as pharmaceutically active compounds or synthetic polymers (polyamides) are also known (Pitzer and Steiner, 2016).
Liu et al. (2012) reported that polyacrylamide was investigated as an ultralow fouling surface coating to highly protein adsorption, cell adhesion and bacterial attachment. Polyacrylamide was grafted on gold surfaces via surface-initiated atom transfer radical polymerization.
Mohanraj et al. (2014) presented an anticancer activity of starch/poly[N-(2-hydroxypropyl)methacrylamide] copolymers.
Zhang et al. (2016) reported N-(2-hydroxypropyl)methacrylamide copolymer – drug conjugates for combination chemotherapy of acute myeloid leukemia.
In this paper, synthesis, structure and properties of hybrid microspheres with amide functionalization, based on methacrylamide (MA), 1,4-divinylbenzene (DVB) and trimethoxyvinylsilane (TMVS), are presented (Figure 1). The microspheres were prepared by suspension–emulsion polymerization in the presence of pore-forming diluents. Different amounts of MA were applied. In order to confirm the chemical structure of new materials, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and elemental analysis were used. The influence of the comonomer on physico-chemical properties of the obtained copolymers was investigated. Porous structures of the obtained microspheres in dry state were studied by means of nitrogen adsorption–desorption measurements (Podkościelna and Sobiesiak, 2016).
Chemical structures of monomers used for the synthesis of microspheres.
Experimental
Chemicals
DVB, MA, TMVS, bis(2-ethylhexyl)sulfosuccinate sodium salt (DAC, BP), toluene and decan-1-ol were from Sigma-Aldrich (Buchs, Switzerland). α,α′-Azoiso-bis-butyronitrile was obtained from Merck (Darmstadt, Germany). Acetone, tetrahydrofuran, chloroform, acetonitrile, methanol and toluene were from POCh, Gliwice (Poland).
Copolymerization
Redistilled water (75 mL) and bis(2-ethylhexyl)sulfosuccinate sodium salt (0.75 g) were stirred for 1 h at 80℃ in a three-necked flask fitted with a stirrer, a water condenser and a thermometer (Gawdzik et al., 2006; Podkościelna and Lutomski, 2015). Then, the solution containing DVB, MA and TMVS (1 % wt.) of α,α′-azoiso-bis-butyronitrile (initiator) in a mixture of pore-forming diluents (toluene and decan-1-ol) was added while stirring to the aqueous medium. Copolymerization was performed at 80℃ for 18 h. The obtained copolymers were filtered off washed with distilled hot water, dried and extracted in a Soxhlet apparatus (with acetone). The chemical structures of monomers used for the synthesis of microspheres are shown in Figure 1.
Characterization
ATR spectra were obtained using a Bruker FTIR spectrophotometer TENSOR 27. The spectra were acquired in the spectral region 400–4000 cm−1. The resolution was set as 4 cm−1, and the number of scans per spectrum was 32.
Elementary analysis of the copolymers was carried out using the Perkin–Elmer CHN 2400 apparatus.
Porous structure of copolymers was investigated by nitrogen adsorption at 77 K using the adsorption analyzer ASAP 2405 (Micrometrics Inc., USA). Before measurements, the copolymers were outgassed at 150℃ for 2 h. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, assuming that the area of a single-nitrogen molecule in the adsorbed state is 16.2 Å2.
Thermogravimetric/derivative thermo-gravimetric (TG/DTG) analysis was performed on an STA 449 Jupiter F1, Netzsch (Selb, Germany) under the following operational conditions: heating rate, 10℃ min−1; dynamic atmosphere of helium (50 ml min−1) in the temperature range of 25°–600℃. The sample mass was approximately 3 mg; sensor thermocouple type, S TG-DSC; and as a reference, an empty alumina crucible was used.
Scanning electron microscopic (SEM) photos of microspheres were obtained by means of an optical microscope Morphologie G3 Malvern (UK).
Results and discussion
The molar ratio of the monomers used in copolymerization.
DVB: 1,4-divinylbenzene; MA: methacrylamide; TMVS: trimethoxyvinylsilane.
ATR-FTIR spectra.
tretching vibrations
ATR-FTIR spectra.
DVB-MA (1:2)
In the spectra of the studied microspheres, C–H stretching vibrations of the aromatic ring backbone methyl group are observed at 2924 cm−1. Moreover, stretching vibrations of the –NH group are visible at about 3358 cm−1. The amide (primary, Io) band is observed at 1662 cm−1 for the C=O stretching vibration and the amide (secondary, II°) band occurs at 1598 cm−1 for the N–H bending vibration. The C–N group gives a shape signal at 1207 cm−1 for DVB-MA. The =CH bending vibration is observed at 897 cm−1 and that of the –CH bending at 1449 cm−1. The vibrations of the C–H aromatic group are visible at about 708 and 793 cm−1.
DVB-MA-TMVS (1:1/3:2/3)
In the spectra of the studied microspheres, C–H stretching vibrations of the aromatic ring backbone methyl group at 2924 cm−1 can be observed. In the spectra, stretching vibrations of the –NH group are visible at about 3397 cm−1. The amide (I°) band is observed at 1672 cm−1 for the C=O stretching vibration and the amide (II°) band occurs at 1597 cm−1 for the N–H bending vibration. At 1118 cm−1, a strong vibration from the C–O group is found. The =CH bending vibration occurs at 900 cm−1 and that of –CH at 1448 cm−1. The signals from the C–H aromatic group are present at 709 and 792 cm−1.
DVB-TMVS (1:1)
In the spectra, C–H stretching vibrations of the aromatic ring backbone methyl group are observed at 2924 cm−1. The aromatic skeletal absorption is visible at about 1600 cm−1 for DVB-TMVS. At 1101 cm−1, a strong vibration from the C–O group is found. The =CH bending vibration occurs at 899 cm−1 and that of –CH at 1447 cm−1. The strong C–H deformations in the aromatic unit are clearly visible at 709 and 793 cm−1, respectively.
Elemental analysis of copolymers obtained with methacrylamide.
Parameters of the porous structure of the studied copolymers.
Specific surface areas are of high values for polymeric materials being in the range 400–ca. 511 m2/g. The total pore volume ranges from 0.83 to 1.02 cm3/g. All the materials are mesoporous and the content of micropores is about 2%. Analyzing the data of SBET presented in Table 4, one can see that the addition of TMVS has a positive effect on porosity development of the obtained microspheres. The highest values (511 m2/g) are obtained for the DVB-TMVS derivative. With the increase of MA amount in the copolymer, a slight decrease of SBET is observed.
Figure 3 shows the pore size distribution curves. Two maxima in the ranges of 35–38 Å and 200–550 Å can be observed for the most probable pore diameter. Only in the case of DVB-MA, the maximum is about 150 Å. The addition of TMVS to the structure of microspheres causes that the maxima of the peaks shift toward higher pore diameter values. This is due to the spatial structure of TMVS.
Pore size distribution curves.
Figure 4 presents the SEM photos of DVB-TMVS and DVB-MA-TMVS (1:2/3:1/3). These photos show well-spherical shapes of the obtained microspheres. Their size in the range of 10–25 µm for DVB-TMVS and 15–30 µm for the DVB-MA-TMVS (1:2/3:1/3) is received. The DVB-TMVS copolymer (obtained without the addition of MA) has smaller size of particles by about 30% in comparison with DVB-MA-TMVS copolymer.
SEM photos of DVB-TMVS (a) and DVB-MA-TMVS (1:2/3:1/3) (b) microsphere.
The obtained hybrid microspheres were also tested for their tendency to swell in common solvents such as acetone, tetrahydrofuran, chloroform, acetonitrile, methanol, toluene and distilled water. The values of swellability coefficient (B) (Gawdzik et al., 2006; Podkościelna and Lutomski, 2015) for all microspheres vary from 0% to 33%. The highest value was obtained for the sample DVB-MA-TMVS (1:2/3:1/3), in tetrahydrofuran as a solvent. The lowest value was found for DVB-TMVS (obtained without the addition of MA), in which the values of the swellability coefficient was 0%. None of the tested samples were swollen in distilled water.
The DSC curves for three, more representative copolymers are presented in Figure 5. There are two endothermic effects: the first is probably associated with the process of partial destruction of amide derivatives, and the second appearing in the range 400℃–480℃ (with the maximum about 440℃) is due to thermal degradation of copolymers. For the DVB-MA sample obtained with the largest content of MA, this endothermic effect is clearly visible in the temperature range 260℃–330℃. In turn, in the DVB-TMVS sample prepared without the addition of amide the effect does not occur.
DSC curves.
This phenomenon can be confirmed by looking at TG curves. Figure 6 presents the mass loss of the tested samples during the temperature increase. It can be observed that the DVB-MA sample starts to degrade at 260℃, whereas two other samples start to degrade at 350℃. Based on the obtained data, we can conclude that the DVB-MA sample has the lowest thermal resistance. Large contribution of MA to the structure of the obtained hybrid materials results in faster partial decomposition of the samples. In turn, in the materials obtained with larger contribution of TMVS, destruction of samples is more homogenous and proceeds at a temperature of maximal decomposition. Based on the DTG analysis, it was found that the degradation maximum of all tested samples appeared at the same temperature about 455℃. The residual mass at 600℃ after the degradation process for the DVB-MA sample is about 10%, for the DVB-MA-TMVS is 25% (and is the highest) and for the DVB-TMVS one is 20%.
TG (a) and DTG (b) curves.
Conclusions
Based on MA, DVB and TMVS hybrid microspheres are obtained. Chemical structure of the obtained microspheres was confirmed by ATR-FTIR and elemental analysis. The copolymer DVB-TMVS (obtained without the addition of MA) is characterized by the most developed porous structures SBET = 511 m2/g. Two maxima in the ranges of 35–38 Å and 200–550 Å can be observed for the most probable pore diameter for the MA-based microspheres. The values of swellability coefficient for all tested microspheres vary from 0% to 33%. Based on the TG/DTG/DSC studies, it was concluded that the sample DVB-MA-TMVS has the highest thermal resistance. With the addition of MA to the structure of hybrid microspheres, their decomposition is more heterogeneous and proceeds in two stages (285℃ and 455℃).
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n° PIRSES-GA-2013-612484.