Structure, Thermal, and Morphological Properties ... - ACS Publications

Article pubs.acs.org/IECR

Structure, Thermal, and Morphological Properties of Novel Macroporous Amino-Functionalized Glycidyl Methacrylate Based Copolymers Ivan S. Stefanović,† Bojana M. Ekmešcǐ ć,† Danijela D. Maksin,‡ Aleksandra B. Nastasović,† Zoran P. Miladinović,§ Zorica M. Vuković,† Darko M. Micić,§ and Marija V. Pergal*,† †

Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia Institute of Nuclear Science “Vinča”, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia § Institute of General and Physical Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ‡

S Supporting Information *

ABSTRACT: Novel macroporous functionalized copolymers with different cross-linker concentrations and porosity parameters were synthesized by reaction of the pendant epoxy groups of poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) (poly(GMA-co-EGDMA)) with hexamethylene diamine, 1,3-bis(3-aminopropyl)tetramethyldisiloxane, and α,ω-diaminopropyl poly(dimethylsiloxane). The copolymers were prepared in forms of spherical beads and characterized by Fourier transform infrared (FTIR), 13C and 29Si solid-state NMR, mercury porosimetry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Copolymers prepared with the higher cross-linker concentrations have better thermal stability, higher glass transition temperatures, higher specific surface areas, and smaller pore diameters that correspond to half of the pore volumes. Our results show that functionalization significantly changed porosity parameters, mechanism of thermal degradation, and increased thermal stability in comparison with the initial copolymers. These macroporous copolymers could potentially have many applications, i.e. for sorption of heavy and precious metals or as material for gas chromatography columns.

1. INTRODUCTION

properties. They found that the type of amines significantly affects porosity parameters of functionalized copolymers. Furthermore, it has been shown that the type of amine and porosity strongly influence the heavy metal sorption by crosslinked macroporous amino-functionalized GMA copolymers. Functionalization of GMA copolymers with amines yields good sorbents with high capacity, fast kinetics, and good selectivity for the heavy metal ions as well as good chemical stability.2 In order to design copolymers with desired porosity parameters and sorption performances, as well as selectivity toward individual metal ions, research on synthesis and characterization of new amino-functionalized GMA-based copolymers continues. Some properties of GMA-based copolymers, such as thermal stability and poor physical performance, need to be improved. In recently published work,16 incorporation of various organosilyl groups with different steric hindrance into GMA copolymer structures has been utilized to modify some copolymer properties such as thermal stability, weather resistance, and mechanical and surface properties. Furthermore, Grama et al.17 reported that coating of poly-GMA and poly(2,3-dihydroxypropyl methacrylate) by silanization with tetraethoxysilane and (3-aminopropyl)triethoxysilane improved biocompatibility in relation to starting polymers. In addition, siloxane grafting on the polymer chain was shown to add some very useful properties to

Cross-linked macroporous copolymers of glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate (EGDMA; poly(GMA-co-EGDMA)) have been widely used as sorbents for chromatography,1 adsorbents for the removal of heavy and precious metals, 2 support for enzymes, 3 and even as biomaterials,4 due to their unique chemical and physical properties. These copolymers can be synthesized in the shape of resin beads by suspension copolymerization in the presence of low-molecular weight inert components. During the polymerization process, cross-linking and phase separation occur, leading to the development of a porous structure. This porous structure of the copolymers is created by varying the type and concentration of inert components, amount of cross-linking agent, temperature, and stirring speed during synthesis.5,6 After the polymerization, the inert component can be easily extracted from the resulting polymer beads. Over past years, GMA-based copolymers have attracted a lot of attention since the presence of epoxy groups allows a number of chemical modifications on the initial polymer to suit a variety of applications. Numerous studies on preparation of GMA-based copolymers and their modifications have been reported.7−12 Švec et al.13 and Kalal et al.14 reported preparation of poly(GMA-coEGDMA) modified with different amines such as ammonia, primary and secondary amines, diamines, and hydroxyalkylamines. Malović et al.15 prepared poly(GMA-co-EGDMA) functionalized with ethylene diamine, diethylene triamine, or triethylene tetramine and investigated their metal sorption © XXXX American Chemical Society

Received: April 5, 2015 Revised: June 13, 2015 Accepted: June 22, 2015

A

DOI: 10.1021/acs.iecr.5b01285 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research the copolymer matrix, such as high flexibility, low surface energy, hydrophobicity, and excellent thermal, oxidative, and hydrolytic stability.18,19 Other important effects of siloxane incorporation into the various networks include surface modification, biocompatibility, and increase in the oxygen permeability.18 Owing to our interest in developing new materials with improved properties and extending the range of their applications, the purpose of this study was to synthesize two series of novel cross-linked macroporous functionalized copolymers by reaction of the pendant epoxy groups of poly(GMA-co-EGDMA) with hexamethylene diamine (HD), 1,3-bis(3-aminopropyl)tetramethyl disiloxane (TMDS), and α,ω-diaminopropyl poly(dimethylsiloxane) (PDMS). To our knowledge, this category of functionalized copolymers has not been reported in the literature. For comparison purposes, initial copolymers were also synthesized and characterized. We believe that the modification of poly(GMA-co-EGDMA) especially with siloxane diamines (TMDS and PDMS) would improve thermal stability and modified surface properties of the functionalized copolymers. We can also specify that the idea for this manuscript was also to continue research related to the amino functionalization of macroporous copolymer poly(GMA-co-EGDMA) with ethylene diamine, diethylene triamine, and triethylene tetramine.15 Introduction of the HD enables the examination of influence of the longer alkyl groups (in relation to the ethylene diamine) on the porosity and other properties of these copolymers. Moreover, previous research2,20 has shown that poly(GMA-co-EGDMA) copolymers functionalized with aforementioned amines are efficient sorbents for removal of toxic pollutants like precious and heavy metals as well as radionuclides. Therefore, the main goal of this study was to obtain functionalized copolymers with enhanced thermal properties and retained macroporosity. The effects of type of functionalized amines and GMA concentration on the structure and properties of functionalized copolymers were investigated by Fourier transform infrared (FTIR), solid-state 13C and 29Si NMR spectroscopy, mercury porosimetry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM).

tion. Reaction mixtures consisted of the monomer phase suspended in the aqueous phase. The monomer phase (75.0 g) contained a monomer mixture (19.5 g of GMA and 13.0 g of EGDMA for sample GMA 60 and 26.0 g of GMA and 6.5 g of EGDMA for sample GMA 80), AIBN as an initiator (0.3 g), and 42.5 g of inert component (34.0 g of cyclohexanol and 8.5 g of tetradecanol). The aqueous phase contained 225.0 g of deionized water and 2.25 g of PVP. Copolymerization was carried out in a nitrogen atmosphere, at 70 °C for 2 h and at 80 °C for 6 h with a stirring rate of 300 rpm. After completion of the reaction, copolymer particles were washed with water and ethanol, kept in ethanol for 12 h, and dried in a vacuum oven at 40 °C. Copolymer particles were purified by Soxhlet extraction with ethanol. The resulting cross-linked beads were sieved, and a fraction of beads with an average particle diameter range of 150− 300 μm was used for functionalization with different amines. The initial copolymers were marked as GMA 60 and GMA 80 (the numbers 60 and 80 indicate mass % of GMA in copolymers). 2.3. Functionalization of poly(GMA-co-EGDMA) with Different Amines. Functionalized macroporous copolymers were synthesized by the reaction of pendant epoxy groups of poly(GMA-co-EGDMA) with HD, TMDS, and PDMS. Functionalization of the initial copolymers with different amines was carried out in a mixture of DMAc/toluene (1:3 v/v) at 80 °C for 10 h under an argon atmosphere, in a four-necked roundbottom flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and an argon inlet. Compositions of the reaction mixtures for preparation of functionalized poly(GMA-co-EGDMA) copolymers are presented in Table S1 (Supporting Information). The functionalized copolymers were filtered, washed with ethanol, and dried in a vacuum oven for 24 h at 40 °C. The functionalized samples were labeled as GMA 60 HD, GMA 60 TMDS, and GMA 60 PDMS and GMA 80 HD, GMA 80 TMDS, and GMA 80 PDMS, where HD, TMDS, and PDMS designate additional functionalization with corresponding amines. The yields of the functionalized copolymers were from 21.2 to 82.0% for the samples in the first series and from 17.3 to 79.5% for the samples in the second series (Table S1 in Supporting Information). 2.4. Characterization. Fourier transform infrared (FTIR) spectra were recorded using attenuated total reflection (ATR) mode on a Nicolet 6700 FTIR spectrometer (Madison, USA). The scanning range was from 500 to 4000 cm−1 at a resolution of 2 cm−1. Thirty-two scans were collected for each sample. Solid-state magic angle spinning (MAS) NMR spectra were obtained using Bruker MSL 400 NMR spectrometer, Tecmag console upgraded (Texas, USA), operating at a 400.13 MHz for 1 H, 100.63 MHz for 13C, and 79.49 MHz for 29Si nuclei. Copolymer samples were packed in a 7 mm zirconia rotor with Kel-F end-caps and spun at 4 kHz in a Bruker HP WB 73A DB MAS probe. 13C MAS NMR spectra were acquired using a 10 μs (90°) pulse with 1H decoupling in the inverse-gated mode accompany with recycle delay of 30 s to fulfill optimum quantitative condition. Chemical shifts δ(13C) were externally referenced to the glycine carbonyl signal (176.4 ppm). 29Si MAS NMR spectra were collected using a pulse width of 4 μs and recycle delay of 120 s, to allow complete spin relaxation, yielding quantitative peak intensities. Approximately, 1000 scans were acquired in each experiment. Chemical shifts δ(29Si) were externally referenced to 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) used as a standard. Deconvolution analysis of all obtained NMR spectra was performed using DMFit application.21

2. EXPERIMENTAL SECTION 2.1. Materials. Glycidyl methacrylate (GMA), purity 97%, and the cross-linker ethylene glycol dimethacrylate (EGDMA), purity ≥98%, were obtained from Merck and Fluka, respectively, and used as received. 1,3-Bis(3-aminopropyl)tetramethyl disiloxane (TMDS), purity 97%, and α,ω-diaminopropyl poly(dimethylsiloxane) (PDMS) (purity >99%, Mn = 1000 g/mol; polymerization degree of PDMS block is 11.2) were purchased from ABCR and dried over molecular sieves (0.4 nm) before use. Hexamethylene diamine (HD), purity >99%, was supplied by Across Organics. 2,2′-Azobis(isobutyronitrile) (AIBN), purity >98%, was obtained from Sigma-Aldrich and was purified by recrystallization in methanol before use. The stabilizer poly(Nvinylpyrrolidone) (PVP; purity 99%, Kollidone 90, Mw = 360 000 g/mol, BASF), cyclohexanol, purity 98%, and N,N-dimethylacetamide (DMAc), purity 99%, were supplied by Sigma-Aldrich. Tetradecanol, purity ≥98%, was purchased from Merck. Toluene (purity 99.3%) and ethanol (purity 99.8%), were supplied by Zorka Pharma. 2.2. Preparation of poly(GMA-co-EGDMA) Copolymers. Two initial macroporous poly(GMA-co-EGDMA) samples, with different concentrations of GMA (60 and 80 mass %), were prepared by a radical suspension copolymerizaB


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Figure 1. FTIR spectra of poly(GMA-co-EGDMA) and functionalized copolymers.

EGDMA) copolymers with 60 and 80 mass % of GMA were confirmed by FTIR and solid-state NMR spectroscopy (Figures 1 and 2 and S1 in the Supporting Information). In the FTIR spectra of the initial copolymer, the characteristic apsorption bands are 2990 and 2950 cm−1 (νC−H), 1730 cm−1 (νCO), 1390 and 1450 cm−1 (δC‑Hasym and δC‑Hsym), and 1150 cm−1 (νC−O−C). Also, the epoxy peaks found in the spectrum of the initial copolymers are at 850 and 910 cm−1 (epoxy ring vibrations) and 1255 cm−1 (δC−H epoxy). 13 C MAS NMR spectra of homopolymers, poly(glycidyl methacrylate) (poly-GMA), poly(ethylene glycol dimethacrylate) (poly-EGDMA), and initial copolymers poly(GMA-coEGDMA) with 60 and 80 mass % of GMA are presented in Figures 2a,b,c and S1 (Supporting Information), respectively. The assignments of the NMR resonance signals have been made by comparison with spectra of analogous chemical groups taken from the previously reported investigation.22−24 The 13C MAS NMR spectrum of homopolymer poly-GMA, presented in Figure 2a, shows resonance peaks at 16 ppm for 3 (CH3−), 55 ppm for 1 (H2CCCH3COCH2), 45 ppm for 2 (H2CCCH3COCH2), 67 ppm for 5 (−O−CH2−), and 177 ppm for 4 (−O(CO)C−).22 The resonance peaks of 6 at 49 ppm and 7 at 44 ppm25 were assignable to CH and CH2 of the epoxy ring. The 13C NMR spectrum of the solid poly-EGDMA shown in Figure 2b has almost the same chemical shifts for the carbonyl and backbone methyl and tertiary carbon resonance as the corresponding signals in the spectrum of poly-GMA. The resonance peak at 62.5 ppm belongs to 5 and 6 (O−CH2−CH2− O) carbons. Resonances identified in Figure 2b around 100−130 ppm are due to double bonds of unreacted EGDMA, and peaks around 160−180 ppm are attributable to different CO groups that can be found in EGDMA. This implies the existence of unreacted pendant methacrylate groups indicating incomplete cross-linking. Overlapping between signals of spinning side bands (SSB) of carbonyl groups (4, 7) and resonances of unreacted vinyl groups (8, 9), present in this spectrum, could be significantly reduced by changing a sample’s rotation speed. In the poly-GMA sample, unreacted vinyl groups are almost totally absent, due to possible longer linear polymer chains affecting the ratio of terminal and reacted vinyl groups inside the chain that is comparable smaller than for the poly-EGDMA. Analysis of the poly-EGDMA homopolymer spectrum shows that upon polymerization, two signals of the olefinic carbons at 125 and 136 ppm in Figure 2b were replaced by signals at 55 and 45 ppm

The porosity parameters were determined by mercury porosimetry (Carlo Erba Porosimeter 2000, Washington, USA, software Milestone 200). Differential scanning calorimetry (DSC) measurements were carried out on a DSC Q1000 V9.0 Build 275 thermal analyzer (New Castle, USA). The DSC scans were recorded under a nitrogen atmosphere, in the temperature range from −90 to 200 °C, at heating and cooling rates of 10 and 5 °C/min, respectively. The thermal stabilities were determined using a TGA Q500 V6.3 Build 189 thermal analyzer (New Castle, USA) under a nitrogen atmosphere, in the temperature range from 25 to 700 °C and at a heating rate of 10 °C/min. The scanning electron microscopy (SEM) micrographs were obtained on JEOL JSM-6460LV instrument (Tokyo, Japan), at a working distance of ca. 14 mm and an accelerating voltage of 20 kV. Functionalized samples were coated with gold in a highvacuum evaporator.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Functionalized Copolymers. Novel macroporous functionalized copolymers with different concentrations of a cross-linking agent and different porosity parameters were synthesized by a ring opening reaction of the pendant epoxy group of poly(GMA-co-EGDMA) with hexamethylene diamine, 1,3-bis(3-aminopropyl) tetramethyldisiloxane, and α,ω-diaminopropyl poly(dimethylsiloxane). EGDMA is a difunctional methacrylic monomer used in this study as a cross-linking agent for synthesis of poly(GMA-co-EGDMA) copolymers. HD, TMDS, and PDMS were used for additional functionalization, i.e., introduction of amines of different chemical structures. Initial copolymers without amines were also prepared for comparison with the synthesized functionalized copolymers. Furthermore, initial copolymers reacted with at least 10 times larger amount of amines to obtain amino-functionalized copolymers. The critical factor in preparation of functionalized copolymers is the reaction medium type. Some authors reported use of toluene as a reaction medium for synthesis of poly(GMA-coEGDMA) copolymers functionalized with ethylene diamine, diethylene diamine, and triethylene tetramine.15 However, we found that a mixture of DMAc and toluene in a 1:3 v/v ratio was a good choice for the functionalized copolymers prepared in this study, and optimum conditions for their synthesis were found to be a reaction temperature of 80 °C anda duration time of 10 h. 3.2. Characterization of Initial poly(GMA-co-EGDMA) Copolymers. The chemical structures of initial poly(GMA-coC


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Figure 3. 13C and 29Si MAS NMR spectra of poly(GMA-co-EGDMA) copolymers functionalized by (a) HD (GMA 60 HD), (b) TMDS (GMA 60 TMDS), and (c) PDMS (GMA 60 PDMS). Epoxy CH peak at 49 ppm is distinguished from other deconvoluted peaks and shown in all NMR spectra. Asterisks mark the position of spinning side bands (SSB). Figure 2. 13C MAS NMR spectra of (a) poly-GMA, (b) poly-EGDMA, and (c) poly(GMA-co-EGDMA) with 60 mass % of GMA. Asterisks mark the position of spinning side bands (SSB).

Figure 4). The values of specific surface area, SHg, were calculated on the basis of the cylindrical pore model as described in the literature26 and presented in Table 1. The increase of cross-linking agent concentration in the monomers mixture from 20 to 40 mass % caused a slight increase

in the copolymer in Figure 2c, which were found to be signals of methylene and tertiary carbon atoms.24 The percentage of unreacted double bonds could be calculated using the ratio of the integrated intensity of the carbonyl resonance assigned to the unsaturated carbonyl, relative to the total integrated intensity of the saturated carbonyl: e.g., 167 versus 177 ppm (Table S2 in the Supporting Information). The measurement of the relative intensities of the carbonyl or olefinic signals for the pendant methacrylate groups can be used to monitor the variation in the density of cross-linking. For the poly-EGDMA sample presented in Figure 2b, this ratio of unreacted olefinic carbons is estimated to be 0.17. The porosity parameters such as specific pore volume, VS, and dV/2, i.e., the values of pore diameter that correspond to half of the pore volume, of the initial poly(GMA-co-EGDMA) samples were read from cumulative pore distribution curves (Table 1 and

Table 1. Porosity Parameters of the Initial and AminoFunctionalized Copolymers

D

copolymer

VS(cm3/g)

SHg (m2/g)

dV/2 (nm)

P (%)

GMA 60 GMA 60 HD GMA 60 TMDS GMA 60 PDMS GMA 80 GMA 80 HD GMA 80 TMDS GMA 80 PDMS

1.16 1.22 1.17 1.07 1.10 1.04 1.04 0.94

85 83 78 74 39 38 38 95

84 92 90 85 203 208 201 65

64 63 63 61 62 61 62 57


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Figure 4. Cumulative pore size distribution curves for the initial poly(GMA-co-EGDMA) and functionalized copolymers.

of VS value of the synthesized poly(GMA-co-EGDMA) samples. On the other hand, a more expressed increase of SHg (more than double increase) and the decrease of dV/2 (2.4 times decrease) was observed, as expected based on previous investigations.5,27 Higher values of specific surface areas and specific pore volumes were observed for the copolymer GMA 60, while the other copolymers and GMA 80 have similar smaller values. In addition, the difference in the GMA content in copolymers GMA 60 and GMA 80 has caused great discrepancy in pore diameter that corresponds to half of the pore volume. Thermal properties of initial poly(GMA-co-EGDMA) copolymers were examined by DSC and TGA analyses. DSC results showed that initial copolymers have one glass transition temperature, Tg (Table 2 and Figure 5). Thermal degradation Table 2. Thermal Properties of the Initial and Functionalized Copolymers Determined by DSC and TGA Analyses copolymer GMA 60 GMA 60 HD GMA 60 TMDS GMA 60 PDMS GMA 80 GMA 80 HD GMA 80 TMDS GMA 80 PDMS

Tg (DSC) (°C)

T10% (°C)

T50% (°C)

Tmax (°C)

residual weight at 600 °C (%)

64.9 64.7

249 279

308 375

258/315/421 359/426

0.04 3.0

65.4

295

406

335/433

5.2

65.1

276

318

308/401

4.2

61.8 62.0

237 268

327 388

242/340/410 305/413

63.5

289

374

368/423

4.1

62.5

278

354

339/427

1.2

Figure 5. DSC thermograms recorded during the second heating run, of the initial poly(GMA-co-EGDMA) and functionalized copolymers.

The surface morphology of initial poly(GMA-co-EGDMA) copolymers was investigated by SEM. As can be seen from SEM micrographs presented in Figure 7, spherical beads of initial copolymers with porous structures are obtained. Figure 8 shows cross-sectional morphology of poly(GMA-co-EGDMA) copolymers. On these SEM micrographs, the appearance of agglomerates of globules divided by channels and pores, which represent the basic macroporous structure of poly(GMA-coEGDMA) copolymers, can be seen. 3.3. FTIR Results of Functionalized Copolymers. The chemical structures of the functionalized copolymers were determined by FTIR spectroscopy. The results of FTIR analysis of the functionalized copolymers are presented in Figure 1. The characteristic stretching frequencies of the functionalized copolymers appeared at 3060−3650 cm−1 (νN−H + νO−H), 2950 and 2945 cm−1 (νsym and νasym of C−H), 1733 cm−1 (νCO), 1535 and 1260 cm−1 (νC−N + δN−H, i.e., amide II and amide III bands), 1080 and 1160 cm−1 (νSi−O−Si and νC−O−C), and 800 cm−1 (ρC−H in SiCH3). The epoxy peaks in spectra of initial copolymers at 910 cm−1 have not completely disappeared from spectra of copolymers functionalized with PDMS (GMA 60 PDMS and GMA 80 PDMS samples), indicating incomplete conversion of the epoxy groups. Also, it is possible that some epoxy groups remain inside the copolymer, which makes them unavailable for the following reactions with the amines, especially with the high molecular

0.01 13.2

of initial GMA 60 and GMA 80 copolymers, determined by TGA, started at 249 and 237 °C, respectively (Figure 6, Table 2). The T10% (temperature at 10% weight loss) value is considered to represent the beginning of degradation of the copolymers. TGA data showed that the T10% increases with increasing concentration of the cross-linking agent. The obtained results are in agreement with the results obtained for other GMA-based copolymers presented in the literature.6 DTG curves show that initial copolymers display three-step degradation processes in nitrogen. E


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Figure 6. TGA and DTG curves of the initial poly(GMA-co-EGDMA) and functionalized copolymers, determined at a heating rate of 10 °C/min, in a nitrogen atmosphere.

Figure 7. SEM microphotographs of the surface of the initial poly(GMA-co-EGDMA) and functionalized copolymers with 60 and 80 mass % of GMA: (a) GMA 60, (b) GMA 60 HD, (c) GMA 60 TMDS, (d) GMA 60 PDMS, (e) GMA 80, (f) GMA 80 HD, (g) GMA 80 TMDS, (h) GMA 80 PDMS, at different magnifications (300×, top; 10 000×, bottom).

Information). In both series of samples, the lowest yields were obtained for copolymers functionalized with PDMS. These results are in agreement with the NMR results reported below. Yield of functionalized copolymers is calculated as the ratio of mass of functionalized copolymers and theoretical maximal mass.

weight PDMS. Similar results were observed for poly(GMA-coEGDMA) copolymers modified with ethylene diamine2 and triethylene tetramine.15 Also, FTIR results in this study are in agreement with the results obtained for the yields of functionalized copolymers presented in Table S1 (Supporting F


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Figure 8. SEM microphotographs of the cross-section of the initial poly(GMA-co-EGDMA) and functionalized copolymers with 60 and 80 mass % of GMA: (a) GMA 60, (b) GMA 60 HD, (c) GMA 60 TMDS, (d) GMA 60 PDMS, (e) GMA 80, (f) GMA 80 HD, (g) GMA 80 TMDS, (h) GMA 80 PDMS, at magnification 10 000×.

the degree of unreacted epoxy rings of GMA inside the copolymer structure could be estimated for all samples in this work, and results of this analysis are given in Table S2 in the Supporting Information. The lowest intensity of the epoxy CH peak at 49 ppm with a concomitant increase of intensity of peaks around 68 and 52 ppm was observed for the copolymer functionalized with HD, in comparison with the other two samples functionalized with TMDS and PDMS. This clearly indicates that the extent of epoxy ring opening reaction was most pronounced for the copolymer functionalized with HD. Obtained values of the intensity of peaks at 49 and 177 ppm, as well as their ratios (used here for comparison between different NMR spectra), are given in Table S2 (Supporting Information) for all presented samples in Figure 3. The data show that the intensity of the peak at 49 ppm in comparison with the intensity of the carbonyl peak gradually increases from GMA 60 HD to GMA 60 TMDS to GMA 60 PDMS. Similar trends are also observed for series of corresponding GMA 80 copolymers functionalized with the same amines. Such findings are in agreement with the results obtained for the yields of functionalized copolymers presented in Table S1 (Supporting Information). 29 Si MAS NMR spectra for siloxane modified copolymers GMA 60 TMDS, GMA 60 PDMS, GMA 80 TMDS, and GMA 80 PDMS are presented in Figures 3 and S2 (Supporting Information). Two peaks could be identified in the spectrum in Figures 3c and S2c, one due to the siloxane unit (−22.1 ppm) within the chain and one due to the siloxane units (7.5 ppm) at chain ends.29 The ratio of these two peak areas was estimated to be 1:2 for GMA 60 PDMS and 1:2.4 for GMA 80 PDMS, specifying appropriate siloxane chain length. For the copolymers functionalized with TMDS, only one peak could be observed at 7.5 ppm (Figures 3b and S2b). 3.5. Porosity of Functionalized Copolymers. To further investigate the effect of the amine type attached to poly(GMA-coEGDMA) on the porosity parameters, the initial copolymer samples GMA 60 and GMA 80 were modified with HD, TMDS, and PDMS. Porosity parameters of the functionalized copolymers were determined on the same manner as for the initial copolymers26 and are presented in Table 1 and Figure 4.

The resulting mass of functionalized copolymers and maximal theoretical mass are given in Table S1 (Supporting Information). The amount of the GMA unit which reacts with amine (having in mind that one amino group reacts with GMA) is first calculated. From this information, the mass of the GMA unit modified with amine can be obtained. Theoretical mass is obtained by the addition of the mass of the GMA unit modified with amine to the mass of the EGDMA unit. 3.4. 13C and 29Si MAS NMR Analysis of Functionalized Copolymers. 13C MAS NMR spectra of six functionalized copolymers of poly(GMA-co-EGDMA), synthesized by the reaction of the epoxy group with different diamines, are presented in Figure 3 (functionalized copolymers with 60 mass % of GMA) and Figure S2 in the Supporting Information (functionalized copolymers with 80 mass % of GMA). In Figure 3a, resonance peaks at 52 ppm for 1 (H2N−CH2−), 28 ppm for 2 (H2N−CH2CH2−), and 24 ppm for 3 (H2N−CH2CH2CH2−) were assigned to HD carbons. Peak assignation of copolymer samples functionalized with TMDS and PDMS has similar assignation with minor shifting of peaks to 45 ppm for 1 (H2N− CH2−), around 25 ppm for 2 (H2N−CH2CH2−), and 15 ppm for 3 (H2N−CH2CH2CH2−).28 A strong peak, characteristic for siloxanes, positioned at 0 ppm is assigned to CH3 carbons directly connected to Si atoms. The reaction of diamine with the epoxy group on the polyGMA part of the copolymer chain results in shifting of epoxy peaks from 49 ppm (CH) and 44 ppm (CH2) to 68 and 52 ppm, respectively. This is more obvious in the case of copolymer presented in Figure 3a, functionalized with hexamethylene diamine. The intensity of the epoxy CH peak is weaker than that of b and c from the same figure, indicating that the epoxy ring opening in reaction with diamines occurred at different extents, depending on diamines’ structure. Therefore, peaks of epoxy CH and carbonyl carbons of copolymers shown in Figures 3 and 2c were chosen as the most distinctive ones to reflect the extent of reaction of the epoxy group with diamines and the molar homopolymer ratio inside the copolymer structure. The ratio of areas of these peaks could be used to assess the efficiency of reaction of the epoxy group with diamines. Areas of carbonyl peaks comprise all of the corresponding SSB peak areas, too. On the basis of this finding, G


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Industrial & Engineering Chemistry Research The most prominent effect of functionalization on the porosity of synthesized GMA-based copolymers was observed for the GMA 80 PDMS sample: almost 3 times the increase of SHg and more than three 3 the decrease for dV/2. It seems that functionalization with PDMS promotes the formation of smaller mesopores and, consequently, the increase in specific surface area. No significant differences in porosity parameters of samples functionalized with HD and TMDS were observed. The SHg, Vs, and dV/2 values were similar, especially for the samples obtained by functionalization of the initial GMA 60. For example, the functionalization of GMA 60 with PDMS caused an SHg decrease of 13% (from 85 to 74 m2/g) and a VS decrease of approximately 8% (from 1.16 to 1.07 cm3/g), while the dV/2 increase was negligible (from 84 to 85 nm), compared with the initial sample GMA 60. Similar results were obtained for the other types of functionalized copolymers. Total porosity, P, of copolymers was very similar and varied from 61 to 64% excepted for GMA 80 PDMS (57%, Table 1). 3.6. Thermal Analysis of Functionalized Copolymers by DSC and TG. Thermal properties of the functionalized copolymers were also evaluated by DSC analysis. The DSC thermograms of these samples recorded during the second heating run are shown in Figure 5. In the tested temperature region, only the glass transition temperature change of functionalized copolymers was noticed. Values of Tg, listed in Table 2, are very similar and increase with increasing content of cross-linking agent. This can be attributed to the increase in cross-linking density that restricts the molecular motion of the polymer chains and leads to the increase in Tg.30 The glass transition temperatures of the initial copolymers are very similar to the glass transition temperatures of functionalized copolymers (Table 2). Furthermore, Tg values do not seem to depend on the type of amines attached to the poly(GMA-co-EGDMA). The thermal degradation analysis of the functionalized copolymers was also carried out using thermogravimetric analysis under a nitrogen atmosphere. The obtained TGA and thermogram derivative (DTG) curves for the copolymers are shown in Figure 6. Thermal degradation of the functionalized copolymers from the first series started between 276 and 295 °C, while the thermal degradation for the functionalized copolymers from the second series started from 268 to 289 °C (Table 2). Furthermore, our data clearly show that the T10% values of functionalized copolymers were approximately 30−45 °C higher than the T10% values of initial copolymers. These results showed that the thermal stability of the functionalized copolymers increased with the introduction of different types of amines, especially TMDS and PDMS amines. Furthermore, the temperatures of 50% weight loss, T50%, of the functionalized copolymers were also significantly increased compared to the ones of the initial samples. The TGA results indicate that the thermal stability of functionalized copolymers depends on the type of amines. Copolymers functionalized with TMDS showed better thermal stability in comparison with HD- and PDMS-based copolymers. Thermal stability of the synthesized copolymers functionalized with TMDS was shown to be similar to the thermal stability of poly(4-cyanophenyl acrylate-co-glycidyl methacrylate)31 and slightly higher than thermal stability of GMA-based copolymers modified with diethylene triamine or triethylene tetramine,32 as well as copolymers based on 2-hydroxyethyl methacrylate and EGDMA as a cross-linker.33

According to the DTG curves, functionalized copolymers display two-step degradation profiles in nitrogen. According to the literature, the thermal degradation of GMA-based copolymers was suggested to start with the cleavage of poly-GMA into fragments around 220 °C.17 Therefore, the first degradation step most likely represents poly-GMA breaking into smaller fragments. The second step might be associated with the ester bond breakdown in copolymers (around 330 °C), and the third step could be attributed to the total degradation of copolymers (around 420 °C).34 The first DTG peak for copolymers functionalized with TMDS and PDMS corresponds to the temperature at a maximum rate of weight loss during the first step of the thermal degradation, which most likely occurred due to the decomposition of the ester groups. During the second step of thermal degradation, the siloxane bonds were most likely decomposed. Our data show that the residual weights at 600 °C are higher for the functionalized samples than for the initial copolymers (Table 2). The residual weights of initial and all functionalized copolymers originated mainly from the “organic” fraction (GMA and EGDMA for initial copolymers and 2-hydroxypropyl methacrylate, EGDMA, and amines for functionalized copolymers). In addition, for copolymers functionalized with PDMS, the poly(dimethylsiloxane) chains under a nitrogen atmosphere degraded by depolymerization, giving cyclosiloxanes.35 3.7. SEM Analysis of Functionalized Copolymers. The surface and cross-section morphology of functionalized copolymers were also examined by SEM analysis (Figures 7 and 8). The images that are shown in Figure 7 illustrate the shape of individual beads and surface morphology of the functionalized copolymers. The shape of beads was shown to be generally spherical, with rough surfaces and porous structures. As Figure 7 shows, copolymers functionalized with TMDS (especially with PDMS) have more filled out and uniform surface, which might be caused by the ability of siloxanes to migrate to the copolymer surface, due to their low surface energy. The results indicated that the type of amine has an impact on the porous properties of the beads surface. However, the particle size did not change significantly after functionalization with amines in comparison with initial poly(GMA-co-EGDMA) particles. We have noted a broad particle size distribution for all prepared samples with high standard deviations. The SEM micrographs of the beads’ cross-sections depict the internal structure of the beads (Figure 8) at a magnification of 10 000×. These micrographs clearly demonstrate a threedimensional porous matrix of the beads, composed of a large number of globules and interconnected with channels and pores. The SEM-EDX (energy-dispersive X-ray spectroscopy) analysis, performed to identify the type of atoms present in the functionalized copolymers at a depth of 100−1000 nm from the surface, confirmed the presence of all expected elements (C, O, Si, and N). The obtained results are presented in Table S3 (Supporting Information). The silicon content of GMA 60 TMDS, GMA 60 PDMS, GMA 80 TMDS, and GMA 80 PDMS, determined from EDX data, was on the surface 6.3%, 5.9%, 7.4%, and 10.3% and in the cross-section, 4.4%, 7.4%, 5.4%, and 5.0%, respectively. It is worth noting that the N-percentage of the functionalized copolymers, determined by the EDX analysis, was slightly higher on the surface than in the cross-section (except for GMA 60 PDMS sample). The obtained results also indicate that the reaction of epoxy groups with amines occurs on the surface and in the interior of the beads. For copolymers functionalized with HD, TMDS, and PDMS, this reaction occurs slightly higher H


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(2) Nastasović, A.; Jovanović, S.; Đorđević, D.; Onjia, A.; Jakovljević, D.; Novaković, T. Metal Sorption on Macroporous Poly(GMA-coEGDMA) Modified With Ethylene Diamine. React. Funct. Polym. 2004, 58, 139. (3) Miletić, N.; Rohandi, R.; Vuković, Z.; Nastasović, A.; Loos, K. Surface Modification of Macroporous Poly(glycidyl methacrylate-coethylene glycol dimethacrylate) Resins for Improved Candida Antarctica Lipase B Immobilization. React. Funct. Polym. 2009, 69, 68. (4) Herault, D.; Saluzzo, C.; Duval, R.; Lemaire, M. Preparation of Monodisperse Enantiomerically Pure Glycidyl Methacrylate−Ethylene Glycol Dimethacrylate Copolymers in Dispersion Copolymerization: Functionalization. React. Funct. Polym. 2006, 66, 567. (5) Jovanović, S. M.; Nastasović, A.; Jovanović, N. N.; Jeremić, K.; Savić, Z. The Influence of Inert Component Composition on the Porous Structure of Glycidyl Methacrylate/Ethylene Glycol Dimetacrylate Copolymers. Angew. Makromol. Chem. 1994, 219, 161. (6) Rahman, A. U.; Iqbal, M.; Rahman, F. U.; Fu, D.; Yaseen, M.; Lv, Y.; Omer, M.; Garvver, M.; Yang, L.; Tan, T. Synthesis and Characterization of Reactive Macroporous Poly(glycidyl methacrylate-triallyl isocyanurate-ethylene glycol dimethacrylate) Microspheres by Suspension Polymerization: Effect of Synthesis Variables on Surface Area and Porosity. J. Appl. Polym. Sci. 2012, 124, 915. (7) Van Berkel, P. M.; Van Der Slot, S. C.; Driessen, W. L.; Reedijik, J.; Sherrington, D. C. Influence of the Polymer Matrix on the Methal-Ion Uptake Characteristics of Ligand-Modified Poly(glycidyl methacrylateco-trimethylolpropane trimethacrylate) Polymers. Eur. Polym. J. 1997, 33, 301. (8) Verweij, P. D.; Van der Geest, J. S. N.; Driessen, W. L.; Reedijik, J.; Sherrington, D. C. Metal Uptake by a Novel Benzimidazole Ligand Immobilized onto Poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate). React. Funct. Polym. 1992, 18, 191. (9) Hruby, M.; Hradil, J.; Beneš, M. J. Interactions of Phenols With Silver(I), Coper(II) and Iron(III) Complexes of Chelating Methacrylate-Based Polymeric Sorbent Containing Quinolin-8-ol Groups. React. Funct. Polym. 2004, 59, 105. (10) Senkal, B. F.; Bicak, N. Glycidyl Methacrylate Based Polymer Resins With Diethylene Triamine Tetra Acetic Acid Functions for Efficient Removal of Ca(II) and Mg(II). React. Funct. Polym. 2001, 49, 151. (11) Alexandratos, S. D.; Ripperger, K. P. Synthesis and Characterization of High-Stability Solvent-Impregnated Resins. Ind. Eng. Chem. Res. 1998, 37, 4756. (12) Sun, Y.; Rimmer, S. Preparation of Multiphase Polymer Beads Composed of Block Copolymer Amphiphilic Networks. Ind. Eng. Chem. Res. 2005, 44, 8621. (13) Švec, F.; Hrudkova, H.; Horak, D.; Kalal, J. Reaction of the Epoxide Groups of the Copolymer Glycidyl Methacrylate-Ethylene Dimethacrylate With Aliphatic Amino Compaunds. Angew. Makromol. Chem. 1977, 63, 23. (14) Kalal, J.; Švec, F.; Marousek, V. Reaction of Epoxide Groups of Glycidyl Methacrylate Copolymers. J. Polym. Sci. Polym. Sym. 1974, 47, 155. (15) Malović, Lj.; Nastasović, A.; Sandić, Z.; Marković, J.; Đorđević, D.; Vuković, Z. Surface Modification of Macroporous Glycidyl Methacrylate Based Copolymers for Selective Sorption of Heavy Metals. J. Mater. Sci. 2007, 42, 3326. (16) Safa, K. D.; Nasirtabrizi, M. H. Ring Opening Reactions of Glycidyl Methacrylate Copolymers to Introduce Bulky Organosilicon Side Chain Substituents. Polym. Bull. 2006, 57, 293. (17) Grama, S.; Plichta, Z.; Trchova, M.; Kovarova, J.; Benes, M.; Horak, D. Monodisperse Macroporous Poly(glycidyl methacrylate) Microspheres Coated With Silica: Design, Preparation and Characterization. React. Funct. Polym. 2014, 77, 11. (18) McGrath, J. E.; Yilgör, I. Polysiloxane Containing Copolymers: A Survey of Recent Developments. Adv. Polym. Sci. 1988, 86, 1. (19) Briganti, E.; Losi, P.; Raffi, A.; Scoccianti, M.; Munaò, A.; Soldani, G. Silicone Based Polyurethane Materials: a Promising Biocompatible Elastomeric Formulation for Cardiovascular Applications. J. Mater. Sci.: Mater. Med. 2006, 17, 259.

on the surface, except for the GMA 60 PDMS copolymer where the reaction occurs slightly higher in interior of the beads.

4. CONCLUSIONS Two series of novel macroporous copolymers with different cross-linker concentrations and porosity parameters, functionalized with hexamethylene diamine, 1,3-bis(3-aminopropyl) tetramethyl disiloxane, and α,ω-diaminopropyl poly(dimethylsiloxane), were successfully synthesized. The structures of functionalized copolymers were confirmed by 13C and 29 Si solid-state NMR, as well as by FTIR spectroscopy. The NMR and FTIR results showed better efficiency in functionalization of initial copolymers with HD and TMDS amines in comparison with PDMS amine. Functionalized copolymers prepared with higher cross-linker concentrations had higher thermal stability, glass transition temperature, and specific surface area, but lower pore size. The functionalized copolymers showed increased thermal stability in comparison to the initial poly(GMA-coEGDMA) copolymers. Furthermore, the functionalized copolymers based on TMDS and PDMS had better thermal stability than HD-based copolymers. SEM analysis showed the porous surface morphology and the inner structure of the functionalized copolymers. The porous properties of the surface of the beads depend on the type of amines, while the size of the microparticles was unaffected by the type of amines as broad particle size distribution was noted for all samples. Newly, synthesized and characterized, functionalized copolymers described here have retained macroporosity and might find new applications that involve macroporous copolymers. We are currently performing studies to evaluate their potential use for the sorption of heavy and precious metals. Due to their improved thermal properties, these copolymers have potential for use as a column material in gas chromatography.

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ASSOCIATED CONTENT

S Supporting Information *

Composition of the reaction mixtures for preparation of functionalized poly(GMA-co-EGDMA) copolymers and yields of copolymers, NMR analysis, results of deconvolution analysis obtained for peak areas of carbonyl and epoxy CH peaks in 13C MAS NMR spectra, NMR spectra of initial and functionalized copolymers with 80 mass% of GMA, and results of SEM-EDX analysis of the functionalized copolymers. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01285.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project No. 172062, III43009 and 45001).

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REFERENCES

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