Investigation of the thermal properties of glycidyl methacrylate–ethylene glycol dimethacrylate copolymeric microspheres modified by Diels–Alder reaction
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The study describes the thermal properties of functional microspheres composed of glycidyl methacrylate (GMA) and crosslinking agent ethylene glycol dimethacrylate (EGDMA). Copolymeric poly(GMA-co-EGDMA) microspheres were prepared via suspension–emulsion polymerization in the presence of toluene and decan-1-ol as porogens. In order to introduce functional groups, the porous methacrylate network was modified by epoxy ring opening with the use of sodium cyclopentadienide and then the Diels–Alder addition with maleic anhydride. The thermal properties of poly(GMA-co-EGDMA) materials were evaluated by thermogravimetry and differential scanning calorimetry. By TG/FTIR, it was observed that new functional materials exhibited multi-staged decomposition patterns, different from parent poly(GMA-co-EGDMA) microspheres. The synthesized poly(GMA-co-EGDMA) microspheres exhibited rather high thermal stability in inert atmosphere. Their initial decomposition temperature determined at the temperature of 2% of mass loss was about 210 °C; however, after the chemical modification it was slightly lower. The thermal degradation of parent poly(GMA-co-EGDMA) copolymer runs mainly according to the depolymerization mechanism, while functionalized by cyclopentadienyl group and maleic anhydride microspheres decompose through the chain scission mechanism.
KeywordsFunctional polymeric microspheres Glycidyl methacrylate Diels–Alder reaction Maleic anhydride Thermogravimetric analysis TG/FTIR
Polymers having functional groups in their structural units and capable of further transformations are termed as reactive polymers. In some cases, the presence of the reactive glycidyl methacrylate in these copolymers made them quite convenient for further changes. Glycidyl methacrylate (GMA) is an attractive vinyl monomer because of its low toxicity, lower cost compared with other acrylic monomers, versatile properties and especially due to the presence of dual functionality, containing both methacrylic and epoxide groups [1, 2]. The methacrylic group containing the reactive C=C bond readily reacts with a wide range of monomers including multifunctional ones which can create the crosslinked network. On the other hand, the glycidyl fragment is a convenient group which in a mild condition is able to react with a numerous of strong and weak nucleophiles to provide the user with maximum freedom and flexibility in designing the polymer structure. However, the mechanical and thermal stability of polymers synthesized with GMA monomer is not quite up to the mark.
Since the pioneering work of Švec et al.  on macroporous copolymer based on GMA and ethylene glycol dimethacrylate, a large number of papers have been focused on more detailed studies of the synthesis, properties, chemical modification and applications of copolymers based on GMA. For decades, copolymers of glycidyl methacrylate crosslinked with ethylene glycol dimethacrylate have been widely used as sorbents for chromatography [4, 5], solid-supported catalysis , enzyme immobilization [7, 8], ion–resin exchange , solid-phase peptide synthesis (SPPS) and solid-phase organic synthesis (SPOS) . The thermal resistance of materials such as poly(glycidyl methacrylate) [11, 12], its copolymers with styrene , methyl methacrylate [12, 14], vinyl acetate , acrylonitrile  and α-methyl styrene  were reported. However, the use of polymers based on glycidyl methacrylate as adsorbents is limited by their thermal stability. To increase the thermal resistance of porous adsorbents, Maciejewska proposed the synthesis of porous microspheres of GMA crosslinked with trimethylolpropane trimethacrylate and functionalized with pyrrolidone  and diethylenetriamine . The interesting porous copolymer of GMA crosslinked with bis[4(2-hydroxy-3-methacryloyloxypropoxy)phenyl]sulphide and modified by diethylenetriamine and triethylenetetramine  possesses high thermal stability and the chemical structure suitable for use as an adsorbent.
In our study, we intended to obtain porous microspheres of glycidyl methacrylate crosslinked with ethylene glycol dimethacrylate, which were able to further chemical modification. The final microspheres bear anhydride groups that can be transferred in a simple way into carboxyl one. Copolymeric porous beads with carboxyl groups are attractive materials used as column packings for liquid chromatography. On the other hand, the synthesis of crosslinked microspheres via suspension or emulsion polymerization from monomers possessing carboxyl group, e.g. methacrylic or acrylic acid, is ineffective. In this study, we propose the synthesis of functional crosslinked microspheres possessing anhydride moieties which can be used as stationary phases in liquid chromatography and solid-phase extraction. Poly(GMA-co-EGDMA) copolymeric microspheres were prepared via suspension–emulsion polymerization. Their structure was modified firstly by cyclopentadienyl group and finally with the use of Diels–Alder cycloaddition reaction with maleic anhydride. The aim of this work is to present the thermal properties of a series of poly(GMA-co-EGDMA) functional polymeric microspheres. Furthermore, the project shows the influence of chemical structure on the thermal stability of copolymeric microspheres. To the best of our knowledge, we demonstrate for the first time the thermal characterization of copolymers prepared in this way. The TG/FTIR-coupled method and DSC method were employed to study the thermal behaviour of the materials discussed in inert atmosphere.
Glycidyl methacrylate (GMA, 97%), ethylene glycol dimethacrylate (EGDMA, 98%), azobisisobutyronitrile (AIBN, 98%), maleic anhydride (99%), sodium cyclopentadienide (NaCp, 2 M in THF), decan-1-ol (99%) and sodium bis(2-ethylhexyl) sulfosuccinate (97%) were obtained from Sigma-Aldrich. Toluene, ethanol, tetrahydrofuran, acetone, hexane and hydrochloric acid were of analytical reagent grade and were from POCh (Gliwice, Poland). Monomers: GMA and EGDMA, were purified of inhibitor by vacuum distillation and stored in a refrigerator until use. Other reagents were used as received without further purification.
Preparation of poly(GMA-co-EGDMA) polymeric microspheres
Polymeric microspheres were synthesized by the suspension–emulsion polymerization in accordance with the procedure described earlier [21, 22]. Polymerization reactions were carried out in an aqueous solution of sodium bis(2-ethylhexyl) sulfosuccinate. Toluene and decan-1-ol were used as pore-forming agents, AIBN acted as an initiator of polymerization. The monomers: GMA and EGDMA, were used at different molar ratios in the range of (4, 1.5, 1):1, respectively. The polymerization process was carried out at 80 °C and stirring at approx. 300 rpm for 10 h. After the reaction was completed, the obtained spheres were extracted with hot THF for 4 h. Based on the molar ratio of GMA to EGDMA, the obtained materials poly(GMA-co-EGDMA) were denoted as: poly(GMA-co-EGDMA)8/2; poly(GMA-co-EGDMA)6/4; and poly(GMA-co-EGDMA)5/5.
Preparation of functional polymeric microspheres
In the next step, the poly(GMA-co-EGDMA)-Cp material was reacted with maleic anhydride in the Diels–Alder reaction. The process was run in acetone at 50 °C for 2.5 h . Afterwards, microspheres were filtered off, extracted with acetone and dried at 50 °C overnight. As a result, poly(GMA-co-EGDMA)-MA microspheres were produced.
Methods of analysis
ATR-FTIR spectra were recorded using the Tensor 27 spectrometer (Bruker, Germany), equipped with a diamond crystal. The spectra were made in the spectral range of 600–4000 cm−1 with a resolution of 4 cm−1 and 16 scans per spectrum.
Parameters characterizing the porosity of parent copolymers were determined by nitrogen adsorption at − 196 °C using a Micromeritics ASAP 2420 analyser. Before measurements, samples were degassed at 60 °C under vacuum.
Differential scanning calorimetry measurements were performed using the DSC 204 calorimeter (Netzsch, Germany) operating in a dynamic mode. The dynamic scans were performed at a heating rate of 10 K min−1 from 20 to 550 °C under argon atmosphere (20 mL min−1). The empty aluminium crucible was applied as the reference.
Thermogravimetric analysis of materials was carried out on a STA 449 F1 Jupiter (Netzsch, Germany) at the heating rate of 10 K min−1, in the temperature range of 20–850 °C, with the sample mass of ≈ 10 mg in inert (helium) atmosphere. The gas flow was 20 mL min−1. As the reference, the empty Al2O3 crucible was used.
Results and discussion
TG and DTG data for prepared materials
The second mass loss of copolymers is observed in the temperature range of 360–470 °C. The calculated mass losses are dependent on the amount of crosslinker (EGDMA) used for the synthesis of copolymers and are in the range of 19.5–28.9%. The FTIR spectrum corresponding to the T max2 (410 °C) shows an emission of the organic compounds derived from the decomposition of residual crosslinked part (including a small amount of ethylene glycol dimethacrylate) and also release of significant amount of carbon dioxide (absorption bands at 2357 and 670 cm−1) as a product of degradation of ester bonds present in the EGDMA units.
The DSC- and TG/FTIR-coupled methods were used to monitor the thermal behaviour of porous primary and modified copolymeric microspheres of GMA crosslinked with EGDMA. The poly(GMA-co-EGDMA) copolymers are thermally stable up to 210 °C in inert atmosphere. Their thermal resistance is dependent on the amount of the crosslinked monomer. The degradation of parent microspheres starts with the depolymerization process initiated at the glycidyl fragments in the copolymer network and is followed by the isomerization process at gaseous phase. The residual crosslinked part of copolymers decomposes above 360 °C. The modification steps of poly(GMA-co-EGDMA) microspheres including the reaction of epoxide group with sodium cyclopentadienide followed by the Diels–Alder reaction with maleic anhydride changed the thermal properties of starting material. The value of T 2% is slightly lower for modified microspheres; however, the main degradation process is shifted to higher temperature region. The poly(GMA-co-EGDMA)-Cp as well as the poly(GMA-co-EGDMA)-MA materials reveal complex degradation processes which run according to the random chain scission mechanism. Although some part of epoxide groups remained in the interior of microspheres after the modification, the products of the depolymerization are not observed during the thermal degradation. Moreover, the thermal analysis of the copolymer with grafted maleic anhydride shows that the esterification takes place probably at higher temperature on the surface of microspheres, prior to their total thermal degradation. In this study, we present the model reaction of functionalized microspheres with maleic anhydride; however, the presence of cyclopentadienyl group in the structure of poly(GMA-co-EGDMA)-Cp copolymer allows to the addition of any dienophile with the use of Diels–Alder reaction. Moreover, the anhydride group in the structure of poly(GMA-co-EGDMA)-MA microspheres can be easily transferred into carboxyl group. The final microspheres possessing anhydride groups or such with carboxyl one can be used as column packings in liquid chromatography and solid-phase extraction.
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 No. PIRSES-GA-2013-612484. The research was carried out with the equipment purchased thanks to the financial support of the Operational Programme Development of Eastern Poland 2007–2013 (contract No. POPW.01.03.00-06-017/09-00).
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