Analysis of Double Bond Conversion of Photopolymerizable

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Analysis of Double Bond Conversion of Photopolymerizable Monomers by FTIR-ATR Spectroscopy Ana M. Herrera-Gonzaĺ ez,* Martín Caldera-Villalobos, Alma A. Peŕ ez-Mondragoń , Carlos E. Cuevas-Suaŕ ez, and J. Abraham Gonzaĺ ez-Loṕ ez Laboratorio de Polímeros, Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma del Estado de Hidalgo, Ciudad del Conocimiento, Carretera Pachuca-Tulancingo km 4.5, Colonia Carboneras, Mineral de la Reforma, Hidalgo, C.P. 42184, México

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S Supporting Information *

ABSTRACT: An experiment for the calculation of the degree of double bond conversion, after a polymerization reaction, of photopolymerizable liquid monomers using FTIR-ATR spectroscopy is reported. The experiment was successfully conducted with undergraduate students in materials engineering during the Fundamentals of Polymeric Material course. Students synthesized a Bis-GMA and TEGDMA copolymer through radical chain polymerization using visible light as the energy source (photopolymerization). Through the quantitative analysis of two absorption bands in the FTIR spectra of the monomer and the polymer, the νCC of the alkene group at 1638 cm−1 and the νCC of the aromatic ring at 1610 cm−1, students calculated the degree of double bond conversion after the polymerization reaction. Students acquired competencies in the synthesis and qualitative and quantitative characterization of polymeric materials through FTIR spectroscopy. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Polymer Chemistry, Polymerization, IR Spectroscopy

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INTRODUCTION The properties of a polymer depend on several factors such as its molecular weight, structure, number of branches, degree of crystallinity, and degree of polymerization.1,2 When the polymer is obtained by radical chain polymerization, the degree of double bond conversion can be estimated through the relative amount of double bonds remaining at the end of the polymerization reaction.3 The degree of double bond conversion can be analyzed with the polymerization of photopolymerizable monomers; some of them are diene monomers such as triethylene glycol dimethacrylate (TEGDMA) and Bis-phenol A glycerolate dimethacrylate (Bis-GMA), which generate four functionalities during the polymerization reaction. These monomers can also be thermally polymerized and have applications as anaerobic adhesives or in restorative dentistry.4,5 In these monomers, the degree of double bond conversion can be quantified by vibrational spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy.6 Such techniques are able to detect the stretching vibration of carbon−carbon double bonds involved in the polymerization process.7 Recently, FTIR spectroscopy was used for the determination of the degree of double bond conversion of experimental dental composite resins based on these types of monomers.8−10 This analysis methodology has been successfully adapted in the development of a laboratory experiment carried out by © XXXX American Chemical Society and Division of Chemical Education, Inc.

undergraduate students using FTIR spectroscopy with attenuated total reflectance (ATR). This experiment has been carried out for 86 undergraduate students in the materials engineering course over six semesters. Learning objectives for this experiment included the acquisition of competences in the chemical characterization of monomers and polymeric materials by FTIR spectroscopy and the consolidation of concepts like monomer, polymer, radical polymerization, photoinitiator, polymer network, radical, mass polymerization, and double bond conversion.

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BACKGROUND The experiment was carried out during the fifth semester in a course titled Fundamentals of Polymeric Materials. Students at this level have previous knowledge of organic chemistry, chemical characterization of materials, analytical chemistry, and general chemistry. In this experiment, this knowledge is applied to the calculation of the degree of double bond conversion in a Bis-GMA/TEGDMA copolymer, so that students can acquire and apply FTIR spectroscopy knowledge for the quantitative and qualitative characterization of monomers and polymers. In addition, the experiment allows students to consolidate essential concepts of polymer Received: August 29, 2018 Revised: May 9, 2019

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DOI: 10.1021/acs.jchemed.8b00659 J. Chem. Educ. XXXX, XXX, XXX−XXX

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HAZARDS The reagents employed in this experiment must be carefully handled. Students must wear a laboratory coat, glasses, and gloves during the experiment. Bis-GMA, TEGDMA, and CQ can cause eye and skin irritation or even allergic reactions. CQ can also cause respiratory tract irritation by inhalation. 4EDMAB is toxic if swallowed. The light of the lamp should not be observed directly; it can cause glare due to high intensity of the blue emitted light. However, there is not risk of permanent damage to the eyes.

chemistry such as radical chain polymerization, bulk polymerization, copolymerization, cross-linking, and photopolymerization. Before the experiment is performed, a theoretical explanation is presented on some fundamental aspects of FTIR spectroscopy to identify functional groups present in organic and inorganic compounds. This information can be used to show the absence or presence of functional groups, and to calculate their relative ratio from absorbance values. The spectrum of a compound obtained in the synthesis can also be compared to the spectrum of a known standard sample or of the starting materials.11−14 The laboratory sessions last 2 h. Students are divided into teams of 3 or 4 members. Each student performs a photopolymerization reaction and characterizes the resulting polymer by FTIR spectroscopy. The use of ATR is important to simplify the process of sample preparation and reduce the acquisition time to 15−20 min for each team.

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RESULTS AND DISCUSSION Before conducting the experiment, students identified the structures of the monomers, initiator, and co-initiator in the standard mixture (Figure 1), as well as the structure of the

EXPERIMENTAL DETAILS

Sample Preparation and Reaction

A standard mixture (prepared for students; Supporting Information, Notes for Instructors) contains the monomers Bis-GMA and TEGDMA (70:30 wt/wt), camphorquinone (CQ) (2 wt %) as initiator, and ethyl 4-dimethylaminobenzoate (4-EDMAB) (4 wt %) as co-initiator. Due to the corrosive and irritant nature of Bis-GMA and TEGMA, the mixture of these monomers must be handled carefully. A sample of the standard mixture (approx 2−3 mg, enough to cover the entire ATR cell) is placed in the ATR cell of the spectrophotometer (D = 2 mm). A silicon mold was used to standardize the distance at 2 mm between the fiber tip of the photopolymerization lamp and the diamond cell window of the ATR unit. An FTIR spectrum is acquired in the absorbance mode. Subsequently, the mixture is irradiated for 30 s with a photopolymerization lamp with emission in the range 420− 450 nm and emission intensity of 460−1200 mW/mm2 directly on the ATR cell. Once the polymerization is complete, a second FTIR spectrum is obtained in the absorbance mode. Each member of a team completes a polymerization reaction to observe the reproducibility of the experiment.

Figure 1. Structures of the reagents employed during the experiment.

polymer obtained from the copolymerization of the monomers. The copolymer is a cross-linked polymer because both diene monomers generate four functionalities during the polymerization reaction (Scheme 1). Students completed a qualitative analysis of the polymerization reaction using the FTIR spectra (Figure 2) to identify important absorption bands: νCC of alkene at 1638 cm−1, νCC of the aromatic ring at 1610 cm−1, νCO of the carbonyl group at 1715 cm−1, and νC−O of the ester group at 1155 cm−1. After the measurements at 1638 and 1610 cm−1 were obtained (Figure 3), the degree of double bond conversion was calculated using eq 1. As an example, the degree of double bond conversion in Figure 3 from a student experiment is 48%. At the end of the experiment, students identified three pieces of evidence that demonstrated the transformation of monomers into a polymer. First, students observed a change in the physical state from a mixture of liquid monomers to a solid polymer. Second, in the FTIR spectra, the relative intensity of the absorption band of νCC (1638 cm−1) of the alkene group decreased because of the homolytic rupture of the CC bonds (Supporting Information video). Finally, in the FTIR spectrum of the polymer, a widening of the absorption bands was observed due to an increase in the molecular weight during the polymerization reaction (Figure 2). Because the absorption band νCC of alkene does not disappear, the instructor explained to the students that the

Data Analysis

In each spectrum, a straight horizontal line is drawn in the range 1660−1590 cm−1 that functions as a baseline. The height is measured in centimeters from the baseline to the maximum point of the absorption bands of νCC of the alkene bond at 1638 cm−1 and νCC of the aromatic ring at 1610 cm−1. The absorbance of νCC bonds from the aromatic ring does not change after the photopolymerization reaction and can, therefore, be used as an internal reference.13,15 The degree of double bond conversion (DC) is determined according to eq 1. ij h1638/h1610pol yz zz × 100% DC (%) = jjj1 − zz j / h h 1638 1610mon { k

(1)

Here, h1638 is the height of the band at 1638 cm−1, and h1610 is the height of the band at 1610 cm−1. The term “mon” corresponds to the spectrum of the unpolymerized monomer mixture, and the term “pol” refers to the spectrum of the polymerized material. B

DOI: 10.1021/acs.jchemed.8b00659 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Scheme 1. Copolymerization of Bis-GMA and TEGDMA

prevent both CC bonds from suffering homolytic rupture in all monomer molecules, and a high vitrification velocity of the polymeric network inhibits movement. From the explanation given to the students, they deduced that vitrification of the material during the polymerization reaction decreased the diffusion of radicals, which prevented some of the double bonds of BisGMA and TEGDMA monomers from reacting during the polymerization reaction.16,17

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CONCLUSION In general, this experiment could be used in any polymer chemistry laboratory to determine the degree of double bond conversion of monomers that can be polymerizable via radical polymerization or ionic polymerization. Students learned to calculate the degree of double bond conversion of photopolymerizable diene monomers. Students acquired competences in the synthesis and qualitative and quantitative characterization of polymeric materials through FTIR spectroscopy. An experiment such as this can be done quickly, with low consumption of reagents and with the use of a single characterization instrument, contributing to the teaching of polymer chemistry in undergraduate curricula.

Figure 2. FTIR spectra of the mixture of monomers (black) and copolymer (red).

degree of double bond conversion does not reach 100%, due to the high velocity of the polymerization reaction which helps C

DOI: 10.1021/acs.jchemed.8b00659 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 3. FTIR spectra for the calculation of the degree of double bond conversion: (a) reaction mixture and (b) polymer.

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carbonate and Bisacrylic Monomers Useful in the Formulation of Dental Composite Resins and in the Crosslinking of Methyl Methacrylate. J. Appl. Polym. Sci. 2016, 133 (4), 42920. (9) González-López, J. A.; Cuevas-Suárez, C. E.; Pérez-Mondragón, A. A.; Berlanga Duarte, M. L.; Herrera-González, A. M. Photopolymerizable Multifunctional Monomers and Their Evaluation as Reactive Bis-GMA Eluents. J. Appl. Polym. Sci. 2018, 135 (9), 46240. (10) Herrera-González, A. M.; González-López, J. A.; CuevasSuárez, C. E.; García-Castro, M. A.; Vargas-Ramírez, M. Formulation and Evaluation of Dental Composite Resins with Allylcarbonate Monomer as Eluent for Bis-GMA. Polym. Compos 2018, 39, E342. (11) Schuttlefield, J. D.; Grassian, V. H. ATR-FTIR Spectroscopy in the Undergraduate Chemistry Laboratory. Part I: Fundamentals and Examples. J. Chem. Educ. 2008, 85 (2), 279−281. (12) Kneisel, A.; Bellamy, M. K. Measuring Breath Alcohol Concentrations with an FTIR Spectrometer. J. Chem. Educ. 2003, 80 (12), 1448−1450. (13) Bellamy, M. K. Using FTIR-ATR Spectroscopy to Teach the Internal Standard Method. J. Chem. Educ. 2010, 87, 1399. (14) Shepherd, B.; Bellamy, M. K. A Spreadsheet Exercise to Teach the Fourier Transform in FTIR Spectrometry. J. Chem. Educ. 2012, 89 (5), 681−682. (15) Collares, F. M.; Portella, F. F.; Leitune, V. C. B.; Samuel, S. M. W. Discrepancies in Degree of Conversion Measurements by FTIR. Braz. Oral Res. 2014, 28 (1), 1−7. (16) Leprince, J. G.; Palin, W. M.; Hadis, M. A.; Devaux, J.; Leloup, G. Progress in Dimethacrylate-Based Dental Composite Technology and Curing Efficiency. Dent. Mater. 2013, 29 (2), 139−156. (17) Coessens, V. M. C.; Matyjaszewski, K. Fundamentals of Atom Transfer Radical Polymerization. J. Chem. Educ. 2010, 87 (9), 916− 919.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00659. Video of temporal evolution of FTIR spectrum during the polymerization reaction (AVI) Student handout (PDF, DOCX) Notes for instructors (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ana M. Herrera-González: 0000-0002-7534-7004 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Directorate of the Institute of Basic Sciences and Engineering for the financial support provided for the maintenance of the FTIR equipment.

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REFERENCES

(1) νOdian, G. Types of Polymers and Polymerizations. In Principles of Polymerization; John Wiley & Sons: Hoboken, NJ, 2004; pp 1−35. (2) Carraher, C. E. Introduction of Polymer Science. In Seymour/ Carraher’s Polymer Chemistry; Marcel Dekker Inc., CRC Press: New York, NY, 2003; pp 36−54. (3) Eliades, G. C.; Vougiouklakis, G. J.; Caputo, A. A. Degree of Double Bond Conversion in Light-Cured Composites. Dent. Mater. 1987, 3 (1), 19−25. (4) Stansbury, J. W.; Trujillo-Lemon, M.; Lu, H.; Ding, X.; Lin, Y.; Ge, J. Conversion-Dependent Shrinkage Stress and Strain in Dental Resins and Composites. Dent. Mater. 2005, 21 (1), 56−67. (5) Yoshida, K.; Greener, E. H. Effect of Photoinitiator on Degree of Conversion of Unfilled Light-Cured Resin. J. Dent. 1994, 22 (5), 296−299. (6) Lee, K. M.; Ware, T. H.; Tondiglia, V. P.; McBride, M. K.; Zhang, X.; Bowman, C. N.; White, T. J. Initiatorless Photopolymerization of Liquid Crystal Monomers. ACS Appl. Mater. Interfaces 2016, 8 (41), 28040−28046. (7) Rueggeberg, F. A.; Hashinger, D. T.; Fairhurst, C. W. Calibration of FTIR Conversion Analysis of Contemporary Dental Resin Composites. Dent. Mater. 1990, 6 (4), 241−249. (8) Herrera-González, A. M.; Cuevas-Suárez, C. E.; CalderaVillalobos, M.; Pérez-Mondragón, A. A. Photopolymerizable BisallylD

DOI: 10.1021/acs.jchemed.8b00659 J. Chem. Educ. XXXX, XXX, XXX−XXX