Article pubs.acs.org/IECR
Influence of the Nature of Acrylates on the Reactivity, Structure, and Properties of Polyurethane Acrylates Aziz Ahmed, Preetom Sarkar, Imtiaz Ahmad, Neeladri Das, and Anil K. Bhowmick*,§ Department of Chemistry, Indian Institute of Technology Patna, Patna-800013 Bihar, India S Supporting Information *
ABSTRACT: Polyurethanes (PUs) are one of the most versatile classes of polymers, and their demand as high-performance industrial materials continues to grow. In this work, new polyurethane acrylates were prepared from diglycidyl ether bisphenol A (DGEBA), substituted acrylic acids, and 4,4′-diphenylmethane diisocyanate using a two-step polymerization methodology. The polymers were characterized by infrared spectroscopy, 1H NMR spectroscopy, gel permeation chromatography, thermogravimetry, and X-ray diffraction. The influence of the structure of the acrylic acid on the reactivity with DGEBA was investigated. Density functional theory and canonical structures supported the reactivity order of the substituted acrylic acids toward DGEBA in both the gas and solution phases. All of the polyurethane acrylates investigated were found to have a high thermal stability.
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INTRODUCTION Polyurethanes (PUs) are polymers that have urethane groups (−NH−CO−O−) bridging the monomers in their backbone. The demand for and interest in PUs have continued to grow since the discovery of polyurethane in 1937 by Bayer et al.1 PUs can be prepared by either the condensation polymerization of bischloroformates with diamines or the addition polymerization of diisocyanates with di- or polyfunctional hydroxyl compounds. The characteristic structural element of polyurethanes is the urethane group.2 A compound containing two or more isocyanate groups per molecule can be reacted with a polyol for the convenient preparation of polyurethanes (Scheme 1).
methylene diphenyl diisocyanate are perpendicular to each other,6 whereas PUs synthesized using 2,4-toluene diisocyanate and 2,6-toluene diisocyanate are arranged in one plane because of a coplanar arrangement.7 This structural diversity of PU chains gives rise to interactions both within individual macromolecules (intramolecular) and between different macromolecules (interchain). The hard segments can interact with each other through hydrogen bonds or polar urethane groups, which give them a rigid phase.8 Despite the apparent simplicity in this synthetic methodology, the ratio of polyol (monomers with two or more −OH groups) and polyisocyanate (monomers with two or more −NCO groups) is very important, and variations in this ratio can result in different extents of cross-linking of the polymer. It has been reported that, by varying the degree of functionality of the starting materials, a wide range of polyurethane materials can be obtained.9 Thus, a substantial change in molar ratio can lead to morphological changes, leading in turn to modifications of the mechanical properties of the material.10 However, the major disadvantages associated with PUs are their high viscosities and poor exterior durability. To reduce the viscosity due to free hydroxyl groups, these groups are capped, which, in turn, decreases the resin viscosity, often at the expense of thin-film properties.11 Because of their unique properties, applications of polyurethanes (PUs) include their use as coatings, adhesives, toughening agents for thermoplastic surfaces, and sealants.12−19 These urethane polymers are important because of their thermoplastic and thermosetting characteristics imparted by their inherent mechanical, thermal, and chemical properties. These properties can be tuned through the proper selection of appropriate monomers from a huge variety of commercially available isocyanates.20 The synthetic efficiency of the polyur-
Scheme 1. General Scheme for the Synthesis of Polyurethane
In general, PUs are segmented block copolymers composed of alternating soft and hard segments.3−5 The hard domain is composed of polar urethane groups derived from diisocyanate, whereas the soft or flexible domains contain polyester polyols or polyesters, which make linear PUs more flexible. The structural segments are schematically represented in Scheme 1. The structure of the macromolecule thus obtained depends on the spatial arrangement of PU chains in the condensed phase, that is, after the polymerization process. Linear PUs synthesized using 4,4′-methylene diphenyl diisocyanate and 1,4-butanediol form zigzag chains in which the benzene rings of 4,4′© 2014 American Chemical Society
Received: Revised: Accepted: Published: 47
July 26, 2014 December 7, 2014 December 15, 2014 December 15, 2014 DOI: 10.1021/ie502953u Ind. Eng. Chem. Res. 2015, 54, 47−54
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Industrial & Engineering Chemistry Research ethane formation reaction, combined with the versatility of the polymers that can be designed, has thus attracted considerable research attention, leading to newer applications of PUs.21 PU coatings are applied on products to improve their appearance, lifespan, and scratch and corrosion resistance. PU adhesives and sealants provide strong bonding and tight seals in a variety of applications.22 From the biomedical point of view, polyurethanes are frequently used in the biomedical and pharmaceutical fields, in drug delivery systems,23 artificial hearts, vascular prostheses, and various types of catheters in clinical applications. Hydrophilic/hydrophobic microdomain structures, such as those of polyurethane, generally display higher blood compatibility.24,25 The growing commercial importance of polyurethane materials emphasizes the need for the study of polyurethane formation with respect to parameters such as chemical properties; micro-, macro-, and nanostructures; reagent concentrations; reaction media; and catalysts. Maji and Bhowmick reported micro- and nanostructured PUs based on various hyperbranched polymers.26,27 Until now, there are only a few literature references on polyurethane acrylate.11,28,29 Research on acrylate-based polyurethane has gained momentum in the recent past. Because of technological advances, specification of any product is changing very rapidly. For example, high abrasion resistance, toughness, high tear strength, and good low-temperature properties of polyurethanes can be combined with superior optical properties and weatherability of acrylates. Similarly, the processing requirements of any product can be met by modifications of the polymer. To the best of our knowledge, a systematic study of polyurethanes using various substituted acrylic acids that act as the driving force for the cleavage of oxirane rings to generate polyols for subsequent reaction with diisocyanates in successive steps has not yet been reported in the literature. In addition, structure−property relationships of such materials are still obscure. The aim of the present work is to study the reactivity of various methyl-substituted acrylic acids toward the formation of polyurethanes. The present work also reports the preparation of low-viscosity polyurethane acrylates (PUAs) using commercially available diglycidyl ether bisphenol A, substituted acrylic acids, and diphenylmethane-4,4′-diisocyanate (MDI). Our approach is based on the reaction between diglycidyl ether bisphenol A and various substituted acrylic acids to yield prepolymers, which are subsequently used as precursors for the synthesis of polyurethane acrylates using MDI (Scheme 2).
Scheme 2. Synthesis of Polyurethane Acrylates (PUAs) where X, Y, Z = CH3 or H and A = Methacrylic Acid (MACA), trans-2-Methyl-2-butenoic Acid (TMBCA), or 3Methyl-2-butenoic Acid (MBCA)a,b
a
See Table 1 for X, Y, and Z. bReagents and conditions: (a) 3 mol % triphenylphosphine (TPP), tetrahydrofuran (THF), 40 °C, 11 h; (b) diphenylmethane-4,4′-diisocyanate (MDI), THF, 40 °C, 1 h. Yield = 85−90%.
using a Rigaku TT RAX 3XRD diffractometer in the range of 2−60° (2θ) with a Cu Kα (0.15 nm) radiation source, a 50 kV voltage, and a 100 mA intensity. Thermogravimetric analysis (TGA) was carried out using an SDT Q600 (TA Instruments) system at a ramp rate of 10 °C/min under a nitrogen atmosphere (flow rate of 100 mL/min) from room temperature to 800 °C. Gel permeation chromatography (GPC) was performed on an Agilent PL-GPC50 instrument equipped with a refractive index (RI) detector. In this case, dimethylformamide (DMF) was used as the mobile phase for the estimation of the molecular weight distribution at a flow rate of 1.0 mL/ min (temperature of 45 ± 2 °C) and passage through columns from Polymer Lab (PL gel 5 μm Mixed-B and PL gel 3 μm 100 in series). Preparation of Polyurethane Acrylates. The polyurethane acrylates reported in this study were synthesized in steps I and II by a series of addition reactions as depicted in Scheme 2. Step I: Prepolymer Synthesis. A typical synthetic protocol involved the addition of a solution of MACA (methacrylic acid, 0.84 mL, 9.61 mmol) to a solution of DGEBA (diglycidyl ether bisphenol A, 1.70 g, 4.99 mmol) in THF. Subsequently, TPP (3 mol %) was added as a catalyst, and the reaction mixture was stirred for 11 h at 40 °C. Step II: Polyurethane Synthesis. In the second step, 0.50 g (1.99 mmol) of diphenylmethane-4,4′-diisocyanate (MDI) in 2.0 mL of THF was added to the reaction mixture of the prepolymer and stirred continuously for another 1 h. Finally, the reaction mixture was transferred in a Petri dish for casting with the slow evaporation of THF molecules over a few hours at room temperature. A hard, sticky, light yellowish product was obtained. Following the above procedure, other polyurethane acrylates using trans-2-methyl-2-butenoic acid and 3-methyl-2-butenoic acid were synthesized. In this way, three different prepolymers
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EXPERIMENTAL SECTION Materials and Instrumentation. Diglycidyl ether bisphenol A (DGEBA) and diphenylmethane-4,4′-diisocyanate (MDI) were used as received from Sigma-Aldrich. Methacrylic acid, trans-2-methyl-2-butenoic acid, and 3-methyl-2-butenoic acid were obtained from Alfa Aesar. Triphenyl phosphine (TPP) from CDH was of analytical grade and was used without further purification. Fourier transform infrared (FTIR) spectra were recorded in universal attenuated total reflectance mode (UATR) using a Perkin-Elmer Spectrum 400 spectrometer in the spectral range of 4000−650 cm−1 with a total of four scans per sample. 1H NMR spectra were recorded on a JEOL-500 MHz or Bruker AVANCE III-400 MHz spectrometer in DMSO-d6 as the solvent and tetramethylsilane (TMS) as the internal standard. X-ray diffraction profiles were recorded for each sample (to estimate the extent of the crystalline nature of the polymers) 48
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Industrial & Engineering Chemistry Research (1a−1c) and the corresponding polyurethane acrylates (PUAs) (2a−2c) were synthesized, as shown in Table 1. Table 1. Different Types of Prepolymers and Polyurethane Acrylatesa substituted acrylic acid
prepolymer
methacrylic acid trans-2-methyl-2butenoic acid 3-methyl-2butenoic acid
1a 1b 1c
PUA (designationb) 2a (pMACA) 2b (pTMBCA) 2c (pMBCA)
X
Y
Z
CH3 CH3
H CH3
H H
H
CH3
CH3
a
Reaction yields were in the range of 85−90%. bDesignations indicate the respective polymers (“p”) derived from methacrylic acid (MACA), trans-2-methyl-2-butenoic acid (TMBCA), and 3-methyl-2-butenoic acid (MBCA).
Figure 1. Structures of substituted acrylic acids.
Solubility. The synthesized polyurethane acrylates (2a−2c) were insoluble in common organic solvents, whereas they were soluble in polar aprotic solvents such as dimethyl sulfoxide (DMSO) and DMF, as indicated in Table 2. Table 2. Solubilities of Polyurethane Acrylates in Different Organic Solventsa
a
sample
CH3OH
C2H5OH
CHCl3
CCl4
THF
DMSO
DMF
pMACA pTMBCA pMBCA
− − −
− − −
− − −
− − −
± ± ±
+ + +
+ + +
Key: −, insoluble; +, soluble; ±, sparingly soluble.
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RESULTS AND DISCUSSION The carboxylate group acts as a nucleophile that is responsible for opening the epoxide ring of DGEBA to generate a prepolymer through an epoxide ring-opening reaction. The rate of this reaction was found to be very slow (reaction time of 30 h). To enhance the rate of reaction, a catalytic amount of a mild base (TPP) was introduced into the reaction mixture. The reaction time was thus reduced to 10−12 h. In the present study, various substituted acrylic acids were reacted with DGEBA. We were interested in studying the effect of methyl substitution on the reactivity of acrylic acid with DGEBA during prepolymer synthesis. The numbering scheme for carbons of the acrylic acid moiety is shown in Figure 1. The methyl substituents are present at either C2 or C3, across the CC bond of the acrylate moiety. The rate of the prepolymer reaction was studied by monitoring the transmittance intensity of the COC stretching vibration of the epoxy groups at 915 cm−1. The UATR-FTIR spectrum was recorded by quenching a portion of the reaction mixture at regular interval of 1 h. The absorbance of the epoxy group (COstretching vibration at 915 cm−1) is plotted against time in Figure 2. The plot obtained clearly indicates that the reactivity of MBCA with DGEBA is a maximum, followed by TMBCA, and finally MACA. In other words, the degree of consumption of epoxy groups is in the order of MBCA > TMBCA > MACA in due course of prepolymer formation. This order is further supported by DFT calculations and the canonical structures of substituted acrylic acids. It should be mentioned that the absorbance of the acid groups of the pristine acids at 1720 cm−1 (CO of COOH groups) with respect to the peak
Figure 2. Absorbance of epoxy groups vs time.
intensity at 1454 cm−1 (CH3) is almost constant, as is evident from the IR spectrum. Structures. As shown in Figure 3, MBCA has the maximum number of canonical forms. Additionally, in these hyperconjugating structures, the negative charge is delocalized/ stabilized on an electronegative atom (oxygen). Hence, it is expected that MBCA is the most acidic, with its conjugate base having highest degree of stability. Therefore, it is expected to have highest reactivity by deprotonation followed by its attack (as a nucleophile) to open the epoxide ring. Using a similar argument, it is expected that MACA is least reactive and TMBCA has intermediate reactivity. This can be also explained by a very simple logic. The methyl groups are electron donors, and the electron-donor ability should decrease the acidity of the acids. Thus, the lower acidity of MACA and TMBCA should be due to the presence of a methyl group close to the carbonyl group. Computational Study. The electric potentials of the monomers MACA, TMBCA, and MBCA [in the gas and solution (THF) phases] were optimized by density functional theory (DFT) calculations utilizing Becke’s three-parameter exchange-correlation-functional (B3LYP) with the 6-31G(d,p) basis set. The substituted acrylic acids were assigned a charge of −1 and singlet multiplicity for calculation purposes. To simulate the effects of solvent, the conductor-like polarizable continuum model30 (CPCM) was used, in which THF was chosen as the solvent. The CPCM solvation energy was evaluated in gas-phase geometries using the B3LYP/6-31G49
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Figure 3. Various canonical structures of substituted acrylic acids.
(d,p) density functional method. All calculations were performed using Gaussian 09.31 There is a general consensus that the B3LYP method provides excellent low-cost performance for structure optimizations.32 The purpose of assessing the electrostatic potential is to identify the reactive site in a given acrylate.33 The reactive site of a molecule is a charged region that has an affinity for interacting with charged particles. A higher potential energy value indicates the absence of negative charges, which would mean that there were fewer electrons in this region. The converse is also true. Thus, a high electrostatic potential indicates the relative scarcity of electrons; on the other hand, an abundance of electrons suggests a low electrostatic potential. This property can be extrapolated to molecules as well. Our calculations predict that, in the solvent phase (i.e., in THF), the effect of the methyl substituent on the electric potential or electrostatic potential or the reactivity on the negatively charged oxygen atoms (O, O) across the carboxylate moiety ( ) are in the order Oav(MBCA) > Oav(TMBCA) > Oav(MACA) (Figure 4 and Table 3). This
Table 3. Comparison of Average Electric Potentials of Oxygen Atoms Across the Carboxylate Moiety of Substituted Acrylic Acids Calculated at the DFT-B3LYP/6-31G(d,p) Level in Solution Phase and Gas Phase substituted acrylic acid
electric potential (kcal mol−1) THF Phase −14.194 × 103 −14.196 × 103 −14.203 × 103
MACA TMBCA MBCA Gas Phase
−14.188 × 103 −14.206 × 103 −14.207 × 103
MACA TMBCA MBCA
atom remains almost constant in the gas phase as well as in THF solution. Characterization of PUAs. Gel Permeation Chromatography (GPC) Analysis. The number-average molecular weights (Mn), weight-average molecular weights (Mw), and polydispersity indexes (PDI) of the three PUAs derived from different acrylates were determined by GPC (using polystyrene standards). The results are reported in Table 4. All of the Table 4. Number-Average Molecular Weights (Mn) and Polydispersity Indexes (PDI) of Different PUAs as Determined by GPC
Figure 4. Ball-and-stick models of deprotonated substituted acrylic acids (red, oxygen; gray, carbon; white, hydrogen).
ordering is in good agreement with the reactivity of these substituted acrylic acids as observed experimentally (FTIR analysis), as well as predictions from a comparison of the canonical structures for the reactivity factor. In the gas-phase calculations, the effect of the methyl substituent at C2 and C3 across the acrylate moiety was studied. The results suggest that the stability of the ions is in the order Oav(MBCA) > Oav(TMBCA) > Oav(MACA). This indicates that the order of the average electric potentials of the electronegative oxygen
sample
Mn
PDI
pMACA pTMBCA pMBCA
14625 16782 20786
1.14 1.26 1.18
polyurethane acrylates eventually yielded a nearly monodisperse molecular weight distribution (PDI = 1.14−1.26). An increase in the number-average molecular weight was observed down the group, as shown in Table 4. This experimental observation is in line with the predicted extent of reactivity of the three substituted acrylic acids from the computational studies. The same trend is expected in the case of the weightaverage molecular weight. 50
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Industrial & Engineering Chemistry Research FTIR Analysis. The structurally significant IR spectral bands for synthesized PUAs and diglycidyl ether bisphenol A were characterized by UATR-FTIR spectroscopy (Figure 5 and
the CO stretching vibration of the urethane group. The appearance of a band in the range of 1660−1630 cm−1 was attributed to the presence of a CN stretching vibration in the synthesized polyurethane. On the other hand, the band between 1607 and 1600 cm−1 might be due to υstr(CC). The band appearing in the range of 1293−1296 cm−1 could well be assigned to the stretching vibration of the COC group. In other words, the appearance of a band due to the NHCOO group was well connected to the disappearance of the characteristic absorption band for the isocyanate functional group. 1 H Nuclear Magnetic Resonance (NMR) Analysis. All of the PUAs were characterized by proton NMR spectroscopy. 1H NMR spectra for polyurethanes derived from MACA, TMBCA, and MBCA are shown, along with the peak assignments, in Figures S1−S3 of the Supporting Information. The aromatic protons were observed in the range of δ = 6.77−7.10 ppm (b,c,f,g) in the 1H NMR spectrum of the polyurethanes investigated herein. The protons of the COOCH2 unit appeared in the range of δ = 3.91−3.95 ppm (j), whereas the hydrogens of the CH2OPh unit were observed in the range of δ = 4.21−4.25 ppm (h). This proton signal is upfield-shifted for TMBCA and MBCA by 0.04−0.05 ppm in comparison to MACA. This change can be attributed to the shielded ring-current effect from the adjacent aromatic ring in the case of CH2OPh. The protons attached to isopropyl carbons appeared in the range of δ = 3.60−3.68 ppm (i). This isopropyl proton was upfield shifted by 0.60−0.62 ppm in TMBCA and MBCA as compared to MACA. The peak due to the methyl group between two aromatic rings of polyurethane acrylates was observed in the range of δ = 1.50− 1.58 ppm (e). The active methylene protons between two aromatic rings were observed in the range of δ = 3.77−3.85 ppm (d). A very weak singlet signal in the range of δ = 8.12− 8.58 ppm (a) was observed and assigned to the urethane NH proton. This NH proton signal in MACA was downfieldshifted by 0.45−0.46 ppm as compared to those in TMBCA and MBCA. A characteristic singlet was detected in the range of δ = 5.33−5.52 ppm (m) indicating the existence of acrylate groups in pMACA and pTMBCA.39 In the synthesized polyurethane acrylates, the presence of the repeat unit as shown in (Scheme 2) with the desired hydrogen ratio was observed, which resulted from the reaction of prepolymer with MDI. The ratio of aromatic protons (assigned as b,c) due to the diisocyanate moiety with respect to the aromatic protons (assigned as f,g) due to the prepolymer moiety in the final structure was observed in the ratio of 1:1. Thus, in all of the polymerization reactions, there was strong evidence toward significant formation of polyurethane acrylates. X-ray Diffraction Analysis. The crystallinity of the synthesized polymers was investigated by the powder X-ray diffraction (XRD) technique. Peaks at lower angle were observed at 21.1° (pMACA), 20.8° (pTMBCA), and 21.0° (pMBCA), as shown in Figure 6. This indicates that the samples had some degree of crystallinity. The degree of crystallinity in the samples was determined from the XRD patterns using PDXL analysis software and was on the order of 23−29%. These peaks were assigned to the scattering from polyurethane chains with regular interplanar spacing.40 The interplanar spacing (d spacing) of the polyurethanes was in the range of 0.421−0.426 nm. The d spacing corresponding to the large peak(s) in the respective curves was calculated using
Figure 5. UATR-FTIR spectra of diglycidyl ether bisphenol A (DGEBA) and the synthesized PUAs (pMACA, pTMBCA, and pMBCA).
Table S1 of the Supporting Information). The stretching frequency for NH group in the PUAs was observed in the range of 3313−3307 cm−1. This band was absent in the IR spectrum of DGEBA. Hydrogen-bonding interactions are an important source of microphase separation, which significantly affects the thermal and mechanical properties of polyurethanes.34 H-bond formation between the NH hydrogen of the urethane group with either the O atom of a carbonyl group of the hard domain or an O atom of the soft domain was observed in the IR spectra of the polyurethanes. The Hbonding interactions split the stretching vibration band of the carbonyl group into two peaks due to H-bonded and non-Hbonded carbonyl groups. A shoulder was observed in the range of 1768−1775 cm−1 that can be attributed to disordered Hbonded carbonyl groups as compared to the ordered hydrogenbonded carbonyl groups with a peak value in the range of 1689−1718 cm−1.35−37 For the PUAs reported herein, the NH peak was broad in the range of 3313−3309 cm−1. This suggests that the formation of H-bonds was due to the involvement of the carbonyl O atom only.38 Two characteristic absorption bands were observed for the oxirane ring of DGEBA: the first centered at 915 cm−1 due to C−O deformation and the second at 3050 cm−1 for the C−H stretching of the terminal oxirane ring. In the synthesized PUAs, these two band positions remained almost constant in the ranges of 916−914 and 3056−3054 cm−1, respectively (although the intensity changed; Figure 2). The most important feature in the FTIR spectra of all of these PUAs was the absence of an isocyanate (NCO) band in the range of 2270−2280 cm−1 and the appearance of a stretching carbonyl vibration, which was an indication for the formation of urethane (NHCOO) linkages. The CH symmetric and asymmetric stretching frequencies due to methyl and methylene groups were dominated by a band between 2970 and 2920 cm−1. In all of the synthesized PUAs, a band was observed in the range of 1718−1689 cm−1 that was assigned to 51
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function of sample temperature for PUAs under a nitrogen purge. The temperatures corresponding to the maximum degradation (Tmax) and the onset of degradation (Ti), the percentage of residue, and the maximum rate of degradation for all samples are reported in Table S3 (Supporting Information). The TGA results show that the PUAs started to undergo thermal degradation in the temperature range of 347−360 °C and with a total mass loss in the range of 84−96%. A small amount of carbon residue (2−16%) remained after the sample was heated to 800 °C. The thermal degradation of polyurethanes occurs in two steps: The first stage is mainly governed by the degradation of the hard segments, and the second stage correlates well with degradation of the soft segments.41,42 The initial mass loss during the thermal degradation of the PUAs might be due to the relatively low thermal stability of the urethane acrylate terminal group, whereas the later mass loss might be associated with decomposition of the soft segments. The typical weight residue is a carbonaceous material formed during the thermal degradation. Lyon et al. reported that the char yield is an indirect way of measuring fire-retardant properties.43 Tmax (°C) for pMACA was found to be 390 °C, whereas the Tmax values for pTMBCA and pMBCA were recorded at 410 and 460 °C, respectively. Thermogravimetric anlaysis showed that the thermal decomposition temperature (Td = 10% weight loss temperature under nitrogen) was in the range of 300−375 °C. By examining the trends in the 10% weight losses, we can conclude the addition of methyl groups into the soft segments increases the initial thermal stability of these polyurethanes.
Figure 6. X-ray diffractograms of different PUAs.
Bragg’s equation taking the wavelength as 0.154 nm. The value of d (nm) is reported in Table S2 (Supporting Information). Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA). Figure 7 shows the thermal analysis results obtained for the PUAs. The plots show weight loss as a
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CONCLUSIONS New polyurethane acrylates were synthesized efficiently and characterized by the FTIR, NMR, TGA-DTA, XRD, and GPC techniques. We studied the effect of substituted methyl groups on the reactivity of acrylic acid derivatives based on diglycidyl ether bisphenol A. The reactivity was highest when there were two methyl groups on the C3 carbon atom. The experimental results were in line with those predicted from computational studies based on DFT calculations using the B3LYP/631G(d,p) basis set and structures. The GPC molecular weight distribution supported the polymerization of the synthesized PUAs with a polydispersity index in the range of 1.14−1.26. In this reaction, the disappearance of the NCO absorption peak at 2270 cm−1 and the OH absorption peak in the range of 3309−3391 cm−1 and subsequent appearance of a CO vibration were indicative of the formation of urethane linkages. The formation of urethane linkages was further supported by 1 H NMR spectra of polyurethanes. Wide-angle X-ray diffraction suggested that the PUAs had crystallinities in the range 23− 29%. The thermal stability of the synthesized PUAs, as ascertained from Tmax, was high. The obtained polyurethane acrylates could be used as potential candidates in the paint industry.
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ASSOCIATED CONTENT
S Supporting Information *
Band position assignments in UATR-FTIR spectra, d-spacing values, thermal parameters, and 1 H NMR spectra of compounds 2a−2c. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 7. (a) TGA and (b) DTG curves of polyurethane acrylates. 52
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. Present Address §
(for A.K.B.) Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, India. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors express sincere gratitude to IIT Indore and IIT Kanpur for providing analytical facilities required for recording 1 H NMR spectra. The authors also extend thanks to Dr. S. Ranganathan of IIT Patna for useful discussions on DFT calculations.
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