Abstract
Copolymer of tris(methoxyethoxy)vinyl silane (TMEVS) with N-vinyl pyrrolidone (NVP) was synthesized by free radical polymerization in dry benzene at 70°C using benzoyl peroxide (BPO) as initiator. The copolymer was characterized by viscometer, FTIR, and 1H-NMR and its thermal properties were studied by DSC and TGA. The copolymer composition was determined by elemental analysis. The monomer reactivity ratios were calculated by linearization methods proposed by Fineman-Ross and Kelen-Tudos. The intersection method was proposed by Mayo-Lewis and nonlinear method was proposed by curve-fitting procedure. The microstructure of copolymer and sequence distribution of monomers in the copolymer were calculated by statistical method.
1. Introduction
Copolymerization is the most successful and powerful method for effecting systematic changes in polymer properties [1]. Copolymerization modulates both the intramolecular and intermolecular forces exercised between like and unlike polymer segments and consequent properties such as glass transition temperature, melting point, solubility, permeability, dye ability, adhesion, elasticity, and chemical reactivity may be varied within wide limits. The utility of copolymerization is exemplified on one hand by the fundamental investigations of structure property relation and on the other hand by the wide range of commercial and biology applications [2, 3].
Reactivity ratios are among the most important parameters for a composition equation of copolymers, which can offer information such as relative reactivity of monomer pairs and estimate the copolymer composition. Knowledge of the copolymer composition is an important step in the evolution of its utility. Copolymer composition and its distribution are dependent on the reactivity ratios. The calculation of the monomer reactivity ratios requires the mathematical treatment of experimental data on the compositions of copolymers and monomers feed mixtures. Many methods have been used to estimate reactivity ratios of a large number of comonomers [4]. In order to determine the amount of the comonomer that has been incorporated into the copolymer, various analytical methods must be used: nitrogen analysis, Fourier Transform Infrared Spectroscopy, Proton and Carbon Nuclear Magnetic Resonance, and ultraviolet-visible spectroscopy [5–7].
The interests in polymers and copolymers which contain silicon atom have been increasing due to their hydrophobicity and wide applications as optical, semiconductor materials, and electric and ceramic processes. In radical copolymerization of vinyl silane monomers with various other monomers, the reactivity of vinyl silane monomers is strongly depending on the position of the silicon atom relative to the vinyl group [8, 9]; the reactivity ratio of these monomers is zero if the Si atom is beside the vinyl group because of d–p interactions between the vinyl group and the Si atom. On the other hand, it should be possible to increase the reactivity of vinyl silane monomers by copolymerization with comonomers that have poor reactivity with themselves. In fact, vinyl silane monomers have been successfully copolymerized with styrene, butyl acrylate methacrylic acid, methyl methacrylate, acrylonitrile, 2-vinyl pyridine, vinyl sulfonate esters, and other monomers [10–14].
N-Vinyl pyrrolidone (NVP) and its copolymers are attracting much attention and have been widely investigated for applications in different field due to its low toxicity, biocompatibility, and good film form [15–22]. The amide group in NVP is highly polar, which confirms that it is hydrophilic. NVP has been copolymerized with various monomers [23–25] because the amide group has high affinity for many large and small molecules that are known to be good hydrogen-bond acceptors. NVP has been copolymerized with various siloxane derivatives monomers such as vinyl triethoxy silane, vinyl trimethoxy silane, vinyl trimethyl silane, vinyl trichlorosilane, and 3-(trimethoxysilyl)propyl methacrylate. In addition, the reactivity ratios of these copolymers have been determined [26–29], but no one has copolymerized and studied the reactivity relationships of NVP with tris(methoxyethoxy)vinyl silane (TMEVS).
The aim of this work is the copolymerization of TMEVS, a hydrophobic monomer with NVP, a hydrophilic monomer, to find the best synthetic conditions and characterization of the copolymer. To determine the reactivity ratios of TMEVS and NVP, from these parameters, a specific comonomer distribution should be estimated.
2. Experimental Section
2.1. Materials
Commercial samples of NVP and TMEVS from Aldrich chemical were distilled under vacuum before copolymerization. Benzoyl peroxide was recrystallized twice from chloroform and dried in a vacuum. Solvents of 99% purity grade were used as received.
2.2. Synthesis of Copolymers
Copolymerization of NVP with TMEVS was carried out by using (1 × 10−3 mol/dm3) BPO as initiator and dry benzene as solvent at 70°C in a glass tube. The total molar composition of the monomer mixture was maintained at 1 mol/dm3 while the monomer feed ratio was varied in a series of copolymerizations of NVP and TMEVS (NVP-CO-TMEVS). Nitrogen gas was bubbled through the mixture for 10 minutes prior to the reaction in order to remove all oxygen. Copolymerization time was controlled to obtain low conversion (<10%). The copolymers were isolated by precipitation in diethyl ether. The precipitates were filtered off and purified by dissolving again in benzene and precipitated in diethyl ether. Copolymers were dried in vacuum at 40°C until constant weight. The reaction of copolymerization is shown in Scheme 1.
2.3. Copolymer Characterization
Perken Elmer-1650 spectrometer was used to record FTIR spectra of the copolymers on KBr Pellets in the range 200–4000 cm−1. The H-NMR was recorded with a JOEL JMTC-500/54/SS (500 MHz) spectrometer using DMSO as solvent and tetramethyl silane as internal standard. The solubility of the copolymers was studied using various organic solvent and water at room temperature for 24 hours. Intrinsic viscosity [] was determined according to the Solomon Gottesman relationship [30] by using an Ostwald Viscometer with negligible kinetic energy correction. Copolymer compositions were estimated by elemental analysis following the variation of estimated nitrogen content arising from vinyl pyrrolidone comonomers units. Thermal degradability of the polymer was studied by TGA using Perkin Elmer in a nitrogen atmosphere at a heating rate of 10°C/min from 0 to 800°C and glass transition temperature () was determined using a DSC-Mettler calorimetric system.
3. Results and Discussion
The structure of the copolymer is confirmed by FITR as shown in Figure 1. The comparative study between the FTIR spectra of the copolymer and monomers shows disappearance absorption bands of the monomers: stretching C–H of double bond 3010–3100 cm−1, stretching C=C of the vinyl group at about 1610–1680 cm−1, and bending C–H of the vinyl group at about 900–1000 cm−1. In addition, many absorption bands appear in the FTIR spectra of the copolymer which belong to the stretching vibration in different functional groups of corresponding monomers: 2935 (Alkan C–H), 1655 (tertiary amide carbonyl), 1422 (amide C–N), 1278 (ether C–O), 1080 (Si–O), and 751 cm−1 (Si–C).
Figure 2 shows H-NMR spectrum of the copolymer. When the copolymer and monomers spectra are compared in H-NMR study, the signal belongs to the vinyl protons of the monomers that disappeared from the region close to 6.0 ppm. Moreover, for the copolymer, methylene protons in NVP ring resonate in 2.5, 3.2, and 4.4, while CH2 protons main chain backbone of monomers resonates at 1.8–2.4. The signal corresponding to the protons of the ethoxy groups linked to the silicon atom in TMEVS at about 3.6 can be clearly observed and the protons of ethoxy groups linked to methyl groups at 3.4. Signals for CH3 groups of TMEVS appear at 3.3. CH protons of the two monomers resonate at 4.6.
3.1. Copolymer Composition
It is very important to study the comonomer reactivity in the copolymers systems because the composition of these copolymers depends on the monomer feed composition. In TMEVS/NVP polymer, composition of the monomer in the copolymer was determined by estimating N% in the copolymer and this percentage indirectly gave the mole fraction of NVP in the copolymer. The copolymers are insoluble in most of the solvents but they are soluble in some specific solvents, like benzene, DMF, and DMSO. The monomer composition and the results of elemental analysis in addition to intrinsic viscosity values [] for samples of seven different compositions are listed in Table 1. The values of [] should be used in estimating qualitatively degree of polymerization.
3.2. Reactivity Ratio
The most common mathematical model of copolymerization is based on finding the relationship between the composition of copolymers and the composition of the monomer feed in which the monomer reactivity ratios are the parameters to be determined [31].
In our investigation several methods have been proposed for the best fitting of () pair from a set of [], [], , and pair, using linearization methods proposed by Fineman and Ross [32], Kelen and Tüdös [33], and Mayo and Lewis [34] and with the nonlinear method proposed by curve-fitting [35] procedures, data are given in Tables 2 and 3 and showed in Figures 3, 4, 5, and 6, respectively.
One has the following:
For mathematical details of these procedures, the original papers [32–35] should be consulted. The values from the different methods are very close, even those obtained by the inverse Fineman-Ross graph.
Table 4 shows the values of reactivity ratios by different methods. With the values of and , the variation of the instantaneous mole fraction of NVP in copolymer (at low conversion) with the mole fraction in the initial feed may be calculated using the copolymer composition equation in the form
Figure 6 shows the theoretical curve (solid line) follows closely the experimental copolymer composition data, in which we have the following:(a)For (the azeotropic composition) .(b)For , .
An azeotropic composition is possible when and are both greater than 1 and less than 1. This condition is fulfilled in TMEVS/NVP system since and are both less than unity. The corresponding azeotropic feed composition (az.) is given by
A value of 0.41 obtained for (az.) in TMEVS/NVP system.
In this study, TMEVS was copolymerized with NVP in spite of the fact that the copolymerization of vinyl silane derived monomers is difficult [36–38].
The reactivity of TMEVS 0.5 is of the same order as reactivity of NVP 0.32 due to the electron attraction and electronic delocalization effects of NVP and oxygen atoms of TMEVS and for this reason attendance to random copolymer forming can be postulated; that is, both and are between 0 and 1.
There are six electrons-attracting oxygen atoms for each monomer unit of TMEVS, giving rise to a significant attraction of the free electron generated in the growing polymer chain with corresponding stabilization of the […TMEVS•] macroradical; for this reason the value of reactivity of TMEVS (0.5) is more than the reactivity of vinyl trimethyl silane VTMS (0.074), vinyl trimethoxy silane VTMOS (0.407), and vinyl triethoxy silane VTEOS (0.3), when they were copolymerized with NVP [8, 26].
3.3. Copolymer Microstructure
The statistical distribution of the monomers in the formation of the copolymer 1-1, 2-2, and 1-2 is listed in Table 5 and is calculated using the following relations [39]:where and are the mole fractions of TMEVS and NVP in the copolymer, and are the reactivity ratios, and , , and are the mole fractions of 1-1, 2-2, and 1-2 sequences, respectively.
The probabilities of finding the sequence of TMEVS and NVP units are listed in Table 6 and calculated as follows:where , , , and are the probability of a TMEVS or NVP unit to be followed by TMEVS or NVP unit. In these equations and are the mole fractions of TMEVS and NVP in the feed. The average length sequences of TMEVS and NVP are listed in Table 6 and calculated using the following equations:
The mole fractions of 1-1 and 2-2 sequences increase as the mole fractions of TMEVS and NVP increase. On the other hand, the 1-2 sequence is between 47 and 71, which indicates both the monomers have a tendency to react with other monomers in the growing chain and form random copolymer and this is in agreement with the values of (0.5) and (0.32); both are between 0 and 1.
The mean sequence length of TMEVS varied from 2.081 to 1.079 for these copolymer compositions; values of NVP were between 1.097 and 3.204.
3.4. Thermal Properties
The thermal properties of copolymer TMEVS/NVP were studied by TGA and DSC. TGA was carried out in the temperature range of 0–800°C under nitrogen atmosphere and is presented in Figure 7 with initial thermal decomposition at 220°C, 10% at about 395, 50% at about 465, and 30% residual at 800°C. Figure 8 shows the DSC thermogram of copolymer indicating being 290°C.
4. Conclusion
TMEVS/NVP copolymer was synthesized by free radical polymerization. The structure of copolymer was confirmed by FTIR and 1H-NMR techniques. Copolymer compositions were obtained by elemental analysis. The reactivity ratios were obtained by Fineman-Ross, Kelen-Tudo, Mayo-Lewis, and curve-fitting methods and a good agreement was observed between the various methods. The value of is of the same order as due to the electronic delocalization and electronegative oxygen atoms of NVP and TMEVS so this copolymer can be classified as a random copolymer. The microstructure and sequence distribution are in agreement with the values of reactivity ratios.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.