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ORIGINAL PAPER
Synthesis of PNIPAM–PEG Double Hydrophilic Polymers Using
Oleic Acid Macro Peroxide Initiator
Baki Hazer1 · Elif Ayyıldız1 · Faruk Bahadır1
Received: 17 May 2017 / Revised: 7 July 2017 / Accepted: 10 July 2017 / Published online: 19 July 2017 © AOCS 2017
chemical products derived from petrochemical feedstocks [1, 2].
Vegetable oils obtained from the annually harvested seeds of plants are very important raw materials. Soybean oil is an abundant, inexpensive and commercially avail-able vegetavail-able oil consisting of unsaturated fatty acids such as oleic, linoleic and linolenic acids. Allylic hydro-gens, ester groups and double bonds of soybean oil are reactive groups that can be used for derivatization [3–7]. The unsaturated fatty acid content is readily susceptible to autoxidation. This autoxidation process does not need any extra heating or irradiation. To obtain oxidized soybean oil polymer, soybean oil is spread on a glass container and exposed to air oxygen under atmospheric conditions at room temperature for 3–4 weeks [8–12]. During autoxida-tion, oxygen attacks the allylic hydrogen causing hydrogen abstraction, which leads to the formation of radicals onto this carbon atom. Then, air oxygen is attached to this car-bon to form hydroperoxide, peroxide and epoxide groups. During this time, polymerization of soybean oil is car-ried out by peroxide linkages. In our very recent articles, the effect of gold and silver nanoparticles on the autoxi-dation process was studied in detail. Gold nanoparticles were strongly catalyzed during the autoxidation process, while silver nanoparticles did not show any catalyst effect throughout the same process [13, 14]. The autoxidation of pure unsaturated fatty acids such as linoleic acid and oleic acid also gives peroxide oligomers [15–18]. The peroxide derivatives of both unsaturated plant oils and fatty acids behave as macroperoxide initiators to start the free radical polymerization of vinyl monomers leading to graft copoly-mers [19].
Poly (N-isopropyl acrylamide) (PNIPAM) is one of the most widely studied thermo-responsive polymers and undergoes a reversible phase transition at low critical Abstract This work refers to the synthesis of a new
dou-ble hydrophilic thermo-responsive polymer using fatty acid macroperoxide initiator, N-isopropyl acryl amide (NIPAM) and polyethylene glycol with two primary amine ends (PEGNH2). For this purpose, oleic acid was spread out onto a petri dish and exposed to air oxygen at room tem-perature for 2 months. The obtained fatty acid macro-per-oxide initiator was used in the free radical polymerization of NIPAM in the presence of PEGNH2. Poly oleic
acid-g-PNIPAM-g-PEG graft copolymers were successfully
obtained. Lower critical solution temperature (LCST) of the graft copolymer was determined by using UV-Vis spec-trometry with a sensible heating unit. Morphology of the fractured surface of the double hydrophilic polymers was visualized by using SEM micrographs. Graft copolymers with LCST close to body temperature were obtained by changing PEG inclusion. Structural characterization, ther-mal analysis and size exclusion chromatography measure-ments of the obtained products were done.
Keywords Oleic acid · Autoxidation · Double hydrophilic polymer · PNIPAM · Thermoresponsive polymer
Introduction
Renewable resources are popular for the production of chemicals and polymers because of limited raw fossil mate-rials and the environmental issues resulting from many
* Baki Hazer bkhazer@beun.edu.tr
1 Department of Chemistry, Bülent Ecevit University, 67100 Zonguldak, Turkey
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solution temperature (LCST) around 30–32 °C in water [20–24].
Polyethylene glycol is an excellent biocompatible hydro-philic polymer [25]. It is used extensively as a biomate-rial in a variety of drug-delivery vehicles and biomedical implants. PEG in aqueous solution has a low interfacial free energy and exhibits a rapid chain motion. Also, its large excluded volume leads to steric repulsion of approaching molecules [26].
Double-hydrophilic block/graft copolymers (DHBCs) are amphiphilic copolymers containing two different water-soluble hydrophilic blocks. At least one of them is a stim-uli-responsive polymer. They are advantageous in biologi-cal applications and other aqueous systems [27–29]. There are fine reports on them because their stimuli responsive-ness in aqueous solutions is advantageous in biological applications and other aqueous systems. In this manner, Wu et al. reported a facile synthetic pathway to double-hydrophilic miktoarm star copolymers composed of PNI-PAM and PEG with multiple arms by combining reversible addition-fragmentation chain transfer, RAFT, the arm-first technique and aldehyde-aminooxy click reaction. In addi-tion, they also studied the unique thermo-responsive micel-lization behaviors of the resulting new double hydrophilic miktoarm star copolymer [30]. Dordovic et al. studied the coassembly of the star double-hydrophilic diblock copolymer [poly(ethylene oxide)-block-poly(2-methyl oxazoline)]4, [PEO-PMOX]4, linear double-hydrophilic diblock copolymers poly(ethylene oxide)-block-poly(2-ethyl oxazoline), PEO-PEOX and PEO-PEOX(2), and lin-ear double-hydrophilic triblock copolymer poly(2-ethyl oxazoline)-block-poly(ethyleneoxide)-block-poly(2-ethyl oxazoline), PEOX-PEO-PEOX, with the amphiphilic anion [3-cobalt(III) bis(1,2-dicarbollide)](−1). They concluded
that PEO/Na-dicarbollide and PEOX/Na-dicarbollide inter-action strengths are comparable [31]. Zhang et al. pre-pared a thermally sensitive ultra-long multiblock copoly-mer, poly(ethylene oxide)-b-poly(N-isopropylacrylamide), using the oxidative coupling of two mercapto groups at the two ends of triblock PNIPAM-b-PEO-b-PNIPAM. They studied the folding of individual multiblock copolymer chains in an extremely dilute solution by laser light scat-tering [32]. Sahn et al. obtained a series of well-defined poly(N-isopropylacrylamide)-b-poly(2-ethyl-2-oxazo-line-b-poly(N-isopropylacrylamide) triblock copolymers via RAFT polymerization of N-isopropylacrylamide using a new bifunctional cationic ring opening polymerization initiator with a larger biphenyl spacer. The influence of the ratio of the polymer blocks on the thermo-responsive prop-erties is provided by the physicochemical and optical meth-ods [33].
In the 1990s, combination polymerization reactions such as cationic to radical transformation reactions were used
to prepare block copolymers [34–37]. In a similar manner, recently a one-pot polymerization reaction has become very popular for combining the different blocks to prepare poly-mers but to avoid purifying intermediates, which saves time and resources. Hence, this provides facile access to poly-mers with various functionalities and architectures [38–43]. In the present work, we report a novel one-pot synthe-sis of a double hydrophilic copolymer using a novel green macroperoxide initiator via combination of free radical polymerization and amidation condensation reactions. First, oleic acid was separately autoxidized in petri dishes for several weeks at room temperature to obtain unsaturated fatty acid macroperoxide initiator with carboxyl groups. The double hydrophilic poly NIPAM-g-polyoleic acid-g-PEG graft copolymer was produced by the reaction of the mixture containing NIPAM, PEGNH2 and macroperoxide fatty acid under nitrogen atmosphere at 95 °C for 5 h. Phys-icochemical and thermal characterizations of the obtained copolymer were done.
Experimental
Materials
Diamine-ended polyethylene glycol with molar mass, MW ~2000 g/mol (Jeffamine D-2003; PEG2KNH2) [O, O′-bis (2-aminopropyl) polypropylene glycol–block polyethyl-ene glycol–block-polypropylpolyethyl-ene glycol] was a gift from Huntsman Co. (Istanbul). The mol ratio of ethylene gly-col units to propylene glygly-col units was 36/9. Oleic acid (purity: 85–88 wt%) was kindly given by “CHS Endüstriyel Ürünler San. Tic. A.Ş. Büyükdere Caddesi No: 122 A Blok Kat:2 Esentepe Istanbul.” Tetrahydrofuran (THF) and other chemicals used in this work were supplied from Sigma-Aldrich.
Autoxidized Polymeric Oleic Acid (Pole)
Autoxidation of oleic acid was performed according to the modified procedure reported in a recent work [13]. Briefly, 18 g of oleic acid was spread out onto a petri dish (Φ = 14.5 cm, oil thickness: 1.0 mm) and was exposed to daylight in the air at room temperature. After a given time of autoxidation (ca. 4 month), a sticky, pale yellow viscous liquid polymer layer was formed. However, the autoxida-tion time for oleic acid was ten times higher than that of linoleic acid. It was reported that the relative rates of autox-idation of oleic (18:1), linoleic (18:2), linolenic (18:3), arachidonic (20:4), and docosahexaenoic acid (22:6) were found to be relatively 1- to ∼40-, 100-, 200-, and 400-fold higher than that of oleic acid, respectively [44].
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Fractionation of the Oxidized Samples
Fractionation of the oxidized oleic acid polymer was per-formed by the procedure described in our recent study [13]. For example, the crude oxidized oleic acid polymer (40 g) was dissolved in 20 mL of CHCl3. Then, 800 mL of petroleum ether was added to this solution. After stirring for 2 h, it was kept in a refrigerator overnight. The upper solution phase was decanted. Later on, the precipitate was leached with petroleum ether, and then the resulting upper solution was decanted. The residue obtained was dried under vacuum at room temperature for 24 h. The petroleum ether phases were combined, and the solvent was then evaporated. The liquid precipitate was dried in a vacuum oven at room temperature for 24 h and kept as the main fraction.
Peroxygen Analysis of the Oxidized Samples
Peroxide analysis of Pole was carried out according to the modified procedure reported in [45].
Briefly, a mixture of Pole (0.2 g), 2-propanol (30 mL), acetic acid (5 mL), and saturated aqueous solution of KI (1 mL) was refluxed for 10 min. The released iodine was then titrated against to 0.1 N sodium thiosulfate solu-tion. The peroxygen percent of the Pole was calculated by using the following equation (Eq. 1):
where V (mL) is the volume of 0.1 M sodium thiosulfate solution and m (g) is the amount of Pole sample.
Acid Value Determination of the Oxidized Samples Approximately 0.2–0.5 g of oxidized oleic acid sample was dissolved in a mixture of ethanol (96 wt%, 25 mL) and diethyl ether (25 mL). Then, this solution was titrated with 0.1 N KOH ethanol solution in the presence of phe-nolphthalein (10 mg). Oleic acid (wt%) was calculated using the following equation (Eq. 2):
where V (mL) is the volume of 0.1 N KOH ethanolic solu-tion, and m (g) is the oxidized oleic acid sample weight. Free Radical Polymerization of NIPAM Initiated with Pole
Free radical polymerization of NIPAM was initiated by Pole according to the modified procedure described in the cited literature [13]. For a typical polymerization experiment, the mixture of 2.0 g of NIPAM and 0.5 g of (1) Peroxide (wt%) = {[V × 0.1 x 16] /10 × m},
(2) Oleic acid (wt%) = V(mL)
m(g) ×2.82,
Pole was dissolved in 5 mL of toluene in a reaction bot-tle. Argon was introduced through a needle into the tube for about 3 min to expel the air. The tightly capped bottle was then put into a water bath at 95 °C for 6 h. Then, the contents of the tube were precipitated in 100 mL hex-ane. The graft copolymer samples (PNole) were dried overnight under vacuum at 30 °C. Polymer film (approxi-mately 0.5 mm thickness) was cast from their chloro-form solution. For this purpose, a solution of 0.50 g polymer in 15 mL chloroform was poured in a petri dish (Φ = 5.0 cm). The petri dish was covered with a piece of paper, and the chloroform was allowed to evaporate for 2 days.
One‑Pot Synthesis of Double Hydrophilic Copolymers A combination of the free radical and condensation polym-erization was used to prepare the double hydrophilic poly-mer. For example, the mixture of 2.0 g of NIPAM, 2.0 g of Pole, and 2.0 g of PEGNH2 was dissolved in 5 mL of THF in a glass bottle. Argon was introduced through a needle into the tube for about 1 min to expel the air. The tightly capped bottle was then put into a water bath at 95 °C for 5 h. Then, the solvent was evaporated and the crude poly-mer was precipitated in petroleum ether. The graft copoly-mer samples were dried overnight under vacuum at 30 °C. Instrumentation
Molecular weights were determined by using a size exclu-sion chromatography instrument, Viscotek GPCmax Auto sampler system, consisting of a pump, three ViscoGEL GPC columns (G2000H HR, G3000H HR, and G4000H HR), and a Viscotek differential refractive index (RI) detec-tor with a THF flow rate of 1.0 mL/min at 30 °C. A calibra-tion curve was generated with three polystyrene (PS) green standards, 2960, 50,400, and 696,500 Da, of low polydis-persity. The polymer sample solution containing 0.05 g in 10 mL of THF was filtered from a 0.45-um filter and injected automatically into the instrument. Data were ana-lyzed using Viscotek Omni SEC Omni 01 software.
Proton NMR spectra in CDCl3 solutions of the samples
were taken at a temperature of 25 °C with an Agilent NMR 600 MHz NMR (Agilent, Santa Clara, CA, USA) spec-trometer equipped with a 3-mm broadband probe.
Fourier-transform infrared spectroscopy (FTIR) spectra were recorded with a PerkinElmer Spectrum 100 Model FTIR spectrometer in transmissive mode and scan rate 4000 to 650 cm−1.
Cloud points were determined in an Agilent Technolo-gies Cary 60 UV-Vis Spectrometer connected to a Cary Single Cell Peltier Accessory. Prior to starting the measure-ment, all sample solutions were kept at 20 °C for 10 min
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inside the instrument. Subsequently, the solutions were heated and cooled at a rate of 1 K min−1 in a temperature
range between 20 and 40 °C. Three consecutive heating/ cooling cycles were performed without interruption of the measurement using the previously defined heating pro-gram. The cloud point temperature (Tcp) was defined as the
temperature where the transmittance decreased 50% in the second heating run.
Thermal analysis of the obtained polymers was carried out under nitrogen through a TAQ2000 DSC and Q600 Simultaneous DSCTGA (SDT) series thermal analysis sys-tem. Differential scanning calorimeters (DSC) measured temperatures and heat flows associated with thermal transi-tions in the obtained polymer samples. The dried sample was heated from −60 to 190 °C under nitrogen atmosphere. Thermogravimetric analysis (TGA) measured weight loss of the samples under nitrogen atmosphere heating from 20 to 600 °C at a rate 10 °C/min.
Results and Discussion
Synthesis of Oxidized Oleic Acid Polymer
Polyunsaturated vegetable oils and fatty acids (e.g., linoleic acid and linolenic acid) have been used previously in autoxi-dation reactions; however, in this study oleic acid was sub-jected to autoxidation to obtain the oleic acid macroperoxy initiator [8, 9, 14, 16, 22, 45]. The structural characteriza-tion of the oleic acid peroxy initiator was simpler than that of the former ones. In addition to this, in case of the oleic
acid macroperoxy initiator, the higher polymerization tem-peratures (e.g., 95 °C) led to a high yield of graft copolymer without crosslinking while the polyunsaturated fatty acids were polymerized at lower temperature (e.g., 80 °C) to avoid crosslinking. Soybean oil autoxidation was also carried out and used in the copolymerization experiments leading to the graft copolymer causing partial crosslinking. As a result, we completed the autoxidized soybean oil and its fatty acids separately and confirmed that both soybean oil and unsatu-rated fatty acid autoxidation led to a macroperoxide initiator to obtain partially green graft copolymers. Another advan-tage of using oleic acid is the less-complicated autoxidation and graft copolymerization due to the presence of only one double bond and two allylic groups for peroxide formation. In addition, a new one-pot polymerization system was also applied in case of oleic acid. First, oleic acid macroperox-ide (Pole) was prepared via autoxidation of the oleic acid. This process is carried out by the reaction between air oxygen and oleic acid under daylight at room temperature. The proposed autoxidation mechanisms leading to oleic acid macroperoxide can be seen in Scheme 1a. During the autoxidation, oxygen attacks the allylic hydrogen causing hydrogen abstraction, which leads to the formation of the radical on this carbon atom. Then, an air oxygen molecule is attached to this carbon to form hydroperoxides and per-oxides [46]. The calculated weight percent of the peroxide content of the Pole sample was found to be 0.81 wt%.
Oleic acid determination of the oxidized oleic acid was done three times. Taking the precursor oleic acid as 100 wt%, the amount of oleic acid in the Pole4 m was found to be 87 wt%. The decrease in the amount of carboxylic acid
Scheme 1 a The proposed
autoxidation mechanism leading to oleic acid macroperoxide;
b one-pot synthesis of double
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in the oxidized oleic acid can be attributed to some side reactions such as formation of small molecules that can be removed by fractionation reactions and/or esterification of the free carboxylic acid with hydroperoxide. Exact analy-sis is needed to understand what causes the carboxylic acid decrease during autoxidation.
The structural characterization of Pole was performed with the 1H NMR spectroscopy technique. Figure 1 shows
the 1H NMR spectra of pure oleic acid and Pole for
com-parison. The characteristic signals of pure oleic acid [47] were observed in the 1H NMR spectrum of Pole with
addi-tional signals of –CH-OO- at 3.5–4.1 ppm.
Intensity of the signals of the allylic methylene protons at 2.0 ppm and the vinyl protons at 5.3 ppm decreased when compared with those of the pure oleic acid. The decrease in the intensity of allylic –CH = CH-CH2- signal at 2.0 ppm verifies the peroxide formation on this allylic methylene group via autoxidation [46]. As for the signal of the vinyl protons at 5.3 ppm, the decrease in the intensity of the signal can be related to the side reactions of the double bond such as peroxyl radical cyclization [12] and hexanal formation [48].
Oleic acid was autoxidized for several months. In the autoxidation process, the slowest oxidation was observed for oleic acid [44] when compared to the molar masses of linoleic acid (Mn ~4000 g/mol) and linolenic acid (~6000 g/mol) for 1 mm of the fatty acid thickness [17].
The molar masses of oleic acid macro-peroxides were determined by size exclusion chromatography (SEC, also called GPC) in view of the Mn (number average molar mass), Mw (weight average molar mass) and D (dispersity). Their chromatograms were multimodal, as shown in Fig. 2. When their chromatograms were analyzed, the molar masses changed from 1760 to 3620 g/mol. In this work, several oxidized oleic acid macroperoxides were obtained from 2 months to up to 4 months. Four-month-autoxidized oleic acid macroperoxide (Pole4 m) was used as an initiator to obtain block copolymer. Two different molar masses (1. Mn: 3620 g/mol, Mw: 3760 g/mol, D: 1.04; 2. Mn: 1760 g/ mol, Mw: 1800 g/mol, D: 1.03) were observed. This shows that Pole is composed of nearly 6 to 12 oleic acid repeating units. Fractional precipitation was performed by extract-ing the viscouse oil with a solvent mixture of chloroform and petroleum benzene having the volume ratio of 1:20.
Fig. 1 1H NMR spectra of Pole and pure oleic acid
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However, this technique was not useful to separate different molecular weights of the fatty acid fractions.
One‑Pot Synthesis of Double Hydrophilic Polymer Oleic acid macro-peroxide initiator (Pole) was used in the free radical polymerization of N-isopropyl acryl amide (NIPAM) in the presence of poly ethylene glycol with two amine ends (PEG2003) to obtain PNIPAM-g-Pole-g-PEG double hydrophilic polymer via one-pot synthesis. The reaction conditions and results are listed in Table 1.
One-pot polymerization reactions can be designed as shown in Scheme 1b. A series of double hydrophilic poly-mers were synthesized in this work to examine the variation of the molar mass against [Pole]-[PEGNH2]+ salt initiator
in feeding. Figure 3 shows the graphs of molar masses and Ds vs. [Pole]−[PEGNH2]+ salt initiator (mmol) in feeding.
The molar masses of the polymers obtained were found to range between 1757 and 3621 g/mol, while dispersity changed from 1.11 to 1.41. An increase in the concentration of the salt initiator in feeding caused a decrease in molar mass of the double hydrophilic polymer that was obtained. Dispersity also increased with the rise in the salt concentra-tion. One can say that the radical transfer to the salt ini-tiator increased as the salt iniini-tiator concentration increased. As a result, this caused greater dispersity and smaller molar masses.
To comparatively characterize the double hydrophilic polymer, free radical polymerization of NIPAM initiated by Pole was performed in the absence of PEG-NH2. The reac-tion condireac-tions and polymerizareac-tion results of PNIPAM-g-Pole diblock copolymers are collected in Table 2.
The structural characterization of the diblock and dou-ble hydrophilic triblock copolymer was performed by 1H
NMR and FTIR techniques. 1H NMR analysis confirmed
the characteristic signals of double hydrophilic polymer, PNIPAM-g-Pole-g-PEG tri block copolymer. Figure 4 shows the 1H NMR spectra of a PNIPAM-g-Pole and
PNI-PAM-g-Pole-g-PEG samples. The PEG signal at 3.6 ppm was a typical characteristic of the double hydrophilic polymer. PEG contents of double hydrophilic polymers
Fig. 2 GPC chromatogram of Pole4 m
Table 1 Reaction conditions
and results of one-pot synthesis of double hydrophilic polymers (95 °C for 5 h in 5 mL of toluene)
Code NIPAM
(g) mmol Pole (g) mmol PEG-2003 (g) mmol Yield (g) Mn (g/mol) Mw (g/mol) D (g/mol) EPNole-PEG-11 2.00 17.7 0.20 0.15 0.30 0.15 1.20 21,000 28,500 1.35 EPNole-PEG-17 2.02 0.26 0.20 0.35 0.17 1.17 23,900 26,400 1.11 EPNole-PEG-22 1.99 0.29 0.22 0.47 0.23 1.58 16,300 22,100 1.36 EPNole-PEG-12 2.00 0.30 0.45 1.63 EPNole-PEG-21 2.01 0.31 0.46 1.55 EPNole-PEG-18 2.06 0.32 0.46 1.92 EPNole-PEG-13 2.00 0.40 0.30 0.60 0.30 1.49 15,100 20,100 1.33 EPNole-PEG-19 1.99 0.42 0.60 1.45 EPNole-PEG-20 2.01 0.59 0.45 0.93 0.46 2.53 14,600 17,900 1.23 EPNole-PEG-14 2.00 1.00 0.77 1.50 0.75 1.21 14,700 20,700 1.41
Fig. 3 Synthesis of PNIPAM-g-Pole-g-PEG double hydrophilic
polymers. Graphs of molar mass and D vs. [Pole]−[PEGNH2]+ salt (mmol) in feeding
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were calculated from their 1H NMR spectra in the range
of 5–12 mol% of PEG units. The molar masses of the diblock and the double hydrophilic polymers prepared by polymerization of NIPAM (Tables 1 and 2) decreased with increased initiator feed ratio. Presumably, the same
amount of functionalization in the copolymers seemed to be higher in case of copolymers with low molar mass. This needs further detailed kinetics investigations, such as the
Table 2 Free radical
polymerization of NIPAM initiated by Pole (95 °C for 5 h in 5 mL of toluene)
Code NIPAM (g) Pole4 m (g) Yield (g) Mn (g/mol) Mw (g/mol) D
PNole- 3 2.0 0.5 1.65 29,100 42,000 1.44
PNole- 4 2.0 1.0 0.88 26,600 37,400 1.40
PNole- 5 2.0 2.0 0.85 17,800 33,800 1.90
Fig. 4 1H NMR spectra of (a) PNIPAM-g-Pole-g-PEG and (b) PNIPAM-g-Pole block copolymers
Fig. 5 FTIR spectra of PNIPAM-g-Pole-g-PEG triblock copolymer
PEG-20-2) and PNIPAM-g-Pole diblock copolymer (PNole-5)
Fig. 6 The graphs plotted by transmittance versus temperature for
the aqueous solutions of the double hydrophilic polymer samples: a EPNole-PEG-11, b EPNole-PEG-18, c EPNole-PEG-20
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chain transfer constants of the initiator salt and rough GPC measurement of water-soluble copolymer against polysty-rene standards.
As for FTIR spectra of the block copolymers, in the case of PNIPAM-g-Pole-g-PEG triblock copolymer, the car-bonyl signal disappeared while PNole contained carcar-bonyl band at 1710 cm−1 coming from –COOH groups of oleic
acid inclusion. FTIR spectra of PNIPAM-g-Pole-g-PEG tri-block copolymer and PNole can be seen in Fig. 5. During the double hydrophilic polymer synthesis, the carboxylic acids react together with primary amine terminal groups of the PEGNH2 to produce amide groups between oleic acid and polyethylene glycol units. In addition, the FTIR spectrum of the double hydrophilic copolymer contains the characteristic additional PEG signal at 1100 cm−1 (Fig. 5).
The temperature dependence of the aqueous solution of the double hydrophilic polymer was measured as the transmittance % of the polymer solutions in a UV-Vis spec-trometer with a heating unit connected to a computer. The lower critical solution temperatures (LCST) of the aque-ous solution of polymer samples were determined from the graphs plotted by transmittance versus temperature. Fig-ure 6 shows the graphs plotted by transmittance versus tem-perature for the aqueous solutions of the double hydrophilic polymer samples. PEG inclusion caused a higher LCST than that of homo PNIPAM. The increase in PEG units gave a double hydrophilic polymer with LCST up to 36 °C (body temperature), which can impart favorable properties for drug delivery systems [49].
Thermal properties of the double hydrophilic polymers were determined by using the DSC-TGA instrument. The DSC thermograms of some double hydrophilic polymer samples are presented in Fig. 7. The melting temperature (Tm) of PNIPAM disappears while PNIPAM has a Tm at
around 140 °C [50]. Tg’s of the triblock copolymers are at around −8 to −9 °C in their DSC thermograms. Presum-ably, Pole and PEG blocks lower the Tg of pure PNIPAM.
Additionally, the crystallinity of the PEG units disappears in the triblock copolymer structure.
Thermogravimetric analysis is a useful tool to determine the change in the mass of a sample as a function of tem-perature under inert atmosphere. Weight loss of the poly-mer samples obtained was studied in this work. Figure 8 shows the TGA curves of the polymer samples obtained in
Fig. 7 DSC thermograms of the double hydrophilic polymers: a
EPNole-PEG-11; b EPNolePEG-13; c EPNolePEG-14
Fig. 8 TGA curves of the polymer samples: autoxidized oleic acid
polymer (Pole4 m), Pole-g-PNIPAM (PNole5, PNole8, and PNole10), Pole-g-PNIPAM-g-PEG (EPNole-PEG-20, EPNole-PEG-11, and EPNole-PEG-18)
Table 3 Thermo-gravimetric
analysis of the polymers obtained
Polymer sample Td1 (oC) T
d2 (°C) Td3 (°C)
Start End Start End Start End
Pole-4 m 220 290 345 380 390 500 PNole-5 220 340 360 400 400 540 PNole-8 240 360 370 410 400 475 PNole-10 250 400 – – 400 490 EPNole-PEG20 290 405 – – 410 505 EPNole-PEG11 290 405 – – 400 520 EPNole-PEG18 320 405 – – 410 490
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Fig. 9 Fractural surface micrographs of polymer samples: a
EPNole-PEG-20: scale bar: 50 μm; a′ EPNole-EPNole-PEG-20: scale bar: 5 μm; b EPNole-PEG-17, scale bar: 50 μm; b′ EPNole-PEG-17, scale bar:
5 μm; c EPNole-PEG-22, scale bar: 50 μm; c′ EPNole-PEG-22, scale bar: 5 μm; d PNole-4, scale bar: 50 μm; d′ PNole-4, scale bar: 5 μm
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this work. Oxidized oleic acid macro-peroxide decomposes from 220 °C up to 290 °C in the first cycle. The peroxide underwent a slight weight loss of approximately 1 wt% from between 150 and 200 °C (curve of Pole4 m in Fig. 8). Decomposition temperatures of the polymer samples are collected in Table 3. Thermal stability of the double hydro-philic copolymers was increased to higher temperatures from 250 to 290 °C than those of the diblock copolymers (ca. 240 °C for PNole-8).
An approximately 10 wt% of weight loss in residue was observed in all samples between 400 and 500 °C.
The morphological features of the double hydrophilic polymer were evaluated under a scanning electron micro-scope. Figure 9 shows the fractural surface micrographs of the PEG-20, PEG-22, EPNole-PEG-17, and PNole-4 samples. PEG blocks in double hydrophilic polymers show agglomerations (Fig. 9a, b, c), while PNole diblock copolymers have smooth sur-faces (Fig. 9d). Interestingly, in case of EPNole-PEG-22 (Fig. 9c), a layered structure was observed with PEG agglomeration.
Conclusion
Fatty acids have become very important for the chemi-cal industry to produce chemichemi-cals and monomers. A new macroperoxide initiator with free carboxylic acid function-ality was obtained by ecofriendly autoxidation of oleic acid. This type of functional macroperoxide initiator was used in the polymerization of NIPAM in the presence of amine-ter-minated polyethylene glycol to obtain a double hydrophilic thermo-responsive polymer. Based on PNIPAM and PEG blocks, this double hydrophilic polymer has a lower criti-cal solution temperature, which is higher than that of pure PNIPAM and is getting close to body temperature. By this one-pot synthesis, double a hydrophilic polymer with LCST at 36 °C was successfully obtained. In this manner, the type of double hydrophilic polymer obtained in this work will be a highly promising material for drug delivery systems.
Acknowledgements This work was supported by the Bülent
Ece-vit University Research Funds (2016-72118496-9 and #BEU-2016-72118496-10) and was proofread by the Bülent Ecevit Univer-sity Article Proofreading and Editing Office.
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