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Highly cross-linked soluble star copolymers with well controlled molecular weights
European Polymer Journal 67 (2015) 292–303
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Highly cross-linked soluble star copolymers with well controlled molecular weights Cansel Tuncer, Vural Bütün ⇑ Department of Chemistry, Faculty of Arts and Science, Eskisehir Osmangazi University, 26480 Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 19 February 2015 Received in revised form 1 April 2015 Accepted 8 April 2015 Available online 15 April 2015 Keywords: Star copolymers Cross-linking Water-soluble copolymers Self-assembly
a b s t r a c t We report the synthesis and characterization of water soluble star copolymers based on 2-(N-dimethylamino)ethyl methacrylate, 2-(N-diethylamino)ethyl methacrylate, and 2(N-morpholino)ethyl methacrylate monomers via group transfer polymerization. Ethylene glycol dimethacrylate was used as a branching agent. Highly cross-linked heteroarm star copolymers were obtained with narrow molecular weight distributions and controlled molecular architectures. Star copolymers were obtained in good yield with a monomer/branching agent at the appropriate mole ratio without macrogelation. Additionally, analog linear diblock copolymers with similar comonomer ratios were also synthesized to make comprehension. Star copolymers had lower viscosities compared to linear analogs at pH 8.1. Critical micelle concentration and critical micellization pH values were also determined for these star copolymers and compared to linear diblock copolymers. All polymers were characterized using gel permeation chromatography combined with a light scattering and a refractive index detectors and 1H NMR spectroscopy, to determine the molecular weights, molecular weight distributions and comonomer compositions, respectively. Solution behaviors of the polymers were also analyzed with a viscometer, densitometer, and surface tensiometer. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Branched polymers as compared to linear analogs have been the focus of attention because of their unique properties. These properties are better solubility in both organic and inorganic solvents, low solution and melt viscosities, to be quite a lot of functional end groups and adjustable solution behavior. This functional end groups allow high performance modifications. Soluble branched polymers having various functional groups, low viscosity and high solubility have found various applications including various powder coating, flame retarder coatings, coating resins [1], controlled drug and gene delivery. They also have great attention in the field of polymeric electrolytes and ion⇑ Corresponding author. E-mail address: [email protected] (V. Bütün). http://dx.doi.org/10.1016/j.eurpolymj.2015.04.007 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
conducting elastomers due to their great ion-transfer ability and electrochemical stability [2,3]. Branched polyethylene/poly(ethylene oxide) was used as a host for Li-ion battery electrolyte [4]. Xiao et al. was synthesized novel carbamate-containing, ferrocene-terminated organometallic highly branched polymer [5]. They successfully examined this polymer as a receptor in the recognition of ATP and glucose. There are some difficulties or limitations on the synthesis of such polymers. Such as, the control of both molecular weight and molecular weight distributions. However there are also disadvantages of these polymers such as irregular branching, broad molecular weight distribution and the uneven distribution of functional groups along the macromolecule [6]. Hyperbranched polymers were revealed for the first time by Kim and Webster in 1988, as a result of synthesizing soluble polyphenylene [7,8]. Hyperbranched
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polymers can be synthesized by polycondensation reactions of AB2 monomers [9]. However, vinyl monomers cannot be polymerized by this method. This disadvantage was overcome by self-condensing vinyl polymerization of initiator monomers having the general structure AB⁄, where A stands for a double bond and B⁄ represents an initiating group [10]. Thus, these molecules combine features of an initiator and a monomer and have therefore been named ‘‘inimer’’ [11–14]. Inimer system has been used to various types of living polymerization like cationic [10], ring-opening polymerization [15], atom transfer radical polymerization [16,17], nitroxide-mediated radical polymerizations [18], and group transfer polymerization (GTP) [19,20]. AB2 hyperbranched polymer and polypropylene mixtures were synthesized and isothermal crystallization kinetics were examined by differential scanning calorimetry [21]. Armes group reported a new strategy to synthesize branched statistical copolymer without any gelation via GTP. They used ethylene glycol dimethacrylate as bifunctional branching agents, 2-(dimethylamino)ethyl methacrylate (DMA) and 2-(diethylamino)ethyl methacrylate (DEA) as comonomers [22]. GTP provided both the control of first chain length and the narrow molecular weight distribution due to its living character. It also provided advantages compared to branched vinyl polymers synthesized by conventional radical method. The order of monomer addition may vary depending on the polymerization. It is possible to synthesize branched copolymers by starting with a linear block and following by branching the second comonomer or vice versa. They concluded that the first block should be linear homopolymer to achieve the best block efficiency for branched copolymers. pH-sensitive branched polymers were prepared by free radical polymerization in a single step by Weaver et al. [23]. These polymers were produced from a pH sensitive 2-(diethylamino)ethyl methacrylate (DEA) monomer and a hydrophilic poly(ethylene glycol) methacrylate (PEG) macromonomer. They managed to synthesize successfully both linear and cross-linked PEG–PDEA copolymers. Branched polyphenylenes having different optical properties are getting attention as well. These branched polymer were synthesized by Khotina et al. [24] to investigate their absorption spectra and to compare with linear analogs. They determined that optical properties of branched polyphenylenes depended on the length of the segment between the branching cores. One special type of the branched polymer is star (co)polymer. Star polymers are macromolecules formed by connecting many linear polymer to a central core [25,26]. Due to their compact structure, star (co)polymers have unique properties, such as lower melting points and solution viscosity than linear analog (co)polymers [27]. They have a variety of applications, such as drug carriers [28], gene delivery [29] and dispersants of carbon nanotubes [30]. Star polymers can be synthesized with three different methods: core-first, arm-first and coupling-onto. For example, PMMA armed star polymers were well known copolymers synthesized via ‘‘arm-first’’ method. With this method, star structure was obtained by the addition of ethylene glycol dimethacrylate (bifunctional monomer) as cross-linker in GTP process [31]. Such polymers were
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intensively studied by Patrickios et al. [32–42]. They evaluated such star copolymers having ability to transfect human cervical HeLa cancer cells. They concluded that the transfection efficiency was affected by the architecture of star (co)polymer. When the DP of the arms of the starPDMA homopolymer decreased, their transfection efficiency increased [34,37]. In all these studies, the related branched, star and heteroarm star copolymers had relatively high molecular weight distributions. These results are significantly affect the properties of the polymer system. The most important point is the method used in the synthesis of well controlled of the molecular weight, branching number and length of branched polymers. The living polymerization chemistry should be used for the control of the molecular weight. In our study, synthesis and characterization of novel water-soluble star copolymers with better control of molecular weight and branching degrees will be discussed. Moreover, solution behavior of these star copolymers will be compared to linear analog and differences will be underlined. 2. Experimental 2.1. Materials and general protocols All monomers, solvents and other reagents were purchased from Aldrich unless otherwise stated. 2-(Ndimethylamino)ethyl methacrylate (DMA), 2-(N-diethylamino)ethyl methacrylate (DEA), and 2-(N-morpholino)ethyl methacrylate (MEMA, Polymerscience Inc) monomers and ethylene glycol dimethacrylate (EGDMA) branching agent were first passed through alumina column to remove volatile inhibitor, followed by addition of both nonvolatile 2,2-diphenyl-1-picrylhydrazyl hydrate inhibitor and calcium hydride and stored at 20 °C. Each monomers, branching agent and 1-methoxy-1-trimethylsiloxy2-methyl-1-propane (MTS) initiator were distilled under reduced pressure and transferred to three-necked reaction flask by cannula under dry nitrogen. The tetra-n-butylammonium bibenzoate catalyst (TBABB) was synthesized as described in the literature [43]. After initial drying processes with solid sodium, the GTP solvent-THF was dried in the presence of potassium by refluxing for three days. All glassware and transfer needles (cannula) were kept in an oven at 140 °C prior to use. Glass materials were flame out under high vacuum before their usage. Nitrogen gas was dried by passing through two P2O5 columns during the synthesis of star copolymers. 2.2. Characterization techniques 2.2.1. Gel permeation chromatography (THF eluent) Molecular weights (Mn) and polydispersity index (Mw/ Mn) of block copolymers were determined by using gel permeation chromatography (GPC). The GPC consisted of an Agilent Iso Pump, a refractive index detector, either combination of Mixed ‘D’ and Mixed ‘E’ columns for diblock copolymer or Mixed ‘B’ and Mixed ‘D’ columns for star copolymer (ex. Polymer Labs), and calibration
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Table 1 Type and amounts of monomers, branching agent and initiator used in the synthesis of copolymers. Polymer code
Diblock copolymer
DMA (mL)
DEA (mL)
MEMA (mL)
EGDMA (mL)
MTS (mL)
CTI CTII CTIII CTIV
PDMA-star-PMEMA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PDEA
5.0 5.0 16.0 13.0
– 6.0 – 15.0
5.6 – 18.0 –
2.0 2.0 – –
0.35 0.35 0.20 0.20
Table 2 Absolute molecular weights (Mw), polydispersity index values (Mw/Mn), comonomer compositions and degrees of polymerizations (DP) for the synthesized copolymers.
a b c *
Polymer code
Copolymer
Mwa (g/mol)
Mw/Mna
Comonomer compositionb (mol%)
DPc
CTI CTII CTIII CTIV
PDMA-star-PMEMA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PDEA
1.1 106 2.4 106 32,640* 33,490*
1.23 1.25 1.04* 1.04*
43/3/54 44/11/45 48/52 51/49
28/2/35 36/9/37 90/98 99/95
As As As As
determined by GPC (light scattering detector). determined from 1H NMR spectroscopy. calculated from both 1H NMR spectroscopy. determined by GPC (refractive index detector).
was carried out using PMMA standards (ex. Polymer Labs), with Mn ranging from 1100 to 248,000 g mol 1. The GPC eluent was HPLC grade THF stabilized with BHT, at a flow rate of 1.0 mL min 1. A dual angle laser light scattering detector (LS) (Viscotek, k = 670 nm, 90° and 7°) was conducted to measure the absolute molecular weights (Mw) in THF with a flow rate of 1.0 mL/min at 25 °C. These detector was initially set up by using a PS standard having narrow molecular weight distribution (Mn = 115,000 g/mol, Mw/Mn = 1.02, [g] = 0.519 dL/g at 25 °C in THF, dn/dc = 0.185 mL/g) provided by Viscotek company. Data were collected by using Omni-Sec version 4.7 software from Viscotek Company. 2.2.2. Nuclear magnetic resonance spectroscopy (NMR) Linear and star copolymer compositions were determined by comparing well-defined peak integrals assigned to the different comonomers. Proton NMR spectra were obtained in CDCl3 solvent by using a Bruker Avance-II 500 MHz NMR Spectroscopy instrument. The actual DP’s of each blocks were also calculated from the combination of both GPC and 1H NMR spectroscopy data. 2.2.3. Dynamic light scattering studies To determine hydrodynamic diameters and polydispersity index (PDI = l2/C2) values of star copolymer micelles, Dynamic Light Scattering (DLS) studies were carried out by using an ALV/CGS-3 compact goniometer system (Malvern Instruments, Inc, UK). The ALV/CGS-3 system was equipped with a 22 mW He–Ne laser operating at ko 632.8 nm, an avalanche photodiode detector with high quantum efficiency, and an ALV/LSE-5003 multiple tau digital correlator electronics system. All measurements were performed at a fixed scattering angle of 90° on aqueous polymer solutions having concentrations between 0.5 and 2.0 wt%, and the data were fitted using second-order cumulants analysis. All solutions were measured after filtration through 0.2 lm syringe-filters.
PDMA-star-PMEMA
PDMA-star
PDMA
Molecular Weight
Fig. 1. GPC chromatograms with refractive index detector for each steps of PDMA-star-PMEMA copolymer synthesis (CTI, after removal of the PDMA homopolymer contaminants).
2.2.4. Surface tension measurements The surface tension was determined by employing a Kruss K-11 tensiometer using the ring method. The ring employed was a commercial platinum-iridium ring supplied by Kruss. The ring was cleaned and flame dried before each measurement. The surface tension of pure water was determined and compared with the literature to confirm that this method provides reliable results [44]. Surface tension measurements were carried out at 20 °C. In general, for each surface tension value, an average of three measurements was reported.
2.2.5. Density meter and viscometer Densities and viscosities of all polymer solutions were measured at 20 °C with an Anton Paar DMA-4500 Density Meter and an Anton Paar AMVn Viscometer. The automated micro viscometer is based on the rolling ball
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RALS
LALS
RI
Fig. 2. GPC chromatograms of PDMA-star-PMEMA copolymer with both RI and LS detectors (CTI).
principle. The sample to be measured is introduced into a glass capillary in which a steel ball rolls. The viscosity of the polymers can be determined by measuring the rolling time of the steel ball, an automatic AntonPaar AMVn viscometer (capillary internal diameter: 1.6 mm, ball diameter: 1.5 mm). The measurements were performed at an angle of 70° and were repeated four times. 2.2.6. pH measurements pH values of all (co)polymer solutions were measured by using Inolab WTW series pH720 model pH meter. The calibration was carried out using pH 4.0 and 7.0 buffer solutions (Aldrich). 2.3. Polymer synthesis 2.3.1. Synthesis of star copolymers (PDMA-star-PMEMA) For the synthesis of star copolymer, in the first step, DMA monomer was first homopolymerized. Approximately 50 mg of TBABB catalyst was added from the side of the three-necked flask with nitrogen outlet, which was attached to the vacuum line. 125 mL THF was added to the reaction flask via cannula. After addition of 0.35 mL MTS, solution was stirred for 14 min for activation
PDMA-star
of the initiator. After this period, first monomer DMA (5 mL, 29.7 mmol) was added to the reaction flask with cannula. During the addition of the monomer, 4 °C temperature rise was observed. After stirring 35 min, 2.0 mL aliquot was taken from the solution for 1H NMR spectroscopy and GPC analyses, and then branching agent (2 mL, 10.6 mmol) was added to the reaction vessel. Second exotherm was observed. After stirring 15 min, 2 mL aliquot was extracted from the solution for GPC and 1 H NMR spectroscopy analyses. Then, the second monomer MEMA (5.6 mL, 29.5 mmol) was added to the reaction medium via cannula. The third exotherm was also observed with a temperature rise of approximately 5 °C. After polymer solution was stirred for 2 h, polymerization was terminated by addition of 1.0 mL of methanol. The solvent was evaporated with the rotary evaporator. The star copolymer was dried in the vacuum line. Finally, the synthesized PDMA-star-PMEMA copolymer (CTI) was characterized by GPC and 1H NMR spectroscopy. The DMA/MEMA comonomer ratio in PDMA-star-PMEMA copolymer was calculated to be 2.8/2.8 based on per mol EGDMA (1.0 mol). The same procedure was followed for the synthesis of other star polymer (PDMA-star-PDEA, CTII). Only the type and amount of second monomer (6.0 mL DEA: 29.9 mmol) was changed. The DMA/DEA comonomer ratio in PDMAstar-PDEA copolymer was calculated to be 2.8/2.8 based on per mol EGDMA (1.0 mol).
PDMA-star-PDEA PDMA
Molecular Weight
Fig. 3. GPC chromatograms with RI detector for each steps of PDMA-starPDEA copolymer synthesis (CTII, after removal of the PDMA homopolymer and PDMA-star contaminants).
2.3.2. Synthesis of linear diblock copolymers (PDMA-b-PMEMA) The first step given above was repeated by aiming a DP of around 100 for the first block PDMA homopolymer. The quantities of materials used in the synthesis of both star and analog block copolymers were given in Table 1. During the addition of DMA monomer, the temperature increase of the reaction media was determined to be 12.1 °C. After stirring for 35 min, 2 mL aliquot was taken from the solution for proton NMR spectroscopy and GPC analyses and followed by the addition of second monomer MEMA via cannula. The second exotherm caused an increase on temperature of 9.0 °C. After polymer solution was stirred for 2 h, polymerization was terminated with 1.0 mL of methanol. The solvent was evaporated with the
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LALS RALS
RI
Fig. 4. GPC chromatograms of PDMA-star-PDEA with both RI and LS detectors (CTII).
rotary evaporator. The linear diblock copolymer was dried in a vacuum line for 24 h. Finally, the linear PDMA-bPMEMA diblock copolymer was characterized by GPC and 1 H NMR spectroscopy studies. The same procedure was used for the synthesis of other cross-linked and analog-linear (co)polymers. Only the type and amount of second monomers were changed. The synthesized star and analog linear copolymers, type and amount of monomers and initiator are listed in Table 1. Copolymer compositions, absolute molecular weights (Mw), PDI (Mw/Mn) and DP’s for the synthesized copolymers are listed in Table 2. In the synthesis of linear block copolymers, mole ratios of related monomers were chosen to be 1:1. 3. Results and discussion 3.1. Heteroarm star copolymer synthesis Heteroarm star copolymers were synthesized via GTP by starting with homopolymerization of DMA monomer. After completing homopolymerization, in the second step, cross-linking agent second monomer was added to the reaction medium to get PDMA-star copolymer. After 15 min, third monomer was added to the reaction vessel as final step to reach to heteroarm star copolymer. An aliquot was taken from reaction medium after each polymerization step for 1H NMR spectroscopy and GPC analyses. Chromatograms obtained with the RI detector were placed on the top in Fig. 1. Mn and PDI values of PDMA homopolymer (CTI) were determined to be 5100 g/mol and 1.08, respectively. Considering the molecular weight of PDMA homopolymer, the degree of polymerization of the primary PDMA chain was calculated to be 32. After addition of branching agent, the molecular weight of the PDMA-star copolymer increased very sharply as seen in Fig. 1 which is very different than that of the linear polymerization and indicates branching (PDMA-star). The nature of the copolymer is expected to be in the form of core crosslinked structure with linear PDMA corona. After linear copolymerization of third monomer, the corona was enhanced with second type of linear PMEMA blocks
(PDMA-star-PMEMA). Final product heteroarm PDMAstar-PMEMA copolymer contains star copolymer with both PDMA homopolymer and PDMA-star contaminants. Selective-precipitation process was applied to remove both contaminants from the final copolymer. First, polymer is dissolved completely in THF and than precipitated in nhexane. Finally, homopolymer residue remained in solution phase and the PDMA-star-PMEMA copolymer precipitated. Absolute Mw values were evaluated by GPC using a dual angular light scattering detector. Absolute Mw was determined to be 1.1 106 g/mol for PDMA-star-PMEMA copolymer. Proton NMR spectra of PDMA-star-PMEMA copolymer aliquots indicated comonomer compositions of DMA/EGDMA/MEMA to be 28/2/35 by end group analysis (Table 2). This NMR result indicated a Mn of PDMA-starPMEMA copolymer to be 11,770 g/mol. When Mw value was compared with the Mn of the related star copolymer, its branching or cross-linking degree was determined to be approximately 93 which can be accepted as the number of each arm in the star copolymer with related DPs. The related GPC chromatograms were given in Fig. 2. Second type of heteroarm star copolymer was also synthesized in the same way just by changing the final block (CTII). In this case, DEA monomer was added to the reaction medium as the last block, and PDMA-star-PDEA heteroarm star copolymer was synthesized. Initially, when the DMA monomer polymerized, molecular weight (Mn) and PDI of PDMA homopolymer was determined to be 4630 g/mol and 1.08 from refractive index detector (see Fig. 3 for GPC chromatograms). Degree of polymerization of PDMA homopolymer was calculated to be 29 from GPC result. After synthesis, the copolymer contained both PDMA homopolymer and PDMA-star contaminants. To remove these contaminants, this polymer mixture was dissolved in water at pH 2 and precipitated with base addition. In this way nearly monodisperse PDMA-star-PDEA copolymer was successfully obtained. GPC chromatograms from both refractive and light scattering detectors were given in Figs. 3 and 4, respectively. Absolute Mw was determined as 2.4 106 g/mol from light scattering detector for final PDMA-star-PDEA copolymer. When Mw value was compared with the Mn of the PDMA-star-PDEA copolymer,
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E D CH 3 H2 C C
C
(a)
36
C O O
CH2 A CH2 B
A
H3C C
N
E D CH 3 H2 C C
F H2 C
36
H3C C
B
CH3 C
G CH 3 C
9
C O
C O
O
O
CH 2
CH 2
CH 2
CH 2
N
O
CH 3 C
E
D
(b) C
F H2 C C
O C
CH 3 G
A
E D CH 3 H2 C C
36
H3 C C
B
F H2 C
G CH3 C
9
H2 C
CH3 C
C O
C O
O
O
O
CH2
CH2
CH2 H
CH2
CH2
CH2 I
CH3 C
O O C F H2 C C CH3 G
J H2C
N
H3C K
4.0
(c)
CH2 J CH3 K
C
K
B+J
I
A+H
4.5
37
C O
N
E+G
D+F
3.5
3.0
2.5
2.0
1.5
1.0
0.5
δ/ppm Fig. 5. Typical 1H NMR spectra (CDCl3) of each step in PDMA-star-PDEA synthesis: (a) PDMA homopolymer, (b) PDMA-star copolymer and (c) PDMA-starPDEA copolymer (CTI).
its branching or cross-linking degree was determined to be around 168. That was also indication of the presence of 168 PDMA arm and 168 PDEA arm in the PDMA-star-PDEA copolymer. Comonomer composition of PDMA-star-PDEA copolymer (CTII) was determined by using 1H NMR spectroscopy. The actual degree of polymerization of the first block PDMA was also determined by end group analysis. The ‘A’ peak integral of the two hydrogens of the methylene group bounded to oxygen (–OCH2–) in the DMA residues at d 4.1 was compared to that of integrated methoxy signal (–OCH3) in the MTS initiator fragment at d 3.6 (see Fig. 5a).
DP of the PDMA homopolymer was calculated to be 36. In the second stage (Fig. 5b), integral areas of D + F (–CH2–) or E + G (–CH3) peaks of PDMA and PEGDMA backbone protons were compared with that of the six protons of dimethylamino of PDMA block at d 2.3 (peak C) and actual polymerization degree of PEGDMA was determined to be 9. DP of PDEA component of the star copolymer was calculated to be 37 by comparing the integrated oxymethylene proton signal at d 4.0 with that of the six protons of dimethylamino groups in the DMA residues at d 2.3 (Fig. 5c). The DMA, EGDMA and DEA contents were determined to be 44, 11 and 45 mol%, respectively.
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E D CH 3 H2 C C
(a) C
28
C O O
CH2 A CH2 B H3 C C
A
N
E D CH 3 H2 C C
F H2 C
28
H3 C C
B
CH3 C
G CH3 C
2
C O
C O
O
O
CH2
CH2
CH2
CH2
N
O
CH3 C
O C F H2 C C
28
H3C C
G CH3 C
2
H2 C
C O
O
O
O
CH2
CH2
CH2 H
CH2
CH2
CH2 I
O O C F H2 C C
J H2C K H2C
O
C
CH2 J CH2 K
K B+I
A+H
4.0
N
(c)
CH3 G
J
4.5
35
C O
CH3 C
E+G
CH3 C
C O
N
D+F
B
A
F H2 C
(b)
C
CH3 G
E D CH 3 H2 C C
E
D
3.5
3.0
2.5 δ/ppm
2.0
1.5
1.0
0.5
Fig. 6. Typical 1H NMR spectra (CDCl3) of each step in PDMA-star-PMEMA copolymer synthesis: (a) PDMA homopolymer, (b) PDMA-star copolymer and (c) PDMA-star-PMEMA copolymer (CTII).
Similarly, the 1H NMR spectrum ‘C’ in Fig. 6 represents a PDMA-star-PMEMA copolymer (CTI). Firstly, DP of PDMA block was determined as 28 by end group analysis. EGDMA was determined to be 2 in the same way as given for CTI. The peak integral of the DMA residues at d 4.1 was compared to that of four oxymethylene protons in the morpholino groups of the PMEMA block at d 3.7 (peak J). DP of PMEMA block was determined to be 35. The DMA, EGDMA and MEMA contents of PDMA-star-PMEMA copolymer were determined to be 43, 3 and 54 mol%, respectively.
3.2. Linear diblock copolymer synthesis PDMA-b-PMEMA linear block copolymer (CTIII) was also synthesized via GTP. Copolymer Mn and Mw/Mn values were determined by GPC combined with refractive index detector. The chromatograms were given in Fig. S1, Supporting Information. Copolymer had low Mw/Mn (1.04) and molecular weight was determined to be 32,640 g/mol. Comonomer composition of this polymer was determined by 1H NMR spectroscopy (see Fig. S2) by comparing well-defined peak integrals assigned to the
C. Tuncer, V. Bütün / European Polymer Journal 67 (2015) 292–303 1 0.8
f (R h)
(a)
Radius: 15.7 nm μ2/Γ2 : 0.02
0.6 0.4 0.2 0 0.1
1
10
100
Radius (nm) 1
f (Rh)
(b)
Radius: 8.8 nm μ2/Γ2 : 0.24
0.8 0.6 0.4 0.2 0 0.1
1
10
100
Radius (nm) Fig. 7. DLS measurements of polymer solutions (0.5 wt%) at 20 °C (pH 8.1): (a) PDMA-b-PDEA block copolymer (CTIV) and (b) PDMA-star-PDEA copolymer (CTII).
different comonomers. The peak integral of DMA residues at d 2.3 was compared to that of the four –CH2OCH2– protons of the morpholino groups of MEMA residues at d 3.7. The PDMA content was found to be 48 mol%, which compared with the theoretical PMEMA content of 52 mol%. The PDMA and PMEMA contents of PDMA-b-PMEMA copolymer were determined to be 90 and 98, respectively. PDMA-b-PDEA linear block copolymer (CTIV) was also synthesized and characterized in the same methods. GPC chromatogram of this polymer was also given in Fig. S3, Supporting Information. This linear block copolymer had low Mw/Mn (1.04) and molecular weight was determined to be 33,490 g/mol. Fig. S4, Supporting Information represents a PDMA-b-PDEA block copolymer (CTIV) which has a content of 51 mol% DMA. The ‘C’ peak integrals of the six dimethylamino protons in the DMA residues at d 2.3 was compared to that of –OCH2– protons in the DEA residues at d 4.0. The DMA content was found to be 51 mol%, which compared with the theoretical DEA content of 49 mol%. The PDMA and PDEA contents of PDMA-b-PDEA copolymer were determined to be 99 and 95, respectively. 3.3. Solution behaviors of copolymers 3.3.1. Solubility and micelle formation Tertiary amine methacrylate based block copolymers are known as water soluble block copolymers and show great surface activity if they have a stimuli responsive block [45]. As reported before, the micelle diameter depended on both pH responsive block content and block copolymer molecular weight [45]. For example, PDMA-bPDEA block copolymers having around 50 mol% DEA
299
comonomer formed nice PDEA-core micelles with a diameter between 25 and 50 nm depending on molecular weight at around pH 8.1 and at 20 °C as reported by us before [45]. Similarly, we expected PDEA-core micelles under the same conditions with PDEA based branched and linear analog copolymers with the polymer we report in this study. As seen in Fig. 7, DLS measurements indicate good micellization under similar conditions. The hydrodynamic diameter of PDMA-b-PDEA linear block copolymer (CTIV) and PDMA-star-PDEA copolymer (CTII) (0.5 wt%) micelles in aqueous solution at pH 8.1 were determined to be 31.4 nm and 17.6 nm, respectively. The micelle diameter of analog PDMA-b-PDEA diblock copolymer with a molecular weight of 33,490 g/mol was similar to that of block copolymers reported before [45]. Surprisingly, the heteroarm star copolymer formed much smaller micelles than the linear analog block copolymer at same pH as well. As reported by us before, the micelle diameter of PDMA-bPDEA diblock copolymer having the degrees of polymerization similar to heteroarm star copolymer was 25 nm under similar conditions (see VB70 in Ref. [45]). The more compact cross-linked structure of the star copolymers should be the reason why the hydrodynamic size of the star polymer micelle was smaller than that of the linear one. Additionally, as a comparison, this 17.6 nm micellar diameter of heteroarm star copolymer given in this study was almost equal to the micelle size of around 19 nm for branched [crosslinked-PDEA]-b-[linear-PDMA] copolymer which was reported before [45]. Micelle diameters at different polymer concentrations (1.0 and 2.0 wt%) for the star and linear polymers containing PDEA blocks were determined at pH 8.1 and compared with the diameters of the related polymers at 0.5 wt% solutions. It was observed that there is no differences on micelle diameter at different polymer concentration.
3.3.2. Surface tension measurements pH dependence of surface tension of both heteroarm star copolymers and their analog linear block copolymers were given in Fig. 8a. Both heteroarm star copolymers and linear block copolymers reduced the surface tension of water. As reported before, tertiary amine methacrylate based block copolymers had surface active nature due to pH responsive block PDEA [45]. Similarly, when the solution pH was increased the star copolymers adsorbed strongly to the air–water interface which caused a decrease in surface tension of the solvent [45]. Above pH 8.5, the limiting surface tension of star copolymer solutions (0.5 wt%) were determined to be ca. 40 and 45 mN/ m depending of comonomer types. This behavior is similar to that of linear PDMA-b-PDEA and PDMA-b-PMEMA diblock copolymers (35 and 45 mN/m). Critical micelle concentrations (CMC) were also determined from the concentration dependence of the surface tension for these copolymer solutions. Surface tension vs concentration curves were given in Fig. 8b. The CMC for star polymer was estimated to be ca. 0.03 wt%, which is lower than the CMC of the linear block copolymer having similar comonomer compositions (ca. 0.06 wt%, as reported before) [45].
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70
(a)
Surface Tension (mN/m)
65
PDMA-b-PDEA PDMA-b-PDEA
PDMA-star-PDEA PDMA-star-PDEA
60
PDMA-b-PMEMA PDMA-b-PMEMA
55
PDMA-star-PMEMA PDMA-star-PMEMA
50 45 40 35 30
2
4
6
8
10
12
pH
(b)
Surface Tension (mN/m)
75 70
PDMA-b-PDEA PDMA-b-PDEA
65
PDMA-star-PDEA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PMEMA
60
PDMA-star-PMEMA PDMA-star-PMEMA
55 50 45 40 35
0
0.2
0.4
0.6
Polymer Conc.
0.8
1
(g/mLx10ˉ2)
Fig. 8. Variation of surface tension (a) as a function of solution pH for 0.5 wt% aqueous solutions of CTI, CTII, CTIII and CTIV copolymers and (b) as a function of polymer concentration for same copolymers at pH 8.0.
3.3.3. Viscosity and density measurements 2.0, 1.0 and 0.5 wt% aqueous solutions of both star and linear diblock copolymer were prepared. Each solution was divided into two parts and the pH of one part was adjusted to pH 2.8 and the other one was adjusted to pH 8.1. It was observed that all of the solutions had almost the same density values at the same concentrations. The densities were slightly affected from polymer concentration. Densities of all polymer samples with the concentrations of 0.5 wt% and 2.0 wt% were varied between 0.99830 and 1.01125 g/ mL, respectively. Due to weak basic character of each blocks, the nitrogen atoms of all copolymers will be in fully protonated form at pH 2.8. Thus, the polymer was dissolved molecularly and formed more expanded coils at pH 2.8. In the basic conditions, all tertiary amine residues were in neutral form and gave more shrinked coils. In the case of PDEA containing copolymers, they formed nice micellization due to dehydration of PDEA blocks. When we compare the viscosities, in general, the viscosity of star copolymers were higher than that of linear block copolymers at pH 2.8 (see Fig. 9a). It is also worth to mention that all copolymers had higher viscosity values in the acidic conditions than that of the basic conditions (Fig. 9a and b). This indicates that micellization caused a decrease on viscosity of the solution.
By considering steric hindrances and water-solubility, the copolymer containing PMEMA block nature having molecular solubility at all pH values caused more increase on viscosity if we compared with the other copolymers (Fig. 9a and b). So, copolymers containing PMEMA block (CTI and CTIII) have higher viscosity than that of copolymers containing PDEA blocks (CTII and CTVI) under similar conditions. In Fig. 9b, all synthesized copolymers were in micellar form (pH 8.1, not in unimer form) as viscosity measurements indicated. On the other way, star polymers compared with the linear analogs were found to have much lower viscosity than the linear polymer due to their intensely cross-linked core structure. Soluble star polymers having low viscosity have a lot of applications [1]. Dynamic viscosity value was determined to be 1.20 mPa s for PDMA-b-PDEA diblock copolymer (2.0 wt% aqueous solution). This value was determined to be 1.07 mPa s for its star analog at pH 8.1. The same situation was also observed for the other two polymers (both PDMA-b-PMEMA diblock copolymer and PDMA-star-MEMA copolymer). It indicates that branching causes a decrease on the solution viscosity when star copolymers are compared with linear analogs (see Fig. 9b). Before viscosity measurements, densities of all copolymer solutions (0.5, 1.0, 2.0 wt%) were determined
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(a)
Dynamic Viscosity (mPa.s)
5.50 5.00
PDMA-b-PDEA PDMA-b-PDEA
4.50
PDMA-star-PDEA PDMA-star-PDEA
4.00
PDMA-b-PMEMA PDMA-b-PMEMA
3.50
PDMA-star-PMEMA PDMA-star-PMEMA
3.00 2.50 2.00 1.50 1.00
0.5
1
1.5
2
Polymer Conc. (w/v%)
Dynamic Viscosity (mPa.s)
1.30
(b)
PDMA-b-PDEA PDMA-b-PDEA
1.25
PDMA-star-PDEA PDMA-star-PDEA
1.20
PDMA-b-PMEMA PDMA-b-PMEMA
1.15
PDMA-star-PMEMA PDMA-star-PMEMA
1.10 1.05 1.00 0.95
0.5
1
1.5
2
Polymer Conc. (w/v%) Fig. 9. Variation of dynamic viscosity with copolymer solution concentration at different pH values: (a) pH 2.8 and (b) pH 8.1.
at both pH 2 and pH 10. It was observed that while an increase on polymer concentration caused a slight increase on density, the pH change did not caused a change on density of related polymers. 3.3.4. Hydrogen ion titration 1.0 wt% aqueous star polymer solutions were prepared and titrated from pH 2 to 12 using a 0.25 M NaOH solution under stirring. Related titration curves were given in Figs. S5 and S6, Supporting Information. Average pKa values of PDMA-star-PMEMA and PDMA-star-PDEA copolymers were determined to be 6.5 and 7.4, respectively (see Figs. S5 and S6). It is also possible to calculate pKa values of both PDMA and PMEMA blocks of PDMA-starPMEMA copolymer from titration curve as 7.6 and 4.7, respectively (see Fig. S5). But, it is difficult to calculate pKa values for each block of PDMA-star-PDEA copolymer since their pKa values are near to each other [45]. As a conclusion, the measured pKa values agree with our previous results. Star-structure of tertiary amine methacrylate copolymers had little effect on pKa value of each PDMA, PDEA and PMEMA blocks. 4. Conclusion Highly cross-linked PDMA-star-PMEMA and PDMAstar-PDEA star copolymers were successfully synthesized without macrogelation via GTP by using ethylene glycol dimethacrylate as a branching agent. These star polymers
had well controlled molecular weight distributions (Mw/ Mn < 1.26) and molecular architectures (having Mw between 1.1x106 g/mol and 2.4x106 g/mol). Number of arms of PDMA-star-PMEMA and PDMA-star-PDEA copolymers were determined as 93 and 168, respectively. All related heteroarm star copolymers were soluble in water and in various organic solvents. The analog linear diblock copolymers (with similar comonomer ratio) were also synthesized to make comprehension. Both linear and heteroarm star copolymers were surface active and reduced the surface tension of water. After determining the limiting surface tension of polymer solutions, the CMC values were estimated to be ca. 0.03 and 0.06 wt% for the PDEA based star and linear block copolymers, respectively. All PDEAbased copolymers showed micellization behavior at pH 8.1. DLS measurement showed that linear polymer (PDMA-b-PDEA) had a monodispers micelle size (l2/C: 0.02) but star copolymer (PDMA-star-PDEA) had a broad polydispersity index value (l2/C: 0.24). When they are compared with the linear analog diblock copolymers, it is concluded that; (i) Heteroarm star copolymers had much lower viscosity than linear-analogs at pH 8.1. (ii) They had lower CMC values than the linear-analog polymers. (iii) Heteroarm star copolymers formed smaller micelles than linear-analog polymers at the same pH and concentrations.
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Acknowledgements The authors are grateful for the financial supports of Eskisehir Osmangazi University (ESOGU). This work was supported by the Commission of Scientific Research Projects of ESOGU (GR/201319033). VB would like to express his gratitude to the Turkish Academy of Sciences (TUBA) as an Associate Member for their support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2015.04.007. References [1] Gao C, Yan D. Hyperbranched polymers: from synthesis to applications. Prog Polym Sci 2004;29(3):183–275. [2] Ghosh P, Crooks RM. Covalent grafting of a patterned, hyperbranched polymer onto a plastic substrate using microcontact printing. J Am Chem Soc 1999;121(36):8395–6. [3] Nishimoto A, Agehara K, Furuya N, Watanabe T, Watanabe M. High ionic conductivity of polyether-based network polymer electrolytes with hyperbranched side chains. Macromolecules 1999;32(5): 1541–8. [4] Brandell D, Priimagi P, Kasemagi H, Aabloo A. Branched polyethylene/poly(ethylene oxide) as a host matrix for Li-ion battery electrolytes: a molecular dynamics study. Electrochim Acta 2011;57:228–36. [5] Xiao FJ, Gu MQ, Liang Y, Dong MJ, Zhao ZP, Zhi DW. Synthesis, characterization and electrochemical behavior of ferrocenecontaining highly branched polyurethane and its application in biosensing. J Organomet Chem 2014;772:122–30. [6] Unal S, Yilgor I, Yilgor E, Sheth JP, Wilkes GL, Long TE. A new generation of highly branched polymers: hyperbranched, segmented poly(urethane urea) elastomers. Macromolecules 2004;37(19): 7081–4. [7] Kim YH, Webster OW. Hyperbranched polyphenylenes. Abstr Pap Am Chem Soc 1988;196 [104-POLY]. [8] Kim YH, Webster OW. Water-soluble hyperbranched polyphenylene - a unimolecular micelle. J Am Chem Soc 1990;112(11):4592–3. [9] Flory PJ. Molecular size distribution in three dimensional polymers. VI. Branched polymers containing A—R—Bf-1 type units. J Am Chem Soc 1952:2718–23. [10] Frechet JMJ, Henmi M, Gitsov I, Aoshima S, Leduc MR, Grubbs RB. Self-condensing vinyl polymerization – an approach to dendritic materials. Science 1995;269(5227):1080–3. [11] Hazer B. Synthesis of PS–PEG and PMMA–PEG branched block copolymers by macroinimers. J Macromol Sci Chem 1991;A28(1): 47–52. [12] Hazer B. New macromonomeric initiators (macroinimers), 2 Gelation in bulk polymerization of styrene with macroinimers. Die Makromolekulare Chemie 1992;193(5):1081–6. [13] Muller AHE, Yan DY, Wulkow M. Molecular parameters of hyperbranched polymers made by self-condensing vinyl polymerization. 1. Molecular weight distribution. Macromolecules 1997;30(23):7015–23. [14] Puskas JE, Grasmüller M. Star-branched and hyperbranched polyisobutylenes. Macromol Symp 1998;132(1):117–26. [15] Sunder A, Hanselmann R, Frey H, Mülhaupt R. Controlled synthesis of hyperbranched polyglycerols by ring-opening multibranching polymerization. Macromolecules 1999;32(13):4240–6. [16] Gaynor SG, Edelman S, Matyjaszewski K. Synthesis of branched and hyperbranched polystyrenes. Macromolecules 1996;29(3):1079–81. [17] Weimer MW, Fréchet JM, Gitsov I. Importance of active-site reactivity and reaction conditions in the preparation of hyperbranched polymers by self-condensing vinyl polymerization: highly branched vs linear poly [4-(chloromethyl) styrene] by metalcatalyzed ‘‘living’’ radical polymerization. J Polym Sci, Part A: Polym Chem 1998;36(6):955–70. [18] Hawker CJ, Frechet JM, Grubbs RB, Dao J. Preparation of hyperbranched and star polymers by a’’ living’’, self-condensing free radical polymerization. J Am Chem Soc 1995;117(43):10763–4.
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Highly cross-linked soluble star copolymers with well controlled molecular weights Cansel Tuncer, Vural Bütün ⇑ Department of Chemistry, Faculty of Arts and Science, Eskisehir Osmangazi University, 26480 Eskisehir, Turkey
a r t i c l e
i n f o
Article history: Received 19 February 2015 Received in revised form 1 April 2015 Accepted 8 April 2015 Available online 15 April 2015 Keywords: Star copolymers Cross-linking Water-soluble copolymers Self-assembly
a b s t r a c t We report the synthesis and characterization of water soluble star copolymers based on 2-(N-dimethylamino)ethyl methacrylate, 2-(N-diethylamino)ethyl methacrylate, and 2(N-morpholino)ethyl methacrylate monomers via group transfer polymerization. Ethylene glycol dimethacrylate was used as a branching agent. Highly cross-linked heteroarm star copolymers were obtained with narrow molecular weight distributions and controlled molecular architectures. Star copolymers were obtained in good yield with a monomer/branching agent at the appropriate mole ratio without macrogelation. Additionally, analog linear diblock copolymers with similar comonomer ratios were also synthesized to make comprehension. Star copolymers had lower viscosities compared to linear analogs at pH 8.1. Critical micelle concentration and critical micellization pH values were also determined for these star copolymers and compared to linear diblock copolymers. All polymers were characterized using gel permeation chromatography combined with a light scattering and a refractive index detectors and 1H NMR spectroscopy, to determine the molecular weights, molecular weight distributions and comonomer compositions, respectively. Solution behaviors of the polymers were also analyzed with a viscometer, densitometer, and surface tensiometer. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Branched polymers as compared to linear analogs have been the focus of attention because of their unique properties. These properties are better solubility in both organic and inorganic solvents, low solution and melt viscosities, to be quite a lot of functional end groups and adjustable solution behavior. This functional end groups allow high performance modifications. Soluble branched polymers having various functional groups, low viscosity and high solubility have found various applications including various powder coating, flame retarder coatings, coating resins [1], controlled drug and gene delivery. They also have great attention in the field of polymeric electrolytes and ion⇑ Corresponding author. E-mail address: [email protected] (V. Bütün). http://dx.doi.org/10.1016/j.eurpolymj.2015.04.007 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
conducting elastomers due to their great ion-transfer ability and electrochemical stability [2,3]. Branched polyethylene/poly(ethylene oxide) was used as a host for Li-ion battery electrolyte [4]. Xiao et al. was synthesized novel carbamate-containing, ferrocene-terminated organometallic highly branched polymer [5]. They successfully examined this polymer as a receptor in the recognition of ATP and glucose. There are some difficulties or limitations on the synthesis of such polymers. Such as, the control of both molecular weight and molecular weight distributions. However there are also disadvantages of these polymers such as irregular branching, broad molecular weight distribution and the uneven distribution of functional groups along the macromolecule [6]. Hyperbranched polymers were revealed for the first time by Kim and Webster in 1988, as a result of synthesizing soluble polyphenylene [7,8]. Hyperbranched
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polymers can be synthesized by polycondensation reactions of AB2 monomers [9]. However, vinyl monomers cannot be polymerized by this method. This disadvantage was overcome by self-condensing vinyl polymerization of initiator monomers having the general structure AB⁄, where A stands for a double bond and B⁄ represents an initiating group [10]. Thus, these molecules combine features of an initiator and a monomer and have therefore been named ‘‘inimer’’ [11–14]. Inimer system has been used to various types of living polymerization like cationic [10], ring-opening polymerization [15], atom transfer radical polymerization [16,17], nitroxide-mediated radical polymerizations [18], and group transfer polymerization (GTP) [19,20]. AB2 hyperbranched polymer and polypropylene mixtures were synthesized and isothermal crystallization kinetics were examined by differential scanning calorimetry [21]. Armes group reported a new strategy to synthesize branched statistical copolymer without any gelation via GTP. They used ethylene glycol dimethacrylate as bifunctional branching agents, 2-(dimethylamino)ethyl methacrylate (DMA) and 2-(diethylamino)ethyl methacrylate (DEA) as comonomers [22]. GTP provided both the control of first chain length and the narrow molecular weight distribution due to its living character. It also provided advantages compared to branched vinyl polymers synthesized by conventional radical method. The order of monomer addition may vary depending on the polymerization. It is possible to synthesize branched copolymers by starting with a linear block and following by branching the second comonomer or vice versa. They concluded that the first block should be linear homopolymer to achieve the best block efficiency for branched copolymers. pH-sensitive branched polymers were prepared by free radical polymerization in a single step by Weaver et al. [23]. These polymers were produced from a pH sensitive 2-(diethylamino)ethyl methacrylate (DEA) monomer and a hydrophilic poly(ethylene glycol) methacrylate (PEG) macromonomer. They managed to synthesize successfully both linear and cross-linked PEG–PDEA copolymers. Branched polyphenylenes having different optical properties are getting attention as well. These branched polymer were synthesized by Khotina et al. [24] to investigate their absorption spectra and to compare with linear analogs. They determined that optical properties of branched polyphenylenes depended on the length of the segment between the branching cores. One special type of the branched polymer is star (co)polymer. Star polymers are macromolecules formed by connecting many linear polymer to a central core [25,26]. Due to their compact structure, star (co)polymers have unique properties, such as lower melting points and solution viscosity than linear analog (co)polymers [27]. They have a variety of applications, such as drug carriers [28], gene delivery [29] and dispersants of carbon nanotubes [30]. Star polymers can be synthesized with three different methods: core-first, arm-first and coupling-onto. For example, PMMA armed star polymers were well known copolymers synthesized via ‘‘arm-first’’ method. With this method, star structure was obtained by the addition of ethylene glycol dimethacrylate (bifunctional monomer) as cross-linker in GTP process [31]. Such polymers were
293
intensively studied by Patrickios et al. [32–42]. They evaluated such star copolymers having ability to transfect human cervical HeLa cancer cells. They concluded that the transfection efficiency was affected by the architecture of star (co)polymer. When the DP of the arms of the starPDMA homopolymer decreased, their transfection efficiency increased [34,37]. In all these studies, the related branched, star and heteroarm star copolymers had relatively high molecular weight distributions. These results are significantly affect the properties of the polymer system. The most important point is the method used in the synthesis of well controlled of the molecular weight, branching number and length of branched polymers. The living polymerization chemistry should be used for the control of the molecular weight. In our study, synthesis and characterization of novel water-soluble star copolymers with better control of molecular weight and branching degrees will be discussed. Moreover, solution behavior of these star copolymers will be compared to linear analog and differences will be underlined. 2. Experimental 2.1. Materials and general protocols All monomers, solvents and other reagents were purchased from Aldrich unless otherwise stated. 2-(Ndimethylamino)ethyl methacrylate (DMA), 2-(N-diethylamino)ethyl methacrylate (DEA), and 2-(N-morpholino)ethyl methacrylate (MEMA, Polymerscience Inc) monomers and ethylene glycol dimethacrylate (EGDMA) branching agent were first passed through alumina column to remove volatile inhibitor, followed by addition of both nonvolatile 2,2-diphenyl-1-picrylhydrazyl hydrate inhibitor and calcium hydride and stored at 20 °C. Each monomers, branching agent and 1-methoxy-1-trimethylsiloxy2-methyl-1-propane (MTS) initiator were distilled under reduced pressure and transferred to three-necked reaction flask by cannula under dry nitrogen. The tetra-n-butylammonium bibenzoate catalyst (TBABB) was synthesized as described in the literature [43]. After initial drying processes with solid sodium, the GTP solvent-THF was dried in the presence of potassium by refluxing for three days. All glassware and transfer needles (cannula) were kept in an oven at 140 °C prior to use. Glass materials were flame out under high vacuum before their usage. Nitrogen gas was dried by passing through two P2O5 columns during the synthesis of star copolymers. 2.2. Characterization techniques 2.2.1. Gel permeation chromatography (THF eluent) Molecular weights (Mn) and polydispersity index (Mw/ Mn) of block copolymers were determined by using gel permeation chromatography (GPC). The GPC consisted of an Agilent Iso Pump, a refractive index detector, either combination of Mixed ‘D’ and Mixed ‘E’ columns for diblock copolymer or Mixed ‘B’ and Mixed ‘D’ columns for star copolymer (ex. Polymer Labs), and calibration
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Table 1 Type and amounts of monomers, branching agent and initiator used in the synthesis of copolymers. Polymer code
Diblock copolymer
DMA (mL)
DEA (mL)
MEMA (mL)
EGDMA (mL)
MTS (mL)
CTI CTII CTIII CTIV
PDMA-star-PMEMA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PDEA
5.0 5.0 16.0 13.0
– 6.0 – 15.0
5.6 – 18.0 –
2.0 2.0 – –
0.35 0.35 0.20 0.20
Table 2 Absolute molecular weights (Mw), polydispersity index values (Mw/Mn), comonomer compositions and degrees of polymerizations (DP) for the synthesized copolymers.
a b c *
Polymer code
Copolymer
Mwa (g/mol)
Mw/Mna
Comonomer compositionb (mol%)
DPc
CTI CTII CTIII CTIV
PDMA-star-PMEMA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PDEA
1.1 106 2.4 106 32,640* 33,490*
1.23 1.25 1.04* 1.04*
43/3/54 44/11/45 48/52 51/49
28/2/35 36/9/37 90/98 99/95
As As As As
determined by GPC (light scattering detector). determined from 1H NMR spectroscopy. calculated from both 1H NMR spectroscopy. determined by GPC (refractive index detector).
was carried out using PMMA standards (ex. Polymer Labs), with Mn ranging from 1100 to 248,000 g mol 1. The GPC eluent was HPLC grade THF stabilized with BHT, at a flow rate of 1.0 mL min 1. A dual angle laser light scattering detector (LS) (Viscotek, k = 670 nm, 90° and 7°) was conducted to measure the absolute molecular weights (Mw) in THF with a flow rate of 1.0 mL/min at 25 °C. These detector was initially set up by using a PS standard having narrow molecular weight distribution (Mn = 115,000 g/mol, Mw/Mn = 1.02, [g] = 0.519 dL/g at 25 °C in THF, dn/dc = 0.185 mL/g) provided by Viscotek company. Data were collected by using Omni-Sec version 4.7 software from Viscotek Company. 2.2.2. Nuclear magnetic resonance spectroscopy (NMR) Linear and star copolymer compositions were determined by comparing well-defined peak integrals assigned to the different comonomers. Proton NMR spectra were obtained in CDCl3 solvent by using a Bruker Avance-II 500 MHz NMR Spectroscopy instrument. The actual DP’s of each blocks were also calculated from the combination of both GPC and 1H NMR spectroscopy data. 2.2.3. Dynamic light scattering studies To determine hydrodynamic diameters and polydispersity index (PDI = l2/C2) values of star copolymer micelles, Dynamic Light Scattering (DLS) studies were carried out by using an ALV/CGS-3 compact goniometer system (Malvern Instruments, Inc, UK). The ALV/CGS-3 system was equipped with a 22 mW He–Ne laser operating at ko 632.8 nm, an avalanche photodiode detector with high quantum efficiency, and an ALV/LSE-5003 multiple tau digital correlator electronics system. All measurements were performed at a fixed scattering angle of 90° on aqueous polymer solutions having concentrations between 0.5 and 2.0 wt%, and the data were fitted using second-order cumulants analysis. All solutions were measured after filtration through 0.2 lm syringe-filters.
PDMA-star-PMEMA
PDMA-star
PDMA
Molecular Weight
Fig. 1. GPC chromatograms with refractive index detector for each steps of PDMA-star-PMEMA copolymer synthesis (CTI, after removal of the PDMA homopolymer contaminants).
2.2.4. Surface tension measurements The surface tension was determined by employing a Kruss K-11 tensiometer using the ring method. The ring employed was a commercial platinum-iridium ring supplied by Kruss. The ring was cleaned and flame dried before each measurement. The surface tension of pure water was determined and compared with the literature to confirm that this method provides reliable results [44]. Surface tension measurements were carried out at 20 °C. In general, for each surface tension value, an average of three measurements was reported.
2.2.5. Density meter and viscometer Densities and viscosities of all polymer solutions were measured at 20 °C with an Anton Paar DMA-4500 Density Meter and an Anton Paar AMVn Viscometer. The automated micro viscometer is based on the rolling ball
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295
RALS
LALS
RI
Fig. 2. GPC chromatograms of PDMA-star-PMEMA copolymer with both RI and LS detectors (CTI).
principle. The sample to be measured is introduced into a glass capillary in which a steel ball rolls. The viscosity of the polymers can be determined by measuring the rolling time of the steel ball, an automatic AntonPaar AMVn viscometer (capillary internal diameter: 1.6 mm, ball diameter: 1.5 mm). The measurements were performed at an angle of 70° and were repeated four times. 2.2.6. pH measurements pH values of all (co)polymer solutions were measured by using Inolab WTW series pH720 model pH meter. The calibration was carried out using pH 4.0 and 7.0 buffer solutions (Aldrich). 2.3. Polymer synthesis 2.3.1. Synthesis of star copolymers (PDMA-star-PMEMA) For the synthesis of star copolymer, in the first step, DMA monomer was first homopolymerized. Approximately 50 mg of TBABB catalyst was added from the side of the three-necked flask with nitrogen outlet, which was attached to the vacuum line. 125 mL THF was added to the reaction flask via cannula. After addition of 0.35 mL MTS, solution was stirred for 14 min for activation
PDMA-star
of the initiator. After this period, first monomer DMA (5 mL, 29.7 mmol) was added to the reaction flask with cannula. During the addition of the monomer, 4 °C temperature rise was observed. After stirring 35 min, 2.0 mL aliquot was taken from the solution for 1H NMR spectroscopy and GPC analyses, and then branching agent (2 mL, 10.6 mmol) was added to the reaction vessel. Second exotherm was observed. After stirring 15 min, 2 mL aliquot was extracted from the solution for GPC and 1 H NMR spectroscopy analyses. Then, the second monomer MEMA (5.6 mL, 29.5 mmol) was added to the reaction medium via cannula. The third exotherm was also observed with a temperature rise of approximately 5 °C. After polymer solution was stirred for 2 h, polymerization was terminated by addition of 1.0 mL of methanol. The solvent was evaporated with the rotary evaporator. The star copolymer was dried in the vacuum line. Finally, the synthesized PDMA-star-PMEMA copolymer (CTI) was characterized by GPC and 1H NMR spectroscopy. The DMA/MEMA comonomer ratio in PDMA-star-PMEMA copolymer was calculated to be 2.8/2.8 based on per mol EGDMA (1.0 mol). The same procedure was followed for the synthesis of other star polymer (PDMA-star-PDEA, CTII). Only the type and amount of second monomer (6.0 mL DEA: 29.9 mmol) was changed. The DMA/DEA comonomer ratio in PDMAstar-PDEA copolymer was calculated to be 2.8/2.8 based on per mol EGDMA (1.0 mol).
PDMA-star-PDEA PDMA
Molecular Weight
Fig. 3. GPC chromatograms with RI detector for each steps of PDMA-starPDEA copolymer synthesis (CTII, after removal of the PDMA homopolymer and PDMA-star contaminants).
2.3.2. Synthesis of linear diblock copolymers (PDMA-b-PMEMA) The first step given above was repeated by aiming a DP of around 100 for the first block PDMA homopolymer. The quantities of materials used in the synthesis of both star and analog block copolymers were given in Table 1. During the addition of DMA monomer, the temperature increase of the reaction media was determined to be 12.1 °C. After stirring for 35 min, 2 mL aliquot was taken from the solution for proton NMR spectroscopy and GPC analyses and followed by the addition of second monomer MEMA via cannula. The second exotherm caused an increase on temperature of 9.0 °C. After polymer solution was stirred for 2 h, polymerization was terminated with 1.0 mL of methanol. The solvent was evaporated with the
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LALS RALS
RI
Fig. 4. GPC chromatograms of PDMA-star-PDEA with both RI and LS detectors (CTII).
rotary evaporator. The linear diblock copolymer was dried in a vacuum line for 24 h. Finally, the linear PDMA-bPMEMA diblock copolymer was characterized by GPC and 1 H NMR spectroscopy studies. The same procedure was used for the synthesis of other cross-linked and analog-linear (co)polymers. Only the type and amount of second monomers were changed. The synthesized star and analog linear copolymers, type and amount of monomers and initiator are listed in Table 1. Copolymer compositions, absolute molecular weights (Mw), PDI (Mw/Mn) and DP’s for the synthesized copolymers are listed in Table 2. In the synthesis of linear block copolymers, mole ratios of related monomers were chosen to be 1:1. 3. Results and discussion 3.1. Heteroarm star copolymer synthesis Heteroarm star copolymers were synthesized via GTP by starting with homopolymerization of DMA monomer. After completing homopolymerization, in the second step, cross-linking agent second monomer was added to the reaction medium to get PDMA-star copolymer. After 15 min, third monomer was added to the reaction vessel as final step to reach to heteroarm star copolymer. An aliquot was taken from reaction medium after each polymerization step for 1H NMR spectroscopy and GPC analyses. Chromatograms obtained with the RI detector were placed on the top in Fig. 1. Mn and PDI values of PDMA homopolymer (CTI) were determined to be 5100 g/mol and 1.08, respectively. Considering the molecular weight of PDMA homopolymer, the degree of polymerization of the primary PDMA chain was calculated to be 32. After addition of branching agent, the molecular weight of the PDMA-star copolymer increased very sharply as seen in Fig. 1 which is very different than that of the linear polymerization and indicates branching (PDMA-star). The nature of the copolymer is expected to be in the form of core crosslinked structure with linear PDMA corona. After linear copolymerization of third monomer, the corona was enhanced with second type of linear PMEMA blocks
(PDMA-star-PMEMA). Final product heteroarm PDMAstar-PMEMA copolymer contains star copolymer with both PDMA homopolymer and PDMA-star contaminants. Selective-precipitation process was applied to remove both contaminants from the final copolymer. First, polymer is dissolved completely in THF and than precipitated in nhexane. Finally, homopolymer residue remained in solution phase and the PDMA-star-PMEMA copolymer precipitated. Absolute Mw values were evaluated by GPC using a dual angular light scattering detector. Absolute Mw was determined to be 1.1 106 g/mol for PDMA-star-PMEMA copolymer. Proton NMR spectra of PDMA-star-PMEMA copolymer aliquots indicated comonomer compositions of DMA/EGDMA/MEMA to be 28/2/35 by end group analysis (Table 2). This NMR result indicated a Mn of PDMA-starPMEMA copolymer to be 11,770 g/mol. When Mw value was compared with the Mn of the related star copolymer, its branching or cross-linking degree was determined to be approximately 93 which can be accepted as the number of each arm in the star copolymer with related DPs. The related GPC chromatograms were given in Fig. 2. Second type of heteroarm star copolymer was also synthesized in the same way just by changing the final block (CTII). In this case, DEA monomer was added to the reaction medium as the last block, and PDMA-star-PDEA heteroarm star copolymer was synthesized. Initially, when the DMA monomer polymerized, molecular weight (Mn) and PDI of PDMA homopolymer was determined to be 4630 g/mol and 1.08 from refractive index detector (see Fig. 3 for GPC chromatograms). Degree of polymerization of PDMA homopolymer was calculated to be 29 from GPC result. After synthesis, the copolymer contained both PDMA homopolymer and PDMA-star contaminants. To remove these contaminants, this polymer mixture was dissolved in water at pH 2 and precipitated with base addition. In this way nearly monodisperse PDMA-star-PDEA copolymer was successfully obtained. GPC chromatograms from both refractive and light scattering detectors were given in Figs. 3 and 4, respectively. Absolute Mw was determined as 2.4 106 g/mol from light scattering detector for final PDMA-star-PDEA copolymer. When Mw value was compared with the Mn of the PDMA-star-PDEA copolymer,
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E D CH 3 H2 C C
C
(a)
36
C O O
CH2 A CH2 B
A
H3C C
N
E D CH 3 H2 C C
F H2 C
36
H3C C
B
CH3 C
G CH 3 C
9
C O
C O
O
O
CH 2
CH 2
CH 2
CH 2
N
O
CH 3 C
E
D
(b) C
F H2 C C
O C
CH 3 G
A
E D CH 3 H2 C C
36
H3 C C
B
F H2 C
G CH3 C
9
H2 C
CH3 C
C O
C O
O
O
O
CH2
CH2
CH2 H
CH2
CH2
CH2 I
CH3 C
O O C F H2 C C CH3 G
J H2C
N
H3C K
4.0
(c)
CH2 J CH3 K
C
K
B+J
I
A+H
4.5
37
C O
N
E+G
D+F
3.5
3.0
2.5
2.0
1.5
1.0
0.5
δ/ppm Fig. 5. Typical 1H NMR spectra (CDCl3) of each step in PDMA-star-PDEA synthesis: (a) PDMA homopolymer, (b) PDMA-star copolymer and (c) PDMA-starPDEA copolymer (CTI).
its branching or cross-linking degree was determined to be around 168. That was also indication of the presence of 168 PDMA arm and 168 PDEA arm in the PDMA-star-PDEA copolymer. Comonomer composition of PDMA-star-PDEA copolymer (CTII) was determined by using 1H NMR spectroscopy. The actual degree of polymerization of the first block PDMA was also determined by end group analysis. The ‘A’ peak integral of the two hydrogens of the methylene group bounded to oxygen (–OCH2–) in the DMA residues at d 4.1 was compared to that of integrated methoxy signal (–OCH3) in the MTS initiator fragment at d 3.6 (see Fig. 5a).
DP of the PDMA homopolymer was calculated to be 36. In the second stage (Fig. 5b), integral areas of D + F (–CH2–) or E + G (–CH3) peaks of PDMA and PEGDMA backbone protons were compared with that of the six protons of dimethylamino of PDMA block at d 2.3 (peak C) and actual polymerization degree of PEGDMA was determined to be 9. DP of PDEA component of the star copolymer was calculated to be 37 by comparing the integrated oxymethylene proton signal at d 4.0 with that of the six protons of dimethylamino groups in the DMA residues at d 2.3 (Fig. 5c). The DMA, EGDMA and DEA contents were determined to be 44, 11 and 45 mol%, respectively.
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E D CH 3 H2 C C
(a) C
28
C O O
CH2 A CH2 B H3 C C
A
N
E D CH 3 H2 C C
F H2 C
28
H3 C C
B
CH3 C
G CH3 C
2
C O
C O
O
O
CH2
CH2
CH2
CH2
N
O
CH3 C
O C F H2 C C
28
H3C C
G CH3 C
2
H2 C
C O
O
O
O
CH2
CH2
CH2 H
CH2
CH2
CH2 I
O O C F H2 C C
J H2C K H2C
O
C
CH2 J CH2 K
K B+I
A+H
4.0
N
(c)
CH3 G
J
4.5
35
C O
CH3 C
E+G
CH3 C
C O
N
D+F
B
A
F H2 C
(b)
C
CH3 G
E D CH 3 H2 C C
E
D
3.5
3.0
2.5 δ/ppm
2.0
1.5
1.0
0.5
Fig. 6. Typical 1H NMR spectra (CDCl3) of each step in PDMA-star-PMEMA copolymer synthesis: (a) PDMA homopolymer, (b) PDMA-star copolymer and (c) PDMA-star-PMEMA copolymer (CTII).
Similarly, the 1H NMR spectrum ‘C’ in Fig. 6 represents a PDMA-star-PMEMA copolymer (CTI). Firstly, DP of PDMA block was determined as 28 by end group analysis. EGDMA was determined to be 2 in the same way as given for CTI. The peak integral of the DMA residues at d 4.1 was compared to that of four oxymethylene protons in the morpholino groups of the PMEMA block at d 3.7 (peak J). DP of PMEMA block was determined to be 35. The DMA, EGDMA and MEMA contents of PDMA-star-PMEMA copolymer were determined to be 43, 3 and 54 mol%, respectively.
3.2. Linear diblock copolymer synthesis PDMA-b-PMEMA linear block copolymer (CTIII) was also synthesized via GTP. Copolymer Mn and Mw/Mn values were determined by GPC combined with refractive index detector. The chromatograms were given in Fig. S1, Supporting Information. Copolymer had low Mw/Mn (1.04) and molecular weight was determined to be 32,640 g/mol. Comonomer composition of this polymer was determined by 1H NMR spectroscopy (see Fig. S2) by comparing well-defined peak integrals assigned to the
C. Tuncer, V. Bütün / European Polymer Journal 67 (2015) 292–303 1 0.8
f (R h)
(a)
Radius: 15.7 nm μ2/Γ2 : 0.02
0.6 0.4 0.2 0 0.1
1
10
100
Radius (nm) 1
f (Rh)
(b)
Radius: 8.8 nm μ2/Γ2 : 0.24
0.8 0.6 0.4 0.2 0 0.1
1
10
100
Radius (nm) Fig. 7. DLS measurements of polymer solutions (0.5 wt%) at 20 °C (pH 8.1): (a) PDMA-b-PDEA block copolymer (CTIV) and (b) PDMA-star-PDEA copolymer (CTII).
different comonomers. The peak integral of DMA residues at d 2.3 was compared to that of the four –CH2OCH2– protons of the morpholino groups of MEMA residues at d 3.7. The PDMA content was found to be 48 mol%, which compared with the theoretical PMEMA content of 52 mol%. The PDMA and PMEMA contents of PDMA-b-PMEMA copolymer were determined to be 90 and 98, respectively. PDMA-b-PDEA linear block copolymer (CTIV) was also synthesized and characterized in the same methods. GPC chromatogram of this polymer was also given in Fig. S3, Supporting Information. This linear block copolymer had low Mw/Mn (1.04) and molecular weight was determined to be 33,490 g/mol. Fig. S4, Supporting Information represents a PDMA-b-PDEA block copolymer (CTIV) which has a content of 51 mol% DMA. The ‘C’ peak integrals of the six dimethylamino protons in the DMA residues at d 2.3 was compared to that of –OCH2– protons in the DEA residues at d 4.0. The DMA content was found to be 51 mol%, which compared with the theoretical DEA content of 49 mol%. The PDMA and PDEA contents of PDMA-b-PDEA copolymer were determined to be 99 and 95, respectively. 3.3. Solution behaviors of copolymers 3.3.1. Solubility and micelle formation Tertiary amine methacrylate based block copolymers are known as water soluble block copolymers and show great surface activity if they have a stimuli responsive block [45]. As reported before, the micelle diameter depended on both pH responsive block content and block copolymer molecular weight [45]. For example, PDMA-bPDEA block copolymers having around 50 mol% DEA
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comonomer formed nice PDEA-core micelles with a diameter between 25 and 50 nm depending on molecular weight at around pH 8.1 and at 20 °C as reported by us before [45]. Similarly, we expected PDEA-core micelles under the same conditions with PDEA based branched and linear analog copolymers with the polymer we report in this study. As seen in Fig. 7, DLS measurements indicate good micellization under similar conditions. The hydrodynamic diameter of PDMA-b-PDEA linear block copolymer (CTIV) and PDMA-star-PDEA copolymer (CTII) (0.5 wt%) micelles in aqueous solution at pH 8.1 were determined to be 31.4 nm and 17.6 nm, respectively. The micelle diameter of analog PDMA-b-PDEA diblock copolymer with a molecular weight of 33,490 g/mol was similar to that of block copolymers reported before [45]. Surprisingly, the heteroarm star copolymer formed much smaller micelles than the linear analog block copolymer at same pH as well. As reported by us before, the micelle diameter of PDMA-bPDEA diblock copolymer having the degrees of polymerization similar to heteroarm star copolymer was 25 nm under similar conditions (see VB70 in Ref. [45]). The more compact cross-linked structure of the star copolymers should be the reason why the hydrodynamic size of the star polymer micelle was smaller than that of the linear one. Additionally, as a comparison, this 17.6 nm micellar diameter of heteroarm star copolymer given in this study was almost equal to the micelle size of around 19 nm for branched [crosslinked-PDEA]-b-[linear-PDMA] copolymer which was reported before [45]. Micelle diameters at different polymer concentrations (1.0 and 2.0 wt%) for the star and linear polymers containing PDEA blocks were determined at pH 8.1 and compared with the diameters of the related polymers at 0.5 wt% solutions. It was observed that there is no differences on micelle diameter at different polymer concentration.
3.3.2. Surface tension measurements pH dependence of surface tension of both heteroarm star copolymers and their analog linear block copolymers were given in Fig. 8a. Both heteroarm star copolymers and linear block copolymers reduced the surface tension of water. As reported before, tertiary amine methacrylate based block copolymers had surface active nature due to pH responsive block PDEA [45]. Similarly, when the solution pH was increased the star copolymers adsorbed strongly to the air–water interface which caused a decrease in surface tension of the solvent [45]. Above pH 8.5, the limiting surface tension of star copolymer solutions (0.5 wt%) were determined to be ca. 40 and 45 mN/ m depending of comonomer types. This behavior is similar to that of linear PDMA-b-PDEA and PDMA-b-PMEMA diblock copolymers (35 and 45 mN/m). Critical micelle concentrations (CMC) were also determined from the concentration dependence of the surface tension for these copolymer solutions. Surface tension vs concentration curves were given in Fig. 8b. The CMC for star polymer was estimated to be ca. 0.03 wt%, which is lower than the CMC of the linear block copolymer having similar comonomer compositions (ca. 0.06 wt%, as reported before) [45].
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70
(a)
Surface Tension (mN/m)
65
PDMA-b-PDEA PDMA-b-PDEA
PDMA-star-PDEA PDMA-star-PDEA
60
PDMA-b-PMEMA PDMA-b-PMEMA
55
PDMA-star-PMEMA PDMA-star-PMEMA
50 45 40 35 30
2
4
6
8
10
12
pH
(b)
Surface Tension (mN/m)
75 70
PDMA-b-PDEA PDMA-b-PDEA
65
PDMA-star-PDEA PDMA-star-PDEA PDMA-b-PMEMA PDMA-b-PMEMA
60
PDMA-star-PMEMA PDMA-star-PMEMA
55 50 45 40 35
0
0.2
0.4
0.6
Polymer Conc.
0.8
1
(g/mLx10ˉ2)
Fig. 8. Variation of surface tension (a) as a function of solution pH for 0.5 wt% aqueous solutions of CTI, CTII, CTIII and CTIV copolymers and (b) as a function of polymer concentration for same copolymers at pH 8.0.
3.3.3. Viscosity and density measurements 2.0, 1.0 and 0.5 wt% aqueous solutions of both star and linear diblock copolymer were prepared. Each solution was divided into two parts and the pH of one part was adjusted to pH 2.8 and the other one was adjusted to pH 8.1. It was observed that all of the solutions had almost the same density values at the same concentrations. The densities were slightly affected from polymer concentration. Densities of all polymer samples with the concentrations of 0.5 wt% and 2.0 wt% were varied between 0.99830 and 1.01125 g/ mL, respectively. Due to weak basic character of each blocks, the nitrogen atoms of all copolymers will be in fully protonated form at pH 2.8. Thus, the polymer was dissolved molecularly and formed more expanded coils at pH 2.8. In the basic conditions, all tertiary amine residues were in neutral form and gave more shrinked coils. In the case of PDEA containing copolymers, they formed nice micellization due to dehydration of PDEA blocks. When we compare the viscosities, in general, the viscosity of star copolymers were higher than that of linear block copolymers at pH 2.8 (see Fig. 9a). It is also worth to mention that all copolymers had higher viscosity values in the acidic conditions than that of the basic conditions (Fig. 9a and b). This indicates that micellization caused a decrease on viscosity of the solution.
By considering steric hindrances and water-solubility, the copolymer containing PMEMA block nature having molecular solubility at all pH values caused more increase on viscosity if we compared with the other copolymers (Fig. 9a and b). So, copolymers containing PMEMA block (CTI and CTIII) have higher viscosity than that of copolymers containing PDEA blocks (CTII and CTVI) under similar conditions. In Fig. 9b, all synthesized copolymers were in micellar form (pH 8.1, not in unimer form) as viscosity measurements indicated. On the other way, star polymers compared with the linear analogs were found to have much lower viscosity than the linear polymer due to their intensely cross-linked core structure. Soluble star polymers having low viscosity have a lot of applications [1]. Dynamic viscosity value was determined to be 1.20 mPa s for PDMA-b-PDEA diblock copolymer (2.0 wt% aqueous solution). This value was determined to be 1.07 mPa s for its star analog at pH 8.1. The same situation was also observed for the other two polymers (both PDMA-b-PMEMA diblock copolymer and PDMA-star-MEMA copolymer). It indicates that branching causes a decrease on the solution viscosity when star copolymers are compared with linear analogs (see Fig. 9b). Before viscosity measurements, densities of all copolymer solutions (0.5, 1.0, 2.0 wt%) were determined
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(a)
Dynamic Viscosity (mPa.s)
5.50 5.00
PDMA-b-PDEA PDMA-b-PDEA
4.50
PDMA-star-PDEA PDMA-star-PDEA
4.00
PDMA-b-PMEMA PDMA-b-PMEMA
3.50
PDMA-star-PMEMA PDMA-star-PMEMA
3.00 2.50 2.00 1.50 1.00
0.5
1
1.5
2
Polymer Conc. (w/v%)
Dynamic Viscosity (mPa.s)
1.30
(b)
PDMA-b-PDEA PDMA-b-PDEA
1.25
PDMA-star-PDEA PDMA-star-PDEA
1.20
PDMA-b-PMEMA PDMA-b-PMEMA
1.15
PDMA-star-PMEMA PDMA-star-PMEMA
1.10 1.05 1.00 0.95
0.5
1
1.5
2
Polymer Conc. (w/v%) Fig. 9. Variation of dynamic viscosity with copolymer solution concentration at different pH values: (a) pH 2.8 and (b) pH 8.1.
at both pH 2 and pH 10. It was observed that while an increase on polymer concentration caused a slight increase on density, the pH change did not caused a change on density of related polymers. 3.3.4. Hydrogen ion titration 1.0 wt% aqueous star polymer solutions were prepared and titrated from pH 2 to 12 using a 0.25 M NaOH solution under stirring. Related titration curves were given in Figs. S5 and S6, Supporting Information. Average pKa values of PDMA-star-PMEMA and PDMA-star-PDEA copolymers were determined to be 6.5 and 7.4, respectively (see Figs. S5 and S6). It is also possible to calculate pKa values of both PDMA and PMEMA blocks of PDMA-starPMEMA copolymer from titration curve as 7.6 and 4.7, respectively (see Fig. S5). But, it is difficult to calculate pKa values for each block of PDMA-star-PDEA copolymer since their pKa values are near to each other [45]. As a conclusion, the measured pKa values agree with our previous results. Star-structure of tertiary amine methacrylate copolymers had little effect on pKa value of each PDMA, PDEA and PMEMA blocks. 4. Conclusion Highly cross-linked PDMA-star-PMEMA and PDMAstar-PDEA star copolymers were successfully synthesized without macrogelation via GTP by using ethylene glycol dimethacrylate as a branching agent. These star polymers
had well controlled molecular weight distributions (Mw/ Mn < 1.26) and molecular architectures (having Mw between 1.1x106 g/mol and 2.4x106 g/mol). Number of arms of PDMA-star-PMEMA and PDMA-star-PDEA copolymers were determined as 93 and 168, respectively. All related heteroarm star copolymers were soluble in water and in various organic solvents. The analog linear diblock copolymers (with similar comonomer ratio) were also synthesized to make comprehension. Both linear and heteroarm star copolymers were surface active and reduced the surface tension of water. After determining the limiting surface tension of polymer solutions, the CMC values were estimated to be ca. 0.03 and 0.06 wt% for the PDEA based star and linear block copolymers, respectively. All PDEAbased copolymers showed micellization behavior at pH 8.1. DLS measurement showed that linear polymer (PDMA-b-PDEA) had a monodispers micelle size (l2/C: 0.02) but star copolymer (PDMA-star-PDEA) had a broad polydispersity index value (l2/C: 0.24). When they are compared with the linear analog diblock copolymers, it is concluded that; (i) Heteroarm star copolymers had much lower viscosity than linear-analogs at pH 8.1. (ii) They had lower CMC values than the linear-analog polymers. (iii) Heteroarm star copolymers formed smaller micelles than linear-analog polymers at the same pH and concentrations.
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