Reversible Addition?Fragmentation Chain-Transfer Polymerization of

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Article Cite This: Langmuir 2019, 35, 8389−8397

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Reversible Addition−Fragmentation Chain-Transfer Polymerization of Amphiphilic Polycarboxybetaines and Their Molecular Interactions Munziya Abutalip,†,‡ Anam Mahmood,† Raikhan Rakhmetullayeva,‡ Alexey Shakhvorostov,§,∥ Yerbol Dauletov,†,¶ Sarkyt Kudaibergenov,§,∥ and Nurxat Nuraje*,† †

Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409, United States Department of Chemistry and Chemical Technology, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan § Laboratory of Engineering Profile, K.I. Satpayev Kazakh National Research Technical University, Almaty 050013, Kazakhstan ∥ Institute of Polymer Materials and Technology, Almaty 050013, Kazakhstan

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S Supporting Information *

ABSTRACT: In this work, we report the first molecular weight-controlled amphiphilic polybetaine synthesis using various hydrocarbons via reversible addition−fragmentation chain-transfer (RAFT) polymerization. The experimental separation of the alkyl aminocrotonate tautomers, which has been the subject of debate, was completed for the first time. The enamine form of these tautomers was further used as a monomer for the RAFT polymerization of amphiphilic polycarboxybetaines. Self-assembly of the amphiphilic polycarboxybetaines showed micelle structures from spherical, rod-like to fractal in the aqueous media due to the competition between both electrostatic and hydrophobic forces. Hydrophobically dominant interactions among amphiphilic polycarboxybetaines and long-chain hydrocarbon alkane molecules were investigated to understand long-chain hydrocarbon alkane crystallization using alkane crystal deposition and viscosity experiments. Strong hydrophobic forces between poly(hexadecyl-grafted aminocrotonate−methacrylic acid) and long-chain hydrocarbon alkane molecules changed the surface properties of the long-chain hydrocarbon alkane nucleus and inhibited the growth of paraffin crystals.

1. INTRODUCTION The interaction of an amphiphilic polymer with both itself and other molecules is critical to advance basic science in the polymer field and to solve technical issues in real world applications from petroleum to other industries.1−7 For example, both the self-assembly of amphiphilic polymers and their association with small molecules provide guidance to design better drug delivery systems.3,4,8−10 Conditions such as temperature, pH, and ionic strength for association and dissociation of the amphiphilic polymers with small drugs can be applied as stimulus parameters for drug release and uptake at target organs.9,11−14 Amphiphilic polymers with fractal morphology because of their large surface area and homogeneous appearance can be applied in making sensors,15−17 catalysts,18 electronic devices,19 and coatings with specific functionalities.20,21 An understanding of the basic © 2019 American Chemical Society

interaction between polymers and paraffin in crude oil is the basis for the design of paraffin inhibitors in the petroleum industry.22,23 Zwitterionic polymers possess unique self-assembly behaviors in different conditions because they possess positive/ negative ionizable groups in one repeat unit (aka polybetaine) or alternative repeat units (polyampholyte), which can induce various forces in different pHs, ionic strengths, and thermal conditions.24−27 In contrast to polyelectrolytes which precipitate in high salt concentrations,28 polybetaines expand with the addition of salts due to the screening of the electrostatic attractive force.24,29 Recently, hydrophobically modified polybetaines have attracted broad interest because they carry Received: May 6, 2019 Published: June 2, 2019 8389

DOI: 10.1021/acs.langmuir.9b01347 Langmuir 2019, 35, 8389−8397

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Figure 1. (A) Chemical structures of poly(alkyl grafted aminocrotonate−methacrylic acid). From left to right: P(OACRO−MAA); P(TACRO− MAA); and P(HACRO−MAA). (B) RAFT polymerization of poly(alkyl grafted aminocrotonate−methacrylic acid). R = C8H17; C14H29; C16H33.

crotonate−methacrylic acid) [P(OACRO−MAA)], poly(tetradecyl-grafted aminocrotonate−methacrylic acid) [P(TACRO−MAA)], and poly(hexadecyl-grafted aminocrotonate−methacrylic acid) [P(HACRO−MAA)]. The resulting poly(alkyl grafted aminocrotonate−methacrylic acid) with various alkyl groups offers an opportunity to investigate their self-assembly behaviors systematically with ionizable positive/ negative groups in addition to their controlled molecular weight over the same polybetaines produced by random radical polymerization (high PDIs).38 At the same time, prolonged scientific questions regarding the presence of aminocrotonate’s tautomers need to be experimentally confirmed. In addition to the systematic study of the self-assembly of HMPB with varied hydrocarbon chain lengths, an investigation of interactions among the hydrophobically modified polybetaines and longchain hydrocarbons directs us to understand the nucleation and crystallization of paraffin. This study leads to the design of better paraffin inhibitors for the petroleum industry.

three different functional groups (ionizable positive/negative and hydrophobic functional groups).30−32 These groups, in different media, generate strong hydrophobic and electrostatic forces which have various self-assembly structures and specific interactions with small molecules.33 However, these hydrophobically modified polybetaines are not widely studied due to the complicated synthesis routes required to control their molecular weight and because protecting groups are sometimes required to protect for active functional groups during the synthesis.24,26 Nevertheless, hydrophobically modified polybetaines (HMPB) with controlled molecular weight enable us to understand their self-assembly behaviors and address the issues in their targeted applications.32,34 Recent studies have been conducted on the synthesis of HMPB (amphiphilic polybetaines) to reach certain application targets.32 For instance, HMPBs were synthesized to assemble as nanocages for pH-responsive drug delivery to tumor tissues.9 Living radical polymerization approaches were utilized to produce polybetaines with a low polydispersity index (PDI) and controlled molecular weight. Poly(sulfobetaine), which has MW = 10 000 g/mol and PDI < 1.2, was synthesized via atom transfer radical polymerization.35,36 Reversible addition− fragmentation chain-transfer (RAFT) polymerization was utilized to synthesize a carboxybetaine triblock copolymer with controlled molecular weight.37 However, few studies have succeeded in synthesizing hydrophobically modified polycarboxybetaines via RAFT. Therefore, we present a RAFT approach to synthesize poly(alkyl-grafted aminocrotonate−methacrylic acid) containing various alkyl groups with low PDI and controlled molecular weight (Figure 1A). These are poly(octyl-grafted amino-

2. EXPERIMENTAL METHOD The chemicals from Sigma-Aldrich Co.: dodecylamine (99.5%), tetradecylamine (99%), hexadecylamine (99%), methacrylic acid (99%), 4-cyano-4-(thiobenzoylthio)pentanoic acid (98%), 2-cyano-2propyl dodecyl trithiocarbonate (97%), and fluorescence indicator (manganese-doped zinc silicate green 254 nm). Fisher Scientific Co.: ethyl acetoacetate (reagent grade, EAA) and dichloromethane (HPLC grade). Agela Tech Co.: 60 μm TLC silica plate; SiliCycle Co.: 40−63 μm silica gel. Before use, methacrylic acid was distilled to remove the inhibitor and kept at 4 °C. 2.1. Synthesis of Alkylaminocrotonate. Under stirring and 60 °C reaction conditions, ethyl acetoacetate was added dropwise into alkylamines in a 1.1:1 molar ratio and allowed to react for 4 h. The 8390

DOI: 10.1021/acs.langmuir.9b01347 Langmuir 2019, 35, 8389−8397

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Figure 2. 1H NMR spectra of the first collection from the flash column chromatography of hexadecylaminocrotonate. 2.4. FTIR Measurement. A VERTEX 70 Fourier transform infrared spectrometer was employed for both the alkylaminocrotonate and polymer products. 2.5. Molecular Weight Measurement. The molecular weight and PDI of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) were determined via gel permeation chromatography (GPC) using an Agilent PLgel Mixed-D column with a TREOS Wyatt DLS/UV combination detector and a Rex Wyatt refractive index detector. For this measurement, 5 mg/mL of the synthesized polymers including polyethylene oxide standards was prepared in dimethylformamide. 2.6. Zeta-Potential Measurement. A Zetasizer (a Brookhaven NanoBrook Omni) was used to measure zeta-potential of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) at different pH conditions. The concentration of the prepared polymer samples was 0.015%. Sample pH was adjusted using potassium hydroxide (KOH) and hydrogen chloride (HCl). 2.7. Morphological Measurement. Self-assembled structures of the synthesized amphiphilic polycarboxybetaines were observed using a Hitachi H-8100 scanning transmission electron microscope equipped with a high-resolution camera and an electronic brakeforce distribution analytical detector. 2.8. Hydrocarbon Crystal Formation. As shown in Figure S1, an experimental setup was built to study hydrocarbon crystal formation on the surface of a cold tube at 0 °C. Crystals formed due to the temperature difference between the tube and a long-chain hydrocarbon solution, which was equilibrated at 50 °C.22 The longchain hydrocarbon solution was prepared through the addition of either precalculated C24H50 or C36H74 wax (4%) to decane as a paraffin model. P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) were separately added to the above decane solution containing either C24H50 or C36H74 to measure the longchain hydrocarbon deposition amounts at the cold surface. Thus, the effect of the amphiphilic polycarboxybetaines on the long-chain hydrocarbon crystal formation was evaluated at three different concentrations of the polybetaines (100, 250 and 500 ppm). 2.9. Viscosity Measurement. In order to determine viscosity, a water bath was set up at 38 °C and a Cannon Calibrated Ubbelohde Viscometer was inserted into it. Then, the test solution was added to the viscometer and allowed to reach thermal equilibrium with the water bath. The time taken for the solution to travel through the bulb of the viscometer was then recorded for three trials. Next, dilutions of this solution were made by adding more decane in the viscometer: three successive dilutions were made of the original sample and time measurements were taken after each dilution. These measurements

reaction continued at room temperature for an additional 24 h. The aqueous layer was removed after centrifugation, and then the reaction mixture was dried overnight in a vacuum oven (30 °C and 25 in. Hg). The final products were identified via TLC using a TLC plate with 60 μm silica beads and dichloromethane. The corresponding Rf values for chemicals including monomers are reported in Table S1. Identified components were collected via flash column chromatography using dichloromethane and a 1% methanol−dichloromethane solution, which is used for highly polar components. Yields of the obtained alkylaminocrotonates are tabled in Table S2. The separated alkylaminocrotonates were also studied using nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR). 2.2. RAFT Polymerization for Synthesis of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA). Before RAFT polymerization, the separated enamine form of alkylaminocrotonate and methacrylic acid were melted to become their liquid state at 70 and 25 °C, respectively. Required amounts of 4-cyano-4-(thiobenzoylthio)pentanoic [chain transfer agent (CTA)] and 2,2-azobies(2-methylpropionitrile) (AIBN) were dissolved in 1 mL of benzene as shown in Table S3. This solution was deoxygenated under nitrogen flow for 10 min. Next, a mixture of methacrylic acid and alkyaminocrotonate at a molar ratio of 1.1:1 was prepared by adding methacrylic acid to the melted alkylaminocrotonate. The mixture was subsequently purged for 1 min before adding a 1:5 mass ratio mixture of AIBN and CTA. The mixture was then purged for 30 s with nitrogen before the initiation of the polymerization reaction at 70 °C, which lasts 3 days. The final polymer was obtained by precipitating it in acetone and subsequently drying it in the vacuum oven at 30 °C and a pressure of 25 in. Hg. According to the literature,39 common radical initiator efficiency is between 0.2 and 0.7. In the ideal case (70% of initiator efficiency), the designed (target) molecular weight can be 23 459 Da; and at 20% initiator efficiency, the target molecular weight can be 82 105 Da. Hence, with the mentioned concentration of the initiator, the target molecular weight should be between 23 459 and 82 105 Da. 2.3. NMR Characterization. A JEOL ECS 500 MHz NMR Spectrometer was used to characterize alkylaminocrotonate and poly(alkyl-grafted aminocrotonates). A solution of 0.5% alkylaminocrotonate was prepared in deuterated dimethyl sulfoxide for 1H NMR spectra. A Varian Unity Inova 500 MHz spectrometer was used to obtain 13C NMR spectra of both alkylaminocrotonates and amphiphilic polycaboxybetaines [P(OACRO−MAA), P(TACRO− MAA), and P(HACRO−MAA)]. For this measurement, a 2.5% solution was prepared in deuterated dimethyl sulfoxide. 8391

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Langmuir Table 1. Identification of FTIR Spectra of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA)a functional groups −1

frequency, (cm )

polymers

ν(NH)

ν(CH)as

ν(CH)s

ν(CO)

ν(CC)

δ(COO−)as

ν(COC)as

octylaminocrotonate P(OACRO−MAA) tetradecylaminocrotonate P(TACRO−MAA) hexylaminocrotonate P(HACRO−MAA)

3286(vw) none 3284(vw) none 3262(vw) none

2926(m) 2924(s) 2923(s) 2922(s) 2917(vs) 2921(vs)

2856(m) 2854(m) 2853(m) 2852(m) 2849(s) 2852(s)

1650(s) 1696(s) 1651(s) 1698(vs) 1646(s) 1696(s)

1604(vs) none 1606(vs) none 1599(vs) none

none 1557(s) none 1558(m) none 1550(m)

1167(vs) 1179(vs) 1168(vs) 1183(vs) 1170(vs) 1179(vs)

a

Band intensities and vibration types: vs, very strong; s, strong; m, moderate; vw, very weak; s, symmetric; as, asymmetric.

Figure 3. C13 NMR spectra of the enamine form of poly(hexadecyl-grafted aminocrotonate−methacrylic acid). allowed for the computation of inherent viscosities. This process was used to test the viscosities for a pure decane solution for reference; 1% w/v C36H74 solution in decane and 1% w/v solutions of P(OACRO− MAA) and P(HACRO−MAA) paraffin crystals, which were obtained from the hydrocarbon crystal formation experiment.

of the obtained hexadecylaminocrotonate, hexadecyl amine, ethyl acetoacetate, and their pairwise mixtures on a silica plate. Dichloromethane was identified as a good solvent, as three distinguishable components formed and were visible under 254 nm UV light exposure or in potassium permanganate solution. Flash column chromatography was utilized to collect the components, which were subsequently characterized by 1H NMR and FTIR. The first collection was identified as hexadecylaminocrotonate as shown in Figures 2 and S2. The second and third collections were a mixture of hexadecylaminocrotonate with unreacted ethyl acetoacetate or hexadecylamine. It is of significance to note that the first collection is of the enamine form of hexadecylaminocrotonate, which is formed by an intramolecular hydrogen bond.29 The final yield of the monomer was around 62.91−71.19% (Table S2). The obtained enamine form of hexadecylaminocrotonate was further used to perform RAFT polymerization to obtain amphiphilic polybetaines, resulting in purer products relative to random polymerization. Likewise, for the separation of tautomers of the other two aminocrotonates, octyl aminocrotonate and tetradecyl aminocrotonate, the aforementioned approach was successfully applied to obtain the enamine forms of their aminocrotonates. Their characteristic peaks were confirmed by FTIR (Table 1) and 1H NMR. The purified enamine forms of the alkyl aminocrotonates display different physical chemistry states. Octyl aminocrotonate is a viscous liquid, whereas tetradecyl and hexadecyl aminocrotonates are solids at room temperature. In order to obtain P(OACRO− MAA), P(TACRO−MAA), and P(HACRO−MAA) with low PDI, the enamine forms of the alkyl aminocrotonates were added into deoxygenated methacrylic acid to polymerize via

3. RESULTS AND DISCUSSIONS To systematically investigate the self-assembly of hydrophobically modified polycarboxybetaines and their interaction with long-chain hydrocarbon molecules, it is essential to synthesize the amphiphilic polymers with controlled molecular weight and various hydrocarbon lengths. This is because controlled chain lengths provide more ordered and precise structures, and both ionizable carboxyl acid groups and amine groups in the polycarboxybetaines with incremental hydrocarbon chain lengths offer adjustable and inducible forces. Hence, RAFT polymerization was used to obtain controlled molecular weight of amphiphilic polycarboxybetaines, which were P(OACRO− MAA), P(TACRO−MAA), and P(HACRO−MAA) (Figure 1A). In our previous work,38 although we successfully synthesized tridecyl-grafted polycarboxybetaine via random radical polymerization, which provided high molecular weight and high PDI (5.6) and confirmed the betaine structure of the polymers by NMR and FTIR characterization, two main concerns still exist. One is the confirmation of two tautomeric forms of alkyl aminocrotonate (Figure 1B). The second concern is the formation of polybetaines via the Michael addition, not through copolymerization. Therefore, we studied conditions for the separation of the two tautomers of hexadecylaminocrotonate via a thin-layer chromatography approach. First, various solvents were tested for the separation 8392

DOI: 10.1021/acs.langmuir.9b01347 Langmuir 2019, 35, 8389−8397

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Langmuir Table 2. Characteristics of Various Polymers Prepared via RAFT Polymerization monomer octylaminocrotonate−MAA tetradecylaminocrotonate−MAA hexadecylaminocrotonate−MAA

RAFT agent

polymer

4-cyano-4-(thiobenzoylthio)pentanoic acid

P(OACRO−MAA) P(TACRO−MAA) P(HACRO−MAA)

Mn × 104 b

a

3.514 1.579 5.099a 2.724b 2.237a 2.701b

Mw × 104

Mw/Mn

yield wt %

2.289a 5.384a 2.68a

1.450 1.056 1.198

33.5 31.8 36.2

a

Determined by GPC. bDetermined by NMR.

chemical shifts of the following characteristic peaks: 3.371 for primary alkyl groups, 2.2 for tertiary amine, 1.818 for carboxyl group (CH2COOH), and 1.223 for secondary alkyl group from the 1H NMR spectrum. These results confirm the occurrence of the Michael addition reaction before polymerization, consistent with results obtained via the random radical polymerization of methacrylic acid and tridecylaminocrotonate. Next, the molecular weight and PDI of the polycarboxybetaines were determined via GPC. These results were compared with the number average molecular weight obtained via 1 H NMR results because the CTA [4-cyano-4(thiobenzoylthio)pentanoic acid] has a distinct peak in the 1 H NMR spectrum as shown in Figure S4, which allows for an estimate of the number average molecular weight of the polycarboxybetaines. The number average molecular weight (Mn) and the weight average molecular weight (Mw) for P(HACRO−MAA) are 22 370 and 26 800 g/mol, respectively, with PDI = 1.198 (Table 2). The number average molecular weight of P(HACRO−MAA) from 1H NMR was found to be 27 000 g/mol, which is close to the number average molecular weight obtained from GPC. Number-average molecular weight, weight-average molecular weight, and PDI for P(OACRO−MAA) and P(TACRO−MAA) are reported in Table 2. The synthesis via RAFT polymerization as well as isolating the enamine form of hexadecylaminocrotonate from the different tautomers contributed to the low molecular weight polydispersity. The resulting amphiphilic polycarboxybetaines were utilized to investigate their self-assembly behaviors with increasing hydrocarbon chain lengths and pH-induced electrostatic forces. Self-assembly behaviors of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) were studied via zeta potential measurements, and TEM. First, the micelle structures formed from P(OACRO−MAA), P(TACRO− MAA), and P(HACRO−MAA) were observed by TEM. The self-assembled structures for P(OACRO−MAA), P(TACRO− MAA), and P(HACRO−MAA) were found to have from spherical, rod-like, to fractal shapes. Both Figures 4 and 5 show that micelle sizes of P(OACRO−MAA) and P(TACRO− MAA) increase with pH. Sizes of spherical micelles are between 5 and 50 nm. As seen in Figures 4a,b and 5a,b, spherical micelles formed in the range of 5−20 nm at pH 1 and in the range of 20−50 nm at pH 4. At pH 10, seen in Figures 4C and 5C, ∼120 nm rod-like micelle structures were formed. A perfect tree-like fractal morphology was observed at pH 12 (Figures 4D and 5D). The same shape and size ranges were also found for P(HACRO−MAA). The increase in hydrocarbon chain length from tetradecyl to hexadecyl in the polycarbxybetaines does not show a dramatic change in their self-assembly. To understand the phenomena, zeta potential changes of P(OACRO−MAA), P(TACRO−MAA), and P(HACRO− MAA) were studied in the pH range of 1−12 (Figure 6A). Isoelectric points (IEP) for all of the polybetaines were found to be between pH 1 and 2 and did not change with increase of

RAFT. In the random radical polymerization of methacrylic acid with tridecyl aminocrotonate mixture, both NMR and FTIR confirmed the formation of amphiphilic polycarboxybetaines, not polyampholytes (copolymers), which is explained by the Michael reaction.38 However, no direct evidence was given for this polymerization. In the RAFT reaction, the molecular weight of the polybetaines were controlled using a CTA. During synthesis of P(HACRO− MAA), the addition of a low concentration of the initiator (i.e. 0.1%) did not result in polymerization. When the initiator concentration was increased to 1%, RAFT polymerization occurred in 72 h. Acetone was added to the resulting P(HACRO−MAA) for separation. The slow RAFT polymerization is attributed to the fast addition−fragmentation rate of CTA on active chains, which also leads to low yields. The molar ratio of the CTA to the initiator was 2.38:1 for this RAFT approach. The P(HACRO−MAA) synthesized via RAFT polymerization was then characterized via FTIR (Figure S3, Table 1) and 1H NMR. In contrast to the FTIR results for alkyl aminocrotonate, the N−H and CC bonds disappear in the FTIR spectra for poly(alkyl-grafted aminocrotonate methacrylic acid). This occurs because of the Michael addition of methacrylic acid onto the secondary amine group, shown in Table 1. NMR was further applied to investigate the polymerization between alkyl aminocrotonate and methacrylic acid and to demonstrate the occurrence of Michael addition reaction between them. In comparison to 1H NMR, 13C NMR provides more direct information on the C−C bond backbone or side chain structures, as shown in Figure 3. Figure 3 shows the assigned chemical shifts of corresponding carbons in the polymer. The J, d, and d′ peaks confirm the addition of methacrylic acid and the presence of the tertiary amine group in P(HACRO− MAA). Together, the 13C NMR spectra and FTIR experiments confirm the polymerization of alkylaminocrotonate and the occurrence of the Michael reaction. The corresponding peaks obtained from NMR and FTIR are consistent with the same polybetaines synthesized via the random radical polymerization approach. Likewise, the successful synthesis of P(OACRO−MAA) and P(TACRO− MAA) was confirmed by FTIR and NMR using the same characteristic peaks. Their characteristic peaks from FTIR and 1 H NMR are tabulated in Table 1. The highest yields for these polycarboxybetaines are reported in Table 2. Although P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) were identified by both NMR and FTIR, there was still concern for the formation of the polycarboxybetaines via Michael addition. To investigate the reaction path, in the absence of an initiator, methacrylic acid was added into tetradecyl aminocrotonate to react. From previous investigations, it is known that mixture of acrylic acid and alkylaminocrotonates polymerize without the presence of an initiator.40 This route of reaction gives relatively low yield.40 The resulting polymer was confirmed to be poly(tetradecylgrafted aminocrotonate−methacrylic acid), which contains 8393

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hydrocarbon chain lengths. The zeta potential of the amphiphilic polybetaines reached around −50 mV, indicating that the carboxylic acid groups in the polycarboxybetaines are ionized and negatively charged. This means that in the basic environment, the repulsive force is dominant over hydrophobic force, which explains the increase of micelle sizes. When close to the IEP at pH 1, the amphiphilic polycarboxybetaine molecules interact, forming small spherical aggregate structures (Figure 6B). This phenomenon occurs because of strong interand intraelectrostatic interactions, as well as hydrophobic interactions, common for polybetaines. As pH increases, the polycarboxybetaines were negatively ionized, causing swelling and forming large spherical micelles. This occurs as a result of strong repulsive electrostatic forces. A zeta potential study further confirms this mechanism as the zeta potential becomes more negative with increasing pH. Electrostatic forces among the hydrophilic ionized groups compete with hydrophobic interactions among alkyl chains, causing the formation of rodlike micelle structures as pH increases to pH 10. This occurs because of the increasing hydrophobic interactions and is a common occurrence during amphiphilic polymers aggregation. At high pH (∼12), the assembled structure formed fractal/ dendritic aggregates, which can be commonly observed as the amphiphilic polymer contains carboxyl acid groups that are self-assembled in high concentration of NaOH and NaCl. According to the literature,41,42 it was shown that in basic conditions, carboxylic acid groups of polymer blocks are ionized and formed sodium-ionized chains, which can create cluster structures. The drying process drives precipitation of the cluster on the template of the crystalline NaOH and, thus, creates finer brushes. During self-assembly of polyampholytes, such as PDMAEMA-b-PAA and PS-b-PAA, NaOH and NaCl are the main cause for fractal aggregation formation. The above detailed study explains and strongly supports the formation of dodecyl-grafted polycarboxybetaines with fractal/dendritic structures at high pH. To investigate the hydrophobic dominant interaction between long-chain hydrocarbons and amphiphilic polycarboxybetaines, P(OACRO−MAA) and P(HACRO−MAA) were selected as models for two extreme hydrocarbon lengths. This study involved long hydrocarbon crystal deposition and viscosity experiments to determine the effect of polycarboxybetaines on the nucleation and crystallization of long hydrocarbons. In the long hydrocarbon crystallization experiment, we built an experimental setup mimicking the cold finger

Figure 4. TEM images for P(OACRO−MAA) formed in aqueous dispersions at set pH intervals. TEM images were slightly altered to increase the contrast (A) pH 1.0, (B) pH 4.0, (C) pH 10.0, and (D) pH 12.0.

Figure 5. TEM images for P(TACRO−MAA) formed in aqueous dispersions at set pH intervals. TEM images were slightly altered to increase the contrast (A) pH 1.0, (B) pH 4.0, (C) pH 10.0, and (D) pH 12.0

Figure 6. (A) Zeta potential data: black squares: P(OACRO−MAA); green triangles: P(TACRO−MAA); red circles: PHACRO−MAA. (B) Schematic explanation of self-assembly structure of the amphiphilic polycarboxybetaines. 8394

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Figure 7. Paraffin deposition: (A) relationship between paraffin deposition and the increase in hydrocarbons of the amphiphilic polycarboxybetaines at a polymer concentration of 100 ppm, (B) decreased paraffin deposition with increasing concentration of P(OACRO− MAA), and (C) viscosity change with the increasing concentration of P(OACRO−MAA) and (D) P(HACRO−MAA).

Figure 8. Schematic illustration for study of long-chain hydrocarbon crystallization.

experiment (Figure S1) to test the effect of the two amphiphilic polycarboxybetaines on the crystallization of C24H50 or C36H74 hydrocarbons. The temperature of the decane solution containing either C24H50 or C36H74 was kept at 50 °C, and the inserted cold tube was kept at 0 °C. Because of the temperature difference between the surface of the inserted cold tube and the long hydrocarbon solution, both C24 and C36 hydrocarbons were easily crystallized and deposited on the cold tube surface. As shown in Figure 7a, the long hydrocarbon deposition amounts were higher as the hydrocarbon chain length in the amphiphilic polycarboxybetaines was shorter. In contrast to this, as the hydrocarbon chain lengths in the amphiphilic polycarboxybetaines were high, the deposited amount of long hydrocarbon was low. To understand this phenomenon, the viscosity changes of the above solutions were investigated through the addition of P(OACRO−MAA) and P(HACRO−MAA). The viscosities of the long hydrocarbon solutions, as both the hydrocarbon lengths in the amphiphilic polycarboxybetaines and their concentration were increased, were getting less and less. These results were consistent with the long hydrocarbon crystal formation results. These findings can be explained as follows (Figure 8). As the hydrocarbon

chain lengths in the amphiphilic polycarboxybetaines increase, the hydrophobic interactions between the long hydrocarbon molecules and the amphiphilic molecules also increase. Thus, amphiphilic polycarboxybetaines with long hydrocarbon chains are easily attached to the nucleus of the long hydrocarbon during the nucleation formation and can alter the surface properties of the long hydrocarbon nuclei. Ultimately, this prevents the formation of long hydrocarbon crystals further. This explains the observed trend of smaller deposition amounts in the hydrocarbon crystallization experiment and the reduction in solution viscosities when the hydrocarbon chains in the amphiphilic polycarboxybetaines are longer. These hydrophobic interaction studies are beneficial to the design and development of new paraffin inhibitors.

4. CONCLUSIONS Amphiphilic polycarboxybetaines containing three different hydrocarbon groups were synthesized successfully via RAFT polymerization. For the first time, the enamine form of the alkyl aminocrotonate tautomers were separated and further polymerized to produce three different polyalkyl-grafted aminocrotane−methacrylic acids: P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA). Zeta potential 8395

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measurements were used to determine the IEP for these amphiphilic polycarboxybetaines, which was pH 1−2. Selfassembled aggregations for P(OACRO−MAA), P(TACRO− MAA), and P(HACRO−MAA) with variable pH were found to have from spherical, rod-like, to fractal structures. Hydrophobic interactions between long hydrocarbon molecules and P(OACRO−MAA), P(TACRO−MAA), and P(HACRO−MAA) offer new understandings of the crystallization of long-chain hydrocarbon molecules. The changes in the crystal deposition and viscosities of these hydrocarbons is explained by strong hydrophobic forces and the ability for zwitterionic groups to alter the process of long hydrocarbon crystallization. The understanding of the self-assembly of amphiphilic polycarboxybetaines and their association with hydrocarbon molecules provide us with invaluable information for their potential applications in industries ranging from nanomedicine to the petroleum field.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01347. Scheme for long-chain hydrocarbon crystal deposition experiment; FTIR spectra of hexadecylaminocrotonate, FTIR spectra of P(OACRO−MAA), 1H NMR spectra of P(HACRO−MAA), Rf values for ethyl acetoacetate, alkylamine, and monomers, yields for different aminocrotonates, and experimental conditions for the synthesis of P(HACRO−MAA) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nurxat Nuraje: 0000-0002-4335-8905 Present Address ¶

Department of Chemical Technology of Organic Compounds and Polymers, K. I. Satpaev Kazakh National Research Technical University, Almaty 050013, Kazakhstan

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.N. greatly acknowledges the financial support from ACS PRF ((57095-DNI7)). We thank Professor Hongjun Liang’s group at Texas Tech University Health Sciences Center for the help with GPC measurements and Mr. Kydyrmolla Akatan at “Sarsen Amanzholov East Kazakhstan State University” for finding out molecular weight via 1H NMR. All authors thank Robin Dupre for proofreading the entire manuscript.

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