Phase Behavior of Poly (vinylidene fluoride)-graft-poly (diethylene

Feb 9, 2016 - Atanu Kuila , Dhruba P. Chatterjee , Nabasmita Maity , Arun K. Nandi. Journal of Polymer Science Part A: Polymer Chemistry 2017 55 (16),...
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Phase Behavior of Poly(vinylidene fluoride)-graf t-poly(diethylene glycol methyl ether methacrylate) in Alcohol−Water System: Coexistence of LCST and UCST Atanu Kuila, Nabasmita Maity, Dhruba P. Chatterjee,† and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *

ABSTRACT: A thermoresponsive polymer poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) is grafted from poly(vinylidene fluoride) (PVDF) backbone by using a combined ATRC and ATRP technique with a high conversion (69%) of the monomer to produce the graft copolymer (PD). It is highly soluble polymer and its solution property is studied by varying polarity in pure solvents (water, methanol, isopropanol) and also in mixed solvents (water−methanol and water−isopropanol) by measuring the hydrodynamic size (Z-average) of the particles by dynamic light scattering (DLS). The variation of Z-average size with temperature of the PD solution (0.2%, w/v) indicates a lower critical solution temperature (LCST)-type phase transition (TPL) in aqueous medium, an upper critical solution temperature (UCST)-type phase transition (TPU) in isopropanol medium, and no such phase transition for methanol solution. In the mixed solvent (water + isopropanol) at 0−20% (v/v) isopropanol the TPL increases, whereas the TPU decreases at 92−100% with isopropanol content. For the mixture 20−90% isopropanol, PD particles having larger sizes (400− 750 nm) exhibit neither any break in Z-average size−temperature plot nor any cloudiness, indicating their dispersed swelled state in the medium. In the methanol + water mixture with methanol content of 0−30%, TPL increases, and at 40−60% both UCSTand LCST-type phase separations occur simultaneously, but at 70−90% methanol the swelled state of the particles (size 250−375 nm) is noticed. For 50 vol % methanol by varying polymer concentration (0.07−0.2% w/v) we have drawn a quasibinary phase diagram that indicates an approximate inverted hourglass phase diagram where a swelled state exists between two single phase boundary produced from LCST- and UCST-type phase transitions. An attempt is made to understand the phase separation process by temperature-dependent 1H NMR spectroscopy along with transmission electron microscopy.

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INTRODUCTION The phase transition behavior of thermosensitive polymers in aqueous solution is still a major research area due to their applications in sensors, devices, drug delivery, membranes etc.1−6 These polymer solutions often show thermally stimulated phase transitions and these phase transition temperatures (Tp) vary with polymer concentrations yielding a curve having either a maxima or a minima. Depending on the polymer’s nature the Tp−composition plot exhibits lower critical solution temperature (LCST) and upper critical solution temperature (UCST) showing a minima and a maxima along that curve, respectively. Often these phase transitions are triggered by many factors like polymer chain length, pH, ionic strength, etc. of the solution7−9 and of course most importantly by the hydrophobic and hydrophilic natures of polymer and as well as of solvents.10−13 Solvents’ hydrophobic−hydrophilic balance can be tuned by mixing the solvents like alcohols with water as alcohols have lower polarity.14 Depending on the choice of alcohols (e.g., methanol, ethanol, isopropanol, etc.) and mixing at different proportions with water the solvent polarity can be finely tuned, which may lead to a competition among the hydrophobic and hydrophilic components between © XXXX American Chemical Society

the polymer and solvent domains. Some typical examples are there for coexistence of LCST and UCST concomitantly in a polymeric solution.15−17 In these systems polymer concentration is an important factor that is capable of triggering effectively both critical solution temperatures (CSTs) and variation of those CST points toward unconventional phase diagrams.18,19 Poly(vinylidene fluoride) (PVDF)-based graft copolymers are always at the center of attraction because of their smart material properties.20−24 These graft copolymers are generally synthesized from PVDF chain either by introduction of peroxide groups through ozone treatment25,26 or by conventional ATRP technique,26−31 but the questions are still asked about livingness of the grafting and control over grafting. The answer is in our prior work,32 where we reported the anchoring of model ATRP initiating sites from PVDF backbone. The grafting of a wide range of (meth)acrylates or acrylamide monomers from PVDF backbone is now quite possible, leading Received: December 1, 2015 Revised: January 16, 2016

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pyridine (Py) (RANKEM, RFCL Limited, New Delhi, India) were distilled and stored in an argon atmosphere. Synthesis of 4-Hydroxy TEMPO Attached PVDF (PTOH). In a nitrogen -purged tube (8 × 2.5 cm), 0.5 g of PVDF and 1 g of 4-hydroxy TEMPO (5.8 mmol) were taken and dissolved in 4 mL of DMF at 60 °C with vigorous stirring. Next, CuCl (56.4 mg, 0.57 mmol) and DMDP (311.2 mg, 1.68 mmol) were added to the above mixture at room temperature (30 °C) and the tube was sealed with a rubber septum. The reaction mixture was stirred for 48 h in an oil bath at 90 °C. The viscous product was then precipitated into a methanol− water mixture (1:5 v/v) and filtered. The residue obtained was washed with methanol three to four times. The as-synthesized polymer was purified by redissolving and reprecipitating twice more. Finally, the polymer was dried under vacuum for 2 days at room temperature. The 1H NMR spectra of PTOH is shown in Figure S1, where at the inset the NMR peaks corresponding to protons of four methyl and two methylene groups of 4hydroxy TEMPO are clearly observed at δ 1.15−1.23.38 Synthesis of PVDF-2-bromoisobutyrate (PVDFBIB). For the synthesis of PVDFBIB, PTOH (0.25 g) was dissolved in PC (20 mL) in a nitrogen-purged tube (12 × 4 cm) at 80 °C and was allowed to cool to room temperature; after that 2bromoisobutyryl bromide (BIB) (2 mL) was added drop-bydrop into it by a pressure equalizer at 0−5 °C, and the reaction mixture was allowed to stir at low temperature for 6 h. After that it was kept stirring at room temperature (30 °C) for the next 24 h. The reaction mixture was diluted and precipitated into a methanol−water mixture (1:5 v/v), filtered, and washed with methanol three times. For the purification purpose the product was redissolved and reprecipitated twice more. Ultimately, the polymer was dried under vacuum for 2 days at 30 °C. The 1H NMR spectra of PVDFBIB are shown in Figure S1, where at the inset the NMR peaks corresponding to protons of four methyl and two methylene groups of 4-hydroxy TEMPO are clearly observed at δ 1.15−1.23 and that of BIB at δ 1.87 is observed. Synthesis of PVDF-g-PMeO2MA (PD). In a nitrogenpurged tube (8 × 2.5 cm), PVDFBIB (0.1 g) dissolved in 1 mL NMP was taken and the monomer (0.5 mL) [MeO2MA (2.7 mmol)], DMDP (0.044 g, 0.24 mmol), and CuCl (0.01 g, 0.1 mmol) were added one by one and the tube was sealed with a rubber septum. The reaction mixture was stirred at 60 °C in an oil bath for 24 h. The reaction vessel was allowed to cool to 30 °C. The mixture was then diluted with 2 mL of NMP and was poured in PET ether. The precipitated polymer was collected and was purified by repeating the process twice. The assynthesized polymer was stirred in hot water at 50 °C to make PVDF graft copolymer copper free. After that, the PD was washed with chloroform several times to remove any trace of homopolymer of diethylene glycol methyl ether methacrylate (PMeO2MA) if it remained present. Then, the product was dried under vacuum for 3 days at 60 °C and weighed, and the % conversion was calculated. The 1H NMR of PD is shown in Figure S2 along with the characteristic peaks assigned for PD. Characterization. 1H NMR Spectroscopy. The 1H NMR spectra were carried out using a 500 MHz NMR spectrometer (Bruker) and DMSO-d6, D2O, and MeOD (Aldrich) were used as solvents according to requirement of the samples. Microscopy. The morphology of the samples was investigated using a transmission electron microscope (TEM JEOL, 2010EX) operated at 200 kV acceleration voltage. The PD sample was dropcasted from a methanol−water mixture (1:1 v/

to a very significant amount of conversion with low polydispersity. This common strategy gives an elite way for preparing PVDF-based graft copolymers with living chain and controlled graft lengths and graft densities, and with this it is now convenient to prepare a gigantic molecular weight hydrophilic graft copolymer. PVDF-based graft copolymers decorated with hydrophilic thermo responsive poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) and poly(N-isopropyl acrylamide) (PNIPAAm) chains can exhibit interesting solution behavior in aqueous medium due to the presence of LCST of the grafted components.33,34 Although both PNIPAAm and PMeO2MA exhibit the LCST-type phase transition, the phase transition is better reversible for PMeO2MA than for PNIPAAm. which exhibits a larger hysteresis because above LCST PNIPAAm chains remain partially dehydrated.34,35 Furthermore, for the short chain polymer PNIPAAm has a significant influence of the end groups on the thermal behavior.36 To avoid these difficulties we have chosen PMeO2MA to graft from PVDF. In our prior work37 we synthesized PVDF-g-PMeO2MA (PD), and temperature triggered antifouling PVDF membranes are produced utilizing the LCST behavior of the sample. We then observed that the polymer produced at 24 h is highly soluble in water because of its high conversion. This tempted us to study the solution property of the PD graft polymer produced at this high conversion because the solution behavior can be finely tuned with temperature and polarity of the medium. Here we report a thorough study on the solution properties of PD graft copolymer by measuring their sizes in water, isopropanol, and their mixtures in a series by dynamic light scattering (DLS) experiments. In another series we have used a more highly polar alcohol, methanol (relative polarity: water = 1.000, methanol = 0.762, isopropanol = 0.546),14 and the phase transitions are studied in a similar fashion. In the former systems UCST- and LCST-type phase separations occur independently, while in the latter systems apart from the independent UCST- and LCST-type phase transition simultaneous appearance of both UCST- and LCST-type phase transition is observed in few solvent compositions. A quasibinary phase diagram drawn at 50 vol % methanol exhibits an approximate inverted hourglass nature. Temperaturedependent 1H NMR spectra and morphology study using transmission electron microscopy (TEM) are used to understand the above phase behavior in both of the systems. Possible explanations have been put forward from the previous results for understanding the different phase transitions.

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EXPERIMENTAL SECTION Materials. Poly(vinylidene fluoride) (PVDF) from SigmaAldrich (Mn = 7.1 × 104, PDI 2.57, head-to-head (H−H) defect = 4.33 mol %) was recrystallized from its dilute acetophenone solution (0.2% w/w), and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) (Sigma-Aldrich) was used as received. The monomer diethylene glycol methyl ether methacrylate (MeO2MA) purchased from Sigma-Aldrich was purified by passing through a basic alumina column. For purification CuCl (Aldrich) was taken in a Schlenk tube under a nitrogen atmosphere and washed by 10% aqueous HCl solution, followed by methanol and diethyl ether. 4,4′Dimethyl-2,2′-dipyridyl (DMDP, Aldrich) was used as received. Propylene carbonate (PC) (Aldrich Chemical USA), N-methyl2-pyrrolidone (NMP), dimethylformamide (DMF), and B

DOI: 10.1021/acs.jpcb.5b11736 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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bromoisobutyryl bromide (BIB). The graft density can also be rechecked using the integral area of peak y (Supporting Information) corresponding to BIB in the spectra of PVDFBIB. The same graft density value in both of the measurements indicates the BIB is attached exclusively from the −OH group of TEMPO. Then, MeO2MA is polymerized from the above model macroinitiator (PVDFBIB) using a CuCl/DMDP catalyst− ligand system in N-methyl-2-pyrrolidone (NMP) medium at 60 °C. The time of polymerization is 24 h for preparation of PD, as we need a higher conversion graft copolymer for better degree of solubility in polar solvents like water or alcohols. Under this ATRP condition an equilibrium concentration of cupric complex is maintained throughout, and it can act as a quencher for any unwanted radical sites generation in the system. Therefore, during grafting reactions from modified PVDF the formation of homopolymer of MeO2MA due to thermal polymerization is not possible.40 The 1H NMR spectra (Figure S2) of PD in DMSO-d6 shows a high amount of grafted PMeO2MA chains (69% conversion of the monomer). The peaks due to grafted chains and PVDF backbone are shown in Figure S2. The PD graft copolymer has Mn (NMR) 460 000 and PDI 1.2.34 The calculated graft density is 7.8 per PVDF chain, so the average interval of grafting is 142 and the graft length calculated from NMR spectra is 265 (Supporting Information). The solution property of this important graft copolymer is inspected in the following section, where the solvent polarity is changed by mixing alcohols of varying polarity and size in water at different proportions. Behavior in Aqueous Environment. Figure 1a illustrates the variation of hydrodynamic diameters (Z-average size) of PD within the temperature range between 5 and 45 °C, clearly

v) onto a 300 mesh carbon-coated copper grid at 15, 30, and 45 °C, and after drying at those temperatures they were used for TEM analysis directly. Dynamic Light Scattering. The DLS experiments of PD solutions were done in a Malvern instrument in which the laser source was a He Ne-laser at an angle of 173° equipped with a noninvasive back scatter detector using the method of cumulants. For temperature variable study, the solutions of polymer samples in water, alcohol, and their mixture (0.2%, w/ v) were heated from 5 to 45 °C at a heating rate of 5 °C min−1. The results were recorded at each 2 °C interval after equilibrating the solution for 2 min.

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RESULTS AND DISCUSSION The PMeO2MA grafted PVDF chain (PD) produced following Scheme 1 exhibits the unique balance of hydrophobicity from PVDF main chain and hydrophilicity of grafted PMeO2MA chains. The PVDF chain is coupled to 4-hydroxy TEMPO by atom transfer radical coupling (ATRC)39 in the presence of CuCl (catalyst) and DMDP (ligand) in dimethylformamide (DMF) medium at 90 °C. The grafting density of PTOH per PVDF chain is calculated from the 1H NMR spectra (Figure S1) using the equation graft density =

total area of peaks x /16 × DPn of PVDF total area (H−T + T−T) of peaks of PVDF/2

where x denotes the integral value of peaks corresponding to the protons of four methyl groups and two methylene groups of TEMPO moieties and DPn indicates the number-average degree of polymerization. The pendant −OH group of TEMPO moiety is utilized for attaching ATRP initiator, 2C

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revealing the thermoresponsiveness with incredible reversibility at both heating and cooling process. The size of the PD polymer in 0.2% (w/v) aqueous solution is ∼10 nm at 5 °C, and it shows a consistency up to 21 °C of temperature. This lower size of the graft copolymers indicates a molecular level dispersion of the PD graft copolymer in water in the 5−21 °C temperature range, but above 21 °C it starts increasing and crosses 550 nm at 37 °C. An inflection point is observed at 25 °C (obtained from the peak of the derivatogram) that can be called a phase-transition (LCST-type) temperature (TPL) (Figure S3) This LCST behavior of the graft copolymer implies a sharp increase in hydrodynamic diameter upon heating due to self-aggregation of the polymer chains for the lowering of hydrophilic (H-bonding) interaction with the solvent molecules resulting in phase separation. To investigate the actual critical solution temperature and critical concentration of the PD-water system, we determined the TPL values for different PD concentrations by DLS experiments, and the TPL values are plotted in Figure 1b. It is inferred from the plot that this polymeric system in aqueous environment shows an LCST at 23 °C having the critical concentration of 0.5% (w/v). This phase diagram implies that the solution is completely immiscible above the concave upward curve showing the typical characteristic of LCST behavior. From Figure 1a the hydrodynamic diameter monitored below LCST (∼10 nm) clearly signifies the molecular level dispersion of a single PVDF (Supporting Information) chain where the hydrophilic part of the PMeO2MA chains allows water molecules to their closer proximity by means of strong hydrogen bonding enforcing the hydrophobic PVDF chain to become well dispersed in water. The solvation of the grafted side chain is so large that the adhered solvent sphere prohibits the two solvated PVDF coils from approaching each other. Above LCST the olefin main chains of PMeO2MA and PVDF backbone collapse as H bonds between the ethylene glycolic group of grafted chain and water molecules weaken, causing desolvation, and as a consequence

Figure 1. (a) Variation of Z-average sizes of PD (0.2%, w/v) in water with temperatures. (b) Phase diagram of PD in water solution.

Figure 2. 1HNMR spectra of PD solution in D2O at indicated temperature. The shaded region indicates the peaks of methylenic protons of the grafted PMeO2MA of the PD polymer. D

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The Journal of Physical Chemistry B aggregation between the grafted PVDF chains starts due to hydrophobic interaction causing larger sizes. A closer insight into the fact can be envisaged from 1H NMR study in D2O because it is an important tool to understand the molecular interaction in the solution state. Figure 2 demonstrates the variation of signature peaks of PMeO2MA chains with temperatures in the NMR spectra. Because of microaggregation in D2O the proton signals of PVDF and methacrylic chains may not be visible, 34 but signals corresponding to diethylene glycol protons c, d, and e would be visible because of their solvated state. The characteristic peaks of the ethylene glycolic groups are designated in the Figure. At 5 °C all of these prominent peaks show a clear indication of being solvated by D2O molecules by means of strong H bonds with D2O. It is necessary to point out that the signal of “c” protons is more broad than that of “d” and “e” protons because it is adjacent to the carboxylate group. The pattern continues to 20 °C but with a gradual shift to the deshielded zone. The peaks due to methylene protons directly attached to ether bonds (peak d) are shaded and reveal a healthy 0.25 ppm shift. No definite reason for the shifts can be put forward here, and one probable reason behind the unusual fact is that PVDF may have some important role. There may exist an H-bonding interaction between >CF2 and methylene protons of the side chain of PMeO2MA chains. As temperature starts to increase below LCST these interactions weaken; as a result protons would appear at deshielded zone, but at 25 °C water molecules start desolvating the glycolic chain of PD and as a consequence peak intensity starts diminishing, and from 30 °C onward all of those peaks completely vanish. Precisely, at low temperatures (below LCST) two types of H-bonding are possibly active in the solution: first, the H bonding among hydrophilic side parts of PMeO2MA chains and water molecules, which manifests the solvation−desolvation phenomena and mostly reflects in the 1H NMR intensities in D2O environment, and second, H bonding among hydrophilic parts of PMeO2MA chains and some adjacent fluorine atoms of PVDF backbone due to the closer existence of hydrophilic− hydrophobic segments in the PD graft copolymer. Because of the induced solubility of the PVDF chains by the grafted PMeO2MA chains H-bonding interactions between the main and grafted chain might be operative at least to some extent, and it reflects in the gradual shifting of the PMeO2MA signature peaks in 1H NMR in the D2O environment. Behavior in Nonaqueous Environment. To get a clearer idea of the solvation and desolvation processes we have attempted to get into the behavior of PD polymer in nonaqueous solvents like smaller alcohols, for example, methanol and isopropanol having decreasing polarity. In Figure 3a we can clearly observe an invariant Z-average size of 8 nm at a wide range of temperatures (5−40 °C) in the methanolic solution (0.2% (w/v) concentration), but in isopropanol solution (Figure 3b) the picture is quite different, exhibiting a thermal phase transition. At lower temperature (up to 14 °C) the Z-average size of the PD polymer in isopropanol environment exhibits a comparative higher value (∼300 nm) (Figure 3b), after 14 °C the size starts to decrease, and at 28 °C onward it shows constant value of ∼3 nm. These smaller sizes are evidence of molecular level dispersion of the PVDF graft copolymer, whose grafted PMeO2MA chains are likely to be in a solvated state in isopropanol at higher temperature (≥28 °C). A comparison among the average hydrodynamic sizes of PD in solution state are 10, 8, and 3 nm for the same PD

Figure 3. Variation of Z-average sizes of PD (0.2%, w/v) in (a) methanol and (b) isopropanol at different temperatures. (c) Phase diagram of the PD in isopropanol solution.

concentration in water, methanol, and isopropanol, respectively. This is indeed a reflection of the solvated condition of the chain, which is relatively higher for water. At this state some solvent molecules get adhered on the PD chain, and in methanol it is somewhat lower than water, whereas in isopropanol it is significantly lower. The relative polarity of isopropanol is the lowest among the three, and its size is also higher, causing a lower hydrodynamic volume arising from the coiled state of the polymer in this solvent. The exact phasetransition temperature in Figure 3b is obtained from the inflection point calculated from the maximum of the derivatogram (Figure S4). This certainly indicates a UCST-type phase transition (TPU) of PD polymer in isopropanol, and it is further examined by determining a series of cloud points for different concentrations of PD in isopropanol solution Figure 3c illustrates the phase diagram of a convex downward curve characterizing a typical UCST-type phase separation. The UCST of PD is at 27.5 °C with a critical composition of 0.09% (w/v) in isopropanol. The larger Z-average size below UCST E

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The Journal of Physical Chemistry B (e.g., at 14 °C, Figure 3b) arises because polymer−polymer and solvent−solvent interactions are prominent in comparison with polymer−solvent interaction in this medium. This absolutely enthalpy-driven phenomenon is further illuminated when temperature increases and a number of isopropanol molecules penetrate into the side chains of PMeO2MA and help the polymer and solvent molecules interact with each other. It means that the polymer−solvent interaction starts increasing, and above 28 °C this particular interaction becomes more prominent than the other two interactions. As a consequence, the solvation of PMeO2MA side chains by isopropanol molecules reaches a limiting value and Z-average size shows only for a single chain of the PVDF graft copolymer. It is important to comment here that although both water and isopropanol have −OH groups in their structure both can interact with the ethylene glycolic part of the PD polymer via H-bonding process, although the latter has lesser polarity, but the above DLS studies indicate a difference in the types of phase transition associated with the PD polymer, and it may be attributed to the size of the two molecules. Because the size of isopropyl alcohol is much larger than that of water it cannot fully approach to solvate the ethylene glycolic part of PD polymer at lower temperature, causing a much smaller interaction with the PD chains than that in water media. With the increase in temperature the isopropanol molecules gain enough thermal energy to penetrate to the ethylene glycolic part of PD, causing more interaction. It results in a favorable free energy of mixing exhibiting an UCST-type phase separation. The LCST-type phase separation may be originated from the entropy of solvent molecules, which increases more abruptly than that of the polymer molecules with temperature. As a result the entropy of mixing decreases with increase in temperature and also here the enthalpy of H-bonding interaction would likely decrease to some extent due to increased randomness. The overall effect is the unfavorable free energy of mixing causing LCST-type phase separation. In the case of PD−methanol system (Figure 3a) neither UCST- nor LCST-type phase separation is observed because both polarity and size are intermediate of the other two solvents. Effect of Isopropanol Content in Aqueous Solution of PD. It is now interesting to study the effect of mixed-solvent (water + isopropanol) on the phase transition behavior of PD graft copolymer as the phase behavior is of completely opposite nature in the respective pure solvents; the former shows LCST but the other exhibits the UCST-type phase separation, respectively. Figure 4a symbolizes the variation of Z-average size of PD with temperature at lower isopropanol content (0− 30 vol %) of the mixture, where the Z-average sizes have been determined at the same concentration of PD (0.2%). We have previously elaborated that when PD is completely hydrated in pure water it exhibits a sharp LCST-type phase transition. As the amount of isopropanol starts increasing, in the mixture a gradual lowering of favorable polymer−solvent interactions occurs because the isopropyl group obstructs the effective interactions among hydrophilic ether side chains of PMeO2MA and the −OH group of water. As a consequence, a larger interchain interaction of PD is observed that reflects the increase in the Z-average sizes below 20 °C. Further increase in isopropanol (30−90% isopropanol) in the mixed solvent PD does not exhibit any thermal transitions. A noticeable fact comes when isopropanol content crosses the 90% limit. The polymer solution starts to exhibit the UCSTtype behavior as in pure isopropanol environment. Starting

Figure 4. Z-average sizes of PD (0.2%, w/v) at different temperatures in isopropanol−water solutions for different isopropanol content (a) 0, 10, 20, and 30% and (b) 90, 92, 94, 96, 98, and 100%.

with pure isopropanol solution, it is clearly evident that with increasing slight water content the UCST-type phase transition sharpens (Figure 4b). The result can be explained in terms of probability and extent of H bonding. Because a couple of water drops come in polymer solution along with isopropanol, a healthy competition between isopropanol and water for H bonding with PD polymer starts operating. Also, polymer− isopropanol H-bonding probability decreases as some of isopropanol molecules get H-bonded with water molecules. As a consequence, below UCST-like interactions (polymer− polymer) become more prominent than unlike interactions (polymer−solvent) with increasing water content from 0 to 8%. The sharpness of the thermal phase transition decreases with increasing amount of isopropanol (Figure 4a), and it is also more clarified from the derivatogram Figure S5, which also reflects an increase in TPL. As isopropanol content increases less H bonding occurs between polymer and solvent molecules. After a certain temperature those H-bonding interactions become ineffective, which results in the interchain aggregation of PD polymer, causing a gigantic size. Thus, LCST-type transition happens in solution, and the less H-bonding breaking, the less sharp the LCST type transition. In the case of UCST-type phase separation (Figure 4b), the sharpness of the transition increases with increase in water content in the isopropanol rich state (Figure S6). The reason is more favorable H-bonding of PD with water in the solvent mixture above the UCST. At 90 vol % of isopropanol neither LCST nor F

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The Journal of Physical Chemistry B UCST transition is noticed, although the size is very large (∼750 nm) in the whole temperature region. No definite reason can be put forward for this observation, and it may be that the polymer remains in the swelled state at the whole temperature region. At 40% isopropanol also a large size (450− 350 nm) is observed, and we do not observe any break characterizing the phase transition, although a small decrease in hydrodynamic size with increasing temperature is noticed (Figure S7). A complete phase diagram of PD polymer in the isopropanol−water mixed solvents for its 0.2% (w/v) concentration is shown in Figure 5. It is clear from the phase

Figure 6. Optical images of PD polymer in water, isopropanol, and their mixtures at indicated temperatures and composition of the mixed solvent for 0.2% (w/v) PD concentration.

transition and the proper enlightenment of hydrophilicity− hydrophobicity balance, we have also chosen methanol (smallest size alcohol with intermediate relative polarity)14 as a component along with water. Figure 7 illustrates the Zaverage sizes of PD at a wide range of temperature in the solvent mixtures with varying methanol content. In Figure 7a the said property is demonstrated for the polymer in the mixed solvent with methanol content of 0−30%. To start with 0%, that is, pure aqueous environment, polymer shows a distinct phase transition (LCST type) that has been previously described. With increasing methanol content (10−30%) phase transition curves are likely to be in similar fashion. It is clear from the derivatogram (Figure S8) that sharpness of the phase transitions is almost unaffected, although a minor gradual increase in cloud point is observed with increasing amount of methanol content in the solvent mixture. The polymer solutions with major amount of methanol (70−100%) do not show any thermal transitions (Figure 7c). From the figure it is evident that the PD solution in pure methanol shows a unchanged Z-average size throughout a temperature range from 14 to 50 °C, where the polymer is molecularly soluble, exhibiting Z-average size ∼8 nm, but with decreasing the amount of methanol (methanol content 90−70%) the Zaverage size increases abruptly (250−375 nm), indicating that the particles are in a swollen state, as previously discussed. It is noteworthy that results come out of the temperature-dependent DLS experiments for the PD solution in methanol−water mixture at intermediate compositions (methanol content 40, 50, and 60%) (Figure 7b, Figure S9). In these solvent mixtures both UCST- and LCST-type phase transitions coexist. For 40% methanol LCST appears before UCST, but for 50 and 60% methanol content LCST appears after the appearance of UCST. At higher methanol content with increase in temperature the methanol molecules gain enough thermal energy to become nearer to the ethylene glycolic part, causing more interaction and solvating it better, and hence the UCST-type phase transition takes place. The above results are summarized in Figure 8, where both the transition temperatures (TPL and TPU) with the methanol content are plotted at the fixed PD concentration (0.2% w/v). The transition temperatures (TPL) are increasing all the way to 60% methanol content due to the higher amount of methanol content into the polymer solution. A careful look at the Figure indicates that up to 40% methanol content the transition points

Figure 5. Phase diagram of PD polymer (0.2%, w/v) solution in different isopropanol−water mixed solvents for different isopropanol content obtained from DLS experiment.

diagram that there is a window of the solvent composition (20−90 vol %) of isopropanol where the PD polymer is possibly in the swelled state. On the left-hand side of the plot it is LCST-type phase separation, whereas the right-hand side plot characterizes the UCST-type phase separation. Again, below the curve on the left-hand side the polymer is miscible, and below the curve on the right-hand side it is immiscible. The important point noticeable here is that the LCST-type transition temperature increases and the UCST-type transition temperature decreases with isopropanol content in the mixture. The swollen state of PD at the intermediate region of the phase diagram can be well understood from the optical images presented in Figure 6. In Figure 6a, the aqueous PD solution is transparent but the isopropanol solution is hazy at 10 °C which is below the LCST and UCST of PD in water and isopropanol, respectively. At 30 °C (Figure 6c) the aqueous solution is cloudy, whereas the isopropanol solution is transparent because this temperature is above the LCST and UCST of PD in water and isopropanol, respectively. At 30 °C, the intermediate compositions (40, 70, and 90%) (Figure 6b) of the solutions are transparent in visible light, although all of these system exhibit a very large size similar to or even higher than the phase-separated systems. The only cause may come from the internal structure of the particles, and it may be that some solvent molecules remain entrapped within the PD particles, making it a “swelled state”. This causes a lower scattering of the visible light, and no turbidity is observed, as observed in the phase-separated systems. Effect of Methanol in Aqueous Solution of PD. To get an enriched idea of the phenomenon of thermal phase G

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Figure 8. Phase transition of PD solutions (0.2% w/v concentration) in methanol−water mixtures of varying methanol content.

transition is occurring first because here the ethylene glycolic part is not well solvated either by water or by methanol and with increase in temperature the methanol molecules begin to be nearer to the ethylene glycolic part, causing solvation. Above 32 °C the hydrogen bonds of both the water and methanol components begin to break, which causes entropy of mixing to decrease as previously stated. This makes the free energy of mixing positive, causing LCST-type phase separation. For 60% methanol content we expect the TPU to be lower because of the greater availability of methanol molecules, but it shows a higher value than that of 50% concentration. No definite reason can be put forward here, and probably the interaction between the component solvents with composition plays an important role in governing the overall phase transition of the system.17 For a deeper inspection of LCST and UCST of the polymer solution in the mixed solvent the methanol content of 50% has been chosen for morphological and 1H NMR studies. Morphology of PD in MeOH−Water Mixed Solvent (Methanol vol % = 50%). In the temperature-dependent DLS study the variation of Z-average size of PD in 50 vol % methanol of the mixed solvent with temperature shows both UCST and LCST types of phase transitions. At 15 and 45 °C temperature the hydrodynamic size shows higher Z-average values of ∼500 nm, and at 30 °C it shows a lower value around 100 nm. From the HRTEM image (Figure 9) of PD in MeOH−water (1:1) exactly at these temperatures, the fact acquired from DLS experiments has been further established. At 15 °C polymer aggregates form a large size structure of ∼500 nm, and at 45 °C again polymer exhibits an aggregated state, as evidenced by its ∼400 nm size. At 30 °C PD exhibits ∼20 nm size. A smaller size shown at HRTEM images in comparison with light-scattering data (∼100 nm) in solution is very much expected. In solution the size of the polymer should show a higher value than that of the dried polymer because in solution there are chances of the particles remaining solvated. 1 H NMR Study of PD in Mixed Solvent (MeOD+D2O, 250 μL Each). Figure 10 illustrates the signature of 1H NMR spectral peaks of PMeO2MA taken in MeOD and D2O (1:1 by vol) for a wide range of temperatures (16 to 48 °C, with 2 °C interval). For the spectrum taken at 26 °C, the peak at just above 4.0 ppm is assigned to methylene proton next to ester group of PMeO2MA. Three peaks at 3.5 to 3.7 ppm are for the methylenic protons directly attached to ether oxygen, and the methyl group at the end of the diethylene glycol chain shows a

Figure 7. Z-average sizes of PD (0.2%) at different temperatures in methanol−water solutions of different methanol content (a) 0, 10, 20, and 30%, (b) 40, 50, and 60%, and (c) 70, 80, 90, and 100%.

show some resistivity to increase, but 40% onward it goes up with higher gradient. A probable reason is that below 40% methanol mostly water molecules solvate the ethylene glycolic part of PD polymer, and above it methanol molecule starts solvating the ether bonds of PMeO2MA. As a result, after a certain temperature when hydrogen bonds with water begin to be disrupted, a PMeO2MA chain somehow tends to collapse, but the methanol molecules resist it by solvating the ethylene glycolic part of the chain. The process of thermal phase transition becomes hindered due to absorption of larger size methanol, preventing the interchain aggregation and causing a sharper rise of TPL. At 50% methanol content UCST-type phase H

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Figure 9. HRTEM images of PD polymer casted and dried at (a) 15, (b) 30, and (d) 45 °C from methanol−water solution (1:1) (v/v). (c) Enlarged image of panel b.

temperature, and for better understanding the total peak areas of the shaded region are calculated from the NMR spectra and they are plotted against temperature in Figure 11. From 16

Figure 11. Comparison of area of methylene peaks of grafted PMeO2MA from 1H NMR in MeOD-D2O (1:1) and Z-average sizes of PD in methanol−water (1:1) at different temperature showing both UCST and LCST.

to 24 °C, peak areas are almost consistently invariant, but from 26 °C onward peak heights start increasing up to 32 °C. After that, peak height again goes down and gets almost a constant value from 36 to 48 °C. Here again attention may go to the two types of interactions as previously mentioned for PD solution in pure D2O. The hydrophilic part of PMeO2MA chains simultaneously may get involved in H bonding with the solvent molecules and also with the fluorine atoms of PVDF backbone. The former H bonding tells about the extent of solvation, and it can be identified from the peak areas. The more solvation, the greater the peak intensities and, consequently, the greater the peak areas. Hydrodynamic diameters (Z-average values) of the polymer under the same conditions are shown in the same plot for better understanding of the relation between the NMR data with the Z-average size. Both the experimental data are approximately mirror images of each other, supporting the coexistence of UCST- and LCSTtype phase transitions at around 24 and 34 °C. Below 24 °C, enthalpy factor comes into play and polymer molecules interact with each other more effectively than with solvent molecules. As a result, neither methanol nor water molecule enters into the polymer system. From 26 °C onward both of the solvents enter

Figure 10. 1HNMR spectra of PD solution in MeOD-D2O (1:1) at indicated temperature. The shaded region indicates the methylenic protons of grafted PMeO2MA of the PD polymer.

sharp peak at ∼3.3 ppm. For 1H NMR spectra of PD in D2O solvent taken at different temperatures, the existence of two types of interactions showing two outcomes has been firmly demonstrated: first, the change in the intensities and second, the shifting of peaks (cf. Figure 2). Here the only difference is that MeOD is used along with D2O at the ratio of 1:1 (by vol.). In the Figure the three peaks for the three methylene protons directly attached to ether oxygen are shown in a shaded region. For clarity, only those three peaks are taken for the present discussion. It is revealed that peaks do not exhibit any appreciable shift up to 26 °C, but after that temperature peaks start shifting toward higher δ value. Another notable fact is the variation of peak intensities with the increase in I

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diagram. The blue shaded regions as shown in the Figure are one-phase regions, and the red shaded regions are two-phase regions. In the conventional hourglass-type phase diagram, at first a UCST phase boundary appears at lower temperature, then an LCST boundary appears at higher temperature, having a small intermediate region of one-phase system.19 In the present system we are getting a temperature region where onephase region appears in the two white regions for few concentrations of PD. The temperatures at lower arc of each region represent TPU, and those at the upper arc represent TPL. At the intermediate concentrations (0.1−0.17%) we are getting a blue shaded region where particle sizes are large enough, but no phase transition is noticed in the DLS study (Figures S13− S15), so we have called it the swelled state (as earlier), where the solvent molecules coexist in the polymer core. It is completely different from the conventional hourglass system of phase diagram, and hence it may be called an approximate inverted-hourglass-type quasi-binary phase diagram containing one polymer and two solvents at a fixed composition. Here the swelled state may be approximated as a two-phase system where particles are large enough to remain phase-separated but due to the entrapped solvent no observable phase change either from DLS experiment or from the naked eye is observed. A possible reason may lie in the nonlinear architecture of the PD graft copolymer.

into the polymer domains slowly, and the hydrophilic part of the PMeO2MA becomes solvated, causing sharper intensities of the NMR peaks. Again, above 34 °C, desolvation of water molecules happens from polymer domain due to breaking of intermolecular H bonding and results in the decrease in three peak intensities of PMeO2MA. A later type of H-bonding interaction of ethylene glycolic part, that is, with fluorine atoms of PVDF chain, arises for the coexistence of hydrophilic and hydrophobic segments of the polymer with a small number of solvent moieties. From 28 °C onward, when some solvent molecules are present in the polymer domain the H-bonding starts to be more effective among PMeO2MA and fluorine of PVDF backbone. With increasing temperature these H bonds weaken and methylene protons adjacent to ether groups shift to higher δ values. The phenomenon even continues when only some methanol molecules exist into the polymer (>34 °C), although water molecules are absent, adhering to the glycolic segments of the PD polymer. Below 26 °C neither methanol nor water molecules enter the polymer system, and as a consequence, those particular H-bonding interactions are not effective. Phase Diagram of PD in Methanol and Water 1:1 Mixture. It would be interesting to visualize the phase diagram this particular sample follows. The polymer PD is dissolved in a 1:1 (v/v) mixture of methanol and water with a concentration of 0.2% (w/v), and phase transition points are determined. As previously mentioned, UCST-type transition appears before LCST-type transition at 23.5 and 34.5 °C, respectively. After that, the solution has been diluted to 0.19% by the addition of 1:1 (v/v) mixture of methanol and water, and phase-transition points are evaluated again from DLS study. On subsequent dilution to 0.18% the transition points are measured again. This consecutive process is carried out up to 0.07% (w/v), and some pairs of distinct phase-transition points are observed in the case of 0.07, 0.08, 0.09, 0.18, 0.19, and 0.2% (Figures S10, S11, and S17−S19). For some concentrations like 0.1 and 0.17% both UCST- and LCST-type transition points are very close to each other (Figure S12 and S16), and for other concentrations (0.1− 0.17%) these cloud points are hard to realize (Figures S13− S15). A quasi-binary phase diagram in the 1:1 MeOH/water system pointing to the above results is presented in Figure 12. It approximately represents an inverted-hourglass-type phase

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CONCLUSIONS The PD graft copolymer is highly soluble in water and methanol, but in isopropanol it is soluble at higher temperature, showing UCST-type phase transition. Its solution in water, however, shows LCST-type phase separation on heating due to breaking of H bonds with water. The UCST-type phase separation has been attributed to increased interaction by overcoming the steric hindrance of the bulkier isopropanol to solvate the ethylene glycolic group. In the mixed solvent (water + isopropanol) both UCST- and LCST-type phase separations are observed independently in a composition; the isopropanolrich mixture exhibits UCST-type phase separation and the water-rich mixture exhibits LCST-type phase separation. In the mixture the TPL increases, whereas the TPU decreases with increase in isopropanol content, except at the concentration range (20−90%) of isopropanol, where particles exist in the swelled state and remain dispersed in the medium. In contrast with water and isopropanol, pure methanol does not exhibit any phase transition with increase of temperature; however, in the methanol + water mixture with methanol content of 0− 30%, TPL increases with methanol content, and in 40−60% methanol, UCST- and LCST-type phase separation occurs simultaneously, and at 70−90% methanol, swelled state of the particles is noticed for 0.2% (w/v) concentration of the polymer. A quasibinary phase diagram drawn in 50 vol % methanol by varying the polymer concentration (0.07−0.2%,w/ v) indicates an approximate inverted hourglass phase diagram, where a swelled state exists between two single-phase boundaries produced from LCST- and UCST-type phase transitions. The phase-separation process is understood from the intensity variation 1H NMR peaks of methylene protons of grafted PMeO 2MA dictating solvation and desolvation processes, and the downfield shift of the above peaks indicates the H-bonding interaction between PMeO2MA and PVDF chain. The larger size of the swelled state without showing any phase transition both in the size temperature plot and in naked eye observation is interesting, and it may be attributed to the

Figure 12. Quasibinary phase diagram of PD in methanol−water solution (1:1 v/v). J

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long branches of the PD graft copolymer. The wide diversity in the phase diagrams of PD in the mixed solvents suggests a finer tuning on the balance of hydrophobicity and hydrophilicity of the solvent mixtures to the PD graft copolymer, comprising both hydrophobic and hydrophilic chains in nonlinear architecture.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b11736. 1 H NMR spectrum of PVDF, PTOH, PVDFBIB, and PD in DMSO-d6 (Figures S1 and S2), 1st derivatives of the Z average sizes showing thermal phase transitions (UCST type and LCST type) (Figures S3−S6, S8, and S9), variation of Z average sizes of PD polymer (0.2% w/v) in isopropanol−water mixture (isopropanol content 40% v/ v) with temperature (Figure S7), and variation of Z average sizes of PD polymer (0.06 to 0.10, 0.13, and 0.15−0.19%, w/v) (Figures S10−S19) in methanol− water mixture (methanol content 50% v/v). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 91 33 2473 2805 Present Address

† D.P.C.: Department of Chemistry, Presidency University, Kolkata 700073, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge SERB New Delhi (grant no. SB/SI/ OC-11/2013) for financial support. A.K. and N.M. acknowledge CSIR, New Delhi for the fellowship.

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