Article pubs.acs.org/Macromolecules
Unconventional Transitions of Poly(N‑isopropylacrylamide) upon Heating in the Presence of Multiple Noncovalent Interactions Long-Hai Wang, Ting Wu, Ze Zhang, and Ye-Zi You* Key Lab of Soft Matter Chemistry, Chinese Academy of Sciences, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, China S Supporting Information *
ABSTRACT: The reversible coil-to-globule transition of poly(N-isopropylacrylamide) in aqueous solution is a tenet of the physics of polymers. However, most of the studies have been limited on the transition induced only by the variation of hydrogen bonds. How and what the coil structure will develop into are unclear in the presence of other noncovalent bindings besides hydrogen bonds. Here we introduce noncovalent bindings of anion−dipole into poly(N-isopropylacrylamide) aqueous system besides hydrogen bonds and hydrophobic interactions and first have observed that the variant temperature responses of hydrogen bonds and anion−dipole bindings could make the coil structure of poly(N-isopropylacrylamide) unconventionally restructure into micelles, nanorods, vesicles, etc., but not only globule-like structure.
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INTRODUCTION The most studied responsive polymer is poly(N-isopropylacrylamide) (PNIPAM), which undergoes a collapse from an expanded coil-like conformation into a contracted, globule-like structure in response to a change in its solubility upon heating.1−9 Since its unique temperature response was first observed in 1968,10 there has been great interest in this coil-toglobule transition,9,11,12 and currently, this transition lies behind well-established technologies based on responsive PNIPAM as well as molecular imaging,13 emerging applications for drug delivery,2,14 and sensors.15 Despite that the folding behaviors of PNIPAM induced by the change of hydrogen bonds and the effects of ions, pH, and solvent on its lower critical solution temperature (LCST) have been very well studied;11,16−26 however, how and what the combination of different multiple noncovalent interactions drives PNIPAM self-assembly into are unveiled until now. In our previous research, we have found that the anion− dipole interactions can enable polar homopolymers (e.g., poly(propylene oxide), poly(vinyl methyl ether), etc.) selfassemble.27,28 We unveiled the self-assembly mechanism induced by the different salts such as NaSCN, NaI, NH 4 SCN, Na 2 SO 4 , NaCl, etc., and the effect of the concentration of these salts on the formation of nanostructures. Here, the noncovalent bindings of anion−dipole were introduced into PNIPAM system besides hydrogen bonds and hydrophobic interactions. Generally, PNIPAM undergoes a coil−globule transition upon heating the solution temperature above its LCST. However, in current study, it was observed that NIPAM underwent coil-to-micelle-to-nanorod-to-nanowormto-nanovesicle-to-micelle transition upon heating its solution in © XXXX American Chemical Society
the presence of the anion−dipole interactions, which have not observed for homopolymers up to now though these transformations are very easily observed in polymerizationinduced self-assembly as the hydrophobic chain grew.29
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EXPERIMENTAL SECTION
Isothermal Titration Microcalorimetry (ITC) Measurement.30 ITC200 (MicroCal) was used to obtain the binding interactions between ion and PNIPAM. We measured the enthalpy change during the addition of NaSCN to PNIPAM aqueous solution as a function of NaSCN concentration at different temperatures. The cell (Vcell = 204.9 μL) was filled with degassed PNIPAM aqueous solution (0.2 wt %) or with water (blank). NaSCN aqueous solution (100 mM) was injected (1 step of 1 μL and 19 more steps of 2 μL each) into PNIPAM solution. The duration of each injection was 4 s. An interval of 120 s was allowed between each injection. The injector stirred the solution at 1000 rpm to ensure complete mixing within a few seconds. Calorimetric data analysis was carried out using Origin 7.0 software (MicroCal). The details of the ITC titration are as follows: (1) titration of PNIPAM aqueous solution with NaSCN solution, (2) titration of water with the same NaSCN solution (blank I), (3) titration of PNIPAM aqueous solution with water (blank II). Appropriate subtractions of these two blanks were performed in order to explore the possible direct binding interaction between the ion and PNIPAM. Cloud Point Measurements. The aqueous solution containing PNIPAM of 3.0 mg/mL and NaSCN of 0.5 M was prepared. The cloud point of PNIPAM aqueous solution was measured using a Beckman DU 640 UV spectrophotometer equipped with a digital Received: September 23, 2015 Revised: December 11, 2015
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DOI: 10.1021/acs.macromol.5b02106 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules temperature controller. The temperature was increased at a rate of 0.5 °C/min, and transmittance of polymer solutions at 500 nm was monitored as a function of temperature. The LCST was determined as the onset of a sharp decrease in transmittance of polymer solutions at 500 nm (intersection of the tangent of the maximal descending slope and the baseline). Dynamic Light Scattering. DLS analyses were performed on a NanoBrook 90Plus PALS Zeta particle size analyzer with a scattering angle of 90°. The aqueous solutions containing PNIPAM of 3.0 mg/ mL and NaSCN of 0 and 0.5 M were prepared. The solution was added into cell, and the temperature was increased from 25 to 55 °C. The measurement temperature was set as 25, 36, 38, 39, 40, 42, 45, 47, and 52 °C. The equilibration time was set as 300 s. TEM and SEM. Stock solution of PNIPAM was prepared with 18 MΩ·cm purified water at concentration of 10 mg/mL. Then, the solution of PNIPAM was kept at 25 °C overnight to make sure that the PNIPAM was completely dissolved. The solution containing PNIPAM of 3.0 mg/mL and NaSCN of 0.5 M was prepared. The solution aliquots were placed in water bath under 36, 38, 39, 40, 42, 45, 47, and 52 °C for 5 min. Then, the solution aliquots were dripped onto the preheated copper grids on filter paper inside an oven at 25, 36, 38, 39, 40, 42, 45, 47, and 52 °C (the water was absorbed by filter paper immediately; the process of sample preparation was carried out inside an oven). The prepared samples on copper grids were used for transmission electron microscopy (TEM) analysis on a JEM-2011 transmission electron microscope with an accelerating voltage of 100 kV. The prepared samples on copper grids were sputtered with gold and used for scanning electron microscope (SEM) analysis on a SIRION200 scanning electron microscope. Calculations on Distribution of Partial Charges.31−33 The Mulliken atomic charges were calculated based on density functional theory (DFT), which was utilized by Gaussian 09. Ground-state geometry optimizations were carried out by the B3LYP functional coupled with 6-31g(d) basis set. Frequency analysis was then carried out to confirm the stability of the molecular structures. Based on the optimized structures, the charge distributions were obtained.
Figure 1. (a) Calculated distribution of partial charges on PNIPAM based on density functional theory. (b) Schematic illustration of anion−dipole interactions and hydrogen bond in PNIPAM aqueous solution.
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RESULTS AND DISCUSSION PNIPAM is a polar polymer and contains many hybrid polar/ nonpolar sites. The polar sites may have weak bindings with ions, in addition to hydrogen bonds and hydrophobic interactions. We calculated the distribution of partial charges on PNIPAM on the basis of density functional theory; the overall charge on the individual CH groups adjacent to these nitrogen atoms is slightly positive (+0.195 e, Figure 1a) because of the electron withdrawing of the nitrogen atom, whereas all the other CHn units are almost neutral (−0.004 to +0.02 e). The amide unit in PNIPAM exhibits a negative charge of −0.281 e. Based on the calculated results, the repeating unit of PNIPAM is dipolar, and PNIPAM has many “dipoles” in the side unit, as shown in Figure 1b. Generally, a dipole has a positive end and a negative end; the positive end will favorably bind with anions, whereas the negative end favorably interacts with cations. Some large-sized soft anions, such as SCN− and I−, are very weakly hydrated, and they prefer to interact with the hydrophobic positively charged CHn units but not water molecules.31 Thereby, anion−dipole bindings would occur between the SCN− anion and the positive end of a “dipole” (−CHδ+···SCN−) in PNIPAM if NaSCN is added into PNIPAM aqueous solution. To identify this binding, we prepared PNIPAM with a high molecular weight (Mn = 67.0 kg/mol and PDI = 1.8) via a traditional AIBN-initiated free radical polymerization. Evidence of the partially positively charged CH groups binding with SCN− via anion−dipole binding was obtained by a NMR study. A linear variation in the proton chemical shift with the anion
concentration has been reported to correspond to nonbinding between the partially positively charged CH group and SCN−, whereas a nonlinear change in the proton chemical shift would indicate that bindings between the ions and CH units would occur.31 We adopted an NMR-based methodology to probe the binding of SCN− with a CHn unit. Figure 2a displays the changes in the chemical shift for these protons as a function of the NaSCN concentration. The variation in the chemical shift for the CH unit (1) exhibits significant nonlinear behavior, which supports the strong bindings of SCN− with the CH groups next to the nitrogen. Figure 2b shows the linear changes in the proton chemical shift (2−4) with the anion concentration, indicating no binding between the SCN− anion and CHn groups of 2, 3, and 4. Fitting the curve to eq 1 gives the binding constant KA for CH group (1), which is 137.0 M−1 at 27 °C. Na2SO4 is used as a control salt because both Na+ and SO42− are strongly hydrated, and they prefer to bind with water molecules but not hydrophobic CH units.31 In a similar experiment for Na2SO4, it is apparent that the variations in the chemical shifts of all the protons (1−4) in PNIPAM decrease linearly with an increase of the Na2SO4 concentration, as observed in Figure S2, thereby indicating no obvious binding. Δδ = −c[M] + ΔδmaxKA[M]/(1 + KA[M])
(1)
Further evidence for the anion−dipole bindings was obtained using isothermal titration calorimetry (ITC) (Figure 2c). A curve fitting estimates that the number of binding sites (N) is B
DOI: 10.1021/acs.macromol.5b02106 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. (a) Chemical shift changes for the protons of 1 in PNIPAM as a function of the concentration of NaSCN at 27 °C. (b) Chemical shift changes for the protons of 2−4 in PNIPAM as a function of the concentration of NaSCN at 27 °C. (c) ITC titration curve for addition of NaSCN solution to PNIPAM at 27 °C. (d) ITC titration curve for addition of NaSCN solution to PNIPAM at 35 °C.
0.26 and the binding constant K is 465 M−1. K and KA are different for the identical system. This discrepancy may result from the intrinsically different methods used for these experiments. In the similar ITC experiments for Na2SO4, the results reveal that the binding constant K is 0 and the binding number N is 0, as observed in Figure S3. Both the NMR and ITC results clearly demonstrate that anion−dipole bindings occur between SCN− and the positive end of the dipole in PNIPAM when NaSCN is added to PNIPAM solution. Hydrogen bonds and anion−dipole noncovalent bindings have different temperature responses. Hydrogen bonds are very weak (∼5−50 kJ/mol), and they can easily break above the LCST. Compared with hydrogen bonds, anion−dipole bindings (50−200 kJ/mol) are much stronger,34 increasing the temperature to above the LCST only has a slight effect on the anion−dipole bindings of PNIPAM and SCN− anions, as demonstrated by the ITC measurements. For example, fitting the ITC data at 35 °C indicates that the number of binding sites (N) is 0.21 and K is 330 M−1, which only represents a slight decrease from 465 M−1 at 27 °C (Figure 2d). The TEM results indicate that all the aggregates of PNIPAM formed in deionized water above the LCST were globule-like nanoparticles (Figure 3a). However, the TEM and SEM results
indicate that in the presence of multiple interactions PNIPAM can self-assemble into micelles, nanorods, vesicles, etc., upon heating the solution from 25 to 55 °C, as shown in Figures 3b and 3c. Previously, Du and O’Reilly reported that PNIPAM can self-assemble into micelles when PNIPAM has a long hydrophobic tail.35 However, in the current study, the hydrophobic fragments originating from AIBN were very small; moreover, we did not observe similar results in deionized water, as shown in Figure 3a, thereby ruling out the possibility that the self-assembly above LCST is due to the residual hydrophobic fragments of AIBN. On the basis of the above results, we proposed the following mechanism. PNIPAM in 0.5 M NaSCN solution has a LCST of 34 °C (Figure S4), which is slightly higher than that for PNIPAM in deionized water (Figure S5). When the temperature rose above 34 °C, the anion−dipole bindings of SCN− with CH units became dominant because most of the hydrogen bonds would be disrupted above the LCST. Therefore, at room temperature, these two effectsanion−dipole interactions and hydrogen bondssynergistically facilitated the complete dissolution of PNIPAM in NaSCN solution. However, increasing the temperature to above the LCST destroyed the hydrogen bondings, leaving the anion−dipole interactions C
DOI: 10.1021/acs.macromol.5b02106 Macromolecules XXXX, XXX, XXX−XXX
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S4b and S5b, which agree with the TEM and the previous observation.36 According to the proposed mechanism, the ion-bound regions stabilize the formed nanostructure, and the surface of the formed nanostructures should therefore be negative. We traced the changes in the surface charge as a function of temperature. In deionized water, the formed nanoparticles were slightly negative, and the values of the surface charge remained almost constant with increasing temperature, as observed in Figure 4. In the control experiment of Na2SO4 solution, the
Figure 4. Zeta potential results verifying the formed nanostructure with SCN− anions at the surface. (a) Schematic illustration of PNIPAM nanoparticles formed in NaSCN solutions having anions on the particle surface. (b) Schematic illustration of PNIPAM nanoparticles formed in Na2SO4 solutions having no anions on the particle surface. (c) Zeta potential changes of a neutral polar PNIPAM (3.0 mg/mL) with temperature in NaSCN solution and deionized water. (d) Zeta potential changes of a neutral polar PNIPAM (3.0 mg/mL) with different concentrations of NaSCN and Na2SO4.
Figure 3. (a) Schematic illustration of PNIPAM folding into globulelike structure in water, and the TEM images of the nanostructures formed at different temperature stages upon heating. (b) Schematic illustration of PNIPAM folding into multiple nanostructure in the presence of multiple interactions. (c) TEM (up) and SEM (below) images of the nanostructures formed in the presence of multiple interactions at different temperature stages upon heating.
surface charge of PNIPAM nanoparticles in Na2SO4 solution remained constant upon increasing the salt concentration, as observed in Figure 4. However, in NaSCN solution, all the nanostructures exhibited highly negative charges, and the surface charge increased with increasing anion concentration. Moreover, the surface charge decreased with increasing temperature because at higher temperature, KA is smaller, and hence, less anion−dipole bindings occur. As a general rule for the self-assembly of block copolymers, copolymers with
