Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Nucleation of Polyaniline Microspheres and Interconnected Nanostructures from Strongly Coupled Micelle-like Systems Ruijuan Wang,* Qiaofeng Han, Lude Lu, Xiaodong Wu, and Fengli Bei* Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Nanjing, 210094, P. R. China S Supporting Information *
ABSTRACT: A mechanistic study on the nucleation of aggregates exhibiting a weak intermolecular coupling and high molecular mobility for major components, exampled by an entity of aniline and salicylic acid in the preparation of polyaniline microspheres (PANI-NS) and interconnected structures (PANI-NC), is explored by in situ 1H NMR experiments. Three different procedures, namely, hydration of the aniline−salicylic acid (SA) entity, removal of the extra charges to the surroundings, and sphere-to-rod transitions, afford the smooth nucleation of products in characteristic morphologies. At the beginning, water plays a fundamental role in attenuating the high chemical potential system by hydrating both the aniline−SA entity and the in situ formed protons in the reaction, and removing the latter to bulk water when the chemical potential increment from the in situ produced proton is at a low initially. The driving force for this process is the increased intermolecular distances between aniline and SA induced by the electrostatic repulsions between positively charged protons in the entity, which paves the pathway for water in bulk to diffuse into the system. When a large amount of protons have been released in the reaction, the high chemical potential can be lowered down by repulsing both large and small sized positive charges to the external surroundings through electrostatic interactions or a sphere-to-rod structural transition initiated by continuously formed oligomers sheathed at the exterior of the spheres, which affords the formation of PANI-NS and PANI-NC, respectively. The competition of the two depends on the relative amplitude between the releasing rate of the protons and the mechanical strength of the aniline−SA entity in the reaction. Our work demonstrates that in situ dynamic NMR experiments such as measurements of NOE and spin−lattice relaxation times, and line shape analysis, provide new perspective powers for resolving the formation profile and more importantly the driving forces for each procedure at the molecular scale.
1. INTRODUCTION Being one of the ubiquitous operations in industry and daily life, the phenomenon of nucleation has attracted a worldwide research interest.1−4 Particularly, the rapid development of nanoscience and nanotechnology has prompted this progress even further.5,6 Being stabilized by the surfactant molecules, the originally less controllable nuclei incline to adopt some specific nucleation pathways and results in the products to exhibit specific morphologies. Over the past few decades, many spectroscopic measurements have been carried out on a large family of micellar systems in an effort to uncover the step-wised procedures and key factors accounting for the activity and selectivity of the nuclei for nanoparticles.7,8 Recently, the nucleation in micelle-like systems has become another hot topic.9−11 Compared with the conventional micelles, the micelle-like ones are compacted together through weak, short-ranged, and reversible interactions such as ionic and hydrogen bonds, π−π stacking interactions, etc. Lacking the strong protection from the surroundings, the nascent nuclei have a short lifetime and display high activity but low selectivity, and thus may adopt multiple nucleation pathways.12−14 Furthermore, the weak intermolecularly coupled © XXXX American Chemical Society
micelle-like surroundings, in many cases, have limited power to withstand the strong shocks in the reaction and may have a structural transition. Whenever the nuclei have a reasonable lifetime and growth rate, some typical structural features in such transition can be copied into the products to yield some peculiar morphologies that cannot be obtained otherwise.14,15 In this aspect, this system is an ideal platform for a theoretical evaluation of various weak molecular interactions on the stability and the activity of the nascent nuclei, and moreover their interactive communications with the volatile local environment, focusing particularly on the competitive and coordinative couplings of the two. Experimentally anilinium initiated micelles (AIMs) induced by salicylic acid (SA) are a typical micelle-like system, which has been widely applied for preparing polyaniline (PANI) with morphologies ranging from microspheres to interconnected nanostructures.15,16 Although abundant nucleation information has been provided in the preparation of these products, the Received: September 12, 2017 Revised: November 30, 2017 Published: December 1, 2017 A
DOI: 10.1021/acs.jpcc.7b09079 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Herein, the J-resolved spectroscopy and a series of NMR dynamic experiments have been employed to characterize AIMs at a high concentration of SA at first and the most reasonable model for this assembly has been derived. The comparison of the aggregation behavior of AIMs at a high and low concentration of SA helps us to conclude that the former one is a strongly coupled micelle-like system. Subsequently, a series of in situ 1H NMR measurements as well as NOE difference spectroscopy by irritating the desired resonances in SA and aniline have been conducted in an effort to illustrate the spatial orientation changes of the major components. To have an indepth understanding of the competitive interactions between the nascent nuclei and the micelle-like surroundings, the evolution of the water signal, including its shifting and integrated area as a function of the reaction time, is analyzed and discussed. Finally, a complete nucleation scenario for the strongly coupled AIMs has been derived after a complementary evaluation of the evolution of the relaxation parameters of the major species.
complexity from the presence of multiple intermolecular couplings and the likelihood of the structural rearrangement of the micelle-like systems makes a clear interpretation of the whole process difficult. Until now, only an intermediate state involving an external diffusion of the monomer after a flux of water into the micelle-like droplets has been put forward by Shahbazi after a careful comparison of the reaction that occurred through aniline and the modified ones;17 however, the most reasonable packing model for AIMs, the molecular mechanism accounting for each transition, and, more importantly, the mechanism responsible for the formation of microspheres rather than the interconnected nanostructures or vice versa has remained unresolved. A clear answer to these questions can provide us a comprehensive understanding of nucleation dynamics and kinetics in micelle-like systems and will be valuable for the design and control over corresponding procedures in practice. To this end, the molecular interactions as well as their interactive connections with the spatial orientation of the major species in different reaction periods become one of the most essential issues. Technically, the measurement of the Nuclear Overhauser Effect (NOE), originating from the inter- and intramolecular dipolar couplings, is one of the most powerful approaches for solution systems.18,19 Although NOE experiments could be carried out in many different styles,20−22 the one with NOE difference by selective excitation of a specific signal has been adopted due to the following considerations: (1) it takes about two times experimental time as long as one normal 1D 1H NMR experiment, and therefore enables to catch up with the most rapid spatial transitions for interested spin pairs; (2) the presence of isolated signals from aniline or SA allows us to excite each species separately and then to observe the evolution of the local dipolar interactions, a strategy valuable for deriving the driving forces for the major steps; and (3) the measurement of the proton relaxation parameters such as the spin−lattice relaxation times (T1H) and the spin−spin relaxation times (T2H) can serve as the complementary tools to identify the contribution from other nondipolar ways.23 To confirm the effectiveness of this approach, another set of control experiment has been carried out by recording a series of 1D 1H NMR spectra at the exact time interval required for performing one NOE difference experiment. The signal evolution in these two experiments at each two continuous time points were compared, and the results suggest that the signal intensity changes are predominantly from NOE rather than the reaction part. (See section 1 in the Supporting Information for details.) To bring to light the mechanism responsible for the formation of PANI microspheres or interconnected nanostructures, we believe water within the hydration layer of the micelle might provide valuable information. Previously, we have confirmed that such water molecules can serve as the most sensitive probes in exploring the nucleation processes because they can feel and respond to each structural transition as soon as possible.8,12 Although micelles have to grow up into larger sizes to prevent water in the hydration layer from decaying into a senseless averaged signal due to the rapid exchange with water in bulk, a careful analysis of the current results on micelle-like systems has revealed that both the status and the dynamic properties of water in these two locations vary significantly, and therefore, a clearly identified signal for each state is expected to be observed directly in NMR experiments.
2. EXPERIMENTAL SECTION 2.1. Chemical Materials. Deuterium oxide (D2O) was purchased from Alfa Aesar and used as received. Aniline, salicylic acid, and ammonium persulfate (APS) were obtained from Sinopharm Chemical Reagent Co. Ltd. Aniline was distilled under reduced pressure before usage. 2.2. Stock Solutions for NMR Measurements. A series of SA and aniline solution in a molar ratio of 0, 0.50, 0.60, 0.75, 0.90, and 1.00 were prepared with a fixed aniline concentration of 0.20 M. All solutes were mixed well in D2O solution containing 0.05% of trimethylsilyl propanoic acid (TMSP) that acted as an internal standard. 2.3. Preparation of PANI Microspheres. PANI hollow microspheres were synthesized by chemical oxidation according to the reported work.15 SA was dissolved in D2O containing TMSP by magnetic stirring at room temperature for about 10 min, and then aniline monomer (the molar ratio of SA to aniline is 0.60, 0.75, and 0.90) was added in the above solution, and the solution was stirred for 20 min to obtain the uniform solution. APS solution was precooled for 25 min at 0−5 °C, and then quickly added into the solution within 30 s. The molar ratio of APS to aniline was 1:1. The polymerization was carried out at 0−5 °C all the time. 2.4. Morphological Characterization of PANI Interconnected Structures and Hollow Microspheres. The morphologies of final products were observed by JEM-2100 transmission electron microscopy (TEM). 2.5. NMR Characterization of the Oxidative Polymerization of Aniline. Before any NMR measurements, a total volume of about 0.4 mL of the stock solution was first filled in an NMR tube and kept at 278.3 K for 10 min. All NMR experiments were performed on a Bruker DRX 300 Spectrometer (300.13 MHz for 1H), equipped with a BBI gradient probe with a maximum gradient strength of 52.7 G· cm−1. The measurements were performed at 278.3 K, and the temperature was controlled to within ±0.1 °C by a Bruker DVT 300 digital controller. The in situ 1H NMR experiments were designed with a fixed concentration of 0.20 M for aniline and varied ones for SA. The polymerization was performed in an NMR tube after addition of APS, and then a series of 1H NMR spectra were collected continuously in a fixed time interval after the reaction. The line widths at half-height of the interested signals were measured from spectra recorded by B
DOI: 10.1021/acs.jpcc.7b09079 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Table 1. Chemical Shift, the Ratio of Free Aniline, and the Mobility for Aniline and SA in AIMs with 0.20 M of Aniline and Varied Concentrations of SAa T1H/s
chemical shift/ppm [SA]/M
An (meta)
An (para)
An (ortho)
SA (H-3)
SA (H-4)
SA (H-5)
SA (H-6)
ratio of aniline
An
SA
D0/× 10−10 m2/s
0.12 0.15 0.18 0.20
7.42 7.45 7.49 7.50
7.27 7.33 7.41 7.46
7.19 7.25 7.33 7.34
6.92 6.92 6.92 6.94
7.46 7.46 7.45 7.46
6.96 6.97 6.97 6.97
7.81 7.81 7.82 7.83
0.23 0.17 0.03 0
4.62 4.35 3.97 3.67
4.52 4.24 4.25 4.01
6.24 6.05 5.80 5.50
a
The 1H NMR spectra and the J-resolved spectroscopy are available in section 2 in the Supporting Information.
Combined with the pronounced positive NOE signals between SA and water in Figure 1b, it can be derived that it is SA
using 1D Carr−Purcell−Meiboom−Gill pulse sequence which was designed to eliminate the inhomogeneous line-broadening from the external field. Each spectrum was acquired with 16 transients and 4 CPMG echoes with a duration time of 2.0 ms per transient. The proton nuclear spin−lattice relaxation time (T1H) was measured by the inversion recovery method. A total of 8 scans were collected for each slice, and 16 variable delays were used. 1H NMR diffusion coefficients (D0) were measured by using the pulsed-field gradient (PFG) stimulated-echo (STE) procedure, with phase cycling of the radio frequency pulses to remove unwanted echoes. The gradient strength was calibrated from the known diffusion coefficient of HOD at 278.3 K. Typical parameters used in our experiments were a total diffusion encoding pulse duration (δ) of 6.2 ms, a diffusion delay (Δ) of 20 ms. Diffusion coefficient was yielded by monoexponential fitting of the echo intensities as a function of the echo time using the Bruker Topspin 1.3 software package. The NOE difference experiments were performed by interleaved selective excitation of the on-resonance signal at 7.83 or 7.40 ppm and the off-resonance one at 0.50 ppm. In the experiment, a total transient of 128 was adopted and Gaussian shaped pulses were used in the selective excitation. The homonuclear J-resolved spectroscopy was recorded to determine the exact chemical shift value from SA and aniline when the molar ratio of the two changes. The parameters for typical J-resolved experiment were: a sweep width of 3598.0 Hz for F2 and 50.0 Hz for F1, a recycle time of 2.0 s between each transient, 16 transients for each t1 increment. The original data matrices were zero-filled to 2644 × 128 points for F2 and F1, respectively.
Figure 1. 1H NMR spectra (a) and NOE difference spectra for AIMs composed by 0.15 and 0.20 M of SA and aniline in D2O at 278.3 K, (b) the signal of SA at 7.83 ppm, and (c) the signal of anilinium at 7.41 ppm are selectively irradiated, respectively.
molecules that interact with water constantly to form the external layer of AIMs. Meanwhile, as shown in Figure 1, small and quite large positive NOE can be observed for water in AIM when aniline and SA are selectively excited, respectively. This suggests the protonation of aniline into the amphiphilic anilinium has accelerated the latter to be assembled into an aggregate through proton exchange reaction and the π−π stacking interactions, inducing the aromatic rings to form the hydrophobic cores while the amino groups or anilinium ions the hydrophilic outer shells. Compared with the conventional micellar systems, the molecular interactions in this system are not strong and therefore only small spherical micelle-like structures can be formed by aniline molecules alone. Fortunately the presence of a large amount of SA, which serves as the counteranion after release of the proton, can promote the further aggregation of the anilinium initiated micelles due to their strong inclination of the interaction with anilinium through π−π stacking interactions, and simultaneously the self-aggregation with water to form a new sheath of one thick hydrophilic layer. In the end, one or more anilinium initiated micelle-like aggregates can be enclosed to form a multicore in one thick shell structure. In this way, both the molecular interaction from aniline and SA will be in a balanced state and also match well with the NOE experiment results, in which the observed small positive NOE corresponds to anilinium and water within the micelle-like structures while the big one arises from SA and water in bulk. One extra advantage for such a structure is its capability to response to even larger intermolecular couplings by simply adjusting the compactness of the cores and thickness of the shells, indicating the way how weak molecular interactions can be assembled into the strongly coupled systems. Indeed, as
3. RESULTS AND DISCUSSION 3.1. Formation Mechanism of AIMs at High Concentration of SA. The morphology and the dynamic property of AIMs at a low concentration of SA (
