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DOI: 10.1039/C6RA23715K (Communication) RSC Adv., 2016, 6, 105070-105075

Controlled fabrication of polymeric Janus nanoparticles and their solution behaviors†

Wei Zhanga, Jinxin Heab and Xia Dong*ab
aCollege of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, 201620, China. E-mail: dongxia@dhu.edu.cn
bKey Lab of Textile Science & Technology, Ministry of Education, 201620, China

Received 23rd September 2016 , Accepted 28th October 2016

First published on 28th October 2016

Abstract

Well-defined polymeric Janus nanoparticles of different morphologies have been achieved by step-wise self-assembly procedures of simple ABC linear tri-block terpolymers, with Janus balances adjustable simply by regulating the chain length of the hydrophilic block. The influences of Janus balance have been investigated on the solution behaviors of the prepared Janus nanoparticles.

Polymeric Janus particles with asymmetric chemistries or polarities are potentially useful in a wide range of applications, including but not limited to drug carriers, phase transfer catalysts, Pickering emulsifiers, etc.1–6 Recently, many synthetic methods of producing Janus particles have been reported, and increasing efforts have been devoted to the synthesis of soft organic Janus structures particularly on nanoscale.7–11 However, solution-based strategies towards homogeneous polymeric Janus nanoparticles were seldom reported until a variety of self-assemblies from block copolymers/terpolymers in selective solvents were developed.12–14 The preparation method based on self-assembly of precursor polymers in selective solvents would overcome the problems faced with the previously reported bulk-segregation-based methods that call for the necessity of suitable polymer–polymer interaction parameters and thus limit the number of applicable polymers. Furthermore, the impact of Janus balance, i.e., the relative sizes of both hemispheres of a Janus particle, has rarely been addressed experimentally so far. Implementation of Janus particles with adjustable Janus balance would, however, benefit the development and understanding of the unique and structure–dependent properties of Janus particles.

In this communication, we report on a method to fabricate well-defined polymeric Janus nanoparticles with Janus balance adjustable simply by regulating the chain length of the hydrophilic PDMAEMA block of the precursor terpolymers, poly(tert-butyl methacrylate)-block-poly(2-(cinnamoyloxy)ethyl methacrylate)-block-poly(2-dimethylaminoethyl methacrylate)s (PtBMA-b-PCEMA-b-PDMAEMAs), and discuss how Janus balance influences the solution behaviors of the Janus nanoparticles. The strategy process towards Janus nanoparticles with different micro-structures is shown in Scheme 1. In the first step, compartmentalized nanostructures with micro-phase separation were formed by step-wise self-assembly of PtBMA-b-PCEMA-b-PDMAEMAs as the building blocks, in selective solvent. The phase-separated nanostructures could be varied by regulating the chain length of polymer blocks. Then after fixation of the phase-separation state by UV-crosslinking, dis-assembly of these self-assembled nanostructures into individual Janus nanoparticles took place in a common solvent, as long as the fixated PCEMA domain is located at the interface of PtBMA and PDMAEMA. Janus balances and micro-structures of the obtained Janus nanoparticles depend highly on the phase-separated morphologies of the self-assembled nanostructures, which are variable according to the chain lengths of the blocks of precursor polymers.


image file: c6ra23715k-s1.tif
Scheme 1 Schematic fabrication process towards different Janus nanoparticles from various self-assembled morphologies of TCDs (PtBMA-b-PCEMA-b-PDMAEMAs).

Triblock terpolymers of PtBMA-b-PCEMA-b-PDMAEMAs (the chemical structure is as shown in Scheme 2 and the molecular weights are listed in Table 1) with different molecular weights have been used to obtain the initial compartmentalized nanostructures, synthesized via successive atom transfer radical polymerization (ATRP) followed by selective modification of the middle block (see ESI† for the detailed procedures). PtBMA-b-PCEMA-b-PDMAEMAs were first dispersed in a nonsolvent for the PCEMA middle block, isopropanol, to generate core–corona micelles with a PCEMA core and a patchy PtBMA/PDMAEMA corona. Subsequent dialysis of these core–corona micelles into a nonsolvent for both PtBMA and PCEMA (water) initiates self-assembling into compartmentalized nanostructures. During this step, the corona patches (PtBMA/PDMAEMA) rearranged to minimize the energetically unfavorable PtBMA/water interface, inducing coalescence along the exposed PtBMA patches. After the phase-separated state was permanently fixated by selective UV-crosslinking of the PCEMA domains, re-dispersion in a common solvent for all blocks (THF) broke up the compartmentalized nanostructures and liberated single, core-crosslinked polymeric Janus nanoparticles. To gain clear insight into the self-assembly of PtBMA-b-PCEMA-b-PDMAEMA triblock terpolymers (TCDs for short) towards Janus nanoparticles, transmission electron microscope (TEM) was used to probe the samples. Fig. 1 shows the compartmentalized nanostructures self-assembled from different TCDs by gradual solvent exchange.


image file: c6ra23715k-s2.tif
Scheme 2 Chemical structure of PtBMA-b-PCEMA-b-PDMAEMA.
Table 1 Fractions of blocks of TCD polymers and Janus balances of TCD-Janus nanoparticles
Polymer Mn (g mol−1) DP fraction Weight fraction Volume fraction Janus balancea
a Janus balance = Vhydrophilic/Vhydrophobic.
TCD0 168[thin space (1/6-em)]180 178[thin space (1/6-em)]:[thin space (1/6-em)]249[thin space (1/6-em)]:[thin space (1/6-em)]587 15[thin space (1/6-em)]:[thin space (1/6-em)]30[thin space (1/6-em)]:[thin space (1/6-em)]55 17[thin space (1/6-em)]:[thin space (1/6-em)]22[thin space (1/6-em)]:[thin space (1/6-em)]61 —
TCD1 94[thin space (1/6-em)]025 229[thin space (1/6-em)]:[thin space (1/6-em)]170[thin space (1/6-em)]:[thin space (1/6-em)]170 35[thin space (1/6-em)]:[thin space (1/6-em)]37[thin space (1/6-em)]:[thin space (1/6-em)]28 40[thin space (1/6-em)]:[thin space (1/6-em)]28[thin space (1/6-em)]:[thin space (1/6-em)]32 0.8
TCD2 196[thin space (1/6-em)]411 354[thin space (1/6-em)]:[thin space (1/6-em)]146[thin space (1/6-em)]:[thin space (1/6-em)]741 26[thin space (1/6-em)]:[thin space (1/6-em)]15[thin space (1/6-em)]:[thin space (1/6-em)]61 28[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]62 2.2
TCD3 413[thin space (1/6-em)]954 354[thin space (1/6-em)]:[thin space (1/6-em)]146[thin space (1/6-em)]:[thin space (1/6-em)]2124 12[thin space (1/6-em)]:[thin space (1/6-em)]7[thin space (1/6-em)]:[thin space (1/6-em)]81 13[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]82 6.3
TCD4 914[thin space (1/6-em)]836 354[thin space (1/6-em)]:[thin space (1/6-em)]146[thin space (1/6-em)]:[thin space (1/6-em)]5310 6[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]91 6[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]92 15.3


image file: c6ra23715k-f1.tif
Fig. 1 TEM images (RuO4 staining: PCEMA black), and hydrodynamic diameter distributions of the compartmentalized nanostructures self-assembled from TCD polymers.

Obviously, all of the nanostructures shown in Fig. 1 are in spherical shape, while the microphase-separated morphologies are different. With the highest volume fraction of PCEMA block, TCD0 polymers eventually self-assembled into “Y-type” compartmentalized nanostructures, since the PCEMA domains were large enough to draw close and fuse together during the self-assembling along the rearranged PtBMA patches upon the change of solvent from isopropanol to water. With a smaller DP fraction of PCEMA compared to TCD-0 polymers, the nanostructures formed from TCD1 polymers exhibited a “clover” morphology of microphase-separation, in which the fusion event of the PCEMA domains did not take place, and the PCEMA core inside the patchy PtBMA/PDMAEMA corona eventually located discretely at the PtBMA/PDMAEMA interface after the self-assembly of the core–corona micelles in water. After further decreasing in fraction of PCEMA, the region of PCEMA phase in the final nanostructures of TCD2 polymers became less discrete and showed a “dimer” morphology. Then as the PDMAEMA chain length increased, the corona of the compartmentalized nanostructures enlarged, resulting in continuously shrinkage and fusion of the PCEMA phase due to the increasing steric hindrance effect (TCD3 and TCD4 in Fig. 1). In addition, the average hydrodynamic diameters obtained by dynamic laser scattering (DLS) of the five nanostructures from different TCD polymers show positive correlations to both molecular weight of the polymers and chain length of PDMAEMA blocks, as shown in Fig. 1.

After the phase-separated states of the compartmentalized nanostructures had been permanently fixated by selective UV-crosslinking of the PCEMA domains, redispersion in a good solvent for all blocks (THF) could break up the PtBMA compartments and then liberate single, core-crosslinked polymeric Janus nanoparticles. It is worth noting that in the “Y-type” nanostructures of TCD0 polymers, the crosslinkable PCMEA regions located within the PtBMA domains, rather than at the PtBMA/PDMAEMA interface. This kind of morphology of phase-separation, resulting from a relatively high DP fraction of PCEMA, could not dis-assemble into subunits after UV-crosslinking of the PCEMA regions after all.15

However, four types of well-defined Janus nanoparticles obtained by dis-assembly of the UV-crosslinked compartmentalized nanostructures have been observed by TEM imaging. Iodine selectively stains PDMAEMA so that the opposite hemispheres of the particles would be distinguishable.16 As shown in Fig. 2, TCD1-Janus nanoparticles generally exhibit a snowman-like morphology, while TCD2-, TCD3- and TCD4-Janus nanoparticles are sphere-shaped. The difference in shape might result from that the large region of PCEMA block in TCD1 polymer molecules was further stretched as solvation of the PtBMA domains took place upon exposure to the cosolvent. The average hydrodynamic diameter of TCD1-Janus nanoparticles is 40 nm, according to DLS measurement, which is properly the axial length of the snowman-like structure of TCD1-Janus nanoparticles. By reducing the chain length of PCEMA block, spherical Janus nanoparticles were obtained, with varied Janus balances according to the calculated volume fraction ratio of the hydrophilic hemisphere to the hydrophobic one. The latter three TEM images in Fig. 2 reveal that the relative areas of PDMAEMA-hemispheres enlarge with the increase in chain length of PDMAEMA block of TCD polymers. Besides, the average hydrodynamic diameters of TCD2-, TCD3- and TCD4-Janus nanoparticles obtained by DLS measurements show a trend of continuous increase with growing molecular weight of the TCD polymers, which are approximately 27.5 nm, 34.5 nm and 79.5 nm, respectively. And Janus balance of the fabricated spherical Janus nanoparticles is adjustable simply by regulating the chain length of PDMAEMA block. The fractions of TCD polymer blocks and the calculated Janus balances of TCD-Janus nanoparticles are summarized in Table 1.


image file: c6ra23715k-f2.tif
Fig. 2 TEM images (I2 staining: PDMAEMA black), and hydrodynamic diameter distributions (in THF) of Janus nanoparticles.

As the hydrophilic-hemisphere enlarges, Janus balance increases and the Janus particle becomes more hydrophilic.17 Amphiphilicities of the four different TCD-Janus nanoparticles were investigated via examinations of surface activity. From Fig. 3, surface tensions of the four aqueous solutions of Janus nanoparticles are all observed to reduce to some extent at the concentration of only 0.00001 g L−1, among which the reduction in surface tension of TCD1-Janus nanoparticle aqueous solution is the most significant. With a stronger hydrophobicity (Janus balance = 0.8), TCD1-Janus nanoparticles tend to adsorb at the air/water interface rather than remain in the water phase, thus showing a higher surface activity. During the concentration increasement from 0.00001 g L−1 to 0.01 g L−1, surface tensions of the four aqueous solutions of Janus nanoparticles all did not decrease significantly. This might be due to the readily clustering of Janus nanoparticles at low concentrations, which resembles Rainer Erhardt and Axel H. E. Müller et al.'s experimental result that the major population of PS/PMMA-Janus nanoparticles were observed to aggregate into clusters at the concentration of only 0.03 mg L−1.18 However, when the concentration increases to 0.1 g L−1, according to Fig. 3, the four surface tensions decrease noticeably, which probably result from dis-assembling of the clusters near the air/water interface and that the liberated Janus nanoparticles adsorb spontaneously at the interface. At the concentration of 0.1 g L−1, the extents of reduction in surface tensions of the latter three aqueous solutions (of TCD2-, TCD3- and TCD4-Janus nanoparticles, respectively) show a negative dependence on Janus balance of the Janus nanoparticles, in general: the stronger the hydrophobicity of the Janus nanoparticles, the greater extent the surface tension of the aqueous solution decreases to. Meanwhile, TCD1-Janus nanoparticles, with the strongest hydrophobicity (or amphiphilicity) and the lowest Janus balance among the four in this experiment, decrease surface tension only mildly at this concentration, due to their high tendency to form clusters as well as low rate of dis-assembly caused by the large hydrophobic PtBMA-hemispheres.


image file: c6ra23715k-f3.tif
Fig. 3 Equilibrium surface tension as a function of (a) concentration and (b) logarithmic concentration of TCD-Janus nanoparticles in water.

Solution behaviors of the Janus nanoparticles were further investigated via dynamic surface tension examinations of the aqueous solutions and dynamic laser scattering measurements of the particles in water. In the maximum bubble method, the dynamic surface tension as a function of surface age (the time interval between the onset of bubble growth and the moment of maximum pressure) was measured by varying the speed of bubble formation. The dynamic surface tension isotherms of the 0.1 g L−1 aqueous solutions of the four types of Janus nanoparticles as a function of surface age are graphed on the left in Fig. 4, and the DLS size distribution histograms of the Janus nanoparticles are as shown on the right in Fig. 4. When the surface age of a bubble is only within 10 ms (band a), there is no significant reduction in surface tension are observed for the aqueous solutions of TCD2-, TCD3- and TCD4-Janus nanoparticles, while the surface tension of the aqueous solution of TCD1-Janus nanoparticle has been reduced to around 55 mN m−1. This indicates a higher diffusion rate for TCD1-Janus particles and that there were amount of them had adsorbed at the interface of water and air during the beginning of the bubble formation (within 10 ms). However, the fast adsorption of some of TCD1-Janus particles at the interface is not energetically orientated, and desorption of the less favorable orientated particles occurs in order to re-arrange into the more favored orientation, as indicated by the noticeable rise in the surface tension during the time period from 10 ms to 50 nm (interval between band a and band b in Fig. 4).19


image file: c6ra23715k-f4.tif
Fig. 4 Dynamic surface tension isotherms of the Janus nanoparticles; DLS size distribution histograms of the Janus nanoparticles, measured in water at the concentration of 0.1 g L−1; insets are the corresponding TEM images of the clusters of the Janus nanoparticles.

Since the rate of disassembly of the clusters into individual Janus nanoparticles is much slower than the diffusion rate of the dispersed Janus nanoparticles, there is a long time for the amount of clusters to release the Janus nanoparticles to adsorb at the air/water interface, which results in a long induction time of the dynamic surface tension variations, as described in our previous report.16 There is an obvious induction time for all of the four dynamic surface tension isotherms: 50–500 ms (interval between band b and band c) for TCD1-Janus nanoparticles, and 10–2000 ms (interval between band a and band d) for TCD2-, TCD3- and TCD4-Janus nanoparticles. The shorter induction time and the lower surface tension at the same surface age, observed from the dynamic surface tension isotherm of the aqueous solution of TCD1-Janus nanoparticles compared to the other three, indicate the higher tendency to adsorb at the water/air interface for the newly dis-assembled TCD1-Janus nanoparticles from their clusters near the surface. Furthermore, there is an evident and continuous reduction in surface tension as the surface age grows after the induction time period for all of the four aqueous solutions, and the extent of reduction at the same surface age generally shows a positive correlation to the hydrophobicity of the Janus nanoparticles. This means the lower Janus balance, the higher surface activity the Janus nanoparticles exhibit in this study. In addition, the size distribution of the Janus nanoparticles becomes narrow as Janus balance increases, as shown in the four DLS size distribution histograms, which further confirms that the amphiphilicity of the Janus nanoparticles has influences their solution behaviors considerably.

In conclusion, well-defined polymeric Janus nanoparticles of different morphologies have been prepared by step-wise self-assembly of the precursor triblock terpolymers, with Janus balance adjustable simply by regulating the chain length of the hydrophilic PDMAEMA block. All of the four types of Janus nanoparticles prepared are surface-active, and the surface activity shows a negative correlation to Janus balance. There are tendencies for the Janus nanoparticles to aggregate into clusters in water, which depend highly on Janus balance: the stronger the hydrophobicity the Janus nanoparticles have, the more tendencies for them to aggregate. However, the clusters of Janus nanoparticles near the air/water interface were proven to dis-assemble into individuals, by the dynamic surface tension reduction variation with the increasing surface age. The influences of Janus balance (or amphiphilicity) investigated on the solution behavior of Janus nanoparticles are believed to have laid a foundation for further research on pragmatic design and application of the Janus nanoparticles about their attractive surface-active nature.

Acknowledgements

This research was supported by the National Nature Science Foundation of China (20973036, 21303013), Science and Technology Commission of Shanghai Municipality (13ZR1450900), the Fundamental Research Funds for the Central Universities (2232014D3-12), and Chinese Universities Scientific Fund (CUSF-DH-D-2013049).

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Footnote

† Electronic supplementary information (ESI) available: Materials, instruments, detailed procedures and characterization of precursor polymers. See DOI: 10.1039/c6ra23715k
This journal is © The Royal Society of Chemistry 2016
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