Chinese Chemical Letters  2019, Vol. 30 Issue (2): 477-480   PDF    
Self-assembly of homopolymer of PAA-NH4
Tongbing Suna,b, Xiaoli Yangb, Caizhen Zhua, Ning Zhaob,*, Haixia Dongb, Jian Xub,*     
a College of Chemical and Environmental Engineering, Shenzhen University, Shenzhen 518060, China;
b Beijing National Laboratory for Molecular Sciences, Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract: Self-assembly of homo-polymers has rarely been reported. Herein, PAA-NH4 assemblies varying from nanospheres to large particles and yolk-shell vesicles were obtained by adding different amount of HCl solution into the dispersion of PAA-NH4 in ethanol. The changes of zeta potentials, pH value and microstructure of the PAA-NH4 assemblies were characterized, and the influences of molecular weight and different alcohols on the assembly morphologies were studied. A possible assembly mechanism based on the solubility and electrostatic interaction was proposed. Our study offered an interesting example of homo-polymer assembly and may extend the practical application due to the simple polymers used.
Keywords: PAA-NH4 assemblies     Nanospheres     Yolk-shell vesicles     Homo-polymer assembly     Electrostatic interaction    

Polymer assemblies in solution have attracted increasing research interests [1-6], especially for amphiphilic block copolymers which can self-assemble in selective solvents to form comprehensive structures and controlled morphologies. These assemblies can be used in many fields such as drug delivery, cosmetic materials, catalysis and patterning [7-13]. Many efforts have been presented to study the assembly morphology [14-16] and formation mechanism [17-20]. In general, the specific morphology of the aggregates can be attributed to the balance among the chain stretching in the core, the interfacial energy, and the repulsion among corona chains [21]. Factors influencing the above three terms can be utilized to tune the morphology of aggregates [22-28].

Compared to large amount of research on copolymers, little attention has been paid to the studies of homo-polymer selfassembly [29, 30]. To form comprehensive and controlled morphologies by self-assembly of homo-polymers is more desirable due to the simple polymers used. As an example, stable polymeric nanoparticles can be prepared from poly(ethylacrylic acid) or poly-(propylacrylic acid) through heating the polymer solutions to certain temperature [31, 32]. Yan and co-workers synthesized a novel amphiphilic homo-polymer which could self-assemble into spherical micelles in water [33]. By simply changing the homopolymer's chain length or co-solvents, a wide range of nanostructures have been obtained based on hydrogen bonding [34].

The above mentioned homo-polymers are actually amphiphilic macromolecules [35]. Herein, we reported the formation of various stable PAA-NH4 assemblies in alcohols by simply adding HCl solution. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and laser particle size analyser were employed to investigate the morphology transformation of the assemblies upon with the addition of HCl solution. A self-assembly mechanism was proposed. The formation of controllable aggregates by using simple homo-polymers may extend the research of polymer assembly and may find more practical application due to the simple polymers used.

Specifically, 0.085 g PAA (8.50 μmol, Mw = 5.0×103, 50 wt% aqueous solution) was added into 1.5 mL ammonia solution (9.63 mmol, the more ratio of ammonia to carboxyl groups is about 16) under stirring for 30 min, then the resultant PAA-NH4 solution was mixed with 30 mL ethanol. After stirring for another 5 min, dispersion of PAA-NH4 colloids with blue opalescence was obtained. The dispersion was divided into glass bottles with each part of 3 mL, and then different volumes (0–2.0 mL) of 0.1 mol/L HCl solution (0–0.2 mmol) were dropwise added to induce the formation of PAA-NH4 assemblies with various morphologies. Other PAA-NH4 aggregates were prepared through the same process by using PAA of different molecular weights at fixed content, or by using other alcohols instead of ethanol.

Fig. 1A shows the phase transition behaviours of PAA-NH4 in ethanol with the addition of HCl. The pristine PAA-NH4 aqueous solution was transparent and colourless. When the solution was added into ethanol, a poor solvent for PAA-NH4, a blue opalescent dispersion was obtained, indicating nanosized colloids were formed (Fig. 1A, 1). Interestingly, we found that continuously adding HCl solution in the dispersion would change the dispersion from translucent to turbid to transparent again. The dispersion became cloudy but still translucent when HCl solution was increased from 0.05 mL to 0.25 mL (Fig. 1A, 2–4). Further addition of HCl caused the dispersion turn to turbid and precipitates could be observed at the bottom of vial (Fig. 1A, 5–7). When HCl was increased to 2.0 mL, a completely transparent solution was formed again.

Fig. 1. (A) Photograph of the dispersions of PAA-NH4 in ethanol formed by adding (1) 0, (2) 0.05, (3) 0.12, (4) 0.25, (5) 0.50, (6) 1.00 and (7) 1.50 mL of 0.1 mol/L HCl, respectively; (B) and (C) Change of the size of the resultant assemblies upon with the volume of HCl added.

The colloidal size change at different stages was determined by dynamic light scattering (DLS). As shown in Fig. 1B, with the addition of HCl solution, the zeta diameter curve shifted to right and turned narrow, which means that the average diameter of PAANH4 colloids increased and the size distribution decreased, and then increases dramatically when more than 0. 25 mL of HCl was added (Fig. 1C).

The change of the appearance from translucent to turbid to transparent indicated the variation of the assemblies formed. TEM and SEM images of the assemblies at different stages are displayed in Fig. 2. The PAA-NH4 colloids formed in ethanol have a diameter less than 50 nm and the colloids are connecting with each other (Fig. 2A). When 0.05 mL HCl was added, compact colloids with larger size are formed, and each colloid is seemly composed of many tiny units less than 10 nm (Fig. 2B). Further addition of HCl to 0.12 mL caused the formation of bigger and less connecting spherical aggregates (Fig. 2C). When the volume of HCl increased to 0.25 mL, "yolk-shell" structured vesicles can be obtained (Fig. 2D). Many tiny particles can also be observed. The change of the morphologies is in agreed with the results shown in Fig. 1. It seems that the shell thickness increases with the increase of HCl concentration to 1.5 mL (Figs. 2DG). SEM images also confirm the transition from the small compact nanoparticles to the large "yolkshell" vesicles, since "bowler hat" like morphologies derived from the vesicles can be observed, especially for the vesicles with thin shells (insets in Figs. 2BG). The vesicles begin to dissociate when more HCl was added. Broken vesicles can be found in Fig. 2H, and the assemblies disappear almost completely when 2.0 mL HCl was added and thus why the dispersion turns to transparent again (Fig. 2I).

Fig. 2. TEM images of PAA-NH4 colloids in ethanol with 0.1 mol/L HCl solution of different volumes: (A) 0 mL, (B) 0.05 mL, (C) 0.125 mL, (D) 0.25 mL, (E) 0.5 mL, (F) 1.0 mL, (G) 1.5 mL, (H) 1.75 mL and (I) 2.0 mL; Insets in (B–G) are corresponding SEM images, and insets in (H, I) are the enlarged TEM images.

We found that both HCl and water affected the assembling when the dilute HCl solution was added. HCl would influence the electronic interaction between the polymer chains, while water was a good solvent for the polymer thus would change the solubility of the polymer in the solvents. To separate the two factors, strong HCl solution of 5 mol/L and pure water were utilized, respectively, to repeat the above experiments. Compared with 0.1 mol/L HCl solution, the pH changed a little at the beginning and then levelled off when pure water was added. For 5 mol/L HCl solution, the pH decreased sharply (Fig. S1 in Supporting information). The corresponding morphology changes of the assemblies are shown in Figs. 3 and 4. Compared with the pristine PAA-NH4 colloids (Fig. 2A), loose aggregations containing tiny particles were formed and the size of the aggregations increased with the volume of water added (Figs. 3AD). The resultant morphologies were similar to those shown in Figs. 2BC. Further addition of water made the aggregations begin to dissociation (Fig. 3E), and a transparent solution would be formed at last. Compared with using 0.1 mol/L HCl solution, "york-shell" vesicles did not appear.

Fig. 3. TEM images of PAA-NH4 assemblies formed by adding (A) 0.09, (B) 0.18, (C) 0.36, (D) 0.87 and (E) 1.29 mL of water, respectively, into 3 mL of the dispersion in ethanol; Scale bar is 100 nm.

Fig. 4. TEM images of PAA-NH4 assemblies formed by adding (A) 0.01, (B) 0.04, (C) 0.07, (D) 0.08 and (E) 0.18 mL of 5 mol/L HCl, respectively, into 3 mL of the dispersion in ethanol. Scale bar is 100 nm.

The morphology change triggered by using 5 mol/L HCl solution was also different. The pristine PAA-NH4 colloids gradually aggregated to form large core-shell structured assemblies (Figs. 4AE). The assemblies were stable when 0.18 mL of 5 mol/L HCl was added, which led to a pH value of about 8.8. To reach such a low pH value by using 0.1 mol/L HCl solution, a transparent solution was left. Based on the above data, it was proved that the morphology transformation of the assemblies upon with the addition of the dilute HCl solution was the comprehensive effect of both HCl and water: HCl played the dominant role at the beginning while water was the leading influence when large amount of HCl solution was added.

The carboxylic groups of PAA chains turned to be COO-.NH4+ ionic pairs when PAA was added into ammonia solution. Ethanol was a poor solvent for the resultant PAA-NH4, PAA-NH4 colloids would be formed when the PAA-NH4 aqueous solution was added in to large amount of ethanol. The dispersion was stable due to the strong electrostatic repulsion interaction between the colloids. With the addition of dilute HCl solution, PAA-NH4 colloids would be neutralized gradually. Zeta potentials of the dispersions increased quickly from -52±2 mV to -23±2 mV when 0.5 mL of 0.1 mol/L HCl solution was added, accompanied with the pH value decreased from 11.0 to 10.2 (Fig. S2 in Supporting information). The change of zeta potentials indicated the electrostatic repulsion between the assemblies decreased gradually, thus made the pre-existing assemblies began to aggregate to reduce the interface energy. Therefore, large aggregates were formed, as shown in Figs. 2B and C. The smaller the assemblies, the easier to aggregate of PAA-NH4 colloids and lead to a narrower distribution of the dispersion (Fig. 1B). Further addition of HCl solution, the as-formed aggregates began to shrink to form solid cores while the newly deposited polymers from the solution will form the shell. This competition results in the formation of unique "yolk-shell" structures, as shown in Figs. 2DG. When more HCl solution was added, the solubility improved due to enough amount of water was introduced into the solvents, thus the above described aggregates began to destroy gradually and dissolved completely at last (Figs. 2H and I).

Similar morphology transformation was also observed by using different molecular weights of PAA ranging from 1.8 × 103 to 90.0 × 103 (Fig. S3 in Supporting information). The resultant PAANH4 assemblies experienced the transformation from interconnecting nanospheres, to large particles, "yolk-shell" vesicles, and finally disappeared with the addition of HCl. This phenomenon suggests that the assembly of the homo-polymer PAA-NH4 is different from that of many copolymers reported, in which the molecular weight has a profound influence. The results indicate all the assemblies, no matter of the molecular weight of the polymers used, followed a similar assembling mechanism.

The influence of different alcohols on the assembly was also studied. PAA-NH4 colloids could be formed in methanol, isopropanol and propanol while the polymer precipitated in butanol, and the size of the resultant colloids increased in the order of methanol, ethanol, isopropanol and propanol (Figs. S4 and S5 in Supporting information). The results indicated that the solubility of the homo-polymers decreased with the increase of alkyl chains in the alcohols. With the addition of HCl solution, the change tendency of the appearance of the dispersions was similar, namely from translucent to turbid and transparent (Fig. S6 in Supporting information). The morphologies of the assemblies were different because of the different solubility of the polymer in the alcohols. For example, the average diameter of PAA-NH4 assemblies in methanol was smaller than those formed in ethanol. PAA-NH4 colloids gradually aggregated to form large spherical assemblies with the diameter about 200 nm (Figs. S7A–C in Supporting information). The as-formed large assemblies had a rough surface due to the deposition of surrounding smaller polymer aggregates for the purpose to reduce the interface energy. Further addition of HCl solution resulted in the dissociation and irregular rod-like structures were observed (Fig. S7D in Supporting information). Finally the aggregates would dissolve completely and a transparent solution was formed (Fig. S6A). The formation of assemblies with controllable morphologies by using different alcohols and polymers with various molecular weights confirm the assembly mechanism.

In summary, self-assembly of homo-polymer PAA-NH4 in alcohols triggered by adding HCl solution was reported. A possible assembly mechanism was proposed based on the change of electrostatic repulsion and solubility upon with the addition of the HCl solution. This study gives an interesting example of fabrication of various assemblies by using simple homo-polymers and may extend the practical application of polymer assemblies in different fields.


This work was supported by the National Science Foundation of China (Nos. 51522308, 21421061), and Chinese Academy of Sciences (No. QYZDB-SSW-SLH025).

Appendix A. Supplementary data

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