Ice template-assisted assembly of spherical PS-b-PAA micelles into novel layer-by-layer hollow spheres

Novel microscale layer-by-layer hollow spheres are template-assembled by monodispersed spherical PS-b-PAA micelles during the sublimation of the frozen micellar solution.

The aggregation of nanoscale building blocks, such as clusters,1 spheres,2,3 and superlattices,4–6 has attracted intense interest due to their importance in fundamental research and potential wide-ranging applications.

To date, a wealthy of methods have been developed to assemble nanoparticles into two- and three-dimensional ordered aggregates, which offers opportunities to explore their novel collective optical, magnetic, and electronic properties.7–10

For examples, Xia et al7. produced various uniform aggregates of spherical colloids following an approach combining physical template and capillary forces.

Jenekhe and Chen11 obtained periodic structures assembled by poly(phenylquinoline)-block-polystyrene via solution evaporation.

Stucky et al12. gained micro-sized hollow spheres co-assembled by silica and gold nanoparticles based on cysteine–lysine diblock copolypeptides.

These novel structures have potential applications ranging from electro-optical devices 13,14 and separation membranes 15,16 to active biomaterial coating 17,18 and nano-reactors.18,19

Our group has been studying the micellization of amphiphilic block copolymers and the ordered aggregation of the polymeric micelles.20,21

Recently, we have prepared various morphological aggregates of micelles via different methods.21

One strategy is to control ordered flower-like aggregation of micelles on a adjustable template of water polycrystal.

The present study is focused on the formation of novel layer-by-layer hollow polymeric spheres by ice template-assisted aggregation of nanoscale micelles.

To the best of our knowledge, this is the first report of obtaining such novel layer-by-layer hollow aggregates via such a simple and convenient method.

The PS30-b-PAA51 micelles were firstly prepared as described elsewhere21 and characterized by dynamic laser scattering (DLS) and transform electronic microscopy (TEM).

A drop of the aqueous micelle solution was first placed onto a clean glass slide and quickly frozen in liquid nitrogen.

Then the ice in the frozen micellar sample was sublimated by freeze-drying under vacuum and the resultant samples were observed by scanning electron microscopy (SEM).

Furthermore, the crystal formation process of pure water was monitored by polarized light microscopy (PLM).

Fig. 1a shows the hydrodynamic diameter (Dh) distribution f(Dh) of the PS30-b-PAA51 micelles in aqueous solution obtained from DLS.

Obviously, the diameter distribution of the micelles is monodispersed and the average Dh of the micelles is about 58 nm.

Fig. 1b displays the TEM images of the micelles and we can clearly see the spherical shape of the micelles with about 40 nm in diameter.

It must be noted that the hydrodynamic diameter Dh of the micelles measured by DLS is much larger than that observed by TEM.

This is because the micelles are swollen in water due to the soluble PAA block, while TEM observation shows the dried aggregates.

Usually, nanoparticles dispersed in water, such as the micelles shown in Fig. 1, aggregate to form random-shaped clusters when the water is removed.

In order to achieve controlled aggregation of the nanoparticles, physical templates are usually used.7

Can the solvent be used directly as a template to control the aggregation?

To the best of our knowledge, liquid water is not a suitable candidate, but ice may be.

Compared with other templates that can be added, the ice template can be formed and removed easily by freezing and sublimating.

Fig. 2 gives the typical PLM image of the thin layer of ice template formed by freezing a drop of water in liquid nitrogen.

The dark grid-like lines as shown in Fig. 2 divide the ice film into many micro-regions.

These micro-regions are supposed to shrink gradually to form spherical micro-regions during the slow sublimation of H2O, which is similar to the sublimation of ice in winter.

Thus, it is reasonable to think that ice as shown in Fig. 2 can be used as a template to guide the aggregation of the micelles.

Herein, we aim to use this template to fulfil the controlled aggregation of the micelles shown in Fig. 1.

Upon first rapidly freezing the micellar aqueous solution in liquid nitrogen and then freeze-drying, the resultant samples were observed by SEM and the SEM images are shown in Fig. 3.

The resultant sample shown in Fig. 3a looks greatly different from the spheres in Fig. 1b.

The most remarkable difference is the broken hollow spheres containing a cavity encapsulating a smaller spherical particle and the outer shell is about 100 nm in thickness, which can be seen in the insert-enlarged image in Fig. 3a.

The fine structure of the shell can be seen in Fig. 3b.

Obviously, the shell is formed by the packed small particles, which corresponds to the nanoscale spherical PS30-b-PAA51 micelles.

These results confirm that the microscale layer-by-layer hollow spheres are just resulted from the assembly of the nanoscale PS30-b-PAA51 micelles.

Thus, it can be concluded that the ice template plays a key role in the ordered assembly of the dispersed PS30-b-PAA51 micelles in water.

According to what we have obtained above, we propose a schematic illustration of the ice template-assisted assembly of the nanoscale micelles into layer-by-layer hollow spheres in Fig. 4.

The first step is to freeze the micellar solution instantly (A in Fig. 4), where the micelles are uniformly dispersed in the ice template.

It should be noted out the ice template is divided into many micro-regions as schematically magnified in step B. Second, during the H2O sublimation process, the micro-regions in the ice template gradually shrink and become spherical and the micelles adhered to the surface of the micro-regions in the ice template are piled gradually into a three-dimensional (3D) spherical arrangement (B→C→D).

Sublimation continuously, a spherical core-shell structure forms (E), where the shell is composed of the packed nanoscale PS30-b-PAA51 micelles and the core is still the frozen micellar solution.

The further sublimation of H2O molecule existing in the frozen core results in formation of a cavity between the obtained spherical polymeric shell layer and the shrunk frozen core (F).

Further continuing the sublimation of H2O under high vacuum state till the ice template is completely removed, the novel layer-by-layer polymeric spheres (G) are obtained.

In summary, we have presented a convenient and simple approach to assemble nanoscale micelles into novel microscale layer-by-layer polymeric hollow spheres.

This method can be extended to general polymeric micelles dispersed in water.

We believe the method will broad a new way to produce novel structures as shown in this study.