Gelation Inside Block Copolymer Aggregates and Organic/Inorganic Nanohybrids

Gelation Inside Block Copolymer Aggregates and

Organic/Inorganic Nanohybrids

Yongming Chen,* Jianzhong Du, Ming Xiong, Ke Zhang, Hui Zhu

State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials,Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, P. R. ChinaE-mail: [email protected]

Received: February 22, 2006; Revised: March 13, 2006; Accepted: March 14, 2006; DOI: 10.1002/marc.200600128

Keywords: block copolymers; gelation; nanoparticles; polysiloxanes; self-organization

Introduction

When twounfavorable polymer segments are linked together

by a covalent bond, so called block copolymers are produced.

As a result of a balance of entropy and enthalpy, the self-

assembly of block copolymers generates a tremendous well-

defined structure on the nanoscale. This property has

attracted unprecedented attentions of peopleworking in both

academia and industry. Sincemany books and review articles

have summarized the general chemistry and properties of

block copolymers, only the self-assembly of block copoly-

mers in solution is introduced briefly. In a selective solvent

for one segment, block copolymers can form polymer

micelles by self-assembling. Depending upon the block

copolymer architecture, composition, and conditions of self-

organization, a variety of morphologies like star micelles,[1]

vesicles,[2,3] worms,[4] tubules,[5] donut rings,[6,7] multi-

compartment micelles,[8] and many other kinetically con-

trolled structures have been obtained from the spontaneous

self-assembly of amphiphilic block copolymers.[9]

Owing to their unique structures, well-defined self-

assemblies of block copolymers in solution are considered

to be good candidates for applications in nanomaterial

science and technology. However, the fact that block

copolymer aggregates are composed of many polymer

chains and are subject to structural damage when the

temperature and solvents change has significantly limited

their applications in situations where a morphological

stability is needed. In addition, block copolymer aggregates

generally are organic objects, for their precursors are

organic structures: the poor temperature stability and

mechanic stability of organic objects have also made them

Summary: Like in many other cases, block copolymersbearing alkoxysilyl groups in one segment self-assemble intoaggregates. However, they may allow one to study the sol-gelreaction by making use of the gelable groups located in thedomains of the aggregates, therefore, the gelation process canbemade in a selected domain of the nanoscale. As a result, theorganic nanostructure is transformed into an organic/inorganic hybrid. The process of using covalently graftedalkoxysilane groups along the block copolymer precursors isapplicable not only to the aggregates formed in solution, but

also to the other forms of aggregates in melts, thin films, andinterfaces. In this feature article, this emerging field isintroduced,mainly focusing on the gelation that occurs insidethe preformed block copolymer aggregates in solution. Themorphologically fixed hybrid nanoparticles, such as spheres,hollow particles, and complex hollow particles, are presentedand discussed. This technology has great potentials in thefields of nanomaterials and nanotechnologies, since variousorganic/inorganic hybrid nanoparticles and nanostructurescan be effectively generated using this process.

Gelation in preformed block-copolymer aggregates generates controlledstructures.

Macromol. Rapid Commun. 2006, 27, 741–750 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Feature Article DOI: 10.1002/marc.200600128 741

unsuitable for applications where such stabilities are

required. The morphological stability can be fulfilled by

performing intra-aggregate chemical cross-linking. In this

respect, Liu et al. have established a photo-cross-linking

procedure to treat preformed aggregates by using the 2-

cinnamoyl groups contained in block copolymers, and

versatile stable organic nano-objects have been pro-

duced.[10] A few other groups have also performed a

cross-linking reaction to fix the self-assembly by using

polymerizable groups located along the block copolymer

chains.[11] As a result of chemical cross-linking, nano-

particles that are stable against environmental change are

produced. However, all the nanoparticles discussed above

are made of pure organic polymers, the mechanical and

thermal stability issue remains to be addressed.

It is well-known that alkoxysilanes with organic

substituents, like R0–Si(OR)3, easily undergo hydrolysis

and polycondensation. When the alkoxysilane groups are

grafted along organic polymer chains, a sol-gel process of

such a polymer generates an organic polymer/inorganic

cross-linked network.[12] Furthermore, a two-step or

simultaneous sol-gel reaction and radical polymerization

of alkoxysilane with polymerizable groups can also yield

organic polymer/inorganic hybrids.[13] Combining the uni-

que properties of organic polymer and inorganic materials,

organic/inorganic hybrid materials fill the gap between

Yongming Chen completed his Ph.D. degree at the Nankai University in China under the supervision ofProf. Binglin He in 1993. He then worked in the group of Prof. Fu Xi at the Institute of Chemistry, the CAS.From Oct. 1998 to Aug. 2001, he made his postdoc research in Germany with Prof. G. Wulff at theUniversity of Dusseldorf and Prof. M. Schmidt at the University of Mainz, respectively. Afterwards, hereturned to the Institute of Chemistry as a professor. His research interests lie in the areas of controlledpolymerization, dendrimer chemistry and polymeric nanomaterials. His present projects include the novelapproach to synthesis star/branched polymers by using controlled radical polymerization, dendronizedpolymer functionalization, and the gelation process inside reactive block copolymer aggregates.

Jianzhong Du obtained his Ph.D. in polymer chemistry under the supervision of Prof. Yongming Chen atthe Institute of Chemistry, the CAS in 2004. He studied the synthesis and properties of block copolymersbearing alkoxysilyl groups. Then he joined Prof. S. P. Armes’ group at the University of Sheffield as apostdoc. Since Oct. 2005, he switched to the group of Prof. W. A. Goedel at Chemnitz University ofTechnology supported by Alexander von Humboldt Stiftung.

Ming Xiong finished her undergraduate degree of chemistry at the Wuhan University in China. Since2004, she became a postgraduate of the Institute of Chemistry supervised by Prof. Yongming Chen. Herthesis is currently involved in studying the self-assembly of gelable block copolymers.

Ke Zhang completed his undergraduate degree and master degree of materials at the Sichuan Universityin China. Since 2004, he joined the group of Prof. Yongming Chen at the Institute of Chemistry as a Ph.D.candidate. His research projects have involved organic/inorganic hybrids based on novel blockcopolymers.

Hui Zhu studied chemistry at the Tsinghua University in China since 1998 to 2002. He joined the group ofProf. Yongming Chen at the Institute of Chemistry as a Ph.D. candidate. His research projects haveinvolved the synthesis and properties of reactive block copolymers.

742 Y. Chen, J. Du, M. Xiong, K. Zhang, H. Zhu

Macromol. Rapid Commun. 2006, 27, 741–750 www.mrc-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

organic and inorganicmaterials. Though the sol-gel process

has been applied extensively and some organic/inorganic

hybrid materials have been commercialized, the resultant

materials are macroscopic. Gelations in micro- or nano-

domains of these materials have not been achieved with the

above mentioned approaches.

The self-assembly of block copolymers has been used

during the sol-gel process of silicate precursors and, in such

cases, the self-assembly serves as a template to direct the

formation of an ordered structure. After the block copo-

lymers are removed, mesoporous silica is generated.[14]

Amphiphilic block copolymers have also been used as

structure-directing agents to form ceramics by sol-gel

chemistry, and the hydrophilic segments are integrated into

the ceramic phase to generate novel composites described

as a ‘quasi two-phase system’.[15] In these approaches, the

gelation process is made in the spaces of organized block

copolymer micelles or in one phase of the block copolymer

microdomain of the nanoscale. Needless to say, these

achievements have demonstrated great potential applica-

tions of this type of materials for material science and

technology.However, in these cases, the ordered structure is

only generated with the packing of well-defined polymer

micelles or within the microphase separation. Therefore, in

a dilute solution, it is almost impossible to make such

materials without covalent bonding between the gelation

precursor and block copolymer, especially to precisely

control the gelation.

Block copolymers can carry reactive alkoxysilyl groups

in one segment via covalent bonds. Since the alkoxysilyl

groups are located precisely in a certain domain, no matter

whether in bulk or dilute solutions, self-organization of

such block copolymers can be achieved successfully. In

addition, the gelation process can be tailored in a selected

domain because of the precise localization of the covalently

bonded alkoxysilyl groups.

This feature article presents an emerging subject of the

gelation inside aggregates of block copolymers that bear

gelable alkoxysilane groups. The principle and the syn-

thesis of gelable block copolymers are first introduced.

Gelation chemistry in the preformed aggregates from the

gelable block copolymers in solution is then described, and

some nano-objects are reviewed. Last, the paper is

concluded with a summary and an outlook.

The Principle

Block copolymer self-assembly and sol-gel chemistry of

alkoxysilyl groups are the fundamentals of the emerging

approach discussed in this paper. If a block copolymer

contains –Si(OR)3 groups in one segment and it forms well-

defined aggregates in a selective solvent of one segment, the

–Si(OR)3 groups will be located in the core or corona of the

aggregates. When a catalyst for the hydrolysis and

polycondensation is added, a gelation reaction on the

nanoscale occurs. As a result, alkoxylsilane domains

transform into a silica oxide network and the particles are

no longer an aggregate but a nano-object. For example, if an

amphiphilic block copolymer of hydrophobic segments

with –Si(OR)3 pendants self-organizes into micelles in

aqueous solution, the hydrophobic alkoxysilyl segments

become the core while the hydrophilic segments form the

corona, and then gelation can be performed in this core.

Because of corona protection, inter-micellar gelation is

avoided. After gelation, the core is cross-linked and adopts

an inorganic structure, and, a spherical polymer brush with

hydrophilic hairs is produced. This process is schematically

presented in Scheme 1.

If the morphology of the aggregates can be tuned by

adjusting the block copolymer composition and self-

assembly condition, like what has been practiced in the

literature, novel organic/inorganic nanohybrids can then be

made by this simple procedure. Owing to the cross-linking

and inorganic components, the hybrid particles with

improved micromechanical properties are robust and stable

against environmental changes.

Nonetheless, there are several challenges in this process.

It is known that the alkoxysilyl group, especially meth-

oxysilyl, is sensitive to water, especially under acidic or

basic conditions. The hydrolysis of alkoxysilyl groups may

change the polarity of the block copolymer and, therefore,

may hamper the self-organization. A prerequisite for the

process to be successful is that the aggregation should be

faster than the hydrolysis reaction. Another important

issue is related to the gelation step: the hydrolysis and

condensation in the nanometer-scale microdomains should

not destroy themicellar structure. Concern for the first issue

can be easily eased since it is known that the spontaneous

self-assembly of a block copolymer is a faster process

Scheme 1. The principle of gelation inside a block copolymer micelle.

Gelation Inside Block Copolymer Aggregates and Organic/Inorganic Nanohybrids 743


relative to the gelation reaction when no catalyst is present.

As for the second issue, the experimental results demon-

strate that the sol-gel process that occurs in the domains is

successful in fixing themorphology. Readers will learn how

this procedure works and how novel organic/inorganic

nanohybrids are made by our group and others from the

following sections.

Synthesis of Block Copolymer BearingTrialkoxysilyl Segments

For such a target, a family of block copolymers with one

segment that bears alkoxy silanes needs to be prepared. The

simplest way seems to make them via controlled poly-

merization of the alkoxysilane with a polymerizable group.

Scheme 2 lists some gelable monomers and their

block copolymers. 3-(Trimethoxysilyl)propylmethacrylate

(TMSPMA), an important silane coupling agent, is awidely

used gelable monomer in the preparation of organic/

inorganic composites. Although this monomer can be

easily introduced into polymers through conventional

radical copolymerization, the controlling polymerization

process is not an easy task.

As early as 1992, Hirao et al. have studied the

anionic polymerization of TMSPMA, 3-(triethoxysilyl)-

propyl methacrylate, and 3-(tripropoxysilyl)propyl meth-

acrylate in tetrahydrofuran (THF) at�78 8Cwith an anionic

initiator.[16] Well-defined polymers are obtained under

selected conditions and poly(3-(tripropoxysilyl)propyl

methacrylate)-block-poly(methyl methacrylate) is pre-

pared. It is of note that the polymerization of TMSPMA is

not as well controlled as that of the other two monomers.

Since it is known that anionic polymerization is

sensitive not only to air and moisture but also to some

functional groups, therefore, the controlled polymerization

of TMSPMA remains to be explored.

In recent years, controlled radical polymerization has

been developed as a versatile and powerful tool for

preparing block copolymers with predetermined molecular

weights and narrow-molecular-weight distributions. This

process relies on a dynamic equilibrium between a low

concentration of radicals and a predominant amount of

dormant species that do not propagate or terminate. Typical

methods include: atom-transfer radical polymerization

(ATRP), reversible addition-fragmentation chain transfer

(RAFT) polymerization, and nitroxide-mediated polymeri-

zation (NMP). One of the main advantages of controlled

radical polymerization is its tolerance to many func-

tional groups. Many well-defined polymers that carry

pendant or end-functional groups have been prepared by

using functionalized monomers and functional initiators.

The ATRP of TMSPMA mediated by CuBr/N,N,N0,N00,N00-pentamethyldiethylenetriamine using ethyl 2-bromo-

isobutyrate (2-EBiB) and poly(ethylene oxide) methyl ether

2-bromoisobutyrate (PEO-Br) as the initiators have been

explored.[17] The results indicate that the polymerizations of

TMSPMA exhibit first-order kinetics, andmolecular weights

increase linearly with monomer conversion. Molecular

weight distributions remain narrow throughout the polymer-

izations. When the PEO macroinitiator is applied, a series of

reactive diblock copolymers, poly(ethylene oxide)-block-

polyTMSPMA(PEO-b-PTMSPMA), have been synthesized.

Scheme 2. Some gelable monomers and their block copolymers.



This familyofblockcopolymers is composedofahydrophilic

PEO and a hydrophobic PTMSPMA, therefore, such

an amphiphilic copolymer may self-assemble in aqueous

solution. Indeed, this block copolymer has displayed

interesting behaviors, as illustrated in the following sections.

The controlled radical polymerizationofTMSPMAbyATRP

has been also reported by a different group and a

corresponding block copolymer has been obtained.[18]

By the introduction of a second monomer during the

polymerization, a block copolymer that contains random

copolymer as one segment can be prepared. For example, by

random copolymerization with methyl methacrylate (MMA),

a PEO-b-P(TMSPMA-stat-MMA) copolymer is prepared.[17]

Other monomers, such as tert-butyl acrylate (tBA),

N-isopropylacrylamide (NIPAM), 2-(dimethylamino)ethyl

methacrylate (DMAEMA), and 2-(diethylamino)ethyl meth-

acrylate (DEAEMA) may also be incorporated into the

TMSPMA segments. The introduction of the second mono-

mer not only changes the cross-linking densities but may also

endow the copolymers and their aggregates with more

functionalities.

Radical polymerization of the TMSPMA monomer

mediated by 2-cyanoprop-2-yl dithiobenzoate (CPDB), a

chain transfer agent (CTA), using 2,20-azoisobutyronitrile(AIBN) as an initiator has also been reported.[19] Block

copolymers of TMSPMA and MMA are synthesized

starting from either PTMSPMA or PMMA macro-CTA.

Liu et al. have also applied PNIPAMmacro-CTA to prepare

PNIPAM-b-PTMSPMA by RAFT-mediated polymeriza-

tion.[20] The advantage of the RAFT polymerization over

the ATRP is that no metal catalyst is needed, so purification

of the final product is easier.

Besides the examples listed above, Thomas et al.

prepared polystyrene-block-poly(3-(triethoxysilyl)propyl

isocyanate) (PS-b-PIC) by anionic polymerization initiated

with butyllithium in THF at �78 8C.[21] In their study, the

polyisocyanate segment, a stiff helical rod, forms a

lyotropic nematic liquid crystalline phase.

Gelation Inside Block CopolymerAggregates in Solution

Our group has used this approach extensively to prepare

organic/inorganic hybrid nanoparticles that are shape

persistent. Some successful examples from this group and

a few from others are summarized in the following sections,

which are categorized by the morphologies of the aggrega-

tions.

Simple Hollow Particles of BlockCopolymer Vesicles

The PEO-b-PTMSPMA polymer is soluble in methanol.

Therefore, the first attempt to produce the micelles is

performed in a mixed solvent of methanol and water.

Unexpectedly, vesicles, instead of spherical micelles, form

under these conditions.[22] A micelle solution is made upon

adding water dropwise into a PEO45-b-PTMSPMA59 block

copolymer solution in methanol (1.0 g �L�1) to a water

content of 55 wt.-%. A drop of such solution is then taken

out and freeze-dried for transmission electron microscopy

(TEM) analysis. Figure 1A shows a vesicular morphology

with a diameter close to 100 nm. 1HNMRanalysis indicates

that the peaks that correspond to the protons of the

PTMSPMA segments disappear, while the peak of the

PEO protons remains unchanged. This result demonstrates

that PTMSPMA forms the wall of vesicles and PEO forms

the corona. However, the vesicles at this time are easily

deformed at a dry state because of the low glass transition

temperature of the PTMSPMA segments.

When a small amount of gelation catalyst, triethylamine

(TEA), is added into the vesicle solution, a hollow

structured vesicle with a much higher contrast is observed

(shown in Figure 1B). The wall thickness, which is highly

uniform, is ca. 17 nm, a value that corresponds to a typical

bilayer structure. From the released methanol, as observed

in 1H NMR spectra, as well as the mass losses that result

from the hydrolysis and polycondensation observed from

Figure 1. TEM images of PEO45-b-PTMSPMA59 vesicles in methanol/waterA) before and B) after gelation, ref.[22]



static light scattering (SLS) analysis, the gelation that occurs

in the vesicle walls is proven. TEM, SLS, and dynamic light

scattering (DLS) indicate that gelation does not affect

the vesicular morphologies. Therefore, the organized

PTMSPMA segments gelate only in the vesicle wall, which

transforms into a cross-linked polysilsesquioxane as

demonstrated in Scheme 3. The vesicle is no longer an

aggregate of many polymer chains but a cross-linked robust

hollow nano-object. Scanning electron microscopy (SEM)

images have displayed the periphery of the clasped vesicles.

The detailed conditions for the formation of the vesicles,

including the water content in a binary solvent, initial

copolymer concentration in methanol, and compositions of

the diblock copolymers, have been explored. The results

demonstrate that the vesicles could be generated under a

variety of conditions, such as over a broad range of water

content at the concentration of 5.0 mg �mL�1 PEO45-b-

PTMSPMA59. Before the exclusive vesicles appear,

spheres, short rods, and lamellae are observed as coexistant

morphologies when the water content increases gradual-

ly.[22b] For the block copolymers with a fixed PEO length

and different PTMSPMA lengths, vesicles are obtained

under the conditions used. Longer PTMSPMA blocks

generate vesicles with a thicker wall. However, when the

length of the PEO segments varies, particles of other

morphologies are obtained.

What is interesting in this system is that the vesicles can be

formed in a solution with a rather high solid content. It is

found that exclusive vesicles are produced when adding

water into a methanol solution with the concentration of

PEO45-b-PTMSPMA59 as high as 100mg �mL�1. Since one

important application of polymer vesicles is to encapsulate

guest molecules, this special property is important for

practical applications. In case of a dilute solution, the

majority of the guest molecules are not encapsulated inside

vesicles, unless the vesicles form in a highly concentrated

polymer solution. Self-assembly of PEO45-b-PTMSPMA59

in aTHF/watermixture is also explored. Figure 2Ashows the

vesicles produced after gelation. The diameter is around

1 mm, which is much larger than the vesicles from a

methanol/water mixture [unpublished result].

Both acid and base have been evaluated in terms of their

influence on the formation of vesicles. It indicates that a

base, such as TEA, only catalyzes gelation but has no effect

on the vesicular morphology, whereas, the acid destroys the

vesicles to give macroscopic gels. This result is easily

understood when one refers to the catalytic mechanisms of

gelation by acid and base. The hydrolysis rate catalyzed by

the acid is higher than the polycondensation rate, as a result,

the hydrolyzed PTMSPMA blocks are switched from

hydrophobic to hydrophilic and the vesicular morphologies

are destroyed. In contrast, the hydrolysis reaction catalyzed

by the base is slower in comparison to the polycondensation

reaction. Once the partial hydrolysis reaction occurs in the

PTMSPMA domain, the polycondensation proceeds simul-

taneously. Therefore, the vesicle structure remains and

becomes more stable as a result of gelation.

The above approach has been extended to PEO-b-

P(DMAEMA-stat-TMSPMA), with the expectation of

generating a stable vesicle with pH sensitivity. This block

copolymer forms vesicles even though a lot of different

monomers are introduced. Figure 2B shows the gelated

vesicles obtained from PEO45-b-P(DMAEMA58-stat-

TMSPMA51) in methanol/water [unpublished result]. Very

recently, a similar gelated vesicle has been reported by

performing the self-assembly of PEO-b-P(DEAEMA-stat-

TMSPMA) in THF/water mixtures by other groups.[23]

The authors claim that the walls of the hybrid vesicles

are pH-tunable. Furthermore, these vesicles are decorated

with gold nanoparticles by using the DEAEMA units

located in the vesicle walls. Figure 3 illustrates the TEM

images of hybrid vesicles and their decoration with gold

particles.

Complex Hollow Particles

Eisenberg et al. have found that the common solvent of

block copolymers plays an important role in the self-

Scheme 3. Formation of PEO-b-PTMSPMAvesicle and gelation of the wall.

Figure 2. TEM images of gelated vesicles of A) PEO45-b-PTMSPMA59 in THF/water andB) PEO45-b-P(DMAEMA58-stat-TMSPMA51) in methanol/water, unpublished results.



assembly of their crew-cut micelles.[9] Even with the same

block copolymer, the morphologies vary when different

common solvents are used.WhenN,N0-dimethylformamide

(DMF)/water is used as the assembling solvent of PEO-b-

PTMSPMA, compound vesicles are obtained. Inside the

particles there are many cavities insulated by cross-linked

hybrid bilayers of uniform thickness (Figure 4A and B).[24]

When increasing the copolymer concentration in DMF,

both the size of the particles and the number of cavities

increase. The polymer solution with a concentration as high

as 10 or 20 mg �mL�1 generated sunflower-like particles

with some vesicle-like mastoids tethering from the multi-

cavity core (shown in Figure 4C). It is observed that the

cavities are separated by uniform walls whose thickness

is the same as that of the petals (ca. 15 nm). The honey-

comb inner structure is confirmed by the microtomed

sample. An SEM image of the flowers (Figure 4D) shows a

clear three-dimensionalmorphology.With the support of an

internal skeleton, the hollow core of the flowers does not

collapse in the dry state, although the diameter reaches

500 nm.

This result is different from the simple vesicles in the

methanol/water mixture described in the previous section.

This can be explained by the properties of common

Figure 3. TEM image ofA) pH sensitive vesicles of PEO-b-P(DEAEMA-stat-TMSPMA)and B) supported gold nanoparticles thereof, ref.[23]

Figure 4. Gelated complex vesicles of PEO45-b-PTMSPMA59 from DMF/water at lowpolymer concentration, A) TEM and B) SEM images, and at high polymer concentration,C) TEM and D) SEM images, ref.[24]



solvents. The vesicles in methanol/water solution are

stabilized by the repulsive interaction of the PEO corona.

However, the repulsion of PEO chains can not stabilize the

individual vesicles in the DMF/water system because of the

high interfacial potential. As a result, the spontaneously

formed vesicles tend to adhere and fuse to decrease the

interfacial potential. Because of the self-assembly, the

thickness of the wall of the outermost particle and that of

the membrane between the cavities inside the core are

uniform and they adopt a bilayer structure. When the

gelation catalyst is introduced, a cross-linking reaction

occurs and themorphologies are fixed. Therefore, this result

supplies a facile way to generate hollow particles with

patterned structures by performing gelation inside the

compound vesicles.[24]

Hybrid Nanospheres of Block Copolymer Micelles

When the PEO segment of PEO-b-PTMSPMA is longer,

PEO113-b-PTMSPMAx, the aggregation in a methanol/

water mixture produces simple spherical micelles exclu-

sively.[25] After gelation in the core, nearly monodisperse

spherical particles with a polysiloxane core embedded in a

PEO corona are obtained, as indicated in Figure 5A. In

comparison with the vesicles that result from using shorter

PEO segments, copolymers with longer PEO segments

form spherical micelles because of the larger core-corona

curvature caused by the stronger chain repulsion between

PEO segments. Spherical particles can also be formed in a

highly concentrated block copolymer solution.

Thermoresponsive hybrid nanospheres have been pre-

pared based on gelation in the core of the preformed

micelles of PNIPAM-b-PTMSPMA in water. PNIPAM

hairs of this particle show two stages of stimulus collapse

upon heating through the lower critical solution temper-

ature (LCST) of the PNIPAM hairs.[20] Koh et al. have

prepared hybrid nanoparticles with the surface of a block

copolymer micelle being coated with a thin silica layer.[26]

An amphiphilic block copolymer that bears some

TMSPMA units in the poly[poly(ethylene oxide) methyl

ether methacrylate] hydrophilic block is synthesized by

ATRP. This block copolymer forms a micelle, and the

methoxysilyl groups situated at the outermost layer of the

micelle gelates in the presence of active silicate. In this

research, block copolymer micelles serve as a template for

silica shell formation. This group has also prepared the core

gelated spheres in organic solvent by heating a PMMA-b-

P(TMSPMA-stat-MMA) solution in the presence of a base

catalyst. PMMA hairy spheres with a cross-linked siloxane

core ca. 30 nm in size are obtained.[18]

Large particles that range from 200–600 nm have also

been obtained by performing the gelation process with the

self-assembly of PEO45-b-P(TMSPMA19-stat-MMA67) in

a DMF/water mixture (Figure 5B).[17] Since the size is

much larger than the simple spherical micelles with the

core-corona structure, their precursors should be classified

as the so-called large compound micelles (LCMs) with a

reverse micellar structure.[9] A large MMA unit content

may increase the hydrophobicity of the copolymer segment,

which generates LCMs instead of the vesicles or LCVs

under the conditions used.

The Gelation of Other Forms of BlockCopolymer Self-Assembled Structures

Block copolymers undergo self-assembly not only in

selective solvents but also in bulk, at an interface, or in a

confined space, etc. Therefore, onemay extend this gelation

process to different forms of block copolymer assemblies to

generate well-defined organic/inorganic nanocomposites

on different length scales. Based on the microphase sepa-

ration of PS-b-PIC, prism-like PIC domains that organize in

Figure 5. A) TEM image of core gelated micelles of PEO113-b-PTMSPMA206,ref.[25] B). SEM image of spherical gelated particles of PEO45-b-P(TMSPMA19-stat-MMA67), ref.

[17]



a smectic pattern in the PS matrix are obtained, and are

further thermally cross-linked through polycondensation of

triethoxysilyl groups of PIC segments. The gelated film can

be dissolved in toluene, and prism like nanoparticles have

been generated as indicated in Figure 6. This approach,

based on the gelable coil-rod block copolymer, provides a

new method to create non-spherical particles.[21b]

Summary and Outlook

If gelable block copolymers bearing alkoxysilyl groups self-

organize into well-defined aggregates in a selective solvent,

sol-gel chemistry can be performed inside these self-

assemblies. As a result, the morphology of the polymer

aggregates is permanently fixed by the cross-linked

polysiloxane, and novel robust hybrid objects are obtained.

Owing to the gelation, the morphology of the particles is

stable against drastic changes in conditions. This approach

precisely controls thegelation only in the domainswhere the

alkoxysilyl groups are located. It not only fixes the

morphologies of block copolymer aggregates, but also

allows the preparation of organic/inorganic nanoparticles in

an effectiveway, which otherwise are difficult to achieve. In

principle, aggregates in any form obtained from the block

copolymers could be transformed to the hybrid structure by

this approach. Therefore, gelation in a block copolymer self-

assembly presents a powerful approach for the preparation

of organic/inorganic hybrid nanoparticles and materials.

PEO-b-PTMSPMA is a fascinating block copolymer in

terms of its self-assembly in aqueous solution. By controlling

the block copolymer composition, solvent property, and

mixed solvent composition, various self-assemblies with

different morphologies (spherical micelles, vesicles, com-

pound vesicles and other morphologies) have been produced

using only one family of block copolymers. It is found that

gelation performed inside these aggregates in solution does

not effect themorphologies. Furthermore, the gelationmakes

TEM analysis of the aggregation much easier. Special

techniques such as cryo-TEM and staining are not needed to

perform the analysis since the high contrast results from the

silica-rich domain and the stability. These organic/inorganic

hybrid particles are a new class of nanomaterials, which

cannot be generated by other means. Thanks to the presence

of inorganic components, calcination at high temperature

may produce purely inorganic structure. SEM images in

Figure 7 show themorphology of gelated vesicles before and

after calcination at high temperature.[22b] It reveals that the

inorganic structures are leftwhen theorganic components are

removed.

In particular, for the PEO-b-PTMSPMA system, the

organization medium used in the preparation and the

dispersion is water, this is the precondition of biorelated

application. Furthermore, attention should be paid to the

PEO hairs grafted on the surfaces of these nanoparticles. It

is well-known that PEO has very important applications in

Figure 7. SEM images of the hybrid vesicles of PEO45-b-PTMSPMA59 beforeA) and after calcination B), ref.[22b]

Figure 6. Bright field TEM images of the nanoobjects bydissolving gelated micro-phase separation of PS-b-PIC, ref.[21b]



biomaterials and biotechnology since it resists protein

adsorption and cell adhesion. In addition, silica oxide is

non-toxic and biocompatible. All these unique structures

permit the application of these PEO/silica hybrid particles

in biomedical areas such as protein/peptide drug encapsu-

lation, delivery, and tissue engineering. If a non-cross-

linkable monomer is introduced into the PTMSPMA

segments of the PEO-b-PTMSPMA polymer, the cross-

linking density can be finely tuned and, therefore, the

selective transport of molecules into or out of the vesicles

based on their size will become possible. Furthermore, in

the case that the second monomer is stimulus responsive to

environmental changes, like temperature, pH, and light, the

cross-linking density of the vesicular wall will be sensitive

to the stimulus. The capsule wall will function as a gate,

which opens or closes upon applying a stimulus. This can be

very important for developing intelligent carriers for

medicines and catalysts. The potential application of the

multicavity nanomaterials in encapsulation is also promis-

ing. For example, as there are many cavities inside the

particles, it may be possible to encapsulate different thera-

peutic compounds into different cavities of a particle for a

controlled stepwise-release. To broaden their applications,

these spherical and hollow particles can be further modified

with bioactive or other functional species on the siloxane

surface.

This approach can also be extended to block copolymer

self-assemblies in different forms and length scales such as

in block copolymer melts, gels, droplets, at interfaces, and

in a confined environment. Therefore, the gelation process

can be performed in these different forms of block

copolymer self-assemblies and, as a result, an enormous

amount of new robust organic/inorganic hybrid materials

are expected to be generated in the future. These materials

patterned with soft organic and hard inorganic micro-

domains may possess unique properties that are difficult to

find in materials produced from other approaches. Fur-

thermore, by sacrificing the organic components, inorganic

nanomaterials are expected to be generated by this

approach.

Acknowledgements: The authors thank Professor ManfredSchmidt, Dr. Karl Fischer, and Dr. Michael Maskos for theirsupport and discussions, and they thank Professor Charles C. Hanand Professor Anchang Shi for valuable discussions. Financialsupport from theChinese Academy of Sciences and theNSFChina(50473056 and 20534010) is gratefully acknowledged.

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Gelation Inside Block Copolymer Aggregates and Organic/Inorganic Nanohybrids

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