Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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List of Contributors XIX

Achutharao Govindaraj

International Centre for

Materials Science

New Chemistry Unit and CSIR

Centre of Excellence in

Chemistry

Jawaharlal Nehru Centre for

Advanced Scientiﬁ c Research

Jakkur P. O.

Bangalore 560 064

India

and

Solid State and Structural

Chemistry Unit

Indian Institute of Science

Bangalore 560 012

India

Jose E. Herrera

The University of Western

Ontario

Department of Civil and

Environmental Engineering

London, ON N6A 5B9

Canada

Ishenkumba A. Kahwa

The University of the West

Indies

Chemistry Department

Mona Campus

Kingston 7

Mona

Jamaica

Johnson Kasim

Nanyang Technological

University

School of Physical and

Mathematical Sciences

Division of Physics and Applied

Physics

Singapore 637371

Singapore

Kazunori Kataoka

The University of Tokyo

Department of Materials Engineering

Graduate School of Engineering

7 - 3 - 1 Hongo

Bunkyo - ku

Tokyo 113 - 8656

Japan

and

The University of Tokyo

Center for Disease Biology and

Integrative Medicine

Graduate School of Medicine

7 - 3 - 1 Hongo

Bunkyo - ku

Tokyo 113 - 0033

Japan

and

The University of Tokyo

Center for NanoBio Integration

7 - 3 - 1 Hongo

Bunkyo - ku

Tokyo 113 - 8656

Japan

Yonggang Ke

Arizona State University

Department of Chemistry and

Biochemistry & The Biodesign

Institute

Tempe, AZ 85287

USA

Harm - Anton Klok

Ecole Polytechnique Fédérale de

Lausanne (EPFL)

Institut des Matériaux, Laboratoire des

Polymères

STI - IMX - LP

MXD 112 (B â timent MXD), Station 12

1015 Lausanne

Switzerland

XX List of Contributors

Arjun S. Krishnan

North Carolina State University

Department of Chemical &

Biomolecular Engineering

Raleigh, NC 27695

USA

Masashi Kunitake

Kumamoto University

Department of Applied

Chemistry and Biochemistry

2 - 39 - 1 Kurokami

Kumamoto 860 - 8555

Japan

Fernando Langa

Universidad de Castilla - La

Mancha

Facultad de Ciencias del

Medio Ambiente

45071 Toledo

Spain

Massimo Lazzari

University of Santiago de

Compostela

Department of Physical

Chemistry

Faculty of Chemistry and

Institute of Technological

Investigations

15782 Santiago de Compostela

Spain

Sebastien Lecommandoux

University of Bordeaux

Laboratoire de Chimie des

Polym è res Organiques (LCPO)

UMR CNRS 5629

Institut Polytechnique de

Bordeaux

16 Avenue Pey Berland

33607 Pessac

France

Zhi Li

Colorado School of Mines

Department of Chemistry and

Geochemistry

1500 Illinois St.

Golden, CO 80401

USA

Jiang - Jen Lin

National Taiwan University

Institute of Polymer Science and

Engineering

Taipei 10617

Taiwan

Guojun Liu

Queens University

Department of Chemistry

50 Bader Lane

Kingston Ontario K7L 3N6

Canada

Yan Liu

Arizona State University

Department of Chemistry and

Biochemistry & The Biodesign

Institute

Tempe, AZ 85287

USA

Watoru Nakao

Yokohama National University

Department of Energy and Safety

Engineering

79-5 Tokiwadai

Hodogaya-ku

Yokohama 240-8501

Japan

List of Contributors XXI

Dhriti Nepal

Gwangju Institute of Science

and Technology (GIST)

Department of Materials Science

and Engineering

1 Oryong - dong, Buk - gu

Gwangju 500 - 712

South Korea

and

School of Polymer

Textile and Fiber Engineering

Georgia Institute of Technology

Atlanta, GA 30332

USA

Jean - Fran ç ois Nierengarten

Universit é de Strasbourg

Laboratoire de Chimie des

Mat é riaux Mol é culaires

(UMR 7509)

Ecole Europ é enne de Chimie

Polym è res et Mat é riaux

25 rue Becquerel

67087 Strasbourg, Cedex 2

France

Hiroyuki Nishide

Waseda University

Department of Applied

Chemistry

Tokyo 169 - 8555

Japan

Christopher K. Ober

Cornell University

Department of Materials Science

and Engineering

Ithaca, NY 14853

USA

Akihiro Ohira

National Institute of Advanced

Industrial Science and Technology

(AIST)

Polymer Electrolyte Fuel Cell Cutting -

Edge Research Center (FC - Cubic)

2 - 41 - 6 Aomi, Koto - ku

Tokyo 135 - 0064

Japan

Makoto Onaka

The University of Tokyo

Department of Chemistry

Graduate School of Arts and Sciences

Komaba, Meguro - ku

Tokyo 153 - 8902

Japan

Ryan R. Otter

Middle Tennessee State University

Department of Biology

Murfreesboro, TN 37132

USA

Kenichi Oyaizu

Waseda University

Department of Applied Chemistry

Tokyo 169 - 8555

Japan

Munetaka Oyama

Kyoto University

Graduate School of Engineering

Department of Material Chemistry

Nishikyo - ku

Kyoto 615 - 8520

Japan

XXII List of Contributors

Elisa Passaglia

University of Pisa

Department of Chemistry and

Industrial Chemistry

Via Risorgimento 35

56126 Pisa

Italy

Thathan Premkumar

Department of Materials Science

and Engineering

Gwangju Institute of Science

and Technology (GIST)

1 Oryong - dong, Buk - gu

Gwangju 500 - 712

South Korea

Andrea Pucci

University of Pisa

Department of Chemistry and

Industrial Chemistry

Via Risorgimento 35

56126 Pisa

Italy

Jeremy J. Ramsden

Cranﬁ eld University

Bedfordshire MK43 0AL

and

Cranﬁ eld University at

Kitakyushu

2 - 5 - 4F Hibikino

Wakamatsu - ku

Kitakyushu 808 - 0135

Japan

C.N.R. Rao

International Centre for Materials

Science,

New Chemistry Unit and CSIR Centre

of Excellence in Chemistry

Jawaharlal Nehru Centre for Advanced

Scientiﬁ c Research

Jakkur P. O.

Bangalore 560 064

India

and

Solid State and Structural Chemistry

Unit

Indian Institute of Science

Bangalore 560 012

India

Marina Resmini

Queen Mary University of London

School of Biological and

Chemical Sciences

Mile End Road

London E1 4NS

Ryan M. Richards

Colorado School of Mines

Department of Chemistry and

Geochemistry

1500 Illinois St.

Golden, CO 80401

USA

Aaron P. Roberts

University of North Texas

Department of Biological Sciences &

Institute of Applied Sciences

Denton, TX 76203

USA

List of Contributors XXIII

Kristen E. Roskov

North Carolina State University

Department of Chemical &

Biomolecular Engineering

Raleigh, NC 27695

USA

Giacomo Ruggeri

University of Pisa

CNR - INFM - PolyLab

c/o Department of Chemistry

and Industrial Chemistry

Via Risorgimento 35

56126 Pisa

Italy

Helmut Schlaad

Max Planck Institute of Colloids

and Interfaces

MPI KGF Golm

14424 Potsdam

Germany

Evan L. Schwartz

Cornell University

Department of Materials Science

and Engineering

Ithaca, NY 14853

USA

Tsunetake Seki

The University of Tokyo

Department of Chemistry

Graduate School of Arts and

Sciences

Komaba, Meguro - ku

Tokyo 153 - 8902

Japan

Ze Xiang Shen

Nanyang Technological University

School of Physical and Mathematical

Sciences

Division of Physics and Applied

Physics

Singapore 637371

Singapore

Young - Seok Shon

California State University, Long Beach

Department of Chemistry and

Biochemistry

1250 Bellﬂ ower Blvd

Long Beach, CA 90840

USA

Richard J. Spontak

North Carolina State University

Department of Chemical &

Biomolecular Engineering

Raleigh, NC 27695

USA

and

North Carolina State University

Department of Materials Science &

Engineering

Raleigh, NC 27695

USA

Koji Takahashi

Kyushu University

Hakozaki

Higashi-ku

Fukuoka 812-8581

Japan

Akrajas Ali Umar

Universiti Kebangsaan Malaysia

Institute of Microengineering and

Nanoelectronics

43600 UKM Bangi Selangor

Malaysia

XXIV List of Contributors

Hao Yan

Arizona State University

Department of Chemistry and

Biochemistry & The Biodesign

Institute

Tempe, AZ 85287

USA

Ting Yu

Nanyang Technological

University

School of Physical and

Mathematical Sciences

Division of Physics and Applied

Physics

Singapore 637371

Singapore

Jingdong Zhang

Huazhong University of Science and

Technology

College of Chemistry and Chemical

Engineering

Wuhan 430074

China

Phase - Selective Chemistry in Block Copolymer Systems

Evan L. Schwartz and Christopher K. Ober

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

1.1

Block Copolymers as Useful Nanomaterials

1.1.1

Introduction

Despite our best efforts to chemically design functional nanomaterials, we cannot

yet match the brilliance of Nature. One striking example of this fact comes from

a tethering structure known as a byssus created by the bivalve, Mytilus edulis .

Byssal threads are the highly evolved materials that M. edulis uses to provide secure

attachments to rocks and pilings during ﬁ lter feeding. The threads begin at the

base of the mussel ’ s soft foot and attach to a hard surface by an adhesive plaque.

Under strong tidal forces, an ordinary material would not be able to withstand the

contact stresses that would result from the meeting of such soft and hard surfaces.

Recent studies have shown that M. edulis solves this materials design problem

through the creation of a “ fuzzy ” interface that avoids abrupt changes in the

mechanical properties by gradually changing the chemical composition of the

thread [1] . The chemistry that it uses to accomplish this graded material involves

the elegant use of collagen - based self - assembling block copolymer s ( BCP s) [2] . The

ventral groove of the mussel ’ s foot contains several pores that act as channels for

a reaction – injection - molding process that creates the copolymer. For this, central

collagen blocks are mixed with a gradient of either elastin - like (soft) blocks, amor-

phous polyglycine blocks (intermediate), or silk - like (stiff) threads to form “ di -

block ” copolymers of gradually decreasing mechanical stiffness as M. edulis moves

farther away from the rock interface. Spontaneous self - assembly of the biopolymer

seems to occur by the metal - binding histidine groups found in between each block

interface that may act as ligands for metal - catalyzed polymerizations. The transi-

tion metals used for these polymerizations, such as Zn and Cu, are extracted from

the ambient water through ﬁ lter feeding.

M. edulis byssal thread is not the only example of a self - assembling chemical

system found in Nature that seems perfectly suited to its environment. Self -

assembly such as that found in M. edulis can be found in nearly every level of

2 1 Phase-Selective Chemistry in Block Copolymer Systems

nature, from cellular structures such as lipid bilayers [3] , the colonization of bac-

teria [4] , and the formation of weather systems [5] . The concept of self - assembly

is deﬁ ned by the automatic organization of small components into larger patterns

or structures [6] . As small components, nature often uses various molecular inter-

actions, such as hydrophilic/hydrophobic effects and covalent, hydrogen, ionic and

van der Waals bonds to construct nanomaterials with speciﬁ c macroscale func-

tionalities. As scientists, we have learned an extraordinary amount about how to

construct better synthetic materials from careful studies of how structure ﬁ ts func-

tion in natural materials [7] .

In the ﬁ eld of soft matter, one type of self - assembling synthetic material that

has already been introduced in the M. edulis example is the BCP. BCPs are com-

posed of different types of polymer connected by a covalent bond [8] . Apart from

their interesting physical properties that have resulted in their use in byssal

threads, upholstery foam, box tape, and asphalt [9] , BCPs are also interesting due

to the ability of each polymer block, or phase , to physically separate on the nanom-

eter scale into various self - assembled morphologies such as spheres, cylinders,

and sheets. These structures are attractive to scientists for several reasons.

• First, if one of the phases is removed from the periodic, ordered lattice, then thin

ﬁ lms of the material could be used as stencils to etch patterns into semiconductor

substrates such as silicon or gallium arsenide. This application is of great

interest to the semiconductor industry, which is currently searching for

alternative technologies for sub - 20 nm lithography.

• Second, chemists are interested in BCP templates because they provide the

power to carry out chemical reactions within speciﬁ c phases of the material.

This ability opens up many new areas of chemistry for nanomaterial design,

including the growth of functional nanoparticle arrays for catalytic applications,

the selective sequestration of chemicals for drug delivery, and the creation of

mesoporous monolithic structures as low - k dielectric materials.

• Third, chemical functionalities attached to one phase within BCPs can be driven

to segregate to the surface, where they can be affected by external stimuli such

as ultraviolet ( UV ) light. These surface - responsive materials could be

lithographically patterned to control the selective adsorption of biomolecules for

biosensor applications.

All of the above applications use phase - selective chemistry to effect changes to the

BCP microstructure and create useful nanostructured materials. In this chapter,

we will discuss not only the recent investigations in these areas but also many

other new and interesting applications.

The chapter is organized into three sections. In the ﬁ rst section we will discuss

the basics of BCP self - assembly, and include a more detailed analysis of the mor-

phologies possible with this class of material, along with an overview on how they

are made and modiﬁ ed. The second section will provide a literature review of

relevant studies in the ﬁ eld, including descriptions of BCPs as lithographic materi-

als, as nanoreactors , as photo - crosslinkable nanobjects, and as surface - responsive

1.1 Block Copolymers as Useful Nanomaterials 3

materials. The third section will conclude with a summary of the most important

contributions, together with a few additional insights on the future direction of

the ﬁ eld of phase - selective BCP systems.

1.1.2

Self - Assembly of Block Copolymers

The thermodynamics of polymer mixing plays a large role in the self - assembly of

BCPs [10] . In typical binary polymer mixtures, it is entropically unfavorable for

two dissimilar homopolymers to mix homogeneously, as both components feel

repulsive forces that result in the formation of large “ macrophases ” of each com-

ponent in the mixture, akin to the mixing of oil and water. In diblock copolymers,

however, the two component polymer “ blocks ” are chemically attached with a

covalent bond. Here, the covalent bond acts as an elastic restoring force that limits

the phase separation to mesoscopic length scales, thus resulting in “ microphase ”

separated structures. The size of these phases, which are also known as microdo-

mains , scale directly as the two - thirds power of the copolymer molecular weight

[11] . The speciﬁ c shape of the microdomains relies on a number of factors that

control how each of the blocks interacts with each other. In the simplest argument,

if there are equal amounts of each polymer, the microdomains will form into

distinct layers with planar interfaces. However, if there is more of one block than

the other, then curved interfaces will result. This curvature minimizes the repul-

sive interfacial contact between the A and B block, which also minimizes the free

energy of the system. The bend that forms can be characterized by the curvature

radius, R , as shown in Figure 1.1 . Therefore, the equilibrium morphology of the

BCP can usually be predicted based on differential geometry.

Other, more complicated, ‘ self - consistent mean ﬁ eld ’ theoretical treatments can

be used to calculate the equilibrium morphology of the BCP. These theories sum

the free energy contributions between (i) the repulsive polymer – polymer interac-

tions versus (ii) the elastic restoring force energy for a particular microphase

structure. The microphase structure with the lowest free energy sum will be the

ﬁ nal equilibrium morphology. These theoretical equilibrium morphologies can be

mapped out on a phase diagram, as shown in Figure 1.2 . A typical BCP phase

diagram plots the product χ N on the ordinate versus the volume ratio, f

, on the

independent axis. χ is known as the Flory – Huggins interaction parameter, which

quantiﬁ es the relative incompatibility between the polymer blocks, and is inversely

related to the temperature of the system. N is called the degree of polymerization ,

which is the total number of monomers per macromolecule. The volume fraction

is represented by f

= N

/ N , where N

is the number of A monomers per molecule.

For very low concentrations of A monomer, no phase separation will occur and

the two polymers will mix homogeneously. However, at slightly higher composi-

tions, where f

< < f

, the A blocks form spherical microdomains in a matrix of B.

The microdomains arrange on a body - centered cubic ( BCC ) lattice. Increasing the

volume fraction to f

< f

leads to an increase in the connectivity of the microdo-

mains, triggering the spheres to coalesce into cylinders that arrange on a hexago-

4 1 Phase-Selective Chemistry in Block Copolymer Systems

nal lattice. A roughly equal amount of both A and B blocks ( f

≈ f

) will result in

the formation of alternating layered sheets, or lamellae , of the A and B blocks. Any

further increase in f

( f

> f

), will cause the phases to invert, which means that

the B block forms the microdomains in the matrix of A.

Thus, by tailoring the relative amount of A, the chemist can control the con-

nectivity and dimensionality of the global BCP structure: spheres essentially rep-

resent zero - dimensional points in a matrix; cylinders represent one - dimensional

lines; and lamellae represent two - dimensional sheets. Additionally, narrow regions

of f

exist in between the cylindrical and lamellar phase space where the two mor-

phologies interpenetrate each other to form three - dimensional ( 3 - D ) “ gyroid ” [12,

13] network structures. Some reports of these morphologies have been published,

and efforts have been put forth to take advantage of the added dimensionality with

new applications [14, 15] .

1.1.3

Triblock Copolymers

Adding extra polymer blocks to the BCP chain introduces additional levels of

complexity into the self - assembled phase behavior. Core – shell morphologies [16] ,

“ knitting pattern ” [17] and helical structures (Figure 1.3 ) are just a few of the exotic

Figure 1.1 (a) Equal volume fractions of A

and B blocks form layered structures called

lamellae with curvature radius approaching

inﬁ nity. Unequal volume fractions of A and B

cause a curvature at the intermaterial dividing

surface ( IMDS ) to minimize interfacial

contact between the blocks and cause

decrease of the curvature radius;

(b) Schematic representing the application of

this model in a sphere - forming (PS - b - PMMA)

block copolymer system. Adapted from Ref.

[29] .