Guodong Sui, Miodrag Micic, Qun Huo and Roger M. Leblanc- Studies of a novel polymerizable...

8/3/2019 Guodong Sui, Miodrag Micic, Qun Huo and Roger M. Leblanc- Studies of a novel polymerizable amphiphilic dendrimer

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Colloids and Surfaces

A: Physicochemical and Engineering Aspects 171 (2000) 185197

Studies of a novel polymerizable amphiphilic dendrimer

Guodong Sui, Miodrag Micic, Qun Huo, Roger M. Leblanc *

Department of Chemistry, Center for Supramolecular Science, Cox Science Building-Room 315, Uni6ersity of Miami,

PO Box 249118, Coral Gables, FL 33124, USA

Abstract

In the present study, we have synthesized a polymerizable amphiphilic dendrimer by attaching 10,12-pentcosadiynoic acid (PDA) to a third generation poly(amidoamine) (PAMAM) dendrimer core (PDA-PAMAM). Th

dendrimer was characterized by 1H NMR and MALDI-TOF mass spectroscopy. The amphiphilic property of th

dendrimer has been studied. Surface pressure and surface dipole momentarea isotherm measurements have show

that PDA-PAMAM forms a stable monolayer at the airwater interface with a limiting molecular area of 460 A, 2 p

molecule. The compressed monolayer can be readily polymerized upon UV irradiation, as like other PDA derivative

The topography of the monolayer was observed by Brewster angle microscopy (BAM) as well as by the environmen

tal scanning electron microscopy (ESEM). We found a good correlation between the size and shape of monolay

domains observed with the BAM and ESEM techniques. To our knowledge, this is the first time wet mode ESEM

has been used to characterize LB film. The wet mode ensures that LB films remain intact without being damage

by the high vacuum system of traditional SEM. As a preliminary study, we have found that this new dendrimer form

colloidal particles in chloroform solution and can be readily polymerized by UV irradiation. 2000 Elsevier Scienc

B.V. All rights reserved.

Keywords: Dendrimer; ESEM; PAMAM; PDA; PDA-PAMAM

www.elsevier.nl/locate/colsur

1. Introduction

Dendrimers represent a new class of synthetic

macromolecules characterized by a regularly

branched treelike structure [1]. Recently the den-

dritic structures have attracted increasing atten-

tion for their potential applications in molecular

recognition [2], building blocks for self assembly

process [3], catalysis [4], encapsulation and con-

trolled release [5], chemical sensors [6], biomimet

materials [7,8], metal nanoclusters [9] and so on

The end functional groups of dendrimers a

confined in space and are present at the peripher

of the molecule. Therefore, dendrimers becom

unique starting materials for numerous chemic

modifications. End group modification is particu

larly attractive for dendrimers made by the divegent approach. From this class of compounds, th

poly(amidoamine) dendrimer (PAMAM den

drimer) [10,11], the arborols [12], and th

poly(propylene imine) dendrimers (DAB-dend

(NH2)x) [13] are among those most frequent

* Corresponding author. Tel.: +1-305-2842282; fax: +1-

305-2844571.

E-mail address: [email protected] (R.M. Leblanc)

0927-7757/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.

P I I : S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 5 5 3 - 1


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G. Sui et al. /Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 185197186

studied on account of their availability on a rea-

sonable large scale, even for the higher-generation

dendrimers.

Recently it has been reported on the prepara-

tion of dendritic inverted unimolecular micelles as

a new class of macromolecular structures by at-

taching polar end groups to the PAMAM or

DAB-dendr-(NH2)x dentritic cores. Barrs et al.[1417] have synthesized a series of amphiphilic

dendrimers by PAMAM dendrimer and the DAB-

dendr-(NH2)x with a variety of hydrophobic alkyl

chains. These compounds were able to encapsu-

late guest molecules and could be used as very

effective extractants in liquid liquid extractions.

As a result of their amphiphilic nature, these types

of dendrimers are suitable building blocks for the

construction of supramolecular architectures in

both solution and airwater interface. They also

represent a novel class of surfactants. Langmuir

film studies at the airwater interface demon-

strated that these molecules are able to arrange

themselves in monolayers in which the dendrimer

part of the molecule is in contact with the water

subphase and all the alkyl chains point to the air.

Although the ambivalent nature of dendrimers

make it possible to form monolayers on surface of

solid substrate or aqueous solution, work in this

field is not extensively investigated yet. Much of

the fundamental knowledge and application as-

pects of amphiphilic dendrimer remain unclear.

Beyond the PAMAM and DAB-dendr-(NH2)xmonolayers, carbosilane dendrimer [18],

polystyrene dendrimer [19] and polyether den-

drimer [20] Langmuir or LB films have also been

studied by techniques such as the surface pressure

and surface dipole moment area isotherm mea-

surements, Brewster angle microscope and scan-

ning force microscope methods.

Polydiacetylenes represent an important class of

polymers, which have attracted continuous atten-

tion due to their unique electronic and optical

properties. Diacetylenes substituted with variousside chains readily undergo photopolymerization

to form an ene-yne alternated polymer chain upon

UV irradiation (254 nm) in a wide range of orga-

nized structures, such as single crystals, Lang-

muir Blodgett films, self-assembled monolayers,

liposomes or vesicles, and solutions [21]. The re-

sulting one-dimensional conjugated polyd

acetylene backbone shows large nonlinear optic

susceptibilities comparable to inorganic semicon

ductors with ultrafast response time. The chr

matic property of polydiacetylenes is of particul

interest in the recent years. The highly sensitiv

chromatic exchange of polydiacetylene betwee

blue and red forms can be potentially used in thdevelopment of optical biosensors [21].

In the present study, we report the synthes

and study of an amphiphilic dendrimer with poly

merizable diacetylene alkyl chains attached to th

dendrimer core. The free amino groups on th

third generation PAMAM dendrimer are cappe

with sixteen 10,12-pentacosadiynoic acid chain

(Fig. 1). We have two main purposes behind th

study. Firstly, we would like to reveal more abou

the fundamental aspects of a monolayer propert

of this macromolecular dendrimer at the airw

ter interface by using different thermodynami

spectroscopic and microscopic techniques. Se

ondly, it will be interesting to examine the photo

polymerization behavior of this macromolecu

both in solution and at the air water interfac

The polymerization may lead to the design

novel macromolecules with unique electronic an

optical properties as exhibited in other polyd

acetylene compounds.

2. Experimental section

2.1. Materials

Starburst PAMAM (generation 3) dendrim

and other chemicals were purchased from Aldric

Chemical (Milwaukee, WI), and were used

organic synthesis directly. All organic solvents fo

the synthesis were purchased from Fisher Scie

tific (Norcross, GA), as reagent grade and wer

used without further purification. 10,12-Pent

cosadiynoic acid was purchased from GSF Chemical (Powell, OH) and recrystallized fro

petroleum ether before use. The 1H NMR spectr

were recorded on a Varian VXR 400 MHz spe

trometer. The IR spectrum was obtained from

Perkin Elmer PE-GRAMS/2000 FT-IR spe

trometer. UV-VIS spectra were performed fro


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G. Sui et al. /Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 185197 1

HP8452A Hewlett Packard 8452A diode array

spectrophotometer. University of Illinois, School

of Chemical Sciences, conducted the MALDI-

TOF mass spectrum.

2.2. Synthesis

To the 7 ml dichloromethane solution of thePDA succinimidyl ester (150 mg, MW 471 g

mol1, 0.3 mmol), 300 ml starburst PAMAM

dendrimer (generation 3, 20 wt% in methyl alco-

hol, 0.015 mmol) was added within 20 min and

the solution was stirred vigorously for 24 h. The

solution was concentrated en vacuo. To the left

white solids, 50 ml 1 N NaOH solution was adde

and the suspension was maintained at 60C for

h. The suspension was filtered, and the solids wer

washed with 1 N NaOH solution, followed b

distilled water and demineralized water. Aft

vacuum dried, the product was obtained as whi

solids with a yield of 110 mg, 65.6%. 1H NM

(CDCl3): l 0.85 (t, 48H); 1.23 (m, 578H); 1.50 (m170H); 2.2 (m, 88H); 3.3 (m, 44H). IR (cm

CHCl3 as solvent): 3490 s; 2900 s; 2100 w; 1650

800 s. MALDI-TOF MS calculated fo

C452H928O44N58, m/z 8948.0, found 8981.7 (com

pound was dissolved in CHCl3/CF3COOH mi

ture solvent (v:v, 80:1)).

Fig. 1. Structure of new dendrimer PDA-PAMAM.


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2.3. General methods for surface chemistry studies

All the surface chemistry studies were con-

ducted in a clean room class 1000 where tempera-

ture (2091C) and humidity (5091%) were

controlled. The chloroform used in monolayer

studies was purchased from Fisher Scientific, as

HPLC grade. The water used for the monolayerstudy was purified by a Modulab 2020 water

purification system (Continental Water System,

San Antonio, TX) with a specific resistivity of 18

MV cm and a surface tension of 72.6 mN m1 at

2091C. The injection volume of the dendrimer

solution (0.1 mg ml1, CHCl3/CF3COOH mix-

ture solvent (v:v, 80:1)) was 30 ml for the surface

pressure measurements, 70 ml for the surface po-

tential measurements and 40 ml for the Brewster

angle experiments. After the sample was spread, a

15 min period was allowed to pass by for the

complete evaporation of solvent before compres-

sion. The compression rate was set up at 5 A, 2

molecule1 min1 for all the experiments.

Three different Langmuir troughs were used.

The Langmuir trough used for the surface pres-

sure measurements was a KSV minitrough (KSV

Instrument, Helsinki, Finland). Two computer

controlled symmetrically movable barriers were

used to regulate the surface area. The trough

dimensions are 7.530 cm. The surface pressure

was measured by the Wilhelmy method. The sen-

sitivity of the Wilhelmy plate is 90.01 mN m1

.The isotherm presented is an average of three

measurements. An UV-Vis spectrophotometer

(Hewlett Packard, Wilmington, DE) can be slid

over the KSV trough for recording the absorption

spectra of the dendrimer monolayer directly at the

air water interface. The UV lamp of the spec-

trophotometer is also the light source for the

irradiation. The power of the light on the mono-

layer at 254 nm is 0.6 W m2. The Langmuir

trough used for the surface potential measure-

ments is a home-made trough. Two symmetricallymovable barriers controlled by the computer were

used to regulate the surface area. The dimensions

of this trough are 0.612100 cm. The surface

potential was measured using the ionizing elec-

trode method. A reference platinum electrode was

immersed in the reference trough compartment

and an americium electrode (Am241) was place

about 12 mm above the monolayer under stud

For the Brewster angle microscopy studies,

Nippon trough (547 cm) equipped with a mov

ing wall system (NL-LB 140SMWC, Nippo

Laser and Electronics Lab., Nagoya, Japan),

helium-neon laser (wavelength 632.8 nm an

power 10 mW), and a CCD camera. The imagefrom the CCD was captured and digitized using

digital video capture mode (Snappy video Snap

shot, Rancho Cordova, CA) for further analysi

The 3-D representations of the BAM images wer

also presented using a software NIH 1.62 imag

that allow noise reduction using a low pass filte

The dark areas of the image represent the valley

whereas the bright areas represent high peaks i

the 3-D figures. All the images presented in th

work have dimensions of 400400 mm.

For the ESEM studies, we used a Philips/Ele

troscan environmental scanning electron micr

scope, XL 30, with a field emission electron gun

Horizontal lift method was used to deposit PDA

PAMAM monolayer to the surface of the high

oriented pyrolytic graphite (HOPG) which w

purchased from Adv. Ceramic, OH. Two differen

LB films were prepared, one at a surface pre

sure of 15 and the other of 35 mN m1. All th

images were obtained at an ambient temperatu

of 20C. In order to get the optimal pictu

quality in terms of resolution and contrast rati

we varied humidity and pressure (water vapoatmosphere with pressure varying from 4 to

Torr) inside the chamber.

3. Results and discussions

3.1. Surface pressure and surface dipole

momentarea isotherms

The surface pressure area isotherm (Fig.

shows that the PDA-PAMAM dendrimer formsmonolayer at the air water interface whic

evolves from a liquid expanded phase betwee

surface pressure 0 and 8 mN m1 to a liqu

condensed phase at surface pressure between

and 40 mN m1. The limiting molecular area wa

obtained by the extrapolation of the liquid con


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Fig. 2. Surface pressure (y)area (A) and surface dipole

moment (v

)area (A) isotherms of the PDA-PAMAM den-

drimer monolayer at airwater interface.

DAB-dendr-(NH2)16 dendrimer [13], of which th

obtained limiting molecular area is about 480 A

molecule1.

From the modeling of the PDA-PAMAM den

drimer (Hyperchem Software), the whole molecu

adopts a flat shape with the hydrophobic alk

chains fully extended toward the outside of th

dendrimer core (Fig. 3). The molecular area PDA-PAMAM in this conformation is abo

8200 A, 2 molecule1 and the molecular area of th

hydrophilic core (the PAMAM dendrimer part)

1880 A, 2 molecule1. None of these numbers co

responds to the limiting molecular area. In th

other hands, the limiting molecular area of on

PDA alkyl chain is :2530 A, 2 molecule1, an

the total molecular area of sixteen PDA alk

chains would be around 480 A, 2 molecule1. Th

limiting molecular area we obtained is approx

mately equivalent to the total molecular area o16 alkyl chains. This result indicates that when th

PDA-PAMAM dendrimer monolayer is com

pressed to a liquid condensed phase, the who

molecule is highly compact. Not only all th

hydrophobic chains stand up, but also the h

drophilic core is distorted and compacts.

The surface potential technique measures th

dipole moment changes of the monolayer [22

Surface potential values are frequently expresse

alternately in terms of surface dipole moments, v

calculated, from the equation, v=A DV/12[23]. The surface dipole moment is a more realist

indicator of the monolayer reorientation proces

We have obtained the vA curve (Fig. 2) of th

PDA-PAMAM dendrimer monolayer from th

surface potentialarea isotherm. The dipole mo

ment of the monolayer is directly related to th

packing density and molecular orientation withi

the film. Before compression, the amphiphi

molecules exist randomly in a gas phase at th

airwater interface. The total dipole moment con

tribution from the amphiphile molecules is zer

due to the random distribution of the amphiphi

molecules. With the compression, the surfa

dipole moment of the monolayer starts to increa

and the amphiphiles start to orient themselves t

form an organized monolayer. When the mon

layer reaches a liquid condensed phase, the su

Fig. 3. Structure of the PDA-PAMAM dendrimer molecularmodel.

densed part of the isotherm at surface pressure nil

with a value of 460 A, 2 molecule1. This result

agrees with the previous measurements on a

palmitoyl alkyl chain modified third generation


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face dipole moment reaches its maximum value.

The difference between this maximum value and

the surface dipole moment at clean water repre-

sents the total surface dipole moment contribu-

tion from the monolayer molecules. From the

surface dipole moment area isotherm of PDA-

PAMAM, one can see that the surface dipole

moment starts to increase at molecular area of1700 A, 2 molecule1. That means that all the

molecular dipoles of the dendrimer are being ori-

ented vertically and the dendrimer molecules be-

gin to organize themselves on the surface by

orienting their hydrophobic chains directed into

air. After increasing at a relatively low speed from

molecular area of 1400900 A, 2 molecule1, the

surface dipole moment increases steadily, which

indicates that all the hydrophobic chains have

been orientated vertically into air. The surface

dipole moment reaches its highest value at molec-

ular area of 400 A, 2 molecule1. This result corre-

sponds to what has been observed from the

surface pressure area isotherm. At molecular

area of 400 A, 2 molecule1, the monolayer is in a

liquid condensed phase and all the molecules are

tightly packed. The total surface dipole moment

contribution from the monolayer reaches its

highest value and the surface dipole moment re-

mains unchanged with continuous compression.

There are several structure models [2427] such

as the edge-on and face-on models [24,25]

which were quite useful in analyzing the mono-layer structures. By comparing with the molecular

dimensions of the CPK model of PDA-PAMAM

molecule, we have proposed a model to describe

the orientational and conformational change of

PDA-PAMAM monolayer during the compres-

sion, as shown in the cartoon picture of Fig. 2.

A noteworthy point that needs to be addressed

here is the surface dipole moment increase of the

PDA-PAMAM monolayer at molecular area of

1700 A, 2 molecule1. Before compression, the

dendrimers are lying flat at the air water inter-face. When compressed to molecular area about

1700 A, 2 molecule1, which is approximately

equivalent to the size of the dendrimer core, the

hydrophobic long tail starts to lift up and the

surface dipole moment of the monolayer starts to

increase. After all the alkyl chains are oriented

towards the air, the surface dipole moment in

creases very slowly for a certain time until th

area is further decreased. At this stage, the h

drophilic core of the molecule also starts to b

compressed and the surface dipole moment in

creases again until a highly compact monolayer

formed. At this final stage, the hydrophilic de

drimer core must be highly distorted to a non-planar structure as illustrated in Fig. 2. This al

indicates that dendrimer core is a very flexib

structure at the airwater interface.

3.2. Brewster angle microscopy and en6ironmenta

scanning electron microscopy

In the recent years, the Brewster angle m

croscopy is frequently used in the characterizatio

of monolayers at the airwater interface [2830

The topography of the PDA-PAMAM monolay

at the air water interface has been observe

through Brewster angle microscope. Previou

study [15] has pointed out the formation of pr

aggregates of amphiphilic dendrimers on the w

ter surface before compression due to the stron

van der Waals interactions between the hydropho

bic alkyl chains. This was also observed in th

present study, as indicated in the BAM images o

the monolayer at nil surface pressure (Fig. 4A

Particularly, large patches of monolayer (brig

areas) are seen with some small holes (dark area

randomly distributed in the patches. The holhave an average diameter of 1020 mm. Wit

continuous compression, the topography of th

monolayer becomes more and more homog

neous, as shown in the BAM images obtained

surface pressure of 15 and 35 mN m1 (Fi

4B,C). The dark holes become less and less obv

ous. We attribute the appearance of the dar

holes to the self-assembling pattern of the PDA

PAMAM molecules at the air water interfa

such as illustrated in Fig. 5. The hydrophobic va

der Waals interactions must be the driving forcbehind this self-assembling.

As an interesting preliminary investigation, w

have used the environmental scanning electro

microscope (ESEM) to observe the topography o

one layer PDA-PAMAM Langmuir Blodge

film. ESEM represents a variation of the scannin


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Fig. 4. Brewster angle microscope images of PDA-PAMAM dendrimer monolayer observed at different surface pressures: (A) 0 m

m1; (B) 15 mN m1; (C) 35 mN m1.


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electron microscope [31] in which high vacuum

chamber is separated from the sample compart-

ment by a system of small apertures, allowing

sample to be examined under controlled atmo-

sphere instead of high vacuum conditions. More-

over, non-conducting samples do not need

conductive coating in order to be observed. These

technical advantages make ESEM ideal for theinvestigations of thin organic films. Traditional

SEM has been used to observe the topography of

the Langmuir Blodgett films. However, tradi-

tional SEM requires high vacuum (:108 Torr)

to operate and under such a high vacuum, the

films are very easily damaged and the observed

image may not reflect the real topography of the

films. As a new detection configuration for con-

ventional SEM, ESEM retains the main advan-

tages of SEM while allowing the imaging of a

sample in its natural state by maintaining a mini-mum water vapor pressure in the specimen cham-

ber. Since ESEM is operated in a relatively low

vacuum, the risk of losing or damaging the film

by the high vacuum system is greatly reduced.

In this study, we have successfully used the

ESEM to observe the topography of one layer

PDA-PAMAM LB film with water vapor pres-

sure of 46 Torr, as presented in Fig. 6. First, we

have found good correlation between size and

shape of monolayer domains observed with BAM

and ESEM techniques. In the ESEM images (Fi

6A C), the topography of the monolayer o

tained at surface pressure of 15 mN m1 exhibi

similar domains as well as black holes as observe

from the BAM images. With increased surfa

pressure, ESEM images also indicate that th

topography of monolayer becomes more homog

neous and compact (Fig. 6DF vs. Fig. 6ACMore importantly, we have found that ESEM ca

have a better resolution to study the doma

structures of monolayer. As shown in Fig. 6C, w

can clearly see hundreds of nm scale structures o

the monolayer domains. At surface pressure of 3

mN m1, even though the monolayer seems qui

homogeneous as observed in the 100s mm scal

we can still see small black holes with 100 n

scales existing in the monolayer patches when w

observe the surface topography at magnific

tion20 000 times. BAM technique obvious

lacks such a high resolution. Our study sugges

that ESEM can become a very reliable, conv

nient and precise microscopic technique to stud

the topography of Langmuir Blodgett films

the near future.

3.3. Photopolymerization of the PDA-PAMAM

monolayer

As one of our purpose to design PDA-PA

MAM, we would like to study the photopolyme

ization of PDA-PAMAM monolayer at thairwater interface. As presented in Fig. 7, th

PDA-PAMAM monolayer can be readily pol

merized upon UV irradiation. The polymerize

monolayer exhibits a typical blue form absorptio

band at 630 nm as well as absorption at 570 nm

and the polymerization reaches a maximum exten

very quickly. From the previous discussion, w

know that at surface pressure of 35 mN m1, th

hydrophobic chains of the PDA-PAMAM a

lifted towards the air and are highly compact i

the monolayer. With such close packing, thtopochemical photopolymerization between th

diacetylene groups can readily take place. As

matter of fact, we can easily see the polymerize

blue film on the water surface with the naked ey

A noteworthy point is the chromatic property o

PDA-PAMAM monolayer upon prolonged UFig. 5. An illustration to explain the black holes in the

PDA-PAMAM monolayer domains.


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Fig. 6. ESEM images of one layer PDA-PAMAM dendrimer LB film obtained at different surface pressures: AC: 15 mN m

DF: 35 mN m1. The image magnifications (bar) are the following: (A), 10 mm; (B), 1 mm; (C), 500 nm; (D), 20 mm; (E), 10 mm

(F), 1 mm.


10/13


Fig. 6. (Continued)


11/13


Fig. 7. UV-Vis absorption spectra of polymerized PDA-PA-

MAM dendrimer monolayer (surface pressure 15 mN m1)

during different photo-irradiation times.

Since the PDA-PAMAM dendrimer contain

sixteen hydrophobic alkyl chains surrounding th

hydrophilic dendrimer core, we feel it will be ver

interesting to see whether this dendrimer can form

a supramolecular self-assembly in solution as i

lustrated in Fig. 8. As shown from the modelin

structure of this molecule, the distance betwee

the diacetylene groups on different alkyl chains at least about 7 A, away from each other. Th

means it is impossible for the diacetylene group

from the same dendrimer molecule to polymeriz

together in solution, since the topochemical pho

topolymerization requires a distance between d

acetylene groups within 45 A, . However, if th

dendrimer molecules self-assemble to form

supramolecule as illustrated in Fig. 8, we shoul

be able to polymerize the supramolecule along th

self-assembling direction and at the end, th

supramolecular assembly will become a unimacromolecule with a long fiber-like structure. The six

teen polydiacetylene backbones thus created

the same macromolecule may lead to some poten

tial applications based on the unique properties o

polydiacetylenes.

We have found that in chloroform at a concen

tration of 105 M, the PDA-PAMAM forms a

opalescent solution and the dispersion can rema

unprecipitated for one month when kept in dark

It is certain that the PDA-PAMAM forms co

loidal particles in chloroform solution as found i

irradiation. In contrast to most of the previously

studied polydiacetylenes [21], there is no obvious

red form absorption band appearing with in-

creased irradiation time on the PDA-PAMAM

monolayer. Since all the alkyl chains from the

dendrimer molecules are closely packed, the prop-

agation of the polymer backbone extends from

one dendrimer to another dendrimer, leading to

the formation of highly branched polymerized

macromolecules at airwater interface.

3.4. Photopolymerization of PDA-PAMAM insolution

It has been previously reported that am-

phiphilic dendrimers aggregate to form nanoscale

particles in aqueous solution [26]. The van der

Waals interactions between the large number of

hydrophobic alkyl chains should be the driving

force behind the formation of these aggregates.

While hydrogen bonding has been used exten-

sively as a non covalent tool in the design of

supramolecular self-assemblies, the hydrophobicvan der Waals interactions have not receive

enough attention in similar studies. A few groups

have recently noticed that the hydrophobic molec-

ular interactions may become a dominant factor

like hydrogen bonding in the formation of

supramolecular self-assemblies in solution [32].Fig. 8. A plausible supramolecular structure of the PDA-PA

MAM dendrimer colloidal particles in CHCl3

solution.


12/13


Fig. 9. UV-Vis absorption spectra of the PDA-PAMAM den-

drimer during different photo-irradiation times in CHCl3 solu-

tion (103 M).

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other amphiphilic dendrimers [26]. We have triedto irradiate this opalescent solution in a UV

quarts cell and the absorption spectra with differ-

ent irradiation time are presented (Fig. 9). The

PDA-PAMAM particles can be easily polymer-

ized within a time period of a few s to give a blue

form absorption band at 640 nm and a side band

at 590 nm. The opalescent solution became deep

blue almost immediately upon exposure to UV

light. With prolonged irradiation, the absorption

at 590 nm became stronger and stronger and at

last a strong absorption at 530 nm as well asabsorption at 490 nm appeared. And the solution

became deep purple, as indicated in the appear-

ance of a red form absorption band in the UV

spectra after irradiation of 500 s. Since we know it

is very unlikely that the diacetylene groups from

the same dendrimer polymerize together, the ob-

served polymerization must extend from one den-

drimer to another dendrimer. We are currently

working on the more detailed characterization of

the nature of PDA-PAMAM particles before and

after polymerization. We expect these studiesmight provide a new approach in the design of

special macromolecules with well-defined three di-

mensional structures along the direction of single

moleculesupramolecular self-assembly

macromolecule.


13/13


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