Preparation of Different Dendritic-Layered SilicateNanocomposites

Amal Amin,1 Rajarshi Sarkar,2 Charles N. Moorefield,3 George R. Newkome2,3,4

1Department of Polymers and Pigments, National Research Center, Dokki, Giza, Egypt

2Department of Chemistry, The University of Akron, Akron, Ohio 44325-4717

3The Maurice Morton Institute for Polymer Science, The University of Akron, Akron, Ohio 44325-4717

4Department of Polymer Science, The University of Akron, Akron, Ohio 44325-4717

Several dendrimer–clay nanocomposites have beenprepared. Firstly, the dendrimer (DE1)/clay nanocompo-site was obtained via an in situ free radical polymeriza-tion of a double bond-ended dendrimer (DE1), derivedfrom Behera’s amine by using 2,20-azobisisobutyroni-trile (AIBN), as initiator, and Cloisite 30 B, as nanofiller.Further free radical in situ copolymerization processeswere conducted between DE1, methyl methacrylate(MMA), and styrene (St). Two other dendrimer/claynanocomposites were prepared by the reaction ofsecond generation (G2)–36-acid dendrimer (DE2) andN,N0,N0,N0-tetrakis[2-hydroxy-1,1-bis(hydroxylmethyl)ethyl]-a,a,x,x-alkane-tetracarboxamide [6]-10-[6] Arbor-ols (DE3) with montmorillonite clay (MMT). POLYM. ENG.SCI., 53:2166–2174, 2013. ª 2013 Society of Plastics Engineers

INTRODUCTION

Since Toyota researchers first published [1–3] their

work on nylon 6-clay hybrid materials, many types of

polymer–clay nanocomposites (PCNs) [4] such as polyi-

mide [5], epoxy resin [6, 7], polystyrene [8–10], polycap-

rolactone [11], polypropylene [12], polyvinylalcohol [13],

and polyurethane [14] have been reported. The three main

categories of PCN that have been described include inter-

calated, flocculated, and exfoliated–delaminated nanocom-

posites. PCNs are generally prepared using a low-load

percentage of clay nanofiller that is homogeneously

dispersed within the polymer matrix. PCNs are currently

prepared by in situ polymerization, solution exfoliation,

and melt intercalation. PCN incorporation has been shown

to impart superior mechanical, thermal, and barrier prop-

erties when compared to materials strictly composed of

conventional polymers or composites [15–23].

Herein, it was thought to incorporate dendrimers as the

vital polymeric category in such nanocomposites. Den-

drimers are considered as an ubiquitous type of precisely

defined polymers [24, 25]. Dendrimers are a class of

regularly branched macromolecules with unique structural

and topological features whose properties are attracting

considerable interest from both scientists and technolo-

gists [26–28]. Dendrimers are different from traditional

polymers in that they possess a multibranched, three-

dimensional architecture with very low polydispersity and

high polyfunctionality. The dendrimers are potentially

usable in numerous applications such as catalysts, photo-

active and electronic materials, medicinal and functional

materials, biomedical materials, and specifically as drug

carriers [29–39]. Dendrimers are synthesized via a fully

controllable stepwise series of reactions. Two major

synthetic strategies are used to construct dendritic struc-

tures, namely, either a divergent or convergent approach.

Herein, three different dendritic materials with three

different functionalities were chosen and prepared such as

DE1, DE2, and DE3. The three dendritic architectures

were involved in forming nanocomposites with layered

silicates or clay to invest the impact of organic dendritic

moieties/inorganic clay hybrids in different potential

applications and mostly in biomedical ones. DE1 was dou-

ble bond-ended Behera’s amine-based 1 ? 3 C-branched

construct where Behera’s amine is considered as impor-

tant dendritic brick because of several reasons including

facile acylation of the amino moiety and quantitative

removal of the carboxylic acid protecting group and hence

transformation to versatile chemical active structures [40,

41]. Also, possible polymerization reactions can be

carried out on the double bond end of DE1 with other

Additional Supporting Information may be found in the online version of

this article.

Correspondence to: Amal Amin; e-mail: aamin_07@yahoo.com

Contract grant sponsor: US-Egypt Grant.

DOI 10.1002/pen.23485

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2013 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2013

vinyl monomers as was done actually in the current com-

munication with methyl methacrylate (MMA) and styrene

(St) to obtain several new functionalities and hence new

dendritic architectures. Two other water-soluble dendritic

architectures were prepared such as DE2 (3-oxo-6-oxa-2-

azaheptylidyne): (3-oxo-2- aza-pentylidyne): propionic

acid which ends with COOH [42] and DE3 (N,N0,N0,N00-tetrakis [2-hydroxy -1,1-bis(hydroxyl-methyl)ethyl]-a,a,x,

x-alkanetetracarboxamide or better a [6]-10-[6] Arborols)

which ends with OH [43].

EXPERIMENTAL

Materials

Cloisite (30 B) and montmorillonite (MMT) were

kindly provided by Southern clay products (Texas, USA).

All other chemicals were purchased from Sigma-Aldrich

and were used as received, unless further modified as

noted below [40–43]. Methyl methacrylate (MMA) and

styrene (St) monomers were purified by passing through

alumina columns, and then stored under argon.

Instrumentation

Molecular weights (�MnGPC) of the prepared polymers

were determined by gel permeation chromatography

(GPC) using an Agilent GPC-1100 with a refractive index

detector with 100, 104, and 105 A ultrastyragel columns

connected in series. Tetrahydrofuran (THF) and N,N-

dimethylformamide (DMF) were used, as the eluents, with

flow rates ca. 1 mL/min. Commercially available linear

polystyrene standards were used to calibrate the columns.

Proton nuclear magnetic resonance (1H NMR) spectra

were recorded using a Varian Mercury 300 MHz NMR;

tetramethylsilane (TMS) was used as the internal stand-

ard, and deuterated chloroform (CDCl3) and dimethyl

sulfoxide [(CD3)2SO)] were used as solvents. FT-IR spec-

tra were recorded using KBr pellets on a Digilab Excali-

bur FTS 3000.

X-ray diffraction (XRD), for measurement of the basal

spaces (d) between the layers of clay, was obtained on a

1D WAXD (wide angle X-ray diffraction) powder diffrac-

tion pattern using a Rigaku MultiFlex 2kW X-ray genera-

tor coupled to the diffractometer, which had a hot stage

for studing the phase transitions, as a function of tempera-

ture. The hot stage was calibrated with a deviation range

of 658C and the samples were scanned across a 2y range

of 3–358 at a scanning rate of 1 deg/min. Peak positions

were calibrated using silicon powder in the high-angle

region (\158).Transmission electron microscopy (TEM) measure-

ments were acquired with JEM 1200XII electron micro-

scope operating at 60 kV. Samples were prepared by drop

casting a polymer suspension onto carbon-coated copper

grids, followed by solvent evaporation in air.

Thermal gravimetric analysis (TGA) and differential

scanning calorimetry (DSC) data were obtained under a

nitrogen atmosphere using the TA Instruments Q50 Ther-

mogravimetric Analyzer and Q200 Differential Scanning

Calorimeter, respectively.

SYNTHETIC PROCEDURES

Dendritic DE1

Behera’s Amine. The nitrotriester (50 g, 112 mmol)

was added to an EtOH solution of Raney nickel, as previ-

ously described elsewhere (Scheme 1) [40]. The hydro-

genation was maintained for 45–75 min, while external

cooling was used so that the temperature does not exceed

558C. The catalyst was removed by filtration through a

Celite pad; the desired amine is soluble in EtOH. Then,

the solvent was removed in vacuo maintaining the

temperature below 558C to give an oil, which solidified

in vacuo to give (89%) pure Behera’s amine, as white crys-

tals: 1H NMR (CDCl3) [40]: d 1.44 (s, CH3, 27 H), 1.78

(m, CH2, 12 H); 13C NMR: d 27.8 (CH3), 29.8 (CH2CO),

34.2 (CCH2), 52.2 (CNH2), 80 (CMe3), 172.8 (CO2).

Synthesis of Di-tert-butyl 4-acryloylamino-4-(2-tert-

butoxycarbonylethyl)-heptanedioate (DE1). Acryloyl

chloride (1 g, 11 mmol) dissolved in CH2Cl2 (20 mL)

was added dropwise over a period of 15 min to a stirred

solution of Behera’s amine (4.6 g, 11 mmol) and Et3N

(3.1 mL, 22 mmol) in dry CH2Cl2 (100 mL) at 08C [41].

The resulting mixture was stirred for 2 h at 258C. Then, it

was washed with water and saturated brine, dried

(MgSO4), filtered, and concentrated in vacuo to give a

crude solid, which was chromatographed (SiO2) eluting

with an EtOAc/CHCl3 (1:9 v/v) solvent mixture. DE1 was

obtained (96%) as a white solid: m.p. 144–1458C; 1H

NMR (CDCl3) [41]: d 1.44 (27 H, s), 2.04 (6 H, t), 2.26

(6 H, t), 5.59 (1 H, dd), 6.03 (1 H, dd), 6.20 (1 H, s),

6.22 (1 H, dd); 13C NMR (CDCl3): d 27.9, 29.6, 29.9,

57.4, 80.3, 125.5, 131.7, 164.7, 172.7; IR 3290, 1710,

1654, 1622 cm21. Elemental analyses: calcd for

C25H43NO7: C, 63.94; H, 9.23; N, 2.98. Found: C, 63.68;

H, 9.30; N, 2.84.

Synthesis of Dendrimer DE2. (3-Oxo-6-oxa-2-azahep-

tylidyne): (3-oxo-2-azapentylidyne): propionic acid (DE2)

was prepared by hydrolysis of the corresponding ester

[(3-oxo-6-oxa-2-azaheptylidyne): (3-oxo-2-azapentyli-

dyne): tert-butyl propanoate] (E) via the described proce-

dure [42] by stirring (13.55 g, 2.22 mmol) of E in formic

acid (95%, 100 mL) for 12 h at 258C (Scheme 2). After-

ward, toluene (50 mL) was added to the concentrated

solution. The solution was evaporated in vacuo to give

a crude solid that was dissolved in a water (200 mL)/

acetone (10 mL) mixture and subsequently washed with

CH2Cl2 (50 mL) and EtOAc (50 mL). The aqueous phase

was boiled with charcoal (50 mg), filtered through Celite

and then concentrated in vacuo to give DE2, as white

solid: m.p. 132–1348C; 1H NMR (5% NaOD/p-dioxane/

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 2167

3.54 ppm) [42]: d 1.75, 1.99 (br, CH2CH2CO, 192 H),

2.32 (br, OCH2CH2CO, 8 H), 3.19 (br, CH2O, 8H), 3.48

(br, OCH2, 8 H); 13CNMR (5% NaOD/p-dioxane/66.4

ppm): d 29.7, 30.1 (CH2CH2CO), 37.0 (O CH2CH2CO),

67.7 (CH2O), 69.9 (OCH2), 173.1 (CONH), 179.7 (CO2);

IR 3363 (br, acid OH), 1718 (acid C¼¼O), 1648 (amide

C¼¼O) cm21. Elemental Analysis: (C177H268N16O92),

calcd: C, 51.95; H, 6.60, N, 5.48. Found: C, 51.74; H,

6.71; N, 5.30.

Synthesis of N,N0,N0,N0-Tetrakis[2-hydroxy-1,1–bis

(hydroxylmethyl)ethyl]-a,a,x,x-alkanetetracarboxamide

[6]-10-[6] Arborols (DE3). A mixture of tris (hydroxy-

methyl)aminomethane (8.0 mmol), tetraester (T, 2.0

mmol), and anhydrous K2CO3 (8.64 mmol) in Me2SO (5

mL) was stirred at 258C for 24 h (Scheme 3) [43]. The

solution was filtered and the solid was washed with

Me2SO (10 mL). The combined filtrate was concentrated

in vacuo. The residue was dissolved in water, precipitated

by the addition of acetone, and then filtered to give the

desired tetraamide (DE3), as a white solid, which was

washed with anhydrous EtOH. The arborol was obtained

in purity 97%. 13C NMR d 26.7 (b-CH2), 53.8 (a-CH),

60.5 (CH2OH), 171.3 (C¼¼O).

Preparation of Dendrimer/Clay Nanocomposites

p-DE1/Cloisite 30 B Nanocomposites. To a glass vial,

Cloisite 30 B (250 mg) and xylene (10 mL) were inserted

with continuous stirring, then DE1 (1 g, 22 3 1024 mol)

was added. The vial was closed with added argon. The

reaction mixture was placed at thermostated oil bath

adjusted at 708C. Afterward, AIBN (100 mg, 61 3 1025

SCHEME 1. Synthesis of DE1, p-DE1/MMA, and p-DE1/St nanocomposites.

2168 POLYMER ENGINEERING AND SCIENCE—-2013 DOI 10.1002/pen

mol) was dissolved in low amount of xylene (�2 mL)

and injected into the reaction. The polymerization proce-

dure was left for 44 h, then the vial was opened and the

nanocomposite was isolated, dried, and characterized (see

Supporting Information).

p-DE1/MMA or St Copolymers/Cloisite 30 B Nanocom-

posites. The previous sequence for addition of ingre-

dients was followed but DE1 and MMA or St were added

in 50/50 wt%. Also, MMA or St was added after closing

the vial under argon before adding AIBN solution. The

copolymerization was maintained in case of MMA and St

for 48 h to obtain �60% conversion. The amount of reac-

tants used were Cloisite 30 B (250 mg), xylene (10 mL),

DE1 (1.17 g, 25 3 1024 mol), MMA or St (25 3 1024

mol), and AIBN (100 mg, 61 3 1025 mol). To character-

ize the structure of the p-DE1 inside the formed polymer/

clay nanocomposite with 1H NMR and GPC, the nano-

composite was dissolved in THF to separate the polymer

solution from clay with centrifugation and filtration (see

Supporting Information).

DE2 or DE3/Montmorillonite/Nanocomposites. Clay

(250 mg, MMT) was dispersed in H2O (60 mL) with

continuous stirring for 24 h at 608C. At the same time, DE2

(500 mg, 12 3 1025 mol) or DE3 (66 3 1025 mol) was

separately stirred in H2O (40 mL) for 3 h at 608C. After-

ward, the dendrimer suspension was added to the clay, then

the total mixture was maintained for 24 h. The resulting

nanocomposites were separated, dried, and analyzed.

RESULTS AND DISCUSSION

The field of PCNs is a rapidly growing field of

research extending to comprise different types of

polymers [5–14]. Recently, efforts were exerted to form

nanocomposites by using new polymeric architectures,

such as hyperbranched polymers [44, 45]. Accordingly,

some dendritic compounds DE1, DE2, and DE3 were

involved in forming new nanocomposites with two types

of clay such as Cloisite 30 B and montmorillonite clay

(MMT) according to the functionalities of the dendritic

structures where Cloisite 30 B was chosen for DE1 and

its derivatives because it is organically modified nanoclay

[46] which fits with the hydrophobic nature of DE1 and

its derivatives with MMA and St. On the other hand,

montmorillonite clay with its hydrophilic OH ends was

suitable for DE2 and DE3 with COOH and OH polar end

groups, respectively.

Firstly, DE1 with double bond end was synthesized

from the reaction of Behera’s amine (Scheme 1) with

acryloyl chloride at 08C in dry CH2Cl2 [41]. Poly-DE1

(p-DE1)/clay nanocomposite was obtained by in situ free

radical polymerization of DE1 using AIBN and Cloisite

30 B at 708C. The same procedure was followed in case

SCHEME 2. Synthesis of DE2.

SCHEME 3. Synthesis of DE3.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 2169

of p-DE1/MMA and p-DE1/St nanocomposites by an

in situ polymerization of DE1 with MMA or St under the

same reaction conditions using AIBN in the presence of

Cloisite 30 B at 708C. The polymerizations were left for

about 48 h and an almost 60% conversion was obtained

in each case. p-DE1 and the copolymers p-DE1/MMA and

p-DE1/St were separated from their nanocomposites and

identified with GPC and 1H NMR to confirm their struc-

tures as in situ formed polymers in the presence of clay.

GPC indicated �MnGPC of p-DE1, p-DE1/MMA, and

p-DE1/St as �2740, 4180, and 6320 g/mol, respectively.

The 1H NMR of p-DE1 (CDCl3) showed the chemical

shifts of (CH3)3 and CH3CHCO appeared at 1.3–1.4 and

1.8 ppm, respectively. The chemical shifts of

CH2CH2COOC and CH2CH2COOC ranged from 2 to 2.2

ppm. CH and NH bands were observed at 2.3 and 6.20

ppm, respectively. 1H NMR of p-DE1/MMA (CDCl3)

revealed some similar chemical shifts as in the parent

polymer p-DE1 for some functionalities such as in case of

CH3, CH2, CH, and NH which were observed at 1.3–2.2

and 5.4 ppm, respectively. Distinguishing characteristic

band for MMA unit appeared at 3.5 ppm referring to

OCH3. The 1H NMR of p-DE1/St (CDCl3) showed, in

addition to the similar bands of the parent p-DE1, 2.3

(CHph) and 7.2 ppm (ph).

On the other hand, the G2–DE2 was prepared from the

36-acid dendrimer by the hydrolysis of the corresponding

ester (E) as previously shown (Scheme 2) [42]. The third

dendritic compound DE3 was a 6-10-6 arborol, which was

prepared as shown in Scheme (3) by the reaction of tris

FIG. 1. XRD of p-DE1, p-DE1/MMA, and p-DE1/St nanocomposites.

FIG. 2. XRD of DE2 and DE3/MMT nanocomposites.

2170 POLYMER ENGINEERING AND SCIENCE—-2013 DOI 10.1002/pen

with tetraester (T) [43]. Both of DE2 and DE3/clay nano-

composites were prepared, as previously formed using

montmorillonite (MMT). Because of the hydroxyl and

carboxyl functionalities of both of DE2 and DE3, respec-

tively, polar–polar interactions were expected and hence

successful formation of nanocomposites with no need for

pretreatment of clay surface. The prepared nanocompo-

sites were characterized via XRD, TGA, DSC, and TEM.

Analyses of the Nanocomposites

XRD exhibited broadened peaks for p-DE1, p-DE1-

MMA, and p-DE1-St/nanocomposites, as shown in Fig. 1.

The practically recorded interlayer spacing (d) ¼ 18.8,

19.7, and 19 nm for p-DE1, p-DE1/MMA, and p-DE1-St/

nanocomposites, respectively, at lower 2y value with

respect to the pristine clay, whose d ¼ 1.85 nm at higher

2y [23]. Thereby, the interlayer distance between clay pla-

telets enlarged than the pristine clay by the effect of

inclusion of the new polymeric materials inside the clay

gallery. The previous fact revealed the disturbance of clay

ordering led to intercalated to semi-exfoliated structure of

the resulting nanocomposites.

In case of DE3-MMT nanocomposites, as shown in

Fig. 2, a sharp band appeared with interlayer spacing (d) ¼1.5 nm, which was more than that of the neat clay (MMT)

whose d spacing ¼ 1.25 nm [45]. This behavior was

expected to lead to intercalated morphology of the resulting

nanocomposite. On the other hand, XRD of DE2-MMT

nanocomposites recorded no peaks where exfoliation struc-

FIG. 3. TGA of p-DE1 parent, p-DE1, p-DE1/MMA, and p-DE1/St cloisite 30 B nanocomposites.

FIG. 4. TGA of DE2 and DE2/MMT nanocomposites.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 2171

FIG. 5. TGA of DE3 and DE3/MMT nanocomposites.

FIG. 6. TEM of (a) p-DE1, (b) p-DE1/MMA, and (c) p-DE1/St cloisite nanocomposites.

ture was estimated. For the DE2 and DE3-MMT nanocom-

posites, it was expected that polar–polar interactions would

predominate in the formation of nanocomposites; however,

some repulsion forces, to some extent, were expected

between the polar ends whether on the clay surface or that

of the dendrimers DE2 (COOH) and DE3 (OH) themselves.

Therefore, that repulsion effect might be responsible for

widening of the interlayer spaces between the clay platelets

as in case of DE3/MMT nanocomposites or might cause

destruction of internal ordering of clay stacks, as in case of

DE2/MMT nanocomposites and hence led to exfoliation

structure of the resulting nanocomposite.

In the case of TGA, as shown in Fig. 3, the parent p-

DE1 was the least stable where 2.5% weight loss was

recorded at 1758C, then a gradual decomposition was

observed until 90% weight loss was observed at 4508C. p-

DE1/Cloisite nanocomposite lost about 40% of its weight

sample up to 7508C. With respect to p-DE1/MMA and p-

DE1/St–MMT nanocomposites, almost 2.5% loss happened

until 1758C, then gradual decomposition occurred on two

stages in case of p-DE1-MMA/Cloisite nanocomposite re-

cording 21% and 70% weight loss at 2408C and 4758C,

respectively. On the other hand, p-DE1/St/Cloisite nano-

composite gradually decomposed at 1758C up to almost

75% weight loss up to 8008C. It was clear that p-DE1-St/

Cloisite nanocomposite was more stable than p-DE1, p-

DE1/Cloisite, and p-DE1-MMA/Cloisite nanocomposites.

The DE2-MMT nanocomposite Fig. 4 demonstrated

higher thermal stability than its parent DE2 where the

nanocomposite lost 12.5% of its weight up to 5158C,

where a sharp decomposition began to reach 72.5%

weight loss, then another stage of decomposition was

indicated until 80% consumption of the total weight of

sample at 6758C. DE2 itself began its actual decomposi-

tion at 1968C, where it lost about 11% of its weight up to

2508C, then the sample completely degraded or 100% of

its weight at 7008C. In case of DE3 and its nanocomposite

(i.e., DE3/MMT, Fig. 5), DE3 started gradual decomposi-

tion at 1758C and continued up to 85% weight loss at

5008C, but in case of DE3-MMT nanocomposite, only

20% weight loss was recorded up to 8008C.

The DSC measurements were conducted to give the Tg

values of the polymers in the prepared nanocomposites.

The recorded Tg values of p-DE1, p-DE1/MMA, and

p-DE1-St nanocomposites were 131, 75, and 798C,

respectively. The Tg values of DE2/MMT and DE3-MMT

nanocomposites were 99 and 758C, respectively.

The prepared nanocomposites were also characterized

by TEM. The TEM images as shown in Figs. 6 and 7

mostly demonstrate the presence of intercalated clay pla-

telets with relative uniform distribution in the polymer

matrices where intercalated nanocomposites formed, in

almost all cases, except DE2/MMT nanocomposite where

exfoliated nanocomposite was resulted. The TEM micro-

graph of DE2-MMT nanocomposite (Fig. 7) indicates that

the destructed clay platelets or the individual exfoliated

platelets are embedded in the polymer matrices.

CONCLUSION

Three different dendritic moieties DE1, DE2, and DE3

were involved in forming polymer/clay nanocomposites.

The DE1 was a monomeric dendritic compound

FIG. 7. TEM of (a) DE2 and (b) DE3/MMT nanocomposites.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2013 2173

terminated with a double bond, where it was subjected to

in situ free radical homopolymerization in the presence of

Cloisite 30 B. However, the other dendritic molecules

DE2 and DE3 formed nanocomposites with MMT without

further treatment as their skeletons include ¼¼COOH

and ��OH functional polar groups. Generally, the result-

ing nanocomposites demonstrated higher thermal stability

than the original polymers. Intercalated structures were

observed in almost all nanocomposites, except in case of

DE2/MMT nanocomposite, where exfoliation was

observed. The resulting dendrimers/clay nanocomposites

can be actively applied in several applications such as

drug delivery systems, which can be further developed in

future research.

ACKNOWLEDGMENT

The authors thank all workers at Department of Polymer

Science at the University of Akron, Akron, Ohio, USA.

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