study of organic phase mobility in nanocomposite organic-inorganic coatings

Study of Organic Phase Mobility in Nanocomposite

Organic-Inorganic Coatings

Giuseppina Ceccorulli, Elisa Zini, Mariastella Scandola*

University of Bologna, Department of Chemistry ‘‘G. Ciamician’’ and INSTM UdR Bologna, via Selmi 2,40126 Bologna, ItalyFax: þ39 051 2099456; E-mail: [email protected]

Received: February 2, 2006; Revised: March 22, 2006; Accepted: March 23, 2006; DOI: 10.1002/macp.200600053

Keywords: coatings; hybrid; nanocomposites; relaxation

Introduction

Increasing interest is presently devoted to the development

of new materials containing both organic and inorganic

components that may usefully combine the individual

materials’ characteristics (low density, flexibility and

processability of the organic phase with thermal stability,

strength and rigidity of the inorganic phase). A large body

of literature is available[1–11] on organic-inorganic compo-

site materials obtained by blending polymers with variously

functionalized clays, using solvent casting or melt proces-

sing techniques, or by the synthesis of polymers from

monomers containing dispersed clay particles.

The main problems connected with this approach are

associated with phase distribution (exfoliation and even

dispersion of the inorganic phase) and domain size. With

the aim to overcome such difficulties the scientific interest

has recently focused on organic-inorganic hybrids, where

the two phases are bonded through covalent chemical

linkages or strong hydrogen bonds, and domain dimensions

in the range of nanometers can be easily obtained.

Examples of organic-inorganic hybrids are abundant in

nature (bones, shells, stalks etc.) where inorganic (silica,

calcium carbonate) deposits reinforce various types of

polymer matrices. Synthetic hybrids, comprising a number

of polymers as the organic phase and metal oxides or silica

Summary: Organic-inorganic hybrids synthesized by a dualphotopolymerization and condensation process from (i) twoorganic precursors, either poly(ethylene glycol) a,o diacry-late (Mw ¼ 600) (PEGDA) or bisphenol-A-ethoxylate(15EO/phenol)-dimethacrylate (BEMA), (ii) the organic-inorganicbridging monomer (methacryloyl-oxypropyl-trimethoxysi-lane, (MEMO)) and (iii) the inorganic precursor tetraethoxy-silane (TEOS) were investigated by differential scanningcalorimetry (DSC) and dynamic mechanical spectroscopy(DMTA). It is found that progressive formation of thecrosslinked network during the different steps of hybridproduction results in changes of molecular mobility that showup in changes of the glass transition of the organic phase. Whilemoving from the organic precursor to the final hybrid throughthe subsequent photopolymerization and condensation reac-tions, the transition is seen to broaden, decrease in intensity andshift to higher temperature. Excellent agreement of DSC andDMTA results is obtained. Dynamic mechanical analysis of thehybrids coated on PET film (coating thickness 10 and 40 mm)show an additional up-shift of Tg, more marked in the case ofthe thinner hybrid coating. This result is attributed to molecularinteractions at the substrate-coating interface that locally hindermolecular mobility. The consequent increase of Tg is moreevident when the coating layer is thin. The results show the

potential of the DMTA technique in coating-polymer substrateadhesion studies. Finally, the relaxation spectrum of the hybridsis sensitive to humidity absorbed from the environment andreversibly changes in absorption-desorption cycles.

DMTA loss spectrum of‘free’ PEGDA-based hybridfilm, PET film with thick coating and PET film with thincoating.

Macromol. Chem. Phys. 2006, 207, 864–869 � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

864 DOI: 10.1002/macp.200600053 Full Paper

as the inorganic component, were prepared in recent years

by the sol-gel process.[12–20] The sol-gel approach consists

in low temperature hydrolysis and condensation reactions

that yield controlled composition nanostructured organic-

inorganic materials.

A further approach to the synthesis of the hybrids (also

called ceramers, to account for their ceramic and polymeric

dual nature) was recently developed using organic mono-

mers with double bond functionalities and combining a UV

curing process to the sol-gel reaction.[21–24] In particular,

the synthesis of hybrids containing poly(ethylene glycol)

(PEG) segments linked to an acrylic-methacrylic network

by a dual photopolymerization-condensation reaction

process was recently reported.[23] The inorganic particles

in the hybrids had an average dimension of 20 nm (by TEM)

and were optically transparent.

The aim of this work is to analyze two such hybrids in

order to investigate the changes of organic phase mobility

caused by the progressive formation of a crosslinked

network, during the steps of hybrid preparation through the

dual-curing process. This study was carried out by means of

calorimetric (DSC) and dynamic-mechanical (DMTA)

techniques.

The hybrids under investigation were developed in view

of applications as polymer film coatings, and among the

aims of this work was the study of coating adhesion. The

hybrids coated on poly(ethylene terephthalate) (PET) film

were therefore analyzed by dynamic mechanical spectro-

scopy after sandwiching between aluminum supporting

plates. The results of this work demonstrate the suitability

of the DMTA technique to reveal adhesion between the

ceramer coating and the polymer substrate through changes

in the viscoelastic relaxation behavior.

Experimental Part

The synthesis of the organic-inorganic hybrids was describedpreviously.[23] In brief, starting from the mixture of an organicprecursor, of an organic-inorganic bridging monomer (metha-cryloyl-oxypropyl-trimethoxysilane, (MEMO)) and of theinorganic precursor tetraethoxysilane (TEOS), two reactionsteps were performed, as shown in Table 1. As organicprecursors either poly(ethylene glycol a,o-diacrylate)(Mw ¼ 600) (PEGDA) or bisphenol-A-ethoxylate (15EO/phenol)-dimethacrylate (BEMA) were used. In the first stepthe precursor (PEGDA or BEMA, 80 wt.-%) and MEMO(20 wt.-%) react by UV photopolymerization, leading to a

partially crosslinked polymer (PCP). Subsequently, a condensa-tion reaction between the PCP (70 wt.-%) and TEOS (30 wt.-%)yields the organic-inorganic fully crosslinked hybrid (Hyb).

The hybrids were coated on PET film (35 mm thick, Mossi &Ghisolfi commercial product) by means of a wire-woundapplicator, obtaining two different coating thicknesses: 10 mmand 40 mm. All investigated samples were kindly provided byA. Priola and G. Malucelli (Politecnico Torino, Italy).

Differential scanning calorimetry (DSC) was carried outusing a TA-DSC 2010. The following procedure was applied toprepare samples for DSC measurements: the sample (5–7 mg)was introduced in the aluminum pan, dried overnight in adessicator under vacuum over phosphorous pentoxide, then thepan was hermetically sealed and weighed. DSC scans wereperformed in the temperature range from �120 to 80 8C, at20 8C/min. The results of the first heating scan are reported inthis work. Subsequent scans after quench cooling showedexcellent reproducibility. The melting temperature (Tm) andcrystallization temperature (Tc) were taken at the peak of theDSC endo- and exotherm, respectively. The glass transitionwas taken at the mid-point of the specific heat increment,whereas the width of the transition (DTg) was calculated as thetemperature difference between the intercepts of the baselinesbefore and after the transition and the steepest tangent to thestep.

Dynamic mechanical measurements were carried out onfilm samples using a DMTA MkII (Polymer Laboratories Ltd.)in the temperature range from �150 8C to above the highestglass transition of the analyzed system (heating rate: 3 8C/min,frequency: 3 Hz). Two rectangular film samples (40 mm�7 mm) – of either the ‘free’ hybrid or of the hybrid coated onPET – and three equally sized supporting aluminum plateswere alternated, and the obtained sandwich was tested in dualcantilever (three point bending) geometry.

Results and Discussion

Differential Scanning Calorimetry (DSC)

Figure 1 shows DSC curves of PEGDA and BEMA organic

precursors (liquids at ambient temperature) during heating

from �120 to 80 8C. The calorimetric behavior of PEGDA

is typical of a polymer with a conspicuous crystalline phase

developed during cooling in the DSC. The enthalpy

associated with the melting process is very high (DHm¼119 J/g) while the increment of specific heat at the glass

transition is very low and barely visible in the DSC curve of

Figure 1. The insert in the same Figure shows a

magnification of the glass transition of the semicrystalline

PEGDA precursor.

Table 1. Synthesis of the organic-inorganic hybrids.

Step Reactants Reaction Product

1 PEGDAþMEMOor

BEMAþMEMO

photochemical curing Hg UVlamp (20 mW/cm2, in N2)

organic-inorganic partially crosslinked polymer (PCP)

2 PCPþTEOS condensation (4 h at T¼ 75 8C) fully crosslinked organic-inorganic hybrid (Hyb)

Study of Organic Phase Mobility in Nanocomposite Organic-Inorganic Coatings 865

Macromol. Chem. Phys. 2006, 207, 864–869 www.mcp-journal.de � 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Unlike PEGDA, the DSC curve of BEMA clearly

displays the glass transition, followed by a cold crystal-

lization exotherm and a sharp melting endotherm. The

magnitude of the crystallization and melting phenomena is

comparable, suggesting that quenching prevents BEMA

crystallization during cooling in the DSC and yields an

amorphous glass at low temperature. The calorimetric data

are collected in Table 2.

The different thermal behavior of PEGDA and BEMA is

clearly related to different chain flexibility of the two

organic compounds (see Tg values in Table 2). During the

cooling scan, the high molecular mobility of PEGDA

allows development of the three-dimensional order neces-

sary for crystalline phase formation. Conversely, in the case

of BEMA, the partially aromatic character of the chain

inhibits any significant crystallization and the polymer is

frozen in the disordered glassy state.

The effect of the dual curing process applied to produce

the hybrids (photopolymerization and condensation,

Table 1) on the organic phase mobility was investigated

by analyzing the changes of glass transition. Figure 2

compares the DSC curve of the PEGDA precursor

(truncated after the glass transition for the sake of clarity),

with that of the partially crosslinked polymer obtained after

UV-curing (P-PCP, curve A) and that of the final hybrid

after the sol-gel reaction (P-Hyb, curve B). When photo-

curing is applied to the PEGDA/MEMO mixture to yield the

partially crosslinked polymer (P-PCP), the most striking

features are the increase of intensity of the glass transition,

its shift to higher temperature and the absence of any

crystallization/melting process. The glass transition is

30 8C higher than that of the PEGDA precursor (Table 2)

and the PEG sequences have lost their ability to crystallize

above Tg.

The sol-gel reaction yielding the final hybrid (step 2)

creates crosslinks between the organic and inorganic phases

that cause an additional 12 8C shift of Tg (compare curves B

and A). The width of the glass transition (DTg in Table 2)

increases reflecting a broader relaxation time spectrum of

the cooperative molecular motions responsible of the glass

transition.

Figure 1. DSC curves of the BEMA and PEGDA organicprecursors. Insert: enlargement of the PEGDA curve in the glasstransition region.

Table 2. DSC and DMTA results.

Samplea) Tg DTgb) Ta

c) tandmaxd) W1/2

e)

8C 8C 8C 8C

PEGDA �69 7 – – –P-PCP �39 17 �45 0.171 10P-Hyb �27 21 �27 0.067 12P-Hyb coated

on PET(thick)– – �20 0.048 15

P-Hyb coated onPET(thin)

– – �15 0.028 18

BEMA �56 5 – – –B-PCP �29 15 �41 0.169 7B-Hyb �19 19 �21 0.079 10B-Hyb coated on

PET(thick)– – �16 0.033 13

B-Hyb coatedon PET(thin)

– – �7 0.027 14

a) Sample labeling in Table 1. Initial P- and B- denote the PEGDAand BEMA system respectively.

b) Width of the DSC glass transition.c) Temperature of a relaxation, from DMTA.d) Intensity of DMTA a-relaxation peak.e) Half-width of DMTA a-relaxation peak.

Figure 2. DSC curves of the PEGDA organic precursor, of thepartially crosslinked UV-cured polymer (P-PCP, curve A) and ofthe final hybrid (P-Hyb, curve B).

866 G. Ceccorulli, E. Zini, M. Scandola


An analogous set of DSC measurements was performed

on the other hybrid, based on the BEMA precursor, in order

to investigate the changes of molecular mobility during the

two synthetic steps. The thermal data on the BEMA system

are collected in Table 2. Figure 3 shows the relevant DSC

curves, that mimic the behavior described above for the

PEGDA-based hybrid: the glass transition of the organic

BEMA phase progressively shifts to higher temperature and

broadens, as a consequence of the sequential curing

reactions (photopolymerization, curve A, and condensa-

tion, curve B).

Both investigated hybrids were transparent and WAXS

measurements confirmed the absence of a crystal phase.

Dynamic Mechanical Thermal Analysis (DMTA)

Dynamic mechanical spectroscopy was used to analyze the

viscoelastic behavior of the organic-inorganic hybrids

during their sequential two-step synthesis. For the two

systems investigated Figure 4 compares the DMTA tan dcurve of the photochemically cured partially crosslinked

polymer (PCP) with that of the corresponding final hybrid

(Hyb). Details of the experimental sample setting are

provided in the Experimental section. In both the PEGDA

(Figure 4a) and BEMA-based systems (Figure 4b) the main

viscoelastic relaxation (a-peak), associated with the glass

transition of the organic phase, is seen to change in a very

similar manner as a consequence of the second-step

condensation reaction. The a peak parameters (tempera-

ture, intensity and half-width) are collected in Table 2. In

response to the new crosslinks generated in the reaction

between the PCP and TEOS, the a peak shifts to higher

temperature and becomes broader and smaller. The DMTA

results closely agree and confirm the above DSC data.

In addition to the main relaxation, the DMTA spectra of

Figure 4 also show a poorly resolved dissipation region at

temperatures lower than the glass transition peak. In the

literature[25] the viscoelastic spectrum of poly(ethylene

glycol) shows a broad and low-intensity relaxation (g-peak)

in a comparable temperature range. Hence the low-

temperature dissipation in the present hybrids is tentatively

attributed to local molecular motions of the PEG sequences in

the organic phase of both PEGDA and BEMA.

Dynamic mechanical spectroscopy was also applied in

this work to investigate adhesion between the hybrids and

PET films used as coating substrates. In the presence of

adhesion, molecular interaction at the substrate-coating

interface is expected to cause additional limitation to the

hybrid organic phase mobility that should show in changes

of the glass transition. Hence the relaxation spectrum of the

hybrids in the ‘‘free’’ state was compared with that obtained

after coating on PET. Figure 5 and 6 report the relevant

DMTA curves for the PEGDA- and BEMA-based hybrids,

respectively. Two different coating thicknesses were used,

indicated in this work as ‘thin’ (10 mm) and ‘thick’ (40 mm).

The DMTA curves of Figure 5 and 6 clearly show a shift to

higher temperatures of the tan d peak associated with the

ceramer glass transition when the hybrids are coated on

Figure 3. DSC curves of the BEMA organic precursor, of thepartially crosslinked UV-cured polymer (B-PCP, curve A) and ofthe final hybrid (B-Hyb, curve B).

Figure 4. Comparison of the DMTA loss spectrum of thepartially crosslinked UV-cured polymer (PCP) with that of thecorresponding final hybrid (Hyb): a) PEGDA-based system,b) BEMA-based system.



PET, more marked for the thin than for the thick coating.

Also the shape of the a peak changes: the intensity

decreases, as expected with decreasing hybrid content in

the tested sample, and peak width increases (peak

parameters are reported in Table 2).

The observed behavior is taken as an indication of good

adhesion between coating and substrate that restricts the

cooperative conformational rearrangements that set in at

the glass transition of the hybrid organic phase. In line with

this interpretation, the effect on the DMTA spectrum of

such interfacial motional hindrance is strong when the

coating is thin while it becomes less significant (i.e. it tends

to be ‘diluted’) by increasing coating thickness (see Ta in

Table 2). As regards the low temperature dissipation region

shown in Figure 5 and 6 by the coated PET films, it may

result from local motions of both the hybrid and the

supporting PET, that exhibits a secondary relaxation in the

same temperature range.[26]

The present results show that dynamic mechanical

spectroscopy can be usefully applied to investigate

adhesion of ceramers coated on polymeric film substrates,

provided that the substrate itself does not show large

viscoelastic relaxations in the temperature range of interest

that might overlap and interfere with the coating main loss

process.

The hybrids investigated in this work contain PEG

sequences and ester bonds, which are expected to confer to

the material some sensitivity to environmental humidity.

This point is illustrated in Figure 7, where two subsequent

DMTA measurements on the same B-Hyb coated on PET

are compared: (i) 1st scan up to 120 8C on sample stored in

room conditions and (ii) immediate re-run after cooling

from 120 8C under dry nitrogen purge. The curve for the

room conditioned sample is the same as that for the thin B-

Hyb coating of Figure 6, with the additional temperature

range from 20 to 120 8C, where the glass transition of the

PET substrate around 85 8C (same Ta as plain PET film) is

observed together with a broad dispersion region centered

around 50 8C.

The DMTA curve markedly changes after heating to

120 8C (compare 1st and 2nd scan). The hybrid glass

transition relaxation shifts to higher temperature and the

broad dissipation located between the glass transitions of

coating and substrate disappears. It is suggested that the

mentioned broad relaxation is associated with the presence

of absorbed moisture in the hybrid coating, and specifically

to water evaporation from the sample during the 1st heating

run. This interpretation is confirmed by the total reversi-

bility of the process: after keeping the same sample in room

humidity conditions for some time, a DMTA curve

analogous to that of the 1st scan is obtained. The plasticizing

effect of absorbed water is also responsible for the down

shift of the hybrid a-relaxation temperature in the 1st scan.

As for the low-temperature secondary relaxation, the

Figure 5. DMTA loss spectrum of: (~) ‘free’ PEGDA-basedhybrid (P-Hyb) film, (*) PET film coated with P-Hyb (thickcoating, 40 mm) and (*) PET film coated with P-Hyb (thincoating, 10 mm).

Figure 6. DMTA loss spectrum of: (~) ‘free’ BEMA-basedhybrid (B-Hyb) film, (*) PET film coated with B-Hyb (thickcoating, 40 mm) and (*) PET film coated with B-Hyb (thincoating, 10 mm).

Figure 7. Effect of absorbed humidity on the DMTA lossspectrum of the BEMA-based hybrid (B-Hyb, thin coating,10 mm): 1st scan from room humidity conditions, 2nd scanafter cooling from 120 8C under dry nitrogen purge.

868 G. Ceccorulli, E. Zini, M. Scandola


DMTA curves of Figure 7 show a slight shift to higher

temperature in the 2nd scan (i.e. as a consequence of water

loss), according to a behavior frequently found in polymers

containing polar groups that may interact with absorbed

water molecules.[27–31]

Conclusion

This work shows that the glass transition of the organic

phase, which marks the onset of segmental mobility of the

polymeric component, can be taken as a sensitive indicator

of the progressive formation of the crosslinked hybrid

structure. In the investigated ceramers the intensity of the

glass transition process gradually decreases with increasing

mobility limitations imposed by the crosslinks. In parallel,

the transition as a whole shifts to higher temperature and

broadens. The results from the two characterization

techniques employed (DSC and DMTA) very closely agree

and support each other.

This work also shows the potential of dynamic

mechanical analysis to reveal interfacial adhesion when

the investigated ceramers are coated on a polymeric (PET)

film. Coating-substrate adhesion results in a further up-shift

of the coating glass transition temperature, whose magni-

tude reasonably correlates with coating thickness.

Acknowledgements: Financial support by INSTMInteruniversityConsortium (Florence, Italy) in the framework of a PRISMA 2003Project, as well as useful discussions withA. Priola andG.Malucelliof Politecnico Torino (Italy) are gratefully acknowledged.

[1] A. Okada, M. Kawasumi, T. Kurauchi, O. Kamigaito,Polym.Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1987, 28, 447.

[2] T. Lan, P. Kaviartna, T. Pinnavaia, Polym. Mater. Sci. Eng.Proc. 1994, 71, 527.

[3] Z. Wang, T. Pinnavaia, Chem. Mater. 1998, 10, 3769.[4] J.-C. Huang, Z.-H. Zhu, J. Yin, X.-F. Qian, Y.-Y. Sun,

Polymer 2001, 42, 873.

[5] R. Vaia, H. Ishii, E. Giannelis, Chem. Mater. 1993, 5,1694.

[6] H. Ishida, J. Miller, Macromolecules 1984, 17, 1659.[7] K. A. Carraio, L. Xu, Chem. Mater. 1998, 10, 1440.[8] M. Okamoto, S. Morita, H. Taguchi, Y. Kim, T. Kotaka,

H. Tateyama, Polymer 2000, 41, 3887.[9] C. Zeng, L. J. Lee, Macromolecules 2001, 34, 4098.

[10] M. Kawasumi, J. Polym. Sci., Part A:Polym.Chem. 2004,42,819.

[11] C. Park, J. G. Smith, Jr., J. W. Connell, S. E. Lowther, D. C.Working, E. J. Siochi, Polymer 2005, 46, 9694.

[12] C. J. Brinker, G. W. Scherer, ‘‘Sol-Gel Science’’, AcademicPress, Boston 1990.

[13] S. Yano, K. Iwata, K. Kurita, Mater. Sci. Eng. 1998, C6,75.

[14] D. Tian, S. Blacher, Ph. Dubois, R. Jerome, Polymer 1998,39, 855.

[15] D. Tian, S. Blacher, R. Jerome, Polymer 1999, 40, 951.[16] R. L. Ballard, J. P. Williams, J. M. Njus, B. R. Kiland, M. D.

Soucek, Eur. Polym. J. 2001, 37, 381.[17] C. J. Cornelius, E. Marand, Polymer 2002, 43, 2385.[18] M. Messori, M. Toselli, F. Pilati, E. Fabbri, P. Fabbri,

L. Pasquali, S. Nannarone, Polymer 2004, 45, 805.[19] S. L. Huang, W. K. Chin, W. P. Yang, Polymer 2005, 46,

1865.[20] D. Deriu, A. Di Venere, G. Mei, R. Cantucci, N. Rosato,

J. Non-Cryst. Solids 2005, 351, 3037.[21] K. Gigant, U. Posset, G. Schottner, L. Baia, W. Kiefer,

J. Popp, J. Sol-Gel Sci. Technol. 2003, 26, 369.[22] K. Zou, M. D. Soucek,Macromol. Chem. Phys., Suppl. 2004,

205, 2032.[23] G. Malucelli, A. Priola, M. Sangermano, E. Amerio, E. Zini,

E. Fabbri, Polymer 2005, 46, 2872.[24] J. D. Cho, H. T. Ju, J. W. Hong, J. Polym. Sci., Part A: Polym.

Chem. 2005, 43, 658.[25] N. G. McCrum, B. E. Read, G. Williams, Anelastic and

Dielectric Effects in Polymeric Solids, Wiley, New York1967, chapter 14.

[26] N. G. McCrum, B. E. Read, G. Williams, ‘‘Anelastic andDielectric Effects in Polymeric Solids’’, Wiley, New York1967. chapter 13.

[27] M. Pizzoli, M. Standola, ‘‘Polymeric Materials Enciclope-dia’’, J. C. Salamone, Ed., CRC Press Inc. 1996, p. 5301 ff.

[28] J. Kolarik, J. Janacek, J. Polym. Sci., Part C 1967, 16,441.

[29] H. W. Starkweather, Jr., ‘‘Water in Nylon in Water inPolymers’’, S. P. Rowland, Ed., American Chemical Society,Washington 1980.

[30] H. Jacobs, E. Jenckel, Makromol. Chem. 1961, 47, 72.[31] J. Kolarik, Adv. Polym. Sci. 1982, 46, 119.



study of organic phase mobility in nanocomposite organic-inorganic coatings

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