Progress in Organic Coatings 65 (2009) 366–374

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Progress in Organic Coatings

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Coatings of PDMS-modified epoxy via urethane linkage: Segmental correlationlength, phase morphology, thermomechanical and surface behavior

S.K. Ratha, J.G. Chavana, S. Sasanea, Alips Srivastavaa, M. Patri a, A.B. Samuia,∗,B.C. Chakrabortya, Shilpa N. Sawantb

a Naval Materials Research Laboratory, Shil-Badlapur Road, Addl. Ambernath, Ambernath 421506, Maharastra, Indiab Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Maharastra, India

a r t i c l e i n f o

Article history:Received 11 November 2008Received in revised form 5 February 2009Accepted 27 February 2009

Keywords:PDMSEpoxyPhase morphologySurface behaviour

a b s t r a c t

Modification of an aliphatic epoxy resin with silicone was carried out through urethane route.Hydroxy-terminated polydimethylsiloxane (HTPDMS) was reacted with toluene diisocyanate to form anisocyanate-capped prepolymer. The –NCO group of the prepolymer was further reacted with –OH group ofan aliphatic epoxy adipate resin, obtained by reaction of adipic acid with an epoxy resin, to obtain silicone-modified epoxy resin. Silicone-modified epoxy resins containing 15 and 30 wt% of silicone prepolymerhave been synthesized by this procedure. TGA analysis of the silicone-modified epoxy resin showedconsiderable improvement in thermal stability over the unmodified epoxy resin. Modulated differentialscanning calorimetry (MDSC) was used to find the segmental correlation length of the silicone-modifiedpolymers. The cured films of silicone-modified resins showed biphasic morphology as evidenced fromscanning electron microscopy. Further, the thermomechanical property of the samples was investigated

by dynamic mechanical analysis. The surface properties of the silicone-modified matrices were studied byatomic force microscopy and contact angle goniometry. The silicone-modified samples showed consid-erably lower surface energy, surface roughness and contact angle hysteresis compared to the unmodifiedresins.

1

o[[cototawHvnoio

0d

. Introduction

Silicones have become important commercial polymers becausef a combination of properties, including high thermal stability1], low surface tension [1,2], low glass transition temperature1,2], etc. The materials however have relatively poor mechani-al strength [3,4]. The poor strength is usually attributed to flawsr microcracks that readily grow because of the high mobility ofhe chains [4,5]. In order to improve the mechanical propertiesf the silicone materials while retaining the inherent proper-ies of silicone moiety, numerous synthetic pathways have beenttempted. The methods adopted are usually blending of siloxanesith hydrocarbon-based polymers, more notably epoxy resins [6,7].owever the non-polar nature of the siloxanes together with theery low levels of intermolecular attractions lead to thermody-

amically and mechanically incompatible blends with virtually allther polymeric systems. This is reflected in the very low solubil-ty parameter of siloxanes (ı = 14.9 J1/2 cm−3/2) when compared tother polymers (ı = 17.6–28.6 J1/2 cm−3/2) [8]. Therefore, it is more

∗ Corresponding author. Tel.: +91 251 2623036; fax: +91 251 2623004.E-mail address: absamui@rediffmail.com (A.B. Samui).

300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2009.02.007

© 2009 Elsevier B.V. All rights reserved.

useful to synthesize a polymer that possesses both the polysiloxaneand other hydrocarbon-based polymers. Thus, polydimethylsilox-ane (PDMS) has been introduced to various polymeric systems, suchas polystyrene [9], poly(methyl methacrylate) [10], polycarbonate[11] and polyurethanes [12,13] through copolymerization methods.

Another potential alternative is to attach polysiloxanes asside chains to hydrocarbon polymers. When polysiloxanes wereattached to polymethacrylate backbones, contact angle measure-ments showed migration of the side chains to the polymer surface inair [14]. Baum et al. reported synthesis of side loop polymers basedon hydrosilylation of polybutadiene [15]. Holohan et al. reportedsynthesis of well defined siloxane graft copolymers of siloxanes bycoupling of appropriate functional groups on preformed polymers[8]. There have been numerous reports of such polydimethylsilox-ane modified epoxy resins [16–19].

The aim of the present investigation is threefold. Firstly, syn-thesis of a silicone-modified epoxy resin through isocyanate route.In the present system we have chosen a neopentyl-based epoxy

resin as the base material and a flexible polyether diamine as thecurative. This is in contrast to the majority of the hitherto stud-ied systems dealing with diglycidyl ether of bisphenol-A (DEGBA)and aromatic diamine-based systems [16–19]. The silicone modifi-cation was carried out by introducing toluene diisocyanate capped

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ag

S.K. Rath et al. / Progress in Or

olydimethylsiloxane into adipic acid modified epoxy resin. Secondim was to carry out an extensive structure–property correlationtudy in terms of the phase morphology of the cured matrices andonsequences of the same on thermomechanical properties. Ourhird objective was to evaluate the surface properties of the cured

atrices by contact angle goniometry and atomic force microscopy.

. Experimental

.1. Materials

Diglycidyl ether of neopentyl glycol, XR-87 (epoxy equiv-lent weight 180 ± 5 g/equiv.), was purchased from Atul Ltd.ndia. Hydroxyl-terminated polydimethylsiloxane (–OH content.7 wt%) (DC-3563) was procured from Dow Corning Indiavt. Ltd. Toluene diisocyanate (TDI) was purchased from E-erck India Pvt. Ltd. and used as received. Adipic acid (99%)as purchased from E-Merck India Pvt. Ltd. and used as

eceived. Triphenyl phosphine, purchased from S.D. Fine Indiatd. was used as received. Triethylamine purchased from E-erck India Pvt. Ltd., was vacuum distilled before use. The

iamine curative, O,O′-bis-(2-aminopropyl) polypropyleneglycol-lock-polyethyleneglycol-block-polypropylene glycol (Mr ≈ 600)as purchased from Fluka.

.2. Synthesis of TDI-capped hydroxyl-terminatedolydimethylsiloxane

The prepolymer was synthesized by reacting TDI and hydroxyl-erminated polydimethylsiloxane (in the molar ratio of 2:1) at 60 ◦Cn a three-necked R.B flask equipped with a mechanical stirrernder nitrogen atmosphere. Triethylamine (0.003 mol%) was useds a catalyst for the reaction. The reaction was carried out untilhe isocyanate content reached the theoretical value. Isocyanateontent in the prepolymer, as determined by titration with diethylmine, was found to be 3.18% compared to the theoretical value of.23%.

.3. Synthesis of epoxy adipate resin

Epoxy adipate resin was prepared by reaction of 300 g ofpoxy resin (1.66 moles) with 23 g adipic acid (0.157 moles) usingriphenyl phosphine as catalyst. Reaction was carried out in a three-ecked RB flask at 140 ◦C with constant stirring. The reaction isasically a carboxyl-epoxide esterification. The material synthe-ized is nomenclatured as EPA.

.4. Synthesis of silicone-modified epoxy resin

Isocyanate-capped siloxane was reacted with epoxy adipateesin in a three-necked R.B. flask equipped with a mechanical stirrernder nitrogen atmosphere. As the reaction progressed, viscos-

ty of the reaction mixture increased. The reaction mixture washen diluted with toluene, adjusting the solid content to 40%. Theeaction was carried out until complete consumption of isocyanateroup. Through this route, epoxy adipate resins modified with 15nd 30 wt% silicone prepolymer were prepared. The 15 and 30%ilicone-modified samples were nomenclatured as EPASi-15 andPASi-30, respectively.

.5. Epoxy equivalent weight (EEW) determination

The epoxy, epoxy adipate and silicone-modified samples werenalysed for their epoxy content by standard titration with hydro-en bromide in acetic acid [20].

Coatings 65 (2009) 366–374 367

2.6. Isocyanate content determination

The free isocyanate content in the siloxane prepolymer wasdetermined by the titration method (ASTM D 2572-97). Accord-ing to this method, the prepolymer was reacted with an excessof di-n-butylamine in toluene for 15 min to ensure completion ofthe reaction. The excess di-n-butylamine was determined by backtitration with standard hydrochloric acid.

Calculation of NCO content is as follows:

NCO% = [(B − V)N 0.042]W

× 100

where, B is the volume of HCl for titration of the blank (ml); V,volume of HCl for titration of the sample (ml); N, normality of HCl;and W, sample weight (g).

2.7. Curing of the silicone epoxy resin

The silicone-modified epoxy resins, unmodified epoxy resinand epoxy adipate resin were mixed separately with stoichiomet-ric amount of the amine hardener, O,O′-bis-(2-aminopropyl)polypropyleneglycol-block-polyethyleneglycol-block-polypropylene glycol in toluene, adjusting the initial solid contentto 30% for total materials. (For example, 6.9 g of the curative wasadded to 10 g of unmodified epoxy resin in 50 ml toluene.) Thesolution mixtures were poured into a Teflon mold and allowed tocure at 25 ◦C and RH of 50% for 7 days to ensure complete solventevaporation and curing. Thus, cured films of the samples with athickness of 500 ± 5 �m were obtained. The cured materials werenomenclatured as EPJ (cured pure epoxy resin), EPAJ (cured epoxyadipate resin), EPASi-15J (cured 15% silicone-modified epoxy resin)and EPASi-30J (cured 30% silicone-modified epoxy resin).

2.8. Characterizations

2.8.1. FTIRInfrared spectroscopic experiments of the uncured samples

were performed with a Nicholet Omnic 3 Fourier transform infraredspectrometer with a resolution of 4 cm−1. Signals of 16 scans wereaveraged before Fourier transform.

2.8.2. TGAThe thermal stability of the uncured epoxy, epoxy adipate and

silicone-modified samples was studied in nitrogen environmentusing a thermogravimetric analyzer (TA instruments; Hi Res 2950).The measurements were carried out from 30 to 600 ◦C at a heatingrate of 20 ◦C/min.

2.8.3. MDSCModulated differential scanning calorimetry was performed

using a modulated DSC (TA instruments: model Q-100) instrument.Samples were cooled to the glassy state at 5 ◦C/min and then testedusing a heating rate of 5 ◦C/min with a superimposed temperaturemodulation having an amplitude of 0.5 ◦C and a period of 40 s. Priorto these measurements an indium sample was used to calibrate theheat capacity signal of the instrument at the same modulated tem-perature profile. MDSC measurements were carried out only for theuncured samples.

2.8.4. Density measurementsThe density measurements of the uncured samples were carried

out by standard pycnometers.

2.8.5. DMADynamic mechanical analysis of the cured samples was per-

formed on a dynamic mechanical analyzer (Gabo Eplexor 150N).

368 S.K. Rath et al. / Progress in Organic Coatings 65 (2009) 366–374

ped H

Tyow

2

efw

2

us(wot

R

w

Z

2

ctwd1wu

coLr

�

Fl

wadw

order to generate hydroxyl functionality in the epoxy resin, whichcould subsequently be utilized for reaction with the isocyanate-capped product. The product obtained was characterized by epoxyequivalent and acid value measurements. The acid value was found

Scheme 1. Synthesis of TDI cap

he samples (0.5-mm thick, 5 mm wide and 10 mm long) were anal-sed within the temperature range of −140 to 50 ◦C at a heating ratef 2◦C/min. The measurements were carried out in tensile modeith a frequency of 1 Hz.

.8.6. SEMA JEOL JSM-840 scanning electron microscope was used to

xamine the morphology of the cured silicone-modified samplesractured cryogenically in liquid nitrogen. The fracture surfacesere sputter coated with gold.

.8.7. Atomic force microscopyAFM measurements for the cured samples were carried out

sing a scanning probe microscope (SPM-Solver P47, NT-MDT, Rus-ia) in contact mode. Rectangular cantilevers of silicon nitridelength 200 �m and width 40 �m) having a force constant of 3 N/mere employed for the measurement. Roughness of the films was

btained using the software provided with the AFM. It calculateshe average roughness (Ra) using the formula

a = 1NxNy

Nx∑i=1

Ny∑j=1

|z(i, j) − Zmean|

here z is the Z-coordinate value and

mean = 1NxNy

Nx∑i=1

Ny∑j=1

Zij

.8.8. Contact angle goniometryThe surface free energies of cured samples were determined by

ontact angle goniometry using a Kruss G10 goniometer interfacedo image capture software. Advancing and receding contact anglesere measured for droplets (2–10 �l) of double distilled water andiidomethane. The spatial size probed (in the plane of the surface) is–2 mm in diameter. The images of three droplets of each test liquidere taken by a CCD camera and the contact angles determinedsing automated image analysis.

Surface energies were evaluated using the surface tension-omponent theory. According to this approach, the surface tensionf a phase could be divided into independent components, viz.ondon dispersions contribution �d and polar contribution �p, asepresented below.

= �d + �p

or a drop of liquid at equilibrium with a solid surface, theiquid–solid contact angle (�) is given by the following equation:

�L(1 + cos �)

2(�d)1/2

= (�PS )

1/2(�P

L �dL )

1/2 + (�dS )

1/2

L

here �L is the surface tension of the liquid and subscript S = solidnd L = liquid. �d

L , �pL can be found in literature [21]. In this study

iiodomethane and water were used as the probing liquids. Forater �L = 72.8 mJ/m2, �p

L = 51.0 mJ/m2 and �dL = 21.8 mJ/m2. For

TPDMS urethane prepolymer.

diidomethane �L = 50.76 mJ/m2, �pL = 0 mJ/m2, �d

L = 50.76 mJ/m2

at 20 ◦C. Hence by measuring the contact angle for two well char-acterized liquids two equations with two unknowns i.e. �d

s and�p

s are generated. The surface energy �s = �ds + �p

s can thus beobtained.

3. Results and discussion

3.1. Synthesis of TDI capped HTPDMS

The capping reaction of hydroxyl-terminated polydimethyl-siloxane with toluene diisocyanate was carried out usingtriethylamine as catalyst (Scheme 1). The criterion of selectionof an amine-based catalyst is twofold. Firstly, triethylamine (TEA)is a moderate urethane catalyst, which prevents a runaway reac-tion. Secondly, TEA, also a curing catalyst, would accelerate thecuring reaction of silicone-modified epoxy with amine hardener.The reaction progress was monitored by titrimetric determina-tion of isocyanate content. The isocyanate content of the cappedproduct was found to be 3.18% as against the theoretical valueof 3.23%, suggestive of the completion of the reaction. Ho et al.reported synthesis of PDMS-based urethane prepolymer by reac-tion of HTPDMS with methane diphenyl diisocyanate (MDI) usingstannous octoate as catalyst [18]. In present case, HTPDMS has beencapped with TDI. Fig. 1 shows the FTIR spectra of HTPDMS and thecapped product. The formation of urethane linkage is evidenced byappearance of the peak at 1715 cm−1 corresponding to the carbonylgroup of the urethane linkage. The peak at 2270 cm−1 in the cappedproduct spectrum corresponds to the free isocyanate functionalgroup.

The reaction of epoxy resin and adipic acid was carried out, in

Fig. 1. FTIR spectra of HTPDMS, TDI capped HTPDMS, epoxy adipate and silicone-modified epoxy resin (15%).

S.K. Rath et al. / Progress in Organic Coatings 65 (2009) 366–374 369

ne-m

tits1prp

itsc2tefiTrot

taw

section. It was observed that silicone-modified epoxy adipate sys-tems cured faster compared to epoxy and epoxy adipate systems.This is clearly due to the catalytic action of residual triethylamineused in the capping reaction.

Scheme 2. Synthesis of silico

o be zero, indicating complete consumption of adipic acid dur-ng the reaction. The epoxy equivalent of the product was foundo be 206 ± 7 as against a value of 180 ± 5 for neat epoxy. The FTIRpectrum (Fig. 1) of the epoxy adipate product shows a peak at733 cm−1 corresponding to the ester group. The oxirane linkageeak at 836 cm−1 is observed for both the epoxy and epoxy adipateesin. Further, the formation of hydroxyl groups is confirmed by theresence of a broad peak at 3491 cm−1.

The next step involves the reaction of the isocyanate-cappedntermediate with epoxy adipate shown in Scheme 2. The reac-ion was monitored by measurement of isocyanate content viatandard diethylamine titration procedure and FTIR until completeonsumption of the isocyanate group. Disappearance of the peak at270 cm−1 in the FTIR spectrum suggests completion of the reac-ion. The reaction was also followed by measurement of the epoxyquivalent values. The epoxy equivalents for 15 and 30% modi-ed samples were found to be 237 ± 7 and 269 ± 10, respectively.he values calculated from weight percentage and titrimetry cor-espond well. This indicates that successful silicone modificationccurs through reaction of the epoxy adipate hydroxy group and

he isocyanate of the urethane prepolymer.

The resulting siloxane modified epoxy resins show similar reac-ivity as other epoxy resins and can be cured with various curinggents. The epoxy, epoxy adipate and silicone-modified materialsere cured as per the procedure mentioned in the experimental

odified epoxy adipate resin.

Fig. 2. TGA thermogram of EP, EPA, EPASi-15 and EPASi-30.

3 rganic Coatings 65 (2009) 366–374

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3

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afiodSrssispTosTbroPw

sphi

70 S.K. Rath et al. / Progress in O

.2. Thermal analysis

.2.1. TGAThe effect of silicone modification on the thermal stability of

esins has been investigated by TGA. The results from TGA ther-ogram (Fig. 2) of the uncured epoxy and silicone-modified epoxy

ystems reveal that the initial decomposition temperature of epoxydipate system has increased marginally compared to the unmodi-ed resin. Significant improvement in thermal stability is observedpon silicone modification. The initial decomposition temperatureIDT) increases from 179 ◦C for EPAJ to 223 ◦C for EPAJSi-15. Howeverhere is no further improvement in the initial decomposition tem-erature for 30% silicone-modified epoxy adipate resin comparedo EPAJSi-15. The increase in thermal stability could be attributedo higher bond dissociation energy of Si–O bond. In all cases, a dis-inct single step decomposition is observed. Further it is seen thathe weight loss rate becomes smaller and the decomposition tem-erature range becomes broader for the silicone-modified epoxydipate systems.

.2.2. MDSCFig. 3 shows the DSC thermogram of the uncured epoxy, epoxy

dipate and the silicone-modified epoxy adipate samples. From thegure it is seen that epoxy resin has a glass transition temperaturef −21.4 ◦C. The Tg increases to −15.2 ◦C for the epoxy adipate resinue to the introduction of polar ester groups into the epoxy matrix.ilicone-modified samples show two transitions in the temperatureange of scan. Peak at −46 ◦C corresponds to the melting of PDMSegment while the Tg is observed at −16 and −11 ◦C for 15 and 30%ilicone-modified samples, respectively. The peak area for EPASi-30s higher compared to EPASi-15, as expected, owing to the increasediloxane percentage. The Tg of EPASi-15 remains unchanged com-ared to EPA, whereas the Tg of EPASi-30 is 5 ◦C higher than EPA. Theg for PDMS segment could not be seen in the temperature rangef scan. The observation of higher Tg of the 30% silicone-modifiedample could be because of the stiff urethane junctions. The value ofg depends on the mobility of the polymer chain, the more immo-ile the chain, the higher the value of Tg. In particular, anything thatestricts rotational motion within the chain should raise Tg. Basedn the above study, it seems that, there is a critical threshold ofDMS content or conversely the urethane hard segment contenthich results in a higher Tg of the silicone-modified samples.

In order to gain further insight into the thermal behaviour of theamples, the dCp/dt versus temperature plot is shown in Fig. 4. Theeak area of dCp/dt versus temperature is equal to the increment ofeat capacity Cp at glass transition temperature. From the figure it

s seen that the incremental heat capacity of the silicone-modified

Fig. 3. DSC thermogram of EP, EPA, EPASi-15 and EPASi-30.

Fig. 4. Rev.Cp/dT vs. temperature plot of EP, EPA, EPASi-15 and EPASi-30.

samples is lower compared to the reactants. This indicates that cer-tain immobilization near the urethane hard segment and epoxy softsegment exists for silicone-modified epoxy segments. Furthermore,thermal fluctuation theory was used by Donth and co-workers [22]to derive an expression for length scale of cooperativity, �˛ and itstemperature dependence from calorimetry.

�˛ =(

kT2�(1/Cv)

�(ıT)2

)1/3

: �(

1Cv

)≈ �

(1Cp

)

In the above equation, �(1/Cp) = (1/Cp,glass) − (1/Cp,liquid) and ıTis the half-width of the thermal response which is regarded asthe mean fluctuation associated with the cooperatively rearrangingregions, k is the Boltzman constant, � is the density of the sample,�(1/Cv) is the step of reciprocal specific heat capacity at constantvolume at the glass transition temperature and Cp is at constantpressure.

The thermal fluctuation parameters are presented in Table 1.Using the above equation, a segmental length scale of cooperativ-ity of 3.2 nm is obtained for neat epoxy resin. Upon modificationof the epoxy resin with adipic acid, the value increases to 3.4 nm.There are two possible consequences of such a modification. Firstis the introduction of polar ester groups, which increases the Tg, asalready seen. Second is the introduction of four methylene spacerswhich flexibilize the epoxy chain. As of the effect on the length scaleof coopertivity, the first factor should have a tendency to decreasethe �, whereas the methylene spacer can increase it. Hence we pre-sume that the second factor predominates resulting in a marginallyhigher value of �˛. In case of silicone-modified epoxy resins, it isobserved that the length scale of coopertivity decreases (2.7 and2.2 nm for 15 and 30% silicone-modified epoxy resins) as a resultof increased ıT values. This may be attributed to the presence ofurethane junctions in the silicone-modified samples. Crosslinks

are known to modify the segmental relaxation behaviour of poly-mer networks [23,24]. The urethane junction in the present systemfor silicone-modified samples could well be regarded as physicalcrosslinks. The network junction may be viewed as geometries of

Table 1Thermal fluctuation parameters at T = Tg from MDSC.

Sample � (g/cm3) Tg (K) ıT (K) �Cp (J/g K) �(1/Cp) (gK/J) � (nm)

EP 1.17 251.6 2.29 0.643 0.243 3.2EPA 1.18 257.8 2.23 0.586 0.272 3.4EPASi-15 1.13 257 2.8 0.379 0.198 2.7EPASi-30 1.10 262.3 3.9 0.499 0.232 2.2

S.K. Rath et al. / Progress in Organic Coatings 65 (2009) 366–374 371

gen fr

rrai

3

o3mm

Ft(

Fig. 5. Cross-sectional SEM micrograph of liquid nitro

estriction similar to those in polymer nanocomposites [25]. Theesults are in agreement with calorimetric experiments on a widessortment of molecular glass formers showing that the character-stic length of glass transition near Tg is typically 1–3.5 nm [22].

.2.3. SEMFig. 5 shows scanning electron micrographs of the cross-section

f the fractured surfaces of EPJ, EPAJ, EPASi-15J and EPASi-0J samples, respectively. We observe a smooth and featurelessorphology for EPJ and EPAJ. The fractured surfaces of the silicone-odified samples, on the other hand, are observed to consist of

ig. 6. Temperature dependence of tan ı of EPJ, EPAJ, EPASi-15J, EPASi-30J. Inset:emperature dependence of tan ı (−140 ◦C to 50 ◦C) of EPJ, EPAJ, EPASi-15J, EPASi-30Jsub-room temperature relaxation behaviour).

actured (A) EPJ; (B) EPAJ; (C) EPASi-15J; (D) EPASi-30J.

two distinct phases; globular PDMS rubber particles dispersed in acontinuous epoxy adipate matrix. More number of such particlesis observed for EPASi-30J as is expected for higher PDMS con-tent in the matrix. The rubbery siloxane particles (particle size:1–5 �m) are distributed uniformly throughout the matrix. Furtherfrom the micrographs one can see broken rubber particles and thestress whitened zone. Stress whitening is due to scattering of visi-ble light from the layer of the scattering centre—in this case, voids[26]. The generation of the voids is due to the cavitation of rubberswhich is the most widely observed energy dissipating mechanismin case of rubber toughened epoxies. Furthermore stress whiteningis more prominent for EPASi-30J. Uniform distribution of rubberparticles in the matrix is very important as it allows the yieldingprocess to operate throughout the matrix [26,27]. Quite notably wedo not observe any agglomeration of rubber particles even at 30%PDMS content. This is in stark contrast to observation of agglomer-ated rubber particles upon use of external toughening agents usedas impact modifiers for epoxy resins [27,28]. In the present casesince the PDMS phase is covalently bonded to the epoxy chain byurethane linkages, it allows more uniform distribution of rubberparticles in the matrix [29].

3.3. Dynamic mechanical analysis

Fig. 6 shows the temperature dependence of tan ı of the curedepoxy, epoxy adipate and the silicone-modified samples in thetemperature range −135 to 60 ◦C. The measurements could not becarried out at higher temperatures due to the flexible nature ofthe materials, apparently because of low glass transition temper-atures. From the results it is observed that the EPJ shows a sharp

tan ı peak at 11.7 ◦C corresponding to its glass transition tempera-ture. A shift in the Tg to a higher temperature (14.7 ◦C) is seen forthe EPAJ sample suggestive of the stiffening of the matrix becauseof the introduction of polar ester groups. Silicone modification ofthe samples leads to contrasting effects in EPASi-15J and EPASi-

372 S.K. Rath et al. / Progress in Organic

F3

3at1o3TPu

higher Tg value for EPASi-30J. The sub-room temperature modulus

ig. 7. Temperature dependence of storage modulus of EPJ, EPAJ, EPASi-15J, EPASi-0J.

0J. EPASi-15J shows a Tg value (13.2 ◦C) intermediate between EPJnd EPAJ, whereas EPASi-30J shows a higher Tg of 17.2 ◦C. Thushe Tg of the cured samples follows the order EPJ (11.7 ◦C) < EPASi-5J (13.2 ◦C) < EPAJ (14.7 ◦C) < EPASi-30J (15.2 ◦C), in contrast to therder of Tg for uncured samples, i.e., EPJ < EPAJ < EPASi-15J < EPASi-

0J. Introduction of soft PDMS segments is expected to lower theg of the matrix, which is the case for EPASi-15J. However at 30%DMS content, the Tg increases, possibly due to more number ofrethane junctions which are expected to decrease the segmental

Fig. 8. AFM topographic image of air-side surfa

Coatings 65 (2009) 366–374

mobility. We are investigating the length scale of cooperativity ofthe crosslinked samples through MDSC to gain further understand-ing of the relaxation behaviour of the silicone-modified samples.The intensity of the tan ı peaks, a measure of the damping efficiency,decreases for the silicone-modified samples with slight broadeningas well.

The low temperature relaxation behaviour is given in the insetto Fig. 6 in the temperature range −140 to −20 ◦C. We observea broad peak for the epoxy, epoxy adipate as well as silicone-modified samples centered around −80 ◦C. This could be attributedto the transition involving the polyether diamine block copolymerwhich has been used as the curative for all the samples. For EPASi-30J we observe a sub-room temperature transitions at −122 ◦Ccorresponding to the Tg of PDMS [18] and a peak at −45.6 ◦C, corre-sponding to melting of PDMS phase which is absent in EPJ and EPAJ.However for EPASi-15J the Tg peak is noticeable but no melting peakis observed.

Fig. 7 shows the temperature dependence of storage modu-lus for the epoxy and silicone-modified matrices. From the figureit is seen that in the glassy region the modulus is in the orderEPJ > EPAJ > EPASi-15J > EPASi-30J, which is quite reasonable. Inter-estingly though, the room temperature modulus trend remains thesame for EPJ and EPAJ, but is somewhat altered for silicone-modifiedsamples. EPASi-30J shows a higher modulus compared to EPASi-15J.This could be a direct manifestation of the glass transition tem-perature trend as seen in the tan ı–T plot, where we observed a

for EPASi-30J shows a drop in modulus corresponding to the glasstransition and melting of the PDMS phase. (It’s not very clear thoughto us as to why the modulus begins to rise after the glass transitiontemperature.) Noticeably no such trend is observed for EPASi-15J.

ce of (A) EPJ; (B) EPASi-15J; (C) EPASi-30J.

S.K. Rath et al. / Progress in Organic Coatings 65 (2009) 366–374 373

e topo

3

refapcaArtPtiswosisW3fihocPl

Fig. 9. Cross-section profile along the line drawn of th

.4. AFM

AFM is a widely used technique to determine the surface topog-aphy of polymer films. Fig. 8 depicts the AFM topography of curedpoxy adipate and the 15, 30% silicone-modified samples. The sur-ace morphology of the epoxy adipate film shows large domains ofbout 500–1000 nm width and 50–80 nm height (cross-sectionalrofile along the line drawn in the Fig. 9A). The domains observedould be due to the phase separated polar ester groups. The aver-ge roughness of the scanned surface was found to be 16.9 nm. TheFM topography of EPASi-15J (15% silicone-modified epoxy adipateesin) sample was found to be planar with a discernible phase con-rast wherein the darker area might correspond to the relatively softDMS-rich domains. These domains have less than 500 nm diame-er and 6–8 nm height (cross-sectional profile along the line drawnn Fig. 9B). The average roughness was found to be 2.5 nm. Thisuggests that there is a drastic reduction in the surface roughnessith silicone modification. Further, the phase contrast is indicative

f surface chemical heterogeneity. We believe macroscopic phaseeparation could be the reason for such biphasic behaviour as seenn the SEM studies. In a recent investigation Majumdar and Web-ter reported a biphasic structure at a PDMS content of 10 wt% [12].

ith further increase in the degree of silicone modification (EPASi-0J) the surface roughness decreased to 2.2 nm. The surface wasound to be highly planar with a relatively larger dark area as seenn Fig. 8C. From the cross-sectional profiles (Fig. 9C) the average

eight of the raised region is found to be only about 2–4 nm. Thusn silicone modification, the domain size decreases at 15% PDMSontent compared to EPAJ and becomes almost featureless at 30%DMS content. This clearly indicates the surface enrichment of theow surface energy component (PDMS segments) in order to min-

graphic image of (A) EPJ; (B) EPASi-15J; (C) EPASi-30J.

imize the overall surface free energy of the system. Consequently,the overall roughness was found to decrease drastically comparedto EPAJ.

3.5. Surface wettability

Table 2 shows the advancing and receding contact angles and thesurface energy values for the samples. From the results it is seen thatthe surface energy for EPJ is 36.8 ± 0.35 mJ/m2. Upon modificationwith adipic acid, for the EPAJ sample, the advancing contact anglewith both the probing liquids decreases, which in turn increases thesurface energy to 39.1 ± 0.37. This could be attributed to the pres-ence of polar ester groups in EPAJ. Further, it is seen that the contactangle hysteresis increases on modification. This could be a directmanifestation of the increased surface roughness of the samples asseen in the AFM results. Silicone modification of the samples resultsin a higher advancing and receding contact angles for both the prob-ing liquids compared to EPJ and EPAJ with a reduced contact anglehysteresis. It is noteworthy that the present system is quite com-plex, in that it comprises of three different soft segments namelyepoxy adipate main chain, the siloxane chain linked through theurethane group and the polyether diamine block copolymer whichhas been used as the curative. Hence, in a competitive process ofsurface enrichment by the three soft segments, the governing fac-tors are evaporation rate of the solvent, the solubility parameterdifference between the solvent and the soft segments, the surface

energy of the segments and the mobility or the Tg of the segments[30]. In the present study, toluene has been used as the solvent forall the systems as it is a good solvent for PDMS segments. Of allthe segments, it is well established that PDMS segments has thelowest surface energy [31]. Moreover, the Tg of the segments are

374 S.K. Rath et al. / Progress in Organic Coatings 65 (2009) 366–374

Table 2Contact angle, surface energy and contact angle hysteresis (w.r.t. water) of air-side surface of cured systems using the two-liquid method.

Material Contact angle (◦) water Contact angle(◦) diiodomethaneadvancing (�a) (◦)

Contact angle hysteresis (CAH)w.r.t. water

Surface energy (�) mJ/m2

Advancing (�a) Receding (�r)

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PJ 83.5 ± 1.7 62.2 ± 1.2 46 ± 1.1PAJ 78.2 ± 1.4 49.1 ± 1.3 43 ± 1.1PASi-15J 89 ± 1.5 70.5 ± 1.7 55 ± 1.3PASi-30J 94 ± 2.2 80.1 ± 2 69.4 ± 1.2

n the order PDMS (−122 ◦C) < polyether diamine (−82 ◦C) < epoxydipate main chain (10.2 ◦C for EPASi-15J and 16.3 ◦C for EPASi-30J)s seen in DMA results. It is well known that soft blocks, which haveow glass transition temperatures (Tg), concentrate in the surfaceecause of their more favorable air/interface energetics [32]. Hence,n account of the lower Tg and surface energy of the PDMS seg-ents, it is expected to enrich the surface. The surface energy value

or EPASi-15J is found to be 28 mJ/m2, conforming to the expecta-ion. Further, the contact angle hysteresis (CAH) w.r.t. water alsoecreased to 18.5 ± 1.7◦ compared to EPJ and EPAJ. At 30% PDMSontent the surface energy further reduces to 23 mJ/m2 with a con-act angle hysteresis of 13.9 ± 1.2◦. However the water CAH is muchreater compared to a pure PDMS surface (Made from hydrosi-ane and vinyl silane) which is reported to be usually less than0◦ [33,34]. Relatively higher CAH for the present system coulde due to the combination of the presence of the hydrophilic cur-

ng agent as well as the polar urethane groups. The reduced CAHalues for the PDMS-modified films compared to the neat sam-les, correlate well with the observed surface smoothness for theilicone-modified samples. Such observation of decreased surfacenergy in PDMS containing block copolymers and blends are wellstablished [31,35]. However reports of systems with PDMS in theide chain in a crosslinked matrix are relatively less. We believehe phase separated PDMS segment, as seen from DMA and SEMtudies, with a Tg of −122 ◦C, retains sufficient mobility to enrichhe surface. However it must be noted that the PDMS segmentsre anchored at both the ends to the main chain via stiff urethaneinkages, which in turn would impede the mobility to a certainxtent compared to blended systems. We are investigating the sur-ace chemical composition of the systems by angle resolved X-rayhoton electron spectroscopy to gain further insight into surfaceehaviour.

. Conclusion

Silicone modification of epoxy adipate resins were carried outhrough urethane route. The silicone-modified samples showed

onsiderable improvement in the thermal stability. The characteris-ic length scale of cooperativity, found from MDSC, decreased uponilicone modification apparently due to the restriction in relax-tion posed by stiff urethane junctions. The cured silicone-modifiedpoxy resins showed biphasic morphology with a uniform distribu-

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21.3 ± 1.8 36.8 ± 0.3529.1 ± 1.9 39.1 ± 0.3718.5 ± 1.7 28.2 ± 0.3613.9 ± 1.2 23.7 ± 0.30

tion of PDMS rubber particles even at 30% rubber content. Furtherevidence of phase separation was provided by dynamic mechanicalanalysis. The silicone-modified epoxy resins showed considerablylower surface energy, contact angle hysteresis and surface rough-ness compared to the unmodified resins.

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