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Polymer 55 (2014) 3107e3119

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Polymer

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Effect of ultraviolet/ozone treatment on the surface and bulkproperties of poly(dimethyl siloxane) and poly(vinylmethyl siloxane)networks

A. Evren Özçam1, Kirill Efimenko, Jan Genzer*

Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA

a r t i c l e i n f o

Article history:Received 16 February 2014Received in revised form30 April 2014Accepted 7 May 2014Available online 17 May 2014

Keywords:Silicone elastomersSurface modificationPDMS

* Corresponding author.E-mail address: Jan_Genzer@ncsu.edu (J. Genzer).

1 Present address: 3M Purification Inc., 3M Center,

http://dx.doi.org/10.1016/j.polymer.2014.05.0270032-3861/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

We present a comparative study aiming at comprehending the effect of ultraviolet/ozone treatment onthe modification of poly(dimethyl siloxane) (PDMS) and poly(vinylmethyl siloxane) (PVMS) siliconeelastomers networks (SENs). Both PDMS and PVMS SENs undergo dramatic changes in their propertieswhen exposed to UVO. The surface chemical composition of both PDMS and PVMS at long UVO treatmenttimes changes substantially and features a high density of hydrophilic groups. There are two majordifferences in behavior in the two classes of materials. First, relative to PDMS, the PVMS-based SENs getmodified throughout the entire bulk. Second, the physico-chemical changes detected in PVMS take placeon much shorter time scale relative to PDMS. These results are in accord with our earlier reports thatindicated that when exposed to UVO, the topmost z5 nm of PDMS gets converted into a silica-likematerial, which then acts as a barrier for diffusion of atomic oxygen. In this case, the bulk of PDMSmaintains its elasticity. In contrast, both the surface and bulk of PVMS films undergo substantial changesin properties when exposed to UVO. First, the surface modification of PVMS SENs takes place after only afew seconds of the UVO treatment. In addition, we register substantial modification of bulk properties,including the complete densification accompanied with increased bulk modulus. Likely, the suscepti-bility of the vinyl bonds to radical reactions is responsible for this effect.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Silicones, polysiloxanes, silicone elastomers (SEs) representunique polymeric materials comprising an inorganic SieOeSibackbone with two pendant functional groups attached to eachsilicon atom. The low energy barriers for rotation, asymmetric SieOeSi bond angles, and longer bond lengths relative to CeOeC bondendow silicones with exceptionally high flexibility, which results invery low glass transition temperatures (z150 K). Poly(dimethylsiloxane) (PDMS) is the most commonly utilized silicone, in whichboth of pendant functionalities about the Si atom feature methylgroups. The presence of the two stable methyl groups on eachrepeat unit provides PDMS with high chemical resistance and lowsurface energy.

St. Paul, MN 55144, USA.

Individual silicone chains can be crosslinked chemically toform silicone elastomer networks (SENs) with elastic moduliranging from several kPa to a few MPa while maintaining theliquid-like nature of the parent individual polymer chains be-tween the crosslink junctions intact. SENs have been appliedactively in numerous technological applications, including butnot limited to, insulation, soft contact lenses, and advancedmedical screening devices. PDMS has been employed as a majorcomponent in the aforementioned applications due to its wideavailability, relatively low cost, biocompatibility, and chemicalinertness. Many technological applications demand that SE sur-faces are hydrophilic or can be modified to attach variouschemical moieties. The surfaces of PDMS-based SENs are inher-ently hydrophobic and are difficult to alter chemically due totheir high chemical stability. Over the years, numerous physicaland chemical routes have been developed that facilitate tuningthe SEN surface properties [1e5].

Modifying the surfaces of PDMS by chemical means is limited tostrong base or acid exposures that lead to uncontrollable and non-

A.E. Özçam et al. / Polymer 55 (2014) 3107e31193108

uniform surface modification. Additional difficulty arises from thefact that the PDMS surface gets contaminated with the modifierand often cannot be easily cleaned. Finally, exposing the SENs tosolutions of extreme pH causes network degradation [6]. Physicalsurface modification techniques, such as plasma, corona or ultra-violet/ozone (UVO) treatment, allow for relatively well-controlledchemical modification and in-plane uniformity of the surface.Prior studies have shown that the exposure of SE surfaces to plasmaof various gases (N2, O2) leads to rapid conversion of the methylgroups on the PDMS backbone to various hydrophilic moieties anda breakdown of the SieO linkage accompanied with the formationof “secondary crosslinks” [4]. The type of plasma gas affects greatlythe chemical composition of the surface. Oxygen plasma is themostcommonly used treatment; it renders PDMS surfaces highly hy-drophilic as evidenced by corresponding changes in the watercontact angle from 110� to 10e20� [7e9]. Polar moieties (e.g.,silanols, carboxylic acids) present on such physically-modifiedPDMS surfaces facilitate carrying out a variety of chemical modifi-cation strategies to generate surfaces with tailored functionalities.Previous research has shown that plasma treatment propagatesseveral hundreds of nanometers below the surface causing irre-versible chemical changes to the base material in the near-surfaceregion of PDMS [3]. A brittle silica-like layer at the material sur-face is formed due to accumulation and condensation of silanolgroups on the SE surface whose mechanical properties differssignificantly compared to the elastomer bulk. The thickness of thesilica layer increases with increasing oxidation time, power of theplasma, chamber pressure, and gas chemical composition. Thisoften undesirable effect is accompanied by instantaneous forma-tion of microscopic cracks in the silica like layer due to themismatch between the mechanical properties of the hardened toplayer and elastic bulk of PDMS [3,10e12]. The chemical compositionof the plasma-treated PDMS surfaces changes over time due touncontrollable diffusion of uncrosslinked silicone oligomersthrough the surface cracks [10,4]. This phenomenon results inrecovering the original hydrophobic nature of material (so-called“hydrophobic recovery”) [5,13e15]. It is very challenging to controlthe extent of physical modification with plasma since the surfacemodification of SENs happens very quickly (a few seconds,depending on the dosage).

Alternatively, the plasma techniques can be substituted suc-cessfully by the UVO treatment that produces polar functionalitieson the PDMS surface. The UVO treatment represents a milder typeof physical modification compared to plasma treatment withsimilar surface changes but with approximately an order ofmagnitude increase in processing time [4,13]. The UVO treatmentinvolves a photosensitized oxidation process, in which the mole-cules of the treated material are excited and/or dissociated by theabsorption of short-wavelength UV radiation and atomic oxygen.Even though it is more controlled than the plasmamodification, theUVO treatment of PDMS still causes uncontrollable and irreversiblechanges to the surface of SE. At short modification times a variety ofhydrophilic groups are formed on the surface of the SE, includinghydroxyls, carboxyls, aldehydes, peroxides, and other hydrophilicgroups [4]. Long UVO treatments of PDMS lead to the formation of asilica-like layer on the surface of the PDMS which hardensconsiderably the original soft SEN surface. Previous work by ourgroup has shown that extended UVO treatment times (>30 min)result in the formation of a 5 nm thick layer whose density isz50%of that of pure silica [4,16]. Yet another advantage of the UVOtreatment relative to the plasma and corona techniques is that thesample temperature during the treatment rises only slightly, thusavoiding any non-desirable thermal treatment effects.

From the previous discussion it is apparent that altering thesurface chemical composition of PDMS is a challenging task. Given

the need for novel silicone-based materials whose surfacecomposition can be easily altered requires utilization of alternativeSE materials. Poly(vinylmethyl siloxane) (PVMS) possesses similarmechanical and thermal properties to PDMS, yet it offers the flex-ibility with regard to the number of alternative chemical pathwaysleading to the modification of pendant vinyl groups. Therefore,PVMS represents a unique class of SENs that offers the samefunctions as PDMS with the additional ability to tune the chemicalnature of the SEN via vinyl-based reactions. While PVMS polymershave been known and synthesized for many decades, limited in-formation is available on their modification with various physicaltreatments, such as plasma or UVO. In this work we present acomprehensive study aiming at understanding the properties ofPVMS based SENs after exposing them to the UVO treatment. Wealso employ the conventional PDMS-based SENs in the sametreatment in order to provide a benchmark.

2. Experimental section

2.1. Preparation of poly(vinylmethyl siloxane) and its networkformation

Poly(vinylmethyl siloxane) was synthesized using step-growthpolymerization of short hydroxyl-terminated oligomeric vinylmethyl siloxane chains as reported previously by Efimenko andcoworkers [17]. The precursor monomer for the reaction was ob-tained by slow hydrolysis of methylvinyldichlorosilane in thepresence of dilute aqueous solution HCl. The reaction productscomprised variousmethylvinyl siloxane cycles (VD3, VD4, VD5) andlinear hydroxyl-terminated chains. The cyclic products were sepa-rated by vacuum distillation to obtain linear chains with a yield of25e35% depending on the amount of HCl and water present in thereaction mixture. All polymers used in the experiments were pre-pared from linear siloxane chains. The solvent-free siloxane poly-merization was initiated by a small amount of lithium hydroxide(10e20 ppm) at 120 �C for various reaction times under constantflow of nitrogen that facilitated the removal of water moleculesformed during the polymerization. The reaction was terminated bythe addition of carbon dioxide, which resulted in formation ofa,u�hydroxy-terminated PVMS chains. This method allows for thesynthesis of a broad range of molecular weight PVMS that can beeasily monitored by the change in the viscosity in the reactionapparatus. The final polymer was vacuum filtered using Celite 545filter aid system. The unreacted short oligomeric chains wereremoved by extraction in methanol. First, PVMS was dissolved indiethyl ether and then added drop wise to chilled methanol. Thehigh molecular weight polymer portion was collected and driedunder vacuum for 72 h to remove any traces of ether and methanol.This procedure was repeated two times. Size exclusion chroma-tography (SEC) equipped with light scattering and refractive indexdetectors has verified the complete removal of low molecularweights compounds. The monomer conversion yields were veryclose to 98%. IR spectroscopy confirmed that the amount of vinylfunctional groups remained unchanged throughout the experi-ment, which suggested that no backbone branching took placeduring the polymerization. While PVMS of various molecularweights were prepared and characterized, the experimentsdescribed hereafter were carried out using only hydroxy-terminated PVMS with molecular weight of 35 kDa.

The a,u-hydroxy-termination of PVMS enables controlledcrosslinking reaction at the terminal ends of the chains throughalkoxy condensation of silanols in the presence of Sn catalyst asreported earlier [18,19]. PDMS networks with two different for-mulations were also prepared in order to compare the effect of UVOtreatment on both PVMS and PDMS modification. The first set of

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samples was prepared from a commercially available Sylgard-184PDMS kit, which was used as a standard material inmanufacturing of siloxane-based devices. Sylgard-184 contains alarge amount (up to 60% bymass) of silica fillers that serve as elasticreinforcers; those particulates may have a strong influence on themodification reaction taking place during the UVO irradiation [4].In addition, we prepared amodel PDMS SENwith no fillers by usinga,u-vinyl-terminated PDMS with molecular weights of 28 kDa bymeans of hydrosilation reaction with tetrakis(dimethylsiloxy)silane (TDSS) in the presence of Pt(II) catalyst as described previ-ously [20].

2.2. Ultraviolet/ozone (UVO) treatment setup

Ultraviolet/ozone treatment of all SENs was carried out using acommercial UV/ozone chamber (Jelight Corp. Model 42) that em-ploys the low-pressure grid mercury lamp to produce the UV ra-diation with a maximum emission in the ultraviolet region (fusedquartz UV lamp having transmission about 65% at 284 nm) [4]. AllSENs tested in our study were placed under the lamp at the dis-tance ofz5 mm tomaximize the exposure of the treated surface tothe UVO radiation since the concentration of the modifying speciesdecreases exponentially with increasing distance from the lamp.The power output of the lamp at this distance was z8.2 mW/cm2.

2.3. Fourier transform infrared spectroscopy

Chemical transformation of SENs after exposure to the UVOtreatment was monitored by Fourier transform infrared spectros-copy (FT-IR) in the attenuated total reflection mode (ATR-FTIR)equipped with Ge crystal (Nicolet 6700). The ATR-FTIR system waspurged with extra dry air to minimize the effect of environmentalwater vapor and CO2 on the collected spectra. For each samplestudied, 128 scans were collected after recording the backgroundsignal using a DTGS TEC detector with a scan resolution of 4 cm�1

between 600 and 4000 cm�1. In order to insure a good contactbetween the treated surface and the Ge crystal, all samples werefirst places onto microscope glass slides and then brought in con-tact with the Ge crystal surface by applying a constant force on thetested sample. The Ge crystal was thoroughly cleaned between theexperiments to ensure that surface stays clean of any residualcontamination. All data processing was done using the OMNICsoftware package supplied by the instrument manufacturer.

2.4. Gas permeation measurements

Gas permeationmeasurements were performed on the in-housebuild constant-volume and variable pressure setup [21]. The SENswith thickness (l) 750e800 mm were sandwiched between twoaluminum tapes which had openings of a known area (A) allowinggas to permeate through membrane. After applying moderatevacuum (z200 mTorr) to both sides of the membrane, the test gaswas introduced at a known pressure on the upstream side of themembrane. The downstream pressure was recorded upon gaspermeation through the SE membrane. Since the downstreamvolume is known (V), the permeability (P) of individual gases can becalculated using Equation (1):

P ¼ V,lA,R,T,Dp

dpdt

(1)

where R is the universal gas constant, T is the absolute temperature,Dp is the difference between upstream and downstream pressuresand dp/dt is the steady rate of pressure increase on the downstreamside. The permeability of O2 and CO2 through UVO-modified SE

membranes was calculated for the upstream pressure of 2 atm at23 �C.

2.5. Contact angle measurements

Contact angle measurements were performed using the Ramé-Hart contact angle goniometer (model 100-00) equipped with adigital camera and image processing software. High purity deion-ized water was used as the probing liquid. The contact angles wererecorded by dispensing 6 ml of liquid with subsequent advancingand receding it to 8 and 4 ml correspondingly in order to obtaininformation about surface energy and non-uniformity. The contactangle data presented in this paper represent an average value for atleast five different measurements that were performed across thesample surface with error of ca. �1.5�.

2.6. Differential scanning calorimetry

Differential scanning calorimetry (DSC) was performed using aTA Instruments Q2000 commercial system equipped with a liquidnitrogen cooling system. All experimental data were recorded ac-cording to following two-step cycling procedure. The samples werefirst cooled from 25 to �150 �C with a rate of �5 �C/min, and thenheld at �150 �C isothermally for 5 min. Heating was carried out byincreasing the temperature to 25 �C with the rate of 5 �C/min. Thisprocedure was repeated two times to ensure the reproducibility ofthe measurements. The glass transition temperatures (Tg) weremeasured at the at the inflection point on the heating cycle of thethermograms.

2.7. Thermo gravimetric analysis

The thermal stability of the PDMS, PVMS and Sylgard-184 sub-strates were studied using a TA Instruments Q500 Thermo Gravi-metric Analyzer (TGA). In order to decouple the thermal andthermo-oxidative degradation behavior of the SENs, we havemeasured extracted and unextracted samples under oxygen andnitrogen atmospheres. All studies were performed between 25 and900 �C in platinum pans with a heating rate of 10 �C/min.

2.8. Ultraviolet-visible spectroscopy

The UV absorption of silicone elastomers was carried out onNicolet Evolution 300 (Thermo Electron Corporation) between 200and 800 nm with the resolution of 1 nm and a scan speed of120 nm/min.

3. Results and discussion

As pointed out earlier, plasma and corona treatment have beenutilized extensively to render the hydrophobic SE surface hydro-philic. Compared to plasma or corona, the modification using UVOis much milder for physical modification of polymer surfaces[15,22]. This is largely due to the smaller radiation dosage that getsdelivered to the sample surface resulting in slower chemicalalteration of the material and longer processing times. The increasein the magnitude of UVO dosage (controlled by the irradiation timeexposure time; generally a few minutes) relative to plasma/corona(seconds to tens of seconds, depending on the power) allows forbetter control over the surface chemical composition and conse-quently the degree of wettability. The time needed to convert froma hydrophobic to hydrophilic SEN surface depends not only on theprocessing conditions but also on the chemical nature of the parentSEN material. Based on the chemical structures of PDMS and PVMSbased SENs, one can make an intuitive assumption that vinyl-

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functionalized SENs would be more susceptible to UVO modifica-tion compared to the traditional PDMS networks because of thepresence of the vinyl groups that are chemically less stable thanmethyl moieties. To illustrate this point, in Fig. 1 we plot watercontact angles (WCA) collected from UVO-modified PDMS (a,squares) and PVMS (b, circles) samples. In both instances, theaverage molecular weight between the crosslinks in the networkwasz30 kDa. A few important observations can be made from thedata. First, the hydrophobicity of the parent PDMS is higher thanthat of PVMS due to the presence of the slightly more hydrophobictwo methyl groups in PDMS relative to the more hydrophilic vinylgroup present in PVMS. Second, the rate of surface conversion ismuch faster for PVMS compared to PDMS. This behavior is associ-ated with different chemical changes on the two SENs during theUVO radiation, as will be discussed in detail below [6,7]. Finally, thepresence of two chemically stable methyl groups minimizes themodification of PDMS initially because those groups shield suc-cessfully the SieO backbone; only the prolonged UVO treatmentcauses dramatic changes in the SE material. From our previouswork we know that the primary sites in the PDMS SEN that getmodified first are the crosslink points. The vinyl groups are lessstable than methyls and more susceptible to UVO modification, asdiscussed elsewhere [7]. Therefore the PVMS SENs undergo morerapid modification relative to PDMS. After z20 min of UVO expo-sure the PVMS surfaces become completely hydrophilic(WCA < 20�).

UVO treatment causes not only changes in wettability of SENsbut also leads to modification of mechanical properties. We alreadymentioned earlier that plasma and corona treatments producerather thick, silica-like layers resting on top of the unmodified SEbase [15,23,24]. Contrary to these observations the UVO treatmenttimes up to 5e10 min (depending on the UVO dosage) result inrelatively small changes in surface wettability and do not affectdramatically the mechanical properties of the PDMS SEN. However,the prolonged exposure to UVO radiation leads to densification ofthe SEN due to oxidation, chain scission, and elimination ofoxidized short molecules similar to ozone treatment. Sinceoxidized silicon atoms cannot be easily removed from the UVO-treated SEN surface by evaporation, they start accumulating onthe surface while short volatile organic fragments of the parentchain are removed. The surface immobilized oxygen enriched

Fig. 1. Water contact angles collected from UVO-modified PDMS network (a) andPVMS network (b) after exposure to UVO. The closed and open symbols represent theadvancing and receding water contact angles, respectively.

silicone groups may undergo a condensation reaction to form thesilica-like layer. For PDMS, the thickness of the formed silica-likelayer is z5 nm after extensive UVO exposure for 2 reasons: 1)stable pendent methyl groups of PDMS do not let the modificationpropagates deep into the material, and 2) as formed silica-like layeracts as a protective barrier layer by hardening the surface layer,restricting the surface mobility and hindering the diffusion ofreactive [O] and O3 deeper into the PDMS network (50% density ofsilica, as determined by x-ray reflectivity [4]). These changes inPDMS with increasing UVO exposure can be followed convenientlywith mechanical measurements. In Fig. 2 we plot the storage (cf.Fig. 2a) and elastic modulus (cf. Fig. 2b) determined by dynamicmechanical thermal analysis (DMTA) test and nanoindentation,respectively. While the storagemodulus, which is ameasure of bulkproperties, remains virtually unchanged for all UVO times studied,there is a small increase in the elastic modulus that reflects changesin the mechanical response of the surface. These data support ourearlier x-ray reflectivity studies indicating the formation of a thinrigid layer (z5 nm) formed on top of PDMS SEN [4]. This layer is notbeing sensed by the storage modulus measured by DMTA, whichprobes the properties of the entire specimen (z700 mm), but isdetected by the elastic modulus obtained from surface-sensitivenanoindentation. The mechanical properties of UVO-modifiedPVMS SENs are quite different from those of PDMS. While a briefexposure of PVMS to UVO (2e5min) only causes changes in surfacechemistry, extended UVO treatment results in a dramatic variationof the bulk PVMS SEN. After z20 min, the PVMS SEN hardensconsiderably; this hardening occurs not only on the surface butpropagates throughout the entire specimen, as documented by thebulk (storage) and surface (elastic) moduli plotted in Fig. 2. Thegenerated radicals in PVMS react with neighboring vinyl groupsand initiate formation of a “secondary network” between the vinylgroups starting at the air/PVMS interface. The “secondary network”propagates deeper in the PVMS film with increasing UVO exposuretimes and leads to an increase in modulus of the PVMS network. Asa consequence, prolonged UVO treatment converts the originallyflexible SE into a glass-like material. The schematic in Fig. 3 depictspictorially the different mechanisms for UVOmodification of PDMSand PVMS SENs.

A series of gas permeation experiments were carried out tounderstand the effect of UVO treatment on the formation of silica-like layer and the free volume of the SENs. Gas transport in poly-meric materials typically occurs by the solution-diffusion

Fig. 2. Storage modulus (a) and elastic modulus (b) collected from PDMS (squares) andPVMS (circles) SENs after exposure to UVO for various times.

Fig. 3. Schematic depicting the modification of PDMS (a) and PVMS (b) SENs after exposure to UVO. The photographs in sections a) and b) demonstrate the flexibility and rigidity ofthe UVO-modified PDMS and PVMS networks, respectively.

A.E. Özçam et al. / Polymer 55 (2014) 3107e3119 3111

mechanism. Thus the permeation of gas molecules is defined as theproduct of their solubility and diffusivity in the polymermatrix. Thesolubility of gas molecules in the polymer matrix depends on theinteractions between the penetrant and the matrix while thediffusion depends on the mobility and size of the gas molecules aswell as the transient voids present in thematrix due to the availablefree volume in the polymer. The permeability results for PVMS(circles), Sylgard-184 (open squares) and model PDMS (closedsquares) for carbon dioxide (CO2) and oxygen (O2) as a function ofthe UVO treatment time of the base SEN are plotted in Fig. 4. Beforethe UVO exposure all SENs possess high permeability for bothgases. Slight differences are detected for the permeability of CO2and O2 and this effect can be attributed to the differences in thechemical structure of the PVMS and PDMS since the crosslinkdensities of all networks were kept constant. While Sylgard-184and model PDMS networks are based on identical a,u-vinylterminated poly(dimethyl siloxane), the presence of a large per-centage of fillers and additives present in the commercial PDMSformulation decreases effectively the free volume of the elasto-meric membrane. Therefore the permeation through Sylgard-184 islower compared to that of a model PDMS SEN.

Upon exposure to UVO, surface modified PVMS networksexhibit rapid decrease in permeation for both gases until the net-works become fully impermeable at z30 min of UVO treatment.Remarkably, this significant drop in gas diffusion can be easilyregistered even for very short treatment times (<10 min). Contraryto the UVO-treated PVMS, the gas permeation properties of PDMSand Sylgard-184 SENs do not change considerably up to 20 min ofthe UVO exposure. As can be seen in Fig. 4 both PDMS and Sylgard-184 SENs exhibit similar behavior and become practically imper-meable only after prolonged exposures exceeding 60 min. Theintriguing difference in gas permeation of PVMS and PDMS elas-tomers upon exposure to UVO suggests that fundamentallydifferent processes take place inside the networks during the UVOtreatment. As discussed earlier [4], PDMS SENs exposed to UVO

irradiation are modified only near the polymer/air interface, asdiscussed earlier. The formation of the dense silica-like layercommences only afterz30 min of the UVO treatment and does notchange substantially with increasing the UVO time. The permeationdata obtained for model PDMS and Sylgard-184 systems clearlyindicate that the initial decrease in permeability can be observedafter 30 min of treatment and it reaches plateau at z60 min, inaccordwith previous observations pertaining to the extent of PDMSmodification. Contrary to this, the rapid drop in permeability inPVMS system provides additional experimental confirmation thatmodification takes place throughout the entire bulk of the PVMSnetwork.

In order to comprehend these results we have carried out aseries of UVeVis experiments in which we can compare the ab-sorption spectra of modified SENs. This comparison allowed us todraw general conclusions about the degree of modification of theSENs under the UVO treatment. In Fig. 5 we plot the UVeVisspectra of the three classes of materials as a function of wave-length for different UVO exposure times. UVO-treated PDMS SENsshow evidence of a slight increase in UV absorption around210 nm only after prolonged exposures (100 min) that can beassociated with the aforementioned formation of an ultrathinsilica-like crust. Contrary to this, the behavior of the UVO-treatedPVMS is quite different. Specifically, the unmodified PVMS net-works absorb UV radiation at 230 nm and show a very strongbroadening of the peak toward 300 nm upon increasing UVOtreatment time. Additionally, the formation of the absorption peakz260 nm is observed, which is due to the formation of ketonesand aldehydes. These two effects clearly suggest that we have anextensive modification of PVMS material; moreover the newlycreated material should resemble a glass-like structure. While theUVeVis spectroscopy cannot provide quantitative measure of theprocesses that took place inside PVMS networks after the UVOexposure, these experiments confirm that the UVO-treatment ofPVMS takes place throughout the bulk of the material. The PVMS

Fig. 6. Differential scanning calorimetry from Sylgard-184 (top), model PDMS (mid-dle), and PVMS (bottom) SENs after modification with UVO for different times.

Fig. 4. Permeability of O2 (a) and CO2 (b) of Sylgard-184 (open squares), model PDMS(closed squares) and PVMS (closed circles) SENs as a function of the UVO treatmenttime.

A.E. Özçam et al. / Polymer 55 (2014) 3107e31193112

networks are thus significantly more susceptible to the UVOirradiation compared to the PDMS systems.

Differential scanning calorimetry (DSC) experiments were car-ried out to elucidate the details of the chemical changes took placein the SENs as a result of UVO treatment. In Fig. 6 we plot the heatflow as a function of temperature for the three SENs under inves-tigation. The glass transition temperature (Tg) is a key parameterthat allows one to determine segmental mobility of polymer chains

Fig. 5. Ultraviolet/visible spectrograms for PDMS (red) and PVMS (blue) modified forvarious UVO-treatment times. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

and the information fromDSC can be used as ameasure of chemicalprocesses that take place in the bulk of the SENs as a result of UVOtreatment. The Tg of Sylgard-184, model PDMS, and PVMS networkswere determined based on the inflection points at �124.5, �124.3and �130.1 �C, respectively. These values are in accord with pre-vious findings in the literature [25,26]. The Tg values of the modelPDMS and Sylgard-184 are identical in spite of the large loading ofsilica nanoparticles in the Sylgard-184 formulation. Model PDMSnetworks exhibit an endothermic peak at z�47 �C that can beassociated with cold crystallization of PDMS chains [20]. This peakis completely diminished in the case of Sylgard-184 due to highcontent of silica particles that disturb the crystallization in thesystem due to constrains induced by the filler [27,28]. We haveobserved that the Tg values of Sylgard-184 and model PDMS net-works do not change even after prolonged UVO exposure, whichsuggests that the modification of the networks due to the UVOtreatment takes place only in a thin layer present exclusively closeto the air/polymer interface and does not have significant impact onthe bulk behavior. Contrary to this observation, the heat flow inUVO-treated PVMS networks started to decrease substantially after30 min of UVO treatment and disappeared almost completely forexposure times exceeding 50 min. These observations reveal that atlonger exposure times, the entire bulk of the PVMS networksturned into a “glass-like”material. As mentioned earlier, during theexposure to UV light, monatomic oxygen and ozone predominantlyattack the vinyl groups present in PVMS leading to the formation ofa large number of radicals throughout the material: these radicalsreadily recombined either with each other or with neighboringvinyl groups creating additional crosslink points in the system. This

Fig. 7. IR spectra from PDMS (a) and PVMS (b) networks modified for various UVO-treatment times.

A.E. Özçam et al. / Polymer 55 (2014) 3107e3119 3113

process, in turn, reduces the ability of local free segmental motionof the individual chains after prolonged UVO exposure times, whichresults in completely diminished heat flow. This finding combinedwith the UVeVis spectroscopy and permeation data clearly sug-gests that the modification caused by UVO irradiation propagatesthroughout the entire PVMS network. This is in contrast to theUVO-modification of PDMS substrates that get modified only closeto the sample/air interface.

The different behavior upon UVO treatment confirms that themechanisms of modification are different for the PVMS and PDMS-based SENs. ATR-FTIR measurements were carried out to investi-gate the chemical changes took place on the UVO-treated SE surfaceto a depth of several micrometers. We monitored the characteristicsilicone IR peak intensities as a function of treatment time (cf. Fig. 7)and the summary of these peaks is tabulated in Table 1. The mostnoticeable change in the chemical structure for both PDMS andPVMS based SENs that take place upon exposure to UVO is theincrease in the SieOeSi signal (1055e1090 cm�1) with increasingUVO treatment time. The SE surfaces get converted efficiently to a

Table 1Assignment of IR spectra of, Sylgard-184, model PDMS, and PVMS.

IR region (cm�1) Description

785e815 eCH3 rocking and hSieCh stretch in hSieCH3

825e865 hSieO stretch in hSieOH875e902 hSieO stretch in hSieOH960 C]C twist/]CH2 wagging1015e1150 In-phase and out-of-phase wagging vibrations of e(CH2)e

and hSieCH2eSih1055e1090 Asymmetric hSieOeSih stretch1245e1270 Symmetric eCH3 deformation in hSieCH3

1407 ]CH2 scissors1587 C]C stretch1900 C]C wagging2950e2970 Asymmetric eCH3 stretch in hSieCH3

3050e3700 eOH stretching in hSieOH, possibly also in hCeOH (361

silica-like material due to removal of organic groups and conden-sation of silanol groups with each other. The peak intensity of ReOH (3050e3040 cm�1) increases and broadens until it reaches aplateau atz30min for both systems. This peak intensity increase isaccompanied by the increase in the intensity of SieOH signal (875e920 cm�1). This behavior is expected since the contact angle valuesindicate clearly an enhancement of the polarity of the surface. Wehave registered the same trend for both systems, but the rate isconversion is quite different. There are no noticeable changes in theSieOH signal for PVMS material after 30 min of treatmentcompared to the PDMS SENs, where we can see the continuingenrichment of polymer with silanols up to 180 min. This observa-tion can be explained from the fundamentally different behavior ofthese systems that we outlined previously. The modification ofPDMS networks is much slower due to very high stability of methylgroups on the backbone of the polymer. All signals correspondingtoeCH3 (785e815,1245e1270 and 2950e2970 cm�1) exhibit smallchanges with increasing UVO treatment time. Since the probingdepth of ATR-FTIR equipped with Ge crystal is on the order of 0.2e

Sylgard-184 PDMS PVMS

O O OO O OO O O

Oin hSie(CH2)2eSih O O O

O O OO O O

OOO

O O O0e3640 cm�1) O O O

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1.5 mm the majority of the methyl functionalities remain undis-turbed in the bulk of the polymer due to protective silica-like layer,which is consistent with our previous studies showing that modi-fication takes place at very interface of the material (z5 nm) [4].

The presence of the vinyl group alters completely the modifi-cation pathways for PVMS networks upon UVO irradiation. Wehave monitored all signals associated with vinyl group as functionof UVO exposure time. As expected, the trends for C]C twist/]CH2wagging (960 cm�1), ]CH2 scissors (z1405 cm�1), C]C stretch(1587 cm�1) show a reduction in the intensity with increasing UVOtreatment time. The data presented in Fig. 7 allow us to make a fewimportant conclusions. First, the modification of vinyl groups canbe registered even for very brief exposure times (<3 min) due tohigh susceptibility of the vinyl group to UVO treatment. Second, thesignal associated with the vinyl functionality diminishescompletely for UVO times longer than 30 min. This suggests thatthe modification through vinyl bond alteration in the PVMS SEtakes place and it propagates deep into the bulk of the UVO-treatedPVMS SE with increasing UVO exposure.

Control experiments were performed in order to elucidate thedetails of modification mechanism for the PVMS- and PDMS-basedSENs upon UVO treatment. Specifically SE specimens were exposedto low pressure mercury lamp in an argon-purged glove box tocomprehend the effect of UV light at ambient conditions with asiliconwafer on top of the SE just to block the UV light to investigatethe effect of ozone/atomic oxygen. In addition, we carried outannealing experiments at 150 �C under air and nitrogen atmo-sphere to determine the effect of heat on PVMS and PDMS basedSENs. The extent of PVMS modification during the UVO is furtherdetailed in Fig. 8, where we plot the ATR-FTIR peak areas of theSENs as a function of UVO-treatment time.

Fig. 8. Intensities of selected IR signals in PVMS treated for various UVO times. The colors coN2 purge (green lines) and heat treatment (blue lines). (For interpretation of the references

We carried out the thermo-gravimetric studies of SENs treatedwith UVO irradiation for 0, 15, 30 and 60min. The thermal responseof PDMS materials was independent of the UVO exposure time; itexhibits the classical thermal stability characteristic for these sys-tems. PDMS is stable up to 300 �C and shows only small weight loss(<5%) with increasing temperature up to 450 �C. Afterwards thePDMS networks undergo rapid thermal decompositionwith weightloss close to 60%. In contrast, the TGA data of PVMS networks aremore complex. As shown in Fig. 9, the weight loss is stronglydependent on the initial UVO treatment time and the extent ofextraction of uncrosslinked chains. The unmodified system clearlydisplays three distinct regions corresponding to different weightloss mechanisms. Initially, PVMS is stable up to 100 �C at whichpoint a slight decrease in weight is detected (z0.4%). This phe-nomenon can be explained by the evaporation of low molecularweight cyclic compounds from the bulk of the network. One cansuppress this loss significantly by careful solvent extraction andvacuum distillation of the precursor PVMS polymer (right panel inFig. 9). The effect of cyclic siloxane on the thermal response ofPVMS networks is not very pronounced for the PVMS SENs exposedto UVO for 30 and 60 min. The weight loss decreased withincreasing treatment time due to the capability of the cyclics toincorporate themselves into the network structure. Interestingly,we have registered slight weight gain for unmodified PVMS sys-tems above 200 �C, which we associate with the attack of oxygen tothe vinyl groups of the PVMS SEN. This gain is smaller for the UVO-treated networks for two reasons: 1) the limited number of avail-able vinyl groups available for oxygen attachment since some ofthem are consumed during secondary network formation and 2)reduced oxygen diffusion due to the densification of the networkduring the UVO exposure. All PVMS SENs studied are relatively

rrespond to UVO treatment (black), UVO treatment with UV filter (red lines), UVO withto colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Thermo gravimetric analysis of SE. PDMS measured in air (a), Sylgard-184 measured in air (c), PVMS with cyclic marker measured in air (b) and PVMS measured in air (d).

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stable between 200 and 450 �C and show the thermal decompo-sition at higher temperatures, similarly to PDMS. The main differ-ence between the PVMS and PDMS based SENs is in the overallweight loss. The absolute value of weight loss in PVMS never ex-ceeds 30e35% of the original weight, which is approximately half of

Fig. 10. Elemental concentration of oxygen (top row), carbon (middle row) and silicon (bovarious times as determined by XPS.

the value for PDMS SENs. This observation is another direct evi-dence of the densification of the PVMS networks throughout thebulk upon the UVO treatment that we described earlier.

The surface atomic concentration of carbon, oxygen and siliconfor PVMS and PDMS based SENs was determined by XPS as a

ttom row) in PDMS (left panel) and PVMS (right panel) samples treated with UVO for

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function of the UVO exposure time. The oxygen and carbon con-centrations for all the SENs investigated in this work increased anddecreased, respectively, (cf. Fig. 10) due to the introduction of morehydrophilic moieties (i.e., eOH, carboxyls, and ketones) and theelimination of carbon-containing groups from the sample surface.The increase in the oxygen atomic concentration on the SE surfacesas a result of the UVO treatment is in agreement with a concurrentdecrease of WCA (cf. Fig. 1). Surface oxygen concentration of PDMSSENs increased steadily up to z53% for the first 30 min of UVOtreatment and then it reached z60% at the end of 60 min of UVOexposure. In contrast, the surface oxygen concentration on PVMSSEN increased for the first 10 min and then leveled-off z56%.However, after 60 min of UVO treatment the oxygen concentrationin both samples reached approximately the same level due toincorporation of hydrophilic moieties and silanol groups. The sur-face carbon concentration followed the opposite trend for bothSENs, where the values went down due to volatilization of carbonfragments. The rate of hydrophilization is much faster for PVMSSEN due to presence of vinyl groups, which are more labile tooxidation and this data is in accord with WCA and ATR-FTIR. Wefound no dependence of the extent of surface modification on themolecular weight of the SEs.

The positions of the various peaks obtained in high-resolutionXPS (HR-XPS) experiments reveal information about chemicalenvironment of the corresponding atoms (cf. Fig. 11). For instance,the binding energy of Si 2p provides information about the silicon-to-oxygen coordination on the surface of SENs and the changes takeplace on the surface due to UVO exposure can be monitored byanalyzing Si 2p binding energy. The silicon atom of PDMS has two

Fig. 11. High resolution XPS spectra of PDMS (a) and PV

neighboring oxygen atoms with a Si 2p binding energy of 101.5 eV.In contrast, the silicon atom in silica (SiOx) has, on average, 4neighboring oxygen atoms and a binding energy of 103.6 eV [11].The Si 2p peaks of unmodified PVMS and PDMS networks arecentered at 101.9 eV (normalized with respect to 284.6 eV C 1 s)which is in close agreement with the reported Si 2p binding energyof unmodified PDMS. The examination of HR-XPS Si 2p spectra ofthe UVO-modified SENs reveals the conversion of organo-silicon toSiOx as a result of the UVO exposure, as shown in Fig. 11. The shift inthe Si 2p binding energy for both SENs indicates that the organo-silicon at the air interface of the SENs has been converted to SiOx.The Si 2p binding energy of PVMS-based SEN shifts to higherbinding energies up to 10 min of UVO exposure and then saturatesat z103.3 eV, which corresponds to z1.4 eV peak shift. On theother hand, the shift of the Si 2p binding energy of PDMS proceedsmuch slower compared PVMS based SE and reached. As reported byOuyang, Si 2p peak shifts of 1.4 and 1.2 eV for PDMS and PDMS-co-PVMS copolymers respectively after 120 min of UVO treatment[15]. Therefore, having lower peak shift of 1.3 eV for the UVOmodified PVMS layer is reasonable.

One of the key properties of interest in any surface-modifiedSENs is their stability over prolonged periods of time upon expo-sure to various environmental factors. While we cannot estimatethe bulk stability of the UVO-modified networks, we canmeasure itfor the air/polymer interface. In order to accomplish this we employcontact angle measurements to assess the surface energy variationafter the UVO irradiation and exposure to vacuum for 7 and 30 days.We first examined the surfaces of freshly treated networks bydispensing a droplet of DI water and recording value of advancing

MS (b) samples treated with UVO for various times.

Fig. 12. Water contact angle data (advancing: solid symbols, receding: open symbols) from, PDMS (top row) and PVMS (bottom row) samples treated with UVO for various times atvarying degree of aging.

Fig. 13. Elemental concentration of oxygen (top row), carbon (middle row) and silicon(bottom row) in PDMS (blue) and PVMS (red) samples treated with UVO for varioustimes after exposure to air as determined by XPS. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of this article.)

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(closed symbols) and receding (open symbols) contact angles (cf.Fig. 12). The difference between these two values (the contact anglehysteresis, CAH) provides information about the variation in thechemical heterogeneity of the surfaces [3,11]. Both networksexhibit similar trends in the contact angle as a function of the UVO-treatment time. However, the PVMS networks demonstrate muchfaster kinetics of surface conversion compared to the PDMS net-works. Complete conversion to hydrophilic materials is accom-plished in the first 10 min of the UVO treatment, while the PDMSsystems display similar conversion only after 60 min. Interestingly,the CAH is quite different for both systems; the CAH for PDMSnetworks increases after prolonged time exposure to the UVO. Incontrast, the CAH for UVO-modified PVMS is smaller except forshort treatment times (<10 min) due to the unrestricted rapidsurface reconstruction of the modified networks. While we detec-ted different degree of modification for PDMS and PVMS SENs, theaged systems show practically undistinguishable behavior. Thewater contact angle values for both systems become nearly inde-pendent of the initial treatment time and reach a plateau at z70�.This important finding allows us to conclude that originallychemically different systems exhibit nearly identical surface char-acteristics upon aging. These findings are further supported by XPSdata presented in Fig. 13.

We have also tested the properties of UVO-treated PVMS sub-strates after exposing them to a physiological solution. In Fig. 14 weplot the WCA data for PVMS SENs in their as-cast form as well asthose that have been treated with UVO for 1 min (middle panel)and 10 min (right panel). The data suggest that when exposed tohydrophilic environments, the UVO-treated PVMS substrates retaintheir hydrophilicity for many hours after the UVO exposure. Evenafter prolonged incubation times, the hydrophilicity of PVMS-UVOis much higher when exposed to hydrophilic media, relative to air.This result is not surprising given that the presence of hydrophilic

Fig. 14. Water contact angle data from PVMS (left), PVMS treated for 1 min (middle) and 1 h (right) and incubated in air (blue open squares) and physiological solution (red solidsquares) for various times. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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environment diminishes the hydrophobic recovery associated withthe diffusion of free, uncross-linked chains from the network. TheWCA measurements reveal that aging behavior in physiologicalsolution is directly affected by the initial UVO exposure time. Weexplain this behavior after evaluating all the data represented so far.To this end, the PVMS SEN exposed to UVO for 1 min has lesssecondary cross-link points and therefore has more mobile chainsin the surface region compared to the sample treated with UVO for10 min. As we stated earlier, the modification of the SEN com-mences at the air interface and propagates deeper in the networkwith increasing UVO exposure time. At short UVO exposure timeswe only oxidize the vinyl groups without introducing a largenumber of secondary crosslink points as evidenced fromWCA, DSC,nanoindentation and permeability measurements. The absence ofsecondary crosslink points results in more mobile chains in thePVMS network. Fewer secondary crosslink points in the 1min UVO-treated PVMS network allow the hydrophilized chains tomigrate tothe surface, which, in turn, decreases the water contact angle withlonger exposure to physiological solution. This attribute might beuseful in many applications that require soft elastomeric hydro-philic surfaces, such as soft contact lenses.

4. Conclusions

In this work we offered a comparative study pertaining to ul-traviolet/ozone treatment of PDMS and PVMS silicone elastomernetworks. The PDMS materials tested involved both commercial,silica-filled elastomers (i.e., Sylgard-184) as well as model PDMSnetworks formed by end-chain crosslinking. The PVMS was syn-thesized in house and end-chain crosslinked by protocols devel-oped earlier in the group. Our study revealed that both PDMS andPVMS SENs underwent dramatic changes in their properties whenexposed to UVO. Both materials exhibited substantial changes insurface chemistries when exposed to UVO. Specifically, a largenumber of hydrophilic groups was found as a result of the in-teractions with reactive atomic oxygen and UV radiation in bothmaterials. However, these changes occurred faster in PVMS relativeto PDMS. In addition, we found that PDMS was modified primarily

close to the surface while PVMS underwent dramatic changesthroughout the entire bulk when exposed to UVO. These results arein accord with our earlier reports that indicated that when exposedto UVO, the topmost z5 nm of PDMS gets converted into a silica-like material, which then acts as a barrier for diffusion of atomoxygen. In this case, the bulk of PDMS maintains its elasticity. Incontrast, both the surface as well as bulk of PVMS films underwentsubstantial changes in properties when exposed to UVO. First, thesurface modification of PVMS SENs took place after only a fewseconds of the UVO treatment. In addition, we registered sub-stantial modification of bulk properties, including the completedensification accompanied with increased bulk modulus. The sus-ceptibility of the vinyl bonds to radial reactions was responsible forthis effect.

Acknowledgment

The authors thank the Office of Naval Research for supportingthis work under contract No. N000141210642.

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