Journal of Colloid and Interface Science 367 (2012) 494–501

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Synthesis of a thermosensitive surface by construction of a thin layer of poly(N-isopropylacrylamide) on maleimide-immobilized polypropylene

Thelma S.P. Cellet a, Marcos R. Guilherme a, Rafael Silva b, Guilherme M. Pereira a, Marcos R. Mauricio a,Edvani C. Muniz a, Adley F. Rubira a,⇑a Grupo de Materiais Poliméricos e Compósitos, Departamento de Química, Universidade Estadual de Maringá, Av. Colombo, CEP 87020-900 Maringá, PR, Brazilb Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, USA

a r t i c l e i n f o

Article history:Received 28 July 2011Accepted 24 October 2011Available online 29 October 2011

Keywords:Graft copolymersLayer growthPhotopolymerizationStimuli-sensitive polymersSurfaces

0021-9797 � 2011 Elsevier Inc.doi:10.1016/j.jcis.2011.10.052

⇑ Corresponding author. Fax: +55 44 3011 4125.E-mail address: afrubira@uem.br (A.F. Rubira).

Open access under the Els

a b s t r a c t

Thermosensitive surfaces were developed by the grafting of a thin layer of PNIPAAm through an UV-induced photopolymerization reaction of vinyl monomers with a free radical-activated polypropylene(PP) surface. PNIPAAm layer covering the PP surface corrected, to some extension, both depressionsand fissures of the previously modified PP surfaces. The layered surfaces have morphological character-istic different from those of the non-layered surfaces, and their thickness was dependent on irradiationtime. Water contact angles of the layered surfaces revealed a transition at approximately 33.5–36.5 �Cas a result of a response to the variation of temperature. There was an increase in the values of the contactangles with an increase in temperature from 26 �C to 44 �C, revealing the nature both hydrophilic andhydrophobic of the surfaces due to a conformational rearrangement of PNIPAAm exposing its isopropylgroups to the liquid drop. This work offers a chemically stable thermosensitive surface (because it is cova-lently structured) with great potential for use as sensors and actuators.

� 2011 Elsevier Inc. Open access under the Elsevier OA license.

1. Introduction

The performance of polymer-based materials for both tradi-tional and modern applications depends not only on structure-re-lated properties, such as malleability, thermal stability, mechanicresistance, but also on their surface properties and interface behav-ior. Applications in which the surface properties play a protagonistrole, such as wettability, adhesion, adsorption, lubrication, biocom-patibility, and permeability [1–5], depend substantially on chemi-cal nature of the polymer device. Chemical modification of apolymer surface is an interesting strategy to extend the use of de-vices that exhibit both properties and structures of well-definedvolume. There are many methodologies that have been developedover last decades with the purpose of producing devices that exhi-bit specified surface properties [6–12]. Physical methods, such asplasma modification, ultraviolet irradiation, and corona discharge,can be used to modify the surface properties of polymers [13–15].These methods are excellent tools to produce functionalized sur-faces with a high covering degree but they require specific operat-ing conditions, such as high energy discharge, ambient withcontrolled atmosphere and low pressure. The chemical modifica-tion of inert surfaces can also be performed by both the function-alization of the polymer surface with the use of oxidizing agents

evier OA license.

in aqueous solution or gas phase and the grafting of molecules witha specific activity [13–17]. Usually the functionalization is enoughto produce surfaces with specific characteristics. However, in somecases, it is the initial step of the modification process that is fol-lowed by the grafting. Modification based on production and onimmobilization of macromolecules is also a useful and interestingway to activate surface of polymers. Polypropylene (PP) is a ther-moplastic polymer of hydrophobic nature with inert surface. Fur-thermore, it is insoluble at room temperature even in organicsolvents, and complete solubility is possible only at high tempera-tures in some organic solvents. PP possesses interesting properties,such as mechanical resistance, flexibility, low specific weight, ther-mostability, and good transparency [18–20], which make it animportant device for uses in biotechnology, and is cheap to pur-chase. Furthermore, PP is also appropriate for in vivo tests becauseit neither produces nor releases fragments of toxic substances in abiological environment. In biotechnology, poly(N-isopropylacryl-amide) (PNIPAAm) has been of great interest because of a transi-tion from a hydrophilic to hydrophobic macromolecule as thetemperature is raised above 30–35 �C. In water, it exhibits revers-ible phase transition at lower critical solution temperature (LCST)with resulting dramatic shrinkage [21–24]. Furthermore, PNIPAAmhas been used in controlled drug release, and cell culture [25,26].The grafting of PNIPPAm on the PP surface to produce smart mate-rials means that this device would have somewhat impact on bio-technology. The grafting of PNIPAAm layer on surfaces has been

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described in the literature [27,28]. In this work, our attention hasbeen focused on developing temperature-sensitivity surfaces bythe grafting of a thin layer of PNIPAAm on PP with a free radical-activated surface. This type of device has not been reported yetand can be recognized as a smart material in terms of state-of-the-art applications as sensors and actuators. The process consistsof anchoring free functional groups on PP, creating a free radical-activated (functionalized) surface, for a further UV-induced photo-polymerization reaction of vinyl monomers through a five-stepsurfacing technique: (1) modification of PP with maleic anhydride(APP), (2) amination of maleic anhydride-modified PP (NH2APP),(3) reaction of maleic anhydride with NH2/anhydride-modifiedPP (ANHAPP), (4) cyclization reaction for immobilizing maleimideon anhydride/NH2/anhydride-modified PP (MALPP), and (5) UV-in-duced polymerization reaction of NIPAAm with the maleimideimmobilized on PP (PNIPAAm-g-MALPP). This work proposes achemically stable thermosensitive surface (because it is covalentlystructured) with great potential for use as sensors and actuators.

2. Materials and methods

2.1. Materials

Polypropylene (Aldrich, isotactic, average Mw � 50.000 g mol�1,melting point 160–165 �C, density 0.9 g mL�1 at 25 �C), acetic anhy-dride (Aldrich, P99%), maleic anhydride (Aldrich, P99%), ethylene-diamine (Aldrich, P99%), acetone (F. Maia, P99.5%), chloroform(Synth, P99.8%), sodium acetate anhydrous (Nuclear, P99%), N-iso-propylacrylamide, NIPAAm, (Acros Organics P98%). The analyticalgrade reagents, expect NIPAAm, were used without furtherpurification.

2.2. PP processing

Isotactic PP surfaces were prepared from PP-based pellets usinga 4 cm-length, 3 cm-width dimension rectangular plate in amechanical heat press. The samples were submitted to pressureof 30.1 MPa at 165 �C for 3 min and subsequently cooled in anice bath. Surface-cleaning process was performed in a glass bal-loon-fitted Soxhlet extractor by passing acetone through the sam-ples for 24 h.

2.3. Synthesis and immobilization of maleimide on the PP surface forphotopolymerization

2.3.1. Grafting reaction of maleic anhydride on the PP surface (APP)PP samples were immersed into a solution consisting of 80 mL

acetic anhydride and 4 g maleic anhydride at temperature of100 �C under strong stirring. After that, 0.44 g of benzoyl peroxidewas introduced to mixture that was kept under stirring for 6 h.After being taken from solution, the samples were washed withacetone (three times, 500 mL each) in order to remove the maleicanhydride in excess and subsequently washed with water by Soxh-let extraction for 24 h.

2.3.2. Amination of the APP surface with ethylenediamine (NH2APP)APP samples were immersed into 50 mL of ethylenediamine for

5 h at 100 �C. After that, the samples were taken from the liquidand washed with acetone by Soxhlet extraction for 24 h.

2.3.3. Treatment of the NH2APP surface with maleic anhydride(ANHAPP)

NH2APP samples were immersed into a solution consisting of2 g maleic anhydride and 30 mL chloroform. The resulting solution

was kept under reflux for 24 h, and subsequently the samples weretaken to be washed with water in the Soxhlet extractor for 24 h.

2.3.4. Cyclization reaction for immobilizing maleimide on the ANHAPPsurface (MALPP)

ANHAPP samples were immersed into 30 mL of acetic anhy-dride containing 0.1 g of sodium acetate anhydrous at 60 �C for24 h. Later, the resulting samples were washed with water in theSoxhlet extractor for 48 h.

2.3.5. UV-induced polymerization reaction of NIPAAm with MALPP(PNIPAAm-g-MALPP)

MALPP samples were introduced into a 1.0 mm-internal diame-ter quartz cuvette that was filled up with aqueous-NIPAAm solu-tion of 1.40 mol L�1 concentration. The mixture-containingcuvette was exposed to the UV irradiation of a low-pressure Hg va-por lamp 250 W at different photopolymerization times. The dis-tance from the lamp to the cuvette was 30 cm. Thereafter,resulting products were washed with water by dispersion in anultrasonic bath 1440 A (Ondontobrás) by applying a frequency of44 kHz. Subsequently the products were washed with water inthe Soxhlet extractor for 48 h in order to remove unreacted NIP-PAm or any other residues that could be only physically sorbedon the samples.

2.3.6. Characterization studies of the final and intermediate productsChemical characterization of the products was performed by

attenuated total reflection in Fourier Transform Infrared (ATR-FTIR) using Bomem model MB-100 equipped with a Pike MIRacleATR accessory at an incident angle of 45� and a ZnSe crystal undernitrogen stream. Atomic force microscopy (AFM) images and val-ues of roughness mean square (RMS) of the surfaces were obtainedin Shimadzu SPM-9500J3. Morphological characterization was per-formed in a scanning electron microscope (Shimadzu, model SS550 Superscan). SEM images were made applying an acceleratingvoltage 15 kV and a current intensity of 30 mA.

Thermosensitive properties of the layered surfaces were evalu-ated by static contact angle variations of water drops on the sur-faces of films as a function of temperature using a contact anglemeter (Tantec, Model Cam-Micro) with a controlled-temperaturemetal cell having a 5-cm2 smooth surface on which the films werecarefully fixed, and equilibrated at 25 �C for 5 min. The contact an-gle measurements of the surfaces layered at different polymeriza-tion times were performed at temperatures of 26 �C, 28 �C, 30 �C,32 �C, 34 �C, 36 �C, 38 �C, 40 �C, 42 �C and 44 �C. Data of contact an-gles were calculated as the average over four points on the surfaceof three different samples, totalizing twelve determinations perfilm.

3. Results and discussion

Maleimides are a special class of vinyl compounds with a strongelectron-acceptor characteristic due to its chemical structure thathas two carbonyl groups bonded to a vinyl group (>C@C<) [29–31]. Vinyl group of the maleimide undergoes electronic transitionto an excited state when exposed to UV irradiation at a specificwavelength. It is important to report that the hydrogen-donatinggroups in its structure are essential to start off polymerizationthrough radical ions resultant from proton–electron transferences[32]. A simple photopolymerization reaction of NIPAAm with MAL-PP is demonstrated in Fig. 1. After the UV light exposition, there isthe formation of a radical ion on the vinyl group of the maleimidethat attacks the carbon–carbon p-bonds of NIPAAm, and thus thephotopolymerization process starts off with the addition of newNIPAAm radicals to the ever-increasing chain. It is important to

Fig. 1. A schema for five-step photopolymerization reaction of NIPAAm with maleimide immobilized on the PP surface: (1) modification of PP with maleic anhydride (APP),(2) amination of maleic anhydride-modified PP (NH2APP), (3) reaction of maleic anhydride with NH2/anhydride-modified PP (ANHAPP), (4) cyclization reaction forimmobilizing maleimide on ANHAPP surface (MALPP), and (5) photopolymerization reaction of NIPAAm with the maleimide immobilized on the PP (PNIPAAm-g-MALPP).

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report that UV-induced polymerization using maleimides as a sen-sitizer allows the control of growth of the polymer layer even whenthe process is performed at room temperature.

Fig. 2. FTIR-ATR spectra of PP, APP, APP treated in a KOH solution (1 mol L�1),NH2APP, ANHAPP, and MAPP.

3.1. ATR-FTIR characterization of the compounds

3.1.1. APPGrafting reactions are versatile techniques for introduction of

functional groups onto polymer-based substrates, including apolarpolymers with low reactivity as PP. The grafting mechanism occursthrough reaction of monomers with a free radical-activated sur-face. The reaction starts off with the formed radicals attackingthe PP, mainly in the branch points where the tertiary carbonsshow stability higher than the secondary carbons. When benzoylperoxide initiator is introduced to reaction medium at the hightemperatures, there is a high selectivity for chemical groups ofPP due to high reactivity of the radical formed from thermaldecomposition of the initiator. The polymer chain reacts then withvinyl carbons of the maleic anhydride. This is followed by anarrangement of the polymerizing system, over which there is theformation of carbon–carbon double bonds between the maleicanhydride and PP chains due to the breakdown of the branchedchains [33]. By means of experimentation, the grafting reactionof maleic anhydride with PP surface can be verified by FTIR-ATRspectroscopy. This technique provides chemical information onthe surface. In Fig. 2, it is shown the FTIR-ATR spectra of PP, APP,

APP treated in a KOH solution (1.0 mol L�1), NH2APP, ANHAPP,and MAPP. The formation of PP was evidenced by the appearanceof the bands at 2950 cm�1 and 2868 cm�1, attributed to asymmet-ric CAH and symmetric CAH stretching frequencies of CH3 groups,

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respectively. The bands at 1456 cm�1and 1370 cm�1 were corre-sponded, respectively, to asymmetric CAH and symmetric CAHbending of CH2 groups.

The bands at 1772 cm�1 and 1740 cm�1 in the spectrum of APPwere assigned, respectively, to asymmetric C@O and symmetricC@O stretching frequencies of functional groups coming fromanhydride, a strong indicative of the grafting. Other relevant bandsthat also indicate the formation of APP were identified as follows:at 1230 cm�1 was attributed to CAO stretching frequency, and at1715 cm�1 was corresponded to C@O stretching frequencies of car-boxylic acids that are groups resultant of the hydrolysis reaction ofthe anhydride. The reactivity of APP was evaluated by treating thesample with a KOH solution of 1.0 mol L�1 concentration for 2 h at50 �C, and subsequently analyzed by FTIR-ATR spectroscopy. Therewas a complete disappearance of the bands at 1772 cm�1 and1740 cm�1 in the spectrum of APP/KOH (Fig. 2); both bands werecorresponded to the anhydride. On the other hand, a weak bandstill appears at 1715 cm�1, referring the formation of carboxylicacids by the hydrolysis reaction of the anhydride. The presenceof such a band can be also justified by the movement of someanhydride molecules inward the APP sample, for which there areno hydrolysis. The functionalization of PP via a heterogeneousreaction results in a distribution of anhydride molecules at bothsurface and interior of PP. The appearance of an intense band at1560 cm�1 was attributed to C@O stretching frequency of carbox-ylate groups resultant of the hydrolysis reaction. The treatment ofAPP with KOH induces the complete hydrolysis of anhydridegroups.

3.1.2. NH2APPThe reaction of a cyclic anhydride with a primary amine issued

from ethylenediamine, for further amination of PP, results in aproduct with a functional amine group and an amine salt, as repre-sented in Fig. 3. The complete disappearance of the bands at1772 cm�1 and 1740 cm�1, owing to asymmetric C@O stretchingfrequencies of both carbonyl groups in the anhydride molecule,in the spectrum of NH2APP (Fig. 2), is an indicative of consumptionof the anhydride over the amination process with ethylenedia-mine. Correspondingly, the band at 1545 cm�1 (NAH bendingmode of primary amines) and 1658 cm�1 (C@O stretching fre-quency of carbonyl groups) are additional evidences of the attach-ment of ethylenediamine moieties on APP. The appearance of theband at 1704 cm�1 in the spectrum of NH2APP, corresponded toC@O stretching frequency of carbonyl groups in the carboxylicacids, indicates that the negatively charged carboxylate groupsformed over the reaction were converted to carboxylic acid groups.This also suggests that the amine salts were removed during theextraction process.

3.1.3. ANHAPPThe intensity of the band at 1545 cm�1 in the spectrum of

NH2APP (Fig. 2) decreased in the spectrum of ANHAPP. This meansa significant consumption of amide groups during the reaction. Thepresence of carboxylic acids is confirmed by the stretching band at1715 cm�1, and the amine groups by the bending band at1635 cm�1.

Fig. 3. A drawing of the reaction between a cyclic anhydride and a primary amineissued from ethylenediamine.

3.1.4. MALPPThe cyclization reaction for maleimide is a dehydration process

that occurs more efficiently at high temperatures. At low temper-atures, it is required a catalyst agent. In such case, the maleimidering can be formed with the use of sodium acetate as catalyst inan acetic anhydride medium [34]. The specimens that have beenformed on the substrate surface over the cyclization are detectableby FTIR-ATR spectroscopy. There was a complete disappearance ofthe band at 1635 cm�1 in the spectrum of MALPP (Fig. 2), owing toNAH bending mode of amide groups, but a discrete band was ob-served at 1654 cm�1 that corresponds to C@O stretching frequencyof amide groups, indicatives of formation of the maleimide. On theother hand, it was expected a decrease in intensity of the band at1715 cm�1 that corresponds to C@O stretching frequency of car-boxylic groups, predicted to be consumed over the reaction. How-ever, the unpredicted increase in its intensity was associated to anoverlap of the band that corresponds to C@O stretching frequencyof the maleimide [35].

3.1.5. PNIPAAm-g-MALPPFig. 4 shows the FTIR-ATR spectra of MALPP, PNIPAAm-g-MALPP

for photopolymerization times of 30 min, 45 min, 60 min, and90 min.

The bands at 1643 cm�1 and 1541 cm�1 in the spectrum of PNI-PAAm-g-MALPP are characteristic stretching frequencies of C@Oand NAH groups, respectively, indicating the formation of PNI-PAAm. There was also an important increase in intensity of bothbands for longer photopolymerization times. The intensity of bothbands may be viewed as a parameter of the formation of a PNI-PAAm-structured layer. High-intensity bands can indicate an in-crease in the PNIPAAm layer on PP. Furthermore, the completedisappearance of the band at 1620 cm�1, owing to C@C stretchingfrequency of NIPAAm monomers, is an evidence of both the poly-merization reaction and the total consumption of NIPAAm [36].

3.2. Surface characteristics

Fig. 5 shows the scanning electronic microscopy (SEM) imagesof PP (a), APP (b), MALPP (c), and PNIPAAm-g-MALPP for irradiationtimes of 30 min (d), 45 min (e), 60 min (f), and 90 min (g). PPshowed smooth surface with homogenous mass distribution, aclearly different appearance from those of APP (surface with nar-row openings) and MALPP (surface with depressions). The graftingof NIPAAm monomers on MALPP has significantly changed the sur-face morphology of the substrate, as demonstrated in Fig. 5d–g. It

Fig. 4. FTIR-ATR spectra of MALPP, and PNIPAAm-g-MALPP for photopolymeriza-tion times of 30 min, 45 min, 60 min and 90 min.

Fig. 5. Scanning electronic microscopy (SEM) images of PP (a), APP (b), MALPP (c), and PNIPAAm-g-MALPP for photopolymerization times of 30 min (d), 45 min (e), 60 min (f),and 90 min (g).

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can be suggested that the PNIPAAm layer covering the PP surfacecorrects, to some extension, both depressions and fissures of thepreviously modified samples. Furthermore, this effect becamemore prominent for longer photopolymerization times. Morpho-logical characterization of the samples was followed by atomicforce microscopy 3D images. This type of AFM images gives an in-sight onto surface modification and allows for a better evaluationof the morphological structure.

Fig. 6 shows the AFM 3D images of PP (a), APP (b), MALPP (c),and PNIPAAm-g-MALPP for irradiation times of 30 min (d),45 min (e), 60 min (f) and 90 min (g).

AFM images of Fig. 6b-c reveal irregular surfaces with localizedmass distribution (APP) and depressions (MALPP), as observed bySEM imaging. The morphological irregularities on the surfaceswere somewhat corrected by the grafting of NIPAAm, dependingon photopolymerization time. There is, in fact, the formation of asurface-improving PNIPAAm layer by the structural modifications.Roughness mean square (RMS) of the surfaces was calculated toprovide better compression of the structural changes. RMS was cal-culated using the same application software which controls theatomic force microscope. The values of RMS in each of the fivereaction steps are the average over four different AFM images.

In Fig. 7, it is shown a RMS curve of PP, APP, NHAPP, ANHAPP,MALPP, and PNIPAAm-g-MALPP for irradiation times of 30 min,

45 min, 60, and 90 min. The RMS curve was elaborated classifyingthe materials obtained in each of the five reaction steps, as de-scribed in Fig. 1. The RMS increases to a maximum value withthe progression of the surface modification (reaction steps rangingfrom 1 to 4) but it decreases appreciably with the grafting time ofNIPAAm (step 5) on the previously modified surfaces: 73 nm for30 min, 52 nm for 45 min, and 32 nm for 60 min.

The thickness of the PNIPAAm layer covering MALPP surfacewas visualized by SEM images of the fractured samples after beingfrozen in liquid nitrogen. Fig. 8 shows the SEM images of PNI-PAAm-g-MALPP for irradiation times of 45 min (a), 60 min (b),and 90 min (c). In all micrographs, it is observed a thin layer withmorphological characteristic different from that of the non-layeredsurfaces (Fig. 5), and its thickness was 1880 nm for 45 min,2510 nm for 60 min and 3810 nm for 90 min, showing an explicitdependence on irradiation time. This relation was also verified bygravimetry: 500 nm (30 min), 1890 nm (45 min), 2700 nm(90 min), and 3650 nm (120 min). These values were estimatedapplying the following equation [35]:

Tl ¼Dw

A:d

where Dw is the variation on the substrate mass after the photopo-lymerization, Tl and A are, respectively, the thickness and film

Fig. 6. Atomic force microscopy (AFM) 3D images of PP (a), APP (b), MALPP (c), and PNIPAAm-g-MALPP for photopolymerization times of 30 min (d), 45 min (e), 60 min (f)and 90 min (g).

Fig. 7. Roughness mean square (RMS) as a function of the chemical nature of PP,APP, NH2APP, ANHAPP, MALPP, and PNIPAAm-g-MALPP for photopolymerizationtimes of 30 min, 45 min, 60 min, and 90 min. The order of the films was consideredfollowing the reaction steps of Fig. 1.

Fig. 8. Scanning electronic microscopy (SEM) images of PNIPAAm-g-MALPP forphotopolymerization times of 45 min (a), 60 min (b) and 90 min (c).

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surface area before and after the photopolymerization, and d is thedensity of PNIPAAm (1.269 g cm�3).

3.3. Thermosensitive properties of PNIPAAm-g-MALPP surface

Fig. 9 shows the static contact angles of water drops on the sur-faces of PP, APP, NH2APP, ANHAPP, and MALPP. The contact angleof PP was 92� ± 1�, corresponding to a hydrophobic surface. ForAPP, contact angle decreased to 78� ± 1� due to the presence of car-boxylate groups of the anhydride that gives a somewhat hydro-philic character to the PP surface. This effect becomes moreevident with the introduction of ethylenediamine to the APP sur-face (NH2APP) that showed a contact angle of 75� ± 1�, revealinga less hydrophobic nature. This suggests that the nitrogen groups,which can establish interactions with water by way of hydrogen

bonds, are localized at the air/substrate interface. It follows that,with the introduction of more anhydride groups (ANHAPP), therewas an appreciable decrease in the angle contact to 64� ± 1� thatindicates the presence of both free nitrogen and carboxylategroups at the air/substrate interface. However, after the formationof the maleimide by the cyclization (MALPP), the contact angle in-creased to 76�, indicating a character less hydrophilic than that of

Fig. 9. Static contact angles of water drops on the surfaces of PP, APP, NH2APP,ANHA PP, and MALPP. Fig. 10. Temperature-dependent contact angle variations on the surfaces of MALPP

and PNIPAAm-g-MALPP for irradiation times of 30 min, 45 min, 60 min, and 90 min.

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ANHAPP due to the absence of both free nitrogen and carboxylategroups on the surface.

Fig. 10 shows the temperature-dependent contact angle varia-tions on the surfaces grafted by a PNIPAAm layer for irradiationtimes of 30 min, 45 min, 60 min, and 90 min. For comparison, thecontact angle of MALPP (before coating with a PNIPAAm layer)was also showed in Fig. 10 as a function of temperature. It didnot exhibit any variation on the surface properties for any of thetemperatures measured.

The LCST of the layered surfaces was determined from theinflection point on the first derivative curves of contact angle ver-sus temperature (dCA/dt). On the other hand, with temperatureranging from 26 �C to 44 �C, the contact angle of the layered sur-faces exhibited sigmoid-like curves with a transition at

Fig. 11. A drawing illustrating the effect of grafting layer on surface wettability for PNIPA

35.0 ± 1.0 �C (30 min), 33.5 ± 1.0 �C (45 min), 36.5 ± 1.0 �C(60 min), and 35.5 ± 1.0 �C (90 min), showing the effect of graftinglayer on surface wettability. Furthermore, there was an increase inthe values of contact angles with an increase in temperature,revealing the nature both hydrophilic and hydrophobic of the lay-ered surfaces. Upon contact with a drop of water above the LCST,PNIPAAm undergoes a conformational rearrangement exposingits isopropyl groups to the liquid. Isopropyl groups are hydropho-bic and tend to displace water. The hydrophobic effect of the lay-ered surface was attributed to these groups in particular, becausePNIPAAm possesses hydrophilic domains even at temperaturesabove LCST [37]. A drawing illustrating the effect of grafting layeron surface wettability for PNIPAAm-g-MALPP substrate at thetemperatures below and above the LCST is shown in Fig. 11. The

Am-g-MALPP substrate at the temperatures below and above the LCST of PNIPAAm.

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PNIPAAm-g-MALPP substrates showed thermosensitive surfacesand have a great potential for use as sensors and actuators.

4. Conclusions

Thermosensitive surfaces were developed by the grafting of aPNIPAAm layer on the PP substrate though a five-step surfacingtechnique: (1) modification of PP with maleic anhydride, (2) ami-nation of maleic anhydride-modified PP, (3) reaction of maleicanhydride with NH2/anhydride-modified PP, (4) cyclization reac-tion for immobilizing maleimide on the anhydride/NH2/anhy-dride-modified PP, and (5) UV-induced polymerization reactionof NIPAAm with the maleimide immobilized on PP. The mechanismreaction involved in each of the five reaction steps was character-ized by FTIR-ATR spectroscopy. The grafting mechanism of the PNI-PAAm layer occurs through reaction of vinyl monomers with thefree radical-activated PP surface. PNIPAAm layer covering PP sur-face corrected, to some extension, both depressions and fissuresof the previously modified samples. Furthermore, this effect be-came more prominent for longer photopolymerization times. Thelayered surfaces have morphological characteristic different fromthose of the non-layered surfaces, and their thickness was depen-dent on irradiation time. Temperature-dependent contact anglevariations of the layered surfaces exhibited sigmoid-like curveswith a transition at 35.0 ± 1.0 �C (30 min), 33.5 ± 1.0 �C (45 min),36.5 ± 1.0 �C (60 min), and 35.5 ± 1.0 �C (90 min), revealing the nat-ure both hydrophilic and hydrophobic of the layered surfaces dueto a conformational rearrangement of PNIPAAm exposing its iso-propyl groups to the water drop. The variation of contact anglesas a function of temperature shows that the ultrathin layers of PNI-PAAm have thermosensitive properties, demonstrating that the PPfilms covered with PNIPAAm have a great potential for applicationsin which the PNIPAAm is frequently explored. This work proposes achemically stable thermosensitive surface (because it is structuredon covalent bonds) with great potential for use as sensors andactuators.

Acknowledgments

TSPC and GMP are grateful to Conselho Nacional de Desenvolvi-mento Tecnológico (CNPq-Brazil) for master degree fellowships.MRM thanks the CNPq for doctoral fellowship. A.F.R acknowledgesthe financial supports given by CNPq, Coordenação de Aperfeiçoa-mento de Pessoal de Nível Superior (CAPES-Brazil), and Fundação

Araucária-Brasil. M.R.G. thanks the CAPES/PNPD for post-doctoralfellowship.

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