Development of Styrene-Grafted Polyurethane by Radiation-Based Techniques

materials

Article Development of Styrene-Grafted Polyurethane by Radiation-Based Techniques

Jin-Oh Jeong, Jong-Seok Park * and Youn-Mook Lim

Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute, 1266 Sinjeong-dong, Jeongeup-si, Jeollabuk-do 580-185, Korea; [email protected] (J.-O.J.); [email protected] (Y.-M.L.) * Correspondence: [email protected]; Tel./Fax: +82-63-570-3067

Academic Editor: Volker Altstädt Received: 23 February 2016; Accepted: 31 May 2016; Published: 2 June 2016

Abstract: Polyurethane (PU) is the fifth most common polymer in the general consumer market, following Polypropylene (PP), Polyethylene (PE), Polyvinyl chloride (PVC), and Polystyrene (PS), and the most common polymer for thermosetting resins. In particular, polyurethane has excellent hardness and heat resistance, is a widely used material for electronic products and automotive parts, and can be used to create products of various physical properties, including rigid and flexible foams, films, and fibers. However, the use of polar polymer polyurethane as an impact modifier of non-polar polymers is limited due to poor combustion resistance and impact resistance. In this study, we used gamma irradiation at 25 and 50 kGy to introduce the styrene of hydrophobic monomer on the polyurethane as an impact modifier of the non-polar polymer. To verify grafted styrene, we confirmed the phenyl group of styrene at 690 cm´1 by Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) and at 6.4–6.8 ppm by 1H-Nuclear Magnetic Resonance (1H-NMR). Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), Thermogravimetric Analysis (TGA) and contact angle analysis were also used to confirm styrene introduction. This study has confirmed the possibility of applying high-functional composite through radiation-based techniques.

Keywords: polyurethane; styrene; grafting polymerization; gamma-irradiation

1. Introduction Due to their excellent physical properties and low price, polyolefin-based polymers have recently gained wide use in daily life. This class of general-purpose polymers forms the largest section of the consumer market, partly because they are commonly used in household goods and industrial structural materials [1–3]. Polyolefin-based polymer resins, such as Polypropylene (PP), Polyethylene (PE), Polyvinyl chloride (PVC), and Polystyrene (PS), have been most frequently used. Their excellent performance mechanical strength and workability make them quite versatile, but their low impact resistance is a disadvantage. Accordingly, studies are actively underway to improve this property [4]. In contrast, polyurethane (PU) has excellent impact resistance, mechanical strength, thermal stability, and chemical resistance. PU has been used in the production of flexible foams for beds, automobiles and sofas, rigid foams used as an insulating material of refrigerators and buildings, paints, adhesives, spandex fibers, and thermoplastic polyurethane [5–7]. PU is the fifth most commonly used polymer in consumer markets, following PP, PE, PVC, and PS, and is the most commonly used polymer in thermosetting resins. PU has excellent physical properties, such as elasticity and flexibility similar to rubber, and is relatively stable in the body. To improve heat resistance, blending of PU and polyolefin-based polymers was required [8]. These blended composites were developed for use in branching and to improve upon disadvantages of the individual polymers [9].

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Materialscomposites2016, 9were, 441 developed for use in branching and to improve upon disadvantages of 2the of 9 individual polymers [9]. However, difficulties arise in the blending of hydrophobic polyolefin-based polymers and the However, difficulties arise in the blending of hydrophobic polyolefin-based polymers and the hydrophilic polyurethane polymer. This blending reduces problems with mechanical strength and hydrophilic polyurethane polymer. This blending reduces problems with mechanical strength and can can be achieved using chemical additives or surfactants or polymer modification techniques [10]. be achieved using chemical additives or surfactants or polymer modification techniques [10]. The grafting polymerization reaction is a method of polymerizing a monomer onto a polymer The grafting polymerization reaction is a method of polymerizing a monomer onto a polymer used for branching a polymer chain onto a substrate polymer. The polymers made through grafting used for branching a polymer chain onto a substrate polymer. The polymers made through grafting polymerization are durable and maintain their physical properties [11,12]. polymerization are durable and maintain their physical properties [11,12]. The typical polymer modification techniques use chemical modification, plasma, and radiation The typical polymer modification techniques use chemical modification, plasma, and [13]. Disadvantages of chemical modification include long process time, complicated process, and radiation [13]. Disadvantages of chemical modification include long process time, complicated process, required chemical additives, such as initiators and catalysts. Conversely, radiation technology, which and required chemical additives, such as initiators and catalysts. Conversely, radiation technology, works through formation of free radicals due to ionization of the materials and the monomers with which works through formation of free radicals due to ionization of the materials and the monomers gamma-irradiation and electron beam energy has the advantage that the monomers can be grafted with gamma-irradiation and electron beam energy has the advantage that the monomers can be without chemical additives [14,15]. In addition, radiation can be used in solid state or at low grafted without chemical additives [14,15]. In addition, radiation can be used in solid state or at low temperatures. Radiation can also be employed in a short period of time, which has the advantage of temperatures. Radiation can also be employed in a short period of time, which has the advantage of minimized energy consumption [16,17]. minimized energy consumption [16,17]. In this study, the blending of polyolefin-based polymers and PU was completed by grafting In this study, the blending of polyolefin-based polymers and PU was completed by grafting hydrophobic monomers of styrene onto PU using gamma irradiation. Polymer grafting was verified hydrophobic monomers of styrene onto PU using gamma irradiation. Polymer grafting was verified through characterization with ATR-FTIR, SEM, NMR, XPS, and TGA analysis. The possibility of high- through characterization with ATR-FTIR, SEM, NMR, XPS, and TGA analysis. The possibility of performance composite materials using radiation techniques has been confirmed. high-performance composite materials using radiation techniques has been confirmed. 2. Experimental 2. Experimental

2.1. Materials Methylene diphenyldiphenyl diisocyanate diisocyanate (MDI)-based (MDI)-based polyurethane polyurethane (PU) was(PU) purchased was purchased from Songwon from IndustrialSongwon Industrial Co., Ltd. Co., (Ulsan, Ltd. Korea).(Ulsan, Korea). The average The average molecular molecular weight weight was approximately was approximately 80,000. Styrene80,000. Styrene monomer monomer was purchased was purchased from from Showa Showa Chemical Chemical Co. Co. (Tokyo, (Tokyo, Japan). Japan). MethanolMethanol and tetrahydrofuran (THF) (THF) were were purcha purchasedsed from from Duksan Duksan reagent reagent (Ansan, (Ansan, Korea). Korea). All Allother other reagents reagents and andsolvents solvents were were of analytical of analytical grade grade and andwere were used used as received. as received.

2.2. Preparation of Styrene Grafted Polyurethane The 1010 wtwt %% PU PU was was dissolved dissolved in in THF THF solution solution at roomat room temperature. temperature. Styrene Styrene monomer monomer was laterwas immersedlater immersed in PU/THF in PU/THF solution solution at different at different concentrations concentrations (5 wt %, (5 10 wt wt %, %, 10 and wt 20 %, wt and %). 20 The wt prepared %). The mixturesprepared weremixtures exposed were to exposed a gamma to 60a gammaCo sources 60Co (ACEL sources type (ACEL C-1882, type Korea C-1882, Atomic Korea Energy Atomic Research Energy Institute)Research withInstitute) a total with absorbed a total doseabsorbed ranging dose from ranging 25 to from 50 kGy 25 (10to 50 kGy/h). kGy (10 To kGy/h). remove To the remove remaining the solvent,remaining the solvent, PU was the precipitatedPU was precipitated three times three in times methanol in methanol and subsequentlyand subsequently vacuum-dried vacuum-dried for 48for h.48 Theh. The overall overall pattern pattern diagrams diagrams are are shown shown on on Figure Figure1 and1 and Scheme Scheme1. 1. The The characteristics characteristics of of styrene-grafted polyurethane areare shownshown inin TableTable1 1..

Figure 1. Schematic diagrams of styrene grafted polyurethane.

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Table 1. Characteristics of styrene-grafted polyurethane. Table 1. Characteristics of styrene-grafted polyurethane. Concentration of Styrene Irradiation Dose Samples Samples Concentration(%) of Styrene (%) Irradiation(kGy) Dose (kGy) 5SPU255SPU25 55 25 25 10SPU2510SPU25 1010 25 25 20SPU2520SPU25 2020 25 25 5SPU505SPU50 55 50 50 10SPU5010SPU50 1010 50 50 20SPU5020SPU50 2020 50 50

SchemeScheme 1. 1. Radiation-inducedRadiation-induced grafting grafting polymerization. polymerization.

2.3.2.3. Attenuated Attenuated Total Total Reflection Reflection Fourier Fourier Transform Transform Infrared Spectroscopy (ATR-FTIR) InfraredInfrared spectra spectra of of the the PU PU and and styrene styrene grafted grafted PU PU (SPU) (SPU) were were recorded recorded using using Bruker Bruker TEMSOR TEMSOR ´1 3737 (Bruker (Bruker AXS. Inc.,Inc., Karlsruhe,Karlsruhe, Germany) Germany) equipped equipped with with ATR ATR mode mode over over the rangethe range of 650 of to650 4000 to cm4000 ´1 cmat a−1 resolutionat a resolution of 6 cmof 6 cmaveraged−1 averaged over over 64 scans. 64 scans. The conversionThe conver ratesion ofrate phenyl of phenyl was calculatedwas calculated using usingthe following the following equation: equation: ConversionrateConversion ratep%%q “= r11−´ pPu´−Pmqs ˆ×100% 100% (1) (1) where Pu and Pm represent the peak intensity (670–725 cm−1) of unmodified PU and modified PU, where P and P represent the peak intensity (670–725 cm´1) of unmodified PU and modified respectively.u m PU, respectively. 2.4. Scanning Electron Microscope (SEM) 2.4. Scanning Electron Microscope (SEM) To observe the high-resolution images of the PU and SPU with increasing the concentration of To observe the high-resolution images of the PU and SPU with increasing the concentration of styrene and irradiation dose, the polymers were coated with a layer of gold for 60 seconds by sputter styrene and irradiation dose, the polymers were coated with a layer of gold for 60 seconds by sputter coating. The morphology of the polymers was investigated using SEM (JSM-6390, JEOL, Tokyo, coating. The morphology of the polymers was investigated using SEM (JSM-6390, JEOL, Tokyo, Japan) Japan) with an electron beam of 10 kV and a working distance of 10 to 12 mm. with an electron beam of 10 kV and a working distance of 10 to 12 mm.

2.5.2.5. Blue Blue Ink Ink Staining Staining TheThe samples samples stained with slowslow shakingshaking in in a a 24 24 well well plate plate containing containing 1 mL1 mL of of water-based water-based blue blue ink ink(PILOT, (PILOT, 30cc, 30cc, Seongnam, Seongnam, Korea) Korea) for 4 hfor at 4 room h at temperature.room temperature. Each sample Each sample was washed was washed three times three for times12 h in for deionized 12 h in deionized water (DIW). water (DIW).

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2.6.2.6.1 1H-NuclearH-Nuclear MagneticMagnetic ResonanceResonance (1H-NMR)H-NMR) TheThe structure structure of PUof PU and SPUand wereSPU determinedwere determined by 500 MHzby 5001H-NMR MHz (ECA1H-NMR 500 MHz(ECA spectrometer, 500 MHz JEOL,spectrometer, Tokyo, Japan) JEOL, with Tokyo, averaged Japan) over with 32 averaged scans. Next, over 3.5 32 mg scans. PU andNext, SPU 3.5 were mg PU dissolved and SPU in 0.5were mL THF-ddissolved8 solution, in 0.5 mL and THF-d product8 solution, spectra and were product obtained. spectra were obtained.

2.7.2.7. X-ray X-ray PhotoelectronPhotoelectron SpectroscopySpectroscopy (XPS)(XPS) AnalysisAnalysis of of PU PU and and SPU SPU was was performed performed on on X-ray X-ra photoelectrony photoelectron spectra spectra (XPS, (XPS, Theta Theta Probe Probe AR-XPS AR- System,XPS System, Thermo Thermo Fisher Fisher Scientiﬁc, Scientific, Waltham, Waltham, MA, USA)MA, USA) ﬁtted withfitted anwith Al an Kα Alsource Kα source (soft X-ray (soft sourceX-ray atsource 1486.6 at eV, 1486.6 which eV, is which monochromated). is monochromated). The anode The was anode operated was operated at 150W at (15150 kV).W (15 kV).

2.8.2.8. Thermogravimetric Thermogravimetric AnalysisAnalysis (TGA)(TGA) ThermogravimetricThermogravimetric analysisanalysis was performed using using TA TA Q600 Q600 (TA (TA Instrument, Instrument, New New Castle, Castle, PA, PA, USA).USA). Next,Next, 15.015.0 mgmg PUPU andand SPUSPU werewere preparedprepared andand placedplaced in a platinum pan. The The analysis analysis was was performedperformed at at a a heating heating raterate 1010˝ °C/minC/min from from 40 40 to to 700 700 °C˝C under under nitrogen nitrogen flow. ﬂow.

2.9.2.9. Contact Contact AngleAngle TheThe staticstatic waterwater contactcontact anglesangles werewere measuredmeasured according to to the the sessile sessile drop drop method method using using a a PhoenixPhoenix 300 300 contact contact angleangle analyzeranalyzer (Surface(Surface Electro Optics Co., Suwon, Korea). Korea). Regarding Regarding the the water water contactcontact angle, angle, at at least least ﬁvefive specimensspecimens werewere tested,tested, andand thethe average value was taken.

3.3. Results and Discussion AsAs shown shown in in Scheme Scheme1, 1, it it is is possible possible to to infer infer two tw possibleo possible routes routes of theof the styrene styrene grafting grafting onto onto PU PU via radiation.via radiation. Initially, Initially, the double the double bond ofbond the styreneof the styrene was broken was bybroken radiation, by radiation, causing radicalcausing formation radical information the branch in and the styrenebranch graftingand styrene to the grafting carbonyl to group the carbonyl of PU. The group second of reactionPU. The demonstrates second reaction how styrenedemonstrates could be how grafted styrene onto could the aminebe grafted group onto of PU,the amine whereas group polystyrene of PU, whereas could be polystyrene produced bycould free radicalbe produced polymerization, by free radical from polyme non-graftedrization, styrene. from non-grafted styrene. FigureFigure2 2illustrates illustrates the the chemical chemical properties properties ofof PUPU andand SPU composites investigated by by ATR-FTIR ATR-FTIR analysis.analysis. AsAs shownshown inin thethe SPUSPU spectra,spectra, aa newnew signalsignal fromfrom aa phenylphenyl groupgroup waswas observedobserved at 690 cm´−1 1 duedue to to grafting grafting polymerization polymerization of styreneof styrene on the on PU. the In PU. addition, In addition, the intensity the ofintensity peak was of signiﬁcantlypeak was increasedsignificantly with increased increases with in the increases concentration in the concentration of styrene and of radiation styrene and dose. radiation Table2 shows dose. thatTable the 2 conversionshows that ratethe conversion of phenyl grouprate of wasphenyl 56.6% group in the wa 5s 56.6% wt % styrene-graftedin the 5 wt % styrene-grafted PU, whereas PU, with whereas 20 wt % styrene-graftedwith 20 wt % styrene-grafted PU, it increased PU, to 82.8% it increased under ato radiation 82.8% under dose a of radiation 25 kGy. Thedose conversion of 25 kGy. rate The of styrene-graftedconversion rate PU of styrene-grafted at a radiation dose PU at of a 50 radiation kGy also dose increased. of 50 kGy The also styrene increased. content The increases styrene content with the conversionincreases with rate the of the conversion phenyl group. rate of the phenyl group.

FigureFigure 2. SurfaceSurface chemical chemical properties properties of styrene of styrene grafted polyurethane grafted polyurethane by Attenuated by Attenuatedtotal reflectance- total reﬂectance-FourierFourier transform transforminfrared spectro infraredscopy spectroscopy (ATR-FTIR) (ATR-FTIR) at dose rate: at dose (a) 25 rate: kGy; (a )and 25 kGy;(b) 50 and kGy. (b ) 50 kGy.

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Materials 2016, 9, 441 5 of 9 Table 2. Conversion rate of aromatic (phenyl ring) of SPU from ATR-FTIR. Table 2. Conversion rate of aromatic (phenyl ring) of SPU from ATR-FTIR.

SamplesSamples Conversion Conversion Rate Rate (%) (%) 5SPU255SPU25 56.6 56.6 10SPU2510SPU25 60.6 60.6 20SPU2520SPU25 82.8 82.8 5SPU505SPU50 59.8 59.8 10SPU5010SPU50 74.4 74.4 20SPU50 87.1 20SPU50 87.1

Figure 33 showsshows the SEM micrographs of of surface surface morphologies morphologies of of PU PU and and SPU SPU composites composites at differentat different styrene styrene contents contents and and radiation radiation doses. doses. The surface The surface morphologies morphologies of unmodified of unmodiﬁed PU were PU almostwere almost smooth smooth but became but became rougher rougher with increasing with increasing styrene styrene content content and radiation and radiation dose dose[5,6]. [The5,6]. surfaceThe surface morphology morphology of the of theSPU SPU composites composites were were dependent dependent on on the the styrene styrene content content and and radiation radiation dose [18,19]. [18,19]. Unmodified Unmodiﬁed PU PU was was stained stained blue blue wi withth increasing increasing styrene styrene content, content, while while the the SPU SPU was only stained light blue due to presence of hydrophobic styrene groups.

Figure 3.3.Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) images images of (a) polyurethane, of (a) polyurethane, styrene grafted styrene polyurethane grafted polyurethanedose rate at 25 dose kGy; rate (b) at 5 25 wt kGy; %; (c ()b 10) 5 wt wt %; %; ( (dc)) 2010 wtwt %; dose(d) 20 rate wt %; at 50dose kGy; rate (e at) 550 wt kGy; %; ((fe)) 10 5 wt wt %; %; (andf) 10 ( gwt) 20 %; wt and %. (g) 20 wt %.

Figure 4 shows the 1H-NMR spectrum of PU and 20SPU50. The peaks at 4.1 and 2.6 ppm Figure4 shows the 1H-NMR spectrum of PU and 20SPU50. The peaks at 4.1 and 2.6 ppm corresponded to the characteristic methylene protons of COOCH2CH2, respectively. Additionally, the corresponded to the characteristic methylene protons of COOCH2CH2, respectively. Additionally, the peak at 3.6 ppm assigned to the methyl proton of OCH3 was observed [20]. After grafting, the peak peak at 3.6 ppm assigned to the methyl proton of OCH was observed [20]. After grafting, the peak of aromatic ring of styrene at 6.4–6.8 ppm was observed3 [21,22]. Moreover, the peak of methylene of aromatic ring of styrene at 6.4–6.8 ppm was observed [21,22]. Moreover, the peak of methylene protons of styrene appeared at 2.1 ppm. This result suggest that the SPU has been successfully protons of styrene appeared at 2.1 ppm. This result suggest that the SPU has been successfully grafted. grafted.

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1 1 FigureFigure 4. 1H-Nuclear 4. H-Nuclear magnetic magnetic resonance resonance ( 1(H-NMR)H-NMR) spectra spectra of of (a () apolyurethane;) polyurethane; and and(b) 20% (b) styrene 20% styrene grafted polyurethane at 50 kGy. grafted polyurethane at 50 kGy.

1 1 FigureThe XPS 4. H-Nuclear spectra of magnetic the PU resonanceand SPU (areH-NMR) shown spectra in Figure of (a )5 polyurethane; according to and styrene (b) 20% content. styrene The Thestrong XPSgrafted carbon spectra polyurethane peak of thetogether PUat 50 and kGy.with SPU oxygen are shownpeak of in PU Figure and SPU5 according indicated to binding styrene energies content. of The287.5 strong carbonand peak 533 together eV, respectively with oxygen [6]. The peak atomic of PUconcentrat and SPUions indicated with the styrene binding content energies and ofradiation 287.5 and dose 533 eV, The XPS spectra of the PU and SPU are shown in Figure 5 according to styrene content. The respectivelyare presented [6]. The in atomicTable 3. concentrationsThe carbon and withoxygen the co styrenentent of contentPU were andfound radiation to be 76.4% dose and are 21.2%, presented strongrespectively. carbon Carbonpeak together content with was oxygensignificantly peak incrof PUeased and after SPU styrene indicated grafting; binding it wasenergies found of to 287.5 be in Table3. The carbon and oxygen content of PU were found to be 76.4% and 21.2%, respectively. and78.1%, 533 80.1%eV, respectively and 81.1% for[6]. the The 5 atomicwt %, 10 concentrat wt %, andions 20 wtwith % thestyrene styrene reacted content under and a radiationradiation dose dose Carbonareof contentpresented25 kGy. Conversely, was in Table significantly 3. the The oxygen carbon increased content and oxygen afterwas foun styrenecontentd to ofbe grafting; PU19.6%, were 17.0%, itfound was and to found be15.4%, 76.4% to respectively, be and 78.1%, 21.2%, 80.1% and 81.1%respectively.indicating for the that Carbon 5 the wt hydrophobicity %,content 10 wt was %, significantly and of SPU 20 wtwas % incrsignificantly styreneeased after reacted increased styrene under due grafting; to a radiationthe itradiation-induced was found dose ofto 25be kGy. Conversely,78.1%,styrene 80.1% thegrafting. oxygen and 81.1%It was content for possible the was5 wt to found%, verify 10 wt tothat %, be andthe 19.6%, 20carbon wt 17.0%, % contentstyrene and isreacted 15.4%,higher under and respectively, composeda radiation indicating ofdose a that theofhydrophobic 25 hydrophobicity kGy. Conversely, structure of [14,23].the SPU oxygen The was carboncontent significantly content was foun of increased styrened to be grafted 19.6%, due PU to17.0%, at the 50 and radiation-inducedkGy 15.4%, also increased, respectively, but styrene grafting.indicatingoxygen It was content that possible the decreased hydrophobicity to verify at 25 kGy. that of theThe SPU carbonamount was si gnificantly contentof carbon isin increased higher20SPU50 and dueincreased composed to the by radiation-induced 6.2% of more a hydrophobic than styrene grafting. It was possible to verify that the carbon content is higher and composed of a structurefor 20SPU25, [14,23]. but The the carbon amount content of oxygen of decreased styrene grafted by 4.9%. PUThis at finding 50 kGy is most also likely increased, observed but due oxygen hydrophobicto grafting of structure styrene into [14,23]. the carbonylThe carbon group content of polyurethane of styrene grafted through PU radiation. at 50 kGy also increased, but content decreased at 25 kGy. The amount of carbon in 20SPU50 increased by 6.2% more than for oxygen content decreased at 25 kGy. The amount of carbon in 20SPU50 increased by 6.2% more than 20SPU25,for 20SPU25, but the but amount the amount of oxygen of oxygen decreased decreased by 4.9%.by 4.9%. This This finding finding isis mostmost likely observed observed due due to graftingto grafting of styrene of styrene into the into carbonyl the carbonyl group group of polyurethane of polyurethane through through radiation. radiation.

Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of styrene grafted polyurethane at dose rate: (a) 25 kGy; and (b) 50 kGy.

Table 3. Atomic concentration with the styrene content and radiation dose. Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of styrene grafted polyurethane at dose rate: Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of styrene grafted polyurethane at dose rate: (a) 25 kGy; and (b) 50 kGy.Samples Carbon (%) Oxygen (%) (a) 25 kGy; and (b) 50 kGy. PU 76.4 21.2 Table 3. Atomic5SPU25 concentration with78.1 the styrene content 19.6 and radiation dose. Table 3. Atomic10SPU25 concentration with80.1 the styrene content 17.0 and radiation dose. Samples Carbon (%) Oxygen (%) 20SPU25 81.1 15.4 Samples5SPU50PU Carbon76.477.6 (%) 20.3Oxygen 21.2 (%) 5SPU2510SPU50 78.180.5 16.5 19.6 PU 76.4 21.2 10SPU2520SPU50 80.187.4 10.6 17.0 5SPU25 78.1 19.6 20SPU25 81.1 15.4 10SPU255SPU50 77.6 80.1 20.3 17.0 20SPU25 81.1 15.4 10SPU50 80.5 16.5 5SPU50 77.6 20.3 20SPU50 87.4 10.6 10SPU50 80.5 16.5

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As shown in Figure Figure 6 and Table4 4,, thethe thermalthermal stability of of the the PU PU and and SPU SPU was was determined determined by byTGA. TGA. At Atthe theinitial initial degradation degradation temperature temperature of approximately of approximately 300 300°C, when˝C, when the styrene the styrene content content was wasincreased, increased, thermal thermal stability stability was wasalso alsoincreased increased [24]. [In24 ].addition, In addition, the new the newdegradation degradation peaks peaks in SPU in SPUcomposites composites were were obviously obviously observed observed at 400–460 at 400–460 °C ˝dueC due to tothe the decomposition decomposition of of styrene styrene [21]. [21]. Remaining PU content was 11.0% at 410 ˝°C,C, and the remaining amount of SPU confirmed conﬁrmed for 5 wt %, 1010 wt %, and 20 wt % styrene styrene content content was was 14.9%, 14.9%, 15.1%, 15.1%, and and 17.8% 17.8% at at 25 25 kGy, kGy, respectively. respectively. In addition, addition, whenwhen thethe radiationradiation dose was 50 kGy, thethe remainingremaining SPUSPU contentcontent was 12.4%,12.4%, 15.2%, and 17.8%, respectively. ThermalThermal stabilitystability increasedincreased asas styrenestyrene contentcontent increased,increased, mostmost likelylikely duedue toto the excellent heat resistance andand machinabilitymachinability ofof styrene.styrene.

Figure 6.6. Thermogravimetric analysis (TGA) curves of styrene grafted polyurethane at dose rate: ((aa)) 2525 kGy;kGy; andand ((bb)) 5050 kGy.kGy.

Table 4.4. RemainingRemaining amountsamounts (410(410 ˝°C)C) of SPU from TGA.TGA. Samples Weight Percent (%) SamplesPU Weight11.0 Percent (%) PU5SPU25 14.9 11.0 5SPU2510SPU25 15.1 14.9 10SPU2520SPU25 17.8 15.1 20SPU255SPU50 12.4 17.8 5SPU5010SPU50 15.2 12.4 10SPU50 15.2 20SPU50 17.8 20SPU50 17.8

Comparison of the water contact angles of PU and SPU can reveal changes in the hydrophobic characterizationComparison on of the the PU water that contact was grafted angles to of the PU PS and after SPU the can radiation-induced reveal changes in grafting. the hydrophobic Figure 7 characterizationshows the water contact on the PU angles that of was PU grafted and SPU to compos the PS afterites at the different radiation-induced styrene contents grafting. and radiation Figure7 showsdoses. theThe water water contact contact angles angle ofincreased PU and for SPU increa compositessing the at radiation different doses styrene and contents the styrene and radiation content. doses.As shown The in water Figure contact 7, the angle water increased contact angle for increasing of unmodified the radiation PU was doses 86.6°, and whereas the styrene at 25 kGy, content. the ˝ Aswater shown contact in Figure angle 7of, theSPU water for 5 contactwt %, 10 angle wt %, of and unmodiﬁed 20 wt % was PU 88.8°, was 86.6 91.4°,, whereasand 94.3°, at respectively. 25 kGy, the ˝ ˝ ˝ waterIn addition, contact when angle the of radiation SPU for 5 dose wt %, was 10 50 wt kGy, %, and the 20 water wt % contact was 88.8 angle, 91.4 of SPU, and for 94.3 5 wt, respectively. %, 10 wt %, Inand addition, 20 wt % whenwas 91.9°, the radiation 95.5°, and dose 98.8°, was respectively 50 kGy, the. It water can be contact attributed angle to of the SPU higher for 5 hydrophobicity wt %, 10 wt %, ˝ ˝ ˝ andof the 20 PS wt due % was to the 91.9 radiation-induced, 95.5 , and 98.8 ,grafting. respectively. It can be attributed to the higher hydrophobicity of the PS due to the radiation-induced grafting.

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FigureFigure 7. 7. ContactContact angles angles of of (a (a) )polyurethane, polyurethane, styrene styrene grafted grafted polyurethane polyurethane dose dose rate rate at at 25 25 kGy; kGy; ((bb)) 5 5 wt wt %; %; ( (cc)) 10 10 wt wt %; %; ( (dd)) 20 20 wt wt %; %; dose dose rate rate at at 50 50 kGy; kGy; ( (ee)) 5 5 wt wt %; %; ( (ff)) 10 10 wt wt %; %; and and ( g)) 20 wt %.

4.4. Conclusions Conclusions InIn this this study, study, to to modify modify a a hydrophilic hydrophilic structur structuree into into hydrophobic hydrophobic structure, structure, styrene-grafted styrene-grafted PU PU waswas generated generated by by radiation-based radiation-based techniques. techniques. When When the the absorbed absorbed styrene styrene content content was was 20 20 wt wt % % and and radiationradiation dose dose was was 50 50 kGy, kGy, the the surface surface morphology morphology of of the the SPU SPU was was rough. rough. According According to to NMR, NMR, the the carbonylcarbonyl group group of of SPU SPU peak peak intensity intensity is is decreased decreased compared compared to to PU, PU, whereas whereas the the benzylic benzylic group group peak peak intensityintensity is is increased. increased. It It was was established established that that styrene styrene was was grafted grafted to to carbonyl carbonyl group group of ofPU. PU. SPU SPU has has a hydrophobica hydrophobic structure structure due due to increased to increased presence presence of benzylic of benzylic groups. groups. The possibility The possibility of applying of applying high- functionalhigh-functional composites composites by radiation-ba by radiation-basedsed techniques techniques has been has confirmed. been conﬁrmed.

Acknowledgments:Acknowledgments: ThisThis work was supported byby NationalNational ResearchResearch Foundation Foundation of of Korea Korea (NRF) (NRF) grant grant funded funded by bythe the Ministry Ministry of Science, of Science, ICT (Information ICT (Information & Communication & Communication Technology) Technology) and Future an Planning,d Future KoreaPlanning, government Korea (No. 2013M2A2A6042482). government (No. 2013M2A2A6042482). Author Contributions: Jong-Seok Park designed the experiment. Jin-Oh Jeong performed the experiment. AuthorYoun-Mook Contributions: Lim anaylyzed Jong-Seok the data Park and designed discussed the experiment.experiment. Jin-OhJin-Oh JeongJeong and performed Jong-Seok the Park experiment. worte the Youn-Mookmanuscript. Lim The anaylyzed manuscript the was data reviewed and discussed by all authors. the experiment. Jin-Oh Jeong and Jong-Seok Park worte the manuscript.Conﬂicts of The Interest: manuscriptThe authors was reviewed declare no by conﬂict all authors. of interest.

Conflicts of Interest: The authors declare no conflict of interest. References

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