Grafting Polytetrafluoroethylene Micropowder Via in Situ Electron

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Article Grafting Polytetraﬂuoroethylene Micropowder via in Situ Electron Beam Irradiation-Induced Polymerization

Hui Wang 1, Yingfeng Wen 1, Haiyan Peng 1, Chengfu Zheng 2, Yuesheng Li 3, Sheng Wang 3, Shaofa Sun 3,*, Xiaolin Xie 1,2 and Xingping Zhou 1,*

1 Key Laboratory for Material Chemistry of Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (H.W.); [email protected] (Y.W.); [email protected] (H.P.); [email protected] (X.X.) 2 National Anti-counterfeit Engineering Research Center, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] 3 School of Nuclear and Chemistry & Biology, Hubei University of Science and Technology, Xianning 437100, China; [email protected] (Y.L.); [email protected] (S.W.) * Correspondence: [email protected] (S.S.); [email protected] (X.Z.); Tel.: +86-715-824-3545 (S.S.); +86-27-8755-8194 (X.Z.) Received: 11 April 2018; Accepted: 4 May 2018; Published: 6 May 2018

Abstract: Decreasing the surface energy of polyacrylate-based materials is important especially in embossed holography, but current solutions typically involve high-cost synthesis or encounter compatibility problems. Herein, we utilize the grafting of polytetrafluoroethylene (PTFE) micropowder with poly (methyl methacrylate) (PMMA). The grafting reaction is implemented via in situ electron beam irradiation-induced polymerization in the presence of ﬂuorinated surfactants, generating PMMA grafted PTFE micropowder (PMMA–g–PTFE). The optimal degree of grafting (DG) is 17.8%. With the incorporation of PMMA–g–PTFE, the interfacial interaction between polyacrylate and PTFE is greatly improved, giving rise to uniform polyacrylate/PMMA–g–PTFE composites with a low surface energy. For instance, the loading content of PMMA–g–PTFE in polyacrylate is up to 16 wt %, leading to an increase of more than 20 degrees in the water contact angle compared to the pristine sample. This research paves a way to generate new polyacrylate-based ﬁlms for embossed holography.

Keywords: electron beam irradiation-induced grafting; polytetraﬂuoroethylene; methyl methacrylate; interfacial interaction

1. Introduction Currently, fake and shoddy products are regarded as the second largest public nuisance after the drug trade in the world; they endanger the development of the economy and people’s health. In the face of this problem, anti-counterfeiting techniques need to provide more reliable solutions. To this end, many common anti-counterfeiting methods such as holography, bar coding, magnetic recording, and radio frequency identiﬁcation (RFID) are employed [1–4]. Techniques involving optical manipulation have attracted considerable attention because of their optical features readily recognizable with the naked eye without the need for special equipment [5,6]. For instance, holograms [7], since the seminal work of Gabor in 1948 [8], have taken the top choice in daily optical anti-forgery owing to their angle-dependent blazing effect or three-dimensional dynamic images, visible to the naked-eye [9]. Embossed holograms, which can be easily reproduced on an industrial

Polymers 2018, 10, 503; doi:10.3390/polym10050503 www.mdpi.com/journal/polymers Polymers 2018, 10, 503 2 of 16 scale by transferring holographic images from a hot metal master plate to flexible polymer films under a press, are particularly important and have been increasingly applied for security in certificates, currency, bank cards, and other valuable goods. Thermoplastic polyacrylate is commonly used for recording the transferred embossed holograms thanks to its outstanding transparency, good weather resistance, and high flexibility [10]. Nickel plate is typically employed as the master plate because of its ease of use in hologram production and relatively low cost. During the embossing process, holograms on the surface of a nickel master plate are transferred to the softened area of a polyacrylate film under pressure. The embossed parts on the polyacrylate film can be softened and reshaped through glass transition by heating to 160–180 ◦C[11]. The problem is that the softened polyacrylate always displays strong adhesion to the hot nickel plate. Consequently, it is hard to separate the embossed polyacrylate with the nickel master without reducing the quality of the embossed holograms [12]. To avoid strong adhesion of the polyacrylate to the hot metal plate, surface modification of the metal plate with an anti-sticking agent [13] and reducing the surface energy of the polyacrylate via copolymerization or blending [14–16] have been employed. To do this, fluoropolymers are considered to be the best materials to produce anti-adhesive performance owing to their excellent hydrophobic and oleophobic properties [17–19]. Their low surface energy and outstanding inertness primarily arise from the molecule structure. Because of the high electro-negativity and large atomic radius compared to hydrogen atoms, fluorine atoms in fluoropolymers exhibit a strong intramolecular repulsion force and thus arrange spirally along the carbon backbone as a protective sheath [20]. Moreover, high bond energy and low polarizability of C–F bonds give rise to weak intermolecular actions with fluoropolymer themselves or other materials [21]. As expected, the fully fluorinated polymer, i.e., polytetrafluoroethylene (PTFE), shows an extremely low surface energy (γs = 2.02 × 10−2 N/m) [22]. Nevertheless, the low surface energy of PTFE leads to inherently poor adhesion and compatibility with other materials [23,24]. To improve the compatibility of PTFE with polyacrylates, surface modification including chemical etching and high-energy irradiation is a good solution. Some fluorine atoms are removed from the PTFE surface, simultaneously generating new hydroxyl, carboxyl, and other functional groups [25]. The new grafted polymer surface could enhance adhesion of the fluoropolymers to other materials. For example, by grafting glycidyl methacrylate, the T-peel adhesion strength between PTFE film and aluminum foil increases significantly [26]. Among the techniques used to modify PTFE, high-energy irradiation such as plasma, ion beam, gamma ray, X ray, vacuum ultraviolet, and electron beam, are efficient and controllable [27–32]. No initiator or catalyst is required and a large-scale production without affecting the bulk properties can be achieved for meeting the criteria of practical applications. Particularly, electron beam irradiation has been well recognized to be advantageous for its high dose rate, uniform beam energy, less wettability loss over time, and large penetration depth [33,34]. Since the original research of electron beam irradiation-induced grafting on PTFE by Chapiro [35,36], pre-irradiation-induced grafting and in situ irradiation-induced grafting have been developed. In the former method, PTFE is firstly exposed to electron beam irradiation to produce trapped radicals or peroxy/hydroperoxy radicals, and then the radicals initiate monomers to polymerize on the PTFE surface [37,38]. In the latter method, grafting polymerization is carried out through irradiating both PTFE and monomers simultaneously [39,40]. Compared with pre-irradiation-induced grafting reaction, the in situ irradiation-induced grafting polymerization gives a higher degree of grafting (DG) because of a higher radical yield and less radical loss. Various vinyl monomers such as acrylic acid, styrene, N-isopropylacrylamide, and 4-styrenesulfonate can be grafted onto PTFE [29,41]. A water contact angle decrease on the PTFE surface is an evidence of successful grafting [39]. The modified PTFE has also been widely applied to adsorbent materials [42], proton exchange membranes [38], and biomedical materials [43]. Despite extensive work on the grafting process, the compatibility improvement of PTFE with other polymer matrices remains a challenge [44,45]. Severe aggregation usually occurs which is hard to avoid even when employing Polymers 2018, 10, 503 3 of 16 large irradiation dosage or monomer concentration during grafting [46–48]. Thus, in order to meet the requirements of embossed holograms, the optimization of DG and PTFE loading is highly anticipated. In this work, in situ irradiation-induced grafting polymerization of methyl methacrylate (MMA) onto PTFE micropowder was performed, generating poly (methyl methacrylate) (PMMA) grafted PTFE micropowder (PMMA–g–PTFE). The PMMA–g–PTFE was then incorporated into a polyacrylate matrix to fabricate composite films. To achieve better dispersion of the micropowder and to reduce aggregation, we employed fluorosurfactant to disperse the micropowder into a monomer solution prior to electron beam irradiation [49]. Optimization of DG and PMMA–g–PTFE loading in composite films was exerted to decrease the surface energy of the polyacrylate-based composites while maintaining a good interfacial action. The results pave the way for generating new polyacrylate-based films for use in embossed holography.

2. Materials and Methods

2.1. Materials Commercially available PTFE micropowder (Dyneon™ TF-9205, 3M, Neuss, Germany) and fluorosurfactant ((C2H4O)n(CF2CF2)mF Zonyl™ FSN-100, Dupont, Wilmington, DE, USA) were used as received. Methyl methacrylate (MMA, purity: 99%), butyl acetate (purity: 99%), toluene (purity: 99.5%), butanone (purity: 99.5%), acetone (purity: 99.5%), Mohr’s salt (FeH8N2O8S2·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China and utilized directly without any further purification. Polyethylene terephthalate (PET) film and polyacrylate resin (50% solid content) were supplied by Huagong Image Technology Development Co., Ltd., Wuhan, China. Ultrapure water with a conductivity of 18.3 S/cm was used as the grafting medium.

2.2. Grafting PTFE Micropowder with PMMA Grafting PTFE micropowder with PMMA was implemented through in situ electron beam irradiation-induced polymerization of MMA. Prior to grafting polymerization, 3 g PTFE micropowder, 3 mL MMA, Mohr’s salt, and fluorosurfactant were mixed and diluted with ultrapure water. The contents of Mohr’s salt and fluorosurfactant were controlled to be 0.3 wt % and 0.05 wt % of the amount of MMA, respectively. To dial the DG, the MMA concentration in the mixture was controlled to be 3, 6, 9, 12, and 15 wt %, respectively, by controlling the amount of added water. Emulsions were obtained through shearing the oil–water mixture for 5 min at a speed of 8000 r/min. The resulting emulsion was then degassed in a thin polyethylene bag to remove air and finally sealed. The grafting reaction was initiated at room temperature by irradiating the samples with accelerated electrons under an Electron Beam Processing System (Wasik Associates Inc., Dracut, MA, USA). The electron energy and beam current were set as 1.0 MeV and 18 mA, respectively. The irradiation dose was controlled to be 20, 40, 60, and 80 kGy, separately, at a dose rate of 20 kGy/pass. Subsequent to the irradiation-induced grafting reaction, the PMMA–g–PTFE micropowder was centrifugalized for 6 min at a speed of 8000 r/min. To remove the residual monomer and homopolymer formed during irradiation, the centrifuged PMMA–g–PTFE micropowder was first washed with deionized water and then Sohxlet-extracted with acetone for 72 h at 75 ◦C until a constant weight was achieved. Finally, the PMMA–g–PTFE micropowder was dried at 65 ◦C in vacuum until a constant weight was achieved.

2.3. Formulation of Polyacrylate/PMMA–g–PTFE Composite Films Commercially available polyacrylate resin in 2 g was first dissolved in a mixed solvent composed of 2.8 mL of toluene and 1.9 mL of butanone. Then, the PMMA–g–PTFE micropowder in a predesigned amount was dispersed in 2.8 mL of butyl acetate, followed by ultrasonication for 30 min. Subsequently, the individual PMMA–g–PTFE suspension and polyacrylate resin were mixed by ultrasonication for 1 h. To form composite films, 200 µL of resulting mixture was spin-cast onto PET films (24 mm length × 24 mm width), and then dried at 120 ◦C for 2 min. The content of PMMA–g–PTFE in these composite Polymers 2018, 10, 503 4 of 16

films was controlled to be 2, 4, 6, 8, 10, 12, 14, and 16 wt %, respectively, relative to polyacrylate. As a control, neat polyacrylate films and polyacrylate/PTFE composite films with the same micropowder contents were also fabricated following the aforementioned steps.

2.4. Characterization Fourier transform infrared spectroscopy (FTIR, Equinox 55, Bruker, Karlsruhe, Germany) was used to identify the functional group. KBr pellets were employed to support the samples during characterization. Data were collected in the transmission mode after cumulating 32 scans with a resolution of 2 cm−1. Thermogravimetric analysis (TGA, 4000, Perkin Elmer, Waltham, MA, USA) was performed under nitrogen atmosphere from 30 to 700 ◦C at a ramp rate of 10 ◦C/min. X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, Manchester, UK) was used to analyze the chemical bonding in the PMMA–g–PTFE and PTFE micropowder. Photoemission was excited by a monochromatic Al-Kα radiation. Survey scans were carried out from 1200 to 0 eV with an interval of 1.0 eV. Narrow scans were exerted with 0.05 eV steps. All XPS spectra were charge-compensated to C1s at 285 eV. Linear baseline for background subtraction and Gaussian- Lorentzian function were used for peak fitting. The morphology of the pristine PTFE, PMMA–g–PTFE, and their composite films with polyacrylate were studied using field-emission scanning electron microscopy (FE-SEM, Sirion 200, FEI, Hillsboro, OR, USA) at an accelerating voltage of 10 kV. PTFE and PMMA–g–PTFE micropowders for SEM characterization were prepared by placing a drop of the acetone-suspended micropowder onto a silicon wafer and dried in an ambient atmosphere. The composite films for SEM characterization were prepared by placing the polyacrylate/micropowder suspension onto PET films and dried at 120 ◦C. A duration of 5 min was chosen to fully remove the solvent prior to SEM characterization. Composite films were peeled off from the PET substrate when conducting the SEM characterization to avoid the background signal from PET. All samples were coated with platinum on the top surface before the test. The dispersion stability of the polyacrylate/PMMA–g–PTFE mixture and the polyacrylate/PTFE mixture were studied by ultraviolet-visible spectrophotometer (UV-vis, UV 2600, Shimadzu, Kyoto, Japan) at a wavelength of 600 nm. Data were collected in the transmission mode at a rate of 10 scan/min over the course of 60 min. Water contact angles were measured by the static contact angle test (CA, JC2000C1, Zhongchen Powereach Company, Shanghai, China). Around 0.5 µL of deionized water was dropped on the sample surface at room temperature. For each sample, ten independent tests on different surface parts were exerted to give mean values with standard deviations. Atomic force microscopy (AFM, SPM 9700, Shimadzu, Kyoto, Japan) was conducted on the surface of composite films in the tapping mode with a resonant frequency of 300 kHz. The root-mean-square- roughness (RMS) was calculated from the roughness profile determined by AFM.

3. Results and Discussion

3.1. Determination of DG in PMMA–g–PTFE micropowder Initially, to conﬁrm the successful grafting reaction of PMMA to PTFE, FTIR spectra of the pristine PTFE and PMMA–g–PTFE were captured, individually. As illustrated in Figure1, both pristine PTFE and PMMA–g–PTFE display the clear characteristic bands at 1155 and 1215 cm−1, because of the −1 symmetrical stretching and asymmetrical stretching of CF2, respectively. The peaks at 507-636 cm are ascribed to CF2 rocking, wagging, and bending vibrations. The spectrum of PMMA–g–PTFE micropowder shows an absorption at 1741 cm−1, which is associated with C=O stretching vibration of the ester group in grafted PMMA. This absorption is also clear for the PMMA homopolymer, while −1 is absent in PTFE. In addition, peaks appear at 1398 and 833 cm arising from CH3 on the grafted PMMA chain [50]. To exclude the possible formation of carbonyl and hydroxyl groups on PTFE under Polymers 2018, 10, 503 5 of 16 irradiation, the pristine PTFE micropowder was directly irradiated at the same dose in the absence Polymersof monomers. 2018, 10, x Results show that there is no carbonyl or hydroxyl group absorption in the irradiated5 of 15 PTFE.Polymers These 2018, 10 FTIR, x results imply the possible chemical coupling of PTFE with grafted PMMA5 in of the 15 absorptionPMMA–g–PTFE in the micropowder.irradiated PTFE. These FTIR results imply the possible chemical coupling of PTFE withabsorption grafted in PMMA the irradiated in the PMMA PTFE. –Theseg–PTFE FTIR micropowder. results imply the possible chemical coupling of PTFE with grafted PMMA in the PMMA–g–PTFE micropowder.

Figure 1. Fourier transform infrared spectroscopy (FTIR) spectra of the pristine Figure 1. Fourier transform infrared spectroscopy (FTIR) spectra of the pristine polytetraﬂuoroethylene polytetrafluoroethyleneFigure 1. Fourier transform(PTFE), poly infrared (methyl methacrylate)spectroscopy (PMMA(FTIR)) , irradiatedspectra PTFE of the and PMMA pristine (PTFE), poly (methyl methacrylate) (PMMA), irradiated PTFE and PMMA grafted PTFE micropowder graftedpolytetrafluoroethylene PTFE micropowder (PTFE (PMMA), poly–g (methyl–PTFE) formedmethacrylate) under a( PMMAdosage )of, irradiated 80 kGy. PTFE and PMMA (PMMA–g–PTFE) formed under a dosage of 80 kGy. grafted PTFE micropowder (PMMA–g–PTFE) formed under a dosage of 80 kGy. DG is simply the mass ratio of grafted PMMA to PTFE. To determine the DG in PMMA–g– PTFEDG, TGA isis simply simplywas exerted. thethe mass mass Asratio shownratio of of graftedin grafted Figure PMMA 2,PMMA the todegradation PTFE.to PTFE. To determineTo temperature determine the DGrangethe inDG of PMMA–g–PTFE, pristinein PMMA PTFE–g– isTGAPTFE 490–, was 610TGA °C exerted. was while exerted. that As shownfor As PMMA shown in Figure is in 180 Figure2–,420 the 2,°C, degradation the in degradationgood agreement temperature temperature with rangeprevious range of pristinereports of pristine [ PTFE51, 52PTFE]. is It ◦ ◦ is490–610is 490clear–610 thatC °C whilethe while degradation that that for for PMMAPMMA of PMMA isis 180–420180 and–420 PTFE °C,C, in in are good good independent agreement agreement withfrom with previouseach previous other. reports reports Thus, [51 for [,5152 ,the]52. It]. irradiationItis isclear clear that that-generated the the degradation degradation PMMA –ofg of –PMMAPTFE PMMA micropowder, and and PTFE PTFE are are the independent independent DG of PMMA from from can each each be other.calculated other. Thus, Thus, from for for the weightirradiation-generatedirradiation loss- generatedat two distinct PMMA PMMA–g–PTFE degradation–g–PTFE micropowder,stages, micropowder, the the DG DG of of PMMA can be calculated from the weight loss at two distinct degradation stages, DG=[(m -m )/m ] 100% (1) f i i DGDG=[(m= [(m ff− -mm i )/mi)/m i ]i] × 100%100%(1 (1)) where mf and mi signify the mass of PMMA–g–PTFE micropowder and PTFE micropowder core, respectively. where mf and mi signify the mass of PMMA–g–PTFEPMMA–g–PTFE micropowder and PTFE micropowder core, core, respectively.

Figure 2. Thermogravimetric analysis (TGA) curves of the pristine PTFE, PMMA, PMMA–g–PTFE withFigure varied 2. Thermogravimetric degree of grafting analysis (DG) of 7.5%,( (TGA)TGA 17.8%) curves , and of 27.9%, the pristine respectively PTFE, .PMMA, PMMA, PMMA–g–PTFEPMMA–g–PTFE with varied degree of grafting (DG)(DG) ofof 7.5%,7.5%, 17.8%,17.8%, andand 27.9%,27.9%, respectively.respectively. The DG in PMMA–g–PTFE can be dialed by varying the initial monomer concentration and irradiationThe DG dose. in PMMAAs depicted–g–PTFE in Figure can be 3, dialedthe DG by is varyingfound to the increase initial with monomer monomer concentration concentration and andirradiation irradiation dose. dose. As depictedIt is worth in noting Figure that 3, the approximate DG is foundly twice to increase the DG with is achieved monomer when concentration increasing theand irradiationirradiation dose. dose It from is worth 20 to noting 40 kGy. that Furtherapproximate increasingly twice the the irradiationDG is achieved dose when gives incr lesseasing DG increments.the irradiation Radical dose concentration from 20 to 40 and kGy. monomer Further diffusion increasing are the considered irradiation to dose be thegives two less main DG increments. Radical concentration and monomer diffusion are considered to be the two main

Polymers 2018, 10, 503 6 of 16

The DG in PMMA–g–PTFE can be dialed by varying the initial monomer concentration and irradiation dose. As depicted in Figure3, the DG is found to increase with monomer concentration and irradiation dose. It is worth noting that approximately twice the DG is achieved when increasing the irradiationPolymers 2018, dose 10, x from 20 to 40 kGy. Further increasing the irradiation dose gives less DG increments.6 of 15 Radical concentration and monomer diffusion are considered to be the two main reasons. The grafting reactionreasons. The is expected grafting to reaction happen is asexpected follows. to Whenhappen exposed as follows. to electron When exposed beams, to the electron PTFE main beams, chain the cleavagePTFE main occurs chain to cleavage generate occurs free radicals.to generate The free carbon radicals. center The radicalscarbon center are believed radicals to are contribute believed to thecontribute grafting to polymerization the grafting polymerization via initiating the via surrounded initiating the monomers surrounded to polymerize. monomers Thus,to polymerize. it is easy toThus, understand it is easy thatto un aderstand higher concentration that a higher concentration of radicals or of monomers radicals or would monomers accelerate would the accelerate grafting polymerization,the grafting polymerization, in accordance in withaccordance previous with reports previous [53 ].report Meanwhile,s [53]. Meanwhile, the liquid the MMA liquid is a MMA good solventis a good for solvent PMMA, for thus PMMA, providing thus provid bettering mobility better formobility the growing for the growing PMMA chainsPMMA which chains leads which to higherleads to propagation higher propagation efﬁciency. efficiency. Although Although increasing increasing the dose canthe increasedose can the increase DG, a the higher DG, dose a higher may alsodose leadmay to also more lead bimolecular to more bimolecular termination termination that consequently that consequently causes crosslinked causes crosslinked polymers whichpolymers are notwhich soluble are not in anysoluble solvent. in any As solvent. a consequence, As a consequence, the DG increment the DG increment may be impeded may be [ 54impeded]. [54].

Figure 3. Overall DG for the PMMA–g–PTFEPMMA–g–PTFE micropowder formed at different methyl methacrylate (MMA)(MMA) concentrationsconcentrations andand underunder variedvaried doses.doses.

In addition, the sample temperature was noted to rise rapidly at a high irradiation dose, In addition, the sample temperature was noted to rise rapidly at a high irradiation dose, although although it is hard to measure this precisely. The temperature increase is also expected to accelerate it is hard to measure this precisely. The temperature increase is also expected to accelerate grafting grafting reaction through improving the MMA molecular diffusion [55]. reaction through improving the MMA molecular diffusion [55]. To further verify the chemical bonding between PTFE and PMMA in the PMMA–g–PTFE To further verify the chemical bonding between PTFE and PMMA in the PMMA–g–PTFE micropowder, XPS surveys of the pristine PTFE and PMMA–g–PTFE micropowder were exerted. micropowder, XPS surveys of the pristine PTFE and PMMA–g–PTFE micropowder were exerted. For instance, the XPS data for the micropowder with DG of 7.5%, 17.8% and 27.9%, respectively, are For instance, the XPS data for the micropowder with DG of 7.5%, 17.8% and 27.9%, respectively, are presented in Figure 4. For the pristine PTFE micropowder, C1s core level at 290 eV and F1s core level presented in Figure4. For the pristine PTFE micropowder, C1s core level at 290 eV and F1s core level at 686 eV assigned to the C–F bond are clear. By contrast, the carbon doublet peaks at 285.0 eV and at 686 eV assigned to the C–F bond are clear. By contrast, the carbon doublet peaks at 285.0 eV and 292.1 eV are observed in the PMMA–g–PTFE micropowder, while the pristine PTFE survey has only 292.1 eV are observed in the PMMA–g–PTFE micropowder, while the pristine PTFE survey has only a a single carbon peak. single carbon peak. To get deeper insights into the chemical bonding of PMMA with PTFE in the PMMA–g–PTFE, XPS C1s narrow scans were analyzed. Here, to make the comparison much clearer, we focused on the PMMA–g–PTFE formed from the oil-in-water emulsion with 3 wt % MMA. The irradiation dose was varied from 20 to 80 kGy to tailor to the DG value, as discussed in Figure3. As shown in Figure5a, the ungrafted PTFE micropowder exhibits only one peak at 292.5 eV attributing to CF2 bonds in the repeating unit of -(CF2–CF2)-. By contrast, for the PMMA–g–PTFE micropowder, the C1s core level exhibits broad peaks centered at 285.0 eV and 292.1 eV, respectively, as illustrated in Figure5b–e, which can be deconvoluted to several peaks associated to C–C (284.9 eV), C–O (286.2 eV), C=O (288.9 eV), C–F (291.9 eV), and CF2 bonds (292.5 eV), respectively [56,57]. The peaks assigned to C–C (284.9

Figure 4. X-ray photoelectron spectroscopy (XPS) survey scans of the pristine PTFE and PMMA–g– PTFE micropowder with an individual DG of 7.5%, 17.8%, and 27.9%.

Polymers 2018, 10, x 6 of 15

reasons. The grafting reaction is expected to happen as follows. When exposed to electron beams, the PTFE main chain cleavage occurs to generate free radicals. The carbon center radicals are believed to contribute to the grafting polymerization via initiating the surrounded monomers to polymerize. Thus, it is easy to understand that a higher concentration of radicals or monomers would accelerate the grafting polymerization, in accordance with previous reports [53]. Meanwhile, the liquid MMA is a good solvent for PMMA, thus providing better mobility for the growing PMMA chains which leads to higher propagation efficiency. Although increasing the dose can increase the DG, a higher dose may also lead to more bimolecular termination that consequently causes crosslinked polymers which are not soluble in any solvent. As a consequence, the DG increment may be impeded [54].

Polymers 2018, 10, 503 7 of 16 Figure 3. Overall DG for the PMMA–g–PTFE micropowder formed at different methyl methacrylate (MMA) concentrations and under varied doses. eV), and C=O (288.9 eV) bonds become clearer when the irradiation dosage increases (Figure5b–e). The atomicIn addition, ratios and the bond sample proportions temperature calculated was noted from the to XPS rise C1s rapidly narrow at scansa high are irradiation listed in Table dose,1. Thealthough F/C atomic it is hard ratio to ofmeasure pristine this PTFE precisel is 1.82,y. inThe agreement temperature with increase the theoretical is also value.expected A small to accelerate amount ofgrafting oxygen reaction (atomic through concentration improving of 0.45%) the MMA in the molecular pristine PTFE diffusion micropowder [55]. might be caused by H2O or oxygenTo further from airverify [58 ]. the The chemical concentrations bonding of between C–C, C–O, PTFE C=O, and and PMMA C–F bonds in the clearly PMMA increase–g–PTFE in themicropowder, PMMA–g–PTFE, XPS surveys in comparison of the pristine with the PTFE pristine and PTFE,PMMA and–g– increasePTFE micropowder with the augmentation were exerted. of Polymers 2018, 10, x 7 of 15 DGFor instance, in the PMMA–g–PTFE. the XPS data for Whereas, the micropowder the CF2 content with DG decreases of 7.5%, signiﬁcantly.17.8% and 27.9%, As arespectively, result, the F/C are presented in Figure 4. For the pristine PTFE micropowder, C1s core level at 290 eV and F1s core level atomicTo ratio get deeper in the PMMA–g–PTFE insights into the micropowder chemical bonding drops fromof PMMA 1.69 to with 1.52 PTFE when in increasing the PMMA the– DGg–PTFE from, at 686 eV assigned to the C–F bond are clear. By contrast, the carbon doublet peaks at 285.0 eV and 1.4%XPS C1s to 9.8%, narrow while scans the O/Cwere atomicanalyzed. ratio Here, increases to make from the 0.01 comparison to 0.05. It should much beclearer, pointed we outfocused that theon 292.1 eV are observed in the PMMA–g–PTFE micropowder, while the pristine PTFE survey has only increasethe PMMA in C–F–g–PTFE content formed with thefrom augmentation the oil-in-water of irradiation emulsion dosagewith 3 indicateswt % MMA. the chemicalThe irradiation attachment dose a single carbon peak. ofwas PMMA varied to from PTFE 20 through to 80 kGy the to CF tailor2 bonds to the breaking. DG value, as discussed in Figure 3. As shown in Figure 5a, the ungrafted PTFE micropowder exhibits only one peak at 292.5 eV attributing to CF2 bonds in the repeating unit of -(CF2–CF2)-. By contrast, for the PMMA–g–PTFE micropowder, the C1s core level exhibits broad peaks centered at 285.0 eV and 292.1 eV, respectively, as illustrated in Figure 5b– e, which can be deconvoluted to several peaks associated to C–C (284.9 eV), C–O (286.2 eV), C=O (288.9 eV), C–F (291.9 eV), and CF2 bonds (292.5 eV), respectively [56,57]. The peaks assigned to C–C (284.9 eV), and C=O (288.9 eV) bonds become clearer when the irradiation dosage increases (Figure 5b-e). The atomic ratios and bond proportions calculated from the XPS C1s narrow scans are listed in Table 1. The F/C atomic ratio of pristine PTFE is 1.82, in agreement with the theoretical value. A small amount of oxygen (atomic concentration of 0.45%) in the pristine PTFE micropowder might be caused by H2O or oxygen from air [58]. The concentrations of C–C, C–O, C=O, and C–F bonds clearly increase in the PMMA–g–PTFE, in comparison with the pristine PTFE, and increase with the augmentation of DG in the PMMA–g–PTFE. Whereas, the CF2 content decreases significantly. As a result, the F/C atomic ratio in the PMMA–g–PTFE micropowder drops from 1.69 to 1.52 when

increasing the DG from 1.4% to 9.8%, while the O/C atomic ratio increases from 0.01 to 0.05. It should be pointedFigure 4.out4. X-rayX -thatray photoelectronthe increase in spectroscopysp Cectroscopy–F content ( (XPS)XPS with) survey the augmentation scans of the pristine of irradiation PTFE and dosage PMMA–g–PMMA indicates–g– the chemicalPTFE micropowder attachment withwith of anPMMAan individualindividual to PTFE DGDG ofofthrough 7.5%,7.5%, 17.8%,17.8% the CF, andand2 bond 27.9%.27.9%.s breaking.

FigureFigure 5. 5. The XPS C1s narrow scan spectra of (a)) the the pristine pristine PTFE; PTFE; andand ( (bb––ee)) PMMA–g–PTFEPMMA–g–PTFE micropowdermicropowder formed under varied doses of (b) 20 kGy;kGy; ( c) 40 kGy;kGy; ( d) 60 kGykGy;; and and (ee)) 80 80 kGy, respectively.respectively. TheThe DGDG forfor eacheach isis listedlisted inin TableTable1 .1.

Table 1. Atomic ratios and bond proportions of the pristine polytetrafluoroethylene (PTFE) and poly (methyl methacrylate) (PMMA) grafted PTFE micropowder (PMMA–g–PTFE) obtained from X-ray photoelectron spectroscopy (XPS)1.

Atomic Ratio Bond Proportion (%) Entry Dose (kGy) DG (%) F/C O/C C–C C–O C=O C–F CF2 1 0 0 1.82 0.005 1.37 0 0 0 98.63 2 20 1.4 1.69 0.01 3.59 2.34 2.56 38.67 52.84 3 40 5.1 1.67 0.02 3.65 2.31 2.78 49.65 41.61 4 60 7.7 1.65 0.03 7.28 2.75 3.39 52.89 33.69 5 80 9.8 1.52 0.05 11.56 4.18 3.98 53.14 27.14

Polymers 2018, 10, 503 8 of 16

Table 1. Atomic ratios and bond proportions of the pristine polytetraﬂuoroethylene (PTFE) and poly (methyl methacrylate) (PMMA) grafted PTFE micropowder (PMMA–g–PTFE) obtained from X-ray photoelectron spectroscopy (XPS)1.

Atomic Ratio Bond Proportion (%) Entry Dose (kGy) DG (%) F/C O/C C–C C–O C=O C–F CF2 1 0 0 1.82 0.005 1.37 0 0 0 98.63 2 20 1.4 1.69 0.01 3.59 2.34 2.56 38.67 52.84 Polymers 20183, 10, x 40 5.1 1.67 0.02 3.65 2.31 2.78 49.65 41.61 8 of 15 4 60 7.7 1.65 0.03 7.28 2.75 3.39 52.89 33.69 1 Entry5 1: pristine 80 PTFE 9.8 micropowder; 1.52 entry 0.05 2–5: 11.56PMMA–g 4.18–PTFE micropowder 3.98 53.14; The 27.14 bond 1proportionEntry 1: pristine and PTFEelemental micropowder; ratio were entry calculated 2–5: PMMA–g–PTFE from the micropowder; peak areas Theshown bond in proportion Figure and5; degree elemental of ratiografting were ( calculatedDG) values from were the peak determined areas shown from in Figure thermogravimetric5; degree of grafting analysis (DG) values (TGA were) determinedand dialed from by thermogravimetric analysis (TGA) and dialed by varying the irradiation dosages on the emulsion with 3 wt % of varying the irradiation dosages on the emulsion with 3 wt % of methyl methacrylate (MMA). methyl methacrylate (MMA).

The generated ∙CF- active sites after destroying the CF2 bond are well accepted to

simultaneouslyThe generated initiate·CF- the active grafting sites afterpolymerization destroying the(Figure CF2 bond6) [53]. are The well break accepteding of tohydrophobic simultaneously CF2 initiategroup and the formation grafting polymerization of hydrophilic groups (Figure, 6such)[ 53 as]. C=O The, are breaking able to of increase hydrophobic the surface CF2 energygroup and formationwettability of of hydrophilic PMMA–g– groups,PTFE micropowder. such as C=O, To are prove able to this, increase a water the contact surface angle energy investigation and wettability on ofmicropowder PMMA–g–PTFE tablets micropowder. was implemented. To prove These this, a watertablets contact were anglemade investigation from micropowder on micropowder under a tabletspressure was of implemented. 200 MPa. As Theseillustrated tablets in were Figure made S1, fromgrafting micropowder PMMA chains under on a pressurethe surface of 200 of PTFE MPa. Asmicropowder illustrated ingives Figure rise S1, to graftinga decrease PMMA in the chains water on contact the surface angle of from PTFE 101.8 micropowder ± 1.9° to 96.7 gives ± 1.0°, rise toimplying a decrease an increase in the water in hydrophilicity. contact angle The from grafting 101.8 ± modification1.9◦ to 96.7 of± 1.0 PTFE◦, implying with PMMA an increase is also inbelieved hydrophilicity. to increase The the grafting miscibility modiﬁcation between of PTFE PTFE and with the PMMA polyacrylate is also matrixbelieved as to the increase solubility the miscibilityparameter betweenimbalance PTFE decrease and thes. polyacrylate matrix as the solubility parameter imbalance decreases.

Figure 6. 6.In In situ situ irradiation-induced irradiation-induced grafting grafting polymerization polymerization mechanism mechanism of PMMA–g–PTFE of PMMA micropowder–g–PTFE. micropowder.

It is well known that the glass transition temperature, Tg, of polymer is mainly related to the It is well known that the glass transition temperature, Tg, of polymer is mainly related to the segmental chain mobility and free volume. The grafted PMMA chains on PTFE exhibit a higher Tg segmental◦ chain mobility and free volume. The grafted◦ PMMA chains on PTFE exhibit a higher Tg of of 121 C than that of the PMMA homopolymer (103 C). The primary reason for raising the Tg is 121 °C than that of the PMMA homopolymer (103 °C). The primary reason for raising the Tg is the the limited segmental chain mobility of grafted PMMA. Meanwhile, the strong polarity of C–F bonds limited segmental chain mobility of grafted PMMA. Meanwhile, the strong polarity of C–F bonds leads to larger dipole–dipole forces between the PTFE and PMMA molecules [59]. Lastly, because of leads to larger dipole–dipole forces between the PTFE and PMMA molecules [59]. Lastly, because of the aggregation of PMMA–g–PTFE micropowder, the free volume of PMMA chains decreases with an the aggregation of PMMA–g–PTFE micropowder, the free volume of PMMA chains decreases with increase of DG [60], thus leading to constrained segmental mobility. an increase of DG [60], thus leading to constrained segmental mobility.

3.2. Micromorphology of PMMA–g–PTFE Micropowder Crosslinking between different micropowders could cause aggregation, that is, clustering. For the pristine PTFE micropowder, irregular shapes and severe aggregation are clear under SEM (Figure 7a,e), mainly because of its hydrophobic and oleophobic nature and poor dispersity in solvents. After electron beam irradiation, smaller PMMA–g–PTFE micropowders with a relatively narrow size distribution (from 130 to 160 nm) are given; this is because of the grafted PMMA chains on the PTFE surface (Figure 7b,f). Nevertheless, crosslinking probably occurs between PMMA chains on the surface of PTFE particles, especially under a high dose of irradiation, which results in clusters (Figure 7c,d,g,h). Less clusters with a lower DG indicates that the crosslinking primarily occurs between PMMA chains on different PTFE particles. It should be noted that the addition of fluorosurfactant greatly enhanced the dispersion of pristine PTFE micropowder in the monomer solution (Figure S2). Without fluorosurfactant,

Polymers 2018, 10, 503 9 of 16

3.2. Micromorphology of PMMA–g–PTFE Micropowder Crosslinking between different micropowders could cause aggregation, that is, clustering. For the pristine PTFE micropowder, irregular shapes and severe aggregation are clear under SEM (Figure7a,e), mainly because of its hydrophobic and oleophobic nature and poor dispersity in solvents. After electron beam irradiation, smaller PMMA–g–PTFE micropowders with a relatively narrow size distribution (from 130 to 160 nm) are given; this is because of the grafted PMMA chains on the PTFE surface (Figure7b,f). Nevertheless, crosslinking probably occurs between PMMA chains on the surface of PolymersPTFE particles, 2018, 10, x especially under a high dose of irradiation, which results in clusters (Figure7c,d,g,h).9 of 15 Less clusters with a lower DG indicates that the crosslinking primarily occurs between PMMA chains crosslinkingon different PTFEbetween particles. PMMA on the surface of different particles is significant and hard to extract (Figure S3).

Figure 7.7. SEMSEM imagesimages ofof ( a(,ae,)e the) the pristine pristine PTFE, PTFE, and and PMMA–g–PTFE PMMA–g–PTFE micropowder micropowder with with a DG a ofDG (b of,f) (7.5%;b,f) 7.5% (c,g); 17.8%;(c,g) 17.8% and; (andd,h) ( 27.9%,d,h) 27.9%, respectively. respectively.

3.3. Polyacrylate/PMMA–g–PTFE Composite Films It should be noted that the addition of fluorosurfactant greatly enhanced the dispersion of pristine PTFEPolyacrylate/ micropowderPMMA in the–g monomer–PTFE composites solution (Figure were formed S2). Without by dispersing fluorosurfactant, the PMMA crosslinking–g–PTFE micropowderbetween PMMA into on the the commercially surface of different available particles polyacrylate is significant resin. and To hardmake to a extractclear comparison, (Figure S3). the PMMA–g–PTFE micropowders with DG of 7.5%, 17.8%, and 27.9% were investigated, respectively. To3.3. understand Polyacrylate/PMMA–g–PTFE the dispersity of Composite PMMA– Filmsg–PTFE in these polyacrylate, fracture surfaces of the pristinePolyacrylate/PMMA–g–PTFE polyacrylate film, polyacrylate composites composites were with formed untreated by dispersingPTFE and polyacrylate/ the PMMA–g–PTFEPMMA– gmicropowder–PTFE composite into thefilms commercially with 10 wt available% micropowder polyacrylate were resin.evaluated To make using a SEM. clear comparison,As displayed the in FigurePMMA–g–PTFE 8a, a smooth micropowders surface for with the DG polyacrylate of 7.5%, 17.8%, is and clear. 27.9% When were doped investigated, with untreated respectively. PTFE, To aggregatesunderstand show the dispersitying a poor of dispersity PMMA–g–PTFE in polyacrylate in these polyacrylate,are clear (Figure fracture 8b). surfacesPoor interfac of theial pristine action polyacrylatebetween PTFE film, and polyacrylate polyacrylate composites is clear from with the untreated cracks as PTFE highlighted and polyacrylate/PMMA–g–PTFE by the red arrows [27]. In contrast,composite the films polyacrylate/ with 10 wtPMMA % micropowder–g–PTFE composite were evaluated films exhibit using SEM.a much As better displayed interfacial in Figure action8a, (Figurea smooth 8c– surfacee), because for of the the polyacrylate close solubility is clear. parameters When between dopedwith grafted untreated PMMA PTFE,and polyacrylate. aggregates 3 1/2 Theshowing solubility a poor of dispersity PMMA was in polyacrylate calculated to are be clear 18.6 (Figure (MJ/m8b).) while Poor interfacial that for polyacrylate action between was 3 1/2 estimatedPTFE and polyacrylateto be 17.8 (MJ/m is clear) fromfrom thethe cracksmixed as solvent. highlighted No significant by the red aggregated arrows [27 ].PMMA In contrast,–g–PTFE the polyacrylate/PMMA–g–PTFEclusters were shown when the composite DG equals films 7.5%.exhibit Yet, a much such better low interfacial degree of action grafting (Figure provides8c–e), insufficientbecause of theinterfacial close solubility action between parameters PTFE between and polyacrylate, grafted PMMA thus small and polyacrylate.white domains The of solubilityPTFE are clear.of PMMA Further was increasing calculated tothe be DG 18.6 gives (MJ/m rise3) 1/2 to while better thatinterfac for polyacrylateial action and was good estimated dispersity. to be 17.8 The (MJ/mPMMA3–)g1/2–PTFEfrom with the mixed a DG solvent.of 17.8% Noresults significant in the best aggregated dispersion PMMA–g–PTFE and interfacial clusters adhesion were between shown whenPTFE with the DG polyacrylate. equals 7.5%. Compared Yet, such with low PMMA degree– ofg–PTFE grafting micropowder provides insufficient with a low interfacial DG, PMMA action–g– PTFEbetween micropowder PTFE and polyacrylate,with 27.9% DG thus prefers small to white aggregate domains as large of PTFE clusters are clear.in polyacrylate. Further increasing the DG gives rise to better interfacial action and good dispersity. The PMMA–g–PTFE with a DG of 17.8% results in the best dispersion and interfacial adhesion between PTFE with polyacrylate. Compared

Polymers 2018, 10, x 9 of 15 crosslinking between PMMA on the surface of different particles is significant and hard to extract (Figure S3).

Figure 7. SEM images of (a,e) the pristine PTFE, and PMMA–g–PTFE micropowder with a DG of (b,f) 7.5%; (c,g) 17.8%; and (d,h) 27.9%, respectively.

3.3. Polyacrylate/PMMA–g–PTFE Composite Films Polyacrylate/PMMA–g–PTFE composites were formed by dispersing the PMMA–g–PTFE micropowder into the commercially available polyacrylate resin. To make a clear comparison, the PMMA–g–PTFE micropowders with DG of 7.5%, 17.8%, and 27.9% were investigated, respectively. To understand the dispersity of PMMA–g–PTFE in these polyacrylate, fracture surfaces of the pristine polyacrylate film, polyacrylate composites with untreated PTFE and polyacrylate/PMMA– g–PTFE composite films with 10 wt % micropowder were evaluated using SEM. As displayed in Figure 8a, a smooth surface for the polyacrylate is clear. When doped with untreated PTFE, aggregates showing a poor dispersity in polyacrylate are clear (Figure 8b). Poor interfacial action between PTFE and polyacrylate is clear from the cracks as highlighted by the red arrows [27]. In contrast, the polyacrylate/PMMA–g–PTFE composite films exhibit a much better interfacial action (Figure 8c–e), because of the close solubility parameters between grafted PMMA and polyacrylate. The solubility of PMMA was calculated to be 18.6 (MJ/m3)1/2 while that for polyacrylate was estimated to be 17.8 (MJ/m3)1/2 from the mixed solvent. No significant aggregated PMMA–g–PTFE clusters were shown when the DG equals 7.5%. Yet, such low degree of grafting provides

Polymersinsufficient2018, 10interfacial, 503 action between PTFE and polyacrylate, thus small white domains of PTFE10 ofare 16 clear. Further increasing the DG gives rise to better interfacial action and good dispersity. The PMMA–g–PTFE with a DG of 17.8% results in the best dispersion and interfacial adhesion between withPTFE PMMA–g–PTFE with polyacrylate. micropowder Compared with with PMMA a low DG,–g–PTFE PMMA–g–PTFE micropowder micropowder with a low DG, with PMMA 27.9% DG–g– prefersPTFE micropowder to aggregate with as large 27.9% clusters DG prefers in polyacrylate. to aggregate as large clusters in polyacrylate.

Polymers 2018, 10, x 10 of 15

Figure 8. SEM images of fracture surfaces of (a) the pristine polyacrylate film; (b) polyacrylate Figure 8. SEM images of fracture surfaces of (a) the pristine polyacrylate film; (b) polyacrylate composite film with 10 wt % of untreated PTFE; and (c–e) polyacrylate composite film with 10 wt % composite film with 10 wt % of untreated PTFE; and (c–e) polyacrylate composite film with 10 wt % of PMMA–g–PTFE. The DG in the PMMA–g–PTFE was controlled to be (c) 7.5%; (d) 17.8%; and (e) of PMMA–g–PTFE. The DG in the PMMA–g–PTFE was controlled to be (c) 7.5%; (d) 17.8%; and (e) 27.9%, respectively. 27.9%, respectively.

ToTo betterbetter understandunderstand thethe improvedimproved interfacialinterfacial adhesionadhesion between PMMA–g–PTFEPMMA–g–PTFE micropowder andand thethe polyacrylatepolyacrylate matrix, the dispersion stability of grafted PTFE with a DG of 7.5%,7.5%, 17.8%,17.8%, andand 27.9%,27.9%, respectively, respectively, were were compared compared with with the pristine the pristine PTFE (Figure PTFE9 )[ (Figure61,62]. For 9) all[61 polyacrylate/,62]. For all PMMA–g–PTFEpolyacrylate/PMMA and– polyacrylate/PTFEg–PTFE and polyacrylate/PTFE mixtures in solution, mixtures close in solution, initial transmission close initial intensities transmission are shown,intensities indicating are shown, a similar indicating homogeneity. a similar After homogeneity. being left toAfter stand being for aroundleft to stand 10 min, for the around transmission 10 min, intensitythe transmission gradually intensity increases gradual becausely increases of the micropowder because of the precipitation. micropowder Clearly, precipitation. the transmission Clearly, intensitiesthe transmission of the polyacrylate/PMMA–g–PTFEintensities of the polyacrylate/ mixturePMMA– withg–PTFE different mixture DG with values different are always DG values lower thanare always that of lower the polyacrylate/PTFE than that of the polyacrylate/PTFE mixture. The untreated mixture. PTFE The micropowder untreated PTFE is almost micropowder completely is precipitatedalmost completely after beingprecipitated stood forafter 2 being h (Figure stood S4). for It2 h is (Figure worth S4). pointing It is worth out that pointing the transmission out that the intensitytransmission of the intensity polyacrylate/PMMA–g–PTFE of the polyacrylate/PMMA mixture–g–PTFE with 27.9% mixture DG with increases 27.9% much DG increases faster than much that offaster mixtures than that with of a lowermixtures DG. with It means a lower that DG. the PMMA–g–PTFEIt means that the with PMMA a DG–g of–PTFE 27.9% with tends a toDG precipitate of 27.9% moretends readilyto precipitate because more of the readily formed because aggregation, of the formed as aforementioned. aggregation, as aforementioned.

FigureFigure 9.9. TransmissionTransmission atat 600600 nmnm ofof polyacrylatepolyacrylate solution when adding 10 wt % of untreated PTFE, andand PMMA–g–PTFEPMMA–g–PTFE withwith aa DGDG ofof 7.5%,7.5%, 17.8%,17.8%, andand 27.9%,27.9%, respectively.respectively.

3.4. Surface Properties of the Polyacrylate/PMMA–g–PTFE Composite Films It is commonly accepted that a material with a higher solid surface energy will have a better wettability with a given liquid. According to the Young–Dupré equation, a decrease in the contact angle indicates an increase in the work of solid–liquid adhesion, namely better wettability [63]. Accordingly, a hydrophobic solid surface with lower surface energy always exhibits higher water contact angle. This can be used as an important measurement criterion of wettability. The following section focuses on the PMMA–g–PTFE with a DG of 17.8%. With incorporated PMMA–g–PTFE micropowder, the wettability of the polyacrylate composite films was studied by measuring the static water contact angle (Figure 10). As expected, the water contact angle increases with the augmentation of PMMA–g–PTFE. The water contact angle on the top surface of pristine polyacrylate film is 77.7°, while that of polyacrylate composite films increases by 26% to 97.8° when loaded with 16 wt % of PMMA–g–PTFE. It has been known that the surface contact angle is related to the chemical composition and surface morphology [64]. To further explain the water contact angle results, the surface roughness of polyacrylate/PMMA–g–PTFE composite films was investigated using AFM (Figure 11). The composite films display “humps” in different sizes composed of aggregated PMMA–g–PTFE micropowder clusters. The roughness (RMS) of the composite film is found to increase from 3.4 to 37.2 nm with an augmentation of PMMA–g–PTFE from 4 wt % to 16 wt %. For the composite film with 4 wt % PMMA–g–PTFE, the size of aggregated PMMA–g–PTFE clusters on the surface is clearly

Polymers 2018, 10, 503 11 of 16

3.4.Polymers Surface 2018, Properties10, x of the Polyacrylate/PMMA–g–PTFE Composite Films 11 of 15 It is commonly accepted that a material with a higher solid surface energy will have a better wettabilitysmaller than with that a givenof the liquid.films with According higher toPMMA the Young–Dupr–g–PTFE contents.é equation, The a decreasedensity of in micropowder the contact angle on indicatesthe film surface an increase increases in the si workgnificantly of solid–liquid at a high adhesion, PMMA–g namely–PTFE betterloading wettability. These facts [63 ].are Accordingly, consistent awith hydrophobic those observed solid surface in SEM with of fracture lower surface surfaces energy of polyacrylate/ always exhibitsPMMA higher–g–PTFE water composite contact angle. films This(Figure can 12). be usedThe PMMA as an important–g–PTFE measurementmicropowder criteriontended to of aggregate wettability. at a loading of 16 wt %, which resultedThe in following large aggregated section focuses PMMA on–g the–PTFE PMMA–g–PTFE clusters and rough with a surfaces DG of 17.8%. like the With “lunar incorporated surface”. PMMA–g–PTFEWith micropowder micropowder, loadings of 12 the wt wettability % and 14 ofwtthe %, polyacrylatethe PMMA–g composite–PTFE disperse films homogeneously was studied by measuringin polyacrylate the staticand the water raising contact surface angle roughness (Figure 10 contributes). As expected, to a large the water water contact contact angle angle increases to some withextent. the Thus, augmentation the optimization of PMMA–g–PTFE. of both DG and The loading water of contact PMMA angle–g–PTFE on the are top important surface to of increase pristine polyacrylatethe interfacial film action is 77.7 between◦, while thatPTFE of and polyacrylate polyacrylate composite and also films to increases dial the by surface 26% to property 97.8◦ when of loadedpolyacrylate. with 16 wt % of PMMA–g–PTFE.

Figure 10. (a) Photographs of a water droplet on the top surface of composite films; ﬁlms; and ( b) water contact angle as a function of PMMA–g–PTFEPMMA–g–PTFE micropowdermicropowder content.content.

It has been known that the surface contact angle is related to the chemical composition and surface morphology [64]. To further explain the water contact angle results, the surface roughness of polyacrylate/PMMA–g–PTFE composite films was investigated using AFM (Figure 11). The composite films display “humps” in different sizes composed of aggregated PMMA–g–PTFE micropowder clusters. The roughness (RMS) of the composite film is found to increase from 3.4 to 37.2 nm with an augmentation of PMMA–g–PTFE from 4 wt % to 16 wt %. For the composite film with 4 wt % PMMA–g–PTFE, the size of aggregated PMMA–g–PTFE clusters on the surface is clearly smaller than that of the films with higher PMMA–g–PTFE contents. The density of micropowder on the film surface increases significantly at a high PMMA–g–PTFE loading. These facts are consistent with those observed in SEM of fracture surfaces of polyacrylate/PMMA–g–PTFE composite films (Figure 12). The PMMA–g–PTFE micropowder tended to aggregate at a loading of 16 wt %, which resulted in large aggregated PMMA–g–PTFE clusters and rough surfaces like the “lunar surface”. With micropowder loadings of 12 wt % and 14 wt %, the PMMA–g–PTFE disperse homogeneously in polyacrylate and the raising surface roughness contributes to a large water contact angle to some Figure 11. Surface morphology obtained from atomic force microscopy (AFM) for spin-coated extent. Thus, the optimization of both DG and loading of PMMA–g–PTFE are important to increase the composite films with (a) 4 wt %; (b) 10 wt %; (c) 12 wt %; (d) 14 wt %; and (e) 16 wt % PMMA–g– interfacial action between PTFE and polyacrylate and also to dial the surface property of polyacrylate. PTFE micropowder, respectively. The DG of PMMA–g–PTFE is 17.8% in all samples.

Polymers 2018, 10, x 11 of 15 smaller than that of the films with higher PMMA–g–PTFE contents. The density of micropowder on the film surface increases significantly at a high PMMA–g–PTFE loading. These facts are consistent with those observed in SEM of fracture surfaces of polyacrylate/PMMA–g–PTFE composite films (Figure 12). The PMMA–g–PTFE micropowder tended to aggregate at a loading of 16 wt %, which resulted in large aggregated PMMA–g–PTFE clusters and rough surfaces like the “lunar surface”. With micropowder loadings of 12 wt % and 14 wt %, the PMMA–g–PTFE disperse homogeneously in polyacrylate and the raising surface roughness contributes to a large water contact angle to some extent. Thus, the optimization of both DG and loading of PMMA–g–PTFE are important to increase the interfacial action between PTFE and polyacrylate and also to dial the surface property of polyacrylate.

PolymersFigure2018 ,10.10, 503(a) Photographs of a water droplet on the top surface of composite films; and (b) water12 of 16 contact angle as a function of PMMA–g–PTFE micropowder content.

Figure 11. SurfaceSurface morphology morphology obtained obtained from from a atomictomic force force microscopy ( (AFM)AFM) for spin-coatedspin-coated composite ﬁlmsfilms withwith (a(a)) 4 4 wt wt %; % (;b ()b 10) 10 wt wt %; % (c; )( 12c) 12 wt wt %; % (d;) ( 14d) wt14 %;wt and%; and (e) 16 (e wt) 16 % wt PMMA–g–PTFE % PMMA–g– PTFEmicropowder, micropowder, respectively. respectively. The DG The of DG PMMA–g–PTFE of PMMA–g– isPTFE 17.8% is in17.8% all samples. in all samples. Polymers 2018, 10, x 12 of 15

Figure 12. SEM imagesimages ofof fracturefracture surfacessurfaces of of composite composite ﬁlms films with with (a ()a 4) wt4 wt %; % (b; )(b 10) 10 wt wt %; % (c;) ( 12c) 12 wt wt %; (%d;) ( 14d) wt14 %;wt and%; and (e) 16(e) wt 16 %wt of % PMMA–g–PTFE of PMMA–g–PTFE micropowder, micropowder, respectively. respectively. The DGThe of DG PMMA–g–PTFE of PMMA–g– isPTFE 17.8% is 17.8% in all samples.in all samples.

4. Conclusions PMMA grafted grafted PTFE, PTFE, referred referred to asto PMMA–g–PTFE,as PMMA–g–PTFE was, successfully was successfully prepared prepared via in situ viaelectron in situ beamelectron irradiation-induced beam irradiation- radicalinduced polymerization radical polymerization of MMA on of the MMA PTFE on surface. the PTFE The formationsurface. The of oil-in-waterformation of emulsion oil-in-water using emulsion a fluorosurfactant using a fluorosurfactant was found to be was critical found during to be the critical grafting during reaction. the Thegrafting chemical reaction. bonding The ofchemical PMMA bonding to PTFE wasof PMMA identified to byPTFE FTIR was and identified XPS. The by degree FTIR of and grafting, XPS. The DG, degree of grafting, DG, was readily dialed by tuning the irradiation dosages and MMA was readily dialed by tuning the irradiation dosages and MMA concentration in the initial emulsion, andconcentration easily determined in the initial by TGA.emulsion, The PMMA–g–PTFEand easily determined was then by TGA. added The into PMMA commercially–g–PTFE availablewas then added into commercially available polyacrylate to decrease the surface energy. Optimization of both polyacrylate to decrease the surface energy. Optimization of both DG and PMMA–g–PTFE loading wasDG recognizedand PMMA to–g be–PTFE important loading to improve was recognized the uniformity to be of important polyacrylate/PMMA–g–PTFE to improve the uniformity composite of polyacrylate/PMMA–g–PTFE composite films. With a 14 wt % PMMA–g–PTFE loading, the films. With a 14 wt % PMMA–g–PTFE loading, the micropowder shows the optimized dispersity micropowder shows the optimized dispersity in the polyacrylate matrix and surface energy. This in the polyacrylate matrix and surface energy. This finding paves a way to formulate new polymer finding paves a way to formulate new polymer composites for advanced applications in embossed composites for advanced applications in embossed holograms. holograms. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/5/503/s1, FigureSupplementary S1: Photographs Materials: of a water The droplet following on the aresurface available of (a) PTFE online tablet at and www.mdpi.com/ (b) PMMA–g–PTFExxx/s1 tablet, Figure with a DG S1: ofPhotographs 27.9%; Figure of S2:a water Photographs droplet on of suspensionsthe surface of of (a) the PTFE pristine tablet PTFE and micropowder (b) PMMA– dispersedg–PTFE tablet in MMA with monomer a DG of 27.9%; Figure S2: Photographs of suspensions of the pristine PTFE micropowder dispersed in MMA monomer solution (a) with 0.05 wt % fluorosurfactant and (b) without fluorosurfactant; Figure S3: SEM images of the PMMA–g–PTFE micropowder obtained with MMA monomer solution (a) with 0.05 wt % fluorosurfactant and (b) without fluorosurfactant; Figure S4: Photographs of suspensions of (a) the pristine PTFE and (b) PMMA–g– PTFE micropowder dispersed in polyacrylate solution after being stood for 2 h.

Author Contributions: Xingping Zhou and Xiaolin Xie conceived and designed the experiments; Hui Wang performed the experiments, analyzed the data and wrote the paper; Haiyan Peng and Xingping Zhou provided suggestion for the designing of experiments and corrected the paper; Yingfeng Wen and Sheng Wang helped do the irradiation experiments; Shaofa Sun and Yuesheng Li contributed Electron Beam Processing System; Chengfu Zheng contributed materials.

Acknowledgments: The authors gratefully appreciate the funds from the National Natural Science Foundation of China (Key Program 51433002) and the Key Technology R&D Program of Hubei Province of China (2014BAA103). The technical assistance from the Analytical and Testing Center of HUST is also appreciated.

Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

Polymers 2018, 10, 503 13 of 16 solution (a) with 0.05 wt % fluorosurfactant and (b) without fluorosurfactant; Figure S3: SEM images of the PMMA–g–PTFE micropowder obtained with MMA monomer solution (a) with 0.05 wt % fluorosurfactant and (b) without fluorosurfactant; Figure S4: Photographs of suspensions of (a) the pristine PTFE and (b) PMMA–g–PTFE micropowder dispersed in polyacrylate solution after being stood for 2 h. Author Contributions: Xingping Zhou and Xiaolin Xie conceived and designed the experiments; Hui Wang performed the experiments, analyzed the data and wrote the paper; Haiyan Peng and Xingping Zhou provided suggestion for the designing of experiments and corrected the paper; Yingfeng Wen and Sheng Wang helped do the irradiation experiments; Shaofa Sun and Yuesheng Li contributed Electron Beam Processing System; Chengfu Zheng contributed materials. Acknowledgments: The authors gratefully appreciate the funds from the National Natural Science Foundation of China (Key Program 51433002) and the Key Technology R&D Program of Hubei Province of China (2014BAA103). The technical assistance from the Analytical and Testing Center of HUST is also appreciated. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

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