Free Volume Effect Via Various Chemical Structured Monomers On

polymers

Article Free Volume Eﬀect via Various Chemical Structured Monomers on Adhesion Property and Relative Permittivity in Acrylic Pressure Sensitive Adhesives

Jung-Hun Lee 1,2, Ji-Soo Kim 2 , Hyun-Joong Kim 2,3,* , Kyujong Park 4, Jungwoo Moon 4, Jinyoung Lee 4 and Youngju Park 4

1 Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeonbuk 55324, Korea; [email protected] 2 Lab. of Adhesion and Bio-Composites, Program in Environmental Materials Science, Department of Agriculture, Forestry and Bioresources Seoul National University, Seoul 08826, Korea; [email protected] 3 Research Institute of Agriculture and Life Sciences, College of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea 4 Samsung Display Co., Ltd., Yongin 17113, Korea; [email protected] (K.P.); [email protected] (J.M.); [email protected] (J.L.); [email protected] (Y.P.) * Correspondence: [email protected]; Tel.: +82-2-880-4783

Received: 12 October 2020; Accepted: 5 November 2020; Published: 10 November 2020

Abstract: Acrylic pressure-sensitive adhesives (PSAs) are used as fixatives between layers of a display. PSAs’ function is an important factor that determines the performance of the display. Of the various display types available, the touch screen panel (TSP) of smart devices is firmly related to the relative permittivity of the elementals. Therefore, adjusting the relative permittivity of the PSA is indispensable for driving the TSP. Accordingly, selected acrylic pre-polymers were polymerized and the pre-polymer was blended and cross-linked with monomers with different chemical structure to adjust the relative permittivity. The monomers were hexametyldisiloxane (HMDS), N-vinylcaprolactam (NVC), tert-butyl acrylate (TBA), and isooctadecyl acrylate (ISTA). The gel fraction and transmittance as a function of the monomers show a similar result to the pure acrylic PSA. However, the gel fraction value decreased to about 90% and the transmittance decreased to about 85%, due to the immiscibility between nonpolar HMDS and acrylic PSA. On the other hand, the adhesion properties were improved when NVC was added because of the polarity of the nitrogen group. In addition, the relative permittivity of the PSA decreased regardless of the monomer chosen. There was, however, a difference in the optimal content of each monomer, and NVC decreased from 4 phr content to about 3.4 in reducing relative permittivity. Through the above results, it was confirmed that NVC having a nitrogen group is most advantageous in lowering adhesion properties and relative permittivity, and necessitates further research based on the findings.

Keywords: acrylic pressure-sensitive adhesive; monomer; adhesion performance; relative permittivity; free volume

1. Introduction Pressure-sensitive adhesives (PSAs) are widely used in various industrial fields, such as medical products, aircraft, space shuttles, electrical devices, optical products, and automobiles. In particular, the demand for functional PSAs is rapidly increasing in the display market [1]. Displays are composed of a plurality of layer types and PSAs play an important fixative role. The PSA can fix the base material by applying slight pressure and becomes easy to adjust,

Polymers 2020, 12, 2633; doi:10.3390/polym12112633 www.mdpi.com/journal/polymers Polymers 2020, 12, 2633 2 of 14 Polymers 2020, 12, x FOR PEER REVIEW 2 of 14 basewhile material the processing by applying rate slight can be pressure increased and [2 ].becomes Recently, easy to imto proveadjust, the while technological the processing development rate can beof increased displays, br[2].oader Recently, properties to improve of the PSA the technolo functiongical are required development including of displays, transparency broaderhigh properties elongation, ofhigh the recovery,PSA function and heatare required dissipation including [3]. transparency high elongation, high recovery, and heat dissipationOptical [3]. clarity means a transmittance of 90% or more in the range of 400 to 800 nm and PSA satisfies thisOptical criteria definingclarity means the utility a transmittance of optical clear of adhesives90% or more (OCAs). in the PSA range includes of 400 rubber, to 800 acrylic, nm and silicone, PSA satisfiesand urethane, this criteria and acrylic defining PSA the is widelyutility of used optical in terms clear of adhesives their optical (OCAs). clarity. PSA Mo reover,includes the rubber, process acrylic,time can silicone, be significantly and urethane, reduced, and andacrylic UV crosslinkingPSA is widely is considerablyused in terms economical of their optical as a techni clarity.que. Moreover,As the transmittance the process of time PSA can is a verybe significantly important factorreduced, in dis andplay UV qua crosslinlity, a lokingw transmittance is considerably may economicallimit its application as a technique. in the displayAs the transmittance industry [4,5]. of PSA is a very important factor in display quality, a low Atransmittance touch screen may panel limit (TSP) its isapplication a major technology in the display for increasing industry [4,5]. input convenience and reducing productA touch weight screen by panel replacing (TSP) theis a keyboardmajor technology in the display for increasing of electronic input convenience devices. The and demand reducing for productTSP has weight soared by owing replacing to the the spread keyboard of smart in the phones display and of tabletelectronic PCs, devices. and it is The essential demand to improvefor TSP hastouch soared sensitivity. owing to A the capacitive spread touchof smart screen phones panel and is tablet composed PCs, and of a topit is andessential bottom to improve plate, on touch which sensitivity.a transparent A capacitive electrode touch (indium screen tin oxide,panel ITO)is composed is deposited of a andtop and each bottom layer isplate, fixed on using which PSAs a transparent(see Figure 1electrode). As part (indium of the capacitive tin oxide, method,ITO) is deposited a current flowand operateseach layer through is fixed ITO using on onePSAs side (see of Figurethe glass 1). orAs apart transparent of the capacitive film and amethod, touch operation a current is flow recognized operates by through sensing ITO a change on one in side capacitance of the glassthat occursor a transparent when a finger film orand a toucha touch pen operation touches theis recognized touch screen by surface. sensing a change in capacitance that occurs when a finger or a touch pen touches the touch screen surface.

FigureFigure 1. 1. StructureStructure of of capacitance capacitance type type touch touch screen screen pa panelnel (TSP). (TSP). PSA, PSA, pressu pressure-sensitivere-sensitive adhesive; adhesive; ITO,ITO, indium indium tin tin oxide. oxide.

TheThe relative relative permittivity isis aa measuremeasure of of the the degree degree of polarizationof polarizati ofon electric of electric charges charges inside inside when whenan electric an electric field isfield applied is applied to the to non-conductor the non-conduc surfacetor surface from from the outside. the outside. From From this pointthis point of vie ofw, view,the non-cond the non-conductoructor is called is called the dielectric the dielectric (ε). The (ε relative). The relative permittivity permittivity (εγ, dielectric (ɛγ, dielectric constant) constant) refers to refersthe ratio to the of theratio permittivity of the permittivity of the material of the base material relative base to therelative permittivity to the permittivity in a vacuum. in I na vacuum. other wo rds,In otherthe relative words, permittivitythe relative ispermittivi determinedty is bydetermined the permittivity by the ofpermittivity a substance of (see a substance Figure2)[ (see6] andFigure can 2) be [6]expressed and can bybe expressed the following by the equation: following equation: εγ = ε/ε0 (1) ɛγ = ɛ/ɛ0 (1) Over the past decade, the performance of microelectronic integrated circuits (ICs) that make-up Over the past decade, the performance of microelectronic integrated circuits (ICs) that make-up electronic devices has been improved owing to an increase in transistor speed and a decrease in size. electronic devices has been improved owing to an increase in transistor speed and a decrease in size. ICs have become faster and more complex because smaller transistors run faster. However, these efforts ICs have become faster and more complex because smaller transistors run faster. However, these are causing a decrease in the speed of signal propagation. The signal delay can increase with the size efforts are causing a decrease in the speed of signal propagation. The signal delay can increase with of the electronic device, which can degrade the performance of the final product. In other words, the size of the electronic device, which can degrade the performance of the final product. In other continuous shrinkage increases the speed of the transistor, but decreases the interconnection between words, continuous shrinkage increases the speed of the transistor, but decreases the interconnection the transistors. A way to improve this is to use a non-conductor with a low dielectric constant [7]. between the transistors. A way to improve this is to use a non-conductor with a low dielectric Non-conductors with a relative permittivity of 4.2 or less are called low-k dielectrics [8]. In addition, constant [7]. Non-conductors with a relative permittivity of 4.2 or less are called low-k dielectrics [8]. the relative permittivity of the capacitive TSP is closely related to the touch sensitivity. In order for PSA In addition, the relative permittivity of the capacitive TSP is closely related to the touch sensitivity. to be applied to TSPs, the relative permittivity of the material that affects touch sensitivity becomes In order for PSA to be applied to TSPs, the relative permittivity of the material that affects touch a very important factor. If the permittivity is not adjusted, noise may be easily detected or signal sensitivity becomes a very important factor. If the permittivity is not adjusted, noise may be easily detected or signal transmission time may be delayed [9]. Therefore, the factor controlling the relative

Polymers 2020, 12, 2633 3 of 14 Polymers 2020, 12, x FOR PEER REVIEW 3 of 14 transmissionpermittivity time of the may dielectric be delayed itself [9]. is Therefore, very important. the factor The controlling relative permittivity the relative permittivitycontrol method of theincludes dielectric a method itself is veryof lowering important. the Thepolarity relative using permittivity polarizable control groups method such as includes C-H, C-C, a method C-O, C-Si, of loweringand C-F the bonds polarity and usinga method polarizable of lowering groups the suchdensity as C-H,by introducing C-C, C-O, C-Si,free volume and C-F and bonds nano-pores and a method[10–23]. of loweringHowever, the research density result by introducings have also free been volume reported and nano-pores that such [voids10–23 ].can However, affect mechanical research resultsproperties have also[24–29]. been reported that such voids can aﬀect mechanical properties [24–29].

FigureFigure 2. 2.Schematic Schematic of of polarization polarization of of electric electric charges charges and and permittivity permittivity in in vacuum vacuum and and PSA. PSA. In this research, the possibility of adjusting the relative permittivity of monomers composed of In this research, the possibility of adjusting the relative permittivity of monomers composed of various chemical structures was analyzed and mapped to behavioral changes in adhesion properties. various chemical structures was analyzed and mapped to behavioral changes in adhesion properties. An acrylic pre-polymer was synthesized via UV polymerization with acrylate monomers. In addition, An acrylic pre-polymer was synthesized via UV polymerization with acrylate monomers. In addition, four types of monomers were chosen and mixed with the pre-polymer. An acrylic PSA sheet was four types of monomers were chosen and mixed with the pre-polymer. An acrylic PSA sheet was prepared via photo-crosslinking as a function of the monomer content. The crosslinking degree prepared via photo-crosslinking as a function of the monomer content. The crosslinking degree of of the PSAs was confirmed via gel fraction and the transmittance was measured using a UV/vis the PSAs was confirmed via gel fraction and the transmittance was measured using a UV/vis spectrophotometer. The adhesion property was measured via the peel strength, probe tack, and lap spectrophotometer. The adhesion property was measured via the peel strength, probe tack, and lap shear strength. The relative permittivity was measured using a micro vacuum probe station to obtain shear strength. The relative permittivity was measured using a micro vacuum probe station to obtain reliable data as a function of chemical structure and content of the monomer. Through these results, reliable data as a function of chemical structure and content of the monomer. Through these results, we tried to confirm the applicability of the low-k monomer in controlling the relative permittivity, we tried to confirm the applicability of the low-k monomer in controlling the relative permittivity, adhesion performance, and optical property of the acrylic PSA for TSP. In addition, it was attempted to adhesion performance, and optical property of the acrylic PSA for TSP. In addition, it was attempted secure a reliable relative permittivity value of semi-solid samples at room temperature as a result of a to secure a reliable relative permittivity value of semi-solid samples at room temperature as a result low glass transition temperature (Tg). of a low glass transition temperature (Tg). 2. Experimental Section 2. Experimental Section 2.1. Materials and Chemicals 2.1. Materials and Chemicals Reactive acrylic monomers, 2-hydroxyethyl acrylate (2-HEA, 99.0%), 2-ethylhexyl acrylate (2-EHA,Reactive 99.0%), acrylic and isobutyl monomers, acrylate 2-hydroxyethyl (IBA, 99.0%), acrylate were purchased (2-HEA, from99.0%), Samchun 2-ethylhexyl Pure Chemicalacrylate (2- (REHA,epublic 99.0%), of Korea). and isobutyl They w acrylateere used (IBA, as received99.0%), were without purchased further from purification Samchun toPure synthesize Chemical the(Republic acrylic pre-polymers.of Korea). They were Hexamethyldisiloxane used as received without (HMDS, further Sigma-Aldrich, purification to St. synthesize Louis, MO, the U acrylicSA), N-vinylcaprolactampre-polymers. Hexamethyldisiloxane (NVC, Tokyo Chemical (HMDS, Industry Sigma-Aldrich, Co., Ltd., Tok yo,St. Japa Louis,n), tert-butyl MO, USA), acrylate N- (TBA,vinylcaprolactam Sigma-Aldrich, (NVC, USA), Tokyo and isooctadecyl Chemical Industry acrylate (ISTA,Co., Ltd., Nippon Tokyo, Shokubai Japan),Co., tert-butyl Ltd., Os acrylateaka, Jap (TBA,an) wereSigma-Aldrich, selected as the USA), monomers and isooctadecyl to adjust relative acrylate permittivity (ISTA, Nippon of the Shokubai acrylic PSA Co., via Ltd., introduction Osaka, Japan) of freewere volume selected (see as Figure the monomers3). 2-hydroxy-2-methyl-1-phenyl-1-one to adjust relative permittivity (Irgacure of the acrylic 1173, BASF,PSA via Ludwigshafen, introduction of Germany)free volume and (see Phosphine Figure 3). oxide 2-hydroxy-2-methyl-1 (Irgacure 2100, BA-phenyl-1-oneSF, Germany) were(Irgacure used 1173, as the BASF, photoinitiator Ludwigshafen, for Germany) and Phosphine oxide (Irgacure 2100, BASF, Germany) were used as the photoinitiator for

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Polymers 2020, 12, x FOR PEER REVIEW 4 of 14 the acrylic pre-polymer synthesis and UV crosslinking, respectively. Crosslinking for the pre-polymer thewas acrylic used 1,6-hexanediolpre-polymer synthesis diacrylate and (HDDA, UV Sigma-Aldrich,crosslinking, respectively. USA). Crosslinking for the pre- polymer was used 1,6-hexanediol diacrylate (HDDA, Sigma-Aldrich, USA).

FigureFigure 3. 3.ChemicalChemical structure structure of ofmonomers. monomers. HMDS, HMDS, hexa hexamethyldisiloxane;methyldisiloxane; NVC, NVC, N-vinylcaprolactam; N-vinylcaprolactam; TBA,TBA, tert-butyl tert-butyl acrylate; acrylate; IS ISTA,TA, isooctadecyl isooctadecyl acrylate. acrylate.

2.2.2.2. Acrylic Acrylic Pre-Polymer Pre-Polymer Synthesis Synthesis TheThe acrylic acrylic monomers monomers were were blended blended with with an Irga ancure Irgacure 1173 inside 1173 insidea 500 mL a 500four-neck mL four-neck round- bottomedround-bottomed flask equipped ﬂask equipped with a stirrer, with a thermometer, stirrer, thermometer, and N2 andpurging N2 purging tube (the tube formulation (the formulation of the synthesizedof the synthesized acrylic pre-polymer acrylic pre-polymer is listed isin listedTable in1). Table The mixture1). The mixture was continuously was continuously stirred for stirred 20 min for at2 0room min temperature at room temperature with N2 gas. with The N2 gas.monomer The monomer mixture was mixture synthesized was synthesized by UV irradiating by UV irradiating using a UV-spotusing a cure UV-spot system cure (SP-9, system USHIO, (SP-9, Tokyo, USHIO, Japan) Tokyo, under Japan) a N2-rich under atmosphere a N2-rich until atmosphere the temperature until the oftemperature the mixture ofrose the by mixture 5 °C. The rose above by 5 ◦processC. The wa aboves iterated process five was times iterated and the ﬁve product times and was the stored product in a waswide-mouth stored in bottle a wide-mouth to protect bottle the acrylic to protect pre-polymer the acrylic from pre-polymer light and fromair. light and air.

TableTable 1. 1. FormulationFormulation of of acrylic acrylic pre-poly pre-polymer.mer. 2-HEA, 2-HEA, 2-hydroxyethyl 2-hydroxyethyl ac acrylate;rylate; 2-EHA, 2-EHA, 2-ethylhexyl 2-ethylhexyl acrylate;acrylate; IBA, IBA, isobutyl isobutyl acrylate. acrylate.

ReactiveReactive Monomers Monomers SampleSample Names Names 2-HEA2-HEA (wt.%) (wt.%) 2-EHA 2-EHA (wt.%) (wt.%) IBA (wt.%) IBA (wt.%) AcrylicAcrylic Pre-Polymer Pre-Polymer 20 20 60 60 20 20 Photoinitiator:Photoinitiator: 2-hydroxy-2-methyl 2-hydroxy-2-methyl-1-phenyl-1-one,-1-phenyl-1-one, Irgacure Irgacure 1173. 1173.

2.3.2.3. UV UV Crosslinking Crosslinking of ofAcrylic Acrylic PSAs PSAs with with Monomers Monomers PSAPSA sheets sheets with with monomers monomers were were prepared prepared by blending by blending 100 wt.% 100 wt.% of the of synthesized the synthesized acrylic acrylic pre- polymerpre-polymer with with1 part 1 partper hundred per hundred resin resin (phr) (phr) of HDDA of HDDA and and Irgacure Irgacure 2100 2100 as asa afunction function of of the the monomermonomer content content (see (see Table Table 2).2 ).The The mixture mixture was was combined combined and anddeformed deformed using using a paste a paste mixer mixer (SR- 500,(SR-500, Thinky, Thinky, Tokyo, Tokyo, Japan) Japan) for 4 for min. 4 min. The The blends blends were were coated coated onto onto the the surface surface of of corona-treated corona-treated polyethylenepolyethylene terephthalate terephthalate (PET) (PET) films ﬁlms about about 50 50 μµmm in in thickness thickness and and cross-linked cross-linked by by UV UV light, light, approximatelyapproximately 1300 1300 mJ/cm mJ/cm2.2 .

2.4. CharacterizationsTable 2. Formulation of cross-linked acrylic pre-polymer/monomer blends. PSA, pressure-sensitive Toadhesive; confirm HMDS,the influence hexamethyldisiloxane; of monomers on NVC, the cros N-vinylcaprolactam;slinking degree of TBA, the PSA, tert-butyl the gel acrylate; fraction was measuredISTA, isooctadecyl as a function acrylate. of the monomer content. The gel fractions of cross-linked PSA as a function of HDDA content were determined by soaking about 5 gMonomers of the PSA in toluene for 24 h at Acrylic Pre-Polymer room Sampletemperature Names with shaking. The soluble part was removed by filtration and dried at 80 °C for 6 (wt.%) HMDS NVC TBA ISTA h to obtain a constant weight. The gel fraction was(phr) calculated by(phr) the following(phr) equation: (phr)

PSA-HMDS Gel fraction (%)2/4/6 =/8 (W/101/W0) × 100 - - - (2) PSA-NVC - 2/4/6/8/10 - - 100 where WPSA-TBA0 and W1 are the weights before and after soaking - and filtration, - respectively. 2/4/6/8/10 - PSA-ISTA - - - 2/4/6/8/10 Crosslinking agent: 1,6-hexanediol diacrylate, HDDA; Photoinitiator: phosphine oxide, Irgacure 2100.

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2.4. Characterizations To confirm the influence of monomers on the crosslinking degree of the PSA, the gel fraction was measured as a function of the monomer content. The gel fractions of cross-linked PSA as a function of HDDA content were determined by soaking about 5 g of the PSA in toluene for 24 h at room temperature with shaking. The soluble part was removed by filtration and dried at 80 ◦C for 6 h to obtain a constant weight. The gel fraction was calculated by the following equation:

Gel fraction (%) = (W /W ) 100 (2) 1 0 × where W0 and W1 are the weights before and after soaking and filtration, respectively. Visible light transmittance of the specimens as a function of monomer type and content was measured in the wavelength of 400 to 800 nm using UV/vis spectrophotometry (Cary 100, Agilent Technologies, Santa Clara, CA, USA). The PSA specimens were prepared to a thickness of approximately 400 µm. The peel strengths of the PSAs according to the type and content of monomer were investigated using a texture analyzer (TA-XT2i, Micro Stable Systems, Godalming, UK). Firstly, 25 mm width PSA films were attached to a stainless steel (SUS) substrate and pressed twice by a 2 kg rubber roller. The peel strength was determined at an angle of 180◦ with a crosshead speed of 300 mm/min at room temperature based on ASTM D3330. The specimens were pressed onto stainless steel (SUS) substrates by two passes of a 2 kg rubber roller and then stored at room temperature for 24 h. Each sample was repeatedly measured three times, and the average value was calculated with N/25 mm. Lap shear testing was investigated using a texture analyzer. The tested specimens were cut into smaller pieces with a width of 25 mm. After being removed from a silicone release film, each PSA film was attached to another PET substrate (the adhesion cross-sectional area was equal to 25 25 mm2 × and a 2 kg rubber roller was passed over the film surface three times). The lap shear tests were performed at a crosshead rate of 5 mm/min. The shear strain rate values were calculated using the following equation: Shear strain rate (%) = ∆L/t 100 (3) × where ∆L is the moving distance and t is the thickness of the PSA film. The shear stress property is one of the important factors of PSAs for display applications. Therefore, researching the relationship between the shear strain rate and the thickness of the applied PSA film is imperative for future use in the display industry [30]. Figure4 is an evaluation of the reliability of the relative permittivity data of PSAs using a surface/surface probe. Although general polymers have very high data reliability, the nature of PSA is semi-solid and the thickness of the specimen is sensitively changed by the pressure of the surface/surface probe. General polymers have very high data reliability, but the semi-solid nature of the PSA against the thickness of the specimen may interfere with the sensitively of the touch pressure at the surface/surface probe interface. Hence, for this reason, the reproducibility of the data was a concern and it was difficult to establish a reliable correlation. Figure5 shows the analysis equipment for obtaining data with a higher reliability of PSA. It consisted of (a) an impedance analyzer (1260 Impedance Analyser, Solartron Analytical, UK), (b) a dielectric interface (1296 Dielectric Interface, Solartron Analytical, UK), and (c) a chamber (micro vacuum probe station, NEXTRON Co., Ltd., Republic of Korea). The permittivity of a PSA sheet was investigated using the more delicate micro vacuum probe station. The frequency range was set from 1 to 1000 kHz and the thickness of all PSA films was approximately 400 µm with the sample diameter detector fixed at a circular cross-section of 10 mm. Polymers 2020, 12, 2633 6 of 14

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Figure 4. Difference in relative permittivity results using a surface/surface probe as a function of PSA FigureFigure 4. 4.Di Differenceﬀerence inin relative permittivity permittivity results results usin usingg a surface/surface a surface/surface probe probe as a function as a function of PSA of distance. PSAdistance. distance.

FigureFigure 5. 5.Equipment Equipment andand fixture fixture scheme scheme to to measure measure the the relative relative permittivity permittivity for the for PSA the films: PSA films:(a) Figure 5. Equipment and fixture scheme to measure the relative permittivity for the PSA films: (a) (a)impedance impedance analyzer, analyzer, (b) (b) dielectric dielectric interfac interface,e, and and (c) (c)micro micro vacuum vacuum probe probe station. station. impedance analyzer, (b) dielectric interface, and (c) micro vacuum probe station. 3. Results3. Results and and Discussion Discussion 3. Results and Discussion 3.1.3.1. Gel Gel Fraction Fraction 3.1. Gel Fraction TheThe cross-linking cross-linking degree degree of of the the acrylic acrylic PSAPSA waswas indirectly confirmed confirmed by by calculating calculating the the insoluble insoluble portionportion inThe in toluene. cross-linking toluene. Figure Figure degree6 is6 is a grapha ofgraph the of acrylicof the the gel gel PSA fraction fraction was indirectly resultsresults according confirmed to to theby the calculatingtype type and and content content the insoluble of of monomer.monomer.portion Thein The toluene. gel gel fraction fraction Figure value value 6 is ofa of graph the the cross-linked cross-link of the geled fr PSAsaction decreased results according up up to to 90% 90% toas as the the HMDStype HMDS and content contentcontent of increased.increased.monomer. This This The result result gel fraction is is thought thought value to to be ofbe duethe due cross-link toto thethe lacklacked ofof PSAs functional decreased groups groups up capableto capable 90% asof of thereacting reacting HMDS with withcontent thetheincreased. acrylic acrylic pre-polymer pre-polymer This result and isand thought the the low low to interfacial interfacial be due to surfacesurf theace lack energy of functional between between the thegroups acrylic-silane acrylic-silane capable groups of groups reacting [31]. [31 with]. TheThethe silane acrylicsilane group group pre-polymer HMDS HMDS was wasand selected theselected low because interfacialbecause itit isissurf reportedreportedace energy to be be between excellent excellent the in in acrylic-silanereducing reducing the the relative groups relative [31]. permittivity.permittivity.The silane The group The effect effectHMDS of the of wasresultthe selectedresult on adhesion on because adhesion will it be iswill describedreported be described to in anotherbe excellent in another session. in reducingsession. It was confirmed, Itthe was relative however,confirmed,permittivity. that however, the The gel fractioneffect that theof value thegel fraction (inresult accordance onvalue adhesion (in with accordance thewill content be with described and the all content monomers in another and all except monomerssession. HMDS) It was except HMDS) was about 97% or more and did not inhibit the degree of crosslinking of the acrylic wasconfirmed, about 97% however, or more andthat did the notgel inhibitfraction the value degree (in ofaccordance crosslinking with of the acryliccontent PSA.and Thisall monomers result PSA. This result is attributed to the monomer C=C bond participation in the crosslinking process, is attributedexcept HMDS) to the was monomer about 97% C=C or bond more participation and did not i innhibit the crosslinkingthe degree of process, crosslinking preventing of the acrylic gel preventing gel disintegration via bond weakening. disintegrationPSA. This result via bond is attributed weakening. to the monomer C=C bond participation in the crosslinking process, preventing gel disintegration via bond weakening.

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FigureFigure 6. 6. Gel GelGel fractions fractionsfractions of ofof crosslinked crosslinkedcrosslinked acrylic acrylic PSAs PSAs as as a a function function of of monomer monomer type type and and contents. contents.

3.2.3.2. Transmittance TransmittanceTransmittance FigureFigure 77 7 isis is the the measurement measurement datadata of ofof the thethe transmittance transmittance in in the the visible visible visible light light light region region region as as as a a function afunction function of of monomerofmonomer monomer contentcontent content type.type. type. TransmittanceTransmittance Transmittance ofof of thethe the PSAPSA PSA isis is oneone one ofof of thethe important important properties properties because becausebecause it itit isis closelyclosely related related toto to thethe the imageimage image qualityquality quality of of of the the the display. display. display. Irrespective Irrespective Irrespective of of of the the the content content content type, type, type, the the the transmittance transmittance transmittance of ofallof all monomersall monomers monomers was was was similar similar similar to to thatto that that of of of the the the neat neat neat acrylic ac acrylicrylic PSA, PSA, PSA, with with with the the the exceptionexception exception ofof of HMDS.HMDS. HMDS. In In the the case case ofooff HMDS, HMHMDS,DS, however,however, comprising comprising thethe samesame silanesilane group,grgroup,oup, itit was was confirmedconfirmed confirmed that that the the transmittance transmittance was was notnot significantlysignificantly significantly didifferent differentfferent from from from the the the gel gel gel fraction fraction fraction result, re result,sult, but but but was was was observed observed observed to decreaseto to decrease decrease to aboutto to about about 85% 85% 85% of the of of theoriginalthe original original size size size at 10 at at phr10 10 phr phr content. content. content. It wasIt It was was noted noted noted that th th theatat the the transparency transparency transparency of of theof the the acrylic acrylic acrylic PSA PSA PSA decreased decreased decreased as as as a aresulta result result of of of immiscibility immiscibility immiscibility between between between the the the acrylate acrylate acrylate and an an silanedd silane silane group group group in in HMDS.in HMDS. HMDS. This This This problem problem problem was was was deemed deemed deemed to tobeto be limitedbe limited limited to totransparent to transparent transparent display display display applications applications applications and and and largely largely largely dependent dependent dependent on on theon the the application application application site. site. site.

FigureFigure 7. 7. Transmittance Transmittance fromfrom from aa a UVUV/vis UV/vis/vis spectrophotometer spectrophotometer spectrophotometer of of of a a crosslinkeda crosslinked crosslinked acrylic acrylic acrylic PSA PSA PSA as as aas functiona a function function of ofmonomerof monomer monomer type type type and and and content: content: content: (a )( a PSA-HMDS;(a) )PSA-HMDS; PSA-HMDS; (b )( b( PSA-NVC;b) )PSA-NVC; PSA-NVC; (c) ( PSA-TBA;c(c) )PSA-TBA; PSA-TBA; (d )( d( PSA-ISTA.d) )PSA-ISTA. PSA-ISTA.

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3.3. Adhesion Performances The underlying behavior of PSAs is related to adhesion, cohesion, and tack-related performance. Adhesion refers to adhesion to the substrate, cohesion refers to internal bonding strength, and tack refers to contact adhesion to the substrate [2]. Thus, in order to measure the adhesion properties of the PSA mapped by the content type of the monomer composition, peel strength, probe tack, and lap shear tests were conducted (see Figure8). The peel strength and the probe tack value of HMDS with increasing PSA showed a decreasing trend with HMDS owing to the silane group content. In lap shear test results, maximum stress and strain at maximum stress decreased with increasing HMDS content. In general with PSAs, when a network is formed by crosslinking, the cohesion force increases and the tacky and peel strength values of the PSA decrease [32–35]. However, in the case of HMDS, as a result of measuring the gel fraction, the degree of crosslinking decreased, and the adhesion properties decreased as the content increased. In addition, one of the disadvantages of general acrylic PSAs is that it has very low adhesion properties relative to low surface energy substrates such as polyethylene (PE), polypropylene (PP), and polydimethylsiloxane (PDMS). For this reason, a silicone-coated PET film is used as a protective film. Unlike other monomers, HMDS has a low polarity, so its adhesion properties to a substrate with low surface energy are very low. That is, the attraction between the molecular chains of the acrylic PSAs and the HMDS is weak, and this also contributes to lowering the adhesion properties of the PSAs. The peel strength of PSAs increased consistently with the NVC content as a result of the improvement in the intermolecular interaction with a relatively high polarity caused by the presence of the NVC nitrogen atom. Its inclusion played a role in improving the cohesion of the acrylic PSA [36,37]. For this reason, probe tack values and lap shear results also increased with the increasing content. However, the effect of polarity had relatively little effect on the shear direction. On PSA with TBA and ISTA, peel strength slightly increased as the content increased with no significant difference. When comparing TBA and ISTA in terms of chemical structure, ISTA with a comparatively larger molecular weight decreased the wettability and its presence was marked by an overall decrease compared with the branch structured TBA configuration. Probe tack results also showed a similar tendency with NVC as the TBA content increased, whereas ISTA having a long alkyl chain group did not significantly affect the value. Similar to NVC, TBA and ISTA also did not have a significant impact on lap shear values. However, it should be noted that the value was higher than that of the PSA to which HMDS was added, and the standard deviation accordingly decreased. This result shows a more stable result because NVC, TBA, and ISTA have C=C groups capable of reacting with residual acrylic monomers, crosslinking agent, and acrylic pre-polymers unlike HMDS. On the other hand, the HMDS monomer that did not participate in the reaction instead generated a bubble-like content during the crosslinking process of the acrylic PSA. The resulting phenomenon is ascribed to the adhesion strength decrease accompanied by an increase in the standard deviation (see Figure9).

3.4. Relative Permittivity The relative permittivity in the low frequency region according to the type and content of the monomer was investigated as shown in Figure 10. Regardless of the chemical structure of monomer, the relative permittivity of the acrylic PSA to which the monomer was added began to decrease in all measured frequency ranges investigated. This may arise as a result of the participation of the monomer in a crosslinking reaction in the acrylic pre-polymer resulting in a form a free volume between the acrylic molecular chains [24–29]. However, the relative dielectric constant of the acrylic PSA was observed to rise gradually above a certain threshold of the monomer content. The threshold marked diﬀerences that related to the functional groups of the monomer. HMDS and TBA began to increase at 8 phr or more, and NVC and ISTA at 4 phr or more. This result can be explained in two ways. Firstly, the monomer (especially HMDS) that did not react with the acrylic pre-polymer was not present in the acrylic PSA, as shown in the gel fraction and adhesion performances. Secondly, when the content of the monomer exceeds a certain level, it becomes excessive compared with the capacity and monomer Polymers 2020, 12, 2633 9 of 14

Polymersaggregation 2020, 12, x FOR results PEER in REVIEW a decrease in the efficiency in the free volume formation. Through this result,9 of 14 the monomers were effective in lowering the relative permittivity of the acrylic PSAs (see Figure 11). In addition, it was confirmed that there is a difference in the optimal content according to the chemical Polymers 2020, 12, x FOR PEER REVIEW 9 of 14 structure of the monomers.

FigureFigure 8. Adhesion 8.8.Adhesion Adhesion performances performances performances ofof PSAsof PSAs PSAs as aas as function aa fufunctionnction of monomer of monomermonomer type and type type contents: and and contents:( acontents:) peel strength(a) peel(a) ;peel strength;b (b) probec d tack; (c,d) maximum stress and strain at maximum stress from lap shear test. strength;( ) probe (b) probe tack; ( tack;, ) maximum (c,d) maximum stress and stress strain and at maximumstrain at maximum stress from stress lap shear from test. lap shear test.

Figure 9. Pictures of crosslinked acrylic PSA films: (a) pure acrylic PSA and (b) PSA-HMDS (10 phr).

Figure3.4. RelativeFigure 9. Pictures 9.PermittivityPictures of crosslinked of crosslinked acrylic acrylic PSA PSA films: ﬁlms: ( (aa)) pure pure acrylicacrylic PSA PSA and and (b ()b PSA-HMDS) PSA-HMDS (10 phr).(10 phr). The relative permittivity in the low frequency region according to the type and content of the 3.4. Relativemonomer Permittivity was investigated as shown in Figure 10. Regardless of the chemical structure of monomer, Thethe relative relative permittivity permittivity of the in acrylicthe low PSA frequency to which theregion monomer according was added to the began type toand decrease content in allof the monomermeasured was investigatedfrequency ranges as shown investigated. in Figure This 10. may Regardless arise as ofa theresult chemical of the structureparticipation of monomer,of the monomer in a crosslinking reaction in the acrylic pre-polymer resulting in a form a free volume the relative permittivity of the acrylic PSA to which the monomer was added began to decrease in all between the acrylic molecular chains [24–29]. However, the relative dielectric constant of the acrylic measured frequency ranges investigated. This may arise as a result of the participation of the PSA was observed to rise gradually above a certain threshold of the monomer content. The threshold monomermarked in differencesa crosslinking that related reaction to the in functionalthe acryli groupsc pre-polymer of the monomer. resulting HMDS in a andform TBA a freebegan volume to betweenincrease the acrylicat 8 phr molecular or more, and chains NVC [24–29]. and ISTA However, at 4 phr or the more. relative This dielectricresult can beconstant explained of thein two acrylic PSA was observed to rise gradually above a certain threshold of the monomer content. The threshold marked differences that related to the functional groups of the monomer. HMDS and TBA began to increase at 8 phr or more, and NVC and ISTA at 4 phr or more. This result can be explained in two

Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 ways. Firstly, the monomer (especially HMDS) that did not react with the acrylic pre-polymer was ways.not present Firstly, in the the monomer acrylic PSA, (especially as shown HMDS) in the that gel didfraction not react and adhesionwith the acrylicperformances. pre-polymer Secondly, was notwhen present the content in the acrylicof the monomerPSA, as shown exceeds in athe certai gel nfraction level, it and becomes adhesion excessive performances. compared Secondly, with the whencapacity the contentand monomer of the monomer aggregation exceeds results a certai in an delevel,crease it becomesin the efficiency excessive comparedin the free with volume the capacityformation. and Through monomer this aggregationresult, the monomers results in were a de effectivecrease inin loweringthe efficiency the relative in the permittivity free volume of formation.Polymersthe acrylic2020 Through,PSAs12, 2633 (see this Figure result, 11). the In monomersaddition, it werewas confirmed effective in that lowering there is the a difference relative permittivity in the optimal10 of of 14 thecontent acrylic according PSAs (see to Figure the chemical 11). In addition,structure itof was the confirmedmonomers. that there is a difference in the optimal content according to the chemical structure of the monomers.

Figure 10.10. RelativeRelative permittivity permittivity of of cross-linked acryli acrylicc PSAs as a function of monomer type and Figure 10. Relative permittivity of cross-linked acrylic PSAs as a function of monomer type and contents: ( a) HMDS; ( b) NVC; (c) TBA; and (d) ISTA. contents: (a) HMDS; (b) NVC; (c) TBA; and (d) ISTA.

Figure 11. Schematic of free volume distribution in acrylic PSAs as a function of monomer contents. FigureFigure 11. 11. SchematicSchematic of of free free volume volume distribution distribution in in acry acryliclic PSAs PSAs as as a a function function of of monomer monomer contents. contents.

In the PSA display, the value of the relative permittivity at a particular frequency is very important. In particular, the relative permittivity that accompanies the touch sensitivity of the TSP is related to a low frequency region. In this region, a degree of variability in the relative permittivity is expected and deviation from the required value varies with frequency. Figure 12 is a graph that summarizes the relative permittivity at a specific frequency. As mentioned above, the relative Polymerspermittivity2020, 12, 2633was observed to decrease upon addition of the monomer and this tendency11 varied of 14 depending on the chemical structure of the monomer. As in the lap shear test result, it is believed that air bubbles may occur by evaporation of the aggregated monomers from heat generation via chemicalultraviolet structure exposure of the during monomer. the crosslinking As in the lapprocess. shear In test addition, result, it the is believedair bubbles that had air a bubbles greater mayeffect occuron the by evaporationrelative permittivity of the aggregated than the monomersadhesion pe fromrformances. heat generation According via ultraviolet to previous exposure studies, during it has thebeen crosslinking reported process.that the relative In addition, permittivity the air bubbles of polymer had acan greater vary egreatlyﬀect on depending the relative on permittivity the air gap than[38–40]. the adhesion performances. According to previous studies, it has been reported that the relative permittivity of polymer can vary greatly depending on the air gap [38–40].

FigureFigure 12. 12.Relative Relative permittivity permittivity at specificat specific frequency frequency (1, 100, (1, 100, 200, 400,200, 1000)400, 1000) of cross-linked of cross-linked acrylic acrylic PSA asPSA a function as a function of monomer of monomer type and type contents. and contents. 4. Conclusions 4. Conclusions A study was conducted on the relative permittivity of PSA, which is closely related to the touch A study was conducted on the relative permittivity of PSA, which is closely related to the touch sensitivity of TSP. To lower the relative permittivity of PSA, four types of monomers were selected. sensitivity of TSP. To lower the relative permittivity of PSA, four types of monomers were selected. The gel fraction value and transmittance according to the chemical structure and content of the The gel fraction value and transmittance according to the chemical structure and content of the monomer did not show a significant difference compared with the pristine acrylic PSA. However, at monomer did not show a significant difference compared with the pristine acrylic PSA. However, at 10 phr of HMDS comprising a silane group, the gel fraction value decreased to about 90% and the 10 phr of HMDS comprising a silane group, the gel fraction value decreased to about 90% and the transmittance decreased to about 85%. As a result, the adhesion performances deteriorated owing to transmittance decreased to about 85%. As a result, the adhesion performances deteriorated owing to immiscibility between the nonpolar HMDS and the acrylic molecular chain. In contrast, the adhesion immiscibility between the nonpolar HMDS and the acrylic molecular chain. In contrast, the adhesion properties improved as a result of the polarity reaction by the nitrogen group of NVC, TBA, and ISTA. properties improved as a result of the polarity reaction by the nitrogen group of NVC, TBA, and ISTA. Branched and long alkyl chain grouped monomers did not significantly affect the adhesion properties. Branched and long alkyl chain grouped monomers did not significantly affect the adhesion properties. In addition, it was confirmed that the monomers devoid of functional groups capable of reacting with In addition, it was confirmed that the monomers devoid of functional groups capable of reacting with the acrylic pre-polymer remained generating bubbles by heat evaporation and impacting the adhesion properties. Regardless of the type of monomer, the relative dielectric constant of the PSA decreased and the relative permittivity began to rise above a certain content. This is because of the generation of air bubbles due to heat generation in the cross-linking process, likely resulting from monomer aggregation. In hindsight, the optimum content of the monomer was effective in lowering the relative permittivity as a result of the formation of free volume, but the adhesion performances and the relative dielectric constant were affected by bubbles caused by evaporation. Polymers 2020, 12, 2633 12 of 14

As shown in this study, the adhesion performance changed according to the type of monomer, and as a result, the relative permittivity can be adjusted according to monomer content. In the future, we plan to further investigate the application methods for monomer control, adhesion properties, and relative permittivity of acrylic PSA. We aim to study the behavior of adhesion performance and relative permittivity when the dispersibility of monomer is enhanced by producing a pre-polymer by simultaneously reacting reactive acrylic monomer and monomer via in situ polymerization.

Author Contributions: J.-H.L., conceptualization, methodology, validation, writing and revision of the article; J.-H.L. and J.-S.K., investigation, data curation; H.-J.K., supervision of the article; H.-J.K., K.P., J.M., J.L., and Y.P., writing (review & editing); K.P., J.M., J.L., and Y.P., project administration. All authors have read and agreed to the published version of the manuscript. Funding: This project is an industry-academia-research assignment, and no grant number is assigned. Acknowledgments: This work was financially supported by Samsung Display Co., Ltd., Republic of Korea. Conflicts of Interest: The authors declare no conflict of interest.

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