Flexible Polycarbonate and Copoly(Imide-Carbonate)s-Based Frequency Selective Surface for Electromagnetic Shielding Application

Optically transparent polycarbonates (PCs) and Copoly(Imide-Carbonate)s (Co-PICs) were synthesized by the melt poly-condenzation method. Rigid (imide) and fexible (-O- and –C(CH 3 ) 2 − ) moieties were incorporated in the structure of bisimide diol comonomer using 4-aminophenol and 4,4 ′ -(4,4 ′ -isopropylidenediphenoxy) bis(phthalic anhydride). Te structural properties of synthesized comonomers and polymers were confrmed by 1 H, 13 C-NMR and FT-IR spectra. Termal properties of polycarbonates and copolycarbonates were examined using DSC and TG analysis. Termal properties (glass transition ( T g ) and thermal decomposition (T d ) temperature) of copolymers were enhanced without sacrifcing properties of BPA-based PC (high transparency, ductility, and processability) by the incorporation of active functional bisimide diol comonomer (5–10mole %) in the polycarbonate backbone. Diferent sets of PCs and Co-PICs thin flm substrates were prepared by the solvent casting method and used to design frequency selective surface. Te proposed fexible FSS ofers shielding of 20dB at 8.8 GHz. In addition, the FSS ofers polarization independent operation with its symmetrical unit cell geometry.


Introduction
During the recent decades, diferent aspects of the synthesis of new copolymers are very important and potential class of materials which fnd wide applications as high-performance engineering polymers [1][2][3].Aromatic polycarbonates (PCs) constitute continue to receive much attention in the FSS substrate for electromagnetic shielding application because of their good optical transparency, high glass transition temperature, good solubility in halogenated solvents (CHCl 3 ), processability, and thermal and mechanical properties [4][5][6][7][8][9][10][11][12][13].In recent years, there has been extensive research to improve the heat resistance of PCs.Tis can be achieved by either restricting the conformational mobility of the carbonate moiety or increase in persistent length of the polymer.Use of tetra methyl bis-phenol A as a monomer is an example of the former approach, whereas copolymer with ester monomer is an example of the latter approach [14].Bisphenol-A (BPA)-based PCs (BPA-DPC (diphenyl carbonate)) are having Tg around 145 °C, which can be modifed in the incorporation of the diferent functional and bulky groups in the derivate of the BPA.Design of mechanically robust high T g (180-230 °C) polymers has been accomplished by copolymerization with 1, 4-cyclohexanedicarboxylic acid [15].Te most successful approach to high heat PCs is by incorporating 4, 4`-(3, 3, 5 -trimethylcyclohexylidene diphenol) (T g (240-283 °C)) as the comonomer [16,17].Te gem-dimethyl (T g (198-275 °C)) group at the 3-position is believed to be responsible for the observed property improvement.High heat properties are achieved without sacrifcing properties of PC, such as high transparency, high ductility, and processability [18][19][20][21][22][23][24][25][26][27][28].In corporation of rigid (imide), fexible (cyclic ring), and side chain (bulky groups) via commoner might be an efective way to achieve higher thermal and mechanical properties, and this could be focused in the article.Introduction of heterocyclic imide (BPA derivative) moiety with fexible (ether, methyl, phenyl, etc.) and rigid (imide) groups into the structure of the highperformance polymers to improve the thermal properties as well as processability of the copolymers.To overcome the issue, a new molecule is required to enhance the thermal and mechanical properties of polymers.[29][30][31].
Electromagnetic pollution is of serious concern nowadays due to intensive networking.Te impact of which tends to increase with the concept of Internet of Tings (IoT) gathering momentum.Frequency selective surfaces (FSSs) are extensively used for shielding applications.Flexible substrate-based FSSs have gained much attention owing to its conformal geometry.However, so far, glassy epoxy resins (GER) have been used extensively as the substrate material to design the FSS, but these materials are highly rigid, opaque, and brittle in nature.A new versatile material is needed to solve this problem.Flexible polymer substrate has been seemed as the alternative and promising material to replace the currently used rigid GER [32][33][34][35].
In this research work, the easy synthetic accessibility to a class of imide containing bisphenol A in the course of our study on thermal rearrangement led us to explore the utility of this class of monomers for polycarbonate.Such monomers have not been reported as comonomers for polycarbonates.Incorporation of fexible (-O-and -C(CH 3 ) 2 −) and rigid (imide) and moieties in the PCs' backbone has improved the thermal (T g and thermal degradation) properties.Te excellent Co-PICs are used as a substrate to design a fexible FSS for conformal electromagnetic shielding applications.

Measurements.
Melting points were determined by the open capillary tube method.FT-IR analysis was carried out using KBr pellet and the spectral range of 4000-450 cm −1 on a Bruker Tensor 27 spectrometer.NMR spectra were recorded on a Bruker 400 MHz spectrometer at a resonance frequency of 400 MHz for 1 H and 13 C using CDCl 3 and DMSO-d 6 as a solvent.Termogravimetric analysis was performed on a Perkin-Elmer TGA-7 system at a heating rate of 10 °C/minute under nitrogen atmosphere.Diferential scanning calorimetry (DSC) analysis was carried out on TA Instruments DSC Q10 at a heating rate of 10 °C/minute in nitrogen atmosphere.Te intrinsic viscosities were measured with an Ubbelohde viscometer at 25 ± 0.1 °C in chloroform.GPC (gel permeation chromatography) type of liquid chromatography was used to determine the number (M n ) and weight (M w ) average molecular weight of the polymers and as well as poly dispersive index (PDI).Te analysis was performed with Copoly(Imide-Carbonate)s [Co-PICs-1 and Co-PICs-2] samples in THF solvent, and the solution is injected into a chromatography column to measure the parameters.Te optical transparency of PC (1), Co-PICs-1 (2), and Co-PICs-2 (3) was measured using UVspectroscopy with wavelength from 200 to 800 nm.

Synthesis of Bisimide Diol
Comonomers.Synthesis of comonomer reaction is represented in Scheme-I.4aminophenol (2.099 g, 19.204 mmol) was dissolved in Nmethyl pyrrolidone (NMP) (50 mL) and toluene (50 mL) in a 500 mL round bottom fask ftted with a Dean-Stark trap, refux condenser, and nitrogen purge.4,4′-(4,4′-isopropylidenediphenoxy) bis (phthalic anhydride) (5.0 g, 9.6063 mmol) was added under stirring at 0-5 °C for 1 h.A small exotherm was observed during formation of the amic acid.Te reaction was allowed to proceed at 30 °C for 1 h and then refuxed (160 °C) for 12 h to remove the water of imidization as a toluene-water azeotrope 1 .Excess toluene was then removed by distillation.After cooling, the fask contents were slowly poured into methanol/water (200 mL, 1 : 1 mixture by volume).Te white precipitate was collected by fltration and washed with water.Te product was recrystallized from ethyl acetate and dried at 100 °C for 48 h in a vacuum.Te bisimide diol was used as a comonomer for the synthesis of copolycarbonates and which is not yet reported.Yield 6.7 g, 99%; mp 262-263 °C (white).

Synthesis of Polycarbonates and Co-Poly(Imide-Carbonate)s (Co-PICs).
Te polycarbonates were synthesized according to the following general procedure, and the polymerization reaction is shown in Scheme-II.A glass reaction tube equipped with a Claisen distiller and a nitrogen gas inlet and outlet were flled with a homogeneous solid mixture composed of (1 g, 4.380) mmol bisphenol A, 2 International Journal of Chemical Engineering (1.0134 g, 4.7308 mmol) diphenylcarbonate, NaOH (2.19 μl), and tetramethylammonium carbonate (2.628 μl).Te reaction mixture was heated in a salt bath at 180 °C under a stream of nitrogen gas for 2 h.Te temperature of the bath was increased to 200 °C with pressure at 180 mmHg for 25 min, the pressure was reduced to 100 mmHg, and the reaction was kept for 45 min.Te reaction was then heated at 220 °C for 5 min after that the pressure reduced to 15 mmHg and then reaction continue for 45 min.Te reaction was then heated at 250 °C for 5 min after that the pressure reduced to 2 mmHg and then the reaction continued for 45 min.Te temperature was gradually increased to 300 °C, and the pressure was reduced to 0.1 mmHg, and then the reaction continued for 2 h to remove the byproduct phenol in the polycondenzation reaction.After 1 h, the glass reaction tube was cooled, and the formed polymer was dissolved in chloroform.Te solution was fltered, and the polymer was precipitated by dropwise addition to methanol as a nonsolvent.Te resulting polymer was fltered and dried in a vacuum at 80 °C for 24 h.Yield: 1.02 g (97.27%) (white); the copolycarbonates (Co-PICs-1) were synthesized according to the abovementioned general (synthesis of polycarbonates) procedure 2-5 , and the polymerization reaction is shown in Scheme 3. 1.8999 g, (8.3225 mmol (95%)) bisphenol A (BPA), 0.3078 g, (0.4380 mmol (5 mole %)) of the abovesynthesized bisimide diol comonomer, 2.0268 g (9.4614 mmol) diphenylcarbonate (DPC), sodium hydroxide (NaOH) solution (4.38 μl, 0.001 M), and tetramethylammonium hydroxide (TMAH) solution (5.250 μl (0.221 M)).Yield 2.30 g, 99% (white coloured fber); Similarly, Co-PICs-3 [1.7000 g (8.3223 mmol (85%)) of bisphenol A, 0.9234 g, (0.8760 mmol (15%)) of comonomer, 2.0268 g (9.4617 mmol) of DPC, (NaOH) (4.38 μl, 0.001 M), and (TMAH) (5.250 μl (0.221 M))] were synthesized, Yield 2.01 g, 91% (pale yellow); but it thin flm also pale yellow in nature and loss the PC properties.Tere is a limitation in the incorporation of comonomers in the structure of copolymers, after 20 mole percentage of comonomer incorporation loss of the properties of PCs.Due to such reasons, Co-PIC-3 was not used for further process.International Journal of Chemical Engineering elemental analysis.From the 1 H-NMR spectrum of monomer (Figure 1), the -OH and aromatic proton peaks were observed at δ value 9.79 and 6.78-7.97ppm, respectively.4aminophenol "c" and "d" protons are overlapped with "c" and "d" protons of anhydride due to proton shielding efects, which is mentioned in Figure 1.CDCl 3 was used as a cosolvent, whose peaks are observed at δ value 9.79 ppm.DMSO-d 6 solvent and moisture (water) proton peaks were obtained at δ value 2.53 and 3.5 ppm, respectively.From the 13 C-NMR spectrum (Figure 2) of monomer, carbonyl (C�O), ether linkage (-C-O-C-), and hydroxyl group (-C-OH) carbon peaks were observed at δ value 166.9, 162.9-157.From the FT-IR spectrum of polycarbonate, disappearance of phenolic peaks (3350 and 1304 cm −1 ) and appearance of all other peaks (carbonate, ArCH, (-(CH 3 ) 2 −) could confrm that the respective functional groups are present in the structure of polycarbonate in the polymer backbone.Te structure of the polycarbonate could be further confrmed by the NMR spectrum.

Results and Discussion
Te 1 H-NMR spectrum of bisphenol A is shown in Figure S1.Te assignments of the peaks of bisphenol A are 1 H-NMR analysis of bisphenol A confrms that exact protons are appearing in the structure of bisphenol A. Similarly, diphenyl carbonate structure is also analyzed by 1 H-NMR analysis.Figure S2 shows the 1 H-NMR spectrum of diphenylcarbonate.From this spectrum, aromatic proton peaks only observed in the range of δ � 6.96-7.25 ppm (10H, ArCH).Te 1 H-NMR analysis of DPC confrms the structure of diphenyl carbonate.

Synthesis and Characterization of Co-PICs.
Copoly(Imide-Carbonate)s [Co-PICs-1 and Co-PICs-2] were synthesized with DPC, BAP, and bisimide diol comonomer (5 and 10 mole %) by melt polycondenzation method sodium hydroxide and tetramethylammonium hydroxide as a catalyst [36][37][38].Te 1 H-NMR spectra of the Co-PICs-1 and 2 are shown in Figures 4 and 5, respectively.From the 1 H-NMR spectra of PC and Co-PICs, the ratio of aromatic and aliphatic protons was integrated.Te obtained integration values of aromatic and aliphatic protons of polycarbonates and Copoly(Imide-Carbonate)s are nearly close to the calculated values (values are calculated according to the mole ration of commoners).Te 1 H-NMR peak intensity results show that the appropriate percentage of the imide functional moiety was incorporated in the polymer backbone.Te intrinsic viscosity [η] of Co-PICs was determined at a diferent concentration such as 0.1, 0.2, 0.3, 0.4, and 0.5 g•dl −1 in CHCl 3 as a solvent at 25 °C for Co-PICs-1 (0.66) and Co-PICs-2 (0.64).Te molecular weights of Co-PICs-1 and Co-PICs-2 are found to be 380189 and 363078 g/ mol, respectively, using the Mark Houwink empirical equation.Te PDI values of Co-PICs-1 and Co-PICs-2 are found to be using number average molecular weight (M n , derived from GPC result) 6.33 and 5.10, respectively (Table 1).

International Journal of Chemical Engineering
Te spectrum of gas permeable chromatograms was analyzed using the THF solvent and is shown in Figure 6.Two diferent narrow distributions of sharp peaks were observed in the spectra, which confrm that diferent molecular weight polymers such as imide and carbonate moieties are present in the structure of the copolymers.Tis result is also a strong evidence to confrm the incorporation of imide and carbonate functional moieties in the structure of the copolymers and as well as random polymerization.From the GPC analysis, M n , M w , and PDI were determined and data are listed in Table 1.Te M n values of Co-PICs-1 and Co-PICs-2 are found to be 6000 and 7110 g/mol, respectively.Similarly, M w was measured to be 376009 and 35800 g/mol, respectively.Te PDI (M w /M n ) was calculated from obtained M w and M n values, which is found to be 6.28 and 5.98, respectively.Te high-PDI indicated that broad molecular weight distributions are observed in the structure of copolymers.Almost same PDI and average molecular weight were obtained from GPC and intrinsic viscosity methods.Tis result also suggests that random polymerization reaction occurred via the polycondenzation process, which makes high molecular weight copolymers.International Journal of Chemical Engineering and T g were evaluated under N 2 atmosphere.Te glass transition temperatures (T g ) of Co-PICs are increased with the addition of comonomer mole fractions from 143 °C for homopolymers (PCs) to 165 °C for copolymers.DSC results show that T g value of copolymers is higher than homopolymers.Slight signifcant changes occurred in the transparency of homo to copolymer flms with 5 to 10 mole % incorporation of comonomers in the polymer structure.So, high heat properties (T g ) are achieved without sacrifcing the transparency of PCs.TG analysis shows that PCs (Figure 8) began to decompose at near 400 °C which are less as the thermal decomposing behaviour of Co-PICs (430 °C).TG analysis result reveals that the decomposition behaviour of Co-PICs is improved by the incorporation of rigid imide functional moieties in the polymer chain.Based on the DSC and TG analysis, both PCs and Co-PICs are promising materials for FSS substrate due to its more thermal stability.

Fabrication of Polymer Films.
PCs and Co-PICs flms were prepared by the solvent casting method and shown in Figure 9. 5% of polymer solution was prepared by dissolving the synthesized polymers into CHCl 3 .Te solution was fltered by using 1 mm PTFE syringe flter and cast on a glass Petri dish.Te cast solution was dried at 30 °C for 24 hrs to allow slow evaporation of the solvent.Increase the comonomer ratio above 10%, slightly yellow colour was appeared due to the presence of imide moieties in PCs,   International Journal of Chemical Engineering which is shown in Figure 9 (Co-PICs-3 comonomer ratio is 15%).Te optical transparency of PCs and Co-PICs, Co-PICs-2 was found to be 97.65,95.75, and 92.64%, respectively (Figure 10).In this study, 2 and 5% optical transparency has decreased for Co-PICs-1, Co-PICs-2, respectively, which is due to the incorporation of comonomers in the polymer structure.Te UV results suggest that the comonomer is infuenced by optical transparency.Based on the result of PCs and Co-PICs-1, Co-PICs-2 was chosen as a FSS substrate due to its high thermal properties and fexible and optical transparency.
3.6.FSS Design and Analysis.Unit cell design of the FSS geometry was constructed on a polymer substrate of thickness 0.05 mm and is illustrated in Figure 11.Te FSS unit cell is of 0.5 mm in thickness with dimensions P � 15 mm and D � 20 mm.Te dielectric constant of the proposed PCs and Co-PICs is found using the dielectric probe kit, and the results are shown in Figure 12.It is evident that the increase in mole percentage of comonomer decreases the dielectric constant (ε r ).Hence, for the PCs, Co-PICs-1 and Co-PICs-2 ε r are found to be 5.8, 5.5, and 4.5 at 8.8 GHz, respectively.Te variation in the dielectric value is attributed to the imide linkages which reduces the dielectric constant for increased comonomer ratio.Te copolycarbonates (Co-PICs-2) substrate with dielectric constant 4.5 at 8.8 GHz is considered for the analysis.FSS is simulated using the commercially available CST microwave studio software, and its shielding efectiveness (SE) curve is shown in ( It is apparent from the simulated results that the proposed fexible polymer substrate-based FSS provides band stop response at 8.8 GHz with SE of more than 50 dB for both TE and TM modes of operation.Te similar response for TE and TM modes is attributed to the symmetrical geometry of the FSS unit cell, hence providing the advantage of polarization independency.International Journal of Chemical Engineering comparison of the proposed polymer-based FSS with the existing FSSs is provided in Table 2.It is observed that the proposed polymer-based FSS provides shielding up to 20 dB in the desired spectrum.Te fabricated fexible polymerbased FSS is found to be the potential candidate for shielding conformal surfaces providing good transparency against its conventionally used FR-4 substrate.

Conclusions
Flexible PCs and Co-PICs were successfully synthesized by the melt polycondenzation process.Te imide, ether, and methyl functional moieties were incorporated in the structure of copolycarbonates, which was confrmed by FT-IR and NMR analysis.DSC and TG analyses were used to examine the thermal properties of PCs and Co-PICs.Te results showed that the T g and T d of Co-PICs were improved from 143 to 165 °C and from 400 to 430 °C, respectively, compared to PCs.Tis indicates that Co-PICs are more stable even at high temperatures without compromising the standard properties of PCs.Te polymer thin flm was prepared by the solvent casting method using CHCl 3 .
Based on the dielectric constant result, Co-PICs-2 is chosen as a substrate in FSS design.A prototype of the FSS is fabricated, and the measured results are found to be in good agreement with simulated results.Te good thermal stability, fexibility, transparency, and simulation measurement of Co-PICs indicated that the Co-PICs could ofer excellent
Figure 13.Te SE is obtained from the transmission characteristics using the following formula: SE(dB) � 20 log Transmission characteristics without FSS Transmission characteristics with FSS .

FrequencyFigure 15 :
Figure 15: Comparison of the simulated and measured results.
PCs and Co-PICs.Termal properties of the PCs and Co-PICs were examined by DSC (exo efect graph) and TG analysis and shown in Figures7 and 8, respectively.Te thermal decomposition behaviour Figure 3: FT-IR spectra of bisphenol A (BPA), diphenylcarbonate (DPC), and polycarbonate (PC).

Table 1 :
Average molecular weight and poly disperse index of copolymers.

Table 2 :
Comparison of the proposed polymer-based FSS with FSSs existing in the literature.International Journal of Chemical Engineering substrate and stable polymeric materials to be used for the design of fexible FSS, which fnds wide applications in the feld of electromagnetic shielding.