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Communication Characteristics of Thermosetting Nanocomposites: Siloxane-Imide-Containing Benzoxazine with Silsesquioxane

Chih-Hao Lin 1 , Wen-Bin Chen 2, Wha-Tzong Whang 1 and Chun-Hua Chen 1,*

1 Department of Materials Science and , National Chiao Tung University, Hsinchu 300093, Taiwan; [email protected] (C.-H.L.); [email protected] (W.-T.W.) 2 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-3513-1287

 Received: 16 September 2020; Accepted: 26 October 2020; Published: 28 October 2020 

Abstract: A series of innovative nanocomposites comprising of polysiloxane-imide-containing benzoxazine (PSiBZ) as the matrix and double-decker silsesquioxane (DDSQ) epoxy or polyhedral oligomeric silsesquioxane (POSS) epoxy were prepared for improving thermosetting performance. Thermomechanical and dynamic mechanical characterizations indicated that both DDSQ and POSS could effectively lower the coefficient of thermal expansion by up to approximately 34% and considerably increase the storage modulus (up to 183%). Therefore, DDSQ and POSS are promising materials for low-stress encapsulation for electronic packaging applications.

Keywords: polysiloxane-imide-containing benzoxazine; polyhedral oligomeric silsesquioxane epoxy; double-decker silsesquioxane epoxy; polymer nanocomposite

1. Introduction Compared with pristine polymer nanocomposites, hybrid organic–inorganic nanocomposites comprising of functional polymers as the matrix and nanoscale inorganic constituents have attracted greater interest in both academia and because of their tunable and generally more favorable thermal, mechanical, electrical, and barrier properties [1–3]. Upgrading current thermosetting polymers has become critical because of their utilization in various applications. Such as fuel cells [4], catalysis [5], membranes [6], CO2 adsorption [7], metal uptake [8], [9], thermal insulation [10], and super capacitor [11]. Studies using nanoparticles for modifying thermosetting polymers have revealed the feasibility of hybrid strategies [12–14]. (PBZ) is considered a high-performance thermosetting because of its excellent thermal stability, high modulus, low water absorption, low surface free energy, and near-zero shrinkage upon [15–17]. Those properties are superior to some other thermosetting polymers. Crucially, PBZ requires simple procedures of thermal activated ring opening polymerization without the need for catalysts, and do not form byproducts. The low surface free energy property would allow it to have many potential applications as mold release material, lithographic patterning, and super hydrophobic surface material [18–24]. However, high curing temperatures and poor mechanical properties (brittleness) severely limit PBZ applications. Modification of benzoxazine structures, blending with other polymers, and addition of nanoscale fillers or fibers are the currently major approaches to overcoming the drawbacks of PBZ [25–28]. Chen et al. demonstrated that siloxane-imine-containing benzoxazine (SiBZ) [29,30] could be used to synthesize PBZ with improved flexibility and weather resistance and reduced surface free energy. However, the conjunction

Polymers 2020, 12, 2510; doi:10.3390/polym12112510 www.mdpi.com/journal/polymers Polymers 2020,, 12,, 2510x FOR PEER REVIEW 2 of 10 surface free energy. However, the conjunction of the siloxane segment and benzoxazine can directly ofincrease the siloxane the coefficient segment andof thermal benzoxazine expansion can directly (CTE) increaseand thus the pose coeffi acient problem of thermal of dimensional expansion (CTE)instability. and thus pose a problem of dimensional instability. Silsesquioxane, with a general formula of (RSiO ) is a representative organic–inorganic Silsesquioxane, with a general formula of (RSiO33/2/2)n is a representative organic–inorganic compound. Cubic polyhedralpolyhedral oligomericoligomeric silsesquioxane silsesquioxane (POSS), (POSS), a a silsesquioxane silsesquioxane derivative, derivative, consists consists of anof an inorganic inorganic silsesquioxane silsesquioxane core with with a pore a pore diameter diameter of of 0.3–0.4 0.3–0.4 nm nm and and organic organic functional functional groups groups at theat the vertex vertex of of the the cubic cubic silsesquioxane silsesquioxane frame. frame. The The introduction introduction ofof organicorganic substituents,substituents, such as alkyl, aryl, epoxy, oror groups, into POSS has improvedimproved compatibility between the POSS molecule and polymer matrices [[31–35].31–35]. Another silsesquioxane derivative, double-decker silsesquioxane (DDSQ) contains two substituents, such as epoxy or allyl groups,groups, in diagonal positions [[36–39].36–39]. Incorporation of POSS or DDSQ into polymer matrices is an eeffffectiveective strategystrategy to enhanceenhance thermal stability and mechanical and dielectric properties [[40,41].40,41]. Regarding thethe enhancement enhancement of theof dimensionalthe dimensional stability, stability, although although several studies several have studies introduced have cubicintroduced POSS tocubic various POSS polymer to various matrices, polymer the resultingmatrices, CTEs the resulting are not satisfactory. CTEs are not For instance,satisfactory. the useFor ofinstance, POSS considerablythe use of POSS reduced considerably the CTE reduced of a the CTE (PI) of a matrix polyimide [42]. (PI) By contrast,matrix [42]. the By presence contrast, of the presence octa-functionalized of the octa-functionalized POSS epoxy in POSS a thermosetting epoxy in a thermosetting epoxy/HHPA matrixepoxy/HHPA exhibited matrix less exhibited influence onless the influence CTE, which on the was CTE, attributed which was to the attributed flexible organicto the flexible tethers organic around tethers the POSS around cage the forming POSS a cage soft interphaseforming a soft [43]. interphase [43]. Previous research also incorporate POSS or DDSQDDSQ with conventional PBZ and demonstrate the enhancement of thermal oxidation stability, mechanical properties [44–46]; [44–46]; however, a high rigid structure of PBZ, POSS, or DDSQ make the nanocomposites become brittle. In this study, study, we focused on the improvementimprovement of thethe dimensionaldimensional stabilitystability ofof moremore flexibleflexible polysiloxane-imide-containingpolysiloxane-imide-containing benzoxazine (PSiBZ). (PSiBZ). Two Two potential potential candidates, candidates, namely, namely, octa-functionalized octa-functionalized POSS POSS epoxy epoxy and 3,13- and 3,13-diglycidyloxypropyloctaphenyldiglycidyloxypropyloctaphenyl DDSQ DDSQ epoxy, epoxy, were selected were selected as the organic–inorganic as the organic–inorganic hybrid dopants hybrid dopantsfor the PSiBZ for the matrix PSiBZ because matrix of because their caged of their silsesquioxane caged silsesquioxane structures structures and chemical and chemicalaffinities awithffinities the withmatrix. the In matrix. the molecular In the molecular structure, structure, DDSQ has DDSQ larger has cage larger dimensions cage dimensions than cubic than POSS cubic and POSS thus and is thusexpected is expected to provide to provide desired desired thermal thermal dimension dimension stability stability (reduced (reduced CTEs) CTEs) for for organic–inorganic organic–inorganic nanocomposite materials. The design concept is shown in Scheme1 1..

Scheme 1. The design design concept concept of ( (aa)) DDSQ DDSQ epoxy/PSiBZ epoxy/PSiBZ and ( b)) POSS POSS epoxy/PSiBZ epoxy/PSiBZ nanocomposites.

2. Materials and Methods 2.1. Materials 2.1. Materials Phenyltrimethoxysilane (97%), dichloromethylsilane (97%), triethylamine (99.5%), isopropyl alcohol Phenyltrimethoxysilane (97%), dichloromethylsilane (97%), triethylamine (99.5%), isopropyl (99.5%), and allyl glycidyl ether (99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). alcohol (99.5%), and allyl glycidyl ether (99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium hydroxide was purchased from UniRegion Bio-Tech (Hsinchu, Taiwan).

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Polymers 2020, 12, x FOR PEER REVIEW 3 of 10 Sodium hydroxide was purchased from UniRegion Bio-Tech (Hsinchu, Taiwan). Tetrahydrofuran (95%)Tetrahydrofuran was purchased (95%) fromwas purchased Fisher Chemical from Fisher(Pittsburgh, Chemical PA, (Pittsburgh, USA). Toluene PA, USA). (99.5%) Toluene was obtained (99.5%) wasfrom obtained J.T. Baker from(Phillipsburg, J.T. Baker NJ,(Phillipsburg, USA). Tris(dibutylsulfide)rhodium NJ, USA). Tris(dibutylsulfide)rhodium trichloride was trichloride obtained fromwas obtainedGelest (Morrisville, from Gelest PA, (Morrisville, USA). N,N-dioctadecylmethylamine PA, USA). N,N-dioctadecylmethylamine was supplied by was Fluka supplied (Tokyo, by Japan).Fluka Activated(Tokyo, Japan). charcoal Activated powder charcoal was obtained powder from was Showa obtained (Saitama, from Showa Japan). (Saitama, Siloxane-imide-containing Japan). Siloxane- imide-containingbenzoxazine monomer benzoxazine (SiBZ) was mo providednomer by(SiBZ) Industrial was provided Technology by Research Industrial Institute Technology (Hsinchu, Research Taiwan). InstituteOcta-functionalized (Hsinchu, POSSTaiwan). epoxy Octa-functionalized (70 wt%) in cyclopentanone POSS epoxy was (70 purchased wt%) in fromcyclopentanone Hybrid was (Hattiesburg,purchased from MS, Hybrid USA). Plastics DDSQ (Hattiesburg, epoxy was prepared MS, USA). with DDSQ reference epoxy towas previous prepared literature with reference [38,40]. 1 toThe previous following literature is the structure [38,40]. The of the following as-synthesized is the structure DDSQ epoxy of the was as-synthesized confirmed through DDSQ epoxyH nuclear was confirmedmagnetic resonance through 1H (NMR): nuclear 0.31 magnetic (s, Si-CH resonanc3), 0.74e (t,(NMR): Si-CH 20.31-), 1.70 (s, Si-CH (m, Si-CH3), 0.742-CH (t, Si-CH2-), 2.452-), and1.70 2.67(m, (m,Si-CH -O-CH2-CH22-,-), ), 2.45 and 2.98 2.67 (m, (m, -O-CH-, -O-CH epoxide),2-, epoxide), 3.17 2.98 and 3.45(m, -O-CH-, (m, Si-(CH epoxide),2)3-O-CH 3.172-], andand 3.343.45 ppm (m, (m,Si- (CHSi-CH2)32-O-CH-CH2-CH2-], 2-O-).and The3.34 chemicalppm (m, structures Si-CH2-CH of the2-CH SiBZ2-O-). monomer, The chemical DDSQ epoxy, structures and POSS of epoxythe SiBZ are monomer,displayed in DDSQ Figure epoxy,1a–c, respectively. and POSS epoxy are displayed in Figure 1a–c, respectively.

Figure 1. Chemical structure of (a) siloxane-imide-containing benzoxazine monomer (SiBZ), Figure 1. Chemical structure of (a) siloxane-imide-containing benzoxazine monomer (SiBZ), (b) (b) double-decker silsesquioxane (DDSQ) epoxy, and (c) polyhedral oligomeric silsesquioxane double-decker silsesquioxane (DDSQ) epoxy, and (c) polyhedral oligomeric silsesquioxane (POSS) (POSS) epoxy. epoxy. 2.2. Preparation of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites 2.2. Preparation of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites The weight ratios of the prepared DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites are displayedThe weight in Tableratios1 .of A the mixture prepared of the DDSQ POSS epox or DDSQy/PSiBZ epoxy and and POSS SiBZ epoxy/PSiBZ monomers wasnanocomposites dissolved in aretoluene displayed to form in a Table homogeneous 1. A mixture solution of the with POSS a solid or DDSQ content epoxy of 50 and wt%. SiBZ The monomers mixture was was then dissolved poured in toluene to form a homogeneous solution with a solid content of 50 wt%. The mixture was then into an aluminum pan to remove the solvent through heating at 100 ◦C for 5 h. According to the results pouredof differential into an scanning aluminum calorimetry pan to remove (DSC) the analysis, solvent pure through SiBZ heating and DDSQ at 100 epoxy °C for/SiBZ 5 h. wereAccording cured to at the results of differential scanning calorimetry (DSC) analysis, pure SiBZ and DDSQ epoxy/SiBZ were 200 and 230 ◦C for 2 h. POSS epoxy/SiBZ were cured at 200 ◦C for 2 h and 230 ◦C for 6 h. cured at 200 and 230 °C for 2 h. POSS epoxy/SiBZ were cured at 200 °C for 2 h and 230 °C for 6 h. Table 1. Composition of double-decker silsesquioxane (DDSQ) epoxy/ polysiloxane-imide-containing Tablebenzoxazine 1. Composition (PSiBZ) and of double-decker polyhedral oligomeric silsesquioxane silsesquioxane (DDSQ) epoxy/ (POSS) polysiloxane-imide-containing epoxy/PSiBZ nanocomposites. benzoxazine (PSiBZ) and polyhedral oligomeric silsesquioxane (POSS) epoxy/PSiBZ nanocomposites. Sample SiBZ * DDSQ Epoxy POSS Epoxy Sample SiBZ * DDSQ Epoxy POSS Epoxy PSiBZ 100 wt% - - PSiBZ 100 wt% - - 20 wt% DDSQ epoxy/PSiBZ 80 wt% 20 wt% - 4020 wt% wt% DDSQ DDSQ epoxy epoxy/PSiBZ/PSiBZ 80 wt% 60 wt% 20 wt% 40 wt% - - 2040 wt% wt% POSS DDSQ epoxy epoxy/PSiBZ/PSiBZ 60 wt% 80 wt% 40 wt% - - 20 wt% 4020 wt% wt% POSS POSS epoxy epoxy/PSiBZ/PSiBZ 80 wt% 60 wt% - - 20 wt% 40 wt% 40 wt% POSS epoxy/PSiBZ 60 wt% - 40 wt% * Siloxane-imide-containing benzoxazine monomer. * Siloxane-imide-containing benzoxazine monomer. 2.3. Instrumentation 2.3. Instrumentation Proton nuclear magnetic resonance (1H-NMR) analysis was performed using an NMR spectrometer Proton nuclear magnetic resonance (1H-NMR) analysis was performed using an NMR (Varian 500 MHz; Agilent, Santa Clara, CA, USA) with CDCl3 as the solvent and tetramethylsilane (TMS) spectrometeras a reference. (Varian Differential 500 scanningMHz; Agilent, calorimetry Santa (DSC Clara, Q10; CA, TA Instruments,USA) with CDCl New3 Castle, as the DE, solvent USA) wasand tetramethylsilaneconducted under a(TMS) nitrogen as a atmosphere. reference. Differential Approximately scanning 10 mg calorimetry of the testing (DSC sample Q10; TA was Instruments, sealed in an New Castle, DE, USA) was conducted under a nitrogen atmosphere. Approximately 10 mg of the aluminum pan. The samples were heated from 40 to 250 ◦C at a heating rate of 10 ◦C/min. The onset testingtemperature, sample peak was temperature, sealed in an and aluminum enthalpy pan. of the The exothermic samples peakwere ofheated the cured from compositions 40 to 250 °C were at a heatingrecorded. rate Thermal of 10 °C/min. degradation The propertiesonset temperature, were measured peak usingtemperature, a thermogravimetric and enthalpy analyzer of the (TGA,exothermic Q500; peak of the cured compositions were recorded. Thermal degradation properties were measured using a thermogravimetric analyzer (TGA, Q500; TA Instruments, New Castle, DE, USA) from 30 to 800 °C at a heating rate of 10 °C/min. Dynamic mechanical analysis (DMA, Q800; TA Instruments, New

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Polymers 2020, 12, x FOR PEER REVIEW 4 of 10 TAInstruments, New Castle, DE, USA) from 30 to 800 ◦C at a heating rate of 10 ◦C/min. Dynamic mechanical analysis (DMA, Q800; TA Instruments, New Castle, DE, USA) was performed at a heating rate of 5 ◦C/min Castle, DE, USA) was performed at a heating rate of 5 °C/min from −80 to 280 °C with a fixed from 80 to 280 C with a fixed frequency of 1 Hz in the single cantilever mode. The dimensions of the frequency− of 1◦ Hz in the single cantilever mode. The dimensions of the test species were 17.5 mm (L) test species were 17.5 mm (L) 10.0 mm (W) 0.5 mm (T). Thermomechanical analysis (TMA, Q400; × 10.0 mm (W) × 0.5 mm (T). ×Thermomechanical× analysis (TMA, Q400; TA Instruments, New Castle, TA Instruments, New Castle, DE, USA) was conducted at a heating rate of 10 C/min from 30 to 250 C, DE, USA) was conducted at a heating rate of 10 °C/min from 30 to 250 °C, and◦ a force of 0.05 N was◦ andapplied. a force of 0.05 N was applied. 3. Results and Discussion 3. Results and Discussion 3.1. Curing Behavior of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites 3.1. Curing Behavior of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites The curing reactions of the pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites and POSS The curing reactions of the pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites and POSS epoxyepoxy/PSiBZ/PSiBZ nanocomposites nanocomposites are are illustrated illustrated inin SchemeScheme 22.. TheThe SiBZ monomer can can self-cure self-cure through through thermalthermal activated activated ring-opening ring-opening polymerizationpolymerization and co-cure with with DDSQ DDSQ or or POSS POSS epoxy epoxy to toform form DDSQDDSQ epoxy epoxy/PSiBZ/PSiBZ and POSSand POSS epoxy /epoxy/PSiBZPSiBZ thermosetting thermosetting nanocomposites nanocomposites under a highunder temperature a high conditiontemperature [47]. condition [47].

SchemeScheme 2. 2.Cross-linking Cross-linking structurestructure of of (a (a) )pure pure polysiloxane-imide-containing polysiloxane-imide-containing benzoxazine benzoxazine (PSiBZ), (PSiBZ), (b) (b)DDSQ DDSQ epoxy/PSiBZ, epoxy/PSiBZ, and and (c () cPOSS) POSS epoxy/PSiBZ epoxy/PSiBZ nanocomposites. nanocomposites.

TheThe DSC DSC thermograms thermograms of of thethe purepure PSiBZ,PSiBZ, thethe DDSQ epoxy/PSiBZ, epoxy/PSiBZ, and and POSS POSS epoxy/PSiBZ epoxy/PSiBZ nanocompositesnanocomposites are are shown shown the the exothermic exothermic curvecurve andand displayed in in Figure Figure 22 andand the the summary summary of of the the curingcuring characteristics characteristics are are listed listed in Tablein Table2. In 2. the In DDSQthe DDSQ epoxy epoxy/PSiBZ/PSiBZ system, system, both both the exothermicthe exothermic onset andonset peak and temperatures peak temperatures increased increased with the with percentage the percentage by weight by ofweight DDSQ of epoxy.DDSQ Aepoxy. similar A tendencysimilar wastendency observed was for observed POSS epoxy for POSS/PSiBZ; epoxy/PSiBZ; however, the however, steric hindrance the steric ofhindrance the eight of epoxy the eight group epoxy in the vicinitygroup position in the vicinity of the position POSS core of the was POSS higher core thanwas higher two epoxy than two group epoxy in diagonalgroup in diagonal position position of DDSQ core,of DDSQ which core, resulted which in resulted the exothermic in the exothermic onset and on exothermicset and exothermic with a maximum with a maximum temperature temperature of POSS of POSS epoxy/PSiBZ higher than those of DDSQ epoxy/PSiBZ and thus the curing time at 230 °C of epoxy/PSiBZ higher than those of DDSQ epoxy/PSiBZ and thus the curing time at 230 ◦C of POSS epoxyPOSS/PSiBZ epoxy/PSiBZ was set longerwas set thanlonger DDSQ than DDSQ epoxy/ epoxy/PSPSiBZ toiBZ ensure to ensure the cured the cured reaction reaction completely. completely. The chain mobility of PSiBZ was restricted by the incorporation of the rigid DDSQ or POSS cage into its structure; therefore, the onset and peak temperatures increased with the DDSQ or POSS

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content. Liu et al. also found a similar curing phenomenon in a PBZ-DDSQ . The onset and peak temperatures of the PBZ-DDSQ copolymer increased with the DDSQ content [44]. Polymers 2020, 12, 2510 5 of 10

Figure 2. Exothermic curves of (a) DDSQ epoxy/PSiBZ and (b) POSS epoxy/PSiBZ nanocomposites. Figure 2. Exothermic curves of (a) DDSQ epoxy/PSiBZ and (b) POSS epoxy/PSiBZ nanocomposites. Table 2. Curing characteristics of pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites, and POSS epoxyTable/PSiBZ 2. Curing nanocomposites. characteristics of pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites, and POSS epoxy/PSiBZ nanocomposites. Sample Tonset (◦C) Tmax (◦C) Pure PSiBZSample T 173onset (°C) Tmax 218(°C) 20 wt% DDSQPure epoxy PSiBZ/PSiBZ 186173 218 229 40 wt%20 wt% DDSQ DDSQ epoxy epoxy/PSiBZ/PSiBZ 190186 229 241 20 wt%40 wt% POSS DDSQ epoxy epoxy/PSiBZ/PSiBZ 198190 241 238 40 wt%20 wt% POSS POSS epoxy epoxy/PSiBZ/PSiBZ 199198 238 248 40 wt% POSS epoxy/PSiBZ 199 248 The chain mobility of PSiBZ was restricted by the incorporation of the rigid DDSQ or POSS cage into3.2. itsThermal structure; Stability therefore, of DDSQ the onsetEpoxy/PS andiBZ peak and temperatures POSS Epoxy/PSiBZ increased Nanocomposites with the DDSQ or POSS content. Liu etThe al. alsothermal found stability a similar of the curing pure phenomenon PSiBZ, the DDSQ in a PBZ-DDSQ epoxy/PSiBZ copolymer. nanocomposites The onset and and the peakPOSS temperaturesepoxy/PSiBZ ofnanocomposites the PBZ-DDSQ was copolymer analyzed increased by TGA with under the DDSQair atmosphere content [ 44as]. displayed in Figure 3a,b. The found 5% mass loss temperature (Td5%) was 419 °C for the pure PSiBZ and slightly decreased 3.2. Thermal Stability of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites to 413 °C and 410 °C for 20 wt% and 40 wt% DDSQ epoxy/PSiBZ, respectively. The decrease in Td5% shouldThe originate thermal stabilityfrom the of reacted the pure two PSiBZ, epoxy the grou DDSQps in epoxy the DDSQ/PSiBZ Epoxy/PSiBZ nanocomposites nanocomposites. and the POSS In epoxythe case/PSiBZ of the nanocomposites POSS epoxy/PSiBZ was analyzed nanocomposites, by TGA under Td5% for air 20 atmosphere wt% and as40 displayedwt% POSS in epoxy/PSiBZ Figure3a,b. Thewas found 392 °C 5% and mass 396 loss°C, respectively, temperature (reasonablyTd5%) was 419due ◦toC the for thepresence pure PSiBZof eight and reacted slightly epoxy decreased groups to of 413POSS◦C with and 410PSiBZ,◦C forleading 20 wt% to a andlower 40 thermal wt% DDSQ stability epoxy than/PSiBZ, the DDSQ respectively. epoxy/PSiBZ The decrease nanocomposites. in Td5% shouldThe char originate yield of from the DDSQ the reacted epoxy/PSiBZ two epoxy and groups POSS in epoxy/PSiBZ the DDSQ Epoxy nanocomposites/PSiBZ nanocomposites. also increased In with the casethe ofincrease the POSS of the epoxy DDSQ/PSiBZ or nanocomposites,POSS. As a result,Td5% the forDDSQ 20 wt% epoxy/ andPSiBZ 40 wt% nanocomposites POSS epoxy/PSiBZ exhibited was 392better◦C thermal and 396 stability◦C, respectively, than the POSS reasonably epoxy/PSiBZ due to nanocomposites. the presence of eight reacted epoxy groups of POSS with PSiBZ, leading to a lower thermal stability than the DDSQ epoxy/PSiBZ nanocomposites. The char yield of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites also increased with the increase of the DDSQ or POSS. As a result, the DDSQ epoxy/PSiBZ nanocomposites exhibited better thermal stability than the POSS epoxy/PSiBZ nanocomposites.

3.3. Dynamic Mechanical Properties of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites The dynamic mechanical properties of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites from 100 to 280 C were characterized through DMA. The storage modulus and − ◦ tanδ of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites are presented in Figure4 and Table3. The storage modulus of pure PSiBZ was 960 MPa at 25 ◦C. After 20 and 40 wt% DDSQ epoxy was added, the storage modulus increased to 1506 and 1516 MPa, respectively. In the 40 wt% POSS epoxy/PSiBZ, a higher storage modulus of 1753 MPa was obtained. The increases were approximately 158% and 183% for DDSQ and POSS, respectively, indicating superior reinforcement with POSS.

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content. Liu et al. also found a similar curing phenomenon in a PBZ-DDSQ copolymer. The onset and peak temperatures of the PBZ-DDSQ copolymer increased with the DDSQ content [44].

Figure 2. Exothermic curves of (a) DDSQ epoxy/PSiBZ and (b) POSS epoxy/PSiBZ nanocomposites.

Table 2. Curing characteristics of pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites, and POSS epoxy/PSiBZ nanocomposites.

Sample Tonset (°C) Tmax (°C) Pure PSiBZ 173 218 20 wt% DDSQ epoxy/PSiBZ 186 229 40 wt% DDSQ epoxy/PSiBZ 190 241 20 wt% POSS epoxy/PSiBZ 198 238 40 wt% POSS epoxy/PSiBZ 199 248

3.2. Thermal Stability of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites The thermal stability of the pure PSiBZ, the DDSQ epoxy/PSiBZ nanocomposites and the POSS epoxy/PSiBZ nanocomposites was analyzed by TGA under air atmosphere as displayed in Figure 3a,b. The found 5% mass loss temperature (Td5%) was 419 °C for the pure PSiBZ and slightly decreased to 413 °C and 410 °C for 20 wt% and 40 wt% DDSQ epoxy/PSiBZ, respectively. The decrease in Td5% should originate from the reacted two epoxy groups in the DDSQ Epoxy/PSiBZ nanocomposites. In the case of the POSS epoxy/PSiBZ nanocomposites, Td5% for 20 wt% and 40 wt% POSS epoxy/PSiBZ was 392 °C and 396 °C, respectively, reasonably due to the presence of eight reacted epoxy groups of POSS with PSiBZ, leading to a lower thermal stability than the DDSQ epoxy/PSiBZ nanocomposites. The char yield of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites also increased with the increase of the DDSQ or POSS. As a result, the DDSQ epoxy/PSiBZ nanocomposites exhibited better thermal stability than the POSS epoxy/PSiBZ nanocomposites. Polymers 2020, 12, x FOR PEER REVIEW 6 of 10 Polymers 2020, 12, 2510 6 of 10

Figure 3. TGA thermograms of (a) DDSQ/PSiBZ nanocomposites and (b) POSS epoxy/PSiBZ nanocomposites.

3.3. Dynamic Mechanical Properties of DDSQ Epoxy/PSiBZ and POSS Epoxy/PSiBZ Nanocomposites The dynamic mechanical properties of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites from −100 to 280 °C were characterized through DMA. The storage modulus and tanδ of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites are presented in Figure 4 and Table 3. The storage modulus of pure PSiBZ was 960 MPa at 25 °C. After 20 and 40 wt% DDSQ epoxy was added, the storage modulus increased to 1506 and 1516 MPa, respectively. In the 40 wt% POSS epoxy/PSiBZ, a higher storage modulus of 1753 MPa was obtained. The increases were approximately 158% and 183% for DDSQ and POSS, respectively, indicating superior reinforcement with POSS. Figure 3. TGA thermograms of (a) DDSQ/PSiBZ nanocomposites and (b) POSS epoxy/

PSiBZ nanocomposites.

Figure 4. (a) Storage modulus and (b) tanδ of DDSQ epoxy/PSiBZ nanocomposites as functions of temperature.Figure 4. (a) (Storagec) Storage modulus modulus and and (b ()d tan) tanδ δofof DDSQ the POSS epoxy/PSiBZ epoxy/PSiBZ nanoco nanocompositesmposites as asfunctions functions of oftemperature. temperature. (c) Storage modulus and (d) tanδ of the POSS epoxy/PSiBZ nanocomposites as functions of temperature. Table 3. Storage modulus and tanδ of DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites. Table 3. Storage modulus and tanδ of DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites. Sample Storage Modulus (MPa) @25 ◦C Tg (◦C) Pure PSiBZSample Storage Modulus 960 (MPa) @25 °C Tg (°C) 157 20 wt% DDSQPure epoxy PSiBZ/PSiBZ 1506 960 157 155 40 wt%20 wt% DDSQ DDSQ epoxy epoxy/PSiBZ/PSiBZ 1516 1506 155 140 20 wt%40 wt% POSS DDSQ epoxy epoxy/PSiBZ/PSiBZ 1407 1516 140 226 40 wt%20 wt% POSS POSS epoxy epoxy/PSiBZ/PSiBZ 1753 1407 not 226 obvious 40 wt% POSS epoxy/PSiBZ 1753 not obvious The peak temperature from the tanδ curves of the DDSQ/PSiBZ nanocomposites revealed that The peak temperature from the tanδ curves of the DDSQ/PSiBZ nanocomposites revealed that the addition of 20 and 40 wt% of DDSQ lowered Tg from the 157 ◦C of pure PSiBZ to 155 and 140 ◦C, the addition of 20 and 40 wt% of DDSQ lowered Tg from the 157 °C of pure PSiBZ to 155 and 140 °C,

Polymers 2020, 12, x FOR PEER REVIEW 7 of 10 respectively. The eight bulky phenyl groups of the DDSQ epoxy disturbed the chain stack to increase Polymerslarger free2020, volume12, 2510 in the cross-linked structure and thus reduce the Tg [44]. By contrast, the addition7 of 10 of 20 wt% POSS epoxy resulted in a higher Tg of 226 °C. Notably, Tg becomes not obvious when the POSS epoxy content was 40 wt%. The increased Tg can be attributed to the increased cross-linking respectively. The eight bulky phenyl groups of the DDSQ epoxy disturbed the chain stack to increase caused by the presence of eight epoxy groups of POSS. Furthermore, at 40 wt% POSS epoxy, the larger free volume in the cross-linked structure and thus reduce the T [44]. By contrast, the addition of apparent nano effect caused by strong interaction between nanoscaleg POSS epoxy and PSiBZ was 20 wt% POSS epoxy resulted in a higher Tg of 226 ◦C. Notably, Tg becomes not obvious when the POSS found; consequently, Tg became not obvious. Similar nano effect phenomena have been previously epoxy content was 40 wt%. The increased T can be attributed to the increased cross-linking caused by observed [48,49]. g the presence of eight epoxy groups of POSS. Furthermore, at 40 wt% POSS epoxy, the apparent nano There are two major factors that led to the reinforcement of the DDSQ/PSiBZ and POSS/PSiBZ effect caused by strong interaction between nanoscale POSS epoxy and PSiBZ was found; consequently, nanocomposites: (i) the rigidity of DDSQ and POSS and (ii) the Tg (directly related to the crosslinking T became not obvious. Similar nano effect phenomena have been previously observed [48,49]. density)g of the DDSQ/PSiBZ and POSS/PSiBZ nanocomposites networks. In the case of DDSQ/PSiBZ There are two major factors that led to the reinforcement of the DDSQ/PSiBZ and POSS/PSiBZ nanocomposites, the storage modulus increased with increasing the DDSQ content due to the rigidity nanocomposites: (i) the rigidity of DDSQ and POSS and (ii) the Tg (directly related to the crosslinking of DDSQ; however, the increased DDSQ simultaneously resulted in a lower Tg, which would lead to density) of the DDSQ/PSiBZ and POSS/PSiBZ nanocomposites networks. In the case of DDSQ/PSiBZ the decrease of modules because of the decreased crosslinking density. The very close modulus found nanocomposites, the storage modulus increased with increasing the DDSQ content due to the rigidity from the specimen of the 20 or 40 wt% of DDSQ could thus be considered as the competition of DDSQ; however, the increased DDSQ simultaneously resulted in a lower Tg, which would lead to of these two factors. On the other hand, in the POSS/PSiBZ system, both of the rigidity and Tg increase the decrease of modules because of the decreased crosslinking density. The very close modulus found with the increase of the POSS epoxy. The eight epoxy groups in POSS could provide more from the specimen of the 20 or 40 wt% of DDSQ epoxies could thus be considered as the competition crosslinking points in the POSS/PSiBZ network. As a consequence, the superior reinforcement effect of these two factors. On the other hand, in the POSS/PSiBZ system, both of the rigidity and T increase could be obtained for the POSS/PSiBZ nanocomposites. g with the increase of the POSS epoxy. The eight epoxy groups in POSS could provide more crosslinking points3.4. Thermomechanical in the POSS/PSiBZ Properties network. of DDSQ As aEpo consequence,xy/PSiBZ and the POSS superior Epoxy/PSiBZ reinforcement Nanocomposites effect could be obtained for the POSS/PSiBZ nanocomposites. The CTEs of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites were investigated 3.4.through Thermomechanical TMA and compared Properties with of DDSQ that of Epoxy pure/PSiBZ PSiBZ. and Figure POSS 5 Epoxy illustrates/PSiBZ the Nanocomposites dimensional changes and CTEs of the pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites, and POSS epoxy/PSiBZ The CTEs of the DDSQ epoxy/PSiBZ and POSS epoxy/PSiBZ nanocomposites were investigated nanocomposites below the transition temperature (Tg). Figure 5a revealed that CTE 1 (below Tg through TMA and compared with that of pure PSiBZ. Figure5 illustrates the dimensional changes and at a temperature range of 40–75 °C) considerably decreased with an increase in DDSQ content. From CTEs of the pure PSiBZ, DDSQ epoxy/PSiBZ nanocomposites, and POSS epoxy/PSiBZ nanocomposites 0 to 40 wt% of DDSQ, the decreases in CTE 1 were approximately 34%. Notably, unlike the DDSQ below the temperature (Tg). Figure5a revealed that CTE 1 (below Tg at a temperature epoxy system, the POSS epoxy/PSiBZ nanocomposites show the estimated changes in CTE 1 were range of 40–75 ◦C) considerably decreased with an increase in DDSQ content. From 0 to 40 wt% of approximately 14% (Figure 5b). The present results indicate that as the temperature decreased below DDSQ, the decreases in CTE 1 were approximately 34%. Notably, unlike the DDSQ epoxy system, Tg, the molecules lose energy under the glassy state, only short-range molecular movement occurred, the POSS epoxy/PSiBZ nanocomposites show the estimated changes in CTE 1 were approximately 14% and the larger core dimension and the phenyl substituents at the vertex of the DDSQ frame efficiently (Figure5b). The present results indicate that as the temperature decreased below Tg, the molecules restricted the chain mobility of PSiBZ. Thus, noticeable variation was observed in CTE 1. The results lose energy under the glassy state, only short-range molecular movement occurred, and the larger core indicate that both adding 40 wt% DDSQ epoxy and POSS epoxy demonstrated the significant CTE 1 dimension and the phenyl substituents at the vertex of the DDSQ frame efficiently restricted the chain reduced the effect of PSiBZ; furthermore, a DDSQ epoxy exhibited more of a CTE 1 reduce effect than mobility of PSiBZ. Thus, noticeable variation was observed in CTE 1. The results indicate that both the POSS epoxy. adding 40 wt% DDSQ epoxy and POSS epoxy demonstrated the significant CTE 1 reduced the effect of PSiBZ; furthermore, a DDSQ epoxy exhibited more of a CTE 1 reduce effect than the POSS epoxy.

Figure 5. (a) Dimension change and coefficients of thermal expansion (CTEs) of the DDSQ epoxy/PSiBZ nanocomposites and (b) dimension change and CTEs of the POSS epoxy/PSiBZ epoxy nanocomposites.

Polymers 2020, 12, 2510 8 of 10

4. Conclusions In this study, DDSQ epoxy and POSS epoxy were added to PSiBZ to achieve superior thermomechanical and dynamic mechanical properties. The addition of 40 wt% DDSQ epoxy resulted in a 34% reduction in CTE 1 and 158% increase in the storage modulus. The introduction of the 40 wt% POSS epoxy increased the storage modulus by 183%. This study not only serves as proof of the design concept but also experimentally demonstrated the potential of the DDSQ and POSS epoxy as functional additives for PSiBZ for lowering the CTE while enhancing the thermomechanical properties.

Author Contributions: C.-H.L. conceived the original concept and conducted experiments and the investigation as well as wrote the manuscript. W.-B.C. provided SiBZ monomer. W.-T.W. and C.-H.C. directed and revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to express their appreciation for the technical support from Material and Chemical Research Laboratories, ITRI. Conflicts of Interest: The authors declare no conflict of interest.

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