Zero Thermal Expansion Polyarylamide Film with Reversible

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Zero Thermal Expansion Polyarylamide Film with Reversible Conformational Change Structure Yuhui Huang,† Xingyuan Shen,†,‡ Zhao Wang,† Ke Jin,† Jennifer Q. Lu,‡ and Changchun Wang*,† †

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State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, 220 Han Dan Road, Shanghai 200433, China ‡ Materials Science and Engineering, School of Engineering, University of California, Merced, 5200 North Lake Road, Merced, California 95343, United States S Supporting Information *

ABSTRACT: Interface failure due to thermal mismatch is one of the culprits that shorten life cycle of devices. In this work, we prepared a series of free-standing cross-linked polyarylamide films with tunable thermal responsive behaviors. By incorporating thermal contractile units, s-dibenzocyclooctadiene (DBCOD), which shrink as a result of thermally induced upconversion from twist boat to chair, negative, zero, and positive coefficients of thermal expansion (CTE) have been realized with the values of CTE ranging from −17.7 to 57.1 ppm/K (20−150 °C). This wide tunable CTE window allows the as-prepared cross-linked polyarylamide films to match desired thermal performance for specific applications. Zeroexpansion polymer film without the need for inorganic dopants has also been obtained. Such a series of free-standing polyarylamide films with tunable thermal responses showed great potential for many practical applications.

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INTRODUCTION

Through years of intensive research, a number of methods have been developed to suppress the positive CTE values of polymers, one of which is to add inorganic fillers with low CTE or negative CTE, such as ZrW2O8 (CTEl = −9 ppm/K, 0− 1050 K),15 ScF3 (CTEl = −14.0 ppm/K, 60−100 K),16 and PbTiO3 (CTEv = −19.9 ppm/K, 298−763 K),17 and the antiperovskite manganese nitrides family.18−20 These materials could be employed as compensators to reduce the comprehensive CTE values of the resin matrix composites.21−25 However, the lack of interaction between polymer matrix and inorganic fillers limits the effectiveness of this approach.26,27 To avoid these issues, the most straightforward way is to directly incorporate thermal contractile units into polymer chains. Very recently, we have reported that polyarylamides incorporated with s-dibenzocyclooctadiene (DBCOD) exhibited negative thermal expansion (NTE).28−31 The unique contraction mainly originated from the reversible conformational change of DBCODs in the polymer chains. From the aspect of application, zero thermal expansion (ZTE) materials might be of greatest importance, as many practical conditions called for strict size preciseness, such as high-precision instruments,32 microelectronics,33,34 and protection shields for inflammables and explosives.35 In these

Because of the anharmonic nature of chemical bond potential, most of the materials exhibit positive thermal expansion (PTE),1−5 and various materials offer different coefficients of thermal expansion (CTE). For real-world applications, the large CTE of materials sometimes is rather detrimental, especially in certain fields that emphasize much on the dimensional stability, such as standard rulers, precision optical instruments, and so on. Furthermore, in the case of devices constructed by multiple materials, CTE mismatch within different components upon temperature variation might lead to internal thermal stress6,7 or even structural failure,8 as the thermally driven size changes could cause interfacial stress accumulation and descended precision, which are sure to sharply reduce the reliability and service life of a certain device.9 Today, polymers have played an increasingly important role in practical applications, owing to their advantages of good performance, ease to process, and cheap price.10−12 However, compared with inorganic materials such as silica (SiO2, CTE ≈ 0.54 ppm/K), bulk polymers usually possess much higher positive CTE values,13,14 for example, polyethylene (PE, CTE ≈ 200 ppm/K), polyoxymethylene (POM, CTE ≈ 100 ppm/ K), and epoxy resin (EP, CTE ≈ 60 ppm/K). Such large CTEs might hinder their applications in composite materials and precision equipment. Therefore, it is of great importance to find solutions to modulate the thermal expansion of polymers. © XXXX American Chemical Society

Received: September 1, 2018 Revised: October 1, 2018

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DOI: 10.1021/acs.macromol.8b01890 Macromolecules XXXX, XXX, XXX−XXX

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1; a large amount of white precipitate was obtained. The precipitate was separated by centrifugation and then was washed with water for several times. After drying, 0.4 g (2.1 mmol) of target solid product with yield about 95% was obtained. 1H NMR (400 MHz, DMSO-d6): δ (ppm) 13.09 (s, 2H), 7.73 (s, 2H), 3.33 (s, 4H) (Figure S1). 13C NMR (101 MHz, DMSO-d6): δ (ppm) 166.6, 148.8, 129.0, 128.3, 31.3 (Figure S2). Elementary analysis for [C10H8O4]: calculated C (62.50), H (4.20), N (0.00); found C (62.51), H (4.14), N (0.00). Maldi-TOF m/z for C10H8O4 [M + Ag], calculated: 299.0; found: 299.0. Preparation of Linear BCB-Containing Polyarylamide with Four-Membered Ring Units. The Yamazaki−Higashi phosphorylation polycondensation was applied to prepare a series of linear polyarylamides. The same molar amount of total amine groups as that of total carboxyl groups was kept in all polymerizations with triphenyl phosphite (TPP) and pyridine (Py) as condensation agents. A typical experiment for preparation of S-2/1 as an example: a 10 mL Schleck tube equipped with a magnetic stirrer was charged with 192.0 mg (1 mmol) of DDA, 129.1 mg (0.5 mmol) of OBBA, 300.0 mg (1.5 mmol) of 3,4′-ODA, and 525 mg of CaCl2. Then the Schleck tube was degassed three times and filled with nitrogen. 2 mL of TPP, 1 mL of Py, and 4 mL of NMP were mixed and added into the Schleck tube. All monomers can be dissolved in the solvent. After sealing the Schleck tube, the reaction system was heated to 120 °C with vigorous stirring for 24 h. The reaction solution gradually became viscous. After the solution cooled to room temperature, 50 mL of methanol was dropped in to induce the precipitation of polymer from NMP. The precipitated fiber-like product was collected by centrifugation and then purified by repetitive methanol washing, followed by drying under vacuum at 40 °C for 12 h. All the other linear polyarylamide precursors were prepared through a similar operation. Preparation of the Cross-Linked Polyarylamide Films. Silicon substrates coated with 100 nm of thermally grown oxides were cleaned by treatment with a freshly prepared piranha solution (3/1, v/v, concentrated H2SO4/30% aqueous H2O2) at 90−100 °C for 1 h and then rinsed with deionized water; all silicon substrates were treated before use. For the film preparation, the dried linear polymer was dissolved in NMP at a concentration of 50 mg/mL, and then 200 μL of polymer solution was drop cast onto the pretreated silicon substrates (20 mm × 8 mm). After 2−3 days of drying at 50 °C, the films were annealed in a tube oven under argon at 310 °C for 8 h and 350 °C for 16 h. Buffered hydrofluoric acid etching deattached the cross-linked films from the substrates. The thickness was measured as 66 ± 3 μm. The as-obtained films were used in other measurements and characterizations. Monomer and Polymer Structure Characterization. Liquid 1 H NMR and 13C NMR spectra were recorded at ambient temperature on a Varian Mercury plus 400M. In a typical procedure, approximately 5−8 mg of samples (either monomers or polyarylamides) was fully dissolved in an appropriate solvent before measurements. A typical scan time for a sample was 32 times for 1H NMR. Data were reported as the chemical shift (δ) measured in ppm downfield from tetramethylsilane (TMS). Elemental analysis was recorded on Analysemsysteme GmbH, Vario EL III elemental analyzer. The MALDI-TOF mass spectrum of DDA was obtained on AB SCIEX 5800 with silver trifluoroacetate as salt. Molecular Weight Characterization. Molecular weights (Mn, Mw) and polydispersity (Mw/Mn) of the as-synthesized linear polyarylamide samples were measured at 55 °C on an Agilent 1260 gel permeation chromatograph (GPC) equipped with refractive index detector (RI). Dimethylformamide (DMF) was utilized as the eluent at a flowing rate of 1.0 mL/min, with a series of commercial poly(methyl methacrylate) (PMMA) standards. Determining of the Cross-Linking Temperature for the Linear Polymer Films. The annealing temperature was determined by a TA Q2000 differential scanning calorimetry (DSC) instrument. 3−5 mg linear polymer powder was sealed in an aluminum pan and heated from 40 to 400 °C at a heat rate of 10 °C/min under nitrogen (flow rate 50 mL/min).

cases, the deformation ratio and CTE value were expected to be close to zero within a certain temperature range, which required a more precise balance between thermal expansion and contraction within a material. Among all these explorations of novel ZTE materials, two strategies are widely employed.36,37 The first approach was to build composites by blending thermal expansive materials and thermal contractive materials together, while the problem might be the internal stress caused by CTE mismatch as mentioned above.36,38 The other approach was chemically doping the NTE materials to tailor the CTE value from negative to zero. However, this method was usually applied only in inorganic materials and might be limited by the complex doping process. Thus, only a few inorganic compounds, such as YbGaGe,39 N(CH3)4CuZn(CN)4,40 Mn3AN (A = Cu/Sn, Zn/Sn),41 and Fe[Co(CN)6],42 have been found to have ZTE feature. To make the things worse, some of these compounds have a narrow effective temperature window (generally below room temperature43), and most of them do not have sufficient mechanical properties to produce macroscale parts, which definitely hindered their applications in optical, energy, and microelectric fields.44,45 Therefore, finding more ZTE materials with good mechanical properties was still challenging. Meanwhile, another important thing should be considered was whether the thermal response was isotropic. It has been reported that some inorganic thermal contractive materials have anisotropic CTE values; for example, they may contract in one direction while remain thermally expansive in other directions.46,47 Such anisotropic thermal responsive behaviors were not favorable in practical applications, as it might lead to interfacial and local stress or even microcracks during repetitive cycles of heating and cooling,35 and finally resulted in deteriorated performance and reduced service life. So isotropic CTEs were vital and highly demanded as well, and this performance could be obtained in amorphous organic polymers. Herein, we reported a series of cross-linked amorphous polyarylamide films which offered a wide range of CTE values for the first time. First, we successfully prepared a series of benzocyclobutene (BCB)-containing linear precursors by introducing an aromatic diacid, 4,4-oxybisbenzoic acid (OBBA), as a third monomer. This monomer not only realized the regulating of BCB content but also greatly improved the solubility and film formation ability of the linear polyarylamides. A following thermal annealing step caused BCB dimerization and led to the formation of DBCOD. The as-prepared free-standing cross-linked films with different amounts of thermal contractile unit DBCOD offered a range of CTEs, from negative to positive, and even zero expansion.

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EXPERIMENTAL SECTION

Materials. 3,4′-Oxydianiline (3,4′-ODA, 97%) and N-methyl-2pyrrolidone (NMP, anhydrous) were purchased from J&K, and 4,4′oxybibenzoic acid (OBBA, 98%) was obtained from Aladdin. All above chemicals were used as received. 1,2-Dihydrocyclobutabenzene-3,6-dicarboxylic chloride (DDC) was achieved from a cooperative group. All other reagents, unless otherwise stated, were purchased from commercial sources and used without further purification. Synthesis of 1,2-Dihydrocyclobutabenzene-3,6-dicarboxylic Acid (DDA). 0.5 g of 1,2-dihydrocyclobutabenzene-3,6dicarboxylic chloride (DDC, 2.2 mmol) was added to 5 g of sodium hydroxide (NaOH) aqueous solution (10 wt %) at room temperature. After being stirred overnight, the reaction solution became transparent. Then diluted hydrochloric acid was added to adjust the pH to B

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Figure 1. (a) Synthetic route of 1,2-dihydrocyclobutabenzene-3,6-dicarboxylic acid (DDA). (b) Yamazaki−Higashi phosphorylation polycondensation of DDA, OBBA, and ODA.

Table 1. Chemical Compositions, Molecular Weights, and Thermal Properties of As-Synthesized Polyarylamide Copolymers sample

monomer feeding ratioa

m:nb

BCB contentc (wt %)

Mwd (kg mol−1)

Mnd (kg mol−1)

Mw/Mnd

Tge (oC)

S-DDA S-9/1 S-7/1 S-5/1 S-2/1 S-1/2 S-1/4 S-OBBA

1:0:1 9:1:10 7:1:8 5:1:6 2:1:3 1:2:3 1:4:5 0:1:1

1:0 9.01:1 6.99:1 5.02:1 2.00:1 1:1.97 1:3.99 0:1

26.5 23.5 22.7 21.5 16.7 7.95 4.68 0

f 79.4 63.1 43.0 62.2 68.6 54.7 81.1

f 50.7 41.3 22.9 43.2 50.3 32.3 36.8

f 1.57 1.53 1.88 1.44 1.36 1.70 2.20

g 237 231 229 236 225 231 192

a Represent feeding molar ratios of DDA:OBBA:ODA. bm:n was calculated by 1H NMR through the integration of double peak signals (at δ 10.10 ppm and δ 10.06 ppm) and single peak signal (at δ 10.28 ppm) (Figure 2). cBCB content was calculated by the weight ratio of BCB unit to the whole polymer chain. dThe data of weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (Mw/ Mn) were measured by GPC with DMF as eluent (1 mL/min) calibrated by PMMA standard. eGlass transition temperatures were recorded by DSC with a heating rate of 10 °C/min under a nitrogen atmosphere after eliminating thermal history (representative curves are shown in Figure S3). fS-DDA is insoluble in DMF and cannot be characterized by GPC. gS-DDA is in crystalline state so that no Tg was observed.

load was 5 mN, and nitrogen flow rate was 40 mL/min. The ramp rate was 2 K/min unless otherwise mentioned. For the investigation on reversibility, the same film sample was tested for several runs of heating under the same procedure. The film recovered under room temperature for 24 h each time before it was tested again. The heating rate was set as 2, 20, and 50 K/min to study the effect of heating rate on thermal response, while all other parameters were kept the same. The linear thermal expansion coefficient of a film under constant pressure can be calculated according to eq 1.

Thermal Properties Evaluation of the Polymer Films. DSC (TA Q2000) was applied to test the glass transition temperature (Tg) of the linear polyarylamide percursors. 4−6 mg samples encapsulated in aluminum pan were first heated to 300 °C and isothermed for 5 min to eliminate thermal history. Then they were cooled to 25 °C and heated to 300 °C again with a rate of 10 °C/min and nitrogen flow of 50 mL/min. Tg was measured by the midpoint of the heat capacity jump during the second scan. To evaluate thermal stability, thermal gravimetric analysis (TGA) was performed on a PerkinElmer Pyris 1 TGA instrument. Approximately 1−3 mg of a sample was heated under nitrogen with a flow rate of 40 mL/min at a heating rate of 10 °C/min. Characterization of the Cross-Linked Films by WAXD. Wideangle X-ray diffraction (WAXD) was performed by a Rigaku X-ray diffractometer with a Ni-filtered Cu Kα radiation (λ = 0.154 nm) at room temperature. The scan rate was 5°/min from 5° to 40°. The selected voltage and current were 40 kV and 200 mA, respectively. The morphology of the polymer films was detected by an atomic force microscope (AFM) on Multimode 8 (Bruker, USA) with tapping mode. Study on the Thermal Expansion/Contraction Behavior of the Cross-Linked Films. Thermal expansion/contraction behaviors of the polymer films were analyzed by a Mettler Toledo-SDTA841e thermomechanical analyzer (TMA). Polymer films were tailored to a uniform size of 15 mm × 4 mm (effective length 10 mm). The tensile

α=

1 ΔL L0 ΔT

(1)

where ΔL/ΔT is the change rate of the sample length with temperature and L0 is the initial length of a sample. The deformation ratio can be calculated according to eq 2.

deformation ratio (‰) =

ΔL × 1000‰ L0

(2)

Study on the Microscopic Conformational Change of the Cross-Linked Films. DSC (TA Q2000) was used to study the DBCOD conformational change. The preannealed cross-linked polymer films (3−5 mg) were encapsulated in an aluminum pan and then were heated from −30 to 150 °C at a heating rate of 2 °C/ min with the nitrogen flow rate of 50 mL/min. For reversibility study, C

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Macromolecules the same samples were measured with the same procedures after 24 h resetting at room temperature in a DSC pan. FTIR spectra at different temperatures were measured on a spectrometer (Thermofisher, Nicolet 6700) equipped with heating and temperature controlling accessories. Detailed parameters were as follows: 32 scans, spanning a spectral range of 4000−400 cm−1 with resolution of 4.0 cm−1. Films were directly used in FTIR measurements.

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RESULTS AND DISCUSSION Preparation of Linear BCB-Containing Polyarylamides. Our previous work has revealed that the conformational change of DBCOD units in polyarylamide could lead to giant thermal shrinkage.28,31 One route to form thermal contractive unit DBCOD is dimerization of benzocyclobutene (BCB). To prepare BCB-containing polyarylamide, we first converted 1,2-dihydrocyclobutabenzene-3,6-dicarboxylic chloride (DDC) to 1,2-dihydrocyclobutabenzene-3,6-dicarboxylic acid (DDA), as shown in Figure 1a. Yamazaki−Higashi phosphorylation polycondensation (Figure 1b) was used to prepare BCB-containing polymers. Then a series of polyarylamides with different amounts of BCB units (Table 1) were prepared by adjusting the molar ratio of DDA, 3,4′oxidianiline (3,4′-ODA), and a third monomer 4,4′-oxybisbenzoic acid (OBBA). OBBA was employed to adjust BCB content in the polymer chain, while the ratio of total amine groups to total carboxyl groups was always kept equimolar to obtain high molecular weight polymers. Metasubstituted 3,4′-ODA was used instead of para-substituted 4,4′-ODA to reduce dense packing of polymer chains and to favor both solubility and film formation. The feeding molar ratios of DDA:OBBA:ODA were set as 1:0:1, 9:1:10, 7:1:8, 5:1:6, 2:1:3, 1:2:3, 1:4:5, and 0:1:1. The corresponding linear polymer was named as S-DDA (containing BCB), S-9/1, S-7/1, S-5/1, S-2/1, S-1/2, S-1/4, and SOBBA (none BCB). After polymerization, a series of clear, homogeneous, and highly viscous polymer solutions were obtained. Gel permeation chromatography (GPC) was used to measure both the weight-average molecular weight (Mw) and number-average molecular weight (Mn) of this series of linear polyarylamides with poly(methyl methacrylate) as standards. As summarized in Table 1, we have achieved high molecular weight polymers with Mw and Mn ranging from 43.0 to 81.1 kg/mol and 22.9 to 50.7 kg/mol, respectively. Such high molecular weight macromolecules allowed the formation of free-standing films with good mechanical properties. 1 H NMR spectra of BCB-containing polymers and S-OBBA are displayed in Figure 2. Peaks with chemical shift ranging from 8.10 to 6.70 ppm were assigned to hydrogen atoms on benzenes, while the peak at 3.45 ppm was from the hydrogens of −CH2− units. The doublet with the chemical shift of 10.10 and 10.06 ppm was ascribed to the amide from DDA and 3,4′ODA, while the single peak at chemical shift of about 10.28 ppm was ascribed to the amide from OBBA and 3,4′-ODA. The ratio of these integrated areas was used to quantify BCB content in the polyarylamides, as listed in Table 1. There was little difference between feeding ratio and calculation results, which further convinced the excellent polymerization and the high molecular weight of the polymers. Another purpose of introducing the third monomer was to improve both solubility and the film formation. The polyarylamide sample of S-DDA, without a third monomer of OBBA, was not soluble in N,N-dimethylformamide (DMF), whereas the rest of polyarylamides that contained OBBA were

Figure 2. 1H NMR spectra of the as-prepared linear polyarylamide copolymers (inset: an enlarged part of the spectra with chemical shift from 10.5 to 9.9 ppm).

soluble in common polar solvents such as DMF or NMP. For film preparation, the polymer solution with a concentration of about 50 mg/mL in NMP was drop-cast on a silicon substrate. After solvent evaporation, all these samples could be peeled off from the substrates to form free-standing films except the sample of S-DDA (Figure S4). This result implied the improved film formation ability by the incorporation of OBBA. Preparation and Characterization of DBCOD-Containing Cross-Linked Polymer Films. After solvent evaporation, the polymer films were annealed to dimerize BCB groups to form DBCOD-containing cross-linked films. Figure 3a illustrates a schematic process flow. It was known that BCBs would undergo dimerization to form DBCOD.28 As shown in the differential scanning calorimetry (DSC) curves (Figure S5), the exothermal peak with an onset temperature at about 310 °C was the result of [4+4] cycloaddition, which led to the formation of thermal contractile unit DBCOD (Figure 3b). Thus, a two-step annealing, 8 h at 310 °C and 16 h at 350 °C under an argon atmosphere was used to ensure the completeness of dimerization reaction. Because BCB also functioned as cross-linking group, the reaction between BCB units would indicate that a cross-linking network was obtained at the same time. The cross-linked samples were named as XS, for example, XS-2/1 was the corresponding film made from linear polymer S-2/1 through thermal annealing. After thermal annealing, a series of flexible, free-standing cross-linked films were obtained (Figure S6). Thermal gravimetric analysis (TGA) curves (Figure S7) revealed that the thermal stability of the films was greatly improved after dimerization, which was about 5% weight loss at 470 °C (Table S1). On the contrary, the decomposition temperature of non-cross-linked films was about 368 °C. The enhanced thermal stability about 100 °C might be attributed to the formation of cross-linking network, verifying the occurrence of the dimerization reaction of BCB units. There were no sharp peaks observed in the wide-angle X-ray diffraction (WAXD) spectra (Figure S8) for all the cross-linked D

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Figure 3. (a) Process flow of the formation of DBCOD-containing cross-linked polymer films from the BCB-containing linear polymers. (b) Dimerization of BCB units during thermal annealing to produce thermal contractile unit DBCODs. (c) Conformational change of DBCOD units from twist boat to chair during heating leading to volume shrinkage.28

Figure 4. Thermal contraction behaviors of the as-prepared DBCOD-containing cross-linked polyarylamide films. TMA plots for the series of films (a) S-OBBA, (b) XS-9/1, (c) XS-5/1, (d) XS-2/1, (e) XS-1/2, and (f) XS-1/4 under N2 with a heating rate of 2 K/min (effective size of all samples is 10 mm × 4 mm × 66 μm).

containing cross-linked polyarylamide films exhibited different thermal responses as indicated by TMA characterization shown in Figure 4b−f and Figure S10. The data in Table 2 are the summary of TMA analysis, indicating the CTE values can be tailored from −17.7 to 15.7 ppm/K depending on the amount of DBCOD units in the films. Equivalently, the deformation ratio could be controlled within ±2.5 ‰ (ranging from −2.3‰ to +2.0‰). The tunable CTE values from negative to nearly zero and positive allowed them to be used for a variety of applications. For example, it could match well with the CTE value of Cu (CTE = 16−18 ppm/K) for electronic packaging, which was of great value as they would effectively avoid the problems caused by CTE mismatch within different materials.

polymer films, indicating that these films were in amorphous state. Atomic force microscope (AFM) images (Figure S9) in agreement with WAXD results showed no apparent ordered structure. It can be deduced that these materials should possess isotropic thermal behaviors. Thermal Contraction/Expansion Behaviors of the DBCOD-Containing Cross-Linked Polyarylamide Films. Thermal mechanical analysis (TMA) was utilized to study thermal behaviors of this series of films. First of all, the length change of the control sample S-OBBA (non-DBCOD) upon heating was recorded. The linear polyarylamide film, S-OBBA, underwent normal thermal expansion in the temperature range 20−150 °C (Figure 4a). The calculated CTE value was 57.1 ppm/K (20−150 °C). On the other hand, the DBCODE

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films gradually decreased when the temperature increased. That was rare, especially in polymers. Based on our former work, such unique thermal contraction behavior was attributed to the conformational change of thermal contractile unit DBCOD (formed from the dimerization of the BCB units in the linear polyarylamides during thermal annealing) from twist boat to chair (Figure 3c).28,30 Because the conformational change could be driven by low energy, the macroscopic thermal contraction all started at a temperature very close to room temperature. Such low energy induced responses and wide range of CTEs rendered this series of polyarylamide films to be qualified candidates for many practical uses. Study on the Mechanism of Thermal Contraction. To verify that the origin of the unique thermal contraction was the conformational change of DBCOD, both DSC and variable temperature Fourier transform infrared spectroscopy (VTFTIR) were employed in further mechanism study. Taking the cross-linked film of XS-5/1 as an example, there was an endothermic peak with an onset point of about 40 °C in DSC curve (Figure 5a), which did not exist in the DSC figure of control polymer S-OBBA or corresponding linear BCBcontaining polyarylamide S-5/1 (Figure 5b), suggesting that the endothermic peak should originate from the conformational change of DBCOD. Meanwhile, VT-FTIR analysis of the crossslinked XS-2/1 film at different temperatures (Figure 5c) also pointed to the conclusion that the macroscopic volume shrinkage was induced by the conformational change of thermal contractive unit DBCOD. As shown in Figure 5c, there was a pronounced −NH− stretching band at 3634 cm−1 at a lower temperature, indicating that the cross-linked film contained more free amide groups. In addition, there was also a broader −NH− peak that contained multiple −NH− stretching bands. On the basis of that, it was expected that

Table 2. Thermal Expansion/Contraction Analyses and Comparison for the As-Prepared DBCOD-Containing Polyarylamide Films with the Control Film by TMA at a Heating Rate of 2 K/min under N2 samplea XS-9/1 XS-7/1 XS-5/1 XS-2/1 XS-1/2 XS-1/4 S-OBBA

average CTE (20−150 oC) (ppm/K)b −2.9 −6.4 −17.7 −12.3 −1.3 15.7 57.1

± ± ± ± ± ± ±

0.3 1.0 0.5 0.3 0.1 1.2 2.4

deformation ratio (20−150 oC) (‰)c −0.38 −0.83 −2.31 −1.59 −0.16 2.04 7.17

± ± ± ± ± ± ±

0.04 0.12 0.07 0.05 0.02 0.15 0.33

Samples with the effective size of 10 mm × 4 mm. The lengths of films were obtained from TMA measurement. bAverage CTE = (L150 °C − L20 °C)/(150 − 20)/L20 °C × 106, where L150 °C is the length value at 150 °C and L20 °C is the length value at 20 °C. cDeformation ratio (‰) = (L150 °C − L20 °C)/L20 °C × 1000‰. a

Materials with an absolute CTE value below 2 ppm/K can be considered as zero expansion (ZTE).48,49 In the temperature range 20−150 °C, the CTE value of XS-1/2 was about −1.3 ppm/K, with an extremely low deformation ratio of only −0.16 ‰, close to zero. Therefore, a new free-standing ZTE polymer film without any fillers was created, which might be of great potential value in practical applications, since developing free-standing noncomposite polymer films with isotropic zero expansion was rather a challenging task yet with significant importance. It could be noted that some DBCOD-containing crosslinked films offered negative CTE (such as XS-5/1 with the lowest CTE of −17.7 ppm/K), which meant they underwent a peculiar thermal contraction. In other words, the length of the

Figure 5. Mechanism study of the thermal contraction behavior. (a) DSC plot of XS-5/1 film with a heating rate of 2 °C/min for the first time and after 24 h recovery. (b) DSC plot of control S-OBBA and linear S-5/1 prepolymer film with a heating rate of 2 °C/min. (c) FTIR spectra for XS-2/ 1 equilibrated at different temperatures from 25 to 150 °C. (d) FTIR spectra for XS-2/1 at 25 °C, equilibrated at 150 °C and cooled to 25 °C. F

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polymerization. Compared with dual-component polyarylamides, both solubility and film formation ability of the linear tricomponent polyarylamides were greatly enhanced. During thermal annealing at high temperature, the BCB units in the polyarylamides underwent dimerization to produce thermal contractive unit DBCOD, and at the same time, corresponding free-standing, isotropic, cross-linked polyarylamide films were obtained. Attributed to the low-energy driven, reversible, shrinkage-inducing conformational change of thermal contractive structure DBCOD from twist boat to chair, these annealed films showed unique macroscopic thermal contraction. More importantly, different responses to thermal stimuli were realized by just tuning the BCB contents in linear polyarylamide films. In a temperature range of 20−150 °C, this series of cross-linked polyarylamide films with different DBCOD concentration offered a set of low CTE values no more than 16 ppm/K, varying from −17.7 to 15.7 ppm/K with deformation ratios under 2.5‰ (−2.3‰ to 2.0‰). Notably, it also provided a new solution to prepare size-stable ZTE polymers without any inorganic fillers, and the CTE absolute values could be tailored lower than 1.5 ppm/K. Because all polyarylamide films were amorphous and isotropic, the thermal responses were also expected to be the same in all directions and could effectively avoid various problems such as internal stress accumulation that occurred in anisotropic materials or composites. Therefore, this intriguing series of free-standing, amorphous polyarylamide films with tunable CTE values from negative to zero and positive were of great importance and practical potential in many fields. In addition, the low-energy driven, quick, and reversible response to thermal stimuli also made them qualified candidates for many practical applications, such as packing industry, optical instruments, etc.

most DBCOD entities would be in twist boat conformation at lower temperature. With the temperature increasing, there appeared a great difference in the spectra, that was, the free −NH− gradually disappeared, and at the same time, the hydrogen-bonded −NH− stretching peaks became narrower. That trend implied that twist boat conformers were gradually converting to chair conformation state, leading to volume shrinkage. Therefore, in a macroscopic view, the film would undergo a contraction process, which was well consistent with TMA data. When the temperature reached about 120 °C, there was no further significant change, signaled that the conformational conversion of DBCOD had reached an equilibrium. Both DSC and VT-FTIR results corresponded well with our former findings.28,30 We further studied how heating rate affected the thermal response of the DBCOD-containing polyarylamide films. It was found that their thermal contraction was heating rate dependent (Figure S11). When the heating rate increased from 2 to 50 K/min, the CTE values and deformation ratio of XS-5/ 1 film in the same temperature region (20−150 °C) decreased from −17.7 to −2.6 ppm/K and −2.3 to −0.3 ‰, respectively (Table S2). That was, the film shrunk less under higher heating rate in the contraction region, while in the expansion region, the CTE value almost remained the same. Thus, it can be supposed that when the temperature increased faster than the conformational change of DBCOD, part of the thermal contractive units might not be able to catch up with heating and finally resulted in the reduced contraction. This also indicated that the thermal contraction could be attributed to the conformational change of thermal contractile unit DBCOD. It was also impressive that the contractive unit DBCOD could quickly respond to the environmental temperature changes. Revealed by the VT-FTIR spectra (Figure S12a), it took only a few minutes (∼10 min) to reach the equilibrium state of conformational change when temperature fluctuation was about 10 °C, as the spectra showed no significant difference after 10 min. When the temperature went higher, its thermal response would become even faster, so that no more than 5 min was required (Figure S12b). Such a fast thermal response might be of great importance in certain applications like actuators. Furthermore, the conformational change was reversible in both macroscopic and microscopic views. As indicated by Figure S13, there was no obvious difference in thermal response of sample XS-5/1 after several cycles of heating and cooling, proving the reversibility of conformational change. Meanwhile, in a microscopic view, a second DSC scan was taken again on the same film that had already undergone a cycle of thermal contraction and recovery. It could be found that after cooling at room temperature for 24 h the figure can be completely restored (Figure 5a), including the peak position and peak area, which illustrated its full reversibility and was consistent with macroscopic TMA results as well. Moreover, FTIR spectra also further proved its reversibility. When the film cooled to low temperature (25 °C), the new spectrum and the initial one obtained before heating can coincide (Figure 5d), also suggesting that conformational change was reversible.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01890. 1

H NMR and 13C NMR spectra of the monomer and the characterization of the polyarylamide films (photographs, DSC, TGA, TMA, WAXD, and AFM) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.W.). ORCID

Changchun Wang: 0000-0003-3183-2160 Author Contributions

The concept was designed by C.W. and Y.H., and the experiments were conducted by Y.H. and X.S. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was financially supported by the National Science Foundation of China (Grants 51633001 and 51721002), State Key Project of Research and Development (Grant 2016YFC1100300), and NSF-Biomaterials 1309673 and NASA NNX15AQ01.

CONCLUSION In this paper, a series of linear polyarylamides with different contents of BCB units were successfully prepared by introducing a third monomer during the condensation G

DOI: 10.1021/acs.macromol.8b01890 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01890 Macromolecules XXXX, XXX, XXX−XXX