Novel design, facile synthesis and low infrared emissivity properties of single-handed helical polysilanes

Abstract

The novel single-handed helical polysilanes (HPS), poly[di-n-hexylsilane-co-(methoxycarbonyl ethyl propyl ether)methylsilane] were synthesized by the Wurtz-type coupling of the chiral functional monomer, dichloro(methoxycarbonyl ethyl propyl ether)methylsilane (DCMMS). The monomer was prepared by the hydrosilylation reaction of methyl (S)-2-(allyloxy)propanoate. The chemical structure of HPS was confirmed by ¹H, ¹³C and ²⁹Si nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis) and circular dichroism (CD). The weight distributions and crystallinity of the polysilanes were demonstrated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and polarized light microscopy (POM), respectively. The single-handed helical characterization and polymerization degree of the new-typed polysilanes could be controlled by the co-monomers of DCMMS and di-n-hexyldichlorosilane (DCDHS), respectively. Furthermore, the infrared emissivity values (0.598 at 8–14 μm) of HPS were investigated by an Infrared Emissometer.

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1. Introduction

The low infrared emissivity polymers have been gradually attracting great interest in the past decade.^1,2 Polymers not only exhibit extraordinary properties like solubility,³ tractability and anti-corrosion,⁴ but also fulfill the adjustable infrared emissivity properties by switching structures to realize various applications. Hence, we took actions to prepare helical polymers and studied their intramolecular cooperative interaction. In previous works of our group, various kinds of helical polymers have been synthesized including polyacetylene, polyurethane–urea and polyurethane which derived from optical active amino acid and 1,1′-binaphthyl-2′,2-diol. Their low infrared emissivity properties are resulted from the highly absorptive groups C=O and N–H and polymers' intramolecular hydrogen. Moreover, the intramolecular hydrogen bond improves the generation of orderly secondary structure and hydrogen deficiency simultaneously. Whereas, the orderliness and helicity would dramatically weaken when the temperature rises above 100 °C. The limit of temperature restricts their high temperature application such as military stealth. To solve this problem, HPS are devised to improve the infrared emissivity performance at high temperature. Besides the high temperature stability, polysilanes with particular substituents adopt helical conformation.⁵ The helical conformation of backbone doesn't merely rely on the intramolecular hydrogen bond induced by side chain functional groups.

Helical polysilanes have been synthesized and applied in various fields like molecular recognition and information storage.⁶ Typically, most of the optical active polysilanes are modified by alkyl or aromatic side groups.^7,8 To be more specific, poly(di-n-hexylsilane) (PDHS) with hexyl functional groups possesses a typical 7/3 helix backbone which consists of both P- and M-screw senses.^9,10 Polysilane with randomly distributed coiled segment doesn't show conformational orderliness. Hence, the preparation of helical polysilanes with enantiomerically pure conformations is reasonable.^11,12 Optically active polysilanes with enantiopure chiral functional groups can be obtained by introducing chlorosilane chiral monomer.¹³ Since the reactivity of silicon–chlorine group, polysilanes with organo-functional groups are rarely reported.¹⁴ Polar polysilanes containing randomly distributed –OH groups haven't been prepared until multihydroxy functional polysilanes are synthesized by Reuss and his co-workers.¹⁵ The acetal protecting strategy offers a novel pathway to entirely new class of functional polysilanes. Thus, functional groups like methoxycarbonyl and ether can be employed in preparing functional polysilanes as well.^16–20 The addition of unsaturated group C=O makes the synthesis of HPS meaningful. Moreover, due to the chiral structure of functional group, lactic acid-based polymers exhibit excellent optical and physical properties.²¹

In this work, we present the synthesis and characterization of helical polysilane copolymers consisting of randomly distributed methoxycarbonyl and ether groups (Scheme 1). Methyl-(S)-2-(allyloxy)propanoate, applies as one of the raw materials, is originated from methyl-(S)-(−)-lactate. The other reactant dichloromethylsilane and the corresponding monomer demand low temperature and inert gas environment for preservation. Tetrahydrofuran (THF) offers the mild condition of copolymerization in which monomer can be preserved. The crystallinity and polymerization degree of helical polysilanes can be controlled by the copolymerization ratio of DCMMS and DCDHS. Additionally, the influences of helical conformation and unsaturation are investigated. The incorporation of chirality in the resulting polymers provides a promising prospect in low infrared emissivity application. Poly[di-n-hexylsilane-co-(methoxycarbonyl ethyl propyl ether)methylsilane] with copolymerization ratio of 10%, 30%, 50% MMS content are abbreviated as HPS-1, HPS-2, HPS-3 for the reason of conciseness.

Scheme 1 Synthesis of HPS copolymers.

2. Experimental

2.1 Materials

All reagents and solvents were purchased from Aladdin Industrial Corporation or J&K and used without further purification unless otherwise stated. Solvents for the polymerizations (THF, xylene) were purified according to procedures of literature. Allyl bromide was purchased from Adamas-beta. Karstedt's catalyst, platinum(0)-1,3-divinyltetramethyldisiloxane complex (19.0–21.5% as Pt), L(+)-lactic acid and dichlorodi-n-hexylsilane (DCDHS) were obtained from Tokyo Chemicals Industry. DCDHS was purified by fractionating distillation prior to use. Methyl-(S)-2-(allyloxy)propanoate was prepared according to the literature.²⁹ Deuterated solvents were purchased from Beijing Seaskybio Technology Co., Ltd and stored over molecular sieves (4 Å). Dialysis bag (MD44) was bought from Spectrum Laboratories Inc. and pretreated by sodium bicarbonate and LEDTA.

2.2 Measurements

¹H NMR spectra (300 MHz) and ¹³C NMR spectra (75 MHz) were recorded using a Bruker AV 300. All spectra were referenced internally to residual proton signals of the deuterated solvent. ²⁹Si NMR spectra (99.35 MHz) were obtained on a Bruker DRX 500. GPC measurements were performed on a setup consisting of a Polymer Laboratories' PL-GPC220, and PLgel-5 μL-MIXED-C, 300 × 7.5 mm. Tetrahydrofuran was used as an eluent at 30 °C and at a flow rate of 1 mL min⁻¹. UV absorptions were detected by a SHIMADZU UV3600 Series spectrophotometer. Calibration was carried out using poly(styrene) standards provided by Polymer Standards Service. DSC measurements were carried out using a Perkin-Elmer DSC 8000 in the temperature range from −75 to +100 °C using heating rates of 40 and 20 K min⁻¹ for the copolymer samples and 40 and 5 K min⁻¹ for the PDHS sample. Sodium dispersions were prepared using a DF-101S magnetic stirrer purchased from YUHUA company. CD spectra were determined with a Jasco J-810 spectropolarimeter using a 10 mm quartz cell at room temperature. Polarized light microscopy was carried out using a Olympus BX53P microscope with Hot and Cold Stages. Infrared emissivity values of the samples were investigated on a silicon substrate by using an IRE-2 Infrared Emissometer of Shanghai Institute of Technology and Physics, China. Thermal analysis experiments were performed using a TGA apparatus operated in the conventional TGA mode (TA Q-600, TA Instruments) at a heating rate of 10 K min⁻¹ in a nitrogen atmosphere, and the sample size was about 5 mg.

2.3 Monomer synthesis

The synthesis was carried out under argon atmosphere. Methyl-(S)-2-(allyloxy)propanoate (2.80 g, 15.1 mmol, 1 equiv.) was placed in a Schlenk flask. Chlorobenzene (3.5 mL), 90 μL of Karstedt's catalyst solution, and 3.5 mL of dichloromethylsilane (33.5 mmol, 2.2 equiv.) were added via syringe. After stirring at room temperature overnight, excess dichloromethylsilane and chlorobenzene were removed under reduced pressure (135 °C per 0.001 mbar).

DCMMS: yield: 54%, colorless oil. ¹H NMR (300 MHz, CDCl₃): δ [ppm] = 3.96 (q, 1H, ³J = 6.8 Hz, CH), 3.72 (s, 3H, CH₃–O), 3.55 (dd, 1H, J_AB = 15.2 Hz, ³J = 6.4 Hz, O–CH′H′′–CH), 3.40 (dd, 1H, J_AB = 15.2 Hz, ³J = 6.4 Hz, O–CH′H′′–CH), 1.85–1.73 (m, 2H, CH₂–CH₂–CH₂), 1.38 (d, 3H, CH–CH₃), 1.19 (m, 2H, Si–CH₂–CH₂), 0.77 (s, 1H, Si–CH₃). ¹³C NMR (75 MHz, CDCl₃): δ [ppm] = 173.53 (s, CH–COO–CH₃), 74.80 (s, CH₃–CH–O), 71.14 (s, CH₂–CH₂–O), 51.71 (s, CH₃–O), 22.80 (s, Si–CH₂–CH₂), 18.51 (s, CH₂–CH₂–CH₂), 17.96 (s, CH–CH₃), 5.07 (s, Si–CH₃). ²⁹Si NMR (99.35 MHz, CDCl₃): δ [ppm] = 32.97 (s, Si–CH₃).

2.4 Copolymer synthesis

In a glovebox, 1.035 g (45 mmol, 2.5 equiv.) sodium and 25 mL xylene were placed in a Schlenk flask, transferred to the vacuum line, and heated to 110 °C. The mixture was transformed into a fine dispersion using a preheated homogenizer (DF-101S, YUHUA). After evaporation of xylene in high vacuum, 20 mL of THF and 20 mmol of dichlorodiorganosilane were added via syringe. The mixture turned purple after 30 min and was vigorous stirring for 24 h at room temperature. The reaction was then quenched with 120 mL of methanol. The precipitated polymer was filtered, washed with water and methanol, and dried in high vacuum. The cyclic fraction was separated via fractional precipitation from THF/2-propanol. The preliminary treated product was moved in the pretreated dialysis bag. After 72 h dialysis, the pure product was obtained.

HPS: yield: 10–55%. ¹H NMR (300 MHz, CDCl₃): δ [ppm] = 4.01–3.88 (CH), 3.79–3.65 (CH₃–O), 3.57–3.41 (O–CH′H′′–CH), 3.35–3.23 (O–CH′H′′–CH), 1.80–1.55 (CH₂–CH₂–CH₂), 1.43–1.35 (CH–CH₃), 0.91–0.78 (Si–CH₃), 1.78–0.91 (C(CH₃)₂, Si–(CH₂)₅–CH₃, and CH₃–Si–CH₂). ¹³C NMR (75 MHz, CDCl₃): δ [ppm] = 104.67 (CH–COO–CH₃), 59.44 (CH₃–CH–O), 45.80 (CH₃–O), 45.94 (Si–CH₂–CH₂), 33.05 (Si–(CH₂)₂–CH₂–Pr), 31.55 (Si–(CH₂)₃–CH₂–Et), 22.57 (Si–CH₂–CH₂–Bu), 31.62 (CH₂–CH₂–CH₂), 29.64 (CH–CH₃), 16.61 (Si–(CH₂)₄–CH₂–Me), 16.17 (Si–CH₂–(CH₂)₂–O), 16.11 (Si–(CH₂)₅–CH₃), 14.04 (Si–CH₂–CH₂–Bu), −0.08 (Si–CH₃). ²⁹Si NMR (99.35 MHz, CDCl₃): δ [ppm] = −21.6 to −23.34 (MeSiR), −24.0 to −27.0 (Hex₂Si), FT-IR (cm⁻¹, KBr): 2957 (asymmetrical C–H stretching), 2920 (symmetrical C–H stretching), 2856 (C–H stretching), 1750 (asymmetrical C=O stretching), 1740 (symmetrical C=O stretching), 1570 (C=C stretching), 1464 (CH₂ scissoring), 1405 (asymmetrical C–H bending), 1378 (symmetrical C–H bending), 1340 (symmetrical C–H bending), 1258 (C–O–C stretching), 1186 (CH₃ rocking), 1075 (C–O–C stretching), 1015 (Si–O–C stretching), 962 (Si–OH stretching), 841 (Si–CH₃ rocking), 770 (asymmetrical Si–C stretching), 722 (CH₂ rocking), 688 (symmetrical Si–C stretching).

3. Results and discussion

3.1 Monomer synthesis

Vinyl compounds are commonly used as raw materials to synthesize dichlorodiorganosilanes. The reaction usually carries out in inert gas environment to protect Si–Cl from reacting with water to generate hydrogen chloride. The hydrosilylation synthetic route is not suitable for the compounds contain amino and hydroxyl groups due to the reactivity of Si–Cl. Kaverin and his coworkers²¹ reported that polar olefins with ester moieties and could be easily converted to dichlorodiorganosilanes by hydrosilylation with dichloromethylsilane under the catalyzing of nickel catalytic systems at 60 °C. According to the previous literature, we synthesized the monomer of DCMMS bearing the functional group of methoxycarbonyl. Besides ester group, ether moiety was selected due to its stability in hydrosilylation and Wurtz-type coupling reaction. The addition of allyl improved the nonpolarity of methyl-(S)-2-(allyloxy)propanoate which demonstrated better solubility in n-hexane than methyl lactate. The synthesis of monomer was conducted according to Scheme 1.

Methyl-(S)-2-(allyloxy)propanoate was prepared by reacting methyl lactate with allyl bromide in the presence of Ag₂O. Then the product was hydrosilylated with dichloromethylsilane after the addition of Karstedt's catalyst, platinum(0)-1,3-divinyltetramethyldisiloxane, resulted in the predicted chiral silane monomer with a yield of 54% after optimization of reactions. The structure of DCMMS was characterized by ¹H, ¹³C and ²⁹Si NMR spectroscopy. Fig. 1 illustrates the ¹H NMR spectra of methyl-(S)-2-(allyloxy)propanoate and the corresponding hydrosilylated product in CDCl₃. Raw material and DCMMS showed similar peaks at 3.96, 3.72 and 1.38 ppm which were assigned to the chiral functional group. The signals assignable to the protons of vinyl (–CH=CH₂) around 5.43 and 6.07 ppm disappeared after reacting with dichloromethylsilane, which demonstrating the formation of the carbon silicon bond. Due to the hydrosilylation reaction, the signal of CH₂ shifted from 3.95 to 3.45 ppm. Fig. S2† displays characteristic peak assignable to the methyl silicon group at 33 ppm of the DCMMS. The ¹³C NMR spectrum also supports the structure of the monomer.

Fig. 1 ¹H NMR spectra (CDCl₃, 300 MHz) of methyl-(S)-2-(allyloxy)propanoate (blue) and DCMMS (red).

Due to the formation of by-product at elevated temperature (≥155 °C), the hydrosilylated product is hard to be purified, fractional distillation is accompanied with a color change to black or brown and decreasing the yield as well as purity of prepared monomer. This phenomenon is widely reported among dichlorodiorganosilanes and had already been studied among dichlorosilane monomers containing ether linkages.²² This detrimental behavior is caused by inter and intramolecular reactions of the polar Si–Cl bonds with the oxygen atoms in the substituents, leading to thermodynamically favored Si–O bonds as well as C–Cl compounds. The minor peak around 32 ppm in ²⁹Si NMR spectrum results from degradation between synthesis and recording of spectra. The peak position corresponds to that expected from degradation, as reported in the research work of Reuss.¹⁵ Since the limited preservation time, distilled DCMMS has to be utilized in all further reactions and characterization immediately. After 3 days of storage under argon at 0 °C, color change of the product from orange to black was observed.

3.2 Polymer synthesis

Sodium is commonly utilized to synthesize polysilane in the Wurtz-type coupling reaction. In order to achieve higher yields compared to the classical reagent toluene, the polymerization was performed in THF at room temperature. The dispersion procedure was carried out as the method of literature.²³ Fig. 2(A) shows the FT-IR spectra of PDHS and HPS, the strong absorptions at 2957, 2920, and 2856 cm⁻¹ were assigned to the C–H bond of hexyl groups. The bands at 1405 and 1378 cm⁻¹, in the spectrum, were associated with asymmetric and symmetric bend of the methyl groups. Absorption of methoxycarbonyl C=O group appeared around 1750 cm⁻¹. Moreover, the characteristic band at 1258 cm⁻¹ demonstrated the existence of ester group. The ¹H NMR spectra of HPS-1 and PDHS are shown in Fig. 2(B) and the peak assignment of HPS-1 is presented in ESI Fig. S3.† The newly emerged signals of HPS-1 from 3.2 to 4.0 ppm are assignable to the protons of chiral functional group which are similar to the chiral center peaks of DCMMS in Fig. 1, confirming the formation of copolymerized polysilane. Compared to DCMMS, CH₂ signal of HPS-1 was more close to 0 ppm. These results indicated that the reaction had taken place as expected. Fig. 2(C) shows the ²⁹Si NMR spectrum of HPS-3. The broad signal between −24.0 to −27.0 ppm was generated by longer runs of directly connected di-n-hexylsilane repeating units. The broad resonance around −22 ppm can be assigned to the MMS units. The ¹³C NMR spectrum of the HPS-1 is presented in ESI Fig. S4.†

Fig. 2 (A) IR spectra of PDHS and HPS-1, (B) ¹H NMR spectra (CDCl₃, 300 MHz) of PDHS and HPS-1, (C) inverse gated ²⁹Si NMR spectrum (CDCl₃, 99.35 MHz) of HPS-3 and peak assignment.

3.3 Optically activity and secondary structure of polymers

The secondary structure of HPS-3 was examined by CD and UV-vis spectroscopies. Fig. 3 depicts the CD and UV-vis spectra of HPS-3 measured in THF at room temperature (c = 0.05 g dL⁻¹). The peak at 243 nm corresponded to the n–p* transition of the methoxycarbonyl, and the broad absorption peak between 270–370 nm was assigned to the σ-conjugated length along the silicon backbone. The CD spectroscopic analysis is significant for the investigation of helical structure.^24,25 Hence, it can be used to confirm the helicity of the polysilane. In Fig. 3(A), HPS-3 exhibits a combined positive intense CD signal with two peaks at 245 and 290 nm respectively, due to the existence of Cotton effects, indicated that the polysilane adopts single-handed helical backbone. The positive peak from 223 to 267 nm was assigned to the chiral center of the methoxycarbonyl functional group. In analogy to previous report, the CD absorption from 267 to 360 nm resulted from σ-conjugation Si–Si bonds of the HPS-3 helical backbone.

Fig. 3 (A) CD spectra of HPS-3 measured in THF; (B) UV-Vis spectra of HPS-3 measured in THF. The concentration of measured sample is about 0.01 g dL⁻¹.

3.4 Cooperative interaction analysis

Cooperative interaction between functional groups is responsible for the rigid rod-like helical polysilane with the single-handed screw sense. In polymer science, the noncovalent interactions especially hydrogen bonding have been utilized to synthesize the helical polymers. Intermolecular interaction like C–F⋯Si²⁶ could be amplified and form the helical conformation. The single-handed helical conformation could also be related with the chain stiffness affected by the intermolecular weak interaction between the pedant multihydroxy functional group and hexyl substituent. The pendant functional group contains medium electron withdrawing groups like ester and ether. On the contrary, the alkyl substituent is a kind of weak electron donating group. The weak interaction between electron withdrawing and donating groups brings about the amplification effect on helical conformation. The brief illustration of cooperative interaction and helical structure of the HPS is presented in Fig. 4.

Fig. 4 Cooperative interaction and helical structure of the HPS.

The intermolecular weak C=O⋯H interaction influenced the C=O stretching vibration. IR spectrum of HPS-1 demonstrates the splitting band at 1750 and 1740 cm⁻¹. The former and later characteristic bands could be respectively assigned to the free C=O band and C=O interacting with alkyl group. The intermolecular interaction like hydrogen bonding causes the blue-shift effect. Besides, the peak of ether group also emerged at a lower frequency band 1075 cm⁻¹, which could be attributed to the weak interaction between C–O–C and alkyl as well.

The ²⁹Si NMR spectroscopy is an efficient instrument for the detection of HPS copolymers' microstructure and noncovalent interaction. The helical conformation of backbone induced by weak interaction could be proved by ²⁹Si NMR spectrum as well. The linewidth is related to the rigidity of polysilane, the broader linewidth reflects the enhanced rigidity of HPS-3. The splitting resonance of di-n-hexylsilane repeating units might results from the electron withdrawing effect of O atoms. Hence, the minor and higher signal around −26.2 ppm is attributed to the upfield chemical shift of di-n-hexylsilane repeating units. From the analyses of IR and ²⁹Si NMR, the cooperative interaction affects the functional groups of HPS, thus leading to the helical conformation and other improved properties.

3.5 Crystallinity and thermal properties

The molecular weights and molecular weight distributions of HPS copolymers and the respective homopolymers are listed in Table 1. The corresponding GPC elugrams can be seen in Fig. 5. Molecular weights are distributed in the range from 1300 to 140 000 g mol⁻¹. As the addition of DCMMS fraction, the copolymers' molecular weights decreased at the same time. This phenomenon could be attributed to the solution equilibrium alteration at the sodium surface caused by the polar substituents.²⁷ The addition of glyme or diglyme to the heterogeneous chemical synthesis of polysilane will result in a great reduction of molecular weight.

Table 1 Characterization data for polysilane homo- and copolymers

Polymer	Content^a/%	Mw^b/g mol^−1	PDI^b	Yield^c/%
a The content denotes the DCMMS fraction in the monomer feed, the value in parentheses determined via ^1H NMR spectroscopy after separation of the cyclic fraction by fractionating precipitation.b Determined by GPC in THF, polystyrene standards, RI detection.c After separation of the cyclic fraction by fractionating precipitation.
PDHS	0	146 100		36
HPS-1	10(9)	47 800	2.10	55
HPS-2	30(28)	12 100	1.27	21
HPS-3	50(47)	6100	1.20	33
PMMS	100	1300	0.99	10

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Fig. 5 GPC elugrams of the polysilane copolymer series (THF, PS standards).

Symmetrical alkyl substituted polysilanes usually exhibit special mesomorphic behavior, PDHS is a kind of conformationally disordered liquid crystalline with hexagonal columnar phase. Thus, HPS with MMS content from 0 to 100% are characterized by DSC to investigate the thermal properties. The characteristic appearance can be seen in the Fig. S1,† the PDHS turned to be colorless crystalline powder. As the addition of DCMMS in copolymerization increases, the products gradually lead to a yellow, visquous material. The respective DSC curves of HPS with varying MMS content are depicted in Fig. 6, the corresponding calorimetric data are summarized in Table 2. Around 45 °C, PDHS powder displayed the distinctive thermochromic order–disorder transition from crystalline to a hexagonal columnar mesophase.²⁸ As the Fig. 6 describes, the melt point and intensity of HPS decreased with broader linewidth, which result from comonomer-induced irregularities and crystalline order disturbance compared to PDHS. The gradual rising transition temperatures with increasing MMS content reflects the existence of the HPS copolymers. In order to eliminate the interference aroused by PDHS homopolymer fraction, PDHS mixed with HPS-1 and HPS-3 at the weight ratio of 1 : 1 were measured additionally. In line with literature, the DSC curves showed the combined thermal property of both proportions and immiscibility induced by crystallization,¹⁷ disclosing the absence of PDHS homopolymer.

Fig. 6 DSC traces of the second heating run for the polysilane copolymer series (THF, PS standards). The arrow indicates the decrease of the transition temperature to the conformationally disordered mesophase with increasing fraction of MMS.

Table 2 Calorimetric data for polysilane homo- and copolymers (DSC)

Polymer	MMS content^a/%	Tg/°C	Tp^b/°C	ΔH^c/J g^−1
a DCMMS fraction determined via ^1H NMR spectroscopy after separation of the cyclic fraction by fractional precipitation.b Peak temperature of the endothermic transition observed in the second heating run of the DSC measurements.c Enthalpy of the endothermic transition determined via DSC (second heating run). The determination of the exact enthalpy is impeded by the broadness of the phase transition.
PDHS	0		45	79
HPS-1	10	−40	26	36
HPS-2	30	−41	1	18
HPS-3	50	−42
PMMS	100	−41

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The detection of anisotropic character of HPS was conducted on polarized light microscopy with crossed polarizers. PDHS and HPS-1 are able to form mesophases above their respective transition temperatures and demonstrate birefringence above their disordering temperatures. Kinetically controllable nucleation causes the hysteresis of the transition temperatures and the phase transition procedure could be reversed completely. The amorphous fraction increased with the increase addition of DCCMS comonomer. Meanwhile, a glass transition temperature was observed around −41 °C. HPS with more than 30% MMS content were unable to form a mesophase and showed amorphous isotropic property above the glass transition temperature. The microscopy images of HPS-3 observed by polarized microscopy are presented in ESI Fig. S5.†

3.6 Infrared emissivity analysis

After the completion of polysilanes syntheses, the HPS demonstrated the property of infrared low emissivity which clearly related to the addition of methoxycarbonyl group. The oxygen atom in C=O provides the lone pair electrons, thus improves the unsaturated degree of the helical polysilane. In addition, the stretch of C–O–C alter the original vibration mode of molecules, hence the infrared low emissivity could be attributed to the addition of ether group as well. To be more specific, infrared emissivity values of polysilanes at wavelength of 8–14 μm were investigated at room temperature and shown in Fig. 7 and Table 3. PDHS and PMMS homopolymers demonstrated the infrared emissivity values of 0.972 and 0.816 respectively, meanwhile the HPS-3 displayed the lowest value of 0.612 at 20 °C. HPS-3 with 50% MMS content optimized the weak interaction between functional groups and showed the superior infrared emissivity property.

Fig. 7 Infrared emissivity values of the polysilane copolymer series (THF, PS standards).

Table 3 Lowest infrared emissivity values of polysilanes

Polymer	MMS content/%	Infrared emissivity (8–14 μm)
PDHS	0	0.972
HPS-1	10	0.826
HPS-2	30	0.681
HPS-3	50	0.598
PMMS	100	0.782

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To further evaluate the relationship between temperature and infrared emissivity, the infrared emissivity of HPS-3 was measured from 0 to 200 °C and presented in Fig. 8. The infrared emissivity value of HPS-3 increased slowly from 20 to 135 °C, then rose sharply above 140 °C and became smoothly as high as 190 °C. These data indicating the interaction between chiral functional group and main chain, and noncovalent forces gradually weaken with the rising temperature. Vice versa, the lower temperature makes randomly distributed backbone orderly which leads to the improvement of low infrared emissivity. According to this phenomenon, we further evaluated the performance of HPS in low temperature condition. At 5 °C, the value of infrared emissivity reached a minimum number of 0.598. For comparison, the infrared emissivity values of PMMS were also measured, similar pattern of value curve is shown on the Fig. 8.

Fig. 8 Infrared emissivity values of (A) PMMS and (B) HPS-3 at 0 to 200 °C.

TG curve of HPS-3 is presented in Fig. 8 and observed dramatically thermal decomposition above 300 °C. HPS-3 based on chiral monomer possesses more regular secondary structure which favors the transmission of heat. Therefore, the molecule's thermal restraining and vibrational state modification would facilitate the reduction of infrared emissivity.¹ The infrared emissivity is clearly relevant to the addition of methoxycarbonyl and ether group according to above analyses.

4. Conclusions

DCMMS derived from chiral methyl lactate has been synthesized and then copolymerized with DCDHS in the presence of sodium, providing the corresponding optically active polysilanes (HPS) in high yields. The HPS and PMMS exhibited the property of low infrared emissivity values at the wavelength of 8 to 14 μm. The asymmetric force field in the chiral methoxycarbonyl induces orderliness in new region. They possess helical configuration and higher degree of unsaturation compared to the PDHS which demonstrate randomly coiled backbone. The anisotropy and viscosity of HPS provide further proofs of regular secondary structure and more inter-chain interactions. All these characters result in the reduction of infrared emissivity. The optically active polysilanes based on lactic acid could be regarded as a kind of promising material for stealthy technology and the approach to adjust their emission rate is presented. As the HPS possess low infrared emissivity property at the wider temperature range, it is significant to the infrared emission source in both military and civilian application.

Acknowledgements

The authors are supported by National Nature Science Foundation of China (51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province (BA2011086), the Fundamental Research Funds for the Central Universities (3207045301), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1417). Scientific Innovation Research Foundation of College Graduate in Jiangsu Province (KYLX_0161).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17239j

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