Organized Molecular Interface-Induced Noncrystallizable Polymer Ultrathin Nanosheets with Ordered Chain Alignment Haili Qin,†,‡ Fujin Li,† Dong Wang,† Hongzhen Lin,† and Jian Jin*,† †
i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech & Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China ‡ School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China S Supporting Information *
ABSTRACT: The orientation and state of organization of polymer chains play significant roles in determining the final properties and functions of these materials. Unlike most semicrystalline polymers, which have an inherent driving force toward crystallization, the means to control chain packing in noncrystallizable polymers is still restricted and remains a challenge. We report herein a 2D soft template-directed fabrication for ultrathin polyacrylamide nanosheets with a thickness as low as 3.5 nm and large dimensions (>20 μm). More importantly, the polymer chains in the nanosheets produced are well aligned with a clear interchain spacing. The formation of polymer nanosheets with ordered chain alignment was performed in a special solution containing a periodic sandwich structure of lamellar bilayer membranes and water layers that are hundreds of nanometers thick. It functions as a 2D orientation template to align the monomers in an orderly manner along the in-plane direction of the bimolecular membrane via hydrogen bonding. KEYWORDS: 2D polymer, chain alignment, ultrathin nanosheet, 2D orientation template, bimolecular membrane
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limitations of stereoregular polymerizations, several other strategies have been developed, such as mechanical rubbing, the design of supramolecular action and topochemical polymerizations, and template-induced, space-confined crystallization.20−23 Recently, a new strategy that relies on host−guest “ordered cross-links” was reported to produce polymeric materials that exhibit a crystalline arrangement.24 However, these strategies lack generality and are usually targeted at particular polymers or monomers. A novel and versatile methodology to induce ordered chain packing and create anisotropy at the molecular level in amorphous polymer materials is still required. In addition, polymer thin films are widely used in various practical applications, such as in adhesives and lubricants.25−27 With the rapid development of nanoscience and nanotechnology, the design and fabrication of ultrathin polymer films is becoming increasingly important.28−34 The alignment of molecular chains within an ultrathin polymer film with a molecular-level thickness remains a significant challenge and has not been demonstrated until now.
n the many applications of polymer materials, the orientation and state of organization of polymer chains play significant roles in determining their final properties and functions.1−9 Many studies have shown that chain alignment has a significant influence on the macroscopic properties of polymeric materials.10−14 For instance, increasing the crystallinity and chain orientation of a polymer could significantly improve its thermal conductivity.12 It has been shown that chain alignment along the longitudinal direction of a fiber is an effective method to improve the mechanical strength of these materials;15,16 other polymers exhibit improved electrical, optical, and elastic properties after chain alignment. In principle, chain alignment could be produced in semicrystalline polymers upon cooling from a liquid state. During this process, polymer chains rearrange to form energetically more favorable, ordered regions called lamellae. Unlike semicrystalline polymers, controlling chain packing in noncrystallizable polymers is still restricted, and external forces are typically required to induce chain alignment. By designing specific polymerizations and using solid-state catalysts, stereoregular polymers that have a highly ordered arrangement of pendant groups along their chains and therefore a high degree of crystallinity can be produced.17−19 To overcome the © XXXX American Chemical Society
Received: September 30, 2015 Accepted: December 21, 2015
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DOI: 10.1021/acsnano.5b06149 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano Scheme 1. Schematic of the Organized, Molecular-Membrane-Induced Polymerization Processa
a (A) Molecular structure of the amphiphile, HGM. (B) Optical photographs and corresponding reflection spectra of a 2 wt % HGM lamellar bilayer membrane solution and the solution after the addition of 2 M AAm with an inset that shows a schematic of the HGM system with a periodic, ordered structure consisting of a HGM lamellar bilayer membrane and water layer. (C) Schematic illustration of the formation of freestanding and ultrathin PAAm nanosheets in the 2D confined space constructed by HGM.
accomplished in a supramolecular system with an HGM concentration of 2 wt % in this study. Under these conditions, the HGM solution is a royal purple color, as shown in Scheme 1B. The UV−vis reflectance spectrum of the solution displays a sharp peak at 465 nm, indicating the presence of a periodic layered structure. On the basis of Bragg’s equation, the layer spacing was 177 nm, which was approximately equal to the thickness of the water layer considering the nanometer-scale thickness of the HGM lamellar bilayer. Such a special solution with well-defined water layers may provide a 2D, confined reaction space for the preparation of 2D nanomaterials, which has been demonstrated in our previous study.36,37 To produce the PAAm nanosheets, the acrylamide (AAm) monomer was first introduced into the HGM solution; the reflectance peak position then shifted to 496 nm in the mixed solution, indicating that the periodic structures had been maintained and that the interlayer spacing was marginally broadened due to the addition of the monomer. After the addition of N,N′methylenebis(acrylamide) (MBAA), which acted as the crosslinking agent, and hydrogen peroxide (H2O2), which acted as the photoinitiator, into the system, polymerization was initiated with exposure to UV irradiation at 365 nm (Scheme 1C). After 30 min, an opaque gel was obtained, and this product was freeze-dried to remove the absorbed water. A scanning electron microscopy (SEM) image of the dried gel clearly shows the large amount of lamellae that piled up in close formation, suggesting that the 2D layered structures are maintained throughout the entire polymerization process (Figure 1A). Individual freestanding PAAm nanosheets could be created by dispersing the dried gel in polar organic solvents (e.g., tetrahydrofuran (THF)) to remove the HGM. SEM and dark field scanning transmission electron microscopy (DF-STEM) showed the presence of numerous polymeric nanosheets with large areas (e.g., lateral size >20
In this study, we report a soft 2D template-oriented fabrication of ultrathin (i.e., 3.5 nm) polyacrylamide (PAAm) nanosheets with an ordered polymer-chain alignment. The polymerization of acrylamide into polyacrylamide was performed in a special iridescent solution composed of a periodic structure of lamellar bilayer membranes that were selfassembled by an amphiphilic molecule called hexadecyl-glyceryl maleate (HGM) and were sandwiched by hundred nanometerthick water layers. We used surface sum frequency generation (SFG) vibrational spectroscopy to demonstrate that the organized bimolecular membrane provides a 2D orientation template whereby acrylamide monomers are arranged in an orderly fashion along the in-plane bimolecular membrane; this ordering is driven by the formation of hydrogen bonds between the headgroup of HGM and acrylamide. Such an orientation effect can result in the formation of polyacrylamide ultrathin nanosheets with an ordered arrangement of polymer chains. The polyacrylamide ultrathin nanosheets produced had uniform thicknesses, smooth surfaces, and large dimensions (>20 μm). This simple 2D template-oriented method could be extended to fabricate other types of amorphous polymers with chain alignment by predesigning the order of the monomers.
RESULTS AND DISCUSSION The preparation of PAAm nanosheets was conducted in a unique 2D supramolecular system whereby lamellar bilayers composed of a self-assembled nonionic surfactant called hexadecylglyceryl maleate (HGM) (Scheme 1A) and hundred nanometer-thick water layers formed periodic ordered structures in the form of a sandwiched lamellar bilayer/ water/lamellar bilayer structure at a 1−2 wt % concentration of HGM.35,36 The ordered structure gives the solution an iridescent color due to its Bragg reflectance of visible light. The synthesis of ultrathin polyacrylamide nanosheets was B
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spectrum of the PAAm nanosheets. The amide ν(CO) band at 1665 cm−1 and the ν(NH2) bands at 3421 and 3201 cm−1 are also found in the PAAm nanosheets spectrum. It is noted that there is no characteristic peaks of HGM throughout the spectrum of the PAAm nanosheets, indicating the complete removal of HGM from the nanosheets. In the 13C NMR spectra of the AAm monomer and PAAm nanosheets (Figure S2), three peaks are observed at 179.6, 41.8, and 30.5 ppm that correspond to the CO, CH, and CH2 groups in the PAAm nanosheets spectrum, respectively. In addition, the peaks at 129 and 128 ppm that can be attributed to the CC of the AAm monomer disappear after polymerization. These results prove the successful polymerization of the AAm monomer into PAAm nanosheets. Wide-angle XRD (WAXRD) was applied to characterize the internal structure of the PAAm nanosheets (Figure 3A); a set of two broad diffraction peaks was observed. The first order peak occurred at 2θ = 21.6°, corresponding to a d-spacing of 0.41 nm. This implies that the polymer chains in the PAAm nanosheets were aligned. The d-spacing corresponds to the adjacent interchain distance. High-resolution TEM (HR-TEM) was used to directly visualize the polymer chain arrangement within a PAAm nanosheet (Figure 3B).24 The presence of chain-like structures parallel along one direction can be clearly seen. The interchain distance of 0.41 nm agrees with the value obtained from the WAXRD analysis. It is worth noting that a certain fraction of amorphous domains is also observed in the figure. The amorphous domains coexist with ordered domains in the PAAm nanosheets. Differential scanning calorimetry (DSC) was performed on the PAAm nanosheets to investigate the effect of chain alignment on the thermal transitions of the polymer (Figure 4). The DSC of bulk PAAm synthesized by direct polymerization of AAm using the same amount of cross-linking agent was also measured for comparison. Upon heating, a weak peak at 192 °C was found in both the DSC of the PAAm nanosheets and the bulk PAAm. This peak represents the glass transition temperature (Tg) of PAAm and is a consequence of its amorphous structure. Contrary to the properties of the bulk PAAm, a new endothermic peak at 142 °C with a melting enthalpy of ΔH = 53.1 J/g appeared in the DSC of PAAm nanosheets during the first heating step. Although the origin of the appearance of the new endothermic peak is still unclear at
Figure 1. Morphology characterization of the ultrathin PAAm nanosheets. (A) SEM image of the as-prepared PAAm after freezedrying. (B) SEM image of monodispersed PAAm nanosheets deposited onto a silicon wafer. (C) DF-STEM image of PAAm nanosheets on a copper grid. (D) SEM image of a PAAm nanosheet spanning on a copper grid.
μm) (Figure 1B and C); these nanosheets could easily span micrometer holes (Figure 1D). The nanosheets are also thin and transparent to electron beam irradiation. Atomic force microscopy (AFM) was applied to identify the surface conditions and thicknesses of the sheets. As shown in Figures 2A, the PAAm nanosheets are particularly smooth and homogeneous with a thickness of 3.5 nm. Using small-angle X-ray diffraction (XRD), a sharp and strong diffraction peak at 2θ = 2.38° is found, which corresponds to a d-spacing of 3.5 nm (Figure 2B); this value matches the thickness of the nanosheets that was measured via AFM. The successful polymerization and composition of the PAAm nanosheets was confirmed by Fourier-transform infrared spectroscopy (FTIR) analysis and nuclear magnetic resonance (NMR) spectroscopy. From the FTIR analysis, the band at 1609 cm−1, which could be attributed to ν(CC in amide) of AAm, disappears in the PAAm nanosheets spectrum (Figure S1). Two new peaks at 2922 and 2852 cm−1, which could be attributed to νas(CH2) and νs(CH2), respectively, appear in the
Figure 2. (A) Topological AFM image of a typical PAAm nanosheet and (inset) corresponding height profile showing a thickness of 3.5 nm. (B) Small-angle XRD profile of PAAm nanosheets. C
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Figure 3. Chain alignment in the PAAm nanosheets. (A) WAXRD patterns of the PAAm nanosheets and the substrate used. (B) HR-TEM image of a typical PAAm nanosheet showing regular patterns with a 0.41 nm periodicity attributed to the regular PAAm interchain spacing.
mz = 4n13 sin 2 ψ /n2 3 con ψ
where n1, n2, and ψ correspond to the refractive index of air and the polymer film, and the angle of incidence in the GIRA-FTIR measurement, respectively. The transmission FTIR and GIRA-FTIR spectra of the PAAm nanosheets are shown in Figure 5B. In the high frequency region of the transmission FT-IR spectrum, the bands at 3421 and 3201 cm−1 that can be attributed to ν(NH2), and the bands at 2922 and 2852 cm−1 that can be attributed to ν(CH2) are prominent; however, these bands are nearly invisible in the GIRA-FTIR spectrum. This implies that the C− H and N−H bonds are nearly parallel to the substrate surface. In the low frequency region of the GIRA-FTIR spectrum, the ν(CO) absorption at 1665 cm−1 and the ν(C−N) absorption at 1400 cm−1 are much stronger than those in the transmission FTIR spectrum. The absolute intensity of these two bands can be directly acquired from the two spectra shown in Table 1. On the basis of eqs 2 and 3, the θ values of the CO and C−N bands are calculated to be 65 and 55°, respectively; these results demonstrate that the polymer chains in the nanosheets are aligned to a certain extent at a given angle. On the basis of the θ values, the molecular orientation of the PAAm chains in the nanosheets is schematically shown in Figure 5A. In-plane N−H bending and N−H stretching vibration of the amide II at 1598 cm−1 appears in the GIRA-FTIR spectrum but is not visible in the transmission FTIR spectrum. This observation further supports the presence of a certain orientation of polymer chains in the PAAm nanosheets. Our reaction system for the preparation of PAAm nanosheets can be regarded as a series of thin, liquid layers of an AAm aqueous solution with a confined thickness on the nanometer scale that are sandwiched between lamellar layers of HGM. It is thought that the AAm monomers are preferentially oriented and align in an orderly fashion along the flat surfaces of the HGM lamellar layers via the formation of hydrogen bonds between the amide groups of the AAM and the hydroxyl groups of the head groups of HGM. To better understand the interfacial interaction between AAm and HGM and to confirm the preferential orientation of AAm along the HGM layers, surface sum frequency generation (SFG) vibrational spectroscopy was used as a probing tool with a high interface sensitivity.40−42 To perform SFG measurements, a monolayer of HGM that was present on top of an AAm solution was first
Figure 4. DSC traces of PAAm bulk materials and nanosheets.
present, it indicates that the chain alignment in PAAm nanosheets is different from that of bulk PAAm. To quantitatively evaluate the molecular orientation used in the PAAm nanosheets in more detail, grazing incidence reflection absorption Fourier-transform infrared (GIRA-FTIR) spectroscopy was examined by depositing PAAm nanosheets onto a Au-evaporated glass slide, and the grazing reflection absorption spectra at an incident angle of 85° were recorded (Figure 5A).38,39 The molecular orientation could be determined by comparing the absolute intensity of the absorption band in the GIRA-FTIR spectra with the normal FTIR transmission based on the following calculation, which was previously reported by Takenaka (eqs 1−3):39 A T /AR = sin 2 θ /(2mz cos2 θ + mx sin 2 θ )
(1)
where AT and AR are the absorbance of a definite vibrational mode in the transmission and GIRA-FTIR spectra, respectively; θ is the angle for a normal vibrational mode from the surface normal; and mz and mx are described as the intensity enhancement factors in the GIRA-FTIR and transmission FTIR spectra, respectively. Generally, mx can be neglected because it is much smaller than mz. Therefore, eq 1 can be simplified to A T /AR = sin 2 θ /2mz cos2 θ
(3)
(2)
The intensity enhancement factor in the GIRA-FTIR spectrum can be calculated using the following equation: D
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Figure 5. Quantitative evaluation of the molecular orientation in the PAAm nanosheets. (A) Schematic drawing of the GIRA-FTIR spectroscopic study on the molecular orientation of the PAAm nanosheets, where θ1 and θ2 correspond to the angles for the CO and C−N vibrational modes from the surface normal, respectively. (B) GIRA-FTIR spectrum of PAAm nanosheets deposited onto Au-evaporated glass (curve a) and the transmission FTIR spectrum of the same polymer in a KBr pellet (curve b).
Table 1. Absolute Intensities of the Absorption Bands in the Transmission FTIR and GIRA-FTIR Spectra; Their Ratios; Intensity Enhancement Factors; the Refractive Indices of the Air and the Polymer; the Angle of Incidence in the GIRA-FTIR Spectra; and the Orientation Angle of the Primary Infrared Bands ν (cm−1)
assignment
AT
AR
AT/AR
mz
ψ (deg)
n1
n2
θ (deg)
1665 1440
CO C−N
0.0265 0.0090
0.1451 0.1200
0.1826 0.0750
13.495 13.495
85 85
1.0 1.0
1.5 1.5
65 55
Figure 6. (A) Schematic representation of a typical experimental arrangement for surface sum frequency generation vibrational spectroscopy. (B) SFG spectra (ssp configuration) of the aqueous solution/dispersion of HGM (curve a), AAm (curve b), HGM/AAm (curve c), HGM/ AAm irradiated by UV light in the presence of a catalytic amount of H2O2 (curve d), and PAAm nanosheets (curve e) at the vapor/liquid interface. The solid lines are the fitted curves with Lorentzian line shape functions.
formed to eliminate the optical interference between various HGM/AAm interfaces (Figure 6A). Briefly, a small volume of a
HGM chloroform solution was dropped onto the surface of an aqueous solution of AAm; then, the chloroform was allowed to E
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and the adsorbed amide groups so that they are forced to adopt a more ordered packing. The appearance of the two peaks in the amide II region suggests that there may be two different adsorption modes for the amide groups: one that is strongly involved in hydrogen bonding and that possesses a lower vibration energy (∼1524 cm−1), another that does not participate in hydrogen bonding to the same extent and thus appears at a higher energy (∼1570 cm−1). Theoretical simulations are required to clarify the mechanism, which are beyond the scope of this article. The SFG response of the PAAm nanosheets after the removal of HGM was also investigated for comparison. The PAAm nanosheets shows peaks similar to those of the randomly polymerized PAAm in the C−H stretching region (not shown), whereas in the 1300− 1900 cm−1 region (Figure 6B-e), they possess several unique bands at approximately 1503, 1549, and 1676 cm−1, which are not observed for traditional PAAm (Figure S4). Similarly, the ∼1676 cm−1 peak can be attributed to CO stretching, whereas the peaks at ∼1503 and 1549 cm−1 can be attributed to amide II vibrations. Because the nanosheet layer had two sides with the same groups, the SFG signal from the vapor/ nanosheets interface and that from the nanosheets/water interface may interfere with each other, leading to unexpected spectral shapes. This complicates the proposed spectral analysis. However, we could qualitatively confirm the ordered orientation of the amide groups within the nanosheet surface. The SFG and GIRA-FTIR results provide clear evidence that the HGM molecular layer could serve as a soft template on which the AAm molecules are adsorbed and polymerized with the amide side groups preferentially oriented; the polymer backbone would therefore tend to grow in parallel to the HGM layer rather than in other directions. The nanospaces within the lamellar structure of the HGM may further exert a confinement effect on the PAAm polymer chains, leading to their orderly packing during the formation of the nanosheets. To evaluate the role of the cross-linking agent (MBAA) on the formation of the PAAm nanosheets, polymers with different amounts of MBAA were prepared via photopolymerization under the same conditions. The obtained PAAm samples deposited onto silica wafers were characterized by AFM (Figure S5). The AFM results show that lower quantities of MBAA (i.e., 20 mg) leads to the formation of aggregates on the polymer sheets, indicating the importance of optimizing the quantity of the cross-linking agent used to prepare PAAm nanosheets with a complete and smooth structure. However, the spacing of the water layers also has a significant effect on the formation of the polymer nanosheets. A wider spacing (i.e., a water layer thickness >400 nm) was shown to decrease the stability of the system during the polymerization process. A smaller spacing (i.e., a water layer thickness
