Low-Frequency Vibrational Modes of Nylon 6 Studied by Using Infrared and Raman Spectroscopies and Density Functional Theory Calculations
Low-Frequency Vibrational Modes of Nylon 6 Studied by Using Infrared and Raman Spectroscopies and Density Functional Theory Calculations
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利用红外光谱和拉曼光谱以及密度泛函理论计算研究尼龙 6 的低频振动模式
DOI：
10.1021/acs.jpcb.9b04347
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发表时间：
2019
期刊：
影响因子：
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通讯作者：
Ozaki Yukihiro
中科院分区：
文献类型：
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作者：
Yamamoto Shigeki;Ohnishi Erika;Sato Harumi;Hoshina Hiromichi;Ishikawa Daitaro;Ozaki Yukihiro
通过对实验光谱和量子力学计算的比较，对α晶型尼龙6在90-400 cm-1范围内的远红外（FIR）和低频拉曼谱带进行了归属。一个片段的方法应用在明确考虑链间相互作用和晶体对称性的计算。在拉曼光谱和FIR光谱的主要特征被很好地再现，这使得基于密度泛函理论的能带归属和显着改进的传统的归属，有很大的争议。实验结果表明，在222和111 cm-1处的两个谱带均为α型结构。它们的强度随温度的升高而线性下降，有两个明显的转变点，分别对应于玻璃和Brill转变。这两个谱带都可以作为α型尼龙6晶格长度的指标。在此基础上，成功地将近红外和拉曼谱带归属于亚甲基的扭转和酰胺基的横向运动，其中NH和O原子移出酰胺平面.对计算的光谱进行分解，发现两个光谱中100 cm-1处的强度主要来源于酰胺基，其次才是亚甲基.此外，在100 cm-1处的FIR强度几乎完全由酰胺基团控制，这可能是该FIR带对低频带中的氢键特别敏感的原因。在222 cm-1处的FIR谱带归属于亚甲基的扭转和NH基团的横向运动。在222和111 cm-1处的FIR谱带都含有亚甲基和酰胺基的垂直运动。这将是它们对α型尼龙6中链间相互作用敏感的原因。在294 cm-1处的远红外吸收峰平行于链的方向，归属于C-CH 2-CH 2的形变和C-O在酰胺平面的弯曲运动。这就是为什么该带对尼龙6的结构转变不敏感的原因。我们以前的工作表明，在125和70 cm-1的区域，有特定的振动峰的结晶聚酯主要由酯基的面外运动产生。我们发现结晶聚酯和尼龙6在125和70 cm-1区域有相似之处，都在100 cm-1附近显示出特定的面外振动峰，这对聚合物链之间的晶格长度很敏感。
Far-infrared (FIR) and low-frequency Raman bands in the 90–400 cm–1region of crystalline nylon 6 in α form were assigned based on comparisons of experimental spectra and quantum mechanical calculations. A fragment methodology was applied in the calculations for explicit consideration of interchain interactions and crystal symmetry. The main features in both Raman and FIR spectra were reproduced well, which enabled the band assignments based on density functional theory and the significant improvement of the conventional assignments for which there had been a big dispute. Temperature dependence of the experimental FIR spectra has revealed that both bands at 222 and 111 cm–1are characteristic of the α-form structure. Their intensities linearly decreased with increasing temperature with marked two transition points, which correspond to glass and Brill transitions. Both bands can be indicators of the lattice length of α-form nylon 6. On the basis of the calculations, the FIR and Raman bands at ∼100 cm–1were successfully assigned to methylene torsion and transverse motion of amide groups in which NH and O atoms move out of the amide plane. Decomposition of the calculated spectra revealed that the intensities at ∼100 cm–1in both spectra mainly originate from the amide groups and only secondarily from the methylene groups. Moreover, the FIR intensities at ∼100 cm–1were nearly perfectly governed by the amide groups, which could be a reason why this FIR band is particularly sensitive to hydrogen bonds among the low-frequency bands. The FIR band at 222 cm–1was assigned to methylene torsion and transverse motion of NH groups. Both FIR bands at 222 and 111 cm–1contain perpendicular motions of methylene and amide groups. This will be a reason for their sensitivity to interchain interactions in α-form nylon 6. Contrarily, the FIR band at 294 cm–1is in parallel polarization to the chain direction and assigned to a deformation of C–CH2–CH2and bending motion of C═O in the amide plane. This is the reason why this band is not sensitive to the structural transitions of nylon 6. Our previous works revealed that in regions of 125 and 70 cm–1, there are specific vibrational peaks of crystalline polyesters primarily arising from out-of-plane motion of ester groups. We can find a similarity in 125 and 70 cm–1regions between crystalline polyesters and nylon 6 that both polymers show specific out-of-plane vibrational peaks around 100 cm–1, which are sensitive to the lattice length among polymer chains.