Prokhorov General Physics Institute of the Russian Academy of Sciences, Vavilov Str. 38, 119991 Moscow, Russia.
Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences, Leninsky Av. 29, 119991 Moscow, Russia.
Spectrochim Acta A Mol Biomol Spectrosc. 2021 May 5;252:119494. doi: 10.1016/j.saa.2021.119494. Epub 2021 Jan 22.
The degradation of the ubiquitous polyvinylchloride (PVC) material under the influence of various factors is known to result commonly in polyene formation. Such polyene defects occur in the form of conjugated aliphatic chains with different lengths and contents, and their sensitive and length-specific monitoring is important for the assessment of PVC degradation. Here, we report on the resonance-enhanced Raman signatures of polyene sequences of varying lengths in photo- and thermally degraded PVC films. The lengths of polyene segments have been estimated based on their selectively enhanced and spectrally resolved contributions to the Raman bands assigned to the stretching vibrations of conjugated double carbon bonds. Using deconvolution analysis of a characteristic Raman band of polyenes, we especially demonstrate that the spectral signatures of polyene segments corresponding to other various electronic resonances contribute to the Raman spectral envelope observed at a given resonant excitation. In most cases, we observe an asymmetric band profile, indicating an asymmetric length distribution of polyene defects formed in PVC films upon a mild degradation extent less than 1% mass loss. We also demonstrate that the wavenumber (ν) of the stretching vibrations of single carbon bonds in the polyene sequences of degraded PVC is inversely related to the number (n) of double carbon bonds by an empirical equation n=476·cm/ν-1082·cm. To the best of our knowledge, while considering different laser excitations spanning the range of possible electronic resonances from blue to near-infrared for Raman investigations, the present work includes (i) the first Raman spectral deconvolution study for the 532.0 nm excitation wavelength used in most portable Raman probes nowadays and (ii) the screening of polyene defects also beyond the red edge of the visible spectrum and the evidence of a resonance-enhanced polyene signal at 647.1 nm. Important new information has been obtained about polyene lengths and spectral distribution for PVC, whose critical physical properties ranging from flexibility to electrical resistance are severely affected by polyene formation.
聚氯乙烯（PVC）材料在各种因素的影响下会降解，通常会形成多烯。这种多烯缺陷以不同长度和含量的共轭脂肪链的形式存在，对其进行敏感且具有长度特异性的监测对于评估 PVC 的降解非常重要。在这里，我们报告了在光和热降解的 PVC 薄膜中不同长度的多烯序列的共振增强拉曼特征。多烯段的长度是基于它们对特征拉曼带的选择性增强和光谱分辨贡献来估计的，这些拉曼带分配给共轭双键的伸缩振动。通过对多烯的特征拉曼带进行去卷积分析，我们特别证明了对应于其他各种电子共振的多烯段的光谱特征有助于观察到在给定共振激发下的拉曼光谱包络。在大多数情况下，我们观察到不对称的带型，这表明在轻微降解程度（质量损失小于 1%）下，在 PVC 薄膜中形成的多烯缺陷的长度分布不对称。我们还证明，降解 PVC 中多烯序列中单碳键的伸缩振动的波数（ν）与双键的数量（n）之间存在经验关系，即 n=476·cm/ν-1082·cm。据我们所知，在考虑从蓝到近红外的可能电子共振范围内的不同激光激发进行 Raman 研究时，本工作包括（i）首次对当今大多数便携式 Raman 探头中使用的 532.0nm 激发波长进行拉曼光谱的去卷积研究，以及（ii）筛选超出可见光谱的红色边缘之外的多烯缺陷，并证明在 647.1nm 处存在共振增强的多烯信号。获得了有关 PVC 中多烯长度和光谱分布的重要新信息，其关键物理性能从柔韧性到电阻都受到多烯形成的严重影响。
