Moldovan Dumitrița, Fechete Radu
Physics and Chemistry Department, Technical University of Cluj-Napoca, Memorandumului 28, R-400114 Cluj-Napoca, Romania.
Polymers (Basel). 2021 Oct 13;13(20):3523. doi: 10.3390/polym13203523.
The H DQ Fourier and Laplace-like spectra for a series of cross-linked natural rubber (NR) samples naturally aged during six years are presented and characterized. The DQ build-up curves of these samples present two peaks which cannot be described by classical functions. The DQ Fourier spectra can be obtained after a numeric procedure which introduces a correction time which depends less on the chosen approximation, spin-½ and isolated CH and CH functional groups. The DQ Fourier spectra are well described by the distributions of the residual dipolar coupling correlated with the distribution of the end-to-end vector of the polymer network, and with the second and fourth van Vleck moments. The deconvolution of DQ Fourier spectra with a sum of four Gaussian variates show that the center and the width of Gaussian functions increase linearly with the increase in the cross-link density. The Laplace-like spectra for the natural aged NR DQ build-up curves are presented. The centers of four Gaussian distributions obtained via both methods are consistent. The differences between the Fourier and Laplace-like spectra consist mainly of the spectral resolution in the favor of Laplace-like spectra. The last one was used to discuss the effect of natural aging for cross-linked NR.
本文给出并表征了一系列在自然环境中老化六年的交联天然橡胶（NR）样品的H DQ傅里叶谱和类拉普拉斯谱。这些样品的DQ累积曲线呈现出两个无法用经典函数描述的峰。通过引入一个校正时间的数值程序可以得到DQ傅里叶谱，该校正时间对所选近似、自旋-½以及孤立的CH和CH官能团的依赖性较小。DQ傅里叶谱可以通过与聚合物网络端到端矢量分布以及第二和第四范弗莱克矩相关的残余偶极耦合分布很好地描述。用四个高斯变量之和对DQ傅里叶谱进行去卷积表明，高斯函数的中心和宽度随交联密度的增加而线性增加。本文还给出了天然老化NR DQ累积曲线的类拉普拉斯谱。通过两种方法得到的四个高斯分布的中心是一致的。傅里叶谱和类拉普拉斯谱之间的差异主要在于类拉普拉斯谱的光谱分辨率更高。最后利用类拉普拉斯谱讨论了天然老化对交联NR的影响。
