Segmental anisotropy in strained elastomers by 1H NMR of multipolar spin states
Segmental anisotropy in strained elastomers by 1H NMR of multipolar spin states
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DOI：
10.1021/ma020532v
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发表时间：
2002-07-30
期刊：
影响因子：
5.5
通讯作者：
Blümich, B
中科院分区：
文献类型：
--
作者：
Fechete, R;Demco, DE;Blümich, B
导论.橡胶弹性理论的一个众所周知的结论是键取向。1，2弹性体的变形引起聚合物线圈的骨架键的各向异性。近年来，在橡胶弹性的核磁共振研究中，橡胶的变形机理和网络链的取向越来越受到人们的关注。在过去的十年中，2 H和1H的一维和二维NMR光谱被广泛用于测量偶极相关效应、同向和异向剩余偶极耦合以及相应的动力学序参数。3-6在单轴机械力的存在下，氘NMR线分裂，揭示了在聚合物网络中诱导的链段的分子各向异性。7-14有两种不同的机制导致这种诱导的局部各向异性：10，12（i）由于网络连接的长度和方向的变化，每条链的剩余偶极相互作用的各向异性增加，并且与链之间的相互作用无关。(ii)链各向异性的第二种机制是由属于不同链的链段之间的各向同性短程取向相互作用给出的。在轴向变形下，链将产生屏蔽短程排斥体积相互作用，并产生诱导吸引势。除了2D NMR，用于研究拉伸天然橡胶带中残余偶极耦合的其他NMR方法是基于1H Hahn和固体回波，15，16受激回波，5和1H双量子（DQ）相干和偶极编码纵向磁化（DELM）。[17，18]对于偶极耦合自旋-1/2对的情况，后面的多极自旋态很容易理解。在这种情况下对自旋系统响应的评估表明，DQ激发期结束时的密度算符包含形式为（Iz 1 + Iz 2）sin（DDQτ）的项，其中Izi（i）1，2）是自旋矢量算符的z分量，DDQ是由DQ实验编辑的偶极耦合常数，τ是激发期的持续时间。17，18与基于偶极相关函数的方法相比，通过多极自旋态（如DQ和DELM）和谱线分裂确定1H和2 H剩余偶极和四极相互作用是无模型的。通过测量残余偶极耦合与延伸率λ的关系及其对取向角θ的依赖性（其中θ是单轴作用力FB与静磁场方向BB 0之间的角度），可以区分变形过程中链拉伸和链重取向的影响。15在本文中，我们报告了拉伸弹性体中1H剩余偶极耦合的角度依赖性，确定了不受1H DELM衰减和DQ相干建立曲线的初始激发状态影响的模型。
Introduction. A well-known consequence of the theory of rubber elasticity is bond orientation. 1, 2 Deformation of an elastomer induces anisotropy of the backbone bonds of the polymer coil. In recent NMR studies of rubber elasticity the mechanism of deformation and the orientation of network chains has received increasing attention. One-and two-dimensional NMR spectroscopy of 2H and 1H has been used intensively in the past decade to measure the dipolar correlation effect, homonuclear and heteronuclear residual dipolar couplings, and corresponding dynamic order parameters. 3-6 In the presence of a uniaxial mechanical force the deuterium NMR line splits, revealing that a molecular anisotropy of the chain segments is induced in the polymer network. 7-14 There are two distinct mechanisms responsible for this induced local anisotropy: 10, 12 (i) The anisotropy of the residual dipolar interactions is increased for each chain due to changes in the length and orientation of the network junctions and is independent of the interactions between the chains.(ii) The second mechanism responsible for the chain anisotropy is given by the isotropic short-range orientational interactions between segments belonging to different chains. Under the axial deformation the chains will produce a screened short-range excluded-volume interaction with an induced attractive potential. Besides 2D NMR, other NMR methods used for investigation of the residual dipolar couplings in stretched natural rubber bands are based on 1H Hahn and solid echoes, 15, 16 the stimulated echo, 5 and 1H double-quantum (DQ) coherences and dipolar encoded longitudinal magnetization (DELM). 17, 18 The latter multipolar spin states can be easily understood for the case of dipolar coupled spin-1/2 pairs. The evaluation of the spin system response in this case shows that the density operator at the end of DQ excitation period contains a term of the form (Iz1+ Iz2) sin (DDQτ), where Izi (i) 1, 2) are the z components of the spin vector operator, DDQ is the dipolar coupling constant edited by the DQ experiment, and τ is the duration of the excitation period. 17, 18 Determination of 1H and 2H residual dipolar and quadrupolar interactions by multipolar spin states (like DQ and DELM) and line splitting are model free compared to the methods based on dipolar correlation functions. 5, 15, 16 By measuring the residual dipolar couplings vs the extension ratio λ and its dependence on the orientation angle θ, where θ is the angle between the uniaxial applied force FB and the static magnetic field direction BB0, it is possible to distinguish the influences from chain stretching and from chain reorientation in the deformation process. 15 In this paper, we report on the angular dependence of the 1H residual dipolar couplings in stretched elastomers determined model free from the initial excitation regime of 1H DELM decay and DQ coherence build-up curves.