Rheological behavior of polymeric liquids has been considered to be universally determined by a few time-independent molecular parameters (such as the Rouse relaxation time and the entanglement number per chain) irrespective of the chemical structure of the polymer and the solvent (if any) and the polymer concentration. This universality has been supported by extensive experiments under shear conducted over 50 years and the successive theoretical development of coarse-grained molecular models. Nevertheless, recent experimental and theoretical studies have revealed the breakdown of this universality in the extensional flow behavior of both entangled and unentangled polymers. A key concept resulting from those studies is the reduction of segmental friction of highly oriented/stretched polymer chains in melts under fast extensional flow. The magnitude of this friction reduction changes with the chemical structure of the polymer and solvent as well as the polymer concentration even if the time-independent molecular parameters remain the same, which results in the non-universality in the extensional behavior. This review article summarizes these experimental and theoretical findings for linear polymers under uniaxially extensional flow, and discusses remaining problems in particular for the concept of friction.

聚合物液体的流变行为已被普遍认为是由一些与时间无关的分子参数（如Rouse弛豫时间和每条链的缠结数）决定的，而与聚合物和溶剂（如果有）的化学结构以及聚合物浓度。这种普遍性得到了50多年来在剪切作用下进行的广泛实验以及随后的粗粒度分子模型理论开发的支持。然而，最近的实验和理论研究揭示了这种普遍性在缠结和未缠结聚合物的拉伸流动行为中的破坏。这些研究得出的一个关键概念是在快速拉伸流动下降低熔体中高取向/拉伸聚合物链的链段摩擦。即使与时间无关的分子参数保持不变，这种摩擦减小的幅度也会随聚合物和溶剂的化学结构以及聚合物浓度而变化，这导致伸展行为的非普遍性。这篇综述文章总结了线性聚合物在单轴拉伸流动下的这些实验和理论发现，并讨论了特别是摩擦概念方面尚存的问题。