Small-angle X-ray scattering data from stretched UHMWPE samples

Scattered intensity (arbitrary units) versus scattering vector (in reciprocal Angstroms) data for drawing ratios 2.5, 5, and 7.5.Supplementary information for 'Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene.'Abstract:Thermally conductive polymer crystals are of both fundamental and practical interest for their high thermal conductivity that exceeds that of many metals. In particular, polyethylene fibers and oriented films with uniaxial thermal conductivity exceeding 50 Wm−1K−1 have been reported recently, stimulating interest into the underlying microscopic thermal transport processes. While ab-initio calculations have provided insight into microscopic phonon properties for perfect crystals, such properties of actual samples have remained experimentally inaccessible. Here, we report the direct observation of thermal phonons with mean free paths up to 200 nm in semicrystalline polyethylene films using transient grating spectroscopy. Many of the mean free paths substantially exceed the crystalline domain sizes measured using small-angle x-ray scattering, indicating that thermal phonons propagate ballistically within and across the nano-crystalline domains, with those transmitting across domain boundaries contributing nearly a third of the thermal conductivity. Our work provides the first direct determination of thermal phonon propagation lengths in molecular solids, providing insights into the microscopic origins of their high thermal conductivity.

散布强度（任意单位）与散射矢量（倒数埃）数据，用于绘制2.5、5和7.5的比例图。为《沿取向聚乙烯中的弹道热声子穿越纳米晶粒域》提供补充信息。摘要：具有超越众多金属的热导率的聚合物晶体，在基础研究和实际应用中均具有极大兴趣。特别是，近期报道的单轴热导率超过50 Wm−1K−1的聚乙烯纤维和取向薄膜，激发了人们对其微观热传输过程的兴趣。尽管对完美晶体的微观声子性质的原位计算提供了洞见，但实际样品的此类性质仍实验上难以获取。在此，我们利用瞬态光栅光谱直接观察到半结晶聚乙烯薄膜中热声子的平均自由程可达200 nm。许多平均自由程显著超过使用小角X射线散射测量得到的晶粒尺寸，表明热声子能够在纳米晶粒域内及域间进行弹道传播，且跨越域边界的那些传播贡献了近三分之一的导热率。本研究首次直接测定了分子固体中热声子传播长度，为理解其高热导率的微观起源提供了洞见。