Stress-strain curves derived from tensile specimens are the primary characteristic of bulk polymers' mechanical properties. Current tools, however, cannot provide molecular insights from this single bulk measurement. Hence, we use Fourier transform infrared (FT-IR) spectroscopic imaging to optically and nondestructively measure molecular structure and its spatial dependence in tensile specimens in high density polyethylene homopolymers. To overcome the limitations of FT-IR imaging, we use an emerging approach involving the use of tunable quantum cascade lasers that allows imaging through thick samples and facile polarized light imaging. Crystal structure and orientation are obtained from spatially varying measurements of molecular properties in the necking region. Local molecular (re)arrangements to characterize mechanical properties of drawn samples are deduced from spectral data. A modified Eyring model was developed to quantitatively understand spatial dependence in terms of a conformational volume. We report the strain rise in high density polyethylene homopolymers is governed by the degree of association between the crystalline domains. Together, the new measurement technology and analysis reported here can relate molecular composition, microscopic gradients, and orientation to bulk mechanical properties of semicrystalline polymers.