| Summary: | The transient elongational viscosity <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="sans-serif">η</mi><mi mathvariant="normal">e</mi></msub><mrow><mo>(</mo><mi mathvariant="normal">t</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> of the polymer melt is known to exhibit strain hardening, which depends on the strain rate <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="sans-serif">ε</mi><mo>˙</mo></mover></mrow></semantics></math></inline-formula>. This phenomenon was elucidated by the difference of chain stretching in the entanglement network between extension and shear. However, to date, the microscopic evolution of polymer melt has not been fully statistically analyzed. In this work, the radial size distributions <i>P</i>(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="normal">R</mi><mi mathvariant="normal">g</mi></msub><mrow><mo>,</mo><mi mathvariant="normal">t</mi></mrow></mrow></semantics></math></inline-formula>) of linear polymers are explored by dissipative particle dynamics during the stretching processes. In uniaxial extensional flow, it is observed that the mean radius of gyration <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover><mi mathvariant="normal">R</mi><mo>¯</mo></mover></mrow><mi mathvariant="normal">g</mi></msub><mrow><mo>(</mo><mi mathvariant="normal">t</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> and standard deviation <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mrow><mi mathvariant="sans-serif">σ</mi><mo>(</mo><mi mathvariant="normal">t</mi><mo>)</mo></mrow></mrow></semantics></math></inline-formula> remain unchanged until the onset of strain hardening, corresponding to linear viscoelasticity. Both <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover><mi mathvariant="normal">R</mi><mo>¯</mo></mover></mrow><mi mathvariant="normal">g</mi></msub></mrow></semantics></math></inline-formula> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi mathvariant="sans-serif">σ</mi></semantics></math></inline-formula> rise rapidly in the non-linear regime, and bimodal size distribution can emerge. Moreover, the onset of strain hardening is found to be insensitive to the Hencky strain (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mover><mi mathvariant="sans-serif">ε</mi><mo>˙</mo></mover></mrow><mi mathvariant="normal">H</mi></msub><mi mathvariant="normal">t</mi></mrow></semantics></math></inline-formula>) and chain length (N).
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