Determining Entanglement Molar Mass of Glassy Polyphenylenes Using Mechanochromic Molecular Springs.

Molecular force transduction in tough and glassy poly( meta , meta , para -phenylene) (P mmp P) was investigated as a function of M n using covalently incorporated mechanochromic donor-acceptor torsional springs based on an ortho -substituted diphenyldiketopyrrolopyrrole ( o DPP). Blending o DPP-P mmp P probe chains with long P mmp P matrix chains allowed us to investigate molar-mass-dependent mechanochromic properties for a series of specimens having mechanically identical properties. In the strain-hardening regime, the mechanochromic response (Δλ max,em ) was found to be a linear function of the acting stress and fully reversible, making o DPP-P mmp P a real-time and quantitative stress sensor. For entangled and nonentangled probe chains, distinctly different values of Δλ max,em were observed, yielding a critical molar mass of M c ≈ 11 kg mol -1 for P mmp P. Once physical cross-linking of o DPP in the network of P mmp P was ensured, Δλ max,em was found to be independent of M n . The resulting value of M c is in very good agreement with results from rheology.