Molecular Motion Below the Glass Transition: A Solid‐State NMR Study of Siloxane Polymer Dynamics

Molecular mobility below the glass transition temperature ( T g ) governs physical aging, mechanical relaxation, and long‐term stability in polymer glasses, yet remains difficult to quantify with chemical and timescale specificity. We use solid‐state NMR relaxometry to probe sub‐ T g dynamics in a sequence‐ and composition‐tuned series of diphenyl‐substituted siloxane polymers spanning 14%–100% phenyl sidechain content. Variable‐temperature 1 H lineshapes establish the onset of motional averaging near T g , while 1 H T 1 , 1 H T 1 ρ , and site‐resolved Lee–Goldburg cross‐polarization T 1 ρ (LGCPH T 1 ρ ) measurements disentangle nanosecond to microsecond motions across methyl siloxane and aromatic environments. Above T g , all phenylated copolymers exhibit similar reduced‐temperature behavior in 1 H T 1 , with a common T 1 minimum at ~70°C ± 10°C above T g , indicating that fast local dynamics track T g without strong composition‐dependent changes. In contrast, rotating‐frame relaxometry reveals a composition‐dependent divergence below T g : methyl siloxane‐rich materials retain temperature‐responsive microsecond‐scale dynamics, whereas increasing phenyl incorporation progressively suppresses sub‐ T g segmental mobility and produces flattened T 1 ρ profiles. Site‐specific LGCPH T 1 ρ confirms that these trends reflect intrinsic differences in segmental dynamics rather than proton spin‐diffusion averaging. Together, these results map how phenyl substitution and sequence regulate residual glassy‐state mobility in polysiloxanes and provide a chemically resolved, multi‐timescale view of sub‐ T g polymer dynamics.