Dynamic crossover of the sub-Rouse modes in the glass–rubber transition region in poly(n-alkyl methacrylates) with different side chain lengths

Multiple relaxation dynamics under electric field enables tunable viscoelastic response of poly(methyl methacrylate) above glass transition temperature

Optimizing mechanical performance is crucial for the practical utilization of stimuli-responsive polymers, while complex viscous and elastic behavior hinders a deep understanding of functional polymers under external field excitation. Here, we demonstrate the in situ dynamic and static mechanical responses under electric stimuli of poly(methyl methacrylate) (PMMA) above glass transition temperature (Tg) by applying a direct current electric field vertically to the mechanical loading. The results show that the electro-mechanical response of PMMA is directly correlated to chain relaxation modes with different length scales: for local segments, polarization provides resistance for molecular motion, manifested by enhanced moduli, increased transient viscosity, and a wider linear viscoelastic range, whereas in a larger spatial range, polarization-induced conformation change causes faster relaxation, reduced elastic modulus, and a lowered modulus plateau. Moreover, flow viscosity is reduced because of weaker friction between chain segments under polarization. Our results suggest effective strategies for precisely tuning the viscoelastic behavior of polymers above Tg through electric stimuli.

Mediating the slow dynamics of polyacrylates by small molecule-bridged hydrogen bonds

This report studied the changes in the slow dynamics of polyacrylate by adding a hindered phenol (CA) capable of forming three intermolecular hydrogen bonds (inter-HBs) per molecule with the polymer chain. The CA molecule apparently diminishes the slow modes (with lower peak temperatures and peak heights) of the polyacrylate, although it has a higher glass transition temperature (T_g) than the acrylic matrix, and the rigid CA-bridged HB network significantly amplifies the α-relaxation near T_g (with higher peak temperatures and peak heights). Consequently, the mixtures exhibit a diminishing slow mode that gradually merges with the prominent α-peak with increasing CA loadings. The anomalous dynamics concerning the opposite behaviors of the slow mode and α-relaxation was further rationalized in terms of dissociation of inter-HBs when the temperature is higher than T_g, together with the small molecule-alleviated macromolecular connectivity. This work provides essential insights into the slow dynamics of such HB-driven hybrids, and paves the way for tailoring the viscous flow properties of the hybrid material from a molecular level perspective.

Correlation between Locally Ordered (Hydrogen-Bonded) Nanodomains and Puzzling Dynamics of Polymethysiloxane Derivative

We examined the behavior of poly(mercaptopropyl)methylsiloxane (PMMS), characterized by a polymer chain backbone of alternate silicon and oxygen atoms substituted by a polar pendant group able to form hydrogen bonds (-SH moiety), by means of infrared (FTIR) and dielectric (BDS) spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and rheology. We observed that the examined PMMS forms relatively efficient hydrogen bonds leading to the association of chains in the form of ordered lamellar-like hydrogen-bonded nanodomains. Moreover, the recorded mechanical and dielectric spectra revealed the presence of two relaxation processes. A direct comparison of collected data and relaxation times extracted from two experimental techniques, BDS and rheology, indicates that they monitor different types of the mobility of PMMS macromolecules. Our mechanical measurements revealed the presence of Rouse modes connected to the chain dynamics (slow process) and segmental relaxation (a faster process), whereas in the dielectric loss spectra we observed two relaxation processes related most likely to either the association-dissociation phenomenon within lamellar-like self-assemblies or the sub-Rouse mode (α'-slower process) and segmental (α-faster process) dynamics. Data presented herein allow a better understanding of the peculiar dynamical properties of polysiloxanes and associating polymers having strongly polar pendant moieties.

Evolution of high-temperature molecular relaxations in poly(2-(2-methoxyethoxy)ethyl methacrylate) upon network formation

Copolymers of 2-(2-methoxyethoxy)ethyl methacrylate (poly(MEO2MA)) are regarded as bioinert replacements of poly(N-isopropylacrylamide) in some biomedical applications. Networks of poly(MEO2MA) of various architecture form thermo-responsive hydrogels. Here, we present dielectric and mechanical spectroscopy studies on segmental motions and network relaxation processes in linear poly(MEO2MA) and its networks - bare network and the network grafted with short poly(MEO2MA) chains. We show that the α process assigned to the segmental motions of poly(MEO2MA) is independent on the polymer topology and the glass transition temperature, Tg, associated with this process equals 235-236 K for all investigated systems. The α' relaxation observed above Tg by dynamical mechanical analysis is assigned to the sub-Rouse process. It strongly depends on the polymer network architecture and slows down by four orders of magnitude upon network formation.