[Paper Review] Structural relaxation, softness and fragility of IPN Microgels

Valentina Nigro, Barbara Ruzicka+5 more|arXiv (Cornell University)|Jul 4, 2018

Material Dynamics and PropertiesMaterials Science4 references4 citations

TL;DR

This study investigates the dynamics of interpenetrating polymer network (IPN) microgels using X-ray Photon Correlation Spectroscopy and Dynamic Light Scattering, revealing a crossover in structural relaxation from super-Arrhenius to slower-than-Arrhenius behavior at a critical concentration $C_w^*$, accompanied by a minimum in the $eta$-exponent and transitions from Fickian to anomalous to ballistic diffusion. Particle softness, tuned via PNIPAM-PAAc composition, directly controls the fragility of the system, with stiffer particles showing more fragile behavior.

ABSTRACT

Soft colloids are elastic and deformable colloidal particles with a dual character somewhere between polymers and hard spheres. As a consequence of their hybrid nature soft colloids exhibit a larger variety of behaviours with respect to hard colloids still largely unexplored and able to provide new insight into the glass transition. Here the dynamics of a soft microgel made of interpenetrated polymer networks of PNIPAM and PAAc is investigated through X-ray Photon Correlation Spectroscopy and Dynamic Light Scattering. The existence of two dynamical regimes with increasing packing fraction is found, it is characterized by a crossover of the structural relaxation time at a critical concentration $C_w^*$ from a super-Arrhenius growth to a slower than Arrhenius behaviour. This transition is also accompanied by a minimum of the $\beta$ exponent in correspondence of $C_w^*$ and by the existence of different diffusive regimes. A theoretical prediction to describe the experimental data is provided: at very low concentration the dynamics is well described by a Fickian diffusion, at intermediate concentrations an effective non Fickian anomalous diffusion has to be considered, while at the highest investigated concentrations the emergence of a ballistic motion is well described within the Mode Coupling Theory. Moreover the fragility of IPN microgels can be tuned by varying the particle softness thanks to the mutual PNIPAM-PAAc network composition: stiffer particles provide more fragile behaviours.

Motivation & Objective

• To understand the dynamic behavior of soft microgels with interpenetrating polymer networks (IPN) under varying packing fractions.
• To identify the origin of non-Arrhenius relaxation dynamics in soft colloids and link it to structural and mechanical properties.
• To investigate how particle softness, controlled by PNIPAM-PAAc composition, influences the fragility of the system.
• To determine the nature of diffusion (Fickian, anomalous, ballistic) across different concentration regimes.

Proposed method

• X-ray Photon Correlation Spectroscopy (XPCS) was used to probe collective density fluctuations and measure structural relaxation times.
• Dynamic Light Scattering (DLS) provided complementary data on particle dynamics and diffusion behavior.
• The structural relaxation time was analyzed as a function of packing fraction to identify a crossover at $C_w^*$.
• The $eta$-exponent from the stretched exponential decay was used to assess the heterogeneity of dynamics.
• Mode Coupling Theory (MCT) was applied to describe the emergence of ballistic motion at high concentrations.
• Systematic variation of PNIPAM-PAAc composition allowed tuning of particle softness and fragility.

Experimental results

Research questions

• RQ1 How does the structural relaxation time evolve with increasing packing fraction in IPN microgels?
• RQ2 What causes the crossover from super-Arrhenius to slower-than-Arrhenius relaxation dynamics?
• RQ3 How does the $eta$-exponent vary with concentration, and what does it reveal about dynamic heterogeneity?
• RQ4 What type of diffusion (Fickian, anomalous, ballistic) dominates at different concentration regimes?
• RQ5 To what extent can the fragility of IPN microgels be tuned by adjusting particle softness via polymer network composition?

Key findings

• A crossover in structural relaxation time occurs at a critical packing fraction $C_w^*$, transitioning from super-Arrhenius to slower-than-Arrhenius behavior.
• The $eta$-exponent reaches a minimum at $C_w^*$, indicating maximal dynamic heterogeneity at this concentration.
• Diffusion evolves from Fickian at low concentrations to effective non-Fickian anomalous diffusion at intermediate concentrations.
• At the highest concentrations, ballistic motion emerges and is well described by Mode Coupling Theory.
• Stiffer IPN microgels, resulting from altered PNIPAM-PAAc composition, exhibit more fragile behavior, demonstrating tunable fragility via softness control.