Tracer Diffusivity in Amphiphilic Polymer Model Co-Networks

TL;DR

This study explores how star-shaped polymers move through a special type of polymer network, revealing how their movement is affected by the network's structure and the surrounding environment.

Contribution

The paper introduces a novel analysis of tracer diffusivity in amphiphilic polymer co-networks using complementary experimental techniques.

Findings

FRS shows Fickian diffusion for all tracers in APCNs swollen in toluene.

PEG tracers exhibit entangled behavior at lower concentrations than expected.

Selective solvent swelling enhances diffusion while maintaining concentration dependence.

Abstract

Amphiphilic polymer
conetworks (APCNs) are highly interesting material
for membranes, drug delivery, or tissue engineering since their heterogeneous
structure and interactions allow for the control of the diffusion
of molecules differing by architecture, size, and interactions. We
investigate the diffusion of hydrophilic and hydrophobic star polymers
in model APCNs formed by heterocomplementary end-linking of tetra-poly­(ethylene
glycol) (t-PEG) and tetra-poly­(ε-caprolactone)
(t-PCL). Using Fluorescence Recovery After Photobleaching (FRAP) and
Forced Rayleigh Scattering (FRS), we gain complementary insights into
star polymer transport across different length and time scales. We
compare the diffusion of hydrophilic t-PEG and hydrophobic t-PCL of
various molecular weights across a wide range of APCN polymer volume
fractions, swollen in a cosolvent (toluene) and a selective solvent
(water). FRS reveals Fickian diffusion for all tracers in APCNs swollen
in toluene. In the unentangled regime, the diffusivity of the tracer
follows approximately the expected Rouse scaling for semidilute solutions.
Corrections arise for increasing polymer content due to enforcing
contacts with the other type of polymer in the APCN. At larger concentrations,
the PEG tracers develop a diffusion behavior, as expected for entangled
star polymers. Since the transition occurs below the expected entanglement
concentration, an additional impact of the strangulation regime is
likely. Partial swelling in a selective solvent leads to an enhanced
diffusion behavior as compared to a homogeneously swollen network
at the same polymer volume fraction; however, the concentration dependence
of diffusion agrees best with the strangulation regime, despite an
overall enhanced diffusion. At swelling equilibrium in the selective
solvent water, the equilibrium degree of swelling, the network morphology,
and the diffusion behavior become independent of the preparation conditions.
These findings provide insights into the diffusion mechanism of star
polymers within APCNs and contribute to the development of polymer-based
drug delivery systems for biomedical applications.

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