Phase transition behavior of cationic diblock copolymers

Abstract

Introduction: This study focused on characterizing the phase transition behavior in water of
thermoresponsive cationic diblock copolymers comprised of a cationic and a non-ionic block.

Background: Thermoresponsive polymers, especially block copolymers, have long been of
interest to researchers within the field of drug delivery systems. However, more research into
their phase transition behavior is needed to understand how these polymers could be optimized
for such use.

Aim: The aim of this study was to investigate the phase behavior, i.e., the association behavior,
of two thermoresponsive cationic diblock copolymers with different block lengths, in aqueous
solution on their own as well as mixed with a bile salt biosurfactant.

Methods: The thermoresponsive block copolymers, in aqueous solution on their own as well
as mixed with the bile salt, were analyzed using dynamic and static light scattering to study the
relaxation time distributions as well as the total intensity of scattered light at different
temperatures. Additionally, ultraviolet-visible spectroscopy was utilized to track the changes in
turbidity of the samples at different temperatures.

Results: On their own, both polymers exhibited thermoresponsive behavior with a phase
transition temperature slightly above 40 °C. It was not possible to distinguish different phase
transition behaviors for the two polymers. When mixed with the bile salt in sufficient
concentration, larger co-assembled structures were obtained. No thermoresponsive behavior
was observed for these within the investigated range.

Conclusion: In conclusion, the pure block copolymers exhibited thermoresponsivity within the
investigated temperature range. Upon bile salt addition, the block copolymer and the bile salt
co-assembled into larger stable structures. These did not, as indicated by the analytical
techniques used in this study, exhibit any pronounced thermoresponsivity within the
investigated range, indicating a possible use for this type of block copolymer as a bile salt
sequestrant which could be used in treatment of bile salt related diseases.

Popular Abstract

Polymers are versatile materials made up of smaller molecules known as monomers. They can,
for example, be used to encapsule medications for delivery to target sites within the body.
Certain polymers are unique in the sense that they respond to changes within the body. Polymers
carrying a therapeutic substance and exhibiting lower critical solution temperature (LCST)
behavior, meaning that their solubility is greatly reduced above a certain temperature, may
release the encapsulated substance upon reaching for example an infection site where the
temperature is heightened.

By combining two different types of monomers in the synthesis of a polymer, what is known
as a copolymer is obtained. The arrangement and chemistry of the monomers will influence the
physical behavior of the copolymer, such as its solution properties. Thus, the resulting
copolymer will integrate properties of both monomers and possibly develop additional ones.
Because of the latter, copolymers are of particular interest to researchers within the field of drug
delivery systems. However, due to their richer phase behavior, more research is required to
fully understand their properties. Such aspects would be the conditions under which phase
transitions will occur in aqueous solution, their interactions with different drugs, and their
biocompatibility.

In this study, the phase transition behavior in water of a type of block copolymer comprised of
a non-ionic block and a cationic block was investigated, both on its own as well as mixed with
the bile salt sodium deoxycholate, a surfactant naturally found in the human digestive system
that helps in the digestion of fats. Light scattering methods and ultraviolet-visible spectroscopy
were employed to gain information about changes in the association behavior of the block
copolymers at different temperatures.

The obtained results indicated that the block copolymer on its own exhibits a thermoresponsive
behavior in water with a phase transition temperature slightly above 40 °C, where aggregates
begin to form. This is due to the temperature-dependent phase behavior of the non-ionic block,
which as a homopolymer displays LCST behavior. When mixed with the bile salt in sufficient
concentrations, larger structures were obtained. These were not observed to exhibit
thermoresponsivity within the investigated temperature range.

• author: Johansson, Emma
• supervisor: Karin Schillén; Jaume Adrover Forteza
• organization: Department of Chemistry
• course: KEMK10 20252
• year: 2025
• type: M2 - Bachelor Degree
• subject: Chemistry
• keywords: cationic diblock copolymers, bile salt, physical chemistry, thermoresponsivity
• language: English
• id: 9214827
• date added to LUP: 2025-11-04 14:25:42
• date last changed: 2025-11-04 14:25:42