Synthesis and Characterization of a Thermoresponsive Copolymer with an LCST-UCST-like Behavior and Exhibiting Crystallization

Abstract

In this work, the diblock copolymer methoxy-poly(ethylene glycol)-block-poly(ε-caprolactone) (MPEG-b-PCL) was synthesized with a block composition that allows this polymer in aqueous media to possess both an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) over a limited temperature interval. The value of the UCST, associated with crystallization of the PCL-block, depended on heating (H) or cooling (C) of the sample and was found to be CP_UCST^H = 32 °C and CP_UCST^C = 23 °C, respectively. The LCST was not affected by the heating or cooling scans; assumed a value of 52 °C (CP_LCST^H = CP_LCST^C). At intermediate temperatures (e.g., 45 °C), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM) showed that the solution consisted of a large population of spherical core-shell particles and some self-assembled rodlike objects. At low temperatures (below 32 °C), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) in combination with SAXS disclosed the formation of crystals with a cylindrical core-shell structure. Cryo-TEM supported a thread-like appearance of the self-assembled polymer chains. At temperatures above 52 °C, incipient phase separation took place and large aggregation complexes of amorphous morphology were formed. This work provides insight into the intricate interplay between UCST and LCST and the type of structures formed at these conditions in aqueous solutions of MPEG-b-PCL diblock copolymers.

Figure 1

Synthetic route for the preparation of the diblock copolymer MPEG-b-PCL via a ring-opening polymerization procedure.

Figure 2

NMR characterization of MPEG-b-PCL diblock copolymer by using (a) ¹H NMR spectrum (CDCl₃-d as a solvent, 400 MHz) and (b) ¹³C NMR spectrum (CDCl₃-d as a solvent, 150 MHz).

Figure 3

Turbidity curves of 1 wt % of MP(EG)₁₇-b-P(CL)₉ in water. The blue curve represents the cooling curve of the sample, and the red curve represents the heating curve. In both series, a heating/cooling rate of 0.2 °C/min was used.

Figure 4

DSC heating (a) and cooling (b) curves of 1 wt % of MP(EG)₁₇-b-P(CL)₉ in water. The heating and cooling rates were set at 0.2 °C/min.

Figure 5

(a) Autocorrelation functions of the scattered intensity for 1 wt % of MP(EG)₁₇-b-P(CL)₉ aqueous solution at the temperatures indicated. (b) Intensity-weighted distributions of the apparent hydrodynamic radii at different temperatures.

Figure 6

Time evolution of the fast and slow hhydrodynamic radii corresponding to the fast (R_H,f) (black) and the slow mode (R_H,s) (red) in 1 wt % of aqueous solutions of MP(EG)₁₇-b-P(CL)₉ at the temperatures indicated: (a) 45 °C, (b) 40 °C, (c) 37 °C, and (d) 30 °C.

Figure 7

(a) Evolution of the apparent hydrodynamic radius (R_H) upon heating 1 wt % of aqueous solutions of MP(EG)₁₇-b-P(CL)₉ from 45 to 65 °C. The inset plot shows significant increase of the count rate from 54 °C and above. (b) Intensity-weighted size distributions at the selected temperatures showing the presence of only one population in the sample. A slight increase in R_H can be observed at higher temperatures.

Figure 8

SAXS curves (every 4th point is skipped) for 1 wt % of MPEG–b-PCL aqueous solution at 30 and 45 °C.

Figure 9

WAXS scattering curves of 1 wt % of MPEG–b-PCL aqueous solution at 45 °C (red), 30 °C (green), and 20 °C (blue).

Figure 10

Cryo-TEM images of 1 wt % of MPEG–b-PCL aqueous solution at (a) 45 °C and (b) 20 °C.