Microstructure-Thermal Property Relationships of Poly (Ethylene Glycol- b-Caprolactone) Copolymers and Their Micelles

Abstract

The crystallinity of polymers strongly affects their properties. For block copolymers, whereby two crystallisable blocks are covalently tethered to one another, the molecular weight of the individual blocks and their relative weight fraction are important structural parameters that control their crystallisation. In the case of block copolymer micelles, these parameters can influence the crystallinity of the core, which has implications for drug encapsulation and release. Therefore, in this study, we aimed to determine how the microstructure of poly(ethylene glycol-b-caprolactone) (PEG-b-PCL) copolymers contributes to the crystallinity of their hydrophobic PCL micelle cores. Using a library of PEG-b-PCL copolymers with PEG number-average molecular weight (Mn) values of 2, 5, and 10 kDa and weight fractions of PCL (fPCL) ranging from 0.11 to 0.67, the thermal behaviour and morphology were studied in blends, bulk, and micelles using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WXRD), and Synchrotron wide-angle X-ray scattering (WAXS). Compared to PEG and PCL homopolymers, the block copolymers displayed reduced crystallinity in the bulk phase and the individual blocks had a large influence on the crystallisation of one another. The fPCL was determined to be the dominant contributor to the extent and order of crystallisation of the two blocks. When fPCL < 0.35, the initial crystallisation of PEG led to an amorphous PCL phase. At fPCL values between 0.35 and 0.65, PEG crystallisation was followed by PCL crystallisation, whereas this behaviour was reversed when fPCL > 0.65. For lyophilised PEG-b-PCL micelles, the crystallinity of the core increased with increasing fPCL, although the core was predominately amorphous for micelles with fPCL < 0.35. These findings contribute to understanding the relationships between copolymer microstructure and micelle core crystallinity that are important for the design and performance of micellar drug delivery systems, and the broader application of polymer micelles.

Keywords: copolymer; crystallinity; micelle; microstructure; thermal behaviour.

Figure 1

Representative DSC thermograms of PEG (M_n = 2, 5 and 10 kDa) and PCL (M_n = 2 kDa) homopolymers, and PEG/PCL homopolymer physical blends (1:1 w/w) showing the first heating, first cooling, and second heating profiles (ramp rate 10 °C /min).

Figure 2

Representative DSC thermograms of the PEG₂ and PCL₂ homopolymers, PEG₂PCL_y copolymer series, and PEG₂/PCL₂ (1:1 w/w) blend, showing the first heating, first cooling, and second heating profiles (ramp rate 10 °C/min).

Figure 3

Stacked WXRD diffraction patterns of the PEG₂ and PCL₂ homopolymers recorded at 25 °C, and the PEG₂PCL_y block copolymers recorded before thermal treatment at 25 °C (as precipitated), in the molten state at 80 °C, and at T_cryst upon cooling.

Figure 4

(a) T_m and ΔH_m, (b) T_cryst and ΔH_cryst of the PEG₂ homopolymer and PEG₂PCL_y copolymer series recorded during the first and second heating cycles as a function of f_PCL (n = 5).

Figure 5

Representative DSC thermograms of the PEG₅ and PCL homopolymers and the PEG₅PCL_y polymer series, showing the first heating, first cooling, and second heating profiles (ramp rate 10 °C/min).

Figure 6

Stacked WXRD diffraction patterns of the PEG₅ and PCL₂ homopolymers recorded at 25 °C, and the PEG₅PCL_y copolymer series at room temperature (25 °C), in the molten state (80 °C) and at the T_cryst values upon cooling.

Figure 7

Representative DSC thermograms of the PEG and PCL homopolymers, and PEG₂PCL_y, PEG₅PCL_y, and PEG₁₀PCL_y copolymer micelles (lyophilised powder), showing the first heating profiles (ramp rate 10 °C/min).

Figure 8

WAXS scattering profiles of the PEG₂PCL_y, PEG₅PCL_y, and PEG₁₀PCL_y copolymer micelles (lyophilised powder), and PEG₅ and PCL₂ homopolymers.