Pooch, Fabian
(Helsingin yliopisto, 2019)
Poly(2-oxazoline)s consist of a -(CH2-CH2-N)- main chain and an N-acyl substituent. They were reported for the first time in 1966/67. They have been investigated in the bulk, in solutions and in dispersions. The recent interest lies primarily in their chemical versatility and their potential for nanomedical applications. Tailoring materials for such specific applications requires a sound knowledge of their phase behaviors, which depends on intensive parameters. Amongst others, composition and temperature are of particular interest. The phase behaviors of poly(2-propyl-2-oxazoline)s (PPOxs) will be the main focus of this thesis. PPOx homopolymers are investigated as well as block copolymers (BCPs) and blends of poly(2-isopropyl-2-oxazoline) (PiPOx) and poly(lactide) (PLA).
The first part describes the synthesis of the polymers. The PPOxs are prepared by cationic ring opening polymerization. They are linear, narrowly dispersed, and bear at the termini one methyl- and one azide-group. Semi-crystalline as well as amorphous PLA is synthesized by ring opening polymerization of L-lactide and DL-lactide, respectively. The linear PLAs are terminated with one propargyl- and one hydroxyl-group. Coupling of the azido- and alkyne-functional homopolymers gives a library of 18 PiPOx-b-PLA BCPs. This approach allows to compare the phase behaviors of the BCPs with those of the individual components.
The next part is dedicated to a study of the solution properties of three PPOx homopolymers, namely poly(2-n-propyl-2-oxazoline) (PnPOx), poly(2-cyclopropyl-2-oxazoline) (PcyPOx) and PiPOx in water, in methanol and in water/methanol mixtures. Nuclear magnetic resonance (NMR) spectroscopy of the three polymers reveals significant differences in the side-group’s rotational freedom. Unexpectedly, these differences are reflected in the calorimetric assessment of the coil-to-globule phase transition. The phase diagrams in respect to the water/methanol composition are constructed based on transmittance measurements. Methanol is a good solvent up to its boiling point for the three PPOxs. The solubility of PnPOx in water decreases when up to 40 vol% methanol is added. This behavior termed “cononsolvency” was first reported for ternary polymer/water/methanol mixtures of poly(N-isopropyl acrylamide), a structural isomer of PnPOx and PiPOx. PiPOx and PcyPOx do not exhibit cononsolvency in the investigated ternary system. The PPOxs’ solution behaviors depend on the rotational freedom of the side-groups.
In the third part, the bulk phase behavior of PiPOx, its blends with PLA, and PiPOx-b-PLA BCPs is studied. The PiPOx volume fractions in the BCPs varies from 14 to 82 %. PiPOx and PLA are miscible based on the single glass transition criterion and small angle x-ray scattering at a temperature above the melting points of the two polymers. Infrared spectroscopy indicates an attractive dipole-dipole interaction between the carbonyl moieties of the PiPOx amide and the carbonyl of the PLA ester. PiPOx and the stereo-regular PLLA are semi-crystalline. The influence of the miscibility on the crystallization is investigated by polarized optical microscopy, differential scanning calorimetry and wide-angle x-ray scattering. It is found that the presence of PLA increases the crystallization rate of PiPOx. In contrast, PLLA remains amorphous in most of the BCPs.
The last part focuses on aqueous dispersions of the self-assembled PiPOx-b-PLA BCPs. The dispersions were prepared by adding a solution of a BCP in THF to water, a non-solvent of PLA but a solvent of PiPOx at low temperature. Contrary to expectation PiPOx resides in the particle interior, together with PLA. It does not form a shell of hydrated chains around the PLA core. This conclusion was attained on the basis of NMR spectroscopy and evaluation of the thermo-responsive properties of the BCP particle dispersions in water. At room temperature the particles are colloidally stable for 20 days at least. The particle morphology is investigated by cryogenic transmission electron microscopy, light scattering and small angle neutron scattering. The particles are spherical and permeated with water over the wide PiPOx volume fraction. Short segments of PiPOx reside on the particle/water interface and stabilize the dispersion. The thermo-responsive properties of the dispersions depend on the configuration and length of these segments. Attractive interactions between soluble and insoluble block are an important factor for the self-assembly of amphiphilic BCPs.