Inversion of crystallization rates in miscible block copolymers of poly(lactide)-block-poly(2-isopropyl-2-oxazoline)

Abstract

a Miscible block copolymers (BCPs) are rarely studied. When one or both components of such BCPs are semi-crystalline polymers, strong effects on the crystallization behavior can be expected. We present a study of 18 miscible BCPs comprised of poly(lactide) (PLLA, semi-crystalline and PDLLA, amorphous) and poly(2-isopropyl-2-oxazoline) (PiPOx, semi-crystalline) with PiPOx volume fractions of 0.14 < ϕ PiPOx < 0.82. All BCPs exhibit a single glass transition and form a homogeneous melt. Mixing has a plasticizing effect on PiPOx and increases its crystallization rates (DSC). In contrast, the crystallization rates of PLLA are dramatically reduced, or in most cases entirely prevented. During isothermal crystallization at 130°C, the crystallization rates of the BCPs were inverted in comparison with those of the parent homopolymers. Crystallization drives the BCPs to phase separate and the formed crystalline structure is that of the parent homopolymers. The fast crystallization of PiPOx confines the observed superstructure. The BCPs were studied on multiple length scales -from the atomic level (WAXS, IR spectroscopy) to the meso level (AFM, SAXS) and the macroscopic superstructure ( polarized optical microscopy). A mechanism of the structure evolution is presented.

Poly(2-isopropyl-2-oxazoline)-b-poly(lactide) (PiPOx-b-PLA) Nanoparticles in Water: Interblock van der Waals Attraction Opposes Amphiphilic Phase Separation

“…When a solvent-selective BCP comprises two blocks miscible in the bulk phase, the attraction between the two blocks becomes an important parameter to consider when assessing its assembly in a selective solvent. We recently reported that poly­( l -lactide)- b- poly­(2-isopropyl-2-oxazoline) (PiPOx - b - PLLA) BCPs are miscible in the bulk . Dipolar interactions of the carbonyl groups of the PiPOx and PLA blocks are responsible for the miscibility, as confirmed experimentally in the FTIR spectra of PiPOx- b -PLLA, which presents shifts of the PiPOx carbonyl-stretching vibrations as a function of the PLLA content.…”

“…We recently reported that poly( l -lactide)- b- poly(2-isopropyl-2-oxazoline) (PiPOx - b - PLLA) BCPs are miscible in the bulk. 3 Dipolar interactions of the carbonyl groups of the PiPOx and PLA blocks are responsible for the miscibility, as confirmed experimentally in the FTIR spectra of PiPOx- b -PLLA, which presents shifts of the PiPOx carbonyl-stretching vibrations as a function of the PLLA content. The close values of the solubility parameters of PiPOx and PLA (δ PiPOx : 24.0 J 0.5 /cm 1.5 ; δ PLA : 22.7 J 0.5 /cm 1.5 ) calculated by the method of Fedors 4 support the experimental observations.…”

“…Dialysis units (Pur-A-Lyzer, molecular weight cutoff 3500 g/mol, Sigma-Aldrich) were washed in deionized water before use. The diblock copolymers PiPOx- b -PLA (Scheme and Table , right) were prepared via click coupling of preformed azide-terminated PiPOx with propargyl-terminated PLA using the homopolymers listed in Table , left …”

“… a Nomenclature of homopolymers adopted from ref . b In kg/mol, PS calibration, THF as eluent. c Nomenclature block copolymers: “2L1” = “PiPOx2- b -PLLA1”. d Absolute molecular weight in kg/mol, calculated by relating M n MALDI (PiPOx2: 7.1 kg/mol, PiPOx3: 12.5 kg/mol) to the ratio of monomeric units obtained from 1 H NMR spectra of the diblocks. e In %, mole fraction of PiPOx repeating units in a BCP. f Ratio n PiPOx / m PLA of monomeric units in the BCPs as defined in Scheme . …”

“…The diblock copolymers PiPOx- b -PLA (Scheme 1 and Table 1, right) were prepared via click coupling of preformed azide-terminated PiPOx with propargyl-terminated PLA using the homopolymers listed in Table 1, left. 3…”