Thermally modulated nanostructure of poly(ε-caprolactone)–POSS multiblock thermoplastic polyurethanes

Abstract

A series of multiblock polyurethanes with alternating sequence structures of a poly(ε-caprolactone) (PCL) segment of 2600 or 3600 g/mol and a polyhedral oligomeric silsesquioxane (POSS) segment with multiple POSS moieties (TPU2.6k_1-x or TPU3.6k_1-x, respectively; the molar ratio of PCL:POSS is 1: x; x = 2, 3, or 4) were synthesized through two-step polymerization to assure quantitative conversion of reactants. Differential scanning calorimetry and simultaneous wide- and small-angle X-ray scattering measurements were performed to study the nanostructures of those samples. The multiblock and alternating sequence structures provided nano-confined environments for PCL and POSS domains, which significantly suppressed crystallinity of the PCL phase, while nano-sized crystallites were formed in the POSS phase. The samples in series TPU2.6k and TPU3.6k were also proven to display either lamellar, cubic, or cylindrical hexagonal phase-separated nanostructures depending on the molecular weight of the PCL segment, as well as the PCL/POSS ratio. It was also found that repeated thermal cycling under a nitrogen atmosphere low enough in temperature not to alter molecular weight caused larger and more ordered PCL and POSS crystalline structure to form for the TPU3.6k series. Apparent reconfiguration of the PCL and POSS moieties along the backbone by exchange reactions associated with reversibility of urethane bonds led to increases in PCL and POSS block lengths in the TPU chains. We envision an opportunity of future research and applicability in the areas of tailored toughness and rigidity in biodegradable polymer coatings, devices with enlarged data storage capacity, drug delivery systems and tissue engineering.