Crystallization of a polyphosphoester at the air-water interface

Nazmul Hasan, Christian Schwieger, Karsten Busse, and Jörg Kressler

Department of Chemistry, Martin Luther University Halle-Wittenberg, D-06099 Halle (Saale), Germany

We are interested in studying the crystallization of synthetic polymers on the water surface [1]. Also water soluble polymers can be studied on the water surface when they have some amphiphilic character as e.g. poly(ethylene oxide) (PEO) with the hydrophilic ether oxygen and the two hydrophobic CH2 groups in its repeat unit. Thus, PEO remains on the water surface of a Langmuir trough when spread from solution and evaporation of the solvent. Even crystallization of PEO can be studied when the water subphase is replaced by certain aqueous salt solutions [2]. But usually hydrophobic polymers are spread from solution onto the water subphase having some polar anchor groups as the most prominent example of poly(-caprolactone) (PCL). Here we report about the crystallization on the water surface of a polyethylene-like material synthesized by acyclic diene metathesis (ADMET) polymerization [3]. The precision polymer contains a phenylphosphate group separated by exactly 20 CH2 groups in its repeat unit (PPE). It was dissolved in chloroform and spread on the water surface of a Langmuir trough. The surface pressure vs area per monomer unit Langmuir isotherm together with epifluorescence and Brewster angle microscopy indicated polymer crystallization upon film compression. The extended plateau region of the Langmuir isotherm corresponds to the 2D crystallization of most polymer chains. Brewster angle and epifluorescence microscopy show that during the crystallization of PPE in the Langmuir film single crystal like hexagonal entities are formed with lateral dimensions of up to 20 µm. These entities break upon compression beyond the limiting area per monomer unit which leads to a decrease of the elasticity modulus of the Langmuir film. The morphology of the single crystals and their failure upon compression are also observed in Langmuir-Blodgett films by atomic force microscopy. The polymer crystallization on the water surface is also confirmed by infrared reflection absorption spectroscopy (IRRAS).

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[4] N. Hasan, C. Schwieger, H.T. Tee, F.R. Wurm, K. Busse, J. Kressler, Crystallization of a polyphosphoester at the air-water interface, Eur. Polym. J., in press.