Ultra-High Molecular Weight Nonlinear Bisphenol A Polycarbonates by Solid State Polymerization in Micro-Layers
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The solid-state polymerization of bisphenol A polycarbonate (BPAPC) has been studied in amorphous and partially crystallized micro-layers (SSPm) of low molecular weight prepolymers in presence of LiOH.H2O catalyst at a temperature between the glass transition temperature and the melting point. When the prepolymers (14,000 g/mol) in micro-layers of a thickness range from 5 μm to 35 μm were solid-state polymerized at 230 °C, the polymer molecular weight increased rapidly to above 100,000 g/mol, exceeding the highest molecular weight obtainable by the conventional solid-state polymerization in micro-particles. It has also been observed that the final molecular weight reached as high as 600,000 g/mol even in presence of significant stoichiometric imbalances of end group mole ratios when the prepolymer having 21,000 g/mol is used at 230°C under low pressure (10 mmHg). Most notably, amorphous prepolymer micro-layers showed significantly higher increase in molecular weight than partially crystallized prepolymer micro-layers.
The chain branching and partially cross-linked structures in high molecular weight polycarbonates have been confirmed by 1H-NMR spectroscopic analysis as well as pyrolysis-gas chromatography mass spectrometry (Py-GC/MS). 13C-NMR analysis and SSP theoretical model simulation have shown that conventional linear step-growth polymerization is not responsible for the additional increase in molecular weight beyond 50,000 g/mol of polycarbonate MW. The ultra-high molecular weight is contributed to the formation of branched and partially cross-linked structures via Fries or Kolbe-Schmitt rearrangement reactions and radical recombination reaction, respectively. Micro-radical polycarbonate species can be produced via chain scission reaction and hydrogen abstraction at the solid-state polymerization temperature. The formation of cross-linked polymers by radical recombination reactions was attributed to the near complete removal of phenol (i.e. radical scavenger) from the micro-layers during the solid state polymerization. Branched structure polycarbonate was also confirmed by atomic force microscopy (AFM).
The presence of branched and cross-linked polymers contributed to the insolubility of the polymer in solvents such as chloroform, tetrahydrofuran (THF), and methlylene chloride. As SSPm process extends for a long reaction time at 230°C, about 95% of the polymer was insoluble with excellent transparency (90-93% light transmission). Properties of ultra-high molecular weight nonlinear polycarbonates (SSPm PCs) have been investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and rheometer.
The development of Multi-Layer Deposit and Reaction (MLDR) technique has shown that the SSPm process is not limited to 5-35μm scale. The layer thickness can be expanded while keeping the merits (e.g. high transparency, good solvent resistance, and obtaining high molecular weight in short reaction time) of the SSPm technique developed in this study.