Influence of Long-Chain Branching on the Crystallization and Melting Behavior of Polycarbonates in Supercritical CO₂

Abstract

The crystallization behavior of two long-chain branched bisphenol A polycarbonates (PC-Bs) was studied in supercritical CO2 using differential scanning calorimetry (DSC), wide-angle X-diffraction, and atomic force microscopy (AFM), with a linear polycarbonate (PC-L) as reference. All the PCs had similar molecular weights and molecular weight distributions. Positron annihilation lifetime spectroscopy measurement indicated that the increase in free volume fraction of 13.1% (PC-B1) and 11.8% (PC-B2) was obtained with incorporating long-chain branches compared to PC-L. This increased segmental mobility decreased the energy barrier for PC crystallization and hence increased the crystallization kinetics for PC-Bs. Two melting peaks (T m1 and T m2) observed during DSC heating scan were associated with the melting of secondary and primary crystallization, respectively. The T m2 of PC-Bs was about 2−4 °C lower than that of PC-L of at the same treatment time, pressure, and temperature. These results, qualitatively consistent with AFM results, mean that the incorporation of long-chain branches decreased the perfection or increased the lattice disorder of primary crystallites. The T m1 of crystallized PC-Bs was also much lower than that of PC-L under the same treatment conditions, which was attributed to the decrease in primary crystallization process and the resultant reduction in conformational constraints in the residual amorphous regions. AFM results also indicate that the branching structure might increase the crystallite density and sequentially the crystallinity degree of PC-Bs.

A Study of the Crystallization, Melting, and Foaming Behaviors of Polylactic Acid in Compressed CO2

“…However, this phenomenon was very different in the melting behavior of PC in supercritical CO 2 [26], where the melting peak usually increased with the temperature. It is known that the increase in melting peak is associated with an increase in lamellar thickness [35].…”

“…Therefore, transformation of the amorphous phase of polymer to a lower free energy crystalline structure is kinetically favorable [26]. However, increased CO 2 pressure did not obviously affect the crystallization thermodynamics of PLA.…”

Cosolvent effect of water in supercritical carbon dioxide facilitating induced crystallization of polycarbonate

“…It has been shown that supercritical CO 2 (scCO 2 ) with nontoxic, nonflammable, and environmentally benign characters can swell and plasticize glassy polymers, leading to a depression on their glass-transition temperature (T g ) to almost the same extent as affected by vapors or liquids [11]. The plasticization effect of scCO 2 increases the mobility of polymer chains, reduces the energy-barrier for crystallization, and hence induces crystallization of PC under moderate conditions [12][13][14][15][16][17]. However, scCO 2induced crystallization of PC has its limitation.…”

Antisolvent crystallization and solid‐state polymerization of bisphenol‐A polycarbonate

“…The results of T g , T m , H m , and X c are listed in Table . As shown in Table and Figure (a), T g of crystallized samples were higher than the neat sample, indicating that the existence of crystal region restricted the chain mobility of amorphous region . In addition, the T g of crystallized samples were very close.…”