Solid Polycondensation Research Articles

Challenges in the experimental investigation of the caloric and thermomechanical behaviour of semi-crystalline polymers: A study on the example of polyethylene terephthalate (PET)

Investigations on industrial produced polyethylene terephthalate (PET) were carried out in the temperature range from the glassy to the liquid state. In differential scanning calorimetry (DSC) experiments, the sample passed through this temperature range several times at different speeds, interrupted by isothermal segments. Thereby the caloric behaviour observed in earlier publications could be confirmed. A closer look reveals variations in the behaviour, depending on the initial state of the sample. The thermal and mechanical boundary conditions seem to have a long-term influence on the semi-crystalline state. Independent of the initial state, a fundamental decrease in the crystallization tendency could be observed after each temperature loop. The most likely explanation for that is an increase in molecular weight in the higher temperature ranges. This leads to an increase in viscosity, as it occurs in the solid poly-condensation (SSP) process. Mechanical experiments were also carried out in the temperature range investigated, but only at different isothermal levels due to experimental limits. For these experiments, theoretical considerations were made on the basis of the viscoelastic generalization of Hooke’s law. These show the influence of the significant decrease of the ratio between shear- and bulk stiffness in the glass-transition range. While the uniaxial tensile mode fail, the uniaxial compression mode with suppressed transversal expansion needs the fluid like behaviour to determine the bulk modulus instead of the longitudinal modulus. The experimental investigations confirm these theoretical results. Concerning the actual purpose of the mechanical investigation, Struik’s protocol is used at higher isothermal levels. As a result, crystallization-related changes in the mechanical behaviour could be observed.

Charged Porous Polymers using a Solid C-O Cross-Coupling Reaction.

Herein, we report a green, fast, efficient mechanochemical strategy for charged porous polymers (CPPs). A cationic CPP with basic anions and an anionic CPP with Li(+) cations were fabricated by solid grinding under solvent-free conditions. Compared with solution-based synthesis, mechanochemical grinding can shorten the reaction time from dozens of hours to several minutes (60-90 min) to form polymers possessing a high molecular mass and low polydispersity. During the construction of CPPs, a Pd-catalyzed solid polycondensation based on unactivated organic linkers was introduced. In particular, CPPs with basic phenolic or proline anions showed good activity and stability in SO2 capture, and Li(+) -functionalized CPPs can be post-modified to CPPs with other metal ions by ion exchange, highlighting the tailorable feature of ionic-modified CPPs.

SYNTHESIS AND PROPERTIES OF HIGH MOLECULAR WEIGHT POLY(LACTIC ACID) AND ITS RESULTANT FIBERS

Direct melt/solid polycondensation of lactic acid (LA) was carried out to obtain high molecular weight poly(lactic acid) (PLA) by a process using various catalysts in the first-step melt polycondensation, and followed solid polycondensation by using p-toulenesulfonic acid monohydrate (TSA) as the catalyst in the second step. Effects of various catalysts and reaction temperature on the molecular weight and crystallinity of resulting PLA polymers were examined. It was shown that SnCl2·2H2O/TSA, SnCl2·2H2O/succinic anhydride, and SnCl2·2H2O/maleic anhydride binary catalysts should be effective binary catalysts to obtain high molecular weight PLA of more than 1.2 × 105. A conventional melt spinning method was used to spin PLA fibers, which displayed tensile strength of (382.76 ± 1.41) MPa and tensile modulus of (4.36 ± 0.07) GPa.

Synthesis of Poly-L-Lactide for Scaffold Materials by Melt-Solid Polycondensation

Poly-L-lactide (PLLA) with low viscosity average molecular mass for scaffold materials was synthesized by melt-solid condensation polymerization and characterized by IR, XRD, and 1H-NMR. The influences of catalyst (LaCl3/C7H8SO3) concentration, polymerization temperature and polymerization time on the viscosity average molecular mass of poly-L-lactide acid (PLLA) were investigated. Poly-L-lactide with a viscosity average relative molecular mass of about 7.2 × 104 was obtained when melt-solid polycondensation was conducted with first preheating at 110°C for 4 h and solid polycondensation at 150°C for 20 h; the catalyst concentration was 0.4 wt. %. PLLA was made into porous materials by using sodium hydrogen carbonate particulates as the porogen to foam. Scanning electron microscope observation indicates that the sample is highly porous and well distributed with good interconnections between pores and the pore size of porous materials in the range of 300–500 μm and it can be used as scaffold for bone tissue engineering.

The Effect of Crystallization on the Solid State Polycondensation of Poly(L‐lactic Acid)

Melt solid polycondensation is an approach to increase the molecular weight of poly (L‐lactic acid) (PLLA). For this report, the effect of crystallization time of PLLA prepolymer on the molecular weight of the biomaterial was studied. In this process, PLLA prepolymer with a molecular weight of 18,000 was first prepared by the ordinary melt‐polycondensation process. The prepolymer was crystallized at 105°C for various times, and then heated at 135°C for 15–50 h for further solid state polycondensation (SSP). The differential scanning calorimetry (DSC) and viscosity measurements were used to characterize the crystalline properties and molecular weight of the resulting PLLA polymers, respectively. The results showed that the molecular weight of PLLA reached a maximum value under the condition of a crystallization time of 30 min and SSP of 35 h.