Crystallization and morphology of polymerized cyclic butylene terephthalate

Abstract

The crystallization behavior and morphology of polymerized cyclic butylene terephthalate (pCBT) were investigated by thermal differential scanning calorimetry (DSC) and polarized light microscopy (PLM). The spherulite growth rate was analyzed based on the Hoffman and Lauritzen theory to better understand the crystallization behavior. We found four typical morphologic features of pCBT corresponding to the crystallization temperature spectrum: usual negative spherulite, unusual spherulite, mixed birefringence spherulite coexisting with boundary crystals, and highly disordered spherulitic crystallites. The Avrami crystallization kinetics confirmed the occurrence of combined heterogeneous nucleation accompanied by a change in the spherulitic shape of pCBT, which also agreed with the PLM results. The equilibrium melting temperature and glass transition temperature of pCBT were 257.8 C and 41.1 C, respectively. A regime II-III transition occurred at 200.9 C, which was 10 C lower than that reported for poly(butylene terephthalate) (PBT). Coinciding with and attributed to the regime transition, the boundary crystal disappeared at temperatures above 200 C and the morphology changed from the mixed type to highly disordered spherulitic crystallites.

A Review of the Recent Advances in Cyclic Butylene Terephthalate Technology and its Composites

“…Similarly, Wu et al 47 In a further step, the water absorption of CBT was measured. First, CBT was dried in the circulating air oven at 100 °C for 5 h. Then the dried CBT was conditioned in ambient atmosphere with a relative humidity of 60% and the water uptake was measured again using Karl-Fischer-titration.…”

Effects of crystalline morphologies on the mechanical properties of carbon fiber reinforcing polymerized cyclic butylene terephthalate composites

“…However, as shown in Figure 2, spherulites abounded in the pCBT matrix and not affected by the crystallization temperature. This finding implies that the nucleation rate of pCBT resin shall be quite high and cause the difficulty on determining spherulitic growth rate and kinetics analysis [14]. It is noteworthy that all transcrystalline morphology might occur in the carbon/pCBT composites due to higher fiber volume fraction.…”

“…This was followed by quenching until the desired crystallization temperatures (T c =185, 195, and 210°C) were reached. Based on crystallization kinetics studies [14], the time for complete crystallization is 20 min for T c = 185°C, 40 min for T c = 195°C, and 120 min for T c = 210°C. The composite samples were then slowly cooled to room temperature and removed from the mold.…”

“…The fiber mass fraction of the composite samples was accurately determined by thermogravimetric analysis after DSC analysis and the measured melting enthalpy was renormalized to the actual amount of pCBT presence in the sample. The degree of crystallinity (X c ) of the pCBT samples was calculated from Equation (1): (1) where M f mass fraction of reinforcement, H f heat of fusion of the fully perfect crystal of PBT with a value of 142 J/g [14,15], #H absolute value of the heat of crystallization or fusion, measured as the area underneath the peaks.…”

“…Additionally, the transition from crystalline to amorphous is more pronounced in the pCBT samples, which might be an indication of a reduction in tie molecules [13]. Our previous study [14,15] on the isothermal crystallization of pCBT polymerized at 230°C catalogues four morphologic features: usual negative spherulites; unusual spherulites; mixed type birefringence spherulites coexisting with boundary crystals; and highly disordered spherulitic crystals corresponding to the crystallization temperatures. Thus, it can be seen visually that the processing condition like temperature strongly affects the crystal morphology, and perhaps even the final properties of the polymer and its composites.…”