The Annealing of Crystals

141

The crystallization behavior of polylactic acid (PLA) was studied in the presence of a crystal nucleating agent, ethylenebishydroxystearamide (EBH). The crystallization rate and crystallinity were significantly increased with addition of EBH. The isothermal crystallization halftime at 105 C was decreased from 18.8 minutes for neat PLA to 2.8 minutes for PLA with 1.0 wt % of EBH. The crystallinity of PLA with 1.0 wt % EBH was about 35% after 5-minute annealing at 105 C. Like neat PLA, the double melting peaks were also observed for nucleated PLA. The changes of the double melt peaks were investigated with various crystallization temperatures, heating rates, and annealing times. The heat deflection temperature (HDT) of nucleated PLA was up to 93 C after annealing. The correlation between crystallinity and HDT was demonstrated. A percolation threshold of crystallinity was found corresponding to HDT. The crystal size of nucleated PLA was significantly decreased with addition of EBH. The mechanical properties of annealed PLA blends simultaneously; showed improved modulus and impact strength.

Section: Resultsmentioning

confidence: 72%

The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent

Tang

Zhang

Liu

et al. 2011

141

“…It is believed that melting at lower temperature is associated with thinner lamellar in the composites. Other workers29–34 have reported the dependence of T m on lamellar thickness. HA filler has higher specific heat capacity, thus making it to be a better heat conductor.…”

Section: Resultsmentioning

confidence: 89%

High density polyethylene/ultra high molecular weight polyethylene blend. II. Effect of hydroxyapatite on processing, thermal, and mechanical properties

Lim

Ishak

Ishiaku

et al. 2006

Hydroxyapatite (HA) is part of bone mineral composition. Several attempts have been made to incorporate HA into high density polyethylene (HDPE) to produce bone replacement biomaterials since neat HDPE is not suitable as bone replacement. The blending of HDPE with ultra high molecular weight polyethylene (UHMWPE) up to 50% by weight was performed with the aim of improving the toughness of composites. Reinforcement of blend with HA of up to 50% by weight was carried out. Methods of characterizing the composites included density, differential scanning calorimetry, thermal gravimetric analysis, ash content, and morphological examination using scanning electron microscope. For the mechanical properties of the composites, tensile, flexural, and impact tests were carried out. Incorporation of HA into HDPE has resulted in the brittleness of the composites. Blending of HDPE with UHMWPE in the presence of HA was found to improve the mechanical properties and promote a ductile failure of the resulting composites.

“…Nevertheless, in the absence of a more suitable model, the JMA equation can be used to predict the salient features of cold‐crystallization kinetics 1. As shown in Figure 2, two distinct stages are manifest during isothermal crystallization.…”

Section: Resultsmentioning

confidence: 99%

“…The properties of semicrystalline polymers largely depend on the crystal structure and morphological evolution, the latter being closely related to external conditions such as the crystallization temperature 1, 2. Since the successful synthesis of syndiotactic polystyrene (s‐PS) was reported in 1986,3 s‐PS has been used in a variety of applications in many fields and has been widely investigated.…”

Section: Introductionmentioning

confidence: 99%

Cold‐crystallization kinetics of syndiotactic polystyrene

Nutt

2003

ABSTRACT:The kinetics of the isothermal and nonisothermal cold crystallization of syndiotactic polystyrene (s-PS) were characterized with differential scanning calorimetry. A Johnson-Mehl-Avrami analysis of the isothermal experiments indicated that the cold crystallization of s-PS at a constant temperature followed a diffusion-controlled growth mode with a decreasing nucleation rate. Furthermore, the slow nucleation rate was the controlling step of the entire kinetic process. For nonisothermal cold-crystallization kinetics, we used a simple model based on a combination of the well-known Avrami and Ozawa models. The analysis revealed that, unlike for melt crystallization, the Avrami and Ozawa exponents were not equal. The activation energies for the isothermal and nonisothermal cold crystallizations of s-PS were 792.0 and 148.62 kJ mol Ϫ1 , respectively, indicating that the smaller motion units in cold crystallization had a weaker temperature dependence than those in melt crystallization.