3.2. The solubility of BHAD in PLLA and their binary phase behavior
3.2.1. The influence of Tf on the crystal morphologies of BHAD
It is found that Tf of sample has influence on Tc (Fig. 4), which
indicates that the solubility of BHAD in PLLA matrix is related to
temperature as well as BHAD concentration. Tc of the samples are
summarized in Table 1, and the influences are concluded into three
types, which is consistent with the crystal morphology diversity.
For the first type, the BHAD concentration is from 0.1 wt% to
0.2 wt%, Tc almost remains unchanged, which is the same as neat
PLLA. This concentration range is defined as region I. In this
range, BHAD completely dissolves in PLLA when temperature is
above 200 C. And in the cooling, BHAD cannot separate from
PLLA. Compared to neat PLLA, Tc of PLLA/BHAD-0.1 and PLLA/
BHAD-0.2 slightly depress. Under the circumstance, BHAD plays
a role of solvent to PLLA and has no nucleating effect on PLLA.
Similar phenomenon has been found in bis(3,4-
dimethylbenzylidene)sorbitol/iPP system. Kristiansen et al. [27]
suggested that very low content additive would be inactive due
to “complete solubility” in the polymers. So the crystal morphologies
of PLLA/BHAD-0.1 and PLLA/BHAD-0.2 are similar to
that of neat PLLA (Fig. 3).
For the second type, the BHAD concentration is from 0.3 wt% to
0.5 wt%, Tc is in inverse proportion to Tf. This concentration range is
defined as region II. In this range, BHAD is partially dissolved in
PLLA matrix. When Tf is low, BHAD concentration will slightly
exceed its saturated solubility. Those undissolved BHAD crystals
become nuclei for the dissolved BHAD and for PLLA in cooling.
However, if Tf rises, the solubility of BHAD will increase. The more