The effect of molecular weight (MW) on the polymorphous crystallization and melting behavior
of poly(l-lactide) (PLLA) were systemically studied by differential scanning calorimetry (DSC), polarized optical
microscopy (POM), wide-angle X-ray diffraction (WAXD), and time-resolved Fourier transform infrared (FTIR)
spectroscopy. It was found that the polymorphism of PLLA is not influenced much by MW, and the α‘- and
α-form crystals are produced at low and high crystallization temperature (T
c), respectively, regardless of the
MW. However, MW significantly affects the crystallization kinetics, and the crystallization rate reduces greatly
with MW increasing. Moreover, the T
c- and MW-dependent melting behavior of PLLA was clarified with combining
the DSC and FTIR results. It was found that the α‘- to α-crystalline phase transition occurs prior to the dominant
melting in both the low- and high-MW PLLA crystallized at low T
c. Unlike the high-MW PLLA, in low-MW
PLLA crystallized at low T
c, the α‘-form crystals only partially transform into the α-one, and some amounts of
α‘-form crystals melt directly without transition during the heating process. With increasing T
c, the melting of
PLLA with various MWs changes from the phase transition + melting mechanism to the usual melt−recrystallization mechanism.
Stacking of poly(3-alkylthiophene)s P3RThs and
poly(4-alkylthiazole)s P4RTzs has been studied.
Light scattering analysis indicates that head-to-tail (HT) type
HT-P3HexTh (R = n-C6H13) gives a
degree of
depolarization (ρv) of 0.26 in CHCl3, which
reveals that HT-P3HexTh takes a stiff structure even in the
good
solvent. Addition of CH3OH to CHCl3
solutions of HT-P3HexTh and head-to-head (HH) type
HH-P4HepTz
(R = n-C7H15) leads to
π-stacking of the polymer molecules to form stable colloidal
particles. The light
scattering analysis of the colloidal solution of HT-P3HexTh in a 2:1
solution of CHCl3 and CH3OH
reveals
that HT-P3HexTh is aggregated in a parallel style. Results of
filtration experiments using membranes with
0.20 and 0.02 μm pores agree with the degree of the aggregation.
P3HexThs with irregular structures (P3HexTh
(Fe) and P3HexTh (Ni) with HT/HH ratios of about 7/3 and 1/2,
respectively) show a weaker trend to aggregate;
however, P3HexTh (Fe) is considered to stack in a surface region of a
stretched poly(ethylene terephthalate)
PET film. A dichroism observed with the stretched PET film
indicates that the π−π* absorption as well as
photoluminescence of the stacked P3HexTh molecules have a transition
moment along the direction of the
polymer main chain. X-ray diffraction analysis of HT-P3RThs and
HH-P4RTzs reveals that they take a face-to-face stacked structure with an end-to-end packing mode, except for
HT-P3MeTh (R = Me). HT-P3MeTh
forms a face-centered lattice with an interlayer distance of 3.51 Å.
An alternative copolymer of bithiazole
and 4,4‘-dibutylbithiazole is packed in an interdigitation mode.
At temperatures below 0 °C, the HT-P3HexTh
molecules are π-stacked in CHCl3, and the 1H
NMR spectrum shows a severe magnetic effect on the
thiophene
ring. Solid 13C NMR data are also consistent with the
π-stacking.
On the basis of estimations in this mathematical model of the natural history of chronic hepatitis C, treating mild chronic hepatitis with interferon-alpha 2b should prolong life expectancy at a reasonable marginal cost per year of life gained, particularly in younger patients.
Effects of annealing conditions and molecular weight (MW) on the crystalline phase transition in poly(L-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). The disordered crystal (R′-form) of PLLA was found to transform into the R one during annealing process at elevated temperatures. The R′-to-R transition is quite dependent on the annealing period (t a : 0-1440 min) and annealing temperature (T a : 120-160°C). With increasing T a , the polymorphic transition progresses much more rapidly. The R′-to-R transition is mainly involved by the slight rearrangement of the chain conformation (especially related to the side groups) and packing manner in the unit cell to the more energy-favorable state, corresponding to the reduction of unit cell dimension. Besides, it was proposed that the R′-to-R transformation mainly proceeds by the direct solid-solid transition mechanism. Moreover, it was found that MW affects the crystalline phase transition significantly. In the low-MW PLLA sample, the R′-to-R transition becomes much faster, and it can proceed prominently even when annealed at relatively lower temperature.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.