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Stereocomplex crystallization and homo-crystallization of enantiomeric substituted poly(lactic acid)s, poly(2-hydroxy-3-methylbutanoic acid)s
Accepted Manuscript Stereocomplex Crystallization and Homo-Crystallization of Enantiomeric Substituted Poly(lactic acid)s, Poly(2-hydroxy-3-methylbutanoic acid)s Hideto Tsuji, Tadashi Sobue PII:
S0032-3861(15)30020-3
DOI:
10.1016/j.polymer.2015.05.056
Reference:
JPOL 17894
To appear in:
Polymer
Received Date: 26 March 2015 Revised Date:
28 May 2015
Accepted Date: 30 May 2015
Please cite this article as: Tsuji H, Sobue T, Stereocomplex Crystallization and Homo-Crystallization of Enantiomeric Substituted Poly(lactic acid)s, Poly(2-hydroxy-3-methylbutanoic acid)s, Polymer (2015), doi: 10.1016/j.polymer.2015.05.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Stereocomplex Crystallization and Homo-Crystallization of
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Enantiomeric Substituted Poly(lactic acid)s, Poly(2-hydroxy-3-methylbutanoic acid)s
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Hideto Tsuji* and Tadashi Sobue
Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
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E-mail: [email protected]
*
To whom correspondence should be addressed.
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Keywords: poly(2-hydroxy-3-methylbutyric acid); poly(hydroxybutanoic acid); stereocomplexation
Abstract: In both solution- and melt-crystallization of blends of poly(L-2-hydroxy-3-methylbutanoic [P(L-2H3MB)]
and
poly(D-2-hydroxy-3-methylbutanoic
acid)
[P(D-2H3MB)],
solely
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acid)
stereocomplex (SC) crystallites were formed without formation of homo-crystallites, excluding the melt-crystallization at crystallization temperature (Tc) = 100°C, wherein both SC and homo-crystallization took place.
In solution-crystallization of neat P(L-2H3MB) and P(D-2H3MB), In melt-crystallization of the neat
SC
the same type of homo-crystallites as reported earlier were formed.
P(L-2H3MB) and P(D-2H3MB), for the Tc range of 0–150°C, a new type of homo-crystallites having
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WAXD profiles similar to the α- or δ-form homo-crystallites of neat PLLA and PDLA were formed. The melting temperature (Tm) values of SC crystallites for the blend (189–193°C) were slightly higher than those of homo-crystallites for the neat P(L-2H3MB) and P(D-2H3MB) (163–178°C). The radial growth rate of spherulites (G) and the spherulitic number per unit area of SC crystallites for the blend
and P(D-2H3MB).
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were respectively much lower and higher than those of homo-crystallites for the neat P(L-2H3MB) Despite not so high Tm and lower G of SC crystallites for the blend compared
with those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB), the critical Tc of the blend
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above which the induction period for spherulite growth (ti) becomes positive was higher by 30°C than those of the neat polymers.
These results indicate that in the blend samples, the large side chains of
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P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the nucleation process, compared with those of the spheruites of homo-crystallites in the neat polymers.
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1. Introduction Poly(L-lactide), i.e, poly(L-lactic acid) (PLLA) is a biodegradable polymer produced from Stereocomplex (SC) crystallization
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renewable resources of carbohydrates such as corn starch [1–6].
in blends of PLLA and poly(D-lactide), i.e., poly(D-lactic acid) (PDLA) or in block copolymers PLLA-b-PDLA is effective to improve mechanical properties, thermal/hydrolytic degradation resistance, and gas barrier properties of poly(lactide), i.e., poly(lactic acid) (PLA)-based materials [6– SC formation is reported for enantiomeric substituted poly(lactide)s (PLAs) such as
poly(2-hydroxybutyrate),
i.e.,
poly(2-hydroxybutanoic
SC
10].
acid)
[P(2HB)]
[11–13]
and
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poly(2-hydroxy-3-methylbutyrate), i.e., poly(2-hydroxy-3-methylbutanoic acid) [P(2H3MB)] [14], which are biodegradable polyesters with the structures of PLAs which methyl groups are substituted with ethyl groups and isopropyl groups, respectively (Figure 1).
Hetero-stereocomplex is formed
between L- and D-forms (and vice versa) of PLA and P(2HB) [15–18] and those of P(2HB) and
L-form
Also, ternary and quaternary SC formation is reported to take place in ternary
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P(2H3MB) [19].
P(2HB) [P(L-2HB)]/D-form P(2HB) [P(D-2HB)]/D-form PDLA blends [20,21] and in
quaternary
P(L-2HB)/P(D-2HB)/L-form
[P(D-2H3MB)] blends [22].
formation
of
SC
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Simultaneous
P(2H3MB)
crystallites
and
[P(L-2H3MB)]/D-form
homo-crystallites
was
P(2H3MB)
observed
for
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solution-crystallized PLLA/PDLA blend when Mw values of the constituent polymers were higher than 1.5×105 g mol-1 [23].
For melt-crystallized blends of P(L-2HB) and P(D-2HB), SC crystallites were
sole crystalline species without formation of homo-crystallization excluding the case for crystallization temperature (Tc) = 70°C, whereas at Tc = 70°C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for Mw exceeding 1.5×104 g mol-1 [13].
For solution-crystallized neat
P(L-2HB) and P(D-2HB), the normal type of homo-crystallites were formed for Mw above 1×104 g mol-1 and the new type of homo-crystallites were formed for Mw below 1×104 g mol-1.
Here, the
normal type of homo-crystallites have the wide-angle X-ray diffraction profiles similar to those of α-
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or δ (α’)-form homo-crystallites of PLLA or PDLA [24–30].
On the other hand, for the
melt-crystallized neat P(L-2HB) and P(D-2HB), only new type, normal and new types, and only normal type of homo-crystallites were formed for Mw below, around, and above 9×103 g mol-1,
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respectively [13].
As stated above, Anderson et al. synthesized P(L-2H3MB) and P(D-2H3MB) with number-average molecular weight (Mn) values of 3.5 and 3.8×103 g mol-1 and confirmed SC formation during solvent
SC
evaporation of a mixed solution of P(L-2H3MB) and P(D-2H3MB) by the use of differential scanning calorimtery (DSC) and wide-angle X-ray diffractometry (WAXD) [14]. The melting temperature
P(L-2H3MB) and P(D-2H3MB).
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(Tm) of P(L-2H3MB)/P(D-2H3MB) stereocomplex (185°C) was higher than 169 and 166°C of neat However, to the best of our knowledge, the effects of
crystallization conditions such as crystallization methods and temperature on SC crystallization and homo-crystallization of P(L-2H3MB)/P(D-2H3MB) blends have not yet been reported. The purpose of the present study is to elucidate the effects of crystallization conditions on SC
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crystallization and homo-crystallization of equimolar blends of P(L-2H3MB) and P(D-2H3MB), together with those on homo-crystallization of neat P(L-2H3MB) and P(D-2H3MB).
Crystallization
methods include solution-crystallization during solvent evaporation and melt-crystallization at a
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constant Tc from the melt. The crystallization was monitored by DSC, WAXD, and polarized optical microscopy (POM). As the crystallization behavior is crucial for biodegradable materials because
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the crystallinity and crystalline size will largely affect their hydrolytic degradation, biodegradation, and drug release behavior [1–6], the information regarding crystallization behavior obtained in the present study will facilitate designing and processing biodegradable materials of neat P(L-2H3MB), P(D-2H3MB), and their blends with a wide variety of physical and hydrolytic degradation properties for aiming at biomedical, pharmaceutical, and environmental applications.
Considering the
hydrophobic large side chains (isopropyl groups), the neat P(L-2H3MB), P(D-2H3MB), and their blends are expected to give the biodegradable materials having the relatively low degradation rate or high hydrolytic degradation-resistance.
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2. Experimental Section
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2.1. Materials P(L-2H3MB) and P(D-2H3MB) were synthesized by the polycondensation of (S)- and (R)-2-hydroxy-3-methylbutanoic acids (2-hydroxy-3-methylbutyric acids or α-hydroxyisovaleric acids) (≥99 and 98 %, respectively, Sigma-Aldrich, Japan K.K., Tokyo, Japan), respectively, were
SC
carried out at 130°C in the presence of p-toluenesulfonic acid (5wt% of monomer, monohydrate, guaranteed grade, Nacalai Tesque Inc., Kyoto, Japan) as a catalyst, according to a previously reported The reaction times of the first and second step polycondensation under
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method [11,15,31,32].
atmospheric and reduced (1.7–1.8 kPa) pressure were 5 and 13 h, respectively.
The purification of
synthesized polymers was performed by reprecipitation using chloroform and methanol, as solvent and precipitant, respectively [19,31]. The purified polymers were dried under reduced pressure for at Figure 1 shows the 1H NMR spectrum of the synthesized P(L-2H3MB), together with
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least 6 days.
molecular structure of P(L-2H3MB). P(L-2H3MB).
P(D-2H3MB) had the spectrum identical with that of
The 1H NMR peaks at 1.0 and 2.3 ppm are ascribed to methyl protons (a) and methine
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proton of isopropyl group (b), respectively, whereas those at 4.1 and 5.0 ppm are attributed to methine protons directly bound to the main chain and located at hydroxyl terminal (d) and inside the chain (c), The peak positions of the latter are in good agreement with those reported in the
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literature [14]. The fact that peak (c) was much higher than peak (d) strongly suggests the successful synthesis of P(L-2H3MB) and P(D-2H3MB). The samples of neat polymers and their blend (thickness of ca. 10 µm) were prepared by solution casting according to a previously reported method [11,15].
Each solution of the purified and
P(L-2H3MB) and P(D-2H3MB) was separately prepared with dichloromethane as the solvent to have a polymer concentration of 0.4 g dL-1.
For the preparation of a P(L-2H3MB)/P(D-2H3MB) blend
sample, the P(L-2H3MB) and P(D-2H3MB) solutions were mixed with each other equimolarly under
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vigorous stirring. The mixed solution was cast onto a petri-dish, followed by solvent evaporation at 25°C for approximately one day.
Neat P(L-2H3MB) and P(D-2H3MB) samples were also prepared
by the same procedure without mixing solutions. The solvent remaining in the as-cast samples was
thus
prepared
samples
"solution-crystallized
samples".
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removed under reduced pressure for at least 6 days and stored in a desiccator before use. We call the Melt-crystallization
of
the
solution-crystallized samples, which were sealed under reduced pressure, was carried out at crystallization temperature (Tc) of 50–150°C for 1 h after melting at 230°C for 2 min.
After
SC
melt-crystallization, the samples were quenched at 0°C in iced water for 3 min to stop further
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crystallization. We call the thus prepared samples "melt-crystallized samples". "Melt-quenched samples" (i.e., Tc = 0°C) were prepared by quenching at 0°C in iced water for 3 min directly after melting at 230°C for 2 min.
2.2. Physical measurements and observation
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The weight- and number-average molecular weights (Mw and Mn, respectively) of the polymers were evaluated in chloroform at 40°C using a Tosoh (Tokyo, Japan) GPC system (refractive index monitor: RI-8020) having two TSK Gel columns (GMHXL) using polystyrene standards.
The
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molecular characteristics of the polymers used in the present study are summarized in Table 1. The [α]25589 of the polymers was measured in chloroform at a concentration of 1 g dL-1 and 25°C using a
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JASCO (Tokyo, Japan) P-2100 polarimeter at a wave length of 589 nm.
The glass transition, cold
crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and the enthalpies of cold crystallization and melting of homo-crystallites and SC crystallites [∆Hcc, ∆Hm(H), and ∆Hm(S), respectively] of the samples were determined using a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter. The samples were heated at a rate of 10°C min-1 from -30 to 230°C under a nitrogen gas flow of 50 mL min-1 for DSC measurements. The Tg, Tcc, and Tm values of the samples were calibrated using tin, indium, and benzophenone as standards.
The crystalline species and
crystallinity (Xc) values of the samples were estimated by WAXD. The WAXD measurements were
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performed at 25°C using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu-Kα source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA.
In a 2θ range = 7.5–27.5°, the crystalline
diffraction peak areas for SC crystallites at 2θ values around 9.8, 12.8, 13.7, 16.6, 19.4° and so on, and
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for P(L-2H3MB) or P(D-2H3MB) homo-crystallites at 2θ values around 13.8, 17.1, 18.9, 21.1, 23.9° and so on (and/or around 9.8, 12.8, 16.6, 19.4°, and so on for a new type of homo-crystallites) relative to the total area between a diffraction profile and a baseline were used to estimate the Xc values of SC
SC
crystallites [Xc(S)] and P(L-2H3MB) or P(D-2H3MB) homo-crystallites [Xc(H)], respectively.
3.1. Wide-angle X-ray diffraction
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3. Results and Discussion
To estimate the crystalline species and Xc of the solution- and melt-crystallized samples, WAXD measurements were performed.
Figure 2 shows the WAXD profiles of the solution- and
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melt-crystallized or quenched neat P(L-2H3MB), P(D-2H3MB) and their equimolar blend samples. For the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples, the diffractions of homo-crystallites were observed at 2θ values of 9.8, 12.8 (main), 13.7, 16.6, 19.4°, and so on, which
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profiles are very similar to those of homo-crystallites reported for the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) [14].
It should be noted that the X-ray wave length used in the
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present study (λ = 1.5418 Å) is different from that used in the reported study (λ = 2.2898 Å) [14] and, therefore, the comparison between the 2θ values in the present and reported studies were performed after the 2θ values reported for λ = 2.2898 Å were converted to those for 1.5418 Å.
For all the
melt-crystallized and quenched neat P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0–100°C), the diffractions of homo-crystallites were observed at 2θ values of 13.8 (main), 17.1, 18.9, 21.1, 23.9°, and so on, irrespective of Tc values. These crystalline diffraction profiles are different from those of the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB), excluding that at 13.8°, and similar to those of α- or δα’)-form of PLLA or PDLA homo-crystallites [24–30]. The finding here exhibits
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the formation of a new type of homo-crystallites in neat P(L-2H3MB) and P(D-2H3MB). All the solution- and melt-crystallized P(L-2H3MB)/P(D-2H3MB) blend samples, excluding melt-quenched sample, had the crystalline peaks only at 2θ values of 9.7, 16.8, 17.7, 19.4° and so on,
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which are very similar to those reported for P(L-2H3MB)/P(D-2H3MB) SC crystallites [14], indicating the formation of only SC crystallites without the formation of P(L-2H3MB) or P(D-2H3MB) homo-crystallites.
Here, again, the comparison between the 2θ values in the present
SC
and reported studies was carried out after the 2θ values reported for λ = 2.2898 Å [14] were converted to those for λ = 1.5418 Å.
For the P(L-2H3MB)/P(D-2H3MB) blend sample crystallized at Tc =
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100°C, in addition to the crystalline diffractions of SC crystallites, the crystalline diffractions were observed at around 12.0, 13.7, and 15.4°. The crystalline peak at 13.7° is attributable to that of the same type of homo-crystallites as observed for the melt-crystallized neat P(L-2H3MB) and P(D-2H3MB), reflecting the formation of both SC and homo-crystallites in the blends crystallized at Tc = 100°C.
However, we could not ascribe the origins of the crystalline peaks at 12.0 and 15.4°.
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For PLLA/PDLA and P(L-2HB)/P(D-2HB) blends, formation of both SC and homo-crystallites was observed when the Mw values were higher than 1.5×105 and 2.3×104 g mol-1, respectively [13,23]. Therefore, the formation of both SC and homo-crystallites in the P(L-2H3MB)/P(D-2H3MB) blend at
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Mw values of 2.4×103 g mol-1 means that the critical Mw value below which both SC and homo-crystallization occur becomes lower with an increase in the size of side chains. The broad
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diffraction profile of melt-quenched P(L-2H3MB)/P(D-2H3MB) blend sample (Tc = 0°C) indicates that the blend was amorphous. The Xc(S) and Xc (H) values evaluated from the WAXD profiles in Figure 2 are plotted in Figure 3 as a function of Tc, in this figure the Xc(H) and Xc(S) values of the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) and their blend samples are shown with broken lines. The Xc(H) values of the neat P(L-2H3MB) and P(D-2H3MB) samples solution-crystallized and melt-crystallized at Tc = 50–150°C were in the range = 65–78 % and those of melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples were independent of Tc.
Again, positive values of the melt-quenched neat
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P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0°C) indicate that the samples were crystallized during quenching from the melt, reflecting the rapid crystallization of the neat polymer samples. The solution-crystallized blend sample had the similar Xc value with those of the neat polymer samples.
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As expected from Figure 2(c), the Xc(H) values of the melt-crystallized blend samples has positive values only at Tc = 100°C, whereas their Xc(S) values increased monotonically with increasing Tc. The result here is indicative of the fact that SC crystallization and homo-crystallization are sensitive to Tc and SC crystallization is facilitated at higher Tc, probably due to the higher Tm values of SC
SC
crystallites (189–193°C) than those of homo-crystallites (163–178°C). The nil crystallinity of the
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melt-quenched blend sample indicates that the slow crystallization of SC crystallites during quenching compared with that of neat polymer samples.
The overall crystallinity values of blend samples
(Table 2) at Tc = 50 and 100°C were much lower than those of the neat polymer samples, again reflecting the low crystallizability of the blend samples.
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3.2. Differential scanning calorimetry
Thermal properties of the samples were estimated by DSC.
Figure 4 shows the DSC
thermograms of the neat P(L-2H3MB), P(D-2H3MB), and their blend samples and thermal properties
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obtained from the DSC thermograms in Figure 4 are summarized in Table 2. The solution- and melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples had glass transition, cold
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crystallization, and melting peak (homo-crystallites) in the ranges of 31–48, 58–108, and 105–188°C, respectively.
A large cold crystallization peak was observed for the melt-quenched neat
P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0°C), confirming the presence of crystallizable amorphous region before DSC heating and the crystallizability of the neat polymers during DSC heating, whereas a very small cold crystallization peak was seen for the neat P(D-2H3MB) sample melt-crystallized at Tc = 50°C, reflecting imperfect crystallization during isothermal crystallization at this temperature.
For the neat polymer samples solution-crystallized and melt-crystallized at Tc =
150°C, a complicated double or multiple melting peak was observed, whereas other neat polymer
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samples had a broad melting peak.
Such a double or multiple melting peak is normally ascribed to
the melting of original and re-crystallized crystallites. On the other hand, solution- and melt-crystallized blend samples had glass transition, cold
respectively.
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crystallization, melting (SC crystallites) peaks in the ranges of 31–48, 58–148, and 189–193°C, For the melt-quenched blend sample (Tc = 0°C), a double cold crystallization peak was
observed, confirming the presence of crystallizable amorphous region before DSC heating and the crystallizability of the blend sample during DSC heating.
Considering the cold crystallization
SC
temperature (Tcc) values of the melt-quenched neat P(L-2H3MB) and P(D-2H3MB) samples (59 and
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64°C) and the lower spherulites growth rate of SC crystallites compared to that of homo-crystallites as stated below, the lower temperature cold crystallization peak of the melt-quenched blend sample at 59°C is attributable to the cold crystallization of homo-crystallites, whereas another higher temperature cold crystallization peak of the melt-quenched blend sample at 95°C can be ascribed to that of SC crystallites.
For the blend sample melt-crystallized at Tc = 50°C, a small cold
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crystallization peak at 96°C, which is ascribed to the cold crystallization of SC crystallites, was seen. The Tm values of SC crystallites in the blend samples (189–193°C) are slightly higher than those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB) samples (165–187°C), although the Tm
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values of SC crystallites in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends (about 230 and 200°C, respectively) are much higher than those of homo-crystallites in the neat polymers (about 180 and However, rather low Tm of P(2H3MB) SC compared with that of
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100°C, respectively) [8,11–13].
PLA SC should be due to the low molecular weights of P(L-2H3MB) and P(D-2H3MB) used in the present study.
∆H(tot) = ∆Hcc + ∆Hm can be used as an indicator of crystallinity. However, in the
present study, a very broad melting peak and a non-linear baseline disturbed the accurate estimation of transition enthalpies and, therefore, ∆H(tot) can be only used as an approximate indicator of crystallinity.
Actually, the melt-quenched blend sample with zero crystallinity by the WAXD
measurement, had a large positive ∆H(tot) value (17.0 J g-1). Due to the similar melting range of SC and homo-crystallites, only one broad melting peak was
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observed for the blend sample melt-crystallized at Tc =100°C, wherein both SC and homo-crystallites were contained. Since a very sharp melting peak was only observed for SC crystallites of the blend sample melt-crystallized at Tc = 150°C, the sharp peak observed at 191°C in the melting peak for the
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blend sample melt-crystallized at Tc = 100°C is attributable to the melting of SC crystallites. Therefore, the broad melting part of the melt-crystallized blend samples can be ascribed to the melting of homo-crystallites.
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3.3. Spherulitic morphology and growth
isothermally crystallized from the melt.
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To investigate the spherulitic morphology and growth, POM was performed on the samples Figure 5 shows the typical polarized photomicrographs of
the neat P(L-2H3MB), P(D-2H3MB), and their blend samples crystallized at 150°C for the shown crystallization times.
All the samples had the morphologies of Maltese-crosses. The number of
spherulites per unit area was larger for the blend sample than for the neat P(L-2H3MB) and
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P(D-2H3MB) samples. This result reveals that blending P(L-2H3MB) and P(D-2H3MB) elevates the number of SC spherulite nuclei per unit mass. The radial growth rate of spherulites (G) and the induction period for spherulite growth (ti) of the samples are plotted in Figure 6(a) and (b) as a As evident from Figure 3, in the Tc range utilized for crystallization of the blend
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function of Tc.
sample (140–180°C), only SC crystallites as crystalline species should be formed.
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that the vertical axis for G is logarithmic [Figure 6(a)].
It should be noted
The G values of SC crystallites in the blend
sample (1.0–30 µm min-1) were much lower than those of homo-crystallites in the neat P(L-2H3MB) (157–532 µm min-1) and P(D-2H3MB) samples (67–288 µm min-1).
Surprisingly, this result is in
marked contrast with those reported for PLLA and PDLA or P(L-2HB) and P(D-2HB) wherein the G values of SC crystallites in the blends are much higher than those of homo-crystallites in the neat polymers [12,33]. It is probable that alternate packing of L- and D-polymer segments during SC crystallization can be readily occur in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends with small side chains
(methyl
and
ethyl
side
groups,
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respectively)
but
becomes
difficult
in
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P(L-2H3MB)/P(D-2H3MB) blends because large side chains (isopropyl groups) should have disturbed alternate packing of L- and D-polymer segments during SC crystallization. The G values of the neat P(L-2H3MB) were higher than those of the neat P(D-2H3MB).
The Mw value of the neat
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P(L-2H3MB) (2.93×103 g mol-1) was lower than that of P(D-2H3MB) (3.71×103 g mol-1) (Table 2). This should have increased the segmental mobility of P(L-2H3MB), resulting in the higher G values of the neat P(L-2H3MB).
The result that the spherulitic number per unit area of SC crystallites for the
SC
P(L-2H3MB)/P(D-2H3MB) blend was higher than those of homo-crystallites in the neat polymers is
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in agreement with the result reported for PLLA/PDLA blend compared with those of homo-crystallites in the neat polymers [33]. The higher Tm of SC crystallites of the blend (189–191°C) than that of homo-crystallites of the neat P(L-2H3MB) and P(D-2H3MB) (171–187°C) should have elevated the degree of super cooling and, thereby, the nuclei number of spherulites per unit area when compared at the same Tc.
The induction period for spherulite growth (ti) of the neat P(L-2H3MB) and
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P(D-2H3MB) and their blend samples was nil for Tc = 130–140°C and 140–170°C and became positive at Tc = 150 and 180°C, respectively.
Despite the small difference in Tm values between the
melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples and the melt-crystallized blend
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samples, the critical Tc of the blend sample above which ti becomes positive was higher by 30°C than those of the neat P(L-2H3MB) and P(D-2H3MB) samples.
These results indicate that in the blend
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sample, the large side chains of P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the nucleation process (as evidenced by the elevated the number per unit area of SC spherulites and the critical Tc), compared with those of the spheruites of homo-crystallites in the neat polymers.
4. Conclusions The following conclusions can be derived from the present study for the effects of crystallization conditions on the crystallization of the neat P(L-2H3MB), P(D-2H3MB), and their blend.
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In both
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solution- and melt-crystallization of the blends, solely SC crystallites were formed without formation of
homo-crystallites,
excluding
melt-crystallization
homo-crystallization took place.
at
100°C
wherein
both
SC
and
The critical Mw value below which both SC and
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homo-crystallization occur in the P(L-2H3MB)/P(D-2H3MB) blend (Mw = 2.4×103 g mol-1) is much lower than those of PLLA/PDLA and P(L-2HB)/P(D-2HB) blends (1.5×105 and 2.3×104 g mol-1, respectively) [13,23].
In solution-crystallization of the neat P(L-2H3MB) and P(D-2H3MB), the In melt-crystallization of the
SC
same type of homo-crystallites as reported earlier [14] were formed.
neat P(L-2H3MB) and P(D-2H3MB), for the Tc range of 0–150°C, the new type of homo-crystallites
formed.
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having WAXD profiles similar to the α- or δ-form homo-crystallites of neat PLLA and PDLA were The Tm values of SC crystallites (189–193°C) were slightly higher than those of
homo-crystallites (163–178°C), in marked contrast with much higher Tm values of PLLA/PDLA and P(L-2HB)/P(D-2HB) SC crystallites compared with those of homo-crystallites in the neat polymers [8,11–13].
The G and the spherulitic number per unit area of SC crystallites for the
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P(L-2H3MB)/P(D-2H3MB) blend were respectively much lower and higher than those of homo-crystallites for neat P(L-2H3MB) and P(D-2H3MB). The lower G values of SC crystallites in the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers
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are in marked contrast with much higher G values of SC crystallites in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends compared with those of homo-crystallites in the neat polymers [12,33].
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The higher spherulitic number per unit area of SC crystallites for the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers is in agreement with the result reported for PLLA/PDLA blend compared with those of homo-crystallites in the neat polymers [33]. Despite not so higher Tm and lower G of SC crystallites in the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers, the critical Tc of the P(L-2H3MB)/P(D-2H3MB) blend above which ti becomes positive was higher by 30°C than those of homo-crystallites in the neat polymers.
These results indicate that in the blend samples, the large
side chains of P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the
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nucleation process, compared with those of the spheruites of homo-crystallites in the neat polymers.
Acknowledgements: This research was supported by JSPS KAKANHI Grant Number 24550251 and a
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Grant-in-Aid for Scientific Research on Innovative Areas "Plasma Medical Innovation" (24108005)
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from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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References [1]
Kharas GB, Sanchez-Riera F, Severson DK. In: Mobley DP, editor. Plastics from microbes. New York: Hanser Publishers; 1994. p. 93–137. Hartmann MH. In: Kaplan DL, editor. Biopolymers from renewable resources. Berlin, Germany:
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[2]
Springer; 1998. p. 367–411. [3]
Tsuji H. Doi Y, Steinbüchel A, Biopolymers. Polyesters III, vol. 4. Weinheim, Germany: Wiley-VCH; 2002. p. 129–77. Södergård A, Stolt M, Prog Polym Sci 2002; 27: 1123–63.
[5]
Albertsson A-C, editor, Degradable aliphatic polyesters (Advances in Polymer Science, Vol.157); Berlin (Germany): Springer, 2002.
[6]
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[4]
Auras R, Lim L-T, Selke SEM, Tsuji H, editors., Poly(lactic acid): Synthesis, structures, properties, processing, and applications (Wiley Series on Polymer Engineering and Technology), New Jersey: John Wiley & Sons, Inc., 2010.
Slager J, Domb AJ, Adv Drug Delivery Rev 2003; 55: 549–83.
[8]
Tsuji, H. Macromol. Biosci 2005; 5: 569–97.
[9]
Fukushima K, Kimura Y. Polym Int 2006; 55: 626–42.
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[7]
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[10] Tsuji H, Ikada Y, In: Yu L, editor, Biodegradable polymer blends from renewable resources, New Jersey: John Wiley & Sons, Inc., 2009; p. 165–90.
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[11] Tsuji H, Okumura A. Macromolecules 2009; 42: 7263–6. [12] Tsuji H, Okumura A. Polymer J 2011; 43: 317–24. [13] Tsuji H, Shimizu S. Polymer 2012; 53: 5385–92. [14] Andersson SR, Hakkarainen M, Albertsson A-C, Polymer 2013; 54: 4105–11. [15] Tsuji H, Yamamoto S, Okumura A, Sugiura Y. Biomacromolecules 2010; 11: 252–8. [16] Tsuji H, Shimizu K, Sakamoto Y, Okumura A. Polymer 2011; 52: 1318–25. [17] Tsuji H, Deguchi F, Sakamoto Y, Shimizu S. Macromol Chem Phys 2012; 213: 2573–81. [18] Tsuji H, Suzuki M. Macromol Chem Phys 2014; 215, 1879–88.
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[19] Tsuji H, Hayakawa T. Polymer 2014; 55: 721–6. [20] Tsuji H, Hosokawa M, Sakamoto Y. ACS Macro Lett 2012; 1: 687–91. [21] Tsuji H, Hosokawa M, Sakamoto Y. Polymer 2013; 54: 2190–8.
[23] Tsuji H, Ikada Y, Polymer 1999; 40: 6699–708. [24] Miyata T, Masuko T. Polymer 1997; 38: 4003–9.
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[22] Tsuji H, Tawara T. Polymer 2015; 68: 57–64.
[25] Pan P, Kai W, Zhu B, Dong T, Inoue Y. Macromolecules 2007; 40: 6898–905.
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[26] Pan P, Zhu B, Kai W, Dong T, Inoue Y. J Appl Polym Sci 2008; 107: 54–62.
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[27] Kawai T, Rahma N, Matsuba G, Nishida K, Kanaya T, Nakano M, Okamoto H, Kawada J, Usuki A, Honma N, Nakajima K, Matsuda M. Macromolecules 2007; 40: 9463–69. [28] Zhang J, Tashiro K, Tsuji H, Domb AJ. Macromolecules 2007; 40: 1049–54. [29] Wasanasuk K, Tashiro K. Polymer 2011; 52: 6097–109.
[30] Tsuji H, Tashiro K, Bouapao L, Hanesaka M. Macromol Chem Phys 2012; 213: 2099−112.
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[31] Tsuji, H., Matsuoka, H., Itsuno, S. J Appl Polym Sci 2008; 110: 3954–62. [32] Tsuji H, Eto T, Sakamoto Y. Materials 2011; 4: 1384–98.
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[33] Tsuji H, Tezuka, Y. Biomacromolecules 2004; 5: 1181–6.
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Table 1.
Molecular characteristics of P(L-2H3MB) and P(D-2H3MB).
Mw a) [α]25589 b) Mw/Mn a) -1 (g mol ) (deg dm-1 g-1 cm3) P(L-2H3MB) 2.93×103 1.71 -72.2 3 P(D-2H3MB) 3.71×10 1.71 75.4 a) Mn and Mw are number- and weight- average molecular weights, respectively. b) [α]25589 values were measured in chloroform.
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Polymer
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Table 2.
Thermal properties during heating and crystallinity of neat P(L-2H3MB), P(2-2H3MB), and their
blends.
Blend
Tccb) (°C)
Tmb) (°C)
∆Hccc)
∆Hmc)
∆H(tot)d)
(J g )
(J g )
(J g )
As-cast 0 50 100 150 As-cast 0 50 100 150 As-cast 0 50 100 130f) 150
48.1 31.6 34.8 36.4 38.7 30.6 47.9 31.2 -
58.9 63.9 72.3 58.3, 108.3 137.9 59.2, 95.2 95.9 148.1
104.5, 164.6, 170.4, 188.1 178.0 174.5 176.1 155.6, 187.3 162.5, 170.4, 184.5 172.4 171.3 171.0 147.1, 162.7, 178.4 193.4 188.5 189.1 191.2
0.0 -14.7 0.0 0.0 0.0 0.0 -26.7 -1.4 0.0 -2.8 -4.3 -27.2 -14.6 -4.1
47.2 40.5 44.8 43.7 37.8 42.1 41.5 41.7 37.9 36.3 52.6 44.2 39.9 31.2
47.2 25.8 44.8 43.7 37.8 42.1 14.8 40.3 37.9 33.5 48.3 17.0 25.3 27.2
-
-
190.1
0.0
53.8
53.8
a)
-1
-1
Tc is crystallization temperature. Tg, Tcc, and Tm are glass transition, cold crystallization, and melting temperatures, respectively. c) ∆Hcc and ∆Hm are enthalpies of cold crystallization and melting, respectively. d) ∆H(tot)= ∆Hcc + ∆Hm. e) Overall crystallinity estimated by WAXD. f) No thermal data.
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b)
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-1
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P(D-2H3MB)
Tg b) (°C)
SC
P(L-2H3MB)
Tc a) (°C)
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Code
Xce) (%) 74.6 19.7 81.6 74.2 79.9 64.6 11.2 73.2 73.7 73.0 74.1 0.0 2.4 52.3 67.1 76.9
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Figure Captions
Figure 1.
1
H NMR spectrum of the synthesized P(L-2H3MB), together with molecular structure of
Figure 2.
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P(L-2H3MB).
WAXD profiles of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples. Figure 3.
Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] of
SC
solution-crystallized (the values are shown with broken lines) and melt-crystallized or quenched neat
Figure 4.
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P(L-2H3M B) (a), P(D-2H3M B) (b), and their equimolar blend (c) samples as a function of Tc. DSC thermograms of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3M B) (b), and their equimolar blend (c) samples. Figure 5. Polarized optical photomicrographs of neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples crystallized Tc = 150°C for the shown crystallization times from the melt.
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Figure 6. Radial growth rate of spherulites (G) (a) and induction period for spherulite growth (ti) of
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neat P(L-2H3MB), P(D-2H3MB), and their equimolar blend samples as a function of Tc.
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Figure 1.
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H NMR spectrum of the synthesized P(L-2H3MB), together with molecular structure of
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P(L-2H3MB).
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Figure 2.
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WAXD profiles of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
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quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples.
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Figure 3. Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] of solution-crystallized (the values are shown with broken lines) and melt-crystallized or quenched neat
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P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples as a function of T
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Figure 4.
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DSC thermograms of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
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quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples.
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Figure 5. Polarized optical photomicrographs of neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their
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equimolar blend (c) samples crystallized Tc = 150°C for the shown crystallization times from the melt.
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Figure 6. Radial growth rate of spherulites (G) (a) and induction period for spherulite growth (ti) of
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neat P(L-2H3MB), P(D-2H3MB), and their equimolar blend samples as a function of Tc.
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Highlights • Stereocomplex (SC) crystallites of L- and D-poly(2-hydroxy-3-methylbutanoic acid)s are formed.
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• The effects of crystallization conditions on the crystallization behavior are studied.
• A low crystallization temperature gives both SC and homo-crystallites in the blend.
• Two types of homo-crystallites are formed in neat polymers depending on crystallization procedure.
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• Large side chains lowered G of SC but facilitates the nucleation of SC spherulites.
S0032-3861(15)30020-3
DOI:
10.1016/j.polymer.2015.05.056
Reference:
JPOL 17894
To appear in:
Polymer
Received Date: 26 March 2015 Revised Date:
28 May 2015
Accepted Date: 30 May 2015
Please cite this article as: Tsuji H, Sobue T, Stereocomplex Crystallization and Homo-Crystallization of Enantiomeric Substituted Poly(lactic acid)s, Poly(2-hydroxy-3-methylbutanoic acid)s, Polymer (2015), doi: 10.1016/j.polymer.2015.05.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Stereocomplex Crystallization and Homo-Crystallization of
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Enantiomeric Substituted Poly(lactic acid)s, Poly(2-hydroxy-3-methylbutanoic acid)s
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Hideto Tsuji* and Tadashi Sobue
Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
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E-mail: [email protected]
*
To whom correspondence should be addressed.
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Keywords: poly(2-hydroxy-3-methylbutyric acid); poly(hydroxybutanoic acid); stereocomplexation
Abstract: In both solution- and melt-crystallization of blends of poly(L-2-hydroxy-3-methylbutanoic [P(L-2H3MB)]
and
poly(D-2-hydroxy-3-methylbutanoic
acid)
[P(D-2H3MB)],
solely
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acid)
stereocomplex (SC) crystallites were formed without formation of homo-crystallites, excluding the melt-crystallization at crystallization temperature (Tc) = 100°C, wherein both SC and homo-crystallization took place.
In solution-crystallization of neat P(L-2H3MB) and P(D-2H3MB), In melt-crystallization of the neat
SC
the same type of homo-crystallites as reported earlier were formed.
P(L-2H3MB) and P(D-2H3MB), for the Tc range of 0–150°C, a new type of homo-crystallites having
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WAXD profiles similar to the α- or δ-form homo-crystallites of neat PLLA and PDLA were formed. The melting temperature (Tm) values of SC crystallites for the blend (189–193°C) were slightly higher than those of homo-crystallites for the neat P(L-2H3MB) and P(D-2H3MB) (163–178°C). The radial growth rate of spherulites (G) and the spherulitic number per unit area of SC crystallites for the blend
and P(D-2H3MB).
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were respectively much lower and higher than those of homo-crystallites for the neat P(L-2H3MB) Despite not so high Tm and lower G of SC crystallites for the blend compared
with those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB), the critical Tc of the blend
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above which the induction period for spherulite growth (ti) becomes positive was higher by 30°C than those of the neat polymers.
These results indicate that in the blend samples, the large side chains of
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P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the nucleation process, compared with those of the spheruites of homo-crystallites in the neat polymers.
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1. Introduction Poly(L-lactide), i.e, poly(L-lactic acid) (PLLA) is a biodegradable polymer produced from Stereocomplex (SC) crystallization
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renewable resources of carbohydrates such as corn starch [1–6].
in blends of PLLA and poly(D-lactide), i.e., poly(D-lactic acid) (PDLA) or in block copolymers PLLA-b-PDLA is effective to improve mechanical properties, thermal/hydrolytic degradation resistance, and gas barrier properties of poly(lactide), i.e., poly(lactic acid) (PLA)-based materials [6– SC formation is reported for enantiomeric substituted poly(lactide)s (PLAs) such as
poly(2-hydroxybutyrate),
i.e.,
poly(2-hydroxybutanoic
SC
10].
acid)
[P(2HB)]
[11–13]
and
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poly(2-hydroxy-3-methylbutyrate), i.e., poly(2-hydroxy-3-methylbutanoic acid) [P(2H3MB)] [14], which are biodegradable polyesters with the structures of PLAs which methyl groups are substituted with ethyl groups and isopropyl groups, respectively (Figure 1).
Hetero-stereocomplex is formed
between L- and D-forms (and vice versa) of PLA and P(2HB) [15–18] and those of P(2HB) and
L-form
Also, ternary and quaternary SC formation is reported to take place in ternary
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P(2H3MB) [19].
P(2HB) [P(L-2HB)]/D-form P(2HB) [P(D-2HB)]/D-form PDLA blends [20,21] and in
quaternary
P(L-2HB)/P(D-2HB)/L-form
[P(D-2H3MB)] blends [22].
formation
of
SC
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Simultaneous
P(2H3MB)
crystallites
and
[P(L-2H3MB)]/D-form
homo-crystallites
was
P(2H3MB)
observed
for
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solution-crystallized PLLA/PDLA blend when Mw values of the constituent polymers were higher than 1.5×105 g mol-1 [23].
For melt-crystallized blends of P(L-2HB) and P(D-2HB), SC crystallites were
sole crystalline species without formation of homo-crystallization excluding the case for crystallization temperature (Tc) = 70°C, whereas at Tc = 70°C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for Mw exceeding 1.5×104 g mol-1 [13].
For solution-crystallized neat
P(L-2HB) and P(D-2HB), the normal type of homo-crystallites were formed for Mw above 1×104 g mol-1 and the new type of homo-crystallites were formed for Mw below 1×104 g mol-1.
Here, the
normal type of homo-crystallites have the wide-angle X-ray diffraction profiles similar to those of α-
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or δ (α’)-form homo-crystallites of PLLA or PDLA [24–30].
On the other hand, for the
melt-crystallized neat P(L-2HB) and P(D-2HB), only new type, normal and new types, and only normal type of homo-crystallites were formed for Mw below, around, and above 9×103 g mol-1,
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respectively [13].
As stated above, Anderson et al. synthesized P(L-2H3MB) and P(D-2H3MB) with number-average molecular weight (Mn) values of 3.5 and 3.8×103 g mol-1 and confirmed SC formation during solvent
SC
evaporation of a mixed solution of P(L-2H3MB) and P(D-2H3MB) by the use of differential scanning calorimtery (DSC) and wide-angle X-ray diffractometry (WAXD) [14]. The melting temperature
P(L-2H3MB) and P(D-2H3MB).
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(Tm) of P(L-2H3MB)/P(D-2H3MB) stereocomplex (185°C) was higher than 169 and 166°C of neat However, to the best of our knowledge, the effects of
crystallization conditions such as crystallization methods and temperature on SC crystallization and homo-crystallization of P(L-2H3MB)/P(D-2H3MB) blends have not yet been reported. The purpose of the present study is to elucidate the effects of crystallization conditions on SC
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crystallization and homo-crystallization of equimolar blends of P(L-2H3MB) and P(D-2H3MB), together with those on homo-crystallization of neat P(L-2H3MB) and P(D-2H3MB).
Crystallization
methods include solution-crystallization during solvent evaporation and melt-crystallization at a
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constant Tc from the melt. The crystallization was monitored by DSC, WAXD, and polarized optical microscopy (POM). As the crystallization behavior is crucial for biodegradable materials because
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the crystallinity and crystalline size will largely affect their hydrolytic degradation, biodegradation, and drug release behavior [1–6], the information regarding crystallization behavior obtained in the present study will facilitate designing and processing biodegradable materials of neat P(L-2H3MB), P(D-2H3MB), and their blends with a wide variety of physical and hydrolytic degradation properties for aiming at biomedical, pharmaceutical, and environmental applications.
Considering the
hydrophobic large side chains (isopropyl groups), the neat P(L-2H3MB), P(D-2H3MB), and their blends are expected to give the biodegradable materials having the relatively low degradation rate or high hydrolytic degradation-resistance.
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2. Experimental Section
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2.1. Materials P(L-2H3MB) and P(D-2H3MB) were synthesized by the polycondensation of (S)- and (R)-2-hydroxy-3-methylbutanoic acids (2-hydroxy-3-methylbutyric acids or α-hydroxyisovaleric acids) (≥99 and 98 %, respectively, Sigma-Aldrich, Japan K.K., Tokyo, Japan), respectively, were
SC
carried out at 130°C in the presence of p-toluenesulfonic acid (5wt% of monomer, monohydrate, guaranteed grade, Nacalai Tesque Inc., Kyoto, Japan) as a catalyst, according to a previously reported The reaction times of the first and second step polycondensation under
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method [11,15,31,32].
atmospheric and reduced (1.7–1.8 kPa) pressure were 5 and 13 h, respectively.
The purification of
synthesized polymers was performed by reprecipitation using chloroform and methanol, as solvent and precipitant, respectively [19,31]. The purified polymers were dried under reduced pressure for at Figure 1 shows the 1H NMR spectrum of the synthesized P(L-2H3MB), together with
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least 6 days.
molecular structure of P(L-2H3MB). P(L-2H3MB).
P(D-2H3MB) had the spectrum identical with that of
The 1H NMR peaks at 1.0 and 2.3 ppm are ascribed to methyl protons (a) and methine
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proton of isopropyl group (b), respectively, whereas those at 4.1 and 5.0 ppm are attributed to methine protons directly bound to the main chain and located at hydroxyl terminal (d) and inside the chain (c), The peak positions of the latter are in good agreement with those reported in the
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literature [14]. The fact that peak (c) was much higher than peak (d) strongly suggests the successful synthesis of P(L-2H3MB) and P(D-2H3MB). The samples of neat polymers and their blend (thickness of ca. 10 µm) were prepared by solution casting according to a previously reported method [11,15].
Each solution of the purified and
P(L-2H3MB) and P(D-2H3MB) was separately prepared with dichloromethane as the solvent to have a polymer concentration of 0.4 g dL-1.
For the preparation of a P(L-2H3MB)/P(D-2H3MB) blend
sample, the P(L-2H3MB) and P(D-2H3MB) solutions were mixed with each other equimolarly under
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vigorous stirring. The mixed solution was cast onto a petri-dish, followed by solvent evaporation at 25°C for approximately one day.
Neat P(L-2H3MB) and P(D-2H3MB) samples were also prepared
by the same procedure without mixing solutions. The solvent remaining in the as-cast samples was
thus
prepared
samples
"solution-crystallized
samples".
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removed under reduced pressure for at least 6 days and stored in a desiccator before use. We call the Melt-crystallization
of
the
solution-crystallized samples, which were sealed under reduced pressure, was carried out at crystallization temperature (Tc) of 50–150°C for 1 h after melting at 230°C for 2 min.
After
SC
melt-crystallization, the samples were quenched at 0°C in iced water for 3 min to stop further
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crystallization. We call the thus prepared samples "melt-crystallized samples". "Melt-quenched samples" (i.e., Tc = 0°C) were prepared by quenching at 0°C in iced water for 3 min directly after melting at 230°C for 2 min.
2.2. Physical measurements and observation
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The weight- and number-average molecular weights (Mw and Mn, respectively) of the polymers were evaluated in chloroform at 40°C using a Tosoh (Tokyo, Japan) GPC system (refractive index monitor: RI-8020) having two TSK Gel columns (GMHXL) using polystyrene standards.
The
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molecular characteristics of the polymers used in the present study are summarized in Table 1. The [α]25589 of the polymers was measured in chloroform at a concentration of 1 g dL-1 and 25°C using a
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JASCO (Tokyo, Japan) P-2100 polarimeter at a wave length of 589 nm.
The glass transition, cold
crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and the enthalpies of cold crystallization and melting of homo-crystallites and SC crystallites [∆Hcc, ∆Hm(H), and ∆Hm(S), respectively] of the samples were determined using a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter. The samples were heated at a rate of 10°C min-1 from -30 to 230°C under a nitrogen gas flow of 50 mL min-1 for DSC measurements. The Tg, Tcc, and Tm values of the samples were calibrated using tin, indium, and benzophenone as standards.
The crystalline species and
crystallinity (Xc) values of the samples were estimated by WAXD. The WAXD measurements were
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performed at 25°C using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu-Kα source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA.
In a 2θ range = 7.5–27.5°, the crystalline
diffraction peak areas for SC crystallites at 2θ values around 9.8, 12.8, 13.7, 16.6, 19.4° and so on, and
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for P(L-2H3MB) or P(D-2H3MB) homo-crystallites at 2θ values around 13.8, 17.1, 18.9, 21.1, 23.9° and so on (and/or around 9.8, 12.8, 16.6, 19.4°, and so on for a new type of homo-crystallites) relative to the total area between a diffraction profile and a baseline were used to estimate the Xc values of SC
SC
crystallites [Xc(S)] and P(L-2H3MB) or P(D-2H3MB) homo-crystallites [Xc(H)], respectively.
3.1. Wide-angle X-ray diffraction
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3. Results and Discussion
To estimate the crystalline species and Xc of the solution- and melt-crystallized samples, WAXD measurements were performed.
Figure 2 shows the WAXD profiles of the solution- and
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melt-crystallized or quenched neat P(L-2H3MB), P(D-2H3MB) and their equimolar blend samples. For the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples, the diffractions of homo-crystallites were observed at 2θ values of 9.8, 12.8 (main), 13.7, 16.6, 19.4°, and so on, which
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profiles are very similar to those of homo-crystallites reported for the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) [14].
It should be noted that the X-ray wave length used in the
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present study (λ = 1.5418 Å) is different from that used in the reported study (λ = 2.2898 Å) [14] and, therefore, the comparison between the 2θ values in the present and reported studies were performed after the 2θ values reported for λ = 2.2898 Å were converted to those for 1.5418 Å.
For all the
melt-crystallized and quenched neat P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0–100°C), the diffractions of homo-crystallites were observed at 2θ values of 13.8 (main), 17.1, 18.9, 21.1, 23.9°, and so on, irrespective of Tc values. These crystalline diffraction profiles are different from those of the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB), excluding that at 13.8°, and similar to those of α- or δα’)-form of PLLA or PDLA homo-crystallites [24–30]. The finding here exhibits
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the formation of a new type of homo-crystallites in neat P(L-2H3MB) and P(D-2H3MB). All the solution- and melt-crystallized P(L-2H3MB)/P(D-2H3MB) blend samples, excluding melt-quenched sample, had the crystalline peaks only at 2θ values of 9.7, 16.8, 17.7, 19.4° and so on,
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which are very similar to those reported for P(L-2H3MB)/P(D-2H3MB) SC crystallites [14], indicating the formation of only SC crystallites without the formation of P(L-2H3MB) or P(D-2H3MB) homo-crystallites.
Here, again, the comparison between the 2θ values in the present
SC
and reported studies was carried out after the 2θ values reported for λ = 2.2898 Å [14] were converted to those for λ = 1.5418 Å.
For the P(L-2H3MB)/P(D-2H3MB) blend sample crystallized at Tc =
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100°C, in addition to the crystalline diffractions of SC crystallites, the crystalline diffractions were observed at around 12.0, 13.7, and 15.4°. The crystalline peak at 13.7° is attributable to that of the same type of homo-crystallites as observed for the melt-crystallized neat P(L-2H3MB) and P(D-2H3MB), reflecting the formation of both SC and homo-crystallites in the blends crystallized at Tc = 100°C.
However, we could not ascribe the origins of the crystalline peaks at 12.0 and 15.4°.
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For PLLA/PDLA and P(L-2HB)/P(D-2HB) blends, formation of both SC and homo-crystallites was observed when the Mw values were higher than 1.5×105 and 2.3×104 g mol-1, respectively [13,23]. Therefore, the formation of both SC and homo-crystallites in the P(L-2H3MB)/P(D-2H3MB) blend at
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Mw values of 2.4×103 g mol-1 means that the critical Mw value below which both SC and homo-crystallization occur becomes lower with an increase in the size of side chains. The broad
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diffraction profile of melt-quenched P(L-2H3MB)/P(D-2H3MB) blend sample (Tc = 0°C) indicates that the blend was amorphous. The Xc(S) and Xc (H) values evaluated from the WAXD profiles in Figure 2 are plotted in Figure 3 as a function of Tc, in this figure the Xc(H) and Xc(S) values of the solution-crystallized neat P(L-2H3MB) and P(D-2H3MB) and their blend samples are shown with broken lines. The Xc(H) values of the neat P(L-2H3MB) and P(D-2H3MB) samples solution-crystallized and melt-crystallized at Tc = 50–150°C were in the range = 65–78 % and those of melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples were independent of Tc.
Again, positive values of the melt-quenched neat
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P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0°C) indicate that the samples were crystallized during quenching from the melt, reflecting the rapid crystallization of the neat polymer samples. The solution-crystallized blend sample had the similar Xc value with those of the neat polymer samples.
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As expected from Figure 2(c), the Xc(H) values of the melt-crystallized blend samples has positive values only at Tc = 100°C, whereas their Xc(S) values increased monotonically with increasing Tc. The result here is indicative of the fact that SC crystallization and homo-crystallization are sensitive to Tc and SC crystallization is facilitated at higher Tc, probably due to the higher Tm values of SC
SC
crystallites (189–193°C) than those of homo-crystallites (163–178°C). The nil crystallinity of the
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melt-quenched blend sample indicates that the slow crystallization of SC crystallites during quenching compared with that of neat polymer samples.
The overall crystallinity values of blend samples
(Table 2) at Tc = 50 and 100°C were much lower than those of the neat polymer samples, again reflecting the low crystallizability of the blend samples.
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3.2. Differential scanning calorimetry
Thermal properties of the samples were estimated by DSC.
Figure 4 shows the DSC
thermograms of the neat P(L-2H3MB), P(D-2H3MB), and their blend samples and thermal properties
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obtained from the DSC thermograms in Figure 4 are summarized in Table 2. The solution- and melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples had glass transition, cold
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crystallization, and melting peak (homo-crystallites) in the ranges of 31–48, 58–108, and 105–188°C, respectively.
A large cold crystallization peak was observed for the melt-quenched neat
P(L-2H3MB) and P(D-2H3MB) samples (Tc = 0°C), confirming the presence of crystallizable amorphous region before DSC heating and the crystallizability of the neat polymers during DSC heating, whereas a very small cold crystallization peak was seen for the neat P(D-2H3MB) sample melt-crystallized at Tc = 50°C, reflecting imperfect crystallization during isothermal crystallization at this temperature.
For the neat polymer samples solution-crystallized and melt-crystallized at Tc =
150°C, a complicated double or multiple melting peak was observed, whereas other neat polymer
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samples had a broad melting peak.
Such a double or multiple melting peak is normally ascribed to
the melting of original and re-crystallized crystallites. On the other hand, solution- and melt-crystallized blend samples had glass transition, cold
respectively.
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crystallization, melting (SC crystallites) peaks in the ranges of 31–48, 58–148, and 189–193°C, For the melt-quenched blend sample (Tc = 0°C), a double cold crystallization peak was
observed, confirming the presence of crystallizable amorphous region before DSC heating and the crystallizability of the blend sample during DSC heating.
Considering the cold crystallization
SC
temperature (Tcc) values of the melt-quenched neat P(L-2H3MB) and P(D-2H3MB) samples (59 and
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64°C) and the lower spherulites growth rate of SC crystallites compared to that of homo-crystallites as stated below, the lower temperature cold crystallization peak of the melt-quenched blend sample at 59°C is attributable to the cold crystallization of homo-crystallites, whereas another higher temperature cold crystallization peak of the melt-quenched blend sample at 95°C can be ascribed to that of SC crystallites.
For the blend sample melt-crystallized at Tc = 50°C, a small cold
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crystallization peak at 96°C, which is ascribed to the cold crystallization of SC crystallites, was seen. The Tm values of SC crystallites in the blend samples (189–193°C) are slightly higher than those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB) samples (165–187°C), although the Tm
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values of SC crystallites in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends (about 230 and 200°C, respectively) are much higher than those of homo-crystallites in the neat polymers (about 180 and However, rather low Tm of P(2H3MB) SC compared with that of
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100°C, respectively) [8,11–13].
PLA SC should be due to the low molecular weights of P(L-2H3MB) and P(D-2H3MB) used in the present study.
∆H(tot) = ∆Hcc + ∆Hm can be used as an indicator of crystallinity. However, in the
present study, a very broad melting peak and a non-linear baseline disturbed the accurate estimation of transition enthalpies and, therefore, ∆H(tot) can be only used as an approximate indicator of crystallinity.
Actually, the melt-quenched blend sample with zero crystallinity by the WAXD
measurement, had a large positive ∆H(tot) value (17.0 J g-1). Due to the similar melting range of SC and homo-crystallites, only one broad melting peak was
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observed for the blend sample melt-crystallized at Tc =100°C, wherein both SC and homo-crystallites were contained. Since a very sharp melting peak was only observed for SC crystallites of the blend sample melt-crystallized at Tc = 150°C, the sharp peak observed at 191°C in the melting peak for the
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blend sample melt-crystallized at Tc = 100°C is attributable to the melting of SC crystallites. Therefore, the broad melting part of the melt-crystallized blend samples can be ascribed to the melting of homo-crystallites.
SC
3.3. Spherulitic morphology and growth
isothermally crystallized from the melt.
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To investigate the spherulitic morphology and growth, POM was performed on the samples Figure 5 shows the typical polarized photomicrographs of
the neat P(L-2H3MB), P(D-2H3MB), and their blend samples crystallized at 150°C for the shown crystallization times.
All the samples had the morphologies of Maltese-crosses. The number of
spherulites per unit area was larger for the blend sample than for the neat P(L-2H3MB) and
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P(D-2H3MB) samples. This result reveals that blending P(L-2H3MB) and P(D-2H3MB) elevates the number of SC spherulite nuclei per unit mass. The radial growth rate of spherulites (G) and the induction period for spherulite growth (ti) of the samples are plotted in Figure 6(a) and (b) as a As evident from Figure 3, in the Tc range utilized for crystallization of the blend
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function of Tc.
sample (140–180°C), only SC crystallites as crystalline species should be formed.
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that the vertical axis for G is logarithmic [Figure 6(a)].
It should be noted
The G values of SC crystallites in the blend
sample (1.0–30 µm min-1) were much lower than those of homo-crystallites in the neat P(L-2H3MB) (157–532 µm min-1) and P(D-2H3MB) samples (67–288 µm min-1).
Surprisingly, this result is in
marked contrast with those reported for PLLA and PDLA or P(L-2HB) and P(D-2HB) wherein the G values of SC crystallites in the blends are much higher than those of homo-crystallites in the neat polymers [12,33]. It is probable that alternate packing of L- and D-polymer segments during SC crystallization can be readily occur in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends with small side chains
(methyl
and
ethyl
side
groups,
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respectively)
but
becomes
difficult
in
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P(L-2H3MB)/P(D-2H3MB) blends because large side chains (isopropyl groups) should have disturbed alternate packing of L- and D-polymer segments during SC crystallization. The G values of the neat P(L-2H3MB) were higher than those of the neat P(D-2H3MB).
The Mw value of the neat
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P(L-2H3MB) (2.93×103 g mol-1) was lower than that of P(D-2H3MB) (3.71×103 g mol-1) (Table 2). This should have increased the segmental mobility of P(L-2H3MB), resulting in the higher G values of the neat P(L-2H3MB).
The result that the spherulitic number per unit area of SC crystallites for the
SC
P(L-2H3MB)/P(D-2H3MB) blend was higher than those of homo-crystallites in the neat polymers is
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in agreement with the result reported for PLLA/PDLA blend compared with those of homo-crystallites in the neat polymers [33]. The higher Tm of SC crystallites of the blend (189–191°C) than that of homo-crystallites of the neat P(L-2H3MB) and P(D-2H3MB) (171–187°C) should have elevated the degree of super cooling and, thereby, the nuclei number of spherulites per unit area when compared at the same Tc.
The induction period for spherulite growth (ti) of the neat P(L-2H3MB) and
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P(D-2H3MB) and their blend samples was nil for Tc = 130–140°C and 140–170°C and became positive at Tc = 150 and 180°C, respectively.
Despite the small difference in Tm values between the
melt-crystallized neat P(L-2H3MB) and P(D-2H3MB) samples and the melt-crystallized blend
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samples, the critical Tc of the blend sample above which ti becomes positive was higher by 30°C than those of the neat P(L-2H3MB) and P(D-2H3MB) samples.
These results indicate that in the blend
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sample, the large side chains of P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the nucleation process (as evidenced by the elevated the number per unit area of SC spherulites and the critical Tc), compared with those of the spheruites of homo-crystallites in the neat polymers.
4. Conclusions The following conclusions can be derived from the present study for the effects of crystallization conditions on the crystallization of the neat P(L-2H3MB), P(D-2H3MB), and their blend.
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In both
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solution- and melt-crystallization of the blends, solely SC crystallites were formed without formation of
homo-crystallites,
excluding
melt-crystallization
homo-crystallization took place.
at
100°C
wherein
both
SC
and
The critical Mw value below which both SC and
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homo-crystallization occur in the P(L-2H3MB)/P(D-2H3MB) blend (Mw = 2.4×103 g mol-1) is much lower than those of PLLA/PDLA and P(L-2HB)/P(D-2HB) blends (1.5×105 and 2.3×104 g mol-1, respectively) [13,23].
In solution-crystallization of the neat P(L-2H3MB) and P(D-2H3MB), the In melt-crystallization of the
SC
same type of homo-crystallites as reported earlier [14] were formed.
neat P(L-2H3MB) and P(D-2H3MB), for the Tc range of 0–150°C, the new type of homo-crystallites
formed.
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having WAXD profiles similar to the α- or δ-form homo-crystallites of neat PLLA and PDLA were The Tm values of SC crystallites (189–193°C) were slightly higher than those of
homo-crystallites (163–178°C), in marked contrast with much higher Tm values of PLLA/PDLA and P(L-2HB)/P(D-2HB) SC crystallites compared with those of homo-crystallites in the neat polymers [8,11–13].
The G and the spherulitic number per unit area of SC crystallites for the
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P(L-2H3MB)/P(D-2H3MB) blend were respectively much lower and higher than those of homo-crystallites for neat P(L-2H3MB) and P(D-2H3MB). The lower G values of SC crystallites in the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers
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are in marked contrast with much higher G values of SC crystallites in PLLA/PDLA and P(L-2HB)/P(D-2HB) blends compared with those of homo-crystallites in the neat polymers [12,33].
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The higher spherulitic number per unit area of SC crystallites for the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers is in agreement with the result reported for PLLA/PDLA blend compared with those of homo-crystallites in the neat polymers [33]. Despite not so higher Tm and lower G of SC crystallites in the P(L-2H3MB)/P(D-2H3MB) blend compared with those of homo-crystallites in the neat polymers, the critical Tc of the P(L-2H3MB)/P(D-2H3MB) blend above which ti becomes positive was higher by 30°C than those of homo-crystallites in the neat polymers.
These results indicate that in the blend samples, the large
side chains of P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the
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nucleation process, compared with those of the spheruites of homo-crystallites in the neat polymers.
Acknowledgements: This research was supported by JSPS KAKANHI Grant Number 24550251 and a
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Grant-in-Aid for Scientific Research on Innovative Areas "Plasma Medical Innovation" (24108005)
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from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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References [1]
Kharas GB, Sanchez-Riera F, Severson DK. In: Mobley DP, editor. Plastics from microbes. New York: Hanser Publishers; 1994. p. 93–137. Hartmann MH. In: Kaplan DL, editor. Biopolymers from renewable resources. Berlin, Germany:
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[2]
Springer; 1998. p. 367–411. [3]
Tsuji H. Doi Y, Steinbüchel A, Biopolymers. Polyesters III, vol. 4. Weinheim, Germany: Wiley-VCH; 2002. p. 129–77. Södergård A, Stolt M, Prog Polym Sci 2002; 27: 1123–63.
[5]
Albertsson A-C, editor, Degradable aliphatic polyesters (Advances in Polymer Science, Vol.157); Berlin (Germany): Springer, 2002.
[6]
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[4]
Auras R, Lim L-T, Selke SEM, Tsuji H, editors., Poly(lactic acid): Synthesis, structures, properties, processing, and applications (Wiley Series on Polymer Engineering and Technology), New Jersey: John Wiley & Sons, Inc., 2010.
Slager J, Domb AJ, Adv Drug Delivery Rev 2003; 55: 549–83.
[8]
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[9]
Fukushima K, Kimura Y. Polym Int 2006; 55: 626–42.
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[7]
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[10] Tsuji H, Ikada Y, In: Yu L, editor, Biodegradable polymer blends from renewable resources, New Jersey: John Wiley & Sons, Inc., 2009; p. 165–90.
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[11] Tsuji H, Okumura A. Macromolecules 2009; 42: 7263–6. [12] Tsuji H, Okumura A. Polymer J 2011; 43: 317–24. [13] Tsuji H, Shimizu S. Polymer 2012; 53: 5385–92. [14] Andersson SR, Hakkarainen M, Albertsson A-C, Polymer 2013; 54: 4105–11. [15] Tsuji H, Yamamoto S, Okumura A, Sugiura Y. Biomacromolecules 2010; 11: 252–8. [16] Tsuji H, Shimizu K, Sakamoto Y, Okumura A. Polymer 2011; 52: 1318–25. [17] Tsuji H, Deguchi F, Sakamoto Y, Shimizu S. Macromol Chem Phys 2012; 213: 2573–81. [18] Tsuji H, Suzuki M. Macromol Chem Phys 2014; 215, 1879–88.
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[19] Tsuji H, Hayakawa T. Polymer 2014; 55: 721–6. [20] Tsuji H, Hosokawa M, Sakamoto Y. ACS Macro Lett 2012; 1: 687–91. [21] Tsuji H, Hosokawa M, Sakamoto Y. Polymer 2013; 54: 2190–8.
[23] Tsuji H, Ikada Y, Polymer 1999; 40: 6699–708. [24] Miyata T, Masuko T. Polymer 1997; 38: 4003–9.
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[22] Tsuji H, Tawara T. Polymer 2015; 68: 57–64.
[25] Pan P, Kai W, Zhu B, Dong T, Inoue Y. Macromolecules 2007; 40: 6898–905.
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[26] Pan P, Zhu B, Kai W, Dong T, Inoue Y. J Appl Polym Sci 2008; 107: 54–62.
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[27] Kawai T, Rahma N, Matsuba G, Nishida K, Kanaya T, Nakano M, Okamoto H, Kawada J, Usuki A, Honma N, Nakajima K, Matsuda M. Macromolecules 2007; 40: 9463–69. [28] Zhang J, Tashiro K, Tsuji H, Domb AJ. Macromolecules 2007; 40: 1049–54. [29] Wasanasuk K, Tashiro K. Polymer 2011; 52: 6097–109.
[30] Tsuji H, Tashiro K, Bouapao L, Hanesaka M. Macromol Chem Phys 2012; 213: 2099−112.
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[31] Tsuji, H., Matsuoka, H., Itsuno, S. J Appl Polym Sci 2008; 110: 3954–62. [32] Tsuji H, Eto T, Sakamoto Y. Materials 2011; 4: 1384–98.
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[33] Tsuji H, Tezuka, Y. Biomacromolecules 2004; 5: 1181–6.
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Table 1.
Molecular characteristics of P(L-2H3MB) and P(D-2H3MB).
Mw a) [α]25589 b) Mw/Mn a) -1 (g mol ) (deg dm-1 g-1 cm3) P(L-2H3MB) 2.93×103 1.71 -72.2 3 P(D-2H3MB) 3.71×10 1.71 75.4 a) Mn and Mw are number- and weight- average molecular weights, respectively. b) [α]25589 values were measured in chloroform.
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Polymer
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Table 2.
Thermal properties during heating and crystallinity of neat P(L-2H3MB), P(2-2H3MB), and their
blends.
Blend
Tccb) (°C)
Tmb) (°C)
∆Hccc)
∆Hmc)
∆H(tot)d)
(J g )
(J g )
(J g )
As-cast 0 50 100 150 As-cast 0 50 100 150 As-cast 0 50 100 130f) 150
48.1 31.6 34.8 36.4 38.7 30.6 47.9 31.2 -
58.9 63.9 72.3 58.3, 108.3 137.9 59.2, 95.2 95.9 148.1
104.5, 164.6, 170.4, 188.1 178.0 174.5 176.1 155.6, 187.3 162.5, 170.4, 184.5 172.4 171.3 171.0 147.1, 162.7, 178.4 193.4 188.5 189.1 191.2
0.0 -14.7 0.0 0.0 0.0 0.0 -26.7 -1.4 0.0 -2.8 -4.3 -27.2 -14.6 -4.1
47.2 40.5 44.8 43.7 37.8 42.1 41.5 41.7 37.9 36.3 52.6 44.2 39.9 31.2
47.2 25.8 44.8 43.7 37.8 42.1 14.8 40.3 37.9 33.5 48.3 17.0 25.3 27.2
-
-
190.1
0.0
53.8
53.8
a)
-1
-1
Tc is crystallization temperature. Tg, Tcc, and Tm are glass transition, cold crystallization, and melting temperatures, respectively. c) ∆Hcc and ∆Hm are enthalpies of cold crystallization and melting, respectively. d) ∆H(tot)= ∆Hcc + ∆Hm. e) Overall crystallinity estimated by WAXD. f) No thermal data.
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b)
18
-1
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P(D-2H3MB)
Tg b) (°C)
SC
P(L-2H3MB)
Tc a) (°C)
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Code
Xce) (%) 74.6 19.7 81.6 74.2 79.9 64.6 11.2 73.2 73.7 73.0 74.1 0.0 2.4 52.3 67.1 76.9
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Figure Captions
Figure 1.
1
H NMR spectrum of the synthesized P(L-2H3MB), together with molecular structure of
Figure 2.
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P(L-2H3MB).
WAXD profiles of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples. Figure 3.
Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] of
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solution-crystallized (the values are shown with broken lines) and melt-crystallized or quenched neat
Figure 4.
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P(L-2H3M B) (a), P(D-2H3M B) (b), and their equimolar blend (c) samples as a function of Tc. DSC thermograms of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3M B) (b), and their equimolar blend (c) samples. Figure 5. Polarized optical photomicrographs of neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples crystallized Tc = 150°C for the shown crystallization times from the melt.
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Figure 6. Radial growth rate of spherulites (G) (a) and induction period for spherulite growth (ti) of
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neat P(L-2H3MB), P(D-2H3MB), and their equimolar blend samples as a function of Tc.
19
Figure 1.
1
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H NMR spectrum of the synthesized P(L-2H3MB), together with molecular structure of
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P(L-2H3MB).
20
Figure 2.
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WAXD profiles of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
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quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples.
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Figure 3. Crystallinity of SC and homo-crystallites [Xc(S) and Xc(H), respectively] of solution-crystallized (the values are shown with broken lines) and melt-crystallized or quenched neat
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P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples as a function of T
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Figure 4.
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DSC thermograms of solution-crystallized and melt-crystallized (Tc = 50–150°C) or
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quenched (Tc = 0°C) neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their equimolar blend (c) samples.
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Figure 5. Polarized optical photomicrographs of neat P(L-2H3MB) (a), P(D-2H3MB) (b), and their
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equimolar blend (c) samples crystallized Tc = 150°C for the shown crystallization times from the melt.
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Figure 6. Radial growth rate of spherulites (G) (a) and induction period for spherulite growth (ti) of
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neat P(L-2H3MB), P(D-2H3MB), and their equimolar blend samples as a function of Tc.
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Highlights • Stereocomplex (SC) crystallites of L- and D-poly(2-hydroxy-3-methylbutanoic acid)s are formed.
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• The effects of crystallization conditions on the crystallization behavior are studied.
• A low crystallization temperature gives both SC and homo-crystallites in the blend.
• Two types of homo-crystallites are formed in neat polymers depending on crystallization procedure.
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• Large side chains lowered G of SC but facilitates the nucleation of SC spherulites.