Photon Factory Activity Report 2019 #37 (2020)
BL-6A/2018G027

Confined and Hidden Crystallization of Poly(ethylene glycol) in Spherulites ofPoly(L-lactic acid) in a PLLA/PEG Blend
Apisit BANPEAN1, Hideaki TAKAGI2, Nobutaka SHIMIZU2, Noriyuki IGARASHI2,Sono SASAKI1 and Shinichi SAKURAI1,*1
Department of Biobased Materials Science, Kyoto Institute of Technology, Matsugasaki, Kyoto,606-8585, Japan2
Photon Factory, Institute of Materials Structure Science,High Energy Research Organization, 1-1 Oho, Tsukuba, 305-0801, Japan
1 Introduction
Intriguing crystallization behaviors of poly(ethyleneglycol) (PEG) in a blend specimen of poly(L-lactic acid)(PLLA) with PEG were studied by the polarizing optical
microscopy (POM) and the differential scanningcalorimetry (DSC).
The PLLA/PEG (50/50) blend
specimen was heated up to 180˚C and kept at thistemperature for 5 min to obtain complete melt withoutliquid-liquid phase separation.
Then, a two-step
temperature-jump was conducted as 180˚C→ 127˚C→45˚C. It was found that PEG can crystallize only in theprecedingly grown spherulites of PLLA, as nocrystallization of PEG was found in the matrix of themixed PLLA/PEG amorphous phase. This is because ofthe depression of the freezing temperature of PEG (Tf)due to the complete mixing of PLLA/PEG, by noting thatthe neat PEG can crystallize at 45˚C which is lower than
Tf of the neat PEG (49.8˚C).
Upon the PLLA
crystallization at 127˚C, the PEG content in theamorphous region inside of the PLLA spherulite isincreased because of the formation of the pure solid phaseof PLLA (crystalline phase). Namely, the PLLA content
is decreased in the amorphous phase of the PLLA/PEGmixture. Then, Tf of PEG increases so that PEG cancrystallize but this crystallization is only allowed insideof the preceded PLLA spherulite. Thus, the confined andhidden crystallization of PEG can be accounted for. It
was further found that the ultimate degree of crystallinity(ϕ∞) of PEG was increased and the induction period of
the confined and hidden crystallization of PEG wasdecreased as a function of the PLLA crystallization timeat 127˚C because of the increase in the PEG content in theamorphous phase due to the increase of the PLLA amountcrystallized, which was more clearly recognized as afunction of the size of the preceded PLLA spherulite. The
direct evidence of the PEG crystallization was obtainedby the wide-angle X-ray scattering (WAXS). The kinetics
was also studied for the confined and hidden
crystallization of PEG. Based on the results of the kinetic
study, the effect of the space confinement is discussed.
2 Experimental
The PLLA sample (4032D) used in this study was theproduct of Nature Works LLC. Its weight-averagemolecular weight (Mw) is 1.66 × 105 with the D content of1.4 %. The PEG sample was purchased from Wako Pure
Chemical Industries, Ltd., of which Mw is 2.0 × 104. The
PLLA/PEG (50/50) blend specimen was prepared by thesolution casting method. Given amounts of PLLA andPEG were dissolved in dichloromethane (DCM) at room
temperature (~25˚C) to obtain a solution with ca. 5 wt%of the total polymer concentration. The polymer solution
was then poured into a Petri dish (diameter of 5 cm) forcomplete evaporation of DCM.
POM observations were conducted by using a Nikon
Eclipse Ci-POL polarizing optical microscope equippedwith the Linkam THMS600 hot stage (Linkam Scientific,UK). The retardation plate (of which optical retardation is530 nm) was inserted in the optical path. The specimen
was sandwiched between two cover slips with a spacer ofwhich thickness is 20 µm. The temperature protocol for
the POM observations is shown in Fig. 1. The PLLA/PEGFig. 1 Temperature protocol for the POM observation
for the PLLA/PEG (50/50) blend specimens
isothermally crystallized by the two-step T-jump. Thesolid (black) and broken (red) lines are the protocols for
the PEG crystallization after the PLLA crystallization at
127˚C for 0 min (one-step T-jump directly from 180˚C)
and for 15 min (as an example of the two-step T-jumpvia 127˚C for 15 min), respectively.

Photon Factory Activity Report 2019 #37 (2020)

Fig. 2
POM micrographs showing the crystallization processes of PLLA/PEG (50/50) blend specimens at
45˚C (40 min) after the temperature jump from 127˚C crystallization of PLLA for (a) 0, (b) 5, (c) 10, and (d) 15min. Subsequently, the specimens were subjected to reheating with 50˚C/min up to 67˚C. POM images shownat the right column were taken after 10 sec of the temperature equilibrium at 67˚C.
(50/50) blend specimen was heated up to 180˚C and kept
at this temperature for 5 min to obtain complete melt
without liquid-liquid phase separation. Then, two-step Tjump was conducted as 180˚C → 127˚C → 45˚C. The
isothermal crystallization time at 127˚C was controlled as0, 5, 10, and 15 min where the PLLA spherulite grew.
After that, the specimen was quenched to 45˚C and kept
at this temperature for 40 min to induce the crystallizationof PEG.
WAXS measurements were conducted to obtain thedirect evidence of the PEG crystallization at 45˚C byusing the synchrotron radiation as an X-ray source at thebeamline BL-6A of Photon Factory at the KEK (HightEnergy Accelerator Research Organization) in Tsukuba,
Japan. The wavelength of the incident X-ray beam was0.150 nm. The PLLA/PEG blend specimen was packed
into an aluminum cell with a diameter of 4 mm andthickness of 1 mm and was sandwiched by a couple ofpieces of the polyimide (Kapton) film, obtained fromDuPont-Toray Co., Ltd., Tokyo, Japan [1,2]. In theWAXS measurement, a specimen was heated up to 180˚Cand kept at this temperature for 5 min to completely melt
the specimen and then the sample cell was quickly moved
to another heater block which was maintained at 127˚C.

The specimen was annealed at this temperature for 15
min to grow the PLLA spherulite and then the sample cell
was quickly transferred into the heater block settled at thesample position for the WAXS measurement where theincident X-ray beam was irradiated. This heater block
temperature was maintained at 45˚C to detect the PEG
crystallization. Time-dependent WAXS patterns weremeasured with a step of 15s by using PILATUS 100K
(Dectris, Switzerland) as a two-dimensional detector.
Note here that this two-step T-jump was conductedaccording to the same temperature protocol for the POM
observation and for the DSC measurement, which wasshown in Fig. 1. The 1d-WAXS profiles were extracted
from the 2d-WAXS patterns.
3 Results and Discussion
Fig. 2a shows the POM result showing no PEG
crystallization at 45˚C due to the depression of the Tf,PEG
in the complete mixture of the PLLA/PEG (50/50) blend
specimen. Fig. 2a also confirms the formation of thePEG spherulite at 41˚C, where PLLA cannot crystallize
because of its extraordinarily slow crystallization rate. InFigs. 2b-d, the PLLA spherulites in the first-step of Tjump to 127˚C (for 5, 10, and 15 min) were clearlyobserved. Interestingly, at the secondary T-jump in Figs.

Photon Factory Activity Report 2019 #37 (2020)
2b-d, POM images in the middle column suggest that thecolors changed into more brilliant ones in the interior of
all PLLA spherulites. Moreover, the POM images shownin the right column being taken at 67˚C, which is above
Tm,PEG, clearly indicated almost the same images ofspherulites observed at 127˚C. These results imply thePEG crystallization in the PLLA spherulites at 45˚C.
The direct evidence of the PEG crystallization wasprovided by the time-resolved WAXS measurement. Thechange in the WAXS profiles during the isothermal
crystallization at 45˚C after the PLLA crystallization for
15 min at 127˚C was show in Fig. 3a. Here, q denotes themagnitude of the scattering vector, as defined by q =(4π/λ) sin(θ/2) whit λ and θ being the wavelength of Xray and the scattering angle, respectively. The WAXSprofiles measured at t = 315 s to t = 405 s with a step of
15 s (in total 6 profiles) are shown in Fig. 3a and those forthe neat PLLA (crystallized isothermally at 127˚C) andfor the neat PEG (crystallized isothermally at 45˚C) are
shown in Fig. 3b for comparison. In Fig. 3a, there were
no PEG crystalline reflection peaks in the first profile at t= 315 s, which shows only the PLLA crystallinereflection peaks at q = 11.95, 13.43, and 15.45 nm-1 forthe (200)/(110), (203), and (210) plane reflections,respectively [3]. As time goes on, the PEG crystalline
reflection peaks appeared at q = 13.56, 15.94, 16.12,
16.30 nm-1 with increasing their intensity.
From thelattice parameters for PEG [2] which are a = 0.805 nm, bFig. 3 (a) Time-resolved 1d-WAXS profiles along thePEG crystallization at 45̊C in the PLLA/PEG(50/50) blend specimen after PLLA crystallized at127̊C for 15 min and (b) 1d-WAXS profiles for theneat PLLA and PEG crystallized at 127̊C and 45̊C,respectively.

= 1.304 nm, c = 1.948 nm, and β = 125.4˚ of themonoclinic crystal with 7/2 helix conformation of thepolymer chains, it is confirmed that those reflections are
assigned to (120), (20-2), (12-4)/(-124), and (112)/(1-12)/(-204) of PEG, respectively, although the q range of from
15.8 to 16.4 nm-1 is very crowded with a bunch of PEG
crystalline reflection peaks [4]. The evolution of thosereflection peaks is the clear evidence of the PEG
crystallization.
We conducted peak decomposition to determine thepeak area. Although the peak area for PLLA seems toslightly decrease as a function of time, we found that theintensity level due to the amorphous halo decreased withtime along the progress in the PEG crystallization. This
explains the apparently decreasing tendency of the PLLA
reflections. As a matter of fact, the peak area stays
constant as plotted in Fig. 4. On the contrary, the peakarea of all of the PEG reflections shows the similar
increasing tendency in Fig. 4. The onset time of the peakevolution can be considered as the induction period which
was about 5 min for this particular case. This value is ingood accord with the differential scanning calorimetry(DSC) results. The termination of the PEG crystallization
where the curves in Fig. 4 leveled off was evaluated to be
6.5 min. Therefore, the total crystallization time is about
1.5 min, which is also in good accord with the DSC
results.
Although the peak positions of the PEG reflections in thesecond-step of T-jump of the blend specimen is the sameas those for the neat PEG, it is specific to recognize thetremendous suppression of the (12-4)/(-124) reflection
peak for the blend specimen as compared to that for theneat PEG. This may indicate the effect of the space
confinement that the direction of the PEG crystal growthis suppressed, which is the [12-4]/[-124] direction almost
parallel to the c-axis, in turn, the polymer chain direction.
On the other hand, the (120) reflection peak is not
suppressed, indicating that the PEG crystal growth in thedirection perpendicular to the polymer chains was notaffected. Assuming the folded-chain crystal of the PEG
crystalline lamella, these results suggest the suppressionof the lamellar thickening due to the space confinement in

Fig. 4 Plots of the area of the crystalline peaks as afunction of time, which was evaluated from WAXS
results shown in Fig. 3(a) after the peakdecomposition.

Photon Factory Activity Report 2019 #37 (2020)
the amorphous phase sandwiched by the PLLA crystalline
lamellae. This further suggests the orientation of the PEG
lamellae parallel to those of PLLA. Huang et al. havereported this kind of the crystal orientation for the PEG
confined crystallization in the PEG-b-PS (polystyrene)
diblock copolymer forming the lamellar microdomains at
the isothermal crystallization temperature higher than
20˚C for Mn,PEG = 2.31 × 104 [5], which is close to ourPEG specimen’s Mw used in the current study. Thus, ourresult implying the parallel orientation of the PEG
lamellae (parallel to the preceding PLLA crystallinelamellae) as a consequence of the space confinement canbe explained by the previous work of Huang, et al. [5].

References[1] P. T. N. Diep, M. Mochizuki, M. Doi, H. Takagi, N.
Shimizu, N. Igarashi, S. Sasaki, S. Sakurai, Polymer
Journal, 51, 283 (2019).
[2] P. T. N. Diep, H. Takagi, N. Shimizu, N. Igarashi, S.
Sasaki, S. Sakurai, J. Fiber Sci. Technol., 75, 99(2019).
[3] P. De Santis and A. J. Kovacs, , 6, 299 (1968).
[4] Y. Takahashi and H. Tadokoro, Macromolecules 6,672 (1973).
[5] P. Huang, et al., Macromolecules 37, 3689–3698(2004).
* shin@kit.ac.jp