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Poly(lactic acid) stereocomplex microspheres asthermally tolerant optical resonators†
Cite this: Nanoscale, 2023, 15, 19062
Published on 13 November 2023. Downloaded on 9/25/2024 4:34:25 AM.

Received 21st October 2023,
Accepted 10th November 2023

Suharman,a,b

Wey Yih Heah,c Hiroshi Yamagishi

a,cand Yohei Yamamoto*a,c

DOI: 10.1039/d3nr05318k
rsc.li/nanoscale

Thermally tolerant polymer optical resonators are fabricated froma stereocomplex of poly(L-lactic acid) and poly(D-lactic acid)through the oil-in-water miniemulsion method. The thermal stability of the microspheres of the stereocomplex poly(lactic acid)(SC-PLA) is superior to that of the homochiral poly(lactic acid)(HC-PLA). As a result of the high thermal stability, the optical resonator properties of the SC-PLA microspheres are preserved at anelevated temperature of up to 230 °C, which is 70 °C higher thanthat of microspheres formed from HC-PLA.

Poly(lactic acid) (PLA) is a synthetic polyester made from lactic
acid.1–4 Due to its biodegradable, biocompatible, and nontoxic
properties, PLA is widely used in biomedical applications,5–7
drug transportation,8 textile,9 and packaging.10,11 PLA has
three types of stereoisomers: poly(L-lactide acid) (PLLA), poly(Dlactide acid) (PDLA), and their atactic polymer, poly(DL-lacticacid) (PDLLA).12 The homopolymers of PLLA and PDLA form
semicrystalline aggregates with a melting temperature ofaround 170 °C, while the atactic PDLLA copolymer forms anamorphous aggregate with a lower melting temperature.13
One of the drawbacks of PLA is that its thermal stability is
not so high due to its low crystallinity, which limits its use in
optical and electronic applications.14 Some techniques havebeen applied to enhance the thermal stability of PLA byadding a nucleating agent,15–18 physical modification throughfibre reinforcement,19,20 the addition of inorganic particles,21
chemical modification,22 and blending it with a material withhigh Tg and high heat resistance.23,24 In particular, stereocom-

plexation by mixing PLLA and PDLA has received much attention due to its enhanced thermal and mechanicalproperties.25,26 A mixture of PLLA and PDLA, prepared from its
molten state or solution state, induces the formation of stereocomplex PLA (SC-PLA) driven by the intensive intermolecularhydrogen (H)-bonding and dipole–dipole interactions between
PLLA and PDLA (Fig. 1).27–29 Compared with homochiral PLA(HC-PLA), SC-PLA has a melting temperature (Tm) of around
230 °C, which is higher than that of pure HC-PLA (∼170 °C),since the intermolecular H-bonding interaction between PLLA
and PDLA chains increases the rigidity of the PLA chains.30–35
Optical resonators confine light in a small volume thatinterferes with itself and shows up sharp resonant opticalsignals.36 Typical optical resonators are Fabry–Pérot (F–P) resonators and whispering gallery mode (WGM) resonators.37 For
F–P resonators, light is confined between counter mirrors or
crystalline facets.38 On the other hand, WGM resonators
confine light circularly by total internal reflection (TIR) at theinterface between the inner and outer media with diﬀerent
refractive indices. Typical WGM resonators have shapes ofaDepartment of Material Innovation, Graduate School of Pure and Applied Science,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8573, Japanb
Department of Chemistry, Faculty of Mathematics and Natural Science, Universitas
Sumatera Utara, Jl. Dr. T. Mansur No. 9, Padang Bulan, Medan Baru, Medan,
Sumatera Utara 20222, Indonesiac
Department of Material Science, Institute of Pure and Applied Science, University ofTsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8573, Japan.
E-mail: yamamoto@ims.tsukuba.ac.jp† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d3nr05318k

19062 | Nanoscale, 2023, 15, 19062–19068

Fig. 1 Schematic illustration of the fabrication of microspheres from
HC- and SC-PLA by the O/W miniemulsion method.

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Nanoscalespheres, rings, polyhedra etc.39 WGM is highly sensitive to thesurface morphology because a rough surface tends to scatter
light, which reduces the light confinement eﬃciency in theresonator.40 Therefore, a highly smooth surface is one of theimportant factors for high quality factor (Q) microresonators.
The sharp PL peaks from the resonators are utilized for highly
sensitive chemical and biosensing applications by monitoring
the peak shift.41,42
In this work, we investigated the optical resonator properties of the microspheres formed from PLAs. The whispering
gallery mode (WGM) microresonators are fabricated from a
blend of PLLA and PDLA through the oil-in-water (O/W) miniemulsion method. We expect that the microresonators formed
from SC-PLA have better thermal stability than the microresonators formed from pure HC-PLA. We found that the WGM
resonators from SC-PLA are much more stable even at 230 °C
in comparison with those from HC-PLA (∼170 °C).
Microspheres of HC-PLA composed of PLLA (the number
average molecular weight, Mn = 40 kg mol−1) or PDLA (Mn =90 kg mol−1) were prepared by the O/W miniemulsion method.
Similarly, microspheres of SC-PLA composed of a blend of
PLLA and PDLA were fabricated with mixing ratios of PLLA to
PDLA (L/D) of 9/1, 7/3, 5/5, 3/7, and 1/9. The miniemulsion
method is chosen because of the eﬃcient formation of SC-PLA
with a high reaction rate compared to other methods such as
solution blending, supercritical fluid mixing, and melted
blending.32,43 During the emulsification and subsequent
solvent evaporation processes, PLLA and PDLA form astereocomplex.
Fig. 2 shows scanning electron microscopy (SEM) images of
the resultant microspheres formed from HC- and SC-PLA. All
the microspheres have high sphericity but have diﬀerent
surface morphologies. The microspheres of HC-PLA with an
L/D ratio of 10/0 containing only PLLA (Fig. 2a and b) and
SC-PLA with L/D ratios of 9/1 and 7/3 (Fig. 2c and d, respectively) have rather smooth surfaces with the values of the rootmean-square (RMS) roughness between 4 and 5 (for details on
the calculation of the RMS roughness, see the ESI†). The
surface of the microsphere becomes rough with the increase of
the content of PDLA with L/D ratios of 5/5, 3/7 and 1/9 withthe values of the RMS roughness being 6.23, 11.5, and 14.6,
respectively (Fig. 2e–g). As for the microparticles made fromonly PDLA (L/D = 0/10), the surface morphology is quite roughwith an RMS roughness value as large as 46.3 (Fig. 2h). The
diﬀerence in the surface morphologies of the microspheresfrom PLLA and PDLA is possibly due to the diﬀerence in themolecular weights of these polymers, which causes thediﬀerent crystallinity of the microspheres.44
We investigated the change in the morphology of the microsphere by heating. As shown in Fig. S1,† the microspheres of
HC-PLA (L/D = 10/0 and 0/10) melt when heated at 200 °C for 2s. However, microspheres of SC-PLA with L/D ratios of 9/1, 7/3,
5/5, 3/7, and 1/9 keep their spherical morphologies after being
heated at 200 °C, indicating that the thermal stability of PLA isremarkably improved by SC formation. Even for a microspherewith an L/D ratio of 9/1, where the composition of HC crystal-This journal is © The Royal Society of Chemistry 2023

Communication

Fig. 2 SEM micrographs of HC-PLA and SC-PLA microspheres preparedby the O/W miniemulsion method with diﬀerent L/D ratios of 10/0 (aand b), 9/1 (c), 7/3 (d), 5/5 (e), 3/7 (f ), 1/9 (g), and 0/10 (h). The graphs on
the right in (b–h) show the proﬁles of the diﬀerence in the gray values
versus the distance (d ) at the cross section of the microspheres shownas yellow-dotted lines in the corresponding micrographs.

lites is greater than that of SC crystallites, the microsphere preserves its spherical morphology upon heating at 200 °C.
The thermal stability of HC- and SC-PLA microspheres wasinvestigated in more detail by diﬀerential scanning calorimetry(DSC). Fig. 3 shows the DSC thermograms (first heating) of the
HC and SC crystallites with diﬀerent mixing ratios (L/D = 10/0,
9/1, 7/3, 5/5, 3/7, 1/9, and 0/10). Several characteristic endo-/
exothermic peaks were observed: endothermic peaks at
60–70 °C due to the glass transition (Tg), exothermic peaks at
90–110 °C due to the crystallization (Tc), and endothermic

Fig. 3

DSC traces of HC- and SC-PLA with diﬀerent L/D ratios.

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Nanoscalepeaks at >170 °C due to the melting of the HC and SC crystallites. For HC-PLA (L/D = 10/0 and 0/10), Tm appears at 174 and
177 °C, respectively. In contrast, for SC-PLA, another peak
appeared at 227–230 °C, which is attributed to the Tm of the
SC crystallites.25 These results indicate that blending PLLA
and PDLA forms both SC and HC domains.
The thermal properties of HC- and SC-PLA are summarizedin Table 1. From the peak area of Tm in the DSC thermograms,
the melting enthalpies (ΔHm) of the HC and SC crystallites are
determined. By changing the L/D ratio from 10/0 to 5/5, theΔHm of HC-PLA (ΔHHC) decreases from 54 to 24 J g−1 and thenincreases to 71 J g−1 by further changing the L/D ratio to 0/10.
Concomitantly, the ΔHm of SC-PLA (ΔHSC) significantly
increases from 9 to 45 J g−1 and then decreases to 15 J g−1
upon increasing the PDLA content. The ΔHHC and ΔHSC
values show minimum and maximum at an L/D ratio of 5/5,
indicating that PLA mostly forms SC domains in the 5/
5 mixture of PLLA and PDLA.45 Moreover, the decomposition
temperatures (Tdec) of both HC- and SC-PLA were determinedusing thermogravimetry diﬀerential thermal analysis (TG/DTA,
Fig. S2†). For pure HC-PLLA, Tdec was at around 280 °C. As theratio of PDLA to PLLA increased, Tdec increased and reached
337 °C for an L/D ratio of 1/9. For pure HC-PDLA, Tdec reached
354 °C. Therefore, Tdec is mainly aﬀected by the Mn of PLA,
where a large Mn has a high Tdec. However, the TGA profiles of
SC-PLA show a single decomposition step, indicating that thestereocomplex state is not only a simple mixture of PLLA and
PDLA but also has a strong interaction between PLLA and
PDLA via H-bonding (Fig. 1).
To understand the crystalline states of HC- and SC-PLA,
powder X-ray diﬀraction (PXRD) measurements were conducted. As shown in Fig. 4a, powder samples of HC-PLA with
L/D ratios of 10/0 and 0/10 show diﬀraction peaks at 2θ = 14.9,
16.7, 19.1, and 22.4°, which are assigned to the (010), (110)/(200), (203), (015) planes of the HC crystallites, respectively. In
comparison, powder samples with the blend of PDLA and
PLLA exhibit three additional diﬀraction peaks at 12.0, 20.8,
and 24.1°, which are assigned to the (110), (300)/(030), (220)planes of the SC crystallites, respectively.25,26,46 For the sample
with an L/D ratio of 5/5, diﬀractions from the HC crystallites
mostly disappeared. The percentages of the crystallites (Xc) of
HC- and SC-PLA are calculated from the intensity ratios of thediﬀraction peaks of the HC and SC crystallites to the entire

Table 1 Summary of the thermal behaviour of HC- and SC-PLA microspheres with diﬀerent L/D ratios

Sample

Tg
(°C)

Tc
(°C)

Tm, HC(°C)

Tm, SC(°C)

ΔHHC(J g−1)ΔHSC(J g−1)

HC-PLA 10/0
SC-PLA 9/1
SC-PLA 7/3
SC-PLA 5/5
SC-PLA 3/7
SC-PLA 1/9
HC-PLA 0/10

—
65
66
66
67
67
—

109
93
95
96
98
98
—

174
176
175
177
177
179
177

—
227
229
230
229
227
—

54
47
24
24
24
33
71

—
9
42
45
41
15
—

19064 | Nanoscale, 2023, 15, 19062–19068

Fig. 4 (a) PXRD patterns of HC- and SC-PLA microspheres. (b) The relative degree of crystallinity (Xc) of the HC and SC crystallites. (c–f )Relative degree of Xc versus temperature in the range of 25 to 220 °C for
HC and SC crystallites of PLA with L/D ratios of 10/0 (c), 9/1 (d), 7/3 (e),and 5/5 (f ).

diﬀraction. The Xc value of HC decreases when the L/D ratiochanges from 10/0 to 5/5 and then increases when the L/Dratio reaches 0/10 (Fig. 4b, blue). Conversely, the Xc value of SC
increases (decreases) when that of HC decreases (increases)(Fig. 4b, red).
PXRD measurements were further conducted by elevating
the temperature from 25 to 220 °C (Fig. S3†). For samples with
L/D ratios of 10/0, 1/9, 9/1, and 0/10, the diﬀraction peaks of
the HC crystallites disappear at 200 °C due to the melting of
the HC crystallites. For samples with L/D ratios of 7/3, 5/5, and3/7, small diﬀraction peaks from the HC crystallites remain at
200 °C, but they completely disappear at 220 °C. In contrast,
the diﬀraction peaks of the SC crystallites remain even at
220 °C. These results are consistent with the DSC results,
where the Tm of HC-PLA is around 180 °C while that of SC-PLA
is around 230 °C.47 Fig. 4c–f plot the Xc values versus temperature for the HC and SC crystallites. The Xc value of the HC crystallites abruptly drops at 200 °C, and simultaneously, the Xc
value of the SC crystallites increases at 200 °C. These results
indicate that the melting of the HC domain subsequently
induces the formation of the SC domains.
The microspherical structure with a smooth surface isadvantageous for use as a WGM optical resonator.48–50 As
schematically drawn in Fig. 5a, photoluminescence (PL) gener-

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NanoscaleCommunication

ated at the surface of the polymer microsphere is confined via
total internal reflection at the medium/air interface and interferes with itself showing up sharp and periodic resonant PLlines.51–58 To investigate the optical resonator properties of the
PLA microspheres, HC- and SC-PLA microspheres are dopedwith a fluorescent dye, zinc(II) tetraphenylporphyrin (ZnTPP),
with Tm (∼350 °C) higher than that of PLA. The ZnTPP-doped
HC- or SC-PLA microspheres are dispersed on a quartz substrate by a spin coating method. PL spectra of a single HC- andSC-PLA microsphere are recorded upon focused laser excitation with a 405 nm continuous wave (cw) laser to a single
microsphere.59–63 At 30 °C, the PL spectra of a HC-PLA microsphere show periodic PL lines, attributed to WGMs (Fig. 5b).
As shown in Fig. S4,† these resonant peaks are assigned totransverse electric (TE) and transverse magnetic (TM) modes.
The free spectral range (FSR) of four microspheres with
diﬀerent sizes is plotted in Fig. S5.† The FSR is measured
using the following equation:
FSR ¼ ðλ 2 =nπÞ  ð1=dÞ;which indicates that the PLA microspheres certainly act as an
optical resonator.64 The clear WGM peaks in the PL spectra aremaintained upon thermal heating of the microspheres up to
160 °C (see Fig. S6a† for more details). By further heating at

Fig. 5 (a) Schematic representation of the light conﬁnement and WGM
PL. (b–d) PL spectra of a single PLA microsphere after being annealedfor 2 s upon photoexcitation at 405 nm with a cw laser. (e) Plots of thenormalized PL intensity of the microspheres with L/D = 10/0, 9/1, and 7/
3 versus annealing temperature. (f ) Plots of the Q factor of the microspheres with L/D = 10/0, 9/1, and 7/3 versus annealing temperature.

This journal is © The Royal Society of Chemistry 2023

170 °C, these WGM PL peaks disappear due to the melting of
HC-PLA that causes the deformation of the microsphericalmorphology.
The optical resonator properties are observed for the microspheres of SC-PLA with L/D ratios of 9/1 and 7/3 (Fig. 5c and d,respectively). Intriguingly, the thermal stability of the microresonator is preserved upon thermal heating up to 230 °C. The
WGM PL peaks disappear by heating the microspheres at
240 °C (see Fig. S6b and c† for more details). In Fig. 5e, temperature dependencies of the WGM peak intensity, normalized
by the PL intensity of the background unconfined PL, areplotted. It is obvious that the thermal stability of the opticalresonator is higher by ∼70 °C for the microspheres of SC-PLA
with L/D ratios of 9/1 and 7/3 than that of the HC-PLA microspheres (L/D = 10/0). It is worth noting that the phase transition of PLA aﬀects the shape of PL spectra, possibly causedby the change in the aggregation manner of ZnTPP in the PLA
microspheres. We confirm that the ratio of the PL intensities
at 603 and 643 nm (I603/I643) from a cast film of the PLA microspheres varies at the phase transition temperature (Fig. S7†).
To gain insight into the resonant properties in more detail,the Q factor, defined as the wavelength of the resonant peaks
divided by their full width at half maximum (FWHM), is evaluated.65 Fig. 5f plots the Q factor of the PL peaks versus theheating temperature. The Q factor gradually increases uponheating, possibly due to the improvement of the surface roughness. However, phase transition causes the loss of the Q factor
caused by the scattering of the confined light by the crystalline
domains. For example, the Q factor of the HC-PLA microsphere(L/D = 10/0) dropped from 500 to 360 at 110 °C, where phasetransition occurs from glass to the crystalline state. A further
drop in the Q factor occurs at 170 °C, where melting of the
HC-PLA microsphere takes place with the collapse of its microspherical morphology. In the case of SC-PLA with L/D ratios of
9/1 and 7/3, the Q factor drops once at ∼200 °C, where HC-PLA
crystallites melt and SC-PLA crystallites form, and finally drops
oﬀ at 240 °C, at which the SC-PLA completely melts.
For comparison, SC-PLA microspheres with L/D ratios of 5/5, 3/7, and 1/9 display quite poor WGM PL (Fig. S8†), because
the surface morphology of the microspheres is rather roughwith the RMS roughness greater than 6 (Fig. 2). The light confinement is sensitive to the surface roughness, where therough surface causes a scattering of the confined light, leading
to the loss of the optical resonator properties.65 Similarly,
HC-PLA microspheres from PDLA (L/D = 0/10) do not show
WGM PL due to the ill-defined spherical morphology(Fig. S8†).
In conclusion, we successfully prepared thermally tolerant
optical resonators from stereocomplex crystallites of PLA
formed by blending PLLA and PDLA through the oil-in-water
miniemulsion method. The SC-PLA microspheres exhibit
higher thermal stability in comparison with the HC-PLA microspheres, where the melting temperature of SC-PLA is morethan 50 °C higher than that of HC-PLA. The relative degree ofcrystallinity of SC-PLA is maximum when the content of PLLA
and PDLA is 5/5, as evaluated by PXRD studies. The tempera-

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Communicationture-dependent PXRD results clearly show that PLAs maintainthe stereocomplex structure even when the temperaturereaches 220 °C. The temperature-dependent µ-PL spectra showthat the SC-PLA resonators exhibit higher thermal tolerance incomparison with the HC-PLA resonators, where the WGM resonance properties of SC-PLA are preserved even at 230 °C. This
work demonstrates a powerful strategy of stereocomplex formation toward thermally tolerant bio-related materials for
optical applications.

Conﬂicts of interest
Published on 13 November 2023. Downloaded on 9/25/2024 4:34:25 AM.

There are no conflicts to declare.

Acknowledgements
This work was financially supported by CREST (JPMJCR20T4)
and ACT-X (JPMJAX201J) from the Japan Science andTechnology Agency (JST), Grant-in-Aid for Scientific Research
for Early-Career Scientists (JP22K14656) from the Japan Society
for the Promotion of Science (JSPS) and the New Energy andIndustrial Technology Development Organization (NEDO), andthe Kato Memorial Bioscience Foundation.

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