Syn hesis, Mo phology, and C ys alliza ion Kine ics o
Polyhep alac one (PHL)
Ma ia Rosa ia Capu o, Asie Olmos, Bo Li, Jo ge L. Olmedo-Ma ínez, Anna Mala on e,
Claudio De Rosa, Ha i z Sa don, Rachel K. O’Reilly, And ew P. Do e, and Alejand o J. Mulle *
Ci e This: Biomac omolecules 2023, 24, 3256−3267
Read Online
ACCESS Me ics & Mo e A icle Recommenda ions *
sı Suppo ing In o ma ion
ABSTRACT: Alipha ic polyes e s a e widely s udied due o hei
excellen p ope ies and low-cos p oduc ion and also because, in
many cases, hey a e biodeg adable and/o ecyclable. The e o e,
expanding he ange o a ailable alipha ic polyes e s is highly
desi able. This pape epo s he syn hesis, mo phology, and
c ys alliza ion kine ics o a sca cely s udied polyes e , polyhep a-
lac one (PHL). Fi s , we syn hesized he η-hep alac one monome
by he Baeye −Villige oxida ion o cyclohep anone be o e se e al
polyhep alac ones o di e en molecula weigh s (in he ange
be ween 2 and 12 kDa), and low dispe si ies we e p epa ed by ing-
opening polyme iza ion (ROP). The in luence o molecula weigh on p ima y nuclea ion a e, sphe uli ic g ow h a e, and o e all
c ys alliza ion a e was s udied o he i s ime. All o hese a es inc eased wi h PHL molecula weigh , and hey app oached a
pla eau o he highes molecula weigh samples employed he e. Single c ys als o PHLs we e p epa ed o he i s ime, and
hexagonal-shaped la single c ys als we e ob ained. The s udy o he c ys alliza ion and mo phology o PHL e ealed s ong
simila i ies wi h PCL, making PHLs e y p omising ma e ials, conside ing hei po en ial biodeg adable cha ac e .
1. INTRODUCTION
Plas ic ma e ials a e widely used o hei e sa ili y, low
p oduc ion cos , and easy manu ac u ing. Fu he mo e, plas ic
ma e ials possess a wide ange o p ope ies, so i is possible o
ind hem in almos all p oduc ion sec o s, such as packaging,
clo hing, medicine, and elec onics.
1−3
On he o he hand, he
massi e p oduc ion and use o plas ics ha e led o one o he
main p oblems o he las decades: plas ic was e and i s
disposal.
4−7
Ano he nega i e aspec is ha hese plas ic
ma e ials a e pe oleum de i a i es, which inc eases he
emission o CO2in o he a mosphe e and a ec s clima e
change.
8
To o e come his p oblem, ecycling hese plas ic
ma e ials o ob ain alue-added p oduc s has been used o
manage issues associa ed wi h hei disposal.
9−14
Addi ionally,
p oducing and using ma e ials wi h lowe en i onmen al
impac is an al e na i e.
15,18,16,17
Polyes e s a e con en ionally p ocessed in a ious o ms,
such as ibe s, ilamen s, esins, e c., and ha e b oad
applica ions ac oss packaging, ex iles, au omo i e, medical,
elec onic, and cons uc ion ields. Some polyes e s can be
ecycled h ough physical (mechanical) and chemical (hyd ol-
ysis, me hanolysis, and glycolysis eac ion) me hods, and hei
ecycled pa s can also be used in he packaging and
cons uc ion ields. E icien and e ec i e ecycling o
polyes e s could lead o he educ ion o CO2emissions and,
consequen ly, o global wa ming.
17,18
A e sa ile alipha ic polyes e ha is also biodeg adable is
polycap olac one (PCL), which is usually ob ained by ing-
opening polyme iza ion (ROP) o ε-cap olac one o by
polycondensa ion o hyd oxycap oic acid.
19
PCL is a
biocompa ible and biodeg adable polyme ; i is miscible wi h
se e al o he polyme s,
20−22
and he cos s associa ed wi h i s
p oduc ion a e e y low. These ad an ageous p ope ies make
PCL one o he mos used polyes e s and sui able o di e en
ields o applica ion, such as issue enginee ing, d ug deli e y
sys ems, o as an addi i e o polyu e hanes.
23−26
Simila al e na i e polyme ic ma e ials a e being in es-
iga ed, conside ing he ad an ages o alipha ic polyes e s.
Poly(η-hep alac one) (PHL), a polyes e de i ed om η-
hep alac one (lac one ha con ains one mo e ca bon a om
han ε-cap olac one), has been la gely unde s udied hus a , as
i is no comme cially a ailable. Gi en he simila i y in
s uc u e o ε-cap olac one (PCL), PHL could be expec ed o
exhibi compa able p ope ies in p ocessabili y, mel ing, and
c ys alliza ion empe a u es. Ano he po en ially in e es ing
aspec could be he p esence o one mo e ca bon a om han
Recei ed: Ma ch 23, 2023
Re ised: June 6, 2023
Published: June 21, 2023
A iclepubs.acs.o g/Biomac
© 2023 The Au ho s. Published by
Ame ican Chemical Socie y 3256
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
Downloaded ia UNIV DEL PAIS VASCO on May 23, 2024 a 13:16:52 (UTC).
See h ps://pubs.acs.o g/sha ingguidelines o op ions on how o legi ima ely sha e published a icles.
PCL, which could a ec he c ys alliza ion. Fo example, in a
ecen ly published s udy,
26
i was epo ed ha PEB (poly-
(e hylene b assyla e)), a sho -long alipha ic polyes e (wi h 13
me hylene g oups in i s epea uni ), exhibi s a peculia
c ys alliza ion beha io due o a simila sel -poisoning e ec o
ha obse ed p e iously in long-chain alkanes.
26−29
The sel -
poisoning e ec consis s in he display o a c ys alliza ion a e
minima upon dec easing he iso he mal c ys alliza ion empe -
a u e. In he case o long-chain alkanes, ex ended-chain
lamellae o m i s a high c ys alliza ion empe a u e, hen
segmen s wi h he olded chain con o ma ion a ach, upon
dec easing Tc alues, o he g ow h on , p e en ing u he
g ow h un il hey de ach, and g ow h can again p oceed in he
ex ended o m.
26−29
The highe a io o me hylene g oups
e sus es e g oups in PEB along he alipha ic chain leads o
p ope ies ha come close o ha o polye hylene (PE), as he
pola g oup is “dilu ed” along he nonpola es o he chain. In
his sense, PEB is close o PE, while PHL is close o PCL.
Only a ew epo s ha e shown he abili y o syn hesize
PHL; all ha e esul ed in polyme s wi h la ge dispe si y alues
(2.8).
30
The syn hesis and cha ac e iza ion o low polydispe -
si y (PHL) wi h p edic able molecula weigh s and a s udy o
he c ys alliza ion o hese in e es ing ma e ials ha e no been
epo ed p e iously in he li e a u e. The e o e, in his wo k,
we epo he syn hesis and he mal and s uc u al cha ac e -
iza ion o polyhep alac one (PHL). The e ec o he PHL
molecula weigh on i s he mal p ope ies, nuclea ion, and
c ys alliza ion kine ics is s udied o he i s ime. Addi ionally,
we epo , also o he i s ime, he p epa a ion o single
c ys als o his sca cely s udied semic ys alline polyme .
S udying c ys alliza ion, mo phology, and c ys alliza ion
kine ics is e y impo an as hese aspec s in luence p ope ies
such as he mal p ope ies, mechanical beha io , and
biodeg ada ion po en ial.
2. EXPERIMENTAL SECTION
2.1. Ma e ials. Chemicals and sol en s we e pu chased om
Sigma-Ald ich, Ac os, Fluka, Fishe Chemical, Al a Aesa , o VWR.
D y sol en s we e pu i ied using he MBRAUN SPS sol en
pu i ica ion sys em. η-Hep alac one was d ied o e calcium hyd ide
o 24 h be o e acuum dis illa ion. The dual- unc ional chain ans e
agen (CTA), 4-cyano-4-(((e hyl hio)ca bono hioyl) hio)pen anoic
acid (CEPA), was p epa ed acco ding o a p e iously epo ed
p ocedu e.
36
2.2. Syn hesis o η-Hep alac one. As η-hep alac one is no
comme cially a ailable, i was syn hesized he e by he Baeye −Villige
oxida ion me hod acco ding o p e iously epo ed li e a u e.
31
B ie ly, he cyclohep anone (223 mmol) and m-chlo ope benzoic
acid (275 mmol) we e mixed in CH2Cl2(250 mL). The suspension
was hea ed unde e lux o 3 days. The eac ion mix u e was cooled
in an ice ba h, and he solids we e il e ed o e Celi e and washed wi h
CH2Cl2(2 ×50 mL). The il a e was washed wi h 10% Na2S2O3
solu ion (2 ×200 mL), sa u a ed Na2CO3solu ion (2 ×200 mL),
and sa u a ed NaCl solu ion (1 ×200 mL). The o ganic laye was
d ied wi h MgSO4, il e ed, and e apo a ed in acuo. The esul ing
liquid was dis illed o e CaH2 o a o d he lac one in yields o a ound
70%.
1H NMR (400 MHz, 298 K, CDCl3): δ= 4.28 ( , 2H, CH2O), 2.48
( , 2H, CH2C�OO), 1.75 (m, 4H, CH2), 1.52 (m, 4H, CH2) ppm.
13C NMR (125 MHz, 298 K, CDCl3): δ= 176.4 (OCOCH2), 64.3
(OCOCH2), 31.0 (CH2COO), 30.5 (OCH2CH2), 28.0
(CH2CH2COO), 25.4 (CH2CH2CH2COO) and 23.5
(OCH2CH3CH2) ppm.
2.3. Syn hesis o Poly(η-hep alac one). Fou di e en
homopolyme s o polyhep alac one (PHL) wi h di e en molecula
weigh s we e p epa ed in his s udy. B ie ly, in a ni ogen- illed
glo ebox, solu ions o diphenylphospha e (10 mg, 0.04 mmol) in d y
oluene (1 mL) and CEPA (9.92 mg, 0.04 mmol) in d y oluene (1
mL) we e added o η-hep alac one (490 μL, 4 mmol). A e s i ing a
oom empe a u e o a de ined ime pe iod, he solu ion was
emo ed om he glo ebox, p ecipi a ed h ee imes in o ice-cold
me hanol, and collec ed by cen i uga ion. 1H NMR (400 MHz,
CDCl3)δ/ppm: 4.04−4.11 ( , CH2OH), 3.63 (m, C(CN)CH2CH2),
3.33 (q, SCH2CH3), 2.28 ( , OCOCH2), 1.61−1.35 (m,
OCOCH2(CH2)3CH2OH).
2.4. 1H Nuclea Magne ic Resonance (NMR). Unless o he wise
s a ed, 1H nuclea magne ic esonance (NMR) spec a we e eco ded
a 400 MHz on a B uke DPX-400 spec ome e in CDCl3. Chemical
shi s a e epo ed as δin pa s pe million (ppm) down ield om he
in e nal s anda d ime hylsilane.
2.5. Size Exclusion Ch oma og aphy (SEC). Size exclusion
ch oma og aphy (SEC) was pe o med on an Agilen 390-MDS on
PLgel Mixed-D ype columns in se ies wi h e ac i e index (RI)
de ec ion. Weigh s we e calcula ed using a calib a ion cu e
de e mined om poly(s y ene) s anda ds wi h chlo o o m (0.5%
NE 3) as eluen lowing a 1.0 mL.min−1and sample concen a ion o
3 mg mL−1.
2.6. The mog a ime ic Analysis. To de e mine he empe -
a u e a which he samples he mally deg ade in ai , a Pe kinElme
he mog a ime ic analyze was used. To pe o m his expe imen , 10
mg o each sample we e placed in a pla inum c ucible and we e hea ed
om 30 o 600 °C a 10 °C/min.
2.7. Di e en ial Scanning Calo ime y (DSC). A Pe kinElme
Py is 8000 DSC wi h an In acoole 2P was employed o cha ac e ize
he mal p ope ies. The ins umen was p e iously calib a ed wi h
indium and in s anda ds. Samples o 5 mg o each PHL we e used,
and he expe imen s we e ca ied ou unde ul apu e ni ogen low
by placing he ma e ials in sealed aluminum pans. Non-iso he mal and
iso he mal expe imen s we e pe o med.
Du ing he non-iso he mal expe imen s, he samples we e hea ed
up o 90 °C a 20 °C/min and le a his empe a u e o 3 min o
e ase he he mal his o y; hen, hey we e cooled a 20 °C/min down
o 25 °C and held o 1 min a his empe a u e. A e wa d, hey we e
hea ed o 90 °C/min a 20 °C/min.
Du ing he second hea ing scan, i was possible o calcula e he
deg ee o c ys allini y o each PHL sample as ollows
x
H
H
100
c
m
m
0
= ×
whe e ΔHm(J/g) is he mel ing en halpy o he samples and ΔHm
0is
he en halpy o mel ing o a ully c ys alline sample (195 J/g)
calcula ed acco ding o he g oup con ibu ion me hod.
32
To in es iga e he o e all c ys alliza ion kine ics, iso he mal
expe imen s we e ca ied ou . In he i s pa o he expe imen ,
he minimum iso he mal c ys alliza ion empe a u e Tc,min was
de e mined by he ial and e o me hod p oposed by Lo enzo e
al.
33
Samples o PHL we e cooled om he mel o Tc alues a 60
°C/min and immedia ely ehea ed a 20 °C/min up o 90 °C. This
p o ocol was epea ed cyclically a dec easing Tcun il no mel ing
phenomenon was ound in he ehea ing scan. In his way, i was
possible o de e mine a Tc ange. The iso he mal c ys alliza ion
expe imen s we e pe o med ollowing he p ocedu e sugges ed by
Lo enzo e al.:
33
(I) hea ing om oom empe a u e o 30 °C abo e
he mel ing poin a 20 °C/min, i.e., 90 °C; (II) holding he sample
o 3 min a 90 °C o e ase he mal his o y; (III) quenching he
sample o a p ede e mined c ys alliza ion empe a u e (Tc) a 60 °C/
min; (IV) iso he mal c ys alliza ion un il maximum sa u a ion (in ou
case be ween 15 and 60 min o each Tc); and (V) hea ing om Tc o
90 °C a 20 °C/min o eco d he mel ing beha io a e he
iso he mal c ys alliza ion. This inal mel ing un p o ided he alues
o appa en mel ing poin s employed o pe o m he Ho man−Weeks
ex apola ion o calcula e he equilib ium mel ing empe a u e o each
ma e ial. These expe imen s we e pe o med on as-p epa ed samples,
which we e in powde o m.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3257
2.8. X- ay Powde Di ac ion o he As-P epa ed and Mel -
C ys allized Samples. X- ay powde di ac ion p o iles o he as-
p epa ed (p ecipi a ed om he polyme iza ion solu ion) and mel -
c ys allized samples o PHL we e ob ained a oom empe a u e wi h
Ni- il e ed Cu Kα adia ion (λ= 1.5418 Å) by using an au oma ic
Philips di ac ome e ope a ing in he e lec ion geome y. Mel -
c ys allized comp ession-molded samples we e p epa ed by hea ing
he as-syn hesized samples a 30−40 °C abo e he mel ing
empe a u e be ween he pla es o a p ess and hen cooling o
oom empe a u e by ci cula ing cold wa e inside he p ess pla es
(es ima ed cooling a e ≈20 °C/min).
2.9. P epa a ion and Cha ac e iza ion o Single C ys als o
PHL. A 0.012 w % solu ion o he sample PHL 66 in 1-hexanol was
p epa ed by dissol ing he polyme (0.3 mg) in o 3 mL o sol en .
The solu ion was placed a 85 °C and main ained a his empe a u e
o 1 h o comple ely dissol e he polyme . The solu ion was hen
slowly cooled o 50 °C (es ima ed cooling a e ≈1.2 °C min−1) and
kep a his empe a u e o 21 h o allow c ys alliza ion. A e wa d,
he solu ion was slowly cooled o oom empe a u e (es ima ed
cooling a e lowe han 1 °C min−1). D ops o c ys al suspension we e
deposi ed on ca bon-coa ed g ids and allowed o d y be o e
ansmission elec on mic oscopy (TEM) analysis. B igh - ield TEM
images o c ys als we e acqui ed using an FEI TECNAI G2 200 kV S-
TWIN mic oscope (elec on sou ce wi h LaB6emi e ) ope a ing a
120 kV.
2.10. Pola ized Ligh Op ical Mic oscope (PLOM) Analysis.
A pola ized ligh op ical mic oscope, Olympus BX51 (Olympus,
Tokyo, Japan), equipped wi h an Olympus SC50 digi al came a and
wi h a Linkam-15 TP-91 ho s age (Linkam, Tadwo h, U.K.)
(coupled o a liquid ni ogen cooling sys em) was used o obse e he
mo phology o he samples, a e c ys alliza ion om he mel . Films
wi h app oxima ely 50 μm hickness we e p epa ed by mel ing he
samples be ween wo glass slides.
The samples we e hea ed o 90 °C, kep a his empe a u e o 3
min o e ase hei he mal his o y, and hen cooled om he mel a
20 °C/min o 25 °C; using he same equipmen , he iso he mal
sphe uli ic g ow h a e was measu ed. The samples we e hea ed
be ween wo glass slides o 90 °C and kep a his empe a u e o 3
min o e ase he he mal his o y. The samples we e hen cooled a 50
°C/min o a empe a u e whe e he sphe uli es began appea ing. The
sphe uli e g ow h was ollowed iso he mally as a unc ion o ime by
eco ding mic og aphs. The p ocedu e was epea ed o di e en
empe a u es, and o each empe a u e, he adius o he sphe uli es
was measu ed and plo ed as a unc ion o ime. F om hese da a, he
g ow h a e (G) o he sphe uli es was de e mined, and he
expe imen al alues o G e sus Tcwe e i ed using he Lau i zen−
Ho man equa ion.
34
2.11. T ansmission Elec on Mic oscopy (TEM). TEM analysis
was conduc ed o obse e he mo phology a he lamella le el. A
RuO4solu ion was used o s aining PHL ilms o oughly 1 mm
hickness. Thin s ips o samples we e pu in o his solu ion o 16 h.
A e wa d, ul a- hin sec ions we e cu a −90 °C wi h a diamond
kni e wi h a Leica EMFC6 ul a-c yo-mic o ome de ice. The ul a-
hin, 90 nm hick sec ions we e moun ed on 200 mesh coppe g ids.
Finally, hey we e examined using a TECNAI G2 20 TWIN TEM
equipped wi h a LaB6 ilamen ope a ing a an accele a ing ol age o
120 kV.
3. RESULTS AND DISCUSSION
As η-hep alac one (HL) is no comme cially a ailable, i was
i s syn hesized and cha ac e ized. The i s aim was o ob ain
Figu e 1. (a) Syn hesis o he PHL polyme s by ROP o η-hep alac one ca alyzed by DPP and (b) 1H NMR spec um o PHL 35 (CDCl3, 400
MHz).
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3258
poly(η-hep alac one) (PHL) wi h p edic able molecula
weigh and low dispe si y. The e o e, ROP kine ics o he
HL polyme iza ion was s udied as ca alyzed by an o gano-
ca alys , namely, diphenylphospha e. A ange o di e en DPs
o PHL homopolyme s we e a ge ed o u he in es iga ion
o c ys alliza ion kine ics.
3.1. Syn hesis o Poly(η-hep alac one). η-Hep alac one
was syn hesized by Baeye −Villige oxida ion ollowing a
li e a u e p ocedu e. The pu e p oduc was con i med wi h 1H
NMR spec oscopy and was consis en wi h p e ious li e a u e
da a (Figu e S1).
31
A e he success ul p epa a ion o he
monome , he homopolyme iza ion o η-hep alac one was
ca ied ou . Diphenylphospha e (DPP) was selec ed as he
ca alys as a esul o he supe io abili y o his ca alys o
p o ide polylac ones wi h na ow dispe si ies and end-g oup
ideli y.
35
The polyme iza ion was ini ia ed by a dual-head
ini ia o and chain ans e agen o be consis en wi h ou
p e iously epo ed PCL wo k
36
(Figu e 1a).
The ROP o η-hep alac one was a emp ed a a monome
concen a ion o 4 M in oluene as sol en a oom
empe a u e in an N2- illed glo ebox, wi h ini ia o (DP o =
100) and 1 mol % DPP ca alys . 1H NMR spec oscopic
analysis was employed o moni o he eac ions and cha ac e -
ize he esul ing polyme s (Figu e 1b). The disappea ance o
he cha ac e is ic signals o he me hylene p o ons o he HL
monome a δ= 4.28 ppm and he concomi an appea ance o
hese me hylene p o ons in he polyme chain a δ= 4.05 ppm
pe mi us o calcula e he con e sion. The polyme iza ion
eached 70% con e sion a e 4 h while s ill exhibi ing i s -
o de kine ics (Figu e S2). Using hese condi ions, di e en
DPs o PHL we e a ge ed o u he c ys alliza ion s udy.
Di e en con e sion deg ees we e ob ained depending on he
eac ion ime, and he e o e ou di e en samples o PHL wi h
di e en molecula masses we e syn hesized (Table 1).
The a e age molecula weigh was measu ed by 1H NMR
spec oscopy and SEC. The polyme molecula weigh s we e
de e mined by end-g oup analysis by 1H NMR spec oscopy,
compa ing he a io be ween he polyme CH2OC�O
esonances o (δ= 4.05) and he chain ans e eagen
SCH2CCN(CH3) esonance (δ= 3.69). Size exclusion
ch oma og aphy (SEC) analysis is epo ed in he SI and
e eals low dispe si y (ĐM< 1.2) and good o e lap o he
e ac i e index (RI) and ul a iole (UV) (λ= 309 nm,
co esponding o he π−π*elec onic ansi ion o he
hioca bonyl moie y) peak in he SEC aces, which signi ies
he e en ion o he hioca bonyl g oup in he ini ia o (Figu e
S3).
3.2. X- ay Powde Di ac ion. The X- ay powde
di ac ion p o iles o he as-p epa ed and mel -c ys allized
samples o PHL o highe molecula mass (samples PHL 66
and PHL 90), acqui ed in he 2θ egion be ween 5 and 40°, a e
shown in Figu e 2a,b, espec i ely.
The di ac ion p o iles o bo h as-p epa ed and mel -
c ys allized samples (Figu e 2a,b) a e cha ac e ized by wo
s ong and sha p e lec ions a 2θ≈21 and 24°and o he
mino di ac ion peaks o lowe in ensi y in he 2θ egion
be ween 25 and 40°. Simila di ac ion p o iles ha e been
ob ained o he di e en samples in Table 1. The X- ay
di ac ion p o iles o PHL (Figu e 2) appea simila o ha o
polye hylene (PE)
37
and o he linea alipha ic polyes e s,
―(―O―(CH2)m―CO―)n―, such as poly-
(11-undecalac one) (PUDL, m= 10),
3838
poly(δ- ale olac-
Table 1. Polyme iza ion Time, Deg ee o Con e sion, A e age Molecula Masses, and Polydispe si y o he Syn hesized
Samples o PHL
ime (h) sample con e sion (%) Mn
a
(GPC) (kDa) Mw
a
(GPC) (kDa) ĐMMn [1H (NMR)]
b
(kDa)
1 PHL 15 15 6.2 7.0 1.11 2.2
2 PHL 35 35 9.9 11.4 1.11 4.7
4 PHL 66 70 16.6 19.6 1.15 9.2
5 PHL 90 90 20.8 22.7 1.17 11.8
a
Measu ed by SEC.
b
Measu ed by 1H NMR spec oscopy.
Figu e 2. X- ay powde di ac ion p o iles o he as-p epa ed (A) and mel -c ys allized (B) specimens o samples PHL 66 (a) and PHL 90 (b).
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3259
one) (PVL, m= 4),
39
poly(β-p opiolac one) (PPL, m= 2),
40
poly(16-hexadecalac one) (PHDL, m= 15),
41
poly(12-
dodecalac one) (PDDL, m= 11),
42
poly(15-pen adecalac one)
(PPDL, m= 14),
43
and poly(ε-cap olac one) (PCL, m=
5).
44,45
3.3. TGA Resul s. Figu e 3 shows he weigh loss p o iles o
he ou PHL samples e ealed by TGA. In he case o he wo
samples wi h he lowes molecula weigh (PHL 15 and PHL
35), he TGA cu es a e cha ac e ized by a wo-s ep weigh
loss a abou 210 and 380 °C. This phenomenon disappea s in
he wo samples wi h he highes molecula weigh (PHL 66
and PHL 90), wi h only one weigh loss s ep a abou 400 °C.
This pa icula beha io has al eady been obse ed p e iously
in he case o PCL samples wi h di e en molecula weigh s,
46
in which he eason o he i s s ep o weigh loss o low-
molecula -weigh samples was ini ially a ibu ed o he
de elopmen o ε-cap olac one in he he mal decomposi ion
p ocess, and FT-IR spec oscopic analyses subsequen ly
con i med his. Since PCL is chemically simila o PHL, i is
possible ha he beha io de ec ed by he TGA could be
a ibu ed o he same cause, i.e., he p oduc ion o η-
hep alac one du ing he he mal deg ada ion p ocess.
3.4. Non-Iso he mal DSC. Figu e 4 shows DSC esul s
ob ained du ing cooling om he mel and subsequen hea ing
scans o he di e en molecula weigh PHL samples. I is
obse ed ha he c ys alliza ion empe a u e inc eases as a
unc ion o molecula weigh , om 36 °C o he lowes
molecula weigh o 42.5 °C o he highes molecula weigh .
The mel ing empe a u e has a simila beha io , inc easing as a
unc ion o molecula weigh om 55 o 61.5 °C. Figu e 5
shows he change in Tmas a unc ion o molecula weigh . As
expec ed, he mel ing empe a u e inc eases wi h molecula
weigh bu does no each sa u a ion in he ange o molecula
weigh s explo ed he e. The same end is obse ed o Tc, as
epo ed in Figu e S4.
The alues o he mel ing and c ys alliza ion empe a u es
wi h he ela i e en halpies a e shown in Table S1 o he
Suppo ing In o ma ion. The same able also shows he glass-
ansi ion empe a u e (Tg), and deg ee o c ys allini y (xc)
alues ob ained in he second hea ing scan. Figu e S5 epo s
an enla gemen o he second hea ing scans o app ecia e he
glass ansi ion be e .
3.5. Mo phology and Sphe uli ic G ow h. The samples
we e analyzed by PLOM o s udy hei supe s uc u al
mo phology. Figu e 6 shows he PLOM mic og aphs using
he same magni ica ion scale ba (500 μm) collec ed a 25 °C
a e non-iso he mal c ys alliza ion om he mel a 20 °C/
min. The mic og aphs o he samples wi h he lowes and
highes molecula weigh (PHL 15 and PHL 90) a e epo ed
since he mo phology does no show signi ican di e ences o
Figu e 3. Weigh loss (%) as a unc ion o he empe a u e (°C) o
PHL samples.
Figu e 4. (a) DSC he mog ams (cooling scans) a 20 °C/min and (b) subsequen DSC he mog ams (hea ing scans) a 20 °C/min o PHL
samples.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3260
he o he samples. Samples c ys allize, o ming mic osphe -
uli es, ega dless o hei molecula weigh . The esul s imply
ha he samples possess a e y high nuclea ion densi y,
p obably coming om ca aly ic deb is, which can ac as e y
ac i e nuclea ing he e ogenei ies.
When he samples a e iso he mally c ys allized, he
mo phology can be be e app ecia ed, as he e ogeneous
nuclei ac i a ion is known o dec ease when he iso he mal
c ys alliza ion empe a u e inc eases. Figu e 7 shows he
PLOM mic og aphs acqui ed du ing iso he mal c ys alliza ion
expe imen s a he indica ed Tcand wi h he same supe -
cooling alue ( he equilib ium mel ing empe a u e needed o
calcula e he supe cooling was also ob ained in his wo k; see
below). The samples we e cooled om he mel (a 50 °C/
min) o a chosen c ys alliza ion empe a u e in he ange o
45−60 °C. Al hough he c ys als appea la ge , hey do no
ha e he classic ci cula shape ypical o sphe uli es: hey
esemble mo e axiali es o lamella agg ega es wi h wo-
dimensional symme y. As hei a e age sizes a e close o one
ano he , he nuclea ion is p obably ins an aneous, as was la e
con i med by he o e all c ys alliza ion kine ics measu emen s
(see below).
Gi en he shape o he c ys als, o measu e he g ow h a e,
G(μm/s), he leng h a ia ion om one ex eme o he o he
was measu ed as a unc ion o ime o di e en c ys alliza ion
empe a u es ( he plo s we e always linea ).
The ypical g ow h a e (G) end as a unc ion o
empe a u e is ep esen ed by a bell-shaped cu e, gi en by
he compe i ion o wo opposi e phenomena.
47,48
On he igh -
hand side o he plo , he g ow h a e inc eases as Tcdec eases
because he seconda y nuclea ion inc eases wi h he supe -
cooling un il a maximum (i.e., Gis con olled by seconda y
nuclea ion in his high- empe a u e ange). This maximum
co esponds o he poin a which he iscosi y o he mel is so
high ha he g ow h o he c ys als is domina ed by he slow
di usion o he chains o he c ys alliza ion on . Sub-
sequen ly, he g ow h a e dec eases and becomes ze o a Tg.
In he case o he PHL samples in ol ed in his s udy, i was
possible o measu e g ow h a es only on he igh side o he
G e sus Tccu e a high c ys alliza ion empe a u es. Any
a emp o analyze g ow h measu emen s a lowe empe -
a u es ailed, as he ma e ial c ys allized du ing he as cooling
o Tc.
Figu e 8a shows he esul s o he sphe uli ic g ow h a e as a
unc ion o Tc o he ou samples in ol ed in his wo k,
de e mined by PLOM expe imen s. The end is simila o all
samples and is he ypical end whe e seconda y nuclea ion
domina es he supe s uc u al g ow h. I is possible o no ice
ha he molecula weigh o each sample in luences he g ow h
a e alues. Indeed, he g ow h a e is as e as he molecula
weigh inc eases, and he sphe uli es o he highe molecula
weigh samples g ow as e a he same c ys alliza ion
empe a u e in he molecula weigh ange s udied in his
wo k. This beha io is e iden om he g aph shown in Figu e
8b, in which he g ow h a e is ep esen ed as a unc ion o Mn
a he same c ys alliza ion empe a u e. This esul is consis en
wi h he con ol o seconda y nuclea ion. I has al eady been
epo ed in he case o o he low-molecula -weigh polyes e s,
such as PCL,
49−51
PEO,
52
and PDEO.
53
I highe molecula
weigh samples could be ob ained, e en ually, di usion ac o s
will ake o e , and he g ow h a e would e en ually dec ease
as he molecula weigh inc eases.
The solid lines epo ed in Figu e 8a a e i s o he Lau i zen
and Ho man heo y.
The Lau i zen−Ho man equa ion is gi en by
54
Ä
Ç
Å
Å
Å
Å
Å
Å
Å
Å
Å
Å
É
Ö
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ä
Ç
Å
Å
Å
Å
Å
Å
Å
Å
Å
Å
Å
É
Ö
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
G G U
R T T
K
T T T
exp ( ) ( )
0
c 0
g
G
m
0c
=
*
whe e G0is a cons an ha includes all e ms ha do no
depend on empe a u e, U*is he anspo ac i a ion ene gy
o he polyme chains di usion (in his s udy, a alue o 1500
cal/mol was employed), Ris he uni e sal gas cons an , Tcis
he c ys alliza ion empe a u e, T0is he empe a u e a which
he mo emen o he chains is ozen, and i is 30 °C deg ees
lowe han he Tg,Tm
0is he equilib ium mel ing empe a u e,
Figu e 5. Mel ing empe a u es as a unc ion o molecula weigh o
PHL samples; he solid ed line is a line o guide he eye.
Figu e 6. PLOM mic og aphs o PHL samples. Mic og aphs we e aken a 25 °C a e mel ing o 1 min a 90 °C and cooling a 20 °C/min.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3261
and is a empe a u e co ec ion ac o , gi en by he
exp ession 2Tc/(Tm
0+Tc). Kg
Gis a cons an p opo ional o
he ene gy ba ie o he sphe uli ic g ow h o seconda y
nuclea ion
Kjb T
k H
g
G 0 e m
0
m
0
=
whe e jassumes he alue o 2 o he Regime II c ys alliza ion,
a egime whe e he seconda y nuclea ion and he sp ead o he
nucleus on he g ow h on a e equi alen ,
55
b0is he chain’s
wid h, σis he la e al su ace ee ene gy, σeis he old su ace
ene gy, kis he Bol zmann cons an and, inally, ΔHm
0is he
equilib ium la en hea o usion. When
Gln U
R T T( )
c 0
+
is
plo ed e sus
T T
1
( )
c
, i is possible o ob ain a s aigh line in
which Kg
Gis he slope, and G0is he in e cep . F om he Kg
G
alue, he σσe alue can be ob ained, and, using he exp ession
H a b0.1 m
0
0 0
=
, whe e a0b0is he chain c oss-sec ional
a ea, i is possible o calcula e he alues o σand σe. Mo eo e ,
i is also possible o calcula e he wo k ha he mac omolecule
does o old as q= 2a0b0σe.
56
The de ailed analysis o he pa ame e s can be ound in
Table S2, whe e di e ences in he Kg
G,σe, and q alues a e
no ed: hese pa ame e s dec ease wi h inc easing molecula
weigh . This indica es ha he ene gy ba ie o he PHL
sphe uli es o g ow dec eases as Mninc eases, in he molecula
weigh ange explo ed in his wo k. A simila beha io has
al eady been obse ed p e iously o PCL samples in a low o
medium molecula weigh ange.
49,51
Figu e 7. PLOM mic og aphs aken a he indica ed Tc alues and a he same supe cooling (ΔT= 5 °C). The a e o cooling om he mel was 20
°C/min.
Figu e 8. Sphe uli ic g ow h a e (G) as a unc ion o : (a) c ys alliza ion empe a u e and (b) molecula weigh a Tc= 54.5 °C o samples PHL
15, PHL 35, PHL 66, and PHL 90. The solid lines in he le g aph a e i s o he Lau i zen and Ho man equa ion.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3262
3.6. O e all C ys alliza ion Kine ics Ob ained by DSC.
The o e all c ys alliza ion p ocess is he ans o ma ion om
he mel o he semic ys alline s a e, including bo h p ima y
nuclea ion and supe s uc u al g ow h. DSC is a con enien
and p ecise echnique o de e mine he o e all iso he mal
c ys alliza ion kine ics. Rep esen a i e DSC iso he ms a e
epo ed in Figu es S6a,b and S7a−d. The expe imen al da a
can be i ed wi h he A ami and he Lau i zen and Ho man
heo y.
57,58
The in e se o he induc ion ime ( 0) is epo ed as a
unc ion o Tcin Figu e 9. The induc ion ime is he ime ha
elapses be o e he DSC de ec s any c ys alliza ion p ocess.
The e o e, he in e se o he induc ion o incuba ion ime is
p opo ional o he p ima y nuclea ion a e (be o e c ys al
g ow h s a s om he c ea ed p ima y nuclei). The p ima y
nuclea ion a e inc eases (i compa ed a a cons an empe -
a u e) as he PHL molecula weigh inc eases. S ill, he
di e ence be ween PHL 66 and PHL 90 is e y small,
indica ing a sa u a ion e ec , which was no obse ed o he
sphe uli ic g ow h a es.
The in e se o he hal c ys alliza ion a e (1/τ50%) is plo ed
in Figu e 10 as a unc ion o Tc(a) and molecula weigh a a
cons an Tc(b). This alue is he in e se o he ime polyme s
need o achie e 50% o hei ela i e c ys allini y du ing an
iso he mal p ocess. I ep esen s an expe imen al alue o he
o e all c ys alliza ion a e. The end is simila o ha seen o
he in e se o he induc ion ime, i.e., samples wi h highe
molecula weigh c ys allize as e and a lowe supe cooling
han samples wi h lowe molecula weigh . Howe e , he
di e ence be ween PHL 66 and PHL 90 is minimal in he
empe a u e ange whe e he expe imen al da a was ga he ed.
Figu e 10b plo s 1/τ50% alues as a unc ion o he molecula
weigh a he same alue as he c ys alliza ion empe a u e.
I should be no ed ha o he PHL 60 and PHL 90 cases,
he alues we e ex apola ed wi h he Lau i zen and Ho man
i s, as no expe imen al da a o hese samples could be
measu ed a Tc= 47 °C. In any case, we ha e ob ained a
simila end o p ima y nuclea ion a e, sphe uli ic g ow h,
and o e all c ys alliza ion a e; as in all cases, hese a es
inc ease wi h PHL molecula weigh , bu hey a e close o
s a ing o sa u a e a he highes molecula weigh alues
employed he e. A simila beha io has al eady been obse ed
in he case o PCL,
49,50
PLLA,
59
and PHB
60
in he low o
medium molecula weigh ange.
The expe imen al da a o he o e all DSC c ys alliza ion a e
was i ed by he A ami equa ion,
61−63
which can be exp essed
as
V k 1 ( ) exp( ( ) )
n
c 0 0
=
In his equa ion, Vc ep esen s he ela i e olume ic
ans o med ac ion, is he expe imen al ime, 0is he
induc ion ime, kis he o e all c ys alliza ion a e cons an , and
Figu e 9. The in e se o induc ion ime (1/ 0) as a unc ion o Tc.
Figu e 10. O e all c ys alliza ion a e (1/τ50%) as a unc ion o (a) c ys alliza ion empe a u e and (b) molecula weigh a Tc= 47 °C, o PHL 15,
PHL 35, PHL 66, and PHL 90. The solid lines in he le g aph a e i s o he Lau i zen and Ho man equa ion.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3263
nis he A ami index, ela ed o he nuclea ion a e and he
g ow h dimensionali y o he c ys als; he alue o ncan
luc ua e be ween 2 and 4 in he case o bulk polyme s. A ami
index alues close o 2 a e co ela ed wi h ins an aneously
nuclea ed axiali es. I n= 3, wo possibili ies exis , ei he
spo adically nuclea ed axiali es o ins an aneously nuclea ed
sphe uli es, and inally, n= 4 indica es he gene a ion o
spo adically nuclea ed sphe uli es. I he nuclea ion is be ween
ins an aneous and spo adic, ac ional alues o he A ami
index can be ob ained.
34
Figu e 11a shows an example o he PHL 35 sample, whe e
he expe imen al da a o he iso he mal DSC is plo ed
oge he wi h i s A ami i , and Figu e 11b shows he ypical
A ami plo in he con e sion ange used o pe o m he i .
The ag eemen be ween he expe imen al da a and he A ami
i is excellen , as in his case, he A ami equa ion desc ibes no
only he p ima y c ys alliza ion ange (which is ypical) bu
also he seconda y c ys alliza ion ange, a e he supe -
s uc u al en i ies (i.e., axiali es mos likely in his case) ha e
impinged on one ano he , a expe imen al imes beyond he
peak alue.
Tables S4−S7 in he SI lis he i ing pa ame e s o all o
he samples employed he e; i is possible o no ice ha he
i ings ob ained a e always excellen wi h co ela ion
coe icien s la ge han 0.999. The expe imen al alues o
τ50% a e also in excellen ag eemen , as expec ed by he quali y
o he i , wi h hose p edic ed by he A ami heo y. Tables
S4−S7 also epo he A ami index, and i is no ed ha alues
be ween 2.2 and 3 we e ob ained. These alues can be
in e p e ed as ep esen ing axiali es whose nuclea ion a ies
om close o ins an aneous (n= 2) o spo adic (n= 3). In he
cases whe e n= 3, he alues could also ep esen sphe uli es
ins an aneously nuclea ed; howe e , acco ding o he PLOM
obse a ions p esen ed in Figu e 8, hey we e no obse ed.
Mo eo e , in his wo k, du ing he iso he mal c ys alliza ion
p ocess, he equilib ium mel ing empe a u e o he ou PHL
samples was calcula ed, and o his pu pose, he Ho man−
Weeks ex apola ion was used. Figu e S7 shows he
ex apola ions ob ained. No mono onic change wi h he
molecula weigh was ob ained, and he equilib ium mel ing
empe a u es a e in he ange o 71−75 °C, which conside ing
he e o o he ex apola ions pe o med, could be ega ded as
simila o one ano he . The a e age Tm0 alue ob ained is close
o 73 °C, simila o some o he alues epo ed o PCL (i.e.,
78 °C).
64
3.7. T ansmission Elec on Mic oscopy (TEM). TEM
analysis was pe o med on he PHL 15 and PHL 35 samples;
he co esponding esul s a e shown in Figu e 12a,b. In he
p esen s udy, he TEM analysis was conduc ed a e s aining,
so he da k a eas a e he amo phous a eas, and he b igh e
a eas a e he c ys alline a eas. The p esence o s aigh and
long lamellae (whi e lines in Figu e 12a,b) is obse ed, and
hei hickness was measu ed manually using ImageJ so wa e.
Figu e S8 shows he hickness dis ibu ion his og ams. Fo
bo h samples, he lamella hickness is e y simila , wi h an
Figu e 11. The A ami i equa ion using he ee O igin plug-in de eloped by Pe ez-Cama go e al.
34
o PHL 35 a Tc= 48 °C.
Figu e 12. TEM mic og aphs aken a oom empe a u e. Be o e c yo-cu ing a −90 °C, he samples we e c ys allized by cooling om he mel a
20 °C/min.
Biomac omolecules pubs.acs.o g/Biomac A icle
h ps://doi.o g/10.1021/acs.biomac.3c00305
Biomac omolecules 2023, 24, 3256−3267
3264
