Ci a ion: Díaz, E.; León, J.;
Mu illo-Ma odán, A.; Ribei o, S.;
Lance os-Méndez, S. In luence o
GO on he C ys alliza ion Kine ics,
Cy oxici y, and Elec ical and
Mechanical P ope ies o Poly
(L-lac ide-co-ε-cap olac one)
Sca olds. Ma e ials 2022,15, 7436.
h ps://doi.o g/10.3390/ma15217436
Academic Edi o : F ancesco Baino
Recei ed: 21 Sep embe 2022
Accep ed: 19 Oc obe 2022
Published: 23 Oc obe 2022
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published maps and ins i u ional a il-
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Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
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ma e ials
A icle
In luence o GO on he C ys alliza ion Kine ics, Cy oxici y,
and Elec ical and Mechanical P ope ies o Poly
(L-lac ide-co-ε-cap olac one) Sca olds
Espe anza Díaz 1,2,* , Joseba León3, Albe o Mu illo-Ma odán3, Syl ie Ribei o 4,5 and
Senen xu Lance os-Méndez 2,6
1
Escuela de Ingenie ía de Bilbao, Depa amen o de Ingenie ía de Mine a, Me alú gica y Ciencia de Ma e iales,
Uni e sidad del País Vasco (UPV/EHU), 48920 Po ugale e, Spain
2BcMa e ials, Basque Cen e o Ma e ials, Applica ions and Nanos uc u es, (UPV/EHU) Science Pa k,
48940 Leioa, Spain
3
Depa men o Mechanics, Design and Indus ial Managemen , Uni e si y o Deus o, A da Uni e sidades 24,
48007 Bilbao, Spain
4Cen o de Física, Uni e sidade do Minho, 4710-058 B aga, Po ugal
5Cen e o Molecula En i onmen al Biology (CBMA), Uni e sidade do Minho, 4710-057 B aga, Po ugal
6Ike basque Basque Founda ion o Science, 48013 Bilbao, Spain
*Co espondence: [email p o ec ed]
Abs ac :
Biodeg adable sca olds o poly (L-lac ide-co-
ε
-cap olac one) (PLCL) and educed g aphene
oxide ( GO) we e p epa ed by TIPS ( he mally induced phase sepa a ion). The noniso he mal cold
c ys alliza ion kine ics we e in es iga ed by di e en ial scanning calo ime y (DSC) wi h a ious
cooling a es. The expe imen al alues indica e ha noniso he mal c ys alliza ion imp o es wi h
cooling a e, bu he inc easing GO concen a ion delays c ys alliza ion a highe empe a u es. The
ac i a ion ene gies we e calcula ed by he Kissinge equa ion; he alues we e e y simila o PLCL
and o i s compounds wi h GO. The elec ical conduc i i y measu emen s show ha he addi ion o
GO leads o a apid ansi ion om insula ing o conduc i e sca olds wi h a pe cola ion alue o
≈
0.4 w/w. Mechanical comp ession es s show ha he addi ion o GO imp o es he mechanical
p ope ies o po ous subs a es. In addi ion, i is an aniso opic ma e ial, especially a composi ions
o 1% w/wo GO. All o he samples wi h di e en GO con en up o 1% a e cy o oxic o C2C12
myoblas cells.
Keywo ds:
PLCL/ GO; sca olds; c ys alliza ion; elec ical p ope ies; mechanical p ope ies;
cy o oxici y
1. In oduc ion
O iginally, only ma e ials capable o wi hs anding mechanical s esses equi alen o
hose o bone ha e been used in bone implan s. Me allic ma e ials, such as s ainless s eels
and i anium alloys, me hese equi emen s as bone ixa ion sys ems, bu hese equi e
cos ly and isky su gical ope a ions o hei implemen a ion and emo al when hey ha e
ul illed hei pu pose [1].
In he las decade, se e al bioma e ials ha e been de eloped ha can suppo he
issue egene a ion p ocess a he de ec si e and a e subsequen ly eso bed and eplaced
wi h he newly gene a ed issue [
2
,
3
]. Many o hese bioma e ials belong o he amily o
alipha ic polyes e s and a e biodeg adable and biocompa ible, such as PLLA [
4
], PLG [
5
],
PCL [
6
], and hei copolyme s PLGA [
7
] and PLCL [
8
]. These polyme s allow ela i ely
easy ab ica ion in he o m o sca olds in which he issues o be epai ed a e implan ed,
a ached, di e en ia ed, and p oli e a ed.
PLCL is a biodeg adable copolyme o PLLA and PCL, belongs o he amily o
alipha ic polyes e s, and i s deg ada ion a es can be manipula ed by a ying he a io
Ma e ials 2022,15, 7436. h ps://doi.o g/10.3390/ma15217436 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2022,15, 7436 2 o 15
o he cons i uen polyme s. I is app o ed by he Uni ed S a es Food and D ug Admin-
is a ion (U.S. FDA) o clinical applica ion [
9
]. The addi ion o GO o hese biopoly-
me s can signi ican ly modi y hei elec ical, physical, and mechanical p ope ies [
4
,
10
].
Qi e al. [4]
s udied he e ec s o he addi ion o GO in PLLA and ound ha he e was an
inc ease in he c ys allini y o PLLA and an imp o emen in i s piezoelec ic p ope ies.
Seyedsalehi e al. [10]
analysed he in luence o he addi ion o Go o PCL and ound ha i
inc eases he s eng h, s i ness, and oughness o he sca olds and p oduces mechanically
esis an s uc u es.
Many g oups ha e s udied he sca olds o PLCL o hei applica ion in egene a i e
medicine [
4
,
6
,
8
,
9
], bu none o he wo ks ha e ca ied ou an in-dep h s udy o noniso he -
mal c ys alliza ion, conduc i i y, and cy o oxici y. This knowledge is essen ial because
many o he polyes e s’ p ocessing ope a ions (ex usion, injec ion molding) ake place
unde noniso he mal condi ions [
11
] and, du ing hese p ocesses, c ys alliza ion is oo
slow o de elop signi ican c ys allini y. The addi ion o some pa icles may, in same cases,
accele a e he a e o c ys alliza ion.
In he li e a u e, we can ind some wo ks in which he e ec o he addi ion o a
nuclea ing agen in he c ys alliza ion o PLLA and PCL is s udied. The addi ion o hese
nuclea ing agen s no only in luences he c ys alliza ion, bu also modi ies he mechanical
and elec ical p ope ies. Qui e al. [
12
] s udied he e ec on he c ys alliza ion o PLLA
when alc is added. Zhou e al. [
13
] and Neja i e al. [
14
] analyzed he c ys alliza ion wi h
nHA. Zhao e al. [
15
] s udied he e ec o he addi ion o CNTs on he mal, mechanical,
and elec ical p ope ies.
In his wo k, he e ec o GO addi ion on c ys alliza ion, conduc i i y, cy o oxici y,
mechanical, and elec ical p ope ies o PLCL sca olds ab ica ed by he mally induced
phase sepa a ion is s udied.
2. Ma e ials and Me hods
Medical g ade Poly(L-lac ide-co-cap olac one) (70/30) was supplied om Pu ac Bio-
ma e ials (Pu aso b PLC 7015, Ams e dam, The Ne he lands). Gel pe mea ion ch oma og-
aphy (GPC, Pe kin Elme 200, T iad Scien i ic, Manasquan, NJ, USA) was used o cal-
cula e he weigh -a e age ela i e molecula weigh : M
w
= 130,489; M
n
= 79759; and
Mw/Mn= 1.63.
2.1. Fab ica ion o Sca olds
The ab ica ion o PLCL and PLCL/ GO composi e sca olds was pe o med by he -
mally induced phase sepa a ion (TIPS) and he eeze–d ying echnique and hey we e
la e dispe sed by sonica ion.
Po ous sca olds we e p epa ed wi h 2.5% (w/ ) o PLCL in 1,4-dioxane. The mix u e
was s i ed a 50
◦
C o 2 h o ob ain a homogeneous polyme solu ion. Reduced g aphene
oxide ( GO) was mixed wi h he solu ion o PLCL in a pe cen age o 0, 0.3, 0.6, and 1 w %
o he o al polyme mass a oom empe a u e. The esul ing solu ions we e ozen and
eeze-d ied (LyoQues o Tels a , Ba celona, Spain) a 60
◦
C and 0.5 mmHg o 10 days o
emo e he sol en . Sca olds wi h a po osi y o up o 90% we e ab ica ed (see Figu e 1).
Ma e ials 2021, 14, x FOR PEER REVIEW 3 o 16
PLCL
PLCL/ GO 0.3 w %
PLCL/ GO 0.6 w %
PLCL/ GO 1 w %
Figu e 1. F om le o igh , samples o pu e PLCL and PLCL/ GO a 0.3, 0.6, and 1 w % GO.
2.2. Scanning Elec on Mic oscopy (SEM)
The sca olds we e coa ed wi h gold, in a JEL Ion Spu e JFC-1100 (Ami on Machin-
e y, Oxna d, CA, USA) a 1200 V and 5 mA. Scanning elec on mic oscopy (SEM) (HITA-
CHI S-3400N, Tokyo, Japan) was used o pe o m PLCL and PLCL/ GO mic og aphs.
2.3. Elec ical Conduc i i y
The elec ical conduc i i y (σ) a 37 °C was measu ed in a sel -buil he mos a cham-
be wi h a digi al mul ime e (SMU-236, Kei hley Ins umen s, Cle eland, OH, USA). The
elec ical conduc i i ies o sca olds we e ob ained wi h a ou -p obe echnique. Fi e spec-
imens we e elec ically es ed om each sample and hei alues we e a e aged.
2.4. Di e en ial Scanning Calo ime y (DSC)
A DSC Q-200 TA Ins umen s calo ime e (Wa e , NC, USA) was used o de e mine
he he mal p ope ies o sca olds. All es s we e pe o med unde ni ogen pu ge o p e-
en oxida ion, and he weigh o he samples a ied be ween 4 and 6 mg. The samples
we e cooled o −40 °C un il eaching he amo phous s a e and hen hea ed o 200 °C and
cooled again o −40 °C.
2.5. Mechanical Cha ac e iza ion
An Ins on 5967 Uni e sal Tes ing Machine (Ins on, No olk Coun y, MA, USA) was
used o cha ac e ize he mechanical p ope ies o po ous suppo s ollowing he ASTM
D695 s anda d. Fo each composi ion, a o al numbe o 10 cylind ical samples was ex-
ac ed om he sca old. Then, a se o uniaxial comp ession es s was pe o med a oom
empe a u e and a cons an speed o 0.5 mm·min−1, using a load cell o 500 N. F om he
es s, he ma e ial elas ic modulus and elas ic limi we e de e mined o each composi ion.
To analyze he mechanical aniso opy, o each composi ion, hal o he samples we e
es ed in he axial di ec ion and he o he hal in he adial di ec ion o he sca old. The
esul s we e ob ained by a e aging hei alues.
2.6. Cy o oxici y
Fo he cy o oxici y assays, sample s e iliza ion is essen ial and, o ha , memb anes
wi h 0.1 g.mL−1 we e cu and s e ilized by UV o 2 h be o e cell seeding (1 h each side).
A e ha , he samples we e washed i e imes wi h phospha e bu e saline (PBS) solu-
ion o 5 min o emo e any esidual sol en . This s udy is based in he indi ec cy o ox-
ici y e alua ion o he samples in adap a ion o he ISO 10993-5 s anda d es me hod.
The medium in con ac wi h he samples (condi ioned medium) was p epa ed by
imme sing he samples in a 24-well issue cul u e polys y ene pla e wi h DMEM (con ain-
ing 4.5 g·L−1 glucose (Gibco, Mad id, Spain) supplemen ed wi h 10% e al bo ine se um
(FBS, Bioch om, Camb idge, UK) and 1% penicillin/s ep omycin (P/S, Bioch om, Cam-
b idge, UK), a 37 °C in a 95% humidi ied ai con aining 5% CO2 and incuba ed o 24 h.
Figu e 1. F om le o igh , samples o pu e PLCL and PLCL/ GO a 0.3, 0.6, and 1 w % GO.
Ma e ials 2022,15, 7436 3 o 15
2.2. Scanning Elec on Mic oscopy (SEM)
The sca olds we e coa ed wi h gold, in a JEL Ion Spu e JFC-1100 (Ami on Machine y,
Oxna d, CA, USA) a 1200 V and 5 mA. Scanning elec on mic oscopy (SEM) (HITACHI
S-3400N, Tokyo, Japan) was used o pe o m PLCL and PLCL/ GO mic og aphs.
2.3. Elec ical Conduc i i y
The elec ical conduc i i y (
σ
) a 37
◦
C was measu ed in a sel -buil he mos a chambe
wi h a digi al mul ime e (SMU-236, Kei hley Ins umen s, Cle eland, OH, USA). The elec-
ical conduc i i ies o sca olds we e ob ained wi h a ou -p obe echnique.
Fi e specimens
we e elec ically es ed om each sample and hei alues we e a e aged.
2.4. Di e en ial Scanning Calo ime y (DSC)
A DSC Q-200 TA Ins umen s calo ime e (Wa e , NC, USA) was used o de e mine he
he mal p ope ies o sca olds. All es s we e pe o med unde ni ogen pu ge o p e en
oxida ion, and he weigh o he samples a ied be ween 4 and 6 mg. The samples we e
cooled o
−
40
◦
C un il eaching he amo phous s a e and hen hea ed o 200
◦
C and cooled
again o −40 ◦C.
2.5. Mechanical Cha ac e iza ion
An Ins on 5967 Uni e sal Tes ing Machine (Ins on, No olk Coun y, MA, USA)
was used o cha ac e ize he mechanical p ope ies o po ous suppo s ollowing he
ASTM D695 s anda d. Fo each composi ion, a o al numbe o 10 cylind ical samples
was ex ac ed om he sca old. Then, a se o uniaxial comp ession es s was pe o med
a oom empe a u e and a cons an speed o 0.5 mm
·
min
−1
, using a load cell o 500 N.
F om he es s, he ma e ial elas ic modulus and elas ic limi we e de e mined o each
composi ion. To analyze he mechanical aniso opy, o each composi ion, hal o he
samples we e es ed in he axial di ec ion and he o he hal in he adial di ec ion o he
sca old. The esul s we e ob ained by a e aging hei alues.
2.6. Cy o oxici y
Fo he cy o oxici y assays, sample s e iliza ion is essen ial and, o ha , memb anes
wi h 0.1 g
·
mL
−1
we e cu and s e ilized by UV o 2 h be o e cell seeding (1 h each side).
A e ha , he samples we e washed i e imes wi h phospha e bu e saline (PBS) solu ion
o 5 min o emo e any esidual sol en . This s udy is based in he indi ec cy o oxici y
e alua ion o he samples in adap a ion o he ISO 10993-5 s anda d es me hod.
The medium in con ac wi h he samples (condi ioned medium) was p epa ed by
imme sing he samples in a 24-well issue cul u e polys y ene pla e wi h DMEM (con aining
4.5 g
·
L
−1
glucose (Gibco, Mad id, Spain) supplemen ed wi h 10% e al bo ine se um (FBS,
Bioch om, Camb idge, UK) and 1% penicillin/s ep omycin (P/S, Bioch om, Camb idge,
UK), a 37
◦
C in a 95% humidi ied ai con aining 5% CO
2
and incuba ed o 24 h. Fu he ,
20% o dime hylsul oxide (DMSO, Sigma Ald ich, S . Louis, MO, USA) was used as a
posi i e con ol and he cell cul u e medium was employed as a nega i e con ol.
In a 96-well issue cul u e polys y ene pla e, C2C12 cells we e seeded a he densi y o
2
×
10
4
cells
·
mL
−1
, a he same ime, and incuba ed o 24 h. A e 24 h, he cul u e medium
om he 96-well issue cul u e polys y ene pla e was emo ed and he condi ioned medium
was added o he wells (100
µ
L). A e wa ds, he cells we e incuba ed o 72 h and, a e
each ime, cell iabili y assessmen was quan i ied wi h he (3-(4,5-dime hyl hiazol-2-yl)-5-
(3-ca boxyme hoxyphenyl)-2-(4-sul ophenyl)-2H- e azolium) (MTS) assay. A his ime,
he MTS eagen was added in o each well (p opo ion 1/5 o MTS/DMEM medium) and
incuba ed a 37
◦
C o 2 h. The abso bance was de ec ed a 490 nm wi h a
mic opla e eade
.
All quan i a i e esul s we e ob ained om ou eplica es o he samples and con ols
and we e analyzed as he a e age o iabili y ±s anda d de ia ion (SD).
Ma e ials 2022,15, 7436 4 o 15
The pe cen age o cell iabili y was calcula ed om he ollowing o mula:
Cell iabili y (%) = Abso bance o sample
Abso bance o neega i e con ol ×100 (1)
3. Resul s and Discussion
3.1. Mo phology
The po osi y and in e connec i i y o he po ous suppo s is a c i ical pa ame e as
i allows cellula in il a ion inside he ma e ial; exchange wi h he medium whe e i is
implan ed; and cellula adhesion, di e en ia ion, and p oli e a ion. I has been ex ensi ely
epo ed ha g ea e po osi y imp o es os eogenesis [11,16–18].
In he mic og aphs shown in Figu e 2, i can be obse ed how he po e size inc eases
wi h he addi ion o GO (20
µ
m o pu e PLCL, 40–60
µ
m o samples wi h 0.3 and
0.6 w %
GO, and be ween 80 and 100
µ
m o hose wi h 1 w %). The e ec o GO on
he mo phology o he sca olds is s iking, as i no only inc eases he po e size, bu also
makes he sca olds mo e sphe oidal and o de ed in shape and he po e walls hicke . This
hickness eaches a maximum alue o he 0.3 w % GO samples and dec eases when
inc easing he g aphene concen a ion. These walls become mo e po ous and less consis en
wi h he addi ion o GO. An ex eme case is ep esen ed by he mic og aphs in Figu e 2d
co esponding o he samples wi h 1 w % GO, in which i can be seen ha he ubula
s uc u e is los , gi ing ise o a e y po ous bu i egula s uc u e wi hou good de ini ion
o he po ous walls. I seems ha , om he mo phological poin o iew, he mos desi able
s uc u e would be achie ed wi h he addi ion o 0.3–0.6 w % GO.
Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 16
(a)
(b)
(c)
(d)
Figu e 2. Rep esen a i e SEM images o he su ace mo phology o PLCL composi es. (a) PLCL ×
250. (b) PLCL/ GO 0.3w % × 250. (c) PLCL/ GO 0.6 w % × 250. (d) PLCL/ GO 1w % × 250.
3.2. Elec ical Conduc i i y S udies
Reduced g aphene oxide has eno mous po en ial as a ein o cing ma e ial in poly-
me ic composi es, as well as imp o ing i s elec ical p ope ies, which enables hem o be
used in a wide ange o applica ions. The key challenge in he p epa a ion o he sca olds
is o ob ain a homogeneous dispe sion o GO, which is incompa ible wi h mos hyd o-
phobic polyme s in which i ends o pile up, hus p e en ing he o ma ion o a con inu-
ous ne wo k o elec on anspo . Fab ica ion echniques a e c i ical, wi h solu ion
blending as well as in si u polyme iza ion being he mos commonly used [20–22].
Figu e 3 shows he a ia ion in elec ical conduc i i y as a unc ion o GO con en .
PLCL is an insula ing polyme , bu when GO is added, he elec ical conduc i i y in-
c eases signi ican ly. The polyme unde goes a apid ansi ion om insula o o a mo e
conduc i e ma e ial ( ypical pe cola ion p ocess). This is due o he o ma ion o mul iple
conduc i e ne wo ks in he educed g aphene oxide. The so-called pe cola ion h eshold
is he c i ical alue a which an ab up ansi ion in conduc i i y occu s wi h a sligh in-
c ease in he concen a ion o he conduc i e cha ge [20,21]. The alue o he pe cola ion
h eshold is low (0.4 w %). This alue is ela ed o he p ope ies o he composi e ma e i-
als; ha is, high elec ical conduc i i y o GO (80 Scm−1), ai ly uni o m dispe sion, and
la ge size o he g aphi e shee s [22,23].
Figu e 2.
Rep esen a i e SEM images o he su ace mo phology o PLCL composi es. (
a
) PLCL
×
250.
(b) PLCL/ GO 0.3w % ×250. (c) PLCL/ GO 0.6 w % ×250. (d) PLCL/ GO 1w % ×250.
Ma e ials 2022,15, 7436 5 o 15
When la ge amoun s o GO (
≥
1%) a e added, he dis ibu ion o GO h oughou he
polyme solu ion is less homogeneous, hus he s uc u e o he sca olds is mo e i egula .
This beha io has also been ound o PLLA sca olds wi h MWCNTs [
19
], whe e he
addi ion o nano ubes a concen a ions o 1 and 5 w % p oduces an o de ed, uni o m
s uc u e. Howe e , a highe le els, he p oduc ion and dispe sion echnique is ine ec i e,
wi h he nano ubes ending o o m agg ega es.
3.2. Elec ical Conduc i i y S udies
Reduced g aphene oxide has eno mous po en ial as a ein o cing ma e ial in polyme ic
composi es, as well as imp o ing i s elec ical p ope ies, which enables hem o be used
in a wide ange o applica ions. The key challenge in he p epa a ion o he sca olds is o
ob ain a homogeneous dispe sion o GO, which is incompa ible wi h mos hyd ophobic
polyme s in which i ends o pile up, hus p e en ing he o ma ion o a con inuous
ne wo k o elec on anspo . Fab ica ion echniques a e c i ical, wi h solu ion blending
as well as in si u polyme iza ion being he mos commonly used [20–22].
Figu e 3shows he a ia ion in elec ical conduc i i y as a unc ion o GO con en .
PLCL is an insula ing polyme , bu when GO is added, he elec ical conduc i i y inc eases
signi ican ly. The polyme unde goes a apid ansi ion om insula o o a mo e conduc i e
ma e ial ( ypical pe cola ion p ocess). This is due o he o ma ion o mul iple conduc i e
ne wo ks in he educed g aphene oxide. The so-called pe cola ion h eshold is he c i ical
alue a which an ab up ansi ion in conduc i i y occu s wi h a sligh inc ease in he
concen a ion o he conduc i e cha ge [
20
,
21
]. The alue o he pe cola ion h eshold is
low (0.4 w %). This alue is ela ed o he p ope ies o he composi e ma e ials; ha is,
high elec ical conduc i i y o GO (80 Scm
−1
), ai ly uni o m dispe sion, and la ge size o
he g aphi e shee s [22,23].
Ma e ials 2021, 14, x FOR PEER REVIEW 6 o 16
Figu e 3. Va ia ion o he elec ical conduc i i y o he composi es wi h inc easing GO concen a-
ion in %.
To in es iga e he dispe sion o he educed g aphene oxide ilms, he conduc i i y
o he sca olds in he pa allel and pe pendicula di ec ion o he po es was s udied. The
esul s can be seen in Table 1. I can be seen ha he conduc i i ies o he composi es con-
aining 0.6 w % GO a e e y simila in he pa allel and pe pendicula di ec ions. I indi-
ca es a homogeneous dispe sion o GO and an iso opic beha io o he ma e ial. How-
e e , o concen a ions o 1 w % o GO, e y di e en alues o conduc i i y a e ob-
se ed in he wo di ec ions s udied, in e ms o o de o magni ude. A he la e concen-
a ion, he ma e ial is aniso opic, wi h di e en p ope ies depending on he di ec ion
s udied. Simila beha io has been ound by Na iman Youse i e al. [22] o polyu e-
hane/ GO nanocomposi es a composi ions o 2 o 5 w %. I can be concluded ha , when
an elec ic ield is applied, elec ons can low in o he sca olds h ough in e connec ed
po e channels, wi h a pe cola ion h eshold alue o 0.4 w %.
Table 1. Elec ical conduc i i y o PLCL/ GO sca olds in he di ec ion pe pendicula o he po es
and in he pa allel di ec ion.
GO Con en (w %)
Conduc i i y (S·cm−1)
Pe pendicula o Po es
Conduc i i y (S·cm−1)
Pa allel o Po es
0
1 × 10−12
1 × 10−12
0.3
2 × 10−6
5 × 10−7
0.6
3.5 × 10−5
8 × 10−5
1
7 × 10−8
9.2 × 10−5
3.3. The mal P ope ies
C ys alline polyme s can c ys allize be ween he glass ansi ion empe a u e (Tg)
and mel ing empe a u e (Tm). Depending on wha he ini ial s a e is, he c ys alliza ion
p ocess can be classi ied in o wo ca ego ies; one would be mel c ys alliza ion, whe e he
polyme mus ini ially emain a a empe a u e abo e hei Tm, and he o he would be
cold c ys alliza ion, which means ha he ini ial s a e is he amo phous s a e and he pol-
yme samples mus emain a a empe a u e lowe han hei Tg25. In his wo k, he c ys-
alliza ion o pu e PLCL and i s composi es wi h 0.3, 0.6, and 1% GO om he amo phous
s a e was s udied.
The samples we e subjec ed o di e en cooling a es o 5, 10, 15, and 20 °C·min−1.
They we e i s cooled down o −40 °C and hen hea ed up o 200 °C and again cooled
down o −40 °C (a di e en cooling a es). Fi s , he e ec o GO on he c ys alliza ion
Figu e 3.
Va ia ion o he elec ical conduc i i y o he composi es wi h inc easing GO concen a ion
in %.
To in es iga e he dispe sion o he educed g aphene oxide ilms, he conduc i i y
o he sca olds in he pa allel and pe pendicula di ec ion o he po es was s udied. The
esul s can be seen in Table 1. I can be seen ha he conduc i i ies o he composi es
con aining 0.6 w % GO a e e y simila in he pa allel and pe pendicula di ec ions. I
indica es a homogeneous dispe sion o GO and an iso opic beha io o he ma e ial.
Howe e , o concen a ions o 1 w % o GO, e y di e en alues o conduc i i y a e
obse ed in he wo di ec ions s udied, in e ms o o de o magni ude. A he la e
concen a ion, he ma e ial is aniso opic, wi h di e en p ope ies depending on he
di ec ion s udied. Simila beha io has been ound by Na iman Youse i e al. [
22
] o
polyu e hane/ GO nanocomposi es a composi ions o 2 o 5 w %. I can be concluded ha ,
Ma e ials 2022,15, 7436 6 o 15
when an elec ic ield is applied, elec ons can low in o he sca olds h ough in e connec ed
po e channels, wi h a pe cola ion h eshold alue o 0.4 w %.
Table 1.
Elec ical conduc i i y o PLCL/ GO sca olds in he di ec ion pe pendicula o he po es
and in he pa allel di ec ion.
GO Con en (w %) Conduc i i y (S·cm−1)
Pe pendicula o Po es
Conduc i i y (S·cm−1)
Pa allel o Po es
01×10−12 1×10−12
0.3 2×10−65×10−7
0.6 3.5 ×10−58×10−5
17×10−89.2 ×10−5
3.3. The mal P ope ies
C ys alline polyme s can c ys allize be ween he glass ansi ion empe a u e (Tg)
and mel ing empe a u e (Tm). Depending on wha he ini ial s a e is, he c ys alliza ion
p ocess can be classi ied in o wo ca ego ies; one would be mel c ys alliza ion, whe e
he polyme mus ini ially emain a a empe a u e abo e hei Tm, and he o he would
be cold c ys alliza ion, which means ha he ini ial s a e is he amo phous s a e and
he polyme samples mus emain a a empe a u e lowe han hei Tg
25
. In his wo k,
he c ys alliza ion o pu e PLCL and i s composi es wi h 0.3, 0.6, and 1% GO om he
amo phous s a e was s udied.
The samples we e subjec ed o di e en cooling a es o 5, 10, 15, and 20
◦
C
·
min
−1
.
They we e i s cooled down o
−
40
◦
C and hen hea ed up o 200
◦
C and again cooled
down o
−
40
◦
C (a di e en cooling a es). Fi s , he e ec o GO on he c ys alliza ion
p ocess was s udied. Figu e 4shows how, o he same cooling a e (15
◦
C
·
min
−1
), he
c ys alliza ion empe a u e eaches highe alues as he GO con en inc eases, excep o
he sample wi h 1 w % GO.
Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 16
p ocess was s udied. Figu e 4 shows how, o he same cooling a e (15 °C·min−1), he c ys-
alliza ion empe a u e eaches highe alues as he GO con en inc eases, excep o he
sample wi h 1 w % GO.
Figu e 4. Noniso he mal cold c ys alliza ion beha io o nea PLCL and i s composi es a 15 °C·min−1
om he amo phous s a e (in he e ical axis, he uni s a e missing).
The e ec o he cooling a e on noniso he mal cold c ys alliza ion is obse ed in Fig-
u e 4, wi h he c ys alliza ion peak becoming wide and mo ing o lowe empe a u es
wi h he inc easing cooling a e o he same GO composi ion. This beha io is obse ed
o all o he composi ions s udied.
Acco ding o he ob ained expe imen al esul s, Table 2 shows how, a all o he
es ed speeds and o he same GO composi ion, he empe a u e o he noniso he mal
c ys alliza ion peak (Tp) dec eases as he cooling speed inc eases. The e alua ion o he
in luence o he GO concen a ion a all o he es ed speeds (Figu e 5) shows ha he Tp
inc eases wi h he addi ion o GO, ega dless o he amoun inco po a ed.
Figu e 5. Noniso he mal cold c ys alliza ion beha io o PLCL/ GO 0.3 w % a di e en cooling
a es.
Figu e 4.
Noniso he mal cold c ys alliza ion beha io o nea PLCL and i s composi es a 15
◦
C
·
min
−1
om he amo phous s a e (in he e ical axis, he uni s a e missing).
The e ec o he cooling a e on noniso he mal cold c ys alliza ion is obse ed in
Figu e 4, wi h he c ys alliza ion peak becoming wide and mo ing o lowe empe a u es
wi h he inc easing cooling a e o he same GO composi ion. This beha io is obse ed
o all o he composi ions s udied.
Ma e ials 2022,15, 7436 7 o 15
Acco ding o he ob ained expe imen al esul s, Table 2shows how, a all o he
es ed speeds and o he same GO composi ion, he empe a u e o he noniso he mal
c ys alliza ion peak (Tp) dec eases as he cooling speed inc eases. The e alua ion o he
in luence o he GO concen a ion a all o he es ed speeds (Figu e 5) shows ha he Tp
inc eases wi h he addi ion o GO, ega dless o he amoun inco po a ed.
Table 2.
Summa y o ele an he mal pa ame e s o nea PLCL and i s nanocomposi es: cooling
a e (
Φ
), noniso he mal peak empe a u e (Tp), c ys alliza ion en halpy (
∆
Hc), and noniso he mal
c ys alliza ion hal - ime ( 1/2).
Samples Φ(◦C/min) Tp (◦C) ∆Hc (J/g) 1/2 (min)
Nea PLCL 5 98.13 20.10 10.15
10 86.31 19.50 5.26
15 76.71 13.52 4.25
20 72.77 5.27 3.46
PLCL/0.3% GO 5 118.38 8.47 13.45
10 110.10 12.53 6.60
15 96.46 13.60 4.42
20 87.54 12.79 3.36
PLCL/0.6% GO 5 118.36 6.55 13.92
10 109.38 13.44 6.87
15 99.87 15.51 4.63
20 95.71 12.27 3.45
PLCL/1% GO 5 103.57 18.95 13.17
10 89.91 19.10 6.74
15 78.92 13.56 4.72
20 71.86 5.591 3.72
Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 16
p ocess was s udied. Figu e 4 shows how, o he same cooling a e (15 °C·min−1), he c ys-
alliza ion empe a u e eaches highe alues as he GO con en inc eases, excep o he
sample wi h 1 w % GO.
Figu e 4. Noniso he mal cold c ys alliza ion beha io o nea PLCL and i s composi es a 15 °C·min−1
om he amo phous s a e (in he e ical axis, he uni s a e missing).
The e ec o he cooling a e on noniso he mal cold c ys alliza ion is obse ed in Fig-
u e 4, wi h he c ys alliza ion peak becoming wide and mo ing o lowe empe a u es
wi h he inc easing cooling a e o he same GO composi ion. This beha io is obse ed
o all o he composi ions s udied.
Acco ding o he ob ained expe imen al esul s, Table 2 shows how, a all o he
es ed speeds and o he same GO composi ion, he empe a u e o he noniso he mal
c ys alliza ion peak (Tp) dec eases as he cooling speed inc eases. The e alua ion o he
in luence o he GO concen a ion a all o he es ed speeds (Figu e 5) shows ha he Tp
inc eases wi h he addi ion o GO, ega dless o he amoun inco po a ed.
Figu e 5. Noniso he mal cold c ys alliza ion beha io o PLCL/ GO 0.3 w % a di e en cooling
a es.
Figu e 5.
Noniso he mal cold c ys alliza ion beha io o PLCL/ GO 0.3 w % a di e en
cooling a es.
The expe imen al esul s ob ained indica e ha he GO concen a ion and cooling a e
s ongly a ec he c ys alliza ion p ocess. Tha is, he highe he cooling a e, he lowe he
Tp and, when GO is added, i inc eases. The e o e, he noniso he mal c ys alliza ion p o-
cess imp o es wi h he cooling a e, bu he inc ease o GO slows down he c ys alliza ion
p ocess a highe empe a u es.
The in eg a ion o he exo he mic peaks e sus empe a u e du ing noniso he mal
c ys alliza ion allows o ob ain he ela i e c ys allini y e sus c ys alliza ion empe a u e
o cooling a es o 5–20
◦
C
·
min
−1
. In Figu e 6, i is obse ed how he c ys alliza ion
empe a u e dec eases wi h he inc easing cooling a e. This beha io has been obse ed
Ma e ials 2022,15, 7436 8 o 15
o all o he samples s udied and indica es ha a highe a e does no slow down he
c ys alliza ion p ocess, bu a he has he opposi e e ec . In ac , he opposi e beha io has
been epo ed in he li e a u e, such as Zhao e al. [
15
] and Wang e al. [
15
,
24
,
25
], al hough
hey we e epo ing on MWCNTs and GO, espec i ely. In o he wo ds, i seems ha he
ype o ein o cemen used has a s ong in luence on he c ys alliza ion p ocess.
Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 16
Table 2. Summa y o ele an he mal pa ame e s o nea PLCL and i s nanocomposi es: cooling
a e (), noniso he mal peak empe a u e (Tp), c ys alliza ion en halpy (Hc), and noniso he mal
c ys alliza ion hal - ime ( 1/2).
Samples
(°C/min)
Tp (°C)
Hc (J/g)
1/2 (min)
Nea PLCL
5
98.13
20.10
10.15
10
86.31
19.50
5.26
15
76.71
13.52
4.25
20
72.77
5.27
3.46
PLCL/0.3% GO
5
118.38
8.47
13.45
10
110.10
12.53
6.60
15
96.46
13.60
4.42
20
87.54
12.79
3.36
PLCL/0.6% GO
5
118.36
6.55
13.92
10
109.38
13.44
6.87
15
99.87
15.51
4.63
20
95.71
12.27
3.45
PLCL/1% GO
5
103.57
18.95
13.17
10
89.91
19.10
6.74
15
78.92
13.56
4.72
20
71.86
5.591
3.72
The expe imen al esul s ob ained indica e ha he GO concen a ion and cooling
a e s ongly a ec he c ys alliza ion p ocess. Tha is, he highe he cooling a e, he
lowe he Tp and, when GO is added, i inc eases. The e o e, he noniso he mal c ys al-
liza ion p ocess imp o es wi h he cooling a e, bu he inc ease o GO slows down he
c ys alliza ion p ocess a highe empe a u es.
The in eg a ion o he exo he mic peaks e sus empe a u e du ing noniso he mal
c ys alliza ion allows o ob ain he ela i e c ys allini y e sus c ys alliza ion empe a u e
o cooling a es o 5–20 °C·min−1. In Figu e 6, i is obse ed how he c ys alliza ion em-
pe a u e dec eases wi h he inc easing cooling a e. This beha io has been obse ed o
all o he samples s udied and indica es ha a highe a e does no slow down he c ys al-
liza ion p ocess, bu a he has he opposi e e ec . In ac , he opposi e beha io has been
epo ed in he li e a u e, such as Zhao e al. [15] and Wang e al. [15,24,25], al hough hey
we e epo ing on MWCNTs and GO, espec i ely. In o he wo ds, i seems ha he ype
o ein o cemen used has a s ong in luence on he c ys alliza ion p ocess.
Figu e 6. Rela i e c ys allini y e sus empe a u e o PLCL/ GO 0.6 w % a di e en cooling a es.
Figu e 6. Rela i e c ys allini y e sus empe a u e o PLCL/ GO 0.6 w % a di e en cooling a es.
The ela ionship be ween he c ys alliza ion ime and he co esponding empe a-
u e Tdu ing he noniso he mal cold c ys alliza ion p ocess is ela ed o he ollowing
exp ession [26]:
=Tc −T
Φ(2)
whe e Tc is he empe a u e a c ys alliza ion ime, To is he ini ial c ys alliza ion empe a-
u e, and
Φ
is he cooling a e. The in eg a ion o he (exo he mic) c ys alliza ion peaks
du ing he noniso he mal c ys alliza ion p ocess allows o ob ain he ela i e c ys allini y
e sus c ys alliza ion ime o cooling a es o 5, 10, 15, and 20
◦
C
·
min
−1
(Figu e 7). I
is obse ed ha he c ys alliza ion ime inc eases wi h he dec easing cooling a e. Fo
example, he PLCL/ GO 1 w % samples needed 7 min o comple e he c ys alliza ion
p ocess a a as cooling a e o 20
◦
C
·
min
−1
. Howe e , 30, 19, and 11 min we e used o
a es o 5, 10, and 15 ◦C·min−1, espec i ely.
Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 16
The ela ionship be ween he c ys alliza ion ime and he co esponding empe a-
u e T du ing he noniso he mal cold c ys alliza ion p ocess is ela ed o he ollowing
exp ession [26]:
𝑡 = 𝑇𝑐 − 𝑇
(2)
whe e Tc is he empe a u e a c ys alliza ion ime, To is he ini ial c ys alliza ion empe -
a u e, and
is he cooling a e. The in eg a ion o he (exo he mic) c ys alliza ion peaks
du ing he noniso he mal c ys alliza ion p ocess allows o ob ain he ela i e c ys allini y
e sus c ys alliza ion ime o cooling a es o 5, 10, 15, and 20 °C·min−1 (Figu e 7). I is
obse ed ha he c ys alliza ion ime inc eases wi h he dec easing cooling a e. Fo ex-
ample, he PLCL/ GO 1 w % samples needed 7 min o comple e he c ys alliza ion p ocess
a a as cooling a e o 20 °C.min−1. Howe e , 30, 19, and 11 min we e used o a es o 5,
10, and 15 °C·min−1, espec i ely.
Figu e 7. Rela i e c ys allini y e sus c ys alliza ion ime o PLCL/ GO 1 w % a di e en cooling
a es.
The mean c ys alliza ion ime ( 1/2) is he ime equi ed o he suppo o acqui e 50%
o he o al c ys allini y and can be ob ained di ec ly om he alues o he ela i e c ys-
allini y e sus c ys alliza ion ime plo s. These alues a e e y impo an o discuss he
noniso he mal cold c ys alliza ion a e o pu e PLCL and PLCL/ GO s udied a di e en
cooling a es. Table 1 shows all 1/2 alues o pu e PLCL and i s h ee composi es wi h
GO a di e en cooling a es. I is obse ed ha 1/2 dec eases wi h he inc easing cooling
a e o bo h pu e PLCL and i s GO composi es, sugges ing ha he a e o gene al noni-
so he mal c ys alliza ion becomes as e wi h he inc easing cooling a e. I is shown ha
he 1/2 alues a e highe in he sca olds made wi h GO han hose o pu e PLCL a a
gi en cooling a e. This end sugges s ha he noniso he mal cold c ys alliza ion beha -
io o PLCL is no enhanced by he p esence o GO owing o he nuclea ion e ec . This
beha io is e y di e en om ha ound by Wang e al. [26] in hei s udies wi h
PLLA/GO, bu i should be no ed ha hey a e dealing wi h GO and no GO, as in he
p esen pape ; in addi ion, a copolyme is used.
To quan i a i ely e alua e he e ec o GO on he c ys alliza ion a e, he CPR pa-
ame e [27–30], which is he c ys alliza ion a e pa ame e o he polyme , has been p o-
posed. The CRP can be calcula ed om he slope o he lines ep esen ing 1/ 1/2 e sus he
cooling a e (see Figu e 8). A highe slope alue would indica e a highe c ys alliza ion
Figu e 7.
Rela i e c ys allini y e sus c ys alliza ion ime o PLCL/ GO 1 w % a di e en
cooling a es.
Ma e ials 2022,15, 7436 9 o 15
The mean c ys alliza ion ime (
1/2
) is he ime equi ed o he suppo o acqui e
50% o he o al c ys allini y and can be ob ained di ec ly om he alues o he ela i e
c ys allini y e sus c ys alliza ion ime plo s. These alues a e e y impo an o discuss he
noniso he mal cold c ys alliza ion a e o pu e PLCL and PLCL/ GO s udied a di e en
cooling a es. Table 1shows all
1/2
alues o pu e PLCL and i s h ee composi es wi h
GO a di e en cooling a es. I is obse ed ha
1/2
dec eases wi h he inc easing cooling
a e o bo h pu e PLCL and i s GO composi es, sugges ing ha he a e o gene al
noniso he mal c ys alliza ion becomes as e wi h he inc easing cooling a e. I is shown
ha he
1/2
alues a e highe in he sca olds made wi h GO han hose o pu e PLCL
a a gi en cooling a e. This end sugges s ha he noniso he mal cold c ys alliza ion
beha io o PLCL is no enhanced by he p esence o GO owing o he nuclea ion e ec .
This beha io is e y di e en om ha ound by Wang e al. [
26
] in hei s udies wi h
PLLA/GO, bu i should be no ed ha hey a e dealing wi h GO and no GO, as in he
p esen pape ; in addi ion, a copolyme is used.
To quan i a i ely e alua e he e ec o GO on he c ys alliza ion a e, he CPR pa am-
e e [
27
–
30
], which is he c ys alliza ion a e pa ame e o he polyme , has been p oposed.
The CRP can be calcula ed om he slope o he lines ep esen ing 1/
1/2
e sus he cooling
a e (see Figu e 8). A highe slope alue would indica e a highe c ys alliza ion a e. The
alues ob ained we e 0.991 o PLCL and 0.985, 0.980, and 0.999 o i s compounds wi h
0.3, 0.6, and 1 w % o GO, espec i ely. The alues ob ained o CRP a e e y simila o
all composi es.
Ma e ials 2021, 14, x FOR PEER REVIEW 10 o 16
a e. The alues ob ained we e 0.991 o PLCL and 0.985, 0.980, and 0.999 o i s com-
pounds wi h 0.3, 0.6, and 1 w % o GO, espec i ely. The alues ob ained o CRP a e
e y simila o all composi es.
Figu e 8. E ec o GO con en on he PLCL c ys alliza ion a e o he ou composi ions s udied.
C ys alliza ion a e pa ame e .
Khanna e al. [31] p oposed o compa e he c ys alliza ion a e o di e en polyme
sys ems using a c ys alliza ion a e coe icien (CRC), which ep esen s a change in he
cooling a e equi ed o p oduce a 1 °C change in he supe cooling o he mel ing polyme .
The CRC can be used as a guide o ank he polyme on a single scale o c ys alliza ion
a es. CRC alues should be highe o as e c ys allizing sys ems. The CRC alue could
be de e mined om he slope o he linea plo o he cooling e sus Tm–Tp, whe e Tm
and Tp ep esen he mel ing poin and maximum noniso he mal c ys alliza ion empe -
a u e, espec i ely. In his wo k, he c ys alliza ion beha io o PLCL and i s composi es
wi h he GO has been s udied om he mel s a e; he e o e, he CRC de e mina ion has
been modi ied using Tp–Tg ins ead o Tm–Tp, whe e Tp and Tg a e he maximum noni-
so he mal c ys alliza ion empe a u e and he glass ansi ion empe a u e, espec i ely,
ep esen ing he change in he cooling a e equi ed o cause a 1 °C change in he supe -
cooling o he mel phase o he polyme (see Figu e 9). The CRC coe icien o he PLCL
sample was 0.999 and ha o i s composi es wi h GO 0.3, 0.6, and 1 w % we e 0.996, 0.977,
and 0.991, espec i ely.
Figu e 8.
E ec o GO con en on he PLCL c ys alliza ion a e o he ou composi ions s udied.
C ys alliza ion a e pa ame e .
Khanna e al. [
31
] p oposed o compa e he c ys alliza ion a e o di e en polyme
sys ems using a c ys alliza ion a e coe icien (CRC), which ep esen s a change in he
cooling a e equi ed o p oduce a 1
◦
C change in he supe cooling o he mel ing polyme .
The CRC can be used as a guide o ank he polyme on a single scale o c ys alliza ion
a es. CRC alues should be highe o as e c ys allizing sys ems. The CRC alue could
be de e mined om he slope o he linea plo o he cooling e sus Tm–Tp, whe e Tm and
Tp ep esen he mel ing poin and maximum noniso he mal c ys alliza ion empe a u e,
espec i ely. In his wo k, he c ys alliza ion beha io o PLCL and i s composi es wi h
he GO has been s udied om he mel s a e; he e o e, he CRC de e mina ion has been
modi ied using Tp–Tg ins ead o Tm–Tp, whe e Tp and Tg a e he maximum noniso he mal
c ys alliza ion empe a u e and he glass ansi ion empe a u e, espec i ely, ep esen ing
he change in he cooling a e equi ed o cause a 1
◦
C change in he supe cooling o he
mel phase o he polyme (see Figu e 9). The CRC coe icien o he PLCL sample was
