polyme s
A icle
No el Poly inyl Alcohol/S a ch Elec ospun Fibe s
as a S a egy o Dispe se Cellulose Nanoc ys als
in o Poly(lac ic acid)
Ca ol López de Dicas illo 1,*, Ka ina Roa 1, Luan Ga ido 1, Alejand o Pe ei a 2and
Ma ia Jose Galo o 1
1Food Packaging Labo a o y (Laben), Depa men o Science and Food Technology, Facul y o Technology,
Cen e o he De elopmen o Nanoscience and Nano echnology (CEDENNA), Uni e sidad de San iago de
Chile (USACH), 9170201 San iago, Chile; [email p o ec ed] (K.R.); [email p o ec ed] (L.G.);
[email p o ec ed] (M.J.G.)
2Facul y o Physics, Cen e o he De elopmen o Nanoscience and Nano echnology (CEDENNA),
Uni e sidad de San iago de Chile (USACH), 9170201 San iago, Chile; [email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +56-2-27184520
Academic Edi o : An onio Pizzi
Recei ed: 18 Janua y 2017; Accep ed: 17 Ma ch 2017; Published: 7 Ap il 2017
Abs ac :
In his wo k, elec ospun ibe s o poly inyl alcohol (PV) and s a ch (ST) we e ob ained
o imp o e dispe sion o cellulose nanoc ys als (CNC) wi hin a poly(lac ic acid) (PLA) ma ix wi h
he aim o enhancing mechanical and ba ie p ope ies. The de elopmen and cha ac e iza ion
o elec ospun ibe s wi h and wi hou CNC, ollowed by hei inco po a ion in PLA a h ee
concen a ions (0.5%, 1% and 3% wi h espec o CNC) we e in es iga ed. Mo phological, s uc u al,
he mal, mechanical and ba ie p ope ies o hese nanocomposi es we e s udied. The pu pose o
his s udy was no only o compa e he p ope ies o PLA nanocomposi es wi h CNC embedded
in o elec ospun ibe s and nanocomposi es wi h eeze-d ied CNC, bu also o s udy he e ec
o elec ospinning p ocess and he inco po a ion o CNC on he PV and s a ch p ope ies. SEM
mic og aphs con i med he homogenous dispe sion o ibe s h ough PLA ma ix. X- ay analysis
e ealed ha he elec ospinning p ocess dec eased he c ys allini y o PV and s a ch. The p esence
o CNC enhanced he he mal s abili y o elec ospun ibe s. Elec ospun ibe s showed an
in e es ing nuclea ing e ec since c ys allini y o PLA was s ongly inc eased. Nanocomposi es
wi h elec ospun ibe s con aining CNC p esen ed sligh ly highe lexibili y and duc ili y wi hou
dec easing ba ie p ope ies.
Keywo ds: elec ospinning; poly inyl alcohol; s a ch; poly(acid lac ic); cellulose nanoc ys als
1. In oduc ion
O e he las se e al yea s, he e o s o imp o e he p ope ies o biodeg adable ma e ials
ha e inc eased due o he e ec s ha esidues o con en ional ma e ials ha e on he en i onmen [
1
].
The in oduc ion o nano echnology in he de elopmen o new ma e ials has opened up a g ea
numbe o possibili ies. Adding nano ille s has p esen ed an in e es ing way o ex end and o imp o e
some aspec s, p incipally mechanical and ba ie p ope ies [
2
]. A nanocomposi e is a mul iphase
composi e whe e a leas one o he phases p esen s nanoscale dimension.
The e a e many
nano ille s
( h ee-dimensional sphe ical and polyhed al, wo-dimensional nano ibe s o one-dimensional
shee -like nanopa icles) ha ha e been s udied, bu du ing he las decade, “cellulose nanoc ys als”
(CNC) ha e a ac ed signi ican in e es in o de o p oduce ully enewable and biodeg adable
nanocomposi es. CNC is a na u al nano ille ob ained om cellulose, a ib ous, ha d and
Polyme s 2017,9, 117; doi:10.3390/polym9040117 www.mdpi.com/jou nal/polyme s
Polyme s 2017,9, 117 2 o 16
wa e -insoluble subs ance ha plays an essen ial ole in main aining he s uc u e o plan cell walls.
The mul iple ne wo ks be ween cellulose chains h ough hyd ogen bonding cons i u e cellulosic ib ils
ha ha e highly o de ed (c ys alline) and uno de ed (amo phous) egions. Amo phous egions can
be selec i ely hyd olyzed h ough acid hyd olysis, ob aining nanosized c ys alline egions called
nanocellulose o “cellulose nanoc ys als” [
3
,
4
]. As compa ed o ino ganic ein o cing ille s, CNC ha e
many addi ional ad an ages including wide a ailabili y o sou ces, low-ene gy consump ion, ease o
ecycling by combus ion, high aspec a io and good mechanical p ope ies [
5
,
6
]. Se e al s udies ha e
associa ed he inco po a ion o CNC wi h imp o emen s in dynamic mechanical he mal p ope ies,
ensile s eng h, oughness and elonga ion a b eak [
7
–
10
]. Ne e heless, one o he main di icul ies
associa ed wi h he use o CNC as ein o cing agen s is i s high hyd ophilici y and s ong hyd ogen
bond in e ac ions, which make i di icul o dispe se in hyd ophobic media, including mos widely
esea ched he moplas ic biopolyme s, such as poly(lac ic acid), PLA. Al hough PLA is one o he mos
popula bio-based plas ics and inds wide indus ial use nowadays, his biopolyes e s ill p esen s
some d awbacks as low he mal esis ance, excessi e b i leness and high oxygen pe meabili y. These
issues a e mainly due o hei low c ys allizing na u e, and di e en p ocesses ha e been employed o
imp o e hese p ope ies [
11
]. Thus, some s a egies ha e been de ised in o de o imp o e dispe sion
o CNC in o polyme ma ices, such as g a ing and chemical su ace modi ica ion o CNC, mas e ba ch
in si u polyme iza ion, he use o su ac an s and pa ial silyla ion [
12
–
16
]. Ne e heless, mos o hese
modi ica ions a e complica ed p ocesses, and esul s ha e demons a ed ha he modi ied CNC ha e
less ein o cing e ec s. In his wo k, he al e na i e p oposed was he inco po a ion o CNC in o
poly(acid lac ic), PLA, by means o elec ospinning wi h poly inyl alcohol, PV, and s a ch, ST. The
p incipal aim o his wo k was he enhancemen o mechanical and ba ie p ope ies o PLA h ough
he de elopmen and he s udy o hese elec ospun ibe s as an e icien s a egy o success ully
dispe se CNC in o his hyd ophobic biopolyme and compa e he p ope ies o hese ma e ials wi h
PLA nanocomposi es wi h eeze-d ied CNC.
Elec ospinning is an economical, simple and e sa ile echnique o deposi polyme ibe s
wi h dimensions om mic ome e s down o nanome e s on o a a ge using an elec ic ield o
egula e he ejec ion o he polyme ic luid je om he sy inge [
17
]. Poly inyl alcohol (PV) and
s a ch (ST) we e selec ed due o hei hyd ophilic and wa e soluble na u e. PV is a semi-c ys alline
polyme wi h good chemical and he mal s abili y. Elec ospun ibe s p oduced om PV and mix u es
wi h o he polyme s ha e been widely s udied o e he pas ew yea s because o i s non- oxic and
biodeg adabili y [
18
,
19
]. On he o he hand, s a ch is among he mos abundan and inexpensi e
biopolyme , since i is ound in plan issues, such as lea es, s ems, seeds and oo s. I is composed
o epea ing glucose monome s, and is ound in i s linea o m as amylose, and in a b anched o m
as amylopec in. Howe e , pu e s a ch lacks he s eng h, wa e esis ibili y, he mal s abili y and
p ocessabili y. Se e al a emp s ha e been made o ab ica e s a ch ibe s, such as p ocessing only
amylose ibe s, u ilizing modi ied s a ches, changing sol en s, o including plas icize s, c osslinke
esin o o he polyme s [
20
–
22
].
The combina ion
PV-ST is ideal because PV is usually modi ied wi h
o he polyme s, such as s a ch,
o imp o e
i s biodeg adabili y and pe o mance. Addi ionally, ew
wo ks ha e cen e ed hei a en ion on he elec ospinning o PV and ST [
23
,
24
]. Se e al wo ks ha e
used his echnique wi h he pu pose o imp o e physical p ope ies o di e en polyme s. Na ayanan
e al. s udies ha e shown ha he ein o cemen o poly(
ε
-cap olac one) wi h cyclodex in esul ed in
imp o emen s in mechanical and he mal p ope ies [
25
–
27
]. Ma inez-Sanz e al. [
28
] ha e al eady
inco po a ed bac e ial cellulose nanoc ys als in o PLA h ough PLA elec ospun ibe s esul ing on
ma e ials wi h highe alues o ensile s eng h and elas ic modulus, bu lowe elonga ion and ba ie
p ope ies. In his wo k, he ad an age by using hyd opillic polyme s, PV and s a ch, is o a oid he
need o eeze-d ying he CNC solu ion, which is one o he longes and mos ene ge ically cos ly
p ocess in he CNC ob aining p ocedu e. In addi ion o compa e he p ope ies o PLA nanocomposi es
wi h CNC embedded in o PV/ST elec ospun nano ibe s and nanocomposi es wi h di ec addi ion o
Polyme s 2017,9, 117 3 o 16
eeze-d ied CNC, he s udy o he e ec o elec ospinning p ocess and he inco po a ion o CNC in
he PV and s a ch p ope ies was also in es iga ed.
2. Ma e ials and Me hods
2.1. Ma e ials and Nano ein o cemen s
2.1.1. Polyme s and Chemicals
Poly(lac ic acid) (PLA), 2003D (speci ic g a i y
1
4
1.24; MFR g/10 min (210
◦
C, 2.16 kg)) was
pu chased om Na u ewo ks
®
Co. (Minne onka, MN, USA). Gohsenol ype AH-17 poly inyl alcohol
(PV) (saponi ica ion deg ee 97%–98.5% and iscosi y 25–30 mPa s) was ob ained om he Nippon
Syn he ic Chemical Co. (Osaka, Japan). S a ch (ST), cellulose ibe s (CF) (powde 80–145
µ
m), and
polye hylene glycol (PEG) we e supplied by Sigma Ald ich (San iago, Chile). Chlo o o m and sul u ic
acid 95%–97% we e supplied by Me ck (San iago, Chile). Low- low PES (polye he sul one) 170 dialysis
memb anes (35
µ
m hickness, 20.000 Da po e size) we e pu chased om he Nip o Medical Co po a ion
(San iago, Chile).
2.1.2. P epa a ion o Cellulose Nanoc ys al Solu ion
Ten g ams o cellulose ibe s (CF) was mixed wi h 50 mL o deionized wa e and pu in an ice ba h
and s i ed while 50 mL o concen a ed sul u ic acid we e added d opwise un il he solu ion achie ed
9 M concen a ion. The suspension was hen hea ed a 45
◦
C and s i ed o 120 min, ollowed by he
addi ion o wa e o s op he hyd olysis. The esul ing mix u e was cen i uged a 4000 pm o 20 min,
and he clea supe na an con aining acid esidues and amo phous egions o he cellulose ibe was
emo ed. Subsequen ly, successi e washings we e pe o med adding 50 mL o dis illed wa e and he
ubes we e shaken again and cen i uged a 4000 pm o 12 min. This ope a ion was epea ed un il
he supe na an was a u bid suspension con aining he CNC [
29
,
30
]. The suspension ob ained was
dialyzed un il he washing wa e main ained a cons an pH. A known olume o he p e ious CNC
suspension was eeze-d ied o calcula e he concen a ion o CNC and o ob ain d y CNC o cas CNC
nanocomposi es used as con ol nanocomposi es.
2.1.3. Elec ospun PV/S a ch Nano ibe s
CNC solu ion (CNC-A) ob ained om dialysis was concen a ed h ough e apo a ion un il a
inal concen a ion o 2% (w/ ) (CNC-B) wi h he pu pose o achie e a high inco po a ion deg ee o
CNC in o he elec ospun ibe s and lowes in luence o PV/s a ch polyme s in he blends wi h PLA.
Fu he mo e, 1.6 g PV and 0.6 g s a ch we e added o 20 mL o CNC-B solu ion and s i ed a 90
◦
C
un il polyme s we e dissol ed. In o de o s udy he inco po a ion o CNC in o he ibe , PV/s a ch
solu ion a he same concen a ion wi hou CNC was also p epa ed o be elec ospun. Solu ions we e
ans e ed o 5 mL plas ic sy inges and connec ed h ough a PTFE (poly e a luo oe hylene) ube o an
18-gauge blun s ainless s eel needle cha ged by a high ol age powe supply wi h a ange
o 0–30 kV.
The collec o pla e was ixed a a wo king dis ance o 8.5 cm below he needle ip and connec ed o he
g ounded coun e elec ode o he powe supply. A ol age o app oxima ely 15 kV and a low a e o
1.5 mL/h we e used. CNC con aining PV/s a ch ibe s we e named “(PVST/CNC)
“ and PV/s a ch
nano ibe s “(PVST) ”.
2.2. PLA Nanocomposi e P epa a ion
PLA based ilms we e ob ained by solu ion-ex ension-e apo a ion p ocess (“cas ing”).
Elec ospun ibe s (PVST/CNC)
we e mixed wi h PLA solu ion in chlo o o m in o de o ob ain
blends wi h a inal concen a ion o 0.5, 1 and 3 w % CNC espec PLA weigh ( ilm codes:
“0.5PLA(PVST/CNC) , 1PLA(PVST/CNC) and 3PLA(PVST/CNC) ”, espec i ely). PEG was added
a 5% (w/wpolyme ) o all o mula ions o acili a e he cas ing p ocess. Two di e en con ol
Polyme s 2017,9, 117 4 o 16
nanocomposi e ilms we e also cas ed: (i) wi h elec ospun ibe s (PVST)
owing o cla i y i he
di e ences on ma e ial p ope ies we e due o he ibe s o o he p esence o CNC: “0.5PLA(PVST)
,
1PLA(PVST)
and 3PLA(PVST)
”; and (ii) wi h lyophilized CNC: “0.5PLACNC, 1PLACNC and
3PLACNC”) in o de o e i y he hypo he ical imp o emen expec ed due o he encapsula ion
o nanocellulose in o hese biodeg adable elec ospun ibe s. Table 1shows he con en o each
componen (in %) o de elop e e y ilm in o de o each CNC concen a ions o 0.5%, 1% and 3%, and
he co esponding con ol ilms. PLA blank (only wi h PEG) was also cas ed and named “PLA”.
Table 1. Composi ion (%) o de eloped ilms.
Film Samples PLA (PVST/CNC) (PVST) CNC
PLA 100 - - -
0.5PLA(PVST/CNC) 96.75 3.25 - -
0.5PLA(PVST) 97.25 - 2.75 -
0.5PLACNC 99.5 - - 0.5
1PLA(PVST/CNC) 93.5 6.5 - -
1PLA(PVST) 94.5 - 5.5 -
1PLACNC 99 - - 1.0
3PLA(PVST/CNC) 80.5 19.5 - -
3PLA(PVST) 83.5 - 16.5 -
3PLACNC 97 - - 3.0
Abb e ia ions: PV: poly inyl alcohol; ST: s a ch; PLA: poly(acid lac ic); CNC: cellulose nanoc ys al; (PVST) :
elec ospun nano ibe s; (PVST/CNC) : elec ospun nano ibe s con aining CNC.
2.3. Scanning Elec onic Mic oscopy (SEM) Analysis
The mo phology o he elec ospun ibe s (PVST/CNC)
and (PVST)
and he nanocomposi es
we e s udied using a scanning elec on mic oscope (SEM) JSM-5410 Jeol (Tokyo, Japan) wi h
accele a ing ol age a 10 kV. Films we e ac u a ed using a Tensile Tes e because i was no possible
o ob ain he samples h ough c yo- ac u e. Then, samples we e coa ed wi h gold palladium using
a Spu e ing Sys em Humme 6.2., and SEM mic og aphs o he su ace and he c oss-sec ion o he
ma e ials we e aken.
2.4. X- ay Di ac ion (XRD)
S uc u es o CNC, elec ospun ibe s and PLA nanocomposi es we e e alua ed wi h X- ay
di ac ion (XRD). XRD pa e ns we e measu ed using a Siemens Di ac ome e D5000 (Siemens AG,
E langen, Ge many) (30 mA and 40 kV) using CuKa (
λ
= 1.54 Å) adia ion a oom empe a u e.
All scans we e pe o med in a 2θ ange 2◦–12◦a 0.02◦/s.
2.5. The mal P ope ies
The mog a ime ic analyses (TGA) o CNC, elec ospun ibe s and PLA composi es we e ca ied
ou using a Me le Toledo Gas Con olle GC20 S a e Sys em (Schwe zenbach, Swi ze land) TGA/DCS.
Samples (ca. 9 mg) we e hea ed om 20 o 600
◦
C a 10
◦
C min
−1
unde ni ogen a mosphe e ( low
a e 50 mL min−1).
Di e en ial Scanning Calo ime y (DSC) analyses we e also pe o med wi h a Me le Toledo
DSC-822e calo ime e (Schwe zenbach, Swi ze land). The mog ams we e ob ained om
−
20 o
220 ◦C,
cooling o
−
20
◦
C, and a second hea ing p ocess o 220
◦
C wi h 10
◦
C min
−1
hea ing a e. Sample
weigh was abou 8–10 mg. The deg ee o c ys allini y (X
c
) o he PLA ma e ials was deduced using
Equa ion (1):
Xc= % c ys allini y o PLA = 100 ×[(∆Hm−∆Hcc)/∆H0m], (1)
whe e
∆
H
m
is he speci ic mel ing en halpy o he sample (J g
−1
);
∆
H
cc
is he speci ic cold c ys alliza ion
en halpy o he sample (J g
−1
) and
∆
H
0m
is he speci ic mel ing en halpy o a wholly c ys alline PLA
(93.1 J g−1) [31].
Polyme s 2017,9, 117 5 o 16
2.6. Op ical P ope ies
Films opaci y measu emen s we e pe o med acco ding o he me hod o Pa k e al. [
32
]. Films
we e cu in o ec angula shapes (9 mm
×
30 mm) and placed inside he spec opho ome e cell a
600 nm. Fi e eplica es o each ilm we e es ed. The opaci y o he ilms we e calcula ed ollowing
Equa ion (2):
O=
Abs600
X(2)
whe e Ois he opaci y, Abs600 is he alue o abso bance a 600 nm and Xis he ilm hickness (mm).
2.7. Tensile Tes ing
Tensile es ing o each ma e ial was measu ed using a Zwick Roell model (Ulm, Ge many) BDOFB
0.5 TH Tensile Tes e , acco ding o ASTM D-882 (Ame ican Socie y o Tes ing and Ma e ials o Tensile
p ope ies o Thin Plas ic Shee ing). S ips (10 cm
×
2.5 cm) o ilms we e cu using a die cu e and
kep a 25
◦
C and 50% RH ( ela i e humidi y) o 48 h be o e he es . Analyses we e ca ied ou wi h a
1 kN load cell. The ini ial g ip sepa a ion was 10 cm and he c osshead speed used was 50 mm min
−1
.
Resul s a e he a e age o 8 specimens o each ilm.
2.8. Oxygen Pe meabili y
The oxygen pe mea ion a es o he PLA ma e ials we e de e mined a 0% RH and 23
◦
C using
an Ox an model 2/21 ML Mocon (Lippke, Neuwied, Ge many). Films we e p e iously pu ged wi h
ni ogen o a minimum o 16 h in he RH desi ed, p io o exposu e o an oxygen low o 10 mL/min.
Pe mea ion alues we e de e mined e e y 45 min un il cons an .
2.9. S a is ical Analysis
A andomized expe imen al design was conside ed o he expe imen s. Da a analysis was
ca ied ou using S a g aphics Plus 5.1 (S a Poin Inc., He ndon, VA, USA). This so wa e was used o
implemen a iance analysis and Fishe ’s LSD (Leas Signi ican Di e ence) es . Di e ences we e
conside ed signi ican a p< 0.05.
3. Resul s and Discussion
3.1. Mo phological Resul s o Nanos uc u es and Nanocomposi es
Elec ospun ibe s (PVST/CNC) we e success ully ob ained wi h a inal composi ion o 15.38%,
61.54% and 23.27% (w/w) o CNC, PV and ST, espec i ely. Con ol elec ospun ibe s had a
composi ion o 72.73% and 27.27% o PV and s a ch, espec i ely. As an example, Figu e 1A shows he
image o he ob ained ibe ma elec ospun (PVST/CNC)
, since, o bo h ibe s, he image was he
same. The a e age sample diame e was 10 cm. As i was al eady men ioned, he low a e used was
1.5 mL/h, ob aining a inal p oduc ion a e o 0.165 and 0.195 g/h o ibe s (PVST)
and (PVST/CNC)
,
espec i ely. The dis ibu ion o ibe diame e s’ measu emen s ob ained om elec ospun ibe s
(PVST) and (PVST/CNC) a e p esen ed as his og ams in Figu e 1B,C, espec i ely.
SEM mic oscopy was a use ul ool o obse e he mo phology o he elec ospun ibe s (PVST)
and (PVST/CNC)
and he esul ing PLA nanocomposi es. As Figu e 2shows, i was possible o
ob ain homogeneous ibe s wi h a e age diame e s o (211.8
±
18.6) nm and (160.4
±
18.4) nm o
(PVST/CNC)
and (PVST)
, espec i ely. The inco po a ion o cellulose nanoc ys als sligh ly enhanced
he size o esul ing ibe s. As i was al eady obse ed in o he s udies, CNC could ha e been aligned
along he ibe axis unde he elec ical ield p oduced du ing elec ospinning p ocess [33,34].

Polyme s 2017,9, 117 6 o 16
Polyme s 2017, 9, 117 5 o 16
600 nm. Fi e eplica es o each ilm we e es ed. The opaci y o he ilms we e calcula ed ollowing
Equa ion (2):
=


 (2)
whe e O is he opaci y, Abs600 is he alue o abso bance a 600 nm and X is he ilm hickness (mm).
2.7. Tensile Tes ing
Tensile es ing o each ma e ial was measu ed using a Zwick Roell model (Ulm, Ge many)
BDOFB 0.5 TH Tensile Tes e , acco ding o ASTM D-882 (Ame ican Socie y o Tes ing and
Ma e ials o Tensile p ope ies o Thin Plas ic Shee ing). S ips (10 cm × 2.5 cm) o ilms we e cu
using a die cu e and kep a 25 °C and 50% RH ( ela i e humidi y) o 48 h be o e he es . Analyses
we e ca ied ou wi h a 1 kN load cell. The ini ial g ip sepa a ion was 10 cm and he c osshead speed
used was 50 mm min−1. Resul s a e he a e age o 8 specimens o each ilm.
2.8. Oxygen Pe meabili y
The oxygen pe mea ion a es o he PLA ma e ials we e de e mined a 0% RH and 23 °C using
an Ox an model 2/21 ML Mocon (Lippke, Neuwied, Ge many). Films we e p e iously pu ged wi h
ni ogen o a minimum o 16 h in he RH desi ed, p io o exposu e o an oxygen low o 10 mL/min.
Pe mea ion alues we e de e mined e e y 45 min un il cons an .
2.9. S a is ical Analysis
A andomized expe imen al design was conside ed o he expe imen s. Da a analysis was
ca ied ou using S a g aphics Plus 5.1 (S a Poin Inc., He ndon, VA, USA). This so wa e was used
o implemen a iance analysis and Fishe ’s LSD (Leas Signi ican Di e ence) es . Di e ences we e
conside ed signi ican a p < 0.05.
3. Resul s and Discussion
3.1. Mo phological Resul s o Nanos uc u es and Nanocomposi es
Elec ospun ibe s (PVST/CNC) we e success ully ob ained wi h a inal composi ion o 15.38%,
61.54% and 23.27% (w/w) o CNC, PV and ST, espec i ely. Con ol elec ospun ibe s had a
composi ion o 72.73% and 27.27% o PV and s a ch, espec i ely. As an example, Figu e 1A shows
he image o he ob ained ibe ma elec ospun (PVST/CNC) , since, o bo h ibe s, he image was
he same. The a e age sample diame e was 10 cm. As i was al eady men ioned, he low a e used
was 1.5 mL/h, ob aining a inal p oduc ion a e o 0.165 and 0.195 g/h o ibe s (PVST) and
(PVST/CNC) , espec i ely. The dis ibu ion o ibe diame e s’ measu emen s ob ained om
elec ospun ibe s (PVST) and (PVST/CNC) a e p esen ed as his og ams in Figu e 1B,C,
espec i ely.
Figu e 1. (A) mac oscopic pho og aph o (PVST/CNC) ma ; (B) his og am o ibe diame e o
(PVST) ; and (C) his og am o ibe diame e o (PVST/CNC) .
Figu e 1.
(
A
) mac oscopic pho og aph o (PVST/CNC)
ma ; (
B
) his og am o ibe diame e o (PVST)
;
and (C) his og am o ibe diame e o (PVST/CNC) .
Polyme s 2017, 9, 117 6 o 16
SEM mic oscopy was a use ul ool o obse e he mo phology o he elec ospun ibe s (PVST)
and (PVST/CNC)
and he esul ing PLA nanocomposi es. As Figu e 2 shows, i was possible o
ob ain homogeneous ibe s wi h a e age diame e s o (211.8 ± 18.6) nm and (160.4 ± 18.4) nm o
(PVST/CNC)
and (PVST)
, espec i ely. The inco po a ion o cellulose nanoc ys als sligh ly
enhanced he size o esul ing ibe s. As i was al eady obse ed in o he s udies, CNC could ha e
been aligned along he ibe axis unde he elec ical ield p oduced du ing elec ospinning p ocess
[33,34].
Figu e 2. Mo phology o elec ospun ibe s: (A) (PVST/CNC)
, 10,000×; (B) (PVST/CNC)
, 40,000×;
(C) (PVST)
, 10,000×; (D) (PVST)
, 40,000×.
Scanning elec on mic oscopy was also used o obse e he mo phology o de eloped ma e ials
and he dis ibu ion o he ibe s and CNC in o he PLA ma ix. Mic og aphs o ma e ial su aces
and c oss sec ions o blank PLA and he ma e ials wi h maximum concen a ion o ibe s and CNC,
as an example, a e p esen ed in Figu e 3. PLA blank exhibi ed a smoo h su ace and appa en ly a
compac and homogeneous s uc u e (Figu e 3A1,A2). Mic og aphs o PLA wi h 3% o
(PVST/CNC)
, (Figu e 3B1,B2), showed clea ly he homogenous dis ibu ion o ibe s along he
hickness o he ilm, while ilms wi h ibe s wi hou CNC showed some agglome a ions and a
he e ogeneous ibe dis ibu ion (Figu e 3C1,C2). Films ein o ced wi h elec ospun ibe s a low
concen a ion had a smoo h su ace, bu , a highe concen a ions, mainly a 3% (w/w), composi es
p esen ed a ough su ace, al hough ma e ials wi h CNC con aining ibe s we e isually mo e
uni o m.
Meanwhile, as Figu e 3D1,D2 shows, nanocomposi es wi h CNC p esen ed a smoo h su ace
simila o a blank sample, al hough i was possible o obse e ce ain agglome a ions when CNC
concen a ion inc eased. Thus, mic og aphs con i med ha he dispe sion o hese nano ille s was
imp o ed when hey we e embedded in he elec ospun ibe s.
Figu e 3. SEM images o PLA composi es: (A) PLA nea a : (A1) 2000×; (A2) 4000×; (B)
3PLA(PVST/CNC)
a : (B1) 4000×; (B2) 10,000×; (C) 3PLA(PVST)
a (C1) 2000×; (C2) 4000×; and (D)
3PLACNC a (D1) 2000×; (D2) 10,000×.
3.2. X- ay Analysis Resul s
X- ay di ac ion (XRD) pa e ns o ille s, CNC and elec ospun ibe s (PVST)
and
(PVST/CNC)
, and PLA composi es a highes concen a ion o ille s, as an example, a e plo ed in
Figu e 4A–C. Di ac ion pa e n o CNC exhibi ed a sha p peak a 2θ = 22.34°, co esponding o he
Figu e 2.
Mo phology o elec ospun ibe s: (
A
) (PVST/CNC)
, 10,000
×
; (
B
) (PVST/CNC)
, 40,000
×
;
(C) (PVST) , 10,000×; (D) (PVST) , 40,000×.
Scanning elec on mic oscopy was also used o obse e he mo phology o de eloped ma e ials
and he dis ibu ion o he ibe s and CNC in o he PLA ma ix. Mic og aphs o ma e ial su aces
and c oss sec ions o blank PLA and he ma e ials wi h maximum concen a ion o ibe s and CNC,
as an example, a e p esen ed in Figu e 3. PLA blank exhibi ed a smoo h su ace and appa en ly a
compac and homogeneous s uc u e (Figu e 3A1,A2). Mic og aphs o PLA wi h 3% o (PVST/CNC)
,
(Figu e 3B1,B2), showed clea ly he homogenous dis ibu ion o ibe s along he hickness o he
ilm, while ilms wi h ibe s wi hou CNC showed some agglome a ions and a he e ogeneous ibe
dis ibu ion (Figu e 3C1,C2). Films ein o ced wi h elec ospun ibe s a low concen a ion had a
smoo h su ace, bu , a highe concen a ions, mainly a 3% (w/w), composi es p esen ed a ough
su ace, al hough ma e ials wi h CNC con aining ibe s we e isually mo e uni o m.
Meanwhile, as Figu e 3D1,D2 shows, nanocomposi es wi h CNC p esen ed a smoo h su ace
simila o a blank sample, al hough i was possible o obse e ce ain agglome a ions when CNC
concen a ion inc eased. Thus, mic og aphs con i med ha he dispe sion o hese nano ille s was
imp o ed when hey we e embedded in he elec ospun ibe s.
Polyme s 2017, 9, 117 6 o 16
SEM mic oscopy was a use ul ool o obse e he mo phology o he elec ospun ibe s (PVST)
and (PVST/CNC)
and he esul ing PLA nanocomposi es. As Figu e 2 shows, i was possible o
ob ain homogeneous ibe s wi h a e age diame e s o (211.8 ± 18.6) nm and (160.4 ± 18.4) nm o
(PVST/CNC)
and (PVST)
, espec i ely. The inco po a ion o cellulose nanoc ys als sligh ly
enhanced he size o esul ing ibe s. As i was al eady obse ed in o he s udies, CNC could ha e
been aligned along he ibe axis unde he elec ical ield p oduced du ing elec ospinning p ocess
[33,34].
Figu e 2. Mo phology o elec ospun ibe s: (A) (PVST/CNC)
, 10,000×; (B) (PVST/CNC)
, 40,000×;
(C) (PVST)
, 10,000×; (D) (PVST)
, 40,000×.
Scanning elec on mic oscopy was also used o obse e he mo phology o de eloped ma e ials
and he dis ibu ion o he ibe s and CNC in o he PLA ma ix. Mic og aphs o ma e ial su aces
and c oss sec ions o blank PLA and he ma e ials wi h maximum concen a ion o ibe s and CNC,
as an example, a e p esen ed in Figu e 3. PLA blank exhibi ed a smoo h su ace and appa en ly a
compac and homogeneous s uc u e (Figu e 3A1,A2). Mic og aphs o PLA wi h 3% o
(PVST/CNC)
, (Figu e 3B1,B2), showed clea ly he homogenous dis ibu ion o ibe s along he
hickness o he ilm, while ilms wi h ibe s wi hou CNC showed some agglome a ions and a
he e ogeneous ibe dis ibu ion (Figu e 3C1,C2). Films ein o ced wi h elec ospun ibe s a low
concen a ion had a smoo h su ace, bu , a highe concen a ions, mainly a 3% (w/w), composi es
p esen ed a ough su ace, al hough ma e ials wi h CNC con aining ibe s we e isually mo e
uni o m.
Meanwhile, as Figu e 3D1,D2 shows, nanocomposi es wi h CNC p esen ed a smoo h su ace
simila o a blank sample, al hough i was possible o obse e ce ain agglome a ions when CNC
concen a ion inc eased. Thus, mic og aphs con i med ha he dispe sion o hese nano ille s was
imp o ed when hey we e embedded in he elec ospun ibe s.
Figu e 3. SEM images o PLA composi es: (A) PLA nea a : (A1) 2000×; (A2) 4000×; (B)
3PLA(PVST/CNC)
a : (B1) 4000×; (B2) 10,000×; (C) 3PLA(PVST)
a (C1) 2000×; (C2) 4000×; and (D)
3PLACNC a (D1) 2000×; (D2) 10,000×.
3.2. X- ay Analysis Resul s
X- ay di ac ion (XRD) pa e ns o ille s, CNC and elec ospun ibe s (PVST)
and
(PVST/CNC)
, and PLA composi es a highes concen a ion o ille s, as an example, a e plo ed in
Figu e 4A–C. Di ac ion pa e n o CNC exhibi ed a sha p peak a 2θ = 22.34°, co esponding o he
Figu e 3.
SEM images o PLA composi es: (
A
) PLA nea a : (
A1
) 2000
×
; (
A2
) 4000
×
;
(
B
) 3PLA(PVST/CNC)
a : (
B1
) 4000
×
; (
B2
) 10,000
×
; (
C
) 3PLA(PVST)
a (
C1
) 2000
×
; (
C2
) 4000
×
; and
(D) 3PLACNC a (D1) 2000×; (D2) 10,000×.
3.2. X- ay Analysis Resul s
X- ay di ac ion (XRD) pa e ns o ille s, CNC and elec ospun ibe s (PVST)
and (PVST/CNC)
,
and PLA composi es a highes concen a ion o ille s, as an example, a e plo ed in Figu e 4A–C.
Polyme s 2017,9, 117 7 o 16
Di ac ion pa e n o CNC exhibi ed a sha p peak a 2
θ
= 22.34
◦
, co esponding o he c ys allog aphic
plane 002, and he cellulose shoulde a 2
θ
= 15.5
◦
, which is no mally assigned o he cellulose I
s uc u e [
35
,
36
]. PV and s a ch blank polyme s (no p ocessed h ough elec ospinning) we e also
analyzed, aiming o s udy he e ec o he elec ospinning p ocess in he c ys allini y o hese polyme s.
As Figu e 4A shows, he PV di ac ion pa e n p esen ed cha ac e is ic peaks a 2
θ
= 11.5
◦
, 19.5
◦
,
22.6
◦
, 32.1
◦
and 40.5
◦
ha we e a ibu ed o he semi-c ys alline na u e o he polyme [
37
–
39
].
The c ys al s uc u e
o s a ch can be associa ed wi h wo c ys alline polymo phic o ms: A- and
B- ype, depending on he composi ion. These c ys al s uc u es ha e been ex ensi ely s udied and
consis o le -handed pa allel s anded double helices packed in monoclinic and hexagonal uni cells
o he A and B- ype c ys alli es, espec i ely. Typical A- ype-X- ay di ac ion pa e ns p esen s peaks
a 15
◦
, 18
◦
and 23
◦
[
40
,
41
]. On he o he hand, X- ay s udies o elec ospun ibe s we e pe o med
no only o obse e he e ec o he elec ospinning p ocess, bu also he inco po a ion o CNC on PV
and s a ch. As Figu e 4B shows, XRD o elec ospun ibe s (PVST)
p esen ed a conside able loss o
c ys allini y, showing only a b oad shoulde wi h he maximum a ound 19.5
◦
co esponding ce ainly
o PV. P e ious X- ay di ac ion s udies ha e al eady shown a comple e des uc ion o c ys alli e
in eg i y as a unc ion o mois u e con en and empe a u e. Undoub edly, howe e , he main ac o s
ha a ec ed c ys allini y o bo h polyme s we e he in e ac ion be ween bo h polyme s and he
elec ospinning p ocess. Rapid p ocesses, such as elec ospinning, gene ally hinde de elopmen o
c ys allini y. Thus, elec ospun ibe s p esen ed a low deg ee o c ys allini y. Se e al au ho s a gue
ha c ys allini y dec eases as a consequence o he as sol en e apo a ion a e ha leads o a low
molecula a angemen . Fu he mo e, he s a ing ma e ial o elec ospinning is a solu ion whe e he
polyme has no c ys al s uc u e. Due o he e y la ge spinning a e, p ac ically no c ys alliza ion
occu ed. I is, howe e , possible ha he elec ospun polyme may unde go some c ys alliza ion, as
was he case o ibe s wi h CNC ha p esen ed a sligh enhancemen on cha ac e is ic peaks om bo h
polyme s and clea ly a peak co esponding o he p esence o CNC a 2
θ
= 22.3
◦
. As Figu e 4B shows,
al hough he elec ospinning p ocess educed he c ys allini y o PV and s a ch, he inco po a ion
o CNC in he ibe s implied a ise in he in ensi ies o peaks a 15
◦
, 19.5
◦
and 22.6
◦
. XRD analysis
con i med ha CNC showed some nuclea ing e ec .
Polyme s 2017, 9, 117 7 o 16
c ys allog aphic plane 002, and he cellulose shoulde a 2θ = 15.5°, which is no mally assigned o
he cellulose I s uc u e [35,36]. PV and s a ch blank polyme s (no p ocessed h ough
elec ospinning) we e also analyzed, aiming o s udy he e ec o he elec ospinning p ocess in he
c ys allini y o hese polyme s. As Figu e 4A shows, he PV di ac ion pa e n p esen ed
cha ac e is ic peaks a 2θ = 11.5°, 19.5°, 22.6°, 32.1° and 40.5° ha we e a ibu ed o he
semi-c ys alline na u e o he polyme [37–39]. The c ys al s uc u e o s a ch can be associa ed wi h
wo c ys alline polymo phic o ms: A- and B- ype, depending on he composi ion. These c ys al
s uc u es ha e been ex ensi ely s udied and consis o le -handed pa allel s anded double helices
packed in monoclinic and hexagonal uni cells o he A and B- ype c ys alli es, espec i ely. Typical
A- ype-X- ay di ac ion pa e ns p esen s peaks a 15°, 18° and 23° [40,41]. On he o he hand, X- ay
s udies o elec ospun ibe s we e pe o med no only o obse e he e ec o he elec ospinning
p ocess, bu also he inco po a ion o CNC on PV and s a ch. As Figu e 4B shows, XRD o
elec ospun ibe s (PVST) p esen ed a conside able loss o c ys allini y, showing only a b oad
shoulde wi h he maximum a ound 19.5° co esponding ce ainly o PV. P e ious X- ay di ac ion
s udies ha e al eady shown a comple e des uc ion o c ys alli e in eg i y as a unc ion o mois u e
con en and empe a u e. Undoub edly, howe e , he main ac o s ha a ec ed c ys allini y o bo h
polyme s we e he in e ac ion be ween bo h polyme s and he elec ospinning p ocess. Rapid
p ocesses, such as elec ospinning, gene ally hinde de elopmen o c ys allini y. Thus, elec ospun
ibe s p esen ed a low deg ee o c ys allini y. Se e al au ho s a gue ha c ys allini y dec eases as a
consequence o he as sol en e apo a ion a e ha leads o a low molecula a angemen .
Fu he mo e, he s a ing ma e ial o elec ospinning is a solu ion whe e he polyme has no c ys al
s uc u e. Due o he e y la ge spinning a e, p ac ically no c ys alliza ion occu ed. I is, howe e ,
possible ha he elec ospun polyme may unde go some c ys alliza ion, as was he case o ibe s
wi h CNC ha p esen ed a sligh enhancemen on cha ac e is ic peaks om bo h polyme s and
clea ly a peak co esponding o he p esence o CNC a 2θ = 22.3°. As Figu e 4B shows, al hough he
elec ospinning p ocess educed he c ys allini y o PV and s a ch, he inco po a ion o CNC in he
ibe s implied a ise in he in ensi ies o peaks a 15°, 19.5° and 22.6°. XRD analysis con i med ha
CNC showed some nuclea ing e ec .
Figu e 4. X- ay di ac ion pa e ns o CNC (cellulose nanoc ys als), elec ospun ibe s and PLA
based composi es.
Figu e 4.
X- ay di ac ion pa e ns o CNC (cellulose nanoc ys als), elec ospun ibe s and PLA
based composi es.
Polyme s 2017,9, 117 8 o 16
X- ay di ac ion pa e ns o de eloped composi es we e also pe o med o ob ain in o ma ion
abou ma e ial c ys allini y (Figu e 4C). PLA con ol exhibi ed cha ac e is ics di ac ion peaks a a
small peak a 2
θ
= 14.9
◦
, 16.4
◦
, 19.1
◦
and 22.5
◦
, which ag eed wi h da a ob ained by
Pagés e al. [42].
The inco po a ion o ille s did no a ec c ys allini y alues, and, al hough his me hod is no
quan i a i e, i was possible o obse e some enhancemen on c ys allini y deg ee, which was
con i med by DSC measu emen s.
3.3. The mal P ope ies o Nano ille s and De eloped Nanocomposi es
The mal analyses we e pe o med o s udy he e ec on he he mal p ope ies o : (i) elec ospinning
p ocess on PV and s a ch; (ii) he inco po a ion o CNC; and (iii) he p esence o hese nano ille s in
PLA. Figu e 5A shows he mass (%) and he de i a i e o mass wi h espec o he empe a u e (DTGA)
o all componen s used in he de elopmen o he elec ospun ibe s. The ini ial mass dec easing
below 100
◦
C was a ibu ed o wa e loss. The he mog am o CNC epo ed cellulose deg ada ion
p ocesses, such as depolyme iza ion, dehyd a ion and decomposi ion o glycosyl uni s ha occu ed
a ea lies empe a u es. The i s p ocess co esponded o he deg ada ion o he mos accessible
egions, which we e highly sulpha ed, and he second p ocess co esponded o he b eakdown o
he c ys alline ac ion no a acked by he sulphu ic acid, obse ed in he DTGA cu e as a small
shoulde [
43
,
44
]. A he mog am o s a ch p esen ed he main peak o deg ada ion a 307
◦
C, which
co esponds o he py oly ic decomposi ion phase o amylose and amylopec in. In he case o pu e
PV, a shoulde was obse ed ollowed by a peak be ween 220 and 415
◦
C, which we e ela ed o he
de achmen o side g oups ha o ms wa e , ace ic acid and ace aldehyde as by-p oduc s. The main
peak a 370
◦
C was associa ed wi h he mal deg ada ion o c ys alline PV, and he he mal deg ada ion
in he mol en s a e occu ed as a shoulde o he main peak [
45
]. Ba e a e al. [
46
] sugges ed ha he
de achmen o he side g oups is he main mechanism o he he mal deg ada ion o PV. The peak
obse ed a 468 ◦C co esponded o he mal deg ada ion o he PV backbone.
Elec ospun ibe s (PVST)
p esen ed wo deg ada ion p ocesses ela ed o he s uc u al
decomposi ion o s a ch ollowed by PVs. Deg ada ion o PV o elec ospun ibe s was shi ed
o lowe empe a u es compa ed o he pu e PV, p obably due o he in e ac ion wi h s a ch and due o
he elec ospinning p ocess ha caused changes in he polyme s uc u e, as XRD s udies sugges ed
p e iously. Al hough h ee deg ada ion p ocesses we e obse ed in he case o (PVST/CNC)
,
he main
deg ada ion p ocess was shi ed o highe empe a u es when compa ed o (PVST)
, p obably because
he inco po a ion o CNC imp o ed i s he mal s abili y. This esul indica ed ha CNC ac ed as
a he mal ba ie due o he o ma ion o hyd ogen bonds be ween CNC and polyme s, indica ing
ce ain compa ibili y be ween he componen s [47].
Figu e 5B shows he de i a i e o he cu es o mass loss wi h he empe a u e o PLA ma e ials
wi h lowes and highes concen a ion o ibe s, as an example, and Table 2p esen s he empe a u es
o maximum deg ada ion o all composi es. The i s aspec o be men ioned is ha all composi es
p esen ed one unique deg ada ion p ocess ha indica ed a good compa ibili y be ween he componen s.
Ne e heless, as i is clea ly obse ed in Figu e 5B, he inco po a ion o CNC h ough elec ospun
ibe s (PVST/CNC)
sligh ly dec eased he he mal s abili y o composi es, and his e ec enhanced as
he concen a ion o CNC inc eased. As con ol ma e ials wi h (PVST)
also p esen ed his dec ease,
i was
ce ainly due o he ea lie deg ada ion o elec ospun ibe s based on PV and s a ch polyme s.
The p esence o CNC on he ibe s also p o ec ed he ma e ials om his ad ancemen on deg ada ion,
showing i s he mal ba ie e ec al eady obse ed.
Table 2summa izes he signi ican he mal p ope ies o all PLA composi es ob ained by DSC
du ing he second hea ing p ocess. En halpies o calcula e c ys allini y deg ee we e co ec ed
o PLA con en . In gene al, glass ansi ion empe a u es we e no signi ican ly al e ed by he
inco po a ion o CNC h ough elec ospun ibe s. Howe e , he p esence o bo h ibe s, (PVST/CNC)
and (PVST)
, a o ed he cold c ys alliza ion p ocess, dec easing signi ican ly he empe a u e a which
c ys alliza ion began.
Polyme s 2017,9, 117 9 o 16
Polyme s 2017, 9, 117 9 o 16
Figu e 5. (A) TGA ( he mog a ime ic analysis) cu es o indi idual componen s. Inse : DTGA
(de i a i e o he TGA cu e) o cu es; (B) DTGA o PLA based composi es.
Glass ansi ion empe a u es o composi es p esen ed alues simila o hose epo ed in Lizundia
e al. s udies o PLA [48]. These alues we e lowe han Tg ob ained in he li e a u e. The eason o
he di e ence could be ela ed o he di e en ob aining p ocess, he use o plas icize and he
o igins o he samples. In addi ion, he nanocomposi es wi h a low concen a ion o eeze-d ied
CNC and embedded CNC in o ibe s p esen ed a g ea dec ease on Tg. Fo una i e al. s udies ha e
al eady showed his e ec in PLA including cellulose nanoc ys als and a su ac an o dispe sion due o
he plas icizing o PLA by deso bed su ac an s, o he modi ica ion o CNC by lac ide oligome s [49,50].
As was al eady obse ed by Ma inez-Sanz e al. [28], a good dispe sion o nano ille s ac ed as
nuclea ing agen , p omo ing a as e c ys alliza ion o PLA du ing hea ing. As Table 2 shows, alues
o cold c ys alliza ion peaks o hese composi es and con ol samples wi h elec ospun ibe s (PVST)
we e educed compa ed o he PLA and CNC con aining PLA nanocomposi es, indica ing he
nuclea ing ac ion o elec ospun ibe s [12]. Fu he mo e, as Table 2 shows, c ys allini y deg ee o
ma e ials was s ongly enhanced, and his imp o emen inc eased as he concen a ion o ibe s
inc eased. This esul has also been obse ed by Espino-Pé ez e al. [12] using CNC chemically
modi ied wi h n-oc adecyl-isocyana e. The imp o emen on CNC dispe sion linked o hei
inco po a ion h ough elec ospun ibe s inc eased he numbe o in e ace/con ac ille wi h he
polyme ma ix. O he wo ks ha e named he ansc ys alini y e ec , he ac ha ibe s e ealed a
nuclea ing ole, inducing a obus c ys alline mo phology o PLA [51]. The p esence o eeze-d ied
CNC also p omo ed c ys alliza ion, bu a a lowe a e.
Figu e 5.
(
A
) TGA ( he mog a ime ic analysis) cu es o indi idual componen s. Inse : DTGA
(de i a i e o he TGA cu e) o cu es; (B) DTGA o PLA based composi es.
Glass ansi ion empe a u es o composi es p esen ed alues simila o hose epo ed in Lizundia
e al. s udies o PLA [
48
]. These alues we e lowe han T
g
ob ained in he li e a u e. The eason o
he di e ence could be ela ed o he di e en ob aining p ocess, he use o plas icize and he o igins
o he samples. In addi ion, he nanocomposi es wi h a low concen a ion o eeze-d ied CNC and
embedded CNC in o ibe s p esen ed a g ea dec ease on T
g
. Fo una i e al. s udies ha e al eady
showed his e ec in PLA including cellulose nanoc ys als and a su ac an o dispe sion due o he
plas icizing o PLA by deso bed su ac an s, o he modi ica ion o CNC by lac ide oligome s [49,50].
As was al eady obse ed by Ma inez-Sanz e al. [
28
], a good dispe sion o nano ille s ac ed as
nuclea ing agen , p omo ing a as e c ys alliza ion o PLA du ing hea ing. As Table 2shows, alues o
cold c ys alliza ion peaks o hese composi es and con ol samples wi h elec ospun ibe s (PVST)
we e
educed compa ed o he PLA and CNC con aining PLA nanocomposi es, indica ing he nuclea ing
ac ion o elec ospun ibe s [
12
]. Fu he mo e, as Table 2shows, c ys allini y deg ee o ma e ials
was s ongly enhanced, and his imp o emen inc eased as he concen a ion o ibe s inc eased.
This esul has also been obse ed by Espino-Pé ez e al. [
12
] using CNC chemically modi ied wi h
n-oc adecyl-isocyana e. The imp o emen on CNC dispe sion linked o hei inco po a ion h ough
elec ospun ibe s inc eased he numbe o in e ace/con ac ille wi h he polyme ma ix. O he
wo ks ha e named he ansc ys alini y e ec , he ac ha ibe s e ealed a nuclea ing ole, inducing
a obus c ys alline mo phology o PLA [
51
]. The p esence o eeze-d ied CNC also p omo ed
c ys alliza ion, bu a a lowe a e.
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