Surface Roughness Enhances Self-Nucleation of High-Density Polyethylene Droplets Dispersed within Immiscible Blends

Su ace Roughness Enhances Sel -Nuclea ion o High-Densi y
Polye hylene D ople s Dispe sed wi hin Immiscible Blends
Sei Eddine Fenni, Ma ia Rosa ia Capu o, Alejand o J. Mulle ,*and Da io Ca allo*
Ci e This: Mac omolecules 2022, 55, 1412−1423
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ABSTRACT: Highly linea o high-densi y polye hylenes
(HDPEs) ha e an in insically high nuclea ion densi y compa ed
o o he polyolefins. Enhancing hei nuclea ion densi y by sel -
nuclea ion is he e o e difficul , leading o a na ow sel -nuclea ion
Domain (i.e., he so-called Domain II o he empe a u e Domain
whe e sel -nuclei can be injec ed in o he ma e ial wi hou he
occu ence o annealing). In his wo k, we epo ha when HDPE
is blended (up o 50%) wi h immiscible ma ices, such as a ac ic
polys y ene (PS) o Nylon 6, i s sel -nuclea ion capaci y can be
g ea ly inc eased. In addi ion, empe a u es highe han he
equilib ium mel ing empe a u e o he HDPE phase a e needed
o e ase he significan ly enhanced c ys alline memo y in he blends. Mo phological e idence ga he ed by Scanning and
T ansmission Elec on Mic oscopies (SEM and TEM) indica es ha hese unexpec ed esul s can be explained by he modifica ion
o he in e ace be ween blend componen s. The filling o he solid HDPE su ace aspe i ies by he low iscosi y polys y ene du ing
hea ing o he sel -nuclea ion empe a u e, o he c ys alliza ion o he ma ix in he case o Nylon 6, enhances he in e ace
oughness be ween he wo polyme s in he blends. Such oughe in e aces can ema kably inc ease he sel -nuclea ion capaci y o
he HDPE phase ia su ace nuclea ion.
1. INTRODUCTION
Polyme blending is a use ul way o p epa e polyme sys ems
ha exhibi an a ac i e combina ion o he p ope ies o he
nea polyme componen s.
1,2
As an ou come o he blending
p ocess, wo ca ego ies o polyme blends can be ob ained, i.e.,
miscible and/o immiscible blends. In immiscible blends,
mixing a small amoun o a semic ys alline polyme wi h a
second immiscible polyme (ei he amo phous o semic ys al-
line) o en leads o he o ma ion o a sea-island mo phology,
in which mic odomains (MDs) o d ople s o he mino
c ys alline phase will be dispe sed in he ma ix o he majo
phase.
3,4
The c ys alliza ion beha io and supe s uc u e o he
mixed polyme s, in he case o semic ys alline componen (s),
a e affec ed by he blending p ocess.
3−5
The obse ed change
is mainly ela ed o he nuclea ion beha io (including bo h
he mechanism and kine ics) o he mino phase. Gi en he
ac ha d ople s a e ypically small (i.e., ew mic ome e s o
less), nuclea ion can be conside ed as he a e-de e mining
s ep in he o e all c ys alliza ion p ocess.
6−8
F ac iona ed c ys alliza ion is o en encoun e ed du ing
c ys alliza ion o he mino semic ys alline componen in
immiscible blends. The ac iona ed c ys alliza ion phenomen-
on has been he subjec o a ecen comp ehensi e e iew, see
e 6. I a ises because o he lack o ac i e he e ogenei ies in
e e y single d ople .
6
Hence, du ing mel -c ys alliza ion, a
diffe en se o d ople s will c ys allize a diffe en deg ees o
supe cooling. D ople s ha con ain a leas one highly ac i e
he e ogenei y will c ys allize a a c ys alliza ion empe a u e
simila o close o ha o he nea componen , while he o he
se s o d ople s will c ys allize a la ge supe coolings. Clean
d ople s o d ople s wi h ine he e ogenei ies will c ys allize a
he highes supe cooling ia su ace nuclea ion o homoge-
neous nuclea ion.
6,9,10
Su ace nuclea ion was ound o play an impo an ole in
he c ys alliza ion o immiscible blends (bo h o he ma ix
and he dispe sed mic o-domains).
5,9
Se e al esea che s
epo ed ha su ace nuclea ion could ini ia e om solid
polyme su aces as well as om mol en su aces/poly-
me s.
11,12
Fenni e al.
12
epo ed he nuclea ion effec o
mol en poly(ε-cap olac one) (PCL) and mol en poly(bu ylene
succina e) (PBS) on poly(lac ic acid) (PLA) sel -assembled
d ople s in hei 45/10/45 PCL/PLA/PBS immiscible e na y
blend. While in ano he wo k, Fenni e al.
13
showed ha in 45/
10/45 PLA/PCL/PBS e na y blends, he PBS con inuous
phase was able o nuclea e a he su ace o he p e iously
c ys alline PLA. Se e al ac o s we e claimed o con ol and
Recei ed: Decembe 6, 2021
Re ised: Janua y 21, 2022
Published: Feb ua y 11, 2022
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affec su ace nuclea ion, such as pola i y o he polyme s,
14
affini ies be ween diffe en componen s,
12
s a es o he
in e ace, and su ace oughness.
7,15
Rega ding his las ac o ,
Dalnoki-Ve ess and Ca alho ound a di ec co ela ion
be ween he nuclea ion mechanism o he d ople s and he
oughness o he subs a e in hei polys y ene/poly(e hylene
oxide) (PS/PEO) sys ems.
15
Ca meli e al.
7
epo ed a clea
change in he c ys alliza ion kine ics o he dispe sed high-
densi y polye hylene (HDPE) d ople s induced by changing
he su ace oughness o he polyp opylene (PP) ma ix ia
sel -nuclea ion (SN).
Sel -nuclea ion (SN), which is conside ed as one o he
possible nuclea ion mechanisms in polyme s, is he p ocess o
he p oduc ion o sel -nuclei and/o sel -seeds by applying a
specific he mal p o ocol based on pa ial mel ing o he
polyme , i.e., ei he by using lowe mel ing empe a u e o
sho e mel ing imes. SN is a use ul s a egy o p omo e
polyme nuclea ion. Howe e , he exac na u e o he p oduced
sel -nuclei is no uni ocally assessed.
16−18
Mulle e al.
17,18
ha e ex ensi ely in es iga ed he sel -nuclea ion o polyme s,
and ecen ly, i s applica ion, he majo expe imen al a iables
ha could affec i , i s in e p e a ions, and he ecen
expe imen al combined echniques used o cha ac e ize and
in e p e i ha e been summa ized.
18
Th ee sel -nuclea ion domains can be defined based on he
DSC cooling and hea ing scans du ing a SN p o ocol.
16
Domain I (DI) o he iso opic mel domain is encoun e ed
when he c ys alliza ion beha io o he polyme is d i en
exclusi ely by high- empe a u e- esis an he e ogeneous nuclei.
Domain II (DII)o hesel -nuclea iondomain is he
empe a u e egion in which he applied sel -nuclea ion
empe a u e (Ts) is (i) low enough o lea e some sel -nuclei,
which will accele a e he c ys alliza ion du ing he subsequen
cooling scan, bu (ii) no enough o lea e any c ys al agmen
ha anneals and affec s he final mel ing beha io o he
polyme . The lowes empe a u e in Domain II is defined as he
ideal sel -nuclea ion empe a u e (Ts ideal). I is he empe a u e
a which a maximum inc ease o he c ys alliza ion empe -
a u e, du ing subsequen cooling, is eco ded while no change
in he mel ing beha io is obse ed. Any u he dec ease o he
Tsbelow he Ts ideal will lead o annealing and hickening o
some c ys al agmen s, leading o he appea ance o annealing
mel ing peaks a highe empe a u e wi h espec o he
con en ional mel ing poin o he polyme . This sel -nuclea ion
empe a u e ange is called Domain III (DIII). Mulle e
al.
17−19
p oposed a u he di ision o he DII in o wo sub-
domains, i.e., (a) he mel -memo y subdomain (DIIa) ha
occu s a he highe empe a u e ange o he DII, whe e he
applied Tsis high enough o mel c ys als wi hou ully e asing
he mel memo y, and (b) he sel -seeding subdomain (DIIb)in
which he applied empe a u es a e capable o mel he
polyme c ys als bu low enough o lea e c ys al agmen s
called sel -seeds.
Unlike mos o he polyme s ha exhibi h ee sel -
nuclea ion domains, high-densi y polye hylene (HDPE)
exhibi s a e y peculia sel -nuclea ion beha io . In ac , no
clea acco dance abou he numbe o SN domains in HDPE
was eached up o da e. Indeed, in se e al wo ks, he HDPE
homopolyme and he polye hylene block wi hin copolyme s
p esen ed only DI and DIII.
20,21
T ujillo e al.
20
epo ed a
di ec ansi ion be ween DI and DIII in hei HDPE
homopolyme . The au ho s a ibu ed he o al absence o
he Domain II o he ex emely high numbe o ac i e
he e ogenei ies ha o iginally exis in he HDPE, which hinde
SN om showing any u he inc ease in he nuclea ion
densi y. On he o he hand, addi ional wo ks epo ed a e y
na ow DII o he HDPE and polye hylene in copolyme s.
7
In e es ingly, Alamo e al.
22
epo ed a s ong sel -nuclea ion
effec , e en a empe a u es abo e he equilib ium mel ing
poin (Tm°) in andom e hylene copolyme s. Such effec is no
obse ed in linea homopolyme s and is hus a ibu ed o he
complex mel opology c ea ed by sequence leng h selec ion
du ing copolyme c ys alliza ion.
22
Sel -nuclea ion was widely used o in es iga e he c ys al-
liza ion o d ople s in immiscible blends. Among o he
applica ions, SN was applied in o de o o e come he
ac iona ed c ys alliza ion o he mino c ys alline phase in
immiscible blends.
23,24
In he p esen wo k, SN o he HDPE
dispe sed d ople s in immiscible blends, ei he in amo phous
o semic ys alline ma ices, has been in es iga ed. I is shown
o he fi s ime ha he sel -nuclea ion Domain DII o HDPE
can be la gely ex ended only by di iding he HDPE in o mic o-
domains in con ac wi h o eign ma ices in e aces.
2. MATERIALS AND EXPERIMENTS
2.1. Ma e ials. Two high-densi y polye hylene (HDPE) g ades
we e used in o de o demons a e he gene ali y o he findings. The
fi s HDPE (MB7541) was a comme cial g ade p o ided by Bo ealis,
and i was cha ac e ized by a mel ing poin (Tm) o a ound 130 °C, a
mel flow a e (MFR) o abou 4 g/10 min, and a densi y o 0.954 g/
cm3. I is indica ed in he ollowing as HDPE-1. A second HDPE
(Rigidex HD6070EA), a comme cial g ade p o ided by Ineos
Polyolefins, has a mel ing poin (Tm) o a ound 133 °C, a mel
flow a e (MFR) o 7.6 g/10 min, and a densi y o 0.960 g/cm3. The
second HDPE is coded as HDPE-2.
Polys y ene (PS), wi h an MFR o 1.3 g/10 min, was pu chased
om Sigma Ald ich. I had a densi y o 1.04 g/cm3, a weigh -a e age
molecula weigh (Mw) o 350 kg/mol, and a dispe si y (Mw/Mn)o
2.05.
The Nylon 6 used in his s udy was Du e han B30S p o ided by
LANXESS. I had a densi y o 1.14 g/cm3and a mel ing poin o
a ound 220 °C.
We no e ha HDPE-1 was blended wi h PS, while HDPE-2 was
used o he blend wi h Nylon 6.
2.2. Blend P epa a ion. PS/HDPE-1 blends we e p epa ed in a
B abende - ype in e nal mixe . Mel mixing was pe o med a 200 °C
using a o o speed o 100 pm o 10 min. Meanwhile, he Nylon 6/
HDPE-2 blend was p epa ed in a Collin ZK25 co- o a ing win sc ew
ex ude -kneade , wi h a o o speed o 180 pm and a mixing
empe a u e o 230 °C. All he p epa ed blends a e summa ized in
Table 1.
2.3. Blend Cha ac e iza ion. 2.3.1. SEM Analysis. The
mo phology o he ac u ed su ace o he diffe en blends was
in es iga ed using a field-emission scanning elec on mic oscope
(Sup a 40 VP model, Zeiss, Ge many) a an accele a ing ol age o 5
kV.
Table 1. Composi ion o he P epa ed Samples
sample HDPE (w %) PS (w %) Nylon 6 (w %)
HDPE-1 100
Nylon 6 100
90/10 PS/HDPE-1 10 90
85/15 PS/HDPE-1 15 85
80/20 PS/HDPE-1 20 80
70/30 PS/HDPE-1 30 70
50/50 PS/HDPE-1 50 50
90/10 Nylon 6/HDPE-2 10 90
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Two me hods we e applied du ing he p epa a ion o he
in es iga ed samples. In he fi s one, specimens we e di ec ly
subme ged in liquid ni ogen o 30 min and ac u ed c yogenically.
Meanwhile, in he second me hod, samples we e subjec ed o he
he mal p o ocol shown in Figu e S1 p io o he c yogenic ac u e.
All samples we e finally hinly spu e -coa ed wi h ca bon using a
Pola on E5100 spu e -coa e .
The numbe -a e age (dn) and olume-a e age (d ) diame e s we e
calcula ed using he equa ions shown in e 23 by measu ing a ound
200 d ople s om diffe en egions o he samples.
2.3.2. TEM Analysis. To be e cla i y he mo phological s uc u e
o some o he samples in ol ed in his s udy, a TEM analysis was
pe o med. Since he samples a e essen ially composed o ca bon and
hyd ogen, hey do no ha e much diffe ence in e ms o elec on
densi y; he e o e, o be obse ed by TEM, hey mus unde go he
s aining p ocess. Fo his pu pose, a RuO4solu ion was used. 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 on a Leica
EMFC6 c yo-ul amic o ome de ice. The ul a hin sec ions o 90 nm
hickness 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
filamen ope a ing a an accele a ing ol age o 120 kV.
2.3.3. The mal Beha io by Means o DSC. 2.3.3.1. Non-
iso he mal Analyses. Diffe en he mal cha ac e iza ions we e done
in wo diffe en labo a o ies and using wo diffe en DSCs. Nea
HDPE-1 and PS/HDPE-1 blends we e cha ac e ized using a DSC1
STARe Sys em (Me le Toledo, Swi ze land). All measu emen s
we e pe o med using sample masses in he ange o 3−5 mg and
unde a con inuous ni ogen flow (20 mL/min). In his DSC analysis,
nea HDPE-1 and PS/HDPE-1 blends we e fi s hea ed om oom
empe a u e o 170 °Ca 10°C/min and held a 170 °C o 3 min, o
e ase he he mal his o y o he HDPE-1 componen . The samples
we e hen cooled a a cooling a e o 10 °C/min om 170 o 20 °C
while he cooling scan was eco ded. Finally, a second hea ing scan a
a hea ing a e o 10 °C/min was pe o med and acqui ed.
On he o he hand, a Pe kinElme Py is I DSC equipped wi h an
In acoole 2P was employed o cha ac e ize he he mal p ope ies o
nea Nylon 6 and he Nylon 6/HDPE-2 blend.
All he expe imen s we e pe o med unde an ul apu e ni ogen
flow, and he ins umen was calib a ed wi h indium and in s anda ds.
Samples o 10 mg o he blend, i.e., 1 mg o he nea PE and nea
Nylon 6 (wi h espec o he composi ion in he o al blend), we e
used. Measu emen s we e pe o med by placing he samples in sealed
aluminum pans. Be o e being subjec ed o hea ea men s, he
samples we e kep in a acuum o en a 100 °C o e nigh , o
elimina e any ace o mois u e abso bed du ing s o age. Non-
iso he mal expe imen s o nea polyme s and he blends we e ca ied
ou ollowing he same he mal p o ocol bu wi h diffe en
empe a u es. The nea HDPE-2 was fi s hea ed a 20 °C/min up
o 180 °C and le a 180 °C o 3 min o e ase he he mal his o y;
hen, i was cooled a 20 °C/min down o 25 °C and held o 1 min a
his empe a u e. Finally, i was ehea ed a 20 °C/min up o 180 °C.
The same me hod was used o nea Nylon 6 and Nylon 6/HDPE-2
bu employing a maximum mel empe a u e o 250 °C since Nylon 6
has a highe mel ing empe a u e han HDPE-2.
2.3.3.2. Sel -Nuclea ion Expe imen s (SN). PS/HDPE-1 samples
we e analyzed using he sel -nuclea ion p ocedu e desc ibed
below.
16−18
(1) The c ys alline his o y was e ased by mel ing he sample a 170
°C o 3 min (40 °C abo e he mel ing poin o he nea
HDPE-1 componen ).
(2) The sample was cooled o 0 °C, a a cooling a e o 10 °C/min,
o c ea e a s anda d c ys alline s a e.
(3) Pa ial (o comple e) mel ing o he sample was pe o med by
hea ing a 10 °C/min o he SN empe a u es (Ts) and holding
i he e o 5 min.
(4) Cooling o 0 °C was pe o med, a a cooling a e o 10 °C/
min, o c ys allize he sample and de ec he effec o he
annealing a he diffe en SN empe a u es.
(5) A final hea ing scan om 0 o 170 °C o he ec ys allized
sample was pe o med a a a e o 10 °C/min. Rega ding he
Nylon 6/HDPE-2 blend, a ela i ely diffe en he mal p o ocol
was applied in which he mel ing poin used in s eps (1) and
(5) was se a 200 °C while he applied scan (bo h cooling and
hea ing) a e was 20 °C/min ins ead o he 10 °C/min used
o he PS/HDPE-1 blends.
3. RESULTS AND DISCUSSION
3.1. Mo phological Cha ac e iza ion. Figu e 1 and
Figu e S1 (in he SI) p esen mic og aphs o he c yogenically
ac u ed su ace o all he in es iga ed blends. In addi ion o
he 50/50 PS/HDPE-1 blend, which exhibi s a co-con inuous
mo phology, and he 70/30 PS/HDPE-1, which displays a
mix u e o sea-island and co-con inuous mo phologies (see
Figu e S2), all he o he blends exhibi a sea-island mo phology
in which he mino HDPE phase is p esen in he o m o
dispe sed d ople s o mic o-domains (MDs) in he amo phous
PS o semic ys alline Nylon 6 ma ices. The mo phology o
each blend confi ms he immiscibili y o he s udied sys ems.
Table 2 epo s he size o dispe sed mic o-domains in he
a ious blends. The size o he dispe sed d ople s changes wi h
he HDPE-1 con en and wi h he ype o ma ix. Fo ins ance,
by inc easing he HDPE-1 con en om 10 o 20 w % in he
PS/HDPE-1 blends, he size (dn/d ) o he HDPE-1 d ople s
inc eases om 0.63/0.92 o 1.66/2.61 μm, espec i ely. On he
o he hand, a la ge mic o-domain size was ound in he 90/10
Figu e 1. Mo phologies o diffe en bina y blends. (a) 90/10 PS/HDPE-1, (b) 80/20 PS/HDPE-1, and (c) 90/10 Nylon 6/HDPE-2.
Table 2. Numbe -A e age (dn) and Volume-A e age
Diame e s (d ) and Dispe si y (D) o he In es iga ed
Blends
blend dn[μm] d [μm] D
90/10 PS/HDPE-1 0.63 0.92 1.46
80/10 PS/HDPE-1 1.66 2.61 1.57
90/10 Nylon 6/HDPE-2 3.23 3.61 1.12
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Nylon 6 /HDPE-2 blend, e en hough he HDPE-2 con en
was only 10 w %.
The ob ained diffe ence in he size o he HDPE mic o-
domains a equi alen weigh con en s, in he PS/HDPE-1 and
Nylon 6/HDPE-2 blends, should be a ibu ed o he
diffe ences in he mel − iscosi y a io, shea a e, in e acial
ension be ween he wo componen s, o p ocessing
condi ions.
25
As p e iously known, he size o he dispe sed mic o-
domains in immiscible blends is c ucial and has a s ong effec
on he final c ys alliza ion beha io o he mino
phase.
13,23,24,26−28
3.2. DSC Noniso he mal Analysis. Resul s o he DSC
s anda d cooling and hea ing scans a e shown in Figu e 2 and
Figu e S3. I should be no ed ha PS/HDPE-1 blends ha e
been analyzed using a scan a e o 10 °C/min (Figu e 2a,b and
Figu e S3), while o he sys em Nylon 6/HDPE-2, a scan a e
o 20 °C/min has been applied (Figu e 2c,d). The
c ys alliza ion empe a u es (Tc) and mel ing empe a u es
(Tm) o he HDPE phase in all in es iga ed blends a e
summa ized in Table S1 o he Suppo ing In o ma ion (SI).
A fi s , nea componen s will be conside ed. Nea HDPE-1
displays one single sha p c ys alliza ion peak a a ound 116 °C
and mel s a a ound 130 °C. Nea Nylon 6 exhibi s a
c ys alliza ion peak a abou 186 °C and mel s wi h a b oad
peak a a ound 221 °C. The nea PS, which is amo phous, has
aTga a ound 105 °C.
Rega ding he HDPE phase in he a ious blends, a clea
co ela ion be ween he c ys alliza ion beha io and he
mo phology is ound. Fi s , o PS/HDPE-1 blends whe e
he sea-island mo phology is s ill p ese ed (blends wi h an
HDPE-1con en o less han30w %), ac iona ed
c ys alliza ion is obse ed. Fo ins ance, o he 90/10 PS/
HDPE-1 blend, mul iple c ys alliza ion e en s a diffe en
supe coolings (a 115.7, 90, and 77.5 °C) a e encoun e ed. As
discussed in he In oduc ion, du ing cooling om he mel ,
each se o d ople s will c ys allize a diffe en supe cooling,
e.g., he d ople s ha con ain highly ac i e impu i ies will
c ys allize a a low supe cooling (i.e., a he empe a u e a
which he bulk polyme c ys allizes). In compa ison, mic o-
Figu e 2. (a) DSC cooling scans and (b) subsequen DSC hea ing scans o he nea HDPE-1 and PS/HDPE-1 blends a a cooling and hea ing a e
o 10 °C/min. (c) DSC cooling scans and (d) subsequen DSC hea ing scans o he nea HDPE-2 and Nylon 6/HDPE-2 blends a a scan a e o 20
°C/min.
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domains con aining less ac i e he e ogenei ies solidi y a
highe supe cooling. Clean d ople s will c ys allize a a e y
high supe cooling ia su ace nuclea ion o homogeneous
nuclea ion.
3,9,10,29−32
Ano he eason ha could lead o his
c ys alliza ion beha io is he mig a ion o mo e ac i e
impu i ies/he e ogenei ies om he HDPE-1 phase o he PS
du ing mel mixing. This will esul in a meaning ul lowe ing in
he nuclea ion a e inside he dispe sed HDPE-1 d op-
le s.
31,33,34
In he p esen case, o he 90/10 PS/HDPE-1
blend, only a limi ed numbe o d ople s c ys allize a a high
c ys alliza ion empe a u e (namely, a 115.6 °C), and hei
c ys alliza ion en halpy is e y low (nea ly negligible wi h
espec o he o al c ys alliza ion en halpy o he HDPE-1
phase). On he o he hand, he la ges po ion o he HDPE-1
mic o-domains solidifies a a highe supe cooling: a main la ge
c ys alliza ion peak is obse ed a 90 °C, and i s c ys alliza ion
en halpy is he la ges among all peaks. Las ly, a hi d
c ys alliza ion e en occu ed a he highes supe cooling ( ha
is, a 77 °C). This should be a ibu ed o d ople s ee om
impu i ies ha c ys allize, mos p obably, ia su ace nuclea ion
mechanism (homogeneous nuclea ion is excluded because he
c ys alliza ion empe a u e o his se o mic o-domains is well
abo e he Tgo HDPE-1).
35
By inc easing he HDPE-1 con en in he blends (in he 85/
15 and 80/20 PS/HDPE-1 blends), he en halpy o he high-
empe a u e c ys alliza ion peak inc eases a he expense o he
low- empe a u e c ys alliza ion peaks. The c ys alliza ion
empe a u e o he low- empe a u e c ys alliza ion peaks, as
well, is ound o shi owa d highe empe a u es. The
obse ed changes a e a ibu ed o he inc ease in he mic o-
domain sizes, which leads o a highe numbe o d ople s
con aining ac i e he e ogenei ies. Simila esul s, in which
ac iona ed c ys alliza ion o he HDPE phase was obse ed,
Figu e 3. (a) DSC cooling scans (a 10 °C/min) o he nea HDPE-1 a e 5 min a he indica ed Ts, (b) hea ing scans (a 10 °C/min) a e he
cooling uns shown in (a), and (c) ep esen a ion o he SN domains o he nea HDPE-1 supe imposed on he s anda d DSC hea ing cu e. Red
squa es ep esen he Tc( igh -hand-side y-axis) as a unc ion o Ts(x-axis).
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ha e been p e iously epo ed o he sys ems PS/
HDPE,
24,36,37
PET/HDPE,
38
and PMMA/HDPE.
39
In blends whe e he HDPE-1 con en is equal o o abo e 30
w %, he co-con inuous mo phology (o a mix u e o co-
con inuous and sea-island mo phologies) is obse ed and he
size o he HDPE-1 phases is la ge enough; hence, a bulk-like
c ys alliza ion beha io is obse ed.
Conce ning he 90/10 Nylon 6/HDPE-2 sys em and e en
hough he HDPE-2 is p esen ed only in 10 w %, a bulk-like
c ys alliza ion beha io is obse ed. The ob ained bulk-like
c ys alliza ion could be a ibu ed o he la ge size o he
HDPE-2 mic o-domains, which may allow mos o he HDPE-
2 d ople s o con ain a leas one highly ac i e impu i y.
Ano he ac o ha could be he eason behind his bulk-like
c ys alliza ion beha io is he p esence o he p e iously
c ys allized Nylon 6 in e ace, which can lowe he ene gy
ba ie needed o nuclea ion and hence accele a e he
nuclea ion s ep and he o e all c ys alliza ion a e o he
dispe sed d ople s.
The DSC mel ing aces o he HDPE-1 phase in he PS/
HDPE-1 blends a e shown in Figu e 2b. I is clea ha he
mel ing poin o he HDPE-1 phase dec eases wi h he HDPE-
1 con en in he blend. The ob ained esul s a e logical because
he lowe he HDPE-1 con en , he lowe he size o he mic o-
domains, which in u n will be eflec ed in he size and
hickness o he o med lamellae. Hence, a lowe mel ing poin
will be eco ded. In he case o he Nylon 6/HDPE-2 blend
(see Figu e 2d), he HDPE-2 phase p esen s a mel ing poin
e y simila o he one o nea HDPE-2; hus, he
co esponding mel ing empe a u e has no changed.
3.3. Sel -Nuclea ion. 3.3.1. Sel -Nuclea ion o he Nea
HDPE-1. Figu e 3a,b shows DSC cooling and hea ing scans
ob ained a e sel -nuclea ing he nea HDPE-1 a diffe en SN
empe a u es (Ts). In Figu e 3c, he s anda d DSC hea ing
cu e o he nea HDPE-1 is plo ed oge he wi h he
Figu e 4. (a) DSC cooling scans (a 10 °C/min) o he 90/10 PS/HDPE-1 blend a e 5 min a he indica ed Ts, (b) hea ing scans (a 10 °C/min)
a e he cooling uns shown in (a), and (c,d) collec ion o Tc(s) and ΔHc(s) as a unc ion o he employed Ts(x-axis) supe imposed on op o he
s anda d DSC mel ing ace.
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c ys alliza ion empe a u es (Tc) eco ded a e diffe en SN
ea men s, and he bo de s be ween he h ee cha ac e is ic
SN domains a e indica ed as e ical lines. Fo cla i y, DSC
cu es om diffe en SN domains a e plo ed in diffe en colo s
( ed o Domain I, blue o Domain II, and g een o Domain
III) as sugges ed by Mulle e al.
17,18
The nea HDPE-1 p esen s a classical SN beha io wi h
h ee SN domains. By applying Tsin he ange o 132−170 °C,
bo h c ys alliza ion and mel ing aces a e unchanged, and he
Tc eco ded was 116 °C. This empe a u e ange (i.e., 132 °C
and abo e) co esponds o Domain I (o DI) in which only
high- empe a u e- esis an he e ogenei ies/impu i ies a e e-
sponsible o he ob ained c ys alliza ion beha io . By lowe ing
Tsin he ange o 131−128.5 °C, a g adual inc ease in he Tc
alues upon dec easing Ts alues is ob ained, a beha io ha
co esponds o Domain II o he sel -nuclea ion Domain (i.e.,
DII). In pa allel o ha , no changes in he mel ing
cha ac e is ics ha e been eco ded while he sample is in DII
(see Figu e 3b). The maximum inc ease in he c ys alliza ion
empe a u e Tcwi hou inducing any change in he mel ing
beha io was 128.5 °C; ha is he ideal sel -nuclea ion
empe a u e (Ts ideal).
Wi h he help o Figu e 3c ( he supe posi ion o he Tc s Ts
alue on op o he DSC mel ing scan o he nea HDPE-1), we
can (i) say ha he mel memo y domain (DIIa) does no exis
and (ii) deduce ha he DII in he p esen case is only a sel -
seeding domain (DIIb), in which he obse ed SN nuclea ion
beha io and he inc ease in Tca e only due o some c ys al
agmen s exis ing in he polyme mel .
18,40
The wid h o his
ob ained sel -nuclea ion domain is 2.5 °C. As we men ioned
p e iously, o he bes o ou knowledge, no clea ag eemen
on he p esence/absence as well as he wid h o he DII o
HDPE has been achie ed. Fo ins ance, in he wo k o T ujillo
e al.,
20
a o al absence o DII was obse ed, while Ca meli e
al.
7
showed a sel -nuclea ion domain o 1.5 °C wid h in hei
HDPE. This is p obably because hey a e diffe en HDPE
samples, and each sample has a cha ac e is ic numbe o
he e ogenei ies ha depends on he ca aly ic deb is con en
and o he ypes o impu i ies p esen ha can ac as
he e ogeneous nuclei.
A u he dec ease in he applied Tsbelow 128.5 °C leads o
he sel -nuclea ion and annealing domain (DIII), whe e clea
changes in he mel ing beha io o he HDPE-1 (s ep (5) o
he he mal p o ocol desc ibed in he expe imen al sec ion) a e
obse ed in pa allel wi h he g adual inc ease in he Tc.A Ts
below 128.5 °C, he sample unde goes pa ial mel ing; hence,
he emaining unmol en c ys als will hicken (du ing he
annealing p ocess o 5 min a Ts), esul ing in an addi ional
mel ing peak a a highe empe a u e.
3.3.2. Sel -Nuclea ion o he HDPE-1 in he 90/10 PS/
HDPE-1 Blend. Figu e 4a shows he DSC cooling scans a e
sel -nuclea ion o he HDPE-1 mino phase wi hin he 90/10
PS/HDPE-1 a he indica ed Ts, while Figu e 4b shows he
subsequen hea ing scans. As we desc ibed p e iously, when
he 90/10 PS/HDPE-1 blend is cooled om he iso opic mel ,
he HDPE-1 phase unde goes ac iona ed c ys alliza ion
whe e wo majo peaks Tc1 and Tc2 a e obse ed, a 115.6
and 90 °C, espec i ely. To a oid such ac iona ed
c ys alliza ion beha io , se e al s a egies ha e been applied,
such as he addi ion o nuclea ing agen s (NA) and he
applica ion o sel -nuclea ion ea men .
18,23,31,41
In he p esen 90/10 PS/HDPE-1 blend and a Tshighe
han 154 °C, no app eciable changes in he c ys alliza ion
ea u es (Tc(s), en halpies, shape o he exo he ms, and
p opo ion o ela i e magni ude o each c ys alliza ion peak)
a e obse ed. This empe a u e ange (170−155 °C)
co esponds o he comple e mel ing domain (DI).
Su p isingly, s a ing om a e y high Ts(namely, 154 °C)
down o 126 °C(DII), clea changes in he c ys alliza ion
beha io a e eco ded, and he en halpy o he high
c ys alliza ion peak Tc1 s a s o inc ease a he expense o
he low c ys alliza ion peak Tc2 (see Figu e 4a,d). In his ange
o empe a u es, he he mal ea men applied du ing SN
c ea ed/injec ed some sel -nuclei (in he empe a u e ange o
154−129 °C, which is he mel memo y domain (DIIa)) and
sel -seeds ( om 129 o 126 °C, co esponding o he sel -
seeding domain (DIIb)) inside he HDPE-1 d ople s. The
subdi ision o he DII and he edge be ween DIIa and DIIb,
which a e shown in Figu e 4c,d, a e defined on he basis o he
obse ed changes in he c ys alliza ion and mel ing beha io , as
well as on he DSC mel ing endo he m o he HDPE-1 (a e
cooling om he s anda d mel ). Fo ins ance, he uppe limi
o DIIa is defined as he Ts alue a which changes in he
c ys alliza ion beha io a e obse ed, while he lowe limi is
fixed a he empe a u e a which all c ys als a e mol en ( he
poin whe e he DSC mel ing ace eaches he baseline). On
he o he hand, in he empe a u e ange o DII (154−126
°C), no no iceable changes ha e been obse ed in he mel ing
beha io . Fo he fi s ime, HDPE-1 shows a sel -nuclea ion
domain (DII) wi h a wid h o 28 °C and an uppe limi well
abo e i s equilib ium mel ing poin Tm°(30 °C abo e he Tm
o he dispe sed HDPE-1 d ople s).
The lowes Ts empe a u e a which he maximum change in
he c ys alliza ion beha io (in he magni ude o he wo main
c ys alliza ion peaks Tc1 and Tc2) is eco ded was 126 °C. This
is he ideal sel -nuclea ion empe a u e (Ts ideal), and a his SN
empe a u e, mos o he HDPE-1 d ople s c ys allized a a
empe a u e iden ical o he one o bulk HDPE-1, and only a
e y small po ion o he d ople s c ys allized a a highe
supe cooling (i.e., a 75 °C). This small d ople popula ion
ha c ys allizes displaying e y small magni ude exo he ms a
peak c ys alliza ion empe a u es close o 75 °C needs e en
lowe Ts alues o become sel -nuclea ed. This could be ela ed
o he d ople s’small dimensions. We ha e no s udied in
de ail hei beha io , as hey ep esen a mino ac ion o he
o al c ys alliza ion en halpy o he HDPE-1 phase.
Below Ts= 126 °C, mo e d ople s unde go sel -nuclea ion,
and he a ea o he exo he m loca ed a Tc2 con inues o
dec ease g adually un il i disappea s comple ely a Ts= 124
°C. In pa allel, clea changes in he final hea ing scans ha e
occu ed whe e he appea ance o ano he small mel ing peak
a a highe empe a u e is obse ed a e SN a Tso 125 and
124 °C. The addi ional small endo he ms obse ed a e SN a
125−124 °C (indica ed by an a ow in Figu e 4b) a e due o
he mel ing o he c ys als ha we e annealed and hickened
du ing 5 min a Ts. F om he p e iously men ioned
obse a ion, i can be concluded ha he sel -nuclea ion and
annealing domain (DIII) o mos o he HDPE-1 d ople s is
loca ed a Ts alues below 126 °C.
To confi m ha he peculia SN beha io was no ela ed o
any change occu ing in he HDPE-1 du ing mel blending, he
HDPE-1 phase in he 90/10 PS/HDPE-1 blend was eco e ed
by ex ac ing PS wi h ho oluene and sepa a ing he HDPE-1
phase ia cen i uga ion. The eco e ed ma e ial was hen
subjec ed o SN s udy. The ob ained esul s a e shown in
Figu e S5. I is clea ha he eco e ed HDPE-1 exhibi s SN
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beha io simila o he one o he nea HDPE-1 wi h a e y
na ow DII (only 0.5 °C wid h), which s a s a a low Ts(129
°C). These esul s sugges ha he peculia SN beha io
obse ed in Figu e 4 is due o some changes in he s a e o he
in e ace be ween PS and HDPE-1 in he blend. Fu he
explana ion will be gi en in he Discussion sec ion.
Resul s o he SN o he HDPE-1 dispe sed phase in he 85/
15, 80/20, 70/30, and 50/50 PS/HDPE-1 blends (blends wi h
highe HDPE-1 con en s) a e shown in Figu es S6−S9,
espec i ely. On he o he hand, Figu es S10−S15 p esen he
SN o he HDPE d ople s in o he sys ems (o he immiscible
blends p epa ed using diffe en HDPE g ades and/o diffe en
amo phous ma ices), which we e es ed o assess he
gene ali y o his obse a ion. All he es ed blends exhibi ed
simila SN beha io o he 90/10 PS/HDPE-1, in pa icula
wi h DII s a ing a a e y high Ts(always ≥150 °C).
A e examining he sel -nuclea ion o HDPE d ople s in
immiscible blends wi h amo phous ma ices, a semic ys alline
ma ix is used, and he SN beha io o he dispe sed HDPE-2
mino phase is in es iga ed.
3.3.3. Sel -Nuclea ion o he HDPE-2 in he 90/10 Nylon
6/HDPE-2 Blend. Figu e 5a,b p esen s DSC cooling and
hea ing scans o a 90/10 Nylon 6/HDPE-2 blend a he
indica ed Ts alues. Meanwhile, he collec ion o he Tcand
ΔHcas a unc ion o he applied Tsis supe imposed on op o
he s anda d DSC mel ing endo he m o he HDPE-2 phase
and shown espec i ely in Figu e 5c,d.
A Ts> 160 °C, bo h c ys alliza ion and mel ing o he
HDPE-2 mino phase a e in a ian . In his empe a u e ange,
which ep esen s DI, he c ys alliza ion beha io is con olled
only by high- empe a u e- esis an he e ogenei ies.
Again, he SN DII began a a e y high Ts alue o a linea
polye hylene sample, namely, 160 °C. In ac , by lowe ing Ts o
Figu e 5. (a) DSC cooling scans (a 20 °C/min) o he 90/10 Nylon 6/HDPE-2 blend a e 5 min a he indica ed Ts, (b) hea ing scans (a 20 °C/
min) a e he cooling uns shown in (a), and (c,d) collec ion o Tcand ΔHcas a unc ion o he employed Ts(x-axis) supe imposed on op o he
s anda d DSC mel ing ace.
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160 °C, he Tco he HDPE-2 phase inc eases om 114 o
a ound 115.5 °C. By eaching he lowe limi o he DII (a
130 °C), a Tco abou 117.5 °C was achie ed. Meanwhile, no
no iceable change in he mel ing beha io is obse ed.
DIII (sel -nuclea ion and annealing domain) s a s a Ts alues
lowe han 130 °C. In his empe a u e ange and e en hough
he Tcis s ill inc easing wi h he dec ease in Ts, he sample is
only pa ially mol en, and he unmol en c ys al agmen s
expe ience annealing, become hicke , and esul in a second
mel ing peak a highe empe a u es in he final hea ing scan.
4. DISCUSSION
In his pa , he peculia sel -nuclea ion beha io o he HDPE
d ople s and he injec ion o sel -nuclei a empe a u es well
abo e he equilib ium mel ing poin will be conside ed.
As we men ioned p e iously, a e sel -nuclea ing he
eco e ed HDPE-1 ( om 90/10 PS/HDPE-1), i was ound
ha i s beha io is simila o he one o nea HDPE-1. This
means ha he obse ed SN beha io in he blends could be
a ibu ed o he exis ence o a nuclea ing in e ace wi h he
ma ix and, mos p obably, o he oughness o he su ace.
In a p e ious wo k, Mulle e al.
42
in es iga ed he
c ys alliza ion o he 80/20 PLA/PCL immiscible blend om
he glassy s a e. They analyzed he cold c ys alliza ion o PLA
a e c ys allizing he PCL phase a diffe en Tc alues. They
epo ed a di ec co ela ion be ween he c ys allini y deg ee o
he dispe sed PCL d ople s and he cold c ys alliza ion a e o
PLA, in which he highe he c ys allini y deg ee o PCL, he
lowe he PLA cold c ys alliza ion empe a u e Tcc. A highe
c ys allini y deg ee o he PCL d ople s means mo e sh unk
d ople s, which induced some addi ional oughness and s ess
a he PLA/PCL in e ace and led o a as e PLA nuclea ion.
In addi ion, Gałeski and Ba czak
43
epo ed significan
changes in he su ace s a e in immiscible blends (o in a
sandwich o wo immiscible componen s) a e c ys allizing
one componen . The in e ace be ween he wo componen s
shi s om fla o an in e ace ull o ca i ies and g oo es. The
eason behind he modifica ion o he in e ace is he sh inkage
o he c ys allizing componen s (when i con e s om a mel
o he semic ys alline s a e), which induces significan
de o ma ion o he in e ace and pushes he mel o he
second componen o flow and fill hose g oo es and ca i ies.
As a consequence, mo e su ace a ea and mo e con ac
be ween he wo componen s will be ob ained. On he o he
hand, he appea ance o a new ough su ace will in p inciple
a o su ace nuclea ion and accele a e he nuclea ion
p ocess.
7,15
I should be no ed ha ough o w inkled su ace
opog aphies esul ing om he so-called buckling ins abili ies
a e indeed ob ained in a a ie y o sys ems, om elec ospun
polyme fibe s
44
o films on a subs a e,
45
as a consequence o
a de o ma ion misma ch be ween wo phases ( ela ed o
ins ance o he mal expansion o sh inkage).
Fo he PS/HDPE-1 immiscible blends p epa ed he e,
du ing cooling om he mel (s ep (2) in he SN p o ocol),
he HDPE-1 d ople s unde go an ini ial c ys alliza ion p ocess
s a ing om a empe a u e o abou 119 °C. The HDPE-1
d ople su ace will hen become ough, as a consequence o
lamellae o ma ion. Howe e , his oughness canno be
imp in ed on he PS su ace because o he low empe a u e,
close o i s Tg, and ela ed high iscosi y. On he o he hand,
upon hea ing om he s anda d s a e o Ts(s ep 3 o he SN
p o ocol) he PS becomes less iscous and can adhe e o he
HDPE-1, eplica ing i s ough su ace opog aphy.
44
We
assume ha he ob ained addi ional oughness can be e ased
Figu e 6. (a,b) SEM mic og aphs and (c,d) TEM mic og aphs o he SN 90/10 PS/HDPE-1 blend; (a,c) SN a 170 °C and (b,d) SN a 140 °C.
The a ows show poin s om which some HDPE-1 c ys alline lamellae s a . The applied he mal p o ocol be o e his SEM/TEM analysis is shown
in Figu e S1.
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