Enhanced and Reusable Poly(hydroxy urethane)-Based Low Temperature Hot-Melt Adhesives

Enhanced and Reusable Poly(hyd oxy u e hane)-Based Low
Tempe a u e Ho -Mel Adhesi es
Al a o Gomez-Lopez, Na oa Ayensa, B uno G igna d, Lou des I us a, Inigo Cal o, Alejand o J. Mulle ,
Ch is ophe De embleu , and Ha i z Sa don*
Ci e This: ACS Polym. Au 2022, 2, 194−207
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ABSTRACT: Poly(hyd oxy u e hane)s (PHUs) based on 5-
membe ed cyclic ca bona es ha e eme ged as sus ainable
al e na i es o con en ional isocyana e-based polyu e hanes.
Howe e , while om he poin o iew o sus ainabili y hey
ep esen an imp o emen , hei p ope ies a e s ill no compe i i e
wi h con en ional polyu e hanes. In his wo k, he po en ial o
PHUs as e e sible ho -mel adhesi es is discussed. We ound ha
wi h a judicious choice o eagen s (i.e., he dicyclic ca bona e and
diamine), he de imen al hyd ogen bonding be ween he so
segmen o he chains and he pendan hyd oxyl g oups was
pa ially a oided, hus impa ing PHUs wi h ho -mel adhesion
p ope ies (i.e., adhesion a ele a ed empe a u es and cohesi e-
ness a a empe a u e lowe han Tg/Tm). The impo ance o a balanced ha d o so segmen a io, along wi h he ele ance o he
chain ex ende in he final p ope ies, is highligh ed. Addi ion o alipha ic diamines (HMDA, 1,12-DAD) esul ed in ubbe y
ma e ials, while he employmen o cycloalipha ic (CBMA) o a oma ic ones (MXDA, PXDA) led o ma e ials wi h ho -mel
adhesi e p ope ies. The he mo e e sibili y o all composi ions was assessed by ebonding specimens a e lap-shea es s. Lap-shea
s eng h alues ha we e compa able o he i gin adhesi es we e obse ed. The b eaking and e o ma ion o hyd ogen bonding
in e ac ions was demons a ed by FTIR measu emen s a diffe en empe a u es, as well as by heological equency sweep
expe imen s. In o de o mi iga e he nega i e impac o he low mola mass PHUs and o enhance he se ice empe a u e o he
adhesi es, a hyb id PHU was p epa ed by adding a small amoun o an epoxy esin, which ac s as a c oss-linke . These hyb id PHUs
main ain he he mo e e sibili y displayed by he moplas ic PHUs while p o iding be e adhesion a ele a ed empe a u es. We
belie e ha his wo k p o ides some impo an insigh s in o he design o PHU-based ho -mel adhesi es.
KEYWORDS: non-isocyana e polyu e hanes, poly(hyd oxy u e hane)s, ho -mel , adhesi es, sus ainabili y, g een chemis y
1. INTRODUCTION
Ho -mel adhesi es (HMAs) a e ypically sol en - ee he mo-
plas ic ma e ials o ligh ly c oss-linked he mose s, which a e
cha ac e ized by hei solid s a e a low empe a u es, while
p esen ing low iscosi y and good flowing abo e his
empe a u e.
1
They p o ide g ea bond s eng hs in sho
pe iods upon cooling, which makes hem e y a ac i e
ma e ials when as p ocessing is equi ed. Mo eo e , hey a e
ela i ely easy o handle, economical, and clean unning. Thus,
ho -mel adhesi es a e used in a la ge ange o applica ions
including he au omo i e indus y, packaging, bookbinding,
shoe making, ex iles, labeling o bo les, disposable p oduc s,
s amps, and en elopes.
1−3
Indeed, he global ho -mel adhesi e
ma ke is p ojec ed o each USD 9.46 billion by 2022.
4
Typical o mula ions o HMAs consis o polyme s (∼33%),
low mola mass esins (∼33%), waxes (∼32.5%), and
an ioxidan s (∼0.5%).
5
Polyme s impa s eng h and ho
ack, esins p o ide a lowe iscosi y, imp o e we abili y, and
adjus he Tgo he sys em, and waxes enhance se ing speed
and p o ide hea esis ance.
6
Among he polyme s used o
HMAs, polyu e hanes (PUs) possess high popula i y as hey
show be e low empe a u e p ope ies and g ea e flexibili y
han hose based on e hylene inyl ace a e and polyamide.
7
Mo eo e , hey p esen excellen adhesion on su aces ha a e
difficul o adhe e, such as low su ace oughness ma e ials.
PUs a e syn hesized in wo s eps. Fi s , an isocyana e-
e mina ed p epolyme is p epa ed by eac ing a long chain
polyol wi h an excess o diisocyana es. The p epolyme is
u he eac ed in a second s ep wi h a low mola mass diol,
known as a chain ex ende . The esul ing polyme al e na es
so segmen s, mainly con aining he polyol, and ha d
Recei ed: No embe 10, 2021
Re ised: Decembe 21, 2021
Accep ed: Decembe 22, 2021
Published: Janua y 10, 2022
A iclepubs.acs.o g/polyme au
© 2022 The Au ho s. Published by
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h ps://doi.o g/10.1021/acspolyme sau.1c00053
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segmen s, o med h ough he eac ion o he isocyana e and
he sho chain diols. The incompa ibili y be ween he wo
phases leads o a phase-sepa a ed s uc u e consis ing o so
and ha d domains. This phase sepa a ion, which is b ough by
hyd ogen bond-based c oss-linking o he ha d segmen ,
impa s unique mo phological and physical p ope ies.
8
Ne e heless, adi ionally, he syn hesis o PUs equi es he
use o oxic isocyana es, which may p o oke as hma and
de ma i is.
9−11
The chemicals employed o he p epa a ion o
he isocyana es hemsel es a e also highly oxic, o en in ol ing
he use o phosgene,
12
and he e o e, issues ela ed o oxici y
ep esen majo d awbacks in he use o con en ional PUs. To
o e come hese issues, in he pas decade in ensi e effo s ha e
been di ec ed owa d he p epa a ion o PUs using sa e and
mo e sus ainable app oaches ha a oid he use o isocyana es.
One o he me hods ha is conside ed o be compe i i e
wi h con en ional isocyana e-based PUs is he s ep-g ow h
polyme iza ion o dicyclic ca bona es wi h diamines. Ini ially
de eloped in 1957 by Dye and Sco
13
as a me hod o
p oduce polyu e hanes a oiding he use o mois u e sensi i e
isocyana es, he cu en ends o g eene chemis ies ha e
g ea ly boos ed he de elopmen o his chemis y.
14,15
Indeed,
his p ocess o he p oduc ion o non-isocyana e polyu -
e hanes (NIPUs) is 100% a om economic, and he cyclic
ca bona e monome s a e easily accessible by he acile [3 + 2]
chemical inse ion o CO2in o (na u al) epoxy esins.
16−20
Howe e , he polyme iza ion also has some d awbacks in he
p epa a ion o ho -mel s.
On he one hand, he ing-opening o he cyclic ca bona e
gene a es hyd oxyl g oups along he polyme ic chain,
p o iding he so-called poly(hyd oxy u e hane)s (PHUs).
These hyd oxyl g oups can es ablish s ong hyd ogen bonds
wi h he so segmen , enhancing he miscibili y be ween bo h
phases and supp essing he dis inc i e phase sepa a ion o
con en ional PUs. Some au ho s ha e ackled his issue
employing diffe en app oaches, selec ing ca e ully he
monome s
21−24
o he syn he ic condi ions.
25
Howe e , he
esul ing polyme s showed elas ome ic-like beha io , and
adhesion pe o mance was no epo ed. On he o he hand,
while high mola mass PUs can be easily achie ed, in he case
o PHUs, he inhe en slow aminolysis o he cyclic ca bona es
and he in e - and in amolecula hyd ogen bonding o he
PHU chains gi es ise o low mola masses, which limi s hei
use as The moplas ic Polyu e hane (TPU) ma e ials.
26
Recen ly Sukuma an Nai e al. epo ed he o ma ion o
PHU-based ho -mel adhesi es wi h a he mal ansi ion close
o 80−100 °C by eac ing a oma ic and cycloalipha ic dicyclic
ca bona es wi h amino- e mina ed oligo(p opylene glycol).
27
The au ho s demons a ed he he mo e e sibili y o he
ma e ials a e manually debonding and hen ebonding he
subs a es wi h no no iceable loss o he adhesion alues. While
hey ound a ela ion be ween he hyd ogen bonding and he
he mo e e sibili y, a a ional s udy o unde s and he
s uc u al needs o design PHU ho -mel adhesi es was no
p o ided.
He ein, we aim o p o ide an in-dep h s udy o he s uc u al
ea u es needed o p epa e PHU-based HMAs. A se ies o
PHUs was p epa ed using a ious ha d ( eso cinol-based) and
so (PPG-based) dicyclic ca bona es and diamines (alipha ic,
cycloalipha ic, o a oma ic). The influence o he mola
composi ion o blends o dicyclic ca bona es, as well as he
na u e he eo , on he adhesion p ope ies is add essed by
heology, p obe ack, lap-shea , shea adhesion ailu e empe -
a u e (SAFT), and s a ic shea esis ance measu emen s.
Finally, in o de o mi iga e limi a ions a ising om he low
mola mass PHUs, hyb id PHUs we e p epa ed by combining
he non-isocyana e chemis y wi h he epoxy esin one. The
addi ion o he epoxy esin chemis y endowed PHU ho -mel
adhesi es wi h be e se ice empe a u es making hem
aluable al e na i es o adi ional isocyana e-based PU HMAs.
2. EXPERIMENTAL SECTION
2.1. Reagen s and Ma e ials
Poly(p opylene glycol) diglycidyl e he (Mn∼640 g mol−1)
(PPGDGE), Reso cinol diglycidyl e he (RDGE), 1,4-bu anediol
diglycidyl e he (BDGE), e abu ylammonium iodide (98%) (TBAI),
1,12-diaminododecane (98%) (1,12-DAD), p-xylylenediamine
(PXDA) (99%), and hexame hylenediamine (HMDA) (98%) we e
pu chased om Me ck KGaA, Ge many. 1,3-Cyclohexanebis-
(me hylamine) (cis and ans mix u e) (CBMA) (98%) was
pu chased om TCI Eu ope N.V., Belgium. m-Xylylenediamine
(99%) (MXDA) was pu chased om Ac os O ganics, Belgium. 1,3-
Bis(2-hyd oxyhexafluo oisop opyl)benzene (97%) (1,3-bis-HFIB)
was pu chased om Fluo ochem, Uni ed Kingdom. Solid epoxy
esin based on bisphenol A ( ade name: D.E.R. 671) was kindly
supplied by O ibay G oup Au omo i e S.L., Spain. Deu e a ed
dime hyl sul oxide (DMSO-d6) was pu chased om Me ck KGaA,
Ge many. The cyclic ca bona es used in his s udy we e syn hesized
by CO2coupling wi h he comme cial p ecu so s using a homemade
ca alys epo ed elsewhe e by some o he au ho s.
28
All eagen s
we e used wi hou u he pu ifica ion.
Plexiglas XT 20070 [poly(me hyl me hac yla e) (PMMA), hick-
ness o 3 mm], wood (oak, hickness o 5 mm), and high densi y
polye hylene (PE-HD, hickness o 3 mm) subs a es we e pu chased
om Rocholl GmbH, Ge many. S ainless s eel AISI 316 (SS,
hickness o 1.95 mm) subs a es we e kindly supplied by O ibay
G oup Au omo i e S.L., Spain.
2.2. Cha ac e iza ion Techniques
1H NMR spec a we e eco ded on a B uke Ad ance DPX 300
spec ome e a 25 °C. Deu e a ed dime hyl sul oxide (DMSO-d6)
was used as sol en . FT-IR spec a we e ob ained using an FT-IR
spec opho ome e (Nicole is20 FT-IR, The mo Scien ific Inc., USA)
equipped wi h a enua ed o al eflec ance (ATR) wi h a diamond
c ys al. Spec a we e eco ded be ween 4000 and 600 cm−1wi h a
spec um esolu ion o 4 cm−1. All spec a we e a e aged o e 16
scans. The spec a a high empe a u es we e ob ained using an FT-IR
spec opho ome e (Nicole 6700FT-IR, The mo Scien ific Inc.,
USA) equipped wi h a specap a iable empe a u e ansmission
cell. Spec a we e eco ded be ween 4000 and 400 cm−1wi h a
spec um esolu ion o 4 cm−1, and 64 scans we e signal a e aged.
Samples we e p epa ed by dissol ing in THF and cas ing on KB
windows. A diffe en ial scanning calo ime e (DSC-Q2000, TA
Ins umen s Inc., USA) was used o analyze he he mal beha io o
he samples. 6−8 mg o he samples was scanned om −70 o 120 °C
a a hea ing a e o 20 °C min−1. The glass ansi ion empe a u es
(Tg) we e aken om he inflec ion poin in he hea capaci y cu e.
Dynamic mechanical empe a u e analysis (DMTA) expe imen s
we e pe o med using a ec angula sample o he c oss-linked
ma e ials (2 ×3.5 ×1 mm), using a T i on 2000 DMA om T i on
Technology in bending mode. Tes s we e pe o med a 1 Hz, a a
hea ing a e o 4 °C min−1 om −35 o 160 °C. Size exclusion
ch oma og aphy (SEC) was pe o med in THF a 35 °C(flow a e o
1 mL min−1) using a Wa e s ch oma og aph equipped wi h h ee
columns in se ies (S y agel HR1, HR2, and HR4) wi h inc easing
po e sizes ( om 100 o 106Å). Toluene was used as a ma ke .
Polys y enes o diffe en mola masses, anging om 106 o 436,000 g
mol−1, we e used o he calib a ion. A omic o ce mic oscopy (AFM)
measu emen s we e ca ied ou unde ambien condi ions using an
AFM Dimension ICON (B uke ). Topog aphy AFM images we e
collec ed in apping mode employing TEST-V2 ips wi h a esonance
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equency o 320 kHz and a sp ing cons an o 37 N m−1. Samples
we e p epa ed cas ing 0.15 mL o a 20 mg mL−1solu ion o he
polyme on glass and spin-coa ing a 1000 pm o 30 s, ob aining a
coa ing o he polyme o e he glass.
2.2.1. Rheological Measu emen s. Tempe a u e and equency
sweep expe imen s we e pe o med in a s ess-con olled An on Paa
Physica MCR101 heome e . Tempe a u e sweep measu emen s we e
ca ied ou om −10 o 120 °C (some expe imen s we e finished
be o e due o inconsis ency o he da a), a a equency o 1 Hz and a
s ain in he linea iscoelas ic ange o he ma e ials, which alue
depended on he s udied polyme , using he 15 mm disposable
pa allel pla e geome y. F equency sweep expe imen s we e ca ied
ou a a cons an empe a u e in he linea iscoelas ic egime o he
ma e ials, om 0.01 o 100 Hz (0.0628 o 628 ad s−1). The
expe imen s we e ca ied ou using he 15 mm disposable pa allel
pla e geome y. The ypical e minal beha io o nons uc u ed
polyme s is cha ac e ized wi h eqs 1 and 2.
29,30
ω′∼G
2
(1)
ω′′ ∼G(2)
whe e G′is he s o age modulus, G′’is he loss modulus, and ωis he
angula equency in ad s−1.
The e o e, he linea fi o he double loga i hm plo s gi es ise o
slopes o 2 o G′and 1 o G′’(eqs 3 and 4).
ω′∼Gm
l
og( ) log( )
(3)
whe e G′is he s o age modulus, ωis he angula equency in ad s−1,
and mis equal o 2.
ω′′ ∼Gm
l
og( ) log( ) (4)
whe e G′’is he loss modulus, ωis he angula equency in ad s−1,
and mis equal o 1.
Linea fi s o he cu es a low equencies we e pe o med
employing he da a analysis ools o O igin 2020b.
2.2.2. Shaping o Ho -Mel Adhesi es. To p epa e he ho -mel
adhesi e samples, ∼190 mg ( heology), ∼90 mg (p obe ack), ∼250
mg (lap-shea ), o ∼500 mg (SAFT and shea esis ance) o he
polyme was shaped in an o en be ween wo subs a es (Teflon pape
shee s we e used o a oid adhesion o he subs a es) wi h space s o
con ol he hickness (0.6−0.8 mm). Ci cula samples o 15 mm
diame e ( heology), a ound 8−10 mm diame e ( ack p obe),
ec angula samples (12.5 ×25 mm2), and squa es o 25 ×25 mm
(625 mm2) (SAFT and shea esis ance) we e p epa ed. In he case o
hyb id ho -mel adhesi es made by adding 15 w % D.E.R. 671, a 1 kg
weigh was placed o e he second subs a e o accele a e he shaping
o he ho -mel adhesi e.
2.2.3. P obe Tack Tes s. P obe ack measu emen s we e ca ied
ou using a TA.HDPlus ex u e analyze (Tex u e Technologies,
Hamil on, MA, USA) unde con olled empe a u e in an o en. A 5
mm s ainless s eel cylinde p obe was mo ed downwa d a a speed o
0.1 mm s-1 un il i was b ough in o con ac o he adhesi e su ace.
Immedia ely a e he con ac (10 s o dwelling ime, wi h a
comp essi e o ce o 1 N), he c osshead was allowed o mo e upwa d
a a speed o 300 mm min−1un il he p obe was comple ely sepa a ed
om he adhesi e. Tes s we e pe o med on 3 samples o each
o mula ion o de e mine he a e age lap-shea s eng h, and he
s anda d de ia ion was used as he e o .
2.2.4. Lap-Shea Tes s. The adhesion p ope ies o he PHUs
we e e alua ed a oom empe a u e using an Ins on 5569 and
applying a pa allel o ce o he adhesi e bond wi h a displacemen a e
o 50 mm min−1. Co esponding subs a es wi h dimensions o 100
mm ×25 mm ×( hickness o each subs a e) mm we e bonded wi h
an adhesi e con ac a ea o 312.5 mm2(25 mm ×12.5 mm). The
g ipping leng h on bo h sides o he es specimens was 25 mm. Tes s
we e pe o med on 5 samples o each o mula ion o de e mine he
a e age lap-shea s eng h, and he s anda d de ia ion was used as he
e o . The na u e o adhesion ailu e was also eco ded based on
isual inspec ion o he sample ollowing he es .
Lap-shea es specimens we e p epa ed as ollows. The su aces o
he subs a es we e cleaned ollowing he p ocedu e desc ibed
elsewhe e.
31
Ho -mel adhesi e samples we e applied a e shaping
on o one o he pai s o he cleaned subs a es and we e in oduced
in o an o en (Memme Vacuum D ying O en VO200, The mo
Scien ific Inc., USA) a 80 o 100 °C o 5 min, allowing he so ening
o he polyme . A e wa d, bo h subs a es we e pu oge he and
placed again in o he o en o 5 min a he co esponding empe a u e
(80 o 100 °C). The adhesi e join s we e s o ed a ambien
empe a u e o 24 h p io o es ing.
Fo hyb id ho -mel adhesi es, polyme s we e pu oge he wi h he
subs a es and di ec ly placed in he o en a 120 °C o 10 min wi h 1
kg o e he glue line o imp o e he we ing o he adhe ends.
A e finishing he lap-shea es , he he mo e e sible adhesion o
he adhesi es was es ed, o p o e he efficiency o he ma e ial o
epea ed use. Each pai o subs a es was s uck again and was placed
in o he o en unde he same condi ions o he fi s applica ion.
Following he same p ocedu e, lap-shea measu emen s we e
epea ed.
Subsequen ly, a second ebonding was done. In his case, adhesi es
we e hea ed and emo ed om he subs a es manually. The es
specimens we e p epa ed, and lap-shea measu emen s we e ca ied
ou ollowing he same p ocedu e, employing he ecycled ma e ial o
shaping he adhesi es.
2.2.5. Shea Adhesion Failu e Tempe a u e (SAFT) Tes s.
Shea es s we e pe o med on s ainless s eel panels using SAFT
equipmen . Specimens we e p epa ed ollowing he p ocedu e
desc ibed o lap-shea es s. Specimens we e condi ioned a 298 K
wi h 50% o R.H. o 1 day. A mass o 1000 g was hung o each panel,
and hey we e placed in o an o en. The empe a u e was inc eased
om 30 o 217 °C a a hea ing a e o 1 °C min−1. The empe a u e o
ailu e was epo ed oge he wi h he na u e o adhesi e ailu e. Tes s
we e pe o med on 4 samples o each o mula ion o de e mine he
a e age lap-shea s eng h, and he s anda d de ia ion was used as he
e o .
2.2.6. Shea Resis ance Tes s. Shea esis ance es s we e
pe o med ollowing he same p ocedu e and using he same
equipmen as he one desc ibed o SAFT expe imen s. Ins ead o
applying a hea ing a e, empe a u e was se a 30 °C and he ime o
ailu e was eco ded. The na u e o adhesion ailu e was also eco ded
based on isual inspec ion o he sample ollowing he es .
3. RESULTS AND DISCUSSION
3.1. P epa a ion o Model Ho -Mel PHU adhesi es
In ou ques o ho -mel adhesi es, a PHU made o a igid
diamine such as m-xylylenediamine (MXDA) combined wi h
eso cinol dicyclic ca bona e (RdiCC) was ini ially syn hesized
a 80 °C o 24 h (Scheme 1).
Scheme 1. Syn hesis o PHU Homopolyme s om RdiCC
and MXDA
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The comple ion o he eac ion was ollowed by FTIR-ATR
and 1H NMR, which a e desc ibed in he Suppo ing
In o ma ion (Figu es S1 and S2, espec i ely).
The mal cha ac e iza ion o he MXDA-RdiCC PHU was
pe o med by diffe en ial scanning calo ime y (Figu e S3).
The glass ansi ion empe a u e (Tg) was calcula ed om he
second hea ing scan and es ima ed o be 51 °C, confi ming i s
solid s a e a ambien condi ions. As a esul , his ma e ial is
oo s iffa oom empe a u e o be used as ho -mel adhesi es.
We ha e ecen ly epo ed he impo ance o he balance
be ween ha d and so segmen s o designing adhesi es wi h
op imal p ope ies.
31,32
The e o e, polyp opylene glycol
dicyclic ca bona e (PPGdiCC) was in oduced in he
RdiCC/MXDA o mula ion o b ing so ness o he PHU
HMAs,
In o de o s udy he effec o he so segmen on he
adhesi e p ope ies, h ee o mula ions a ying he
PPGdiCC:RdiCC mola a ios (70/30, 60/40, and 50/50)
we e used and we e eac ed wi h MXDA a 80 °C o 24 h
(samples named 70/30-MXDA, 60/40-MXDA, and 50/50-
MXDA, Scheme 2). Fo compa a i e pu poses, a homopol-
yme based on pu e PPGdiCC was also p epa ed. The
copolyme iza ions we e also moni o ed using FTIR-ATR and 1
H NMR spec oscopy. As expec ed he cha ac e is ic bands o
he ca bona e g oup bo h in he FTIR (1780−1790 cm−1)
(Figu e S4) and in he 1H NMR (5.02, 4.62−4.53, and 4.23−
4.13 ppm) disappea ed and new bands a ibu ed o he
o ma ion o he u e hane ca bonyl a 1696 cm−1and a 7.6−
7.8 ppm (-NH-C(O)O) and a 4.15 ppm (-CH2NH-) we e
obse ed, which is in ag eemen wi h he expec ed ing-
opening o he cyclic ca bona es (Figu es S5, S6, S7, and S8).
In Figu e S3 he DSC cu es o all he copolyme s and he
homopolyme based on PPGdiCC ca bona e a e shown. As
expec ed, he homopolyme s based on PPGdiCC exhibi ed he
lowes Tg(−11 °C) while he es o he ma e ials ha e
in e media e Tg alues anging om 0 o 9 °C which a e mo e
app op ia e o low empe a u e ho -mel adhesi e applica-
ions.
33
Then, dynamic heological measu emen s we e pe o med
o ge a be e unde s anding o he iscoelas ic esponse o he
PHUs. Bo h empe a u e sweep as well as equency sweep
expe imen s we e ca ied ou o elucida e he beha io o he
adhesi es as ho -mel s. Figu e 1 shows he e olu ion wi h
empe a u e o he s o age (G′) and loss (G′’) moduli and he
an δ o he diffe en copolyme composi ions.
As obse ed in he DSC he mog ams, he Tgdec eased as
he PPGdiCC mola a io was inc eased because he chain
mobili y was enhanced. Thus, o he 70/30-MXDA
composi ion iche in flexible PPGdiCC, a Tg alue o 10.3
°C was measu ed. This alue inc eased up o 30.5 °C when an
equimola amoun o bo h dicyclic ca bona es was employed
(50/50-MXDA). Mo eo e , he 50/50-MXDA o mula ion
showed a slowe decay o he moduli a e he Tg, p obably due
o he g ea e densi y o hyd ogen bonds. Sho e leng hs
be ween u e hane g oups gi es ise o a highe densi y o
hyd ogen bonds.
24
In addi ion, his decay was no as
p onounced as i should be o polyme s ha do no p esen
any sup amolecula in e ac ions.
In o de o confi m he p esence o absence o hese
in e ac ions wi hin he o mula ions, he adhesi es we e
cha ac e ized by equency sweep measu emen s a 0, 10, 25,
50, 80, and 100 °C(Figu e 2).
F equency sweep measu emen s showed ha below he Tg,
50/50-MXDA p esen ed a solid-like beha io , and a 0 °C, G′
emained abo e G′′ o e he whole ange o equency. Wi h
he inc ease o empe a u e, he ansi ion c osso e poin om
solid-like o liquid-like beha io (G′=G′′) mo ed o highe
equencies. This is due o he g ea e mobili y o he chains a
highe empe a u es, and he e o e, he ma e ial exhibi s a
liquid-like beha io a sho e imes. On he o he hand, he
Scheme 2. P epa a ion o PHU Copolyme s om PPGdiCC, RdiCC, and MXDA
Figu e 1. S o age (G′) and loss (G′’) moduli and an δ alues o 70/
30-MXDA (blue), 60/40-MXDA (g een), and 50/50-MXDA ( ed)
composi ions be ween −10 and 100 °C.
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mo e liquid-like beha io o he 70/30-MXDA was confi med,
as his ansi ion occu ed a highe equency when compa ed
wi h 50/50-MXDA a he same empe a u es. Abo e he Tg,
bo h composi ions p esen ed a liquid-like beha io a low
equencies, whe e G′′ was always highe han G′.Fo
nons uc u ed ma e ials his e minal zone is cha ac e ized by
slopes o −2 and −1 o G′and G′′, espec i ely, in he double
loga i hmic ep esen a ion and co esponds o a flow
egime.
29,30,34,35
Howe e , in he case o he PHU adhesi es,
he slope o he cu es diffe ed om hese alues, e en a 100
°C(Figu e S9). This sugges s he o ma ion o s uc u ed
polyme ne wo ks h ough hyd ogen bonds be ween he
poly(hyd oxy u e hane)s chains, which hinde he flowing o
he ma e ial. Mo eo e , his p ocess is ully e e sible as when
ca ying a second equency sweep, G′as well G′′ we e
p ac ically equal o he fi s measu emen , suppo ing he
p esence o e e sible hyd ogen bonds (Figu e S10).
As addi ional p oo o hyd ogen bond o ma ion, FTIR
spec a we e eco ded a diffe en empe a u es (Figu e 3). I is
well es ablished ha hyd ogen bonding and debonding can be
ollowed h ough he displacemen o he u e hane CO
s e ching ib a ion, N−H bending ib a ion, and O−H
s e ching ib a ion o he hyd oxyl g oups.
35−39
The CO
s e ching band egion o he in a ed spec a is shown in
Figu e 3a. A oom empe a u e, signals a 1720 and 1705
cm−1, which a e ela ed o ee and hyd ogen bonded
ca bonyls, espec i ely, can be obse ed. The ela i e
abso bance o he hyd ogen bonded band is highe , indica ing
ha he majo i y o he ca bonyl g oups a e aking pa in he
o ma ion o hyd ogen bonds ( ha can happen wi h he N−H
and OH).
36−38
When he empe a u e was inc eased, he
abso bance o he peak a 1720 cm−1, ela ed o he ee
ca bonyl g oups, inc eased. Howe e , he emaining shoulde
a 1705 cm−1shows ha hyd ogen bonds we e s ill p esen ,
e en a 130 °C. Following he same end, he N−H bending
band ed-shi ed om 1534 o 1517 cm−1(Figu e 3b) when
empe a u e inc eased due o he disappea ance o he
ancho ing es ic ion om hyd ogen bonding.
39
Finally, he
in ensi y o he maximum OH band a 3344 cm−1( ela ed o
hyd ogen bonded hyd oxyl g oups) blue-shi s wi h empe -
a u e (Figu e 3c). Because o he weakening o he hyd ogen
bonds wi h empe a u e, a g ea e ene gy was necessa y o
exci e he O−H bonds.
35
In e es ingly, when he sample was
cooled down, hese signals e u ned o he ini ial wa enumbe
alues, demons a ing he e e sibili y o he hyd ogen bonds.
Cha ac e iza ion o he 50/50-MXDA composi ion by AFM
showed ha he copolyme p esen ed some phase sepa a ion a
he nanoscale, confi ming ha he igid diamine can pa ly
p e en hyd ogen bonding o ma ion, educing he phase
mixing o ha d and so domains (Figu e S11).
In sum, combina ion o bo h so and ha d segmen cyclic
ca bona es allowed p epa a ion o good PHU candida es o
ho -mel adhesi es. Inco po a ion o g ea e amoun s o ha d
segmen hinde ed he ansi ion o liquid-like beha io
Figu e 2. (a) G′and G′′ alues be ween 0.01 and 100 Hz a diffe en empe a u es o (a) 50/50-MXDA and (b) 70/30-MXDA.
Figu e 3. Va iable empe a u e FTIR spec a in he egion o he (a) CO s e ching band, (b) N−H bending band, and (c) O−H s e ching
band o he 50/50-MXDA composi ion.
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ma e ial. Rheological measu emen s confi med he o ma ion
o s uc u ed ma e ials h ough hyd ogen bonding, which was
u he confi med by FTIR-ATR spec a.
3.2. Influence o he Monome S uc u e on he
Rheological Beha io o PHUs
3.2.1. Copolyme s Based on Alipha ic Dicyclic
Ca bona es. In o de o elucida e i he phase sepa a ion in
PHUs was due o he a oma ici y o he employed cyclic
ca bona e, eso cinol dicyclic ca bona e was subs i u ed o an
alipha ic one, 1,4-bu anediol dicyclic ca bona e (BdiCC). The
syn hesis was pe o med ollowing he p ocedu e employed o
he p epa a ion o he p e ious copolyme s (Scheme 3). FTIR
and 1H NMR cha ac e iza ion o he PHU ob ained is epo ed
in Figu es S12 and S13, espec i ely.
3.2.1.1. Rheological Beha io o 50/50BdiCC-MXDA.
Aiming o elucida e he iscoelas ic beha io o he copolyme
based on alipha ic dicyclic ca bona es, empe a u e and
equency sweep measu emen s we e ca ied ou . In a fi s
s ep, he s o age (G′) and loss (G′′) moduli as well as loss
angen ( an δ) we e e alua ed as a unc ion o empe a u e
(Figu e 4). As expec ed, he subs i u ion o he a oma ic
RdiCC o he alipha ic BdiCC ga e ise o less igid ma e ials
wi h a 2- old dec ease o he glass ansi ion empe a u e om
30.5 o 15.5 °C(Table S2). The decay o he moduli was also
as e , showing a g ea e liquid-like beha io when BdiCC was
employed. F equency sweep measu emen s co obo a ed he
as e decay o he moduli o e he whole ange o
empe a u es and a highe dependency o hese moduli wi h
equency (Figu e S14). Ne e heless, al hough he moduli
alues we e qui e diffe en , HMAs could be ob ained wi h
diffe en p ope ies when alipha ic cyclic ca bona es we e
employed o p epa e he PHU.
3.2.2. Tuning PHU Ho -Mel Adhesi es by Changing
he Diamine S uc u e. 3.2.2.1. Syn hesis and Cha ac e -
iza ion o Copolyme s Based on Diffe en Diamines. A e
confi ming ha HMA could be ob ained wi h bo h a oma ic
and alipha ic dicyclic ca bona es, we in es iga ed he impac o
he diamine on he HMA beha io . To do so, new copolyme s
based on a 50/50 mola a io o he dicyclic ca bona es a ying
he diamine we e p epa ed ollowing he p ocedu e o he
MXDA-based copolyme s (Scheme 4). The o ma ion o he
PHU was confi med by FTIR-ATR (Figu e S15) and 1H NMR
(Figu es S16 and Figu e S17). Su p isingly, he samples
p epa ed using HMDA and 1,12-DAD alipha ic diamines we e
no soluble in common sol en s such as THF, alcohols
(MeOH, E OH, o IPOH), DMF, DMAc, and DMSO, which
easily dissol ed he syn hesized a oma ic-based PHUs.
Tempe a u e sweep expe imen s we e ca ied ou o add ess
he iscoelas ic beha io o he ma e ials (Figu e 5). When
using he wo alipha ic diamines, HMDA as well as 1,12-DAD,
we did no obse e any HMA beha io as bo h ma e ials
exhibi ed la ge alues o he s o age (E′o G′) han he loss
modulus (E′′ o G′′) o e he whole ange o empe a u es,
which is ypical o c oss-linked ma e ials (Figu e 5a and b).
Mo eo e , when equency sweeps we e ca ied ou , E′and E′′
exhibi ed a low dependence on equency, demons a ing he
pe manen elas ic beha io o he ma e ial (Figu e S18a).
These samples also p esen ed high gel con en a e Soxhle
ex ac ions in efluxing THF (Table S2). I is hough ha in
his case he in e ac ions be ween hyd oxyl g oups and
u e hanes g oups a e s onge han in he case o a oma ic
diamines due o he g ea e mobili y and lowe s e ic
hind ance o he alipha ic chains, which dec ease he likelihood
o he ma e ial o flow (Figu e 5d). This esul shows ha igid
diamines a o ed selec i e o ma ion o hyd ogen bonding
ideal o p epa ing HMAs (Figu e 5e).
Scheme 3. Syn hesis o he Copolyme s Based on PPGdiCC, BdiCC, and MXDA
Figu e 4. G′,G′′, and an δ alues o 50/50-MXDA (blue) and 50/
50BdiCC-MXDA (g een). 50/50-MXDA was ep esen ed again o
easie compa ison o he da a. The cu e o 50/50BdiCC-MXDA was
s opped a lowe equencies due o he high signal- o-noise a io
caused by he low moduli alues.
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In o de o e alua e i his selec i e hyd ogen bonding could
be de i ed om π−πin e a c ions, bo h a oma ic and
cycloalipha ic diamines we e in es iga ed, including 50/50-
CBMA and 50/50-PXDA (Figu e 5c). These wo diamines
Scheme 4. P epa a ion o PHU Copolyme Composi ions Reac ing a 50/50 Mola Ra io o PPGdiCC/RdiCC wi h he
Co esponding Diamine, i.e. m-Xylylenediamine (MXDA), p-Xylylenediamine (PXDA), 1,3-Cyclohexanebis(me hylamine)
(CBMA), Hexame hylenediamine (HMDA), and 1,12-Diaminododecane (1,12-DAD)
Figu e 5. (a) G′,G′′, and an δ alues o 50/50-HMDA om empe a u e sweep measu emen s. (b) E′,E′′, and an δ alues o 50/50-DAD om
DMTA analysis. (This composi ion was s udied h ough DMTA analysis due o he easie sample p epa a ion.) (c) G′,G′′, and an δ alues o 50/
50-MXDA (blue), 50/50-CBMA (g een), and 50/50-PXDA ( ed). 50/50-MXDA was ep esen ed again o easie compa ison o he da a.
Schema ic ep esen a ion o he hyd ogen bonding (in o ange) hypo hesis o (d) non es ic ed mo emen alipha ic diamine-based PHUs and (e)
es ic ed mo emen igid ing-based diamines.
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Figu e 6. P obe ack s ess−s ain cu es pe o med a 100 °C o (a) 70/30-, 60/40-, and 50/50-MXDA composi ions and (b) 50/50-MXDA,
-CBMA, and -PXDA composi ions.
Figu e 7. Schema ic ep esen a ion o he p ocess o bonding s ainless s eel adhe ends o (a) he fi s ime, (b) a fi s ebonding a e b eaking he
bond line, and (c) a second ebonding a e b eaking wo imes he join . Lap-shea s eng h alues when samples we e applied on o he subs a es
a 100 °C o he (d) 70/30-, 60/40-, and 50/50-MXDA o mula ions and (e) 50/50-MXDA-, -CBMA-, and -PXDA-based composi ions. 50/50-
PXDA*(pu ple) was applied a 120 °C. The lap-shea s eng h alues a e he a e age o fi e es specimens, and he e o ba ep esen s he
s anda d de ia ion.
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showed simila beha io o 50/50-MXDA, and abo e he glass
ansi ion empe a u es bo h o mula ions suffe ed om a
p og essi e dec ease o he moduli. I has o be no ed ha his
dec ease o he moduli was a bi mo e p onounced when
cycloalipha ic diamine, CBMA, was employed, whe eas when
PXDA was used, he moduli emained highe wi h G′≈G′′,
indica ing a s onge in e ac ion o he chains. F equency
sweep measu emen s we e ca ied ou o u he analysis o
he heological beha io o he adhesi es (Figu e S19).
F equency sweep measu emen s a 80 °C o 50/50-CBMA
exhibi ed a simila decay o he moduli o he 50/50-MXDA
sample. Thus, possible π−πcon ibu ions we e disca ded o be
he dominan cause o he in e ac ions be ween PHU chains.
On he o he hand, 50/50-PXDA showed a c osso e be ween
G′and G′′ (Figu e S19d, ed spo s). A high equencies, his
ma e ial p esen ed a solid-like beha io (G′>G′′), while a
low equencies, he adhesi e beha ed like a liquid (G′′ >G′).
In he whole ange o equencies a 80 °C, adhesi es based on
PXDA exhibi ed la ge s o age as well as loss moduli han
MXDA-based ma e ials, demons a ing a key ole o he
a oma ic ing subs i u ion in hyd ogen bond in e ac ions. We
can conclude ha his selec i e hyd ogen bonding is mo e
ela ed o he con o ma ional effec o he diamine ha we use.
Thus, igid diamines limi o some ex en hyd ogen bonding in
PHUs, a o ing he o ma ion o HMAs.
3.2.3. Adhesi e P ope ies o PHU Copolyme s. The
adhesi e p ope ies o all PHU o mula ions we e e alua ed
h ough p obe ack, lap-shea , shea adhesion ailu e empe -
a u e (SAFT), and s a ic shea esis ance expe imen s. The
influence o he so o ha d segmen a io in samples based on
MXDA ha ing 50/50, 60/40, and 70/30 o PPGdiCC and
RdiCC, espec i ely, was in es iga ed. Subsequen ly, he
impac o he na u e o he dicyclic ca bona e and he diamine
was u he explo ed.
3.2.3.1. Dynamic Adhesi e P ope ies. Fi s , he ackiness
o he ho -mel PHUs was p obed by ack measu emen s a
wo empe a u es, 80 and 100 °C. The ack o ce was e alua ed
by he maximum s ess and capaci y o PHU adhesi es o
we ing he su ace by he e olu ion o he cu es and
fib illa ion o he adhesi e. Tes s we e pe o med using a 5 mm
s ainless s eel cylinde , allowing 10 s o con ac be ween he
p obe and he adhesi e a he co esponding empe a u e,
employing a debonding speed o 300 mm s−1. S ess−s ain
cu es o he composi ions a 80 and 100 °C a e depic ed in
Figu e S20 and Figu e 6a, espec i ely.
I was ound ha ega dless o he igid/so a io, he
o mula ions based on MXDA mani es ed a simila beha io ,
exhibi ing simila maximum s esses, al hough a 100 °C he
alue was lowe due o he highe liquid-like beha io o he
adhesi es a his empe a u e. A 80 °C, 50/50-MXDA
p esen ed a sha p dec ease a e he maximum s ess and an
adhesi e ailu e a e he es . The cohesion o he ma e ial was
highe han adhesion o ces, hinde ing fib illa ion o he
adhesi e. The e o e, a highe empe a u e would a o
applica ion o he adhesi e and he we ing o he adhe ends.
On he con a y, 70/30- and 60/40-MXDA showed a sligh
s abiliza ion o he s ess (pla eau), which was indica i e o he
c ea ion o a fib illa ing s uc u e. The la e p esen ed a
g ea e capaci y o fib illa ion since a highe pla eau was
achie ed. A 100 °C, all composi ions exhibi ed fib illa ion, i.e.
capaci y o we ing he su ace. The 50/50-MXDA o -
mula ion was able o e ain cohesi e p ope ies o a g ea e
ex en , which co ela ed wi h he highe G′ alues a 100 °Cin
he heological expe imen s.
On he o he hand, he influence o he diamine s uc u e on
he ackiness o he adhesi es was u he examined. P obe ack
s ess−s ain cu es o MXDA-, CBMA-, and PXDA-based
PHUs a e shown in Figu e 6b. Acco ding o heology
equency sweeps, he 50/50-PXDA composi ion showed a
g ea e cohesi eness, and no o ma ion o he pla eau was
obse ed. Due o hese highe cohesi e o ces, adhesi e ailu e
was eco ded while no esidue emained on he s ainless s eel
cylinde p obe. As he o mula ion did no p esen liquid-like
beha io a his empe a u e, a u he expe imen was
pe o med a 120 °C(Figu e S21). The maximum s ess o
he 50/50-PXDA was sligh ly lowe , bu he pla eau was
ex ended, exhibi ing highe capaci y o we he su ace.
None heless, adhesi e ailu e was s ill eco ded, and no
fib illa ion was obse ed, showing simila solid-like beha io
a hese es condi ions.
A e assessing he ackiness o he adhesi es, he lap-shea
s eng h was de e mined. Lap-shea specimens o samples wi h
a ying so o ha d segmen a io we e p epa ed as shown in
Figu e 7a, by bonding he s ainless s eel subs a es a 80 and
100 °C(Figu e S22 and Figu e 7d, espec i ely).
When he ma e ials we e applied a 80 °C, he 70/30-
MXDA was cha ac e ized by a low lap-shea s eng h, i.e., 1.9
±0.6 MPa, while he 60/40-MXDA and 50/50-MXDA
showed simila and be e pe o mance (Figu e S22) wi h a
lap-shea adhesion o 2.5−3 MPa. On he o he hand, aising
he empe a u e up o 100 °C enhanced he pe o mance o
he adhesi es, especially in he case o 50/50-MXDA (Figu e
7d) o 7MPa.
This obse a ion co ela ed wi h p obe ack measu emen s,
whe e he 50/50-MXDA exhibi ed a g ea e liquid-like
beha io and, he e o e, imp o ed we ing o he su ace o
he adhe ends. Adhesi e ailu e was p edominan o all he
composi ions ega dless o he applica ion empe a u e,
indica ing g ea e cohesi e o ces o he polyme han adhesion
be ween in e aces (Figu es S23a−c and S24a−c).
A e e alua ing he impac o he so o ha d segmen a io,
he effec o BdiCC on he adhesion p ope ies was
in es iga ed, eplacing RdiCC by his monome . Acco ding
o he heological beha io showed by his composi ion, he
adhesion p ope ies (2 ±0.4 MPa) we e a below he
adhesion pe o mance o 50/50-MXDA in all he measu e-
men s (Figu e S25 and Table S3). Adhesi e ailu e o he PHU
was eco ded showing g ea e cohesi e o ces han in e ac ion
wi h he adhe end (Figu e S26a).
Finally, he influence o he diamine s uc u e on lap-shea
expe imen s was add essed. Based on heology and be e lap-
shea esul s o he ini ial s udy, ho -mel adhesi es we e
applied a 100 °C. The MXDA-based o mula ion p esen ed
sligh ly be e lap-shea s eng h han he CBMA-based
composi ion (7 MPa s 5.8 MPa), while he diffe ence
be ween he 50/50-PXDA when i was applied a 100 and 120
°C demons a ed he necessi y o good we ing o he su ace
o ob ain op imal adhesi e pe o mance (Figu e 7e) as a es ed
by a nea ly 2- old inc ease o he lap-shea adhesion alues.
Adhesi e ailu e a e lap-shea es s was p edominan ly
obse ed o he composi ions (Figu e S27), demons a ing
he g ea e cohesi e o ces o he polyme s han hei adhesion
affini y o he adhe end.
In b ie , dynamic adhesi e p ope ies we e dependen on he
balance be ween so and ha d segmen s, inc easing he
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