Chemical Enginee ing Jou nal 483 (2024) 149292
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Pho on Densi y Wa e spec oscopy o moni o he pa icle size in seeded
semiba ch emulsion copolyme iza ion eac ions
Usue Ola z Aspiazu, Ma ia Paulis
*
, Jose Ramon Leiza
*
POLYMAT, Kimika Aplika ua saila, Kimika Fakul a ea, Uni e si y o he Basque Coun y UPV/EHU, Joxe Ma i Ko a zen oa, 20018 Donos ia-San Sebas i´
an, Spain
ABSTRACT
The pe o mance o Pho on Densi y Wa e spec oscopy (PDW) is assessed o inline moni o he pa icle size du ing he seeded semiba ch emulsion copolyme iza ions
o me hyl me hac yla e, bu yl ac yla e, me hac ylic acid (MMA/BA/MAA =51/47/2 w % composi ion) wi h pa icle size and solids con en anges ele an in
indus ial o mula ions. Mo eo e , he sui abili y o he echnique o ea ly de ec some o he mos p obable de ia ions, such as pa icle agg ega ion and seconda y
nuclea ion, is s udied. I is shown ha he PDW can be success ully used o inline moni o he educed sca e ing coe icien , and consequen ly he pa icle size
e olu ion in emulsion polyme iza ion eac ions o a b oad ange o pa icle sizes and solids con en s. Ne e heless, o pa icle sizes highe han 350 nm and solids
con en abo e 40 %, he low signal o noise a io oge he wi h he cha ac e is ic mul iple solu ions egime o he heo e ical educed sca e ing e sus he pa icle
size cu e, p oduces unce ain ies in he de e mina ion o he pa icle size by PDW.
1. In oduc ion
Pa icle size and pa icle size dis ibu ion a e impo an p ope ies o
polyme la exes because hey s ongly a ec he ilm o ma ion and he
heological p ope ies o he la exes. Thus, moni o ing he pa icle size is
o pa amoun impo ance du ing he p oduc ion o wa e bo ne
polyme ic dispe sions, o which eal ime online/inline measu emen s
o he pa icle size a e equi ed.
Pho on Densi y Wa e spec oscopy (PDW), based on pho on ans-
po heo y (inco po a ing mul iple sca e ing), Mie heo y and heo y o
ime dependen ligh sca e ing, de e mines he abso p ion and sca -
e ing p ope ies o highly u bid samples, wha allows a dilu ion ee
pa icle size de e mina ion, by he in oduc ion o a p obe di ec ly in he
non-dilu ed samples [1–4]. A summa y o he PDW heo y can be ound
in he Suppo ing In o ma ion and elsewhe e [4].
PDW has been used in he las decade o moni o ing he pa icle size
o a ange o di e en p ocesses dealing wi h highly u bid media.
Zimme man e al. epo ed he sui abili y o he PDW o i ania pa icle
syn heses analysis [5], while H¨
ane e al. used he PDW o moni o he
zeoli e pa icles syn hesis [6]. Münzbe g e al. in es iga ed he use o
PDW o moni o he o ma ion o o mazine om di e en s a ing
poin s and a di e en condi ions [7]. Mo eo e , Reich e al. moni o ed
an emulsi ica ion p ocess o oil-in-wa e using SDS as su ac an [8].
Some bio p ocesses we e also analyzed by Hass e al. Fo his aim,
di e en kind o milks we e analyzed as a s a ing poin and once he
sui abili y o he echnique o bio p oduc s was es ed, bee , blood and
o he p ocesses we e also analyzed, such as he enzyma ic
decomposi ion o he bee a 67 ◦C o he oxygena ion p ocess o he
blood, among o he s [9]. Tanguchi e al. also analyzed he sui abili y o
PDW o de ec agg ega ion and o he phenomenon in milk samples [10].
Va gas e al. add essed he abili y o PDW o moni o he a phase
ansi ion o he esh milk [11] and Ha wig e al. i s use o de ec
lac ose c ys alliza ion [12]. The abili y o he echnique o analyze cell
cul i a ion p ocesses in pho obio eac ion was also es ed by Sandmann
e al. [13] and i s applica ion in bioenginee ing was epo ed by
Gu schmann e al. [14].
Rega ding he sui abili y o he PDW o analyze polyme iza ion
eac ions, se e al wo ks can be ound in li e a u e. Hass e al. moni o ed
he syn hesis o polyac yla e nanopa icles (50 % o s y ene and 50 % o
o he ac ylics) du ing s a ed eed polyme iza ion, wi h di e en
amoun o emulsi ie s and pa icle sizes up o 200 nm a 40 % solids
con en (SC) in a 25 L eac o [15]. Hass e al. also analyzed he
possibili y o using PDW o inline moni o he pa icle size o se e al
polyme s such as PS, PMMA and P- e -BuMA in hei indus ial
applica ion [16]. Pa icle sizes up o 350 nm and SC up o 30 % we e
analyzed by he au ho s. The o ma ion o sol en bo ne ac ylic
copolyme s ob ained by he ace one p ocess was also s udied o an
ac ylic copolyme wi h high ac ylic acid (AA) con en by Ku lug e al.
[17]. Du ing he s udy he wa e eed and he sol en emo al we e
analyzed. Schlappa e al. moni o ed he syn hesis o poly inyl ace a e
* Co esponding au ho s.
E-mail add esses: [email p o ec ed] (M. Paulis), [email p o ec ed] (J.R. Leiza).
Con en s lis s a ailable a ScienceDi ec
Chemical Enginee ing Jou nal
jou nal homepage: www.else ie .com/loca e/cej
h ps://doi.o g/10.1016/j.cej.2024.149292
Recei ed 26 Sep embe 2023; Recei ed in e ised o m 25 Janua y 2024; Accep ed 31 Janua y 2024
Chemical Enginee ing Jou nal 483 (2024) 149292
2
emulsions a solids con en s up o 54 % ( inal la exes wi h SC om 40 o
54 % we e syn hesized), whe e only he educed sca e ing coe icien
was epo ed, due o he la ge disc epancies as compa ed wi h DLS in
pa icle size calcula ions [18]. In a ecen publica ion [19], hey
a ibu ed he pa icle size calcula ion e o s by PDW o he
wa e -swelling o poly inyl ace a e pa icles s abilized wi h poly inyl
alcohol. Fu he mo e, he abili y o PDW o moni o solids con en s up
o 70 % was also s udied by Jacob e al. [20,21]. These high solids
con en analyzed by Jacob e al. we e ob ained by seeded semiba ch
emulsion polyme iza ion p ocesses wi h di e en s ages and o pa icle
sizes up o 300 nm. Fo pa icle sizes o e 300 nm, agglome a ion was
obse ed and ul asound ea men was needed [21]. Al hough he
eac ion aced some p oblems, PDW was able o de ec he
agglome a ion p ocess. This eac ion was scaled up o 100 L eac o
[20]. Al hough he ep oducibili y o he eac ion was challenging due
o i s endency o agglome a e, la ge pa icle sizes and high solids
con en s we e analyzed and good ep oducibili y o he expe imen and
PDW da a was achie ed. All he men ioned analysis we e ca ied ou o
monomodal dispe sions. Bimodal dispe sions ha e been also s udied,
bu i s eal ime moni o ing was no p o en, due o he high numbe o
unknown pa ame e s o be conside ed [22].
As discussed abo e, he sui abili y o PDW o analyze se e al
ma e ials has been es ed du ing he las yea s, om ce amics o
polyme s, unde di e en condi ions o size and syn hesis me hods and
ge ing p omising esul s. Ne e heless, he e a e no in dep h
assessmen s o he moni o ing o emulsion polyme iza ion eac ions
co e ing a b oad ange o pa icle sizes and solids con en du ing
semiba ch ope a ion. No e ha , he u bidi y o a sample is ela ed no
only o he pa icle size o he dispe sed phase, bu also o he solids
con en o he medium [23]. Thus, o s udy he sui abili y o PDW o
moni o pa icle size in emulsion polyme iza ion p ocesses, a wide ange
o pa icle sizes (dp)and SC samples ha e o be analyzed. This way, he
abili y o he echnique o measu e he pa icle size a di e en u bid-
i ies can be s udied and he limi a ions can be add essed. The e o e, he
objec i e o his wo k is o assess PDW as an inline moni o ing senso in
emulsion polyme iza ion p ocesses unde condi ions ypically used in
indus ial p oduc ion plan s; speci ically, seeded semiba ch emulsion
polyme iza ions o ac ylic monome s. Mo eo e , he sui abili y o he
PDW o de ec di e en phenomena ha can occu du ing emulsion
polyme iza ion eac ions will also be s udied; namely, he agg ega ion/
coagula ion p ocesses and he c ea ion o new polyme pa icles.
2. Expe imen al sec ion
2.1. Ma e ials
Me hyl me hac yla e (MMA) (Quimid oga), n-bu yl ac yla e (BA)
(Quimid oga) and me hac ylic acid (MAA) (Ald ich) we e used as
ecei ed. Sodium Dodecyl Sulpha e (SDS, Ald ich) was used as emulsi-
ie . Dow ax 2A1 was kindly supplied by Dow Chemical and used as
emulsi ie . Ammonium Pe sul a e (APS, Ald ich) was used as ini ia o .
Deionized high-pu i y wa e ob ained h ough a Me ck Millipo e sys em
(DI-wa e ) was used as con inuous medium in he emulsion
polyme iza ions and o dilu e he samples o DLS analysis (wa e con-
duc i i y was lowe han 1µS/cm in all cases) and hyd oquinone
(Ald ich) was used as inhibi o o s op he eac ion in he samples
wi hd awn om he eac o o o line analysis.
2.2. Syn hesis o la exes moni o ed by PDW
Seeded semiba ch emulsion polyme iza ion eac ions we e moni-
o ed using he PDW equipmen . Fi s a seed was syn hesized and hen
he pa icles o he seed we e g own un il he a ge ed pa icle size ( his
s ep was moni o ed wi h he PDW). A seed wi h a ge mola
Table 1
Reac ions pe o med o e alua e he PDW.
Reac ion SC seed /% dp seed* /nm PDI seed SC a ge ed /% dp a ge ed /nm Feeding ime /min
R1 2.73 45 0.08 45 175 180
R2 0.3 45 0.08 40 350 180
R3 7.54 175 0.012 40 425 180
R4 3 45 0.08 40 80 / 350 240
Agg ega ion 33 119 0.05 – – 65
*
All pa icle sizes a e numbe -a e age pa icle sizes measu ed by DLS using he cumulan s me hod.
Fig. 1. Calcula ed e olu ion o he e ac i e index o e he wa eleng h o he
e polyme o MMA/BA/MAA wi h a weigh composi ion o 51/47/2 %.
Fig. 2. E olu ion o he ins an aneous and o e all con e sions o expe i-
men R1.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
3
composi ions o MMA/BA 50/50 was syn hesised by ba ch emulsion
polyme iza ion. Fo ha , wa e , comonome s and emulsi ie we e mixed
in he eac o . Once all he mix u e was hea ed o 85 ◦C, a sho o
ini ia o was added. The mix u e was le o eac o 4 h. The ecipe o
he syn hesis o he seed is p esen ed in he Suppo ing In o ma ion
Table SI 1.
The seeded semiba ch emulsion polyme iza ion eac ions we e
pe o med as ollows: Fi s , he seed was loaded in he eac o and i was
dilu ed o each he needed solids con en be ween 0.3 and 8 %,
depending on he ini ial and a ge ed pa icle sizes. Then, he mix u e
was hea ed o each he eac ion empe a u e (85 ◦C). Subsequen ly,
monome s and ini ia o we e ed o he eac o in wo eeding s eams
du ing 180 min a cons an low a e o R1-3 eac ions. The ecipes o
he eac ions a e p esen ed in he Suppo ing In o ma ion Table SI 2.
A e he eeding, he la ex was main ained a he eac ion empe a u e
o 30 min o emo e he esidual monome .
Fo mimicking he o ma ion o new polyme pa icles (i.e., sec-
onda y nucla ions) (R4 eac ion), a sho o seed pa icles was in oduced
du ing he monome eeding s age in a seeded semiba ch emulsion
polyme iza ion ( ecipe in he Suppo ing In o ma ion Table SI 2). This is
a ypical s a egy used o p oduce high solids low iscosi y la exes in he
p oduc ion o wa e bo ne polyme dispe sions [24–29]. The e o e, a
p e o med seed wi h 0.3 % o solids con en and a pa icle size o 45 nm
was hea ed o 85 ◦C, he eac ion empe a u e, and he monome s we e
di ec ly ed o e he seed o 240 min. To mimick he o ma ion o a new
c op o pa icles, and hence a bimodal PSD, a e 142 min o eeding, a
sho o he seed la ex (45 nm pa icles) was injec ed o he eac o . The
pa icle g ow h du ing he 240 min o eeding and he 30 min o
pos polyme iza ion was moni o ed by PDW.
All hese eac ions we e ca ied ou in a 1 L calo ime e eac o
(RC1e) om Me le Toledo, equipped wi h a jacke ed glass eac o
i ed wi h a sampling de ice, a ni ogen inle , eeding inle s, a P -100
p obe, he PDW p obe and a s ainless s eel ancho ype s i e ( o
mo e de ails see Suppo ing In o ma ion). Reac ion empe a u e and
inle low a es o semicon inuous eeds we e con olled by an au oma ic
con ol sys em, iCon ol RC1e So wa e. In all he eac ions, samples
we e wi hd awn e e y 10–15 min du ing he i s 60 min o he eac ion
and e e y 20–30 min a e ha ime o analyse he e olu ion o mono-
me con e sion and pa icle size o line by DLS. De ailed in o ma ion
abou he sampling (amoun , ime, ex ac ed mass) in all eac ions and
he ep oducibili y o PDW esul s can be ound in he Suppo ing In-
o ma ion as well.
Fo he agg ega ion de ec ion es , a sodium chlo ide solu ion was
p epa ed wi h di e en sal concen a ions (0.05 and 0.15 M). These
solu ions we e ed o a p e o med la ex o 33 % o SC and a pa icle size
o 119 nm du ing 65 min and wi h a speed o 1 g/min. The agg ega ion
de ec ion es was ca ied ou in a 0.5 L beake equipped wi h a magne ic
s i e , a eeding inle and he PDW p obe. The eac ion was pe o med
a oom empe a u e and he addi ion o he sal solu ion was done wi h
a bu e e.
A summa y o he condi ions o he eac ions including solids con en
(SC) and seed and a ge ed pa icle diame e s, as well as eeding imes
a e p esen ed in Table 1.
2.3. Cha ac e iza ion
2.3.1. Con e sion calcula ions
As samples we e wi hd awn om he eac o o measu e pa icle size
by DLS, con e sions we e measu ed by g a ime ic analysis. The
ins an aneous con e sion is de ined as he amoun o polyme p oduced
di ided by he mass o monome s ed un il he sampling ime (Eq. (1)).
The o e all con e sion is he amoun o polyme di ided by he o al
amoun o monome in he o mula ion (Eq. (2)).
Xa ins an =Polyme ized monome ( )
To al ed monome ( )(1)
Fig. 3. Inline moni o ing o pa icle size by PDW in expe imen R1: a) Inline educed sca e ing coe icien e olu ion, and b) pa icle size e olu ion o each o he
lase wa eleng h.
Fig. 4. Inline moni o ing o pa icle size by PDW in expe imen R1. A e age
pa icle size e olu ion ob ained by PDW compa ed o he numbe -a e age
pa icle size ob ained by o line DLS.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
4
Xa o e all =Polyme ized monome ( )
To al monome (end o eac ion)(2)
No e ha con e sions can be also calcula ed om he hea o eac-
ion measu ed by he calo ime ic eac o . A compa ison o he o e all
con e sions measu ed by bo h me hods as well as he hea eleased
du ing he polyme iza ion o eac ion R1 a e included in he Suppo ing
In o ma ion.
2.3.2. Pho on Densi y Wa e spec oscopy
A Pho on Densi y Wa e spec ome e de eloped by PDW Analy ics
GmbH (PDWA) (Po sdam, Ge many) was used o he pa icle size
moni o ing [1,18,21]. The Pho on Densi y Wa e spec ome e was
equipped wi h ou lase s o 636, 778, 855 and 973 nm wa eleng hs, 27
possible dis ances om 3 o 25 mm and 41 possible equencies om 10
o 610 MHz. To calcula e he pa icle size a so wa e de eloped by
innoFSPEC depa men om he Uni e si y o Po sdam was used, based
on Mie-Theo y and he heo y o dependen sca e ing. To calcula e he
pa icle size he e ac i e indices o he con inuous and dispe sed phase
(shown below), as well as he olume ac ion o he dispe sed phase a e
needed.
2.3.3. Dynamic ligh sca e ing
A Ze asize Nano Se ies (Mal e n Ins umen ) dynamic ligh sca -
e ing (DLS) was used as e e ence echnique in his wo k. Fo he
analysis, he la ex was dilu ed in DI-wa e un il an a enua ion numbe
o 6–7 was achie ed a 4.65 mm posi ion and a PDI alue below 0.1 was
ob ained, o ensu e single sca e ing condi ions. DLS measu emen s
we e ca ied ou a oom empe a u e and h ee epea ed measu emen s
we e ca ied ou o each sample. All e e ence pa icle sizes shown in
his sec ion a e numbe -a e age pa icle sizes measu ed by DLS, excep
o eac ion R4, whe e in ensi y-a e age pa icle sizes a e also epo ed.
2.3.4. Capilla y hyd odynamic ac iona ion (CHDF)
Pa icle size dis ibu ions we e ob ained by capilla y hyd odynamic
ac iona ion. A CHDF3000 ins umen (Ma ec Applied Sciences) was
used o he analysis, wi h i s p op ie a y CHDF3000 PDA 2.1 so wa e
and C-204 column (in e nal diame e 20 µm). The samples we e dilu ed
in ca ie luid (Ma ec Applied Sciences) o 0.05 % SC and he
measu emen s we e done a a low a e o 1.4 mL/min a 35 ◦C.
2.3.5. Re ac i e index measu emen
The e ac i e index o he e polyme o MMA/BA/MAA wi h
weigh composi ion o 51/47/2 % was expe imen ally measu ed. Fo
ha , an expe imen al me hod de eloped by Hass e al. [2] was used. By
measu ing he e ac i e index o a liquid dispe sion o he polyme
ndispe sion(λ)as a unc ion o wa eleng h and olume ac ion o he
dispe sed phase, φ, Eq. (3) allows o he ex apola ion o he e ac i e
index o he pu e ma e ial by global analysis:
ndisp.phase(λ) = [n2
dispe sion(λ) + n2
con .phase(λ)[φ−1]
φ]1/2
(3)
The spec al in e pola ion o he e ac i e index is achie ed based
on a Cauchy polynomial. Fig. 1 shows he calcula ed e ac i e index
o e he wa eleng hs o he syn hesised e polyme o MMA/BA/MAA
Fig. 5. Inline moni o ing o pa icle size by PDW in eac ion R2: a) e olu ion o ins an aneous and o e all con e sions, b) educed sca e ing coe icien e olu ion,
and c) pa icle size e olu ion o each o he lase wa eleng h.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
5
wi h a weigh composi ion o 51/47/2 %.
3. Resul s and discussion
3.1. Inline moni o ing o pa icle size in semiba ch emulsion
polyme iza ion: E ec o pa icle size and solids con en ange
Th ee seeded semiba ch emulsion copolyme iza ion (MMA/BA/
MAA: 51/47/2 w %) eac ions we e used o assess he pe o mance o
he PDW; speci ically, eac ions R1, R2 and R3.
Reac ion R1 aims o p oduce a la ex wi h an a e age pa icle size o
175 nm and a solids con en o 45 w % s a ing om a seed (S1, see
Suppo ing In o ma ion) wi h a pa icle size o 45 nm. Fig. 2 p esen s he
e olu ion o he ins an aneous and o e all con e sions o his expe i-
men , which indica es ha he eac ion p oceeded unde s a ed con-
di ions wi h ins an aneous con e sions abo e 90 % du ing almos he
whole eac ion; which means ha he concen a ion o comonome s in
he polyme pa icles was low and i was main ained ela i ely cons an
du ing he whole eac ion.
Fig. 3a shows he inline educed sca e ing coe icien e olu ion o
eac ion R1 moni o ed by he lase s wi h 636, 778 and 855 nm wa e-
leng hs. As can be seen, he educed sca e ing coe icien inc eases as
he eac ion p oceeds due o he inc ease in he pa icle size and he
solids con en du ing he monome eeding s age o he semiba ch p o-
cess. Mo eo e , he highe he wa eleng h used o pe o m he analysis,
he lowe is i s sensi i i y o he changes ha a e occu ing in he
eac o . No e ha al hough he p obe used had a lase a highe wa e-
leng h, λ =973 mn, he signal o noise a io was oo low and hence i has
no been included in he igu e. F om he educed sca e ing coe icien
e olu ion, he pa icle size was e ie ed, as shown in Fig. 3b (see
Suppo ing In o ma ion o de ails on he pa icle size calcula ions). As
can be seen, a di e en pa icle size is ob ained o each wa eleng h. The
sca e ed ligh depends on he wa eleng h o he emi ed ligh and hence
he di e en pa icle sizes ob ained do ep esen he b oadness o he
dis ibu ion o he la ex samples analysed du ing he eac ion.
An a e age diame e can be compu ed om he alues calcula ed a
each lase wa eleng h and he alue ob ained as well as he o line alue
ob ained by DLS a e p esen ed in Fig. 4. The e is a e y good ag eemen
(di e ences lowe han 10 %, see Suppo ing In o ma ion) be ween he
inline a e age pa icle size ob ained by PDW and he o line numbe -
a e age alue measu ed by DLS. The e o e, i can be said, ha he
syn hesis o MMA/BA/MAA polyme pa icles by seeded semiba ch
emulsion polyme iza ion up o 175 nm and 45 % SC was p ope ly
moni o ed by PDW.
In eac ion R2, a b oade e olu ion o he pa icle size was assessed.
The eac ion s a ed wi h he same seed la ex as R1 (bu subs an ially
mo e dilu ed) and he inal pa icle size a ge ed was 350 nm wi h a
solids con en o 40 w %. Fig. 5a shows he e olu ion o he ins an a-
neous and o e all con e sions. In his case, an inhibi ion pe iod o 30
min can be obse ed a he beginning o he eac ion, bu a e ha , he
polyme iza ion p oceeded unde s a ed condi ions. Fig. 5b p esen s he
e olu ion o he inline measu emen s o he educed sca e ing. No ably
he educed sca e ing coe icien alues a e subs an ially highe han
Fig. 6. Theo e ical dependence o he educed sca e ing coe icien on he
pa icle size o a monodispe sed dispe sion o sphe ical pa icles wi h 40 % o
SC o wo wa eleng hs (636 and 855 nm, da k ed and da k blue lines,
espec i ely). Dashed ligh ed and dashed ligh blue lines show he
expe imen ally ob ained educed sca e ing coe icien alue o he inal la ex
o R2 expe imen measu ed wi h 636 and 855 nm wa eleng hs, espec i ely.
See ex o he discussion on he lines d awn in he plo . (Fo in e p e a ion o
he e e ences o colou in his igu e legend, he eade is e e ed o he web
e sion o his a icle.)
Fig. 7. Inline moni o ing o pa icle size by PDW in eac ion R3. a) E olu ion o ins an aneous and o e all con e sions, b) e olu ion o he educed sca e ing
coe icien du ing he en i e eac ion.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
6
hose measu ed o eac ion R1 o he h ee wa eleng hs, indica ing a
as e g ow h o he seed pa icles (see Fig. 5c). Howe e , he sca e o
he educed sca e ing measu emen inc eased wi h eac ion ime and
u he mo e he lowe he wa eleng h, he highe he sca e . In e es -
ingly, abo e 120 min o eac ion he educed sca e ing alues o all
wa eleng hs we e high and mos ly o e lapped. The pa icle sizes
e ie ed om he educed sca e ing alues a each wa eleng h a e
p esen ed in Fig. 5c, which ini ially ma ch pe ec ly wi h he DLS da a
(up o 60 min). Howe e , in o de o unde s and he e olu ion o he
pa icle size ob ained by PDW om ha poin on, he heo y behind
PDW should be u he conside ed.
Fig. 6 shows he heo e ical dependence o he educed sca e ing
coe icien o e he pa icle size o a monodispe sed la ex o MMA/BA/
MAA 51/47/2 w % composi ion and 40 % o solids con en . As can be
seen, in a b oad ange o pa icle sizes (no e ha in Fig. 6 pa icle size
spans om ew nanome e s o se e al mic ons) he educed sca e ing
shows di e en egimes. A small pa icle sizes ( he limi depends on he
wa eleng h o he lase used), he e is an inc ease o he educed
sca e ing wi h he inc ease o he pa icle size. A in e media e pa icle
sizes, he educed sca e ing eaches an oscilla ing pla eau; namely, he
educed sca e ing is simila o a b oad ange o pa icle sizes (up o ew
mic ons). In he hi d egime, u he inc easing he pa icle size he
educed sca e ing dec eases and he alues a di e en wa eleng hs
end o o e lap. In e es ingly, he end o he i s egime is eached a
smalle pa icle sizes, he smalle is he wa eleng h o he lase (250 nm
a 636 nm and ci ca 350 nm a 855 nm).
The pa icle size is e ie ed om he expe imen ally measu ed
educed sca e ing coe icien using he heo e ical ela ionship plo ed
in Fig. 6 o he op ical p ope ies o he sample analysed (see Sup-
po ing In o ma ion). This means ha in a blind analysis, mul iple so-
lu ions a e expec ed o a gi en measu ed educed sca e ing alue (see
dashed lines in e cep ing in he cu es). The e o e, o ob ain a unique
alue o pa icle size he PDW analysis mus be done a leas a wo
wa eleng hs as shown in Fig. 6, expec ing a solu ion ha is equal o he
measu emen done in he wo wa eleng hs (see Suppo ing In o ma ion
Figu e SI2).
On he o he hand, i one has ce ain knowledge abou he sample
like in his case, whe e emulsion polyme iza ion eac ions a e
conside ed and he expec ed pa icle size should be submic on, one can
limi he maximum pa icle size and hence limi he ambigui y. In
p ac ice, howe e , and as obse ed in he pa icle sizes e ie ed in e-
ac ion R1, he pa icle sizes e ie ed a e no exac ly he same because o
he dispe si y o he samples. Anyhow, om he pa icle size ga he ed an
a e age alue (PDW a e age) can be calcula ed.
Fig. 8. Pa icle size e olu ion o each wa eleng hs (636, 778, 855 and 973 nm) du ing he inline moni o ing o eac ion R3 by PDW.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
7
None heless, expe imen R2 b ough ano he limi a ion o he ech-
nique. This is he ac ha , e en in he submic on egion, he educed
sca e ing coe icien can be in he second egime, whe e he e is mul-
iplici y o solu ions o a gi en educed sca e ing alue. As discussed
abo e, his mul iple solu ion egime is eached a smalle pa icle sizes
he smalle is he wa eleng h o he lase employed (see Suppo ing
In o ma ion Figu e SI2 and Figu e SI3 o mo e insigh ). This explains
ha o wa eleng h 636 nm, eliable esul s we e only ob ained ill 60
min o eac ion (see Fig. 5c and Fig. 6), whe eas he whole eac ion was
moni o ed o he 855 nm wa eleng h.
Fig. 6 shows he pa icle size e ie ed om he heo e ical depen-
dence cu e o he educed sca e ing coe icien o he inal la ex o R2
expe imen a 636 and 855 nm wa eleng hs. As can be seen, he educed
sca e ing alue ob ained o 636 nm is in he mul iple solu ion egime
(ligh ed line), which gi es ou possible solu ions (232, 317, 369 and
1245 nm), while o 855 nm (ligh blue line) only wo possible solu ions
would be possible (306 and 2171 nm). As can be obse ed, he 317 and
306 nm a e he closes ones, which a e also close o he DLS alue, 355
nm.
The e o e, i can be seen in Fig. 5c ha he inline alues measu ed a
855 nm by PDW a e in e y good ag eemen wi h he DLS alues
(di e ences lowe han 10 %, see Suppo ing In o ma ion). The inline
alues ga he ed a he lowe wa eleng hs only o e lapped up o he
egions whe e he educed sca e ing coe icien was in he i s egime,
sugges ing ha jus he i s pa icle size esul was gi en by he PDW.
The e o e, in he hi d eac ion, as a la ge pa icle size was a ge ed,
he lase o 973 nm wa eleng h was inco po a ed o he PDW analysis.
Reac ion R3 was designed o p oduce an ac ylic copolyme la ex wi h
425 nm and 40 w % solids con en . The seed la ex ( he la ex syn hesized
in eac ion R1) had a pa icle size o 175 nm and i was dilu ed o 7.5 w
%.
Fig. 7a shows ha he polyme iza ion p oceeded unde s a ed
condi ions du ing he en i e expe imen ; namely, wi h monome con-
cen a ion ela i ely low in he polyme pa icles and mos ly cons an
du ing he eeding pe iod. Fig. 7b p esen s he e olu ion o he educed
sca e ing coe icien du ing he whole p ocess. Con a y o wha was
obse ed in he p e ious wo expe imen s (R1 and R2), in his expe i-
men he educed sca e ing coe icien s a e in he pla eau egion
(mul iple solu ion egime) almos om he e y beginning o he
eac ion.
As can be seen in Fig. 7b, in he i s 40 min, whe e he educed
sca e ing coe icien inc eases wi h eac ion ime (o pa icle size), he
di e ence in he educed sca e ing wi h he wa eleng h a e less clea
han in expe imen s R1 and R2. This is due o a lack o signal ha comes
om he p obe con igu a ion used in his wo k. As mul iple sca e ing
condi ions a e analysed, he in ensi y o he ligh dec eases wi h he
dis ance om he emission poin , and i he ligh is los be o e eaching
he de ec ion ibe (mo e p obable a highe pa icle size and solids
con en condi ions), noise is measu ed in he de ec ion poin . I he
p obe was buil wi h o he dis ances (wi h smalle emission/de ec ion
dis ances in his speci ic case), he quali y (signal o noise a io) o he
measu ed da a would ha e imp o ed. This means ha he con igu a ion
o he emission/de ec ion ibe dis ances in he p obe can be op imized
o each expec ed e olu ion o he pa icle size. In o he wo ds, he p obe
con igu a ion included in he cu en PDW se -up is no uni e sal.
A e he i s 40 min, all da a s a o o e lap. The 778 and he 855
wa eleng hs con inue o inc ease a bi , due o hei capaci y o analyse
highe u bidi y condi ions, bu a e 60 min o eac ion, all wa eleng h
each he mul iple solu ion egime and all da a is o e lapped. A his
poin , each educed sca e ing da a co esponds o mo e han one
Fig. 9. No malized numbe a e age pa icle size dis ibu ion e olu ion
measu ed by CHDF du ing eac ion R4, a e 90, 140 and 240 minu es
o eac ion.
Fig. 10. Inline moni o ing o pa icle size in expe imen R4: a) Inline educed sca e ing coe icien e olu ion esul s and b) pa icle size e olu ion e ie ed om he
educed sca e ing coe icien and o line measu ed by DLS.
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
8
possible pa icle size. Thus, o calcula e he eal pa icle size, he
di e en solu ions e ie ed om each wa eleng h whe e compa ed
be ween hem and o he pa icle size measu ed o line by DLS (Fig. 8).
As can be seen in Fig. 8, wi h he smalles wa eleng h (636 nm) he
eac ion can only be moni o ed o he i s 40 min, up o a pa icle size
o abou 250 nm. Fo he es o he eac ion, he educed sca e ing
alue is in he mul iple solu ion egime, hus he pa icle size canno be
di ec ly e ie ed. Fo he second wa eleng h (778 nm), conside ing all
he possible pa icle size solu ions, almos all he eac ion can be
moni o ed, bu , selec ing he co ec pa icle size is e y challenging,
since he compa ison wi h o he wa eleng hs is e y di icul o apply,
due o he polydispe si y o he samples. Fo he hi d wa eleng h (855
nm), a subs an ial pa o he eac ion can also be moni o ed. The e is a
ime egion whe e he pa icle size could no be e ie ed due o e y
low signal o noise a io in he measu ed da a. Fo he highes wa e-
leng h (973 nm) e en i he educed sca e ing coe icien seems o be
ela i ely cons an , he e o o he measu emen is oo big compa ed o
he o he wa eleng hs and he signal o noise a io is oo low o eliably
e ie e he cu en pa icle size.
As can be seen, conside ing all he possible diame e s, he e is always
one ha i s he da a ob ained om he DLS. Ne e heless, he co ec
selec ion o he diame e is e y challenging. A e hese obse a ions i
can be concluded ha he e a e se e al easons why all wa eleng hs
show ce ain limi a ions, which a e ela ed o he se ings o he PDW
p obe ha ha e been used in his wo k. Fo he smalles wa eleng hs
(636, 778 and 855 nm), he sensi i i y o he u bidi y o he medium is
oo high and mul iple solu ion egime is eached in polyme iza ion wi h
apid inc ease o pa icle size and solids con en . Fo he bigges
wa eleng h (973 nm), he e a e no enough sho dis ances o ge
accu a e signals wi h a high signal o noise a io. To o e come hese
p oblems, he con igu a ion o he lase ibe s in he p obe should be
modi ied.
3.2. Inline moni o ing o bimodal PSD by PDW
The sui abili y o PDW o de ec seconda y nuclea ions ha migh
c ea e bimodal PSD was assessed in his sec ion. Seconda y nuclea ion is
some imes desi ed in o de o gene a e a bimodal la ex because his
allows inc easing solids con en wi h li le o low impac on he iscosi y
o he dispe sion [25]. Howe e , o he imes seconda y nuclea ions a e
no desi ed and hey migh be an indica ion o a de ia ion o he a ge
g ow h o he pa icles o igina ed om a ailu e in he eeding o
su ac an , ini ia o o e en monome . In any case, ea ly de ec ion o he
o ma ion o a new c op o polyme pa icles du ing an emulsion
polyme iza ion p ocess is desi ed.
Reac ion R4 was ca ied ou o mimick he gene a ion o a new
Fig. 11. Theo e ical dependence o he educed sca e ing coe icien on he
pa icle size o a monodispe sed dispe sion o sphe ical pa icles wi h 40 % o
SC a 636 nm wa eleng h and polyme composi ion o MMA/BA/MAA 51/47/
2 w % (da k ed). Ligh ed dashed line ep esen s he expe imen ally measu ed
educed sca e ing coe icien alue a he end o R4 expe imen , 15.5 mm
−1
.
(Fo in e p e a ion o he e e ences o colou in his igu e legend, he eade is
e e ed o he web e sion o his a icle.)
Fig. 12. Polyme pa icle agg ega ion de ec ion es . a) Inline educed sca e ing coe icien (636 nm wa eleng h) e olu ion and b) pa icle size e olu ion, du ing he
agg ega ion expe imen o wo di e en sal concen a ions: 0.05 M in black and 0.15 M in blue. The o ange line ep esen s he momen we e he coagula ion
becomes massi e o he case o 0.15 M NaCl addi ion. (Fo in e p e a ion o he e e ences o colou in his igu e legend, he eade is e e ed o he web e sion o
his a icle.)
U.O. Aspiazu e al.
Chemical Enginee ing Jou nal 483 (2024) 149292
9
popula ion o small pa icles du ing a seeded semiba ch polyme iza ion
p ocess. This eac ion s a ed wi h a seed la ex wi h a pa icle size o 45
nm and a a ge pa icle size o 350 nm, like in expe imen R2. Howe e ,
a an in e media e poin du ing he eeding o he monome (a 142
min), an amoun o he same seed la ex was injec ed o he eac o o
c ea e he bimodal la ex.
PDW was used o moni o inline he emulsion polyme iza ion.
Fu he mo e, in addi ion o o line DLS analysis, his ime samples we e
also analysed by CHDF (Capilla y Hyd oDynamic F ac iona ion),
because bimodal o mul imodal popula ion a e be e de ec ed by his
ch oma og aphic echnique han wi h DLS [30]. Fig. 9 p esen s he
no malized numbe -a e age pa icle size dis ibu ions measu ed by
CHDF o he samples wi hd awn om he eac o a di e en eac ion
imes (90, 140 and 240 min) (no malized weigh -a e age pa icle size
dis ibu ion can be ound in he Suppo ing In o ma ion). I was
con i med ha up o 140 min, a g owing monomodal pa icle size
dis ibu ion was p esen in he eac o , whe eas a he end o he eac-
ion a bimodal la ex was clea ly obse ed.
Fig. 10a p esen s he inline e olu ion o he educed sca e ing
coe icien a each o he wa eleng hs o he p obe. A he lowes
wa eleng h (636 nm) he educed sca e ing coe icien inc eases
linea ly up o a ound 100 min, whe e a pla eau alue is achie ed which
is main ained a e he sho o he seed la ex o s a dec easing a he
end o he eeding o he monome s. The end o he o he wa eleng hs
is simila , he educed sca e ing coe icien inc eases linea ly up o 240
min o eac ion o each a pla eau alue ( he alue is highe he lowe is
he wa eleng h) wi h almos no no iceable e ec o he sho o he seed
la ex.
Acco ding o he discussion be o e, his educed sca e ing e olu ion
should allow o calcula e a single pa icle size o all he wa eleng hs
du ing almos he whole polyme iza ion eac ion. Fig. 10b displays he
pa icle sizes e ie ed om he educed sca e ing alues and he o line
alues measu ed by DLS. No e ha wo a e ages, numbe and in ensi y
based ones, a e plo ed o he DLS measu emen s. The DLS pa icle sizes
(bo h a e ages) inc ease up o he poin he seed sho is injec ed (142
min). Beyond his ime he wo a e ages di e ge, indica ing, as shown
by he CHDF measu emen s in Fig. 9, ha a bimodal la ex has been
o med. Since DLS is unable o dis inguish he wo popula ions, he
a e age alues di e ge in o highe sizes ( o d
I,DLS
) and smalle ones ( o
d
N,DLS
). In e es ingly, he inline pa icle size alues measu ed by PDW
o he highes wa eleng hs (778, 855 and 973 nm) seem o ollow close
he in ensi y-a e age alues du ing he whole p ocess. On he con a y,
o he smalles wa eleng h (636 nm) a di e en beha iou is app eci-
a ed. The alue ob ained om he smalles wa eleng h ag ees wi h he
numbe -a e age pa icle size o he DLS, he one showing he e ec o
he added pa icles. This is also shown when s udying how he Mie cu e
and he expe imen al alue o he educed sca e ing coe icien
in e cep .
Fig. 11 shows how he expe imen ally measu ed educed sca e ing
coe icien a 636 nm a he end o R4 eac ion (15.5 mm
−1
) is in e -
cep ing he heo e ical cu e in he i s egime (linea egime). Thus, i
can be concluded ha e en i o he highes wa eleng hs he e is no
no iceable change in he measu ed size de ec ed a e he sho o he
seed la ex, he smalles wa eleng h seems o be mo e sensi i e o he
sca e ing o he small pa icles han o he la ge ones.
3.3. De ec ion o agg ega ion o polyme pa icles by PDW
Ano he impo an phenomenon ha migh occu in emulsion
polyme iza ion eac ions, and ha is no desi ed, is he agg ega ion o
coagula ion o pa icles ha migh cause impo an economical losses in
indus ial p ocesses. Ea ly de ec ion o agg ega ion is he e o e desi ed
in indus ial p oduc ion o polyme la exes. In his sec ion, he
sui abili y o he PDW o inline de ec changes in he educed sca e ing
(and hence pa icle size o he dispe sion) du ing an agg ega ion p ocess
will be e alua ed.
Fo his pu pose, he s abili y o a p e o med ionically s abilized
la ex was modi ied by he addi ion o a solu ion o sal (NaCl), and he
educed sca e ing coe icien and he pa icle size we e moni o ed by
PDW du ing he addi ion o he sal . Two sal concen a ions we e es ed
o des abilize he la ex a di e en speeds. The la ex used was a MMA/
BA/MAA copolyme wi h a 51/47/2 w % composi ion s abilized wi h
SDS and a pa icle size o 119 nm and solids con en o 33 w %. The NaCl
solu ions employed we e 0.05 M and 0.15 M. The solu ions we e ed o
he beake a a low a e o 1 g/min du ing 65 min. The ob ained esul s
a e p esen ed in Fig. 12.
Fig. 12 displays he e olu ion o he educed sca e ing ( he
sca e ing measu ed a he 636 nm wa eleng h is only shown) and he
e ie ed pa icle size o he wo concen a ions o NaCl solu ions. As
can be seen, o he lowes sal concen a ion, he coagula ion was li le
and he pa icle size change was p ope ly moni o ed du ing he en i e
agg ega ion p ocess, o each a inal alue o 137 nm. Fo he highe sal
concen a ion, an exponen ial inc ease o he educed sca e ing coe -
icien is obse ed a e 50 min o sal addi ion, showing clea ly he
coagula ion. In his second case, he en i e la ex was coagula ed o ming
a solid body a 60 min, bu an abno mal inc ease o he educed sca -
e ing coe icien s a ed o occu ea lie (a 40–50 min). Thus, i was
shown ha PDW is a sui able ool o inline de ec coagula ion/agg e-
ga ion phenomena and o ea ly de ec coagula ion, lea ing some ime o
ake ac ions o a oid i , du ing emulsion polyme iza ion eac ions.
4. Conclusions
In his wo k, he sui abili y o he PDW o moni o he pa icle size
e olu ion o polyac yla e la exes syn hesized by semiba ch emulsion
polyme iza ions was discussed. Fi s , a b oad ange o pa icle sizes and
solids con en s we e analysed ( om 45 o 425 nm pa icle size and om
0.3 o 45 % o SC, espec i ely) and he limi s o he submic on size
analysis o he desc ibed PDW equipmen was shown and how his is
a ec ed by he Mie cu e. The highe he wa eleng h, he highe he
pa icle size ha can be p ope ly analysed. Rega ding he analysed
MMA/BA/MAA 51/47/2 e polyme sys em, pa icle sizes up o 350 nm
a solids con en s up o 40 % we e p ope ly analysed. Fo pa icle sizes
o e 400 nm, mul iple solu ion egime o he heo e ical educed sca -
e ing cu e was eached o all he used wa eleng hs and low signal o
noise alues we e ob ained o he used dis ances, making challenging o
e ie e he pa icle size in eal ime.
Then, he abili y o he PDW o de ec he o ma ion o new
nuclea ions and agg ega ion was s udied, showing ha he de ec ion o
new nuclea ions is e y challenging. Ne e heless, i could be in e ed
om he sca e ing a lowe wa eleng hs, ha his wa eleng h is mo e
sensi i e owa ds seconda y nuclea ions (636 nm). On he o he hand,
coagula ion o polyme pa icles is quickly moni o ed, be o e a
ca as ophic eac ion akes place, by inline moni o ing he e olu ion o
he educed sca e ing coe icien .
CRediT au ho ship con ibu ion s a emen
Usue Ola z Aspiazu: Me hodology, In es iga ion, W i ing – o iginal
d a , Da a cu a ion. Ma ia Paulis: Concep ualiza ion, Funding acqui-
si ion, Supe ision, W i ing – e iew & edi ing. Jose Ramon Leiza:
Concep ualiza ion, W i ing – e iew & edi ing, Funding acquisi ion,
Supe ision.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
U.O. Aspiazu e al.
