CU 41）TER 4
（）pti〃ial」spinning condition／br 9「adient sかuctu「e
elongation at break or tensile strength at break were seen as an effect of the different spinning
conditions．
Table 4．2
1）oe・and・li uid血ow rates used to adjust血ber di劉meter jbr eac血fiber．
㎜
concentration
of bore liquid （w重％）
Air gap d【istance
（mm）
Flow rate（m／min） Take up
Fiber
Dope
Bore liquid speed（m／min）PSFOO−400
CHAPTER 4
0ptima1 spinning conditionプわr gradient・structure
Although the necessary flow rate of dope solution in order to obtain target fiber
diameter was almost constant fbr each spirming condition， the necessary flow rate of bore
liquid decreased with higher bore liquid NMP concentration． For example， bore liquid flow
rate of 13．09 m／min was required when bore liquid was water， but the necessary bore liquid
且ow rate was about half that at 6．48 m／min in the case of 70 wt％NMP solution． On the other
hand， there was not any change for bore liquid flow rate at different AG distances． Regarding
the relation between bore liquid NMP concentration and spinning stability， at AG distances of
50㎜orless， stablehollow−fiber spinning was achieved．
4．3．2」Pure waterプZzoc
In Figure 4．1， pure water permeate且ux of the obtained hollow−fiber membranes is
plotted against NMP concentration of bore liquid． The membrane made with O wt％NMP
solution as bore liquid had a flux of 1431・m−2・h一1， and that made with 60 wt％of NMP
solution as bore liquid had nearly equivalent flux of 1551・m−2・h−1， values that are in the
normal range fbr UF perfbrmance． Flux of the membranes made with 65 wt％of NMP
solution and with 70 wt％NMP solution j umped sharPly to 4，8211・m−2・h−1 and 7，8821・m−2・h−1，
re spectively， value s that are in the normal range fOr MF performance．
in Figure 4．2， pure water permeate flux is plotted fbr hollow−fiber membranes obtained
with 70 Wt％NMP solution as bore liquid and various AG distances from 400 mm to 50 mm．
The influence of AG distance on pure water permeate flux was quite strong， with water
permeate f【ux sharply decreasing with lower AG distance．
CE4」PTER 4
ρρ伽al S碑伽g oo励 加for grad蜘t struc 膨
8000
6000
4000
2000
0
0 55 60 65 70 75
NMP concentration of bore曜i叩id（Wt％》
Figure 4・1 Effect of NMI concentration of bore liquid on pure water permeate血ux at AG distance of 400mm and AG temperature of 80°C．
8000
6000
4000
2000
0
0 100 200 300 AG dista臨ce（mm》
400 500
Figure 4．2 Effect of air gap distance on pure water permeate fiux with 70 wt％
NMP so且ution as bore liquid at AG temperature of 8°C．
CHAPTER 4
の〃〃ia1 spinning conditわnプ〜）r gradient structure
4．3．3 SEM anal／sis
SEM images of cross sections， outer surfaces， and imer surfaces of hollow−fiber
membranes obtained with each set of spinning conditions are shown in Figures 4．3，4．4， and
45，respectively． Distance from the outer surface to the largest pore layer（W1）， distance from
the outer surface to the smallest pore layer（W2）， and the ratios of these distances to
membrane thickness（W・）analyzed fr・m Figure 4・3 are sh・wn in Table 4・4・P・r・sity and
mean p ore size of the outer surfaces and imer surfaces analyzed from Figures 4．5 and 4．6 are
shown in Table 4．5 and Table 4．6．
Table 4．4
Distance from the outer surface to t血e l劉rgest pore layer（W1）， distance from the euter surface to the sma1亘est ore layer（W2）， and the ratios of these distances to membrane thickness（Wo）．
Fiber
Wo（μm） W1（pm） W2（μm）W1バVo W2バVo SEM photo
no．（Fi．4）
PS］F OO−400 PSF 60−400 PSF 65−400 PSF 70−400 PS］F 70−200 PSF 70−050
178 189 189 178 171 171
138 157 164 163 162 162
0．78 0．83 0．87 0．92 0．95 0．95
0．16 0．15 0．13 0．14 0．11 0．05
の切0ののO
CHA1）TEIR 4
qρ 〃7a1 s gin〃加g oo〃dilio〃ノわr 9アadie〃t structure
（a）
（d）
（b）
（e）
（c）
（D
Figure 4．3 SEM photographs of cross section of PSF hollow fibers． Distance from the outer surface to the largest pore l町er（W1）， distance血om the outer surface to the smallest pore layer（W2）， and membrane t血ic㎞ess（Wo）．
（a）ho夏low fiber PSmO−400 250；（b）h・11・w fib・r PSF60−400・a50；（・）h・ll・w五b・r PSF65−400・a50；（d）血・盈1・w肋。r PSF70−400 x250；（e）血ollow血ber PSF70−200 x250；（i）hollow fiber PSF70−050 x250．
CHAPTER 4
0ρtima1 spinning conditionf（）r gradient structzare
（a） （b） （c）
（d） （e）
①
Figure 4．4 SEM photographs of inner surface ofPSF hollow fibers．（a）hollow血ber PSmO−400 x1，000；（b）血0110w fiber PSF60−400 x1，000；（c）ho且10w fiber PSF65−400 x1，000；（d）hollow fiber PSF70−400 x1，000；（e）血ollow fiber PSF70−200 x1，000；① hollow fiber PSF70−050 x1，000．
CHAPTE1〜4
ρρ伽α1遡刀痂9 conditionfor gradient stア〃cture
（a） （b） （c）
（d） （e） （f）
Figure 4．5 SEM photographs of outer surface of PSF hollow血bers．
（a）hollow fiber PSFOO−400 x10，000；（b）ho110w血ber PSF60−400 x10，000；（c）
血ol夏ow fiber PSF65−40 x10，000；（のhollow fiber PSF70−400 x10，000；（e）hollow 血ber PSF70−200 x10，000；（責）ho1蚤ow fiber PSF70−050 x10，000．
CHAPTE1〜4
0pti〃ial spinn〃zg eonditionプわr gradient s伽 o z e
Table 4．4 shows that the W1／Wo ratio increases with higher NMP concentration of bore
liquid． ln other words， the largest pore layer moves toward inner surface with higher NMP
concentration of bore liquid． Specifically， from PSF60−400 to PSF65−400， WI／Wo increased
丘om O．83 to O．87， and with PSF70−400 it increased to O．92．
The ratio WI／Wo also increased with shorter AG distance， and at AG of 50 mm the
ratio W1／Wo becomes nearly 1．00． SEM analysis also reveals that the smallest pore layers
obtained in each case were located slightly beneath the outer surface． The distance丘om the
outer surface to smallest pore layer was almost unaffected by NMP concentration of bore
liquid， with the W2／Wo ratio being O．13−0．16． However， W2／Wo decreased with shorter AG
distance， from O．14 in PSF70−400 to O．11in PSF70−200， and finally to O．05 in PSF70−50．
Figure 4．4 shows that pores were not observed in the inner surface of P SFOO−400 and
PSF60−400，0btained by ushlg O wt％and 60 wt％㎜solution of bore liquid， respectivel）乙
However， large pores were fbrmed on the inner surface with more than 65 wt％NMP
concentration of bore liquid． The appearance ofthese pores was that of an irregular， lacerated
structure，
る
The porosity of PSF70−4000btained using 70 wt％NMP solution of bore liquid was
greater than that of PSF65−4000btained using 65 wt％㎜solution of bore liquid． We find
these observations of the inner−surface structures to be consistent with the relation between
NMP concelltration of bore liquid and water permeability results shown in Figure 4．2．
As AG distance was shortened from 400 mm to 200 mm with the same 70 Wt％NMP
bore liquid condition， porosity of the obtained membranes increased． Yet as AG distance was
α孟4P7ER 4
ρρ伽al S卿痂9 condit加か97磁θ廊tructure
fUrther shortened from 200 mm to 50 mm with the same 70 Wt％NMI bore liquid， membrane
porosity did not change．
Regarding outer surface structure， the pore sizes and porosity on the outer surface of
PSFOO−400， PSF60−400， and PSF65−400 were almost the same． However， the pore sizes on
the outer surface of PSF70−400 were clearly larger than those of other membralles． As AG
distance was shortened while using same 70 wt％NMP bore solution， outer surface pore size
decreased． We find these observations of the outer−surface structures to be consistent with the
relation betWeen AG distance and water permeability results shown in Figure 4．2
Table 4．5
Porosity and mean ore size of inner surface．
Fiber
Porosity（％） Mean pore size（μm） SE］M【photo no．（Fig．5）PSF OO−400 PS］F 60−400 PS］F 65−400 PSF 70−400 PS］F 70−200 pS］F 70−050
Not analyzab艮e Not analyzab且e
20．3 45．1 55．5 56．1
Not ana且yzab蓋e Not ana且yzab且e 2．6 2．4 2．7 3．1
のい0ののO
Table 4．6
Porosity and mean Ore SiZe Of OUter SUrfaCe．
Fiber
Porosity（％） Mean pore siZe（pm） SEM photo no．（Fig．6）PSF OO−400 PSF 60−400 PSF 65−400 PSF 70−400 PSF 70−200 1⊃SF 70−050
28．3 29．7 30．3 29．3 36．2 37．7
0．22 0．23 0．23 0．30 0．25 0．21
ωω◎ω⑨①
CHAPTER 4
ρρ 〃〃al spinning condゴtionプ〜）r gradient structure
4．4Discussion
4．4．1Mechanis〃z（）fme〃7brane cro∬se（rtionプ〜）r〃lation
hthe case of dry−wet sp㎞，ing， AG conditions play a very important role in the
fbrmation of the hollow fiber membrane structure． On the inner surface side of the hollow一
且ber membrane， phase separation and solidi丘cation are immediately induced by the non−
solvent ofthe bore liquid after discharge from the spinneret． On the other hand， on the outer
surface side， phase separation and solidification may occur within the AG when water vapor
is present． However， because ofthe short residence time in the AG the region where
membrane structure is fbrmed by vapor ofAG is limited to the neighborhood of the outside
surface． Therefbre， maj or membrane formation ofthe outer surface region must be promoted．
upon immersion血the coagulation bath， a丘er pass血g through the AG．
Control of inner−surface morphology is generally affected by adj usting the mixture
ratio of bore liquids comprising water， being a strong non−solvent of PSF and PES， and a
solvent such as NMP［19，20， and 13］．1n the case of using water as bore liquid， because the
time丘om beginning of phase separation to reach coagulation composition is very short，
nucleation and growth ofpolymer lean phase advances only slightly befbre the structure is
fixed． Therefbre， when water is used as bore liquid， a dense layer may be fbrmed on the ilmer
surface first．
After the dense layer is formed， water ofthe bore liquid diffuses into the nascent
hollow−fiber until the difference between the water concentration ofthe 1）ore liquid and that in
the nascent hollow fiber disapPears． Inevitabl）x， the concentration ofwater will decrease from
the bore liquid side of the hollow fiber toward the outer surface side， and the time until
CIL4PTE1〜4
0ptimal spinning（フonditわ〃プb7 97adient st7uctu「e
coagulation composition is reached becomes longer toward the outer surface side． The
growth ofpolymer lean phase advances by just that time and pore size will grow．
However， the difference betWeen the water concentration of the bore liquid and that of
the nascent hollow丘ber immediately becomes small because bore liquid is shut out by the
hollow fiber inner surface， and mutual diffUsion ends in a limited period of time． Phase
separation from the inner surface side thus comes to an end．
ln industrial spinning processes， water is generally used as coagulation bath． Because
amount of water in the coagulation bath is almost in丘nite compared to the amount of bore
liquid， water concentration ofthe coagulation bath is constant after the nascent hollow fiber is
immersed． Phase separation and coagUlation on the outer surface side ofnascent hollow fiber
discharged from spimeret do not proceed as much as on the inner surface side in the Aq but
theypr・ceed much飴ster than・n the inner surface side after entering the bath．
Even though the dope solution flow rate required to ol）tain target fiber diameter was
almost constant fbr all sphming conditions， the required bore liquid flow rate decreased with
higher bore liquid NMP concentration．
As shown in Table 4．2， the necessary flow rate of bore liquid in order to o1）tain target
fiber diameter was high（13．09 m／min）when bore liquid was wateL However， the necessary
flow rate of bore liquid decrease to 9．03 m／min（PSF60−400），7．90 m／min（PSF65−400）， and
6．48m／min（P SF70−400）， according with decreases of water concentration of bore liquid．
These results indicate that as bore liquid water collcentration becomes higher， there is
more di血sion of water fヒom the bore liquid into the inside of the nascent hollow−fiber from
the iimer surface toward the outer surface． It was considered that value of Wl／Wo in Table 4．4
CH 4PTER 4
qρ ゴ〃7a1 spi〃ning eo刀ditionプわr gradient struct〃「e
shows the ratio of membrane fbrmation by the coagulation bath， and that the value of（1−
W1／Wo）in Table 4．4 shows the ratio of membrane fbrmation by the bore liquid．
When bore liquid was water， the value of Wl／Wo was O．780r（1−Wl／Wo）was O．22．
However， when bore liquid was 60 wt％NMP solution（PSF60−400）the value of（1−
W1／Wo）was O．17， when it was 65 wt％NMP solution（PSF65−400）this value was O．13， and
when it was 70 wt％NMP solution（PSF70−400）this value was O．08． It was clear that the
ratio of structure fbrmed by bore liquid decreases with lower water concentration of bore
liquid， and that the structure is mostly formed from the outer surface．
The value of（1−W1／Wo）of PSF70−200， with which AG distance was shortened from
400 mm to 200 mm， was O．05， indicating that membrane formation was almost completely
from the outer surface． This implies that because the residence time in the AG was reduced
by half， coagulation from the outer surface begins earlier， and the ratio of membrane
formation f止om the inner surface decreased．
It became clear that the smallest−pore layer of each PSF hollow−fiber membrane was
♂
located j ust below the outer surface， and that the distance of this layer from the outer surface
ら
increased with longer AG distance， as shown in Table 4．4． This phenomenon could be
explained as an effect of nascent hollow fiber discharged from spinneret adsorbing water from
the Aq and nucleation and growth ofthe polymer Iean phase progressing during the period of
passage through the AG
Tsai et al． carefUlly investigated the effect ofAG relative humidity on the structure of
PSF hollow−fiber membrane， particularly in the vicinity ofthe outer surface［21］． They
concluded that penetration ofwater vapor into the membrane dominated the effect of solvent
CHAPTER 4
0ρtima1 spin痂g oo〃ditわnfor gradient structure
evaporation during the short period of time when nascent fiber passes through the AG
Especially in case ofhigh boiling poj血t solvent such as NMP（boiling point，202°C）， they
fbund evaporation induced phase separation to be negligible compared to vapor induced phase
separatlon．
Ih this study， with the AG at 80°C and relative humidity of 100％， absolute humidity
was determined to be 292 g／m3 using Equations l and 2． In accordance with Tasi et al．， we
believe that NIP S occurred in our study due to penetration ofwater vapor in the AG．
hl that case， as take−up speed was constant in this study， AG distance l）ecome an
important factor fbr nucleation and growth． As phase separation on the outer surface of
nascent hollow fiber discharged from the spinneret was initiated by adsorption of water from
the Aq progress of nucleation and growth of the polymer lean phase corresponded to the
length of residence time in the AG h addition， as the amount of absorbed water increased
with longer AG distance， the de gree to which the domain of phase sep aration extended into
the membrane wall from the outer surface toward the inner surface corresponded to the length
ofthe AG distance．
The value of W2 shown幽in Table 4．4 is considered to indicate the extent of NIPS
occurrence by water absorbed from the AG We also considered the boundary betWeen the
region where NPS did not occur and the region where NrPS occurred to be the region where
phase separation and coagulation occurred after immersion in the coagulation bath， and that
this region became the smallest pore layer． Formation of the desired gradient structure
required that the smallest pore layer be proximate to the outer surface， and this apparently
CHAPTER 4
0pti〃7al sフinning conditわnプ〜）r 9γadient structure
In short， the fbrmation of a gradient structure having a filter layer in the region of the
outer surface and gradually increasing pore size across the thickness of the membrane wall
toward the inner required raising the ratio of structure fbrmation on the outer surface side，
which means high NMP concentration of bore liquid and short distance ofAG are desirable．
4．4．2・Vechanis〃1 qプ〃2θ〃zbrane inner surfaceプわr〃2ation
Figure 4．4 shows that pores were not observed in the inner surface of P SFOO−400 and
PSF60−400，0btahled by using O wt％and 60 wt％N1》IP solution of bore liquid， respectively。
However， large pores were formed on inner surface when NMP concentration of bore liquid
was 65 wt％or more． The shape of the pores was irregular， with the membrane structure
having a lacerated apPearance．
Ohya et． al． studied the fbrmation of PSF hollow−fiber when various molecular
weights of PEG in a PSF／NMP／PEG temary dope system were used． They reported that when
PSF hollow且ber sp皿with 70侃％｝㎜solution as bore liquid，400 mm of AG distance，
and using more than 20 kD PEG as additive， pores were formed on the inner surface， and their
shapes were irregular， with a lacerated appearance of the structure． The reason fbr the
fbrmation of this structure is also explained to be due to a partial ripping and pulling apart of
the irmer surface under drawing tension． The results of this study are consistent with that
c・ncept・As sh・wn血Table 4・4・the rati・・fmembrane f・m・ati・n by b・re liquid decrgased as
the NMP concentration of bore liquid increased．
From this perspective， the value of（1−W1／Wo）being O．13 when 65 wt％NMP
solution was used as bore liquid and it being O。08 when 70 wt％NMP solution was u3ed
CH4P7ER 4
の〃〃zal spinning conditionノわr 9「adient st「uctu「e
indicates that a very thin layer was fbrmed in inner surface region． The very thin layer
formed by l）ore liquid was partially ripPed and pulled apart under drawing tension． We
consider丘om Figure 5 that the ratio of ripPing and pulling increased as NMP concentration of
bore liquid was raised from 65 wt％to 70 wt％． Furthermore， this thin layer was not strong
enough to support the nascent hollow−fiber under drawing tension when bore liquid of NMP
concentration was 75 wt％．
As Ohya et． al pointed out， such irregular irmer−surface structures would generally be
undesirable in practical MF application． h Chapter 5， we will study the method to form an
inner surface with a more uniform structure．
4．43」Ve（rhan is〃z qプ〃ze〃zZ）rane outer surfa（re／b7〃zatlon
Regarding outer surface structure， the pore sizes and porosity on the outer surface of
PSFOO−400・PSF60−400・and PSF65−400 were alm・st the same・H・wever・the p・re sizes・n
the outer surface ofPSF70−400 were apparently larger than those of other membranes． As A G
distance was 唐?盾窒狽?獅?п@while using the same 70 wt％NMP bore solution， outer surface pore
size clearly decreased．
As described in section 4．1，this phenomenon was explained as phase separation of the
outer surface of nascent hollow fiber discharged from spineret being initiated by adsorption
of water from the Aq and nucleation and growth of polymer lean phase progressing to the
extent that AG residence time is long． Average pore size of PSF70−400， PSF70−200， and
CHAPTER 4
0pti〃7al spinning conditionプ〜）r gradient structure
PSF70−50 were O．30μm，0．25 pm， and O．21 pm， respectively， becoming smaller as AG
distance deceased．
On the other hand， bore liquid NMP concentration might have a role in the formation
of outer surface pores． However， when PSF70−400 is compared with PSF65−400， both with
the same AG distance， average outer surface pore size of PSF70−400 is larger than that of