MODEL POLYMERS WITH DIMETHYLAMINE AND SULFOZWITTERIONIC END-GROUPS. SYNTHESIS AND SELF ASSEMBLY IN SOLUTION AND IN BULK
MODELNI POLIMERI Z DIMETILAMINSKIMI IN SULFOZWITTERIONSKIMI KONČNIMI SKUPINAMI
NIKOS HADJICHRISTIDIS, S. PISPAS, M. PITSIKALIS
University of Athens, Department of Chemistry, Panepistimiopolis Zografou, 15771 Athens, Greece
Prejem rokopisa - received: 1997-10-01; sprejem za objavo - accepted for publication: 1997-10-21
The dilute solution and bulk properties of a variety of vvell-defined polymers of different architectures (linear homopolymers, diblock and triblock copolymers and star homopolymers) having dimethylamine and sulfobetaine end groups mainly synthesized in our laboratory are reviewed. The end functionalized polymers were synthesized by means of anionie polymerization high vacuum techniques, using 3-dimethylaminopropyl-lithium as initiator for the introduction of the dimethylamine group at the chain end. This group was converted to sulfozwitterionic by reaction with cyclopropanesultone. Extensive molecular characterization proved the high homogeneity of these model materials. Their aggregation properties in dilute solutions, of solvents of different polarity and selectivity tovvards different parts of the molecules, vvere studied by viscometry, static and dynamic light scattering. The co-functionalized polystyrenes and the block copolymers of styrene have a much lovver degree of association, than the homopolydienes, probably due to the polarizability of the phenyl group. The bulk properties of end-functionalized homopolymers and diblocks vvere studied by SAXS, reology and dielectric spectroscopy revealing nevv features of self organization at this lovv ionic content and extraodinary phase behavior at high temperatures. The adsorption behavior of stars vvith different number of functionalized arms in dilute solutions vvas also investigated by ellipsometry shovving different behavior compared to linear polymers.
Key vvords: end-functionalized polymers, anionie po!ymerization, macromolecular arehiteeture, association, phase separation, adsorption
V delu je podan pregled lastnosti vrste dobro definiranih polimerov z različnimi arhitekturami (linearni homopolimeri, diblok in triblok kopolimeri, zvezdasti homopolimeri) z dimetilaminskimi in sulfobetainskimi končnimi skupinami, v razredčenih raztopinah in v masi. Polimere s funkcionalnimi končnimi skupinami smo sintetizirali z anionsko polimerizacijo z visoko-vakuumsko tehniko. Z iniciatorjem 3-dimetilaminopropiIitijem smo na konec verige uvedli dimetilaminsko skupino, ki smo jo z reakcijo s ciklopropansultonom prevedli v sulfozvvitterionsko. Temeljita molekularna karakterizacija je potrdila, da so sintetizirani modelni materiali zelo homogeni. Z viskozimetrijo ter statičnim in dinamičnim sipanjem svetlobe smo raziskali njihove agregacijske lastnosti v razredčenih raztopinah, v topilih različne polarnosti in selektivnosti do določenih delov molekul, co-funkcionalizirani polistireni in blok kopolimeri stirena imajo, verjetno zaradi polarizabilnosti fenilne skupine, mnogo nižjo stopnjo asociacije kot homopolidieni. S SAXS reologijo in z dielektrično spektroskopijo smo študirali lastnosti co-ftinkcionaliziranih homopolimerov in diblokov v masi ter ugotovili nove oblike samoorganiziranosti pri tej nizki koncentraciji ionov ter nenavadno obnašanje faz pri visoki temperaturi. Z elipsometrijo smo raziskali tudi adsorpcijske lastnosti zvezdastih polimerov z različnim številom funkcionaliziranih vej v razredčenih raztopinah in ugotovili, da se razlikujejo od lastnosti linearnih polimerov.
Ključne besede: polimeri s funkcionalnimi končnimi skupinami (to-funkcionalizirani polimeri), anionska polimerizacija, makromolekularna arhitektura, asociacija, fazna separacija, adsorpcija
1 INTRODUCTION
The presence of even a few highly polar groups dis-tributed along or fixed at the ends of nonpolar chains changes dramatically the properties of polymers'-7. These changes are caused by association of polar groups in the nonpolar environment of the hydrocarbon chains in the bulk or of the aliphatic solvents in solution.
The least complicated examples of polymeric associ-ating species are chains vvith one polar group89. These simple materials offer an essential starting point for testing theories and establishing the basic strueture-proper-ties relationships, vvhich vvill help to design associating polymers for practical applications.
Although many routes exist for the synthesis of end functionalized polymersUH2, living anionie polymeriza-tion has been proven to be the most efficient method for synthesizing vvell defined macromolecules1314, since it
gives the possibility to control many struetural variables including placement of the ionic groups.
This revievv vvill be focused on the dilute solution and bulk properties of dimethylamine and sulfozvvitterionic end-functionalized polymers having different architectures (linear homopolymers, diblock and triblock copoIy-mers and star polymers vvith a different number of functional groups), synthesized mainly in our laboratory.
2 SYNTHESIS AND CHARACTERIZATION Homopolymers
3-Dimethylaminopropyl-lithium (DMAPLi) vvas used as initiator for the introduction of the dimethylamine group at the end of the polymer chain in ali cases. DMAPLi vvas prepared by reaction of the corresponding chloride (DMAPCl) and Li dispersion according to Ste-vvart et al.15. Styrene (St), Isoprene (Is) and butadiene
(Bd) were polymerized with DMAPLi. The molecular vveight distributions are low in the čase of polydienes. Stoichiometric molecular weights are in very good agreement vvith the number average molecular vveights, measured by osmometry. These results indicate that DMAPLi is an efficient initiator for the polymerization of isoprene and butadiene.
The microstructure of the polydienes vvas investigated using 'H-NMR spectroscopy. The results clearly show an increase of the vinyl content with decreasing chain length, due to the presence of the polar dimethy-lamine group in the initiator.
The slow initiation rate vvas readily observed during the polymerization of St by the gradual appearance of the orange color16, vvhich is characteristic of the living polystyIyllithium chains. This fact in combination vvith the very fast propagation rate for the polymerization of St resulted in very broad molecular vveight distributions (1=1.23-1.27). In addition, the M„(SEC) of samples vvith stoichiometric molecular vveights Ms<9300 vvere much higher than the stoichiometric ones, vvith the difference being higher by decreasing Ms. This behaviour can be explained by partial consumption of the initiator. This vvas verified by the subsequent addition of Is, vvhich pro-duced polymers having bimodal distribution (figure 1). The lovver molecular vveight peak, not seen by the UV detector, corresponds to NPI homopolymer, produced by the DMAPLi, vvhich remained unreacted during the po-lymerization of St. The higher molecular vveight peak is attributed to the NPSt-b-PI block copolymer vvith the amine group at the PSt chain end, produced by the cross-over reaction of NPStLi vvith Is. Addition of THF to the mixture of St and DMAPLi ([THF]/[Li] > 3) gave poly-mers vvith close agreement betvveen Ms and Mn and lovver polydispersities.
3 DIBLOCK AND TRIBLOCK COPOLYMERS
Block copolymers of styrene and isoprene having di-methylamine end-groups at the one or the other chain end vvere prepared using DMAPLi and sequential addition of monomers17. When Is is polymerized first, a small amount of THF is added after the polymerization of the diene is completed to accelerate the crossover reaction vvith St. The reaction scheme is the follovving:
(CH3)2NCH2CH2CH2Li + n CH2CHC(CHj)CH2 —benzene >
ch3	ch3
I	I
(CH3)2NCH2CH2CH2-eCH2—CH-C—CH2-^,CH2-CH=C- CH2LI mCH2 CH
O	™3
-► (CH3)2NCH2CH2CH2-(CH2—CH=C—CH2->-tCH2—CH-VLi
THF	n	i
CH,OH
---► NIŠ
Triblock copolymers, having dimethylamine groups at both chain ends vvere prepared by coupling diblock chains vvith dimethyldichlorosilane, (CH3)2SiCl2. The products are characterized by compositional and chemical homogeneity and lovv molecular vveight distributions. The samples are designated vvith the letter N corresponding to the end-amine group, follovved by the sequence of blocks starting vvith the block at vvhich the functional group is attached.
4 co-FUNCTIONALIZED STAR POLYISOPRENES AND Mono-, Di- AND Tri-co-FUNCTIONALIZED THREE ARM STAR POLYBUTADIENES
Three and tvvelve arm PI stars18 and three arm star PBd vvith ali ends functionalized vvith dimethylamine groups19 vvere synthesized by the reaction of end-func-tionalized living polymers with suitable chlorosilanes. As an example the 3-arm polybutadienes have been prepared according to the follovving scheme:
Elutlon volumt (mL)
Figure 1: SEC chromatograms of (a) NS-3B (purifted DMAPLi; polymerization in pure benzene) and (b) NS-3B after the addition of isoprene (first peak, diblock formed; second peak, homopolyisoprene formed by reaction of isoprene vvith unreacted DMAPLi). The UV detector sees only the PS part
Slika 1: SEC kromatogrami (a) NS-3B (očiščen DMAPLi; polimerizacija v čistem benzenu) in (b) NS-3B po dodatku izoprena (prvi vrh ustreza diblok kopolimeru; drugi vrh ustreza homopoliizoprenu, ki nastane v reakciji med izoprenom in nezreagiranim DMAPLi). UV detektor zazna samo polistirenski del blok kopolimera
Bd +DMAPLi
3NPBdLi + CH3SiCl3
(-) (+)
• (CH3)2NaaaaaaA ; Lf ' NBdLi
<fH3
-*■ NPBd —Si—NPBd
-3 LiCl
NPBd 3N-3PBd
Three arm stars PBd with one or two end-amine groups were prepared using controlled chlorosilane chemistry19. The presence of one or two functional groups is denoted by the symbols IN- and 2N- respec-tively, whereas the symbol 3PBd denotes a three arm star PBd. So lN-3PBd is a three arm star PBd with one end-amine group. The following numbers differentiate sam-ples of the same series. A schematic representation of the reaction sequence used for the synthesis of samples 1N-3PBd is shown below:
(-) (+)
Bd + DMAPLi-+ (CH3)2m'Ww Li
NPBdLi
yH3
(CH3)2NAWvW(")Li(+) + CH3SiCl3(excess)-► C1—<ji—C1 +
NPBd
CHjSiCljt + LiCl
(-) (+)
Bd + s-BuLi -► s-Bu'VWWV Li
PBdLi
In the čase of samples with low arm molecular weight (M„<104) and in order to prevent the formation of the diadduct the steric hindrance of the living arm PB vvas inereased by reaction vvith diphenylethylene (DPE). A fevv drops of THF were added to accelerate the cross-over reaction. The coupling reaction vvas minimized using this procedure giving less than 3% of the byproduct.
The molecular characteristics of representative samples synthesized as deseribed above are given in Table I.
Table I: Molecular characteristics of diblock, triblock and three-arm PBd stars with one, two, or three dimethylamino end groups Tabela I: Molekularne lastnosti zvezdastih PBd polimerov z dvema blokoma, s tremi bloki in tremi vejami, z eno, dvema ali tremi dimetilaminskimi končnimi skupinami
Sample	Mwxl0"4	M„x 10"4	I=M„/M„	%wt
			(SEC)	PS
NPI	4.56	4.53	1.04	
3NP1	7.2	6.6	1.05	
NIS-3	2.44	2.25	1.06	28
NSI-1	6.96	6.12	1.06	30
NSISN-1	7.63	7.02	1.05	36
NISIN-1	6.98	6.27	1.05	27
lN-3PBd30	11.1	10.4	1.06	
2N-3PBd30	62.4	61.8	1.06	
3N-3PBd40	93.1	91.4	1.06	
yH3
2 PBdLi + C1—!ji—C1 NPBd
■ 2LiCl
f"3
PBd—f|i—PBd NPBd lN-3PBd
5 POST-POLYMERIZATION REACTION OF THE AMINE-FUNCTIONALIZED POLYMERS
The amine end groups can be easily transformed to ionic dipoles by reaction vvith 1,3 cyclopropane sul-tone20 21, illustrated in the follovving scheme:
A living end-funetionalized PBd chain vvas prepared in benzene using DMAPLi as initiator. The living polmer solution vvas added to a large excess of methyl-trichlorosilane (Si-Cl/C-Li =100/1) in order to prepare the methyldichlorosilane-capped PBd. The excess of linking agent vvas removed under vacuum line conditions. The polymer vvas repeatedly redissolved and pumped to extract traces of silane from the bulk polymer. Finally benzene vvas distilled into the reactor to dissolve the co-methyldichlorosilane PBd arm.
The next step involved the synthesis of the unfunc-tionalized arm, using s-BuLi as initiator. A small excess of this living polymer vvas coupled vvith the maero-molecular linking agent to produce the final product. Termination of the residual aetive anions vvith degassed MeOH and subsequent fraetionation to remove the ex-cess PBd arm gave the pure lN-3PBd star polymer.
A similar procedure is follovved for the synthesis of 2N-3PBd stars, starting from the reaction of the living unfunctionalized arm vvith excess methyltrichlorosilane follovved, after the removai of the excess linking agent, by the coupling reaction of the dichlorosilane-capped arm vvith a small excess of the amine-funetionalized living arm. Ali these procedures vvere monitored by SEC.
-*. v™™™ M (+)CH 2CH 2CH jS OjC)
A	1
O o	ch3
The reaction takes plače in dilute THF solutions (2-3 w/v%) at 70°C for several days using an excess of the sultone over the amine groups (sultone/amine = 10/1). For the PBd samples inert atmosphere vvas used. Under these conditions this post-polymerization reaction is free of side reactions (crosslinking, degradation etc.) as vvas verified by SEC. Similar peaks vvith the corresponding amine-capped polymers vvere observed in CHC13 in ali cases.
It is difficult to determine the extent of the conver-sion of the t-amine groups to sulfobetaines due to the lovv concentration of these groups in the polymer chains. Hovvever, qualitative results by 'H-NMR shovv that the reaction yield is very high17 22. In figure 2 the 'H-NMR spectra of linear block copolymer NIS-5 and the corresponding zvvitterion sample are given. The peak at 2.2 ppm is assigned to the methyl protons of the carbons at-tached to the nitrogen atom. This peak has completely disappeared after the reaction vvith 1,3 cyclopropane sul-
7i7JUUMU«iUUUU2JI1i
U
U
L
-L.

-L.
-L.


-L.
_U
_1_
7.5 7.0 iS ».0 S.S S.0 4J 4.0 SJ S.0 2J 2.0 1J
Figure 2: 'H-NMR spectra of samples NIS-5 (top) and ZwIS-5 (bottom) in CDCI3
Slika 2: 'H NMR spektri vzorcev NIS-5 (zgornji) in ZwIS-5 (spodnji spekter) v CDCI3
diene homopolymers probably because of the solvat-ing effect of phenyl rings on the dipolar groups
c)	The aggregates are polydisperse as concluded by LALLS, MO and DLS measurements
d)	The associates behave hydrodynamically as star poly-mers as evidenced by the increasing kH values with increasing degree of association and by the good agreement betvveen experimental aggregation num-bers and those calculated assuming the star model
e)	The linear head packing model describes fairly well the structures of the associates.
Detailed studies by small angle x-ray scattering (SAXS) vvere performed on lovv molecular vveight zvvit-terion-capped polyisoprenes26. For samples having 14000 < Mw < 28000 the scattering proftles shovv that the aggregates form a body-centered cubic lattice.
Figure 3 shovvs the corresponding scattering proftles for the lovver molecular vveight samples (2200 < Mw < 4650). The peaks can be indexed on a tvvo dimensional hexagonal lattice of tubes. In other vvords the aggregates have a tubular structure vvith the tubes closed packed on a tvvo dimensional hexagonal lattice vvith crystalline order. The core is formed by dipoles vvhich are arranged in an antiparallel conftguration as shovvn in figure 4.
It is characteristic that a very small vveight fraction of ionic species (3.5-7.5%) is able to promote a hexagonal cylinder morphology vvith long range order in contrast to usual block copolymers.
The viscoelastic behavior in the melt state of end-functionalized polyisoprenes vvas also investigated27. The amine-capped samples behave more or less as conven-tional polyisoprenes indicating that only weak associa-
tone and tvvo nevv peaks at 3.15 and 2.95 ppm have emerged. These peaks are assigned to the methyl protons attached to the positively charged nitrogen atom of the zvvitterionic group and to the methylene protons of the carbon attached to the sulfur atom respectively21'23.
6 DILUTE SOLUTION AND BULK PROPERTIES Homopolymers
The dilute solution properties of co-functionalized linear homopolymers vvere studied by membrane os-mometry (MO), lovv angle laser light scattering (LALLS) viscometry, and dynamic light scattering (DLS) in various non-polar solvents18-24'25. The conclusions obtained from this study are the follovving:
a)	The dimethylamine-cappped samples present no evidence of association in non-polar solvents (cyclohex-ane, CCL, toluene)
b)	The zvvitterion-capped samples form large aggregates in these solvents vvith aggregation numbers increasing vvith decreasing molecular vveight of the parent material. Hovvever, aggregation numbers for PS ho-mopolymers are lovver than those obtained for poly-
T—i—i—|—i—i—i—i—|—i—i—i—r
(1.01	a. Hw«2200
b.	Mw"3850
c.	rv*t6SD
13.0)
a q(A-»)
Figure 3: X-ray scattering profiles of zvvitterion-capped polyisoprene samples vvith molecular vveights of 4650, 3850, and 2200. The profiles of the last tvvo samples vvere shifted by a factor (a) to make the first peak positions overlap
Slika 3: Krivulje sipanja rentgenskih žarkov vzorcev poliizoprena s končnimi zvvitterionskimi skupinami z molskimi masami 4650 (c), 3850 (b) in 2200 g/mol (a). Zaradi lažje primerjave krivulj sta krivulji zadnjih dveh vzorcev premaknjeni za faktor (a)
Figure 4: A schematic representation of the formation of two-dimensional lattices of close-packed tubuiar aggregates Slika 4: Shematska predstavitev nastanka dvodimenzionalnih mrež tesno zloženih cevastih agregatov
tion may exist in the melt state. The situation is very different for the zvvitterion-cappped polymers with the dy-namic moduli broadened and shifted to much lower fre-quencies. For samples vvith high base molecular vveights the viscoelastic behaviour more closely resembles the behaviour of conventional star polymers.
Samples with intermediate and lower molecular vveights show a second relaxation regime at very low fre-quencies. A characteristic example is given in figure 5. It is observed that some resemblance exist betvveen the zvvitterion and star polymer only at intermediate and high frequencies.
The viscosities of the zvvitterion polymers, especially of lovv and intermediate molecular vveights are much larger than those predicted assuming the star model. Consequently, it is reasonable to consider that aggregates
Figure 5: Comparison of dynamic moduli for a four-arm polyisoprene star and a monofunctional zvvitterion polyisoprene in the melt state at 25°C. Data for the star (Ma=4.4 x 104) are shown by the solid lines; the points are data for a sample vvith M0=4. 61 x 104 Slika S: Primerjava dinamičnih modulov zvezdastega poliizoprena s štirimi vejami in monofunkcionalnega zvvitterionskega poliizoprena v talini pri 25°C. Neprekinjena črta - zvezdasti poliizopren (Ma=4,4 x 104), točke - vzorec z M„=4,61 x 104
have extended morphologies (lamellae, strings etc.). Only in the čase of lovv aggregation numbers, observed for samples of high base molecular vveight, the behavior is similar to those of star polymers because the core size is rather small and can be considered as the star's center. The extended structures are delicate in a mechanical sense making it possible to explain the remarkable strain sensitivity observed at lovv frequencies. It is noted that there is close aggrement vvith the results obtained by melt rheology and SAXS.
7 co-FUNCTIONALIZED BLOCK COPOLYMERES OF STYRENE AND ISOPRENE
The association behavior of end-functionalized diblock and triblock copolymers of isoprene and styrene vvas studied in CC14, vvhich is a nonpolar and good sol-vent for both blocks1728. The aggregation numbers, Nw are almost the same vvhether the zvvitterion group is linked at the PI or the PS chain end. Their value depends strongly on the Mw of the base polymer. Nw decreases vvith increasing molecular vveight of the precursor polmer, M0. The variation of Nw vvith M() for the čase of ZvvPI in cyclohexane18 and CC1417, PS in CC1427 di- and triblock copolymers in CC1417 is given in figure 6. The aggregation numbers for ZvvPI are lovver in CC14 than in cyclohexane due to the higher polarizability of the former solvent. Another point to be noticed is that the de-grees of association of copolymers are closer to those determined for the ZvvPS samples than to those of ZvvPI samples in CC14. The aromatic rings, due to their high polarizability cause some kind of solvation, and lead to reduced Nw values. The aggregation numbers are almost the same for the monofunctional and difunctional samples. This is rather surprising, since the difunctional polymers form gels at concentrations lovver than c8Ci
10-
—I—
so
100
M.. 10-3
—I—
15«
200
Figure 6: Dependence of the vveight average aggregation number, Nw from the base molecular vveight, M0 for various polymer series Slika 6: Odvisnost utežnega povprečja števila agregacije Nw od molekulske mase M0 za različne serije polimerov
(cgCi=0.5 c*) and can be seen as evidence of intramolecu-lar association in very dilute solutions.
DLS was used to study the hydrodynamic properties of the end-functionalized copolymers. The zwitterionic polymers have a substantially different behaviour than their precursors, due to the formation of aggregates in CC14. The values of diffusion coefficient at infinite dilu-tion, Do are lower, the RH values higher, and the aggregates are polydisperse. The kD values are negative in most cases, due to the aggregation process and are con-sistent vvith the lovv A2 values obtained by LALLS.
Viscosity measurements vvere also performed to com-plement the DLS data. The [r|] values for the zvvitte-rionic samples are considerably higher than those for the amine-capped samples and the reduced viscosity vs concentration plots are not always linear. The Huggins plots are concave upvvards in some cases and especially for the difunctional samples.
The nonlinear dependence of the reduced viscosity on concentration is an indication that the association number changes by increasing concentration, something vvhich is expected to be more pronounced in the čase of the difunctional triblocks.
The stability of aggregates vvas tested by adding small amounts of an alcohol, namely 2-methylcyclohex-anol (at 1% and 5% content). The association is reduced in the presence of the alcohol, but even by 5% alcohol samples remained aggregated. With increasing alcohol content the aggregation numbers decreased, the A2 values increased and the kH values are decreased also.
SAXS, rheology, and dielectric spectroscopy vvere used to study the statics and dynamics of the end-functionalized block copolymer29'30. SAXS profiles from amine and the corresponding zvvitterion-capped samples confirm the existence of ionic aggregation. A characteris-tic example is given in figure 7. The follovving features vvere observed:

m v T-*»K
\
10
D
.4.
,5.

. v
v
• v ■ v
• i
104
-2w-S<r5
OJ	0J>
Q (nm-1)
OJ	0^5
Q (nm"1)
Figure 7: SAXS profiles for tvvo (o-functionalized IS diblock copolymers at T=303 K. Data vvere corrected for density fluctuations and the intensities are given in absolute units
Slika 7: SAXS krivulje dveh co-funkcionaliziranih izopren-stirenskih (IS) diblok kopolimerov pri T=303 K. Pri podatkih so upoštevane fluktuacije gostote, intenzitete so podane v absolutnih enotah
a)	a background originating from density and concentration fluctuations
b)	an excess intensity at lovv Q related to heterogeneities vvith long correlation lengths in the čase of ionom-
ers31
c)	the microdomain peak32 characteristic of the micro-phase separation process betvveen PI and PS phases and
d)	the peak related to the polar groups, vvhich emerged in the čase of the zvvitterionic sample.
The last three characteristics are temperature depend-ent vvith the aggregate peak intensity being much less sensitive to changes of temperature for the specific ex-periment temperature range. The microdomain peak in-tensity has a similar temperature dependence for both the amine and the zvvitterion-capped copolymers.
A completely different behavior is observed vvhen the functional group is attached to the PS chain-end, as shovvn in figure 8. The microdomain peak dominates the scattering pattern in this čase. The peak increases in in-tensity, sharpens, and moves to slightly higher Q values vvith increasing temperature. The absence of any dissolu-tion process clearly indicates that the microdomain structure is stabilized by the ionic aggregates. In the čase of ZvvSI samples the ionic groups are trapped vvithin the PS phase vvithout being able to aggregate. The increase of temperature increases the mobility of the polar groups leading to the formation of aggregates vvithin the "hard" phase. This is schematically shovvn in figure 9 for both systems, ZvvIS and ZvvSI. As a consequence the incom-patibility of PI and PS is enhanced and a completely different phase behavior is observed. So only by changing the position of the polar group, from the PI to the PS chain-end it is possible to change the phase diagram.
10a
CO
| lO4-D
aj
103
• ° • °
' \
\\
\\
•	: NHS*5
•	:ZwHS#5
• \
Q (nm-1)
1
Figure 8: Comparison of the SAXS profiles for the dimethyIamino-and zvvitterion-substituted to-functionalized copolymers at tvvo temperatures
Slika 8: Primerjava SAXS krivulj dimetilamino- in zvvitterionsko substituiranih co-funkcionaliziranih kopolimerov pri dveh temperaturah
Zw-IS
Zw-SI
(g) Low T
(b) Hgh T

(O) t<t"
illfllftll
jifci
(b) T>T„m
rrinL|__jJlll
Figure 9: Schematic illustration of the microstructures in to-functionalized SI block copolymers, showing the Zw-IS (left) and Zw-SI (right) cases at low (upper) and high (lower) temperatures. The corresponding electron density distributions are also shown Slika 9: Shematska predstavitev mikrostrukture co-funkcionaliziranih SI blokkopolimerov pri nižji (zgoraj) in višji temperaturi (spodaj) za Zw-IS (levo) in Zw-SI (desno). Prikazana je tudi ustrezna porazdelitev elektronske gostote
The conclusions derived by SAXS studies vvere confirmed by rheology. In the čase of the zvvitterionic co-polymers an extension of the rubbery plateau is observed. This behaviour is explained considering that the aggregates act as physical crosslinks vvithin the PI phase. Furthermore, vvithin the temperature range investigated no sign of an order-disorder transition vvas observed in agreement vvith SAXS results, meaning that the cubic microdomain structure is stable up to high temperatures.
Dielectric spectroscopy also offers the means to ver-ify the conclusions dravvn so far through the selective probing of the Pl chains. In the čase of ZvvIS copolymers in addition to the fast segmental and the slovv normal mode an intermediate process, vvith activation parameters which are reminiscent of the segmental process, is observed. This intermediate process arises from regions of the reduced mobility created around the aggregates impending the motion of the PI chains in their immediate environment. Hovvever differential scanning calorimetry, DSC is not so sensitive and the size of these regions very small in order to detect an intermediate Tg value, al-though an increase on the Tg of the polyisoprene block has been observed at lovv molecular vveights33.
The combination of the association process caused by the presence of polar groups in a nonpolar solvent vvith the micellization procedure, promoted in selective solvents leads to interesting solution behavior. The dilute solution properties of ©-functionalized diblock copoly-mers having dimethylamine or zvvitterion groups at the PS chain-end vvere studied in n-decane a nonpolar selective solvent for the PI blocks34.
The presence of the polar groups introduces another factor capable to enhance the aggregation numbers for the zvvitterionic samples in n-decane. Much lovver Nw
I
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Oj) -
Tonparetui* fC)
Figure 10: Kc/ARe vs concentration plots given for (a) sample NSI-4F and (b) sample ZwSI-4F in n-decane at 25°C. (c) Intensity vs temperature plot for the same samples at c=2.800 x 10"3 g/mL for NSI-4F and c=1.629 x 10"3 g/mL for ZwSI-4F Slika 10: Kc/ARe v odvisnosti od koncentracije za vzorca (a) NSI-4F in (b) ZwSI-4F v n-dekanu pri 25°C; (c) odvisnost intenzitete od temperature za NSI-4F pri c=2,8 x 10'3 g/mL in za ZwSI-4F pri c= 1,629 x 10"3 g/mL
values were observed for the amine-capped copolymers, meaning that the amine groups are not polar enough to enhance the association process. TVpical LALLS plots are given in figure 10.
From DLS measurements negative kD values vvere obtained for the amine-capped polymers as expected having in mind the negative A2 values. For the zvvitte-rionic samples the kD values vvere positive meaning that the equilibrium is shifted in favor of the micelles.
Viscometry measurements vvere also performed. The Huggins coefficients increase vvith increasing molecular vveight for the amine-capped polymers. This behavior is consistent vvith a star-like structure. For the zvvitterionic samples constant kH values, around 1.1 vvere obtained, meaning that rather compact structures exist in solution.
The Rv and RH values are identical vvithin experimen-tal error for the amine polymers but for the zvvitterionic polymers RH is much higher than Rv. The former result is consistent vvith star-like structures, vvhereas the latter can be explained considering the high sensitivity of DLS to large structures and/or to the development of shear forces in the capillary tube able to disrupt the larger ag-gregates. The fact that the polar core probably has an elongated structure vvith antiparallel placement of the zvvitterionic groups is able to support the above assump-tion, since a break of the association at one point can cause a large reduction of the micelle's size.
Intensity vs temperature measurements at concentra-tions vvhere micelles are the dominant species revealed that the micelles formed by the zvvitterionic copolymers are stable at much higher temperatures than the ones made of amine-capped precursors. It seems that in the former čase the high temperature resistive ionic cores stabilise the micelles (figure 10(c)).
8 STAR SHAPED POLYBUTADIENES VVITH END-FUNCTIONAL GROUPS
The amine-capped star polymers provide no evidence of association in cyclohexane, vvhereas strong association is observed in the čase of zvvitterionic samples. It is evident that (a) among the different series of polymers the aggregation number decreases vvith increasing num-ber of functional groups and (b) among the samples vvith the same number of polar groups the degree of association decreases vvith increasing molecular vveight of the precursor polymer, due to excluded volume repulsions. These results are given schematically in figure 11.
Multifunctional samples, especially the trifunctional stars form gels even at lovv concentrations. This result connected vvith the lovv aggregation numbers for these samples leads to the conclusion that in very dilute solution intramolecular association dominates and by increasing concentration there is a rather sharp transition from intramolecular to intermolecular association able to produce stable gels.
20 15
I 10
S
o
0 SOOOO 100000 190000 200000 (Mw)N
Figure 11: VVeight-average aggregation number, Nw vs base molecular vveight (MJN of the star polymers: Zw-lN-3PBd (o), Zw-2N-3PBd (■). and Zw-3N-3PBd (A)
Slika 11: Utežno povprečje števila agregacije Nw v odvisnosti od molske mase (MW)N zvezdastih polimerov: Zw-lN-3PBd (o), Zw-2N-3PBd (■) in Zw-3N-3PBd (A)
The degrees of association of monofunctional stars are lovver than those measured for the linear co-function-alized PBd, meaning that the star structure prevents the association due to the steric hindrance caused by the un-functionalized arms.
The hydrodynamic behavior of the functionalized stars vvas studied by DLS and viscometry35. The increased values of p2/T2 (>0.2) indicate that the aggre-gates produced by the zvvitterionic samples are polydis-perse in agreement vvith the MO and LALLS results. Lovv kD values vvere observed in most cases as a con-cequence of the decreased second virial coefficients.
The strongly negative kD values for the trifunctional stars indicate the existence of strong hydrodynamic in-teractions betvveen macromolecular chains eventhough these samples have lovv aggregation numbers and shovv only small increases in RH compared to their precursors. This behavior can be seen as evidence of intramolecular association in very dilute solutions. For the čase of di-functional stars the above analysis is not straightforvvard. It is clear that intermolecular association cannot be ruled out.
For monofunctional samples there is no possibility of intramolecular association. The star model can be used for these samples considering that the aggregates corre-spond to star polymers and their precursors to the arms of these stars. Consequently, it is possible to calculate the aggregation numbers from DLS measurements, Ndls-The results shovv that the aggregates formed from the monofunctional samples behave hydrodynamically as star polymers vvith functionality equal to 2NW. It seems that the tvvo unfunctionalized arms anchored at the pe-riphery of the aggregates are responsible for the overall size of micelles.
The conclusions drawn by DLS are verified by vis-cometry for the amine-capped polymers. Zwitterionic tri-functional samples have lovver intrinsic viscosities than their precursors but the kH values are extremely high, in-dicating the presence of strong hydrodynamic interactions. This behavior implies that in very dilute solutions compact structures are formed through intramolecular association. This result is in agreement vvith LALLS and DLS data.
Comparative examination of Rv and RH values shovvs that R„<RH for the zvvitterionic polymers, meaning that the aggregates dissociate to some extent in the capillary tube, due to the shear forces applied therein. It seems that the increased steric repulsions introduced by the un-functionalized arms lead to the formation of not so strong associates as in the čase of Iinear polymers.
The adsorption behavior of functionalized Iinear and three arm star PBd vvas studied by e!lipsometry at 20°C in a mixed solvent of cyclohexane and toluene (50% by volume)36. A mixture of cyclohexane and toluene vvas used. In this mixture association is not detected up to the concentration of 2.0 mg/ml from DLS measurements and the dn/dc values (0.050 ml/g at 589.4 nm at 20.0°C) pro-vide enough contrast for accurate measurements.
Various parameters of the adsorption behavior of samples are reported in Table II. The adsorbed amount A is increased vvith decreasing molecular vveight for the Iinear samples. The longer the PBd chains adsorbed the bigger space they occupy and the stronger the repulsion betvveen them. The ratio 8=Dinlcr/Dover of the interchain distance (Diin) over the space needed to accomodate a svvollen polymer coil in a good solvent in it's unpertur-bed state on the surface (D„ver) is much lovver than unity. This indicates that the adsorbed chains are stretched adopting a brush-like conformation.
In the čase of the zvvitterionic stars the adsorbed amount increases vvith increasing number of functionalized arms. The grafting density, defined as g=ANa/Mw, vvhere A is the adsorbed amount, NA the Avogadro number and Mw the vveight average molecular vveight of the
Table II: Various adsorption parameters of co-functionalized Iinear (L) and mono-(lN)-, Di-(2N)-, and tri-(3N)-co-functionaIized polybutadienes
Tabela II: Adsorpcijski parametri co-funkcionaliziranega linearnega (L) in mono- (IN)-, di- (2N)- in tri- co-funkcionaliziranih polibutadienov
Sample	Aplatcau (mg/m2)	O Dinter (chains/ (nm) nm2)		Dover (nm)	§=Dimer/ Dover
Zw-L-PBdl 2	2.47+0.01	0.125	2.8	10.5	0.27
Zw-L-PBd20	2.410+.25	0.07	3.8	14.5	0.26
Zw-L-PBd80	1.69+0.18	0.012	9.1	33.3	0.27
Zw-lN-3PBd30	1.77+0.17	0.0096	10.2	25.2	0.4
Zw-2N-3PBd30	1.86+0.06	0.0179	7.5	19.1	0.39
Zw-2N-3PBd40	1.90+0.12	0.0128	8.8	25.2	0.35
Zw-3N-3PBd25	2.14+0.18	0.0191	7.2	19.7	0.37
Zw-3N-3PBd40	2.3+0.2	0.0148	8.2	24.3	0.34
star seems to present stronger dependence on the molecular vveight than the functionality of the stars. The o values of the samples Zw-2N-3PBd30 and Zw-3N-3PBd25 are very close indicating that despite the fact that the adsorption energy is high the entropic loss in-volved in the attachment of the third arm vvhen tvvo arms are already attached may be very high.
The adsorption kinetics, studied by time-ressolved el-lipsometry shovvs tvvo processes. In the initial stages the adsorption is diffusion controled. By longer times the polymers must penetrate the barrier formed by the in-itially adsorbed chains. It vvas found that the star poly-mers penetrate this barrier faster than the Iinear chains, due to the different conformations adopted by the stars.
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