Langmuir 1994,10, 3035-3042
3035
Synthesis and Characterization of Step Growth Hydrophobically-Modified Ethoxylated Urethane Associative Thickeners? J. Philip KaczmarskiS and J. Edward Glass* Department of Polymers and Coatings, North Dakota State University, Fargo, North Dakota 58105 Received April 14, 1993. In Final Form: May 26, 1994@ The influences of molecular weight, the presence of internal alkyl groups, and the size of the external alkyl group among hydrophobically-modified, ethoxylated urethanes (HEURs)are examined through the synthesis of compositionally defined thickeners. The polymers were prepared by a step growth polymerization of poly(oxyethy1enes)with diisocyanates and possess broad molecular weight distributions. HEURs synthesized with only internal alkyl groups do not effect viscosity increases despite variances in the structure of the alkyl group sizes of the diisocyanates, and evidence of hydrophobic aggregation is not observed by fluorescence spectroscopy. The size of the terminal alkyl groups is important. Dramatic increases in hydrophobic associations and viscosity as a function of thickener concentration are observed with increasing “effective”terminal hydrophobe size. The ‘effective” hydrophobe size can be altered by increasing the hydrophobicity of the alkyl amine or the alkyl diisocyanate used to couple the amine to the poly(oxyethy1ene) units. For example, with a constant amine hydrophobe size a larger diisocyanate, dicyclohexylmethane diisocyanate (HI~MDI), increases the effective hydrophobe size of the terminal alkyl group to a greater extent than the smaller hexamethylene diisocyanate (HDI). Contrary to viscosity build through chain entanglements, lowering the molecular weight of HEUR thickeners effects larger viscosity increases. This also is related to hydrophobic aggregation through the increase in concentration of hydrophobes.
Introduction When highly extended, the chemical bonds of macromolecules are broken,l and their viscosifjmg properties a t low shear rates are not fully regained. Hydrophobe modification of water-soluble polymers provides a means of minimizing this loss.2 Aqueous solutions of high molecular weight polymers exhibit high elastic behavior, and in high deformation rate processing, this can be detrimental. Hydrophobe modification of water-soluble polymers can provide viscosities equivalent to high molecular weight polymers a t low shear rates with a significantly lower elasticity a t higher deformations rates. In concept, the extent of the rheological changes is dependent upon the size ofthe hydrophobe, the proximity of their placement, and the overall architecture of the water-soluble p ~ l y m e r . ~ , ~ Relating structural aspects to the performance of the modified water-soluble polymer is difficult because of the competing reactions and complexity of polymerization processes. For example, in chain growth polymerizations in aqueous media (i.e., hydrophobically modified polyacrylamides), the tendency for micellar behavior of the ~~
~
~~
surfactant containing monomer results in sequence r ~ n s , ~ , 6 even if the modified monomer is only a minor percentage of the total monomer charge. Sequence runs can have a beneficial influence on the synergy of associations but this also is difficult to quantify.’ This question has only recently been addressed in a n expanded quantitative manner.8 The hydrophobes can be placed randomly by postreaction of the preformed polymergor by incorporating small amounts of the hydrophobe monomer in a n alternating copolymer pair.1° None of these approaches provides a true model thickener for predicting behavior in applications. In the preceding paper1’ model hydrophobe-modified, water-soluble polymers were prepared by stepwise addition of an excess of diisocyanates to poly(oxyethy1ene) diols, followed by the reaction of the terminal isocyanate groups with hydrophobic alcohols or amines. With these model HEUR results in hand, this study will focus on the synthesis of HEURs by a step growth polymerization process, which increases the complexity of reaction products. Broad molecular weight distributions are produced and intermediates in the step growth propagation may be terminated by hydrophobic moieties or remain a t this molecular weight, unreacted.I2 The extent of modification among the different, distinct molecular weights is unknown and two types of potentially hydro-
t Presented in part at the American Chemical Society Meeting,
Washington, DC. Proc. ACS Div. Polym. Mater.: Sci. Eng. 1992, 67, 282. Current address: GE Plastics, 1 Lexan Lane, Mount Vernon, IN 47620. * Abstract published inAdvance ACSAbstracts, August 15,1994. (1) Odell, J. A.; Keller, A.; Muller, A. J. In Polymers in Aqueous Media: Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry223; American Chemical Society: Washington,DC, 1989; p 193, and references therein. (2) Evani, S. R.; Rose, G. D. Polym. Mater.: Sci. Eng. 1987,57,477. (3) Lundberg, D. J.; Glass, J. E.; Eley, R. R. J . Rheol. 1991,35 (6), 1255. (4) Karunasena, A.; Brown, R. G.; Glass, J. E. Polymers in Aqueous Media: Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989; p 495.
*
0743-7463/94/2410-3035$04.50/0
(5) Peer, W. J. Polymers in Aqueous Media: Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989; p 381. (6) Valint, P. L., Jr.; Bock, J.; Schulz, D. N. Polymers in Aqueous Media: Performance Through Association; Glass, J. E., Ed.; Advances in Chemistry 223;American Chemical Society: Washington, DC, 1989; p 399. (7) Biggs, S.; Selb, J.; Candau, F. Langmuir, 1992, 8 , 838. (8) Biggs, S.; Hill, A.; Selb, J.; Candau, F. J . Phys. Chem. 1992,96, 1505. (9) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7,617. (10)Evani, S.; Lalk, R. H. US.Patent 3 779 970, 1973. (11) Lundberg,R. D.;Brown, R. G.;Glass, J. E.;Eley, R. E.Langmuir, preceding paper in this issue. (12) Glass, J. E.;Kaczmarski,J. P. Polym. Mater. Sci.Eng. 1991,65, 175.
0 1994 American Chemical Society
3036 Langmuir, Vol. 10, No. 9, 1994
u
u
CH?
(IPDI)
Figure 1. Structures of diisocyanates reacted with POE t o form internal alkyl units. Table 1. Molecular Weights of Starting POEs SEC Mn material (OHno.) Mn M, PDI designation 6 156 7 048 7 605 1.08 HO(Et0)iseH POE 6000 POE 8000 batch 1 8 140 8 935 10 354 1.16 HO(Et0)ienH POE 8000 batch 2 8 828 9 275 11031 1.19 HO(Et0)zooH 29 219 33 408 37 237 1.12 HO(Et0)sesH POE 35000
phobic alkyl groups are present, an internal alkyl group within the poly(oxyethy1ene) backbone and an external alkyl group present a t the termini of the HEUR chain. Two synthetic procedures are presented which allow the relative contributions of the internal and external alkyls on viscosity build to be examined. The HEUR polymers, prepared with well-defined components, vary in molecular weights (17000-54000),molecular weight distributions, and aqueous solution behavior.
Experimental Section Materials. Poly(oxyethy1ene)s of molecular weight 6000 (Fluka), 35 000 (Fluka), and 8000 (Union Carbide, Carbowax 8000) were used as received. Octadecyl isocyanate (Aldrich, technical grade) was gravity filtered and stored under argon before use. Hexamethylene diisocyanate (HDI, Aldrich, 98%) and isophorone diisocyanate (IPDI, Aldrich, 98%) were stored under argon prior to use. Dicyclohexylmethane diisocyanate(HizMDI) and tetramethylxylylene diisocyanate (TMXDI)(Milesand American Cyanamid, respectively, purities unknown) were distilled under vacuum before use and stored under argon. The structures of these diisocyanates are illustrated in Figure 1. Hexylamine (99%), octylamine (97%), dodecylamine (98%), tetradecylamine (96%), and octadecylamine (99%), dibutyltin dilaurate (98%),8-anilinonaphthalenesulfonicacid (ANS, 97%), andNjV'-dimethylformamide (DMF, 99%)were used as received from Aldrich. Toluene and tetrahydrofuran (both ACS Reagent Grade) were dried with anhydrous sodium sulfate before use. Acetone (Baxter Scientific, 99.5%)and petroleum ether (Mallinkrodt, purity unknown) were used as received. Water (DDI water) was distilled and passed through a Milli-Q (Millipore) ion exchange and filtration system. Triton X-100 (Rohm & Haas), an ethoxylated octylphenol surfactant with an average of 9-10 oxyethylene units ( C S H ~ ~ C ~ H ~ O ( E ~was O )used ~ O Has) ,received. Characterization of Starting Materials. Poly(oxyethy1ene)s (POEs) were analyzed for molecular weight by end group analysis and size exclusion chromatography; the results and designations are summarized in Table 1. The hydroxyl number titration was performedaccordingto the pyromellitic dianhydride imidazole method13 for determining hydroxyl numbers. Synthesis of HEURs without TerminalAlkyYAryl Units. The first synthetic procedure (Scheme 1, with x moles of diisocyanate to x + 1mol of POE), using a slight excess of POE to obtain hydroxyl endgroups, involved the reaction of HO(13)Kingston, B.H.; Garry, J. J.; Hellwig, W. B. Anal. Chem. 1969,
41 (11, 86.
Kaczmarski and Glass (Et0)lsgHwith one of the four diisocyanates (Figure 1)in a mole ratio of 7:6 (HO(Et0)13gH to diisocyanate). This gave HEUR polymerswith an average of six internal alkyl groups and allowed an analysis of the contribution of the internal alkyl group to hydrophobic domain formation. The following synthetic procedure was used. The polyol, HO(Et0)139H(-70 g), was weighed into a three-neck 500-mLround-bottomed flask with a magnetic stir bar. The poly01melted at about 70 "C,was dewatered under vacuum for 2 h, and cooled to room temperature under vacuum. The vacuum was broken and the flask was purged immediately with argon. (No decrease in the molecular weight was observed before and after vacuum drying, as measured by SEC and hydroxylnumber.) Athermometer, mechanical stirrer, and reflux condenser were connected t o the flask and 300 mL of dry tetrahydrofuran was introduced. The solution was heated to 45 "C and dibutyltin dilaurate catalyst (0.2 w t %) and the stoichiometric amount of diisocyanate was weighed into a 20-mL vial, diluted with 15 mL of dry THF, and added to the flask. Completion of the reaction was monitored (FT-IR) by the disappearance of the isocyanate band between 2250 and 2300 cm-l (cumulated double bond stretching of the isocyanate) and the appearance ofbands between 1675 and 1525cm-l (urethane All thickeners were isolated by carbonyl and N-H ~tretch1.l~ precipitation of the polymer into petroleum ether (3:l volume petroleum etherholume of thickener solution), collecting on a sintered glass funnel, and drying using a water aspirator. Synthesis of HEUR Thickeners with Terminal AlkyY ArylUnits. Two synthetic procedures were examined to obtain HEURs with both internal and terminal alkyl groups. In the first synthesis, HO(Et0)lezH was employed in slight excess to the diisocyanate t o obtain hydroxyl endgroups. The mole ratios of HO(EtO)l82H to diisocyanate were 2:1, 3:2, and 4:3 and the same reaction conditions were used as described in the previous section. These polymers were isolated, characterized by SEC, and partitioned into several batches for subsequent reaction with a monoisocyanate. Monoisocyanatemodificationofthe HEURs, prepared from HO(Et0)182H and TMXDI in mole ratios of 2:1, 3:2, and 4:3, was accomplishedusing the followingprocedure. A three-necked 500-mL round-bottom flask was equipped with a mechanical stirrer, argon inlet, thermometer, Dean Stark water trap, and reflux condenser. About 50 g of the HEUR with internal alkyl groups and terminal hydroxyl groups were weighed into the reaction flask and 300 mL of dry toluene was added to the flask and trap. Water was azeotropically distilled by heating the toluene to reflux and removing a total of 150 mL of toluene. The flask was cooled to 40 "C and 150 mL of dry, distilled THF and dibutyltin dilaurate (0.2%by weight) was added. A 4-fold excess of the stoichiometric amount of octadecyl isocyanate was filtered by gravity and introduced into the flask. The reaction was allowed to stir for 6 h a t 40-45 "C at which time the HEURs were isolated by precipitation into petroleum ether as described in the previous section. Molecular weights were recorded before and after monoisocyanate modification. Longer reaction times were required for reactions performed with TMXDI; this is probably due to the increased steric hindrance of the isocyanate groupby the isopropylideneunit separatingthe isocyanate group from the aromatic ring. A n alternative, direct synthesis of HEURs utilized a higher ratio of diisocyanates to POE to obtain terminal isocyanate groups. This intermediate product was reacted with an alkylamine prior to isolation of the final HEUR material (Scheme 1). For example, the mole ratios of HDI to POE were 3:2, 5 4 , and 7:6 to obtain a range of molecular weights. The diisocyanate also was changed from HDI to HlzMDI and the mole ratio of HlzMDI to POE was kept at 3:2. The same reaction setup was used as described in the previous section. About 60 g of HO(EtO)zooH was added to the argon purged flask. Dry toluene (350 mL) was filtered through an anhydrous sodium sulfate pad into the flask. The solution was heated to reflux and approximately 150 mL of toluene was removed through the water trap. The solution was cooled to 50 "C and dibutyltin dilaurate (0.2 w t %) added. The stoichiometric amount of diisocyanate was weighed into a 20-mL scintillation (14)Infrared Spectroscopy Committee of the Chicago Society for Coatings Technology An Infrared Spectroscopy Atlas for the Coatings Industry;Federation of Societiesfor CoatingsTechnology: Philadelphia, PA, 1980.
Step Growth HEUR Thickeners
Langmuir, Vol. 10, No. 9, 1994 3037
Scheme 1. Direct Addition Synthetic Route to Model, Broad Molecular Weight, Hydrophobically-modified,Ethoxylated Urethane (HEUR)Thickeners" (X+1) OCN-R-NCO
+
=
X HO-(Et0)2oo-H
+ 2 R-N&
R"-N-C-N-R HI
ti,'
PI fa":,
N-C-O-(Et0)2oo-C-N-R
N-C-N-R
H
X
R , alkyl amines added to terminal isocyanates; R, diisocyanate structures are illustrated in Figure 1. a
vial, diluted with 15 mL of dry toluene and added to the flask. The reaction was allowed to proceed for 30 min at which time the residual isocyanate groups were reacted with a slight stoichiometric excess (1.05x ) ofalkylamine (diluted with toluene as described above,but the longer chain amines required heating for dissolution). The reaction was allowed to proceed for 4 more h, at which time FT-IR analysis demonstrated that the isocyanate bands had disappeared. These HEURs were isolated by precipitation into petroleum ether as described above. MolecularWeight Determinations. Molecularweights and molecularweight distributions were determined by size exclusion chromatography (SEC), using a Waters Model 510 pump, a Waters M730 data module, a Waters Model R401 differential refractometer, tetrahydrofuran as the mobile phase, a flow rate of 1.0 mumin, a column temperature of 31 "c,two Ultrastyragel columns (10 000 A, 1000A) and twop-Styragel columns (10 000 A and 10000 A) (for HEURs without terminal alkyls and monoisocyanate modified HEURs) or a set of four Ultrastyragel columns, 100 000 A, 2 x 10 000 A, and 1000 A (for HEURs prepared by the direct addition process). A calibration curve was prepared with poly(styrene) standards. Spectroscopy. Infrared spectra were recorded on a Mattson Sirius 25 Fourier transform infrared spectrophotometer. Thirtytwo scans were recorded at a spectral resolution of 4 cm-1. Fluorescence emission spectra of the purified aqueous HEUR thickeners were recorded on a SPEX 2T2 Fluorologfluorometer using a xenon lamp as a light source. Solutions were prepared using a 2 x lO+M stock solution of8-anilinonaphthalenesulfonic acid. The stock solution was degassed with argon before use. The excitation spectrum of ANS was recorded to determine the optimum excitation wavelength. By use of this wavelength (377 nm) to excite the probe, the emission spectra were recorded from 400 to 600 nm for (CsH17CGHrO(EtO)loHsurfactant solutions above and below the critical micelle concentration (CMC)of the surfactant. The data were plotted in various ways t o evaluate the probes ability to estimate CMC. The method which gave the most reliable and reproducible results for the surfactants was used in studying the formation of hydrophobic domains of the HEURs. Aqueous probe solutions of surfactants or thickeners were prepared according t o the following procedure. The degassed surfactant or thickener solution was weighed into a 20-mL vial and diluted to a 2 g total weight with argon purged distilled water. This solution was diluted to 10gwith the degassed probe solution, so that the concentration of probe was kept constant. Fluorescence spectra were recorded within an hour after preparing these solutions. Rheological Measurements. Intrinsic viscosities were determined at 25.3 & 0.2 "C in DDI water and DMF with an Ubbelholde capillary viscometer. Higher concentration HEUR solutionswere measured with cone and plate viscometersat shear rates between 2 and 40 s-l afier a steady-state viscosity was achieved.
Results and Discussion
HEURs without Terminal Alkyl Groups. Step growth polymerizations lead to broad distributions in
L
i
IS.0
1
I
30.0
320
I i90
1
I
36.0
38.0
-0 0
Elution Volume, ml Figure 2. SEC chromatographs of HO-[(EtO)ls~-TMXDI)zO(Et0)lazH thickeners (from top to bottom): x = 1;x = 2; x = 3; HO(Et0)lszH.
molecular weights.16,16 In the polymerizations described in this contribution, small quantities of the diisocyanate are used relative to the POE (1-2 g of diisocyanate to 50-70 g of POE) t o obtain a given stoichiometric ratio. Small fluctuations in measuring the amounts of starting materials may lead to a large difference in molecular weights; therefore, synthetic conditions must be established which allow reproducible molecular weights to be obtained. "he step growth nature of the polymerization leads to two types ofhydrophobicalkyl groups in the HEUR thickener, an internal alkyl group present within the poly(oxyethylene) backbone and external alkyl groups present at the termini of the polymer chain. In the first synthetic procedure, HEURs without terminal alkyl groups were synthesized by reacting HO(EtO)lszH with four diisocyanates, varying in size. As the mole ratio of diisocyanate t o HO(EtO)lazH is changed from 1:2to 2:3to 3:4,a progressive increase in the polydispersity index and molecular weights is observed. Chromatographs of one series of three thickeners with different mole ratios of TMXDI to HO(Et0)lSzH are illustrated in Figure 2. A peak with retention time close to 35.8 min, corresponding to the unreacted HO(EtO)ls2H, is observed in each chromatograph, but the amount of this material
Kaczmarski and Glass
3038 Langmuir, Vol. 10,No. 9, 1994 Table 2. Molecular Weights of HO(EtO),WDiisocyanate H E W with Terminal Hydroxyl Group General HEUR Structure: HO(EtO)~~~-[CONH-R-NHCOO-(Et0)18~-LH R= n X M,(theory) M, M, PDI HlzMDI 182 1 16542 20496 27507 1.34 Hl2MDI 2 24944 25178 34660 1.38 HlzMDI 3 33346 28585 40810 1.43 IPDI 182 1 16502 17393 24603 1.42 IPDI 2 24864 24465 38714 1.58 IPDI 3 33226 27826 44968 1.62 HDI 182 1 16448 18409 26515 1.44 HDI 2 24756 24172 37409 1.55 HDI 3 33064 31423 49791 1.58 TMXDI 182 1 16524 18748 25012 1.33 TMXDI 2 24908 23976 33132 1.38 TMXDI 3 33292 27107 38087 1.40
IPDI HDI
TMXDI HlzMDI
139 139 139 139
i 3
6 6 6 6
31150 35264 35090 33415
I
49520 56482 55795 53000
1.59 1.60 1.59 1.59
20
I O wt? T
3 d
ckener
Figure 4. Intensity of emission of ANS as a function of model ( 0 ) HOHEUR concentration, concentration of A N S these data over( E t 0 ) ~ s H(*) ; HO-[(EtO)l3g-HDI]6-O(EtO)l~gH; lap with the IPDI and TMXDI materials and are not given;
1
I
>
r
i
I
l
Figure 3. Intensity of emission of A N S as a function of C ~ H ~ & ~ H ~ O ( E ~concentration O)~OH (1 CMC = 0.019 wt %), concentration of A N S 2 x IO+.
01
l
C
I
rixa-----+
1 ! -
7n ."
Wt% Thickener
decreases as the ratio ofTMXD1 to HO(Et0)lszH is changed from 1:2 to 2:3 to 3:4. The molecular weights ofthe HEURs with the same mole ratios but different internal alkyl units are very similar (Table 2), although they are not in agreement with the target molecular weights as calculated by the reaction stoichiometry. The discrepancy can be attributed to two factors, the step-growth nature of the polymerization and the difference in structures between the HEURs and the polystyrene standards used in the calibration curve. To minimize the amount of unreacted poly(oxyethy1ene) present and to optimize the possible aggregation of internal alkyl groups, a 7:6 mole ratio of a poly(oxyethy1ene)(of a lower degree of polymerization, n = 139) to diisocyanate was used to obtain HEURs of 31000-35000 M , (Table 2). This lower POE molecular weight was used in this study to enhance hydrophobic differences that may arise from the internal groups with variations in the alkyl group size of the diisocyanate. The fluorescent probe 8-anilinonaphthalenesulfonicacid (ANS) is sensitive to the local environment. When the probe enters a region of different polarity, the wavelength a t which a maximum in emission intensity occurs will shift and the intensity of the emission also will increase with the formation of a hydrophobic domain.17-19 The (17)Ikemi, M.; Noboyoki, 0.;Shinohara, I.; Chiba, A. Macromolecules 1982, 15, 281.
Figure 5. Newtonian shear viscosity (Pa-s) as a function of HEUR concentration: (0)HO-[(EtO)139-HDI]6-0(Et0)13gH; (A) HO-[(Et0)~~g-IPDI]~-O(Et0)~~gH; (*) HO-[(E~O)I~~-TMXDIIGO(Et0)13gH;(0)HO-[(EtO)i3g-HizMDII6-O(EtO)i3gH.
fluorescence spectra of A N S is very broad and the wavelength a t which a maxima in fluorescence intensity occurs is not well-defined. To establish validity, the critical micelle concentration (CMC) of the surfactant, CaH1,CsH40(EtO)loH, is estimated using this technique. When the intensity is plotted as a function of C B H ~ ~ C ~ H ~ O (EtO)loH concentration (Figure 3), a very definitive transition is evident for solutions of the surfactant, approximate to its CMC. The intensity with the unmodified HO(Et0)663H does not change sharply with increasing concentration. Although minor differences are observed in the emission intensities of the HEURs with different internal alkyl groups (Figure 4), the sharp intensity increase (indicative of hydrophobic aggregation) is not observed with the HEURs containing only internal alkyl groups. These HEURs also do not exhibit viscosity increases within moderate concentration increases (Figure 5). The viscosity data correlate with fluorescence data, indicating t h a t hydrophobic aggregates are not formed. HEURs with Terminal Alkyl Groups. I t was envisaged that the most uniform approach to preparing HEURs
(18)Van Alstine, J. M.; Sharp, K. A,; Brooks, D. E. Colloids Surf. 1986, 17, 115.
(19)Boussouira, B.; Ricard, A. Polym. Bull. 1988,19, 193.
Langmuir, Vol. 10, No. 9, 1994 3039
Step Growth HEUR Thickeners Table 3. Molecular Weights of HO(EtO)ls&/ Diisocyanate HEURa with Terminal Hydroxyl Groups and Monoisocyanate Modification R= X M,(theo~) Mn Mw PDI 18748 25012 1.33 TMXDI 1 16 524 23976 33 132 1.38 TMXDI 2 24 908 27 107 38087 1.40 TMXDI 3 33 292 Monoisocyanate (Cp3H37)Modified HEURs 1 12724 17516 1.38 TMXDI TMXDI 2 19847 28366 1.43 TMXDI 3 16240 25017 1.54
of a given molecular weight would be through the preparation of a central batch of hydroxyl terminated material. This product would then be partitioned and reacted with monoisocyanates to produce uniform molecular weight HEURs with variable terminal hydrophobe sizes. In the TMXDI/HO(EtO)l~&lseries the molecular weights of the HEURs after monoisocyanate modification (Table 3) are noticeably lower than the hydroxyl-terminated precursors. The lower molecular weights indicate that degradation occurred either before or during monoisocyanate modification due to reversal of the equilibrium reaction for urethane formation during the azeotropic distillation step (-110-120 "C) and/or degradation of the oxyethylene chain by dissolved oxygen. Despite expectations, the monoisocyanate modification did not give reliable, reproducible syntheses, and the products of this synthetic procedure were discarded. HEURs with terminal hydrophobe also were prepared directly by reversing the stoichiometry to an excess of diisocyanate to poly(oxyethy1ene) (POE)in the step-growth polymerization. These intermediates were reacted with a n alkylamine without isolation of the intermediate. The potential disadvantage of this direct addition process is t h a t each thickener must be prepared independently and the molecular weights may vary slightly since no central batch is utilized. Fortunately, this was not observed. To obtain a range of molecular weights, the mole ratios were 3:2,5:4, and 7:6 ofHDI to POE. The terminal isocyanates were reacted with amines varying in alkyl sizes from hexyl to octadecyl. The influence of the diisocyanate coupler was examined by changing the diisocyanate from HDI to HI2MDI,while keeping the mole ratio of diisocyanate to HO(Et0)200H constant a t 3:2. In thickeners prepared by this direct addition method, a small quantity of a white precipitate forms upon addition ofthe hydrophobic amine to the reaction. This precipitant was marginally soluble in tetrahydrofuran. This also occurs when the isolated HEURs are dissolved in water. The cause of the impurity is the reaction ofthe hydrophobic amine with unreacted, monomeric diisocyanate. Unreacted diisocyanate is present in the reaction mixture as a consequence of the stoichiometry used in the step-growth polymerization. Independent synthesis of the model impurities and FT-IR analysis established that the impurity was the NJV'-dialkylurea. The impurities were removed by dissolving the HEUR in warm acetone, DMF, ethyl acetate, or absolute ethanol, gravity filtering while warm, removing part ofthe solvent under reduced pressure (Rotovap), and precipitating into petroleum ether. The products of these independent syntheses have similar SEC molecular weights (Table 4). When the mole ratio of HDI t o HO(Et0)zooH is changed from 3:2 to 7:6, progressive increases in the molecular weights are observed, and the direct addition of the alkylamines did not significantly alter HEUR molecular weights (Figure 6). For materials with a constant mole ratio of HDI to HO(Et0)zooH (5/4) but different external alkyl groups, the
Table 4. SEC Molecular Weights of Direct Addition HEURa Mn MW PDI R-NHCONH-[H12MDI-NHCOO-(E ~O)ZOO-CONH-IZH12MDI-NHCONH-R 25648 36846 1.43 R = C6H13 24198 34564 1.43 C8H17 24138 34311 1.42 C12H23 23007 31926 1.38 C14H29 R-NHCONH-[HDI-NHCOO-(E~O)~OO-CONH-I~HDI-NHCONH-R 26499 37914 1.43 R= 52592 1.49 C8H17 run la 35377 run 2a 29668 41 739 1.41 33730 49294 1.46 C12H23 run la run 2a 28429 39817 1.40 25874 36318 1.40 c14H29 29249 41 750 1.43 C18H37
R-NHCONH-[HDI-NHCOO-(E~O)~O~-CONH-I~HDI-NHCONH-R R = C6H13 C8H17
C12H23 C14H29 C18H37
29588 31 188 32286 32450 35365
45744 47753 49804 49358 54 166
1.55 1.53 1.54 1.52 1.54
R-NHCONH-[HDI-NHCOO-(EtO)~~-CONH-16HDI-NHCONH-R R = CsH13 C8H17 C12H23
41 755 40 105 38273 39812
63396 61403 57736 59902
1.52 1.53 1.51 1.50
c14H29 T w o synthetic runs were made of each HEUR. Run 1 was performed according to the procedures described in the Experimental Section except that the diisocyanate was not diluted with toluene. Run 2 was performed diluting the diisocyanate with toluene. By not diluting the diisocyanate with toluene, a lower quantity of diisocyanate was added to the reaction, causing a lowering of the diisocyanaWOE stoichiometric ratio and an increased molecular weight for the HEUR. The HEURs from run 2 only were used in all subsequent studies. a
SEC chromatographs are similar in shape (Figure 7) and number and weight average molecular weights (Table 4).
A. Influence of Effective Terminal Hydrophobe Size. When large alkyl groups are positioned at the termini of the ethoxylated urethanes (e.g., C12H25-NH-
CONH-[HDI-NHC00-(Et0)~~~CONH-l~-HDI-NHCONHC12H25) a sharp upturn in fluorescence intensity is observed (Figure 8) with increasing concentration, indicating that the external alkyl groups are forming hydrophobic aggregates. The upturn occurs a t lower concentrations in the R-NHCONH-[HDI-NHCOO-(E~O)~OOCONH-12-HDI-NHCONH-Rseries with a n increase in the size of the alkylamine (i.e., R is varied from C6H13- to C14H29-). Similar results are observed with the R-NH-
CONH-[H~~MDI-NHC00-(EtO)~~-CONH-l~-H~~MDICONH-R series (Figure 9); the alkyl amine size is varied from C6H13- to C14H29-. Intrinsic viscosities on the products of these two series were conducted in NJV-dimethylformamide (DMF), a solvent that disrupts micelle formation. The data (Table 5 ) demonstrate the reproducibility of the synthetic procedure. The intrinsic viscosities in DMF increase as the molecular weights are increased, but within a given molecular weight series, increases in intrinsic viscosity are not observed as the size of the external hydrophobe is varied. Intrinsic viscosities are typically measured a t dilute concentrations where polymer-polymer interactions can be neglected.20 If the internal and/or terminal alkyl groups associate in aqueous media at very low ~
~~
(20) Billmeyer, F. W., Jr. Textbook of Polymer Science; John Wiley and Sons: New York, 1971; pp 84-90.
Kaczmarski and Glass
3040 Langmuir, Vol. 10, No. 9, 1994
28.0
30.0
32.0
34.0
36.0
38.0
40.0
Elution Volume, ml Figure 6. SEC chromatographs of CsHl3-[HDI-(EtO)zoo-k-HDICsH13 thickeners (from top t o bottom) a, X = 2; b, X = 4;c, X = 6.
I
I
I
280
300
320
I 340
I
1
I
36 0
380
400
Elution Volume, ml
Figure 7. SEC Chromatographs of R-[HDI-(E~O)~OO]~-HDI-R thickeners (from top to bottom) R = hexyl, octyl, dodecyl, tetradecyl, and octadecyl.
concentrations, polymer-polymer interactions through hydrophobic aggregation negate the dilute concentration conditions. Fluorescence measurements (Figures 8 and - _ _ :Am3 I 9) indicate that such associations occur in water a t very I low concentrations for HEURs with terminal alkyl groups. I I In view of these observations, specific viscosities at concentrations of 1.0 g/dL were measured in water rather than intrinsic viscosities and are compared with specific viscosities in DMF. A n increase in the specific viscosity is observed in aqueous media relative to the organic media.21 In the R-NHCONH-[HDI-NHCOO-(EtO)zooCONH-I2-HDI-NHCONH-Rseries (Table 5), the HEURs with terminal alkyl group R = C6H13 and R = C ~ H (12 I~ and 14 carbons, respectively, if the alkyl group of the HDI is considered) do not build viscosity a t the concentrations studied. As the terminal alkyl groups, R, are increased to C12H25-, &H29-, andCI8HZ7- in this series, adeviation I n from unity is observed. ‘3.09’ il 3 0 2.’CO ;,cd The low shear rate viscosity data as a function of HEUR Log W t i . +EL? concentration for the R-NHCONH-[HDI-NHCOO-(EtO)2~-Figure 8. Intensity of emission ofANS as a function ofR-[HDICONH-12-HDI-NHCONH-Rand the H & D I series paral(EtO)2oo-]2-HDI-Rconcentration: 0,C&3; A, CsH17; 0 ,C12H23; lels the specific viscosity data. When the interconnecting *, C14H29. unit is changed from HDI to H12MDI and the alkylamine alkyl size. The C14H29-NHCONH-[H12MDI-NHCOOis kept constant, the influence of the diisocyanate on the (E~O)~O~-CONH-]~-H~~MDI-NHCONH-C~~H~~ thickener is hydrophobicity ofthe external alkyl unit is evident (Figure the only discrepancy noted in these comparisons. The 10 and 11). The importance of the “effective” size of the effect of incomplete hydrophobe modification on the external alkyl group (defined as the combination of the resulting rheological behavior has been described previlong chain alkylamine and the alkyl group of the diisoously.22 The data in total suggest t h a t the C14H29cyanate used to couple the amine to the poly(oxyethy1ene)) NHCONH-[H~zMDI-NHC00-(EtO)~oo-CONH-l~-H~~MDI is evident in comparison of the viscosity and fluorescence N H C O N H - C ~ ~ Hthickener Z~ may have a lower level of data (Figures 8 and 9) among the two series. The hydrophobe. The lH NMR technique to evaluate extent aggregation phenomenon occurs at lower concentrations of hydrophobe modification cannot be used in these with HEURs containing the larger external “effective” systems because the internal alkyl groups have similar structures to the external alkyl groups. (21)Sau, A. C.; Landoll, L. M. Polymers in Aqueous Media: Per,
formance Through Association;Glass, J. E., Ed.; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989; p 343.
~
(22) Kaczmarski, J. P.; Glass, J. E. Macromolecules 1993,26, 5149.
Step Growth HEUR Thickeners
Langmuir, Vol. 10, No. 9, 1994 3041 -~
__..
- ._ -~
....
II
f
c )IC
C GI3
-05
N
% *E
0. [I 0
0 51-
?
Figure 9. Intensity of emission ofANS as a function of R-[H12MDI-(E~O)~OO]~-HIZMDI-R concentration: 0, C6H13; A, C8H17; 0 , C12H23.
10
:fJ
Wt% Thickener
Figure 10. Newtonian viscosity (Pa-s)as a function of RNH[HDI-(E~O)~OO~Z-HDI-NHR concentration: 0, C6H13; A, C8H17; 0 , C12H23; *, C14H29.
Table 5. Reduced and Intrinsic Viscosities of Direct Addition Thickeners %plC
thickener
DMF
v.plC ratio
H~O ~ F_ ,~_ D M H,O/DMF ~
~~
~
~~
R-NHCONH-[H~~MDI-NHCOO-(E~~)~O~-CONH-I~H12MDI-NHCONH-R 0.40 1.01 0.47 0.47 R = C6H13 0.64 0.40 1.42 CsHi0.45 0.40 3.30 0.46 1.50 C12H23 0.36 2.60 0.42 1.10 CliH29 R-NHCONH-[HDI-NHCOO-(EtOh00-CONH-12HDI-NHCONH-R 0.51 0.40 1.00 R = C6H13 0.52 0.69 0.70 0.52 C8H17 run 1 1.03 0.51 r u n 2 0.51 0.45 0.99 1.15 0.53 1.86 C12H23 run 1 0.62 0.85 0.40 1.64 r u n 2 0.52 0.41 3.06 0.54 1.67 C14H29 0.46b 3.53 C18H37 0.4g4 1.74" R-NHCONH-[HDI-NHCOO-( Et0)~oo-CONH-lcHDI-NHCONH-R 0.64 0.53 1.03 0.66 R= 0.64 0.51 1.02 C8H17 0.62 0.80 0.52 1.21 C12H23 0.65 1.75 0.53 2.70 c14H29 0.65 15.90 0.55 22.26 C18H37 0.70 0.62" 1.124 1.79
.-& s o
x
5
I
1 'J
20
Wt% Thickener
Figure 11. Viscosity (Pes at a shear rate of 20 s-l) as a function of RNH-[H~~MDI-(E~O)~O~-I~-H~~MDI-NHR concentration: 0, C6H13; A, C8H17;0 ,C12H23; *, C14H29. C&3 and C8H17 solutions were Newtonian.
R-NHCONH-[HDI-NHCOO-(E~O)~O~-CONH-I~HDI-NHCONH-R R = C6H13 CSH17 C12H23 C14H29 a
splc at 0.5 gldL.
0.80 0.78 0.72 0.74
0.82 0.80 0.80 1.86
0.67 0.62 0.58 0.58
1.01 1.03 1.11 2.51
[VI at 0.5 gldL and lower concentrations.
B. Molecular Weight Influence. A molecular weight influence also is observed in the specific viscosity data in water. As the stoichiometric ratio of HDI to POE is changed from 3:2 to 5 4 to 7:6 (Table V), the specific I1 IO ?I 1 viscosity ratio decreases toward unity if the terminal alkyl Wt% Thickener size is constant. The low shear rate viscosities parallel Figure 12. Newtonian viscosity (Pass) as a function of RNHthe specific viscosity data. With increasing HEUR [HDI-(E~O)~O~-I~-HDI-NHR concentration: 0, C6H13; A, C8H17; molecular weight, the concentration of the external alkyl 0,C12H23; *, C14H29. groups decreases, the extent of association is lower, and CONH-L-HDI-NHCONH-R and R = CcH13 and higher concentrations of the thickener are required before increasing the molecular weight imparts higher viscosities. the aggregation phenomenon and viscosity build occur The increases in viscosity a t very high concentrations for (Figures 8, 10,12, and 13). these thickeners are due to the overlap of the polyIn these well-defined HEURs, the higher hydrophobe (oxyethylene)chains and not to hydrophobic aggregation. content in the lower molecular weight HEURs in constant In our previous studies fully modified HEURs with weight precent studies effects a greater formation of CleH3+7HC00 hydrophobes and narrow molecular weight hydrophobic aggregates. In the series of HEURs with distributionszzexhibited phase separated solutions in the the general structure R-NHCONH-[HDI-NHCOO-(EtO)~~-
3042 Langmuir, Vol. 10, No. 9,1994
/
VJ
m
LL
I
Kaczmarski and Glass be expected to exhibit phase separation. The differences observed arise from differences in molecular weight distribution. Our studies in this area will be reported in the near future.
Conclusions
The reproducible syntheses of HEURs, prepared by step growth polymerizations, can be accomplished but the > products are a mixture of components, varying in molecular weight and hydrophobe content. Degradation of HEURs was observed in one synthetic procedure; the synthetic procedure must be chosen with stability of the products as a final concern. Fluorescence (using an extrinsic probe) and viscosity measurements reveal that 0 internal alkyl groups (the alkyl group of the diisocyanate) do not build viscosity through hydrophobic aggregation. Wt% Thickener A dramatic increase in viscosity as a function of thickener Figure 13. Newtonian viscosity as a function of R-NH-[HDIconcentration is observed with increasing effective terNHCOO-(EtO)zoo]6-HDI-NHR concentration: 0, C6H13; A, minal hydrophobe size, defined as the combination of the CBH17; 0 , C12H23; *, C14H29. long chain alkylamine and the alkyl group of the diisocyanate used to couple the amine to the poly(oxyethy1ene). absence of surfactant. None of the HEURs in this study Increasing the molecular weights of the HEUR while exhibited phase separated solutions, even though the HEUR, CIsH3,-NHCONH-[HDI-NHCOO-(EtO)zoo-CONH- keeping the terminal hydrophobe size constant effects lower viscosity efficiency in well-defined HEUR polymers. IB-HDI-NHCONH-C~~H~~, possesses a significantly greater effective terminal hydrophobe size. The extent of hydroAcknowledgment. The financial support of these phobe modification of the step growth HEURs presented studies by the James River Co. and the E.I. DuPont de in this study cannot be measured. Based on an analysis Nemours and Co. is gratefully acknowledged. of the size of the external alkyl groups, this HEUR would .-t g [I M ._
