Synthesis and rheological properties of hydrophobically modified poly(vinyl alcohol)

ORIGINAL PAPER

Synthesis and rheological properties of hydrophobicallymodified poly(vinyl alcohol)

Qiaoman Hu & Guangsu Huang & Jing Zheng &

Heng Su & Chao Guo

Received: 27 March 2012 /Accepted: 12 October 2012 /Published online: 25 November 2012# Springer Science+Business Media Dordrecht 2012

Abstract A novel series of water-soluble hydrophobi-cally modified poly(vinyl alcohol) (HMPVA) with var-ious hydrophobe contents was prepared by graftingpoly(vinyl alcohol) (PVA) using 1-dodecanol andtoluene-2,4-diisocyanate as hydrophobic monomer andcoupling agent, respectively. The chemical structure ofHMPVA was analyzed by Fourier Transform InfraredSpectrometer (FTIR) and 1H NMR. Rheological prop-erties of the aqueous solutions also confirmed the in-corporation of hydrophobic groups into PVA. In diluteconcentration regime, HMPVAs exhibited lower intrin-sic viscosity than PVA, suggesting that HMPVA mole-cules were more shrunken. While the aqueous solutionviscosity was enhanced due to hydrophobic modifica-tion at a high concentration, and HMPVAs with higherhydrophobe contents exhibited lager values of apparentviscosities. Over a frequency range of 1 to102 rad/s,the dynamic storage modulus of PVA solution wassmaller than the dynamic loss modulus whereas thedynamic storage modulus of HMPVAs solutions wasgreater than the dynamic loss modulus, indicating theevolution of viscoelastic solid properties in HMPVAssolutions. The yield stress of PVA was nearly zerowhereas that of HMPVAs represented positive values,implying that networks were present in HMPVAssolutions.

Keywords Hydrophobically modified poly(vinyl alcohol) .

Rheological properties . Viscoelastic solid properties .

Network

Introduction

Hydrophobically modified water-soluble polymers haveattracted appreciable interests in recent years primarily fromapplications in enhanced oil recovery, which mainly focuson polymers with pendant hydrophobes distributed alongthe backbone [1]. PVA, due to its unique nonionic backboneand hydroxyl functional group for subsequent reaction, isconsidered as an excellent starting material for hydrophobicmodification [2].

Generally, the synthesis method of HMPVA includesesterification, etherification and transesterification.Yahya et al. [3] grafted PVA with acid chlorides of longchain fatty acids to form ester linkages. Marstokk et al.[4] followed Williamson’s ether synthesis method, andincorporated an alkyhalide and propanesultone into thepolymer backbone to yield HMPVA with anionic func-tionality. Shedge et al. [5] reported the hydrophobicmodification of PVA using methyl 3,4,5-tris(n-octyloxy)benzoate and methyl 3,4,5-tris(n-dodecyloxy) benzoateby transesterification reaction. All HMPVAs exhibitedsuperior viscosification performance due to the forma-tion of a transient physic network which is induced bythe strong intermolecular hydrophobic interaction be-tween the hydrophobic groups in the structure [6, 7].The microstructure of this interaction was demonstratedprimarily using environment scan electronic microscope,fluorescence spectrum and confocal laser scanning mi-croscopy [5, 8].

Q. Hu :G. Huang (*) : J. Zheng (*) :H. Su : C. GuoCollege of Polymer Science and Engineering, State KeyLaboratory of Polymer Materials Engineering, Sichuan University,Chengdu 610065, Chinae-mail: [email protected]: [email protected]

J Polym Res (2012) 19:6DOI 10.1007/s10965-012-0006-3

However, the chemical structure of HMPVA formed byesterification and transesterification is unstable because ofthe hydrolysis property of ester bond. In addition, the pro-cess of preparing HMPVA by Williamson ether synthesis isextremely complex though ether bond is much more stable.Is there a way that can easily prepare stable HMPVA?Meanwhile, the process of preparing samples for themicro-structural characterization mentioned above is alsocomplicated. Is there another method to study the networkconstruction?

In this paper, a novel HMPVA was prepared by a poly-addition reaction through grafting PVA. 1-dodecanol andtoluene-2,4-diisocyanate were used as hydrophobic mono-mer and coupling agent, respectively. To improve the hy-drolysis resistance and salt tolerance, we choose theurethane bond as hydrolyzed stable structure and long-chain fatty alcohol as nonionic group. We make simpleassumption regarding the deal reaction as follow, onlyalkylate-mono-isocyanate was yield as shown in Scheme 1,and the side reaction as shown in Scheme 2 was consideredto be very little. Because the para and ortho NCO groups inthe toluene-2,4-diisocyanate exhibited significantly differentreactivity due to the electronic effect of methyl group.Reactivity of the NCO group in para position was found tobe 10 times higher than that for the NCO group located inortho position at room temperature [9, 10]. More impor-tantly, the reactivity of second NCO group within toluene-2,4-diisocyanate is lower 2–5 times after the first group isconverted because the NCO group is a stronger electronacceptor than the -NH-COO- group [11, 12]. To investigatethe hydrophobic association in more simple and directmethod, the influence of hydrophobic modification onthe rheological behavior of HMPVA solution was also studied.

Furthermore, the network construction of polymolecularentanglements was verified in theoretically by testing themacroscopic rheological behaviors of HMPVAs solutions.

Experimental

Materials

The degree of saponification of PVA (Hongxinyuan Chem-ical Factory, China) is 99 % and the number-average degreeof polymerization (Pn) is 2,600, and it was washed withdeionized water and dried in vacuum at 70 °C before use. 1-Dodecanol was purchased from Bodi Chemical Factory(Tianjin, China) and used after removing water by molecularsieve. Dimethyl sulfoxide (DMSO) was purchased fromBodi Chemical Factory (Tianjin, China) and used afterremoving water by reduced pressure distillation after a re-action with calcium hydride. Toluene-2,4-diisocyanate waspurchased from Baiyinyinguang Chemical Factory(Gansu,China) and used as received. Ethanol was purchased fromJingxi Chemical Factory (Shanghai, China) and used asreceived.

Preparation of Alkylate-mono-isocyanate

Alkylate-mono-isocyanate was prepared from the reactionof toluene-2,4-diisocyanate and the equal molar of 1-dodecanol. Toluene-2,4-diisocyanate was added to a three-necked flask and 1-dodecanol was added dropwisely intothe flask at 40 °C. The reaction process was followed by thecontent determination of isocyanate group through dibutyl-amine back titration [13].

Scheme 1 Preparation of alkylate-mono-isocyanate

Scheme 2 The side reaction of preparation of alkylate-mono-isocyanate

Page 2 of 9 J Polym Res (2012) 19:6

Preparation of HMPVAs

The hydrophobic modification step was conducted by dis-solving 4 g of the PVA in 40 mL of DMSO at 80 °C. After ahomogeneous solution was obtained, the temperature wascooled down to 50 °C and various ratios of alkylate-mono-isocyanate in DMSO were added dropwisely to the reactionmixture with vigorous stirring. After 2 h of reaction time,the reaction mixture was cooled and the resulting polymerwas precipitated into ethanol, purified three times in excessethanol, and then dried at 70 °C under vacuum. The reactionscheme and structure of the HMPVA were given inScheme 3. The reaction stoichiometry and the solubility ofthe polymers were given in Table 1. The grafting ratio wascalculated by means of 1H NMR by following the appear-ance of the benzene ring in reference to the methenyl at-tached to the hydroxyl involved in the addition reaction, asseen in Scheme 3. Grafting ratio was calculated by theequation

grafting ratio %ð Þ ¼ i= i þ bð Þ½ � � 100

where i and b were the peak area of the Hi and Hb, respec-tively, as shown in Fig. 4.

Characterization

Intrinsic viscosity ([η]) was measured at 25 °C by Ubbe-lohde viscometer at a concentration of 0.5 g/dL. FTIRanalysis of the polymer composition was conducted with aNicolet-560 infrared spectrometer (USA). 1H NMR spectrawere recorded on a spectrometer (Varian INOVA-400) at25 °C using DMSO-d6 as the solvent and tetramethylsilane(TMS) as the internal standard.

Preparation of PVA s and HMPVAs solution

PVA and HMPVAs were hard to dissolve in water atroom temperature, so the polymer solutions of desiredconcentrations were prepared by dissolving a knownamount of polymer in deionized water with gentle stir-ring at 98 °C,and were kept for 2 h to ensurehomogenization.

Rheological measurements

The rheological properties of the polymer solutions weremeasured by an AR 2000 rheometer(TA Instruments,American) at 25 °C. Cone and plate was adopted whosediameter and degree were 40 mm and 1°. Measurementtype were viscosity-shear rate test over the shear raterange 10−1–102 s−1 and dynamic frequency sweep testover the frequency range 1–102rad/s.

Results and discussion

1H NMR spectra of alkylate-mono-isocyanate

As shown in Fig. 1, in the case of 1-dodecanol, thecharacteristic peaks at 4.30 and 3.38 ppm, arising fromhydroxyl group and methylene group attached to it,respectively, confirmed the presence of 1-dodecanol.

Scheme 3 Preparation of HMPVA

Table 1 The solubility of various HMPVAs (2 g/dL polymer insolution)

Sample code -NCO/-OH(mol)

Solubilityin DMSO

Solubilityin water

5 % HMPVA 5:95 Soluble Insoluble



1.5 % HMPVA 1.5:98.5 Soluble Insoluble

1 % HMPVA 1:99 Soluble Soluble

0.75 % HMPVA 0.75:99.25 Soluble Soluble



J Polym Res (2012) 19:6 Page 3 of 9

Due to urethane reaction between 1-dodecanol and toluene-2,4-diisocyanate, the 1H NMR spectra exhibited several newpeaks, such as the methyl of toluene-2,4-diisocyanate at2.20 ppm, the benzene ring region at 7.2–7.5 ppm, the NHprotons at 8.5–9.6 ppm and the methylene attached to theurethane groups at 4.05 ppm. The signal of terminal hydroxylgroups of 1-dodecanol were completely disappeared, indicat-ing that isocyanation of all the terminals of 1-dodecanol wasalmost consumed. The increased area of the NH protons peaksand the chemical of urethane were shown in the Figs. 1 and 2,respectively. Seen from Fig. 2, the formation of (C) and (D)

was attributed to toluene-2,6-diisocyanate mixed into thetoluene-2,4-diisocyanate participating in the addition reaction[10, 11, 14]. In any case, the peak area of NH proton at9.63 ppm was dominated compared with other NH proton,consistent with our assumption as discussed before.

FTIR spectra of HMPVA

As presented in Table 1, only when the hydrophobecontent was less than 1 mol%, the samples were solublein water. An excess percentage of the hydrophobic side

Fig. 1 1H NMR spectra of 1-dodecanol and alkylate-mono-isocyanate

Fig. 2 Chemical structures ofurethane corresponding to 1HNMR of alkylate-mono-isocyanate


chains gave rise to a decrease in solubility in water. Asimilar conclusion is also mentioned by Shaikh et al. [7]and Lin Ye et al. [8]. All of the subsequent detailedstudies were conducted using following four samples,0.25 % HMPVA, 0.5 % HMPVA, 0.75 % HMPVA, 1 %HMPVA.

Figure 3 showed the FTIR of PVA and HMPVA.Compared to PVA, the absorption peaks at 1,601 cm−1

and 1,540 cm−1 were present in HMPVA; the absorptionpeaks at 1,000–1,200 cm−1, 1,430 cm−1, 1,718 cm−1 and2,900 cm−1 were considerably enhanced in HMPVA. Thepeaks at 1,601 cm−1 and 1,540 cm−1 that represented thestretching vibration of benzene ring and bending vibra-tion of aromatic secondary amine were attributed to theincorporation of toluene-2,4-diisocyanate into PVA

molecules. The peaks at 1,000–1,200 cm−1 and1,718 cm−1 that represented stretching vibration of C-Oand carbonyl were attributed to the incorporation ofcarbamate group into PVA molecules. The peaks at1,430 cm−1 and 2,900 cm−1 that represented the bendingvibration and stretching vibration of C-H were attributedto the incorporation of alkanes group into PVA mole-cules. From the above description, it can be confirmedthat the 1-dodecanol was introduced to the PVA mole-cules using toluene-2, 4-diisocyanate as coupling agent.

1H NMR spectra of HMPVA

The labels of the protons of HMPVA were shown inScheme 3. As depicted in Fig. 4, upon comparison of

Fig. 3 FTIR of PVA and HMPVA

Fig. 4 1H NMR spectra of PVA and 1 % HMPVA

Fig. 5 Plot of intrinsic viscosity against hydrophobe contents at 25 °C


spectra of PVA and HMPVA, it can be readily seen thatHMPVA showed the distinct peaks of the 1-dodecanoland toluene-2,4-diisocyanate. The terminal methyl pro-tons of the hydrophobic side chain of 1-dodecanolappeared at 0.86 ppm. The methyl protons of toluene-2,4-diisocyanate appeared at 2.15 ppm. The aromaticprotons of toluene-2,4-diisocyanate appeared at 7.0–7.5 ppm (Hi, Hj, Hh). The amide NH protons oftoluene-2,4-diisocyanate appeared at 8.65 ppm (Hg)and 9.2 ppm (Hk) [14]. The enlargement of the peakscoming from alkylate-mono-isocyanate was shown inFig. 4. This clearly indicated the formation of the graftcopolymers PVA-graft-dodecanol using toluene-2,4-dii-socyanate as a coupling agent.

The grafting ratio was calculated by taking 1 % HMPVAas example because the hydrophobe content of 0.25 %HMPVA/0.5 % HMPVA/0.75 % HMPVA was too few tocalculate the grafting ration through 1H NMR. As exempli-fied in Fig. 4, the integration of Hi was 1.00 when theintegration of Hb was 105.34. These values confirm thatthe grafting ratio of the 1 % HMPVA was 0.94 % and thegrafting efficiency was 94 %.

Intrinsic viscosity measurements

To estimate the state of individual molecule of PVA andHMPVA in aqueous solution, respectively, intrinsic vis-cosity ([η]) at 25 °C at a concentration of 0.5 g/dL wasmeasured and presented in Fig. 5. It can be seen thatPVA gave greater value of [η] than HMPVAs andHMPVAs with higher hydrophobe contents exhibitedlower values of [η], indicating that the chain ofHMPVAs in water solution were more shrunken, com-pared with the PVA. And the extent of shrinkage wasgreater with increasing the hydrophobe contents. Asexpected, the hydrophobes induce an intramolecular in-teraction owing to the repulsion between non-polar moi-eties and polarity media to decrease their direct contactwith water [15–17]. The decrease of [η] with increasinghydrophobe contents was attributed to the fact that theintramolecular association is stronger and stronger with

increasing hydrophobe contents [18]. As depicted in theScheme 4, the hydrophobic groups from the same mol-ecule tended to associate in aqueous solutions to mini-mize exposure to the hydrophilic medium, resulting inthe contraction of the molecular chain.

Steady-shear measurements

To investigate intermolecular interaction in PVA andHMPVAs solutions, the high concentrations such as2 g/dL, 5 g/dL and 8 g/dL had been chosen. Figure 6depicted apparent viscosity curve of PVA and HMPVAssolutions with shear rate at different concentration. Allsystems exhibited that the HMPVAs showed muchhigher viscosities, compared with PVA over the wholeshear rate range. Furthermore, HMPVAs with higherhydrophobe contents had higher apparent viscosities.At high polymer concentrations, some micro-scale phaseseparation may take place through the formation ofhydrophobic domain due to the repulsion mentionedabove, and thus the HMPVAs solutions are more het-erogeneity, compared with the PVA solution. Mean-while, a transient physic network could also take placebecause the hydrophobic domain acted as cross-linkersbetween the polymeric backbone, which resulted in anincrease in hydrodynamic volume and, consequently,caused an increase in the apparent viscosity ofHMPVAs. Scheme 5 schematically represents the forma-tion of network in HMPVA solution. Nevertheless, thenetwork was stronger with increasing hydrophobe con-tents, and thus the HMPVAs with higher hydrophobecontents had greater values of apparent viscosity. Allspecimens exhibited shear thinning behavior, on theone hand, which could be attributed to the disentangle-ment of the polymolecular chains under shear. A similarphenomenon and interpretation were also mentioned byYahya et al. [3]. On the other hand, what’s more im-portant was that the progressive rupture of the physicnetwork established by intermolecular associations uponincreasing the shear rate [19–21]. Additionally, Com-pared with that of 5 g/dL and 8 g/dL, the viscosity of

Scheme 4 The model ofintramolecular interaction.


HMPVAs solution at 2 g/dL decreased more sharplywith shear rate, which could be explained that thenetwork established at 2 g/dL was so unstable that tobe easily destructed and deformed by shear.

Oscillatory measurements

Linear viscoelastic behavior of the polymer solutionmay provide an important clue to elucidating the vis-cosity differences of PVA and HMPVAs solutions be-cause the relative value between dynamic storagemodulus (G′) and dynamic loss modulus (G″) is aquantitative measure of solid-like elastic body orliquid-like viscous fluid of a system. In principle, fluidcharacter is dominant when the loss tangent value isgreater than 1 and solid character is dominant when thevalue is smaller than 1 [22]. Among three concentra-tions above, the concentration of 8 g/dL was chosen tomake the differences among PVA and HMPVAs solu-tions more distinct. Figure 7 showed the variation of G′and G″ of PVA and HMPVAs solutions with frequency(ω) at 8 g/dL. It is worth mentioning in the figure thatHMPVAs solution with higher hydrophobe contents hadlarger values of G′ and G″, and all the values of G′ andG″ were greater than those of PVA solution. In PVAsolution, the G′ was smaller than G″ over the entirefrequency examined, indicating that the solution wasviscoelastic fluid properties. In the case of HMPVAssolutions, the G′ was larger than G″, implying that thesesolutions were viscoelastic solid properties. The networkconstruction would be accounted for the HMPVAs ofhigher values of G′ and G″ and of viscoelastic solidproperties.

Upon the network existing in the HMPVA heterogeneitysystem, it should exhibit the yield stress because the de-struction of the network needed some energy. Bingham flowbehavior gave rise to a nonzero yield stress (τ0), whichrepresents the minimum energy required to break networksuch as gels and microdomains [23]. τ0 of heterogeneoussystems can be determined from the Casson plot, the plot ofthe square root of shear stress (τ) against square root of shearrate (γ) in steady shear flow. K is constant. τ0 is obtainedfrom [24].

t1=2 ¼ t01=2 þ Kg1=2

Although this equation holds strictly for Newtonian sus-pensions only, it is also considered valid for non-Newtoniansystems at low shear rates. Similarly, the intercept of the G″axis (G″0) on the plot of the square root of G″ against the

Fig. 6 Variations of apparent viscosity with shear rate for PVA andHMPVAs at 25 °C at different concentrations: a at 2 g/dL, b at 5 g/dLand c at 8 g/dL

�


square root of frequency (ω) may be considered as a mea-sure of yield stress in dynamic shear measurement as [25].

G001=2 ¼ G0001=2 þ K 0w1=2

Figure 8 presented plot of the square root of G″s ofPVA and HMPVAs solutions with the square root of ω.In the case of PVA solution, the intercept nearly re-duced to zero in the so-called Casson-type plot, show-ing that the PVA formed little network in the solution.However, all HMPVAs solutions represented the nonze-ro intercepts on the Casson-type plot, indicating thatnetworks existed in the systems. The intercept valuesincreased with the increasing in hydrophobe contents,implying that HMPVAs of higher hydrophobe contentshad stronger networks development. As mentionedabove, the higher hydrophobe contents resulted in morestable networks, which needed greater energy to breakthem.

The presence of network by the physical junctionsmay affect the relaxation behavior as well. If there issome molecular order or a mesophase, much longerrelaxation time is expected [26]. For the polymeric

systems in which some pseudostructures are involved,the relaxation time (λ) under dynamic shear may becalculated as [22, 27].

J 0 ¼ G0

η�½ �wð Þ2 ¼ lη�½ �

Where J′ and η* are the compliance and complex viscos-ity, respectively. As reported by Song Ie Song et al. [22], inthe case of relaxation times, the effects of hydrogels ruledover the individual molecular motions. Thus, the relaxationtimes were used to characterize the relaxation process ofnetwork. As presented in Fig. 9, HMPVAs solutions gavelonger λ than PVA solution, and similar speculation of thepresence of network was responsible for this. The order ofmagnitude of λ and the frequencies that were used to extractλ were both close to the previous research by Won SeokLyoo et al. [28, 29] and Song Ie Song et al. [22]. The λ ofHMPVA solution was slightly increased with increasinghydrophobe contents, which could be considered that thespecimens with higher hydrophobe content had a morestable structure.

Scheme 5 The dissolutionprocess of HMPVA solutiona the initial stage of dissolution;b the formation of hydrophobicdomain; c the formation ofnetwork

Fig. 7 Storage and loss modulus of 8 g/dL PVA and HMPVAs sol-utions with frequency at 25 °C

Fig. 8 Square root of loss modulus of 8 g/dL PVA and HMPVAssolutions with the square root of frequency at 25 °C


Conclusion

1-dodecanol and toluene-2,4-diisocyanate were used to syn-thesize HMPVAs. To keep good water solubility ofHMPVAs, the hydrophobe contents must not exceed1 mol%. Thus, HMPVAs with different hydrophobe con-tents (0.25, 0.5, 0.75, and 1 mol%) were prepared, and therheological properties were studied. In the dilute regime,HMPVAs gave lower intrinsic viscosity than PVA. At highpolymer concentrations, the η, G′ and G″ of HMPVAssolutions increased with increasing hydrophobe contents,greater than those of the PVA solution. Meanwhile, theHMPVAs solutions exhibited viscoelastic solid propertiessince the G′ was greater than G″ over the entire frequencyexamined, higher τ0 and longer λ. Some extents of micro-scale phase separation and physical network, arising fromthe repulsion between hydrophobic groups and polaritymedia, are responsible for these. Furthermore, the higherthe hydrophobe contents were, the stronger the networkdeveloped.

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Fig. 9 Relaxation time of 8 g/dL PVA and HMPVAs solutions at25 °C


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Synthesis and rheological properties of hydrophobically modified poly(vinyl alcohol)