Sundar Saimani,Kannan Tharanikkarasu,Ganga Radhakrishnan

Advanced Centre in Polymers,Central Leather Research Institute,Adyar,Chennai600020,India

Received18December2001;accepted5June2002

ABSTRACT:Polyurethane macroiniferter(PUMI)includ-ing tetraphenylethane was synthesized and used to prepare polyurethane–polyacrylic acid multiblock copolymers.Film-forming aqueous dispersions without any added external emulsifiers were prepared from polyurethane–polyacrylic acid multiblock copolymers.The effect of varying PUMI content,polymerization time,and percent ionization on the properties of multiblock copolymeric dispersions were stud-ied in detail.Interfacial tension of the dispersions and crit-ical surface tension measurements of thefilms formed thereof have shown that the polymers exhibit a hydrophilic character in the dispersed phase and a hydrophobic charac-ter in the solid phase.©2002Wiley Periodicals,Inc.J Appl Polym Sci87:1109-1115,2003

Key words:polyurethanes;aqueous dispersions;macro-iniferter;block copolymers;living radical polymerization

INTRODUCTION Polyurethanes(PUs)are of commercial interest due to their excellent properties such as high abrasion resis-tance,high chemical resistance,high strength,and low temperatureflexibility.Normally PUs are water in-compatible and are prepared using organic solvents. They can be dispersed,however,in water by incorpo-rating hydrophilic segments.1,2Due to the upcoming regulations in volatile organic emission coupled with high solvent price,these types of aqueous PU disper-sions are slowly replacing conventional solvent-based systems.Aqueous PU dispersions are of three types—viz.,nonionic,cationic,and anionic—depending upon the type of hydrophilic segments present in PU back-bone.The synthetic methods and applications of aque-ous PU dispersions are well documented.3–7The main advantages of aqueous PU dispersions are that they are nontoxic,nonflammable,and eco-friendly.More-over,these dispersions can be prepared with high molecular weights,without much change in viscosity, and with excellent scope for coating applications. Block copolymers,which evince a lot of commercial interest,8exhibit a high degree of microphase separa-tion.In block copolymers,change in the length of the two blocks affects the degree of phase separation, phase mixing,and hard segment domain organiza-tion,which in turn leads to variation in the proper-ties.9,10In the synthesis of block copolymers through conventional radical polymerization techniques,there is no effective control over the length of blocks.This drawback can be overcome by adopting living radical polymerization,which can be achieved by incorporat-ing iniferter groups11,12in the PU chain.Our group has reported several block copolymers using tetraphe-nylethane-based PU macroiniferters.13,14

A block copolymer,having one of the blocks soluble in water and the other insoluble,forms micelles.15,16 This type of block copolymer can be prepared by incorporating vinyl blocks having ionic groups into a PU backbone.Such a block copolymer can be used to prepare aqueous dispersions and the properties of the dispersions depend on the ratio of the hydrophobic PU to hydrophilic vinyl segments.Since the block length of the ionic segment plays a major role in deciding the properties of the dispersions,the control over the block length is important.The controlled incorporation of the ionic block into PU backbone has been successfully achieved in our lab and many iono-meric PU block copolymers have been prepared.17,18 In this article we report the synthesis and character-ization of aqueous dispersions of polyurethane–poly-acrylic acid(PU-PAA)multiblock copolymer where polyacrylic acid is the ionic segment.By using PU macroiniferter(PUMI),which follows living radical polymerization,the length of the ionic blocks has been altered by just varying the polymerization time.

EXPERIMENTAL

Materials

Poly(tetramethylene oxide)glycol(Aldrich,Milwau-kee,USA)of molecular weight2000(PTMG2000)was

Correspondence to:G.Radhakrishnan(clrieco@md3. vsnl.net.in).

Journal of Applied Polymer Science,Vol.87,1109-1115(2003)©2002Wiley Periodicals,Inc.dried under reduced pressure at100°C before use. Toluene diisocyanate(TDI;mixture of80%2,4-and 20%2,6-isomers)and dibutyltindilaurate(DBTDL;Al-drich,Milwaukee,WI,USA)were used as received. Acrylic acid(AA;SISCO,Mumbai,India)was distilled under reduced pressure and the middle portion was stored at4°C until use.Methyl ethyl ketone(MEK) and N,N-dimethylformamide(DMF;Merck,Mumbai, India)were distilled and DMF was stored over molec-ular sieves(4Å)until use.1,1,2,2-Tetraphenylethane-1,2-diol(TPED)was prepared from2-propanol and benzophenone,and the detailed procedure has been reported elsewhere.19

Characterization technique

Gel permeation chromatography(GPC;Waters,USA) attached with410differential refractometer and four ultrastyragel columns(106,105,104,and103Å)con-nected in a series was used to determine the average molecular weight and molecular weight distribution. Chromatographic grade DMF(0.01%LiBr added)was used as an eluent at aflow rate of1.0mL/min and molecular weight calibration was done using polysty-rene standards.Viscosity of the dispersions was mea-sured using Advanced Rheometer(AR500,TA Instru-ments,USA).Particle size was measured using Master sizer2000,(Malvern Instruments,UK).Interfacial ten-sion of the dispersions was determined by Wilhelmy plate technique.Critical surface tension(␥)of the polymer surfaces was measured from static contact angle.Contact angle was measured by a custom-built instrument and was directly read off a projected im-age of the liquid(volume20␮L),placed on thefilm. Strips(20ϫ10ϫ0.20mm3)of eachfilm were char-acterized with Dynamic Mechanical Analyzer(DMA 2980,TA Instruments,USA)using the tensionfilm mode in the temperature range ofϪ100toϩ100°C at a heating rate of5°C/min,strain amplitude of20␮m, and frequency of1Hz.The Microtensile specimen (three each)for stress–strain analysis was cut at a size of40ϫ10mm2and kept for conditioning at a tem-perature of20Ϯ2°C and relative humidity of65Ϯ2% for24h before testing.The specimen conformed to ASTM D6385.The tensile testing was done using an Instron Universal Testing machine model4501at a crosshead speed of100mm/min.

Synthesis of PUMI

PTMG2000(0.04mol,80g)and TDI(0.08mol,13.93g) were reacted at75°C in nitrogen atmosphere for3h. To this,TPED(0.04mol,14.65g)was added after cooling the reaction mixture to room temperature.At the same temperature,2mol%(based on NCO con-tent)of DBTDL and183mL of MEK(based on40% solid content)were added and the mixture was stirred for another24h.The resulting PUMI was precipitated by pouring the reaction mixture into a tenfold excess of methanol.PUMI wasfiltered,dried under reduced pressure,and stored at4°C until use.The detailed procedure to prepare PUMI is reported elsewhere.20 Synthesis of PU-PAA multiblock copolymers and their anionomers

Calculated amounts of PUMI,DMF,and acrylic acid (Table I)were taken in a polymerization tube and degassed by passing nitrogen for15min.The reaction mixture was kept in a thermostated water bath at75°C for a stipulated period.Then the reaction was arrested by quenching in ice salt mixture and calculated amount of triethylamine(Table I)was added.The solvent and the unreacted monomer were removed

TABLE I

Synthesis and GPC Results of PU-PAA Multiblock Copolymeric Anionomers

Sample no Polymer

code

PUMI

(g)

AA

(g)

Time

(h)

TEA

(g)

%Ionization

(%)

GPC Results Conversion a

(%)

M៮nϫ10Ϫ4M៮w/M៮n

1BV8/28248 1.12407.26 1.3674.57 2BV6/448 2.24407.81 1.2768.82 3BV4/8 3.37408.24 1.2453.32 4BV2/82848 4.49408.56 1.241.79 5TV24h24 2.24407.34 1.2546.32 6TV36h36 2.24407. 1.2755.54 7TV48h48 2.24407.81 1.2768.82 8TV72h72 2.24407.94 1.2879.93 9IV0%4800———

10IV20%48 1.1220———

11IV40%48 2.2440———

12IV60%48 3.3760———

13IV80%48 4.4980———

14IV100%48 5.61100———

a ConversionϭWeight of Block CopolymerϪWeight of PUMI/Weight of AAϫ100.

1110SAIMANI ET AL.from the reaction mixture by applying vacuum.Ho-mopolymer,if any,present in the ionic block copoly-mer was removed by washing thoroughly with ice cold water.The product was then dried and stored at 4°C until use.

Preparation of aqueous dispersions

A calculated amount of PU-PAA multiblock copoly-meric ionomer was dissolved in DMF and the reaction mixture was homogenized by stirring for30min.To this,a calculated amount of water(Table II)was added dropwise at a constant rate while the stirring speed was maintained at1000rpm.A portion of the resulting dispersion was cast in a teflon plate and dried at120°C in an air-circulating oven.

RESULTS AND DISCUSSION

Several PU–polyvinyl block copolymers13,14and PU–polyvinyl block copolymeric ionomers17,18were syn-thesized using PUMI,and the reactions were shown to follow controlled radical mechanism.In this series, synthesis and properties of aqueous dispersions of PU-PAA multiblock copolymeric anionomers are pre-sented.Since the amount of hydrophilic segment is the crucial factor in deciding the properties of the disper-sion,living radical polymerization technique has been adopted to incorporate hydrophilic vinyl blocks into PU backbone.The synthetic route to prepare aqueous dispersion of PU-PAA multiblock copolymeric an-ionomers is given in Scheme1.

Table I gives the synthetic formulations and GPC results of various block copolymeric anionomers.It is seen from the table that for a specific composition of PUMI and acrylic acid,as the polymerization time increases there is increase in conversion and molecular weight.This is because the polymerization of acrylic acid follows controlled radical mechanism.With the decrease in PUMI content the molecular weight of the multiblock copolymers increases because as the PUMI content decreases,initiating species also decreases, thereby resulting in higher molecular weight of PU-PAA block copolymers.

Particle size and viscosity

Aqueous polyurethane dispersions are mainly used in coatings.Particle size and viscosity are important

pa-

Scheme1

TABLE II

Interfacial Tension and Critical Surface Tension Values For PU-PAA Multiblock Copolymeric Dispersions a

Sample no Polymer

code

Interfacial

tension

(mN/m)

Critical surface

tension

(mN/m)b

1BV8/242.426.23 2BV6/445.527.29 3BV4/6.527.80 4BV2/848.528.45 5TV24h44.025.05 6TV36h44.726.95 7TV48h45.527.29 8TV72h47.528.36 9IV0%——10IV20%42.526.04 11IV40%45.527.29 12IV60%48.428.44 13IV80%55.529.29 14IV100%59.731.50

a(PU-PAA)ϭ7g;DMFϭ21mL,H

2Oϭ59.5mL.

b Results for thefilms obtained from the dispersions.

AQUEOUS DISPERSIONS OF PU-PAA COPOLYMERS1111

rameters in deciding the type of coating required.For surface coatings,larger particle size is preferred to ensure faster drying.If penetration into the substrate is required,smaller particle size is preferred.A suit-able viscosity range is required to avoid sagging (in case of low viscosity)and practical difficulty in appli-cation (encountered with high viscosity).

Preparation of PU-PAA multiblock copolymeric dis-persions is presented in Table II.It is interesting to note that when PU-PAA multiblock copolymers are dispersed without ionization,the dispersion is not stable.Hence all the dispersions were prepared from block copolymeric anionomers.

In general,ionic content is inversely proportional and molecular weight is directly proportional to par-ticle size.21In our case there is an increase in molecular weight as well as ionic content when PUMI content is decreased.Hence it is interesting to see the dual effect of PUMI content on the particle size of the dispersions.If ionic content plays a major role,particle size is expected to decrease with decreasing PUMI content,whereas particle size is expected to increase with de-creasing PUMI content when molecular weight plays a major role.The effect of PUMI content variation on particle size and viscosity is given in Figure 1.The results show that the particle size is inversely propor-tional to PUMI content,which implies that the molec-ular weight plays a major role in this system.A similar trend was observed in the case of PU–polymethacrylic acid block copolymeric dispersions.22

For constant molecular weight,ionic content should play a major role in deciding particle size.The effect of ionic content on particle size and viscosity is illus-trated in Figure 2.It is evident from Figure 2that at constant molecular weight,particle size decreases

with increasing ionic content from 20to 100%.Figure 3gives the effect of polymerization time on particle size and viscosity.When polymerization time in-creases,molecular weight increases and hydrophilic-ity of the polymer also increases.Here,as in PUMI content variation particle size increases due to in-crease in molecular weight irrespective of the increase in hydrophilicity of the polymer.

When PUMI content decreases,molecular weight increases and hydrophilicity of the polymer also in-creases.It is well known that viscosity increases with increase in hydrophilicity.Though in aqueous disper-sion the viscosity is not very much influenced by the molecular weight,here it is expected to have an indi-rect effect by affecting particle size.The relation

be-

Figure 1Effect of PUMI content on particle size and vis-

cosity.

Figure 2Effect of percent ionization on particle size and

viscosity.

Figure 3Effect of polymerization time on particle size and viscosity.

1112SAIMANI ET AL.

Interfacial and critical surface tension

Coating is an area where aqueous dispersions are mainly used.The ideal dispersion is one in which the polymer is hydrophilic when it is dispersed to en-hance the pot life and thefilms derived from the dispersions are hydrophobic to have water-resistant qualities.These properties can be assessed by interfa-cial and critical surface tension(CST)measurements. In order to understand the forces between the hy-drophobic and hydrophilic segments of the polymers and their distributions at the water/polymer interface, and solid/liquid interface,studies on interfacial ten-sion(␥)using the Wilhelmy plate technique and the critical surface tension using static contact angle mea-surements have been carried out.The results are given in Table II.The values clearly indicate that the poly-meric dispersions show a hydrophilic character with more polar groups at the interface and thus give rise to high␥values.However,thefilms obtained from the dispersions indicated that the domains on the surface are hydrophobic due to reorganization and ordering of the polymers at the solid/liquid interface.This is borne out by the low CST value ofϳ26m N/m. When the polymer is dispersed in water,the inter-facial energy shows a high value compared to the surface energy of the castfilm.The CST value in-creases with a decrease in PUMI content and an in-crease in polymerization time.There is also a notable increase in CST value when there is an increase in percent ionization.When the ionic percentage is in-creased from20–100%,the interfacial tension in-creases,indicating the increase in hydrophilic nature of the polymer.As the polymerization time increases, interfacial tension also increases.This is because the increase in polymerization time increases the incorpo-ration of the hydrophilic acrylic acid segments.When PUMI content decreases,the interfacial tension in-creases because of the fact that the incorporation of acrylic acid group increases when PUMI content is decreased.

Mechanical properties

Static mechanical analysis

Static and dynamic mechanical data of thefilms ob-tained from PU-PAA block copolymeric dispersions are presented in Table III.Generally tensile strength increases and elongation decreases with an increase in hard segment content of the polymer.When PUMI content decreases,more acrylic acid segments are in-corporated in the PU backbone,thereby increasing the hard segment content of the polymer chain and hence the tensile strength increases and the elongation de-creases.When percent ionization increases,ionic in-teractions between the polymer chains increase,lead-ing to an increase in tensile strength and elongation. When polymerization time is increased,since the sys-

TABLE III

Mechanical Properties of PU-PAA Multiblock Copolymeric Anionomers

Sample no.Polymer code

Static Dynamic

Tensile strength

(N/mm2)Elongation(%)T g1T g2

Initial storage modulus

(MPa)

1BV8/2 5.41412Ϫ57.946.5796

2BV6/4 5.71098Ϫ61.851.51424

3BV4/6 6.01048Ϫ60.256.32243

4BV2/812.5583Ϫ61.562.83780

5TV24h 3.21487Ϫ56.330.41379

6TV36h 4.81374Ϫ60.443.71424

7TV48h 5.71098Ϫ61.851.61499

8TV72h7.4968Ϫ60.560.91581

9IV20% 3.1983Ϫ59.349.02935

10IV40% 5.71098Ϫ61.851.51424

11IV60% 5.81304Ϫ62.958.51129

12IV80%7.31621Ϫ.463.2707

13IV100%9.611Ϫ66.3.7436 AQUEOUS DISPERSIONS OF PU-PAA COPOLYMERS1113

tem follows living radical polymerization,more acrylic acid units are incorporated,which results in an increase of the tensile strength and a decrease of elon-gation as in the case of PUMI variation.Dynamic mechanical analysis

Figure 4shows the effect of PUMI content variation on the dynamic mechanical characteristics of the block copolymer,and the transition temperatures are pre-sented in Table III.All the block copolymers exhibit two transitions in the tan ␦vs temperature plot corre-sponding to T g of the PU and that of the polyacrylic acid segment.Although the T g at the high temperature region (corresponding to polyacrylic acid)increases with a decrease in PUMI content,there is not much variation in the low temperature region (correspond-ing to PU).One can also observe that the intensity of the tan ␦peak corresponding to the PU segment de-creases while that of the polyacrylic acid segment increases with increasing acrylic acid content.This is also reflected in the initial storage modulus value (Ta-ble III),where the initial storage modulus increases with increase in acrylic acid content,suggesting the increasing rigidity of the system.Also when polymer-ization time increases,initial storage modulus in-creases,revealing that more acrylic acid units are in-corporated with time.

With an increase in polymerization time,the T g corresponding to the polyacrylic acid segment in-creases without much variation in the T g of the PU segment.With increase in ionic content the T g corre-sponding to PU decreases whereas T g corresponding

to the acrylic acid segment increases due to increased microphase separation.

CONCLUSION

PU-PAA multiblock copolymeric dispersions with dif-ferent block lengths and percent ionizations were pre-pared.GPC results confirm that the polymerization follows controlled radical mechanism.Studies show that particle size and viscosity increase with an in-crease in polymerization time and decreasing PUMI content.Particle size decreases and viscosity increases with an increase in percent ionization.When more acrylic acid groups are incorporated,the tensile strength increases due to more rigidity imparted by polyacrylic acid segments.The dynamic mechanical curves show two transitions corresponding to PU and acrylic acid segments.Critical surface tension and in-terfacial tension studies reveal that the polymers are highly hydrophilic,thus enhancing the stability of the dispersion and as films show a low critical surface energy profile,indicating the surface to be water re-pellent.

One of the authors (SS)thanks the Council of Scientific and Industrial Research (CSIR),India,for the award of a research fellowship.

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