interpenetrating thermo and ph stimuli-responsive polymer networks of paac/pnipaam grafted onto pp

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1176

Interpenetrating Thermo and pHStimuli-Responsive Polymer Networksof PAAc/PNIPAAm Grafted onto PP

Juan-Carlos Ruiz, Guillermina Burillo,* Emilio Bucio

A new strategy was used to prepare an interpenetrating polymer network of two stimuli-responsive polymers, a thermosensitive poly N-isopropylacrylamide (PNIPAAm) and a pHsensitive poly(acrylic acid) (PAAc) grafted onto poly(propylene) (PP) films (PP-g-PAAc). Threeconsecutive steps were used: graft copolymerization of PAAc onto PP films by gammaradiation, crosslinking of PP-g-PAAc to form the first network, and synthesis of a secondnetwork of PNIPAAm in situ within the cross-linked PP-g-PAAc by chemical polymerizationand crosslinking. The phase transitiontemperature (LCST), measured by swellingmeasurements and DSC, the pH critical point,obtained by swelling, and the SEM derivedmorphology of network samples are reported.

Introduction

‘‘Stimuli-responsive’’ polymers, also called ‘‘smart’’ polymers,

exhibit relatively large and sharp physical or chemical

changes in response to small physical or chemical

stimuli.[1–4] These polymers are being used as hydrogels[5–7]

or copolymers for technical applications in chemical and

mechanical engineering systems such as mass separation,

chemical valves, temperature or pH indicators, biomedical

and drug delivery systems.[8–9] For these applications, a rapid

response and good mechanical properties are necessary.

Interpenetrating polymer networks consist of a combination

of two or more polymers in network form, with at least one

such polymer polymerized and/or crosslinked in the

immediate presence of the other(s).[10] In a sequential IPN,

the first polymer network is synthesized; then a monomer

plus crosslinker and activator are swollen into the

network and polymerized in situ. Formerly when poly(N-

isopropylacrylamide) (PNIPAAm) and poly(acrylic acid)

J.-C. Ruiz, G. Burillo, E. BucioDepartamento de Quımica de Radiaciones y Radioquımica,Instituto de Ciencias Nucleares, Universidad Nacional Autonomade Mexico, Ciudad Universitaria, 04510 Mexico, D. F. MexicoE-mail: [email protected]

Macromol. Mater. Eng. 2007, 292, 1176–1188

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

(PAAc) were chemically combined, their sensitivity was

often altered or eliminated and their copolymer had poor

mechanical properties.[11] Attempts to solve this problem by

creating IPNs with a reduced gel size or by using a

macroporous structure were successful in preserving sensi-

tivity but failed to produce adequate mechanical proper-

ties.[12] In an attempt to get better mechanical properties,

PNIPAAm and PAAc were grafted in poly(tetrafluoroethyl-

ene) (PTFE), but the low critical solution temperature of

30–33 8C due to PNIPAAm shifted to 20–27 8C.[13–14]

This paper reports results of work undertaken to prepare

and evaluate an interpenetrating network of PNIPAAm

and PAAC grafted onto poly(propylene) (PP, Scheme 1). The

object of this investigation has been to improve the rate of

response, to preserve LCST, or increase the thermo and pH

sensitivity, and to enhance the mechanical properties of

systems of this type.

Experimental Part

Materials

Isotactic PP films (PEMEX, Mexico), with a 71% of crystallinity and

60 mm thickness respectively, were cut in 1.2 cm�4 cm pieces.

N-isopropylacrylamide (NIPAAm) supplied by Aldrich Co., USA, was

further purified by dissolving and recrystallizing in hexane/toluene

DOI: 10.1002/mame.200700178

Interpenetrating Thermo and pH Stimuli-Responsive Polymer Networks . . .

Scheme 1. (A) Graft copolymerization of PAAc onto PP; (B) crosslinking of PP-g-PAAc; and (C)interpenetrating network of PNIPAAm within crosslinked PP-g-PAAc.

(50:50 vol.-%;). Acrylic acid (AAc) supplied by Aldrich Co., USA, was

vacuum distilled for purification. N,N0-methylenebisacrylamide

(MBAAm), N,N,N0,N0-tetramethylethylenediamine (TEMED), and

Scheme 2. Mechanism of radiation graft initiation (PP-g-PAAc).



ammonium persulfate (APS) were used as

supplied by Sigma–Aldrich Co., USA. Hex-

ane and toluene from Baker were used as

received.

Grafting of PAAc onto PP Films

(PP-g-PAAc)

PP films were irradiated in air with a 60Co gamma source (Gamma

Beam 651 PT of Nordion Co.) at a dose rate of 2.66 kGy �h�1 and a

total radiation dose of 10 kGy (pre-irradiation method). The

www.mme-journal.de 1177

J.-C. Ruiz, G. Burillo, E. Bucio

Scheme 3. Mechanism of chemical PNIPAAm polymerization and crosslinking, (a) crosslinked PP-g-AAc film in solution with NIPAAmþMBAAm and APS; (b) addition of TEMED to swollen film; (c) turn down of ampoule; (d) heat at 70 8C.

Scheme 4. Mechanism of chemical polymerization and crosslinking initiation.

1178Macromol. Mater. Eng. 2007, 292, 1176–1188

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200700178


irradiated samples were placed in glass

ampoules containing aqueous solutions of

AAc 40 vol.-%. The ampoules were saturated

with argon for 20 min, sealed and heated to

50 8C at reaction times of 20–75 min. The

grafted films were washed with water during

24 h and then dried. The grafted percentage (%g)

was calculated as follows: %g¼ [(Wf �Wi)/

Wi]� 100; where Wf and Wi are the weight of

PP film after and before grafting respectively.

The mechanism of graft polymerization is

showed in Scheme 2.

Figure 1. Grafting of PAAc onto PP, as a function of time.

Crosslinking of the PAAc Grafted onto

PP (First Network)

The PP-g-PAAc films were placed in glass

ampoules with distilled water (7 mL) and

crosslinked by two different methods. In the

first method, the ampoules were saturated with argon, sealed and

irradiated at a dose rate of 2.86 kGy �h�1 and a total radiation dose

of 30 kGy; in the second method, the samples were crosslinked as

in the first method, but with the addition of the crosslinking agent

MBAAm (1.5 wt.-%;). The crosslinked films were then washed with

water (24 h) and then dried under vacuum.

IPNs of PNIPAAm and the Crosslinked PAAc Grafted

onto PP [net-(PP-g-PAAc)-inter-net-PNIPAAm]

The second network of PNIPAAm was synthesized inside the

crosslinked PP-g-PAAc by chemical polymerization and cross-

linking. The feed composition to form this second network was

based on a successful experiment by Zhang et al. in which

PNIPAAm hydrogels were synthesized and their ratios determined

gravimetrically.[15] They prepared full IPN-PNIPAAm hydrogels to

increase the polymer mass per unit volume to get better

Figure 2. Swelling percentages for two different grafting percentages of PP-g-PAAc:(~) 100% and (^) 300%, as a function of pH.

mechanical properties without reducing the

thermal sensitivity. In a typical experiment,

NIPAAm (0.6 M in distilled water) was combined

with the crosslinking agent MBAAm (3%), the

redox initiator, APS (1.5%), and TEMED (1.5%).

The concentration of the additives was based on

the mass of NIPAAm. The crosslinked PP-g-PAAc

films were put in ampoules and swelled for 24 h

with NIPAAm, MBAAm, and APS [Scheme 3(a)].

After this time TEMED was added [Scheme 3(b)];

the ampoules were sealed, turned down to

separate the excess of solution and left to stand

at room temperature during 2 h [Scheme 3(c)].

After this time the reaction was ended by heating

at 70 8C for an additional 24 h. The grafted films

were washed with water during 24 h to eliminate

the additives, homopolymer and unreacted

monomer, and then vacuum dried. The amount

of PNIPAAm incorporated into the film, was

calculated as follows: PNIPAAm¼Wf,IPN �WG;



where Wf,IPN is the weight of IPN and WG is the weight of

crosslinked PP-g-PAAc. The mechanism of polymerization and

crosslinking is showed in Scheme 4, the initiation step was taken

from Guilherme et al.[16]

Film Characterization

In each step of experimentation, the films were characterized by

their pH and/or thermal sensitivity, and their time of limited

swelling to get the pH critical point, and/or the lower critical

solution temperature (LCST). Chemical composition was deter-

mined by Fourier Transform Attenuated Total Reflection IR

spectroscopy (FTIR-ATR), morphology by scanning electron

microscopy (SEM), and thermal properties by DSC and TGA.

For the limited swelling time measurements, the films were

immersed in distilled water (pH 7) at a constant temperature of

20 8C for various periods of time after which they were removed



Figure 3. Influence of the swelling history on the swelling percentages as a functionof pH, for the same crosslinked PP-g-PAAc with 70%: (^) first time, (~) second time,and (~) third time.

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from the water and dried with a filter paper to

remove excess surface water, weighed, and

reimmersed until the hydrated weight reached

a constant value. The percentage of swelling was

determined gravimetrically by the following

equation: Swelling percent¼100[(Ws�Wi)/Wi],

where Ws and Wi are weights of the swollen and

initial film respectively. The pH sensitivity of

films was studied by swelling experiments

in Na2HPO4/citric acid buffer solutions in the

range of pH from 2.2 to 8 at the equilibrium

swelling time (limited swelling time); pH solu-

tions were calibrated with a HANNA Instruments

Potentiometer HI 4212. The pH critical point was

evaluated at the inflection point of the plot of

swelling percentage as a function of pH. The

thermo-sensitive response was studied from 10

to 50 8C by swelling with distilled water (pH 7) at

different temperatures, and the LCST was calcu-

lated from the inflection point of the plot of

swelling percentage as a function of temperature.

Thermosensitivity was defined as the ratio of the

swelling percentage of samples at 10 and 40 8C, and pH sensitivity

was defined as the ratio of swelling percentages in buffer

solutions of pH 3.0 and 8.0. The pH sensitivity (SpH) or

thermosensitivity (ST) are expressed as SpH or ST¼ SH/SL; where

SL is the lowest swelling and SH is the highest swelling in the range

studied. Swelling–deswelling reversibility of the films was

determined by immersing them during 24 h in a buffer solution

of pH 7.4. They were then weighed and reimmersed at pH 1.2 and

weighed again. The experiment was repeated several times at

each pH.

Infrared spectra were obtained a Perkin-Elmer PARAGON

500 FTIR-ATR (attenuated total reflection) spectrometer with a

SeZn glass in contact with the sample surface. Thermogravimetric

analysis in nitrogen atmosphere was performed with a TGA Q50

(TA Instruments, New Castle, DE) to determine the decomposition

temperatures. DSC studies were made on TA Instruments Model

2010; glass transition (Tg) determinations were performed under a

nitrogen atmosphere (flow rate of 60 mL �min�1)

and a heating rate of 10 8C �min�1 in a temperature

range from �20 to 220 8C, and LCST determinations

were made at a heating rate of 1 8C �min�1, in a

range from 5 to 50 8C using IPNs swelled with

water (at 10 8C during 24 h). Scanning electron

micrographs were obtained using a (SEM) JEOL

model JSM 5200 scanning electron microscope.

Figure 4. Swelling percentages of PP-g-PAAc (30% graft): (A) PP-g-PAAc, (B) cross-linked PP-g-PAAc, and (C) crosslinked PP-g-PAAc in presence of MBAAm.

Results and Discussion

Figure 1 shows the results of grafting of

PAAc onto PP films by the oxidative pre-

irradiation method. The amount of grafted

PAAc increased with the reaction time which

is normal for radiation induced grafting; in

this system, 800% grafting was obtained in

only 75 min. The film thickness also increases



with the grafting percent, e.g., 0.08 mm for 100% and

0.22 mm for 500%. This increase in thickness indicates

surface grafting.[17]

Determination of the pH critical point for two different

grafts with the same swelling history is shown in Figure 2.

The pH critical point at about 5.4 and SpH of 2 are the same

in both 100 and 300 graft percentages. The swelling history

on poly[NIPAAm-co-(methacrylic acid)] has been studied

and is consistent with the results shown in Figure 3.[18] In

this figure, the swelling history for PP-g-PAAc (70% graft) is

presented; each time that the films were exposed to

pH-induced swelling changes, the maximum swelling

percent increased.

In Figure 4, the change from hydrophobic to hydrophilic

behavior, at about pH 5.4 and a pH sensitivity SpH of 3, is

similar in PP-g-PAAc (70% graft) and crosslinked PP-g-PAAc

DOI: 10.1002/mame.200700178


Figure 5. Reversible swelling behavior of crosslinked PP-g-PAAc by gamma radiation: (^)210% graft and (~) 520% graft.

Figure 6. Swelling percentage of IPN 1 in water as a function of temperature; (88%/molPAAc, 12% mol PNIPAAm).

(70% graft) with and without the crosslinking agent

MBAAm. In spite of the same pH critical point and pH

sensitivity, different swelling percentages were found:

PP-g-PAAc (70% graft) has a maximum swelling of 62% at

pH 8; the crosslinked PP-g-PAAc at a radiation dose of 30

kGy in water has a lower swelling of about 45% at the

Table 1. Composition of the synthesized IPNs.

PP-g-PAAc Grafting Crosslinking method

wt.-%

IPN 1 69% Radiation

IPN 2 70% RadiationRMBAAm



same pH; this behavior is due to the

increased network structure after the

crosslinking process. Finally, crosslinked

PP-g-PAAc prepared in the presence of

MBAAm shows a maximum swelling of

23% at the same pH; this lower swelling

is due to higher crosslinking density

because of the use of the crosslinking

agent, MBAAm. The reversible swelling

behavior is shown in Figure 5 for two

crosslinked PP-g-PAAc: 210 and 520%

with an average SpH of 1.5 and 2 respec-

tively. These films were immersed at pH

7.4 and then at pH 1.2 during 24 h in

each case.

A lower critical solution temperature,

LCST, was found at about 35 8C for IPN 1

with a composition of 88/12% mol/mol

of PAAc/PNIPAAm (Figure 6); this value is

similar to the LCST of the PNIPAAm

hydrogel, it seems that there are not

important interactions between the two

networks because of the low PNIPAAm

ratio. Hydrogen bond arrangements

between the carboxylic group and amide

group have been detected depending on

their ratios.[18] It is very important that

an LCST behavior of PNIPAAm was

conserved in the IPN grafted system. A

LCST was not observed in the IPN 2, with a

composition of 92/8% mol/mol of PAAc/

PNIPAAm. In this last IPN 2, hydrophilic

groups which are sensitive to tempera-

ture decrease from 12 to 8%, and a more

compact structure was formed, because

of the high crosslinking density of the

PAAc caused by the presence of cross-

linking agent, MBAAm. This higher

crosslinking density prevents good diffu-

sion of both the monomer of NIPAAm and the necessary

additives for the crosslinking, inhibits the thermosensitive

properties, and favors intermolecular complexes between

PAAc, attributed to hydrogen bond formation. The

composition of both IPNs studied is presented in

Table 1. Figure 7 shows a limited swelling time for IPN

PAAc/PNIPAAm ratio PP-g-IPN

mol-% wt.-%

88/12 77%

92/8 81%



Figure 7. Swelling percentage (~) IPN 1 and (^) IPN 2, as a function of time.

1182

1 and IPN 2 at about 5 min. It is interesting to note that the

limited swelling time for PP-g-PAAc was obtained in 25

min; this time was not changed with the crosslinking of

PP-g-PAAc films.

A maximum swelling value of 200% was found for IPN 1,

whereas it was only about 80% for IPN 2 because of the

smaller voids in this last IPN. The swelling percentages of

IPNs were higher than grafted and crosslinked PP-g-PAAc.

This higher swelling percent for IPN 1 is due to several

swelling and drying steps before forming the IPN, and

appears to be related to the swelling history.

The pH critical point (Figure 8) is similar in both IPNs

and independent of their polymer composition. Again, the

higher swelling percentage for the IPN 1 is due to a low

density of crosslinking and therefore an increased prob-

ability of intermolecular complex formation between

Figure 8. Swelling percentage as a function of pH; (^) IPN 1; (~) IPN 2.



PNIPAAm and PAAc attributed to hydrogen

bond formation. This type of associations

was proved by Aoki et al.[19] with IPNs of

PAAc and poly(N,N-dimethylacrylamide)

without grafting. The LCST of IPN 1 at pH

3 and pH 8 (Figure 9), below and above the

critical pH point of 5.2, shows just one LCST

of 32 8C at pH 3. At pH 3 it exhibits

hydrophobic behavior corresponding to a

PAAc hydrogel, with predominant hydro-

gen bonding between –COOH groups

within the PAAc and between the COOH

groups of PAAc and the amino groups of

PNIPAAm which leads to a low swelling

percentage. At low temperatures the max-

imum swelling of the thermosensitive

PNIPAAm is observed. With an increase in

temperature, the PNIPAAm has a change of

phase from hydrophilic to hydrophobic

behavior at LCST of about 32 8C. At a pH 8 where the

PAAc hydrogel has a hydrophilic behavior and a maximum

swelling (ionizing groups), this process competes with the

decrease in swelling because of the increase in tempera-

ture of the PNIPAAm. As a result the LCST of the PP-g-IPN

was not observed.

Figure 10 exhibits the DSC thermograms of the IPN 1,

which was swelled in a buffer solution (pH 3). The onset

point of the endothermal peak, determined by the inter-

secting point of two tangent lines from the baseline and

slope of the endothermal peak, was used to determine

LCST.[20] IPN 1 shows a LCST of 33 8C for water and 36 8C for

buffer solution (pH 3). The obtained LCST by DSC, is almost

equal to that which was obtained by swelling measure-

ments.

DSC thermal transitions for different systems (shown in

Figure 11) are summarized in Table 2.

In Figure 11, the thermograms exhibit

both a Tg of PAAc and Tm of PP for the

PP-g-PAAc for both the 33 and 70% grafts. In

the case of the film with 300% graft, only a

broad Tm band occurs with higher enthalpy.

The Tg of PP-g-PAAc increased when it was

crosslinked due to its more rigid and compact

structure (Table 2); this Tg is better defined

than when there is only grafting, and it

appears to be very near to the reported values

for PAAc and PNIPAAm (131.4 8C).[15] When

the grafted films undergo a second heating,

these films exhibits just one transition and

the Tm of PP shifts to lower temperature.

Figure 12 shows thermo-gravimetric

curves for several systems; a less than 10%

weight loss is seen up to about 300 8C for all

curves except for PAAc which appears to lose

DOI: 10.1002/mame.200700178


Figure 10. DSC thermograms of swelled IPN 1 with water and buffer solution (pH 3) at a heating rate of 1 8C �min�1 from 25 to 45 8C.

Figure 9. Swelling percentage as a function of temperature for IPN 1 in two pH different solutions: (~) pH¼ 3.0 and (^) pH¼0 8.0.


� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mme-journal.de 1183


Figure 11. DSC thermograms of PP-g-PAAc: (A) 33% graft; (B) 70% graft; and (C) 300% at a heating rate of 10 8C �min�1.

1184

residual water around 100 8C PAAc has two decomposition

temperatures at 304 and 400 8C; PNIPAAm starts decom-

position at 401 8C. The crosslinked PP-g-PAAc exhibits

considerably increased thermal stability with a decom-

position temperature at about 450 8C, and this thermal

resistance was kept by IPN synthesized with PNIPAAm.

The morphology of the representative systems studied

is shown in Figure 13. PP shows a homogeneous



microphase; PP-g-PAAc (70% graft) showed a well-defined

oriented diagonal structure, and in PP-g-PAAc (290% graft)

a network structure was observed in the film surface. In

the crosslinked film irradiated in the presence of the

crosslinking agent MBAAm, a more compact structure was

obtained. In this last structure the diffusion of the second

monomer was difficult, and this factor inhibits the

formation of a second network. The morphology of the

DOI: 10.1002/mame.200700178


Table 2. Glass transition (Tg) and melting point (Tm) of different systems.

System Tg Tm

-C -C

PP 169.5

PP-g-PAAc (33% graft) 134.6 (PAAc) 170.2 (PP)

PP-g-PAAc (70% graft) 119.6 (PAAc) 174.1 (PP)

PP-g-PAAc (300% graft) 172.6

Crosslinked PP-g-PAAc (70% graft) by g 124.5 (PAAc) 166.3 (PP)

IPN 1 117.1 (IPN) 166.8 (PP)

Second heating of PP-g-PAAc (70% graft) 159.1

Second heating of crosslinked PP-g-PAAc (70% graft) by g 152.2

Second heating of IPN 1 154.5

IPNs films synthesized without MBAAm showed a porous

surface with variably sized voids, and in the lateral view of

IPN 1, the film showed the IPN graft porous structure on

the surface of the PP films. The voids defects enhance the

water uptake (bound and free water) in the continuous

part of the gel and also the diffusion of the second

monomer in the first network.

Zhang observed FTIR spectra of PNIPAAm hydrogels

with a typical amide I band (�1 644 cm�1) consisting of

the C––O stretch and the amide II band (�1 538 cm�1),

including N–H vibration.[20] Sciarratta studied iPP covered

Figure 12. TGA of different systems: (a) PAAc, (b) PNIPAAm, (c) crossl



with PAAc by plasma and observed the main band

characteristics of PAAc and PP: the –CH3, –CH2, –CH

stretching bands, between 3 030 and 2 740 cm�1, and

the carbonyl C––O stretching band approximately

1 715 cm�1.[21] FTIR spectra of the synthesized samples

(Figure 14) confirm the presence of PAAc and PNIPAAm in

the interpenetrating network with the characteristic peaks

at 1 641 and 1 715 cm�1 of the C––O groups of NIPAAm and

AAc respectively, and at 1 538 cm�1 corresponding to NH

of the NIPAAm amide group, in addition of the character-

istic peaks of PP.

inked PP-g-PAAc, and (d) IPN 1.



Figure 13. SEM micrographs of the representative systems: (a) PP film; (b) PAAc hydrogel 70% graft, crosslinked by gamma irradiation; (c)PAAc hydrogel 290%graft, crosslinked by gamma irradiation; (d) PAAc hydrogel 290%graft, crosslinked by gamma irradiation in presence ofMBAAm; (e) IPN 1; (f), (g), and (h) lateral view of IPN 1 film at different magnifications.

1186Macromol. Mater. Eng. 2007, 292, 1176–1188

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mame.200700178


Figure 14. FTIR for different systems: (A) PP, (B) crosslinked PP-g-PAAc by gamma radiation with MBAAm, (C) crosslinked PP-g-PAAc bygamma radiation without MBAAm, and (D) IPN1.

Conclusion

Elastic films with good mechanical properties were

obtained on combining PAAc and PNIPAAm in the form

of IPNs grafted onto PP. The velocity of response (swelling)

was very fast at about 5 min, compared with grafted films

of PP-g-PAAc of about 25 min. The properties of these IPNs

are attributed to the phase separation and small interac-

tion between PAAc and PNIPAAm components and to the

properties of the matrix PP films. PAAc hydrogels cross-

linked only by gamma radiation had better conditions to

form IPNs due to higher dimension of the pores which

enhance diffusion of the second monomer to form a second

network inside the first one. These IPNs grafted onto PP

retain the thermal and pH sensitivity, LCST, and pH critical

values. LCST behavior was not observed at pH values above

the pH critical point. Swelling history influences the

swelling measurements of all the films.

Acknowledgements: The authors wish to thank C. Magana fromIF-UNAM, F. Garcıa and S. Castillo-Rojas from ICN-UNAM fortechnical support and DGAPA-UNAM grant IN200306 for financialsupport.

Received: June 1, 2007; Revised: August 7, 2007; Accepted: August17, 2007; DOI: 10.1002/mame.200700178



Keywords: DSC; IPNs; LCST; PAAc; pH critical; PNIPAAm

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interpenetrating thermo and ph stimuli-responsive polymer networks of paac/pnipaam grafted onto pp

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