research article phase diagrams of smart copolymers poly(n...

Research ArticlePhase Diagrams of Smart CopolymersPoly(N-isopropylacrylamide) and Poly(sodium acrylate)

Iwona Zarzyka1 Maria Laura Di Lorenzo2 and Marek Pyda13

1 Department of Chemistry Rzeszow University of Technology 35-959 Rzeszow Poland2 Consiglio Nazionale delle Ricerche Istituto di Chimica e Tecnologia dei Polimeri co Compresorio OlivettiVia Campi Flegrei 34 80078 Pozzuoli Italy

3 ATHAS-MP Company Knoxville TN 37922 USA

Correspondence should be addressed to Iwona Zarzyka izarzykaprzedupl

Received 25 January 2014 Revised 17 July 2014 Accepted 17 July 2014 Published 14 August 2014

Academic Editor Franck Cleymand

Copyright copy 2014 Iwona Zarzyka et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The phase behavior of linear poly(N-isopropylacrylamide) (PNIPA) linear copolymer poly(N-isopropylacrylamide) andpoly(sodium acrylate) (PNIPA-SA) and chemically cross-linked PNIPA in water has been determined by temperature modulateddifferential scanning calorimetry (TM-DSC) Experiments related to linear polymers (PNIPA and PNIPA-SA) indicated nontypicaldemixingmixing behavior with a lower critical solution temperature (LCST) which do not correspond to the three classical typesof limiting critical behavior Some similarities and differences are observed in comparison to our literature data using standardTM-DSC for PNIPAwater Furthermore no influence of composition cross-linked PNIPAwater system on demixingmixingtemperature was observed

1 Introduction

Nowadays stimuli-sensitive polymers also called smart poly-mers are the focus of our attentionThese polymers give theirown responses to external stimuli such as temperature pHelectric field light biochemical compounds or specific ionsScientists attach a lot of importance to these properties asthe key to technological development The features could beadapted precisely to the demands of any user The polymersfind applications in many industrial areas For this reasonsmart polymers are a part of devices both nanodevices andmicrodevices used in medicationrsquos delivery tissue engineer-ing bioseparation sensors and actuators and even artificialmuscles [1ndash5] Intelligent polymers are known for theirresponses to small variations such as the external stimulimentioned above These variations result in both on themolecular scale (changes of properties of the surface eitherin hydrophilic or hydrophobic) and on the macroscopicscaleThese include precipitation (linear polymers) a volumechange transition (cross-linked polymers) and a change inthe water content (hydrogels) [6]

Poly(N-isopropylacrylamide) (PNIPA) is representativefor many stimuli-sensitive polymers as well as hydrogelsPNIPA characterizes the discontinuous volume transition [7]

The most important aspect of PNIPA is its phase transi-tion from a hydrophilic to a hydrophobic state at the LCST[8 9] These changes are connected to temperature so aboveLCST there are hydrophobic interactions between polymerchains that entail participation or deswelling BelowLCST theswelling or solubility and the separation of the polymer chainsrise and PNIPA is more hydrophilic [10 11]

Thebehavior of PNIPAdepends on structure of polymersespecially on the kind of comonomer and copolymer [12ndash18]as well as the character of end groups [19ndash23]

Solubility of linear PNIPA has a minimum of about45wt polymer in dependence on molecular weight of poly-mer at temperature 26-27∘C [24] or under 25∘C according to[25]

The demixingmixing of water soluble polymers fromtheir solution can be subdivided into three types The typeof demixing is related to a specific swelling behavior ofthe corresponding chemical network [26ndash28] These three

Hindawi Publishing Corporatione Scientific World JournalVolume 2014 Article ID 516076 8 pageshttpdxdoiorg1011552014516076

2 The Scientific World Journal

Table 1 Molecular weights and molecular weight distributions of linear gels

Gel Mn [gmole] Mw [gmole] Mz [gmole] Molecular weight distribution 119875 = MwMnPNIPA 343200 (plusmn03)lowast 731800 (plusmn03) 1389000 (plusmn1) 2132 (plusmn04)PNIPA-SA 505700 (plusmn03) 1097000 (plusmn05) 2274000 (plusmn2) 2169 (plusmn06)lowastError in

types of phase behavior are described in detail and areschematically illustrated in [28] They are explained by thetheory developed by Flory [29] Huggins [30] and Stavermanand Van Santen [31]

Afroze et al studied the influence of molecular weightand concentration of polymer on the demixing tempera-ture [24] They indicated the type II demixing behavior ofPNIPAwater A limiting critical situation at infinite molarmass at off-zero concentration is around 50wt polymer[32]

A type II LCST demixing behavior of PNIPA was con-firmed by investigations of van Mele group [33]

The LCST can be measured by turbidimetry or ultravi-oletvisible (UVVIS) spectroscopy dynamic light scatteringdifferential scanning calorimetry (DSC) small-angle neutronscattering (SANS) and other techniques [34]

In our work TM-DSC was used to determine a phasediagram for linear PNIPA linear copolymer PNIPA-SAand cross-linked PNIPA (PNIPA-BIS)water system Thephase diagram of PNIPA was shown many times by otherscientists but this time a somewhat different shape of thecurve was found Next the phase diagram of PNIPA-SA wasconstructed and compared with the curve of PNIPA withoutSA Copolymer PNIPA-SA is known and investigated earlier[35ndash39] An introduction of SA increases water absorptioncapacity of PNIPAThe gel swelling properties weremeasuredas a function of pH ionic strength temperature salt pres-ence and other stimuli [40ndash42] but none have shown thecompleted phase diagram of this copolymer Preliminarilyonly results for the range of 10ndash35wt gels PNIPA-SA werepresented in the paper [43]

2 Experimental Part

21 Materials N-Isopropylacrylamide (NIPA) 97 NNN1015840N1015840-tetramethylethylenediamine (TEMED) 99 andsodium acrylate (SA) 97 were purchased from Sigma-Aldrich NN1015840-methylenebisacrylamide (BIS) ge 995 wasprovided by Fluka Ammonium persulfate (APS) 98 wasobtained from Riedel-de Haen All chemicals were used asreceived

22 Hydrogel Synthesis PNIPA gels were prepared by freeradical polymerization in water which is a solvent for allcomponents of the mixture [7 43] The preparation of thelinear copolymer PNIPA-SA and cross-linked copolymerPNIPA-BIS was similar but in the case of this later one (thecross-linking degree defined as the ratio of moles of cross-linking agent and the moles of repeating unit equals 147)

Themolar weight distribution of the linear polymer sam-ples was investigated by the gel permeation chromatography

minus025

minus045

minus065

minus085

minus105

minus1250 10 20 30 40 50

Hea

t flow

(Wg

)Temperature (∘C)

1Kmin2Kmin

3Kmin5Kmin

10Kmin

20Kmin

Figure 1 DSC thermograms of PNIPA-SAwater (2080) systemin equilibrium swelling degree Heating was performed with 1 2 35 10 and 20Kmin after cooling with 10Kmin

Hea

t flow

(Wg

)

10 20 30 40 50

Temperature (∘C)

minus07

minus08

minus09

minus10

minus11

Total heat flow

Reversing heat flow

Figure 2 TM-DSC thermograms of PNIPA-SAwater (2080)system in equilibrium swelling degree upon heating rate of 5 Kminand cooling rate of 10 Kmin

(GPC) with NN1015840-dimethylformamide as the solvent Themolar weight results for the three polymer samples are shownin Table 1

23 Preparation of PolymerWater Mixtures Gels were driedunder vacuum for a few days at 70∘C after which the watercontent was less than 15 wt as determined by thermogravi-metric analysis Polymerwater mixtures containing from 5to 90 of water were prepared by direct addition of the

The Scientific World Journal 3

Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

Total heat flow

Reversing heat flow

0

minus007

minus008

minus009

minus010

minus011

minus012

1Kmin

(a)

0 40 50 60

minus040

minus045

minus050

minus055

Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

5Kmin

Total heat flow

Reversing heat flow

(b)

Figure 3 TM-DSC thermograms of PNIPA-SAwater (4060) system in equilibrium swelling degree upon heating rate of (a) 1 Kminand (b) 5 Kmin and cooling rate of 10 Kmin

appropriate amount of water to the dried gels and then storedin a refrigerator overnight The homogeneous mixtures wereencapsulated in aluminum hermetic pans for calorimetricanalysis Simultaneously sample weight loss wasmeasured byTGA to check the preparation procedure and determine theeffective water content (error le 1 wt)

24 Thermal Analysis

241 Thermogravimetry Water content of the hydrogels wasdetermined with a Perkin Elmer Pyris Diamond TGDTanalyzer Samples were placed in aluminum open samplepans (capacity 90120583L) and heated at 20∘Cmin from roomtemperature to complete evaporation of water High puritynitrogen gas was fluxed into the furnace at a flow rateof 50mLmin As mentioned above thermogravimetry wasused to check the water content of the hydrogels simulta-neously with DSC analyses as well as to determine eventualresidual water in the dried polymers

242 Calorimetry MTDSC measurements were performedon a TA Instruments Q1000 DSC with the modulated DSCoption and an RCS cooling accessory High purity nitrogengas was fluxed at 50mLmin during all measurements andthermal treatments Indium and sapphire were used fortemperature heat of fusion and heat capacity calibrationrespectively Samples (swollen gels) of 1-2mgwere placed intoTzero hermetic aluminum pans The standard temperaturemodulation DSC conditions were used with an amplitudemodulation of 119860

119879= 08∘C and a period of p = 60 s at an

underlying heating rate of 5∘Cminminus1 The measurementswere carried out from minus50 to 80∘C Samples were cooledinside of DSC cell at a rate of 10 Kmin using standard DSCmethod No noticeable difference in the sample mass beforeand after each DSC run was detected thus indicating that no

water evaporation took place during DSC analysis using theTzero hermetic aluminum pans

3 Results and Discussion

For the obtained polymers-linear PNIPA linear copolymerPNIPA-SA (gel contains 5wt of sodium acrylate in the drystate) and cross-linked PNIPA (PNIPA-BIS) the influence ofhydrogel content on the LCST is analyzed as function of waterswelling in the range from 10 to 95wt of gel using DSC andTM-DSC

As was illustrated in Figure 1 the ldquophase separationrdquo pointdetermined byDSC depends on the temperature heating rate

TM-DSC measurements of the same sample have givensomewhat different results (Figure 2) Only one peak hasbeen noticed both on curve of total heat flow and on curveof reversing heat flow in contradistinction to DSC curvedespite the same heating rate (5 Kmin) (Figures 1 and 2)Figure 2 shows a comparison of total and reversing heat flowof demixing where endotherms are similar and the process isreversing The demixing temperature (LCST) was estimatedas onset temperature

Due to the fact that this work was mostly qualitative anal-ysis of phase separation of PNIPA and its comonomerswatersystems all measurements were performed at 5 Kmin heat-ing rate This heating rate was selected based on the furtherpreliminary investigations which showed the most conve-nient heating rate to such type of analysis (Figure 3)

For better results the heating rate should be as slow aspossible Figures 4(a) and 4(b) show two examples of resultsof reversing heat flow and total heat flow fromTM-DSCmea-surements for demixingmixing process of polymerwatersystem PINIPA (10 90) (Figure 4(a)) and PNIPA-SA(40 60) (Figure 4(b))


Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus6

minus4

minus2

0

2

4

10 20 30 40 50

Temperature (∘C)

Temperature (∘C)

Heating

Mixing

Demixing

Cooling

10wt of PNIPA

(a)

Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus2

0

2

20 40

Temperature (∘C)

Temperature (∘C)Heating

Mixing

Demixing

Cooling1

minus1

40wt of PNIPA-SA

(b)

Hea

t cap

acity

(JKminus

1gminus

1)

20

15

10

5

0minus100 minus50 0 50 100

Melting

Demixing

Cp (experimental)

Cp (vibrational)

Temperature (∘C)

PNIPAwater (1090) wt

(c)

Figure 4 ((a) (b)) TM-DSC thermogram of gelwater system in equilibrium swelling degree (a) PNIPAwater (1090) and (b) PNIPA-SA (4060) Onset values of LCST were taken from the TM-DSC plots (reversing heat flow) upon first and second heating at 5∘CminCooling was performed with 10Kmin by standard DSC Endotherms around 0∘C on heating and exotherms around minus30∘C on cooling arerelated to melting and crystallization of water respectively (c) Experimental reversing heat capacity of the 1090 PNIPAwater and theirvibrational baseline 119862

119901

(vibrational) [44 45]

Next Figure 4(c) shows an example of reversing heatcapacity for the raw data presented in Figure 4(a)The revers-ing heat flow of PNIPAwater (1090) was converted toreversing heat capacity119862

119901(experimental) and comparedwith

vibrational heat capacity 119862119901(vibrational) of PNIPAwater

system Experimental and calculated vibrational 119862119901 data

were in good agreement on a short range of temperaturebetween minus35 and minus20∘C The only contribution to exper-imental heat capacity from below melting and demixingendotherms is given from vibrationmotion of PNIPAmacro-molecules and molecules of water The solid vibrational heatcapacity 119862

119901(vibrational) (see Figure 4(c)) of 1090wt

PNIPAwater was estimated as a linear combination of thevibrational heat capacity of PNIPA 119862

119901

119901 (vib) and water119862119901

119908 (vib) listed as follows 119862119901(vibrational) = 119891

119901119862119901

119901 (vib)+ 119891119908119862119901

119908 (vib) where 119891119901and 119891

119908are the weight fractions

of polymer and water respectively This vibrational heat

capacity is a line that together with liquid heat capacity canconstruct a baseline which possibly separates a true heatcapacity from excess heat or latent heat ofmelting and demix-ing processes of PNIPAwater system Similar separation wasperformed for poly(vinyl methyl ether)water in [46] Fulldescription of vibrational and liquid heat capacity PNIPAand PNIPAwater including table of vibration spectrum ispresented in the other papers which are under review andpreparation [44 45] Other interpretations of behaviors ofapparent heat capacity PNIPAwater in terms of hydrophilicand hydrophobic interactions between PNIPAM and thesurrounding water molecules are out of scope from TMDSCmethod

In this paper the measured values of LCST are plotted inFigures 5 6 7 and 8 as function of gel contentThree distincttwo-phase areas are observed in case of PNIPA and PNIPA-SA


24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n

Polymer concentration (wt)

1st run

A

B C

2nd run

Figure 5 Phase diagramof linear PNIPAwater system onset valuesof LCST were taken from the TM-DSC plots (reversing heat flow)upon first and second heating at 5∘Cmin lines are guides for theeye

Three critical points (A B and C) appear at around 3652 and 70wt hydrogel defined at approximately 7 151 and149∘C respectively and are observed on the phase diagramofPNIPA in Figure 5 Two first two-phase areas (A and B) haveclearly marked extrema in comparison to the third area (C)which has rather a plateau Also in Figure 5 two intersectionsbetween these three regions at around 44 and 55wt gel weredefined at 173 and 1665∘C where three phases coexist TheseLCST values are repeated in the subsequent heating as we cansee in Figure 5 Phase diagrams drawn on the basis of 1st and2nd heating are congruous

There was only one limiting critical concentration atapproximately 40wt gel observed at a temperature of about23∘C so van Durme and coworkers [33] classified it like IItype demixing behavior Our investigation shows the phasediagram as a type III demixing behavior There are two off-zero limiting critical concentrations [28] but in addition thereis also a plateau

The resulting phase diagram of the PNIPAwater systemobtained in this paper is similar in a low concentrationof polymer and different in high range compared to thephase diagram obtained for this system by van Durme andcoworkers in [33] In Figure 4(a) differences start above36wt PNIPA One of the differences is the high rangeof PNIPA concentration in PNIPAwater system which canbe due to different placing of water by polymeric materialIn our case it was by mixing and adding water to PNIPAwhile for van Durme and coworkers [33] it was preparedby evaporation of water Two different critical points (Band C) can be a result of an unexpected morphology ofpolymer leading to a specific combination of hydrophilicand hydrophobic interactions between PNIPAM and watermolecules

24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B

C

2nd run

Figure 6 Phase diagram of linear copolymer PNIPA-SAwatersystem onset values of LCST were taken from the TM-DSC(reversing heat flow) plots upon first and second heating at 5∘Cminlines are guides for the eye

A similar phase diagram is gained during measurementsof LCST of copolymer PNIPA-SA gelwater system (Figure 6)for first (1st run) and second (2nd run) heating

On the phase diagram (1st run) three minima (A B andC) at about 34 39 and 70wt are present which correspondto the temperatures 10 7 and 154∘C respectively As shownin Figure 6 the third area (C) of phase diagram has theplateau tooThe intersections appear at about 35 and 45wtappropriately at temperatures 134 and 165∘C respectively Insubsequent heating (2nd run) similar results were producedas shown in Figure 6

Comparing the shapes of both phase diagrams PNIPAandPNIPA-SAwater systemwe can see some similarities anddifferences (Figure 7)

The first critical point on both curves is situated quitesimilarly although an introduction of 5 wt of SA intostructure of PNIPA causes somewhat of an earlier occurrenceof the critical point at 34wt gel and at a slightly highertemperature of 10∘C in comparison to the first minimumcritical point of PNIPAwater system located at 36wt and7∘C (Figure 7)

Definitely larger changes are observed in case of secondminimumof the limiting critical concentration On the phasediagram of PNIPA-SAwater system the second minimumof critical point is shifted from 52wt of PNIPA to 39wtof PNIPA-SA Furthermore there is a visible change of theminimum in temperature with a decrease from 151 to 7∘C

The plateau of both phase diagrams is very similar(Figure 7) It should be noticed that the intersections arelocated in different gel content but the distance between themis almost identical The intersections on phase diagram ofPNIPA-SAwater system appear to be about 10wt of gelcontent less than in case of PNIPAwater system The secondintersection of PNIPA-SAwater system (45wt 165∘C)


24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run

Figure 7 A comparison of phase diagrams of PNIPA and PNIPA-SAwater systems

1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run

Figure 8 Phase diagrams of PNIPA PNIPA-SA and PNIPA-BISwater systems onset values of LCST were taken from the TM-DSC plots (reversing heat flow) upon first and second heating at5∘Cmin

nearly overlaps to the first intersection of PNIPA (44wt173∘C)

The shape of the phase diagram of cross-linked PNIPA-BIS is completely different (Figure 8) Present is almost astraight line at the level of temperature 295∘C We canobserve no dependence of the LCST on gel content in systemPNIPA-BISwater system

4 Conclusions

In summary the presented study indicated nontypical demix-ing behavior of PNIPA and PNIPA-SA The phase diagramof PNIPAwater system determined that basis of TM-DSC

measurements consisted of three two-phase areas separatedat a specific temperature at which three phases coexist It wasshown that the introduction of ionic comonomer (SA) intothe structure of PNIPA results in a similar shape of phasediagram and differences are related to the shift of minimumcritical points and intersections to the lower content of gel Inthe contrast it was shown that cross-linking of PNIPAmakesthe LCST of the PNIPA-BISwater system become insensitiveon changes of the gel content in sample

The purpose of this paper is only paying attention tonontypical shape of well-known phase diagrams of PNIPAand its copolymerswater systems by TM DSC Therefore abare description of the thermoresponsive phase behavior ofPNIPA gels without giving any molecular interpretation isonly given Detailed explanation and quantitative analysisof the described phenomena will be the subject of the nextpapers

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] D C Coughlan and O I Corrigan ldquoDrug-polymerinteractions and their effect on thermoresponsive poly(N-isopropylacrylamide) drug delivery systemsrdquo InternationalJournal of Pharmaceutics vol 313 no 1-2 pp 163ndash174 2006

[2] J Gu F Xia YWu XQu Z Yang and L Jiang ldquoProgrammabledelivery of hydrophilic drug using dually responsive hydrogelcagesrdquo Journal of Controlled Release vol 117 no 3 pp 396ndash4022007

[3] S Lue J-J Hsu C-H Chen and B-C Chen ldquoThermallyon-off switching membranes of poly(N-isopropylacrylamide)immobilized in track-etched polycarbonate filmsrdquo Journal ofMembrane Science vol 301 no 1-2 pp 142ndash150 2007

[4] Y K Yew T Y Ng H Li and K Y Lam ldquoAnalysis of pHand electrically controlled swelling of hydrogel-based micro-sensorsactuatorsrdquo Biomedical Microdevices vol 9 no 4 pp487ndash499 2007

[5] D Kuckling A Richter and K F Arndt ldquoTemperature and pH-dependent swelling behavior of poly(N-isopropylacrylamide)copolymer hydrogels and their use in flow controlrdquo Macro-molecularMaterials and Engineering vol 288 no 2 pp 144ndash1512003

[6] I Y Galaev and B Mattiasson ldquorsquoSmartrsquo polymers and whatthey could do in biotechnology and medicinerdquo Trends inBiotechnology vol 17 no 8 pp 335ndash340 1999

[7] B Strachotova A Strachota M Uchman et al ldquoSuper porousorganicndashinorganic poly(N-isopropylacrylamide)-based hydro-gel with a very fast temperature responserdquo Polymer vol 48 no6 pp 1471ndash1482 2007

[8] S Hirotosu Y Kirokawa and T J Tanaka ldquoVolumeminusphasetransitions of ionizedNminusisopropylacrylamide gelsrdquoThe Journalof Chemical Physics vol 87 pp 1392ndash1395 1987

[9] E Matsuo and T J Tanaka ldquoKinetics of discontinuous volumendashphase transition of gelsrdquoThe Journal of Chemical Physics vol 89pp 1695ndash1697 1988


[10] H G Schild ldquoPoly(N-isopropylacrylamide) experiment the-ory and applicationrdquo Progress in Polymer Science vol 17 no 2pp 163ndash249 1992

[11] H E Teal Z Hu and D D Root ldquoNative purification ofbiomolecules with temperature-mediated hydrophobic modu-lation liquid chromatographyrdquoAnalytical Biochemistry vol 283no 2 pp 159ndash165 2000

[12] F Meunier C Pichot and A Elaıssari ldquoEffect of thiol-containing monomer on the preparation of temperature-sensitive hydrogel microspheresrdquo Colloid and Polymer Sciencevol 284 no 11 pp 1287ndash1292 2006

[13] L Y Bao and L S Zha ldquoPreparation of poly(Nminusisopropylacrylamide) microgels using different initiators under variouspH valuesrdquo Journal of Macromolecular Science A Pure andApplied Chemistry vol 43 no 11 pp 1765ndash1771 2006

[14] G Chen and A S Hoffman ldquoGraft copolymers that exhibittemperature-induced phase transitions over a wide range ofpHrdquo Nature vol 373 no 6509 pp 49ndash52 1995

[15] S Furyk Y Zhang D Ortiz-Acosta P S Cremer and DE Bergbreiter ldquoEffects of end group polarity and molecularweight on the lower critical solution temperature of poly(N-isopropylacrylamide)rdquo Journal of Polymer Science A PolymerChemistry vol 44 no 4 pp 1492ndash1501 2006

[16] C M Schilli M Zhang E Rizzardo et al ldquoA new double-responsive block copolymer synthesized via RAFT polymeriza-tion poly(N-isopropylacrylamide)-block-poly(acrylic acid)rdquoMacromolecules vol 37 no 21 pp 7861ndash7866 2004

[17] N Singh and L A Lyon ldquoSynthesis of multifunctional nanogelsusing a protected macromonomer approachrdquo Colloid and Poly-mer Science vol 286 no 8-9 pp 1061ndash1069 2008

[18] K Tauer D Gau S Schulze A Volkel and R DimovaldquoThermal property changes of poly(119873-isopropylacrylamide)microgel particles and block copolymersrdquo Colloid and PolymerScience vol 287 no 3 pp 299ndash312 2009

[19] J E Chung M Yokoyama T Aoyagi Y Sakurai and TOkano ldquoEffect of molecular architecture of hydrophobicallymodified poly(N-isopropylacrylamide) on the formation ofthermoresponsive core-shell micellar drug carriersrdquo Advancesin Colloid and Interface Science vol 53 no 1ndash3 pp 119ndash1301998

[20] T Lopez-Leon A Elaıssari J L Ortega-Vinuesa andD Bastos-Gonzalez ldquoHofmeister effects on poly(NIPAM) microgel par-ticles macroscopic evidence of ion adsorption and changes inwater structurerdquoChemPhysChem vol 8 no 1 pp 148ndash156 2007

[21] T Lopez-Leon J L Ortega-Vinuesa D Bastos-Gonzalezand A J Elaı1ssari ldquoCationic and anionic poly(N-isopropylacrylamide) based submicron gel particles electrokinetic prop-erties and colloidal stabilityrdquo The Journal of Physical ChemistryB vol 110 no 10 pp 4629ndash4636 2006

[22] J RikaMMeewes RNyffenegger andT Binkert ldquoIntermolec-ular and intramolecular solubilization Collapse and expansionof a polymer chain in surfactant solutionsrdquo Physical ReviewLetters vol 65 no 5 pp 657ndash660 1990

[23] A Yamazaki J M Song F MWinnik and J L Brash ldquoSynthe-sis and solution properties of fluorescently labeled amphiphilic(N-alkylacrylamide) oligomersrdquo Macromolecules vol 31 no 1pp 109ndash115 1998

[24] F Afroze E Nies and H Berghmans ldquoPhase transitions inthe system poly(N-isopropylacrylamide)water and swellingbehaviour of the corresponding networksrdquo Journal of MolecularStructure vol 554 no 1 pp 55ndash68 2000

[25] K Van Durme G Van Assche V Aseyev J Raula H Tenhuand B VanMele ldquoInfluence of macromolecular architecture onthe thermal response rate of amphiphilic copolymers based onpoly(n-isopropylacrylamide) and poly(oxyethylene) in waterrdquoMacromolecules vol 40 no 10 pp 3765ndash3772 2007

[26] RMoerkerke R Koningsveld H Berghmans K Dusek and KSole ldquoPhase transitions in swollen networksrdquo Macromoleculesvol 28 no 4 pp 1103ndash1107 1995

[27] H Schafer-Soenen R Moerkerke H Berghmans and R Kon-ingsveld ldquoZero and off-zero critical concentrations in systemscontaining polydisperse polymers with very highmolarmasses2 The system water-poly(vinyl methyl ether)rdquoMacromoleculesvol 30 pp 410ndash416 1997

[28] R Moerkerke F Meeussen R Koningsveld et al ldquoPhasetransitions in swollen networks 3 Swelling behavior of radi-ation cross-linked poly(vinyl methyl ether) in waterrdquo Macro-molecules vol 31 no 7 pp 2223ndash2229 1998

[29] P J Flory ldquoThermodynamics of high polymer solutionsrdquo TheJournal of Chemical Physics vol 9 no 8 pp 660ndash661 1941

[30] M L Huggins ldquoSolutions of long chain compoundsrdquo Journal ofChemical Physics vol 9 no 5 p 440 1941

[31] A J Staverman and J H Van Santen ldquoThe miscibility of waterand alkylhalidesrdquo Recueil des Travaux Chimiques des Pays-Basvol 60 pp 836ndash841 1941

[32] K Solc K Dusek R Koningsveld and H Berghmans ldquolsquoZerorsquoand ldquoOff-Zerordquo critical concentrations in solutions of polydis-perse polymers with very high molar massesrdquo Collection ofCzechoslovak Chemical Communications vol 60 pp 1661ndash16881995

[33] K van Durme G van Assche and B van Mele ldquoKinetics ofdemixing and remixing in poly(119873-isopropylacrylamide)waterstudied by modulated temperature DSCrdquo Macromolecules vol37 no 25 pp 9596ndash9605 2004

[34] M J Hore B Hammouda Y Li and H Cheng ldquoCo-Nonsolvency of Poly(n-isopropylacrylamide) in DeuteratedWaterEthanol Mixturesrdquo Macromolecules vol 46 pp 7894ndash7901 2013

[35] W Xue S Champ and M B Huglin ldquoObservations on somecopolymerisations involving N-isopropylacrylamiderdquo Polymervol 41 no 20 pp 7575ndash7581 2000

[36] Y Liu J L Velada and M B Huglin ldquoThermoreversibleswelling behaviour of hydrogels based on N-isopropylacrylamide with sodium acrylate and sodium methacrylaterdquoPolymer vol 40 no 15 pp 4299ndash4306 1999

[37] E Matsuo and T Tanaka ldquoKinetics of discontinuous volumendashphase transition of gelsrdquoThe Journal of Chemical Physics vol 89no 3 pp 1695ndash1697 1988

[38] Patent EP 0693 508 A1 1996[39] C Ni and X Zhu ldquoSynthesis and swelling behavior of ther-

mosensitive hydrogels based on N-substituted acrylamides andsodium acrylaterdquo European Polymer Journal vol 40 no 6 pp1075ndash1080 2004

[40] S Beltran J P Baker H H Hooper H W Blanch andJ M Prausnitz ldquoSwelling equilibria for weakly ionizabletemperature-sensitive hydrogelsrdquo Macromolecules vol 24 no2 pp 549ndash551 1991

[41] K Kratz T Hellweg andW Eimer ldquoInfluence of charge densityon the swelling of colloidal poly(N-isopropylacrylamide-co-acrylic acid)microgelsrdquoColloids and SurfacesAPhysicochemicaland Engineering Aspects vol 170 no 2-3 pp 137ndash149 2000


[42] H Chen and Y Hsieh ldquoDual temperature- and pH-sensitivehydrogels from interpenetrating networks and copolymeriza-tion of119873-isopropylacrylamide and sodium acrylaterdquo Journal ofPolymer Science A Polymer Chemistry vol 42 no 13 pp 3293ndash3301 2004

[43] I Zarzyka M Pyda and M L Di Lorenzo ldquoInfluence ofcrosslinker and ionic comonomer concentration on glass tran-sition and demixingmixing transition of copolymers poly(N-isopropylacrylamide) and poly(sodium acrylate) hydrogelsrdquoColloid and Polymer Science vol 292 no 2 pp 485ndash492 2014

[44] A Czerniecka I Zarzyka and M Pyda ldquoVibrational heatcapacity of poly(N-isopropylacrylamide)rdquo Analytical Chem-istry In press

[45] A Czerniecka I Zarzyka and M Pyda Polymer In press[46] M Pyda K van Durme BWunderlich and B VanMele ldquoHeat

capacity of poly(vinylmethyl ether) in the presence and absenceof waterrdquo NATAS Notes vol 37 pp 7ndash13 2004

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


Table 1 Molecular weights and molecular weight distributions of linear gels

Gel Mn [gmole] Mw [gmole] Mz [gmole] Molecular weight distribution 119875 = MwMnPNIPA 343200 (plusmn03)lowast 731800 (plusmn03) 1389000 (plusmn1) 2132 (plusmn04)PNIPA-SA 505700 (plusmn03) 1097000 (plusmn05) 2274000 (plusmn2) 2169 (plusmn06)lowastError in

types of phase behavior are described in detail and areschematically illustrated in [28] They are explained by thetheory developed by Flory [29] Huggins [30] and Stavermanand Van Santen [31]

Afroze et al studied the influence of molecular weightand concentration of polymer on the demixing tempera-ture [24] They indicated the type II demixing behavior ofPNIPAwater A limiting critical situation at infinite molarmass at off-zero concentration is around 50wt polymer[32]

A type II LCST demixing behavior of PNIPA was con-firmed by investigations of van Mele group [33]

The LCST can be measured by turbidimetry or ultravi-oletvisible (UVVIS) spectroscopy dynamic light scatteringdifferential scanning calorimetry (DSC) small-angle neutronscattering (SANS) and other techniques [34]

In our work TM-DSC was used to determine a phasediagram for linear PNIPA linear copolymer PNIPA-SAand cross-linked PNIPA (PNIPA-BIS)water system Thephase diagram of PNIPA was shown many times by otherscientists but this time a somewhat different shape of thecurve was found Next the phase diagram of PNIPA-SA wasconstructed and compared with the curve of PNIPA withoutSA Copolymer PNIPA-SA is known and investigated earlier[35ndash39] An introduction of SA increases water absorptioncapacity of PNIPAThe gel swelling properties weremeasuredas a function of pH ionic strength temperature salt pres-ence and other stimuli [40ndash42] but none have shown thecompleted phase diagram of this copolymer Preliminarilyonly results for the range of 10ndash35wt gels PNIPA-SA werepresented in the paper [43]

2 Experimental Part

21 Materials N-Isopropylacrylamide (NIPA) 97 NNN1015840N1015840-tetramethylethylenediamine (TEMED) 99 andsodium acrylate (SA) 97 were purchased from Sigma-Aldrich NN1015840-methylenebisacrylamide (BIS) ge 995 wasprovided by Fluka Ammonium persulfate (APS) 98 wasobtained from Riedel-de Haen All chemicals were used asreceived

22 Hydrogel Synthesis PNIPA gels were prepared by freeradical polymerization in water which is a solvent for allcomponents of the mixture [7 43] The preparation of thelinear copolymer PNIPA-SA and cross-linked copolymerPNIPA-BIS was similar but in the case of this later one (thecross-linking degree defined as the ratio of moles of cross-linking agent and the moles of repeating unit equals 147)

Themolar weight distribution of the linear polymer sam-ples was investigated by the gel permeation chromatography

minus025

minus045

minus065

minus085

minus105

minus1250 10 20 30 40 50

Hea

t flow

(Wg

)Temperature (∘C)

1Kmin2Kmin

3Kmin5Kmin

10Kmin

20Kmin

Figure 1 DSC thermograms of PNIPA-SAwater (2080) systemin equilibrium swelling degree Heating was performed with 1 2 35 10 and 20Kmin after cooling with 10Kmin

Hea

t flow

(Wg

)

10 20 30 40 50

Temperature (∘C)

minus07

minus08

minus09

minus10

minus11

Total heat flow

Reversing heat flow

Figure 2 TM-DSC thermograms of PNIPA-SAwater (2080)system in equilibrium swelling degree upon heating rate of 5 Kminand cooling rate of 10 Kmin

(GPC) with NN1015840-dimethylformamide as the solvent Themolar weight results for the three polymer samples are shownin Table 1

23 Preparation of PolymerWater Mixtures Gels were driedunder vacuum for a few days at 70∘C after which the watercontent was less than 15 wt as determined by thermogravi-metric analysis Polymerwater mixtures containing from 5to 90 of water were prepared by direct addition of the


Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

Total heat flow

Reversing heat flow

0

minus007

minus008

minus009

minus010

minus011

minus012

1Kmin

(a)

0 40 50 60

minus040

minus045

minus050

minus055

Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

5Kmin

Total heat flow

Reversing heat flow

(b)



24 Thermal Analysis













Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus6

minus4

minus2

0

2

4

10 20 30 40 50

Temperature (∘C)

Temperature (∘C)

Heating

Mixing

Demixing

Cooling

10wt of PNIPA

(a)

Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus2

0

2

20 40

Temperature (∘C)


Mixing

Demixing

Cooling1

minus1

40wt of PNIPA-SA

(b)

Hea

t cap

acity

(JKminus

1gminus

1)

20

15

10

5

0minus100 minus50 0 50 100

Melting

Demixing

Cp (experimental)

Cp (vibrational)

Temperature (∘C)


(c)


119901









119901

119901 (vib) and water119862119901


119901119862119901

119901 (vib)+ 119891119908119862119901

119908 (vib) where 119891119901and 119891






24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B C

2nd run





24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B

C

2nd run









24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run


1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run




4 Conclusions






References
























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

Total heat flow

Reversing heat flow

0

minus007

minus008

minus009

minus010

minus011

minus012

1Kmin

(a)

0 40 50 60

minus040

minus045

minus050

minus055

Hea

t flow

(Wg

)

10 20 30

Temperature (∘C)

5Kmin

Total heat flow

Reversing heat flow

(b)



24 Thermal Analysis













Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus6

minus4

minus2

0

2

4

10 20 30 40 50

Temperature (∘C)

Temperature (∘C)

Heating

Mixing

Demixing

Cooling

10wt of PNIPA

(a)

Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus2

0

2

20 40

Temperature (∘C)


Mixing

Demixing

Cooling1

minus1

40wt of PNIPA-SA

(b)

Hea

t cap

acity

(JKminus

1gminus

1)

20

15

10

5

0minus100 minus50 0 50 100

Melting

Demixing

Cp (experimental)

Cp (vibrational)

Temperature (∘C)


(c)


119901









119901

119901 (vib) and water119862119901


119901119862119901

119901 (vib)+ 119891119908119862119901

119908 (vib) where 119891119901and 119891






24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B C

2nd run





24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B

C

2nd run









24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run


1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run




4 Conclusions






References
























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus6

minus4

minus2

0

2

4

10 20 30 40 50

Temperature (∘C)

Temperature (∘C)

Heating

Mixing

Demixing

Cooling

10wt of PNIPA

(a)

Hea

t flow

(Wg

)

Total heat flow

Reversing heat flow

minus40 minus20 0 20 40 60 80

minus2

0

2

20 40

Temperature (∘C)


Mixing

Demixing

Cooling1

minus1

40wt of PNIPA-SA

(b)

Hea

t cap

acity

(JKminus

1gminus

1)

20

15

10

5

0minus100 minus50 0 50 100

Melting

Demixing

Cp (experimental)

Cp (vibrational)

Temperature (∘C)


(c)


119901









119901

119901 (vib) and water119862119901


119901119862119901

119901 (vib)+ 119891119908119862119901

119908 (vib) where 119891119901and 119891






24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B C

2nd run





24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B

C

2nd run









24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run


1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run




4 Conclusions






References
























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B C

2nd run





24

22

20

18

16

14

12

10

8

6

0 20 40 60 80 100

Tem

pera

ture

of p

hase

sepa

ratio

n


1st run

A

B

C

2nd run









24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run


1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run




4 Conclusions






References
























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




24

22

20

18

16

14

12

10

8

6

Tem

pera

ture

of p

hase

sepa

ratio

n

0 20 40 60 80 100


1st run1st run

PNIPA

PNIPA-SA

2nd run 2nd run


1st run

1st run1st run

PNIPA PNIPA-SA

PNIPA-BIS28

24

20

16

12

8Tem

pera

ture

of p

hase

sepa

ratio

n (∘

C)

0 20 40 60 80 100


2nd run2nd run

2nd run




4 Conclusions






References
























































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials

















































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article phase diagrams of smart copolymers poly(n...

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