POLYMER NETWORK-POLY(ETHYLENE GLYCOL) COMPLEXES WITH SHAPE MEMORY EFFECT

Chinese Journal of Polymer Science Vol. 21, No. 1, (2003), 29-33Chinese Journal ofPolymer ScienceC2003 Springer-VerlagPOLYMER NETWORK-POLY(ETHYLENE GLYCOL) COMPLEXES WITHSHAPE MEMORY EFFECTYi-ping Cao', Ying Guan*, Juan Du", Yu-xing Peng*C.W. Yip° and Albert S. C. Chan"I/nstitute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China"Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic Univerity,Hung Hom, Kowloon, Hong Kong, ChinaAbstract The complexes of poly(methacrylic acid-co-methyl methacrylat) network with poly(ethylene glycol) stabilized bybydrogen bonds were prepared. By introducing the poly(ethylene glycol), a large dffrence in storage modulus below andabove the glass lransition temperature occurred and the complexes exhibited shape memory behaviors. The morphology ofcomplexes was studied by using DSC, WAXD, and DMA. The results indicate that the fixed phase of this kind of novelsbape memory materials is the network, and the reversible phase is the amorphous statc of PEG:PMAA complex phase. Theshape recoverability almost reaches 100% This type of complexes can be regarded as a novel shape memory network.Key words Hydrogen bonded complex, Shape memory efct, Storage modulus ratioINTRODUCTIONPolymer complexes are macromolcular structures formed by the noncovalent association of polymers with theafinity for one another. The major classes of polymer complexes are stereocomplexes, polyelectrolytecomplexes, and hydrogen bonded complexes". Among these complexes, the hydrogen bonded poly(methacrylicacid) (PMAA) network-poly(ethylene glycol) (PEG) complexes have atracted the attention of many researchersdue to their special character over the past two decades'21. However, research work on the mechanical propertiesofPMAA-PEG cormplexes has not been presented.As is well known, polymer networks generally could not show a satisfactory shape memory behavior, since alarge dfferenrce in modulus below and above the glass transitin temperature and sharp gass-ubber transitionscan not be obtained for network. A reasonable way to produce shape memorized networks is the preparation ofnetworks containing crystallizable polymer chainsl}-6l. In our previous paper'(, we discovered the shape memoryeffects of poly(acrylic acid-co-methyl mthacylae)-ctyltrimethylammonium bromide complexes. The shapememory principle of this complex is based on a reversible ordered-disorder transition due to the formation ofcsalline aggregates among the long alkyl chains of cetytrimethylammonium in the complex. All thesenetworks can exhibit shape memory behaviors because their melting transition is sharper and the difference inmodulus below and above the melting transition temperature is large enough.We report here another type of shape memory interpolymer complexes stabilized by hydrogen bonds thoughcrystalline aggregates in this complex cannot be found. The complexes form between chemically crosslinkedcopolymer of methacrylic acid (MAA) with methyl methacrylate (MMA) (P(MAA-CO-MMA)) acting as a proton-donating polymer and poly(ethylene glycol) (PEG) acting as a proton accentor. Bv introducing the PMMAcomponent, a softening effect can be expected for the comple:中国煤化工the glass transitionYHCNMHG'Corresponding author: Yu-xing Peng, E-mail: yxpeng@cioc.ac.cn; Albert S.C. Chan, E-mail: bcachan@poiyu.edu.hkReceived February 1, 2002; Revised April 18, 2002; Accepted April 23, 200230Y.P. Cao et al.temperature Tg of the PEG:PAA cormplex is 180C)T. We have found that the complex exhibits shape memoryeffects due to a high elasticity ratio of the glass state modulus (E]) to the rubbery modulus (E) and the ratio ofrecovery almost reaches 100%， In this paper, the structure and morphology of the complex are brieflycharacterized and the mechanism of shape memory behaviors with temperature preliminarily discussed.EXPERIMENTALMaterialsMethacrylic acid (MAA), methyl methacrylate (MMA), 2,2'-azobis(isobutyronitrile) (AIBN) and N,N.methylenebis(acrylamide) (MBAA) were commercially available. MAA and MMA were distilled under reducedpressure before use. AIBN, used as a radical initiator, was recrystallized from ethanol solution. MBAA used as across-linker, and poly(ethylene glycol) (PEG, molecular weight 1000 and 4000) were used without furtherpurification.PreparationP(MAA-c0-MMA) network was prepared by radical copolymerization of 1.0 mol/L MAA with 1.0 molL MMAin the presence of 0.01molL AIBN as initiator and 0.02mol/L MBAA as crosslinker in dimethyl sulfoxide at60'C for 24 h. After polymerization, the crosslinked P(MAA-co-MMA) was immersed in a large amount ofethanol-water mixture for 1 week to remove the monomers and uncrosslinked polymers, then in water for2 weeks. The sample was divided into two parts. One represented as P(MAA-co-MMA) network was stillimmersed in water. The other part was immersed in different molecular weight PEG solutions with 6.0 wt% at25'C for 1 week, then immersed in water to remove PEG absorbed on the surface of P(MAA-co-MMA)-PEGcomplex. According to the molecular weight of PEG, the latter samples were represented as P(MAA-Co-MMA)-PEG complex 1000 and complex 4000. AlI specimens were dried under vacum at room temperature for 3 days.MeasurementsFTIR spectra were obtained with a Nicolet 200SXV FTIR spectrometer. The glass transition temperature wasmeasured using a differential scaning calorimeter (Du Pont 9900) within a temperature range of--50C to 150Cat a heating rate of 10 K/min. The dynamic mechanical analyses were carried out with a Du Pont 983 instrumentat a fixed oscillation amplitude of 0.1 mm and under nitrogen gas purging. The specimens were heated from 30Cto 150C using a heating rate of 5 K/min.The method of evaluating the shape memory effect of the shape-memory alloy was adopted to investigatethat of our specimens. The shape memory effect was examined by a bending test as follows: a flat strip specimenwas folded at 100C and cooled to keep the deformation. Then the deformed sample was heated again at a fixedheating rate of 2 K/min from 10C to140C and the change of the angle b with temperature was recorded. Theratio of the recovery was defined as 6/180.RESULTS AND DISCUSSIONVarious studies of the complexation mechanism have been carried out using pH, viscosity and potentiometricmeasurementsi. These studies show that complexation occurs due to the formation of hydrogen bonds betweenthe carboxyl protons of PMAA and the ether group of PEG, and that the hydrophobic interactions between theCH2 groups of PEG and the CH3 groups of PMAA stabilize the complex. Figure 1 is the FTIR spectra ofP(MAA-co-MMA) network before and after complexation with PEG 4000. The intensity of peak V-CH2 at2955 cm-1 of P(MAA-co MMA)-PEG complex was clearly higher than that of P(MAA co-MMA) network. Thisspectral result resembles the case of poly(acrylic acid co methyl methacrylate)-cetyltrimethylammonium bromidecomplex'可lThe vibrational frequency v=o of carboxylic acids is also known to be affected by hydrogen bonding. TheV-o of carboxylic acids usually appears at 1750 cm'l for the n中国煤化工:ric state and shifts to1700 cm"'l for the hydrogen bonded dimeric structure. On tl:Y片CN M H Garboxylic aeids in H .complex with the ether oxygen of PEG usually appears at 1130 cmn ”. inus, Ine relative intensity of thePMMA-CO-MMA)-PEG Complex with Shape Memory Effect31absorption at 1730 cm' and 1700 cm^' may be taken as the ratio of hydrogen bonds of the carboxylic acid:ethercomplex to those of the dimeric carboxylic acid structurel10].In Fig. 1, the心=o of carboxylic acids appeared at 1700 cm:' rather than 1750 cm:' for P(MAA-co MMA)network, indicating that the carboxylic acid groups may have a bydrogen bonded dimeric structure. Aftercomplexation, the absorption at 1700 cm' disappeared, and the C=0 stretching absorption was only observed at1730 cm', implying that the C=0 stretching absorption of PMAA was overlapped with the C=O stretchingabsorption of PMMA. This suggests that complexation occurs due to the formation of hydrogen bonds betweenthe PMMA carboxylic protons and the PEG ether group. The conclusion was confrmed by a new absorption at1110 cm ascribed to the C- 0- -C absorption stretching ofPEG.The thermograms of P(MAA-C0-MMA) network, P(MAA-c0-MMA)-PEG complex 1000 and the P(MAA-CO-MMA)-PEG complex 4000 are shown in Fig. 2. A glass transition temperature at 100C atributed to thePMAA segment was obtained for both the polymer network and complex 1000. Meanwhile, a melting point at15C and a crystalline point at -30C were found in complex 1000, indicating that a fair amount ofPEG 1000 isnot complexed with PMAA. This is because the chain length of PEG 1000 is insufficient for the cooperativecomplexation, However, endothermic peaks at 50C due to the melting of PEG 4000 crystallites cannot beobserved for complex 4000, and only one glass transition was exhibited at 75"C. These facts mean that thecrystaline phase of PEG 4000 is destroyed in the complex and there is only the PEG:PMAA complex phase incomplex 4000 where the segments are molecularly mixed. This result was confirmed by wide-angle X-raymeasurements (WAXD). The dynamic mechanical analysis result that will be discussed later also suggests thatthe PEG:PMAA complex phase is amorphous due to a single glass transition. We assume the complexes to behomogeneous ta a few hundred angstroms and that motional heterogeneity does not exist in the complexes.P(MAA-C0-MMA)wLWPOMAA-co-MA)Complex 10002955700Complex 4000m' PMAA.co-MMA)-PBGV1730! 1101400 3000 2000 1500 1000 500古80 120 160v(cm')T(°C)Fig. 1 FTIR spectra of P(MAA-C0-MMA) network andFig. 2 DSC thermograms of P(MAA-C0-MMA)P(MAA-co-MMA)-PEG complex 4000network, complex 1000 and complex 4000A high elasticity ratio (Eg/E), preferably a difference of two orders of magnitude, allows easy shaping at T>T, (shape memory temperature) and great resitance to deformation at T< T2. One or two of these requirementscan be satisfed easily with many of the existing polymeric materials. However, glass transitions are generally notpreferred because the transition is not sharp for polymer networks.The storage modulus E of the P(MAA-C0-MMA) network and P(MAA-C0-MMA)-PEG complex 4000 as afunction of temperature is shown in Fig. 3 (complex 1000 behaves like a soft elastomer even in the dry state andcould not provide its DMA result). A clear decrease in E for complex 4000 occurred in a temperature range of50- 90C and E values above 90"C became as low as 107 Pa which was approximately 2 orders magnitude lessthan the values below the Tg However, E values for P(MAA-c0-MMA) network gradually decrease during theglass transition and only a small difference of E[E, was discc: ratio (defined wasE，_20oc/ E'r,+20°C ) of P(MAA-c0-MMA)-PEG complex was中国煤化工c0-MMA) networkwas only 3.8. A large difference in modulus below and aboveTH.CNMHGhemoststatalproperty underlying the materials shape memory function. With a large elastic modulus ratio (E]E), high32Y.P. Cao et al.temperature deformation becomes easy while keeping the resistance to low temperature deformation great. Thus,the P(MAA-cO-MMA)-PEG complex shows shape memory effects due to this high modulus ratio. We assume thedifferent dynamic mechanical properties between P(MAA-CO-MMA) network and P(MAA-CO-MMA)-PEGcomplex could be interpreted in terms of cooperative inter-polymer hydrogen bonds which alter the dynamics andstructures of the component polymers.Shape memory materials have two phase structures, namely, the fixing phase remembers the initial shapeand the reversible phase shows a reversible soft and rigid transition with respect to temperaturel!31!. Consideringthe P(MAA-CO-MMA)-PEG complex, the fixing phase is the P(MAA-cO-MMA) network, while the reversiblephase is the PEG: PMAA complex phase.Figure 4 shows the tan&temperature curves of P(MAA-cO-MMA) network and P(MAA-CO-MMA)-PEG4000 complex. The tanδ of complex reached its maximum at 80C corresponding to the Tg of complex, while themaximum of tanδ for network appeared at 93C and is significantly lower. Since tanδ corresponds to the strainenergy disspated by viscous friction, a large tanδ implies that the material is more likely viscous than elastic..6-◆PMAA-CO-MMA)0-■P(MAA-CO MMA)-PEG0.5-外-.4-星810.3-0.2-5t0.1|◆P(MAA-Co-MMA)4080 7205040 6080100 120 140T(C)T('C)Fig.3 Temperature dependence of tensile storageFig. 4 Plot of tanδ versus temperature formodulus E for P(MAA-C0-MMA) network andP(MAA-co-MMA) network and PMAA-cO-P(MAA-c0-MMA)-PEG complex 4000MMA)-PEG complex 4000The shape memory effect of P(MAA-co-MMA)-PEG complex 4000 and P(MAA-co-MMA) network is :demonstrated in Fig. 5. The deformed complex could be recovered rapidly when it is heated again to hightemperature. The recoverability could almost reach 100%. But the network shows a slow recovery rate, and it stillhas some residual deformation that could not be recovered under the conditions of the testing process.103020一r -Nerworkr - Complex 400010 305070 90 110 130 150Fig. 5 Shape memory efct of comp)中国煤化工Figure 6 shows the shape memory phenomenon of P(MAA:TYTHC N M H Go. The spring shapeP(MMA-c0-MMA)-PEG Complex with Shape Memory Effect3sample was heated to 90C and deformed to a string shape and then cooled to room temperature. The string wasrigid and retained it shape. On heating the string again to 90"C, it became soft and returned to its original springshape. This intresting phenomenon is reversible and cyclcally reproducible by repeated temperature changes.Initial shape0[10S20s30s40s1Fig. 6 Shape memory phenomenon of P(MAA-cO-MMA)-PEG complex 4000The initial spring shape sample was heated to 90C and deformed as a string shape, then cooled to roomtemperature under constrained conditions. After withdrawal of the external force, the strip shape samplewas put in a glass container kept at a constant temperature of 90C. The photos were takcn by a digitalcamera every 2 s.CONCLUSIONThe morphology of P(MAA-CO-MMA)-PEG complex was studied by DSC, WAXD, and DMA. The resultsindicate that the fixed phase of this kind of novel shape memory materials is the network, and the reversiblephase is the amorphous state of PEG:PMAA complex phase. The complexes show shape memory properties dueto a large difference in storage modulus below and above the glass transition temperature.Acknowledgement The Hong Kong Poltechnic University is gratefully acknowledged for financial support.REFERENCES1 Lowman, AM., Cowans, B.A. and Peppas, A., J, Polym. Sci, Polym. Phys. Ed, 2000, 38: 28232 Miyoshi, T, Takegoshi, K. and Terao, T, Macromolecules, 1999, 32: 89143 Osada, Y. and Matsuda, A, Nature, 1995, 376: 2194 Kagami, Y, Giong, J.P. and Osada, Y, Macromol. 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