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Supramolecular polyurethane networks containing pyridine moieties for shape memory materials
Materials Letters 63 (2009) 1462–1464
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Supramolecular polyurethane networks containing pyridine moieties for shape memory materials Shaojun Chen, Jinlian Hu ⁎, Chun-wah Yuen, Laikuen Chan Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon Hong Kong, China
a r t i c l e
i n f o
Article history: Received 10 February 2009 Accepted 19 March 2009 Available online 25 March 2009 Keywords: Shape memory materials Polyurethane Pyridine Supramolecular
a b s t r a c t Fabricating smart materials with supramolecular switch is an attractive research topic in recent years. In this communication, shape memory polyurethane supramolecular networks are synthesized from BINA, HDI, MDI and BDO. DSC results show that amorphous soft phase and hard phase are formed through different intermolecular hydrogen bondings at the regions of pyridine units and urethane groups in the resulted BINA containing polyurethanes (BIN-PUs). Moreover, DMA results show that the BIN-PUs have higher modulus ratio (Eg / Er N 400) and much higher maximum tanδ. These results determine that the BIN-PUs exhibit excellent shape memory effect: higher shape fixity (N97%) and higher shape recovery (N 91.7%). © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental part
Shape memory materials (SMMs) are those materials that have the capability of fixing a temporary shape and recovering to its original shape by applying an external stimulus. Generally, two components are required on a molecular level for polymers exhibiting shape memory effects (SMEs), one is the net-point by either chemical crosslink or physical crosslink to determine the permanent shape; and the other is the switching segment with a suitable transition temperature (Ts) to fix the temporary shape[1]. Over the last decades, the molecular designing of shape memory polymers (SMPs) is mainly focused on the crystallization or the glass transition of soft segment. Until recently, the thermal reversible switching of supramolecular networks based on the non-covalent interactions have been introduced to control shape fixing and shape recovering in SMPs [2–4]. In addition, the hydrogen-bonded systems formed by pyridine moieties as H-acceptors are widely studied [5,6]. Supramolecular polymers and supramolecular liquid crystalline polymers can be achieved through the strong hydrogen bonding between carboxyl or phenolyl and pyridine moieties. However, until now, there is no report about SMPs based on the hydrogen bonding of pyridine moieties. In this experiment, a pyridine derivative, N,N-bis(2-hydroxylethyl) isonicotinamine (BINA), is employed to prepare polyurethane supramolecular networks. Brief communication about its supramolecular structure, thermal properties, dynamical mechanical properties and SMEs is presented in this communication.
2.1. Preparation Materials used in this study were all bought from Sigma-Aldrich Chemical Co., St. The polymerization reaction of BINA and HDI (hexamethylene diisocyanate) was carried out at 80 °C for 2 h in a 500-mL flask filled with nitrogen and equipped with a mechanical stirrer, a thermal meter. If necessary, BDO (1, 4-butanediol) and MDI (diphenylmethane diisocyanate) were added to the HDI–BINA prepolymer for another 2 h. During the reaction process, DMF was added into the reaction to control the viscosity of solution occasionally. Thereafter, the 10.0 wt.% PU/DMF solution was poured on PTFE mould for film casting at 100 °C oven for 48 h. 2.2. Characterization FT-IR spectra at various temperatures were recorded on a Nicolet 760 FT-IR spectrometer equipped a temperature-controlled chamber with thin film prepared from the 5% PU/DMF solution. DSC testing was carried out on a Perkin-Elmer DSC-7 at a scan rate of 10.0 K/min with a sample of 5–7 mg. DMA curves were determined by using a Perkin-Elmer DMA at a heating rate of 2.0 K/min in 1.0 Hz with a film of 0.5 mm thickness and 5.0 mm width. Cyclic tensile test was done in an instron 4466 apparatus with a temperature-controlled chamber with a thin film of 0.5 mm thickness, 5.0 mm width and 40 mm length; and a personal computer was used to control and record all data. 3. Results and discussion
⁎ Corresponding author. Tel.: +852 27666437; fax: +852 2775 1432. E-mail address: [email protected] (J. Hu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.028
Infrared spectroscopy is a powerful tool for the study of specific interactions in the hydrogen-bonded system; and the frequency shift
S. Chen et al. / Materials Letters 63 (2009) 1462–1464
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Fig. 1. FT-IR spectra of PUPy at various temperatures (T1 = 25 °C; T2 = 50 °C; T3 = 60 °C; T4 = 80 °C; T5 = 100 °C; T6 = 120 °C; T7 = 150 °C).
has been generally accepted as a measure of the strength of hydrogen bonding [7]. Fig. 1 presents the FT-IR spectra of BINA–HDI copolymer (coded as PUPy) at various temperatures upon heating. At ambient temperature (see T1), it can be observed that there are two typical strong absorption peaks at 3328 cm− 1 and 1706 cm− 1 which are attributed to the stretching vibration of amide (N–H) and carboxyl (CfO), respectively; and they delegate the formation of urethane groups (–NHCOO–) [5]. Additionally, it was reported that the absorption peaks resulting from the stretching vibration of the pyridine ring appear at the frequencies of 1601 cm− 1 and 998 cm− 1 [5,6]. Thus, it is found that as the temperature increases from 25 °C to 150 °C, the N–H stretch frequency shifts from 3328 cm− 1 to a higher frequency 3349 cm− 1, while the frequency of the pyridine ring at 998 cm− 1 shifts to a lower frequency 991.8 cm− 1. This frequency variation is obvious particularly at the temperature range of 25 °C– 60 °C. Thus, it is confirmed that strong hydrogen bondings are formed between the CfO and N–H of urethane groups and between the pyridine ring and urethane N–H (i.e. CfO…H–N and N–H…N respectively.). Moreover, these hydrogen bondings are sensitive to the stimulus of temperature, particularly at the temperature range of 25–60 °C.
4. Thermal properties
obvious glass transition appearing at about 98 °C, there are also a slight glass transition appears at about 45 °C, which gets clear on the second heating curves (not shown here). It implies that two phases may occur in the resulted BIN-PUs. Moreover, if the HDI–BINA precopolymer is extended by BDO to polymer PUPy–BDO, the content of BINA will be diluted; while more regular HDI–BDO segment, which was widely reported as the hard segment in polyurethane structure [8], will be congregated. As a result, micro-phase separation in PUPy– BDO is improved greatly; and the HDI–BDO segment will be developed to hard phase, even crystalline phase when HDI–BDO reaches to a critical content. As shown in Fig. 2 for sample PUPy-BDO containing 30 wt.% BINA, a clear crystal melting peak and recrystallization peak appear on the heating curve B and cooling curve b, respectively. It means that the PUPy–BDO polymer has an amorphous soft phase and crystalline hard phase. In addition, it is well known that there are not only intermolecular hydrogen bonds, but also dipole– dipole interaction and induced dipole–dipole interaction among the MDI formed hard segment [9]. If the HDI of PUPy–BDO system is replaced partly by MDI to polymer PUPy–MDI, the congregated MDI– BDO segment will reinforce the whole hard phase greatly due to its more rigid structure. Therefore, besides of the glass transition of amorphous soft segment at about 40 °C, another significant transition with a big heat enthalpy (ΔCp ) resulting from the hard phase also appear at about 95 °C on the heating curve C in Fig. 2. This implies that
Fig. 2 shows the thermal properties of BINA containing polyurethanes (BIN-PUs) after annealing at 50 °C vacuum oven for 24 h. It was observed on the curve A for sample PUPy that beside of an
Fig. 2. DSC curves of BIN-PUs (A, a-PUPy; B, b-PUPy–BDO; C, c-PUPy–MDI) for first heating scan and first cooling scan.
Fig. 3. DMA curves of BIN-Pus.
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S. Chen et al. / Materials Letters 63 (2009) 1462–1464
Fig. 4. Strain–stress curves of sample PUPy–MDI.
the phase separation is enhanced greatly; and the hard phase gets more clear or perfect. 5. Dynamical mechanical properties Fig. 3 presents the dependency of storage modulus and tanδ on temperature. It can be observed that the glassy storage modulus (Eg) in all samples reaches 4.0 GPa. It means that higher stiffness or higher modulus is achieved in this kind of polyurethanes. As the temperature increases to above 45 °C, significant decrease in modulus is observed in PUPy sample, e.g. only 9.0 MPa at 65 °C. Comparatively, the rubber storage modulus (Er) in the PUPy–BDO and PUPy–MDI sample is higher, whereas their modulus ratio (Eg/Er) is still higher than 400. Thus, higher shape fixity is expected in the three samples. Additionally, it is also observed that higher energy loss exists in this system at glass-transition temperature range, e.g. the maximum tanδ of sample PUPy is higher than 2.2. However, as the BINA content decreases by the addition of BDO or MDI, the maximum tanδ decreases to 1.5 and 0.8 in sample PUPy–BDO and sample PUPy–MDI, respectively. This implies that hydrogen bonding affects the damping properties of polymer. That is, more number of hydrogen bonding at the pyridine ring, more energy loss at the glass-transition temperature range. The reason is that strong hydrogen bondings are formed among the urethane groups as well as between pyridine group and NH of urethane group as discussed above. The association of hydrogen bonding at lower temperature results in higher Eg; while the dissociation of hydrogen bonding at higher temperature results in the lower Er. 6. Thermal-induced SMEs The phase separation structure and higher modulus ratio (Eg/Er) described above suggest that the resulted BIN-PUs can be used as
SMMs [1]. In this experiment, rectangle specimens (5.0 mm × 40 mm) cut from the films of PUPy, PUPy–BDO and PUPy–MDI were tested by cyclic tensile testing. Results show that all the three samples show the typical thermal-induced SMEs. Fig. 4 presents the typical strain–stress curves of sample PUPy–MDI. It is observed that original length can be elongated easily to a second length (e.g. 100% elongation) since the polymer is very soft after it is heated to above 80 °C; and the second length is fixed under the load due to the higher glassy storage modulus after it cools down to ambient temperature. When the temperature increases to 80 °C, it is observed that the fixed second shape recovers to its original shape again. From the fixed strain and recovered strain, shape fixity and shape recovery of polymer can be calculated. Comparatively, the PUPy sample shows very good shape fixity (N99%); while the shape fixities in PUPy–BDO and PUPy–MDI sample decrease to about 97%. For the shape recovery, as the cyclic time increases, the shape recovery decreases gradually. Whereas in the first cycle, more than 91.7% shape recovery can be achieved in all samples, e.g. 92.7% for PUPy, 91.7% for PUPy–BDO and 97.2% for PUPy– MDI. Therefore, it is believed that the BINA containing polyurethanes can be used as excellent SMMs. 7. Conclusions In this communication, novel SMPU supramolecular networks were synthesized from the pyridine derivative and diisocyanate. DSC results show that the BIN-PUs supramolecular networks have microphase separation structure formed through different hydrogen bondings at the region of pyridine ring and urethane group; and DMA results show that they have much higher maximum tanδ at the glass transition temperature range; and higher modulus ratio (Eg/Er) due to their higher Eg. Finally, the higher shape fixity (N97%) and shape recovery (N91.7%) obtained by cyclic tensile testing support that the polyurethane supramolecular networks containing pyridine moieties can be used as good SMMs. Acknowledgement This work was financially supported from the PhD studentship of Hong Kong Polytechnic University. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Behl M, Lendlein A. Materials Today 2007;10:20. Liu G, Ding X, Cao Y, Zhang Z, Peng Y. Macromol Rapid Commun 2005;26:649. Li J, Viveros JA, Wrue MH, Anthamatten M. Adv Mater 2007;19:2851. Zhu Y, Hu JL, Liu YJ. Eur Phys J, E 2009;28:3. Wan L, Kan D, Lu Y, Yu X. Polym Mater Sci Eng 2004;20:9. Ambrozic G, Mavri J, zigon M. Macromol Chem Phys 2002;203:439. Brunette CM, Hsu SL, Macknight WJ. Macromolecules 1982;15:71. Li Y, Ren Z, Zhao M, Yang H, Chu B. Macromolecules 1993;26:612. Lee BS, Chun BC, Chung YC, Sul K, Cho JW. Macromolecules 2001;34:6431.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Supramolecular polyurethane networks containing pyridine moieties for shape memory materials Shaojun Chen, Jinlian Hu ⁎, Chun-wah Yuen, Laikuen Chan Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Kowloon Hong Kong, China
a r t i c l e
i n f o
Article history: Received 10 February 2009 Accepted 19 March 2009 Available online 25 March 2009 Keywords: Shape memory materials Polyurethane Pyridine Supramolecular
a b s t r a c t Fabricating smart materials with supramolecular switch is an attractive research topic in recent years. In this communication, shape memory polyurethane supramolecular networks are synthesized from BINA, HDI, MDI and BDO. DSC results show that amorphous soft phase and hard phase are formed through different intermolecular hydrogen bondings at the regions of pyridine units and urethane groups in the resulted BINA containing polyurethanes (BIN-PUs). Moreover, DMA results show that the BIN-PUs have higher modulus ratio (Eg / Er N 400) and much higher maximum tanδ. These results determine that the BIN-PUs exhibit excellent shape memory effect: higher shape fixity (N97%) and higher shape recovery (N 91.7%). © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental part
Shape memory materials (SMMs) are those materials that have the capability of fixing a temporary shape and recovering to its original shape by applying an external stimulus. Generally, two components are required on a molecular level for polymers exhibiting shape memory effects (SMEs), one is the net-point by either chemical crosslink or physical crosslink to determine the permanent shape; and the other is the switching segment with a suitable transition temperature (Ts) to fix the temporary shape[1]. Over the last decades, the molecular designing of shape memory polymers (SMPs) is mainly focused on the crystallization or the glass transition of soft segment. Until recently, the thermal reversible switching of supramolecular networks based on the non-covalent interactions have been introduced to control shape fixing and shape recovering in SMPs [2–4]. In addition, the hydrogen-bonded systems formed by pyridine moieties as H-acceptors are widely studied [5,6]. Supramolecular polymers and supramolecular liquid crystalline polymers can be achieved through the strong hydrogen bonding between carboxyl or phenolyl and pyridine moieties. However, until now, there is no report about SMPs based on the hydrogen bonding of pyridine moieties. In this experiment, a pyridine derivative, N,N-bis(2-hydroxylethyl) isonicotinamine (BINA), is employed to prepare polyurethane supramolecular networks. Brief communication about its supramolecular structure, thermal properties, dynamical mechanical properties and SMEs is presented in this communication.
2.1. Preparation Materials used in this study were all bought from Sigma-Aldrich Chemical Co., St. The polymerization reaction of BINA and HDI (hexamethylene diisocyanate) was carried out at 80 °C for 2 h in a 500-mL flask filled with nitrogen and equipped with a mechanical stirrer, a thermal meter. If necessary, BDO (1, 4-butanediol) and MDI (diphenylmethane diisocyanate) were added to the HDI–BINA prepolymer for another 2 h. During the reaction process, DMF was added into the reaction to control the viscosity of solution occasionally. Thereafter, the 10.0 wt.% PU/DMF solution was poured on PTFE mould for film casting at 100 °C oven for 48 h. 2.2. Characterization FT-IR spectra at various temperatures were recorded on a Nicolet 760 FT-IR spectrometer equipped a temperature-controlled chamber with thin film prepared from the 5% PU/DMF solution. DSC testing was carried out on a Perkin-Elmer DSC-7 at a scan rate of 10.0 K/min with a sample of 5–7 mg. DMA curves were determined by using a Perkin-Elmer DMA at a heating rate of 2.0 K/min in 1.0 Hz with a film of 0.5 mm thickness and 5.0 mm width. Cyclic tensile test was done in an instron 4466 apparatus with a temperature-controlled chamber with a thin film of 0.5 mm thickness, 5.0 mm width and 40 mm length; and a personal computer was used to control and record all data. 3. Results and discussion
⁎ Corresponding author. Tel.: +852 27666437; fax: +852 2775 1432. E-mail address: [email protected] (J. Hu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.028
Infrared spectroscopy is a powerful tool for the study of specific interactions in the hydrogen-bonded system; and the frequency shift
S. Chen et al. / Materials Letters 63 (2009) 1462–1464
1463
Fig. 1. FT-IR spectra of PUPy at various temperatures (T1 = 25 °C; T2 = 50 °C; T3 = 60 °C; T4 = 80 °C; T5 = 100 °C; T6 = 120 °C; T7 = 150 °C).
has been generally accepted as a measure of the strength of hydrogen bonding [7]. Fig. 1 presents the FT-IR spectra of BINA–HDI copolymer (coded as PUPy) at various temperatures upon heating. At ambient temperature (see T1), it can be observed that there are two typical strong absorption peaks at 3328 cm− 1 and 1706 cm− 1 which are attributed to the stretching vibration of amide (N–H) and carboxyl (CfO), respectively; and they delegate the formation of urethane groups (–NHCOO–) [5]. Additionally, it was reported that the absorption peaks resulting from the stretching vibration of the pyridine ring appear at the frequencies of 1601 cm− 1 and 998 cm− 1 [5,6]. Thus, it is found that as the temperature increases from 25 °C to 150 °C, the N–H stretch frequency shifts from 3328 cm− 1 to a higher frequency 3349 cm− 1, while the frequency of the pyridine ring at 998 cm− 1 shifts to a lower frequency 991.8 cm− 1. This frequency variation is obvious particularly at the temperature range of 25 °C– 60 °C. Thus, it is confirmed that strong hydrogen bondings are formed between the CfO and N–H of urethane groups and between the pyridine ring and urethane N–H (i.e. CfO…H–N and N–H…N respectively.). Moreover, these hydrogen bondings are sensitive to the stimulus of temperature, particularly at the temperature range of 25–60 °C.
4. Thermal properties
obvious glass transition appearing at about 98 °C, there are also a slight glass transition appears at about 45 °C, which gets clear on the second heating curves (not shown here). It implies that two phases may occur in the resulted BIN-PUs. Moreover, if the HDI–BINA precopolymer is extended by BDO to polymer PUPy–BDO, the content of BINA will be diluted; while more regular HDI–BDO segment, which was widely reported as the hard segment in polyurethane structure [8], will be congregated. As a result, micro-phase separation in PUPy– BDO is improved greatly; and the HDI–BDO segment will be developed to hard phase, even crystalline phase when HDI–BDO reaches to a critical content. As shown in Fig. 2 for sample PUPy-BDO containing 30 wt.% BINA, a clear crystal melting peak and recrystallization peak appear on the heating curve B and cooling curve b, respectively. It means that the PUPy–BDO polymer has an amorphous soft phase and crystalline hard phase. In addition, it is well known that there are not only intermolecular hydrogen bonds, but also dipole– dipole interaction and induced dipole–dipole interaction among the MDI formed hard segment [9]. If the HDI of PUPy–BDO system is replaced partly by MDI to polymer PUPy–MDI, the congregated MDI– BDO segment will reinforce the whole hard phase greatly due to its more rigid structure. Therefore, besides of the glass transition of amorphous soft segment at about 40 °C, another significant transition with a big heat enthalpy (ΔCp ) resulting from the hard phase also appear at about 95 °C on the heating curve C in Fig. 2. This implies that
Fig. 2 shows the thermal properties of BINA containing polyurethanes (BIN-PUs) after annealing at 50 °C vacuum oven for 24 h. It was observed on the curve A for sample PUPy that beside of an
Fig. 2. DSC curves of BIN-PUs (A, a-PUPy; B, b-PUPy–BDO; C, c-PUPy–MDI) for first heating scan and first cooling scan.
Fig. 3. DMA curves of BIN-Pus.
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S. Chen et al. / Materials Letters 63 (2009) 1462–1464
Fig. 4. Strain–stress curves of sample PUPy–MDI.
the phase separation is enhanced greatly; and the hard phase gets more clear or perfect. 5. Dynamical mechanical properties Fig. 3 presents the dependency of storage modulus and tanδ on temperature. It can be observed that the glassy storage modulus (Eg) in all samples reaches 4.0 GPa. It means that higher stiffness or higher modulus is achieved in this kind of polyurethanes. As the temperature increases to above 45 °C, significant decrease in modulus is observed in PUPy sample, e.g. only 9.0 MPa at 65 °C. Comparatively, the rubber storage modulus (Er) in the PUPy–BDO and PUPy–MDI sample is higher, whereas their modulus ratio (Eg/Er) is still higher than 400. Thus, higher shape fixity is expected in the three samples. Additionally, it is also observed that higher energy loss exists in this system at glass-transition temperature range, e.g. the maximum tanδ of sample PUPy is higher than 2.2. However, as the BINA content decreases by the addition of BDO or MDI, the maximum tanδ decreases to 1.5 and 0.8 in sample PUPy–BDO and sample PUPy–MDI, respectively. This implies that hydrogen bonding affects the damping properties of polymer. That is, more number of hydrogen bonding at the pyridine ring, more energy loss at the glass-transition temperature range. The reason is that strong hydrogen bondings are formed among the urethane groups as well as between pyridine group and NH of urethane group as discussed above. The association of hydrogen bonding at lower temperature results in higher Eg; while the dissociation of hydrogen bonding at higher temperature results in the lower Er. 6. Thermal-induced SMEs The phase separation structure and higher modulus ratio (Eg/Er) described above suggest that the resulted BIN-PUs can be used as
SMMs [1]. In this experiment, rectangle specimens (5.0 mm × 40 mm) cut from the films of PUPy, PUPy–BDO and PUPy–MDI were tested by cyclic tensile testing. Results show that all the three samples show the typical thermal-induced SMEs. Fig. 4 presents the typical strain–stress curves of sample PUPy–MDI. It is observed that original length can be elongated easily to a second length (e.g. 100% elongation) since the polymer is very soft after it is heated to above 80 °C; and the second length is fixed under the load due to the higher glassy storage modulus after it cools down to ambient temperature. When the temperature increases to 80 °C, it is observed that the fixed second shape recovers to its original shape again. From the fixed strain and recovered strain, shape fixity and shape recovery of polymer can be calculated. Comparatively, the PUPy sample shows very good shape fixity (N99%); while the shape fixities in PUPy–BDO and PUPy–MDI sample decrease to about 97%. For the shape recovery, as the cyclic time increases, the shape recovery decreases gradually. Whereas in the first cycle, more than 91.7% shape recovery can be achieved in all samples, e.g. 92.7% for PUPy, 91.7% for PUPy–BDO and 97.2% for PUPy– MDI. Therefore, it is believed that the BINA containing polyurethanes can be used as excellent SMMs. 7. Conclusions In this communication, novel SMPU supramolecular networks were synthesized from the pyridine derivative and diisocyanate. DSC results show that the BIN-PUs supramolecular networks have microphase separation structure formed through different hydrogen bondings at the region of pyridine ring and urethane group; and DMA results show that they have much higher maximum tanδ at the glass transition temperature range; and higher modulus ratio (Eg/Er) due to their higher Eg. Finally, the higher shape fixity (N97%) and shape recovery (N91.7%) obtained by cyclic tensile testing support that the polyurethane supramolecular networks containing pyridine moieties can be used as good SMMs. Acknowledgement This work was financially supported from the PhD studentship of Hong Kong Polytechnic University. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Behl M, Lendlein A. Materials Today 2007;10:20. Liu G, Ding X, Cao Y, Zhang Z, Peng Y. Macromol Rapid Commun 2005;26:649. Li J, Viveros JA, Wrue MH, Anthamatten M. Adv Mater 2007;19:2851. Zhu Y, Hu JL, Liu YJ. Eur Phys J, E 2009;28:3. Wan L, Kan D, Lu Y, Yu X. Polym Mater Sci Eng 2004;20:9. Ambrozic G, Mavri J, zigon M. Macromol Chem Phys 2002;203:439. Brunette CM, Hsu SL, Macknight WJ. Macromolecules 1982;15:71. Li Y, Ren Z, Zhao M, Yang H, Chu B. Macromolecules 1993;26:612. Lee BS, Chun BC, Chung YC, Sul K, Cho JW. Macromolecules 2001;34:6431.