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Surface organization of single hyperbranched polymer molecules, as studied by atomic force microscopy
Materials Science and Engineering C 15 Ž2001. 311–314 www.elsevier.comrlocatermsec
Surface organization of single hyperbranched polymer molecules, as studied by atomic force microscopy P. Viville a,) , A. Deffieux b, M. Schappacher b, J.L. Bredas ´ a, R. Lazzaroni a a
SerÕice de Chimie des Materiaux NouÕeaux, Centre de Recherche en Sciences des Materiaux Polymeres ´ ´ ` (CRESMAP), UniÕersite´ de Mons-Hainaut 20, Place du Parc, 7000 Mons, Belgium b Laboratoire de Chimie des Polymeres ` Organiques, UMR ENSCPB-CNRS 5629, UniÕersite´ de Bordeaux 1, AÕenue Pey Berland BP108, 33405 Talence, France
Abstract The recent emergence of hyperbranched polymers has opened the door to the design of a large variety of new chain architectures that promise to be strong competitors for dendrimers in a variety of potential applications. For instance, «comb-like» polymers can be obtained from polyŽchloroethyl vinyl ether.-g-polystyrene ŽPCEVE-g-PS. copolymers, with excellent control over the dimensions of the polystyrene branches and the PCEVE backbone. In this work, the nanometer scale organization of these materials is studied by means of Atomic Force Microscopy ŽAFM.. We focus on the influence of the intrinsic molecular architecture of the hyperbranched PCEVE-g-PS on the organization of the material. Several parameters such as sample preparation conditions Žthe nature of the solvent and the substrate, the solution concentration. and annealing of the samples are also taken into account. In the case of very thin deposits, we observe a layer-by-layer organization Žthe height of one monolayer corresponding to ; 7 nm.. On the free surface, it is possible to image single polymer molecules and to analyze their size in terms of the polymer molecular weight. In thicker deposits, the molecules are found to adopt an extended conformation and to form lamellar arrangements. q 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM; Self-assembled molecules; Hyperbranched polymers
1. Introduction Polymer chemistry has long focussed on linear macromolecules, which occasionally contain some smaller or longer branches. In recent years, highly branched Ždendritic. polymers have gained widespread attention due to their unique properties, which differ significantly from their linear counterparts, e.g. the large number of terminal functional groups, the intrinsic globular structure Žleading to specific rheological properties., and the possibility to accommodate guest molecules within the macromolecule. Compared to dendrimers, hyperbranched macromolecules are characterized by a lower degree of branching, but still possess a non-linear architecture and a high number of reactive end groups. Moreover, they can be prepared much more rapidly and economically than dendrimers and are
)
Corresponding author. Tel.: q32-65-373-868; fax: q32-65-373-861. E-mail address: [email protected] ŽP. Viville..
therefore considered as very good candidates to replace dendrimers in a number of applications w1–3x. This paper deals with the characterization of the molecular organization of new hyperbranched polyŽchloroethyl vinyl ether-g-styrene. ŽPCEVE-g-PS. copolymers in thin solid films. The dimensions of the polystyrene branches and the polyŽchloroethyl vinyl ether. backbone can be controlled so that both the length and the number of branches of the graft copolymers can be tuned w4,5x. As a consequence, different molecular chain architectures can be generated, which are expected to arrange into different supramolecular assemblies. We characterize the surface molecular organization of PCEVE-g-PS thin films by means of Atomic Force Microscopy ŽAFM.. AFM allows to accurately probe the topography of the surface with very high lateral resolution and is thus an ideal technique to reveal the morphology on very small length scales. Special emphasis is given to the influence of the molecular architecture of these macromolecules on their surface organization. The influence of other parameters, such as sample preparation conditions, e.g. nature of the substrate and annealing of the samples, are also considered.
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 3 4 - X
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P. ViÕille et al.r Materials Science and Engineering C 15 (2001) 311–314
2. Experimental section PCEVE-g-PS copolymers are synthesized by living polymerization via a grafting reaction of polystryryl lithium segments onto polyŽchloroethyl vinyl ether. chains. The selectivity of the coupling reaction is very high and allows for almost complete substitution of the chloride group of the initial CEVE units by PS chains, thus allowing the synthesis of comb-like polymers w4,5x. The two molecular architectures investigated in this work are:
Samples for AFM analysis are prepared by solvent casting at ambient conditions starting from solutions in tetrahydrofuran ŽTHF.. Typically, 50 ml of a dilute solution Žfrom 0.01 up to 0.1 wt.%. is cast on a 1 = 1-cm2 surface of a freshly cleaved mica substrate. The samples are analyzed after complete evaporation of the solvent at room temperature. All AFM images are recorded in air with a Nanoscope IIIa microscope ŽDigital Instruments, Santa Barbara, California. operated in Tapping Mode ŽTM.. TM-AFM is known to minimize the sample distortion, which may be caused by permanent mechanical interactions between the AFM tip and the surface. It thus appears to be particularly convenient for studies of soft materials such as polymers w6,7x. Both the topography and the phase signal are recorded. The probes are commercially available silicon tips with a spring constant of 24–52 Nrm and a resonance frequency lying in the 264–339 kHz range. Images are recorded with the highest sampling resolution, i.e. 512 = 512 data points.
3. Results and discussion Fig. 1 shows two 750 = 750 nm2 TM-AFM phase images of a Ž800r83. and a Ž800r83r50r40. PCEVE-gPS copolymer prepared from a 0.1 wt.% solution in THF, respectively. For the two copolymers, we observe a welldefined morphology on the nanometer scale, made of long lamellae assembled parallel to each other along with re-
gions where the lamellae fold Žfor instance, the bright feature marked by an arrow in the left image.. Such an ordered structure can be attributed to the intrinsic Acomb-likeB molecular architecture of the PCEVE-g-PS copolymers that forces the macromolecules to adopt a conformation of wormlike cylindrical brushes Žscheme in Fig. 2, left.. This conformation is caused by the sterical overcrowding due to the numerous grafted seg-
Fig. 1. TM-AFM phase images Ž750 = 750 nm2 . of the surface morphology of a Ž800r83. PCEVE-g-PS copolymer Žleft. and a Ž800r83r50r40. PCEVE-g-PS copolymer Žright.. The gray scale is 88.
P. ViÕille et al.r Materials Science and Engineering C 15 (2001) 311–314
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Fig. 2. Schematic view of the cylindrical brush conformation. The inset shows a schematic representation of the more complex molecular architecture of the Ž800r83r50r40. copolymer.
ments that increase the chain stiffness of the PCEVE backbone. Consequently, the conformation of a comb-like polymer changes from that of a statistical coil toward a wormlike chain. The intermolecular ordering observed on such flat layers is controlled by the repulsion of the grafted lateral segments. This observation is in good agreement with recent publications that report similar arrangement of grafted macromolecules w8,9x. Under these conditions, the interdistance between two neighboring lamellae in the images of Fig. 1 corresponds to the distance between two neighboring PCEVE chains. A dramatic increase in the lamellar thickness is observed when the structural complexity of the lateral side chain is modified from that of a first generation Ž800r83. to a second generation Ž800r83r50r40. grafted copolymer. Indeed, in this last case, each lateral segment consists of a «comb-like» structure, bound to a common backbone Žsee inset in Fig. 2.. The higher steric hindrance of these longer and bulkier side chains causes the average lamellar thickness to increase from 13 nm for the Ž800r83. copolymer up to 26 nm for the Ž800r83r50r40. copolymer. Considering Žas an extreme case. that a fully extended PCEVE backbone with 800 monomer units yields a value of ; 150 nm for the chain contour length, we conclude that the
objects appearing within the lamellae in the two images of Fig. 1 are indeed single molecules. With the aim of determining the surface organization of isolated molecules, we then investigated the surface morphology of copolymer samples prepared from more dilute solutions Ž0.01 wt.%.. However, under the conditions we used, the solution dewets from the mica substrate during the drying process and heterogeneous deposits are formed ŽFig. 3, left.. It is interesting to notice that the large scale dewetting process leads to the formation of layered deposits. The gray areas in Fig. 3 Žleft. correspond to a perfectly flat, 7-nm thick, layer while the brightest zones are two-layer deposits. Within the monolayer ŽFig. 3, right., the chains are densely packed in the typical cylindrical brush conformation. In contrast, the areas located between the deposits reveal the presence of very small isolated structures that seem to have been Aleft behindB on the mica during the dewetting process. When zooming-in on such an area ŽFig. 4, left., it clearly appears that all these small objects are similar in size and shape: they are round-shaped, with what appears to be a AholeB in the middle. We have checked that the appearance of this «doughnut-like» shape is independent of the parameters used for the imaging, which
Fig. 3. Left: TM-AFM topographic image Ž25 = 25 mm2 . of the multilayer organization for a PCEVE-g-PS Ž800r83. copolymer prepared from a 0.01 wt.% solution. Right: TM-AFMq phase image Ž1.5 = 1.5 mm2 . of one monolayer.
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Fig. 4. Left: TM-AFMq phase image Ž500 = 500 nm2 . of single Ž800r83. PCEVE-g-PS copolymer molecules. Right: Statistical analysis of the molecule diameter distribution.
indicates that it represents the true structure of the object. It is worth noting here that the height of these objects corresponds to the height of one monolayer, i.e. ; 7 nm. The diameter distribution of these Adoughnut-likeB objects is statistically determined for a set of different AFM images. Fig. 4 shows the Gaussian distribution obtained by measuring the diameters of 220 different objects, observed on two separate samples prepared in the same conditions. The average diameter 38.8 " 2.9 nm is found to be in very good agreement with the hydrodynamic radius ŽRh. value of the copolymer single molecules, experimentally determined in solution by Light Scattering. Therefore, one can reasonably conclude that the features shown in Fig. 4 are isolated molecules. The Adoughnut-likeB morphology is believed to be the result of Ži. the weak affinity of the PCEVE-g-PS copolymer molecules towards a hydrophilic surface such as mica and Žii. the absence of neighboring polymer molecules. This would force the isolated molecules to coil to a certain extent. However, due to the rigidity of the backbone, the chain cannot fully collapse and a void remains in the center. Assuming such a conformation, the contour length of the PCEVE backbone giving rise to the measured diameter is around 130 nm, which is in reasonable agreement with the expected length of the elongated backbone.
4. Conclusions The organization of hyperbranched polyŽchloroethyl vinyl ether-g-styrene. copolymers in thin films is charac-
terized by means of Atomic Force Microscopy. We observe that the degree of lateral ordering of the chains depends on their intrinsic molecular architecture. In particular, the structural complexity of the lateral side chains strongly affects the surface morphology. In submonolayer deposits, AFM images show very small objects whose size is coherent with that of single copolymer molecules. The Adoughnut-likeB morphology of these individual molecules probably originates from partial coiling of the PCEVE backbone because of the weak affinity between the PS lateral chains and mica. In order to verify this behavior, the molecular organization of other substrates, such as graphite, is currently investigated.
References w1x Y.H. Kim, J. Polym. Sci., Part A: Polym. Chem. 36 Ž1998. 1685. w2x M. Freemantle, Chem. Eng. News 6 Ž1999. 37. w3x A. Sunder, R. Mulhaupt, R. Haag, H. Frey, Adv. Mater. 12 Ž2000. ¨ 235. w4x A. Deffieux, M. Schappacher, Macromol. Symp. 45 Ž1998. 132. w5x A. Deffieux, M. Schappacher, Macromolecules 32 Ž1999. 1797. w6x Q. Zhong, D. Inniss, K. Kjoller, V.B. Elings, Surf. Sci. Lett. L 688 Ž1993. 290. w7x Ph. Leclere, J.M. Yu, Ph. Dubois, R. ` R. Lazzaroni, J.L. Bredas, ´ Jerome, Langmuir 12 Ž1996. 4317. ´ˆ w8x S.S. Sheiko, M. Gerle, K. Fisher, M. Schmidt, M. Moller, Langmuir ¨ 13 Ž1997. 5368. w9x M. Gerle, K. Fischer, S. Roos, A.H.E. Muller, M. Schmidt, S.S. ¨ Sheiko, S. Prokhorova, M. Moller, Macromolecules 32 Ž1999. 2629. ¨
Surface organization of single hyperbranched polymer molecules, as studied by atomic force microscopy P. Viville a,) , A. Deffieux b, M. Schappacher b, J.L. Bredas ´ a, R. Lazzaroni a a
SerÕice de Chimie des Materiaux NouÕeaux, Centre de Recherche en Sciences des Materiaux Polymeres ´ ´ ` (CRESMAP), UniÕersite´ de Mons-Hainaut 20, Place du Parc, 7000 Mons, Belgium b Laboratoire de Chimie des Polymeres ` Organiques, UMR ENSCPB-CNRS 5629, UniÕersite´ de Bordeaux 1, AÕenue Pey Berland BP108, 33405 Talence, France
Abstract The recent emergence of hyperbranched polymers has opened the door to the design of a large variety of new chain architectures that promise to be strong competitors for dendrimers in a variety of potential applications. For instance, «comb-like» polymers can be obtained from polyŽchloroethyl vinyl ether.-g-polystyrene ŽPCEVE-g-PS. copolymers, with excellent control over the dimensions of the polystyrene branches and the PCEVE backbone. In this work, the nanometer scale organization of these materials is studied by means of Atomic Force Microscopy ŽAFM.. We focus on the influence of the intrinsic molecular architecture of the hyperbranched PCEVE-g-PS on the organization of the material. Several parameters such as sample preparation conditions Žthe nature of the solvent and the substrate, the solution concentration. and annealing of the samples are also taken into account. In the case of very thin deposits, we observe a layer-by-layer organization Žthe height of one monolayer corresponding to ; 7 nm.. On the free surface, it is possible to image single polymer molecules and to analyze their size in terms of the polymer molecular weight. In thicker deposits, the molecules are found to adopt an extended conformation and to form lamellar arrangements. q 2001 Elsevier Science B.V. All rights reserved. Keywords: AFM; Self-assembled molecules; Hyperbranched polymers
1. Introduction Polymer chemistry has long focussed on linear macromolecules, which occasionally contain some smaller or longer branches. In recent years, highly branched Ždendritic. polymers have gained widespread attention due to their unique properties, which differ significantly from their linear counterparts, e.g. the large number of terminal functional groups, the intrinsic globular structure Žleading to specific rheological properties., and the possibility to accommodate guest molecules within the macromolecule. Compared to dendrimers, hyperbranched macromolecules are characterized by a lower degree of branching, but still possess a non-linear architecture and a high number of reactive end groups. Moreover, they can be prepared much more rapidly and economically than dendrimers and are
)
Corresponding author. Tel.: q32-65-373-868; fax: q32-65-373-861. E-mail address: [email protected] ŽP. Viville..
therefore considered as very good candidates to replace dendrimers in a number of applications w1–3x. This paper deals with the characterization of the molecular organization of new hyperbranched polyŽchloroethyl vinyl ether-g-styrene. ŽPCEVE-g-PS. copolymers in thin solid films. The dimensions of the polystyrene branches and the polyŽchloroethyl vinyl ether. backbone can be controlled so that both the length and the number of branches of the graft copolymers can be tuned w4,5x. As a consequence, different molecular chain architectures can be generated, which are expected to arrange into different supramolecular assemblies. We characterize the surface molecular organization of PCEVE-g-PS thin films by means of Atomic Force Microscopy ŽAFM.. AFM allows to accurately probe the topography of the surface with very high lateral resolution and is thus an ideal technique to reveal the morphology on very small length scales. Special emphasis is given to the influence of the molecular architecture of these macromolecules on their surface organization. The influence of other parameters, such as sample preparation conditions, e.g. nature of the substrate and annealing of the samples, are also considered.
0928-4931r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 2 3 4 - X
312
P. ViÕille et al.r Materials Science and Engineering C 15 (2001) 311–314
2. Experimental section PCEVE-g-PS copolymers are synthesized by living polymerization via a grafting reaction of polystryryl lithium segments onto polyŽchloroethyl vinyl ether. chains. The selectivity of the coupling reaction is very high and allows for almost complete substitution of the chloride group of the initial CEVE units by PS chains, thus allowing the synthesis of comb-like polymers w4,5x. The two molecular architectures investigated in this work are:
Samples for AFM analysis are prepared by solvent casting at ambient conditions starting from solutions in tetrahydrofuran ŽTHF.. Typically, 50 ml of a dilute solution Žfrom 0.01 up to 0.1 wt.%. is cast on a 1 = 1-cm2 surface of a freshly cleaved mica substrate. The samples are analyzed after complete evaporation of the solvent at room temperature. All AFM images are recorded in air with a Nanoscope IIIa microscope ŽDigital Instruments, Santa Barbara, California. operated in Tapping Mode ŽTM.. TM-AFM is known to minimize the sample distortion, which may be caused by permanent mechanical interactions between the AFM tip and the surface. It thus appears to be particularly convenient for studies of soft materials such as polymers w6,7x. Both the topography and the phase signal are recorded. The probes are commercially available silicon tips with a spring constant of 24–52 Nrm and a resonance frequency lying in the 264–339 kHz range. Images are recorded with the highest sampling resolution, i.e. 512 = 512 data points.
3. Results and discussion Fig. 1 shows two 750 = 750 nm2 TM-AFM phase images of a Ž800r83. and a Ž800r83r50r40. PCEVE-gPS copolymer prepared from a 0.1 wt.% solution in THF, respectively. For the two copolymers, we observe a welldefined morphology on the nanometer scale, made of long lamellae assembled parallel to each other along with re-
gions where the lamellae fold Žfor instance, the bright feature marked by an arrow in the left image.. Such an ordered structure can be attributed to the intrinsic Acomb-likeB molecular architecture of the PCEVE-g-PS copolymers that forces the macromolecules to adopt a conformation of wormlike cylindrical brushes Žscheme in Fig. 2, left.. This conformation is caused by the sterical overcrowding due to the numerous grafted seg-
Fig. 1. TM-AFM phase images Ž750 = 750 nm2 . of the surface morphology of a Ž800r83. PCEVE-g-PS copolymer Žleft. and a Ž800r83r50r40. PCEVE-g-PS copolymer Žright.. The gray scale is 88.
P. ViÕille et al.r Materials Science and Engineering C 15 (2001) 311–314
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Fig. 2. Schematic view of the cylindrical brush conformation. The inset shows a schematic representation of the more complex molecular architecture of the Ž800r83r50r40. copolymer.
ments that increase the chain stiffness of the PCEVE backbone. Consequently, the conformation of a comb-like polymer changes from that of a statistical coil toward a wormlike chain. The intermolecular ordering observed on such flat layers is controlled by the repulsion of the grafted lateral segments. This observation is in good agreement with recent publications that report similar arrangement of grafted macromolecules w8,9x. Under these conditions, the interdistance between two neighboring lamellae in the images of Fig. 1 corresponds to the distance between two neighboring PCEVE chains. A dramatic increase in the lamellar thickness is observed when the structural complexity of the lateral side chain is modified from that of a first generation Ž800r83. to a second generation Ž800r83r50r40. grafted copolymer. Indeed, in this last case, each lateral segment consists of a «comb-like» structure, bound to a common backbone Žsee inset in Fig. 2.. The higher steric hindrance of these longer and bulkier side chains causes the average lamellar thickness to increase from 13 nm for the Ž800r83. copolymer up to 26 nm for the Ž800r83r50r40. copolymer. Considering Žas an extreme case. that a fully extended PCEVE backbone with 800 monomer units yields a value of ; 150 nm for the chain contour length, we conclude that the
objects appearing within the lamellae in the two images of Fig. 1 are indeed single molecules. With the aim of determining the surface organization of isolated molecules, we then investigated the surface morphology of copolymer samples prepared from more dilute solutions Ž0.01 wt.%.. However, under the conditions we used, the solution dewets from the mica substrate during the drying process and heterogeneous deposits are formed ŽFig. 3, left.. It is interesting to notice that the large scale dewetting process leads to the formation of layered deposits. The gray areas in Fig. 3 Žleft. correspond to a perfectly flat, 7-nm thick, layer while the brightest zones are two-layer deposits. Within the monolayer ŽFig. 3, right., the chains are densely packed in the typical cylindrical brush conformation. In contrast, the areas located between the deposits reveal the presence of very small isolated structures that seem to have been Aleft behindB on the mica during the dewetting process. When zooming-in on such an area ŽFig. 4, left., it clearly appears that all these small objects are similar in size and shape: they are round-shaped, with what appears to be a AholeB in the middle. We have checked that the appearance of this «doughnut-like» shape is independent of the parameters used for the imaging, which
Fig. 3. Left: TM-AFM topographic image Ž25 = 25 mm2 . of the multilayer organization for a PCEVE-g-PS Ž800r83. copolymer prepared from a 0.01 wt.% solution. Right: TM-AFMq phase image Ž1.5 = 1.5 mm2 . of one monolayer.
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P. ViÕille et al.r Materials Science and Engineering C 15 (2001) 311–314
Fig. 4. Left: TM-AFMq phase image Ž500 = 500 nm2 . of single Ž800r83. PCEVE-g-PS copolymer molecules. Right: Statistical analysis of the molecule diameter distribution.
indicates that it represents the true structure of the object. It is worth noting here that the height of these objects corresponds to the height of one monolayer, i.e. ; 7 nm. The diameter distribution of these Adoughnut-likeB objects is statistically determined for a set of different AFM images. Fig. 4 shows the Gaussian distribution obtained by measuring the diameters of 220 different objects, observed on two separate samples prepared in the same conditions. The average diameter 38.8 " 2.9 nm is found to be in very good agreement with the hydrodynamic radius ŽRh. value of the copolymer single molecules, experimentally determined in solution by Light Scattering. Therefore, one can reasonably conclude that the features shown in Fig. 4 are isolated molecules. The Adoughnut-likeB morphology is believed to be the result of Ži. the weak affinity of the PCEVE-g-PS copolymer molecules towards a hydrophilic surface such as mica and Žii. the absence of neighboring polymer molecules. This would force the isolated molecules to coil to a certain extent. However, due to the rigidity of the backbone, the chain cannot fully collapse and a void remains in the center. Assuming such a conformation, the contour length of the PCEVE backbone giving rise to the measured diameter is around 130 nm, which is in reasonable agreement with the expected length of the elongated backbone.
4. Conclusions The organization of hyperbranched polyŽchloroethyl vinyl ether-g-styrene. copolymers in thin films is charac-
terized by means of Atomic Force Microscopy. We observe that the degree of lateral ordering of the chains depends on their intrinsic molecular architecture. In particular, the structural complexity of the lateral side chains strongly affects the surface morphology. In submonolayer deposits, AFM images show very small objects whose size is coherent with that of single copolymer molecules. The Adoughnut-likeB morphology of these individual molecules probably originates from partial coiling of the PCEVE backbone because of the weak affinity between the PS lateral chains and mica. In order to verify this behavior, the molecular organization of other substrates, such as graphite, is currently investigated.
References w1x Y.H. Kim, J. Polym. Sci., Part A: Polym. Chem. 36 Ž1998. 1685. w2x M. Freemantle, Chem. Eng. News 6 Ž1999. 37. w3x A. Sunder, R. Mulhaupt, R. Haag, H. Frey, Adv. Mater. 12 Ž2000. ¨ 235. w4x A. Deffieux, M. Schappacher, Macromol. Symp. 45 Ž1998. 132. w5x A. Deffieux, M. Schappacher, Macromolecules 32 Ž1999. 1797. w6x Q. Zhong, D. Inniss, K. Kjoller, V.B. Elings, Surf. Sci. Lett. L 688 Ž1993. 290. w7x Ph. Leclere, J.M. Yu, Ph. Dubois, R. ` R. Lazzaroni, J.L. Bredas, ´ Jerome, Langmuir 12 Ž1996. 4317. ´ˆ w8x S.S. Sheiko, M. Gerle, K. Fisher, M. Schmidt, M. Moller, Langmuir ¨ 13 Ž1997. 5368. w9x M. Gerle, K. Fischer, S. Roos, A.H.E. Muller, M. Schmidt, S.S. ¨ Sheiko, S. Prokhorova, M. Moller, Macromolecules 32 Ž1999. 2629. ¨