- Email: [email protected]
Dissolution and thermochromism of polydiacetylenes with chiral pendent groups
2 October 1998
Chemical Physics Letters 295 Ž1998. 63–69
Dissolution and thermochromism of polydiacetylenes with chiral pendent groups David Bloor
)
Lehrstuhl fur ¨ Experimentalphysik II, UniÕersity of Bayreuth, 95440 Bayreuth, Germany Received 9 March 1998; revised 12 August 1998
Abstract The absorption spectra of soluble polydiacetylenes with chiral pendent groups, – ŽCH 2 . nOCONHC)HŽCH 3 .Ph, are reported. When n G 4 dissolution in chloroform gives yellow solutions of disordered, worm-like chains. The polymers with n s 3 give brown to red solutions in chloroform and, on heating, in tetrahydrofuran. The spectra show that both disordered and ordered chains are present. The polymer fraction dissolved in tetrahydrofuran depends on the dissolution temperature. On heating there is a gradual transition to a yellow solution that is metastable at room temperature, the ordered chains being recovered slowly. Absorption bands associated with ordered chains having different structures are observed. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Polydiacetylenes ŽPDAs. are conjugated polymers obtained by the solid-state polymerisation of di-substituted diacetylene monomer crystals with the general structure
When R is an alkyl urethane ŽR s – ŽCH 2 . nOCONHCH 2 COOBu. soluble nBCMU polymers are obtained. The dramatic colour changes associated with order–disorder transitions in solutions w1x and solids w2x of these PDAs have been known for many )
Permanent address: Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK. E-mail: [email protected]
years. This phenomenon continues to attract interest with reports of the photo-induced transition in crystals w3x, the thermochromism of liquid crystalline polymers w4x and layered single crystals w5x and the solvato- and thermochromism of known and new soluble polymers w6–8x. The generality of the impact of changes of order on the spectra of conjugated polymers has been shown experimentally and discussed theoretically w7,9x. Two models have been proposed to explain the solvato- and thermochromism of PDA solutions. On the basis of the concentration independence of the transition, particularly at low concentrations where molecular contact was reduced, it was identified as a random coil to rigid rod transformation occurring in single polymer molecules w10x. Conversely, results from light and neutron scattering indicated that the solutions contained worm-like chains with quite long persistence lengths that formed ordered aggregates at
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 9 3 8 - 5
64
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
the transition w11,12x. Theoretical studies indicated that both were physically reasonable w13–15x. A clear distinction between the models was difficult to achieve as rigid rods aggregate readily at modest concentrations. Very low concentrations, which are not accessible by absorption spectroscopy are required to eliminate the effect completely. However, Raman spectra were recorded for concentrations at which the molecular contact time was much longer than the time for the formation of ordered polymer w16x. The Raman spectrum of such ordered, isolated 4BCMU polymer chains was found to be similar to that of a disordered crystalline phase of the polymer. Thus, it was concluded that the polymer chains were in a similar environment in both situations and that the transition in solution was due to a process of intramolecular collapse similar to crystallisation. This model is consistent with the coexistence of ordered and disordered forms over a finite temperature range at the thermochromic transition w7,10x and the long times associated with some solvatochromic transitions w17,18x. Thus, at very low polymer concentrations intra-chain collapse is the principal mechanism with inter-chain aggregation becoming increasingly important as the concentration is increased. Chiral conjugated polymers have not been as extensively studied as their achiral counterparts despite their potentially useful properties w19x. Only one report of chiral PDAs has appeared in the literature w20x. These are related to the nBCMU polymers with an optically pure, chiral carbon at the end of the pendent group, ŽR s – ŽCH 2 . nOCONHC)HŽCH 3 .ŽC 6 H 5 ... Circular dichroism spectroscopy w20x shows that these polymers are pure enantiomers. A brief account of the dissolution and thermochromism of the n equals 3, 4 and 6 chiral Ž nRMBU and nSMBU. polymers in chloroform ŽCHCl 3 . and tetrahydrofuran ŽTHF. is presented here.
with acetone. Solutions were prepared in Merck pro analysie solvents either at room temperature or by heating to a known temperature. Solutions were filtered prior to use to remove any undissolved polymer. Spectra were recorded with a Perkin Elmer Lambda 2 spectrophotometer in a cuvette heated by water circulated from a Lauda M3 constant temperature bath. The temperature of the cuvette holder was measured with a Pt 100 resistance thermometer. A thermometer immersed in the solution was used to determine the temperature offset between holder and solution.
3. Results and discussion CHCl 3 is a good solvent for soluble PDAs giving orange to yellow solutions with a single broad absorption band due to the presence of disordered worm-like chains. This is also the case for the chiral polymers when n G 4, Fig. 1 shows typical spectra for 4RMBU and 6RMBU. On heating the solutions there is a small thermo-solvatochromic shift of the absorption maximum to higher energy with increasing temperature. A more detailed study of this effect for a wider range of PDAs will be the subject of a forthcoming publication. For 4RMBU there is a weak
2. Experimental The diacetylene monomers were prepared by the reaction of either the R-Žq.- or the S-Žy.-methylbenzyl isocyanate with an appropriate diacetylene diol w20x. Polymers were obtained by 60 Co g-ray irradiation of the monomer with a typical dose of 50 Mrad. Residual monomer was removed by washing
Fig. 1. Absorption spectra of chloroform solutions of Ža. 4RMBU, Žb. 6RMBU and two samples of 3SMBU of different number average molecular weight, Žc. high n w and Žd. low n w . The spectra are displaced vertically for clarity.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
absorption tail at low energy, which suggests that a small amount of ordered polymer is present in the solution. In contrast dissolution of the 3RŽS.MBU polymers in CHCl 3 at room temperature gives brown solutions. The spectra of these solutions are stable over many months with no visible signs of precipitation. The colouration is due to the presence of additional absorption below 19 000 cmy1 with a low energy peak at about 16 000 cmy1 . The spectra of 3SMBU samples as polymerised and degraded in chloroform solution w21x are shown in Fig. 1. The energy of the maximum absorption for the disordered polymer is related to the weight average chain length Ž n w ., which for high n w results from the dependence of persistence length on n w w22x. This relationship varies from one PDA to another but can be used to obtain approximate values of n w for the chiral polymers. Thus, the spectra indicate that n w is of the order 500 for the degraded low n w sample and in excess of 2000 for the as polymerised high n w sample. The low frequency absorption weakens as the solution is heated to just below its boiling point, but does not disappear completely, for both low and high n w . There is no isosbestic point as observed for thermochromic transitions in other PDAs, see Fig. 2. However, the transition is gradual and spectral shifts
Fig. 2. Absorption spectra of a chloroform solution of high n w 3SMBU over the temperature range from 26 to 608C, the absorbance in the range from 16 000 to 20 000 cmy1 falls with increasing temperature. The second derivatives of the spectra below 19 000 cmy1 are shown in the inset.
65
Fig. 3. Absorption spectra of low n w and high n w samples of 3SMBU dissolved in THF, Ža. low n w sample dissolved at 258C, Žb., Žc. and Žd. high n w sample dissolved at 25, 40 and 508C respectively. The spectra were recorded at room temperature Ž22"28C..
due to thermo-solvatochromism, which are negligible for other PDAs where the transition occurs over a narrow temperature range w10,23x, obliterate the isosbestic point. The residual absorption at low energy is removed by prolonged heating. The yellow solution obtained is metastable when cooled to room temperature with the low energy absorption reappearing slowly over a period of many days. The second derivatives of the spectra below 19 000 cmy1 , inset in Fig. 2, show clearly the absorption at 16 200 cmy1 , the associated vibrational sideband at 17 500 cmy1 and a weak absorption at 15 750 cmy1 , which is not visible in the absorption spectra and disappears when the solution is heated above 358C. Thus, the transition is attributed to the gradual transformation of two types of ordered, pseudo-crystalline, chains into worm-like chains. The behaviour of the two fractions of 3SMBU in THF is complex. For dissolution at room temperature the low n w sample has an absorption below 19 000 cmy1 identical to, but weaker than, that for the CHCl 3 solution, Fig. 3. Under the same conditions the nominally higher n w sample has negligible absorption below 19 000 cmy1 , with an absorption maximum at a higher energy Ž23 040 cmy1 . than that of the low n w sample Ž22 730 cmy1 .. In general a linear relationship between n w and the position of the absorption maximum is valid in the low molecu-
66
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
The structure of the low energy absorption for THF solutions of high n w 3SMBU prepared at 508C, and above, is very sensitive to the time–temperature history of the solution. Two heating cycles for a single sample, stored at room temperature for 32 days between the cycles, are shown in Fig. 4. There are obvious differences in the two sets of spectra and in their second derivative plots, see Fig. 5. The absorption at ca. 15 750 cmy1 seen in the second derivative plot for CHCl 3 solutions appears just below 16 000 cmy1 . A common origin for these absorptions is indicated by the identical temperature dependence, i.e. both disappear at 358C. The feature at ca. 16 500 cmy1 weakens and moves to lower energy on heating, a weak peak grows in at 17 500 cmy1 and a less temperature sensitive feature is seen at 18 800 cmy1 . The features at ca. 16 000, 17 500 and 18 800 cmy1 are all much stronger for the stored solution. These spectra indicate that polymer chains
Fig. 4. Thermochromism of high n w 3SMBU sample dissolved in THF at 508C, Ža. initial heating cycle and Žb. second heating cycle after storage at room temperature for 32 days. The spectra were recorded over the temperature range 26 to 618C.
lar weight region w22x. Hence, the fraction of the high n w sample dissolved at room temperature has an average n w about ten times smaller than that of the low n w sample, i.e. n w ( 50. For dissolution at 408C the strength of the low energy absorption is increased and for dissolution in the range 50 to 608C it becomes a major feature, see Fig. 3. For dissolution at 40 and 508C the main absorption maximum occurs at 22 320 and 20 040 cmy1 respectively. This indicates that the average n w of the dissolved polymer increases with increasing dissolution temperature with complete dissolution of the n w ) 2000 sample at 508C, as evidenced by the absence of residual polymer in this case. The much stronger absorption below 19 000 cmy1 for this sample could reflect a greater probability of intramolecular collapse into ordered nano-particles for higher n w polymer w16x.
Fig. 5. The second derivatives of the spectra shown in Fig. 4, Ža. initial heating cycle and Žb. second heating cycle after storage at room temperature for 32 days.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
with four different structures are present in THF solutions. In addition to the reappearance of the ordered polymer chains in solutions stored after heating, two other effects were observed. First, on long term storage the ordered polymer forms larger aggregates, which reduce the transmission due to scattering and eventually precipitate as a fine red deposit. Secondly, after washing with THF cuvettes used to heat and store solutions have a weak spectrum due to polymer adsorbed on the internal surfaces. Both precipitate and adsorbate have an absorption maximum at ca. 19 000 cmy1 . The energy of the absorption maximum of the polymer backbone depends on the polymer structure and a shift due to the dielectric properties of the surrounding medium w24–26x. The small differences in the energies of the absorptions below 19 000 cmy1 in different solvents can be attributed to the latter effect. The larger energy differences between the features are, as indicated above, predominantly due to differences in polymer structure. In achiral PDAs the principal structural differences are reordering of the pendent groups and deviation of the polymer backbone from an ideal extended planar conformation w10–18,24,25x. The same effects will occur in chiral PDAs but the ideal extended structure will be helical w20x. In the solid state a complex phase structure has been observed for nBCMU polymers w24,25x. However, while multiple phases have been observed for 4-, 6- and 9BCMU this is not the case for 3BCMU, which melts without any evidence of intermediate phases and freezes to give polymer with a structure similar to that in the original crystal. The solvatochromic transition from disordered, worm-like chains to quasi-crystalline chains results, under different conditions, in two chain structures for 9BCMU w17x. However, only one structure is observed in solvato- and thermochromism of 3- and 4BCMU solutions w1,2,25x. Thus, while there are precedents for the existence of multiple polymer chain structures in related polymers only one chain structure has been observed for the ordered form of 3BCMU in either the solid state or solutions. Thus, the behaviour of the 3SŽR.MBU polymer solutions is unusual. For both the nBCMU and the nRŽS.MBU polymers the behaviour of the n s 3 polymers is distinct
67
from that of the other members of the series. Raman spectra and molecular models indicate that for 3BCMU the alkyl groups adjacent to the polymer chain from an all-trans alkyl sequence in both the as polymerised material and samples obtained from either solution or melt w24,25,27x. For other nBCMUs, while the as polymerised samples have all-trans alkyl sequences, this ordered structure is lost in samples obtained from either solution or melt w27,28x. Thus, the formation of the hydrogen bonds between adjacent pendent groups is strongly favoured by an all-trans alkyl sequence in 3BCMU. By analogy the difference in behaviour of 3RŽS.MBU relative to other nRŽS.MBU polymers can be attributed to this property of the alkyl sequence. However, the steric hindrance between the bulky chiral moieties adjacent to the hydrogen-bonding sites is a complicating factor, which seems to have two effects. First, when hydrogen bonds are formed the structure is more stable than that found in 3BCMU. Secondly, a variety of other structures occur where the alkyl groups do not form an all-trans sequence, as has been shown by Raman spectroscopy w27x. On the basis of the spectroscopic data models can be suggested for some of the different chain structures observed. The peak below 16 000 cmy1 is always weaker than the other features. The low absorption energy suggests extended polymer chains with a structure close to that found in as polymerised crystals, which are associated in loose, easily disrupted aggregates. Evidence for the formation of easily disrupted networks in PDA solutions, even at low concentrations, is provided by measurements of flow induced birefringence w29x. The peak just above 16 000 cmy1 weakens gradually as the solutions are heated. It is removed by prolonged heating just below the boiling point in CHCl 3 , but persists at similar temperatures in THF. In CHCl 3 it moves to higher energy and in THF to lower energy as the temperature is increased. These properties are appropriate for less well ordered chains associated in quasi-crystalline, sub-micron size aggregates, formed by intra- and inter-molecular interactions. The lower order raises the absorption energy slightly and the structure is more difficult to disrupt on heating. Swelling by solvent ingress will produce an increase in the absorption energy w24,26x. In THF annealing of the persistent aggregates at elevated
68
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
temperature could give greater order shifting the absorption to lower energy, as observed. There is no obvious explanation for the broad feature that grows in at 17 500 cmy1 on heating THF solutions. The feature near 19 000 cmy1 occurs as a weak shoulder for low n w 3SMBU solutions in CHCl 3 and THF and a peak in aged THF solutions of high n w 3SMBU. Absorption at this frequency occurs for the ‘red’ form of PDAs in both solutions and crystals w3,4,10,24,25x. A structure of this form has been proposed to be one in which the chains are extended and the hydrogen bonds between the pendent groups remain intact but the inner most alkyl groups are disordered w24,25x. The ‘red’ form is absent in 3BCMU, presumably because of the difficulty in deforming the all-trans alkyl sequence while retaining the H-bonds between adjacent pendent groups. Its appearance in 3SMBU suggests that, as noted above, the bulky chiral moieties hinder reformation of the H-bonds between the pendent groups allowing a less ordered structure to occur.
4. Conclusions The chiral RŽS.MBU PDAs with four or more alkyl units in the pendent group dissolve in CHCl 3 to give solutions of disordered, worm-like chains. The behaviour of the polymers with three alkyl units in the pendent group, 3RŽS.MBU is more complicated with a series of additional absorption peaks appearing in the spectral region between 15 500 and 19 000 cmy1 . Within this region four absorption peaks due to polymer chains with different microscopic morphology can be identified. For 3SMBU the relative abundance of the four forms of ordered polymer were found to be dependent on the mean molecular weight of the polymer, the solvent used, the dissolution temperature and the thermal history of the solution. The observation of several different forms of ordered polymer for 3SMBU contrasts sharply with that of only a single ordered phase for the achiral 3BCMU. The extended temperature ranges over which the polymer aggregates dissolve also contrasts with the narrow thermochromic transitions and rapid onset of dissolution observed for other soluble, achiral PDAs. Steric hindrance of the bulky chiral group,
which will inhibit the formation of hydrogen bonds between the pendent groups in the chiral polymers, is the most likely origin of this atypical behaviour. Models, consistent with the experimental data, have been proposed for the microstructure of three of these ordered forms. These are produced by the intra- and inter-molecular aggregation of polymer chains with different conformations, to give structures comparable to those known to exist for other, achiral PDAs. Further studies of solutions and films of 3RŽS.MBU and molecular modelling are in hand in an effort to obtain a more detailed picture of the different microstructures that occur for these polymers. Acknowledgements This research was performed during the tenure of an Alexander von Humboldt Fellowship. Thanks are due to the AvH Foundation for its support, to Prof. M. Schwoerer, Lehrstuhl fur ¨ Experimentalphysik II, Universitat ¨ Bayreuth for access to research facilities, to I. Rystau and T. Fehn for assistance in performing the experiments, to D.J. Ando for the provision of the chiral PDAs and the University of Durham for granting research leave. References w1x G.N. Patel, R.R. Chance, J.D. Witt, J. Chem. Phys. 70 Ž1979. 4387. w2x R.R. Chance, R.H. Baughman, H. Mueller, C.J. Eckhardt, J. Chem. Phys. 67 Ž1977. 3616. w3x S. Koshihara, Y. Tokura, K. Takeda, T. Koda, Phys. Rev. B 52 Ž1995. 6265. w4x P.T. Hammond, M.F. Ruebner, Macromolecules 30 Ž1997. 5733. w5x L.S. Li, S.I. Strupp, Macromolecules 30 Ž1997. 5313. w6x N.B. Kodali, W. Kim, J. Kumar, S.K. Tripathy, S.S. Talwar, Macromolecules 27 Ž1994. 6612. w7x S.D.D.V. Rughooputh, Synth. Met. 80 Ž1996. 195. w8x Y. Zhang, T. Wada, H.A. Sasabe, J. Wen, J. Fluorine Chem. 77 Ž1996. 27. w9x C. Singh, D. Hone, Synth. Met. 62 Ž1994. 61. w10x K.C. Lim, A.J. Heeger, J. Chem. Phys. 82 Ž1985. 522. w11x G. Wenz, M.A. Mueller, M. Schmidt, G. Wegner, Macromolecules 17 Ž1984. 837. w12x M. Rawiso, J.P. Aime, J.L. Fave, M. Schott, M.A. Mueller, M. Schmidt, H. Baumgartl, G. Wegner, J. Phys. France 49 Ž1988. 861. w13x K.S. Schweizer, J. Chem. Phys. 85 Ž1986. 1156.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69 w14x K.S. Schweizer, J. Chem. Phys. 85 Ž1986. 1176. w15x G. Allegra, S. Brueckner, M. Schmidt, G. Wegner, Macromolecules 19 Ž1986. 399. w16x M.A. Taylor, J.A. Odell, D.N. Batchelder, A.J. Campbell, Polymer 31 Ž1990. 1116. w17x D. Bloor, D.J. Ando, J.S. Obhi, S. Mann, M.R. Worboys, Makromol. Chemie. Rapid Commun. 7 Ž1986. 665. w18x B. Chu, R. Xu, Acc. Chem. Res. 24 Ž1991. 384. w19x L. Pu, Acta Polymer. 48 Ž1997. 116. w20x A.F. Drake, P. Udvarhelyi, D.J. Ando, D. Bloor, J.S. Obhi, S. Mann, Polymer 30 Ž1990. 1063. w21x D. Bloor, M.R. Worboys, J. Mater. Chem. 8 Ž1998. 903.
69
w22x A.J. Campbell, C.K.L. Davies, Polymer 35 Ž1994. 4787. w23x G.N. Patel, J.D. Witt, Y.P. Khanna, J. Polym. Sci. Polym. Phys. Ed. 18 Ž1980. 1383. w24x A.J. Campbell, C.K.L. Davies, Polymer 36 Ž1995. 675. w25x A.J. Campbell, Ph.D. Thesis, University of London, 1992. w26x J.P. Aime, H.E. King, M.W. Kim, R.R. Chance, Synth. Met. 41–43 Ž1991. 203. w27x D. Bloor, Polymer Ž1998. in press. w28x A.J. Campbell, C.K.L. Davies, D.N. Batchelder, Macromol. Chem. Phys. 199 Ž1998. 109. w29x M.A. Taylor, D.N. Batchelder, J.A. Odell, Polymer 29 Ž1988. 253.
Chemical Physics Letters 295 Ž1998. 63–69
Dissolution and thermochromism of polydiacetylenes with chiral pendent groups David Bloor
)
Lehrstuhl fur ¨ Experimentalphysik II, UniÕersity of Bayreuth, 95440 Bayreuth, Germany Received 9 March 1998; revised 12 August 1998
Abstract The absorption spectra of soluble polydiacetylenes with chiral pendent groups, – ŽCH 2 . nOCONHC)HŽCH 3 .Ph, are reported. When n G 4 dissolution in chloroform gives yellow solutions of disordered, worm-like chains. The polymers with n s 3 give brown to red solutions in chloroform and, on heating, in tetrahydrofuran. The spectra show that both disordered and ordered chains are present. The polymer fraction dissolved in tetrahydrofuran depends on the dissolution temperature. On heating there is a gradual transition to a yellow solution that is metastable at room temperature, the ordered chains being recovered slowly. Absorption bands associated with ordered chains having different structures are observed. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Polydiacetylenes ŽPDAs. are conjugated polymers obtained by the solid-state polymerisation of di-substituted diacetylene monomer crystals with the general structure
When R is an alkyl urethane ŽR s – ŽCH 2 . nOCONHCH 2 COOBu. soluble nBCMU polymers are obtained. The dramatic colour changes associated with order–disorder transitions in solutions w1x and solids w2x of these PDAs have been known for many )
Permanent address: Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK. E-mail: [email protected]
years. This phenomenon continues to attract interest with reports of the photo-induced transition in crystals w3x, the thermochromism of liquid crystalline polymers w4x and layered single crystals w5x and the solvato- and thermochromism of known and new soluble polymers w6–8x. The generality of the impact of changes of order on the spectra of conjugated polymers has been shown experimentally and discussed theoretically w7,9x. Two models have been proposed to explain the solvato- and thermochromism of PDA solutions. On the basis of the concentration independence of the transition, particularly at low concentrations where molecular contact was reduced, it was identified as a random coil to rigid rod transformation occurring in single polymer molecules w10x. Conversely, results from light and neutron scattering indicated that the solutions contained worm-like chains with quite long persistence lengths that formed ordered aggregates at
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 9 3 8 - 5
64
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
the transition w11,12x. Theoretical studies indicated that both were physically reasonable w13–15x. A clear distinction between the models was difficult to achieve as rigid rods aggregate readily at modest concentrations. Very low concentrations, which are not accessible by absorption spectroscopy are required to eliminate the effect completely. However, Raman spectra were recorded for concentrations at which the molecular contact time was much longer than the time for the formation of ordered polymer w16x. The Raman spectrum of such ordered, isolated 4BCMU polymer chains was found to be similar to that of a disordered crystalline phase of the polymer. Thus, it was concluded that the polymer chains were in a similar environment in both situations and that the transition in solution was due to a process of intramolecular collapse similar to crystallisation. This model is consistent with the coexistence of ordered and disordered forms over a finite temperature range at the thermochromic transition w7,10x and the long times associated with some solvatochromic transitions w17,18x. Thus, at very low polymer concentrations intra-chain collapse is the principal mechanism with inter-chain aggregation becoming increasingly important as the concentration is increased. Chiral conjugated polymers have not been as extensively studied as their achiral counterparts despite their potentially useful properties w19x. Only one report of chiral PDAs has appeared in the literature w20x. These are related to the nBCMU polymers with an optically pure, chiral carbon at the end of the pendent group, ŽR s – ŽCH 2 . nOCONHC)HŽCH 3 .ŽC 6 H 5 ... Circular dichroism spectroscopy w20x shows that these polymers are pure enantiomers. A brief account of the dissolution and thermochromism of the n equals 3, 4 and 6 chiral Ž nRMBU and nSMBU. polymers in chloroform ŽCHCl 3 . and tetrahydrofuran ŽTHF. is presented here.
with acetone. Solutions were prepared in Merck pro analysie solvents either at room temperature or by heating to a known temperature. Solutions were filtered prior to use to remove any undissolved polymer. Spectra were recorded with a Perkin Elmer Lambda 2 spectrophotometer in a cuvette heated by water circulated from a Lauda M3 constant temperature bath. The temperature of the cuvette holder was measured with a Pt 100 resistance thermometer. A thermometer immersed in the solution was used to determine the temperature offset between holder and solution.
3. Results and discussion CHCl 3 is a good solvent for soluble PDAs giving orange to yellow solutions with a single broad absorption band due to the presence of disordered worm-like chains. This is also the case for the chiral polymers when n G 4, Fig. 1 shows typical spectra for 4RMBU and 6RMBU. On heating the solutions there is a small thermo-solvatochromic shift of the absorption maximum to higher energy with increasing temperature. A more detailed study of this effect for a wider range of PDAs will be the subject of a forthcoming publication. For 4RMBU there is a weak
2. Experimental The diacetylene monomers were prepared by the reaction of either the R-Žq.- or the S-Žy.-methylbenzyl isocyanate with an appropriate diacetylene diol w20x. Polymers were obtained by 60 Co g-ray irradiation of the monomer with a typical dose of 50 Mrad. Residual monomer was removed by washing
Fig. 1. Absorption spectra of chloroform solutions of Ža. 4RMBU, Žb. 6RMBU and two samples of 3SMBU of different number average molecular weight, Žc. high n w and Žd. low n w . The spectra are displaced vertically for clarity.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
absorption tail at low energy, which suggests that a small amount of ordered polymer is present in the solution. In contrast dissolution of the 3RŽS.MBU polymers in CHCl 3 at room temperature gives brown solutions. The spectra of these solutions are stable over many months with no visible signs of precipitation. The colouration is due to the presence of additional absorption below 19 000 cmy1 with a low energy peak at about 16 000 cmy1 . The spectra of 3SMBU samples as polymerised and degraded in chloroform solution w21x are shown in Fig. 1. The energy of the maximum absorption for the disordered polymer is related to the weight average chain length Ž n w ., which for high n w results from the dependence of persistence length on n w w22x. This relationship varies from one PDA to another but can be used to obtain approximate values of n w for the chiral polymers. Thus, the spectra indicate that n w is of the order 500 for the degraded low n w sample and in excess of 2000 for the as polymerised high n w sample. The low frequency absorption weakens as the solution is heated to just below its boiling point, but does not disappear completely, for both low and high n w . There is no isosbestic point as observed for thermochromic transitions in other PDAs, see Fig. 2. However, the transition is gradual and spectral shifts
Fig. 2. Absorption spectra of a chloroform solution of high n w 3SMBU over the temperature range from 26 to 608C, the absorbance in the range from 16 000 to 20 000 cmy1 falls with increasing temperature. The second derivatives of the spectra below 19 000 cmy1 are shown in the inset.
65
Fig. 3. Absorption spectra of low n w and high n w samples of 3SMBU dissolved in THF, Ža. low n w sample dissolved at 258C, Žb., Žc. and Žd. high n w sample dissolved at 25, 40 and 508C respectively. The spectra were recorded at room temperature Ž22"28C..
due to thermo-solvatochromism, which are negligible for other PDAs where the transition occurs over a narrow temperature range w10,23x, obliterate the isosbestic point. The residual absorption at low energy is removed by prolonged heating. The yellow solution obtained is metastable when cooled to room temperature with the low energy absorption reappearing slowly over a period of many days. The second derivatives of the spectra below 19 000 cmy1 , inset in Fig. 2, show clearly the absorption at 16 200 cmy1 , the associated vibrational sideband at 17 500 cmy1 and a weak absorption at 15 750 cmy1 , which is not visible in the absorption spectra and disappears when the solution is heated above 358C. Thus, the transition is attributed to the gradual transformation of two types of ordered, pseudo-crystalline, chains into worm-like chains. The behaviour of the two fractions of 3SMBU in THF is complex. For dissolution at room temperature the low n w sample has an absorption below 19 000 cmy1 identical to, but weaker than, that for the CHCl 3 solution, Fig. 3. Under the same conditions the nominally higher n w sample has negligible absorption below 19 000 cmy1 , with an absorption maximum at a higher energy Ž23 040 cmy1 . than that of the low n w sample Ž22 730 cmy1 .. In general a linear relationship between n w and the position of the absorption maximum is valid in the low molecu-
66
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
The structure of the low energy absorption for THF solutions of high n w 3SMBU prepared at 508C, and above, is very sensitive to the time–temperature history of the solution. Two heating cycles for a single sample, stored at room temperature for 32 days between the cycles, are shown in Fig. 4. There are obvious differences in the two sets of spectra and in their second derivative plots, see Fig. 5. The absorption at ca. 15 750 cmy1 seen in the second derivative plot for CHCl 3 solutions appears just below 16 000 cmy1 . A common origin for these absorptions is indicated by the identical temperature dependence, i.e. both disappear at 358C. The feature at ca. 16 500 cmy1 weakens and moves to lower energy on heating, a weak peak grows in at 17 500 cmy1 and a less temperature sensitive feature is seen at 18 800 cmy1 . The features at ca. 16 000, 17 500 and 18 800 cmy1 are all much stronger for the stored solution. These spectra indicate that polymer chains
Fig. 4. Thermochromism of high n w 3SMBU sample dissolved in THF at 508C, Ža. initial heating cycle and Žb. second heating cycle after storage at room temperature for 32 days. The spectra were recorded over the temperature range 26 to 618C.
lar weight region w22x. Hence, the fraction of the high n w sample dissolved at room temperature has an average n w about ten times smaller than that of the low n w sample, i.e. n w ( 50. For dissolution at 408C the strength of the low energy absorption is increased and for dissolution in the range 50 to 608C it becomes a major feature, see Fig. 3. For dissolution at 40 and 508C the main absorption maximum occurs at 22 320 and 20 040 cmy1 respectively. This indicates that the average n w of the dissolved polymer increases with increasing dissolution temperature with complete dissolution of the n w ) 2000 sample at 508C, as evidenced by the absence of residual polymer in this case. The much stronger absorption below 19 000 cmy1 for this sample could reflect a greater probability of intramolecular collapse into ordered nano-particles for higher n w polymer w16x.
Fig. 5. The second derivatives of the spectra shown in Fig. 4, Ža. initial heating cycle and Žb. second heating cycle after storage at room temperature for 32 days.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
with four different structures are present in THF solutions. In addition to the reappearance of the ordered polymer chains in solutions stored after heating, two other effects were observed. First, on long term storage the ordered polymer forms larger aggregates, which reduce the transmission due to scattering and eventually precipitate as a fine red deposit. Secondly, after washing with THF cuvettes used to heat and store solutions have a weak spectrum due to polymer adsorbed on the internal surfaces. Both precipitate and adsorbate have an absorption maximum at ca. 19 000 cmy1 . The energy of the absorption maximum of the polymer backbone depends on the polymer structure and a shift due to the dielectric properties of the surrounding medium w24–26x. The small differences in the energies of the absorptions below 19 000 cmy1 in different solvents can be attributed to the latter effect. The larger energy differences between the features are, as indicated above, predominantly due to differences in polymer structure. In achiral PDAs the principal structural differences are reordering of the pendent groups and deviation of the polymer backbone from an ideal extended planar conformation w10–18,24,25x. The same effects will occur in chiral PDAs but the ideal extended structure will be helical w20x. In the solid state a complex phase structure has been observed for nBCMU polymers w24,25x. However, while multiple phases have been observed for 4-, 6- and 9BCMU this is not the case for 3BCMU, which melts without any evidence of intermediate phases and freezes to give polymer with a structure similar to that in the original crystal. The solvatochromic transition from disordered, worm-like chains to quasi-crystalline chains results, under different conditions, in two chain structures for 9BCMU w17x. However, only one structure is observed in solvato- and thermochromism of 3- and 4BCMU solutions w1,2,25x. Thus, while there are precedents for the existence of multiple polymer chain structures in related polymers only one chain structure has been observed for the ordered form of 3BCMU in either the solid state or solutions. Thus, the behaviour of the 3SŽR.MBU polymer solutions is unusual. For both the nBCMU and the nRŽS.MBU polymers the behaviour of the n s 3 polymers is distinct
67
from that of the other members of the series. Raman spectra and molecular models indicate that for 3BCMU the alkyl groups adjacent to the polymer chain from an all-trans alkyl sequence in both the as polymerised material and samples obtained from either solution or melt w24,25,27x. For other nBCMUs, while the as polymerised samples have all-trans alkyl sequences, this ordered structure is lost in samples obtained from either solution or melt w27,28x. Thus, the formation of the hydrogen bonds between adjacent pendent groups is strongly favoured by an all-trans alkyl sequence in 3BCMU. By analogy the difference in behaviour of 3RŽS.MBU relative to other nRŽS.MBU polymers can be attributed to this property of the alkyl sequence. However, the steric hindrance between the bulky chiral moieties adjacent to the hydrogen-bonding sites is a complicating factor, which seems to have two effects. First, when hydrogen bonds are formed the structure is more stable than that found in 3BCMU. Secondly, a variety of other structures occur where the alkyl groups do not form an all-trans sequence, as has been shown by Raman spectroscopy w27x. On the basis of the spectroscopic data models can be suggested for some of the different chain structures observed. The peak below 16 000 cmy1 is always weaker than the other features. The low absorption energy suggests extended polymer chains with a structure close to that found in as polymerised crystals, which are associated in loose, easily disrupted aggregates. Evidence for the formation of easily disrupted networks in PDA solutions, even at low concentrations, is provided by measurements of flow induced birefringence w29x. The peak just above 16 000 cmy1 weakens gradually as the solutions are heated. It is removed by prolonged heating just below the boiling point in CHCl 3 , but persists at similar temperatures in THF. In CHCl 3 it moves to higher energy and in THF to lower energy as the temperature is increased. These properties are appropriate for less well ordered chains associated in quasi-crystalline, sub-micron size aggregates, formed by intra- and inter-molecular interactions. The lower order raises the absorption energy slightly and the structure is more difficult to disrupt on heating. Swelling by solvent ingress will produce an increase in the absorption energy w24,26x. In THF annealing of the persistent aggregates at elevated
68
D. Bloor r Chemical Physics Letters 295 (1998) 63–69
temperature could give greater order shifting the absorption to lower energy, as observed. There is no obvious explanation for the broad feature that grows in at 17 500 cmy1 on heating THF solutions. The feature near 19 000 cmy1 occurs as a weak shoulder for low n w 3SMBU solutions in CHCl 3 and THF and a peak in aged THF solutions of high n w 3SMBU. Absorption at this frequency occurs for the ‘red’ form of PDAs in both solutions and crystals w3,4,10,24,25x. A structure of this form has been proposed to be one in which the chains are extended and the hydrogen bonds between the pendent groups remain intact but the inner most alkyl groups are disordered w24,25x. The ‘red’ form is absent in 3BCMU, presumably because of the difficulty in deforming the all-trans alkyl sequence while retaining the H-bonds between adjacent pendent groups. Its appearance in 3SMBU suggests that, as noted above, the bulky chiral moieties hinder reformation of the H-bonds between the pendent groups allowing a less ordered structure to occur.
4. Conclusions The chiral RŽS.MBU PDAs with four or more alkyl units in the pendent group dissolve in CHCl 3 to give solutions of disordered, worm-like chains. The behaviour of the polymers with three alkyl units in the pendent group, 3RŽS.MBU is more complicated with a series of additional absorption peaks appearing in the spectral region between 15 500 and 19 000 cmy1 . Within this region four absorption peaks due to polymer chains with different microscopic morphology can be identified. For 3SMBU the relative abundance of the four forms of ordered polymer were found to be dependent on the mean molecular weight of the polymer, the solvent used, the dissolution temperature and the thermal history of the solution. The observation of several different forms of ordered polymer for 3SMBU contrasts sharply with that of only a single ordered phase for the achiral 3BCMU. The extended temperature ranges over which the polymer aggregates dissolve also contrasts with the narrow thermochromic transitions and rapid onset of dissolution observed for other soluble, achiral PDAs. Steric hindrance of the bulky chiral group,
which will inhibit the formation of hydrogen bonds between the pendent groups in the chiral polymers, is the most likely origin of this atypical behaviour. Models, consistent with the experimental data, have been proposed for the microstructure of three of these ordered forms. These are produced by the intra- and inter-molecular aggregation of polymer chains with different conformations, to give structures comparable to those known to exist for other, achiral PDAs. Further studies of solutions and films of 3RŽS.MBU and molecular modelling are in hand in an effort to obtain a more detailed picture of the different microstructures that occur for these polymers. Acknowledgements This research was performed during the tenure of an Alexander von Humboldt Fellowship. Thanks are due to the AvH Foundation for its support, to Prof. M. Schwoerer, Lehrstuhl fur ¨ Experimentalphysik II, Universitat ¨ Bayreuth for access to research facilities, to I. Rystau and T. Fehn for assistance in performing the experiments, to D.J. Ando for the provision of the chiral PDAs and the University of Durham for granting research leave. References w1x G.N. Patel, R.R. Chance, J.D. Witt, J. Chem. Phys. 70 Ž1979. 4387. w2x R.R. Chance, R.H. Baughman, H. Mueller, C.J. Eckhardt, J. Chem. Phys. 67 Ž1977. 3616. w3x S. Koshihara, Y. Tokura, K. Takeda, T. Koda, Phys. Rev. B 52 Ž1995. 6265. w4x P.T. Hammond, M.F. Ruebner, Macromolecules 30 Ž1997. 5733. w5x L.S. Li, S.I. Strupp, Macromolecules 30 Ž1997. 5313. w6x N.B. Kodali, W. Kim, J. Kumar, S.K. Tripathy, S.S. Talwar, Macromolecules 27 Ž1994. 6612. w7x S.D.D.V. Rughooputh, Synth. Met. 80 Ž1996. 195. w8x Y. Zhang, T. Wada, H.A. Sasabe, J. Wen, J. Fluorine Chem. 77 Ž1996. 27. w9x C. Singh, D. Hone, Synth. Met. 62 Ž1994. 61. w10x K.C. Lim, A.J. Heeger, J. Chem. Phys. 82 Ž1985. 522. w11x G. Wenz, M.A. Mueller, M. Schmidt, G. Wegner, Macromolecules 17 Ž1984. 837. w12x M. Rawiso, J.P. Aime, J.L. Fave, M. Schott, M.A. Mueller, M. Schmidt, H. Baumgartl, G. Wegner, J. Phys. France 49 Ž1988. 861. w13x K.S. Schweizer, J. Chem. Phys. 85 Ž1986. 1156.
D. Bloor r Chemical Physics Letters 295 (1998) 63–69 w14x K.S. Schweizer, J. Chem. Phys. 85 Ž1986. 1176. w15x G. Allegra, S. Brueckner, M. Schmidt, G. Wegner, Macromolecules 19 Ž1986. 399. w16x M.A. Taylor, J.A. Odell, D.N. Batchelder, A.J. Campbell, Polymer 31 Ž1990. 1116. w17x D. Bloor, D.J. Ando, J.S. Obhi, S. Mann, M.R. Worboys, Makromol. Chemie. Rapid Commun. 7 Ž1986. 665. w18x B. Chu, R. Xu, Acc. Chem. Res. 24 Ž1991. 384. w19x L. Pu, Acta Polymer. 48 Ž1997. 116. w20x A.F. Drake, P. Udvarhelyi, D.J. Ando, D. Bloor, J.S. Obhi, S. Mann, Polymer 30 Ž1990. 1063. w21x D. Bloor, M.R. Worboys, J. Mater. Chem. 8 Ž1998. 903.
69
w22x A.J. Campbell, C.K.L. Davies, Polymer 35 Ž1994. 4787. w23x G.N. Patel, J.D. Witt, Y.P. Khanna, J. Polym. Sci. Polym. Phys. Ed. 18 Ž1980. 1383. w24x A.J. Campbell, C.K.L. Davies, Polymer 36 Ž1995. 675. w25x A.J. Campbell, Ph.D. Thesis, University of London, 1992. w26x J.P. Aime, H.E. King, M.W. Kim, R.R. Chance, Synth. Met. 41–43 Ž1991. 203. w27x D. Bloor, Polymer Ž1998. in press. w28x A.J. Campbell, C.K.L. Davies, D.N. Batchelder, Macromol. Chem. Phys. 199 Ž1998. 109. w29x M.A. Taylor, D.N. Batchelder, J.A. Odell, Polymer 29 Ž1988. 253.