Rigby

Communications and Notes Thermal Transitions o f Synthetic and Biological Polymers in Bulk and in Solution Ir~ A recent communication to this Journal Moraglio et al. ~ reported some very small thermal transitions in polystyrene occurring in the bulk phase as well as in solution. Using a sensitive dilatometric technique Moraglio and Danusso located a transition region some 8°C wide and centred at about 50°C both for the bulk polymer and for toluene solutions of 0"5 to 1-0 per cent concentration; one of their published curves for the bulk polymer is reproduced as curve P in Figure 1. Bianchi and Rossi confirmed the location of the solid-state transition by measuring the internal pressure at different temperatures, whilst Liquori and Quadrifoglio observed optical density anomalies around 50°C in solutions of isotactic polystyrene in decalin. Moraglio et al. recognized the essentially intramolecular nature of these phenomena, but considered that the basic molecular process would not be a major conformational change 'analogous to helix-coil shape transitions .... in proteins'. They suggested instead some unspecified molecular change whose effects would appear equally in solution or in the bulk material. In view both of the smallness of the transitions in polystyrene and of their location well below the glass-transition point (ca. 105°C) it seems likely that only very local molecular motions are involved, e.g. the /3- and -/-mechanisms studied in polymer viscoelasticity2. In polystyrene the phenyl gro,ups are thought to be responsible for the small/3-transition and the even smaller -/-transition has been attributed 3 to the presence of adjacent ----CH2-- groups resulting from head-to-head polymerization. The measurements of Illers and Jenckel" show the fl-transition (at 1 cycle/see) lying between 40 ° and 50°C and it may be suggested that this mechanism underlies the phenomena discussed above. However, the main point we wish to make in this Note is that the phenomena of solution- and bulk-transitions occurring at the same temperatures is also found in the protein systems collagen-saline 3 and keratinwater s where the solution mechanism at least is certainly a helix-coil transition. Optical rotation or viscosity measurements on dilute solutions prepared from mammalian collagen r show a pronounced helix-coil transition over a very narrow range of temperature; yon Hippel and Wong ~ have shown that AT, the difference in temperature between the points at which the transition is one quarter and three quarters complete, is only 2 +0-5°C. The location of this transition is in the range 35 ° to 40°C for dilute saline solutions near to neutrality. Measurements of the apparent specific volume of the bulk material in dilute saline show a small and narrow volume transition in the 90

COMMUNICATIONS AND NOTES

same region of temperature. The curve C in Figure 1 is an example of such a 'minor' transition for kangaroo tail tendon in 0"9 per cent saline solution (the 'major' transition in this system occurs at a higher temperature and consists of melting of the crystalline phase). There is thus a formal identity with the situation in polystyrene. The comparable behaviour of keratin is less dear, partly because the native material is insoluble, and measurements of the thermal stability of solutions have therefore to be carried out on a-helix-forming proteins extracted after reduction of the disulphide bonds responsible for the original insolubility. In this way optical rotatory dispersion measurements 9 showed a very broad helix-coil transition at pH 9"1, centred on 55°C and with a A T of approximately 23°C. Specific volume measurements on solid keratin immersed in water s also showed a transition region between about 45 ° and 65°C, having a qualitative resemblance to the behaviour reported by Moraglio and Danusso for bulk polystyrene. Curve K in Figure 1 illustrates a typical result for Corriedale wool in water.

C

E 3

..d

Figure /--Minor thermal expansion transitions in bulk polymer. C - Collagen-saline solution; P--Polystyrene (after Moraglio et al.1); K - Keratin-water

L nO

)(3

40 60 Temperature, °C

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Notwithstanding the obvious structural differences between the three polymer systems considered, it is seen that in polystyrene and collagen, and perhaps also in keratin, the solution behaviour can be detected in the solid state. It may be considered that on statistical grounds there will be regions where the molecular packing is exceptionally open and the local free volume so large that the molecular segments experience little interaction from neighbouring molecules. The material in these regions will undergo any transition which occurs in a dilute solution, and this will be detected if a sufficiently sensitive technique is used. The extent of these regions may be small and dependent on the thermal history of the specimen, varying for 91

C O M M U N I C A T I O N S A N D NOTES

instance between specimens which have been abruptly quenched from a higher temperature or more slowly annealed. This would account for the smallness and variability of the phenomena described. Nevertheless, phenomena of this type may be expected to be found quite generally in either glassy or partly crystalline polymers. One difference between the situations in polystyrene and in collagen is that the solution transition for collagen is a first-order effect involving helix-coil transformations of every tropocollagen molecule in the solution. The smallness of the bulk transition thus indicates that only a small fraction of the molecules can be involved. Nevertheless, if this fraction is directly involved in carrying the mechanical stresses, it may be crucial in certain physiological situations. For example, in collagen the phenomena of stress relaxation and length recovery show marked changes TM at approximately 38°C, the temperature of the transition in question. Permanent damage to the tissue may then be produced by straining at temperatures much above this value, which is significantly very close to normal deep-body temperatures. A polymer which could provide a clear test of the foregoing ideas is poly(cyclohexyl methacrylate). This gives a well defined viscoelastic transition below the glass-transition point which has been shown 11 to arise from alternation of the cyclohexane ring between two isomeric chair forms of the molecule. Other polymers containing the cyclohexyl group exhibit similar '/~-mechanisms' at the same frequency-temperature combinations, indicating that the mechanism is operating independently of the environment. On this supposition, dilatometry of the polymer in bulk or in dilute solution should reveal the same transition, as with the materials discussed above. P. MASON B. I. RIGBY

Division of Textile Physics, C.S.I.R.O. Wool Research Laboratories, Ryde, Sydney, Australia. (Received October 1964) REFERENCES I MORAGLIO, G., DANUSSO, F., BIANCHI, V., RoSSl, C., LIQUORI, A. M. and QUADRIFOGLIO, F. Polymer, Lond. 1963, 4, 445 FERRY, J. D. Viscoelastic Properties of Polymers. Wiley : New York, 1961 a ILLERS, K. H, and JENCKEL, E. J. Polym. Sci. 1959, 41, 528 4 ILLERS, K. H. and JENCKEL, E. Rheol. Acta, 1958, 1, 322 5 MASON, P. and RIGBY, B. J. Biochim. biophys. Acta, 1963, 66, 448 s MASON, P. Textile Res. J. In press r DOlY, P. and NlSmHARA, T. in Recent Advances in Gelatin and Glue Research (ed. G. STAINSBY).Pergamon : London, 1958 8 VON HIPPEL, P. H. and WONG, K-Y. Biochemistry, 1963, 2, 1399 9 HARRAP, B. S. Austral. J. biol. Sci. 1963, 16, 231 10 RIGBY, B. J., HIRAI, N., SPIKES, J. n . and EYRING,, H. J. gen. Physiol. 1959, 43, 265 11 HEUBOER, J. Kolloidzschr. 1956, 148, 36

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