Wortmann Et Al 2006

F.-J. Wortmann1 M. Stapels1 R. Elliott2 L. Chandra2

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DWI at RWTH Aachen University, Pauwelsstrasse 8, D-52074 Aachen, Germany

The Effect of Water on the Glass Transition of Human Hair

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Unilever Research, Port Sunlight, CH63 3JW Wirral, UK Received 19 August 2005; revised 25 November 2005; accepted 8 December 2005 Published online 15 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/bip.20429

Abstract: The glass transition of human hair and its dependence on water content were determined by means of differential scanning calorimetry (DSC). The relationship between the data is suitably described by the Fox equation, yielding for human hair a glass transition temperature of Tg ¼ 1448C, which is substantially lower than that for wool (1748C). This effect is attributed to a higher fraction of hydrophobic proteins in the matrix of human hair, which acts as an internal plasticizer. The applicability of the Fox equation for hair as well as for wool implies that water is homogeneously distributed in a-keratins, despite their complex morphological, semicrystalline structure. To investigate this aspect, hair was rendered amorphous by thermal denaturation. For the amorphous hair neither the water content nor Tg were changed compared to the native state. These results provide strong support for the theory of a quasi-homogeneous distribution of water within a-keratins. # 2005 Wiley Periodicals, Inc. Biopolymers 81: 371–375, 2006 This article was originally published online as an accepted preprint. The ‘‘Published Online’’ date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at [email protected] Keywords: equation

human hair; differential scanning calorimetry; water content; glass transition; Fox

INTRODUCTION The glass transition of keratin fibers and particularly its dependence on water content is of distinct practical relevance for textile as well as for cosmetic applications. For wool textiles the glass transition plays a major role for the understanding of the (undesirable) phenomenon of wrinkling1,2 while for human hair it

is a decisive factor for an analogous phenomenon, namely, the (desirable) stability of the water wave in human hair. Various approaches have been made to determine the humidity dependence of the glass transition temperature Tg in wool by mechanical tests1 as well as by differential scanning calorimetry (DSC).3,4 Though it may be argued that, in view of its relevance for me-

Correspondence to: F.-J.Wortmann, University of Manchester, School of Materials, P. O. Box 88, Manchester MQ60 1QD, UK; e-mail: [email protected] Biopolymers, Vol. 81, 371–375 (2006) # 2005 Wiley Periodicals, Inc.

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chanical properties, Tg should best be measured by mechanical means, DSC is a fast and readily applied method offering the additional advantage that constant conditions of water content for a sample are more easily maintained. The objective of the current investigation was to determine the glass transition temperature for human hair and its dependence on water content by DSC as well as to find a suitable analytical description for the data. Furthermore, the investigation was extended to thermally denatured human hair in order to contribute to our understanding of the distribution of the water in the morphological structure of human hair.

EXPERIMENTAL All experiments were conducted on commercial, mediumbrown, untreated European human hair (Fa. Kerling, Backnang, Germany) provided in the form of tresses. From these tresses, fiber snippets (length 2–4 mm) were obtained. All experiments were conducted with a DSC-7 instrument (Perkin Elmer) using large-volume, pressure-resistant, stainless steel capsules (Perkin Elmer). It is well known that the physical history of a glassy material, namely aging and annealing, has a major influence on the measurement of the glass transition by DSC, especially by inducing an excess enthalpy peak. Therefore, a procedure was chosen to control the history of the hair as far as experimentally feasible by establishing stable conditions of the water content for the samples, on the one hand, while avoiding overly strong effects of aging due to long storage on the other. To preserve this status, all measurements were furthermore limited to conditions of less than 20% water content to ensure that Tg was well above the storage temperature under ambient room conditions (20–258C). Samples of about 100 mg of hair snippets were stored in a desiccator first over P2O5 for 24 h and then for four days at 208C over a suitable, concentrated salt solution, providing a constant relative humidity. For the DSC test, 5–10 mg of material was rapidly transferred into a DSC capsule; 10
L of silicone oil was added and the capsule sealed. The silicon oil was added to suppress as far as possible water desorption from the sample during the measurement, which is considered the major source of certain experimental problems, as particularly observed by Kure et al.4 The DSC was calibrated, prior to each series of experiments, using indium and palmitic acid, both of high purity. For the measurements an empty container without the Oring rubber seal was used as reference. The optimum heating rate was found to be 58C/min, providing adequate signal intensity while being on the safe side with respect to nonlinearities of the measurement, which are expected at higher heating rates. The water content of hair was determined on parallel samples by DSC. Conditioned hair snippet samples were

weighed into gas-tight aluminum pans. Prior to the measurement, the lid of the pan was perforated. The sample was heated to 1358C (208C/min) and held at that temperature for 30 min, then cooled to room temperature under a flush of nitrogen. In view of the discussion of the dry weight of keratin,5 this procedure was chosen because the completion of the desorption process of water was readily observed in the DSC curve. Weighing the sample before and after desorption yielded its water content, which is the weight percentage of water in the conditioned sample. This parameter is not to be confused with the term regain, which relates to the dry weight of the sample. To enable the measurement of the glass transition for totally amorphous hair material, the crystalline -helical structures were irreversibly denatured.6 To achieve complete denaturation, hair fiber snippets were submitted to a DSC measurement in water in the temperature range 80– 1708C (58C/min),6 beyond the denaturation peak at around 1508C. Neither on cooling nor in a further DSC experiment in water was any indication of a reformation of -helical material found. After cooling, the capsules were opened; the denatured hair snippets were removed, dried, and stored under ambient conditions until used in DSC experiments for measuring Tg in the same way as the native hair material.

RESULTS AND DISCUSSION Figure 1 shows a typical DSC curve for human hair with a water content of 15%. The temperature range of the transition is given by the onset and the offset temperature, respectively, giving a span of about 158C. Following the considerations for the definition of Tg, as reviewed by Peyser7 and as prescribed in DIN-53765,8 the glass transition temperature is defined as the midpoint between the lower and the upper level of the DSC curve. This definition of Tg is expected to correlate well with mechanical data.7 Due to the experimental procedure, the peaks for excess enthalpy were generally small in the curves so that this determination could be maintained for all experimental curves without introducing a relevant bias and without the need to take recourse to integral methods for the determination of Tg, as proposed by Richardson and Savill.9 For the comparison of Tg measurements of keratin fibers by DSC, in general it is important to note that this standard practice for the determination of Tg is at variance with the work by Phillips,3 Kure,4 and Huson10 for wool and Ota et al.11 for human hair. They determined the onset of the glass transition in the DSC-curve as Tg, a practice covered by ASTM D3418.12 Biopolymers DOI 10.1002/bip

Effect of Water on Glass Transition of Hair

FIGURE 1 Typical DSC curve of human hair with 15% water content (heating rate 58C/min). The graphical definition of the glass transition temperature is given.

An initial observation of the systematic change of the glass transition of the keratin fiber wool with water content was made by Wortmann et al.1 based on measurements of torsional recovery of wool fibers, corroborated by data from a variety of other tests. It was shown that the data for water content between 9 and 20% were well described by the Fox equation13: 1=Tg ¼ w1 =Tg1 þ w2 =Tg2 ; where w is the weight fraction and the subscripts 1 and 2 refer to dry wool and pure water, respectively. Fox13 expected his equation to apply to systems that are compatible and not too strongly polar. Equation (1) implies that the depression of Tg with solvent concentration is fully explained by the Tg differences of the constituents of the system, regardless of specific interactions. Features such as crystallinity or crosslinking only play a role insofar as they affect diluent distribution.14,15 For keratins, the size of the step in the DSC curve, which is the change of heat capacity DCp at the glass transition, is experimentally quite variable (0.03– 0.2 J g
1 K
1) and shows a mean of only about 0.1 J g
1 K
1 over the humidity range considered. This is quite small compared to DCp  0.4 J g
1 K
1 found for other proteins.16 For this reason, the DSC values for Tg for hair are somewhat more variable than those obtained for the mechanical measurements for wool.1 The size and variability of the change in heat capacity at Tg furthermore constrained the use of other multiparameter approaches, such as the Couchman–Karasz17 or the Gordon–Taylor18 equation. Biopolymers DOI 10.1002/bip

373

To stabilize the fit of the Fox equation to the data and for ease of comparison with wool, the Tg value for water 125 K (
1488C) is taken from Ref. 1, yielding for the dry hair Tg1 ¼ 417 6 3 K (6 standard error), equivalent to 1448C.19 The solid line in Figure 2 shows the fit through the DSC results. The value for the glass transition temperature of amorphous solid water has been subject of considerable debate, as reviewed by Kalichevsky et al.20 There appears to be a general consensus, however, that the Tg of water falls into the region between 130 and 145 K at normal DSC heating rates. Considering the wide range for the extrapolation, the applied value for the Tg of water is in acceptable agreement with the literature data. The glass transition temperature of hair (1448C) is 308C smaller than for wool (1748C). As a consequence, the Fox equation curve for Tg lies systematically lower compared to wool as shown in Figure 2. Looking at the shift between the curves in terms of water content yields a value of 2–3%. Human hair thus ‘‘softens’’ at lower water content and temperatures than wool. This is a rather unexpected result, because human hair is generally considered more hydrophobic than wool, due to a higher fraction of hydrophobic high-glycine-tyrosine (HGT) proteins.21,22 The applicability of the Fox equation for the human hair/water as well as for the wool/water system,1 without the need to consider, e.g., crystallinity, implies that water, despite the complex morphological, semicrystalline structure of keratin fibers, acts as if it is homogeneously distributed throughout the fiber structure.

FIGURE 2 Glass transition temperature Tg vs. water content for native (*) and amorphous (^) hair samples. The curves relate to the fit of the Fox -equation for native hair (solid line) and sheep’s wool (broken line, Ref. 1), respectively. The lines are limited on the x-axis by the anticipated maximum water content of the materials.

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In this context it is important to note that the effects of water on a somewhat analogous polymer, namely nylon 6, do not follow the Fox equation.23 In nylon 6 as well as 66, water sorption drops with the degree of crystallinity.24 The same principle seems to apply for a variety of partly -helical proteins, such as lactoglobulin, zein, and others.25 This does not apply for ovalbumin, silk fibroin, and wool, for which Mellon et al.26 showed that even severe treatments, that lead to denaturation of the higher protein structures, have very little effect on water sorption. As early as 1959, Feughelman27 proposed a twophase filament/matrix model for the interpretation of the mechanical properties of keratins. The axially oriented, mechanically effective filaments have been identified as the -helical material (30 vol %) in the intermediate filaments (IF). The matrix comprises the remainder of the morphological components, including the nonhelical fraction of the IFs, the intermediate filament associated proteins (IFAPs), which form the classical27 matrix, as well as other morphological components, such as cuticle, cell membrane complex, nuclear remnants, etc. (70 vol %).28 It is well established29,30 that water has a strong plasticizing effect on the matrix phase, while the modulus of the filaments remains unaffected by the uptake of water. In this view there appears to be a conflict with the assumption of effectively homogeneously distributed water, as derived from the applicability of the Fox equation. Following previous investigations,1,3,10 Kure et al.4 again determined by DSC the glass transition temperature of wool as a function of regain. Applying the implication of the two-phase model, they introduced the hypothesis that water is only located in the matrix phase of the fiber, while the filaments take up no water. Consequently, they calculated the water content that was relevant for the analysis of their Tg data for wool on the basis of the matrix fraction only, thus introducing a bias compared to the usual values. The same bias was subsequently introduced by Pierlot31 for the analysis of the water sorption of wool. Such a bias would obviously have rather far-reaching consequences beyond representing just a calculational issue, affecting the description and interpretation of a wide variety of water-dependent properties of keratins such as mechanical, thermal, sorption, etc. To further investigate the question of the water distribution in human hair, Tg measurements were conducted on thermally denatured, amorphous hair. If the crystalline phase (30%) is transferred into an amorphous state, it is expected to absorb water similarly as the matrix and thus increase the water content

at a given relative humidity. This would be expected to lead to significant changes of the humidity dependence of the glass transition. However, the determination of water content for native and amorphous hair, conducted in the context of the Tg determination, showed no significant differences between the two materials. This experimental result is in conflict with the hypothesis put forward by Kure et al.4 and Pierlot,31 but in agreement with the investigations by Mellon et al.26 The results for Tg for amorphous hair are summarized in Figure 2. The data fit well into the data set for the native hair, showing that hair crystallinity has no effect on water sorption and on Tg.

CONCLUSIONS Hair takes up systematically less water than wool at equal relative humidity and should thus be generally less susceptible to the effects of water. In contrast to this expectation, it is observed that the glass transition for hair is systematically about 308C lower for hair compared to wool. For both materials, the change of Tg with water content is well described by the Fox equation. The decreased glass transition of human hair compared to wool is attributed to a difference in composition of the matrix proteins. These contain a higher fraction of hydrophobic HGT proteins in the case of hair, which may induce a lower amount of hydrogen bonding in the matrix that stabilizes the glassy state. These proteins are thus acting as ‘‘internal’’ plasticizer. That the partly -helical, native, and the denatured amorphous hair shows the same dependence of Tg with water content leads to the conclusion that water is in fact quasi-homogeneously distributed in native hair, where the adsorption to the surface of the -helical filaments is similar to the amount absorbed into the matrix. Some insight into the interaction between -helical proteins and water has been given by Manas et al.32 on the basis of IR analysis. The special performance of -keratin is attributed to the filamentous arrangement of the -helical material in the IFs, leading to a large specific surface with good accessibility for water.

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Effect of Water on Glass Transition of Hair 4. Kure, J. M.; Pierlot, A. P.; Russell, I. M.; Shanks, R. A. Text Res J 1997, 67, 18. 5. Watt, I. C.; Kennett, R. H.; James, J. F. P. Text Res J 1959, 29, 975. 6. Wortmann, F.-J.; Deutz, H. J Appl Polym Sci 1993, 48 137. 7. Peyser, P. In Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1989; Chap VI, p 209. 8. DIN 53765, 1994. 9. Richardson, M. J.; Savill, N. G. Polymer 1975, 16, 753. 10. Huson, M. G. Polym Int 1991, 26, 157. 11. Ota, Y.; Fukumashi, A.; Nishimura, Y.; Nakamura, K. Sen-i Gakkaishi 1996, 52, 1. 12. ASTM D3418, 1982 (re. 1988). 13. Fox, T. G. Bull Am Phys Soc 1956, 1, 123. 14. Jin, X.; Ellis, T. S.; Karasz, F. E. Makromol Chem 1985, 186, 191. 15. Agarwal, N.; Hoagland, D. A.; Farris, R. J. J Appl Polym Sci 1998, 63, 401. 16. Rouilly, A.; Orliac, O.; Silvestre, F.; Rigal, L. Polymer 2001, 42, 10111. 17. Couchman, P. R.; Karasz, F. E. Macromolecules 1978, 11, 117, 1156. 18. Gordon, M.; Taylor, J. S. J Appl Chem 1952, 2, 493.

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19. Stapels, M. M.Sc. thesis, RWTH Aachen University, Germany, 1997. 20. Kalichevsky, M. T.; Jaroszkiewics, E. M.; Blanshard, J. M. V. Int J Biol Macromol 1992, 14, 257. 21. Gillespie, J. M.; Frenkel, M. J. Comp Biochem Physiol 1972, 47B, 339. 22. Wortmann, F.-J.; Wortmann, G.; Zahn, H. Text Res J 1995, 65, 669. 23. Kettle, G. J. Polymer 1977, 18, 742. 24. Puffr, R.; Sebenda, J. J Polym Sci 1966, C16, 79. 25. Breuer, M. M. J Soc Cosmet Chem 1972, 23, 447. 26. Mellon, E. F.; Korn, A. H.; Hoover, S. R. J Amer Chem Soc 1949, 71, 2761. 27. Feughelman, M. Text Res J 1959, 29, 223. 28. Wortmann, F.-J.; Zahn, H. Text Res J 1994, 64, 737. 29. Feughelman, M. Mechanical Properties and Structure of Alpha-Keratin Fibres; UNSW Press: Sydney, 1997. 30. Wortmann, F.-J.; De Jong, S. Text Res J 1985, 55, 750. 31. Pierlot, A. P. Text Res J 1999, 69, 97. 32. Manas, E. S.; Getahun, Z.; Wright, W. W.; DeGrado, W. F.; Vanderkooi, J. M. J Am Chem Soc 2000, 122, 9883.

Reviewing Editor: Laurence Nafie