Viscoelasticity of Biomaterials - American Chemical Society

Chapter 9

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 16, 2014 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0489.ch009

Viscoelasticity of In Situ Lignin as Affected by Structure Softwood vs. Hardwood A-M. Olsson and L. Salmén Swedish Pulp and Paper Research Institute, Stockholm S-114 86, Sweden The viscoelastic properties of various wood species under water-saturated conditions have been studied by mechanical spectroscopy. Tests were made at temperatures from 20° to 140°C at frequencies ranging from 0.05 to 20 Hz. It is shown, contrary to earlier data, that hardwood lignins have lower softening temperatures than softwood lignins. For the hardwood lignins the softening process also has a lower apparent activation energy. Increased cross-linking of the lignin, achieved by heating in an acid environment, raises the softening temperature and increases the apparent activation energy of the softening process. In all cases the softening follows a WLF type of behavior, indicating that under wet conditions the viscoelastic properties of the lignin govern the properties of the wood fiber. The differences noted between softwood and hardwood lignin are discussed in terms of structural parameters. The properties of wet wood reflect to a large extent the properties of the water-saturated lignin within the wood (1). This is due to the fact that the carbohydrates, both the hemicelluloses and the amorphous cellulose, are highly softened under water-saturated conditions already at 20°C, leaving only the cellulose crystals and the stiff lignin as load-transferring materials. The cellulose crystals have mainly an elastic response and may thus be viewed as an inert filler material in a lignin matrix. It has also been shown that the viscoelastic properties of water-saturated spruce wood follow a W L F type of behavior (1), indicating that the lignin behaves as a normal amorphous polymeric material under these circumstances. Thus, studies on water-saturated wood may make it possible to deduce something about the specific properties of the native lignin in that particular wood species. 0097-6156y92A)489-0133$06.00A) © 1992 American Chemical Society

In Viscoelasticity of Biomaterials; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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V I S C O E L A S T I C n Y O F BIOMATERIALS

Viscoelastic measurements on various wood species have earlier indicated that there is no difference in the softening behavior between softwood and hardwood species (2), which suggests that structural differences between hardwood and softwood lignins have no bearing on the viscoelastic properties of the lignin. More recent measurements with differential scanning calorimetry on moist lignin samples (3,4) have, however, indicated some differences in softening behavior between wet lignins from hardwood and softwood. The present study was undertaken in order to clarify the influence of the native lignin structure on its viscoelastic properties and to see whether the structural differences between hardwood and softwood lignins have any effect on the mechanical properties of the wood. The viscoelastic properties were studied under water-saturated conditions between 20° and 140°C for spruce, pine, birch, and aspen, and also for a spruce sample deliberately cross-linked under acidic conditions. Experimental Procedure Materials. Four wood species have been used: two softwoods, Norwegian spruce (Picea abies) and Scandinavian pine (Pinus silvestns), and two hardwoods, Scandinavian birch (Betula verrucosa) and European aspen (Populus tremula). The wood samples were cut with the longitudinal direction across the grain with a length of 70 m m and a cross-section of 15 x 50 m m . Before being tested, the samples were saturated with water and then preconditioned with a steam treatment for 30 min at 135°C. One sample of spruce wood was instead impregnated with HC1 at a p H of 1.8 and then heated for 30 m i n at 135° C in order to cross-link the lignin within the wood. Mechanical Spectroscopy. Dynamic mechanical properties were measured at temperatures between 20° and 140°C, keeping the relative humidity at 100% in the testing autoclave. Forced sinusoidal vibrations with zero mean stress in the frequency range from 0.05 to 20 Hz were applied using a Material Testing System ( M T S ) servohydraulic testing machine. The deformation was measured with an extensometer attached to the specimen. In all cases the amplitude was kept within the viscoelastic range, up to 0.1% deformation (1), and data have been obtained by extrapolation to zero deformation. Evaluation. Dynamic mechanical properties for each temperature and frequency were calculated at each of three amplitudes from the means of about 50 sinusoidal loops. The storage modulus E ' and the loss coefficient t a n i were determined from the complex modulus E * , obtained from the maximum and minimum values of the stress and strain and the area of the hysteresis loop. The moduli are based on the macroscopic dimensions of the wood specimens at 20°C and are thus corrected for changes in specimen volume with temperature (1). Master curves were constructed by shifting log E ' - l o g frequency curves for a specific temperature horizontally with respect to the mean values of


9.

Viscoelasticity of In Situ Lignin

OLSSON & SALMÉN

135

Table I. The glass transition temperatures at different frequencies for the wood species tested Softening Temperature, T^, °C


Frequency Hz 0.05 0.1 0.5 0.6 2.0 4.0 5.0 6.0 20

Aspen

Birch

Pine

Spruce

—

— — —

— —

83

64

— 68

—

92.5

70 75

— — —

80 85

—

— —

73.5

— — 82

99

103

Spruce Crosslinked

— —

—

88.5

113

96

120.5

100

124

— — —

— — —

—

—

the linear regression curves for two consecutive temperatures, Figure 1. In order to apply the W L F concept [Williams, Landel and Ferry (5)] to the master curve and then more accurately evaluate the WLF-constants, these were determined separately by first fitting fifth order polynomial curves to the experimental log E'-temperature curves as illustrated in Figure 2. Shift factors log αχ were then determined from calculated log E'-frequency curves taken every fourth degree. The W L F constants, C7 and C2, were determined by rewriting the W L F equation in the form: (T - T

r e /

)/loga

T

= l / C i ( T - Trej) + C /d 2

(1)

and then taking the linear part of the graph of ( Γ — T / ) / l o g arvs. T—T j. The reference temperature, T / , is taken to correspond to the maximum value of the W L F - t y p e behavior. The W L F equation thus obtained is well in line with the experimentally determined shift factor as illustrated in Figure 3 where log αχ is given as a function of Τ — T / based on the experimental determination, with the curve representing the W L F equation given as determined from the polynomeric fits. re

re

r e

r e

Results The softening temperature obtained by mechanical spectroscopy is most easily defined by the maximum in the mechanical loss coefficient, t a n i . Table I gives the glass transition temperatures of water-saturated native lignin, defined in this way, at the various frequencies for the wood species tested. These tests refer to measurements across the grain, but no major difference in glass transition temperature is noticed for measurements along the grain (6).



136

VISCOELASTICITY OF BIOMATERIALS

50 h 10 Frequency ( Hz ) Figure 1. Master curve for the storage modulus of birch wood tested under water-saturated conditions i n the temperature range from 23 to 130°C.

Figure 2. The storage modulus at different frequencies (0.6, 2.0, 6.0 and 20 Hz) for birch wood as a function of temperature. Fifth order polynomial curves are fitted to the experimental values.


9.


OLSSON & SALMÉN

137

It is obvious that softening in the hardwood species occurs at a lower temperature than i n the softwood species, i n contrast to the results of some measurements performed in torsion (2 J ) . The frequency dependence of the glass transition for the different wood species is illustrated i n Figure 4, where the logarithmic frequency is plotted vs. the reciprocal glass transition temperature i n an Arrhenius diagram. The slope of the lines is proportional to the apparent activation energy for the softening process, AH , which is evidently lower for the hardwood than for the softwood lignins. The apparent activation energies for the various lignins are given i n Table II as determined by: Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 16, 2014 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0489.ch009

a

A^a(Arrheniue) = 2.303 · R(A log / ) / Δ ( 1 / Γ )

(2)

#

For the spruce wood in which the lignin had been cross-linked by the action of acid, both the softening temperature and the apparent activation energy of the softening process were higher than for all other samples tested. Table II. Data for the softening process for the various wood species tested

Aspen

Birch

Pine

Spruce

Spruce Crosslinked

A.Ha(Arrh.) (kJ/mol)

290

240

400

380

420

A.ff (WLF) (kJ/mol)

330

360

440

390

460

TrefCC)

50

62

66

71

90

e

« / ( d e g " ) 1.86 x l O " /, 0.0212 1

4

0.79 x 1 0 " 0.0142

4

1.11 χ 1 0 " 0.82 χ 1 0 " 0.29 x 10 0.0144 0.0084 0.0155 4

4

Figure 5 shows the relation between the softening temperature and the apparent activation energy of the softening process, AH together with the empirical relations obtained by Lewis (8) for sterically restricted and unrestricted polymers. Apparently, the general trend found for polymers that AH increases with increasing softening temperature also holds for the lignin i n different wood species. It is, however, difficult from this figure to discuss the structural restrictions of the lignin polymer, since at high temperatures the experimental data fall between the two equations even for synthetic polymers. Earlier investigations (1,9) have shown the possibility of constructing time-temperature correspondence curves, master curves, according to the W L F concept for water-saturated wood samples. Such master curves show the relation for the stiffness over a larger frequency interval at a given temperature and have here been constructed from the measurements on the different wood species, as described earlier. ai

a


138

VISCOELASTICITY OF BIOMATERIALS


ο

Figure 3. The shift factor, log ay, vs. the temperature difference, T — T / , for the master curve of birch. The curve is calculated from the polynomial fit and the points are taken from experimentally shifted data. r e

2.5

2.6

2.7 1000 / T

2.8 ( Κ ) 1

g

Figure 4. The glass transition temperature at various frequencies for the different wood species plotted in an Arrhenius diagram. The temperature decreases to the right in the diagram.


9.

OLSSON & SALMÉN


139

Figure 6 shows master curves for the spruce and aspen specimens at a reference temperature of 100°C. In order to compare the softening processes of the two samples, the curves have been shifted vertically to the same modulus at 20°C. These curves clearly show that the softening of the aspen occurs at higher frequencies, which correspond to lower temperatures, than the spruce sample. This is yet another confirmation that the glass transition temperature of hardwood lignins is lower than that of softwood lignins. The modulus drop over the softening region in Figure 6, taken as the relative decrease Embber/Egiass, * about 72%, determined as the vertical difference between the tangents to the master curve at 1 0 " and 1 0 Hz, respectively. This decrease is somewhat larger than the drop calculated for wood across the grain using modulus values for isolated lignin (10) but, considering the approximations, it is quite a reasonable value for the type of cross-linked polymer in a composite here considered. When the W L F equation is expressed in the linear form, the range of its validity and its parameters may be determined, as discussed under Experimental. The lowest temperature where the W L F equation is valid is considered to be equal to the glass transition temperature at very low frequencies and is given in Table II as T / . In all cases, the W L F equations are valid up to about 120°C. This is somewhat lower than the usual validity range of T + 100°C, which may be because degradation at high temperatures affects the measurements. The fractional free volume, f and the volumetric expansion coefficient of the free volume, a y , determined from the W L F constants, are also given in Table II. Both f and otf decrease for both hardwood and softwood with increasing glass transition temperature, and there is only a small difference between the two types of wood species at about the same softening temperature. From the shift factors of the master curve, the limiting apparent activation energy, Aif (wLF)> t T / may also be determined as: s


5

10

r e

g

gi

g

a

A#a(WLF) = 2.303 · Λ(ί log a ) / 6 ( l / T ) T

a

r e

(3)

This value for the different wood species is given in Table II. The relation between the apparent activation energy determined from the W L F equation and T / is given in Figure 7 together with the data based on the Arrhenius plot given in Figure 5. In this case there is also a general tendency for the apparent activation energy to increase with i n creasing softening temperature as given by T y . The increase with tem perature is also very similar for the two determinations, whether by the W L F equation or by the Arrhenius plot. This is as expected, of course, for the behavior of synthetic polymers, as the apparent activation energy for the glass transition increases at lower frequencies. The effect of cross-linking of the lignin on the viscoelastic properties of the wood is illustrated in Figure 8 with master curves for spruce samples, with the reference temperature taken as 100°C. It is obvious that crosslinking shifts the softening region towards lower frequencies, which implies a higher softening temperature. It is evident that neither the modulus value in the glassy region at high frequencies nor that in the rubbery region at low r e

r e


140


VISCOELASTICnY OF BIOMATERIALS

1-10

1

HO

5

1-10

11

F r e q u e n c y ( Hz )

Figure 6. Master curves for spruce and aspen wood, shifted vertically to the same modulus at 20°C. The reference temperature is taken as 100°C and the frequency is obtained from the shift factor log αχ.


9.


500 ι

141

Viscoelasticity ofIn Situ Lignin

OLSSON & SALMÉN

1

—

Figure 7. The apparent activation energy for the softening process, as a function of the softening temperature, T .

AH , a

g

500 400 ~ 300 o_ 2 r 2oo LU CO

crosslinked lignin

jA

Ο

*

_D

3 Ό Ο Φ

Ο

*

α» Ο ο*

Viscoelasticity of Biomaterials - American Chemical Society

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