Brittle-Ductile Transitions in High Density Polyethylene Films

Lubricants and Lubrication / D. Dowson 1995 Elsevier Science B.V.

et al.

(Editors)

153

Brittle-Ductile Transitions in High Density Polyethylene Films B J. Briscoe and P.S. Thomas Department of Chemical Engineering and ChemicalTechnology, Imperial College of Science, Technologyand Medicine, London, SW7 2BY.

The paper describes a peculiar transition in the interfacerheology or frictional characteristicsof high density polyethylene thin films. This transition is unusual in the sense that the transition is apparently from a ductile phase to a brittle phase as the temperature increases.The transition is observed to be a function of the contact pressureand sttain rate. Again, andpeculiarly. the transition is observed to decrease with an increase in both of these contact mechanical parameters.In addition, the vibrational spectraof HDPE as a function of temperature are examined in order to identify a mechanism for this transition. Due to the unusual nature of the transition, the mechanism sought is not at the molecular level, but is rationalisedin term of the melting dynamics of the crystalline lamellae of HDPE.

1. Introduction The interfacialrheological properties of organic polymers have been subject of investigation for several decades. In this time, their properties in shear have been characterised and may be summarised by a series of simple, and reasonably

is observed to be characteristicof a transition from a ductile phase to a brittle morphology. This transition was first observed by Briscoe and Tabor (1) and was found not to occur in low density polyethylene (LDPE) systems. This transition is the subject of the present paper and is discussed in term

the interfacial shear strength on several contact

of its contact pressure and strain rate dependence. In addition,the architecturalpropertiesat the molecular

mechanical parameters some of which are described below. These relationships have been shown to be

level are also investigated using vibrational spectroscopy in order to identify the origins of the

general for a wide variety of organic, both short chained surfactant monolayers and high molecular

transition and to identify the origin of its peculiarity to the HDPE system.

accurate,empirical relationships for the dependence

weight polymers, as well as inorganic materials (13).

1.1 Rheology

The currentpaper concentratesupon the interface rheology of a particular organic polymer, that is,

The rheologicalproperties of thin polymer films have been characterised by the measurement of the

high density polyethylene (HDPE). HDPE is chosen

due to the observation of a peculiar “phase

interface shear strength, ‘5, which is defrnedas the frictional force, F, per unit area, A, of contact. It is

transition” in its interface rheology as a function of temperature (1). By a comparison with the

synonymous to the magnitude of the energy dissipated in sliding friction per unit areaof contact

dependence of the contact mechanical parameters on the brittle-ductile phase transition in amorphous

per unit sliding distance. ‘5 is a strong function of several contact mechanical variables including; the

polymers (4), the nature of the transition in HDPE

mean contact pressure, P, the temperature, T, and

3

I

2.5

I

2.7

I

2.9 1000 Km

I

3.1

I

3.3

1

3.5

Figure 1. The temperaturedependence of the interfaceshear stress for: poly(terrafluoroethy1ene) (a), calcium HDPE (0). LDPE (A), anthracene (A), sebasic acid (V) poly(methy1 methacrylate)(V), stearate (0). poly(cetosteary1methacrylate) (0).Contact pressure 6x10' Pa, sliding speed 0.2 mm/s. the sliding velocity, V, or the contact time [1-4]. These functionalities have been observed to have,

where f0. T~',and TO", are material constants, a is the pressure coefficient, Q is an 'activation energy',

experimental error. the approximate forms:

R is the gas constant and 8 is the velocity index. These functionalitiesare observed to be quite general

within

following

for polymers, although there are some exceptions which have been fully discussed elsewhere [3.5]. The current paper concentrateson the temperature dependenceof T for HDPE. The generality of the temperature dependence for a variety of organic polymers is shown in Figure 1 where it can be seen that the temperaturedependencefollows the form of Eq. 2. The 'activation energy' takes on the value of zero or some positive value usually between 10 to 20 W/mol(l). The value of zero is regardedas being indicative of interfacial brittle fracture and is

155 associated with a glassy polymer morphology (5). Positive values of Q are considend to be indicative

1.3 Molecular Structure

of ductile flow within the polymer film. If a phase transition is incorporated in the temperature range studied, two values of Q arenoted. Normally, the brittle phase is pmxded by the ductile morphology;

structure of the varying types of polyethylene (PE). Linear PE is simply (-CH2-)”. This material is difficult to synthesise due to the synthetic process. It is usually a free radical synthesis which often

see for example poly(methy1 methacrylate)in Figure 1. HDPE is observed to be the only known

produces irregularities in the polymer chain in the form of chain branching. For HDPE, there arc

exception to this norm. From Figure 1, for HDPE, it can be seen that a ductile morphology, denoted by

generally found to be around 3 alkane branches per thousand carbon atoms. LDPE contains around ten

15 kl/mol), precedesa a positive value of Q (a. brittle morphology showing a temperatm

times this proportion. The substituent alkane groups are important in the crystallisation p m s s

independenceand hence a value of Q equal to zero. It is the nature andorigin of this transition that is the

as they disrupt the crystalline lamellae producing dislocations (7). It is these dislocations that are

subject of this paper.

considend below to be responsible for the “peculiar transition” in HDPE.

1.2 Polymer Morphology In addition to

the interface rheological

measurements, the morphology of HDPE is also characterisedasa function of the temperatureusing

It is also important to note the molecular

2. Experimental Methods and Materials 2.1 Rheology The interface shear strength of HDPE was

vibrational spectroscopy. This study limits the use of this technique to the investigation of the effectof

measured using a model contact geometryof a glass sphere on a flat. The apparatus has been fully

temperature on the two crystallinemodes (6),one of

described elsewhere[131 and is only briefly Fepeated

which is split into two at 723 cm-’and730 cm-l.

here. The polymer films were prepared by film

These modes occur due to correlationsplitting in the 725 cm-l amorphous -CH2- rocking mode. Effectively, the intensities of these modes m proportional to the percentage crystallinity in the sample. It is the temperaturerange in which the melting transition occurs which is used to characterise the melting transition properties. The second of the crystalline vibrational modes is at 2845 cm‘ l. This mode is a CH stretching mode.

transfer onto smooth float glass slides by rubbing the polymer directlyonto the glass plates (8). An hemispherical indenter, of known radius, was loaded against the organic layer which was subsequently slid over the film under load, W, at constant velocity and the frictional force, F. was measured.In the loading regime (10’ to lo8 Pa), the deformation of the glass substrates is wholly elastic. It is assumed that the presence of the organic layer does not significantly affect the deformation of the substrates and the area of contact, A, is calculated using classical elasticity equations [9]. It is further

156

I

0

20

I

40

I

60

I

80

Tempemtud'C

I

100

I

120

I

140

Figure 2. The temperature dependenceof the interfaceshear stress for HDPE as a function of pressure: 200 MPa, 0, 180 MPa, 120 MPa, W, 63 MPa, 0 and 43 MPa, 0. The nominal strain rate was calculated at 2000 s-1.

+,

assumed that this areais equal to the molecular a m of the film in the contact and that there is no indenter-substratecontact which was borne out by the fact that on carefulexaminationof the slider, no

out using a Bomem Ramspec 152 spectrometer in which was placed a modifiedLinkam THMS 600

damage was apparent. Thus, the mean contact pressure (=W/A) and the interface shear strength

microscope temperature stage. The polymer films were cast from dilute perchloroethylene solution

(=F/A) may be calculated.The sliding velocity, was maintained constant at 30, 40, 100 and 200 pm/s

onto potassium bromide discs which were placed in the temperature stage for spectral acquisition over

and was considered not to induce frictional heating within experimental error. The samples were placed

the temperature range 30 to 220°C in increments of 10°C.

on a temperature stage with a control of +2"C. For the purposeof the calculation of the nominal strain

3. Results and Discussion

rate (=V/h), the film thickness, h, was assumed to be 1onm.

The temperaturedependenceofthe interfaceshear stress, T, for HDPE is shown in Figures 2 and 3 as

2.2 spectroscopy The spectroscopic characterisation was canid

a function of the contact pressure and the nominal strain rate. An unusual transition is observed in each

157

5-

8

4.5-

4-

3.5-

E

3-

3

2-

f

1.5-

a

1-

m

3 2*50)

0.5-

0

I

I

I

100 150 Temperatud"C

50

I

200

1

250

Figure 3. The temperaturedepndenceof the interfaceshear stress as a function of nominal strain rate: 2.2~104, 0, l.lx104, 0,3.7x103, 0 and 3 . 3 ~ 1 0s-', ~ for a contact pressure of 72 MPa.

+,

curve. For a variety of contact pressures and strain rates a rather monotonic declinein the shear stress is

to expectation. It is normally expectedthat for any phase transition, the transition is shifted to higher

observed as the temperature is increased. This behaviour is in accoTdancewith typical amorphous

temperature with increasing pressure or increasing strain rate. The shift of a transition to a higher

or semicrystalline polymers above their respective glass transition temperatures (4). At a critical

temperature, detected by changes in the contact mechanical properties, may be rationalised in terms

temperaturethe shear stress rapidly increases.Above this temperature the shear stress is also apparently

of shear stress activation at the molecular level in terms of molecular motions. Increasing a nominal

temperature independent. This behaviour is more characteristic of a brittle interfacial rheological

hydrostaticpressure across a shear activation site, where a molecular flow entity travels from one site

response (5). This transition is also observed to be a function

to another through a rearrangement of the molecular environment, increases the activation energy for the

of both the contactpressure and the strain rate. For both of these parameters, the transition is found to

molecular flow rearrangement (5). This in turn increases the macroscopically sensed shear stress.

be shifted to lower temperatures as the magnitude of the variable increases. This featureis again contrary

For the case of the strain rate dependence, shear stress activation may only occur if the activated

158

2854

720

Q 2852,

E Y p19

.-(

\

h

8

f 2850

6)

ff

L4

CL

718

I

I

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50 100 150 TemperatunPC

1

200

Figure 4. The temperature dependence of the frequency of one of a pair of split crystallinebands around 725 cm-lfor HDPE, and linear PE, 0.

730 2729 Y

b28 6)

._ $27

"

4

2848 -

sb

160

140

TempemtmPC

2 0

Figure 6. The temperature dependenceof the frequencyof one of a pair of split crystalline bands around 2845 cm-lfor HDPE, and linear PE, 0. relaxation modes whose relaxation times are within the defonnation time frame. The thermal activation of these modes therefore occurs at higher temperatures thus increasing the corresponding transition temperature. These trends are commonly encountered with virtually all organic films save for the case under examination. Due to the fact that the observed behaviour is the reverse of the expected, it is thereforeunlikely that the process responsible for the perceived transition is associated with these short length scales. The temperature dependenceof the fresuencyof three vibrational bands which are sensitive to the crystallinity in PE are shown in Figures 4 to 6.

726k I

725 0

50

100

150

200

TemperaWT

Figure 5 . The temperature dependence of the Erequency of one of a pair of split crystallinebands and linear PE, 0. around 725 cm-lfor HDPE,

Two of these bands (718 and 730 cm-l)are the

relaxation times of the molecularentities that are to be murangedare within the rate at which at which

result of correlation splitting, due to the crystallinity in PE, of the CH, rocking mode

the deformationis being applied. An increasein the strain rate, reduces the number of activated

(Figures 4 and 5 ) . Their frequencies are directly related to the dimensions of the unit cell (10). The

third band is a CH, stretching mode at 2845 cm‘l Figure 6). In addition to the modes of HDPE the temperaturedependence of these modes for linear PE are shown as a reference. In all cases, there is evidenceof a transition brought about by melting. There is a marked differencebetween the behaviour of the reference and HDPE. The transition in linear PE is sharp and occurs in the range 130 to 140°C at

the expected temperature (For LDPE, T, = 142°C). HDPE, however, shows a progressive transition over a range of 20’. The onset is in the region of l0O’C fl, = 120’C). The broad nature of the transition in HDPE is associated with the existence

of alkane branches off the main backbone chain. The alkane chain substituents reduce the cohesion of the crystal cell causing the onset of melting at temperatures below the expected transition

to the increase of intermolecular “ties”. These ‘‘ties’’ may be though of as producing a three dimensional

random structure from the two dimensions of the lamellae system thus increasing the rigidity of the system. As there is no current evidence for a particular post-(pre-melting)mechanism, it will not

be further speculated upon here. The the proposal for the pre-melting mechanism as the cause for the observed rheological transition, however, does conform to the available evidence. The rheological transition is observed to demase with increasing pressure and increasing strain rate. Simplistically, both of these actions increase the energy input into the system thereby reducing the the thermal energy required to induce pre-melting and hence the transition. If this transition is considered to be instant (the evidence suggests that this is the case as there is no time c 4 x x h c e

temperature.This behaviour is sometimes referredto as pre-melting.

observed in the transition) then;

The nature of the transition observed in the interfacial rheology is evidently not specifically

dP/dT = AS,/AV,

dependent upon the molecular relaxation modes, as is, for example, the glass transition (the glass transition for HDPE is at -125°C). The origin of this peculiar transition is more likely to be associated with the pre-melting of the crystalline lamellae. The following briefly describesa possible

(4)

where P is the pressure, T the temperature, AS, is the change in entropy on melting and AV, is the volume change on melting. The slope of the dP/dT curve is negative and of the form

+ 92.

mechanism as to how the pre-melting process causes an increase in the shear stress and an apparent

T, = -2.5 x lO-’P

temperature independence (the “brittle” nature of the post transition phase). The pre-melting process

where T, is the transition temperature in “C. The negative slope is indicative of either a decrease in

reduces the size of shear slip planes associated with the crystalline lamellae. The resultant is an

the entropy (unlikely) or a decrease in the volume. Again, speculatively, a decreasein the volume on

amorphous material where the chain orientation becomes random and the resulting intermolecular

melting provides an amorphous structure of greater density than the semicrystalline system below the

interactionshinder the flow of molecular entities k

transition.

(5)

160 4. Conclusions

References

An unusual transition in the interfacial rheology of HDPE is discussed. This transition has been

1. Briscoe B.J., Scruton B. and Willis R.F. (1973) Proc. Roy. SOC.Lond. A333, 99

found to decrease with increasing contact pressure and strain rate. The mechanism for the transition is

2. B0wersR.C. (1971). J, Applied Polymer Physics 42(12). p4961

ascribed to the pre-melting process in the crystalline lamellae caused by the existence of defectsalong the polyethylene chains in the form of chain branching. The pre-melting process conveys an amorphous

3. BriscoeB.J. and Smith A.C. (1982) J. Phys. D.: Applied Phys., 15,579 4. Brisc0eB.J. and Smith A.C. (1983) J. Appl. Polym. Sci. 28, 3827

characterto the original ductile film. Although the transition is restricted to a certain type of

5. Briscoe B.J., Thomas P.S. and Williams D.R. (1992) Wear 153, 263; (1993) in “Thin Films in

polyethylene, HDPE, it does underline the commonly stated observation that interfacial

Tribology”, Tribology Series 2 5 (Leeds-Lyon 19), 453

rheologicalcharacteristics of thin films are generally in accord with the observed bulk response. The

6. Hagmann H., Snyder R.G., Peacock A.J. axl Mandelkern L. (1989), Macromolecules, 2 2, 3600;

rather high contact pressuresproduced in thin films, which are quasi hydrostatic in nature, does however

Spells S.J. and Sadler Macromolecules, 2 2, 3948

reveal unusual effects such as the pre-melting to an amorphous state as speculatedupon in the current

7. Basset D.C. (1981) ‘Principles of Polymer

paper. Although such effects have not been seen in macroscopic HDPE we may speculate that this

D.M.

(1989),

Morphology’, Cambridge University Press

transition will occur. It is also realised that such effectswhich may not be seen in monolithic friction

8. Amuzu J.K.A., Briscoe B.J. and Smith A.C. (May 1984) 3rd International Conference on Solid Lubrication 9. Timoshenko S.P. and Goodier J.N. (1951)

experimentsas the contact pressures are likely to be too low to suppress the transition. However, in

‘Theory of Elasticity’ (New York McGraw-Hill) 10. Hsu T.S. (1980), J. Polym. Sci.: Polym. Phys.

films, at higher pressures, they may exist in other

Ed, 18, 2379; Bower D.I. and Maddams W.F.

semicrystalline polymeric systems.

(1989) ‘The Vibrational Spectroscopy of Polymers’,

Cambridge University Press