Poly(aryl ether ketone)s containingdibenzoylbiphenyl groups: dynamicmechanical and physical ageing behaviourAA Goodwin,1* JA Campbell1 and ZY Wang2

1Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia2Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6, Canada

Abstract: The dynamic mechanical and physical ageing behaviour of a series of isomeric poly(aryl

ether ketone)s containing 2,2'- and 3,3'-dibenzoylbiphenyl groups has been determined. A secondary

g-process, which was detected in all polymers by dynamic mechanical analysis, was both broader and

located at higher temperatures in the 3,3' polymers; this was attributed to greater intermolecular

cooperativity and a higher effective relaxation time, arising from the more densely packed chains. The

enthalpy relaxation of a pair of isomers was analysed using a common multi-parameter model for

structural relaxation. The equilibrium enthalpy lost by the aged glass showed good agreement with the

value extrapolated from the liquid state. Although the enthalpy lost by the equilibrium glass was

identical for each isomer, the 3,3' substituted polymer required a longer time to reach equilibrium and

showed a greater dependence of physical ageing on structure. The higher density of the 3,3 isomer

resulted in a slowing of both the physical ageing and the mechanical relaxation process.

# 1999 Society of Chemical Industry

Keywords: poly(aryl ether ketone)s; dibenzoylbiphenyl; dynamic mechanical analysis; physical ageing; enthalpyrelaxation

INTRODUCTIONHigh-temperature thermoplastic polymers are of great

scienti®c and technological interest because of their

range of outstanding properties and ease of proces-

sing.1 They have applications in both un®lled and

reinforced forms, while the potential for controlling

their morphology by changing their thermal history

makes them of interest in the study of structure±

property relationships. Common high temperature

polymers such as PEEK consist of rigid aromatic

groups, polar ketone units and ¯exible ether units in

the chain backbone. Increasing the proportion of

ketone linkages in the backbone increases chain

stiffness and raises the crystal binding energy, these

effects combine to increase the glass transition

temperature and the melting temperature.2 A higher

concentration of ketone linkages also increases the rate

of `cold' crystallization on heating from the amorphous

glass.3 Although thermal properties are signi®cantly

altered by changes in the ether/ketone ratio, it has little

effect on mechanical behaviour; a comparison of

injection-moulded semicrystalline samples revealed

that an increase in the concentration of ketone linkages

slightly raised the elastic modulus while tensile

strength was unaltered.4 A modi®cation that has been

applied to the structure of aromatic poly(ether

ketone)s is the incorporation of meta linkages in the

polymer backbone of PEEK. The disruption to the

amorphous chain and crystal packing caused by this

modi®cation result in a decrease of glass transition

temperature, melting temperature and crystallization

rate.5 The changes in thermal behaviour are greatest

for the melting process, and thus the practical

signi®cance of meta substitution is that the polymer

can be melt-processed within a suitable temperature

range while retaining a high glass transition tempera-

ture. Other modi®cations to aromatic poly(ether)s and

poly(ether ketone)s have aimed at increasing the glass

transition temperature while retaining other desirable

properties, through the incorporation of dibenzoyl-

benzene and dibenzoylbiphenyl (DBBP) groups.6,7

We report here the investigation of the dynamic

mechanical and physical ageing behaviour of series of

amorphous poly(aryl ether ketone)s containing 2,2'-DBBP and 3,3'-DBBP groups and a range of

connecting units, to gain an insight into the effect of

chemical structure and substitution pattern on relaxa-

tion behaviour.

EXPERIMENTALMaterialsThe synthesis and characterization of the polymers

studied here have been reported elsewhere.8 The

polymer structures are shown in Fig 1.

Dynamic mechanical analysisThe dynamic mechanical response of the polymers

Polymer International Polym Int 48:353±360 (1999)

* Correspondence to: AA Goodwin, Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia(Received 25 May 1998; accepted 23 October 1998)

# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50 353

was examined using a Perkin Elmer DMA 7 (CT,

USA) using a penetration probe method. Samples

were heated between ÿ150°C and 250°C at

3°Cminÿ1 with an applied frequency of 1Hz. The

main parameter of interest was tand. The temperature

response of the instrument was calibrated using high

purity indium and n-octane standards. The glass

transition temperature was de®ned as the temperature

corresponding to the peak maximum. To reduce the

in¯uence of absorbed moisture on the dynamic

response, particularly in the sub-ambient region, all

samples were throughly dried at elevated temperature

under vacuum before scanning.

Differential scanning calorimetryA Perkin Elmer DSC 7 was used for the physical

ageing studies. All samples were subjected to a

standard thermal history before ageing. This consisted

of holding a sample of 5±10mg in a sealed aluminium

pan at 50°C above the Tg for 5min, followed by rapid

cooling of the sample to 100°C. The sample was then

heated at 200Kminÿ1 to the chosen ageing tempera-

ture and isothermally aged for a speci®ed time. After

the ageing time had elapsed, the sample was then

rapidly cooled to 100°C and immediately scanned at

10°Cminÿ1 through the glass transition region to

reveal the endothermic overshoot associated with

physical ageing. To obtain an accurate baseline for

quantitative analysis of the ageing process the unaged

samples were also scanned from 100°C through the

glass transition. This procedure was repeated for each

of the chosen ageing times and temperatures. To

determine the cooling rate dependence of the Tg,

samples were cooled at various rates from 50K above

the quenched Tg to 50K below the Tg, and were then

immediately re-scanned to the initial temperature at

20Kminÿ1. The temperature and power response of

the calorimeter was calibrated using high purity

indium, tin and zinc standards.

DensityDensity measurements were carried out at room

Figure 1. Chemical structures of poly(aryl ether ketone)s.

354 Polym Int 48:353±360 (1999)

AA Goodwin, JA Campbell, ZY Wang

temperature using a Micromeritics AccuPyc 1330

pycnometer (GA, USA). All samples were thoroughly

dried under vacuum at elevated temperature before

testing.

Curve fittingNon-linear least squares ®tting of the dynamic mechan-

ical data was accomplished with PeakFit 4.04 software

(SPSS Science, IL, USA) which utilizes the iterative

Marquedt±Levenberg ®tting algorithm. The con®-

dence intervals for the ®tting parameters were deter-

mined at the 95% limit and a graphical pre®tting

method was used to manipulate the ®tting function on

the screen to match the experimental data before ®tting.

RESULTS AND DISCUSSIONDynamic mechanical analysisFigure 2 shows the dynamic mechanical spectrum of

polymer 2e taken at 1Hz in a temperature scanning

mode. The presence of three well-separated relaxation

processes can be seen, two of which occur in the sub-

Tg region. The secondary peaks are typically broad and

of low strength, with only the g transition being

observed in all polymers while the glass transition is, as

expected, much sharper and more intense. The peak

locations for each polymer are listed in Table 1. The

dynamic mechanical glass transition temperatures are

in good agreement with those reported previously for

the same polymers measured by DSC.8 The higher Tg

values observed for the 2,2'-substituted polymers

result from the steric hindering effect of the ortho

substitution on chain rotation, and this effect has been

correlated with the core angle between the points of

substitution for similar polymers.9 Increasing the bulk

and stiffness of the connecting unit shifts the Tg to

higher temperatures, and this is particularly signi®cant

in the case of the ¯uorenyl connector group (e) where

the Tg exceeds 200°C. It is interesting to note that

incorporation of hexa¯uoro units into the connector

group (d) produces a slight upward shift in Tg,

compared with the bisphenol-A connector group (a).

This contrasts with a previous study of hexa- and tri-

¯uorinated polyimides where a modest decrease in Tg

with increase in ¯uorine content was reported,10 while

we have shown that a signi®cant drop in Tg occurs with

per¯uorophenylene substitution.11 The decrease in Tg

is generally attributed to changes in free volume and

polarization induced by the highly electronegative

¯uorine.12 The absence of internal plasticization in

this case is probably due to the very low concentration

together with the position of substitution of the

¯uorine present in the polymer repeat unit.

The polymers showed some interesting relaxation

behaviour below the Tg. All polymers displayed a low

temperature g transition, the peak location of which

varied over a signi®cant temperature range, while a

higher temperature b transition was observed in some

of the 3,3'-substituted polymers, but was not very

apparent in the 2,2'-substituted polymers. The proxi-

mity of these two processes in 2a, which have been

resolved using a Fuoss±Kirkwood (FK) ®t to the data,

is shown in Fig 3. The FK expression is given by13

"00 � "00max sech m lnfmax

f�1�

where f is the frequency of measurement and m is a

parameter ranging from zero to unity which charac-

terizes the symmetric broadness of the relaxation.

Temperature dependence was introduced into this

equation by allowing the relaxation time to vary

according to the Williams-Landel-Ferry equation.

A comparison of the g transition of 1a and 2a, shown

in Fig 4, shows that the process occurs at a signi®cantly

higher temperature in the 3,3'-substituted polymer. A

summary of the location of the secondary peaks and

the corresponding ®ts to the temperature-dependent

FK equation is shown in Table 2. An important

observation is that although 2,2'-substitution results in

a higher glass transition temperature, compared with

3,3'-substitution, the g transition of the 2,2' polymers

Figure 2. Dynamic mechanical spectrum of 2e taken at 1Hz and withheating of 2°Cminÿ1. Primary and secondary transitions are labelled on theplot.

Table 1. Temperature location (in°C) ofrelaxation peaks observed by dynamicmechanical analysis at 1Hz

Peak

Sample a b g

1a 176 ± ÿ105

1b 178 ± ÿ90

1c 205 ± ÿ100

1d 188 ± ÿ95

1e 240 ± ÿ110

2a 160 32 ÿ70

2b 152 60 ÿ80

2c 156 ± ÿ94

2d 167 80 ÿ85

2e 210 70 ÿ85

Polym Int 48:353±360 (1999) 355

Ageing of poly(aryl ether ketone)s

is shifted to relatively lower temperatures, which

indicates that the mechanism underlying this process

is more easily facilitated by ortho substitution.

Previous dynamic and dielectric relaxation studies of

meta-substituted aromatic polyketones has shown that

the shape, strength and location of the b peak is

unaffected, compared with related para-substituted

polymers,14 whereas others have reported increased

mechanical broadness in meta-substituted PPS, com-

pared with para-PPS.15

The subambient secondary relaxation of polycarbo-

nate is probably the most commonly studied sub-Tg

process in polymers, and it has been reported that

while the carbonate group cannot move independently

of the phenyl ring, substitution on the phenyl ring

leads to a shift in mechanical relaxation temperature.16

However, it has also been shown that the relaxation

occurs even when the phenyl rings are totally ®xed in

position, and this is taken as an indication that motions

of the carbonate linkage do contribute to the process.17

It has been concluded that the process arises from both

carbonate (low temperature) and phenyl (high tem-

perature) motions which combine to form a broad

damping peak.17

Fits to the temperature dependent FK equation,

listed in Table 2, show that the 3,3'-substituted

polymers have broader g transitions than the equiva-

lent 2,2'-substituted polymers. This, and the fact that

the g transition in the 3,3' polymers is shifted to higher

temperatures, is indicative of increased intermolecular

coupling raising the barriers to local rotation and

leading to a greater range of relaxing environments. A

possible explanation for increased coupling in the 3,3'polymers is the tighter packing of the polymer chains.

The room temperature densities of 1e and 2e were

found to be 1.3130�0.0068gcmÿ3 and 1.3326�0.0023gcmÿ3, respectively. The higher density of

the 3,3' polymer suggests that a lower local free volume

is available for small-scale segmental motion, thus

increasing intermolecular coupling. This is consistent

with the study referred to above17 where it was shown

that the polycarbonates with higher density (ie

increased chain packing) exhibited broader damping

peaks at higher temperatures. The above-ambient

secondary relaxations (b process) which were observed

in the 3,3'-substituted polymers (with the exception of

2c) were located between 30°C and 80°C, and were of

intermediate broadness to that of the g and atransitions. b processes are frequently observed in

thermosetting polyimides, and have been ascribed to

various relaxation mechanisms,18 but they are not

thought to be common in aromatic thermoplastics,

except in quenched polymers where it is believed that

the process is caused by the relaxation of chain packing

defects.19 We have recently established the presence of

a b process in a series of aromatic poly(ether)s and

poly(ether ketone)s which contain per¯uorinated

aromatic groups, by using dynamic mechanical analy-

sis and dielectric relaxation spectroscopy.11,20

Physical ageingPhysical ageing is a reversible effect that produces an

Figure 4. Dynamic mechanical spectrum of 1a (*) and 2a (*) showing theg-relaxation region.

Table 2. Temperature location (in°C) of dynamicmechanical g peaks and broadness parameter mdetermined by fits to the Fuoss–Kirkwood equation

Sample Location of g-peak (°C) m

1a ÿ105 0.50

1b ÿ93 0.39

1c ÿ105 0.47

1d ÿ91 0.45

1e ÿ105 0.54

2a ÿ69 0.48

2b ÿ78 0.36

2c ÿ91 0.42

2d ÿ83 0.37

2e ÿ82 0.40

Figure 3. Dynamic mechanical spectrum of 2a showing the sub-Tg region.Solid lines are fits to the Fuoss–Kirkwood equation.

356 Polym Int 48:353±360 (1999)

AA Goodwin, JA Campbell, ZY Wang

observed change in a property of a glassy material as a

function of storage time at a temperature below the

glass transition temperature. Physical ageing occurs as

a result of the non-equilibrium state of the glass and,

because of the temperature dependence of the rate of

molecular motions, its effects are usually most

important over a narrow temperature range below

the Tg. Physical ageing has engineering signi®cance,

because it can result in profound changes in mechan-

ical behaviour and other properties, and it is a topic

which has received wide attention.21

The effects of chemical structure on physical ageing

have recently been investigated for a series of related

polymers. Brunacci and co-workers22,23 reported that

there was no detectable difference in the time-scale of

physical ageing and the relaxation broadness for

polystyrenes containing different para substituents.

Physical ageing was signi®cantly slower, however, in

polystyrene containing an a-methyl group. There were

observable differences in the enthalpy lost by the fully

relaxed glass with changes in chemical structure, and

this parameter was considered to be independent of

any internal barriers to relaxation. It was concluded

that structural relaxation of glassy polymers is not

in¯uenced by the nature of a rigid side group unless it

introduces strong speci®c interactions, for example in

the case of a hydroxy group. In this study we have

examined the physical ageing behaviour of 1e and 2e

to establish whether the substitution pattern has any

effect on the kinetics and thermodynamics of ageing.

Physical ageing is both a non-linear and a non-

exponential phenomenon, with the non-linearity aris-

ing from the dependence of the relaxation time on the

temperature and structure of the glass. A widely used

analytical expression to de®ne the non-linearity of

physical ageing is present in the Tool±Narayana-

swamy±Moynihan model21

��T ;Tf � � �0 expx�h�

RT� �1ÿ x��h�

RTf

� ��2�

where Dh* is an apparent activation energy, Tf is the

®ctive temperature and x is the structure parameter.

The development of an endothermic overshoot in a

DSC heating scan for different cooling rates can be

used to determine the apparent activation energy. The

structure parameter is evaluated from DSC heating

scans after isothermal ageing to produce a well-

annealed glass (Fig 5). The kinetics of physical ageing

of each isomer were compared by evaluating the time

required to reach equilibrium te. This was estimated

from plots showing the development of the ageing

enthalpy as a function of ageing time (Fig 6). The

results of this analysis are shown in Fig 7, where it can

be seen that te exhibits a linear dependence on ageing

time for both isomers over the narrow temperature

range studied, while the 3,3 isomer requires a relatively

longer time to reach equilibrium. This is consistent

with the dynamic mechanical behaviour of the gtransition, where we have tentatively suggested that

the higher temperature, ie higher effective relaxation

time, of this process results from closer packing of the

3,3' polymer chains, as re¯ected in the higher density.

In contrast, the enthalpy lost by the fully relaxed glass,

as a function of ageing temperature, shown in Fig 8, is

identical for both isomers, this is to be expected since

the repeat unit of the two polymers is identical.

The apparent activation energy Dh* can be evalu-

ated using the relationship

Figure 5. (a) Development of enthalpy overshoot with time for physicalageing of 2e at 15°C below the DSC glass transition temperature. (b)Development of enthalpy overshoot during DSC heating scan for 2e cooledat the rates (K/min) shown on plots.

Figure 6. Time dependence of physical ageing for 1e at DT =5K (*),DT =10K (*) and DT=15K (&) with data displaced for clarity.

Polym Int 48:353±360 (1999) 357

Ageing of poly(aryl ether ketone)s

�h�

R� ÿ @ ln jq1j

@ 1Tf

" #�H�0;q2

�3�

where q1 is the cooling rate and q2 the constant heating

rate. The ®ctive temperature corresponding to each

cooling rate was found by equating the glass and liquid

state enthalpy, and determining the temperature of

intersection from a linear regression. A linear depen-

dence of ln/q1 on 1/Tf was observed in both cases, and

from the slope Dh* was calculated to be in the range

2200±2400KJmolÿ1 for each isomer. These values are

much higher than those calculated for the glass

transition process in aromatic thermoplastics such as

PEEK24 and PES,25 although these polymers do not

have such a high concentration of aromatic groups in

the chain backbone, nor are they as rigid.

The structure parameter x de®nes the relative

contributions of temperature and structure to the

relaxation time, and can be evaluated from plots of

peak endotherm temperature (Tp) against enthalpy

loss during isothermal ageing (Fig 9) if the same

heating rate is employed throughout.30 To do this a

function F(x) was calculated from

F�x� � �Cp

�Tp

��H

� ��4�

and x was taken from a master curve of F(x) against xreported by Godard et al.30 Using this method, values

of x of 0.73�0.02 for the 2,2'-substituted polymer and

0.64�0.02 for the 3,3'-substituted polymer were

calculated. Of interest is the fact that the value of xremained constant over the ageing temperature range

employed, while in other cases it has been reported to

vary with ageing time and temperature.21 The higher

value of x for the 2,2'-substituted polymer implies a

greater effect of temperature on the ageing kinetics,

compared with 3,3' substitution. The relationship

between the value of x and chain interactions in

polymers has been highlighted in a recent report on the

physical ageing of thermoplastic polyesters.24 It was

reported that a small value of x indicates the

dominance of intramolecular forces, while larger

values of x are associated with strong intermolecular

forces. In addition, it has been commonly found with

other polymers and also inorganic glasses, that there is

an inverse relationship between the apparent activa-

tion energy and the structure parameter.

The distribution of relaxation times for physical

ageing, represented by the parameter b, can be

evaluated from the values of normalized peak height

of the cooling-dependent scans. The normalization is

achieved by

Figure 7. Temperature dependence of the time to reach equilibrium forageing of 1e (*) and 2e (*).

Figure 8. Temperature dependence of enthalpy lost by equilibrium glassfor 1e (*) and 2e (*).

Figure 9. Variation of Tp with lost enthalpy at DT=15K for 1e (*) and 2e(*).

358 Polym Int 48:353±360 (1999)

AA Goodwin, JA Campbell, ZY Wang

CNp �

jCp ÿ CpgjTjCpl ÿ CpgjT

�5�

where the numerator represents the height of the peak

relative to Cpg, the heat capacity of the glass, and Cpl is

the heat capacity of the liquid. Thus the denominator

is DCp at the peak temperature. From the values of CNp

we obtained estimates of the non-exponentiality

parameter b, from the theoretical dependence of the

peak height on the ratio of cooling rate to heating rate

and the value of x.24 The value of b for the 2,2 isomer

lies close to 0.46 whereas b for the 3,3 isomer the value

was somewhat higher in the range 0.46±0.6, which

suggests the 2,2 isomer has a broader distribution of

relaxation times associated with physical ageing. Plots

of DHmax plotted against ageing temperature were

linear, and slopes of 0.188JKÿ1gÿ1 and

0.184JKÿ1gÿ1 were found for 1e and 2e, respectively.

This was consistent with DCp (Tg) of 0.170JKÿ1gÿ1

and 0.186JKÿ1gÿ1 as measured for each polymer by

DSC. The plots extrapolated to zero enthalpy loss at

Tg, with 241°C and 212°C recorded for the 2,2'- and

3,3'-substituted polymers, respectively. These values

are in excellent agreement with the DSC Tg and,

accordingly, the relationship

�Hmax�Ta� � �Cp�Tg��Tg ÿ Ta� �6�is valid over the temperature range studied. This

relationship has been observed for aromatic polymers

such as PEEK, PET and polycarbonate, and also the

non-aromatic poly(hydroxybutyrate).25 For other

polymers such as PMMA, PS and PVAc26,27 eqn 3

does not hold. In each case where there was a

discrepancy between the experimental and extrapo-

lated values of lost enthalpy, the experimental values

were found to be signi®cantly lower. Cowie et al27

concluded that, from experimental as well calculated

data, enthalpy relaxation in the glass state of PVAc can

be accounted for by a combination of main-chain

conformational changes and reduction in free volume,

with the side chains remaining mobile below the glass

transition.

CONCLUSIONSThe glass transition temperature of the studied

polymers has been shown by dynamic mechanical

analysis to be strongly dependent on both chemical

structure and substitution pattern, in line with

previous DSC measurements. All polymers exhibited

a typical sub-ambient g process, while a less common

sub-Tg b process was detected in some of the polymers

with 3,3' substitution. The g transitions of each of the

3,3' polymers were both broader and shifted to higher

temperatures, compared with the 2,2' polymers; this

can be attributed to a higher effective relaxation time,

increased intermolecular coupling, and a greater range

of relaxing environments. This situation may have its

origin in the increased packing density of the 3,3'polymer chains which was revealed in the two

polymers for which density values were obtained.

The temperature dependence of the enthalpy lost on

ageing by 1e and 2e compared favourably with the

values extrapolated from the equilibrium liquid, and

the experimental data con®rmed that the enthalpy lost

by the equilibrium glass did not depend on the

substitution pattern of the isomers, although the rate

of physical ageing was reduced in the more closely

packed 3,3 isomer. The physical ageing process was

analysed by a multi-parameter model for structural

relaxation, and while the apparent activation energy

for the ageing process was similar for both isomers, the

relaxation broadness and structure parameters were

quite different. The higher effective relaxation time for

physical ageing and dynamic mechanical relaxation of

the 3,3 isomer is suggestive of a link between the two

processes.

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Ageing of poly(aryl ether ketone)s