Effect of the processing history of amorphous polymeric film specimens on their relaxation properties

Polymer Science U.S.S.R. Vol. 27, No, 9, pp. 2165-2172, 1985 Printed in Poland

0032-3950/85 $10.00+.00 © Pergamon Journals Ltd.

EFFECT OF THE PROCESSING HISTORY OF A M O R P H O U S POLYMERIC FILM SPECIMENS ON THEIR RELAXATION PROPERTIES* M. S. MATEVOSYAN,A. A. ASKADSKII, G. L. SLONIMSKII and YA. V. GENIN A. N. Nesmeyanov Institute for Elemento-Organic Compounds, U.S.S.R. Academy of Sciences (Received 8 February 1984)

It has been shown with PS and PMMA as examples that the mechanical relaxation processes and strength properties of glassy polymers depend on the specimens' processing history. The reasons for the effect of the history are connected with differences in the thermodynamic affinity between the polymer and the solvent as well as differences in the microporosity of the structure, which depends on the volume of the solvent molecules. TItE rupture strength and deformation properties of amorphous polymeric specimens are known to depend to a certain extent on the method of producing the specimens [1-8]. The processing history in the folmation of films from solutions is known to have a substantial effect on their rupture lives [9] and even on the deformation compliance of melts of PS specimens obtained by lyophilic drying from solutions of various concentrations [10]. In this respect, the relaxation properties of amorphous polymers have not been studied in detail. It is of interest to clarify how the parameters for the mechanical relaxation processes in amorphous polymers vary as a function of their processing history (the type of solvent from which the film was obtained and the technology of forming (casting from solution or moulding from the melt)). These changes should depend on the nature of the supermolecular stxucture, which is determined by the type of solvent used to obtain the film, that is, by its thermodynamic affinity towards the polymer, etc. The question of the effect of the processing history of amorphous specimens on their relaxation properties is of interest both theoretically and also piactically. Firstly, a chartge in the parameters of the relaxation processes is, although an indirect indication, nevertheless in a number of cases a convincing indication of the effect of the supermolecular structure of amorphous polymers on their properties. Secondly, the undertaking of an investigation of this type is of important practical significance since its results may Foint the way to the technology of obtaining film specimens with optimum values of the relaxation properties that determine the mechanical durability of the polymer [11 ]. This is especially important for heat-resistant polymers, films of which *Vysokomol. soyed. A27: No. 9, 1925-1931, 1985. 2165

2166

M. S. MATEVOSYANet

al.

are, in most cases, obtained only through a solution stage. Thirdly, there are methods that enable the various physical properties of substances to be calculated on the basis of the chemical structure of the polymer's repeat link [12]. Many of these properties hardly depend at all on the processing history of the specimen, that is, on the supermolecular structure. Amongst such properties are the refractive index, glass transition temperature, thermal coefficient of volumetric expansion, solubility parameter (the cohesive energy density), etc. However, such limiting mechanical characteristics as the rupture stress or strain and the elastic modulus depend on the specimens' processing history. It is precisely for this reason that calculation schemes to determine the modulus of elasticity of polymers on the basis of their chemical structure have not been considered in the work referred to (there are only a few attempts of this nature [13]). In order to develop a calculation scheme enabling a polymer's elastic modulus to be assessed on the basis of its chemical structure, it is therefore necessary to take account of the fact that the modulus also depends on the processing history. The establishment of this relationship is a first but an important step on the road to creating such a scheme of calculation (we are here not talking about the temperature dependence of the modulus, or the effect of the rate of mechanical loading on the modulus, or about the number of other factors which must be taken into account). It should also be remembered that on the whole, the moduli of elasticity of polymers in the glassy state do not vary very much on going from one polymer to another and it therefore becomes necessary to take account of the processing history. PS and P M M A were selected as model polymers for the analysis of the effect of the processing history of film specimens of amorphous polymers on their relaxation properties. Films of PS and PMMA were obtained by casting from solutions in various solvents on to a smooth cellophane substrate with subsequent removal of the solvent and heating the specimens in vacuum (at a residual pressure of 3 mmHg) at 40-50°C for 16 hr. IR spectroscopy and also periodic weighing of the specimens to constant weight were used to monitor the completion of the solvent elimination from the films. The tensile curves of all the specimens obtained were determined preliminarily before carrying out the relaxation measurements. The measurements were made with a Polyani dynamometer having a rigid force-measuring system, the rate of movement of the lower grip being 0'0071 mm/sec. The tensile curves for the films are shown in Fig. 1. It may be seen that the rupture strain e, which is small for these polymers at r o o m temperature, hardly depends at all on the type of solvent. The specimens obtained from the melt by direct pressing are all characterized by practically the same value of rupture strain. The rupture stress and the elastic modulus E depend quite substantially on the film's previous history (Fig. 1, Table). On the whole, the value of E in the systems studied varies by approximately 25 and the value of ~r by 20 ~ for PS and by 35 ~ for PMMA. Lower values of E are found to correspond to lower values of a. Without considering the reasons for the differences in E and ~r as a function of the specimens' processing history, we shall discuss the cor-

Effect of processing history of a m o r p h o u s polymeric film specimens STRUCTURE AND PHYSICO-MECHANICAL PROPERTIES OF

PS

AND

PMMA

2167

FILMS OBTAINED FROM VARIOUS

SOLVENTS

Solvent

tr, MPa

e, %

E, MPIi

-- log C

d, g/cm 3

n

6, (kcal/cma) * experi- I calc. ment I

l, A

NAZ'al V, cruZ/ /mole

Polystyrene Moulded Chloroform Benzene Dioxane

38.0 35.0 32.0 30.0

3"5 3.4 3.5 3.2

1350 1310 1120 1030

1-33 1.40 1.17 1.23

1-11 1-30 1.11 1-18

1.055 1.054 1.049 1.052

4'60 4'60 4"60 4'60

8"8*

{

9'3 9'15 10; 10"4

9'1 8"7 9'3 10'8

41 '9 53"1 52"8

Polymethylmethacrylate Moulded Chloroform Diehloroethane Benzene Toluene Dioxane

60-0

4-8

1500

1.14

1.16

1.204

6"40

48.0 46.0 41.0 38.0 35.0

4.6 4.5 3.8 4"0 3.5

1270 1230 1170 1160 1150

1-20 1.19 1.20 1.20 1-19

1.16 1.19 1.17 1.18 1-17

1.229 1.207 1.175 1.173 1.178

6"32 6"15 6"32 6"32 6"32

9"1; 9"5; 9"4* 9"8 9"2 9'15 8"9 10; 10'4

9"3* 9"4 9"3 9"3 8"9 10"8

41 "9 45"9 53"1 63"3 52"8

* For the polymer.

~,MPa

I

i !

50;-

a

?5

30

3

7O I

I

I

I

t

I

2

2

}

# ~,%

FiG. 1. Tensile curves of: a - P S and b - P M M A film specimens. Here and in Fig. 2-5, the specimens were obtained as follows: / - b y moulding; the rest from the following solvents: 2 - chloroform; 3 - benzene; 4 - dioxane; 5 - dichloroethane and 6 - toluene.

~MPo

2O

20 l

10

,

20

qO

1~

10

20

qO

60

"/-[me, rain FIG. 2. Stress relaxation curves for: a - P S

and b - P M M A

film specimens at 20°C.

M . S. MATEVOSYAN et aL

:2168

responding relaxation behaviour. Stress relaxation curves for all the specimens investigated were obtained for analysis over a wide range of temperatures and strains. Two series of experiments were carried out: in the first series, the stress relaxation curves were determined at approximately the same specified strains (corresponding to 0-6 of the limiting elastic strain) and at various temperatures within the glassy condition; in the second series, the curves were determined at one temperature but with various initial strains including the regions of linear and non-linear mechanical behaviour. As an example, Fig. 2 shows the stress relaxation curves determined at 20°C. It may be seen that, depending on the type of solvent used, the stress relaxation curves are arranged in the same order as are the tensile curves in Fig. 1. Consequently, the greater ,the initial value of the modulus Er, the higher is its value throughout the entire per iod of measurement. Similar behaviour is also observed at all the other temperatures (the scatter in the experimental data being taken into account). In order to demonstrate this conclusion more graphically, Fig. 3 shows the temperature dependence of the initial and current values of the relaxed modulus (E6o, corresponding to 60 rain relaxation). Specimens obtained from the melt have the highest initial values of modulus and

r:L, "fPa .7 2.

a

E~o~MPa

b

7000 4

I000

508

500

1

1

i

I

7 ISO0

1500

C

d l

lO00 !

~

~

~

500

I

20

1000 •

I

500

I

60

2 5

I

To

$4

20

EO

T°

~:Io. 3. Temperature dependence of: a and e - t h e initial and b and d - t h e final values of ehc relaxed modulus of: a and b - P S specimens; c and d - P M M A specimens.

Effect of processing history of amorphous polymeric film specimens

2169

those obtained from dioxane have the lowest (this is true both for PS and for PMMA). The change in properties caused by differences in the specimens' processing history is consequently a stable characteristic that is apparent for a long time and at all temperatures. The temperature-time analogy principle was used to generalize the relaxation curves. For this purpose, the relaxation curves plotted with the coordinates log E, vs. log t were displaced along the log t axis until they coincided and tormed the generalized relaxation curve of Fig. 4. Generalized relaxation curves for the films with different processing histories are considerably scattered especially in the region of high values of log t/a T. For the PS specimens obtained from chloroform or benzene, the generalized relationships in the region of low values of log t/aT are thus positioned relatively close to one another but are considerably scattered at high values of log t/a T whereas foI specimens obtained from dioxane or by direct moulding from the melt, the scatter begins even at low values of log t/a T and increases as the relaxation period is increased. In the case of PMMA, the specimen obtained from dioxane has the lowest values of Er over the entire range of values of log t/a T . The moulded specimen is found to have the highest value of Er. The temperature dependence of the shift factor also depends to some extent on the specimens' processing history.

logE,[MpaJ 3

6/

2

1

i t

4

8

t

q

I

I

8 lo9 i/ctr

Flo. 4. Generalized relaxation curves for: a - P S and b-PMMA film specimens. The temperature dependence of log a.r for PS and PMMA, like those for other polymers in the glassy state, may be approximated well by a powei function [14] log a T= C ( T - To)n where To is a reduction temperature and C and n are material parameters that also depend on the value of strain. Values of the parameters C and n are shown in the Table. C and n may be seen to change hardly at all with the processing history of the film specimens.

2170

M. S. MATEVOSY'AN e t al.

The generalized curves have thus been found to depend on the processing history of the film specimens of amorphous polymers; this may be taken as evidence that the structure-formation processes occurring during the detbrmation of film obtained fiom different media are different.

G'o,MPa

~0, MPa

[ a

c

/2

d

!

30-

23 q

20

2 3 4

20 - i t ZO

I

2

E~%

"?/_IF' 70

2

E~%

1

2

3

I

1

2

:

3

1~6. 5. Isochronal dependence of: a and c-~ro; b and d-the relaxed stress on the value of strain imposed for: a and b-PS; c and d-PMMA specimens. The effect of temperature on the characteristics of the relaxation behaviour of PS and PMMA specimens has been considered above. We shall now turn to the effect of the magnitude of the imposed strain on the relaxation characteristics. Figure 5 shows the isochronal dependence of the initial and the relaxed stress on the magnitude of strain imposed (the initial stress is taken to be the stress developed in the specimen when the rapid imposition of the strain has just been completed); ~r6o is the stress required to maintain the specified strain at the end of the relaxation process the length of which was 60 min. It may be seen fiom the Figure that the processing history of the film specimens has a similar effect on the trend of the relaxation piocesses at all strains. The highest values of cr and ~r6o are characteristic of specimens obtained by direct moulding and the lowest, of specimens obtained from solution in dioxane. These differences are larger, both for ~ro and for O'6ofor higher values of the strain e. Very considerable changes in the relaxation properties of amorphous polymers are thus observed, depending on the conditions of their formation. A number of additional measurements and calculations were made to explain the relationships obtained. The X-iay diffraction patterns of all the specimens were found to be practically identical and independent of the processing history. The mean spacing, l, between the PS and PMMA chains was calculated from these diffraction patterns and the corresponding values are shown in the Table. The Table also gives values of the density, d, for all the specimens studied. The various PS specimens differ from one another only very slightly in density and the mean spacings between the chains are practically identical. In the case of PMMA, the densities differ more significantly. The mean spacing between chains are practically the same for specimens obtained from the melt and from the various

Effect of processing history of amorphous polymericfilm specimens

2171

solvents, with the exception of the specimen obtained from dichlorethane. Compared with the moulded specimen, the mean spacing between chains for this specimen is less, although its density is also less rather than greater. This is explicable only by the formation of a more developed ultramicroporous structure, which leads to a reduction in the macroscopic density as compared with the specimen obtained by moulding but with a shorter spacing between chains. In addition to these data, it is of interest to compare the values of the solubility parameters 3 for PS and PMMA and for all the solvents from which specimens were obtained (see Table). In the case of PS, the values of 6 for the polymer and for such solvents as chloroform and benzene are practically the same and a film with the highest value of E is formed from these solvents. The value of 6 for dioxane is substantially different from that for PS and the relaxed modulus of the film is found to be very low. A similar relationship between the solubility parameters and the relaxation properties is also observed in the case of PMMA. In order to explain the features noted above concerning the effect of the films' processing history on its strength and relaxation properties, let us analyse the data ot" the measurements and calculations shown in the Table. In the case of PS, the modulus of elasticity and a are found to decrease with the solvent in the following series: chloroform-benzene-dioxane. The density of the specimens changes only very slightly and the mean spacing between the chains remains the same. Nevertheless, the density of the specimens decreases somewhat with an increase in the van der Waals volume of the solvent molecl le. This volume was calculated using tabulated data for van der Waals atomic volumes, dVi [15]. This is entirely in order, since the greater the volume of the solvent molecules removed from the film during its drying, the greater will be the volume of the micropores [16]. However, since there is no corlelation between the density, mean spacing between chains and the mechanical property parameters, these factors clearly do not play an important part in the case of PS. We may consider it to be established that the greatest change in mechanical properties towards a reduction in their values is found when a solvent is used whose solubility parameter differs most from that of the polymer. In the case of PMMA, the specimens are also found to have a high density when the film is formed trom solvents having a smaller van der Waals molecular volume, so that the density may even exceed the density of the moulded specimen. When solvents with a large van der Waals molecular volume are used, the density of PMMA takes on its lowest value and, corresponding to this, the mechanical properties are lowered. In view of the fact that the mean spacing between the chains is approximately the same and is in no way connected with the magnitude of the density, it may be concluded that the reason for the diffelence in properties is the formation of different types of ultramicroporosity in the structure with average micropore dimensions that differ one from another when the different solvents are used. As is also the case for PS, the worst mechanical characteristics are observed for the specimen obtained from dioxane whose value of ~ differs most from that of the polymer. The different values of density found for specimens of one particular polymer but with different processing histories are evi-

2172

M . S . MATEVOSYANet aL

d e n c e t h a t the s p e c i m e n s ' s t l u c t u r e s a r e in a state o f m e t a s t a b l e e q u i l i b r i u m and, u n d e r c e r t a i n conditions, the structures m a y p o s s i b l y t r a n s f o r m into the e q u i l i b r i u m state c h a r a c t e r i z e d b y the highest density. A special study o f the c o n d i t i o n s for such t r a n s i t i o n s has n o t b e e n m a d e in the p r e s e n t w o r k . H e a t i n g the specimens at 50°C in v a c u u m d o e s not, however, l e a d to any e q u a l i z a t i o n o f the density. E x p e r i m e n t s t h a t were carr i e d o u t s h o w e d t h a t a change in density does n o t occur either when specimens are h e l d at i o o m t e m p e r a t u l e fo~ two years. The p r o b l e m ot the change in density a n d o f t h e a s s o c i a t e d v o l u m e r e l a x a t i o n requires the setting-vp o f special experiments, which d i d n o t f o r m p a r t o f the objective o f the p r e s e n t work. Translated by G. F. MODLEN

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