A kinetic study of crazing in deformation of blends based on poly(vinyl chloride) in liquid media

Polymer Science U.S.S.R. Vol. 26, No. 11, pp. 2663-2670, 1984 Printed in Poland

0032-3950/84 $10.00+.00~ © 1985 Pergamon Press Ltd.

A KINETICSTUDY OF CRAZING IN DEFORMATION OF BLENDS BASED ON POLY(VINYL CHLORIDE) IN LIQUID MEDIA* L. M. YARYSHEVA,L. Yu. PAZUKHINA, T. A. BORODULINA, T. B. ZAVAROVA,A. L. VOLYNSKII,N. F. BAKEYEV and P. V. KOZLOV M. V. Lomonosov Moscow State University (Received 3 June 1983)

The processes of craze formation and growth have been investigated, which accompany deformation in ethanol of PVC and its blends with PMMA or with the graft copolymer of methyl methacrylate and poly (butadiene-costyrene). Elongation at a constant rate as well as creep were investigated. The distribution of growth velocity of crazes and data on the number of crazes were analysed taking into consideration the microheterogeneous structure of the blends. WHEN polymers are deformed in adsorption-active media, both the mechanism of deformation and the physico-ehemical properties of polymers are substantially affected. These problems have been studied in a number of papers [1-3]. Although films and other products made of polymeric blends find nowadays many practical applications, their mechanical behaviour upon deformation in adsorption-active media has not been widely investigated so far, and even in the existing studies the properties of blends have not been compared with those of the constituent homopolymers, so that it is difficult to reach unambiguous conclusions concerning the effect of the second polymeric component present upon the behaviour of the blend in an adsorption-active medium [4-6]. Deformation of polymers in adsorption-active media is accompanied by the formation and growth of a considerable number of crazes. A clear-cut relationship has recently been found to exist between the mechanical parameters of the polymer immersed in the medium and the formation or rate of growth of crazes, which are formed in the specimen upon elongation at a constant rate [7-11 ] or in deformation under conditions of a creep experiment [12, 13]. N o t only it is possible to estimate the mean velocity of craze growth either from the dynamometric response of the polymer deformed at a constant rate [9, 10] or from the creep [ 12], but a microscopic investigation of the number of crazes and of their growth velocity can be conversely used for predicting the mechanical characteristics of the polymer in the given medium. A similar approach to the problem of crazing, where the number of crazes formed and the velocity of their growth are juxtaposed with the mechanical properties of the polymer in the given adsorption-active medium, has been adopted in the present study; * Vysokomol. soyed. A26: No. I1, 2380-2386, 1984. 2663

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L.M.

YARYSHEVA e t al.

fillms made of poly (vinyl chloride) (PVC) and of its blends with h~/rd, glassy poly (methyl methacrylate) (PMMA) and with rubbery graft copolymer of styrene and poly (methyl methacrylate-co-butadiene) (MBS) were investigated. Special attention was devoted to the kinetics of crazing in the blend PVC]MBS, which shows enhanced impact strength in air in comparison with PVC itself [14]. Conditions employed in blending a n d in the preparation of samples of PVC a n d its mixtures -with 30 wt. ~o M B C or 30 wt. ~ P M M A were analogous as in [14]. Mechanical properties were studied with specimens punched in the form of dumb-bells with the dimensions of the middle part 6 x 22 ram; the films were 100 to 120 n m thick. The instruments employed in studying the elongation at a constant rate (v = const.) and creep constant stress, (tr=const.) with a simultaneous determination of the craze length by optical microscopy have been described elsewhere [10, 15]. The specimens were elongated at a constant rate equal to 8.0 × 10 -4 m/see. A constant load of 23.5 M P a was used in the creep experiments. The specimens were elongated up to 50 ~ deformation. Absolute ethanol served as the adsorption-active medium. The methodology used for determining the distribution c u r v e s of growth velocity of crazes is described in [15].

Pi OG1 3

(~.21_!II1 t

04

I

ZO C;% F~G. 1

0

& ._

lO

20

30

u,

lOa,m/sec

FIG. 2

FIG. 1. Activity of the medium for blends of different compositions: 1 - PVC/PMMA; 2 - PVC] /MBS. FiG. 2. Distribution of crazes according to linear velocity of growth. PVC (•); PVC/PMMA (2); PVC/MBS (3). When a polymer immersed in an adsorption-active medium is deformed at a low elongation rate, the limit of forced elasticity ar, is substantially reduced. Consequently, the activity of the medium can be estimated by comparing the values o'f(~) (in the reed-

Crazing in deformation of blends based on PVC

2665

ium) and ,,fv--tat(in air): ~"fet"~a~--"fe~tm~X/~t~JZ"fe" The data in Fig. 1 show that the activity of the medium increases when P M M A is blend with PVC, while the opposite is true for the blends with MBS. A similar effect of a rubbery filler upon the efficiency of an adsorption-active medium with respect to reducing the limit of forced elasticity and also the critical rate of deformation (above which the effect of the medium on o-re vanishes) has been already observed [16]. The mechanical characteristics of the polymer deformed in an active medium are closely connected with the number of formed crazes and with the velocity of their growth [7-12]. Thus, it is necessary to compare these two characteristics with the data on the efficiency of adsorption-active media. It has been established in studying the process of crazing with polymers such as polycarbonate [17] or poly (ethylene terephthalate) (PETP) [ 10] that crazes formed during deformation of the specimen in a liquid medium differ in their growth velocity. Distribution of crazes with respect to their growth velocity has been analysed [ 10, 12] in connection with the conditions of deformation. The existence of this distribution is related to microdefects and microheterogeneities in polymers. In polymeric blends the original structure is modified, and these changes result in a concentration of additional inner stress at the interphases. The existence of a two-phase structure has been established in blends of PVC and MBS, with PVC forming the continuous phase and MBS the dispersed phase [ 14, 18]. Although PVC and P M M A belong to the calss of partially compatible polymers, their blends at the content of P M M A also show a two-phase structure with P M M A concentrated mostly in the dispersed phase [ 19, 20]. Let us consider the distribution of crazes according to the growth velocity as given in Fig. 2 for PVC (curve 1) and for its blends with P M M A (curve 2) and with the rubbery modifier MBS (curve 3); these curves were obtained in experiments where the specimens were elongated at a constant rate in ethanol. First of all it must be noted that the character of the distribution curves is similar to that observed in [ 10] with PETP: the distributions are asymmetric, with a long tail in the range of large growth velocities, and shows a very distinct maximum corresponding to the most probable velocity of craze growth. Similarly as in the case of PETP the majority of crazes in PVC and its blends are formed at the beginning of elongation in the region of small deformations, and with progressing elongation the number of newly formed crazes drops rapidly (Fig. 3). Comparing the formation of crazes upon elongation at a constant rate in PVC and in its blend with P M M A in the medium, we see that the incorporation of P M M A increases NUMBER

OF CRAZES

AND

VELOCITY

OF

THEIR

GROWTH

CONDITIONS

Polymer PVC PVC/PMMA

FOR

POLYMERIC

BLENDS

UNDER

DIFFERENT

OF DEFORMATION

N

vm× 106, m/see

23-3[163 35-2[18.2

1.5/0.005 1.5/0.045

Note. N u m e r a t o r - - at v = c o a s t . , d e n o m i n a t o r - a t a = e o n s t .

Polymer PVC+ MBS

N 15.8/19-2

Vm× 106, m/sec 0.625/1.255

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L . M . YARYSHEVAet al.

the number of crazes N (per 1 mm of specimen l e n g t h - see Fig. 3, curve 2 and Table 1). The distribution of crazes according to the growth velocity for the blend PVC/PMMA spans a broad interval of velocities indicating a pronounced occurence of defects and inhomogeneities in the metarial. The most probable velocity of growth Vm practically does not change when PVC is blended with PMMA. Thus, the enhanced activity o f the medium observed for the blend PVC/PMMA in comparison with PVC itself is accompanied by an increased number of formed crazes and by broadening of the distribution curve towards the region of large velocities. ~, MPa 3O

20

lO I

4'

20

12

FIG. 3

e, */,

t

I0

I

30

i

!

£,% 50

FIG. 4

FIG. 3. Number of crazes per 1 mm length as a function of deformation in PVC (1), PVC/PMMA '(2), and PVC/MBS (3). FIO. 4. Dynamometric curves of PVC (1), PVC/PMMA (2), PVC/MBS (3). When the behaviour of PVC is compared with that of the blend containing the rubbery modifier MBS, we see that the incorporation of the latter leads to a decrease in the number of formed crazes (see Fig. 3, curve 3 and Table 1) regardless of the increased inhomogeneity of the mixture due to its two-phase structure [ 14, 18]. The distribution of crazes according to growth velocity gets narrower end the most probable linear velocity vm decreases upon incorporation of MBS. Thus, a smaller number of formed crazes and a decreased velocity of craze growth are characteristics for a blend for which the activity of the medium is lower in comparison with PVC. Consequently, the efficiency of the medium is seen to represent some integral characteristics of the number of crazes formed on elongation and also of their growth velocity. In analysing the craze formation in PVC blends with MBS and with P M M A during elongation at a constant deformation rate one must take into consideration that the elongation proceeds at different levels of stress. Figure 4 shows the dynamometric curves obtained for PVC and for its blends with P M M A and MBS. The values of trf. and the stress corresponding to stationary deformation, as, are close for PVC and its blends with PMMA, while the curve for the blend PVC/MBS is shifted to lower values of stress.

Crazing in deformation of blends based on PVC

2667

It has been shown m a n y times in investigations of both the growth velocity of unit crazes [21-25] and the most probable growth velocity for a large number of crazes that these two characteristics are extremely sensitive to the value of stress attained during the deformation of the sample.

pz 0'4

3

0.2

0.05

0.15

1"25

11.25 u,lO6,m/sec

FIc. 5. Distribution of crazes according to linear velocity of growth in creep experiments at a constant load a=23-5 MPa: PVC (•); PVC/PMMA (2); PVC/MBS (3). For this reason, in order to compare the behaviour of PVC and its blends in ethanoI we performed experiments with constant o- so as to eliminate the effect of stress on the craze formation. Figure 5 shows the distribution curves of crazes according to the growth velocity as obtained in creep experiments with constant stress of 23-5 MPa, which lies between afe and the stress at the stationary deformation for PVC elongated at a constant rate, v = const. Similarly as in the case of deformation at a constant rate, the individual crazes differ in their growth velocity, and the shape of the distribution curves is also similar. The most probable linear velocity of craze growth has been determined from the distribution curves, and the over-all number of crazes has been found for P V C and for its blends (see Table 1). The number of crazes and their growth velocity decrease when the specimen is deformed in the medium under conditions of constant stress. This can be explained by considering that initially the stress acting in PVC deformed in the medium at constant rate is higher than that applied in the creep experiment, and the majority of crazes originate in PVC and also in the blends at this initial stage (Fig. 3). A higher value of stress results in a larger number of formed crazes and also the most probable linear velocity increasses. Let us consider the incorporation of the second polymer, viz., the glassy P M M A , influences the crazing in the regime of constant load. Figure 5 and the data in Table ! show that both v,, and N increase upon introduction of P M M A into PVC. Similarly as in the case of elongation at constant rate in the medium, the increase in N is apparently related to the microscopic inhomogeneity of the two-phase blend P V C / P M M A . The ratio of values (number of crazes as well as the most probable linear growth velocity) observed in the two regimes of deformation is apptoximately the same for the blend P V C / P M M A and for PVC alone, since the value of the constant load selected in the

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L. M. YARYSrmVA et al.

creep experiment (23-5 MPa) lies for the blend also between the respective values o f arc (27-2 MPa) and a, (19 MPa). A comparison of the distributions of crazes according to the growth velocity obtained in the creep experiment in ethanol for PVC and its blend with the rubbery modifier MBS however shows that the resulting effect is the opposite to that observed in elongation at constant rate (cf. Fig. 2, curves 1, 3 and Fig. 5, curves 1, 3). The number o f formed crazes increases upon incorporation of MBS into PVC (Table 1); this finding does not contradict the assumption that the inhomogeneity of the polymer is increased as a result of the formation of a two-phase structure in the blend, although a more rapid rise in the number of crazes might be expected on the basis of published results, since the incorporation of rubbery particles into glassy polymers leads to the formation of a large number of crazes during deformation in air [26]. Nevertheless, the most dramatic is the change in the value of vm, which is by several orders of magnitude higher for the blend than for PVC. The crazes formed in PVC and in its blend with MBS are of different shape (Fig. 6). While the crazes in PVC are narrow and straight, those in the blend are wider at the base or more open, and wedge-shaped. The edges of crazes in the blend PVC/MBS are uneven, and the tip often deviates from a straight trajectory. In addition, the increased number of crazes formed in the blend PVC/MBS results in a frequently observed merging of individual crazes~

FIG. 6. Photomicrographs of crazes formed in the blend PVC/MBS during elongation at a constant rate v = 8.0 x 10-6 m/sec (a) and under a constant load a = 23"5 MPa (b). The mechanical characteristics of PVC and its blends deformed at constant stress in the medium fully agree with the observed process of crazing. The rate of deformation o f the investigated polymers increases with the number of crazes and with their growth velocity. The time dependence of deformation at constant load is shown in Fig. 7 for PVC and its blends. The time required for the deformation to reach 100 ~o increases in the series PVC/MBS, PVC/PMMA, PVC, in accord with the observed trends in growth velocity and in the numbeI of crazes (Table 1). Thus, the experiments carried out in the regime of constant load revealed the contribution to crazing due to the second polymer incorporated into PVC. In comparison

Crazing in deformation of blends based on PVC

2669

with PVC a larger number of crazes are formed in the blends PVC/PMMA and PVC/ /MBS, the velocity of their growth is higher (in particular in the latter case) and, as a result, the rate of deformation increases.

50

1

30

IL7

S

5

!~5 f ..rse<7

FIO. 7. Time dependence of deformation in ethanol at a constant load of 23-5 MPa : PVC (1); PVC/PMMA (2); PVC/MBS (3).

A consideration of experiments with deformation in the medium at constant velocity and constant load shows that the incorporation of rubbery particles can either increase or decrease both the velocity of growth and the number of crazes, depending on the level of stress acting in the polymer; different results on growth velocity, number and shape (openness) of crazes can be obtained under different experimental cone ditions. Translated by M. KUBIN REFERENCES 1. R. P. KAMBOUR, J. Polym. Sci., Macromolec. Rev. 7: 1, 1973 2. A. N. TYNNH, Proclmost' i razrushenie polimerov pri vozdeistvii zhidkikh sred (Strength and Destruction of Polymers under the Action of Liquids). Naukova dumka, Kiev, 1975 3. V. N. MANIN and A. N. GROMOV, Fiziko-khimicheskaya stoikost' polimernykh materialov v usloviakh ekspluatatsii (Physico-chemical Stability of Polymer Materials in Use). Khimiya 1980 4. Y. W. MM, J. Mater. Sci. 11: 303, 1976 5. Y. W. MAI and A. J. ATKINS, J. Mater. Sci. 11: 677, 1976 6. K. IISAKA and K. SHIBAYAMA, J. Appl. Polym. Sci. 24: 2113, 1979 7. N. BROWN and S. FISHER, J. Polym. Sci., Polym. Phys. Ed. 13: 1315, 1975 8. N. BROWN, Phil. Mag. 32: 1041, 1975 9. A. G. ALESKEROV, A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. B19: 213, 1977 (Not translated in Polymer Sci. U.S.S.R.) 10. A.L. VOLYNSKII, V. D. SMIRNOV, R. N. STOCHES, V. I. GERASIMOV, A. G. ALESK~ ROV and N. F. BAKEYEV, Vysokomol. soyed. AI8: 940, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 4, 1074, 1976)

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