Mechanochemical reactions on poly(vynilchloride) - Springer Link

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chemical reactions between benzidine and PVC. The modification of polymer chemical structure will affect the behaviour under the conditions of both dynamic.
Colloid & PolymerSci 263:738-743 (1985)

Colloid & Polymer Science

Mechanochemical reactions on poly(vynilchloride) 16. Behaviour under static tensile stress of PVC modified with benzidine

C. V. Opreal), M. Popal), A. Ioanid2), and P. Bfrs~nescu ~) 1) Polytechnic Institute of Jassy, Department of Organic and Macromolecular Chemistry, Jassy, Romania 2) Institute of Macromolecular Chemistry "P. Poni" Jassy, Romania Abstract: The creep of a rigid material based on poly(vinylchloride) modified with benzidine is studied under tensile stress conditions. Based on the proposed rheological model, the creep equation is derived. The modifications induced by the mechanical stress on some physical properties of the material are studied. The analysis of the facture surfaces shows a "brittle" character of the fracture, especially in the final stage of the process.

1. Introduction

2. Experimental

Poly(vinylchloride) (PVC) is a polymer with particularly wide applications, which may be processed for obtaining semi-rigid plasticized and rigid products, the latter being particularly important for their superior physico-mechanical characteristics. Some technical fields require rigid PVC products with physico-mechanical properties superior to those of the materials usually obtained. A reinforcing of PVC with o-toluidine [1] and bis-phenol A [2] was thus achieved to obtain high values of tensile strength. Good results were also obtained by PVC processing in the presence of aromatic diamines (o-, p-phenylendiamine, benzidine) [3-6]. Thus, one has to mention that by simukaneous two roll mixing of the polymer with benzidine under certain conditions, the tensile strength (or) may increase from 54 N/mm 2 (unmodified polymer) to 84 N/mm 2. This effect is a consequence of polymer crosslinking due to the mechanochemical reactions between benzidine and PVC. The modification of polymer chemical structure will affect the behaviour under the conditions of both dynamic and static stress. The present paper is a study of the tensile creep of PVC modified with benzidine in order to derive the creep equation and to identify the modifications of some physical properties induced under different static tensile stresses.

The studies were performed with PVC obtained by suspension polymerization (K = 70), having the physico-mechanical and morphological characteristics previously described [7-9], and modified by two roll mixing in the presence of benzidine [3]. The samples were made from the films resulting from PVC two roll mixing with stabilizers and benzidine at 170 ~ for 15 minutes. During the two roll mixing the reaction between the reinforcing agent (benzidine) and the macromolecular compound also took place. The films thus obtained were pressed for 10 minutes at 170 ~ and 172 bars (following a preheating for 10 min) and plates of 15 mm thickness were obtained. The shape and size of the samples are illustrated in figure 1. The static tensile stress was performed on the device presented in its principal parts in figure 2, permitting the simultaneous loading of 5 samples. The deformation in time of the stressed samples was registerd with a precision of micron order by means of tensile marks placed at the extremities of their active sector. Three values of stress (o), at room temperature (20 + 2 ~ and with atmospheric humidity of 60-65 % were used. The stress dura-

K 933

150 60_+0.5

~~

_

50+-0'5 91

Fig. 1. Sample for the study of creep of the modified PVC with benzidine

Oprea et al., Mechanochemical reactions on poly (vynilchloride)

739 The variation of the amount of dissolved polymer with time was followed by maintaining sample fragments of 0.5 g in 100 ml cyclohexanone for periods ranging between 0 and 144 hours (20~

3. Results and discussion

Based on the experimental results the coefficients of the creep equation were established and some modifications of the material properties under the conditions of static tensile stress analysed. Fig. 2. Outline scheme of the device for the tensile creep study

don was between 310-360 hours, then the stress action ceased, the deformation variation being followed for 350 hours. For each value of the stress 5 samples were stressed and the average value of the deformation at different times was taken into account. In one experiment 5 samples were stressed at a value of stress of 45 N/ram 2, the macroscopic breaking of the material being noticed after 75 hours. These samples were examined by electronic microscopy. Thus, the fracture surfaces of the fragments of the broken samples were analysed with a Tesla BS-513 A (RSC) microscope. The replicas with carbon in one stage taken from these surfaces were microphotographed after a previous shading with palladium. The modification of some physical properties with stress was analysed. The swelling behaviour was analysed by the Dogatkin method, with acetone as swelling agent [10]. The maximum swelling degree (or..... %, g acetone/g polymer) and the swelling rate constant (K, s-~), both characteristic values for limited swelling, were determined.

3.1 Cre~ equation The experimental tensile creep curves are given in figure 3 (full line). Their shape permits some remarks on the principal material properties: elasticity, creep, "inverse creep". Consequently, a rheological model was proposed (Biirgers variant) [11] reflecting these fundamental characteristics (fig. 4). Thus, the arc E1 expresses the material elasticity due to the modifications of inter-atomic distances and bond angles in macromolecules. The viscoelasticity is expressed by the Voigt generalized model where E2 Kexpresses the chain conformational modification, and the piston cylinders 22/cthe environment response. The irreversible plastic deformations caused particularly by scission of chemical bonds in the polymer main chain are characterized by a piston cylinder ,i3.

~.10 3 ~m/mm} 32

28

2s

2(

,,.0

'0

12

-----~.--__._o._____~2 8

/,

0

I

loo

I

200

I

300

I

Loo

s~o

6~o

' t [ h )" 700

Fig. 3. Experimental (full line) and theoretical creep curves (dashed line) for the modified PVC with benzidine. 1 - o = 14.08 N/ruing; 2 - o = 23.13 N/mm2; 3 - o = 25.3 N/mm 2

Colloid and PolymerScience, VoL 263. No. 9 (1985)

740

~E2 El

1

~60 E

E22

"o 40 @J

o

E2 N

~

.~ o2 0

E2n 0

,

l

48

I

I

96

I

I

l,,

144 t(h)

Fig. 4. Rheological model for the tensile creep of the modified PVC with benzidine

Fig. 5. Variation of dissolved polymer amount with time. 1 = PVC blank sample; 2 = PVC modified with benzidine, stressed (o = 25.3 N/mm2); 3 = PVC modified with benzidine (unstressed)

The proposed rheological model is described by the equation:

curve (fig. 3, dashed line) and the experimental one (full line) are in good agreement, proving that the proposed rheological model is adequate.

Ig(l+o) +o(l_e

C--

E

-pt) D + ~

+ Bo n

(1) where: o e E

= stress (N/mm2), = deformation (%, mm/mm), = Young's modulus.

In this form, the equation contains all the components of the deformation, specific to polymers. The coefficients and exponents in (1) were calculated for each term of the equation by the trial and error method till the best agreement between the theoretical and experimental curves was obtained. The final form of the creep equation becomes: F1 e 0/0 = 10-4 12 lg (1 + o) + o(1 -

(

e - t / 3 x 10 - 6 )

1)

0.5+ t x 1 0 - b + l

+ 0.01256 9o165].

J

(2)

The proposed rheological model and the established equation are valid for a time interval varying between 0.5-300 h. On this domain, the theoretical

3.2 Modification of some physical properties

During the mechanical loading, the polymer suffers important modification affecting its structure at each organizational level. Thus, conformational changes of the macromolecules appear, the segments being oriented in the diretion of stress action even at temperatures below the glass transition temperature. The absorbed meachnical energy is concentrated in the defect zones of the polymer and consumed at least partially for the deformation of the chemical bonds and valence angles: the polymer passes in a mechano-excited state. During stress relaxation a part of the accumulated elastic energy is consumed for the homolytical scission of the chemical bonds and formation of free radicals. Consequently, the molecular weight decreases [12-15], fractions of lower molecular weight are accumulated in the polymer and the solubility in the specific solvents increases. This effect is clearly made evident by the results presented in figure 5. The lower solubility of PVC modified with benzidine (compared to the unmodified sample) is noticed as a consequence of some crosslinking reactions during the synthesis [4-6]. PVC modified with benzidine, submitted to mechanical stress, shows an increased solubility (curve 2) compared to the standard sample (curve 3). This effect is probably due to the molecular weight decrease caused by mechano-chemical degra-

Oprea et al., Mechanochemical reactions on poly (vynilchloride)

741

analysed by means of electron microscopy (fig. 7 a-c). The pronounced relief shown by the obtained microi/) photographs leads to the conclusion that the response ,,,,,I"" 0 x of the material to the mechanical stress is "brittle". It ~2 E may be appreciated that the unanimously accepted 1.04 general mechanism of polymer fracture under stress [18-28] is also valid in this case. The mechanical stress'is concentrated on the chemi2.3 1.0 cal bonds of the macromolecules in the polymer defect zones. Following the complete stretch of some chain segements compressed between two "nodes" of the 0.96 network, conformatinal modification are no longer possible and the homolytical scission of some chemical /2 bonds takes place. These have the tendency to accui mulate in microvolumes which constitute destruction 300.92 la microcenters. Under the combined action of the me~-(N/mm2} chanical effort, thermal fluctuations and "mechano-raFig. 6. Maximum swelling degree - static tensile stress profiles for dicals" generated in the incipient stages of the stress, PVC modified with benzidine (1) and swelling constant rate - static the great number of the broken bonds in a microtensile stress profiles (2) volume of the material join together leading to the appearance of microcracks. In the case under study these may be evidenced by analysing the microphotodation reactions promoted by the mechanical graph 7 a. They increase by either subsequent scission stress. of the macromolecules or by coalescence until the criMacroradical formation in polymers is possible tical size is attained, according to the Griffith criterion even at a deformation representing 20 % from the [29]. During this stage the effort of mechanical stress is breaking one [16,17]. PVC modified with benzidine maintained at values below the breaking stress so that presents a breaking deformation of 6.5 %. For static the relaxation times of the polymer are comparable stressing at o = 25.3 N/mm 2 its deformation is about with the stress times. Consequently, the macromole3.2 %, which represents about 50 % from the breaking cules may rearrange under load so that the relief of one. The scission of a great number ofmacromolecular these zones on the fracture surfaces is less pronounced. chains and the formation of lower chains, soluble in The appearance of the magistral crack by nucleation cydohexanone, is thus very possible. or coalescence ofmicrocracks constitutes the first stage The process of macromolecular orientation under of the macroscopic breaking. Since the stress is less stress results in an increase in the polymer packing than that required for macroscopic breaking the crack degree and the corresponding modification of its phy- progresses slowly within the sample, creating new sursical properties. faces of a rather smooth aspect. The energy is partially The polymer behaviour in swelling agents (acetone) dissipated within the material by unelastic deformais affected as shown by the results in figalre 6. The tions which do not involve the scission of the chemical increase in the packing degree under increasing stress bonds. results in the intensification of the interactions between When the material section attains a critical value and the macromolecules. Consequently, the swellingagent the stress becomes comparable to the breaking stress penetrates with difficulty into the polymer and the the propagation of the magistral crack is suddenly maximum swelling degree and swelling rate constant "catastrophically" self-accelerated within the sample. decrease. In this case the chemical bonds are broken, due The prolonged action of a mechanical stress lower almost entirely to the mechanical effort, the participathan thee breaking stress leads to delayed polymer tion of the thermal fluctuations being greatly dimifracture. To obtain some information on the breaking nished. The propagation of the magistral crack character of this rigid material, a set of 5 samples was becomes rapid, being controlled by the unelastic propsubmitted to mechanical stress at o = 45 N/mm 2. The erties of the polymer. Since its relaxation times are macroscopic breaking took place at durations between much longer than the stress ones the stressed macro70-80 hours. The fracture surfaces of the samples were molecules cannot rearrange under load so that the 2.4.,

1.08,.7

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Colloid and Polymer Science, Vol. 263 9No. 9 (1985)

Fig. 7. Microphotographs of the fracture surfaces of modified PVC with benzidine (o = 45 N/mm 2, 75 hours, x 9900)

relief of the fracture surface becomes smooth (fig. 7 b), a characteristic of the polymers which do not show energy losses due to the plastic and viscoelastic properties. The transition from the regime characterized by slow propagation of the crack to that of rapid propagation thus becomes evident. Figure 7 c shows a clear demarcation of the two zones which constitutes the front of the magistral crack in the moment of its "catastrophic" propagation. Based on the results obtained by electron microscopy it may be appreciated that the PVC samples modified with benzidine show a "brittle" response to the mechanical stress, more pronounced in the final stages of the breaking. 4. Conclusions 1. The creep of PVC modified with benzidine may be described by the equation corresponding to the modified Biirgers model.

2. The mechanical stress of PVC modifiedwith benzidine induces changes in the physico-mechanical and chemical properties of the polymer. 3. The breaking behaviour under constant load of PVC modified with benzidine is of the "brittle" type, more pronounced in the final stages of the breaking. References 1. Porter W, Scott G (1971) Eur Polytu J 7:489 2. Voskresenskii CA, Shakirzyanova SF (1962) J Prikl Him 35:1145 3. Oprea CV, Petrovan S, Popa M (1979) Meh kompoz mater 6:977 4. Oprea CV, Popa M (1980) Coil & Polym Sci 258:371 5. Oprea CV, Popa M (1981) Rev roum Chim 26:2, 291 6. Oprea CV, Popa M (1983) Mater plast 20:3,185 7. Oprea CV, Negulianu C, Popa M, Simionescu C (1977) Haste und Kautschuk 9:604 8. Simionescu C, Oprea CV, Negulianu C, Popa M (1977) Haste und Kautschuk 10:689 9. Oprea CV, Popa M (1981) Bull IH, XXVII, 1-2:115 10. Voinitkii SS, Panici RM (1974) Praktikum po koloidnoi himii i electronnoi tuikroskopii Himiia, Moskva, p 151

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Oprea et aL, Mechanochemical reactions on poly(vynilchloride)

11. Volintiru T, Ivan Gh (1980) Introducere in fizico-chimia polimerilor, Ed Tehnic~l, Bucuresti 12. Casale A, Porter RS, Johnson JF (1971) Rubber Chem and Techn 44:534 13. Mehta RE (1972) J Macromol Sci, B 8:961 14. Birch WM, Williams JG (1978) Int J Fract 14:169 15. Becht I, Fisher H (1970) Kolloid Z u Z Polymere 240:775 16. Nagamura T, Takayanaghi M (1974)J Polym Sci, Polym Phys 12:10, 2019 17. Nagamura T, De Vries KL (1978) Pol Eng Sci, 19:2 18. Jurkov SN, Narsulaev BN (1953)J Tech Phys, USSR 23:1677 19. Jurkov SN (1965) Intern J Fract Mech 1:311 20. Peterlin A (1970) J Polym Sci C32 21. Peterlin A (1979) Pol Eng Sci 2:19 22. Andrews EH, Reed PE (1978) Adv in Polymer Sci, Vol 27, Springer Verlag, Berlin-Heidelberg 23. Andrews EH (1979) Developments in Polymer Fracture, Applied Science Publishers Ltd, Barking, England

24. 25. 26. 27.

Kausch HH, Becht J (1972) Kolloid Z u Z Polymere 250 Kausch H H (1979) Pol Eng Sci 2:19 Sohma J, Sakaguki M (1976) Adv Polym Sci 20 Murakami K, Ono K (1979) Chemorheology of Polymers, Hsevier, Amsterdam 28. Williams JG (1984) Fracture Mechanics of Polymers, Ellis Horwood Ltd, Chichester 29. Griffith AA (1920) Trans Roy Soc, London, series A 221:163 Received August 20, 1984; accepted March 11, 1985 Authors' address: C. Vasiliu Oprea Dept. of Organic and Macromolecular Chemistry Calea 23 August Nr. 11A 6600 Jassi, Romania