Mesophase formation in crosslinked polydiethylsiloxanes under uniaxial stretching

Polymer Science U.S.S.R. Vol. 30, No. 2, pp. 329-335", 1958 Printed in Poland

0032-3950/88 $10.00+.00 .~ 1989 Pergamon Prms plc

MESOPHASE FORMATION IN CROSSLINKED POLYDIETHYLSILOXANES UNDER UNIAXIAL STRETCHING* Yu. K. GODOVSKII, I. A. VOLmOVA, L. A. VALETSKAYA, A. V. REBROV, L. A. NOVITSKAYA and S. ]. ROTF-NaURO L. Ya. Karpov Physicochemical Scientific Research Institute

(Received 21 July 1986) It is established by a study of crosslinked polydiethylsiloxane specimens using deformational calorimetry, optical microscopy and X-ray diffraction analysis, that at a specific degree of stretching the formation of an oriented mesophase is observed, which is sensitive to the stretching temperature and the quantity of crosslinking agent. POLYDIETHYLSILOXANE(PDES) which is a representative of a small group of thermotropic liquid crystal polymers, which do not contain classical mesogenic groups, but which are none the less able to form mesophases, has recently become the subject of special attention. In the case o f PDES, information has already been obtained on the thermodynamics o f phase transition [l], the structuxe o f the crystalline and mesomorphous phases [2], and on the kinetic morphological features of mesophase formation [3]. These papers also contain preliminary information on tbe effect o f chemical crosslinking on phase transitions in PDES. In particular, it was established that the ability of crosslinked PDES to f o r m a mesophase is sharply decreased, but that under ten.~ion the proportion o f mesophase in it increases significantly. The object of this work was to undertake a thorough optical, X-ray diffraction and calorimetric study of the mesophase formation process in crosslinked PDES specimens under uniaxial tension. Two PDES specimens were used, i.e. a specimen crosslinked with respect to the end hydroxyl groups, and a polyfunctional siloxane resin (PSS), in quantity 15 and 45 wt. % respectively. Both normal and polarized light was used in the optical studies, the m,croscope having a tensioning device and a heating table, which enabled a PDES specimen containing 15 wt. ~o PSS to be observed at various degrees of tension and at increased temperatures (up to 100°C). In the case of the specimen containing 45 wt.~o PSS, because of the considerable turbidity optical observations m transmitted light were impossible. The state of the mesomorphous phase in the crosslinked PDES in the isotropic and tensioned states was evaluated from the melting thermograms, obtained by means of a "Pcrkin-Elmer" model DSK-2 scanning calorimeter, and also from analysis of large angle X-ray diffraction data. The energy effects in the deformation of crosslinked PDES specimens were investigated on a deformational micro-calorimeter [4] at temperatures between room temperature and 100°C. The * V y s o k o m o l . soyed. A30: N o . 2, 359-364,

1988.

329

330

Yu. K GODOVSKI!et at.

12-15 mm specimens were drawn m the calorimeter at a constant rate of 80 ram/rain up to the assigned deformation, after which drawing was stopped, and after a certain time necessary for complete relaxation of the thermal effect, the specimens were returned to their original state. Simultaneous recording of the mechanical stress and the thermal effects as a function of the strain enabled the work and heat of elastic tension and contraction to be determined by integration of the corresponding areas. The accuracy of determining the thermal effects and the work of tension and contraction was 2%. Microscopy observation o f crosslinked PDES containing 15 wt. % PSS in polarized light showed that under 200--300 % tension narrow, long light bands appear, oriented in the direction o f tension, the intensity o f which is a maximum when located at 45 ° to the direction o f vibration of the light in the polarizer.

F,o. 1. Photo-X-ray diffraction patterns of crosslinked PDES containing 15 wt. % PSS, in the un-

drawn (a) and drawn (e = 200 %) state (b). Vertical orientation axis.

Figure 1 shows photo-X-ray diffraction patterns for this PDES specimen in the isotropic and drawn ( t = 300 %) states, and indicates that under tension a high degree o f orientation o f the macromolecule is attained in crosslinked PDES: the intensity o f the fundamental reflection 2 0 = 11"03° (which is characteristic of a m e s o m o r p h o u s PDES structure [2]) in the equatorial direction for an oriented specimen is significantly higher tb.an in the meridional direction. The optical observations and the X-ray diffratlon data are m good agreement w~th the calorimztric studies. Figure 2 shows heating t h e r m o g r a m s for both crosslinked PDES specimens in the region o f the mesomorphous transition (curves 1 and 2). For comparison, Fig. 2 also shows a t h e r m o g r a m for isotropization o f non-crosslinktd PDES (curve 5). It follows from Fig. 2 that with increase in the PSS content the heat TABLE 1. HEAT OF ISOTROPIZATIONOF CROSSLINKEDPDES $PECIM£NS Quantity of crosslinking agent, wt. ~o 15 45

j I

Heat of isotropization, Jig crosslinked PDES i non-crosslinked PDES (e = 200 %) undrawn drawn 0.70 2-0 / 2.7 0"35 0.8

i

331

Mesophase formation in crosslinked PDES

of lsotropization ofcrosslinked PDES is sharply decreased (Table 1), and its temperature range is displaced in the direction of lower temperatures. Curves 3 and 4 on Fig. 2 are thermograms for the isotropization of oriented (e = 200 ~ ) crosslinked PDES. Comparison of these with curves 1 and 2 shows that the temperature range is wider in the oriented specimens than in the isotropic specimens, and is displaced in the direction

~a/dt

_A

,

..A

o"/vPa 0-,5-

2

0.3-

0.1 I

20

I

50 FIG. 2

7"*

100

200

300 ~,%

FIG. 3

FIG. 2. Heating thermograms in the region of the mesomorphic transition of the crosslinked PDES specimens in the undrawn (1, 2) and drawn (e=200~) states (3, 4), containing 15 (1, 3) and 45 wt. PSS (2, 4); 5-original non-crosslinked PDES. Heating rate 20 deg/min. FIG. 3. Tension curves (1-4) and contraction curves (1'-4') of a crosslinked PDES specimen containing 15 wt.y. PSS, at 20 (1, 1'), 30 (2, 2'), 40 (3, 3'), and 50°C (4, 4'). of high. temperatures. The heat of isotropization of the crosslinked specimens under tension increases, but still remains less than the value for the heat of isotropization of the non-crosslinked PDES (Table I). Comparison of curves 3 and 4 shows that increase in the quantity of PSS has an unfavourable effect on formation of an oriented mesophase on tensioning of crosslinked PDES, so that this process was then studied mainly in specimens having a small PSS content. The data obtained on visual observation of the tensioning and contraction of the given PDES specimen will now be considered. In contrast to conventional elastomers which remain uncrystallized under tension, crosslinked PDES, on attaining a degree of tension of about 100 ~o begins to be drawn with only slight heterogeneity, which gradually disappears at a degree of tension exceeding 200 ~. On contraction from higher degrees of drawing on attaining e = 2 0 0 ~ , a neck is formed in the PDES specimen, which on further contraction moves along the specimen and disappears at 10-20~ tension. The photo-X-ray diffraction pattern of PDES at 50 ~o tension is similar to that for the same specimen in the original isotropic state (Fig. la). On the other hand, the photo-X-ray diffraction pattern of the inhomogeneous section, formed in the PDES specimen at less than 100~o tension, diff.~rs little from the photo-X-ray diffraction pattern of the oriented specimen (Fig. lb). Consequently, at ~ 100 K tension, an oriented me-

Yu. K. GODOVSKUet

332

al.

sophase is formed locally in crosslinked PDES, which on further tension is distributed over the whole length of the specimen. The formation of a neck on contraction is the result of fracture of this mesophase. Continued holding of the PDES specimen in the tensioned state for several days results in such a degree of stabilization of the mesophase that even after removing the load the specimen does not contract. To completely restore the specimen length, the temperature must be raised to 50--60°C. TABLE 2. DATA FROM VISUAL OBSERVATION OF THE UNIAXIAL TENSION PROCESS AND CONCENTRACTION AT A RATE OF 18

T° 20 30 40

ram/rain,

90 I00 110

I 20 35 55

IN THE CASE OF A CROSSLINKED P D E S SPECIMEN CONTAINING 15 WT. ~o

5°

200

60 7O

% 120 130 140

60 70 80

200119080

200 200

PSS

rs,,

7, 82, %[e~, 7, 160 100 I 200 190 110 I 200

95

Visual observation of the tensioning and contraction of crosslinked PDES at increased temperatures showed that with rise in temperature the values of the degree of tensioning for which the specimen begins to be drawn unevenly et and for which the

0-4

e,% 0"0.

oi /,2 A3 o# o5 "6

1"2

Q,~'/b' FfG. 4. Plots of the deformation of PDES specimens against the mechanical work and the heat Q. Here and in Figs. 5 and 6 the PSS contents are 15 (1-4) and 45 wt.~. (5, 6). Tffi20 (1, 5), 30 (2), 5"0(3), 70--100 (4) mad 80° (6). neck in the specimen disappears on contraction e2 gradually increase, and the degree of tensioning at which a neck is formed in the specimen on contraction e3 remains unchanged (Table 2). It must be emphasized that with increase in temperature the inho-

333

Mesophase formation in crosslinked PDES

mogeneity of specimen deformation under tension and contraction becomes less noticeable, and at about 100°C almost disappears. It thus follows that with rise in the temperature of tensioning crosslinked PDES the beginning of oriented mesophase formation is displaced in the direction of large degrees of tension, and its proportion is gradually decreased. 2OO

300 ~,% I

ol "2 0.5

I

a5 "6

~4Uo/W)p~T tOO

200

300 ~ %

oq

1.0 o4

1"5

AUp,T,J/g

-6

• 6

F~o. 5 1~o. 6 FIo. 5. Changes in internal energy of crosslinked PDES specimens as a function of strain at different temperatures. FIG. 6. Plots of the strain ratios (AU/W)p.r of crosslinked PDES specimens at different temperatures. The broken line gives the values of the intramolecular PDES components.

The qualitative picture obtained of the formation of an oriented mesophase must be supplemented by quantitative analysis of the thermodynamic parameters (work, thermal effects, and change in internal energy) of uniaxial tension of crosslinked PDES. Figure 3 shows a plot of stress against strain in a tension-contraction cycle for PDES specimens containing 15 wt.~o PSS, and indicates that the deformational behaviour of this specimen depends essentially on the temperature. It must be emphasized that the form of the cycle is independent of the number of cycles for the same temperature. It xs clear from the tension curves that at room temperature and 100 to 200~o deformation, as a result of mesophase formation there is a certain hardening of the PDES specimen, which, with increase in test temperature, is displaced in the direction of higher degrees of tension and becomes less noticeable. The contraction curves show a similar change. The rapid fall in stress in the PDES specimen on contraction from large strains is associated with delay of the oriented mesophase destruction process, and the l~ateau on the contraction curves corresponds to rupture of this mesophase with neck formation. With increase in test temperature, the contraction curves tbr the crosslinked PDES arc higher on Fig. 3, and the hysteresis phenomena in the tension-contraction cycle are gradually decreased. Figure 4 shows the mechanical work done and the thermal cffect~ as a function of ,train and temperature [or a PDES specimen containing 15 wt. ~o PSS. The work done changes little with temperature and gradually increases with strain. The heat evolved on tension of this specimen at room temperature increases sharply with strain, whicll

334

Yu. K. GODOVSKII el al.

is associated with. the formation of an oriented mesophase, but the heat evolution ~s inv.'rsely related to the test temperature, and at test temperatures above 60°C the plots of the thermal effects against strain differ little from each other. Figure 4 also shows plots for the mechanical work done and the thermal effects for a PDES specimen containing 45 wt. 70 PSS. Comparison of these plots for both PDES specimens shows that with increase in the quantity of PSS there is no noticeable hardening of the PDES, but the thermal eff.'cts accompanying deformation of the PDES specimen of high PSS content are noticeably less at room temperature. On the other hand, at temperatures above the temperature of existence of the oriented mesophase tension of both specimens is accompanied by the same temperature effects. Tlxe change in internal energy AUp,r on deformation of both PDES specimens as a function of temperature is shown in Fig. 5. The nature of the change in internal energy as a function of the dfformation of the crosslinked PDES specimens also reflects the formation in them of an oriented m:sophase, sensitive to the test temperature and the PSS content. At test temperatures above 60°C, with both PDES specimens the values of ,4 Up,r are the same, and are gradually d.'creased under tension. This change in the internal energy obviously refbcts only the special features of the conformational change energetics on dfformation of PDES macromolecules, and does not include a contribution from the decrease in A U~,r on formation of a mesophase. With decrease in test temperature the zfUo,r plots become rapidly inclined downwards, this inclination, which is associated with the formation of an oriented mesophase, being inversely related to the temperature and the PSS content. Analysis of the deformational (strain) plots of the ratio of the change in internal energy to the work done (AU/W)o,r in the case of the crosslinked PDES specimens (Fig. 6) is the next stage. Over the 100 to 20070 deformation region the value of (/IU/ /W)o,r for elastomers is almost ind.~p~ndent of the deformation and tends to a constant value of the energy component (A U/W)v,r, which is characteristic of the chemical nature of the given elastom.~r and denotes the proportion of the intramolecular energy changes determined only by conformational changes. It follows from Fig. 6 that the plots of (AU/W)o,r against the strain ~ for both PDES spzcimens at test temperatures above 60°C are unique, and are typical of elastomers; the energy component is equal to -1.1. A value for the energy component as high as this has not so far been obtained foc any known elastomer, and must be analysed separately. It must bz noted that on lowering the test temperature, even at small deform~ions of the crosslinked PDES specimens, the values of (AU/Br)p,r are sharply decreased, as observed in the crystallization of elastomers under tension. In the case of the PDES specimen of lower PSS content and at room temperature this decrease is maximum. As the formation of the oriented m:sophase on the deformation of crosslinked PDES finishes, the ratio (,ffU/W)~,r begins to increase and tends to the value of the energy component. Accordingly, analysis of the entropy and energy effects on tension of crosslinked PDES indicates that their bzhaviour at test temperatures above 60°C obeys the main laws of change of thermoelastic properties characteristic of elastomer behaviour. The

335

Ionization equilibrium of amine groups in eopolymers

elasticity of PDES specimens at these temperatures is not simply an entropy factor, but is accompanied by extremely unfavourable intramolecular energy changes. The formation of an oriented mesophase on tension of crosslinked PDES at temperatures below 60°C introduces its own contribution into their mechano-thermal behaviour, this effect being inversely related to the temperature and the content of crosslinking agent. The a u t h o r s wish t o t h a n k A. N. Ozerin f o r his h e l p in discussing t h e results o f t h e X - r a y diffraction studies.

Translated by N. STANDBN REFERENCES !. V. S. PAPKOV, Yu. K. GODOVSKY, V. S. SVISTUNOV, V. M. LITVINOV and A. A. ZHDANOV, J. Polymer Sci. Polymer Chem. Ed. 22: 3617, 1984 2. D. Ya. TSVANKIN, V. S. PAPKOV, V. P. ZHUKOV, Yu. K. GODOVSKY, V. S. SVISTUNOV and A. A. ZHDANOV, J. Polymer Sci. Polymer Chem. Ed. 23: 1043, 1985 3. V. S. PAPKOV, V. S. SVISTUNOV, Yu. K. GODOVSKY and A. A. ZHDANOV, J. Polymer Sci. Polymer Phys. Ed. 25: 1858, 1987 4. Yu. K. GODOVSKY, Teplofizicheskie metody issledovamya polimerov (Thermophysical Methods of Investigating Polymers). Moscow, 1976

PolymerSctence U.S.S.R. Vol. 30, No Printed m Poland

2, pp.

335-340. 1988

0032-3950/88 $10.00+.00 © 1989Pergamon Presspie

FEATURES OF THE IONIZATION EQUILIBRIUM OF AMINE GROUPS IN MACROMOLECULES OF COPOLYMERS OF N-VINYLAMIDES WITH ALLYL AND VINYLAMINES IN AQUEOUS SOLUTIONS* Yu. E. K.IRSH, M. V. BATRAKOVA, I. YU. GALAYEV.

A. I. AKSENOVand T. M. KARAPUTADZE All-Union Scientific Research Institute for Blood Substitute Technology and Hormonal Preparations

(Received 21 August 1986) A significant decrease in amino group basicJty (2-3-5 pK, units) in copolymers of Nvinyl-N-methylacetamide, N-vinyl pyrrolidone and N-vinyleaprolactam with allyl or vinyl amines, when these are present in the chain in small amounts (0' 25-10 ~o), compared with the basicity of their low-molecular weight analogues is observed. The nature of the relation * Vysokomol. soyed. A30: No. 2, 365-369, 1988.