Structure formation in a poly-p-benzamide-sulphuric acid system

PolymerscienceU.S.S.R.Vol. 20, pp. 742-748. Pe~~monPreenLtd. 1979.Printedin Poland

0032-3960/78/0301-0742207.50/0

0

STRUCTURE FORMATION IN A POLYq-BENZAMIDE-SULPHURIC ACID SYSTEM* S. P. PAPKOV, N. A.

M. M.’ IOVLEVA,

IVANOVA,

S. I. BANDURYAN,

I. N. ~DREYEVA, and

A.

V.

D. KALMYKOVA

V. VOLOK~A

All-Union Scientific Research and Design Institute for the Manufacture of Synthetic Fibres (Received 10 June 1977) Polarization-optical studies of poly-p-benzemide (PB) solutions in sulphuric acid established that relatively concentrated solutions of this polymer (ll-13%) at ordinary temperatures may be in non-equilibrium state for a long period of time, which is characterized by the coexistence of three phases: isotropic mobile liquid crystalline and solid. The separation of a solid phase from the isotropic solution corresponds to the equilibrium state. A study of possible types of phase equilibrium and results of electron microscope studies of these systems suggest that PB crystal solvates may be separated in certain temperature end concentration ranges in the form of a solid equilibrium phase.

SEVERAL studies

are concerned

with poly-p-benzamide

(PB)

solutions

in DMAA

in the literature about phase transformations and structure formation in a PB-sulphuric acid system, although bearing in mind the universality of sulphuria acid as a solvent for most rigid chain polymers, the study of this system is of considerable interest. Results are given in this paper which were obtained by polarization-optical and electron microscope investigations of PB solutions in sulphuric acid. [l-5].

However,

there

is hardly

any information

The initial polymer had an intrinsic viscosity [q] = 1.4 dl/g and therefore, a molecular weight calculated from results [6] previously obtained of the order of 11,000. f3-13°h PB solutions in 98% H,SO, were prepared from a reprecipitated polymer while heating to 40-50’ and transparent homogenoous solutions obtained. While cooling at room temperature solutions of a concentration of 11 T/oand higher became turbid and acquired a motherof-pearl shine typical of PB solutions in the liquid-crystalline (LC) state and after a given length of time became pasty. Solutions placed into sealed capillaries or hermetic cells with plane parallel walls were observed using an optical microscope (MIN-8) provided with a heating stage. DTA was also applied to study phase transformations in solutions during heating. The rate of heating was 2 deg/min. Observations on cooling formed.

to

show that

B-IO%

15” are spherulitic

ll-13%

* Vysokomol.

solutions

PB solutions refractive

at normal

solid

temperature

soyed. A20: No. 3, 668-682, 742

1978.

are isotropic particles

at 20-24’ with

are anisotropic,

and only

a Maltese while

cross on in-

Structure formation in poly-p-benzamide-sulphuric

creasing

temperature,

ture-concentration an isotropic

they

change

dependence

one, determined

into the isotropic

of the transition

acid system

solutions.

The tempera-

of an anisotropic

by a polarization-optical

method

743

solution

into

and by DTA

is

-- n ’I c 60

I,

L

I

8

FIG. 1. Relation

between

from the anisotropic results

PIG. 2. Transitions i.e. mobile LA

of

12 c, %

the transition

temperatures

of the PB-H,SO,

into the isotropic state and PB concentration

polarization-optical

measurements

in phase in the system

(left side), isotropic

the mobile LA phase (bottom);

PBA-H,SO,:

(dark field) and

c-system

u-the

solid

at equilibrium

(I)

(rigid

and

system

according t,o

DTA

existence

(2).

of

3 phases,

side): b--deformation

of

(solid particles on the background

of the isotropic solution (polarized light photographs).

WIG. 5. Isolation of the solid phase from the So/b PBA (crossed nicols) .

solution at reduced temperatures

744

f?.. P.

PAPKOV

et cd.

shown in Fig. 1; it can be seen that with an increase in polymer concentration, the temperature of transition increases. It should be noted that these solutions tend to undergo supercooling. On heating these solutions to a temperature which is 10-1~5~ higher than the temperature of transition into the isotropic state, followed

by cooling, the solutions remain isotropic

for 12-14 hr and anisotropic

phase se&ions are only formed in them after a given period of time. T

I

1

1’

To TCP

?

G

P

FIG. 3

FIG. 4

FIU. 3. Diagram of phase equilibrium in the rigid chain polymer (P) - solvent (S) system. See text for explanations. FIU. 4. Mutual superposition of phase diagrams: l,l’-LC equilibrium; 2-isotropic solution- crystal solvate equilibritim (see text for explanations).

This process was observed using an optical microscope with crossed Polaroids and the following results were obtained. A mobile anisotropic phase distributed in the form of luminescent particles on the background of a non-luminous isotropic solution was isolated initially. This was followed by the formation and extension of a dendritic anisotropic solid phase, which gradually increased as a result of the disappearance of the liquid anisotropic phase. All the three phases may simultaneously be observed in the visual field of the microscope in the intermediate st.age. Figure 2a shows the LC range of the mobile phase on the left hand side and dendrites of the solid phase, on the right hand side. The dark background corresponds to the isotropic phase. The following experiment proves that one of the luminescent phases is mobile. If the glass of a plane cell containing the system is displaced, the liquid phase undergoes deformation, while the solid phase remains unchanged (Fig. 2b). The mobile anisotropic phase completely disappears as time goes on and the solid phase expands (Fig. 2~). This phase breaks down to fine solid fragments under mechanical action (compression of the glass).

Structure formation in poly-p-benzamide-sulphuric

acid system

745

If the system is reheated at the intermediate stage of coexistence of three phases, the liquid anisotropic phase first changes into an isotropic phase and then the solid phase melts with transition into the isotropic solution. Experiments therefore suggest that PB solutions in H&JO,, on transition to the region situated below the curve shown in Fig. 1, may within a certain period of time be in the non-equilibrium state. Equilibrium in this range of concentration and temperature, reached during prolonged standing of the system (2-15 days), corresponds to the coexistence of only two phases: the isotropic and the solid (dendritic) phase. When the intermediate mobile phase may be identified as LC, which is also typical of DMAA of PB solutions, the problem concerning the type of solid anisotropic phase requires explanation. This may be the LC phase with high concentration of the polymer or PB en H&30, crystal solvate. The formation of a new LC phase highly concentrated in relation to the polymer, which is different from mobile LC, that has been studied in fair detail for the PB-DMAA system [4, 51, may be hypothetically explained proceeding from the type of phase equilibria in the rigid chain polymer-solvent system, as proposed in the Flory theory [7]. Two equilibrium sections of LC phases with the isotropic solution may be found in these systems, as shown by t,he diagram in Fig. 3. Concentrations of the isotropic and anisotropic phases differ little (theoretically, the ratio of these concentrations does not exceed 1.6) in a narrow range of coexistence of two phases. On changing the parameter of interaction of the polymer and solvent, or accordingly on changing temperature, the system may change into the region of coexistence of the isotropic and highly concentrated LC phases. According to Flory’s calculations, the concentration of the isotropic phase in this wide region of the diagram is very low, whereas the concentration of the LC phase approximates to the composition of the pure polymer (higher than 0.9 vol. fractions of the polymer). On transition from an isotropic initial solution with a concentration x,, and a temperature T, to final temperature T,, from T,, the system passes through the region of decomposition to isotropic and low concentration LC phases. Nuclei of the LC mobile phase are fairly quickly formed and then extended. The formation of nuclei of this phase is made easier by the fact that owing to little differences between the concentration of this phase and the concentration of the initial solution, it is comparatively likely that nuclei are formed by fluctuation. At the same time, formation of sections of highly concentrated LC phase in the temperature range T,-T, takes place at a much lower rate since the probability of fluctuations corresponding to the composition of the concentrated LC phase (xx*) is considerably lower. This explains the simultaneous formation of three phases at the intermediate stage: isotropic, non-equilibrium mobile and solid LC phases. An alternative is the assumption that the PB-H,SO, system is characterized by another phase equilibrium, namely; equilibrium of the isotropic solutionPB*n H&30, crystal solvate (with a multiple component ratio), as well as the phase equilibrium described by a narrow ‘corridor’ of coexistence of isotropic

8. P. PApKov et at.

746

and mobile LC phases. The mutual superposition of these two independent phase equilibria is diagrammatically shown by Fig. 4, where curves 1, 1’ correspond to boundaries of transitions of the isotropic solution x’-low concentration LC phase x”, while curve 2 is the liquidus of the crystal solvate. The formation of nuclei of the crystal solvate phase is similar to the case of conventional crystal. lization, which may take place with a long induction

period. Therefore,

tion with initial concentration

x,,, cooled to temperature

T’p,, without the simultaneous

formation

the solu-

T, passes through point

of nuclei of phase x** (crystal solvate).

A mobile LC phase of x” composition is only formed in this case. On keeping the system for a prolonged period of time nuclei of phase x** are formed and the polymer begins to change into the equilibrium isotropic phase x* and a crystal s)! vate x** composition.

FIG. 0. Electron photomicrographs ca-

produced by sedimenting the PBA solution with water:

isotropic solution as orginal; b -

FIG. 7. Electron at 300%

a -

diffraction

picture

solid phase as original; c of films

precipitated

mobile LA phase as original.

with

isotropic solution and solid phase as original; b -

water

and

heat

treated

mobile LA phase as original.

With a concentration of the initial solution lower than the boundaries of formation of a LC mobile phase, a solid phase is only formed in the equilibrium state in both systems, this phase being a highly concentrated LC phase (according to the first system), or a crystal solvate (according to the second system.),

Structure formation in poly-~-benzamide-suphuric Indeed,

the 8%

solid formations

solution

cooled

to temperatures

acid system

of the order

12-15”

747 isolates

of spherulitic type (Fig. 5).

A direct electron microscope study of the system in sulphuric acid was impossible. An attempt was therefore made to examine the structure of phases during the separation

of the polymer

by the action of a precipitant.

formation while isolating PB by electron microscopy, ing system. Films suitable for electron microscope

Structure

took place by the followstudy were prepared by

applying a drop of solution between two glasses, followed by shifting the glasses until a very thin layer had formed and immersing the glass into water. Results obtained for films formed from S-9% PB solutions indicate that a dense homogeneous structure is formed in this case without any symptoms of heterogeneity on the level of the resolving power level of the electron microscope. Similar results are obtained if a film is formed of 11-13°~ solutions at a temperature of 65-70”, i.e. above the transition temperature of the system into the isotropic state. A different pattern is observed for anisotropic solutions with nonsteady-state equilibrium. Three types of structure arc found in the films: a dense homogeneous structure formed from isotropic sections of the film (Fig. &), large ovoid structural elements with strict geometry and similar dimensions, which may correspond to disintegration of solid dendritic form&ions (Fig. fib) acd lamellar network structure which, according t,o origin, may be c~l;tsxifirtl 2~s Dhe initial mobile LC part of the system (Fig. 6~). Analysis of electron diffraction curves plotted for each of the sections tlescribc>d gave the following respective result s. The isotropic phase shows the existence of an amorphous halo and a reflection, which corresponds to an interplanar distance of 2.05 A. After heating to 300” the electron diffraction curve of this section has meridional reflections of 2.05; 3.11 and 6.27 A and no equatorial reflexes (Fig. 7~). This means that isotropic sections change into the LC state during heat’ing, which is generally typical of rigid chain amorphous polymers; being fixed in the amorphous state as a result of rapid precipitation from isotropic solutions, when imparting mobility to macromolecules as a result of heating to high temperatures, these polymers tend to change into the ordered LC state. These reflexes were established for liquid crystals previously obtained from PB solutions in DMAA [4]. For sections with lamellar network structure the electron diffraction curves obtained had an intense equatorial reflex corresponding to an interplanar distance of 3.69 A. This reflex points to the beginning of crystallization of PB. Heat treatment intensifies polymer crystallization and reflexes identified as crystalline PB reflexes appear on the electron diffraction curve (Fig. 7h). Relatively rapid crystallization of LC of the mobile phase has been noted previously when investigating the PB-DMAA system [5]. Mobility and ordering of the LC phase, apparently, contribute to a more rapid formation of nuclei of a truly crystalline structure than in the case of a precipitated isotropic phase, where ordering during heating only reaches the stage of liquid crystal.

S. P. PAPKOV et al.

748

It seems that if elements of the solid phase on the electron diffraction curves are strictly separated from these sections, new reflexes are formed, which correspond to the crystal solvate lattice, or the new LC phase. However, an amorphous halo and a reflex corresponding to an interplanar distance of 2.05 A (ordering along the chain of main valencies) are only observed in this case. The same pattern as that observed for isotropic sections is repeated during heating, i.e. transition to the LC state without crystallization of PB (Fig. 7~). This specific behaviour of solid phase sections, namely the existence of a strict geometrical shape of solid phase elements in the absence of reflexes indicating three dimensional ordering, may be hypothetically interpreted as decomposition (amorphi,zation) of the PB *n H,SO, crystal solvate with the retention of the morphology of these elements when the solvent is removed. Low mobility of the solid phase does not promote this transition which is carried, out for the mobile LC phase, while heating only ensures the transformation of the amorphous structure into a conventional LC order. Investigations seeking to define more accurately the problem of the type of solid equilibrium phase continue. Tralzelated by E. SEMERE

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in