INTRODUCTION:

It is well known that that certain additives, commonly known as nucleating agents when added to a semicrystalline polymer, significantly affect the crystallization behavior and related morphology of the polymer. Several attempts have been made to control the rate of crystallization and the morphology of the polymers by the addition of finely divided substances which promote abundant nucleation^1-5 The majority of the nucleating agents used are salts of organic and inorganic acids with very high melting points. These nucleating agents do not behave as heterogeneous substrates in lowering the energy barrier towards crystal nucleation but melt along with the polymer while being processed and react as true chemical reagents with the molten polymer to form the nucleating species. Mitra and Mishra⁶ have evaluated the effect of such a nucleating agent, dibenzylidene sorbitol (DBS), on cystallization and morphology of polyethylene terephthalate (PET). It has been found that DBS when incorporated into the polyester at a 0.5−1.0 wt% level significantly lowers the induction period and reduces half-life of crystallization. Mercier and coworkers⁷ established the new mechanism of nucleation on the basis of the effect of sodium−2−chlorobenzoate on PET.They also observed similar behavior between a range of reactive alkali metal salts and PET.

In each case, the infra-red spectra indicated the formation of ionic end groups from which the nucleated polymer remelted. Mitra and Mishra⁸ have also established the fact that the same nucleating agent when used beyond a certain level does not nucleate the crystallization process. This is probably due to the formation of agglomerates of the nucleating agent beyond this level of incorporation.

In the present study, a very small amount (0.2−2.0 wt%) of sodium−2−chlorobenzoate was used as a nucleating agent in the crystallization process of a liquid crystalline polyester synthesized by a low temperature polycondensation process⁹. The texture of the nucleated polymer was described by their photomicrographs and was found to exhibit texture unlike the original polymer.

MATERIALS AND METHODS:

Sodium−2−chlorobenzoate was used as a nucleating agent for the liquid crystal polyester. The nucleating agent was mixed with the polymer in different proportions varying from 0.2− 2.0%. The mixture was then heated above the melting temperature of the polymer (i.e. 210⁰C). In order to ensure the completion of the reaction the polymer and the nucleating agent were kept in the molten state for 2, 5, 10 and 15 minutes. The nucleated sample was then cooled naturally to room temperature. The texture of the nucleated sample was visualized by optical microscopy. A polarizing microscope (LeitzLaborlux 12 Pol S) fitted with a Wild Leitz MPS 46 /52 Photoautomat was used for this purpose.

RESULTS AND DISCUSSION:

Study of morphology of nucleated polyester

Morphology of the liquid crystal polymers can be controlled by the use of certain additives, often known as nucleating agents. Though there is a wide variety of nucleating agents available, the selection of right type of these agents, their concentration and the corresponding morphological changes of the polymers should be investigated. The nucleating agents provide additional nucleation sites which are responsible for an improved crystallization behavior. Sometimes these nucleating agents do not behave as heterogeneous substrates but react with the polymer as true chemical reagents to form the nucleated species via the formation of ionic end groups. In the present study varying proportions of the nucleating agent, sodium−2−chlorobenzoateand the polyester were melted together above the melting temperature of the polyester, kept there for varying time period and then cooled to room temperature. On cooling recrystallization of the polyester was expected to occur. The texture of the nucleated polyester was described by the photomicrographs. The molecular structure of the polyester is as follows

(a)

(b)

(c)

(d)

(e)

Figure1.

(a) crystals of Na-chlorobenzoate; (b) discontinuous threads for a smectic C phase in the polyester ; (c) birefringent clusters of nucleating agent in polymer melt after 2 minutes of incorporation; (d) appearance of rodlike crystals of nucleated polymer after 5 minutes; (e) rodlike crystal growth completed in 10 minutes

The micrographs shown in Figure 1 revealed that the nucleation was complete in 10 minutes and rodlike texture for a smectic A mesophase appeared in the recrystallization process. If the nucleation reaction is continued only for 2 minutes, birefringent agglomerated species covering the whole area was observed without the appearance of any specific crystalline texture. After 5 minutes of reaction, the rod like texture started appearing along with the discontinuous threads of a smectic C phase of the original polyester. The rods were found to cover the whole mass after 10 minutes of reaction ensuring complete nucleation, which has started almost immediately after the dissolution of the nucleating agent in the molten polyester.

The chemistry involved in this process occurring between the polyester and the nucleating agent may be described as follows,

Above 200⁰C, sodium−2−chlorobenzoate dissolved in the molten polyester sample and within few minutes sodium salt of the polyester was formed through hydrolysis. This salt was insoluble in the reaction medium and responsible for the appearance of birefringent clusters in the thin film of recrystallized polyester. Subsequently, regeneration of sodium−2−chlorobenzoate and the nucleated polyester takes place within 10 minutes of dissolution of the nucleating agent in the polymer melt. The entire reaction path, involving the formation of ionic end groups and nucleated polyester is shown below,

Formation of ionic end group:

Regeneration of the nucleating agent:

Recrystallization of the nucleated polyester was found to result in the formation of improved crystalline texture as shown by the photomicrographs in Figure 1.

After certain reaction period the recrystallized products were analyzed by infra-red spectroscopy in the 1500 −1600 cm⁻¹ region. The sodium salt of the polyester was identified with the characteristic absorption at 1557 cm⁻¹ and 1602 cm⁻¹ in the product recrystallized after 2 minutes of dissolution. At longer reaction time, i.e. about 10 minutes, strong absorption at 1585cm⁻¹was observed in the spectrum which showed the dismutation of the sodium carboxylate end group with concomitant regain of the sodium salt of chlorobenzoic acid. The absorption at 1585cm⁻¹ associated with the sodium salt of chlorobenzoic acid was found in the spectrum of the nucleating agent, which disappeared in the spectrum taken after 2 minutes of dissolution and recrystallization. Thus, it confirmed the reaction between the polymer and the nucleating agent and the formation of the ionic end groups of the polyester. Reappearance of the peaks at 1585cm⁻¹ revealed the regeneration of the nucleating agent after 10 minutes of nucleation reaction and recrystallization. Corresponding IR spectra are shown in Figure 2.

Figure 2. FTIR scan for

(a) Na- 2- chlorobenzoate (NA)

(b) Polyester with NA after 2 minutes of dissolution

(c) Polyester with NA after 10 minutes of dissolution

Hence, it is established that, the mechanism of nucleation of polyesters by alkali metal salts of chlorobenzoic acid involved a true chemical reaction which takes place through the formation of ionic end groups an degeneration of nucleated polymer.

Study on agglomeration of nucleating agents

The incorporation of nucleating agent improve the crystallization of the polyester, however, beyond certain level of incorporation no further improvement in crystallization behavior was noticed. The present study also shows that the nucleating agent tends to agglomerate and reduces the number of nucleating sites beyond certain level of incorporation. Photomicrographs of the nucleated polyester with varying proportion of nucleating agent ranging from 0.2−2.0% are presented in Figure. 3.

Figure 3. Textures formed in the recrystallization process of nucleated polyester with (a) 0.2% NA (b) 0.5% NA (c) 1.0% NA (d) 2.0% NA

With only 0.2% nucleating agent, though development rod like crystals has been noticed, transformation of original threads into rods was not complete. For a further change in crystal structure the dosage was increased to 0.5%. This time the rods were prominent in the photograph with a few spots of clustered nucleated polymer. With still higher dosage of nucleating agent, i.e.1.0% incorporation, completion of nucleation reaction was established by the distinct rods covering the whole mass. The rods were not so distinct and in agglomerated form with 2.0% incorporation of nucleating agent, though the rods were being identified clearly. But beyond 2.0% incorporation, the molecules of the nucleating agent agglomerated in the polymer matrix forming large particles which scattered visible light, thereby reducing transparency. Incompatibility of the agglomerated nucleating agent and the polyester produced in homogeneity in the system due to which the effect of nucleating agent in improving the crystallization behavior of the polyester was achieved. The fact was supported by Mitra and Mishra⁸.

CONCLUSION:

Sodium−2−chlorobenzoate has been found to act as a chemical reagent in the nucleation of the crystallization process of polyester. It has been established that sodium−2−chlorobenzoate, when incorporated in the polyester at a 0.2−2.0% weight percent level, significantly improved the crystalline structure of the polymer. Beyond 2.0% incorporation of the nucleating agent, effect of nucleating agent on improving the crystallization behavior of the polyester was not achieved.

ACKNOWLEDGMENT:

The author takes the opportunity to express her gratitude to Professor P. K. Sengupta, Department of Plastics and Rubber Technology, University of Calcutta, India, for his valuable guidance, encouragement and inspiration through the course of this work.

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Received on 10.01.2012 Modified on 28.01.2012

Asian J. Research Chem. 5(2): February 2012; Page 295-298