Miscibility of poly(tetrahydropyranyl-2-methacrylate) and poly(cyclohexyl methacrylate) with styrenic polymers

Eur. Polym. J. Vok 27, No. 6, pp. 501-504, 1991

0014-3057/91 $3.00+0.00 Copyright © 1991 PergamonPress plc

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MISCIBILITY OF POLY(TETRAHYDROPYRANYL-2METHACRYLATE) AND POLY(CYCLOHEXYL METHACRYLATE) WITH STYRENIC POLYMERS Y. F. CHONG and S. H. GoH* Department of Chemistry, National University of Singapore, Singapore 0511 (Received 27 July 1990)

Abstract--The glass transition behaviour for blends of poly(tetrahydropyranyl-2-methacrylate) (PTHPMA) with poly(~-methylstyrene) (Pa MS), polystyrene (PS) and poly(p-methylstyrene) (Pp MS) has been studied by differential scanning calorimetry. The results indicate that PTHPMA/PaMS blends are fully miscible, whereas PTHPMA/PpMS blends are immiscible. The miscibility of PTHPMA/PS blends depends on their compositions. Lower critical solution temperature (LCST) behaviour was observed for all the miscible blends containing PTHPMA. Poly(cyclohexyl methyacrylate) was found to be miscible with PS, PaMS and PpMS, and none of the blends showed LCST bchaviour.

INTRODUCTION There have been m a n y studies on the miscibility behaviour of homologous polymethacrylates with other polymers such as poly(vinyl chloride) (PVC) [1-3], polyepichlorohydrin (PECH) [4], poly(vinylidene fluoride) (PVDF) [5], styrene/acrylonitrile copolymer (SAN) [6, 7], ~-methylstyrene/acrylonitrile copolymer (~MSAN) [8] and poly(chloromethyl methacrylate) ( P C M M A ) [9]. In general, the miscibility decreases with increasing bulkiness of the pendant groups of the polymethacrylate. However, poly(cyclohexyl methacrylate) (PCHMA) is miscible with several other polymers in spite of the bulkiness of the cyclohexyl group [4, 10, 11]. In a recent series of studies, we reported that poly(tetrahydrofurfuryl methacrylate) ( P T H F M A ) , a polymethacrylate with bulky pendant groups, is miscible with a variety of polymers [9, 12-14]. The good miscibility of P T H F M A has been attributed to the presence of additional interacting moieties, viz. ether oxygen atoms, in the pendant groups. In the present series of studies, we investigate the miscibility behaviour of poly(tetrahydropyranyl-2methacrylate) (PTHPMA). It is of interest to compare the miscibility behaviour of P T H P M A with those of P C H M A and P T H F M A . In this communiCH3

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cation, we report the miscibility of P T H P M A and P C H M A with three styrenic polymers, viz. polystyrene (PS), poly(ct-methylstyrene) (P~tMS) and poly(p-methylstyrene) (PpMS). It has been claimed in several patents that blends of P C H M A with PS and P~MS are "homogeneous and glass-clear" [15, 161. EXPERIMENTAL

PROCEDURES

Materials

Tetrahydropyranyl-2-methacrylate was obtained from Polysciences, Inc. and purified by fractional distillation at 83°/2 mmHg. PTHPMA was prepared by solution polymerization in 2-butanone at reflux temperature for 6 hr, using 0.25% by weight of azobisisobutyronitrile as initiator. The polymer was obtained by precipitation in excess methanol. It was then dried in vacuo at 90° for 48 hr. The numberaverage (M,) and weight-average (Mw) molecular weights (PS equivalent) of PTHPMA as determined by GPC were 44,000 and 65,000 respectively. The following commercial polymers were used: PS (BDH Chemicals, -~w = 150,000), PctMS (Scientific Polymer Products, /f/w= 50,000) PpMS (Polysciences, -~/w= 240,000) and PCHMA (Scientific Polymer Products, ~3w= 66,000). Polymer blends

All the blends were solution-cast from tetrahydrofuran. Solvent was allowed to evaporate at room temperature. The resulting films were further dried in vacuo at 110° for 3 days. Glass transition temperature (Tg ) measurements

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Measurements of T~ were carried out on a Perkin-Elmer DSC-4 differential scanning calorimeter, using a heating rate of 20°/min. The initial onset of the change of slope in the DSC plot was taken as T8. The reported Tg value is the average based on the second and subsequent runs. Cloud point measurements

Scheme 1 *To whom all correspondence should be addressed.

All the miscible blends were examined for the existence of LCST behaviour. The film was sandwiched between two microscopic cover glasses and heated in a Fisher-Johns melting point apparatus with a heating rate of about 10°/min. The optical appearance of the film was observed with a magnifying glass attached to the apparatus. A 501

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Fig. 1. DSC curves for PTHPMA/PS blends. transparent film which turns cloudy upon heating indicates the existence of LCST. The temperature at which the film first showed cloudiness was taken as the cloud point. The reported cloud point is the average value from several measurements. RESULTS

AND

DISCUSSION

PTHPMA /PS blends Figure 1 shows the DSC curves for various PTHPMA/PS blends. Blends containing 20, 25 and 30 wt% of PS showed two distinct glass transitions, indicating phase-separation. However, each of the other blends showed only a single glass transition, indicating single-phase nature. The Tfcomposition curve for PTHPMA/PS blends is shown in Fig. 2. Therefore, the miscibility of PTHPMA/PS blends depends on their compositions. Such behaviour has been observed for other blends also [17-20]. The blend containing 40% of PS shows a very broad glass transition. It has been observed that there

is an abrupt increase in the glass transition width just prior to the miscibility limit [21-23]. The present result can be taken to indicate that the blend containing 40% of PS is near the verge of immiscibility. All the miscible blends showed LCST behaviour and the cloud point curve is shown in Fig. 3.

PTHPMA /P~MS blends All the PTHPMA/Pct MS blends were transparent and each showed only one composition-dependent glass transition. The Tfcomposition curve is shown in Fig. 4. All the blends showed LCST behaviour and the cloud point curve is shown in Fig. 3. Thus PTHPMA is miscible with PctMS.

PTHPMA /PpMS blends All the blends were cloudy. DSC measurements revealed the existence of two glass transitions in each blend. The Tfcomposition curve is shown in Fig. 5. The lower Tg values are generally close to that of PTHPMA except for the blend containing 25% of PpMS, which has a Tg value several degrees lower than that of pure PTHPMA. The upper Tg values are 120

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Fig. 4. Tg-compositioncurve for PTHPMA/P~tMS blends.

Miscibility of PTHPMA and PCHMA with styrenic polymers

clarity and the enthalpy relaxation behaviour of PCHMA/PS blends, we conclude that PCHMA is miscible with PS in all proportions.

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All the blends of PCHMA with P~ MS were clear and each blend showed a single composition-dependent Ts as shown in Fig. 7, indicating single-phase nature, The positive deviation in the Ts-composition curve may indicate a strong interaction between the component polymers [26, 27]. All the blends remained transparent when heated to 300 ° .

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Fig. 5. T~-composition curve for PTHPMA[PpMS blends. substantially lower than that of PpMS, indicating the presence of PTHPMA in the PpMS-rich phase.

PCHMA/PS blends All the blends were transparent and showed no LCST behaviour even when heated to 300 °, the limit of the melting point apparatus. Conventional DSC measurements cannot be used to ascertain the miscibility of the blends as the Ts value of PCHMA (95 °) is quite close to that of PS (102°). Instead, the blends were subjected to an annealing process and their enthalpy relaxation bebaviour was examined as suggested by some recent studies [24, 25]. All the blends were first annealed at 150° for 5 min to eliminate the solvent effect and thermal history. They were then annealed at 90 ° for one week; for comparison, PS and PCHMA were similarly annealed. Figure 6 shows the DSC curves of annealed PS, PCHMA and PCHMA/PS blends. A single enthalpy relaxation peak is observed in each of the annealed blend samples. The enthalpy relaxation peak also shifts to a higher temperature with increasing content of PCHMA in the blends. Therefore, from the optical

The proximity of the Tss of PCHMA and PpMS (107 °) precludes the use of conventional DSC measurements to ascertain the miscibility of the blends. Moreover, the enthalpy relaxation peaks of annealed PCHMA and PpMS occur at about the same temperature, making it difficult to ascertain the miscibility of the annealed blends. In fact, an annealed two-phase PCHMA/PpMS (1:1) mixture shows only one enthalpy relaxation peak as shown in Fig. 8. However, all the blends were transparent and did not show LCST behaviour even when heated to 300°C. The optical clarity of the blends can be taken to indicate miscibility. Although the refractive index of PpMS is not available, its value is expected to be close to that of poly(o-methylstyrene) (1.5874) [28]. This value is significantly larger than that of PCHMA (1.5075) [28]. The transparency, therefore, does not arise from the matching of refractive indices of PCHMA and PpMS.

Miscibility behaviour The present study shows that the miscibility of PTHPMA with the three styrenic polymers follows the order of P~MS > P S >PpMS. The order for PS and Pp MS is consistent with reports that the miscibility of PS is generally better than that of PpMS [29-321. The miscibility of PTHPMA with PS and P~tMS may arise from interaction between ether oxygen atoms and benzene rings by analogy with miscible PS/poly (vinyl methyl ether) blends [33]. It is 120

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Fig. 6. DSC curves for annealed PCHMA/PS blends. (a) 100% PCHMA, (b) 75% PCHMA, (c) 50% PCHMA, (d) 25% PCHMA and (e) 100% PS.

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Fig. 8. DSC curves for annealed polymer samples. (a) 50/50 mixture of PCHMA/PpMS, (b) PpMS and (c) PCHMA. also interesting that, while P T H F M A is immiscible with any of the three styrenic polymers [12], P T H P M A is miscible with P~tMS at all compositions and with PS at certain composition. Apparently, in addition to the presence of ether oxygen atoms, other factors such as ring size may also affect the miscibility behaviour. As mentioned earlier, P C H M A is miscible with several polymers including PVC [10], P E C H [4] and vinylidene chloride/vinyl chloride copolymer [11], but there is no suitable explanation to account for its good miscibility. It is rather puzzling that PS is miscible with P C H M A but immiscible with polymethacrylates such as poly(methyl methacrylate), poly(ethyl methacrylate), poly(n-propyl methacrylate), poly(isopropyl methacrylate) and poly(n-butyl methylate) [6]. Further work on P C H M A , P T H P M A and P T H F M A is in progress in order to understand better their miscibility behaviour.

Acknowledgement--Financial support of the National University of Singapore is gratefully acknowledged. REFERENCES

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