결정성 고분자의 결정화도와 모폴로지에 따른 동적 기계적 특성 연구

박찬석, 이기준, 남재도*, 김성우**

서울대학교 응용화학부

성균관대학교 고분자공학과*

경기대학교 화학공학과**

Effects of Crystallinity and Morphology on the Dynamic Mechanical Characteristics of a Semicrystalline Polymer

Chan-Seok Park, Ki-Jun Lee, Jae-Do Nam*, Seong Woo Kim**

Department of Chemical Engineering, Seoul National University

Department of Polymer Science and Engineering, Sung Kyun Kwan University*

Department of Chemical Engineering, Kyonggi University**

INTRODUCTION

Crystallinity of semicrystalline polymers affects the viscoelastic characteristics specifically above its glass transition temperature, Tg, seemingly by at least two mechanisms[1]. First, the crystallites act as cross-links by tying segments of many molecules together. Second, the crystallites have very high moduli compared to the rubbery amorphous parts, and they behave like rigid fillers in the semicrystalline polymer matrix. Hard particles may stiffen a soft matrix. Since a glass-state polymer has a modulus nearly as great as that of an organic crystal[2], crystallinity has only a slight effect on the modulus below Tg. The stiffening effect is most pronounced in the rubbery region of response. The modulus of semicrystalline polymer in the rubbery state increases very rapidly with the degree of crystallinity. The cross-linking and filler effects of the crystallites are observed up to the melting point of polymer. The melting point often increases slightly as the degree of crystallinity increases. Above the melting point, the typical rubbery behavior of an amorphous polymer is found. Between the Tg and the melting point, the modulus-temperature curves often show an appreciable negative slope. This gradual change in modulus is due partly to some melting of small or imperfect crystallites below the melting point, which reduces both the rigid filler effect and the cross-linking effect, and due partly to a loosening of the structure as a result of thermal expansion.

In this study the crystalline morphology of poly(butylene terephthalate)(PBT) was investigated using a polarized optical microscope(POM), and the degree of crystallinity was measured by the wide angle X-ray diffraction(WAXD) method. The effects of morphology and crystallinity on the dynamic mechanical behavior of PBT were also investigated by the experiments of DMTA and analyzed by the dual-phase polymer composite model.

EXPERIMENTS

A dried pellet of PBT was initially melted by a hot-press between a cover glass and a Teflon film at 270oC for 10min and then compressed at 2000psi for 2min. The resulting sample was then quickly quenched into a silicone oil bath, which was adjusted to a specific temperature ranging from 20oC to 200oC. And ice-water bath for preparation of an amorphous sample was also used. The thickness of films was typically below 100□m. Solution casted films for morphological observations were also prepared from a 10% solution of tetrafluoro acetic acid(TFA) in CCl4.

The crystalline structures of the samples were examined by using a polarized optical microscope. The morphological changes during melting and crystallization were observed by using a Mettler hot-stage equipped on the microscope..

For the crystallinity determination of the films crystallized at the isothermal temperatures the wide angle X-ray diffraction(WAXD) method was utilized. The WAXD experiments were performed with a general area diffraction detector(GADDS) (Bruker) equipped with a collimator having the diameter of 0.3mm, which enables focus on very small area of the specimen. CuK□ (□=1.541□) radiation was utilized and scans were made at 5o to 45o(2□). The degree of crystallinity of the PBT samples were obtained by using the Ruland' analysis[3]:

[eqn: w sub c = {int from s sub 0 to s sub 1 { s sup 2 I sub c (s) ds }} over {int from s sub 0 to s sub 1 { s sup 2 I (s) ds }} K]                             (1)

Dynamic mechanical thermal analysis(DMTA) was performed by using a Rheometric Scientific Mark IV in tension mode. The storage modulus, G', and loss modulus, G'', were recorded automatically by the system for data acquisition and analysis. A heating rate of 5oC/min and fixed frequencies of 1 and 5Hz were used.

Perkin-Elmer 7 differential scanning calorimeter (DSC) was used for investigating the melting behavior of samples crystallized at each crystallization temperature. All the measurements were carried out in a nitrogen atmosphere. Specimens of 10 to 12mg were weighted into standard aluminum pans and sealed for the DSC experiments.

RESULTS AND DISCUSSION

Figure 1 shows the possible features of PBT spherulites crystallized at various crystallization temperatures, Tc. In this figure it should be noted that PBT in polarized light show dark maltes cross in the spherulitic structure: when Tc<40, the maltese crosses exhibit about 45o to the polarizers, known as an unusual type(Fig. 1(a)), while in the solvent crystallized films the maltes crosses were along the polar directions (0o-90o)(Fig.1(c)) known as a usual type. Finally, when 160<Tc<200, the cross-patterns seemed to be a mixture of usual and unusual types, referred as mixed type(Fig.1(b)). These results indicate that the bulk structure or macrostructure of PBT depends on the Tc and whether the crystallization occurs from solutions or from melts. These morphological structures may give different behaviors of melt characteristics and mechanical properties[4].

From the WAXD experiments and Ruland's analysis for the crystallinity, the degree of crystallinity as a function of Tc can be obtained in Fig. 2. The degree of crystallinity of PBT has a maximum value of about 35%.

Figure 3 shows the dynamic mechanical behavior of PBT crystallized at various Tc. It can be seen from the figure that the crystallinity has only a slight effect on the modulus below Tg, while the stiffening effect is most pronounced in the rubbery region of response. In rubber region, the modulus increases very rapidly with the degree of crystallinity. The cross-linking and filler effects of the crystallites are observed up to the melting temperature. In addition to the main crystalline melting at around 230oC, another distinct modulus drop is observed for the samples crystallized above Tc □80oC.

The effects of crystallinity on the modulus are illustrated in the Fig. 4. The lines in the figure represent the predictions using a dual-phase continuity model showing the moduli raised to the one-fifth power[5]:

[eqn: G sup 1/5 = w sub a G sub a sup 1/5 + w sub c G sub c sup 1/5]                       (2)

where Ga and Gc are the moduli of the amorphous and crystalline phases, respectively.

The damping behavior of PBT is shown in the Fig. 5. Crystalline polymers have a damping peak corresponding to the glass transition between 50 to 100oC. As can be seen, the damping intensity of tan□ is decreased as Tc is increased, indicating that the increased crystalline phase in the sample may interrupt the physical relaxation of amorphous phase.. As also shown in the figure, the sample crystallized at higher Tc low another peak, which is related to melting of the crystalline phase.

CONCLUSIONS

Depending on the crystallization conditions three different types of spherulites were observed, which could lead to multiple melting endotherms during the heating process. The dynamic moduli were increased with the degree of crystallinity because of filler effects of crystallites. The damping peaks of specimens exhibited the glass transition and crystalline melting characteristics, both affected by crystallization conditions.

REFERENCES

1. L. E. Nielsen and F. D. Stockton, J. Polymer Sci., A1, 1995 (1963).

2. J. J. Joseph, J. L. Kardos, and L. E. Nielsen, J. Appl. Polym. Sci., 12, 1151

(1968).

3. U. W. Gedde, Polymer Physics, Champman & Hall, London, 1995.

4. L. E. Nielsen and R. F. Landel, Mechanical Properties of Polymers and

Composites, 2nd Ed., Marcel Dekker, New York, 1994.

5. W. E. A. Davies, J. Phys. D, 4, 318 (1971).

(a)                    (b)                   (c)

Fig. 1. Maltese-cross patterns of PBT spherulites crystallized at different conditions;(a)

unusual, (b)mixed, and (c) usual types, respectively.

Fig. 2 Crystal size and the degree of       Fig. 3 Storage modulus as a function of

crystallinity of PBT spherulites             temperature.

as a function of Tc.

Fig. 4 Storage moduli at each region as    Fig. 5 Damping behaviors of the PBT.

a function of crystallinity.