European Polymer Journal 39 (2003) 1311–1317

www.elsevier.com/locate/europolj

Melting behavior of isotactic polystyrene revealed by di erential scanningff calorimetry and

transmission electron microscopyTianxi Liu *

Molecular & Bio-Materials Cluster, Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore

Accepted 7 January 2003

Abstract

The triple melting behavior and lamellar morphologies of isotactic polystyrene isothermally crystallized from the glassy state have been investigated by di erential scanningff calorimetry (DSC), temperature-modulated DSC and transmission electron microscopy (TEM). The combination of thermal analysis measurements and morphological observations indicates that: (1) The lowest endothermic peak, the so-called ‘‘annealing peak’’ (Ta ), is not associated with the melting of the subsidiary crystals formed by secondary crystallization as often suggested in the literature, but probably with a constrained interphase between the amorphous and crystalline regions; (2) Within spherulites two lamellar populations with di erentff degrees of perfection (or thermal stability) are confirmed by direct TEM observa- tions following partial melting experiments, which are responsible for the so-called double melting peaks (Tm;1 and Tm;2 ) at higher temperatures observed in DSC curves; (3) The highest endothermic peak (Tm;2

) is partially originated from the melting of the recrystallized lamellae formed during heating process in DSC.© 2003Elsevier Science Ltd. All rights reserved.

Keywords: Isotactic polystyrene; Melting; Lamellar morphology; Di erentialff scanning calorimetry; Transmission electron microscopy

1. Introduction

Many semicrystalline polymers exhibit double or multiple melting behavior upon heating in di erentialff scanning calorimeter (DSC). However, there are con- siderable controversies in the literature concerning their origins [1]. In this regard, several mechanisms have been proposed to explain this complex phenomena [1,2], such as melting–recrystallization–remelting during the DSC heating process, presence of more than one crystal modifications (polymorphism), variation in morphology (e.g., lamellar thickness, distribution, perfection or sta- bility), physical aging and/or relaxation of the rigid amorphous fraction, di erentff molecular weight species, and so on. Obviously, there exist more or less diver-

* Tel.: +65-68748594; fax: +65-67744657.E-mail address: liu-tx@imre.a-star.edu.sg (T. Liu).

gences among these proposed melting models, and up to now, no consensus has been reached. A unique model that is reasonably compatible with all physical obser- vations has not yet been put forward. Therefore, a fur- ther understanding of the origins of double or multiple melting behavior is of great significance and should provide a new insight into the crystallization and melt- ing processes of crystalline polymers [1].

Additionally, the melting processes of polymers areusually studied on bulk materials by techniques such as DSC, X-ray scattering, and density measurements, which do not yield direct information on the lamellar level during the melting process. Although transmission electron microscopy (TEM) technique is a powerful tool frequently used to probe the morphology of the crys- talline polymers, the melting studies on the lamellar scale using TEM are surprisingly scarce. It is expected that a more complete description on the multiple melting behavior may be derived from the direct morphological

0014-3057/03/$ - see front matter © 2003Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00017-X

1312 T. Liu / European Polymer Journal 39 (2003) 1311–1317

observations using the microscopic techniques, such as TEM. In this report, the combination of DSC and TEM is applied to the investigation of melting behavior of isotactic polystyrene (iPS) with very low isotacticity. This polymer is selected as the studied system due mainly to its slow crystallization rate, absence of poly- morphism, and higher stability to electron irradiation than most other polymers [3].

2. Experimental

2.1. Material

The granular iPS sample (Mw ¼ 851 000, Mw=Mn ¼ 5:93, isotacticity: 60%) was purchased from Polysciences Inc. The granules were heated to 250 °C for 5 min, and subsequently quenched into ice water to get amorphous samples (which was confirmed by a di useff halo from wide-angle X-ray scattering experiments). The obtained amorphous samples were then isothermally cold-crys- tallized at di erentff temperatures for 6 h for the use of DSC measurements.

2.2. DSC measurements

Conventional DSC experiments were performed in a DSC-2920 from TA Instruments coupled with a TA- 2000 control system. The temperature was accurately calibrated to be ±0.5 °C in error, with gallium, indium and tin using the standard procedure. Most samples were heated with a scanning rate of 10 °C/min. To study the e ectff of heating rate on multiple melting behavior of iPS, the samples (isothermally cold-crystallized at 170 °C for 6 h) were scanned from 2.5 to 40 °C/min. The weights of all the samples were in the range of 5 ± 0.1 mg. All crystallization and subsequent melting treat- ments were performed under nitrogen atmosphere in order to diminish oxidation. Temperature-modulated DSC experiments were conducted under modulated mode. A heating rate of 5 °C/min was used with mod- ulation period of 60 s and modulation amplitude of ±1°C. Fourier transformation of the modulated heat flowwas performed during data collection, yielding separa- tion of reversing and non-reversing phenomenon.

2.3. TEM observations

Thin films for direct TEM observations were obtained by the following procedures. First, the dilute polymer- xylene solution (with concentration of 0.1% w/v) was dropped onto mica covered with a carbon film. Then, the polymer films on the carbon-support film were floated onto water surface and transferred onto electron microscope copper grids. After that, the obtained thin

films on the grids were melted at 250 °C for 5 min in DSC and subsequently took out by rapidly quenching into ice water to get amorphous samples. Finally, the amorphous thin films were isothermally crystallized at170 °C for 6 h (under which the resulting multiplemelting peaks can be well separated on the DSC curve). The obtained crystalline thin film samples were further partially melted or annealed at 212 °C for di erentff times (1 and 30 min). For surface replica experiments, the partially melted bulk sample was etched in 1% potas- sium permanganate in a mixture of sulfuric acid, phos- phoric acid and water (5:2:2 v/v/v) for 2 h at room temperature. After subsequent washing, following the prescribed procedures by Olley and Bassett [4], the etched surface was examined under TEM using a stan- dard one-stage replication process, i.e., making a first replicate in cellulose acetate, subsequently shadowing it with platinum, finally coating it with a carbon film. Morphological observations were carried out using a Philips CM200 TEM operated at 200 kV. The contrast in the bright-field (BF) electron micrographs of the thin films was obtained by defocusing the objective lens [5].

3. Results and discussion

3.1. Multiple melting behavior by DSC

The e ectff of crystallization temperature (Tc) on the melting behavior of iPS (cold-crystallized at di erentff temperatures for 6 h) is illustrated in Fig. 1(A). It can be observed that: (i) Besides the glass transition tempera- ture (Tg ) around 100 °C, there exist three additional endothermic peaks which are labelled as Ta (the so-called ‘‘annealing peak’’), Tm;1 and Tm;2 in the order of tem- peratures from low to high. (ii) When Tc ¼ 140 °C, only the melting of the crystals associated with the higher endotherm, Tm;2 , can be detected upon heating in the DSC scan, and no appreciable melting events corre- sponding to Ta and Tm;1 can be observed. With the in- crease of Tc , the Tm;2 almost remains constant around 220 °C. (iii) For the case of Tc ¼ 150 °C, besides the Tm;2 , the lower endotherm (Tm;1 ) was observed. As increasing the Tc, the Tm;1 shifts to higher temperatures, and finally becomes the dominant melting event indicating that the perfection of pre-existing crystals related to the Tm;1 is significantly enhanced. (iv) When Tc P 160 °C, the Tm;1 becomes apparent and the Tm;2 is well developed, and moreover, the annealing peak is now beginning to form and shifts to higher temperatures with increasing Tc . The position of the Ta always appears at ca. 10–15 °C above the Tc . (v) As increasing the Tc , the perfection of the lamellae is gradually unified, finally only one melting peak can be observed (for instance, when Tc P 200 °C). Accordingly, it is evident that the triple melting peaks