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Introduction to Polymer Crystallization

N.M. Nurazzi1,2, M.N.F. Norrrahim3, S.S. Shazleen4, M.M. Harussani5, F.A. Sabaruddin6 and M.R.M. Asyraf7,8

1Universiti Sains Malaysia, School of Industrial Technology, Bioresource Technology Division, 11800 Penang, Malaysia

2Universiti Sains Malaysia, School of Industrial Technology, Green Biopolymer, Coatings & Packaging Cluster, 11800 Penang, Malaysia

3Universiti Pertahanan Nasional Malaysia (UPNM), Research Centre for Chemical Defence, Kem Perdana Sungai Besi, 57000 Kuala Lumpur, Malaysia

4Universiti Putra Malaysia, Institute of Tropical Forestry and Forest Products (INTROP), 43400 Serdang, Selangor, Malaysia

5Tokyo Institute of Technology, School of Environment and Society, Department of Transdisciplinary Science and Engineering, Meguro, Tokyo, 152-8552, Japan

6Universiti Putra Malaysia, Faculty of Biotechnology and Biomolecular Sciences, 43400 Serdang, Selangor, Malaysia

7Universiti Teknologi Malaysia, Engineering Design Research Group (EDRG), Faculty of Engineering, School of Mechanical Engineering, 81310, Johor Bahru, Johor, Malaysia

8Universiti Teknologi Malaysia, Centre for Advanced Composite Materials (CACM), 81310, Johor Bahru, Johor, Malaysia

1.1 Introduction

Long-chain molecule polymeric materials have benefited from the use of crystallization as a fundamental thermodynamic phase transition in condensed matter physics of pure substances. Keller made the electron microscope findings on polyethylene (PE) single crystals grown in diluted solutions in 1957, following the synthesis of high-density PE with the development of Ziegler-Natta catalysts, thus developed the chain-folding model [1]. Since then, the discovery of diverse polymer crystal morphologies has been aided by the chain-folding concept. Nowadays, semi-crystalline polymers, such as polyolefins, polyesters, and polyamides, account for more than two thirds of all synthetic polymer products produced worldwide due to their numerous uses in our everyday lives. The degree of crystallinity, which normally ranges between 10% and 80%, describes the proportion of organized polymer molecules [2]. Only small-molecule materials, which are often brittle materials, can attain the greater value of crystallinity.

Hu asserts that the chemical structures of repeating units of polymer can be categorized using two distinct contributions to the perseverance of melting points: intramolecular interactions of collinear connection energy of bonds on the chain for thermodynamic adaptability and intermolecular interactions of local bond-bond interactions for the parallel-packing of two neighboring bonds in the conventional lattice models for parallel-packing order [3]. As a result, the melting temperatures of polymers with repeating units that favor greater stiffness or more dense/stronger packing are typically higher. Techniques used to evaluate the crystallinity of polymers include density measurement, X-ray diffraction (XRD), infrared spectroscopy, differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) [4, 5].

Referring to Zhang et al., the mechanical and optical performance of crystalline polymers like PE and polyethylene terephthalate (PET) corresponds with molding parameters that are strongly influenced by their crystallinity [6]. Crystalline polymers undergo stress at freezing and retain stress from crystallization, according to Kato et al. [7]. Due to the lack of appropriate methods for quantitatively evaluating these transitions, the micro-mechanical forces during polymer crystallization remain a highly discussed topic. Up until now, the forms of proof have been theoretical, indirect experimental, or empirical discussions [7]. There are several experimental methodologies and approaches to estimate the amplitude of micro-mechanical forces during polymer crystallization to limit and avoid material failure owing to these forces. This includes non-destructive test [8], destructive test [9], and computer simulation [10]. Between these, non-destructive techniques have been employed to examine the physical relaxation of components during heating and determine their initial stress state, such as holographic interferometry and synchrotron XRD research. Despite the benefits of these techniques being non-destructive, neither a qualitative computation nor a stress visualization can be completed instantly.

Approximately 30-60% of the substance was comprised of polymer crystals, which ranged in size from a few nanometers to several, randomly oriented in space. Because crystalline polymers could withstand loads and act in diverse directions like reinforced rubber, as well as because macromolecules were often much longer than the crystal dimensions. The fundamental understanding that crystals might function as cross-linkers similar to those in cross-linked rubbers [11]. The tensile, microhardness, and compression behavior patterns of semi-crystalline polymers (Figure 1.1b) have been significantly influenced by micro-mechanical forces throughout polymer crystallization through tie chain portions, which appear to be molecular connections between individual crystallites from the perspective of the molecular topology of the amorphous phase (Figure 1.1a). Additionally, tie chain polymer crystallization improves fracture toughness and slow crack propagation resistance [12, 13].

Most molecular-level descriptions of the semi-crystalline phase are based on topological properties, including the theories of tie chain segments, loop segments, tails, and the alternating of crystalline and amorphous domains [14]. Olsson et al. claim that interface Monte Carlo moves are utilized to relocate sites and change chain connections on the atoms and chains in the amorphous domain to produce new loops, tails, and bridges. The resulting samples' crystalline components are still faultless, that is, devoid of twins or dislocations. According to reports, these faults weaken the critical shear stress and weaken slide processes. As a result, the models under consideration are idealizations of a true semi-crystalline PE material, and the anticipated resistance to crystal yielding is anticipated to be larger than what has actually been empirically observed [15].

Figure 1.1 The arrangement of polymer molecular chains (a) in amorphous and (b) in semi-crystalline polymers state.

1.2 Degree of Crystallinity

The degree of crystallinity determines how ordered a solid is structurally; the more crystalline a polymer is, the more regularly its chains are aligned, and the arrangement of atoms or molecules is repeatable and consistent. The degree of crystallization of polymer materials has a big impact on their characteristics. In terms of performance, a molded part is stiffer, stronger, but also more brittle the more crystallization there is. Hardness, density, transparency, and diffusion are all significantly influenced by the degree of crystallinity. Chemical composition and thermal history, such as cooling conditions during manufacturing fabrication process and post-thermal treatment, have an impact on the degree of crystallization. However, the characteristics are also influenced by the size of the structural units or the molecular orientation in addition to the degree of crystallinity [16, 17]. In general, a higher degree of crystallinity is typically the result of variables that make polymers more regular and organized because fewer short branches allow molecules to pack more tightly together. Syndiotactic and isotactic polymers have a higher degree of stereoregularity than atactic polymers, but the polymers are also more organized and have regular copolymer structures [18]. Based on the study by Yao et al., it was discovered that a rise in crystallinity directly correlated with an improvement in mechanical characteristics by examining the effects of various crystallization parameters, such as crystal shape ratio and crystallinity [19]. The PET crystal structure ratios did not, however, substantially enhance the mechanical characteristics. Furthermore, at a higher isothermal temperature, considerably higher than the Tg, the crystallinity of PET foam will be strongly increased. Slow crystallization can be used to explain the increase in crystalline content at higher temperatures, which promotes regular chain folding and subsequently reduces topological disorder at the surface of the crystallites. According to Jonas et al., the relationship between the service temperature and crystallinity is strong within the experimental range of 10-150 °C. When the operating temperature is close to or higher than Tg, migration causes isothermal-induced crystal perfection, and rejection of the structural faults at the crystal's surface causes a rise in the crystalline phase content [20]. The mechanism of crystallization enhanced development from the amorphous state to the crystalline state in isothermal treatment above Tg is schematically depicted in Figure 1.2. The delayed crystallization promotes better crystal lamella development and chain refolding, as seen in Figure 1.2.