Topic 7: Directly Observing Nucleation and Producing Nano-Oriented Crystal (NOC) Polymers
Supercritical Elongation-Induced Crystallization Creates Plastic Stronger than Steel
“Why is plastic weaker than steel?” Although common knowledge prevents most people from posing this question, Dr. Masamichi Hikosaka (Professor, Hiroshima University, Japan) and colleagues set out to answer this question. They employed the highly brilliant and precise X-rays at SPring-8, which can closely analyze processes of polymer crystallization and the internal structures of polymers at the nanometer level (10-9 m), along with quiet dedication to successfully develop a plastic that is much stronger than steel in terms of unit weight. They termed this plastic “nano-oriented crystals (NOCs).” Amazingly, the production cost of NOCs is equivalent to that of commodity plastic. Hence, their efforts may cause a new paradigm in the basic materials industry.
Unraveling the Mystery of Polymer Crystallization - A Reckless Attempt
Since the 1930s, the theory that when a material crystallizes, a nanometer-sized nucleus (nano-nucleus) is initially created, which then grows into a larger crystal, has been commonly accepted. Moreover, when a liquid is cooled below its freezing point (supercooling state), it solidifies, specifically by crystallization. However, the mechanisms of nucleation remained a mystery. A nano-nucleus, or “a baby crystal,” is extremely small and rare, and exceedingly difficult to demonstrate.
Most researchers have long given up demonstrating the existence of a nano-nucleus. However, Dr. Hikosaka has continued his quest. In 1987, he hypothesized the sliding diffusion theory of polymer crystallization; entangled long string-like molecules untangle themselves, slide along a crystal lattice like a snake, and arrange themselves to form a crystalline polymer. In general, crystalline polymers such as polyethylene and polypropylene, which are composed of long string-like molecules, are finely folded in equal intervals to form a plate structure (Fig. 1). Dr. Andrew Keller (University of Bristol, UK), a leading authority in polymers, discovered a folded chain-polymer crystal in 1957, and an “extended chain-polymer crystal” was identified in 1964. Dr. Hikosaka’s sliding diffusion theory of polymer crystallization attempts to explain both types of crystals in an integrated manner. Originally his theory was a global controversy, but today is widely accepted with the support of Dr. Keller.
Dr. Hikosaka was 43 years old when he proposed the sliding diffusion theory of polymer crystallization. However, he himself could not confirm the initial state of polymer crystallization. “It was 1992 when I decided to try to confirm the entire process; from nucleation to crystallization. However, everybody said I was crazy,” recalls Dr. Hikosaka. Major properties of solid materials are determined in the early stage of crystallization. However, if the crystallization process can be controlled at the nano-nucleus level, then the development of previously unknown materials would be possible. Hence, revealing of the mechanisms of nucleation is industrially significant.
In polymers, polymer chains are often folded to form a 10-nm thick folded chain crystal. This crystal further forms an amorphous layered structure where the layers grow to construct a 100-μm sized golf ball-like giant crystal, called a spherulite. Herein, the crystallinity of a spherulite is less than 50%, which results in a weak solid.
Revealing the Behavior of a Nano-Nucleus for the First Time
Small-angle X-ray scattering (SAXS) is the only promising observation technique to detect nano-nuclei. However, the X-ray intensity available at the existing synchrotron X-rays was too low to distinguish valid data from noise. Thus, Dr. Hikosaka put his hope on the highly brilliant synchrotron X-rays at SPring-8. In 2002, he examined nano-nucleation of a supercooled polyethylene melt by irradiating X-rays in the Structure Biology II Beamline (BL40B2) at SPring-8. The melt is liquid where only pure material is being melted. As expected, SPring-8 yielded much clearer data than ever before. However, the number density of the produced nano-nuclei was so low that the data statistics were poor.
Thus, innovation was needed to produce sufficiently high-density nano-nuclei. Dr. Hikosaka thought of utilizing nucleating agents (NAs), which are crystals to promote nucleation. If nuclei are created on NA particles, they can easily start growing. This is the same mechanism where dust promotes cloud formation. However, it is very difficult to uniformly distribute NAs because a polymer melt is highly viscous, and a non-uniform production of nano-nuclei would result in unreliable data. However, Dr. Kiyoka Okada (Hiroshima University), a former fourth-year undergraduate student in Dr. Hirosaka’s lab, volunteered to research producing a uniform distribution of NAs. Eventually, she developed a technique to mix solvent, polymers, and NAs using ultrasound. “I dedicated myself to this research for more than a year beginning in 2003, and finally succeeded in increasing the production of nuclei more than 10,000 times,” recalls Dr. Okada. Ultimately, highly brilliant X-rays were irradiated on the polyethylene melt in which NAs were uniformly distributed.
The increased scattering intensity confirmed in real time that various sized nuclei (> 1 nm) are produced, and this is the first observation of nano-nucleation. This analysis revealed that nano-nuclei are being continuously produced, but most disappear instantaneously. Only one in one million nuclei survives. Additionally, nucleus size measurements indicate that initially the number of nano-nuclei rapidly increases, but as the larger nuclei are produced the rate decreases (Fig. 2). This research became Dr. Okada’s doctoral dissertation, and was published in Polymer (2007), drawing international attention.
a Size distribution of nuclei with time where the longitudinal axis represents the size distribution of nuclei f (N, t) and the horizontal axis represents time t. Nuclei sizes (N) are represented as repeating molecular units. Initially the number of small nuclei (N = 20) rapidly increases but slowly increases with large nuclei, revealing nucleation processes for the first time.
b Schematic views of nucleation in a melt. Numerous small nuclei are initially created, but as large nuclei form the rate decreases.
Brilliant Idea - Crushing Polymer Melt
Because a crystalline polymer is a long string-like molecule, the molecules tend to become entangled in the melt. Thin plates with folded chain-structures form a layered conformation, which becomes a golf ball-like crystalline body called a “spherulite” (Fig. 1). More than half of the content inside a spherulite cannot crystallize, and it becomes a solidified amorphous instead (Fig. 1). This high amorphous fraction is thought to cause the low strength and low heat resistance of plastic. Hence if the amorphous fraction can be reduced to nearly zero, the properties of the polymer should significantly improve. However, reducing the amorphous fraction proved to be extremely difficult.
Then Dr. Hikosaka conceived the idea to “stretch” the polymer, which causes randomly directed crystals or molecules to point in the same direction (or “orientation”) because an entangled string can assume a linear form by pulling it left and right. Stretched and oriented string-like molecules are easily crystallized, significantly reducing the amorphous fraction. However, it is impossible to pull liquid. “So we tried crushing the melt,” recall Drs. Hikosaka and Okada. They instantaneously applied high pressure to a melt that was poured into a long and narrow channel spanning to the left and right. This produced a rapid current in the melt, which expanded to the left and right, stretching the string-like molecules (analogous to clothes stretching when exposed to a rapid stream), and realizing a high orientation. In their experiments, high pressure, which corresponded to a stretching force with 1,000-fold stretch per second, was applied to the melt poured in a channel. The flow rate of the melt, which was induced by this stretching force, was termed “elongational strain rate.”
This experiment revealed an interesting phenomenon; above a critical elongational strain rate, the crystallization speed increases one million-fold, even at the same temperature. It is hard to imagine such a drastic change occurring within the same material. To unravel the mechanisms of this phenomenon, Dr. Hikosaka and colleagues stretched a polypropylene melt at rates above the critical elongational strain rate to create a crystalline solid, which they examined at SPring-8 in BL40B2. They found that polymer chains in the melt arranged in parallel to form an almost perfectly oriented melt, and infinite number of nuclei were created, which grew to form nanocrystals on the millisecond order. The final product was a solid of which 90% were crystallized. This is the birth of NOCs.
Although NOCs are paper thin, their stretching fracture strength is 2-5 times higher than that of steel with the same weight. In addition to a higher transparency, its heatproof temperature is 176 °C, which is more than 50 °C higher than that of common polypropylene. Furthermore, its production costs are comparable to existing plastics, and more than 90% of the body may be recyclable. Hence, NOCs are an economical, ultra-high performance polymeric material.
Nanocrystals are stronger than steel because they are rigidly bound by a “string” (Fig. 3). Dr. Hikosaka has attributed some of his success to SPring-8, “X-ray diffractometry at SPring-8 has revealed that more than 100 crystals measuring 20-30 nm are bound by a single 2-μm long string-like molecule chain, which is formed with a strong force (covalent bond).” Details of this NOC production were reported in Polymer Journal (June 2010), and patent applications were filed in Japan and internationally. NOCs can replace steel and ceramics in various products, including automotive steel sheets, and have the potential to realize energy and resource conservation. Thus, researchers are working toward practical applications of NOCs in a wide range of areas with the support of the Japan Science and Technology Agency(JST). The 40 years of effort that Dr. Hikosaka and his colleagues have dedicated to polymer crystallization is beginning to produce revolutionary results.
a NOC has a structure where a 2-μm long string-like molecule chain, which has the equivalent strength of diamond, binds about 100 nanocrystals. This structure is called an “armor model.” NOCs exhibit almost 100% crystallinity.
b X-ray diffraction images reveal NOC creation (left: WAXD, right: SAXS). Nanocrystal has a diameter of 26 nm.
References
1. K. Okada, K, Watanabe, I. Watanabe, A. Toda, S. Sasaki, K. Inoue and M. Hikosaka; Polymer, 48, 382 (2007)
2. K. N. Okada, J. Washiyama, K. Watanabe, S. Sasaki, H. Massunaga and M. Hikosaka; Polymer J., 42, 464 (2010)