Investigation on the growth of snowflake-shaped Poly(l-Lactic acid) crystal by in-situ high-pressure microscope

Highlights

• An in-situ system was developed to observe PLLA crystal growth in pressurized CO₂.

• The whole growth process of PLLA snowflake crystal was recorded for the first time.

• The branch root from twin crystal induced from self-generated concentration field.

• The pressurized CO₂ plays a nucleation-limit role in crystallization of polymer film.

Abstract

Crystallization of polymer film is a widely concerned condensed matter phenomenon directly determining the film properties and performances. However, challenging on understanding of the crystal growth process of polymer thin film still remains. Here, we developed an in-situ high-pressure observing system to record the crystal growth process of Poly(l-Lactic Acid) (PLLA) film in pressurized CO₂ environment with different film thicknesses, molecular weights, temperatures and CO₂ pressures. The preliminary stage of morphological evolution of snowflake-shaped crystal was recorded for the first time. Based on the morphological feature and microscopic structure, we found that the snowflake-shaped crystal with six main branches is constructed by multilayer rhombic terrace crystal originated from screw dislocation, and the six main branches are not formed at the same time. Two of them are preferentially formed on the long-axis directions of rhombic crystal, while the other four main branches are subsequently formed. The subsequently formed branches root from twin crystal induced from self-generated concentration field. The growth process of snowflake-shaped PLLA crystal is quite different from that of snowflake-shaped H₂O crystal, but still meets the self-similarity principle and iterative generation principle of the fractal theory.

Introduction

Crystallization of polymer molecules from melt film is a widely concerned condensed matter phenomenon [1] that can realize the closely packing of the long-chain molecular and functional group. The crystal morphology directly influences the chemical and physical characters of polymer film [1,2], and the morphology control plays a key role in functional film engineering [3]. So, understanding of the dependence of polymer film morphology on controlled variables and the crystal formation mechanism from melt film is of great significance.

The crystal of polymer film generally shows various morphologies including spherulite [4], seaweed-shape crystal [5], petal-like crystal [6] crystallographic dendritic crystal [7], polygonal single crystal [3,8] and so on. Among these crystal morphologies, the crystallographic dendritic crystal, believed as the transition morphology from non-crystallographic crystal to single crystal [[8], [9], [10], [11]], is receiving considerable attention. Snowflake-shape crystals were prepared by controlling thickness and temperature of isotactic Polystyrene (iPs) film [9] and Poly(l-Lactic Acid) (PLLA) film [15]. Wang et al. [[11], [12], [13], [14]] prepared Poly(ethylene oxide) (PEO) dendritic crystal with four growth directions through tune film thickness, temperature, and molecular weight. Based on the understanding on the interplay between crystallization and phase separation [16,17], crystallographic dendritic crystal was formed in the polymer blend system, like PEO/Poly(methyl meth acrylate) (PMMA) [18], PLLA/Poly(1,4-butylene adipate) [19,23], Tannin/Poly(ethylene succinate) [20], Poly(d-lactic acid)/Poly(1,4-butylene adipate) [21] and PLLA/PEO [22]. Furthermore, the crystallographic dendritic crystal of block copolymer was also induced by phase separation [[24], [25], [26]].

The formation process of these aesthetic structures also elicits a widespread interest. It is well known that the six main branches of snow originated from the six preferentially growing directions of hexagonal water single crystal [27]. Same with the growth mode of water molecule, the faceted structures assembled by PEO, Poly-2-vinylpyridineblock-polyethyleneoxid [28], iPS [29] and Polyoxymethylene [30] also developed from the preferentially growing directions of quadrangular crystal or hexagonal crystal. But there is an exception to this rule. Based on the previous research on PLLA crystal, at a relative low temperature of crystallization, the PLLA molecules will assemble a rhombic single crystal with two preferentially growing directions [[31], [32], [33]]. Interestingly, the rhombic crystals assemble a snowflake-shaped crystal with six main branches [19,21,22,34]. Through phase-separation-induced crystallization, Nurkhamidah and Woo [19] proposed a growth and packing mode of single crystals into snowflake-shaped dendritic crystal. Generally, the research method to speculate the growth process of polymer crystal according to the final crystal morphology is called as inversion method (sometimes called as archaeological method). The existing understanding of crystal morphology evolution of PLLA thin film is based on this inversion method, and the growth mode of snowflake-shaped PLLA crystal is still questionable.

In order to study the growth of snowflake-shaped PLLA crystal, it is needed to develop new methods and new tools. For many polymer materials, CO₂ is a kind of poor solvent which can swell but not dissolve the most of polymer [35]. Previous researches indicated that CO₂ can influence the glass transition temperature, melt temperature, supercooling degree [36,37], crystallization rate [38], crystallinity [39] and mesophase [40,41] of polymer bulk. For the polymer thin film, CO₂ molecules can quickly embed into the film or escape from the film due to the feature of gas-like viscosities and liquid-like densities, especially in the supercritical state (temperature higher than 31 °C and pressure higher than 7.38 MPa) [35]. Based on the previous research, CO₂ has been proved as a gentle tool to tune the crystallization behavior of polymer. Moreover, the pressurized CO₂ can slow down the growth rate of polymer crystal [36] which provides a window to present more detail of the crystal growth process. However, the morphology and the crystal growth of polymer film are influenced by many factors including polymer molecular structure and environmental variants. In this way, to reveal the whole formation process of snowflake-shaped crystal, a reasonable crystallization condition should be firstly determined by considering PLLA degradation, observable crystal size, and recordable growth time.

In this paper, we developed an in-situ high-pressure observing system with special designed sample cell whose temperature and pressure can be close-loop controlled. The crystal growth process of Poly(l-Lactic Acid) (PLLA) film was recorded by this system in pressurized CO₂ environment with different film thicknesses, different molecular weights, different temperatures and different CO₂ pressures. With a reasonable selection of experimental parameters, the preliminary stage of formation process of snowflake-shaped crystal was recorded by the self-established system. With the help of atomic force microscope, we tentatively concluded the growth mode of the snowflake-shaped PLLA crystal.