The nucleation, crystallization and dispersion behavior of PET–monodisperse SiO2 composites
Introduction
Poly(ethylene terephthalate) (PET) has molecular chains connected with aromatic, ethyl and ester groups. The twist or torque motion between PET rigid aromatic segments and ethyl connector greatly slows down its crystallization rate and prolongs its processing cycle. Comparatively, the maximum grown rate of the spherulites for PE is 5000 μm min−1, while it is only 10 μm min−1 for PET [1]. So, the diverse traditional nucleators, several nanoparticles and fibers [2], [3], [4], [5], [5](a), [5](b), [6] were taken into consideration to improve PET crystallization behavior. Montmorillonite (MMT) of layered silicates was taken as nucleation agent to prepare PET–MMT nanocomposites through in situ intercalation polymerization. These composites have 100–300% higher crystallization rate than that of pure PET [7]. So far, several similar composites (e.g., PBT–MMT, PE–MMT, PP–MMT, PA–MMT, PEO–MMT, etc.) [8], [8](a), [8](b) have been prepared for practical applications.
While, it was found that this or other problems occurred when these PET–MMT composite products were applied to functional films, barrier bottles, etc. How to balance between enhancing PET crystallization rate and keeping its original overall properties is thought to be one of the key problems, which relies on the control of the nucleation and growth process of crystallization (N & G) [5], [5](a), [5](b). In many practices of using multi-scale silica particles as functional fillers [4], [5], [5](a), [5](b), [6], [7], [8], [8](a), [8](b), [9], [10], it was found that, as silica surface rich in hydroxyl groups, they easily formed chain-like structure through different hydrogen bonds and/or 3D net-like structure or aggregated particles. So, SiO2 particle surface needs covering with some organic compounds, oligomer or polymers in order to improve their dispersion behavior.
It was seen that good dispersion of SiO2 particles created the effective nucleation and growth of crystals in PET. Thus, the monodisperse core–shell structured SiO2–PS (polystyrene) composite particles were designed and prepared by dispersion polymerization techniques [11]. The covered PS shell layer (ca. 10 wt.%) in the core–shell composite particles serves as media for SiO2 particles to disperse well in PET and especially balance the silica nucleation effect. Their crystallization behavior under the isothermal or non-isothermal crystallization process [9], [10], [11] was designed to be quantitatively characterized with differential scanning calorimeter (DSC). With the established dynamics, Avrami equations [12], [13] quantitatively describe the isothermal crystallization process and its corrected forms, or Jeziorny [14], [15] and Ozawa theories [16], [17] are for the non-isothermal crystallization process.
In this paper, to more accurately investigate the particle nucleation and dispersion behaviors, the monodisperse SiO2 particles were specially adopted to prepare the core–shell SiO2–PS particles, and their nanocomposites with PET through simple mixing method. Their nucleation and crystallization behaviors under isothermal and non-isothermal conditions were quantitatively investigated. Their dispersion and order patterns were observed with SEM and POM. It is greatly expected that, through monodisperse particles with strictly controlled size deviation and their core–shell structure, the nucleation and crystallization behaviors of PET would be more accurately presented and thus more valuable references to advanced PET–silica composites would be provided.
Section snippets
Materials
Polymer of poly(ethylene terephthalate) (PET) with intrinsic viscosity of 0.650 dL g−1 (MW = 47,000, MW/Mn = 2.4) was acquired from LiaoYang Petrochemical, PetroChina, and γ-methacrylic propyl trimethoxysilane (MPS) from C.P, Beijing ShengDa Fine chemical. Co. Ltd. The related reagents, e.g., polyvinyl pyrrolidone (PVP), and solvents were acquired from Beijing Yili Fine Chemical Co. Ltd.
Preparation of monodisperse core–shell structured particles
Preparing monodisperse SiO2: monodisperse SiO2 particles with size of 35–380 nm, homemade in our lab [18]. The
Multi-melting and equilibrium melting (Tm0)
When annealing at temperatures between Tg and Tm, DSC patterns of PET–silica composites appeared double melting behavior as reported [23], [24], which was thought to be resulted from multi-scale crystallites formed during the annealing process. Such double melting behavior disappeared when the sample annealed at Tm0 for enough time. At Tm0 temperature, polymer crystallites melt completely and have the minimum free energy. Tm0 is related to the multi-scale crystallites and defects formed during
The effect of SiO2 size on the crystallization and melting behaviors
The physical and thermal parameters of DSC patterns for SPET and SNPET samples are summarized in Table 1. Generally, the superheat temperature ΔTcc and the supercool temperature ΔTmc can characterize the sample crystallization and nucleation effect. ΔTcc = Tcc − Tg, where Tg, glassy transition temperature; Tcc, cold crystallization peak. ΔTmc = Tm − Tmc, where, Tmc, melting crystallization peak; Tm, melting peak.
Usually, the lower ΔTcc or ΔTmc predicts the higher nucleation and crystallization rates.
Conclusion
In quantitative investigation of monodisperse core–shell SiO2–PS structured particles and their nanocomposites with PET, conclusions were reached as below:
At isothermal crystallization, the crystallization rate G for SNPET was greater than that of PET, while its ΔE was smaller than that of PET. At non-isothermal crystallization, G for SNPET was also greater than PET, while its ΔE was smaller than that of PET. Ozawa number m for PET was 2.4–2.7, compared with m of 2.1–2.6 for SNPET, both of
Acknowledgement
This research financially supported by PetroChina under contract no. 030407-06WT is greatly appreciated.
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