The nucleation, crystallization and dispersion behavior of PET–monodisperse SiO2 composites

doi:10.1016/j.polymer.2007.03.059

Polymer

Volume 48, Issue 11, 21 May 2007, Pages 3324-3336

https://doi.org/10.1016/j.polymer.2007.03.059 Get rights and content

Abstract

To more accurately investigate the nucleation, crystallization and dispersion behaviors of silica particles in polymers, the composites of PET with monodisperse SiO₂–PS core–shell structured particles were prepared with SiO₂ size from 380 nm to 35 nm.

For these SNPET samples, DSC results showed that the nucleation rate of silica particles increased as their size decreased, in which 35 nm SiO₂ particles produced the most obvious nucleation effect. At 2.0 wt.% load of 35 nm silica, Avrami equation proved that the isothermal crystallization rate G of SNPET was ca. 30% higher than that of pure PET and the crystallization activation energy for SNPET was −218.7 kJ mol⁻¹ lower than −196.1 kJ mol⁻¹ for PET. While, the non-isothermal crystallization ΔE for SNPET was −199.8 kJ mol⁻¹ lower than −185.5 for PET.

On non-isothermal crystallization, Jeziorny equation presented the primary and secondary crystallization stages in PET and SNPET, in which nano SiO₂ accelerated the crystallization rate. Their Ozawa number m was from 2.1 to 2.7, which was smaller than that of Avrami number n.

The nucleation and dispersion behaviors of SiO₂ particles were directly observed. POM results demonstrated that SNPET samples crystallized more quickly from melt and their crystallization rate increased as silica load increases but accelerated at 2–3 wt.%. The spherulites grew well in PET but their size was smaller in SNPET due to the silica barrier on their growth. SEM and TEM observed the homogeneous silica dispersion morphology and the vivid ordered patterns formed in SNPET. The monodisperse particles are highly expected to give more accurate and valuable references than multi-scale ones in obtaining novel advanced PET 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 (M_W = 47,000, M_W/M_n = 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 (T_m⁰)

When annealing at temperatures between T_g and T_m, 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 T_m⁰ for enough time. At T_m⁰ temperature, polymer crystallites melt completely and have the minimum free energy. T_m⁰ 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 ΔT_cc and the supercool temperature ΔT_mc can characterize the sample crystallization and nucleation effect. ΔT_cc = T_cc − T_g, where T_g, glassy transition temperature; T_cc, cold crystallization peak. ΔT_mc = T_m − T_mc, where, T_mc, melting crystallization peak; T_m, melting peak.

Usually, the lower ΔT_cc or ΔT_mc 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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The nucleation, crystallization and dispersion behavior of PET–monodisperse SiO2 composites

Abstract

Introduction

Section snippets

Materials

Preparation of monodisperse core–shell structured particles

Multi-melting and equilibrium melting (Tm0)

The effect of SiO2 size on the crystallization and melting behaviors

Conclusion

Acknowledgement

Polymer

Polymer

Polymer

Eur Polym J

Acta Polym Sinica

J Chem Phys

Polymer

Eur Polym J

Macromolecules

Macromol Theory Simul

Anal Chem

J Polym Sci Part A2 Polym Phys

J Polym Sci Part A2 Polym Phys

Polym Compos

J Appl Polym Sci

Chem Mater

Appl Surf Sci

J Colloid Interface Sci

J Appl Polym Sci

J Appl Polym Sci

Polymer

The nucleation, crystallization and dispersion behavior of PET–monodisperse SiO₂ composites

Multi-melting and equilibrium melting (T_m⁰)

The effect of SiO₂ size on the crystallization and melting behaviors