Kinetics of the thermally induced precursor curing to polymer via statistical analysis of TEM images
Introduction
The curing is a process of deliberately cross-linking a low molecular weight precursor (e.g., liquid oligomer) to a hard and stiff polymer [1]. The term is practically referred to the cross-linking of unsaturated polyester [2] and epoxy resins [3], which used in many industrial applications, especially as matrices in reinforced composites. Recent investigations have been paid to create novel precursor compositions exhibiting improved properties after their curing. One of such a composition is known as the Rolivsan [4].
Of course, any changes in the chemical structure of a precursor during its transformation to a cured polymer cause variation of its morphology. It has long been realized that morphology is a key to understanding material properties. At the moment, microscopic techniques seem to reveal detailed micro-structure of a sample very carefully. Yet, quantitative interpretation of microscopic images is not always easy though many useful mathematical procedures for the image analysis have been elaborated and a qualitative correlation of material properties and parameters of mathematical morphology have been established [5].
The goals of the work are: (i) identification and size evaluation of the morphological entities in the Rolivsan precursor during its conversion to the polymer, polyrolivsan (PR), under heating; (ii) description of statistical size distributions of the morphological entities using principles of irreversible thermodynamics; (iii) analysis of the distribution parameters as a function of the curing temperature; and (iv) finding a correlation of micro-structural and macroscopic parameters varying during the precursor curing.
Section snippets
Experimental
The Rolivsan is a liquid precursor mixture containing the monomers M1 (14%), M2 (27%), and M3 (25%); the oligomer Ol (32%); and an antioxidant (2%). Chemical structures of the precursor components are presented below:
Being thermally cured, the Rolivsan precursor transforms to the thermally stable cross-linked polymer, PR. The primary polymer network obtained at 150 °C is converted upon further heating (⩾170 °C) to the final network. Its chemical structure can be presented as follows [4]:
Samples
Model
Based on principles of irreversible thermodynamics, the model of reversible aggregation gives a general characterization of micro-structure in different systems [6], [7], [8]. According to the model, stationary micro-structures are created by linking the energy-equivalent dynamic units in metastable clusters called aggregates. Aggregates are characterized by a definite lifetime, i.e., they are permanently formed and decomposed; this is a condition of the aggregates reversibility. In the liquid
Results
Fig. 1 shows representatively TEM images of the replicated PR samples cured at Tc = 150 and 250 °C. The PR morphology is seen to be formed by the ordered regions with strong molecular interactions (a set of densely packed micro-domains) and disordered regions with weak interactions (inter-domain space).
Fig. 2 depicts the resulting treatment of the TEM images in the form of the statistical distribution of the micro-domain diameter d. The histograms demonstrate that micro-domains form a statistical
Discussion
Kinetics of the thermally induced precursor curing is evidently indicated by the evolution of the statistical distribution parameters with the curing temperature (Fig. 3). A change in the distribution parameters demonstrates an improvement of the supramolecular structure of the polymer upon heating: the reduced aggregation energy, Δu0/kT, increases (i.e., statistical distribution becomes narrower) whereas both the mean micro-domain size, 〈d〉, and the embryonic size, d0, decrease with increasing
Conclusions
Thermally induced transformation of the monomer–oligomer mixture to the cured polymer was studied. Using TEM, the cured polymer was shown to have micro-domain morphology. The analysis of the frequency distributions of micro-domain diameters allowed the conclusion that at each curing temperature, micro-domains form a statistical ensemble described subsequently using the model of reversible aggregation. This means that the polymer morphology during the processing is optimized in sense of
References (16)
Sov. Phys. Sol. State (Fiz. Tverd. Tela Trans.)
(1983)Unsaturated Polyesters: Structure and Properties
(1964)- et al.
Russ. J. Appl. Chem. (Zh. Prikl. Khim., Trans.)
(2003) Image Anal. Stereol.
(2004)- et al.
J. Chem. Phys.
(1997) - et al.
Progr. Colloid Polym. Sci.
(2001)
Cited by (5)
Nature of the liquid-crystalline phase in supramolecular linear polymers
2008, Polymer Science - Series BKinetics of the growth of the ordered phase at isotropic liquid-nematic phase transition in mesomorphic dimers
2007, Russian Journal of General Chemistry